Titania nanoarchitectures have been intensively studied as a mean of light-harvesting molecular system. In particular, the production of molecular hydrogen from aqueous suspension of sensitized titania has received much attention in the aspect of solar fuel. Despite simplicity and scientific interest, however, such system has suffered from instability and low efficiency; the latter results primarily from the fast charge recombination. Very recently, we synthesized well-aligned titania nanofibers (TNFs) and nanotubes (TNTs) and found that they exhibited surprisingly high photocatalytic activities for hydrogen production from water under ultraviolet. Such the high activities were attributed to the inhibited charge recombination by spatially separating a charge carrier (e.g., hole) from a counter charge carrier (e.g., electron). We have further studied the enhanced charge transfer occurring at TNFs and TNTs by employing dye and Q-dot-sensitized systems and applied them to visible-light induced hydrogen production. Similarly to the titania/UV case, TNFs and TNTs had much larger amounts of hydrogen than titania nanoparticles. More detailed study indicated that the primary factors for such enhancement were facilitated interpartricle charge transfer and many specific active sites for proton reduction.

Titanium dioxide (TiO₂) is one of the most widely studied photocatalysts for energy applications. It is also a promising candidate for the development of photochemical electrodes for water splitting and solar hydrogen production¹ as well as dye sensitized solar cells.² For energy applications, nanostructured TiO₂ is a very attractive material due to its large surface area and high electrochemical catalytic activity. In addition, TiO₂ is cheaper than other photosensitive materials and has also outstanding resistance to photocorrosion in aqueous solutions over a long period of time.³ Recently, one-dimensional TiO₂ nanostructures such as nanorods, nanotubes, and nanowires have emerged as promising building blocks for the new generation of nanoscale devices.⁴ Among them, we focused on extremely small diameter (~8 nm dia), high-aspect-ratio (~1-10 μm Length) TiO₂ nanotubes. The 8 nm dia nanotubular structure we fabricated is suitable to achieve larger enhancement of the surface area without increasing the geometric area.⁵ We have investigated vertically aligned TiO₂ nanotubes with different lengths and densities for water splitting. Non-aligned TiO2 nanotube array layers were also fabricated by using hydrothermal processing under different conditions. These TiO₂ films were characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM).⁶ In this water splitting experiment, we have conducted a comparative study of the electric properties of titania based materials under chemical bias as well as a investigation of their photo-responses. High density TiO₂ nanotubes were active photocatalysts well suited to solar fuel production. This study allowed us to tailor nanostructured electrode to the solar energy applications. References 1. A. Fujishima, and K. Honda, Nature, 1972, 5358, 37. 2. B. Oâ?TRegan, and M. Grâ?³atzel, Nature, 1991, 6346, 737. 3. J. Nowotny, T. Bak, M. K. Novotny and L. R. Sheppard, Int. J. Hydrogen Energy, 2007, 32, 2609. 4. Z. Zhang, M. F. Hossain, and T. Takahashi, Int. J. Hydrogen Energy, 2010, 35, 8528. 5. H. Kim, K. Noh, C. Choi, J. Khamwannah, D. Villwock, and S. Jin, Langmuir, 2011, 27, 10191. 6. K. S. Brammer, H. Kim, K. Noh, M. Loya, C. J. Frandsen, L. Chen, L. S. Connelly, and S. Jin, Adv. Eng. Mater., 2011, 13, B88.

TiO2 nanorod (TNR) photoelectrode is one of attractive materials for solar water splitting into hydrogen and oxygen because it has an efficient charge transfer property along the one-dimensional nanostructure.[1, 2] However, only a few papers with solar water splitting using TNR photoelectrode are reported so far. Therefore, we have investigated the influence of TNR photoelectrode structure on water splitting properties of them. The vertically aligned TNR (Rutile) array on FTO glass was prepared by the hydrothermal synthesis according to the literature.[3] Different kinds of TNR photoelectrodes were prepared by changing preparation conditions such as concentration of precursors, synthetic temperature and time, and calcination temperature and time. Water splitting property of TNR photoelectrode was determined by observed water splitting photocurrent in the photoelectrochemical cell which composed of TNR photoelectrode, Pt wire counter electrode, Ag/AgCl reference electrode and 0.1M-NaOH aqueous electrolyte solution. Irradiation source was a solar simulator with 100mW/cm2 and AM1.5. XRD, UV-vis., SEM and XPS were used for characterization of TNR photoelectrode. Water splitting property was dramatically changed by preparation condition of TNR photoelectrode. The best water splitting photocurrent was 1.1 mA/cm2 at the applied bias of 1.5V vs NHE. This photocurrent was about 9 times higher than that of mesoporous TiO2 (Anatase) photoelctrode prepared by TiO2 particles with diameter of 20 nm under the same measurement condition. Solar to hydrogen conversion efficiency (ηSTH) was 0.5% in this case. The positive correlation between photocurrent and the intensity of (101) plane of TNP photoelectrode determined by XRD was observed. It was suggested the decrease of photocurrent with the increase of (002) plane intensity was due to decrease of surface area of TNR photoelectrode with a compact-packing-structure. The tandem cell composed of the optimized TNR photoelectrode and a two-series-connected dye-sensitized solar cell, which could afford 1.5 V to TNR photoelectrode, was applied to solar water splitting. Overall efficiency for water splitting (ηSTH) was 1.3%. It has proved that TNR (Rutile) photoelectrode is very effective for solar water splitting compared with the mesoscopic TiO2 (Anatase) particulate photoelectrode. References [1] X.Feng, K.Shankar, O.K.Varghese, M.Paulose, T.J.Latempa, C.S.Grime, Nano Lett.,8, 3781(2008). [2] J.Hensel, G.Wang, Y.Li, J.Z.Zhang, Nano Lett., 10, 478(2010). [3] B.Liu, E.S.Aydil, J. Amer. Chem. Soc., 131, 3985(2009).

Photocatalyzed water splitting has received considerable attention as a clean, abundant and renewable strategy in which to address both the energy crisis and environmental concerns over the use of fossil fuels. Efficient, stable, chemically inert, low-cost, and nontoxic photoactive semiconductor materials are essential to the success of water splitting. Titanium dioxide (TiO2), due to its excellent solid-state physical-chemical properties, has demonstrated a wide range of applications in hydrogen production, lithium-ion batteries, fuel cells, gas sensors, detoxification, photovoltaics, photocatalysts, and supercapacitors. The one-dimensional (1D) morphology, such as a TiO2 nanowire (NW), is considered as a superior candidate for achieving higher performance in those applications compared to the bulk form. The high crystal quality of the NWs is essential to reduce the scattering effect and hence improve the electron mobility compared to the porous TiO2 films composed of nanosized particles. Recently, we developed a surface reaction-limited pulsed chemical vapor deposition (SPCVD) technique that can grow highly uniform anatase TiO2 nanorods (NRs) over a large area, even inside highly confined submicrometer-sized spaces or on densely-packed Si NW forest. Here, we report the growth of unique TiO2 nanostructure using this technique by simply modifying purging and pulsing conditions in the SPCVD process. A necklace TiO2 nanostructure was obtained. The resulting TiO2 nanostructure exhibited high-quality crystallinity and large surface areas with exposure of high-index surfaces. Control experiments illustrated the influence of deposition conditions on the TiO2 growth behavior. Photoactivity of the TiO2 nanostructure and the stability of the necklace structure under intensive and long light illumination have been also demonstrated. This research enriched our knowledge of the SPCVD technique and demonstrated the capability of using this technique to control the TiO2 morphology and improve its photo response.

We fabricated highly crystalline anatase thin films on single crystalline Nb-doped TiO2 (NTO) substrates via two-step sol-gel process. The epitaxy and morphology of anatase films were controlled by changing pH of initial precursor solution. At relatively high pH (over pH 10), the anatase films were found to possess epitaxy with NTO substrate, characterized by field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD). At pH 9, polycrystalline anatase thin films were observed. The water splitting properties of both polycrystalline and epitaxial anatase films were also characterized using gas chromatographic analyzer, which was discussed in terms of charge transport and surface area of active sites on each film.

Titania nanoparticles have been extensively studied as the most popular photocatalyst for solar energy utilization. TiO2 has established itself as the most popular environmental photocatalyst among many semiconducting materials because of its practical merits such as high oxidation power of holes, photochemical and chemical stability, abundance and easy availability, non-toxicity, and low material cost. Despite their popularity as photocatalytic nanomaterial, breakthroughs in materials development have yet to be achieved for practical applications. In particular, the low photonic efficiency and the lack of the visible light activity are the most critical drawback of TiO2. A variety of chemical approaches have been investigated to modify titania using various inorganic and organic materials for the higher photocatalytic efficiency and visible light absorbing ability. In this talk, various methods related with visible light activation and modification of TiO2 will be introduced. The specific examples include the dye-sensitized TiO2, doped TiO2, fullerol-anchored TiO2, graphene/TiO2, charge transfer complexation on TiO2, semiconductor coupling, and nafion-coated TiO2. Each chemical modification method has a different effect on the visible light activity and the photocatalytic reaction mechanism. Different modification methods can be integrated into titania nanoparticles to get synergic effects in the overall photocatalytic activity. The applications of the chemically modified titania systems to the degradation of pollutants, hydrogen production, and photocurrent generation under UV and visible light will be introduced and discussed.

With band edges that bracket the redox potential for reduction and oxidation of water, titania (TiO₂) would be an appealing photocatalyst for generation of H₂ from water splitting, but the solar energy conversion efficiency of this oxide is limited by rapid charge recombination and a large bandgap. The objective of our work is to design and synthesize titania aerogels with high surface area, 3D accessible active sites, optimized charge separation/transfer, and increased response to visible light to improve photoefficiency for water splitting. We have synthesized Auâ?"TiO₂ composite aerogels, with highly dispersed Au, controlled Au particle size, and a high Au||TiO₂ interfacial density. The Auâ?"TiO₂ aerogels show improved excited state lifetimes and lower rates of recombination because photoexcited electrons can diffuse rapidly through the interconnected nanoparticle network to reach the Au||TiO₂ interfaces, where they are effectively localized. In that the incorporated Au nanoparticles have a broad surface plasmon feature centered around 550 nm, plasmonic enhancement in the photoactivity at visible wavelengths is expected for the Auâ?"TiO₂ composite aerogel and under investigation. We will report on the structural and functional characterization of these materials including: optical response, charge carrier-density, recombination rates, and photocurrent. We will also report H₂ yields from water splitting and overall energy conversion efficiency achieved with the modified aerogels.

The development of photochemical systems capable of splitting water into hydrogen and oxygen has attracted significant interest motivated by the need to secure the future supply of clean and sustainable energy [1]. In this context it is important to realize that it is particularly the water oxidation reaction which is the major obstacle hampering the efficiency of water-splitting devices [2]. Accordingly, one of the fundamental challenges in photoelectrochemical water splitting is the development of highly efficient and stable photoanodes with suitable optical (bandgap), photoelectrochemical (position of band edges on the energy scale), and surface catalytic properties [3, 4]. Recently, we have been developing a novel class of visible-light photoactive inorganic/organic hybrid materials â?" TiO₂ with the surface modified by polyheptazine compounds bound to the TiO₂ surface [5]. Notably, polyheptazine-type materials represent a very stable system of conjugated Ï?-bonds and their utilization as photocatalysts has been suggested recently [6]. However, in contrast to pristine polyheptazines showing only weak absorption in the near visible, the TiO₂/polyheptazine hybrids exhibit strong red shift in visible light absorption, which is based on formation of interfacial charge-transfer complex between polyheptazine (donor) and TiO₂ (acceptor) [5]. Visible (λ> 420 nm) light-induced water splitting at moderate bias potentials (0.5 V vs. Ag/AgCl, pH 7) was evidenced by oxygen evolution when IrO₂ nanoparticles were deposited as a co-catalyst [5]. Current attempts at coupling the hybrid materials with low-cost oxygen evolving co-catalysts will be discussed. References [1] N.S. Lewis, D.G. Nocera, Proc. Natl. Acad. Sci. U.S.A., 103 (2006) 15729-15735. [2] H. Dau, C. Limberg, T. Reier, M. Risch, S. Roggan, P. Strasser, ChemCatChem, 2 (2010) 724-761. [3] R. van de Krol, Y. Liang, J. Schoonman, J. Mater. Chem., 18 (2008) 2311-2320. [4] B.D. Alexander, P.J. Kulesza, I. Rutkowska, R. Solarska, J. Augustynski, J. Mater. Chem., 18 (2008) 2298-2303. [5] M. Bledowski, L. Wang, A. Ramakrishnan, O.V. Khavryuchenko, V.D. Khavryuchenko, P.C. Ricci, J. Strunk, T. Cremer, C. Kolbeck, R. Beranek, Phys. Chem. Chem. Phys., in press, DOI:10.1039/C1CP22861G. [6] X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, Nat. Mater., 8 (2009) 76-80.

Hydrogen is often used as a reducing agent and/or to modify the electronic properties and reactivity of TiO2. We carried out density functional theory calculations to investigate hydrogen adsorption and diffusion on surface and subsurface sites of the majority (101) surface of anatase, the technologically most relevant polymorph of TiO2. Hydrogen diffusion into the subsurface is kinetically favored over H2 recombinative desorption even though the latter is energetically favored. High hydrogen coverages can lead to surface oxygen vacancy formation. This may contribute to the surface disorder observed in TiO2 nanocrystals exposed to high-pressure hydrogen.

We demonstrate a simple and effective method to fundamentally improve the performance of TiO2 nanowires for photoelectrochemical (PEC) water splitting by hydrogen treatment. Hydrogen-treated rutile TiO2 (H:TiO2) show substantially enhanced photocurrent compared to pristine TiO2, and a exceptionally low photocurrent saturation potentials of 0.4 V vs. RHE. The optimized H:TiO2 nanowire sample yields a photocurrent density of 1.97 mA/cm2 at 0.4 V vs. RHE, in 1 M NaOH solution under the illumination of simulated solar light. IPCE analyses confirm the photocurrent enhancement is mainly due to the improved photoactivity of TiO2 in the UV region. The solar-to-hydrogen efficiency is calculated to be â^¼1.1%. Electrochemical impedance investigation shows hydrogen treatment increases the donor density of TiO2 nanowires by 3 orders of magnitudes, via creating a high density of oxygen vacancies that serve as electron donors which is believed to be beneficial for charge transportation and separation. The capability of making highly photoactive H:TiO2 nanowires opens up new opportunities in PEC water splitting and photocatalysis.

Nanostructured fiber-mats have large surface area, high reactivity, low weight and low agglomeration tendency. These are advantages if compared with nanoparticles for photocatalytic application. Fiber-mats can be used not only as a photocatalytic material on their own, but also incorporated in different surfaces or fabrics and as well as a filtration membrane. In this work, high temperature stable anatase titanium dioxide fiber-mats doped with silica (0.5 to 30%) or doped with tin (0.5 to 15%) were produced by electrospinning technology. The precursors used were titanium propoxide (TiP), tetrapropoxysilane (TPS) and Tin 2-ethylhexanoate. They were hydrolyzed in acetic acid and mixed with an alcoholic solution of 10wt% polyvinylpirrolidone. The effect of heat treatment on the microstructure characteristics and the photocatalytic activity of the fiber-mats in comparison with a commercial TiO2 powder (Degussa P-25) were studied. After the electrospinning process, a thin, porous fiber-mat was obtained. This material was dried in air at room temperature for 24h. These fibers were then heat treated from 500 to 800°C for 3 hours at a heating rate of 1.4°C/min. The fiber-mats were then, characterized using N2 adsorption - BET surface area, X-ray diffraction for phase, SEM and TEM analyses for morphological characterization. The photocatalytic activity under acid and basic pH was studied using as model system the degradation of methyl orange in water (20ppm) under UV-A and visible light. Raw fibers are amorphous but become crystalline after heat treatment. As the heat treatment temperature increases the surface area decreases significantly. Quite the opposite happens with the rutile to anatase ratio and the anatase and rutile crystallite sizes, which increase with higher heat treatment temperatures. The photoactivity increases with the increment in heat treatment temperature until 650°C, when the fibers start to become densified and the surface area drops significantly due to sintering. Fibers produced at higher temperatures and with higher amounts of silica are predominantly anatase and are generally more photoactive under UV-A radiation.

Titanium dioxide (TiO2), also known as Titania has been extensively studied in the context of material properties as well as for its photocatalytic properties. Titania can be prepared by several techniques including sol-gel, cold deposition, and Atomic Layer Deposition (ALD). A significant challenge to-date is the formation of highly controlled and photocatalytic films on high aspect ratio nanometric scaffolds and templates, together with high molecular permeability, or porosity and preferably without the addition of surfactants. In particular, such materials are desirable for the formation high surface area structures with controlled surface properties and functionality at the nanometric scale leading to highly favorable catalytic activity of nanometric hybrid structures. A notable property of the ALD approach is the high quality, conformal, and pin-hole free films readily formed by this method. However, in the context of catalysis, and photocatalysis in particular, the pin-hole free molecularly impermeable films that are beneficial for many applications are disadvantageous as compared to porous or molecularly permeable films due to the limited surface area and available catalytic sites at the film surface and interfaces. Recently there is great interest is using ALD as well as Molecular Layer Deposition (MLD) for the formation of organic-inorganic hybrid thin film materials in a well-controlled manner. Here, we present solvent-free MLD approach for highly controlled, molecularly permeable, photo-catalytically active organic-inorganic hybrid films. Thermal anneal of the hybrid films result in decomposition of the organic portion of the and gradual formation of crystalline TiO2 anatase phase with increasing temperature. Notably, the thermally annealed films show crystalline regions as well as retain the molecular permeability properties for a wide temperature range, which is a valuable for designing highly photocatalytic films by combining high surface area with photocatalytic crystalline regions embedded within the same thin film. Our results demonstrate the feasibility and high potential of using MLD process to form nanometric hybrid structures with high aspect ratio and surface area for photocatalysis.

A novel polymer dispersant with imide functionalities and poly(oxyethylene)-segment in the structure was incorporated in the nanocrystalline TiO2 film as the electrode. The uses of the dispersants could dispersed TiO2 by decreasing the van der waals force among the nanoparticles, observed by TEM. The resultant TiO2/POEM film as the photoanode rendered the dye-sensitized solar cell (DSSC) with enhanced performance. By comparing to the traditional photoanode composing of polyethylene glycol dispersed TiO2, the POEM dispersed TiO2 provided large surface area and high roughness in the dye adsorbed film. Furthermore, the fabricated TiO2/POEM photoanode has a better light-scattering property which contributes to the improvement for the short-circuit current density (Jsc) and the power-conversion efficiency (η) of the DSSC to be 19.1 mA cm-2 and 8.7% , respectively. The performance is superior to 13.2 mA cm-2 and 7.34% for a DSSC with the photoanode containing TiO2/PEG.

As an excellent photocatalyst, TiO2 has triggered broad interest studies. Many studies devoted to enhancing the photocatalytic activity of TiO2 via doping, compound, or exposure of reactive facets. However, besides photocatalytic activity, the lifetimes of TiO2 is also very crucial in photocatalysis. Therefore, some researches tent to explore the deactivation and regeneration mechanism from the transformation of functional groups or characterizing the chemical reaction at the surface of TiO2. In this work, in order to reveal the microstructural evolution of TiO2 during the photocatalytic process, six samples were examined directly by using HRTEM, including pristine P25 nanoparticles, methylene blue (MB) degraded nanoparticles, deactivation nanoparticles after degradation of MB for 20 times, and the deactivation samples calcined at 200, 400, 600 °C for 2h in Ar atmosphere. Deactivation The results revealed that: 1) The pristine TiO2 nanoparticles exhibited a perfect crystal lattice with a clear HRTEM image; 2) After degradation, there were many MB molecular absorbed on the surface of nanoparticles, which resulted in a fuzzy HRTEM image. When the TiO2 was exposed in air for a period of time, MB molecular disappeared and the TiO2 lattice image again became integrated as the pristine one; 3) If the nanoparticles became deactivated after degradation of MB for more than 20 cycles, the HRTEM lattice image was fuzzy fully and could not recover even it was exposed in air for a long time. We suppose that the lattice distortion on anatase TiO2 (101) surface induced by chemical adsorption of MB molecules is a crucial intermediate step during photocatalyzing. This lattice distortion essentially changes local density of electronic states and increases the surface chemistry potential energy of TiO2. Regeneration Heat treatment was adopted to recover the deactivated photocatalysts. The results showed that: 1) when the temperature was 200 °C, there was an amorphous layer coated on the surface of TiO2 nanoparticles, and the nanoparticles exhibited a perfect crystal lattice again; 2) As the temperature rose to 400 °C, the amorphous layer transferred into graphite-like carbon layers; 3) When the temperature got up to 600 °C, graphite-like carbon layers disappeared, whereas a few of carbon quantum dots were attached to the surface of TiO2 nanoparticles. Photocatalytic measurements confirm that TiO2 nanoparticles with a graphite-like carbon shell or combined with carbon quantum dots showed excellent photocatalytic activity which is much higher than that of pristine TiO2 nanoparticles under UV light irradiation. The mechanism is based on the high migration efficiency of electrons at the graphite-like carbon/TiO2 interface. In addition, a high activity under visible light irradiation is also observed. The response of TiO2 is extended into the visible range due to the electronic coupling of Ï? states of the graphite-like carbon and conduction band states of TiO2.

Due to its strong catalytic activity, high chemical stability, biologic compatibility, and anti photo-corrosion ability, TiO2 has become one of the most promising photocatalysts and triggered broad interest studies. Photocatalysis is a reaction that happens on the surface of catalysts, so enlarging the specific surface area of TiO2 can also improve the photocatalytic efficiency. The titanium dioxide nanotube (TNT) â?oin-suitâ? synthesized by anodic oxidation on the surface of Ti substrate have been widely concerned recently, because of its larger specific surface area to regular TiO2-based materials and can avoid the troublesome recycling problems in the use of nanoparticles. However, the pristine TiO2 nanotube with large band gap (such as Eg = 3.2 eV for anatase) make it absorb only ultraviolet light with a wavelength shorter than 387 nm, which accounts for only a small fraction (4%) of the sunlight spectrum. Therefore, variant methods have been focused on doping TiO2 with both transition metals and non-metal impurities, such as N, S, C, F, Cr, Fe and Mo, for enlarging the TiO2 absorption band border to the visible light range. However, these impurities doping still remains insufficient in TiO2 and can expand the band border only dozens of nanometers to red shift. Recently, the black hydrogenated titanium dioxide nanocrystal with increasing solar absorption has been reported by Chen and his coworkers, which has aroused widespread attention in photocatalysis circles. There have been present various efforts to obtain black TiO2 in divers morphologies. While, the black hydrogenated TiO2 nanotube, as one of the most efficient photocatalysts, has not been reported in the application of photocatalysis. We present a valid approach to synthesize the black TiO2 nanotubes by annealing the pristine nanotubes in hydrogen atmosphere at 500 °C for 10 hours in this paper. The X-ray diffraction spectrometer (XRD) results reveal that the black TiO2 nanotube contain mainly anatase and small amount of TiHx. The X-ray photoelectron spectroscopy (XPS) of black TiO2 nanotube shows the bond of Ti-O-H, which demonstrates the hydrogen has been doped into TiO2 lattices. Compared with the transparent pristine TiO2 nanotube, the hydrogenated present super higher absorption in full ultraviolet-visible spectrum. Its photocatalytic activity has also enhanced nearly triple than the pristine TiO2 nanotubes under uv-vis light illumination.

Dye-sensitized solar cells (DSCs) have been pursued as next-generation photovoltaic devices with simple and low cost fabrication requirements. We report herein a facile approach to enhance the open-circuit voltage (V_OC) resulting improvement in overall device efficiency. A ZnO-treated TiO₂ electrode was fabricated by multiple coating of ZnO precursor solution onto the surface of 3D ordered porous TiO₂ structures. The presence of ZnO over TiO₂ was confirmed by Energy-dispersive X-ray spectroscopy (EDX) and we observed the current-voltage characteristics, the dark current measurements, and the electrochemical impedance spectra. The increase of V_OC was observed, which was attributed to negatively shifted conduction band edge of TiO₂ due to the formation of a surface dipole layer. At the surface of the ZnO-treated TiO₂ electrodes, protons released from dye molecules form the surface dipole layer with higher electron density in TiO₂, shifting the conduction band edge negatively. We demonstrated the performance of 3D ordered porous TiO₂ electrodes with ZnO treatment as possible candidates for use in DSCs.

There exist various methods of producing titania nanoparticles, such as sol-gel process, electrochemical coating, hydrothermal process, hydrolysis, microemulsion method, thermolysis, flame aerosol process, chemical vapor deposition, etc. The inert gas condensation (IGC) technique is another route for the synthesis of metal and ceramic nanoparticles initiated by Gleiter, Siegel, et al. In IGC method, contamination of nanoparticles can be minimized during synthesis because high-purity metal vapor source from high-purity metal powder and high-purity inert gas atmosphere are used. Anatase TiO2 nanoparticles were successfully synthesized by post-heat treatments of partially crystalline Ti and amorphous TiOx nanoparticles, respectively produced by inert gas condensation and subsequent oxidation. The nanoparticles condensed on a liquid-nitrogen containing cooling finger (sample LN) were identified to be partially crystalline Ti phase with ~10-20 vol% amorphous TiOx. On the other hand, those condensed on a room-temperature cooling finger (sample RT) were almost completely amorphous TiOx phase. Differential scanning calorimetry scan curves of as-oxidized samples were interpreted using Kissinger analysis, the non-isothermal kinetics, and activation energy for the anatase formation was determined as ~455 and 865 kJ/mol for samples LN and RT, respectively. As-oxidized samples LN and RT were heat treated at 400 oC for 2 h, respectively (samples LN-H and RT-H). Samples LN-H and RT-H showed the onset of UV-visible light absorption near 400 nm and the optical band gap of 3.12 and 3.21 eV, respectively, corresponding to anatase. The sample LN-H showed faster photocatalytic decomposition of methylene blue and rhodamine B dyes compared to the sample RT-H due to high crystallinity of anatase and rutile phases.

The main purposes of this study are replacing conventional hydro-thermal method by microwave heating using water as reaction medium to rapidly synthesize TiO2.Titanium tetraisopropoxide (TTIP) was hydrolyzed in water. The solution is subsequently processed with microwave heating for crystal growth. The reaction time could be shortened into few minutes. Then we chose different acids as dispersion agents to prepare TiO2 paste for investigating the effects of dispersion on the power conversion efficiency of dye-sensitized solar cells (DSCs). The photovoltaic performance of the microwave-assisted synthesized TiO2 achieved power conversion efficiency of 6.31% under AM 1.5 G condition (100 mW/cm2). This PCE value is compatible with that of the devices made from commercial TiO2.

Systematic defect chemical studies of lithium storage electrodes are â?" in spite of the paramount importance â?" almost absent in the literature. One exception is the detailed study of the redox-pair LiFeO₄ â?" FePO₄ for which the charge carrier chemistry has been explained as a function of lithium activity, temperature and doping concentration [1-4]. We investigate the point defect chemistry of lithium storage in anatase TiO₂ and emphasize its great importance to lithium battery applications. As an additional degree of freedom, the anatase nanoparticles were treated in hydrogen atmosphere, giving rise to increased electronic conductivity and therefore greatly improved battery performance [5] due to electrons compensating the introduced oxygen nonstoichiometry. Not only charge carrier concentration but also lithium diffusivity are successfully interpreted in terms of lithium content and activity as well as oxygen stoichiometry. A detailed defect chemical analysis reveals that ionic-electronic defect associations play a significant role in determining the concentration of defects and lithium diffusivity, and hence can have large impact on overall lithium storage performance of oxide based electrode materials [6]. [1] R. Amin, C. Lin, J. Maier, Phys. Chem. Chem. Phys. 10 (2008) 3524. [2] J. Maier, R. Amin, J. Electrochem. Soc. 155 (2008) A339. [3] R. Amin, C. Lin, J. Peng, K. Weichert, T. Acartürk, U. Starke, J. Maier, Adv. Funct. Mater. 19 (2009) 1697. [4] K. Weichert, W. Sigle, P. van Aken, J. Jamnik, R. Amin, T. Acartuerc, U. Starke, J. Maier, J. Am. Chem. Soc, 2011, submitted. [5] J.-Y. Shin, J.H. Joo, D. Samuelis, J. Maier, Chem. Mater. 2011, submitted. [6] J.-Y. Shin, D. Samuelis, J.Maier, Solid State Ionics 2011, submitted.

Colloidal sol-gel method is a powerful route to obtain stable and homogeneous sols of nanoparticles of controlled size, which can be used to prepare thin films by different techniques such as dip-coating, electrophoretic deposition and spin-coating. The use of nanocrystalline based structures provides an additional control on the gap of the material, thus allowing a supplementary control on the excitation properties. Since direct excitation of the parity-forbidden intra-f-shell rare-earth ion transitions has low efficiency, methods to enhance the excitation efficiency have been investigated. One of them is the incorporation of the RE ions in a wide-band-gap material (e.g. ZnO and TiO2) so they could be sensitized efficiently by exciton recombination in the host. In this work the optical properties of Er-doped TiO2 nanoparticulate sols and thin films prepared from the sols are investigated. Thin films were prepared by spin-coating on Si and glass substrates using different rotating speeds with the aim to obtain the largest homogeneity. Optical spectroscopy techniques (absorption, reflection and ellipsometry) and photoluminescence are used to monitor the properties of the films in order to determine structural and morphological features. The use of post-annealing treatments to optimize their optical response in the IR will be discussed.

Aligned rutile TiO2 nanorod (NR) array were grown on the FTO substrate by hydrothermal method, and then the branches of the TiO2 nanodendrites (ND) were formed using an aqueous chemical bath deposition (CBD) without any capping agent. TEM characterizations reveal that both nanorod and branch are single crystalline. The three-dimension (3D) TiO2 nanodendrite (ND) array offers not only a large surface area for electrochemical reactions but also a direct conducting channels for charge transport. The highly ordered TiO2 ND array with 2 μm in length yield a photoelectrochemical efficiency of 1.05% with an applied bias of -0.2 V versus Ag/AgCl under AM 1.5 at 100 mWcm-2, which is nearly three times higher than that of the bare TiO2 nanorod array. ND array produces a photocurrent density of 1.6 mA/cm2 at 0.9 V versus Ag/AgCl, which is nearly thirty times higher than that of the bare TiO2 nanorod array. The results suggest that the dense and aligned three-dimensional TiO2 ND arrays are a promising photocatalyst for hydrogen generation from water splitting based on PEC cells.

Tuning photonic band gap by electric field is an emerging field for device application such as display and sensor. In order to make electric field-dependent photonic band gap materials, polystyrene and silica particles were used due to their simple synthesis with monodispersed size. These materials showed good responsibility in electric field but their low refractive index limit the performance of device. Titania is one of the materials having high refractive index. Therefore, there have been lots of efforts using titania to make photonic band gap materials, but difficulty to make homogeneously dispersible titania particles and their low electrophoretic mobility in electric field have restricted their usage in this interesting applications. Here we report the synthesis of core/shell and hollow titania nanostructures with narrow size distribution. Density of each particle is reduced compared to normal titania particle by exchanging or removing titania core. Surface charges could be controlled by further surface modification. As a result, the mobility (or responsibility) of titania in electric field was increased by both reducing density and controlling surface charge. Photonic band gap generated by new structured titania nanoparticles was varied by their size with rapid electrophoretic mobility as expected, and more details on the synthesis and device characteristics will be discussed.

Highly energetic surfaces possess excellent catalytic property, and therefore maximum exposure of specific highly-energetic facets is beneficial for designing new substances at atomic level and moreover exploiting novel properties. However, because of their thermodynamic instabilities, it still remains a challenge as to chemically preparing nanocrystals having high-energetic facets, let alone the rational property control. In this work, {001} faceted TiO2 microspheres were taken as a prototypal example to investigate the importance of high energy facets. All microspheres were constructed by bundles of nanowires that grew along direction [001] from the centers of microspheres to outward surface, leading to the terminated faces (001). By manipulating the growth kinetics, building blocks of TiO2 nanowires were tailored to show diameters ranging from 4.2 to 14.6 nm. As a consequence, the resulting microspheres have the combined features of highly surface hydration, ionicity enhancement, and lattice expansion. In particular, lattice expansion occurred within the building blocks of nanowires may promote the axial electron transfer to the terminated facet (001) to produce an optimum catalytic activity. The methodology reported here may offer opportunities for exploring highly energetic surfaces of micro-architectures by simply manipulating the nanoscale building blocks to active the surface reactions, potentially useful in various catalytic applications.

For several decades biomineralizing systems have been viewed as potentially valuable models for developing mild synthetic approaches towards technological materials. Among a variety of solid-state materials examined, titanium dioxide has been shown to be a particularly attractive candidate for bioinspired syntheses, as novel aqueous routes to several crystalline titania polymorphs have been demonstrated. However, all such previously reported routes have required the use of mineralizing biomacromolecules (e.g. synthetic polypeptides or naturally extracted proteins). Here we present a scalable approach that yields crystalline titanium dioxide and other technological materials from aqueous solution in the complete absence of DNA or other proteins. Mineralization is conducted in water-in-oil emulsion droplets generated via a microfluidic platform, and combinatorially selected chemistry at the water-oil interface controls crystalline product formation from a water-soluble titania precursor. Thus, the present system utilizes the biomimetic features of micro-compartmentalization and chemical templating at liquid-liquid interfaces. With this method we have produced spherical mineral shells with unique photoluminescence properties, and have established a general platform for targeting a variety of technological crystals via protein-free bioinspired aqueous reactions.

Abstract not available.

High performance energy devices such as fuel cells, solar cells and batteries which involve reactions and transport problems require nanostructured functional materials which are highly ordered mesoporous and crystalline materials. The self-assembly based structure direction of functional inorganic materials by block copolymers is a promising route to energy devices because it combines mesoporosity at the 2-50 nm length scale, high surface areas and full control over morphology. Titania is a particularly interesting transition metal oxide and it has been applied in many areas such as photocatalysis, membranes, optics, photovoltaics and so forth. Although block copolymer based mesoporous and crystalline titania materials have been explored for more than a decade, they are mostly limited to water and alcohol soluble polymers which only give the structures of small dimensions and few morphologies. In this work, we present solubility design guidelines which facilitate the coassembly with PI-b-PEO (polyisoprene-block-polyethylene oxide) which is a highly amphiphilic block copolymer. This approach along with CASH method enabled the fabrication of highly ordered crystalline titania with an inverse hexagonally arranged cylindrical morphology. We also explored the pore size control of mesoporous crystalline titania by applying different molecular weight PI-b-PEOs and we report the largest reported cylindrical pores (>30 nm) of highly ordered and crystalline titania without the use of pore-expanders. Furthermore, a more complex triblock terpolymer system is demonstrated which enables the fabrication of crystalline titania with an ordered 3D network structure.

Developing high-performance electrode architectures has been an essential component of the current endeavor in electrical energy storage. We report a general strategy towards high-performance electrodes from assembling conductive scaffolds of carbon nanotubes (CNTs) and active nanocrystals (NCs) building blocks. To demonstrate this concept, anatase TiO2 was used as model NCs due to its abundance, low cost and environmentally benignity. To make such structure, uniformed NCs were first synthesized and well dispersed in nonpolar solvent. Upon contact of the NCs solution with a pre-formed CNTs scaffold, such hydrophobic NCs rapidly coat onto the hydrophobic scaffold as driven by the hydrophobic interaction. A subsequent sintering process removes the capping ligands from the NCs, creating mesoporous conformal coatings of crystalline TiO2 on the scaffold. Alternatively, another efficient strategy is to disperse both building blocks in solvent in which a micro-phase separation forms and NCs assemble with CNTs. Unlike previous reports of using surface-functionalized CNTs to support active materials, our strategy utilizes the hydrophobic interactions between the NCs and the CNTs to confine the NCs around the CNTs, which offers effective electron transport, effective ion transport and high active-material loading. As resulted electrodes show exceptionally high capacity and high rate-capability. Considering the vast library of synthesized NCs, our strategy provides a general approach towards better electrodes from assembling NC building blocks.

We report the nonaqueous surfactant-assisted synthesis of highly uniform TiO2 nanocrystals with tailorable morphology in the 10-100 nm size regime, prepared through a seeded growth technique developed by Cozzoli, et al. It is observed that the nanocrystal morphology is dramatically altered based on the titanium precursor utilized. By modifying the metal precursor and using combined precursors, tetragonal bipyramids and nanoplates of anatase phase can be synthesized with only the {101} and {001} facets exposed. Thus, we are able to prepare uniform nanocrystals with precise ratios of {001}/{101}. In addition, this synthetic technique allows for nonmetal doping of TiO2 nanocrystals. A broad visible/NIR absorption is observed when certain nonmetal dopants are introduced (causing the TiO2 to appear blue in color) and the absorption intensity scales with the dopant concentration. X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) data suggest that the cause of the optical absorption is substitutional nonmetal doping, as opposed to the formation of oxygen vacancies. Activity towards photocatalytic hydrogen evolution in the presence of sacrificial methanol is determined after the nanocrystals are activated through ligand exchange. Within the series of anatase nanocrystal morphologies, the activity is observed to increase when higher percentages of {101} facets are exposed, while brookite nanocrystals show the highest overall rate of hydrogen evolution. This nonaqueous synthetic technique allows for the production of monodisperse doped TiO2 nanocrystals with tunable size and shape, in a size regime relevant for photocatalytic applications.

TiO2 exhibits a variety of interesting physical properties, including resistive switching, catalytic activity, and large dielectric permittivity. It has been suggested that coupled electron-ion dynamics could play an important role in these intriguing properties. Manipulating and characterizing the surface charges and resulting potential distribution are crucial for investigating the physics underlying all the peculiar physical phenomena. Scanning probe microscopy (SPM) is one of the best tools for these purposes because of its versatility and superior resolution compared with other techniques. In this research, we used electrostatic force microscopy (EFM) to investigate how an SPM-tip-induced electric-field influences the surface potential distribution of rutile TiO2 (100) single crystals. To study the role of the ambient gas, comparative SPM characterizations were performed in atmospheres consisting of H2/Ar, Ar, and O2. The SPM tip-induced electrical stress resulted in reversal in the surface potential, Vsurf, polarity only in H2/Ar (Î"Vsurf = 0.30 eV), and not in Ar and O2. Quantitative measurement of the influence of ambient gas on the surface potential led us to develop a model where both of the adsorbed oxygen molecules and oxygen vacancies can influence the surface potential at TiO2 surface.

Homogeneous arrays of TiO₂ nanotubes fabricated by anodic oxidation are utilized in a variety of applications. Besides hydrogen sensors, lithium-ion-batteries and water photolysis, specifically thin-film solar cells benefit of the arrays' high surface area and their regular pattern. In many cases, electron conductivity along the tubes is crucial. Thus, deeper knowledge of the material properties within the layer is necessary. Since methods like X-ray diffraction or scanning electron microscopy can only provide global information about crystalline appearance and morphology respectively, we employed diverse transmission electron microscopy (TEM) techniques to obtain information about local characteristics within the array. By using cross-sectional TEM sample preparation methods, the arrays can be investigated in context, avoiding a limited view on only few tubes. Amongst a verification of the tube dimensions as well as the anatase crystal phase, we found large crystallites which reached several hundred nanometers along the tubes. Their maximal length depends on the voltage at which the anodization was performed. Furthermore, adjacent tubes often possess the same crystalline orientation, so large areas of about 12 or more interconnected tubes have been found. Taking into account that polycrystalline materials with grain sizes of about 20 nm contain many grain boundaries which have to be crossed by conduction electrons, large crystallites facilitate functions like leading away electrons after charge separation in solar cells since the number of grain boundaries is reduced considerably.

Developing renewable, sustainable and green energy resources and securing the environment by reducing the emission pollutants are two of the largest challenges for the human civilization within the next 50 years. Besides the energy resources that power the world today - petroleum, coal, and natural gas - active research and development is done exploring alternative energy resources such as solar, biomass, wind, and hydrogen. Research and innovation within the area of the rapidly expanding field of nanoscience and nanotechnology, multi-disciplinary by nature involving physics, chemistry, biology, molecular biology, is mandatory to make the vision of a clean society and our vision of plentiful, low cost sustainable energy, a reality. In this talk, we will summarize recent surface science studies on a prototypical model oxide system â?" the rutile TiO2(110)â?"(1 Ã- 1) surface [1-3]. We present clear-cut identification of surface defects such as oxygen vacancies, hydroxyl groups, and near surface defect such as Ti interstitials [1]. Based on these assignments we discuss the complex oxygenâ?"TiO2(110) interaction [1,2]. Specifically, the role of bulk defects in the oxygen chemistry on reduced rutile TiO2(110) is studied by means of scanning tunnelling microscopy (STM) and temperature-programmed desorption (TPD) measurements. An ionosorption model is proposed to explain the obtained STM and TPD results [2]. In addition, we show that these results help to improve our understanding of the use of TiO2 as photo-catalysts. By means of high resolution STM and TPD/TPR spectroscopy, we studied the desorption and thermal decomposition of ethanol and ethoxide groups on differently prepared TiO2(110) surfaces. Furthermore, direct evidence for ethanol dissociation at bridging O vacancies will be presented [3]. Accompanying density functional theory (DFT) calculations support the assignments made in the STM studies and rationalize the observed distinct diffusion behaviors of molecularly and dissociatively adsorbed ethanol species. Finally, the photo-reaction of ethanol on differently prepared TiO2(110) surfaces will be discussed. [1] S. Wendt, P. T. Sprunger, E. Lira, G. K. H. Madsen, Z. Li, J. Ã~. Hansen, J. Matthiesen, A. Blekinge-Rasmussen, E. Lægsgaard, B. Hammer, F. Besenbacher, Science 320, 1755 (2008). [2] E. Lira, S. Wendt, P. Huo, J. Ã~. Hansen, R. Streber, S. Porsgaard, Y.Y. Wei, R. Bechstein, E. Lægsgaard, and F. Besenbacher, J. Am. Chem. Soc. 133, 6529 (2010). [3] J. Ã~. Hansen, P. Huo, U. Martinez, E. Lira, Y.Y. Wei, R. Streber, E. Lægsgaard, B. Hammer, S. Wendt, F. Besenbacher, Phys. Rev. Lett. 107, 136102 (2011).

The flatband potential of nanocrystalline TiO₂ electrodes is a function of contacting media: e.g., organic or aqueous electrolytes and potential-determining ions. In addition, the existence of intra-band gap trap states results in electron transfer to acceptors at potentials more positive than the nominal redox potential of the conduction band edge. We have previously investigated the photoluminescence (PL) from intra-band gap states of nanocrystalline anatase and assigned the spectrum to spatially isolated hole and electron trap distributions, which give rise to peaks at green and red wavelengths, respectively. In the present work, this PL is investigated as a function of applied bias and for different inert electrolytes in a three-electrode electrochemical cell. In aqueous media at different pH values, there is an abrupt onset of trap state emission as the applied bias is made more negative, followed by a blue-shift in the PL spectrum. The onset of the red PL follows the well-known Nernstian behavior of the flatband potential associated with the surface amphoterism of TiO₂. In nonaqueous electrolyte, the bias-dependence of green- and red-emitting traps appears to be a function of the transport properties of the film. The relationship of traps and carrier transport to nanoparticle and film morphologies was investigated through spectroelectrochemical photoluminescence measurements of TiO₂ nanoparticles, nanosheets, and nanotubes in different electrolyte solvents and as a function of empirical surface treatment. Results will be discussed in terms of a model for the chemical nature of electron and hole traps and their interactions with contacting solvent.

In past work, we have interpreted the broad photoluminescence (PL) spectra of nanocrystalline TiO₂ to be a superposition of hole trap emission, peaking in the green, and broad red PL arising from electron traps. In this work, intra-band gap states of dispersed single titanium dioxide (anatase) nanotubes, prepared by anodization of titanium foil followed by sonication, were investigated by means of micro-photoluminescence spectroscopy and imaging at an excitation wavelength of 350.7 nm. Photoluminescence (PL) spectra of individual nanotubes in inert environment showed a broad emission with peaks at around 560 to 610 nm. While the emission spectra of single nanotubes are similar to those of anatase nanoparticles, both differ greatly from the more blue-shifted PL of ordered nanotube films. PL images and the locations of the emissions were shown to be concentrated in the area of excitation, but the peaks in the red and green components of the PL are not spatially coincident, showing the contribution of charge transport to the observed PL. Furthermore, PL occurring away from the excitation point is shown to peak in the blue-green (~510 nm). While the PL from ensembles of TiO₂ nanotubes is fairly insensitive to contacting media, exposure of single-nanotubes to air and ethanol changes the PL spectrum as well as the shape of the emissive region. These results will be discussed in terms of the spatial and energetic distributions of hole and electron traps and their relation to carrier transport. While hole traps appear to derive from the more stable (101) anatase surfaces, and the electron traps from minority (001) planes, the spectroscopic activity of these traps is strongly modified by transport properties of the sample.

A study was made to investigate the surface morphology of titania films processed by plasma electrolytic oxidation (PEO) using different electrolytes with and without ammonium fluoride. We also measured the photovoltaic properties of two different PEO-coated titania films. During the PEO coating in the electrolyte with ammonium fluoride, the time when the micro discharges were generated in the electrolyte was significantly shortened than that without ammonium fluoride, due to its different electrolyte conductivity. Also, the fluoride-induced oxide dissolution in the electrolyte with ammonium fluoride caused not only numerous nano-pores of titania film but also improved surface roughness of the structure of titania film and these results were analyzed via SEM and AFM. From X-ray diffraction patterns, the anatase and rutile phases were clearly detected due to the complicated mechanism such as anodic oxidation and oxide dissolution in the electrolyte. The relatively high photovoltaic properties were obtained in titania films coated in the electrolyte with ammonium fluoride.

TiO2 has many distinct polymorphs that depend upon extreme conditions such as temperature and pressure. In the bulk form the stable form of these is often identified as being the rutile phase. However in a TiO2 nanopaticle the stable phase is no longer rutile but an anatase structure. Of then this observation has been atributed to a "surface tension" phenomena where essentially the surface is distorting the pressure thereby leading to a pressure induced phase transformation. This interpretation is consistent with observations that the size effect of the nanoparticle affects the overall internal structure. Molecular dynamics simulations are performed on several distinct structures of TiO2 nano particles with the view to elaborating and visualizing the behavior of the nanoparticle under exreme conditions of temperature and pressure but at the atomic level. Variations with size of the nano particle are examined and conclusions drawn over the role played by the surface and also defect stuctures within the nano particle. Surface amorphization and densification is especially significant.

O-vacancies on oxide surfaces have long been thought to be important in the chemistry. Indeed, for TiO2(110) we find that vacancies provide the dominant contribution to the reactive band-gap state. Reactions of vacancies of this model photocatalyst surface have been directly visualised in the case of reactions with H2O/O2. The vacancies have been assumed to be neutral in calculations of the surface properties. However, by comparing experimental and simulated scanning tunnelling microscopy images and spectra, we show that oxygen vacancies act as trapping centres and are negatively charged. We demonstrate that charging the defect significantly affects the reactivity by following the reaction of dioxygen with surface hydroxyl formed by water dissociation at the vacancies. Calculations with charged hydroxyl favour a condensation reaction forming water and surface O adatoms, in line with experimental observations. This contrasts with simulations using neutral OH where peroxide is found to be the most stable product.

The ability to reduce and vary the thermal properties in nanosystems is immensely important for future development of thermoelectric and photovoltaic materials and can have far reaching implications for realization of novel, nano-based ideas such as solid-state thermal rectification, thermal memory devices, and thermal storage. However, this subsequent decrease in thermal conductivity often comes at a cost; for example, fabrication and construction of nanoparticle composite films can be intricate and costly, thereby proving difficult to reap the benefits of the unique thermal properties for larger area applications. In this work, we demonstrate an approach to fabricate films of closely packed nanoparticles that exhibit exceptionally low, and tunable, thermal conductivity. We report on the thermal conductivity of a series of convectively assembled, anisotropic titania (TiO_2) nanoparticle films. The TiO_2 films are fabricated by flow coating a suspension of ellipsoidal colloidal nanoparticles, resulting in structured films with tailored orientational order. The thermal conductivities depend on nanoparticle orientation and can be less than amorphous TiO_2 films due to inter-nanoparticle boundary scattering. This nanoparticle ordering presents a unique method for manipulating the thermal conductivity of titania nanocomposites.

Understanding the effects of wide bandgap semiconductor surface chemistry/structure on sensitizer function is a challenging but crucial step towards better dye-sensitized solar cells (DSSCs), photocatalysts and molecular electronics. Unfortunately, the subtleties of electron transfer (ET) kinetics at these interfaces are often lost in the stretched exponential and multicomponent fits required for even simple systems. In contrast, we elucidate the dependence of photoinduced electron transfer on semiconductor surface properties without ensemble averaging - through single particle spectroscopy of quantum dot chromophores bound to relevant TiO2 surfaces. When comparing the traditional nanoparticle platform (np-TiO2) to oriented polycrystalline thin films (pc-TiO2) grown by atomic layer deposition (ALD) we observe a striking difference. Further systematic variation of the surface defect density affords an experimental handle to deduce the structure-function relationship underlying the different ET distributions. We find that pc-TiO2 films result in significantly reduced ET heterogeneity compared to the np-TiO2 films. As a result of this reduced heterogeneity we distinctly resolve two peaks in the ET kinetics, each of which rivals the width (heterogeneity) measured on ET-inactive substrates. We are further able to assign the faster rate to TiO2 surface defects by using sub-monolayer ALD growth of aluminum oxide, which selectively binds to the defect sites and â?oturns offâ? the fast component of the ET rate distribution.

It is desirable to remove NOx exhausted in the stationary emission source and the mobile emission source because the emission of NOx which is causative of town smog and acid rain is brought under control strictly. In the case of the stationary emission source such as a power station, waste incinerator, and industrial boiler, NOx in exhaust gas is removed ordinarily by the selective catalytic reduction (SCR) process with NH3 (NH3â?"SCR) in the presence of O2 at high temperature (from 573 to 673 K) over V2O5â?"WO3/TiO2 and V2O5â?"MoO3/TiO2. The NH3â?"SCR process was developed by three Japanese corporations (Hitachi Ltd., Babcockâ?"Hitachi K.K., Mitsubishi Petrochemical Corp.). Nowadays, almost stationary emission sources are loaded with the NH3â?"SCR system such as a strong deâ?"NOx process. The exhaust gas contains various polluted gases and materials, e.g., NOx, SOx, halogen compounds, particulate matter (PM) and fly ash, etc. The several removal systems are arranged in outlet of industrial plants to eliminate these materials efficiently. In the case of the waste incinerators which exhaust the high concentration of SOx and halogen compounds, the NH3â?"SCR process is often located downstream of the deâ?"halogen and/or deâ?"SOx processes because the catalysts used in the NH3â?"SCR process (V2O5â?"WO3/TiO2 or MoO3â?"WO3/TiO2) are deactivated rapidly by halogen compounds and highly concentrated SOx. The outlet temperature of the deâ?"halogen and/or deâ?"SOx processes falls below 453 K because they are based on wet mixed shotcrete which was carried out by spray of Ca(OH)2 and Mg(OH)2 solutions. In the NH3â?"SCR process, it is necessary to reâ?"heat the catalyst bed and gas in order to activate the catalyst. In addition, unreacted NH3 elimination (ammonia slip) is now a significant problem to be solved urgently. Selective catalytic oxidation (SCO) of NH3 to N2 is a potentially available method in order to reduce the ammonia pollution and accordingly SCO technology attracts interest recently. The SCO process may also be applied to the NH3â?"SCR process for removing unreacted ammonia. Because of this situation, some authors reported novel catalysts activated at low temperatures. Photocatalysts are known to promote the reaction at ambient temperatures and low pressures. TiO2 is the most wellâ?"known photocatalyst and many studies have been done using it. We carried out photocatalytic SCR of NO with NH3 (photoâ?"SCR) and SCO of NH3 (photo-SCO) over TiO2 under mild condition, identified adsorbed species and intermediates derived from NO, NH3, and O2 on TiO2 by means of FTâ?"IR and ESR spectroscopies, and proposed the reliable reaction mechanisms.

The synthesis of 1D TiO2 nanomaterials is one of the main topics of interest in the development of new photocatalysts for environmental issues. In this respect, due to its low cost, and easy scale up, hydrothermal techniques based on the treatment of TiO2 powder under alkaline conditions have been developed in the last years. This treatment leads to titanate materials containing large amounts of sodium (or potassium) which have be removed to obtain after calcination TiO2 nanotubes or nanowires with high specific surface areas. Experimental conditions of the hydrothermal treatment and of the calcination step strongly influence the final composition and morphology. Control of structural and textural properties of the final nanostructured TiO2 solids has recently opened possibilities to evaluate the importance of morphology and composition of TiO2 nanostructures in the photocatalytic degradation of organic pollutants (formic acid, phenol). Herein, UV-visible, Raman, XRD, and TEM characterizations were first carried out to determine precisely the influence of the calcination step on the final composition and morphology. The TiO2 nanomaterials were then evaluated in the photocatalytic degradation of formic acid (FA) showing that the combination of a high surface area and of the formation of an anatase phase on nanotubes is the key parameter leading to very active photocatalysts materials 4 times more active than P25. A direct correlation was found between surface area and photocatalytic activity showing that both external and internal surfaces of nanotubes participate in the reaction. Interestingly, in the degradation of phenol, an opposite trend was observed with an increase of the photocatalytic efficiency with the decrease of the surface area, the higher photocatalytic efficiency being found for TiO2 nanowires obtained by calcination at 700°C. This shows a complete different photocatalytic mechanism to the one observed for the degradation of FA. Finally, TiO2 nanomaterials were tested to determine their photocatalytic efficiency in the degradation of FA in the presence or not of Fusarium Solani fungi (FS12) under visible light conditions. Results show a high activity for the degradation of FA using TiO2 nanotubes. Degradation of FS12 was also observed. However, in the presence of both FA and FS12, while on P25, the competition for active sites between FA and FS12 leads to a retarded kinetics of degradation of FS12 compared to FA, TiO2 nanotubes are able to degrade efficiently and simultaneously FA acid and FS12 making these materials well suited for photocatalytic wastewater treatment

A sandwich-structured photocatalyst shows an excellent performance in degradation reactions of a number of organic compounds under UV, visible light, and direct sunlight (see picture). The catalyst was synthesized by a combination of nonmetal doping and plasmonic metal decoration of TiO2 nanocrystals, which improves visible-light activity and enhances light harvesting and charge separation, respectively. Furthermore, faceted TiO2 nanocrystal with exposed (001) facets have been successfully synthesized. The synthetic mechanism as well as the enhanced photocatalytic performance have been systematically studied.

One-dimensional (1-D) TiO₂ nanostructures such as nanowires, nanotubes, and nanobelts have been studied extensively for their potential photocatalysis usage in elimination of pollutants and water splitting for hydrogen generation. In particular, the controllable synthesis of 1-D TiO₂ nanostructures on a supporting substrate has been of great interest. Another desirable aspect of a synthesis method is that it allows effective modification of TiO₂ by multi-element doping in a simple and low-cost manner, such as in-situ doping. The wide bandgap energy (~3 eV) of TiO₂ limits its photocatalytic reactivity to ultraviolet light. In order to extend the photocatalytic reactivity to visible light regime, transition metals or nonmetals such as carbon and nitrogen should be doped into TiO₂. An appropriate combination of two or more dopants has also been known to be very effective because of their combined or synergetic photocatalytic effects. Among a number of synthesis methods, chemical vapor deposition (CVD) is potentially the most attractive technique for the direct formation of 1-D TiO₂ nanostructures, along with in-situ doping of multi-elements on a substrate, because of its merits of vapor phase deposition and chemical flexibility. In this presentation, we report a simple metalorganic CVD (MOCVD) technique to synthesize C and Fe co-doped TiO₂ (denoted as C:Fe:TiO₂) nanobelts on a substrate. This MOCVD-based synthesis not only provides self-doped C but also facilitates the simultaneous controllable in-situ doping of Fe, which are utilized effectively to enhance both UV and visible-light photocatalytic reactivity. Moreover, the C:Fe:TiO₂ nanobelts are self-catalytically grown without the use of any metal catalysts such as Au. We reveal for the first time a unique growth mechanism that mediates the TiO₂ nanobelt growth through a self-formed nanographitic layer. As a consequence, the vertically aligned nanobelts are a hybrid form with a nanographitic layer, a structure which holds significant promise for the electrode applications of solar cells and Li-ion batteries as well. We will further discuss the self-catalytic growth mechanism and photocatalytic characteristics of the vertically-aligned C:Fe:TiO₂ nanobelts.

Thin TiO₂ coatings can be used to create transparent, photocatalytically active, self-cleaning, antifogging, and superhydrophilic surfaces and therefore have a very high industrial relevance. For high scale industrial applications, it is important to develop cheap, fast and environmentally friendly deposition techniques. We developed two different types of aqueous TiO₂ precursors that can be used for the ink-jet printing of transparent TiO₂ coatings and performed jetting test in different types of ink-jet printers. A first type of ink contains Ti⁴⁺ ions stabilized in solution by selected chelating agents. After heat treatment at 500 °C, wet printed layers of this precursor are converted into anatase TiO₂ coatings. These layers are 80 to 500 nm thick, transparent and exhibit high photocatalytic activity. Yet, for many applications, it is important to reduce the minimal conversion temperature in order to be able to use a wider range of substrates. Therefore, we developed a second type of titania precursor ink which contains preformed TiO₂ nanoparticles suspended in water. These particles are synthesized by a microwave-assisted hydrothermal synthesis to optimize production efficiency. Preliminary tests reveal that TiO₂ layers obtained from this second type of precursor can be converted into functional layers at temperatures lower than 500 °C.

The interaction of hydrogen, water and titanium dioxide nanoparticles will be discussed.

A novel method for producing hierarchically porous TiO2 has recently been developed that allows a tailored combination of macroscopic (50-1000 um) highly interconnected pores, with wall struts containing highly interconnected nano-pores/channels. The fabrication method, gel-casting, involves preparation of a slurry mix with tailored rhealogical properties and foaming by mechanical whipping. It is then cast into a mold where it is set by polymerisation of one of its' monomeric components. The method allows the designed porosity at the nano-scale (both intra-strut channel size and percentage) via altering the ceramic to polymer loading and sintering temperature, and at the macroscale by altering the pressure during foam setting. The resulting foams can either be used as TiO2 superstructures, or using an electrochemical reduction process converted into metallic (CP2 Ti) foam. Porous TiO2 structures find numerous applications such as in sorption media, photo catalysis, water and gas purification filters and sensors. It has been deemed as a suitable tissue engineered bone implant material (Haugen H. et el, J Eur. Ceram. Soc., 24 (2004) 661-68); metallic titanium foam is a preferred material of choice among orthopaedics. The nano-millimeter scale hierarchical structure of both types of the foam (i.e., ceramic or metallic) is considered advantageous for most of the applications. Thus, its nano-scale characterisation and relationship to macroscopic properties is of paramount importance. In all the applications, the unique feature is the easily tuned nano scale porosity. This talk focuses on the application of laboratory and synchrotron nano-tomography to characterize the tortuous, three dimensional structures. X-ray microscopy has the unique ability to quickly characterize the foams with minimal sample preparation. Furthermore, the non-destructive nature of the imaging method enables repeated characterization of the same sample to better understand, for example, the effects of different processing conditions on the foam nano-structures. In this work, the application of state of the art, laboratory- and synchrotron-based x-ray microscopy is presented to studies of the TiO2 foam pore networks. The novel system architecture enables 3D resolution as high as 50 nm in the laboratory and 30 nm in the synchrotron, lending itself naturally to studies of both the micro- and nanostructures. Methods of quantifying these structures, and algorithms for measuring the accessible reaction area per unit volume are presented. The voxel data is also used to calculate the permeability of the structure at multiple length scales, enabling a more comprehensive understanding of these materials than seen previously.

Dye Solar Cells (DSCs) based on dye sensitized mesoporous TiO2 active layers are a promising low cost PV technology. High power over large areas can be obtained through series connection of cells in modules. Efficiency and stability are fundamental not only at the cell but also at the module level where new issues appear which need to be tackled carefully. In a module cells have to be matched to optimize power output under illumination and to avoid significant imbalance. These mismatches can be due to shadowing, differences in cell production or degradation rates. When the performance of a cell is impaired it operates in reverse bias regime due to the output of the other cells of the module. Reverse bias effects are thoroughly studied in traditional photovoltaic technologies, and protection schemes put into place, but have not yet been deeply investigated in DSCs. In this work we show that prolonged reverse bias stress can cause the operating point of the fully shadowed/mismatched cell to drift with time leading eventually to device degradation. A detailed analysis was performed on both single cells and on modules consisting of 5 series-connected cells. Single cells were stressed in the reverse bias regime by applying a fixed current, equal in module to the short circuit current. The modules were aged inside an ageing chamber for 2500h at 55°C under 0.8 Sun illumination. In order to gauge effects of reverse bias, one of the five cells of the module was completely shaded for the duration of the experiment. The I-V characteristics in dark and under 1 Sun, the IPCE, the Electrochemical impedance spectroscopy (EIS) data and cell absorbance were monitored in time intervals to obtain the stability trend. Chrono-potentiometries revealed that the module of the reverse bias voltage Vrb, at fixed applied current, across the shaded cells increased over time. This occurred through two phases: an initial slow increase which produced loss of fill factor, and thus efficiency, and later a much quicker increase which led to the complete breakdown of the reverse-biased cell. EIS and dark I-V characteristics showed that the first phase was accompanied by the decrease of the triiodide I3- concentration inside the electrolyte. Spectral investigations showed a reduced IPCE and a blue shifted spectrum. Prolonged high negative voltages across the cell caused decomposition of the electrolyte and vapour formation within the cell, likely due in part to an electro-catalytic effect operated by the dye covered TiO2 active layer. This was followed by leakage of electrolyte and dye degradation/desorption off the TiO2 and device breakdown. The electrolyte composition was changed in order to see how the I3- concentration and presence of water affects the stability of cells under reverse bias. The thorough investigation of reverse bias regime is important for outdoor module stability in real operating conditions and also to suggest protection strategies for this technology.

Electrophoresis, the motion of charged colloids relative to a fluid under the constant electric field, is frequently used for film deposition and display. In these applications, mobility of colloids and the origin of charging are important in their performance because they are closely related to the response time. Charge mechanism of colloid in ion-contented polar solvent is explained by zeta potential; when colloids are in fluid, their surfaces react with the fluid and have certain charge, and the potential generated from the counter ions which are attracted by surface charge is zeta potential. For cases of TiO₂ in polar solvent, it has negative surface charge in fluid because oxygen of the surface uptakes H⁺ ion and turns to OH^-. However, the origin of surface charge of TiO₂ in the non-polar solvent is not known because under the non-polar solvent, charge from the solvent is not available. As non-polar is preferred for electrophoretic applications because of its non-reactive properties and low density, understanding of charge mechanism of TiO₂ in non-polar, ion-free solvent are needed. We studied the electrophoretic characteristics of TiO₂ in ion-free solvent. Cyclohexanone was used for dispersion solvent colorized by blue dye and solvent with TiO₂ colloids injected between two ITO glasses. After sealing glasses, electric field was applied between the ITO glass and the movement of colloids was monitored by the change of reflectivity of top ITO glass. Difference of color between solvent and colloids caused the change of reflectivity according to the movement of colloids. The change of reflectivity was monitored as a function of time while in various voltages, so the response time and mobility of particle can be measured. Various sizes and crystal structures of colloids were studied as well. The change of the reflectivity in the top ITO glass has monitored in the case of anatase TiO₂ when the electric field changes, while no change occurs in amorphous TiO₂. Because there are no ions in the solvent, the surface charge of TiO₂ must be driven from intrinsic properties of TiO₂ such as crystal structure and size. The charge occurs not only in surface but also in inner part of colloids, therefore small-sized colloids which have less surface charge are neutralized earlier and show fast reflectivity decrease. The origin of surface charge and behavior of TiO₂ colloids in various filed condition are discussed as well.

We report an alternative process to produce free-standing dual-layer TiO2 nanotube arrays (TNTs). In this process, a thin TNT layer was fabricated via anodization of pure titanium foil in fluorine solution containing ethylene glycol. Then further anodization at a different voltage was performed to grow a second layer below the first anodized layer. After annealing the dual layers and followed by a final step anodization, free-standing and dual-layer crystalline TNTs were self-detached from the Ti substrate. The morphology of the free-standing dual-layer TNT with thickness equal to 9.2 µm have been investigated by Scanning Electron Microscopy (SEM). The SEM images show that the TNT diameters are 75 and 50 nm for the first and second layers respectively. An opposite structure with a smaller diameter for the first layer arrays is difficult to be fabricated due to strong electrochemical etching from the electrolyte. Meanwhile, the first layer serves as a sacrificial layer to protect the continuous growth of the second layer from chemical dissolution. The second TNT layer grows below or among gaps of the first layer TNT arrays. The total thickness of the first layer (40V, 2hrs) was decreased from 25 to 7.6 µm after an additional anodization at 30 V for 2 hrs during which the second TNT layer is formed with thickness equal to 1.6 µm. By taking advantage of different mechanical stability and etching contrast between amorphous and anatase layers, crystallized dual-layer TNTs were effectively detached from the underlying substrate at final anodization step and identified by X-ray diffraction technique. This process provides a powerful approach to tailor the vertical properties of self-organized TNTs, which have far-reaching implications in the design of future nanoscale systems.

TiO2 is among the most well-known materials used as a photocatalyst. However, the wide band gap of TiO2 (3 to 3.2 eV) limits its photocatalytic utility to the Ultra Violet (UV) region of the electromagnetic spectrum, which is less than 5% of the total range. TiO2 has three main phases: anatase, rutile and brookite; among which anatase and rutile are more stable phases. Titanium dioxide can be doped in order to shorten its band gap so that visible sunlight can be used to induce this reaction. This photocatalytic technology can lead to a low cost, environmentally friendly method for purification of water. Among the different methods that have been suggested for the band gap reduction of TiO2, cation or anion doping offers a potential route. Here, we test the effects of low atomic weight transition metal doping on the optoelectronic and photocatalytic properties of TiO2 both in theory and experiment. Density functional calculation of doped samples is provided to support the trend of band gap reduction in the prepared samples. We use Perdue Burke Ernzerhof (PBE) pseudopotentials which were implemented in Vienna ab initio simulator (VASP) [1, 2]. The cut-off energy is set at 400 eV. The study is done on two main types of supercells of doped and non-doped TiO2 to distinguish the optoelectronic effects of low-atomic-weight transition metals. The calculation is performed for spin-up and down and the possible ferromagnetic properties are studied. Fe, Mn and Cr doped TiO2 have been prepared through a hydrothermal approach using metal salts as a source of the dopants. Phase and structural characterization of the synthesized powder were performed using X-ray diffraction, Scanning Electron Microscopy and Energy Dispersive Spectroscopy. The optoelectronic properties of the resulting materials have been investigated through UV-vis absorption spectra. We demonstrate a band gap reduction and red-shift up to 600 nm. On the other hand, we investigate the a low-temperature, high pressure and environmentally friendly method to synthesize single phase and co-phase TiO2 in different morphologies which can lead to synthesis of high surface area, high porosity and one dimensional TiO2 . The recent product can be the template for our doping methodology and provide a low band gap, high surface area, multi-phased, doped TiO2. [1] G. Kresse, J. Hafner, â?oAb initio molecular dynamics for liquid metalsâ?, Phys. Rev. B 47, 1993, pp 558â?"561. [2] J.P. Perdew, K. Burk, M. Ernzerhof, â?oGeneralized Gradient Approximation Made simpleâ?, Phys. Rev. Lett,77, 1996, pp 3865â?"3868.

Proton exchange membrane fuel cells (PEMFCs) provide the highest power density in comparison with all other fuel cell types currently available. In order to improve the fuel cells efficiency, further development into enhancing the thermal stability of the PEMFCs membrane has been a continued focus for decades. Unfortunately, the polymer structure of Nafion membranes falls short of structural stability at elevated temperatures. However, based on our preliminary data, intercalated titanate nanobelt/Nafion composites may hold the key to overcoming these technological drawbacks. Titanate nanomaterials harbor a lattice matrix of negatively charged edge-shared TiO6-octahedra. The location of intercalated cations within the interlayer space may dictate the charge-conductions. This environment may in turn bestow the nanocompositeâ?Ts elevated temperature, ion-conduction, and interfacial interaction between Nafion and titanate nanobelts. Here synthesis and characterization of the nanobelt-Nafion composite membranes with varied loading weight ratio of nanobelts will be discussed. Successful fabrication of new nanocomposite membranes will be shown.

Nano sized monodispersed carbon doped Titanium Dioxide powders were synthesized in a low pressure premix flame system. The phase structure of the synthesized TiO2 powders was controlled by varying the operating conditions of the premix flame system and thus it was possible to obtain any of the three natural phases of TiO2 namely Anatase, Rutile and Brookite in this system. The crystallinity of the samples were confirmed by X-Ray Diffraction and Raman spectroscopy. The Transmission Electron Microscope and Scanning Electron Microscope were utilized to analyze the morphology, and had shown that the particles sizes ranged from 3-7nm and exhibit agglomerated particle size in the order of micron. UV-Vis spectroscopy was employed to study the shift in band gap of carbon doped TiO2 powders, it was noted that Carbon doped Anatase, Rutile and Brookite had a bandgap of 2.9 eV, 2.5 eV and 2.2 eV respectively, compared to a bandgap of 3.2 eV of undoped Anatase TiO2 . Thus carbon doped TiO2 showed visible light absorption. Furthermore, XPS was employed to show that our sample was carbon doped with the Ti-O-C type chemical structure. Finally, it was demonstrated that the as synthesized powder was capable of photo-degrading phenol. Carbon doped Brookite showed higher photo degradation compared to Rutile and Anatase because of its lower band gap and increased visible light absorption.

Currently, the main component of dental bleaching agent is hydrogen peroxide. In order to improve the bleaching effect, it is always associated by the light irradiation, such as LED and halogen lamp. TiO2 nanopaticles are widely used as photocatalysts in industry. Recent studies have shown that the addition of titanium dioxide activated hydrogen peroxide, and made the dental bleaching more effective. The activation degree may be different by the wavelength of light, because the TiO2 nanopaticles initiate photocatalytic reactions effectively with UV. So the purpose of this study is to find the proper wavelength of light which can maximize the activation of hydrogen peroxide containing TiO2 nanopaticles. We prepared 3.5% hydrogen peroxide solution (Group A), 0.01% TiO2 nanopaticle colloid (Group B) and 3.5% hydrogen peroxide solution contained 0.01% TiO2 nanopaticles (Group C). Methylene blue was added to each group as an indicator of the bleaching effect. For the three different wavelength of light sources, LED (430-480 nm peak: 460 nm), UVA (300-425 nm peak: 356 nm) and UVB (275-350 nm peak: 306 nm) were chosen. UV/Vis spectrometer was used to measure the absorbance of each experimental group on every minute for 12 minutes after the irradiation of each light source to determine the concentration of methylene blue. As the result, on the effects of the wavelength of light, the order of reduction rate of methylene blue concentration was UVA>UVB>LED in all groups(p<0.05). On the effects of experimental groups, the Group C caused the most significant reduction in methylene blue concentration in all light sources(p<0.05). Therefore, it is expectant that using the hydrogen peroxide containing TiO2 nanopaticles as dental bleaching agent associated with UVA irradiation will make tooth whitening more effective.

Titanium dioxide (TiO2) has received special interest as an anode material for lithium ion batteries due to its abundance, low cost, and its structural stability during Li insertion/extraction. However, the poor rate capability of TiO2 electrodes limits their practical use. We synthesized nanosized, surface functionalized TiO2 hollow fibers that are capable of decreasing the diffusion length of Li-ion and improving electron conductivity. Nanosized TiO2 hollow fibers with outer diameters of 200 nm and inner diameters of 160 nm were prepared via electrospinning, and the surface of the resultant TiO2 hollow nanofibers were sunsequently functionalized by nitridation under NH3 atmosphere. During the nitridation treatment, a uniform electron conducting layer composed of TiN formed along the inner and outer surface of TiO2 hollow nanofiber, which improved the rate capability of the TiO2 hollow nanofiber anode. The nitridated TiO2 hollow nanofibers showed twice higher rate capability compared to that of pristine TiO2 nanofibers. This dramatic increase in a rate capability signficantly advances the potential use ofTiO2 anode materials. Furthermore, our electrode design strategy could be expanded and applied to other metal oxide materials such as Cu2O, SnO2, and Co3O4 for the realization of high performance lithium ion batteries.

The discovery of photoelectrochemical splitting of water on TiO₂ electrodes by Fujishima and Honda in 1972 started the fast development on semiconductor-based photocatalysts. Due to its high chemical stability, good photoactivity, relatively low cost and nontoxicity, TiO₂ has appeared as the leading candidate among various semiconductor-based photocatalysts, especially for industrial use. Recently, anatase TiO₂ single crystals with controlled facets attract a lot of research interests. Both theoretical and experimental studies show that anatase TiO₂ with {001} facets is much more active than anatase TiO₂ with thermodynamically stable facts. Currently, most reported anatase TiO₂ samples with {001} facets are in the micrometer size range, so their specific surface area values are quite small which is not desirable for a good photocatalytic performance. Another intrinsic limitation of anatase TiO₂ with {001} facets is that it could not be activated without the ultraviolet light (λ400 nm) illumination due to its relatively large band gap (~ 3.2 eV), which seriously limits its solar efficiency. Since Asahi et al. reported that nitrogen doping of TiO₂ extended the optical absorbance of TiO₂ into the visible-light region, anionic nonmetal dopants, such as nitrogen, carbon, sulfur, or fluorine, have been entensively explored for visible-light photocatalysis. In this work, we combined the morphology control technique and the anion-doping technique, and successfully created nitrogen-doped TiO₂ nanocrystals with a high percentage of {001} facets. By the adoption of a solvent-thermal process to replace the hydrothermal process, nano-sized anatase TiO₂ crystals with a high percentage of {001} facets were obtained which largely enhanced their specific surface areas. The further nitrogen-doping process introduced the visible-light-activity to this material system. Their enhanced photocatalytic performance under visible light illumination was demonstrated by the degradation of Methylene blue (MB), compared with P25 TiO₂ nanoparticles.

The ultraviolet (UV) photon-stimulated reactions in oxygen adsorbed on reduced TiO₂(110) are studied. For chemisorbed O₂, the photochemistry depends on the O₂ coverage. For small coverages (e.g. less than 1 O₂ per bridging oxygen vacancy), only ~14% desorbs while the rest either dissociates during UV irradiation, or remains molecularly adsorbed on the surface. For the maximum coverage of chemisorbed oxygen, the fraction of O₂ that photodesorbs is ~40%. However when physisorbed O₂ is also present, ~70% of the initially chemisorbed O₂ photodesorbs. Experiments using O₂ isotopologues show that UV irradiation results in exchange of atoms between the chemisorbed and physisorbed oxygen. Annealing chemisorbed oxygen to 350 K maximizes these exchange reactions, while dissociatively adsorbing oxygen on TiO₂(110) at 300 â?" 350 K does not lead to the exchange reactions. The exchange products photodesorb in the plane perpendicular to the bridge-bonded oxygen rows at an angle of 45° to the surface normal. Remarkably, the chemisorbed species is stable under multiple cycles of UV irradiation with physisorbed O₂, and the atoms in the chemisorbed species can be changed from ¹⁸O to ¹⁶O and then back to ¹⁸O via the exchange reactions. The results show that annealing oxygen adsorbed on TiO₂(110) to ~350 K produces a stable chemical species with interesting photochemical properties. Possible forms for the photoactive species include O₂ adsorbed in a bridging oxygen vacancy or tetraoxygen. We propose that both hole- and electron-mediated reactions are important for O₂ adsorbed on TiO₂(110). A simple model based on the oxygen coverage and the charge of the chemisorbed oxygen, which accounts for the observations, is presented.

The photoelectric properties of oxygen-deficient titanium dioxide (TiO2) nanotube arrays are investigated in this study. The TiO2 nanotube arrays are prepared by anodization, followed by annealing at 450 to 750 C for 3 h in air to form different crystalline phase mixtures. When the annealing temperature is increased, several phenomena are observed: (1) the ratio of anatase to rutile decreases, (2) the anatase nanotubes are shortened and (3) the thickness of the dense rutile film layer underneath the anatase nanotubes increases. The efficiency of visible light absorption of the nanotube arrays is enhanced with increasing annealing temperature. This is believed to be caused by the ionic defects, especially the oxygen vacancies, generated during the annealing procedure, enabling the absorption of low-energy radiations. The X-ray photoelectron spectroscopy (XPS) depth profile analysis provides the supporting evidence on the chemical nonstoichiometry (i.e., oxygen-deficiency) of the TiO2 nanotube arrays annealed at high temperatures. With increasing annealing temperature, a decrease and an increase in the photocurrent density of the nanotube arrays under UV and visible light (wavelength > 500 nm) irradiations, respectively, are detected. The decrease of the photocurrent density under UV irradiation is caused by the reduction in the specific surface area (i.e., anatase nanotubes transform into rutile film with vigorous annealing). In contrast, the increase of the photocurrent density under visible light irradiation is contributed to the oxygen vacancies in the nanostructure, providing extra energy levels (locating below the conduction band of TiO2) within the band structure.

Photo-catalytic conversion of CO2 into hydrocarbon fuels is an ideal avenue for mitigating the CO2 emission. Ever since the photo-catalytic reduction of CO2 was discovered to take place at TiO2 surface a few decades ago, many have been extensively working to improve the photo-catalytic reaction on the TiO2 surface. It is commonly believed that the photo-catalytic reduction of CO2 into methanes/methanols could take two different reaction routes through an intermediate step forming formic acid or carbon mono-oxide. Using first principles calculations, we investigated that both reaction mechanisms on the surfaces of bulk Anatase TiO2(101) and a small TiO2 nano-cluster. We will discuss significant differences we observed for these two cases and elucidate role of quantum confinement and the tetrahedrally-coordinate Ti atoms.

The stability of nano-structured TiO₂ Dye-Sensitized solar cells (DSCs) is an important issue faced by those working on the up-scaling and industrialisation of the technology. One aspect of DSC stability in need of addressing is the UV photostability of devices intended for long term outdoor usage in areas of high insolation. DSC devices exposed to UVA radiation (0.64 Wm^-2 at a λ_max of 354 nm) undergo degradation resulting in failure within 400 hours total exposure. Initial changes to J_SC and V_OC point to a positive shift in the TiO₂ conduction band. In addition, an initial decrease in the recombination resistance suggests a removal of species from the TiO₂ surface. At the point of cell failure, J_SC collapses and the recombination resistance undergoes a dramatic increase, suggesting that there has been a removal of charge carriers from the electrolyte. UV-Vis measurements show a decrease in the absorbance of the cells at 450 nm indicating that it is the triiodide that is being consumed. This is contrasted with apparently little change in the absorbance of the cell at 530 nm, suggesting that UV exposure leaves the dye unaffected, at least in the time frame of the experiments conducted. It is shown that cells degrade more quickly when placed under an electrical load (than under open circuit) and it is therefore proposed that the cause of triiodide removal is photooxidation via photogenerated TiO₂ holes, as when an external circuit is present, electrons in the TiO₂ conduction band will be exported, resulting in less TiO₂ electron-hole recombination and thus an increase in the concentration of oxidative holes. Further evidence of the photocatalytic nature of cell failure is obtained by the fact that cell degradation can be slowed considerably by filtering out those wavelengths that cause direct TiO₂ photoexcitation. Following UV exposure it has subsequently been shown that the triiodide can be regenerated by application of a reverse bias and that photocurrents can be restored to near original levels in cells that had previously suffered a collapse in J_SC. The long term effects of periodic triiodide regeneration on UV exposed cells is currently under investigation with the results having obvious implications for long term cell stability.

Dye-sensitized nanostructured solar cells (DSSCs) are considered to be one of the alternative solar cell technologies to compete with the traditional silicon based solar cells in the future. However, for DSSCs to be relevant to industrial applications, their long-term stabilities have to be fully understood. Water (or moisture) is one of the various factors that have been generally perceived as harmful to the efficiency and long-term stability of DSSCs. However, a recent study suggests that high water content in the electrolyte may not be linked to the poor efficiency and stability of water-containing DSSCs [1]. To fully understand the effect of water on the cell performance and stability, it is critical to understand the impact of water on the charge carrier dynamics during the solar conversion processes. In this presentation, we discuss the results of our studies on the effect of water on the performance, stability, and charge carrier dynamics of DSSCs based on commonly used I3â?"/Iâ?" electrolytes. Charge transport and recombination properties of DSSCs were studied by electrochemical impedance spectroscopy. Adding water is found to affect strongly the energetic positions of the electrode/electrolyte interface and the charge carrier dynamics in DSSCs. The underlying mechanisms that determine the relationship between the charge carrier dynamics and the photovoltaic characteristics of DSSCs are discussed. The effect of adding water on the long-term stability of the cells is also discussed. References [1] Law, C. H.; Pathirana, S. C.; Li, X. O.; Anderson, A. Y.; Barnes, P. R. F.; Listorti, A.; Ghaddar, T. H.; O'Regan, B. C. Advanced Materials, 2010, 22, 4505.

In spite of size and shape as a decisive factor to identify the photocatalytic activity of TiO2, TiO2 nanoparticlulate catalysts precisely controlled in size and shape have never been systematically synthesized, because of many technological barriers to obtaining well-defined TiO2 catalyst materials. The objective of the present study is to investigate effects of morphology and surface crystal plane of anatase-type TiO2 nanoparticles, prepared by the Gel-Sol method, upon the photocatalytic activity. The effect of morphology, that is, exposed crystal planes of anatase-type TiO2 nanoparticles on the photocatalytic activity has been systematically investigated. Anatase-type TiO2 nanoparticles with specific size and shape, used in the present study, were obtained by the Gel-Sol method with using ammonia and sodium oleate as a shape controller. Initially, titanium(IV) isopropoxide and triethanolamine were mixed at a molar ratio of 1 : 2 under dry atmosphere, then, final concentration of the titanium(IV) was fixed to 0.50 mol dm-3 by addition of ion exchange water to obtain an aqueous stock solution of titanium(IV). Next, this stock solution and ammonia or sodium oleate aqueous solution were mixed. The resulting solution was placed into a screw-capped Pyrex bottle and aged at 100 °C for 24 h to obtain gel. Then, the gel was transferred to a Teflon-lined autoclave and aged at 140 °C for 3-4.5 days. The Ni-loaded TiO2 nanoparticles with a cubic morphology showed the highest photocatalytic activity for H2 evolution from ethanolic aqueous solution. Judging from HRTEM and FFT images, the cubic shaped TiO2 nanoparticles were found bounded by {100} and {001} faces. Hence, {001} surface promotes H2 evolution over the photocatalytic reaction. As a result, the precise control of morphology and crystal surface plane of nano-sized particles is critically decisive for photocatalytic activity. Enhancement of photocatalytic activity of other oxides is probably achieved by control of shape and surface plane in monodispersed nanoparticles.

Rutile TiO2 is a wide-gap semiconductor with possible applications within fields such as optics, electronics, catalysis and photo-electrochemistry. Recently it has been speculated whether the photo catalytic properties of TiO2 are due to metal vacancies which may be formed under oxidizing/humidifying conditions: 2H_2 O(g)+2O_O^(x)=v_Ti^(4/)+4OH_O^(*) (1) In this work we have studied the defect chemistry of rutile TiO2, including defect pair formation, by combining DFT calculations with thermodynamic modeling. To address the effect of exchange and correlation, all bulk calculations were performed using the GGA, GGA+U (U=4.2 eV) and HSE06 (intermixing 20% exact exchange) functionals. All defect formation energies were evaluated as function of supercell size and extrapolated to the infinite dilution limit. By combining the formation energies with tabulated gas chemical potential, and applying the electroneutrality condition, we have calculated the thermal equlibrium defect chemistry of bulk rutile TiO2 under various conditions, involving all common point defects and their mutual complexes. The segregation tendancies of the defect are also studied towards a (110) terminated surface using a slab model consisting of 336 atoms. With both the GGA+U and HSE06 functionals, the bulk donor defects OH_O, H_O, v_O and Ti_i with excess electrons lead to in-gap states due to trapping of one or more electrons at bulk Ti4+, whereas GGA only yields the delocalized solutions. The corresponding charge transition levels also fall within the band gap when calculated using the +U formalism. However, with HSE06 (and GGA) the transition levels appear close to the conduction band, indicating that the before mentioned donor defects behave as shallow donors in rutile TiO2. From the thermal equilibrium defect chemistry, bulk rutile TiO2 is predicted to be dominated by OH_O^(*) and its complexes with v_Ti^(4/). At moderate temperatures, the complexes dehydrate, and v_Ti^(4/) come up as the majority defect, compensating OH_O^(*) according to Eq. 1. At higher temperatures, v_O^(**) and its complexes with v_Ti^(4/) come up as the majority defects, while OH_O^(*) leaves the material. Further, n > p at all temperatures and the material will thus be n-type. The surface calculations show that v_O^(**), OH_O^(*), vTi^(4/) and the complexes all have negative segregation energies towards the (110) surface, but the segregation properties are most pronounced for v_O^(**). Finally, the possible precence various defects and their impact on the reactivity of TiO2 with H2O is discussed.

We present a joined experimental and first-principle investigation of the TiO₂ polymorphism effects on dye-sensitized solar cells (DSSC) photovoltaic properties. TiO₂ building blocks based on pure anatase, pure rutile and pure brookite stabilized phases with various sizes and shapes have been prepared by growth techniques in solution in order to evaluate their properties in photovoltaic devices. For a valuable comparison, these various nanoparticles have been used to construct identical solar cells, i.e. fabricated in a similar manner. Their properties have been finely estimated and analyzed by impedance spectroscopy measurements and computed data using an ab-initio density functional theory (DFT) approach. We show that the open circuit voltage (V_OC) of the solar cells ranges in the following order rutileâ?¤anataseâ?¤brookite which is explained in the light of DFT calculations by the conduction band edge position depending on the TiO₂ phase. Also important are the quantifications of electron lifetimes, transfer times, diffusion coefficient in the various TiO₂ photoanodes. For the best cells the polymorph conductivity order is rutileâ?¤brookiteâ?¤anatase and the present study illustrates that for similar dye loadings, the porous layer conductivity is a key parameter for explaining the cell performances.

Nanowire/nanorods have demonstrated a great potential to achieve high diffusion coefficient of carriers in electric devices, due to their unique one-dimensional (1-D) structure. When the nanoparticle-based photoanode of DSSC is substituted with the nanorods-based photoanode, it is expected that the nanorods provide a ballistic pathway to the carriers and enhance the carrier transport. It is reported that the stable high fill factor (FF) and increasing short circuit current as the length of TiO2 nanowire gets longer are accredited to the elongated lifetime of the carriers. Nb doped TiO2 is semiconductor material which has attracted increasing interests for its application to the photoanode of dye sensitized solar cells (DSSCs). When Nb id doped, the electronic structure of TiO2 modified to promote carrier extraction and transport in DSSCs. Therefore, the replacement of current mesoporous nanoparticles-based films with Nb doped TiO2 nanowire arrays provices an arena to fully exploit the unique properties of 1-dimensional nanomaterials such as fast carrier transport and less recombination. In this presentation, we report the electrical properties of Nb doped single crystalline rutile nanowires and their effect on the performance of DSSCs. It is found Nb-doping enhances carrier lifetime and electron injection efficiency and of DSSCs by 400% and 60%, respectively. Nanoscale conductive atomic force microscopy (c-AFM) measurement and electrochemical analysis are performed to disclose the underlying mechanism of longer lifetime of carriers and larger carrier injection efficiency. This considerable improvement is ascribed to an increase in electron concentration and a subsequent change in Fermi energy (EF) level. By changing the work function of conductive film coated on c-AFM tips, we conclude that the Schottcky type junction barrier is present at TiO2/electrode interface and dye/TiO2 interface. The height of this barrier is found to depend on EF of TiO2 nanowires. As Nb concentration increases, the electron concentration in TiO2 nanorods increases, which, in turn, decreases the height of barriers at the surface of TiO2 nanorodes and suppresses the loss of electrons during a transport process.

It is demonstrated that incorporation of metallic double-walled carbon nanotubes (DWCNTs) into a TiO2 photoanode results in a significant improvement in the overall energy conversion performance of a dye-sensitized solar cell (DSSC). Under identical materials and process parameters as well as measurement conditions, a DSSC with composite anode containing TiO2 nanoparticles and 0.2 wt % DWCNTs has a short-circuit photocurrent density (Jsc) that is 43% greater than a cell with a standard TiO2 anode. The DSSC power conversion efficiency is also improved noticeably with the addition of DWCNTs (to ~6.4%). The observed enhancement in the solar cell performance is attributed primarily to noticeably reduced microcracking of the TiO2 layer when DWCNTs are incorporated. The carbon nanotubes provide mechanical reinforcement, and the electrical conductivity of the film improves with fewer cracks. Some contribution of the electrically conductive filler material (DWCNTs) to the improved DSSC properties may also be possible, and the scattering characteristics of the photoanode may also be influenced with the presence of carbon nanotubes. Optimized amount and configurations of embedded nano- or micro- sized conducting elements can improve the DSSC performance. The use of continuous metallic fillers[1] for improved solar cell performance and FTO-glass-free DSSC assembly will be described. New approaches for enhanced DSSC using free-standing nanocomposite anode sheets with internally conducting metallic electrical paths will also be discussed. [1]. C. S. Rustomji, C. J. Frandsen, S. Jin, M. J. Tauber, J Phys Chem B 114, 14537 (2010).

Dye-sensitized solar cells (DSCs) are promising next-generation alternatives to conventional silicon-based photovoltaic devices owing to their low manufacture cost. In general, a DSC comprises a nanocrystalline titanium dioxide (TiO2) electrode modified with a dye fabricated on a transparent conducting oxide, a platinum (Pt) counter electrode, and an electrolyte solution with a dissolved iodide ion/tri-iodide ion redox couple between the electrodes. As a power generation device, improvement of its energy conversion efficiency (η) is an important task. In this presentation, we will discuss the strategy for improving η of DSCs. It is well known that the cell performance can be divided into three parts, short circuit density (Jsc), open circuit voltage (Voc) and fill factor (FF), as η corresponds to the product of these. To find ways to improve the η of DSCs, we investigated the internal resistance of DSCs through electrochemical impedance spectroscopy (EIS) measurement as a means of researching DSC mechanisms. According to our analysis, there are four resistance elements, related to the charge transfer processes at the Pt counter electrode (R1), the charge transportation at the TiO2/dye/electrolyte interface (R2), ionic diffusion in the electrolyte (R3), and the sheet resistance of TCO (Rh), in DSCs. Then, researches aimed at achieving high efficiency were carried out based on this equivalent circuit. To increase Jsc, we introduced the concept of haze factor, defined as the ratio of diffused light to the total light transmitted through the electrodes, to quantitatively analyze the dependence of Jsc on haze factor in TiO2 electrodes. The haze factor can be tuned by accurately controlling the size and its distribution of TiO2 particles in the TiO2 mesoporous film to improve the Jsc of DSCs. Furthermore, the adsorption of dye on TiO2 film was also investigated. It is found that Jsc can be increased by preventing adsorbed dyes from aggregation through the introduction of co-adsorbents. Hence, we designed and synthesized new β diketonate Ru complexes with long alkyl chain in order to suppress the dye aggression and gain high Jsc. In order to improve the fill factor (FF), series resistance, composed of the three resistance elements of R1, R3 and Rh were reduced by increasing the roughness factor of the counter electrodes, decreasing the thickness of the electrolyte layer and the sheet resistance of the transparent conducting oxide. Finally, a cell with black dye and new co-adsorbent was fabricated. Current-voltage characteristics were measured by Research Center for Photovoltaic, National Institute of Advanced Industrial Science and Technology (AIST, Japan) using a metal mask and with an aperture area of 0.231 cm2 under standard AM 1.5 sunlight (100.0 mW/cm2). An overall conversion efficiency of 11.4% was achieved which is the highest confirmed efficiency.

This presentation will discuss new results on the physics and chemistry of the mesoporous-TiO2/electrolyte interface including surface charge, trap density, electron diffusion coefficient, and electron transfer time. We also look at the site to site variation in some of the values. Changes in surface charge are invoked to explain a multitude of observations involving TiO2 electrochemistry and photochemistry. Yet surface charge is itself rarely measured. In part this is because the available instruments are often not appropriate in one way or another. We have designed a miniature streaming potential measurement instrument for thin films of nanostructured oxides. With this system we have measured the surface charge of TiO2 in DSSC electrolytes, and the affect of different dyes, adsorbates, and treatments such as the "TiCl4" treatment. In comparing oxide materials with different morphologies, or different sources, one would like to separate effects of changes in trap density from those of changes in charge mobility. We show that the temperature dependence of transport, along with charge density measurements, allows this to be quantified. We have previously pointed out the binding of iodine to dyes is important in DSSCs. Continuing the theme of molecular association, we show that mesopores in oxides have a strong stabilizing effect on reactions that give rise to charged products (e.g. ACN-I+ ), and how this impacts measurement of iodine binding to dyes in-situ. Lastly, we present new femtosecond to nanosecond transient absorption results showing that in standard high efficiency DSSCs electron injection from excited dyes occurs in the 10-100s ps time scale, not in femtoseconds as been found for many model systems.

We present hierarchical periodic crystalline titania nanostructures [1] consisting of macroporous inverse opal titania layers [2, 3] and a mesostructured titania phase within the interstitial voids of the macroporous scaffold [4, 5]. The formation of the mesostructure in the confined space of the macroporous scaffold upon thermal treatment was investigated with in-situ grazing incidence small angle X-ray scattering (GISAXS). The macroporous scaffold strongly influences the mesostructure assembly of the Pluronic P123 containing sol-gel solution. The obtained mesopore dimensions are larger within the hierarchical films than in the mesoporous reference film [4]. The inverse opal scaffolds with 100 and 200 nm large macropores act as a stabilizing matrix and limit the shrinkage of the mesopores upon heating. Additionally, the crystalline walls of the macroporous host beneficially affect the crystallization of the incorporated mesophase into anatase, compared to the reference sol-gel film. The 5 μm thick hierarchical titania structure has a fully accessible porous system and a high total surface area of 154 m2/g. The average mesopore size was 6.1 nm, which is about 20 % larger than the pore size of the reference mesoporous film obtained on a flat substrate. The hierarchical titania structures were used to construct dye-sensitized solar cells (DSCs), showing a conversion efficiency of 4 % under one sun illumination. [1] B. Mandlmeier, J. M. Szeifert, D. Fattakhova-Rohlfing, H. Amenitsch, and T. Bein J. Am. Chem. Soc., 2011, 133 (43), 17274â?"17282. [2] Galusha, J. W.; Tsung, C.-K.; Stucky, G. D.; Bartl, M. H. Chem. Mater. 2008, 20, 4925. [3] Nishimura, S.; Abrams, N.; Lewis, B. A.; Halaoui, L. I.; Mallouk, T. E.; Benkstein, K. D.; van de Lagemaat, J.; Frank, A. J. J. Am. Chem. Soc. 2003, 125, 6306. [4] Alberius, P. C. A.; Frindell, K. L.; Hayward, R. C.; Kramer, E. J.; Stucky, G. D.; Chmelka, B. F. Chem. Mater. 2002, 14, 3284. [5] J. M. Szeifert, D. Fattakhova-Rohlfing, D. Georgiadou, V. Kalousek, J. Rathousky, D. Kuang, S. Wenger, S. M. Zakeeruddin, M. Grätzel, T. Bein, Chem. Mater., 2009, 21, 1260-1265.

Nanocrystalline titanium oxide film has been used as an excellent substrate for energy conversion technologies, such as dye-sensitized solar cell and photocatalytic water splitting. Mesopores in the nanocrystalline TiO2 layer plays a crucial role in such a dye-sensitized solar cell. Dye-sensitized solar cell is composed of a mesoporous TiO2 film, a dye and a redox electrolyte. The dye plays a role in absorbing light, thereby generating electrons and holes, the TiO2 layer acts as an electron accepting and transporting material and the redox electrolyte plays a role of hole transporting. Nanoparticle size, pore size and porosity were found to have significant influence on electron diffusion rate and mass transport of redox species. For the case of cobalt complex electrolyte, increase of porosity from 52% to 59% led to increase in efficiency from 1.4% to 4.8% in spite of 24% decreased dye loading. Panchromatic absorption was realized by selective positioning of three different dyes, which was possible by controlling pore size of TiO2 film. Mesoporous TiO2 film was also used as a substrate for quantum dot sensitizer. Perovskite-type inorganic sensitizer (RNH3)PbI3 was deposited on TiO2 surface by spin coating method. The inorganic sensitizer deposited 3.6 μm-thick TiO2 film demonstrated 6.5% efficiency at AM 1.5G 1 sun illumination, which is by far the highest efficiency among the reported quantum dot sensitizers.

Highly mesoporous TiO2 nanoparticles were synthesized by using a combination of aero-sol-gel and aqueous washing processes. By varying the mass fraction of NaCl templates in the TiO2-NaCl composite nanoparticles, we examined the formation of mesoporous TiO2 nanoparticles with optimized surface area and pore volume distributions. Then, the photovoltaic properties of the resulting mesoporous TiO2 nanoparticles were systematically investigated by forming a photovoltaic active layer of these nanoparticles on the photoanode of dye-sensitized solar cells (DSSCs). The mesoporous TiO2 nanoparticle-based DSSCs fabricated in this study showed an improved short circuit current densities and power conversion efficiencies compared to solid TiO2 nanoparticle-based DSSCs (the values increased from 2.02 ± 0.11 to 8.16 ± 0.51 mA/cm2 and from 1.07 ± 0.21% to 3.88 ± 0.33%, respectively). This improvement is due to an increase in the amount of inorganic dye (N719) adsorption in the mesoporous TiO2 nanoparticles with a specific surface area of 184 m2/g and an average pore size of 5 nm. These specially designed mesoporous TiO2 nanoparticles have great potential as an effective dye-supporting and electron-transfer medium and can improve the photovoltaic performance of DSSCs.

Organic solar cells have gained remarkable interest during the past two decades and have reached high power conversion efficiencies of 8%. This has become possible due to broadly absorbing conjugated materials exhibiting absorption spectra up into the near-IR. Nevertheless, device absorption is still incomplete especially at the margins of the spectra owing to very thin active layers. Further increased efficiencies demand for thicker active layers allowing more complete photon harvesting. However, the active layer thickness of conventional organic solar cells is conceptually limited to 100-200 nm due to the relatively uncontrolled morphology of their active layers. Charge trapping and recombination becomes more and more prominent for increasing layer thicknesses due to inconsistent charge transport channels. One of the most elegant ways to overcome these limitations is the substitution of the organic acceptor material with wide band-gap semiconductors like TiO2 in so-called hybrid solar cells. TiO2 also acts as electron acceptor but can be nanostructured in order to meet the competing demands: 1) high interfacial area for efficient exciton separation and 2) consistent pathways for charge transport. TiO2 nanotubular arrays with dimensions comparable to exciton diffusion lengths in typical organic materials are synthesized on conductive glass substrates via electrochemical anodization of thin Ti films and can be readily infiltrated with donor material to realize ordered hybrid heterojunctions. Growth of these structures on glass substrates is a great advantage over anodized Ti foils since loss-free solar cell illumination through the glass becomes possible, whereas solar cells on foils have to be illuminated through semi-transparent metal contacts. The properties of the metal oxide-organic junction can be further tuned by introduction of interfacial modifiers like dye molecules and are investigated in detail.

New metal@TiO2 core@shell nanomaterials have been conceived for both regular and photo catalysis applications. Progress will be reported in four areas of research: (1) the development of synthetic strategies for the making of the core@shell nanostructures, (2) the photophysical characterization of the electronic properties of those samples; (3) the chemical characterization of our catalysts in terms of diffusivity of gases through the shells and access of the metal and in terms of photocatalytic activity; and (4) their catalytic activity for CO oxidation. One of the main conclusions reached so far is that a higher degree of crystallinity of the titania in the shell structure, accomplished by a multistep synthesis involving several silica sacrificial layers, leads to longer fluorescence lifetimes and higher photocatalytic activity. Also, these catalysts can promote CO oxidation at temperatures much below room temperature.

The modification of TiO2 surfaces with organic and organometallic molecules in dye-sensitized solar cells (DSSCs) with high energy conversion efficiencies is of great interest to address the rapidly increasing energy demands meeting the requirements of sustainability and low production costs. However, despite the widespread use of TiO2 in DSSCs, there has been little work investigating the structural and electronic properties of dye molecules on TiO2 surfaces at the atomic level. The interfacial characteristics are of paramount importance for the charge injection efficiencies. Here, we study the adsorption of N3 dye molecules on an anatase (101) surface by means of scanning tunneling microscopy and spectroscopy methods in ultra high vacuum to address the following aspects on a fundamental level: (i) geometric configuration of photosensitizers on TiO2 surfaces for optimal light absorption and electron transfer, (ii) influence of the semiconductor surface on metal-to-ligand charge transfer, (iii) TiO2 band structure and quasi-Fermi-level dependence, (iv) influence of substrate defects, (v) degree of aggregation and lateral interaction between photosensitizers and influence on electronic properties, and (vi) co-sensitization and concerto effect. The presented results reveal the structure-property relationship of N3 dyes on TiO2 at the molecular level.

Titanium dioxide nanocrystals attract increased interest as important photoactive candidates for solar light conversion industry. In contrast with doped TiO2 nanoparticles, TiO2 nanocrystals having a smaller sizes < 1nm keep excitons out of recombination due to size confinement. At the same time, unsaturated bonds create sufficient amount of additional defects thus narrowing the HOMO-LUMO band gap and let to absorb the whole solar spectrum. The [1] investigation of the electronic properties of a series of titanium dioxide clusters (TiO2)n conducted by PES binding energy evolution analysis revealed minimum of HOMO-LUMO energy gap of 2.2 â?" 2.6 eV for clusters n = 1- 4 sizes. Authors also concluded the existence of stable TiO2 nanocrystal structures with cluster size n = 2. Thus, it may be reasonable to fabricate stable TiO2 nanostructures with specific crystal phases, which can facilitate charge separation and hinder recombination. Here we are presenting the dynamic changes in XPS and AES spectra of titanium dioxide thin films with mass thicknesses up to monolayer formed by redox reactions of stepwise sputtered Ti onto Si (100) substrate with native oxide conducted in situ under UHV conditions. Each deposition was followed by XPS and AES analysis and was repeated a few times on each sample. The depositions were implemented in three different deposition exposures but with the same deposition rate 0.1 nm/min. The increasing of Ti2p3/2 XPS intensities with deposition time reveals sections of fast and slow growth for all three curves of different deposition exposures. The sections of the slow growth were accompanied by maximum fluctuations in XPS intensity for Si 2p / Siox2p relation. Also, there were observed almost the same fluctuations on the intensity curve for O1s XPS. We concluded that such behavior is caused by increasing in roughness of partly reduced native silicon oxide. Just the opposite slow growth of titanium dioxide XPS signal at those thicknesses demonstrates the height titanium dioxide nanocrystal growth. The STM image taken from the sample 1 fabricated with the short deposition exposures shows hills with small spikes of ~ 0.8nm size on hillsâ?T tops, which could be sharp TiO2 nanocrystals on the tops of partly reduced native silicon oxide hills. At those mass thickness sections, even though all XPS titanium spectra were in oxide form of TiO2, there was observed clear shift of Ti2p3/2 towards high binding energies. Beside that, the Ti2p3/2 peak shape became narrow with clear visible two shifted Ti2p3/2 peaks. The monitoring of Auger peaks L3M23M23 and L3M23M45 of titanium dioxide versus mass thickness demonstrated increasing of binding energies for 3p and 3s core levels. 1. Zhai, H.- J., Wang, L.- S. J. Am. Chem. Soc. 2007, 129, 3022

Hydrogen is considered to be a potential candidate for a clean energy source due to its environmental friendly and renewable characteristics. Many studies have been dedicated to the development of photocatalytic materials or photoelectrochemical cells for conversing solar energy into hydrogen [1-3]. At present, titanium dioxide (TiO2) is the most widely used and most intensely studied photocatalyst, due to its low cost, nontoxicity, and high efficiency. However, TiO2 can only be activated with UV light, which accounts for only 4% of the solar energy that reaches the Earthâ?Ts surface [4]. There is therefore an urgent need to develop photocatalysts that respond to visible light, and this important field of research has attracted considerable attention in recent years. Sensitization of TiO2 with semiconductors which absorb light in the visible region is one of the promising ways to enhance the visible light activity of the TiO2. In this study we sensitized hydrothermally grown TiO2 single crystalline nanorods on FTO/glass substrate with the CdS and CdSe quantum dots. Quantum dots have been formed on TiO2 by SILAR process. Moreover, to achieve the better charge separation and reduce the recombination, CdS/CdSe co-sensitized TiO2NRs composite structures also have been studied. Before sensitization, Pt nanoparticles were deposited onto TiO2 NRs to form a co-catalyst layer for enhanced H2 generation. Photocurrent measurements of the CdS, CdSe and CdS/CdSe co-sensitized NRs showed increased current density in the both white and visible light conditions. Higher photocurrent density under the visible light of the TiO2/CdS and TiO2/CdS/CdSe samples will be discussed and explained in terms of microstructural and electrical properties. REFERENCES [1] A. Fujishima, et al., Nature 238 (1972) 37 [2] A. Kudo, et al., Chem. Soc. Rev., 38 (2009) 253 [3 ] Y. Lee, et al., Chem. Mater. 22 (2010), 922â?"927 [4] L. Zhou, et al., Mol. Catal. A: Chem., 252 (2006) 120

Many researchers have been focused on utilization of solar energy in dye sensitized solar cells (DSSC) employing mainly TiO2 nano powders or thin films due to their advantages of low cost and chemical stability [1]. However, the efficiency for solar energy harvesting is still low and need further development. Recently, narrow-band-gap semiconductor quantum dots (QDs) such as CdS, CdSe, PbS, and InAs, have also been the subject of considerable interest as promising candidates for replacing the sensitizer dyes in DSSCs [2]. The use of semiconductor QDs as sensitizers has some unique advantages over the use of dye molecules in solar cell, such as controllable band gap, extinction coefficients due to the quantum confinement effect. [3] In this study, we report photocatalytic properties of highly ordered TiO2 nanorod arrays on FTO glasses to address the issues of improving the photo-conversion efficiency for QDSSC under visible light irradiation. Ordered arrays of single crystalline nanorods possess better charge transport and higher light penetration through the active layer compared to nanoparticle film. Hydrothermally grown TiO2 nanorod arrays on FTO /glass substrate were sensitized with CdS/CdSe quantum dots by SILAR process with aim to increase the visible light response. To suppress the surface trapping of photoexited electrons and holes additional ZnS layer was coated. Photocurrent measurements of the CdS/CdSe and CdS/CdSe/ZnS co-sensitized NRs showed increased performance in the both white and visible light conditions. Higher photoresponse under the visible light of the CdS/CdSe and CdS/CdSe/ZnS samples was explained and discussed according to their microstructural and electrical properties. . REFERENCESS [1] B. O'Regan, Michael Grätzel, Nature, 353 (1991), 737 [2] Qing Shen, et al., Appl. Phys. Lett. 97 (2010), 123107 [3] A. J. Nozik, Nano Lett., 10 (2010), 2735

TiO2 is of great interest due to its various applications such as dye-sensitized solar cells, photoelectrochemical water splitting, antireflection coatings, Li-ion batteries, and chemical sensors. For these applications, since the device performance largely depends on the surface-to-volume ratio of the TiO2 electrodes or catalysts, tremendous efforts have been devoted to synthesize TiO2 nanomaterials with large specific surface areas. Among them, TiO2 nanotubes have attracted great interest owing to the facile synthesis of them using anodic oxidation processes. Most of studies on TiO2 nanotubes were based on anodization of Ti or Ti-alloy foils, which results in TiO2 nanotubes on thick metal substrates. For high-quality gas sensors or solar cells, the separation and transfer of TiO2 nanotube arrays from the metal substrates onto Si or glass substrates are needed. However, using the synthesis-and-transfer method, obtaining vertical TiO2 nanotubes on the Si or glass substrates is challenging because TiO2 nanotubes are very fragile and thus the separation of large-area TiO2 nanotubes is difficult. Alternatively, vertically synthesis of TiO2 nanotubes on Si or glass substrates have been reported, but the quality of the TiO2 nanotubes fell far behind those from anodization of Ti foils. Here we report synthesis-in-place of vertically aligned TiO2 nanotubes. By anodizing Ti films on patterned Si or glass substrates, we could obtain vertical TiO2 nantubes on the substrates. In synthesis-in-place skill, TiO2 nanotubes grow in situ at the patterned Si and glass substrates fabricated through photolithography. The orderness of the TiO2 nanotubes is unparalleled with those of the previously reports. Furthermore, the vertically synthesized TiO2 nanotubes on the Si or glass substrates could be used as chemical sensors without additional processes. The experimental results reveal that the chemical sensors based on the high-quality TiO2 nanotubes exhibits ultra-sensitive gas sensing properties to flat TiO2 thin films. In addition, it will be shown that the TiO2 nanotube sensors are very promising as high-quality UV sensor.

During the past few years, antireflective, self-cleaning, hygienic and germicidal sol-gel coatings using photocatalytic titanium dioxide (TiO2), especially the anatase crystalline form, have stimulated widespread interest and research activity in both academy and industry. In particular, disinfection and air purification systems based on particulate TiO2 are considered to be ideal systems for protecting buildings against environmental contamination. Preparation of hygienic coatings is generally based on a sol-gel deposition, for instance by coating particle layers on a substrate. Many of these substrates are organic materials that are thereafter subjected to degradative processes due to photocatalytic activity of TiO2. As a consequence, important durability problems on such organic supports are reported [1, 2]. This work is a fundamental study, focus on understanding and localization of the photocatalytic activity in organic environment. The model architecture used in this work is composed of an underlying nanoparticles TiO2 dense film, an intercalated protective mesoporous SiO2 layer and a top fully covered organic varnish layer. Elipsometric measurements show that organic coatings can be successfully protected against photocatalytic effect by intercalating a SiO2 mesoporous ultrathin film. Nanostructured coatings were obtained through a facile sol-gel liquid deposition of two successive TiO2 and SiO2 films on top of a silicon substrate. The thickness, refractive index, geometry and the mesoporous size of SiO2 interlayer was controlled by selecting the proper chemical and processing conditions to adjust the protective properties. Interestingly, photoactivity of the mesoporous protective silica film was investigated by following the refractive index of the layer, contaminated by impregnation with a model pollutant such as lauric acid. The study shows that the decomposition of polluants, takes place inside the mesoporous SiO2 protective interlayer. This effect is most likely due to (i) the diffusion of pollutants towards TiO2 layer and/or (ii) the diffusion of radical species towards the pollutants. As a consequence, the capability of SiO2 and TiO2 layer-by-layer system and organic varnish layer architecture above presented is two-fold. Thus, showing an excellent ability to photodecompose adsorbed organic compounds in mesoporous SiO2 layer and to protect the organic coating under UV radiation. [1]Chem. Mater., 2000, 12 (11), pp 3501â?"3508 [2]J. Phys. Chem. B, 2005, 109 (23), pp 11667â?"11674

Recent research on TiO2 has focused on its incorporation into new technologies. Although TiO2 is currently used as a catalyst it is also a promising candidate for energy devices and other nanotechnology applications. The implementation of TiO2 is related to the optimizing the properties of the three main polymorphs, anatase, rutile and brookite. Phase stability, phase transformations and interfaces between these phases are therefor a matter of growing interest, not only for the bulk materials but more importantly for materials at the nanoscale. For example, recent studies have reported the presence of more than one phase could affect potential applications when related with specific orientation. In view of these observations, it is important to have a better understanding of the relation orientation between the different crystal structures since this could provide valuable information on how to control or estimate a desirable response. In order to further understand these internal interfaces, the coexistent phases present in TiO2 crystals is being studied in the present work. Sections are cut, cross-sectioned, and polished for SEM analysis. The microstructure has been observed using SEM, and the chemical composition was determined using XEDS. For example, EBSD analysis has confirmed the presence of the TiO2 polymorphs, rutile and brookite, and indicate the morphological changes that occur during the transformation to rutile. TEM samples have been produced using FIB. The orientation relationship between the brookite and rutile phases will be discussed in detail with attention paid to particle stability at the nanoscale.

Cooling of a hot surface by evaporation of a liquid such as water brought in contact with the surface is a well known method of passive cooling. It was reported in a press release by NEDO (www.nedo.go.jp) on August 29, 2007 that super hydrophilicity of titanium dioxide (TiO2) surface could be utilized to enhance evaporative cooling. This approach can be considered a method of solar cooling, since hydrophilicity of TiO2 was activated by the absorption of solar radiation. In this presentation, another example of solar cooling by TiO2 is suggested where solar radiation absorbed by TiO2 generates superoxide (O2-) ions which serve as condensation nuclei for water droplets used in the cooling process. In a typical water droplet molecules are bonded to each other by hydrogen bonds which has a bonding energy of ~0.2 eV. One can create a more rugged droplet by using an ion as a condensation nucleus. In that case, water molecules are held together by the interaction between the ion and the dipole moments of the water molecules surrounding the ion, in addition to any hydrogen bonding. Oxygen molecule which exhibits positive electron affinity is a good candidate to serve as the ionic condensation nucleus, because addition of an electron to O2 creates an energetically more stable state than the neutral O2. Interaction of O2 with the surface atoms of a TiO2 photocatalyst was investigated using first principle quantum mechanical calculations. Atomic model of the photocatalyst was chosen as a nanocrystal of anatase type TiO2. Optimal position of the O2 molecule with respect to the nanocrystal was calculated using the DFT method with B3LYP hybrid functional and Pople type basis sets augmented with polarization and diffuse functions. It was found that the O2 molecule preferred Ti sites on the TiO2 surface. The distance between the oxygen of O2 and Ti of the nanocrystal was found to be ~1.9 angstrom, which is about the same as the Ti-O bond length in the nanocrystal. Atomic model of the rugged water droplet consisted of a superoxide ion surrounded by water molecules. Energy of interaction between the ion and the nearest water molecule was calculated to be ~0.7 eV which is about 3 times the bonding energy of a hydrogen bond. One can dissipate more thermal energy and hence create a greater cooling effect by evaporation from a charged droplet as compared to a neutral one. The cooling arrangement is such that the photocatalyst is placed at a low temperature zone where it can absorb solar radiation and generate superoxide ions. Water molecules in the ambient atmosphere agglomerate around the ions and form droplets. Then, these droplets are transferred to the high temperature zone where water molecules evaporate from the surfaces of the droplets and create the desired cooling effect.

Realizing Ge channel MOS devices requires synthesis of ultra-thin gate dielectrics that are thermally stable and form an electrically-passive interface with Ge. In addition, sufficiently large band offsets to suppress gate leakage current are required. However, metal oxide band gaps (E_g) tend to decrease with increasing dielectric permittivity (k). To obtain high gate capacitance density on Ge, one solution is to interpose an ultrathin large E_g insulator with reasonable k as an interface layer between a higher-k dielectric and the channel. Al2O3 layers can have stable interfaces with Ge with k~8 and a large E_g (~8.8 eV for α-Al₂O₃). On the other hand, TiO₂ can have a high k (~60) when in the Rutile phase, but its conduction band offset with Ge is less than 1 eV, resulting in high gate leakage when used as a single-layer gate insulator. Previous reports have shown that TiO₂/Al₂O₃ bilayers deposited on Ge(100) by ALD can achieve low interface trap density with small gate leakage current after post-metal forming gas anneal. An increase in k of TiO₂ after FGA at 350 °C has also been observed, which may be due to crystallization of initially amorphous TiO₂. Therefore, the focus of this contribution is on the annealing effect on TiO₂ phase evolution on Ge(100). In-situ transmission electron microscopy (TEM) was conducted on plan-view TEM samples of ALD-grown TiO₂ (7.5 nm)/Al₂O₃ (2.5 nm)/Ge to observe the TiO₂ phase transformation. The nucleation of crystallites in the TiO₂ layer was observed at 300 °C. At 350 °C, the size of the crystallites increased to ~10 nm, which were identified as anatase phase by selected area electron diffraction (SAED). From 450 °C to 500 °C heating, the transformation into predominantly Rutile phase TiO₂ was observed. Post-metal FGA temperature used in processing Pt/TiO₂/Al₂O₃/p-Ge(100) MOS capacitors was varied to determine the TiO₂ phase transition effect on C-V characteristics. From capacitance-voltage measurements, the maximum capacitance at a given frequency increases for the 450 °C FGA sample, indicating k _TiO2 increased to ~50. In order to measure the channel transfer characteristics for this TiO₂ (7.5 nm)/Al₂O₃ (2.5 nm)/Ge(100) stack, pMOSFETs with long channels (L_g = 2 â?" 30 um) were fabricated. The devices show a subthreshold swing of 115 mV/dec and on state current of 60 μA/μm. Measured peak hole mobility reaches 370 cm²/Vs, which suggests the potential of TiO₂/Al₂O₃/Ge gate stacks for high performance MOSFETs.

There is an alarming increase of a variety of new chemicals that are now being discharged into the wastewater system. This is predominantly due to the rapid emergence of technological developments within industries (e.g., pharmaceuticals) and is causing increased concern for public health and safety because many are not removed by typical wastewater treatment practices. Degradation of organic compounds by oxidation via hydroxyl radicals (OHâ-) is a new potential treatment technology that degrades a wide range of organic compounds to complete mineralization with no selectivity. Titanium Dioxide (TiO₂) is of significant interest due to its semiconducting properties that enable its use as a heterogeneous photocatalytic material, rapidly and completely mineralizing organics without harmful byproducts. TiO₂ is synthesized by various methods such as chemical and physical vapor deposition, which require high temperatures or extreme atmospheric conditions (e.g., high vacuum) to achieve the desired phase, shape, and size of the material. Solution routes such as chemical bath deposition, sol-gel and hydrothermal routes afford environmentally friendly processing with low cost. However, these solution-based technologies lack the necessary control of crystal size, phase, and morphological features that yield optimized semiconductor materials. Mineralizing biological organisms demonstrate how nature can produce elegant structures at room temperature through controlled organic-mineral interactions. These organics exist as either soluble forms or as insoluble scaffolds that are often used to control size, shape, and phase of deposited mineral. We utilize biologically-inspired scaffolds to template the nucleation and growth of inorganic materials such as TiO₂, which aid in controlling the size and phase of these particles and ultimately, their properties. Understanding the fundamental nucleation and growth mechanism is critical to control the microstructure and thus function. Nanosized rutile and anatase particles were synthesized at relatively low temperatures and mild pH conditions. The effects of reaction conditions on phase and grain size were investigated and discussed from coordination chemistry and coarsening mechanisms. By modifying solution conditions, we have been able to produce a self-supporting porous, high surface area TiO₂ nanoparticle membrane with controlled crystallite size, phase and porosity. These bulk porous TiO₂ membranes can be utilized in photocatalytic applications, eliminating the need for nanoparticle recovery systems, thereby reducing processing costs and increasing amount of viable applications of photocatalytic systems.

In recent years, dyeâ?"sensitized solar cells (DSSCs) based on nanocrystalline titanium dioxide have emerged as a viable technology for the conversion of sun light into electricity. The potential for high efficiency, low cost, and light weight devices makes DSSCs a promising system for many different solar power applications. Their limited spectral response is a key factor that needs to be addressed to realize increased device efficiencies. One approach to the problem is to use cosensitization of the TiO₂ using a combination of dyes to adsorb different parts of the solar spectrum, but this technique has not achieved real benefits since the two dyes are competing for space on the metal oxide surface to form the active monolayer. Our strategy to obtain a broad optical absorption throughout the visible and near IR region is based on multilayer dye sensitization. In DSSCs the dyes need to be close enough to the semiconductor oxide surface to be able to inject electrons into the semiconductor upon excitation. They should be firmly anchored to the nanoparticle surface and chemically stable with respect to their interaction with the electrolyte or hole conducting material. We have developed a process which uses two complementary dyes that are self assembled onto the semiconductor surface in order to achieve panchromatic sensitization. Using electrostatic layerâ?"byâ?"layer assembly we have successfully coated the internal surfaces of a mesoporous TiO₂ film in a conformal fashion and with a controllable thickness. The film growth can be monitored by UVâ?"vis spectroscopy and results in a linear increase of the amount of dye deposited on the mesoporous TiO₂ film with each deposition cycle. The resulting spectrum is a combination of the spectra of the individual components. The effect of multilayer dye sensitization on solid-state dye-sensitized solar cell performance is presented.

Several reports have highlighted interesting photocatalytic properties of Zn-Ti oxide systems, prepared either as mixed oxides or as composites of the binary oxides [1]. Furthermore, high ease of formation and visible light photoactivity have been reported for N-doped Zn,Ti oxide spinels [2]. Thus a good basic understanding of the behavior of such complex systems is of interest. Here DFT-type periodic calculations (at the GGA+U or hybrid functional levels) of the electronic structure and energetics of some of these systems will be presented. First, hybrid functional calculations on the mixed oxides will show that the spinel mixed oxide phases, with composition between Zn2TiO4 and Zn2Ti3O8, can have bandgaps above or approaching that of anatase, depending on the type of order present in the octahedral cation sites, while rhombohedral ZnTiO3 seems to have a much larger gap. In all these systems the conduction band is made mainly of Ti 3d orbitals, which implies a chemical behaviour of the photoexcited electrons similar to that of TiO2 retaher than ZnO. Secondly, N-doping achieved through substitution of O by N anions (as is possible via high temperature treatment with ammonia), that rises the top valence band with N-centered states, is shown to be energetically more favourable in the spinel phases than in pure TiO2, thanks to the flexibility of charge compensation provided by the presence of different cationic charges and to the charge inbalance produced by the fourfold cationic charge in Ti-rich compositions. Finally, the behaviour of TiO2-ZnO junctions in composites can be modeled with periodic alternating layers, thanks to the occurrence of a favourable epitaxial relationship. By using recently suggested methods for accurately studying band alignments in semiconductor interfaces [3] the nature of such alignment in anatase-ZnO composites is clarified, and its influence on facilitating electron-hole charge transfer after photon absorption, which increases photocatalytic efficiency, is discussed. The same model may be applied to study other biphasic photocatalysts where this type of charge separation can be relevant. [1] See e.g. a) S. Liao et al. J. Photochem. Photobiol. A 168 (2004) 7; b) S.A. Mayén-Hernández et al. Solar Energy Mater. Solar Cells 91 (2007) 1454; c) A.A. Ismael, Appl. Catal. B 85 (2008) 33; d) T. A. Khalyavka et al. Theor. Exp. Chem. 45 (2009) 234. [2] F. Grasset et al. Adv. Maters. 17 (2005) 294; T. Hisatomi et al. Bull. Chem. Soc. Jpn. Vol. 81 (2008) 1647. [3] A. Alkauskas et al. Phys. Status Solidi B 248 (2011) 775.

TiO2 is widely used as a UV-light photocatalyst. To make a visible light sensitive photocatalyst, methods like coupling with narrower bandgap semiconductors, metal or nonmetal doping to TiO2 have been extensively researched. For example, semiconductor coupling of CdS with TiO2 is often used to produce visible light photocatalysts. While CdS has a suitable bandgap and conduction band position, it is unstable and needs hole-scavengers (usually S2-/SO32-) due to accumulation of photogenerated holes. MnO2 is a stable oxide semiconductor and has a small bandgap and a more negative valence band maximum compared to CdS. Holes generated at the CdS may flow to MnO2 which protects CdS from photocorrosion but also benefits for electron-hole pair separation. A novel MnO2/CdS/TiO2 nanocomposite is being produced as a potentially more efficient and stable visible light photocatalyst. CdS/TiO2 is synthesized by mixing TiO2 powder into CdS sols of different concentrations. MnO2/CdS/TiO2 nanocomposites are synthesized by impregnating the catalyst with different concentrations of manganese acetate solutions which are then dried. XRD, UV-Vis, TEM, N2 adsorption-desorption are utilized to characterize the materials. A novel form of atomic resolution environmental TEM with in-situ light irradiation will be used to explore the nanoscale structure of titania based nanocomposites under reactive gas conditions. A discussion about the photocatalytic properties and nanostructures will be presented.

Titania nanotubes have been shown to have wide range of application such as in hydrogen sensor, solar cells, etc by growing tubes out of Ti foil. With the ability to grown these tubes out of thin films on wafer scale, regions where the tubes need to be grown can be well defined by photolithography and integration of these tubes into devices becomes simplified. We present a method to grow titania nanotubes on Si substrates by anodizing Ti thin films in an organic solution. Thermally oxidized (100 nm) n-type Si wafers were used as substrates to deposit 1 μm Ti films by e-beam evaporation. A mixture of water (10 wt%), ammonium fluoride (0.5 wt%), and ethylene glycol was used as the anodization solution, and platinum foil (1.5 cm Ã- 1 cm) was used as the cathode. The anodization was carried out at voltages varying from 10 to 60V (D.C.) with an initial current of about 5 mA (the current drops to 0 on complete anodization of the film) for 1 hour with magnetic stirring of the anodizing solution. The tubes were annealed in air for 2 hours at 550 °C with the temperature ramp rate of 1.5 °C/min. Characterization of the titanium film and titania tubes before and after annealing was performed using SEM, AFM, UV-vis diffuse reflectance spectroscopy, and XRD to measure the morphology, surface morphology, optical properties, and microstructure, respectively. The lengths of the nanotubes observed in SEM micrographs were 1.5 μm. The nanotubes were longer than the thickness of Ti film due to addition of oxygen atoms during anodization and did not elongate further irrespective of longer anodization time. The nanotube diameter linearly increased from 40 to 240 nm with increase in anodization voltage from 10 to 60V. AFM micrographs collected using PeakForce QNM corroborated the nanotube diameter, and also showed that the tubes had slightly (10-20%) tapered structure with the smaller diameter at the top. There was no difference in the morphology of the tubes before and after annealing. Spectrophotometry showed that the tubes before and after annealing had maximum absorbance edge at wavelengths around 360-375 nm. XRD showed that the before annealing the tubes were amorphous with presence of 2 theta degree peaks at 69 showing the presence of (004) Si. Annealed samples showed the presence of anatase titania with 2 theta degree peaks at 25, 48, 54, 55 and 62. Low intensity peaks were present at 38 showing traces of Ti metal which was not anodized. Annealed sample showed peaks at 33 showing the presence of (002) Si which may be due to the stressed (004) Si.

One of the most common methods for the synthesis of nanoparticles and the formation of thin films is the colloidal sol-gel process. It consists of different steps among which peptization is one of the most important. This has been achieved by using optical techniques, including Laser Difraction (LD), Dynamic Light Scattering (DLS) and Multiple Light Scattering in the near infrared (MLS). In order to confirm this methodology, in this work the synthesis of nanoparticulate TiO2 sols with Ti(iPrO)4 as a precursor and water in molar ratio 1:50, respectively, is studied. In addition, the influence of doping with Er3+ to relative contents of 1, 2 and 3 mole % is described. The evolution of peptization has been followed by measurements of particle size by LD and DLS and the stability of the sols has been study by zeta potential and viscosity measurements. The structural, microstructural, and optical characterization of the xerogels obtained under different conditions has been performed and hence, the influence of doping with Er3+ in the TiO2 matrix has been established. Finally, the photocatalytic activity of the different sols under UV light has been evaluated.

Predicting reaction pathways in heterogeneous photocatalysis is difficult due to a general lack of knowledge regarding interfacial charge transfer mechanisms. In this presentation, selectivity in photodecomposition of three adsorbates on the rutile TiO₂(110) surface will be examined. For trimethyl acetate (TMA), the oxygen partial pressure controls the partitioning between the two immediate C₄ reaction products: isobutene and isobutane. Under anaerobic conditions, the photoreaction occurs with pseudo first order kinetics yielding a near 1:1 ratio of these C₄ products, whereas in excess of O₂ there is high selectivity toward isobutene. The varying selectivity is linked to the formation of surface oxygen-containing products (from O₂ photoreduction) that convert t-butyl radicals preferentially into isobutene. O₂ shifts the reaction from pseudo first order to one occurring at the boundaries between hydrophilic and hydrophobic domains. Similar examples of photocatalytic selectivities in carbonyl molecules and in methanol will also be discussed. These examples of varying selectivity in photocatalysis on TiO₂(110) provide motivation for learning how to predict and guide heterogeneous photocatalytic reactions to desired products. This work was supported by the US Department of Energy, Office of Science, Division of Chemical Sciences and performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energyâ?Ts Office of Biological and Environmental Research.

Titanium dioxide (TiO2) is one of the most important materials for future application to photocatalysts. At the same time, TiO2 is a highly polymorphic material. Therefore, much attention has been given not only to improvements of photocatalytic properties but also to understanding its fundamental physical properties. Especially, relative stabilities of various phases and the impurity effects in TiO2 are both scientifically and technologically important. We study the effects of nitrogen doping into the TiO2 lattice in the framework of the density-functional theory and the thermodynamic stabilities of various TiO2 phases based on the density-functional perturbation theory. We employ the pseudopotential plane-wave method to perform the electronic-structure calculation. In order to reveal the nitrogen-doping effects on the energetics and the electronic properties, we use the supercell with various dopant concentrations[1]. It is found that the nitrogen doping into both rutile and anatase phases significantly reduces the minimum photo-excitation energy and expands the photoresponse energy region down to the visible-light energies. From the total energies obtained, on the other hand, it is suggested that the dopants tend to cluster in the TiO2 lattice. This â?oclusteringâ? is known to often occur in doping into semiconductor device materials and might cancel the above-mentioned doping effects on the photoresponse energies. From the optimized geometries, it is found that the average length of the Ti-N bonds in the nitrogen-doped TiO2 lattice is longer than that of the Ti-O bonds in the pristine TiO2 lattice. Therefore, the volume of the nitrogen-doped region should be larger than that of the pristine TiO2 region, giving rise to the clustering of impurities to share the volume- expansion region. We study the effects of the interstitial doping as well and the difference from the substitutional doping case will be discussed. Next, we consider the finite-temperature effect by introducing phonons within the quasi-harmonic approximation to investigate the thermodynamic phase stabilities. We compare the free energy of four phases, rutile, anatase, brookite, and TiO2-II having a columbite-like form. We also compare structural and elastic properties among these four phases. We use both these results between the local density approximation and the generalized gradient approximation and clarify the importance of the exchange-correlation functional to be used in this issue. We also pay attention to the treatment of semicore states in constructing Ti pseudopotentials. [1] Y. Aoki and S. Saito, J. Phys: Conf. Ser. 302, 012034 (2011)

Titania is an essential component of new generation devices for photocatalysis and solar energy conversion. Its special behavior under illumination is at the basis of several practical applications like self-cleaning and self-sterilizing surfaces, superhydrophilicity, corrosion protection, etc. Most of these effects are observed under UV light and efforts are now oriented to the preparation of visible light photoactive titania. In this talk we will review the most recent advances in this field, and we will discuss in more detail the nature of doped TiO2 and of the interplay of intrinsic defects (like oxygen vacancies resulting in the formation of â?oTi3+â? ions) and states induced by transition and non-transition metal dopants like Cr, Sb, B, C, N, and F. The addition of heteroatoms, of crucial importance to improve the photactivity of the material, results in new states in the gap of titania and in paramagnetic centers. These centers may contribute to improve the photoactivity of the material under visible light but can also act as recombination centers for electrons and holes, thus resulting in a reduced activity. The description of these systems with advanced theoretical methods presents problems connected to the need to correctly reproduce the band gap of the material and the localized nature of the defects created by doping.

TiO2 nanotubes have attracted increasing interest over the past few years due to their excellent biocompatibility and photocatalytic properties. Various applications are currently envisaged or developed: biomedical devices, sensors, photocatalysis for hydrogen production, solar cells... The fabrication of biofiltration nanotubular membranes is certainly noteworthy among biomedical applications. As of today, the simplest way of producing highly ordered TiO2 nanotubes is to anodize a metallic Ti foil in an organic electrolyte solution (using for example a Pt foil as the counter electrode). The impact of the experimental conditions of this process (electrolyte composition, anodization voltage and timeâ?¦) on the structural parameters of TiO2 nanotubes are very well documented in the literature (see articles from Craig Grimes and Patrick Schmuki). Each of these nanotubes presents an open top end and a closed bottom end (barrier layer in between the tube and the underlying metallic Ti). In order to produce self-standing TiO2 nanotubular membranes, the nanotubes have to be detached from the Ti foil and their bottom ends need to be opened. Despite many different techniques reported so far, these steps are still very difficult to realize in practice (especially for large surface area membranes). Moreover, the opening of the closed pores usually involves highly corrosive and hazardous chemicals. We report here a new fabrication technique for free-standing TiO2 nanotubular membranes with through-hole morphology without using any chemical etchant. It consists in a 3-step anodization procedure. The Ti foil is first pre-anodized (80V, 1h) in an ethylene glycol solution containing NH4F and H2O. The TiO2 tubes are then removed from the Ti foil by sonicating the sample in a diluted sulfuric acid aqueous solution. This produces a patterned Ti surface that is subsequently anodized (80V, 24h) in the electrolyte solution. The Ti/TiO2 sample is then rinsed with water and ethanol and stored in ethanol. A post-anodization procedure is then carried out using a fresh electrolyte solution and a much higher voltage (180V). After a few minutes, the TiO2 layer starts to detach from the underlying Ti (especially on the edges). A scalpel can then be used to assist the separation of the TiO2 membrane. We believe a very thin layer of tubes grows in between Ti and TiO2 during the post-anodization step and causes the detachment of the TiO2 membrane. The examination of these membranes by SEM shows that their bottom ends are fully opened over a large portion of the surface. The pores present a conical shape across the membrane thickness ranging from â?^200 nm (inner diameter of the tubes) at the top to â?^50 nm at the bottom. Such through-hole TiO2 membranes could be very useful in biofiltration applications (virusesâ?T blockage, immunoisolation of transplanted cellsâ?¦).

The electronic properties of materials change substantially when size and shape are reduced to the nanometre scale. In this work, the dependence of the dielectric function on size and shape is investigated in novel TiO2 nanostructures which were prepared by solvothermal and non-hydrolytic synthesis [1],[2]. Herein, we report for the first time the dielectric functions of thin anatase nanoplatelets and high aspect ratio anatase nanorods obtained using energy-loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM). High resolution TEM structural characterization shows that the platelets have a measured thickness of 5 nm and edge length of 20 â?" 60 nm, while the rods exhibit an average diameter and length of 4 and 40 nm, respectively. STEM-EELS data were collected from the platelets aligned along two perpendicular, [200] and [002], crystallographic orientations, and the nanorods in various zone axes. The size dependence of the dielectric function was studied for different thicknesses of the platelets by varying the number of overlapping structures, as well as for different diameters and lengths of the rods. We will discuss the results by conducting a comparison with the bulk dielectric function obtained for the various orientations, as well as density functional theory simulations. Our observations suggest that both size and orientation affect the shape of the dielectric function of the nanostructures. In this contribution we will also present a first attempt to characterize the surface structure of the platelets through exit wavefunction reconstruction (EWR) using focal series of aberration corrected high resolution TEM images. The obtained data give an insight into the novel intrinsic properties of the platelets due to their exposed facets [1],[3], and besides, provide a reminder to discuss the advantages of aberration corrected TEM for very thin structures. [1] J.S. Chen and X.W. Lou 2009 Electrochem. Commun. 11, 2332. [2] U. VukičeviÄ?, S.C. Ziemian, A. Bismarck and M.S.P. Shaffer 2008 J. Mater. Chem. 18, 3448. [3]X. Han , Q. Kuang, M. Jin, Z. Xie and L. Zheng 2009 J. Am. Chem. Soc. 131, 3152.

Advances in the development of more efficient TiO₂ particle systems for photochemical applications rely on comprehensive characterization work which is based on a close connection between synthesis, the incisive characterization of materialsâ?T properties and physico-chemical in-depth experiments. These should aim at the establishment of key figures to quantitatively describe the various steps comprising the overall photochemical process. We explored the effect of solid-solid interfaces between vapour phase grown TiO₂ nanoparticles on the spectroscopic and photoelectronic ensemble properties using electron paramagnetic resonance, FT-IR and other spectroscopic techniques.[1] Of primordial importance for the evaluation of the photoelectronic properties of wet chemistry-derived TiO₂ nanostructures, it was found that solvent removal and additional vacuum based procedures which aim at bare and adsorbate free particle surfaces are inevitably associated with the emergence of solid-solid interfaces. As will be shown in this contribution, the associated recombination loss of photogenerated charge carriers can be eliminated by intentional attachment of other metal oxide particles, such as ZrO₂ or SnO₂, to the TiO₂ nanoparticle surface.[2] As another example, the effect of unintentional doping which arises from synthesis related impurities on charge separation will be discussed.[3] During the process of annealing synthesis related and surface adsorbed nitrogen remnants at a level of less than 0.5 atom percent were found to penetrate into the TiO₂ bulk and transform into photoactive nitrogen species that give rise to characteristic spectroscopic features.[3] On the basis of quantitative UV charge separation effects we evaluated the impact of dopant induced charge carrier recombination effects and provide a rationale for the fair evaluation of doping effects on the photocatalystâ?Ts spectral response. [1] Baumann et al. Langmuir 2011, 27, 1946. [2] Siedl et al. J. Phys. Chem. C 2009, 113, 15792. [3] Dâ?~Arienzo et al. J. Phys. Chem. C. 2010, 114, 18067.

Perovskite-type inorganic sensitizers based on (C_nH_2n+1NH₃)PbI₃ (n=1, 2) are synthesized and their photovoltaic properties are investigated using nanocrystalline TiO₂-based photoelectrochemical cell. Deposition of the precursor solution containing alkylammonium iodide and lead iodide on the TiO₂ surface leads to semi-spherical crystal with diameter of about 2-3 nm. For the case of (CH₃NH₃)PbI₃ quantum dot (QD), effects of precursor concentration, post-annealing temperature and TiO₂ film thickness on photovoltaic performance are investigated. From the systematic investigation, solar-to-electrical conversion efficiency as high as 6.5% has been achieved at AM 1.5G 1sun illumination (100 mW/cm²), which is by far the highest efficiency among the reported QD sensitizers. Although the QD sensitizers are not fully covered on TiO₂ surface, electron transport rate and electron life time are almost identical with those of the fully covered N719 dye. The longer alkyl chain ethylammonium cation is stabilized in A site of the ABI₃ perovskite structure for the first time. The as-deposited (C₂H₅NH₃)PbI₃ on TiO₂ surface forms an orthorhombic crystal phase with a=8.7419(2) Ã., b=8.14745(10) Ã., and c=30.3096(6) Ã.. The bandgap energy is determined to be about 2.3 eV from ultraviolet photoelectron spectroscopy (UPS) and UV-vis spectroscopy. The 1.8 nm-sized (C₂H₅NH₃)PbI₃ QD sensitizer shows the conversion efficiency of 2.4% at 1 sun condition.

Energy storage is an important component for future renewable energy applications. Among all available options for energy storage, electrochemical energy storage, such as fuel cells, supercapacitors, and batteries, will find wide usages in current and future devices. Among these systems, supercapacitor has the advantages of high power density, excellent reversibility, and long cycle life. Since the electrode is one of the most important parts of a supercapacitor or a battery device, the need for developing better methods to fabricate highly conductive electrode with 3D network is critical. Moreover, the fundamental understanding of the chemistry and materials science behind the electrode material processing-structure-property relationships also reveal to us the underlying principle of electrochemical properties of these materials. Here, we have proposed an approach to optimize the supercapacitor electrodes by preparing a well-connected conductive 3D carbon/oxide (TiO2) network in nanocomposite structure using a solution route. This well linked carbon/oxide structure improves the overall conductivity of the materials and at the same time, allowing access to the metal oxide for electrochemical reactions in a conductive network. Various parameters such as TiO2 to carbon ratio, pH value before gelation, annealing temperature and duration were systematically tailored to improve the 3D network structures and properties including surface area, conductivity and crystallization phase etc. This composite material is promising for energy storage systems as initial results with C/TiO2 nanostructures show high specific capacity of ~400 mAh/g after 50 cycles. The combination of carbon with inorganic nanostructures probably offers a good way forward to design hierarchical systems offering high stability, excellent functionality and moderate cost. Thus, making them as promising materials for supercapacitors, rechargeable batteries, advanced catalyst support for fuel cells, and adsorbents. Experimental details and electrochemical, microstructural results of C/TiO2 nanostructures will be presented.

Mesoporous TiO2 shell nanostrucutures with controllable anatase crystallinity have been successfully synthesized by using novel synthetic method including silica-protected calcination, partial etching and re-calcination process. This method involves several synthetic steps as follows : 1) synthesis of SiO2@TiO2@SiO2 colloidal composites through sol-gel process and silica-protected calcination, 2) partial etching to preferentially remove portions of the SiO2 layers contacting the TiO2 surface and 3) re-calcination to improve the TiO2 crystallinity and finally etching of the inner and outer SiO2 to produce mesoporous anatase TiO2 shells. The silica-protected calcination step allows crystallization of the amorphous TiO2 layer into anatase nanocrystals, while simultaneously limiting the growth of anatase grains to within several nanometers. The partial etching step produces a small gap between SiO2 and TiO2 layers which allows space for the TiO2 to further grow into large crystal grains. The re-calcination process leads to well developed crystalline TiO2 which maintains the mesoporous shell structure due to the protection of the partially etched outer-silica layer. When used as photocatalysts for the degradation of Rhodamine B under UV irradiation, mesoporous TiO2 shells with enhanced crystallinity show significantly improved catalytic activity. We will discuss further our synthetic methodologies, characteristics and photocatalytic activity of mesoporous TiO2 shell nanostructure in this presentation

Solar energy can be converted to electricity and chemical fuels for energy use and storage. There are emerging technologies of using nanostructured semiconductors for light harvesting assemblies; and charge transfer processes to solar cells. The ability to control the particle size, morphology and composition of nanoparticles is of crucial importance nowadays both from the fundamental and industrial point of view considering the tremendous amount of high-tech applications of nanostructured 3d metal compounds in the applications of solar photovoltaic and photoelectrochemical cells, catalysts etc. Controlling the crystallographic structure and the arrangement of atoms along the surface of nanostructured material will determine most of its physical and chemical properties. In general, electronic structure ultimately determines the properties of matter. In the soft xray region, the question tends to be, what are the electrons doing as they migrated between the atoms? The electronic structure of the reaction intermediates of the catalysts will be obtained from L-edge x-ray absorption (XAS) and x-ray emission spectroscopy (XES) of the transition metals. Soft X-ray absorption spectroscopy probes the local unoccupied electronic orbitals, while soft x-ray emission probes the occupied electronic orbitals. In addition, resonant inelastic soft xray scattering (RIXS) opens a new field of study, including the charge-transfer transitions and energy transfer. Transition metal (TM) L-edge soft x-ray spectroscopy has been rarely applied to dilute, wet samples and/or biological nanostructured materials. RIXS in combination with XAS, has demonstrated the metal-to-ligand charge transfer occurring between a Co nanoparticle catalyst and surfactant ligands on its surface. The results reveal the electronic structure of the 3d metal compounds in pure form and their variations upon doping. Also, in-situ characterization of metal nanocrystal suspensions demonstrated the way for real-time studies of nanomaterial growth and chemical reactions. Soft x-ray spectroscopy is an element-specific method that is unique for probing electronic and geometric structures of an element of interest within a complex environment. Ti, Mn, Fe, and Co L-edge XAS spectra of transition metal compounds (soft x-ray spectroscopy) provide direct information of 3d electrons and 3d-orbital while K-edge XAS spectra (hard x-ray spectroscopy) probe the information of 4p-electrons and 4p-orbitals owing to the dipole selection rule governing the transition processes. Mn L-edge XAS spectra can be simulated well by ligand-field theory, and it provides electronic state information with significantly better resolution. The charge transfer in TiO₂, Fe₂O₃, and Co₃O₄ nanoclusters grown in silica nanopores that act as efficient and robust catalysts for water oxidation has been revealed.

There is always a demand to develop environmental sustainable and cheaper technique to purify water against chemical and biological materials. The use of metal oxides such as MnO2 and Fe2O3 to remove organic compounds and heavy metal ions from water has been studied expensively. ZnO coated multiwalled carbon nanotubes (MWNTs) are used in hydrolysis of organic chemical and converting it to p-nitrophenol. Particularly, the composites of TiO2 and carbon (TiO2- C) are currently being considered as potential photocatalysts for water purification purposes. We know that Rhamnolipid (example: Emulsan, Rhamnolipid, Surfactin), a biosurfactantâ?T is a good anionic material with no toxicity and being used to clean the petroleum contaminants and heavy metals from water and soil. The use of biosurfactant is environmentally compatible and economical than using Ligands or modified metal chelators. In fact, there is no single material, which satisfies the remediation strategy in a wide range of pollutants, several materials and/or methods, have to be employed in order to achieve the desired adsorption and purification of water. Keeping in view the complete remediation of organic and metallic contaminants, we have integrated two technologies for removal of organic pollutants using the biosurfactant (Rhamnolipid) and TiO2 containing graphene nanoparticles. The Rhamnolipid will be able to remove all the organic compounds and heavy metallic, whereas the complete removal of organic compounds will be made possible by photocatalysts graphene containing TiO2 particles. The TiO2â?"graphene nanoparticles have been synthesized and optimized by using Scanning Electron Microscopy (SEM), Raman spectroscopy, FTIR spectroscopy, X-ray-diffraction, electrochemical, and electrical measurement techniques. The G-TiO2 nanocomposite shows metal oxide coated platelet characteristics much larger in size than the graphene nano-platelets. The graphene doped TiO2 nanomaterial shows interesting optical properties. The rhamnolipidâ?"TiO2 nanocomposite structures have been optimized using similar techniques. The water purification has been studied by immobilizing the nanomaterials on fiber, glass and ceramics with wide range of wavelengths. Our novel material shows the water purification at visible wavelength. Based on our results the integrated material could be a transformable and viable material for water purification in day light and be commercially exploited.

A promising route towards the optimization of PEC cells is proposed based on the use of hierarchical nanostructures which are produced via a double or multistep anodisation of titanium foils using different electrolytes and controlled conditions such as anodisation potential. It allows obtaining straigthforward significant improvement of the generated photocurrent versus stardard via. One of the advantages of anodisation is the good control of the nanostructure and at the same time its facile scale up of the process. To achieve it, we have used two different electrolytes. The first anodisation step is used to create a pattern of holes in the titanium foil, with diameters ranging from 300 to 500 nm. The second one is used to create nanotubes of 50nm inside the prepatterned structure. Using this strategy, it is possible to increase, at least, by a factor of 3 the photocurrent generated compared to the conventional anodisation.

It has been reported that low level doping of TiO2 with Sn(IV) enhances its photocatalytic activity under UV light and visible light. Based on electronic structure studies of rutile SnxTi1-xO2 solid solutions, the improved photocatalytic activity has been attributed to a narrowing of the bulk bandgap at low Sn concentration and surface states associated with segregated Sn (II). The latest lie above the top of the main valence band and can therefore act as trapping sites for holes produced under photoexcitation [1]. The chemical state of surface Sn may also have influence on the photocatalytic properties of Sn-doped TiO2 due to differences in surface chemistry of SnO2 and SnO [2]. In this contribution we report a study of the reduction profile of Sn-doped TiO2 anatase powders based on temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). The influence of the extent of reduction of the samples on their surface chemistry has been studied on the basis of zeta potential and CO2 adsorption measurements, aiming conditions for optimal photocatalytic properties. TPR profiles of hydrothermally synthesized undoped and Sn-doped samples in a range from 0% to 10% Sn indicate that H2-consumption occurs at temperatures as low as 200°C for Sn-doped TiO2, but pure TiO2 can only be reduced at temperatures over 400°C. H2-consumption at low temperature for Sn-doped samples can therefore be assigned to surface Sn reduction, which suggests that reduction of Sn(IV) ions is more favourable than Ti(IV) reduction. XPS measurements of Sn-doped samples clearly show electronic states associated to surface Sn(II) arising above the valence band upon in situ reduction at temperatures up to 350°C; however, no signal assigned to Ti(III) states could be detected in the spectra. These new states introduced within the band gap of TiO2 induce strong visible light absorption and enhance the photocatalytic activity of the material under visible light irradiation. Zeta potential titrations show similar profiles for undoped and Sn-doped samples. However, the isoelectric point of Sn-doped samples increases after reduction treatments. This observation has important implications on the behaviour of samples as photocatalysts in aqueous media, e.g. strength and nature of the interaction with organic compounds in aqueous solutions. Ongoing studies, e.g. by means of CO2 temperature-programmed desorption measurements, aim at determining the potential of Sn-doped samples with different extent of reduction as photocatalyst for CO2 reduction. The concentration of Sn(II) at the surface of the solid is expected to influence the interaction with adsorbed species as well as the surface basicity, which is a crucial factor for CO2 adsorption and activation. ____________ [1] F. E. Oropeza, B. Davies, R. G. Palgrave, R. G. Egdell, Phys. Chem. Chem. Phys. 2011, 13, 7882. [2] M. Batzill, U. Diebold, Prog. Surf. Sci. 2005, 79, 47â?"154.

Epitaxial oxide heterostructures present an ideal platform to explore electronic and magnetic properties, and when integrated with semiconductors, potentially have applications in advanced electronics, hyperspectral sensors, and persistent surveillance and radar technologies. Chemical routes, such as atomic layer deposition (ALD) need to be explored to enable lower cost manufacturing routes, growth over large area substrates, and potentially easier insertion of multifunctional oxide technology applications into the commercial sector. In the present work, epitaxial titanium dioxide (TiO2) is grown by ALD on 20 x 20 mm2 Si(001) substrates using strontium titanate (STO) grown by MBE as a buffer layer and surface template. The ALD growth of TiO2 was achieved using titanium isopropoxide (TTIP) and water as the co-reactants at a substrate temperature of 225°C. To preserve the quality of the MBE-template, the samples were transferred in-situ to a custom-built ALD chamber. After the ALD growth, the samples were annealed in-situ at 600°C in vacuum (10-8 torr) for 1-2 hrs. Reflection high-energy electron diffraction (RHEED) was performed in-situ during the MBE growth of STO on Si(001), as well as after the growth of TiO2 by ALD. The ALD films were shown to be highly ordered and reflected the quality of the MBE template. It was found at least four unit cells of STO must be present to create a stable template on the silicon substrate for ALD growth. Both x-ray photoelectron spectroscopy (XPS) and x-ray diffraction (XRD) were performed ex-situ to determine composition and structure of the ALD films, respectively. XPS analysis indicates that the titanium ions are in the Ti4+ oxidation state, where the binding energy for the Ti 2p3/2 peak is 458.8 eV. X-ray diffraction revealed that the TiO2 films were anatase with only the (004) reflection present at 2Theta~38.15°. This indicates that the a-axis has increased and the c-axis is slightly shrunken from that of anatase powder (2Theta~37.8°), which slightly reduces the lattice mismatch of anatase-type TiO2 to the substrate. Rocking curve of the anatase (004) reflection for a 20 nm thick film grown by ALD revealed a full-width half maximum (FWHM) of 1.40°. Presently, we have grown up to 100 nm thick TiO2 films that remain highly ordered in the (001) direction. Currently we are working to gather photocatalytic data on our films; the talk will present the epitaxial TiO2 growth and characterization and provide initial data for the photocatalytic testing of these films.

TiO2/graphene nanocomposite material is very promising for photocatalytic applications. Graphene, with its unique electronic properties, large specific surface area and high transparency, contributes to facile charge separation and adsorptivity in this hybrid structure. In our work, hydrothermal method was utilized to synthesize the graphene-TiO2 nanoparticle (G/NP) and nanowire (G/NW) hybrid structures. The structural, chemical, electrical, and optical properties of the hybrid materials are studied based on SEM/TEM, micro-Raman spectroscopy, FTIR, X-ray photoelectron spectroscopy (XPS), and electrochemical impedance spectroscopy (EIS). Their photocatalytic performance is evaluated based on photo degradation of methylene blue under visible light. Graphene sheets were achieved by graphene oxide reduction in a Teflon-lined autoclave, together with TiO2 nanoparticles (NPs). The autoclave was maintained at 200C for 24 hours and then cooled down to room temperature naturally. The obtained products were successively washed with dilute hydrochloric acid, deionized water and absolute methanol for several times until the pH value equals to 7. After recovered by filtration, the products were dried at 70 C for 6 hours, and eventually, the gray powder was attained. For the G/NP structure, TiO2 NPs were decorated on graphene sheets, while for the G/NW structure, graphene sheets were observed to be densely wrapped around by TiO2 NWs with a width of about 30 nm, which was self-assembled from the TiO2 NPs during the hydrothermal process. Comparative studies of properties and photocatalytic performance of G/NP and G/NW structures will be reported.

Anatase TiO2/Ti-based perovskite oxide heterostructures are of great interest in a number of research areas, ranging from photocatalytic water splitting [1] to oxide spintronics [2]. Moreover, as a result of the trend of miniaturization of devices, the reduced dimensionality makes interface effects more important for determining device functionality. However, oxide heterointerfaces are not as well-understood as those between metals and semiconductors [3]. In this talk, using ab-initio density functional calculation and electron energy loss spectroscopy (EELS) we discuss the electronic structure and the interface properties of the anatase TiO2/SrTiO3 (001) heterostructure eptiaxially grown on Si (001) by molecular beam epitaxy (MBE). In the local density approximation (LDA), we show that there is a chemical bonding induced charge transfer from SrTiO3 to anatase TiO2 to form a double layer at the interface. We show that O lattice polarization is the main dielectric response at the interface. The valence band top of anatase film is found to be lower in energy by 0.74 eV than that of SrTiO3. By comparing the local electronic structure obtained from the calculation to the O K edge spectra taken at the physical interface with atomic resolution, we resolve how the valence band evolves in response to the change in the symmetry and bonding configuration across the interface. We also perform GW calculation for SrTiO3 and anatase TiO2 and show that the quasi-particle correction is large for the conduction band edge while the valence band edge remains largely the same, supporting that the LDA valence band offset is reliable. Finally, we discuss that the valence band offset could be varied depending on the interface stoichiometry by considering interfacial O vacancy. References [1] X. Chen, S. Shen, L. Guo, and S. S. Mao, â?oSemiconductor-based photocatalytic hydrogen generationâ?, Chem. Rev. 110, 6503 (2010) [2] T. C. Kaspar, T. Droubay, V. Shutthanandan, S. M. Heald, C. M. Wang, D. E. McCready, S. Thevuthasan, J. D. Bryan, D. R. Gamelin, A. J. Kellock, M. F. Toney, X. Hong, C. H. Ahn, and S. A. Chambers, â?oFerromagnetism and structure of epitaxial Cr-doped anatase TiO2 thin filmsâ?, Phys. Rev. B 73, 155327 (2006). [3] O. Sharia and A. A. Demkov, â?oTheoretical study of the insulator/insulator interface: Band alignment at the SiO2/HfO2 junctionâ?, Phys. Rev. B 75, 035306 (2007)

The deposition, growth and aerosolization of microbes in the built environment have adverse public health and economic impacts. Illuminated surfaces of TiO2 have potential for photocatalytic destruction of even the hardiest microbes. TiO2 photocatalytic coatings for indoor applications have seen tremendous commercialization in recent years. Their primary application is self-cleaning coatings in hospitals to combat viruses, bacteria and mold. However, TiO2 alone is effective only with opaque and heavy coatings that obscure the appearance of the underlying materials. A translucent, antimicrobial, photocatalytic coating was developed using polyhydroxy fullerene (PHF) as an enhancer for TiO2 photocatalysis on surfaces. Photocatalytic activity was optimized by varying the ratio of PHF to TiO2, as indicated by decolorization of organic dye (Procion red MX-5B). The optimized TiO2-PHF nanocomposite achieved double the rate of dye degradation under UVA illumination, as compared to TiO2 alone. The TiO2-PHF nanocomposite coatings were then tested against spores of Aspergillus niger, a household fungus commonly implicated in asthma and surface degradation. The TiO2-PHF nanocomposite coatings achieved close to three-fold enhancement of spore inactivation over a 3-hour exposure period.