Symposium EE14 : Titanium Oxides---From Fundamental Understanding to Applications

Nanostructured TiO2 has been recognized as key elements in diverse functional applications and devices including energy conversion and storate, photocatalysis, biomedical theranostics, etc. In this talk, I overview our recent efforts and accomplishments to exploit hybrid TiO2 nanomaterials for electrodes in energy conversion devices, solar fuel generation via water splitting, and environmental remediation. Thin films of carbon-TiO2 were prepared by direct calcination of UV-stabilized diblock copolymer inverse micell templates and integrated into the interface between the FTO and photoanode as well as between the photoanode and electrolytes in dye-sensitized solar cells. We interprete the markedly enhanced cell performance in terms of the decrease in barrier resistance. Doped-TiO2 via carbonization of polymer-TiO2 precursors was shown to exhibit visible light activity in photocatalytic water splitting and pollutant degration. To this end, we developed inverse opal nanostuctures consisting of metal oxides or multimetallic oxides decorated with carbon moieties. Enhanced hotocatalysis mediated by Fenton-photon reaction and hydrogen peroxides will be introduced along with electron paramagnetic resonance analysis. Importantly, we will propose a novel analytical tool to investigate the photocatalytic environmental remediation process based on in-situ surface plasmon resonance spectroscopy.

The common picture of excess electrons - originating from photoexcitation/ photo-injection, intrinsic defects or dopants - in TiO₂ is based on the polaron model, where the electron becomes localized by inducing a lattice distortion that stabilizes the localized state. The location and energies of the trapping sites are of fundamental importance in essentially all applications of this material, but are still rather controversial. We present first principles molecular dynamics simulations of the localization and dynamics of excess electrons on bare TiO₂ anatase surfaces and anatase-water interfaces. Our results provide insights into the effects of different surface terminations, hydroxyls, impurities, and the interaction between localized carriers and water at the interface.

The conduction band (CB) engineering of TiO₂ is of fundamental implication for photocatalysis, solar cells and solar fuel generation. The position of CB edge controls the driving force for photocatalytic hydrogen evolution from water, potential of dye-sensitized solar cell (DSC) and recombination blocking in perovskite solar cells. The staggered alignment in mixed anatase/rutile phases is assumed to enhance photocatalytic activity, but it is unclear whether the CB of rutile or that of anatase is higher. XPS and most DFT simulations support the former, but the flat-band potential measurements provide just opposite results. The controversy can be explained by taking into account the adsorption of OH^– and H⁺ from the electrolyte solution on the electrode surface. XPS indicates that the CB edge of (001)-anatase is upshifted by 0.1 eV referenced to (101)-anatase in agreement with the DFT calculation and with the electrochemical flatband potentials (upshift of CB by 60 meV).

About one billion people worldwide do not have access to safe drinking water. The World Health Organization (WHO) has estimated that 80 percent of illnesses in the developing world are the results from waterborne diseases. Effective and low-cost water disinfection methods are urgently needed. Chlorination, ozonation, and UV irradiation are common methods for water disinfection. While these methods can control the proliferation of waterborne infectious agents, they are not perfect technologies. Chlorination and ozonation often generate toxic disinfection byproducts, and UV has limited penetration power. TiO₂-based photocatalysis represents a promising alternative to the traditional techniques for water disinfection. It works by generating powerful but short-lived hydroxyl radicals upon irradiation. These radicals can inhibit the growth of bacteria. In addition to the advances in basic research, practical photocatalytic disinfection products will also be introduced in this presentation.

References
W.J. Wang, G.C. Huang, J.C. Yu, P.K. Wong, "Advances in Photocatalytic Disinfection of Bacteria: Development of Photocatalysts and Mechanisms," Journal of Environmental Sciences 2015, 34, 232-247.
W.J. Wang, J.C. Yu, P.K. Wong, "Photocatalysts for Solar-Induced Water Disinfection: New Developments and Opportunities," Chapter 2 in Photocatalytic Materials & Surfaces for Environmental Cleanup-II, Ed.: Rajesh J. Tayade, pp. 63-89, Trans Tech Publications Inc., Switzerland, 2013.
D.Q. Zhang, G.S. Li, J.C. Yu, "Inorganic Materials for Photocatalytic Water Disinfection," Journal of Materials Chemistry 2010, 20, 4529-4536.
T.Y. Leung, C.Y. Chan, C. Hu, J.C. Yu, P.K. Wong, "Photocatalytic Disinfection of Marine Bacteria Using Fluorescent Light," Water Research 2008, 42, 4827-4837.
J.C. Yu, W.K. Ho, J.G. Yu, H.Y. Yip, P.K. Wong, J.C. Zhao, "Efficient Visible-Light-Induced Photocatalytic Disinfection on Sulfur-Doped Nanocrystalline Titania," Environmental Science and Technology 2005, 39, 1175-1179.

White titanium dioxide (TiO₂) has a large bandgap energy (3.03 eV for rutile (R-TiO₂) and 3.20 eV for anatase (A-TiO₂)); thus, it can only absorb UV light. As a consequence, the theoretical limit of hydrogen production by white TiO₂ is less than 5% under air mass (AM) 1.5 global solar illumination. In this presentation, I will report a various chemical reduction methods to maximize the efficiency of solar water splitting in TiO₂ nanomaterials on a transparent conducting oxide (TCO) substrate. The controlled reduction leads to oxygen vacancies in the TiO₂ arrays, which subsequently induce a cathodic on-set potential shift and sharp photocurrent evolution. Moreover, controlled morphologies of TiO₂ is also very effective for Li ion storage, which will be applicable for anode materials of Li-ion batteries.

Flame synthesis, a $15 billion/year industry is the most widely used method for the global production of TiO₂. Industry leaders include, Cristal Global, DuPont, and Evonik. A key challenge in industry is that flame reactors are usually built for a single product and the possibility of modifying the reactor’s operational parameters is limited. It is, therefore, important to develop flame reactors that will allow for a wide range of growth parameters and that can be easily scaled, specifically for the production of advanced materials, particularly next-generation TiO₂. The most commonly used flame types for material synthesis include premixed, normal diffusion, inverse diffusion, and co-flow flames. The use of multiple diffusion flames offers several key advantages, such as uniform temperature and chemical species profiles and many of the limitations related to premixed flames such as flashback and flame speed are avoided. Using the multiple-diffusion burner in a single-step, we have demonstrated the growth of carbon-coated titanium dioxide (TiO₂) nanoparticles, TiO₂ doped/coated with different metals (vanadium, silicon, and iron), and SiO₂ doped with carbon.

In lab scale experiments, laminar flames remain the dominant approach for the flame synthesis of nanomaterials. However in industry, turbulent flames are commonly utilized as it enables larger-scale production of nanomaterials. Recently we investigated the use of a novel turbulent burner for the flame synthesis of nanoparticles. It involves the use of an axisymmetric curved-wall jet (CWJ) burner, where the Coanda effect enables flame stability. Key advantages of this method include the enhancement of turbulence due to the radially inward velocity component of the jet. The vaporized precursor mixes with the gases delivered in the axisymmetric curved-wall jet burner. The combined effects enable enhanced mixing of the precursor, which is important in gas-phase synthesis. Results suggest we can effectively control the growth of anatase and rutile particles based on the operating conditions of the flame. Such a setup can be used to burn low cost chemical precursors due to the high temperature of the turbulent flame. It is possible this setup can provide new pathways for scalable synthesis of advanced TiO₂ nanoparticles.

High dielectric constant materials are critical for applications in nanoscale microelectronics, and as capacitors for energy storage and memory devices. Both Al₂O₃ and TiO₂ have been extensively investigated as high-k materials to replace SiO₂ as a gate and for high-capacitance capacitors for electronics. The dielectric constants of Al₂O₃ and TiO₂ are approximately 7 and 80, respectively. Our previous studies showed that amorphous Ti_xAl_1-xO_y films exhibit dielectric constant of ~30, with 4.8 eV bandgap. Here we report that giant dielectric constants (> 800) can be achieved with Al₂O₃/TiO_x nano-laminates, synthesized by atomic layer deposition (ALD) and with sub-layer thickness ≤ 1 nm, for frequencies up to 10⁶ Hz. The high dielectric constant is attributed to Maxwell-Wagner (M-W) relaxations, resulting from electrical heterogeneity of the multilayers. Al oxidation is favored over Ti-oxide since the Gibbs free energy for Al oxidation is more negative than for Ti. It appears that the Ti-oxide sub-layers are not stoichiometric TiO₂, but semiconducting TiO_x. The difference in electrical conductivities of the TiO_x and Al₂O₃ layers results in surface charge accumulation at the interfaces. The surface charges relax with AC field and cause M-W relaxation. An interface layer inserted at the interface between top electrode and Al₂O₃/TiO_x nanolaminate is critical to yield high-k up to about 1 MHz with low dielectric losses (~0.02) and low leakage current (~ 10^-9 A/cm²). We used ALD to produce large area capacitors via conformal coating of large area ridge arrays fabricated on Si surfaces. These capacitors can yield ≥ 10 µF/cm² capacitance.
We also report the feasibility of controlling the dielectric properties - high dielectric constant (k) and substantially extended relaxation frequency of thin film nanolaminates (NLs) with various sublayer thicknesses, uniquely realized by ALD process. For 150 nm thick TiO_x/Al₂O₃ NLs with sub-nanometer thick sublayers, few Angstrom change in sublayer thickness dramatically increases relaxation cut-off frequency by more than 3 orders of magnitude (from kHz to MHz) with high dielectric constant (> 800 ).
The nano-laminates are also explored for applications such as energy storage embedded capacitors in a Si microchip implantable in the human retina to restore sight to people blinded by genetically-induced degeneration of photoreceptors, for supercapacitors integrated with ferroelectric-based high-efficiency photovoltaic devices for energy generation/storage systems, and for high-k gate oxide with low leakage current and losses for next generation DRAMs and nanoscale CMOS devices.

TiO₂ is touted as a material capable of use in photo-catalytic environmental remediation strategies. However it exhibits efficient recombination processes and exhibits a bandgap congruent with ultraviolet light which represents ~5-6% of the solar spectrum. Engineering of TiO₂’s bandgap by doping with a variety of cations and anions has allowed visible light, which represents a far larger proportion of solar irradiation, to be utilised whilst reducing the recombination of charge carriers.

However, serious long term testing of dopant stability under irradiation has yet to feature within the literature. We present long term testing using XPS and photo-catalytic tests to demonstrate the lability of dopants in doped TiO₂’s which have been previously characterised. Depending on the dopant, such as phosphorus or tantalum, and its location (interstitial versus substitutional) some exhibit better stabilities and relevance for environmental applications than others. We discuss the issues and consequences of these findings for the field, with big potential impacts in solar cells, H₂ production and photo-catalysis in general. We go on to stress the importance of long term testing within the field of TiO₂ and photo-catalysis in general so that decisions regarding its use for environmental applications are made using clear design rules.

Metal halide perovskite solar cells have seen an incredible surge in efficiency to over 20 % in recent years. In the first incarnation, the perovskite was used as a sensitizer on a mesoporous TiO₂ scaffold, and functioned in a manner similar to the dye sensitized solar cell. The technology rapidly transferred to a solid state system which offered advantages in terms of stability. The current state of the art consists of a both planar and mesostructured architectures the vast majority of which continue to employ TiO₂ as the electron selective contacts.

This talk will focus on recent work from our group investigating the role of TiO₂ as the electron selective contact in solar cells using the commonly employed CH₃NH₃PbI₃ semiconductor. Our results suggest that while TiO₂ is currently used in the state of the art architectures, TiO₂ may not be the ideal material in terms of its effects on absolute efficiency, hysteresis phenomena, and perhaps most importantly, device stability. The origins of hysteresis in perovskite solar cells will be discussed through an analysis of device behavior with varying selective contacts at different temperatures and scan rates. The role of band alignment plays a large role in both absolute efficiency and the extent to which hysteresis can be manifested in devices, and can be improved by changing contact materials. We find that fullerenes form an improved contact with the perovskite, allowing us to obtain improved performances with minimal hysteresis. This is quantified by means of photoluminescence quenching experiments with varying biases and contact layers. These demonstrate clearly that electron transfer to planar TiO₂ layers is hindered by an energetic barrier whose height can be temporarily tuned by poling induced ion migration, resulting in strong hysteretic behavior.

On an equally important note, we address the impact of TiO₂ layers on the stability of perovskite solar cells. We focus particularly on the effect of UV light exposure on the stability of the devices. We have found that the complex interaction of adsorbed oxygen, UV light, and TiO₂ surface states leads to the formation of deep trap sites which result in a rapid reduction in performance of TiO₂ containing solar cells. This effect can be remedied by altering the recombination mechanisms of the solar cells or simply by replacing the TiO₂ with fullerenes, which do not suffer from such UV activated states. It appears that while TiO₂ can be a useful material to make high performance perovskite solar cells, it may not be the ideal material for reaching the efficiencies and stability ultimately required of the technology.

In this work, we focus on developing various novel nanostructured TiO2 for enhancing photoelectrocatalytic activity in applications of clean environments and energy. An ultrasonication-assisted sequential chemical bath deposition (S-CBD) method was developed to modify anatase TiO2NTAs with Cu2O nanoparticles to form p-n heterojunction photoelectrodes for enhancing visible light photocurrent and photocatalytic performance through manipulating the size and content of deposited Cu2O nanoparticles. It was shown that a small amount of Cu2O nanoparticles significantly improved the photocatalytic activity. The as-prepared heterojunction photoelectrodes obtained from ultrasonication-assisted (S-CBD) method for 4 min possessed the highest photocurrent and photocatalytic degradation rate of Rhodamine B under both UV and visible illumination. When applied a 0.5 V bias potential, photocatalytic electrodes exhibited the superior photoelectrocatalytic activitydue to the synergistic effect from the photocatalysis and electrochemistry. The ZnFe2O4-decoratedanatase TiO2 NTA electrodes was constructed via a hydrothermal technique. The size and loading amount of ZnFe2O4 nanoparticles were adjusted by different hydrothermal process. It was displayed that the photocurrent and photocatalytic efficiency of TiO2 NTA electrode after loading of ZnFe2O4nanoparticles under visible light irradiation were incredibly increased. Among the photocatalystssynthesized for different times, the electrode obtained from the hydrothermal reaction for 6 h presented the optimal photocatalytic performance in degrading Acid Orange II under visible light illumination. Electrochemical measurements (i.e., electrochemical impedance spectra and Mott-Schottky test) revealed that the modification of ZnFe2O4 nanoparticles promoted the charge carrier transfer at the interface of the photocatalyst and the electrolyte, which efficiently inhibited the recombination of photogenerated charge carriers. The SrTiO3-modified rutile TiO2 nanorod array electrodes were fabricated via the three consecutive facile hydrothermal techniques. In the first two hydrothermal reactions, rutile TiO2 NRAs were synthesized and etched to increase the specific surface area of the electrodes. During the subsequent hydrothermal treatment, rutile TiO2 NRAs served as both the structure-directing scaffold and Ti source and rutile TiO2 at the surface layer of nanorods was converted into SrTiO3. The conversion content of rutile TiO2 to SrTiO3 can be adjusted through tuning the hydrothermal time. It was demonstrated that after partially turning into SrTiO3, the prepared photocatalytic electrodes showed not only boosted UV light response but also enhanced photocatalytic degradation rate of methylene blue under UV illumination. The heterojunction electrodes obtained from the hydrothermal reaction for 12 h possessed the best photocatalytic performance. It was found from the electrochemical impedance measurements that the SrTiO₃ modification suppressed the recombination of electron-hole pairs and improved the carrier mobility at the interface of electrode and electrolyte.

Here, we would like to introduce our recent work on the green TiO₂ photocatalysts, including the discovery, properties, and photocatalytic performances in photocatalytic hydrogen generation and pollutant removal.

The development of new color switching systems that can reversibly change color in response to external stimuli, such as light or heat, has attracted a great deal of attentions for their important applications in sensing devices, display and signage technologies, rewritable media, and security features. Here we discuss a new color switching system based on reversible redox reaction that could be initiated by photocatalytic response of TiO₂ nanocrystals. With the assistance of TiO₂ nanocrystal-based photocatalysts, UV light irradiation can rapidly reduce the imaging materials and result in obvious color change, while the recoloration can be achieved by re-oxidizing the system with the assistance of visible light irradiation or heating. The excellent performance of the new color switching system promises their potential applications as attractive rewritable media to meet our society’s increasing needs for sustainability and environmental conservation.

One-dimensional nanoarrays such as nanorods, nanowires, or nanotubes have attracted significant interest for photovoltaic applications. However, the power conversion efficiency of dye-sensitized solar cells (DSSCs) based on one-dimensional nanoarray photoelectrode is still lower than the commercial nanoparticle due to the lower surface area for the dye loading. Hence, the current challenge is to synthesize the nanoarray photoelectrode possessing larger internal surface area for larger amount of dye loading. Here, we report the facile hydrothermal fabrication of hierarchical and hyperbranched anatase TiO₂ nanowire array for efficient DSSCs and quantum dot-sensitized solar cells (QDSSCs). For example, ultra-long (47 um in length) anatase TiO₂ nanowire array was prepared via a multi-step hydrothermal reaction which leading to an impressive power conversion efficiency of 9.40 %. Furthermore, a series of TiO₂ nanowire array-nanosheet (or nanorod) branch-tiny nanorod hyperbranched structures have been also successfully prepared via the simple hydrothermal process which exhibit more than 11% photovoltaic performance in DSSCs. The higher efficiency for such hierarchical TiO₂ nanoarrays can be mainly attributed to the fast electron transport and slow electron recombination, efficient dye loading and superior light scattering ability, which was investigated by intensity modulate photocurrent spectroscopy (IMPS), intensity modulate photovoltage spectroscopy (IMVS), electrochemical impedance spectroscopy (EIS), and UV-vis diffused spectroscopy etc. Such novel TiO₂ nanowire arrays are anticipated to exhibit the significant applications in photovoltaic, photocatalysis, water splitting, CO₂ conversion, optoelectronics, energy conversion and storage.

Keywords: Solar cells, TiO₂, nanowire, photovoltaic performance

References
[1] Wu, W. Q.; Feng, H. L.; Rao, H. S.; Xu, Y. F.; Kuang, D. B.;^* Su, C. Y., Nat. Commun, 2014, 5, 3968,
[2] Wu, W. Q.; Xu, Y. F.; Rao, H. S.; Kuang, D. B.;^* Su, C. Y., J. Am. Chem. Soc, 2014, 136 (17), 6437–6445
[3] Wu, W. Q.; Xu, Y. F.; Rao, H. S.; Feng, H. L.; Su, C. Y.; Kuang, D. B.,^* Angew. Chem, 2014, 126 (19), 4916-4921; Angew. Chem. Int. Ed, 2014, 53(19), 4816-4821
[4] Wu, W. Q.; Xu, Y. F. ; Su, C. Y.; Kuang, D. B. ^* Energy Environ. Sci., 2014, 7(2), 644-649.
[5] Wu, W. Q.; Lei, B. X.; Rao, H. S.; Xu, Y. F.; Wang, Y. F.; Su, C. Y.; Kuang, D. B. ^* Sci. Rep. 2013, 3,1352.
[6] Liao, J. Y.; Lei, B. X.; Chen, H. Y.; Kuang, D. B.; ^* Su, C. Y. Energy Environ. Sci. 2012, 5, 5750.

Photoelectrochemical (PEC) splitting of water represents a promising strategy for the low cost and environmentally friendly production of solar fuel. A key challenge in PEC water splitting is to fabricate nanostructured photoelectrodes with desirable architectures and properties. In this work, heterostructured TiO₂ nanorod@nanobowl (NR@NB) arrays consisting of rutile TiO₂ nanorods grown on the inner surface of arrayed anatase TiO₂ nanobowls were designed and fabricated as a new type of photoanodes for PEC water splitting. The unique heterostructures with a hierarchical architecture were readily fabricated by interfacial nanosphere lithography followed by hydrothermal growth. Owing to the two-dimensionally arrayed structure of anatase nanobowls and the nearly radial alignment of rutile nanorods, the TiO₂ NR@NB arrays provide multiple scattering centers and hence exhibit enhanced light harvesting ability. Meanwhile, the large surface area of the NR@NB arrays enhances the contact with the electrolyte while the nanorods offer direct pathways for fast electron transfer. Moreover, the rutile/anatase phase junction in the NR@NB heterostructure improves charge separation because of the facilitated transfer of photogenerated electrons from rutile to anatase. Accordingly, the PEC measurements of the TiO₂ NR@NB arrays on the FTO substrate showed significantly enhanced photocatalytic properties for water splitting. Under AM1.5G solar light irradiation, the TiO₂ NR@NB array photoelectrode yielded a photocurrent density of 1.24 mA/cm² at 1.23 V with respect to the reversible hydrogen electrode (RHE), which is almost two times higher than that of the TiO₂ nanorods grown directly on the FTO substrate. This work may open new avenues towards building complex semiconductor nanostructures with desirable architectures for efficient harvesting and utilization of solar energy.

The trapping and mobility of electrons in nanocrystalline oxide materials underpins a diverse range of applications in areas such as solar energy generation, catalysis, gas sensing and nanoelectronics. However, directly probing the properties of electrons in such complex nanocrystalline systems is extremely challenging. Here, we provide insight into these important issues through first principles based modeling of the interaction of electrons with surfaces and grain boundaries in rutile TiO2. We show that different surface orientations exhibit markedly different electron affinities: some preferring to trap electrons with others repelling electrons. The equilibrium nanocrystal morphology exposes both electron trapping and electron repelling facets and therefore is predicted to posses highly anisotropic electron trapping properties [1]. Interfaces between nanoparticles (grain boundaries) are associated with high concentrations of strong electron trapping sites which hamper electron transport between grains. However, we show how this effect is partially ameliorated at high current densities (>0.01 mA/cm2) as a result of a highly nonlinear trap filling effect [2]. We discuss how with atomistic insight into the electron trapping properties of nanocrystalline materials one can suggest ways to improve the performance of materials for applications, for example by designing optimal nanocrystal morphologies.

[1] S. Wallace and K. P. McKenna, Journal of Physical Chemistry C 119, 1913 (2015)

[2] S. Wallace and K. P. McKenna, Advanced Materials Interfaces 1, 1400078 (2014)

The use of TiO₂ photoelectrodes (or TiO₂ suspensions) to produce hydrogen from various electrolytes (with or without sacrificial agents) has been highly investigated over the past decades and still remains the most investigated semiconductive material. [1] The advantage of anodic TiO₂ nanotubes (NTs) is not only that they provide a one dimensional charge-transfer path, but also they are back-contacted on metal substrate that can be used as electrodes directly. [2] Pathways to create noble-metal-free H₂ evolution activity on TiO₂ NTs are not only of considerable scientific but also of high economic interest. [3-4] Here we introduce an efficient way to introduce a sub-surface configuration of a broad range of lattice defects (namely vacancy/interstitial pairs) into any crystalline material using high energy ion implantation. [5]
In this study, we report the creation of sub-surface configuration of defects containing TiO₂ NTs that strongly acts as photo co-catalysts. These new class of catalysts are obtained by low-dose nitrogen ion-implantation that can be tuned to obtain a highly defined ion and defect distribution within the TiO₂ NTs substrate. A particular advantage of creating these defect centers by N-implantation is that the active centers can be placed as a coherent layer at a defined sub-surface location. The catalysts present considerable photocatalytic activity for hydrogen evolution under solar light illumination in the absence of noble metals and external bias. Thus these results provide a concept for a new generation of TiO₂ based photocatalysts.


References
[1] A. Fujishima and K. Honda, Nature, 1972, 238, 37-38.
[2] X. Zhou, N. T. Nguyen, S. Özkan, P. Schmuki, Electrochem. Comm., 2014(46,) 157–162.
[3] X. Zhou, N. Liu and P. Schmuki, Electrochem. Commun., 2014 (49), 60–64.
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[5] X. Zhou, V. Häublein, N. Liu, N. T. Nguyen, E. M. Zolnhofer, H. Tsuchiya, M. S. Killian, K. Meyer, L. Frey, and P. Schmuki, 2015, submitted.

We report that the valence and conduction band energies of TiO₂ can be tuned over a 4 eV range by varying the local coordination environments of Ti and O. We examine the electronic structure of eight known polymorphs and align their ionization potential and electron affinity relative to an absolute energy reference, using an accurate multiscale quantum-chemical approach. For applications in photocatalysis, we identify the optimal combination of phases to enhance activity in the visible spectrum. The results provide a coherent explanation for a wide range of phenomena, including the performance of TiO₂ as an anode material for Li-ion batteries, allow us to pinpoint hollandite TiO₂ as a new candidate transparent conducting oxide, and serve as a guide to improving the efficiency of photo-electrochemical water splitting through polymorph engineering of TiO₂.

Nanoscale conformal layers of TiO₂ can be coated on the inner pore walls of porous silicon films by room-temperature infiltration of a sol-gel precursor consisting of titanium t-butoxide and triethanolamine in ethanol. The composite films can act as label-free optical interference sensors for chemicals (VOCs) and biological agents, and the photoactivity of the TiO₂-coating provides a self-cleaning function when irradiated with UV light. Selectivity for biological agents is achieved by physical adsorption of protein A, followed by the specific binding of rabbit anti-sheep immunoglobulin (IgG) and then specific capture of sheep IgG. The specificity of the protein A, rabbit anti-sheep IgG construct on the sensor is confirmed by tests with non-binding chicken IgG. The sensitivity is 8210±170 nm/refractive index unit (RIU).

The current dependency on non-renewable and polluting fossil fuels is responsible for many social, environmental, and health problems such as air pollution, greenhouse effects and climate changes. It is therefore essential that alternative means of generating energy and fuels be developed. Of all alternatives, conversion of solar energy to chemical fuels is one of the most attractive routes because of the unlimited supply and easy accessibility of sunlight. In this talk, I will focus on our efforts in the synthesis and characterization of visible-light responsive photocatalytic materials based on TiO₂. Several families of TiO₂ based materials with visible light response have been synthesized and their photocatalytic properties have been characterized. These materials include self-doped TiO₂ with Ti³⁺ introduced by hydrothermal and combustion methods as well as post-synthetic treatments by different reducing agents. The research also shows that different Ti precursors can also have a great impact in the materials’ photocatalytic properties. A systematic study of the structural defect, Ti³⁺ concentrations and their impact to the materials photocatalytic properties will be discussed. The materials with different morphologies and facets have been obtained and will be presented.

Thin-film semiconducting anatase TiO₂ finds several applications, including photocatalysis, wherein control over carrier concentration and other material properties would be useful. Literature tends to pursue relationships between specific synthesis conditions and film morphology with little attention given to carrier concentration. Meanwhile, attempts to significantly vary carrier concertation have centered heavily around the use of dopants such as nitrogen. Both bodies put little focus on annealing and other post processing conditions.

The present work employs atomic layer deposition to fabricate thin amorphous films of TiO₂, which are then annealed into polycrystalline anatase. Variations in the temperature and growth rate for deposition, as well as in the ramp rate and final temperature employed during the annealing, were used to determine how carrier concentration can be affected without the use of dopants. Effects of annealing in oxygen scavenging atmospheres (H₂ and Cl₂), as well as super band gap photostimulation, are reported as well.

These variations propagate into changes in one or more of the final carrier concentration, crystallite size and bulk density. Many of these surprising findings can be interpreted in terms of medium range atomic order existing in the initial amorphous films. The findings further demonstrate that medium range order may be a physical property capable of being manipulated in metal oxide systems

Improving the photocatalytic (PC) properties of titania has been the focus of this research. In particular, increasing titania’s PC activity in the visible region of the spectrum is crucial for increasing its widespread use as a catalyst. Two recent methods have been shown to increase titania’s PC activity. First, codoping anatase titania nanoparticles with transition metals and nitrogen increases PC activity in the visible region, however, the usual route for making these nanoparticles, the hydrothermal method, creates particles with widely-varying sizes. This variation in size is thought to hinder charge transport. To address this, the second method creates uniformly-sized (also known as monodisperse) anatase titania nanoparticles. These particles are produced by a two-step method in which monodisperse titania nanospheres are produced via a sol gel route, and these particles are then used a template for growth of mesoporous, anatase nanospheres. The question naturally arises as to whether combining of these two techniques to create monodisperse, codoped titania would exhibit even higher activity. Here we describe the process for making monodisperse, codoped, mesoporous titania nanoparticles and compare their photocatalytic performance with those having a large variance in size.

Titanium oxide is an interesting material for microelectronic, opto-electrical, and photochemical applications due to its stability, high-K dielectric properties, and high refractive index. Novel applications include passivation layers for production of solar fuels [1], resistance switching material in non-volatile memory ReRAM [2] and as tunnel dielectrics for MIS contacts [3]. These applications require precise control of thickness, composition, and crystallinity and low temperature processes are desired. In this presentation we compare results obtained from ALD deposited titanium oxide films using a variety of precursors and a wide range of process temperature on 300 mm native-oxide silicon wafers with an ASM Pulsar^® 3000 cross-flow ALD reactor.

Titanium precursors studied include titanium isopropoxide (TTIP), titanium methoxide (TMOT), or titanium tetrachloride (TiCl₄) with oxygen sources of water (H₂O), ozone (O₃), or TTIP [4]. Deposition temperatures from 50°C to 300°C were studied, with the specific range depending on the precursor used. Films were characterized for thickness and refractive index by ellipsometry. The effect of precursor and temperature on film composition was analyzed by SIMS and ERD. Density and crystallinity was also characterized by XRR and XRD.

A wide range of growth rates were observed depending on temperature and precursors used. Growth at temperatures as low as 50°C was observed for TiCl₄ and water. Ozone provides higher growth rate than water. The highest growth rate was achieved using the metal alkoxide TTIP as the oxygen source with TiCl₄ [4]. Refractive index and density of the films increased with temperature. Slight changes in O/Ti ratio was observed depending on precursors used. TiCl4-based processes can result in Cl contamination at lower temperatures, but higher temperatures reduce Cl to negligible levels. Films deposited at 300°C have crystalline anatase form. Amorphous films can be deposited at 200°C, but readily crystallize upon subsequent anneal at 400°C. High quality films are deposited with high RI and density with multiple precursors in the temperature range of 250-300°C.

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[3] J.Y.J. Lin, A.M. Roy, K.C. Saraswat, IEEE Electron Device Lett., 33, 11, p. 1541-1543 (2012)
[4] M. Ritala, K. Kukli, A. Rahtu, P. Räisänen, M. Leskelä, T. Sajavaara, J. Keinonen, Science, 288, 5464, p. 319-321 (2000)

Large-band gap metal oxide TiO₂ have suitable band positions for photoelectrochemical cells (PECs) for solar-driven water splitting, but uses only UV light region in the solar spectrum which represent only about 5 % of the energy. On the other hand, α-Fe₂O₃ with suitable bandgaps for efficient absorption in the solar spectrum require an external bias to drive hydrogen generation at the cathode due to the conduction band of α-Fe₂O₃ below the H₂ evolution potential and have short carrier diffusion lengths. Synthesizing the metal oxide nanomaterials which have both suitable band position to drive reaction and visible light absorbed band gap is one of the major challenge in PECs for water splitting field. Hetero structure of α-Fe₂O₃ and TiO₂ offer a potential solution to improve this problem. However, the inherent low electrical conductivity resulting in the high electron-hole pair recombination rate and short carrier diffusion length of α-Fe₂O₃ limit its practical use. Here we report a novel hierarchical heterostructure of α-Fe₂O₃ ultrathin nanoflakes branched on TiO₂ nanotube strategy for PEC water splitting. On the basis of the detailed experimental results and associated theoretical analysis, we demonstrate that suitable morphological control of α-Fe₂O₃ and TiO₂ plays an important role in enhancing the photoelectrochemical water splitting performance.

Photoelectrochemical (PEC) water splitting can be employed for the production of hydrogen by exploiting solar radiation; however, the development of innovative photoanode and cathode materials and architectures still represents a key challenge for the realization of an efficient and competitive PEC cell. In this context, this study focuses on quasi-1D hydrogen-treated titanium oxide nanostructures as photoanodes.
Titanium dioxide is one of the most studied materials as photoanode for water splitting, nonetheless it presents some limitations that are currently addressed following two main strategies, i.e., first, doping or sensitization, to shift the absorption to the visible; second, controlling the material structure and morphology at the nanoscale, to enhance light harvesting and quantum efficiency. A recent innovative approach towards the first goal consists in the hydrogenation or reduction of titanium dioxide, leading to the so-called black titania, which exhibits remarkable efficiency for the water splitting reaction, although being not fully understood [1]. On the other hand, quasi-1D hierarchical nanostructures have been considered because of their advantages, such as large surface area, effective light scattering, mesoporous structure and anisotropic morphology.

In this work an explorative combined approach, i.e. extension of the photoresponse to the visible range as well as optimization of morphology and structure, has been investigated. Amorphous hierarchical or quasi-1D TiO₂ nanostructures were synthetized by Pulsed Laser Deposition (PLD) in an O₂ background atmosphere [2], and different strategies were explored to achieve hydrogenation or reduction, namely changing the deposition background gas (from pure O₂ to Ar/O₂ or Ar/H₂ mixtures), or substituting/combining the air annealing process at 500°C (necessary to induce crystallization to the anatase phase) with Ar/H₂ annealing. SEM, Raman spectroscopy and UV-vis-IR spectroscopy were employed to investigate and understand the material morphology, structure and optical properties. A photoluminescence background and a tail absorption towards the visible region emerged for hydrogen-treated samples.

Photocurrent measurements under solar simulator illumination showed a noteworthy increase of photocatalytic response for the Ar/O₂ deposited samples followed by a double air+Ar/H₂ thermal treatment with respect to the O₂ deposited/air annealed stoichiometric TiO₂ samples. It is suggested that this effect could be ascribed to an optimized morphology characterized by large surface area, good electron transport, reduced recombinations, combined with oxygen vacancy-related tail states in the bandgap. Further investigations for a deeper understanding of the physical/chemical mechanisms involved in the hydrogenation process at the atomic scale are currently being carried out.

[1] Chen, X. et al. Chem. Soc. Rev. 2015, 44 (7), 1861–1885.
[2] Matarrese, R. et al. Chem. Eng. Trans. 2014, 41, 313–318.

Abstract
Among various metal oxides, TiO₂ has attracted much attention due to its potential to be used in wide range of applications in the field of light harvesting, photocatalysis, gas sensing, and so on. However, in most cases its performance is limited due to high bandgap (~3-3.2 eV), even by fabricating nanostructures, and thus it requires bandgap engineering. In this scenario, ion beam irradiation is known to be a promising technique for tuning bandgap through introduction of defects, especially vacancies and/or interstitials in TiO₂ matrix. Here we will show how the optical bandgap of self-assembled TiO₂ nanorods on chemically etched Si pyramids can be tuned by 50 keV Ar⁺-ions with fluences in the range of 5×10¹⁴ to 1×10¹⁷ ions/cm² at room temperature. In particular, we will demonstrate the appearance of degenerate states in TiO₂ matrix at a critical fluence of 1×10¹⁷ ions/cm² using ultraviolet-visible spectroscopy. This intriguing phenomenon will be discussed in the light of Burstein-Moss effect due to the gradual increase of oxygen vacancies in TiO₂ as established by transmission electron microscopy and further supported by x-ray photoelectron spectroscopy.

Titanium dioxide, TiO₂, is a very versatile and cheap material that can be used in a lot of different applications like (photo)catalysis, sensing and self cleaning coatings.¹ Many of these applications take advantage of a high specific surface area and a high degree of crystallinity. As there is always the trade-off between these two parameters, obtaining both in the same material is not straightforward. By using microwave irradiation we were able to achieve this goal. A highly mesoporous titania material with a specific surface area above 330 m²/g,which is among the highest numbers presented in literature, was obtained by adding an additional microwave irradiation step during a normal evaporation-induced self-assembly (EISA) synthesis. The degree of crystallinity which is often neglected in literature, was determined using Rietveld refinement and an increased degree of crystallinity with more than 10 % was achieved. The isoelectric point, particle size and surface groups remained the same after the addition of amicrowave irradiation step to the synthesis, thus retaining the material properties that are crucial for applications.

One step further in obtaining outstanding materials is the addition of noble metal nanoparticles on mesoporous titania. This kind of materials has a lot of applications in the (photo)catalyst sector. Many synthesis routes are already developped to prepare these kind of materials, but little is known about the oxidation state of the metal ions present in these materials even though it can be an important factor when one studies the reaction mechanisms of a reaction. Therefore we performed a study of the oxidation state of gold atoms of Au/TiO₂ materials reduced in four different ways, using X-ray adsorption spectroscopy (XAS).² We found that different particle sizes, oxidation states and interactions of the gold with the titania support are obtained when one uses different reduction methods.

When one uses these composite materials in photocatalytic remediation reactions or alcohol oxidation reactions, one can observe differeneces between these samples even though the gold loading stays the same.

1. M. Arin, P. Lommens, et al., J. Eur. Ceram. Soc., 2011, 31, 1067-1074.
2. M. Meire, P. Tack, et al., Spectrochim. Acta B, 2015, 110, 45-50.

As a typical intercalation material, various nanostructured TiO₂ has been extensively studied as an alternative and safer anode material for use in high power density Li-ion batteries. However, the studies have been mainly dedicated to the development of powder type electrode materials and relatively little attention has been paid to studies of other electrode architectures. While composite electrodes containing a mixture of the TiO₂ particles, binders and conductive additives still are commonly used, such electrodes often yield poor material utilization, undefined material/component arrangements and a lot of complex interfaces. Binder- and additive-free nanostructured oxide electrodes can, on the other hand, provide a pristine model electrode system for careful evaluation of the battery performance of various electrodes.[1] In the present work, we demonstrate that highly ordered, free-standing anodic TiO₂ nanotube array electrodes, can be either be directly used as high energy and power density electrodes for Li-ion microbatteries or as model monolithic electrodes for electrode engineering and Na-ion battery studies. By using anatase TiO₂ nanotube electrodes, an areal capacity of 0.37 mAh cm^-2 (i.e., 40 mAh g^-1) at a rate of 10C (using a (dis-)charge current density of 9 mA cm^-2), and 1 mAh cm^-2 (i.e., 91 mAh g^-1) at a rate of C/5, can be achieved. [2] In addition, well-defined monolithic TiO₂ nanotube electrodes with fine-tuned nanotube size gradients (e.g., tube length, diameter and wall thickness) can also be manufactured using a bipolar electrochemistry approach. [3] The gradient nanotube electrodes can provide excellent rate performance, with capacities of 0.16 mAh cm^-2 or 169 mAh g^-1 at a rate of C/5 to of 0.04 mAh cm^-2 and 42 mAh g^-1 at a rate of 50C. A careful comparison of the Li-ion and Na-ion battery performances of anatase TiO₂ nanotube and amorphous nanotube electrodes will also be presented.[4] Refs: [1] W. Wei, et al., J. Mater. Chem. A 2013, 1, 8160; [2] W. Wei, et al., submitted, 2015 ; [3] W. Wei, et al., Electrochim. Acta 2015, 176, 1393; [4] W. Wei, et al., in preparation, 2015

Titanium dioxide is well known for its ability to absorb and resist radiation, especially in the form of ultraviolet light. However, it is a soft material and is not suitable to apply as a protective barrier by itself. The addition of zirconia to titania in order to form a nanocomposite film is being investigated for use as a protective coating on steel in a range of industrial applications. Zirconia, usually stabilized by yttria, is desirable in protective coatings as it is chemically inert, but it can still be fractured in stressful conditions; preliminary work has shown that zirconia is further stabilized when blended with other oxides. The purpose of this work is to investigate a range of TiO₂/ZrO₂ nanocomposite films in order to understand the effect of composition changes on their performance under mechanical, chemical, or radiological stress and how they can be combined in order to simultaneously enhance their beneficial properties and while mitigating the individual component’s shortcomings. Physical vapor deposition and co-sputtering of TiO₂ and ZrO₂ were the primary deposition techniques used for this study. Initial single layer coatings were fabricated and characterized to fully understand the properties associated with the individual component thin films. Following monolayer deposition, a range of TiO₂/ZrO₂ compositions was created and characterized. Before making measurements of the functional properties and mechanical, chemical, and radiological performance of the as deposited coatings, they were characterized for composition, microstructure, and morphology and these properties were correlated to coating composition so that performance could be further correlated back to deposition. The single component titania and zirconia coatings performed as expected with respect to hardness, corrosion, and radiation attenuation and in context of what is widely seen in literature for these materials. Composite performance varied as expected across the range of component concentrations and showed alternating enhancement and mitigating effects, leading to the conclusion that individual composite formulations may find use depending on application needs.

Abstract:
Functional oxides are very promising materials for next-generation electronics, as they can undergo transitions from insulator to semiconductor and further to metal, as the charge carrier density increases. Nanocrystalline titanium dioxide (TiO₂) is one of the most investigated oxide materials that already found applications in sensing, electrochromics, photovoltaic, and photocatalytic devices. [1]
We fabricated, using an unconventional patterning process, polycristalline TiO₂ electrolyte gated (EG) transistors making use of high surface area activated carbon (AC), as a gate electrode, and the ionic liquid (IL) 1-Butyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide [EMIM][TFSI], as the gating media.
To explore the effect of the double layer capacitance on device performance we investigated bottom-contact top-gated transistor where we varied the area of the active layer in contact with electrolyte and the area of overlap of source/drain electrodes with gate. The operating voltage of the devices is lower than 2 V and the ON/OFF ratio as high as 10⁴. To shed light on the doping mechanisms of such transistors we performed electrical measurements, cyclic voltammetry and electrochemical impedance spectroscopy. The electron mobilities were in the range of 0.05 cm²/Vs - 1 cm²/Vs and correlate with double layer capacitance.
We believe that these simple architecture devices working at low voltages are promising for low cost and large area electronics.
[1] C. Wei and C. Chang, 2011, Materials Transactions, 52 (3), 554- 559.

Titanium dioxide (TiO₂) is a well-known photocatalytic material for oxidative degradation of various organic contaminants and water splitting. TiO₂ has outstanding photocatalytic properties along with good chemical stability, nontoxic nature and low production cost. The rutile phase of TiO₂ is thermodynamically stable and it is able to absorb violet light along with a proportion of visible light but the fast recombination rate prevents its photocatalytic performance. So, many methods have been studied to modify the properties of rutile TiO₂ to enhance photocatlytic performance. Among them, the creating defects on the TiO₂ structures and incorporation of Ti³⁺ show good performance due to the decrease of the recombination rate.
In this study, we demonstrate a facile synthesis method to modify rutile TiO₂ properties via microwave irradiation with hydrogen peroxide (H₂O₂). The original and modified rutile TiO₂ samples will be discussed using by X-ray diffraction, Raman spectroscopy and X-ray photoelectron spectroscopy. The surface modifications will be reported using Transmission electron microscopy. Finally, the photocatalytic performance using degradation of methylene blue and disinfection of anti-bacterial under visible light will be highlighted.

Visible light photocatalytic activity of doped TiO2 has been widely investigated for the application of solar water splitting; and nitrogen has been intensively studied as a dopant where it has been introduced into the TiO2 lattice in either the pre-synthesis or post-synthesis step. My work focuses on the spatial distribution of nitrogen dopants in the TiO2 lattice. It involves doping single crystals of TiO2 Rutile, which are depth profiled through using X-ray Photoelectron Spectroscopy, Secondary Ion Mass Spectrometry and Low Energy Ion Scattering. These advanced characterisation techniques, in conjunction with each other, are used to identify the chemical states of dopants and the spatial distribution of these dopants.The distribution of dopant ions (and therefore photoactivity) can be highly dependent on the method used for nitrogen incorporation and associated synthesis parameters. The effect of the TiO2 single crystal orientation on the concentration of nitrogen dopant incorporated using solid-state diffusion was investigated. The TiO2 facets in order of their increasing nitrogen dopant ion incorporation in the top ~ 2-3 nm are (110) > (001) > (100). While interstitially doped Nitrogen species predominates the near-surface region, only substitutionally doped Nitrogen species is seen below the near-surface region. These results were compared against the samples doped using Ammonolysis. In addition, it was found that employing a reduction step prior to the doping step resulted in greater nitrogen ion incorporation. Effect on TiO2 surface topography upon nitrogen doping was also investigated. The solid-state diffusion of dopants into the single crystals is favoured by low temperatures as higher temperatures results in surface topographical effects such as roughening. This was investigated further by employing lower temperatures and longer annealing times. The Atomic Force Microscopy images show smoothening of the (100) TiO2 surface upon nitrogen doping, which was not observed with (110) and (001) facets. This work involving a detailed investigation of the structure of TiO2 is key in designing new and improved photocatalytic materials.

Titanium dioxide and doped titanium dioxide offer many potential uses for various photocatalytic processes, such as water splitting, antimicrobial coatings and pollutant degradation. Previous studies suggest that antimony doping could improve the photocatalytic activity of TiO₂ though there has been minimal investigation into the structural composition of Sb doped TiO₂.

The work to be presented focuses on the effect of synthesis method and amount of dopant on the integration of Sb into the TiO₂ lattice. Doped TiO₂ has been achieved through solid-state reaction of TiO₂ and Sb_xO_y powders both in air and under vacuum. Interestingly, no doping can be achieved under vacuum- all Sb leaches out of the starting mixture- whereas, at certain temperatures, Sb is successfully doped into TiO₂ powders heated in air.

Diffusion of the dopant into the lattice has been studied by doping single crystals and analysis with X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD) and Secondary Ion Mass Spectrometry (SIMS). Spatial distribution of the dopant ions and their behaviour upon secondary thermal treatment has been studied, to further understand the stability of these materials. It appears that Sb does not deeply penetrate into the lattice and tends to segregate substantially to the surface of the material.

Lattice parameter changes indicate that the solubility limit of Sb in TiO₂ has been determined as 17%, above which secondary phases of Sb_xO_y are consistently observed in XRD. Furthermore, the implication of Sb surface segregation and the effect of this on other dopants when used in combination with Sb is interesting. When used as a codopant, it appears that Sb can restrict diffusion of other dopant ions into the TiO₂ lattice and could therefore be used as a method to control the distribution of other ions.

Memristors are described as the future of non-volatile devices for novel computing because of their fast operation and multistep storage capabilities. Memristors, where a few nanometers thin transition metal oxide film is sandwiched between two metal electrodes, are operated on the principle of dynamic resistive switching (RS) due to the electromigration of oxygen vacancies. Recent studies for the Pt/TiO₂/Pt memristor devices reveal that the switching from high resistance state (HRS) to low resistance state (LRS) and vice versa, is associated with the electroforming process at high applied bias. Electroforming process in TiO₂ based memristors are generally dominated by oxygen vacancies evolution through Joule heating and therefore can be destructive in nature and also uncontrollable. It can damage the device or even can destroy it completely. Therefore it’s necessary to fabricate a device with the electroforming voltage as low as possible to avoid Joule heating effect and to achieve an electric field dominating resistive switching in TiO₂ based memristors while maintaining the high endurance and stable electronic behavior.
Here, we exploit the switching behavior of a low bias electroforming memristor device based on Pt/TiO_x/Pt, fabricated at room temperature, using a combination of maskless photolithography and high vacuum magnetron sputtering. In this case, TiO_x is an oxygen deficient phase of titanium oxide with a high density of in-built vacancies available for electromigration. The physical characterization of the TiO_x film is performed using SEM, AFM, XRD and SIMS, and the electronic behavior (I-V characteristics) of the memristor devices are analyzed using a semiconductor analyzer. Before electrically tested, the device stays in its virgin state exhibiting a Schottkey behavior. An electroforming voltage as small as ±1.5 V is required to switch the device to LRS from HRS state where the device changes its behavior from Schottkey to Ohmic. Once the irreversible electroforming step is over, the device exhibits a stable ON and OFF switching under a low sweep voltage (±1.0 V) with remarkable ON/OFF ratio over 260 at 0.5 V. Junction size dependency is observed for four different junctions (5×5, 10×10, 20×20 and 50×50 µm²). High retention capacities are observed for 10⁵ s while high endurance and fast switching are achieved up to 150 switching cycles.
This study describes a promising process for fabricating efficient nanostructured metal oxide based memristors to improve the performance. Next step towards improvement will be incorporation of new hybrid nanostructured materials with appropriate structural modification, which will lead to a high-performance memristor device with even higher endurance and lower energy consumption.

Carbon coated P-25 TiO₂ nanoparticles were utilized for synthesis of titanium carbide (TiC) and titanium diboride (TiB₂) nanoparticles via carbothermal reduction process. The synthesis procedure was optimized for narrow size distribution of the product. As synthesized TiC and TiB₂ nanoparticles were coated onto surface of commercial 304, 430, and P91 steel substrates with HVOF thermal spray method to form a dense coating layer as a cost efficient method to improve high temperature corrosion resistance of the steels. Coated steel samples were tested for corrosion resistant properties at 550°C to 750°C in simulated flue gas conditions and in air. Crystallinity of the nanoparticles were examined with X-Ray Diffration (XRD). Surface and structural morphology of the coated steels were studied using scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS). Improved corrosion resistance including retarded oxidation on surface of steel and lower oxidation penetration were observed for the TiC and TiB₂ coated steel substrates both under oxygen and sulfur attack.

The synthesis of titanium carbide nanofibers using a carbothermal process was investigated. Titanium oxide nanofibers were fabricated by electrospinning method from a sol containing a mineral acid for peptizing, followed by calcination at temperatures ranging from 250 - 800°C. Carbon was coated on the nanofibers by cracking of propylene (C₃H₆). Carbon coated titania fibers were reacted at 1400°C and 1500°C for 1 or 2 hours under argon atmosphere to form titanium carbide nanofibers. The titanium carbide nanofibers were characterized using scanning electron microscope (SEM), transmission electron microscope (TEM), and X-Ray diffraction (XRD). The average diameter of the TiO₂ fibers and TiC fibers averaged at 150 nm and 350nm respectively. Due to the high theoretical conductivity, chemically and electrochemically stability in strong acid and high potential environment, the synthesized TiC fibers show great potential for fuel cell applications such as support of bipolar plates and catalyst support. The electrical and thermal properties of the synthesized TiC will be reported and compared with the theoretical values.

Scattering effects of crystalline mesoporous titanium dioxide (TiO₂) scatters have been widely utilized in various optoelectronic devices. Various structures of mesoporous TiO₂ scatters would induce complex interactions between photons and scatters to show specific optical properties especially in the cases of more than one kind of scatters embedded in the scattering medium. At the longer wavelength regime, the multiple-scattering effects become important and change with the inter-particle spacing and scatter size. Until now, with different analysis techniques, to clarify the scattering effect of various scatters is always a work of necessity. In previous researches, most analyses of the scattering effects are based on Mie scattering model which is limited to a single spherical scatter with single-scattering effect. In this work, we utilize a fiber based low-coherence interferometry to analyze the samples of mesoporous TiO₂ scatters. The setup is based on Michelson interferometer with a broadband low-coherence super-luminance diode source (853nm). This optical system provides advantages of high axial resolution, and deeper probing range. Furthermore, the depth signals have been fitted by the extend Huygens-Fresnel model with further consideration of multiple-scattering effects. Here, a series of mesoporous TiO₂ scatters with average sizes of 20, 150, 300, 500 nm have been prepared on the glass substrate with sol-gel process. These samples were measured by UV-Vis spectrophotometer at the wavelength regime of 300-1100 nm. For the mesoporous TiO₂ scatters size of 20 nm, the results of normalized diffused-reflectance spectrum show the highest reflectance peak at 400 nm and decrease to 40% at 800 nm. By increasing the scatter size up to 300 nm, the resul show the highest reflectance peak at 450nm and decrease to 70% at 800 nm. In comparison, the red-shift of reflectance peak is resulted from the Mie scattering effect. On the other hand, for the bigger scatters, higher reflectance intensities at the longer wavelength regime are shown and may be caused by enhancement of multiple-scattering effects. Simultaneously, the multiple-scattering effects had also been quantified and confirmed for each samples by low-coherence interferometry. After fitting algorithm based on EHF model, the percentage sharing of single- and multiple-scattering effects had been studied. By increasing the sizes of mesoporous TiO₂ scatters from 20, 150, 300, to 500 nm, the percentages of multiple-scattering effects are enhanced from 5%, 10%, to 40%, and then decreased to 20%. The scattering coefficient could also be extracted from 13.5±0.6, 16.5±0.6, to 20.2±0.6 mm^-1, and then decreased to 18.5±0.6 mm^-1. In this work, the size-dependent multiple-scattering effects had been first analyzed by low-coherence interferometry. The mesoporous TiO₂ scatters with size of 300 nm have the largest reflectance intensity due to the larger contribution percentage of multiple-scattering effects.

Titanium dioxide (TiO₂) has been attracting extensive attention because of its importance in several technological applications including catalysis, nano-electronic devices, and sensors. A well-understood and atomically flat TiO₂ surface is prerequisite for investigating the atomistic details of catalysis on this surface, as well as for the growth of ultra-thin rutile transition metal oxide heterostructures. Among the low-index surfaces, rutile TiO₂(001) is less intensively studied because of its morphological instability due to its high surface energy; the standard approach of sputtering and annealing usually introduces faceting. Here we demonstrate a facile method to create atomically flat, non-faceted TiO₂(001) surfaces. A step-terrace surface morphology is achieved through buffered hydrofluoric acid (BHF) etching and subsequent annealing at 810 °C in air or in flowing O₂.Low energy electron diffraction (LEED) and reflection high energy electron diffraction (RHEED) show sharp (1x1) diffraction spots accompanied by predominant c(7√2 X 5√2)R45⁰ reconstruction with two perpendicular domains for the as-prepared substrate. The reconstructed surface persists even after O₂ plasma treatment at 600 °C, which is higher than the traditional substrate temperature used for epitaxial films growth. Periodic stripes, corresponding to c(7√2 X 5√2)R45⁰ reconstruction, are observed by scanning tunneling microscopy (STM). Furthermore, a homoepitaxial TiO₂(001) film of 6 monolayers is successfully grown via pulsed laser deposition (PLD). These results show that relatively stable and atomically flat TiO₂(001) surfaces can be obtained, which could serve as an ideal template for subsequent rutile transition metal oxide heterostructure growth and the investigation of the catalytic properties of TiO₂(001).

Titanium Nitride is known for its corrosion resistance, wear-resistance, high hardness, and extremely high melting temperatures, making it an optimal material system for applications at elevated temperatures. However, most high temperature applications also occur in oxygen rich environments, favoring the formation of oxide scales which lack the desirable properties of the base material. While there has been extensive experimental and computational work done to characterize the initial steps of oxidation little has been done to characterize the nature of fully formed oxide scales on TiN. This work uses Density Functional Theory (DFT) calculations to analyze the energetics and stability of TiN/TiO2 interfaces. Specifically, the work of adhesion and migration energy barriers for oxygen diffusion through the interface are calculated for multiple possible interface geometries and terminations.

Protonated titanate nanotubes were chosen as precursor in hydrogenation process. Owing to the high capacity for molecular hydrogen storage of naotubes, TiO₂ nanotubes can be hydrogenated through thermal treatment under N₂ and H₂ mixed flow at lower temperature. A series of hydrogenated TiO₂ naotubes and nanobelts were synthesized and characterized by XRD, UV-Vis, TEM, EPR and XPS. The results showed that the hydrogenated TiO₂ nanotubes possess tiny and uniform diameters of 8-10 nm and the walls thickness of 2-3 nm, and were mainly anatase. The anatase TiO₂ nanotubes transformed to TiO₂-B nanobelts at the higher hydrothermal temperature. The light absorption of hydrogenated TiO₂ nanotubes was expanded to visible light. However, air-TiO₂ and hydrogenated TiO₂ nanobelts only absorbed ultraviolet light. According to XPS and EPR analysis, hydrogenated TiO₂ nanotubes displayed stable core-shell structures, in which the surface was mainly stoichiometric TiO₂ and the core was non-stoichiometric TiO₂ with Ti³⁺ and oxygen vacancies. The adsorption and photocatalytic performance were evaluated by removal rate of phenol. Based on pseudo-first order kinetic model, the degradation rate constant was obtained with the regression analysis. The highest degradation rate constant of hydrogenated TiO₂ nanotubes was 5.2 times higher than air-TiO₂. In comparison, the degradation rate constants of hydrogenated TiO₂ nanobelts were much lower than air-TiO₂. The results showed that the precursor with nanotubes structure can be hydrogenated easily in lower temperature comparing with nanobelts, resulting in the photocatalytic activity of hydrogenated TiO₂ nanotubes enhanced drastically.

Mesoporous titanium dioxide (titania) thin films with 2D Hexagonal Close Packed (HCP) cylindrical nanopores have been synthesized by the evaporation-induced self-assembly (EISA) technique with Pluronic surfactant F127 as template. To provide orthogonal alignment, surface modification of substrates with crosslinked F127 has been used. GISAXS studies show that aging at 4 °C is indeed necessary for ordered mesostructure development and that aging at this temperature helps to provide orthogonal orientation of the cylindrical nanopores which forms directly by a disorder-order transition. F127 provides pores with 8-9 nm diameter, which is precisely the structure expected to provide short carrier diffusion length and high hole conductivity required for efficient bulk heterojunction solar cells. Anatase titania is a n-type semiconductor with the valence and conduction bands located at +3.1 and -0.1 eV relative to the Fermi level, thus giving a band gap of +3.2 eV. Therefore, titania readily absorbs UV light with a wavelength below 387 nm. Because of this, these titania films can be used as a window layer with a p-type semiconductor incorporated into the pores and at the top surface of the device to synthesize a photovoltaic cell. The pores provide opportunities to increase the surface area for contact between the two semiconductors, to align organic semiconductors, and to induce quantum confinement effects. Using these films as window layer and CdTe as absorber, a power conversion efficiency of 5.53% has been obtained which is 3 times higher than that obtained using planar titania films and is attributed to high surface area of nanoporous TiO₂ and reduced recombination at the interface. The charge carrier concentration is higher in case of nanoporous TiO₂ as compared to planar titania. These films are also used as negative electrodes in Li-ion batteries where their high surface area and thin walls allow for rapid Li ion diffusion and efficient insertion without capacity fading. Capacity as high as 300 mAh/g has been achieved after 50 cycles which is pretty close to theoretical capacity of titania (330 mAh/g). These films has been used to develop nafion-titania composite membranes which can be used both as separator and electrolyte in solid state batteries. Interfacial modification between nafion and titania has been achieved using silane chemistry and been studied using FTIR and impedance spectroscopy.

A novel methodology has been recently developed for the fabrication of structured and functionalized composite polymeric micro- and nano-spheres. This unique approach utilizes a thermally-induced fluid instability occurring at the interfaces within multimaterial fibers by which the internal structuring and composition of the fibers are imparted upon the generated spheres. Applications of such spheres span a variety of industries and fields including pharmaceutical therapies, imaging and biomedical diagnostics, paints and coatings, and cosmetics. Moreover, the process can be applied to a wide range of thermally compatible materials and additives, potentially producing spheres with previously unattainable compositions, structuring, and geometric allocation of functionalities. We present here one such realization in the form of heterogeneous composite microspheres composed of a polycycloolefin matrix integrated with a random distribution of TiO₂ nanoparticles. In homogeneous microspheres of the same size (tens to hundreds of microns) composed of a dielectric, one expects, upon irradiation, that the microsphere exhibits the characteristic Mie scattering. However, due to the large index contrast, high transparency, and randomized, discrete nature of the nanoparticle distribution in the polymer matrix, irradiation of the composite microspheres produced speckle patterns indicative of diffusive optical scattering. This behavior was observed by tightly focusing a 632-nm-wavelength laser beam onto a solitary composite microsphere suspended in-fiber and imaging the scattered field via transmission microscopy. It is readily determined from the speckle pattern that the multiply-scattered waves exiting the microsphere vary randomly in amplitude and phase independent of the incident polarization, thus exhibiting behavior beyond the known Mie scattering typical of a homogenous microsphere.

Bulk doping and use of different substrates for deposition can induce significant variation in the surface electronic structures of the film. The variation in the surface defect structure is expected due to mismatch between the lattice parameters of titanium oxide and substrate under deposition. Also, in case of doped titanium oxide, the mismatch came due to the differences in the ionic radii, electro-negativity and valence states between dopant and the host ions that can induce different bulk as well as surface active defects. Titanium oxide is very well known photo-catalyst thereby expected to be an excellent gas sensor as both are exclusively surface active phenomena.^[1,2] We attempted here to enhance the H₂S sensitivity by using titanium oxide films on engineering the surface defect structure.

The titanium oxide thin films were prepared using Pulsed Laser Deposition (PLD) technique. The target pellet used for the ablation was defect-free phase-pure stoichiometric TiO₂. Recently, we observed that mere change in ablation energy of laser pulse from 200 mJ to 500 mJ can induce substantial variation in surface defect chemistry of the film.^[3] The different substrates such as Si, LAO, STO, c-Al₂O₃, polycrystalline Al₂O₃, soda-lime glass etc. were used for the deposition to get variation in the defect-richness in the film surface. Furthermore, thin films of titanium oxide doped by different elements such as Cu, Zn, Al, Na, Fe etc. were deposited on LAO substrate and have attempted towards enhancement in H₂S gas sensitivity by modification in the film surface activity. The characterization techniques such as XPS, UPS, synchrotron XAS, EXAFS, SPM, GIXRD etc. were employed to reveal the surface electronic structure responsible for H₂S gas response. Gold pads of 1mm apart were made on these films using thermal evaporation technique which were used to make ohmic contacts required in testing H₂S gas response using a laboratory made gas sensing set-up.

It was observed that films deposited on Si and LAO were found to be highly sensitive to H₂S than any other gas at relatively lower sensor operating temperature (100^oC) with faster response (few seconds) and quicker recovery (~4 min) at 50 ppm of H₂S. Also, Cu-doped titanium oxide film exhibited extra-ordinary H₂S response than any other doped titanium oxide systems. The H₂S sensitivity observed is the best in the literature for pure titanium oxide film on Si (001) and Cu-doped titanium oxide film on LAO and it is SR % ~ 1,12,000 and SR % ~ 1,36,000 respectively at 50 ppm of H₂S. The same films were found be considerably sensitive to H₂S at sub-ppm level. XPS, UPS and synchrotron XAS measurements show that film having extra-ordinary H₂S response has richness in surface defects. The valence states of titanium in film were +4 and 0. The film composition concluded to be Ti/TiO_x and AFM imaging showed increasing surface roughness with vertically aligned short nano-wires on highly H₂S sensitive films. It is inferred that the enhanced H₂S sensitivity is due to the excessive over layers of chemi-sorbed oxygen on the film surface. These data will be discussed and presented.
One of the authors TCJ would like to acknowledge DST, Govt. Of India for INSPIRE Faculty Award.

References:
(1) Chem. Rev., 2007, 107, 2891;
(2) J. Phys. Chem. C 2008, 112, 14595;
(3) RSC Adv., 2015, 5, 93081

We demonstrate that is possible to finely tune the electrical and optical properties of TiO₂ and Ta-doped TiO₂ (TaTO) thin films by engineering their structure and morphology at the nanoscale. Donor doped TiO₂-based Transparent Conducting Oxides (TCO) [1] are receiving increasing attention because of the particular properties of TiO₂, such as photoactivity or chemical stability in reducing atmospheres; the use of TiO₂ as an electron selective layer or photoanode in electrochemical or perovskite solar cells motivates the goal of controlling functional properties in these TiO₂-based materials.
TiO₂ and TaTO films were deposited at room T via pulsed laser deposition, followed by thermal annealing in a reducing atmosphere, which is necessary to obtain conducting polycrystalline anatase. The best functional properties (resistivity ~5×10^-4 Ωcm, mean transmittance in the visible >80% for a 150 nm film) were obtained when depositing at 1 Pa O₂ followed by annealing in vacuum at 550°C [2]. X-ray diffraction, Raman spectroscopy and positron annihilation spectroscopy have been employed to examine the microstructure and defect chemistry in terms of deposition and annealing atmospheres, thus opening the possibility to finely tune the functional properties (e.g. carrier density) for both doped and undoped TiO₂.
We also demonstrate the possibility to crystallize the films exploiting an ultra-fast thermal treatment at ambient pressure in N₂, yielding virtually the same conductivity as a conventional anneal carried out in vacuum (for doped and undoped TiO₂). By monitoring the crystallization threshold via in-situ electrical measurements we were able to investigate the effects of O incorporation on the electrical properties. This process, which is industrially scalable, reduces the annealing time from about 3 hours to a few minutes and allows the electrical properties to be uncoupled from the influence of the annealing environment.
Finally, by increasing the O₂ pressure during the deposition process we are able to obtain hierarchically nanostructured mesoporous layers, which could effectively act as large surface area photoanodes with tunable functional properties [3]. This opens the way to the realization of an all-TiO₂ transparent electrode/selective layer/photoanode with a reduced number of interfaces and thus of recombination centers, which could be beneficial for electron transport in real devices.

1. Y. Furubayashi et al.. Appl. Phys. Lett. 86, 252101 (2005)
2. P. Mazzolini et al., J. Phys. Chem C 119, 6988 (2015)
3. L. Passoni et al., ACS Nano 7, 10023 (2013)

Interest in the integration of TiO₂ within microfluidic devices has grown considerably since the first demonstration of a TiO₂-based photocatalytic microreactor nearly a decade ago. Examples of device applications reported since then include those for water purification, water-splitting, and photochemistry. The photocatalytic properties of TiO₂ have also been exploited as a novel means for achieving microfluid control, and site-specific nanoparticle synthesis in situ. In many of these applications, miniaturization affords opportunity for significantly enhancing performance relative to conventional bulk reactors, due to increased surface area, reduced diffusion length, and greater uniformity of irradiation. However, low volumetric throughput remains a critical limitation in many applications, as does the difficulty associated with integrating TiO₂ uniformly within complex microfluidic device geometries.

As we have reported earlier, growth of nanoporous TiO₂ (NPT) within Ti-based microfluidic devices may provide a new means for addressing these issues. In this approach, NPT is grown in situ, directly from the Ti channel surfaces, using a peroxide-based oxidation process. The advantages of this approach include: a) conformal coverage of complex geometries that would be difficult, if not impossible to coat uniformly using conventional sol-gel or thin-film deposition routes; b) reticulated, open-framework porosity that provides higher surface area than dense films, and greater fluidic accessibility than nanoparticle-based films; and c) potential for fabrication of robust, large-area photocatalytic devices that can provide volumetric throughputs approaching those required for commercial feasibility in many applications. However, while this approach shows promise, our current reliance upon NPT growth conditions originally developed for other applications suggests potential for improvement via optimization of growth conditions specifically for use in photocatalytic microreactors.

Herein, we report our recent efforts to explore this potential through use of a Taguchi-based experimental design. Given the relatively large parameter space involved, the use of design of experiments techniques is needed to ensure efficient optimization. Using an orthogonal array composed of three parameters (oxidation time, temperature, and concentration) with three levels, an experimental design of nine trials was developed, with the reaction rate constant for methylene blue degradation serving as the quality reporter. Our studies helped identify the growth parameters that most strongly influence photocatalytic performance, as well as the optimal combination thereof. Furthermore, our studies also showed that optimally-grown NPT provided performance approaching that of Degussa P25 TiO₂ nanoparticle films with comparable surface area. Collectively, these results represent important steps towards our goal of developing robust, high-performance photocatalytic microreactors.

Synthesis of highly-ordered nanostructures of valve metal oxides has recently attracted huge scientific and technological interest motivated by their possible use in many applications.
It is the nanotubular TiO₂ that has received the highest attention after porous Al₂O₃, motivated by its range of applications, including photo-catalysis, solar cells and biomedical uses. However, the TiO₂ nanotube potential for a range of advanced devices, in particular when considering all possible shapes and geometries, has not at all been exploited. One of the major issues to extend the functional range of nanotubes is to coat homogenously tube interiors by a secondary material, potentially until the complete tube filling.
The presentation will focus in detail on the filling, decoration or coating of the nanotubes by various means. The deposited materials influence strongly optical, electrical and mechanical properties of nanotubes. Experimental details and some very recent results will be presented and discussed.

Dye sensitized solar cell (DSSC) has attracted considerable attention and is being considered as a promising candidate for environment friendly photovoltaic technologies. A typical DSSC utilizes fluorine doped tin oxide (FTO) glass as the transparent conductive substrate. However if the heavy, fragile glass could be replaced by plastic conductive substrate, it enables to meet the requirements of light-weight and flexibility. The flexible DSSC modules can be tailored for specific applications while satisfying the special shape and surfaces needs. In addition, the industrial large-scale production of flexible DSSC module will become possible by roll-to-roll processing, which will result in low cost module production. As the plastic conductive substrate (ITO/PEN) cannot withstand high temperature process, herein, a novel binder-free TiO₂ paste is developed that enables the fabrication of flexible TiO₂ photo-anode at room temperature. Moreover, in order to realize the fully printable flexible module, low-temperature, non-Pt printable materials for flexible counter electrodes were employed. Cold isostatic press (CIP) technique was utilized to not only improve the film quality but also increase the TiO₂ film thickness. We will present the DSSC evolution from laboratory cell to glass module and finally to flexible module. The efficiency loss from small cell to photovoltaic module was studied and analyzed. Based on the employment of binder-free TiO₂ paste, flexible DSSC module with size of 100 mm × 100 mm was prepared and optimized. With the treatment of cold isostatic press (CIP) technique and augment of film thickness, conversion efficiency of 3.27% was achieved for flexible DSSC module. The practical application for mobile phone charging under indoor light was also demonstrated by flexible DSSC module (100 mm × 100 mm) with W type series connection

Excitonic solar cells including dye-sensitized solar cells, quantum dot-sensitized solar cells, bulk heterojunction organic photovoltaics, are built upon nanostructures of various functional materials. Nanostructures are essential for the high power conversion efficiency, for example, in dye-sensitized solar cells and quantum dot-sensitized solar cells, mesoporous TiO₂ or ZnO photoanodes made of nanoparticles offer large specific surface area for loading a large amount of dyes or quantum dots so as to capture a sufficient fraction of photons. However, the large surface area of such nanostructures also provide easy pathways for charge recombination, and surface defects and connections between adjacent nanoparticles may retard effective charge injection and charge transport, leading to a loss of power conversion efficiency. Surface facets and chemistry may also affect the conformal coating and adhesion of dye molecules and polymer layers. In this presentation, I will present and discuss our recent work on the design and control of (1) nanostructures and surface chemistry of photoanodes for dye- or quantum dot-sensitized solar cells and (2) nonstoichiometric composition, doping, and allignment of quantum dots in quantum dot-sensitized solar cells, (3) introduction of semiconductor passivation layers to minimize the charge recombination, and (4) incorporation of plasmonic nanocrystals to improve the light absorption. Our research has demonstrated that the power conversion efficiency can be significantly enhanced with excellent device stability when both nanostructures and interface chemistry are properly controlled.

TiO2 as an important semiconducting material play a key role in a variety of photoelectrochemical (PEC) energy conversion systems including PEC water splitting and PEC solar cells. In the past few years, we have been focusing the following two aspects of TiO₂ based materials; 1) exfoliation of layered titanate to prepare single TiO₂ nanosheets and their re-assembled composites for photocatalysis and photoanodes in dye (quantum dots) sensitised solar cells; and 2) the design of nanosized TiO₂ as compact layers or scaffold layers for new generation perovskite solar cells, which all exhibited excellent performance for solar fuels and solar electricity generation.

Titanium dioxide (TiO₂) is one very important metal oxide semiconductor having a wide range of applications such as solar cells, photocatalysis, lithium storage, and sensor. The interaction between the surface/interface of TiO₂ and ions/molecules is an essential and basic process controlling the performances. Modulating the surface-interface structures of TiO₂ to steer such interaction represents a long-lasting active research topic. In this talk, two aspects in modulating surface-interface structures of TiO₂ for photocatalytic solar energy conversion and lithium energy storage will be introduced. One is tailoring the exposure of facets of TiO₂ crystals to decrease the barrier of electron transport across the interface between TiO₂ and metal contact. It was found that the electric conductivity along the [001] direction is one order of magnitude higher than that along the [010] direction. On the basis of this finding, an efficient anode containing TiO₂(001)-W interfaces for lithium storage was constructed. The other is constructing crystalline-amorphous core-shell structure with Ti³⁺ contained interface region. With the assist of such core-shell structure, the photocatalytic hydrogen evolution of rutile TiO₂ was activated. The reason for this activation is attributed to the promoted charge separation of photogenerated electrons and holes in the bulk as a result of hole storing capability of the interface region and regulated abundant major oxidizing and minor reducing reaction sites on the photocatalyst surface. These results are representative of using crystal facet engineering and amorphization to tailor surface-interface structures of rutile TiO₂. It is anticipated that such structure design and construction could be extendable to other metal oxides based functional materials.

Titanium dioxide is the most studied photocatalytic material because of its abundance, non-toxicity, stability and high activity. Nevertheless, electron-hole recombination is the biggest problem limiting its efficiency. Several approaches have been previously explored to improve charge separation, including structural modification, doping, or by formation of heterostructures. Here, we clearly show the power of anisotropy in boosting the photocatalytic activity; specifically that H₂ production in uniform, 1-dimensional brookite nanorods is highly enhanced by engineering their length. By using complimentary characterization techniques to probe excited electron and holes, we demonstrate that the high rates are due to their anisotropic structure, which favors electron-hole separation and efficient carrier utilization. Quantum efficiency for hydrogen production from ethanol, glycerol, and glucose as high as 65, 35, and 6%, respectively, demonstrate the promise and generality of this approach for improving the photoactivity of semiconducting nanostructures.

Titanium dioxide is one of the most important semiconductor-based photocatalysts and had been widely studied for environmental applications. Most previous studies focused on the photocatalytic oxidation of various environmental pollutants, while the studies on the photocatalytic reduction of environmental pollutants were relatively limited. For the photocatalytic reduction of environmental pollutants, they react with photogenerated electrons. Thus, in order to enhance the photocatalytic reduction activity of TiO₂, proper material design and modification are needed to efficiently separate photogenerated electron/hole pairs and achieve their oriented migrations to enrich the presence of photogenerated electrons on the surface of TiO₂.

Crystal facet engineering is becoming an important approach to obtain photocatalysts with higher activity. It was found that the photocatalytic activity of anatase TiO₂ could be modulated by incorporating different exposed crystal facets. Because there are slight surface energy differences of the valence and conduction bands between different facets, photogenerated electrons and holes could be driven to different facets. So the separation of photogenerated electrons and holes could be more effective and oriented in space. Noble metal modification is another effective way to initiate the oriented migration of photogenerated electrons due to their higher work functions compared with that of TiO₂. Noble metal nanoparticles deposited on the surface of TiO₂ could serve as trapping centers for photogenerated electrons and subsequently as active centers for photocatalytic reactions. Thus, it would be desirable to selectively deposit noble metal nanoparticles onto {101} facets of anatase TiO₂ nanocrystals with co-exposed {001} and {101} facets, which could combine the advantages from both the crystal facet effect and noble metal modification to initiate the oriented migration of photogenerated electrons and further enrich their presence onto TiO₂ surface for the enhanced photocatalytic reduction activity.

Herein, we reported a newly developed process to selectively deposit silver nanoparticles onto {101} facets of anatase TiO₂ nanocrystals with co-exposed {001}/{101} facets through a modified photo-deposition process. The synergistic effect of crystal facet engineering and the selective deposition of silver nanoparticles on {101} facets largely enha