Photosynthesis converts carbon dioxide (CO2) into hydrocarbons that can be utilized as food and fuel. We have developed a synthetic photosynthesis system that combines plant pigments and a titanium dioxide (TiO2) photocatalyst to convert CO2 into hydrocarbons using a broader range of solar energy. Synthetic pigment components were assembled by partially replacing magnesium ions in chlorophyll with TiO2. The TiO2 photocatalyst utilizes both UV and green light, changing the landscape of photosynthesis by allowing plants to scavenge additional wavelengths. TiO2 nanowires and nanotubes also resemble the natural antenna systems formed by plant pigments that concentrate light energy. Hybrid pigment structures were formed by self-assembly on silk protein substrates and analyzed using UV-Vis absorption spectroscopy, tensile strain, and structure analysis. The hybrid pigment structures were also confirmed for biocompatibility and physiology compatibility. Water and CO2 were converted to hydrocarbons using a microfluidic and capillary approach under simulated light and atmospheric and compressed gas extraction pressure. The synthetic photosynthesis system efficiently produces hydrocarbons not possible in natural plants with unprecedented efficiency and applicability.

Photolytic water splitting as a method of hydrogen production has attracted a lot of attention since Honda et al. first demonstrated this concept with TiO2 in 1972 [1]. TiO2 has since been used as a reference material in numerous experimental as well as in theoretical studies in the field of solar hydrogen production[2-4]. The most promising performance is observed with nanostructured films consisting of predominantly the anatase TiO2 phase[2]. The atomic scale structure of such films is not well understood, and could be essential to further the understanding of the mechanisms involved in this complex process. Being able to predict which anatase surfaces are likely to be exposed in experimental conditions is, therefore, essential to achieve this goal. With the aim of gaining a better understanding of anatase surfaces, Li et al studied a vicinal surface of anatase[5]. The experimental techniques used did not allow for the determination of the atomic structure of the surface or distinguish between the (514) and (516) planes. In this work we present hybrid-exchange density functional theory calculations of low-index anatase surfaces as well as the vicinal surfaces possibly observed by Li et al.[5]. We show that the (516) surface is very stable and, in fact, has a surface formation energy comparable to the (101) surface. The (514) surface is also relatively stable with respect to other low-index surfaces and both could provide ways of forming low energy steps. These could be important when forming nanostrucured films. Electronic structure calculations were used to simulate STM images (in the constant current mode) to allow for the comparison of the computed structures with experimentally obtained images. References 1. Fujishima K. and Honda, A. Nature, 1972 238, 37. 2. Graetzel, M. Nature, 2001, 414, 338-344 3. Lazzeri, M. and Selloni, A. Phys. Rev.Lett. 2001, 87, 266105 4. Labat, F., Baraneka, P. Domain, C., Minot, C., Adamo, C. J. Chem. Phys. 2007 126, 154703 5. Li, S.C., Dulub, O. and Diebold, U., J.Phys.Chem.C Lett. 2008 112, 16166-16170

TaON is considered as a potential candidate as a visible-light responsive photocatalyst. We report the results of ab initio studies of electronic structure of TaON alloys. Specifically, we show that the position of conduction and valence band can be modified by varying the oxygen and nitrogen concentrations in TaO1-xN1+x. We find that the band gap decreases monotonically with the increase of N/O ratio. The band gap energy is decreased in monoclinic TaON from near 2.7eV to just over 1.1eV (i.e. by 230%) when N/O ratio is reduced from 1/3 to 3/1. The band gap reduction is mostly associated with the change in the position of the valence band due to the hybridization of N 2p states, while the conduction band consisting mostly of Ta 5d-states is not sensitive to N content. The calculated optical absorption spectra show reduction in the optical band gap with increasing N/O ratio. Our calculations show that the band gap reduces as function of the N/O ratio in a series of experimentally fabricated alloys ZrTa3O5N3->TaON->YTa7O7N8 where the latter has the cubic, and the formers have the baddeleyite crystal structure.

Photoelectrochemical water splitting using semiconductor electrodes offers a promising route towards harvesting solar energy and storing it in chemical bonds. However, efficient performance requires several traits which one single material cannot itself satisfy, foremost being the generation of a photovoltage sufficient to drive both the reduction and oxidation half-reactions of electrolysis (1.23 V plus overpotentials). For instance, the metal oxide hematite (α Fe₂O₃) has a band gap suitable for substantial solar absorption, yet with a conduction band too positive to achieve water reduction, hematite-only electrodes require applied bias to achieve water splitting. No degree of catalyst treatments, doping, or nanostructuring can solve this fundamental problem of band misalignment. We have therefore performed studies on the under-investigated aspect of hematite electrode energetics, using combinations of materials design and high-quality synthesis by atomic layer deposition (ALD). Both n-p homojunction hematite and silicon nanowires / hematite heterojunction devices produced enhanced photovoltages, manifested as cathodic shifts in the photocurrent onset potentials signifying a decreased requirement for applied anodic bias. These results represent some of the lowest turn-on potentials observed on hematite devices and were achieved without hematite doping, catalysts, or surface treatments, pointing towards an important new direction of study for enhancing the efficiency of metal oxide photoelectrodes.

The anticipated production of fuels as H2 or CH-compounds with solar light would provide a sustainable and secure primary energy source by producing a storable and transportable fuel. For this reason oxide photocatalysts to mimic photosynthesis have intensively been studied in the past. However, the systems identified so far are limited in their conversion efficiencies. Advanced heterostructure photocatalysts are suggested in this presentation based on nanosized Janus type structures. We suggest the combination of a wide band gap light absorber (bandgap 2-3eV) and adapted contact materials (co-catalysts) in a photovoltaic arrangement which breaks the spherical symmetry of the absorber and provide the conditions for quantum efficiencies approaching 100%. Central part is the (nanoscale) synthesis of Janus type heterostructures either by combining two different semiconductor materials with a well defined mis-alignment of band edges (type II alignment) or by forming asymmetric contacts by different metals providing different work functions. Efficient charge carrier separation is provided by a well-defined vectorial separation of electron-hole pairs and transport of electrons and holes to the different co-catalyst particles ideally deposited on opposite sides of the semiconductor. The additionally needed minimization of photovoltage (chemical potential) losses can be expected for isoenergetic electronic coupling of the highly efficient HER/OER catalyst to the conduction/valence band of the semiconductor. Possible ways to manufacture such Janus structures will be presented.

Current fuel cell technology requires the use of large amounts of platinum in the cathode for the oxygen reduction reaction (ORR). A major roadblock to the commercialization of fuel cells is the high cost of Pt. Current efforts to reduce the cost of producing fuel cells have focused on the minimization of the amount of Pt, thus bringing down the overall cost. Prior work has demonstrated the creation of stable conductive titanium oxide films grown using atomic layer deposition (ALD). ALD is a self-limiting technique used to grow single layers of a film, or well dispersed particles of a material. These ALD-grown supports have demonstrated themselves to be stably conductive in oxidizing environments. This allows for noncontinuous coatings of Pt catalysts for the ORR. Current work focuses on ALD Pt on titanium oxide grown in a multi-chamber ALD cluster tool. Pt was deposited using methylcyclopentadienyltrimethyl platinum (IV) (MeCpMe3Pt) as the precursor with NH3 and H2 as reactant gases, and a recipe was developed to optimize the the Pt with respect to growth rate and conductivity. In addition, post-deposition annealing in a reducing atmosphere was used to increase conductivity of ALD Pt films. Films were characterized using scanning electron microscopy (SEM), Auger electron spectroscopy (AES), Rutherford backscattering spectrometry (RBS), four point probe analysis, and cyclic voltammetry (CV). It was found that Pt nanoparticle size could be controlled by adjusting the number of ALD growth cycles. In the first 50 cycles of Pt deposition, particles in the 3-5 nm regime are visible. After 200 cycles, the particles have grown and coalesced into full films. After this point no further increase in catalytic activity is expected.

Cuprous oxide (Cu₂O) has emerged as a promising p-type thin film semiconductor for solar cells as its bandgap is well matched to the solar spectrum. However, as it is not possible to stably dope the material n-type, it is necessary to use heterojunction cells rather than homojunctions. There are several published reports that heterojunctions of Cu₂O deposited onto ZnO have on/off current rectification ratios of over 100 [Gershon, T., et al., Sol. Energy Mater. Sol. Cells, 96, 148-154 (2012); Mittiga, A., et al., Appl. Phys. Lett., 88, 163502 (2006)]. In these previous studies, a number of deposition techniques have been employed, including sputtering, thermal oxidation and electrodeposition. In all cases, the junction has been exposed to atmosphere between the deposition of the two layers. In this work, ZnO/Cu₂O heterojunctions were fabricated using a low temperature sputtering technique (HiTUS) which allows the ZnO and Cu₂O thin films to be deposited consecutively without the need to break vacuum [Li, F.M., et al., Thin Solid Films, 520, 1278-1284 (2011)]. These heterojunctions were found to have a rectification ratio of 10, which compares poorly to cells published in literature fabricated with atmospheric exposure to the heterojunction interface. Hence, a series of experiments were performed to investigate this apparent effect. It is known that ZnO is prone to adsorption of both oxygen and water vapour. Therefore this study aimed to understand if either or both of these were responsible for the improved cell rectification. It was found that when the ZnO thin film was exposed to atmospheric conditions briefly (8 minutes) before the Cu₂O layer was deposited, the resulting heterojucntions had improved rectification ratios of up to 300. This improvement came from a reduction in the reverse saturation current, suggesting that the ZnO/Cu₂O interface defect density had been reduced by the exposure. Further devices were also fabricated with prolonged exposure to atmosphere, but this did not noticeably improve the junction quality. In order to investigate whether H₂O or O₂ was the likely cause of the improvement, cells were fabricated where the ZnO layer was dipped in water before the Cu₂O deposition. These were found to produce diodes with a rectif