Visible light active, nano-architectured metal oxide photo-catalysts for solar fuel applications

Large-scale implementation of renewable energy sources such as solar requires the development of an efficient energy capture, conversion and storage scheme. Harnessing solar energy to create storable fuels, i.e., solar fuels, provides a unique strategy to meet this objective. In this regard, hydrogen generation through water photoelectrolysis and methane generation via the photocatalytic conversion of carbon dioxide and water vapor are investigated. The primary motivation of this work lies in the development of efficient, low cost materials for solar fuel applications.;Metal oxide semiconductors such as n-type titanium dioxide (TiO 2) have generated significant interest in the scientific community due to their low cost, stability and high photocatalytic activity under band gap illumination. The implementation of nano-structured materials has significantly enhanced the conversion efficiency obtained with TiO2 in applications such as water photoelectrolysis. Despite these advancements, TiO2 has an inherently poor photoresponse due its wide band gap (3.0-3.2 eV), which accounts for ≈ 5% of the solar spectrum energy. Therefore, the primary objective of this work is to develop materials with a photocatalytic activity approaching that of TiO2, while shifting the photo-response to harness the visible light portion of the solar spectrum. Two differing approaches are evaluated in this work to meet this objective.;Hematite (alpha-Fe2O3) has a band gap ≈ 2.2 eV, well suited for capturing solar energy, but suffers from intrinsically poor electrical characteristics. To overcome these limitations, iron oxide nanotubes were developed using a temperature controlled anodization technique. This provides greater control over the film morphology to create high aspect ratio nano-structures approximately 1-4 mum in length, sufficient to harness solar energy, with a wall thickness approaching 10 nm to improve the electrical characteristics for photocatalytic application. The performance of hematite nanotubes, formed after thermal annealing, were characterized using incident photon conversion efficiency measurements (IPCE). A maximum IPCE of 3.5% was obtained under partial bias conditions, with a photo-response extending to ≈ 600 nm.;Alternatively, modification of a nano-structured material with an intrinsically poor photoresponse was evaluated. Amorphous tantalum oxide nanotubes were synthesized using an anodization technique, providing great control over the film morphology. Nanotubes of varying film thickness in the range of 240 nm -- 15 mum, and wall thickness as small as 7 nm were obtained. Subsequent crystallization and nitridation through thermal annealing in ammonia ambient was evaluated to create tantalum nitride (Ta3N5) nanotubes. Tantalum nitride has a band gap ≈ 2.1 eV, similar to that of hematite. Water photoelectrolysis performance was evaluated and a maximum IPCE of 5.3% was obtained under partial bias conditions, with a red shift in the photoresponse of ≈ 300 nm towards the visible relative to Ta2O5 nanotubes.;Finally, the photocatalytic conversion of carbon dioxide and water vapor into hydrocarbons such as methane was evaluated for TiO2, alpha-Fe 2O3 and Ta3N5 nanotube arrays. A microwave assisted, solvothermal approach to load platinum nanoparticle catalysts within the nanotube structure is evaluated. Catalyst sensitization is necessary to achieve measurable yields for carbon dioxide reduction, and the composite nanotube photocatalysts were evaluated under simulated solar AM 1.5G conditions. Methane generation is achieved for TiO2 and Ta3N5 composite photocatalysts at the rate of 25 ppm / cm2-hr and 9 ppm / cm2-hr, respectively.