Development of highly transparent Bi4Ti3O12 thin-film electrodes for the photoelectrochemical reduction of CO2

Photo-electrochemical reduction of carbon dioxide can be currently considered as the most efficient approach for CO2 transformation into clean and storable fuels and chemicals. In a Photo-Electrochemical Cell (PEC) rapid spatial separation of the photo-generated carriers and their transport kinetics within the photoelectrodes materials are fundamental to achieve high-performance devices avoiding charges recombination. From this perspective Aurivillius systems, thanks to their unique layered structure which allows the migration of photo-generated holes and electrons within different areas of the materials, represent a promising option as PEC electrodes, facilitating charge separation and incrementing cells efficiency. Among the different Aurivillius compounds, bismuth titanates such as Bi4Ti3O12 (BiTO) are of great interest owing to their excellent photocatalytic activity. The use of BiTO powder for the photocatalytic reduction of CO2 has recently been reported1-4 but its utilization as photo-electrode for the photo-electrochemical reduction of carbon dioxide as never been exploited. In this work, highly transparent BiTO-based thin film photo-cathodes were fabricated coupling a sol-gel and a spin coating process. An initial detailed optimization of the main process parameters such as deposition cycle, temperature and type of baking process, and thermal annealing profile was carried out to obtain homogeneous and defect free BiTO films. The influence of the photo-electrodes thickness on layer morphology and on the photo-electrochemical properties was accurately investigated and an optimized photo-electrode was thus obtained which registered a photocurrent-density of -4.1 mA· cm2 at -1.5V vs Ag/AgCl. In addition, the electrode band structure was studied confirming the applicability of the developed highly transparent BiTO thin film electrode in the photo-electrochemical reduction of CO2.