(45d) TiO2 Nanotubes Supported Gold Photoanodes for Photoelectrochemical Hydrogen Production
Photoelectrochemical cells (PEC) combine a photovoltaic device for light harvesting and an electrolyzer for water splitting into a one single device. In its simplest form, a PEC cell is composed of a photoanode for water oxidation where oxygen is evolved, a cathode where hydrogen is evolved, and an electrolyte. A major obstacle for the wide-spread use of PEC cells is the low photoanode efficiency. By some accounts, the rate of water oxidation has to be increased by more than an order of magnitude to keep pace with the production of electrons and holes. In addition, recombination losses can limit performance. We explored two strategies for improving performance of the photoanode: producing the photocatalyst in the form of nanotubes to improve collection efficiencies and incorporating nanocrystalline gold onto the nanotubes to improve activity.
Highly ordered TiO2 nanotube (TiNT) arrays were fabricated using an anodization process. Gold nanoparticles were supported onto the TiNTs using a modified deposition precipitation (DP) method. The pH, aging time and Au precursor concentration were manipulated to increase the Au dispersion and loading on the nanotubes. Cyclic voltammetry was used to measure the total and Au electrochemical surface areas. The performance of photoanodes was evaluated in a 3-electrode cell with a 1.0 M KOH electrolyte using a solar simulator (1.5 AM) and potentiostat. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction were used to characterize key properties of the materials, and optical absorption measurements were used to determine their bandgaps.
Longer nanotubes provided higher photocurrents compared to the TiO2 powders and shorter nanotubes. This was attributed to enhanced capture of the light and better separation of the charge carriers. The deposition of Au nanoparticles resulted in a slight reduction of the bandgap, which we attributed to the existence of impurity levels between the band edges of the oxide. Introduction of the gold nanoparticles resulted in a significant improvement in the electrocatalytic properties. In addition the intrinsic activity increased as the Au particle size decreased, in a manner similar to that observed for CO oxidation. We observed a three-fold increase in activity on reducing the average Au particle size from 28 to 2.9 nm. These and other results will be presented in this paper.