CO2 Reduction to Methanol on TiO2-Passivated GaP Photocatalysts

In the past, the electrochemical instability of III-V semiconductors has severely limited their applicability in photocatlaysis. As a result, a vast majority of the research on photocatalysis has been done on TiO2, which is chemically robust over a wide range of pH. However, TiO2 has a wide band gap (3.2 eV) and can only absorb similar to 4% of the solar spectrum, and thus, it will never provide efficient solar energy conversion/storage on its own. Here, we report photocatalytic CO2 reduction with water to produce methanol using TiO2-passivated GaP photocathodes under 532 rim wavelength illumination. The TiO2 layer prevents corrosion of the GaP, as evidenced by atomic force microscopy and photoelectrochemical measurements. Here, the GaP surface is passivated using a thin film of TiO2 deposited by atomic layer deposition (ALD), which provides a viable, stable photocatalyst without sacrificing photocatalytic efficiency. In addition to providing a stable photocatalytic surface, the TiO2 passivation provides substantial enhancement in the photoconversion efficiency through passivation of surface states, which cause nonradiative carrier recombination. In addition to passivation effects, the TiO2 deposited by ALD is n-type due to oxygen vacancies and forms a pn-junction with the underlying p-type GaP photocathode. This creates a built-in field that assists in the separation of photogenerated electron-hole pairs, further reducing recombination. This reduction in the surface recombination velocity (SRV) corresponds to a shift in the overpotential of almost 0.5 V. No enhancement is observed for TiO2 thicknesses above 10 nm, due to the insulating nature of the TiO2, which eventually outweighs the benefits of passivation.