CO 2 Reduction to Methanol on TiO 2 ‐ Passivated GaP Photocatalysts

6 ABSTRACT: In the past, the electrochemical instability of III−V semi7 conductors has severely limited their applicability in photocatlaysis. As a result, 8 a vast majority of the research on photocatalysis has been done on TiO2, which is 9 chemically robust over a wide range of pH. However, TiO2 has a wide band gap 10 (3.2 eV) and can only absorb ∼4% of the solar spectrum, and thus, it will never 11 provide efficient solar energy conversion/storage on its own. Here, we report 12 photocatalytic CO2 reduction with water to produce methanol using TiO213 passivated GaP photocathodes under 532 nm wavelength illumination. The TiO2 14 layer prevents corrosion of the GaP, as evidenced by atomic force microscopy 15 and photoelectrochemical measurements. Here, the GaP surface is passivated 16 using a thin film of TiO2 deposited by atomic layer deposition (ALD), which 17 provides a viable, stable photocatalyst without sacrificing photocatalytic efficiency. In addition to providing a stable photocatalytic 18 surface, the TiO2 passivation provides substantial enhancement in the photoconversion efficiency through passivation of surface 19 states, which cause nonradiative carrier recombination. In addition to passivation effects, the TiO2 deposited by ALD is n-type 20 due to oxygen vacancies and forms a pn-junction with the underlying p-type GaP photocathode. This creates a built-in field that 21 assists in the separation of photogenerated electron−hole pairs, further reducing recombination. This reduction in the surface 22 recombination velocity (SRV) corresponds to a shift in the overpotential of almost 0.5 V. No enhancement is observed for TiO2 23 thicknesses above 10 nm, due to the insulating nature of the TiO2, which eventually outweighs the benefits of passivation. 24