(Invited) Enhanced Photocatalysis on TiO2-Passivated III-V Compounds for Water Splitting and CO2 Reduction

Abstract

We report enhanced photocatalysis for H2 evolution and CO2 reduction using TiO2-passivated InP and GaAs photocathodes.1-3 The TiO2 layer makes the InP semiconductor photochemically stable. This represents a major step forward in photocatalysis, which has typically been limited to metal oxide materials. In addition to making these surfaces stable, the TiO2 film, deposited by atomic layer deposition (ALD), also provides a substantial enhancement in the efficiency of H2 evolution. We find that passivating GaAs with just a few nm of TiO2 produces a shift in the onset potential of H2 evolution by +0.35 V at 1 mA/cm2 and enhances the photocurrent by 32-fold over bare GaAs (at 0 V vs. RHE). Here, thinner TiO2 films produce a larger enhancement than thicker films, which correlates with the higher density of O-vacancies (i.e., Ti3+ surface states) observed in these thinner films using X-ray photoemission spectroscopy (XPS). While TiO2 films 1-5nm thick produce large enhancements, no enhancement is observed for TiO2 thicknesses above 10 nm, which are crystalline and, therefore, considerably more insulating than thinner amorphous TiO2 films. We also report photocatalytic CO2 reduction with water to produce methanol using TiO2-passivated InP nanopillar photocathodes under visible wavelength illumination.2 Again, the TiO2 passivation layer provides a stable photocatalytic surface and substantial enhancement in the photoconversion efficiency and selectivity through the introduction of O-vacancies associated with the nonstoichiometric growth of TiO2 by ALD. Plane wave-density functional theory (PW-DFT) calculations confirm the role of oxygen vacancies in the TiO2 surface, which serve as catalytically active sites in the CO2 reduction process. PW-DFT shows that CO2 binds stably to these oxygen vacancies and CO2 gains an electron (−0.897e) spontaneously from the TiO2 support. The TiO2 film increases the Faraday efficiency of methanol production by a factor 5.7X under an applied potential of −0.6 V vs NHE, which is 1.3 V below the E o (CO2/CO2 −) = −1.9 eV standard redox potential. In order to further understand the strong dependence of these photocatalysts on TiO2 thickness over the range of 0−15 nm, we performed cross-sectional high resolution transmission electron microscopy (HRTEM) of GaAs/TiO2 heterojunctions.3 Thinner films (1−10 nm) are amorphous and show enhanced catalytic performance with respect to bare GaAs. HRTEM images and electron energy loss spectroscopy (EELS) maps show that the native oxide of GaAs is removed by the TiCl4 ALD precursor, which is corrosive. Thicker TiO2 films (15 nm) are crystalline and have poor charge transfer due to their insulating nature, while thinner amorphous TiO2 films are conducting.