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 TiO₂films.

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 TiO₂films are conducting.

We also report measurements of hot electron-driven photocatalytic water splitting using Au films with and without TiO₂ coatings.⁴ In these structures, a thin (3-10nm) film of TiO₂is deposited using atomic layer deposition (ALD) on top of a 100nm thick Au film. We utilize an AC lock-in technique, which enables us to detect the relatively small photocurrents (~µA) produced by the short-lived hot electrons that are photoexcited in the metal. Under illumination, the bare Au film produces a small AC photocurrent (<1 µA) for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) due to hot electrons and hot holes, respectively, that are photoexcited in the Au film.

References:

1. Qiu, J., G. Zeng, M. Ge, S. Arab, M. Mecklenburg, B. Hou, C. Shen, A.V. Benderskii and S.B. Cronin, Correlation of Ti³⁺ states with photocatalytic enhancement in TiO₂-passivated p-GaAs. Journal of Catalysis, 337, 133 (2016).

2. Qiu, J., G.T. Zeng, M.A. Ha, M.Y. Ge, Y.J. Lin, M. Hettick, B.Y. Hou, A.N. Alexandrova, A. Javey and S.B. Cronin, Artificial Photosynthesis on TiO₂-Passivated InP Nanopillars. Nano Letters, 15, 6177-6181 (2015).

3. Qiu, J., G.T. Zeng, M.A. Ha, B.Y. Hou, M. Mecklenburg, H.T. Shi, A.N. Alexandrova and S.B. Cronin, Microscopic Study of Atomic Layer Deposition of TiO₂ on GaAs and Its Photocatalytic Application. Chemistry of Materials, 27, 7977-7981 (2015).

4. Hou, B., L. Shen, H. Shi, R. Kapadia and S.B. Cronin, Hot Electron-Driven Photocatalytic Water Splitting. Physical Chemistry Chemical Physics, DOI: 10.1039/C6CP07542H (2017).