(Invited) Hot-Electron Driven Photocatalysis and Photoexcited Charge Carrier Dynamics in TiO2-Passivated Photocatalysts
Authors: Cronin Stephen B. 
Published: February 2020

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).