(Invited) Investigation of the Si/TiO2/Electrolyte Interface Using Operando Tender X-ray Photoelectron Spectroscopy

Abstract

Photoelectrochemical (PEC) cells based on semiconductor-liquid interfaces provide a method of converting solar energy to electricity or fuels 1-3. Recently, we have demonstrated operational systems that involved stabilized semiconductor-liquid junctions 4,5. We have employed operando ambient-pressure X-Ray photoelectron spectroscopy (AP-XPS) on these highly stable semiconductor-liquids junctions to directly characterize the semiconductor-liquid junction at room temperature under real-time electrochemical control 6,7. The escape depth of photoelectrons that originate from tender X-ray synchrotron radiation — 2.3 to 5.2 keV — allows for sampling of the sample simultaneously near the sample surface but through an electrolyte layer as the inelastic mean free path of such photoelectrons is on the order of 5 to 10 nanometers. Accordingly, tender X-ray AP-XPS has enabled simultaneous monitoring of the semiconductor surface, the semiconductor-electrolyte interface, and the bulk electrolyte of a PEC cell as a function of the applied potential, E. Accumulation, depletion and Fermi level pinning were observed. The observed shifts in the core level emission binding energies with respect to the applied potential directly revealed ohmic and rectifying junction behavior on metallized and semiconducting samples, respectively. 1. H. J. Lewerenz et al., Energ. Environ. Sci., 3, 748–760 (2010). 2. H. J. Lewerenz, in Photoelectrochemical Materials and Energy Conversion Processes, R. C. Alkire, D. M. Kolb, J. Lipkowski, and P. N. Ross, Editors, vol. 12, p. 61–181, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany (2010). 3. Z. Huang, C. Xiang, H. J. Lewerenz, and N. S. Lewis, Energ. Environ. Sci., 7, 1207–1211 (2014). 4. S. Hu et al., Science, 344, 1005–1009 (2014). 5. M. F. Lichterman et al., Energy Environ. Sci., 7, 3334–3337 (2014). 6. S. Axnanda et al., Meet. Abstr., MA2013-02, 921–921 (2013). 7. S. Axnanda et al., (2014), submitted.