Resonant and Selective Excitation of Photocatalytically Active Defect Sites in TiO2
By Hou, Bingya; Shen, Lang; Shi, Haotian; Chen, Jihan; Zhao, Bofan; Li, Kun; Wang, Yi; Shen, Guozhen; Ha, Mai-Anh; Liu, Fanxi; Alexandrova, Anastassia N.; Hung, Wei Hsuan; Dawlaty, Jahan; Christopher, Phillip; Cronin, Stephen B.
Published in ACS Applied Materials & Interfaces
2019
Abstract
It has been known for several decades that defects are largely responsible for the catalytically active sites on metal and semiconductor surfaces. However, it is difficult to directly probe these active sites because the defects associated with them are often relatively rare with respect to the stoichiometric crystalline surface. In the work presented here, we demonstrate a method to selectively probe defect-mediated photocatalysis through differential alternating current (ac) photocurrent (PC) measurements. In this approach, electrons are photoexcited from the valence band to a relatively narrow distribution of subband gap states in TiO2 and then subsequently to the ions in solution. Because of their limited number, these defect states fill up quickly, resulting in Pauli blocking, and are thereby undetectable under direct current or continuous wave excitation. In the method demonstrated here, the incident light is modulated with an optical chopper, whereas the PC is measured with a lock-in amplifier. Thin (5 nm) films of TiO2 deposited by atomic layer deposition on various metal films, including Au, Cu, and Al, exhibit the same wavelength-dependent PC spectra, with a broad peak centered around 2.0 eV corresponding to the band-to-defect transition associated with the hydrogen evolution reaction (HER). While the UV