Passivation layers for nanostructured photoanodes: ultra-thin oxides on InGaN nanowires

Abstract

An experimental strategy for systematically assessing the influence of surface passivation layers on the photocatalytic properties of nanowire photoanodes by combining photocurrent analysis, photoluminescence spectroscopy and high resolution transmission electron microscopy with a systematic variation of sample structure and the surrounding electrolyte is demonstrated. Following this approach we can separate the impact on recombination and transport processes of photogenerated carriers. We apply this strategy to analyze the influence of ultra-thin TiO₂, CeO₂ and Al₂O₃ coatings deposited by atomic layer deposition on the photoelectrochemical performance of In_xGa_1−xN/GaN nanowire (NW) photoelectrodes. The passivation of surface states results in an increase of the anodic photocurrent (PC) by a factor of 2.5 for the deposition of 5 nm TiO₂. In contrast, the PC is reduced for CeO₂- and Al₂O₃-coated NWs due to enhanced defect recombination in the passivation layer or increased band discontinuities. Furthermore, photoelectrochemical oxidation of the In_xGa_1−xN/GaN NW photoelectrode is attenuated by the TiO₂ layer and completely suppressed for a layer thickness of 7 nm or more. Due to efficient charge transfer from the In_xGa_1−xN NW core a stable TiO₂-covered photoanode with visible light excitation is realized.