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

Neuderth, Paula et al.
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 InxGa1−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 InxGa1−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 InxGa1−xN NW core a stable TiO2-covered photoanode with visible light excitation is realized.
Financial support is provided by the DFG via the GrK (Research training group) 2204 “Substitute Materials for sustainable Energy Technologies.” MC thanks Ram ́on y Cajal program RYC- 2013-12448. We also acknowledge nancial support from the Spanish Ministry of Economy and Competitiveness, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV-2015-0496). This article is based upon work from COST Action MP1402 “Hooking together European research in atomic layer deposition (HERALD)”, supported by COST (European Cooperation in Science and Technology). SMS acknowledges funding from “Programa Internacional de Becas “la Caixa”- Severo Ochoa”. JA, MdlM. and SMS also acknowledge funding from Generalitat de Catalunya 2014 SGR 1638 and the Spanish MINECO e-TNT (MAT2014-59961-C2-2-R). ICN acknowledges support from the Severo Ochoa Program (MINECO, Grant SEV- 2013-0295) and is funded by the CERCA Programme/Generalitat de Catalunya. The atomic resolution HAADF-STEM microscopy was conducted in the Laboratorio de Microscopias Avanzadas at the Instituto de Nanociencia de Aragon-Universidad de Zaragoza. JA and SMS thank them for offering access to their instruments and expertise. Part of the present work has been performed in the framework of Universitat Aut`onoma de Bar- celona Materials Science PhD program. RM gratefully acknowledges funding in the Emmy-Noether program (MA 5392/3-1) of the German Research Foundation DFG.