Metal Oxide Protection Layers for Enhanced Stability and Activity of WO<sub>3</sub> Photoanodes in Alkaline Media
Svenja Speldrich, Michael Wark, Günther Wittstock
Abstract
High Resolution Image Download MS PowerPoint Slide Photoelectrochemical water splitting using complex oxides has been envisioned as an avenue toward sustainable fuel production. The reaction is usually performed in neutral or alkaline solutions to enable the use of earth-abundant transition metal oxides. However, those materials, such as widely studied WO 3, may still suffer from photocorrosion or chemical dissolution. Application of protective layers to the photoabsorber constitutes one way to combine favorable absorption characteristics with stability in the working solution. Protective layers must also have electrocatalytic properties and charge carrier transport characteristics, which, in turn, are modified by application as a thin layer on a semiconducting absorber material. A combinatorial approach was used here, in which protection layers of Cr, Fe, Co, Ni, Ga, and Bi oxides were applied to WO 3 as a photoabsorber by material printing from liquid precursors and screened by scanning photoelectrochemical microscopy (SPECM). The layers prepared with Bi oxides exhibited the best combination of stability and activity followed by protection layers of Fe, Co, and Ni oxides. The most promising layered W/Bi oxide system was further characterized by chopped light voltammetry, electron and light microscopy, Raman spectroscopy, grazing incidence X-ray diffraction, X-ray photoelectron spectroscopy, and inductively coupled plasma optical emission spectroscopy of ions released to the working solution. The Bi oxide contains Bi 2 WO 6 and Bi 2 O 3 phases. The calcination temperature influences the stability and phase formation, with the best results obtained for samples calcined at 600 °C. In the W/Bi oxide system, the charge separation is supported by the formation of a heterojunction between the absorber and the protection layer, which contributes to the enhanced photoactivity of the layered W/Bi oxide.