Oxygen-vacancy-induced photoelectrochemical water oxidation by platelike tungsten oxide photoanodes prepared under acid-mediated hydrothermal treatment conditions

Abstract

Introduction of oxygen vacancies in tungsten oxide photoanodes has been reported as an effective route towards enhancing the solar-driven water oxidation photocurrent. Therefore, it is reasonable to seek facile methods for controlling the number of oxygen vacancies in these photoanodes. Herein, a simple acid-mediated hydrothermal treatment method followed by calcination is utilized to fabricate tungsten oxide photoanodes. It is found that the variation of acid treatment temperature prior to calcination could influence the concentration of oxygen vacancies in the prepared platelike tungsten oxide photoanodes. These defects act as electron donors thus, they result in enhanced photoelectrochemical performance. Using XRD and Raman analyses, an insight is gained into the structure of the samples treated by acid-mediated hydrothermal method. Based on that, a correlation is made between the number of the intercalated water molecules in WO₃·nH₂O (n = 1 or 2) with the formation of oxygen vacancies during the calcination step. This finding is confirmed by XPS and Mott–Schottky analyses. At the optimum acid-mediated hydrothermal treatment temperature (75 °C), a photocurrent of 1.06 mA cm⁻² at 1.23 V vs. RHE is obtained. This is six times larger than the photocurrent produced by the sample fabricated at a higher temperature of 175 °C. An IPCE of 36.2% is acquired for this sample under irradiation with 350 nm light and at an applied potential of 1.8 V vs. RHE. The IPCE of the optimum sample is three times higher than that of the sample treated at 175 °C. This indicates that the mild acid-mediated hydrothermal treatment enhances photocurrent and photoelectrochemical water oxidation performance due to the increased formation of beneficial oxygen vacancy defects.