In situ optical spectroscopic understanding of electrochemical passivation mechanism on sol–gel processed WO₃ photoanodes

A prevailing understanding on electrochemical activation of photoelectrodes is that electrochemical treatment leads to increased charge carrier densities thereby improved photoelectrode performances. Contrary to this understanding, in this study enhanced photoactivity of WO₃ photoanode upon electrochemical treatment is ascribed to an extraordinary mechanism of surface trap passivation. The associated mechanism is analyzed by in situ optical spectroscopy, using which the optical property changes of WO₃ electrode during electrochemical treatment are monitored. The results suggest surface W⁵⁺ species, the origin of surface traps on WO₃ photoanodes, are converted to W⁶⁺ ions by electrochemical treatment. This study demonstrates the particular ability of the electrochemical strategy to passivate surface traps of photoanodes, and also shows the advantages of in situ optical spectroscopy to investigate the real-time electronic structure variations of electrodes during electrochemical treatment.