Ultrathin and Conformal TiOx Overlayers on WO3 Photoelectrodes for Simultaneous Surface Trap Passivation and Heterojunction Formation

Nanoporous structures facilitate the exposure of active sites and allow a high ratio of the space charge region to the bulk in water-splitting photoelectrodes. However, unfavorable surface defects may develop on nanoporous photoelectrodes, which deteriorate the band bending (built-in electric field) and trigger serious charge carrier recombination. To maximize the advantages of nanoporous structures in photoelectrodes, one common strategy is the introduction of ultrathin overlayers to passivate undesirable surface defects and traps, which usually require advanced deposition technologies such as atomic layer deposition. In this study, a process of drop-casting followed by O2 plasma treatment is employed to realize ultrathin and conformal TiOx overlayers on WO3 photoelectrodes. Notably, the ultrathin TiOx overlayer demonstrates dual effects of surface trap passivation and heterojunction formation on WO3 photoelectrodes, which result in suppressed surface charge recombination and enhanced band bending. The as-derived TiOx-modified WO3 photoanode shows an increase in water-splitting photocurrent (increased by 81% at 1.6 V vs the reversible hydrogen electrode), along with a 160 mV cathodic shift in photocurrent onset potential. The proposed approach here provides valuable insights into the room-temperature fabrication of uniform and ultrathin overlayers for nanostructure modification.