Visible-Light-Responsive Oxyhalide PbBiO₂Cl Photoelectrode: On-Site Flux Synthesis on a Fluorine-Doped Tin Oxide Electrode

The performance of photoelectrodes is hugely affected by the preparation method. Although a flux synthesis is useful to endow semiconductor particles with the desired properties such as high crystallinity, there are only a few reports on its application to photoelectrode fabrication, probably because relatively high temperatures are necessary. In the present study, we introduce a new concept for on-site flux synthesis of semiconductor crystals on a commonly used fluorine-doped tin oxide (FTO) substrate; a seed layer is predeposited and then treated with an appropriate flux containing other required elements at a right temperature lower than the limit temperature of FTO but sufficiently high to transform the seed layer to the target material with the aid of flux. Here, an oxyhalide PbBiO2Cl, one of the promising semiconductors for achieving visible-light water splitting, is selected as a target material. Combination of a BiOCl seed layer and the NaCl–PbCl2 flux containing other precursors enables the seed layer to transform into PbBiO2Cl crystals even at 450 °C. The thickness of the PbBiO2Cl layer can be controlled by changing the thickness of the BiOCl seed layer for efficient photon-to-current conversion. Owing to a good contact at the semiconductor–substrate interfaces as well as the high quality of PbBiO2Cl crystals, the flux-synthesized PbBiO2Cl photoelectrode shows a significantly improved PEC performance compared with those prepared from the particulate PbBiO2Cl samples via the conventional squeegee method. In addition, the present PbBiO2Cl photoelectrodes exhibit both anodic and cathodic photoresponses with substantially high current values depending on the applied potentials; the unusual phenomenon is affected by the conditions in flux-assisted synthesis. The present study provides a new and effective way for fabricating efficient photoelectrodes of various semiconductors on various substrates and a possible option to control their morphologies and p/n types for further improvement in performance.