pH-dependence in facet-selective photo-deposition of metals and metal oxides on semiconductor particles

Abstract: Facet-engineering and the deposition of co-catalysts lead to improvement in efficiency of semiconductors in photocatalytic applications. While facet-based charge separation is a known phenomenon and it is reported for a range of semiconductors, there is still a limited understanding of it and the mechanism behind remain elusive. Moreover, the proposed simplistic mechanism based on an offset of the valence band and conduction band of different facets is debated. Here, we show that facet-selective, photo-induced reductive or oxidative deposition of co-catalysts onto plate-like bismuth-oxy-bromide (BiOBr) nanoparticles is strongly pH-dependent. High resolution atomic force microscopy and spectroscopy measurements demonstrate that the effect of pH is caused by a reversal of the surface charge of the 001 facets upon increasing the pH from 3 to 9 (isoelectric point 5), while the side facets remain (increasingly) negatively-charged. We propose a different mechanism and we suggest that differences in surface charge/band bending that vary for different surface orientations is the dominant element causing selective charge transport between different facets and metal-oxide deposition