Facet-engineering and the deposition of co-catalysts lead to significant improvement in efficiency of semiconductors in photocatalytic applications. Here, we demonstrate, using the specific example of bismuth-oxy-bromide (BiOBr) particles, that facet-selective, photo-induced reductive or oxidative deposition of co-catalysts onto plate-like semiconductor particles 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  facets upon increasing pH from 3 to 9 (isoelectric point ≈5), while the side facets become increasingly negatively-charged. We discuss the effect of facet-surface-charge on particle distributions by band-bending, favoring either electron transfer and metal deposition, or hole transfer and metal-oxide deposition. This finding opens up new ways to design highly effective, photocatalytic composite architectures, containing spatially separated catalytic particles of multiple compositions.