The point of zero charge and solid-state properties of silver bromide

Abstract

The point of zero charge (p.z.c.) of AgBr depends on properties of the AgBr crystals. It is demonstrated that the p.z.c. of AgBr shifts to a higher pAg in aqueous solution if lead is incorporated in the AgBr lattice, i.e., by the creation of silver vacancies in the lattice. On the other hand, the p.z.c. of AgBr shifts to a lower pAg if sulfur is incorporated in the lattice, i.e., by the creation of interstitial silver ions in the lattice. By tempering AgBr crystals between 250° and 400°C prior to the p.z.c. measurement, the p.z.c. is shifted reproducibly to lower pAg values. This indicates some kind of “freezing in” when the AgBr crystals are cooled rapidly after tempering. It is argued that this freezing in is not a bulk effect, but a surface effect; presumably a surface charge is frozen in. Good quantitative agreement can then be obtained among the various experimental data on the p.z.c. of doped crystals. From these p.z.c. measurements is it is deduced that in pure AgBr in contact with an aqueous solution at the p.z.c. the surface has a potential of −60 mv with respect to the bulk. Furthermore, the fraction of lattice defects in pure AgBr is found to be 10 −7,2, in good accordance with solid-state measurements.