The surface charging at low density of protonatable surface sites

The point of zero charge (PZC) of a sparingly soluble metal oxide depends on the density of protonatable surface oxygen atoms. The shift in the PZC is due to protonation/deprotonation of water in the regions free of protonatable surface oxygen atoms originating from the solid. The PZC of alumina increases when the density of protonatable surface oxygen atoms increases. In contrast, the PZC of titania is rather insensitive to the density of protonatable surface oxygen atoms. In surfaces of many materials the regions free of protonatable surface oxygen atoms dominate. These materials have a PZC at pH about 4.