Experimental and Theoretical Investigations of Surface Electrical Charges of Different Faces of ZnO Nanorods for Environmental Remediation

The surface electrical charges in ZnO nanostructures kept in an aqueous medium have an overriding role on their nucleation, growth, and doping processes, as well as on any interactions with their surrounding environment. Despite the significance of their anisotropic character in the chemical and environmental applications made of ZnO nanostructures, they are still highly debated. Here, we investigate the surface electrical charges both experimentally in ZnO nanorods grown by chemical bath deposition, and theoretically for several faces of ZnO crystals, using the Multi-Site Complexation (MUSIC) model. The overall experimental isoelectric point (IEP) value of ZnO nanorods as deduced from zeta potential measurements is found to be 9.4. Correlatively, their minority top Zn-polar +c-plane faces and their majority lateral nonpolar m-plane sidewalls exhibit an experimental IEP value in the range of 9.0–9.4 and of 10.1, respectively, as inferred from latex nanobead adsorption processes. Interestingly, the MUSIC model reports the theoretical IEP values of 6.5, 8.7, and 10.9 for the O-polar –c-, nonpolar m-, and Zn-polar +c-planes, respectively, in the case of perfect ideal surfaces. The apparent discrepancy between the experimental and theoretical IEP values is reconciled by considering the roughening process operating on the surfaces of ZnO nanorods when grown by chemical bath deposition, resulting in the high concentration of VZn and in the formation of numerous surface steps. The evolution of the surface electrical charges around the IEP value is further shown to strongly depend on the nonpolar or polar character of each face of ZnO crystals. The quantitative values of the surface electrical charges for the O-polar –c-, nonpolar m-, and Zn-polar +c-planes in the case of perfect ideal surfaces are eventually inferred from the MUSIC model in the range of pH from 3.0–14.0. These findings cast a detailed light into the anisotropic nature and magnitude of surface electrical charges in ZnO, as a valuable insight for any engineering devices in the fields of chemistry and environment where surface and interface processes are predominant phenomena.