Cobalt was successfully doped into the lattice of unique ZnO platelets for the first time. Huge reductions in photocatalytic activity (PCA) were observed compared to that of undoped powders in UVA, UVB, and UVC regions. PCA reduction was explained via tracking the surface hydroxyl groups using XPS analyses.

Abstract

Platelet-shaped zinc oxide (ZnO) particles exhibit certain advantages compared to conventional micron and nano forms. Their superior surface coverage and excellent optical properties make these particles very attractive as UV filters in cosmetics and personal care applications. Although ZnO is non-toxic, it exhibits photocatalytic activity (PCA), which increases when the size of particles approaches the nanoscale. High PCA of mineral/inorganic filters is not desired in cosmetics and personal care applications. Therefore, it is essential to develop strategies to reduce PCA of such materials under UV exposure. Accordingly, the research objective of this study is to develop an understanding of the effects of Co-doping on the optoelectronic and crystal structure of platelet shaped ZnO particles with Zn_1-xM_xO stoichiometry (x=0-4 wt % cobalt). XRD-based Rietveld refinement and WD-XRF reveal that Co was successfully doped into the ZnO lattice. SEM and particle size analyses confirmed that the shape and size of the designed ZnO platelets did not change significantly after doping. ~85 % reduction in PCA has was achieved by 3 wt % Co doping which was attributed to reduced OH and O₂ free radical concentration. These results clearly show that Co-doping can be used to effectively tailor the bandgap and optical properties of the designed ZnO particles.