Influence of pH-control in phosphoric acid treatment of zinc oxide

Abstract

Zinc oxide is often used as a white pigment for cosmetics; however, it shows photocatalytic activity that causes decomposition of sebum on the skin when exposed to the ultraviolet radiation in sunlight. In this work, zinc oxide was reacted with phosphoric acid at various pH values to synthesize a novel white pigment for cosmetics. The chemical composition, powder properties, photocatalytic activities, colors, and smoothness of these pigments were studied. The obtained materials exhibited X-ray diffraction peaks relating to zinc oxide and phosphate after phosphoric acid treatment. The ratio of zinc phosphate to zinc oxide was estimated from inductively coupled plasma -atomic emission spectroscopy results. Samples treated at pH 4-7 yielded small particles with sub-micrometer sizes. The photocatalytic activity of zinc oxide became lower after phosphoric acid treatment. Samples treated at pH 4-7 showed the same reflectance as zinc oxide in both the ultraviolet and visible ranges. Adjustment of the pH was found to be important in the phosphoric acid treatment of zinc oxide. Keywords: zinc oxide, phosphoric acid treatment, photocatalytic activity, particle size. Resumo

Preparation of novel cosmetic white pigment by pyrophosphate treatment of zinc oxide

“…5 shows UV-vis reflectance spectra of samples treated in various P/Zn ratios for 6 h. All samples indicated high reflectance at the visible light region. Zinc phosphate had high reflectance at the ultraviolet range, therefore samples were expected to have some reflectance at that range [19,20]. However, the reflectance of all the samples prepared in this study was low in the ultraviolet region due to the low reaction rate.…”