Adsorption of Photosensitive Dyes to Zinc Oxide and Spectral Sensitization of Its Photoconductivity

Abstract

The nature of interaction between the surface of zinc oxide grains and adsorbed dye molecules has been examined by measuring the amount of adsorbed dyes, adsorption isotherm, heat of adsorption and visible reflection spectra of adsorbed dyes on zinc oxide grains. It has been found that the adsorption of xanthene dyes to zinc oxide grains is caused by the coulombic force and also by van der Waals' forces, and that the coulombic force arises from the localized electrostatic interaction between carbonyl groups of the adsorbed dye molecule and zinc ions at the surface of zinc oxide grains. It has been also found that dyes in the molecular state mainly take part in spectral sensitization of photoconductivity of zinc oxide grains. Then, the authors have examined the possibility that the small amount of dyes strongly affected by imperfections of zinc oxide grains have large influences on the electron or energy transfer from the optically excited dyes to zinc oxide grains, i.e., spectral sensitization, by use of the experimental method designed in the previous work. No evidence was obtained for the presence of the influence of imperfections of zinc oxide grains on spectral sensitization of their photoconductivity. Based on these results, the relation between adsorbability and sensitizing power of photosensitive dyes for zinc oxide grains has been examined, and the dye with the strong adsorbability has been found to show the strong sensitizing power. This result is thought to support the electron transfer mechanism for spectral sensitization of photoconductivity of zinc oxide grains.