Photochromic property of cerium oxide nanoparticles prepared by dialysis of solution of cerium nitrate

Abstract

CeO2 nanoparticles were obtained by heating 50 mL of 0.1 M Ce(NO3) 3·6H2O solution mixed with 5 mL of 1 M NH3 aq. at 368 K for 24 h. When H2O and ethanol were used as solvents for cerium nitrate hydrate, obtained CeO2 nanoparticles were aggregated. On the other hand, in case that ethylene glycol was used for the solvent, stable sol of CeO2 nanoparticles dispersed in ethylene glycol. Furthermore, dialysis of the sol with H2O successfully enabled the exchange of the dispersion solvent from ethylene glycol to H2O and the average particle size was 23 nm, which was less than those of the other solvents. UV and visible light were irradiated onto the obtained CeO2 powders to examine their photochromic characteristics. The CeO2 nanoparticles obtained with the ethylene glycol solution showed change of absorbance spectra after irradiation of light whose wavelength was less than 600 nm, that is, the visible light irradiation caused photochromism on the CeO2 nanoparticles. According to Ce 3d XPS spectra of the CeO2 nanoparticles, valence state of the Ce ions in the CeO2 nanoparticles was mixed valence state of Ce3+ and Ce4+, which causes inter valence charge transfer absorption. Therefore, the change of the optical absorption spectra, photochromism, occurred with the optical absorption due to the charge transfer absorption between Ce3+ and Ce4+. Furthermore, the photochromic characteristics depended on heat treatment of the CeO2 nanoparticles. Heating the nanoparticles at more than 473 K decreased amount of oxygen vacancies, which affected the mixed valence state of Ce ions so that the photochromism by irradiation of the light within the wavelength range from 400 nm to 600 nm was greatly affected.