Emergence of defect fluorite structure in nano-sized thoria through doping with some divalent transition-metal ions

With the objective of incorporating some divalent transition-metal ions in thoria and to comprehend its effect on the crystal structure, electronic as well as catalytic properties, Ni2+, Cu2+ and Cd2+ substituted thoria samples were synthesized by the epoxide gel method. Of the two concentrations investigated, 10mol% of Ni2+, Cu2+, and Cd2+ could be substituted retaining the fluorite structure and phase separation into individual oxides was noticed for 15mol%. The average crystallite size of thoria and 10mol% substituted samples was 14nm. Le-Bail structural refinements of Powder X-ray diffraction (PXRD) patterns indicated marginal increase in unit cell constant for the Cd2+ substituted sample and a decrease for Ni2+ and Cu2+ substituted samples. In addition to broadening of the band at around 460cm(-1) (F-2g vibration of the fluorite), less intense band near 560-590cm(-1) emerged for all the transition-metal ion-containing samples in the Raman spectra implying the formation of oxygen defects. The absorption edge in the UV-visible spectra moved toward higher wavelength for Cd2+, Ni2+ and Cu2+ containing samples as compared to pure thoria. In addition, d-d transition was observable for Ni2+ and Cu2+ containing samples. By virtue of these changes in the electronic structure of transition-metal ion-containing samples, they were examined as catalysts for the degradation of aqueous Rhodamine-6G (Rh-6G) dye solutions under visible radiation.