Effects of catalyst/zinc mole fraction on ZnAl2O4:0.01% Cr3+ nanocrystals synthesized using sol-gel process

A report on the sol-gel preparation of nanosized ZnAl2O4:0.01% Cr3+ at a relatively low temperature (similar to 80 degrees C) is presented. The catalyst/Zn mole fraction in the solution was varied from 0-3 during the synthesis. The x-ray diffraction (XRD) data show that the annealed samples were pure cubic crystalline structures at the lower catalyst/Zn mole fractions. However, minor diffraction peaks associated with ZnO were detected at the higher catalyst/Zn mole fractions. An increase in the mole fraction led to transformation of morphology from spherical particles to rod-like-structures. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis and elemental mapping revealed the presence of Zn, Al, O and Cr. In addition, the catalytic mole fraction influenced the band-gap of the ZnAl2O4 host. Photoluminescence (PL) spectra indicated that the undoped and 0.01% Cr3+-doped powders exhibit a bluish-violet emission at slightly different peak positions, suggesting that the emission could either be originating from the host or the Cr3+ dopant ion. The emission from the host was attributed to oxygen vacancies (V-o*), while emissions from the Cr3+ were attributed to the transitions from different energy levels in the Cr3+ ion. It was also noted that the PL emission intensity was dependent on the catalyst/Zn mole fraction. The colour chromaticity showed that the emission colour of the ZnAl2O4:0.01% Cr3+ phosphor could be tuned by varying the catalyst/Zn mole fraction.