The Size-dependent variation of the optical properties of Bi-doped nickel tungstate

Nickel tungstate samples doped with bismuth are synthesized by the method of direct chemical precipitation. The variation in the calcination duration leads to the size variation of Bi-doped NiWO₄ samples. It is widely recognised that calcination duration affects the structural parameters and changes their characteristics and functionalities. In the present work, it is found that the calcination duration affects the structural properties. X-ray diffraction (XRD) analysis is carried out to find the average crystallite sizes of the doped samples. The average crystallite size of the samples is found to increase with the increase in calcination duration, which is also confirmed with the W–H Analysis. The change in the structural parameters leads to a variation in the optical properties. UV–visible spectra of the samples show a shift in the absorption intensity and a slight change in the absorption peak. The variation in the spectra is due to the variation in the nanoparticle size. The energy gap also gets modified with the variation in the average crystallite size. An increase in the calcination duration leads to an increase in average crystallite size, and hence, the optical band gap reduces. This is by quantum confinement. The photoluminescence property is also modified by the change in the crystallite size. The emission colour of the sample varies significantly with the change in particle size. The emission spectrum of the sample with a longer calcination duration is found to have noteworthy differences from that of the sample with a shorter calcination duration. In that condition, transitions in the Bi³⁺ ions start dominating, and the emission spectrum has a peak corresponding to the Bi transition.