Microwave assisted citrate gel combustion synthesis of ZnO Part-II: Assessment of functional properties

In this study, the functional properties of nanocrystalline ZnO derived through a microwave assisted citrate gel decomposition technique were examined by UV-Vis and PL spectroscopy. The prepared ZnO and ZnO containing Al³⁺, Ag⁺ and a mixture of (Al³⁺+ Ag⁺) dopants were subjected to photocatalytic degradation of methylene blue dye under UV irradiation and photoluminescence analysis. The band gap reduction and photo-oxidation reaction kinetics were assessed. The functional property studies revealed that ZnO nano clusters showed a UV absorption edge at 393 nm with band gap energy of 3.159 eV. A significant band gap reduction is found in mixed ions doping [Al³⁺+Ag⁺] compared to single Ag⁺ and Al³⁺ dopants. The photo-oxidation reaction follows pseudo- first-order reaction kinetics and a k_obs constant of 0.273 was determined with mixed dopants. An enhanced photoactivity was confirmed by the ultrasonication of a ZnO-dye suspension. The PL measurements showed blue and red emission in Ag⁺ and Al³⁺ doped ZnO, respectively. The mixed ions [Al³⁺ + Ag⁺] resulted in a pronounced intense orange-red emission in the visible region. In addition to this, the photoactivity of microwave-derived ZnO clusters was confirmed in a kaolin clay matrix. The kaolin modified with 2 wt% ZnO was found to be photoactive and it degraded the methylene blue dye within 3 h under UV irradiation.