Influence of pH tuning at the precursor-preparation process on the structural characteristics and catalytic performance of hydrothermally synthesized ZnAl₂O₄ nanoparticles

The relationship between the structural characteristics of hydrothermally synthesized ZnAl2O4 nanoparticles and the pH of the source solution used during the precursor preparation process was investigated. A pH of approximately 9 was the turning point between large and small crystallite-sized regimes, and between stoichiometric and nonstoichiometric compositions. This difference can likely be attributed to the formation process of ZnAl2O4, because the stabilities of the Zn2+ and Al3+ hydroxides in the source solution change at pH 9. Additionally, we compared the catalytic performance with regards to the decomposition of methylene blue for the nanoparticles made from source solution with pH 7 (NP7) and 10.5 (NP10.5). The smaller-sized NP7 did not show any catalytic performance; whereas, the larger NP10.5 accelerated the decomposition of methylene blue. The critical difference between NP7 and NP10.5 is the Zn2+ deficiency, rather than the particle size. This result is important for future investigations in order to improve the catalytic properties of ZnAl2O4 nanoparticles.