Aqueous Chemical Synthesis of Nanosized ZnGa₂O₄ Using Mild Reaction Conditions: Effect of pH on the Structural, Morphological, Textural, Electronic, and Photocatalytic Properties

The effect of pH on the structural, textural, morphological, and electronic properties of ZnGa2O4 nanoparticles obtained by coprecipitation using mild reaction conditions (25 degrees C; 30 min) was studied. The pH ranges in which coprecipitation reactions occurred and the chemical species associated with the reaction mechanism were identified. It was determined that the samples synthesized at pH values between 6 and 10 consisted of Zn-Ga oxide blends, with spinel ZnGa2O4 being the majority phase. Conversely, the material prepared at pH 12 was constituted by Zn-Ga layered double hydroxide phase along with wurtzite ZnO traces. The synthesis pH determined the reaction product yield, which decreased from 51 to 21% when the reaction medium turned from softly acidic (pH 6) to strongly alkaline conditions (pH 12). The bandgap energies of the synthesized materials were estimated to be in the range of 4.71-4.90 eV. A coprecipitation-dissolution-crystallization mechanism was proposed from the precipitation curve, with specific mononuclear and polynuclear species being involved in the formation of the different precipitates. Phenol was employed as a probe molecule to evaluate the photocatalytic performance of the synthesized samples. Among the samples, the one prepared at pH 6 showed the largest photodegradation efficiency (similar to 98%), which was superior to commercial TiO2-Degussa P25 (similar to 88%) under the same process conditions, which can be attributed to both its high specific surface area (140 m(2) g(-1)) and the formation of a Zn2xGa2-2xO3+x/ZnGa2O4 heterojunction.