Synthesis, Optical, and Photocatalytic Properties of Cobalt Mixed-Metal Spinel Oxides Co(Al1-xGax)2O4

Cobalt mixed-metal spinel oxides, Co(Al_1-xGa_x)₂O₄, have been predicted to exhibit promising properties as photocatalysts for solar energy conversion. In this work, Co(Al_1-xGa_x)₂O₄ were synthesized with a range of 0 ≤ x ≤ 1 via both single-source and multi-source routes. Single-source molecular precursors, [Co{M(O^tBu)₄}₂] (M = Al or Ga), were decomposed at 300 °C to form amorphous oxides. Multi-source precursors, stoichiometric mixtures of metal acetylacetonate (acac) complexes, were used to form nanocrystalline spinel materials. Both were subsequently converted to bulk spinel products by annealing at 1000 °C. The properties of materials fabricated from the single-source and multi-source synthetic routes were compared by analysing data from X-ray diffraction, scanning electron microscopy, transmission electron microscopy, UV-vis spectrophotometry, inductively coupled plasma-optical emission spectroscopy, and gas sorption measurements. The X-ray diffraction data of the materials showed ideal solid solution behavior that followed Vegard's law for both routes, with the multi-source route giving more crystalline bulk material than the single-source route. UV-vis absorbance data revealed that the absorption onset energies of Co(Al_1-xGa_x)₂O₄ decreased monotonically with increasing x (from 1.84 eV for x = 0 to 1.76 eV for x = 1 from the single-source method; 1.75 eV for x = 0 to 1.70 eV for x = 1 from the multi-source method). The photocatalytic activities of the spinel oxides were evaluated via the photodegradation of methyl orange and phenol, which showed that the photoactivity of Co(Al_0.5Ga_0.5)₂O₄ was dependent on both pH and substrate. Remarkably, under appropriate substrate binding conditions (pH 3 with methyl orange), low energy (<2.5 eV) ligand-field transitions contributed between 46 and 72% of the photoactivity of Co(Al_0.5Ga_0.5)₂O₄ prepared from the multi-source route.