Synthesis, optical, and photocatalytic properties of cobalt mixed-metal spinel oxides Co(AlGa)O

Cobalt mixed-metal spinel oxides, Co(AlGa)O, have been predicted to exhibit promising properties as photocatalysts for solar energy conversion. In this work, Co(AlGa)O were synthesized with a range of 0 ≤ x ≤ 1 via both single-source and multi-source routes. Single-source molecular precursors, [Co{M(OBu)}] (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(AlGa)O 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(AlGa)O 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(AlGa)O prepared from the multi-source route.