Synthesis of Gallium Zinc Oxynitride Solid Solution by Flame Spray Pyrolysis
Photocatalytic water splitting is seen as a viable alternative to steam reforming for hydrogen fuel generation. One of the major keys to exploiting photocatalytic technologies as a commercial energy source lies in the development of catalytic materials that can drive photocatalytic reactions using the largest portion of the solar energy spectrum – the visible light region. Gallium zinc oxynitride solid solution has been recommended as a promising candidate for photocatalytic water splitting, however, the current synthesis methods are largely multi-steps and batch wet chemistry techniques that are not easily amenable to scale up. Flame spray pyrolysis (FSP) is a continuous, single step, fast, relatively cheaper and easily scalable nanomaterial synthesis method. This work reports the deployment of flame spray pyrolysis to the synthesis of gallium zinc oxynitride solid solutions. The solid solutions were prepared using two different precursor compositions expressed as urea to total metal ratio, solvent mixtures and synthesis conditions, and X-ray diffraction and UV-Vis spectra were employed for preliminary characterizations. Based on solvent screening results, no single solvent satisfied the solubility, boiling point and combustion enthalpy requirements. The XRD data shows that solvents of weak combustion enthalpy resulted in either amorphous or very low crystalline, bimodal and inhomogeneous phase solid solution. However, solvent mixtures of adequate combustion enthalpy yielded single phase and highly crystalline solid solutions with all peaks indexed to the hexagonal wurtzite ZnO structure irrespective of synthesis conditions. The absence of extra peaks coupled with the slight lattice contraction relative to pristine ZnO satisfied the necessary conditions to qualify the as-prepared samples as solid solutions of GaN and ZnO. Lattice contraction was favored as urea to total metal ratio decreased while peak broadening occurred as the urea to total metal ratio in the precursor solution increased while the crystallinity lowered. This could be possibly due to increased dopant concentration in the ZnO lattice. Samples made with DI H2O + butanol solvent mixture were more crystalline and of larger crystallite size than those made from methanol + butanol mixture which is attributed to the fact that the solvent mixture supplied higher rate of energy (1.17 kJ/s) than the methanol-butanol (1.12 kJ/s) to the flame.
Tauc’s plots were used to estimate the direct bandgaps of the samples. With DI H2O + butanol mixture, the bandgaps are 3.08 and 3.12 eV with smaller bandgap belonging to the sample with higher urea to total metal ratio in the precursor solution. Comparing with the XRD data, the smaller bandgap corresponds with lattice contraction. But for the methanol-butanol mixture, the bandgaps are 3.07 eV and 3.16 eV with the smaller bandgap belonging to the sample with lower urea to total metal ratio. This perhaps, support the claims as reported in Literatures that bandgap is not solely defined by dopant content. Overall, based on bandgaps, the synthesized samples can drive photocatalytic activity in the UV region. Hence, optimization of the synthesis technique is necessary to further narrow the bandgap to the visible light range.