In situ X-ray diffraction study of the solvothermal formation mechanism of gallium oxide nanoparticles

Gallium oxides are of broad interest due to their wide band gaps and attractive photoelectric properties. Typically, the synthesis of gallium oxide nanoparticles is based on a combination of solvent-based methods and subsequent calcination, but detailed information about solvent based formation processes is lacking, and this limits the tailoring of materials. Here we have examined the formation mechanisms and crystal structure transformations of gallium oxides during solvothermal synthesis using in situ X-ray diffraction. γ-Ga₂O₃ readily forms over a wide range of conditions. In contrast, β-Ga₂O₃ only forms at high temperatures (T > 300 °C), and it is always preceded by γ-Ga₂O₃, indicating that γ-Ga₂O₃ is a crucial part of the formation mechanism of β-Ga₂O₃. The activation energy for formation of β-Ga₂O₃ from γ-Ga₂O₃ is determined to be 90-100 kJ mol^-1 in ethanol, water and aqueous NaOH based on kinetic modelling of phase fractions obtained from multi-temperature in situ X-ray diffraction data. At low temperatures GaOOH and Ga₅O₇OH form in aqueous solvent, but these phases are also obtained from γ-Ga₂O₃. Systematic exploration of synthesis parameters such as temperature, heating rate, solvent and reaction time reveal that they all affect the resulting product. In general, the solvent based reaction paths are different from reports on solid state calcination studies. This underlines that the solvent is an active part of the solvothermal reactions and to a high degree determines different formation mechanisms.