Structure, Property, and Function of Gallium/Urea and Gallium/Polyethylene Glycol Composites and Their Sintering Products: β- and γ-Gallium Oxide Nanocrystals

The first part of the study is devoted to the comparison between the doping effect of urea (a small molecule) and polyethylene glycol (PEG, a long-chain polymer) on the physical property of metallic gallium (Ga). The physical properties of the Ga composited in the two materials, Ga/urea and Ga/PEG, were investigated by scanning electron microscopy, X-ray powder diffraction, X-ray photoelectron spectroscopy, differential scanning calorimetry, superconducting quantum interference device, and surface-enhanced Raman scattering spectra and compared with our previous results for the effect of macrocyclic hosts (e.g., cyclodextrins, calixarenes) on the physical modification of metallic Ga. Our data provide new direct evidence that the modification of physical properties of Ga is highly dependent on the nature of dopants used. For example, the addition of a small amount of urea causes a fundamental change in the crystallization behavior of Ga, and the presence of PEG results in the occurrence of a weak paramagnetism of Ga at high fields, both of which are completely different from the effect of other dopants. The other part of the study is devoted to demonstrating whether there is a significant difference in the oxidation process of metallic Ga and its composites. Our result gives a strong positive answer to the question. β- and γ-gallium oxide nanocrystals were obtained by sintering the Ga/urea composite at different temperatures and exhibited distinctive photoluminescence and photocatalysis properties. These results gave a strong impression that the introduction of different dopants leads metallic Ga to generate different features in microstructure, physical property, and especially chemical reactivity. We believe that the findings of this study have important implications for the development of inorganic materials.