Structural Origin of Semiconductor Optical Size Effect through In2O3 Nanocubes and Use in Photocatalytic Benzothiazole Formation.

In2O3 nanocubes with sizes of 10, 72, and 95 nm have been synthesized solvothermally in ethanol. Synchrotron X-ray diffraction (XRD) patterns reveal ultrasmall peak shifts among these samples. Moreover, while 10 nm cubes give symmetric diffraction peaks, those of 72 and 95 nm cubes can be deconvoluted into bulk and surface layer lattice components. Fast Fourier transform (FFT) processing of their high-resolution transmission electron microscopy (HR-TEM) images presents similar lattice point appearances throughout the crystal for a 10 nm cube, while 72 and 95 nm nanocubes show greater surface lattice deviations compared with inner lattice points. These lattice differences should be the structural basis for the observed optical size effect, with band gaps of 10 and 95 nm cubes differing by 0.13 eV. Heating the nanocubes to high temperatures changed their color from white to yellowish due to cell constant changes and significant lattice point distortion. This simple experiment demonstrates that lattice variations can produce color changes in semiconductor crystals. The In2O3 nanocubes were employed to photocatalyze the condensation of 2-aminothiophenol and benzaldehyde, forming benzothiazole. Active species have been identified. Both intrinsic and thermally induced lattice deviations can cause light absorption shifts, explaining why the same semiconductor material displays diverse colors.