2.2. Structure Characterization of SnO2 Nanoaggregates

SnO₂ powder samples were characterized using X-ray diffraction (XRD; Rigaku RINT Ultima/PC with monochromated Cu–Kα radiation, Tokyo, Japan). The crystallite size of the SnO₂ aggregates was estimated using the Scherrer equation (D = Kλ/βcosθ) based on the XRD data, where D, K, λ, and θ indicate the crystallite size, Scherrer constant (0.90), X-ray wavelength (1.54 Å), and Bragg angle, respectively. The SnO₂ sample surface was analyzed by XPS (XPS; ULVAC, Quantera SXM, Kanagawa, Japan). Nanostructure analysis was carried out by TEM (JEM-2100, Tokyo, Japan). The Brunauer–Emmett–Teller (BET) surface area was evaluated by N₂ physisorption measurement at 77 K, using the Micromeritics TriStar II 3020 (Kyoto, Japan). Photoabsorption spectra were measured using a Hitachi U-4000 spectrophotometer. Ultraviolet–visible (UV–Vis) spectra were obtained from the diffuse reflectance of the dry powder samples.