*2.3. Characterization*

The crystal structures of the samples were determined by X-ray diffraction (XRD, Bruker D8 Advance, Germany) using Cu Kα radiation (λ = 1.5418 Å) and scanning over a 2*θ* range of 10◦ to 80◦ . The surface morphologies of the samples were observed using a scanning electron microscope (SEM, Hitachi S3400, Tokyo, Japan) equipped with an energydispersive X-ray spectroscopy (EDS) system. The UV–visible (UV–Vis) absorption spectra of the samples were obtained using a Scan UV–Vis spectrophotometer (Hitachi U-3900, Tokyo, Japan). The spectra were recorded in the range of 200–800 nm at room temperature in air. The compositions of the samples were inferred following X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250Xi, Waltham, MA, USA) results, which were obtained using an ESCALab MKII X-ray photoelectron spectrometer with Al Kα X-ray radiation as the excitation source. The photoluminescence spectra (PL) of the samples were obtained using the Edinburgh FLS 980 (Edinburgh, UK) fluorescence spectrometer. The electron spin resonance (ESR) signals of radicals trapped by 5,5-dimethyl-1-pyrroline *N*-oxide (DMPO) were detected at ambient temperature using a Bruker (E580, Rheinstetten, Germany) spectrometer under visible-light irradiation (*λ* > 400 nm).
