2.2. Characterizations

The micro-morphologies of VG, MoO₃, and VG/MoO₃ nanosheets were observed by scanning electron microscopy (SEM, Supra 60, Zeiss, Jena, Germany) and transmission electron microscopy (TEM, Titan3 G² 60–300, FEI Electron Optics B.V., Hillsboro, OR, USA). The analysis of material composition was conducted with energy dispersive spectrum (EDS) and corresponding mapping images, embedded in the SEM and TEM systems. The crystal structures of the samples were measured by powder X-ray diffraction (XRD, D-max 2200 VPC, Rigaku, Tokyo, Japan) and micro-Raman spectrometer (Raman, In Via Reflex, Renishaw plc, Gloucestershire, UK) with a 532 nm laser excitation. The valence states of elements in the samples were further analyzed by X-ray photoelectron spectroscopy (XPS, Escalab 250Xi, Thermo Fisher Scientific, Waltham, MA, USA).