X-ray photoelectron spectroscopy (XPS) was performed on the pristine powders using a SPECS XPS system, equipped with a monochromatic Al Kα X-ray source (hν = 1487 eV) and PHOIBOS 150 spectrometer. High-resolution scans of the C 1s, O 1s, V 2p and N 1s transitions were acquired at 400 W, 30 eV pass energy, and 0.1 eV energy step. Calibration of the binding energy was carried out using the adventitious carbon signal in the C 1s region (C-C/C-H) at 284.8 eV as a reference. The collected spectra were fitted by CasaXPS software using a nonlinear Shirley-type background and a 70 % Gaussian / 30 % Lorentzian line shape. Thermogravimetric analyses (TGA) were carried out in the temperature range from room temperature to 550 °C at a heating rate of 5 K min -1 under constant oxygen flow in aluminum crucibles loaded with ca. 8 mg of material using a TGA209 F1 Libra (Netzsch).