EPR investigations of polymeric and H2O2-modified C3N4-based photocatalysts

The C3N4-based nanopowders prepared by thermal condensation of melamine (MCN) with subsequent thermal etching (MCN-TE) and H2O2-treatment were investigated by Q- and X-band EPR spectroscopy in dark and upon in situ UVA or visible-light exposure. Lorentzian signal at g = 2.003, more pronounced in the case of the thermally etched material, dominates EPR spectra of MCN and MCN-TE. More complex spectra were found for H2O2-treated photocatalysts revealing the presence of signals attributed to the radicals produced via H2O2 interaction with C/N sites in the C3N4 polymeric network. The X-band spectra monitored upon in situ irradiation of the C3N4-based photocatalysts evidenced the intensity growth of the single line at g = 2.0033 indicating the photoinduced generation of electrons in localized paramagnetic states with the Curie dependence on temperature in the temperature range 100-180 K. The response towards UV or visible-light exposure was significantly limited in the case of H2O2-treated photocatalysts. EPR spin trapping experiments performed in aqueous suspensions demonstrated the formation of HO2 center dot and HO center dot spin-adducts, and the increased stability of the primary photo generated O-2(center dot-) in aprotic media was well documented by the irradiation of the photocatalysts in the dimethylsulfoxide/water mixed solvent. The highest activities in the production of the non-persistent radical species spin-adducts were found for the thermally etched and pristine photocatalysts, confirming the negative effect of H2O2-treatment