Modifications of graphitic C3N4 with multi-metal oxides for enhanced visible-light active photocatalysis

D. Masih, S. Rohani
University of Western Ontario,

Keywords: g-C3N4, heterojunction, photocatalysis


Bulk graphitic C3N4 (g-C3N4) is a p-type semiconductor with band gap of 2.7 eV. For photocatalytic water splitting, valance and conduction band gap positions of g-C3N4 sandwich both oxygen and hydrogen evolution potentials, making g-C3N4 an important material for further investigation. In recent years, g-C3N4 has attracted much attention towards harvesting solar energy for applications in environment and energy sectors. However, separation of photo-excited electron and hole pairs is an intrinsic problem with bulk g-C3N4. Control of g-C3N4 morphology helps in increasing the number of active sites upon formation of a mesoporous structure, and separation of electrons and holes in its graphene analogous nanosheets in the UV range. Doping and fabrication of heterojunctions are the main approaches for enhanced separation followed migration of electron and hole pairs onto the surface, and consequently their use in photocatalytic reactions. Moreover, band gap engineering of g-C3N4 with suitable plasmonic and semiconductor materials helps in extended optical absorption of visible-light and efficient harvesting of solar energy. Multi-metal oxides in d0 and d10 electronic configurations are interesting narrow band gap semiconductor for harvesting solar energy. But, poor separation of electron and hole pair along with their instability under experimental conditions pose serious problems. Formation of multi-metal oxides of group V (d0) with copper/silver (d10) metals, and fabrication of their heterojunctions with g-C3N4 may help in development of stable and efficient visible-light active photocatalyst systems. In this study, g-C3N4 was synthesized by solid-state conversion of precursors like melamine and urea at around 823 K. The as-synthesized g-C3N4 was suspended in a suitable solvent and impregnated with solutions of metal precursors under reflux. Thus obtained mixture was taken into a Teflon liner enclosed in a steel reactor, and solvo/hydrothermally treated at around 453 K. In another approach, the mixture was dried in oven and the obtained powder was annealed at various temperatures. The synthesized materials were characterized by XRD, DR-UV-Vis, FT-IR, TG-DTA, BET, and electron microscopy techniques. From the solid-state reaction of melamine, a yellow color material was obtained with a typical XRD pattern demonstrating formation of g-C3N4. DR-UV-Vis and FT-IR characterizations further confirmed the formation of g-C3N4. Copper vanadate heterojunctions were fabricated by in-situ growth, and deposition of pre-formed CuVOx onto g-C3N4 by solution process. Essentially, graphitic structure of C3N4 was maintained after heterojunctions with copper vanadate. And, new X-ray diffraction peaks for copper vanadate appeared in the heterojunctions. Over these materials, photo-degradation of environmental pollutants was carried out under visible-light irradiation.