Understanding carbon nitride photocatalysts : materials and mechanism in photocatalytic energy conversion

Abstract

Selected functional carbon nitride-based photocatalysts were synthesized and applied to visible or near-infrared (NIR) light-driven hydrogen generation. The as-synthesized catalysts not only showed the robust photon absorption under visible-light excitation but also exhibited NIR-driven photoactivity. Femtosecond/nanosecond transient absorption spectroscopies (fs/ns-TA) were also used to determine the photocatalytic processes mechanisms and photo-induced charge transfer pathways. A related study about graphitic carbon nitride (g-C3N4) was reported by Prof. James R. Durrant from Imperial College London. As reported, the overall photoactivity of g-C3N4 was greatly inhibited due to the deep trapping of the charges, which was proved to be photocatalytically inactive. Chapter 3 showed a simple method to fabricate copper phosphide (Cu3P)/g-C3N4 photocatalysts with high-efficiency inhibition of the intrinsic electron trapping and resulted in a great increase of photoactivity. The as-prepared Cu3P/g-C3N4 exhibited a higher visible-light-driven photocatalytic water splitting performance for hydrogen production (277.2 µmol•h-1•g-1) than that of pure g-C3N4 (0.75 µmol•h-1•g-1). The fs/ns-TA results demonstrated that the long-lived active electrons and excited state appear attached to the enhanced hydrogen generation activity under visible-light excitation. The research project here clearly displayed a practicable method to suppress the intrinsic electrons trapping of carbon nitride and discerned the important role of the transition metal phosphides as the electron acceptor for photo(electro)catalysis. It’s still hard to obtain full-spectrum photoactivity due to the narrow light absorption of g-C3N4. Both visible and NIR show the high occupation (total 95%) in the solar spectrum. Therefore, it’s a hot research point to construct Visible-NIR-driven g-C3N4-based photocatalysts. Chapter 4 demonstrated that the visible-NIR-driven photocatalytic hydrogen evolution reaction (HER) activity of as-synthesized copper-nickel sulfide/g-C3N4 composite. Copper-nickel sulfide can effectively restrain the high-frequency occurrence of deep electron trapping of carbon nitride and widen the photon absorption range to the NIR area. The intimate chemical interactions (C-S bond) between copper-nickel sulfide and carbon nitride were observed and influence the ultrafast interfacial charge transfer (~ 1.67 ps). Compared to carbon nitride (1.6 μmol•h-1•g-1), about 470-fold improvement of visible-driven photocatalytic HER (752.8 μmol•h-1•g-1) was obtained over copper-nickel sulfide/carbon nitride composite. Fs-TA measurement reveals the long-lived charge separation states (~ 4896 ps). This research exhibited great potential for the noble-metal-free photo(electro)catalytic energy conversion. The suppression of the deep trapping process in g-C3N4 was proved to efficiently obtain better photoactivity. In contrast, the long-lived shallow trapping is favor of photocatalytic performance enhancement. In Chapter 5, the cubic In2O3-induced electromagnetic field was used to efficiently prevent deep charge trapping in g-C3N4. The relationship between shallow and deep trapping processes was further determined by using fs/ns-TA measurements with TD-DFT simulations of the absorption spectra. Compared to pure carbon nitride, the In2O3-cube/g-C3N4 composite can display a 34-time improvement of visible-light-driven HER. The shortened lifetime (from 173.2 to 85.2 ns) for the deep trapping, the long-lived shallow trapping (~ 434.8 ps), and the interfacial electron transfer (~ 71.5 ps) directly created enough time window for photocatalytic reaction.