Rational design of graphitic carbon nitride with sustainable energy production

Abstract

Since photocatalysis is an attractive approach for energy artificial synthesis, developing a suitable photocatalyst is highly required. Material with a promising band structure can ensure effective light-harvesting and high photoreduction kinetics. Many metal-free photocatalysts of graphitic carbon nitride (g-C3N4) have been developed for photocatalysis since Wang et al. first reported it in 2009. Despite the poor charge transfer and restrained carrier separation ability of g-C3N4, this polymeric photocatalyst is a strong competitor for renewable energy production. Element doping and novel heterojunction construction are two main methods to regulate photogenerated carrier movement. Except for achieving superior photocatalytic performance, the mechanism also requires sophisticated investigation to discover behind carrier dynamics. Nevertheless, the photogenerated carrier movement happens within nanoseconds or picoseconds. The transient charge transfer dynamic is essential to disclose the carrier movement route. This study has been targeted on developing g-C3N4-based photocatalysts to enhance the photocatalytic energy production efficiency and the investigation of the relevant mechanisms. The followings are the findings of this study: • Several approaches have been adopted to improve the photocatalytic H2 evolution performance of g-C3N4. The salt template approach provides a feasible way for generating mesoporous that can enhance charge movement. As a result, the Na and O co-doped photocatalyst of Na20-CNNR (20wt% Na2S2O3•5H2O with respect to the g-C3N4 precursor) exhibits an excellent H2 generation rate of 7.46 mmol/h/g, leading to 84 times higher compared with the g-C3N4 counterpart. • To improve the quasiparticle transfer in g-C3N4, ruthenium phosphide (RuxP) can act as the electron acceptor, while hydroxide ions (OH⁻) can promote hole exhaustion. As a result, The unique heterojunction of RuxP incorporated in the bulk CN (B-RuxP-CN) shows a superior quasiparticle movement performance and a high photocatalytic H2 evolution rate of 6.37 mmol/h/g, which is comparable to most of the g-C3N4-based photocatalysts when Pt-free material is used as cocatalyst. • Another approach is the doping of O elements in the bulk graphitic carbon nitride through a precursor pretreatment using ultraviolet (UV) light irradiation. This modified non-metal photocatalyst of g-C3N4 have increased visible light absorption, enhanced carrier density, and suppressed the deep trap of electrons. With a UV irradiation for 60 hours, the UV60-CN sample enhanced its CO2 photoreduction rate (9.3 μmol/h/g) and selectivity (77.0%) toward CH4. • Vacuum ultraviolet (VUV) light generated hydroxyl radical (●OH) that decompose bulk graphitic carbon nitride into small CN fragments (CNF) and further wrap onto single-walled carbon nanotube (SWNT), which served as the rope to string up g-C3N4 fragments. The above modification method contributes to the improved CO2 photoreduction activity of SWNT/CNF, with a competitive CH4 production yield of 45.3 μmol/g and a superior selectivity of 95.4%. The above-conducted research works demonstrate a novel route to efficient photocatalysis energy production. Future research perspectives will centralize on building a feasible photocatalysis model (i.e., novel photocatalyst fabrication, spatial engineering, and reaction environment regulation) for efficient artificial photosynthesis of energy to realize practical application. (476 words)