Abstract
Graphitic carbon nitride is an exemplar material for metal-free photocatalytic hydrogen production, essential to drive the change to a greener economy. However, its bandgap is too large, at 2.7 eV, for visible light harvesting, which hinders uptake in applications. From two sets of independent quantum mechanical simulations, we have determined the effect of two representative interstitial (hydrogen and fluorine) dopants on the electronic structure and optical properties of this material. From defect analysis, we have found that for a significant range of chemical potential the anionic fluorine dopant is favored. This dopant has significant effects on the optical absorption with the valence band edge shifted up by 0.55 eV, which extends light absorption into the visible. In contrast, hydrogen prefers to be cationic, with the conduction band edge shifted down by 0.45 eV, which strongly reduces hydrogen production as the thermodynamic driving force for proton reduction is significantly reduced. Fluorine is a...
Highlights
Low cost and effective hydrogen (H2) production via photocatalysis of water is urgently required to leverage the free energy of the sun to synthesize clean renewable fuel
Experimental realization of compression-induced bandgap reduction and associated optical absorption deeper into the visible range will only occur for carbon nitrides grown on substrates such as scandium, which limits their applicability for cheap photocatalysts
Graphitic carbon nitride is composed of sheets of heptazine molecules (C6N8) linked together at their corners by C−N bonds, as shown in Figure 1, panel a
Summary
Low cost and effective hydrogen (H2) production via photocatalysis of water is urgently required to leverage the free energy of the sun to synthesize clean renewable fuel. Several different prospective dopants have been studied to improve the optical absorption by reduction of the band gap This includes nitrogen,[10] oxygen,[11] boron,[12,13] phosphorus,[14] sulfur,[15−17] iron,[18] iodine,[19] fluorine,[20−22] and precursors infiltrated with methyl orange dyes,[23] or 2,6-diaminopyridine.[24] Additional nitrogen atoms can dope into carbon lattice sites, which results in a minor decrease of the band gap of less than 0.1 eV (from 2.72−2.65 eV).[10] A greater reduction in the bandgap is observed for oxygen-doping, with a reduction of 0.21 eV measured experimentally.[11] This is believed to arise from oxygen atom insertion on a nitrogen site, which results in a lowering of bandgap position due to the formation of a partially empty occupied state ∼0.22 eV below the conduction band edge (CBE).[25] Boron doping was found to improve the photocatalytic activity of g-C3N4 toward rhodamine B photo-oxidation, partially associated with a reduction of the band gap by 0.04 eV and, by providing a reaction site for photocatalysis.[12,13] Sulfur
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