Global environmental issues, in addition to limited fossil fuel resources, are being addressed by quests in search of efficient visible-light-driven water splitting catalysts for hydrogen production. The photocatalytic water splitting activities of CdX/C 2 N (X = S, Se) heterostructures have been investigated here using hybrid density functional theory calculations. The calculated band gaps of CdS/C 2 N and CdSe/C 2 N heterostructures are 1.48 and 2.12 eV, respectively. These are ideal band gap values that make possible harvesting of more visible light from the solar spectrum, which will result in high solar to energy conversion efficiencies. Charge density difference analysis shows that the charge redistributions mainly occur in the interface regions and that the charges transfer from the C 2 N to CdX layers. It is interesting to note that the CdX/C 2 N heterostructures possess a type-II band alignment, where the relative band alignment of the C 2 N and CdX monolayers promotes a spatial separation of the electrons (that resides in C 2 N) and holes (that resides in CdX). Importantly, the band edges of the heterostructures straddle the water redox potential under different pH conditions. This study demonstrates that the CdS/C 2 N and CdSe/C 2 N heterostructures are suitable materials to split water (from various sources) in different ranges of pH values.

Two-dimensional (2D) semiconductors have shown great promise as efficient photocatalysts for water splitting. Tailoring the band gap and band edge positions are the most crucial steps to further improve the photocatalytic activity of 2D materials. Here, we report an improved photocatalytic water splitting activity in a C N monolayer by isoelectronic substitutions at the C-site, based on density functional calculations. Our optical calculations show that the isoelectronic substitutions significantly reduce the band gap of the C N monolayer and thus strongly enhance the absorption of visible light, which is consistent with the observed redshift in the optical absorption spectra. Based on the HSE06 functional, the calculated band edge positions of C Si N and C Ge N monolayers are even more favorable than the pristine C N monolayer for the overall photocatalytic activity. On the other hand, for the C Sn N monolayer, the conduction band minima is more positive than the oxygen reduction potential and, hence, Sn substitution in C N is unfavorable for the water decomposition reaction. In addition, the isoelectronic substitutions improve the separation of e -h pairs, which, in turn, suppress the recombination rate, thereby leading to enhanced photocatalytic activity in this material. Our results imply that Si-, and Ge-substituted C N monolayers will be a promising visible-light photocatalysts for water splitting.

Exploiting earth-abundant and low-cost photocatalysts for high efficiency photocatalytic water splitting is of profound significance. Herein, we report an improved photocatalytic water splitting activity by P and As substitution at the N-site in the C2N monolayer using state-of-the-art hybrid density functional calculations. Our results show that the band gap can be reduced in C2N by increasing the concentrations of P and As substitution, and at the same time the obtained band gap value is higher than the free energy of water splitting except for As with concentrations of x = 0.333. This indicates that these new compositions of P/As substituted C2N monolayers are thermodynamically suitable to drive hydrogen evolution reaction. The calculated effective mass of charge carriers illustrates that charge transfer to the reactive sites would be easier in the substituted system than the pure C2N, and also our results suggest that the recombination rate would be lower in the substituted system, indicating the enhancement in the efficiencies of photocatalytic water splitting. The band edge position with respect to the redox potentials of water shows that P/As substituted C2N monolayers are the potential photocatalysts for water splitting than the pristine C2N monolayer. From the optical absorption spectra, we found that P/As substituted C2N monolayer shows optical absorption extended more into the visible region, indicating enhanced energy harvesting. Our results reflect that the P/As substituted C2N monolayer could be the potential visible-light photocatalyst for overall water splitting.

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