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Density functional theory study on a nitrogen-rich carbon nitride material C3N5 as photocatalyst for CO2 reduction to C1 and C2 products

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Abstract

A new-type nitrogen-rich carbon nitride material C3N5 has been synthesized recently, in which the C:N ratio increases from 3:4 in g-C3N4 to 3:5 due to the introduction of azo linkage (single bondNdouble bondNsingle bond) connecting segments in two C6N7 units. Herein, C3N5 as a photocatalyst for CO2 reduction was investigated by density functional theory methods. The electronic and optical properties indicate that C3N5 has a longer visible-light region with 2.0 eV of band gap in comparison with g-C3N4. The spatial distributions of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) show that the π network of C3N5 is extended by introducing —Ndouble bondN— linkage, which results in much higher photocatalytic efficiency than g-C3N4. The Gibbs free energies for possible CO2 reaction paths on C3N5 were computed. The results show that CO2 can be reduced to CH4 with a low limiting potential of −0.54 V and to CH3CH2OH with a low limiting potential of −0.61 V, which all driven by solar energy. The present work is expected to provide useful guide for new-type nitrogen-rich C3N5 as promising photocatalyst for CO2 reduction reaction (CO2RR).

Introduction

The emission of carbon dioxide (CO2), a greenhouse gas, from the excessive use of fossil fuel can cause severe global warming and environmental pollution. Photocatalytic CO2 reduction into hydrocarbon fuels is a promising strategy that not only lowers the atmospheric CO2 level, but also provides clean fuels, such as carbon monoxide (CO), formic acid (HCOOH), methane (CH4) and methanol (CH3OH), ethanol (CH3CH2OH) and ethylene (C2H4) to replace the conventional fossil resources. Various metal-based materials (such as Cu [1] Pd [2], Ag [3] Sn [4] and Au [5]) as catalysts for CO2 reduction reaction (CO2RR) have been intensively studied. Although these catalysts can convert CO2 into various chemicals, they still have some challenges as follows [6], [7]: (1) their catalytic efficiencies are low due to high overpotential, low stability and poor selectivity; (2) some catalysts are expensive, thereby increasing the cost. Therefore, searching for highly efficient and low-cost metal-free photocatalysts is extremely demanding.

Carbon nitride materials (CNx), metal-free catalysts, have attracted more attentions because of unique properties e.g., semi-conductivity, high stability, simplicity of synthesis, etc. CNx can be divided into nitrogen-doped (N-doped) carbon materials (CNx, x  < 1) and carbon nitrides (CNx, x  > 1) [8]. The catalytic activities of CNx are found to be higher than those of carbon materials. Zhao et al. indicated that N-doping can modify the electronic properties of graphene for enhancing CO2 reduction and pyrrolic N-doped graphene has a higher catalytic activity on CO2RR than that of graphene [9] Therefore, the introduction of nitrogen into CNx not only results in a small band gap, but also helps enhancing their catalytic properties [10]. Developing nitrogen-rich CN materials becomes an effective strategy for multifunctional properties. Graphitic carbon nitride (g-C3N4) with 3:4 C:N stoichiometry is one of the representative nitrogen-rich CNx materials, which has been intensively investigated as a promising photocatalyst [11]. Except for the band gap (2.70 eV) [12], the presence of numerous nitrogen-rich sites and ordered defects provide active centers for the substrate interaction, which increases catalytic ability [13]. However, like other semiconductors, pure g-C3N4 has some internal disadvantages such as carrier recombination. Doping with metal (Co, Cu, Fe) [14], [15], [16] or non-metal (S, P, B, O) [17], [18], [19], [20] atoms and constructing heterojunctions (g-C3N4/SnS2, [21] g-C3N4/Ag3PO4 [22]) have been utilized to improve photocatalytic ability effectively. After all, there is a limit to improve the photocatalytic properties of g-C3N4 by structural modification. In our previous work, we designed (C6N7)n, [C6N7(C2)1.5]n, [C6N7(C4)1.5]n and [C6N7(C3N3)]n via C6N7 unit of g-C3N4 connected with C—C, C—Ctriple bondC—C, C—Ctriple bondC—Ctriple bondC—C and C3N3 ring. Density functional theory (DFT) calculations indicated that the band gaps of designed structures decrease and absorption spectrum edges all red-shifted compared with g-C3N4 [23]. In 2013, Kroke et al. speculated that —Ndouble bondN— linkage replacing the connecting bridge carbon atoms based on C6N7 motif is an efficient way to increase N atoms content, which the C:N ratio is increased from 3:4 in g-C3N4 to 3:5, named as C3N5 [24]. Due to the combination of azo chromophores (—Ndouble bondN—) with aromatic compounds, azo-bridged aromatic compounds are well known for visible light absorption [25]. Recently, Shankar et al. have successfully synthesized the C3N5 modified carbon nitride framework by thermal deammoniation of 2,5,8-trihydrazino-s-heptazine and proved that C3N5 exhibits remarkable photocatalytic activity [26]. The catalytic activity of C3N5 for CO2RR, however, needs to be clarified.

Recently, many studies proposed that the CO2RR can not only generate the C1 products (such as CO, HCOOH, CH4 and CH3OH), but also generate the multi-carbon products, especially C2 products (such as CH3CH2OH and C2H4) [27], [28], [29], [30]. Yeo et al. showed that CuAg composite catalysts enhanced CO2 reduction selectivity toward ethanol [27]. Zhao et al. reported that boron doped graphdiyne exhibit high-electrocatalytic activity for the conversion of CO2 to C2H4 by DFT [28], while single iron atom supported on graphdiyne can provide an active site for coupling with CO to generate C2H5OH [29]. Xu et al. simulated that FeFe-grafiN6 and FeCu-grafiN6 could reduce CO2 to C1 and C2 products [30]. Based on these studies, we also investigated the possible C2 products generated.

In the present work, the CO2RR mechanism on C3N5 is investigated by DFT calculations. It is found that C3N5 is a locally stable structure and has a more suitable band gap (2.0 eV) than g-C3N4. C3N5 can efficiently catalyse CO2 to CH4 with a rather low limiting potential of −0.54 V and to CH3CH2OH with a rather low limiting potential of −0.61 V driven by solar energy.

Section snippets

Computational details

All the geometric optimizations and single-point energy calculations were performed using the DMol3 code [31], [32]. The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) exchange–correlation functional [33] was used to describe exchange and correlation effects. To accurately describe weak interactions between species involved in the CO2RR and catalysts, the PBE + D2 method with the Grimme van der Waals (vdW) correction was employed [34]. A conductor-like screening

Geometric structures and molecular dynamics

Azofra et al. [47] suggested corrugated g-C3N4 is more stable than planar configuration. In our case, we found the optimized corrugated C3N5 (a = b = 14.97 Å, Fig. 1a) is also more stable than planar configuration (a = b = 15.12 Å, Fig. 1a). The total energy difference (ΔE) between planar and corrugated reaches 0.24 eV in the 1 × 1 unit cell. According to the different chemical environment, N atoms are categorized into three N types, namely N1, N2 and N3. N1 and N2 represent 2-fold and 3-fold

Conclusions

In conclusion, we systematically studied the catalytic mechanism of CO2RR on new-type nitrogen-rich carbon nitride materials, namely C3N5 by DFT calculations. The ab initio MD simulations show that C3N5 is at least locally stable 2D structure. Due to the introduction of —Ndouble bondN— linkage, C3N5 possesses a more suitable band gap (2.0 eV) than that of g-C3N4 and extends the π network resulting in more electron transfer, which effectively separates the photogenerated e/h+ pairs. Compared with g-C3N4,

CRediT authorship contribution statement

Yuelin Wang: Conceptualization, Investigation, Methodology, Data curation, Writing - review & editing, Software. Thanh Ngoc Pham: Investigation, Writing - review & editing, Software. Yu Tian: Investigation, Writing - review & editing. Yoshitada Morikawa: Investigation, Methodology, Writing - review & editing, Supervision. Likai Yan: Conceptualization, Investigation, Methodology, Writing - review & editing, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

The authors gratefully acknowledge financial support by NSFC (21403033, 21571031). This work was also partly supported by JSPS KAKENHI (Grant No. 20H02569) and MEXT Element Strategy Initiative for Catalysts and Batteries (Grant No. JPMXP0112101003). We acknowledge Institute of Theoretical Chemistry, Jilin University for providing the computational resources for this work.

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