DFT Study on Regulating the Electronic Structure and CO2 Reduction Reaction in BiOBr/Sulphur-Doped G-C3N4 S-Scheme Heterojunctions

Photocatalytic CO₂ reduction is a promising method to mitigate the greenhouse effect and energy shortage problem. Development of effective photocatalysts is vital in achieving high photocatalytic activity. Herein, the S-scheme heterojunctions composed by BiOBr and g-C₃N₄ with or without S doping are thoroughly investigated for CO₂ reduction by density functional theory (DFT) calculation. Work function and charge density difference demonstrate the existence of a built-in electric field in the system, which contributes to the separation of photogenerated electron-hole pairs. Enhanced strength of a built-in electric field is revealed by analysis of Bader charge and electric field intensity. The results indicate that S doping can tailor the electronic structures and thus improve the photocatalytic activity. According to the change in absorption coefficient, system doping can also endow the heterojunction with increased visible light absorption. The in-depth investigation indicates that the superior CO₂ reduction activity is ascribed to low rate-determining energy. And both of the heterojunctions are inclined to generate CH₃OH rather than CH₄. Furthermore, S doping can further reduce the energy from 1.23 to 0.44 eV, indicating S doping is predicted to be an efficient photocatalyst for reducing CO₂ into CH₃OH. Therefore, this paper provides a theoretical basis for designing appropriate catalysts through element doping and heterojunction construction.

Introduction

With the development of society, the greenhouse effect has posed a great threat to human life due to excessive CO₂ emission. Numerous solutions have been explored, including electrochemical Liu et al. (2016), Albo et al. (2017), thermochemical Erb and Zarzycki (2016), Gong et al. (2016), and photochemical methods (Deng et al., 2020; Deng et al., 2021; Gogoi et al., 2021; Meng et al., 2021; Zhang et al., 2021b). Among them, photocatalytic methods are promising due to the sustainability of solar light (Chen et al., 2020; Liu et al., 2020; Xu et al., 2020b; Zhen et al., 2020; Liu et al., 2021a; Wei et al., 2021a; Zhang et al., 2021c). Specifically, photocatalytic CO₂ reduction can convert CO₂ into usable hydrocarbon fuels (Huo et al., 2021; Kang et al., 2021; Ke et al., 2021; Yang et al., 2021; Yang et al., 2021b; Zhang et al., 2021d). Even though photocatalysis shows great advantages, its application in CO₂ reduction is still greatly limited because of the chemical inertness of CO₂ and low visible-light utilization. Therefore, exploration of effective photocatalysts is necessary. In the past few years, various photocatalysts have been explored for photocatalytic CO₂ reduction such as metals (Dong et al., 2020), metal sulfides (Suzuki et al., 2018; Ge et al., 2019; Wang et al., 2020; Xu et al., 2020a), metal oxides (Wang et al., 2020; Chen et al., 2021; Wang et al., 2021), and nonmetals (He et al., 2020; Fei et al., 2021). Despite the great progress, low visible-light absorption and poor catalytic activity are still common problems faced by these photocatalysts. Therefore, modification, especially element doping and heterojunction construction, have been widely implemented to improve the photocatalytic activity (Truc et al., 2019; Ren et al., 2020; Lian et al., 2021; Liu et al., 2021b). Through element doping and heterojunction construction, the electronic structure and recombination of photogenerated electron-hole pairs can be effectively regulated and inhibited.

As a promising metal-free polymeric photocatalyst, graphitic carbon nitride has features of good physicochemical stability, a narrow bandgap, and appropriate band potential (Wang et al., 2009; Li et al., 2020a; Xia et al., 2020; Xie et al., 2020; Li et al., 2021b; Zhang et al., 2021a). In addition, the CO₂ molecule exhibits a strong affinity to pyridine nitrogen in g-C₃N₄, which is beneficial for CO₂ reduction (Zhu et al., 2017). Construction of hybrids has also proven to be effective for g-C₃N₄ (Li et al., 2020b; Li et al., 2020c; Li et al., 2021a; Li et al., 2021c; Li et al., 2020d; Mei et al., 2021; Tao et al., 2021). For instance, Fu et al. designed a 2D/2D WO₃/g-C₃N₄ composite, in which atomic-level thickness of each is realized (Fu et al., 2019). The ultrathin 2D/2D WO₃/g-C₃N₄ is proven to be S-scheme heterojunction, exhibiting high redox capacity and improved photocatalytic activity. In addition, element doping can also have a great influence on the properties of g-C₃N₄ through tailoring the electronic structures (Chen et al., 2021). Tian et al. fabricated P-doped g-C₃N₄ by mixing melamine and diammonium hydrogenphosphate (Tian et al., 2020). It was found that the light absorption is redshifted with the increase of doping concentration which is due to the electronic redistribution by P ion doping. It can be deduced that appropriate element doping and heterojunction building is fruitful in regulating the electronic structures and improving the photocatalytic performance.

Among available photocatalysts, BiOBr is deemed to be a prominent candidate for constructing a heterojunction with g-C₃N₄. It has appropriate bandgap and layered structure where one (Bi₂O₂) slab is surrounded by the upper and lower chlorine atoms. More importantly, its unique layered structure allows the formation of 2D/2D Van der Waals heterojunction with g-C₃N₄. In addition, the BiOBr/g-C₃N₄ hybrid has already been investigated by theoretical and experimental research as photocatalysts (Jiang et al., 2018; Qu et al., 2020). It has been proven that the BiOBr/g-C₃N₄ hybrids show superior photocatalytic activities in degradation of dyes and organic pollutants. Even though theoretical and numerous experimental studies have been done on the BiOBr/g-C₃N₄, reports on CO₂ reduction are lacking, and further investigation is still needed to figure out the intrinsic photocatalytic mechanism of CO₂ reduction on BiOBr/g-C₃N₄. In addition, previous studies indicate that element doping such as S doping can promote the performance of photocatalytic CO₂ reduction (Raziq et al., 2018; Wang et al., 2018; Ojha et al., 2019). It can be inferred that the introduction of S atom into BiOBr/g-C₃N₄ (BiOBr/S-g-C₃N₄) can improve the photocatalytic CO₂ reduction performance.

Herein, the effect of sulfur doping on the BiOBr/g-C₃N₄ heterojunction is investigated by exploring electronic, optical properties, and CO₂ reduction reaction. Through the theoretical calculation, comprehensive understanding of the CO₂ reduction over BiOBr/g-C₃N₄ systems will be acquired. Furthermore, electron distribution, visible light adsorption, and CO₂ reduction will also be investigated comprehensively. It is expected that this research can provide a basis for the design of novel CO₂ reduction materials.

Calculation Details

Vienna ab initio simulation package (VASP) was employed for calculations (Hafner, 2008). The geometry structures were optimized utilizing the generalized gradient approximation (GGA) Perdew-Burke-Ernzerhof (PBE) as exchange–correlation function (Grimme, 2006; Wu and Cohen, 2006). The DFT-D method of Grimme was selected to treat van der Waals interaction (Le et al., 2012). A 500 eV cutoff energy was adopted for the plane-wave expansion. For Brillouin-zone, a 4 × 2 × 1 Monkhorst–Pack k-point mesh was used in the geometry optimization and other properties calculation. A vacuum distance of 15 Å was used to eliminate periodic interactions between adjacent images. The convergence criteria of the geometry optimization for the energy change and maximum force were set to be 10⁻⁵ eV and 0.01 eV/Å, respectively.

In the process of constructing heterojunction, the lattice match is the key point. It is necessary to choose two components with similar cell parameters. Herein, a 1 × √3 single layered g-C₃N₄ is placed at the top of a 2 × 3 single-layered BiOBr for constructing 2D/2D BiOBr/g-C₃N₄ heterojunction. The 2D/2D BiOBr/S-g-C₃N₄ heterojunction is built by stacking a 1 × √3 S-g-C₃N₄ monolayer above a 2 × 3 BiOBr monolayer.

For CO₂ reduction, the reaction processes of each step were evaluated by calculating their Gibbs free energy change (ΔG) (Han and Sohn, 2005; Yan et al., 2016), which is expressed by the following equation:

$\Delta G=\Delta H-T\Delta S+ZPE(1)$

In this formula, ΔH denotes the energy difference of each reaction step gained from DFT calculation. T represents the temperature at 298.15 K ΔS and ZPE are the change of entropy and zero-point energy, respectively.

Results and Discussion

Geometric Structures

Prior to the investigation of heterojunctions, the geometric structures of monolayered g-C₃N₄, S-doped g-C₃N₄ (S-g-C₃N₄) and BiOBr along the (001) facet were first studied. As shown in Figure 1, the optimized lattice parameters are a = b = 7.13 Å for monolayered g-C₃N₄; the parameters are a = b = 3.95 Å for monolayered BiOBr. These results agree well with the experimental and theoretical results (Zhao and Dai, 2014; Bai et al., 2016; Zhu et al., 2018). According to previous research, S atom is more inclined to substitute pyridine nitrogen on g-C₃N₄ (Wang et al., 2018; Ghashghaee et al., 2020). Therefore, the doped g-C₃N₄ is constructed by replacing the 2-fold coordinated N atom with S atom (Figure 1B). The calculated lattice constants are a = b = 7.15 Å. Compared with the pristine g-C₃N₄, the S−C bonds in S-doped g-C₃N₄ are slightly longer than the corresponding N−C bonds because of the larger atomic radius of S atom than that of N atom. Figures 2A,B are the top and side view of optimized BiOBr/g-C₃N₄ heterojunction; Figures 2C,D depict the top and side view of optimized BiOBr/S-g-C₃N₄ heterojunction. The equilibrium distance between g-C₃N₄ and BiOBr in BiOBr/g-C₃N₄ is 2.66 Å, while it is 2.64 Å in BiOBr/S-g-C₃N₄. The equilibrium distance of these two heterojunctions exhibits the feature of van der Waals (vdW) heterojunction, indicating that a vdW interaction is established between (S-doped) g-C₃N₄ and BiOBr. Moreover, it can be seen clearly that the g-C₃N₄ in both heterojunctions changes from a planar structure to a curved structure, indicating there is an interaction between g-C₃N₄ and BiOBr.

FIGURE 1

FIGURE 1. Optimized geometric structures of monolayered g-C₃N₄, S-g-C₃N₄, and BiOBr. The C, N, S, Bi, O, and Br atoms are represented by the grey, blue, yellow, purple, orange, and brown balls, respectively.

FIGURE 2

FIGURE 2. Optimized geometric structures of BiOBr/g-C₃N₄ and BiOBr/S-g-C₃N₄ heterojunctions. (A) and (C) are the top view; (B) and (D) are the side view.

In addition, the thermodynamic stability of BiOBr/(S-doped)g-C₃N₄ heterojunctions is evaluated by calculating the formation energy based on the following equations:

$E_{\text{F}}=E_{\text{H}}-E_{\text{A}}-E_{\text{B}}(2)$

where E_H is the total energy of BiOBr/g-C₃N₄ or BiOBr/S-g-C₃N₄ heterojunction, E_A represents total energy of pure or S-doped g-C₃N₄, and E_B represents the total energy of BiOBr. A more negative value of binding energy suggests a more stable structure. The calculated formation energies of BiOBr/g-C₃N₄ and BiOBr/S-g-C₃N₄ are −0.52 and −0.63 eV, respectively, demonstrating that both the constructed heterojunctions are stable.

Work Function

As an important criterion to judge charge transfer, the work function is equivalent to the gap between Fermi level and vacuum level. It is expressed by the following formula:

$\mathrm{\Phi }=E_{\text{vac}}-E_{\text{F}}(3)$

where E_vac and E_F represent vacuum level and Fermi level, respectively. The work functions of g-C₃N₄, S-doped g-C₃N_4, and BiOBr are obtained and shown in Figure 3. The calculated Φ of g-C₃N₄ is 4.62 eV, consistent with the previous results (Mahmood et al., 2020). After the introduction of S atom, the work function is reduced to 3.96 eV. This is mainly due to the fact that S atoms own more valence electrons than N atoms, which can raise the Fermi level. In addition, it is found that the Fermi level of BiOBr (6.40 eV) is lower than those of g-C₃N₄ and S-g-C₃N₄. Therefore, it can be deduced that the electrons will flow from the pure and S-doped g-C₃N₄ to the BiOBr until reaching the uniform Fermi level.

FIGURE 3

FIGURE 3. The calculated work functions of BiOBr, g-C₃N_4, and S-g-C₃N₄. The red and black dotted lines signify the Fermi level and vacuum energy level, respectively.

Charge Density Difference

To intuitively reflect the charge transfer and separation between different constituents, the charge density difference is calculated by:

$\Delta \rho =\rho \left(\text{BiOBr/}\left(\text{S}‐\text{doped}\right)-\text{g}‐{\text{C}}_3{\text{N}}_4\right)-\rho \left(\left(\text{S}-\text{doped}\right)‐\text{g}‐{\text{C}}_3{\text{N}}_4\right)-\rho \left(\text{BiOBr}\right)(4)$

where ρ[BiOBr/(S-doped)-g-C₃N₄], ρ[(S-doped)-g-C₃N₄], and ρ(BiOBr) are the charge densities of S-doped or pure BiOBr/g-C₃N₄ hybrids, S-doped or pure g-C₃N_4, and BiOBr, respectively. Figure 4 depicts the charge density difference of hybrid systems along Z axis. The charge depiction and accumulation are marked by blue and yellow regions, respectively. It is obvious that the surface of S-doped and pure g-C₃N₄ are mainly covered by the blue region, while the BiOBr surface is dominant by the yellow region. Therefore, the electrons transfer from S-doped and pure g-C₃N₄ to BiOBr in the heterojunctions, which agrees well with the analysis of aforementioned work function. Particularly, the blue and yellow coverage areas in the S-doped hybrids are broader than those in the pure hybrids, indicating that S doping brings about a stronger interface interaction. In addition, the Bader charge is calculated to quantitatively investigate the charge transfer. It is found that there are 0.08 electrons transferring from g-C₃N₄ to BiOBr. As for the S doping counterpart, the number of charge transfers from S-doped g-C₃N₄ to BiOBr increases to 0.15 e. This further confirms that the introduction of S atom to the hybrid can have a great effect on increasing interfacial electron transfer.

FIGURE 4

FIGURE 4. The top and side view of charge density difference in (A) BiOBr/g-C₃N₄ and (B) BiOBr/S-g-C₃N₄. The blue and yellow areas represent charge consumption and accumulation, respectively. The isosurface value was 0.3 e⋅nm⁻³.

Strength of Built-in Electric Field

Electron transfer results in uneven charge distribution at the interface, thus forming a polarized electric field at the hybrid’s interface. As an important physical quality, the electric field has a great correlation with band bending and separation of photogenic electrons and holes. The strength of the built-in field can be evaluated by the following equation:

$E=\frac{P}{\epsilon Sd}(5)$

Herein, P is the dipole moment; ε is the dielectric constant which is equal to 8.85 × 10⁻¹² Fm⁻¹; S represents the surface area of heterojunctions; and d represents the interfacial distance of heterojunctions. The calculated E value for doped and non-doped hybrid is 2.56 × 10⁹ and 0.64 × 10⁹ Vm⁻¹, respectively. Obviously, the electric field intensity of S-doped heterogeneous junction is greatly improved. This improvement is mainly attributed to the magnitude of p value, which is related to the number of electron transfers at the interface. Following the above analysis of charge transfer, the BiOBr/S-g-C₃N₄ exhibits more electron transfer at the interface, thus leading to a larger dipole moment and stronger electric field intensity.

Optical Absorption

To explore the influence of doping and heterojunction construction on light absorption, the absorption coefficient α(ω) is calculated according to equation:

$\alpha \left(\omega \right)=\sqrt{2}\omega \left[ \sqrt{{\epsilon }_1{\left(\omega \right)}^2 + {\epsilon }_2{\left(\omega \right)}^2} \mathrm{-} {\epsilon }_1\left(\omega \right) \right]{ }^{1/2}(6)$

where ε₁ and ε₂ denote the real and imaginary parts of dielectric function, respectively. ω is the optical frequency which determines the dielectric functions. Figure 5 describes the calculated absorption spectra of g-C₃N₄, S-g-C₃N₄, BiOBr/g-C₃N_4, and BiOBr/S-g-C₃N₄. In the visible light range (1.5–3.1 eV), the light absorption intensity is ordered by BiOBr/S-g-C₃N₄ > BiOBr/g-C₃N₄ > S-g-C₃N₄ > g-C₃N₄. There are obvious red shift and enhanced light absorption intensity for S-g-C₃N₄, BiOBr/g-C₃N₄ relative to pure g-C₃N₄. Moreover, the strongest visible light absorption occurs in BiOBr/S-g-C₃N₄ heterojunction which further verifies the positive effect of element doping and heterojunction constructing on optical property. Therefore, both the S doping and heterojunction construction of g-C₃N₄ with BiOBr can improve the visible light absorption.

FIGURE 5

FIGURE 5. Optical absorption coefficients of single layered g-C₃N₄, single layered S-g-C₃N₄, BiOBr/g-C₃N₄, and BiOBr/S-g-C₃N₄ heterojunction.

Photocatalytic Mechanism

Based on the aforesaid results, it is found that the S atom can elevate the Fermi level of g-C₃N_4, thus enhancing the strength of the built-in electric field of BiOBr/g-C₃N₄ heterojunction. To further explore the reasons for photocatalytic activity enhancement, the photocatalytic mechanism is interpreted comprehensively (Figure 6). According to the analysis of work function, before contact, the g-C₃N₄ possesses a higher Fermi level than the BiOBr. Upon contact, the electrons will transfer from g-C₃N₄ to the BiOBr until Fermi level is equalized. The electron flow leads to a built-in electric field pointing to BiOBr and band bending of each component. Upon light illumination, the electrons on the valance band (VB) are excited to the conduction band (CB), leaving the photogenerated holes in the VB. Under the effect of internal electric field and band bending, the holes in the g-C₃N₄ VB will combine with the electrons in the BiOBr CB at the interface. The electrons with strong reduction ability are reserved in the g-C₃N₄ CB for CO₂ reduction; holes with superior oxidation capacity in the BiOBr VB survive for oxidation reactions such as pollutant degradation. Therefore, BiOBr/g-C₃N₄ heterojunction follows the S-scheme photocatalytic mechanism. After the introduction of S atom, the doped heterojunction still follows the S-scheme photocatalytic mechanism. And the main difference between the S-doped and undoped one is their interface interaction.

FIGURE 6

FIGURE 6. (A) and (B) are the charge transfer mechanism in BiOBr/g-C₃N₄ before and after contact, while (C) and (D) are the charge transfer mechanism in BiOBr/S-g-C₃N₄ before and after contact.

Specifically, the S atom enlarges the Fermi level difference between BiOBr and g-C₃N₄ by 0.12 eV, which induces more interfacial electron transfer. Consequently, an increased interfacial field (2.56 × 10⁻⁹ V/m) is established. In general, the elevated Fermi level of S-doped heterojunction generates a stronger interfacial field, which provides a stronger force for the recombination of electrons and holes than the non-doped counterpart. Additionally, it also leads to more effective separation of electrons and holes. To sum up, the introduction of S atom can promote the photocatalytic activity by facilitating charge separation.

CO₂ Reduction

The above analysis indicates that the S atom doping can regulate electric fields and increase light absorption. In this regard, the heterojunction with S doping is expected to be a preferable photocatalyst for CO₂ reduction. Through the investigation of the reaction mechanisms, as well as identification of active sites and reduction products of BiOBr/g-C₃N₄ with or without S doping for CO₂ reduction, we will have a profound understanding of CO₂ reduction reaction.

As the initial step for CO₂ reduction, the absorption of CO₂ attaches importance to the further reduction processes. It is critical to find the active sites for CO₂ molecules. According to the photocatalytic mechanism of the BiOBr/(S-doped)g-C₃N₄ heterojunction, the photogenerated electrons for CO₂ reduction are mainly on the surface of g-C₃N₄. Thus, the CO₂ reaction process proceeds on the (S-doped) g-C₃N₄ side of BiOBr/g-C₃N₄. As shown in Figure 7, various possible adsorption sites on the g-C₃N₄ side of BiOBr/g-C₃N₄ are considered by comparing their adsorption energy. For convenience, different kinds of C and N atoms are labeled as C1, C2, C3, N1, and N2, respectively. It is found that the C2 and N2 atoms exhibit more negative adsorption energy than the rest, indicating higher affinity to CO₂. In addition, after the structure optimization, the CO₂ molecule adsorbed on the C2 site inclines to move over the N2 atom, indicating that the N2 position is the most favorable site for CO₂ adsorption. Thus, the two coordination N atoms are selected to be the active sites for the initial CO₂ adsorption, in agreement with the previous studies (Zhu et al., 2017). For the BiOBr/S-g-C₃N₄, the C2, N2, and S positions are selected as potential active sites. The calculated absorption energy for C2, N2, and S are −0.10, −0.17, and −0.14 eV, respectively. After S doping, judging from absorption energy, the two coordination N atoms still preserve the strongest affinity for CO₂ molecules. Thus, the N2 atom in the BiOBr/S-g-C₃N₄ is also treated as the initial active site. In general, both the pure and S-doped BiOBr/g-C₃N₄ exhibit strong CO₂ adsorption capacity, which is beneficial for the subsequent reduction reaction.

FIGURE 7

FIGURE 7. The adsorption energy of CO₂ at different adsorption sites in BiOBr/g-C₃N₄.

After the CO₂ adsorption, the following reduction processes can proceed by a hydrogenation step on the C or O atom. All the possible intermediates and reaction paths are listed in Figure 8. The most favorable reaction pathways of CO₂ reduction to CH₃OH or CH₄ are obtained by comparing their free energy of each step. For the pure BiOBr/g-C₃N₄, the scheme and the most stable structure of the optimal path for CO₂ reduction are depicted in Figure 9. The first step of the reaction is the hydrogenation of CO₂ into COOH* with a free energy of 1.23 eV. In terms of the whole barrier diagram, this step is deemed to be the rate-limiting step with the highest energy barrier, which is due to the inertness of CO₂ molecules, and similar results have been found in other studies (Zhi et al., 2019). After the generation of COOH*, the following step is related to the acquisition of CO and HCOOH by a dehydroxylation or hydrogenation step of COOH*, respectively. The respective free energy for CO and HCOOH formation is calculated to be −0.51 and −1.12 eV, indicating that both processes are exothermic and spontaneous. Moreover, the more negative ΔG of HCOOH generation manifests that the formation of HCCOH is more competitive relative to that of CO. The desorption ability of the two intermediates is further investigated to evaluate whether they are intermediates or final products. The calculated adsorption energy for HCOOH and CO are −0.78 and −0.14 eV, respectively. The interaction between HCOOH and catalyst is strong due to the more negative adsorption energy, indicating that the HCOOH is more likely to be intermediate for the next reduction. By contrast, the CO inclines to desorb from the catalyst surface. Therefore, we focus the subsequent reaction on the hydrogenation and dehydration of HCOOH. The calculated ΔG from HCOOH to HCO* is 0.64 eV, suggesting an endothermic process. During the process of HCO* protonation, the reaction free energy of H₂CO and CHOH* differs considerably. The energy barrier for H₂CO formation is −0.22 eV, while that for CHOH* is 1.93 eV, which is apparently a non-spontaneous process. Thus, the H₂CO intermediate is more favorable for the next reaction. After comparing the free energy in generating H₃CO* (1.23 eV) and CH₂OH* (0.05 eV), the CH₂OH* is selected for the latter reaction. It is worth noting that the two products, CH₃OH and CH_4, share the same reaction path before the formation of CH₂OH*. After that, the product of CH₃OH is obtained through a hydrogenation at the C of CH₂OH*; while the CH₄ is generated by experiencing the CH₂OH*, CH₂*, CH₃*, and CH₄ reaction path. Moreover, the rate-determining step for CH₄ and CH₃OH generation on the pure and S-doped heterojuncions is the same, to explore the reaction selectivity, the free energy of each reaction step is further compared. It is obvious that the protonation of CH₂OH to CH₃OH is a spontaneous exothermic process with a free energy of −0.51 eV, while the highest energy barrier of CH₂OH to CH₄ is 0.98 eV. Thus, compared with CH₄, CH₃OH is more liable to be the final product. On the whole, among the three reduction products, i.e. CO, CH₃OH, and CH₄, the BiOBr/g-C₃N₄ heterojunction showed stronger selectivity to CO and CH₃OH than CH₄.

FIGURE 8

FIGURE 8. The possible reaction paths and intermediates of CO₂ reduction by hydrogenation of C and O. The red part is the optimal reaction path on the BiOBr/g-C₃N₄ and BiOBr/S-g-C₃N₄.

FIGURE 9

FIGURE 9. (A) Optimized structure of reaction intermediates and (B) free energy diagram on the optimal path of CO₂ reduction on BiOBr/g-C₃N₄. The blue and red dotted lines indicate optimal and infeasible reaction paths, respectively. The white spheres represent H atoms.

Figure 10 shows the favorable free energy scheme and the most favorable adsorption geometries of the optimal path for CO₂ reduction on BiOBr/S-g-C₃N₄. It is found that the reaction pathways for CO₂ reduction to CO, CH₃OH, and CH₄ on S-doped hybrid are the same as those on the undoped counterpart but the reaction energy of each step is quite different. The respective optimal reaction path for the CO and CH₃OH formation on the doped system still proceeds sequentially in the following order: CO₂, COOH*, CO; CO₂, COOH*, HCOOH, HCO*, H₂CO, CH₂OH*, and CH₃OH. However, the formation of CH₄ is obtained according to CO₂, COOH*, HCOOH, HCO*, H₂CO, CH₂OH*, CH₂*, CH₃*, and CH₄. For CO generation, each step of the reaction is endothermic with a rate-determining energy of 0.46 eV. It is clear that the rate-determining step for CH₃OH formation on doped hybrid is the last step, namely, conversion of CH₂OH* into CH₃OH. Unlike it, the highest energy barrier for CH₃OH on pure hybrid occurs at the first step. And the BiOBr/S-g-C₃N₄ possesses lower rate-determining energy (0.44 eV) than the undoped hybrid (1.24 eV). Thus, S-doped hybrid is more conductive to the CH₃OH formation. Moreover, in the process of CO, CH₃OH, and CH₄ generation, the energy barrier of the rate-determining step of CO and CH₄ is larger than that of CH₃OH. Therefore, the BiOBr/S-g-C₃N₄ also inclines to reduce CO₂ into CH₃OH rather than CO and CH₄.

FIGURE 10

FIGURE 10. (A) Optimized structure of reaction intermediates and (B) free energy diagram on the optimal path of CO₂ reduction on BiOBr/S-g-C₃N₄.

Conclusion

In this work, DFT calculations are adopted to investigate the geometric structure, electronic properties, and CO₂ reduction mechanism of pristine and S-dopedBiOBr/g-C₃N₄. The charge density difference demonstrates the existence of a built-in electric field pointing to BiOBr, and the formation of S-schemeheterojunction is validated. Bader charge analysis indicates that more electrons transfer in the doped hybrids compared with the pristine one. Integrated with the calculation of the strength of thebuilt-infield, it can be inferred that S doping can enhance the interfacial electric field intensity. The improved electronic interaction is on account of the elevation of the Fermi level by S doping. Also, the computed optical absorption coefficient indicates improved visible-light absorption after S doping and construction of heterojunction. Through the investigation of CO₂ reduction, it is found that both the pure and S-doped hybrids prefer to reduce CO₂ to CH₃OH rather than CH₄. Moreover, compared with the pure heterojunction, the BiOBr/S-g-C₃N₄ exhibits a more superior CO₂ reduction activity towards CH₃OH due to the lower limiting energy. Therefore, the heterojunctions construction and nonmetal doping can lower CO₂ reduction energy barrier, which will ultimately improve the photocatalytic activity. This work will provide a theoretical basis for the design of g-C₃N₄-based photocatalysts.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.

Author Contributions

LZ: Conceptualization, Writing-Reviewing and Editing, Supervision. JY: Conceptualization, Methodology, Supervision. BZ: Software, Validation.

Funding

This work was supported by the National Key Research and Development Program of China (2018YFB1502001), the National Natural Science Foundation of China (51872220, 21905219, 51932007, U1905215, 21871217, and U1705251), the Fundamental Research Funds for Central Universities (2021IVA137), National Postdoctoral Program for Innovative Talents (BX20180231), China Postdoctoral Science Foundation (2020M672432), and the Hubei Postdoctoral Program for Innovative Research Post.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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