DFT Study on Sulfur-Doped g-C3N4 Nanosheets as a Photocatalyst

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 3

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A DFT Study on Sulfur-Doped G-CN Nanosheets as Photocatalyst for CO Reduction Reaction 2

Yuelin Wang, Yu Tian, LiKai Yan, and Zhongmin Su J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00098 • Publication Date (Web): 21 Mar 2018 Downloaded from http://pubs.acs.org on March 27, 2018

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The Journal of Physical Chemistry

A DFT Study on Sulfur-Doped g-C3N4 Nanosheets as Photocatalyst for CO2 Reduction Reaction Yuelin Wang, Yu Tian, Likai Yan* and Zhongmin Su* Institute of Functional Material Chemistry, National & Local United Engineering Lab for Power Battery, Key Laboratory of Polyoxometalate Science of Ministry of Education, Faculty of Chemistry, Northeast Normal University, Changchun 130024, P. R.

China,

Fax:

+86-431-5684009;

E-mail:

[email protected];

[email protected].

ABSTRACT: Graphitic carbon nitride (g-C3N4) can be used as photocatalyst to reduce CO2. Doping is an efficient strategy for improving the photocatalytic activity and tuning the electronic structure of g-C3N4. The sulfur-doped g-C3N4 (S-doped g-C3N4) as a promising photocatalyst for CO2 reduction was investigated by density functional theory (DFT) methods. The electronic and optical properties indicate that doping S enhances the catalytic performance of g-C3N4. From the reduction Gibbs free energies, the optimal path for CO2 reduction reaction to CH3OH production catalyzed by S-doped g-C3N4 is CO2 → COOH* → CO → HCO* → HCHO → H3CO* → CH3OH. In comparison with g-C3N4, doping S can alter the rate-determining step and reduce the Gibbs free energy from 1.43 eV to 1.15 eV. CO2 reduction activity of S-doped g-C3N4 is better than that of g-C3N4, which is in well agreement with the experimental results. Our work provides useful insights into designing nonmetal-doped g-C3N4 for photocatalytic CO2 reduction reactions. 1 ACS Paragon Plus Environment

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1. INTRODUCTION Fossil fuels are main energy sources and materials for a variety of chemical products. However, the use of fossil fuels leads to increasing accumulation of greenhouse gas, CO2, resulting in the serious environmental problems.1,2 Therefore, finding effective way to reduce CO2 emissions or efficiently capture CO2 and CO2 reduction is the key to solve the problem.2 Nevertheless, CO2 is a stable and relatively inert compound, the conventional methods (adsorption, ultrafiltration, coagulation, etc.)

1−4

rely on

high-energy input for high-temperature or high-pressure conditions. In addition, the use of alternatives or carbon-free energy sources to reduce CO2 emissions and capturing CO2 are expensive and difficult. Recently, photocatalytic CO2 reduction by using semiconductors has become hot issue as photocatalysts occur under relatively mild conditions with lower energy input, especially the reaction is activated by solar energy or other easily obtained light sources.5−7 The mechanism of photocatalysis can be described as following: (1) electrons transfer from the valence band (VB) to the conduction band (CB) under light irradiation, (2) the electron-hole pairs (e-/h+) are generated, and (3) CO2 can be reduced into various reducing substances by electron−hole pairs.2,5,6 The products for CO2 reduction generally depend on the properties of catalysts and the reaction conditions.8−10 The reduction of CO2 to products such as methanol (CH3OH), formaldehyde (HCHO), formic acid (HCOOH) and trace amounts of methane (CH4) by using semiconductors TiO2, 8 ZnO,11 WO3,12 Fe2O3,13 GaN,14 and g-C3N415−18 have been reported and extensively investigated. 2 ACS Paragon Plus Environment

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Graphitic carbon nitride (g-C3N4) with the band gap of 2.7 eV19 is a metal-free photocatalyst for CO2 reduction.15−18 Unfortunately, there are few concerns on CO2 reduction by using pristine g-C3N4 due to the fast recombination of photogenerated holes and electrons5 and low photocatalytic efficiency. For improving the photocatalytic efficiency of pristine g-C3N4, many works have been carried out including structure optimization, doping modification and composite semiconductor.20 The nonmetal doping of g-C3N4 keeps the metal-free photocatalytic system.5 The experimental studies demonstrated that nonmetal doping, such as P, B, C, O, and S doping was an effective way to enhance the photocatalytic performance of g-C3N4.21 Sarga et al.22 found that boron-doped g-C3N4 coated with Rh as a co-catalyst show higher photocurrent response under solar light irradiation, and its photocurrent is about 10-times larger than that of g-C3N4. The H2 evolution photoreactivity of S-doped g-C3N4 are 7.2 and 8.0 times higher than g-C3N4 under λ > 300 nm and 420 nm, respectively.23 Wang et al.24 reported that the CH3OH was the main product for CO2 reduction by S-doped g-C3N4 catalyst, and the CH3OH yields catalyzed by S-doped g-C3N4 and g-C3N4 were 1.12 and 0.81 μmol·g−1, respectively. Based on these results, six-electron CO2 reduction reaction (CO2RR) by S-doped g-C3N4 for yielding CH3OH as main product in aqueous solution has been considered,25 while the detailed mechanism needs to be resolved. In this work, we investigated the possible reaction paths for CO2 reduction to CH3OH using S-doped g-C3N4 catalyst. The Gibbs free energy changes (ΔG) were calculated to understand the optimal reaction path for CO2 reduction to CH3OH. And 3 ACS Paragon Plus Environment


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the influence of dopant S atom on the structure, electronic and optical properties as well as photocatalytic CO2 reduction activity was investigated and discussed. 2. CALCULATION METHODS Density functional theory (DFT) calculations provide a unique tool for investigating catalytic reaction paths by examining individual elementary reaction steps.25 The mechanism for CO2RR catalyzed by S-doped g-C3N4 has been studied by DFT methods, which are implemented in DMol3 code.26,27 Supercell (2 × 2 × 1) was repeated periodically on the x-y plane, and a vacuum region of 20 Å was applied along the z-direction and enough to avoid the spurious interactions between repeating slabs. The generalized gradient approximation (GGA) with the functional of Perdew−Burke−Ernzerhof functional (PBE)28 was utilized for all geometric optimization. Effective core potentials with double-numeric quality basis were employed for the description of core electrons. The PBE+D2 method with the Grimme van der Waals (vdW) correction29 was employed due to the weak interactions between CO2RR species and catalyst. The convergence criteria for structure optimization were set to: (1) energy tolerance of 1.0 × 10-5 Ha, (2) maximum force tolerance of 0.002 Ha/Å, (3) maximum displacement tolerance of 0.005 Å and (4) Monkhorst-Pack k-point sampling: 5 × 5 × 1. The change in ΔG30,31 was defined as: ΔG = ΔE + ΔEZPE - TΔS + ΔGpH + ΔGU

(1)

where ΔE is the change of reaction energy directly obtained from DFT total energies, ΔEZPE is the change of zero-point energy, T is the temperature (298.15 K), and ΔS is the change in entropy. ΔGU = –neU, where n is the number of transferred electrons 4 ACS Paragon Plus Environment

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and U is the electrode potential. ΔGpH is the correction of the H+ free energy by the concentration, ΔGpH = kBT × ln10 × pH, where kB is the Boltzmann constant, and the value of pH was set to be zero for acidic condition. Zero-point energies and entropies of the CO2RR intermediates were computed from the vibrational frequencies. The entropies and vibrational frequencies of molecules in the gas phase were taken from the NIST database, while the vibrational frequencies of adsorbed intermediates were computed to obtain ZPE contribution in the free energy expression. The vibrational modes of adsorbate were computed explicitly, while the catalyst sheet was fixed (assuming that vibrations of the substrate are negligible). The adsorption energies (Eads)32 of the adsorbed CO2 , CH3OH and intermediates on g-C3N4 and S-doped g-C3N4 were calculated by following equation: Eads = EA-S - ES - EA

(2)

where EA−S, ES, and EA are the total energies of adsorbate−substrate (A−S) complex, substrate (S), and absorbate (A), respectively. The band gap, partially density of states (PDOS) and the absorption spectra were simulated with plane-wave ultrasoft (PWUS) pseudopotential method as implemented in the Cambridge Sequential Total Energy Package (CASTEP) code.33,34 In order to obtain more accurate band gaps, the band structures and partially density of states (PDOS) were performed with the Heyd-Scuseria-Ernzerhof (HSE06)35,36 hybrid functional. The absorption spectra were obtained with PBE functional. 3. RESULTS AND DISCUSSION

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3.1. Geometric Structures. The optimized structures of g-C3N4 and S-doped g-C3N4 are shown in Figure 1. In experiments, S-doped g-C3N4 was synthesized by replacing N atoms of g-C3N4 with S atoms.23,24 In this work, the substituted doped models of g-C3N4 were constructed with three different doped sites (Ncen, NAro, and NTet) in which Ncen atom is 3-fold coordinated with three C atoms in C6N7-unit, NAro is 2-fold coordinated with two C atoms in C6N7-unit, and NTet atom is connecting with three nearest heptazine rings. As NTet atom plays an important role in mediating three nearest heptazine rings,37 so NTet is not considered as doping site. To evaluate the possible doping sites, the formation energies (Eform)21, 38, 39 of the doping systems by substituting Ncen or NAro with S atom were calculated. Eform is defined as follows: Eform = E(S-C3N4) - E(g-C3N4) - μ(S) - μ(N)

(3)

where E(S-C3N4) is the total energy of S-doped g-C3N4 system, E(g-C3N4) is the total energy of g-C3N4, μ(S) and μ(N) are the chemical potentials of single S and N atoms, respectively. In addition, μ(N) or μ(S) is the energy per N or S atom in its reference phase, μ(N) is defined as μ(N2)/2. The Eform values of NAro-site and NCen-site doping systems are −2.98 eV and −1.59 eV, respectively. So the formation of NAro-site doping system is more stable than NCen-site doping system, which is in well agreement with the theoretical and experimental study.23 Thus, the NAro-site doping system is considered as a catalyst for CO2RR in present work. The first step for CO2RR is CO2 hydrogenation to generate COOH*. In order to investigate the influence of S-doped g-C3N4 layers on the adsorption, the adsorption energies of COOH* on monolayer and bilayer S-doped 6 ACS Paragon Plus Environment

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g-C3N4 were computed and the results show that the adsorption energy difference between monolayer and bilayer of S-doped g-C3N4 is only 0.1 eV. So the monolayer S-doped g-C3N4 is considered in present work.

Figure 1. Top and side views of geometric structure of g-C3N4 and top view of the possible doping sites (NCen, NAro, NTet) (a). Top and side views of geometric structure of S-doped g-C3N4 and the possible absorption sites (1, 2 and 3) (b). The gray, blue and yellow balls represent C, N and S atoms, respectively. 3.2. Electronic properties of g-C3N4 and S-doped g-C3N4. The band structures and PDOS of g-C3N4 and S-doped g-C3N4 were calculated by HSE06 method and shown in Figure 2 and 3. It should be mentioned that the calculated band gap of g-C3N4 (2.80eV) by HSE06 functional is similar with experimental data (2.70 eV)19. The band gap of S-doped g-C3N4 is 2.42 eV, which is closed to the experimental data



(2.55eV)40, which is smaller than that of g-C3N4. Comparing to g-C3N4, the band gap of S-doped g-C3N4 is decreased by doping S atom. For g-C3N4, the PDOS shows that the VB is mainly dominated by N atoms, and the CB is mainly contributed by C atoms and small amounts of CB are from N atoms (Figure 3a). For S-doped g-C3N4, the VB is composed of C, N and S atoms and the CB is composed of C and N atoms (Figure 3b). From the PDOS of g-C3N4, it can be seen that the Fermi level adjoins the VB. However, the Fermi level of S-doped g-C3N4 shifts to the bottom of CB, indicating that S-doped g-C3N4 possesses an n-type doping system, which may have more electron with reduction ability to improve CO2 reduction ability.41,42 The lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of g-C3N4 are shown in Figure 4. The LUMO mainly distributes on C atoms and moderately consists of N atoms, and the HOMO localizes on all N atoms, which conforms to the PDOS. In other words, no electrons would be excited from bridging N atoms under light irradiation, and the photogenerated electrons neither migrate to bridging N atoms nor transfer from one heptazine (C6N7) unit to the adjacent unit through bridging N atoms. It means that the excited e-/h+ is limited in per triazine unit, so g-C3N4 has weak photocatalytic efficiency. The LUMO and HOMO of S-doped g-C3N4 are shown in Figure 4. The HOMO mostly locates on N atoms except the N atoms around S atom. On the contrary, the LUMO mainly locates on S atom. The results are consistent with the analysis on PDOS that the S atom is involved in CB. Overall, the orbital distributions of g-C3N4 significantly change upon S doping, and


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thus it will have better ability to separate the photogenerated e-/h+ pairs and enhance the photocatalytic efficiency.

Figure 2. Band structure of g-C3N4 (a), S-doped g-C3N4 (b).

Figure 3. The PDOS of g-C3N4 (a) and S-doped g-C3N4 (b).



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Figure 4. The LUMOs and HOMOs of g-C3N4 and S-doped g-C3N4. 3.3. Band edge positions of g-C3N4 and S-doped g-C3N4. The semiconductor acting as a photocatalyst for CO2 reduction must have suitable band edge positions that match the potential of CO2/hydrocarbons. The VBM and CBM edge positions of the g-C3N4 and S-doped g-C3N4 with relative to the NHE potential were calculated using the work function method43,44: EVBM = -Φ + 0.5 Eg

(4)

ECBM = -Φ - 0.5 Eg

(5)

E'CBM/VBM = -ECBM/VBM (pH = 7) - 4.5


(6)

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where Φ is the work functions, Eg is the band gap, E'CBM/VBM is the potential vs. NHE (PH=7). The work functions of g-C3N4 and S-doped g-C3N4 calculated by using HSE06 functional are 4.85 eV and 4.60 eV, respectively (Figure 5a and b). As shown in Figure 5c, the CBM of g-C3N4 and S-doped g-C3N4 are at −1.05 and −1.11 V, which are above the CO2/CH3OH potential. However, the VBM of g-C3N4 and S-doped g-C3N4 are at 1.75 and 1.31 V, which are below the CO2/CH3OH potential. Therefore, g-C3N4 and S-doped g-C3N4 can be photocatalysts for CO2 reduction to CH3OH. The CBM of S-doped g-C3N4 is 0.06 V above the CBM of g-C3N4. Thus, the S-doped g-C3N4 is better for CO2 reduction than g-C3N4 due to its strong reducing ability.

Figure 5. The work function for g-C3N4 (a) and S-doped g-C3N4 (b). Band edge position of the CBM and VBM with CO2/CH3OH potential calculated by HSE06 functional (c).



3.4. Optical properties of g-C3N4 and S-doped g-C3N4. The optical absorption curves of g-C3N4 and S-doped g-C3N4 are plotted in Figure 6. For g-C3N4, there is a strong absorption peak near 350 nm and the wavelength of optical absorption edge (λedge) is near 475 nm. Compared to g-C3N4, the optical absorption edge of S-doped g-C3N4 red-shifts by 75 nm, indicating that S-doped g-C3N4 can expend the visible-light response range. From the optical absorption intensity of g-C3N4 and S-doped g-C3N4 in visible light, the integrated area of optical absorption curves in visible light (about 400 nm−780 nm) are analyzed. Compared with the g-C3N4, the absorption intensity of doping S increases in the visible-light region due to the large dispersion of CB and impurity level in the band gap induced S atom. The absorption curve area of S-doped g-C3N4 is larger than that of g-C3N4, predicting that more electrons of S-doped g-C3N4 could be generated upon the visible light irradiation. It suggests that the photocatalytic ability is enhanced by S doping.

Figure 6. The optical absorption behaviors of g-C3N4 (a) and S-g-C3N4 (b). 3.5. Catalytic performance for the CO2 reduction reaction on g-C3N4. Azofra et al.45 proposed that the reaction paths for CO2 reduction catalyzed by g-C3N4 is as


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following, CO2 → COOH* → CO → HCO* → HCHO → H3CO* → CH3OH. According to this reaction path, the Eads of CO2, COOH, CO and CH3OH on g-C3N4 were computed by Dmol3 code and shown in Table 1. All possible adsorption sites of CO2, COOH, CO and CH3OH were considered and the most stable sites were determined. The results show that the COOH and HCO prefer to bind on the NAro atom, while CO2, CO, HCHO, CH3O and CH3OH adsorb on C site linking with S atom, which is in well agreement with the previous theoretical study.45 The negative Eads indicates that the adsorption is an exothermic process in all instances. In generally, the Eads of adsorbates are all larger than 1 eV, corresponding to strong chemisorption46,47. As shown is in Table 1, the Eads of CO2, CO, CH3OH are −0.09 eV, −0.23 eV and -0.41 eV, respectively, indicating that the adsorption are between weak chemisorption and strong physisorption, which can be proved by long distance between CO2, CO or CH3OH and g-C3N4. Table 1. The calculated Eads and the shortest distance (d) of CO2, COOH*, CO and CH3OH with g-C3N4 and S-doped g-C3N4.

CO2

COOH

CO

CH3OH

Eads (eV) d (Å)

Eads (eV) d (Å)

Eads (eV) d (Å)

Eads (eV) d (Å)

g-C3N4

−0.09

3.03

−1.61

1.43

−0.23

3.18

−0.41

2.76

S-C3N4

−0.15

2.99

−1.86

1.58

−0.13

3.13

−0.24

2.84

The calculated free energy diagrams to CO2 reduction through above reaction path on g-C3N4 is summarized in Figure 7a. For CO2RR, the initial reduction through a proton−electron transfer step results in O−H bond formation to produce a COOH* 13 ACS Paragon Plus Environment


intermediate. As shown in Figure 7a, the ΔG for CO2 hydrogenation to COOH* is uphill by 1.41 eV. Once COOH* is obtained, the thermodynamic profile indicates that the formation of CO, HCHO, and CH3OH along the second, fourth, and sixth steps are characterized by the release of energy. On the contrary, the formation of HCO* and CH3O* demand the input of energy by 0.49 and 1.43 eV, respectively.

Figure 7. Calculated free energy diagram to the reaction paths followed by the CO2 conversion on g-C3N4 (a) and S-doped g-C3N4 (b). 3.6. Catalytic performance for the CO2 conversion path on S-doped g-C3N4. S-doped g-C3N4 was synthesized and applied to CO2 photocatalytic reduction into CH3OH.23,24 Based on the experiments, the four possible reaction paths for CO2 reduction to CH3OH by S-doped g-C3N4 were considered and shown in Scheme 1. CO2 → COOH* → CO → COH* → CHOH* → CH2OH* → CH3OH (Path I), CO2 → COOH* → CO → HCO* → CHOH* → CH2OH* → CH3OH (Path II), CO2 → COOH* → CO → HCO* → HCHO → CH2OH* → CH3OH (Path III) and CO2 → COOH* → CO → HCO* → HCHO → CH3O* → CH3OH (Path IV). In these four


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paths, the first and second steps are CO2 → COOH* and COOH* → CO, respectively. Both of these two steps involve O−H bond formation. For Path I and II−IV, the major branching point for determining whether CO is reduced to a COH* or a HCO* intermediate through the formation of a O−H or C−H bond, respectively. The HCO* is reduced to CHOH* through the formation of a O−H bond in Path II, while the HCO* is reduced to HCHO through the formation of a C−H bond in Path III and IV. For Path III and IV, the major branching point depends whether CH2OH* or CH3O* is generated. The last step of four paths is the formation of CH3OH.

Scheme 1. Proposed four reaction paths for CO2 reduction on S-doped g-C3N4, producing CH3OH.

*

symbol shows that the intermediate can be adsorbed on the

substrate. For S-doped g-C3N4, three absorption sites are considered including 1 (S), 2 (N) and 3 (C) site, respectively (see Figure 1b). By calculating and screening the Eads, the CO, HCHO, CH3O prefer to absorb on C site, while N site favorably facilitates the adsorption of the other intermediates, which are same as the absorption sites on g-C3N4. The shortest distance between CO2, COOH, CO, CH3OH and two catalysts are shown in Table 1 and Figure 8. Compared with g-C3N4 and S-doped g-C3N4, the OA−CS bond distance between CO2 and substrate is 3.03 Å (g-C3N4) and 2.99 Å



(S-doped g-C3N4), respectively, indicating that the adsorptions of CO2 on two substrates are similar. For COOH* adsorption, the CA−NS bond distance increases from 1.43 Å for g-C3N4 to 1.58 Å for S-doped g-C3N4, while the CA−CS distance for CO adsorption decreases from 3.18 Å (g-C3N4) to 3.13 Å (S-doped g-C3N4), and the CA−OS bond distance between CH3OH and two substrates increases from 2.76 Å (g-C3N4) to 2.84 Å (S-doped g-C3N4), where A and S are absorbate and substrate, respectively. It implies that CO easily adsorb on S-doped g-C3N4 and CH3OH easily desorbs from S-doped g-C3N4 in comparison with g-C3N4. The Eads of CO is -0.13eV, suggesting that the CO can desorb from the catalysts. While the main product of CO2 reduction was CH3OH in the experiment24, so the CO can be further hydrogenated. Thus, the desorption and hydrogenation of CO on S-doped g-C3N4 nanosheets are considered.

Figure 8. Calculated the structures corresponding to the optimal reaction path followed by the CO2 conversion on g-C3N4 (a) and S-doped g-C3N4 (b). Selected distances are shown in Å. Chemisorbed (bound) species are indicated by full bonds, while physisorbed species are indicated by dashed bonds. 16 ACS Paragon Plus Environment

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As is shown in Figure 7b, the common steps for four reaction paths are CO2 → COOH* and COOH* → CO with free energy changes by 1.15 eV and −0.57 eV, respectively. In addition, COOH* can also be hydrogenated to HCOOH, the calculation free energy change of COOH* → HCOOH is -0.93eV, indicating HCOOH is one of the CO2RR product. The key step is the hydrogenation of CO to COH* (Path I) versus to HCO* (Path II−IV). The free energy change of CO → COH* is 1.99 eV and larger than that of CO → HCO* (0.47 eV). Hence, the reactions of Path II−IV are relatively easier than Path I. The hydrogenation position of HCO* for Path II−IV is different. The ΔG for the formation of CHOH* in Path II is uphill by 0.33 eV, while the ΔG of HCO* → HCHO step in Path III and IV is −0.65 eV. The result suggests that the HCO* intermediate will be reduced to HCHO rather than CHOH*. The key selectivity step between Path III and Path IV is the hydrogenation of HCHO to CH2OH* (Path III) versus to CH3O* (Path IV). The ΔG of HCHO → CH2OH* reaction in Path III is uphill in the free energy profile by 0.45 eV, while HCHO → CH3O* reaction in Path IV is uphill by 0.03 eV. So the CO2RR follows Path IV is easier than Path III. The ΔG of the last step, CH3O* → CH3OH is −0.68 eV. As shown in Figure 7b, The rate-determining step in Path IV is CO2 → COOH* with the free energy change by 1.15 eV. Among the proposed four paths, the Path IV is easier. Apart from the catalytic activity and overpotential reduction, the competing hydrogen evolution reaction (HER)46 should be considered. The hydrogen evolution reactions in three different sites (N, C, S) was considered. In Figure 9a, we can see that the optimal adsorption site is C atom with the HER free energy of -0.08eV. The 17 ACS Paragon Plus Environment


free energy is lower than that of the rate-determining step of CO2 reduction, indicating that HER side reaction is easy to react. However, the HER side reaction can be suppressed by adjusting the electrolyte pH47. Therefore, it is possible to minimize the influence of the HER under experimental adjustments.

Figure 9. The energy profile for HER on the S-doped g-C3N4. To verify the effect of sulfur doping, the CO2RR path by g-C3N4 and S-doped g-C3N4 are compared and shown in Figure 10. It can be seen that the rate-determining step catalyzed by g-C3N4 is HCHO → CH3O* with ΔG of 1.43 eV. Sulfur doping alters the rate-determining step as CO2 → COOH* and the ΔG of rate-determining step is reduced to 1.15 eV, indicating that the CO2RR catalyzed by S-doped g-C3N4 is more favorable than g-C3N4. So the sulfur doping can enhance the photocatalytic activity of g-C3N4.


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Figure 10. Calculated free energy diagram corresponding to the optimal reaction path followed by the CO2 conversion on g-C3N4 and S-doped g-C3N4. 4. CONCLUSION In summary, this work represents a comprehensive study of CO2 reduction reaction on g-C3N4 and S-doped g-C3N4 based on DFT calculations. The influence of S doping on electronic, optical properties and catalytic performance in CO2 reduction reaction was investigated. The band gap of S-doped g-C3N4 is smaller than that of g-C3N4, predicting that the adsorption spectra of S-doped g-C3N4 red-shifts compared to pristine g-C3N4. The HOMO of S-doped g-C3N4 mainly locates on N atoms, which are away from S atom, while the LUMO mainly locates on S atom. The result suggests that S-doped g-C3N4 as a photocatalyst is favorable to promote the charge separation, inhibits the electron−hole recombination, and prolongs the life time of charge carriers, and thus enhances the photocatalytic efficiency in comparison with g-C3N4. The S-doped g-C3N4 exhibits stronger and extended light absorption in the visible light range than g-C3N4. Corresponding to the narrow band gap of S-doped



g-C3N4, more electrons could be generated upon the visible light irradiation, which would enhance the photocatalytic ability for CO2RR. The DFT calculations demonstrate that the optimal reduction path for CO2 reduction into CH3OH catalyzed by S-doped g-C3N4 is CO2 → COOH* → CO → HCO* → HCHO → CH3O* → CH3OH. The rate-determining step is CO2 → COOH* with the ΔG of 1.15 eV. Comparing with g-C3N4, the photoactivity is enhanced by S doping. The present findings could shed light on the design of efficient photocatalysts and provide important information for the photocatalytic CO2 reduction fuel production. ACKNOWLEDGMENTS The authors gratefully acknowledge financial support by NSFC (21403033, 21571031). We acknowledge Institute of Theoretical Chemistry, Jilin University for providing the computational resources for this work. REFERENCES (1) Li, Q. J.; Wei, Z.; Guo, H. T.; Hong, G. F. Surface Tuning for Oxide-Based Nanomaterials as Efficient Photocatalysts. Chem. Soc. Rev. 2013, 42, 9509−9549. (2) Phairat, U.; Dena, M.; Amornvadee, V.; Paitoon, T. Photocatalytic Process for CO2 Emission Reduction from Industrial Flue Gas Streams. Eng. Chem. Res. 2006, 45, 2558−2568. (3) Hideaki, N. Recent Organic Pollution and Its Biosensing Methods. Anal. Methods 2010, 2, 430−444.


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DFT Study on Sulfur-Doped g-C3N4 Nanosheets as a Photocatalyst

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