g-C3N4 heterojunction with enhanced oxygen evolution reaction activity: A theoretical insight

Journal Pre-proofs Full Length Article Constructing MoS2/g-C3N4 Heterojunction with Enhanced Oxygen Evolution Reaction Activity: A Theoretical Insight Zhe Xue, Xinyu Zhang, Jiaqian Qin, Riping Liu PII: DOI: Reference:

S0169-4332(20)30245-2 https://doi.org/10.1016/j.apsusc.2020.145489 APSUSC 145489

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Applied Surface Science

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Please cite this article as: Z. Xue, X. Zhang, J. Qin, R. Liu, Constructing MoS2/g-C3N4 Heterojunction with Enhanced Oxygen Evolution Reaction Activity: A Theoretical Insight, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc.2020.145489

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© 2020 Published by Elsevier B.V.

Constructing MoS2/g-C3N4 Heterojunction with Enhanced Oxygen Evolution Reaction Activity: A Theoretical Insight Zhe Xue†, Xinyu Zhang*,†, Jiaqian Qin ‡, and Riping Liu† †State

Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, P. R. China

‡Metallurgy

and Materials Science Research Institute, Chulalongkorn University, Bangkok 10330, Thailand

ABSTRACT Developing and designing highly efficient oxygen evolution reaction (OER) electrocatalysts is full of significance to achieve complete water splitting. In this work, using density functional theory (DFT) calculations, we performed a systematic theoretical study of the OER electrocatalytic activity of MoS2/g-C3N4 heterojunction. We find that the construction of heterojunction can efficiently modify the electronic properties of MoS2 phase, facilitate electron transfer, and thereby improving OER activity. In comparison to the pristine MoS2 monolayer, the theoretical overpotential of the MoS2/g-C3N4 heterojunction was significantly reduced nearly 69% from 2.52 to 0.78 V. Such outstanding OER activity mainly originates from the electronic coupling between MoS2 and g-C3N4. Overall, the present findings not only provide a vital insight into the catalytic mechanism of the enhanced OER activity of MoS2/g-C3N4 heterojunction, but also provide a new pathway to develop high-performance OER electrocatalysts for future energy conversion applications.

Keywords: MoS2/g-C3N4 Heterojunction; Oxygen evolution reaction; First-principles calculation; Electrocatalyst; Electrochemical water splitting;

1. Introduction To meet the growing demand for renewable and cleaner energy, electrocatalytic water splitting[1, 2] into hydrogen and oxygen gases with zero emission of carbon dioxide, has been recognized as one of the most attractive and competitive technologies in energy conversion and storage fields. As well known, electrocatalytic water splitting involves two independent half-reactions, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Until now, the state-of-the-art electrocatalysts are mainly Pt based materials[3, 4] for HER and RuO2 and IrO2 based compounds[5-7] for OER, respectively. However, the intrinsic characteristics such as extreme scarcity and high cost of these catalysts, greatly hindered their large-scale application. In this context, developing and designing highly efficient viable alternatives especially made from the cheap and earth abundant elements is extremely desriable. Most importantly, in comparison to the HER, OER is commonly considered as a major bottleneck to achieve highly efficient water-splitting owing to its high overpotential and sluggish kinetics, which are mainly caused by its multistep proton-coupled electron transfer process[8, 9]. In this regard, the rational design of high-performance OER electrocatalysts is full of significance to achieve complete water splitting. Therefore, tremendous efforts have been made to design and discover novel OER catalysts to meet the requirement of renewable and cleaner energy.

At the same time, two-dimensional (2D) Molybdenum disulfide (MoS2), as highefficient and cost-effective HER electrocatalysts has attracted considerable attention in recent years[10-18]. As well known, the coupling of HER and OER catalysts into a single-material for water splitting has become research hotspot in electrocatalytic field during the past decades. However, both theoretical and experimental investigations demonstrated that the layered MoS2 is an excellent HER electrocatalyst[19] but possessing poor OER activity[20]. In this respect, designing and discovering MoS2based electrocatalysts with superior OER activities is highly desirable. Attractively, the construction of MoS2-based heterojunction structures with inherited advantages of each single-phase has been widely regarded as an effective way to accomplish overall water splitting. Among these, numerous MoS2-based heterostructures as highly active bifunctional electrocatalysts were successfully synthesized and utilized for over water splitting.

For

instance,

MoS2/Ni3S2,[21-23]

CoS2/MoS2,[24]

MoS2/NiS,[25]

Co9S8/MoS2[26], Co3O4/MoS2[27, 28] systems. Furthermore, considering the excellent chemical stability, simple production procedure, low cost and earth abundance of 2D layered g-C3N4, in this work we mainly focused on the electrocatalytic activity of the MoS2/g-C3N4 heterojunction for OER. Currently, the application of MoS2/g-C3N4 heterojunction was mainly focused on the field of photocatalysts[29-35]. For example, Yuan et al.[33] recently reported that 2D-2D MoS2/g-C3N4 nanosheets shows an outstanding photocatalytic activity with the H2 generation rate of 1155 μmol·h-1g-1. Moreover, Wu et al.[34] proposed that the superior photocatalytic properties of MoS2/g-C3N4 mainly attributed to the excellent

interfacial heterostructure between MoS2 and g-C3N4. More specifically, their results indicate that the MoS2/g-C3N4 system could significantly extend the visible light adsorption range, inhibit the recombination and meanwhile facilitate the transfer rate of photoproduced charge. In addition, Majumder and co-worker[36] successfully fabricated the mesoporous MoS2/g-C3N4 nanosheets as the sulfur host for lithium-sulfur batteries. On the other hand, the heterostructure of MoS2/g-C3N4 also was successfully utilized as highly efficient HER electrocatalysts[37, 38]. Unfortunately, to our knowledge, the research of the MoS2/g-C3N4 heterojunction as OER electrocatalyst is still lacking. With these considerations in mind, in the present work, a MoS2 based heterojunction electrocatalyst for OER was successfully constructed by coupling the layered g-C3N4 on the vertical direction of MoS2 with consideration of vdW interactions[39, 40]. Moreover, a systematic density function theory (DFT) calculations were performed to investigate the interface structure, electronic properties and electrocatalytic mechanism of the MoS2/g-C3N4 heterojunction. As expected, our theoretical results demonstrate that the construction of the MoS2/g-C3N4 heterojunction can efficiently modify the electronic properties of MoS2 phase by reducing its band gap, which is beneficial for electron transfers. As a result, the OER efficiency of MoS2 has been significantly enhanced by forming the MoS2/g-C3N4 heterojunction.

2. Computational methods In this work, all DFT calculations were performed with the Vienna Ab initio

Simulation Package (VASP)[41-43] using the generalized gradient approximation (GGA) and the Perdew-Burke-Eznerhof (PBE) function[44]. The ion-electron interactions were described by the projector augmented wave (PAW) method[45, 46] with the plane-wave kinetic energy cutoff of 500 eV. Specifically, DFT-D3 correction method in Grimme’s scheme[39, 40] was employed to accurately describe the longrange vdW interactions. The Brillouin zone was sampled using the Monkhorst-Pack method[47] with a 5  5 × 1 -centered k-points mesh. For geometry optimization, all atomic positions, cell shape and volume were fully relaxed for bulk crystals, while only atomic positions relation was allowed for surface models. The convergence criterion for force and total energy were set as 0.02 eV/Å and 10-5 eV per atom, respectively. Considering that PBE functional[44] generally underestimated the band-gap due to the effects of artificial self-interaction, a much denser k-points mesh and HSE06 hybrid functional[48] were applied to accurately obtain the band-edge positions of the MoS2/gC3N4 Heterojunction. Moreover, the Bader charge analysis[49, 50] was employed to evaluate electronic transfer/ redistribution before and after adsorption of intermediates. Herein, the valence electron configurations of 1s1, 2s22p2, 2s22p3, 2s22p4, 3s23p4 and 4p5s4d were treated explicitly for H, C, N, O, S and Mo atoms, respectively. In electrochemistry, the overall OER in acid solution involves four-electron transfer steps, which can be summarized as: 2𝐻2𝑂 → 𝑂2 + 4𝐻 + + 4𝑒 ―

(1)

That is, the elementary reaction steps can be expressed as[51]: 𝐻2𝑂 + ∗ →𝑂𝐻 ∗ + 𝐻 + + 𝑒 ―

(2)

𝑂𝐻 ∗ → 𝑂 ∗ + 𝐻 + + 𝑒 ―

(3)

𝐻2𝑂 + 𝑂 ∗ → 𝑂𝑂𝐻 ∗ + 𝐻 + + 𝑒 ―

(4)

𝑂𝑂𝐻 ∗ → ∗ + 𝑂2 + 𝐻 + + 𝑒 ―

(5)

where * represents an active site on the bare catalysts surface, and OH*, O*, OOH* represent three different catalytic intermediates. Based on these reaction proceeds, the change of free energies ∆𝐺𝑖 (𝑖 = 1, 2, 3, 4) can be obtained by the following expression[52]: (6)

Δ𝐺𝑖 = Δ𝐸 + Δ𝑍𝑃𝐸 ― TΔS + ∆𝐺𝑈 +∆𝐺𝑝𝐻

here ∆E is the adsorption energies of adsorbed intermediates obtained from DFT computations. ZPE and S are the changes of zero-point energy and the entropic contribution, T is the temperature (298.15 K). ∆𝐺𝑢 = ―𝑒𝑈, where U is the electrode potential. ∆𝐺𝑝𝐻 = 𝑘𝐵𝑇𝑙𝑛10 × 𝑝𝐻, where 𝑘𝐵 is the Boltzmann constant and in the present work, pH = 0 was employed. More detailed, the values of ZPE could be derived by the vibrational frequency results, as given: ZPE = 1 2 ∑ℎ𝑣𝑖

(7)

Subsequently, the entropy S of the adsorbed species can be evaluated using the following equation[53]: S(𝑇) =

[

3𝑁 ∑𝑖 = 1

(

―𝑅𝑙𝑛 1 ― 𝑒

ℎ𝑣𝑖 𝐵𝑇

―𝑘

)+

𝑁𝐴ℎ𝑣𝑖 𝑇

𝑒

―ℎ𝑣𝑖 𝑘𝐵𝑇

1―𝑒

―ℎ𝑣𝑖 𝑘𝐵𝑇

]

(8)

Moreover, applying the method proposed by Nørskov et al., the thermodynamic overpotential OER for a given electrocatalyst was determined by[54]: 𝐺𝑂𝐸𝑅 = 𝑚𝑎𝑥{∆𝐺1, ∆𝐺2, ∆𝐺3,∆𝐺4} 𝜂𝑂𝐸𝑅 = 𝐺𝑂𝐸𝑅 𝑒 ―1.23 𝑉

(9) (10)

3. Results and discussion 3.1. Optimized atomic configurations of the MoS2/g-C3N4 heterojunction Before constructing the the MoS2/g-C3N4 heterojunction, the geometry relaxations of the g-C3N4 and MoS2 were first performed in the present work. Based on the settings in Section 2, the calculated lattice constants for MoS2 and g-C3N4 unit cell were 3.18 and 7.17 Å, which are in agreement with other theoretical results. The optimized configurations for MoS2 and g-C3N4 were displayed in Fig. 1a and b. Moreover, to simulate the pristine MoS2 surface, a 4 × 4 supercell including 16 Mo and 32 S atoms was established with a vacuum region of 20 Å, which was sufficiently large avoid the interaction between periodically repeated images. In addition, a 1 × 1 g-C3N4 supercell and a 3 × 3 MoS2 supercell were chosen to construct the MoS2/g-C3N4 heterojunction. Here, the MoS2/g-C3N4 heterojunction was achieved by g-C3N4 and MoS2 monolayers stacked in c-axis direction with consideration of vdW interactions. Noteworthy, in order to obtain a satisfying lattice mismatch, the average values of the two slabs were adopted as the lattice parameters of the MoS2/g-C3N4 heterojunction. Top and side views of the optimized MoS2/g-C3N4 heterojunction are shown in Fig. 1c and d, respectively.

Fig. 1. Schematic illustrations of the optimized structures. (a) g-C3N4 monolayer; (b) MoS2 monolayer; (c) top view of the MoS2/g-C3N4 heterojunction; (d) side view of the MoS2/g-C3N4 heterojunction. The grey, brown, yellow and purple balls refer to N, C, S and Mo atoms, respectively.

3.2. Electronic structures of the MoS2/g-C3N4 heterojunction Electrical conductivity, is a quite important index for an electrode material, which is closely related to many electrochemical reactions. As know, the major hurdle for the applications of MoS2-based materials in the electrochemical field can be attributed to their poor electrical conductivity. Therefore, insight into the intrinsic electronic structure of the MoS2/g-C3N4 interface has a great significance in understanding its electrochemical activity. As displayed in Fig. 2a-c, the total density of states (TDOS) and the projected density of states (PDOS) of the pristine MoS2 and g-C3N4 monolayers as well as the MoS2/g-C3N4 heterojunction were firstly examined using HSE06 functional. The upper panel of Fig. 2a reveals that the pristine MoS2 monolayer is a semiconductor with the band gap Eg of 2.08 eV, which is consistent with previous DFT results [55-57]. Surprisingly, after constructing the heterojunction, the bottom panel of Fig. 2a clearly shows that the band gap of the MoS2 in the supercell is dramatically reduced, and this narrowing bandgap could be interpreted by the down-shift of the valence band maximum (VBM). Similarly, from the PDOS analysis of g-C3N4 monolayer (Fig. 2b), compared to the pristine g-C3N4 monolayer, one can clearly see that both VBM and CBM were noticeably down shift, while this down-shifting of the CBM is much more prominent. As a result, the band gap of the individual g-C3N4 layer in the MoS2/g-C3N4 heterojunction was significantly decreased from 2.65 to 0.98 eV,

as shown in Fig. 2b. Moreover, the TDOS and PDOS analysis of the MoS2/g-C3N4 heterojunction were performed, as illustrated in Fig. 2c. Here, the VB and CB are highlighted by red and blue shadows, respectively. Obviously, the CBM of the g-C3N4 in the supercell lies on the band gap region of the MoS2 layer, resulting in an extremely narrow Eg of 0.21 eV for the MoS2/g-C3N4 heterojunction. That is to say, the electrical conductivity as well as the electron transport properties of the MoS2 phase have been pronouncedly improved by contacting with the g-C3N4 layer, which is beneficial for its application in many electrochemical reactions. In addition, the band edge positions of the pristine MoS2 and g-C3N4 monolayers as well as the combined MoS2/g-C3N4 heterojunction were collected, as depicted in Fig. 2d. In more detail, the 𝐸𝑔 of the pristine g-C3N4 monolayer spans from -5.28 to -2.63 eV with a bandgap of 2.65 eV, which is in excellent agreement with other DFT results[58, 59], whereas the 𝐸𝑔 spans from -3.40 to -1.32 eV for the pristine MoS2 monolayer. As expected, we note that the band gap was dramatically reduced for the case of MoS2/g-C3N4 heterojunction. These results demonstrate that the construction of the MoS2/g-C3N4 heterojunction is certainly an effective way to tune the electronic properties of the MoS2 phase.

Fig. 2. Calculated TDOS and PDOS of (a) the pristine MoS2 monolayer, (b) the pristine g-C3N4 monolayer, and (c) the MoS2/g-C3N4 heterojunction using the HSE06 functional. The Fermi level was set to zero, as marked by the black dash-dotted line. (d) Band-edge positions of the free-standing g-C3N4 and MoS2 monolayers as well as that of the MoS2/g-C3N4 heterojunction. Herein, the vacuum level is taken as zero reference.

Furthermore, the charge transfer and redistribution across the MoS2/g-C3N4 heterojunction were quantitatively analyzed by the planar-averaged differential charge density , which is given as ∆ρ = 𝜌𝑀𝑜𝑆2 here 𝜌𝑀𝑜𝑆2

𝑔 ― 𝐶3𝑁4

𝑔 ― 𝐶3𝑁4

― 𝜌𝑀𝑜𝑆2 ― 𝜌𝑔 ― 𝐶3𝑁4

(11)

, 𝜌𝑀𝑜𝑆2, and 𝜌𝑔 ― 𝐶3𝑁4 are the charge densities of the combined

MoS2/g-C3N4 heterojunction, isolated MoS2 and g-C3N4 monolayers in the same supercell, respectively. According to this definition, the positive/negative values of  denote that the charge accumulation/depletion. The profile of ∆ρ along the z-direction

(perpendicular to the interface) was displayed in Fig. 3a. Here, the charge-accumulation and charge-depletion are depicted as the blue and red regions, respectively. Meanwhile, the purple-dotted lines distinguish the edge position of the MoS2 (4.93 Å) and g-C3N4 (8.27 Å) monolayer in the supercell. Fig. 3a clearly shows that significant charge transfer and redistribution were observed at the MoS2/g-C3N4 interface. More specifically, the charge is transferred from the top S atoms of the MoS2 layer to the N atoms in g-C3N4 layer. In addition, the charge reorganization at the MoS2/g-C3N4 interface was further corroborated by the Bader charge analysis. The results indicated that a considerable amount of charge, about 0.09 e, is transferred from MoS2 to g-C3N4, which is mainly attributed to the strong electronegativity of the N atoms in g-C3N4 slab. Similar charge transfer and redistribution were also been observed in other 2D semiconductors/g-C3N4 heterojunctions. Most importantly, the negatively charged gC3N4 layer can provide active sites for the intermediates (OH*, O* and OOH*) adsorption, which is beneficial for improving the OER performance. Overall, from the viewpoint of electronic structures, the MoS2/g-C3N4 heterojunction can be used as a better candidate OER electrocatalyst.

Fig. 3. (a) Planar average charge density difference along z-direction for the MoS2/g-C3N4 heterojunction. (b) Charge density difference of the MoS2/g-C3N4 heterojunction; the blue and red color denote the charge accumulation and depletion area, respectively, with isosurface values of 0.003 e/Å3.

3.3. OER catalytic activity of the MoS2/g-C3N4 heterojunction As known, an ideal catalyst requires that all energy differences between the two adjacent intermediate states are approximately 1.23 eV. Only in this way can the water oxidation reaction proceed spontaneously just above the equilibrium potential. But in reality, the reaction free energies often exhibit noteworthy differences, which significantly limits the OER process. Of course, as discussed in eqn.(9) and (10), the theoretical overpotential OER for a given electrocatalyst is determined by the largest free-energy barrier. To begin with, the OER performance of the pristine MoS2 monolayer was systematically investigated by the free energy profiles, as depicted in Fig. 4a. Remarkably, in the absence of any external potential (U = 0 V), both the second and the fourth step are downhill with negative values of ∆𝐺2 and ∆𝐺4, which indicates

that the conversions of OH*→O* and OOH*→O2 can spontaneously occur at U = 0 V. Also, we found that the adsorption of O* is relatively strong, whereas the binding strength of OOH* is overly weak when compared with that of the thermochemically ideal OER process, thereby resulting in the third step O*→OOH* being the potential determining step (pds). At equilibrium potential of 1.23 V, the first and the third step still need to overcome a large free-energy barrier of 1.54 and 2.52 eV, respectively. Untill U = 3.75 V was applied, all elementary steps turn out to be downhill and exothermic, yielding a overpotential of 2.52 V. Such large overpotential futher implies that the OER activities of the pristine MoS2 monolayer are extremely poor. Moreover, to fully understand these catalytic properties, the charge density difference, Bader effective charges as well as the relevant bond length parameters for different adsorbed intermediates were summarized separately, as displayed in Fig. 4b. Combining the analysis of the charge density difference (upper panel) and Bader effective charges (middle panel), we note that in the cases of OH* and O*, a considerable number of the electrons were transferred from S to O atoms, suggesting a strong S-O interaction. In contrast, for OOH* adsorption on the pristine MoS2 surface, only a few electron transfers were occurred between the adsorbed OOH* intermediate and the MoS2 substrate, further indicating the overly weak OOH* adsorption. Subsequently, the relevant bond length parameters for the optimized configurations were collected, as seen in the bottom panel of Fig. 4b. Obviously, the extremely short S-O bond length of 1.48 Å for O* adsorption indicates exceptionally strong interaction between the intermediate and the catalyst. Also, we noticed that the OOH* intermediate displays an

exceedingly long S-O bond of 2.82 Å, which is mainly caused by the overly weak OOH* adsorption.

Fig. 4. (a) The free energy profiles of the OER on the pristine MoS2 monolayer; (b) Side views of charge density difference, Bader effective charges and the relevant bond length parameters for different adsorbed intermediates. Here, the electron accumulation and depletion are described by the blue and red regions with the isovalues of 0.003 eÅ-3. The grey, brown, yellow and purple balls refer to N, C, S and Mo atoms, respectively.

Next, we focused our analyses on the optimal reaction sites of the MoS2/g-C3N4 heterojunction, as portrayed in Fig. 5. The predicted free-energy changes at different external potentials for the N active site was plotted in Fig. 5a. Clearly, from Fig. 5a, it could be seen that at U = 0 V, only the ∆𝐺4 of the last step is less negative. At equilibrium potential of 1.23 V, both the first and the third step are uphill with the energy barriers of 0.55 and 0.78 eV, respectively. Apparently, the third step O*→OOH* is the potential determining step (pds), which is essentially caused by the relative weak OOH* adsorption on N site (∆𝐺𝑂𝑂𝐻 ∗ = 5.04 𝑒𝑉). When a larger electrode potential of U = 2.01 V was applied, all elementary steps turn out to be downhill and exothermic, yielding a overpotential of 0.78 V. Similarly, the optimized

free energy profiles of the C@Site at different external voltages of 0, 1.23 and 2.01 V were constructed, as shown in Fig. 5b. Interestingly, when the external potential is 0 V, it is clearly seen that ∆𝐺1 of the first step is negative, which implies that the formation of OH* from adsorbed H2O molecule at C active site can spontaneously occur. Moreover, at equilibrium potential of 1.23 V, the first two steps become downhill, but for the other two steps, they remain uphill with the energy barriers of 1.46 and 1.60 eV, respectively. Even the external potential is increasing to 2.01 V, the conversions of O*→OOH* and OOH*→O2 still need to overcome a large free-energy barrier of 0.68 and 0.82 eV, resulting in a relatively high overpotential of 1.60 V. These results indicate that the C sites would be passivated by O*or OOH* species during the OER process. Therefore, the dominant active sites of the MoS2/g-C3N4 heterojunction for the OER process are N sites. Overall, the construction of MoS2/g-C3N4 heterojunction structure is certainly an effective way to enhance the OER activities of MoS2 monolayer.

Fig. 5. The free energy diagrams of the OER pathway for (a) N@Site, (b) C@Site at different external potentials. Specifically, the potential determining steps (pds) are highlighted by the black arrows.

3.4. Electrocatalytic mechanism of the MoS2/g-C3N4 heterojunction

A clear understanding of the electrochemical mechanism of OER on the MoS2/gC3N4 heterojunction is of great importance. Therefore, based on the optimized N active site, the charge density difference, Bader effective charges as well as the relevant bond length parameters for different adsorbed intermediates (OH*, O* and OOH*) were comprehensively analysed, as shown in Fig. 6. From Fig. 6a-c, we can see that the charge transfer and redistribution were certainly occurred between the adsorbed intermediates and the g-C3N4 slab of the MoS2/g-C3N4 heterojunction. Furthermore, electron transfers were also evaluated quantitatively using Bader charge analysis, as depicted in Fig. 6d-f. The results indicate that the O and N atoms actually gain some charges, and meanwhile the C and H atoms lose corresponding charges. More specifically, O atoms obtaining electrons from the directly connected N and/or H atoms were 0.69e, 0.41e and 0.37e on average for OH*, O* and OOH* adsorption, respectively. Controlling the electron transfers at a moderate level would ensure that the adsorption strengths of the oxidation intermediates are neither too strong nor too weak, which are beneficial for the superior OER performance. In addition, the bond lengths are also important indicators, which are closely related to the bonding strengths. Herein, the relevant bond length parameters for the optimized configurations were collected, as illustrated in Fig. 6j-l. Clearly, the N-O bonds were located on the region of 1.26-1.38 Å, and the O-H bonds were constant with the value of 1.02 Å. Overall, the construction of the MoS2/g-C3N4 heterojunction can efficiently modify the electronic properties of MoS2 phase, facilitate electron transfer, and thereby improving the OER activity.

Fig. 6. The schematic diagram of optimized configurations for OH*, O* and OOH* intermediates. Top view (a), (b), (c) and side view (g), (h), (i) of charge density difference of the MoS2/g-C3N4 heterojunction after adsorption of OH*, O* and OOH* intermediates, respectively. Here, the electron accumulation and depletion are described by the blue and red regions with the isovalues of 0.003 eÅ-3. (d), (e) and (f) the distribution of Bader effective charges; (j), (k) and (l) the relevant bond length parameters. The grey, brown, yellow and purple balls refer to N, C, S and Mo atoms, respectively.

4. Conclusions In summary, a systematic DFT calculations were carried out to insight into the OER electrocatalytic mechanism of MoS2/g-C3N4 heterojunction. The results reveal that the pristine MoS2 monolayer exhibits an exceptionally high overpotential of 2.52 V, which implies that the water oxidation reaction is extremely difficult to conduct. This

originates from the fact that the poor electrical conductivity of MoS2 monolayer. Moreover, the construction of the MoS2/g-C3N4 heterojunction can efficiently modify the electronic properties of MoS2 phase by reducing its band gap, which is beneficial for electron transfers, and thereby facilitating OER activity. It is noteworthy that the C sites of the MoS2/g-C3N4 heterojunction would be passivated by O*or OOH* species owing to the overly weak adsorption strength, and meanwhile the N sites play a critical role during the OER process with a relatively lower overpotential of 0.78 V. Overall, compared to the pristine MoS2 monolayer, the overpotential for the MoS2/g-C3N4 heterojunction was significantly reduced nearly 69%. These results strongly suggest that the MoS2/g-C3N4 heterojunction could be used as a promising OER catalyst. Our work may provide a new way to exploration and development of the highly efficient MoS2-based OER catalysts. Acknowledgments This work is supported by the National Science Foundation for Distinguished Young Scholars for Hebei Province of China (grant E2016203376), Thailand Research Fund (RSA6080017), and the Energy Conservation Promotion Fund from and the Energy Policy and Planning Office, Ministry of Energy.

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Highlights 1. A systematic theoretical study of the OER electrocatalytic activity of MoS2/g-C3N4 heterojunction was performed. 2. The construction of the MoS2/g-C3N4 heterojunction can efficiently modify the electronic properties of MoS2 phase. 2. The theoretical overpotential of the MoS2/g-C3N4 heterojunction was significantly reduced nearly 69% from 2.52 to 0.78 V compared with the pristine MoS2. 4. The outstanding OER activity of the MoS2/g-C3N4 heterojunction mainly originates from the electronic coupling between MoS2 and g-C3N4.

Declaration of interests ☒ 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.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

The authors declare no conflict of interest.

Zhe Xue carried out the theoretical calculations and wrote the manuscript. Xinyu Zhang conceived the idea. Jiaqian Qin and Riping Liu contributed to the project design. All authors were involved in the analysis and discussion of the results.