Graphitic Carbon Nitride Hybrid Electrocatalysts

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p-Doped Graphene/Graphitic Carbon Nitride Hybrid Electrocatalysts: Unravelling Charge Transfer Mechanisms For Enhanced HER Performance Xin Tan, Hassan A. Tahini, and Sean C. Smith ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01951 • Publication Date (Web): 12 Sep 2016 Downloaded from http://pubs.acs.org on September 17, 2016

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p-Doped Graphene/Graphitic Carbon Nitride Hybrid Electrocatalysts: Unravelling Charge Transfer Mechanisms For Enhanced HER Performance

Xin Tan, Hassan A. Tahini, Sean C. Smith∗

Integrated Materials Design Centre (IMDC), School of Chemical Engineering, UNSW Australia, Sydney, NSW 2052, Australia

∗

Corresponding author: [email protected] 1

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ABSTRACT Recently, hybrid electrocatalyst systems involving an active layer of g-C3N4 on a conductive substrate of N-doped graphene (g-C3N4@NG) have been shown to achieve excellent efficiency for the hydrogen evolution reaction (HER) [e.g., Zheng, Y.; Jiao, Y.; Zhu, Y.; Li, L. H.; Han, Y.; Chen, Y.; Du, A.; Jaroniec, M.; Qiao, S. Z. Nat. Commun. 2014, 5, 3783]. We demonstrate here through first principle calculations examining various hybrid g-C3N4@MG (M = B, N, O, F, P and S) electrocatalysts that the N-doped case may be regarded as an example of a more general modulation doping strategy - by which either electron donating or electron withdrawing features induced in the substrate can be exploited to promote the HER. Despite the intrinsically cathodic nature of the HER, our study reveals that all of the graphene substrates have an increasingly electron withdrawing influence on the g-C3N4 active layer as H atom coverage increases, modulating binding of the H-atom intermediates, the overpotential and the likely operational coverage. In this context, it is not surprising that p-doping of the substrate can further enhance the effect. Our calculations show that B is the most promising doping element for g-C3N4@MG (M = B, N, O, F, P and S) electrocatalysts due to the predicted overpotential of 0.06 eV at full coverage and a large interfacial adhesion energy of -1.30 eV, offering prospects for significant improvement over the n-dopant systems such as g-C3N4@NG that have appeared in the literature to date. These theoretical results reveal a more general principle for rational design of hybrid electrocatalysts, via manipulation of the Fermi level of the underlying conductive substrate.

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KEYWORDS: p-doped graphene, hybrid electrocatalysts, charge transfer effects, hydrogen evolution reaction, first principle calculations

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1. INTRODUCTION Hydrogen (H2) is considered a prospective alternative to fossil fuels for various energy applications because of its abundance, high energy content, clean burning, and potentially renewable nature.1,2 Electrocatalytic reduction of water to molecular hydrogen through the hydrogen evolution reaction (HER) provides a promising route for scalable hydrogen production with high purity, however, an efficient electrocatalyst is critical to achieve energy efficiency and cost effectiveness.3 It is well known that platinum (Pt) is the most active and stable electrocatalyst for HER, with extremely high exchange current density and small Tafel slope.4 However, its low abundance and consequently high cost inhibit large-scale commercial applications. Therefore, exploring low-cost, efficient and durable alternatives of Pt is a key technological task in the development of the hydrogen economy. One class of candidate electrocatalyst materials for the HER is based on transition metals (Mo, W, Co, Ni, Fe) and their derivative components (sulfides, phosphides, nitrides, borides, carbides),5-13 which suffer inherent corrosion and oxidation susceptibility during the acidic proton exchange membrane electrolysis. Another class of candidate materials comprises of various carbon-based materials as metal-free electrocatalysts,14-17 with some unique advantages of tunable molecular structures, abundance and strong tolerance to acid/alkaline environments. Graphitic carbon nitride (g-C3N4) has attracted extensive attention due to its facile synthesis, high stability in both acidic/alkaline conditions, and moderate band gap leading to good visible-light response. Thus, g-C3N4 has found multifunctional

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catalysis applications such as photocatalysis,

photocatalytic

H2 evolution,

photocatalytic O2 evolution, CO2 reduction, and other energy conversion processes.1827

However, the moderate band gap also implies inherently low electrical conductivity,

which has severely limited electrochemistry-related applications of of g-C3N4.28 A solution for the lack of conductivity may be found by creating a hybrid system in which the catalyst g-C3N4 is supported by a conductive substrate such as graphene (gC3N4@G). While this significantly improves the electrical conductivity and electrocatalytic performance of g-C3N4 for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER),29-32 the electrocatalytic performance of hybrid gC3N4@G for the HER is poor.33 Recently, Zheng et al.34 demonstrated that by doping the support graphene with nitrogen atoms (N-graphene; NG), the coupled g-C3N4 / N-graphene hybrids (gC3N4@NG) show surprisingly good HER activity - comparable to some welldeveloped metallic catalysts such as nanostructured MoS2. By complementing the experiment observations with density functional theory (DFT) calculations, they concluded that the doped nitrogen atoms strengthen the chemical and electronic coupling between g-C3N4 and the (N-) graphene substrate, which synergistically promotes the proton adsorption and reduction kinetics. In addition, the hybrid electrocatalyst system exhibits enhanced stability under both acidic and alkaline conditions. Subsequently, Gao et al.35 reported a computational study of the HER on a pure g-C3N4 monolayer under a compressive/tensile strain, and found that the Gibbs free energy of H* adsorption ( ∗ ) on g-C3N4 is very sensitive to mechanical strain,

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suggesting that the experimentally-observed high HER activity in g-C3N4@NG hybrids may actually originate from charge-transfer induced strain in g-C3N4 layer. Shinde et al.36 have investigated the HER performance of g-C3N4 on another graphene-based support, i.e. phosphorous-doped graphene (P-graphene, PG), and found that the coupled g-C3N4 / P-graphene hybrids (g-C3N4@PG) show even better HER performance than that of g-C3N4@NG hybrids. Thus, there still remain questions as to the reason why the doped N/P atoms in the graphene support enhance the HER catalysis in g-C3N4@NG hybrids. In this work, we use first principle DFT computations to systematically examine the electrocatalytic activities of g-C3N4 supported by various doped graphenes summarized as g-C3N4@MG (M = B, N, O, F, P and S) – in an attempt to gain a more comprehensive understanding of the catalysis and thence a predictive paradigm. We find that both strain in the g-C3N4 layer and coupling between g-C3N4 and the support layer contribute to the performance of these hybrid g-C3N4@MG (M = B, N, O, F, P and S) electrocatalysts for the HER. An intriguing observation arising from our analysis is that – regardless of the intrinsic charge transfer between the conductive (doped) graphene support layer and the active g-C3N4 layer - as the H-atom coverage increases all of the graphene supports tend to draw more electron density away from the H-bound active layer. The differing extent to which this occurs modulates the crucial H-atom binding free energies at different coverages and hence modulates the catalysis. Our results show that this effect can be enhanced by introducing a p-dopant boron into the graphene layer and that the hybrid g-C3N4@BG electrocatalyst is

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predicted to have a low overpotential of 0.06 eV, high electrical conductivity and large interfacial adhesion energy of -1.30 eV. This suggests that the g-C3N4@BG electrocatalyst may display significantly improved catalytic HER performance compared to g-C3N4@NG and g-C3N4@PG. The theoretical results not only clarify the origin of the high HER activities of these hybrid g-C3N4@MG (M = B, N, O, F, P and S) electrocatalysts, but also provides a possibility for obtaining even more efficient and metal-free electrocatalysts for the HER.

2. MODELS AND COMPUTATIONAL METHODS Our spin-polarized DFT calculations were performed using the VASP program37 using a plane-wave basis set and a projector augmented wave method (PAW) for the treatment of core electrons.38 The generalized gradient approximation of Perdew, Burke, and Ernzerhof (GGA–PBE)39 with van der Waals (vdW) correction proposed by Grimme (DFT-D2)40 was used in all the calculations due to its good description of long-range vdW interactions. For the expansion of wavefunctions over the planewave basis set, a converged cutoff was set to 500 eV. In order to simulate g-C3N4@G hybrid, a 3×3 graphene supercell was used to match the 1×1 g-C3N4 unit cell, and the slab models of hybrid g-C3N4 and doped graphene electrocatalysts were established based on a g-C3N4@G hybrid model by replacing one of the carbon atoms in graphene with doped heteroatom (B, N or P) or adding doped heteroatom (O, F or S) adsorbed on graphene. The vacuum space was set to larger than 15 Å in the z direction to avoid interactions between periodic

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images. Considering the ∗ on g-C3N4 is very sensitive to mechanical strain,35 the lattice constants of all the hybrids were optimized in this paper. Moreover, our test calculations show that the calculated free energy diagram of HER on pure g-C3N4 monolayer with optimized lattice constants is quite different from that with fixed lattice constant (see Figure S1). In geometry optimizations, all the atomic coordinates were fully relaxed up to the residual atomic forces smaller than 0.001 eV/Å, and the total energy was converged to 10−5 eV. The Brillouin zone integration was performed on the (8×8×1) Monkhorst– Pack k-point mesh,41 and polarization effect was considered in all cases. The interaction between the g-C3N4 layer and the support layer (graphene or doped graphene) can be evaluated by the interfacial adhesion energies defined as = − −

(1)

where , , and present the total energy of the hybrid, g-C3N4 monolayer, and support monolayer, respectively. The electron distribution and transfer mechanism are determined using the Bader charge analysis.42 According to this definition, a more negative adhesion energy indicates a stronger binding of the g-C3N4 layer and the support layer. The overall HER mechanism is evaluated with a three-state diagram consisting of an initial H+ state, an intermediate H* state, and 1/2 H2 as the final product. The ∗ is proven to be a key descriptor to characterize the HER activity of the electrocatalyst. A electrocatalyst with a positive value leads to low kinetics of adsorption of hydrogen, while a catalyst with a negative value leads to low kinetics of

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release of hydrogen molecule.43 The optimum value of | ∗ | should be zero; for instance, this value for the well-known highly efficient Pt catalyst is near-zero as | ∗ | ≈ 0.09 eV.43 The ∗ is calculated as14,34 ∗ = ∗ + %&' − ()

(2)

where ∗ is the hydrogen absorption energy, and %&' and ) are the difference in zero point energy and entropy between the adsorbed hydrogen and hydrogen in the gas phase, respectively. As the contribution from the vibrational entropy of hydrogen in the adsorbed state is negligibly small, the entropy of hydrogen adsorption is *

) ≈ − + ), , where ), is the entropy of H2 in the gas phase at the standard conditions. Therefore, Eq. (2) can be rewritten as14,34 ∗ = ∗ + 0.24/0

(3)

Here, we defined the hydrogen absorption energy ∗ as *

∗ = ∗ − 1 *2 ∗ − + ,

(4)

where ∗ , 1 *2 ∗ , and , represents total energies of hybrid plus n adsorbed hydrogen atoms, total energies of hybrid plus n − 1 adsorbed hydrogen atoms, and gas H2 molecule, respectively. According to this definition, a more negative adsorption energy indicates a stronger binding of atomic hydrogen to adsorbent.

3. RESULTS AND DISCUSSION 3.1. Electronic Structure, Electron-Transfer Properties, and HER Free Energy Diagram of g-C3N4@G Hybrid. It is necessary to establish a reference point for the doped graphene support systems examined below by studying first the effect of a

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pristine graphene support on electrical conductivity and electrocatalytic activity of gC3N4. As will be seen, this not only cross-validates some existing results but also leads to some remarkable conclusions regarding the HER electrocatalytic mechanism and optimal design of new electrocatalyst systems. Figure 1a shows the lowest-energy configuration for the g-C3N4@G hybrid, with optimized lattice constant a = 7.30 Å. The distance d between g-C3N4 layer and graphene layer is 3.24 Å, and the calculated Eadhesion is -0.75 eV, which indicates that the g-C3N4@G hybrid is stable. The calculated band structure and projected density of states (PDOS) of g-C3N4@G are shown in Figure 1b. Contrary to the semiconductive pure g-C3N4 monolayer,28 hybrid g-C3N4@G shows no band gap, and the Dirac cone at the Γ point guarantee an enhanced electrical conductivity, which is significant for the electrocatalytic HER. Moreover, after coupling g-C3N4 with pristine graphene support, the charge density in hybrid’s interlayer is redistributed in the form of 0.05 etransfer from conductive graphene to g-C3N4 layer, which is expected to have significant effects on the HER activity of g-C3N4@G electrocatalyst. These results are consistent with the previous calculations.44

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Figure 1. (a) Top (upper) and side (lower) views of the optimized g-C3N4@G hybrid. The blue and grey balls represent N and C atoms, respectively, and the unit cell of gC3N4@G is indicated by solid grey lines. The charge density difference of g-C3N4@G is also shown in (a). Yellow and cyan refer to electron-rich and -deficient area, respectively. The isosurface value is 0.0005 e/au. The red arrow shows the charge transfer between support layer and g-C3N4 layer. (b) The calculated band structure (left) and PDOS (right) of g-C3N4@G. The blue dashed line denotes the Fermi level. (c) The calculated free energy diagram of HER at the equilibrium potential for gC3N4@G under different H* coverage (θ = 0.33, 0.67 and 1). The free energy diagram of HER for Pt is also shown for comparison.

Figure 1c shows the calculated free energy diagram of HER at the equilibrium potential for g-C3N4@G under different H* coverage (θ = 0.33, 0.67 and 1), and the corresponding lowest-energy configurations are shown in Figure S2. Clearly, the insertion of pristine graphene support reduces the ∗ values of g-C3N4 at various H* coverage (see Figure S1). The main reason for these reduced ∗ is that the lattice constants of g-C3N4@G hybrids are much larger than those of pure g-C3N4 monolayer (see Table 1), which is consistent with the previous conclusion.35 This allows slightly more space within the g-C3N4 pores, alleviating steric interactions of

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the H* intermediates. The ∗ of g-C3N4@G at H* coverage of 0.33 is -0.42 eV, which indicates the release of hydrogen gas is quite unfavourable. However, the ∗ values for g-C3N4@G at H* coverage of 0.67 and 1 are -0.14 and 0.15 eV, respectively. These suggest that at equilibrium, the low coverage state is fully occupied, and g-C3N4@G at high H* coverage is the most likely model for the electrocatalytically active species. Note that this is quite different from pure g-C3N4 monolayer, whose smallest calculated | ∗ | value corresponds to a low coverage of 0.33 (see Figure S1). The smallest | ∗ | value for g-C3N4@G hybrid is 0.14 eV, which is however still relatively large and this may be the reason why the experimentally observed HER performance of g-C3N4@G hybrid is not especially good. Even though the pure graphene support is not optimal, these calculations further support the concept32,34 that coupling g-C3N4 with a graphene-based support is an attractive technique to modify the electrical conductivity and the electrocatalytic HER activity of g-C3N4. As always in catalysis, it is a matter of finding the sweet spot.

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Table 1. The optimized lattice constant (a, Å); the distance between g-C3N4 layer and support layer (d, Å); the interfacial adhesion energy (Eadhesion, eV); the corresponding Bader charge on the support layer (Qsupport, e); and the Gibbs free energy of H* adsorption (∆GH*, eV) calculated for various hybrids with different H* coverage.

H* coverage

a

d

Eadhesion

Qsupport

∆GH*

7.30 7.32 7.32 7.32

3.24 3.19 3.19 3.14

-0.75 -0.80 -0.84 -0.91

0.05 -0.03 -0.09 -0.13

-0.42 -0.14 0.15

7.28 7.30 7.30 7.32

3.23 3.18 3.14 3.11

-0.81 -0.82 -0.81 -0.87

0.17 0.09 0.01 -0.03

-0.37 -0.09 0.18

7.37 7.37 7.37 7.37

3.21 3.04 3.04 3.04

-0.76 -1.19 -1.30 -1.42

0.05 -0.25 -0.31 -0.36

-0.82 -0.22 0.06

7.37 7.35 7.35 7.37

3.21 3.16 3.15 3.13

-0.73 -0.84 -0.87 -0.97

0.06 -0.10 -0.17 -0.23

-0.50 -0.14 0.08

g-C3N4@G θ=0 θ=0.33 θ=0.67 θ=1

g-C3N4@NG θ=0 θ=0.33 θ=0.67 θ=1

g-C3N4@BG θ=0 θ=0.33 θ=0.67 θ=1

g-C3N4@PG θ=0 θ=0.33 θ=0.67 θ=1

A closer inspection of the charge analysis data in Table 1 reveals an important new feature regarding charge transfer between the graphene support and the active g-C3N4 layer. Focusing for the present on the g-C3N4@G system, one identifies that although there is an intrinsic transfer of electron density from the graphene support to the active

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layer at zero coverage (consistent with earlier work44), this reverses itself in the presence of the H* intermediates. In fact, the higher the H* coverage, the stronger is the reversal effect. This apparently correlates with a strengthening of the interfacial adhesion energy and must also play a role in the coverage-dependence of ∗ . Each of the substitutional dopant supports in Table 1 (considered in more detail below) displays a correlated trend. This observation suggests two related mechanistic principles. (i) The electron donating / withdrawing properties of the support layer should be considered not only for the pristine interface, but also for different coverages of H* in order to understand how to enhance interfacial adhesion and electrocatalytic properties. (ii) Modulation doping of the conductive graphene substrate tunes the Fermi level and band structure of the material, which in turn tunes the charge transfer properties of the interface, the adhesion energy and the coveragedependent ∗ . This latter principle, coupled with the observed reversal of the “intrinsic” interfacial charge transfer in the presence of bound H* intermediates, then leads one to explore an interesting possibility: that electron withdrawing properties in the conductive support – corresponding to p-doping – might in fact promote cathodic HER reduction by synergistically tuning the adhesion energy and ∗ . At first glance, this proposal seems somewhat counterintuitive, since the direction of interfacial charge transfer would be opposite to the direction of electron flow. However, the charge analysis data of Table 1 tells us that, in fact, this is what we already have for the pure graphene support – but the resultant tuning doesn’t quite hit

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the sweet spot for ∗ . The following discussion illustrates that, indeed, modulation p-doping of the conductive graphene support with boron leads to highly promising predicted electrocatalytic properties for HER. 3.2. The 345∗ and the Electrical Conductivity of Hybrid g-C3N4@MG systems (M = B, N, O, F, P and S). Previous studies have established that heteroatom doping is an effective way to change the chemical composition and modulate the electronic structure of graphene-based materials.45,46 Important advances have been made recently34,36 to demonstrate that g-C3N4@NG and g-C3N4@PG hybrids show very good HER activity. To explore this issue, we examine the computationally predicted HER activities of a range of hybrid g-C3N4@MG (M = B, N, O, F, P and S), as shown in Figure 2. For B, N and P dopants (Table 1), the substitution doping model is appropriate, while the O, F, and S dopants are adsorbed onto the graphene surface.

Figure 2. Top (upper) and side (lower) views of the optimized (a) g-C3N4@BG, (b) gC3N4@NG, (c) g-C3N4@OG, (d) g-C3N4@FG, (e) g-C3N4@PG, and (f) g-C3N4@SG

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hybrids. The blue, grey, light magenta, red, cyan, purple and yellow balls represent N, C, B, O, F, P and S atoms, respectively, and the unit cells of various hybrids are indicated by black dot lines. The distance between g-C3N4 layer and doped graphene layer of these hybrids are shown, and the optimized lattice constants of these hybrids are listed below.

Figure 3 shows the computed variation of ∗ across the series g-C3N4@MG (M = B, N, O, F, P and S) for different H* coverage (θ = 0.33, 0.67 and 1). The ∗ of pure g-C3N4 monolayer and g-C3N4@G hybrid are also shown for comparison. Generally, the ∗ values of pure g-C3N4 monolayer, g-C3N4@G, and g-C3N4@MG (M = B, N, O, F, P and S) have the same trend under different H* coverage: ∗ 16 = 0.332 < ∗ 16 = 0.672 < ∗ 16 = 12, indicating slower kinetics for adsorption of the intermediate H-atom and more facile release of molecular hydrogen as the H* coverage increases. Like g-C3N4@G, the ∗ values of g-C3N4@MG (M = B, N, O, F, P and S) are significantly more negative than those of the pure g-C3N4 monolayer. As explained above, this is related to the relatively larger lattice constants for the interfacial systems compared with the g-C3N4 monolayer. Consistent with the “modulation doping” picture, we also find that the ∗ values vary across the dopant series. Considering that the changes in lattice constants are minor across the series (ranging from 7.28 to 7.37 Å), we may ascribe the variation in ∗ across the series to modulation of charge transfer between the support layer and the g-C3N4 layer in the presence of H* intermediates.

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Figure 3. The calculated ∗ of various g-C3N4@MG (M = B, N, O, F, P and S) hybrids under different H* coverage (θ = 0, 0.33, 0.67 and 1). The grey area denotes | ∗ | ; 0.09 eV. The ∗ of pure g-C3N4 monolayer and g-C3N4@G hybrid are also shown for comparison.

The smallest | ∗ | values – in each case falling within the range (shaded in gray in Figure 3) comparable to or less than Pt at 0.09 eV – are found for g-C3N4@BG 16 = 12 , g-C3N4@NG 16 = 0.672 and g-C3N4@PG 16 = 12 . Boron, nitrogen and phosphorous are all commonly used as dopant heteroatoms in graphene experiments, which suggests that these three doped graphenes are very feasible support materials for the g-C3N4 active layer and may be anticipated to display high HER activity with overpotentials similar to or less than Pt. This expectation is borne out experimentally for the N-doped34 and P-doped36 systems, while the B-doped hybrid electrocatalyst system has not yet been reported. To evaluate the most stable phase (i.e. the coverage of H*) of various doped hybrid electrocatalyst systems at the standard conditions (1 atm H2 and 298 K), we used ab 17


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initio thermodynamics47 to determine the Gibbs free energy of adsorption on gC3N4@BG, g-C3N4@NG and g-C3N4@PG with different H* coverages at a specific temperature and pressure, as shown in Figure S3. One can see that at low hydrogen pressure, low H* coverage is the most stable phase, while at high hydrogen pressure, high H* coverage is the most stable phase. At ambient pressures, the most stable phase for g-C3N4@BG, g-C3N4@NG, and g-C3N4@PG are 6 = 0.67, 0.33, and 0.33, respectively. Importantly, for g-C3N4@NG and g-C3N4@PG, the Gibbs free energies of adsorption are negative when 0.33 ; 6 ; 0.67, while the Gibbs free energies of adsorption for g-C3N4@BG are all negative when 0.33 ; 6 ; 1. It should be noted that the Gibbs free energy of adsorption for g-C3N4@BG at 6 = 1 and 0.33 are only 0.25 and 0.02 eV higher than that of the most stable phase (6 = 0.67), indicating gC3N4@BG with different H* coverages (0.33 ; 6 ; 1) have comparable stability at ambient pressures. For HER in acidic condition, there are two types of possible pathways for reducing protons

to

hydrogen,

namely,

the

Volmer-Heyrovsky

or

Volmer-Tafel

mechanism.34,48,49 In order to simulate the detailed HER paths and the associated barriers on doped hybrid electrocatalyst systems, we adopted an approach previously used by Skulason et al.48 and Tang et al.49 in elucidating the reaction mechanism of HER on Pt(111) and 1T-MoS2 monolayer. Our results show that if we put excess H atoms into the water layer on g-C3N4@BG surface, the excess H atoms automatically move to the g-C3N4 layer of g-C3N4@BG during the geometry optimization, which suggest no activation barrier for Volmer reaction. However, this hinders us to simulate

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the following Heyrovsky reaction. Considering that g-C3N4@NG have been shown to demonstrate excellent efficiency for the HER in experiment, we believe the activation barriers for HER on doped hybrid electrocatalyst systems are small. Since the high electrical conductivity is one of the prerequisite for high performance HER electrocatalysts, we calculated the band structures of g-C3N4@BG, g-C3N4@NG, and g-C3N4@PG hybrids with/without H* adsorption (see Figure S4).50 For the bare hybrids, g-C3N4@BG and g-C3N4@NG are conductive while gC3N4@PG is semiconductor with a small band gap about 0.1 eV. The band structures show that g-C3N4@BG with θ = 0.67, g-C3N4@NG with θ = 0.33, and g-C3N4@PG with θ = 0.67 are conductive, indicating that all these three hybrid electrocatalysts have high electrical conductivity.

3.3. The ?@ABCD of Hybrid g-C3N4@MG (M = B, N, O, F, P and S). The stability of electrocatalysts in both acidic/alkaline conditions is an important criterion for their electrocatalytic performance. The of the g-C3N4@MG (M = B, N, O, F, P and S) hybrids are the determining factor of the stability, which is significant for the electrocatalytic HER. Therefore, we examined the of various gC3N4@MG (M = B, N, O, F, P and S) hybrids under different H* coverage (θ = 0, 0.33, 0.67 and 1), as shown in Figure 4. Generally, the of all g-C3N4@MG (M = B, N, O, F, P and S) hybrids become stronger (more negative value) as the H* coverage increases, indicating the stability of these g-C3N4@MG (M = B, N, O, F, P and S) hybrids is enhanced as the H* coverage increases. This apparently correlates

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with the amount of charge transfer amount between the support layer and g-C3N4 layer, evidenced by the fact that the B-doped system displays the largest enhancement of charge transfer as well as the largest enhancement of adhesion energy as the H* coverage increases (see Table 1). The calculated values for g-C3N4@BG at the H* coverage θ ≥ 0.33 are much stronger than other hybrids, which indicates that g-C3N4@BG should display good stability and tolerance to acid/alkaline environments. Considering the highest stability and the low overpotential for HER (the smallest | ∗ | is 0.06 eV), we expect that the HER activity of g-C3N4@BG should be better than the synthesized efficient hybrid electrocatalysts, namely gC3N4@NG and g-C3N4@PG. Note that, the of g-C3N4@PG with θ = 0.67 is stronger than that of g-C3N4@NG with θ = 0.33 or 0.67, we believe this is the reason why the previous experiment observations demonstrated that g-C3N4@PG hybrids show even better HER performance than that of g-C3N4@NG.

Figure 4. The calculated interfacial adhesion energies of various gC3N4@MG (M = B, N, O, F, P and S) hybrids under different H* coverage (θ = 0, 20


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0.33, 0.67 and 1). The interfacial adhesion energy of g-C3N4@G hybrid is also shown for comparison.

3.4. The Origin of the Stability of Hybrid g-C3N4@MG (M = B, N, O, F, P and S). In order to understand the trend of the stability of various hybrid g-C3N4@MG (M = B, N, O, F, P and S), we compared the charge transfer between support layer and gC3N4 layer of these hybrids. Here, we focused on four hybrid systems at configurations corresponding to H* coverages one site short of achieving the smallest | ∗ | values, i.e. g-C3N4@G (θ = 0.33), g-C3N4@NG (θ = 0.33), g-C3N4@BG (θ = 0.67), and g-C3N4@PG (θ = 0.67), according to the generally adopted rationale that the “vacant” site with the smallest | ∗ | acts as the available site for optimal reactivity.50 The charge density difference of these hybrids are shown in Figure 5. From Figure 5 and the Bader charge of the support layer (Table 1), we can see that the charge transfer between support layer and g-C3N4 layer of g-C3N4@BG with θ = 0.67 is very large (0.31 e-), which leads to the large negative value of . Moreover, we also found that the charge transfer between support layer and g-C3N4 layer of these hybrids follow the order of g-C3N4@G (θ = 0.33) ≤ g-C3N4@NG (θ = 0.33) < gC3N4@PG (θ = 0.67) < g-C3N4@BG (θ = 0.67). These results are very consistent with our calculated values of these hybrids, indicating the enhanced stability of hybrid g-C3N4@MG (M = B, N, O, F, P and S) correlates with the charge transfer between support layer and g-C3N4 layer.

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Figure 5. The charge density difference of (a) g-C3N4@G with θ = 0.33, (b) gC3N4@NG with θ = 0.33, (c) g-C3N4@BG with θ = 0.67, and (d) g-C3N4@PG with θ = 0.67. Yellow and cyan refer to electron-rich and -deficient area, respectively. The isosurface value is 0.0005 e/au. The blue, grey, white, light magenta, and purple balls represent N, C, H, B and P, respectively. The red arrows show the charge transfer between support layer and g-C3N4 layer.

4. CONCLUSIONS In summary, we have carried out a comprehensive study of the stability and HER activity of hybrid g-C3N4@MG (M = B, N, O, F, P and S) electrocatalysts, by means of DFT computations. Analysis of the data leads to several important mechanistic conclusions. (i) Charge transfer between the conductive support and the active gC3N4 layer plays a crucial role in modulating the interfacial adhesion as well as the 22


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crucial free energy of the surface-bound intermediate H*, ∗ . (ii) The electron donating / withdrawing properties of the support layer should be considered not only for the pristine interface, but also for different coverages of H* in order to understand how to enhance interfacial adhesion and electrocatalytic properties. In fact, the intrinsic charge transfer from graphene to g-C3N4 is reversed as H* coverage increases. (iii) Modulation doping of the conductive graphene substrate tunes the Fermi level and band structure of the material, which in turn tunes the charge transfer properties of the interface, the adhesion energy and the coverage-dependent ∗ . These conclusions provide a rationale - based on modulation doping concepts - to design optimal hybrid electrocatalyst systems. Somewhat counter-intuitively, following this line of reasoning we find strong predictive indications that p-doping of the graphene substrate can very effectively enhance cathodic HER reduction. Our results show that B is the most promising doping element for g-C3N4@MG (M = B, N, O, F, P and S) electrocatalysts. The conductive g-C3N4@BG hybrid system is calculated to have very good stability and HER activity, with calculated overpotential (| ∗ | = 0.06 eV) and interfacial adhesion energy (-1.30 eV) significantly better than the best experimentally reported g-C3N4@NG and g-C3N4@PG electrocatalysts. This study not only helps to clarify the origin of the high HER activities of these hybrid g-C3N4@MG (M = B, N, O, F, P and S) electrocatalysts, but also provides a possibility for obtaining even more efficient metal-free electrocatalysts for HER. We note that co-doping of carbon-based materials can enhance the HER activity compared to single element doping case.14-16 It will therefore be beneficial going

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forwards to theoretically investigate the impact of such co-doping, e.g., co-doping with N and P, on HER activity of hybrid g-C3N4@MG electrocatalysts.

ASSOCIATED CONTENT Supporting Information The lowest-energy configurations of pure g-C3N4 monolayer with different H* coverage and the calculated free energy diagram of HER at the equilibrium potential for pure g-C3N4 monolayer under different H* coverage (Figure S1); The lowestenergy configurations of g-C3N4@G hybrid with different H* coverage (Figure S2); Gibbs free energy of adsorption at 298K as a function of hydrogen chemical potential or H2 pressure for various doped hybrid electrocatalysts with different H* coverages (Figure S3). The calculated band structures of g-C3N4@G, g-C3N4@BG, gC3N4@NG, and g-C3N4@PG hybrids with/without H* adsorption (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected] (S. S.)

Notes The authors declare no competing financial interest.

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ACKNOWLEDGEMENTS This research was undertaken with the assistance of resources provided by the National Computing Infrastructure (NCI) facility at the Australian National University; allocated through both the National Computational Merit Allocation Scheme supported by the Australian Government and the Australian Research Council grant LE120100181 (“Enhanced merit-based access and support at the new NCI petascale supercomputing facility, 2012-2015).

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(50) At equilibrium, the low coverage state, i.e. H* coverage one site short of achieving the smallest |∗ | value, is fully occupied. The addition of a new proton and electron add a hydrogen atom in the high coverage state, i.e. H* coverage corresponding to the smallest | ∗ | value. In catalytic process, the high coverage state is the catalytically active species, which is not always occupied. Therefore, when we study the stabilities, the band structures and the charge distributions of these hybrid electrocatalysts, we focus on the low H* coverage. For example, the smallest | ∗ | value for g-C3N4@BG is θ = 1, and we study the stability, the band structure and the charge distribution of this electrocatalyst at θ = 0.67.

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Table of Contents (TOC)

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Graphitic Carbon Nitride Hybrid Electrocatalysts

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