Hexagonal CuCl Monolayer for Water Splitting: A DFT Study | ACS

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Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

Hexagonal CuCl Monolayer for Water Splitting: A DFT Study Priyanka Garg,† Kuber Singh Rawat,† Gargee Bhattacharyya,‡ Sourabh Kumar,† and Biswarup Pathak*,†,‡ †

Discipline of Chemistry and ‡Discipline of Metallurgy Engineering and Materials Science, Indian Institute of Technology (IIT) Indore, Indore, M.P. 453552, India

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S Supporting Information *

ABSTRACT: In the past few years, enormous efforts have been put to develop earthabundant semiconductor based photocatalysts for overall water splitting. In the present work, we predict a hexagonal CuCl monolayer (ML) to be promising for such semiconductor based photocatalytic reactions. Density functional theoretical (DFT) calculations have been employed to understand the overall water splitting reactions on the CuCl ML and CuCl bulk structure. The electronic structure, band edge alignment, and overpotential studies indicate that CuCl ML is a promising material for overall water-splitting reaction compared to the CuCl bulk structure. This is due to the stabilization of the valence band of the ML structure over the CuCl bulk structure. KEYWORDS: DFT, CuCl (111), CuCl monolayer, overpotential, photocatalysis

1. INTRODUCTION Solar hydrogen production by means of semiconductor based photocatalysts via water splitting reaction is a promising strategy to address energy and environmental problems.1,2 The water splitting reaction on semiconductor based photocatalysts initiates with the absorption of photons followed by the formation of photogenerated electrons and holes. These photogenerated charge carriers (electron and hole) migrate to the surface of the photocatalysts and then react with water under the suitable condition of valence and conduction band positions with respect to the water oxidation and reduction potentials. The photogenerated hole initiates the oxygen evolution reaction (OER; 2H2O → O2 + 4e− + 4H+; E = 1.23 V vs NHE). On the other hand, the electron initiates the hydrogen evolution reaction (HER; 2e− + 2H+ → H2; E = 0.0 V vs NHE). The overall reaction free energy change for the water-splitting process is 1.23 eV. Consequently, a suitable band gap and appropriate redox potential of the photocatalysts are prerequisites for an efficient photocatalytic water splitting reaction.3,4 In the recent past, late first-row transition metal oxide,5,6 phosphide,7,8 and sulfide9,10 based catalysts have been used for water-splitting because of their abundance and stability. Copper (Cu) is one of such interesting elements and relatively more environmentally friendly compared to cobalt and nickel.11 Among Cu based catalysts, cuprous oxide (Cu2O) is one of the widely studied Cu based heterogeneous catalysts for the hydrogen evolution reaction due to its direct band gap (2.1−2.2 eV) and appropriate band edge potential.12,13 Nevertheless, Cu2O shows low photostability during light illumination in aqueous solution.12 In order to search other cost-effective copper based catalysts, Cu3P nanosheets have been synthesized on conductive nickel foam © XXXX American Chemical Society

as a Janus catalyst for water-splitting reaction, which has shown a current density of 10 mA/cm2 at an external potential of ∼1.67 V.8 After that, Cu2S structures on 3D Cu foam have been synthesized for OER with persistent stability (>10 h) and low reaction overpotential of 336 mV at the current density of 20 mA cm−2, which is comparable to that of a IrO2/Cu foam electrode.9 Furthermore, cuprous chloride (CuCl) has been used as a source material for Cu and Cl ions to enhance the catalytic activities of the oxide based catalysts such as Cu2O, NaTaO3, and K2La2Ti3O10.14,15 However, the catalytic activities of pure CuCl based catalysts toward HER and OER are yet to be investigated. The copper halide based materials (CuCl, CuBr, and CuI) are an interesting class of materials16,17 because of their direct band gap (3.39 eV for CuCl, 2.91 eV for CuBr, and 2.95 eV for CuI) and optoelectronic applications.18,19 Interestingly, the band structure of these materials shows similar dispersion though the valence band edge position changes with the nature of the halide.19,20 However, the conduction band edge position remains more or less the same irrespective of the nature of the halide with respect to the absolute vacuum scale (AVS).20,21 Interestingly, among all these Cu based halides and some of the copper oxide based materials (such as Cu2O and CuO), CuCl has a more stabilized valence band, which is certainly promising for the water oxidation reaction.21−23 Therefore, it would be more enthusiastic to investigate whether CuCl nanostructure based catalysts can be promising for the overall water splitting reaction as CuCl has already been synthesized Received: April 16, 2019 Accepted: June 17, 2019 Published: June 17, 2019 A

DOI: 10.1021/acsanm.9b00699 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials

3. RESULTS AND DISCUSSION This section is discussed in four different parts: (1) structure modeling and bonding nature of CuCl monolayer (ML), (2) stability analysis to validate the CuCl structure, (3) the electronic properties of CuCl ML and its bulk structure, and (4) band edge alignment of CuCl ML and bulk structures (to locate the valence and conduction band position with respect to water redox potential). Such band edge alignment is then combined with the OER and HER overpotential to see the effect of band edge position on reaction overpotential. 3.1. Structure Modeling and Nature of Bonding. The bulk structure of CuCl has zinc blende structure (Figure 1),

in different nanostructural forms (nanoplatelets, quantum cubes, and quantum dots).24−26 Inspired from these previous literature reports, we have modeled a two-dimensional (2D) CuCl monolayer (ML) to understand its catalytic behavior toward water splitting reaction. 2D materials are believed to be an ideal catalyst for such reactions due to their large surface area, fast charge transfer, and great strength/flexibility/tunability compared to their 3D bulk counterparts.27,28 We have also considered the periodic CuCl (111) structure to compare its catalytic activity with the CuCl ML structure. In this context, a systematic investigation of all the important factors such as stability, electronic structure, band edge alignment, reaction free energy, and overpotential has been employed to understand the water splitting reaction on the CuCl ML and CuCl bulk structures. On the other hand, the potentials of photogenerated electron and holes at the valence and conduction band affect the OER and HER overpotential. Therefore, we have investigated all these factors to understand whether CuCl ML can be a promising photocatalyst compared to the CuCl bulk or any other reported ML based structures.

2. COMPUTATIONAL DETAILS First-principles calculations have been performed by using the projector augmented wave (PAW)29 method which is implemented in the Vienna Ab initio Simulation Package (VASP).30 The exchange−correlation potential is well-defined at the level of the GGA using Perdew−Burke Ernzerhof (GGA-PBE).31,32 To describe the electronic wave function, the PAW method was employed with an energy cutoff of 470 eV. The energy convergence limit for selfconsistent calculations is set to 10−4 eV between two consecutive steps, and the atomic forces of all the systems are minimized to less than 0.01 eV Å−1 without any symmetry constraints. The computational calculations have been carried out in a 4 × 4 × 1 supercell of CuCl ML. The Brillouin zone of CuCl ML is integrated with 7 × 7 × 1 and 27 × 27 × 1 Gamma-centered k-mesh for geometry optimization and density of states calculations, respectively. ∼16 Å of vacuum has been added in the z-direction to avoid the spurious interaction between two periodic layers. The dispersion energy corrections for van der Waals interactions have been computed via Grimme’s D3-type of semiempirical method.33 Furthermore, the hybrid functional methods (PBE0 and HSE06)34,35 have been used to validate the electronic properties of the CuCl ML and periodic CuCl (111). Bader charge analysis is done using the Henkelman code with a near grid algorithm refine-edge method.36−38 The free energy of proton coupled electron transfer oxidation/ reduction reaction has been computed through the method developed by Nørskov and co-workers according to which the free energy change of an electrochemical reaction is calculated as39,40 ΔG = ΔE + ΔZPE − T ΔS + ΔGU

Figure 1. Schematic illustration of modeling of CuCl ML from its zinc blende bulk structure.

and the lattice constant of the optimized structure is a = b = c = 5.42 Å, which is in good agreement with the previous reports.46,47 In the growth morphology of the bulk CuCl crystal, the (111) surface is predominated.48 On that basis, we have constructed CuCl ML by cutting along the (111) plane (Figure 1) of the zinc blende phase of the bulk CuCl structure. The relaxed two-atom (one Cu and one Cl) unit cell structure of the CuCl ML (Figure 2) has hexagonal symmetry. The optimized lattice constant of CuCl ML is a = b = 3.88 Å. Figure 2 shows that the CuCl ML structure forms a planar honeycomb structure. The Cu−Cl bond length is 2.24 Å, remarkably shorter than that in the bulk CuCl (2.35 Å), which could be due to change from tetrahedral to planar geometry. Furthermore, the modeling of the most active periodic CuCl (111) 49 has been discussed in Text S1 (Supporting Information). The bulk phase of the CuCl system has ionic nature,50 so it is necessary to know whether the ionic nature changes from bulk to ML. For this, the electron localization function plot (ELF) has been plotted (Figure 2b) to understand the bonding between Cu and Cl in the ML structure. Figure 2b shows that the electrons are completely localized on Cl compared to Cu, which indicates that CuCl ML has ionic bonding interaction between Cu and Cl atoms. To check this discrepancy, we have calculated the Bader charges of atoms in the monolayer, bilayer, and bulk structure. Interestingly, Cu transfers 0.56 |e| electrons to Cl in the CuCl ML, while 0.57 |e| and 0.59 |e| in bilayer and bulk CuCl, respectively. Therefore, charge transfer increases in bilayer and bulk, which indicates that CuCl ML is less ionic compared to the bulk.

(1)

where, ΔE, ΔZPE, and ΔS are the difference in total energy, zeropoint energy, and entropy between final and initial states, respectively. The ΔGU = −eU, where U is the potential of the photogenerated electrons/holes with respect to the NHE; e is the number of transferred electrons. Here, T denotes the temperature (300 K). The entropy (ΔS) and ΔZPE correction for all the free gaseous molecules have been taken from the NIST database,41 while the ZPE correction for adsorbed intermediates is computed via a vibrational frequency calculation using the density functional perturbation theory (DFPT)42 as implemented in the VASP. As the bond energy of molecular oxygen (O2) cannot be accurately calculated using PBE, we have taken the O2 bond energy referenced from water formation (2H2 + O2 = 2H2O).43−45 B


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Figure 2. (a) Top and side views of the CuCl ML showing different adsorption sites. The black dotted line indicates a two-atom unit cell. (b) Contour plots of electron localization function for CuCl ML.

3.2. Stability of ML. The stability of CuCl monolayer can be evaluated using thermodynamic (energetic), dynamic, thermal, and mechanical stability based calculations. First, the thermodynamic stability of the system indicates the thermodynamic plausibility of formation of materials.51−53 In this context, cohesive and formation energies (Details in Text S2, Supporting Information) have been calculated. The formation energies of CuCl ML and periodic CuCl (111) have been calculated with respect to the bulk face centered cubic Cu and isolated chlorine molecule, whereas cohesive energies have been calculated with respect to their discrete atoms. Furthermore, the negative value of Ef means that the formation of CuCl ML is possible. On the other hand, the negative values of Ecoh means, once formed, CuCl ML would not be decomposed into atoms spontaneously. Thus, CuCl ML can be possible to synthesize using the chemical vapor deposition or molecular beam epitaxy methods as used for synthesis of ML based structures.51,52 Furthermore, we have also calculated the formation and cohesive energies of bilayers (BL) and triple layers (TL) of CuCl systems, and the calculated values are shown in Table 1. As expected, these

bond breaking abnormalities do not occur during vibrations. This further indicates that it can be stabilized over supported materials as such monolayer based structures have always been synthesized using an appropriate substrate.54,55 For example, a thin film of copper iodide has been synthesized over a Cu(111) surface.56 Furthermore, the monolayer structure of ionic MgCl2 has been synthesized on Pd, Pt, and Rh metal surfaces.57 Therefore, we expect that such ML structure can also be experimentally realizable. The thermal stability of CuCl ML has been calculated for observing the effect of temperature (Text S4, SI) on the phase stability. There is no appreciable energy change (Figure S2) throughout the simulation not only at 300 K but also at higher temperatures (500 and 700 K). Furthermore, we have also studied the displacements of atoms with time by plotting the root-mean-square displacement (RMSD) as a function of time step (Figure S2) for CuCl ML. Our RMSD plot indicates that atomic displacement is small at 300−500 K temperature, whereas it is quite high at 700 K (∼0.8 Å), respectively. However, the honeycomb lattice retains its structure at the end of simulation (Figure S3), which indicates that there are no structural reconstructions even after heating up to 700 K for 10 ps. Thus, we can say that CuCl ML can be thermally stable. Moreover, the stability of the CuCl ML structure has been further verified using the AIMD simulations at higher temperatures (500 and 700 K). For this, we have placed two CuCl MLs in a box separated by 3.9 and 4.4 Å, which are significantly higher than the interlayer distance (2.34 Å) in bulk CuCl. Figure S4 shows the AIMD simulated results, and we find that the CuCl MLs do not interact at 500 K temperature though they interact at 700 K. This suggests that such ML structure can be stable up to 500 K temperature and there is a significant barrier for the formation of bilayer from ML. Adsorption of intermediate species may induce some distortion on the surface of CuCl ML. Therefore, it is required to examine the effect of lattice deformation on CuCl ML for their synthesis and applications. Here our main concern is to find how much stress the lattice can sustain without forming another new phase under the application of external strain since such kind of ML structure generally synthesized over a support material which generates stress due to lattice mismatch between ML and substrate. Therefore, it is very important to calculate the mechanical stability of such ML structure. A detailed study of mechanical stability and properties has been

Table 1. Calculated Formation (Ef/atom) and Cohesive Energies (Ecoh/atom) of CuCl ML, BL, TL, and (111) Surface System CuCl CuCl CuCl CuCl

ML BL TL (111)

Ef (eV)

Ecoh (eV)

−0.14 −0.21 −0.23 −0.23

−3.04 −3.11 −3.13 −3.13

values show that formation of multilayers of the CuCl system is more favorable over monolayer. Such negative formation energy values have been reported for several synthesized ABO3 perovskite structures.53 The dynamic stability of the ML structure has been evaluated using the phonon dispersion plot (Text S3, SI) to observe the lattice vibrations. There are three acoustic phonon modes in the phonon dispersion plot of CuCl ML, out of which one mode gives an imaginary frequency in the order of ∼22i cm−1 (Figure S1) within Γ→ Γ high symmetry k-point path. Furthermore, we have seen that the soft mode has been observed due to out-of-plane (along z-axis) lattice vibration (Figure S1); however, the change in the bond length between Cu−Cl during vibration varies up to 0.02 Å. As a result, the C


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Figure 3. Total and partial density of states (TDOS/PDOS) plots of (a) CuCl ML and (b) CuCl bulk. The electronic band structures of (c) CuCl ML and (d) bulk. Both calculations have been done using the PBE0 method. Here, the Fermi level is shifted to zero and indicated by a black dashed line. Brillouin zones of hexagonal CuCl ML and CuCl bulk (zinc blende structure) with high symmetry k-point paths.

underestimates the band gap energy of semiconductors,31 we have used hybrid functionals (such as PBE0 and HSE06) to calculate the electronic properties of bulk CuCl (Table S1). The calculated band gap values for bulk CuCl are 2.50 and 3.18 eV using the HSE06 and PBE0 functionals, respectively. The band gap calculated by PBE0 (3.18 eV) comes closer to the experimental value. Henceforth, we have considered the PBE0 functional for electronic structure calculations. The calculated band gap of CuCl ML is 3.66 eV. Furthermore, the partial density of states (PDOS) indicates that the valence band edges of CuCl systems (bulk and ML) are mainly composed of copper 3d orbitals along with Cl 3p, while the conduction band edges (Figure 3(a−b)) are mainly constituted by Cu 4s with slight mixing of Cl 3s and 3p orbitals. The valence bands of CuCl systems split into three different bands. The first two zones of the valence band show slight p-d mixing, but the lower lying valence band contains only Cl-3p character which is separated from the upper valence band by ∼1.16 and ∼2.65 eV in the CuCl ML and bulk counterpart, respectively. Figure 3(c−d) shows band structure plot of CuCl ML and bulk structure. As the CuCl ML and bulk systems are hexagonal and cubic, we have considered a highsymmetry k-point path for these two types of lattices as shown in Figure 3. Hence, our proposed ML is a direct band gap semiconductor, where the lowest band gap is obtained at the gamma point. The flat bands in the valence band edge (Figure 3(c)) indicate that they do not participate in the bond formation. However, the PDOS of the ML and bulk show a similar kind of bonding though the gap between upper and lower lying valence bands has reduced from bulk to ML. This indicates that the Cl-3p orbital density is shifted toward the Cu-3d orbital density in ML over bulk because the ML structure has less ionic character as compared to the bulk

discussed in Text S5. We generate stress in the lattice by gradually increasing/decreasing the lattice parameters.58,59 Generally the total energy of the optimized strained structure gradually increases as we increase the % of strain. The maximum point at which the structure cannot withstand the stress and thus forms a new phase with more lower energy indicates the critical point of that system. At this critical point we observe a sudden drop of stress (Figure S5 (a)) due to formation of new phase. In our case we observe this critical point at 10% and 15% uniaxial and biaxial strains, which suggests that CuCl ML can sustain up to 10% and 15% uniaxial and biaxial strains, respectively. The maximum value of stress is 7.76 GPa. In spite of this, the calculated values of elastic constants such as C11, C12, and C44 are 37 N/m, 26 N/m, and 5 N/m, respectively. It is noteworthy here to mention that observed values of elastic constants indicate that CuCl ML follows the thumb rule for mechanical stability (C11 > C12 and C44 > 0).52 Therefore, we predict that CuCl ML is a mechanically stable sheet. The calculated values (in plane) of Young’s modulus (Y) and Poisson’s ratio (PR) are 17 N/m and 0.70, respectively. Interestingly, the PR value of CuCl ML is higher compared to that in graphene (0.22).60 Thus, CuCl ML has large shear motion over graphene, which indicates that CuCl ML can be a mechanically soft material. 3.3. Electronic Structure. We have thoroughly discussed the band structure, along with the total and partial density of states (DOS) to examine the electronic properties of CuCl ML and its bulk counterpart. First, we have calculated the band gap of CuCl systems (bulk and ML) using the GGA-PBE level of theory. However, the GGA-PBE calculated band gaps are 0.8 and 1.1 eV for bulk and ML, respectively. Therefore, the GGA calculated value (0.8 eV) for bulk CuCl is far from the experimental reported value of 3.39 eV.18 Since GGA D


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tunable electronic properties. In the case of CuCl ML, there is a highly stabilized valence band, which indicates that such material can be promising for hole transfer, which in turn favors the water oxidation process. Hence, CuCl ML can be used as a promising Z-scheme photocatalytic material. The band alignment study indicates that the CuCl ML system is a good photocatalytic material because it satisfies the thermodynamic requirement of the water splitting reaction. Moreover, the conduction/valence band edge potential of CuCl ML is more appropriate for the water oxidation/ reduction reaction compared to some of the previously reported efficient photocatalysts.66 However, to fulfill the band edge requirement for overall water-splitting, some materials still require an additional potential to carry out the water-splitting reaction, and such additional potential is referred to as overpotential. Besides, it is also important to know whether the potential of the VBM and CBM is good enough to overcome such overpotential. Thus, we have studied the reaction free energies for oxygen evolution and hydrogen evolution reactions on the CuCl ML and its bulk surface. Though our band edge alignment study indicates that the bulk surface is not very promising for water oxidation over the ML structure, still we have studied the OER reaction mechanism on the bulk surface to understand the reasons behind this. 3.5. Oxygen Evolution Reaction (OER). The possible reaction steps of the OER have been studied on the surface of the CuCl ML and the bulk CuCl (111) surface to calculate the reaction overpotential. The OER is a four-electron water oxidation process, and the probable reaction mechanism steps are as follows:39,40

counterpart. This indicates that the planar structure of ML leads to an increase of the interaction of orbitals over the bulk counterpart. Therefore, the electronic properties of such monolayer structures can be easily tuned, which is beneficial for the photocatalytic applications. 3.4. Band Edge Alignment. The band edge alignment study has been done to find the position of the conduction band minimum (CBM) and valence band maximum (VBM) potential with respect to the water redox potential. Thus, the position of the VBM and CBM can be deduced using the following equation:61,62 1 ECBM/VBM = BGC ± Eg (2) 2 where ECBM/VBM, BGC, and Eg denote the energy of the CBM, VBM, band gap center energy, and band gap of the system. The BGC defines the Fermi position for undoped materials, and the position of the BGC can be estimated from the electrostatic potential (V(r)) value.61 Furthermore, the position of the VBM and CBM can be calculated using eq 2 at the AVS. Figure S6 shows the electrostatic potential plot averaged over the plane parallel to the surface, where the positions of the VBM and CBM have been evaluated at the AVS.63 Furthermore, the band edge potential values of CuCl bulk and ML have been calculated (Figure 4) using the PBE0

* + H 2O → HO* + (H+ + e−)

(3)

HO* → O* + (H+ + e−)

(4)

O* + H 2O → HOO* + (H+ + e−)

(5)

HOO* → * + O2 + (H+ + e−)

(6)

where the asterisk (*) sign denotes the CuCl ML/periodic CuCl (111) surface and O*, HO*, and HOO* denote an adsorbed oxygenated species, respectively. We have tried to adsorb intermediates in all the possible sites of ML (Figure 2), and the bulk surface (Figure S7) and the energetically most stable structure has been considered for the study of OER/ HER. The most stable structures are given in Figure 5, and their adsorption energies (Text S6, SI) are listed in Table S2. The adsorption pattern of all intermediates species of OER on CuCl ML is like that on the CuCl bulk except for O* adsorption as it interacts with three Cu atoms and therefore is more strongly adsorbed on the ML surface than any other OER intermediates. On the other hand, HO* more strongly adsorbs on the bulk surface than that on the ML though their adsorption type is similar. The calculated Bader atomic charges show that O of HO carries more negative charge (1.18 |e|) when it adsorbs on the CuCl bulk surface than that on the ML (1.15 |e|). Thus, the HO* intermediate strongly adsorbs on the bulk surface. Adsorption of intermediate will decide the free energy change of individual steps of OER. The bulk CuCl (111) surface shows some corrugation after optimization (Figure S7), which is consistent with the previous studies too.67 Such corrugated structure could be the reason for strong adsorption. However, in case of OER, there are four

Figure 4. Band edge alignment of CuCl ML and CuCl bulk with respect to water oxidation and reduction potential. Here, EVBM, ECBM, and Eg are valence band maxima, conduction band minima, and band gap (eV), respectively.

functional. Figure 4 shows that CuCl bulk and ML have a perfect band position for the overall water splitting reaction. The valence and conduction band offset values are 1.2 and 0.72 eV, respectively, which indicates that the VBM of ML is 1.2 eV more stabilized than the bulk, whereas the CBM of the bulk lies 0.72 eV above the CBM of the ML. Thus, the ML has a more stabilized valence band compared to that of the bulk structure as the Cl-3p orbitals shifted toward the Cu-3d orbitals. However, CuCl is the wide band gap semiconductor, and such wide band gap based materials can be important for Z-scheme based photocatalysis.64 The combination of two photocatalysts in the Z-scheme approach is one of the most promising strategies for enhancing the performance of the photocatalytic water splitting reaction due to spatial separation of electron−hole pairs.65 Furthermore, ML based structures are very much flexible for making heterostructure and also have E


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Figure 5. Adsorption of OER intermediates on (a) CuCl ML and (b) CuCl (111) surface. Adsorption free energy thermodynamics for OER under different applied potentials (U) for (c) CuCl ML and (d) CuCl(111). Here, red and light pink color balls represent oxygen and hydrogen atoms, respectively.

Figure 6. Adsorption of HER intermediate on (a) CuCl ML and (b) periodic CuCl (111) surface. (c) Reaction free energy profile of HER.

applied electrode potentials on both the CuCl ML and (111) surface are shown in Figure 5. At U = 0, all the OER steps are endergonic on the ML surface. The strong adsorption of O* is the reason for the HOO* formation step to be the rate limiting step (ΔG= 2.76 eV). Furthermore, Figure 5 shows that equilibrium potential (1.23) is not enough for all steps to be thermodynamically favorable. At U = 2.76 V, all the elementary steps are exergonic with an OER overpotential (ηOER) of 1.53 V (ηOER = ΔG - 1.23 V). Therefore, CuCl ML shows better activity toward OER compared to some of the previously reported MLs such as g-C3N4 (1.56 V)68 and transition metal doped C2N (1.58−2.93 V).69 In comparison to CuCl ML, all the OER reaction steps are endergonic on the CuCl(111) surface except for HO* formation, as HO* strongly adsorbs on the bulk surface. As a result, O* formation becomes highly endergonic with a free energy change of 2.84 eV. Thus, it gives an OER overpotential (ηOER) of 1.61 V, which is higher than that on the ML. Furthermore, we have considered the situation of irradiation when the potential of the photogenerated holes is treated as

electrochemical steps. Therefore, the strong adsorption energy of OH means that one of the four steps is favorable. This does not mean that the whole OER reaction is favorable. In this study, the feasibility of the OER reaction is evaluated based on the overall free energy change for all the four electrochemical steps rather than for a single electrochemical step. In this work, we have considered overpotential as the important activity descriptor since it is of considerable interest from experimental perspectives. The overpotential has been identified from free energy calculations, which are expected to follow the same trend as that of kinetic barriers owing to the BEP relation between free energy and activation barriers. We have considered Nørskov’s computational hydrogen electrode model to understand the energetics of OER elementary steps under the effect of applied potential.40 This model has been well standardized for electrocatalytic reactions such as OER in explaining the activity of catalysts.6,39,40 Hence we believe that our free energy based analysis is good enough to explain the OER activity of the systems under consideration. The reaction free energy profiles for OER in the presence/absence of F


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the reaction overpotential, but the strong adsorption of intermediate in the bulk surface makes CuCl ML a better catalyst for HER too. Interestingly, CuCl ML has better band edge position compared to some of the previously reported catalysts, for example TiO2, MoS2, Cu2O, and Cu2S and so on. The calculated OER and HER activity is even better compared to that of some previously reported MLs such as g-CN, g-C3N4, and transition metal doped C2N. Hence, we predict that the proposed CuCl ML can be a promising material for water splitting.

the valence band potential. The values of the VBM for the ML and bulk are 2.55 and 1.35 eV with respect to the NHE. The holes in the valence band of the CuCl ML provide enough driving force to compensate the OER overpotential, whereas an external bias of 1.49 V (2.84−1.35) requires for OER in the bulk surface. Therefore, the CuCl bulk surface requires a very high overpotential as the valence band position of the bulk surface is not much stabilized for water-oxidation reaction. That means both band alignment and OER reaction overpotential study are commensurate with each other. Therefore, from the above discussion, we have observed a different notable point which confirms that CuCl ML is a better candidate for the water-oxidation half-cell reaction over the bulk counterpart. 3.6. Hydrogen Evolution Reaction (HER). The electrocatalytic activity of CuCl ML has been further confirmed by studying the mechanism of the second half-cell reaction named as HER with respect to the periodic CuCl (111) surface. Therefore, all the possible HER mechanisms have been considered:68,70 * + (H+ + e) → H*

(7)

H* + (H+ + e) → H 2 + *

(8)

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.9b00699.

Figure 6 shows the most stable HER intermediate based structures. The adsorption behaviors of HER intermediates on a CuCl ML and periodic CuCl (111) surface are quite different as H binds with one Cu atom on a CuCl ML, while it binds with two Cu atoms on the periodic CuCl (111) surface. As a result, H* shows high adsorption energy when it adsorbs on the periodic CuCl (111) surface over a CuCl ML. The reaction free energy of HER has been calculated using eq 1. The ΔG is the major descriptor of the electrocatalytic activity of materials toward HER.68−70 The free energy profile (Figure 6) suggests that CuCl ML requires high free energy for H* adsorption compared to that on the bulk surface. Therefore, the calculated overpotential for HER on CuCl ML is 0.95 V, which is quite high compared to that on the periodic CuCl (111) surface (0.56 V). On the other hand, the photogenerated electrons in the conduction band of the ML and bulk surface provide enough potential (1.11 and 1.83 eV with respect to the NHE) to compensate the HER overpotential,68 which indicates that both the CuCl based systems are promising for HER. Therefore, we have shown that reducing the dimension from bulk to monolayer significantly affects the overall watersplitting process, and the proposed monolayer can be a potential candidate for the overall water splitting reaction.

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The modeling of periodic CuCl (111) surface and computational details, energetic, dynamic, thermal, mechanical stability, and adsorption energy calculations. The relaxed structure of the periodic CuCl (111) surface, phonon dispersion plot of CuCl ML using displacement eigenvectors of the CuCl ML corresponding to the soft phonon modes, AIMD simulation, and RMSD plot, snapshot of the CuCl ML after AIMD, stress−strain curve, and the electrostatic potential averaged in the z-direction of the CuCl ML and bulk surface. The band gap value (eV) of CuCl bulk and ML calculated by the different levels of theories and adsorption energies of OER/HER intermediates. (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Priyanka Garg: 0000-0002-9852-929X Kuber Singh Rawat: 0000-0002-7308-4204 Gargee Bhattacharyya: 0000-0002-8512-6854 Sourabh Kumar: 0000-0003-2488-8529 Biswarup Pathak: 0000-0002-9972-9947 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank IIT Indore for the lab and computing facilities. This work is supported by DST-SERB [Grant number: EMR/2015/ 002057] and CSIR [Grant number:01(2886)/17/EMR(II)]. P.G. and G.B. thank MHRD, and K.S.R. and S.K. thank UGC for the research fellowship.

4. CONCLUSION In conclusion, the photocatalytic water splitting reaction has been systematically studied over the CuCl ML and its bulk surface. The stability of the CuCl monolayer has been verified via energetic, dynamic, thermal, and mechanical studies. Our band edge alignment study indicates that the overall water splitting is very much favorable on the CuCl ML surface with respect to the bulk surface. Furthermore, the reaction overpotential study indicates that CuCl ML provides enough driving force to compensate the reaction overpotential for OER compared to that of the bulk. This could be due to stabilization of the valence band in the ML. On the other hand, photogenerated electrons of the conduction band in the CuCl ML and bulk surface provide enough potential to compensate

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REFERENCES

(1) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37−38. (2) Kudo, A.; Miseki, Y. Heterogeneous Photocatalyst Materials for Water Splitting. Chem. Soc. Rev. 2009, 38, 253−278. (3) Garg, P.; Kumar, S.; Choudhuri, I.; Mahata, A.; Pathak, B. Hexagonal Planar CdS Monolayer Sheet for Visible Light Photocatalysis. J. Phys. Chem. C 2016, 120, 7052−7060. (4) Zhang, T.; Lin, W. Metal−organic frameworks for artificial photosynthesis and photocatalysis. Chem. Soc. Rev. 2014, 43, 5982− 5993. G


Article

ACS Applied Nano Materials (5) Jin, H.; Wang, J.; Su, D.; Wei, Z.; Pang, Z.; Wang, Y. In situ Cobalt−Cobalt Oxide/N-Doped Carbon Hybrids as Superior Bifunctional Electrocatalysts for Hydrogen and Oxygen Evolution. J. Am. Chem. Soc. 2015, 137, 2688−2694. (6) Valdes, A.; Qu, Z.-W.; Kroes, G.-J.; Rossmeisl, J.; Nørskov, J. K. Oxidation and Photo-Oxidation of Water on TiO2 Surface. J. Phys. Chem. C 2008, 112, 9872−9879. (7) Popczun, E. J.; McKone, J. R.; Read, C. G.; Biacchi, A. J.; Wiltrout, A. M.; Lewis, N. S.; Schaak, R. E. Nanostructured Nickel Phosphide as an Electrocatalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 9267−9270. (8) Han, A.; Zhang, H.; Yuan, R.; Ji, H.; Du, P. Crystalline Copper Phosphide Nanosheets as an Efficient Janus Catalyst for Overall Water Splitting. ACS Appl. Mater. Interfaces 2017, 9, 2240−2248. (9) He, L.; Zhou, D.; Lin, Y.; Ge, R.; Hou, X.; Sun, X.; Zheng, C. Ultrarapid in Situ Synthesis of Cu2S Nanosheet Arrays on Copper Foam with Room-Temperature-Active Iodine Plasma for Efficient and Cost-Effective Oxygen Evolution. ACS Catal. 2018, 8, 3859−3864. (10) Giovanni, C. D.; Reyes-Carmona, Á .; Coursier, A.; Nowak, S.; Grenèche, J.−M.; Lecoq, H.; Mouton, L.; Rozière, J.; Jones, D.; Peron, J.; Giraud, M.; Tard, C. Low-Cost Nanostructured Iron Sulfide Electrocatalysts for PEM Water Electrolysis. ACS Catal. 2016, 6, 2626−2631. (11) Zhao, J.; Tran, P. D.; Chen, Y.; Loo, J. S. C.; Barber, J.; Xu, Z. J. Achieving High Electrocatalytic Efficiency on Copper: A Low-Cost Alternative to Platinum for Hydrogen Generation in Water. ACS Catal. 2015, 5, 4115−4120. (12) Luo, J.; Steier, L.; Son, M.-K.; Schreier, M.; Mayer, M. T.; Gratzel, M. Cu2O Nanowire Photocathodes for Efficient and Durable Solar Water Splitting. Nano Lett. 2016, 16, 1848−1857. (13) Kwon, Y.; Soon, A.; Han, H.; Lee, H. Shape Effects of Cuprous Oxide Particles on Stability in Water and Photocatalytic Water Splitting. J. Mater. Chem. A 2015, 3, 156−162. (14) Yang, R.; Lu, X.; Huang, X.; Chen, Z.; Zhang, X.; Xu, M.; Song, Q.; Zhu, L. Bi-Component Cu2O−CuCl Composites with Tunable Oxygen Vacancies and Enhanced Photocatalytic Properties. Appl. Catal., B 2015, 170−171, 225−232. (15) Iwashina, K.; Iwase, A.; Kudo, A. Sensitization of Wide Band Gap Photocatalysts to Visible Light by Molten CuCl Treatment. Chem. Sci. 2015, 6, 687−692. (16) Kawazoe, T.; Masumoto, Y. Luminescence Hole Burning And Quantum Size Effect Of Charged Excitons In CuCl Quantum Dots. Phys. Rev. Lett. 1996, 77, 4942−4945. (17) Valenta, J.; Moniatte, J.; Gilliot, P.; Honerlage, B.; Ekimov, A. I. Hole-Filling of Persistent Spectral Holes in the Excitonic Absorption Band of CuBr Quantum Dots. Appl. Phys. Lett. 1997, 70, 680−682. (18) Ahn, D.; Park, S.-H. Cuprous Halides Semiconductors as a New Means for Highly Efficient Light-Emitting Diodes. Sci. Rep. 2016, 6, 20718. (19) Ullah, H.; Reshak, A. H.; Inayata, K.; Alia, R.; Murtaza, G.; Sheraz, S.; Din, H. U.; Alahmed, Z. A. Structural, Elastic, Optoelectronic and Optical Properties of CuX (X= F, Cl, Br, I): A DFT study. J. Optoelectron. Adv. M. 2014, 16, 1493−1502. (20) Lee, J.; Park, D.; Heo, I.; Yim, S. Effect Of Cuprous Halide Interlayers On The Device Performance of ZnPc/C60 Organic Solar Cells. Mater. Res. Bull. 2014, 58, 132−135. (21) Huang, D.; Xu, J.-P.; Jiang, J.-W.; Zhao, Y.-J.; Peng, B.-L.; Zhou, W.-Z.; Guo, J. Difficulty of Long-Standing n-Type Conductivity in Equilibrium and Non-Equilibrium Γ-CuCl: A First-Principles Study. Phys. Lett. A 2017, 381, 2743−2747. (22) Hossain, M. I.; Alharbi, F. H.; Tabet, N. Copper Oxide As Inorganic Hole Transport Material For Lead Halide Perovskite Based Solar Cells. Sol. Energy 2015, 120, 370−380. (23) Janczarek, M.; Kowalska, E. On the Origin of Enhanced Photocatalytic Activity of Copper-Modified Titania in the Oxidative Reaction Systems. Catalysts 2017, 7, 317. (24) Taubert, A. CuCl Nanoplatelets from an Ionic Liquid-Crystal Precursor. Angew. Chem. 2004, 116, 5494−5496.

(25) Edamatsu, K.; Iwai, S.; Itoh, T.; Yano, S.; Goto, T. Subpicosecond Spectroscopy of The Optical Nonlinearities of CuCl Quantum Dots. Phys. Rev. B 1995, 51, 11205. (26) Kawazoe, T.; Kobayashi, K.; Lim, J.; Narita, Y.; Ohtsu, M. Direct Observation of Optically Forbidden Energy Transfer between CuCl Quantum Cubes via Near-Field Optical Spectroscopy. Phys. Rev. Lett. 2002, 88, 067404. (27) Miro, P.; Audiffred, M.; Heine, T. An Atlas of TwoDimensional Materials. Chem. Soc. Rev. 2014, 43, 6537−6554. (28) Ci, L.; Song, L.; Jin, C.; Jariwala, D.; Wu, D.; Li, Y.; Srivastava, A.; Wang, Z. F.; Storr, K.; Balicas, L.; Liu, F.; Ajayan, P. M. Atomic Layers of Hybridized Boron Nitride and Graphene Domains. Nat. Mater. 2010, 9, 430−435. (29) Blochl, P. E. Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953−17979. (30) Kresse, G.; Furthmuller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using A Plane-Wave Basis Set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169−11186. (31) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (32) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Fiolhais, C. Atoms, Molecules, Solids, And Surfaces: Applications of The Generalized Gradient Approximation For Exchange And Correlation. Phys. Rev. B: Condens. Matter Mater. Phys. 1992, 46, 6671−6687. (33) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) For The 94 Elements H-Pu. J. Chem. Phys. 2010, 132, 154104. (34) Perdew, J.; Ernzerhof, M.; Burke, K. Rationale for Mixing Exact Exchange with Density Functional Approximations. J. Chem. Phys. 1996, 105, 9982. (35) Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Hybrid Functionals Based on A Screened Coulomb Potential. J. Chem. Phys. 2003, 118, 8207−8215. (36) Henkelman, G.; Arnaldsson, A.; Jonsson, H. A Fast and Robust Algorithm for Bader Decomposition of Charge Density. Comput. Mater. Sci. 2006, 36, 354−360. (37) Posternak, M.; Baldereschi, A.; Freeman, A. J.; Wimmer, E.; Weinert, M. Prediction of Electronic Interlayer States in Graphite and Reinterpretation of Alkali Bands in Graphite Intercalation Compounds. Phys. Rev. Lett. 1983, 50, 761−764. (38) Rawat, K. S.; Mahata, A.; Pathak, B. Thermochemical and Electrochemical CO2 Reduction on Octahedral Cu Nanocluster: Role of Solvent Towards Product Selectivity. J. Catal. 2017, 349, 118−127. (39) Rossmeisl, J.; Logadottir, A.; Nørskov, J. K. Electrolysis of Water On (Oxidized) Metal Surfaces. Chem. Phys. 2005, 319, 178− 184. (40) Skùlason, E.; Tripkovic, V.; Björketun, M. E.; Gudmundsdóttir, S.; Karlberg, G.; Rossmeisl, J.; Bligaard, T.; Jónsson, H.; Nørskov, J. K. Modeling the Electrochemical Hydrogen Oxidation and Evolution Reactions on the Basis of Density Functional Theory Calculations. J. Phys. Chem. C 2010, 114, 18182−18197. (41) NIST. Computational Chemistry Comparison and Benchmark Database, NIST Standard Reference Database Number 101, Release 19, April 2018, Editor: Johnson, Russell D., III (http://cccbdb.nist. gov/). (42) Baroni, S.; Giannozzi, P.; Testa, A. Green’s-Function Approach to Linear Response In Solids. Phys. Rev. Lett. 1987, 58, 1861−1864. (43) Nørskov, K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jónsson, H. Origin of the Overpotential for Oxygen Reduction at a Fuel-Cell Cathode. J. Phys. Chem. B 2004, 108, 17886−17892. (44) Shin, D.; Sinthika, S.; Choi, M.; Thapa, R.; Park, N. Ab Initio Study of Thin Oxide−Metal Overlayers as an Inverse Catalytic System for Dioxygen Reduction and Enhanced CO Tolerance. ACS Catal. 2014, 4, 4074−4080. (45) Shin, D.; Thapa, R.; Park, N. An Oxygen Reduction Catalytic Process Through Superoxo Adsorption States On N-Type Doped HBN: A First-Principles Study. Curr. Appl. Phys. 2015, 15, 727−732. H


Article

ACS Applied Nano Materials (46) Wang, X.; Chen, W.-K.; Lu, C.-H. A Periodic Density Functional Theory Study of The Dehydrogenation of Methanol Over CuCl (111) Surface. Appl. Surf. Sci. 2008, 254, 4421−4431. (47) Hull, S.; Keen, D. A. High-Pressure Polymorphism of The Copper(I) Halides: A Neutron-Diffraction Study To ∼ 10 GPa. Phys. Rev. B 1994, 50, 5868. (48) Wang, Q.; Chen, G.; Pei, J. Morphological Evolution of CuCl Microstructures From Nano/Micro-Rod Arrays To Thin Film With A Preferred Orientation (1 1 1) Plane. J. Alloys Compd. 2013, 549, 341− 347. (49) Xie, T.; Gong, M.; Niu, Z.-Q.; Li, S.-A.; Yan, X.-Y.; Li, Y.-D. Shape-controlled CuCl Crystallite Catalysts for Aniline Coupling. Nano Res. 2010, 3, 174−179. (50) Hsueh, H.-C.; Maclean, J. R.; Guo, G. Y.; Iee, M.-H.; Clark, S. J.; Ackland, G. J.; Crain, J. Pressure-induced Polymorphism in CuCl: An Ab Initio Study. Phys. Rev. B 1995, 51, 12216. (51) Shi, Z.; Zhang, Z.; Kutana, A.; Yakobson, B. I. Predicting TwoDimensional Silicon Carbide Monolayers. ACS Nano 2015, 9, 9802− 9809. (52) Ding, Y.; Wang, Y. Density Functional Theory Study of the Silicene-like SiX and XSi3 (X = B, C, N, Al, P) Honeycomb Lattices: The Various Buckled Structures and Versatile Electronic Properties. J. Phys. Chem. C 2013, 117, 18266−18278. (53) Emery, A. A.; Wolverton, C. High-throughput DFT calculations of formation energy, stability and oxygen vacancy formation energy of ABO3 perovskites. Sci. Data 2017, 4, 170153. (54) Allard, A.; Wirtz, L. Graphene on Metallic Substrates: Suppression of the Kohn Anomalies in the Phonon Dispersion. Nano Lett. 2010, 10, 4335−4340. (55) Mannix, A. J.; Zhou, X.-F.; Kiraly, B.; Wood, J. D.; Alducin, D.; Myers, B. D.; Liu, X.; Fisher, B. L.; Santiago, U.; Guest, J. R.; Yacaman, M. J.; Ponce, A.; Oganov, A. R.; Hersam, M. C.; Guisinger, N. P. Synthesis of borophenes: Anisotropic, Two-Dimensional Boron Polymorphs. Science 2015, 350, 1513−1516. (56) Andryushechkin, B. V.; Eltsov, K. N.; Shevlyuga, V. M. Cui Growth on Copper Surfaces under Molecular Iodine Action: Influence of the Surface Anisotropy in the Iodine Monolayer. Surf. Sci. 2004, 566−568, 203−209. (57) Fairbrother, D. H.; Roberts, J. G.; Rizzi, S.; Somorjai, G. A. Structure of Monolayer and Multilayer Magnesium Chloride Films Grown on Pd (111). Langmuir 1997, 13, 2090−2096. (58) Wang, H.; Liu, E.; Wang, Y.; Wan, B.; Ho, C. H.; Miao, F.; Wan, X. G. Cleavage tendency of anisotropic two-dimensional materials: ReX2 (X = S, Se) and WTe2. Phys. Rev. B: Condens. Matter Mater. Phys. 2017, 96, 165418. (59) Liu, S.; Liu, B.; Shi, X.; Lv, J.; Niu, S.; Yao, M.; Li, Q.; Liu, R.; Cui, T.; Liu, B. Two-dimensional Penta-BP5 Sheets: High-stability, Strain tunable Electronic Structure and Excellent Mechanical Properties. Sci. Rep. 2017, 4, 2404. (60) Kalosakas, G.; Lathiotakis, N. N.; Galiotis, C.; Papagelis, K. InPlane Force Fields and Elastic Properties of Graphene. J. Appl. Phys. 2013, 113, 134307. (61) Toroker, M. C.; Kanan, D. K.; Alidoust, N.; Isseroff, L. Y.; Liao, P.; Carter, E. A. First Principles Scheme to Evaluate Band Edge Positions in Potential Transition Metal Oxide Photocatalysts And Photoelectrodes. Phys. Chem. Chem. Phys. 2011, 13, 16644−16654. (62) Kanan, D. K.; Carter, E. A. Band Gap Engineering of MnO via ZnO Alloying: A Potential New Visible-Light Photocatalyst. J. Phys. Chem. C 2012, 116, 9876−9887. (63) Haastrup, S.; Strange, M.; Pandey, M.; Deilmann, T.; Schmidt, P. S.; Hinsche, N. F.; Gjerding, M. N.; Torelli, D.; Larsen, P. M.; RiisJensen, A. C.; Gath, J.; Jacobsen, K. W.; Mortensen, J. J.; Olsen, T.; Thygesen, K. S. The Computational 2D Materials Database: HighThroughput Modeling and Discovery of Atomically Thin Crystals. 2D Mater. 2018, 5, 042002. (64) Liu, J.; Liu, Y.; Liu, N.; Han, Y.; Zhang, X.; Huang, H.; Lifshitz, Y.; Lee, S.-T.; Zhong, J.; Kang, Z. Metal-Free Efficient Photocatalyst for Stable Visible Water Splitting via A Two-Electron Pathway. Science 2015, 347, 970−974.

(65) Low, J.; Jiang, C.; Cheng, B.; Wageh, S.; Al-Ghamdi, A. A.; Yu, J. A Review of Direct Z-Scheme Photocatalysts. Small 2017, 1, 1700080. (66) Tamirat, A. G.; Rick, J.; Dubale, A. A.; Su, W.-N.; Hwang, B.-J. Using Hematite for Photoelectrochemical Water Splitting: A Review of Current Progress and Challenges. Nanoscale Horiz. 2016, 1, 243− 267. (67) Suleiman, I. A.; Radny, M. W.; Gladys, M. J.; Smith, P. V.; Mackie, J. C.; Kennedyc, E. M.; Dlugogorskid, B. Z. Thermodynamic Stability and Structure of Cuprous Chloride Surfaces: A DFT Investigation. Phys. Chem. Chem. Phys. 2015, 17, 7038−7045. (68) Wirth, J.; Neumann, R.; Antonietti, M.; Saalfrank, P. Adsorption and Photocatalytic Splitting of Water on Graphitic Carbon Nitride: A Combined First Principles and Semiempirical Study. Phys. Chem. Chem. Phys. 2014, 16, 15917−15926. (69) Zhang, X.; Chen, A.; Zhang, Z.; Jiaoa, M.; Zhou, Z. Transition Metal Anchored C2N Monolayers as Efficient Bifunctional Electrocatalysts for Hydrogen and Oxygen Evolution Reactions. J. Mater. Chem. A 2018, 6, 11446−11452. (70) Srinivasu, K.; Ghosh, S. K. Photocatalytic Splitting of Water on S-Triazine Based Graphitic Carbon Nitride: An Ab Initio Investigation. J. Mater. Chem. A 2015, 3, 23011−23016.

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