Chemical interaction between nitrogen-doped graphene defects and a copper (1 1 1) surface: Effects on water molecule adsorption

Applied Surface Science 502 (2020) 144149

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Chemical interaction between nitrogen-doped graphene defects and a copper (1 1 1) surface: Effects on water molecule adsorption

T

Victor A. Cardozo–Mataa, J.A. Pescador–Rojasa, A. Hernández–Hernándeza, L.A. Hernández–Hernándeza, A. Miralriob, F.J. Martínez–Faríasa, E. Vallejo–Castañedaa, ⁎ E. Rangela, a b

Escuela Superior de Apan, Universidad Autónoma del Estado de Hidalgo, Carretera Apan-Calpulalpan Km.8, Col. Chimalpa, 43920 Apan, Hgo, Mexico Tecnologico de Monterrey, Campus Toluca, Departamento Bioingeniería y Ciencias, Eduardo Monroy Cardenas 2000, San Antonio Buenavista, 50110 Toluca, Mexico

A R T I C LE I N FO

A B S T R A C T

Keywords: Nitrogen doped graphene Copper surface Water adsorption Nitrogen defects

The electronic properties of nitrogen–doped graphene (pyridinic, pyrrolic and graphitic defects) supported by a copper substrate and their effects on water molecule adsorption have been studied by applying density functional theory, together with the vdW–DF correction. We discovered that pyrrolic and pyridinic defects strongly interact with the copper substrate due to covalent chemical bonds between the nitrogen atoms and the underlying metal. The binding energy and charge transfer from the copper to nitrogen atoms induces a formal oxidation state in the copper [copper (I) or copper (II)], depending on the type of defect which interacts. The effects of the metallic support on pyridinic, pyrrolic and graphitic defects influence the adsorption properties of water molecules. For nitrogen–doped graphene without substrate, the interaction of the water molecule with the pyridinic or pyrrolic defects consists mostly of hydrogen bonds and once a water molecule is adsorbed into a nitrogen site, this promotes the adsorption of the next molecule. Likewise, for the sheet bound to the copper substrate, an increase in van der Waals interactions for the water molecule adsorption occurs, due to the proximity between the sheet and the substrate.

1. Introduction Nitrogen (N) doping has proven to be an effective way to adjust graphene properties and facilitate their potential use in different applications [1–9]. Conceptually, N contains one additional electron compared to a carbon (C) atom, and graphene N–doping changes electron density, thus increasing its reactivity [10]. Several groups have also observed three different nitrogen doped graphene (NG) systems which are apparent defects (1) pyridinic, (2) pyrrolic, (3) graphitic [3,11–16]. Depending on the synthesis method and the parameters employed, it is possible to modify the proportion of the various defects. Furthermore, Rangel et al. performed the optimization for different types of NG [17]. The theoretical N concentration in our super–cells was calculated to range from 4.1 to 9.7%; whereas the corresponding experimental values [15] (NG sheets containing pyridinic and graphitic defects) range from 5.7 to 11.3% respectively. Apparently, N atoms may stabilize single, double, and triple vacancy formation in graphene. Moreover, we have found that the formation energy NxVy defects (x representing the number of carbon atoms replaced by nitrogen atoms,

⁎

and y representing the number of vacancies V) diminished with an increase in the number of nitrogen atoms [17]. Moreover, our results suggest that the concentration of pyridinic defects (N3V1, N4V2) and pyrrolic defects (N3V1, N3V3) will predominate over other defects [17]. However, defect induction via substrate and its relationship with the modification of the properties of the NG is still far from clearly understood [18–21]. A simple method can be applied to synthesize large areas of NG grown on copper foil [22] (order of centimetres), via chemical vapour deposition (CVD). These films consist of a single atomic layer that can be transferred to other surfaces [11]. Unfortunately, the most recent literature [21–23], states that these layers can be only grown on specific substrates, prompting the question of whether a chemical interaction occurs between the NG and the Cu surface. Bulushev et al. [24] in particular, who synthesized Cu clusters on N–doped mesoporous carbon via CVD, discovered that N doping leads to a strong interaction with the Cu species on the substrate, in the form of clusters around 5 nm. The Cu clusters on the N–doped carbon were smaller than those particles obtained on N free C, subsequent to the reaction.

Corresponding author. E-mail address: [email protected] (E. Rangel).

https://doi.org/10.1016/j.apsusc.2019.144149 Received 17 June 2019; Received in revised form 27 August 2019; Accepted 21 September 2019 Available online 18 October 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

Applied Surface Science 502 (2020) 144149

V.A. Cardozo–Mata, et al.

(a = b = 12.20 Å; c = 30 Å) defect models were chosen. The magnitude of c for both systems was large enough to avert un-desired interactions between adjacent layers, along this plane. To investigate the interactions of NG on the Cu(1 1 1) surface, a Cu super-cell was employed, conforming three atomic (5 × 5) Cu layers stacked in the z direction, which concurs with the system reported by L. Ferrighi et al. [2]. Experimentally, graphene grows on Cu(1 1 1), forming Moiré patterns as a consequence of the mismatch between Cu(1 1 1) and graphene, (2.556 Å and 2.461 Å, respectively). The Cu(1 1 1) super-cell was adapted to the graphene lattice parameter, so as to more closely resemble the experimental conditions [31]. Employing the vdW-DF correction, we reproduced lattice parameter values of 2.46 Å for graphene and 2.52 Å for Cu(1 1 1), corresponding to a 3.12% lattice mismatch. The binding energy Ebind of the Cu(1 1 1) on the NG was accounted as follows:

Similarly, understanding the hydrophilicity or hydrophobicity of NG is fundamental to the development of devices such as catalysts, adsorbents, storage-separators and others, but the properties of NG remain poorly understood. In only one theoretical work [25], the authors show the existence of anomalous water behaviour on N–doped carbon. They found that hydrophilicity is conditioned by pore-size; type of N defects and N concentration [25]. However, this work did not study N–doped nanoporous carbon on substrates such as copper, which implies that the underlying substrate may influence the hydrophilicity properties at the C surface. Therefore, some important questions emerge: What kind of interaction occurs between pyridinic (N3V1, N4V2), and pyrrolic (N3V1, N3V3) defects at the copper surface? At a fundamental level, how is the interaction between the water molecule and each one of the N defects induced by the N–doping in graphene? Is the interaction between the defects and water molecule modified in the presence of the copper substrate? Significantly, there are no first-principle calculations for either the chemical interaction between the nitrogen-doped graphene defects; pyridinic (N3V1, N4V2) and pyrrolic (N3V1, N3V3) and the copper substrate or their effects on water molecule adsorption. Considering that copper as substrate is widely used in the industrialized production of graphene due to its low cost and catalyst ability; additionally to largearea monolayer graphene growth, high nitrogen content incorporation and wide variety of related defects. The focus of this work is to perform theoretical calculations with DFT using the vdW–DF functional, in order to study the nature of binding and magnetic and electronic properties of pyridinic and pyrrolic defects, both free and in contact with the copper substrate and their effects on water molecule adsorption. This study is presented as follows; in Section 1, we present introduction. In Section 2, we describe our theoretical method and in Section 3, we present the results and discussion, in this section, we studied the nitrogen doped graphene defects on a supporting Cu substrate (1 1 1), and then we introduced the interactions between the water molecule and the nitrogen doped graphene defects, both free and supported by the copper substrate. Finally, conclusions can be found in Section 4.

Ebind = EGNx Vy + Esubstrate − EGNx Vy + substrate where EGNx Vy + substrate is the total energy per supercell of the monolayer in its ground state. The adsorption energies of H2O molecules on NG defects and H2O molecules on NG defects on Cu(1 1 1) were calculated as follows:

Eads = EGNx Vy + E H2 O − EGNx Vy + H2 O Eads = EGNx Vy + substrate + E H2 O − EGNx Vy + substrate + H2 O In this context, GNxVy represents the defect of graphene (G), x is the number of carbon atoms substituted by N atoms and y is the number of V vacancies. x + y is the number of total carbon atoms removed from the perfect graphene sheet to form the defects. EGNx Vy is the energy of the NG system with y-vacancy. EGNx Vy + substrate is the total energy of the NG on the copper surface with the y-vacancy system and E H2 O is the total energy of water. 3. Results and discussion 3.1. Nitrogen-doped graphene (NG) systems

2. Theoretical method Optimization for the different NG defects was performed previously by us (see supplementary material relating to reference 14). The formation energies for pyridinic N3V1 and N4V2 defects are 3.30 and 3.65 eV, respectively, whereas for pyrrolic N3V1 and N3V3, defects are 5.52 and 8.07 eV, respectively; the magnetic moments conform to 0.35, 0.00, 0.92 and 0.10 μB, respectively. The total charges for the N atom are −0.19, −0.18, −0.15, and −0.22 e, respectively. These results suggest that the concentration of pyridinic N3V1 and N4V2, as well as pyrrolic N3V1 and N3V3, will predominate over NxVy defects (see supplementary material relating to reference 14). Contrastingly, dipole moments are established in NG, in relation to the electronegativity difference between N and C, as well as its sp2 hybridization, permitting the establishment of a potential difference compared to the surface of the copper substrate.

2.1. Computational methods Density functional theory (DFT) was applied within the general gradient approximation (GGA) [26] and the Quantum Espresso code [27]. Spin–unrestricted calculations were accomplished within the dispersion-corrected formalism. Specifically, exchange–correlation functional of Perdew-Burke-Ernzerhof [28] (PBE) was applied and the dispersion corrections were included by employing the vdW-DF approach, as proposed by the Thonhauser group [29,30]. The former vdW-DF approach is non-empirical spin-density formalism for the Van der Waals interaction and highlights the importance of spin for adsorption of several molecules on magnetic systems. This formalism was chosen because of the magnetic nature of most of our N-doped graphene systems. Ultrasoft pseudopotentials (USP), generated from scalar-relativistic calculations by the Rappe Rabe Kaxiras Joannopoulos (RRKJUS) method were employed on all atoms, which take into account nonlinear core-corrections for the Cu atom. Plane waves basis were used and the cut-off energy was set at 1360 eV and a total of 68 k points within the Monkhorst-Pack special k point scheme were used. Energy was converged by up to 1.0 × 10−7 a.u. The convergence criteria for the geometry optimization contemplated a threshold of 1.0 × 10−5 a.u., and 1.0 × 10−4 a.u., in terms of total energy for the residual forces on each atom. Electronic valence states considered in this work included: hydrogen (H) 1 s1, carbon (C) 2s22p2, oxygen (O) 2s22p4, copper (Cu) 3d104s14p0, and nitrogen (N) 2 s2 2p3. Periodic boundary conditions within a hexagonal unit cell, for pyridinic (a = b = 12.20 Å; and c = 30 Å) and pyrrolic

3.2. Adsorption on Cu(1 1 1) We optimized the NG system, with pyridinic (N3V1, N4V2) and pyrrolic (N3V1, N3V3) defects, deposited on Cu(1 1 1) substrate (see Fig. 1). The NG can be placed on the Cu(1 1 1) substrate with each N atom superimposed on a single Cu atom. Furthermore the Cu slab was modelled and optimised into three layers, each one composed of 25 atoms. Results from the chemical interaction (binding energy) between three, four, and five copper layers with NG defects were similar, which suggests that three layers are sufficient. We found that the graphitic defect energy adsorption on the Cu(1 1 1) is 0.17 eV, with a distance of 3.84 Å between the graphitic defect and the substrate, applying the vdW-DF functional. These results are consistent with previous 2

Applied Surface Science 502 (2020) 144149

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Fig. 1. Perspectives and details of the different systems interacting with an isolated water molecule, NG sheet on Cu(1 1 1), (a) pyrrolic N3V1, (b) pyridinic N4V2, (c) pyridinic N3V1 and (d) pyrrolic N3V3 defect. At the centre, the lateral and top views of the corresponding systems are shown and the mean distance from the copper substrate to the N-doped graphene sheet is presented (black font). On top of the lateral view is shown a zoom around the N defect and the copper substrate, the radial distance of the H bond is presented by a red dotted line. Within this frame, we have labelled the radial distances related to NeH bond (black font) from the water molecule linked to the nitrogenated defect in the graphene sheet and the CueN bond (blue font) from the N-doped graphene defect to the copper substrate. At the bottom, the related frame for the pyridinic N4V2 system is presented and the CueN bonds are labelled (black font). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

calculations by Ferrighi et al. [2] of 0.14 eV and 3.36 Å, respectively, applying vdW-DF2C09x methodology. Subsequently, we described the chemical interaction between the pyridinic and pyrrolic defects on the substrate. The binding energies of the pyridinic N3V1, pyrrolic N3V1 and pyrrolic N3V3 defects on the substrate are 4.013, 2.130, and 3.008 eV, respectively. The calculations using PBE with the vdW-DF functional increase values by about 12%. The binding energies and their nature depend strongly on the defect types. Fig. 1 shows the CueN bond lengths between 1.97 and 2.11 Å. In particular, there is evidence of the Cu(II) complexes coordination polyhedral, with CuN5 and CuN3O1 chromophores, having CueN bond lengths close to 2.7 Å [32], whereas other structures were reported with CueN bond lengths between 2.05 and 2.13 Å [33]. Notably, NG growth by CVD is occasionally polycrystalline [34], with point and line defects that may strongly interact with the Cu substrate; however this subject will be dealt with in future work. Charge density provides information about the bonding between NG and the substrate. Fig. 2 shows the charge density of a Cu(1 1 1) transversal section. The most intense colours indicate regions of maximum density charge, where NG bonds with the substrate. The existence of NeCu bonds is also indicated in Fig. 2 by the presence of closed spherical isolines that suggest the existence of covalent bonding between these atoms. There is experimental evidence via XPS spectra of the strongly interacting Cu species with N-doped porous carbon networks [24]. One new line at 399.0 eV is attributed to the interaction of Cu species with pyridinic N-doped carbon [24]; likewise other lines are attributed to pyridinic, 398.2 eV, pyrrolic 399.8 eV and graphitic, 400.9 eV, N species. Lines corresponding to the pyridinic, pyrrolic and graphitic species strongly resemble the CueN species, which makes the detection of lines by experimental analysis of the XPS spectra very difficult. The interaction between the electronic clouds of π-orbitals (NG) and the 3d-orbitals (Cu) occurs when the distance between the NG and the Cu substrate is below 4 Å [35]. Overall, these systems show small

differences on their electronic levels, since Cu 4p orbitals are the most dominant above the Fermi level (Ef) (vicinities C in Fig. S5). Small differences among them appear near Ef, being more noticeable a sp2 contribution of nitrogen in case of N3V1-Pyrrolic defect (vicinities B in Fig. S5). In case of N3V1 Pyridinic and N3V3 Pyrrolic, nitrogen sp2 orbitals appear deeper in energy and overlapped with Cu 3d orbitals (vicinities A in Fig. S5). We found that along with the former interaction, an oxidative addition [36,37] also occurs, and there is a charge transfer from the Cu to the N atom via covalent bond (net charges 0.4 e for Cu) breaking the π bond between N and the nearest C atom (see Fig. 3). The above was confirmed by means of the electron density-based Bader charge calculations, which obtained values ranging from 0.254 to 1.035 e (Table 1). Likewise, the mechanism described creates a free radical (unpaired electron) on C (α-nitrogen) as shown in Fig. 3(a) and (b) [38–40]. Therefore, the unpaired electron can be delocalized into a π cloud of the nearest pyrrolic or pyridinic defect, until this is positioned next to an N atom; this promotes an oxidative addition mechanism for the covalent NeCu bond, see Fig. 3(c). The aforementioned NeCu bond occurs for the pyrrolic N3V3, pyrrolic N3V1 and pyridinic N3V1 defects with a formal Cu oxidation state [Cu(I)]. Overall the states in the valence band near to the Fermi level, in the case of bonded Cu and N atoms are dominated by the contributions from the Cu 4p orbitals, hybridized with nitrogen 2sp2 (vicinity A in Fig. 4). Contributions derived from those N atoms with no bonds to Cu become more important around 1 eV below the, whereas no contributions to Cu are apparent (vicinity B in Fig. S3). In contrast, states derived from the Cu 3d orbitals, below 1 eV the, appear hybridized with nitrogen 2sp2. Likewise, remaining N atoms contribute, more to sp2 states (vicinity C in Fig. 4). Following the previously described procedure, pyridinic N4V2 interacts strongly with Cu(1 1 1); however, due to the electronic structural symmetry of the defect, a Cu atom is detached from the substrate

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Fig. 2. Lateral view and zoom-in around the nitrogenated defect of the charge density distribution (according to the logarithmic distribution, Log10) for the different systems interacting with an isolated water molecule, NG sheet on Cu(1 1 1) are presented. (a) pyridinic N3V1 on the CueN interaction plane, (b) pyridinic N3V1 on the water-system interaction plane, (c) pyridinic N4V2 on the water-system interaction plane, (d) pyrrolic N3V1 on the CueN interaction plane and (e) pyrrolic N3V1 on the water-system interaction plane. Top view of the charge density distribution (see Fig. S2).

Table 1 Bader charge calculated for the closest pair of Cu and N atoms in case of N3V1 (Pyridinic), N3V1 (Pyrrolic), N3V3 (Pyrrolic) and N4V2 (Pyridinic) defects. Also, total Bader charge calculated for a single water molecule adsorbed on each defective NG surface. System

Q(Cu)

Q(N)

Q(H2O)

N3V1 N3V1 N3V1 N3V1 N3V3 N3V3 N4V2 N4V2

0.254 0.266 0.326 0.325 0.319 0.236 1.035 1.058

−0.753 −0.76 −0.517 −0.529 −0.54 −0.557 −0.534 −0.527

– −0.016 – −0.022 – −0.019 – 0.003

(Pyridinic) (Pyridinic)–H2O (Pyrrolic) (Pyrrolic)–H2O (Pyrrolic) (Pyrrolic)–H2O (Pyridinic) (Pyridinic)–H2O

and positioned at the centre of the defect vacancy (see Fig. 1b), with a formal Cu(II) oxidation state and a 1.4 e net charge, furtherly confirmed by means of the 1.035 e value calculated for the Bader charge (Table 1). These results are consistent with the oxidation state and square planar geometry observed in NeCu(II) complexes [41,42]. This effect averts the chemical interaction between the N4V2-Cu defects and Cu substrate, leading only to the dispersive interactions presented in Fig. 1b). This is noticeable in case of its density of states, since near the Fermi level (vicinities B and C in Fig. S5) a marked overlap between copper atom and N4V2 defect is clearly shown. As result of the important interaction N4V2-Cu defect, copper orbitals 3d effectively overlap with nitrogen sp2. Deeper levels of the atom detached from the substrate appear up to −2

Fig. 3. (a) Pyridinic, (b) pyrrolic defects, contact between π electronic cloud and d electron orbitals of copper atom. (c) Oxidative addition from metallic Cu (in the substrate) to the N atom. It is proposed that in the transition state, the π electronic cloud undergoes a disruption that leads to the formation of a radical type reactive intermediate.

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of another H2O molecule. When a water molecule is adsorbed onto an N site, it promotes the adsorption of the next molecule via H bond (see Fig. 5). Water clusters are formed by cooperative adsorption of water molecules on the nearest N sites interacting via H bonds with other water molecules that are already adsorbed (see information about molecular dynamics in SI-1). V. Kumar et al. [25] found that these molecules, at higher relative pressures act as polynucleation sites and promote the formation of water nanowires if the N defects are close enough. The water adsorption energies on NG supported on Cu(1 1 1), can be explained in terms of van der Waals interactions , with a decrease in the interaction hydrogen bond (N—HeO); van der Waals interactions play an important role in the mechanism of water adsorption on pyridinic and pyrrolic defects (see Table 2). These assumptions have been confirmed by means of projected density-of-states (Fig. S5). In all cases, orbitals belonging water do not effectively overlap with defect’s orbitals. Instead of this, only a minimal shift of the contributions coming from the nitrogen atom closest to water is appreciated due to van der Waals interactions. In addition, the most stable site in all cases was also found above the vacancy with an H nearest to the one N defect as shown in Fig. 1, except for the water adsorption on pyridinic N4V2, for which the O atom is on top of the Cu atom. In case N3V1 pyridinic, N3V1 pyrrolic and N3V3 pyrrolic defective systems, water received a small amount of charge, since Bader charge was calculated ranging from −0.016 to −0.022 e (Table 1). However, in case of N4V2 pyridinic defect, water donated a minimal amount of charge, obtaining Bader charge of about 0.003 e (Table 1). As previously concluded, N3V1 and N3V3 pyrrolic defects behave similar, whereas N4V2 differs notoriously. The percentage increase in van der Waals interactions for water molecule adsorption is due to the proximity between NG and Cu(1 1 1): 1.97 Å for pyrrolic N3V1, 2.02 Å for pyrrolic N3V3, and 2.11 Å for pyridinic N3V1, respectively (see Fig. 1). The average adsorption energies of water at other sites distanced from the vacancy are about 0.27 eV (except for N4V2, ~0.15 eV), three times more than the corresponding values for water adsorption energy distanced from the vacancy on NG without substrate. Thus the proximity between NG and Cu(1 1 1) distanced from the vacancies also promotes the increase in the percentage of the Van der Waals interactions at those sites. This effect increases the water-NG interaction which may facilitate the spreading of water molecules on the surface, whilst also playing an important role involving interference with the water-water (H bonding) interaction resulting in the generation of nanodrops. It is significant that the water adsorption energy on a graphitic defect; both free and in contact with the substrate are 0.2406 and 0.3456 eV, respectively (see Fig. S4). The analyses discussed here suggest that a percentage of the Van der Waals interactions between the water and the Cu(1 1 1) substrate are transmitted through the NG, due to the proximity between the sheet and the substrate, thereby promoting certain wetting transparency in the hydrophilicity phenomena. Thus, we show that the substrate and the N-doping may act to tune the hydrophilicity of the graphene by modulating both the distance of a NG sheet supported by a Cu(1 1 1) substrate and also its metallic properties. However, it is still not clear whether the graphene surfaces supported on the substrate have complete wetting transparency [44] or complete wetting opacity [45]. For example, recent studies have suggested that the monolayer graphene on hydrophilic substrates is more transparent to wetting and more opaque to wetting, on hydrophobic substrates [46]. Other studies suggest that both substrate and dopants can change the Fermi level of graphene, which in turn affects the interaction between the graphene and the water molecules [47]. A number of authors found that supported graphene forms ripples and corrugations to maintain thermodynamic stability, which affects water adsorption [48,49]. Moreover, Zhang et al. showed that the surfaces of Cu films are

Fig. 4. Density of states of bonded CueN calculated for the 5 × 5 NG (pyrrolic N3V1) supported on Cu(1 1 1) super-cell, as well as PDOS projected over 2p, 2s orbitals of the nitrogen, 4s, 3d and 4p of the Cu atoms. Orbitals for regions: A, CueN bond from hybridized 4p and 2sp2; B, non-bonded N atoms; C, CueN bond from hybridized 3d and 2sp2. Fermi energy set at 0 eV. (For density of states of non-bonded CueN, see Fig. S3). For N3V1, N4V2 pyridinic and N3V3 pyrrolic systems see Fig. S5 in supplementary information.

eV below Fermi level (vicinity A in Fig. S5). 3.3. Adsorption of water molecule on free standing NG and on the Cu substrate Adsorption of the water molecule on free standing NG defects was studied at different sites. The most stable site of all (pyridinic and pyrrolic) was identified above the vacancy. The full relaxation yielded the adsorbed configurations, with the hydrogen nearest to the N defects (pyrrolic and pyridinic), with binding energies ranging from 0.36 to 0.48 eV (0.49–0.66 eV, taking into account the dispersive correction by the vdW-DF functional) and hydrogen bond (N—HeO) distances ranging from 2.07 to 2.31 Å (see Fig. S1 and Table S1). Water adsorption energies on other sites, distanced from the vacancy by 0.025 to 0.018 eV (~0.1 eV with dispersive correction). One configuration has the O atom above the centre of the C hexagonal ring with the two H equidistant to these C atoms, and the other configuration has the OH bonds directly on top of the C atom. Notably, other authors have used various theoretical methodologies to obtain values for the energies, ranging from 0.070 to 0.098 eV for water adsorption on graphene, revealing configurations that concur with the way our systems are distanced from the vacancy [43]. An analysis of the charge density (see Fig. 2), as well as of the density of states indicates the absence of any additional impurity states close to the Dirac point, meaning that single water molecules on free NG defects do not result in any doping. Furthermore, these results suggest the existence of a hydrogen bond (N—HeO), between the hydrogen (from the nearest water molecule) to the sole nitrogen atom (pyrrolic or pyridinic defects); roughly 65% of the interaction corresponds to the H bond, and the rest is derived from dispersive effects. The water adsorption energies were obtained using the PBE and vdW-DF (dispersive correction) functional, and the dipole of the water molecule is oriented at an angle θ with respect to the Z axis, as shown in Table 2. We also found that the negatively charged N atom attracts the H atoms of one water molecule, whereas the O atom attracts the H atom 5

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Table 2 Water adsorption (Ead - H2O), Dispersive Energy (vdW), Dispersive Correction Energy (error %), Angle normal direction to N defect plane ϕ(H2O), H2O angle (θ), Angle normal direction to plane (XY) φ(H2O)-Z angle. System

Pyrrolic N3V1 Pyrrolic N3V1 – Cu Pyridinic N3V1 Pyridinic N3V1 – Cu Pyrrolic N3V3 Pyrrolic N3V3 – Cu Pyridinic N4V2 Pyridinic N4V2 – Cu

Ead

- H2O

(eV)

Error %

PBE

vdW-DF

vdW

vdW-DF/PBE

0.363 0.246 0.312 0.141 0.363 0.264 0.480 0.149

0.492 0.389 0.467 0.455 0.476 0.442 0.660 0.293

0.129 0.143 0.155 0.314 0.114 0.178 0.181 0.144

35.6 58.3 49.9 223.0 31.3 67.3 37.7 96.92

not flat: they have ridges and valleys [50], which would thus affect the growth of nitrogen doped graphene and its chemical interaction with the Cu(1 1 1) substrate. All these facts and controversial results clearly indicate a dependency on experimental conditions, impelling the need for a better understanding of the physics and chemistry of pure and doped graphene on substrates in order to enhance experimental control and avoid incurring mistaken interpretations of the facts, regarding hydrophilicity properties.

ϕ(H2O)

θ

φ(H2O)-Z angle

46.75 40.32 57.83 66.73 56.99 49.96 45.81 56.14

103.74 103.35 103.57 104.97 103.92 102.77 103.29 103.89

44.99 38.47 66.10 66.35 59.58 48.17 55.70 60.08

hybridized with nitrogen sp2. In contrast, the states derived from Cu 3dorbitals, hybridized with nitrogen sp2 are mostly 1 eV below the Fermi level. Similarly, the adsorption of the water molecule on freestanding NG defects was studied at different sites. The most stable site in all cases appeared to be above the vacancy, with the hydrogen nearest to the nitrogen atom defects showing binding energies between 0.49 and 0.66 eV and an N—HeO distance that ranged between 2.07 and 2.31 Å. These results suggest the existence of an N—HeO hydrogen bond between hydrogen (from the water molecule) and the sole nitrogen atom, with roughly 65% of the interaction corresponding to the hydrogen bond, and the rest resulting from dispersive effects. Analysis of density of states shows the absence of any additional impurity states close to the Dirac point, which indicates that single water molecules on free NG do not result in any doping. The effects of metallic support on pyridinic, pyrrolic and graphitic defects influences the adsorption properties of water molecules. Water adsorption energies on NG supported on Cu (1 1 1) can mostly be explained by van der Waals interactions accompanied by a decrease in the hydrogen bond interaction. Again, the most stable site out of all the cases evidently referred to the vacancy, where hydrogen is nearest to the sole nitrogen atom defect, apart from water adsorption on N4V2, where the oxygen atom is on top of the Cu atom. The increase in the percentage of Van der Waals interactions causing water molecule adsorption is due to the proximity of NG to the substrate, which thus promotes certain wetting transparency in the hydrophilicity phenomena. Results obtained in this work are important because they will improve understanding of the chemical and physical properties of nitrogen doped graphene supported on a Cu(1 1 1) substrate, as a way of

4. Conclusion Chemical interactions of nitrogen doped graphene (NG); pyridinic defects (N3V1, N4V2), and pyrrolic defects (N3V1, N3V3) with a Cu(1 1 1) substrate and their effects on water molecule adsorption have been studied by applying density functional theory (DFT) with a vdW-DF correction. We found that pyridinic (N3V1, N4V2), and pyrrolic defects (N3V1, N3V3) strongly interact with the Cu(1 1 1) substrate, due to oxidative addition from the copper to the nitrogen atom occurs as Bader analysis indicated. The chemical interaction from copper to nitrogen atoms induces a formal oxidation state Cu(I)/Cu(II) in the copper atom. The binding energies of the pyridinic N3V1, pyrrolic N3V1 and pyrrolic N3V3 defects on the substrate are 4.013, 2.130, and 3.008 eV, respectively. N4V2 defect strongly interacts with the Cu(1 1 1) substrate; however, due to the electronic structural symmetry of the defect, a Cu atom is detached from the substrate and positioned at the centre of the defect vacancy with a formal oxidation state Cu(II) and Bader charge of about 1.035 e. Overall, the states in the valence band near to the Fermi level are dominated by the contributions of the Cu 4p orbitals,

Fig. 5. Perspectives and details of the different systems interacting with two water molecules, N-doped graphene sheet, (a) pyrrolic N3V1, (b) pyridinic N3V1, (c) pyrrolic N3V3 and (d) pyridinic N4V2 defect. A lateral view of the corresponding systems is shown at the centre; on top these perspectives are presented, tilted at 40 degrees from the lateral view. At the bottom a zoom around the N defect and the region of interaction between water molecules is shown, the radial distance of the H bond is presented by a dotted red line. The radial distances related to NeH bond from the water molecule linked to the nitrogenated defect in the graphene sheet, as well as the bonding distance between the water molecules are labelled with black and blue font, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 6

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enhancing experimental control. Erroneous interpretations regarding the properties of hydrophilicity will thus be avoided.

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Acknowledgments [20]

This research was conducted at the Escuela Superior de Apan, Universidad Autónoma del Estado de Hidalgo (UAEH). Also, this research used resources of the National Energy Research Scientific Computing Center (NERSC), a U.S. Department of Energy Office of Science User Facility operated under Contract No. DE-AC0205CH11231. Also, L.A. Hernandez-Hernandez acknowledge financial support from PRODEP, under grant 511-6/18-8661.The authors acknowledge to Ph D. J. Jesús Pelayo for the fruitful comments on this work.

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Appendix A. Supplementary material

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Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.144149.

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