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Jul 24, 2017 - Qatar Environment and Energy Research Institute, Hamad Bin Khalifa ... Department of Chemistry, King Fahd University of Petroleum and ...
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Effect of Surface Symmetry on the Dissociative Adsorption of Water on Gallium Oxynitride Golibjon R. Berdiyorov, Ahsanulhaq Qurashi, Gofur Eshonqulov, and Fedwa El-Mellouhi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b03968 • Publication Date (Web): 24 Jul 2017 Downloaded from http://pubs.acs.org on July 27, 2017

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The Journal of Physical Chemistry

E↵ect of Surface Symmetry on the Dissociative Adsorption of Water on Gallium Oxynitride G. R. Berdiyorov,1, ⇤ A. Qurashi,2, 3 G. Eshonqulov,4 and F. El-Mellouhi1 1

Qatar Environment and Energy Research Institute, Hamad Bin Khalifa University, Qatar Foundation, Doha, Qatar 2 Center of Research Excellence in Nanotechnology, King Fahd University of Petroleum and Minerals, Dhahran, 31261, Saudi Arabia 3 Department of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran, 31261, Saudi Arabia 4 Department of Physics, National University of Uzbekistan, Tashkent, Uzbekistan (Dated: July 21, 2017) Recently, gallium oxynitride was shown to be a promising material for photoelectrochemical watersplitting applications. Herein, we perform density functional theory (DFT) calculations to study the e↵ect of surface symmetry and chemistry on the adsorption and dissociation processes of water molecules on spinel type gallium oxonitride (Ga3 O3 N). We found strong dissociative adsorption of water molecules for both N- and O-rich surfaces with di↵erent surface symmetries (i.e. (100), (010), (001) and (111)). In addition, DFT molecular dynamics simulations reveal that water molecules undergo dissociation on all considered Ga3 O3 N surfaces at room temperature. These interesting findings indicate a promising potential of Ga3 O3 N for water-splitting applications. I.

INTRODUCTION

Photocatalytic water splitting using sunlight is considered to be one of the e↵ective methods of producing clean energy without environmental impact [1, 2]. Among various metal oxide photocatalysts, gallium oxynitrides (GaON) were recently shown to be a promising material for developing visible-light active photocatalysts for water splitting applications [3–5]. GaON can be synthesized from either GaN or Ga2 O3 by oxidation and nitridation/ammonolysis, respectively [6, 7]. Hu et al., proposed Ga(OH)3 as a more suitable precursor for synthesis of GaON than Ga2 O3 due to structural advantages [3]. Very recently, Iqbal et al. synthesized GaON from gallium metal and organic diamine via low temperature direct solvothermal approach [5]. GaON showed high chemical stability in di↵erent environments, which may reduce catalytic degradation of the material [5]. In addition, photocatalytic activity of GaON can further be enhanced due to the presence of equal amount of N and O atoms, which promotes enhanced charge mobility [3]. Most importantly, GaON has a band gap energy around 2 eV [3–5], which enables exploiting visible-light photon energy for photocatalytic evolution of H2 and O2 gases. Here, we use density functional theory (DFT) to study the e↵ect of surface orientation and chemistry on the adsorption and dissociation of water molecules on spinel type (Imm2 space group, no. 44) gallium oxonitride Ga3 O3 N. We found that, regardless of the surface conditions, water molecules are strongly adsorbed on the surface of the material with adsorption energies larger than 2 eV. Interestingly, the water molecule tend to dissociate during the geometry optimization in all considered sys-

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tems. DFT molecular dynamics (MD) simulations also show the dissociation of water molecules to OH radical and H atom on the surface of Ga3 O3 N at room temperature. These promising results also indicate the large potential of Ga3 O3 N for water splitting applications. II.

COMPUTATIONAL DETAILS

Simulations are conducted using DFT within the generalized gradient approximation of Perdew-BurkeErnzerhof (PBE) for the exchange-correlation energy [8]. The Brillouin zone is integrated using 12⇥12⇥12 Monkhorst-Pack k-point sampling for the unit cell of Ga3 O3 N (see Fig. 1(a)). k-point sampling is reduced to 12⇥12⇥1 for slab geometries due to the presence of a vacuum spacing of more than 10 ˚ A perpendicular to the slabs (Figs. 1(b-i)). All the atoms are described using double-zeta-polarized basis sets of numerical orbitals and the electrostatic potential is determined on a realspace grid with mesh cuto↵ energy of 150 Ry. van der Waals interactions are introduced by Grimme’s empirical dispersion correction PBE-D2 [9]. The convergence criterion for Hellman-Feynman forces is 0.01 eV/˚ A. The adsorption energy of a water molecule is defined as Eads = EGaON+H2 O

EGaON

EH 2 O ,

(1)

where EGaON+H2 O is the total energy of Ga3 O3 N slab with adsorbed water molecule, EGaON is the total energy of Ga3 O3 N slab and EH2 O is the total energy of isolated water molecule. The total energy calculations are performed only for the fully optimized structures. DFT MD simulations are conducted using the canonical ensemble with a time step of 0.5 fs. Nose-Hover chain thermostat is used for temperature control. All calculations are conducted using the computational package Atomistix toolkit [10].

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FIG. 2: Calculated surface energies for Ga3 O3 N slabs with di↵erent surface symmetries.

timized bulk geometry with di↵erent surface orientations (see Fig. 1(b-g)): 100 (b,c), 010 (d,e), 001 (f,g) and 111 (h,i). Surface cleavage was performed in a such a way that the surfaces are either N-rich (Fig. 1(c,e,g,h)) or Orich (Fig. 1(b,d,f,i)). The resulting slab geometries are optimized further by fixing some of the atoms at the bottom layer, which are highlighted in Fig. 1. The surface energy was calculated as Esurf =

FIG. 1: (a) Optimized unit cell of Ga3 O3 N. (b-g) Adsorption of a water molecule on Ga3 O3 N slabs with di↵erent Nand O-rich surfaces (indicated in each panel). Atoms of the water molecule are shown by larger balls. Highlighted (selected) atoms at the bottom of the slabs are fixed during the optimization.

III.

Etot

N Ebulk , 2A

(2)

where Etot and A are the total energy and the surface area of the slab geometries, respectively. Ebulk is the total energy of the bulk system and N refers to the number of formula unit, consisting of 1 N atom, 3 Ga atoms and 3 O atoms. Figure 2 shows the calculated surface energy for all considered surface symmetries. It is seen from this figure that the lowest surface energy is obtained for N-rich 111 surface orientation, whereas 001(N) surface cleavage resulted in the largest surface energy. The formation energies also depend on the surface termination except for 010 surface, for which both N- and O-terminations resulted in the same surface energy. Di↵erence in energies between the 111 surface and other surfaces such as 100 and 010 are of the order of the thermal energy (less than 0.025 eV) which justifies well the possible coexistence of other orientations under operating conditions. Hence justifying very well the rational behind the study

RESULTS AND DISCUSSIONS

As a typical example, we considered spinel structure (Imm2, no. 44) gallium oxonitride (Ga3 O3 N) with lattice parameters a, b=5.8534 ˚ A and c=8.2780 ˚ A (see Refs. [11, 12]). We have fully relaxed the positions of all atoms keeping the lattice parameters fixed. After optimization, we have created slab geometries from the op-

FIG. 3: Adsorption energies of water molecule on di↵erent surfaces of Ga3 O3 N.

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we undertook here. To study the water adsorption on Ga3 O3 N, we introduced a single water molecule in the vacuum region at a distance more than 5 ˚ A from the surface. After that we optimized the system further using LBFGS optimizer method [13]. During the optimization, some atoms at the bottom of the slab were kept fixed (as highlighted in Fig. 1). For all the considered slab geometries, the adsorption occurs through the formation of Ga-O bond (see supplemental online video [14]). Water molecule attachment also results in a local structural changes of the Ga3 O3 N slabs. Interestingly, regardless of the surface orientation and chemistry, the water molecule dissociates into one hydroxyl and one hydrogen (see Figs. 1(b-g)). The dissociated hydrogen atom of the water molecule may transfer to neighboring either oxygen (Fig.1(g)) or nitrogen (Fig.1(f)) atom. No molecular adsorption has been obtained during the structural optimization for the considered surface symmetries and the density of the water molecules. Figure 3 shows the adsorption energies Eads of the water molecule on di↵erent surfaces of Ga3 O3 N. In all the considered systems the water molecule is strongly chemisorbed with adsorption energy exceeding 2 eV. The strongest adsorption is obtained on the N-rich 001 surface of Ga3 O3 N (Eads = 7.09 eV), where the hydrogen atoms is transferred to the N atoms (see Fig. 1(f)). However, this surface is less stable than the other surfaces (see Fig. 2). Strong adsorption is also obtained for N-rich 111 surface (Eads = 3.48 eV). For the other two surface orientations (100 and 010) O-rich surface shows slightly smaller adsorption energy. To characterize the systems with the dissociated water molecules, we present in Fig. 4 H-H (a) and Ga-OH (b) distances for all the considered surface geometries. The results are obtained for the adsorption sites of the water molecules for which the system takes the lowest energy, as shown in Fig. 1.

FIG. 4: H-H (a) and Ga-OH (b) distances for dissociative adsorption of water molecules on di↵erent surfaces of Ga3 O3 N.

H-H distance is rather scattered, ranging from 1.9 ˚ A (for 010(O) surface) to 4.46 ˚ A (for 111(N) surface). As for the Ga-OH distance, it is smaller than 1.9 ˚ A when the oxygen atom is bond to a single Ga atom (see Fig. 1(c) for the case of 100(O) surface) and it is larger than this value when the oxygen atom is connected to two neighboring Ga atoms (see Fig. 1(b)). Next, we conduct finite temperature MD simulations to study water adsorption and dissociation processes on the surface of Ga3 O3 N. We first increased the temperature of the system (water molecule is initially located at 6 ˚ A above the surface) from T=0 to T=300 K with heat rate of 149.95 K/ps. In order to reduce the chemical reactions during the thermalization, we set the thermostat timescale to 10 fs. Even with such small damping parameter, we found the splitting of the water molecule on some of the surface geometries (see right inset in Fig. 5). To characterize the water splitting process, we calculated the radial distribution functions g(r) between the hydrogen atoms, which are shown in Fig. 5 in the cases of N-rich 001 and 100 surfaces. In the former case, g(r) is characterized by a strong peak at 1.55 ˚ A (solid-black curve in Fig. 5), which corresponds to the average distance between the H atoms of water (see left inset in Fig. 5). This peak reduces considerably when the water dissociates in the system (dashed-red curve in Fig. 5). The dissociation results in the appearance of extra peaks in the distribution curve larger distances. In this particular case the separation between the H atoms is around 3.5˚ A (see right inset in Fig. 5). After the thermalization, we have conducted room temperature MD simulations for 4 ps using the thermostat timescale of 100 fs. Figures 6(a,b) show the radial distribution functions between the hydrogen atoms when the water molecule is adsorbed either N-rich (a) or O-rich (b) surfaces of Ga3 O3 N with di↵erent symmetries. It is seen from this figure that the peak on the g(r) curves

FIG. 5: Radial distribution function (g(r)) between H atoms of the water molecule adsorbed on N-rich 001 (solid-black curve) and N-rich 100 (dashed-red curve) surfaces during 2 ps long MD simulations when the system temperature is increased from 0 to 300 K. Insets show snapshots of the systems at the end of the simulation time. Dashed arrows indicate the peaks on the g(r) curves corresponding to H-H bonds in the insets.

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tween the hydrogen atoms when the water molecule is adsorbed on 111 surface of the material. Even in this case, the water molecule dissociates and, consequently the maxima in the g(r) curves shift to larger distances. Thus, fast water dissociation takes place on the surface of GaOH irrespective the surface symmetry and chemistry. As we have mentioned above, non-dissociative adsorption of water molecules was obtained during the thermalization only for 001 surface of the materials (see dashed areas in Figs. 6(a,b)). In order to find the origin for this phenomenon, we have conducted electrostatic potential calculations, the results of which are shown in Figs. 6(c-e). It is seen from these figures that all considered surfaces (except 001 surface) are rich with both N and O atoms, whereas 001(N) surface lacks of O atoms (see Fig. 6(e)). This in turn a↵ects the electrostatic potential at the surface and consequently to the adsorption process of water molecules. Finally, we consider the case when more than one water molecules are present in the simulation cell. For that purpose we have chosen Ga3 O3 N slab with N-rich 111 surface, which has lowest surface energy as compared to

FIG. 6: Radial distribution function (g(r)) between H atoms of the water molecule adsorbed on (a) N-rich and (b) O-rich surfaces of Ga3 O3 N with di↵erent surface symmetry during 4 ps long MD simulations at T=300 K. Shaded areas highlight the peaks on the g(r) curves corresponding to H-H bonding in water molecule. Insets show snapshots of the systems at the end of the simulation time. (c-f) Contour plots of the electrostatic di↵erence potential for 100(N) (c), 010(N) (d), 001(N) (e) and 111(N) (f) surfaces of Ga3 O3 N.

corresponding to the H-H distance of the water molecule remains only for 001 surfaces of GaOH (see shaded areas in Figs. 6(a,b)). However, the amplitude of these peaks are considerably reduced as compared to the case when no dissociation of the water molecule occurs (see solid black curve in Fig. 5). In addition, extra peaks appear on the g(r) curves at larger distances, indicating the dehydrogenation of water (see dotted-blue curves in Figs. 6(a,b)). Indeed, the snapshots on the system at the end of 4 ps MD simulations show the dissociation of the water molecule to OH radical and H atom (see rightmost insets in Figs. 6(a,b)). Interestingly, the first peak on the radial distribution curves disappear in all other considered surface geometries (see solid-black and dashed-red curves in Figs. 6(a,b)). These additional peaks are located at di↵erent distances for di↵erent surface symmetries. Note that in all the considered systems the hydrogen atom of the water molecule is transferred to neighboring oxygen atoms (see insets in Fig. 6). Dash-dotted green curves in Figs. 6(a,b) show the radial distribution function be-

FIG. 7: (a) Adsorption energies of water molecules on 111(N) surface of Ga3 O3 N as a function of number of adsorbed water molecules (NH2 O) ). (b) The number of splitted water molecules (Nsplit ) as a function of NH2 O) . (c-e) Optimized geometry of 111(N) Ga3 O3 N slab with di↵erent number of adsorbed water molecules. Water molecules are highlighted.

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the other considered structures (see Fig. 2). The surface area of the simulation cell is 76.09 ˚ A2 . We have gradually increased the number of water molecules adsorbed on the surface (NH2 O ) and conducted structural optimization using the same convergence criterion and the same number of fixed atoms as in Fig. 1(h). Figure 7(a) shows the adsorption energy per water molecule as a function of NH2 O . In general, the adsorption energy increases with increasing the density of water molecules. However, Eads is correlated with the number of splitted water molecules (Nsplit , see Fig. 7(b)); the adsorption becomes stronger with increasing Nsplit (compare adsorption energies for NH2 O =2 and NH2 O =3 and for NH2 O =5 and NH2 O =6 (see Figs. 7(d,e))). For the considered densities of the water molecules, we obtained a saturation limit of 3 water molecules splitted during the adsorption process. This is mainly due to high surface coverage.

analysis and electrostatic potential calculations were also performed to explain the obtained results. We have considered N- and O-rich 100, 010, 001 and 111 surfaces and found out that 111(N) surfaces has the lowest surface energy, while the other surfaces can coexist at room temperature. However, regardless of surface symmetry and chemistry, water molecules are dissociatively adsorbed on the surface of Ga3 O3 N. In addition, DFT MD simulations reveal fast dissociation of the water molecule to hydroxyl and hydrogen atom at room temperature. Dissociative adsorption of water molecules are also obtained for larger densities of water molecules. These findings indicate a great potential of Ga3 O3 N for water splitting applications.

V. IV.

ACKNOWLEDGMENTS

CONCLUSIONS

DFT calculations were performed to study water adsorption on di↵erent surfaces of Ga3 O3 N. Structural

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Computational resources have been provided by the research computing center at Texas A&M University in Qatar.

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