DOI: 10.1002/chem.201400006

Full Paper

& Density Functional Calculations

DFT Simulations of Water Adsorption and Activation on LowIndex a-Ga2O3 Surfaces Xin Zhou,[a] Emiel J. M. Hensen,[b] Rutger A. van Santen,*[b, c] and Can Li*[a]

icant. Calculations of electron local density of states indicate that the electron-energy band gaps for the four investigated surfaces appears to be less related to the difference in coordinative unsaturation of the surface atoms, but rather to changes in the ionicity of the surface chemical bonds. The electrochemical computation is used to investigate the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) on a-Ga2O3 surfaces. Our results indicate that the (100) and (110) surfaces, which have low stability, are the most favorable ones for HER and OER, respectively.

Abstract: Density functional theory (DFT) calculations are used to explore water adsorption and activation on different a-Ga2O3 surfaces, namely (001), (100), (110), and (012). The geometries and binding energies of molecular and dissociative adsorption are studied as a function of coverage. The simulations reveal that dissociative water adsorption on all the studied low-index surfaces are thermodynamically favorable. Analysis of surface energies suggests that the most preferentially exposed surface is (012). The contribution of surface relaxation to the respective surface energies is signif-

Introduction

e phases, whose formation can be controlled by preparative means.[16] In these phases, b-Ga2O3 has drawn most attention as the most stable crystal phase.[11, 13, 17] The b phase also exhibits higher photocatalytic activity toward the decomposition of aromatic compounds than a and g phases.[14] However, several works report that a-Ga2O3 can be easily obtained when nanosizing Ga2O3 at relatively low temperatures.[18–20] At temperatures higher than 500 8C, it is usually converted to the more stable b-form. Recently, Li et al. reported that Ga2O3 with tunable a–b phase junctions can stoichiometrically split water into H2 and O2 with drastically enhanced activity over those with a pure a or b phase.[12] The results show that the original aGa2O3 undergoes gradual a-to-b phase transformation upon increasing the calculation temperature.[17] The photocatalytic water-splitting activity of the pure a phase is higher than that of the b phase. Another work found that a-Ga2O3 microspheres fabricated by a self-assembly process display higher photocatalytic activity than TiO2 in the degradation of organic dyes.[15] These experimental observations point to the importance of the surface structure of photocatalytically active metal oxides for their performance in catalyzing the surface redox processes. Understanding of these aspects is crucial to further optimize these materials and develop better ones. Theoretical studies based on first-principles electronic structure calculations have proven useful in illuminating the relationship between the surface structure and physical and chemical properties of oxide surfaces.[21–23] With respect to Ga2O3, few such studies are available therefore its surface properties remain far from understood.[24–26] Most of these works have focused on the more stable b-Ga2O3 phase, with little efforts being made for a-Ga2O3.[27] a-Ga2O3 has the corundum structure (rhombohedral with a R3¯c space group). To better understand the surface structure and chemical properties of a-

Being considered one of the most attractive approaches to solving energy and environmental issues at a global level, photocatalytic water-splitting is drawing increasing attention, mainly from academia.[1–6] Besides many experimental efforts focusing on the development of new promising materials, computational chemistry is also increasingly contributing to this burgeoning field. Among photocatalytically active metal oxides, semiconductors with a d10 electronic configuration exhibit superior photocatalytic activity,[7–10] mainly because their conduction bands are formed by hybridized sp orbitals with large dispersion able to generate photoexcited electrons with large mobility.[4] Gallium oxide (Ga2O3) is an example of such d10 metal oxides, exhibiting high activity for water splitting and degradation of organic pollutants.[11–15] Ga2O3 can adopt five different crystalline structures, designated as a, b, g, d, and

[a] Dr. X. Zhou, Prof. Dr. C. Li State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics Chinese Academy of Sciences Dalian National Laboratory for Clean Energy 457 Zhongshan Road, Dalian, 116023 (P.R. China) E-mail: [email protected] [b] Prof. Dr. E. J. M. Hensen, Prof. Dr. R. A. van Santen Schuit Institute of Catalysis, Laboratory of Inorganic Materials Chemistry Eindhoven University of Technology Den Dolech 2, 5612 AZ Eindhoven (The Netherlands) [c] Prof. Dr. R. A. van Santen Institute for Complex Molecular Systems Eindhoven University of Technology Den Dolech 2, 5612 AZ Eindhoven (The Netherlands) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201400006. Chem. Eur. J. 2014, 20, 1 – 13

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Full Paper Method and Computational Details Optimization of bulk and surfaces a-Ga2O3 All the DFT calculations were performed with the VASP (Vienna Ab initio Simulation Package) code.[45, 46] The exchange correlation potential was described by the Perdew–Burke–Ernzerhof functional within the generalized gradient approximation.[47] The projector-augmented wave method was applied to describe electron–ion interactions.[48, 49] Full optimization of the cell parameters for the bulk a-Ga2O3with a corundum structure (Figure 1 a) has been carried out by using the cutoff energy of 400 eV and 9  9  3 Monkhorst– Pack-type k-point sampling. The calculated lattice parameters, a = b = 5.032, c = 13.504 , and g = 120 8, are in good agreement with the experimental data.[50] Four a-Ga2O3 surfaces of interest are created by cleaving the optimized bulk structure through the corresponding planes, and the resultant structures are depicted in Figure 1. Illustration of bulk a-Ga2O3 crystal and stoichiometric surfaces before and after relaxation: a) bulk, Figure 1 b–e. The rhombic cell is b) (001), c) (100), d) (110), and e) (012). The red spheres represent oxygen atoms, whereas brown spheres repreutilized for the (001) surface: sent gallium atoms. 10.065  10.065 , involving a total of 120 atoms. Rectangular cells are Ga2O3, we have carried out first-principles density functional utilized for the other three surface slabs: 5.032  13.504  for (100) with 90 atoms; 13.504  8.717  for (110) with 120 atoms, and theory (DFT) calculations. We determined the structures and 5.032  16.072  for (012) with 120 atoms. The separation distances formation energies of four different surfaces that appear to be between the slabs are set at 15 . A 3  3  1, 6  2  1, 2  3  1, and frequently exposed and extensively studied for corundum6  2  1 Monkhorst–Pack k-point mesh are applied on structural phase materials, namely, (001), (100), (110), and (012) as depictand energy calculations for (001), (100), (110), and (012), respectiveed in Figure 1.[28–31] In photocatalytic applications, most reacly. As shown in Figure 1, the (001) surface is described by a stoichiotions are carried out in aqueous solution or, at least, need the metric Ga-termination, including six complete GaOGa layers. participation of water, with surface hydroxyl radicals derived Each atomic layer of the (100) plane is stoichiometric and neutral, from water decomposition being an important reaction interand a nine-layer slab is used in our calculation. The (110) surface displays a corrugated structure terminated by oxygen atoms. It mediate.[32, 33] Therefore, we have investigated the adsorption consists of a 12-layer-thick slab with four complete OGaO layers. of water and its dissociation on these a-Ga2O3 semiconductor The (012) surface slab contains 8 Ga layers and 12 O layers. Accordsurfaces in detail as a function of water coverage. The overall ing to Tasker’s classification, the (100) surface belongs to type I, water-splitting reaction is divided into two half-redox reacwhich involves a stacking of neutral ionic planes.[51] The (001), tions, namely, the hydrogen evolution reaction (HER) and the (110), and (012) surfaces are referred to as type II, which are characoxygen evolution reaction (OER), which are closely interrelated terized by a sequence of charged ionic planes with no net electric to photo-generated electrons and holes, respectively. Recent dipole moment in the slab due to the symmetrical stacks. For the investigations on crystal facet engineering of semiconductors surface energy calculation, all the slabs are fully relaxed. During the optimization of the surface attached with adsorbates, the top have demonstrated that photo-excited electrons and holes half of the slab and the adsorbates are allowed to fully relax and may be driven to different crystal facets. Certain facets of the bottom half of the slab is held fixed for (001), (110), and (012) a semiconductor prefer reduction, whereas others favor oxidasurfaces. With respect to the (100) surface, the five topmost layers [34–38] tion. To probe the roles of different facets of a-Ga2O3 on and any adsorbates are allowed to relax, and the four bottom the water reduction and oxidation reaction, we have analyzed layers were kept fixed at their optimized bulk positions. Symmetrithe mechanism of HER and OER on every surface, according to zation is switched off for the slab models, and dipolar corrections the thermochemistry of the reactions, using the method develare included. Structures are relaxed until all forces on atoms are oped by Nørskov et al.[39–44] less than 0.01 eV 1. The vibrational frequencies of adsorbates are

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Full Paper energy, DG = DE + DZPETDS, is calculated as follows: The reaction energy DE is obtained from DFT calculations. The differences in zero point energies (DZPE) and the change in entropic contribution DS are calculated by using computed vibrational frequencies and standard tables for the reactants and products in the gas phase.[54] The entropies for the atoms and molecules adsorbed to the surface active site are assumed to be zero. The temperature dependence of the enthalpy is neglected in these calculations. Applying an external bias U on each proton-coupled electron-transfer step is accounted for by including a eU term in the reaction free energy. For simplicity, the effect of pH is not considered here and we restrict the calculations to pH 0. Therefore, the reaction free energies are expressed as follows:

calculated within the harmonic approximation. The Hessian matrix is calculated by the finite difference approach, with a moving step size of 0.02  around the equilibrium position.

Free-energy calculation scheme for the HER and OER We focus on the thermodynamic process of the surface reactions that generate H2 and O2. Thus, we do not explicitly model the photo-absorption event or the kinetic process of the subsequent charge migration. Photo-excitation and charge migration are considered to generate the electrochemical potentials for electrocatalytic H2O decomposition. The HER involves binding of H atoms to the catalyst surface and recombinative desorption of molecular hydrogen, which plays an essential role in hydrogen fuel cells, electrodeposition, corrosion of metals, and hydrogen energy storage.[52–54] These applications have led to a large body of research relevant to the HER, covering a broad range of systems including metals, alloys, hydrogenases, and semiconductors.[42–44, 55–58] From these works, it has become evident that the HER activity can be correlated to the adsorption energy of a single H atom, that is to say, a good catalyst for the HER should exhibit a small adsorption energy for H (j DGH j  0). In this work, we elucidate the thermochemistry of this reaction by computing the free energy of atomic hydrogen bonding to the aGa2O3 surface and compare the hydrogen evolution activity of different surfaces. The free energy of the adsorbed state is calculated as: DGH ¼ DE H þ DZPETDSH

in which EH is the hydrogen chemisorption energy obtained from DFT calculations and DZPE is the difference in zero point energy between the adsorbed and the gas phase. Since the vibrational entropy in the adsorbed state is small, the entropy of adsorption of 1/2 H2DSH is approximately equal to the negative value of half the entropy of H2 in the gas phase at standard conditions.

HO* ! O* þ Hþ þ e

ðBÞ

O* þ H2 O ! HOO* þ Hþ þ e

ðCÞ

HOO* ! * þ O2 þ Hþ þ e

ðDÞ

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ð3Þ

DGC ¼ EðHOO* ÞEðO* ÞE H2 O þ 1=2 E H2 þ ðDZPETDSÞC eU

ð4Þ

DGD ¼ Eð* ÞEðHOO* Þ þ E O2 þ 1=2 E H2 þ ðDZPETDSÞD eU

ð5Þ

DE O ¼ EðO* ÞEð* ÞE H2 O þ E H2 *

ð6Þ

DE HO ¼ EðHO* ÞEð* ÞE H2 O þ 1=2 E H2 *

ð7Þ

DE HOO ¼ EðHOO* ÞEð* Þ2 E H2 O þ 3=2 E H2 *

ð8Þ

Results and Discussion Bulk and surface structure Figure 1 a shows that the hexagonal unit cell of a-Ga2O3 contains six Ga2O3 formula units. Each Ga cation is bonded to six O anions in the form of a distorted octahedron, and each O anion is bonded to four Ga cations in the form of a distorted tetrahedron. The atomic layer stacking sequence along the c axis consists of six O layers and twelve Ga layers. The Ga atoms are staggered along the c axis from their ideal position centered between the oxygen layers. As listed in Table 1, this leads to two sets of GaO bond lengths; Ga atoms have three long GaO1 bonds (2.076 ) to the oxygen layer above and three short GaO2 bonds (1.95 ) to the oxygen layer below, which are in good agreement with the experimental values of 2.077 and 1.921 , respectively.[50]

in which * denotes a surface site and X* represents an adsorbed X intermediate on the surface. We obtain the energy of H + + e implicitly by referencing it to the energy of H2 using the standard hydrogen electrode. This implies that at standard conditions (pH 0, p = 1 bar and T = 298 K) the free energy of H + + e can be taken equal to be half the formation energy of H2. The reaction free Chem. Eur. J. 2014, 20, 1 – 13

DGB ¼ EðO* ÞEðHO* Þ þ 1=2 E H2 þ ðDZPETDSÞB eU

The adsorption energies of the O*, HO*, and HOO* species at the surface site are computed as:

In the overall water-splitting reaction, the OER is considered to be more difficult than the HER because the former involves a fourelectron-transfer process. The mechanism of the OER is complicated and not yet fully established. Insights into the thermodynamics of the reaction can be obtained by using the scheme developed by Nørskov and co-workers,[39–41] in which the molecular oxygen is formed through a surface HOO* intermediate and the reaction takes place at the coordinatively unsaturated surface sites. In this scheme, the OER is assumed to consist of four elementary reaction steps, each involving the electron transfer accompanied by proton removal:

ðAÞ

ð2Þ

in which E(*), E(HO*), E(O*), and E(HOO*) are the calculated DFT energies of the clean surface and surfaces with adsorbed HO*, O*, and HOO*, respectively. EH2O, EH2, and EO2 are the calculated energies for the isolated gaseous molecules H2O, H2, and O2, respectively. The free-energy change of the total reaction H2O!1/2 O2 + H2 is fixed at the experimental value of 2.46 eV per water molecule. This means that in the reaction step involving the formation of O2, we consider that DG(2 H2O!O2+2 H2) = 4.92 eV = EO2 + 2 EH22 EH2O + (DZPETDS)(2 H2O!O2+2 H2). The reaction overpotential can be calculated from the difference between the voltage at which all freeenergy steps become downhill and the minimum voltage required for the OER.

ð1Þ

H2 O þ * ! HO* þ Hþ þ e

DGA ¼ EðHO* ÞEð* ÞE H2 O þ 1=2 E H2 þ ðDZPETDSÞA eU

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Full Paper Table 1. Structural parameters [] of bulk a-Ga2O3 and bulk-truncated (before) and optimized (after) surfaces; different surface atoms are labeled as in Figure 1. Bulk Ga-O1 Ga-O2

2.076 1.950

Ga3c-O3c Ga1-O3c Ga2-O3c

(001) before

after

1.950 1.950 2.076

1.815 1.925 2.013

Ga4c1-O3c1 Ga4c1-O3c2 Ga4c2-O3c2 Ga4c2-O2c

(100) before

after

1.950 2.076 2.076 1.950

1.817 1.935 1.911 1.841

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after

2.076 2.076 1.950 1.950

2.044 1.951 1.921 1.851

Ga5c-O3c1 Ga5c-O3c2

(012) before

after

2.076 1.950

1.948 1.89

Surface energy

The (001) surface is the most studied surface of materials with a corundum structure, such as a-Al2O3 and a-Fe2O3.[59–65] In this work, we focus on the stoichiometric half-metal termination of (001), which exposes a top layer of coordinatively unsaturated threefold Ga atoms (Ga3c) and a hexagonal array of second-layer threefold coordinated O atoms(O3c) as depicted in Figure 1 b. In the optimized surface, the Ga3c atom relaxed toward the O3c atoms, decreasing the interlay spacing from 0.83 to 0.06  and creating a nearly trigonal environment of O3c around the Ga3c. As shown in Table 1, the Ga3cO3c bonds are all equivalent and have a calculated length of 1.815 , whereas O3c bind to the two subsurface Ga layers with distances of 1.925 and 2.013 , respectively. As shown in Figure 1 c, the (100) surface is the only neutral surface, and only one type of termination is possible. All the exposed Ga atoms are fourfold-coordinated (Ga4c). Two-thirds of the surface O atoms are threefold-coordinated (O3c) and one-third of them are twofold coordinated (O2c). The optimized structure shows that the topmost two layers exhibit appreciable relaxations in the atomic positions. In the top layer, the Ga atoms move inward toward the bulk and the O atoms move outward toward the vacuum. As a result, the intra-layer distance of the top layer increases by 0.30  relative to that in the bulk, and the interlayer distance between the first and second layer decreases by 0.23 . As can be seen from Table 1, four different surface GaO bond lengths are obtained, namely, 1.817, 1.841, 1.911, and 1.935 . The stoichiometric termination of the (110) surface is shown in Figure 1 d. The exposed surface consists of two kinds of fivefold-coordinated Ga atoms (Ga5c1 and Ga5c2), two different threefold-coordinated O atoms (O3c1 and O3c2) and twofold-coordinated O atoms (O2c). The surface largely keeps the original corrugated conformation upon relaxation. The bonds between Ga5c1 and three coordinating O3c atoms decrease from 2.076  on the bulk-truncated surface to 2.044 and 1.951 , respectively. The distance between the Ga5c2 and the adjacent O3c2 and O2c decreases from 1.950 to 1.921 and 1.851 , respectively. The (012) surface can be built up from neutral OGaOGaO units as shown in Figure 1 e. Our slab is comprised of four such stacking units. The zigzag oxygen ridges that characterize the (012) surface are clearly seen. The exposed Ga and O atoms are five- and threefold-coordinated, respectively. The magnitude of the surface relaxation of the (012) surface is smaller than those of other surfaces. The Ga5cO3c1 and Ga5cO3c2 bonds decrease by 0.028 and 0.052 , respectively. &

Ga5c1-O3c1 Ga5c1-O3c2 Ga5c2-O3c2 Ga5c2-O2c

(110) before

To estimate surface energies, slabs with different numbers of stoichiometric repeated layers were constructed for each surface. The computed total energies of the slabs, together with their corresponding numbers of Ga2O3 units, were then fitted into the following Equation to calculate the surface formation energy g, g = (EslabnEGa2O3)/2A, in which Eslab is the total energy of the slab, n is the number of Ga2O3 units in the slab, and 2A is the total exposed area of the two identical sides of the slab. As shown in Figure 1, for the (001) model, a stoichiometric repeated layer is composed of three planes (GaOGa), as for the (110) model (OGaO), whereas only one plane forms the repeated layer in the (100) model. In the (012) model, a layer consists of five planes (OGaOGaO). We investigate the dependence of surface energy on the number of layers. The surface energies converged to within 0.01–0.03 J m12 for the all the surfaces when n reaches 4. The computed surface energy is 0.96 J m12 for (001), 1.43 J m12 for (100), 1.23 J m12 for (110), and 0.94 J m12 for (012). Based on the calculated surface energies, the relative stability trend of the surfaces is predicted to be (012)  (001) > (110) > (100). Similar trends of surface stability were also determined for a-Fe2O3 and a-Al2O3 surfaces by experimental and theoretical means.[28–31] We have further computed the relaxation energy per Ga2O3 unit for every surface, which equals to the energy difference before and after optimization. The relaxation energy is 0.57 eV for (001), 0.73 eV for (100), 0.31 eV for (110), and 0.16 eV for (012). Considering the large change in the (001) surface energy before and after relaxation, we argue that the (012) surface will be preferentially exposed. The (012) surface was indeed observed during the phase transformation process of a-Ga2O3 by using high-resolution transmission electron microscopy.[17] To ensure the reliability of our calculation, we also computed the surface energy of the a-Al2O3 (001) surface. The corresponding surface energies with a thickness of six, nine, and twelve repeated layers are 1.66, 1.67, and 1.68 J m12, respectively. They are close to the experimental value of 1.69 J m12.[66] Electronic structure of bulk and surfaces The calculated total density of states (DOS) and partial density of states (PDOS) for a-Ga2O3 bulk and surfaces are plotted in Figure 2. The Fermi level is set to zero and marked by a vertical dashed line. We can see that the band gap of bulk a-Ga2O3 is computed to be 2.74 eV. Although the computed band gap is 4

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Full Paper

Figure 2. Density of states for the: a) bulk a-Ga2O3, b) (001), c) (100), d) (110), and e) (012) surfaces. The Fermi level is shown the by the vertical dashed line.

lower than the experimental value of about 4.5 eV[14] due to the well-known limitation of the generalized gradient approximation (GGA),[67, 68] the character of the band structure and the energy gap variations are expected to be reasonable and reliable. As shown in Figure 2 a, the lower valence bands located in the range of 20 to 16 eV are predominantly composed of O 2s states, slightly hybridized with Ga 4s, 4p, and 3d states. Chem. Eur. J. 2014, 20, 1 – 13

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The upper valence band shows a mixture of O 2p, Ga 4s, 4p, and 3d states, yielding a band width of 7.5 eV. The valence band maximum (VBM) is mainly composed of O 2p states. The strong mixing of O 2p and Ga 4s, 4p, and 3d states is indicative of the high degree of covalent bonding in this compound. In this case, the band gap between the valence band and the conduction band can be considered as being mainly deter5

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Full Paper Water adsorption on the perfect surfaces mined by the difference between GaO bonding and antibonding orbitals. The conduction band minimum (CBM) conWe considered the structure and energetics of molecular and sists mostly of Ga 4s states with slight contributions of O 2p dissociative adsorption of water on surfaces at different water and Ga 4p orbitals. The upper CB mainly exhibits the charactercoverage. Here, we refer to the coverage q as the ratio beistic of mixed GaO antibonding orbitals with Ga 4p atomic ortween the number of adsorbed H2O molecules and the bitals. Generally, one expects a decrease in the band gap of the number of surface unsaturated Ga sites. For a surface with n surface compared with the bulk value due to the dangling adsorbed waters, we define the adsorption energy per molebonds on the surface. As shown in Figure 2 b, the band gap of cule (Eads) as the total energy difference Eads = [Esurf + the (001) surface is indeed narrowed by about 0.2 eV comnEH2OEtotal]/n, in which Esurf is the energy of the bare surface pared with the bulk. There is a new band located at the Fermi slab, EH2O is the energy of one free water molecule, n is the level, which mainly consists of surface O3c 2p states with number of adsorbed H2O, and Etotal is the total energy of the a slight contribution from the surface Ga3c states. Based on the clean surface with n adsorbed waters. Therefore, a positive Eads uppermost-exposed surface structure, the (001) surface has an value corresponds to exothermic adsorption. The optimized oxygen-rich termination. So the surface states at the Fermi geometries, key structural parameters, and adsorption energies level originate from the unsaturated oxygen atoms on the surare shown in Figure 3 and Table 2. The following abbreviations face. It implies slightly less electron-transfer from Ga atoms to O atoms. The calculated band gap of the (100) surface is 1.96 eV, which is smaller than that of (001) surface. This is unexpected because the surface atoms on the (100) surface have a higher coordination than those on the (001) surface. However, on the (001) surface, an excess of lowcoordinated O atoms leads to the increase of the band-gap through the increased polarity of the surface chemical bonds. An increased band gap is the signature of additional Madelung energy stabilization, which also may explain the exceptionally low energy of the (001) surface. For the (110) and (012) surfaces, the new bands close to the CBM are responsible for the further Figure 3. Optimized structures of water molecular and dissociative adsorption on (001), (100), (110), and (012) surfaces. The red spheres represent the oxygen atoms of surfaces, the brownspheres represent gallium atoms, the band gap reduction. These two yellow spheres represent the oxygen atoms of water, and the white spheres represent hydrogen atoms. surfaces have an excess of coordinatively unsaturated Ga atoms. This leads to reduced charges on Ga, and as a consequence, lowered surface Madelung energies are used in the text to describe the structure: Os are the as follows from the narrowed band gaps. The increased relaoxygen atoms in the exposed surface, Gas are the gallium tive stability of these surfaces now relates to decreased ionicity atoms in the topmost layer, Ow, H1, and H2 are the oxygen of surface chemical bonds, so that electrostatic cost of charge and hydrogen atoms of water. separation is reduced. As can be seen from PDOSs in Figure 2 d and e, the new surface states are primarily composed of surMolecular and dissociative adsorption on the (001) surface face Ga 4s orbitals hybridized by Ga 4p orbitals. To sum up, the results in Figure 2 show that the band gaps for the four inWe start our discussion with the adsorption of a single water vestigated surfaces are narrower than that of the bulk in varymolecule. As shown in Figure 3 and Table 2, the stable strucing degrees, which appears to be less related to a difference in ture of water molecular adsorption on (001) surface is denoted coordinative unsaturation of the surface atoms, but are possias 001-M. An isolated water molecule adsorbs molecularly to bly related to changes in the ionicity of the surface chemical a surface gallium site through its O atom, with an adsorption bonds. energy of 0.89 eV. Water adsorption lifts up Gas slightly from the surface, increasing the GasOs distance by 0.03  and cre&

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Full Paper atom are less deep than those of the Os atom with higher Ga coordination. Consequently, the interaction between H 1s orbital and O 2p orbital is very different. In the case of interacting with the highly coordiAdsorption Gas-Ow Gas-Os Os-H1 Ow-H1 Ow-H2 Eads Erelax wOsH wOwH2 nated Os atom, low-energy bonding and antibonding 1 1 [] [] [eV] [eV] [cm ] [cm ] [] [] [] type OH orbitals are formed. The high occupation of anti001-M 2.099 1.845 2.356 0.986 0.986 0.89 0.22 bonding orbitals implies a relatively weak OH bond 001-D 1.838 2.016 0.980 2.739 0.973 1.19 1.54 3627 3755 energy. The interaction of H atom and singly coordi100-M1 2.139 1.864 1.771 1.010 0.975 0.67 0.09 nated Ow atom gives a bonding orbital of higher 100-M2 2.123 1.969 2.114 0.987 0.976 0.69 0.09 100-D1 1.892 1.955 1.001 1.837 0.974 1.45 0.81 3196 3750 energy and as a consequence also an antibonding or100-D2 1.898 3.325 1.048 1.525 0.976 1.66 0.43 2417 3726 bital of higher energy, which now appears partially 110-M 2.296 1.893 1.722 1.016 0.975 0.82 0.17 above the Fermi level. In this case the OH bond 110-D1 1.990 2.025 0.978 2.466 0.976 1.64 1.41 3691 3721 energy will be higher. This difference in bond ener110-D2 1.885 3.508 1.037 1.561 0.973 1.72 1.00 2579 3753 012-M 2.129 2.016 1.847 1.003 0.980 0.77 0.12 gies is also reflected in the different vibrational ener012-D1 1.865 2.176 0.979 2.370 0.975 0.83 1.29 3660 3734 gies of the respective OH bonds. The vibrational anal012-D2 1.901 2.027 1.023 1.668 0.972 1.25 0.89 2812 3771 ysis of the stretching frequencies of the two types of OH groups is reported in Table 2. The highest calculated frequency is at 3755 cm1 and corresponds to ating a flattened tetrahedral environment of Gas. The GasOw OwH2. The lower-frequency mode at 3627 cm1 corresponds bond length is 2.099 . The distances between the H atoms to the OsH1 group. The hydrogen atom attached to the and the nearest Os are 2.356 and 2.390 , respectively, which bridging Os atom is expected to be less basic than one are short enough to generate hydrogen bonds to further enbonded to a single-coordinated Ow atom, which agrees with observed differences in OH bond energies. In contrast to exhance the stability of the final structure. The molecular water pectation, in the electronic ground state the more acidic state is metastable to dissociated water. From the equilibrium proton has to have a slightly higher negative charge. Upon configuration 001-D in Figure 3 a, we can see that water dissocontact with a reagent, the proton in contact with the O atom ciates on one gallium site and on the adjacent oxygen site, bridging with several Ga atoms induces the larger polarization leading to the formation of two new hydroxyl groups at the of the OH bond, such that electron density is more increased surface. Compared with molecular water adsorption, dissociaon this proton, which reveals its acidic nature.[69] tion further draws Gas away from the surface plane and the GasOs bond length is increased to 2.016 . The calculated reThe results above correspond to a 1/4 monolayer (ML) covsults show that 001-D is energetically more favorable than erage of water. When the water coverage is increased to 1 ML, 001-M by 0.3 eV. the adsorption geometries for both molecular and dissociated To shed more light on the dissociative mode, we performed configurations do not exhibit essential changes (Figure 5 a). analyses on DOS of 001-D. It is notable that PDOSs of the H1 The distance between the Gas atom and Ow atom is larger for and H2 atoms are quite different, as shown in Figure 4. This re1 ML molecular adsorption (2.145 ) than at low coverage lates to the different PDOSs of the O atoms as a function of co(2.099 ), indicating the weaker bond. As a result, the molecuordination. The 2s and 2p bands of the singly coordinated Ow lar adsorption mode is slightly destabilized by 0.08 eV based on the 1/4 ML value. As for dissociative adsorption, the calculated GasOw bond length (1.836 ) is close to that at low coverage (1.838 ), whereas the bond length of GasOs is predicted to be 1.970 , which is shorter than that at low coverage (2.016 ). The adsorption energy at this coverage decreases modestly to 1.11 from 1.19 eV at the 1/4 ML coverage. The fact that adsorption energy does not vary significantly with coverage indicates that the interactions between the adsorbed molecules are weak. The results show that, on the (001) surface, the dissociative state is always energetically more stable than the molecular state, whereas the energetic difference between them is 0.3 eV, independent of the water coverage. Table 2. Structural parameters and adsorption energies of water adsorbed on different a-Ga2O3 surfaces at the lowest water coverage, and calculated stretching frequencies of various OH groups as a function of surface orientation.

Molecular and dissociative adsorption on the (100) surface Two stable geometries of the single molecular adsorption state denoted 100-M1 and 100-M2 are depicted in Figure 3, with selected distances compiled in Table 2. For 100-M1, the Ow atom of H2O interacts with the Ga4c2 atom bonded to O3c2 and O2c atoms in Figure 1 c, with the H1 atom pointing toward neigh-

Figure 4. Density of states for the dissociative adsorption of H2O on the (001) surface, 001-D. The Fermi level is shown by the vertical dashed line. Chem. Eur. J. 2014, 20, 1 – 13

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Full Paper pected because the atoms on the (100) surface are less coordinatively unsaturated than those on the (001) surface. It is most likely because there is less relaxation of the GasOs bond length when H attaches (0.114  on (100) versus 0.201  on (001)). The stretching frequencies of OwH2 for 100-D1 and 100D2 are estimated at 3750 and 3726 cm1, respectively. These values are close to the value calculated for 001-D. The OsH1 frequency for 100-D1 at 3196 cm1 is lower than that for 001D, because the interaction between Gas atom and Os atom is stronger in 100-D1 than in 001-D. An even lower-frequency mode (2417 cm1) is found for OsH1 in 100-D2, exhibiting a strong hydrogen bond between Ow and H1. As shown in Figure S1 (in the Supporting Information), the DOS plots for 100D1 and 100-D2 look very similar in both distribution and composition, and are similar with the case in the (001) surface. A high coverage of water with  = 1 ML has been studied and the relaxed structures are shown in Figure 5 b and Figure S2 (in the Supporting Information). As for the molecular adsorption configuration, the orientations and adsorbed sites of water molecules are a mixture of 100-M1 and 100-M2. The distance between Gas and Ow is 2.155 and 2.354 , respectively, which are longer than those in 100-M1 and 100-M2. The weaker interaction between H2O molecules and the surface decreases the adsorption energy to 0.55 eV. Different initial structures of dissociative adsorption have been optimized. The most stable configuration with the adsorption energy of 1.34 eV is shown in Figure 5 b, and other modes with lower adsorption energies are displayed in the Supporting Information (Figure S1d–f). As shown in Figure 5 b, the dissociated H atom and O atom in H2O are far away from each other in the dissociative adsorption configuration at 1 ML. The interaction of intramolecular hydrogen bonds is essentially weakened. The intermolecular hydrogen bond length is calculated to be 1.889 , which is also longer than hydrogen bonds in 100-D1 and 100-D2. Consequently, the addition of more water molecules decreases the stability of dissociative adsorption. Nevertheless, the dissociative adsorption is always energetically more favorable than the molecular adsorption at different coverages of water.

Figure 5. Side views of the optimized geometries of molecular and dissociative adsorption of water at high coverages and the corresponding adsorption energy (Eads) attached for: a) (001), b) (100), c) (110), and d) (012) surface. The Ga atoms, surface O atoms, O atoms of water, and H atoms are represented by brown, red, yellow, and white spheres, respectively.

boring O2c atom to form a hydrogen bond. Adsorption in this mode induces only small geometric relaxation of the (100) substrate. The GasOs bond increases by about 0.02 . The distances of the formed GasOw bond is calculated to be 2.139 . The calculated H2O adsorption energy for this site is 0.67 eV. With respect to 100-M2, the water molecule binds to the Ga4c1 site with a GasOw bond length of 2.123 . The distance between Os and H1 is computed to be 2.114 . The shorter Gas Ow bond and longer OsH1 distance results in an adsorption energy of 0.69 eV for 100-M2, which is similar to that for 110M1. In the case of the dissociative adsorption on (100) surface, five stable configurations have been obtained. The two most stable adsorption types, denoted as 100-D1 and 100-D2 are shown in Figure 3. The structures of the adsorption modes with lower adsorption energies are shown in the Supporting Information (Figure S1a–c). The difference between 100-D1 and 100-D2 is the bonding of the H1 atom to different surface O atoms. The GasOw bond is calculated to be 1.892  for 100D1 and 1.898  for 100-D2, respectively. They are shorter than the GasOw bond in 100-M by about 0.04–0.05 , which essentially stabilizes the dissociative adsorption. The adsorption energies of 100-D1 and 100-D2 are predicted to be 1.45 and 1.66 eV, respectively. Due to the strong hydrogen bond (1.525 ) formed between Ow and H1, 100-D2 is more stable than 100-D1 by 0.21 eV. The dissociation energy of H2O on this surface is higher than that on the (001) surface, which is unex&

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Molecular and dissociative adsorption on the (110) surface The adsorption of an isolated H2O molecule on the (110) surface has been investigated for all possible configurations. The only stable molecular adsorption site, 110-M, is found on the Ga5c2 site. The distance between Gas atom and Os atom is calculated to be 2.296 , which is the longest of all the molecular adsorption structures on different surfaces. Yet, the adsorption energy of 0.82 eV is intermediate among the four investigated surfaces. The reason of the relatively stronger interaction between water and the (110) surface is that the formed hydrogen bond between H1 and Os is computed to be 1.722 , which is the shortest for all the surfaces. Direct transfer of H1 atom in 110-M to the Os atom generates 110-D1, and a new bond between the surface Ga atom and Ow atom (2.027 ) forms at the same time, which is an important reason for the high stability 8

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Full Paper ble site of adsorbing molecular water is the Ga5c2 atom displayed in Figure 1 d, and the hydrogen atom of water tends to bind to O2c atom to form the dissociative configuration. Due to the selective adsorption and dissociation of water molecules on (110) surface, mixed molecular/dissociative adsorption is favored at high coverages ( > 1/4 ML).

of this dissociative state with an adsorption energy of 1.64 eV. When the water molecule attaches to the Ga5c1 site, the dissociation will occur spontaneously to form two hydroxyl groups, labelled as 110-D2 in Figure 3 c. From the equilibrium configuration, we can find that the adsorbed Ow atom could be attached to the Gas atom to form a GasOw bond with a length of 1.885 , which is shorter than that in 110-D1. Due to the corrugated conformation of the (110) surface, the Ow atom and H1 atom are close enough to each other to generate a strong hydrogen bond (1.561 ), enhancing the stability. So, the adsorption energy of 110-D2 is even higher than that of 110-D1 by 0.08 eV. As shown in Table 2, the calculated stretching frequencies of the two hydroxyl groups in 110-D1 at 3721 and 3691 cm1 are similar, because both OH groups are twofold coordinated. The analysis of the OH-stretching modes of 110-D2 indicates that two frequencies are present at 3753 and 2579 cm1. The lowfrequency mode corresponds to the OsH1 group, exhibiting a strong hydrogen bond between Ow and H1. The exceptional high frequency of the OwH2 group is observed in both dissociative adsorption structures. The DOS plots in Figure S3 (in the Supporting Information) indicate that the PDOSs of Os, Ow, H1, and H2 exhibit similar characters for 100-D2 and 110-D2 due to the similar local structure. The adsorption of a single water molecule on the (110) surface corresponds to the coverage of 1/12 ML. To simulate the high coverage adsorption (1 ML) on the (110) surface, 12 H2O molecules were introduced in the slab model. As can be seen from the Supporting Information Figure S2a, the proximity of the surface Ga atoms leads to steric hindrance. As a result, only 8 water molecules can be physically absorbed. The adsorption energy for the mixed molecular and dissociative adsorption state is calculated to be 0.84 eV. We examined a few initial starting configurations for modeling the molecular adsorption structure for q = 1/2 ML. No matter how we adjust the position of water molecules, the relaxed configuration is always a mixture of the molecular and dissociative adsorption (Supporting Information, Figure S2b). The highest coverage at which only molecular adsorption occurs is 1/4 ML. The optimized structure is shown in Figure 5 c. The distance between the Gas atom and Ow atom is 2.322 , which is larger than that at 1/12 ML coverage. The weaker interaction between water molecules and surface results in a decrease of the adsorption energy from 0.82 to 0.77 eV. As shown in Figure 1 d, the number of exposed Ga atoms is larger than the number of under-coordinated O atoms on the (110) surface, so the simulation of dissociative adsorption with monolayer water is not possible. Accordingly, two higher coverages (1/4 and 1/2 ML) have been studied and the relaxed structures are shown in Figure 5 c and the Supporting Information Figure S4c, respectively. At the 1/4 ML coverage, the surface structure of the configuration is a mixture of 110-D1 and 110-D2 structures, with an adsorption energy of 1.70 eV. For  = 1/2 ML, because the formed hydroxyl groups are close to each other, it is easy to reform water. As shown in the Supporting Information Figure S4c, the formation of one water molecule decreases the adsorption energy to 1.40 eV. The calculated results show that the favoraChem. Eur. J. 2014, 20, 1 – 13

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Molecular and dissociative adsorption on the (012) surface As shown in Figure 3 d, the molecular adsorption of an isolated water molecule on the (012) surface is found to be stable in an approximately parallel configuration (012-M). In this case, the Ow atom interacts with the Gas atom and the H1 atom points toward the neighboring Os atom with an adsorption energy of 0.77 eV. The length of the newly formed GasOw bond is 2.129 . The distance between H1 and Os is computed to be 1.847 , which belongs to the hydrogen-bond interaction. For dissociative adsorption, two stable configurations denoted as 012-D1 and 012-D2 were obtained, in which H1 adsorbed onto the different surface O atoms (O3c2 and O3c1 in Figure 1 e, respectively). The calculated adsorption energies for these sites are 0.83 and 1.25 eV, respectively. The hydrogen bond formed between Ow and H1 (1.668 ) makes 012-D2 more stable. The results indicate that dissociative adsorption is more favorable than the molecular adsorption on the (012) surface. Due to the presence of a hydrogen bond, the stretching frequency of OsH1 group for 012-D2 is estimated at 2812 cm1, which is much lower than that for 012-D1 (3660 cm1). Another possible reason is that the longer bond length of GasOs by 0.15  and the shorter bond length of OsH1 by 0.05  in the 012-D1.The analysis of the OwH2 stretching modes indicates that they are present at 3734 cm1 in 012-D1 and 3771 cm1 in 012-D2, respectively. The slight difference between them is related to the similar distance between the Gas atom and Ow atom. The PDOSs for 012-D1 and 012-D2 plotted in Figure S5 (in the Supporting Information) suggest that the overlap between Os 2p, and H1 1s orbitals is deeper than that between Ow 2p, and H2 1s orbitals, as is the case for 001-D. In addition to the investigation on the single molecule water adsorption, we considered the behavior of a water monolayer on the (012) surface. As shown in Figure 5 d, the orientation of molecular adsorbed water at  = 1 ML is the same as that at  = 1/6 ML. The distance between Gas atom and Ow atom increases by 0.065 . The weaker interaction between water molecules and the surface decreases the adsorption energy to 0.66 eV. For dissociative adsorption, the intermolecular distance between hydroxyl groups is short enough to form the hydrogen bond of 1.858 . Therefore, as the number of the hydroxyl groups increases at the high coverage, the formation of hydrogen-bonding network further stabilize the dissociative adsorption, with an adsorption energy of 1.32 eV. The relaxation energies We carried out a detailed theoretical analysis of the interaction between the water molecules and surfaces of a-Ga2O3. Our calculations show that, for all the investigated surfaces, both mo9

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lecular adsorption and dissociative adsorption are exothermic processes. Dissociative adsorption is energetically more favorable than molecular adsorption at low and high coverages of water, which is beneficial to the water-splitting reaction. However, only consideration of the coordination of unsaturated atoms cannot provide us a reasonable prediction of the adsorption strength of water on various surfaces. To understand the results in more detail, we computed the relaxation energies for all the water-adsorbed complexes at the lowest coverage. Here, the relaxation energy is defined as Erelax = Esurf@compEsurf, in which Esurf@comp is the single-point energy of clean surface at the optimized geometry in water-adsorbed complexes, and Esurf is the energy of relaxed clean surface. The calculated relaxation energies are listed in Table 2. For molecular adsorption, the relaxation energy is 0.22 eV for (001), 0.09 eV for (100), 0.17 eV for (110), and 0.12 eV for (012), respectively. The relation between the relaxation energy and adsorption energy for molecular adsorption is shown in Figure 6.

relaxation energy of the surface and the formation of hydrogen bond in water-adsorbed complexes.

Figure 6. The relationship between the relaxation energy and the adsorption energy for the molecular adsorption configuration on four surfaces.

Figure 7. Calculated free-energy diagram for hydrogen evolution at a potential U = 0 relative to the standard hydrogen electrode at pH 0.

The variation of the relaxation energy trends well with adsorption energy. In other words, the molecular adsorption mode with higher adsorption energy undergoes larger relaxation. As shown in Table 2, the relaxation energy of dissociative adsorption configurations for every surface is higher than that of the molecular adsorption configuration, which is in good agreement with the comparative result of adsorption energies. However, this rule is not suitable to explain the difference between two dissociative modes in the same surface. Taking the (110) surface as an example, the relaxation energies of 110-D1 and 110-D2 are predicted to be 1.41 and 1.00 eV, whereas the adsorption energies of 110-D1 and 110-D2 are 1.64 and 1.72 eV, respectively. The configuration with a larger relaxation has relatively lower adsorption energy. By examining the structures of two configurations, one finds that there is a strong hydrogen bond formed between the Ow atom and the H1 atom in 110D2, which importantly stabilizes the adsorbed mode. Similar strong hydrogen bonds also exist in 100-D2 and 012-D2 with higher adsorption energies and smaller relaxation energies. Therefore, our results indicate the most important factors determining the adsorption energies of water on surfaces are the

hydrogen adsorption reaction DGH are predicted to be 0.15 eV for the (001) surface, 0.08 eV for the (100) surface, 0.58 eV for the (110) surface, and 0.21 eV for the (012) surface. If the j DGH j is close to zero, the catalyst will be near its optimum performance composition.[42–44, 55–58] Therefore, the results indicate that (100) is the most favorable surface for the HER. After elucidating the mechanism of the HER, we now turn to the analysis of the oxygen evolution reaction (OER). Figure 8 and the Supporting Information (Figures S6–S8) show the calculated free-energy diagrams of the OER and the relaxed structures for intermediates on the (001), (100), (110), and (012) surfaces of a-Ga2O3 at U = 0, pH 0, and T = 298 K, respectively. The results demonstrate that, for no applied bias, U = 0 V, all steps in all the surfaces are uphill. At standard equilibrium potential for oxygen evolution at U = 1.23 V, some of the steps become downhill but some still remain uphill. It is necessary to impose a bias on all the four surfaces to have every step downhill. As shown in Figure 8, the most favorable case is for the (110) surface, which needs an overpotential of 0.6 V (= 1.83–1.23 V). The (110) surface has been experimentally observed as part of the a–b phase junction structure by using high-resolution

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Hydrogen and oxygen evolution on the different surfaces We will start to discuss the hydrogen evolution reaction (HER) mechanism. Initially, we considered all possible active sites for hydrogen adsorption, including Ga and O sites. Our calculations show that the adsorption energies of hydrogen atoms on O sites are energetically more favorable than those on Ga sites by 1.8–2.4 eV depending on the different surfaces. Among the different O sites, hydrogen atoms tend to adsorb to the lowcoordinated O atoms for all the surfaces studied. Based on the most stable adsorption configuration for every surface, the free-energy diagram for hydrogen evolution at a potential U = 0 relative to the standard hydrogen electrode at pH 0 is determined and shown in Figure 7. The free-energy changes of the

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Figure 9. Adsorption energies of HOO* versus adsorption energies of HO* on (001), (100), (110), and (012) surfaces.

0.56, and an intercept of 3.44 eV, as indicated in Figure 9. This implies that the absolute error of the linear fit is 0.07 eV. There is a strong correlation between the two species. The constant intercept suggests that HO* and HOO* prefer the same type of binding site. From the point of view of the optimized structures shown in Figure 8 and the Supporting Information (Figures S6–S8), HO* and HOO* configurations on every surface look very similar.

Conclusion

Figure 8. Free-energy diagram at pH 0 and T = 298 K for the four steps of the OER at the different applied potentials (U = 0, 1.23, and 1.83 V) for the (110) surface.

We have performed a comprehensive investigation on the water adsorption and activation on various stoichiometric aGa2O3 surfaces by means of first-principles DFT calculations. The conclusions are as follows:

transmission electron microscopy.[12] The computed overpotential for the (110) surface is higher than that calculated result of RuO2 by about 0.2 V,[34] but comparable to the value of Co3O4,[70] and lower than the calculated results for a-Fe2O3, WO3, and TiO2 surfaces.[34, 35, 71–73] Combining the results of HER with OER, one can find that water reduction and oxidation reactions on pure a-Ga2O3 surfaces have reasonable energetic requirements, which does not agree with the experimental observations, since experiments show that without co-catalysts (NiOx), the water-splitting reaction on the surfaces of a-Ga2O3 hardly takes place.[12] There are two possible reasons for the disagreement: 1) The surfaces that we discover to be active have low stability and are not present in the experimental particles; 2) Activation barriers of the elementary reactions, which are not included in the electrochemical approach we used in this work, are responsible for the low electrocatalytic activity of a-Ga2O3 without co-catalysts. A universal scaling relationship between the adsorption energies of HO* and HOO* has been proposed by Koper and coworkers from studying various transition-metal oxide surfaces.[74] They found that the adsorption energies of HO* and HOO* are linearly correlated to each other by a constant of 3.2 eV. A similar scaling relationship is also found to hold for pure or doped-hematite surfaces by Liao et al.[71] This linear relation is found for the four surfaces of a-Ga2O3, with a slope of Chem. Eur. J. 2014, 20, 1 – 13

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1) On the basis of the computed surface energies, the stability of various surface planes has been assessed. The trend of relative stability is predicted to be (012) > (001) > (110) > (100). 2) At both low and high water coverages, molecular and dissociative adsorption of water are exothermic reactions. The dissociative adsorption is always thermodynamically more favorable than molecular adsorption on all the investigated surfaces. 3) The two factors that determine the adsorption strength of water on surfaces are the formation of hydrogen bonds with surface atoms and the relaxation energy of surface. 4) A linear relationship between the adsorption energy of HOO* and HO* configurations is found. 5) The (100) and (110) surface with relatively high surface energy are predicted to be the most favorable surfaces for HER and OER, respectively.

Acknowledgements This work is financially supported by National Science Foundation of China under Grant 21003115, 21090340, and 11

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Full Paper 21061140361, the National Basic Research Program of China (2014CB239400), the Solar Energy Initiative of the Knowledge Innovation Program of the Chinese Academy of Sciences (Grant. No. KGCX2-EW-311). The authors also acknowledge the Program Strategic Alliances between China and The Netherlands funded by the Royal Netherlands Academy of Arts and Sciences and the Chinese Ministry of Science and Technology. Keywords: density functional calculations photocatalysis · surfaces · water adsorption

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Chem. Eur. J. 2014, 20, 1 – 13

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Received: January 2, 2014 Published online on && &&, 2014

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Full Paper

FULL PAPER & Density Functional Calculations

Beneath the surface: Chemisorption energies of water on different surfaces of a-Ga2O3 do not relate to differences in the respective surface energies or coordinative unsaturation of the surface atoms, but instead with the respective adsorption-induced surface relaxation energies (see figure).

Chem. Eur. J. 2014, 20, 1 – 13

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These are not the final page numbers! ÞÞ

X. Zhou, E. J. M. Hensen, R. A. van Santen,* C. Li* && – && DFT Simulations of Water Adsorption and Activation on Low-Index a-Ga2O3 Surfaces

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