Removal of Water Adsorbates on GaN Surfaces via Hopping

Article pubs.acs.org/JPCC

Removal of Water Adsorbates on GaN Surfaces via Hopping Processes and with the Aid of a Pt4 Cluster: An Ab Initio Study Yun-Wen Chen,* Yaojun Du, and Jer-Lai Kuo* Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan S Supporting Information *

ABSTRACT: In one of our previous works (Du, Y. A.; Chen, Y.-W.; Kuo, J.-K. Phys. Chem. Chem. Phys. 2013, 15, 19807−19818), it had been found that nonpolar and polar facets of gallium nitride (GaN) nanoparticles may play different roles in water splitting processes because of their different redox characters for O2 and H2 evolution. The mechanism of removing other water adsorbates on different facets was left undetermined. In this work, we investigate the hopping processes of water adsorbates on GaN (101̅0) nonpolar and (0001) polar surfaces and also the effect of a Pt4 cluster on nonpolar surface for water splitting. The hopping of O adatoms from polar to nonpolar surface is found to be feasible at room temperature and hence can facilitate the O2 evolution. However, the mobility of H adatoms from nonpolar to polar surface is relatively limited for higher barriers. On the other hand, the Pt4 cluster offers mediate states to facilitate water splitting and H2 evolution on GaN nonpolar surface. The features of Pt4 projected density of states show suitable properties of being a cocatalyst for H2 evolution but also a shortcoming as an electron−hole recombination center.

1. INTRODUCTION The urgency of finding replacements for diminishing fossil fuels is one of the most important issues nowadays that propels scientists to investigate possible solutions for renewable energy resources. Among various proposed strategies, sunlight harvesting through water splitting is now considered as a feasible way for gaining clean and renewable energy without producing undesired hazards or pollution.2,3 Since Fujishima and Honda discovered that the illuminated titanium dioxide (TiO2) electrode has the ability of driving water electrolysis,4 many scientists have devoted themselves to finding efficient photocatalysts for water splitting and understanding the underlying mechanism. The overall reaction is recognized as H2 and O2 evolution happening at different sites on photocatalyst surfaces by water adsorbates adsorbing separated photoexcited electrons and holes.5,6 In the past decades, many photocatalysts have been studied. Some of them can only adsorb photons with energies higher than visible light because of wide energy band gaps,5,6 while others are only proper for either water reduction or oxidation because of inappropriate band-edge positions.5,6 To acquire the maximum power of sunlight, an efficient photocatalyst should satisfy the following general criteria: (1) the positions of conduction and valence band-edges are suitable for overall water redox, (2) the energy band gap is between 1.23 and 3 eV, and (3) the photocatalyst is stable during photocatalytic reaction.5−7 To improve the efficiency, different physical designs (including etching, nanorod, nanoparticle) and various combinations with cocatalysts and electrolytes were investigated.5,6 Among various investigated photocatalysts, GaN doped with zinc oxide (ZnO) solid solution was found to be a potential material for harvesting sunlight energy with tunable © XXXX American Chemical Society

band gap and right band-edge positions for visible light-driven water splitting.5−7 Besides the efforts of looking for efficient photocatalytic water splitting systems, recently more and more elaborate experiments tried to discover the details happening in water splitting processes like those using GaN, ZnO, or GaN/ZnO as photocatalysts.8−16 Most of these experiments were concerning the dynamics of photon-excited electrons and holes; hence, they focused on the topics of electron−hole recombination,8−10 band bending and Fermi level pinning,10−13 the electron states of photocatalyst and cocatalyst,10−15 and the charge accumulation or transfer at interfaces.9,10,12,13,16 To obtain a microscopic picture of the intrinsic mechanism and hence predict suitable photocatalysts, many theoretical calculations estimated the band gaps and band edge positions of known and potential materials.1,17−27 Some of these works even analyzed band structures and/or predicted the light absorption spectra.17−22 Many other theoretical works studied the water/ photocatalyst interface structure and/or thermal dynamics of water molecules and adsorbates to understand the water splitting processes from an atomic point of view.1,28−43 In one of our previous works,1 we observed that it is much easier to produce oxygen gas molecules than H2 on (101̅0) nonpolar surface, while the polar surface behaves in a reverse way. The paths for removing other water adsorbates (hydroxyl groups, hydrogen, and oxygen adatoms) from GaN surfaces are still undetermined. The proper removing mechanism is important in photocatalytic water splitting systems for steady H2 and O2 Received: June 4, 2014 Revised: August 13, 2014

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dx.doi.org/10.1021/jp5055149 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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implemented in VASP. Intermediate images of each cNEB simulation are relaxed until the perpendicular forces are smaller than 0.05 eV/Å.

evolution. The photocatalytic water splitting reactions will cease if those water adsorbates are not able to be removed and making the photocatalyst surfaces poisoned. Hence, in this work we extend our previous studies to investigate the hopping processes of water adsorbates on GaN (101̅0) nonpolar and (0001) polar surfaces by using density function theory (DFT) and climbing nudged elastic band (cNEB) method.44−47 Our calculations show that the hopping barriers of water adsorbates on the polar surface are generally smaller than 1 eV; however, the corresponding hopping barriers on nonpolar surface are generally larger than 1.5 eV. The results indicate O adatoms are able to diffuse from polar to nonpolar surface for O2 evolution at room temperature, but the diffusion of H adatoms from a nonpolar to a polar surface is much limited. In addition, we find that a Pt4 cluster deposited on the nonpolar surface could offer mediate stable states for the dissociation of water adsorbates and also reduce the barrier heights for H2 evolution processes. This Article is organized as follows. In section 2, we describe the details of simulation models and parameters used in all of our calculations. In section 3, we present the simulation results and discuss the hopping processes on the nonpolar and polar surfaces. The effect of a Pt4 cluster deposited on nonpolar surface is inspected in terms of energy barriers of water splitting processes and also the projected density of states (PDOS) of stable adsorption models. Finally, a brief conclusion is given in section 4.

3. RESULTS AND DISCUSSION A. Hopping of Water Adsorbates on the Nonpolar Surface. On the (101̅0) nonpolar surface, the landscapes along the [001] and [010] directions are quite different. To qualitatively investigate the transport behavior of water adsorbates on this surface, we calculate the hopping barriers of a H adatom, an O adatom, and an OH group along two mentioned directions on a large slab model (Figure 1a) with

Figure 1. Top views (upper pictures) and side views (lower pictures) of GaN (101̅0) and (0001) surfaces. In (a), Ga (red) and N (blue) atoms in the first double layer of a GaN (1010̅ ) surface are shown with darker and bigger spheres; the atoms in the second double layer are brighter and smaller spheres. In (b), only the first double layer Ga (red) and N (blue) atoms of a GaN (0001) surface are shown. The green frames and lines indicate the periodic boundaries of simulation cells.

2. SIMULATION METHOD AND PARAMETERS To discuss various hopping situations of water adsorbates on GaN surfaces, we used four GaN surface slab models with a 15 Å vacuum in the z direction with different sizes of surface areas for the nonpolar and polar surfaces. The nonpolar surfaces are modeled by slabs with 1 × 2 × 4 and 3 × 3 × 4 unit cells; that is, the slabs have four double layers in the z direction (the same as 4 cation−anion bilayers in ref 1). The polar surfaces are modeled by slabs with 2 × 2 × 3 and 4 × 4 × 3 unit cells (the large polar slab is reshaped into the same orthorhombic cell as ref 28); that is, the slabs have six double layers in the z direction. The lattice parameters are set to the experimental values of a = b = 3.19 Å and c = 5.189 Å.48 The bottom of the surface models are all passivated with pseudohydrogen atoms with 0.75e for dangling N atoms or 1.25e for dangling Ga atoms. The positions of pseudohydrogen atoms are optimized with fixed perfect slabs. The optimizations of water adsorption on surface models then are done with the bottom 2 or 4 double layers of nonpolar or polar surfaces, and also pseudohydrogen atoms always fixed. The simulation tool used is the Vienna ab initio simulation package (VASP),49−52 which applies plane-wave basis sets to expand electron wave functions in conjunction with pseudopotentials by the projector augmented-wave method.53 The valence electron wave functions of Ga, N, O, Pt, and original H atoms are approximated with 4s4p3d, 2s2p, 2s2p, 6s5d, and 1s orbitals. The Perdew−Wang 91 generalized gradient approximation (GGA)54 is applied to calculate the electron exchangecorrelation energies. The cutoff energy for plane-wave expansion is set to 400 eV, and 7 × 7 × 1, 3 × 4 × 1, 7 × 7 × 1, and 3 × 3 × 1 Γ-centered Monkhorst−Pack55 k-point meshes are applied for small and large slabs of nonpolar and polar surface models. Optimizations of all simulation models are done until the force components on unfixed atoms are less than 0.01 eV/Å. The energy barriers are estimated via the climbing nudged elastic band (cNEB) method44−47 as

the aid of the cNEB method. The hopping of one water molecule on the nonpolar surface is omitted because of the extremely small barrier for an intact water molecule splitting into a H adatom and an OH group.1,33 For simplicity, we also ignore the effect of additional H, O adatoms, or OH groups on the hopping processes. Figure 2 summarizes the calculated six hopping energy curves with the x direction representing [001] and the y direction representing [010]. The hopping barrier of a H adatom along the [001] direction is about 0.5 eV lower than that along [010], which indicates the hopping along [001] is easier than that along [010]. However,

Figure 2. Hopping energy curves of water adsorbates on the GaN (101̅0) surface along the [001] and [010] directions. The x and y directions represent the [001] and [010] directions, respectively. Hence, the HX, HY, OX, OY, OHX, OHY curves represent the hopping of one H, O adatom, and OH group along [001] and [010]. The reaction coordinate moving from 0.0 to 1.0 represents a translation along [001] or [010] by one unit cell. B



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both hopping barriers are larger than 2 eV and much higher than the strength of water hydrogen bonds, which is about 0.7 eV/H2O in GGA.56 The H adatoms will be hard to diffuse on the (1010̅ ) nonpolar surface at room temperature with the aid of thermal fluctuation of water molecules only. On the other hand, the barriers for H2 evolution are more than 5 eV on (101̅0) nonpolar surface,1 which are even much higher than the hopping barriers (≤2.5 eV). Hence, the H2 evolution on the nonpolar surface will be a much rarer event as compared to a H adatom hopping. To estimate the probability of a H adatom hopping through a hydrogen-bond network, we also study the process of one H adatom detaching from a surface N atom to form H3O+ species with another water molecule using cNEB method with six intermediate images in a small slab model. The configuration of one H adatom and two OH groups adsorbed on surface plus one H3O+ molecule H-bonding to one of the OH groups is not stable during optimization; several tries always converge to the same final configuration with one monolayer (ML) water dissociation (two H adatoms and two OH groups) on surface and one water molecule H-bonding with the OH groups. An artificial and unstable configuration by manually fixing the oxygen atom of the H3O+ is 4.6 eV higher than the optimized state. With the help of cNEB calculations, there is no transition state between the optimized state and the artificial state. This test suggests that it is extremely rare for a H adatom to detach from a surface N atom and hop through a hydrogen-bond network at room temperature. The calculated hopping barriers of one O adatom are 1.54 and 1.68 eV along the [001] and [010] directions (the third and fourth curves in Figure 2), respectively. However, the O2 evolution has an even lower barrier (our result in this work is 1.1 eV instead of 1.35 eV in ref 1), which makes two O adatoms on a nonpolar surface more likely to combine with each other to form O2 rather than diffuse on nonpolar surface. Unlike H and O adatoms, the mobility of hydroxyl groups hopping along the [001] direction is very different from that along [010]. The hopping barrier of one OH group is about 2 eV along [001], but it is just 0.31 eV along [010]. Because the latter one is less than hydrogen-bond strength, 0.7 eV/H2O, OH groups will primarily transport along the [010] at room temperatures, although the hydrogen bonds between OH groups and bulk water might play certain roles and complicate the hopping process. The strength of hydrogen bonds between OH groups was estimated to be about 0.3 eV/OH group in ref 1. Meanwhile, the adsorption energy of one water molecule adsorbed on the nonpolar surface with 1 ML water dissociation is 0.62 eV by using the small slab, in which the water molecule has one donor and one acceptor H-bonding with the OH groups. To catch the conditions close to experiments, we are planning to investigate the effect of bulk water on surface water adsorbates in a water channel model. On the other hand, an OH group is actually hard to dissociate into H and O adatoms on the (101̅0) nonpolar surface for a high energy barrier of 3.36 eV as shown in the first curve of Figure 3. At the same time, the backward reaction only needs to overcome 0.54 eV for one O adatom recombining with one neighbor H adatom. Hence, the removal of OH groups on nonpolar surface is mainly dependent on the hopping along the [010] direction. B. Hopping of Water Adsorbates on the Polar Surface. Because the surface energy difference between a (2 × 1) surface reconstruction and a pristine (1 × 1) GaN polar surface is small

Figure 3. Energy curves of dissociating an OH group into a H and an O adatom on 3 × 3 nonpolar, 2 × 2 polar, and 4 × 4 polar surface models. The 0.0 in reaction coordinate represents the adsorbed OH group state; the 1.0 is the dissociated state.

in theory (less than 0.06 and 0.094 eV/Ga in refs 28 and 57, respectively) and the estimated transition barrier is also small as 0.11 eV/Ga in this work, we calculate the hopping barriers of water adsorbates on the (1 × 1) surface (see Figure 1b) without considering the effect of surface reconstruction. That means we assume the hopping processes along the [100], [010], and [110] on the hexagonal polar surface are all equivalent. Actually, water adsorption will induce the surface buckle (reconstruction); hence, it is difficult to distinguish adsorbates/surface interaction from the effect of the surface reconstruction in total adsorption energies and also the hopping barriers. We calculate the hopping barriers of water adsorbates on GaN polar surface with 2 × 2 and 4 × 4 slab models to get clear conclusions. Because the water dissociation on the polar surface has to obey the electron counting rule,28,58 the dissociation number of water molecules is limited. Hence, the possible water adsorbates on a polar surface include intact water molecules, H, O adatoms, and hydroxyl groups. Although the surface models in consideration have a simple landscape (when omitting the surface reconstruction), the arrangements of adsorbates could be complicated because each Ga site can absorb various types of water adsorbates. In this study, we also consider the hopping in chemically overbinding systems28 to make this investigation more complete. The hopping energy curves of various water adsorbates are summarized in separated charts of Figure 4. An intact water molecule will be adsorbed on top of a Ga site; hence, the hopping vector is one of the unit-cell vectors shown in the surface top view of Figure 1b. As shown in Figure 4a, one hop of an intact water molecule only needs to overcome a barrier about of 0.36−0.4 eV, which is smaller than hydrogen-bond strength. Hence, our results suggest that an intact water molecule is able to diffuse on the polar surface at room temperatures. Because the intact water hopping is not the focus of water splitting processes, we do not calculate the barriers in the presence of other water adsorbates, such as another water molecule and OH group, on the surface. Yet it is expected that the hopping of an intact water molecule will be affected by the hydrogen bonds forming with other intact water molecules, OH groups, and bulk water. The adsorption energy of one water molecule adsorbed on a small GaN polar slab with 1 ML water dissociation is 0.6 eV when the water molecule has one donor and one acceptor H-bonding with the OH groups on surface. The effect of bulk water to surface water adsorbates will need further effort by using a water channel model. C



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Figure 4. Hopping barrier cureves of water adsorbates on GaN (0001) 2 × 2 and 4 × 4 surface models. In (a), (b), (c), and (d), one intact water molecule, one H adatom, one O adatom, and one OH group hopping barriers are shown in various situations. In (b) and (d), “xH, yOH” indicates the adsorbates are x H adatoms and y OH groups. “8w_7” means 8 water molecules adsorbed with 7 of them dissociated, which follows the same convention in ref 28. In (c), “H” means a hollow site and “N-top” means a site above a N atom of the first double layer. “xO 4 × 4” indicates there are x O adatoms adsorbed on a 4 × 4 polar surface model.

than one O adatom adsorbed, the energy barriers vary from 0.59 to 1.81 eV according to the last three curves in Figure 4c. There are 5, 6, and 7 O adatoms adsorbed on the 4 × 4 polar surface model for those three curves, respectively. According to the electron counting rule, the 6-O adatom adsorption reaches the maximum adsorption energy; further O adatom adsorption will cause energy penalty. Our calculated adsorption energies agree with the argument of electron counting rule as well. A ̈ guess is the hopping barriers would be around 1 eV for naive not overbinding systems. However, the hopping barriers of 5-O and 6-O adatom adsorption systems dramatically increase as compared to the first four curves for one O adatom hopping. With careful inspections, we find that O adatoms like to stay away from each other on the polar surface. The adsorption energies become smaller and the barriers rise by 0.8 and 0.66 eV when the number of neighboring O adatoms increases after hopping in the 5-O and 6-O adatom cases examined. For the chemically overbinding system, 7-O, the barrier is about 0.59 eV only. In that particular model, the number of neighbors remains the same after hopping; hence, the energies of end points are close. Because a chemically overbinding system has less average adsorption energy, the hopping barrier is expected to be smaller. One hydroxyl group can be adsorbed on top of a Ga site or across two Ga sites. The first curve in Figure 4d shows the energy landscape that one hydroxyl group may experience when moving from one Ga site to a cross site (the fourth point from left) and then to its adjacent Ga site. The energy barrier is less than 0.3 eV, which is smaller than the hydrogen-bond strength. Actually, all of the OH group hopping processes we tested experience small barriers on polar surface (Figure 4d); the barriers vary only from 0.11 to 0.31 eV. On the other hand, the estimated OH group dissociation barrier is between 0.73 and 0.93 eV (Figure 3) on the polar surface, which is smaller than the energy barrier of 1.42 eV for a H adatom forming a H2 gas molecule with an OH group and

On the other hand, the hopping barriers of a H adatom range from 0.8 to 1 eV, which is more than twice the intact water hopping. The hopping vector is one of the unit-cell vectors as well because a H adatom is bonding on top of a Ga site. According to Figure 4b, the hopping of a H adatom is about 1 eV when the GaN slab is not chemically overbinding. For the hopping in an overbinding system, 8w_7 (7 water molecules dissociated when 8 molecules adsorbed, the same convention in ref 28), three of the four tries we have made are mediated by water molecules, which involves recombination of the H adatom with an OH group. The only valid try has the barrier still around 1 eV (the last curve of Figure 4b), similar to other under-binding systems. Because the 8w_7 system is chemically overbinding and is less stable than full-binding condition, 8w_6 (6 water molecules dissociated when 8 molecules adsorbed), by about 4 eV,28 a H adatom hopping in 8w_7 system will easily be mediated by a water molecule, like the other three cases we tried. Hence, the H adatom hopping with in 8w_7 system should not be the dominant process in experiments. In the three failed trials, energy barriers are ranging from 0.2 to 0.35 eV when one OH group absorbs the hopping H adatom to form a water molecule. The most stable adsorption site for an O adatom on the polar surface is the hollow site, which is at the center of three top Ga atoms and N atoms; the other metastable site is N-top site. Hence, the hopping direction of an O adatom could be along the unit-cell vectors or the combination of them, like [120] (see Figure 1b). However, the N-top site is a very shallow absorption site because there is no transition state found for one O adatom hopping from a hollow site to a N-top site, although it is found as an adsorption site during structure optimization. When there is only one O adatom on the model surface, the energy barriers are about 1 eV, and the transition state is always located near the neighborhood of an N-top site (the second and fourth curves in Figure 4c). However, when there is more D



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leaving one O adatom.29 Hence, an OH group on a polar surface is easy to hop from site to site; it has less chance to be dissociated and even lower opportunity to form a hydrogen molecule with a H adatom. When the system is chemically overbinding (8w_7), the energy barriers are all less than 0.2 eV (see the last two curves in Figure 4d). According to the results in Figure 4d, there is no obvious tendency in the barriers of OH group hopping with respect to whether the surface is chemically overbinding or not. In contrast to the cases of H adatom hopping, all three OH group hopping trials of chemically overbinding systems (only two barriers are shown in Figure 4d) are not mediated by water molecules. According to the three failed trials of H adatom hopping on the polar surface in the 8w_7 system, the energy barrier for one OH group combining one H adatom to form a water molecule is comparable with one OH group hopping (0.2−0.35 eV). Hence, OH groups could either hop among Ga sites or form intact water molecules on a chemically overbinding GaN polar surface. C. Pt4 Cluster Decorated GaN (101̅0) Nonpolar Surface. Cocatalysts are commonly used in many photocatalytic water splitting systems to enhance the H2 evolution.5,6 It is suggested that an effective cocatalyst can collect the photoexcited electrons and suppress the unwanted electron− hole recombination. In experiments, nanoparticles of transition metals or their oxides were used as cocatalysts in (Ga1−xZnx)N1−xOx photocatalytic systems.6 Among these cocatalysts, Platinum (Pt) is a common catalyst used in various chemical reactions including water splitting and H2 evolution. Recently, many theoretical studies investigated the interactions between Pt clusters and adsorbates involved in hydrogen fuel cell and water splitting.59−70 Generally, Pt has been considered as a catalyst for chemical reactions involving proton transfer. Here, we try to explore the possible characters of cocatalysts by investigating the effects of a Pt4 cluster on water splitting processes on GaN nonpolar surface. The optimized structure of the Pt4 cluster on the GaN nonpolar surface (see Figure 5a and

adsorption models, the most stable one is the dissociation form with the OH group binding to a Ga atom site (A site) and the H adatom binding to a N atom site (B site) on the nonpolar surface. The adsorption energy in this configuration is less than on a pure nonpolar surface by 0.12 eV (it is 1.78 eV in this work; 2.19 eV in ref 1). The Pt4 cluster does alter the surface environment near it and the relative adsorption energies among different water adsorption configurations. It is observed the adsorption energy increases to 1.76 eV when the OH group and H adatom both are one more unit cell distant away from A and B sites. In contrast, the adsorption energy of one intact water on A site is increased to 1.11 eV, which is twice more than on a pure nonpolar surface (0.49 eV; it is 0.5 eV in ref 1). The energy barrier curves concerning the water splitting processes, H2 evolution, and H adatom hopping on Pt4 cluster are shown in Figure 6. As compared to the results in Figure 3

Figure 6. Energy barrier curves for the first, second hydrogen atom dissociation from a water molecule onto the Pt4 cluster, the H adatom hopping on Pt4, and the H2 generation by combining two H adatoms on the D site of Pt4 cluster.

and ref 1, the barriers for OH group dissociation (the second H atom dissociation of water) and one H2 evolution are much reduced from 3.36 to 1.24 eV and 5.031 to 0.95 eV, respectively (the third and fourth curves in Figure 6). Hence, the Pt4 cluster offers mediate states able to lower barriers for water splitting and H2 evolution processes, although these mediate states may have higher configuration energies as compared to the counterparts on pure nonpolar surface. On the other hand, the dissociation barrier of the first H atom from a water molecule is raised but is still much lower than the two mentioned processes. According to our calculations, the first H atom dissociation onto Pt4 cluster and its hopping on Pt4 cluster are relatively easy processes for only overcoming 0.28 and 0.37 eV energy barriers (the first and second curves in Figure 6). By analyzing the projected density of states (PDOS) of these Pt4 aided water adsorption systems, we find that Pt cocatalyst has proper conduction states for receiving photoexcited electrons, but it also behaves as an electron−hole pair recombination center. The PDOS are shown in Figure 7 and Figure S1 of the Supporting Information including some results of pure nonpolar and polar surfaces as well. Please notice that the DOS scales are changing in the y axis of Figure 7b−d and also Supporting Information Figure S1 to emphasize the small peaks of PDOS from Pt4, O, and H atoms. The correct scale axis for a curve is the one near its label. The Fermi levels are indicated by the boundaries between white and yellow regions in x axis except for the DOS of bulk in Figure 7a, which has its

Figure 5. (a) Top view and (b) side view of the optimized Pt4 cluster adsorption on GaN nonpolar surface. The olive spheres are Pt atoms; the Ga, N atoms are presented in the same convention as Figure 1. In simulation models, the water molecule, OH group, or O adatom could be adsorbed on A site; single H adatom on B or C sites; two H adatoms on D site.

b) is distorted with Pt−Pt bonds varying between 2.52 and 3.74 Å in contrast to the 2.6 Å Pt−Pt bond lengths of an isolated Pt4 cluster. The absorption energy of one Pt4 cluster on GaN nonpolar surface is 5.56 eV. Near the Pt4 cluster, various water adsorbates are manually planted on the A, B, C, and D sites as shown in Figure 5 to study the effect of Pt4 cluster to water splitting and H2 evolution on nonpolar surface. The A site in Figure 5 is a Ga atom site, which can adsorb a water molecule, OH group, or O adatom; the B site and C site are a N atom and a Pt atom site for one H adatom adsorption; the D site is a Pt atom site at the top of Pt4 cluster for two H adatoms adsorption. Among the E



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Figure 7. PDOS of (a) GaN bulk, Pt4 decorated nonpolar surface, and pure polar surface; (b) the dissociated water adsorption with one OH group adsorbed at the A site, one H adatom at B site; (c) one OH group adsorbed at the A site, one H adatom at C site on Pt4 decorated nonpolar surface; and (d) the dissociated water adsorption (1 OH and 1 H adatom) on pure polar surface.

to Pt4 cluster. At the same time, the PDOS of O atom have overlapping at both conduction and valence states of Pt4 between 0 and 3 eV as well; the overlap states become bigger and the Pt4 conduction states go to lower energies when more H atoms are dissociated (Supporting Information Figure S1b). The low Pt4 conduction states are actually at proper positions for collecting photoexcited electrons because they are overlapping the lowest nonpolar surface conduction states. The excited electrons will have a chance to transfer to Pt4 cluster and then to H adatoms for H2 evolution because of the overlapping PDOS of the Pt4 cluster, H adatoms, and GaN nonpolar surface. The similar PDOS features are also observed in the models decorated with only one Pt atom (not shown), although the relative energies between adsorption models are different. On both pure nonpolar and polar surfaces, the conduction states of H adatom do not strongly overlap the GaN conduction band edge (Figure 7b and d) for nonpolar and polar surfaces. Hence, in these two cases, the opportunities for H adatoms gaining excited electron should be low. However, the positions of the highest Pt4 valence states also suggest that Pt4 will collect photoexcited holes as well for its highest valence states just right above 0 eV, which is the GaN bulk valence band edge as shown in Figure 7a. Hence, Pt4 cluster behaves as an electron−hole recombination center on the GaN nonpolar surface. This could be the reason that Pt was not a very efficient cocatalyst among the tested metals and metal oxides on the (Ga1−xZnx)N1−xOx photocatalyst surface in previous experimental studies (Table 1 in ref 6). Additional experimental support comes from the work done by Yoshida et al.71 measuring the attenuated total reflection surface-enhanced infrared absorption spectroscopy of CO molecules adsorbed on

Fermi level at 0 eV. All of the PDOS are calculated with spin unrestricted, but only spin 1 curve is shown if there is no significant difference between the PDOS of two spins. Some Pt4 decorated nonpolar surface systems have metallic surface states coming from Pt4 cluster; however, the same situation is not found for a nonpolar surface decorated with one Pt atom. We think it is because the surface area of our simulation models is not big enough to avoid the wave function of Pt4 cluster coupling between periodic images in calculations. However, we believe the following observations and conclusions should be still valid. In Figure 7a, the Pt4 cluster has its highest valence states higher than 0 eV, which is the original valence band edge of GaN bulk and pure nonpolar surface. Just below 0 eV, the extra surface valence states of nonpolar surface mainly from surface N atoms retain the same features as there is no Pt4 cluster (indicated by the gray region with green strips). With the effect of Pt4, some small portions of surface valence states are overlapping the highest valence states of Pt4 cluster as shown in the gray region above 0 eV. Above the Fermi level, the Pt4 cluster has conduction states resident in the band gap of GaN nonpolar surface and also near the lowest conduction surface states mainly from surface Ga atoms. When there is no H adatom binding to Pt4 cluster (Figure 7b and Supporting Information Figure S1a), the PDOS of water adsorbates are not strongly overlapping the valence and conduction states of Pt4 between 0 and 3 eV. Actually, the PDOS profiles of water adsorbates are very similar to that on pure nonpolar surface. In contrast, additional conduction states from the H adatom will overlap those low conduction states of Pt4 (Figure 7c and Supporting Information Figure S1b) when the H adatom binds F



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Table 1. Hopping Energies of Water Adsorbatesa water adsorbates surface nonpolar

a

H adatom x-direction y-direction

polar

2.05 2.48 0.82−0.96

Pt4 on nonpolar

0.37

O adatom 1.53 1.68 0.88−0.99 for single O adatom, 1.66 for 5O, 1.81 for 6O, 0.59 for 7O

OH group

intact water

1.94 0.31 0.11−0.31

0.36−0.4

The numbers are given in electronvolts.

Figure 7d are actually near the original GaN bulk valence band edge. On the other hand, that H adatom is gaining electrons (∼0.3 e−) from the GaN polar surface instead of losing electrons according to Bader charge analysis.72,73 Similar phenomena are found for PDOS and charge transfer of the 8w_6 case (6 water molecules dissociated when 8 molecules adsorbed) on the polar surface, which is a chemically fullbinding system without extra surface states, and hence the Fermi level is at 0 eV. All of these observations together make us wonder if the H2 evolution mechanism on GaN polar surface is actually an oxidation process like O2 evolution on a polar surface. D. Further Discussion: The Removal of Water Adsorbates on Nonpolar and Polar Surfaces. In general, the hopping processes of water adsorbates are easier on GaN (0001) polar surface than on (1010̅ ) nonpolar surface. The only exception is the OH group hopping on the nonpolar surface along the [010] direction. In Tables 1 and 2, we

Pt particles deposited on GaN photocatalyst. They suggested that some Pt particles behave like electron−hole recombination centers because of the estimated positive potential shift on Pt (Figure 2b in ref 71). In addition, the gradually evolving negative potential shift on Pt may reflect the competitive electron and hole immigrations from GaN to Pt.71 In a theoretical work by Muhich et al., similar PDOS analyses were done by using a more realistic Pt cluster size (Pt37) as a cocatalyst deposited on TiO2 (101) surface.43 They also suggest that Pt is an electron−hole recombination center on TiO2 surface according to their calculated PDOS,43 although they made this conclusion with a different reason based on the metallic Pt PDOS. However, if Pt particles on GaN photocatalyst behave as what we just mentioned, it is easy to conclude that when Pt loading is getting higher, the effective electron−hole recombination centers increase accordingly, and hence the total electron−hole recombination rate will be enhanced. Therefore, we still agree with the inference by Muhich et al. that when Pt loading is too high, the rate of H2 evolution will decrease when electron−hole recombination rate is higher than the rate for holes migrating to photocatalyst surface. Although it may be important to estimate the ability of grabbing electrons and holes of Pt at low and high coverage before making the above suggestion, we believe that the trend mentioned should be true. On the other hand, O2 evolution will not be a favorable process near Pt cocatalyst because both the Pt4 highest valence and lowest conduction states overlap the states of a single O adatom near Pt4 (Supporting Information Figure S1b); even though there are chances for the excited holes transfer to O adatom, the excited electrons also like to hop onto O adatom and recombine with holes. In contrast, the highest valence states of one O adatom adsorbed on pure nonpolar surface are above the valence band edge of the GaN surface (Supporting Information Figure S1c). That means an O adatom has opportunities to absorb excited holes from GaN nonpolar surface. The lowest conduction states of O adatom overlap the lowest conduction bands of nonpolar surface but with higher energy states as compared to the case with Pt4 decorated (Supporting Information Figure S1b). This indicates an O adatom on pure nonpolar surface has higher probability than that on a site near Pt4 to gain excited holes. The situation on a polar surface is more complicated for excited electrons and holes transferring to H and O adatoms because the position of Fermi level will largely change between chemically under-binding and full-binding since the additional surface valence states are ranging between GaN conduction band edge to about 1 eV below that (Figure 7a). We do not discuss the possible electron and hole transfer processes but leave related PDOS in Figure 7d and Supporting Information Figure S1d. One special feature we still want to point out here is the top valence states of the first H adatom on Ga site in

Table 2. Energies of Water Dissociation, H2, and O2 Evolutiona water dissociation

a

gas evolution

surface

first H

second H

H2

O2

nonpolar polar Pt4 on nonpolar

0.04b 0.1c 0.28

3.36 0.73, 0.93 1.24

>5b 1.42,c 1.94c 0.95

1.11, 1.35b >8

The numbers are given in electronvolts. bReference 1. cReference 29.

summarize the hopping energies and water dissociation energies, energies of H2 and O2 evolution for comparison. In Figure S2 of the Supporting Information, the top views of our simulation models mentioned in previous subsections are offered. In our previous work,1 we found that the H2 evolution is nearly forbidden on nonpolar surfaces with an energy barrier more than 5 eV, but it is much easier on polar surfaces with energy barriers of 1.42 and 1.94 eV for a different mechanism.29 We suggested that the H2 evolution mainly takes place on the polar surfaces of GaN nanoparticles. According to our results, the hopping of H adatom on nonpolar needs to overcome barriers more than 2 eV to transfer to the polar surface, which is a possible but slow process around room temperature as compared to the diffusion of bulk water. In contrast, the H adatom can hop on the polar surface much faster because the hopping barrier is more comparable (∼1 eV) with the hydrogen-bond strength (about 0.7 eV/H2O56). In contrast, the H adatom can hop on the polar surface much faster because the hopping barrier is more comparable (∼1 eV) with the hydrogen-bond strength. Hydrogen molecule evolution may take place on a polar surface via the combination of two H G



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adatoms (1.94 eV energy barrier) or interaction between one H adatom and one OH group (1.42 eV energy barrier). On the other hand, the evolution of O2 gas molecules was found to take place much easily on the (1010̅ ) nonpolar surface instead of the polar surface.1 The hopping barriers of an O adatom on the polar surface that is not chemically overbinding are just about 1 eV, similar to H adatom hopping barriers and comparable to hydrogen-bond strength. Hence, an O adatom is very likely to hop from the polar to nonpolar surface and then to combine with another O adatom to form O2. One restriction is O adatoms do not like to stay next to each other on the polar surface; hence O adatoms have to move one by one to transfer to nonpolar surface. O adatoms will not diffuse too deep on nonpolar surfaces because the energy barriers of O2 evolution (1.35 eV in ref 1, 1.1 eV in this work) and the recombination of H and O adatoms (0.54 eV) are lower than the hopping barriers (1.53 and 1.68 eV). Especially the low barrier of recombination of H and O adatoms may hinder O2 evolution. The dissociation of an OH group is much harder (3.36 eV energy barrier) on the nonpolar surface than on the polar surface (0.73−0.93 eV energy barrier). At the same time, it is found to be much harder for an OH group moving along the [001] direction, but OH groups still can migrate to polar surfaces of a GaN nanoparticles via hopping along [010] to other facets. Once the OH groups transfer to the polar surface, they can easily hop and also possibly be dissociated into H and O adatoms. The Pt4 cluster is found be able to offer different mediate states and lower barriers for water splitting and H2 evolution processes. Hence, the deposition of Pt can help the removal of H adatom on nonpolar surface. The features of Pt4 PDOS suggest that it could help the collection of photoexcited electron; however, it is also a recombination center for excited electron−hole pairs. Hence, we would like to propose that a proper cocatalyst should have its low conduction states overlapping the counter parts of photocatalyst and also H adatoms binding on it. Meanwhile, the highest valence states of cocatalyst should be obviously lower than the counter parts of photocatalyst as well to avoid holes collecting.

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*Phone: +886 (2) 2362 4913. E-mail: [email protected]. sinica.edu.tw. *Phone: +886 (2) 2362 4961. E-mail: [email protected]. edu.tw. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by Academia Sinica under Research Program on NanoScience and NanoTechnology, the Ministry of Science and Technology (NSC101-2113-M-001-023-MY3). We want to acknowledge Prof. Ming-Kung Tsai at the Chemistry Department of National Taiwan Normal University for very intuitive suggestions in our simulation works. We would also like to thank the support from the National Center for Theoretical Sciences (South) Physics Division in various academic activities. Our computational resources in part are supported by the National Center for High Performance Computing.

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REFERENCES

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4. CONCLUSIONS In this work, we apply ab initio calculations to study the hopping processes of water adsorbates on (101̅0) nonpolar and (0001) polar surfaces. Among the water adsorbates, the hopping of O adatoms from the polar surface to nonpolar surface for O2 evolution is very likely to take place at room temperature but will be hindered by the recombination with H adatoms on nonpolar surface. On the other hand, the hopping of H adatoms from the nonpolar surface to polar surface is limited due to large barriers around 2 eV. We also find that a Pt4 cluster on nonpolar surface could reduce the barriers of water splitting and H2 evolution processes. At the same time, the PDOS of Pt4 cluster suggests Pt could improve the H2 evolution rate, but it is also an electron−hole recombination center implying that high loading of Pt will eliminate photoexcited electrons and decrease the H2 evolution rate. The PDOS feature of a proper cocatalyst is proposed.

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AUTHOR INFORMATION

Corresponding Authors

ASSOCIATED CONTENT

* Supporting Information S

More PDOS of water adsorbates on other surface models and the arrangements of water adsorbates on these surfaces. This H



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Removal of Water Adsorbates on GaN Surfaces via Hopping

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