DFT Study of Hydrogen Adsorption On the Monoclinic WO3 (001

Article pubs.acs.org/JPCC

DFT Study of Hydrogen Adsorption On the Monoclinic WO3 (001) Surface Fenggong Wang, Cristiana Di Valentin,* and Gianfranco Pacchioni Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, Centro MIB-SOLAR, Via Cozzi 53 20125 Milano Italy ABSTRACT: Hydrogen adsorption on the WO3 (001) surface has been studied by performing DFT calculations with the B3LYP functional and periodic slab models. We show that in the case of WO3 (001) surface the common practice to fix the bottom layers of the oxide slab results in spurious surface states and in the incorrect description of the band structure. The problem can be removed by full relaxation of all of the slab layers, resulting in converged values of the surface energy (0.33 J/m2) and of the band gap (2.75 eV vs computed bulk value of 3.10 eV). When H is adsorbed on the surface, we find that the under-coordinated O or the surface in-plane O1,p,y sites have about the same binding energy, 2.5 eV. We find comparable adsorption energies also for one subsurface in-plane O site (O2,p,y), which suggests an easy migration of the H atom on the surface or into the bulk. In all cases, H binds to the O anions as a proton and donates the valence electron to the 5d level of one or more W ions (formal change of oxidation state from W6+ to W5+). The corresponding occupied W 5d states lie high in the gap and sometimes even merge with the conduction band. The more or less localized nature of the donated electron depends on the site where the proton is bound, suggesting a high mobility of the excess electrons. Dissociative adsorption of H2 is a weakly exothermic process at low H coverage but becomes weakly endothermic at high coverage. The partial modification and occupation of W 5d states, by hydrogen adsorption on the surface or introduction in the lattice of WO3, can explain the experimentally observed change in optical and electric properties of this material when exposed to H2 gas, also known as the “chemichromic” effect because of the analogies with the well-known “electrochromic” effect.

1. INTRODUCTION Tungsten trioxide (WO3), an n-type semiconductor, has been used in several technological applications in the past decades, such as smart windows, dye-sensitized solar cells, sensors, high TC superconductivity, and photoelectrochemical (PEC) water splitting.1−3 As a photocatalytic material, it is characterized by (i) a smaller band gap than other semiconductors (e.g., the extensively studied and more popular TiO2), (ii) a consequent inherent absorption of solar light and (iii) an ability to generate photocurrents.4,5 Furthermore, WO3 is well-known for its photosensitivity, good electron transport properties, and stability against photocorrosion in aqueous solutions.6−10 However, the band gap of WO3 is still too large for an efficient use of the solar spectrum and the bottom of the conduction band (CB) is too low for photocatalytic hydrogen production.11,12 Therefore, extensive efforts from both experimental and theoretical perspectives have been made to understand the electronic properties and to improve the photocatalytic properties of WO3, e.g., by engineering its band gap and band edge positions.13−24 The active sites at the catalyst surface and the underlying reaction mechanisms in photocatalytic processes of WO3 are still far from being understood. Depending on the temperature and pressure, bulk WO3 can exhibit a series of polymorphs, including ε-monoclinic,25 triclinic,26 γ-monoclinic,27 orthorhombic,28 and tetragonal29 structures. At room-temperature (RT), the stable WO3 is the γ-monoclinic structure, which can be viewed as a distorted 2 × 2 × 2 superstructure of the simple cubic ReO3-type unit cell,30 by tilting the WO6 octahedra and displacing the central W atom. In this structure, the bond © 2012 American Chemical Society

lengths are nearly symmetric along the [100] direction, whereas alternating long−short bonds exist along both [010] and [001] directions.31 This antiferroelectric distortion leads to a layering of the crystal along the [001] direction, which facilitates the cleaving of single crystals to produce thin films.32,33 Several experimental studies have been dedicated to the surface structures of WO3 using scanning tunneling microscopy (STM), low energy electron diffraction (LEED), etc.34−43 Both (100) and (001) WO 3 thin films have been experimentally prepared and several kinds of surface reconstructions were found depending on the growth conditions. The (001) surface of γ-monoclinic WO3 has the lowest energy,40 and thus is the cleavage plane, which, in principle, can be terminated either by the WO2 plane or by the O plane. However, the WO3 thin films tend to lose oxygen in the surface in order to cancel the electrostatic dipole. For example, (√2 × √2)R45° reconstruction of the (001) surface can be accomplished by removing half of a monolayer of the O atoms alternatively along the [100] and [010] directions, as seen in Figure 1(a), corresponding to a periodicity √2 times that of the underlying ReO3-like simple cubic framework. On the theory side, several studies on the WO3 surfaces have been reported. Oliver et al. calculated the surface energies of the (110), (111), and five reconstructions of the (001) surface of the idealized simple cubic WO3,44 and found that the experimentally observed (√2 × √2)R45° reconstruction of Received: March 7, 2012 Revised: April 18, 2012 Published: April 18, 2012 10672

dx.doi.org/10.1021/jp302210y | J. Phys. Chem. C 2012, 116, 10672−10679

The Journal of Physical Chemistry C

Article

readily adsorbs at the monocoordinated O site and that dissociative adsorption of H2 occurs at this O site. Hydrogen molecule adsorption on WO3 can lead to a metallic character due to hydroxyl formation and dehydration.54 Despite the considerable amount of work done so far, there are still important aspects of the electronic and structural properties of the WO3 surface and of hydrogen adsorption which are unclear, especially for the RT γ-monoclinic WO3 phase. Furthermore, the theoretical work done so far is based on standard DFT methods which seriously underestimate the semiconductor band gap. This problem can be improved by using DFT in combination with hybrid functionals which provide a satisfactory option for the description of both the electronic and structural properties of WO3.13,14 It should also be mentioned that in most of previous studies, the (001) surface of WO3 is modeled by a slab where the bottom layers are fixed; we will show below that this leads to some unphysical and artificial gap states which may affect to some extent the description of the surface reactivity.

2. COMPUTATIONAL DETAILS The calculations were carried out within the linear combination of atomic orbitals (LCAO) approach with periodic boundary conditions combined with the B3LYP55,56 hybrid functional, as implemented in CRYSTAL09 code.57,58 The all-electron Gaussian-type basis set 8−411(d1) was adopted for oxygen,59 while for tungsten we used an effective core pseudopotential (ECP) combined with a modified Hay-Wadt double-ζ basis set.13,60,61 The optimized lattice parameters of RT γ-monoclinic WO3 (space group P21/n) were obtained from our previous work.13 The (001) surface was modeled by a periodic slab with several trilayers cleaved from the optimized bulk structure; each trilayer (from now on simply “layer”) consists of three atomic monolayers, namely OWO2−O. For example, a schematic side view of the slab model including six layers directly cleaved from the bulk is shown in Figure 1. For all of the slab calculations, the lattice constants were fixed and only the atomic relaxations were allowed. We performed two sets of calculations: (1) all the atoms were free to relax: “free” slab model; (2) one or two bottom layer(s) were fixed at the bulk position while all the others are free: “fixed” slab model. For the adsorption of hydrogen on the surface, also spin polarized calculations were performed. At least 20 k points were sampled for the reciprocal irreducible Brillouin zone (BZ). The equilibrium structure was determined by using a quasi-Newton algorithm with a BFGS Hessian updating scheme.62 The thresholds for the maximum and the root-mean-square (rms) forces were set to 0.00045 and 0.00030 au, and those for the maximum and the rms atomic displacements to 0.00180 and 0.00120 au, respectively. For convenience, we will use the following symbols: W1,6C and W1,5C to denote the surface 6- and 5-coordinated W atoms, Wil,6C, Wir,6C, Oi,t, Oi,p, Oi,p,x, and Oi,p,y to represent the W atoms along the left and right [001] chains, the top and bottom (outof-plane) O atoms, the in-plane O atoms along [100] (x) and [010] (y) directions, of the ith layer from the top to the bottom (see Figure 1), respectively. For example, W1,6C denotes the 6fold W atom of the first (top) layer while W2r,6C denotes second layer W atom along the right [001] chain.

Figure 1. Schematic atomic structures of (√2 × √2)R45° reconstruction of the (001) surface of γ-monoclinic WO3. (a) Top view; (b) Side view of one layer; (c) and (d) Side views of the slab consisting of six layers before and after optimization. The small spheres represent the O atoms and the large spheres represent the W atoms, respectively. The green arrows in (d) indicate the left row moves upward while the right row downward after optimization.

the (001) surface has the lowest surface energy and thereby is the most stable one. Levy et al. studied the O atom adsorption on γ-monoclinic WO3 slab,45 showing that O is adsorbed in a neutral state on the perfect oxide. Gutowski et al. investigated the driving force for the relaxation of ε-monoclinic WO3 (001) surface with various types of termination.46 It was found that the surface relaxation is accompanied by a dramatic redistribution of the density of states (DOS) near the Fermi level which is suggested to be the driving force of the surface relaxation. More recently, these authors studied the reactivity of hydrogen and methanol on the (001) surfaces of ε-monoclinic WO3 and of the mixed ReO3/WO3 system.47 Valdés and Kroes investigated the photo-oxidation of water on the (200), (020), and (002) surfaces of γ-monoclinic WO3 by using DFT calculations.48 Jin et al. have performed DFT calculations on the structural and electronic properties of the (001) surface of γ-monoclinic WO3.49 The oxygen defects on the WO3 surface were also studied by ab initio methods, showing that O vacancy is the predominant defect.50 Hydrogen interaction with WO3 plays a pivotal role in several phenomena and applications. For example, WO3 layers coated with Pt catalyst may change the optical and electrical properties when exposed to H2, making it suitable for chemical sensor applications.51,52 Due to the analogy with the electrochromic effect, the hydrogen effect is also called “chemichromic” effect.3 Nonstoichiometric hydrogen tungsten bronzes HxWO3 may form with hydrogen incorporation into the WO3 structure.53 Hydrogen adsorption on WO3 surface has also been investigated theoretically,47 showing that atomic hydrogen 10673



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3. RESULTS AND DISCUSSION 3.1. Description of the WO3 (001) Surface Using “Fixed” Slab Models. As mentioned above, the WO3 (001) surface can be terminated either by O or by a WO2 layer, forming a (O−WO2)n slab. This slab, however, has a perpendicular dipole and thus is energetically unstable causing the surface reconstruction. The reconstruction can occur, for instance, by transferring half of the oxygen atoms alternatively along [100] and [010] directions from the top to the bottom layer, forming a nonpolar [O−(WO2)2−O]n slab. This corresponds to the (√2 × √2)R45° reconstruction of the ReO3-type cubic framework, which has been observed experimentally and indicated theoretically to be the most stable surface.40,44,49 We will investigate only the most stable (√2 × √2)R45° reconstruction of the (001) surface of γmonoclinic WO3 [Figure 1(a)]. Note that the γ-monoclinic unit cell corresponds to a 2 × 2 × 2 cubic unit cell.13 Compared to simple cubic WO3, an alternating long−short O−O distance along the [110] direction appears because of the distortion of monoclinic WO3. In slab calculations, it is common to fix the bottom layer at the truncated bulk positions and to allow the other layers to relax. We have followed this procedure to investigate the WO3 (001) surface by using the slab models with fixed bottom layer(s). In this case, the surface energy (Esurf) is defined as follows: Esurf =

⎤ 1 ⎡ relax,up 1 unrelaxed Eslab − (Eslab + nE bulk )⎥ ⎢ ⎣ ⎦ A 2

The total DOS of WO3 (001) slab shows that some occupied states appear in the gap region, Figure 2(a). A very small energy

Figure 2. The total (a) and projected (b, c, and d) DOS (TDOS and PDOS) of WO3 (001) slab consisting of six layers with the bottom layer fixed. Parts (b), (c), and (d) correspond to the PDOS on the first (top), third, and sixth (bottom) layer, respectively.

separates the highest occupied level (HOMO) from the lowest unoccupied state (LUMO), 0.21 eV, see also Table 1. The origin of these gap states becomes evident by plotting the DOS for each layer of the slab, Figure 2. The gap states originate mainly from the atoms of the fixed bottom layer, Figure 2(d). In fact, the band gap of the top layer is about 2.83 eV (Table 1), which is a bit smaller than that of bulk WO3. If we consider the two top layers (fully relaxed), then the band gap becomes 2.63 eV. Next, we projected the DOS of the bottom layer on the O atoms. As shown in Figure 3, the top of the valence band (VB)

(1)

unrelaxed Erelax,up , slab ,Eslab

where and Ebulk are the total energies of the surface with only the upper layers free to relax, the unrelaxed surface, and the optimized bulk unit, A is the surface area, and n stands for the number of WO3 units in the surface. In Table 1, we report results for slab models consisting of 3, 4, 5, 6, and 8 layers with only the bottom layer kept fixed at the Table 1. Surface Energy, HOMO−LUMO Energy Gap, Gap of the Top Layer and of the Top Two Layers of Various WO3 (001) Slabs with the Bottom Layer Fixed slab

surface energy (J/m2)

HOMO−LUMO Gap (eV)

top layer Gap(eV)

top two layers Gap (eV)

3-layers 4-layers 5-layers 6-layers 8-layers

0.96 0.81 (1.33)a 0.73 0.69 0.65

0.61 0.37 0.38 0.21 0.27

2.54 2.51 2.83 2.83 2.83

2.52 2.43 2.42 2.63 2.81

a

The surface energy is 1.33 J/m2 for the 4-layers slab with two bottom layers fixed.

bulk positions; in the case of 4 layers we also considered a model where the two bottom layers were fixed but, not surprisingly, the surface energy is the largest (1.33 J/m2), indicating that this model is not accurate for calculation of the surface energy. When only the bottom layer was fixed, the surface energy decreases gradually from 0.96 to 0.65 J/m2 as the thickness of the slab increases (Table 1). However, even with 8 layers values are not at convergence. In order to better understand the reasons for the slow convergence of the surface energy with the number of layers in the slab, we have analyzed the electronic structure of the WO3 (001) surface. We restrict the analysis to the 6-layers model.

Figure 3. The PDOS of the fixed bottom layer in the 6-layers slab. (a) On the top O atoms; (b) on the in-plane O atoms along [100] (x) direction; (c) on the in-plane O atoms along [010] (y) direction; (d) on the bottom O atoms.

is mainly due to the pπ orbitals of the top and in-plane oxygen atoms. There is an artificial position of the energy levels associated with these O atoms due to their tendency to relax. The gap states are mainly due to the O 2p orbitals of the two bottom O atoms. Clearly, the absence of relaxation results in 10674



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where and Ebulk represent all layers free to relax and the optimized bulk unit, respectively; A stands for the surface area, and n stands for the number of WO3 units in the surface. Table 2 shows the calculated surface energy and band gap values for the (001) slabs with different thickness. One can

bulk system,13 and no gap states are present. The top of the VB is mainly composed of O 2p orbitals, hybridized with the W 5d states, while the bottom of the CB arises mainly from the W 5d orbitals with some contribution from O. The band gap reduction can be attributed to the surface states locating at the region near the band edges due to the structural and charge redistribution near the surface. Table 3 shows the atomic displacements along the [001] direction for the relaxed 6-layers slab with respect to the corresponding truncated bulk positions. The top O atoms (O1,t) move slightly upward; also the 6-fold W atoms of the top layer (W1,6C) move significantly upward, toward the top O atoms (O1,t), so that their distance decreases from about 2.224 to about 1.689 Å (6-layers, Table 4). The O atoms (O2,t) below

Table 2. Surface Energy and Band Gap of Various WO3 (001) Slab with All the Layers Free to Relax

Table 4. Bond Lengths along The [001] Chains in the 6Layers (001) Slab before and after Optimization

spurious effects that can severely affect the electronic structure of the slab. 3.2. Description of the WO3 (001) Surface Using “Free” Slab Models. Here we consider the electronic properties of the WO3 (001) surface by allowing all the layers of the slab to relax. The surface energy is defined as follows: Esurf =

1 relax (Eslab − nE bulk ) 2A

(2)

Erelax slab

layers number

4

6

8

10

surface energy(J/m2) gap (eV)

0.35 2.77

0.33 2.77

0.33 2.75

0.33 2.71

distance (left) O1,t−W1,6C W1,6C−O2,t O2,t−W2 l,6C W2 l,6C−O3,t O3,t−W3 L,6C W3 L,6C−O4,t

clearly see that the surface energy is almost identical when the thickness of the slab increases from four to ten layers. The surface energy of the 6-layers slab is 0.33 J/m2, a value similar to that of the relaxed cubic ReO3-like, c(2 × 2)O terminated (001) surface (0.36 J/m2),46 and slightly smaller than the previously reported value of the γ-monoclinic WO3 (001) surface (0.448 J/m2).49 Also for the band gap the results are much more realistic. The gap is about 2.77 eV for a slab consisting of six layers, a value smaller than the bulk one (3.10 eV).15 Thus, both the surface energy and the fundamental band gap values show a good convergence as a function of the slab thickness. The DOS of the 6-layers slab is shown in Figure 4. The overall VB and CB characteristics are similar to those of the

unrelaxed relaxed (Å) (Å) 2.224 1.752 2.222 1.753 2.224 1.753

1.689 2.367 1.742 2.266 1.746 2.240

distance (right)

unrelaxed (Å)

relaxed (Å)

W1,5C−O1,b O1,b−W2r,6C W2r,6C−O2,b O2,b−W3r,6C W3r,6C−O3,b

2.224 1.753 2.222 1.752 2.224

1.730 2.248 1.740 2.235 1.745

the surface 6-fold W atoms move only slightly. Thus, the distance between the W atoms and the underneath O atoms (W1,6C−O2,t] significantly increases. At the same time, the 5coordinated W atoms of the top layer (W1,5C) move downward by as much as 0.488 Å, while the bottom O atoms of the top layer (O1,b) remain almost at the original position. As a result, the (W1,5C−O1,b] distance decreases from 2.224 to 1.730 Å. The atoms in the other layers including the W atoms at the central layer (W3 l,6C and W3r,6C) have similar displacements. Therefore, as seen in Figure 1(c),(d), all of the W atoms at the left side move upward while those at the right side move significantly downward. Correspondingly, the alternating long− short bond lengths change their order: the initially long bond becomes the short one and vice versa (Table 4 and Figure 1). We also analyzed the relaxed atomic structures as a function of the slab thickness, including atomic displacements, bond lengths and angles (Table 3). All of the aspects of the structure are virtually the same for 6- and 10-layers slabs. Therefore, we conclude that the 6-layers slab is sufficiently thick to describe both structural and electronic properties of the γ-monoclinic WO3 (001) surface. 3.3. Atomic Hydrogen Adsorption on the WO3 (001) Surface. Hydrogen adsorption was investigated by placing one H atom at different sites of the (1 × 1) slab unit cell. The

Figure 4. The total and partial DOS of the 6-layers WO3 (001) fully relaxed slab.

Table 3. Atomic Displacements with Respect to Truncated Bulk Positions, Bond Lengths and Angles of WO3 (001) Slabs Including Six and Ten Layersa atomic displacements (Å) O1,t W1,6C W1,5C Wml,6C Wmr,6C a

0.173 (0.255) 0.698 (0.778) −0.488 (−0.426) 0.518 (0.513) −0.506 (−0.506)

bond lengths (Å) O1,t−W1,6C W1,6C−O2,t W1,5C−O1,b

1.689 (1.689) 2.367 (2.389) 1.730 (1.731)

angles (deg) W1,6C−O1,p,y−W1,5C W1,6C−O1,p,x−W1,5C Wm,6C−Om,p,y−Wm,6C Wm,6C−Om,p,x−Wm,6C

177.2 178.8 172.1 168.9

(176.4) (178.6) (171.1) (169.3)

The “m” stands for the middle layer of the slab. The numbers in the parentheses are for 10-layers. 10675



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binding energy Eb for H bound to the slab is calculated as follows: E b = (ESlab + E H) − E H/Slab

Table 6. Binding Energy, Spin Density and Band Gap for Different Configurations with H Adsorbed on WO3 (001) Slab and on γ-Monoclinic Bulk WO3

(3)

where ESlab and EH/Slab are the total energies of the slab without and with H adsorbed on it, respectively, and EH is the total energy of atomic H (or of the H2 molecule when two H atoms are adsorbed). Several configurations have been considered with the H atom bound to: 1-coordinated top O (H−Ot), 2coordinated in-plane surface O (H−O1,p,x and H−O1,p,y), subsurface O (H−O2,p,y and H−O2,t). The existence of different O sites is related to the different O−W bond lengths along different directions (Table 5). In particular, O1,p,x and O1,p,y Table 5. Optimized O−W and O−H Bond Lengths for Different Configurationsa bonds

H−Ot

H−O1,p,x

H−O1,p,y

H−O2,p,y

H−O2,t

O−W1 O−W2 O−H

1.896

2.123 2.054 0.975

2.140 2.110 0.971

2.079 2.130 0.980

2.000 2.428 0.973

0.970

configuration

binding energy (eV)

spin density (e)

gap (eV)

H−Ot H−O1,p,x H−O1,p,y H−O2,p,y H−O2t H−Oz (Bulk) H−Ox (Bulk) Hc (Bulk) 2H−Ot H−Ot + H−Ot H−Ot + H−Op

2.55 2.22 2.53 2.56 2.10 2.17 1.90 −0.23 −0.32 −0.10 −0.27

0.82 0.52; 0.25; 0.20 0.67; 0.19; 0.13 0.58; 0.28; 0.12 0.77 0.66; 0.18 0.34; 0.21; 0.15; 0.12 0.95 1.26; 0.46 0.85; 0.85 1.32; 0.33; 0.17

2.76 2.74 2.75 2.72 2.58 3.07 2.99 3.09 2.71 2.76 2.74

these results indicate that the dipole has no significant effect, in the rest of the work we will only consider H atoms adsorbed only on one side of the slab. A partly different situation is found when the H atom is adsorbed on the surface in-plane O atom along the [100] direction (H−O1,p,x). In fact, in this case the gap state is more delocalized and merges with the bottom of the CB, leading to a metallic character. Correspondingly, the spin density resides on three W atoms (0.52, 0.25, and 0.20 e), two directly bound to the O1,p,x atom. The binding of H to this site is slightly smaller than that in the previous case, 2.22 eV (Table 6). The O1,p,x atom moves slightly away from the original W−O−W chain, resulting in longer O−W bond lengths. For the H−O1,p,y configuration, the spin density on the three W atoms is 0.67, 0.19, 0.13 e, respectively, i.e., slightly more localized than the H−O1,p,x configuration. The corresponding state is in contact with the bottom of the CB. On the O1,p,y site H binds by 2.53 eV, i.e., practically the same binding energy calculated for the surface Ot site. A similar binding energy is also computed for H interacting with the subsurface O2,p,y site, 2.56 eV, while the second subsurface site is less favorable (O2,t, 2.10 eV, Table 6). Electron transfer from H to W also takes place also when the H atom is bound to the subsurface O atoms (H−O2,p,y and H− O2,t). Overall, the resulting gap state is more delocalized and merges with the bottom of the CB. The corresponding bond lengths and spin densities are listed in Tables 5 and 6, respectively. We wish to stress that, once more, these results are essentially independent of the slab thickness. In fact, we considered H adsorption on the surface Ot site and on the inplane O1,p,y site using a fully relaxed 4-layers slab: all of the features described above are essentially confirmed by this calculation, indicating that the results are converged with respect to the number of layers in the model of the WO3 [001] surface. The similar binding energies found for H interacting with surface or subsurface sites suggests a relatively easy diffusion of the proton on the surface and from the surface to the bulk of WO3. Of course, a precise answer to this point requires the calculation of the diffusion barriers. However, despite our extensive search of the transition state for the diffusion process, we could not succeed in localizing it. 3.4. Atomic Hydrogen Adsorption in Bulk WO3. We have also considered H incorporation into the bulk of γmonoclinic and simple cubic WO3 structures in order to check if there are substantial differences with the surface. For the γ-

a

W1 and W2 are the two W atoms bound to the O atom on which H is adsorbed. Distance is in Å.

refer to the surface in-plane O atoms along the [100] and [010] directions, respectively; O2,p,y is the subsurface in-plane O atom along the [010] direction; O2,t is the subsurface top O atom along [001] direction. When H is adsorbed on the Ot site (H−Ot), one electron is transferred from the H atom to the slab, occupying the initially empty dxz orbital of the W atom neighbor to Ot, with formation of one proton H+ and a W5+ (5d1) ion, as testified by the band structure and spin density plots (Figure 5). The spin density is

Figure 5. The band structure of WO3 (001) slab with one H atom adsorbed on the Ot site. The inset is the spin density distribution.

mainly localized on the W atom (0.81 e). Notice that this localization cannot be described by standard DFT methods due to the well-known self-interaction error and consequent excessive delocalization.63,64 The binding energy is substantial, 2.55 eV (Table 6). Using a PBE functional, Ling et al. have calculated, for the same adsorption site, a binding energy of 2.78 eV.47 The calculated Kohn−Sham band gap at Γ is 2.76 eV, similar to that of the bare slab. However, the presence of W5+ results in an occupied gap state 0.44 eV below the bottom of the CB. Due to the attraction of the proton, the Ot atom moves upward, leading to a longer Ot−W6C bond length. In order to check whether the presence of a dipole normal to the surface affects the results, we also investigated the configuration with two H atoms adsorbed on both sides of the slab, respectively. The two resulting unpaired electrons are localized on the W atom neighbor to the Ot site, and each of them carries about 0.81 e in an orbital with dominant dxz character. Since 10676



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6), when two H atoms are adsorbed on two adjacent Ot sites the value reduces to 2.37 eV. Since the dissociation energy of the H2 molecule in the present approach is 4.84 eV, the coverage effect is sufficient to change the reaction energy from exothermic (low H coverage) to endothermic (high H coverage). The fourth reason for the different results lies in the different functional used. In fact, at the PBE level the energy released by adsorbing on H atom on a single Ot site is 2.78 eV while it is only 2.55 eV at the B3LYP level. In summary, the results indicate that the process of dissociative H2 adsorption is slightly exothermic, coverage dependent, and that the exact value of the energy released depends on the DFT functional used. When two H atoms are adsorbed on the same Ot site, two electrons are transferred from H to W. They are mainly localized on two W atoms (1.26 and 0.46 e), leading to two spin-up occupied states (triplet) appearing in the band gap (2.71 eV). The Ot-W bond length is 2.269 Å, which is stretched with respect to the case without H adsorption (1.704 Å). This configuration can also be viewed as a water molecule adsorbed at the W5C site, as justified by the long Ot−W distance. This means that hydrogenation of the surface can result in an easy loss of water. Also for the adsorption case (b), two electrons are transferred from the two H to the surface leading to two gap states and a triplet ground state. The Ot−W bond lengths are identical (1.902 Å), showing a symmetric charge distribution. In fact, also the spin density is localized on the two W atoms neighboring the Ot site and each of them has 0.85 e. For configuration (c), the two gap states are more delocalized, merging with the bottom of the CB. The spin density resides on three W atoms (1.32, 0.33, and 0.17 e, respectively).

monoclinic structure, we used a supercell containing 64 atoms, corresponding to a hydrogen concentration of 6.25%. Three configurations were considered, (1) H bound to a [001] chain O (H−Oz); (2) H bound to a [100] chain O (H−Ox); (3) H inserted in the center of a W8 cube (Hc). Among the investigated structures, the H−Oz configuration is the most stable one (2.17 eV), followed by the H−Ox configuration (1.90 eV). The interstitial Hc configuration is unstable with a negative binding energy (−0.23 eV). This result is consistent with a previous report on hydrogen tungsten bronze that H is unstable at the center of the cavity,65 suggesting that the proton tends to bind to the O atoms, forming hydroxyl groups, because of the high mobility of the intercalated proton.66 For the H−Oz configuration, one electron is transferred from H to two W atoms neighbor to the Oz atom; the spin distribution is asymmetric, 0.66 and 0.18 e, reflecting the asymmetric W−Oz bond lengths (2.234 and 2.044 Å). The direct band gap is 3.07 eV, i.e., is unchanged with respect to pure WO3. However, an impurity state associated to the W5+ ions appears in the gap at about 0.5 eV below the bottom of the CB. For the H−Ox configuration, the gap state is more delocalized, merging with the CB and leading to a metallic conductivity, in line with the more dispersive spin density on four W atoms (0.34, 0.21, 0.15, 0.12 e). On the contrary, no charge transfer takes place in case of the Hc configuration, explaining the highly unfavorable total energy. For the simple cubic structure we considered two configurations, (1) H bound to O (H−O), (2) H interstitial at the center of a cavity (Hc). Not surprisingly, the Hc configuration is 3.21 eV higher in energy and corresponds to a neutral H atom incorporated into the structure. As such, it will not be further discussed. On the contrary, for the H−O configuration (hydroxyl group) there is a net charge transfer and the corresponding gap state becomes less localized. The spin density is equally distributed on the two neighboring W atoms, 0.46 and 0.44 e, respectively. 3.5. Dissociative Adsorption of H2 on the WO3 (001) Surface. We have also studied a pair of H atoms resulting from the dissociative adsorption of one H2 molecule on the surface. Three different configurations have been considered: (a) in the first case the two H atoms are bound to the same Ot center (2H−Ot); (b) in a second configuration, the two atoms bind to two different 1-coordinated Ot atoms (H−Ot + H−Ot); (c) the last case is with one H atom bound at Ot and the other one at an Op site (H−Ot + H−Op). We find that the H−Ot + H−Ot configuration has the lowest energy, followed by the H−Ot + H−Op configuration (0.17 eV higher in energy) and the 2H− Ot configuration (0.22 eV higher in energy). However, overall H2 dissociation is, according to the B3LYP calculations, slightly endothermic (negative binding energies, Table 6). In particular, the energy of WO3 + H2 is 0.1 eV lower than that of the two adsorbed H atoms on different Ot sites. This is different from what found in ref 47 based on plane waves PBE calculations. In that work, in fact, an energy gain of 0.49 eV is reported for H adsorption on the two different Ot sites. The results are qualitatively different as the previous study finds a favorable dissociative adsorption while here we do not. There are four reasons for this discrepancy. First of all, calculations were done on the (001) surface of a slightly different WO3 polymorph (εmonoclinic). Second, they used a “fixed” slab model. Third, when two H atoms are adsorbed on vicinal sites, there is a coverage effect that reduces the interaction energy. In fact, while the binding of a single H atom on Ot is 2.55 eV (Table

4. CONCLUSIONS We have performed periodic DFT calculations of the hydrogen adsorption on the monoclinic WO3 (001) surface using the B3LYP hybrid functional. Some care is required in representing the WO3 surface by using slab models. In fact, the standard procedure to fix the atoms in the bottom layer(s) of the slab to their positions in the truncated bulk introduces artificial and spurious states in the gap of the material which may result in an incorrect description of the adsorption properties. The problem can be overcome by relaxing all the atoms in the slab model. In this case, both the band gap, 2.75 eV, and the surface energy, 0.33 J/m2, are well converged with respect to the number of layers in the slab. Both atomic and molecular hydrogen adsorptions have been considered. Atomic H adsorbs preferentially on the undercoordinated Ot site of the surface, consistent with previous findings.47 However, adsorption on the in-plane O1,p,y site or on subsurface in-plane O2,p,y sites is isoenergetic, suggesting an easy migration of the H atom on the surface and from the surface into bulk. In terms of electronic structure, H adsorption always results in the formation of a proton bound to an O ion of the WO3 lattice and an electron which is transferred to the empty W 5d states. Depending on the position of the proton and of the W atom, different degrees of localization are found. When H is adsorbed on the Ot site, the unpaired electron is localized on the adjacent W atom. A singly occupied state forms in the gap about 0.4 eV below the CB. When H is adsorbed on a surface in-plane O atom or binds to a subsurface O atom, with similar energy as before, the electron transfer involves more than a single W atom. In this case, the singly occupied state lies very 10677



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high in the gap and can even merge with the CB. Considering the expected high mobility of the protons inside the WO3 structure and the easy hopping of the transferred electron from site to site, one can conclude that H doping of WO3 will, most likely, result in a metallization of the system. Finally, we find that the dissociative adsorption of molecular H2 is almost a thermoneutral process. The exact energetic balance depends on (a) the sites where H is bound, (b) the level of hydrogen coverage and (c) the type of exchange correlation functional used to compute the electronic structure. H2 dissociation on two different Ot sites (H−Ot + H−Ot) is the most stable configuration, but while at low coverage the reaction is slightly exothermic, at high coverage it becomes slightly endothermic. As a concluding remark, we note that the present hybrid functional study shows that the hydrogen adsorption on the surface or insertion in the subsurface layers and in the bulk of WO3 causes a modification of the electronic structure of the material, introducing occupied defect states near the bottom of the CB. This is probably the origin of the observed change in the optical and electrical properties of the material in the presence of hydrogen (e.g., from transparent to colored), also known as the “chemichromic” effect for its similarities with the “electrochromic effect”.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the CARIPLO Foundation through an Advanced Materials Grant 2009, by the Italian MIUR through the FIRB Project RBAP115AYN “Oxides at the nanoscale: multifunctionality and applications” and the PRIN Project “New generation photosensitive semiconducting oxides modified with non metals to enhance solar light harvesting. Design, synthesis, characterisation and testing”, by Regione Lombardia and CILEA Consortium, through a LISA Initiative (Laboratory for Interdisciplinary Advanced Simulation).

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NOTE ADDED AFTER ASAP PUBLICATION This article was published ASAP on May 2, 2012 with the incorrect Table of Contents and Abstract graphic. The correct version was published on May 17, 2012.

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DFT Study of Hydrogen Adsorption On the Monoclinic WO3 (001

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