H2O Adsorption on WO3 and WO3âx (001) Surfaces - ACS Applied

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H2O Adsorption on WO3 and WO3−x (001) Surfaces Elisa Albanese,* Cristiana Di Valentin, and Gianfranco Pacchioni Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, via R. Cozzi 55, 20125 Milano, Italy S Supporting Information *

ABSTRACT: The nature of the interaction of water with the WO3 surface is of crucial importance for the use of this semiconductor oxide in photocatalysis. In this work, we investigate water adsorption and dissociation on both clean and O-deficient (001) WO3 surfaces by means of an accurate DFT approach. The O vacancy formation energy (computed with respect to O2) has been evaluated for all possible surface configurations, and the removal of the terminal O atom along the c axis is found to be preferred, costing about half the corresponding energy in the bulk. The presence of oxygen vacancies leads to a semiconductor to metal transition, confirming the experimental evidence of n-type conductivity in defective WO3 films. H2O preferably adsorbs on WO3 in a molecular undissociated form, due to the presence of W ions at the surface that act as Lewis acid sites. This interaction, about −1 eV per H2O molecule, is not very strong. Contrary to what is usually expected, the presence of oxygen vacancies does not significantly affect H2O adsorption. Finally, we investigated the H2O desorption from a hydroxylated surface. This suggests that the exposure of WO3 to H2 directly results in a hydroxylated surface and the corresponding H2O desorption turns out to be a very efficient mechanism to generate a reduced oxide surface, with important consequences on the electronic structure of this oxide. KEYWORDS: photocatalysis, oxygen vacancy, tungsten oxide, surface, water adsorption

1. INTRODUCTION Photosynthetic water splitting performed by semiconductor photoelectrodes, which are able to efficiently absorb visible light and generate electron−hole pairs to produce H2 and O2 by redox reactions, represents one of the most important goals in solar energy conversion into fuels. The principal issue on the realization of efficient solar-driven-energy conversion is related to the design of suitable semiconductor photoanodes, which could overcome the complex water oxidation process (or oxygen evolution reaction, OER) promoted by photongenerated holes.1,2 Tungsten trioxide is a semiconductor oxide, which has attracted a lot of interest in the past as photoanode and catalytic material.3−7 This is due to its earth abundance and stability against acids and photocorrosion. WO3 also shows good electron transport properties and an appropriate optical gap of 2.6−2.8 eV (0.4 eV smaller than that of TiO2). Furthermore, the position of its valence band maximum (VBM) at about −3.1 V relative to NHE (Normal Hydrogen Electrode, i.e., 7.44−7.54 eV below vacuum) is well below the oxidation potential of O2/H2O (1.23 V vs NHE).1 An accurate knowledge of water−WO3 interaction is of crucial importance for improving OER and therefore for the use of this material as photoanode. So far, however, very few studies on this topic have been reported in literature.8,9 Moreover, all the previous theoretical works are based on standard DFT methods (pure GGA functional), which are not able to correctly reproduce the electronic properties of the material, especially the band gap, because of the self-interaction error inherent to the standard DFT formulation.10−14 For this © XXXX American Chemical Society

reason, we focused our attention on this important topic by adopting a hybrid functional (B3LYP) for all the calculations. It has been indeed proven that the correct description of Odeficient metal oxide systems can be attained only using selfinteraction corrected functionals.10,13,15 WO3 is a reducible oxide and its surface can present high concentrations of oxygen vacancies (VO). Recent works15,16 on oxygen deficient bulk WO3 have addressed the problem of defect states, excess charge, and charge transition levels providing new insights into the electronic and optical properties of this important material. The nature of oxygen vacancies in bulk MoO3, which possess a structure very similar to that of WO3, has also been studied, showing some similarities to WO3.17,18 However, no studies seem to exist in the literature about the influence of VO on the electronic properties of the surface of WO3 and on the interaction of this such defective surface with H2O molecules. In this work, we consider the interaction of water with the surface of WO3 and the role of oxygen vacancies. A deeper understanding of how substoichiometric WO3 interacts with water is clearly relevant as it has been demonstrated that WO3−x formed by creating oxygen vacancies is thermodynamically stable at room temperature and can protect WO3 from photocorrosion, being resistive to the peroxo-species induced dissolution.19 Received: May 2, 2017 Accepted: June 16, 2017 Published: June 16, 2017 A

DOI: 10.1021/acsami.7b06139 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces The paper, divided into two parts, first addresses the anisotropic effect of different types of oxygen vacancies (VO) on the structural and electronic properties of the γ-monoclinic WO3 (001) surface; in the second part, we consider the interaction of H2O molecules with stoichiometric and defective WO3 surfaces. Finally, we also discuss the H2O desorption from a hydroxylated surface. We considered the H2 adsorption to investigate the H2O desorption and then the possible formation of an under-stoichiometric WO3−x surface.

2. COMPUTATIONAL DETAILS We performed periodic DFT calculations employing the B3LYP20,21 exchange and correlation functional as implemented in the CRYSTAL14 program.22 Hybrid functionals are well-known to provide a robust and appropriate description of spin-polarized systems; in particular, being able to correctly describe the localized excess charge generated by the presence of oxygen vacancies.10,16,23 Various forms of hybrid functional have been proposed in the literature. Here we use the B3LYP approach in order to be able to compare the present results with those obtained by us in previous studies on the same (WO3) and on other (TiO2, ZnO, ZrO2, SnO2, etc.) semiconducting oxides.14,24,25 Crystalline orbitals are represented as linear combinations of Bloch functions (BF) and are evaluated over a regular three-dimensions mesh of points in reciprocal space. Each BF is built from local atomic orbitals (AO) resulting from contractions (i.e., linear combinations with constant coefficients) of Gaussian-type-functions which in turn are the product of a Gaussian times a real solid spherical harmonic function. All electron basis sets have been used for O and H atoms: 8− 411(d1) and 511(d1), respectively. For W the Hay and Wadt smallcore effective core potential (ECP) and a modified double-ζ basis set were used. The computed Kohn−Sham band gap (Eg) for the bulk structure is 3.0 eV, close to the experimental values obtained by means of direct and inverse photoemission (3.38−3.39 eV)26,27 and the optical band gap (2.6−3.2 eV).28−31 The Eg value obtained for the slab model is slightly smaller than the bulk one, 2.77 eV.14 For the numerical integration of exchange-correlation term, 75 radial points and 974 angular points (XLGRID) in a Lebedev scheme in the region of chemical interest were adopted. The Pack− Monkhorst/Gilat shrinking factors for the reciprocal space were set to 6 for all the structures, corresponding respectively to 20 real reciprocal space points at which the Hamiltonian matrix was diagonalized. Calculations of stoichiometric (001) WO3 surface were performed on a 96 atoms p(√2 × √2)−R45° cell, corresponding to 6 layers of monoclinic (001) WO3 (thickness of about 24 Å). According to previous work,32 this value ensures a stabilization of the structural and electronic properties of the slab model (for details, see ref 34). The optimized cell of the nondefective system is consistent with the experimental values33 (a = 7.461 Å, b = 7.748 Å, see Table S1 of the Supporting Information, SI). The accuracy of the integral calculations was increased with respect to its default value by setting the tolerances to 7, 7, 7, 7, and 18. The self-consistent field (SCF) iterative procedure converged to a tolerance in total energy of ΔE = 1 × 10−6 a.u. The threshold for the maximum and the root-mean-square forces were set to 0.00045 au and 0.0003 au. The above computational parameters ensured a full numerical convergence on all the computed properties described in this work. All the crystal structures are fully optimized (i.e., both cell parameters and internal coordinates) without symmetry operators in order to allow a complete structural relaxation and without fixing any layer. The common approach to fix some atomic layers of the slab model at the bulk position can, in this case, lead to computational errors.32 The study of oxygen defective structures was performed by spin polarized calculations on a c(4 × 4) supercell, which contains 192 atoms (see Figures 1 and S1). The c(4 × 4) slab models for the simulation of the different types of oxygen vacancy have been reported

Figure 1. Top and side views of enlarged c(4 × 4) supercells of the optimized (001) WO3 surface. The cell axes are also reported. Gray spheres represent W atoms and red spheres O atoms.

in Figure 2. This ensures a sufficiently large distance to avoid fictitious interactions among defects (>10 Å).

3. RESULTS AND DISCUSSION 3.1. Oxygen Vacancies on the (001) WO3 Surface. At room temperature, WO3 adopts a monoclinic γ phase. The (001) surface with a p(√2 × √2)-R45° reconstruction presents the lowest surface energy, as reported by Ping et al.34 This surface structure has also been observed in many STM and LEED experiments35−38 and is commonly considered in theoretical works.9,15,16 The well-known crystal structure of monoclinic WO3 is characterized by a structural anisotropy along the three cell axes that leads to a different behavior of the oxygen vacancy depending on the direction of the WO W chain where it is created.15,16 This anisotropy is related to the distortion of the monoclinic structure with respect to the simple cubic WO3 and implies an alternating long−short W O bond length, especially along the b axis.33,39 The electronic properties of the (001) WO3 surface are similar to those of the bulk system.15,32 The top of the valence band (VB) is mainly composed of O 2p orbitals, while the bottom of the conduction band (CB) is characterized by W 5d states. In the slab model, several oxygen vacancies (VO) can therefore be considered and simulated; O atoms lying on the topmost layer of the surface or in the inner layers can indeed be removed. We verified that VO localized in the inner layers behaves like VO in the bulk, already analyzed in previous papers.15,16 Here, therefore, we focus only on oxygen vacancies at the (001) surface. B


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Figure 3. a) Electronic structures (only spin majority for VbO and VcO), b) top and side view of the excess charge, c) structural features of the defective optimized structures WO3VO, and d) of pristine WO3 surface along the direction indicated in the bottom. In the first column the results concerning WO3VaO, in the second WO3VbO and in the last one those on WO3VcO are reported. The Fermi level is represented as a dashed line. Light green squares represent the oxygen vacancies. Isodensity threshold values were set to 0.003 e/a.u.3 Distances are in Å, angles in degrees. Labels A and B in the electronic structures correspond to (1/2, 0, 0) and (1/2, 1/2, 0) high-symmetry k points.

Figure 2. Top and side view of the three defective models: a) vacancy along a axis; b) VO along b axis, and c) VO along c axis. The structures are unrelaxed. Green squares represent the VO defect centers.

In the following we discuss the nature of VO created along the three crystallographic axes (VaO, VbO, and VcO, respectively). In spite of the rotation of the cell axes applied to build the c(4 × 4) reconstructed supercell (Figure S1), we continue to identify the oxygen vacancies with the common labels: VO along the distorted WOW chain (a axis) is denoted as VaO; that along the linear chain (b axis) VbO and, finally, VO directly bound to one W6c atom along the nonperiodic crystallographic c axis as VcO (see Figure 2). For convenience, we denote the 6- and 5-coordinated W atoms as W6c and W5c, respectively, and the 2- and 1- coordinated O as O2c and O1c. Moreover, O atoms lying on a, b, or c axis are identified as Oa, Ob and Oc. 3.1.1. Oxygen Vacancy along a Axis. Along the distorted WOW chain, the two OaW6c distances are almost equivalent with d(W6cOa) = 1.946 Å (see Figure 3 column a). Both singlet and triplet spin configurations of WO3VaO have been relaxed. The ground state is singlet closed-shell, with the triplet being about 0.8 eV higher in energy, in agreement with previous results for oxygen vacancies in bulk WO3.16 The vacancy formation leads to a strong geometrical rearrangement of the undercoordinated W6cO5 and W5cO4 groups (see Figure 3, first column). During the optimization process, the VaO W6cOc1c angle decreases by about 60°, going from 99.4° (stoichiometric structure) to 38°. Oc1c moves toward the hole generated by removing Oa and almost fills the VaO site, thus contributing to stabilize the overall structure. The reconstruc-

tion of the surface lowers the vacancy formation energy (Eform, with respect to 1/2 O2) with respect to the value obtained for the bulk (5.0−5.3 eV)14 by about 2 eV (Eform = 3.2 eV, Table 1). Table 1. Electronic Configuration (Conf) of the Three VO Structures Considered (Ground State), Energy Difference (ΔEtot in eV) with Respect to the Most Stable One (Vco), and Vacancy Formation Energy (Eform, eV) Vo

conf

ΔEtot

Eform

Vao Vbo Vco

singlet triplet triplet

0.24 1.48 0.0

3.22 4.46 2.98

In the band structure the defect state associated with the vacancy is dispersed along the k path and it merges with the conduction band (CB) leading to a metallic character with the Fermi energy lying in the continuum of states (see stoichiometric (001) WO3 band structure for comparison, Figure S2). Accordingly, the excess of charge is fully delocalized C


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WOW chain along c axis structurally distorted by the formation of the oxygen vacancy. Among the discussed models, the dispersed electronic state associated with this defect results as the most dispersed band, covering the largest range of energies. By comparing these results for the surface with those obtained for O-deficient bulk WO3,16 we note that the ground states of the three defective systems are the same in both cases; however, several differences also exist. In the WO3−VO bulk, both in VaO and VbO defective systems, the excess charge is localized in space (in the void or on the undercoordinated W atoms), and is accompanied by a small geometrical relaxation.15 Only in WO3VcO, one defect state is reported as a very dispersed band. In a similar way, recent works on oxygen deficient bulk α-MoO3, characterized by a structure similar to that of WO3, report comparable results and suggest a higher stability of localized excess electrons over the delocalized ones.17 In general, the atoms lying on the surface are freer to move with respect to those in the bulk. This allows a stronger structural rearrangement resulting in more delocalized states with a larger dispersion with k-points. Moreover, the nature of the conduction band (CB) has to be discussed. The CB minimum (CBM) of WO3 surface is quite different from that of the bulk, resulting in a smaller band gap (from 3.1 eV for the bulk to 2.77 eV for the surface, see Section 2). This difference can be attributed to the surface electronic states locating at the CBM due to the structural and charge redistribution near the surface. This, in turn, could promote the delocalization of the excess electron for the surface. A similar behavior has been observed and discussed for rutile and anatase TiO2.40 The CB of the anatase is wider and lower in energy with respect to rutile favoring the delocalization of the excess electron. On the contrary, the narrower and higher CB of the rutile favors the polaronic solution. We will now briefly mention the charged oxygen vacancies. So far, we discussed only neutral vacancies. However, the simulation of singly and doubly charged VO centers provides important insights into the optical properties of the material.16 As mentioned above, in a previous work16 on O-deficient bulk WO3, the authors reported computed charge transition levels (CTLs) and showed for VcO a shallow level, thus suggesting that singly charged vacancies are the most stable ones at finite temperature. Moreover, the stability and electronic properties of bulk VcO are in good agreement with the present results. Therefore, it is reasonable to expect that the finding about bulk VcO CTLs can be applied also to the present surface VcO center. To summarize, we found that the VO formation leads to a strong structural reorganization that is able to stabilize the defective surface, thus significantly decreasing the oxygen vacancy formation energy and causing a further delocalization of the excess charge. We indeed observe a strong correlation between charge localization and structural changes. The more the band is dispersed the more the structure is stable. The relaxation tends to reconstruct the pristine (001) WO3 surface by filling the VO with the closest Oc atom and to transform the undercoordinated W6c site into a pristine W5c ion of the surface. Only in WO3−VbO minor relaxations are found after the geometrical optimization: here, we observe a localized state occupied by one excess electron that blocks this reconstruction and results in a higher Eform. On the contrary, when this rearrangement can occur, such as in WO3VcO, the defective system results to be quite stable and Eform has the lowest value.

on the 5d orbitals of all the W atoms lying on the topmost layer and partially on the second one (see Figures 3 and S3). 3.1.2. Oxygen Vacancy along b Axis. In the case of the linear WOW chain (b axis), two different VbO can be formed. In fact, the W6cOb2c splitting is more relevant, with two bond lengths of 1.813 and 2.160 Å (see Figure 3). For completeness, an accurate analysis of both defective structures has been performed, but they show a very similar electronic and structural behavior. For this reason, only the VbO generated by removing the Ob2c on the longer distance (d(W6cOb2c) = 2.160 Å) will be discussed. Contrary to VaO, the ground state of VbO is a triplet open shell. Here, the difference with respect to the singlet is 0.5 eV. The two excess electrons rearrange in a different way. One is strongly localized in space, in particular on the 5d states of the now undercoordinated W5c site ion (deriving from the original W6c) nearby VbO that is then reduced from W5c4+ to W5c3+. The second one is delocalized on the fully coordinated W atoms of the surface. Accordingly, the band structure shows two defect bands with very different character. One is rather dispersed following the CB shape, while the other one is well localized 0.5 eV below the Fermi energy. Again, the dispersed state merges with the CB leading to a metallic character. The strong localization of the electron around the defect cavity on the W6c atom hinders the strong geometrical rearrangement observed in the previous case (i.e., VaO). Here, the local structure surrounding the void is just slightly changed, similarly to what observed for the bulk. This prevents the strong stabilization observed in the VaO case. The WO bond lengths of the under-coordinated W atoms show a change of 4%. Only WO along the VO direction shows a higher change: VOW5cOb is shortened by 19%, while VOW6cOb is elongated by 15%, as expected. This explains the computed vacancy formation energy, which is much higher than in the previous case, i.e., 4.46 eV, and similar to the value obtained for the Ob-deficient bulk (4.6 eV).16 3.1.3. Oxygen Vacancy along c Axis. In this case the terminal O atom, singly bound to W6c, is removed. Our calculations indicate the triplet configuration as the ground state, with the singlet about 1.0 eV higher in energy. The removal of the most exposed oxygen leads to a strong structural change of the associated W6cO5 group. With the loss of the Oc atoms, the undercoordinated W6c assumes the same geometry of the W5c neighbors (see Figure 3). The net result is the presence of one additional W5c site at the surface. The remaining W6cOc2c bond of the undercoordinated W6c atom undergoes an important shortening from 2.39 to1.75 Å, thus becoming similar to a pristine W5cOc2c bond (1.749 Å vs 1.732 Å, Figure 3). These changes strongly stabilize this defective system, which results the most stable one among all the WO3VO considered, with a vacancy formation energy (Eform) of 2.98 eV (see Table 1). The WO3VbO and WO3VcO systems show a very similar electronic behavior in contrast to the WO3VaO: one excess electron is delocalized on the 5d orbitals of all the fully coordinated W atoms of the surface, whereas the other one is localized. In particular, the dispersed state clearly follows the dispersion behavior of the CB edge. This suggests a freeelectron-like behavior. The localized state is due to the formation of a large polaron at the third layer below the surface, in particular on a W atom directly below the defective W6cVO center, as shown in Figure 3c. Therefore, the delocalized electron seems to prefer the W ion that lies on the D


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As regards WO3/H2O, two possible coverages have been considered: “half” (low) coverage, with only one H2O adsorbed on one available W5c site (WO3/1H2O), and “full” coverage with two H2O molecules on the two W5c sites (WO3/2H2O). Moreover, different models for each coverage can be explored: H2O can be oriented according to the smaller OWO angle (82°) or to the larger one (93°). The relative stabilities of these bonding modes are very similar (energy differences always below 0.01 eV); therefore, we will discuss only one structure for each coverage regime (Table 2). In Figure 4, the structural features are reported. When the water molecule adsorbs on W5c, we observe an asymmetric elongation of the two OwHw (Xw atoms from water molecule) bond lengths (Table 2). Moreover, the H2OW5c distance is around 2.37 Å, which is comparable with the bond length between W6c of the surface and the Oc2c atom lying along the c axis in the second topmost layer (2.386 Å, see Figure 3). The adsorption of one or two H2O leads, therefore, to a change of W5c centers that tend to assume the structure of the W6c sites. No relevant changes in the surrounding structure are observed. The small changes in the OwHw bond lengths are indicative of a weak interaction with the (001) WO3 surface. Since the computed adsorption energies for both low and high coverage (Table 2) are about the same (−1 eV per H2O molecule, in agreement with previous work9), one may conclude that the W5c-H2O species at high coverage are noninteracting. This is probably due to their distance and to the presence of W6cO bonds in between (Figure 4). Concerning dissociated water, OH adsorbs on one W5c, while H can, in principle, bind on Oa, Ob, or on Oc. Only one low coverage model has been obtained; in particular, the structure with H on Oa, (WO3/1(OHHa)). The structure with H on Ob is expected to be very similar, whereas the configuration with H bound to Oc is not stable. The dissociated low coverage configuration, WO3/1(OH Ha), shows positive ΔEads (+0.19 eV) that implies a slightly endothermic dissociation of H2O on the regular surface. On the contrary, the high coverage structure, WO3/2(OHH), is characterized by two dissociated water molecules forming a stable hydrogen bond network (Figure 4), resulting in an energy gain of −0.57 eV with respect to WO3/1(OHH) (ΔEads = −0.38 eV, see Table 2). Here, the H atoms are all connected to Oc atoms and their H-bonding network allows an overall stabilization. As final result, we obtain a rearranged surface where all the W atoms are bound to a hydroxyl group (Figure 4b). However, the dissociated high coverage structure is still about 0.5 eV less stable than the molecular adsorption configuration (Table 2).

Based on our results, we suggest that VO formation in WO3 mainly concerns the loss of the singly coordinated Oc1c of the surface, which is stable as singly charged VO with one excess electron delocalized in the CB and behaving as a free-electron, in agreement with the experimental evidence of n-type conductivity in γ-monoclinic WO3 films.15,16,41,42 3.2. H2O Adsorption on the WO3 (001) Surface. To evaluate the interaction of water molecules with the stoichiometric (001) WO3 surface, the p(√2 × √2)-R45° cell (96 atoms) has been adopted. The following reaction has been considered for the adsorption of intact (or dissociated) H2O molecules: WO3 + H 2O → WO3 /H 2O(WO3 /(OHH))

(1)

The ΔEads is reported as eV for H2O molecule. Negative ΔEads values represent spontaneous adsorption. Since the topmost layer of the nondefective surface is characterized by two W6c and two W5c atoms composing a quasi-square pattern (with two different OWO angles, see Figure 4a), several

Figure 4. Top view of water molecules adsorbed on the WO3 surface. a) Stoichiometric WO3, b) H2O low coverage WO3/1H2O, c) H2O high coverage WO3/2H2O, d) dissociative H2O high coverage WO3/ 2(OHH), and e) dissociative H2O low coverage WO3/1(OHHa). The blue spheres represent O atoms of the water molecules.

WO3/H2O(OHH) configurations can be considered. In particular, we modeled five configurations with undissociated flat molecules, hereafter denoted as WO3/H2O, and two with dissociated H2O, i.e., WO3/(OH-H).

Table 2. Water Adsorption Energy (per H2O Molecule) (ΔEads) and Relevant Structural Features ΔEads (eV)

d(OwW5c) (Å)

d(HwOwW5c) (Å)

WO3/1H2O WO3/2H2O

−0.95 −0.91

2.360 2.381 2.373

0.975/0.981 0.973/0.983 0.973/0.984

WO3/2(OHH)

−0.38

WO3/1(OHHc)

+0.19

2.015 2.041 1.948

b/a

E

d(HwOW6c) (Å) 0.970 0.973 0.973

d(HwOwW5c) (Å)

0.967 DOI: 10.1021/acsami.7b06139 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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In order to check whether the presence of a dipole moment along the nonperiodical direction of the slab due to VO and the H2O molecule on the surface affects our results, we simulated one configuration with VO and H2O on both sides of the slab. We verified that the dipole moment has no relevant effects on the energetics and the electronic properties. Therefore, in the following, we considered one VO and one H2O located only on one side of the slab. All the structures have a singlet closed shell ground state. Moreover, in all the cases, the excess electrons remain delocalized on the same sites as for the clean surface, with the corresponding energy levels at the same position. To evaluate the water adsorption energy (ΔEads per H2O molecule), we considered the following reaction:

The tendency to preferably adsorb H2O in molecular form is not surprising since the reconstruction of the (001) surface leads to two undercoordinated W5c ions that act as Lewis acid sites able to bind H2O.9 Moreover, this is a common behavior with other metal oxides;43−45 in particular, the binding energies computed for WO3 are comparable to those of TiO2. Recent works on (101) TiO2 anatase performed by different DFT functionals, i.e., PBE45 and B3LYP-D*46 (with dispersion forces included), reports adsorption energy of, respectively, −0.72 eV and −0.95 eV for molecular H2O and of −0.34 eV and −0.59 eV for the dissociated form. 3.2.1. H 2O Adsorption on O-Deficient (001) WO 3−x Surface. Now we discuss the adsorption of H2O molecules on the most stable WO3‑x surface models (see Section 3.1.). According to the computed vacancy formation energies, VcO and VaO defects are expected to be predominant with respect to VbO, as they are more stable by 1.5 and 1.3 eV, respectively. Hence, we simulated four different configurations: (i) water on WO3 VcO (denoted as WO3VcO/H2O); (ii) water on WO3VaO (i.e., WO 3V Oa /H 2O); dissociated water on WO 3 VOc (WO3VOc /(OHH)) and; (iv) dissociated water on WO3VaO (WO3VaO/(OHH)) (see Figure 5).

WO3VO + H 2O → WO3VO/H 2O (or/(OHH)) (2)

This means that we neglect the oxygen vacancy formation energy that for WO3VaO is, as reported above, 0.2 eV higher in energy with respect to WO3VcO. We first discuss the adsorption of a water molecule on the oxygen vacancy at the topmost layer along c (WO3VcO/H2O). This corresponds to the adsorption of a H2O molecule on a W5c ion (Figure 5). We obtain an adsorption energy, ΔEads = −1.05 eV, similar to that obtained for the stoichiometric WO3 surface. Also the local structure (see Table 3 vs Table 2) is almost unchanged. This is due to the nature of the W site at the surface. After the oxygen vacancy formation, the defective W6c undergoes a strong local reorganization that actually transforms it into a surface W5c totally analogous to the regular ones, both structurally and electronically. This is clear from the electrostatic potential reported in Figure 6 and the structural features,

Figure 6. Electrostatic potential (min: −0.032 au; max: 0.09 au) of a) pristine (001) WO3 surface, and b) WO3VcO mapped on top of the charge density isosurface (i.e., 0.002 au). Blue represents positive regions and red negative ones. W5c, Oc and VO positions are also indicated.

Figure 5. Top view of water absorption configuration on O-deficient WO3. a) WO3VaO/H2O, b) WO3VaO/(OH-H), c) WO3VcO/ H2O, and d) WOVcO/(OHH). Blue spheres represent Ow atoms.

Figure 3. It is not surprising that ΔEads is almost the same. A closer analysis of the electrostatic potential, Figure 6, shows

Table 3. Adsorption Energy (per H2O molecule) of H2O on Defective Structures (WO3VO + H2O → [Products] (P), in eV) and of H2 on WO3 (and WO3 + H2 → [Products] (P), in eV) and Relevant Bond Lengths of All Structures (Å) WO3VO + H2O → P

WO3 + H2 → P

ΔEads

ΔEads

d(HwOwW)

WO3VaO/H2O [2HOa2c]

−0.52

+0.21

WO3VaO/H2O [2HOc1c]

−1.05

−0.56

2.365 2.400 2.380

WO3VaO/(OHH) [(HOc1c)(HOa2c)]

−1.01

−0.28

WOVcO/(OHH) [2(HOc1c)]

−0.69

−0.21

P

2.089 2.117 1.873 F

d(HwOW)

d(HwOwW)

d(HwOW)

1.876

0.980 0.980 0.973 0.973 0.975

0.970

1.876

0.970

0.969


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The first model, 2HOa2c, corresponding to a dissociative H2 adsorption on Oa2c, is slightly endothermic (+0.2 eV, Table 3). The double coordinated Oa2c that lies on the a axis should indeed become four-coordinated, causing a strong change in the local structure. On the contrary, H2 dissociation on Oc1c is exothermic, with ΔEads = −0.56 eV. This reinforces the thesis reported by other works47 on the tendency of the H2 molecule to dissociate on the most exposed and reactive O atom (i.e., Oc1c). The other two configurations, (HOc1c)(HOa2c) and 2(HOc1c), show very similar and intermediate adsorption energies, −0.21 eV and −0.28 eV (Table 3). The H2 dissociation on two distant O atoms is, therefore, less favored. These results are different from what reported in two previous works; in refs 32 and 6, the authors found an activated H2 dissociation on the WO3 surface. The reason can be mainly related to the structural model adopted for the calculations in refs 32 and 6, and in particular to the use of a small unit cell. When H atoms are adsorbed on vicinal sites, their repulsive interaction destabilizes the structure. For the sake of completeness we considered also the heterolytic H2 dissociation, characterized by one H bound to Oc1c and the other on vicinal W5c (with formation of a proton and an hydride ion). The calculations have been performed both on small (p(√2 × √2)−R45°) and large (c(4 × 4)) WO3 unit cells. The results show, in both cases, a singlet ground state with two electrons localized on the WH bond (hydride) and the corresponding (flat) electronic state lying 0.5 eV above VB (Figure 7). The triplet configurations are higher in energy by about 1.5 eV and result in the dissociation and desorption of one H atom (Figure S6).

that the blue squares that represent the W5c sites are slightly smaller in WO3VcO with respect to the pristine (001) WO3 surface. Moreover, in the former case the electrostatic potential exhibits an increased negative charge (more yellowish surface) due to the presence of the two delocalized excess electrons associated with VcO. On the contrary, when the water molecule is inserted in the VaO, WO3VaO/H2O, the adsorption energy is smaller than WO3VcO/H2O by 0.5 eV. This can be explained by the fact that once H 2 O is adsorbed on this site the strong reorganization associated with the VaO center (see previous section) cannot occur, destabilizing the structure (Figure 5). Now, we consider the two dissociated forms: WO3VaO/ (OHH) and WO3VcO/(OHH). In WO3VaO/(OH H), one Hw atom is bound to the Ow inserted along the a axis and the other one on the most far O1c atom. ΔEads is similar to that of the WO3VcO/H2O structure (keeping in mind that here the difference in the VO formation energy between VaO and VcO is neglected). Finally, the WO3VcO/(OHH) configuration, characterized by two H atoms bound to two far Oc1c, corresponds to a higher dilution of the WOwHw species on the surface with respect to that obtained by adsorption of dissociated H2O on stoichiometric WO3 (i.e., WO3/2(OHH), see previous section). This allows to compute the ΔEads for one single dissociated water molecule, which is about −0.7 eV. The H2O molecular adsorption is still energetically favorable. For completeness, we also investigated the effect of a subsurface VO on the H2O adsorption. In particular, the oxygen vacancy along the c axis (the most abundant defect also in the bulk) in the second topmost layer, and not directly connected to W5cH2O, has been considered. The energies and the electronic properties obtained are similar to those of the aforementioned configurations. Furthermore, given the higher formation energy of this kind of VO, the probability to realistically observe this configuration is quite low. Therefore, this structure will not be discussed in detail. In conclusion, the interaction of H2O with O-deficient WO3 surface does not show any relevant change compared to the pristine surface. The reason lies in the highly stabilized nature of VcO at the WO3 surface. The particular structure of the WO3 surface allows a geometrical relaxation after the removal of an O atom that results in an almost unchanged topmost layer. This also results in a similar reactivity, as observed experimentally.19 3.3. H2O Desorption from Hydroxylated Surface. By means of the previous modeled structures, we can investigate H2O desorption from the hydroxylated WO3 surface. In fact, the WO3VO/H2O and WO3VO/(OHH) final structures are equivalent to adsorb one H2 molecule on the stoichiometric surface, involving either a single O atom (Oa2c and Oc1c, O atom deriving from the water molecule) or on two distant O atoms. Therefore, in the following we focus the attention on the H atom position, considering that the O atom will simply refill the vacancy. The nomenclature will be changed accordingly. WO3VaO/H2O and WO3VcO/H2O (see Figure 5) are denoted as 2HOa2c and 2HOc1c; while WO3VaO/(OH H) and WO3VcO/(OHH) as (HOc1c)(HOa2c) and 2(HOc1c), respectively (see Figure 5 and Table 3). First, we computed the H2 adsorption energy (per H2 molecule) according to the following equation: ΔEads = E WO3 /H2 − (E WO3 + E H2)

Figure 7. Band structure of H2 dissociation on Oc1c (left side, 2H Oc1c) and W site (right side) (c(4 × 4) supercell). Excess charge is reported as inset (iso-density threshold values 0.003 e/a.u.3). The Fermi energy is set to zero.

The size effect of the cell is clearly visible by considering the adsorption energy obtained for the two singlet solutions: for the small cell we obtain ΔEads = 0.84 eV and for c(4 × 4) a ΔEads = 0.38 eV. In agreement with previous results,47 the heterolytic dissociation is not spontaneous; the O sites are energetically more favorable due to the formation of surface

(3) G


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indeed the most realistic situation. However, the presence of oxygen vacancies does not strongly influence the H2O adsorption. The strong structural rearrangement that stabilizes the O-deficient surface structure leads to a O-deficient surface similar, in the electronic and geometrical characteristics, to the pristine WO3. According to our findings, water adsorption is nondissociative on WO3 (001). The distances between OW and W atoms at the surface are around 2.3 Å, which is similar to the W−O bond length in the bulk. These results can be analyzed also by another point of view. By investigating the dissociation of H2 on the stoichiometric WO3 surface, we observed that the dissociation (homolytic) is spontaneous on the oxygen sites. In particular, the most favored site is the terminal Oc1c atom with an adsorption energy of −0.56 eV, in good agreement with previous results. This suggests that once exposed to H2, WO3 is reduced by simple addition of hydrogen (formation of OH groups and extra electrons on W 5d states). The adsorption of H2 on the WO3 surface implies a band structure modification with the introduction of a defect state near the CB leading, as for the VO formation, to a metallic system. This is likely the reason for the color change observed experimentally in the presence of H2. This process can also be followed by hydrogen migration on the surface, and formation of an H2O molecule that can then desorb with a relatively low energy cost (about 1 eV) leaving behind an O vacancy. The cost to produce a vacancy in this way is considerably lower than by direct O2 desorption (about 3 eV). This is a very efficient mechanism that leads to a reduced oxide surface.

hydroxyl groups and the electron transfer to the W empty 5d states (oxide reduction). Finally, compared to the bridging O2c, the unsaturated terminal Oc1c site is more reactive. However, the heterolytic dissociation is not competitive with the homolytic one, consistent with the reducible nature of WO3. Therefore, on the basis of the present calculations a H2 molecule on the WO3 may undergo dissociation, preferably on Oc1c site, but also other O active sites because of the similar ΔEads values, and migrate on the surface to find the most stable configuration. As mentioned, the configuration with two H atoms adsorbed on the same surface O ion can be viewed as a water molecule adsorbed at the W5C site. Since the adsorption energy of H2O is relatively low (about −1 eV), we can consider the water desorption from such hydroxylated surface with consequent oxygen vacancies formation as a low cost process: c WO3 + H 2 → WO3 /(2HOc ) → WO3 + V O + H 2O

(4)

The two steps of this process have been experimentally demonstrated in a very recent paper.6 It has been reported that the stoichiometric WO3 surface is able to activate the H2 molecule for the hydrogenation of 1-hexene. This means that H2 can dissociate on the surface. Moreover, the reaction leads to a color change of the sample (from yellowish to intense greenish), characteristic of reduced WO3, which, together with the absence of hydroxyl groups, would imply the formation of a considerable number of VO centers on the surface. Finally, the temperature-programmed reduction shows that the WO3 surface can be reduced by H2 even at a temperature as low as 40 °C.6

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ASSOCIATED CONTENT

S Supporting Information *

4. CONCLUSIONS We investigated the water adsorption and dissociation on both clean and O-deficient (001) WO3 surfaces. A detailed study on the oxygen vacancies at the topmost layer has been also performed showing a strong relationship between geometrical and electronic structures. The anisotropic nature of the VO, which has been observed in the bulk system, is present also on the surface. The removal of the terminal O atom along the c axis is highly preferred, with a Eform of about 3.0 eV; this is 1.5 eV lower than the corresponding bulk value. Thus, it is expected that, after thermal annealing, the free surface will be difficult to obtain in stoichiometric form. This defect center has a triplet ground state characterized by two singly occupied defect states, one localized just below the CB and the other one higher in energy in resonance with the CB. Accordingly, the excess of charge generated by the oxygen vacancy is partly localized in space in the inner layers and partly delocalized on the topmost layer of the surface. A strong structural relaxation that tends to restore the pristine WO3 surface structure stabilizes the overall structure. The presence of oxygen vacancies leads to a semiconductor to metal transition for all the three cases (VaO, VbO, and VcO) considered, confirming the experimental evidence of n-type conductivity in WO3 films. The study of H2O interaction with WO3, important for its use as photoanode material, shows the tendency of the WO3 surface to preferably adsorb H2O in molecular, undissociated form, due to the presence of low-coordinated W ions that act as Lewis acid sites. However, as observed also for other semiconductor oxides, this interaction is not very strong, with a value of about −1 eV. The adsorption of water has been investigated also on defective WO3 surfaces; this represents

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b06139. Table S1 with optimized cell parameters of WO3 (001) surface and of the three WO3VO optimized models; Figure S1 schematic top view of the (001) WO3 p(√2x√2)−R45° cell and of the c(4 × 4) unit cell; Figures S2 and S3 electronic properties stoichiometric WO3 surface and of WO3VaO, respectively; Figure S4 vacancy formation energies of the WO3VO as a function of the oxygen pressure at different temperature; Figure S5 band structure of the WO3/H2O model; and Figure S6 spin density plot of the triplet configuration of the heterolytic H2 dissociation. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (E.A.). ORCID

Elisa Albanese: 0000-0002-8255-3658 Cristiana Di Valentin: 0000-0003-4163-8062 Gianfranco Pacchioni: 0000-0002-4749-0751 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the Cariplo Foundation with the grant no. 2013-0615 “Novel heterojunction based photocatalytic materials for solar energy conversion” and by CINECA-LISA projects HPL13PITRY and HPL13PYS1C, H


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2016−2017, for the high-performance computing resources. Financial support from the Italian MIUR through the PRIN Project 2015K7FZLH SMARTNESS “Solar driven chemistry: new materials for photo- and electro-catalysis” is also acknowledged.

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