Doping of WO3 for Photocatalytic Water Splitting: Hints from Density

Apr 2, 2012 - Optimizing the Band Edges of Tungsten Trioxide for Water Oxidation: A .... Shankara S. Kalanur , Il-Han Yoo , Kiryung Eom , Hyungtak Seo...
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Doping of WO3 for Photocatalytic Water Splitting: Hints from Density Functional Theory 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 S Supporting Information *

ABSTRACT: The electronic properties of doped tungsten oxide (WO3) have been studied using DFT calculations with hybrid functionals. While the position of the top of the valence band (VB) in WO3 is good for O2 evolution in water splitting, the conduction band (CB) is too low for H2 production. Furthermore, the band gap should be reduced to improve activity with visible light. Doping can be used to alter the position of the energy levels, thus resulting in a more efficient photocatalyst. Replacing W in the lattice by isovalent Mo or Cr ions narrows the band gap but shifts the CB edge further down. Replacing O by S has the effect to narrow the energy gap by introducing localized occupied states above the VB and shifts the CB minimum upwardtwo effects that go in the right direction. Substitution of W with low-valent Ti, Zr, or Hf ions widens the band gap and shifts the CB edge to higher energies. However, a low-valent ion replacing W induces the formation of compensating defects: in Hf-doped WO3 oxygen vacancies (VO) have negative formation energies. The simultaneous presence of substitutional Hf and of an O vacancy results in a shift of both VB and CB to higher energies and a reduction of the band gap, with potential benefit for photocatalytic hydrogen production. Codoping with Hf + 2F or Hf + VO + S has also been investigated.

1. INTRODUCTION Hydrogen production by photocatalytic or photoelectrochemical (PEC) water splitting using solar energy is an ambitious yet crucial objective for sustainable energy production.1 Relevant problems in this context are the semiconductor band gap as well as the conduction band (CB) and valence band (VB) edges position.1 To make efficient use of the visible light, the semiconductor band gap should be around 2.0 eV and in any case much smaller than 3.0 eV (λ > 415 nm). Furthermore, to facilitate both the reduction and oxidation of water by photoexited electrons and holes, the band gap should match the H2O redox potential. In particular, the CB edge should be more negative than the reduction potential of H+/H2 [0 V versus normal hydrogen electrode (NHE)], whereas the top of the VB should be more positive than the oxidation potential of O2/H2O [1.23 V].1,2 The photoelectrodes should also be stable against photocorrosion in aqueous solutions. Clearly, the search for efficient photocatalytic materials is a key step in this process. Since the report of the first example of water splitting system based on TiO2 and Pt,3,4 over 130 inorganic materials and derivatives have been studied as catalysts for the overall splitting of water or for water oxidation or reduction in the presence of external redox agents.5 Among them, WO3 has attracted a lot of interest as an n-type catalyst partly due to its photosensitivity,6−8 good electron transport properties,9 and stability against photocorrosion.10,11 Furthermore, its smaller band gap (∼2.8 eV)1 than that of other semiconductors (e.g., TiO2) makes it suitable for absorption of visible solar light. However, the gap of WO3 is still too large to realize a sufficient absorption of the solar spectrum. Moreover, experiments © 2012 American Chemical Society

suggest that the conduction band minimum (CBM) of bulk WO3 is about 0.4 eV below the hydrogen redox potential.12,13 For the WO3 surface, one report states that the CBM lies 0.31 eV above the hydrogen redox potential, still too low for hydrogen production.14 Doping and codoping are possible ways to tailor the electronic structure as well as the PEC properties of semiconducting oxides, as shown for instance for TiO2.15 Until now, a variety of dopants have been considered experimentally to improve the photocatalytic properties of WO3. For instance, Mg doping results in an upward shift of the CB edge to a level well above the hydrogen reduction potential.16 Mo-doped WO3 nanowires showed an enhancement of vis-light photoactivity, as the band gap of the MoxW1−xO3 solid solutions was narrowed by 0.48 eV with Mo content increasing from 0 to 0.75.17,18 The effect of several transition metals on the photocatalytic activity under UV irradiation was also studied, suggesting that Ni doping gives the maximum hydrogen production.19 Doping with other metals (e.g., Ti, Zn, Dy, Te, Ta, V, Cu, Ag, Ce) was also reported in the literature,20−28 with some claims of improved WO3 photocatalytic properties. Nonmetal (N, C) doping was reported to decrease the band gap and increase the visible light photoresponse and photocurrent.29−31 Finally, mixed WO3/WS2 systems showed enhanced performance in degrading phenols.32 Received: January 26, 2012 Revised: March 16, 2012 Published: April 2, 2012 8901

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Density functional theory (DFT) calculations have already been used to study the effects of some atomic impurities on the band edges of WO3.33 It was found that the band gap reduction is often due to the formation of impurity bands and that a shift of the CB edge can be induced by foreign atoms. However, the results are somehow questionable due to the severe underestimation of the WO3 band gap by standard DFT methods. For example, in some cases the impurity states merge into the host VB, leading to an artificial metallic character. Moreover, the doping effect on the fundamental band gap of WO3 is still not clear, and the shift of the band edges might be underestimated or incorrect. Hybrid functionals represent a clear improvement and provide a practical, although not perfect, solution to this problem since they allow one to better describe the band gap34 and to correctly describe the localization of electrons and holes. Recently, we found that the electronic and structural properties of WO3−x, and in particular the nonmetal to metal transition as a function of the reduction level, are well described by the B3LYP functional, at variance with standard GGA methods.35,36 Since the aim of the present work is not to compute optical properties, such as excitation energies or excited states, but instead of evaluating relative energy shifts of band edges and degree of localization of eventual impurity states, we can safely rely on ground state DFT. In this paper, doping and codoping effects on the electronic and photocatalytic properties of WO3 are studied by performing B3LYP periodic calculations with the aim of identifying the species that can alter the position of the VB and CB in monoclinic WO3 and, most important, the general rules that govern these electronic structure modifications. Cr, Mo, Ti, Zr, and Hf have been considered for cation doping, S for anion doping, and Hf + S and Hf + 2F as two examples of codoping.

Figure 1. Atomic structures of (a) WO3, (b, c) HfxW1−xO3, and (d) HfxW1−xO3−x. The large gray and small red spheres represent W and O atoms, respectively. (b) Yellow spheres: O atoms that contribute to the top of the valence band; blue spheres: O atoms that contribute to about 0.1 eV below the top of the valence band; red spheres: O atoms that contribute to about 0.25 eV below the top (see text). (c) Large green spheres: W atoms contributing to the conduction band minimum. (d) Small yellow spheres: O atoms contributing to the valence band maximum.

set to 0.000 45 and 0.000 30 au and those for the maximum and the rms atomic displacements to 0.001 80 and 0.001 20 au, respectively. Formation energies reported in the following are estimated from total electronic energies of the species involved in the process. The band gap values discussed in the article are HOMO−LUMO gaps. Band edge shifts for the doped systems are calculated with reference to the position of the VBM and CBM in pristine γ-monoclinic WO3, after alignment of 1s core levels of the O atoms (for the doped systems only the O atoms not in direct contact with the dopant are considered).

2. COMPUTATIONAL DETAILS All the calculations were carried out within the linear combination of atomic orbitals (LCAO) approach with periodic boundary conditions combined with the B3LYP37,38 hybrid functional, as implemented in CRYSTAL09 code.39,40 The allelectron 8-411(d1) Gaussian-type basis set was adopted for oxygen,41 while for tungsten we used an effective core potential (ECP) combined with a modified Hay−Wadt double-ζ basis set.42,43 For Mo, Cr, S, Ti, Zr, Hf, and F, we adopted the following basis sets: Haywsc-311(d31),44 86-411(d41),45,41 86311d1,46 86-411(d31),47 Haywsc-311d31,48 Haywsc-411d31,49 and 7-311,50 respectively. For all cases, the lowest spin state has been determined and will be discussed in the text. The lattice parameters of room temperature (RT) γmonoclinic WO3 (space group P21/n) (Figure 1a) were obtained from the experiment51 and then fully optimized. For each dopant, the lattice parameters were fixed at the WO3 equilibrium values while all the atoms were allowed to relax. Only in some cases which will be explicitly mentioned, also lattice parameters were allowed to relax. For both geometry optimization and density of states (DOS) calculations, the reciprocal irreducible Brillouin zone (BZ) was sampled according to a regular sublattice with a shrinking factor (input IS) of 4 (36 k-points). The Kohn−Sham eigenvalues were computed on each k-point of the mesh and used to estimate the band gap. The equilibrium structure was determined by using a quasi-Newton algorithm with a BFGS Hessian updating scheme.52 For all the atoms, the thresholds for the maximum and the root-mean-square (rms) forces were

3. RESULTS AND DISCUSSION 3.1. Pure WO3. The structure of undoped RT γ-monoclinic WO3 is associated with tilting and distortion of the cornersharing WO6 octahedra. It can be seen as a 2 × 2 × 2 superstructure based on an idealized cubic unit cell or as consisting of pseudo-one-dimensional W−O−W chains (Figure 1a). The calculated direct band gap of pure WO3 is 3.10 eV, and the indirect band gap between Z and Γ is only slightly larger.35 As shown in Figure 2, the upper part of the VB (bandwidth of about 7.5 eV) is mainly derived from the O 2p states, while the lower part is predominantly composed of O 2s states (not shown). The lower part of the CB is mainly composed of W 5d states with some contribution from O 2p orbitals. Because each W atom is surrounded by six O atoms in a distorted octahedral coordination, the W 5d states split into t2g and eg states due to the crystal field. The DOS of WO3 is decomposed into W 5d eg, W 5d t2g (dxy, dxz, dyz), O 2pσ (along W−O bonds), and O 2pπ (perpendicular to W−O bonds) contributions. A schematic representation of the electronic energy levels from the DOS and electron charge density plots (see Figure S1 in Supporting Information) is presented in Figure 2 (top).53 The upper part of the VB can be resolved into three major components: a 8902

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the VB edge, and (3) doping by metals with d states higher in energy than W 5d levels could shift up the CB edge. Because of the large covalent character of the W−O bonding, nonmetal or metal doping may also affect the CB or VB edges as a consequence of the hybridization between O 2p and W 5d states. 3.2. Substitutional Doping with Isovalent Cations (Mo, Cr) or Anions (S). Generally, doping with isovalent elements does not result in charge mismatch problems, thus limiting the formation of other defects like anion or cation vacancies. Mo is isovalent with W but has a slightly larger atomic radius. We replaced one W atom by Mo in the WO3 unit cell, resulting in a 12.5% Mo concentration. This does not induce any significant structural distortion (the largest bond length difference between Mo−O and W−O bonds is 0.06 Å only, Table 1). Mo-doped WO3 also shows a direct band gap Table 1. Optimized W−O Bond Lengths of Undoped WO3 and the Corresponding M−O Bond Lengths in the Substitutional M-Doped WO3 (M = Cr, Mo, Ti, Zr, Hf) (Distances in Å) bond type

+x

−x

+y

−y

+z

−z

W−O Mo−O Cr−O Hf−O Zr−O Ti−O

1.91 1.97 2.05 1.99 2.01 1.79

1.90 1.86 1.65 1.99 2.01 1.97

2.14 2.17 2.19 2.02 2.04 1.95

1.77 1.76 1.62 2.12 2.16 1.99

2.22 2.25 2.26 2.09 2.12 2.07

1.75 1.74 1.60 2.10 2.13 1.95

with the overall band profile similar to that of the undoped crystal, Figure 3a. The band gap of Mo-doped WO3 is 2.91 eV,

Figure 2. DOS of the pure monoclinic WO3. The top of the valence band is taken as the zero of energy, as indicated by the vertical solid line. The vertical dashed line indicates the conduction band minimum. On the top, a summarizing scheme of the electronic energy levels in terms of molecular or atomic orbitals.

lower energy part ranging approximately from −7.5 to −6 eV; a middle energy part from −6 to −2 eV; a higher energy part from −2 to 0 eV (where 0 is set at the valence band maximum, VBM). The lower energy part with σ bonding characteristic arises mainly from the mixing of O 2pσ and W 5d eg and W 6s states. The middle region shows a predominant π bonding character and is composed of O 2pπ and W 5d t2 g states, with minor contributions from O 2pσ and W 5d eg states. The higher energy part is mainly from the O 2pπ states and extends to the VB top. Notice that there is also some hybridization between O 2p and W 5d states in this region. The lower part of the CB can also be decomposed into three main parts. The bottom of the CB (from about 3.1 to 4 eV) consists mainly of W dxy and dxz orbitals, with some contribution of O 2pπ and W dyz states. Above 4 eV, one antibonding π* state arises from the hybridization of W t2g and O 2pπ states. In this energy range, some contribution of the σ* antibonding state also exists. The higher part of the CB with σ* antibonding character (above 7.3 eV) is mainly composed of W eg states hybridized with O 2pσ states; above there is one antibonding σ* state derived from the hybridization of O 2pσ and W 6s states. From this analysis, we can infer that (1) to effectively narrow the band gap, the 2pπ states contributing to the VB top or the 5dxy (5dxz) states contributing to the bottom of the CB need to be modified, (2) doping by nonmetal elements with pπ states higher in energy than O 2pπ levels could in principle shift up

Figure 3. Band structure and DOS of (a) Mo-doped WO3 and (b) Crdoped WO3. Total DOS (TDOS) of the pure WO3 is also shown for comparison and the shift of band edges with respect to the pure case is indicated.

i.e., 0.19 eV smaller than the undoped one, consistent with the experimental observation that the band gap of MoxW1−xO3 micro/nanostructures narrows with increasing Mo content.17,18 Notice that previous DFT studies using standard functionals reported no variation of the band gap value in Mo-doped WO3.33 8903

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Upon Mo doping, the CBM shifts down by 0.16 eV while the VBM shifts up by 0.03 eV only (Figure 3a). The lower part of the CB shows a strong hybridization between W, Mo, and O orbitals, shifting the CBM down, which is undesired and detrimental to hydrogen production. We also checked the effect of concentration. To this end we replaced one W atom with Mo in a supercell containing 64 atoms, corresponding to a Mo concentration of 6.25%. The calculated band gap is 3.00 eV, which is only 0.10 eV narrower than the undoped one. Besides, the shifts of the VBM and CBM are 0.02 and −0.08 eV, respectively. Therefore, both the band gap reduction and the band edge shifts are related with the dopant concentration, in line with experimental findings.18 We now consider W substitution by Cr in the WO3 unit cell (concentration 12.5%). Compared to W6+ (0.62 Å), the Cr6+ ion has a smaller ionic radius (0.52 Å) and induces a small lattice distortion. After full atomic relaxation, three of the six O neighbors move toward Cr by 0.15−0.25 Å, while the other three move outward by 0.05−0.14 Å, leading to larger bond length asymmetries along the three directions (Table 1). As in the undoped case, the top of the VB is mainly composed of O 2p orbitals with no appreciable effect of Cr-doping (Figure 3b). Therefore, the VBM remains almost at its original position. The bottom of the CB becomes very flat due to the more localized nature of Cr 3d compared to W 5d orbitals. With respect to the undoped case, we observe the formation of an impurity Cr 3d band 0.15 eV below the original CBM. As a consequence, the calculated band gap of Cr-doped WO3 is 2.92 eV, 0.18 eV smaller than the undoped one. To summarize, W substitution by Mo and Cr can reduce the band gap of WO3, with a general benefit for the visible light absorption. However, both elements cause a shift of the CBM to lower energy, an effect which is detrimental for PEC hydrogen production. The effect of doping with an isovalent anion has also been investigated by substituting one O atom along the z chain in the unit cell by S, corresponding to a dopant concentration of 4.2%. Because of the relatively larger atomic radius of S compared to O (1.0 vs 0.6 Å), this is expected to induce a large atomic reorganization. Substitution along the z-direction was chosen because it exhibits the largest lattice constant and can, therefore, accommodate the S impurity with less strain. After optimization, the W atoms adjacent to S move outward substantially, while the S atom moves slightly toward the interstitial cavity in the perovskite structure. The W−S bond lengths (2.20 and 2.50 Å) are much larger than the W−O ones (2.00 and 2.22 Å). Furthermore, the W−S−W chain is significantly distorted with an angle of about 131°. S doping creates three additional localized states, one of which merges into the host VB while the other two are fully detached from it (Figure 4). All the S-related states are occupied and strongly hybridized with the O 2p and W 5d orbitals. The VB edge shifts up slightly from the original position, whereas the highest occupied level moves upward significantly, by 1.16 eV, due to the much higher energy of S 3p compared to the O 2p orbitals. Unlike the isovalent-cationdoped case, an indirect band gap arises, with the highest occupied state locating at E0 point of the first Brillouin zone (see Figure 4a). The overall CB profile looks similar to the undoped case. The bottom of the CB is mainly composed of W 5d orbitals which hybridize with O 2p orbitals. However, the CBM shifts upward substantially, by 0.27 eV, with respect to the undoped case. This is likely due to the pressure induced by

Figure 4. (a) Band structure and (b) DOS of substitutional S-doped WO3. TDOS of pure WO3 is also shown for comparison, and the shift of band edges with respect to the pure case is indicated. The horizontal dashed line in (a) represents the Fermi energy of S-doped WO3.

the larger S atom, leading to an upward shift of the antibonding states. The calculated HOMO−LUMO excitation energy (from the localized S 3p states to the CBM) energy gap is 2.21 eV, a value corresponding to the visible light absorption, due to the fact that now there are deep S states in the WO3 energy gap. In this respect, S doping not only reduces the absorption energy but shifts up the CBM significantly, two beneficial effects for PEC hydrogen production. This is a direct consequence of the strain induced by the presence of the bulky S atom in the supercell. In order to check that the strain is not overestimated, we performed a full optimization of the lattice constant. The result is that the a and b lattice parameters decrease by 0.02 and 0.18 Å, respectively, while c increases significantly, by 0.61 Å. As a result, the W−S−W chain becomes less distorted with an angle of 146°, leading to the larger W−W distances (by 0.40 Å). The HOMO−LUMO excitation energy becomes 2.32 eV, and the CBM shifts slightly downward. Thus, while the electronic structure is sensible to pressure effects, the whole picture is not very different from that emerging from calculations with fixed lattice constants. The reduction of the band gap due to the presence of highlying S 3p states, however, does not necessarily translate into an improved photocatalytic activity. In fact, the calculations show that the S 3p states are rather localized, with little dispersion of the corresponding energy level. It is generally believed that localization of the impurity states is detrimental for photocatalysis as localized states can act as recombination centers, although in some cases they may instead be effective trapping sites, lengthening the charge carriers lifetime, as exstensively discussed in the case of TiO2 in ref 54. 3.3. Substitutional Doping with Low-Valence Cations (Ti, Zr, Hf). Doping by Ti, Zr, and Hf has been investigated by substituting one W atom per unit cell (12.5% concentration). The main reason for choosing these elements is that they have less contracted d orbitals and larger ionic radii compared to W, which could result in a shift of the CBM to higher energies. In fact, a shorter metal−oxygen bond length can increase the covalent interaction between W and O, resulting in an upward 8904

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slightly shift up the CB edge but introduces new unoccupied gap states which may act as recombination centers. The ionic radius of Zr4+ ion is 0.79 Å; thus, replacing W with Zr induces larger atomic relaxation than for the case of Ti. Indeed, four O neighbors to Zr move outward by 0.10, 0.11, 0.38, and 0.38 Å while the remaining two move inward by 0.10 Å, respectively (Table 1). Overall, the O−Zr−O bond lengths are more symmetric and the ZrO6 octahedron is less distorted. The ground state of Zr-doped WO3 is also triplet, with the singlet state 1.56 eV higher in energy. The fundamental band gap becomes 3.35 eV, 0.25 eV larger than the undoped case (Figure 5b). Also, here two unoccupied O 2p states are found in the gap region; they are associated with two O neighbors to Zr. The Zr orbitals have no significant effect on the VB edge, and the VBM remains at the same position as for undoped WO3. The CBM shifts upward by 0.26 eV in virtue of the higher energy level of Zr 4d orbitals than W 5d orbitals and of the compression effect. The shift of CBM is more pronounced than for Ti, consistent with the larger size of the Zr dopant. Replacing W with Hf (Hf4+, 0.78 Å) also induces local atomic relaxation. Four O neighbors move outward (0.08, 0.09, 0.35, and 0.35 Å) while the other two move inward (0.12 Å), reducing the distortion of the HfO6 octahedron (Table 1). The electronic structure is reminiscent of that of Zr-doped WO3. The band gap at Γ increases to 3.34 eV due to a shift of the CBM by 0.23 eV for the reasons discussed above and more detailed in ref 53 (the VBM remains practically at the original position) (Figure 5c). Given the large ionic radius of Hf, we also performed a full optimization of the lattice constants. As a result, the a, b, and c lattice parameters increase by 0.08, 0.15, and 0.05 Å, respectively, and the Hf−O−W angle along all directions increases slightly, leading to a less distorted Hf−O− W chain. However, no major difference is observed on the band gap and band edge positions. Since there are two nonequivalent W atoms in the WO3 unit cell, we also considered the possibility to substitute a different W atom with Hf. In this case, the calculated band gap is 3.31 eV, and both the VB and CB edge shifts are identical to what found for the other site. Therefore, substitution of two nonequivalent W sites leads to identical effects. We also replaced one W atom with Hf in a larger supercell of 64 atoms, corresponding to a dopant concentration of 6.25%. The calculated band gap is 3.24 eV. The CBM shifts upward by about 0.13 eV, and the top of the VB does not move. Therefore, Hf substitution for W induces a moderate upshift of the CB edge which depends on the dopant concentration. This shift was not previously observed by using standard DFT calculations33 and can be observed only when an appropriated description of the band gap is performed. A detailed analysis of the projected DOS (PDOS) of HfxW1−xO3 allows us to identify three groups of O atoms, according to their positions: (1) the O atoms in the xy-plane that do not contain Hf (see yellow spheres in Figure 1b); (2) the O atoms in the xz-plane that do not contain Hf (see blue spheres in Figure 1b); (3) all the other O atoms (small red spheres in Figure 1b). The top of the VB is composed only of O 2p orbitals of group (1) since these are the least affected by Hf being the distance between xy-planes the largest (lattice parameter c > b > a). Note that the VBM is unchanged with respect to pure WO3. The 2p orbitals of group (2) O atoms start at about 0.1 eV below the VBM while those of group (3) start at about 0.25 eV below the VBM. Correspondingly, the bottom of the CB is basically made up of W 5d orbitals in the xy-plane of group (1) O atoms because they are also the least

shift of the antibonding empty states with corresponding shift of the CBM. However, replacing the W atom (6s25d4) with Ti, Zr, or Hf with only four valence electrons [(n + 1)s2nd2] introduces two holes in the O 2p states, an effect which is usually compensated by creation of an oxygen vacancy (VO). This has been explicitly considered for the case of Hf, and indeed, we found a very easy formation of one VO per Hf atom (see below; Hf doping has been already reported in a separate study53 and is discussed here for comparative purposes or to add information that has not been reported previously). Before addressing the electronic structure of the system containing both Hf and VO defect centers, however, we discuss the simple effect of replacing W with Ti, Zr, and Hf without vacancy formation. Ti has a slightly larger ionic radius (Ti4+, 0.68 Å) than W (W6+, 0.62 Å), and its presence results in two holes in the O 2p valence band. The singlet state is 1.41 eV higher in energy than the triplet ground state. In the triplet configuration (ground state) three O neighbors to Ti along the +x, +y, and +z directions move inward by 0.12, 0.18, and 0.15 Å, while the other three O neighbors along the −x, −y, and −z directions move outward by 0.07, 0.22, and 0.20 Å, respectively (Table 1). Thus, the asymmetries of the bond lengths increase along the x direction but decrease along y and z directions. Except for two unoccupied states in the spin-down set, the overall band structure remains similar to undoped WO3. The fundamental band gap is 3.22 eV, i.e., 0.12 eV larger than the undoped material. The top of the VB arises mainly from the O 2p orbitals and there is no shift of VBM (Figure 5a). The two

Figure 5. Band structure and DOS of (a) Ti-doped WO3, (b) Zrdoped WO3, and (c) Hf-doped WO3. TDOS of the pure WO3 is also shown for comparison and the shift of band edges with respect to the pure case is indicated.

holes in the spin-down bands are localized on two O 2p states (neighboring to the Ti impurity) slightly hybridized with W and Ti states. As expected, the CBM shifts up by 0.12 eV. The bottom of the CB is composed of W 5d orbitals with some hybridization with the O 2p states. Thus, substitutional Ti can 8905

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affected being the farthest apart from the Hf impurity (Figure 1c). Thus, both the top of the VB and the bottom of the CB show an unexpected “plane” effect according to their constituents. Having discussed the effect of replacing a W atom with Hf, we consider now the simultaneous presence of Hf and a compensating defect, i.e., an oxygen vacancy (Hf + VO, HfxW1−xO3−x). The nature of the O vacancy in WO3 has been recently addressed using the same computational approach adopted here.36 Basically, a VO center introduces new doubly occupied states high in the gap whose nature depends on the vacancy concentration and the crystallographic direction where the defect is created. In Hf-doped WO3 we have considered two kinds of VO, near (adjacent) or far from the Hf impurity. For each case, a VO defect has been created along the x-, y-, and zdirections. First, we analyze the case with one neighbor VO center along the x-chain. The VO formation energy computed with respect to 1/2O2 is only 0.14 eV, much smaller than that computed for undoped WO3 (more than 5 eV),36 indicating that doping with Hf indeed facilitates the creation of oxygen vacancies. During the atomic relaxation the O atom initially along z-chain moves toward the VO site, and the corresponding z-chain further distorts with the W−O−W angle decreasing from about 161° to 124° (Figure 6a). As shown in Figures 6b and 6c, the two holes created by the low-valent Hf impurity are compensated by the two extra electrons associated with VO, so that no unoccupied gap state remains. Unlike the Hf-doped case, here the calculated band gap (2.93 eV) is smaller than the undoped one. Both the VBM and CBM shift upward substantially with respect to the undoped case, by 0.48 and 0.31 eV, respectively (Table 2). We also created a VO center adjacent to Hf along the x-chain in a 64-atom supercell, corresponding to a lower vacancy concentration of about 2%. Also, in this case the VBM and the CBM shift upward by 0.57 and 0.20 eV, respectively. This indicates that the upshift of the CBM is due to the simultaneous presence of the Hf and VO defect and that the changes in band structure are only moderately affected by the defects concentration. When a neighbor VO center is created along the y- or zchains, the atomic relaxation is less pronounced than for the xchain (see Table S1 in Supporting Information). This is another manifestation of the anisotropic nature of reduced WO3−x.36 For the y-chain the calculated band gap is 2.71 eV with VBM and CBM shifting upward by 0.50 and 0.10 eV, respectively (Table 2). In this case the VO formation energy is slightly negative (−0.15 eV, exothermic process). For the zchain the band gap is only 2.26 eV, i.e., 0.84 eV smaller than the undoped case. This large band gap reduction arises from the opposite shifts of the VBM and CBM: the VBM shifts upward by 0.58 eV, whereas the CBM shifts downward by 0.26 eV (Table 2). Also, in this case the VO formation energy is negative, −0.41 eV, suggesting the spontaneous generation of O vacancies under equilibrium conditions. These conclusions do not change significantly if the Hf dopant and the VO defect are sufficiently distant that their interaction can be considered negligible. For the x-chain case, the VBM and CBM shift upward by 0.63 and 0.24 eV (Table 2), respectively, resulting in a narrower band gap of 2.71 eV. The VO formation energy is slightly negative, −0.06 eV. For the far y-chain, there is a relatively large atomic relaxation around VO, inducing distortion and tilting of the corresponding WO6 octahedron. Differently from the previous cases, the band gap, 3.33 eV, increases slightly with respect to pure WO3. This is

Figure 6. (a) Schematic atomic relaxation in HfxW1−xO3−x with neighboring Hf impurity and O vacancy along the x-direction. (b) Band structure and (c) DOS of Hf-doped WO3 in the presence of an O vacancy. TDOS of the pure WO3 is also shown for comparison and the shift of band edges with respect to the pure case is indicated.

Table 2. Formation Energy (Ef) for HfxW1−xO3−x with O Vacancy at Different Sites and Shifts of the VBM and CBM in HfxW1−xO3 and HfxW1−xO3−x with Respect to Pure WO3

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structure

Ef (eV)

VBM shift (eV)

CBM shift (eV)

HfxW1−xO3 VO along x-chain (near Hf) VO along y-chain (near Hf) VO along z-chain (near Hf) VO along x-chain (far from Hf) VO along y-chain (far from Hf) VO along z-chain (far from Hf)

0.14 −0.15 −0.41 −0.06 0.35 0.12

0 +0.48 +0.50 +0.58 +0.63 +0.53 +0.60

+0.23 +0.31 +0.10 −0.26 +0.24 +0.76 +0.47

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LUMO energy gap is only 2.22 eV with both the HOMO and LUMO states shifting upward significantly, by 1.56 and 0.83 eV, respectively. The atomic relaxation is very large, with a displacement of one O atom toward the vacancy site. To test the role of defects concentration, we also create one VO along z- or x-chain in the 64-atom supercell. For the z-chain VO case, the CBM falls by 0.21 eV, whereas the HOMO shifts upward by 0.78 eV (this is similar to what found for the HfxW1−xO3−x case). For the x-chain VO case, both the HOMO and LUMO shift upward by 0.57 and 0.20 eV, respectively. In summary, the results for Hf and S codoping show some features which are similar to those observed for simple Hf-doping, with the additional feature that S-related states appear in the gap.

because both the CBM shift upward significantly, by 0.76 eV, more than the corresponding shift of VMB, 0.53 eV. Also in this case the VO formation energy is slightly negative, −0.35 eV. For the last case considered, VO along the z-chain and far apart from the Hf dopant, the atomic relaxation around VO is large and the calculated band gap, 2.96 eV, is reduced due to the upward shifts of the VBM and CBM by 0.60 and 0.47 eV, respectively. The creation of VO is almost thermoneutral, 0.12 eV (Table 2). This discussion shows that Hf-doping of WO3 (and presumably also Ti and Zr doping) will result in the spontaneous creation of compensating O vacancies. The low formation energies, close to zero or slightly negative, for O vacancies created near or far from Hf along the three crystallographic directions suggest that a statistical distribution of these centers will be present in the doped material. Most important, we observe a clear tendency to move the bottom of the CB and the top of the VB to higher energies, with reduction of the band gap for several of the considered defect configurations.53 3.4. Codoping (Hf + 2F and Hf + S). Besides doping with a single impurity, also codoping can be used to modify in a desired way the properties of oxides semiconductors. Here we briefly discuss two cases: Hf and F and Hf and S codoping. We have seen above that doping with Hf leads to the formation of O vacancies in the structure. There is another possible way to compensate for the incorporation of the low-valent Hf atom in the structure. For instance, since Hf introduces two holes, one can compensate charges by replacing two O2− with two F− ions. Notice that F-doping of semiconducting oxides, with direct replacement of lattice O with F, has been successfully done for TiO2.55,56 To check this, one W atom in the WO3 unit cell has been replaced by Hf, while two O atoms have been replaced by F. Also, here we considered the two F atoms adjacent or far away from Hf, with the first situation thermodynamically preferred. In both cases no gap state is present. When F is adjacent to Hf the calculated band gap is 3.14 eV, i.e., slightly larger than the undoped one. The VB edge remains almost at the original position because the F 2p states are deep in the VB and thus have little effect on the VB edge. Also, the CBM is only slightly shifted by 0.03 eV. Things are different when the F atoms are distant from the Hf impurity. In this case, the VBM and CBM shift upward by 0.55 and 0.17 eV, respectively, with reduction of the band gap by 0.38 eV with respect to the undoped case. This result corroborates the explanation of the changes in electronic structure discussed above for HfxW1−xO3−x. Notice, however, that the two situations where F is adjacent of far from the Hf impurity are energetically nonequivalent, with the adjacent case 2.05 eV more stable than the solution where Hf and F are far apart. The second case is that of Hf + S codoping. The previous discussion has shown that Hf substituting W in the lattice results in a shift of the CBM to higher energies and that this is further reinforced by the creation of O vacancies. S-doping narrows the band gap and shifts up the CB edge due to strain effects. In principle, codoping with both elements could result in a synergistic effect. To test this hypothesis, we replaced a W atom by Hf and one O neighbor along the z-chain by S (Hf + S). In order to take into account the charge compensation problem, we also generated an O vacancy (Hf + S + VO, 32atom supercell). The S 3p-related states are detached from the host VB. The fundamental band gap becomes 3.54 eV, 0.44 eV larger than that of undoped WO3. However, the HOMO−

4. CONCLUSIONS In this work we have investigated with first principles electronic structure calculations the effect of doping and codoping WO3 with various elements in order to improve its photoactivity in water splitting with solar light. The calculations have been performed at the DFT level using the hybrid B3LYP functional which properly describes the band gap. To make efficient use of the visible light, the semiconductor band gap should be smaller than 3.0 eV, and to facilitate both the reduction and oxidation of water by photoexited electrons and holes, it should match the H2O redox potential. This means that the CB edge should be higher than the reduction potential of H+/H2, whereas the top of the VB should be lower than the oxidation potential of O2/H2O. The identification of dopants that induce these modifications is the objective of this paper. Doping with isovalent cations like Cr and Mo which replace W in the lattice has minor effects on the geometrical and electronic structure. Both elements induce a downshift of the bottom of the CB with small reduction of the band gap. This is not desirable as the CBM of the semiconducting oxide should be above (and not below) the hydrogen reduction potential. From this point of view anionic doping with S substituting one O atom in the lattice is more promising. The presence of sulfur introduces new states in the band gap associated with the S 3p levels. This should strongly reduce the energy required to excite electrons from these states into the CB. Being larger than O, the S-dopant has also the effect to induce a strain in the lattice which results in a shift of the CBM to higher energies. Both effects go in the desired direction, but one problem remains due to the rather localized nature of the new states which might accelerate the charge carrier recombination rate. We tested also the possibility of doping WO3 with atoms with lower valence than W and in particular Ti, Zr, and Hf. In all the three cases two holes are formed in the O 2p states which can be compensated by the creation of O vacancies. The most interesting aspect is that Ti, Zr, and Hf have a slightly larger atomic radius than W, and their inclusion in the lattice induces a strain similar to that observed for S and an upward shift of the edge of the CB. However, in absence of other defects, the shift in CBM reflects only in an undesired increase of the band gap. On the contrary, the concomitant presence of dopant and O vacancies changes the situation. This has been studied in detail for Hf. The O vacancy formation becomes energetically favorable, with the important consequence that the excess electrons associated with the VO center saturate the holes present in the O 2p shell adjacent to Hf (Figure 7). This causes an upshift of the top of the VBM that contributes to reduce the gap (Figure 7). The combined effect of the Hf substitutional dopant and the O vacancy is thus to shift CBM to 8907

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by 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, characterization and testing”, and by Regione Lombardia and CILEA Consortium, through a LISA Initiative (Laboratory for Interdisciplinary Advanced Simulation).

Figure 7. Schematic representation of the band edges and impurity gap states in (a) WO3, (b) WO3−x, (c) HfxW1−xO3, and (d) HfxW1−xO3−x. The horizontal dashed lines represent the top of the valence band and the bottom of the conduction band of pure WO3.

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higher energies but at the same time to reduce the gap. These are both desirable to obtain a good photocatalyst for water splitting. We propose a rationalization for these findings related to how the pseudolayered WO3 material responds to the presence of an impurity by differentiating the electronic contribution to the VB and CB of atoms lying in different layers. These effects are long-range and involve entire layers of the material which is the reason why they affect the delocalized electronic band states and do not create localized states at or near the impurity. An analogous result to Hf + VO can be obtained for some special configurations of Hf and F codoping. In fact, if one Hf and two F substitutional impurities are introduced in the supercell, the low-valent Hf is compensated by two F− ions without the need to create an O vacancy. The last case considered, combining Hf + VO and substitutional S, which are the two most promising situations identified in this work, confirms the general trends discussed above. To summarize, we have reported results of state-of-the-art first principle calculations with the objective to derive general concepts and the hope to stimulate the synthesis of new materials with improved properties. Our conclusion is that to truly modify the band edges of an oxide system and not just introduce impurity states in the gap, heavy doping with a foreign metal element or even mixing with another oxide system is required. It is clear however that the synthetic step can introduce various problems like phase segregation of different oxides, low solubility of dopant species, presence of interstitials instead of substitutional impurities, defects diffusion to surfaces and grain boundaries, structural instabilities, etc. Therefore, all the results discussed need to be validated by direct tests with materials prepared via efficient synthetic routes. Nevertheless, the examples reported and the analyses presented provide a general guidance to identify the criteria which can eventually result in successful attempts to catalyze water splitting using solar light.

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ASSOCIATED CONTENT

S Supporting Information *

Charge density plots and additional information on bond length details. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 8908

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