Effect of Alkali Metals Interstitial Doping on Structural and Electronic

Article pubs.acs.org/JPCC

Effect of Alkali Metals Interstitial Doping on Structural and Electronic Properties of WO3 Sergio Tosoni, Cristiana Di Valentin,* and Gianfranco Pacchioni Dipartimento di Scienza dei Materiali, Università di Milano Bicocca, via Roberto Cozzi 55, 20125 Milano, Italy ABSTRACT: The band gap engineering of semiconducting oxide to shift the light absorption edge to lower energies in the visible spectrum is often achieved by doping the bulk material. Intercalation of atomic and small molecular species in the cavities of WO3 is a viable approach to introduce foreign elements in the lattice (Mi, Q.; Ping, Y.; Li, Y.; Cao, B.; Brunschwig, B. S.; Khalifah, P. G.; Galli, G. A.; Gray, H. B.; Lewis, N. S.; J. Am. Chem. Soc. 2012, 134, 18318). In this work we show that introduction of alkali metal cations in room-temperature (RT) monoclinic WO3 interstices causes an anisotropic distortion of the lattice parameters (b/c ratio is smaller than that of the pure bulk value) which is the major cause of a considerable band gap reduction (about 0.5 eV). Analogous intercalation of neutral atoms (i.e., Ne and Xe) induces only a tiny b/c ratio variation in the opposite direction with no effective band gap reduction. Structural differences in WO3 (e.g., from RT monoclinic to cubic phase WO3) are known to largely change the band gap value. We show that the lattice distortions caused by small amounts of interstitial alkali metal atoms, although not large, induce considerable variations in the band gap. metallic. These materials are known as “tungsten bronzes”. For this reason, in our model study, we keep the alkali metal concentration very low, much lower than that typical of tungsten bronzes, and allow for free structural relaxation of lattice parameters and internal coordinates. We observe that room-temperature (RT)-monoclinic WO3 has a remarkable structural flexibility which allows it to host, in the interstitial cavities, even large cations, such as Rb+ or Cs+, with relatively small increase of the cell volume (3.2% in the case of Cs+). The effect of the interstitial doping on the band gap is remarkable, with a reduction going from 0.4 eV in the case of Li+ to 0.6 eV for Cs+. A point charge +1 inserted in the cavity, representing purely electrostatic effects, induces a band gap reduction of 0.2 eV only, suggesting that the distortion of the lattice plays indeed a role in reducing the band gap. Surprisingly, however, the insertion in the lattice of bulky neutral atoms, such as Xe, induces an increase of the cell volume comparable to that observed for Cs+ but leaves the band gap almost unaltered. We rationalize this different effect on the electronic properties by noting that the structural distortion caused by neutral dopants is isomorphic in space, while positive dopants induce a more pronounced elongation along the (001) direction (lattice parameter c). Both structural and electronic properties were computed by using a hybrid exchange-correlation functional (B3LYP7,8), since this approach is more appropriate than the use of standard DFT functionals. For the specific case of tungsten oxides, this choice is corroborated by a recent work where it

1. INTRODUCTION In the past few years, electronic structure modifications which improve the visible light absorption of semiconducting oxides, such as TiO2 and WO3, have been the subject of an intense research with the final goal of a more efficient harvesting of solar light for photochemical and photocatalytic applications.1 In particular, WO3 is widely used in tandem or Z-scheme photocatalysts for the O2 evolution in water splitting.2 Molecular nitrogen intercalation into WO3 was reported to form stable clathrates with a beneficial effect on the absorption properties of WO3, whose absorption threshold is reduced by about 0.8 eV.3 However, the microscopic factors causing this strong band gap reduction are still under debate. Density functional theory (DFT) calculations indicate a much lower band gap reduction (0.2 eV) when one N2 molecule is intercalated in a γ-monoclinic WO3 unit cell.4 Only very large concentrations of N2 (xN2/WO3 with x = 0.125) are found to induce a band gap lowering of 0.7 eV; this could possibly exist only in small portions of the material.4 In general, these observations would suggest that the insertion of atomic or small molecular species into the WO3 interstitial cavities is a promising approach to tailor or engineer the oxide electronic structure, besides the more popular substitutional doping.5,6 Thus, in the present study we investigate by means of DFT calculations the effect of the introduction of alkali metal ions at low concentration (Mx/ WO3 with M = Li+, Na+, K+, Cs+ and x = 0.03) as interstitial dopants in the oxide bulk material. It is well-known that alkali metal atoms, when present in a more abundant concentration (x > 0.1), fill the A site of the ABO3 peroxide structure of WO3, deeply modifying both the structure and the electronic properties of the material, which upon doping becomes © 2014 American Chemical Society

Received: December 17, 2013 Revised: January 15, 2014 Published: January 16, 2014 3000

dx.doi.org/10.1021/jp4123387 | J. Phys. Chem. C 2014, 118, 3000−3006

The Journal of Physical Chemistry C

Article

Figure 1. Top: ball and stick representation of cubic (left) and RT γ-monoclinic (right) WO3. Bottom: corresponding band structures.

Table 1. Ionic or Atomic Radius from Experimental Data, Band Gap Values at Γ, Percentage Volume Increase, Optimized Lattice Parameters, and W−O−W Angles for Pure and Doped RT Monoclinic WO3 W−O−W angle (deg)

lattice parameters (Å and deg)a ionic/atomic radius (Å)b WO3 PC/WO3 H+/WO3 Li*/WO3d Li+/WO3 Na*/WO3d Na+/WO3 K*/WO3d K+/WO3 Ne/WO3 Rb*/WO3d Rb+/WO3 Cs*/WO3d Cs+/WO3 Xe/WO3

0.92 1.18 1.51 1.54 1.61 1.74 2.16

band gap in Γ (eV) 3.13 2.93 2.97 2.65 2.72 2.58 2.65 2.54 2.61 3.01 2.53 2.60 2.44 2.51 3.06

ΔV %c

a

b

c

α

β

γ

min

max

−0.4

7.44 7.44 7.44 7.46

7.73 7.73 7.73 7.62

7.91 7.91 7.91 7.96

90.0 90.0 90.0 90.0

90.2 90.2 90.2 90.3

90.0 90.0 90.0 90.0

165 165 165 157

173 173 173 174

0.2

7.48

7.63

7.98

90.0

90.2

90.0

161

175

1.3

7.50

7.67

8.00

90.0

90.0

90.1

168

180

1.0 1.9

7.47 7.51

7.77 7.68

7.91 8.03

90.0 90.0

90.1 90.0

90.1 90.1

166 169

177 180

3.2

7.51

7.64

8.17

90.0

90.0

90.1

169

180

2.5

7.49

7.81

7.96

90.0

90.0

90.3

165

179

Note that the optimized lattice parameters have been obtained after full relaxation of the 2 × 2 × 1 supercell model. bHandbook of Chemistry and Physics 2010−2011; CRC Press: Boca Raton, FL. cThe %V variation is computed for the 2 × 2 × 1 supercell model used to perform the calculations. d M* stands for M+ + 1e−CB. a

2. COMPUTATIONAL DETAILS

was shown that fundamental gaps obtained with hybrid functional methods compare very well with those obtained with more sophisticated many-body G0W0 calculations.9 The paper is organized as follows: in section 3.1 we present the main structural and electronic features of our models of monoclinic WO3 with respect to the cubic phase. In section 3.2 the doping with a low concentration of either interstitial Na atoms or Na+ cations is presented in detail. In section 3.3, trends observed along the group of alkali metals (H+, Li+, Na+, K+, and Cs+) are discussed in order to rationalize the effect of the cation size on the band gap. Finally, in section 4 we present some general discussion and the main conclusions of the work.

The calculations were carried out within the linear combination of atomic orbitals (LCAO) approach with periodic boundary conditions combined with the B3LYP hybrid functional, as implemented in CRYSTAL09 code.10,11 The all-electron 8411(d1) Gaussian-type basis set was adopted for oxygen,12 while for tungsten we used an effective core potential (ECP) combined with a modified Hay−Wadt double-ζ basis set.13,14 The impurity atoms have been described with the following basis sets: Li, Na, and K with a triple-ζ basis set,15 Rb and Cs 3001



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Figure 2. Top: snapshot of the ball and stick representation of Na+ inserted in an interstitial cavity of RT γ-monoclinic WO3 (form a 2 × 2 × 1 supercell model). Band structures for RT γ-monoclinic WO3 doped with Na (center left), Na+ (center right), Na+ + F− (bottom left), Xe (bottom right).

with Hay−Wadt double-ζ basis set,16 Ne and Xe with doubleand triple-ζ basis set,17,18 and F with 7-31119 basis set. The lattice parameters of RT γ-monoclinic WO3 (space group P21/n) (Figure 1a) were obtained from the experiment20 and then fully optimized. For each dopant, both the lattice parameters and the internal coordinates of all the atoms have been fully optimized. 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 using a quasi-Newton algorithm with a BFGS Hessian updating scheme.21 For all the atoms, 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. Supercell models (128-atoms: 2 × 2 × 1) have been used to represent the single alkali metal atom doping in the interstices of RT monoclinic WO3, including one alkali metal atom every 32 WO3 units. This corresponds to an atomic concentrations of 0.8% which ensures negligible alkali−alkali interaction. In the case of cationic doping, lattice and ionic relaxation have been performed on a formally neutral structure, where the charge of

the cation is compensated by one electron in the conduction band of the solid. The electronic structure is then refined with a single point calculation where the positive charge of the cations has been compensated by a diffused background of charge in the supercell.

3. RESULTS 3.1. Monoclinic versus Cubic Bulk Systems. RT monoclinic WO3 is a slightly distorted cubic phase (Figure 1).22 It consists of a series of WO6 octahedrons in a cubic arrangement, with each oxygen atom being shared between two neighboring octahedra. This creates a series of hollow cubes (calculated length of 3.82 Å vs an experimental value of 3.81 Å) with a tungsten atom at each corner of the cube and an oxygen atom along each edge. In the RT monoclinic structure this arrangement is slightly distorted (Table 1), so that the W−O−W angle is no longer 180° but varies between 165° and 173° (experimental: 155− 165°).20 This angle is often referred to as the “tilt” angle, as it measures the tilt between the neighboring WO6 octahedra. The length of each side of the cube also varies between 3.72 and 3.95 Å (experimental: 3.65−3.85 Å). Finally, the β lattice parameter increases from 90° in the cubic phase to 90.2° (experimental: 90.9°)20 for the RT monoclinic phase. The 3002



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Figure 3. Snapshots of the ball and stick representations and band structures of Li+, K+, Rb+, and Cs+ inserted in an interstitial site of RT γmonoclinic WO3 (from a 2 × 2 × 1 supercell model).

in band gap is more likely due to the different W−O−W angle (165−173° in monoclinic WO3 and 180° in cubic WO3). One could speculate that dopants that induce structural relaxations that go in the direction of a cubic phase could also result in a band gap reduction. 3.2. Sodium Doping. The Na atom (or ion) has a size intermediate in the alkali metal group, Table 1, and has been chosen as a benchmark element. It was initially introduced as a neutral atom in an interstitial site of a 2 × 2 × 1 supercell model of RT monoclinic WO3 (Figure 2). The structure was fully relaxed and then used to perform a single point calculation of a positively charged Na+/WO3 system. The two situations are actually rather similar. This is because, in the neutral structure, the valence electron of the Na 3s state is transferred

reduction in symmetry translates into the fact that the monoclinic unit cell is a distorted 2 × 2 × 2 supercell (32 atoms) of the cubic unit cell (4 atoms). The RT monoclinic band structure (Figure 1, bottom) is characterized by a calculated Kohn−Sham band gap of 3.13 eV (B3LYP), which lies in the reported experimental range (using different techniques) of 2.5−3.2 eV. This is a direct Γtransition. In the case of the simple cubic phase, the B3LYP minimum band gap is 1.89 eV and corresponds to an indirect transition.22 Thus, there is a dramatic decrease of the band gap associated to the structural change from monoclinic to cubic despite the fact that the structural modification is small. Since the lattice constants of the two unit cells are not so different, and the change in the β angle is less than 1°, the drastic change 3003



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we have designed a new specific model and performed a calculation on a neutral supercell in the presence of an additional electron acceptor which can trap the electron donated by the Na atom. To this end, a F atom has been added in another interstitial site of the structure. This forms a stable F− species after trapping the extra electron in the conduction band. Thus, a Na+ and a F− species are formed, allowing us to study the effect of a Na+ ion in an overall neutral system. The band structure is very similar to the one obtained for the Na+ case, with a band gap of 2.56 eV, except for the presence of three localized F− 2p states in the middle of the gap (Figure 2). This fully corroborates our results and shows unambiguously that the band gap reduction in the presence of a Na+ ion in the cavity is not an artifact of the adopted model. Finally, we have analyzed the dependence of the band gap on the dopant concentration by considering a higher density of Na+ impurities. One Na cation was inserted in a unit cell model of RT monoclinic WO3 (32 atoms), corresponding to a 3.1% atomic doping. This results in a larger band gap reduction compared to what is found for the larger supercell model (0.8% doping). The resulting gap in Γ is 2.08 eV (high Na concentration) to be compared with 2.78 eV obtained for low Na loading (Table 1). However, as we mentioned in the Introduction, high Na concentrations result in major structural and electronic changes and in the formation of tungsten bronzes, not considered here. 3.3. Group 1A Elements. As a next step, we investigated the dependence of both structural and electronic effects on the size of the impurity atom by considering other members of the alkali metal group. Since the test calculations described above show the reliability of the approach, consisting in the relaxation of the neutral supercell model, followed by a single point calculation on the charged system, we analyze positive ions spanning from Li+ to Cs+ (Figure 3 and Table 1). In general, there is a clear trend correlating the ionic radius of the dopant to the change in the lattice parameters and the band gap. Li+ induces a small contraction of the supercell volume (−0.4%) and a band gap reduction of 0.41 eV. As shown in Figure 3, Li is the only dopant which moves from the center of the interstitial cavity and preferentially interacts with the oxygen ions on a corner of the hosting distorted cubic cavity. As reported in the previous section, upon doping with Na the cell volume remains almost unaltered, while the band gap reduces to 2.65 eV. For dopants of growing radius, we observe a small increase of the cell volume (1.3% for K+, 1.9% for Rb+, and 3.2% for Cs+) and a corresponding similar reduction of the band gap (2.61 eV for K+, 2.60 eV for Rb+, and 2.51 eV for Cs+). The lattice distortion appears like a transition from the monoclinic to the tetragonal phase, where the β angle gets closer to 90° as the radius of the dopant increases.23 The a lattice parameter remains almost constant, while b shrinks and c shows a significant increase. In order to distinguish between the structural and electrostatic contribution to the band gap reduction, we have considered a simplified model system where one H+ ion is set at the center of the interstitial cavity of WO3 in a 2 × 2 × 1 supercell (Table 1). No geometry relaxation was allowed, and this can be seen as a bulk WO3 crystal where the dopant consists of a positive charge with no steric effects. Notice that this is a totally fictitious model since in reality the proton will bind to one of the O ions of the lattice, with formation of an OH group. The band gap of this artificial system is 2.97 eV, which accounts only partially (exactly for 0.16 eV) for the

to the lattice W ions in the lowest conduction band (CB) state, resulting in a Na+ ion + 1e−CB pair. In real systems, at low cation concentration, the extra electron can be trapped at existing defects. The volume of the 2 × 2 × 1 supercell is almost preserved after doping, with a very small increase of 0.2% (Table 1). No phase transition is observed. However, the lattice parameter b is shortened by 0.1 Å, while c is elongated by 0.07 Å. The W−O−W bond angles are now spread on a larger interval, from 161° to 175°, compared to 165−173° for the undoped structure. Therefore, we may conclude that the structural effects induced by the insertion of Na impurities in the interstitial lattice positions are not pronounced, indicating that RT monoclinic WO3 can easily accommodate them without major structural modifications. As far as the electronic effects are concerned, it is interesting to observe that the highest occupied molecular orbital−lowest unoccupied molecular orbital (HOMO−LUMO) band gap in Γ is significantly reduced upon Na insertion, from 3.13 to 2.65 eV in the case of Na+ and to 2.58 eV in the case of Na+ + 1eCB. Beside the change in band gap (in both cases) and the change in Fermi level (only for the neutral Na case), the band structure maintains the same overall structure as that of bulk RT monoclinic WO3 (compare Figures 1 and 2). In particular, no defect states appear into the band gap. In order to better understand our findings, we have performed some additional calculations. First we wanted to verify whether the structural distortions induced by the impurity have any effect on the band structure. This was done in two ways: (a) by performing a total energy and band structure calculation on the distorted geometry after removal of the Na+ impurity; (b) by introducing a neutral impurity with a size larger than that of Na+ (ionic radius 1.16 Å). We used a Xe atom, with a van der Waals radius of 2.16 Å, since this in principle should induce a larger structural distortion compared to Na+. The direct band gap at Γ obtained on the WO3 structure as deformed by Na+ (but without the interstitial ion, case a) is of 2.61 eV, very close to that observed in presence of the cationic dopant (2.65 eV), proving the importance of the structural rearrangement induced by the doping. The question is whether the distortion caused by the Na+ ion is special or any bulky impurity can analogously work. The answer is given by the analysis of the Xe incorporation, case b. Here the cell volume increases by 2.5%, with the a, b, and c lattice parameters all expanded by less than 0.08 Å, Table 1. These structural changes lead to a band gap of 3.06 eV, only 0.07 eV smaller than that in pure WO3. In the Xe case, the structural deformation does not lead to a band gap reduction such as in Na-doped WO3. The reasons for this different behavior will be further discussed and rationalized in section 4. Note that for Xe-doped WO3 new levels appear in the gap at about 0.7 eV above the valence band maximum. These are rather flat bands which were proven by atom-projected density of states to be localized Xe 5p states and do not alter the positions of valence and conduction band edges. Therefore, this defect-related state in the gap cannot be considered as an alteration of the band gap. A major concern is to prove that the band gap reduction observed for Na+/WO3 is not the consequence of a spurious effect due to the use of a compensating background charge in the supercell. A partial proof is already given by the analogy with the neutral Na/WO3 calculation, but in this case the downward shift could be attributed to the occupation of the lowest conduction band state by the donated electron. Thus, 3004



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reduction observed upon doping with alkali cations. Similarly, if a bare single point charge (PC) is put in the middle of the cavity, keeping the bulk positions fixed, the result is almost the same with a band gap of 2.93 eV. On the basis of these test calculations with H+ and a PC, we can conclude that an electrostatic component in the band gap reduction induced by cationic doping exists; however, it is not the major cause, which should be attributed to some specific structural distortions as discussed in the following.

4. DISCUSSION AND CONCLUSIONS Recent work on the band gap engineering of WO3 has shown that interstitial doping with N2 molecules results in major changes in the band gap of this semiconducting oxide. Given the chemical inertness of the N2 molecule, one could speculate that the band gap decrease is connected with some geometrical distortion induced by the incorporated molecules. A similar argument could apply to alkali metal atoms included in the WO3 structure, with the main difference that alkali metals are easily ionized and can transfer one electron to defects present in the structure (vacancies, grain boundaries, etc.) or, in absence of acceptor states, to the conduction band of the material. Therefore, beside a geometrical effect, alkali metals are expected to have also an electronic effect. Indeed we found, by means of a hybrid density functional study, that alkali metal atoms included in interstitial sites of monoclinic WO3 form stable cations and transfer one electron to the conduction band (CB). A parallel band gap reduction is computed for all the alkali metals considered, which becomes larger for larger cations. This is almost independent of the nature of the added species, neutral or ionized alkali metal. However, a simple picture where the band gap reduction is barely due to the induced distortion on the WO3 lattice, i.e., to steric effects, is not sufficient, as proven by the case of Xe, where the insertion of a large, neutral impurity affects the lattice but leaves the band gap almost unaltered. Electrostatic effects, on the other hand, account for only a small fraction of the band gap reduction, as proved by the calculation on the model system with a point charge (or a proton) hosted in the middle of the undistorted cavity, and do not explain the differences in band gap observed along the alkali metal series from Li+ to Cs+. A more detailed analysis of the interplay between electrostatic and steric factors is required to fully understand the origin of the band gap reduction. To this end, in Figure 4, we plot the band gap value as a function of the percent variation from the pure bulk value of the ratio between the lattice parameters b and c. It is evident that Xe-doped WO3 displays almost the same b/c ratio of the bulk and consequently the same band gap of the pure material. The introduction of a neutral atom with a smaller radius than Xe, namely, Ne, presents a very similar behavior, with a tiny variation in the b/c ratio from the bulk value and a resulting band gap of 3.01 eV (Table 1). Na+-, Rb+-, and K+-doped systems show very similar b/c ratios and consequently similar band gap (between 2.53 and 2.65 eV) which are much lower than the bulk value. Cs+-doped WO3, displays the largest variation of b/c ratio from the bulk value and induces the largest band gap shift (more than 0.6 eV). An exception to this rule is represented by the Li+-doped system with a percent variation of b/c ratio similar to the other larger cations but a smaller band gap reduction (2.72 eV). We explain this as a local effect induced by the close Li−O interactions (see Figure 3).

Figure 4. Plot of the band gap change vs lattice deformation as measured by the ratio between b and c lattice parameters.

K+ and Ne dopants are further analyzed in the following because, although presenting very similar radius (1.52 Å for K+ and 1.54 Å for Ne) and causing a similar volume percent variation (+1.3% and +1.0%, respectively), they induce very different effects on the band gap of WO3, namely, a reduction of almost 0.5 eV for K+ and only 0.1 eV for Ne. The actual difference between the two doped systems lies in the fact that Ne induces a small stress on the lattice, with only a very tiny increase of a, b, and γ parameters and decrease of the β one. The b/c ration is thus only slightly increased with respect to the bulk (+0.47%). On the contrary, K+ acts in a more anisotropic form and in the opposite direction, causing a shrinking of b parameter and an elongation of c, with a resulting b/c ratio which differs from the bulk value by −1.81%. In Figure 5 we

Figure 5. Geometrical parameters projected on the (010) plane of the interstitial sites of K/WO3 (left), WO3 (center), and Ne/WO3 (right).

report some specific geometrical parameters in the (010) plane of the undoped, K+-doped, and Ne-doped interstitial cavities of WO3 in order to prove that the variation in the b/c ratio is the only relevant structural deformation in the lattice which truly differentiates the three systems. It is evident from the picture that the O−W−O angles are totally analogous in the three cases. Moreover, we wish to add that the mean oxygen-dopant and tungsten-dopant distances in K+-doped WO3 are practically the same as in Ne-doped WO3 (2.73 vs 2.75 Å and 3.40 vs 3.36 Å, respectively). Some of the results from this work are corroborated by the comparison with previous computational data from ref 4, where neutral rare gas atoms and molecules have been inserted in RT monoclinic WO3 at the same low concentration (0.8%). Very small changes in the band gap are reported (0.1−0.2 eV) for Xe and Ne4 in good agreement with the values reported in Table 1. It should be pointed out, however, that the authors of ref 4 3005



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(11) Dovesi, R.; Saunders, V. R.; Roetti, C.; Orlando, R.; ZicovichWilson, C. M.; Pascale, F.; Civalleri, B.; Doll, K.; Harrison, N. M.; Bush, I. J.; D’Arco, Ph.; Llunell, M. CRYSTAL09 User’s Manual; University of Torino: Torino, Italy, 2009. (12) Ruiz, E.; Llunell, M.; Alemany, P. Calculation of exchange coupling constants in solid state transition metal compounds using localized atomic orbital basis sets. J. Solid State Chem. 2003, 176, 400. (13) Hay, P. J.; Wadt, W. R. Abinitio effective core potentials for molecular calculationspotentials for K to Au including the outermost core orbitals. J. Chem. Phys. 1985, 82, 299. (14) Durand, P. H.; Barthelat, J. C. Theoretical method to determine atomic pseudopotentials for electronic-structure calculations of molecules and solids. Theor. Chim. Acta 1975, 38, 283. (15) Peintinger, M. F.; Vilela Oliveira, D.; Bredow, T. Consistent gaussian basis sets of triple-zeta valence with polarization quality for solid-state calculations. J. Comput. Chem. 2013, 34, 451. (16) Prencipe, M. Laurea Thesis, University of Torino, 1990; p 91. (17) Nada, R.; Nicholas, J. B.; McCarthy, M. I.; Hess, A. C. Basis sets for ab initio periodic Hartree−Fock studies of zeolite/adsorbate interactions: He, Ne, and Ar in silica sodalite. Int. J. Quantum Chem. 1996, 60, 809. (18) Towler, M. D. CRYSTAL Resources Page, 1995. http://www. tcm.phy.cam.ac.uk/∼mdt26/crystal.html (accessed March 1, 2013). (19) Nada, R.; Catlow, C. R. A.; Pisani, C.; Orlando, R. An ab-initio Hartree−Fock perturbed-cluster study of neutral defects in LiF. Modell. Simul. Mater. Sci. Eng. 1993, 1, 165. (20) Loopstra, B. O.; Rietveld, H. M. The structure of some alkalineearth metal uranates. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1969, B25, 1420. (21) Civalleri, B.; D’Arco, P.; Orlando, R.; Saunders, V. R.; Dovesi, R. Hartree−Fock geometry optimization of periodic systems with the CRYSTAL code. Chem. Phys. Lett. 2001, 348, 131. (22) Wang, F.; Di Valentin, C.; Pacchioni, G. Electronic and Structural Properties of WO3: A Systematic Hybrid DFT Study. J. Phys. Chem. C 2011, 115, 8345. (23) Analogously, we have found that if a K+ ion is inserted into a cubic model which is then fully relaxed, a transition from cubic to tetragonal phase is observed leading to almost the same lattice parameters as when starting from a monoclinic model. The resulting band gap from the two different approaches is also very similar (within 0.1 eV).

report as a band gap reduction what indeed is a localized gap state due to the 5p orbitals from Xe. Once this is taken into account, their data are in good agreement with Table 1, in spite of the different computational approaches adopted in ref 4 and in the present work. In summary, the presence of a low amount of single alkali cations in the interstitial cavities of the WO3 matrix has the effect to induce a band bending which lowers the position of the conduction band states compared to the valence band ones, with a consequent reduction of the band gap. Although not particularly large, from 13% to 19% depending on the radius of the dopant, this effect is clear and measurable. We have reported evidence that it is associated to small but significant lattice distortions, in particular to a change in the ratio of b and c lattice constants.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank Lorenzo Ferraro and Robertson Burgess for technical help. This work has been supported by the Italian MIUR through the FIRB Project RBAP115AYN “Oxides at the nanoscale: multifunctionality and applications”. The support of the COST Action CM1104 “Reducible oxide chemistry, structure and functions” is gratefully acknowledged. We thank Regione Lombardia for the computational resources through a LISA Initiative at CINECA supercomputing center (LI01p_VISFOTOCAT).

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REFERENCES

(1) Di Valentin, C.; Diebold, U.; Selloni, A. Doping and Functionalization of Semiconducting Metal oxides. Chem. Phys. 2007, 339, 44. (2) Kudo, A.; Miseki, Y. Heterogeneous photocatalyst materials for water splitting. Chem. Soc. Rev. 2009, 38, 253. (3) Mi, Q.; Ping, Y.; Li, Y.; Cao, B.; Brunschwig, B. S.; Khalifah, P. G.; Galli, G. A.; Gray, H. B.; Lewis, N. S. Thermally Stable N2Intercalated WO3 Photoanodes for Water Oxidation. J. Am. Chem. Soc. 2012, 134, 18318. (4) Ping, Y.; Li, Y.; Gygi, F.; Galli, G. Tungsten Oxide Clathrates for Water Oxidation: A First Principles Study. Chem. Mater. 2012, 24, 4252. (5) Wang, F.; Di Valentin, C.; Pacchioni, G. Doping of WO3 for Photocatalytic Water Splitting: Hints from Density Functional Theory. J. Phys. Chem. C 2012, 116, 8901. (6) Hameed, A.; Gondal, M. A.; Yamani, Z. H. Effect of transition metal doping on photocatalytic activity of WO3 for water splitting under laser illumination: role of 3d-orbitals. Catal. Commun. 2004, 5, 715−719. (7) Becke, A. D. Density-Functional Thermochemistry 0.3. The Role of exact Exchange. J. Chem. Phys. 1993, 98, 5648. (8) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron-Density. Phys. Rev. B 1988, 37, 785. (9) Ping, Y.; Rocca, D.; Galli, G. Optical properties of tungsten trioxide from first-principles calculations. Phys. Rev. B 2013, 87, 165203. (10) Dovesi, R.; Orlando, R.; Civalleri, B.; Roetti, C.; Saunders, V. R.; Zicovich-Wilson, C. M. CRYSTAL: a computational tool for the ab initio study of the electronic properties of crystals. Z. Kristallogr. 2005, 220, 571. 3006


Effect of Alkali Metals Interstitial Doping on Structural and Electronic

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