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Nb-doped CaO: an efficient electron donor system
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 J. Phys.: Condens. Matter 26 315004 (http://iopscience.iop.org/0953-8984/26/31/315004) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 150.108.161.71 This content was downloaded on 22/08/2014 at 22:52
Please note that terms and conditions apply.
Journal of Physics: Condensed Matter J. Phys.: Condens. Matter 26 (2014) 315004 (6pp)
doi:10.1088/0953-8984/26/31/315004
Nb-doped CaO: an efficient electron donor system Stefano Prada, Livia Giordano and Gianfranco Pacchioni Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, 20125 Milano, Italy E-mail: [email protected] Received 17 April 2014, revised 15 May 2014 Accepted for publication 21 May 2014 Published 17 June 2014 Abstract
Transition metal atoms incorporated into insulating materials (oxides in particular) can deeply modify their adsorption properties. In particular, charge transfer to adsorbed species can be induced by the presence of substitutional dopants, which introduce new electronic states in the band gap of the host crystal. Here we show, by means of density functional theory calculations, that Nb represents an excellent dopant to turn the rather inactive CaO(100) surface into an electron-rich support. The charge transfer ability of the doped material is shown by comparing the adsorption properties of the electronegative Au atoms on pure and Nb-doped CaO. While in the first case the CaO–Au bonding is relatively weak and the Au atom is essentially neutral, in the Nb-doped system a much stronger adhesion is found due to a net charge transfer from the Nb dopant and to the formation of a gold anion. This mechanism occurs also for Nb in high oxidation states. Nb is thus an excellent modifier of the calcium oxide properties. Keywords: oxide, doping, adsorption, charge transfer (Some figures may appear in colour only in the online journal)
1. Introduction
charge at the metal/oxide interface that reinforces the metal adhesion to the oxide surface, similarly to the case of gold on Introducing hetero-atoms in solid materials is a common pro- oxide ultrathin films on metal substrates [6–8]. A similar effect cedure to alter their electronic properties. In some cases, sta- has been predicted also for Nb-doped SrTiO3 [9]. Moreover, bilization of particular phases can also be obtained by specific it has been shown that Au clusters (e.g. Au20) exhibit different doping. Doping is widely used for instance to modify the prop- reactivity in CO oxidation depending on the thickness of the erties of oxide materials used in catalysis [1–4]. In the spe- MgO layer where they are supported [10, 11]. On 2–3-layer cific field of photocatalysis, huge efforts have been dedicated MgO films the clusters are supposed to form 2D islands due to the attempt to improve visible light absorption by doping to a charge transfer from the metal support to the adsorbed and co-doping semiconducting oxides with transition and non- gold, while on 10-layer MgO films the charge transfer is suptransition metal atoms. Recently, it has been shown that the pressed and the reactivity follows a different mechanism [11]. shape of supported gold particles can change completely due to Thus, electron transfer with the oxide support is relevant for the presence of dopants in the oxide support [5]. In particular, the chemical properties of supported nanoparticles. In the latter the presence of Mo impurities in CaO modifies the adsorption example, the charge transfer occurs because the electrons of properties of the Au metal, which grows as two-dimensional the metal support tunnel through the ultrathin film and are (2D) islands on Mo-doped CaO and as three-dimensional (3D) accumulated at the MgO/Au interface. particles on pure CaO. It has been shown by density functional Doping can induce a similar effect via a different mechatheory (DFT) calculations that the change in growth mode of nism. The dopant introduces new states in the oxide gap which Au is related to the occurrence of a charge transfer from the Mo act as donors towards the empty states of adsorbed species. dopant to gold [5]. The resulting negatively charged Au par- Clearly, the charge transfer is strongly dependent on the kind ticles adopt 2D structures due to the presence of extra negative of dopant and nature of the oxide. For instance, if one includes 0953-8984/14/315004+6$33.00
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J. Phys.: Condens. Matter 26 (2014) 315004
Cr in MgO, a parent system to Mo:CaO, no charge transfer takes place and no effect is observed on the growth mode of Au nanoparticles [12]. On the other hand, in the case of Mo:CaO O2 molecules can also become negatively charged and readily dissociate [13]. This is related to the different nature of the dopant and of its redox properties [12, 14]. This brief account shows that there is a great potential for doping simple oxides with transition metal (TM) ions and transforming rather inert supports into chemically active materials. DFT calculations have played a useful role to elucidate the nature of the interaction between the dopant, the oxide host matrix, and the adsorbed gold species [3–5, 12, 14]. The problem is complicated by the fact that heterovalent dopants facilitate the formation of intrinsic defects (e.g. cation or anion vacancies or their aggregates), which compensate for charge imbalance. Here we present a theoretical investigation of a potential transition metal dopant, Nb, which could be used to modify the properties of simple oxides e.g. CaO. This dopant has been introduced in CaO since its dimensions are such that a substitutional doping (Nb ion replacing a Ca ion) should be experimentally feasible. 2. Computational methods Figure 1. Projected density of states (DOS) of Nb2+ (top) and Mo2+
Periodic DFT calculations were performed with a plane–wave basis set (400 eV energy cutoff) and the projector-augmented wave (PAW) method [15], as implemented in the VASP code [16, 17]. The stability of Nb impurities in different oxidation states was investigated at DFT level (PBE functional [18]) with a (3 × 3 × 3) bulk supercell and (2 × 2 × 2) Monkhorst– Pack k-points mesh. For the description of the electronic properties of isolated substitutional Nb, a (2 × 2 × 2) supercell was used and the Brillouin zone was sampled with a (3 × 3 × 3) Monkhorst–Pack mesh. For Au adsorption, a (3 × 3) surface supercell of a five-layer thick oxide slab was employed, where one impurity Nb atom replaced one oxide Ca2+ cation. Au was adsorbed on one side of the slab. A dipole correction was included in order to eliminate the interaction among repeated slabs, separated by more than 10 Å. All atomic positions were optimized until atomic forces were less than 0.01 eV Å−1. In the case of slab calculations, the atoms of the oxide bottom layer were kept fixed. Magnetic moments of impurities have been evaluated from the projection of the spin density into atomic spheres, while atomic charges have been estimated using the Bader decomposition scheme [19]. Nb ions with a formal oxidation state larger than +2 (e.g. Nb3+ or Nb4+) replacing Ca2+ introduce a charge imbalance. This has been treated in two ways. In the first approach, the Nbn+ charge is balanced by introducing compensating defects, such as cationic vacancies (VCa), so that the entire system is charge neutral. This approach has been employed for bulk calculations to evaluate the thermodynamic stability of Nb ions in different charge states. In a second approach, we considered the isolated impurity and we used a uniform background of charge to compensate for the higher oxidation state of the Nb ion [20]. This latter approach has been used in the study of Au adsorption. These and previous results [21] show that the conclusions are not dependent on the approach followed.
(bottom) in bulk CaO. The two panels are aligned at the valence band edge of CaO. The red vertical line locates the highest occupied state.
For the treatment of the exchange–correlation term we adopted the pure DFT PBE functional [18], also used in previous studies on this topic [5, 12, 21]. While the absolute values of the Au adsorption energies depend on the choice of the functional used, recent work performed with hybrid functionals on Cr:MgO and Mo:CaO systems has shown [21] that the physics of the problem is correctly described also using this level of theory. For CaO we used the optimized lattice constant at PBE level 4.83 Å. 3. Electronic structure and thermodynamic stability of Nb dopants in bulk CaO We first considered an Nb ion that enters in CaO as a +2 species. The configuration assumed by Nb in the octahedral environment is high spin, (t2g α)3 (t2g β)0 (eg)0. The occupied component of the 4d t2g states is found at about 2.7 eV above the top of the valence band and is separated by only about 1 eV from the bottom of the conduction band, figure 1. The empty 4d states merge with the CaO conduction band. For comparison, we report in figure 1 also the PDOS of a Mo2+ substitutional dopant in CaO computed at the same level of theory. In this case the configuration is (t2g α)3 (t2g β)1 (eg)0. From the comparison of the two figures, it is apparent that the Nb 4d states are at a higher energy level, suggesting a stronger tendency to donate electrons compared to Mo. The fact that the Mo 4d states are at a lower energy level than the Nb ones is consistent with the increase in effective nuclear charge (Zeff) moving to the right of the periodic table. However, the situation described above refers to an ideal, perfect crystal where no other defects beyond the dopant are 2
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present. In reality, Nb as other TM ions can easily change its oxidation state, in particular assume higher oxidation states, transferring electrons to other acceptor levels associated to specific defects which are essential to maintain electroneutrality. For instance, in MgO Cr enters as Cr3+, and the extra positive charge is compensated by the formation of Mg vacancies, VMg [22–24]. This has been supported by optical and EPR spectroscopies which also indicate that the Cr3+ and the VMg centers are adjacent [25, 26]. An Mg vacancy in pure MgO (VMg center) results in two holes in the O 2p valence band, with formation of two O− species (triplet ground state [27, 28]). In the presence of Cr dopants, the hole centers associated with a VMg defect in MgO are quenched due to an electron transfer from the Cr 3d states. In particular, one VMg center compensates the formation of two Cr3+ ions in the lattice. A similar mechanism is expected also for Nb:CaO, opening a general question about the relative stability of the possible combinations of VCa centers and Nbn+ (n = 3–5) cations. To answer this question we compared the stability of two Nb atoms in the bulk (3 × 3 × 3) supercell by varying the number of cation vacancies. In this way, the system without vacancies corresponds to the Nb2+ oxidation state, while the system with nV cation vacancies creates two Nbn+ ions with n = 2 + nV. We have verified, by analyzing the PDOS on the Nb ions (not shown), that the holes generated by the cation vacancies are transferred to the Nb impurity for all considered oxidation states. The two Nb atoms are located as far as possible in the supercell, while the vacancies are adjacent to the impurity ions in the cases of Nb3+ and Nb4+. Notice that the Ca vacancy formation energy, Eform(VCa), is much lower in doped than in pure oxides [12]. The reason is that, as mentioned above, one electron is transferred from the d states of each Nb ion to the low lying states of O− ions around VCa, with a change of Nb oxidation state from +2 to +3 [12, 21]. This means that by introducing Nb dopants in CaO the number of cation vacancies is expected to increase significantly. To determine the relative stability of the different defective structures containing both Nb dopants in various oxidation states and cation vacancies, we have computed their Gibbs formation energy per unit of volume at pressure p and temperature T [29]:
Figure 2. Change in Gibbs formation energy for the different Nbn+ ions in CaO compensated by Ca vacancies (see text for details). The dependence on the oxygen partial pressure is also reported at T = 1000 K.
ΔμO(p,T) = μO − ½ E(O2) is computed with reference to half of the total energy of an oxygen molecule. The upper value for the oxygen chemical potential is determined by the condition in which oxygen molecules start to condensate, ΔμO = 0; the lower limit is set where metal particles start to crystallize, μM = EM. Using the condition of equilibrium ECaO − ECa < ΔμO, we obtain the following range of validity: −6.10 < ΔμO < 0. The calculated Gibbs formation energy with respect to the oxygen partial pressure is shown in figure 2 for T = 1000 K. We first note that the +2 oxidation state is not stable in the whole range of experimentally accessible oxygen chemical potential. This means that, if not kinetically impeded, Nb doping in CaO is always expected to produce compensating defects. In oxygen poor conditions, the +3 state is more favorable, while the +5 state only occurs in very oxidizing conditions, figure 2. According to our calculations, if we assume that Nb:CaO can be synthetized in similar conditions as previously studied TM:MO [12], by reactive deposition followed by annealing at T = 1000 K in a oxygen partial pressure of 5 × 10–7 mbar (about 5 × 10−10 atm [5, 12]), the most stable oxidation state is expected to be Nb4+. This means that an Nb impurity has already lost 4 of its 5 valence electrons to intrinsic defects that are generated during the thermal annealing to reach the thermodynamic ground state. The question to answer becomes are even dopants that are in such a high oxidation state still able to transfer electrons to species adsorbed on the surface of the material? This is discussed in the next section.
Gf ( T , p ) = [ G ( T , p, { nx } − ∑ nxμx ( T , px ) ] / V x
G is the Gibbs free energy of the solid with volume V; nx, μx and px are the number of particles in the solid, the chemical potential and the partial pressure in the gas phase of the species x, respectively. Using the condition of equilibrium, we can calculate the Gibbs formation energy change with respect to the doped defect-free oxide, ∆Gf, as the function of the oxygen chemical potential:
4. Adsorption properties of Au atoms adsorbed on the (100) surface of Nb-doped CaO The effect of doping on the properties of CaO has been investigated by adsorbing Au atoms on the CaO(100) surface. With an electron affinity of 2.3 eV, Au is an excellent candidate to form surface anions by trapping electrons from the Nb impurity. Nb has been placed in the third oxide layer, figure 3. This choice is driven by the objective to mimic a situation where the dopant lies in the bulk or in subsurface layers of the oxide, thus preventing the formation of a direct bond between
ΔGf = [ Gdef − Gst + n v ECaO − n vΔμO ( p, T ) ] / V
where Gdef and Gst are the Gibbs free energies of the defective and stoichiometric systems respectively and nV is the number of Ca vacancies. The change in the O2 chemical potential 3
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Figure 3. Optimized structures for Au adatoms adsorbed on the oxide surface in the presence of an impurity atom in the third oxide layer (from the surface); (a) Ca-top, (b) O-top and (c) hollow configurations. Ca, O, Nb and Au atoms are represented as light blue, red, black and yellow spheres, respectively. Table 1. Adsorption properties of Au adatoms adsorbed on pristine
and doped CaO surfaces: preferred adsorption site, adsorption energy with respect to gas phase Au (Eads), Au Bader charge (q). Values relative to the Mo:CaO case are also reported for comparison.
CaO Nb2+:CaO Nb3+:CaO Nb4+:CaO Mo2+:CaO (a) Mo3+:CaO (a) Mo4+:CaO (a) (a)
Adsorption site
Eads (Au), eV
q(Au), |e|
O-top hollow hollow hollow hollow hollow hollow
1.35 3.23 3.06 2.60 3.60 2.10 1.75
−0.33 −0.82 −0.79 −0.76 −0.80 −0.79 −0.75
From [21].
adsorbed gold and Nb. Experimentally, this can be achieved by growing a capping layer of un-doped oxide on top of an Nb:CaO film produced via reactive deposition. This method successfully prevents dopant segregation to the surface of Mo-doped CaO [12]. Three different Au adsorption sites have been considered, on-top of Ca, on-top of O and in hollow positions (figure 3). The adsorption properties in the most stable configuration are listed in table 1. As a reference, Au adsorption on pure CaO and on Mo-doped CaO is also reported. On pure CaO the preferred adsorption site is on-top of O with an adsorption energy of 1.35 eV. On the contrary, on Nb-doped CaO, Au prefers to occupy a hollow site, regardless of the oxidation state of the Nb impurity. The Bader charge goes from −0.3 |e| on CaO to about −0.8 |e| on Nb:CaO, table 1. By comparing Au adsorption on pure CaO and Nb:CaO one can observe that on the doped material the magnetic moment on Au is quenched. This, together with the Bader charge, indicates that Au has gained one extra electron, becoming Au−. This is further shown by the PDOS curves, which show that the Au 6s state is doubly occupied, figure 4. The analysis of the Nb2+ states reveals that the electron is transferred from the impurity, which assumes a (t2g2) (eg0) electronic configuration after the charge transfer (Nb3+). The charge transfer strongly reinforces the bond of Au with the surface, and the resulting binding energy, 3.23 eV, is more than twice that on pure CaO, 1.35 eV. Interestingly, the charge transfer occurs for all adsorption sites considered, which have similar binding energies. The less stable Ca-top configuration is 0.14 eV less stable than the hollow site (ground state), while starting from O-top an almost degenerate configuration has been found with the Au atoms slightly tilted with respect to
Figure 4. Density of States for Au adatoms on Nb:CaO in different initial oxidation states. The zero energy reference is set at the highest occupied state.
the surface normal, figure 3. In all cases, due to the negative charge on Au we observe a considerable polaronic distortion, where the lattice cations are attracted and the anions repelled from the adatom as can be observed in figure 3. Nb is able to donate charge to Au also when in an oxidation state larger than +2. In figure 4, the Au PDOS for adsorption on Nb3+:CaO indicate that the Au 6s state is doubly occupied, while Nb oxidizes to Nb4+ and assumes a (t2g1)(eg0) electronic configuration. The energy gain related to the charge transfer is still more than 1.5 eV (Eads = 3.06 eV). This reflects the different position of the Nb 4d levels for the two oxidation states, figure 4. Charging of gold is also possible for Nb4+:CaO which becomes Nb5+ and donates all its valence electrons either to other defects (e.g. Ca vacancies) or to adsorbed species (Au in this case). The electron transfer is net also in this case and the adsorption energy of Au is 2.60 eV, only slightly smaller than in previous cases. These results can be compared with those of Mo:CaO [21]. Also in this case a charge transfer occurs from the Mo 4d states to the Au adsorbate and the Au adsorption energy 4
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J. Phys.: Condens. Matter 26 (2014) 315004 2+
The support of the COST Action CM1104 ‘Reducible oxide chemistry, structure and functions’ is also gratefully acknowledged.
increases from 1.35 eV (pure CaO) to 3.60 eV (Mo ), 2.10 eV (Mo3+), and 1.75 eV (Mo4+). Therefore, the behavior is similar but with a significant difference. In Mo4+:CaO the energy gain is small (the Au adsorption energy, 1.75 eV, is only 0.40 eV larger than for Au adsorbed on pure CaO), while in Nb4+:CaO the adsorption energy is about twice compared to the un-doped material, table 1, indicating a stronger tendency of Nb to oxidize to a +5 oxidation state. The tendency of Nb to be a better donor than Mo when introduced in the lattice can be traced back to the higher position of the t2g occupied levels reported in figure 1. As already pointed out in our previous work, the donor ability correlates with the atomic ionization potential (IP) of the different dopants introduced in the lattice [21]. While Mo is already a good electron donor because of the relatively low IPs, compared for instance to 1st row TMs like Cr, Nb has even lower IPs and electron transfer is indeed found to be more favorable in this case. In particular, the ionization potentials (IP) of the Nb (Mo) ions for the oxidation states relevant in the doped system are: IP(TM2+) = 25.2 (27.1) eV, IP(TM3+) = 38.5 (46.4) eV, IP(TM4+) = 50.8 (54.5) eV [30].
References [1] McFarland E W and Metiu H 2013 Catalysis by doped oxides Chem. Rev. 113 4391–427 [2] Hegde M S, Madras G and Patil K C 2009 Noble metal ionic catalysts Acc. Chem. Res. 42 704–12 [3] Metiu H, Chrétien S, Hu Z, Li B and Sun X Y 2012 Chemistry of Lewis acid−base pairs on oxide surfaces J. Phys. Chem. C 116 10439–50 [4] Shapovalov V and Metiu H 2007 Catalysis by doped oxides: CO oxidation by AuxCe1−xO2 J. Catal. 245 205–14 [5] Shao X, Prada S, Giordano L, Pacchioni G, Nilius N and Freund H-J 2011 Tailoring the shape of metal Ad-particles by doping the oxide support Angew. Chem. Int. Ed. 50 11525–7 [6] Ricci D, Bongiorno A, Pacchioni G and Landman U 2006 Bonding trends and dimensionality crossover of gold nanoclusters on metal-supported MgO thin films Phys. Rev. Lett. 97 036106 [7] Sterrer M, Risse T, Heyde M, Rust H-P and Freund H-J 2007 Crossover from three-dimensional to two-dimensional geometries of Au nanostructures on thin MgO(001) films: a confirmation of theoretical predictions Phys. Rev. Lett. 98 206103 [8] Mammen N, Narasimhan S and de Gironcoli S 2011 Tuning the morphology of gold clusters by substrate doping J. Am. Chem. Soc. 133 2801–3 [9] Zhou M, Ping Feng Y and Zhang C 2012 Gold clusters on Nb-doped SrTiO3: effects of metal-insulator transition on heterogeneous Au nanocatalysis Phys. Chem. Chem. Phys. 14 9660–5 [10] Zhang C, Yoon B and Landman U 2007 Predicted oxidation of CO catalyzed by Au nanoclusters on a thin defect-free MgO film supported on a Mo(100) surface J. Am. Chem. Soc. 129 2228–9 [11] Harding C, Habibpour V, Kunz S, Farnbacher A N S, Heiz U, Yoon B and Landman U 2009 J. Am. Chem. Soc. 131 538–48 [12] Stavale F, Shao X, Nilius N, Freund H-J, Prada S, Giordano L and Pacchioni G 2012 Donor characteristics of transitionmetal-doped oxides: Cr-doped MgO versus Mo-doped CaO J. Am. Chem. Soc. 134 11380–3 [13] Cui Y, Shao X, Baldofski M, Sauer J, Nilius N and Freund H-J 2013 Adsorption, activation, and dissociation of oxygen on doped oxides Angew. Chem. Int. Ed. 52 11385–7 [14] Andersin J, Nevaòaota J, Honkala K and Häkkinen H 2013 The redox chemistry of gold with high-valence doped calcium oxide Angew. Chem. Int. Ed. 52 1424–7 [15] Blöchl P E 1994 Projector augmented-wave method Phys. Rev. B 50 17953–79 [16] Kresse G and Hafner J 1993 Ab initio molecular dynamics for liquid metals Phys. Rev. B 47 558–61 [17] Kresse G and Furthmüller J 1996 Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set Phys. Rev. B 54 11169–86 [18] Perdew J P, Burke K and Ernzerhof M 1996 Generalized gradient approximation made simple Phys. Rev. Lett. 77 3865–8 [19] Bader R F W 1991 A quantum theory of molecular structure and its applications Chem. Rev. 91 893–928 [20] Leslie M and Gillan M J 1985 The energy and elastic dipole tensor of defects in ionic crystals calculated by the supercell method J. Phys. C: Solid State Phys. 18 973–82
5. Conclusion The surface properties of inert oxides can be modified by introducing appropriate dopants in the bulk of the material. In particular, substitutional transition metal impurities in a wide bandgap material as CaO introduce filled states in the band gap from which electrons can be transferred to the adsorbed species. As a result, the doped oxide can act as an electron donor to electronegative species, like gold particles or admolecules. In this paper we have demonstrated by DFT calculations that Nb-doped CaO presents great potential in this respect since its electronic states are higher in energy compared to a parent system, Mo:CaO, which has already revealed to be an excellent electron donor. The thermodynamic analysis of the Nb dopant in different oxidation states, compensated by calcium vacancies, indicates that doping CaO should result in an easy cation vacancy formation and the stabilization of Nb3+ or Nb4+ in the oxide lattice. Our results show that also in such high oxidation states Nb impurities, placed in the inner oxide layers to prevent the direct interaction with the adsorbates, are still able to transfer one electron to Au adatoms. This effect is expected to change the growth mode and, likely, the chemical reactivity of gold nanoparticles adsorbed in this system. Similarly, electronegative molecules like O2 or NO2 can potentially become negatively charged on this surface and be involved in further reactions, thus changing the oxide surface reactivity compared to the un-doped oxide. Acknowledgments We thank Niklas Nilius and Hans-Joachim Freund for many useful discussions. This work has been supported by the Italian MIUR through the FIRB Project RBAP115AYN ‘Oxides at the nanoscale: multifunctionality and applications’. 5
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[26] Imbush G F, Schawlow A L, May A D and Sugano S 1965 Fluorescence of MgO: Cr3+ ions in noncubic sites Phys. Rev. 140A 830–8 [27] Ferrari A M and Pacchioni G 1995 Electronic structure of F and V centers on the MgO surface J. Phys. Chem. 99 17010–18 [28] Baranek P, Pinarello G, Pisani C and Dovesi R 2000 Ab initio study of the cation vacancy at the surface and in bulk MgO Phys. Chem. Chem. Phys. 2 3893–901 [29] Reuter K and Scheffler M 2001 Composition, structure, and stability of RuO2(110) as a function of oxygen pressure Phys. Rev. B 65 035406 [30] Lide D R (ed) 2010 CRC Handbook of Chemistry and Physics (Boca Raton, FL: CRC)
[21] Prada S, Giordano L and Pacchioni G 2013 Charging of gold atoms on doped MgO and CaO: identifying the key parameters by DFT calculations J. Phys. Chem. C 117 9943–51 [22] Schawlow A L 1962 Fine line spectra of chromium ions in crystals J. Appl. Phys. 33 395–8 [23] Larkin J P, Inbush G F and Dravnieks F 1973 Optical absorption in MgO: Cr3+ Phys. Rev. B 7 495–500 [24] Stavale F, Nilius N and Freund H J 2012 Cathodoluminescence of near-surface centres in Cr-doped MgO(001) thin films probed by scanning tunnelling microscopy New J. Phys. 14 033006 [25] Wertz Q E and Auzins P 1957 Crystal vacancy evidence from electron spin resonance Phys. Rev. 106 484–8
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Nb-doped CaO: an efficient electron donor system
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 J. Phys.: Condens. Matter 26 315004 (http://iopscience.iop.org/0953-8984/26/31/315004) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 150.108.161.71 This content was downloaded on 22/08/2014 at 22:52
Please note that terms and conditions apply.
Journal of Physics: Condensed Matter J. Phys.: Condens. Matter 26 (2014) 315004 (6pp)
doi:10.1088/0953-8984/26/31/315004
Nb-doped CaO: an efficient electron donor system Stefano Prada, Livia Giordano and Gianfranco Pacchioni Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, 20125 Milano, Italy E-mail: [email protected] Received 17 April 2014, revised 15 May 2014 Accepted for publication 21 May 2014 Published 17 June 2014 Abstract
Transition metal atoms incorporated into insulating materials (oxides in particular) can deeply modify their adsorption properties. In particular, charge transfer to adsorbed species can be induced by the presence of substitutional dopants, which introduce new electronic states in the band gap of the host crystal. Here we show, by means of density functional theory calculations, that Nb represents an excellent dopant to turn the rather inactive CaO(100) surface into an electron-rich support. The charge transfer ability of the doped material is shown by comparing the adsorption properties of the electronegative Au atoms on pure and Nb-doped CaO. While in the first case the CaO–Au bonding is relatively weak and the Au atom is essentially neutral, in the Nb-doped system a much stronger adhesion is found due to a net charge transfer from the Nb dopant and to the formation of a gold anion. This mechanism occurs also for Nb in high oxidation states. Nb is thus an excellent modifier of the calcium oxide properties. Keywords: oxide, doping, adsorption, charge transfer (Some figures may appear in colour only in the online journal)
1. Introduction
charge at the metal/oxide interface that reinforces the metal adhesion to the oxide surface, similarly to the case of gold on Introducing hetero-atoms in solid materials is a common pro- oxide ultrathin films on metal substrates [6–8]. A similar effect cedure to alter their electronic properties. In some cases, sta- has been predicted also for Nb-doped SrTiO3 [9]. Moreover, bilization of particular phases can also be obtained by specific it has been shown that Au clusters (e.g. Au20) exhibit different doping. Doping is widely used for instance to modify the prop- reactivity in CO oxidation depending on the thickness of the erties of oxide materials used in catalysis [1–4]. In the spe- MgO layer where they are supported [10, 11]. On 2–3-layer cific field of photocatalysis, huge efforts have been dedicated MgO films the clusters are supposed to form 2D islands due to the attempt to improve visible light absorption by doping to a charge transfer from the metal support to the adsorbed and co-doping semiconducting oxides with transition and non- gold, while on 10-layer MgO films the charge transfer is suptransition metal atoms. Recently, it has been shown that the pressed and the reactivity follows a different mechanism [11]. shape of supported gold particles can change completely due to Thus, electron transfer with the oxide support is relevant for the presence of dopants in the oxide support [5]. In particular, the chemical properties of supported nanoparticles. In the latter the presence of Mo impurities in CaO modifies the adsorption example, the charge transfer occurs because the electrons of properties of the Au metal, which grows as two-dimensional the metal support tunnel through the ultrathin film and are (2D) islands on Mo-doped CaO and as three-dimensional (3D) accumulated at the MgO/Au interface. particles on pure CaO. It has been shown by density functional Doping can induce a similar effect via a different mechatheory (DFT) calculations that the change in growth mode of nism. The dopant introduces new states in the oxide gap which Au is related to the occurrence of a charge transfer from the Mo act as donors towards the empty states of adsorbed species. dopant to gold [5]. The resulting negatively charged Au par- Clearly, the charge transfer is strongly dependent on the kind ticles adopt 2D structures due to the presence of extra negative of dopant and nature of the oxide. For instance, if one includes 0953-8984/14/315004+6$33.00
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Cr in MgO, a parent system to Mo:CaO, no charge transfer takes place and no effect is observed on the growth mode of Au nanoparticles [12]. On the other hand, in the case of Mo:CaO O2 molecules can also become negatively charged and readily dissociate [13]. This is related to the different nature of the dopant and of its redox properties [12, 14]. This brief account shows that there is a great potential for doping simple oxides with transition metal (TM) ions and transforming rather inert supports into chemically active materials. DFT calculations have played a useful role to elucidate the nature of the interaction between the dopant, the oxide host matrix, and the adsorbed gold species [3–5, 12, 14]. The problem is complicated by the fact that heterovalent dopants facilitate the formation of intrinsic defects (e.g. cation or anion vacancies or their aggregates), which compensate for charge imbalance. Here we present a theoretical investigation of a potential transition metal dopant, Nb, which could be used to modify the properties of simple oxides e.g. CaO. This dopant has been introduced in CaO since its dimensions are such that a substitutional doping (Nb ion replacing a Ca ion) should be experimentally feasible. 2. Computational methods Figure 1. Projected density of states (DOS) of Nb2+ (top) and Mo2+
Periodic DFT calculations were performed with a plane–wave basis set (400 eV energy cutoff) and the projector-augmented wave (PAW) method [15], as implemented in the VASP code [16, 17]. The stability of Nb impurities in different oxidation states was investigated at DFT level (PBE functional [18]) with a (3 × 3 × 3) bulk supercell and (2 × 2 × 2) Monkhorst– Pack k-points mesh. For the description of the electronic properties of isolated substitutional Nb, a (2 × 2 × 2) supercell was used and the Brillouin zone was sampled with a (3 × 3 × 3) Monkhorst–Pack mesh. For Au adsorption, a (3 × 3) surface supercell of a five-layer thick oxide slab was employed, where one impurity Nb atom replaced one oxide Ca2+ cation. Au was adsorbed on one side of the slab. A dipole correction was included in order to eliminate the interaction among repeated slabs, separated by more than 10 Å. All atomic positions were optimized until atomic forces were less than 0.01 eV Å−1. In the case of slab calculations, the atoms of the oxide bottom layer were kept fixed. Magnetic moments of impurities have been evaluated from the projection of the spin density into atomic spheres, while atomic charges have been estimated using the Bader decomposition scheme [19]. Nb ions with a formal oxidation state larger than +2 (e.g. Nb3+ or Nb4+) replacing Ca2+ introduce a charge imbalance. This has been treated in two ways. In the first approach, the Nbn+ charge is balanced by introducing compensating defects, such as cationic vacancies (VCa), so that the entire system is charge neutral. This approach has been employed for bulk calculations to evaluate the thermodynamic stability of Nb ions in different charge states. In a second approach, we considered the isolated impurity and we used a uniform background of charge to compensate for the higher oxidation state of the Nb ion [20]. This latter approach has been used in the study of Au adsorption. These and previous results [21] show that the conclusions are not dependent on the approach followed.
(bottom) in bulk CaO. The two panels are aligned at the valence band edge of CaO. The red vertical line locates the highest occupied state.
For the treatment of the exchange–correlation term we adopted the pure DFT PBE functional [18], also used in previous studies on this topic [5, 12, 21]. While the absolute values of the Au adsorption energies depend on the choice of the functional used, recent work performed with hybrid functionals on Cr:MgO and Mo:CaO systems has shown [21] that the physics of the problem is correctly described also using this level of theory. For CaO we used the optimized lattice constant at PBE level 4.83 Å. 3. Electronic structure and thermodynamic stability of Nb dopants in bulk CaO We first considered an Nb ion that enters in CaO as a +2 species. The configuration assumed by Nb in the octahedral environment is high spin, (t2g α)3 (t2g β)0 (eg)0. The occupied component of the 4d t2g states is found at about 2.7 eV above the top of the valence band and is separated by only about 1 eV from the bottom of the conduction band, figure 1. The empty 4d states merge with the CaO conduction band. For comparison, we report in figure 1 also the PDOS of a Mo2+ substitutional dopant in CaO computed at the same level of theory. In this case the configuration is (t2g α)3 (t2g β)1 (eg)0. From the comparison of the two figures, it is apparent that the Nb 4d states are at a higher energy level, suggesting a stronger tendency to donate electrons compared to Mo. The fact that the Mo 4d states are at a lower energy level than the Nb ones is consistent with the increase in effective nuclear charge (Zeff) moving to the right of the periodic table. However, the situation described above refers to an ideal, perfect crystal where no other defects beyond the dopant are 2
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present. In reality, Nb as other TM ions can easily change its oxidation state, in particular assume higher oxidation states, transferring electrons to other acceptor levels associated to specific defects which are essential to maintain electroneutrality. For instance, in MgO Cr enters as Cr3+, and the extra positive charge is compensated by the formation of Mg vacancies, VMg [22–24]. This has been supported by optical and EPR spectroscopies which also indicate that the Cr3+ and the VMg centers are adjacent [25, 26]. An Mg vacancy in pure MgO (VMg center) results in two holes in the O 2p valence band, with formation of two O− species (triplet ground state [27, 28]). In the presence of Cr dopants, the hole centers associated with a VMg defect in MgO are quenched due to an electron transfer from the Cr 3d states. In particular, one VMg center compensates the formation of two Cr3+ ions in the lattice. A similar mechanism is expected also for Nb:CaO, opening a general question about the relative stability of the possible combinations of VCa centers and Nbn+ (n = 3–5) cations. To answer this question we compared the stability of two Nb atoms in the bulk (3 × 3 × 3) supercell by varying the number of cation vacancies. In this way, the system without vacancies corresponds to the Nb2+ oxidation state, while the system with nV cation vacancies creates two Nbn+ ions with n = 2 + nV. We have verified, by analyzing the PDOS on the Nb ions (not shown), that the holes generated by the cation vacancies are transferred to the Nb impurity for all considered oxidation states. The two Nb atoms are located as far as possible in the supercell, while the vacancies are adjacent to the impurity ions in the cases of Nb3+ and Nb4+. Notice that the Ca vacancy formation energy, Eform(VCa), is much lower in doped than in pure oxides [12]. The reason is that, as mentioned above, one electron is transferred from the d states of each Nb ion to the low lying states of O− ions around VCa, with a change of Nb oxidation state from +2 to +3 [12, 21]. This means that by introducing Nb dopants in CaO the number of cation vacancies is expected to increase significantly. To determine the relative stability of the different defective structures containing both Nb dopants in various oxidation states and cation vacancies, we have computed their Gibbs formation energy per unit of volume at pressure p and temperature T [29]:
Figure 2. Change in Gibbs formation energy for the different Nbn+ ions in CaO compensated by Ca vacancies (see text for details). The dependence on the oxygen partial pressure is also reported at T = 1000 K.
ΔμO(p,T) = μO − ½ E(O2) is computed with reference to half of the total energy of an oxygen molecule. The upper value for the oxygen chemical potential is determined by the condition in which oxygen molecules start to condensate, ΔμO = 0; the lower limit is set where metal particles start to crystallize, μM = EM. Using the condition of equilibrium ECaO − ECa < ΔμO, we obtain the following range of validity: −6.10 < ΔμO < 0. The calculated Gibbs formation energy with respect to the oxygen partial pressure is shown in figure 2 for T = 1000 K. We first note that the +2 oxidation state is not stable in the whole range of experimentally accessible oxygen chemical potential. This means that, if not kinetically impeded, Nb doping in CaO is always expected to produce compensating defects. In oxygen poor conditions, the +3 state is more favorable, while the +5 state only occurs in very oxidizing conditions, figure 2. According to our calculations, if we assume that Nb:CaO can be synthetized in similar conditions as previously studied TM:MO [12], by reactive deposition followed by annealing at T = 1000 K in a oxygen partial pressure of 5 × 10–7 mbar (about 5 × 10−10 atm [5, 12]), the most stable oxidation state is expected to be Nb4+. This means that an Nb impurity has already lost 4 of its 5 valence electrons to intrinsic defects that are generated during the thermal annealing to reach the thermodynamic ground state. The question to answer becomes are even dopants that are in such a high oxidation state still able to transfer electrons to species adsorbed on the surface of the material? This is discussed in the next section.
Gf ( T , p ) = [ G ( T , p, { nx } − ∑ nxμx ( T , px ) ] / V x
G is the Gibbs free energy of the solid with volume V; nx, μx and px are the number of particles in the solid, the chemical potential and the partial pressure in the gas phase of the species x, respectively. Using the condition of equilibrium, we can calculate the Gibbs formation energy change with respect to the doped defect-free oxide, ∆Gf, as the function of the oxygen chemical potential:
4. Adsorption properties of Au atoms adsorbed on the (100) surface of Nb-doped CaO The effect of doping on the properties of CaO has been investigated by adsorbing Au atoms on the CaO(100) surface. With an electron affinity of 2.3 eV, Au is an excellent candidate to form surface anions by trapping electrons from the Nb impurity. Nb has been placed in the third oxide layer, figure 3. This choice is driven by the objective to mimic a situation where the dopant lies in the bulk or in subsurface layers of the oxide, thus preventing the formation of a direct bond between
ΔGf = [ Gdef − Gst + n v ECaO − n vΔμO ( p, T ) ] / V
where Gdef and Gst are the Gibbs free energies of the defective and stoichiometric systems respectively and nV is the number of Ca vacancies. The change in the O2 chemical potential 3
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Figure 3. Optimized structures for Au adatoms adsorbed on the oxide surface in the presence of an impurity atom in the third oxide layer (from the surface); (a) Ca-top, (b) O-top and (c) hollow configurations. Ca, O, Nb and Au atoms are represented as light blue, red, black and yellow spheres, respectively. Table 1. Adsorption properties of Au adatoms adsorbed on pristine
and doped CaO surfaces: preferred adsorption site, adsorption energy with respect to gas phase Au (Eads), Au Bader charge (q). Values relative to the Mo:CaO case are also reported for comparison.
CaO Nb2+:CaO Nb3+:CaO Nb4+:CaO Mo2+:CaO (a) Mo3+:CaO (a) Mo4+:CaO (a) (a)
Adsorption site
Eads (Au), eV
q(Au), |e|
O-top hollow hollow hollow hollow hollow hollow
1.35 3.23 3.06 2.60 3.60 2.10 1.75
−0.33 −0.82 −0.79 −0.76 −0.80 −0.79 −0.75
From [21].
adsorbed gold and Nb. Experimentally, this can be achieved by growing a capping layer of un-doped oxide on top of an Nb:CaO film produced via reactive deposition. This method successfully prevents dopant segregation to the surface of Mo-doped CaO [12]. Three different Au adsorption sites have been considered, on-top of Ca, on-top of O and in hollow positions (figure 3). The adsorption properties in the most stable configuration are listed in table 1. As a reference, Au adsorption on pure CaO and on Mo-doped CaO is also reported. On pure CaO the preferred adsorption site is on-top of O with an adsorption energy of 1.35 eV. On the contrary, on Nb-doped CaO, Au prefers to occupy a hollow site, regardless of the oxidation state of the Nb impurity. The Bader charge goes from −0.3 |e| on CaO to about −0.8 |e| on Nb:CaO, table 1. By comparing Au adsorption on pure CaO and Nb:CaO one can observe that on the doped material the magnetic moment on Au is quenched. This, together with the Bader charge, indicates that Au has gained one extra electron, becoming Au−. This is further shown by the PDOS curves, which show that the Au 6s state is doubly occupied, figure 4. The analysis of the Nb2+ states reveals that the electron is transferred from the impurity, which assumes a (t2g2) (eg0) electronic configuration after the charge transfer (Nb3+). The charge transfer strongly reinforces the bond of Au with the surface, and the resulting binding energy, 3.23 eV, is more than twice that on pure CaO, 1.35 eV. Interestingly, the charge transfer occurs for all adsorption sites considered, which have similar binding energies. The less stable Ca-top configuration is 0.14 eV less stable than the hollow site (ground state), while starting from O-top an almost degenerate configuration has been found with the Au atoms slightly tilted with respect to
Figure 4. Density of States for Au adatoms on Nb:CaO in different initial oxidation states. The zero energy reference is set at the highest occupied state.
the surface normal, figure 3. In all cases, due to the negative charge on Au we observe a considerable polaronic distortion, where the lattice cations are attracted and the anions repelled from the adatom as can be observed in figure 3. Nb is able to donate charge to Au also when in an oxidation state larger than +2. In figure 4, the Au PDOS for adsorption on Nb3+:CaO indicate that the Au 6s state is doubly occupied, while Nb oxidizes to Nb4+ and assumes a (t2g1)(eg0) electronic configuration. The energy gain related to the charge transfer is still more than 1.5 eV (Eads = 3.06 eV). This reflects the different position of the Nb 4d levels for the two oxidation states, figure 4. Charging of gold is also possible for Nb4+:CaO which becomes Nb5+ and donates all its valence electrons either to other defects (e.g. Ca vacancies) or to adsorbed species (Au in this case). The electron transfer is net also in this case and the adsorption energy of Au is 2.60 eV, only slightly smaller than in previous cases. These results can be compared with those of Mo:CaO [21]. Also in this case a charge transfer occurs from the Mo 4d states to the Au adsorbate and the Au adsorption energy 4
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The support of the COST Action CM1104 ‘Reducible oxide chemistry, structure and functions’ is also gratefully acknowledged.
increases from 1.35 eV (pure CaO) to 3.60 eV (Mo ), 2.10 eV (Mo3+), and 1.75 eV (Mo4+). Therefore, the behavior is similar but with a significant difference. In Mo4+:CaO the energy gain is small (the Au adsorption energy, 1.75 eV, is only 0.40 eV larger than for Au adsorbed on pure CaO), while in Nb4+:CaO the adsorption energy is about twice compared to the un-doped material, table 1, indicating a stronger tendency of Nb to oxidize to a +5 oxidation state. The tendency of Nb to be a better donor than Mo when introduced in the lattice can be traced back to the higher position of the t2g occupied levels reported in figure 1. As already pointed out in our previous work, the donor ability correlates with the atomic ionization potential (IP) of the different dopants introduced in the lattice [21]. While Mo is already a good electron donor because of the relatively low IPs, compared for instance to 1st row TMs like Cr, Nb has even lower IPs and electron transfer is indeed found to be more favorable in this case. In particular, the ionization potentials (IP) of the Nb (Mo) ions for the oxidation states relevant in the doped system are: IP(TM2+) = 25.2 (27.1) eV, IP(TM3+) = 38.5 (46.4) eV, IP(TM4+) = 50.8 (54.5) eV [30].
References [1] McFarland E W and Metiu H 2013 Catalysis by doped oxides Chem. Rev. 113 4391–427 [2] Hegde M S, Madras G and Patil K C 2009 Noble metal ionic catalysts Acc. Chem. Res. 42 704–12 [3] Metiu H, Chrétien S, Hu Z, Li B and Sun X Y 2012 Chemistry of Lewis acid−base pairs on oxide surfaces J. Phys. Chem. C 116 10439–50 [4] Shapovalov V and Metiu H 2007 Catalysis by doped oxides: CO oxidation by AuxCe1−xO2 J. Catal. 245 205–14 [5] Shao X, Prada S, Giordano L, Pacchioni G, Nilius N and Freund H-J 2011 Tailoring the shape of metal Ad-particles by doping the oxide support Angew. Chem. Int. Ed. 50 11525–7 [6] Ricci D, Bongiorno A, Pacchioni G and Landman U 2006 Bonding trends and dimensionality crossover of gold nanoclusters on metal-supported MgO thin films Phys. Rev. Lett. 97 036106 [7] Sterrer M, Risse T, Heyde M, Rust H-P and Freund H-J 2007 Crossover from three-dimensional to two-dimensional geometries of Au nanostructures on thin MgO(001) films: a confirmation of theoretical predictions Phys. Rev. Lett. 98 206103 [8] Mammen N, Narasimhan S and de Gironcoli S 2011 Tuning the morphology of gold clusters by substrate doping J. Am. Chem. Soc. 133 2801–3 [9] Zhou M, Ping Feng Y and Zhang C 2012 Gold clusters on Nb-doped SrTiO3: effects of metal-insulator transition on heterogeneous Au nanocatalysis Phys. Chem. Chem. Phys. 14 9660–5 [10] Zhang C, Yoon B and Landman U 2007 Predicted oxidation of CO catalyzed by Au nanoclusters on a thin defect-free MgO film supported on a Mo(100) surface J. Am. Chem. Soc. 129 2228–9 [11] Harding C, Habibpour V, Kunz S, Farnbacher A N S, Heiz U, Yoon B and Landman U 2009 J. Am. Chem. Soc. 131 538–48 [12] Stavale F, Shao X, Nilius N, Freund H-J, Prada S, Giordano L and Pacchioni G 2012 Donor characteristics of transitionmetal-doped oxides: Cr-doped MgO versus Mo-doped CaO J. Am. Chem. Soc. 134 11380–3 [13] Cui Y, Shao X, Baldofski M, Sauer J, Nilius N and Freund H-J 2013 Adsorption, activation, and dissociation of oxygen on doped oxides Angew. Chem. Int. Ed. 52 11385–7 [14] Andersin J, Nevaòaota J, Honkala K and Häkkinen H 2013 The redox chemistry of gold with high-valence doped calcium oxide Angew. Chem. Int. Ed. 52 1424–7 [15] Blöchl P E 1994 Projector augmented-wave method Phys. Rev. B 50 17953–79 [16] Kresse G and Hafner J 1993 Ab initio molecular dynamics for liquid metals Phys. Rev. B 47 558–61 [17] Kresse G and Furthmüller J 1996 Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set Phys. Rev. B 54 11169–86 [18] Perdew J P, Burke K and Ernzerhof M 1996 Generalized gradient approximation made simple Phys. Rev. Lett. 77 3865–8 [19] Bader R F W 1991 A quantum theory of molecular structure and its applications Chem. Rev. 91 893–928 [20] Leslie M and Gillan M J 1985 The energy and elastic dipole tensor of defects in ionic crystals calculated by the supercell method J. Phys. C: Solid State Phys. 18 973–82
5. Conclusion The surface properties of inert oxides can be modified by introducing appropriate dopants in the bulk of the material. In particular, substitutional transition metal impurities in a wide bandgap material as CaO introduce filled states in the band gap from which electrons can be transferred to the adsorbed species. As a result, the doped oxide can act as an electron donor to electronegative species, like gold particles or admolecules. In this paper we have demonstrated by DFT calculations that Nb-doped CaO presents great potential in this respect since its electronic states are higher in energy compared to a parent system, Mo:CaO, which has already revealed to be an excellent electron donor. The thermodynamic analysis of the Nb dopant in different oxidation states, compensated by calcium vacancies, indicates that doping CaO should result in an easy cation vacancy formation and the stabilization of Nb3+ or Nb4+ in the oxide lattice. Our results show that also in such high oxidation states Nb impurities, placed in the inner oxide layers to prevent the direct interaction with the adsorbates, are still able to transfer one electron to Au adatoms. This effect is expected to change the growth mode and, likely, the chemical reactivity of gold nanoparticles adsorbed in this system. Similarly, electronegative molecules like O2 or NO2 can potentially become negatively charged on this surface and be involved in further reactions, thus changing the oxide surface reactivity compared to the un-doped oxide. Acknowledgments We thank Niklas Nilius and Hans-Joachim Freund for many useful discussions. This work has been supported by the Italian MIUR through the FIRB Project RBAP115AYN ‘Oxides at the nanoscale: multifunctionality and applications’. 5
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