J. Phys. Chem. B 2001, 105, 6191-6193
6191
An ab Initio Study of Hydrogen Adsorption on ZnO(101h0) A. Wander* and N. M. Harrison† CLRC, Daresbury Laboratory, Daresbury, Warrington, WA4 4AD, UK and Department of Chemistry, Imperial College of Science, Technology, and Medicine, London, SW7 2AY, UK ReceiVed: December 31, 2000; In Final Form: April 19, 2001
The interaction of hydrogen with the stoichiometric ZnO(101h0) surface has been studied using ab initio, all-electron total energy calculations. A highly accurate hybrid density functional (B3LYP) implemented within a local Gaussian basis set has been employed. Details of the geometric and electronic structure of the surface have been determined. The results are discussed in terms of the role of this surface as a model system for both catalytic reactions and gas sensing applications. The Zn-O dimers on the (101h0) surface provide a model system for the type I adsorption sites known to be important in surface mediated methanol synthesis reactions. The interaction of hydrogen with the surface does not result in surface metallisation, and hence, the (101h0) surface is unlikely to be an active gas sensing material.
Zincite (ZnO) is an ionic semiconductor which has a wide range of technological uses. These include its use as a white pigment and use in the rubber industry where it shortens the time of vulcanization, and also applications in catalysis and gas sensing systems.1 In combination with copper, ZnO forms an important industrial catalyst for both the water-gas shift reaction2
CO + H2O h CO2 + H2 and for methanol synthesis3
CO + 2H2 f CO3OH Hermann et al.4 proposed that the role of ZnO in the active Cu/ZnO catalyst during methanol synthesis is to provide a site for the dissociative adsorption of hydrogen because H2 is known to dissociate on the surfaces of ZnO at high partial pressures.5 Interestingly, the surfaces of ZnO are inert toward H2 at low partial pressures,6 and hence, the interaction of hydrogen with zincite has not been studied in detail using traditional surface science techniques that are largely confined to ultrahigh vacuum environments. This is an example of the pressure gap inherent in many surface science experiments. The dissociation is believed to occur at Type I sites which consist of a pair of surface Zn and O ions.7
Yates et al.8 have identified the Type I active site on a partially reconstructed and defective polar (0001)-Zn surface. Such defective polar, surfaces would be extremely difficuly to model from first principles. However, such Type I sites occur naturally on the (101h0) surface shown in Figure 1.9 Hence, this surface provides an ideal model for investigating hydrogen adsorption on ZnO and has been used for this purpose in previous semiempirical cluster model calculations.10 † Department of Chemistry, Imperial College of Science, Technology, and Medicine, London.
Figure 1. ZnO(101h0) surface. The surface dimers are an ideal model of the Type I adsorption site for hydrogen dissociation on ZnO.
Zinc oxide crystallizes in the wurtzite structure in which the cations are tetrahedrally coordinated with oxygen ions in the bulk. The atomic planes perpendicular to the (101h0) direction consists of equal numbers of zinc and oxygen ions and can be thought of as consisting of rows of zinc-oxygen dimers. The surface dimers are then bonded to dimers in the second layer as shown in Figure 1. Hydrogen adsorption on this surface can then proceed by saturating the surface dangling “bonds” (which are largely ionic in character unlike the covalent dangling bonds associated with the silicon surface) with hydrogen atoms as shown in Figure 2. This figure displays the basic model used to investigate hydrogen adsorption within the current study. The slab is denoted by the number of bulk Zn-O layers its contains and hence the slab in Figure 2 is referred to as an S6 slab. Our calculations were performed with the CRYSTAL code11 based on the use of the periodic ab initio LCAO (linear combination of atomic orbitals) method. The radial factors of the atomic orbitals were expressed as a linear combination of Gaussian type functions, and high quality basis sets at the all electron level were used throughout. Triple valence basis sets with polarization functions were used, which have been optimized in an early study of bulk ZnO12 giving an 86411d31G basis set for the zinc ion and an 8-411G* basis set
10.1021/jp004627f CCC: $20.00 © 2001 American Chemical Society Published on Web 06/09/2001
6192 J. Phys. Chem. B, Vol. 105, No. 26, 2001
Wander and Harrison TABLE 1: Charges on the Ions of the Slaba ion
ZnO(101h0)-H
HO O1 HZn Zn1 O2 Zn2 O3 Zn3 O4 Zn4 O4 Zn4
1.03 8.55 1.26 29.16 9.06 28.95 9.08 28.92 9.08 28.92 9.07 28.93
ZnO(101h0) 9.00 28.97 9.06 28.97 9.07 28.95 9.07 28.92
a
The clean surface and bulk values are taken from Wander et al.16 The subscripts are the layer in which the ion is located. Figure 2. Hydrogen covered ZnO(101h0) surface used for the current calculations.
for the oxygen ion. The B3LYP exchange and correlational functional was used, as recent studies have shown that this functional produces results for both structural and electronic properties13 that are better than GGA functionals. This hybrid functional consists of Becke’s 3 parameter exchange functional14 together with the Lee-Yang-Parr correlation functional.15 This combination of basis sets and exchange correlation functionals has previously been shown to provide an excellent description of the bulk phase of ZnO,16 its (101h0)16 and (102h0)17 surfaces, and its polar (0001)-Zn and (0001h)-O surfaces.18 The relative charges and bond order populations on the atoms were calculated using a Mulliken partition of the total charge density.19 This is an arbitrary choice, since there is no unique method of performing the partition of the charge density. However, the choice of a given scheme is still extremely useful in comparing the results of calculations performed using similar basis sets. Hence, we can consistently compare the results of bulk and surface calculations. To establish convergence of our results with respect to slab thickness, structural optimizations of S4, S6, and S8 slabs were performed. Double sided adsorption is used and hence the geometric structure of both the top and bottom layers of the slab was optimized. The hydrogen atoms and the first two layers of the slab were fully relaxed subject to the symmetry constraints of the surface. Previous studies of ZnO surfaces have demonstrated that reconstructions are largely confined to the surface of the material and are damped rapidly for layers below the surface plane16-18 and hence, deeper layers of the oxide have been maintained at their bulk positions. The results for the S4, S6, and S8 slab show that the geometric and electronic structure are fully converged for the S8 system. Optimization of the surface structures was performed by energy minimization using an unconstrained Broyden-FletcherGoldfarb-Shanno (BFGS) algorithm as implemented in the DOMIN software20 with gradients of the energy being calculated by numerical differentiation with a finite difference step of 0.002 Å. Extensive tests have shown that the energy surface is smooth on this scale and that the choice of a step of 0.002 Å leads to accurate gradient determinations. The convergence criteria for geometry optimization was that successive structures should differ in energy by less than 10-6 Hartrees and that displacements of ions should be less than 0.01 Å. The electronic structure of the two adsorbed hydrogens differs significantly as shown in Table 1. The H attached to the Zn ion carries significantly more charge (1.260|e|) than the H attached
Figure 3. Notation used to describe the distortions of the surface dimer bond.
to the oxygen ion (1.028|e|). The surface oxygen ion is also significantly less ionic than the bulk oxygen ions (total charge 8.549|e| compared to 9.074|e| in the bulk). Charges in the second and deeper layers of the slab are essentially bulk like. The geometric distortion of the surface dimer is described using the parameters displayed in Figure 3 which is identical to the terminology used in previous studies of the clean surface.16 The results of the structural optimization are displayed in Table 2 along with those determined for the clean surface in an earlier study.16 The most significant differences are a slight opening of the dimer bond (by ∼0.11 Å) and a reversal of the tilt angle although the angle is small for both the clean and adsorbate covered surface. For the clean surface θ ) 5.2°, while for the hydrgen covered surface θ ) -4.7°. A positive θ value corresponds to the oxygen ion being aboVe the zinc ion. The height of the dimer above the surface also significantly increases with hydrogen adsorption. Assuming that the hydrogen acts to saturate the dangling “bonds” of the surface we would expect to observe values for the height of the dimer and its bond length close to the corresponding bulk values. In the current study, this is observed, although fine details of the bonding are dominated by the charge transfer within the surface plane. In line with this picture of hydrogen producing a more “bulk like” ZnO surface, we find that the bond overlap population of the surface Zn-O dimer bond drops from the value of 0.21 for the clean surface16 to a value of 0.11 for the hydrogen covered (101h0) surface, which is identical to the value found in our earlier study of bulk ZnO.16
Hydrogen Adsorption on ZnO(101h0)
J. Phys. Chem. B, Vol. 105, No. 26, 2001 6193
TABLE 2: Reconstruction of the ZnO(101h0) Surfacea parameter
ZnO(101h0)-H
ZnO(101h0)16
bulk16
x1 x2 z1 z2 d ∆d θ O1-H Bond Zn1-H Bond Zn1-O ˆ 1-H O1-Zn ˆ-H 1
0.611 2.621 1.190 1.018 2.017 +0.95 -4.7 0.99 1.59 132.7 136.0
0.705 2.602 0.626 0.777 1.905 -4.9 5.2
0.640 2.605 0.938 0.938 1.998 0.0 0.0
a The parameters used to specify the surface dimer are given in Figure 3. All distances are given in Å, whereas the change in bond length (∆d) is given as a percentage change from the bulk value. The tilt angle θ and the bond angles of the hydrogen atoms are given in degrees. A positive value of θ corresponds to the surface oxygen ion being aboVe the surface Zn ion. The subscripts indicate the layer in which the ion is located.
The observed O-H bond length of 0.99 Å is close to the value obtained for gas-phase water molecule using the same basis set and exchange correlation functional of 0.97 Å. The structurally optimized gas-phase water molecule also has a Mulliken population of 1.061|e| to the H atoms, in line with the value of 1.028|e| found for the H bonded to the surface oxygen ion. The computed Zn-H bond length is 1.586 Å which is close to the value for gas-phase ZnH of 1.59 Å.21 The bond overlap population of the Zn-H bond has a value of 0.268, which is close to the value found for bulk silicon (0.35) and indicates a largely covalent bonding character. The charges found on all atoms within the slab are displayed in Table 1. The charge on each ion has returned to essentially bulk like values for the first subsurface layer. Within the surface layer, we find a large change in the charge of the ions. The charge on the surface OH group is virtually identical to the charge of an isolated OH- group computed using the same calculational scheme. Hence, the surface group can be assigned a formal charge of -1. The change in oxidation state of this surface group from an ion of formal charge O2- to an (O-H)object results in a compensating change in oxidation state of the surface Zn ion. This change is entirely localized within the surface plane. The band gap of the H covered slab is virtually identical to the band gap of the bulk oxide at 3.2 eV and there is no evidence of metallisation of the surface upon exposure to hydrogen which would be expected if the surface was an active hydrogen gas sensor. The density of states of the slab is displayed in Figure 4. Because the calculations were performed using atom centered local Gaussian basis functions, the total density can be projected onto contributions from individual atoms allowing the character of features of interest to be determined. The large density of states just below the Fermi edge is composed of hydrogen and zinc components and is associated with the Zn-H covalent bond. The binding energy of the two hydrogen atoms to the surface is computed to be 0.62 eV relative to an isolated gasphase H2 molecule. Hence, the two atoms are only weakly bound to the surface. This is at variance with the experimental data which shows no binding at low pressure. Hence, it is likely that the adsorption process is kinetically hindered rather than thermodynamically unfavorable. In summary, hydrogen chemisorbs on the (101h0) surface in a Type I adsorption site consisting of a Zn-O surface dimer. The charge states of the two adsorbed hydrogen atoms are very
Figure 4. Density of states of the ZnO(101h0)-2H slab.
different and the nature of the chemisorption can be understood in terms of a simple model in which H addition to a surface O ion leads to the formation of an (O-H)- site. The excess charge is then denoted to a surface Zn ion giving rise to a (Zn-H)+ site. This is at varience with cluster calculations in which the adsorption was described as resulting in Zn2+-H and O2--H sites.10 No evidence of surface states or surface metallisation is apparent within our calculations and hence it appears unlikely that the (101h0) surface of zincite is an active gas sensor for hydrogen adsorption. References and Notes (1) For a general review of properties of zinc oxide, see for example: Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; ButterworthHeinemann: Oxford, 1994. (2) Van Herwijnen, T.; de Jong, W. A. J. Catal. 1980, 63, 83. (3) Denny, P. J.; Whan, D. A. In Catalysis, Specialist Periodical Reports; Dowden, D. A., Kemball, C., Eds.; The Chemical Society: London, 1977; Vol. 2, p 79. (4) Hermann, R. G.; Klier, K.; Simmons, G. W.; Finn, B. P.; Bulko, J. P. J. Catal. 1979, 56, 407. (5) Eishchens, R. P.; Pilskin, W. A.; Low, M. J. D. J. Catal. 1962, 1, 180. (6) Go¨pel, W. Prog. Surf. Sci. 1985, 20, 9. (7) Boccuzzi, F.; Garrone, E.; Zecchina, A.; Bossi, A.; Cania, M. J. Catal. 1978, 51, 160. (8) Griffin, G. L.; Yates, J. T. J. Chem. Phys. 1982, 77, 3744. (9) Figure 1 was produced using the Daresbury Laboratory Visualise package. For further information, see: http://www.cse.clrc.ac.uk/Activity/ DLV. (10) Rodriguez, J. T.; Campbell, C. T. J. Chem. Phys. 1987, 91, 6648. (11) Saunders, V. R.; Dovesi, R.; Roetti, C.; Causa`, M.; Harrison, N. M.; Orlando, R.; Zicovich-Wilson, C. M. CRYSTAL98 Users’s Manual; University of Torino: Torino, 1998. (12) Jaffe, E.; Harrison, N. M.; Hess, A. C. Phys. ReV. 1994, B49, 11 153. (13) Muscat, J.; Wander, A.; Harrison, N. M. Chem. Phys. Lett., submitted. (14) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (15) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. 1988, B37, 785. (16) Wander, A.; Harrison, N. M. Surf. Sci. 2000, 457, L342. (17) Wander, A.; Harrison, N. M. em Surf. Sci. 2000, 486, L851. (18) Wander, A.; Schedin, F.; Steadman, P.; Norris, A.; McGrath, R.; Turner, T. S.; Thornton, G.; Harrison, N. M. Phys. ReV. Lett., in press. (19) Pisani, C.; Dovesi, R.; Roetti, C. Hartree-Fock ab Initio Treatment of Crystalline Systems, Lecture Notes in Chemistry; Vol. 48, SpringerVerlag: Heidelberg, 1988. (20) Spellucci, P.; University of Darmstadt. Available from http:// plato.la.asu.edu/donlp2.html. (21) Handbook of Chemistry and Physics, 73rd ed.; Lides, D. R., Ed. CRC Press: Boca Raton, USA, 1992.
