QIANC SUN et al.: Hydrogen Chemisorption on Supported Semiconductor Films 373

phys. stat. sol. (b) 185, 373 (1994)

Subject classification: 68.45; S1.2; S10.1

Department of Physics, Zhengzhou University ’) ( a ) , Department of Physics, Fudan University, Shanghai’) (b) , and Henan Association, for Science and Technology, Zhengzhou ’) (c)

Hydrogen Chemisorption on Supported Semiconductor Films

BY QIANG SUN (a), JIANJUN XIE (b), and TAO ZHANG (c)

Hydrogen chemisorption on metal supported semiconductor ZnO films is studied by using the Green function method and complex-energy-plane integration approach. The Anderson-Newns model is used to calculate the chemisorption energy A E and the adatom charge transfer Aq. The variations of the adatom state level E,, and the corresponding localized adatom density of states (LDOS) with increasing ZnO film thickness are investigated. It is found that the hydrogen chemisorption energy and the adatom charge transfer depend on both the thickness of the ZnO film and the species of the metal support. The calculations show that ten layers of ZnO film can mirror the chemisorption properties of bulk ZnO. The adatom state level E,,, which corresponds to a peak in LDOS, is fdund shifting upwards to a constant value as the ZnO layer number N increases from 1 to 10.

1. Introduction

The unique activity and selectivity of supported catalysts of the synergetic type has been recognized for many years [l]. Many experimental studies on such composite systems have been performed [2, 31. However, it is only in the last few years that attention has been paid to quantum-mechanical studies on these “dual function catalysts” [4, 51. Davison et al. [4] calculated the chemisorption energy and the charge transfer of hydrogen chemisorption on a metal/semiconductor composite substrate Ni/ZnO using the Anderson-Newns [6,7] theory. By using the complex-energy-plane integration approach and the Green-function method, Liu and Davison [5] studied the chemisorption properties of hydrogen on an “inverse- supported catalyst” within the frame of one-electron theory and reported the dependence of the chemisorption energy on the thickness of the ZnO film. Recently, investigations have been carried out on the effects of impurities on the chemisorption of hydrogen on a contaminated Ni/ZnO catalyst [8]. It has been found that the variations of chemisorption energy and charge transfer are very sensitive to the atomic orbital levels and the locations of impurity atoms. In this paper, we investigate the hydrogen chemisorption on ZnO semiconductor films supported by metals (Ni, Cu, and Pt). Unlike Liu’s treatment, here the charge-consistent Anderson-Newns model is employed and the effects of different metal supports are discussed. The variations of adatom state level and the localized density of states with the thickness of the ZnO film are studied.

The paper is organized as follows. First, the Anderson-Newns model and the formalism are presented in Section 2. Then, in Section 3, the calculation results are discussed, a summary is given in Section 4.

’) 450052 Zhengzhou, Henan, People’s Republic of China. ’) 220 Hadan Road, 200433 Shanghai, People’s Republic of China. ’) 450003 Zhengzhou, Henan, People’s Republic of China.

374 QIANG SUN, JIANJUN XIE, and TAO ZHANG

2. Anderson-Newns Model and the Chemisorption Properties

Similar to Liu and Davison [5], the composite catalyst is described by an N-layer semiconductor ZnO film, and a semi-infinite metal support (Fig. 1). The ZnO film is modelled by a finite chain of alternating s- and p-orbitals with corresponding site energies a, and clp

and bond energies +p2 . The semi-infinite metal support is characterized by a linear chain of d-orbitals with site energy x1 and bond energy PI. The hydrogen adatom with electronic energy E, is bonded to the ZnO surface with energy p.

According to the Anderson-Newns [6, 71 theory, the total Hamiltonian for the system is given by

E&,, + p(la, CJ) (0, 0 1 + h.c.)

- ( p z )2k, CJ) ( 2 k + 1, CJI - pz 12k + 1,0> ( 2 k + 2, C J ~ + h.~.)] a,

+ C [ul Im, a> (m , CJI + P1(Im, .> ( m + 1, CJI + h.c.)I

+ y(ln, a> ( n + ~ C J I + 111 + L o > ( n , 01) + uk,fia-u,

m = n + 1

(1) 1 where CJ denotes the spin, U is the intraatomic Coulomb repulsion on the adatom, ria, is the occupation number operator of the adatom orbital la, 0). y is the bond energy between the ZnO film and the metal base.

Taking gl(g2) as the Green function (GF) for the semi-infinite metal (finite semiconductor film) and g,, as that of the composite system, the surface G F at k = 0 is

g,z(O, 0) = gz(0,O) + Y2g2(o , g 2 h 0) k;'(n + 1 9 n + 1) - y 2 g 2 ( n , 4- l .

ga(E) = [E - Eu + P2g,z(0,0)I-1 >

E, = E, + U(n,-,) 3

(2) The adatom surface GF at site a is

(3)

where

(4) n a - , is the adatom occupation number of spin - CJ, and U(n,_,> is the averaged self-energy introduced for UiZ,-, in the unrestricted Hartree-Fock approximation. The adatom density of states can be obtained from

e(E) = -C1 Im (g,(E)). ( 5 )

H ZnO Fig. 1. Schematic representation of the H-ZnO/metal composite sys- tem showing the hydrogen adatom of electronic energy E, with bond flfo

I I I energy f l to the ZnO surface at a 0 I N N+ I k = O

Hydrogen Chemisorption on Supported Semiconductor Films 375

In the context of the Anderson-Newns chemisorption theory, the chemisorption energy AE is

AE = n-l 7 Im In [l - j2gl2(0, 0) ( E + iO+ - EJ1] dE U - m

and the electron occupation number (n,,) is EF

(n,,) = -n-' Im g,(E)dE. - m

For the nonmagnetic case treated here, the self-consistency requirement is

so (n,,) can be obtained from (3), (7), and (8). The charge transfer to the adatom from the substrate is obtained via

In order to avoid the split-off states, we use the complex-energy-plane integration technique and obtain the chemisorption energy,

m

AE = -2n-' Re In [ l - p2g12(Z) (2 - E J ' ] dy - U(n,)2 + 2E, - E,, (10) 0

and the adatom occupation number,

(n,) = 0.5 + 71-l Re [ Z - E, - p2g12(Z)]-' dy 0

Here 2 = iy + E,, g12(Z) = g12(0,0). In our calculation, the Fermi level E , is taken as the energy zero. The adatom state level E,, is obtained from

E - E, - p2 Re [gl2(0, O)] = 0 . (12)

3. Results and Discussion

In the numerical calculations, the values of parameters are chosen as follows: ct, = 0, up = - 3.4 eV, p2 = 3.755 eV, E, = -9.74 eV, U = 12.9 eV [4]. The H-ZnO bond energy p is chosen to be 3.528 eV, so that the resulting AE for a pure ZnO substrate is very close to the experimental value of -2.67 eV [9]. The site energy m l , bond energy of the metals (Cu, Ni, and Pt) and the ZnO-metal bond strength y are shown in Table 1. The y value is taken as the average of p1 and /I2.

The calculated values of the chemisorption energy AE, the charge transfer Aq, and the adatom state level E,, are listed in Table 2 for Cu, Ni, and Pt supports. The ZnO thickness varies from N = 1 to 10. It is found that the value of the chemisorption energy IAEl increases as the ZnO film grows thicker. In the case of one of two layers of ZnO film, the chemisorption energy AE is significantly modified by the metal basis. Further increase of N yields only slight changes of AE. When N = 6, AE becomes the same as the value of bulk ZnO ( - 2.67 eV) except a very little difference for the Pt support.

376 QIANG SUN, JIANJUN XIE, and TAO ZHANG

Table 1 Values of site energy ul, bond energy /I, of metal supports (Cu [7], Ni [7], and Pt [lo]), and ZnO-metal bond energy y (in eV)

c u - 2.90 0.675 2.16 Ni - 1.7 0.95 2.35 Pt - 2.94 1.825 2.84

Table 2 Chemisorption energy AE, charge transfer Aq and the adatom state energy E,, for hydrogen on N-layer ZnO film supported by Cu, Ni, and Pt supports

1 2 3 4 5 6 7 8 9

10

- 1.915 -2.552 - 2.650 -2.667 - 2.670 - 2.670 - 2.670 - 2.670 - 2.670 - 2.670

0.386 0.328 0.317 0.317 0.317 0.317 0.317 0.317 0.317 0.3 17

- 3.046 - 2.764 - 2.703 - 2.682 -2.678 - 2.677 - 2.676 - 2.676 - 2.676 - 2.676

- 1.862 - 2.464 - 2.608 - 2.648 -2.661 - 2.665 -2.668 - 2.669 - 2.669 - 2.670

0.361 0.331 0.320 0.317 0.317 0.3 17 0.317 0.317 0.317 0.317

-2.956 -2.715 -2.686 -2.686 - 2.677 - 2.676 - 2.676 - 2.676 -2.676 - 2.676

- 1.552 -2.140 -2.415 - 2.542 - 2.602 -2.632 -2.648 - 2.656 -2.661 - 2.664

0.358 0.352 0.333 0.325 0.320 0.317 0.317 0.317 0.317 0.317

-2.882 -2.881 - 2.745 - 2.708 - 2.708 - 2.689 - 2.682 - 2.682 - 2.678 - 2.677

From Table 1 we can see that y (ZnO/Cu) < y (ZnO/Ni) < y (ZnO/Pt), however, the cal- culated AE in Table 2 exhibits a contrary trend, i.e., lAEl (H-ZnO/Cu) > IAEl (H-ZnO/Ni) > /AEl (H-ZnO/Pt) at the same thickness of ZnO layer. Therefore, the stronger metal- semiconductor interaction gives less chemisorption energy. This indicates that the lifetime of an adsorbed hydrogen atom on ZnO film will shorten. However, the short lifetime on the surface does not necessarily mean that this lifetime is insufficient for the adsorbed species to react. On the contrary, the weakened adsorbate-surface interaction sometimes can improve the catalytic properties [ll].

The changes in chemisorptive, catalytic properties on the supported semiconductor surface suggest an electronic interaction at the metal-semiconductor interface. Both experiments and theoretical studies have found the charge transfer between them [12 to 141. From Table 2, we can see that when a single layer of ZnO grows on the metal support, the charge transfer Aq from the substrate to H is largest. Further increase of N gives smaller charge transfer. When N 2 6, Aq decreases to a constant value of 0.317e for all the three supports. This implies that when a single ZnO layer grows on the metal supports, electrons pass from the metal to the ZnO film and provide a source of charge for transfer to the H adatom. When the ZnO film becomes thicker, some electrons from the metal base have sunk in the ZnO film, and therefore the charge transfer to the adatom becomes less. It is interesting that the findings here are in accord with that of Klier [14], whose results suggest that the charge is transferred from the supported Cu film to the semiconductor ZnO base.

Hydrogen Chemisorption on Supported Semiconductor Films 377

Fig. 2. Localized adatom density of states for the a) H-ZnO/Ni and b) H-ZnO/Cu system in the case of N = 1 (solid line), 2 (dashed line), and 3 (dotted line)

When a H atom interacts with the composite substrate, its energy level is shifted upwards from E = E, to E,d. The calculated E,, for the three systems are also listed in Table 2. It is shown that as the ZnO thickness increase, the adatom state level approaches a constant value E,, = -2.676 eV. The variations of the localized adatom density of states (LDOS), with the thickness of ZnO layers N are shown in Fig. 2a (H-ZnO/Ni) and 2b (H-ZnO/Cu) for N = 1, 2, and 3. It can be seen that as N increases, the peak position in LDOS, which corresponds to the adatom state energy level E,,, shifts upwards. Additionally, the peak becomes higher and sharper for larger N . Comparing Fig. 2a and b, we can find that when N = 1 or 2, the peak position of LDOS for Cu support is lower than that of Ni at the same thickness of the ZnO film. When N = 3, the LDOS in Fig. 2a and b show nearly no difference. This indicates that the effect of the metal supports on LDOS decreases rapidly as the ZnO film grows thicker.

4. Summary and Conclusion

Hydrogen chemisorption on the composite substrate ZnO/metal (Ni, Cu, and Pt) has been investigated using the Green function method and the Anderson-Newns model within the

378 QIANG SUN et al.: Hydrogen Chemisorption on Supported Semiconductor Films

tight-binding approximation. The chemisorption energy AE, adatom charge transfer Aq, adatom state level Ead, and the localized adatom density of states for composite systems are calculated. It is found that in the case of one or two layers of ZnO film, the chemisorption energy AE and the charge transfer Aq are significantly different from that of bulk ZnO. The metal-semiconductor interaction weakens the H chemisorption on the supported ZnO films. When N > 6, the effects of the metal supports on the H chemisorption on the ZnO film can be neglected. The peak position in the adatom localized density of states shifts upwards accompanying the movement of the adatom state levels E,, as the ZnO film grows thicker.

Acknowledgement

The work is supported by the State Key Laboratory of Applied Surface Physics, Fudan University, People’s Republic of China.

References

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(Received April 27, 1994)