Photon Factory Activity Report 2005 #23Part B (2006)
Surface and Interface

1C/2004G007

ARPES Study on the Electronic Structure of Deposited Cu on ZnO(10-10)
Yukako OBA1, Kazuyuki EDAMOTO1, Kenichi OZAWA*,2
1
Department of Chemistry, Rikkyo University, Toshima-ku, Tokyo 171-8501, Japan2
Department of Chemistry, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-0033, Japan
Introduction
Zinc oxide (ZnO) is a material that is widely used inmany applications such as optoelectronic devices,chemical sensors, catalysts, etc. As a catalyst, the Cu/ZnO
system show high activity for synthesis of methanol andhigher alcohol, water gas shift reaction and steam
reforming of methanol. Activity of oxide-supported metal
catalysts should be closely related to the morphology andthe electronic structure of metals on oxide surfaces. In thepresent study, we have performed angle-resolved
photoemission spectroscopy (ARPES) measurements ofthe Cu/ZnO(10-10) surface to elucidate the valence
electronic structure of the Cu overlayers at various Cu
coverages.
ExperimentalThe ARPES measurements were carried out at beamline 1C. The ZnO(10-10) surface was cleaned by an Ar+
bombardment and annealing at 1050 K. Cu wasdeposited on the surface at room temperature using acommercial evaporation source (Omicron EFM3).

bending of the ZnO band, which causes the shift of theZnO peaks (the Zn 3d and O 2p-Zn 4sp hybrid peaks),
and the final state screening of the photohole remain in
the Cu cluster affect the Cu 3d and 4sp states equally.
Thus, the extra shift for the Cu 4sp DOS should be caused
by initial state effects. The small noble-metal clusters
have semiconducting electronic structure with an energy
gap between the highest-occupied molecular orbital(HOMO) and the lowest-unoccupied molecular orbital(LUMO), and the HOMO-LUMO gap is reduced withincreasing the cluster size [3]. Since the onset of the Cu
4sp DOS corresponds to the HOMO of the Cu cluster, theextra shift should be due to the change in the HOMOLUMO gap width. Comparison of the coveragedependent shift of the Cu 4sp DOS with that of the Cu 3dpeak suggests that the HOMO-LUMO gap may close atca. 1 ML.
References[1] H. Hovel et al., Phys. Rev. Lett., 81, 4608 (1998).
[2] O. Dulub et al., Surf. Sci., 504, 271 (2002).
[3] C.C. Chusuei et al., Topics Catal., 14, 71 (2001).

Result and Discussions

54

Users' Report

Cu 3d

Intensity (arb.units)

FIG. 1 shows the normal emission spectra from theZnO(10-10) surface at various Cu coverages. Cu
deposition leads to the gradual evolution of the Cu 3dpeak in the band gap region of ZnO. The Cu 3d peaklocates at 3.0 eV at 0.2 ML and shifts to 2.5 eV up to 3.2
ML. In the energy region between the Fermi level (EF)and the Cu 3d peak, the step structure is formed by theemission from the Cu 4sp band, which represents the total
density of state (DOS) of the Cu 4sp states. The onset
position of the 4sp DOS also shifts to the lower binding
energy side with increasing the Cu coverage in a similarmanner to the Cu 3d peak. Formation of the step-like sp
DOS from the low coverage region and the shift of theonset position towards EF with increasing coverage arecharacteristic of noble-metal/oxide systems, where thedeposited noble-metal atoms aggregate to form clusters
and the size of the cluster is enlarged with increasing thecoverage [1]. Thus, growth of Cu on ZnO(10-10) ischaracterized by the cluster formation, in good agreementwith a recent scanning tunnelling microscopy study [2].
An interesting point to note is that the magnitude of theshift as a function of the Cu coverage is larger for theonset of the Cu 4sp DOS than for the Cu 3d peak. For theCu 4sp DOS, the onset position moves from 0.75 eV at
0.2 ML to 0.12 eV at 3.2 ML. Both the Cu-inducedNormal emissionohν = 50 eV, θ i = 45

3.2ML
2.6ML
2.0ML
1.7ML
1.4ML
1.2ML
0.9ML
0.7ML

Zn 3dO 2p–Zn 3d

10

0.5ML
0.2ML
clean

0Binding Energy (eV)FIG.1 Change in the normal emission spectrum of theZnO(10-10) surface as a function of the Cu coverage. One
monolayer (ML) is equivalent to the coverage where thesubstrate surface is covered entirely with the Cu overlayer
with a single atomic thickness.

*kozawa@chem.titech.ac.jp