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A Comparative Study of ZnO, Overlayers on Cu(311), Cu(1 lo), and a High Defect Concentration Cu(11 1) Sabrina S. Fu'st and Gabor A. Somorjai Materials Science Division, Center for Advanced Materials, Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720, and Department of Chemistry, University of California, Berkeley, California 94720 Received May 23, 1991 Zinc and oxygen were deposited in ultrahigh vacuum to form ZnO, overlayers on three different copper faces, Cu(311),Cu(llO), and a high defect concentration Cu(ll1). Up to 1.0 monolayer of ZnO, overlayers were examined. No long-range order was found for ZnO, overlayers on any of these surfaces. We used a combination of CO and COz temperature programmed desorption (TPD), Auger electron spectroscopy (AES),and low-energy electron diffraction (LEED) to follow chemical and structural changes of ZnO, overlayers on copper. Upon heating above 300 K, ZnO, area decreases while copper area increases, indicating clustering of ZnO,. In addition to changes in ZnO, overlayers on copper surfaces, we examined the decomposition of ZnO, to form Zn(g) and O(surface) by a combination of TPD, AES, and LEED. The desorption temperature of zinc vapor from the decomposition of ZnO, depends on the structure of the copper substrate.
Introduction Understanding Cu-Zn-O interaction is an important step to understanding Cu-Zn-0 methanol synthesis catalysts. Our work concerning ZnO, overlayers on copper began with Cu(ll0) as the substrate.' The (110) face of copper was used as it has a small mismatch to a ZnO face; a 2 % mismatch on one side of the unit-cell and a 10% mismatch on the other side to a ZnO(1010) face. Unfortunately, we found no ordered ZnO, overlayer on Cu(ll0). Instead, we found ZnO, to be most stable as threedimensional islands at temperatures greater than 300 K in vacuum. As Cu(110) is an open, flat surface,we decided to investigate ZnO, formation on two surfaces which represent two other distinct types of surfaces, a closepacked surface, Cu(lll), and a high stepped density surface, Cu(311), and compare them to ZnO, formation on Cu(ll0). In this paper, we show that upon heating above 300 K, ZnO, clusterson all three surfaces. The clustering appears tobemostextensiveonthe(ll0) and (111) facesofcopper. Our use of a high defect density Cu(ll1) (>lo% CO temperature programmed desorption peak due to sites other than the (111) face) shows ZnO, prefers to cluster to the defect sites. In addition, we show that the temperature of ZnO, decomposition is dependent on the structure of the copper substrate. Experimental Section The ultrahigh vacuum chamber used in these studies has been described elsewhere.' Briefly, it contains a zinc source and equipment for low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), the temperature programmed desorption (TPD) with a working pressure of 2 X 10-loTorr or less. Zinc is deposited by evaporation of 99.999% pure zinc wires (Aldrich) enclosed in a Knudsen cell. This Knudsen cell is mounted in a stainless steel casing equipped with bellows and a shutter. In addition, the zinc source is equipped with watercooling capabilities and chromel-alumel thermocouples to mon+ Present address: Code 6114, Department of Chemistry, Naval Research Laboratory, Washington, DC 20375-5000. (1)Fu,Sabrina S; Somorjai,G. A. Surf. Sci. 1990,237, 87-98. ( 2 ) MacLaren, J. M.; Pendry, J. B.; Rous, P. J.;Saldin,D. K.; Somorjai,
G. A.; Van Hove, M. A.; Vvedensky, D. D. Surface Crystallographic Information Service; A Handbook of Surface Structures; Reidel: Dordrecht, 1987.
0743-7463/92/2408-0518$03.00/0
itor the temperature of the zinc. Unless otherwise noted, all zinc depositions are done at 150-170 K on copper substrates preadsorbed with half a monolayer of oxygen at 300 K. The O/Cu Auger signal ratio from a sharp p (2 X 1)surface structure of O/Cu(llO)was used to calibrate oxygen coverage, as the p(2 X 1)surface structure of O/Cu(llO)is associated with 0.5 monolayer (ML) ~xygen.~ This O/Cu Auger signal ratio was then used to estimate the oxygen coverages on Cu(ll1) and Cu(311). The different copper surfacedensitieswere not taken into account since the copper Auger peak used is at 918 eV and, hence, has only a small contribution from the topmost layer. This is a rough approximation of the oxygen coverageand hence should be taken as such. The CO and C02coverages were estimatedas previously described.l A typical experimental procedure is as follows: The copper single crystal is cleaned by cycles of argon sputtering at 300 and 910 K. The Cu(311) crystal was then annealed at 770 K for 1h, and the Cu(ll0) and Cu(ll1) crystale were annealed at 910 K for 15 min. Sample cleanness is then checked by AES and ita structure by LEED. Once the sample is clean, half a monolayer of oxygen is deposited onto each copper surface by exposure to 110langmuir of 02(g). Then the sample is exposed to zinc vapor in vacuum at 150-170 K for submonolayer ZnO, coverages and in an ambient of 21 X 10-7Torr 0 2 for 21.0 ML ZnO, coverages. We believe that these ZnO, films are near stochiometric ZnO as further oxidations and heat treatments did not change the properties of ZnO, but did form O/Cu. We had shown previously' that COStitration could be used to calibrate the monolayer point on ZnO, overlayers on Cu(ll0). Once the calibration had been associated with copper, zinc, and oxygenAES ratioson Cu(llO),thoseratioswereusedfordefining the monolayer point for ZnO, on the other two copper surfaces. After the desired amounts of zinc and oxygen have been deposited, the sample is cooled to 150 K (or 130 K for Cu(lll)), positioned 2 mm in front of the mass spectrometer, and dosed with a known amount of gas, and then the sample temperature is ramped linearly at 30 K/s for the Cu(ll1) and Cu(ll0) substrates and at 10K/s for the Cu(311)substrate. AES spectra were obtained after TPD experiments to correlate composition of the surface to CO and C02 TPD spectra. After characterizing each surface by AES, LEED, and CO and COz TPD, the samples were heated to 1100K while monitoring zinc, oxygen, or copper to further characterize these surfaces. Although all TPD data are given with arbitrary unita for desorption rate, all the graphs are shown with the same arbitrary unita, with the multiplication factor for they axis given on the graph or in the figure caption. (3) Ertl, G . Surf. Sci. 1967,6, 208-232.
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Temperature [ K l Figure 1. CO desorption from Cu(llO), a defective Cu(lll), and Cu(311), and the effect of preadsorbed oxygen upon CO adsorption. Enough CO was exposed to each surface to produce saturation coverages: 2.0 langmuirs CO for Cu(llO), 10 langmuirs CO for Cu(lll), and 20 langmuirs CO for Cu(311). The rising baseline from the Cu(ll1) and Cu(311) data are due to desorption of CO from the manipulator parta from the high exposures (110langmuirs) of CO. No COzformationwas detected from any of the CO exposures.
Results 1. COandCOzDesorptionfromCopper (311),(110), and ( l l l ) , and Oxygen-Covered Copper (311), (110), and (111). Figure 1shows CO desorption after saturation exposures on Cu(llO), a high defect concentration Cu(lll),and Cu(311), along with CO desorption from their 0.5 ML oxygen-covered counterparts. The temperature at peak rate of desorption (Tp)for CO from Cu(ll0) occurs a t 218 K. This is in agreement with previous work which shows CO desorption a t Tp = 223 K4from Cu(ll0). CO desorption from Cu(ll1) produces two peaks-one centered at 165 K and the other one centered at 225 K. The peak centered at 165 K is in agreement with previous work which shows CO desorption at 168 K5 from Cu(ll1). We believe that the peak centered at 225 K is due to defects on the Cu(ll1) surface, as other investigators5 did not observe this peak. We shall see later that these defect sites appear to be favored by ZnO, islands. No previous work other than ours has been reported on CO TPD from Cu(311),6 which shows CO desorption centered a t 203 K for saturation coverage at 150 K. From the results above, it can be seen that each of the three copper surfaces has a distinct CO desorption characteristic: Tp= 165 K, 203 K, and 218 K for C u ( l l l ) , Cu(311), and Cu(llO), respectively. Both the (110) and (311) faces of copper adsorb 0.5 ML CO (calibration done by p(2 X 1)CO overlayer on Cu(ll0)') at 150 K. Due to the low binding energy of CO on C u ( l l l ) , even at 130 K, no more than 0.3 ML CO could be adsorbed on Cu(ll1) at low pressures of
