Initial growth of Au on oxides

Transcription

1 SURFACE AND INTERFACE ANALYSIS Surf. Interface Anal. 2001; 32: Initial growth of Au on oxides Qinlin Guo, 1 Kai Luo, 2 Kent A. Davis 2 and D. W. Goodman 2 1 State Key Laboratory for Surface Physics, Institute of Physics, CAS, Beijing , P.R. China 2 Department of Chemistry, Texas A & M University, PO Box 30012, College Station, TX , USA Received 23 October 2000; Revised 14 December 2000; Accepted 12 January 2001 ThegrowthofAuclustersonTiO 2.110/.1 1/ and.1 2/ single-crystal surfaces and on ordered Al 2 O 3 films prepared on Re(0001) has been studied by low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), low-energy ion scattering spectroscopy (LEISS) and high-resolution electron energy-loss spectroscopy (HREELS). The results indicate that Au forms two-dimensional islands on the TiO 2 (110) surface at initial deposition of Au. On the alumina film, Au initially grows as three-dimensional clusters. The coverage of Au at which Au clusters change from non-metallic to metallic properties is found to be >0.3 monolayer equivalent (MLE) of Au deposition based on the energy shift of the surface plasmon measurements. Copyright 2001 John Wiley & Sons, Ltd. KEYWORDS: Au clusters; surface plasmon; Au growth on oxides; Au deposition; ordered film INTRODUCTION The study of gold dispersion on the oxides has increased recently due to its new-found importance in catalysis. When gold is deposited on the metal oxides as nanoscale clusters its chemical and physical properties are changed dramatically. It has been reported that nanoscale Au particles on the single-crystal surface of TiO 2 show an unusual size dependence for the low-temperature catalytic oxidation of carbon monoxide. 1 Some important reactions in chemical industry rely on the gold catalysts in which the gold is highly dispersed on selected oxides. 1 4 The remarkable changes in activity in the catalytic reaction with nanometer scale changes in cluster size reveal a novel phenomenon, that is surprising when compared with the inert properties of bulk Au. It has been found that small nanoscale clusters do not have the characteristics of a uniform state of matter; their properties vary widely as a function of cluster size, presenting many unusual chemical and physical properties. 5 There must be a transition from non-metallic to metallic characteristics at some size range between atom and bulk. To try to understand the nature of this regime, a number of investigations of clusters on various substrates have been carried out in which most were studied by optical spectroscopy Recently, high-resolution electron energyloss spectroscopy (HREELS) has been used on small metal clusters to measure the plasmon excitation: a collective oscillation of the electrons. It has been observed that the Ł Correspondence to: D. W. Goodman, Department of Chemistry, Texas A & M University, PO Box 30012, College Station, TX , USA. Paper presented at APSIAC 2000: Asia Pacific surface and Interface Analysis Conference, October 2000, Beijing, China. Contract/grant sponsor: US Department of Energy. Contract/grant sponsor: US National Science Foundation. Contract/grant sponsor: Chinese National Science Foundation. energy loss of the plasmon shifts as a function of cluster size. 13,14 In other words, one may use electron energy-loss spectroscopy to determine the non-metal to metal transition by measuring the loss energy of the surface plasmon from the materials of interest. In this paper, we will present results to show that Au initially grows on the TiO 2 (110) surface as two-dimensional (2D) islands, whereas on an ordered alumina film Au initially grows as three-dimensional (3D) clusters. A transition of Au clusters from non-metal to metal is observed with increasing coverage. This paper is aimed at gaining a better understanding of the effects of clusters on the (110) rutile and ordered alumina film surfaces, which is important for further understanding of the origin of the catalytic properties of transition metal clusters. EXPERIMENT The experiments were carried out in two ultrahigh vacuum (UHV) chambers with base pressures of 2 ð Torr. One chamber is equipped with reverse-view optics for low-energy electron diffraction (LEED), an Auger electron optics with single-pass cylindrical mirror analyzer (CMA) for Auger electron spectroscopy (AES) and an LK-2000 model spectrometer for HREELS study. The second chamber has AES, LEED, x-ray photoelectron spectroscopy (XPS) and an He C source with a low primary energy for ion scattering spectroscopy (ISS). An Re(0001) single-crystal surface was used as the substrate for alumina film preparation. This substrate was mounted on a probe by spot-welding across two tantalum wires attached to a liquid N 2 cooling reservoir. The substrate can be heated resistively to 1500 K or heated to 2500 K using an electron beam heater at the back side of the sample and cooled down to 90 K using liquid N 2. A W 5%Re/W 26%Re DOI: /sia.1028 Copyright 2001 John Wiley & Sons, Ltd.

2 162 Q. Guo et al. thermocouple was spot welded to the edge of the samples for temperature measurements. The Al 2 O 3 film was prepared by deposition of Al onto the clean and well-ordered Re(0001) surface in O 2 ambient as described previously. 15 The TiO 2 (110) single-crystal sample (10 mm ð 5mm in area and 0.5 mm thickness) supplied by Commercial Crystal Laboratories Inc. was treated and mounted on a molybdenum plate as reported previously. 16 The Au doser is home-made with an Au wire of 99.95% plenty that was tightly wrapped around a tungsten filament. The rate of deposition of Au was calibrated prior to Au growth on the oxide. However, because Au does not necessarily grow on the oxide surface in a layer-by-layer manner, we prefer to use monolayer equivalent units (MLE) for designating Au coverage. For AES measurements, a primary electron beam energy of 3 kev was used. In HREELS, the incident energies, E p,of 25 ev with a typical resolution of mev at full width at half-maximum (FWHM) of the elastic peak were used. For diminishing the damage to the surface by the ion beam during ISS analysis, a primary He C ion beam of relatively low energy (600 ev) was used. RESULTS AND DISCUSSION Gold growth on TiO 2 (110) For clean TiO 2 (110) surfaces, two major surface structures were prepared based on the observation of 1 ð 1 and 1 ð 2 LEED patterns for differently treated crystals. In some cases, a 1 ð 1 surface with streaking along [010] was observed, dependent on the sample preparation procedure. The 1 ð 2 surface is believed to be caused either by a missing row of oxygen or by the formation of Ti 2 O 3. 17,18 The surfaces of TiO 2 (110) with different LEED patterns have been studied using HREELS and ISS prior to Au deposition. Figure 1 shows the EELS data for clean surfaces corresponding to the different LEED structures. For a perfect or near-perfect 1 ð 1 surface, the loss region between 1.0 and 3.5 ev is featureless, which facilitates the study of overlayer electronic structure such as the surface plasmon of Au. A loss at ¾0.8eV is observed for the 1 ð 1 -streaked surface and is even more pronounced for the 1 ð 2 surface, as shown in Fig. 1. This loss indicates the presence of surface defects arising from Ti C3 sites at oxygen vacancies ApreviousstudybyXPS has demonstrated the presence of Ti 3C caused by defects on the 1 ð 2 reconstructed surface. 16 Also shown in Fig. 1 is the spectrum of a sample with a 1 ð 1 LEED pattern that has been annealed in oxygen at 1100 K. This spectrum exhibits a much more abrupt drop from the shoulder of the overtone of the TiO 2 phonon directly to the baseline than is observed for the other surface structures. These data are consistent with the assignment of the 0.8 ev loss feature to be related to Ti 3C at oxygen vacancies. Figure 2 shows EELS spectra of the 1 ð 1 surface as a function of Au coverage at room temperature. For an initial deposition of Au (0.06 MLE coverage) a broad loss at ¾0.9 ev is observed. This loss is more pronounced at 0.1 MLE coverage, as shown in Fig. 2. At 0.2 MLE coverage, the intensity of the loss at 0.9 ev is decreased and a new loss at Figure 1. The HREELS spectra of TiO 2 1 ð 1, 1 ð 1 annealed in oxygen, 1 ð 1 with streak along [010] and 1 ð 2 surfaces E p D 25 ev, i D s D 60. Thedatawere obtained at 300 K. Figure 2. The HREELS spectra of Au deposited on TiO ð 1 at 300 K E p D 25 ev, i D s D ev appears. This loss peak of 2.6 ev remains at the same energy at coverages of MLE. In fact, we have found that this loss does not change position at high coverages for a thick layer of Au. The surface plasmon of bulk Au has been studied; its loss energy is 2.6 ev. 22 From our data, it appears that the Au clusters gain metallic properties when the coverage is >0.2 MLE. It is noteworthy, however, that in this study the Au surface plasmon does not appear at all at coverages of <0.2 MLE. In addition, a study on the

3 Initial growth of Au on oxides 163 Ag/Si system has shown that 2D Ag clusters will not give the surface plasmon. 23 Moreover, another study indicates that the surface plasmon is excited only after the condensed phase in the second monolayer has started to form. 24 This means that there must be 3D layers/clusters formed if surface plasmon is detected. In our studies, the fact that the surface plasmon of Au is not detected indicates the formation of 2D clusters of Au on the surface of TiO 2 (110) at initial growth. Therefore, we conclude that at initial deposition Au growth on the substrate is not 3D islands. This agrees well with a recent scanning tunneling microscopy (STM) investigation. It has been found that STM images represent two different stages of Au clusters on the TiO 2 (110): a quasi-2d mode and a hemispherical 3D mode. Low Au coverages lead to the quasi-2d clusters with diameters of nm and heights of 1 2 atomic layers. 25 For a defect surface, the change in the loss peak as a function of Au coverage is given in Fig. 3. As mentioned above, a loss at ¾0.8 ev is caused by surface defects. Gold coverages of 0.1 and 0.2 MLE result in a shift of this feature or the appearance of a second feature at ¾0.9eV.Atacoverage of 0.4 MLE the loss of the Au surface plasmon is detected at 2.6 ev. The peak position is unchanged compared with the same Au coverage on the 1 ð 1 surface. These data from both 1 ð 1 and 1 ð 2 surfaces coincide well with a recent STM study, from which it has been shown that Au cluster size changes as a function of coverage. 1 At 0.1 MLE coverage of Au the average diameter of the cluster size is 2 nm, and at 0.25 MLE the size becomes 3 nm in diameter, which coincides with the onset of metallic characteristics of the Au clusters. 1 Therefore, it is not surprising that at coverages higher than 0.2 MLE the energy loss of the surface plasmon of Au is at the same position as that of bulk Au. The clusters of Au grown on substrate are 2D islands at <0.2 MLE average, which do not show the characteristics of bulk Au. It is most likely that the introduction of Au on the surface results in a surface modification. One may argue that an introduction of Au might result in more surface defects or an interaction between Au and surface oxygen, resulting in Ti 3C and producing a loss for the defect surface at 0.9 ev. This can be excluded from a study on the growth of ultrathin Au films on the TiO 2 (110) surface by Madey and co-workers, in which no indication of a strong interaction at any Au coverage or substrate temperature is concluded. 26 A recent investigation by STM of Au grown on single crystalline TiO 2 shows that the bandgap of Au changes as a function of Au cluster size. When Au clusters become smaller, the bandgap is increased. 1 Becausethesurfacedefectsgivealoss at ¾0.8 ev, it is difficult to distinguish other loss features in this region. Experimentally, the samples were annealed at high temperature in order to make the sample conductive, which led to a shortage of oxygen atoms. Subsequently, the electronic structure, including the surface electronic structure, must be changed during sample preparations. For a surface that is annealed in oxygen, the 0.8 ev loss feature is absent and, furthermore, no loss feature appears in this region upon deposition of up to 1 MLE of Au. Therefore, although the cause of the loss at 0.9 ev after initial deposition of Au is still an open question, it is related to oxygen vacancies and possibly the perturbation of these vacancies by adsorbed Au clusters. The Au clusters on TiO 2 (110) were studied also using low-energy ion scattering spectroscopy (LEISS). Figure 4 shows the spectra for Au deposited on the 1 ð 1 and 1 ð 2 surfaces at different coverages. Using an He C primary energy of 600 ev, the signals of O and Ti from the clean substrate are observed at 270 and 457 ev, respectively. With increasing Au coverage, the intensities of surface O and Ti are gradually decreased and a new peak appears at 580 ev, which is indicative of Au. However, at a coverage of 5 MLE Au, the O and Ti signals still seen from LEISS are evidence of clustering of Au on both substrates. On the 1 ð 2 surface, Figure 3. The energy shifts of Au surface plasmon as a function of coverage on the TiO ð 2 surface at 300 K (E p D 25 ev, i D s D 60 ). Figure 4. The LEIS spectra of Au deposited on TiO ð 1 and 1 ð 2 surfaces. Argon ions of 600 ev were used. The coverages of Au on both surfaces from bottom to top are 0, 0.2, 0.4, 0.6, 1.0, 2.0 and 5.0 MLE.

4 164 Q. Guo et al. LEISS shows a signal of Au at 0.2 MLE coverage (see Fig. 4) but the surface plasmon of Au is not detected, as shown in Fig. 3. This indicates 2D Au on the defect surface at lower coverages. Gold growth on Al 2 O 3 /Re(0001) An alumina film ¾10 nm thick was prepared on Re(0001) prior to the deposition of Au. This film was characterized by LEED, AES, XPS and HREELS. The results are characteristic of Al 2 O 3. The vibrational frequencies of the surface phonons by HREELS are the same as those reported previously. 15 Using HREELS we have measured also the change of the surface plasmon of Au on the alumina film as a function of the coverage. The results for Au deposition at 300 K are given in Fig. 5. On the clean alumina film surface the spectrum is featureless from 2 to 7 ev, which supplies a window for further study of Au deposition. At a coverage of 0.04 MLE a weak loss is detected at ¾3.0eV,asshownin Fig. 5. For 0.1 MLE coverage, this loss is shifted slightly to 2.9 ev. However, at 0.3 MLE a dominant loss at 2.5 ev was observed. The surface plasmon of Au approaches 2.6 ev at a coverage of >0.3 MLE. This loss does not change position further as the coverage increases. Because the surface plasmon of metallic Au is at 2.6 ev as discussed above, we conclude that at coverages of ¾0.3MLE the Au clusters on the alumina surface are large enough to have the characteristics of bulk Au. However, at lower coverage the Au surface plasmon loss energy increases as the coverage is decreased. The electron energy loss of a surface plasmon depends on many factors. Generally it is related to the momentum transfer parallel to the surface, determined by the incident and scattering angles, the primary beam energy and the loss energy of the surface plasmon. Besides, the energy of loss is also a function of surface structure, the surface temperature and the complex interaction between the cluster and the substrate. Moreover, various cluster shapes may change the loss energy. For instance, recent calculations indicate that cube-shaped metal clusters are considerably lower in energy than that of spherical clusters. 27 Because in our experiments we have used the same primary energy and temperature as well as the same incident and scattering angles, the change in loss energy observed must be caused by either a cluster size effect or interaction between Au clusters and the substrate (alumina film). This is quite different from Au/TiO 2 (110), in which the surface plasmon of Au is not detected at initial growth of Au. The average particle size increases with increasing coverage and ultimately the cluster characteristics coincide with those of the bulk metal. In our study, at a coverage of >0.3 MLE, the appearance of the 2.6 ev energy loss indicates the formation of metal-like Au clusters. At lower coverage, however, Au growth on the substrate is not 2D clusters due to the appearance of the surface plasmon. In fact, the surface plasmon of Au exhibits a blue shift with a decrease of Au deposition. Often a blue shift is caused by a reduced s d screening interaction on the outer regions of the particle, where the s-electrons spill into the vacuum; the s-electrons in this region oscillate with the unscreened plasma frequency. For small cluster sizes this effect becomes important. 28 Although the energy shifts of the surface plasmon of clusters are dependent on many factors, we believe that by using identical experimental conditions these factors will be diminished. The blue shift of the Au plasmon as a function of decreasing Au coverage at 300 K in our experiments is a possible cluster size effect. As discussed above, we conclude that at the initial deposition of Au, 3D Au clusters are more easily formed on alumina films than on rutile. CONCLUSION Growth of Au on TiO 2 (110) with 1 ð 1 and 1 ð 2 structures and ordered Al 2 O 3 films prepared on Re(0001) has been studied by various UHV surface analysis techniques. The results show that the surface plasmon of Au can be used to study the Au clusters. On the TiO 2 (110), Au initially grows as 2D islands at room temperature. A plasmon loss characteristic of metallic Au is found at a coverage of >0.2 MLE. On alumina films, Au initially grows as 3D clusters but the size is too small to exhibit the properties of metallic Au. However, a surface plasmon characteristic of metallic Au appears at a coverage of >0.3 MLE. Acknowledgements We gratefully acknowledge support of this work by the US Department of Energy, the US National Science Foundation and the Chinese Natural Science Foundation. Figure 5. The EELS spectra of Au deposited on Al 2 O 3 /Re(110) at 300 K with different coverage (E p D 25 ev, i D s D 60 ). The Au coverage is given in MLE. REFERENCES 1. Valden M, Lai X, Goodman DW. Science 1998; 281: Haruta M. Catal. Today 1997; 36: 153.

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