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Microstructured Au/TiO2 Model Catalyst Systems S. Kielbassa, M. Kinne, and R. J. Behm* Department Surface Chemistry and Catalysis, University of Ulm, D-89069 Ulm, Germany Received May 29, 2003. In Final Form: March 30, 2004 The preparation of microstructured Au/TiO2 model catalysts as a first step toward micrometer-scale parallel studies on model catalysts and toward studies of mesoscopic effects in catalytic reactions was investigated by atomic force microscopy and X-ray photoelectron spectroscopy. The model systems, which consist of micrometer-size active areas covered with Au nanoparticles that are separated by similarly sized inactive areas free of Au particles, are fabricated by combining optical lithography methods for microstructuring and ultrahigh vacuum evaporation for Au nanoparticle deposition and by applying suitable cleaning steps. It is demonstrated that practically perfect microstructures with Au nanoparticles of catalytically relevant sizes (2-3-nm diameter) on a clean TiO2 substrate can be produced this way and that the processing steps do not affect the deposited Au nanoparticles, neither in size nor in lateral distribution.
1. Introduction Planar oxide-metal model systems have attracted increasing interest over the last years for fundamental studies of catalytic reactions and catalyst properties, both because of their well-defined morphology and because they are easily accessible by modern spectroscopic and microscopic surface science tools.1-5 For various applications, it would be highly desirable if such model systems could be structured in a way that catalytically active areas, functionalized with the same or different catalysts, are separated by inert areas of controlled size and shape. Such model systems would allow, for example, studies of mesoscopic effects during catalytic reactions such as coupling between separate catalytically active areas, spillover effects, or the formation of two-dimensional reaction patterns on surfaces with a controlled structure6-8 as well as parallel studies of various different catalysts, analogous to the high-throughput parallel testing of catalysts on a macroscopic scale.9,10 This is the goal of a project started recently in our laboratory. In the present paper, we report first results on the preparation of microstructured planar Au/TiO2 model catalyst systems. Metal oxide supported Au catalysts, in particular on reducible metal oxide supports, have attracted considerable interest in the past years as highly active catalysts for low-temperature oxidation and hydrogenation reactions;11-13 for the same reason, unstruc* Corresponding chemie.uni-ulm.de.
author.
E-mail:
juergen.behm@
(1) Rainer, D. R.; Xu, C.; Goodman, D. W. J. Mol. Catal. A 1997, 119, 307. (2) Freund, H. J. Angew. Chem. 1997, 109, 445. (3) Lambert, R. M.; Pacchioni, G. In Chemisorption and Reactivity on Supported Clusters and Thin Films; NATO ASI; Kluwer Academic Publishers: Dordrecht, 1997. (4) Henry, C. R. Surf. Sci. Rep. 1998, 31, 231. (5) Freund, H. J. Surf. Sci. 2002, 500, 271. (6) Schu¨tz, E.; Hartmann, N.; Kevrekidis, Y.; Imbihl, R. Catal. Lett. 1998, 54, 181. (7) Esch, F.; Gu¨nther, S.; Schu¨tz, E.; Schaak, A.; Kevrekidis, I. G.; Marsi, M.; Kiskinova, M.; Imbihl, R. Catal. Lett. 1998, 52, 85. (8) Esch, F.; Gu¨nther, S.; Schu¨tz, E.; Schaak, A.; Kevrekidis, I. G.; Marsi, M.; Kiskinova, M.; Imbihl, R. Surf. Sci. 1999, 443, 245. (9) Jandeleit, B.; Schaefer, D. J.; Powers, T. S.; Turner, H. W.; Weinberg, W. H. Angew. Chem. 1999, 111, 2648. (10) Hoffmann, C.; Wolf, A.; Schu¨th, F. Angew. Chem., Int. Ed. 1999, 38, 2800. (11) Haruta, M. Catal. Surv. Jpn. 1997, 1, 61. (12) Bond, G. C. Catal. Rev. Sci. Eng. 1999, 41, 319.
tured planar Au/TiO2 model systems have been prepared and investigated by numerous groups.14-29 The most prominent application, the low-temperature CO oxidation, shall later be used as a test reaction on the microstructured model catalysts. Microstructuring is performed by standard optical lithography techniques; the Au particles are deposited via ultrahigh vacuum (UHV) evaporation. Because of the planned application, the main goal is not to produce as small as possible microstructuressthe minimum size for the planned local mass spectrometric detection of reaction products is expected to be several micrometerssbut to establish procedures that will (i) allow the deposition of catalytically active Au nanoparticles of a few nanometers (2-4 nm) in size as required for highly active Au/TiO2 catalysts and (ii) not affect the activity, size, and distribution of these nanoparticles during subsequent processing. The main questions to be addressed in the present paper are (i) how far is the photoresist film fully removed after development, leaving clean surface areas, on a sub-nanometer scale, for the subsequent Au deposition; (ii) is it possible to produce Au nanoparticles of the correct size and at a high density on the free surface areas; (iii) are the Au particles adsorbed on the TiO2 substrate (in the developed structures) stable during the subsequent lift-off of the remaining photoresist; and (iv) does the subsequent lift-off step completely remove (13) Haruta, M.; Date´, M. Appl. Catal. A 2001, 222, 427. (14) Zhang, L.; Persaud, R.; Madey, T. E. Phys. Rev. B 1997, 56, 10549. (15) Lai, X.; St. Clair, T. P.; Valden, M.; Goodmann, D. W. Prog. Surf. Sci. 1998, 59, 25. (16) Valden, M.; Lai, D.; Goodman, D. W. Science 1998, 281, 1647. (17) Valden, M.; Pak, S.; Lai, X.; Goodman, D. W. Catal. Lett. 1998, 56, 7. (18) Parker, S. C.; Grant, A. W.; Bondzie, V. A.; Campbell, C. T. Surf. Sci. 1999, 441, 10. (19) Lai, X.; Goodman, D. W. J. Mol. Catal. A 2000, 162, 33. (20) Yang, Z.; Wu, R.; Goodman, D. W. Phys. Rev. B 2000, 61, 14066. (21) Kolmakov, A.; Goodman, D. W. Catal. Lett. 2000, 70, 93. (22) Fukui, K.-I.; Iwasawa, Y. Phys. Chem. Chem. Phys. 2001, 3, 3871. (23) Mitchell, C. E. J.; Howard, A.; Carney, M.; Egdell, R. G. Surf. Sci. 2001, 490, 196. (24) Kolmakov, A.; Goodman, D. W. Surf. Sci. 2001, 490, L597. (25) Spiridis, N.; Haber, J.; Korecki, J. Vacuum 2001, 63, 99. (26) Cosandey, F.; Zhang, L.; Madey, T. E. Surf. Sci. 2001, 474, 1. (27) Cosandey, F.; Madey, T. E. Surf. Rev. Lett. 2001, 8, 73. (28) Kielbassa, S.; Kinne, M.; Behm, R. J. J. Phys. Chem. B, in press. (29) Kitchin, J. R.; Barteau, M. A.; Chen, J. G. Surf. Sci. 2003, 526, 323.
10.1021/la0302201 CCC: $27.50 © 2004 American Chemical Society Published on Web 06/29/2004
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Figure 1. Schematic description of the photolithography process used for microstructuring the model catalyst. (1) Application of the photo film (resist) onto the surface by spin-coating, (2) exposure to UV light through a mask, (3) developing of the exposed resist in alkaline solution, (4) evaporation of Au, and (5) lift-off process (removing the remaining resist in acetone).
the remaining film, including the Au particles deposited on the photoresist, leaving a clean TiO2 surface with no remainders on an atomic/molecular scale in the areas between the microstructures. In the end, we are aiming at a microstructured model system which contains wellseparated Au nanoparticles on a clean substrate in the microstructures, inert areas with neither residues of the organic resist nor any Au nanoparticles between the microstructures, and a sharp transition between these areas at the edges of the microstructures. The first point is particularly important in the present case, because TiO2 is well-known for its photocatalytic activity,30 and it cannot be ruled out that the UV exposure leads to photocatalyzed reactions in the film close to the interface, which could affect the removal of the illuminated film in the development stage.
Figure 2. XPS survey scan of a TiO2(110) surface covered with a photo film of 1.5-µm thickness.
2. Experimental Section Surface characterization and Au deposition were mainly performed in a UHV system equipped with facilities for surface characterization, a Au evaporator source, a load lock for rapid sample transfer and a high pressure cell for high pressure processing of planar model catalyst systems in reactive atmospheres at up to ambient pressures, and a fully UHV-compatible atomic force microscope (AFM). The AFM is a home-built design which relies on optical beam deflection for detection.31 The maximum scan range of the instruments is 1 µm × 1 µm, with a vertical resolution of about 0.3 Å. In situ AFM measurements were performed in the attractive noncontact mode (nc-mode) at low force settings to avoid tip-induced displacements of the Au particles. For the AFM measurements, the TiO2 sample was mounted in a special sample holder machined from Mo. The sample (and sample holder) could be heated under UHV conditions via a tungsten filament. In situ surface characterization was possible by X-ray photoelectron spectroscopy (XPS; SPECS EA 200). XPS spectra were obtained with non-monochromatized Al KR radiation (200 W). Because of geometric limitations imposed by the sample holder and the sample manipulator, the spectra had to be taken at normal emission. Large-scale AFM measurements with a scan range of up to 100 µm × 100 µm were performed in air with a commercial AFM (Topometrix Explorer), also in the nc-mode. The TiO2(110) substrates (one side polished) were purchased from TBL Kelpin (Germany). Prior to the experiments, the samples were cleaned in acetone (5 min in a supersonic bath at room temperature) and in 1:1 mixture of 30% H2O2 and concentrated H2SO4, subsequently rinsed in Millipore water, and finally calcined 2-3 h in air at 900 °C. Following this procedure, the TiO2 substrates exhibited a very well-ordered surface with atomically flat terraces of 50-100-nm width separated by monolayer steps. On the basis of the XPS data, the surfaces are (30) Fujishima, A.; Hashimoto, K.; Watanabe, T. In TiO2 Photocatalysis - Fundamentals and Applications, 1st ed.; Bkc. Inc.: Tokyo, 1999. (31) Wiechers, J. Ph.D. Dissertation, University Mu¨nchen, Mu¨nchen, Germany, 1993.-
very clean with only trace amounts of carbon as the only surface impurity (see also Figure 4), which are presumably picked up during transport from the oven to the UHV system, and they are fully oxidized, in contrast to samples prepared in the standard way of sputtering and annealing in a vacuum. Further details on the resulting surfaces are given in ref 28. The different steps of the photolithographic process, which were performed in a “class 10” clean room, are schematically shown in Figure 1. First, the photo film (positive photoresist on DNQ-Novolak basis, AR-P 5350, Allresist, Germany) is deposited on the flat TiO2(110) substrate by spin-coating (film thickness ∼ 1.5 µm) and prebaked for 10 min at 100 °C in an evacuated oven. In the next step, the film is exposed to UV light (275 W, 7 s) through a mask to create the desired microstructures, using a SUSS MJB 3 mask aligner. The masks were produced by electron-beam lithography (x-trem lithography, Ulm). In the present study, we chose square fields of 5-µm side length, which are separated from each other by 10 µm in each direction. The resist was developed in an alkaline developing bath (AR 300-35, diluted 1:2 with water) for 90 s. After removal of the exposed resist, the sample was transferred into the UHV system to deposit the Au particles by evaporation from a homemade evaporator. During evaporation, the sample was kept at room temperature (typical deposition rate: 10 Å/min). Finally, the remaining resist was removed by dipping in acetone (Merck, selectipur) for 10 min.
3. Results and Discussion First the homogeneity and chemical composition of the resist film were controlled by optical microscopy and by XPS. Microscopy images of well-prepared films show homogeneous surfaces. As expected for a clean film surface, the XPS spectra are dominated by the C(1s) and O(1s) signals (see Figure 2). In addition, small sulfur peaks [S(2s) and S(2p)] are apparent, which on the basis of their intensity, correspond to 0.5 atom % and which result from the sulfonyl groups in the resist. During the initial tests,
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it turned out that the spin-coating procedure is very sensitive to contamination and that cleaning the samples in the H2SO4/H2O2 mixture prior to calcining in air reduced the probability for getting defects in the film significantly. In the next two steps, irradiation by UV light and the subsequent developing, it has to be verified that the film is completely removed in the irradiated areas defining the later microstructures. As mentioned in the introduction, this is especially important for TiO2 substrates because they may be affected by the UV light in a photocatalytic reaction. It had been shown recently that Au particles prepared from gold-phosphine complexes are thermally more stable on TiO2(110) surfaces that had been pre-exposed to UV light than on nonexposed samples.22 We tested this by evaporating Au on a clean TiO2(110) crystal, part of which had been exposed to UV light prior to the Au deposition. Using an AFM or XPS, we could not observe any difference between the different areas and the growth and stability of the Au nanoparticles thereon, neither before nor after evaporation and subsequent annealing in air at 350 °C (details on the growth and thermal stability of the nanoparticles on the fully oxidized TiO2 substrates are given in ref 28). The cleanness of the surface after the developing step was evaluated by examining the surface of an unstructured TiO2 sample by the AFM and XPS after it had been covered by a photo film, then exposed to UV light, and finally developed. AFM images of the surface directly after the development step and after a subsequent short time of annealing (3 min) to 500 °C in an UHV, respectively, are shown in Figure 3. After developing, the surface is obviously covered with small adsorbate “particles”, which are removed by the subsequent UHV annealing step. The density and average size of these particles are around 1011 cm-2 and 0.7-nm height, respectively. XPS survey spectra demonstrate that the surface contains no other elements than C, Ti, and O. Comparison of the C(1s) peaks of the XPS spectrum recorded on the sample directly after the development step with that of a freshly prepared sample that was exposed to air shows practically no difference in the C(1s) intensity (Figure 4). The binding energy (BE) of the C(1s) peak of about 285 eV is typical for hydrocarbons. After UHV annealing, XPS spectra indicate a significant loss of carbon. The C(1s) signal decreases by about 30%, similar to what is observed when annealing a freshly cleaned sample in a vacuum after exposure to air, and the Ti/O ratio increases from 0.43 to 0.49. The decrease in oxygen intensity is attributed to the desorption of water from adsorbed water or hydroxyl species and the desorption of oxygen containing hydrocarbon species. Changes in the Ti(2p) XPS signal indicating a reduction of the Ti4+ species due to desorption of O could not be observed. (Thermal loss of oxygen from TiO2 requires temperatures above 600 °C).32 On the basis of these results, it can be ruled out that the “particles” apparent in the AFM images are due to remaining polymers from the resist. Furthermore, adsorbed polymer particles should turn into coke upon UHV annealing rather than disappear. Hence, the photo film is quantitatively removed by the developing process and the remaining adsorbates are assumed to result from impurities in the development solution and from hydrocarbons picked up during transport through air. The following step, evaporation of Au on the prestructured surface, appears uncritical. Therefore, we did not acquire high-resolution AFM images of the Au nanopar(32) Diebold, U.; Anderson, J. F.; Ng, K. O.;Vanderbilt, D. Phys. Rev. Lett. 1996, 77, 1322.
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Figure 3. AFM (nc-mode) image of a TiO2 surface (a) directly after exposure to UV light and development of the resist and (b) after 3 min of annealing to 500 °C in an UHV (both 800 nm × 800 nm).
ticles in the microstructures but will compare the size and distribution of the deposited Au nanoparticles after the lift-off with particles deposited on a clean, unstructured TiO2 substrate. The next critical step is the lift-off of the remaining photo film, which is performed by dipping the sample for 10 min into acetone at room temperature. During this process, Au nanoparticles may be washed away from the TiO2 substrate or be displaced from the microstructures. Furthermore, it has to be verified that the Au nanoparticles deposited on the photo film are completely removed and not transferred to the TiO2 substrate during this process. To ensure that the surface is free of contaminations or organic remainders after the lift-off process, it was characterized by the AFM and XPS, in the same way as described previously for the developing step. The results show that the removal of the photo film by acetone dipping is not as clean as expected. AFM images resolve a large number of adsorbed species/particles on the surface. In contrast to the residues after UV-light exposure and development of the resist, these adsorbates are stable even after annealing in an UHV to 500 °C (see Figure 5a). The XPS survey spectrum recorded after removal of the photo
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Figure 4. XPS spectra of a photo film covered TiO2 substrate surface directly after development of the resist. (a) Survey spectrum and (b) normalized detail spectrum of the C(1s) region (i). In addition, part b also contains a similar spectrum of a freshly prepared sample after annealing in air (ii) and the difference between the C(1s) spectra (iii).
film shows that also in this case no elements other than C, O, and Ti are present on the surface (Figure 6a), indicating that these contaminations are either due to organic adsorbates from the solvent (acetone) or remaining polymer species from the photo film. This is confirmed also by XPS data, which show that after subsequent UHV annealing (500 °C, 15 min) the C(1s) intensity is significantly, by at least 50%, higher than that of a freshly prepared clean sample after similar UHV annealing. This question of the origin of the carbon impurities was investigated by comparing the C(1s) XPS spectrum recorded after the removal of the photo film with the C(1s) peak of a fresh, cleaned sample, which was only dipped in acetone (see Figure 6b). On the latter sample, the C(1s) region exhibits a major peak at 285.3 eV and a shoulder at 287.2 eV, which can be assigned to unpolar carbon in hydrocarbon species and to carbon in CdO bonds such as in acetone, respectively. The BE of the main peak is similar to that of a fresh, clean sample. The situation is different for the sample which had been covered with a photo film first and then dipped in acetone, indicating that the C(1s) intensity on that sample is not due to the pick up of carbon containing contaminations from the solvent (acetone) but results from adsorbed polymer species from the photo film remaining on the surface. The C(1s) intensity is significantly higher than in the former case, and the BE of the dominant peak is slightly shifted to lower energies, to 284.8 eV. The shift to lower BE can tentatively be explained by the presence of phenol-containing polymer residues on the surface.
Figure 5. AFM images of TiO2 surfaces after removal of the photo film by dipping in acetone (10 min, room temperature) and subsequent (a) 15 min of annealing at 500 °C in an UHV or (b) 15 min of annealing at 350 °C in air, respectively (both 800 nm × 800 nm).
This agrees well with the AFM results. Most probably the polymer species remaining on the surface after the lift-off decompose during UHV annealing rather than desorb (“coke formation”), and the carbonaceous particles are still present on the surface and resolved in the AFM images. Performing the lift-off step at higher temperatures of acetone does not affect these findings. A complete removal of the residues could only be achieved by annealing the samples to 350 °C in air (15 min) instead of UHV annealing at 500 °C (15 min; see Figure 5b). Similar conclusions can be drawn also from XPS data. Next, we explored the stability of the Au nanoparticles deposited on the TiO2 substrate during acetone dipping. This was investigated by XPS for three unstructured samples with different Au coverages, that is, with different Au particle sizes. Before dipping, the Au-particle-covered sample was annealed at 250 °C, then dipped into acetone for 10 min, and finally annealed in an UHV for 30 min at 250 °C to remove part of the adsorbed organic species. (The annealing conditions chosen here are a compromise
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Table 1. Influence of Acetone Treatment on the Au Deposit: Normalized Au(4f) Intensities before and after Dipping the Samples in Acetonea sample 1
before acetone treatment after acetone treatment a
sample 2
sample 3
BE (eV) Au(4f)
I[Au(4f)]/ I[Ti(2p)]
BE (eV) Au(4f)
I[Au(4f)]/ I[Ti(2p)]
BE (eV) Au(4f)
I[Au(4f)]/ I[Ti(2p)]
84.3/87.9 84.3/87.9
0.042 0.037
84.2/87.8 84.2/87.8
0.052 0.044
84.0/87.6 84.0/87.6
0.160 0.185
Au coverages: sample 1: 0.2 monolayers; sample 2: 0.3 monolayers; sample 3: 0.9 monolayers.
Figure 7. Au(4f) XPS signal before (i) and after (ii) the treatment in acetone. The difference spectrum (iii) shows no significant influence of the treatment on this spectrum.
Figure 6. XPS spectra of the C(1s) peaks (i) of a TiO2(110) sample directly after removal of the photo film with acetone and (ii) of a freshly prepared sample that was only dipped in acetone.
between maximizing the removal of organic species and minimizing thermal growth of the Au nanoparticles. The initial annealing step was performed to further reduce thermal effects on the Au nanoparticles.) Au(4f) spectra taken before and after the acetone dip of one sample (sample 3) are shown in Figure 7. The resulting normalized Au(4f) intensities of the three samples are collected in Table 1. On average, the changes in relative Au intensity for these three samples are rather small and within the error tolerance of XPS, indicating that very little or no Au is dissolved from the TiO2 substrate surface during acetone dipping. One of the samples (sample 3 with 0.9 monolayer Au) was imaged by an AFM before and after the dipping step. Also, these images show no changes in the surface morphology. After the acetone treatment, the surface is still homogeneously covered by the Au nanoparticles, as evident from the two images in Figure 8. The particle density remains constant at 2.4 × 1012 cm-2, in excellent agreement with the XPS data. On the basis of these data,
we conclude that the lift-off step does not affect the Au nanoparticles deposited in the microstructures on the TiO2 substrate. Finally, we investigated the size and distribution of the Au nanoparticles within the resulting microstructures, after the lift-off process, and explored whether the particle distribution at the edge of the microstructures differs from that in the more central regions. The latter would indicate that the growth of the Au nanoparticles is affected by the presence of the photo film at the border of the prestructured fields. Such effects were observed after depositing thiolstabilized gold particles on lithographically prestructured oxidized silicon surfaces.33,34 Representative AFM images of an entire Au microstructure and of the border region are shown in Figures 9 and 10, respectively. The samples were annealed at 500 °C in air (30 min) after the lift-off step to obtain particles that are big enough to be resolved also on these larger scales. Because of the lower vertical resolution of the ex situ AFM used for recording the image in Figure 9, only the biggest particles (height > 1.5 nm) are visible here. Despite the lower resolution, the image is clear proof that the particles are exclusively found in the microstructure areas and that the Au-free areas separating the Au microstructures are indeed free of Au nanoparticles. The images of the perimeter region of the microstructures in Figure 10, which show the interface between the Au microstructure and the Au free areas, were recorded on the higher resolution UHV AFM. The larger-scale image in Figure 10a shows that the particle distribution within the microstructures is homogeneous, similar to our observations on the unstructured substrates (see previous). The homogeneous distribution of the Au nanoparticles was tested on several samples with always the same positive result. Hence, there is absolutely no evidence for Au particles being washed away from the surface during the lift-off process in acetone. (33) Parker, A. J.; Childs, P. A.; Palmer, R. E.; Brust, M. Appl. Phys. Lett. 1999, 74, 2833. (34) Parker, A. J.; Childs, P. A.; Palmer, R. E.; Brust, M. Nanotechnology 2001, 12, 6.
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Figure 8. AFM (nc-mode) images of the Au-covered surface after room-temperature treatment in acetone. (a) Large-scale overview image (0.75 µm × 0.75 µm), (b) smaller scale detail (150 nm × 150 nm).
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Figure 10. AFM (nc-mode) images of the perimeter of a Au microstructure, resolving the Au nanoparticles within the microstructure. The surrounding surface is free of Au particles. (a) Large-scale image (0.75 µm × 0.75 µm), (b) smaller scale detail image (200 nm × 200 nm).
case (Figure 8), but the annealing step would not cover local losses of Au particles. Finally, the smaller scale image (Figure 10b) provides clear proof that the photo film surrounding the microstructure during Au deposition has no effect on the particle growth. It does not show any agglomeration or depletion at the perimeter of the particle field. 4. Conclusions
Figure 9. Ex situ AFM (nc-mode) image of an entire Au microstructure and the surrounding Au-free area (7.5 µm × 7.5 µm).
Because of the additional annealing step, the particle density (8 × 1011 cm-2) is of course lower than in the latter
Using atomically flat and clean TiO2(110) substrates, we have demonstrated that practically perfect microstructured model catalysts, containing micrometer-size (here 5 µm) catalytically active structures covered by Au nanoparticles of catalytically relevant sizes (2-3-nm diameter) separated by inactive areas free of Au nanoparticles of variable sizes and geometries can be fabricated by combining standard optical lithography techniques and UHV methods for Au nanoparticle deposition and by applying suitable cleaning steps. Using XPS and an AFM for characterizing the morphology and chemical composition of the surface at the subsequent processing steps, we
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could show that (i) the removal of the photo film during developing and during the lift-off of the remaining film, after deposition of the Au nanoparticles, results in rather clean surfaces and carbon impurities remaining after the lift-off can largely be removed by annealing in air above 350 °C after, that (ii) neither the carbon impurities after developing nor the subsequent lift-off process cause any significant effects on the size and distribution of the Au nanoparticles, and that (iii) the lift-off process leads to a complete removal of the Au nanoparticles which had been deposited on the undeveloped film, leaving clean, Au-free areas between the Au-covered microstructures. The feasibility of preparing quasi-perfect microstructured model catalysts with variable, controlled geometries is a first step toward micrometer-scale parallel studies on
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model catalysts and toward studies of mesoscopic effects in catalytic reactions on such systems. Further work, in particular on the local detection of the catalytic activity of individual microstructures and on the use of prefabricated nanoparticles for functionalizing the prestructured surface, is in progress. Acknowledgment. We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft via Sonderforschungsbereich 569 and by the State of BadenWu¨rttemberg via the “Network Functional Nanostructures”. We would like to thank Dr. A. Plettl for his help in the lithographic processing. LA0302201
