Langmuir 1996, 12, 5595-5600
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Catalytic Surfaces from Langmuir-Blodgett Films of Tris(4,4′-diisopropyldibenzylideneacetone)palladium(0) as Precursor. 1. Study of the Thermal Complex Decomposition and Formation of Palladium Particles Ellen Maassen and Bernd Tieke* Institut fu¨ r Physikalische Chemie der Universita¨ t zu Ko¨ ln, Luxemburger Strasse 116, D-50939 Ko¨ ln, Germany
Guntram Jordan and Werner Rammensee Mineralogisch-Petrographisches Institut der Universita¨ t zu Ko¨ ln, Zu¨ lpicher Strasse 49b, D-50674 Ko¨ ln, Germany Received February 13, 1996. In Final Form: August 12, 1996X A detailed study is presented on Langmuir and Langmuir-Blodgett (LB) films of tris(4,4′-(diisopropyldibenzylideneacetone)palladium(0) (1) and the thermal decomposition of these LB films under formation of catalytic palladium particles. Monolayers of 1 at the air-water interface were characterized by their π-A isotherms, while LB film formation was studied using UV-vis spectroscopy. Thermal decomposition was investigated using spectroscopic methods (X-ray photoelectron spectroscopy, UV-vis spectroscopy, and attenuated total reflectance infrared spectroscopy) and microscopic methods (scanning force microscopy). It was found that the spreading of cyclohexane solutions of 1 at the air-water interface leads to stable monolayers, which can be deposited on hydrophobic supports (e.g., silanized quartz, polyarylate) according to the LB technique. Transfer conditions are optimum at a surface pressure of 7.5 mN m-1 and a subphase temperature of 7.5 °C. Monolayers are transferred during the downstroke and upstroke, the transfer ratio being 1. If the LB films are annealed at 100 °C or higher, they gradually decompose under formation of palladium particles whose size is controlled by time and temperature of the annealing process. For example, annealing at 120 °C for up to 3 min leads to formation of palladium clusters with absorption maxima at 280, 380, and 420 nm, while a thermal treatment for 10 min or longer favors the growth of metallic palladium particles with an average diameter of 22 nm. Simultaneously, the ligand molecules are set from the complex and crystallize on the surface under formation of large particles with an average diameter of 250 nm.
1. Introduction Heterogeneous catalysts are of outstanding interest for industrial chemical synthesis. Catalytic activity mostly depends on the presence of specific active sites on definite crystal surfaces. Specific generation of such active sites should thus allow tailoring of new and highly efficient catalyst systems. A relevant base technique could be the Langmuir-Blodgett (LB) deposition method. Using the LB technique it is possible to prepare molecular layers at interfaces with controllable thickness and arrangement of the molecules. In recent years, LB films became very popular in academic and applied surface science,2-4 but they did not yet achieve any importance in the preparation of catalytically active substrates. Indeed, semiconductor and metallic particles have been incorporated5 or generated6 in LB films and their physical properties have been characterized. Also, LB films have been used as precursors for the production of ultrathin metal7 and metal oxide
films.8 However, little is known on the use of the converted films as heterogeneous catalysts. In a recent short communication,7 it has been reported that LB films of the tris(4,4′-di(isopropyldibenzylideneacetone)palladium(0) complex (1) can be used as a precursor for the production of catalytically active substrates in electroless metal plating. However, a precise study of the thermal conversion of LB films of 1 into supported palladium particles and their catalytic activity on the solid support is still lacking. Such LB films could exhibit a high potential for the preparation of new supported catalysts, and therefore we decided to investigate these films in more detail.
* Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, October 15, 1996.
(Dibenzylideneacetone)palladium (dba-Pd) complexes with different stoichiometry and structure have been known for a long time.9 Their use for activation of
(1) Blodgett, K. B. J. Am. Chem. Soc. 1935, 75, 1007. (2) Langmuir-Blodgett Films; Roberts, G. G.; Ed.; Plenum: London, 1990. (3) Ulman, A. An introduction to ultrathin organic films; Academic Press: San Diego, 1991. (4) Kuhn, H.; Mo¨bius, D. Investigations of Surfaces and Interfaces, Part B, 2nd ed.; Rossiter, B. W., Baetzold, R. C., Eds.; Physical Methods of Chemistry Series; John Wiley & Sons, Inc: New York, 1993; Vol. IXB, p 375. (5) (a) Zhao, X. K.; Xu, S.; Fendler, J. H. J. Phys. Chem. 1990, 94, 2573. (b) Du, Z.; Zhang, Z.; Zhao, W.; Zhu, Z.; Zhang, J.; Jin, Z.; Li, T. Thin Solid Films 1992, 210/211, 404. (c) Meldrum, F. C.; Kotor, N. A.; Fendler, J. H. Langmuir 1994, 10, 2035; Chem. Mater. 1995, 7, 1112.
(6) (a) Zylberajch, C.; Ruaudel-Teixier, A.; Barraud, A. Thin Solid Films 1989, 179, 9. (b) Leloup, J.; Maire, P.; Ruaudel-Teixier, A.; Barraud, A. J. Chim. Phys. (Paris), 1985, 82, 695. (c) Perez, H.; RuaudelTeixier, A.; Rouillay, M. Thin Solid Films 1992, 201/211, 410. (d) Sastry, M.; Mandale, A. B.; Badrinarayanan, S.; Ganguly, P. Langmuir 1992, 8, 2354. (7) Tieke, B.; Zahir, S. A.; Mathieu, H. Adv. Mater. 1991, 3, 96. (8) (a) Kalachev, A. A.; Mathauer, K.; Ho¨hne, U.; Mo¨hwald, H.; Wegner, G. Thin Solid Films 1993, 288, 307. (b) Mirley, C. L.; Koberstein, J. T. Langmuir 1995, 11, 1049 and 2837. (9) Takahashi, Y.; Ho, T.; Sakai, S.; Ishii, Y. J. Chem. Soc., Chem. Commun. 1970, 1065.
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polymeric substrates for electroless metal plating has also been described.10 Cross-linkable dba-Pd complexes have been synthesized and incorporated in epoxy resins.11 However, mechanistic studies of the thermal complex decomposition and the relations between structure and catalytic activity of the palladium particles have not yet been described. The present study is concerned with a detailed characterization of monolayers and LB films of 1 and their thermal decomposition under formation of palladium particles on the substrate. Monolayers of 1 at the airwater interface were characterized by their π-A isotherms, while LB films and their thermal decomposition were studied using X-ray photoelectron spectroscopy (XPS), UV-vis and attenuated total reflection infrared (ATR-IR) spectroscopy, and scanning force microscopy (SFM). The study of the catalytic activity and its relation to palladium particle size and concentration as well as to the structure of the original LB film will be reported in a separate part two of this investigation12. 2. Experimental Section Materials. Tris(4,4′-diisopropyl)dibenzylideneacetonepalladium(0) (1) was prepared according to the literature7,13 and obtained as a brown microcrystalline powder melting at 165 °C under decomposition. Anal. Calcd for C69H78O3Pd‚CH3OH (1092.42): C, 77.00; H, 7.5; Pd, 9.74. Found: C, 76.40; H, 7.55; Pd, 9.62. IR (KBr): 1643 cm-1 (ν(CdO)), 981 cm-1 (δ(CHdCH)); UV (CHCl3) 236 nm ( ) 53.0), 340 nm (77.5), 520 nm (8.8). Polyarylate (Mw ) 4.6 × 104 g mol-1, Tg ) 188 °C) was obtained from Amoco (M-100) and used without further treatment. Monolayer Formation and Deposition. For monolayer studies and formation of LB films a commercially available film balance (Lauda FW-1) equipped with a film lift was used. Monolayers were spread from cyclohexane solution (spectroscopic grade, concentration 0.5-1 mg mL-1) onto a pure water subphase (Milli-Q plus). LB films were built up on quartz slides or a zinc selenide crystal hydrophobized with trichlorooctadecylsilane prior to the transfer process. Conditions for transfer were optimum at a surface pressure of 7.5 mN m-1 and a subphase temperature of 7.5 °C (dipping rates were 2 cm min-1 downward and 2 cm min-1 upward). Annealing of the LB films was carried out in an UT 6000 air-circulation drying oven (Heraeus Instruments). Spectroscopic Measurements. ATR-IR spectra were measured using a Nicolet 5 PC FT-IR spectrometer. Zinc selenide crystals (50 × 10 × 3 mm) pretreated with n-octadecyltrichlorosilane were used as substrates onto which the LB films were deposited. UV-vis spectra were recorded using a commercial spectrometer (Perkin-Elmer, Lambda 14). X-ray photoelectron spectra were measured using a M-Probe SSX-100 spectrometer (Fisons Instruments). Elemental binding energies were corrected against the carbon 1s photopeak at 284.6 eV. Structural Studies. X-ray diffraction studies were carried out using a Philips powder diffractometer with Ni-filtered Cu KR radiation. All scanning force microscope (SFM) images were taken in air at room temperature with a Nanoscope II SFM (Digital Instruments) working in contact and equiforce mode. Commercially available Si/N cantilevers with intergrated tips were used.
3. Results and Discussion 3.1. Langmuir Films. If a cyclohexane solution of 1 is spread at the air-water interface, a stable monolayer (10) (a) Wunsch, G.; Deigner, P.; Falk, R.; Mahler, K.; Loeser, W.; Domas, F.; Felleisen, P.; Steck, W. U.S. Patent 4,128,672, 1978; BASF AG; Chem. Sbstr. 1979, 90, 80083. (b) Stabenow, J.; Wunsch, G.; Deigner, P.; Mu¨ller, F.-J.; Loeser, W.; Steck, W. Ger. Offen. 2,451,217, 1976; BASF AG; Chem. Abstr. 1976, 85, 111893. (11) Tieke, B.; Zahir, S. A. Chem. Mater. 1993, 5, 891. (12) Maassen, E.; Tieke, B. Langmuir 1996, 12, 5601. (13) Michaelson, R. C. US Patent 4,347,232, 1982; Chem. Abstr. 1982, 97, 165497.
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Figure 1. π-A isotherms of 1 on a pure water subphase at T ) 7.5, 20, and 30 °C. The broken line indicates the area occupied by a monolayer of 1 with a hexagonal dense packing of the complex molecules.
is formed. In Figure 1, π-A isotherms of 1 are shown on aqueous subphases of different temperatures. The π-A isotherm at 20 °C shows a gradual increase of the surface pressure π once the area A occupied by a single complex molecule becomes smaller than 1.3 nm2. Further compression is accompanied by an increasingly steeper rise in surface pressure until collapse sets in at a π value of 8 mN m-1. The corresponding A value is 0.45 nm2 per complex molecule. Before the collapse point is reached, the isotherm is fully reversible and the monolayer can be kept under constant pressure for several hours without changing the area. Above the collapse point, the monolayer is instable and the isotherm is no longer reversible. If the monolayer is compressed at a subphase temperature of 7.5 °C, the surface pressure begins to rise once a surface area of 1.6 nm2 per molecule is reached. The film collapses at a surface pressure of 9.8 mN m-1 and an A value of 0.52 nm2 per complex molecule. The isotherm at 30 °C exhibits a collapse pressure of 7 mN m-1. Obviously, the film stability decreases with increasing surface pressure, as is usually observed for monolayers at the air-water interface, but the temperature dependence of the isotherms is only very small. It was surprising to us that 1 can be spread at the airwater interface at all, because the molecular shape is very different from the typical shape of a surface-active molecule. Fortunately, the crystal and molecular structure of the tris(dibenzylideneacetone)palladium(0) complex have been determined by X-ray analysis.14 The complex has a spherical shape with the palladium atom in the center surrounded by the hydrophobic ligand molecules. Each ligand molecule attains the s-cis,trans configuration and is bonded to the metal through one olefin group.14 Since the ligand molecules are arranged such that the hydrophilic oxygen atoms of the carbonyl groups point away from the sphere, a certain amphiphilicity is introduced in the complex molecules, which is probably responsible for their spreadability at the air-water interface. Previous studies on the spreading behavior of spherical polymer particles with hydrophilic shell have shown that the particles form monolayers on the water surface, which consist of densely packed two-dimensional domains of either hexagonal (hdp) or quadratic dense packing (qdp).15 Assuming a similar behavior for complex 1, a theoretical area Ahdp can be calculated, which would be occupied by a monolayer of the complex molecules in a hexagonal dense (14) Mazza, M. C.; Pierpont, C. Inorg. Chem. 1973, 12, 2955. (15) Fulda, K.-U.; Tieke, B. Adv. Mater. 1994, 6, 288.
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Figure 2. Plot of the optical density vs the number of transferred complex layers (substrate: glass coated with a 10 µm polyarylate film).
packing at the air-water interface. Using the standard formula
Ahdp )
x32 d
2
(1)
with d being the complex diameter of 0.7 nm derived from the X-ray study,14 an Ahdp value of 0.42 nm2 per complex molecules is obtained. This value is indicated in Figure 1 by the broken line. As also shown in Figure 1, the collapse areas of all isotherms are larger than the Ahdp value; i.e., the monolayers already collapse before a dense packing of all molecules is reached. However, it cannot be ruled out that the complex molecules form denselypacked domains at the interface which are interlaced by free areas. Since the monolayers were very rigid, all efforts to improve the ordering, e.g., by several times compressing and reexpanding the monolayers, were unsuccessful. This was indicated by the invariable shape of the isotherm. 3.2. Langmuir-Blodgett Films. At a surface pressure of 7.5 mN m-1 and a subphase temperature of 7.5 °C, monolayers of 1 could be easily transferred onto various polymeric supports or on silanized glass supports according to the LB technique. Among the polymer supports were polyarylate, polycarbonate, blends of polycarbonate and ABS, and a polyarylamide. Deposition takes place during the downstroke and upstroke with a transfer ratio of 1. The LB films have a characteristic dark red color with absorption maxima at 520, 340, and 236 nm. In Figure 2, the absorbance of 1 at 520 nm is plotted versus the number of complex layers transferred onto a polyarylate support. As shown in the plot, the optical density increases linearly with the number of transferred complex layers. The LB films are fairly stable in air and do not decompose upon exposure to daylight. Storage under ambient conditions for several weeks is accompanied by a decrease of the absorbance of only about 10%. 3.3. Thermal Decomposition. Upon annealing at 100 °C or higher, LB films of 1 are gradually decomposed. Thermal decomposition was studied by various spectroscopic methods such as UV-vis, attenuated total reflection infrared (ATR-IR) and X-ray photoelectron (XPS) spectroscopy, and by scanning force microscopy (SFM). 3.3.1. X-ray Photoelectron Spectroscopy. In Figure 3, an XPS analysis of LB films of 1 annealed for different time periods is shown. LB films with 28 layers in thickness were investigated. In Figure 3a, the XPS spectrum is shown prior to heat treatment. The doublet palladium 3d3/2 and 3d5/2 photopeaks at 336.5 and 341.7
Figure 3. Palladium 3d3/2,5/2 XPS-doublet for an LB film of 1 prior to annealing (a), after annealing at 100 °C for 6 min (b), at 100 °C for 220 min (c), at 150 °C for 20 min (d), and at 270 °C for 60 min (e). Film thickness was 28 layers.
eV reflect the presence of Pd(0). Upon annealing at 100 °C the photopeaks are broadened and shifted to larger binding energies by about 1 eV (Figure 3b,c). Broadening and shift of the palladium photopeaks to larger energies can be ascribed to the formation of palladium clusters.16,17 Since the peak positions reflect the binding situation of the palladium atoms, the broadening indicates the formation of a size distribution of the clusters. Prolonged annealing at 150 °C, and finally at 270 °C, is accompanied by a shift of the palladium photopeaks to the typical position observed for the bulk palladium at 324 and 336.5 eV (Figure 3d,e). Besides, smaller peaks occur which can be ascribed to palladium Auger transitions. These spectral changes indicate an aggregation of the palladium clusters to larger metallic particles upon the prolonged heat treatment. 3.3.2. UV-vis Spectroscopy. Parallel to the XPS studies, the complex decomposition was also studied by UV-vis spectroscopy. In Figure 4a, the optical spectrum of an LB film with 20 layers of 1 is shown prior to annealing and after various annealing times. Prior to heat treatment the complex spectrum exhibits three characteristic absorption maxima at 236, 340, and 520 nm, which can be attributed to the π-π*, n-π* and dπ-pπ transitions, respectively. Short annealing of the LB film leads to a general increase of the absorbance, which is especially pronounced in the visible spectral region. In addition, three new absorption bands occur at 280, 380, and 420 nm. These bands can be attributed to the palladium clusters forming during the thermal treatment. Similar absorption bands have already been observed for palladium clusters by Henglein et al.18 studying the reductive decomposition of Pd(II) salts in aqueous solutions. The slightly different wavelength of the absorption maxima may be explained with the different experimental conditions. The cluster absorption bands can be even better recognized by viewing the difference absorption spectra (16) Mason, G.; Phys. Rev. B 1983, 27, 748. (17) Wertheim, G. K.; Cenzo, S. B. Phys. Rev. B 1986, 33, 5384. (18) Michaelis, M.; Henglein, A. J. Phys. Chem. 1992, 96, 4719.
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a
b
Figure 4. UV-vis absorption spectra of a 20 layer LB film of 1 built up on a quartz substrate monitored before and after annealing at 120 °C for different time periods (a) and difference absorption spectra showing the spectral changes after annealing at 120 °C for 3 min (t1) and 6 min (t2) (b).
shown in Figure 4b. These spectra were calculated by subtracting the absorption spectra at annealing times t1 ) 3 min and t2 ) 6 min from the absorption spectrum monitored at time t0, i.e. prior to annealing. When samples are heated for more than 30 min, the intensity of the cluster absorption bands decreases and finally the bands disappear. The disappearance of the bands is accompanied by a general decrease of the optical absorption. This can be explained (a) by assuming that larger metallic particles are formed, which are no longer transparent and thus do not contribute to the UV-vis spectrum and (b) by the fact that the growth of three-dimensional particles will be accompanied by an increase of the free surface area between the particles giving rise to a higher sample transparency. We now concentrate on the ligand molecules set free from the complex. These molecules either can disappear from the substrate by decomposition or sublimation or crystallize on the substrate and form microcrystals which are as little transparent as the metallic palladium particles and thus cannot be detected by UV absorption spectroscopy. In order to obtain detailed information on the whereabouts of the ligand molecules, ATR-IR measurements of the LB films were carried out. 3.3.3. ATR-IR Spectroscopy. For the ATR-IR measurements, 20 layers of 1 were deposited on a ZnSe support and measured prior to heat treatment and after annealing
Figure 5. ATR-IR spectra of LB films of 1 monitored before (a) and after annealing at 120 °C for 4 h (b). Sample was 10 layers of 1 deposited on both sides of a zinc selenide support.
at 120 °C for 4 h. In Figure 5 the IR spectra of the nonannealed and annealed sample are shown. Spectrum a of the nonannealed LB film indicates strong vibrational bands of the complexed ligand molecules. Band positions of the isopropyl C-H stretching vibrations are at 2963, 2924, and 2851 cm-1, while the CdO stretching frequency occurs at 1643 cm-1 and the aliphatic CdC stretching and wagging frequencies appear at 1622 and 1016 cm-1. After annealing, the band intensities are generally somewhat smaller indicating some loss of material perhaps due to sublimation. Band positions and their relative intensities are only little changed. The CdO stretching mode is shifted to 1653 cm-1, while the intensity of the adjacent CdC stretching vibration is decreased. Previous IR studies of dba-Pd complexes have already shown that the spectral differences between free and bound ligand molecules are only very small,19,20. We therefore conclude from our IR studies that the thermal treatment of the LB films produces free ligand molecules, which are only very slowly sublimed and thus remain on the substrate for a long time. 3.3.4. Scanning Force Microscopy. For a detailed morphological study, the heat-treated LB films were investigated by SFM. From the studies described above it can be concluded that the thermal complex decomposition proceeds according to
Pd(ipdba)3 f Pd + 3ipdba (with ipdba ) 4,4′-diisopropyldibenzylideneacetone). It therefore seems likely that palladium particles and ligand crystallites are formed. In Figure 6a, an SFM surface view of a Langmuir-Blodgett film consisting of 12 complex layers on a quartz support is shown subsequent to annealing at 120 °C for 20 min. Submicrometer-sized particles can be seen which are homogeneously spread over the whole substrate. Their average diameter d can be estimated to be about 250 ((100) nm. From the relative large size it can be ruled out that the particles are palladium particles. To demonstrate that they are ligand particles, it was tried to roughly calculate their total volume V1 and compare it with the volume V2 of the original LB film on (19) Ukai, T.; Kawazura, H.; Ishil, Y.; Bonnett, J. J.; Ibers, J. A. J. Organomet. Chem. 1974, 65, 253. (20) Moseley, P.; Maitlis, P. M. J. Chem. Soc., Chem. Commun. 1971, 1604.
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a
Figure 7. X-ray powder diagrams of 1 before (a) and after annealing at 120 °C for 4 min (b) and 390 min (c).
single complex molecule during its transfer from the airwater interface onto the substrate:
zc ) nF/A
b Figure 6. SFM images of 12 layers of 1 on a quartz support after annealing at 120 °C for (a) 20 min (scan size 11.2 × 11.2 µm2) and (b) 10 min (scan size 1.6 × 1.6 µm2).
the same area. Assuming that the particles are hemispheres, their total volume, V1 on the unit area F ) 11.2 × 11.2 µm2 (which corresponds to the scan area) is given by
(32 π (d2) ) 3
V1 ) N
(2)
where N is the number of particles. For the sample shown in Figure 7a, N is about 260 ( 30, and thus V1 can be calculated to be about (1 ( 0.5) × 109 nm3. V1 has to be compared with V2 of the original LB film. V2 is given by
V2 ) zc Vc
(3)
where Vc is the volume of a single complex and zc is the number of the complex molecules on the unit area. The complex molecule has a spherical shape and therefore Vc can be calculated to be
Vc )
4 π rc3 3
(4)
where rc is the radius of the complex molecule. As derived from the X-ray study,14 rc is about 0.4 nm. zc is proportional to the unit area F and the number n of deposited layers and inversely proportional to the area A occupied by a
(5)
In our case, A is 0.55 nm2 and n is 12. Conclusively, V2 can be calculated to be about 6 × 108 nm3, which is comparable with V1. Looking more closely at the heat-treated LB film, many very small particles become visible. In Figure 6b, an SFM image is shown taken from a scan area of 1.6 × 1.6 µm2. The sample was annealed at 120 °C for 10 min. The average diameter of the particles is 22 ( 5 nm and the number of particles in the unit area is about 600 ( 100. Using a similar calculation as for the ligand particles described above, a total volume V3 of about (1.7 ( 1) × 106 nm is obtained. Assuming that the particles consist of palladium, one has to compare V3 with the total volume V4 of the palladium atoms deposited as palladium complex 1 on the unit area by the LB technique. The calculation is similar to the one for V2, if one takes into account that each complex molecule contains one palladium atom. V4 is then given by
V4 )
4 F π rPd3 n 3 A
(6)
with the radius of the palladium atom being 0.128 nm. With the unit area F being 1.6 × 1.6 µm2 ()scan area), V4 can be calculated to be about 5 × 105 nm3, which is also comparable with V3, indicating that the particles probably consist of palladium. SFM studies of the smaller palladium clusters formed at the beginning of the decomposition failed so far. One reason could be the rough surface morphology of the quartz supports used for our experiments. 3.3.5. X-ray Diffraction Studies. In order to investigate the structure of the LB films of 1, small angle X-ray diffraction studies were carried out. However, the X-ray diffractograms did not clearly show 00l reflections so that
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However, it was possible to measure the X-ray powder patterns of bulk crystalline samples of 1. In Figure 7, such patterns are shown. They were measured before annealing (diagram A) and after heat treatment at 120 °C for 4 min and 6.5 h, respectively (diagrams b and c). Upon heat treatment, new reflections occur at 2θ values from 6 to 9° and 12 to 18°. Their intensity increases with the annealing time, while the original reflections simultaneously disappear. The change in the diffractograms indicates a structural rearrangement, which takes place as soon as the complex is decomposed according to eq 1. Obviously, this decomposition proceeds as a solid state reaction and immediately leads to formation of a new crystalline compound. The IR study discussed above indicates that the new compound is the pure ligand. The also formed palladium particles cannot give rise to intense and sharp X-ray reflections because their concentration is very low and their size is probably too small. 4. Summary and Conclusions
Figure 8. Scheme of the thermal decomposition of the Pd(ipdba)3 complex 1 showing a top view on an LB layer of 1 before annealing (a), after a short annealing at T g 100 °C leading to palladium clusters and ligand (ipdba) crystallites (b), and after prolonged annealing for at least 10 min leading to metallic palladium particles and ligand crystallites (c). The real mutual arrangement of the complex molecules, palladium clusters, and palladium particles is not quite as regular as in our schematic representation.
the layer spacing could not be calculated. One reason could be that the spherical shape hinders the complex molecules to attain a typical layered structure, which would be required for the observation of strong 00lreflections.
Our studies show that the palladium complex 1 forms stable monolayers at the air-water interface, which can be transferred onto various substrates. Thermal treatment of the LB films at 100 °C or higher leads to a complex decomposition as schematically outlined in Figure 8. The original LB film (Figure 8a) is decomposed under formation of palladium clusters and ligand crystallites (Figure 8b). Prolonged heat treatment causes a cluster aggregation and formation of larger particles. After about 10 min., metallic palladium particles with an average diameter of 22 nm are formed (Figure 8c), which probably further grow if the annealing is continued. The ligand molecules form crystallites on the support and sublime only very slowly. The catalytic activity of the palladium particles will be described in the subsequent part 2.12 Acknowledgment. We thank Professor Dr. Th. Kruck, Institut fu¨r Anorganische Chemie der Universita¨t zu Ko¨ln, for kindly allowing us to use his IR equipment and M. Schober for recording the ATR-IR spectra. R. Hansel is thanked for recording the XPS spectra and B. Feist for taking the X-ray powder diagrams. We also thank Degussa for kindly supplying palladium chloride. Financial support by the Deutsche Forschungsgemeinschaft (project II C 1-Ti 219/2-1) is also gratefully acknowledged. LA960133R
