Langmuir 1996, 12, 5601-5605
5601
Catalytic Surfaces from Langmuir-Blodgett Films of Tris(4,4′-diisopropyldibenzylideneacetone)palladium(0) as Precursor. 2. Study of the Catalytic Activity in Electroless Metal Plating of Polymers Ellen Maassen and Bernd Tieke* Institut fu¨ r Physikalische Chemie der Universita¨ t zu Ko¨ ln, Luxemburger Strasse 116, D-50939 Ko¨ ln, Germany Received February 13, 1996X The catalytic activity of palladium particles prepared upon thermal decomposition of Langmuir-Blodgett films of tris(4,4′-diisopropyldibenzylideneacetone)palladium(0) (1) on polymeric supports was investigated. As the catalytic process, the palladium-catalyzed electroless nickel plating was studied. Influences on the nickel plating due to different palladium concentrations on the substrate were studied. It was found that a minimum substrate coverage with four palladium atoms per square nanometer is sufficient to cause homogeneous nickel plating. The presence of a higher palladium concentration has no further effect on the catalytic activity. The influence of a variation of the annealing time of the LB films on the nickel plating was also investigated. Short annealing times favor palladium cluster formation, while long annealing times lead to larger metallic palladium particles (see part 1 of this study). It was found that short annealing times leading to the clusters do not cause a significant catalytic activity, while longer annealing times creating the metallic particles also generate a high catalytic activity. The catalytic activity was also studied for LB multilayer systems in which 1 was not directly exposed to the surface but coated with a number of monolayers of an R-helical copolyglutamate. These systems were only found to be catalytically active when the palladium had time to migrate through the copolyglutamate film. By varying the number of copolyglutamate multilayers, the migration rate of palladium through the polymer could be determined to be 3.6 × 10-11 ms-1 at 120 °C.
1. Introduction In recent years, nanocrystalline metal and semiconductor particles have found a considerable attraction because of their unusual physical properties1 and potential catalytic effects.2,3 Several methods are known to prepare ultrathin films of metal particles on solid supports as for example ion implantation and metal vapor deposition,3 organometallic chemical vapor deposition,4 metal deposition from colloidal solution,5 reductive metal deposition from aqueous salt solution,6 photodecomposition of metal complexes in thin films,7 and photoreductive deposition from Pd(II) complexes in solution.8 Other techniques are based on the film formation of noble metal loaded block copolymers9 or on the Langmuir-Blodgett (LB) transfer of monolayers of surfactant-stabilized metal colloids.10 * Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, October 15, 1996. (1) (a) Halperin, W. P. Rev. Mod. Phys. 1986, 58, 533. (b) Edwards, P. P.; Sienko, M. J. Int. Rev. Phys. Chem. 1993, 3, 83. (2) Vargaftik, M. N.; Zagorodnikow, V. P.; Stolyarow, I. P.; Moiseev, I. I.; Kochubey, D. I.; Likholobov, V. A.; Chuvilin, A. L.; Zamaraev, K. I. J. Mol. Catal. 1989, 53, 315. (3) Che, M.; Bennet, C. O. Adv. Catal. 1989, 36, 55. (4) (a) Sherman, A. Chemical Vapour Deposition for Microelectronics: Principles, Technology and Applications; Noyes Publications: Park Ridge, NJ, 1987. (b) Dryden, N. H.; Kumar, R.; Ou, E.; Rashidi, M.; Roy, S.; Norton, P. R.; Puddephatt, R. J.; Scott, J. D. Chem. Mater. 1991, 3, 677. (5) Schmid, G. Chem. Rev. 1992, 92, 1709. (6) Coulthard, I.; Jiang, D.-T.; Lorimer, J. W.; Sham, T. K.; Feng, X.-H. Langmuir 1993, 9, 3441. (7) Krasnansky, R.; Yamamura, S.; Thomas, J. K.; Dellaguardia, R. Langmuir 1991, 7, 2881. (8) Kondo, K.; Ishikawa, F.; Ishida, N.; Irie, M. Chem. Lett. 1992, 999. (9) (a) Ng Cheong Chan, Y.; Schrock, R. R.; Cohen, R. E. Chem. Mater. 1992, 4, 24. (b) Spatz, J. P.; Roescher, A.; Sheiko, S.; Krausch, G.; Mo¨ller, M. Adv. Mater. 1995, 7, 731. (10) (a) Meldrum, F. C.; Kotov, N. A.; Fendler, J. H. Langmuir 1994, 10, 2035. (b) Meldrum, F. C.; Kotov, N. A.; Fendler, J. H. Chem. Mater. 1995, 7, 1112.
S0743-7463(96)00134-5 CCC: $12.00
A further method to prepare small, supported metal particles is based on the thermal decomposition of LB films of a zerovalent dibenzylideneacetone-palladium complex.11 As recently described, LB films of tris(4,4′diisopropyldibenzylideneacetone)palladium(0) (1)
can be used as a precursor for preparation of supported palladium particles.11 Upon a thermal treatment of the LB film of 1, the particles are set free on the substrate and exhibit a high catalytic activity, e.g. in electroless nickel plating of the substrate. The formation of catalytically active palladium particles proceeds according to
Pd(ipdba)3 f Pd + 3ipdba
(1)
(with ipdba ) 4,4′-diisopropyldibenzylideneacetone).11,12 The use of LB films is advantageous for quantitative studies on the thermal complex decomposition and the catalytic activity of the resulting metal particles, because the layer-by-layer deposition allows us to precisely control the number of metal complex molecules deposited on the substrate. In part 1 of our study, formation and thermal decomposition of LB films of 1 were investigated in detail using spectroscopic and microscopic techniques.12 The LB films are decomposed upon annealing at 100 °C or higher. Annealing at 120 °C, for example, leads to formation of small palladium clusters within 3 min and formation of larger palladium particles with an average diameter of 22 nm within 10-20 min.12 Hence the size (11) Tieke, B.; Zahir, S. A.; Mathieu, H.-J. Adv. Mater. 1991, 3, 96. (12) Maassen, E.; Tieke, B.; Jordan, G.; Rammensee, W. Langmuir 1996, 12, 5595.
© 1996 American Chemical Society
5602 Langmuir, Vol. 12, No. 23, 1996
Maassen and Tieke
of the particles can be taylored by varying the time period of the annealing process. The present work is concerned with a study of the catalytic activity of the supported palladium particles formed upon the thermal treatment of LB films of 1. As the catalytic process, the palladium-catalyzed electroless metalization of polymeric substrates was investigated.13 This process is of great technical importance in the automotive and microelectronic industries, e.g. for the formation of printed circuits and EMI-shielding. The palladium-catalyzed nickel deposition14 proceeds according to [Pd0]
Ni2+ + 2H2PO2- + 2H2O 98 NiV + H2v + H3PO3 (2) The influence of the concentration and size of the palladium particles on the catalytic activity was studied. For this purpose, the annealing time and number of deposited complex layers were varied and the amount of deposited nickel was determined. In addition, the catalytic activity was studied for LB films of 1, which were coated with several monolayers of an R-helical copolyglutamate.15 In that case, the palladium complex is not directly exposed to the surface and the thermally formed palladium particles can only exhibit a catalytic activity after a migration to the surface. This is the situation occurring in technical processes of metalizing a plastic, where the activating palladium compound is often applied in the presence of a polymeric binder material,16 which is used to improve the metal adhesion to the plastic. Studying the relation between the thickness of the copolyglutamate coating and the catalytic activity after the thermal treatment allowed us to estimate the palladium migration rate through the copolyglutamate film. 2. Experimental Section 2.1. Materials. Tris(4,4′-diisopropyldibenzylideneacetone)palladium(0) (1). 1 was prepared according to the literature11,17 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 (d(CHdCH)). UV (CHCl3) 236 nm ( ) 53.0), 340 nm (77.5), 520 nm (8.8). Polyarylate, a polyester consisting of a terephthalic/isophthalic acid mixture and bisphenol A (Mw ) 4.6 × 104 g mol-1, Tg ) 188 °C), was obtained from Amoco (M 100) and used without further treatment. The copolyglutamate was supplied by Dr. H. Tebbe, Max-Plack Institut fu¨r Polymerforschung, Mainz. Its chemical structure is shown in Figure 1. 2.2. Monolayer Formation and Deposition. For the formation and deposition of LB films a commercially available film balance (Lauda FW-1) equipped with a film lift was used. Monolayers of 1 were spread from solutions of the compound in cyclohexane (spectroscopic grade, concentration 0.5-1 mg mL-1) onto a pure water subphase (Milli-Q plus). Monolayers of the copolyglutamate15 were spread from chloroform solution (spec(13) (a) Warshawsky, A.; Upson, D. A. Polymer 1989, 30, 972. (b) Warshawsky, A.; Upson, D. A. J. Polym. Sci. Polym. Chem. Ed. 1989, 27, 2963, 2995, 3015. (14) (a) Gabrielli, C.; Raulin, F. J. Appl. Electrochem. 1971, 1, 167. (b) Flis, J.; Duquette, D. J. J. Electrochem. Soc. 1984, 131, 254. (c) Gutzeit, G. Plating 1959, 46, 1158 and 1275; 1960, 47, 63. (d) Krulik, G. A. J. Chem. Educ. 1978, 55, 361. (15) Duda, G.; Wegner, G. Makromol. Chem., Rapid Commun. 1988, 9, 495. (16) See for example: (a) Rolker, J. H.; Carson, B. A., Bell & Howell Co. U.S. Patent 3, 900, 320, 1975; Chem. Abstr. 1973, 79, 22853. (b) Wada, M.; Konaga, N.; Nishiwaki, K.; Kobori, Y., Okuno Chem. Ind. Co. U.S. Patent 4,469,714, 1984; Chem. Abstr. 1985, 102, 25912. (17) Michaelson, R. C. FMC Corp. U.S. Patent 4,347,232, 1982; Chem. Abstr. 1982, 97, 165497. (18) Hamaya, T.; Kumagai, Y. Chem. Lett. 1989, 1461.
Figure 1. Chemical structure of poly(γ-methyl-L-glutamateco-γ-octadecyl-L-glutamate).15 troscopic grade, concentration 0.5-1 mg mL-1) onto a pure water subphase. LB films of 1 were built up on polyarylate substrates. The preparation of the polyarylate substrate is described below. Conditions for transfer of 1 were optimum at a surface pressure of 7.5 mN m-1 and a subphase temperature of 7.5 °C. LB films of polyglutamate were deposited at a surface pressure of 20 mN m-1 and a subphase temperature of 7.5 °C. In all transfer experiments, the dipping rate was 2 cm min-1. 2.3. Preparation of Polyarylate Substrates. Glass slides (40 × 24 × 1 mm3) silanized with n-octadecyltrichlorosilane were dipped into a solution of polyarylate in 1,1,2,2-tetrachloroethane (15% by weight) and successively dried in air for 12 h and in an oven under reduced pressure (50 mm) at 60 °C. A transparent substrate with a film thickness of approximately 10 µm was obtained. 2.4. Electroless Plating. For electroless metalization, a nickel plating bath of known composition18 was used. The composition was slightly modified by adding glycine and sodium D-Gluconate. A typical plating solution contained 0.1114 mol L-1 NiSO4‚7H2O, 0.186 mol L-1 NaH2PO2, 0.27 mol L-1 glycine, and 0.093 mol L-1 sodium D-gluconate. The temperature of the bath was 90 °C. 2.5. Annealing of the LB films was carried out in an UT 6000 air-circulation drying oven (Heraeus Instruments).
3. Results and Discussion 3.1. Influence of the Palladium Concentration on the Catalytic Activity. In order to study the influence of the palladium concentration on the catalytic activity, samples of the structure schematically shown in Figure 2a were prepared. For this purpose, different numbers of monolayers of the palladium complex 1 were transferred from the air-water interface onto solid supports according to the LB technique. As solid supports, glass substrates coated with a thin polyarylate film were used. Details of the preparation are given in the Experimental Section. Then, in order to set free the catalytically active palladium, the LB films were annealed at 120 °C for 3 h. The formation of the palladium particles has previously been characterized in great detail.12 To study the catalytic activity, the samples were now dipped into the electroless nickel plating bath for 2 min, carefully washed in order to completely remove residual plating solution, and dried in air. Then the quantity of deposited nickel was gravimetrically determined and taken as a measure for the catalytic activity. In Figure 2b, the mass of deposited nickel per square centimeter is plotted as a function of the transferred number of complex monolayers. As indicated by the plot, the transfer of only a single monolayer of 1 is already sufficient to cause the deposition of a considerable amount of nickel on the substrate, i.e. about 0.08 mg cm-2. However, the nickel deposit was not yet homogeneous. To obtain a homogeneous nickel coating, a minimum number of two monolayers of 1 was necessary. If more than six complex layers were transferred, the amount of deposited nickel reached a constant value of 0.15 mg cm-2. If we know the area occupied by a single complex molecule in the monolayer and the number of transferred layers, we are able to calculate the number of complex molecules on the substrate. Each complex molecule contains one
Catalytic Activity in Electroless Metal Plating of Polymers
Figure 2. Scheme of the LB film structure (a) and plot of the amount of deposited nickel versus the number of layers of 1 on a polyarylate substrate (b). For nickel deposition, samples were annealed at 120 °C for 3 h and subsequently dipped into the nickel deposition bath for 2 min.
palladium atom, and thus the number of complex molecules also defines the palladium concentration on the substrate. The number of palladium atoms x per square nanometer is given by
x)
n A
(3)
where n is the number of deposited layers and A is the area occupied by a single complex molecule during LB transfer. Monolayers were transferred at a π-value of 7.5 mN/m and a subphase temperature of 7.5 °C, and thus A can be determined from the corresponding π-A isotherm to be 0.55 nm2.12 As described above, a homogeneous nickel coating could only be obtained if at least two monolayers of 1 were deposited. Hence, with n being 2, x can be calculated to be about 4; i.e., approximately four palladium atoms per square nanometer are needed to catalyze the electroless plating of a homogeneous nickel layer. However, in reality the palladium atoms form three-dimensional particles with an average diameter of 22 nm on the support.12 Taking the particle formation into account, it can be assumed that individual particles contain the palladium atoms in a densely packed array. Since the radius of a palladium atom is 0.128 nm, it can be roughly estimated that each of the 22 nm particles contains about 2 × 105 palladium atoms. This means that about 20 of these particles per square micrometer are necessary to cause a homogenous nickel coating of the substrate. 3.2. Influence of the Particle Size on the Catalytic Activity. For the following experiments, samples of similar structure as in the previous chapter were used. However, in contrast to the previous samples, the number of monolayers of 1 transferred onto the support was kept constant at 14 and instead the annealing time of the LB films at 120 °C was varied. The sample architecture is schematically shown in Figure 3a. As previously reported,12 the variation of the annealing time allows us to
Langmuir, Vol. 12, No. 23, 1996 5603
Figure 3. Scheme of the LB film structure (a) and plot of the amount of deposited nickel versus the annealing time (b). For nickel deposition, samples (14 layers of 1 on a polyarylate substrate) were annealed at 120 °C for different time periods and subsequently dipped into the nickel deposition bath for 2 min.
tailor palladium particles of different size ranging from small clusters, which appear after 2-3 min, to 22 nm sized metallic particles occurring after at least 10 min. The subsequent procedure of electroless nickel plating was the same as in the previous chapter. In Figure 3b, the mass of deposited nickel is plotted versus the annealing time t of the samples. As can be seen in the plot, the nickel deposition sets in once the samples were annealed for 15 min, but the quantity is still fairly low and the nickel coating is not yet homogeneous. Quite homogeneous nickel films are only obtained for samples which were annealed for 30 min at least. Finally, after an annealing time of 100 min, a constant and maximum quantity of deposited nickel is reached. This is a clear indication that the metallic palladium particles rather than the initially formed palladium clusters are the active species catalyzing the nickel deposition. 3.3. Catalytic Activity of LB Multilayer Systems Consisting of Complex 1 and a Copolyglutamate. In this chapter the question will be answered if LB multilayers of 1 are catalytically active when they are coated with polymer films of varying thicknesses. In principle, a catalytic activity can only be expected if the palladium is able to migrate through the polymer layer onto the sample surface or if the polymer coating is inhomogeneous and contains holes in which 1 is directly exposed to the surface. As a suitable polymer, the R-helical copolyglutamate15 of the chemical structure shown in Figure 1 (further on called ‘polyglutamate’) was used. This polymer was chosen because it forms stable monolayers, can be easily transferred onto substrates by the LB technique, and forms Langmuir-Blodgett films with a comparatively low defect concentration. Two effects on the catalytic activity were studied: The influence of the annealing time and the influence of the number of transferred polyglutamate layers.
5604 Langmuir, Vol. 12, No. 23, 1996
Figure 4. Scheme of the structure of the LB multilayer system (a) and plot of the amount of deposited nickel versus the annealing time at 120 °C (b). For nickel deposition, samples (six layers of copolyglutamate plus six layers of 1 plus six layers of copolyglutamate deposited on a glass support) were annealed for different time periods and subsequently dipped into the nickel deposition bath for 2 min.
3.3.1. Influence of Annealing Time on the Catalytic Activity. The structure of the multilayer systems used for this study is schematically shown in Figure 4a. Six layers of polyglutamate, six layers of the palladium complex 1, and again six layers of polyglutamate were transferred onto a silanized glass support. Then the samples were annealed at 120 °C for different time periods and dipped into the nickel plating bath for 2 min. After being thoroughly rinsed with water, the samples were dried in air and the quantity of nickel deposited per square centimeter was gravimetrically determined and taken as a measure for the catalytic activity. In Figure 4b, the mass of nickel is plotted versus the annealing time. It can be seen that the amount of nickel increases with the annealing time. After a time period of 30 min, a value of 0.1 mg of nickel per square centimeter is reached. It is interesting to see that the shape of the curve is not significantly different from that of the curve obtained for the samples without polyglutamate coating, as shown in Figure 3b. A possible explanation is that the copolyglutamate film contains holes. If the hole concentration is high, the thermally created palladium particles could be as effective as if there was no polymer coating. However, morphological studies of the LB films by scanning force microscopy did not indicate holes. Also, a thermogravimetric analysis of mixtures of 1 and the polyglutamate did not indicate a low thermal stability. Even at 200 °C, the thermal weight loss was only 4%, so that a thermal decomposition of the polyglutamate film during the annealing at 120 °C can be ruled out. Therefore, another explanation seems more reasonable: The polyglutamate coating was so thin that even the shortest time period of annealing was long enough for the palladium to migrate through the polymer film and form the catalytically active particles on the surface. XPS studies of the
Maassen and Tieke
Figure 5. Scheme of the structure of the LB multilayer system (a) and plot of the amount of deposited nickel versus the thickness of the upper copolyglutamate multilayer. For nickel deposition, samples (copolyglutamate multilayer of different thickness plus six layers of 1 and six layers of copolyglutamate on a glass support) were annealed at 120 °C for 30 min and subsequently dipped into the nickel deposition bath for 2 min.
palladium concentration on the surface as a function of the annealing time will be carried out in order to clarify this point. 3.3.2. Influence of Copolyglutamate Multilayer Thickness on the Catalytic Activity. In order to find the limit of the catalytic activity, we now varied the thickness of the polyglutamate film on top of the multilayer system. The corresponding sample structure is shown in Figure 5a. It only differs from the system in Figure 4a by the varying number of polyglutamate layers on top of the multilayer system. All samples were annealed at 120 °C for an identical time period of 30 min. Then they were dipped into the nickel plating bath for 2 min and treated as described in the previous chapter. In Figure 5b, the amount of nickel deposited per square centimeter is plotted as a function of the thickness of the polyglutamate multilayer built up on top of the multilayer of 1. The calculation of the polyglutamate film thickness is based on a bilayer spacing of 3.45 nm reported in the literature.19 As shown in the plot, about 0.1 mg cm-2 of nickel are deposited if the film thickness is 20 nm. This value is comparable with the nickel deposition on samples without polyglutamate coating, as shown in Figure 4b. At a higher thickness of the polyglutamate multilayer, the quantity of nickel decreases and the deposition becomes increasingly inhomogeneous. Finally, at a polymer film thickness of 76 nm, the nickel deposition has ceased. At that thickness, the palladium migration rate is obviously not high enough anymore so that the catalyst cannot reach the surface within the given time period of 30 min. Consequently the surface remains catalytically inactive. (19) Tsukruk, V. V.; Foster, M. D.; Reneker, D. H.; Schmidt, A.; Wu, H.; Knoll, W. Macromolecules 1994, 27, 1274.
Catalytic Activity in Electroless Metal Plating of Polymers
On the basis of this result, the migration rate v of palladium through the polymer can be estimated. v is determined by
v)
d (ms-1) t
(4)
with d being the thickness of the polyglutamate multilayer and t the annealing time at 120 °C. It can be derived from Figure 5b that nickel deposition ceases at a film thickness of 65 nm ((10%). Taking this value as d and the annealing time of 30 min as t, a migration rate of about 3.6 × 10-11 ms-1 can be calculated, if the temperature of the sample is 120 °C. The migration is most likely driven by a concentration gradient in the direction normal to the layer plane of the LB multilayer system. The existence of a temperature gradient seems unlikely, because the samples are very thin and the time required for a temperature equilibration of the sample in the oven was much shorter than the time period of annealing. 4. Summary and Conclusions Our studies show that the palladium-coated substrates obtained upon a thermal treatment of LB films of palladium complex 1 exhibit a high catalytic activity in electroless metal plating. The deposition of precise amounts of palladium atoms allowed us to obtain new and quantitative information about the plating process. The number of active sites needed for a homogeneous metalization could be determined, and it could be shown that the main catalytic activity arises from the metallic palladium particles formed after long annealing times rather than from the small palladium clusters formed at the beginning of the complex decomposition. These results
Langmuir, Vol. 12, No. 23, 1996 5605
are in agreement with recent studies on the size-dependent catalytic activity of iridium particles20 but clearly are in contrast to earlier investigations.21 Moreover, a coating of multilayers of 1 with a multilayer of polyglutamate of different thickness allowed us to determine the migration rate of the palladium particles in the polymer matrix. Indeed, influences of palladium concentration and particle size on the catalytic activity have been investigated, but possible effects of the substrate polarity or functionality on the decomposition rate of the complex have been neglected so far. On polar substrates, the activation energy of the first complex layer might be reduced, which could affect the stability of the rest of the film. The study of such interactions and possible changes of the morphology of the resulting palladium particles, of their size, and of their size distribution will be subject to a future investigation. Also, the work will be continued by testing other metal complexes as precursor material for catalytically active substrates and also by studying other catalytic processes as for example hydrogenation reactions. Acknowledgment. Dr. D. Lemke from Shipley GmbH, Esslingen, is thanked for kindly supplying metalization baths, and Dr. H. Tebbe from Max-Planck-Institut fu¨r Polymerforschung, Mainz, is thanked for the copolyglutamate. Financial support by the Deutsche Forschungsgemeinschaft (project II C 1-Ti 219/2-1) is also gratefully acknowledged. LA960134J (20) Xu, Z.; Xiao, F.-S.; Purnell, S. K.; Alexeev, O.; Kawi, S.; Deutsch, S. E.; Gates, B. C. Nature 1994, 372, 346. (21) Hamilton; Logel. J. Catal. 1973, 29, 253.
