Self-Assembly of Polyacrylate-Capped Platinum Nanoparticles on a

Department of Chemistry, American University of Beirut, Beirut 110236, Lebanon; and. National High-Magnetic Field Laboratory, Florida State University...
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Langmuir 2003, 19, 4804-4811

Self-Assembly of Polyacrylate-Capped Platinum Nanoparticles on a Polyelectrolyte Surface: Kinetics of Adsorption and Effect of Ionic Strength and Deposition Protocol Sara Ghannoum,† Yan Xin,‡ Jad Jaber,† and Lara I. Halaoui*,† Department of Chemistry, American University of Beirut, Beirut 110236, Lebanon; and National High-Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310 Received December 23, 2002. In Final Form: February 27, 2003 Polyacrylate-capped Pt nanocrystallites of 2.5 ( 0.6 nm diameter were synthesized by reduction of Pt(IV) with citrate in the presence of polyacrylate and were self-assembled layer-by-layer in poly(diallyldimethylammonium chloride) by virtue of the Coulombic attraction between the negatively charged capping agent and the cationic polyelectrolyte. The self-assembly protocol resulted in a reproducible consecutive surface-charge buildup, leading to a linear incorporation of nano-Pt per layer. TEM imaging revealed a chainlike distribution of the nanoparticles on the polymer surface at submonolayer coverage. The density of assembled nanoparticles per layer can be controlled by varying the dipping time, the Pt solution ionic strength, and its composition or by resorting to an interrupted-deposition protocol involving multiple rinsing/drying cycles per layer. The dynamics of nano-Pt assembly on PDDA was followed ex-situ by UV-visible spectroscopy. The adsorption was found to take place via two kinetics regimes at different time scales. Initially, the adsorption rate is believed to be limited by the nanoparticles availability at the solution/surface interface. At high surface coverage, the slower polyelectrolyte-nanoparticle surface rearrangement necessary for further attachment becomes the rate-determining step, leading to a pseudosaturation within ∼1 h.

I. Introduction The assembly of zero-dimensional building blocks in two- and three-dimensional architectures, and understanding the dynamics of the assembly process, and the factors determining the surface density and distribution are vital for the exploration of the fundamental properties of ensembles of nanoparticles and their coupling to their environment, a much-sought knowledge prior to their incorporation in nanodevices. Such knowledge will pave the way to a future nanotechnology based on the unique properties afforded to this class of matter by spatial confinement. Metal and semiconductor nanoparticles, in particular, have attracted considerable research effort because of their foreseen potential applications in areas of catalysis, photocatalysis, renewable energy, electronics, sensors, and surface-enhanced phenomena such as Raman surfaceenhancement. Pt, the focus of this report, is of particular interest in our work because of its catalytic properties for the goal of (photoinduced) hydrogen fuel generation at semiconductors. The self-assembly of semiconductor and metal nanoparticles in 2D monolayers on surfaces has been achieved and extensively reported in the literature,1-5 * Author to whom correspondence should be addressed. E-mail: [email protected]. † American University of Beirut. ‡ Florida State University. (1) See, e.g.: (a) Schmid, G. Chem. Rev. 1992, 92, 1709. (b) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (c) Ohara, P. C.; Leff, D. V.; Heath, J. R.; Gelbart, W. M. Phys. Rev. Lett. 1995, 75, 3466. (d) Grabar, K. C.; Freeman, R. G.; Hommer, M. B.; Natan, M. J. Anal. Chem. 1995, 67, 735. (e) Alivisatos, A. P. Science 1996, 271, 933. (f) Kiely, C. J.; Fink, J.; Brust, M.; Bethell, D.; Schiffrin, D. J. Nature 1998, 396, 444. (g) Auer, F.; Scotti, M.; Ulman, A.; Jordan, R.; Sellergren, B.; Garno, J.; Liu G.-Y. Langmuir 2000, 16, 7554.

which included reports regarding the assembly of Pt nanoparticles by electrophoretic deposition2 and the selfassembly of dodecanethiol/acrylate-modified Pt nanoparticles4 and dodecanethiol-modified cubic Pt on carbon.5 In recent years, the electrostatically driven assembly of nanoparticles as 3D multilayers in polyelectrolytes has acquired significant momentum. This assembly process is founded on the initial work of Iler6 regarding the deposition of colloidal particles and the following studies by Decher et al. on the assembly of polyelectrolyte multilayers.7,8 Since then, driven by the low cost and relative ease of fabrication and by the stability of the assembled multilayers, this procedure has been extended to incorporate a wide variety of materials in polyelectrolytes including proteins,9 dendrimers,10 semiconductor nanoparticles (e.g., CdS, TiO2),11-14 magnetic nanopar(2) Teranishi, T.; Hosoe, M.; Tanaka, T.; Miyake, M. J. Phys. Chem. B 1999, 103, 3818. (3) Sarathy, K. V.; Raina, G.; Yadav, R. T.; Kulkarni, G. U.; Rao, C. N. R. J. Phys. Chem. B 1997, 101, 9876. (4) Petrovski, J. M.; Green, T. C.; El-Sayed, M. A. J. Phys. Chem. A 2001, 105, 5542. (5) Zhou, S.-Y.; Chen, S.-H.; Wang, S.-Y.; Li, D.-G.; Ma, H.-Y. Langmuir 2002, 18, 3315. (6) Iler, R. K. J. Colloid. Interface Sci. 1966, 21, 569. (7) See, e.g.: (a) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210/211, 831. (b) Decher, G.; Schmitt, J. J. Colloid Polym. Sci. 1992, 89, 160. (c) Lvov, Y.; Decher, G.; Mohwald, H. Langmuir 1993, 9, 520. (d) Lvov, Y. M.; Decher, G. Crystallogr. Rep. 1994, 39, 628. (e) Decher, G. Science 1997, 277, 1232. (8) Decher, G. In Comprehensive Supramolecular Chemistry; Sauvage, J. F., Hosseini, H. W., Eds.; Pergamon Press: Oxford, 1996; Vol. 9, pp 507-528. (9) (a) Lvov, Y.; Decher, G.; Sukhorukov, G. Macromolecules 1993, 26, 5396. (b) Lvov, Y. M.; Lu, Z.; Schenkman, J. B.; Rusling, J. F. J. Am. Chem. Soc. 1998, 120, 4073. (10) Watanabe, S.; Regan, S. L. J. Am. Chem. Soc. 1994, 116, 8855. (11) Kotov, N. A.; Dekany, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065.

10.1021/la0209839 CCC: $25.00 © 2003 American Chemical Society Published on Web 04/05/2003

Pt Nanoparticles on a Polyelectrolyte Surface

ticles,15 metal nanoparticles (e.g., Au and Ag),16-19 silica nanoparticles,12,20-21 and aluminosilicate nanoplatelets.22 Significant research has been directed toward understanding the kinetics of polyelectrolyte adsorption23-26 and the various parameters that control the assembly process and the structure of the resulting multilayers (viz., salt content, pH, rinsing, and drying).7d,8,26-32 On the other hand, the kinetics of nanoparticle adsorption on polyelectrolytes and the factors that govern their self-assembly have not been fully addressed yet, apart from a few investigations regarding, for instance, the effect of ionic strength on the assembly of silica nanoparticles,12,21 the effect of the polymer solution pH on the assembly of modified-Au nanoparticles,18 and the effect of grafting organic modifiers to magnetic nanoparticles on their surface distribution.15 In this paper, we report the layerby-layer electrostatic self-assembly of polyacrylate-capped Pt nanoparticles (〈d 〉 ) 2.5 ( 0.6 nm) in poly(diallyldimethylammonium chloride), PDDA, a cationic polyelectrolyte, and study the nanoparticle assembly dynamics and the dependence of the nanoparticle surface density on several deposition parameters. The multilayered assembly of PAC-stabilized Pt nanoparticles in PDDA led to a consecutive reproducible surface-charge buildup, resulting in a linear incorporation of nanoparticles per layer, despite the submonolayer surface coverage revealed by TEM. The adsorption dynamics was followed ex-situ by UV-visible spectroscopy. The assembly process was found to take place via two regimes exhibiting different kinetics, on separate time scales. Initially at low surface coverage, the adsorption rate is believed to be limited by the transport (or availability) of nanoparticles at the PDDA/solution interface, while at high surface coverage the significantly slower attachment process, necessitating nanoparticleson-surface rearrangement, becomes the rate-determining step. The density of Pt nanoparticles could be controlled by varying the dipping time, ionic strength, or solution composition or by resorting to a protocol we term interrupted-deposition which involved multiple rinsing/drying cycles per layer. In general, the assembly dynamics and the effect of these governing factors were explained by invoking the same model of electrostatic interactions and conformational changes as per polyelectrolyte assembly, a behavior attributed to the soft polymeric capping agent on the nanoparticles surface. II. Experimental Methods Materials. The following materials were used in this work: potassium hexachloroplatinate(IV), K2PtCl6 (Alfa Æsar, Pt (12) Lvov, Y.; Ariga, K.; Onda, M.; Ichinose, I.; Kunitake, T. Langmuir 1997, 13, 6195. (13) Liu, Y.; Wang, A.; Claus, R. J. Phys. Chem. 1997, 101, 1385. (14) Halaoui, L. I. Langmuir 2001, 17, 7130. (15) Ostrander, J. W.; Mamedov, A. A.; Kotov, N. A. J. Am. Chem. Soc. 2001, 123, 1101. (16) Feldheim, D. L.; Grabar, K. C.; Natan, M. J.; Mallouk, T. C. J. Am. Chem. Soc. 1996, 118, 7640. (17) Cassagneau, T.; Fendler, J. H. J. Phys. Chem. 1999, 103, 1789. (18) Hicks, J. F.; Seok-Shon, Y.; Murray, R. W. Langmuir 2002, 18, 2288. (19) Malikova, N.; Pastoriza-Santos, I.; Schierhorn, M.; Kotov, N. A.; Liz-Marza´n, L. M. Langmuir 2002, 18, 3694. (20) Caruso, F.; Mo¨hwald, H. Langmuir 1999, 15, 8276. (21) Sennerfors, T.; Bogdanovic, G.; Tiberg, F. Langmuir 2002, 18, 6410. (22) Kim, D. W.; Blumstein, A.; Kumar, J.; Samuelson, L. A.; Kang, B.; Sung, C. Chem. Mater. 2002, 14, 3925. (23) Miyano, K.; Asano, K.; Shimomura, M. Langmuir 1991, 7, 444. (24) Motschmann, H.; Stamm, M.; Toprakcioglu, Ch. Macromolecules 1991, 24, 3681. (25) Hoogeveen, N. G.; Cohen Stuart, M. A.; Fleer, G. J. J. Colloid Interface Sci. 1996, 182, 133. (26) Dubas, S. T.; Schlenoff, J. B. Macromolecules 1999, 32, 8153.

Langmuir, Vol. 19, No. 11, 2003 4805 40.1%); poly(acrylic acid) 2100 sodium salt, PAC (Fluka Chemica, pH in water 6-8); poly(diallyldimethylammonium chloride), PDDA, 20 wt % in water, average molecular weight ∼200 000350 000, d 1.040 (Aldrich); sodium citrate (AnalaR); sodium chloride (Fluka); and acetonitrile (Fluka, water e 0.05%). All aqueous solutions were prepared with double distilled water. The reported concentration of the polymer solution is for the monomer unit. Synthesis and Precipitation of Pt Nanoparticles. 10 mg of K2PtCl6 was dissolved in a solution of 1.34 g of PAC/50 mL of water (Pt/PAC is 1:31). Aging of the K2PtCl6 solution was not necessary in this synthetic procedure. Upon completely dissolving the Pt salt, a solution of 0.50 g of sodium citrate/50 mL of water was added, and the mixture was refluxed in an oil bath (at 120 °C) for 3.5 h. At this time, the reaction was quenched in an ice/ water bath (so the growth temperature and reaction time are fixed). The resulting Pt solution exhibited a light golden brown color and will be termed Pt as-prepared. The pH of the reaction mixture was initially equal to 7.9-8.0 and remained essentially unchanged at the end of the preparation. In the absence of polyacrylate, this synthetic procedure resulted in the precipitation of Pt black. To prepare Pt solutions with different ionic strengths, Pt nanoparticles were precipitated by adding 1 volume of acetonitrile to 1 volume of the as-prepared Pt solution, followed by centrifugation at 3000 rpm for 5 min. The precipitated nanoparticles were redissolved in 1 volume of double distilled water, and NaCl was added to adjust the ionic strength to the desired value. The pH of the resulting Pt/NaCl solutions was equal to 7.4-7.5. Substrates. Films were assembled on quartz surfaces (G. M. Associates, Inc.) for UV-visible spectroscopy studies. The substrates were cleaned in piranha solution (7:3 H2SO4/30% H2O2) for 30 min and then rinsed with double-distilled water and dried under a slow nitrogen flow. Carbon-coated 300 mesh Cu grids and SiO-coated 300 mesh Cu grids (SPI) were used as sample supports for TEM imaging. Precipitated and dried Pt nanoparticles were mixed with KBr and pressed into a pellet for FT-IR measurements. Deposition Parameters. For Pt/PDDA layer-by-layer assembly, quartz surfaces were dipped in 10 mM PDDA/0.4 M NaCl(aq) solution for 30 min and then immersed in double-distilled water for 1 min, twice, and left to air-dry for 15 min. The surfaces were then dipped in the designated Pt solution for 1 h, immersed in water for 1 min, and left to dry in air for 15 min before building the second layer pair or acquiring a UV-visible spectrum. To study the adsorption kinetics of Pt nanoparticles on PDDA, the PDDA-modified quartz surface was dipped in the designated Pt solution for ∆t ) 10 or 2 min, rinsed with water for 1 min, and air-dried for 15 min, before acquiring a UV-visible spectrum. This dipping protocol was repeated as many times as indicated. Films thus formed were stable; the multiple rinsing/drying steps caused no damage or delamination from the substrate surface. Drying times longer than 15 min (when the experiment was interrupted overnight during the first or second kinetics regimes) did not cause a decrease or increase in the adsorption rate relative to that measured after 15 min of drying. For TEM imaging of nanoparticles, 5 or 10 µL of precipitatedPt(aq) solution was solvent-cast onto Cu grids covered with amorphous carbon, and the solvent was left to evaporate in air. For TEM imaging of Pt/PDDA films, films were deposited on SiO-coated Cu grids, and care was taken not to deposit the films on both sides of the grid. For this purpose, the grids were made to float on the surface of a 10 mM PDDA/0.4 M NaCl solution for 30 min, with the SiO film side down facing the solution, while the other side is dry above the surface (at the air/solution interface). The SiO film surface was then rinsed by letting it float on the surface of water for 1 min, twice, and was dried in air for 15 min. The polymer-modified SiO film was then floated on the Pt solution/air interface for the indicated time (10 min or 1 h) and then rinsed by floating on water for 1 min. Before imaging, the grids were entirely immersed in water for 20-30 s to remove any excess and were left to dry in air. To image films assembled using the interrupted-deposition protocol, the above procedure was repeated as many times as indicated. Instrumentation. TEM images were acquired on a JEOL2010 high-resolution transmission electron microscope operated

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at 200 kV with a point resolution of 0.23 nm and a lattice resolution of 0.14 nm (NHFML, FSU). The d spacings were calculated from the electron diffraction pattern using d ) A/D, where A is a constant determined using an external calibration standard (Al polycrystal) at a fixed magnification of the pattern and D is the measured diameter of the diffraction rings. The measured d spacings were assigned to the corresponding lattice planes after calculation of d spacings for fcc Pt with a lattice parameter a ) 3.924 Å. UV-visible absorption spectra were collected using a Spectronic Genesis 2 spectrophotometer. Fourier transform IR spectra were collected with an Avatar 360 FT-IR (Nicolet) using Omnic E. S. P. Software.

III. Results and Discussion Characterization of Polyacrylate-Capped Pt Nanoparticles. The reduction of Pt(IV) with citrate at boiling temperature, in the presence of polyacrylate (31:1 PAC/ Pt ratio), resulted in the growth of Pt nanocrystallites of mostly spherical or ellipsoidal shapes, as imaged by TEM. The crystallinity of the nanoparticles was determined by means of high-resolution transmission electron microscopy (HRTEM) and electron diffraction (ED) measurements. Figure 1a shows a TEM image of Pt nanoparticles, and Figure 1b shows a HRTEM image, revealing the nanocrystals lattice fringes with d spacings consistent with the fcc structure of the bulk crystal. The ED pattern depicted in the inset of Figure 1b supported the growth of Pt nanocrystals in the fcc structure, having a lattice constant a ) 3.92 Å, in agreement with the bulk lattice parameter. The ED pattern revealed d spacings of 2.27, 1.96, and 1.39 Å, which correspond to the d spacings for the {111}, {200}, and {220} planes, respectively, of facecentered-cubic Pt structure. The average dimension of the nanoparticles was determined to be 2.5 ( 0.6 nm from a sample of 200 particles imaged in a number of micrographs across the sample. The histogram showing the size distribution is presented in Figure 1c. Polyacrylate-capped Pt nanoparticles have been synthesized by El-Sayed and co-workers33,34 according to the method of Rampino and Nord35 and Henglein et al.36 in cubic, tetrahedral, or polyhedral shapes with average dimensions between 7 and 11 nm. Their procedure consisted of reducing PtCl42- with H2 at room temperature under anaerobic conditions in the presence of polyacrylate (PAC/Pt was 1:1-5:1). The growth of Pt nanoparticles under these synthetic conditions, and hence their final shape and size distribution, was shown by El-Sayed and co-workers to be kinetically controlled by the relative rates of growth along the {111} and {100} faces of Pt and the rates of polymer capping and decapping of the surface.37 Cubic nanoparticles were found to dominate at low (27) Lo¨sche, M.; Schmidtt, J.; Decher, G.; Bouwman, W. G.; Kjaer, K. Macromolecules 1998, 31, 8893. (28) Chen, W.; McCarthy, T. J. Macromolecules 1997, 30, 78. (29) Korneev, D.; Lvov, Y.; Decher, G.; Schmidt, J.; Yaradaikin, S. Physica B 1995, 213/214, 954. (30) Arys, X.; Jonas, A. M.; Laschewsky, A.; Legras, R. In Supramolecular Polymers; Ciferri, A., Ed.; Marcel Dekker: New York, 2000; pp 505-563 and references therein. (31) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: London, 1993; pp 343-373 and references therein. (32) Raposo, M.; Pontes, R. S.; Mattoso, L. H. C.; Oliveira, O. N., Jr. Macromolecules 1997, 30, 6095. (33) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. A. Science 1996, 272, 1924. (34) Ahmadi, T. S.; Wang, Z. L.; Henglein, A.; El-Sayed, M. A. Chem. Mater. 1996, 8, 1161. (35) Rampino, L. D.; Nord, F. F. J. Am. Chem. Soc. 1941, 63, 2745. (36) Henglein, A.; Ershov, B. G.; Malow, M. J. Phys. Chem. 1995, 99, 14129. (37) Petrovski, J. M.; Wang, Z. L.; Green, T. C.; El-Sayed, M. A. J. Phys. Chem. B 1998, 102, 3316.

Figure 1. TEM image of Pt nanoparticles deposited on a C/Cu grid (a), HRTEM image and electron diffraction pattern of Pt nanocrystallites (b), and a histogram showing the size distribution determined from a sample of 200 nanoparticles with 〈d 〉 ) 2.5 ( 0.6 nm (c).

Pt Nanoparticles on a Polyelectrolyte Surface

polymer concentration and were larger in size, while smaller tetrahedral nanoparticles dominated at high polymer concentration.33-34, 37 As presented herein, the reduction of Pt(IV) by citrate under reflux in the presence of polyacrylate led to the growth of mostly approximately spherical Pt nanoparticles of 2.5 ( 0.6 nm average dimension. The small size and spherical shape of the nanoparticles is attributed to the high PAC/Pt ratio and possibly to thermodynamic control of the growth mechanism at the temperature employed. Under thermodynamic control, the nanoparticles are thought to grow to spheres by reduction of the metal cation on both the {111} and {100} faces of the Pt nanocrystal, to minimize the surface tension (smallest for spheres) and attain the thermodynamically most stable shape. A transformation in the shapes of the H2-reduced Pt colloids into spheres has been reported by Wang et al. to take place at ∼500 °C, which the authors attributed to surface diffusion or premelting.38 The mechanism and conditions differ in this case, as the synthesis of spherical Pt nanoparticles took place under reducing conditions. The significantly smaller size of our PAC-“citrate-Pt” nanoparticles (2.5 nm) compared to the PAC-“H2-Pt” nanoparticles (7-11 nm) is attributed to the high PAC/Pt ratio employed (31:1 cf. 1:1-5:1 in the work of El-Sayed et al.33-34,37) and a smaller rate of polymer decapping under these reducing conditions (H+ is not produced, which can protonate the polyacrylate and hence result in decapping), thus trapping the nanoparticles in smaller dimensions. The citrate reduction of PtCl62- to Pt colloids has been previously described by Turkevich39 and later by Henglein et al.,36 who reported a mean diameter of 2.5 nm after 1 h of boiling in the absence of polyacrylate. Citrate was reported to act as a stabilizer, in addition to being a reducing agent. The use of polyacrylate in this work was found to enhance the nanoparticle stability. In fact, under the employed conditions, Pt black precipitated from the reaction mixture after less than 3.5 h of reflux in the absence of polyacrylate. Decapping of the particles at ∼100 °C is not expected. Wang et al. reported polyacrylate decapping of the Pt colloids not to take place before an onset temperature of 180 °C.38 Surface capping with polyacrylate was confirmed by Fourier transform infrared spectroscopy. FT-IR spectra of precipitated Pt nanoparticles in KBr pellets exhibited major peaks at the following wavenumbers in order of decreasing intensity: 3420, 1570, 1410, 1660, 2960, and 1460 cm-1, where the peak at 3420 cm-1 is attributed to the vibration of water and the other peaks are attributed to the vibrations of the carboxylate group, with the most intense peak at 1570 cm-1 corresponding to the CO stretch. This was in agreement with the major peaks in the FT-IR spectrum of PAC in a KBr pellet (as we presented previously in ref 14), which appeared at the following wavenumbers in order of decreasing intensity: 3418, 1558, 1408, 1651, 2924, and 1455 cm-1. Capping with polyacrylate is further evidenced in the precipitation of Pt black upon attempting the synthesis of the nanoparticles under the same conditions in the absence of polyacrylate. Layer-by-Layer Assembly of Pt Nanoparticles in PDDA. In addition to its role as a capping agent and surface stabilizer, polyacrylate allowed for the nanoparticles self-assembly in cationic polyelectrolytes driven by electrostatic and van der Waals forces, by imparting a considerable negative charge to their surface and allowing (38) Wang, Z. L.; Petrovski, J. M.; Green, T. C.; El-Sayed, M. A. J. Phys. Chem. B 1998, 102, 6145. (39) Aika, K.; Ban, L. L.; Okura, I.; Namba, S.; Turkevich, J. J. Res. Inst. Catal., Hokkaido Univ. 1976, 24, 54.

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Figure 2. UV-visible absorption spectra of 1-12 bilayers of Pt nanoparticles self-assembled on PDDA from as-prepared Pt solution (a), and a plot of the absorbance at 350 nm vs the number of Pt/PDDA bilayers (b).

for organic group interactions. Figure 2a shows the UVvisible absorption spectra of 1-12 bilayers of PAC-capped Pt nanoparticles assembled layer-by-layer in the cationic polyelectrolyte PDDA, by dipping the polymer-modified surfaces in the as-prepared Pt solution for 1 h per layer. The UV-visible absorption spectra of the Pt films and solutions exhibited no maxima in the UV-visible region, which is in keeping with the absorption spectra reported by Henglein et al. for their “citrate-Pt” colloids.36 The choice of 1 h dipping time was based on the kinetics study presented below, which revealed a loading saturation being almost reached at 60 min of dipping in asprepared Pt solutions (although as we show the density of nanoparticles is dependent on the mode of deposition). A plot of the absorbance at 350 nm versus the number of bilayers (Figure 2b) revealed the linear incorporation of Pt nanoparticles in the polymer, indicative of a reproducible consecutive surface-charge buildup under the deposition parameters employed. TEM images of films (see below) revealed a submonolayer surface distribution, hence indicating that a linear incorporation of nanoparticles does not necessitate, or imply, full surface coverage. Such behavior is attributed to the defect-spanning capability of polyelectrolytes, which can bridge over unoccupied sites. Kotov et al. also observed that the optical density of multilayered films of magnetic nanoparticles in poly-

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Figure 4. Plot of the absorbance at 350 nm vs the total dipping time of Pt nanoparticles assembled on a layer of PDDA via repetitive 2 min dipping in as-prepared Pt solution, followed by 1 min rinsing and 15 min drying. The points marked (4) at times 192-208 min correspond to data collected after replacing the Pt solution with a new one. The inset is a plot of the absorbance at 350 nm vs t1/2.

Figure 3. UV-visible absorption spectra of self-assembled Pt nanoparticles on a layer of PDDA via repetitive 10 min dipping in as-prepared Pt solution, followed by 1 min rinsing and 15 min drying (a), and a plot of the absorbance at 350 nm vs the total dipping time (b). The inset of part b is a plot of the absorbance at 350 nm vs t1/2.

electrolytes increased linearly with the number of bilayers, even while the deposition took place via domain growth rather than a uniform assembly of densely packed nanoparticles on the surface.15 Kinetics of PAC-Pt Self-Assembly on PDDA. The kinetics of adsorption of PAC-capped Pt nanoparticles on PDDA was monitored ex-situ by UV-visible absorption spectroscopy following repetitive dipping of a PDDAmodified quartz surface in as-prepared Pt solution for a time ∆t (10 or 2 min), rinsing in water for 1 min, and drying in air for 15 min. Figure 3a shows the UV-visible absorption spectra of Pt/PDDA self-assembled at ∆t ) 10 min for a 100 min total deposition time, and Figure 3b shows the corresponding film absorbance at 350 nm as a function of the total dipping time. The adsorption of nanoPt on PDDA is thus shown to take place via two processes characterized with different kinetics in effect at wellseparated time scales. A fast initial rate of adsorption resulted in ∼85% of the assembly being completed in 40 min. At times longer than 1 h, the rate of adsorption decreased significantly, resulting in only 1/7 of the initial incorporation per unit time. During this later stage, a pseudosaturation was almost reached. Therefore, controlling the nanoparticle incorporation per layer by varying the dipping time can best be achieved during the first adsorption regime.

The adsorption dynamics was also followed at ∆t ) 2 min, to investigate the effect of the frequency of interruptions (by multiple rinsing/drying cycles) on the assembly dynamics and the saturation levels. A plot of the Pt/PDDA film absorbance at 350 nm as a function of the total dipping time (Figure 4) again revealed two adsorption regimes with different kinetics on similar time scales as in Figure 3b. A fast linear incorporation of nanoparticles per unit time (R2 ) 0.99) took place in the initial 40 min. During the second regime, at times longer than 1 h, the adsorption rate decreased by a factor of 13 in comparison. The leveling off in nano-Pt incorporation with time is a result of the structure and surface-charge buildup of the assembled films opposing further adsorption at high coverage rather than a result of depleting the nanoparticles solution concentration. Two experiments provided evidence to this effect. In a first experiment shown in Figure 4, the Pt solution was replaced with a fresh one at t )190 min, and the deposition was resumed (up to t ) 208 min at ∆t of 2 min) with no measurable effect on the adsorption rate (Figure 4, data points marked with 4). In a second experiment, dipping for ∆t ) 10 min was conducted while replacing the as-prepared Pt solution with a new solution (V ) 10 mL) for every data point collected. Results (not shown) were similar to those presented in Figure 3, indicating two adsorption modes with different kinetics and a pseudosaturation being reached at the same time. The dynamics of PAC-Pt adsorption on PDDA conformed to the general trend of behavior of polymer and polyelectrolyte adsorption, likely because similar forces of interaction are in effect.23-25,31 For instance, Motschmann et al.24 reported that the adsorption of polystyrene-poly(ethylene oxide) diblock copolymer on silicon, studied insitu by ellipsometry, took place via two processes at different time scales: a diffusion-limited process in the initial stage and a significantly slower process at dense surface coverage, when the chain rearrangement on the surface becomes rate-limiting. Hoogeveen et al., studying by reflectometry the rates of adsorption of the strong polyelectrolytes PVP+ (quaternized polyvinylpyridine) and AMA+ (quaternized dimethylaminoethyl methacrylate) and of the weak polyelectrolyte AMA on TiO2 and SiO2 surfaces, also reported a diffusion-limited adsorption process during the initial stage, followed by a smaller

Pt Nanoparticles on a Polyelectrolyte Surface

deposition rate attributed to slower attachment at high surface coverage.25 In the case of a diffusion-limited process, the adsorbed amount is expected to be directly proportional to t1/2, according to the Langmuir-Schaefer (LS) relationship.40 A plot of absorbance versus t1/2 for the kinetics data collected at ∆t ) 10 min is presented in the inset of Figure 3b, revealing a linear dependence on t1/2 only during the first 20 min of deposition (R2 ) 0.9999). This dependence on t1/2 was not observed at all for the case of the assembly interrupted at ∆t ) 2 min (inset of Figure 4). Instead, the amount of assembled nano-Pt increased linearly with time, indicating an adsorption rate independent of the nano-Pt concentration (zero-order or pseudo-zero-order kinetics). This dynamics behavior is attributed to the more frequent interruptions in this case. As a result, the concentration of nano-Pt at the PDDA/solution interface is replenished every 2 min, rendering it seemingly independent of the diffusion process. Furthermore, it is possible that a PDDA conformational rearrangement induced by drying, discussed further below, is creating a more favorable adsorption surface and, hence, contributing to a higher adsorption rate upon interrupting the deposition by rinsing/drying. In the second kinetics regime, the adsorbed amount was found to be an exponential function of time, similar to the adsorption behavior reported by Motschmann et al. for the diblock polymer PS-PE.24 The following picture is presented to explain the observed assembly kinetics. Initially at low surface coverage, Pt nanoparticles present at the PDDA/solution interface will anchor through the capping polyelectrolyte to PDDA adsorption sites by electrostatic and possibly hydrophobic interactions and then relax to a surface position via adsorption of the remaining PAC chains, accompanied by liberation of the counterions, in a similar mechanism to the adsorption of polyions initially described by Hesselink.41 The rate of nanoparticle adsorption is limited at low surface coverage by their availability at the PDDA/solution interface and possibly by the strength of their interactions with the surface. (Adsorption of free polyacrylate at this ionic strength (0.2 M) might also take place in this case.) As the surface density increases, however, the cationic adsorption sites become scarce, and repulsion between the similarly charged nanoparticles increases, thus presenting a potential barrier to adsorption. It follows that the rearrangement of the polyelectrolyte-nanoPt on the surface to accommodate for the incoming nanoparticles becomes rate-limiting, leading to the considerable decrease in the adsorption rate at longer times. The maximum amount adsorbed at this stage is thought to be kinetically determined, which accounts for the reported irreversibility of polyelectrolyte adsorption,30,42 even in solutions of high ionic strength. Effect of the Interrupted-Deposition Protocol on the Assembly Dynamics. The density of self-assembled Pt nanoparticles per PDDA layer was found to increase upon decreasing the dipping interval using an interrupteddeposition protocol,43 which involves inserting multiple rinsing/drying steps in the assembly procedure. This effect is illustrated by comparing the absorbance at 350 nm of Pt/PDDA films assembled by dipping the polymer-modified quartz surface in the as-prepared Pt solution for 1 h using (40) Green, B. W. J. Colloid Interface Sci. 1971, 37, 144. (41) (a) Hesselink, F. J. Colloid Interface Sci. 1977, 60, 448. (b) Hesselink, F. In Adsorption from solution at the Solid/Liquid Interface; Parfit, G., Rochester, C., Eds.; Academic: London, 1983; p 377. (42) Schlenoff, J. B.; Ly, H.; Li, M. J. Am. Chem. Soc. 1998, 120, 7626. (43) Jaber, J.; Xin, Y.; Halaoui, L. I. Unpublished results.

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a single (1 min) rinsing/(15 min) drying step, 6 rinsing/ drying cycles at 10 min dipping intervals, and 30 rinsing/ drying cycles at 2 min dipping intervals, revealing a ratio of 1:1.3:2.2 for A350 nm (1 × 60 min)/A350 nm (6 × 10 min)/ A350 nm (30 × 2 min). A similar result has been reported by Raposo et al., who found that 4-5 times higher amounts of poly(o-methoxyaniline) were adsorbed on surfaces when the deposition was interrupted by rinsing/drying every 5 s relative to interruptions every 30 s,32 which they attributed to drying freeing surface sites previously occupied by water and possibly altering the polymer surface conformation (or doping state), allowing further deposition. The higher incorporation of nano-Pt when the assembly protocol involved smaller dipping time intervals took place primarily during the first adsorption regime [ratio of slopes of absorbance vs time (∆t ) 2 min): (∆t ) 10 min) is ∼ 2:1]. The nanoparticle adsorption rate during the second stage of assembly was independent of the dipping interval (ratio of slopes is equal to 1). The higher adsorption rate induced by more frequent interruptions in the first stage can be attributed to (1) re-establishing bulk nano-Pt concentration at the surface every time the dipping is resumed and (2) making sites previously occupied by water or anions on PDDA instantly available for Pt adsorption. Explaining the difference in surface coverage at which the second stage of deposition sets in, however, also calls for an assumption of some PDDA chain rearrangement taking place upon drying, achieving a surface conformation more favorable to nanoparticles adsorption. Drying has been reported by Lvov and Decher to result in a structural change in assembled poly(vinyl sulfate)/poly(allylamine) multilayers, causing enhanced periodicity of the films (revealed in X-ray diffraction patterns).7d During the second regime, the nanoparticle assembly requires overcoming a potential barrier presented by an increased repulsion and a scarcity of adsorption sites, which necessitates some rearrangement in the nanoparticle distribution on the cationic polyelectrolyte. Such a rearrangement and overcoming the energy barrier to adsorption are processes that take place in-situ (i.e., as the nanoparticles in solution attempt to adsorb), hence the independence of the adsorption rate in the second stage of deposition on the frequency of the rinsing/drying steps. TEM imaging of Pt/PDDA films revealed a submonolayer surface coverage and corroborated the above effect.43 Figure 5 shows TEM images of Pt nanoparticles selfassembled on PDDA upon 1 h of total dipping (in as-prepared Pt solutions) using (a) a single rinsing/drying step and (b) six rinsing/drying cycles at 10 min intervals. The micrographs show a fractal chainlike distribution of assembled Pt nanoparticles in PDDA along a framework on the surface, at submonolayer coverage, and reveal the increase in nanoparticle density incorporated via the interrupted-deposition protocol.43 The nanoparticles are perhaps positioned in troughs (along trains) on PDDA, thereby seeking a minimum potential energy. Effect of Salt Content on Pt Nanoparticle SelfAssembly. The density of PAC-Pt nanoparticles assembled on PDDA was found to depend on the Pt solution ionic strength and its composition. Figure 6 shows plots of (Pt/PDDA)n film absorbance at 350 nm as a function of the number of bilayers assembled from nano-Pt/NaCl (aq) solutions of different ionic strengths and the same Pt concentration. While, in the absence of any salt, Pt nanoparticles did not adsorb on PDDA, increasing the salt content to only 0.05 M resulted in a significant assembly of nano-Pt. The nanoparticle density per layer

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Figure 6. Absorbance at 350 nm vs the number of Pt/PDDA bilayers deposited from a nano-Pt (aq) solution with an NaCl concentration of 0, 0.05, 0.1, 0.2, 0.8, and 1.0 M.

Figure 5. TEM images of Pt nanoparticles deposited on a layer of PDDA using the following deposition parameters: (a) 60 min dipping in as-prepared Pt solution; (b) six times repeated dipping for 10 min in as-prepared Pt solution followed by rinsing for 1 min in water and drying in air for 15 min each time. The scale bars in (a) are 500, 100, and 20 nm, and the scale bars in (b) are 500, 100, and 50 nm (from lowest to highest resolution).

decreased significantly (by 77% cf. to 0.05 M) upon increasing the ionic strength to 0.1-0.2 M and decreased further (by 91% cf. to 0.05 M) upon increasing the salt concentration to 0.8-1.0 M. The linear incorporation of nanoparticles per bilayer was again maintained despite the decrease in incorporation level. The effect of ionic strength on polyelectrolyte adsorption on surfaces, including its role in fine-tuning the thickness of polyelectrolyte multilayers, has been well established and discussed in a large number of reports.8,26-31 The ionic strength has also been reported to influence nanoparticle assembly on polyelectrolyte polymers.12,21 Lvov et al.12 reported a linear increase in the density of SiO2 (25-78 nm) assembled on PDDA upon increasing the ionic strength from 0 to 0.3 M, which they attributed to

Figure 7. Model of the conformation of polyacrylate on the nano-Pt surface in the absence of salt (a) and in the presence of a favorable salt concentration (b).

screening of interparticle repulsion. Sennerfors et al. also recently reported a higher initial adsorption of 12 nm SiO2 particles on cationic polyelectrolytes upon increasing the ionic strength from 1 to 50 mM, also attributed to screening of interparticle repulsion permitting closer packing.21 The dependence of the PAC-capped nano-Pt assembly on the solution ionic strength reported herein is explained by invoking the same model of electrostatic interactions as in polyelectrolyte assembly.8,30,41 We begin by assuming that a number of polyacrylate chains are anchored to the Pt surface during the synthesis through portions of the chains. In the absence of salt ions, segmental repulsion between the polyacrylate units on the Pt surface could lead to tails (and loops) dangling in solution (Figure 7a). The resulting small charge density on PAC-Pt (large effective surface area) and the high repulsion between nanoparticles (excluded volume effect) lead to an insignificant Coulombic attraction term. Furthermore, in the absence of salt, no entropy is gained from liberating counterions that would compensate for the entropy loss

Pt Nanoparticles on a Polyelectrolyte Surface

accompanying the adsorption of PAC-Pt on PDDA. These factors render the adsorption thermodynamically unfavorable. As the salt concentration is increased, screening of similar charges results in decreasing segmental repulsion between the polyacrylate units, allowing them to coil around the Pt surface (Figure 7b), thus increasing the surface-charge density and hence the nano-Pt-PDDA attraction. Furthermore, the decrease in effective volume results in decreasing interparticle repulsion. Accordingly, this “screening-enhanced regime”30 results in a favorable situation, wherein the increase in Coulombic attraction and the increase in entropy from librating the counterions lead to a negative free energy for Pt assembly on PDDA. Upon further increasing the salt content, however, a “screening-reduced regime”30 is reached, where screening of the attraction between nano-Pt and PDDA hinders the assembly process, causing the observed decrease in incorporation. It is noteworthy that the nanoparticle assembly appears highly sensitive to small changes in ionic strength compared to polyelectrolyte multilayered assembly. This is in keeping with the report by Lvov and co-workers of an increase by a factor of 7 in the density of SiO2 (25-78 nm) in PDDA upon increasing the ionic strength from 0 to 0.3 M.12 On the other hand, for instance, Korneev et al. reported an increase in the thickness of PAH/PSS multilayers from 1.09 nm in the absence of salt to 10.4 nm for 3 M NaCl solutions, revealing a smaller dependency on ionic strength.29 The density of nanoparticles incorporated in PDDA from as-prepared Pt solutions, having a pH ) 7.9 and ∼0.2 M of charged species, remained the highest (122% of the Pt density/layer assembled from Pt/0.05 M NaCl solution). This enhanced adsorption, despite the presence of polyacrylate in solution, is attributed to a different solution composition (possibly higher Pt concentration and a high ratio of cationic to anionic species) and not to a pH effect, since deposition from Pt/0.2 M NaCl solution of pH 8 did not result in any increase in incorporation (cf. Figure 6, 0.2 M at pH ) 7.4). The high concentration of cations in as-prepared solutions (0.2 M of mainly Na+ with a small amount of K+) can cause the polyacrylate units on the Pt

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surface to assume a coiled structure, due to screening of intersegmental repulsion, which is a favorable conformation for their adsorption on PDDA. At the same time, the concentration of anionic species (polyacrylate considered as a single charged coiled structure) is 0.027 M, thus causing no screening of Coulombic attraction to the PDDA surface. A comprehensive understanding of this enhanced adsorption, however, requires an additional investigation of the effect of nano-Pt concentration and the different ionic valency on the assembly process. IV. Conclusions The assembly dynamics of PAC-Pt nanoparticles on PDDA and the effect of ionic strength and deposition protocol were investigated. The self-assembly of nano-Pt on PDDA was found to take place via two regimes with different kinetics: an initial fast regime at low surface coverage limited by the nanoparticle availability and the PDDA conformation, and a slower process limited by the surface rearrangement of PAC-Pt at high surface coverage. The density of assembled nano-Pt could be controlled by varying the ionic strength or the dipping time or by resorting to an interrupted-deposition protocol. The dynamics of PAC-capped nano-Pt assembly on an oppositely charged polyelectrolyte and the effect of the above determining parameters were explained by invoking the same model of electrostatic interactions and conformational changes as for polyelectrolyte-on-polyelectrolyte assembly, a behavior attributed to the presence of the soft polyacrylate cap. Acknowledgment. This work was financially supported by the University Research Board (URB) at the American University of Beirut (AUB). Y.X. thanks NHMFL under cooperative agreement DMR-0084173 and NSF Grant No. DMR-9625692. L.I.H. acknowledges financial support for the year 2002-2003 from a Fulbright fellowship and from a William and Flora Hewlett Foundation paid research leave during which time this manuscript was written for publication. LA0209839