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Polymer-Grafted-Platinum Nanoparticles: From Three-Dimensional Small-Angle Neutron Scattering Study to Tunable Two-Dimensional Array Formation Ge´raldine Carrot,*,† Franc¸ois Gal,† Christelle Cremona,†,‡ Je´roˆme Vinas,† and Henri Perez‡ Laboratoire Le´on Brillouin, CEA/DSM/IRAMIS/LLB-CNRS, Baˆtiment 563, CEA/Saclay, 91191 Gif-sur-YVette Cedex, France, and Laboratoire Francis Perrin, CEA/DSM/IRAMIS/SPAM-CNRS, Baˆtiment 522, CEA/Saclay, 91191 Gif-sur-YVette Cedex, France ReceiVed September 1, 2008. ReVised Manuscript ReceiVed October 17, 2008 Nanohybrid objects based on polymer and platinum nanoparticles are of great interest for applications in fuel cells or as biosensors. The polymer part can help first to stabilize and to organize the particles, second to increase the amount of chemical functions available in the organic corona, and, finally, to improve or to mask the properties of the particles. The method to introduce the polymer consists of using both the “grafting from” technique and controlled radical polymerization (atom transfer radical polymerization). Small-angle neutron scattering (SANS) is a well-suited technique for the study of these objects, particularly due to the possibility to use contrast matching to see either the particle or the polymer corona. Polymerization kinetics was followed by SANS and the polymer corona spectra showed a plateau at small q which attested that the objects are individual and well-dispersed. These systems were exempt of free polymers, so the characterization via SANS could lead to quantitative data such as the radius of gyration of the object, the amount of grafted chains and the molecular weight of the chains, using a star model to fit the data. Langmuir films have then been obtained directly from the polymer-grafted-nanoparticles solutions, and compression isotherms have been recorded followed by transmission electron microscopy (TEM) characterization of the films at different pressures. A good correlation has therefore been observed from the distances between objects calculated using the compression isotherms or observed via TEM and the objects’ dimensions determined from SANS study.
Introduction The synthesis of metal nanoparticles has been motivated by their unusual properties and their potential use in electronics, optics, magnetics, catalysts, and sensors.1-4 Similar to gold nanoclusters, platinum nanoparticles (PtNPs)5,6 have a high surface-to-volume ratio and, consequently, a large fraction of the metal atoms exposed at the surface are accessible to reactant molecules and available for catalysis.7-11 Such perspectives require the formation of organized structures in which the size of the nanoparticles and their distance separation are controlled.12 Therefore, different approaches have been investigated to form regular arrays of nanoparticles. Templates based on diblock copolymers or stable latex dispersions loaded with metal salts have produced arrays of core particles of a few nanometers separated by a few tens of nanometers.13-15 More * Corresponding author. E-mail:
[email protected]. † Laboratoire Le´on Brillouin, CEA/DSM/IRAMIS/LLB-CNRS. ‡ Laboratoire Francis Perrin, CEA/DSM/IRAMIS/SPAM-CNRS.
(1) Daniel, M.-C.; Astruc, D. Chem. ReV. 2004, 104, 293. (2) De, M.; You, C.-C.; Srivastava, S.; Rotello, V. J. Am. Chem. Soc. 2007, 129, 10747. (3) Niemeyer, C. M. Angew. Chem., Int. Ed. 2001, 40, 4128. (4) Roucoux, A.; Schulz, J.; Patin, H. Chem. ReV. 2004, 102, 3757. (5) Bratlie, K. M.; Lee, H.; Komvopoulos, K.; Yang, P.; Somorjai, G. A. Nano Lett. 2007, 7(10), 3097. (6) Lee, H.; Habas, S. E.; Kweskin, S. J.; Butcher, D.; Somorjai, G. A.; Yang, P. Angew. Chem., Int. Ed. 2006, 45, 7824. (7) Hashmi, A. S. K.; Hutchings, G. J. Angew. Chem., Int. Ed. 2006, 45, 1896. (8) Narayanan, R.; El-Sayed, M. A. J. Am. Chem. Soc. 2004, 126, 7194. (9) Tsunoyama, H.; Sakurai, H.; Ichikuni, N.; Negishi, Y.; Tsukuda, T. J. Am. Chem. Soc. 2005, 127, 9374. (10) Xiong, Y. J.; Wiley, B.; Xia, Y. N. Angew. Chem., Int. Ed. 2007, 46(38), 7157. (11) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 12663. (12) Balazs, A. C.; Emerick, T.; Russell, T. P. Science 2006, 314, 1107. (13) Forster, S.; Antonietti, M. AdV. Mater. 1998, 10(3), 195. (14) Spatz, J. P.; Herzog, T.; Mossmer, S.; Ziemann, P.; Mo¨ller, M. AdV. Mater. 1999, 11(2), 149.
recent examples included the synthesis of platinum nanoclusters directly in a polymer matrix whose polymer chains contain amide side chains.16 Star copolymers with a carboxylic core have been recently used to prepare PtNPs.17 Particularly, Kanaoka et al.18 demonstrated the interesting use of a star polymer as a scaffold to prevent aggregation of the nanoparticles. In this case, the use of a thermosensitive polymer permitted the isolation and the reuse of the catalyst nanobeads after reaction and precipitation of the polymers. Most of the time, these methods require a posttreatment to produce the arrays of nanoparticles from metal salts. In contrast, the synthesis of core diameter particles of a few nanometers capped with molecules allows the formation of arrays in which the interparticle distance lies in the same range of size. Therefore, other routes are based on a covalent link between the polymer chains and the particles. This includes either the “grafting to” (where functionnalized polymer chains are reacting with preformed particles) or the “grafting from” approaches (where chains are growing from the particles’ surface). The latter method has been widely used since the works of Prucker et al.19,20 due to its main advantage which is the absence of nonreacted polymer chains. In the present paper, the “grafting from” approach is used on 4-mercaptoaniline functionalized platinum21 previously derivatized with an initiator molecule. Indeed, as shown previously, (15) Carrot, G.; Valmalette, J. C.; Plummer, C. J. C.; Scholz, S. M.; Hilborn, J. G. Colloid Polym. Sci. 1998, 276(10), 853. (16) Chen, C.-W.; Tano, D.; Akashi, M. J. Colloid Interface Sci. 2000, 225, 349. (17) Zhang, L.; Niu, H.; Chen, Y.; Liu, H.; Gao, M. Colloid Polym. Sci. 2006, 298, 177. (18) Kanaoka, S.; Yagi, N.; Fukuyama, Y.; Aoshima, S.; Tsukuda, T.; Sukurai, H. J. Am. Chem. Soc. 2007, 18, 1211. (19) Prucker, O.; Ruhe, J. Macromolecules 1998, 31, 602. (20) Prucker, O.; Ruhe, J. Macromolecules 1998, 31, 592. (21) Perez, H.; Pradeau, J.-P.; Albouy, P.-A.; Perez-Omil, J. Chem. Mater. 1999, 11, 3460.
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the amine functionality of 2 nm core diameter platinum can be used to introduce different incoming molecules in the organic crown.22,23 The derivatization rate, i.e., the ratio of reacted-tounreacted amine function, was quite precisely determined using thermogravimetric analysis. The powders obtained before and after chemical modification of the organic crown spontaneously solubilized in solvents and allowed further formation of twodimensional and three-dimensional assemblies using the Langmuir-Blodgett (LB) method. Chemical modification of the crown led to an increase of the interparticle distance, in the powders and in LB films, in the range of 1-2 nm as shown by small-angle X-ray scattering.22,24 The chemical derivatization of a chemically functionalized surface (overgrafting reaction) has also been used to immobilize initiator molecules onto flat surfaces25 or from silica nanoparticles.26 The polymerization technique chosen here was atom transfer radical polymerization (ATRP)27 because it has been shown that it can be conducted in heterogeneous conditions, from particle surfaces without losing control in the polymerization process28-39 and generating any unwanted aggregation of the particles.28,40 One of the main advantage in our system is the absence of free initiator in solution, i.e., all the polymer chains are covalently grafted to the particles. This work reports the synthesis of the initiator-derivatized PtNPs followed by the polymerization of n-butyl methacrylate. Detailed characterization of the powders has been performed using thermogravimetric analysis. Particularly, small-angle neutron scattering (SANS) has been widely used to describe in detail the structure of these objects in solution. The SANS technique is particularly appropriate to characterize this type of object, especially due to the range of scale attainable, and the possibility, using index matching, to study only the polymer layer (grafting density, number of chains, etc.).41,42 The number of polymer branches and chain molecular weights have therefore been obtained from this study. In parallel to this three-dimensional (3D) characterization (in volume), we also report here the formation of two-dimensional (2D) arrays at the air-water interface (Langmuir film) from these polymer-grafted-platinum NPs, in which the interparticle distance may be controlled in the range of tens of nanometers to a few nanometers. Furthermore, the average interparticle distance in the monolayer can be varied (22) Perez, H.; Noe¨l, V.; Cavaliere-Jaricot, S.; Etcheberry, A.; Albouy, P. A. Thin Solid Films, in press. (23) Cavalie`re-Jaricot, S.; Etcheberry, A.; Noe¨l, V.; Herlem, M.; Perez, H. Electrochim. Acta 2006, 51, 6076. (24) Perez, H.; Lisboa de Sousa, R. M.; Pradeau, J.-P.; Albouy, P. A. Chem. Mater. 2001, 13, 1512. (25) Kong, X.; Kawai, T.; Abe, J.; Iyoda, T. Macromolecules 2001, 34, 1837. (26) El Harrak, A.; Carrot, G.; Oberdisse, J.; Eychenne-Baron, C.; Boue´, F. Macromolecules 2004, 37, 6376. (27) Matyjaszewski, K.; Xia, J. Chem. ReV. 2001, 101, 2921. (28) Ejaz, M.; Ohno, K.; Tsujii, Y.; Fukuda, T. Macromolecules 2000, 33, 2870. (29) Gu, B.; Sen, A. Macromolecules 2002, 35–8913. (30) Huang, X.; Wirth, M. J. Macromolecules 1999, 32, 1694. (31) Husseman, M.; Mamstrom, E.; McNamara, M.; Mate, M.; Mecerreyes, D.; Benoit, D.; Hedrick, J.; Mansky, P.; Huang, E.; Russell, T.; Hawker, C. Macromolecules 1999, 32, 1424. (32) Kim, J.-B.; Huang, W.; Bruening, M. L.; Baker, G. L. Macromolecules 2002, 35, 5410. (33) Mori, H.; Mu¨ller, A. H. E.; Klee, J. E. J. Am. Chem. Soc. 2003, 125, 3712. (34) Patten, T. E.; Matyjaszewski, K. AdV. Mater. 1998, 10, 901. (35) Pyun, J.; Jia, S.; Kowalewski, T.; Patterson, G. D.; Matyjaszewski, K. Macromolecules 2003, 36, 5094. (36) Pyun, J.; Matyjaszewski, K. Chem. Mater. 2001, 13, 3436. (37) Radhakrishnan, B.; Ranjan, R.; Brittain, W. J. Soft Matter 2006, 2, 386. (38) Li, D.; He, Q.; Cui, Y.; Li, J. Chem. Mater. 2007, 19, 412. (39) Ohno, K.; Morinaga, T.; Koh, K.-M.; Tsujii, Y.; Fukuda, T. Macromolecules 2005, 38, 2137. (40) Von Werne, T.; Patten, T. E. J. Am. Chem. Soc. 2001, 123, 7497. (41) Carrot, G.; El Harrak, A.; Oberdisse, J.; Jestin, J.; Boue´, F. Soft Matter 2006, 2, 1043. (42) El Harrak, A.; Carrot, G.; Oberdisse, J.; Jestin, J.; Boue´, F. Polymer 2005, 46, 1095.
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reversibly depending on the surface pressure, as shown by transmission electron microscopy (TEM) micrographs. The distances between particles observed via TEM and calculated from the compression isotherms of the Langmuir films are both coherent with the objects’ dimensions at different polymerization conversions, obtained from the SANS study.
Experimental Section Chemicals. The monomer (n-butyl methacrylate) (Aldrich) was distilled from calcium hydride prior to use. All other chemicals were purchased from Aldrich and used without further purification. Preparation of Initiator-Derivatized PtNPs (Overgrafting Reaction). A 150 mg portion of 4-mercaptoaniline-functionalized PtNPs of ca. 2 nm core diameter were synthesized according to a reported procedure21 and dissolved in 50 mL of dimethylacetamide (DMAc). (Dimethylamino)pyridine (DMAP) (107 mg, 0.8 mmol) was added, followed by 0.2 mL (1.5 mmol) of 2-bromoisobutyrate bromide. The reaction was allowed to proceed for 24 h. Then the particles were precipitated in acetonitrile, centrifuged at 3500 rpm, and purified several times by several dissolution and precipitation steps. Thermogravimetric analysis (TGA) was used to determine the percentage of overgrafting (derivatization rate). Atom Transfer Radical Polymerization (ATRP) Procedure. Initiator-derivatized PtNPs in DMAc, CuIBr, and PMDETA were mixed in a three-neck flask under nitrogen. The CuBr/PMDETA/ initiator molar ratio was 2/3/1. The reaction mixture was stirred until it became homogeneous. The monomer (n-butyl methacrylate) was then added before the reaction flask was heated to 60 °C. Theoretical Mn has been calculated from the molar ratio of monomer versus grafted initiator for 100% conversion. Kinetic samples were taken via purged syringes and were used to determine conversion by gravimetry. The polymer-to-platinum ratio was determined by thermogravimetry. Thermogravimetric Analysis (TGA). TGA was performed on a TA Instruments Q50, at a scan rate of 20 °C min-1, up to 800 °C under air. The percentage of overgrafting of the initiator was calculated from the mass fraction of platinum residue, as follows. First, we determined the number of moles of mercaptoaniline per grams of platinum in the functionalized particles,
%w aniline %w platinum naniline ⁄ gplatinum ) Maniline
(1)
The second analysis gave the mass fraction of both the mercaptoaniline and the overgrafted bromide moiety. The number of moles of bromoisobutyrate was then calculated from this analysis and the previous value
%w organic residue %w aniline %w platinum %w platinum nbromide ⁄ gplatinum ) Mbromide function (2) The percentage of overgrafting was finally calculated as follows
%w overgrafting )
nbromide ⁄ gplatinum × 100 naniline ⁄ gplatinum
(3)
Differential Scanning Calorimetry (DSC). DSC was performed on a TA Instruments DSC 2920, at a scan rate of 10 °C min-1. Photon Correlation Spectroscopy (PCS). Particle size and size distribution of particles were first determined using the Zetasizer nanoparticle analyzer (nano-ZS, ZEN3500) from Malvern operating at an angle of 172.8°. Measurements were performed at 25 °C after dilution in DMAc; data treatment was done with the Dispersion Technology Software (DTS) from Malvern. Small-Angle Neutron Scattering (SANS). Experiments have been performed on the PACE and PAXE spectrometer (LLB, Saclay).
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Scheme 1. ATRP Initiator Functionalization of the PtNPs
Two configurations (wavelength λ0 ) 6.0 Å, Dsample-to-detector ) 1 and 5 m) were used, covering a q range of 0.002-0.4 Å-1. Data treatment was done with a homemade program (Pasidur, LLB) following standard procedures43 with H2O used for the normalization of the detector efficiency. Incoherent background was determined with several H/D mixtures and interpolated for the desired concentrations. The absolute values of the scattered intensity (in cm-1) were measured via a determination of the direct beam intensity. Langmuir Films. Solution of 1 mg/mL of polymer-g-PtNPs (∼0.1 mg/mL Pt) were prepared in a mixture of DMAc/chloroform (1/3) just before spreading onto the water surface of a laboratory-made Langmuir-Blodgett (LB) through filled with Millipore-grade water.22,24 The compression isotherms have been recorded at 21 °C by steps of 1 mN/m. Transmission Electron Microscopy (TEM). Images were recorded on monolayers manually deposited onto carbon grid at different film pressures, using a Philips CM12 (120 keV) transmission electron microscope.
Results and Discussion Initiator-Grafted-Platinum Nanoparticles (Br-g-PtNPs). The overall reaction steps that lead to the initiator-grafted-platinum nanoparticles are shown in Scheme 1. Platinum nanoparticles have been synthesized using mercaptoaniline and hexylamine to control both the size and the surface chemistry of the particles. Disulfide of mercaptoaniline was introduced in the reaction mixture, right after the reduction of the platinum salt using sodium borohydride. As shown by TEM analysis and SANS, this synthesis leads to nanoparticles with a ca. 2 nm core size with a low polydispersity, containing amino groups at the surface, from where much organic chemistry may be performed.21,22 The amount of amine functions grafted onto the particle surface has been determined from TGA. The method was well reproducible, and we usually obtained the same ratio of about 15 wt % of the organic part, a value slightly lower than the results given by elemental analysis presumably because of slow oxidation of 4-mercaptoaniline. In previous work21 assuming a cubic shape for the particles in order to simplify calculation, the area occupied by the 4-mercaptoaniline at the surface of the particles was estimated to be 0.14 nm2, which corresponds to a relatively high density amine functionality of 7 functions/nm2. Taking into account the organic content of the powder and the size of the core, an averaged number of 89 amine functions per particle was estimated. The functionalized particles were chemically derivatized through an “overgrafting” reaction (Scheme 1) to generate the (43) Lindner, P. In Neutrons, X-ray and Light Scattering Methods Applied to Soft Condensed Matter; Lindner, P., Zemb, T. , Eds.; North Holland: Amsterdam, 2002; Chapter 2, p 23.
initiator-grafted-platinum nanoparticles (Br-g-PtNPs). The particles were reacted using 2-bromoisobutyrate bromide in DMAc, and the bromine function was introduced through the formation of an amide bond. Particles remained well-dispersed during this reaction step as attested by the PCS analysis which gives an average diameter of 2 nm. TGA recorded on the powder showed, as expected, an increase in the organic content (20%) compared to the functionalized particles. As described before, these analyses allowed us to estimate the rate of amine derivatization (overgrafting percentage) which was found to be 34%. Taking into account the number of 4-mercaptoaniline molecules per particle (i.e., 89 molecules), we deduced a number of bromine functions of 30 per particle, corresponding to an averaged density of about 2.4 bromine/nm2. The Br-g-PtNPs powder spontaneously redissolved in DMAc, which was used as the solvent, to conduct atom transfer radical polymerization. Atom Transfer Radical Polymerization from Br-g-PtNPs. Controlled radical polymerization from hybrid objects always leads to the same questions: could we reach the same control as in solution polymerization, in such heterogeneous media where the overall concentration of initiating species is lower? Could we perform the polymerization without disturbing the colloidal stability? The polymer molecular weight control in ATRP occurs via the persistent radical effect.44,45 Therefore, the concentration of initiating species should be high enough to ensure the formation of deactivator species (copper(II)). In our previous work done on silica nanoparticles, we showed that it was necessary to add a certain amount of sacrificial initiator to ensure a controlled polymerization.26 We were performing the polymerization from 15 nm silica nanoparticles with a grafting density of 0.34 molecules/nm2, corresponding to a total concentration of 0.1 mmol of initiator sites per gram of silica. Many works related to the same type of hybrid objects also showed that the use of free initiator was essential for the controlled radical polymerization from inorganic particles.31,40 Patten et al. shows the influence of the monomer, the grafting density, and the initiator concentrations on the control of the polymerization.40 Another way to reduce the radical population is the addition of copper(II) as shown by Wirth and Matyjaszewski.46,47 In the present case, the PtNPs were much smaller (2 nm in diameter), and both the local and the total concentrations were (44) Fisher, H. Chem. ReV. 2001, 101, 3581. (45) Matyjaszewski, K.; Patten, T. E.; Xia, J. J. Am. Chem. Soc. 1997, 119, 674. (46) Xia, J.; Zhang, X.; Matyjaszewski, K. Macromolecules 1999, 32, 3531. (47) Xiao, D.; Wirth, M. J. Macromolecules 2002, 35, 2919.
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Figure 1. Kinetic plots of the conversion vs time (a) and ln(M0/M) vs t2/3 (b) for the polymerization of n-butyl methacrylate from Br-g-PtNPs. Two different experiments are shown (using the same theoretical molecular weight): PtPMAB7 (9) and PtPMAB8 (2).
dramatically different from the silica particles system. The local concentrations (of initiating species onto the particles) of the two systems were 12.4 and 0.23 mol/L for the PtNPs and the silica nanoparticles, respectively, whereas the global concentrations were 3 × 10-4 and 3 × 10-3 mol/L. This high concentration of grafted initiator onto the PtNPs made possible the polymerization without the presence of free initiator in the reaction medium. The nonuse of sacrificial initiator, and therefore the absence of free chains, represented a great advantage for both the following purification and analysis steps. The polymerization has been performed in a 20 wt % solution of monomer to keep a good dispersion of the Br-g-PtNPs. This yields an initial monomer-to-initiator of 660. Polymerization has been performed in dimethylacetamide where the Br-g-PtNPs could be easily dispersed. The copper (Cu(I)) and the ligand (PMDETA) were added first, followed by the Br-g-PtNPs solution. Then the temperature was increased to 60 °C and the reaction started after the addition of the monomer, n-butyl methacrylate (n-BuMa). Kinetic results of the polymerization were obtained by gravimetric measurements and are shown in Figure 1. Two polymerization experiments (PtPMAB7 and PtPMAB8) have been conducted from different batches of Br-g-PtNPs with roughly the same grafting density (see Table 1). The polymerization conditions were identical in both the initial theoretical degree of polymerization (DP) and the monomer concentration. The polymerization conversion (Figure 1a) increased first linearly with the reaction time before a plateau where the conversion maximum is reached (19.2% for PtPMAB7 and 15.4% for PtPMAB8). The kinetic plots (ln M0/M ) f(t2/3)) are quasi-linear and very similar in each case, attesting that the polymerization is rather controlled during the chain growth (Figure 1b). However, the analysis of the molecular weight of the cleaved chains versus the conversion together with the polydispersity would have given a better idea of the degree of control of the polymerization. The
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degrafting of the chains from the platinum surface is not straightforward, and this is currently under study. From these results, we showed that the three-step synthesis (functionalization, overgrafting, and polymerization) was wellreproducible (5% difference which may be due to the difference in the overgrafting step and errors in the gravimetric measurements). At this stage, we could also attest that the polymerization was controlled without any addition of sacrificial initiator. Some samples have been taken at different polymerization times from the different polymerization batches and characterized. Conversion, percentage of polymer, and Tg are given in Table 1. Interestingly, the glass transition temperature (Tg) was found to be higher when polymer chains are grafted as compared to the bulk polymer (Tg ≈ 25 °C). We have found similar results with polymer tethered from silica particles.26 This result is due to the decrease in chain mobility, which is affected by tethering and confinement. The nonaggregation of the particles in solution during and after polymerization has been checked from both SANS measurements and TEM characterization. SANS Characterization. The SANS technique has been essentially used to characterize the polymer layer. Experiments have been performed in a mixture of 95.7% of deuterated DMAc (FdDMAc ) 6.6 × 1010 cm-2) and 4.3% of hydrogenated DMAc (FhDMAc ) 0.525 × 1010 cm-2) to match the scattering length density of the platinum core (FPt ) 6.34 × 1010 cm-2) and to get the scattering signal of the polymer chains only (Fpolymer ) 0.54 × 1010 cm-2). The four samples described in Table 1 have been studied and prepared in this mixture of deuterated and hydrogenated DMAc solution at a concentration of roughly 7 mg/mL which corresponds to a volume fraction of polymer (for the PtPMAB8-1 sample) of 0.575% (the platinum volume fraction is 4 × 10-3%). Experiments have also been done in the polymer matching conditions (100% of hDMAc) to observe only the signal of the particles. In this case, the intensity of the signal was too low to be detected. This was due to the small volume concentration of the samples (the high density of platinum prevents the preparation of high volume fraction of platinum samples). The spectra obtained from the different samples are shown in Figure 2. The two parts (2a and 2b) represent samples of different polymerization experiments (PtPMAB7 and PtPMAB8). The volume fraction of the scattering objects being a prefactor of the form factor measured at such high dilutions, we have plotted I/ΦPt for direct comparison at different polymer conversions (the Pt concentrations which are directly correlated to the number of objects are close but not exactly the same). First, we could observe that all spectra were similar (this is also a proof that the synthesis is well-reproducible): they both showed a plateau at low q values. The plateau was more visible in Figure 2b because we reached lower q values in this SANS experiment. This plateau meant that the studied objects had a finite size and were not aggregated. Second, the signal in the intermediate q range was also identical for all samples: it showed a decrease in q-2 which is typical of a Gaussian chain. This first result was not obvious as we would rather expect a decrease in q-4, typical of a sharp interface as usually observed in the presence of a compact corona interface. In the present case, we therefore assumed that the polymer corona was not very dense and we would use another model to fit the SANS data. Third, by comparing the two spectra in each figure, we could observe that the SANS intensity increased with the polymer conversion. This makes sense as obviously more diffusing species are present when the amount of polymer increases. Finally, in the intermediate q range, we also observed
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Table 1. Summary of the Kinetic Data and Thermal Analysis for the Different Samples sample
theoretical Mn, g · mol-1
reaction time, min
Molc/nm2
% conversion
polymerization batch
% polymer
Tg, °C
PtPMAB7-1 PtPMAB7-2 PtPMAB8-1 PtPMAB8-2
100000 100000 100000 100000
455 113 334 40
2.75 2.75 2.4 2.4
19.2 15.3 15.4 10.5
PtPMAB7 PtPMAB7 PtPMAB8 PtPMAB8
88.0 83.1 83.6 80.7
34.2 29.5 36.2
the presence of a shoulder (an extra oscillation). This oscillation is even more visible in the Kratky representation (Iq2) of the data (see insert in Figure 2b). In this plot, we could observe a first oscillation at low q values and a second one in the intermediate range (which corresponds to the small shoulder). The peak maximum of the first one was shifted to the lower q values when the conversion was increasing, suggesting that the polymer corona was increasing in size. The bump is characteristic of a branched architecture. Indeed, for a Gaussian chain, we would have a plateau, rather than a peak, which is characteristic of an increase in the density fluctuations. The second oscillation was the same for the two conversion samples, and it corresponded to size ranges of R ∼ 1 nm. This may be due to the scattering of the inner core of the object, as this value is rather close to the PtNPs radius. From the plateau at low q values, we could estimate a radius of gyration of the objects using the Guinier approximation (qRg e 1)
( ) R2 2 g
I(q) ∝ exp q
(4)
3
By plotting ln(I) versus q2, we would determine directly the value of Rg, and as expected, Rg was increasing with the conversion (see Table 2). Now, in order to get more information, we need to use a model which will fit the data in the whole q domain. As mentioned
-1
Figure 2. Relative scattered intensity I/Φ in cm as a function of scattering wave vector q in Å-1 of two different polymerization batches from PtNPs performed in the same conditions. (a) PtPMAB7 with conversion of 19.2% (9) (PtPMAB7-1) and 15.3% (2) (PtPMAB7-2); (b) PtPMAB8 with conversion of 15.4% (2) (PtPMAB8-1) and 10.5% (9) (PtPMAB8-2). Insert: log-log plot q2I(q) vs q of PtPMAB8 for conversion of 10.5% (2) and 15.4% (9).
before, the intermediate domain showed a decrease in q-2, and the Kratky representation (Iq2) shows the presence of a peak characteristic of a star architecture. Indeed, in this case, the core corresponding to the platinum nanoparticle, is very small (R ∼ 1.0 nm), and its contribution may be neglected compared to the polymer corona. Therefore, it can be seen as a multifunctional initiator of a branched structure. This geometry is therefore closer to a star model rather than a compact corona model, such as the one we previously used to describe our polymer-grafted-silica nanoparticles.41 Therefore, the model used here is the form factor of a polymer star where the interactions between chains are neglected (model of Zimm-Stockmayer-Benoıˆt).48,49 Polymer chains are then considered as Gaussian. The form factor corresponding to a polymer (Gaussian) star without interactions is written as
P(q) )
2 d f (f - 1)e-2X⁄f - f(f - 2)e-X⁄f + (f - 3) 2 2 2 X
[
]
(5)
with
X ) (qRgB)2f ) (qRgE)2
f2 3f - 2
with RgE the radius of gyration (Rg) of a star with a functionality f and a molecular mass of M ) Nm and RgB the Rg of one branch. The fit of the experimental SANS plots is shown in Figure 3, and all the parameters that were determined from the fit, for the different samples, are summarized in Table 2. The validity of the model was supported not only by the satisfactory fit of the experimental plot but also by the fact that the number of branches was close but different from one polymerization batch to another (PtPMAB7 and PtPMAB8). However, when the conversion was different for the same batch, the number of chains did not change (Table 2). The results of five or eight chains per particles given by the fit correspond to grafting densities of 0.4 and 0.63 chains per nm2, respectively. This means that particles are covered by a relatively high density polymer layer, with regard to previous work.39 Therefore, the difference with the initial amount of initiators (2.4 molecules/ nm2) could be explained by steric hindrance, and no additional chains could be generated from the surface. This could occur either from an incomplete initiation or a deactivation of the growing chains at the beginning of the polymerization. However, as the number of chains seems to remain constant with the conversion, the lack of initiation efficiency is most probable. This point is currently under study and it will be discussed in more detail in a forthcoming paper. The molecular weight of the star could be estimated from the extrapolation of the scattering intensity (I0) at q ) 0, using the approximation
Mw )
I(q f 0) ⁄ C (∆F)2NA
(6)
From that value and the number of branches, we could estimate the chain molecular weight (Mw). However, the values of Mw (48) Barret, A. J. Macromolecules 1984, 17, 1561. (49) Benoit, H. J. Polym. Sci. 1973, 17, 203.
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Table 2. Data Obtained from the Fit of the SANS Spectra with the Star Model sample
no. of branches
object Rg Guinier, RgG (Å)
object Rg star, RgE (Å)
branch Rg, RgB (Å)
no. of chains per nm2
mol wt of one branch, I0 (g · mol-1)
PtPMA7-1 PtPMA7-2 PtPMA8-1 PtPMA8-2
5 5 8 8
95 75 68 57
86 72 70 61
53 44 42 37
0.42 0.42 0.63 0.63
69000 35000 22500 10500
given here are indicative as they have been calculated for one concentration. Moreover, this star model is satisfactory, but as mentioned before, interactions between chains are neglected in that case, and more sophisticated models such as the Daoud-Cotton one, may be more appropriate.50
Figure 3. Fit with a star model of the SANS spectrum (PtPMAB7-1).
Langmuir Films and TEM. In previous reports, it was shown that the Langmuir-Blodgett method may be used with functionalized and crown-derivatized platinum nanoparticles to build monolayers or multilayers, in which the interparticle distance lay in the same range as the platinum core size.22,23 With the polymer-grafted-PtNPs a different situation was expected. Therefore, it was particularly interesting to compare the features of the 2D arrays with the features determined in solution by SANS studies for the corresponding polymer-grafted-nanoparticles. A solution of PMABu-g-PtNPs (PtPMAB7-1 and PtPMAB7-2) in a solvent mixture (chloroform and DMAc) has been spread onto the surface of water, and compression isotherms have been recorded. Plots of the surface pressure versus the surface area per object are shown in Figure 4. The area per object was estimated by combining the data given by TGA analysis and the average molecular mass calculated for the platinum core (ca. 70000 g/mol). Consistently, the area per particle was found to be larger for the objects with a higher radius of gyration (Rg) in SANS analysis. For the two samples, the compression isotherms exhibited a particular feature with a plateau-like region (more pronounced for polymer-graftedparticles with higher Rg, PtPMAB7-1). The plateau region separates actually two different behaviors of the Langmuir films. At low surface pressure, the compressibility of the nanomaterials was quite high, whereas after the plateau, at high pressure, the layers showed poor compressibility and became stiff. The transition between these two surface pressure domains was characterized by a very important decrease in the area per nanoparticle, suggesting a reorganization of the grafted polymer branches. We found out that the decrease was more pronounced for the particle with polymer chains of higher Mw (or higher Rg). The important variation of the area per particle in the whole compression isotherm prevented to reach the collapse pressure before the limitation which is due to the length-to-width ratio (50) Daoud, M.; Cotton, J. P. J. Phys. (Paris) 1982, 43, 531.
of the Langmuir trough. However, at high pressure, the area per particle recorded for the objects with lower Rg values (PtPMAB72) remained consistently lower than the one recorded for ones with a higher Rg. The area per particle reported on the compression isotherm could be exploited to estimate an average interparticle distance. As an approximation, for the particles being considered as spherical, the interparticle distance is related to the diameter of a sphere (or to the diameter of the circle corresponding to the projection of the sphere on the air-water interface). For the two polymer-grafted-particles, PtPMAB7-1 and PtPMAB7-2 (with polymer conversion of 19.2 and 15.3, respectively), the distances obtained were 37.4 and 23.8 nm at 2 mN/m and 9.4 and 6.2 nm at 26 mN/m, respectively. These latter values (calculated at 26 mN/m at the air-water interface) could be compared to the Rg star (or Guinier) determined by SANS (Table 2). These results suggest that the layer formed at the air-water interface is probably a monolayer. This point was confirmed by further characterization of the Langmuir films by transmission electron microscopy (TEM) manually collected on carbon-coated copper grids. The TEM micrographs are reported in panels a-c of Figure 5 for PtPMAB7-1 and panels d-f of Figure 5 for PtPMAB7-2. Both materials were first characterized at 2 mN/m (Figure 5, panels a and d), then, second, after compression at 26 mN/m (Figure 5, panels b and e), and finally after decompression of the films at 2 mN/m. All the micrographs showed that a monolayer of polymer-grafted-particles was formed in every case. The strong variations of the area per particle versus the surface pressure, as suggested by the compression isotherms, were also directly evidenced by TEM (comparison of panels a and b of Figure 5 for PtPMAB7-1 and panels d and e of Figure 5 for PtPMAB7-2). The differences which were observed between both polymergrafted-particles could also be detected on the TEM micrograph. For a given pressure (2 or 26 mN/m), the density of nanoparticles in the film is larger for samples in which the grafted chain length is lower (PtPMAB7-2). Furthermore, the comparison of panels a and c of Figure 5 as well as panels d and f of Figure 5 indicates that the strong structural change in the polymer capping was reversible. It should be pointed out, at this stage, that the arrays
Figure 4. Compression isotherms of PtPMAB7-1 (2) and PtPMAB7-2 (]) recorded in a static mode (the pressure value is imposed and the Langmuir barrier displacement is measured).
Nanohybrid Polymer-g-Pt Nanoparticles
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Figure 5. TEM micrographs at different degrees of compression for the PtPMAB7-1 sample (a, b, and c) and for the PtPMAB7-2 sample (d, e, and f), at 2 and 26 mN/m, and after decompression at 2 mN/m again, respectively.
of nanoparticles formed at the air-water interface are manually transferred on the carbon grid, and this step may induce some defects in the initial structure. However, a rough estimation of the average interparticle distances in the different cases (grafted chain length and surface pressure) point to a very good consistency with the data obtained from the pressure isotherms. Calculations of the distances between two particles cores, for both polymergrafted-particles at 2 mN/m (PtPMAB7-1 and PtPMAB7-2), gave 37.4 and 23.8 nm, respectively. From the TEM micrographs (Figure 5, panels a and d), we observed that the most frequent interparticle distances were in the range of 20-50 nm. Moreover it was clearly observed that the average interparticle distance in the case of PtPMAB7-1 is higher than that for PtPMAB7-2, as suggested by the isotherm study. This consistency was also observed at high pressures where the interparticle distance for PtPMAB7-1 has roughly a frequent value higher than 10 nm (for 9.5 nm calculated from isotherms) whereas PtPMAB7-2 showed values rather lower than 10 nm (for 6.2 nm calculated). These latter values were also consistent with the Rg values calculated from the 3D SANS study (Rg of objects are 9.5 and 7.5 nm for PtPMAB7-1 and PtPMAB7-2, respectively). Finally, we did perform some dynamic measurements of the compression isotherms (not shown here) where the speed of the Langmuir barrier is imposed, and the film pressure is recorded. We found out that the compression isotherm was identical to the one recorded in the static mode. However, a hysteresis phenomenon was detected during the decompression stage. This hysteresis suggested a change in the chains’ conformation between the low-
and the high-pressure regime. Whereas at low pressure the high compressibility of the film is related to the polymer chain mobility, the high-pressure regime, where the compressibility is lower, corresponds probably to an entangled regime.
Conclusions Polymer chains have been grown from 2 nm core platinum nanoparticles from the initiator-derivatized PtNPs using atom transfer radical polymerization, without any need of sacrificial initiator. From small-angle neutron scattering studies in solution, we could determine the radius of gyration of the afforded objects which was found to increase with the polymer conversion. This study also showed that their overall structure is closer to a star architecture rather than a dense polymer corona surrounding the inorganic core. The number of branches and radius of gyration of the chains could be calculated from the fit with a polymer star model. Compression isotherms of the films made from the polymer-grafted-PtNPs solutions showed, as expected, a dramatic difference compared to the ones obtained from small moleculederivatized PtNPs. The calculated area per particle at the air-water interface is dependent on the polymer conversion and the surface pressure. From a low to high pressure regime, the area per nano-object was divided by a factor of 4, pointing out large structural differences in the grafted chains interactions. Direct evidence of arrays formation was shown by TEM. A good correlation was found between the size of the objects determined by SANS, the interparticle distances estimated from compression
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isotherms, and the direct characterization of the arrays by TEM. It was shown that tunable arrays could be controlled by both the polymer chain length, grafted onto the elementary platinum bricks, and the surface pressure at which they were formed. This control of the distance is particularly important regarding the electrochemical measurements after deposition onto electrodes, which is the next step in the exploitation of these objects as biosensors. Future developments of this work will address different points, such as the organization of the polymer chains in the monolayers, the influence of the monomer involved in ATRP on this
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organization, and the characterization of these arrays deposited onto bulk planar substrates by neutron reflectivity. Acknowledgment. The authors acknowledge Dr. H. Maskrot from CEA-DSM-IRAMIS-SPAM-LFP for the characterization performed by transmission electron microscopy and Dr. A. Brulet from CEA-DSM-IRAMIS-LLB for discussion concerning the neutron scattering data treatment. LA802862Q
