pubs.acs.org/Langmuir © 2010 American Chemical Society
Formation of Catalytic Silver Nanoparticles Supported on Branched Polyethyleneimine Derivatives Aline M. Signori,† Kelly de O. Santos,† Renato Eising,† Brunno L. Albuquerque,† Fernando C. Giacomelli,‡ and Josiel B. Domingos*,† † LaCBio - Laboratory of Biomimetic Catalysis, Chemistry Department, Universidade Federal de Santa Catarina, Campus Trindade, Florian opolis - SC 88040-900, Brazil, and ‡Centro de Ci^ encias Naturais e Humanas, Universidade Federal do ABC, Rua Santa Ad elia 166, Santo Andr e - SP 09210-170, Brazil
Received August 26, 2010. Revised Manuscript Received September 16, 2010 A new and straightforward method for screening highly catalytically active silver nanoparticle-polymer composites derived from branched polyethyleneimine (PEI) is reported. The one-step systematic derivatization of the PEI scaffold with alkyl (butyl or octyl) and ethanolic groups led to a structural diversity correlated to the stabilization of silver nanoparticles and catalysis. Analysis of PEI derivative libraries identified a silver nanoparticle-polymer composite that was able to efficiently catalyze the p-nitrophenol reduction by NaBH4 in water with a rate constant normalized to the surface area of the nanoparticles per unit volume (k1) of 0.57 s-1 m-2 L. Carried out in the presence of excess NaBH4, the catalytic reaction was observed to follow pseudo-first-order kinetics and the apparent rate constant was linearly dependent on the total surface area of the silver nanoparticles (Ag-NPs), indicating that catalysis takes place on the surface of the nanoparticles. All reaction kinetics presented induction periods, which were dependent on the concentration of substrates, the total surface of the nanoparticles, and the polymer composition. All data indicated that this induction time is related to the resistance to substrate diffusion through the polymer support. Hydrophobic effects are also assumed to play an important role in the catalysis, through an increase in the local substrate concentration.
Introduction Colloidal metal nanoparticles (M-NPs) have attracted much attention in several areas due to their exceptional electrical, magnetic, and optical properties that differ considerably from the properties of the bulk metal.1,2 Moreover, due to the large percentage of atoms on the surface,3-5 colloidal dispersions of metal nanoparticles are of special interest for application in catalysis.1 For the preparation of such colloidal M-NPs, the most common method involves the chemical reduction of metal salts in solution.6 The choice of the reducing agent has a great influence on such methods, since its reductive ability determines the kinetics of the M-NP formation. Given that colloidal M-NPs are thermodynamically unstable, their preparation requires the use of a stabilizing agent, an essential component in the control of M-NP growth and in the tuning of their chemical and physical properties.7 Particularly for catalysis, functional polymers are very attractive matrices for NP stabilization, because their multiple possible interactions should result in a catalyst with combined properties from the polymer framework and the metal cluster.8 In fact, the choice of the nature of the functional groups to be built into the polymeric stabilizer for catalytic applications is made on the basis of the roles they are expected to play: (i) binding of metal *To whom correspondence should be addressed. E-mail: jbdomingos@ qmc.ufsc.br. (1) Lu, Y.; Mei, Y.; Schrinner, M.; Ballauff, M.; Moller, M. W.; Breu, J. J. Phys. Chem. C 2007, 111, 7676. (2) Frattini, A.; Pellegri, N.; Nicastro, D.; Sanctis, O. d. Mater. Chem. Phys. 2005, 94, 148. (3) Aiken, J. D.; Finke, R. G. J. Mol. Catal. A: Chem. 1999, 145, 1. (4) Saha, D.; Chattopadhyay, K.; Ranu, B. C. Tetrahedron Lett. 2009, 50, 1003. (5) K€ohler, J. M.; Abahmanea, L.; Wagner, J.; Albert, J.; Mayer, G. Chem. Eng. Sci. 2008, 63, 5048. (6) Zhang, J. Z.; Noguez, C. Plasmonics 2008, 3, 127. (7) Li, G.; Luo, Y.; Tan, H. J. Solid State Chem. 2005, 178, 1038. (8) Toshima, N., Metal nanoparticles for catalysis. In Nanoscale Materials; Liz-Marzan, L. M.; Kamat, P. V., Eds. Springer-Verlag: New York, 2007; p 79.
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ions or complexes, which are the most common precursors of M-NPs; (ii) tuning of the polymer compatibility with different reagents and solvents (a parameter of chief importance for catalyst performance) according to the requirements of the particular reaction under study; (iii) interact attractively or repulsively with substrates, which may result in high selectivity and/or sometimes high activity. Nevertheless, it is not obvious in which proportions these functional groups have to be provided to create the right environment for the nanoparticle stabilization and obtain efficient catalysis. An interesting approach to creating a wide range of microenvironments on a polymer structure was proposed by Hollfelder et al.,9,10 who developed a high-throughput “combinatorial” synthesis in 96 deep well plates to systematically modify polyethyleneimine (PEI) with the introduction of different functional groups in the quest for the generation of efficient catalysts. Making use of this high-throughput derivatization to introduce hydrophobic and polar functional groups, we seek in this study to change, comprehensively, the properties of PEI to identify the best stabilizing agent in the formation of catalytically active silver nanoparticles (Ag-NPs) for nitroaromatic reduction reactions in water. Figure 1 shows the derivatization chemistry used in this work to attach alkyl (butyl or octyl) and ethanolic groups to PEI. Several features of these modified PEI samples suggest that they may be able to stabilize the formation of Ag-NPs and perform efficient catalysis: (i) the PEI framework bearing amine groups will attract and coordinate with the silver ions, an important key feature for the manipulation of silver redox potential; (ii) partially charged PEI (protonated amine groups in aqueous medium) will avoid aggregation of NPs by promoting electrostatic stabilization; (9) Hollfelder, F.; Kirby, A. J.; Tawfik, D. S. J. Org. Chem. 2001, 66, 5866. (10) Avenier, F.; Domingos, J. B.; Van Vliet, L. D.; Hollfelder, F. J. Am. Chem. Soc. 2007, 129, 7611.
Published on Web 10/01/2010
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Figure 1. Derivatization scheme for the PEI framework modification. Polymer amine groups were reacted in the presence of DIPEA with varying amounts of (i) 2-chloroethanol and (ii) 1-bromooctane (or 1-bromobutane, not shown) in DMSO at 25 °C for 5 days.
(iii) the basic characteristics of PEI bearing hydroxyl moieties will promote the action of the metal nanoclusters by interaction with the metal nanoparticle surface, which appears suitable for redox catalysis; (iv) the hydrophobic character resulting from derivatization with alkyl groups may lead to a suitable reaction medium that is different from water (enhancement of substrate recognition and local concentration) and result in the stabilization of NPs by steric repulsion. Herein, we report the derivatization of a PEI framework with only two functional groups (polar and hydrophobic effects) which exerts a synergistic effect on the formation of highly catalytically active Ag-NPs prepared by chemical reduction of AgNO3 with hydroquinone (HQ) in 96-well plates. The screening of the best stabilizer was performed in situ simply by following the formation of Ag-NPs through the appearance of their UV-vis surface plasmon resonance (SPR) band. The analysis of the SPR bands through the use of an innovative method was performed combining some of the most important features of M-NPs (size, polydispersity, and yield), which can be readily estimated from the SPR band. Finally, the Ag-NPs were characterized by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS), and their catalytic activity in the reduction reaction of p-nitrophenol (Nip) using NaBH4 as the reducing agent was verified. The reduction of Nip is a model reaction11 which has been widely used for the quantification and comparison of the catalytic activity of different metal nanoparticles immobilized in a variety of carrier systems. Analysis of PEI derivative libraries identified a silver nanoparticle-polymer composite that was able to efficiently catalyze the p-nitrophenol reduction in water with a rate constant normalized to the surface area of the nanoparticles per unit volume (k1) of 0.57 s-1 m-2 L, which is higher than values reported in the literature (see Table 1). The Ag-NP-PEI derivative composites were found to be stable for months without noticeable changes in their properties.
Experimental Section All reagents and solvents were purchased from commercial sources and used as received. Ultrapure water (resistivity of 18.2 mΩ 3 cm), degassed by ultrasonic treatment, was used in all experiments. All glassware was washed with concentrated nitric acid and rinsed copiously with deionized water prior use. PEI Derivatization. A polymer stock solution was prepared by dissolving commercial branched PEI (25 kDa, Sigma) in dimethyl sulfoxide (DMSO) to give a final concentration of 13.2 mg mL-1 (30 mmol L-1, in monomer residues). Freshly prepared solution mixtures of PEI and N,N-diisopropylethylamine, to give final concentrations of 0.8 and 1.2 mM, respectively, (11) Pradhan, N.; Pal, A.; Pal, T. Colloids Surf., A 2002, 196, 247.
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Table 1. Comparison of Catalytic Activity of Ag-NP Catalytic Systems for Nip Reduction with Values Given in the Literature entry
catalytic system
Dm (nm)b
k1 (s-1 m-2 L)c
1 2 3 4 5 6 7
E11-Ag-NPa E5-Ag-NPa anionic polyelectrolyte brush (SPB)1 PS-NIPA core-shell microgel38 highly branched polymer brush39 PVA polymer11 PVA-PS-PEGMA composite hydrogel40 PVA hydrogel40 chitosan27 Ag-NP-carbon composites41
24.5 ( 4.1 19.5 ( 9.2 3.0 ( 1.2 8.5 ( 1.5 7.5 ( 2 ∼25 35 ( 5
0.57 8.1 10-3 7.81 10-2 5.02 10-2 7.27 10-2 3.78 10-7 7.80 10-5
45 ( 5 ∼3 10 ( 2
7.31 10-5 0.15 1.38 10-4
8 9 10
a This study. b Diameter of the silver particles. c Rate constant normalized to the surface area of the nanoparticles per unit volume, calculated from the data given in the respective papers.
were made up in DMSO. To 1.0 mL of these solutions, 0.3 mL of 2-chloroethanol and 0.3 mL of 1-bromobutane (or 1-bromooctane) solutions (0-1.33 mmol L-1, 0-0.5 equiv per monomer residue of PEI) were added, under vigorous stirring (using microstirring bars with a diameter of 2 mm 5 mm; Aldrich), in 96-well, 2.2 mL polypropylene plates (Axigen, USA). Stirring was continued for 5 days at room temperature. An overview of the synthesis procedure is shown in Figure 1. The Ag-NP stabilizers selected for detailed study after initial screening were synthesized on a larger scale: E5 (0.35 equiv of 2-chloroethanol and 0.4 eq. of 1-bromobutane) and E11 (0.35 equiv of 2-chloroethanol and 0.4 eq. of 1-bromooctane). The synthesis was performed in amber bottles under the same experimental synthetic conditions in the 96-well plates to give a final PEI concentration of 0.237 mmol L-1 (in monomer residues).
Synthesis and Characterization of Colloidal Silver Nanoparticles (PEI-Ag-NPs). The initial library screening, to find the best proportions for the functional groups (ethanolic/alkyl) inserted into PEI for the stabilization of the Ag-NPs, was carried out in situ by acquiring UV-vis spectra at 300-800 nm in transparent 300 μL 96-well plates (NUNC) with a microtiter plate reader (Molecular Devices Spectramax Plus 384). First, aliquots (192 μL) of each of the 96 different reaction mixture combinations were diluted in water to 1.6 mL in deep 96-well plates. The salt precursor (AgNO3) was added to aliquots of this solution, the samples were then incubated for 10 min, and the reducing agent (HQ) was added to give a total volume of 150 μL. Polymer, AgNO3, and HQ concentrations were varied systematically. For detailed determination of the best reaction conditions for the formation of stable Ag-NPs, large scale resynthesized stabilizers E5 and E11 were used. Studies varying the reagent concentration and the ratio between reagents ([AgNO3]/[PEI] and [AgNO3]/[HQ]) were performed in thermostatted quartz cells with 3 mL of total volume. The reagent ratios studied were 0.5, 1, and 2, with the PEI DOI: 10.1021/la103408s
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Article concentration ranging from 0.01 to 0.08 mmol L-1. A waiting period of 120 min for the formation of NPs was used to ensure complete synthesis (time determined by kinetic experiments). Besides the characterization by UV-visible spectrophotometry, the PEI-Ag-NPs were characterized by TEM and SAXS. For the TEM analysis, one drop of reaction mixture was deposited on a 200-mesh Formvar/carbon-coated copper grid, and excess solution was removed by wicking with filter paper to avoid particle aggregation. Electron micrographs were taken with a JEOL JEM-100 electron microscope operating at 100 kV. The particle size was determined by analyzing at least 200 particles considering the maximum length of the particles. The theoretical specific surface area of the Ag-NPs was estimated from the TEM analysis and the density of bulk silver (F = 10.5 g cm-3). The SAXS experiments were performed on the D11A-SAXS beamline of the Brazilian Synchrotron Light Laboratory (LNLS - Campinas, SP, Brazil). The samples were loaded into a temperature controlled vacuum flow-through cell composed of two mica windows separated by a distance of 1 mm, normal to the beam.12 The collimated beam crossed the samples through an evacuated flight tube and was scattered to a 2D CCD marCCD detector with an active area of 16 cm2. The 2D scattering patterns were collected after an exposure time of 600 s. In order to cover the desired q range (from 0.1 to 2.5 nm-1) where n ∼ 1 for X-rays, the sample-to-detector distance was set to 1477.5 mm (silver behenate was used for sample-todetector distance calibration). In all cases, the 2D images were found to be isotropic and they were corrected by taking into account the detector dark noise and normalized by the sample transmission considering the 360° azimuthal scan. The above procedure was carried out using the FIT2D software developed by Hammersley.13 Furthermore, the resulting I(q) vs q scattering curves were corrected by the subtraction of the scattering of the pure solvent, and the I(q) vs q scattering profile of the Ag-NPs could be fitted by using the form factor of homogeneous spheres. The fitting procedures were carried out using the SASfit software which makes use of the least-squares fitting approach to minimize the chi squared (χ2) parameter. The SASfit software package was developed by Kohlbrecher and is available free of charge.14 Catalytic Activity Assays. The catalytic activities of E5 and E11-Ag-NPs were evaluated in the reduction reaction of p-nitrophenol (Nip) to p-aminophenol (Amp) in a quartz cell with 2 mL of final volume at 25 °C. First, the reducer concentration (NaBH4) was varied from 1.2 to 7.2 10-2 mol L-1 while keeping the Nip and Ag-NP (based on amount of silver atoms) concentrations at 6 10-5 and 1 10-5 mol L-1, respectively. Second, the catalyst concentration (Ag-NPs) was varied from 4.0 10-6 to 3.0 10-5 mol L-1 keeping the Nip and NaBH4 concentrations at 6 10-5 and 6 10-2 mol L-1, respectively. Reactions were started after addition of Nip, and they were monitored by the decreasing absorbance at 400 nm on a spectrophotometer incorporating a xenon flash lamp with a thermostatted cell holder.
Results and Discussion Synthesis of Ag-NPs and the Stabilizing Effects of Derivatized PEI. The PEI derivatization using a systematic “parallel” synthesis method10,15 as well the screening for the best functional group combinations in the stabilization of the Ag-NPs formed in situ were performed in the same 96-well format. Figure 2 shows a representation of the response as a function of the derivatization pattern resulting from the PEI modification. With the aim of obtaining a fast and reliable method for the screening of the PEI (12) Cavalcanti, L. P.; Torriani, I. L.; Plivelic, T. S.; Oliveira, C. L. P.; Kellermann, G.; Neuenschwander, R. Rev. Sci. Instrum. 2004, 75, 4541. (13) Hammersley, A. P. Scientific software FIT2D; European Synchrotron Research Facility: Grenoble, 2009; URL: http://www.esrf.eu/computing/scientific/FIT2D/. (14) Kohlbrecher, J. Software package SASfit for fitting small-angle scattering curves; Paul Scherrer Institute: Villigen, 2010; URL: http://kur.web.psi.ch/sans1/ SANSSoft/sasfit.html. (15) Johnson, T. W.; Klotz, I. M. Macromolecules 1974, 7, 149.
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libraries, we made use of the spectroscopy properties of Ag-NPs by following their formation through the appearance of their SPR band in the visible region and applying eq 1. ψ ¼
Amax λmax FWHH
ð1Þ
where ψ is the response, which is a combination of the maximum absorbance (Amax), which reflects the yield of Ag-NPs formed,16,17 the wavelength at Amax (λmax), which is related to the size of the Ag-NPs,16,18 and the full width at half-height (FWHH), which is associated with the size polydispersity of Ag-NPs.16,17 In this equation, the best response would be obtained when the Amax value is maximized and the λmax and FWHH values are minimized, indicating the formation of small narrow Ag-NPs. Clearly, from these 3D graphs, it can be observed that in the absence of the alkyl (butyl or octyl) groups there is little effect on the stabilization of Ag-NPs with addition of the ethanolic group, but there is a significant increase in the response, ψ, upon alcoholic derivatization when PEI was derivatized with 0.4 equiv of hydrophobic groups per monomer of PEI. Thus, the derivatization reagents exert a synergistic effect; that is, combinations of these reagents have a greater effect on the stabilization of Ag-NPs than the sum of the effects of each single reagent. The increases in the effect on the stabilization correlate strongly with the changes in the composition of the derivatization mix, indicating that changes in the properties of the stabilizers occur as a function of the amount of derivatization reagent used. The observed effects validate our derivatization protocol in which a large number of reagent combinations can be efficiently screened. On analyzing these results, two of the best combinations that stabilized the Ag-NPs were chosen, E5 (0.35 equiv of 2-chloroethanol and 0.4 equiv of 1-bromobutane) and E11 (0.35 equiv of 2-chloroethanol and 0.4 equiv of 1-bromooctane), and a detailed large-scale study was performed to verify the effects of the polymer, salt precursor, and reducing agent concentrations. For this, the ratio between the reagents ([AgNO3]/[PEI] and [AgNO3]/[HQ]) and the concentration of each reagent was individually modified. In preliminary experiments, it was observed that the best proportion between salt precursor and polymer concentration was 2.5 with the reducing agent concentration being higher than that of the salt ([AgNO3]/[HQ] = 0.5). These conditions were used in all experiments that follow. Figure 3 shows the experiments carried out varying the polymer concentrations (E5 and E11) while keeping the salt/polymer and salt/reducer concentration ratios constant. It can be observed that the polymer concentration has a strong influence on the response, with the displacement of λmax to lower wavelengths. Such a shift in λmax can probably be attributed to a decrease in the size of the NPs according to the Mie theory.19 The best response in these systems was achieved at 0.08 mmol L-1 for both selected polymers (E11 and E5), that is, with the lowest FWHH and λmax values and the highest Amax. From these experiments, a set of conditions which lead to efficient nanoparticle stabilization were established: 0.08 mmol L-1 polymer (for both E11 and E5), [AgNO3]/[PEI] = 2.5, and [AgNO3]/[HQ] = 0.5, corresponding to 0.2 and 0.4 mmol L-1 AgNO3 and HQ, respectively. The nanoparticles prepared under these conditions showed good temporal stability for months when stored in the refrigerator. Also, manipulation of these nanoparticles could be carried out under a wide range of experimental conditions (16) Wilcoxon, J. P.; Abrams, B. L. Chem. Soc. Rev. 2006, 35, 1162. (17) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 4212. (18) Jain, P. K.; Huang, X.; El-Sayed, I. H.; El-Sayed, M. A. Acc. Chem. Res. 2008, 41, 1578. (19) Mie, G. Ann. Phys. 1908, 330, 377.
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Figure 2. 3D screening graphs: on the left from the library prepared with 1-bromooctane and 2-chloroethanol, and on the right prepared using 1-bromobutane and 2-chloroethanol (equivalents varied from 0 to 0.5, for each reagent, per monomer residue of PEI).
Figure 3. Variation in UV-visible absorption spectra of the solutions containing (a) E11-Ag-NPs and (b) E5-Ag-NPs as a function of polymer concentration: 0.01-0.08 mmol L-1 E11 or E5 (per monomer residue of PEI), maintaining [AgNO3]/[PEI] = 2.5 and [AgNO3]/[HQ] = 0.5.
cannot reduce isolated ions in solution.20 This is because the HQ reduction potential (E° = -0.699 V) is insufficient to reduce the high negative potential of isolated silver ions (E° = -1.8 V),21-23 but it is sufficient to reduce silver in stable clusters, whose potential is þ0.799 V. Thus, it is necessary to introduce a stronger reducer that performs the initial reduction of Agþ to Ag0. Gentry et al.,20 for example, used NaBH4 to seed the process and then used HQ to finish the growth. Nevertheless, in our work, reduction of Agþ did not require the use of any reducing agent other than HQ, even when the reaction was carried out carefully protected from light, which is also known to act as a reducer.20 This shows that the reduction potential of HQ is sufficient to reduce Agþ in the presence of PEI, most likely through an increase in the Agþ reduction potential due to the coordination of the silver ions with the amine groups of PEI. Goia and Matijevic determined a redox potential of þ0.38 V for an Agþ complex with ammonia.24 Ag-NP Characterization. The shape and particle size distribution of E11-Ag-NPs and E5-Ag-NPs were determined by TEM analysis, as shown in Figure 4, under the same colloidal sample conditions shown in Figure 3. Through the Gaussian fits of the data, nanoparticles with mean diameters (Dm) of 19.5 nm (E5-Ag-NPs) and 24.5 nm (E11-Ag-NPs) were determined. Although E5-Ag-NPs have a smaller Dm than E11-Ag-NPs, they have a higher polydispersity, as shown in the histograms in Figure 4. Furthermore, the TEM images show that the majority of nanoparticles have spherical geometry with the appearance of a few rods and triangles. SAXS measurements were also performed in order to probe the size, shape, and dispersity of the scattering particles. Figure 5 shows the SAXS profiles of E11-Ag-NP (a) and E5-AgNP (b) growth under the same conditions used to obtain the TEM images. The SAXS scattering intensity I(q) of an isotropic solution of particles embedded in a matrix with a constant electron density, after normalization considering the background scattering of the
without noting any precipitation of the NPs or turbidity of the aqueous medium, in contrast to the Ag-NPs prepared in the presence of the nonderivatized PEI. It is important to note that some studies have shown that HQ reduces silver in metallic particles that are already present but
(20) Gentry, S. T.; Fredericks, S. J.; Krchnavek, R. Langmuir 2009, 25, 2613. (21) Mulvaney, P.; Henglein, A. J. Phys. Chem. 1990, 94, 4182. (22) Linnert, T.; Mulvaney, P.; Henglein, A.; Weller, H. J. Am. Chem. Soc. 1990, 112, 4657. (23) Henglein, A.; Tauschtreml, R. J. Colloid Interface Sci. 1981, 80, 84. (24) Goia, D. V.; Matijevic, E. New J. Chem. 1998, 22, 1203.
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Figure 4. TEM micrographs and size distribution histograms for (a) E11-Ag-NPs and (b) E5-Ag-NPs [0.2 mmol L-1 AgNO3, 0.08 mmol L-1 polymer (E11 or E5), and 0.4 mmol L-1 HQ].
solvent, is given by IðqÞ ¼ NPðqÞ SðqÞ
ð2Þ
where N is the number of particles per unit volume, P(q) is the form factor of an individual particle, and S(q) is related to the particle interference factor, which arises from long-range correlations between scattering centers. For widely separated systems (as in the current case), S(q) ∼ 1 and I(q) is due to the form factor P(q) of the scattering objects, which is linked to their size and shape. Herein, P(q) values of the Ag-NPs were modeled geometrically as homogeneous spheres: IðqÞ ¼ Vp 2 Δσ 2 Pðq, RÞ ¼
4 3 πR Δσ 3
2
!2 3½sinðqRÞ - qR cosðqRÞ ðqRÞ3
ð3Þ
The sample dispersity was taken into account by using the lognormal distribution, for which the probability density function is given by 1 lnðR=μÞ2 f ðR, μ, σÞ ¼ pffiffiffiffiffiffi exp 2σ 2 2πσR
ð4Þ
where R is the average radius and the parameters μ and σ are the mean and standard deviation of the distribution, respectively. The parameter σ gives quantitative information about the particle dispersity. It can be noted that the fittings approach describes the experimental results reasonably well and leads to R = 8.3 nm and σ = 0.39 (E5-Ag-NP), and R = 14.1 nm and σ = 0.28 (E11Ag-NP). The results match reasonably with the TEM images and confirm that the mean average size of E5-Ag-NP is smaller than that of E11-Ag-NP. They also verify that both samples have a considerable degree of dispersity and that E5-Ag-NP is more 17776 DOI: 10.1021/la103408s
disperse than E11-Ag-NP. The degree of dispersity certainly plays a role in the catalytic activity of the samples (described hereafter). It is also worth mentioning that the high quality of the fittings, particularly in the low-q range of the SAXS profiles, is a fingerprint related to the electrostatic stabilization of the AgNPs provided by the modified PEI since it indicates the complete absence of large aggregates. We also performed SAXS measurements of the Ag-NP growth in the presence of different amounts of PEI, AgNO3, and HQ (not shown here). However, at least structurally, no notable differences were observed in the SAXS profiles. This may mean that even though these variables play a crucial role in the stabilization of the Ag-NP, they do not affect the size, size distribution, or shape of the scattering objects considerably, as evidenced by further SAXS data treatment. The parameters remained roughly the same under all the conditions explored for the different Ag-NPs. Catalytic Activity. The reduction of p-nitrophenol (Nip) to p-aminophenol (Amp) by NaBH4 was used as a model reaction25,26 to evaluate the catalytic activity of Ag-NPs stabilized by the PEI derivatives E5 and E11 (i.e., E5-Ag-NPs and E11-Ag-NPs, respectively). This reaction was monitored spectrophotometrically by measuring the disappearance of Nip, which shows a distinct spectral profile with an absorption maximum at 317 nm in water, but with a shift to 400 nm in the presence of NaBH4 due to the formation of the p-nitrophenolate ion.27,28 Initially, as a control experiment, the catalytic ability of E5 and E11 polymers in the absence of Ag-NPs was examined. The reactions were monitored for 11 days, and the thermodynamically favorable reduction of Nip by NaBH4 was not observed and the peak at 400 nm remained unaltered. In the presence of colloidal dispersions of E5-Ag-NPs and E11-Ag-NPs, the peak at 400 nm gradually (25) Chang, Y.-C.; Chen, D.-H. J. Hazard. Mater. 2009, 165, 664. (26) Harish, S.; Mathiyarasu, J.; Phani, K. L. N. Catal. Lett. 2009, 128, 197. (27) Murugadoss, A.; Chattopadhyay, A. Nanotechnology 2008, 19. (28) Ghosh, S. K.; Mandal, M.; Kundu, S.; Nath, S.; Pal, T. Appl. Catal., A 2004, 268, 61.
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Figure 6. (a) Variation in UV-visible absorption spectra from t0 Figure 5. Experimental SAXS data (open squares) and respective curve fittings (line) for samples (a) E11-Ag-NP and (b) E5-Ag-NP.
decreased with time, fading and ultimate bleaching of the yellow color of Nip in an aqueous solution of NaBH4 occurred, and a new peak appeared at 310 nm, which is known to be due to absorption of Amp (Figure 6a). Because the amount of the AgNPs added is very small, the absorption spectra of Nip are hardly affected by the silver nanoparticles. In these reactions, after all the reactants are mixed together, there is always a time lag (t0) before any visible change in the absorbance values is observed, the socalled “induction time” (Figure 6b). This phenomenon is typical of heterogeneous catalysis and commonly related to the time required for the catalyst activation. After t0, the reaction follows a first-order rate law, as shown in the inset of Figure 6b. The induction time for the reduction of Nip catalyzed by colloidal M-NPs has been attributed to many factors: (i) the diffusion-controlled adsorption of substrates onto the M-NPs surface;29,30 (ii) the presence of dissolved oxygen in water reacting at a faster rate with NaBH4 than with Nip;31 (iii) the coating of a metal oxide layer onto the metal surface upon the addition of BH4-, poisoning the catalyst surface;11,32 and (iv) a slow surface (29) (30) (31) (32)
Kuroda, K.; Ishida, T.; Haruta, M. J. Mol. Catal. A: Chem. 2009, 298, 7. Zeng, J.; Zhang, Q.; Chen, J.; Xia, Y. Nano Lett. 2010, 10, 30. Sharma, G.; Ballauff, M. Macromol. Rapid Commun. 2004, 25, 547. Pal, T.; Sau, T. K.; Jana, N. R. Langmuir 1997, 13, 1481.
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(time interval of 41 s) for the Nip reduction reaction in the presence of E11-Ag-NPs ([Nip] = 3.15 10-5 mol L-1, [E11-Ag-NPs] = 1 10-5 mol L-1, [NaBH4] = 1.0 10-2 mol L-1, 25 °C) and (b) the time dependence of the absorption of p-nitrophenolate ions (Nip) at 400 nm, with an induction period t0. The inset shows the first-order kinetic linearization from t0. ([Nip] = 3.15 10-5 mol L-1; [E11-AgNPs] = 1 10-5 mol L-1; [NaBH4] = 1.0 10-2 mol L-1; 25 °C).
restructuring of the nanoparticles.33 However, since in our work the aqueous reaction medium was carefully degassed before adding NaBH4, we may rule out the possibility of the formation of an oxide layer over the Ag-NPs surface,34 as well the reaction of NaBH4 with dissolved oxygen. Also, as shown in Figure 7, t0 is dependent on the NaBH4 and catalyst concentrations, which is strong evidence that the initial step related to t0 could involve a reaction with borohydride, such as the transfer of a surfacehydrogen species to the metal nanoparticles, or it is associated with the diffusion-controlled process of substrate adsorption onto the Ag-NP surface. There is no evidence, in these catalytic systems, which correlates t0 to a slow surface restructuring of the nanoparticles.33 The effect of the NaBH4 concentration on the apparent firstorder rate constant (kapp) is shown in Figure 8. In a typical set of experiments, the concentrations of Nip and catalyst (E11-Ag-NP or E5-Ag-NP) were kept constant and the NaBH4 concentration (33) Wunder, S.; Polzer, F.; Lu, Y.; Mei, Y.; Ballauff, M. J. Phys. Chem. C 2010, 114, 8814. (34) Saha, S.; Pal, A.; Kundu, S.; Basu, S.; Pal, T. Langmuir 2010, 26, 2885.
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Figure 7. Induction time (t0) as a function of (a) sodium borohy-5
-1
dride concentration at [Nip] = 6 10 mol L , [E5 or E11-AgNPs] = 1 10-5 mol L-1 and (b) catalyst concentration at [Nip] = 6 10-5 mol L-1, [NaBH4] = 6 10-2 mol L-1. E5-Ag-NP, solid spheres; E11-Ag-NP, open spheres.
was varied. All concentrations of NaBH4 were in great excess over the concentration of Nip, assuring pseudo-first-order conditions. It can be observed from Figure 8 that the apparent rate constant increased with increasing concentration of NaBH4 until it leveled off, showing that after a certain NaBH4 concentration the reaction condition is independent of NaBH4 concentration. In the following experiments, we used concentrations above this level, to ensure that the reaction is zero-order with respect to sodium borohydride. Since under microheterogeneous conditions the apparent kinetic rate constant is proportional to the total surface area of all (35) Mei, Y.; Lu, Y.; Polzer, F.; Ballauff, M.; Drechsler, M. Chem. Mater. 2007, 19, 1062. (36) Mei, Y.; Sharma, G.; Lu, Y.; Ballauff, M.; Drechsler, M.; Irrgang, T.; Kempe, R. Langmuir 2005, 21, 12229. (37) Panigrahi, S.; Basu, S.; Praharaj, S.; Pande, S.; Jana, S.; Pal, A.; Ghosh, S. K.; Pal, T. J. Phys. Chem. C 2007, 111, 4596. (38) Lu, Y.; Mei, Y.; Ballauff, M.; Drechsler, M. J. Phys. Chem. B 2006, 110, 3930. (39) Lu, Y.; Mei, Y.; Walker, R.; Ballauff, M.; Drechsler, M. Polymer 2006, 47, 4985. (40) Lu, Y.; Spyra, P.; Mei, Y.; Ballauff, M.; Pich, A. Macromol. Chem. Phys. 2007, 208, 254. (41) Tang, S. C.; Vongehr, S.; Meng, X. K. J. Phys. Chem. C 2010, 114, 977.
17778 DOI: 10.1021/la103408s
Figure 8. Logarithmic plots of the apparent rate constant (kapp) as a function of [NaBH4] for (a) E11-Ag-NP and (b) E5-Ag-NP at [Nip] = 6 10-5 mol L-1, [Ag-NPs] = 1 10-5 mol L-1, and 25 °C.
metal nanoparticles,3,11,35-37 and associated with a zero-order reaction dependence with respect to NaBH4, the rate of the reduction reaction of Nip can be expressed as -
d½Nip ¼ kapp ½Nip ¼ k1 S½Nip dt
ð5Þ
where kapp is the apparent rate constant and k1 is the rate constant normalized to S, the surface area of the metal particles normalized to the unit volume of the reaction system. The rate constant k1 allows the comparison of the catalytic activities of various noble metals for the reduction of Nip. Plots of kapp as a function of the surface area S for both catalytic systems (E11-Ag-NP and E5-AgNP) are shown in Figure 9. It can be observed that the rate constant kapp is indeed proportional to the total surface area of the nanoparticles in the systems; hence, it can be concluded that catalysis takes place on the surface of the nanoparticles. From the angular coefficient for the plots of Figure 9, the rate constants k1 of both catalytic systems were determined. As summarized in Table 1, the E11-Ag-NP system presents higher catalytic efficiency than E5-Ag-NP and also than other silver nanoparticle catalysts reported in the literature. In all cases, Ag-NPs were used for the reduction of Nip with a large excess of NaBH4. Langmuir 2010, 26(22), 17772–17779
Signori et al.
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differences in their catalytic activity must not be mainly related to the surface functional groups, but most probably to the following: (i) Different diffusion rates of the reactants on the Ag-NP surfaces for E11 and E5 PEI derivatives. This hypothesis is supported by observing the induction time, t0, dependence on the concentrations of NaBH4 and the catalyst in Figure 7. It can be readily observed that this dependence is much greater in the case of the E5-Ag-NP system than the E11-Ag-NP system, indicating a slower diffusion of the substrates on the E5-Ag-NP surfaces. This result was to be expected, since PEI derivatives bearing lipophilic groups, such as alkyl chain moieties, have a tendency to form polymeric micelles with a hydrophobic core in water43, smaller and the smaller the hydrophobicity of the lipophilic groups the less compact the colloidal particle should be and, as a consequence, the slower the diffusion of reactants. This is consistent with the difference in the catalytic activities, since the E11-AgNP system offers little resistance to diffusion and therefore a faster reaction is expected. (ii) Different local substrate concentrations in the reaction microenvironment for the PEI derivatives E11 and E5. Frequently, molecular systems presenting a hydrophobic pocket, such as enzymes, are able to draw in substrates from aqueous medium, increasing the local substrate concentration and, consequently, accelerating the reaction rates which are otherwise diffusion-controlled. In this regard, the higher hydrophobicity of E11 compared to E5 would be responsible for the higher local substrate concentration and consequently faster reaction rate.
Conclusions Figure 9. Apparent rate constant (kapp) as a function of the surface area of Ag-NPs normalized to unit volume of the system (S) for (a) E11-Ag-NP and (b) E5-Ag-NP at [Nip] = 6 10-5 mol L-1, [NaBH4] = 6 10-2 mol L-1, [PEI-Ag-NP] = (4-50) 10-6 mol L-1, and 25 °C.
The comparison summarized in Table 1 seems to suggest that the size of the Ag-NPs is an important, but not the only, factor which influences the catalytic activity. For example, the composite E11-Ag-NP exhibited higher catalytic activity than the other catalysts even though the silver nanoparticles were relatively larger, even larger than E5-Ag-NP. However, although E5-AgNP has a small particle size, its degree of dispersity (σ = 0.39) is greater than that of E11-Ag-NP (σ = 0.28), which certainly contributes to an underestimation of the E5-Ag-NP size and its influence on the catalytic activity. In addition, as noted elsewhere,29,35,42 the nature and amount of the stabilizing agent/supporting material have a considerable influence on kapp. Although strong interactions between the M-NP surfaces and the functional groups of the stabilizer/support material can generally prevent M-NPs from aggregating, providing small particle size, they can also weaken the catalytic activity by means of surface catalytic site coverage and deactivation. In this study, however, the two catalytic systems (entries 1 and 2 in Table 1) have similar compositions, differing only in terms of the length of the alkyl chain moiety (butyl for E5 and octyl for E11). Thus, (42) Esumi, K.; Isono, R.; Yoshimura, T. Langmuir 2004, 20, 237.
Langmuir 2010, 26(22), 17772–17779
In this paper, we have reported a new and straightforward method for screening highly catalytically active silver NP-polymer composites for the reduction of p-nitrophenol by NaBH4. The nanocomposite E11-Ag-NP was found to have higher catalytic efficiency than other Ag-NP catalytic systems reported in the literature, with a rate constant normalized to the surface area of the nanoparticles per unit volume (k1) of 0.57 s-1 m-2 L. The detailed kinetic aspects of the reduction reaction showed a distinct difference in the catalytic activity with respect to the polymer composition. The more hydrophobic PEI derivative (E11, ethanolic/octyl derivative) had less dependence on the induction period t0 than E5 (ethanolic/octyl derivative), and the higher rate constant, k1, was mainly attributed to a smaller resistance to substrate diffusion and higher local substrate concentration. The approach of modifying a polymer with mixtures of simple functional groups at a high effective concentration demonstrates that a fairly unsophisticated M-NPs-polymer composite that combines simple M-NP stabilizing features and exploits hydrophobic effects for catalysis can be surprisingly efficient. Acknowledgment. We are grateful to CNPq and CAPES for financial support of this work. We would also like to thank the Central Laboratory of Electron Microscopy (LCME) at UFSC and the Brazilian Synchrotron Light Laboratory (LNLS) for the TEM analysis and beam time usage, respectively. (43) Kuo, P. L.; Chen, C. C.; Jao, M. W. J. Phys. Chem. B 2005, 109, 9445.
DOI: 10.1021/la103408s
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