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Synthesis of Silver/Poly(2-hydroxyethyl methacrylate) Particles via a Combination of Inverse Miniemulsion and Silver Ion Reduction in a “Nanoreactor” Zhihai Cao,†,§ Constanze Walter,† Katharina Landfester,|| Zhenyu Wu,‡ and Ulrich Ziener*,† †
)
Institute of Organic Chemistry III Macromolecular Chemistry and Organic Materials, University of Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany § College of Materials, Chemistry and Chemical Engineering, Hangzhou Normal University, Xuelin Street 16, 310036 Hangzhou, China Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany ‡ Institute of Micro- and Nanomaterials, University of Ulm, Albert-Einstein-Allee 47, 89081 Ulm, Germany
bS Supporting Information ABSTRACT: Silver salt/poly(2-hydroxyethyl methacrylate) (poly(HEMA)) hybrid particles were first prepared by inverse miniemulsion polymerization of 2-hydroxyethyl methacrylate (HEMA) with silver tetrafluoroborate (AgBF4) as a lipophobe. High silver salt loads of up to 13% with respect to the disperse phase were achieved. The silver/poly(HEMA) hybrid particles were subsequently formed via a gas-phase in situ reduction of AgBF4 by hydrazine on the surfaces of silver salt/poly(HEMA) particles. The formation of silver nanoparticles was confirmed by UVvis spectroscopy and X-ray diffraction. The morphology of the hybrid particles of silver salt/poly(HEMA) and silver/poly(HEMA) was fully characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM), and dynamic light scattering (DLS). The influence of the reaction parameters including the type and amount of cosolvent, salt content, and type of surfactant on the particle properties and colloidal stability during the reduction process was thoroughly investigated.
’ INTRODUCTION Metal-containing nanoparticles (metal or metal oxide nanoparticles), exhibiting many excellent properties such as paramagnetism, high catalytic activity, optical properties, and so on, have been intensively investigated in recent years with respect to their potential applications in a variety of fields such as semiconductors, electronics, catalysis, optics, and so forth.1,2 However, the direct application of metal-containing nanoparticles with a size from several nanometers to several tens of nanometers may encounter some difficulties because of their small size and high tendency toward agglomeration. This drawback could be overcome through the encapsulation of metalcontaining nanoparticles in the core or immobilization on the surfaces of polymer particles.310 For instance, Kumacheva et al. prepared metal and magnetic nanoparticles in polymer microgels. They loaded the preformed polymeric microgels with metal salts by ion exchange between the bulk solution of water-soluble salts and the microgels and subsequently metal-containing nanoparticles formed via different reaction mechanisms.3 Wen et al. selectively immobilized noble metal nanoparticles in the outer layers of coreshell microspheres of poly(styrene-co-4-vinylpyridine) to improve the catalytic activity of the hybrid particles.4 Miniemulsion polymerization has been widely used to encapsulate metal-containing compounds (metal and metal oxide nanoparticles and metal salts) in a polymer matrix by taking r 2011 American Chemical Society
advantage of the droplet nucleation mechanism.11 Normally, to encapsulate metal or metal oxide nanoparticles,12,13 surface modification is required for these particles to improve their colloidal stability against coalescence and affinity for the monomer and subsequently polymer matrix. In comparison, the loading of metal salts into the polymer particles is much more convenient. Especially in inverse systems, hydrophilic metal salts are inherently needed to work as a lipophobe to improve the droplet stability and moreover narrow the particle size distribution.1417 In addition, the maximum loading amount of metal salts in inverse miniemulsion is much higher than that in direct systems as a result of the high solubility of hydrophilic metal salts in the polar disperse phase.18,19 The metal salts inside the hybrid particles could be transferred to metal-containing nanoparticles by an in situ reaction in the hybrid particles.20,21 Crespy et al. first synthesized AgNO3-containing polyurea nanocapsules, and subsequently reduced the silver ions by hydrazine by directly introducing the aqueous solution of hydrazine to the dispersion.20 More recently, they reported that polyvinylpyrrolidone/silver nanoparticles were prepared in a nonaqueous miniemulsion at high temperature, and the silver ions were reduced by ethylene glycol via polyol reduction.21 Received: June 6, 2011 Revised: July 5, 2011 Published: July 07, 2011 9849
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Table 1. Formulations, Particle Sizes, and PDIs for All Experimentsa run
surfactant
cosolventb
AgBF4
particle
type
(g)
(mol %)c
size (nm)
PDI
1
P(E/B)PEO1
E/0.50
6.5
248
0.098
2
P(E/B)-
M/0.50
6.5
133
0.334
W/0.50
6.5
153
0.012
E/0.50
3.2
255
0.235
PEO1 3
P(E/B)PEO1
4
P(E/B)PEO1
5
P(E/B)PEO1
E/0.50
9.7
258
0.116
6
P(E/B)-
E/0.50
12.9
242
0.112
E/0.50
6.5
175
0.322
E/0.50
6.5
158
0.199
PEO1 7
P(E/B)PEO2
8
P(E/B)PEO3
9
P(E/B)PEO1
E/0.33
6.5
187
0.198
10
P(E/B)-
E/0.67
6.5
217
0.053
PEO1 a
For all experiments, the masses of HEMA and isopar M were 1.5 and 12.5 g, respectively. b M, E, and W indicate methanol, ethylene glycol, and water, respectively. c On the basis of the monomer.
In the present contribution, silver/poly(2-hydroxyethyl methacrylate) (poly(HEMA)) hybrid particles were successfully synthesized via the combination of inverse miniemulsion and silver ion reduction. A large quantity of silver salts (3.2 to 12.9 mol % to monomer) was first loaded into the poly(HEMA) particles via the inverse miniemulsion of 2-hydroxyethyl methacrylate (HEMA). The resulting silver salt-containing hybrid nanoparticles worked as a nanoreactor, and hydrazine, an efficient reducing agent, was chosen to ensure the fast reduction of silver ions at or close to the surface to obtain the raspberry-like morphology. Hydrazine was introduced into the disperse phase by evaporation from its aqueous solution and subsequent diffusion via the continuous phase. The influence of the type of cosolvent and surfactant, the amounts of silver salt and cosolvent on the particle properties, and the colloidal stability on the process of reduction was investigated.
’ EXPERIMENTAL SECTION Materials. The monomer, 2-hydroxyethyl methacrylate (HEMA, Aldrich, 97.0%), was purified by passage through a column filled with alumina and stored in the refrigerator before use. The block copolymer surfactant, poly(ethylene-co-butylene)-b-poly(ethylene oxide) (P(E/ B)-PEO), was synthesized according to the literature.22,23 The number-average molecular weights of P(E/B)-PEO used in this report were 5350 g 3 mol1 (P(E/B)-PEO1), 6200 g 3 mol1 (P(E/B)-PEO2), and 7000 g 3 mol1 (P(E/B)-PEO3) as determined by 1H NMR spectroscopy. The initiator, R,R0 -azoisobutyronitrile (AIBN, Merck, 98.0%), apolar solvent isopar M (a C12C14 isoparaffinic mixture, Caldic Deutschland), cosolvents methanol (Merck, 99.9%) and ethylene glycol (EG, Aldrich, 99.0%), metal salt silver tetrafluoroborate (AgBF4, Acros
Figure 1. (a) TEM and (b) AFM images of AgBF4/poly(HEMA) hybrid particles prepared via the inverse miniemulsion polymerization of HEMA (b1, height image; b2, phase image; see Table 1, run 1). Organics, 99.0%), and reducing agent hydrazine hydrate (Acros Organics, 55%, hydrazine, 35%) were used as received. Milli-Q-grade demineralized water (resistivity 18 MΩ) was used.
Preparation of Inverse Miniemulsion and Polymerization. Three weight percent surfactant with respect to the disperse phase was dissolved in 12.5 g of isopar M under magnetic stirring. AgBF4 and HEMA were first dissolved in the cosolvent to form homogeneous polar solutions. The polar phase was mixed with the surfactant solution. After 15 min of pre-emulsification under strong magnetic stirring, the mixture was treated with 120 s of ultrasound with a Branson 450W digital sonifier at 90% amplitude in an ice bath to prepare a miniemulsion. The initial miniemulsion was introduced into the reactor and purged with argon for 3 min under magnetic stirring. The argon-protected reaction mixture was placed in a preheated oil bath at 65 °C and stirred for 3 h. The respective amounts and detailed conditions are given in Table 1.
Reduction of Silver Ions by the Gas-Phase Diffusion of Hydrazine. The initial dispersion (0.5 g) was transferred to a 5 mL glass bottle, and then the bottle with the dispersion was put in a closed 50 mL brown-glass bottle containing 2.5 g of an aqueous solution of hydrazine. Hydrazine evaporated from the aqueous solution gradually and diffused into the dispersed phase of the original dispersion via the continuous phase. The reduction reactions were run at room temperature for 6 h with magnetic stirring at 500 rpm. Characterization. Dynamic Light Scattering (DLS). The size and size distribution (as the PDI) were measured by DLS (Nano-Zetasizer, Malvern Instruments) at 20 °C under a scattering angle of 173° at a wavelength of 633 nm. The original dispersions were diluted with isopar M in a glass cuvette before the measurement. Particle sizes and PDIs are given as the average of five measurements. The PDI is a measure of the particle size distribution, and the PDI is a dimensionless number that describes the heterogeneity of the sample; it can range from 0 (monodisperse) to 1 (polydisperse). 9850
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Figure 2. Evolution of (a) UVvis absorption spectra, (b) the absorption maximum, and (c) particle sizes and PDIs during the reduction process of AgBF4 by hydrazine via gas-phase diffusion (Table 1, run 1). Transmission Electron Microscopy (TEM). TEM measurements were performed on a Philips EM 400 Microscope. The dispersion (1.5 μL) was diluted with 3 mL of isopar M, and then 1.5 μL of the diluted sample was placed on a 400-mesh carbon-coated copper grid and dried at 40 °C for at least 4 days. To enhance the contrast, additional carbon coating was used before the measurement. Atomic Force Microscopy (AFM). The original dispersions were diluted with a 3-fold amount of cyclohexane, and the diluted dispersions were cast onto cleaved mica by spin coating. AFM images (height and phase images) were recorded at a scanning rate of 0.5 Hz on a Nanoscope IIIa from Digital Instruments operating in tapping mode by using an NCH cantilever (NanoWorld) with a spring constant of about 42 N/m and a resonance frequency of about 300 kHz. UVVis Spectroscopy. The dispersion (1 μL) was collected during the reduction at specific time intervals and diluted with 3 mL of isopar M. The UVvis spectra were recorded on a Lambda 16 UVvis spectrometer from Perkin-Elmer, working in a spectral range from 200 to 700 nm. X-ray Diffraction (XRD). The X-ray diffraction (XRD) phase analysis of the sample was carried out on a Philips X’Pert Pro X-ray diffraction system with BraggBrentano θ2θ geometry. The generator is a highpowered diffraction tube with a cobalt anode, which was operated at a working power of 1.4 kW (40 kV, 35 mA), and its KR radiation, whose wavelength is 1.789 Å, was used for diffraction scans. Scan patterns were obtained at a resolution of 0.033° ranging from 40 to 120°. The powder samples for XRD were supported on single-crystalline Si, which does not show any additional peaks within the whole measurement range.
’ RESULTS AND DISCUSSION Preparation of AgBF4/Poly(HEMA) Hybrid Particles via Inverse Miniemulsion Polymerization. Particles composed of
Figure 3. XRD patterns of Ag/poly(HEMA) hybrid particles prepared by the reduction with hydrazine via gas-phase diffusion (Table 1, runs 13).
hydrophilic (co)polymers can be conveniently prepared via inverse miniemulsion of the corresponding monomers.1417,24,25 Hydrophilic salts are frequently employed to suppress molecular diffusion among droplets in inverse miniemulsion. In the present contribution, silver tetrafluoroborate (AgBF4) was used as an osmotic pressure agent to prepare stable miniemulsions. The polymerization started in a few minutes under the reaction conditions, judging visually by the change in transparency from transparent (miniemulsion) to milky white (polymer dispersion). As shown in Table 1, the properties of AgBF4/poly(HEMA) particles depend greatly on the content of AgBF4, the type and amount of cosolvent, and the surfactant. The size distribution could be narrowed by increasing the amounts of cosolvent and salt to some extent. The combination of isopar M, ethylene glycol or water, and P(E/B)-PEO1 showed a narrower 9851
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Figure 4. Morphological evolution of hybrid particles during the reduction process of AgBF4 by hydrazine via gas-phase diffusion (a, 15; b, 30; c, 60; d, 120; e, 180; f, 240; g, 300; and h, 360 min; see Table 1, run 1).
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Figure 5. AFM images of hybrid particles after the reduction process of AgBF4 by hydrazine via gas-phase diffusion (a, height image; b, phase image; see Table 1, run 1).
size distribution than did systems with other combinations. This is in agreement with our previous results on poly(HEMA) particles containing other BF4 salts (Fe2+, Co2+, Ni2+, Cu2+, and Zn2+).14,15 Surprisingly, a large number of tiny particles in the range of a few nanometers could be found in the hybrid particles, as seen in the TEM image (Figure 1a, red arrows). This feature was not observed in the poly(HEMA) particles with other BF4 salts.15 The surface investigation by AFM showed a smooth surface of AgBF4/poly(HEMA) particles (Figure 1b,c). Hence, it is reasonable to assume that most of the tiny particles are located inside the poly(HEMA) matrix. Some of the particles seem to
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show a coreshell structure. We assume that the dark area is the contact area between the particles and the carbon film. In addition, some influence of the instability of the polymer toward the electrons, as seen by the irregular shape of the particles, on the contrast cannot be ruled out. UVvis spectroscopy and XRD were employed to determine the chemical composition of the tiny particles in the polymer matrices. Silver nanoparticles display surface plasmon absorption at about 400450 nm, depending on the particle size, shape, and dielectric properties of the surrounding environment.26 No obvious absorption peak could be observed in this range of the UVvis spectrum of the dispersion of run 1 (Figure S1a). In addition, the characteristic peaks of elemental Ag at 2θ = 44.49° (111), 51.76° (200), 76.47° (220), 93.06° (311), and 98.49° (222) were not detected in the X-ray diffraction pattern of the dried hybrid particles of run 1 (Figure S1b). Therefore, we believe that the largest part of Ag in the hybrid particles is not present as nanoparticles of elemental silver but as nanocrystals of silver salt. In other words, the Ag+ ions could not be simply reduced by radicals during the polymerization. The formation of salt crystals of AgBF4 instead of the homogeneous distribution in the polymer matrix as other tetrafluoroborates could be ascribed to the relatively stronger interaction between Ag+ and BF4.15 Preparation of Ag/Poly(HEMA) Hybrid Particles via In-Situ Reduction of Ag+ in Poly(HEMA) Particles by Hydrazine via Gas-Phase Diffusion. Silver ions can be easily reduced to elemental silver by hydrazine at room temperature.27,28 In this contribution, hydrazine was supplied by evaporation from its aqueous solution and by diffusion through the continuous phase to the surface of the AgBF4/poly(HEMA) particles. The reduction of silver ions by hydrazine could be observed visually by the color change of the dispersion from milky white to yellow and then to dark brown over time. The kinetics of the reduction of Ag+ was followed by UVvis spectroscopy. An obvious absorption peak in the spectral range from about 330 to 500 nm appeared in the UVvis spectra of the samples after 30 min of reduction (Figure 2a), which is in agreement with the characteristic surface plasmon absorption of silver nanoparticles, supporting the reduction of silver ions. The absorption maximum (λmax) shifted from 427 nm for the 30 min sample to about 415 nm for the 60 min sample and then stayed almost constant (Figure 2b). In addition, the absorption peak became more symmetrical as the reduction proceeded. This can be regarded as a sign of the homogeneous distribution of silver nanoparticles in the hybrid particles and of a spherical shape.29,30 The formation of silver nanoparticles in the dispersion was further confirmed by XRD measurements (Figure 3). The diffraction peaks in the XRD pattern could be assigned to the (111), (200), (220), (311), and (222) planes of silver nanoparticles. The crystallite size calculated by the Scherrer equation from the baseline-separated (220) peak in the XRD pattern of run 1 amounts to about 15 nm. The number of silver nanoparticles in the dispersion increased over time, as indicated by the increase in the area of the absorption peak in the first 4 h. A slight decrease in the area beyond 4 h of reduction could be ascribed to the formation of some coagulum and a subsequent decrease in the concentration of hybrid particles in the dispersion. The overall particle properties were well maintained during the reduction as indicated by the almost invariant particle size and size distribution (Figure 2c). Compared to the particle morphology in the initial dispersion (Figure 1a), some relatively larger dark dots, which are believed to be silver nanoparticles according to the UVvis and XRD 9853
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Figure 6. Evolution of (a, c) particle sizes and PDIs and (b, d) UVvis absorption spectra during the reduction process of AgBF4 by hydrazine via gasphase diffusion with different cosolvents (a and b, methanol; c and d, water; see Table 1, runs 1 and 3).
results, could be observed by TEM (Figure 4). The size of the silver particles estimated by TEM is in the range of 1035 nm, and the average size of about 20 nm is quite close to the results from XRD (see above). Thus, we assume that at least some of the Ag particles are single crystals. The silver particles were well dispersed in the hybrid particles, which is consistent with the symmetrical peak shape in the UVvis spectra. As roughly estimated by TEM images from Figure 4ah, the number of silver nanoparticles increased significantly in the first 2 h, which coincides with the results from UVvis spectroscopy. All of these results support the formation of silver/poly(HEMA) hybrid particles. It should be pointed out that some tiny particles could still be found in the hybrid particles after reduction, but their chemical composition is unknown for the moment. They could be primary silver nuclei or salt crystals because of the incomplete conversion. Compared with the smooth surface of the AgBF4/poly(HEMA) particles in Figure 1, both the height and phase AFM images show a rough surface of the Ag/poly(HEMA) particles (Figure 5). The phase contrast image in Figure 5b shows bright dots indicating increased hardness. We assign those areas with increased hardness on the surface of the hybrid particles to silver nanoparticles. Attempts to detect elemental silver in the product of run 1 by X-ray photoelectron spectroscopy at 367.9 and 373.9 eV31 failed (Figure S2). This might be attributed to the small amount of silver. According to the AFM results, we believe that most of the silver particles are located on the surface or at least close to the surface instead of deep inside the poly(HEMA) matrix. The formation of silver nanoparticles on or close to the surface could be possibly ascribed to the fast reduction of silver ions by hydrazine and the slow diffusion of hydrazine into the
dispersed phase. Before reduction, the silver salts were homogeneously distributed in the poly(HEMA) matrix because of the presence of the cosolvent. With the diffusion of hydrazine through the continuous phase, the reduction took place immediately when hydrazine reached the particle surface (i.e., the reduction is diffusion-controlled). We assume that most of the hydrazine was consumed before it diffused further into the hybrid particles. The silver ions were also supplied continuously by diffusion from the interior of the hybrid particles to the surface. Consequently, raspberry-like hybrid particles were produced by the formation of silver nanoparticles via the precipitation and aggregation of elemental silver on or close to the particle surface. Variables. Cosolvent Type. According to the UVvis spectra in Figure 6b,d, the reduction of silver ions could also take place in the dispersions with methanol and water as cosolvents. However, the particle properties before and after reduction depend strongly on the type of cosolvent. Both DLS and TEM results indicate a relatively broader size distribution of the particles with methanol as a cosolvent in comparison to the particles with EG and water as a cosolvent. In addition, the size and PDI of the particles increased gradually during the reduction in the first 2 h and then increased drastically for the dispersion with methanol as a cosolvent. This indicates the formation of aggregates during the reduction, supported by the decrease in the absorption peak area after 3 h of reduction. Only a few silver particles could be found in the hybrid particles of the dispersion with methanol as the cosolvent (Figure 7b), which could be ascribed to the relatively high solubility of methanol in isopar M, leading to a significant partitioning of AgBF4 in the continuous phase. Thus, the reduction reaction of Ag+ might mainly take place in the continuous phase. On the other side, the UVvis spectra of 9854
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Figure 7. TEM images of Ag/poly(HEMA) hybrid particles prepared by the reduction with hydrazine via gas-phase diffusion with different cosolvents (a and b, methanol; c and d, water; a and c, before reduction; b and d, after reduction; see Table 1, runs 2 and 3).
the dispersions with methanol and EG as cosolvents, respectively, indicate comparable amounts of Ag particles. This would implicate the presence of most of the silver nanoparticles in the continuous phase, but the stabilization mechanism for those nanoparticles has not yet been fully understood. Visually, the colloidal stability during the reduction was well controlled for the system using water as the cosolvent. The constant peak area in the UVvis spectrum after 4 h confirms this, meaning no loss of hybrid particles (silver nanoparticles) by the formation of coagulums. In addition, the particle size stayed almost constant during the reduction and the PDIs increased only slightly (Figure 6c). According to the TEM image (Figure 7d), the silver/poly(HEMA) hybrid nanoparticles were successfully prepared. As roughly estimated by the TEM images in Figures 4h and 7d, the number of silver particles per hybrid particle of the system with EG as the cosolvent is higher than that of the system with water. This could be ascribed to the obvious difference in the size of hybrid particles (248 nm (EG), and 153 nm (water)). By a simple estimation, the absolute amount of silver salt per particle in the dispersion with EG was increased by a factor of 4.3 compared to that in the dispersion with water, leading to the formation of more silver particles per hybrid particle. Salt Content. The particle size of the silver salt/poly(HEMA) particles is slightly influenced by the variation of the amount of salt in the range from 3.2 to 12.9 mol % with respect to the monomer.
The colloidal stability and particle properties of the dispersion with 3.2 mol % silver salts are less controlled compared to those of the dispersions with higher salt contents. The peak area in the UVvis spectra started to decrease after 2 h, indicating the formation of coagulum. The particle size increased gradually during the reduction, but surprisingly, the PDIs decreased. The particle sizes and PDIs during the reduction in the systems with 6.5 mol % or more silver salt were almost constant, indicating efficient control over the particle properties. The peak area in the UVvis spectra increased expectedly with the increase in the loading amount of silver salts in the original particles. This finding is confirmed by TEM (Figure S3, 4 h). Surfactant Type. The colloidal stability and particle properties strongly depend on the molecular weight of P(E/B)-PEO.14 All of the P(E/B)-PEO used in this contribution could provide sufficient colloidal stability in the stage of inverse miniemulsion polymerization. However, the size distribution of AgBF4/poly(HEMA) particles was broadened significantly with the increase in the molecular weight of P(E/B)-PEO (PDIs in Table 1 and Figure 9a,c). This is consistent with our previous report that the size distribution is strongly dependent on the combination of surfactant, cosolvent, and nonpolar solvent.14 Basically, the reduction of Ag+ by hydrazine could be successfully carried out in systems with different P(E/B)-PEOs, on the basis of the UVvis spectra in Figure 9 and TEM images in Figure S4. The reduction of Ag+ led to the decrease in the 9855
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Figure 8. Evolution of (a, c, and e) particle size and PDIs and (b, d, and f) UVvis absorption spectra during the reduction process of AgBF4 by hydrazine via gas-phase diffusion with different salt contents (a and b, 3.2 mol %; c and d, 9.7 mol %; e and f, 12.9 mol %; see Table 1, runs 46).
colloidal stability of the dispersions, especially for the dispersions with higher-molecular-weight P(E/B)-PEO. This can be reasonably ascribed to the better protection ability of P(E/B)-PEO1 because of its lower HLB values, compared with that of P(E/B)PEO2 and P(E/B)-PEO3. For the systems with P(E/B)-PEO2 and P(E/B)-PEO3, some coagulum appeared after 30 min and the amount of coagulum increased during the reduction. After 180 min, a relatively large amount of coagulum could be found at the bottom of the reactor. The loss of hybrid particles in the dispersion was confirmed by the decrease in the peak area in the UVvis spectrum after 120 min. The particle properties could also not be well controlled in the systems with the highmolecular-weight P(E/B)-PEOs, as indicated by the increase in the particle size during reduction. We assume that the protection by the surfactant is interfered with the interfacial reduction of Ag+ and the presence of silver particles on the surface, leading to the decrease in colloidal stability. Amount of Cosolvent. Normally, the size distribution could be relatively narrowed by increasing the cosolvent amount via
the promotion of the dissociation of salt.14 This rule also works with AgBF4 as a lipophobe. As shown in Table 1, the PDIs decrease significantly from 0.198 to 0.012 by increasing the amount of EG from 0.33 to 0.50 g. Further increasing the amount of cosolvent does not have an obvious influence on the size distribution. The reduction of Ag+ to silver particles was verified by UVvis measurements and TEM (Figures 10 and 11). The colloidal stability during the reduction of Ag+ decreased with the increase in the amount of cosolvent, as indicated by the increase in the amount of coagulum and the decrease in the peak area in the UVvis spectra of silver nanoparticles (Figure 10). As estimated by TEM images, the size of the silver nanoparticles decreases with the increase in the amount of cosolvent. The absorption maximum λmax of silver nanoparticles in the final dispersion (360 min) increased from 412 to 415 nm and to 432 nm for the sample with 0.33 to 0.5 g and 0.67 g of EG. The red shift of λmax supports further increasing in the size of silver particles.32 The higher colloidal stability of silver particles in the hybrid particles with lower 9856
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Figure 9. Evolution of (a, c) particle sizes and PDIs and (b, d) UVvis absorption spectra during the reduction process of AgBF4 by hydrazine via gasphase diffusion with different surfactant types (a and b, P(E/B)-PEO2; c and d, P(E/B)-PEO3; see Table 1, runs 78).
Figure 10. Evolution of (a, c) particle sizes and PDIs and (b, d) UVvis absorption spectra during the reduction process of AgBF4 by hydrazine via gasphase diffusion with different amounts of cosolvent (a and b, 0.33 g of EG; c and d, 0.67 g of EG; see Table 1, runs 910).
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Figure 11. TEM images of Ag/poly(HEMA) hybrid particles prepared by the reduction with hydrazine via gas-phase diffusion with different amounts of cosolvent (a and b, 0.33 g of EG; c and d, 0.67 g of EG; see Table 1, runs 910).
amounts of solvent could be ascribed to the higher viscosity of these particle dispersions.
’ CONCLUSIONS Silver/poly(2-hydroxyethyl methacrylate) (poly(HEMA)) hybrid particles were successfully formed via the in situ reduction of silver tetrafluoroborate (AgBF4) by hydrazine on the surfaces of AgBF4/poly(HEMA) particles. The encapsulation of AgBF4 in the poly(HEMA) hybrid particles via the inverse miniemulsion polymerization of 2-hydroxyethyl methacrylate (HEMA) was confirmed by UVvis spectroscopy and XRD. The successful reduction of silver ions by hydrazine via gas-phase diffusion was confirmed by XRD, TEM, and UVvis measurements. By employing this process, most of the silver nanoparticles were anchored on the surface of the silver/poly(HEMA) hybrid particles. We assume that the formation of raspberry-like hybrid particles is caused by the slow diffusion of hydrazine to the dispersed phase and the fast reduction of silver particles by hydrazine. The particle properties and colloidal stability during the reduction process were investigated in terms of the type and amount of cosolvent, salt content, and type of surfactant. Good colloidal stability during the reduction process could be obtained in the dispersions with lower-molecular-weight P(E/O)-PEO, a relatively greater amount of AgBF4, and a suitable amount of cosolvent. The size of the silver nanoparticles was relatively larger
in the dispersion with EG as a cosolvent than in the dispersion with water. The size of the silver nanoparticles increased slightly with the increase in the amount of cosolvent. Such hybrid particles are of interest for antibacterial coatings in medical applications and will be further investigated.
’ ASSOCIATED CONTENT
bS
Supporting Information. Additional graphs. This material is available free of charge via the Internet at http://pubs.acs. org.
’ ACKNOWLEDGMENT We greatly thank G. Weber for the synthesis of P(E/B)-PEOs, Dr. C. Hoffmann-Richter for the AFM measurements, and L. Han, and Dr. U. Wiedwald for the XPS measurement. Support by the Deutsche Forschungsgemeinschaft (DFG) within Cooperative Research Center SFB 569 is gratefully acknowledged. ’ REFERENCES (1) Daniel, M.-C.; Astruc, D. Chem. Rev. 2004, 104, 293–346. (2) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. Rev. 2005, 105, 1025–1102. (3) Zhang, J.; Xu, S.; Kumacheva, E. J. Am. Chem. Soc. 2004, 126, 7908–7914. 9858
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dx.doi.org/10.1021/la202116s |Langmuir 2011, 27, 9849–9859
