J. Phys. Chem. B 2006, 110, 12311-12317
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Aqueous-Organic Phase-Transfer of Highly Stable Gold, Silver, and Platinum Nanoparticles and New Route for Fabrication of Gold Nanofilms at the Oil/Water Interface and on Solid Supports Xingli Feng,†,‡ Houyi Ma,*,†,‡ Shaoxin Huang,‡ Wei Pan,‡ Xiaokai Zhang,§ Fang Tian,‡ Caixia Gao,‡ Yingwen Cheng,‡ and Jingli Luo| Key Laboratory for Colloid and Interface Chemistry of State Education Ministry, and Department of Chemistry, Shandong UniVersity, Jinan 250100, China, Testing Center, Shandong Normal UniVersity, Jinan 250014, China, and Department of Chemical and Materials Engineering, UniVersity of Alberta, Edmonton, Canada T6G 2G6 ReceiVed: February 16, 2006; In Final Form: May 2, 2006
A simple but effective aqueous-organic phase-transfer method for gold, silver, and platinum nanoparticles was developed on the basis of the decrease of the PVP’s solubility in water with the temperature increase. The present method is superior in the transfer efficiency of highly stable nanoparticles to the common phasetransfer methods. The gold, silver, and platinum nanoparticles transferred to the 1-butanol phase dispersed well, especially silver and platinum particles almost kept the previous particle size. Electrochemical synthesis of gold nanoparticles in an oil-water system was achieved by controlling the reaction temperature at 80 °C, which provides great conveniences for collecting metal particles at the oil/water interface and especially for fabricating dense metal nanoparticle films. A technique to fabricate gold nanofilms on solid supports was also established. The shapes and sizes of gold nanoparticles as the building blocks may be controllable through changing reaction conditions.
Introduction The intrinsic properties of noble metal nanoparticles are mainly determined by their size, shape, composition, and crystallinity.1-3 As a result, the synthesis of size- and shapecontrolled noble metal nanoparticles has attracted worldwide attention in recent years. Various synthetic methods have been developed for preparing nanoparticles with well-controlled sizes and shapes.1,2,4,5 Among the well-developed methods, the waterbased methods are the most frequently used due to water’s ability to solubilize a variety of ions and to stabilize molecules.6 Polymers, surfactants, and coordinative ligands have been widely used as stabilizers in aqueous media, protecting the nanoparticles from agglomeration,5,7,8 to acquire highly stable, well-dispersed metal nanoparticles. Despite great advantages and convenience, the water-based synthetic procedures are still fraught with certain inherent problems,9 including ionic interaction, low reactant concentration, and difficulty in removing residue of stabilizers after synthesis,10 as compared to the oil-based synthesis of nanoparticles. In addition, the nanoparticles prepared in organic phases are usually good for application to catalytic processes and for further fine-tuning surface properties with organic functional groups.6,11,12 Seeing that a variety of well-developed methods are available for the synthesis of well-defined metal nanoparticles in aqueous phases, it is necessary to develop novel and effective nanoparticle phase-transfer techniques, by means of * Corresponding author. Phone: +86-531-88564959. Fax: +86-53188564464. E-mail:
[email protected]. † Key Laboratory for Colloid and Interface Chemistry of State Education Ministry, Shandong University. ‡ Department of Chemistry, Shandong University. § Shandong Normal University. | University of Alberta.
which the nanoparticles prepared in aqueous media can be transferred to organic phases or onto solid supports, keeping the previous monodispersity. In this sense, the phase-transfer techniques of nanoparticles are as important as the synthetic ones of nanoparticles.13 Electrochemical methods have some peculiar advantages over chemical methods in controlling the sizes, shapes, and morphology of highly pure metal nanoparticles.5,7a,14 Recently, by using a water-soluble polymer, poly(N-vinylpyrrolidone) (PVP),5 as the more effective stabilizer for silver and gold clusters than tetraalkylammonium salts employed previously by Reetz and Helbig,7a Rodriguez-Sanchez et al.,14 and Wang et al.,7b,c we prepared size-controlled, spherical gold and silver nanoparticles through direct electroreduction of bulk metallic ions in aqueous solutions.5 The gold and silver nanoparticles stabilized by the long-chain PVP polymers, for example, PVPK30 and PVPK90, displayed a quite high stability and could be stored for several months, without precipitates.5a,b Our electrochemical synthetic method solves the stability problem of nanoparticles well, but a new problem pops up: how to transfer the as-prepared metal nanoparticles with the high stability from aqueous phases to organic phases. Because noble metal nanoparticles are closely wrapped by PVP macromolecules,15 the common phase-transfer methods either cannot meet the requirements of particle separation or destroy the monodispersity of nanoparticles. In this paper, based on the variation of the PVP’s solubility in water with temperature, we achieved the phase-transfer of noble metal particles between aqueous and organic phases only by changing the temperature of oil-water mixtures. The most distinguished features of the new phase-transfer method include ease of operation, high effectiveness, and absence of undesired side impurities. More importantly, electrochemical synthesis of
10.1021/jp0609885 CCC: $33.50 © 2006 American Chemical Society Published on Web 06/06/2006
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noble metal nanoparticles can be directly conducted in the oilwater systems by applying a synthetic technique developed from the phase-transfer method. Experimental Section All chemicals were of analytical grade and were used as received from the suppliers. Two kinds of PVP reagents (BASF) with different chain lengths, K30 (polymerization degree (n) ≈ 360) and K17 (n ≈ 90), were chosen as the stabilizers for Au, Ag, and Pt nanoparticles. Sodium dodecyl benzene sulfonate (SDBS) served as the co-stabilizer for the metal nanoparticles, and KNO3 served as the supporting electrolyte. The electrolytic solution was basically composed of 5 × 10-2 mol dm-3 KNO3, 2 × 10-3 mol dm-3 SDBS, and 30 g dm-3 PVPK30 unless stated otherwise. Before electrochemical synthesis, the concentrated HAuCl4, AgNO3, and H2PtCl6 solutions were added to the electrolytic solution, respectively, to provide the precursors of Au, Ag, and Pt particles. The concentrations of HAuCl4, AgNO3, and H2PtCl6 in the aqueous electrolytes were 5.0 × 10-4, 5.0 × 10-3, and 5.0 × 10-4 mol dm-3, respectively. PVPK30 was used to carry out electrochemical synthesis of gold nanoparticles in an oil-water system. The aqueous phase was the above-mentioned electrolytic solution with 5.0 × 10-4 mol dm-3 HAuCl4, and the oil phase was 1-butanol. The volume ratio between aqueous and 1-butanol phases was 3:2. The instable gold nanocrystal precursors were synthesized in an aqueous solution of 0.1 mol dm-3 KNO3, 20 g dm-3 PVPK17, and 5.0 × 10-4 mol dm-3 HAuCl4, which was used to fabricate the gold nanofilms on the sheet glasses. Electrochemical Synthesis of Noble Metal Nanoparticles. The electrochemical synthesis of Au, Ag, and Pt nanoparticles was carried out in a two-electrode cell. A rotating platinum electrode made from a 2.0 mm diameter platinum rod (Aldrich, 99.9%) was used as the cathode, and a 1.0 cm × 2.0 cm platinum sheet was used as the anode. The electrolysis was conducted in the galvanostatic manner at room temperature (∼22 °C). For electrochemical synthesis of Au and Ag nanoparticles, the current chosen was 100 mA, the electrolysis time was 5 min, and the rotation speed of the cathode was controlled at 1000 rpm. For the synthesis of Pt nanoparticles, the current chosen was 150 mA, the electrolysis time was 10 min, and the rotation speed of the cathode was kept unchanged. Phase-Transfer of Noble Metal Nanoparticles. Twenty milliliters of 1-butanol was added to 30 mL of electrolytic solutions containing as-synthesized nanoparticles (Au, Ag, or Pt particles) under vigorous stirring at room temperature. The aqueous and oil phases separated when the stirring ceased, and the metal nanoparticles were in the aqueous phase. Afterward, the oil-water mixture was gradually heated to 80 °C in a water bath under mechanical stirring. Stopping the stirring and controlling the temperature at 80 °C for 5 min, we observed that the mixture spontaneously separated into aqueous and oil phases with a clear interface (Figure 1), and metal nanoparticles transferred into the upper oil phase. An obvious indication was that the color of both two phases greatly changed as compared to the respective color before heating. The phase-transfer of as-synthesized Au nanoparticles was also performed by using the common phase-transfer procedures. Twenty milliliters of toluene was added to 30 mL of the electrolytic solution containing Au nanoparticles under vigorous stirring at room temperature. The extraction experiments of gold particles were done at room temperature and at 80 °C. To let more gold nanoparticles transfer into the toluene phase, 20 mL of toluene solutions containing 1.66 × 10-3 mol dm-3 tertdodecyl mercaptan (t-C12SH) (or 2.5 × 10-4 mol dm-3 sodium
Figure 1. Photographs showing the color changes of the aqueous phase and the oil one before (left bottle) and after (right bottle) the phasetransfer of gold (a) or silver (b) nanoparticles. The oil-water mixtures consisted of 1-butanol and the gold (or silver) colloidal solution synthesized electrochemically. The upper layer was the 1-butanol phase, and the lower one was the gold (or silver) colloidal solution.
oleate (SO)) were used as the extraction solvents instead of the pure toluene to repeat the same phase-transfer experiments at room temperature and at 80 °C. Characterization of the Nanoparticles. The size and morphology of Au, Ag, and Pt nanoparticles were characterized by transmission electron microscopy (TEM). A drop of the aqueous solution (or 1-butanol) containing the metal particles was placed on a copper grid coated with a thin film of Formvar and dried. The TEM photographs were taken with a Hitachi H-800 TEM at an accelerating voltage of 200 kV. The morphology of the gold nanofilms formed on sheet glasses was observed by a JSM-6700F field emission scanning electron microscope (FESEM) operating at 10 kV. The UV-vis absorption spectra were measured with a Hitachi U-4100 UV-vis spectrophotometer using a 1 cm path-length cell. Results and Discussion Phase-Transfer of PVP-Protected Au, Ag, and Pt Nanoparticles. The freshly prepared gold nanoparticles displayed a vivid rose color, and the color of gold colloid gradually became deep with the prolongation of resting period. PVP has been proven to play a more effective role in controlling sizes and shapes of gold, silver, and other noble metal nanoparticles and in protecting these particles from aggregation1,5,8,15 than other widely used nanoparticle stabilizers, such as ligands, surfactants, and polymers. It was thanks to the outstanding protective ability of PVP against agglomeration between noble metal particles that the gold, silver, and platinum nanoparticles electrochemically synthesized in the electrolytic solutions with the high ionic strength (0.05 mol dm-3 KNO3) were highly stable and could be stored for several months without any sedimentation. However, the PVP’s good stabilization action for metal nanoparticles is disadvantageous to the next collection, separation, and phase-transfer of the nanoparticles. The conventional phasetransfer or separation procedures are not suitable for the PVPprotected noble metal nanoparticles. Seeing that the PVP’s solubility in water decreases with increasing temperature, here we developed a simple, effective phase-transfer procedure of the noble metal nanoparticles protected by PVP polymers. 1-Butanol of appropriate volume was added to a gold colloidal dispersion at room temperature (∼22 °C), followed by strong
Aqueous-Organic Phase-Transfer of Nanoparticles
Figure 2. UV-vis absorption spectra for the aqueous phases measured before and after the phase-transfer of gold (a) and silver (b) nanoparticles.
mechanical stirring. 1-Butanol and water were immiscible with each other; therefore, the oil-water mixture spontaneously separated into two layers with a clear interface once the mechanical stirring stopped. Figure 1a shows that the mixed system consisted of the upper colorless oil phase and the lower red colloidal solution. In contrast, when the mixture was heated to 80 °C, the lower aqueous phase gradually faded, from brick red to pink, whereas the upper oil phase became a brown color. Interestingly, some red substances were observed to crawl upward along the wall of the bottle and enter into the oil phase, leaving a red thin film on the wall. The color changes of both the aqueous and the organic phases indicated the phase-transfer of gold nanoparticles from the aqueous phase to the organic phase. The UV-vis spectroscopy was used to study the phasetransfer process of gold particles originating from the temperature variation. As indicated in Figure 2a, the UV-vis spectrum of the aqueous phase measured before heating presented a symmetric absorption band at 536 nm, corresponding well to the characteristic plasmon band for spherical gold nanoparticles,2a,5b whereas that of the pink aqueous phase obtained after heating to 80 °C only gave a weak adsorption peak at the previous position (536 nm). There is not doubt that the majority of gold nanoparticles were successfully transferred from the aqueous phase to the 1-butanol phase. The newly developed phase-transfer procedure is also suitable for silver and platinum nanoparticles. Here, we took the phasetransfer of silver nanoparticles as an example. When the oilwater mixture composed of a silver colloid and 1-butanol was treated according to the same procedure, similar experimental phenomena were observed. Silver nanoparticles, like gold ones, left the aqueous phase and entered into the organic one as the temperature of the mixture approached 80 °C, the color of the aqueous phase changing from bright yellow before heating to white after heating and that of the organic phase changing in
J. Phys. Chem. B, Vol. 110, No. 25, 2006 12313 the reverse manner (Figure 1b). The UV-vis spectra (Figure 2b) of the aqueous solutions measured before and after heating confirmed the phase-transfer of silver nanoparticles indeed. It should be noted that the phase-transfer process of the platinum nanoparticles was slightly different from that of gold and silver nanoparticles, not involving the formation of platinum particle films on the wall of the container. The platinum nanoparticles seemed to pass directly through the oil/water interface, transferring into the oil phase. PVP is a homopolymer whose individual structural unit contains a hydrophobic vinyl group and a hydrophilic cyclic amide group.5c,15a One of the salient advantages of PVP polymers over other polymers is that PVP may dissolve in water and plenty of organic solvents. Seeing that PVP acts to a great extent as a nonionic surfactant, when heated, the solubility of PVP in water obviously decreases because the hydrogen bonds between the hydrophilic groups of PVP and water molecules are greatly destroyed with the increase in temperature. However, raising temperature is favorable for increasing the PVP’s solubility in oil phases. On the other hand, a PVP macromolecule is positively charged in an aqueous solution because of protonation of amide nitrogen atoms. When SDBS is added to the PVP-containing aqueous solution, some monomers of the PVP polymer should replace water molecules in the vicinity of the head of the DBS negative ions,16 forming the polymersurfactant complexes.17 The electrostatic attraction interaction between the headgroups of DBS anions and polar groups of PVP enhances the PVP’s hydrophobicity in the presence of SDBS, which decreases somewhat the solubility of PVP in water. Thus, when an oil-water mixture composed of the PVPcontaining aqueous solution and 1-butanol was heated to an appropriate temperature, some PVP macromolecules would leave the aqueous phase and enter into the 1-butanol phase due to the changes of the PVP solubility in the aqueous phase and the oil phase. The noble metal particles coated with PVP polymer therefore transferred from the aqueous phase to the 1-butanol phase with the transfer of PVP polymer. It is believed that the driving force for the phase-transfer of metal nanoparticles is attributed to the transfer of PVP. In this way, a simple and easy phase-transfer method is developed on the basis of the transfer of the PVP macromolecules in water toward the organic phase at relatively high temperature. TEM Observation. Figure 3a-f shows TEM micrographs for gold, silver, and platinum nanoparticles before and after the phase-transfer, respectively. For platinum particles, although there still existed the possibility of aggregation between individual particles, the particles transferred to the 1-butanol phase not only kept the previous size and shape but also dispersed better than in aqueous media (Figure 3a and b). Yet for silver nanoparticles (Figure 3c and d), the most obvious difference between the TEM images measured before and after the phase-transfer of the nanoparticles is that the particle number per unit area markedly increased on the TEM grid surface in the latter case, almost forming a dense silver particle film, except that the slight increase in the particle size was observed after the phase-transfer. It is more interesting that a large amount of gold nanocrystals of various size and shapes, including many polyhedral crystallites and a few triangular nanoprisms, were acquired after the phase-transfer (Figure 3f). The electrochemical formation mechanism of PVP-protected gold and silver nanoparticles has been interpreted previously.5 PVP not only serves as both the coordinating agent and the stabilizing one, but also plays an important role in controlling the size and shape of metal nanoparticles.5b The hydrophilic amide groups of PVP are bound to the surface of metal particles due to the strong affinity of N and O atoms for transition metallic
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Figure 3. TEM micrographs of platinum, silver, and gold nanoparticles before (a, c, e) and after (b, d, f) the phase-transfer of the nanoparticles. Micrographs a, c, and e are, respectively, the images of platinum, silver, and gold nanoparticles electrochemically synthesized in 0.05 mol dm-3 KNO3 in the presence of PVPK30 and SDBS, while micrographs b, d, and f show the images of platinum, silver, and gold nanoparticles transferred into 1-butanol phases, respectively.
clusters,5,15b whereas the polyvinyl backbone of PVP forms a hydrophobic domain, which surrounds metal particles and protects the particles from agglomeration.18 At room temperature, the existence of sufficient PVP polymers in the aqueous phase enables the noble metal nanoparticles to be stable in the aqueous media alone or in the aqueous phase of the oil-water mixture of water and 1-butanol. However, when the oil-water mixture composed of a noble metal colloidal solution and 1-butanol was heated, an amount of PVP polymer previously dissolved in the aqueous phase would have to leave the phase and enter into the 1-butanol phase or gather at the oil/water interface, as stated earlier. At the same time, increasing the temperature of the mixture still caused the dynamic shrinkage of polyvinyl chains of the PVP macromolecules. This action led to the possible reaggregation of individual metal nanoparticles, which were previously covered with the PVP chains in the aqueous phase at room temperature. In particular, for certain metal nanoparticles, such as gold particles, when certain suitable crystallographic facets were facing each other, the adhesion and subsequent fusion between the particles would take place.5b,19 By comparing the difference of three kinds of metal nanoparticles in shape, size, and degree of dispersion before and after the phase-transfer, we have inferred that the changes of the particles in shape and size strongly depend on the surface properties of the particles themselves. Although size (or shape)-controlled platinum particles and platinum nanoparticle aggregates have been synthesized,20-22 it is difficult to prepare 1D nanostructures or 2D nanoplates through the oriented aggregation of small platinum particles. Figure 3a shows some loose platinum nanoparticle aggregates electrochemically synthesized in an aqueous solution. Many particles gathered together in the aggregates, but the adhesion or fusion between particles did not occur before and during the phase-transfer; moreover, the nanoaggregates were destroyed and the platinum nanoparticles were dispersed well in 1-butanol after the phase-transfer (see Figure 3b). In contrast, the silver nanoparticles, especially gold nanoparticles, underwent a new aggregation process between particles and even the fusion of preferred facets of the original nanocrystals during the phasetransfer. It is reported that the aldehydes or ketones binding to
Feng et al. the nascent gold nanoparticles makes them “liquidlike” and amenable to sintering at room temperature.4,23 Each monomer of the PVP polymer contains a carbonyl group, and the O atom of this group easily coordinates with the surface atoms of gold nanoparticles. The gold nanoparticles synthesized in the presence of PVP are therefore expected to have such “fluidlike” surface. The same should be true of the PVP-protected silver nanospheres because of the structural similarity between gold and silver crystals. The transformation of spherical gold and silver nanoparticles into nanowires or flat nanoprisms1b,3,5b,8,24 should be associated with the so-called “fluidlike” surface property of the nanoparticles.4 The difference of silver particles in size shown by Figure 2c and d indicates that the slight particle agglomeration took place in the process of the phase-transfer, leading to the slight increase of silver particle size. Perhaps because platinum nanoparticles have no “fluidlike” surface property similar to that of gold nanoparticles, small platinum particles prepared in a PVP-containing aqueous solution did not become larger particles or transform into the nanoprisms and nanowires even if there existed the aggregation between platinum particles during the phase-transfer. Now, we focus on the shape transformation of gold nanospheres in the course of the phase-transfer. Our previous studies indicate that gold nanoparticles electrochemically synthesized under PVP protection,5b,c especially in the case of SDBS as costabilizer, are usually spherical nanoparticles. Figure 3e also shows that only the gold nanospheres existed before the phasetransfer. The comparison of Figure 3e and f clearly manifests that the phase-transfer process involved the growth of precursor gold nanoparticles at the same time. In the oil-water mixture of the gold colloidal solution and 1-butanol, a portion of PVP polymers dissolve in 1-butanol phase and more PVPs are in water at room temperature. When the mixture was heated, the gold nanoparticles gradually transferred into the 1-butanol phase with increasing temperature, but a certain amount of PVP polymers was still in the aqueous phase. Consequently, the PVP concentration used to stabilize the gold particles in the process of the phase-transfer became relatively lower as compared to that in the aqueous phase of the mixture before heating. The decrease of PVP concentration will lower the stability of silver nanoparticles, and even causes the reaggregation of gold particles dispersed well in previous aqueous solution. Yet the key factors leading to the new aggregation between gold particles are (i) the dynamic shrinkage of PVP chains and (ii) the “fluidlike” surface property of gold nanoparticles, as mentioned above. When the aggregation between the particles took place during the phase-transfer, the selective adsorption of PVP on various crystallographic planes of fcc gold played the major role in determining the morphology of final products. Most precursor particles grew into the larger gold polyhedrons through a nonoriented growth, and a small amount grew in the form of oriented aggregation of particles, developing into the micrometer-sized gold nanoprisms. In the latter case, the growth rate of gold nuclei along the 〈111〉 direction was greatly limited and the growth rate along the 〈100〉 direction was enhanced, finally resulting in the formation of single-crystalline gold nanoprisms bounded by large {111} facets.1a,5b,25 Advantage of the Present Phase-Transfer Method. Taking the gold nanoparticles as an example, we also performed the phase-transfer of the nanoparticles from the aqueous phase to the oil phase by means of the commonly used phase-transfer method.21a,26 Toluene and toluene solutions of tert-dodecyl mercaptan (t-C12SH) (or sodium oleate (SO)) were selected as the extraction solvents for gold nanoparticles. The gold colloids
Aqueous-Organic Phase-Transfer of Nanoparticles
Figure 4. A set of UV-vis absorption spectra for the gold nanoparticles in aqueous phases. Curve 1: UV-vis spectrum for gold nanoparticles synthesized electrochemically in aqueous electrolyte. Curves 2, 4, 6: Absorption spectra for the aqueous phases after extraction by pure toluene, a toluene solution of t-C12SH, and a toluene solution of SO at room temperature, respectively. Curves 3, 5, 7: Absorption spectra for the aqueous phases after extraction by pure toluene, a toluene solution of t-C12SH, and a toluene solution of SO at 80 °C, respectively. Curve 8: Absorption spectrum for the aqueous phases after the phase-transfer at 80 °C using the new method described in this paper.
were electrochemically synthesized in advance in the KNO3 aqueous solution and then mixed with the selected extracting agents. Afterward, the transfer effectiveness of gold nanoparticles obtained by using different phase-transfer procedures, including the common procedures and our method, was compared by measuring the UV-vis absorption spectra of the aqueous phases after the phase-transfer. Curve 1 in Figure 4 shows the absorption spectrum of an aqueous solution containing the as-prepared gold nanoparticles. Curves 2-7, respectively, present the UV-vis spectrum of each aqueous solution after being treated by different nanoparticle phase-transfer procedures, for example, extraction by toluene at room temperature (curve 2) and at 80 °C (curve 3), by the toluene solution of t-C12SH at room temperature (curve 4) and at 80 °C (curve 5), and by the toluene solution of SO at room temperature (curve 6) and at 80 °C (curve 7). All absorption curves gave an obvious absorption band around 540 nm, which is attributed to the characteristic surface plasmon excitation of spherical gold nanoparticles,2a,5b although the absorption strengths were different from each other. In contrast, the UV-vis spectrum (curve 8) of the aqueous phase after the phase-transfer by means of our method only displayed a weak characteristic peak for gold nanoparticles. The comparison of curve 8 with other curves demonstrates that our method is more effective in transferring nanoparticles from the aqueous phase to the oil phase than the common phase-transfer methods. The transfer efficiency of the gold nanoparticles is determined in terms of the relation between the absorbance and the particle concentration. A concentrated gold colloidal solution containing 3.5 × 10-4 mol dm-3 particles was electrochemically synthesized in a mixed electrolyte of 5 × 10-2 mol dm-3 KNO3, 2 × 10-3 mol dm-3 SDBS, and 30 g dm-3 PVPK30. A set of gold nanoparticle solutions containing the designated particle concentrations were prepared by diluting the concentrated colloidal solution with a mixed solution of 5 × 10-2 mol dm-3 KNO3, 2 × 10-3 mol dm-3 SDBS, and 30 g dm-3 PVPK30. From the UV-vis spectra of the gold particle solutions, we measured the values of the absorbance (A) at the characteristic absorption peak (about 540 nm). Figure 5 presents the dependence of the absorbance (A) on the nanoparticle concentration (C). Fitting
J. Phys. Chem. B, Vol. 110, No. 25, 2006 12315
Figure 5. Standard curve of the absorbance (A) of spherical gold nanoparticles at their characteristic absorption peak versus the nanoparticle concentration (C).
TABLE 1: Absorbance and Corresponding Concentrations of Gold Nanoparticles in Aqueous Phases after the Phase-Transfer, and the Nanoparticle Transfer Efficiency Obtained Using Different Methods A direct phase-transfer at 80 °C extracted by toluene with SO at 22 °Ca extracted by toluene with SO at 80 °C a
5.14 × 0.295 0.268
10-2
C′/mol dm-3
TE/%
3.21 × 10-5 2.06 × 10-4
90.8 41.1
1.87 × 10-4
46.5
At room temperature.
the data gave a straight line with the regression coefficient of 0.99873:
A ) (6.41 × 10-3) + (1.4 × 103)C
(1)
The gold colloidal solution containing 3.5 × 10-4 mol dm-3 particles was used to carry out the phase-transfer experiments by our method and by extraction with toluene solutions of SO at room temperature (or at 80 °C). On the basis of UV-vis measurements, we obtained the values of A corresponding respectively to the aqueous phase after the phase-transfer by our method and the aqueous phases after extraction with toluene solutions of SO at room temperature or at 80 °C. The concentrations (C′) of the residual gold particles in the aqueous phase can be calculated from eq 1. The values of A and C′ acquired were listed in Table 1. The transfer efficiency (TE) of gold nanoparticles from the aqueous phase to the 1-butanol phase in each case was determined by using the following equation.
TE (%) )
(3.5 × 10-4) - C′ × 100% 3.5 × 10-4
(2)
The values of TE were obtained and also listed in Table 1. Evidently, the new phase-transfer method developed by us is not only simple and convenient, but also gave the higher nanoparticle transfer efficiency than the common methods. Electrochemical Synthesis of Gold Nanoparticles in the Oil-Water System. A simple electrochemical method for synthesizing gold nanoparticles in an oil-water mixture was developed on the basis of the spontaneous phase-transfer of noble metal nanoparticles from the aqueous phase to the 1-butanol phase in the oil-water mixture containing PVP and SDBS at an appropriate temperature. The most obvious feature of this synthetic method is that the PVP-protected gold nanoparticles would transfer from the aqueous phase to the oil/ water interface under the mechanical stirring of the rotating cathode as soon as they were electrochemically synthesized in
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Figure 6. (a) A large-scale FESEM image for the gold nanoparticle films formed on a TEM grid surface. (b) High-magnification FESEM image of the dense gold nanoparticle films. The inset at the lower left corner shows a large, truncated triangular gold nanoprism.
the aqueous phase. The keys to performance of the electrochemical synthesis at the oil/water interface lie on choosing (i) an appropriate reaction temperature and (ii) an ideal surfactant that is favorable for gathering of the PVP polymer at the oil/ water interface. After repeated experiments, SDBS was selected as the surfactant candidate, and the electrochemical synthetic reaction was determined to be performed at 80 °C. The roles of SDBS are that it can combine with PVP to form the necklacelike PVP/SDBS aggregates, which benefit the PVP’s enrichments at the oil/water interface.5a,c The use of SDBS makes the electrochemical synthesis of noble metal nanoparticles and the collection of the particles at the oil/water interface much easier than in the absence of SDBS.27 Besides, SDBS behaves as the co-stabilizer for gold nanoparticles, protecting the nanoparticles formed from agglomeration.5a A purple mirrorlike interlayer gradually formed between the aqueous and oil phases after the electrochemical synthesis finished and the stirring stopped. When a sheet glass was placed in the interlayer, the purple substances automatically covered the whole glass substrate and formed a dense nanoparticle film. Figure 6a displays a large-scale SEM image for the gold nanoparticle film formed on a TEM grid surface. The nanoparticle film was so dense and thick that one could hardly observe its morphology by using TEM. On closer inspection from a high-magnification SEM image (Figure 6b), it was found that such films mainly consisted of polyhedral gold crystallites of different sizes, which range from about 40 nm to more than 120 nm. In addition, a large amount of gold polyhedrons coexisted with a few large nanoprisms. The inset in Figure 6b shows a micrometer-sized, flat nanoprism. At 80 °C, more PVP polymers tended to dissolve in the 1-butanol phase, and only a small amount of PVPs were in the aqueous phase. The agitation of the rotating cathode fully mixed the aqueous and oil phases. Accordingly, the PVP concentration in the aqueous phase was not sufficient for stabilizing the gold nanoclusters electrochemically formed. The gold nanoclusters grew in the form of the nonoriented growth into polyhedral gold crystallites with different size. This is the reason that the nanoparticles shown in Figure 6b were nonuniform. Fabrication of Gold Nanostructures on Solid Substrates. As mentioned above, the phase-transfer process of gold nanoparticles from the aqueous phase to the 1-butanol phase went with the formation of some flat nanoprisms (Figure 3f) due to the oriented aggregation of some precursor particles. The PVP’s protection ability against the particle agglomeration strongly depends on its concentration and the chain lengths. When a short-chain PVP polymer, PVPK17, was used as the stabilizer instead of PVPK30, the gold nanoparticles synthesized electrochemically had a poor stability. The aggregation of the instable nanoparticles resulted in the formation of larger particles, which gradually went down in the solution. The spontaneous sedimentation of PVPK17-protected gold may been used to fabricate the nanofilms on the solid supports.
Figure 7. (a) Large-scale FESEM image for gold nanofilms on a sheet glass. (b) A representative FESEM image indicating aggregation between gold nanoparticles of various sizes and shapes due to insufficient protection of PVPK17. (c) FESEM image for nanosized gold particles obtained by decreasing the HAuCl4 concentration to 5 × 10-5 mol dm-3 under otherwise identical conditions.
A spontaneous sedimentation process of the PVPK17protected gold colloid was observed in a clean glass container. The obvious indications are that the color of the solution changed from rose to almost colorless and a pink thin film formed on the wall and at the bottom after more than 2 days. A large-scale SEM image (Figure 7a) shows that this thin film is constructed by a large amount of gold particles with different shapes and sizes, including a few nanoplates (or nanoprisms) (two flat nanoprisms were marked by a circle). The gold particles were close-packed on the support surface, only leaving a few unoccupied locations. A representative SEM image (Figure 7b) more clearly indicates that the adhesion between gold crystallites of various size and shapes occurred during the film formation. Especially, the presence of two curved gold nanoplates proves that the flat gold nanoplates (or nanoprisms) were flexible. So far, there have been few reports about the flexibility of gold nanoplates. It was found that the shape and size of gold nanoparticles were closely related to the molar ratio between the PVP and the gold precursor. When decreasing the HAuCl4 concentration from 5 × 10-4 to 5 × 10-5 mol dm-3 under otherwise identical conditions, almost all products were small spherical nanoparticles of about 35 nm (mean size) (Figure 7c). They dispersed well and no longer aggregated perhaps because the protection ability of PVPK17 was improved with increasing molar ratio of PVP between the gold precursors. On the basis of the SEM results in Figure 7, we can draw a conclusion that, when the protection ability of PVPK17 for the precursor gold nanoparticles was not high enough, the gold nanocrystal seeds synthesized electrochemically grew into either the single-crystalline nanoprisms in the form of oriented growth or the polyhedrons in the manner of nonoriented growth. The former growth process is a slow one, and the latter process is a rapid one. Usually, the latter growth manner dominates over the former one. This conclusion supports to some extent the growth mechanism of precursor gold nanoparticles in the course of the phase-transfer or during electrochemical synthesis at the oil/water interface. It is generally believed that the formation of flat, singlecrystalline gold nanoprisms requires a slow time-consuming transformation of precursor nanospheres into nanoprisms, which involves the oriented aggregation of particles,1b,3,4 followed by
Aqueous-Organic Phase-Transfer of Nanoparticles
J. Phys. Chem. B, Vol. 110, No. 25, 2006 12317 University. We would like to thank Prof. Guiying Xu and Prof. Yebang Tan from the Key Laboratory for Colloid and Interface Chemistry of the State Education Ministry for their valuable suggestions in preparing the revised manuscript. References and Notes
Figure 8. FESEM image for micrometer-sized, flat gold nanoprisms prepared by first electrolyzing the PVPK17 + KNO3 mixed solution and then adding HAuCl4 to the solution.
the edge-selective fusion growth of nanoprisms.28 Yet here we were able to greatly increase the gold nanoprism yield by simply varying the previous synthetic procedure (i.e., the PVPK17 + KNO3 mixed solution was first electrolyzed, then HAuCl4 was added to the solution), as shown in Figure 8. A small portion of PVP monomers hydrolyzed to form N-vinyl-γ-aminobutyraldehyde (VAB) in the process of electrolysis. The random PVP copolymer with VAB monomers may serve as both stabilizer for gold clusters and reducing agent for Au(III) ions. What is more, the random polymer can direct the reduced gold clusters to grow into nanoprisms through the oriented growth control, producing more single-crystalline gold nanoprism products. Fabricating gold nanofilms on the solid supports based on the spontaneous sedimentation of instable precursor particles can be expected to develop into a special technique for preparation of metal nanoparticle thin films. The shapes and sizes of “building blocks” are controllable through changing reaction conditions. Conclusion An aqueous-organic phase-transfer method of nanoparticles developed on the basis of the decrease of the PVP’s solubility in water with the temperature increase is suitable for the high efficiency transfer of gold, silver, and platinum nanoparticles from the aqueous phase to the 1-butanol phase. Quite different from platinum and silver nanoparticles, gold nanoparticles grew into either large polyhedrons in nonoriented growth manner or the larger flat nanoprisms in oriented growth manner during the phase-transfer, due to the shrinkage of PVP chains. The precursor gold nanoparticles grew more easily into polyhedral products because the nonoriented growth of manner dominated over the oriented growth manner. The same is true of the growth of precursor gold particles during electrochemical synthesis of gold nanoparticles in the oil-water system. The gold nanoparticle seeds electrochemically synthesized under the protection of PVPK17 are unstable because of the low protection ability of PVPK17 and will spontaneously sedimentate. A nanofilm formation technique was established by means of the spontaneous sedimentation of PVPK17-protected precursor gold nanoparticle. Moreover, the shape and size of the particles constructing the nanofilms are changeable depending on the molar ratio between PVP and Au(III) ions and the specific synthetic procedures. Acknowledgment. This work was supported by the National Natural Science Foundation of China (20573068), the Scientific Research Award Fund for Excellent Middle-Aged and Young Scientists of Shandong Province (02BS053), and the Visiting Scholar Foundation of the Key Laboratory at Shandong
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