On the Mechanism of Phase Transfer Catalysis in Brust−Schiﬀrin Synthesis of Metal Nanoparticles Siva Rama Krishna Perala and Sanjeev Kumar* Department of Chemical Engineering, Indian Institute of Science, Bangalore, India S Supporting Information *
ABSTRACT: The two-phase Brust−Schiﬀrin method (BSM) is used to synthesize highly stable nanoparticles of noble metals. A phase transfer catalyst (PTC) is used to bring in aqueous phase soluble precursors into the organic phase to enable particle synthesis there. Two diﬀerent mechanisms for phase transfer are advanced in the literature. The ﬁrst mechanism considers PTC to bring in an aqueous phase soluble precursor by complexing with it. The second mechanism considers the ionic species to be contained in inverse micelles of PTC, with a water core inside. A comprehensive experimental study involving measurement of interfacial tension, viscosity, water content by Karl−Fischer titration, static light scattering, 1H NMR, and small-angle X-ray scattering is reported in this work to establish that the phase transfer catalyst tetraoctylammonium bromide transfers ions by complexing with them, instead of encapsulating them in inverse micelles. The ﬁndings have implications for particle synthesis in two-phase methods such as BSM and their modiﬁcation to produce more monodispersed particles.
INTRODUCTION The two-phase Brust−Schiﬀrin method1 (BSM) proposed for the synthesis of highly stable gold nanoparticles is in widespread use to produce nanoparticles of a number of noble metals, such as gold, silver, copper, platinum, and palladium.2−4 In this two-phase synthesis, the metal precursor (HAuCl4 for gold) is ﬁrst transferred from an aqueous phase into an organic phase using tetraoctylammonium bromide (TOAB) as a phase transfer catalyst (PTC). The particle formation is initiated by contacting the organic phase with an aqueous solution of sodium borohydride, a reducing agent. TOAB plays the critical role of enabling reaction among the precursors in the organic phase. The nucleation of the reduced metal precursor, growth of particles, and their capping by alkanethiol, an organic phase soluble ligand, leads to synthesis of highly stable nanoparticles of a few nanometers in size. Unlike the widely used aqueous synthesis methods,5−7 BSM uses 10 times higher concentration of gold salt and facilitates size control through the concentration of a capping agent.8,9 A number of studies have been carried out8−10 to understand this important synthesis protocol. The recent investigations have, for example, identiﬁed a new reactive intermediate species.10 In this work, we examine yet another important aspect of Brust−Schiﬀrin synthesis. When the organic phase containing TOAB is contacted with the aqueous solution of gold salt, the latter is phase transferred to the organic phase. It was widely held1,9,10 that TOAB phase transfers AuCl4− to the organic phase by forming a complex with it through the following reaction on the interface © 2013 American Chemical Society
H+AuCl4 −(aq) + (R 8)4 N+Br −(org) → (R 8)4 N+AuCl4 −(org) + HBr(aq)
The hydrated TOAB11 thus exchanges its anion (Br−) with the anion of interest (AuCl4− for gold) and forms an ion-pair complex, as shown schematically in Figure 1a. In the second stage, the same mechanism brings in the reductant (BH4−) to the organic phase to initiate particle formation in the bulk. Tong and co-workers12−15 have used 1H NMR to investigate the state of TOAB and water in the organic phase recently. The authors conclude that TOAB is instead present in the form of inverse (reverse) micelles with a water core in them. When an aqueous solution of metal precursor is contacted with the organic phase containing TOAB, the metal precursor is transferred to the water core of inverse micelles of TOAB, as shown schematically in Figure 1b. The water pools inside the inverse micelles are proposed by them to be the active reaction sites for particle synthesis. Clearly, the above two mechanisms for phase transfer catalysiscomplexation vs encapsulation in inverse micellesare quite far apart, and have diﬀerent implications for size control and polydispersity of nanoparticles. It is important to know the correct mechanism of phase transfer to facilitate modiﬁcation of BSM and to develop new particle synthesis strategies in general based on a mechanistic understanding of Received: March 27, 2013 Revised: November 11, 2013 Published: November 11, 2013 14756
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Figure 1. Schematic of the mechanisms for phase transfer of water-soluble ions into organic phase by phase transfer catalyst through (a) ion-pair complex formation and (b) encapsulation of ions in water cores of inverse micelles of phase transfer catalyst. Interfacial Tension and Viscosity Measurements. Interfacial tension (IFT) was measured using the drop weight method. A saturated solution of TOAB was taken in a glass beaker, and DI water saturated with toluene was made to ﬂow into the TOAB solution using a glass Pasteur pipet at a small ﬂow rate of 0.001 mL/s using a syringe pump. The time taken to form one drop of water until its detachment from the pipet was noted. The average of ﬁve such readings was used to calculate the volume of drops. The density of solutions was measured using an Anton Paar DMA 55 densitometer. The toluene− water system was used as the reference. The viscosity measurements were carried out using a modiﬁed Ubbelohde suspended-level viscometer.16 1 H NMR Spectra. Nuclear magnetic resonance (NMR) studies were carried out using a Bruker 400 MHz High Resolution Multinuclear FT-NMR Spectrometer. Instead of using 10 mL of toluene as solvent, 0.5−0.8 mL of benzene-D6 was used as solvent. The data were used to determine (i) the location of 1H peaks for water and TOAB and (ii) the area under these peaks to estimate water content in samples. The moisture content corresponded to that of DI water. The reported error in moisture content is based on the average error in estimating the concentration of diﬀerent protons in TOAB molecules. Karl−Fischer Titrations and SAXS Measurements. Moisture analysis was carried out using a Mettler Toledo DL-32 Karl−Fischer Coulometer. The SAXS analysis was carried out using an Anton Paar SAXSess mc 2 with an accessible q-range of 0.03−28 nm −1 (corresponding diameter range of 200−0.1 nm). Light Scattering. TOAB solutions in toluene did not produce a signal of required intensity for dynamic light scattering analysis (BI200SM, Brookhaven Inc., USA, with BI-9000AT correlator card; 632.8 nm He−Ne laser source, and APD detector). Alternatively, the static light scattering (SLS) mode was used. The light scattered from the sample passed through a pinhole (aperture) wheel and wavelength ﬁlter wheel. The intensity of the light reaching the avalanche photodiode (APD) was measured as the number of photons reaching the detector in kilocounts per second (kcps). For a ﬁxed instrument setting, kcps relates to the presence of structures in a sample. All the SLS measurements were repeated three times, and the average values are reported.
BSM. In the present work, we explore the mechanism of phase transfer by investigating whether TOAB in the organic phase forms inverse micelles or not. A wide range of direct and indirect techniques can be used to probe the existence of micelles. The measurement of aqueous−organic phase interfacial tension at diﬀerent concentrations of TOAB helps in determining the critical micelle concentration (CMC) if micelles are formed. Light scattering, small-angle X-ray scattering (SAXS), small-angle neutron scattering (SANS), cryogenic transmission electron microscopy (Cryo-TEM), etc., are used to directly characterize micelles. The addition of water increases the size of micelles by swelling them and thereby increases the viscosity of the solution that can be measured. The solubility of water in an organic phase containing micelleforming species increases rapidly at concentrations larger than the CMC. The solubility can be determined by using Karl− Fischer titrations and the area under water peaks in 1H NMR spectra. The presence of water in the micelle core or as water of hydration around TOAB molecules provides an additional chemical environment for water molecules. A change in the ratio of chemical environments in which water molecules are present shifts the water peak in 1H NMR spectra, that can also be measured. We have used all the above measurements, except for Cryo-TEM and SANS, to resolve the mechanism of phase transfer in BSM. In the Experimental Section, we provide details of how the measurements were made, followed by a presentation and discussion of the results.
Materials. Toluene (Merck, 99.8% purity), dioctyl sulfosuccinate sodium salt (also known as AOT) (Sigma-Aldrich, 98% purity), tetraoctylammonium bromide (Sigma-Aldrich, 98% purity), benzeneD6 (C6D6) (Sigma-Aldrich, 99.96 atom % D; contains 0.03% v/v TMS), and D2O (Sigma-Aldrich, 99.99 atom % D; contains 1% DSSd6) were used as received, without any further puriﬁcation. Deionized (DI) water (Milli-Q, Millipore) was used in all the experiments. Solution Preparation. The measurements were carried out with 1 μM to 250 mM solutions of TOAB in toluene. Care was taken to prepare these solutions accurately. A 10 mL portion of higher concentration solution was diluted each time with 10 mL of toluene to make a new solution with half the concentration. This process was repeated to prepare all the solutions. These solutions were named unsaturated solutions of TOAB. To prepare water saturated solutions of TOAB, each of these solutions was added with 0.1 mL of water, sonicated for a few seconds, and allowed to settle for a day. All the samples had undissolved water settled at the bottom, which conﬁrmed that TOAB solutions were saturated with water. The toluene phase was separated and centrifuged for 2 min at 10 000 rpm to remove any traces of undissolved water. These solutions were named as saturated solutions of TOAB.
Water Content. The water to surfactant ratio of an equilibrated solution varies with surfactant concentration diﬀerently below and above the CMC, and can point to the formation of micelles. Coulometric Titrations. The coulometric titrations of water saturated TOAB solutions in toluene were carried out using a Karl−Fischer titrator.17 Water is present in the bulk organic phase at its solubility limit. If the additional water is assumed to be the water of hydration around each TOAB molecule, the following balance can be written 14757
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[H 2O] = NH· [T] + [D]
Here, [T] is the concentration of TOAB, NH is the average number of hydrated water molecules around a TOAB molecule, and [D] is the solubility of water in pure tolune. The ratio R, deﬁned as [H2O]/[T], must therefore satisfy the following equation
R = [NH] +
Figure 2 shows the experimental data in terms of R vs TOAB concentration and the model based ﬁt for the data. The ﬁgure
Figure 3. The molar ratio of additional H2O to TOAB at diﬀerent concentrations of TOAB.
NMR Measurements. Tong and co-workers12,15 have recently carried out 1H NMR studies. Their measurements at one concentration of TOAB (35 mM) in deuterated benzene showed that the 0.44 ppm water peak in the absence of TOAB shifts to ∼2.5 ppm for the water saturated TOAB solution. These ﬁndings are attributed to the presence of inverse micelles (please refer to the section “Prior 1H NMR Studies” in the Supporting Information for details). We have carried out similar measurements for a number of TOAB concentrations, ranging from 6 μM to 250 mM to probe into the question of hydrated complexes vs inverted micelles with a water core in them in detail. The 1H NMR measurements, presented in Figure 4, show that the peaks corresponding to the four types
Figure 2. The molar ratio of total H2O to TOAB at diﬀerent concentrations of TOAB.
shows that the hydration model ﬁts the experimental data well. The best ﬁt value of NH is obtained to be 1.67 and that of [D] 18.6 mM. The experimentally measured value of [D] using the same titrations is found to be 18.3 ± 0.3 mM. If the additional water is instead considered to be present in the core of inverse micelles, formed at concentrations larger than the CMC, the following mass balance must be satisﬁed: [H 2O] = NM · ([T] − [T]CMC ) + [D]
where NM is the number of water molecules per TOAB molecule in inverse micelles. The micelle size, and therefore NM, is assumed to be independent of TOAB concentration. The above equation can be rearranged to obtain
R = [NM] +
(2) Figure 4. The 1H NMR spectra of TOAB−C6D6−water showing a shift of water peak (indicated by *) at diﬀerent concentrations of TOAB.
where [D′] = [D] − NM[T]CMC. Clearly, eq 2, identical in form to eq 1, can also ﬁt the experimental data equally well. In order to resolve which model ﬁts the experimental data better, we plot excess water (obtained after subtracting the water present in the bulk solvent) vs TOAB concentration. The data should fall on a straight line passing through the origin for the hydration model and on a line with positive intercept on the x-axis for the inverse micelle model; it is also the value of the critical micelle concentration. Figure 3 shows that the experimental data support both the hydration model and the inverse micelle model with a small value of CMC. This is also evident from eq 2 which reduces to eq 1 in the limit of NM[T]CMC ≪ [D]. Since the value of CMC is not known, a coulometric study based discrimination between the two mechanisms is not possible.
of protons, marked A, B, C, and D in Figure S1 in the Supporting Information, remain nearly unchanged. The 1H NMR peak for water [δO(H2O)], indicated by a star, undergoes a TOAB-concentration-dependent shift. Li et al.12 have attributed this shift to the fast exchange of protons between the water in the core of inverse micelles and the free water in the bulk of the organic phase. Bar and co-workers18,19 have attributed similar shifts (observed in deuterated chloroform) to the fast exchange of protons between the water of hydration and the free water. 14758
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respectively. The parameter values are also consolidated in Table S1 in the Supporting Information. The ﬁgure shows that both of the models ﬁt the experimental data extremely well. A nearly 4 orders of magnitude increase or decrease in the value of [T]CMC did not result in any appreciable change in the ﬁtted value of δC. Also, when [T]CMC is set to zero, the ﬁtted value of δC did not change, and the model based ﬁt remained unchanged. The insensitivity of model ﬁts to parameter [T]CMC for the present system suggests that either the micelles do not exist in the present system ([T]CMC = 0) or, if they exist, they must be formed at extremely low concentrations of TOAB, ﬁtted to be [T]CMC = 4.7 × 10−8 mM. To the best of our knowledge, such low CMC values are not reported in the literature for any system. Thus, even though the NMR water peak shifts can be measured accurately, they cannot lead to an unambiguous resolution of whether the additional water is present as the water of hydration or inside the inverse micelles formed at extremely low CMC. The earlier reports in the literature11,18,19 have indeed attributed similar 1H NMR based ﬁndings to water being present as the water of hydration around PTC molecules. The 1H NMR study presented above also led to estimates for the molar ratio of total water to TOAB by determining the area under various peaks. Similar to the coulometry measurements shown in Figure 2, these measurements show a monotonous increase in ratio R with a decrease in concentration of TOAB (Figure S2 in the Supporting Information). These estimates, with more measurement error in them than in the coulometric measurements, also cannot lead to a resolution of the question posed here. Interfacial Tension. The variation of interfacial tension with surfactant concentration of a micelle-forming surface active species shows a characteristic plot, quite diﬀerent from what is observed for a non-micelle-forming surface active species. According to the Gibbs isotherm,
The experimentally observed shift in water peak (δO) for the hydration model is related to the concentration of two types of protons as ([A] + [D])δO = [A]δ H + [D]δ D
where δD and δH are the NMR shifts for the free water and the water of hydration. The concentration of water of hydration [A], as stated earlier, is equal to NH[T]. A rearrangement of eq 3 thus leads to δO =
A similar balance for the inverted micelle model with a water core inside them leads to ([C] + [D])δO = [C]δC + [D]δ D
Here, [C] is the concentration of water present inside the inverse micelles and δC is the corresponding NMR shift. Two kinds of protons are present inside inverse micelles: (i) those interacting with the polar group of TOAB molecules and (ii) the others inside the water core. Thus, δC is the average shift due to the water present in both of these forms. The amount of water present in micelles is therefore given as [C] = NM([T] − [T]CMC). Replacing [C] with NM([T] − [T]CMC) and rearranging eq 5, we obtain δO =
δD NM[T] ⎡ 1 + [D] ⎣1 −
[T]CMC ⎤ [T] ⎦
δC [D] 1 NM[T] ⎡1 − [T]CMC ⎤ ⎣ [T] ⎦ (6)
Setting the value of the critical micellar concentration to [T]CMC = 0.0 reduces the above model to the hydration model (eq 4), with NM replaced by NH. The variation of the location of water peak with TOAB concentration and the ﬁt produced by the two models (eqs 4 and 6) are shown in Figure 5. The value of hydration number NH in the hydration model and NM in the micellar model is kept constant at 1.67, based on the ﬁndings of the coulometric measurements. The values of [D] are taken to be 32 mM from the literature.12,20 The ﬁtted values of [T]CMC and δC = δH are obtained to be 4.6845 × 10−8 mM and 3.3665 ppm,
Γ2,1 = −
1 dγ RT d ln c 2
Here Γ2,1 is the relative adsorption of component 2 on interface in solvent 1, γ is the interfacial tension, and c2 is the concentration of the surfactant. For a micelle-forming surfactant, the magnitude of slope (dγ/(d ln c2)) should increase continuously until the CMC is reached, followed by an almost constant value of γ at higher surfactant concentrations.21 Figure S3 in the Supporting Information shows the expected behavior for a micelle-forming surfactant. Figure 6 shows the experimentally measured isotherm for the present system. The ﬁgure shows that the slope of the isotherm, instead of increasing, decreases continuously with an increase in TOAB concentration, similar to that expected for a non-micelleforming surface active species.22 Light Scattering. For a micelle-forming system, an increase in W (molar ratio of water to surfactant) increases the micelle size and therefore the intensity of the scattered light. Table 1 conﬁrms this behavior through experimental data for the AOT−toluene−water system, known to form inverse micelles with a water core in them.23 The table also shows that, at each value of W, diluting the micellar solution by half by using unsaturated toluene produces approximately a corresponding decrease in intensity of the scattered light (columns 4 and 5). At W = 0, the diameter of AOT inverse micelles is about 2.5−3 nm,23 in the same range as the micelle size suggested by Li et
Figure 5. Shift in 1H NMR peak for water at diﬀerent concentrations of TOAB solutions, saturated with water, and the ﬁts produced by two models that consider additional water as (i) the water of hydration and (ii) the water encapsulated in inverse micelles. 14759
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These observations are opposite to what is presented in Table 1 for a micelle-forming system which shows a large increase in the intensity of the scattered light. These observations point to the absence of any structures in equilibrated TOAB solutions in toluene. To further conﬁrm that TOAB does not form inverse micelles, we carried out small-angle X-ray scattering (SAXS) measurements, discussed next. Small-Angle X-ray Scattering (SAXS). Figure 7 shows the intensity of the scattered X-ray for pure toluene and 30 mM
Figure 6. Interfacial tension measurements carried out to probe the presence of inverse micelles in the TOAB−toluene−water system.
Table 1. Intensity of Scattered Light for the AOT−Toluene− Water System Obtained Using a Static Light Scattering (SLS) Instrument (2 mm Aperture, 633 nm Wavelength, and 90° Scattering Angle)a
100 mM AOT
intensity (kcps) of 50% diluted
relative change (col 2 - 9.27)
relative change (col 3 - 9.27)
toluene W=0 W=1 W = 1.5 W=2 W=4 W=6 W = 10
9.27 12.77 13.23 14.03 13.97 17 22.5 55.03
9.27 10.73 11.04 11.4 11.7 13.73 16.64 34.63
0.0 3.50 3.96 4.76 4.70 7.73 13.23 45.76
0.0 1.46 1.77 2.13 2.43 4.46 7.37 25.36
Figure 7. SAXS analysis of (i) toluene saturated with water, (ii) 30 mM TOAB solution in toluene, and (iii) 30 mM TOAB solution saturated with water.
unsaturated and water saturated solutions of TOAB in toluene. The data show a mild decrease in the intensity after the solution is equilibrated with water. The Guinier analysis24 is performed after subtracting the background signal due to toluene. The radius of gyration (Rg) decreases from 1.4 nm for the unsaturated solution to 1.1 nm for the saturated solution (Figure S4 of the Supporting Information). If TOAB forms inverse micelles, the equilibration of TOAB solution with water should have increased their size, and thereby increased the intensity of the scattered X-ray signiﬁcantly. The radius of gyration of the structures present should have also gone up. Thus, these measurements also point to the absence of inverse micelles. We further conﬁrm this behavior with the more fundamental viscosity measurements in the next section. Viscosity. For a dilute suspension of spheres in a solvent, the speciﬁc viscosity (ηsp) is related to volume fraction of solute (ϕ) as23 5 ηsp = ϕ (8) 2 An increase in volume of inverse micelles upon addition of water is expected to increase the viscosity of the solution, as suggested by the above equation. The AOT−water−toluene system, which is known to form inverse micelles, shows the expected behavior.23 Figure 8 shows the measurements for the TOAB−water−toluene system. The ﬁgure shows that addition of water for this system instead decreases the viscosity signiﬁcantly. The observed trend for TOAB is thus completely opposite but in agreement with the light scattering and SAXS studies. Starks and Owens25 studied aggregation of PTCs in organic solvent and found that PTC tends to form small aggregates in the organic phase. The aggregation number (