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Tuning the Shape and Size of Gold Nanoparticles with Triblock Polymer Micelle Structure Transitions and Environments¥ Poonam Khullar,§ Vijender Singh,#,§ Aabroo Mahal,#,§ Harpreet Kaur,#,^ Vickramjeet Singh,^ Tarlok Singh Banipal,^ Gurinder Kaur,‡ and Mandeep Singh Bakshi*,† †
Department of Chemistry, Wilfrid Laurier University, Science Building, 75 University Avenue West, Waterloo ON N2L 3C5, Canada Nanotechnology Research Laboratory, College of North Atlantic, Labrador City, NL A2 V 2K7 Canada § Department of Chemistry, BBK DAV College for Women, Amritsar 143005, Punjab, India ^ Department of Chemistry, Guru Nanak Dev University, Amritsar 143005, Punjab, India ‡
bS Supporting Information ABSTRACT: Three block polymers, viz., L31, L64, and P123, were used as reducing agents for the synthesis of gold (Au) nanoparticles (NPs) to determine the effect of their micelle size, structure transitions, and environments on the mechanism of the reduction process leading to the overall morphology of Au NPs. Aqueous phase reduction was monitored with time at constant temperature and under the effect of temperature variation from 20 to 70 C by simultaneous measurement of UVvisible spectra. The ligand to metal charge transfer (LMCT) band around 300 nm, due to a charge transfer complex formation between the micelle surface cavities and AuCl4 ions, and Au NP absorbance around 550 nm, due to the surface plasmon resonance, were simultaneously measured to understand the mechanism of the reduction process and its dependence on the micelle structure transitions and environment of TBPs micelles. L64 micelles showed dramatic shift in the LMCT band from lower to higher wavelength due to an increase in the reduction potential of surface cavities induced by the structure transitions under the effect of temperature variations. This effect was not observed for micelles of either L31 or P123 and is explained on the basis of a difference in their micelle environments. The morphology of Au NPs thus evolved from the reduction process was studied with the help of TEM and SEM studies. Smaller micelle size with few surface cavities, as in L31, produced small NPs in comparison to large micelles with several surface cavities as in P123. Structure transitions of L64 demonstrated direct influence on the final morphology of NPs, and stronger transitions produced fused and deformed NPs in comparison to weaker transitions. The results showed that efficient reduction by the surface cavities and uninterrupted nucleation without structure transitions lead to well-defined morphologies in the presence of P123 micelles.
’ INTRODUCTION Water-soluble poly(ethylene oxide)-b-poly(propylene oxide)b-poly(ethylene oxide) (PEO-b-PPO-b-PEO) triblock copolymers (TBPs) are polymeric surfactants.1 They undergo micelle formation, which is affected by concentration as well as temperature.1k The TBP micelle is made up of a PPO core and a PEO corona (Scheme 1) and demonstrates several interesting shape transitions which are closely associated with a number of factors such as the molar masses of the PEO or PPO units, salt additives, the nature of the solvent, concentration, and temperature.2 Predominantly hydrophilic TBPs with a greater number of PEO units than PPO units are also prone to the formation of compound micelles,2fh but predominantly hydrophobic TBPs usually produce well-defined micelles. Micelles undergo several structure transitions (i.e., micelles f threadlike micelles f vesicles, etc.) with concentration and temperature variations.3 Temperature brings marked dehydration to a greater magnitude of PPO than r 2011 American Chemical Society
PEO blocks which in turn significantly alter the environment (polar/nonpolar) of the micelle. Water from the core is replaced by the polymer as unimers incorporate into the micelle, maintaining the core radius remains same but increasing the aggregation number.4 The corona of the TBP micelle is composed of PEO units arranged in the form of surface cavities (Scheme 1) which are in direct contact with the aqueous phase and constitute the micellesolution interface of a TBP micelle. Thus, any change in the micelle environment brought about by the temperature causes a significant change in the arrangement as well as the number of surface cavities. Micelles with a greater aggregation number thus possess a greater number of surface cavities which act as sites for the site-specific redox reactions Received: February 21, 2011 Revised: April 16, 2011 Published: May 09, 2011 10442
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The Journal of Physical Chemistry C Scheme 1. A TBP Micelle with the Core Occupied by PPO Units and the Corona Constituted by PEO Unitsa
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influence the extent of the reduction process. Likewise, increasing the size of a TBP micelle (L31 < L64 < P123) helps us to understand the soft template effect and its effect on the overall morphology of NPs. All these issues have been addressed by monitoring the reduction of gold ions (AuCl4) into NPs5ad,7 under the effect of concentration and temperature variations.
’ EXPERIMENTAL SECTION
a
Red dotted circle shows a possible surface cavity whose size is related to the number of PEO and PPO units. A larger cavity can easily accommodate a guest molecule in comparison to a smaller cavity.
because of the presence of ether oxygens.5 As one polymer molecule is mainly contributing toward the formation of one surface cavity in a micelle, it accepts one guest ion (oxidizing agent) per cavity where the hostguest fit is very much related to the size of the PEO block (Scheme 1).5 A smaller cavity produced by few PEO units (i.e., a smaller PEO block) is unable to properly accommodate a guest molecule. In contrast, a larger cavity (i.e., a larger PEO block) will form a bucket quite large enough to accommodate a guest molecule. Hence, the reducing ability of a TBP micelle is also very much related to the size of the surface cavities.6 Several surface cavities present on the surface of a micelle can simultaneously reduce an almost equivalent number of oxidizing agents if all oxidizing agents can be accommodated in the cavities. In addition, the micelle environment is also a contributing factor for a proper hostguest fit. Fully hydrated surface cavities with a low aggregation number at low temperature may not accept as many oxidizing agents as that accepted by partially hydrated or dehydrated cavities with a higher aggregation number at high temperature. Once the nucleating centers are created upon reduction of metal ions, they undergo nucleation to produce nanoparticles (NPs).5ad,7 Nucleation can occur between the nucleating centers occupying the adjoining surface cavities or through autocatalytic thermodynamically controlled reduction. In the former case, again micelle environment, shape, and size of the micellar assemblies as soft templates govern the overall morphology of NPs.8 Small micelles with few surface cavities cannot effectively lead the nucleating centers to well-defined morphologies in comparison to large micelles with several surface cavities. Likewise, compact arrangement of surface cavities allows nucleating centers to self-nucleate and allows NPs to replicate as the shape and size of the micelle. Thus, micelle environment, shape, and size of TBP micelles are considered to be the main contributing factors in the overall growth kinetics leading to a specific morphology of NPs. To further our understanding of this we have selected three TBPs, viz., L31, L64, and P123 with EO units = 4, 26, and 40, respectively, and a greater number of respective PO than EO units. A greater number of PO units is essential for the formation of well-defined micelles because it strengthens the hydrophobic environment over the hydrophilicity contributed by EO units. An increasing number of EO units on the other hand helps us to achieve a greater number of surface cavities which directly
Materials. Chloroauric acid (HAuCl4), triblock polymers L31 (PEO2PPO16PEO2), L64 (PEO13PPO30PEO13), and P123 (PEO20PPO70PEO20) were purchased from SigmaAlrdich. Double distilled water was used for all preparations. Synthesis of Au NPs. Aqueous mixtures (total 10 mL) of TBP (2/5/10 mM) and HAuCl4 (0.25/0.5/1.0 mM) were placed in screw-capped glass bottles. After the components were mixed at room temperature, the reaction mixtures were kept in a water thermostat bath (Julabo F25) at precise temperature (below/ above cloud point, cp (0.1 C) for 6 h under static conditions. The color of the solution changed from colorless to pink-purple or purple within 0.5 h and remained the same thereafter in most of the cases. After 6 h, the samples were cooled to room temperature and stored overnight. They were purified from pure water at least two times in order to remove unreacted TBP. Purification was performed by collecting the Au NPs at 10 00012 000 rpm for 5 min after washing each time with distilled water. Methods. UVvisible measurements were simultaneously carried out at various reaction times and temperatures by use of a Shimadzu Model No. 2450 (double beam) instrument equipped with a TCC 240A thermoelectrically temperaturecontrolled cell holder that allows measurement of the spectrum at a constant temperature within (1 C. Transmission electron microscopic (TEM) analysis was performed on a JEOL 2010F at an operating voltage of 200 kV. The samples were prepared by mounting a drop of a solution on a carbon-coated Cu grid and allowed to dry in the air. Scanning electron microscopic (SEM) analysis was carried out on a Zeiss NVision 40 Dual Beam FIB/SEM instrument. Photomicrographs were obtained in bright field scanning/imaging mode, using a spot size of ∼1 nm and a camera length of 12 cm. The cloud point (cp) of each TBP at a specific concentration is determined from the variation in the absorbance of 25 μM methyl orange (MO) in aqueous TBP solution with respect to temperature. A plot of the intensity of MO versus temperature at 460 nm gives a sigmoidal curve where the intensity value is low before the cp is reached because of the solubilization of MO in the micellar phase but increases instantaneously at the cp because of the release of MO during the phase separation. This abrupt change in the intensity provides the cp value within (0.5 C. The cp values are listed in Table S1, Supporting Information.
’ RESULTS L31Au NPs. Reactions above the cp. UVvisible scans of L31þHAuCl4þwater ternary reaction mixtures are shown in Figure 1. Figure 1a illustrates typical UVvisible scans with reaction time at 70 C. Three peaks are evident (indicated by arrows) at 220, 295, and 550 nm due to AuCl4 ions,7 the ligand to metal charge transfer band (LMCT),9 and surface plasmon resonance (SPR)10 of Au NPs. Their intensity variation is demonstrated in Figure 1b. The AuCl4 ions (220 nm peak) produce maximum intensity with no sign of Au NP absorbance 10443
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Figure 1. (a) UVvisible scans of L31 (2 mM) þ HAuCl4 (0.25 mM) þ water ternary mixture with time at 70 C. Black arrows represent three peaks at 220 nm, 290 nm, and 550 nm due to AuCl4 ions, the LMCT complex, and surface plasmon resonance of Au NPs, respectively. (b) Intensity versus time plots of these peaks. (c) Intensity versus time plots of 550 nm peak of Au NPs in the presence of 2, 5, and 10 mM L31. Inset shows the linear variation of “diffusion time” and “limiting time” versus the concentration of L31. (d) TEM micrograph of Au NPs of various shapes and sizes prepared for a ternary mixture of L31 (2 mM) þ HAuCl4 (0.25 mM) þ water at 70 C. Blue arrow shows a thin layer coating around each NP. (e) TEM micrograph of tiny Au NPs entrapped in the fused micellar assemblies prepared with L31 (10 mM) þ HAuCl4 (0.25 mM) þ water at 70 C. (f and g) Similar TEM images of tiny NPs entrapped in small micelles or preaggregates prepared with ternary mixtures of 2 and 5 mM L31, respectively, at 40 C. (h) UVvisible scans of L31 (10 mM) þ HAuCl4 (0.25 mM) þ water ternary mixture with temperature from 20 to 70 C. Wavy scan at 70 C is of a final turbid solution. (i) Plots of intensity at 290 and 550 nm versus temperature for various ternary mixtures with different concentrations of L31. Vertical arrows indicate the cp region. (j) A variation in the intensity of MO at 460 nm versus temperature for ternary mixtures with different concentrations of L31 and without the presence of HAuCl4. Arrows show the respective cp in each case.
(550 nm peak) until 60 min of the reaction (see the solid arrow). Then a sudden decrease in the intensity of the 220 nm peak
with a simultaneous increase in the 550 nm peak is observed until both merge with each other within 180 min of the reaction 10444
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The Journal of Physical Chemistry C (see a dotted arrow). It happens only if nucleation among the Au atoms is a diffusion-controlled process. Every surface cavity of L31 micelle is expected to produce approximately one Au atom,5aand the number of surface cavities is actually related to the aggregation number of the micelle which depends on the hydrophobicity and increases with the number of PPO units.4b Small micelles with few surface cavities will only be able to trigger the nucleation when they undergo intermicelle collisions under the diffusion-controlled process.11 Figure 1c, in fact, demonstrates this mechanism. Increase in the concentration of L31 from 2 to 10 mM at constant HAuCl4 = 0.25 mM regularly decreases the time span of the diffusion process (see the solid arrows at the respective concentrations) in a linear fashion (inset, filled circles). Thus, more micelles at higher L31 concentration undergo intermicelle collisions and trigger nucleation within a shorter period of time which reaches its limiting value within 180 min at 2 mM L31 (see dotted arrow, solid circles curve). The limiting value also decreases linearly with the amount of L31 (inset, empty circles), which means that the maximum nucleation is achieved within a shorter period of time when the number density of micelles is high at higher L31 concentration. Another interesting feature of this figure is that the curve for 10 mM L31 (solid diamonds) starts decreasing soon after the nucleation reaches a maximum value. This is related to a decrease in the SPR of colloidal Au NPs due to their entrapment12 by the micellar phase. It happens only at 10 mM rather than at 2 or 5 mM L31 because a greater amount of L31 will produce not only a greater number of micelles but also micelles of larger size and shorter intermicelle distances4b which will have a higher probability of entrapping NPs during intermicelle collisions. Now we focus our attention on the collective morphology of Au NPs and micelles at different L31 concentrations in the bulk. Figure 1d shows a TEM micrograph of several Au NPs of different sizes (e25 nm) and shapes produced with 2 mM L31 at 70 C. Each NP is capped with a thin layer of L31 (filled block arrow). But on the contrary, tiny NPs (34 nm) entrapped in fused micelles (Figure 1e) are produced when 10 mM L31 is used.7 Presence of large NPs at 2 mM L31 is the outcome of the autocatalytic thermodynamically controlled reduction of AuCl4 ions on the surface of already available nucleating centers,13 because 2 mM L31 will produce a relatively far less number of nucleating centers than 10 mM L31 against a constant amount of HAuCl4 = 0.25 mM. Thus, a smaller number of nucleating centers will grow larger in size (Figure 1d) in comparison to greater number of nucleating centers produced by 10 mM L31 (Figure 1e). Also, 10 mM L31 will not only facilitate the reduction due to the presence of 5 times more surface cavities than 2 mM L31 but will simultaneously entrap all nucleating centers in its micellar phase thereby decreasing the absorbance after reaching a limiting value (Figure 1c). Reactions below the cp. Interestingly, when the same reactions are carried out at 40 C (below cp =52 C), no clear absorbance due to SPR of Au NPs was observed in the presence of L31 = 2, 5, or 10 mM (Figure S1a,b,c, Supporting Information) apart from the absorbances at 220 and 290 nm due to AuCl4 ions and LMCT, respectively. It shows that the reduction of Au(III) to Au(0) is facilitated at 70 C (above the cp) rather than at 40 C (below the cp). TEM micrographs further help us to find out the nature of Au NPs in the aqueous bulk. Figure 1f and 1g shows the TEM images of small micelles (or preaggregates) entrapping tiny Au NPs (23 nm) synthesized with L31 = 2 and 5 mM, respectively, which may lead to
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almost insignificant absorbance in both cases. The sizes of such assemblies (single micelle þ NPs) are 3.3 ( 1.5 nm and 11.7 ( 7.3 nm for 2 and 5 mM L31, respectively. But when 10 mM of L31 is used, then apart from the presence of relatively larger micelles, independent, larger NPs of 48 ( 27 nm are also visible (Figure S1d) which provide weak absorbance around 550 nm (Figure S1c). Effect of Temperature. The above results show that temperature variation has dramatic effect on the synthesis of Au NPs. Figure 1h represents a typical L31(10 mM) þ HAuCl4(0.25 mM) þ water ternary reaction under a temperature variation of 2070 C. No absorbance due to Au NPs is observed up to 54 C (dotted line) around 550 nm (as observed previously with time at 40 C, i.e., below cp). Thereafter, a weak absorbance appears which subsequently becomes quite prominent. The variation in 290 and 550 nm peaks is depicted in Figure 1i. The intensity of the LMCT peak (at 290 nm) runs through a sigmoidal curve over the whole temperature range (solid diamonds) where the middle part of the curve shows a sudden increase representing the cp region. Likewise, the intensity of the Au NP absorbance (at 550 nm) starts rising from 56 C (empty diamonds), which is essentially in the post-cp region and increases continuously thereafter. The same situation arises when this reaction is conducted with 2 and 5 mM of L31 (see the respective curves in Figure 1i). In the case of 2 mM L31, we do not see the curve for Au NPs because it should even lie at a much higher temperature than 70 C according to the present trend. The cp region at three different concentrations of L31 is exactly reproduced by the one in the absence of HAuCl4 (Figure 1j; cp is demonstrated by the variation in the MO absorbance). It means that the reduction of Au(III) into Au(0) is completely controlled by the micellar assemblies7 and facilitated at a higher temperature even beyond the cp region. Before reaching the cp, only weak absorbance due to LMCT complex exists (Figure 1i), but as soon as the cp approaches, intermicelle fusions produce micellar clusters which promote the LMCT complex formation and lead to its limiting value. The LMCT complex then converts into nucleating centers which grow into NPs under the effect of diffusion-controlled process as observed previously in Figure 1c. As more micelles of 10 mM L31 produce a number of intermicelle clusters greater than that of 5 mM L31 because of a diffusion-controlled process, the Au NP peak (empty diamonds curve) appears in a relatively lower temperature range than that of 5 mM L31 (empty circles curve). L64Au NPs. L64 is a larger block polymer than L31 and contains much more PEO as well as PPO units (Experimental Section). A L64 þ HAuCl4 þ water ternary reaction mixture also exhibits absorbances with time (Figure S2a,b, Supporting Information) similar to that observed for L31. However, unlike L31 (Figure 1c), the formation of Au NPs in the present case starts soon after the beginning of the reaction, leaving the remainder of the variation (Figure S2c) similar to that of Figure 1c. However, the most dramatic difference occurs in the variation of the LMCT band (Figure 2a) under a temperature variation of 2070 C; the intensity of this band around 290 nm decreases with temperature and ultimately disappears. Meanwhile, another much weaker band appears at 320 nm, the intensity of which increases from 2 to 5 mM L64 (Figure 2b) and eventually supersedes the first one for 10 mM L64 (Figure 2c). The disappearance of the first and appearance of the second band with a single isosbestic point simply demonstrates the formation of a more stable LMCT complex at 320 nm because a stronger reduction potential of 10445
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Figure 2. (a, b, and c) UVvisible scans of L64 þ HAuCl4 (0.25 mM) þ water ternary mixture with 2, 5, and 10 mM L64, respectively, under the effect of temperature variation from 20 to 70 C. Black arrows indicate the variation in the intensity of two peaks at 290 and 320 nm with temperature. (d) Partial qualitative molecular orbital diagram of a complex formation between Au(III) and ether oxygens of PEO groups depicting the possible LMCT transitions. Inset shows possible multisite interactions between the ether oxygens and the Au(III) center in a square planar AuCl4 complex. “m” number of sites is greater than “n” and hence the former produces a LMCT band at 320 nm in comparison to the latter at 290 nm. (e) Plots of intensity versus temperature for ternary mixtures depicted in a, b, and c for 2, 5, and 10 mM L64, respectively. Filled symbols show the intensity variation in the respective 290 nm peak while empty symbols show the variation of 320 nm peak. Broken arrows indicate the isosbestic point where the 290 nm peak vanishes and the 320 nm peak emerges. Inset shows a plot of variation in the isosbestic point with the amount of L64. (f) Plots of variation in the intensity at 550 nm for the ternary mixtures with different concentrations of L64. Shaded block arrow indicates the cp region. (g) A variation in the intensity of MO at 460 nm versus temperature for ternary mixtures with different concentrations of L64 and without the presence of HAuCl4. (h) Schematic representation of a transition state LMCT complex for a redox reaction between Au(III) and Au(0) whose activation energy decreases with the increase in the amount of L64 from 2 to 10 mM. 10446
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The Journal of Physical Chemistry C PEO surface cavities will shift the LMCT band to a higher wavelength (or lower energy). This all happens because the L64 micelle undergoes three different relaxation processes when subjected to temperature variations.4a,14 In the first process, L64 unimers are rapidly introduced into the micelle, resulting in an increase in the micelle radius while leaving the radius of the core almost the same because water from the core is simultaneously removed by the PPO units.4b Thus, an increase in the micelle size is essentially attributed to the PEO units in the corona. This process produces a thermodynamically unstable micelle and is followed by the restructuring of the micelle (as the second relaxation process) where fresh PEO units tend to generate fresh surface cavities in order to produce a thermodynamically stable micelle. Thus, an unstable micelle produces an unstable LMCT complex which slowly vanishes as a stable micelle emerges with a a stable LMCT complex. In other words, restructuring induces more surface cavities in the corona which then have a stronger reduction potential than previously and hence generate a more stable LMCT complex with a fresh absorbance at 320 nm. Figure 2ac fully supports this mechanism because 2 mM L64 will obviously introduce a smaller number of unimers into the micelle during restructuring in comparison to 10 mM L64, and that is why the relative magnitude of the intensity of the 320 nm peak is much smaller for the former than the latter. A red shift in the wavelength of the LMCT band can further be explained on the basis of the molecular orbital diagram shown in Figure 2d. Accommodation of a square planar AuCl4 ion in a PEO crown cavity triggers the electron transitions from predominantly ligand 3eu(π) to metal 3b1 g(σ*). As the micelle becomes thermodynamically stable after the second relaxations under the temperature effect, the energy difference between predominantly ligand and metal molecular orbitals decreases, which results in the LMCT band shift to lower energy, i.e., 320 nm. Though the exact geometry is unknown at this point, electron donation from oxygen lone pair to metal d-orbitals can occur above and/or below the plane of the molecule and is expected to be carried out by several sites simultaneously in the polyether crown cavity.6 Thus, the stability of the LMCT complex is very much related to the number of electron-donating sites. For instance, a greater number of sites (PEOPPO PEO)m than (PEOPPOPEO)n will shift the LMCT band from 290 to 320 nm (inset, Figure 2d). It is important to determine how variation in the LMCT intensity affects the synthesis of Au NPs because formation of the LMCT complex is the rate-determining step in the reduction process. Figure 2e illustrates the variation in the intensity of the LMCT band versus temperature at different concentrations of L64. Filled symbols of each curve belong to the intensity of the first band while empty symbols belong to the second band. The minimum in each curve (indicated by an arrow) refers to the isosbestic point (the first relaxation process) where the first band disappears and the second one appears. The isosbestic point shifts regularly to a lower temperature as the amount of L64 increases from 2 to 10 mM L64 (inset). This is in accordance with the well-defined micelle formation process that shifts to a lower temperature with an increase in the concentration.1k In 10 mM L64, immediately after the isosbestic point, the curve passes through a strong maximum close to 45 C, suggesting a substantial increase in the LMCT complex formation due to a second relaxation process caused by an increase in the aggregation number.4a,b A larger aggregation number means the presence of more surface cavities, and hence more AuCl4 ions can
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undergo the LMCT formation. Note that the second relaxation process fades away with 5 mM L64 and vanishes in 2 mM L64. In 2 mM, the isosbestic point is the combination of both first and third relaxation processes4a,bbecause it not only indicates the shift in the LMCT band due to the first relaxation process but also leads to clustering of the micelles to achieve the cp. Figure 2f illustrates the variation of Au NP absorbance with temperature which is always maximum for 10 mM and minimum for 2 mM L64 over the whole temperature range even though turbidity is observed in the cp region and turbidity does not overshadow the absorbance of Au NPs. The temperature range between 55 and 60 C belongs to the cp region which is very well reproduced as in the absence of HAuCl4 (Figure 2g). Between 30 and 55 C, no change in the Au NPs absorbance takes place for 10 mM L64 (Figure 2f) contrary to the LMCT band (Figure 2e) which means that no growth of NPs is observed during the second relaxation process. Growth in an NP only happens in two ways, either due to self-nucleation among the nucleating centers or due to an autocatalytic process. In both cases, micelles have to be in their thermodynamically stable state because nucleating centers are actually produced in the surface cavities. The maximum in the 10 mM L64 curve (Figure 2e) indicates that the micelles acquire thermodynamical stability around 45 C, but growth (Figure 2f) starts only with the clustering of the micelles which begins around 55 C before the onset of the cp. Hence, neither self-nucleation nor the autocatalytic process contributes toward growth before the cp is reached; otherwise, we would have not seen the flat portion between 30 and 35 C for the 10 mM L64 curve in Figure 2f. Thus, growth is carried out only during the clustering of the micelles by Ostwald ripening15 that releases the colloidal Au NPs in the bulk due to phase separation, resulting in instantaneous absorbance increase.16 Also, a continuous weak increase in Au NP absorbance from 20 to 55 C in 2 mM L64 indicates the absence of any relaxation process within this temperature range while an instant growth thereafter is again due to the clustering of the micelles before cp. Thus, despite the presence of dramatic structure transitions in the micelles of 10 mM L64, they produce maximum growth in Au NPs because growth is simply related to the number of nucleating centers produced. That, in turn, relates to the greater number of surface cavities, which generate an LMCT complex of greater stability and lower energy of activation in comparison to that of 2 or 5 mM L64 (Figure 2h). SEM and TEM analyses further help us to understand how such micellar transitions ultimately influence the shape and structure of Au NPs. Figure 3a shows a large compound micelle of 5 mM L64 loaded with Au NPs which protrude from the surface of the micelle in different polyhedral morphologies along with some platelike shapes (Figure 3b). Figure 3c shows the corresponding TEM image of free NPs where one can clearly see the deformed shapes. However, if the amount of L64 is increased to 10 mM, such morphologies become less clear from the SEM analysis where one can see mainly large compound micelles (Figure S3a, Supporting Information). But after a proper purification of the sample with aqueous ethanol, NPs can be extracted and their TEM image is shown in Figure 3d. Most of the NPs exist in the form of small fused groups (Figure S3b). On the contrary, when the amount of L64 is decreased to 2 mM, fine spherical micellar assemblies are seen (Figure 3e) and most of the NPs exist in a chainlike arrangement (Figure 3f). Putting all information from various images together, it reveals the dramatic effect of increasing the amount of L64 on the overall morphology of NPs. Clearly, structural transitions in the micelles 10447
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Figure 3. (a) SEM image of a compound micelle loaded with growing NPs for a sample prepared with L64 (5 mM) þ HAuCl4 (0.25 mM) þ water ternary mixture at 70 C. (b) A high resolution image showing NPs of different shapes. (c) TEM image of the same sample showing deformed NPs. (d) TEM image of a sample prepared with 10 mM L64 under the same conditions. Groups of fused NPs are evident. (e) TEM image of a sample prepared with 2 mM L64 under the same conditions, showing several spherical micelles with a few scattered NPs. (f) Chainlike arrangement of NPs of sample from panel e with roughly spherical morphologies.
of 10 mM L64 cause significant Ostwald ripening15 among the nucleating centers, resulting in the generation of interparticle fused morphologies (Figure 3d). As the magnitude of structure transitions is much weaker in 5 mM than 10 mM L64, only deformed morphologies appear (Figure 3c). Similarly, no structure transitions are observed in 2 mM L64 before the cp is reached; instead, distinct, roughly spherical morphologies are formed (Figure 3f). Thus, TEM images fully supplement the conclusions drawn from the UVvisible studies (Figure 2) and indicate the fact that the growth was mainly triggered by the nucleation among the nucleating centers and affected by the structure transitions.
P123Au NPs. The shape and size of NPs were examined when the polymer size was increased from L64 to P123. P123 is an even larger polymer than L64 and L31, with a greater number of PEO units (Experimental Section) that are highly efficient in initiating instant reduction of Au(III) into Au(0).5ad Figure 4a illustrates typical UVvisible scans of a P123 þ HAuCl4 þ water reaction with time at 70 C. Similar scans are obtained when the reaction is conducted at 40 C (Figure S4a, Supporting Information). Interestingly, the peaks at 220 nm (of AuCl4 ions) and 320 nm (of LMCT complex) vanish within 1 h of the reaction (Figure 4b) and result in a simultaneous increase in the 10448
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Figure 4. (a) UVvisible scans of P123 (2 mM) þ HAuCl4 (0.25 mM) þ water ternary mixture with time at 70 C. Dotted arrows represent three peaks at 220 nm, 320 nm, and 550 nm due to AuCl4 ions, the LMCT complex, and surface plasmon resonance of Au NPs, respectively. (b) Intensity versus time plots of these peaks. (c and d) Intensity versus temperature plots of 320 and 550 nm peaks, respectively, for different ternary mixtures with 2, 5, and 10 mM P123. Dotted arrows in both figures indicate the respective cp. (e) Intensity variation of methyl orange at 460 nm versus temperature for ternary mixtures with different concentrations of P123 and without the presence of HAuCl4. Dotted arrows indicate the respective cp.
550 nm peak due to the formation of Au NPs. This was not the case with either L31 (Figure 1b) or L64 (Figure S2c) where reactions took clearly more than 1 h. In addition, note the LMCT peak position at 320 nm instead of 290 nm as observed previously in the case of L31 and L64. It means that there is no issue of LMCT stability due to micelle transitions, and hence no isosbestic point is observed (Figure S4b) under the effect of temperature variation. But the LMCT complex goes through a dramatic variation from 20 to 70 C (Figure 4c). The variation of Au NPs peak at 550 nm is shown in Figure 4d. Between 20 and 30 C in both cases (Figure 4c and 4d, indicated by solids arrows), well-defined micelle formation begins from preaggregates. It is then followed by the intermicelle fusion within 3050 C which ultimately leads to the cp indicated by the dotted arrow in each case and supplemented by Figure 4e. Contrary to the behavior of L64 (Figure 2d,e), the intensity of the LMCT band essentially remains constant between 30 and 50 C (Figure 4c), indicating no structure transitions, while that
of Au NPs shows a rapid increase with more pronounced effects in 10 mM P123 (Figure 4d). It means that a stable LMCT complex is formed at 320 nm as soon as the stable micelle formation begins at 30 C. It simultaneously converts into nucleating centers and is facilitated by the increase in temperature. Such a rapid reduction goes to completion even before the cp is reached (especially for 5 and 10 mM P123) where intensity becomes constant around 40 C, i.e., far below the cp region (Figure 4d). This was again not the case with either L31 (Figure 1i) or L64 (Figure 2e) where intensity still rises even beyond the cp. Thus, a greater number of PEO units with greater surface cavities participates in the rapid reduction only because of the presence of stable LMCT complex. It is therefore interesting to see that how a complete reduction and stable LMCT complex influences the overall morphology of Au NPs. TEM and SEM images of some of the samples prepared at 40 and 70 C are shown in Figure 5. Figure 5a shows a combined image of compound micelles and large Au NPs (78 ( 24 nm, 10449
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Figure 5. (a and b) TEM images of NPs for a sample prepared with P123 (2 mM) þ HAuCl4 (0.25 mM) þ water ternary mixture at 40 and 70 C, respectively. Low contrast morphologies are the micelles or soft templates of micelles. (c) SEM image of sample in panel b showing icosahedral NPs. (d and e) TEM images of the NPs prepared with 5 mM P123 at 40 and 70 C, respectively. (f) SEM image of sample in panel e showing truncated icosahedral NPs. (g and h) Low and high resolution images of 10 mM P123 micelles loaded with growing NPs of different shapes and sizes. (i) SEM image of roughly spherical NPs.
Figure S5a, Supporting Information, for a size distribution histogram) of various shapes of a sample prepared with 2 mM P123 and 0.25 mM gold chloride at 40 C. Some of the NPs appear to be closely related to the overall shape and size of the P123 micelles. A close inspection of different compound micelles (dotted circle in Figure 5a) actually reveals that such assemblies are indeed soft templates where large NPs grow. Solid arrows indicate the growing NPs (with relatively dark contrast) over such templates. A similar situation can be seen for the same sample prepared at 70 C (Figure 5b) where fused large compound micelles are present along with the NPs (89 ( 27 nm, Figure S5b). Both samples provide almost similar shape and size of NPs though they have been conducted at much different temperatures (i.e., 40 and 70 C). This shows that the reduction is completed below the cp and enough surface cavities are available to complete the reduction even at 40 C. The SEM image (Figure 5c) of the sample prepared at 70 C further helps to more clearly understand the morphology of NPs where several NPs with icosahedral geometry occur along with a few nanoplates. However, an increase in the amount of P123 from 2 to 5 mM produces similar morphologies at 40 C (Figure 5d) and 70 C (Figure 5e) of comparable sizes, i.e., 76 ( 24 nm (Figure S5c) and 78 ( 11 nm (Figure S5d), respectively, but the SEM
image (Figure 5f) of the latter suggests that more NPs of truncated icosahedral geometry are present along with a few nanoplates. A further increase in the amount of P123 from 5 to 10 mM produces interesting micellar assemblies (Figure 5g) apart from similar NPs (Figure S6a,b, Supporting Information). Figure 5h shows a close up image of such a compound micelle which contains several growing NPs of different morphologies (indicated by white arrows) that eventually end up in the form of independent NPs shown in Figure 5i. A close scrutiny of roughly spherical NPs of Figure 5i suggests that they are in fact somewhat snub icosidodecahedron. Thus, an increase in the amount of P123 from 2 to 10 mM while keeping the amount of gold chloride constant (i.e., 0.25 mM) leads to a systematic elimination of vertices of icosahedral geometry (Figure 5c) to generate more smooth and roughly spherical NPs (Figure 5i). This is all related to the better capping ability of 10 mM rather than 2 mM P123 because the greater amount of P123 will cap all fcc lattice planes more or less equally to generate roughly spherical NPs rather than a selective capping by 2 mM. Conventional ionic surfactants are highly selective in capping low energy {100} or {110} planes due to their highly surface-active nature and are known for the formation of anisotropic shapes.17 This is not so common in the case of polymeric surfactants such 10450
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The Journal of Physical Chemistry C as P123 where no distinct charge separation between the polar and nonpolar parts can be made. However, Kim et al.18 successfully prepared micrometer sized triangular shaped nanoplates at 70 C by using a solid mixture of P123 and gold salt. Addition of a small amount of water produced long nanowires instead of nanoplates. They explained the formation of such morphologies on the basis of soft template effects of P123. Our studies are limited to the solution phase where a predominantly micellar phase exists rather than a liquid crystalline semisolid phase. Therefore, the present soft template effect (Figure 5h) is only limited to the shape and size of P123 micelles and thus their influence on the morphology of NPs (Figure 5i).
’ DISCUSSION All presently discussed TBPs (i.e., L31, L64, and P123) demonstrate quite unique reduction behaviors with relatively few similarities. The reduction is, of course, entirely carried out by the surface cavities (Scheme 1) but the micelle shape, size, and environments dramatically influence the reduction process and eventually the shape and size of NPs. It is diffusion controlled in L31 when small micelles with a minimum number of PEO units = 4 are employed (Figure 1c, Scheme 2A) but instant and complete in P123 when large micelles with a maximum number of PEO units = 40 are used (Figure 3d, Scheme 2C). L64 with PEO units = 26 demonstrates a quite complicated reduction (Figure 2, Scheme 2B). This is all related to the micelle environment in terms of PPO/PEO ratio which seems to be the most important factor governing the reduction. The PPO/PEO ratio is 4 and 3.5 for L31 and P123, respectively, but close to unity (i.e., 1.15) for L64. This makes the L64 micelle more prone to structural transitions under the effect of temperature variations due to a marked difference in the hydration capacity of PPO and PEO blocks.19 This difference is relatively much less for a high PPO/ PEO ratio (as in the case of L31 and P123) because the PPO block retains less water than PEO but much more water as the PPO/PEO ratio approaches unity (as in the case of L64) or less than unity. PPO blocks loose water more rapidly than PEO and acquire a complete hydrophobic environment which drives more unimers in the micellar phase under the effect of strong hydrophobic interactions. This induces structure transitions in the micelle, as both core and corona are affected in terms of aggregation number and size. On the other hand, a high PPO/ PEO ratio dehydrates both blocks with almost equal rate and hence minimizing the hydrophobic effects. Thus, a greater hydration disparity between PPO and PEO blocks of the L64 micelle makes it more prone to structure transitions in comparison to the micelles of L31 and P123. We have further evaluated this inference by choosing L61 (PEO2PPO30PEO2) with a PPO block identical to that of L64. Unlike L64, L61 is not watersoluble because of a much larger proportion of PPO relative to PEO block. However, it can be made water-soluble by incorporating an anionic surfactant such as sodium perfluorooctanoate (SPO) which is expected to form mixed micelles with L61 due to predominant hydrophobic interactions, and its anionic headgroup will induce the required hydration for the solubilization of L61 in the aqueous phase. A L61/SPO mole ratio = 2 gives a clear water-soluble solution at room temperature and has been used as a model system similar to that of L64. Figure S7a, Supporting Information shows its reduction behavior where a temperature increase from 10 to 20 C clearly induces the much weaker structure transitions around 300 nm in comparison to that of L64
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due to the LMCT complexes which diminish with further increase in the temperature. The TEM image (Figure S7b) of this sample shows the presence of only fused Au NPs of ∼20 nm. Thus, it is the relative hydration index of TBP micelles which, under the effect of temperature variation, controls the nucleation process and we have already seen in Figure 3 for L64. As reduction is site specific and carried out by the micellesolution interfacial arrangement of surface cavities, therefore an appropriate surface arrangement is also a predominant factor toward the formation of a stable and low energy transition state in the form of a LMCT complex. The stability of LMCT complex is very much affected when the micelle goes through structure transitions (Figure 3, Scheme 2B) and ultimately affects the overall morphology of NPs (Figure 4). In contrast, when structure transitions are minimum, a stable LMCT complex proceeds uninterrupted to produce well-defined morphologies (Figure 5, Scheme 2C). In addition, the stability of the LMCT complex also depends on the best fit model of the crown cavity (Scheme 1). L31 with only four PEO units is expected to produce an elliposoidal cavity with two axes of 2.5 and 5 Å.6 A much smaller cavity accommodates an AuCl4 ion of 9.92 Å in comparison to almost a ten times larger cavity of P123 with 40 PEO units (strictly based on geometric factors). Thus, the L31 cavity may not hold and properly accommodate an AuCl4 ion to provide the required stability for the LMCT complex in order to accomplish an effective reduction. That is why it is diffusion controlled in L31. Once a nucleating center is created, it all depends on the dimensions of the cavity to control its growth. A large cavity of a large micelle like that of P123 not only retains the nucleating center but allows it to grow simultaneously while providing a soft template effect (Figure 5a). A large micelle accommodates many nucleating centers, and dehydration brings such centers close enough to self-nucleate with each other to supplement the shape and size of the soft template (Figure 5b). Further growth due to the autocatalytic process (Scheme 2C) on the surface of growing NPs leads to large morphologies with greater probability to exist independently in the colloidal state (Figure 5b,c). The situation is somewhat different with L64 micelles where dehydration along with the micelle transitions (Scheme 2B) brings surface cavities close enough to each other but the soft-template effect does not work, as micelles undergo continuous structural transitions (Figure 2). It facilitates random interparticle fusions and produced disordered morphologies which are rather more prevalent at 10 mM than at 2 mM L64 (Figure 3). Thus, several inter-related factors such as micelle size, number of surface cavities, and micelle environment ultimately define the final morphologies of NPs. The overall size of the NPs is clearly related to the size of the surface cavity (Scheme 2D) in a comparable concentration range in the order of ∼2 nm (L31) < ∼20 nm (L64) < ∼80 nm (P123) though the shape of the NPs is mainly related to the magnitude of structure transitions. Stronger structural transitions during the reduction lead to disordered shapes (Figure 3) without any soft template effects in comparison to minimum structure transitions (Figure 5). Also, a complete reduction with a greater number of surface cavities allows kinetically produced nucleating centers to undergo thermodynamically controlled autocatalytic growth which is the key factor to attain well-defined morphologies (Figure 5). Fewer surface cavities lead not only to the incomplete reduction but also leave the growth on a competing path between kinetically and thermodynamically controlled reductions. Low temperature 10451
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Scheme 2. (A, B, and C) Schematic Representation of the Proposed Mechanism for the Synthesis of Au NPs by Using Micelles of L31, L64, and P123, Respectively (see details in the text). (D) Plot of the Average Size of NPs Estimated from TEM Images versus Cavity Size for L31, L64, and P123a
a
Cavity size was determined from the number of ether oxygens constituting the crown cavity by extrapolating the cavity size from data of ref 6. Extrapolation has been performed by plotting the average of “x” plus “y” parameters of the ellipsoidal cavity.
facilitates the latter over the former (Figure S1d) and hence produces ordered morphologies, but high temperature helps the opposite and hence tiny NPs without clear morphologies are produced (Figure 1e). The reduction carried out by the several surface cavities always allows first the kinetically controlled growth until the completion of the reaction and then is followed by the thermodynamically controlled growth to produce well-defined morphologies at low as well as high temperatures (Figure 5).
’ CONCLUSIONS All results pertaining to the synthesis of Au NPs by using L31, L64, and P123 conclude that the overall reduction process becomes quite complicated when it experiences simultaneous structural transitions in the micelle. The reduction is entirely controlled by the micellar assemblies (i.e., preaggregates, micelles, and their clusters) and especially the surface cavities lining the micellesolution interface. Whenever a structure 10452
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The Journal of Physical Chemistry C transition in the micelle alters the delicate interfacial arrangement of surface cavities, it affects the overall mechanism of the reduction process going through the LMCT complex. A stable LMCT complex with minimum interference by structure transitions generates ordered morphologies when the number of surface cavities with sufficient size is high to accommodate maximum number of AuCl4 ions. A smaller micelle with few surface cavities cannot produce a stable LMCT complex, and hence the nucleation depends on the extent of intermicelle collisions due to diffusion. Structural transitions are significantly influenced by the micellar environment in terms of PPO/PEO ratio. A high ratio strengthens the hydrophobic core of the micelle which is the main driving force for the micelle formation. Thus, a stronger hydrophobic core limits the extent of structure transitions under the effect of temperature change. On the contrary, a PPO/PEO ratio close to or less than unity significantly affects the micelle environment under the effect of temperature variations and hence induces significant structure transitions. Structure transitions not only affect the stability of LMCT complex but also alter the arrangement of surface cavities which in turn triggers the Ostwald ripening among the nucleating centers, and hence disordered morphologies are produced. Hence, in order to get well-defined morphologies, the following factors have to be taken in consideration. First, the choice of TBP should be of high number of PEO units so that instant reduction can be achieved. Second, TBP should have a high value of PPO/PEO ratio so that the micelles should experience minimum structure transitions under the effect of temperature variations. Third, nucleating centers should grow under thermodynamically controlled process to produce ordered morphologies.
’ ASSOCIATED CONTENT
bS Supporting Information. UVvisible spectra, size distribution histograms, and TEM images. This material is available free of charge via the Internet at http://pubs.acs.org. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Author Contributions #
Authors contributed equally in this work.
’ ACKNOWLEDGMENT These studies were partially supported by financial assistance from CSIR [ref no. 01(2219)/09/EMR-II] and [ref no. 01(2102)/ 07/EMR-II] New Delhi.
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