Silver-Assisted Colloidal Synthesis of Stable, Plasmon Resonant

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Silver-Assisted Colloidal Synthesis of Stable, Plasmon Resonant Gold Patches on Silica Nanospheres Huixin Bao,† Benjamin Butz,‡ Zhou Zhou,§ Erdmann Spiecker,‡ Martin Hartmann,§ and Robin N. Klupp Taylor*,† †

Institute of Particle Technology, FAU Erlangen-Nuremberg, Cauerstrasse 4, 91058 Erlangen, Germany Center for Nanoanalysis and Electron Microscopy, FAU Erlangen-Nuremberg, Cauerstrasse 6, 91058 Erlangen, Germany § Erlangen Catalysis Resource Centrer, FAU Erlangen-Nuremberg, Egerlandstrasse 3, 91058 Erlangen, Germany ‡

S Supporting Information *

ABSTRACT: Patchy particles possessing heterogeneous surface composition show great promise as self-organizing building blocks for new classes of hierarchical functional structures. A major hurdle is the scalable synthesis of stable patches on nanosized core particles with arbitrarily defined patch number and coverage. So far, few methods have been reported which could be expected to meet these challenges. Recently, we described the heterogeneous nucleation and growth of silver patches on silica nanospheres via a template free colloidal route. The patches produced, although tunable in size and number and showing interesting plasmon resonant properties, were rather unstable and degraded rapidly during attempts to process them further. In the present work, therefore, we set out to explore if related approaches can be employed to produce patchy particles involving gold, which is known to be more stable. The differences between typical patch precursors Ag+ and [AuClx(OH)4−x]− and their respective interactions with amorphous silica make this a significant challenge. We show that preformed small silver patches in addition to the presence of a reducing agent are necessary for the formation of gold patches conformal to the silica nanosphere surface. Systematic study of the process parameters and their influence on the patchy particle morphology as well as in-depth analytical transmission electron microscopy investigation of the patch composition reveal that patches spread over the silica surface via a cycle of galvanic dissolution and redeposition of silver. The resulting gold patchy particles remain stable during subsequent storage or washing and display tunable plasmon resonances within the visible and near-IR spectrum. functional materials.14 Recently, experimental work demonstrated that the self-assembly of particles with two polar patches leads to a colloidal kagome lattice.27 In that work, the overall size of the patchy particles was rather large (around 1 μm in diameter), in part due to the limitation of such synthesis techniques to core particles of micrometer and millimeter size. However, more effort is required to realize self-organizing patchy particles on the lower submicrometer scale in order to complement the already mature field of small homogeneous or shape anisotropic particle self-assembly. Success in this area is

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articles with anisotropy in size, shape, and chemical functionality have been a major research interest, due to their unique properties and wide range of prospective applications.1−16 Among various classes of anisotropic particles, those having heterogeneously distributed surface properties have received much attention.10,17−20 The simplest example of a particle with such surface anisotropy is the Janus particle, which has distinct hemispheres of two components.21 In the more general case, a patchy particle is defined as a particle with at least one patch covering a certain fraction of the whole surface of the particle. This arrangement endows the particle the ability to experience anisotropic and directional interactions with other particles and physical fields.22 Much work has been done to study the self-assembly of patchy particles.23−26 Simulations predict various possible arrangements, indicating their high potential to act as building blocks for hierarchical © 2012 American Chemical Society

Special Issue: Colloidal Nanoplasmonics Received: December 2, 2011 Revised: February 19, 2012 Published: February 22, 2012 8971

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quest for long-term stable nanoparticles with heterogeneously distributed surface functionality suitable for a range of applications.

expected to unleash an exciting new class of functional nanomaterials based on patchy building blocks with potential applications in fields as diverse as drug delivery and optoelectronics.10 It is clear that before patchy particles can be fully exploited, control over their synthesis should be improved and deeper study of their properties carried out. A common technique used to synthesize patchy particles is based on physical vapor deposition of the patch-forming substance onto a close packed monolayer or multilayer of spherical core particles. In this approach, the number and geometry of patches can be somewhat influenced by the angle of evaporation and the orientation of colloidal layers.24,28,29 However, the general lack of versatility and scalability of this and other reported techniques remains an obstacle to the further exploration of patchy particles. Therefore, a scalable and efficient method for the synthesis of particles with an easily tailored and functionalized patch motif is highly desired. In the effort to find new approaches which overcome these issues, we have recently reported facile routes to produce silver patchy particles with tunable morphologies and plasmon resonances in aqueous solution.30,31 In that work, silver ions accumulating at the surface of native silica nanospheres by electrostatic force were reduced, leading to heterogeneous nucleation and surface conformal growth of thin patches. The surface properties of the silica in addition to the reaction temperature were found to be key factors affecting the yield of patches and their morphology. A fairly serious issue with the materials produced by these approaches is their structural instability which occurs when the particles are stored for longer than a few hours or are washed of their mother solution. In such cases, permanent degradation of both the patch morphology and associated plasmonic properties are observed. Such unstable behavior has been reported for other silver nanostructures and represents a major hurdle to the exploitation of silver nanomaterials with nonequilibrium morphologies.32 On the other hand, experience has shown that gold nanostructures are typically more inert than those based on silver and their associated plasmon resonances have been widely studied.33 However, our route to produce silver patches on silica nanospheres is very unlikely to permit the simple substitution of silver for gold. The most common source of gold is the chloroaurate ion, [AuClx(OH)4−x]− where x depends on the solution pH.34 Due to the expected lack of attraction between the silica surface and such ions, heterogeneous nucleation of gold onto a silica surface and surfaceconformal patch growth is improbable. In the present work, we describe a method which overcomes the problems described above and produces gold patches on silica nanospheres with a similar high yield and potential for upscaling to the previously reported work for silver. We show that a small silver patch formed on the silica can act as the starting point for the surface-conformal growth of gold patches with tunable coverage. We investigate the key parameters of this process and carry out morphological and functional analysis of the gold patchy particles produced. Based on these results, we suggest a mechanism of growth of the gold patches which depends on both galvanic replacement and recycling of silver from the initial patches. Given the weak interaction between gold and the silica surface, the latter is observed to be particularly important for the surface conformal growth of the patch. Importantly, the gold patchy particles produced show superior structural stability compared to those based on silver. Therefore, we anticipate our approach will be significant in the

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RESULTS AND DISCUSSION The approach we have developed to produce gold patches on silica nanospheres is presented in Scheme 1. First, very small Scheme 1. General Approach for the Synthesis of Gold Patches on Silica Nanospheres (k-gold refers to a dark-aged solution of potassium carbonate and chloroauric acid)

silver patches are formed on the silica surface at a certain temperature through heterogeneous nucleation via a similar procedure to that which we previously reported.31 Compared to earlier work, we ensure that a large excess of the reducing agent formaldehyde is used. A dark-aged solution of potassium carbonate and chloroauric acid (hereafter referred to as “kgold”) is then added leading to the formation of gold patches. The coverage of the patches can be tuned by adding further amounts of k-gold. In order to study the development of the gold patches, extinction spectra were measured and the corresponding particle morphologies examined by scanning electron microscopy (SEM). Figure 1 illustrates these data for the growth of gold patches at a reaction temperature of 45 °C following successive addition of different amounts of k-gold (the cumulative total added amount of k-gold is indicated). Each k-gold aliquot was added dropwise over approximately 25 s using a hand-held pipet. For no k-gold added, the SEM image shows that small silver patches form on the silica core after the addition of formaldehyde and ammonium hydroxide. The corresponding extinction spectrum shows a narrow peak at 330 nm and a broad extinction band at 775 nm. These features are typical for thin silver plate-like structures and are consistent with our earlier work.31 Following the addition of 1 mL of kgold, the patches are observed to become thicker and more conical-shaped. At the same time, the extinction maximum redshifts by 50 to 825 nm, presumably due to both the change of the metallic species and size of the patches. The inclusion of gold into the initial silver patches is corroborated by the disappearance of the narrow peak at 330 nm, an out-of-plane quadrupole resonance only observed in silver nanostructures.35 Furthermore, the appearance of holes in the patches is observed in the SEM image (Figure 1 addition of 1 mL), possibly indicating the partial dissolution of silver or inhomogeneous growth of gold. As further k-gold is added to the reaction, the size of the gold patches can be seen to increase to cover approximately half of the silica core (Figure 1, cumulative additions of 3, 5, and 8 mL) along with further red-shifts of the 8972

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Figure 1. SEM images and corresponding optical extinction spectra indicate the growth of gold patches from small silver patches on silica nanospheres at 45 °C following successive dropwise addition of k-gold solution. The cumulative total volume of k-gold added is indicated. The scale bars correspond to 200 nm.

Figure 2. SEM images of gold patchy particles formed at reaction temperatures between 40 and 60 °C with 8 mL of k-gold which had been added dropwise to initial silver patches in the same way as the samples reported in Figure 1. Below the micrographs, the corresponding extinction spectra for increasing cumulative dropwise additions of k-gold are shown. The scale bars correspond to 200 nm.

we use exclusively promote the heterogeneous nucleation and growth of metal patches on the silica nanospheres. In the series of images shown in Figure 1, it can be seen that the patch yield (fraction of particles possessing at least one patch) is maintained at the level observed for the initial silver patches (100%). Indeed, a control experiment where initial silver patches were not formed resulted in no gold patches being obtained (Figure S1c). The use of a silver patch in order to initiate gold patch formation is presumably necessary, since gold in the form of the [Au(OH)4]− complex cannot interact with the silica surface in the way that silver ions (Ag+) can and thus sufficient gold supersaturation for heterogeneous

extinction peak up to 1019 nm. To confirm that the changes in the extinction spectra during k-gold addition were entirely due to changes in the patch morphology, rather than homogeneous nucleation and growth of free metal particles, we first carried out the same reaction in the absence of silica nanospheres. Figure S1a (Supporting Information) shows that metal nanoparticles with a size of about 70 nm were formed in this case. In Figure S1b, the extinction spectrum of these is compared to that of the supernatant of a normal batch of patchy particles which had been sedimented by slow centrifugation. The featureless spectrum of the latter proves the absence of free metal particles and confirms the conditions 8973

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Figure 3. (a) SEM images of gold patchy particles produced for (a) different k-gold addition rates and (b) different periods between silver patch formation and k-gold addition (dropwise over 25 s). The scale bars correspond to 200 nm.

nucleation could not be generated. The extinction spectra of patchy particles show an increase of intensity and red-shift with addition of k-gold (see text for further details). In the experiments described above, the reaction temperature used was 45 °C. Previous experience of the heterogeneous nucleation and growth of silver patches on calcined silica nanospheres showed the best temperature range to be 40−60 °C.31 In the current work, at 45 °C, we found that good yields of initial silver patches could be formed even on noncalcined silica, provided that a 4-fold higher ammonia concentration and 20-fold lower silver ion concentration were used. However, as indicated in Figure 2, if the reaction temperature is reduced to 40 °C, the resulting gold patchy particles have a rather low yield, a single patch, and an extinction maximum which shifts continuously to longer wavelengths during growth. Figure 2 also shows the gold patchy particles formed at 50 and 60 °C. It can be seen that the patch yield and number of patches per particle increases with temperature, with a corresponding decrease in patch size. The latter leads the dispersions to have shorter wavelength extinction peaks and to exhibit a smaller red shift with k-gold addition. Based on these results, it can be concluded that the reaction temperature affects patch growth thermodynamically and kinetically. First, the initial heterogeneous nucleation of silver on the silica surface is expected to be more likely at higher temperatures according to the classical nucleation theory.36 This would lead to an increase in the yield and the number of patches per particle while their size would decrease. This is confirmed by SEM images of the initial silver patches (Figure S2) formed at the three reaction temperatures. On the other hand, diffusion and reduction processes are accelerated at elevated temperatures and so the reduction rate of gold should increase with temperature leading to a different patch morphology. Indeed, subtle, but significant differences in Figure 2 can be observed. At 40 °C, the gold patches appear more conformal to the silica surface, that is, have a low wetting angle. Conversely, at higher temperatures, the wetting of the silica by gold is rather less. These observations are contrary to those made in the case of pure silver patchy particles, where

surface diffusion limited dendritic growth dominated higher reaction temperatures.31 Due to the difference in the interfacial behavior of silver and gold, we suggest that the gold patches become less conformal at higher temperatures because the isotropic enlargement (i.e., growth into the bulk solution) of the metal patch dominates over the surface growth. That the gold patch grows along the silica surface at all can be attributed to the close involvement of silver, as will be discussed later. The morphological differences between patches produced at different reaction temperatures described above can also be correlated with the patchy particle optical properties. The largest red shift during growth is observed for the sample produced at 40 °C, which, as we saw, showed the most conformal growth. Indeed, the strong shift of the extinction peak into the near IR is commensurate with the dipole resonances of metal patches which broaden while maintaining a fairly constant thickness. The fact that this red shift is less for higher reaction temperatures is consistent with the less broad, thicker patches which arise due to poorer wetting of the metal patch on the silica. While such qualitative comparisons can be readily made, a quantitative treatment (i.e., based on optical simulations) is currently prohibitive due to the number of complicating factors such as the patch yield, the distribution of patch widths, heights and roughnesses. This, along with a more exhaustive statistical analysis of the synthesized patchy particles is planned for the future. In addition to the reaction temperature, we determined that reactant concentration also has a profound effect on the patch yield and morphology. The influence of the concentration profile of gold precursor was examined by carrying out the addition of k-gold solution (8 mL) either in one shot or as successive aliquots added quickly (dropwise over 25 s) or slowly (dropwise over 150 s). SEM images of the resulting gold patchy particles are shown in Figure 3a. It can be seen that when k-gold is added in one shot, the patches obtained were relatively small and free gold nanoparticles and detached patches were additionally found. On the other hand, in the case of quick addition of eight 1 mL aliquots, the more constant concentration profile of k-gold apparently results in smoother, 8974

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larger patches with a more conformal appearance. Finally, for slow addition of four 2 mL aliquots, the patches were large and rather irregular, presumably due to the growth occurring over multiple steps. We also observed that the time between initial silver patch formation and k-gold addition influences the gold patchy particles’ yield and morphology (Figure 3b). According to our standard approach, we added k-gold 1 min following formaldehyde and ammonium hydroxide addition. As this time was increased to 10 and 30 min the patch yield clearly decreased and the patches became less conformal (see Figure 3b and insets which show magnified particles). These results suggest that some of the initial silver patches may dissolve during postaging. Furthermore, silver ions or clusters which may initially be distributed over the silica surface will have time to diffuse away, add to the silver patches or form complexes with residual ammonia. This latter process would lead to a depletion of species which otherwise might assist gold in growing as a conformal patch. The above results raise the question as to how the preformed silver patches contribute to forming gold patches which spread conformably around the silica surface. While metallic silver could be regarding as a favorable nucleation site for the formaldehyde reduction of gold, it is also well-known that silver undergoes galvanic replacement in the presence of auric ions.37 To attempt to ascertain the prevalence of these two processes, we studied the composition and morphology of the gold patchy particles in greater detail. The XRD pattern of the patchy particles (Figure S3) shows peaks at 38.2, 44.4, 64.6, and 77.5° which, due to the same crystal structure of gold and silver can be assigned to the (111), (200), (220), and (311) crystal planes of either metal.5,38 Analytical transmission electron microscopical (TEM) analysis of samples before and after adding various amount of k-gold yielded more specific results regarding the role of gold and silver to the patch formation (Figure 4). Due to the high atomic number contrast of annular dark-field scanning TEM (ADF-STEM), the metal patches can be clearly resolved on their silica cores (Figures 4a and S4). On the one hand, these images show that with successive additions of kgold, the diameter of the patches generally increases. On the other hand, the smooth, coarse-grained structure of the silver patches (0 mL k-gold) is first replaced by a rather defective and porous structure comprising finer spheroidal grains (0.5 and 1 mL k-gold). For more added k-gold (4 mL), these defects in the patches have apparently been healed. These results are consistent with our SEM investigation (see Figure 1). Furthermore, they clearly confirm that material from the initial silver patches is removed or restructured following k-gold addition. A direct discrimination of Au and Ag via atomic number contrast mechanisms in the ADF-STEM images is complicated by the locally varying thickness of the patches. Therefore, in order to obtain reliable information on the composition of the patches energy dispersive X-ray spectroscopy (EDXS) was additionally applied, as shown in Figure 4b for a typical patch formed following addition of 0.5 mL k-gold. Both ADF-STEM and EDXS consistently indicate no particular accumulation of either metal apart from a slight enrichment of gold at the edge of the patch. In general, this finding shows that silver as well as gold is included in the laterally growing patch. Assuming the patches are a homogeneous mixture between gold and silver, we carried out EDXS analysis of a wider area of the sample (around 50 particles) in order to obtain good statistical information (see Supporting Information for details

Figure 4. (a) ADF-STEM images of metal patches formed on initial silver patches for 0, 0.5, 1, and 4 mL k-gold dropwise additions. The insets show magnified images of patches from the samples. The scale bars correspond to 100 nm. (b) STEM image and EDXS map of a silver/gold patch formed by addition of 0.5 mL k-gold to initial silver patches. (c) Plot showing the relationship between the ratio of silver and gold (i.e., equivalent to various k-gold addition amounts) used in the patchy particle synthesis, and the EDXS determined average composition of the patches. The dashed black line shows a linear fit of the data, while the dotted red line indicates the theoretical patch composition assuming no losses during the reaction.

of the analysis and Figure S5 for the raw EDXS spectra). Figure 4c shows that the atomic ratio of gold increases roughly linearly as k-gold was added. A slight enrichment of gold for all k-gold addition amounts (compare the dashed fit line with the dotted guide line representing compositions assuming complete inclusion of both metals) suggests a small loss of silver from the patches, for example, due to its higher solubility. These findings demonstrate that the preformed silver patches do not only act as sacrificial nucleation sites, but silver ions released by the galvanic reduction of gold are also recycled into the growing patch, presumably due to the presence of excess reducing agent 8975

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Figure 5. Proposed mechanism that explains the formation of gold patches for (a) conformal growth and (b) nonconformal growth. Further details are given in the text.

by formaldehyde at the edge of the patch (4) as described by eq c.

(formaldehyde) and the high pH. In other words, the silver patches are corroded by the gold precursor, resulting in gold deposition and reintegration of silver into the growing patch. Indeed, we were able to show that this reintegration of silver is essential for the formation of stable patches. To this end we produced silver patches according to our earlier work, that is, without a large excess of formaldehyde (Figure S6a). When kgold was added to this sample, we did not observe the formation of stable gold patches but rather severe degradation and even detachment of the silver patches (Figure S6b). Hence, we conclude that both silver and a reducing agent (formaldehyde) are necessary for the successful growth of surface conformal gold patches on silica nanospheres. Based on our experimental evidence, we propose the following mechanism for silver assisted gold patch formation (Figure 5). Figure 5a considers the case where a small silver patch grows conformably over the silica surface when k-gold is added, that is, as shown in Figure 1 or 4. In addition to the initial silver patch (orange), we expect some silver ions (green) to be distributed over the silica surface and in solution (for clarity, the latter are omitted). We distinguish between three main metal formation paths. In path A, which describes the surface conformal growth of the patches, the silver atoms in the initial patch undergo a galvanic replacement reaction (1) when gold ions are added, as described in eq a:

2Au(OH)4−(aq) + 3HCHO(aq) + OH− → 2Au(s) + 3HCOO−(aq) + 6H2O

This process, as well as the galvanic replacement of silver is also expected to operate on the face of the patch, leading it to thicken (paths B and C, respectively). Although they do not contribute to the broadening of the patch, these paths are will be important for the “filling in” of holes left by the initial corrosion of areas of the silver patch as seen in samples where 1 mL k-gold had been added (see Figures 1 and 4a). Figure 5b explains why gold patches are less conformal at higher temperature or when a long time occurs between silver patch formation and k-gold addition (see Figures 2 and 3b). Under these conditions, silver ions will be lost from the uncoated parts of the silica surface (the concentration of green dots in Figure 5b on the silica is lower than in Figure 5a). They will, for example, desorb into solution or diffuse to and integrate with the silver patch. As a consequence, step (3) in path A will operate less successfully since insufficient supersaturation of silver ions on the surface will be achieved. Therefore, metal will more likely be added to the face of the patch by path B (gold reduction by formaldehyde) or path C (galvanic replacement of silver, not shown for clarity) and so the patch will preferentially grow away from the silica surface, as indicated by the white arrows. Our current work aims to explore this mechanism further. Key to this is the identification of the silver distribution on the silica prior to gold addition. Unfortunately, the elucidation of this effect is not trivial. Ex situ techniques such as electron microscopy are unlikely to be conclusive since evidence from related work shows that silver ions and complexes readily produce anisotropic nanostructures on silica nanospheres following drying. We therefore believe that in situ highly surface specific techniques such as second harmonic generation spectroscopy will be of greater help for the identification of the gold patchy particle growth mechanism.39 Prior to concluding this work, we note that the gold patchy particles produced have improved morphological stability, making them highly suitable as anisotropic building blocks for functionalization and self-organization compared to their silver analogues. This improvement is highlighted by the extinction spectra of silver and gold patchy particles in the freshly

3Ag(s) + Au(OH)4−(aq) → Au(s) + 3Ag+(aq) + 4OH−(aq)

(a)

This leads to gold being deposited on the patch. If this occurs close to the patch edge, then the silver ions released (three ions for each gold atom) may readsorb onto the silica, locally raising the silver concentration (2). The latter makes re-reduction (by formaldehyde) of the silver ions more likely (3) as described according to eq b: 2Ag −(aq) + HCHO(aq) + 3OH− → 2Ag(s) + HCOO−(aq) + 2H2O

(c)

(b)

With this newly deposited silver, the cycle can begin again, that is, with step (1), the galvanic replacement reaction described by eq a. We suggest that such a cyclic reaction allows the patch to grow conformably to the silica surface. A further contribution to patch broadening would be the direct reduction of gold ions 8976

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Figure 6. Extinction spectra of silver and gold patchy particles collected after synthesis and after aging in their mother solutions. was heated to a certain temperature in an oil bath. After 30 min, 8 mL of formaldehyde (37% aqueous solution, Carl Roth GmbH, Germany) (37%) and 800 μL ammonium hydroxide (32%) were added into the solution, the latter by dropwise addition under strong stirring (approximately 2 min). Next, aliquots of a solution of k-gold (25 mg potassium carbonate and 1.5 mL of a 25 mM HAuCl4 aqueous solution mixed with water to 50 mL, dark aged for at least 12 h) were added. The amount and rate of k-gold addition is discussed in the main text. Characterization. An ULTRA 55 field emission scanning electron microscope (Carl Zeiss AG, Germany) operating at 10 kV was used to image the patchy particles following their separation from the mother solution by centrifugation and redispersion in water. Samples were prepared by casting a small amount of suspension onto a piece of clean silicon wafer for SEM and a lacey carbon film for transmission electron microscopy (TEM). To characterize the patch morphology in greater detail annular dark-field scanning TEM (ADF-STEM) was carried out using a Titan 80−300 microscope (FEI Company, Eindhoven, The Netherlands), which was operated at 200 kV. Energy dispersive X-ray (EDX) spectroscopy was used to map the composition of a single patch as well as determine the average patch composition. In the latter case, spectra were recorded by simultaneously illuminating representative ensembles of patchy particles. These, combined with spectra acquired from a reference sample, permitted the average compositions of gold and silver in the patchy particles to be determined. Further details of the quantitative analysis and raw spectra are provided in the Supporting Information. The patchy particles were analyzed by X-ray diffraction (XRD) using an X’pert Pro diffractometer (Philips Analytical, Netherlands) using Cu Kα radiation (λ = 0.15405 nm). The diffraction experiments were conducted at 2θ values ranging from 2° to 80° with a step size of 0.02° and a step time of 1 s. Samples for XRD were washed three times with water and were cast onto an aluminum holder. Optical extinction spectra of as prepared patchy particle dispersions were measured using a Lambda 35 spectrophotometer (Perkin-Elmer GmbH, Germany).

prepared and aged states (Figure 6). In the case of silver, a notable blue-shift and significant broadening of the plasmon resonances can be observed after 2 days. On the other hand, the shape and intensity of the gold patchy particle extinction spectrum remained the same even after 1 week of storage without any treatment. Furthermore, the shape of the gold patchy particle extinction spectrum does not show any change after washing the particles with water for one to three times, confirming the good structural stability of the anisotropic particles (Figure S7).

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CONCLUSION In this work, the colloidal synthesis of morphologically stable gold patches on silica nanospheres using preformed silver patches as starting point has been described. We have investigated the key process parameters and their influence on the product morphology and optical properties. The patchy particles produced were studied by optical spectroscopy, X-ray diffraction, and electron microscopy. Our results indicate that the size, shape, and the number of gold patches can be influenced by the reaction temperature and concentration. Furthermore, we show that various morphologies of gold patches can be obtained by carefully controlling the addition speed and time before addition of k-gold. Based on extensive analytical transmission microscopy evidence, we suggest a mechanism which explains why gold is able to form conformal patches on silica, a rather unexpected result. We also found that the gold patches produced had a superior morphological stability compared to the silver patches of our previous study. This is expected to assist us greatly in our current work to functionalize the metal patches in order to prepare them for studies of their self-organization behavior.

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EXPERIMENTAL METHODS

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Synthesis of Silica Core Particles. Silica nanospheres were synthesized according to the Stöber method.40 Briefly, 5.6 g of tetraethylorthosilicate (VWR International GmbH, Germany) was added rapidly to a vigorously stirred mixture of 74 mL of absolute ethanol (VWR International GmbH, Germany), 10 mL of ultrapure water, and 3.2 mL of ammonium hydroxide (32%, Merck GmbH). Following more than 3 h of gentle stirring, the suspension was washed three times by centrifugation and redispersion in absolute ethanol. The silica particles were then dried under vacuum at 60 °C for at least 12 h. Synthesis of Gold Patches. In a typical synthesis procedure, 200 μL of a 10 mg/mL freshly prepared silica suspension was added to 200 mL of a 5 μM silver nitrate (Carl Roth GmbH, Germany) solution and

ASSOCIATED CONTENT

S Supporting Information *

SEM images of control samples (e.g., reduction of k-gold in the absence of initial silver patches and reaction of silver patches with k-gold in the absence of formaldehyde excess) and initial silver patches, higher resolution STEM images, details of EDX spectra analysis, raw EDX spectra, and a typical XRD spectrum of gold patchy particles. This material is available free of charge via the Internet at http://pubs.acs.org. 8977

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(21) Hong, L.; Jiang, S.; Granick, S. Simple Method to Produce Janus Colloidal Particles in Large Quantity. Langmuir 2006, 22, 9495−9499. (22) Zhang, Z.; Glotzer, S. C. Self-Assembly of Patchy Particles. Nano Lett. 2004, 4, 1407−1413. (23) Kretzschmar, I.; Song, J. H. Surface Anisotropic Spherical Colloids in Geometric and Field Confinement. Curr. Opin. Colloid Interface Sci. 2011, 16, 84−95. (24) Zhang, G.; Wang, D.; Möhwald, H. Pattering Microsphere Surfaces by Templating Colloidal Crystals. Nano Lett. 2005, 5, 143− 146. (25) Noya, E. G.; Vega, C.; Doye, J. P. K; Louis, A. A. The Stability of a Crystal with Diamond Structure for Patchy Particles with Tetrahedral Symmetry. J. Chem. Phys. 2010, 132, 234511−234523. (26) Romano, F.; Sanz, E.; Sciortino, F. Phase Diagram of a Tetrahedral Patchy Particle Model for Different Interaction Ranges. J. Chem. Phys. 2010, 132, 184501−184509. (27) Chen, Q.; Bae, S. C.; Granick, S. Directed Self-assembly of a Colloidal Kagome Lattice. Nature 2011, 469, 381−384. (28) Pawar, A. B.; Kretzschmar, I. Multifunctional Patchy Particles by Glancing Angle Deposition. Langmuir 2009, 25, 9057−9063. (29) Pawar, A. B.; Kretzschmar, I. Patchy Particles by Glancing Angle Deposition. Langmuir 2008, 24, 355−358. (30) Klupp Taylor., R. N.; Bao, H. X.; Tian, C. T.; Vasylyev, S.; Peukert, W. Facile Route to Morphologically Tailored Silver Patches on Colloidal Particles. Langmuir 2010, 26, 13564−13571. (31) Bao, H. X.; Peukert, W.; Klupp Taylor, R. N. One-pot Colloidal Synthesis of Plasmonic Patchy Particles. Adv. Mater. 2011, 23, 2644− 2649. (32) Damm, C.; Segets, D.; Yang, G.; Vieweg, B. F.; Spiecker, E.; Peukert, W. Shape Transformation Mechanism of Silver Nanorods in Aqueous Solution. Small 2011, 7, 147−156. (33) Skrabalak, S. E.; Chen, J. Y.; Sun, Y. G.; Lu., X. M.; Au, L.; Cobley, C. M.; Xia, Y. N. Gold Nanocages: Synthesis, Properties and Applications. Acc. Chem. Res. 2008, 41, 1587−1595. (34) Moreau, F.; Bond, G. C.; Taylor, A. O. Gold on Titania Catalysts for the Oxidation of Carbon Monoxide: Control of pH during Preparation with Various Gold Contents. J. Catal. 2005, 231, 105−114. (35) Hao, E.; Schatz, G. C.; Hupp, J. T. Synthesis and Optical Properties of Anisotropic Metal Nanoparticles. J. Fluoresc. 2004, 14, 331−341. (36) Skripov, V. P.; Faizullin, M. Z. In Nucleation Theory and Applications; Schmelzer, J. W. P., Ed.; Wiley-VCH: Weinheim, Germany; 2005, Ch. 2. (37) Cobley, C. M.; Xia, Y. N. Engineering the Properties of Metal Nanostructures via Galvanic Replacement Reactions. Mater. Sci. Eng., R 2010, 70, 45−63. (38) Maye, M. M.; Zheng, W. X.; Leibowitz, F. L.; Ly, N. K.; Zhong, C. J. Heating-Induced Evolution of Thiolate-Encapsulated Gold Nanoparticles: A Strategy for Size and Shape Manipulations. Langmuir 2000, 16, 490−497. (39) Schneider, L.; Peukert, W. Second Harmonic Generation Spectroscopy as a Method for In Situ and Online Characterization of Particle Surface Properties. Part. Part. Syst. Charact. 2006, 23, 351− 359. (40) Stöber, W.; Fink, A.; Bohn, E. J. Controlled growth of monodisperse silica spheres in the micron size range. Colloid Interface Sci. 1968, 26, 62−69.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +49-9131-8529409. Fax: +49-9131-8529402. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the support of the German Research Foundation (DFG), which, within the framework of its ‘Excellence Initiative’, supports the Cluster of Excellence ‘Engineering of Advanced Materials’ at FAU ErlangenNuremberg.

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REFERENCES

(1) Fejer, S. N.; Chakrabarti, D.; Wales, D. J. Emergent complexity from Simple Anisotropic Building Blocks: Shells, Tubes, and Spirals. ACS Nano 2010, 4, 219−228. (2) Yoshida, M.; Lahann, J. Smart Nanomaterials. ACS Nano 2008, 2, 1101−1107. (3) Grzelczak, M.; Vermant, J.; Furst, E. M.; Liz-Marzan, L. M. Directed Self-Assembly of Nanoparticles. ACS Nano 2010, 4, 3591− 3605. (4) Chen, S.; Wang, Z. L.; Ballato, J.; Foulger, S. H.; Carroll, D. L. Monopod, Bipod, Tripod, and Tetrapod Gold Nanocrystals. J. Am. Chem. Soc. 2003, 125, 16186−16187. (5) Sun, Y. G.; Xia, Y. N. Shape-Controlled Synthesis of Gold and Silver Nanoparticles. Science 2002, 298, 2176−2179. (6) Manna, L.; Scher, E. C.; Alivisatos, A. P. Synthesis of Soluble and Processable Rod-, Arrow-, Teardrop-, and Tetrapod-shaped CdSe Nanocrystals. J. Am. Chem. Soc. 2000, 122, 12700−12706. (7) de Gennes, P. G. Soft Matter. Rev. Mod. Phys. 1992, 64, 645−648. (8) Roh, K. H.; Martin, D. C.; Lahann, J. Biphasic Janus Particles with Nanoscale Anisotropy. Nat. Mater. 2005, 4, 759−763. (9) Halder, A.; Kundu, P.; Viswanath, B.; Ravishankar, N. Symmetry and Shape Issues in Nanostructure Growth. J. Mater. Chem. 2010, 20, 4763−4772. (10) Bianchi, E.; Blaak, R.; Likos, C. N. Patchy Colloids: State of the Art and Perspectives. Phys. Chem. Chem. Phys. 2011, 13, 6397−6410. (11) Lim, B.; Xia, Y. N. Metal Nanocrystals with Highly Branched Morphologies. Angew. Chem., Int. Ed. 2011, 50, 76−85. (12) Sajanlal, P. R.; Sreeprasad, T. S.; Samal, A. K.; Pradeep, T. Anisotropic Nanomaterials: Structure, Growth, Assembly and Functions. Nano Rev. 2011, 2, 5883−5945. (13) Glotzer, S. C.; Anderson, J. A. Nanoparticle Assembly: Made to Order. Nat. Mater. 2010, 9, 885−887. (14) Glotzer, S. C.; Solomon, M. J. Anisotropy of Building Blocks and Their Assembly into Complex Structures. Nat. Mater. 2007, 6, 557−562. (15) Jang, S. G.; Kim, S. H.; Lee, S. Y.; Jeong, W. C.; Yang, S. M. Facile Synthesis of Core-Shell and Janus Particles via 2-D Dendritic Growth of Gold Film. J. Colloid Interface Sci. 2010, 350, 387−395. (16) Lee, K. J.; Yoon, J.; Lahann, J. Recent Advances with Anisotropic Particles. Curr. Opin. Colloid Interface Sci. 2011, 16, 195−202. (17) Romano, F.; Sciortino, F. Colloidal Self-assembly: Patchy from the Bottom up. Nat. Mater. 2011, 10, 171−173. (18) Chastek, T. T.; Hudson, S. D.; Hackley, V. A. Preparation and Characterization of Patchy Particles. Langmuir 2008, 24, 13897− 13903. (19) Mirin, N. A.; Ali, T. A.; Nordlander, P.; Halas, N. J. Perforated Semishells: Far-Field Directional Control and Optical Frequency Magnetic Response. ACS Nano 2010, 4, 2701−2712. (20) Ye, J.; Verellen, N.; Van Roy, W.; Lagae, L.; Maes, G.; Borghs, G.; Van Dorpe, P. Plasmonic Modes of Metallic Semishells in a Polymer Film. ACS Nano 2010, 4, 1457−1464. 8978

dx.doi.org/10.1021/la204762z | Langmuir 2012, 28, 8971−8978