pubs.acs.org/Langmuir © 2010 American Chemical Society
Facile Route to Morphologically Tailored Silver Patches on Colloidal Particles Robin N. Klupp Taylor,* Huixin Bao, Chenting Tian, Serhiy Vasylyev, and Wolfgang Peukert Institute of Particle Technology and Cluster of Excellence “Engineering of Advanced Materials”, Friedrich-Alexander-University of Erlangen-Nuremberg, Cauerstrasse 4, 91058 Erlangen, Germany Received May 6, 2010. Revised Manuscript Received June 30, 2010 Here we demonstrate, for the first time, the heterogeneous nucleation and growth of silver patches on submicrometer silica spheres. While patches can be grown directly onto native silica particles, it is shown that a higher patch yield can be obtained by first treating the silica with a mixture of an alkanolamine and silver nitrate. Variation of the pretreatment and subsequent coating reactions allowed the patch yield, number, size, thickness, and shape to be adjusted. The patchy particles were shown to possess plasmon modes extending from the visible into the near-IR region, making these structures highly interesting for both their asymmetric morphological and functional properties.
Introduction Colloidal nanostructures containing metal components1-4 are currently transforming fields as diverse as industrial catalysis5 and biomedical diagnostics6 where very specific magnetic,7 optical,8 or chemical9 properties are required. Furthermore, applications relying on self-assembled or hierarchical structures demand ever more sophisticated building blocks.10 To meet these needs, attention is increasingly turning to nanostructures with anisotropic morphologies and compositions.11 While single-phase asymmetric1,2 and symmetric composite nanostructures3,6 are well-known, the formation of asymmetric composite nanostructures (e.g., patchy particles12 and Janus particles13) via scalable, tunable techniques remains a challenge. Indeed, due to the breakage in symmetry required to produce such structures, typical methods employ phase boundaries14 or physical vapor deposition.15 In the present Article, we describe a simple colloidal method which circumvents these limitations to produce patchy (and in some cases, Janus) particles by modification of electroless metal coating techniques. Particles comprising a dielectric core and metal shell are being intensively investigated due to their tunable optical properties.16 The preferred method for the synthesis of such nanostructures is *To whom correspondence should be addressed. E-mail: robin.klupp.taylor@ cbi.uni-erlangen.de. (1) Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Angew. Chem., Int. Ed. 2009, 48, 60–103. (2) Hao, E.; Schatz, G. C.; Hupp, J. T. J. Fluoresc. 2004, 14, 331–341. (3) Major, K. J.; De, C.; Obare, S. O. Plasmonics 2009, 4, 61–78. (4) Hu, X.; Dong, S. J. Mater. Chem. 2008, 18, 1279–1295. (5) Semagina, N.; Kiwi-Minsker, L. Catal. Rev. - Sci. Eng. 2009, 51, 147–217. (6) Asefa, T.; Duncan, C. T.; Sharma, K. K. Analyst 2009, 134, 1980–1990. (7) Latham, A. H.; Williams, M. E. Acc. Chem. Res. 2008, 41, 411–420. (8) Khlebtsov, N. G.; Dykman, L. A. J. Quant. Spectrosc. Radiat. Transfer 2010, 111, 1–35. (9) Crossley, S.; Faria, J.; Shen, M.; Resasco, D. E. Science 2010, 327, 68–71. (10) Nie, Z.; Petukhova, A.; Kumacheva, E. Nat. Nanotechnol. 2010, 5, 15–25. (11) Glotzer, S. C.; Solomon, M. J. Nat. Mater. 2007, 6, 557–562. (12) Pawar, A.; Kretzschmar, I. Macromol. Rapid Commun. 2010, 31, 150–168. (13) Jiang, S.; Chen, Q.; Tripathy, M.; Luijten, E.; Schweizer, K.; Granick, S. Adv. Mater. 2010, 22, 1060–1071. (14) Perro, A.; Meunier, F.; Schmitt, V.; Ravaine, S. Colloids Surf., A 2009, 332, 57–62. (15) McConnell, M. D.; Kraeutler, M. J.; Yang, S.; Composto, R. J. Nano Lett. 2010, 10, 603–609. (16) Kalele, S.; Gosavi, S. W.; Urban, J.; Kulkarni, S. K. Curr. Sci. 2006, 91, 1038–1052.
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based on the ripening of “seed” nanoparticles preadsorbed to the core by electroless plating reactions (Figure 1a).17-20 Recent improvements to this technique21,22 have attempted to address its main weaknesses, namely, the dependence of the final shell’s thickness and quality on a high surface density of seed particles and on the simultaneous growth of all seeds at the same rate. However, to our knowledge, an alternative approach whereby the shell is nucleated at a small number of locations and is promoted to grow around the core particle (Figure 1b) has not been explicitly attempted. We believe that if such heterogeneous nucleation and growth conditions can be identified and optimized, it would open up not only the possibility of producing potentially thinner metal shells than is currently possible, but in addition provide a highly scalable route to patchy particles or, in the case of a single patch covering 50% of the core, Janus particles. Here we show that in fact the heterogeneous nucleation and growth of one or more silver patches on spherical silica particles can be achieved rather simply without the need for modification of the silica surface or even explicit attachment of preformed nanoparticles. Moreover, we show that the patch yield, number per core particle, and morphology can be modified by modest changes in the procedure, resulting in significant changes in the optical properties of the particles. Compared to conventional methods for producing plasmonic patchy15 and Janus particles,14,23-25 we believe this work brings closer the widespread exploitation of functional nanostructures based on morphological asymmetry. (17) Oldenburg, S. J.; Averitt, R. D.; Westcott, S. L.; Halas, N. J. Chem. Phys. Lett. 1998, 288, 243–247. (18) Jackson, J. B.; Halas, N. J. J. Phys. Chem. B 2001, 105, 2743–2746. (19) Yong, K. -T.; Sahoo, Y.; Swihart, M. T.; Prasad, P. N. Colloids Surf., A 2006, 290, 89–105. (20) Spuch-Calvar, M.; Pacifico, J.; Perez-Juste, J.; Liz-Marzan, L. M. Langmuir 2008, 24, 9675–9681. (21) Ashayer, R.; Mannan, S. H.; Sajjadi, S. Colloids Surf., A 2008, 329, 134– 141. (22) Brinson, B. E.; Lassiter, J. B.; Levin, C. S.; Bardhan, R.; Mirin, N.; Halas, N. J. Langmuir 2008, 22, 14166–14171. (23) Liu, J.; Maaroof, A. I.; Wieczorek, L.; Cortie, M. B. Adv. Mater. 2005, 17, 1276–1281. (24) Lassiter, J. B.; Knight, M. W.; Mirin, N. A.; Halas, N. J. Nano Lett. 2009, 9, 4326–4332. (25) Lu, Y.; Xiong, H.; Jiang, X.; Xia, Y.; Prentiss, M.; Whitesides, G. M. J. Am. Chem. Soc. 2003, 125, 12724–12725.
Published on Web 07/14/2010
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Figure 1. Schematic representation of (a) seeded growth of a symmetric core-shell structure and (b) heterogeneous nucleation and growth of patchy coatings.
Figure 2. SEM images of patchy silver coatings on (a) dried, and (b) dried and calcined silica spheres and (c) the corresponding optical extinction spectrum.
Experimental Section Core Silica Particles. Two sources of monodisperse silica particles produced by the St€ ober method26 were used in our study: monodisperse silica sphere powder (M500, Merck GmbH, Germany) and particles synthesized in our own laboratory. In the latter case, 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). Stirring was ceased after 10 min, and the reaction was allowed to proceed for 3 h. Following this, 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. A portion of the resulting powder was calcined in air at 800 °C for 6 h. It was confirmed that, provided the silica was calcined, the silica source did not influence the outcome of treatment described in the following. Silica Pretreatment. Portions of silica powder were redispersed in ultrapure water (50 g/L) by sonication (Sonorex ultrasonic bath, Bandelin Electronic GmbH, Germany) for 30 min. The suspensions were then used directly for silver coating or were “seeded” by stirring with an aliquot of an aqueous solution of 8.3 M monoethanolamine (MEA; Sigma-Aldrich GmbH, Germany) (26) St€ober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62–69.
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and 1.4 M silver nitrate (Carl Roth GmbH, Germany) in ambient conditions for 1 h. Seeding was followed by washing three times in water. In all cases, the final silica concentration was 50 g/L. Further details of silica pretreatment are provided in the Supporting Information (Table S1). Silver Coating. Coating was carried out by dispersing nonseeded or seeded silica spheres (diluted to a silica concentration of 5 g/L) in a 0.1 mM silver nitrate solution. Immediately afterward, aliquots of formaldehyde (37% aqueous solution, Carl Roth GmbH, Germany) and ammonium hydroxide (8%) were added under conditions of vigorous stirring. The latter was added dropwise at an approximate rate of one drop every 2 s. In some cases, the order of addition, speed of addition, or time between reactant additions of formaldehyde and ammonium hydroxide was varied (see Table S1 in the Supporting Information for more details). Characterization. An ULTRA 55 field emission scanning electron microscope (SEM; Carl Zeiss AG, Germany) was used to image silver-coated particles dried from suspension onto silicon wafers. High-resolution transmission electron microscopy (HRTEM) and high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) were carried out on an aberration corrected Titan3 80-300 instrument (FEI Company, Netherlands). Samples for TEM were prepared from suspension by evaporating a small droplet onto a copper-grid supported holey carbon film. A PHI 5600-CI XPS instrument (Physical DOI: 10.1021/la102284w
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Figure 3. SEM images (a-c) of patchy silver coatings on silica spheres pretreated with MEA-silver nitrate and (d) their corresponding extinction spectra. Patch size/thickness was controlled by the amount of pretreated silica particles added to the silver plating bath: (a) 0.15 mL, (b) 0.05 mL, and (c) 0.025 mL.
Electronics GmbH, Germany) was used to analyze the surface composition of MEA-silver treated silica spheres. Anodic stripping voltammetry was carried out using an automated device (Metrohm AG, Switzerland) to determine the concentration of silver in aqueous solutions which had been separated from suspensions of seeded silica spheres by centrifugation. Optical extinction spectra were measured using Cary 100 Scan UVvisible (Varian Deutschland GmbH, Germany), Lambda 35 UV-visible, and Lambda 950 UV-visible-NIR (PerkinElmer GmbH, Germany) spectrophotometers. Zeta potentials were measured using a Zetasizer Nano ZS instrument (Malvern Instruments Ltd., U.K.).
Results Patches on Native Silica Particles. As a first example of our approach, amorphous silica particles were synthesized according to the St€ober method,26 were washed in ethanol, and were dried 13566 DOI: 10.1021/la102284w
under vacuum. Part of the resulting powder was then calcined in air. Two suspensions were formed by redispersing the noncalcined and calcined particles in water. Portions of these were mixed with aqueous silver nitrate solution and then aliquots of aqueous formaldehyde and ammonia solutions were added to initiate the reduction of silver. Figure 2 shows SEM micrographs and the optical extinction spectra for the resulting silver coatings on noncalcined (Figure 2a) and calcined (Figure 2b) silica particles. Lower magnification images showing more particles can be found in the Supporting Information (Figure S1). It can be seen in both images that one or two silver patches have grown onto many of the core particles. These patches appear to adhere well to the silica (we found no evidence of detached patches) and possess a conical shape with a circular footprint. The latter suggests that patch growth proceeded outward and sideways from a single point formed by the heterogeneous nucleation of silver. The fact that Langmuir 2010, 26(16), 13564–13571
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Figure 4. Characterization of silica particles seeded in Ag-MEA solutions. (a) Anodic stripping voltammetry determined amount of silver released into aqueous solution when the same batch of pretreated silica was repeatedly dispersed into fresh water and stood for 15 min. (b) XPS spectrum of Ag 3d binding energies of the particles. (c) HAADF-STEM micrograph of particles showing small numbers of silver nanocrystals present on the silica surface. (d) HRTEM micrograph of a silver nanocrystal on the silica particle surface.
the sizes and shapes of the patches are rather distributed suggests that this nucleation occurred over a period of time or mixing during the reaction was inhomogeneous. Furthermore, the exact proportion of particles possessing at least one patch (hereafter referred to as the patch yield) appears to depend on the prior thermal treatment of the silica. The patch yields (determined by counting more than 100 particles for each sample) were 43% and 64% for noncalcined and calcined core particles, respectively. Figure 2b also shows the presence of free silver particles. These were, however, rarely found, indicating that most of the silver had been deposited onto the silica cores. While patches with similar yields could be formed reproducibly by repeating the above procedure, their morphology ranged from the conical patches of Figure 2 to thin, concentric patches (see Supporting Information Figure S2). We believe that this variability can be eliminated by effecting more precise control over the reaction conditions, a subject of ongoing work in our group. The optical properties of the silver patchy particles formed using native silica particles (Figure 2c) are dominated by extinction bands at 330-340, 520-540, and 800-1000 nm. The latter Langmuir 2010, 26(16), 13564–13571
two features are consistent with reports for plasmon resonant nanocaps produced by conventional methods23,24 and correspond to out-of-plane and in-plane dipole plasmon resonances, respectively. The UV-located peak is at a similar wavelength to a resonance observed in thin silver prisms27,28 as well as surface plasmon-polaritons in a thin silver film.29 Finite element simulations, currently being undertaken in our group, will elucidate the physical origin of this resonance for the more complex geometry of the silver patchy particles. Patches on Seeded Silica Particles. Figure 3 indicates that when calcined silica particles were seeded with a mixture of silver and monoethanolamine (Ag-MEA), silver patches could subsequently be grown (using the same coating procedure described above) with patch yields approaching 100%. Similar results were obtained by depositing silver on silica particles seeded with (27) Bastys, V.; Pastoriza-Santos, I.; Rodı´ guez-Gonzalez, B.; Vaisnoras, R.; LizMarzan, L. M. Adv. Funct. Mater. 2006, 16, 766–773. (28) Kelly, K. L.; Coronado, E.; Zhao, L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668–677. (29) Lee, W.; Kim, J.; Park, H.; Lee, M. Opt. Express 2010, 18, 5459–5465.
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complexes of silver and di- or triethanolamines, alkylamines or with ammoniacal silver hydroxide (see Supporting Information Figure S3). Figure 3 shows that the patch size and thickness could be influenced by adjusting the seeded silica concentration in the silver plating reaction. For example, Figure 3a corresponds to the highest seed concentration and consequently has the thinnest and smallest patches. The resulting optical extinction spectra (Figure 3d) show an increase in intensity and slight blue-shift of the visible-IR extinction band as the patches become broader and thicker. This is presumably due to the fact that larger patches can support higher frequency modes of resonance and there may even be weak interactions between patches on the same core particle. The short wavelength peak (Figure 3d inset) exhibits a red-shift which may be indicative of the increasing mean thickness of the patches. In future work, we aim to confirm this relationship by comparing single particle spectroscopic measurements with simulations of the optical properties of the particular patch geometries produced. Silica particles that had been seeded by the Ag-MEA treatment were characterized in order to establish the presence and chemical state of silver. Anodic stripping voltammetry (Figure 4a) was used to determine the amount of silver released from the pretreated particles during cycles of redispersion in pure water. After the first redispersion, a supernatant silver ion concentration of 0.5 g/L was generated per gram of silica and significant amounts of silver continued to be released following five cycles. This magnitude of metal release suggests that silver is associated with the silica as ions rather than as metallic nanoparticles. X-ray photoelectron spectroscopy (XPS) also confirmed the presence of silver at the silica surface (Figure 4b). The negative shift of the 3d binding energy (367.7 eV) compared to that reported for Ag0 (368.2 eV30) indicated the presence of complexed or oxidized silver. XPS elemental analysis (see Supporting Information Table S2) showed that, in addition to silver, nitrogen was found only in the treated particles, confirming that MEA remains associated with the silica, even after repeated washing. Bright spots in HAADF-STEM micrographs of a pretreated silica sample (Figure 4c) were confirmed by HRTEM (Figure 4d) to correspond to silver nanocrystals. The fact that there is a rather small number of these nanocrystals suggests that they act as nucleation points for patch growth. This is corroborated by the morphology of patches in Figure 3 which clearly show a central protrusion on each patch, a feature not observed in patches grown directly onto native silica (Figure 2 and Supporting Information Figures S1 and S2). Further evidence for the role of the amine-silver treatment in generating nucleation points for patch growth was obtained by varying the ratio of MEA to silver nitrate during pretreatment (Figure 5). As this ratio increased, the fraction of resulting silvercoated particles with a single patch (essentially Janus particles) increased from below 20% to over 65%. Tuning of Patch Morphology. In addition to controlling the number, size, and thickness of the silver patches, by adjusting aspects of the final silver plating reaction we could produce a range of patch morphologies. Figure 6 shows the effect of the order of addition of the formaldehyde and ammonia on the coatings’ structure and extinction spectrum, using Ag-MEA seeded silica particles as cores. When the ammonia was added first, immediately followed by the formaldehyde, rapid formation of silver occurred at the seeds on the silica surface. Since the Agþ diffusing from the bulk solution is expected to be more easily reduced than the surface-coordinated silver, the resulting patches acquire a conelike shape (Figure 6a). However, if the seed particles were aged in a solution of ammonia and silver nitrate (30) Hoflund, G. B.; Ha, Z. F. Phys. Rev. B 2000, 62, 11126–11133.
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Figure 5. SEM images of patchy silver coatings on silica spheres formed following pretreatment in solutions of MEA and silver nitrate at different molar ratios: (a) 6 and (b) 30. (c) A map of the number of patches per particle against the monoethanolamine/ silver mole ratio during pretreatment. For all samples, >100 particles were analyzed.
for 30 min before adding the formaldehyde, the resulting patches had a central protrusion and thinner surrounding rim (Figure 6b). Here, the ammonia will have formed complexes with Agþ in solution, thus adding a kinetic barrier to reduction and shifting the balance more toward surface growth. Even more pronounced surface growth occurred when the ammonia was added dropwise after the formaldehyde (Figure 6c), presumably since surfaceresiding silver was already reduced when the pH was still too low for rapid outward growth. The strong morphological dependence of the optical properties is illustrated in Figure 6d. As the patch departs from a mainly conical shape (Figure 6a) and acquires a thin rim (Figure 6b and c) the peak plasmon resonance shifts from 700 to 900 nm, indicating that the patches with more platelike features can support a longer wavelength in-plane dipole mode. In fact, varying the rate of the ammonia addition alone (once formaldehyde had been added) resulted in the formation of patches with morphologies ranging from conical to dendritic (Figure 7), with the latter indicating that surface diffusion limited growth took place. Current work in our group is attempting to unravel the rather complex relationship between the rate of addition of this reactant and the patch yield and morphology.
Discussion Despite several previous studies employing silica core particles and silver plating reactions,18,31,32 to the best of our knowledge, this is the first report of the heterogeneous nucleation and surface growth of metal coatings on submicrometer sized particles. Indeed, Jiang and Liu reported that in the absence of preformed metal seed particles attached to the core, silver coating merely (31) Jiang, Z.; Liu, C. J. Phys. Chem. B 2003, 107, 12411–12415. (32) Kim, J.-H.; Bryan, W. W.; Lee, T. R. Langmuir 2008, 24, 11147–11152.
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Figure 6. SEM images (a-c) of patchy silver coatings on silica spheres pretreated with MEA-silver nitrate and (d) their corresponding extinction spectra. Silver patch morphologies were tailored by varying conditions of the silver coating reaction: (a) Ammonia added a few seconds before formaldehyde. (b) Ammonia added 30 min before formaldehyde. (c) Formaldehyde added a few seconds before ammonia.
resulted in the formation of spheroidal silver particles both on the silica surface and in the bulk solution.31 More recently, Tang and co-workers reported that silver could be nucleated onto the native surface of polystyrene latex and grown into slightly elongated islands.33 That work utilized a silver concentration 2 orders of magnitude higher than that of our study and resulted in a rather large number of silver nuclei formed at the particles’ surface. It was also not clear if surface growth actually occurred or if adjacent nuclei simply coalesced. In general, we can identify three principal differences between our and previous work: (1) we utilize silica particles which have been dried, calcined, and redispersed, (2) we do not functionalize the particles with a moiety which imparts them with a positive surface charge, and (3) we do (33) Tang, S.; Chen, L.; Vongehr, S.; Meng, X. Appl. Surf. Sci. 2010, 256, 2654– 2660.
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not allow the reduction of silver to proceed too rapidly. We postulate that our conditions allow sufficient positively charged silver ions to accumulate at the silica surface such that a small number of heterogeneous nucleation events can occur when the reducing agent and base are added or if pretreatment with AgMEA takes place. In the following, we discuss how the thermal treatment used may enhance this process. Our motivation for vacuum drying the particles was to remove residual ethanol and other byproducts of synthesis. However, according to the widely accepted model of the amorphous silica surface,34 this treatment is not expected to significantly alter the surface’s state of hydroxylation or its ability to physically adsorb water molecules. Therefore, we would expect the surface of a vacuum-dried silica particle following redispersion in water to (34) Zhuravlev, L. T. Colloids Surf., A 2000, 173, 1–38.
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Figure 7. Asymmetric silver coatings on silica spheres with coating morphologies controlled by varying the addition rate of the 0.1 mL 8% ammonium hydroxide added to the silver plating bath after formaldehyde addition: (a) full amount added in
