Article pubs.acs.org/cm
Salt-Mediated Self-Assembly of Metal Nanoshells into Monolithic Aerogel Frameworks Kulugammana G. S. Ranmohotti, Xiaonan Gao, and Indika U. Arachchige* Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23284, United States S Supporting Information *
ABSTRACT: Bimetallic alloy aerogels (Au/Ag, Pd/Ag, Pt/ Ag) have been prepared from a novel approach that involves the salt-mediated self-assembly of metal nanoshells followed by supercritical drying. These materials can be prepared as large (centimeter to inch) self-supported monoliths within a day, and the gelation kinetics can be tuned by engineering the insitu-generated ionic strength of the precursor colloids. Resultant aerogels exhibit a continuous mesopore-to-macropore network that can be altered by the inner diameter of the precursor nanoshells and the accessibility of molecules to the inner surface of the hollows, offering new perspectives for their applications in advanced technologies. The attractive nature of this new strategy is the ability to increase the rate of self-assembly, which otherwise is an intrinsically sluggish process in the oxidation-induced NP assembly reported to date. KEYWORDS: self-assembly, monolithic nanostructures, metal aerogels, metal alloy nanoshells
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INTRODUCTION The novel physical properties observed in metal nanoparticle (NP) colloids are attributed to the dramatic changes in the electronic structure and the large surface area, which make them chemically more reactive than corresponding macroscopic solids.1,2 These unique properties have led to the development of synthetic methods that permit exquisite control of their size, shape, and dispersity on the nanometer length scale.3−5 However, many of the applications envisioned and the devices that are likely to be constructed will utilize the properties of NP superstructures in which individual particles are arranged into nanostructured assemblies. Hence, developing methods for the assembly of NPs into solid-state nanostructures and understanding the physical properties of the resulting architectures are among the important challenges of nanotechnology today. Significant efforts have already been devoted to the assembly of metal NPs utilizing organic ligands,6−9 polymers,10 biomolecules,11,12 and ionic solids.13−15 In each of these cases, the interparticle interactions occur by means of organic or ionic cross-linkers, and the scale of the interconnected assembly is typically limited to micrometers. In contrast, the ability to prepare large (centimeter to millimeter) selfsupported monoliths of metal,16,17 semiconductor,18−23 and their hybrid24−27 NPs has recently been realized through a modified sol−gel synthesis technique. The oxidative removal of the surface ligands from respective NP colloids has been reported to yield monolithic gel frameworks that retain the physical properties of their nanosized objects. The aerogels derived from chalcogenide semiconductor and metal NP colloids represent an emerging class of low-density, highsurface-area conducting superstructures16,17 with enormous potential in catalytic, photonic, optoelectronic, and sensing technologies. Nevertheless, the creation of such nanostructures © 2013 American Chemical Society
has been largely limited to solid NP building blocks, leaving behind opportunities to design novel nanoarchitectures displaying unique and potentially tunable morphologies and physical characteristics. For instance, recently it has been reported that CdSe aerogels prepared from nanorods28 and branched NPs29 exhibit high-surface-area, robust polymeric gel structures whereas those prepared from spherical NPs exhibit low-surface-area, weaker colloidal morphologies. Likewise, additional opportunities to optimize the morphology, surface area, and porosity of the gel structure could arise if the solid NP building blocks are replaced with low-density hollow nanoshells. Metal nanoshells are an interesting class of materials with unique physical characteristics that can be tailored by adjusting the size, shape, and shell thickness.2 They often exhibit plasmonic and catalytic properties distinct from those of their solid NP counterparts but have the advantage of significantly less material segregated into a thin shell of a hollow structure. For instance, Zhang et al. reported the wide tunability of the plasmonic features of Au nanoshells by engineering the size and shell thickness.2,30 In addition, several other reports suggest that the alloy nanoshells exhibit significantly higher activity in a number of catalytic and electrocatalytic applications in comparison to single-element metal NPs.31−34 Moreover, hollow nanoshells of traditionally expensive noble metals are cost-effective, low-density materials with all active surfaces without dead interiors. The self-supported assembly of such nanosized objects into monolithic gel frameworks would result in novel nanostructures with a potentially tunable surface area Received: June 17, 2013 Revised: August 5, 2013 Published: August 6, 2013 3528
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the slow evaporation of the solvent was achieved by a high-vacuum pump with a solvent trap dipped in liquid nitrogen. With this setup, usually within 45−60 min the final volume of the nanoshell sol can be reduced from 144 to ∼5 mL without disturbing the stability of colloids. Once the volume reached the desired value (typically 5−16 mL), the resulting sol was filtered using a 0.22 μm poly(vinylidene difluoride) (PVDF) membrane filter and transferred to a glass vial for gel-formation studies. To prepare concentrated sols by the CF technique, the centrifuge filters (Sartorius, Vivaspin, 20 mL, MWCO 30 000), which are originally filled with 15 mL of as-prepared colloids, were centrifuged at 3500g for 3 to 4 min to reduce the volume to 4 mL. This was performed multiple times to reduce the total volume of the original colloid from 144 mL to a desired value (typically 3−15 mL) while retaining the colloidal stability of metal nanoshells. Au/Ag, Pd/Ag, and Pt/Ag Hydrogel Formation. The concentrated colloidal sols obtained from the RE technique were transferred to glass vials and kept in the dark for spontaneous gel formation. Thin-film like hydrogels were formed within 1−28 days (depending on the concentration of the colloidal sols), leaving a clear supernatant above the gel structures. The resultant hydrogels were carefully washed three times with Millipore water and immersed in methanol for 1−4 days. Upon treatment with methanol, large (centimeter to inch), voluminous wet-gel structures were formed. The resultant wet gels were robust and retained their monolithic gel structures during the subsequent solvent exchange and supercritical drying. In contrast, the concentrated colloidal sols prepared by the CF technique were not successful in spontaneous gel formation even after 2 to 3 months. Gelation Induced by Intentional Addition of NaCl. The concentrated colloidal sols prepared by the CF technique can be transformed into hydrogel structures upon intentional addition of NaCl. In this study, the nanoshell sol was prepared by reducing the original volume of the as-prepared colloid from 144 to 11 mL via CF, and the resultant sol was divided into 1 mL aliquots. The concentrations of NaCl in these aliquots were adjusted in between 0.008 and 0.25 mol/L by the intentional addition of an appropriate volume of 3 M NaCl stock solution. The resultant colloids were vigorously shaken and kept in the dark for 1−2 days for gel-formation studies. The hydrogels prepared by this route were extremely fragile and broke into pieces upon subsequent solvent exchange and supercritical drying. In addition, if the concentration of NaCl is increased above 0.25 mol/L, precipitation of the colloids is observed. Preparation of Au/Ag, Pd/Ag, and Pt/Ag Aerogels and Xerogels. As-prepared wet gels were exchanged with acetone six to eight times over 3 to 4 days (to match the miscibility of liquid CO2), ensuring the minimum condensation of the gel structure. The acetoneexchanged wet gels were supercritically dried at 40 °C for 30 min to yield corresponding metal aerogels. In a typical drying process, the monolithic wet gels were loaded into the supercritical dryer and subsequently immersed in liquid CO2 at 15 °C for 8 h while the system was refreshed with pure liquid CO2 every 2 to 3 h. Finally, the temperature of the system was raised to 40 °C, and the wet gels were dried supercritically for 30 min. As a comparative study, the acetoneexchanged wet gels were also dried on the benchtop to form corresponding xerogels. Characterization. A Philips X’Pert X-ray diffractometer equipped with a Cu Kα radiation source was used to collect powder X-ray diffraction (PXRD) data of all samples. The crystallite sizes were determined by employing the Scherrer calculations.36 A Cary 6000i UV−vis−NIR spectrophotometer (Agilent Technologies) was used for optical absorption measurements. A Hitachi SU-70 scanning electron microscope (SEM) fitted with an energy-dispersive X-ray spectroscopy system (EDS) was used for SEM imaging and elemental composition analyses of all samples. Transmission electron microscopy (TEM) images were recorded using a JEOL JEM-1230 electron microscope equipped with a Gatan ultrascan 4000 camera operating at 120 kV. A drop of nanoshell solution was added to a carbon-coated copper grid, and the solvent was allowed to evaporate under ambient condition for 1−2 days before the analysis. The aerogel samples were prepared on
and pore characteristics. Despite recent success in the formation of monometallic and heterogeneous bimetallic solid NP-based aerogels,16 the creation of such frameworks based on catalytically important metal alloys or nanoshells has not been reported to date. Hence, in this work, we report a facile yet powerful strategy for the self-assembly of Au/Ag, Pd/Ag, and Pt/Ag alloy nanoshells into large (centimeter to inch) self-supported monolithic gel structures by engineering the in-situ-generated ionic strength of the precursor colloids. We show that unlike the solid NP-based gels prepared by oxidative assembly,16,18−23 these novel nanostructures exhibit mesoporosity that can be altered by changing the hollow diameter of the precursor nanoshells and the accessibility of molecules to the inner surface of the hollows, offering new perspectives for their applications. A distinct advantage of this new strategy is the ability to achieve significantly faster gelation kinetics in the absence of external destabilizers, which otherwise is an intrinsically sluggish process for the oxidation-induced NP assembly reported to date.16−22
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EXPERIMENTAL SECTION
Materials. AgNO3 (99.9%), K2PtCl4 (99.9%), and NaBH4 (98%) were purchased from Strem Chemicals. Poly(sodium-p-styrenesulfonate) (PSSS, MW = 70K), citric acid, trisodium salt dihydrate (99%), L(+)-ascorbic acid (99%), K2PdCl4 (32.0% Pd), and HAuCl4 (49.0% Au) were purchased from Acros. Methanol (99+%, extra pure) and acetone (ACS grade) were purchased from Fisher. The water used in all syntheses was 18 MΩ Milli-Q filtered, and all other chemicals were used as received. Synthesis of Au/Ag, Pd/Ag, and Pt/Ag Alloy Nanoshells. The citrate-capped alloy nanoshells were synthesized by following a literature synthesis method35 with several modifications to prepare these particles on a larger scale. In a typical synthesis, alloy nanoshells were prepared in three steps: first, Ag seeds were synthesized, then by using the Ag seeds Ag nanotriangles were produced, and finally the Ag nanotriangles were reacted with a desired metal precursor to prepare corresponding alloy nanoshells. In a typical experiment, Ag seeds were produced by combining aqueous solutions of trisodium citrate (30 mL, 2.5 mM), PSSS (1.5 mL, 500 mg L−1), and NaBH4 (1.8 mL, 10 mM) in a flask followed by the addition of an aqueous AgNO3 solution (30 mL, 0.5 mM) at a rate of 1 mL min−1 using a syringe pump. Upon the introduction of AgNO3, a yellow color appeared gradually, indicating the reduction of Ag+ to Ag seeds. The Ag nanotriangles were produced by combining appropriate volume of as-prepared Ag seeds (9.6 mL), Millipore water (10 mL), and a solution of ascorbic acid (7.2 mL, 10 mM), followed by the addition of another AgNO3 solution (48 mL, 0.5 mM) at a rate of 1.66 mL min−1 using a syringe pump. The resulting mixture was vigorously stirred for 1−2 h until the solution became a deep orange color, indicating the formation of Ag nanotriangles. Au/Ag alloy nanoshells were prepared by combining the as-prepared Ag nanotriangles with an ascorbic acid solution (9 mL, 10 mM), followed by the addition of an aqueous solution of HAuCl4 (60 mL, 0.5 mM) at a rate of 1 mL min−1 using a syringe pump. Pd/Ag and Pt/Ag nanoshells were prepared by employing the same method using K2PdCl4 and K2PtCl4 (60 mL, 0.5 mM) aqueous solutions as metal precursors, respectively. Preparation of Concentrated Colloidal Sols. For the saltmediated self-assembly, concentrated colloidal sols of Au/Ag, Pt/Ag, and Pd/Ag nanoshells were prepared by using either a rotatory evaporation (RE) or centrifuge filtration (CF) technique. In the case of the RE technique, an as-prepared nanoshell solution with an original volume of 144 mL was kept in a 250 mL round-bottomed flask and maintained at 30 °C throughout the evaporation process, and the rotation speed was set to 175 rpm. The condenser of the rotary evaporator was cooled by a water pump circulating ice-cold water, and 3529
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the copper grids by dispersing fine powders in acetone using sonication, followed by depositing a drop of solution onto the grid and evaporating the acetone. A Micromeritics model ASAP 2020 surface area and porosimetry analyzer was used to obtain nitrogen adsorption/desorption isotherms of all samples at 77 K. The aerogel samples were degassed under vacuum at 50 °C for 24 h prior to analysis. The data were fit using a Brunauer−Emmett−Teller (BET) model to determine the surface areas of the aerogels. The Barrett− Joyner−Halenda (BJH) model was used to generate the pore-size distribution plots and to calculate the average pore diameters and cumulative pore volumes. Three independently prepared samples were analyzed for each system.
spontaneously produced from respective concentrated colloids ([M] > 0.0028 M and M = Au, Pd, or Pt) prepared via RE without any additional treatment within 1−28 days (Figure 3A). In contrast, the highly concentrated colloids ([M] = 0.0024−0.012 M and M = Au, Pd, or Pt) prepared by the CF technique were very stable in water even after 2 to 3 months. Hence, we have further investigated the uniqueness of the RE technique in producing monolithic gel structures. The main difference between the colloids prepared by the RE and CF techniques is the ionic strength of the resultant concentrated sols. In the case of RE, the ionic strength of precursor colloids increases rapidly with the evaporation of the solvent, whereas for the CF technique the ionic byproducts of the galvanic displacement (Na+, H+, Cl−, and NO3−) are completely removed from the sol with the removal of water molecules. Hence, the in-situ-generated ionic strength forced by the RE has been successfully utilized as an alternative strategy for the formation of metal hydrogels. Moreover, we found that the rate of gelation can be controlled by adjusting the ionic strength of the precursor colloids. To show how the variation in the strength of ionic byproducts affects the rate of gel formation, Au/Ag colloids with different ionic strengths were prepared by adjusting the final volumes of the colloidal sols using RE because all ions are retained in the system. The initial concentration of the in-situ-generated Na+ ions in the asprepared colloidal sols was found to be 0.00025 M (Supporting Information). When the concentration of Na+ ions is adjusted in between 0.0028 and 0.0073 M after the application of RE, the gelation time varies dramatically. The robust hydrogel monoliths were obtained after 1, 4, 15, and 28 days when the Na+ ion concentrations were 0.0073, 0.0052, 0.0041, and 0.0028 M, respectively. Moreover, consistent with our hypothesis, when the ionic strength of the concentrated colloids prepared by the CF technique is increased by the intentional addition of ionic solids (e.g., NaCl), fragile gel structures were formed within 1−2 days. In contrast, the gelation induced by the oxidative removal of the surface ligands (thiolates or citrates) has been reported to require 1−4 weeks.16,18−25 The resulting hydrogels were immersed in methanol for 1−4 days, and the solvent was exchanged with acetone six to eight times over 3−4 days. Finally, the acetone-exchanged wet-gel structures were dried using supercritical CO2 to form large (centimeter to inch) self-supported monolithic aerogels (Figure 3B and Supporting Information). The initial change in solvent from water to methanol caused the formation of voluminous gel structures resulting from differences in the polarity of the solvents, which can suppress the thin-film-like aggregates and influence the 3D arrangement.38,39 Likewise, the subsequent acetone exchange resulted in a slight shrinkage (∼5%) of the wet-gel structures. In addition to supercritical drying, a few wetgel structures were also dried on the benchtop through ambient solvent evaporation to produce corresponding xerogels. Asprepared metal aerogels were black and showed a 5−10% apparent volume loss when compared to precursor wet gels. Notably, the monoliths of Au/Ag, Pd/Ag, and Pt/Ag aerogels exhibit average densities of as low as 0.07, 0.01, and 0.18 g/cm3, respectively, representing 0.08 to 0.84% of the densities of respective bulk alloys displaying similar elemental composition (Table 1). In contrast, corresponding xerogels (Figure 3C) exhibit much higher average densities (1.5−1.9 g/cm3) owing to the significant collapse of the gel framework upon evaporation of the solvent.
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RESULTS AND DISCUSSION Au/Ag alloy nanoshells were prepared by employing a galvanic displacement reaction of sacrificial Ag nanotriangles with HAuCl4 as reported in the literature.35 In addition, Pd/Ag spherical nanoshells and Pt/Ag triangular nanoshells were prepared by employing a similar synthesis method using K2PdCl4 and K2PtCl4 as metal precursors, respectively. The sacrificial Ag nanotriangles were produced by the reduction of AgNO3 in the presence of PSSS using ascorbic acid as the reducing agent.37 These templates exhibit an average edge length of 34 ± 6 nm and a pronounced surface plasmon resonance (SPR) maximum at ∼550 nm (Figure 1). In contrast,
Figure 1. UV−visible absorption spectra of the (A) Ag seeds and (B) Ag nanotriangles along with (C) Au/Ag, (D) Pd/Ag, and (E) Pt/Ag alloy nanoshells produced by the galvanic displacement of the preformed Ag nanotriangles.
as-prepared Au/Ag alloy nanoshells exhibit an intense SPR maximum at ∼575 nm, whereas the growth of Pd/Ag and Pt/ Ag alloy nanoshells resulted in the gradual loss of Ag SPR over the time of displacement. TEM images show that in a majority of individual Au/Ag alloy particles small divided hollows (3−10 nm) have been created whereas in Pd/Ag and Pt/Ag nanoshells significantly larger single hollows (20−30 nm) have been created (Figure 2A−C insets). Interestingly, in the case of Pt/ Ag nanoshells narrowly dispersed triangular hollows were produced in a rapid and reproducible manner. The elemental compositions, wavelengths of the SPR maxima, and average shell thickness calculated on the basis of SEM/EDS, optical absorption, and PXRD data and the TEM images of the asprepared alloy nanoshells are reported in Table 1. For gel formation, highly concentrated colloidal sols were prepared by using either the CF or the RE technique. We found that the Au/Ag, Pd/Ag, and Pt/Ag hydrogels can be 3530
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Figure 2. TEM images of the (A) Au/Ag, (B) Pd/Ag, and (C) Pt/Ag aerogels prepared by concentrated colloidal sols obtained from a rotary evaporation technique. Insets (scale bar 50 nm) in A−C show the TEM images of the individual nanoshells used in the sol−gel assembly. (D) Magnified TEM image of the Au/Ag aerogel displaying the cross-links between particles. The dark contrast areas in A−D represent the multilayers of nanoshells showing the 3D connectivity of nanoscale building blocks.
Table 1. Comparison of Crystallite Sizes, Average Shell Thicknesses, Elemental Compositions, and Wavelengths of the SPR Maxima of the As-Prepared Au/Ag, Pd/Ag, and Pt/Ag Alloy Nanoshells and the Resultant Aerogels along with the Densities, Surface Areas, Average Pore Diameters, and Cumulative Pore Volumes of the Metal Aerogels Degassed at 50 °C precursor nanoshells
aerogels
sample
Au/Ag
Pd/Ag
Pt/Ag
Au/Ag
Pd/Ag
Pt/Ag
crystallite size (nm)a shell thickness (nm)b elemental composition in atomic percentagesc
10.0 ± 0.2 12.7 ± 3.2 Au 64.3% Ag 33.4% Na 0.6% Cl 1.6% 575 N/A N/A N/A N/A
8.5 ± 0.2 11.4 ± 3.3 Pd 56.1% Ag 36.5% Na 2.4% Cl 4.9% N/A N/A N/A N/A N/A
10.1 ± 0.2 11.9 ± 2.2 Pt 56.6% Ag 36.2% Na 5.3% Cl 1.8% N/A N/A N/A N/A N/A
10.8 ± 0.2 15.4 ± 2.6 Au 64.3% Ag 32.3% Na 2.2% Cl 1.2% N/A 0.07 6.9 × 103 11.9 0.19
8.7 ± 0.2 13.0 ± 2.8 Pd 56.9% Ag 37.8% Na 2.5% Cl 2.8% N/A 0.01 4.3 × 103 14.0 0.14
10.4 ± 0.2 13.6 ± 2.2 Pt 59.2% Ag 35.3% Na 2.9% Cl 2.6% N/A 0.18 5.1 × 103 14.3 0.17
SPR maxima (nm) density (g/cm3) surface area (m2/mol)d average pore diameter (nm)e cumulative pore volume (cm3/g)e
a Crystallite sizes were calculated by employing the Scherrer formula for all diffraction peaks in the PXRD pattern, and the average values are presented. bAverage nanoshell thicknesses were obtained by measuring the inner and outer diameters of more than 100 nanoshells from TEM images. cElemental compositions were obtained by SEM/EDS analysis of multiple samples, and the average values are presented. dMolar surface area values were obtained by applying the BET model to nitrogen adsorption/desorption isotherms and using the molar mass of respective alloys calculated on the basis of their atomic composition. eAverage pore diameters and cumulative pore volumes were obtained by applying the BJH model to the desorption branch of the respective isotherms.
slight shift toward larger 2θ angles owing to the formation of respective alloys. Similarly, the diffraction data of the Pd/Ag aerogel indicates the presence of a cubic Pd phase (PDF no. 000040802) with a slight shift toward smaller 2θ angles owing to the growth of the Pd/Ag alloy. The average shell thickness calculated on the basis of the Scherrer formula36 suggests that
The gel formation has no apparent impact on the structure and crystallinity of nanosized constituents as probed by the PXRD data of the aerogels (Figure 4) and xerogels (Figure S2, Supporting Information). The diffraction patterns of Au/Ag and Pt/Ag aerogels are characteristic of the cubic Au (PDF no. 03-0658601) and Pt (PDF no. 00-0040802) phases with a 3531
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particles. Since the in-situ-generated salt ions are expected to force the nanoshell assembly and not the crystalline salts, we did not observe any crystalline impurities in the PXRD patterns of the resultant gel structures. The low-resolution TEM images of the metal aerogels produced by employing the concentrated colloids obtained from the RE technique are shown in Figure 2A−D. The gel morphology is composed of an interconnected network of triangular or nearly spherical hollow particle building blocks that have typical dimensions on the same scale as the precursor colloids. Therefore, it is apparent that these aerogel structures are formed through the cross-linking of the individual nanoshells without the formation of secondary or tertiary aggregates, as previously reported for metal aerogels prepared by the oxidative aggregation of significantly smaller (3−6 nm) Au and Ag solid NPs.16 Nevertheless, the 3D connectivity between nanosized constituents can be clearly observed, specifically, in the dark contrast areas (multiple layers of nanoshells) of the aerogels. In addition, the presence of a mesoporous (2−50 nm) to macroporous (>50 nm) network with a wide range of pore diameters is clearly visible in both the TEM (Figure 2A−D) and SEM (Figure 5A) images of the asprepared aerogels. In contrast, a larger degree of fusion and densification is apparent in the corresponding xerogel product (Figure 5B). The low-density, highly porous structure of the alloy aerogels is further reflected in the surface area and pore characteristics determined from the nitrogen adsorption/desorption isotherms (Figure 6 and Supporting Information). The isotherms recorded for all samples are similar in shape regardless of the composition and resemble a type IV curve with a sharp upturn in the high-relative-pressure region, with some resemblance to a type II curve.40,41 The hysteresis loops of all isotherms have a combination of H1 and H3 character that corresponds to cylindrical and slit-shaped pore geometries present in the gel structure, respectively.41 The surface-area values determined using the BET model were found to be 6.9 × 103, 4.3 × 103, and 5.1 × 103 m2/mol (32−42 m2/g) for Au/Ag, Pd/Ag, and Pt/Ag alloy aerogels, respectively. These values are comparable to those of previously reported Au/Ag and Pt/Ag heterogeneous bimetallic aerogels (46−48 m2/g) prepared from significantly smaller (3−6 nm) solid NPs,6 taking into account that the alloy nanoshells reported here are not only ∼10 times larger but also are rich in heavy metals (Au and Pt). In contrast, the surface area estimated under the assumption of a long network of Au/Ag disklike solid NPs displaying a 35 nm diameter, 10 nm shell thickness, and average density of 14.9 g/
Figure 3. Photographs showing the comparison of the Au/Ag (A) hydrogel, (B) aerogel, and (C) xerogel. In contrast to xerogel formation (benchtop drying), minimal volume loss is observed in the conversion of hydrogels to aerogels (supercritical drying). The top scale bar is in inches.
Figure 4. PXRD patterns of Au/Ag, Pd/Ag, and Pt/Ag aerogels. The ICDD-PDF overlays of cubic Au (PDF no. 03-0658601), Pd (PDF no. 00-0040802), and Pt (PDF no. 00-0040802) are shown as vertical lines.
the nanoshells within the aerogel framework are slightly larger than those of the precursor colloids (Table 1), which can be attributed to mild crystallite growth during supercritical drying (40 °C/30 min). This observation is consistent with the TEM analyses of aerogels where ultrathin wirelike interconnects (Figure 2D) were observed between the nanoshells, which appeared to increase the shell thickness of the individual
Figure 5. High-resolution SEM images of the as-prepared Au/Ag (A) aerogel and (B) xerogel. 3532
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with cationic surfactants into 1D chains is triggered by the partial shielding of positively charged surfaces by the intentional addition of salt ions.13 Similarly, the bimetallic alloy nanoshells reported in this study are stabilized by anionic surfactants (PSSS and citrate ligands) so that a negatively charged electronic double layer would form along the surface of the colloids, preventing particles from self-assembling (Figure S4, Supporting Information). However, when the in-situ-generated salt ions reach a critical concentration upon RE, the nanoshells become unstable in solution and tend to aggregate as a result of reduced electrostatic repulsions.14,42−44 Moreover, Yin et al. reported that the citrates are weak ligands to metal nanoparticles and they can be partially detached from the surface during extensive washing and centrifugation.14 Likewise, in our systems, the weakly bound surface citrate groups of alloy nanoshells can be partially removed throughout the vigorous shaking caused by rotatory evaporation. Thus, similar to literature reports, the nanoshells that are partially stabilized with citrate groups are likely to be held together by van der Waals forces in the resultant gel structures.14,42−44 This hypothesis is further supported by the irreversibility of saltmediated nanoshell assembly. For instance, when the hydrogel was diluted by the addition of water (decrease in ionic strength), it did not dissociate because there are not enough electrostatic forces to counter the van der Waals forces. Moreover, considering the fact that in-situ-generated salt ions are believed to assist in nanoshell assembly, we did not observe any crystalline impurities in the PXRD patterns. Additionally, the presence of a residual number of salt ions (0.3−1.1% by weight or
