Understanding and Controlling the Morphology of Silica Shells on

Article Cite This: Chem. Mater. 2018, 30, 6249−6258

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Understanding and Controlling the Morphology of Silica Shells on Gold Nanorods Laurel R. Rowe, Brian S. Chapman, and Joseph B. Tracy* Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695, United States

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S Supporting Information *

ABSTRACT: Subtle variations in the conditions for addition of a tetraethyl orthosilicate (TEOS)/methanol (MeOH) solution to gold nanorods (GNRs) stabilized by cetyltrimethylammonium bromide (CTAB) allow for morphological control of silica (SiO2) shells deposited onto the GNRs. The concentration of TEOS in the TEOS/MeOH mixture determines whether the SiO2 shell uniformly coats whole GNRs or forms lobes on the ends of the GNRs. Changes in the optical absorbance spectrum of SiO2-coated GNRs (SiO2-GNRs) after purification with MeOH suggest CTAB can be removed by dissolution through the porous SiO2 shells. The size of the SiO2 lobes can be controlled, but there is a minimum lobe size, below which full encapsulation is favored. The following mechanism of lobe formation is proposed: Initially, a SiO2 shell fully encapsulates the CTAB-stabilized GNR core. Under optimized reaction conditions, determined by the MeOH concentration, the SiO2 shells can reshape into lobes, which requires sufficient solubility of SiO2 and damage or modification of the CTAB coating on the GNRs. SiO2 lobes can be more thermodynamically stable than uniform shells because they reduce the surface energy. Mechanistic insights gained in this study may be applicable to related core materials and the deposition of other kinds of oxides.

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INTRODUCTION

with molecular cargo and functionalized for different kinds of surface chemistry, including click chemistry reactions.8,11−13 There is significant interest in overcoating functional NPs with SiO2, and methods have already been established for overcoating noble metal, semiconductor, and magnetic NPs.14−32 SiO2 shells can improve the stability of NPs and protect them from degradation in harsh thermal and chemical environments, such as high or low pH and high electrolyte concentration.33−35 SiO2 shells also improve the biocompatibility of some kinds of NPs stabilized by cationic surfactants, such as gold nanorods (GNRs), by trapping or providing a barrier between the cationic surfactant and the environment.36−39 Anisotropically shaped core NPs have presented both challenges and opportunities for morphological control of SiO2 overcoatings. SiO2 deposited from tetraethyl orthosilicate (TEOS) onto GNRs stabilized by cetyltrimethylammonium bromide (CTAB) has been reported as both uniform shells12,20 and lobes on the ends of the GNRs.40,41 In a related study, SiO2 lobes were deposited selectively onto the sharp ends of Au nano-bipyramids.42 CTAB has been used to template and guide deposition of SiO2 shells onto many kinds of NPs and also serves as a template for mesoporous SiO2.43 Another

Janus and patchy nanoparticles (NPs) have regions with distinct surface chemistry and are of interest for spatially controlled functionalization and for directing self-assembly. The patches can be functionalized with different chemistry from the underlying core material to impart anisotropic properties.1,2 For example, such dual functionalization can allow NPs to behave as surfactants, if they have hydrophobic and hydrophilic regions.1,2 Similarly, if the patches are oppositely charged, application of an electric field can drive assembly or alignment of patchy NPs.1 This concept can be further extended to complex NPs with patches that respond locally to different kinds of physical and chemical stimuli (e.g., optical, magnetic, catalytic, or pH).3 Silica (SiO2) is of special interest for deposition in patches on inorganic NPs because silane chemistry for depositing SiO2 and for subsequent surface functionalization is well established, and SiO2 is biocompatible. Preceding interest in patchy NPs, the development of the Stöber method in 1968 has inspired half a century of research on the synthesis, manipulation, and applications of silica (SiO2) NPs.4 Addition of cationic surfactants is a common modification of the Stöber method for synthesizing sub-micrometer, mesoporous SiO2 structures with variable pore sizes.5,6 The porosity and surface area of such structures allow for their application in catalysis, biosensing, and theranostics.6−10 Porous SiO2 can be loaded © 2018 American Chemical Society

Received: February 22, 2018 Revised: July 12, 2018 Published: September 12, 2018 6249

DOI: 10.1021/acs.chemmater.8b00794 Chem. Mater. 2018, 30, 6249−6258

Article

Chemistry of Materials

Synthesis and Purification of GNRs. CTAB-GNRs were synthesized using a previously reported method that yields 1.064 L of solution containing approximately 190 mg of GNRs with an aspect ratio of ∼4 (∼80 nm × ∼20 nm) and longitudinal surface plasmon resonances (LSPRs) of ∼800−830 nm.55 Prior to purification, the CTAB concentration was 0.088 M. For each synthesis of SiO2-GNRs, 100 mL of this stock solution was concentrated to 10 mL via a twostep centrifugation process (IEC Centra MP4 with 854 rotor) to bring the final CTAB concentration within a certain range. In the first step, the CTAB-GNR stock solution was divided into aliquots of 40 mL per centrifuge tube (Nalgene Oak Ridge 3119) and centrifuged at 14000g for 20 min. After centrifugation, the nearly colorless, aqueous CTAB supernatant was removed by pipetting, reducing the volume remaining in each tube to 1.5−2.0 mL. Deionized H2O was then added to bring the volume in each tube back to the initial volume of 40 mL, thus diluting the CTAB by a factor of 20−27, while maintaining the same mass of CTAB-GNRs. Adjusting the CTAB concentration in this manner is important for avoiding agglomeration of the GNRs, which occurs if too much CTAB is removed. In the second step, centrifugation was repeated at 14000g for 20 min, and the colorless supernatant was again removed, leaving 1.5−2.0 mL remaining in each tube. Deionized water was added to bring the final volume in each tube to 4 mL, effectively increasing the GNR concentration by a factor of 10 and giving a final CTAB concentration of 1.2−2.2 mM. These values are slightly above the critical micelle concentration of CTAB, which has been reported as 0.90−0.98 mM in water.56 Purification of 100 mL of CTAB-GNR stock solution yielded 10 mL of concentrated CTAB-GNRs. Synthesis and Purification of SiO2-GNRs. Each SiO2 overcoating reaction was conducted using 10 mL of concentrated CTABGNRs, which was kept in a water bath at 29 °C to avoid CTAB crystallization and stirred at 150 rpm. The pH was adjusted to 10.4− 10.5 by adding 0.1 M aqueous NaOH dropwise. TEOS and anhydrous MeOH were quickly and thoroughly mixed to form solutions of different TEOS concentration by volume (specified in Table 1) for injection into the CTAB-GNR reaction mixture. In many instances, a larger volume of the TEOS/MeOH solution was prepared to reduce measurement errors when working with small volumes. The TEOS/MeOH solution was immediately injected at a constant rate into the stirring CTAB-GNR solution via syringe pump over a period of 5 min. In most of the reactions, the amount of TEOS/MeOH

common approach for depositing SiO2 onto NPs with hydrophobic coatings uses nonionic surfactants to form reverse microemulsions, which serve as nanoreactors for SiO 2 deposition. For example, morphological control of SiO2 shells on CdSe/CdS quantum dot nanorods and tetrapods has been reported.28,44−46 Lobed SiO2-GNRs are of interest for spatially controlled functionalization with silanes on the SiO2 lobes and with thiols on the sides with exposed Au. Such spatial control is potentially useful for labeling with dye molecules for fluorescence enhancement47 or molecular tags for surfaceenhanced Raman scattering (SERS)48,49 for fundamental studies of how placement near to or away from the hot spot with the highest field enhancement on the ends of the GNRs affects the emission or enhancement of the Raman signal. Lobed SiO2-GNRs could also potentially be useful for colloidal polymerization,50 where use of silane cross-linkers could favor end-to-end coupling with hot spots for SERS in the gaps between particles. Here, we report a method for controlling the morphology of SiO2-overcoated GNRs (SiO2-GNRs). The concentration of alcohol in the reaction mixture determines whether the SiO2 shell fully encapsulates the CTAB-stabilized GNR (CTABGNR) core or whether patchy lobes of SiO2 form on the ends of the CTAB-GNRs, leaving the Au core exposed on the sides. Intermediate structures are also reported, and the size of the lobes can be adjusted. Similar methods have been reported for depositing SiO2 and TiO2 lobes onto CTAB-GNRs, but insights into the mechanism of morphological control were limited.27,40−42,51 A related study reported anisotropic deposition of periodic mesoporous organosilica onto CTABGNRs. 52 This report addresses a significant need to comprehensively understand the levers that control the morphology of SiO2 deposition and the underlying mechanism. It is important to contrast this work, in which CTAB was the only surfactant on the GNR core, from other studies that used heterogeneous ligand monolayers for spatial templating of SiO2 patches.40,53,54 Many variables affect deposition of SiO2 using sol−gel chemistry. For example, temperature, pH, and water and alcohol content all affect the hydrolysis and condensation of alkoxysilanes.6 Varying the concentrations of TEOS, methanol (MeOH), and GNRs yields significant mechanistic insights into morphological control of the deposited SiO2. Additional experiments also highlight the special role of MeOH. We identify the MeOH concentration as the key lever for controlling the morphology of SiO2-GNRs, which allows or hinders reshaping of uniform SiO2 shells into lobed structures at early stages in the reaction.

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Table 1. Reaction Conditions for Synthesizing SiO2-GNRs Figure

TEOS/MeOH solutiona VTEOS/VMeOH (μL)

vol % TEOS in MeOH

vol % MeOH

1a 1b 1c 1d 1e 1f 1g 1h 3a 3b 3c 4a 4b 4c 4d 5a 5b

50/0 50/50 50/200 50/450 50/664 50/950 50/1617 50/4950 50/950 50/950 50/950 10/190 25/475 10/990 25/975 50/950 50/950

100 50 20 10 7 5 3 1 5 5 5 5 5 1 2.5 5 5

0 0.5 2.0 4.3 6.2 8.6 13.9 33.0 8.6 8.6 8.6 1.8 4.5 9.0 8.9 8.6 8.6

5c

50/950

5

8.6

EXPERIMENTAL SECTION

Chemicals. CTAB (Sigma-Aldrich, 99%, H6269 and VWR Amresco, high purity), KBr (Alfa Aesar, ACS, 99% min), AgNO3 (Alfa Aesar, 99.9995%), HAuCl4·xH2O (Alfa Aesar, 99.999%, where x was estimated as 3), deionized water (Ricca, ACS Reagent grade, ASTM Type I, ASTM Type II), ascorbic acid (AA, J.T. Baker, 99.5%), and NaBH4 (Sigma-Aldrich, 99%, 213462) were used for GNR synthesis. TEOS (Alfa Aesar, 99.9%), NaOH (Sigma-Aldrich 98%), 2[methoxy(polyethyleneoxy)9−12propyl]trimethoxysilane (PEG-silane, Gelest, 90%), and anhydrous MeOH (EMD, DriSolv) were used for the SiO2 overcoating. MeOH (Macron, ChromAR) was used for purification of SiO2-GNRs and PEG-functionalized SiO2-GNRs. Tetrahydrofuran (THF, EMD, OmniSolv, Non-UV) was used instead of MeOH for synthesizing selected samples.

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additional parameters

0.5 × GNRs 1.5 × GNRs 2.0 × GNRs

PEG-silane preadded MeOH THF in place of MeOH

All syntheses included 10 mL of deionized water. DOI: 10.1021/acs.chemmater.8b00794 Chem. Mater. 2018, 30, 6249−6258

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Chemistry of Materials solution added corresponded to 50 μL of neat TEOS. For the samples in which 50 μL of neat TEOS was added, aliquots of 10 μL were manually injected every minute over a period of 5 min. Upon completing the injection, stirring was continued at a reduced rate of 70 rpm for 30 min and then turned off, while the mixture was left unperturbed in the water bath at 29 °C for an additional 20 h. For selected samples, the effects of modifications (specified in Table 1) to this general method were investigated, including varying the CTAB-GNR concentration and amount of TEOS, adding PEGsilane to terminate the reaction early, preadding the MeOH to the CTAB-GNR solution before adding TEOS, and substituting THF in place of MeOH. For one sample, SiO2 shell growth was terminated early by injecting 40 μL of neat PEG-silane 30 min after completing injection of the TEOS/MeOH solution, followed by stirring at 70 rpm for an additional 30 min, and then allowing the reaction solution to sit unperturbed in the water bath at 29 °C for 20 h. In another experiment, MeOH was preadded to the concentrated CTAB-GNR solution and allowed to stir for 5 min, followed by manual injection of 50 μL of neat TEOS over a period of 5 min. After the 20 h reaction period, the SiO2-GNRs were removed from the water bath, transferred to a centrifuge tube, and washed four times with MeOH by filling the remaining volume in the 40 mL centrifuge tube with MeOH and centrifuging at 8400g for 10 min. Between centrifugation cycles, the supernatant was removed as completely as possible by pipet without breaking up the pellet of SiO2-GNRs before adding more MeOH and then briefly sonicating to redisperse the SiO2-GNRs. While each sample presented here was washed and centrifuged four times, in some related samples not reported, agglomeration occurred prior to four purification cycles. For such samples, monitoring by optical absorbance spectroscopy between cycles is helpful for distinguishing the onset of agglomeration. Termination of SiO2 deposition using PEG-silane generally improved the stability during multiple cycles of centrifugation. After the last centrifugation cycle, the SiO2-GNRs were redispersed for storage by sonication in approximately 1 mL of the supernatant remaining in the centrifuge tube. Characterization. The SiO2-GNR products were imaged by transmission electron microscopy (TEM) using a JEOL 2000FX microscope operated at an accelerating voltage of 200 kV. Optical absorbance spectra were acquired using an Ocean Optics CHEMUSB4-VIS-NIR spectrometer and a glass cuvette with a 1 cm path length. All optical absorbance spectra were normalized at 400 nm to the CTAB-stabilized GNRs.

Figure 1. TEM images of SiO2-GNRs prepared using different concentrations of 50 μL of TEOS prediluted in anhydrous MeOH. The scale bar in (a) applies to all panels except the inset in (f), showing a lobed SiO2-GNR at higher magnification.

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RESULTS AND DISCUSSION Morphological Control of SiO2 Shells. CTAB-GNRs employed for this study had approximate dimensions of 80 nm × 20 nm, with minor variations among batches. Adjusting the ratio of TEOS/MeOH added (Table 1) results in SiO2-GNRs with different SiO2 shell morphologies (Figure 1). Completely omitting MeOH, giving neat TEOS, yields fully encapsulated SiO2-GNRs (Figure 1a). Dissolving TEOS in alcohols is a common practice in the aqueous synthesis of SiO2 because TEOS is not soluble in water, causing phase separation at the beginning of the reaction. The alcohol produced from the initial stages of hydrolysis of alkoxysilanes, however, is adequate to fully dissolve the TEOS.57 Therefore, fully encapsulated SiO2-GNRs can be obtained without dissolving TEOS in MeOH. Adding TEOS at concentrations of 50, 20, and 10 vol % in MeOH also yields uniform SiO2 shells (Figure 1b−d). When the concentration is reduced to 7 vol % TEOS, nonuniform SiO2 shells form, resembling peanuts (Figure 1e). Further dilution to 5 vol % TEOS causes formation of distinct SiO2 lobes at both ends of the GNRs (Figure 1f). When the TEOS concentration is further reduced to 3 and 1 vol %, the lobed morphology is lost, and a reversion back to full

encapsulation is observed, albeit with less uniform SiO2 surfaces (Figure 1g,h). Optical Absorbance Spectra. The optical properties of the GNRs were preserved during SiO2 overcoating. Optical absorbance spectra were acquired before and after SiO2 overcoating and before and after purification (Figure 2 and Supporting Information, Figures S1−S4). It should be noted that the optical absorbance is the same as the extinction and was obtained from Beer’s law. Both absorption and scattering processes contribute to the optical absorbance,58 but absorption is expected to be the predominant mechanism of extinction for this size of GNRs.59 The optical absorbance spectra of all SiO2-GNRs, irrespective of the morphology, are red-shifted upon overcoating. This red-shift, which is already known for uniformly coated SiO2-GNRs, is attributed to the higher refractive index of SiO2 than water (1.33).14,19,27,60 The refractive index of porous SiO2 has been reported to range from 1.28 to 1.45 and scales with its density.57,61,62 Therefore, red-shifts after depositing mesoporous SiO2 shells suggest the refractive index of the SiO2 shells is between 1.33 and 1.45. The LSPR of GNRs is highly sensitive to the local dielectric environment.63−65 Increasing or decreasing the refractive index 6251


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the GNRs that templates deposition of SiO2 and remains between the Au core and the SiO2 shell. CTAB readily dissolves in MeOH, and the porous SiO2 structure is expected to allow MeOH to access and dissolve the CTAB coating, which is transported through the porous structure and into solution. CTAB is commonly dissolved from mesoporous silica,68,69 and molecular transport through mesoporous SiO2 is well established.70,71 The porous SiO2 shells of SiO2-GNRs have been loaded with other molecules, such as for drug delivery.72−76 These observations are consistent with both our previous results of a red-shift for unpurified SiO2-GNRs27 and a report by Murphy and co-workers of a minimal red-shift for SiO2GNRs relative to CTAB-GNRs after purification with alcohols.12 CTAB removal during purification of SiO2-GNRs is expected to cause a blue-shift because organic coatings or adsorbates on GNRs generally cause a red-shift due to their higher refractive index than that of water and MeOH.65 While we have not quantified the extent of CTAB removal, the magnitude of the blue-shift suggests substantial CTAB removal. Future work will include quantifying CTAB removal, which is important for biomedical applications because of its toxicity.77 Adjusting the SiO2 Lobe Size. SiO2 lobes of different sizes would offer SiO2 and Au patches of different sizes for functionalization with silanes and thiols, respectively. The SiO2 lobe size can be controlled by adjusting the concentration of CTAB-GNRs in the reaction solution, thereby varying the amount of TEOS per GNR, if the amount of TEOS (5 vol %) is held fixed (Figure 3). Halving the GNR concentration should result in deposition of twice as much SiO2 per GNR, as compared with the standard reaction procedure (Figure 1f), yielding larger SiO2 lobes. This prediction is confirmed by the larger SiO2 lobes that appear to have merged into each other (Figure 3a) when using half the concentration of CTABGNRs. Increasing the GNR concentration by 50% similarly yielded smaller SiO2 lobes due to the reduced amount of TEOS per GNR (Figure 3b). Further increasing the GNR concentration to twice the initial concentration, however, resulted in fully encapsulated SiO2-GNRs (Figure 3c) rather than even smaller lobes. This result suggests SiO2 lobes below a certain size are not stable because thin shells that fully encapsulate the CTAB-GNRs are favored, when depositing smaller amounts of SiO2. The SiO2 lobes can occasionally come off of the CTABGNR core, as shown in the bottom right of Figure 3b. This is a rare occurrence, consistent with the absence of GNRs with

Figure 2. Optical absorbance spectra for CTAB-GNRs in water and SiO2-GNRs before and after purification in MeOH for samples prepared using (a) 20 vol % TEOS, uniform SiO2 shell, and (b) 5 vol % TEOS, lobed SiO2 shell.

of the coating on GNRs or solvent in which they are dispersed causes a red-shift or blue-shift, respectively, in the LSPR. The magnitude of this shift scales with the change in refractive index.64,66 While red-shifts were consistently observed after depositing the SiO2 shells or lobes, purifying the SiO2-GNRs in MeOH caused a blue-shift in the LSPR, bringing the LSPR near to that of CTAB-GNRs without SiO2 shells (Figure 2 and Supporting Information, Table S1, where the maximum absorbance wavelengths are tabulated for CTAB-GNRs and SiO2-GNRs both before and after purification). Water and MeOH have nearly identical refractive indices, which would be expected to result in a negligible shift in the LSPR when varying the proportions of water and MeOH.67 The blue-shift observed after purification therefore suggests additional chemical modification of SiO2-GNRs occurs during purification. We attribute this blue-shift to dissolution of the CTAB coating on

Figure 3. TEM images of SiO2-GNRs prepared using the same volume of 5 vol % TEOS and different concentrations of CTAB-GNRs, where 1× GNRs corresponds to the standard concentration of 1.9 mg/mL (Figure 1f). The scale bar in (a) applies to all panels. 6252


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experiments all yielded fully encapsulated SiO2-GNRs, which suggests a minimum amount of TEOS exceeding 25 μL is required for forming lobes. This is also consistent with the fully encapsulated SiO2-GNRs that resulted when the concentration of GNRs was doubled, which halved the amount of TEOS per GNR (Figure 3c). Additional Experiments To Investigate Lobing. The results reported and discussed thus far suggest that a final concentration of ∼9 vol % MeOH in the reaction mixture after injecting the TEOS/MeOH solution and a sufficiently large amount of TEOS are necessary conditions for obtaining deposition of SiO2 in lobes (Figures 1f and 3a,b). Before proposing a mechanism in the next section, we present three experiments to provide insights into how the deposited SiO2 evolves over time and the role of the solvent:

single SiO2 lobes. The chemical stability of lobed SiO2-GNRs is similar to those with SiO2 shells that fully encapsulate the GNRs; both types remain dispersed in MeOH indefinitely. Additional experiments were conducted to investigate the transition between small lobes and uniform shells by reducing the amount of TEOS (from 50 μL to 25 or 10 μL) added in two different ways. In one case, the concentration of the TEOS/MeOH solution was held at 5 vol % TEOS, which is the standard protocol, and different volumes were injected (Figure 4a,b). Consequently, the amount of MeOH added

(1) Does the deposited SiO2 nucleate as lobes, or does it first completely encapsulate the GNR, followed by reshaping into lobes? PEG-silane was used to terminate SiO2 shell growth, as described previously.27 When terminating the shell growth after 30 min, rough surfaces on tapered SiO2 shells were observed, where the shells were thicker on the ends of the GNRs (Figure 5a). This result suggests SiO2 is initially deposited everywhere and then reshapes into lobes. (2) Is simultaneous addition of TEOS and MeOH by adding a TEOS/MeOH mixture required for lobe formation? Do SiO2 lobes still result, if the MeOH is added to the CTAB-GNRs before adding TEOS? Preadding the same amount of MeOH (950 μL) to the GNRs as in the 5 vol % TEOS solution that gives lobes and then adding 50 μL of neat TEOS yields fully encapsulated SiO2-GNRs (Figure 5b). Therefore, premixing and simultaneously injecting TEOS and MeOH are required for lobe formation. (3) Does the chemical nature of MeOH affect lobing? 950 μL of THF was used as a substitute for MeOH, giving a 5 vol % TEOS injection solution that would otherwise result in SiO2 lobes. This substitution yielded fully encapsulated SiO2-GNRs.

Figure 4. TEM images of SiO2-GNRs prepared using reduced amounts of TEOS at different concentrations prediluted in anhydrous MeOH. The scale bar in (a) applies to all panels.

scaled with the amount of TEOS added. In the second case, the total MeOH concentration in the reaction mixture after adding the TEOS solution was held constant by adding 10 μL of TEOS at 1 vol % and 25 μL of TEOS at 2.5 vol %, which both yielded final MeOH concentrations near 9 vol % (Figure 4c,d). This final MeOH concentration was chosen because adding 1000 μL of a 5 vol % TEOS solution in the standard procedure for synthesizing lobed SiO2-GNRs (Figure 1f) gives a final MeOH concentration of 9 vol % (Table 1). These

Proposed Mechanism of Lobing. Sol−gel synthesis of mesoporous SiO2 from alkoxysilane precursors involves hydrolysis and condensation reactions. The forward reactions are most commonly discussed, but the reverse reactions can also have important effects.78,79 Depolymerization reactions (Scheme 1b,c) can occur, where siloxane bonds are cleaved, and SiO 2 dissolves back into solution. Water drives

Figure 5. TEM images of SiO2-GNRs prepared using 5 vol % TEOS under different reaction conditions: (a) injection of PEG-silane after 30 min to terminate the reaction, (b) preadding the MeOH to the CTAB-GNRs before adding neat TEOS, and (c) substituting THF for MeOH with predilution of TEOS in THF. The scale bar in (a) applies to all panels except the inset in (a), which shows a SiO2-GNR at higher magnification. 6253


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ripening processes. Increasing the alcohol concentration in water−alcohol mixtures generally decreases the solubility of SiO2.80,81 Therefore, at low MeOH concentrations (7−100 vol % TEOS, Figure 1a−e), the CTAB coating remains intact and serves as an adhesive to keep the SiO2 anchored to the GNR, thus inhibiting lobing despite significant SiO2 solubility. At 5 vol % TEOS (Figure 1f), we propose MeOH has begun to partially dissolve the CTAB coating, which reduces its strength for adhering SiO2. The SiO2 solubility is also sufficiently high to allow ripening, causing reshaping into lobes to reduce the surface energy. At higher MeOH concentrations (1 and 3 vol % TEOS, Figure 1g,h), the decreased solubility of SiO2 inhibits reshaping of the SiO2 shell into lobes. The irregularities in these SiO2 shells might arise from regions of greater or lesser damage to the CTAB coating. This proposed mechanism for reshaping of SiO2 is consistent with other results. At the early stage of SiO2 deposition, before complete condensation, SiO2 is capable of reshaping.82 Base catalysis of SiO2 deposition also increases the rate of dissolution.81 Gas chromatographic and 29Si NMR studies have shown that unhydrolyzed monomers exist past the gel point for base-catalyzed silicate reactions, even in solutions of excess water, which proves reverse reactions occur.57,83,84 An alternative, less plausible mechanism is formation of SiO2 lobes driven by nucleation directly onto the ends of the CTABGNRs because the CTAB coatings are less stable on the ends. In our observations and proposed mechanism, the MeOH concentration is the key lever for determining the occurrence of lobes, which are obtained only for sufficiently high amounts of TEOS and at ∼9 vol % MeOH in the reaction mixture. It is not clear how this observation could be explained using a direct nucleation model. Our results further support reshaping of SiO2 shells that initially fully encapsulate CTAB-GNRs rather than direct nucleation of SiO2 lobes. Terminating deposition of SiO2 by adding PEG-silane (Figure 5a) shows that reshaping begins quickly, since tapered, lobed SiO2-GNRs were observed 30 min after starting the reaction. Such restructuring of thin SiO2 shells into spheres has also been observed for CdSe/CdS core/shell tetrapods.44 Preadding MeOH to the GNR solution (Figure 5b) inhibits lobe formation, even though 50 μL of TEOS was added and there was 8.6 vol % of MeOH in the reaction mixturethe same conditions that would otherwise give lobes (Figure 1f). We attribute this different result to the higher starting concentration of MeOH, which is expected to reduce the solubility of SiO2 and could inhibit ripening to form lobes, than when the MeOH is added along with the TEOS. Another possible cause of the difference when preadding MeOH is that more damage to the CTAB coating may occur sooner when MeOH is preadded. The lack of lobing when using THF in place of MeOH (Figure 5c) highlights the role of MeOH in facilitating reshaping of the SiO2; without reshaping, fully encapsulated SiO2-GNRs are obtained. There are, however, some minor indentations in the shells on the sides of the SiO2GNRs. This suggests some SiO2 was pulled toward the ends, as when lobes form, but this process did not significantly proceed toward forming distinct lobes. Indeed, many of the fully encapsulated SiO2-GNRs that we report show similar small indentations. Therefore, we believe MeOH concentration is the primary lever for controlling lobe formation, but removing MeOH does not completely inhibit SiO2 rearrangement. For the synthesis of SiO2-GNRs using 5 vol % TEOS in MeOH, large lobes were achieved by reducing the concen-

Scheme 1. Sol−Gel Reactions for Deposition of SiO2 from TEOS (Adapted from Brinker57)

hydrolysis(2) reactions, and alcohols drive alcoholysis.57 In this system, hydrolysis(2) likely drives depolymerization more strongly than alcoholysis because basic pH favors hydrolysis(2),57 and the concentration of water is higher than of MeOH. Successive polymerization, depolymerization, and repolymerization constitute ripening, which would drive the SiO2-GNRs toward the most thermodynamically stable, equilibrium morphology, rather than kinetically trapped structures. Aprotic species, such as THF, cannot participate in reverse reactions and are comparatively inert in sol−gel reactions.57 Which morphology, full encapsulation versus lobes, is more thermodynamically stable, can be understood by considering the surface energies of the SiO2 network, the SiO2−CTAB interface, and the exposed CTAB coating. For small amounts of SiO2, full encapsulation is always favored, presumably because of an attractive electrostatic SiO2−CTAB interaction. Forming small lobes would leave a large exterior surface area of the CTAB coating exposed, which is avoided with full encapsulation. As the amount of SiO2 is increased, however, the −Si−O−Si− network in the SiO2 has a greater overall contribution to the energy. Forming lobes can reduce the exterior surface area of SiO2 compared with uniform shells containing the same volume of SiO 2 . The energetic stabilization in the SiO2 from forming lobes comes at a cost of exposing part of the CTAB-GNR core, but the total free energy is apparently lower than for a uniform SiO2 shell. The reaction that was terminated by adding PEG-silane after 30 min suggests that SiO2 is deposited first as a uniform SiO2 shell, which then reshapes into lobes, but only under conditions that allow reshaping. Because the amount of MeOH used to dilute the TEOS determines whether reshaping into lobes occurs, we propose two roles for MeOH in the lobing process, summarized in Scheme 2: The amount of MeOH modulates (1) the interactions between SiO2 and the CTAB coating on GNRs and (2) the solubility of SiO2 in the mixed solvent during Scheme 2. Cartoon Depicting Proposed Mechanism of Lobe Formation Facilitated by MeOH

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concentration would tend to stabilize the CTAB coating and thereby counteract the effects of adding a solvent that could dissolve CTAB. Therefore, adjusting the CTAB concentration might have a similar effect to adjusting the amount of added solvent. When using different solvents to dissolve CTAB, some mixtures with water in different proportions might be expected to have similar effects because the solubility of CTAB will differ for each solvent. For example, here, low concentrations of THF do not cause lobing, but this does not preclude the possibility of lobing using different amounts of THF. When using CTAB as a lever, we further suggest that specific effects may be expected above and below the critical micelle concentration (CMC). Use of concentrations below the CMC would be expected to destabilize the CTAB coatings, while concentrations above the CMC would drive partitioning of the SiO227 formed between deposition onto the GNRs and additional SiO2 NPs that are not attached to GNRs. Therefore, higher CTAB concentrations would reduce the amount of SiO2 deposited onto GNRs.

tration of CTAB-GNRs used in the reaction (Figure 3a). The lobes joined as they grew into each other but did not reshape into uniform SiO 2 shells despite the optimal MeOH concentration for reshaping. For this case of lobes that entirely encapsulate the CTAB-GNR, a uniform shell with a lower exterior surface area would reduce the surface energy. This result suggests reshaping into a thick, uniform shell was not possible and that reshaping into lobes occurs at an early stage in the reaction to form a template onto which more SiO2 is deposited. When the lobes grow into each other later in the reaction, a greater extent of condensation of the SiO2 shell and a lower concentration of unreacted monomers might inhibit reshaping. Comparison with Related Studies. It is important to consider the results and proposed mechanism among closely related work that is reported in three papers, which all involved addition of TEOS solutions to CTAB-stabilized Au NP cores dispersed in water. Two papers reported lobed SiO2-GNRs with quite similar structures.40,41 In the first report of lobed SiO2-GNRs, a dilute solution of TEOS in ethanol (EtOH) was added to aqueous GNRs, and the size of the lobes was adjusted by varying the amount of TEOS solution added. The transition between SiO2 lobes and complete encapsulation with SiO2 shells was not reported.40 In the second study of lobed SiO2GNRs, TEOS was diluted in MeOH, and dimethylformamide (DMF) was added as a lever to control the morphology.41 In both studies, formation of lobes of SiO2 was attributed to removal of CTAB from the ends of the GNRs, followed by nucleation of SiO2 onto the ends of the GNRs, but the mechanism was not specifically investigated.40,41 Nucleation of SiO2 onto the ends of GNRs is inconsistent with the ripening of SiO2 that we propose, but results using DMF or EtOH may not be directly comparable with our use of MeOH. Different reaction mechanisms under different reaction conditions are plausible. Another closely related study reported deposition of SiO2 lobes onto the pointed tips of Au nano-bipyramids.42 TEOS was diluted in EtOH, as described above,40 and the size of the lobes and morphology were controlled by varying the concentration of CTAB. Direct nucleation of SiO2 onto the ends of the Au nano-bipyramids was attributed to the weaker CTAB coatings at the sharp tips. Our approach is not directly comparable to these reports, and significant caution is warranted to avoid implying that a single mechanism applies universally. Particularly for the Au nano-bipyramids,42 direct nucleation of SiO2 lobes onto the sharp tips is highly plausible, and that study reported small SiO2 lobes that we were not able to achieve in this study of GNRs. On the basis of several experiments, we propose a mechanism involving ripening and reshaping of SiO2 shells on GNRs that initially encapsulate the entire GNR and then reshape into lobes. If both mechanisms are plausible, this suggests the mechanism is highly sensitive to the specific reaction conditions. A common aspect of our work and these previous studies,40−42 is the high sensitivity to the reaction conditions, specifically the solvent mixture and the CTAB concentration. These two levers are likely related. In general, adding solvents that dissolve CTAB may selectively expose the ends of GNRs to drive direct nucleation of SiO2 onto the ends, but these solvents might also damage the CTAB coating in a manner that would facilitate ripening of a SiO2 shell that fully encapsulates the GNRs into lobes. Increasing the CTAB

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CONCLUSIONS Lobed SiO2-GNRs with control over the lobe size have been obtained using a simple modification of an existing method for synthesizing fully encapsulated SiO2-GNRs. The following mechanism is proposed for lobe formation: Initially, SiO2 shells are deposited that fully encapsulate CTAB-GNRs. If an adequate amount of TEOS is dissolved at a concentration of 5 vol % in MeOH and added, such that the final MeOH concentration is near 9 vol %, then the SiO2 shells may reshape into lobes, which are more thermodynamically stable. Ripening into SiO2 lobes also requires sufficient solubility of SiO2 in the mixed solvent system and some damage or modification of the CTAB coating to reduce the strength of adhesion between the CTAB coating and the SiO2 shell. A blue-shift in the optical absorbance spectrum after purification with MeOH suggests MeOH dissolves CTAB away through the pores in the SiO2 shells of both lobed and fully encapsulated SiO2-GNRs. Lobed SiO2-GNRs are of interest as patchy NPs for applications in self-assembly and spatially controlled functionalization. The synthetic approach reported here may be applicable to related core materials and the deposition of other kinds of oxides.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b00794. Optical absorbance spectra for samples presented in Figures 1 and 3−5 and measurements of the wavelength at the absorbance maximum (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.B.T). ORCID

Joseph B. Tracy: 0000-0002-3358-3703 Notes

The authors declare no competing financial interest. 6255


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ACKNOWLEDGMENTS We acknowledge Jeffrey Brinker for helpful discussions. This research was supported by the National Science Foundation (DMR-1056653 and the Research Triangle MRSEC, DMR1121107). This work was performed in part at the Analytical Instrumentation Facility (AIF) at North Carolina State University, which is supported by the State of North Carolina and the National Science Foundation (ECCS-1542015). The AIF is a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), a site in the National Nanotechnology Coordinated Infrastructure (NNCI).

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