Method To Incorporate Anisotropic Semiconductor Nanocrystals of All

Feb 20, 2014 - This method yields a uniform silica shell, with thickness tunable from 3 to 17 ... This is shown to be essential for the formation of a...
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Method To Incorporate Anisotropic Semiconductor Nanocrystals of All Shapes in an Ultrathin and Uniform Silica Shell Eline M. Hutter,†,§ Francesca Pietra,†,§ Relinde J. A. van Dijk - Moes,† Dariusz Mitoraj,+ Johannes D. Meeldijk,# Celso de Mello Donegá,*,† and Daniel̈ Vanmaekelbergh*,† †

Condensed Matter and Interfaces, Debye Institute for Nanomaterials Science, Utrecht University, Princetonplein 1, 3584 CC Utrecht, The Netherlands + Faculty of Chemistry and Biochemistry, Ruhr University Bochum, Universitätstrasse 150, 44780 Bochum, Germany # Electron Microscopy Utrecht, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands S Supporting Information *

ABSTRACT: In this work, we present a method for the incorporation of anisotropic colloidal nanocrystals of many different shapes in silica in a highly controlled way. This method yields a uniform silica shell, with thickness tunable from 3 to 17 nm. The silica shell perfectly adapts to the shape of the nanocrystals, preserving their anisotropy, a crucial requisite for shape-dependent applications. Our method is based on an adaptation of the reverse microemulsion method. High control over the nucleation and growth of the shell is obtained by slowing down the hydrolysis and condensation rates of the silica precursor by lowering the ammonia concentration. This is shown to be essential for the formation of a uniform silica shell in the case of CdSe/CdS core/shell nanorods. Additionally, the general applicability of this method is demonstrated by coating different anisotropic semiconductor nanocrystals such as nanostars and 2D nanoplatelets. These results thus represent a crucial step toward the fabrication of highly processable and functionalized anisotropic nanoparticles.

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INTRODUCTION Colloidal semiconductor nanocrystals (NCs) show unique sizeand shape-dependent physical properties due to quantum and dielectric confinement.1−3 In the past years, new synthetic procedures have opened the possibility to design colloidal NCs with anisotropic shapes, such as one-dimensional colloidal semiconductor NCs, i.e. nanorods (NRs)4−6 and two-dimensional nanosheets and nanoplatelets (NPLs),7−12 characterized by a remarkable uniformity both in size and shape.13 A major drawback of colloidal semiconductors in applications such as fluorescent biolabels14 or light-emitting devices15 is their poor stability in water- and oxygen-rich environments. A possible strategy to deal with this problem is encapsulation in an inert shell that shields the materials both chemically and physically from the direct environment. In this respect, the incorporation of colloidal NCs in silica is highly interesting, because it increases their photochemical stability while the optical properties are preserved.16 Furthermore, the silica shell can easily be functionalized with organic ligands.17−21 This allows the NCs to be dispersed in both polar and nonpolar solvents, which largely increases their processability. In the past decades, extensive work has been done on the incorporation of spherical NCs (i.e., quantum dots (QDs)) in silica.16,17,19,22−31 The two main approaches to coat nanoparticles with a silica shell are the traditional Stö ber method32,33 and the so-called reverse microemulsion method.34−37 Although the Stöber method is © 2014 American Chemical Society

highly effective in growing silica shells around micrometer-sized colloids33,38 or metal nanoparticles,20 it does not yield uniform silica shells on single semiconductor NCs.39 The reverse microemulsion method however allows for the incorporation of individual QDs located exactly in the middle of silica spheres.17,34 In this approach, the silica shell grows around single QDs that are individually trapped inside an aqueous micelle of a water-in-oil (w/o) microemulsion, together with silica precursor molecules. The silica nucleation and growth are catalyzed by the basic environment resulting from the addition of an ammonia solution. The incorporation mechanism is explained in terms of a ligand exchange process, through which the hydrophobic capping molecules on the surface of the QDs are replaced by the silica precursor tetraethoxysilane (TEOS), prior to silica growth.17,36 Only recently, silica-coating with the reverse microemulsion method was successfully applied to anisotropic NCs like NRs40−42 and tetrapods.43,44 However, the incorporation in silica causes a drastic decrease in the particles’ aspect ratio as the shape changes into dumbbells and ellipsoids.40−42 Moreover, uniform shells thinner than 10 nm cannot be obtained by the currently available methods. Hence, a method to coat highly Received: December 16, 2013 Revised: February 12, 2014 Published: February 20, 2014 1905

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was quenched and washed by precipitation/redispersion with methanol and toluene and stored in a glovebox. Synthesis of CdSe NC Platelets. The CdSe nanoplatelets were synthesized according to Ithurria et al. (Protocol 1).9 Cd(Ac)2 (240 mg), oleic acid (285 μL), and octadecene (15 mL) were mixed in a three-neck flask and degassed at 80 °C under vacuum for 1 h. Then, the mixture was heated to 200 °C, and 150 μL of 1 M TOP-Se was injected. After 1 h the reaction was quenched, and CdSe NPLs were washed twice with a methanol/butanol mixture (1:3 by volume) and stored in 5 mL of hexane. Synthesis of CdSe/CdS Core/Shell NPLs. The CdSe/CdS core/ shell nanoplatelets were synthesized via layer-by-layer growth reported by Mahler et al.50 Therefore, 1.2 mL of the CdSe nanoplatelets solution was diluted in 1.2 mL of hexane, and 50 μL of bis(trimethylsilyl) sulfide (TMS2S) was added. After 1 h, the color changed from yellow to orange. The obtained NCs were washed twice with ethanol and redispersed in hexane. Then, 20 mg of Cd(Ac)2(H2O)2 was added, and another color shift from orange to red was observed as the mixture was sonicated for 10 min. The disaggregation was induced by addition of 200 μL of oleic acid. The NPLs were finally washed with ethanol and redissolved in hexane. Synthesis of PbSe Nanostars. The PbSe nanostars were synthesized by an adapted hot injection method from the literature.51,52 OA (1.6 mL), DPE (2.1 mL), TOP (8.35 mL), and Pb(Ac)2 (0.68 g) were mixed and heated to 150 °C under vacuum for 2 h to form Pb-(OA)2 complexes. Meanwhile, a four-neck flask was filled with 10 mL of DEG and heated to 120 °C under vacuum for 30 min. Next, the temperature was increased to 190 °C followed by the rapid injection of the Pb-(OA)2 precursor and TOP-Se (0.13 g of Se in 1.7 mL of TOP). The NCs were grown at 155 °C for 30 min, then quenched with a methanol/butanol mixture, and after sedimentation finally dispersed in toluene. Silica Coating of NCs. The CdSe/CdS core/shell NRs were coated with silica according to the method described by Pietra et al.41 The microemulsions were prepared by dispersion of 1.3 mL of NP-5 in 10 mL of cyclohexane, followed by the addition of 1.4 nmol NRs in toluene and afterward 80 μL of TEOS. The solutions were continuously stirred, and there were at least fifteen minutes between two consecutive additions. Finally, 150 μL of ammonia solution was added after which the solution was stirred for one more minute and then stored. The reactions were quenched by addition of ethanol followed by three sedimentation/redispersion cycles. The resulting silica-coated NRs were stored in ethanol. The encapsulation of CdSe/ CdS core/shell NPLs and PbSe nanostars in a silica shell was performed following the same protocol. In the case of NPLs, 400 μL of a solution in hexane was added (the OD was measured to be 0.2 at 400 nm, for a 80 times diluted solution). For the nanostars, we added 200 μL of a solution in toluene (the OD was measured to be 0.1 at 400 nm, for a 50 times diluted solution). Different ammonia solutions were prepared by dilution of a stock solution with water, resulting in a series of concentrations from 29.9 wt % to less than 0.6 wt %. The stock solution was stored at 7 °C to prevent ammonia loss by evaporation, and the dilutions were made immediately prior to use. OTMS Coating of NCs in Silica. To make the silica-coated NCs soluble in nonpolar solvents, these were functionalized with OTMS. Therefore, 1 mL of solution of OTMS in cyclohexane (10%, v/v) was added to the reverse microemulsion one week after ammonia (29.9 wt %) addition. One day later, the reaction was quenched with ethanol followed by sedimentation. The resulting particles were washed twice with a toluene/ethanol mixture and finally dispersed in toluene. Characterization. The NR concentration was estimated using inductively coupled plasma atomic emission spectroscopy (ICP-AES), in combination with absorption spectroscopy. Absorption spectra were measured using a Perkin-Elmer Lambda 950 UV/vis/IR absorption spectrophotometer. The purified NCs/silica nanoparticle samples were characterized with Transmission Electron Microscopy (TEM). Samples for analysis were obtained by drop casting the NCs/silica solution onto coated copper TEM grids at room temperature. Prior to extraction of the sample, the solution was sonicated for about 1 min in order to prevent

anisotropic NCs with a uniform and tunable silica shell that preserves the original shape is highly desirable, since the shape anisotropy is indispensable for certain applications, e.g. shapedirected self-assembly,45,46 antibody specificity for cellular uptake,47 and shape-dependent antibacterial activity.48 Here we present a versatile method that allows for high control over the silica growth on semiconductor NCs with different anisotropic shapes. This method leads to the formation of a uniform silica shell with tunable thickness that preserves the original shape anisotropy of the NCs. The shell thickness can be tuned between 3 and 17 nm. The starting point of our method is the reverse microemulsion in which hydrolysis and condensation rates of TEOS are varied. Since these rates depend on the basicity of the environment, we study in detail the role of the ammonia concentration on the silica nucleation and growth mechanism in the case of CdSe/CdS core/shell NRs. We show that the uniformity of the shell is notably dependent on the ammonia concentration. We present the ideal conditions to obtain ultrathin, uniform silica shells. In order to demonstrate that this approach works for all types of shapes, we incorporate highly anisotropic 2D NPLs as well as nanostars in silica.

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EXPERIMENTAL SECTION

Chemicals. Ammonia (29.9 wt % in water, Sigma-Aldrich, stored at 7 °C), bis(trimethylsilyl) sulfide (TMS2S, Sigma-Aldrich), cadmium acetate (Cd(Ac)2, Sigma-Aldrich, 99.99%), cadmium oxide, (CdO, Sigma-Aldrich, 99%), cadmium acetate dihydrate (Cd(Ac)2(H2O)2, Sigma-Aldrich), diphenylether (DPE, Sigma-Aldrich), diethylene glycol (DEG, Sigma-Aldrich), lead acetate (Pb(Ac)2, Sigma-Aldrich, 98%), octadecyl phosphonic acid (ODPA, Sigma-Aldrich, 97%), noctadecyltrimethoxysilane (OTMS, Fluorochem), oleic acid (OA, Sigma-Aldrich) poly(5)oxyethylene-4-nonylphenyl-ether (NP-5, IgePAL CO 520, Sigma-Aldrich), selenium (Strem Chemicals, 99.99%), sulfur (Alfa Aesar, 99%), tetraethyl orthosilicate (TEOS, SigmaAldrich, 99%), trioctylphosphine (TOP, Sigma-Aldrich, 90%), and trioctylphosphine oxide (TOPO, Sigma-Aldrich, 99%) were used for the synthesis of the nanoparticles (NPs). Solvents. Acetone (Merck, anhydrous), butanol (Sigma-Aldrich, anhydrous, 99.8%), cyclohexane (Sigma-Aldrich, anhydrous, 99%), ethanol (Alfa Aesar, anhydrous, 96%), hexane (Sigma-Aldrich, anhydrous, 99.8%), methanol (Sigma-Aldrich, anhydrous, 99.8%), octadecene (Sigma-Aldrich, 90%), and toluene (Sigma-Aldrich, anhydrous, 99.8%) were used as supplied. Synthesis of CdSe NC Seeds. The CdSe NC seeds were synthesized according to the hot injection method reported by Carbone et al.49 TOPO (3.0 g), ODPA (0.290 g), and CdO (0.060 g) were mixed in a 50 mL three-neck flask and heated to 150 °C under vacuum. After 2 h, the solution was heated to 330 °C under nitrogen until Cd-ODPA complexes were formed, indicated by transparency of the solution. Next, TOP (1.5 g) was injected followed by heating to 350−370 °C and injection of TOP-Se (0.058 g Se in 0.360 g TOP). The reaction was quenched by removal of the heating source, cooling down by blowing air toward the flask and finally addition of 5 mL toluene as the temperature dropped below 90 °C. The final size of the NCs depends on the reaction time: longer growth leads to larger NCs. The obtained CdSe NCs were washed three times with methanol, redispersed in toluene, and stored under nitrogen in a glovebox. Synthesis of CdSe/CdS Core/Shell NRs. The CdSe/CdS core/ shell NRs were synthesized using a seeded growth approach.49 TOPO (3.0 g), ODPA (0.330 g), and CdO (0.090 g) were heated to 150 °C in a 100 mL three-neck flask in a Schlenk-line under vacuum for 2 h. Afterward, the solution was heated to 350 °C to form Cd-ODPA complexes, and TOP (1.5 g) was injected. Once the temperature was stabilized, TOP-S (0.12 g S in 1.5 g TOP) and 200 μL CdSe NC seeds in TOP (400 μM) were rapidly injected. After 12 min, the reaction 1906

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agglomeration of the silica-coated particles on the grid. TEM images presented in Figure 1 were obtained with Tecnai microscope operating at 120 kV equipped with a tungsten filament. Images are recorded with a SIS Megaview II CCD-camera in iTEM software. Energy-dispersive X-ray Spectroscopy (EDS) measurements and Scanning Transmission Electron Microscopy (STEM) images are acquired on an FEI Tecnai20FEG instrument with a Fischione High Angle Annular Dark Field (HAADF) detector. The instrument is operated at 200 kV acceleration voltage. EDS-spectra are acquired with an EDAX detector using Tecnai Imaging and Analysis software (TIA).

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for the standard procedure, see Figure 1E. The silica spheres at the opposite tips enlarge by growth, which sometimes results in ellipsoid-shaped particles. After one week, some NRs are completely covered, while others remain dumbbell-shaped (see S1 for a detailed overview of the silica shell evolution over time). Interestingly, a drastic change is observed if the ammonia concentration is decreased to less than 1.5 wt % (see Figure 1A and B). Dumbbell-type particles are no longer observed, and the NRs are completely coated by a uniform ultrathin silica shell (3 nm after 4 h for 1.1 wt %). Since the silica is distributed equally over the surface of each NR, the original aspect ratio is only slightly reduced. Further growth results in a thicker shell with preservation of the original shape. For an ammonia concentration of 2.8 wt %, see Figure 1C, the obtained particles show a conformation intermediate to the dumbbell shape and the rod shape obtained with respectively higher and lower concentrations. The silica starts to grow on both ends of the NRs but appears less spherical and more elongated along the z-axis compared to the characteristic dumbbell-type particles. This is best visible in the early stage (4 h). The effect of the ammonia concentration on the resulting shell thickness was quantified by measuring the temporal evolution of the average shell thickness (Figure 2). The

RESULTS AND DISCUSSION

Influence of the Ammonia Concentration on the Silica Growth. CdSe/CdS core/shell NRs with length 48.5 ± 6.3 nm and diameter 6.2 ± 0.9 nm (according to TEM) were coated with silica by the reverse microemulsion method.39,41 In order to investigate the role of ammonia, we varied the concentration of ammonia in the aqueous phase from 29.9 wt % (standard procedure) to 0.6 wt %, while other parameters were kept constant. The silica-coated NRs were quenched at different stages of growth, i.e. in some cases before the Si-precursor was depleted. The resulting NPs were soluble in polar solvents such as water and ethanol or nonpolar solvents after functionalization with octadecyltrimethoxysilane (OTMS) (see the Experimental Section for details). The silica growth on the NRs was followed in time for several ammonia concentrations, as shown in Figure 1. Figure 1D shows that dumbbell-type structures with two spherical silica shells on the opposite tips are obtained if NRs are coated according to the standard procedure (29.9 wt % ammonia).41 If the ammonia concentration is decreased to 5.7 wt % the resulting particles evolve in a similar way as observed

Figure 2. Semilogarithmic plot of the silica shell thickness evolution over time for different concentrations of ammonia. Growth times of one day, one week, and two weeks are denoted by 1d, 1w, and 2w, respectively. The data points correspond to the samples displayed in Figures 1 and S1.

thickness was measured at the thickest point, perpendicular to the z-axis of the NR. In the early stages (up to 10 h), 29.9 wt % ammonia leads to significantly faster growth than the lower concentrations. The growth is nearly complete after one day, since the shell barely increases in size afterward (the curve reaches a plateau). After one day, an ammonia concentration of 2.8 wt % displays a shell thickness comparable to the standard procedure. On the other hand, for concentrations from 0.6 to 1.5 wt % no silica shell is visible on the NRs in the early stages and an ultrathin shell of 3 nm can be observed after 4 h of growth. The shells remain significantly thinner than those observed for the dumbbells up to 1 week of growth and display a comparable thickness only after two weeks. Note that, although the shell thicknesses are the same, the silica volume around the completely incorporated NRs is larger than the silica volume of the dumbbell-type particles. Importantly, another effect of lowering the ammonia concentration is that the number of self-nucleated silica

Figure 1. A to E: TEM overview of silica growth on CdSe/CdS NRs at different times for several microemulsions equally prepared but with different ammonia concentrations in the aqueous phase. Scale bars correspond to 50 nm. F: Single CdSe/CdS NR isolated after one day from a microemulsion with 1.5 wt % ammonia, scale bar corresponds to 10 nm. 1907

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Figure 3. Schematic representation of the silica growth on CdSe/CdS core/shell NRs under a low (left) and high (right) ammonia concentration. In both cases, the ligand exchange from ODPA to hydrolyzed TEOS (indicated here by the red arrow) starts at the most reactive facets. If the concentration of ammonia is high, this is rapidly followed by growth of silica spheres at the opposite tips. At sufficiently low ammonia concentrations, the ligands are completely exchanged before a uniform silica shell is formed at the entire surface of the NRs. Naturally, there is a gradual transition from the dumbbell- to the rod-type particles as the concentration of ammonia varies between the two extremes.

nanoparticles decreases, as shown in Figure 1. Whereas high ammonia concentrations produce a significant number of empty silica spheres (Figure 1D and E), low ammonia concentrations (Figure 1A to C) rarely lead to the selfnucleation of silica spheres. For the standard procedure, it has been reported that spontaneous nucleation is minimized upon increasing the concentration of NCs.39 This can be understood considering that once there are enough NCs to fill every micelle with a single particle, the silica will only grow around the NCs, and no self-nucleated silica spheres are formed. Interestingly, the same effect is observed in the case of lowering the ammonia concentration (less than 2.8 wt % in this case). Given that the concentration of surfactant molecules and NCs is the same in all microemulsions, we can assume that lowering the ammonia concentration reduces the self-nucleation of silica. High ammonia concentrations are frequently used to obtain spherical silica shells, as low ammonia concentrations yield irregular shapes.32,36,53,54 In the case of anisotropic NCs, reducing the tendency to grow silica into spheres facilitates the formation of uniform anisotropic shells. In order to understand these observations, we first need to consider the chemical processes involved in the nucleation of silica: hydrolysis and polycondensation of TEOS monomers. The first step is the hydrolysis of one to four (x) ethoxy groups of individual TEOS monomers:

In the second step, hydrolyzed TEOS monomers (poly)condensate forming covalent bonds, thereby producing water or ethanol. These condensation reactions involve the attack of a nucleophilic (deprotonated) silanol on a neutral species: =Si−OH + HO−Si= ↔ =Si−O−Si= + H 2O =Si−OH + H5C2O−Si= ↔ =Si−O−Si= + C2H5OH (3)

In our system, hydrolyzed TEOS monomers are either attached to the surface of the NRs or remain dissolved in the aqueous phase. Hence, condensation can occur both in solution and between species on the NR surface. Our results show that the silica shell growth on the NRs as well as the presence of selfnucleated silica are highly dependent on the ammonia concentration. We attribute both effects to a competition between the attachment of TEOS monomers to the NR surface (i.e., ligand exchange) and the polycondensation into a network, which ultimately results in colloidal particles.54,55 Both hydrolysis and condensation steps are base catalyzed by the hydroxide ions from the aqueous ammonia solution. The ammonia concentration thus determines the hydrolysis (eq 1) and condensation (eq 2 and 3) rates and therefore affects the balance between the rates of surface attachment and network formation. With 29.9 wt % ammonia, TEOS monomers are quickly completely hydrolyzed. Since these monomers are deprotonated due to the high pH value (pH ∼ 10.5, pKa of silicic acid: 9.8), rapid polycondensation (eq 2 and 3) leads to fast network formation.55 In the presence of a NR, hydrolyzed TEOS

Si(OC2H5)4 + x H 2O ↔ Si(OC2H5)4 − x (OH)x + xC2H5OH

(2)

(1) 1908

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monomers are attracted toward Cd-atoms on the NR surface and hence replace the original capping molecules, starting at the most reactive facets (i.e., the opposite tips).41 These two processes (network formation and surface attachment) occur simultaneously. Consequently, networks are formed both in empty micelles and at the tips of the NRs. After reaching a critical size these grow into silica spheres, leading to dumbbelltype structures as well as self-nucleated spheres (Figure 1D). The growth behavior remains unchanged for ammonia concentrations down to 5.7 wt % (pH > 9.8), i.e. dumbbellformation and self-nucleation (Figure 1E). As long as the TEOS monomers each have at least two hydrolyzed groups, the polycondensation into networks will be faster than TEOS attachment to the entire surface of the NRs. With less than 1.5 wt % ammonia, the hydrolysis is much slower, and therefore TEOS monomers will be only partly hydrolyzed. In this case, the hydrolysis is a rate-limiting step for the polycondensation into networks, because condensation upon removal of an ethoxy group is less favorable than removal of a hydroxyl group (due to steric factors). Consequently, the attachment of hydrolyzed TEOS monomers to Cd atoms on the NR surface becomes faster than the network formation. We expect that in this regime the original capping ligands are completely exchanged for TEOS monomers before stable networks are formed in solution. The polycondensation will then occur primarily at the surface of the NRs, leading to a uniform silica shell around the entire NR rather than silica spheres at the opposite tips and in empty micelles. The two extreme situations are represented schematically in Figure 3. To further support our model, we performed HAADFSTEM analysis combined with EDS (Figures S2 and S3). Figure S2 shows an image and corresponding EDS analysis on NRs isolated from a microemulsion with 1.1 wt % ammonia after two hours of growth. Although no silica layer could be observed, EDS analysis on a group of these particles confirmed the presence of silicon (Si) and phosphor (P) at the NRs. Thus, at this stage both TEOS monomers and the original capping ligand ODPA are on the surface, which means that the ligand exchange is still in progress. NRs isolated after one day from a microemulsion with 1.5 wt % are completely coated with silica, as shown in Figure 1F. EDS analysis on these particles confirmed that at this stage, ODPA is completely removed (Figure S3). The uniform silica shell adapts not only to the rodshape but also to the bulb originating from the CdSe core embedded in the CdS rod. Hence, these conditions ensure preservation of the original shape anisotropy. The formation of a uniform silica shell can be understood as a surface catalyzed process. To confirm that the uniformity is due to the ammonia concentration and independent of the amount of water, the experiment was repeated with 1.5 wt % ammonia and half the amount of water, which also resulted in thin, uniform shells (see Figure S4). Silica Coating of NCs with Different Shapes. To investigate the applicability of this method to incorporate differently shaped anisotropic semiconductor NCs, we performed similar experiments with nanostars and 2D NPLs. The 2D NPLs are not only highly anisotropic but also relatively large in their lateral dimensions (up to 150 nm). An ammonia concentration of 1.5 wt % was used to promote thin shell formation, and a standard synthesis using 29.9 wt % ammonia was carried out for comparison. Figure 4 shows TEM images of 2D CdSe/CdS NPLs before (Figure 4A) and after silica coating (Figure 4B and 4C). These

Figure 4. (A) TEM image of CdSe/CdS core/shell NPLs and (B) Bright Field TEM and (C) HAADF-STEM images of silica-coated NPLs with 1.5 wt % ammonia after one day. (D) Single silica-coated NPLs considered from different angles. All scale bars correspond to 50 nm.

NPLs were originally less than 2 nm in thickness (five monolayers CdSe and a single monolayer CdS) with 27.6 ± 8.3 nm by 123.6 ± 24.3 nm in their lateral dimensions. After silicacoating, most of the NPLs are upstanding and hence observed from the lateral side. The shell thickness varies from 3.69 ± 0.64 nm after 3 h of growth to more than 10 nm after one day. To confirm that the high intensity lines are upstanding NPLs, single particles were imaged from different angles by tilting the TEM grid with respect to the detector (Figure 4D). These images confirm that all facets are coated with silica. Additionally, HAADF-STEM images show that the shape of the silica shell perfectly adapts to the original proportions of the NPLs, as observed above for the NRs (see Figure 4C). EDS analysis on these NPLs confirms the presence of Cd, Se, S, and Si (see S5). The experiment was repeated with 29.9 wt % ammonia, which also led to the incorporation of NPLs in silica. However, less control over the final shape was obtained (see S6). To the best of our knowledge, we are the first to successfully incorporate 2D semiconductor NCs in a thin silica shell. Finally, NCs with even more complex shapes were successfully coated with a thin silica shell. Figure 5 shows

Figure 5. TEM images of PbSe nanostars prior to (A) and after silica coating, quenched after 2 h (B) and one day (C) of growth. All scale bars correspond to 50 nm.

PbSe nanostars before and after silica coating with 1.5 wt % ammonia. After two hours, a silica shell of less than 3 nm is formed that follows the original shape of the NCs (see Figure 5B). Naturally, this silica shell has a higher tendency to become spherical than the shells around one-dimensional NRs and twodimensional NPLs, since the original shape of the NCs is already more isotropic. Therefore, the silica coated nanostars become gradually more spherical as the silica shell grows thicker (see Figure 5). We expect that our approach is also applicable to obtain thin silica shells around isotropic QDs. General Method To Obtain Ultrathin Uniform Shells. Altogether, these results demonstrate that high control over the 1909

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Author Contributions

silica growth on anisotropic semiconductor NCs is obtained if the ammonia concentration in the reverse microemulsion is substantially decreased to 0.6−1.5 wt %, while all other parameters are kept constant. This method can be widely used to incorporate anisotropic semiconductor NCs with a variety of shapes in silica, provided the native ligands are exchangeable for TEOS. We propose that the following kinetic steps are essential for the formation of a uniform silica shell: (i) slow hydrolysis of TEOS and (ii) complete exchange of the original ligand for (hydrolyzed) TEOS prior to network formation. In this work, we control the thickness of the uniform shell by growth duration before quenching. In the case of CdSe/CdS core/shell NRs, silica shells of 3 nm were achieved after 4 h of growth. Naturally, the exact growth rates depend on the concentration of NC seeds/ TEOS and different parameters characteristic for each NC structure, e.g. the surface energy and the rate of ligand exchange.

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E.M. Hutter and F. Pietra have contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Alfons van Blaaderen for discussions and critical reading of the manuscript. Jantina Fokkema, Stephan Zevenhuizen, and Ward van der Stam are acknowledged for technical support and help with some experiments. The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/20072013)/ERC Grant Agreement n. [291667]. The authors acknowledge financial support from FOMNPS and NWO.

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CONCLUSION In this work, a versatile method to incorporate anisotropic semiconductor NCs in ultrathin, uniform silica shells is developed. This method allows for high control over the shape of the silica shell as well as its thickness, which can be tuned between 3 and 17 nm. The method is based on the reverse microemulsion method, in which a lower concentration of ammonia is used. The crucial role of ammonia was examined with a case study of CdSe/CdS core/shell NRs. We found that 0.6−1.5 wt % ammonia in the aqueous phase results in thin silica shells that perfectly adapt to the original shape of the NRs. This is explained in terms of a relatively slow rate of network formation of TEOS monomers, which allows for a complete ligand exchange on the entire surface of the NRs prior to the formation of a uniform silica shell. Our findings are not only consistent with the proposed silica incorporation mechanism but also enabled us to incorporate highly anisotropic 2D NPLs in silica. The versatility and applicability of the method is shown by the incorporation of zero-, one-, and two-dimensional anisotropic NCs in a thin, uniform silica shell. This method could therefore be widely used to coat anisotropic NCs with a highly controlled silica shell. Additionally, since the key step for the growth of a silica shell is the exchange of the original capping ligand with the silica precursor (TEOS), the possibility to grow a silica shell on specific NCs depends on the surface chemistry of the material and the binding energy of the ligand with the surface of the NCs. Therefore, we expect this method to work on other type of semiconductor materials as well as metals or oxides, as long as the original capping ligand is exchangeable toward hydrolyzed TEOS.

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ASSOCIATED CONTENT

S Supporting Information *

Additional TEM images and EDS data for CdSe/CdS NRs and NPLs. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: (D.V.) [email protected]. *E-mail: (C.d.M.G.) [email protected]. 1910

dx.doi.org/10.1021/cm404122f | Chem. Mater. 2014, 26, 1905−1911

Chemistry of Materials

Article

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dx.doi.org/10.1021/cm404122f | Chem. Mater. 2014, 26, 1905−1911