Method To Incorporate Anisotropic Semiconductor Nanocrystals ofAll Shapes in an Ultrathin and Uniform Silica ShellEline M. Hutter,†,§ Francesca Pietra,†,§ Relinde J. A. van Dijk - Moes,† Dariusz Mitoraj,+

Johannes D. Meeldijk,# Celso de Mello Donega,*,† and Daniel Vanmaekelbergh*,†

†Condensed Matter and Interfaces, Debye Institute for Nanomaterials Science, Utrecht University, Princetonplein 1, 3584 CCUtrecht, The Netherlands+Faculty of Chemistry and Biochemistry, Ruhr University Bochum, Universitatstrasse 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 theincorporation of anisotropic colloidal nanocrystals of manydifferent shapes in silica in a highly controlled way. This methodyields a uniform silica shell, with thickness tunable from 3 to 17nm. The silica shell perfectly adapts to the shape of thenanocrystals, preserving their anisotropy, a crucial requisite forshape-dependent applications. Our method is based on anadaptation of the reverse microemulsion method. High controlover the nucleation and growth of the shell is obtained by slowingdown the hydrolysis and condensation rates of the silica precursorby lowering the ammonia concentration. This is shown to be essential for the formation of a uniform silica shell in the case ofCdSe/CdS core/shell nanorods. Additionally, the general applicability of this method is demonstrated by coating differentanisotropic semiconductor nanocrystals such as nanostars and 2D nanoplatelets. These results thus represent a crucial steptoward the fabrication of highly processable and functionalized anisotropic nanoparticles.

■ INTRODUCTION

Colloidal semiconductor nanocrystals (NCs) show unique size-and shape-dependent physical properties due to quantum anddielectric confinement.1−3 In the past years, new syntheticprocedures have opened the possibility to design colloidal NCswith anisotropic shapes, such as one-dimensional colloidalsemiconductor NCs, i.e. nanorods (NRs)4−6 and two-dimen-sional nanosheets and nanoplatelets (NPLs),7−12 characterizedby a remarkable uniformity both in size and shape.13 A majordrawback of colloidal semiconductors in applications such asfluorescent biolabels14 or light-emitting devices15 is their poorstability in water- and oxygen-rich environments. A possiblestrategy to deal with this problem is encapsulation in an inertshell that shields the materials both chemically and physicallyfrom the direct environment. In this respect, the incorporationof colloidal NCs in silica is highly interesting, because itincreases their photochemical stability while the opticalproperties are preserved.16 Furthermore, the silica shell caneasily be functionalized with organic ligands.17−21 This allowsthe 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 sphericalNCs (i.e., quantum dots (QDs)) in silica.16,17,19,22−31 The twomain approaches to coat nanoparticles with a silica shell are thetraditional Stober method32,33 and the so-called reversemicroemulsion method.34−37 Although the Stober method is

highly effective in growing silica shells around micrometer-sizedcolloids33,38 or metal nanoparticles,20 it does not yield uniformsilica shells on single semiconductor NCs.39 The reversemicroemulsion method however allows for the incorporation ofindividual QDs located exactly in the middle of silicaspheres.17,34 In this approach, the silica shell grows aroundsingle QDs that are individually trapped inside an aqueousmicelle of a water-in-oil (w/o) microemulsion, together withsilica precursor molecules. The silica nucleation and growth arecatalyzed by the basic environment resulting from the additionof an ammonia solution. The incorporation mechanism isexplained in terms of a ligand exchange process, through whichthe hydrophobic capping molecules on the surface of the QDsare replaced by the silica precursor tetraethoxysilane (TEOS),prior to silica growth.17,36

Only recently, silica-coating with the reverse microemulsionmethod was successfully applied to anisotropic NCs likeNRs40−42 and tetrapods.43,44 However, the incorporation insilica causes a drastic decrease in the particles’ aspect ratio asthe shape changes into dumbbells and ellipsoids.40−42 More-over, uniform shells thinner than 10 nm cannot be obtained bythe currently available methods. Hence, a method to coat highly

Received: December 16, 2013Revised: February 12, 2014Published: February 20, 2014

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anisotropic NCs with a uniform and tunable silica shell thatpreserves the original shape is highly desirable, since the shapeanisotropy is indispensable for certain applications, e.g. shape-directed self-assembly,45,46 antibody specificity for cellularuptake,47 and shape-dependent antibacterial activity.48

Here we present a versatile method that allows for highcontrol over the silica growth on semiconductor NCs withdifferent anisotropic shapes. This method leads to theformation of a uniform silica shell with tunable thickness thatpreserves the original shape anisotropy of the NCs. The shellthickness can be tuned between 3 and 17 nm. The startingpoint of our method is the reverse microemulsion in whichhydrolysis and condensation rates of TEOS are varied. Sincethese rates depend on the basicity of the environment, we studyin detail the role of the ammonia concentration on the silicanucleation and growth mechanism in the case of CdSe/CdScore/shell NRs. We show that the uniformity of the shell isnotably dependent on the ammonia concentration. We presentthe ideal conditions to obtain ultrathin, uniform silica shells. Inorder to demonstrate that this approach works for all types ofshapes, we incorporate highly anisotropic 2D NPLs as well asnanostars in silica.

■ EXPERIMENTAL SECTIONChemicals. Ammonia (29.9 wt % in water, Sigma-Aldrich, stored at

7 °C), bis(trimethylsilyl) sulfide (TMS2S, Sigma-Aldrich), cadmiumacetate (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), diethyleneglycol (DEG, Sigma-Aldrich), lead acetate (Pb(Ac)2, Sigma-Aldrich,98%), octadecyl phosphonic acid (ODPA, Sigma-Aldrich, 97%), n-octadecyltrimethoxysilane (OTMS, Fluorochem), oleic acid (OA,Sigma-Aldrich) poly(5)oxyethylene-4-nonylphenyl-ether (NP-5, Ige-PAL CO 520, Sigma-Aldrich), selenium (Strem Chemicals, 99.99%),sulfur (Alfa Aesar, 99%), tetraethyl orthosilicate (TEOS, Sigma-Aldrich, 99%), trioctylphosphine (TOP, Sigma-Aldrich, 90%), andtrioctylphosphine oxide (TOPO, Sigma-Aldrich, 99%) were used forthe 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 byCarbone 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 undervacuum. After 2 h, the solution was heated to 330 °C under nitrogenuntil Cd-ODPA complexes were formed, indicated by transparency ofthe solution. Next, TOP (1.5 g) was injected followed by heating to350−370 °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, coolingdown by blowing air toward the flask and finally addition of 5 mLtoluene as the temperature dropped below 90 °C. The final size of theNCs 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 °Cin 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-ODPAcomplexes, and TOP (1.5 g) was injected. Once the temperature wasstabilized, TOP-S (0.12 g S in 1.5 g TOP) and 200 μL CdSe NC seedsin TOP (400 μM) were rapidly injected. After 12 min, the reaction

was quenched and washed by precipitation/redispersion withmethanol and toluene and stored in a glovebox.

Synthesis of CdSe NC Platelets. The CdSe nanoplatelets weresynthesized according to Ithurria et al. (Protocol 1).9 Cd(Ac)2 (240mg), oleic acid (285 μL), and octadecene (15 mL) were mixed in athree-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 wasinjected. After 1 h the reaction was quenched, and CdSe NPLs werewashed twice with a methanol/butanol mixture (1:3 by volume) andstored 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 reportedby Mahler et al.50 Therefore, 1.2 mL of the CdSe nanoplateletssolution was diluted in 1.2 mL of hexane, and 50 μL ofbis(trimethylsilyl) sulfide (TMS2S) was added. After 1 h, the colorchanged from yellow to orange. The obtained NCs were washed twicewith ethanol and redispersed in hexane. Then, 20 mg of Cd-(Ac)2(H2O)2 was added, and another color shift from orange to redwas observed as the mixture was sonicated for 10 min. Thedisaggregation was induced by addition of 200 μL of oleic acid. TheNPLs were finally washed with ethanol and redissolved in hexane.

Synthesis of PbSe Nanostars. The PbSe nanostars weresynthesized by an adapted hot injection method from theliterature.51,52 OA (1.6 mL), DPE (2.1 mL), TOP (8.35 mL), andPb(Ac)2 (0.68 g) were mixed and heated to 150 °C under vacuum for2 h to form Pb-(OA)2 complexes. Meanwhile, a four-neck flask wasfilled with 10 mL of DEG and heated to 120 °C under vacuum for 30min. Next, the temperature was increased to 190 °C followed by therapid injection of the Pb-(OA)2 precursor and TOP-Se (0.13 g of Se in1.7 mL of TOP). The NCs were grown at 155 °C for 30 min, thenquenched with a methanol/butanol mixture, and after sedimentationfinally dispersed in toluene.

Silica Coating of NCs. The CdSe/CdS core/shell NRs werecoated 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 in10 mL of cyclohexane, followed by the addition of 1.4 nmol NRs intoluene and afterward 80 μL of TEOS. The solutions werecontinuously stirred, and there were at least fifteen minutes betweentwo consecutive additions. Finally, 150 μL of ammonia solution wasadded after which the solution was stirred for one more minute andthen stored. The reactions were quenched by addition of ethanolfollowed by three sedimentation/redispersion cycles. The resultingsilica-coated NRs were stored in ethanol. The encapsulation of CdSe/CdS core/shell NPLs and PbSe nanostars in a silica shell wasperformed following the same protocol. In the case of NPLs, 400 μL ofa solution in hexane was added (the OD was measured to be 0.2 at 400nm, for a 80 times diluted solution). For the nanostars, we added 200μL of a solution in toluene (the OD was measured to be 0.1 at 400nm, for a 50 times diluted solution). Different ammonia solutions wereprepared by dilution of a stock solution with water, resulting in a seriesof concentrations from 29.9 wt % to less than 0.6 wt %. The stocksolution 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 NCssoluble in nonpolar solvents, these were functionalized with OTMS.Therefore, 1 mL of solution of OTMS in cyclohexane (10%, v/v) wasadded to the reverse microemulsion one week after ammonia (29.9 wt%) addition. One day later, the reaction was quenched with ethanolfollowed by sedimentation. The resulting particles were washed twicewith a toluene/ethanol mixture and finally dispersed in toluene.

Characterization. The NR concentration was estimated usinginductively coupled plasma atomic emission spectroscopy (ICP-AES),in combination with absorption spectroscopy. Absorption spectra weremeasured using a Perkin-Elmer Lambda 950 UV/vis/IR absorptionspectrophotometer.

The purified NCs/silica nanoparticle samples were characterizedwith Transmission Electron Microscopy (TEM). Samples for analysiswere obtained by drop casting the NCs/silica solution onto coatedcopper TEM grids at room temperature. Prior to extraction of thesample, the solution was sonicated for about 1 min in order to prevent

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agglomeration of the silica-coated particles on the grid. TEM imagespresented in Figure 1 were obtained with Tecnai microscope operatingat 120 kV equipped with a tungsten filament. Images are recorded witha SIS Megaview II CCD-camera in iTEM software. Energy-dispersiveX-ray Spectroscopy (EDS) measurements and Scanning TransmissionElectron Microscopy (STEM) images are acquired on an FEITecnai20FEG instrument with a Fischione High Angle AnnularDark Field (HAADF) detector. The instrument is operated at 200 kVacceleration voltage. EDS-spectra are acquired with an EDAX detectorusing Tecnai Imaging and Analysis software (TIA).

■ RESULTS AND DISCUSSION

Influence of the Ammonia Concentration on the SilicaGrowth. CdSe/CdS core/shell NRs with length 48.5 ± 6.3 nmand diameter 6.2 ± 0.9 nm (according to TEM) were coatedwith silica by the reverse microemulsion method.39,41 In orderto investigate the role of ammonia, we varied the concentrationof ammonia in the aqueous phase from 29.9 wt % (standardprocedure) to 0.6 wt %, while other parameters were keptconstant. The silica-coated NRs were quenched at differentstages of growth, i.e. in some cases before the Si-precursor wasdepleted. The resulting NPs were soluble in polar solvents suchas water and ethanol or nonpolar solvents after functionaliza-tion with octadecyltrimethoxysilane (OTMS) (see the Exper-imental Section for details). The silica growth on the NRs wasfollowed in time for several ammonia concentrations, as shownin Figure 1.Figure 1D shows that dumbbell-type structures with two

spherical silica shells on the opposite tips are obtained if NRsare coated according to the standard procedure (29.9 wt %ammonia).41 If the ammonia concentration is decreased to 5.7wt % the resulting particles evolve in a similar way as observed

for the standard procedure, see Figure 1E. The silica spheres atthe opposite tips enlarge by growth, which sometimes results inellipsoid-shaped particles. After one week, some NRs arecompletely covered, while others remain dumbbell-shaped (seeS1 for a detailed overview of the silica shell evolution overtime).Interestingly, a drastic change is observed if the ammonia

concentration is decreased to less than 1.5 wt % (see Figure 1Aand B). Dumbbell-type particles are no longer observed, andthe NRs are completely coated by a uniform ultrathin silicashell (3 nm after 4 h for 1.1 wt %). Since the silica is distributedequally over the surface of each NR, the original aspect ratio isonly slightly reduced. Further growth results in a thicker shellwith preservation of the original shape.For an ammonia concentration of 2.8 wt %, see Figure 1C,

the obtained particles show a conformation intermediate to thedumbbell shape and the rod shape obtained with respectivelyhigher and lower concentrations. The silica starts to grow onboth ends of the NRs but appears less spherical and moreelongated along the z-axis compared to the characteristicdumbbell-type particles. This is best visible in the early stage (4h).The effect of the ammonia concentration on the resulting

shell thickness was quantified by measuring the temporalevolution of the average shell thickness (Figure 2). The

thickness was measured at the thickest point, perpendicular tothe z-axis of the NR. In the early stages (up to 10 h), 29.9 wt %ammonia leads to significantly faster growth than the lowerconcentrations. The growth is nearly complete after one day,since the shell barely increases in size afterward (the curvereaches a plateau). After one day, an ammonia concentration of2.8 wt % displays a shell thickness comparable to the standardprocedure. On the other hand, for concentrations from 0.6 to1.5 wt % no silica shell is visible on the NRs in the early stagesand an ultrathin shell of 3 nm can be observed after 4 h ofgrowth. The shells remain significantly thinner than thoseobserved for the dumbbells up to 1 week of growth and displaya comparable thickness only after two weeks. Note that,although the shell thicknesses are the same, the silica volumearound the completely incorporated NRs is larger than thesilica 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 atdifferent times for several microemulsions equally prepared but withdifferent ammonia concentrations in the aqueous phase. Scale barscorrespond to 50 nm. F: Single CdSe/CdS NR isolated after one dayfrom a microemulsion with 1.5 wt % ammonia, scale bar correspondsto 10 nm.

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

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nanoparticles decreases, as shown in Figure 1. Whereas highammonia concentrations produce a significant number ofempty silica spheres (Figure 1D and E), low ammoniaconcentrations (Figure 1A to C) rarely lead to the self-nucleation of silica spheres. For the standard procedure, it hasbeen reported that spontaneous nucleation is minimized uponincreasing the concentration of NCs.39 This can be understoodconsidering that once there are enough NCs to fill every micellewith 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 ammoniaconcentration (less than 2.8 wt % in this case). Given that theconcentration of surfactant molecules and NCs is the same inall microemulsions, we can assume that lowering the ammoniaconcentration reduces the self-nucleation of silica.High ammonia concentrations are frequently used to obtain

spherical silica shells, as low ammonia concentrations yieldirregular shapes.32,36,53,54 In the case of anisotropic NCs,reducing the tendency to grow silica into spheres facilitates theformation of uniform anisotropic shells.In order to understand these observations, we first need to

consider the chemical processes involved in the nucleation ofsilica: hydrolysis and polycondensation of TEOS monomers.The first step is the hydrolysis of one to four (x) ethoxy groupsof individual TEOS monomers:

+

↔ +−

x

x

Si(OC H ) H O

Si(OC H ) (OH) C H OHx x

2 5 4 2

2 5 4 2 5 (1)

In the second step, hydrolyzed TEOS monomers (poly)-condensate forming covalent bonds, thereby producing wateror ethanol. These condensation reactions involve the attack of anucleophilic (deprotonated) silanol on a neutral species:

= − + − = ↔ = − − = +Si OH HO Si Si O Si H O2 (2)

= − + − = ↔ = − − = +Si OH H C O Si Si O Si C H OH5 2 2 5(3)

In our system, hydrolyzed TEOS monomers are either attachedto the surface of the NRs or remain dissolved in the aqueousphase. Hence, condensation can occur both in solution andbetween species on the NR surface. Our results show that thesilica shell growth on the NRs as well as the presence of self-nucleated silica are highly dependent on the ammoniaconcentration. We attribute both effects to a competitionbetween the attachment of TEOS monomers to the NR surface(i.e., ligand exchange) and the polycondensation into anetwork, which ultimately results in colloidal particles.54,55

Both hydrolysis and condensation steps are base catalyzed bythe hydroxide ions from the aqueous ammonia solution. Theammonia concentration thus determines the hydrolysis (eq 1)and condensation (eq 2 and 3) rates and therefore affects thebalance between the rates of surface attachment and networkformation.With 29.9 wt % ammonia, TEOS monomers are quickly

completely hydrolyzed. Since these monomers are deproto-nated due to the high pH value (pH ∼ 10.5, pKa of silicic acid:9.8), rapid polycondensation (eq 2 and 3) leads to fast networkformation.55 In the presence of a NR, hydrolyzed TEOS

Figure 3. Schematic representation of the silica growth on CdSe/CdS core/shell NRs under a low (left) and high (right) ammonia concentration. Inboth cases, the ligand exchange from ODPA to hydrolyzed TEOS (indicated here by the red arrow) starts at the most reactive facets. If theconcentration 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 transitionfrom the dumbbell- to the rod-type particles as the concentration of ammonia varies between the two extremes.

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monomers are attracted toward Cd-atoms on the NR surfaceand hence replace the original capping molecules, starting at themost reactive facets (i.e., the opposite tips).41 These twoprocesses (network formation and surface attachment) occursimultaneously. Consequently, networks are formed both inempty micelles and at the tips of the NRs. After reaching acritical size these grow into silica spheres, leading to dumbbell-type structures as well as self-nucleated spheres (Figure 1D).The growth behavior remains unchanged for ammoniaconcentrations down to 5.7 wt % (pH > 9.8), i.e. dumbbell-formation and self-nucleation (Figure 1E). As long as theTEOS monomers each have at least two hydrolyzed groups, thepolycondensation into networks will be faster than TEOSattachment 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 partlyhydrolyzed. In this case, the hydrolysis is a rate-limiting step forthe polycondensation into networks, because condensationupon removal of an ethoxy group is less favorable than removalof a hydroxyl group (due to steric factors). Consequently, theattachment of hydrolyzed TEOS monomers to Cd atoms onthe NR surface becomes faster than the network formation. Weexpect that in this regime the original capping ligands arecompletely exchanged for TEOS monomers before stablenetworks are formed in solution. The polycondensation willthen occur primarily at the surface of the NRs, leading to auniform silica shell around the entire NR rather than silicaspheres at the opposite tips and in empty micelles. The twoextreme situations are represented schematically in Figure 3.To further support our model, we performed HAADF-

STEM analysis combined with EDS (Figures S2 and S3).Figure S2 shows an image and corresponding EDS analysis onNRs isolated from a microemulsion with 1.1 wt % ammoniaafter two hours of growth. Although no silica layer could beobserved, EDS analysis on a group of these particles confirmedthe presence of silicon (Si) and phosphor (P) at the NRs. Thus,at this stage both TEOS monomers and the original cappingligand ODPA are on the surface, which means that the ligandexchange is still in progress. NRs isolated after one day from amicroemulsion with 1.5 wt % are completely coated with silica,as shown in Figure 1F. EDS analysis on these particlesconfirmed that at this stage, ODPA is completely removed(Figure S3). The uniform silica shell adapts not only to the rod-shape but also to the bulb originating from the CdSe coreembedded in the CdS rod. Hence, these conditions ensurepreservation of the original shape anisotropy. The formation ofa uniform silica shell can be understood as a surface catalyzedprocess. To confirm that the uniformity is due to the ammoniaconcentration and independent of the amount of water, theexperiment was repeated with 1.5 wt % ammonia and half theamount 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 incorporatedifferently shaped anisotropic semiconductor NCs, weperformed similar experiments with nanostars and 2D NPLs.The 2D NPLs are not only highly anisotropic but also relativelylarge in their lateral dimensions (up to 150 nm). An ammoniaconcentration of 1.5 wt % was used to promote thin shellformation, and a standard synthesis using 29.9 wt % ammoniawas 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

NPLs were originally less than 2 nm in thickness (fivemonolayers CdSe and a single monolayer CdS) with 27.6 ± 8.3nm by 123.6 ± 24.3 nm in their lateral dimensions. After silica-coating, most of the NPLs are upstanding and hence observedfrom 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 theTEM grid with respect to the detector (Figure 4D). Theseimages confirm that all facets are coated with silica. Addition-ally, HAADF-STEM images show that the shape of the silicashell perfectly adapts to the original proportions of the NPLs,as observed above for the NRs (see Figure 4C). EDS analysison these NPLs confirms the presence of Cd, Se, S, and Si (seeS5). 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 thebest of our knowledge, we are the first to successfullyincorporate 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

PbSe nanostars before and after silica coating with 1.5 wt %ammonia. After two hours, a silica shell of less than 3 nm isformed that follows the original shape of the NCs (see Figure5B). Naturally, this silica shell has a higher tendency to becomespherical than the shells around one-dimensional NRs and two-dimensional NPLs, since the original shape of the NCs isalready more isotropic. Therefore, the silica coated nanostarsbecome gradually more spherical as the silica shell growsthicker (see Figure 5). We expect that our approach is alsoapplicable to obtain thin silica shells around isotropic QDs.

General Method To Obtain Ultrathin Uniform Shells.Altogether, these results demonstrate that high control over the

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

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

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silica growth on anisotropic semiconductor NCs is obtained ifthe ammonia concentration in the reverse microemulsion issubstantially decreased to 0.6−1.5 wt %, while all otherparameters are kept constant.This method can be widely used to incorporate anisotropic

semiconductor NCs with a variety of shapes in silica, providedthe native ligands are exchangeable for TEOS. We propose thatthe following kinetic steps are essential for the formation of auniform 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 thethickness of the uniform shell by growth duration beforequenching. In the case of CdSe/CdS core/shell NRs, silicashells of 3 nm were achieved after 4 h of growth. Naturally, theexact growth rates depend on the concentration of NC seeds/TEOS and different parameters characteristic for each NCstructure, e.g. the surface energy and the rate of ligandexchange.

■ CONCLUSION

In this work, a versatile method to incorporate anisotropicsemiconductor NCs in ultrathin, uniform silica shells isdeveloped. This method allows for high control over theshape of the silica shell as well as its thickness, which can betuned between 3 and 17 nm. The method is based on thereverse microemulsion method, in which a lower concentrationof ammonia is used. The crucial role of ammonia was examinedwith a case study of CdSe/CdS core/shell NRs. We found that0.6−1.5 wt % ammonia in the aqueous phase results in thinsilica shells that perfectly adapt to the original shape of the NRs.This is explained in terms of a relatively slow rate of networkformation of TEOS monomers, which allows for a completeligand exchange on the entire surface of the NRs prior to theformation of a uniform silica shell. Our findings are not onlyconsistent with the proposed silica incorporation mechanismbut also enabled us to incorporate highly anisotropic 2D NPLsin silica. The versatility and applicability of the method isshown by the incorporation of zero-, one-, and two-dimensionalanisotropic NCs in a thin, uniform silica shell. This methodcould therefore be widely used to coat anisotropic NCs with ahighly 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 silicaprecursor (TEOS), the possibility to grow a silica shell onspecific NCs depends on the surface chemistry of the materialand the binding energy of the ligand with the surface of theNCs. Therefore, we expect this method to work on other typeof semiconductor materials as well as metals or oxides, as longas the original capping ligand is exchangeable towardhydrolyzed TEOS.

■ ASSOCIATED CONTENT

*S Supporting InformationAdditional TEM images and EDS data for CdSe/CdS NRs andNPLs. This material is available free of charge via the Internetat http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Authors*E-mail: (D.V.) d.vanmaekelbergh@uu.nl.*E-mail: (C.d.M.G.) c.demello-donega@uu.nl.

Author Contributions§E.M. Hutter and F. Pietra have contributed equally to thiswork.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors would like to thank Alfons van Blaaderen fordiscussions and critical reading of the manuscript. JantinaFokkema, Stephan Zevenhuizen, and Ward van der Stam areacknowledged for technical support and help with someexperiments. The research leading to these results has receivedfunding from the European Research Council under theEuropean Union’s Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement n. [291667]. The authorsacknowledge financial support from FOMNPS and NWO.

■ REFERENCES(1) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226−13239.(2) Alivisatos, A. P. Science 1996, 271, 933−937.(3) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. Rev.2005, 105, 1025−1102.(4) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.;Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59−61.(5) Son, J. S.; Park, K.; Kwon, S. G.; Yang, J.; Choi, M. K.; Kim, J.;Yu, J. H.; Joo, J.; Hyeon, T. Small 2012, 8, 2394−2402.(6) Shabaev, A.; Efros, A. L. Nano Lett. 2004, 4, 1821−1825.(7) Ithurria, S.; Dubertret, B. J. Am. Chem. Soc. 2008, 130, 16504−16505.(8) Bouet, C.; Tessier, M. D.; Ithurria, S.; Mahler, B.; Nadal, B.;Dubertret, B. Chem. Mater. 2013, 25, 1262−1271.(9) Ithurria, S.; Bousquet, G.; Dubertret, B. J. Am. Chem. Soc. 2011,133, 3070−3077.(10) Li, Z.; Peng, X. J. Am. Chem. Soc. 2011, 133, 6578−86.(11) Son, J. S.; Wen, X.-D.; Joo, J.; Chae, J.; Baek, S.-I.; Park, K.; Kim,J. H.; An, K.; Yu, J. H.; Kwon, S. G.; Choi, S.-H.; Wang, Z.; Kim, Y.-W.; Kuk, Y.; Hoffmann, R.; Hyeon, T. Angew. Chem. 2009, 48, 6861−6864.(12) Wen, X.-D.; Hoffmann, R.; Ashcroft, N. W. Adv. Mater. 2013,25, 261−266.(13) de Mello Donega, C. Chem. Soc. Rev. 2011, 40, 1512−1546.(14) Resch-Genger, U.; Grabolle, M.; Cavaliere-Jaricot, S.; Nitschke,R.; Nann, T. Nat. Methods 2008, 5, 763−775.(15) Anikeeva, P. O.; Halpert, J. E.; Bawendi, M. G.; Bulovic, V. NanoLett. 2009, 9, 2532−2536.(16) Correa-Duarte, M. A.; Giersig, M.; Liz-Marzan, L. M. Chem.Phys. Lett. 1998, 286, 497−501.(17) Koole, R.; Van Schooneveld, M. M.; Hilhorst, J.; de MelloDonega, C.; ’t Hart, D. C.; Van Blaaderen, A.; Vanmaekelbergh, D.;Meijerink, A. Chem. Mater. 2008, 20, 2503−2512.(18) Ito, Y.; Virkar, A. A.; Mannsfeld, S.; Oh, J. H.; Toney, M.;Locklin, J.; Bao, Z. J. Am. Chem. Soc. 2009, 131, 9396−9404.(19) Hu, X.; Gao, X. ACS Nano 2010, 4, 6080−6086.(20) Hu, X.; Gao, X. Phys. Chem. Chem. Phys. 2011, 13, 10028−10035.(21) Zheng, F.; Hu, B. Anal. Chim. Acta 2007, 605, 1−10.(22) Tan, T. T.; Selvan, S. T.; Zhao, L.; Gao, S.; Ying, J. Y. Chem.Mater. 2007, 19, 3112−3117.(23) Wang, S.; Li, C.; Yang, P.; Ando, M.; Murase, N. Colloids Surf., A2012, 395, 24−31.(24) Selvan, S. T.; Patra, P. K.; Ang, C. Y.; Ying, J. Y. Angew. Chem.,Int. Ed. 2007, 119, 2500−2504.(25) Primera-Pedrozo, O. M.; Ates, M.; Arslan, Z. Mater. Lett. 2013,102−103, 116−119.(26) Yang, P.; Ando, M.; Murase, N. Langmuir 2011, 27, 9535−9540.(27) Yi, D. K.; Selvan, S. T.; Lee, S. S.; Papaefthymiou, G. C.;Kundaliya, D.; Ying, J. Y. J. Am. Chem. Soc. 2005, 127, 4990−4991.

Chemistry of Materials Article

dx.doi.org/10.1021/cm404122f | Chem. Mater. 2014, 26, 1905−19111910

(28) Hu, X.; Zrazhevskiy, P.; Gao, X. Ann. Biomed. Eng. 2009, 37,1960−1966.(29) Zhang, M.; Cushing, B. L.; O’Connor, C. J. Nanotechnology2008, 19, 085601.(30) Gerion, D.; Pinaud, F.; Williams, S. C.; Parak, W. J.; Zanchet,D.; Weiss, S.; Alivisatos, A. P. J. Phys. Chem. B 2001, 105, 8861−8871.(31) Darbandi, M.; Lu, W.; Fang, J.; Nann, T. Langmuir 2006, 22,4371−4375.(32) Stober, W.; Fink, A. J. Colloid Interface Sci. 1968, 26, 62−69.(33) Graf, C.; Vossen, D. L. J.; Imhof, A.; Van Blaaderen, A. Langmuir2003, 19, 6693−6700.(34) Selvan, S. T.; Tan, T. T.; Ying, J. Y. Adv. Mater. 2005, 17, 1620−1625.(35) Lopez-Quintela, M. A. Curr. Opin. Colloid Interface Sci. 2003, 8,137−144.(36) Darbandi, M.; Thomann, R.; Nann, T. Chem. Mater. 2005, 17,5720−5725.(37) Arriagada, F. J.; Osseo-Asare, K. Colloids Surf., A 1999, 154,311−326.(38) Marino-Fernandez, R.; Masunaga, S. H.; Fontaina-Troitino, N.;Morales, M. P.; Rivas, J.; Salgueirino, V. J. Phys. Chem. C 2011, 115,13991−13999.(39) Nann, T.; Mulvaney, P. Angew. Chem., Int. Ed. 2004, 43, 5393−5396.(40) Jae Kwon, M.; Jung, H.; Hoon Park, J. J. Phys. Chem. Solids2012, 73, 1448−1451.(41) Pietra, F.; Van Dijk - Moes, R. J. A.; Ke, X.; Bals, S.; VanTendeloo, G.; Donega, C. D. M.; Vanmaekelbergh, D. Chem. Mater.2013, 25, 3427−3434.(42) Kumar, R.; Ding, H.; Hu, R.; Yong, K.-T.; Roy, I.; Bergey, E. J.;Prasad, P. N. Chem. Mater. 2010, 22, 2261−2267.(43) Lian, J.; Xu, Y.; Lin, M.; Chan, Y. J. Am. Chem. Soc. 2012, 134,8754−8757.(44) Xu, Y.; Lian, J.; Mishra, N.; Chan, Y. Small 2013, 9, 1908−1915.(45) Sacanna, S.; Pine, D. J. Curr. Opin. Colloid Interface Sci. 2011, 16,96−105.(46) Ye, X.; Collins, J. E.; Kang, Y.; Chen, J.; Chen, D. T. N.; Yodh,A. G.; Murray, C. B. Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 22430−22435.(47) Barua, S.; Yoo, J.-W.; Kolhar, P.; Wakankar, A.; Gokarn, Y. R.;Mitragotri, S. Proc. Natl. Acad. Sci. U.S.A. 2013, 110, 3270−5.(48) Pal, S.; Tak, Y. K.; Song, J. M. Appl. Environ. Microbiol. 2007, 73,1712−1720.(49) Carbone, L.; Nobile, C.; De Giorgi, M.; Della Sala, F.; Morello,G.; Pompa, P.; Hytch, M.; Snoeck, E.; Fiore, A.; Franchini, I. R.;Nadasan, M.; Silvestre, A. F.; Chiodo, L.; Kudera, S.; Cingolani, R.;Krahne, R.; Manna, L. Nano Lett. 2007, 7, 2942−2950.(50) Mahler, B.; Nadal, B.; Bouet, C.; Patriarche, G.; Dubertret, B. J.Am. Chem. Soc. 2012, 134, 18591−18598.(51) Houtepen, A. J.; Koole, R.; Vanmaekelbergh, D.; Meeldijk, J.;Hickey, S. G. J. Am. Chem. Soc. 2006, 128, 6792−6793.(52) Wehrenberg, B. L.; Wang, C.; Guyot-Sionnest, P. J. Phys. Chem.B 2002, 106, 10634−10640.(53) Pope, E. J. A.; Mackenzie, J. D. J. Non-Cryst. Solids 1986, 87,185−198.(54) Arriagada, F. J.; Osseo-Asare, K. J. Colloid Interface Sci. 1999,211, 210−220.(55) Brinker, C. J. J. Non-Cryst. Solids 1988, 100, 31−50.

Chemistry of Materials Article

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