NANO LETTERS
SiO2-Coated CdTe Nanowires: Bristled Nano Centipedes
2004 Vol. 4, No. 2 225-231
Ying Wang,† Zhiyong Tang,† Xiaorong Liang,‡ Luis M. Liz-Marza´n,*,§ and Nicholas A. Kotov*,† Department of Chemical Engineering, Department of Materials Science and Engineering, 2300 Hayward, UniVersity of Michigan, Ann Arbor, MI, 48109, Department of Chemistry, Purdue UniVersity, West Lafayette, IN, 47907, and Departamento de Quı´mica Fı´sica, UniVersidade de Vigo, 36200 Vigo, Spain Received October 28, 2003; Revised Manuscript Received November 27, 2003
ABSTRACT CdTe nanowires with bristled SiO2 coatings resembling nanoscale centipedes were produced in a modified Sto1 ber process when mercaptosuccinic acid was used as a stabilizer. The reason for the unusual morphology of the coating is believed to be the nonuniform distribution of the stabilizers on the surface of the nanowire. Atomistic calculations of CdTe nanoparticles as a simplified model of a nanowire surface reveal that phase separation of the mercaptosuccinic acid and (3-mercaptopropyl)trimethoxysilane is an energetically favorable process. These data indicate that the heterogeneity of the nanocolloid surface is an important property that may be taken advantage of for preparation of nanostructures of high complexity. From a practical perspective, the centipede nanowires can be a uniquely useful material for high-strength nanoscale composites.
Introduction. One-dimensional nanocolloids, such as nanorods and nanowires (NWs), have received considerable attention owing to their special properties and applications in sensors, optoelectronics, and electronic devices.1-7 Several prototype devices taking advantage of nanoscale confinement effects in NWs have been demonstrated.1,2,8,9 Some technical problems can be raised regarding their possible future technological use. The high sensitivity of the NW conductivity to surface conditions is an advantage for sensor devices, but the same property can cause substantial problems for circuits that require operational stability under various conditions. As well, typical stabilizers used for the preparation of NWs are low molecular weight organic species chemisorbed to the particle surface, which are prone to chemical oxidation. Both of these considerations lead to the necessity of insulated one-dimensional nanoscale colloids with environmentally stable polymerized coatings. It was demonstrated that encapsulation of semiconductor and metal nanoparticles (NPs) with silica in the form of amorphous inorganic polymer prevented their aggregation in liquid and improved their chemical stability.10-15 Silica surface is also easy to functionalize with various coupling reagents, making possible robust covalent and noncovalent attachment of specific ligands, such as biomolecules.15,16 Another advantage is that the silica shell is optically * Corresponding authors. E-mail: uvigo.es † University of Michigan. ‡ Purdue University. § Universidade de Vigo. 10.1021/nl0349505 CCC: $27.50 Published on Web 01/07/2004
[email protected]; lmarzan@
© 2004 American Chemical Society
transparent in the absorption/emission region of semiconductor NWs, so that electronic processes in them could be monitored spectroscopically.15-17 Coating of gold nanorods,18,19 silver NWs,17 single wall carbon nanotubes,20 Bi2S3 nanofibers,21 and recently CdTe NWs22 with SiO2 layer through sol-gel process has been reported to yield coaxial structures. Conductive AFM measurements demonstrated that silica coating on CdTe NWs was electrically insulating with breakdown voltage of at least 108 V/cm.22 The coatings can also be prepared in a wide range of thicknesses sufficient to span both sensor (5-10 nm) and field-effect (50-100 nm) applications. Importantly, polymerization of silica on CdTe NWs occurs preferentially on the sidewalls, which opens a possibility of preparation of insulated nanoconductors with electrically accessible ends.22 At this point in time, one can pose the questions whether it is possible to elevate the complexity of the NW assemblies and to make SiO2 coatings of more sophisticated morphology and function. Different geometry and nanoscale/molecular engineering of the coating will extend the spectrum and the surface effects on NW conductivity and functionalities of NW-based devices. In the past, researchers focused more on controlling the rate of silica deposition by varying the concentration media composition and pH. The effects of SiO2 shell thickness on the physical and chemical properties have been extensively studied as well. However, only very few results have been reported involving the development of novel morphologies
Figure 1. TEM images of some CdTe NWs stabilized by MSA (A) and TGA (B).
of silica films. Concomitantly, very little is also known about the effect of the organic stabilizer adsorbed to the NW surface on the SiO2 nucleation process. Herein, the first synthesis of unique brush-like NW-SiO2 core-shell nanostructures resembling nanoscale centipedes is reported. The analogy is further enhanced by the fact that actual centipedes are coated with a tough layer of chitin providing protection from the environment, similarly to the SiO2 coating of the NWs. The distinctive bristled geometry provides exceptionally large surface area and a structural foundation for highly branched nanocolloid superstructures. As well, it will guarantee superb matrix connectivity in the nanowirepolymer composites for optical materials with stringent requirements to mechanical properties.32 Experimental Section. Chemicals. Cadmium perchlorate hydrate (Cd(ClO4)2, Aldrich), aminoacetonitrile monosulfate (H2NCH2CN‚H2SO4, Aldrich), thioglycolic acid, TGA, (a.k.a. mercaptoacetic acid) (HSCH2CO2H, Aldrich) mercatptosuccinic acid, MSA, (HO2CCH2CH(SH)CO2H, Aldrich), aluminum telluride (CERAC), tetraethoxysilane, TEOS, (Fluka), (3-mercaptopropyl)trimethoxysilane, MPS, (Fluka), and ammonium hydroxide (30%, VWR) were used in their commercial form. Milli-Q water with a resistivity higher than 18.2 MΩ cm and absolute ethanol were used in all the preparations. Synthesis. CdTe NPs were prepared according to the arrested precipitation method,23 in which the H2Te gas reacted with a nitrogen-saturated Cd(ClO4)2 aqueous solution at pH 11.2 in the presence of stabilizing reagents that were either TGA (HSCH2COOH) or mercaptosuccinic acid (HOOCCH2CH(SH)COOH), MSA. Upon NP formation, refluxing of the reaction mixtures was carried out for growth of NPs, and the heating was stopped until desired sizes had been achieved, which could be decided by photoemission peaks of NP solutions. In respect to another paper from our group,7 there was a slight modification for self-assembly process of NPs into NWs. The CdTe colloids were precipitated by methanol to get rid of redundant stabilizer and redissolved in water at pH 10 in the presence of aminoacetonitrile (1 mM). Aging of redispersed NP solutions lasted for 1∼2 days at 60 °C, during which period the color changed from bright 226
Figure 2. TEM images of brush-like CdTe-MSA/SiO2 core/shell nanostructures obtained in the presence of different amounts of TEOS. TEOS concentration: (A) 0.002 M; (B) 0.008 M; (C) 0.012 M.
orange-red to black, indicating the transformation from NPs to NWs. Silica Coating. Core/shell NW structures were prepared by means of a modified Sto¨ber method. The silane coupling agent mercaptopropyl(trimethoxysilane), MPS, was employed as surface primer to render the surface of colloids vitreophilic, i.e., receptive to silica monomers and oligomers.11 A solution of 2 µL MPS in 20 mL ethanol was added under vigorous stirring to 5 mL CdTe NW solution, with final concentration of 3.8 mM referred to Cd for CdTe. After stirring for 1 h, 0.5 mL ammonia solution (30 wt %) and various amount of tetraethyl orthosilicate (TEOS, 0.0020.012 M) were added to the reaction mixtures. The reaction Nano Lett., Vol. 4, No. 2, 2004
Figure 3. Dependence of the bristle length on the TEOS concentration.
Figure 4. X-ray photoelectron spectroscopy spectra from SiO2-coated CdTe NWs. Si 2p spectrum, Cd 3d spectrum, Te3d spectrum. (Red lines are the original experiment data. Green, orange, and purple lines are the deconvoluted curves obtained by Gaussian distribution fitting of the experimental data. Blue and black lines are the resultant fitting and background signal, respectively.
was allowed to proceed at room temperature for 1 day under continuous stirring. Several reaction parameters (such as the growth time, concentration of ammonia catalyst, and water content) could be employed to control the thickness of silica, as was demonstrated in the previous paper.22 Molecular Modeling. The CdTe clusters bearing different organic protective layers were simulated by PC Spartan Pro (Wave function Inc., CA) software package for PC computers (version 2000). The NW surface cluster was approximated by a NP in triplet dianion state. The geometry optimization was done initially after the cluster construction using the software option for geometrical energy minimization. Then thermodynamic parameters of the model were calculated for the equilibrium geometry obtained with molecular mechanics MMFF algorithm. PM3 and AM1 simulations were not successful due to the complexity of the system, even with frozen Cd and Te atoms. Reported MMFF data were obtained without atom freezing. Results and Discussion. The NWs used here were produced by the self-assembly from individual CdTe NPs. Two capping ligands, mercaptoacetic acid also known as thioglycolic acid, TGA, (HSCH2COOH) and mercaptosucNano Lett., Vol. 4, No. 2, 2004
cinic acid, MSA, (HOOCCH2CH(SH)COOH), were used during the synthesis of CdTe NPs. NWs produced from the CdTe-MSA dispersion were slightly larger in diameter than CdTe-TGA NWs and emitted in the red (λmax) 630 nm) and orange (λmax) 600 nm) parts of the spectrum, respectively. The diameter of MSA-stabilized NWs obtained from AFM height measurements was 4.5 ∼ 6.5 nm, while their lengths ranged from several hundred nanometers to 10 µm (Figure 1A). The TGA-stabilized NWs also exhibited high aspect ratios above 2000 and had an average diameter of ∼3 nm (Figure 1B). TEM images showed that both NWs had similarly smooth sidewall surface; no obvious differences in morphology or geometry could be observed. The detailed mechanism of the NW formation is currently under investigation in parallel with the surface modification studies. NW-SiO2 core-shell structures were prepared by means of the Sto¨ber method with a modified procedure as compared to our previous report.22 When MSA-stabilized NWs were used instead of TGA-stabilized ones, a drastic change in the silica shell coating was observed. It was interesting to notice the brush-like appearance of the SiO2-coated NWs with numerous, nearly parallel bristles growing perpendicular to 227
Table 1. Dependence of Coating Morphology of Silica on Experimental Conditions
Figure 5. TEM images of SiO2-coated CdTe NWs bearing TGA as stabilizer. Concentrations of MPS and TEOS were 0.0004 and 0.008 M for (A) and (B), 0.036 and 0.056 M for (C).
the surface of the NW-MSA shaft (Figure 2). Their geometry is similar to 3D assemblies of fibrous proteins.34 The bristles reach about ∼30 nm in length and several nanometers in diameter (Figure 2A). These “centipede legs” are generally thinner and shorter when lower TEOS concentrations are used: their length can be varied from ∼30 nm to 100 nm (Figure 3). The increase in TEOS concentration also results in greater bristle density (Figure 2C). The length of the coated NWs is mainly determined by the original CdTe NWs, although larger quantities of shortened NWs are observed after the coating (Figure 2B). This effect can be attributed to the pressure building up between some bristles, resulting in the break-up of the NW. A manifestation of the same mechanical effects can also be seen when coated NWs acquire a bent conformation (Figure 2C), which was never observed in the “naked” colloid (Figure 1). Some NWs are side-joined to each other through the silica 228
sample
[MPS] (mM)
[TEOS] (mM)
NW-MSA/SiO2 with bristles NW-TGA/SiO2 with bristles NW-TGA/SiO2, smooth coating
0.4 0.4 36
2 ∼ 16 4∼8 56
film (Figure 2B inset). Purely mechanical adherence resembling Velcro tape assisted by some chemical bonding can provide stability to these aggregates. The bristles control the distance between the semiconductor cores in such assemblies, maintaining molecular permeability of the inter-NW space. Somewhat similar superstructures were observed for BaCrO4 nanobelts synthesized in reverse micelles.24 LangmuirBlodgett compression of the nanocolloid monolayer at airwater interface should substantially increase the yield of such assemblies.25-28 X-ray photoelectron spectroscopy (XPS) was employed to investigate the surface elemental composition of the coated NWs (Figure 4). The appearance of a characteristic Si(IV) 2p peak at 102.82 eV, Si 2p1/2 at 99.61 eV, and Si 2p3/2 at 98.97 eV confirms the formation of SiO2 shell over CdTe NWs, in which the molar ratio of O/Si is approximately 1.89. The Cd 3d5/2 peak at 404.5 eV and the Te 3d5/2 peak at 571.79 eV are most probably due to the electron emission through the uncoated or thinly coated parts of the surface, which are located between the spikes. CdS-like signals of Cd 3d5/2 and Cd 3d3/2 peaks at 405.8 and 411.3 eV29,30 have been observed as a result of covalent bonding of the mercapto groups to Cd2+ on the surface as one would expect for thiol-stabilized colloids. The presence of a Te 3d3/2 peak at 585.5 eV indicates the formation of some amount of Te oxide due to the surface oxidation. The mid band gap surface states formed by Te and oxygen are believed to be responsible for the transversal current through the NW.22 The centipede silica shells in MSA-CdTe can be compared to smooth SiO2 coatings (Figure 5C, Table 1) produced when NWs are stabilized with TGA.22 Under different conditions, i.e., after decreasing the concentration of MPS concentration by 90 times to 0.4 mM and TEOS concentration by 7 times to 8 mM, a beaded morphology is observed for SiO2-coated CdTe-TGA NWs (Figure 5). The size of the beads on the shaft is not uniform and they are randomly dispersed along the NWs. Rough and thick SiO2 layers were also obtained when TEOS was taken in excess. The treatment with mercaptopropyl(trimethoysilane) (MPS) partially exchanges the primary stabilizers, i.e., thiocarboxylic acids due to the dynamic nature of the thiol chemical bond to the surface of the semiconductors.31 A mixed layer of the new and old stabilizer with different ratios of components should be expected in most cases due to similar energetics of S-Cd bonds for different thiols. In turn, the composition and distribution of MPS and the original stabilizer around the semiconductor core determines the polymerization conditions of the silica layer, and hence the morphology of the coaxial coating. When the MPS concentration in the exchange step is reduced (compare Figure 5A, B, and C), this Nano Lett., Vol. 4, No. 2, 2004
Figure 6. Results of molecular modeling of mixed stabilizer systems. The dependence of the energy of TGA-MPS (A) and MSA-MPS (B) stabilized model clusters for adjacent (blue) and separate (black) positioning of MPS moieties. Optimized geometry of the TGA-MPS (C, D) and MSA-MPS (E,F) stabilized model cluster for adjacent (C, E) and separate (D, F) positioning of MPS moiety. The stars denote the MPS locations. Color atomic code: yellow-Te, green-Cd, blue-S, gray-C, red -O. Hydrogen atoms are omitted for simplicity.
concomitantly reduces its content in the organic shell of the NWs and results in a less vitreophilic surface. This gives a more heterogeneous growth of the silica shell, i.e., beads. However, even almost 2 orders of magnitude decrease in MPS activity does not produce markedly different shell morphology. This fact makes it clear that it is the specific primary stabilizer, TGA, which determines the unique morphology of the SiO2 shell. The fundamental reason for the bristle formation and brush-like appearance of MSA-CdTe NWs is the nonuniform growth conditions on the semiconductor surface: favorable in some areas and rather adverse in others. The Nano Lett., Vol. 4, No. 2, 2004
characteristic size of both areas should be similar to the diameter of the bristle “root”, i.e., about several nanometers in size (Figure 2). What might be the origin of the variation in silica adherence to the NW surface? It is conceivable that these areas are formed because of the nonuniform charging of the NW surface. pKCOOH for MSA are 3.30 and 4.94, while TGA has a pKCOOH of 3.60. At high pH used for TEOS hydrolysis (pH 11.0), the degree of deprotonation of the thiocarboxilic acids will be high for both molecules. The surface density of this charge determines the eventual degree of deprotonation of the stabilizers also affecting the energy of their bonding to the semiconductor surface.33 Unless there 229
is a significant variation of surface density of the adsorbed thiol, there should be no strong variation of the charge state of the NWs. One of the structural factors that may result in the heterogeneity and surface density of the thiols is the difference in packing of the stabilizer layers. One can suggest that MPS forms patches on the surface of the NWs, which would be particularly suitable for the initiation and subsequent growth of the silica shell. The polymerization of SiO2 on the surface areas containing higher concentration of MSA should be retarded if not prohibited. Overall, this results in the quick growth of the silica columns appearing as bristles in MPS-rich patches and almost no silica deposition on the other areas. Low concentration of TEOS and diffusion limitations to the NW surface can exacerbate the effect. The switch from the smooth SiO2 layer (Figure 5C) to the beaded morphology (Figure 5A, B) when the incorporation of MPS in the TGA coat is small correlates well with this hypothesis. To verify it, the layer of mixed stabilizers on CdTe surface was simulated in the framework of the MMFF molecular mechanics package of Spartan software, which accounted mostly for bond strain due to geometrical factors of a chemical system. To reduce the number of atoms to a manageable quantity (500-700), NW was approximated by a CdTe cluster of a general formula Cd32Te14(stabilizer)36(H2O)4 analogous to those forming with TGA in solutions.35 The question that this model would help to answer is whether there is any thermodynamic predisposition of the mixed thiol stabilizer layer to form a patchy layer on NWs. It was found that the total energy of the cluster with MPS positioned adjacent to each other for TGA-stabilized cluster was consistently higher than energy of the cluster with MPS moieties dispersed over the entire surface for different amount of MPS adsorbed (Figure 6A). The situation is opposite for the cluster stabilized with MSA. The MPS molecules concentrated in one area almost always give systems of lower energy than with separated MPS (Figure 6B), which clearly indicates the preference of mixing in TGA-MPS and separation in MSA-MPS coatings. The analysis of the optimal geometry of the clusters (Figure 6CF) did not reveal any specific interatomic interaction that might be responsible for this effect. One particular regularity has been noticed though. The length of the bond between the water and the cluster (H2O-Cd) in the areas mostly covered by MSA molecules is consistently shorter (2.132.2 A) than the length of the same bond in the areas with mixed MPS-MSA coating (2.46-2.52 A). This structural feature can lead to the energy stabilization of the cluster and the domains, especially in the highly hydrated state in aqueous dispersions. In addition, there are additional forces that favor the separated state, which are not accounted for in the MMFF algorithm but which present in the real colloid. Hydrophobic attraction between propyl backbones of the MPS moieties should further enhance the nonuniform coverage. Packing preferences are also implicated in the phenomenon of 2D phase separation which is well known for LangmuirBlodgett films36-38and self-assembled layer of thiols on gold.39-41 Stranick et al. reported the formation of domains 230
in the mixture of thiols different only by a terminal ether group. The diameter of the domains was strikingly similar to the diameter of the bristle “roots” obtained here.42 The patch formation process in mixed MSA-MPS and TGAMPS adsorption layers can also be characterized as 2D phase separation. Note that all the data on Langmuir-Blodgett and self-assembled monolayers indicate that this process occurs quite rapidly, often during the formation of the film. The same dynamics can also be expected for 2D phase separation in the mixed thiol layers on NWs. In summary, silica-coated CdTe NWs with a unique morphology resembling centipedes have been prepared. This synthetic method is simple and provides the possibility of controlling the brush density and length on the NW surface. The phase separation of the mixed layer of the stabilizer is believed to cause the uneven growth of silica layer on chemically different areas. From a practical point of view, the centipede geometry of the coaxial SiO2 layer could also be very useful in nanodevices where both access to the semiconductor core, for instance by analytes, and insulation from the supporting surface are required. The Velcro-like connectivity between them (Figure 2B) can also be a useful feature for the preparation of NW composites, which would combine optical properties determined by quantum confinement effect and enhanced mechanical properties. Taking a more fundamental approach to these data, they reveal heterogeneity as one of the basic properties of nanocolloid surfaces. One should expect to see its manifestations in other effects and interactions between of NWs and possibly NPs.43,44 These observations also point to the necessity of developing experimental tools for the investigation of the nonuniform distribution of the stabilizers as the source of new nanoscale superstructures, which could be very useful in determination of NP interactions.7 Acknowledgment. Part of this research was done while N.A.K., Y.W., Z.T., and X.R. were affiliated with Oklahoma State University. N.A.K. thanks NSF-CAREER, NSF-Biophotonics, NIH-NASA, AFOSR, and OCAST for the financial support of this project. The authors thank the referees for useful comments. References (1) Duan, X.; Huang, Y.; Agarwal, R.; Lieber, C. M. Nature 2003, 421, 241-245. (2) Cui, Y.; Wei, Q.; Park, H.; Lieber, C. M. Science 2001, 293, 12891292. (3) Hu, J.; Li, L. s.; Yang, W.; Manna, L.; Wang, L. w.; Alivisatos, A. P. Science 2001, 292, 2060-2063. (4) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115-2117. (5) Peng, X.; Manna, U.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59-61. (6) Kwan, S.; Kim, F.; Akana, J.; Yang, P. Chem. Commun. 2001, 447448. (7) Tang, Z.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237-240. (8) Huang, Y.; Duan, X.; Cui, Y.; Lauhon, L. J.; Kim, K. H.; Lieber, C. M. Science 2001, 294, 1313-1317. (9) Law, M.; Kind, H.; Messer, B.; Kim, F.; Yang, P. Angew. Chem., Int. Ed. 2002, 41, 2405-2408. (10) Rogach, A. L.; Nagesha, D.; Ostrander, J. W.; Giersig, M.; Kotov, N. A. Chem. Mater. 2000, 12, 2676-2685. (11) Liz-Marzan, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 4329-4335. Nano Lett., Vol. 4, No. 2, 2004
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