Materials Letters 61 (2007) 5004 – 5009 www.elsevier.com/locate/matlet

Au and Au–Ag bimetallic nanoparticles synthesized by using 12-3-12 cationic Gemini surfactant as template Mandeep Singh Bakshi a,c,⁎, Poonam Sharma a , Tarlok Singh Banipal b a

Department of Ob/Gyn and Biochemistry, University of Western Ontario, 339 Windermere Road, London, ON, Canada N6A 5A5 b Department of Applied Chemistry, Guru Nanak Dev University, Amritsar 143005, Punjab, India c Department of Chemistry, Guru Nanak Dev University, Amritsar 143005, Punjab, India Received 16 October 2006; accepted 26 March 2007 Available online 30 March 2007

Abstract A seed-growth method has been applied to synthesize gold (Au) and Au–silver (Ag) bimetallic nanoparticles (NP) by using 12-3-12, a cationic Gemini surfactant, as a capping agent as well as micellar template. A systematic increase in the [12-3-12] from pre- to post-micellar region (up to 5 times the critical micelle concentration, cmc) produces Au NP from spherical to large plate like structures. Keeping [12-3-12] constant (equal to 1 / 2 cmc) and increasing ascorbic acid (AA) concentration lead to the formation of core shell type Au–Ag bimetallic NP. At maximum AA concentration (i.e. [AA] = 5.6 mM), fused bimetallic Au–Ag NP are obtained. The anisotropic growth of such materials is a key factor for various applications in nanotechnology. © 2007 Elsevier B.V. All rights reserved. Keywords: Gemini surfactant; Micellar templates; TEM; Au–Ag bimetallic nanoparticle

1. Introduction The seed-mediated approach in making large gold (Au) nanoparticles (NP) in aqueous surfactant solutions has become increasingly popular recently [1]. Unlike the use of strong reducing agents, the large growth of Au NP requires weak reducing conditions. Some studies have successfully lead to a control of the size distribution (typically 10–15%) in the range 5–40 nm, and the sizes can be manipulated by varying the ratio of seed to metal salt [2]. Step-by-step particle enlargement is more effective than a single-step seeding method to avoid secondary nucleation [3]. In seeding growth methods, small metal particles are prepared first and later used as seeds (nucleation centers) for the preparation of larger size particles. Such methods have been used for the size control of Au, Ag, Ir, Pd, and Pt particles [4]. A controlled number of seeds leading to a growth condition that inhibits any secondary nucleation, can be achieved simply by varying the ratio of seed to metal salt. ⁎ Corresponding author. Department of Ob/Gyn and Biochemistry, University of Western Ontario, 339 Windermere Road, London, ON, Canada N6A 5A5. E-mail address: [email protected] (M.S. Bakshi). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.03.092

Finding a suitable growth condition that inhibits additional nucleation during the growth generally limits the application of such methods [5].

Fig. 1. UV-visible spectra of aqueous gold nanoparticle solutions in the presence of 12-3-12 ([12-3-12] = 1 /2 cmc, 2 cmc, and 5 cmc) and ascorbic acid ([AA] = 0.7 mM).

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Fig. 2. a. TEM micrographs of gold nanoparticles in the presence of 12-3-12 ([12-3-12] = 1 / 2 cmc) and ascorbic acid ([AA] = 0.7 mM). b. TEM micrographs of gold nanoparticles in the presence of 12-3-12 ([12-3-12] = 2cmc) and ascorbic acid ([AA] = 0.7 mM). Left panel also shows the curved nanorod and right panel shows its enlarged view. The size of curved nanorod has been excluded while calculating the size distribution of gold nanoparticles. c. TEM micrographs of gold nanoparticles in the presence of 12-3-12 ([12-3-12] = 5 cmc) and ascorbic acid ([AA] = 0.7 mM).

Template directed synthesis represents a convenient route to synthesize 1D nanostructures. In this approach, the template mainly acts as a blue print for a nanomaterial which is shaped into a nanostructure with its morphology complementary to that of the template. In a chemical process, the template is usually consumed as the reaction proceeds and it is possible to directly obtain the nanostructure as a pure product. Surfactants have been commonly utilized as templates in the synthesis of NP. Charged surfactants have also been used as stabilizers and templates for

the growth of a variety of semiconductor and metallic nanodots [6]. Mixtures of cationic surfactants have recently been utilized as templates to prepare rod-shaped metallic [6c] and semiconductor [6d] nanoparticles. In most of the these studies ionic monomeric surfactants such as cetyltrimethylammonium bromide (CTAB) and sodium dodecylsulfate (SDS) have been frequently used as capping as well as micellar templates at high surfactant concentrations. Recently, few studies [7] have reported that ionic Gemini surfactants also act as wonderful

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capping agents for the synthesis of Au or silver (Ag) NP. In comparison to ionic surfactants, a relatively much lower concentration of the Gemini surfactant can be used for this purpose due to its dimeric nature which provide efficient capping ability [7c]. In this report, we are presenting a simple, convenient, and room temperature synthesis of large Au and Au–Ag bimetallic NP by simply varying the concentration of trimethylene-1,3-bis (dodecyldimethylammonium bromide), 12-3-12 (a cationic Gemini surfactant). We report that a concentration equal to half cmc [7c] ([12-3-12] = 0.5 × 10− 3 M, i.e. in the low millemolar range) is sufficient to get large NP with uniform size. 2. Experimental details 2.1. Materials Chloroauric acid (HAuCl4), sodium borohydride (NaBH4), and ascorbic acid were obtained from Aldrich. Trimethylene1,3-bis (dodecyldimethylammonium bromide)(12-3-12), were synthesized as reported in the literature [8] and used after repeated crystallization from ethanol. Pure water was used after purification through double distillation. 2.2. Preparation of 12-3-12 capped gold nanoparticles by S-G method In the S-G method, the first step includes the preparation of a seed solution. First of all, 10 ml of HAuCl4 aqueous solution ([HAuCl4] = 0.5 mM) was taken and sodium citrate was added in it so as to make [sodium citrate] = 0.5 mM. Then, 0.6 ml of NaBH4 ([NaBH4] = 0.1 mol dm− 3) solution was added under constant stirring giving rise a ruby red color to the final solution which acts as a seed solution. The growth solution was prepared by taking 30 ml of aqueous HAuCl4 ([HAuCl4] = 0.5 mM) solution in a flask and 12-3-12 was added in order to achieve [12-312] = 0.5 mM. It was stirred for a while so as to completely dissolve the surfactant and then AgNO3 was added to give the final [AgNO3] = 0.25 mM. It was followed by the addition of 0.2 ml ascorbic acid ([AA] = 0.1 M) under constant stirring. This completes our growth solution. After this, the final reaction was performed by taking 9 ml of growth solution, and a 1 ml of seed solution was mixed in it. It was gently shaken for 30 s and left undisturbed for 2 days in the dark. Similar reactions were performed under different surfactant and ascorbic acid concentrations. All final products with different surfactant concentrations gave deep pink color while increase in the ascorbic acid concentration in the growth solution leads to a predominantly yellow color of the final solution. Purification of the NP was carried out by repeated washing with distilled water after performing centrifugation at 10,000 rpm for 5 min each time to remove maximum surfactant. 2.3. Methods UV-visible spectra of as prepared solutions after the reduction of metal ions were taken by UV spectrophotometer (Perkin Elmer Lambda 25) in the wavelength range of 200–900 nm.

Fig. 3. UV-visible spectra of aqueous gold–silver bimetallic nanoparticle solutions in the presence of 12-3-12 ([12-3-12] = 1 / 2 cmc) at different ascorbic acid concentrations. Note a blue shift in the absorbance from [AA] = 0.7 mM to [AA] = 5.6 mM indicating the formation of gold–silver bimetallic nanoparticle. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

The formation of Au nanoparticles was monitored in the visible absorption range of ≈ 530 nm. The shape and size of gold nanoparticles were characterized by transmission electron microscopy (TEM). The samples were prepared by mounting a drop of a NP solution on a carbon coated Cu grid and allowed it to dry in air. They were observed with the help of a Philips CM10 Transmission Electron Microscope operating at 100 kV. 3. Results and discussion 3.1. 12-3-12 concentration effect The synthesis of Au NP in aqueous 12-3-12 solution was confirmed by measuring the UV-visible spectrum at ≈ 530 nm. Each surfactant solution containing Au NP gave a clear absorbance at ≈ 530 nm due to the characteristic surface plasmon resonance. Fig. 1 shows such spectra for Au NP solutions in the presence of 1/2 cmc (0.5 mM), 2 cmc (2 mM), and 5 cmc (5 mM) i.e. ranging from pre- to post-micellar concentration range [1g,7c]. The absorbance peak at 530 nm is quite sharp when [123-12] = 1 / 2 cmc was used (curve 1), increase in the concentration to 2 cmc keeps the absorbance almost at the same place but intensity decreases (curve 2), while increase in the concentration to 5 cmc significantly broadens the absorbance (curve 3). A sharp absorbance band indicates the presence of uniform size Au NP in aqueous monomeric 12-3-12 (at 1 / 2 cmc) as capping agent. As the concentration of 12-3-12 shifts to the micellar phase, the micellar assemblies act as templates for further nucleation of Au NP. A decrease in the intensity (curve 2) suggests the presence of Au NP in the form of aggregates, while the broadening of the absorbance at [12-3-12] = 5 cmc (curve 3) indicates a significant increase in the morphology of NP [9]. TEM micrograph (Fig. 2a) on the other hand shows the presence of fairly monodisperse Au NP (roughly hexagonal) with size distribution around 38 ± 3 nm at [12-3-12] = 1 / 2 cmc. This proves that monomeric surfactant only acts as a capping agent for Au NP in order to achieve a charge and steric stabilization [10]. As the concentration of the surfactant increases further (i.e. at 2 cmc), the size of most of the particles decreases (25.2 ± 3 nm) (Fig. 2b), but it also shows the presence of large horn shaped curved nanorods (CNR) with size close to 125 nm ranging from one limb to another. The micellar structure transitions of 12-3-12

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Fig. 4. a. TEM micrographs of gold–silver bimetallic nanoparticles in the presence of 12-3-12 ([12-3-12] = 1 / 2 cmc) and ascorbic acid ([AA] = 1.4 mM). Details are given in the text. b. TEM micrographs of gold–silver bimetallic nanoparticles in the presence of 12-3-12 ([12-3-12] = 1 / 2 cmc) and ascorbic acid ([AA] = 5.6 mM). Details are given in the text.

might be responsible for a change in the morphology of Au NP to CNR due to the micellar template effects. The mechanism with which cationic Gemini surfactants stabilized the Au NP is considered to be quite similar to that of CTAB [7b,c]. In post-micellar concentration regime, the surfactant apart from preferentially adsorbing at the NP surface in bilayer fashion also exists in the form of micellar assemblies. The negatively charged complex ions of Au(III) as well as Au(I) (an intermediate in the reduction process) bind to the positively charged surfactant head groups available at the micelle-solution interface. This kind of association may change the kinetics and energetics of the redox reaction, and the latter effect even becomes more significant when this association is controlled by the dimeric cationic head group of a Gemini surfactant in comparison to that of a monomeric head group of CTAB.

Thus, a change in the nature of head group significantly alters the micellization process which subsequently influences the nucleation process. Gemini surfactants are known to undergo structure transitions in quite dilute concentration regime [11] (i.e. a few order of their cmc) contrary to their monomeric homologues such as CTAB which demonstrates this effect at relatively much higher concentration. Therefore, it suggests that further nucleation and crystal growth at [12-3-12] is essentially confined to only {111} facets since surfactant monomers preferentially bind to the {100} facets due to the presence of lower atomic density in comparison to that on {111} facets [12]. This mechanism directs Au0 atoms at {111} facets for further nucleation and generally produce straight NR. However, the formation of CNR as in the present study indicates that it is not only {111} facets but {110} facets

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might also be participating in the anisotropic growth and could be the consequences of the kinetics and energetics of Gemini micellar assemblies which act as template for such a growth. Further increase in the concentration up to 5 cmc leads to the formation of large plate like geometries with length and width close to 6 and 1.5 μm, respectively, (Fig. 2c). Gemini surfactants mostly form lamellar phase at high surfactant concentration [11] (mostly in the mM range). Such large micellar structures are quite suitable templates for further nucleation and might lead to 3D plate like geometries. Generally a broad surface plasmon resonance arises due to the aggregation of nanosized NP [9d]. This leads to a decrease in the interparticle distance even than that of the NP diameter resulting in the coupling interactions which broaden and shift absorbance towards the higher wavelength. It gives a blue color to the NP colloidal solution. Interestingly, we do not observe any blue color for Au NP at 5 cmc and instead of it, only large Au NP were obtained at the bottom of the tube. This demonstrates that the plate like geometries mainly originate from the further nucleation of Au NP possibly on the CNR, supported by the lamellar phase of 12-312. 3.2. Ascorbic acid (AA) concentration effect An increase in the amount of ascorbic acid demonstrates a significant effect on the UV absorbance of Au NP. Fig. 3 shows that as the amount of AA is increased from 0.7 mM to 5.6 mM, there is 80 nm blue shift in the absorbance peak from 530 nm to 450 nm [13]. Likewise the color of the NP solution changes from pink to yellow indicating the formation of Au–Ag bimetallic NP. Fig. 4a shows the TEM micrograph of dendritic type roughly spherical bimetallic NP with size distribution 36.3 ± 3 nm and which is synthesized by using [AA] = 1.4 mM. Further increase in the [AA] = 5.6 mM does not increase the size anymore but NP starts merging with each other (Fig. 4b). It seems that the increase in the amount of AA facilitates the reduction of greater amount of Ag+ ions to Ag0 atoms which nucleate on {111} facets of already available Au NP leading to the formation of Au–Ag bimetallic NP [14]. Close inspection of some of the particles indicates that they are clearly made up of Au core and Ag shell (shown by solid block arrows in both Fig. 4a and b). This is also in line with the smaller reduction potential of Auion / Auatom (aq.) (− 1.4 V) than that of the Agion / Agatom (aq.) (− 1.8 V), which predicts an easier conversion of the former than latter. Apart from this, one can identify a significant change from a sharp absorbance at AA = 0.7 mM (yellow line) to a broad absorbance at AA = 5.6 mM (blue line) (Fig. 3) which corresponds to the presence of monodisperse Au NP (Fig. 2a) and interconnected (aggregated) Au–Ag bimetallic NP [14,15] (Fig. 4), respectively. Fig. 4a shows that how large Au–Ag bimetallic NP are interconnected (shown by empty block arrows) with thin bridges when the concentration of AA = 1.4 mM is used. These bridges later on become broad and subsequently lead to the fusion of adjoining NP in certain cases (shown by empty block arrows in Fig. 4b) when the concentration of AA is reached to 5.6 mM. Though, some studies [1j,15] have reported similar bimetallic Au–Ag NP, the present results demonstrates a clear mechanism of conversion of pure Au NP to Au–Ag bimetallic NP upon increasing the amount of AA and that too under ambient conditions. This kind of mechanism can be better explained on the basis of an aggregation growth [16]. The latter term is used when NP aggregates due to some kind of driving force applied by the capping agent. The capping mechanism is expected to be similar in the case of Gemini surfactants to that of CTAB where the surfactant monomer adsorb at the surface of Au NP through electrostatic interactions [7b–d,15]. This happens due to the electrostatic interactions of Br− counterions at the surface of NP which in return interacts with electropositive cationic

head groups. But since the Gemini surfactants are much more hydrophobic (due to their twin tails) than their homologous monomeric surfactants [17], therefore, the stronger hydrophobicity acts as driving force for the aggregation of NP in small aggregates. Once the NP is close enough to each other, any further reduction of a metal ion into metal atom triggers the nucleation on the surface of already available NP [14]. This particularly happens on that crystal plane of anisotropic geometry which is partially occupied by the surfactant monomers. Generally {111} planes are more prone for further nucleation in comparison to {100}. Thus, increase in the amount of AA induces further slow reduction of Ag+ into Ag0 which facilitates the nucleation of Ag0 atoms on {111} planes of already available Au NP. Because the Au NP are available in their close proximity due to the hydrophobic interactions of capping Gemini surfactants, therefore, further anisotropic growth leads to the fusion of adjoining NP by growing Ag shell. Greater reduction by slow reducing agent like AA increases the nucleation process and that in return increases the thickness of the shell. The increase in the thickness of the shells of the adjoining NP present in the aggregates leads eventually to the fusion of the NP as it happens through thin bridges at AA = 1.4 mM (Fig. 4a) and ultimately leading to the overall fusion of NP (Fig. 4b).

4. Conclusions A seed-growth method at ambient conditions has been applied to synthesize the Au and Au–Ag bimetallic NP by using 12-3-12 cationic Gemini surfactant. An increase in the surfactant concentration from pre- to post-micellar region causes anisotropic growth of Au NP. When the concentration of 12-3-12 is twice the cmc, curved nanorods are obtained, and further increase in the concentration to five times the cmc produces large plate like geometries. The micellar structural transitions are considered to be controlling this anisotropic growth. An increase in the ascorbic acid concentration produces Au– Ag bimetallic NP consisting of Au core and Ag shell. A slow reduction of Ag+ ions to Ag0 atoms, and subsequently their nucleation on Au NP surface systematically increases the thickness of Ag shell. The latter phenomenon ultimately leads to the fusion of adjoining NP. The results conclude that how anisotropic growth can be achieved for Au and Au–Ag bimetallic NP by simply varying the Gemini surfactant concentration at ambient conditions. Anisotropic growth of NP is a key factor for various applications in nanotechnology. References [1] (a). N.R. Jana, L. Jana, C.J. Gearheart, Murphy, J. Phys. Chem., B 105 (2001) 4065; (b). N.R. Jana, L. Gearheart, C.J. Murphy, Langmuir 17 (2001) 6782; (c). A. Gole, C.J. Murphy, Chem. Mater. 16 (2004) 3633; (d). B. Nikoobakht, M.A. El-Sayed, Langmuir 17 (2001) 6368; (e). A. Swami, A. Kumar, M. Sastry, Langmuir 19 (2003) 1168; (f). N.R. Zana, Small 1 (2005) 875; (g). M.S. Bakshi, A. Kaura, G. Kaur, K. Torigoe, K. Esumi, J. Nanosci. Nanotechnol. 6 (2006) 644; (h). M.S. Bakshi, A. Kaura, P. Bhandari, G. Kaur, K. Torigoe, K. Esumi, J. Nanosci. Nanotechnol. 6 (2006) 1405; (i). X. Kou, S. Zhang, C.K. Tsung, M.H. Yeung, Q. Shi, G.D. Stucky, L. Sun, J. Wang, C. Yan, J. Phys. Chem. 110 (2006) 16377; (j). M. Liu, P. Guyot-Sionnest, J. Phys. Chem. 109 (2005) 22192;

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