Acrylonitrile-Induced Synthesis of Polyvinyl Alcohol-Stabilized

Sep 12, 2008 - Selenium Nanoparticles. Chetan P. Shah, Manmohan Kumar,* Kumbil K. Pushpa, and Parma N. Bajaj. Radiation and Photochemistry DiVision, ...
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CRYSTAL GROWTH & DESIGN

Acrylonitrile-Induced Synthesis of Polyvinyl Alcohol-Stabilized Selenium Nanoparticles Chetan P. Shah, Manmohan Kumar,* Kumbil K. Pushpa, and Parma N. Bajaj Radiation and Photochemistry DiVision, Bhabha Atomic Research Centre, Trombay, Mumbai-400 085, India

2008 VOL. 8, NO. 11 4159–4164

ReceiVed June 24, 2008; ReVised Manuscript ReceiVed July 21, 2008

ABSTRACT: A new simple wet chemical method has been developed to synthesize selenium nanoparticles (size 50 to 100 nm), by reaction of sodium selenosulphate precursor with acrylonitrile monomer, under ambient conditions. Polyvinyl alcohol has been used to stabilize the selenium nanoparticles. The synthesized nanoparticles can be separated from their sol by using a high-speed centrifuge and can be redispersed in aqueous medium with an ultrasonicator. UV-visible optical absorption spectroscopy, X-ray powder diffraction, energy dispersive X-ray analysis, differential scanning calorimetry, atomic force microscopy, scanning electron microscopy, and transmission electron microscopy techniques have been used to characterize the synthesized selenium nanoparticles. Gas chromatography-mass spectrometry analysis has shown formation of β-hydroxy propionitrile as a byproduct. On the basis of these findings, a mechanism has been proposed for the formation of selenium nanoparticles and β-hydroxy propionitrile. Introduction The synthesis of nanosized materials has received great interest because of their unique optoelectronic, magnetic, and mechanical properties, which differ largely from that of the bulk materials, as well as because of their applications in the fields of catalysis, sensors, and nanodevices.1 A lot of research has been carried out on semiconductors and metals. However, studies on metalloids, like selenium and so forth, are scant. Materials containing selenium, such as ZnSe, CdSe, and so forth, play an important role in many applications because of their photoconductive and photovoltaic properties.2-4 Selenium is also an important element for human beings,5 and deficiency of selenium in the body may lead to many diseases, including cancer.6 Nano selenium has been widely used in electrical rectifiers, photocells, photographic exposure meter, xerography, and so forth.7 It also exhibits glass-forming tendencies.8 Selenium, a metalloid, is present in nature in igneous rocks and fossil fuel. It exists as toxic and soluble selenate (SeO42-) and selenite (SeO32-), and also as insoluble and nontoxic elemental selenium (Se0) forms.9 Nanoselenium is known to be synthesized by various methods, such as chemical reduction,10 sonochemical process,11 γ-radiolytic reduction,12and so forth. Bacterial reduction of selenite (SeO32-) to Se0 has also been studied.9 Selenous acid and sodium selenite are the most frequently used selenium precursors in reduction methods, and the commonly used reducing agents are glutathione, hydrazine, dextrose, ascorbic acid, sodium ascorbate, and so forth. Also, there are a few reports on the formation of selenium nanoparticles by oxidation methods, such as reaction of hydroxyl radicals with selenourea, using a radiolysis technique,13 electrochemical oxidation of selenide ions,14 and so forth. We have recently reported acidinduced formation of selenium nanoparticles from sodium selenosulphate,15 analogous to the well-known acid-induced formation of colloidal sulfur from sodium thiosulphate, and subsequently, it has also been reported by Stroyuk et al.16 In the present study, we report a new simple wet chemical method for the synthesis of polyvinyl alcohol (PVA)-stabilized selenium nanoparticles by reaction of sodium selenosulphate

with acrylonitrile monomer, at ambient conditions. No similar reported work exists in the literature to the best of our knowledge. The synthesized selenium nanoparticles were characterized by UV-visible optical absorption spectroscopy, X-ray powder diffraction (XRPD), energy dispersive X-ray analysis (EDAX), differential scanning calorimetry (DSC), atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques. The gas chromatography-mass spectrometry (GC-MS) technique was used to establish the formation of β-hydroxy propionitrile along with selenium nanoparticles. Experimental Section Materials. High purity PVA of molecular weight 1,25,000, AR grade acrylonitrile obtained from s. d. Fine Chemicals, and selenium powder from Aldrich, were used as received. All the other chemicals used were of GR grade and procured from the local market. Aqueous solutions were prepared using water obtained from a Millipore-Q water purification system. Sodium selenosulphate Na2SeSO3 was prepared by the method reported earlier, using reaction between aqueous Na2SO3 solution and Se powder.17

Na2SO3(aq)+Se(s) f Na2SeSO3(aq)

(1)

Briefly, a mixture of selenium powder (2 g) and Na2SO3 (20 g) in 100 mL of water was refluxed at 70 °C for about 7 h. After completion of the refluxing process, the reaction mixture was filtered to remove unreacted selenium, and the filtrate was kept in dark to prevent photooxidation. This sodium selenosulphate (Na2SeSO3) solution (∼0.25 M), containing unreacted Na2SO3, was used as a stock solution for Se precursor. One % PVA stock solution was prepared by dissolving 1.0 g of PVA into 100 mL of water, while stirring at 80 °C. Both these stock solutions were diluted further with water, to the required concentrations, for different experiments. Synthesis of Se Nanoparticles. PVA-stabilized Se nanoparticles were synthesized by mixing known volumes of aqueous sodium selenosulphate solution (concentration 5 × 10-4 to 1.5 × 10-3 mol

Scheme 1. Tetrahedral Structure of SO42-, S2O32-, and SeSO32- Anions

* To whom correspondence should be addressed. E-mail: manmoku@ barc.gov.in. Phone: 91-22- 25593994. Fax: 91-22- 25505151.

10.1021/cg800669d CCC: $40.75  2008 American Chemical Society Published on Web 09/12/2008

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Shah et al. experiments were carried out on a GCMS-QP2010 model from Shimadzu, using a Rt-QPLOT column of length 30 m and diameter of 0.32 mm, at 170 °C, and helium as a carrier gas at a flow rate of 1.40 mL/min.

Results and Discussion

Figure 1. Aqueous selenium nanoparticle sols obtained at three different starting concentrations of sodium selenosulphate, (A) 5 × 10-4, (B) 1 × 10-3, and (C) 1.5 × 10-3 mol dm-3 Na2SeSO3, in the presence of 0.05% PVA and 9.0 × 10-2 mol dm-3 acrylonitrile. dm-3), containing the required concentration of PVA stabilizer (0.025 to 0.075%), with the required amount of acrylonitrile (concentration ∼ 9.0 × 10-2 mol dm-3). The mixture was stirred in a closed vessel, to avoid evaporation of volatile acrylonitrile at room temperature. The process of formation of selenium nanoparticles was over in less than 1 h. The reaction was also studied at lower concentrations of acrylonitrile, but it required much longer stirring time to complete. Unless mentioned otherwise, all the experiments were carried out in the presence of 9.0 × 10-2 mol dm-3 acrylonitrile. Characterization. UV-visible optical absorption spectra of the selenium nanoparticle sols were recorded, using a double beam spectrophotometer, model Spectroscan 2600 from Chemito. XRPD patterns of the nanoparticles were recorded with a Philips X-ray diffractometer, model PW 1710 system, using a Cu KR source (λ ) 0.15406 nm). DSC measurements were carried out, using a Mettler TA 3000 thermal analysis system. About 10 mg of the synthesized selenium nanoparticles or of standard selenium powder was weighed into an alumina crucible, and the profile was recorded from 45 to 250 °C, in N2 atmosphere, at a heating rate of 10 °C/min (model DSC-30). Selenium nanoparticles, separated from aqueous sols, using high-speed centrifuge at about 14000 rpm, washed with water and dried at room temperature, were used for XRPD, EDAX, and thermal analysis measurements. AFM analysis of the synthesized selenium nanoparticles was carried out, using a Solver P47 model from NT-MDT, Russia. SEM of the synthesized selenium nanoparticle was recorded, using a TESCAN VEGA MV 2300 T/A digital microscope. TEM characterization was carried out with a JEOL-2000 FX electron microscope, using the sample on a copper grid coated with a thin amorphous carbon film. The separated selenium nanoparticles were redispersed into water, using an ultrasonicator, and used for preparing samples for AFM, SEM and TEM experiments. Gas chromatography-mass spectrometry (GC-MS)

During an attempt to make a polyacrylonitrile-selenium nanoparticle composite by radiation-induced simultaneous polymerization of acrylonitrile and formation of selenium nanoparticles from sodium selenosulphate, it was discovered that sodium selenosulphate reacts with acrylonitrile monomer, producing selenium nanoparticles. Literature survey did not show the existence of any such reaction. The ambient reaction conditions and the simplicity of the process encouraged us to study it in detail, to understand its mechanism, and to develop it as a new method for the production of selenium nanoparticles. The anions S2O32- and SeSO32- are the S and Se analogues, respectively, of the SO42- anion and are known to have tetrahedral structure (Scheme 1). Thus, similar to the reaction between sodium selenosulphate and acrylonitrile, a reaction between sodium thiosulphate and acrylonitrile to form sulfur sol may also be expected. However, no such reaction was observed at room temperature up to the studied time period of about 20 days. This may be due to the higher reactivity/lower stability of the selenosulphate anion compared to that of the thiosulphate anion. One of the reasons for this is the bigger size of Se than that of S, which facilitates removal of Se from the SeSO32- anion. To check the role of the nitrile functional group, the reaction was also attempted with acetonitrile instead of acrylonitrile. However, selenium nanoparticles were not formed with acetonitrile. Thus, the nitrile functional group of acrylonitrile is not directly involved in the formation of selenium. An acid can induce formation of selenium nanoparticles from sodium selenosulphate, as reported earlier.15,16 A small amount of hydroquinone is generally added to monomers, as a radical scavenger, to prevent homo polymerization during their storage. To rule out the possibility of acid or hydroquinone-induced formation of selenium nanoparticles, the acrylonitrile monomer, washed with aqueous 2% sodium hydroxide (NaOH) solution, followed by washing with water and drying with anhydrous sodium sulfate, was also used in the reaction with sodium selenosulphate. Similar results were obtained as with the unwashed acrylonitrile. These observations clearly indicate that an acid or traces of a stabilizer present in the monomer are not responsible for the observed formation of selenium nanoparticles

Figure 2. Effect of (A) sodium selenosulphate concentration (a) 5 × 10-4, (b) 1 × 10-3, and (c) 1.5 × 10-3 mol dm-3, in the presence of 0.05% PVA, and of (B) PVA concentration (d) 0.025%, (e) 0.050%, and (f) 0.075% PVA, in the presence of 1.0 × 10-3 mol dm-3 sodium selenosulphate, on the optical absorption spectrum of selenium nanoparticle sols, prepared in the presence of 9.0 × 10-2 mol dm-3 acrylonitrile.

Synthesis of Selenium Nanoparticles

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Figure 3. XRPD pattern of (a) synthesized selenium nanoparticles without heating, and (b) synthesized selenium nanoparticles annealed at 130 °C for 30 min.

Figure 4. EDAX pattern of the synthesized selenium nanoparticles.

Figure 5. DSC thermograms of (a) synthesized selenium nanoparticles, (b) synthesized selenium nanoparticles repeat run, and (c) standard commercial selenium powder.

from aqueous sodium selenosulphate. Probably, it is the reaction involving the double bond of the acrylonitrile monomer and the sodium selenosulphate that results in the formation of selenium nanoparticles. The detailed mechanism of the reaction will be discussed later. Effect of Sodium Selenosulphate and PVA Concentration. The intensity of the selenium nanoparticle sols was found to be dependent on the starting concentration of sodium selenosulphate. The orange/red colored selenium sols of different intensities, obtained at three different concentrations of sodium selenosulphate, are shown in Figure 1.

Figure 2 shows the effect of concentration of selenium precursor and PVA on the UV-visible optical absorption spectrum of the selenium nanoparticle sols, prepared by addition of acrylonitrile. The absorption spectra of selenium nanoparticles do not have any clear maximum in the studied wavelength region. However, it may be present at a wavelength lower than 300 nm. The nature of the spectra matches very well with those reported earlier.18 The reported results indicate that Se sols, with particles of mean diameter 100 nm or more, show an absorption peak at 300 nm or higher wavelength, depending on the size, while those with smaller particles do not show any regular

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Figure 6. AFM images of selenium nanoparticles formed by reaction of 5 × 10-4 mol dm-3 sodium selenosulphate with 9.0 × 10-2 mol dm-3 acrylonitrile, in the presence of 0.05% PVA (a) 2D image and (b) 3D image.

Figure 7. SEM image of the selenium nanoparticles formed by reaction of 5 × 10-4 mol dm-3 sodium selenosulphate with 9.0 × 10-2 mol dm-3 acrylonitrile, in the presence of 0.05% PVA.

absorption peak in the UV-visible region. Comparisons of the observed spectra with the reported ones indicate that the average particle diameter is about 100 nm or smaller. The absorption intensity of selenium sol increases with increase in the concentration of sodium selenosulphate, at a fixed PVA concentration of 0.05% (Figure 2A). It increases in the entire range of 200-600 nm, but the extent of increase is more pronounced at lower wavelengths. However, there is no significant effect of PVA concentration on the spectrum of the sol produced, except for a slightly higher absorbance in the lower wavelength region (Figure 2B), at the highest studied PVA concentration of 0.075%, which may be due to the contribution from PVA. PVA was found to be a very efficient stabilizer for the selenium nanoparticles. Even a small concentration of PVA (0.025% to 0.075%) was sufficient to stabilize the selenium nanoparticles, which could be separated from its sol by using a high-speed centrifuge at ∼10,000 to 14,000 rpm. However, selenium nanoparticles, formed in the sol containing 0.2% or more PVA, could not be separated because of stronger PVA-Se nanoparticle interaction and further reduction in Se particle size; the decrease in particle size with increase in PVA concentration is confirmed by dynamic light scattering experiments. In the absence of PVA stabilizer, a dark reddish black precipitate of selenium was formed. Effect of Acrylonitrile Concentration. The rate of selenium nanoparticles formation reaction was found to decrease drasti-

Figure 8. TEM image of the selenium nanoparticles formed by reaction of 5 × 10-4 mol dm-3 sodium selenosulphate with 9.0 × 10-2 mol dm-3 acrylonitrile, in the presence of 0.05% PVA.

cally with decrease in acrylonitrile concentration. In the aqueous 9.0 × 10-2 mol dm-3 acrylonitrile solution, containing 1.5 × 10-3 mol dm-3 Na2SeSO3 and 0.05% PVA, the reaction was complete in less than 1 h, whereas at 1.0 × 10-2 mol dm-3 acrylonitrile, the reaction took almost ∼ 24 h to complete; at a still lower acrylonitrile concentration (less than 5.0 × 10-3 mol dm-3), no reaction was observed even up to 10 days. The starting concentration of acrylonitrile needed to complete the reaction in less than 1 h is much more than that of sodium selenosulphate. It is expected that sodium selenosulphate will be almost completely consumed and excess acrylonitrile will be left in the reaction mixture, which was confirmed by the observed formation of selenium nanoparticles on addition of a fresh aliquot of sodium selenosulphate to the reaction mixture (after separating the initially formed selenium nanoparticles), while on addition of acid, no such selenium nanoparticle formation was seen. Gas chromatographic analysis also confirmed the presence of the leftover acrylonitrile in the reaction mixture.

Synthesis of Selenium Nanoparticles

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Figure 9. Gas chromatograms of (a) aqueous acrylonitrile (9.0 × 10-2 mol dm-3), (b) sample from the reaction mixture, after completion of the reaction, and mass spectra of (c) the acrylonitrile and (d) the product.

Scheme 2. Proposed Mechanism for Acrylonitrile-Induced Synthesis of Selenium from Sodium Selenosulphate

XRPD Study of Selenium Nanoparticles. Crystal parameters of the selenium nanoparticles were determined by the XRPD technique. Typical XRPD patterns of the two types of selenium nanoparticles, one without heat treatment and another after annealing at 130 °C, for about 30 min, are displayed in Figure 3. These XRPD patterns suggest that the untreated sample (Figure 3a) is probably nanocrystalline in nature, and its crystallinity increases on annealing (Figure 3b). The XRPD

pattern of the synthesized selenium nanoparticles, after annealing at 130 °C, matches very well with that of the standard selenium powder, confirming the formation of selenium particles. All the diffraction peaks are indexed assuming a trigonal phase, with calculated lattice constants, a ) 4.363 Å and c ) 4.952 Å, which are in agreement with the literature values of a ) 4.360 Å and c ) 4.956 Å (JCPDS file No. 06-362).19 Further, chemical composition of the selenium nanoparticles was determined with EDAX analysis. The EDAX spectrum shown in Figure 4 also confirms that the nanoparticles are of pure selenium and do not contain any other element as impurity. Differential Scanning Calorimetry Study. A DSC thermogram of the synthesized selenium nanoparticles, recorded up to 250 °C, is shown in Figure 5a. It shows an exothermic transition at 81 °C, in addition to the endothermic melting peak at higher temperature. The enthalpy of the transition is found to be 48.4 J/g. The repeat DSC thermogram of the same selenium sample, recorded after bringing it to ambient temperature, did not show any exothermic peak at the mentioned temperature (Figure 5b). This clearly indicates that selenium nanoparticles loose their nanocrystalline nature in the first run itself, and the exothermic transition is assigned to the increase in crystallinity of the selenium nanoparticles. This observation is in corroboration with the XRPD results obtained with the synthesized sample annealed at 130 °C. Similarly, the DSC thermogram of the standard selenium powder sample also did not show any such exothermic peak (inset Figure 5c). All the three DSC thermograms showed a melting peak at 220 °C. AFM, SEM, and TEM studies. AFM, SEM, and TEM are the most commonly used techniques that provide actual size, shape, surface topography, and so forth of the nanoparticles.

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Therefore, morphology and structure of the synthesized selenium nanoparticles were studied by these techniques. Typical 2D and 3D AFM images of the nanoparticles are shown in Figure 6. It is clear from the 2D image that the synthesized selenium nanoparticles are almost spherical in shape, while the 3D image indicates that the individual selenium nanoparticles, with size in the range of 50 to 100 nm, are present. Scanning electron microscope and transmission electron microscope images of the synthesized selenium nanoparticles are shown in Figures 7 and 8, respectively. Spherical selenium nanoparticles, ranging from 50 to 100 nm, and smaller aggregates of the particles are seen in the SEM image. Most of the individual particles, seen in the TEM image, seem to be of uniform shape and size. Though, aggregation of the particles appears to be random (non directional) in the SEM image, it seems to be somewhat unidirectional in the TEM image, resulting in nanowires of length of ∼500 nm or more, in certain regions. The conclusion drawn from AFM, SEM, and TEM studies more or less corroborate each other. GC-MS Study. GC-MS was employed to know the fate of acrylonitrile during the selenium nanoparticle formation reaction. Gas chromatographic analysis of the reaction mixture showed that acrylonitrile is not acting as a catalyst and is being consumed during the reaction. Gas chromatograms of aqueous acrylonitrile solution and of a sample of the reaction mixture, after completion of the reaction, are shown, respectively, in the panels a and b of Figure 9. In Figure 9b, two peaks are observed, one for the unreacted acrylonitrile at shorter retention time and another for the byproduct at a relatively longer retention time. The mass spectrum of the later peak (Figure 9d) was found to match with that of β-hydroxy propionitrile, reported in the NIST library of mass spectra. Therefore, the peak was assigned to β-hydroxy propionitrile. However, to rule out the possibility of formation of β-hydroxy propionitrile on the surface of preformed selenium nanoparticles, particles synthesized by an acid were equilibrated with aqueous acrylonitrile solution. The supernatant solution, obtained after centrifugation, was analyzed by GCMS. No peak corresponding to β-hydroxy propionitrile was observed in the chromatogram. This indicates that the reaction of sodium selenosulphate with acrylonitrile results in the formation of selenium nanoparticles and β-hydroxy propionitrile. Proposed Mechanism of the Reaction. On the basis of the GC-MS results, a mechanism has been proposed for the formation of selenium and β-hydroxy propionitrile, which is shown in Scheme 2. In the first step, a selenium anion of selenosulphate attacks at the β-carbon of acrylonitrile via a Michael type of reaction, followed by formation of a bond between the R-carbon and the sulfur atom. This results in a four-membered cyclic transient species. Hydroxide anion can now attack at the R- or β-carbon (with respect to cyanide group) of the four-membered cyclic ring. If it attacks at the β-carbon of the unstable four-membered cyclic ring, separation of selenium and subsequent formation of β-hydroxy propionitrile takes place, on protonation. On the contrary, if the hydroxide anion attacks at the R-carbon, it may result in a cyanohydrin derivative, which can finally be converted to an aldehyde. Because the mass spectrum showed the formation of only β-hydroxy propionitrile, it is concluded that the attack of the hydroxide occurs at the β-carbon, and not at the R-carbon. Conclusions In summary, we have reported, for the first time, the synthesis of selenium nanoparticles by reaction of sodium selenosulphate

Shah et al.

with acrylonitrile. The method has been found to be very simple and can be carried out under ambient conditions. Selenium nanoparticles have been efficiently stabilized by PVA. Optical absorption spectra of selenium nanoparticle sols have shown an increase in absorbance throughout the studied wavelength region of 200 to 600 nm, with increase in sodium selenosulphate concentration, while almost no effect of PVA concentration, in the 0.025 to 0.075% range, has been observed. the nanoscale dimensions of the initially formed selenium particles, and an increase in crystallinity on heating them, were confirmed by both XRPD and DSC experiments. The size of the individual selenium nanoparticles, as determined by AFM, SEM, and TEM techniques, was found to be in the range of 50 to 100 nm. A small fraction of the nanowire aggregates, with aspect ratio of ∼6, was also detected. On the basis of product analysis by GCMS, a mechanism involving attack of the selenium anion on the β-carbon of acrylonitrile, forming a four-membered cyclic transient species, followed by its hydrolysis, has been proposed. The selenium nanoparticles may serve as template to generate other important nano materials and find applications in fabrication of nanoscale optoelectronic devices. Acknowledgment. C.P.S. is grateful to the Department of Atomic Energy for the award of a research fellowship. The authors are thankful to Dr. P.A. Hassan, BARC, for AFM, Dr. S. K. Gupta, BARC, for SEM and EDAX, Dr. D. Shrivastava, BARC, for TEM, and Dr. L. Varshney, BARC, for DSC experiments. The authors also wish to acknowledge Dr. T. Mukherjee and Dr. S.K. Sarkar, for their encouragement during the course of the study.

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