Structure and Photoluminescence Properties of Zinc Sulfide

Dec 30, 2008 - Zinc sulfide nanoparticles (ZnS NPs) were prepared under several precursor concentration conditions in Laponite XLG suspension as clay ...
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J. Phys. Chem. C 2009, 113, 894–899

Structure and Photoluminescence Properties of Zinc Sulfide Nanoparticles Prepared in a Clay Suspension Yasuyuki Arao, Yutaka Hirooka, Katsumi Tsuchiya, and Yasushige Mori* Department of Chemical Engineering and Materials Science, Doshisha UniVersity, 1-3, Miyakodani, Tatara, Kyoutanabe-shi Kyoto 610-0321, Japan ReceiVed: June 11, 2008; ReVised Manuscript ReceiVed: October 6, 2008

Zinc sulfide nanoparticles (ZnS NPs) were prepared under several precursor concentration conditions in Laponite XLG suspension as clay stabilizer to prevent the aggregation of ZnS NPs and to enhance their photoluminescence (PL). We measured their properties such as PL, UV-vis absorption, X-ray diffraction, and the particle image by transmission electron microscopy. The results of the measurements showed that ZnS NPs prepared in the presence of the clay particles were monocrystalline particles, since the dispersed Laponite XLG clay particles had a stabilizing effect on ZnS NPs. ZnS NPs prepared without clay were the aggregated particles of multicrystalline particle. When the precursor concentration of ZnS NPs increased, the particle diameter of ZnS NPs increased. These results agreed with von Weimarn’s law. PL originating from the defect level on the ZnS NPs surface was observed. The PL intensity was enhanced due to the increase of the particle surface area, as the clay particles existed or the particle diameter of ZnS NPs decreased. Introduction Zinc sulfide (ZnS) is a typical II-VI group semiconductor and is used as an ingredient in cosmetics, CRT, and solar batteries. In recent years, its optical properties were improved by decreasing its particle size to nanometers.1,2 Presently, ZnS nanoparticles (NPs) are being tested for applications in inorganic electroluminescence3 and as gas censors.4 As the ratio of the number of atoms on the surface to the volume of NPs increases with decreasing the particle size, the surface activity increases markedly, and the aggregation of NPs could occur to reduce the surface activity energy. The preparation method for making a synthetic semiconductor like ZnS NPs is often taken from the liquid-phase method, which is convenient for synthesis procedures. The following reagents are used as a stabilizer to prevent the aggregation of NPs prepared by the liquid-phase method: acrylic acid,5 polyvinyl alcohol,6 and thiol.7,8 In addition, the reverse micelle method is often used during particle synthesis to prevent aggregation.9-11 On the other hand, zeolite12,13 and layer materials14-19 are used as inorganic stabilizers. The inorganic stabilizers can prevent the aggregation of NPs for a long time because of their stability against external energy. In this paper, Laponite XLG, which is a kind of layered clay mineral, was used as a stabilizer. Since Laponite XLG is a synthetic mineral unlike a natural mineral such as montmorillonite, it has few impurities. Moreover, as the diameter of Laponite XLG particle is much smaller than the absorption and emission wavelength of ZnS particle, Laponite XLG does not disturb photoproperties of ZnS particles. The structure of Laponite XLG particle is disk-shaped and has a face diameter of 20 nm and thickness of 1.5 nm, and its chemical formula is [Mg5.34Li0.66Si8O20(OH)4]Na+0.66. Laponite XLG powder is the aggregated particle stacks with face to face because the face charge is neutral and the edge charge is positive due to its molecular structure. Laponite XLG powder in water can be * To whom correspondence should be addressed. Phone: +81-774-656626. Fax: +81-774-65-6847. E-mail: [email protected].

dispersed into single particles as shown in Scheme 1. Sodium ions on the face dissociate when Laponite XLG powder is added to water and so that the face of Laponite XLG particle is positively charged. The Laponite XLG particles repulse each other through their face charges, and water molecules enter among the Laponite XLG particles. In addition, the face and the edge of Laponite XLG particles attract each other due to the electrostatic attraction forces of the different charges on the face and the edge, and Laponite XLG particles forms the house of cards structure in water.20-22 The house of cards structure limits many reactive places in water. When NPs are prepared in these spaces, the probability of collision among NPs decreases, and then the aggregation of the particle can be prevented. Moreover, the photoabsorption of the ZnS NPs is not obstructed because Laponite XLG particles are transparent when dispersed in water. In this paper, we chose Laponite XLG clay as the stabilizer to prevent the aggregation of ZnS NPs prepared in a clay suspension. The effects of the clay suspension on the photoluminescence (PL) properties, the particle diameter, and the dispersion state of the ZnS NPs were examined by UV-vis absorption, X-ray diffraction (XRD), and transmission electron microscopy (TEM) and discussed. Materials and Methods 1. Materials. Zinc nitrate hexahydrate (Wako Pure Chemical Industries, Ltd., Japan) sodium sulfide enneahydrate (Wako Pure Chemical Industries Ltd., Japan) and synthetic Laponite XLG (Rockwood Additives Ltd., UK) were used as received. Laponite XLG was dried for 12 h at 120 °C before the experiments. Water (18.3 MΩ cm) was obtained from a Milli-Q system (Nihon Millipore Ltd., Japan) and passed through a 0.1-µm filter (mixed cellulose ester type, ADVANTEC Co, Ltd., Japan). 2. Sample Preparation. We prepared ZnS NPs in a clay suspension in the following way: 2 g of Laponite XLG powder were added to 398 g of water and stirred for 3 h. 200 mL of 10, 20, or 40 mM Zn(NO3)2 aqueous solution was added to 400 g of clay suspension and then stirred for 5 min. 200 mL of

10.1021/jp8051449 CCC: $40.75  2009 American Chemical Society Published on Web 12/30/2008

ZnS NPs Prepared in a Clay Suspension

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SCHEME 1: Scheme of Dispersion when Clay Is Added to Watera

a (a) Clay powder. (b) The element on the surface begins to dissociate. (c) Clay particles repulse each other due to their surface charges, and then they disperse. (d) Clay particles form house of cards structure.

Results 1. UV-Vis Absorption. The molar absorption coefficient, σ, of a direct transition type semiconductor such as ZnS is related to the band gap energy of the materials, Eg, by the following equation23

σhν ) K(hν - Ege0)1⁄2

Figure 1. Absorbance spectra of ZnS/clay colloidal solution and ZnS colloidal solution.

Na2S aqueous solution, which was the same concentration as the Zn(NO3)2 aqueous solution used, were rapidly added to the above mixed solution and stirred for 5 min to synthesize ZnS NPs. To analyze the NPs optical properties using UV-vis absorption and PL spectroscopy, a 0.25 mM ZnS NPs suspension sample was prepared by diluting the ZnS NPs suspension with Milli-Q water. To analyze the structure of the ZnS/clay powder by XRD, the ZnS/clay colloidal solution was condensed using a rotary evaporator, and then the white precipitates were obtained by centrifugation at 8000 rpm for 20 min and vacuum drying overnight. 3. Instruments. The UV-vis spectra of samples in the aqueous solution were collected with a UV-2400 (Shimazu Corp., Japan). The PL spectra of the ZnS NPs were measured using a FP-6500 (JASCO Corp., Japan), where the excitation wavelength used was 280 nm. XRD measurements were taken with a Rint-2500 (Rigaku, Japan), with Cu KR radiation (λ ) 0.154 nm) being used at 40 kV and 200 mA in 2θ ranges from 1 to 10° (small angle XRD) and from 20 to 70° (wide angle XRD). TEM images were obtained using an H-8100 (Hitachi Corp., Japan) with an accelerating voltage of 200 kV. The samples for TEM were prepared by putting a drop of the ZnS colloidal suspension on a copper grid. The grids were then dried under a vacuum.

(1)

where h is the Plank constant, ν is the frequency of light, e0 is the charge of an electron, and K is a proportional constant. Eq 1 means that the molar absorption coefficient increases with the frequency of light minus the frequency corresponding to the band gap energy. Therefore the band gap energy is calculated from measurement of the absorption spectrum. Figure 1 shows the absorption spectra of the prepared ZnS NPs with and without clay. The sample names in Figure 1 are referred to in Table 1. As the clay particles do not absorb light due to their very small size, the absorbance profile of ZnS NPs samples with clay can be measured. The band gap energy of the ZnS NPs was calculated from the absorption spectrum by using eq 1. The calculated band gap energies are listed in Table 1. As the band gap energy of bulk ZnS is 3.7 eV, the band gap energies of the prepared ZnS NPs were obtained to be 1.1-1.4 eV larger than that of bulk ZnS. This increase of the band gap energy is due to the quantum size effect, and indicates that the prepared ZnS NPs were nano sized.23 When the diameter of a semiconductor nanoparticle is near in value to the Bohr diameter, the band gap energy can be expressed by a function which relates particle diameter to the quantum size effect. This relationship is known as the equation for effective-mass approximation (EMA).24-26

Eg ) Eg,bulk +

4h2 2 dEMA e0

(

)

3.572e0 1 1 + * - 0.248Eex * 4πε0εpdEMA me mh

(2) where dEMA is the particle diameter, Eg,bulk is the band gap energy of the bulk material, Eex is the bond energy of the exiton, and m*e and mh* are the effective mass of the electron and hole, respectively. ε0 and εp are the electric constant and the dielectric constant of the particles, respectively. We referred to a previous

TABLE 1: Diameter of ZnS NPs on Smectite Obtained by Different Measuring Methods: Calculated from EMA Equation (dEMA), Broadening of the Diffraction Lines (dSherrer), Intercalation Reflection (dBragg), and TEM (dTEM) (PL Intensity at 420 nm and Quantum Efficiency (Φ)) particle diameter (nm)

sample

ZnS precursor concentration

Eg (eV)

dEMA

dSherrer

dBragg

dTEM

10-ZnS/clay 20-ZnS/clay 40-ZnS/clay 10-ZnS

10 mM 20 mM 40 mM 10 mM

3.88 3.86 3.81 3.88

4.6 4.8 5.3 4.6

3.3 4.0 4.3 4.0

3.3 4.1 5.0 6.4

4.8 4.8 5.8

PL intensity at 420 nm

Φ

151 70.7 48.3 29.7

0.078 0.030 0.018 0.0074

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Arao et al.

dSherrer )

Figure 2. Transmittance spectra of ZnS/clay colloidal solution and ZnS colloidal solution converted from UV-vis absorption spectra.

0.9λ B cos θ

where dSherrer is the crystallite diameter of ZnS NPs. B is the full-width at half-maximum of the diffraction peak of (220) face, λ is the wavelength of X-ray, and θ is the Bragg angle of (220) face. The calculated particle diameters, dSherrer, are listed in Table 1. 3. Small-Angle XRD. The small-angle XRD (SXRD) patterns of ZnS powder, clay powder, and nanocomposite powder of ZnS and clay were measured from a range of 1 to 10°. These results were indicated in Figure 4 and were the same tendency as the reported by Nemeth et al., who measured SnO2 NPs prepared in clay suspension. They calculated particle diameter of SnO2 NPs using eq 4.14 The peak of the clay sample was 6.8°, which corresponds to 1.3 nm of basal distance according to Bragg’s law. This basal distance may indicate the thickness of clay powder. The basal distance of the nanocomposite powder was longer than that of clay powder and indicated the intercalation of ZnS NPs among the clay particles, as shown in Figure 5.14,15 The particle diameter of ZnS NPs in the nanocomposite powder, dBragg, was estimated from its basal distance, dL, and the thickness of the clay particles, dL(clay) (1.3 nm)

dBragg ) dL - dL(clay)

Figure 3. WXRD patterns of ZnS/clay nanocomposite powder, ZnS powder, and Laponite XLG powder.

work27 to obtain the parameters of this equation. dEMA was calculated by using eq 2 with the band gap energies, Eg, obtained from eq 1, and shown in Table 1. Figure 2 shows the transmittance spectra of 10-ZnS and 10ZnS/clay converted from their UV-vis absorption spectra using the Lambert-Beer law. The decreases of transmittance at visible region cause by the scattering of ZnS NPs and clay particles. Generally the transmittance becomes high by the decrease of the particle size. Although there was additional existence of clay particles in 10-ZnS/clay sample, the transmittance of 10-ZnS/ clay was higher than that of 10-ZnS. This result indicated that ZnS NPs without clay particles was larger than that with clay particles, that is, ZnS NPs without clay particles aggregated. 2. Wide-Angle XRD. To determine the structure of the mixture of ZnS NPs and clay particles, that is nanocomposite powder, the NPs suspension with clay was dried and measured by wide-angle XRD (WXRD) from 20 to 70°. Figure 3 shows the WXRD patterns of the nanocomposite powder, together with those for ZnS powder and clay powder. The peak angles of ZnS powder appeared at 28.6, 47.9, and 56.7°, which corresponded to (111), (220), and (311) faces of zinc blende type crystal, respectively. Peaks of WXRD pattern of clay powder were 27.8, 35.0, 53.2, and 60.8°. The peaks of nanocomposite powder were found to be both of ZnS NPs and clay powders. These results indicated that the ZnS NPs sample prepared in the clay suspension was also zinc blende type crystals. As diffraction peaks of (111) and (311) faces of ZnS NPs overlapped with the diffraction peaks of clay, the diffraction peak of (220) face of ZnS NPs was used for the size estimation of ZnS NPs with clay by the Sherrer equation

(3)

(4)

The dBragg values are listed in Table 1. Since the 10-ZnS powder sample only consisted of ZnS NPs, the peak observed by SXRD might have originated from the particle diameter of ZnS NPs, as previously reported.28,29 This particle diameter of ZnS NPs estimated using Bragg’s law is also listed Table 1. 4. TEM. The number of particles measured from the TEM images was about 200 in each preparation. Therefore, it is difficult to obtain the exact particle size distribution from this measurement. However, the TEM image is important for

Figure 4. SXRD patterns of ZnS/clay nanocomposite powder, ZnS powder, and Laponite XLG powder.

Figure 5. Schematics illustrating ZnS/Smectite nanocomposite. dL ) basal distance of nanocomposite where ZnS NPs were intercalated in clay, dL(clay) ) basal distance of Laponite XLG (1.3 nm).

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Figure 7. The PL spectra of ZnS/clay colloidal solution and ZnS colloidal solution.

5. PL Spectra. The PL spectra of the colloidal solutions, where the ZnS concentration was adjusted to 0.25 mM by adding Milli-Q water, are shown in Figure 7, when a 280-nm excitation wavelength was used. The PL intensity of the ZnS/clay sample was higher than that of the ZnS sample at the same precursor concentration, and the PL intensity of the ZnS NPs prepared with clay increased when the ZnS NPs were prepared at a low precursor concentration. Discussion Table 1 shows a comparison of particle diameters estimated from the data of UV-vis absorption, XRD, and TEM. The estimated particle diameters increased with the precursor concentration. This result can be explained by von Weimarn’s law, which is known as the relationship between the supersaturation and the precipitated particle size in dispersion system.14,30,31 The main concept of this law is the average size of precipitated particle depends on the supersaturation and has a maximum value. The supersaturation is given by

S)

Figure 6. TEM images and the size distributions of ZnS particles.

comparing the aggregation states of NPs prepared in solution with or without clay. Figure 6 shows TEM images of the prepared ZnS NPs and the particle size distributions calculated from them. The particle size distributions had a range from 2 to 15 nm. But most of particles were in the 2-6-nm size range. In the case of the 10-ZnS sample, however, there were large aggregated particles as shown in Figure 6, so that it was difficult to obtain the particle size distribution of ZnS NPs. The average diameter of the 10-ZnS sample in the 0.25 mM suspension was measured as about 300 nm by using dynamic light scattering. These results indicate that the clay particles dispersed in the solution prevented the aggregation of the ZnS NPs.

(Q - L) L

(5)

where S is the supersaturation of precursor, Q is the amount of dissolved materials, and L is solubility of material. According to von Weimarn’s law, the size of ZnS NPs increases with the supersaturation up to a maximum at a certain supersaturation. A further increase of supersaturation results in the decrease of the size of ZnS NPs because the particle size is decided by a balance between the nucleation rate and the nuclear growth rate. We prepared ZnS NPs without clay suspension at the various supersaturation conditions by increasing of the precursor concentration and showed the effect of the precursor condition on dSherrer in Figure 8. The precipitated particle size was obtained as the maximum value at 3.4 × 108 of the supersaturation corresponding to 40 mM of precursor concentration. This result agrees well with von Weimarn’s law. From this fact, we could explain that the size of ZnS NPs prepared in clay suspension increased with the precursor concentration, because the experimental conditions were less than 3.4 × 108 of the supersaturation. The Sherrer equation is an empirical formula that assumes that the profile of XRD depends only on the crystallite diameter, and quantifies the extension of the profile. From this assumption, the particle diameter given by the Sherrer equation, dSherrer, corresponds to crystalline diameter. The crystalline diameter of 10-ZnS/clay was smaller than that of 10-ZnS. This result indicated that the clay dispersed in the water worked to prevent mass transfer during crystallization.

898 J. Phys. Chem. C, Vol. 113, No. 3, 2009

Arao et al. mechanism of PL did not change to add clay particles and that the roll of the clay addition was only the prevention of the ZnS NPs aggregation. And we also concluded that PL intensities were influenced by the dispersion state of ZnS NPs. The integration value, Fx, of the PL spectrum excited by light of wavelength x nm can be obtained from the product of device constant, K, the absorbance at excitation wavelength, Ax, and the quantum efficiency, Φ

Fx ) KAxΦ

Figure 8. Effect of the supersaturation on crystalline diameter of ZnS particles.

The equation of EMA is expressed from the assumption that exciton energy in a crystal is described by a box type potential, and the particle diameter obtained from this equation also corresponds to crystalline size.24,25 The particle diameter calculated by this equation, dEMA, increased with the precursor concentration regardless of the presence of clay. This tendency was similar to dSherrer, although dEMA was bigger than dSherrer. This size difference might be caused because dEMA was mainly affected by the bigger sized particles in the particle size distribution. To make clear the effect of particle size distribution on dEMA, we examined the following calculations. The particle size distribution of the model sample was assumed a normal distribution with 4 nm mean particle diameter and standard deviation. We applied the EMA equation to convert the particle size into the band gap energy corresponding to the particle size. The absorbance profile of a certain size particle was obtained from eq 1. The total absorbance profile of the model sample was calculated from the sum of the absorbance profile of a certain size particle, if the absorption coefficient of one particle was independent of the particle size. We could draw the tangential line in this total absorbance profile and then estimate the band gap energy of the model sample. By application of the EMA equation to the estimated band gap energy, we obtained dEMA of the model sample could be calculated from this band gap energy using eq 2. We estimated dEMA by the above calculation procedure for two cases, which was that the standard deviation of the model sample was 0.4 or 0.8 nm with 4 nm mean particle size. dEMA was estimated to be 4.7 or 5.4 nm, respectively. Thus dEMA was estimated bigger than the mean particle diameter, which may correspond the particle size calculated using other methods and increased when the particle size distribution became wide. The particle diameter calculated by Bragg’s law, dBragg, of 10-ZnS/clay agreed well with dSherrer. This is because the ZnS NPs existed in clay as a monocrystalline particle. On the other hand, the particles of 10-ZnS were multicrystalline particles according to the results of the comparison between dBragg and dSherrer. The 10-ZnS sample basically consisted of aggregated particles having an orderly structure, which was indicated by XRD at small angles. The particle of 10-ZnS sample might be formed by the aggregation of about four crystallites. In the case of 40-ZnS/clay, dBragg was larger than dSherrer, although there was no difference between dBragg and dSherrer in both cases of 10ZnS/clay and 20-ZnS/clay. This indicates that aggregated particles existed in 40-ZnS/clay, because the clay particles were not enough to prevent aggregation due to the high precursor concentration of ZnS NPs. The PL peak positions of 10-ZnS/clay and 10-ZnS samples were almost same. This may conclude that the emission

(6)

As the device constant can be estimated by using the PL spectrum of quinine sulfate, whose quantum efficiency is known as 55%,23 the quantum efficiency of ZnS NPs can be calculated by eq 6 and is shown in Table 1. The quantum efficiency showed the same tendencies as the PL spectrum. The observed PL at 420 nm wavelengths was reported to be due to the surface defect of ZnS nanoparticles.32 The surface defects of ZnS NPs form the energy level between the valence band and conduction band of ZnS, and consume the excited electrons as PL at 420 nm wavelengths. Many of the excited electrons are consumed at the defect energy level, because the PL lifetime of the defect level is shorter than that of the band gap.33 If the number of defects generated is the same as the unit surface area, a small particle with a large specific surface area has a lot of defects compared with a large particle. Thus, the probability that the excited electrons transfer to the defect level increases with the number of defects due to the increase of the specific surface area, and as a result, the PL intensity increases as shown in Table 1. As the specific surface area decreased due to the aggregation of particles of 10-ZnS, the quantum efficiency of 10-ZnS sample was smaller than that of 10-ZnS/clay sample. Thus the aggregation state is also important for photoproperties of NPs, as well as the particle diameter. Conclusion ZnS NPs were prepared in Laponite XLG clay suspension to prevent from aggregation of ZnS NPs and revealed their properties by several measurement techniques. The band gap energies of the prepared ZnS NPs estimated using the edge of the UV-vis absorption of ZnS NPs were bigger than that of the bulk ZnS due to the quantum size effect. The particle diameters calculated by the EMA equation applying the band gap energy were larger than the crystalline diameter calculated using the WXRD pattern by the Sherrer equation because prepared ZnS NPs had a wide size distribution. The different of the particle diameters estimated using WXRD and SXRD indicated that ZnS NPs prepared in the presence of the clay particles were monocrystalline particle and ZnS NPs prepared without clay consisted of several crystallites. When the precursor concentration of ZnS NPs was increased, the crystalline diameter measured by WXRD increased. This tendency agreed with von Weimarn’s law. TEM images showed the preventing effect of the clay particles from the aggregation of ZnS NPs. PL of ZnS NPs originating from surface defect level was observed. When the particle diameter of ZnS NPs prepared in clay suspension decreased with the increase of the precursor concentration, the quantum efficiency of ZnS NPs estimated using PL spectrum increased because the probability that the excited electrons transferred to the surface defect level increased with the specific surface area. The quantum efficiency of ZnS NPs prepared without clay was lower than that of the ZnS/clay sample due to the aggregation of multicrystalline particles. References and Notes (1) Kagan, C. R.; Murray, C. B.; Bawendi, M. G. Phys. ReV. B 1996, 54, 8633–8643.

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