Template-Free Hydrothermal Synthesis of Novel Three-Dimensional

Mar 25, 2009 - College of Mechanical & Material Engineering, Functional Materials Research Institute, China Three Gorges. UniVersity, Yichang, Hubei ...
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J. Phys. Chem. C 2009, 113, 5984–5990

Template-Free Hydrothermal Synthesis of Novel Three-Dimensional Dendritic CdS Nanoarchitectures Danjun Wang,†,‡ Dongsheng Li,*,†,‡ Li Guo,‡ Feng Fu,‡ Zhiping Zhang,*,§ and Qingting Wei‡ College of Mechanical & Material Engineering, Functional Materials Research Institute, China Three Gorges UniVersity, Yichang, Hubei, 443002, China, Department of Chemistry and Chemical Engineering, Shaanxi Key Laboratory of Chemical Reaction Engineering, Yan’an UniVersity, Yan’an, Shaanxi, 716000, China, Department of Chemistry and Biochemistry, Brigham Young UniVersity, ProVo, Utah, 84602 ReceiVed: NoVember 18, 2008; ReVised Manuscript ReceiVed: February 15, 2009

Novel three-dimensional (3D) dendritic CdS nanoarchitectures were synthesized via a facile template-free hydrothermal process only using CdSO4 and thiourea as precursors and cetylpyridinium chloride (CPC) as a capping reagent. The morphology of CdS nanoarchitecture is strongly dependent on the reaction conditions such as the ratio of thiourea/CdSO4, concentration of CPC, and hydrothermal time. A growth mechanism of dendritic CdS nanoarchitectures is proposed by capturing the shape evolution based on SEM observation, and the detailed formation procedures are as follows: (a) formation of CdS nanoparticles; (b) CdS nanoparticles form larger aggregates via hydrogen-bond interaction; (c) formation of nanopetal by H-bonding effects; (d) formation of the dendritic CdS nanoarchitectures by Ostwald Ripening. Furthermore, the photocatalytic activity experiment illustrated that as-synthesized dendritic CdS nanoarchitectures exhibited an excellent photocatalytic performance to decolorize methyl orange aqueous solution under the visible-light illumination. 1. Introduction The size, shape, and structure of semiconductor nanocrystals are important factors in adjusting their electronic, optical, and other physical properties.1-3 Thus, the synthesis of nano- to microscopic-scale semiconductive materials with controlled size and shape are of great interest owing to their novel properties and potential application in optics, electronics, catalysis, magnetism, and biology.4-8 In past decades, despite many simple shapes (dot, wire, tube, etc.), as well as hierarchical structurebased morphologies, such as comb-like,9 dendron-like,10,11 snowflake-like,12 urchin-like,13,14 and flower-like15,16 patterns and structures, have been obtained. However, it is still a great challenge in material science to understand factors governing the creation of nanocrystal assemblies and develop simple and reliable methods for fabrication of various materials with designed chemical components and controlled morphologies. Nanosized CdS, being one of the most important wide-gap semiconductors, has been extensively investigated because of its tuning emission in the visible-light range with change of size and shape. Also, CdS has attracted increasing interest because of its potential applications as nanoelectronic and photocatalytic materials.17-23 So far, alot of effort has been devoted to the synthesis of CdS nanostructures with different morphologies, including a solvothermal or hydrothermal method,24 vapor-liquid-solid (VLS)-assisted method,25 colloidal micellar method,26 and electrochemical process.27 Recently, based on different driving mechanisms, several different self-assembly processes (e.g., surface tension, capillary effect, hydrophobic interaction, and H-bonding effect) have been advanced in controlling the shape and size of these materials.28,29 * To whom correspondence should be addressed: Phone: +86-7176392538 (D.L.), 1-801-422-5933 (Z.Z.).; e-mail: [email protected] (D.L.), [email protected] (Z.Z.). † China Three Gorges University. ‡ Yan’an University. § Brigham Young University.

Among them, a capping reagent-assisted method is a particular and promising avenue for assembly of diversified semiconductive materials with controlled morphologies. As it is well-known, assembly of nanocrystals is usually driven by interactions between the individual building blocks, and, therefore, control over the surface properties is an important factor to realization of self-assembly. The surfactant molecules usually acting as the capping agents are absorbed onto the surface of nuclei, which will affect and control the final morphology of the crystals. Several studies, in which different capping reagents were employed to control the morphology of the nanocrystals, also revealed that the morphologies of synthesized nanocrystals were greatly dependent on the categories of capping.11,30-33 Two- and three-dimensional CdS nanocrystals were synthesized via a amino acid-mediated process by Chen’s research group.30 Gao and co-workers synthesized three-dimensional CdS nanocrystals using hexamethylenetetramine [(CH2)4N4, HMT] as a capping reagent.31 Yan and Xue successfully synthesized dendritic Cu metal nonostructures by employing cetyltrimethyl ammonium bromide (CTAB) as the capping agent.11 More recently, Ge el al. reported a method for self-assembly of cadmium sulfide (CdS) nanochains through direct deposition of CdS on an unfixed DNA template, in which 2-aminoethanethiol was used as a capping agent.32 Also, Kang et al. studied 2D self-bundled CdS nanorods by capping CdS with tri-n-octylphosphine (TOP) and tetradecyl-phosphonic acid (TDPA).33 A novel capping agent is cetyltrimethyl chloride (CPC), which has unique structural properties in common with cetyltrimethyl ammonium bromide (CTAB). First, CPC exhibits a potentially structural directing effect. Second, it easily dissolves in water and ethanol. These reasons inspired the use of CPC in the preparation of the novel 3D dendritic CdS nanoarchitectures reported here. Specifically, we present a convenient hydrothermal method to prepare dendritic CdS nanoarchitectures in the absence of

10.1021/jp810155r CCC: $40.75  2009 American Chemical Society Published on Web 03/25/2009

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Figure 1. Structure and chemical composition of the product obtained by the reaction of thiourea and CdSO4 (molar ratio of 1:1, concentration of CPC is 5.0 wt%) at 150 °C for 4 h: (A) XRD pattern of CdS, (B) full XPS spectrum of CdS nanostructures, (C, D) high resolution XPS spectra of the CdS nanostructures in the Cd(3d) and S(2p).

templates using CdSO4 and thiourea as precursors and cetylpyridinium chloride (CPC) employed as capping agent. A possible formation mechanism was proposed to account for the production of dendritic CdS nanoarchitectures. The photocatalytic activity of as-synthesized dendritic CdS was also evaluated by the photocatalytic decolorization of methyl orange (MO) aqueous solution under visible-light illumination. 2. Experimental Section 2.1. Sample Preparation. All chemicals used in this work were of analytical grade and commercially available and used without further purification. In a typical procedure, 9 mmol of CdSO4 power was dissolved into 90 mL of ethanol/water solvent (v/v, 1:1). Next, 40 mL of the above solution was mixed with 25 mL of cetylpyridinium chloride (CPC) solution (5.0 wt%), followed by addition of thiourea (9 mmol) under vigorous magnetic stirring. Then 20 mL of the above mixture was transferred into a 25 mL Teflon-lined stainless steel autoclave and maintained at 150 °C for 4 h. After the reaction, the power sample was filtered, washed with distilled water and absolute alcohol several times, and then dried in a vacuum at 80 °C for 2 h. 2.2. Sample Characterization. The phase of as-synthesized products was determined by X-ray diffraction (XRD) using a Shimadzu XRD-7000 X-ray differactometer with Cu KR radiation (λ ) 0.15406 nm). X-ray photoelectron spectroscope (XPS) images were recorded on a PHI-5400 X-ray photoelectron spectrometer. The field emission scanning electron microscope (FE-SEM) images were taken on a JSM-6700 scanning electron microscope. Transmission electron microscope (TEM) images and selected area electron diffraction (SAED) patterns were recorded on a JEM-3010 electron microscope at an accelerating voltage of 300 kV. For TEM characterization, the products were ultrasonically dispersed in ethanol, and then the resulting suspension was dropped on a Cu grid coated with carbon film. High-resoution TEM (HRTEM) micrographs were taken on a JEOL 2010F transmission electron microscope operated at 200

kV. The UV-visible absorption spectrum was recorded on a Shimadzu 2550 UV-visible absorption spectrometry. 2.3. Photocatalytic Testing. The evaluation of photocatalytic performance of as-synthesized samples for photocatalytic decolorization of methyl orange aqueous solution was performed as follows: A 300 W Xe-arc lamp was used as the visible light source with a cutoff filter to cut off the light below 450 nm. The suspension containing powdered catalyst (40 mg) and a fresh aqueous solution of methyl-orange (25 mL) was continuously magnetically stirred in the dark for 20 min to establish an adsorption/desorption equilibrium of the methyl-orange aqueous solution. At given illumination time intervals, a series of aqueous solution was collected for analysis. The photocatalytic performance of the catalyst was evaluated by monitoring the visible absorbance characteristic of the targeted methylorange using a Shimadzu 2550 UV-visible spectrophotometer. 3. Results and Discussion 3.1. Power XRD and XPS. Figure 1A displays the typical XRD pattern of as-synthesized product obtained at 150 °C for 4 h (5.0 wt% CPC). All diffraction peaks in Figure 1A can be indexed to hexagonal wurtzite CdS (JSPDS file No. 41-1049), which can be confirmed by the distinctive reflection peaks at 2θ ) 28.4° and 53°. No obvious diffraction peaks from other impurities, such as 31.5° from cubic blende phase CdS,34 were observed in the XRD pattern. It can be also noted that the (002) reflection is comparatively strong, which can be attributed to the preferential crystal growth orientation along the c-axis.30 The relatively broad peaks should originate from the small secondary structure within nanometer scales. XPS analysis, as given in Figure 1B-D, was employed to determine the chemical composition. Figure 1B-D displays the full XPS spectrum and high-resolution spectra of the Cd(3d) and S(2p) regions, respectively. From Figure 1C and 1D, it is obvious that the binding energy of peaks Cd(3d5/2) and S(2p3/2) are 404.97 and 161.58 eV, respectively. The Cd(3d5/2) and S(2p3/2) peak areas were determined for the quantitative elemental analysis of Cd

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Figure 2. Typical FE-SEM and TEM images of the CdS nanoarchitectures obtained by the reaction of thiourea and CdSO4 (molar ratio of 1.1:1, concentration of CPC is 5.0 wt%) at 150 °C for 4 h: (A-D) FE-SEM images of 3D dendritic CdS nanoarchitectures; (E) TEM image of dendritic CdS nanoarchitectures, inset is SEAD pattern; (F) HR-TEM image of one petal of an individual dendritic CdS nanoarchitectures, the fringe spacing of 0.335 nm corresponds to the separation of (002) lattice plane.

and S in the products, and an atomic ratio of 1:1 was obtained, which further confirms that the products are pure CdS. 3.2. SEM and TEM Observation. The morphology of assynthesized 3D CdS nanoarchitectures was examined by FESEM and TEM. Figure 2A-D shows typical FE-SEM images of the sample by hydrothermal reaction of thiourea and CdSO4 (molar ratio of 1:1) and incubated at 150 °C for 4 h. It clearly indicates that CdS microcrystals exhibit a dendritic CdS architecture with scales of 3-6 µm. Interestingly, the dendritic architectures have many small secondary petals on main branches (Figure 2A-C). Further information about the CdS architectures was obtained from TEM images as given in Figure 2E. From this figure, it can be seen that the branches in CdS nanoarchitectures are built up by nanopetals with diameters of 50-100 nm. The corresponding selected area electron diffraction (SAED) (inset of Figure 2E) pattern was recorded with the electron along the [010] zone axis of wurtzite. From the HRTEM image, as given in Figure 2F, recorded at the area marked in Figure 2E, it is obvious that the petal in an individual dendritic CdS nanoarchitecture is single crystalline and prefers growth along the [010] direction (c-axis), which is consistent with the SAED pattern and the XRD pattern (Figure 1). 3.3. Influence of the Ratio of Thiourea/CdSO4 on the Formation of CdS Nanoarchitectures. To investigate the formation of the dendritic CdS nanoarchitectures, a series of experiments were carried out as discussed below. It is found that the ratio of thiourea/CdSO4 plays an important role in the present synthesis. In a parallel experiment, if the ratio between thiourea and CdSO4 was 4:1, only some spheres with average diameters of 2-4 µm were obtained, which is given in Figure 3A. When 2:1 of thiourea/CdSO4 was added into the reaction system, some dendritic-like nanoarchitectures were obtained as shown in Figure 3B. Among them, there are some overmatured dendritic-like microcrystals, and their shapes are very similar to spheres. With the decrease of the ratio to 1:1, novel dendriticlike CdS nanoarchitectures were obtained (Figure 3C), whereas some urchin-like CdS structures (Figure 3D) form when the ratio between thiourea and CdSO4 is 0.5:1. All results suggested that the morphology of CdS nanostructure depends on the initial ratio of thiourea and CdSO4.

3.4. Influence of CPC on CdS Nanoarchitectures. We also found that the concentration of CPC in the reaction system was also crucial to the growth of dendritic CdS nanoarchitectures. If no CPC was added into the reaction mixture, no dendrite was obtained when other experimental conditions were kept constant, and some spheres and particles with irregular shapes were observed as shown in Figure 4A. As the concentration of CPC was increased to 2.5 wt% (Figure 4B), the morphology changed from spherical shape to cauliflower-like structure. Dendritic CdS nanoarchitectures were formed when the concentration was 5.0 wt% (Figure 4C). As shown in Figure 3D, when the concentration of CPC was 10 wt%, the dendritic morphology vanished. All these observations indicate that the concentration of CPC is a vital factor in controlling the morphology of CdS. To the best of our knowledge, the mechanism by which CPC influences the morphology of the growing CdS nanoarchitectures may involve its selective adsorption on certain crystal faces. The driving force for the growth is similar to that described in a recent report on H-bonding effects.29 At high concentration, the CPC is apparently excessive, and it can effectively cap most of surface of CdS when Cd2+ converts into CdS. The hydrogen bond interaction causes the nanoparticle aggregation, and dendritic CdS nanoarchitectures are unavailable. The petal growth would occur at the appropriate concentration, which causes CdS nanostructure anisotropic growth and results in dendritic CdS nanoarchitectures. 3.5. Influence of Hydrothermal Time on CdS Nanoarchitectures. To investigate the intermediate transition and the growth mechanism of dendritic CdS nanoarchitectures, timedependent experiments were carried out by quenching the Telfon-lined autoclave using cold water at different reaction stages. Figure 5 shows the morphologies from various products obtained at different reaction stages corresponding to the reaction time of 1, 2, 4, 8 h, respectively. The SEM image in Figure 5A reveals that CdS particles exhibit spherical nanoparticles with diameters of about 50 nm in the earlier stage of the reaction (1 h). When the reaction time was prolonged to 2 h, short petals grow out of the original spherical core extending toward directions (Figure 5B). Figure 5D shows the CdS structure

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Figure 3. FE-SEM images of the CdS nanoarchitectures obtained from various thiourea/CdSO4 ratios (concentration of CPC is 5.0 wt%, at 150 °C for 4 h): (A) 4:1; (B) 2:1; (C) 1:1; (D) 0.5:1.

Figure 4. FE-SEM image of CdS crystals obtained from different concentration of CPC (the ratio of thiourea and CdSO4 of 1:1, at 150 °C for 4 h): (A) without CPC; (B) 2.5wt%; (C) 5.0wt%; (D) 10.0wt%.

synthesized at 150 °C for 4 h, and dendritic nanoarchitectures are obtained. When prolonging the reaction time to 8 h, the dendritic nanoarchitectures gradually ripen, and overmatured nanoarchitectures are observed as shown in Figure 5D. At the beginning of the reaction, the CPC is apparently excessive, and it can effectively cap most of surface of CdS when Cd2+ converts into CdS. At the same time, the hydrogen bond interaction causes the nanoparticle aggregation. The petal growth would

occur at the middle reaction stages. At the end of reaction stage, however, CPC has little effect on the surface of CdS, which causes CdS nanostructure anisotropic growth. 3.6. Growth Mechanism of Dendritic CdS Nanoarchitectures. On the basis of SEM observation and experimental process, a possible four-step process mechanism is suggested to explain the formation of dendritic-like CdS nanoarchitectures, which is depicted in Scheme 1. Upon heating, thiourea is

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Figure 5. FE-SEM images of 3D dendritic CdS nanoarchitectures obtained at different reaction times concentration of CPC is 5.0 wt%: (a) 1 h, (b) 2 h, (c) 4 h, and (d) 8 h.

SCHEME 1: Schematic Description of the Growth Process of Dendritic CdS Nanoarchitectures

attacked by the strong nucleophilic nitrogen atoms of cetylpyridinium chlorine (CPC) molecules, which leads to the weakening of CdS double bonds in the structure of thiourea, and forms transitional product thiourea-cetylpyrldinium chlorine (TCPC). At an appropriate temperature, the CdS bond will break, and S2- anion will gradually generate, which then reacts with Cd2+ to form CdS nanoparticles (a). During the growth of CdS crystals, TCPC could be considered as a doubly functional ligand. On the one hand, S atoms can strongly interact with CdS surfaces. On the other hand, the nitrogen atom in the pyridine ring promotes its solubility in ethanol/water solution and is also an important element to produce hydrogen-bond interaction between particles. At the beginning of such a reaction, the number of the CdS particles is few, and TCPC can effectively cap most surfaces of the original formed CdS nanoparticles. Then these nanoparticles interact each other to form larger aggregates via hydrogen interaction (b). When the particles are in close contact, diffusion-controlled growth occurs in the spherical aggregates, and the surface area is reduced by particle fusion and structure rearrangement.35 At this stage, some spheres with a rough surface are obtained (c). With the formation of more and more CdS nanoparticles and the gradual decom-

position of TCPC, TCPC cannot cap most surfaces of CdS crystals, and it will only selectively adsorb onto some faces. Then the CdS crystals selectively absorbed by TCPC via hydrogen-bond interaction, will grow out of the original spherical core, and extend toward favorable directions. As a result, the dendritic architectures with branches are obtained (d). This mechanism is similar to the oriented attachment, which was observed in a system where the small particles were coated with small molecules, and the molecules allowed them to get close to each other and to facilitate attachment.36,37 3.7. Photocatalytic Performance of CdS Nanoarchitectures. The photocatalytic performance of as-synthesized dendritic CdS nanoarchitectures was evaluated by photocatalytic decolorization of methyl-orange aqueous solution. Figure 6A shows the comparison of photocatalytic activities of CdS prepared with various thiourea/CdSO4 ratios. Interestingly, it can be observed that when a 300 W Xe-arc lamp was used as the visible light source with a cutoff filter to cut off the light below 450 nm, as-prepared CdS nanoarchitecture showed an excellent photocatalytic activity for methyl-orange degradation relative to visible-light irradiation (the comparison between them is not shown here). For this reason, in the following experiments

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Figure 6. (A) Photocatalytic activity of the samples obtained from different thiourea/CdSO4 ratios (R): (a) without presence of CPC and R ) 1, (b) with presence of CPC and R ) 4, (c) with presence of CPC and R ) 2; (d) with presence of CPC and R ) 1. (B) Absorption spectra of methyl-orange aqueous solution (10 mg/L) in the presence of dendritic CdS nanoarchitecture (R ) 1) (1600 mg/L) under visiblelight irradiation for different times: (a) 0 min, (b) 20 min, (c) 40 min, (d) 50 min.

TABLE 1: Photodegradation of Methyl Orange Using CdS as Photocatalyst sample no.a

first-order kinetics equation (Ct ) C0e-Kt)

coefficient constant (R2)

half-life, min

a b c d

Ct ) 9.652e-0.0087t Ct ) 9.389e-0.0173t Ct ) 9.978e-0.0248t Ct ) 9.503e-0.0346t

0.9513 0.9602 0.9584 0.9503

80 40 28 20

a

Note: sample number is consistent with Figure 6..

Xe-arc lamp was used as the visible-light source. In order to investigate the activities of as-synthesized samples with various ratios of CdSO4/thiourea, the experiments in the systems with and without presence of CPC were carried out. The change of concentration of methyl-orange solution versus illumination time is shown in Figure 6A, and the kinetics equation is listed in Table 1. The results illustrate the half-life period for the photocatalytic degradation of methyl orange was 80 min (R ) 1, where R means the molar ratio of thiourea/CdSO4, without CPC, sample a), 40 min (R ) 4, with CPC, sample b), 28 min (R ) 2, with CPC, sample c), and 20 min (R ) 1, with CPC, sample d), respectively, which well obey the pseudo-first-order kinetics. Additionally, these results confirm that the dendritic CdS nanoarchitectures obtained from 1:1 of thiourea/CdSO4 in the presence of CPC can greatly promote the photocatalytic degradationofmethyl-orangesolutionrelativetoothermorphologies. Figure 6B shows the comparison of absorption spectra of methyl-orange solution in the presence of dendritic CdS nanoarchitectures under visible-light for various irradiation times. From this figure, it is obvious that methyl-orange aqueous solution is stable in the visible region. However, in the presence of as-synthesized CdS and visible-light illumination, methylorange aqueous solution can be obviously decolorized. With the increase of exposure time, the typical sharp peak at 488 nm gradually diminishes and completely vanishes after 50 min. These results suggest that dendritic CdS nanoarchitecture is an effective catalyst for decolorization of methyl-orange solution, and its photocatalytic activity is superior to that of small CdS nanoparticles.17 However, the excellent photocatalytic performance of as-synthesized dendritic CdS nanoarchitectures is still not clear, and further is needed.

In summary, novel 3D dendritic CdS nanoarchitectures have been successfully synthesized via a facile template-free hydrothermal process in the presence of CPC. By varying the ratio of thiourea/CdSO4 from 4:1 to 0.5:1, the morphology of CdS changes from spherical to overmatured dendritic-like microcrystals. It is also found that CPC plays a crucial role in the formation of dendritic CdS nanoarchitectures. For investigating the growth mechanism of dendritic CdS nanoarchitectures, a series of intermediate morphologies during the shape evolution of dendritic CdS based on SEM observations were examined. On the basis of these observations, a possible four-step formation mechanism was proposed for the dendritic CdS nanoarchitectures, which provides useful information for crystal growth, design, and morphology-controlled synthesis. In comparison to other morphologies (e.g., sphere, overmatured dendritic-like microcrystals), the dendritic CdS nanoarchitectures exhibited a higher photocatalytic performance for the photocatalytic degradation of methyl-orange aqueous solution under visible-light illumination. Because of the facile synthesis and controlled morphology, the obtained 3D dendritic CdS nanoarchitectures may find potential applications in catalysis and other fields. Acknowledgment. This work was supported by the National Natural Science Foundation of China (20773104) and the Program for New Century Excellent Talents in University (NCET-06-0891). This work was also financially supported by the Key Project of the Ministry of Education of China (208143), the Key Project of Key Laboratory of Shaanxi Province (08JZ81), and the Natural Science Foundation of Shaanxi Provincial Education Office (07JK435). References and Notes (1) Yang, P. Nature 2003, 425, 243. (2) Hupp, J. T.; Poeppelmeier, K. R. Science 2005, 309, 2008. (3) Whang, D. S.; Jin, Y. W.; Lieber, C. M. Nano Lett. 2003, 3, 1255. (4) Pardo-Yissar, V.; Katz, E.; Wasserman, J.; Willner, I. J. Am. Chem. Soc. 2003, 125, 622. (5) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122, 12700. (6) Gao, F.; Lu, Q. Y.; Xie, S. H.; Zhao, D. Y. AdV. Mater. 2002, 14, 1537. (7) Lu, Q. Y.; Gao, F.; Zhao, D. Y. Angew. Chem., Int. Ed. 2002, 41, 1932. (8) Park, S.; Lim, J. H.; Chung, S. W.; Mirkin, C. A. Science 2004, 303, 348. (9) Sun, T., J.; Qiu, J. S.; Liang, C. H. J. Phys. Chem. C. 2008, 112, 715. (10) Fang, J. X.; You, H. J.; Kong, P.; Yi, Y.; Song, X. P.; Ding, B. B. Cryst. Growth Des. 2007, 7, 864. (11) Yan, C. H.; Xue, D. F. Cryst. Growth Des. 2008, 8, 1849. (12) Zhang, J.; Sun, L. D.; Yin, J. L.; Su, H. L.; Liao, C. S.; Yan, C. H. Chem. Mater. 2002, 14, 4172. (13) Du, D.; Cao, M. J. Phys. Chem. C 2008, 112, 10754. (14) Zhou, Y. X.; Zhang, Q.; Gong, J. Y.; Yu, S. H. J. Phys. Chem. C 2008, 112, 13383. (15) Dai, Q. Q.; Xiao, N. R.; Ning, J. J.; Li, C. Y.; Li, D. M.; Zou, B.; Yu, W. W.; Kan, S. H.; Chen, H. Y.; Liu, B. B.; Zou, G. T. J. Phys. Chem. C 2008, 112, 7567. (16) Sajanlal, P. R.; Sreeprasad, T. S.; Nair, A. S.; Pradeep, T. Langmuir 2008, 24, 4607. (17) Guo, Y.; Zhang, H.; Wang, Y.; Liao, Z. L.; Li, G. D.; Chen, J. S. J. Phys. Chem. B 2005, 109, 21602. (18) Wang, S. M.; Liu, P.; Wang, X. X.; Fu, X. Z. Langmuir 2005, 21, 11969. (19) Hirai, T.; Bando, Y.; Komasawa, I. J. Phys. Chem. B 2002, 106, 8967. (20) Fox, M. A.; Dulay, M. T. Chem. ReV. 1993, 93, 341. (21) Ludolph, B.; Malik, M. A.; O’Brien, P. Chem. Commun. 1998, 8, 1849. (22) Johnson, B. J. S.; Wolf, J. H.; Zalusky, A. S. Chem. Mater. 2004, 16, 2909.

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