From ZnS·en0.5 Nanosheets to Wurtzite ZnS Nanorods under

Jan 19, 2008 - Then, the small sphere-like ZnS nanoparticles started to dissolve into the solution and grow onto the nanorods with a small aspect rati...
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J. Phys. Chem. C 2008, 112, 1946-1952

From ZnS‚en0.5 Nanosheets to Wurtzite ZnS Nanorods under Solvothermal Conditions Guangcheng Xi,* Chao Wang, Xing Wang, Qing Zhang, and Haiqing Xiao Inspection and Research Center for Nanomaterials and Nanoproducts, Chinese Academy of Inspection and Quarantine, Beijing 100025 P. R. China ReceiVed: October 8, 2007; In Final Form: NoVember 19, 2007

We report here that near-monodispersed wurtzite phase ZnS nanorods can be synthesized with high yield via decomposing a sheet-like ZnS‚en0.5 (en ) ethylenediamine) precursor in a mixed solution of oleic acid/ alcohol/hexadecylamine under mild solvothermal conditions. These as-prepared nanorods are an average of 25 nm in diameter and 300 nm in length. A high-resolution transmission electron microscope (HRTEM) image and fast Fourier transform (FFT) pattern demonstrated that these nanorods were highly single-crystalline and grown along the [001] direction. A series of electron microscopy characterization results suggests that the growth of the present ZnS nanorods is governed by a novel ligand-assisted solid-solution-solid (LSSS) growth mechanism. In this mechanism, 3-10 nm sized sphere-like wurtzite ZnS nanoparticles and a small quantity of wurtzite ZnS nanorods with small aspect ratio were initially generated as the decomposition products of a ZnS‚en0.5 sheet-like precursor in a solvothermal system. Then, the small sphere-like ZnS nanoparticles started to dissolve into the solution and grow onto the nanorods with a small aspect ratio via a process known as Ostwald ripening and gradually developed into longer wurtzite ZnS nanorods with the assistance of a mixed ligand system of hexadecylamine and oleic acid. The effects of ligand, solution composition, and reaction time on the morphology of ZnS are investigated in detail. Photocatalytic experiments showed that the as-synthesized ZnS nanorods performed a high activity for degradation of fuchsine solution under UV irradiation.

Introduction As an important wide-gap semiconductor material (3.68 eV) of the metal sulfides, ZnS has been studied extensively for its wide optical applications.1 In recent years, nanoscaled ZnS crystals attracted more and more attention because of their quantum confinement effect and size- and shape-dependent photoemission characteristics.2 Considerable efforts have been devoted into controlling the size and shape of ZnS nanostructures so that size- and shape-dependent properties can be studied.3 For ZnS, the sphalerite (cubic zinc blende) structure is a stable phase at low temperature, whereas the wurtzite (hexagonal) structure is a high-temperature stable phase.4 Recent studies found that the optical properties of wurtzite ZnS are more excellent than those of sphalerite ZnS.5 However, it is difficult to synthesize pure wurtzite ZnS nanocrystals at relatively low temperatures (below 200 °C); most ZnS nanocrystals synthesized via low-temperature solution-phase methods have the sphalerite structure.6 Exploring the low-temperature synthesis of highly crystalline wurtzite ZnS nanocrystals has been a hot field of current ZnS material research. For example, researchers found that certain biological molecules (such as peptides) are effective media for synthesizing wurtzite ZnS nanocrystals with relatively low crystallinity at room temperature.7 Besides biological molecules, researchers also demonstrated that polyols (such as ethylene glycol) could be used to synthesize wurtzite ZnS nanocrystals at 150 °C.8 As for one-dimensional (1D) wurtzite ZnS nanomaterials, Li et al. reported that wurtzite ZnS nanorods can be synthesized in an octylamine and hexadecylamine system at 150-250 °C.9 Pradhan et al. obtained very uniform wurtzite * Corresponding author. E-mail: [email protected]. Phone: +86-010-85757002. Fax: +86-010-85772625.

ZnS nanorods and nanowires via decomposing zinc xanthate in octadecylamine at 170 °C.10 El-Shall et al. synthesized highly aligned ultra narrow ZnS nanorods via a microwave-assisted low-temperature solution-phase method.11 Ethylenediamine (en) is another important molecule precursor for the synthesis of wurtzite ZnS nanocrystals, which can react easily with Zn and S to form intercalated lamellar compound ZnS‚en0.5. Wurtzite ZnS nanocrystals could be prepared via decomposing the ZnS‚en0.5 precursor in vacuum or solution systems at high temperature. For examples, Li et al. synthesized wurtzite ZnS nanoplatelets by annealing ZnS‚en0.5 precursors at 350-800 °C and proposed a structure-directing coordination molecular template mechanism to explain the formation of the wurtzite ZnS nanocrystals.12 Yu et al. prepared wurtzite ZnS nanosheets via thermal decomposition of the ZnS‚en0.5 precursor at 250 °C in vacuum.13 Wu et al. prepared wurtzite ZnS nanorods via decomposition of ZnS‚en0.5 in a hydrothermal system at 180 °C.14 Recently, Zhao et al. synthesized wurtzite ZnS nanoplanks via decomposing ZnS‚en0.5 in carbon disulfide under solvothermal conditions (160-200 °C).15 More recently, with the assistances of en molecular, Cai et al. and Zhang et al. synthesized wurtzite ZnS nanobelt arrays and monodisperse microspheres in solution, respectively.16 These studies showed that the final morphologies of wuritzite ZnS nanocrystals obtained via decomposition of the ZnS‚en0.5 precursor are different under different conditions. More importantly, though rod-like wurtzite ZnS nanostructures have been synthesized via decomposition of the ZnS‚en0.5 precursor under hydrothermal conditions,14 the growth process from the ZnS‚en0.5 precursor to wurtzite ZnS nanorod is still blurry. To the best of our knowledge, few detailed investigations were reported on the structural evolvement process from the sheet-like ZnS‚en0.5

10.1021/jp709807t CCC: $40.75 © 2008 American Chemical Society Published on Web 01/19/2008

ZnS‚en0.5 Nanosheets to Wurtzite ZnS Nanorods precursor to the rod-like wurtzite ZnS nanostructure. Therefore, it is meaningful to investigate the formation mechanism of wurtzite ZnS 1D nanostructures prepared by decomposing ZnS‚ en0.5 under different experimental conditions, which can provide useful information to the morphology-controlled synthesis of well-defined wurtzite ZnS nanostructures and other inorganic nanostructures. In this paper, a low-temperature solvothermal route was developed to synthesize near-monodispersed wurtzite ZnS nanorods by decomposing the sheet-like ZnS‚en0.5 precursor in a mixed solution of oleic acid (OA)/ethanol/hexadecylamine (HDA). To study the growth mechanism of the nanorods, we have systematically surveyed the growth process of the nanorods by analyzing the samples at different growth stages, which makes it possible to arrest ZnS crystals in different stages of their growth. On the basis of experimental observations, a ligand-assisted solid-solution-solid (LSSS) growth mechanism is proposed to explain the formation of the ZnS nanorods, which may provide more comprehensive insight into the low-temperature formation mechanism of wurtzite ZnS 1D nanostructures. Experimental Section All of the chemicals were used as received from the Shanghai Chemical Reagents Company without further purification. Synthisis of ZnS‚en0.5 Precursor. ZnClB2 (0.25 mmol), 0.25 mmol of 1-hexadecylamine (HDA), 10 mL of oleic acid (OA), 0.5 mL of ethylenediamine (en), and 20 mL of ethanol were added into a three-neck flask. The reaction mixture was at 60 °C for 30 min to form a clear yellow solution. At 60 °C, 0.12 mL of thioacetamide (TAA, 0.5 mmol) aqueous solution was swiftly injected into this solution. Then, the solution was transferred into a Teflon-lined autoclave of 60-mL capacity. The autoclave was sealed, headed to 160 °C for 1 h, and then allowed to cool to room temperature. The resulting white precipitate was retrieved by centrifugation, washed several times with distilled water and absolute ethanol, and dried under vacuum at 50 °C for 5 h. Synthesis of Uniform Wurtzite ZnS Nanorods with a Diameter of 25 nm. The pre-synthesized ZnS‚en0.5 precursor, 0.25 mmol of HDA, 10 mL of OA, and 20 mL of ethanol were added into a Teflon-lined autoclave of 60-mL capacity. The autoclave was sealed, headed to 160 °C for 5 h, and then allowed to cool to room temperature. The resulting white precipitate was retrieved by centrifugation, washed several times with distilled water and absolute ethanol, and dried under vacuum at 50 °C for 5 h. Characterization. The X-ray powder diffraction (XRD) pattern of the products was recorded on a Rigaku (Japan) D/max-γA X-ray diffractometer equipped with graphite monochromatized Cu KR1 radiation (λ )1.54178 Å). The fieldemission scanning electron microscope (FSEM) images of the products were examined by a field emission scanning electron microscope (JEOL-6300F). The transmission electron microscope (TEM) images, energy-dispersive X-ray spectra (EDS), fast Fourier transform (FFT) pattern, and high-resolution transmission electron microscope (HRTEM) images were recorded on a JEOL 2010 microscope. Photoluminescence (PL) spectrum measurement was performed in a Fluorolog-3-TAU fluorescence spectrophotometer with a Xe lamp at room temperature. Nitrogen adsorption measurements were performed using a Micromeritics ASAP 2000 system utilizing BarrettEmmett-Teller (BET) calculations for surface area. Photocatalytic Activity Test. The photocatalytic activities of the ZnS nanorods were evaluated by degradation of acid

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Figure 1. Typical XRD pattern of the as-synthesized ZnS nanorods.

fuchsine in an aqueous solution under UV light from a 60 W high-pressure Hg lamp. A cylindrical flask (capacity ca. 25 mL) was used as the photoreactor vessel. ZnS nanorods as the catalyst (10 mg) was added in the aqueous acid fuchsine solution (CB20HB17NB3OB9SB3NaB2) (1.0 × 10-4 M, 20 mL), was stirred magnetically in the dark for 5 min to reach the adsorption equilibrium of acid fuchsine with the catalyst, and then exposed to UV light. UV-vis absorption spectra were recorded at different intervals to monitor the reaction using a Shimadzu2550 spectrophotometer. Results and Discussion Powder XRD, Morphology, Crystalline Orientation, and EDS of ZnS Nanorods. Figure 1 shows the XRD patterns of the as-synthesized ZnS nanorods. All of the diffraction peaks can be indexed as wurtzite phase ZnS with lattice parameters comparable to that of JCPDS card (36-1450). The relatively broad XRD peaks reveal the small size of the ZnS nanocrystals. It is noticeable that the (002) diffraction peak is enhanced and much stronger than that of the other peaks, suggesting a preferential orientation of the crystals along the (002) plane of the ZnS nanorods,17 which is further demonstrated below by FFT and HRTEM studies. No other crystalline impurities were detected by XRD, indicating that pure wurtzite ZnS can be obtained via the mild reaction conditions. Figure 2a and b shows the FSEM images of the products. Overall, the product is composed of large quantities of nanorods with an average length of 300 nm (Figure 2a). An enlarged FSEM image (Figure 2b) shows that the nanorods have a mean diameter of 25 nm and the surfaces of the rods are considerably smooth. Figure 2c shows a representative TEM image of the product, indicating that only a few ZnS nanoparticles exist in the product. The HRTEM image of a single ZnS nanorod (Figure 2d) exhibits clear fringes perpendicular to the nanorod axis, the spacing of which measures 0.31 nm and agrees well with the interplanar spacing of the (002) planes of wurtzite ZnS. This is supported by the fast Fourier transform (FFT) pattern (inserted in Figure 2d). Clearly, the wurtzite ZnS nanorod grows along the [001] direction. EDS analysis was performed on isolated single rods, with a local probe diameter of 25 nm. Figure 2e shows a representative EDS spectrum of a central part of a rod. Zinc and sulfur peaks can be clearly observed in this spectrum. The Cu and C peaks in the EDS spectrum arise from the copper TEM grid used in the measurements. These results, together with the images in Figure 2a-d and the XRD pattern in Figure 1, demonstrate that the highly crystalline wurtzite ZnS nanorods are structurally uniform with the [001] orientation.

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Figure 2. (a) Low-magnification FSEM image of the ZnS nanorods. (b) High-magnification of the ZnS nanorods. (c) TEM image of the ZnS nanorods. (d) HRTEM image taken from the edge of a single nanorod. The inset shows the corresponding FFT pattern obtained from the single nanorod. (e) EDS analysis of the ZnS nanorods.

Growth Process of ZnS Nanorods. For the synthesis of the ZnS‚en0.5 precursor, a white compound was obtained when the reaction mixtures were treated solvothermally for 1 h at 160 °C. The XRD pattern (Figure 3a) shows that the white material is crystalline ZnS‚en0.5.15 Figure 3b shows that the ZnS‚ en0.5 crystals are flower-like structures in microscale. Highmagnification FSEM and TEM images (Figure 3c and d) displays that the microscaled flower-like structures are composed of many thin nanosheets. To obtain uniform wurtzite ZnS nanorods, the pre-synthesized ZnS‚en0.5 sheet-like precursors, HDA, OA, and ethanol were added into a Teflon-lined autoclave. After 5 h of solvothermal treatment, the product was pure wurtzite ZnS nanorods. The chemical reactions involved in this process are as follows:

Zn2+ + OA f [Zn(OA)]2+

(1)

2[Zn(OA)]2 + + 2S2- + en f 2ZnS‚en0.5 + 2OA

(2)

2ZnS‚en0.5 f 2ZnS + en

(3)

To substantially understand the growth mechanism of the wurtzite ZnS nanorods, we have systematically surveyed the growth process of ZnS nanorods by analyzing samples at different growth stages. Figure 4a-l shows the TEM and HRTEM images of four samples taken at different stages of the solvothermal reaction: (a and b) 2 h, (c and d) 3 h, (e) 4 h,

and (f) 5h. These images clearly exhibit the evolution of ZnS nanostructures from nanoparticles to nanorods over time at 160 °C. The detailed growth process of the ZnS nanorods may be described as follows. After heating for 2 h, a large amount of yellowish solid product was generated; Figure 4a shows the initial product consisted of mainly 3-10 nm sized sphere-like nanoparticles and small quantity of nanorods with small aspect ratios (L/D ≈ 2-3). SAED (inserted in Figure 4a) and XRD patterns (Figure 4b) of the initial products can be indexed as wurtzite phase ZnS, indicating the formation of wurtzite ZnS nanocrystals through the decomposition of ZnS‚en0.5 sheet-like precursors under solvothermal conditions. Figure 4c shows a HRTEM image of a single sphere-like wurtzite ZnS nanocrystal. However, Figure 4e shows a HRTEM image of a single ZnS nanocrystal in which the sphalerite and wurtzite phases exist, indicating vary small quantity of sphalerite phase ZnS nanocrystals have also been generated as the decomposition products of the sheet-like ZnS‚en0.5. Alternatively, the HRTEM image demonstrated that the nanorods with small aspect ratios are pure wurtzite in structure (Figure 4f and g). After 3 h of reaction, the initially generated small sphere-like sphalerite and wurtzite ZnS nanoparticles gradually dissolved into the solution and grew onto large nanoparticles of ZnS (i.e., nanorods with small aspect ratios) via a process known as Ostwald ripening.18 Following this Ostwald ripening process, some ZnS nanorods with larger aspect ratios were observed (Figure 4h). The HRTEM image clearly displays the nanorods grown along the [001] direction (Figure 4i). After 4 h of reaction, most of the ZnS nanoparticles disappeared, and many nanorods with diameters of 15-20 nm appeared (Figure 4j). The XRD pattern of the product obtained after 4 h of reaction shows sharp peaks, indicating that the crystallinity of the ZnS was further improved (Figure 4k). Finally, as the solvothermal reaction proceeded long enough (5 h), uniform ZnS nanorods with diameters of 25 nm and lengths of 300 nm were obtained (Figure 4l). These phenomena could be explained by the fact that longer growth times could lead to the formation of a nanocrystal that is thermodynamically stable.19 The formation process of the nanorods is summarized in Figure 5. Formation Mechanism of ZnS Nanorods. The solidsolution-solid transformation has been used most extensively to explain the growth of 1D nanostructures in solution phase.20 According to the TEM and HRTEM observed results, a possible ligand-assisted solid-solution-solid growth mechanism (LSSS) was proposed to explain the formation of the near-monodispersed ZnS nanorods. The mechanism includes two steps, which just correspond to the practical reaction process: (1) When the ZnS‚en0.5 sheet-like precursors were loaded into the solvothermal system at 160 °C, it decomposed into a mixture of sphalerite and wurtzite phase ZnS nanoparticles under solvothermal conditions. This phenomenon is different from that reported by others. In previous reports, the ZnS‚en0.5 precursor was fully decomposed into pure wurtzite ZnS nanocrystals.12-15 (2) When this colloidal dispersion was constantly solvothermally treated at 160 °C, the small ZnS nanoparticles started to dissolve into the solution and grow onto ZnS nanorods with small aspect ratios via the Ostwald ripening process. It is wellknown that -NHB2 and -COO- can act as a pair of binding sites for metal ions. Because the reaction was processed in a solution containing HDA and OA, Zn2+ ions resulted from the dissolution of the small ZnS nanoparticles have a chance of combining with OA and HDA through the chelate functions. As a result, this combination reduced the concentration of free Zn2+ ions in the reaction solution and leading to a slow growth

ZnS‚en0.5 Nanosheets to Wurtzite ZnS Nanorods

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Figure 3. (a) XRD pattern of the sheet-like ZnS‚en0.5 precursors. (b and c) FSEM image of the sheet-like ZnS‚en0.5 precursors. (d) TEM image displays that the microscaled sheet-like structures are composed of many thin nanoplakes.

rate of ZnS nanorods, which benefits the growth of uniform 1D nanostructures.21 The other possible function for the ligand molecules is to kinetically control the growth rates of different crystalline faces by interacting with these faces though adsorption and desorption. Under solvothermal conditions, the equilibrium constant for HDA and OA binding to certain crystal faces of ZnS may begin to differ, leading to selective loss of HDA and OA on certain crystal faces and allowing the nanoparticle to grow along only one axis. Controlled experiments were carried out to demonstrate the effect of the mixed ligands. Keeping other reaction parameters constant, when no HDA and OA were added into the reaction mixture, wurtzite phase ZnS nanorods also could be prepared via decomposing the sheet-like precursors in ethanol (Figure 6a). However, the size distribution of the nanorods was very broad, which is similar to the products prepared via decomposing ZnS‚en0.5 in water.14 It should be noted that the nanorods became thinner and thinner along their growth direction; therefore, one question is present here: how do the needle-like nanorods form? Because OA and HDA were not added into the reaction mixture, the initial concentration of free Zn2+ and S2- ions resulting from the decomposition of ZnS‚en0.5 increased greatly. The higher precursor concentration would generate nucleation seeds with a larger average size, and subsequent growth results in nanorods with larger sizes. Following the growth of the ZnS nanorods, the concentration of Zn2+ and S2- became lower and lower. Because of the concentration reduction of Zn2+ and S2-, it could not provide enough species for the growth of the nanorods with larger diameters. As a result, the nanorods became thinner and thinner along their growth direction. On the contrary, once OA and HDA were added into the reaction mixture, the Zn2+ and S2- concentrations would keep relatively constant because of the coordination effects, resulting in the formation of nanorods with uniform diameters. On the basis of the controlled experimental results, it is concluded that the existence of mixed ligands was necessary for the formation of uniform ZnS nanorods. On one hand, they acted as complexing agents and stabilizers, keeping the reactant concentration relatively constant and

preventing fast growth and possible aggregation. On the other hand, they made the growth orientation more restricted, suppressing other growth directions. Furthermore, the specific crystal structure of wurtzite ZnS may be another advantageous factor in the formation of a rodshape nanostructure. Figure 6b shows the structural model of wurtzite ZnS, displaying that the wurtzite ZnS structure consists of a serious of Zn2+-S2- polar surfaces along the [001] direction. The energy of the dipole attraction should assist this anisotropic growth and enable the formation of ZnS nanorods. It has been demonstrated that the polar surface plays an important role in the growth of quasi-1D wurtzite-structured ZnO nanostructures;22 therefore, it is rational to conclude that the Zn2+-S2- polar surfaces along the [001] direction in the wurtzite ZnS structure act a basic driving force during the growth process of the ZnS nanorods. Effects of Solvents. It has been found that the volume ratio of OA to ethanol influences the morphology of the final products significantly. Keeping the total volume of OA and ethanol at 30 mL, when the volume ratio of OA to ethanol is 2, nearmonodisperse ZnS microspheres were synthesized, which were composed of a large quantity of small wurtzite ZnS nanoparticles (Figure 7a and b); while the volume ratio of OA to ethanol is 1/5, wurtzite ZnS submicrowires were obtained (Figure 7c and d). The controlled experiments suggested that it was favorable for the formation of uniform ZnS nanorods when the volume ratio (R) of OA to ethanol was the range of 0.4-0.8. We think the action of ethanol is to adjust the viscosity of the mixed solution, which is important to control the diffusion rate of ions in solution. Recently, Li and co-workers reported that tightly aligned wurtzite ZnS nanorods could be synthesized via removing ethylenediamine from the ZnS‚en0.5 precursor under hightemperature conditions.12 Similarly, a tightly aligned ZnS nanorod structure were also synthesized by us via straightly decomposing the as-synthesized sheet-like ZnS‚en0.5 precursors at 500 °C in vacuum (Figure 8a). The FFT pattern (inserted in Figure 8a) of the aligned nanorods shows regular aligned dots,

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Figure 4. TEM and HRTEM images and XRD patterns of ZnS nanocrystals obtained at 160 °C after reaction time of (a-g) 2 h, (h and i) 3 h, (j and k) 4 h, and (l) 5 h.

Figure 5. (a) Sheet-like ZnS‚en0.5 precursors. (b) Sheet-like ZnS‚en0.5 precursors decomposed into ligand-coated sphere-like nanoparticles and a small quantity of nanorods with small aspect ratios. (c and d) Ligandcoated ZnS nanorods with larger aspect ratios were generated via the Ostwald ripening process.

indicating their highly [001] orientation. The HRTEM image (Figure 8b) displays that the nanorods are about 5-15 nm in diameter and highly crystalline. It should be noted that no ZnS nanoparticles were observed during the formation process of the tightly aligned ZnS nanorods. Obviously, the Ostwald ripening process did not involve the formation of the aligned ZnS nanorods. This controlled experiment clearly demonstrated that the present LSSS growth mechanism of wurtzite ZnS

Figure 6. (a) TEM images of the sample obtained when no OA and HDA was added into the reaction system. (b) Crystal structure of wurtzite ZnS.

nanorods is different from that of ZnS nanorods obtained via high-temperature decomposition of the ZnS‚en0.5 precursor in vacuum. Photocatalytic Properties of the As-Synthesized ZnS Nanorods. To demonstrate the potential applicability in photocatalysis of the as-obtained uniform ZnS nanorods, we investigated their photocatalytic activity by choosing photocatalytic degradation of acid fuchsine as a reference and selected the characteristic absorption of acid fuchsine at about 545 nm for monitoring the adsorption and photocatalytic degradation process. Figure 10 indicates the absorption spectra of an aqueous

ZnS‚en0.5 Nanosheets to Wurtzite ZnS Nanorods

Figure 7. (a and b) FSEM and TEM near-monodisperse ZnS microspheres synthesized when the volume ratio of OA to ethanol is 2. (c and d) FSEM images of the wurtzite ZnS submicrowires obtained when the volume ratio of OA to ethanol is 1/5.

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Figure 10. Absorption spectrum of a solution of acid fuchsine (1.0 × 10-4 M, 20 mL) in the presence of ZnS nanorods (10 mg) after its exposure to the UV light: (a) intial solution; (b-h) after adding ZnS nanorods after exposure to UV light for (b) 1, (c) 3, (d) 5, (e) 10, (f) 14, (g) 18, and (h) 22 min.

areas of the nanorods are about 51 m2/g. These results implied that ZnS nanorods have very strong photocatalytic ability possibly because of their high BET surface and thus the efficient absorption of dye molecules. Conclusions

Figure 8. TEM (a) and HRTEM (b) images of the tightly aligned ZnS nanorods obtained via decomposition of ZnS‚en0.5 at 500 °C in vacuum.

In summary, a mixed ligand-assisted solution-phase method has been developed to synthesize uniform wurtzite ZnS nanorods via decomposition of ZnS‚en0.5 nanosheets under mild solvothermal conditions. On the basis of a series of electron microscopy characterization results, a LSSS growth mechanism was found to explain the formation of the ZnS nanorods. Photocatalytic experiments showed that the as-synthesized ZnS nanorods performed a high activity for degradation of the fuchsine solution. Acknowledgment. We acknowledge the financial support from the Dean Foundation of Chinese Academy of Inspection and Quarantine (2007JK014). References and Notes

Figure 9. Adsorption-desorption isotherms of as-obtained ZnS nanorods.

solution of acid fuchsine (intial concentration: 1.0 × 10-4 M, 20 mL) in the presence of 10 mg of ZnS nanorods under UVlight illumination. As shown in Figure 9, the absorbance intensity of the peak corresponding to the acid fuchsine molecule at 545 nm decreased very quickly once ZnS nanorods were added. With exposure time increasing, the typical sharp peak at 545 nm completely vanished after 20 min. The BET surface

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