Facile Synthesis and Hierarchical Assembly of Hollow Nickel Oxide

Sep 5, 2008 - A solvothermal method combining a calcination process was conducted to synthesize NiO hierarchical architectures with controllable morph...
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J. Phys. Chem. C 2008, 112, 15299–15305

15299

Facile Synthesis and Hierarchical Assembly of Hollow Nickel Oxide Architectures Bearing Enhanced Photocatalytic Properties Xuefeng Song and Lian Gao* State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P. R. China ReceiVed: June 4, 2008; ReVised Manuscript ReceiVed: July 24, 2008

A solvothermal method combining a calcination process was conducted to synthesize NiO hierarchical architectures with controllable morphologies and sizes. First, several hollow R-Ni(OH)2 hierarchical architectures assembled by R-Ni(OH)2 nanosheets had been synthesized through the solvothermal method. The nucleationmediated mechanism overwhelmingly determined the morphology of hollow R-Ni(OH)2 architectures through tuning the nucleation rate. The hollow R-Ni(OH)2 spheres could be formed in the case of high nucleation rate. However, the hollow R-Ni(OH)2 tubes would be obtained at low nucleation rate. Second, the hollow NiO hierarchical architecture could be obtained from the precursor without changing their morphologies by a simple calcination procedure. The as-obtained hollow NiO hierarchical architectures showed high photocatalytic property to decompose acid red 1 pollutant and could also be easily recycled under an external magnetic field, displaying great potential in environmental pollutant cleanup applications. Introduction Morphology-controlled synthesis and large scale self-assembly of nanoscale building blocks into three-dimensional (3D) complex structures have been a research hotspot. Some groups have fabricated diverse superstructures with nanoparticles, nanorods, or nanosheets as building blocks.1 It is expected that if the morphology and assembly mode of original building units can be controlled, then those of their secondary architecture would be adjustable to meet different needs. Therefore, the work on morphology-controlled and organization-designed synthesis needs to be developed for more novel properties and applications of nanomaterials. As a p-type wide-bandgap semiconductor, NiO is a very promising material and has attracted increasing attention because of its extensive important applications such as catalysts,2 electrode materials for lithium ion batteries and fuel cells,3 electrochromic films,4 electrochemical supercapacitors,5 magnetic materials,6 and gas sensors.7 To date, many efforts have been attempted to prepare NiO nanostructures, such as nanoplates,3a nanorings,8 nanosheets or nanoribbons,9 nanowires,10 nanotubes,11 and hollow nanospheres.12 Few of the previous works, however, can precisely control the self-assembly of pure NiO nanosheets into curved NiO 3D superstructures (hollow tubes or spheres) using a single-step process without the use of a template, though the hollow NiO nanospheres have been obtained by templating against latex or carbonaceous polysaccharide microspheres.12a,b Furthermore, the template method suffers from the complication of synthesis process and time-consuming removal of the core particles. Therefore, developing a facile and feasible method to prepare the morphology-controlled NiO complex 3D superstructures is still a great challenge to material scientists. In recent years, considerable attention has been given to the environmental problem involving organic pollutants in water. Photocatalysis is a promising technology for the treatment of contaminants, especially for the removal of organic compounds. Many investigations have been * Corresponding author. Tel: +86-21-52412718. Fax: +86-21-52413122. E-mail: [email protected].

SCHEME 1: One-Pot Preparation of Hollow Hierarchical r-Ni(OH)2 Superstructuresa

a (1) Ethylene glycol-mediated self-assembly formation of nickel alkoxide nanorods; (2) the dissolution of primary nanorods and the nuclei formation of R-Ni(OH)2 nanosheets in solution due to high nucleation rate; (3) the nuclei growth of R-Ni(OH)2 nanosheets and the densification of aggregate of nanosheets to form the solid microspheres; (4) formation of hollow hierarchical R-Ni(OH)2 microspheres via the core evacuation process; (5) nuclei formation on the surface of primary nanorods determined by low nucleation rate; (6) the final formation of hollow R-Ni(OH)2 nanotubes self-assembled by a random array of nanosheets.

reported on utilizing metal oxide nanomaterials as photocatalysts to decompose or destroy the organic pollutants in water.13 However, there is little research about the photocatalytic property of NiO superstructures with high specific surface area, though other properties of NiO nanostructures have been widely investigated. In the present work, to synthesize the nickel oxide precursor with different morphologies, we adopted a facile approach involving an ethylene glycol mediated self-assembly process. A possible mechanism for the evolution of a series of superstructures was proposed (see Scheme 1). Ethylene glycol was not only a strong reductant, which was previously used in the polyol process to obtain monodisperse metal or metal oxide nanoparticles and nanocages,14 but also a ligand to achieve the metal oxide microspheres, nanowires, and platelets.15 By

10.1021/jp804921g CCC: $40.75  2008 American Chemical Society Published on Web 09/05/2008

15300 J. Phys. Chem. C, Vol. 112, No. 39, 2008 calcination at an elevated temperature, the as-obtained nickel oxide precursor was transformed to nickel oxide, which maintained its original morphology. More importantly, the hierarchical nickel oxide superstructures had high photocatalytic activity, an easily recycled property due to the special microstructures and magnetism. Experimental Section Preparation of Nickel Oxide Precursor. All of the chemicals were purchased from Shanghai Chemical Regent Co. (Shanghai, China) and J&K Chemical Ltd. (Shanghai, China) and used as received without any purification. For the preparation of hollow spherical nickel oxide precursor, a typical synthesis process was as follows: 0.27 g of NiCl2 · 6H2O and 0.45 g of sodium acetate (NaAc) were dissolved in 7 mL of ethylene glycol (EG) in a warm-water bath at 60 °C, and then 0.15 g of polyethylene glycol (PEG) and 0.4 g of deionized water were added into the mixture under vigorous stirring. The as-formed green transparent solution was introduced into a Teflon-lined autoclave of 40 mL capacity and maintained at 190 °C for varied periods of time. For the synthesis of hollow tubes of nickel oxide precursor, the reaction procedures were similar to those of the hollow spherical precursor, except for the absence of deionized water and polyethylene glycol (PEG), and the reaction was continued with heating at 190 °C for 1-6 h. After cooling to room temperature, the products were collected by centrifugation, washed with ethanol several times to remove ethylene glycol and excess PEG, respectively, and all of the samples were finally dried in a dynamic vacuum at 60 °C for 8 h. Characterizations. The products were characterized by transmission electron microscopy and high-resolution TEM (HRTEM) (TEM, JEOL-2100F), scanning electron microscopy (SEM, JSM 6700F), powder X-ray diffraction (XRD, Rigaku D/max diffractometer using Cu KR radiation, λ ) 0.15406 nm), thermogravimetry-differential thermal analysis (TG-DTA, STA 449C analyzer), Fourier transform IR (FT-IR, Nicolet 7000C), BET analysis (Micromeritics ASAP 2010), and magnetic analysis (PPMS model 6000). Photocatalytic Reaction. Photocatalytic experiments were carried out in a Pyrex photoreactor,13a,b containing the required quantity of as-obtained products with different morphologies and acid red 1 aqueous solution. The suspension was irradiated with a 300 W medium-pressure mercury lamp with a 340 nm cutoff filter for ultraviolet light irradiation experiments. Oxygen was bubbled into aqueous suspensions at a flow rate of 100 mL/min during the whole experiment. The reaction suspension was prepared by adding the sample (0.6 g/L) into 400 mL of acid red 1 solution (20 mg/L). The suspension was sonicated in the dark for about 15 min and then stirred for 30 min to ensure an adsorption/desorption equilibrium prior to ultraviolet light irradiation. The concentration of acid red 1 in the solution was determined using the Lambda 950 UV-vis spectrophotometer by collecting the absorbance of acid red 1 at 505 nm. Results and Discussion The morphology of the nickel oxide precursor obtained in the presence of deionized water and polyethylene glycol was investigated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The images in Figure 1 show the interesting morphology evolution of samples for different solvothermal times from 1 to 8 h. As shown in Figure 1a, the resulting green nickel oxide precursor after 1 h was clubbed in shape with diameters ranging from 200-400 nm

Song and Gao and lengths up to about 9 µm. The nanorods with terracesstructured edge were observed in the SEM image (Figure 1b), indicating that the thicker nanorod might derive from thin nanobundles by self-assembly under van der Waals forces. Interestingly, the hollow spheres would be formed at the expense of nanorods with increasing reaction time, where three obvious evolution stages could be identified. First, accompanying the rod-like precursor, the loose spherical aggregate composed of interlocked nanosheets had also been observed for 1 h (Figure 1c). With increasing reaction time at 3 h, the aggregate continuously grew and densified to form the solid sphere with about 620 nm in size (Figure 1d and e). In the third stage (8 h), the solid sphere was subjected to core evacuation similar to Ostwald ripening, which led to the hollow microsphere ca. 840 nm in size (Figure 1f). According to the distinct color contrast of the TEM image associated with solid spheres, we could detect that the nanosheets in the core region were more compact than those in the exterior, indicating the density variation of the nanosheets inside the solid cores. We proposed that the evacuation of the solid material occurred by diffusion at the core region, resulting in the void space. The nanosheets at a particular core region dissolved to provide the source material for the formation of the swelling of the shell. With the reaction going on, the interior cavity further expanded, eventually forming the hollow spheres. The ability to form the hierarchical hollow spheres could be attributed to the existence of intrinsic density variations inside the solid spheres. Such a core evacuation process was also observed in other metal oxide hollow spheres.16 The hollow interior could be further corroborated by the typically broken hollow microspheres (Figure 1g). As shown in Figure 1h, the spectra of energy dispersive spectroscopy (EDS) demonstrated that the rod precursor had a high O/Ni ratio and that the hollow spheres composed of thin nanosheets had a relatively low O/Ni ratio. When no water and PEG were added into the precursor solution under the same conditions (190 °C for 1 h), the rodlike nickel oxide precursor still formed. However, when increasing the reaction time to 6 h, an astonishing variety of morphology occurred from the original rod to hollow tube (Figure 2a). The external surface of the individual hollow tube, which was covered by a packed random array of nanoflakes with thickness about 20 nm, was extensively rough and porous (Figure 2b). The TEM image (Figure 2b, inset) showed that the nanoflakes on the surface of tubes were very flexible and thin and the interior of the tube was highly hollow. It was noteworthy that the hollowing gradually proceeded from the inner region to the surface region with increasing reaction time (Figure 2c). The EDS spectrum (Figure 2d) confirmed that the hollow tube of the precursor consisted of carbon, nickel, and oxygen and that the chemical composition was similar to that of spherical nickel oxide precursor. To investigate the as-obtained products with various morphologies in more detail, other measurements were employed. The XRD spectra of the rod-like precursor (Figure 3a) showed that the diffraction spectra were strikingly similar to those of other metal oxide precursors reported by other groups,15 especially the strong peak located in the low-angle region. Although a variety of organometallic precursor produced from polyol-mediated processes had been investigated, the exact structure information of such precursors remained unclear.17 Chakroune et al. detected that the main lines in the XRD pattern were indexed as the (00l) reflections. The strong peak around 10° was characteristic for organometallic precursors synthesized by the polyol-mediated process. Xia et al.15c,d proposed that

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Figure 1. (a) General morphology of the rod-like precursor obtained for 1 h. (b)SEM image of the rod-like precursor with the multistep terrace structure at the edge. (c) Magnified TEM image of loose spherical aggregates in the corresponding area of (a). (d and e) TEM and SEM images, respectively, of 3D solid precursor spheres. (f) TEM image of the hollow spherical precursor. (g) SEM image of the typically broken hollow spherical nickel oxide precursor. (h) EDS spectra of the rod-like and hollow spherical precursor.

Figure 2. (a) Low-magnification SEM image of hollow tubes of the nickel oxide precursor obtained at 190 °C for 6 h in the absence of deionized water and polyethylene glycol. (b) High-magnification SEM image of the external surface of a single hollow tube of the nickel oxide precursor. The inset in (b) shows the TEM image of the individual hierarchical hollow tube. (c) TEM image of solid rods with partial cavity in the internal region obtained at 190 °C for 2 h. (d) EDS spectra of hollow tubes of the nickel oxide precursor.

ethylene glycol could serve as a ligand that reacted with metal ions to form linear coordination complexes. The linear complexes could then aggregate into ordered nanobundles to

generate nanowires through van der Waals interactions. The strong peak around 10° was considered to be a typical feature from the coordination and alcoholysis of EG with the center

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Figure 3. XRD patterns of (a) rod-like nickel oxide precursor; (b) hollow R-Ni(OH)2 spheres; and (c) hollow R-Ni(OH)2 tubes.

Figure 4. FT-IR spectra of (a) rod-like nickel oxide precursor; (b) hollow spheres of the nickel oxide precursor; and (c) hollow tubes of the nickel oxide precursor.

metal ions. On the basis of the similarities between our reaction and those reported in the literature, the as-synthesized rod-like precursor might be formed by the layered stacking of organometallic precursors composed of nickel coordination complexes, consistent with the observation of the SEM image (Figure 1b). The hollow hierarchical spheres and tubes possessed analogous crystalline structures (Figure 3b,c), and the diffraction data were in good agreement with R-Ni(OH)2 (JCPDS card: 22-0444). In the FT-IR spectra of rod-like precursors (Figure 4a), the vibrational bands of CH2 at 2946 and 2869 cm-1 as well as the stretching band of C-OH at 1061 cm-1 derived from the ethylene glycol unit were observed, simultaneously, and the strong absorption bands of CdO in acetate ions at 1569 and 1445 cm-1 were also detected. The three bands appearing at around 1034, 883, and 682 cm-1 were assigned to the stretching and bending vibrations of the C-O species in the precursor, also further confirming the presence of acetate ions.18 It should be noted that the acetate groups disappeared in the case of hollow spheres and tubes (Figure 4b,c). On the basis of analysis and results, although the exact crystal structure of the nickel oxide precursor was yet to be determined, we could conclude that the assynthesized rod-like precursor was probably a kind of nickel alkoxide containing acetate ions and that the structure might be similar to those of iron alkoxide and cobalt alkoxide.15a,b,e More work on the exact chemical structure of nickel oxide precursors was underway. The thermogravimetry-differential thermal analysis (TG-DTA) curves of the as-prepared precursor are shown in Figure SI-1 (Supporting Information). The curve of the rod-like precursor (Figure SI-1a, Supporting Information) showed two weight losses (21.2 and 45.6%), corresponding to the departure of water or excess polyol molecules at about 300 °C and the decomposition of the precursor at about 345 °C, respectively, while the hollow spheres and hollow tubes (Figure SI-1b and c,Supporting Information) had weight losses of about 32.4 and 34.2% and a single exothermal peak at about 325 °C,

Song and Gao indicative that the decomposition temperature of the precursor was at about 325 °C. These results also helped us to determine the temperature of the thermal treatment of the various precursors. The introduction of PEG and water must have played a key role to generate uniform microspheres. The addition of 0.4 g of water promoted the hydrolysis of the acetate group to provide more OH- ions for the formation of R-Ni(OH)2 nanosheets. In order to probe the effect of PEG on the morphology, the control experiment was conducted under the same experimental conditions except for the absence of PEG. As shown in Figure SI-2 (Supporting Information), we could obviously detect that a lot of rod-like precursor was retained. However, the solid spheres were exclusively obtained in the presence of PEG. Consequently, PEG might promote the decomposition of the rod-like nickel oxide precursors to release a large number of Ni2+, which made the PEG-Ni2+complex supersaturated in the solution because of the reaction between PEG and Ni2+. According to the classical nucleation theory,19 the rate of homogeneous nucleation is strongly dependent on the degree of supersaturation. With increasing the supersaturation, the nucleation rate increases. Therefore, massive R-Ni(OH)2 nanosheets formed instantaneously in solution via the reaction between the PEGNi2+complex and OH- ion to form R-Ni(OH)2 and then spontaneously attached together to form aggregates of nanosheets to minimize their surface energy by reducing the exposed areas. Additionally, Xu et al.20 reported that the framework structure of the PEG-Ni2+complex and the nature of the crystals played an important role in restricting crystallization of NiS in some directions, which caused the sheet-like primary crystals to aggregate and form 3D flower-like micrometer-sized architectures. In our present work, PEG might also act as the morphology-directing reagent, which favored the R-Ni(OH)2 nanosheets to assemble 3D solid spheres. The amount of water also affected the morphological evolution. A large number of large-size nanodisks were obtained at an EG/H2O ratio of 3:5 (Figure SI-3a, Supporting Information). The result of XRD analysis (Figure SI-3b in Supporting Information) identified them as the β-Ni(OH)2 crystallites. The SAED pattern also showed that the individual nanodisk was single-crystalline β-Ni(OH)2 with [001] thickness direction (c-axis of hexagonal β- Ni(OH)2). Therefore, it could be inferred that more water could accelerate the formation of singlecrystalline β-Ni(OH)2 nanodisks, while inhibiting the formation of R-Ni(OH)2. From the thermodynamic point of view, when the degree of supersaturation was controlled at a low level, heterogeneous nucleation occurred.21 To obtain the low nucleation rate, it was necessary to control the creation rate of R-Ni(OH)2 in the solution. In this study, a low nucleation rate was maintained in the absence of water and PEG. The slower rate of both decomposition of rod-like precursor and hydrolysis of CH3COO- ions resulted in a low degree of supersaturation. Therefore, heterogeneous nucleation and crystal growth took place on the surface, which should be a result caused by the fact that the crystal planes located on the surface of the rod had high surface energy and favored nucleation.22 The formation of rod-like nickel oxide precursors was crucial to achieve the hollow hierarchical R-Ni(OH)2 tube because it was used as the self-template and the slow-release nickel source. Apart from the water content and PEG, the reaction temperature and sodium acetate also had a significant effect on the as-collected nickel oxide precursor. As we all know, the solubility of solutes affect nucleation and the growth of crystals in a solution. Control

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Figure 5. SEM images of the as-obtained NiO (a) hollow sphere and (b)hollow tube. (c) TEM image of hollow NiO spheres. The inset of (c) shows the SAED pattern of the as-obtained NiO polycrystals. (d) High-magnification TEM image of the nanosheet on the surface of the hollow NiO tube. (e) High-resolution TEM (HRTEM) image taken from the as-obtained NiO nanoparticles self-assembling the nanosheet. (f) XRD patterns of various calcined samples: (I) solid sphere; (II) hollow tube; and (III) hollow spheres.

experiments showed the solubility rate of the rod-like precursor decreased with reducing reaction temperature (Figure SI-4a in Supporting Information), while metallic nickel was formed in large quantities with increasing the temperature to 210 °C (Figure SI-4b in Supporting Information), which was attributed to the increase of the reducing power of ethylene glycol at elevated temperature.23 When no sodium acetate was added under the same reaction conditions, no precipitate was obtained. In our case, EG served as a ligand and coordinated with NiCl2 to form nickel alkoxide, and HCl was a byproduct, which was similar to the reaction between EG and FeCl3.15b The increase in HCl amount hampered the formation of nickel alkoxide with the elongated time. When sodium acetate was added, the reaction between sodium acetate and HCl consumed HCl, which prompted the reaction to proceed. It was interesting that the morphologies of as-obtained R-Ni(OH)2 were successfully sustained (Figure 5a-c) during the thermal transformation to NiO at 350 °C in air for 1 h, and the as-annealed NiO hierarchical structures were assembled by well-crystallized nanosheet build blocks. The selected area electron diffraction (SAED) analysis presented the high polycrystalline nature of NiO (Figure 5c, inset). Further insight into the well-crystallized nanosheet was obtained by the highmagnification TEM image (Figure 5d), which evidently revealed that the smooth surface of the nanosheet of unheated samples had been transformed into a highly porous structure consisting of interconnected nanoparticles because of the removal of organic species in the precursor by pyrolysis. The representative high-resolution TEM image taken from the NiO nanoparticle was shown in Figure 5e. The distinct lattice fringe with an interplanar distance of 0.241 nm was observed, corresponding to the spacing of the (111) planes of NiO crystals. Furthermore,

Figure 6. UV/vis absorption spectra of (a) solid spheres, (b) hollow tubes, and (c) hollow spheres.

the XRD patterns of various calcined samples (Figure 5f) showed that diffraction peaks could be assigned to the cubic NiO (JCPDS 78-0643). The average size of the NiO nanoparticles self-assembling solid spheres, hollow tubess and hollow spheres was calculated from Scherrer’s equation to be 17.4, 11.8, 10.6 nm, respectively. The UV-vis absorption spectra of the as-synthesized NiO solid spheres, NiO hollow tubes, and NiO hollow spheres were illustrated in Figure 6. It could be seen that the morphological changes of the NiO semiconductor were accompanied by remarkable changes in the UV-vis spectra because of the significant difference in crystal size associated with NiO

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Song and Gao TiO2 and SnO2) for easily separating the catalyst from the solution by an external magnetic field. The fact that the novel hierarchical NiO photocatalyst with high photocatalytic activity could be easily recovered should promote their application to eliminate the pollutants in water treatment. Conclusions

Figure 7. The relationship plots of photodegradation ratio of acid red 1 with time irradiated by UV-vis light using different catalysts: (a) NiO rod; (b) NiO solid sphere; (c) NiO hollow tube; and (d) NiO hollow spheres. C0 and Ct represent the initial concentration and concentration at time t, respectively.

nanoparticle. It is well known that the bandgap energy of semiconductor nanoparticles increases with the decrease in grain size. In our case, a blue shift of the peak (from ca. 325 to 280 nm) was observed when the average size of NiO nanoparticle composing the various hierarchical structures decreased from 17.4 to 10.6 nm. To investigate the dependence of morphology with photocatalytic property, we implemented photocatalytic activity of the calcined samples using acid red 1 as a probe reaction (Figure 7). The NiO rods showed the lowest activity of 15% after 200 min irradiation, while other hollow hierarchical structures showed significantly higher activities. Especially, the hollow NiO microsphere showed a significant activity as high as 93%. Such dramatic activity enhancement should be due to the change of specific surface area. The specific surface area of hollow tubes and microspheres calculated by the BET method was 69 and 81 m2/g, respectively; however, the value of the rod was 13 m2/g. With a larger surface area, the number of active surface sites increases and so does the surface charge carrier transfer rate in photocatalysis, which can contribute to the high photocatalytic activity. Moreover, the solid microspheres formed after 3 h showed a lower photocatalytic activity than that of hollow structures, which was only 44%, suggesting that the hollow inner volume significantly promoted the photocatalytic activity by allowing multiple reflection of UV light within the internal cavity.24 The variation of the acid red 1 concentration versus irradiating time at 505 nm was presented in Figure SI-5 (Supporting Information), which further demonstrated that the red acid 1 was obviously degraded with hollow NiO spheres as the photocatalysts. The peaks with maxima at 505 and 531 nm were the characteristic bands of the conjugated electron structure (chromophore group) of this dye.25 The obvious decline of peaks at about 505 nm indicated that the azo bond in acid red 1 was reduced and destroyed under the associated function of irradiation of ultraviolet light and NiO photocatalysts. Since there were few reports on the photocatalytic activity of hierarchical NiO superstructures, our findings suggested that hollow NiO superstructures could be used in environmental applications, such as organic pollutant purification, air purification, and waste remediation. Because of the magnetic property of these new NiO photocatalyst (Figure SI-6, Supporting Information), they have taken an important advantage over other photocatalysts (e.g.,

In summary, two different types of hollow R-Ni(OH)2 hierarchical architectures assembled by R-Ni(OH)2 nanosheets were successfully synthesized through a solvothermal method. The important point in hierarchical architectures synthesis was the control of the nucleation rate of product in the solution. The nucleation-mediated mechanism overwhelmingly determined the nucleation mode of R-Ni(OH)2 and finally resulted in changes of morphology and property of hierarchical architectures. The reaction mechanism might be applicable to other metal oxides and throws new light on how to assemble hierarchical architectures. The hollow NiO hierarchical architecture could be obtained from the precursor without changing their morphologies by a simple calcination procedure. The asobtained hollow NiO hierarchical architecture showed high photocatalytic property to decompose acid red 1 pollutant and could also be easily recycled under an external magnetic field, displaying great potential in the application of environmental pollutant cleanup. Acknowledgment. This work was supported by the National Key Project of Fundamental Research (grant no.2005CB623605), the National Natural Science Foundation of China (no.50572116, 5060249), and Shanghai Nanotechnology Promotion Center (no. 0652nm022). Supporting Information Available: TG-DTA data, controlled experiments, UV-vis absorption spectra, and magnetic analysis of the as-prepared sample. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Park, S.; Lim, J. H.; Chung, S. W.; Mirkin, C. A. Science 2004, 303, 348. (b) Wu, C. Z.; Xie, Y.; Lei, L. Y.; Hu, S. Q.; QuYang, C. Z. AdV. Mater. 2006, 18, 1727. (c) Duan, G. T.; Cai, W. P.; Luo, Y. Y.; Sun, F. Q. AdV. Mater. 2007, 17, 644. (d) Wang, J. G.; Xiao, Q.; Zhou, H. J.; Sun, P. C.; Yuan, Z. Y.; Li, B. H.; Ding, D. T.; Shi, A. C.; Chen, T. H. AdV. Mater. 2006, 18, 3284. (2) (a) Park, J.; Kang, E.; Son, S. U.; Park, H. M.; Lee, M. K.; Kim, J.; Kim, K. W.; Noh, H. J.; Park, J. H.; Bae, C. J.; Park, J. G.; Hyeon, T. AdV. Mater. 2005, 17, 429. (b) Li, Y.; Zhang, B. C.; Xie, X. W.; Liu, J. L.; Xu, Y. D.; Shen, W. J. J. Catal. 2006, 238, 412. (3) (a) Wang, X.; Li, L.; Zhang, Y. G.; Wang, S. T.; Zhang, Z. D.; Fei, L. F.; Qian, Y. T. Cryst. Growth Des. 2006, 6, 2163. (b) Mamak, M.; Coombs, N.; Ozin, G. A. Chem. Mater. 2001, 13, 3564. (c) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J. M. Nature 2000, 407, 496. (4) (a) Karlsson, J.; Roos, A. Sol. Energy 2000, 68, 493. (b) Fantini, M. C. A.; Ferreira, F. F.; Gorenstein, A. Solid State Ionics 2002, 152-153, 867. (5) (a) Liu, K.; Anderson, M. J. Electrochem. Soc. 1996, 143, 124. (b) Srinivasan, V.; Weidner, J. J. Electrochem. Soc. 1997, 144, L210. (6) (a) Felic, A. C.; Lama, F.; Piacentini, M. J. Appl. Phys. 1997, 80, 3678. (b) Tiwari, S. D.; Rajeev, K. P. Thin Solid Films 2006, 505, 113. (7) (a) Mattei, G.; Mazzoldi, P.; Post, M. L.; Buso, D.; Guglielmi, M.; Martucci, A. AdV. Mater. 2007, 19, 561. (b) Dirksen, J. A.; Duval, K.; Ring, T. A. Sens. Actuators B: Chem. 2001, 80, 106. (8) Liang, J.; Li, Y. D. Chem. Lett. 2003, 32, 1126. (9) (a) Liang, Z. H.; Zhu, Y. J.; Hu, X. L. J. Phys. Chem. B 2004, 108, 3488. (b) Matsui, K.; Pradhan, B. K.; Kyotani, T.; Tomita, A. J. Phys. Chem. B 2001, 105, 5682. (c) Ni, X. M.; Zhao, Q. B.; Zhou, F.; Zheng, H. G.; Cheng, J.; Li, B. B. J. Cryst. Growth 2006, 289, 29. (10) (a) Wu, Z. Y.; Liu, C. M.; Guo, L.; Hu, R.; Abbas, M. I.; Hu, T. D.; Xu, H. B. J. Phys. Chem. B 2005, 109, 2512. (b) Xu, C. K.; Hong, K. Q.; Liu, S.; Wang, G. H.; Zhao, X. N. J. Cryst. Growth 2003, 255, 308.

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