Structural Evolution of Hydrothermal-Synthesized Ni(SO4)0.3(OH)1.4

Dec 5, 2008 - Structural Evolution of Hydrothermal-Synthesized Ni(SO4)0.3(OH)1.4 Nanobelts During ex Situ Heat Treatment and in Situ Electron Irradiat...
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J. Phys. Chem. C 2009, 113, 142–147

Structural Evolution of Hydrothermal-Synthesized Ni(SO4)0.3(OH)1.4 Nanobelts During ex Situ Heat Treatment and in Situ Electron Irradiation Ke Zhang,†,‡ Jianbo Wang,*,†,‡ Xiaoli Lu,†,‡ Luying Li,†,‡ Yiwen Tang,§ and Zhiyong Jia§ Department of Physics and Key Laboratory of Acoustic and Photonic Materials and DeVices of Ministry of Education, Wuhan UniVersity, Wuhan 430072, P. R. China, Center for Electron Microscopy, Wuhan UniVersity, Wuhan 430072, P. R China, and Institute of Nano-Science and Technology, School of Physics and Technology, Central China Normal UniVersity, Wuhan 430079, P. R China ReceiVed: September 17, 2008; ReVised Manuscript ReceiVed: October 29, 2008

Nickel hydroxyl sulfate (Ni(SO4)0.3(OH)1.4) nanobelts were obtained via a simple template-free hydrothermal reaction. The structural evolution of Ni(SO4)0.3(OH)1.4 nanobelts during ex situ heat treatment and in situ electron irradiation are investigated using transmission electron microscopy (TEM) techniques mainly including bright-field imaging, selected-area electron diffraction combined with kinematic simulations, and high-resolution TEM. The transformation from Ni(SO4)0.3(OH)1.4 to NiO can be observed in both ex situ and in situ experiments. Based on the TEM results, the structural evolution is clarified and the preliminary structural framework of Ni(SO4)0.3(OH)1.4 is proposed. This structural evolution also provides an excellent approach for large-scale production and/or modification of NiO nanoparticles. 1. Introduction One-dimensional nanostructures, such as tubes, wires, rods, ribbons, and belts, have been extensively studied due to their novel properties and potential applications.1-4 In recent years, considerable effort has been devoted to investigating the shape modification of nanostructures, since the morphology, size, and chemical composition of materials have great effects on their physical and chemical properties.5,6 A large number of approaches have been applied to study such characteristics as heat treatment and electron irradiation. Heat treatment on tin monoxide nanobelts,7 nickel oxide nanocrystalline,8 nanodiamond,9 nanostructured coating,10 nanosized graphite,11 and carbon nanotubes12,13 showed thermal instability and morphological change of nanostructures. In addition, electron irradiation on nanorods,14 nanotubes,15-17 nanoparticles,18 and nanocrystalline19 had proved to be an outstanding procedure in material synthesis and modification. Layered hydroxide salts, which can act as catalysts, metal oxide precursors, selective retainers of anions, topotactic incorporators of cations, anticorrosion agents, templates for metal nanoparticles, additives in polymers, electrodes, and polymer electrolytes, etc.,20 have recently attracted the attention of the scientific community. The structures of the layered hydroxide salts derive from the brucite-like structure, with a fraction of the structural hydroxide groups replaced by water molecules and anions. The hydroxide salt compounds are normally formulated as M2+(OH)2-x(Am-)x/m · nH2O, where M2+ stands for the metal cations (e.g., Ni2+, Mg2+, Co2+, Cu2+, Zn2+) and Am- the anions (e.g., SO42-, NO3-, Cl-). Nickel hydroxyl sulfate (Ni(SO4)0.3(OH)1.4), as a typical layered hydroxide salt, was first found in a mineral of paraotwayite from Western Australia, with the monoclinic lattice a ) 7.89 Å, b ) 2.96 Å, * Author to whom correspondence should be addressed. E-mail: [email protected]. † Department of Physics and Key Laboratory of Acoustic and Photonic Materials and Devices of Ministry of Education, Wuhan University. ‡ Center for Electron Microscopy, Wuhan University. § Central China Normal University.

c ) 13.63 Å, and β ) 91.1° (JCPDS 41-1424).21 The needlelike nickel basic sulfate particles were obtained by forced hydrolysis at 100 °C from aqueous solutions, and thermal decomposition was characterized using infrared spectroscopy and other techniques.22 However, detailed structural information of the Ni(SO4)0.3(OH)1.4 phase and its structural evolution during thermal decomposition has not been reported. Recently, we have fabricated uniform Ni(SO4)0.3(OH)1.4 nanobelts at large scale by a simple template-free hydrothermal reaction,23 which facilitates our systematic structural analysis. In this paper, the structure, including crystallographic structure and microstructure such as morphology and defects, and the structural evolution of the Ni(SO4)0.3(OH)1.4 nanobelt during ex situ heat treatment and in situ electron irradiation are investigated. Besides our preliminary analysis results through X-ray diffraction, thermal analysis, etc.,23 systematical structural analysis is applied through employing various transmission electron microscopy (TEM) techniques, mainly including brightfield (BF) imaging, selected-area electron diffraction (SAED) combined with kinematic simulation, and high-resolution TEM (HRTEM). The transformation from Ni(SO4)0.3(OH)1.4 to NiO (Fm3jm, a ) 4.176 Å, JCPDS 78-0643) can be observed during both ex situ heat treatment and in situ electron irradiation, with detailed structural analysis given. 2. Experimental Section The nickel hydroxyl sulfate nanobelts were synthesized by a simple template-free hydrothermal reaction. NiSO4 · 6H2O (0.001 mol) was dissolved in 5 mL of distilled water. Under magnetic stirring, 3 mL of ammonia liquor (35% by v/v) was slowly dropped to form a clear solution at room temperature. The solution was then dropped slowly into 15 mL of ethanol. The final solution was transferred into a Teflon-lined stainless-steel autoclave of capacity 50 mL. The autoclave was then filled with distilled water to 70% of the total volume. Hydrothermal synthesis under the saturated water vapor pressure with 180 °C lasted for 24 h, and the autoclave was then cooled to room temperature. A green product was obtained. The product was

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Ni(SO4)0.3(OH)1.4 Nanobelts

Figure 1. BF images of (a) typical Ni(SO4)0.3(OH)1.4 nanobelts assynthesized at 180 °C and (b) products heat-treated at 500 °C and (c) 750 °C, respectively. (d-f) Corresponding SAED patterns. The subscripts “m” and “c” denote monoclinic Ni(SO4)0.3(OH)1.4 and cubic NiO, respectively.

filtered, washed with distilled water several times, and then dried at 60 °C in a vacuum oven for 24 h. In order to investigate the structural evolution, part of the as-prepared product was heattreated at 500 and 750 °C in air for 2 h, respectively.23 The as-prepared and heat-treated products were sonicated in ethanol and then dropped onto the holey carbon-coated copper grid for TEM observation. If necessary, further surface carboncoated treatment was carried out to reduce electron irradiation damage. TEM observation including BF imaging and SAED was performed using a JEOL JEM-2010(HT) electron microscope with LaB6 filament, and further HRTEM characterization was done on a JEOL JEM-2010FEF(UHR) electron microscope equipped with a field emission gun and an Omega-type in-column energy filter system. Both microscopes were operated with an acceleration voltage of 200 kV. 3. Results and Discussion 3.1. Ex Situ Heat Treatment on Ni(SO4)0.3(OH)1.4 Nanobelts. The structural evolution of the typical Ni(SO4)0.3(OH)1.4 nanobelts during the ex situ heat treatment is followed by recording the BF images (Figures 1a-c) and the corresponding SAED patterns (Figures 1d-f) for different treated products, i.e., as-prepared (Figures 1a, d) and heat-treated at 500 °C (Figures 1b, e) and 750 °C (Figures 1c, f), respectively. The delicate structural features of as-prepared Ni(SO4)0.3(OH)1.4 nanobelts can be unveiled first. A part of the typical as-prepared nanobelt shown in Figure 1a has uniform

J. Phys. Chem. C, Vol. 113, No. 1, 2009 143 width of about 80 nm and length of up to several tens of micrometers. The bend contours marked by short black arrows in the BF image (Figure 1a) and the sharp diffraction spots in the SAED pattern (Figure 1d) indicate the good crystallinity of the as-prepared Ni(SO4)0.3(OH)1.4 nanobelt to some extent. The series of closely spaced spots along the horizontal direction in the SAED pattern (Figure 1d) indicates the large lattice parameter (i.e., c ) 13.63 Å) characteristic of the Ni(SO4)0.3(OH)1.4 structure, and the SAED pattern can be indexed as the [100]m zone axis of Ni(SO4)0.3(OH)1.4. The subscript “m” denotes monoclinic Ni(SO4)0.3(OH)1.4 throughout the paper. The indexed SAED pattern (Figure 1d) along with the BF image (Figure 1a) indicates that the nanobelt is a single crystal grown parallel to [010]m and enclosed by (100)m and (001)m as the top and side planes, respectively. However, planar defects with faults on the (001)m planes exist extensively in the as-prepared nanobelts, which are indicated by long white arrows (Figure 1a). This is consistent with the information revealed by the diffuse streaking extended along c* in the SAED pattern (Figure 1d), also marked by long white arrows. The planar defects indicate that the nanobelts may grow through oriented attachment with smaller nanowires or nanobelts24,25 under mild hydrothermal conditions, which was also noticed by Dong et al.26 In addition, it is very interesting to note that the streaks are located at (1/2(h1l)m, which means that the real lattice parameter border should be twice as large as the b value ()2.96 Å) provided by the literature.21 Therefore, the fault vector is border/2. As the streaking is continuous, the probability of a fault occurring is about 0.5, or 50%. The products heat-treated at both 500 and 750 °C transform into cubic NiO phase according to the SAED patterns (Figures 1e, f), which can be indexed as the [100]c zone axis of NiO. The subscript “c” denotes cubic NiO throughout the paper. After heat treatment at 500 °C, the nanobelts still keep the belt morphology but become short and porous (Figure 1b). The porous mosaic belt contains NiO grains with nearly the same orientation, and the tiny misorientation is about 3° around the [100]c direction (Figure 1e). As the heat treatment temperature increases to 750 °C, the belt is completely destroyed and transforms into agglomerated NiO nanoparticles with random orientation but better crystallinity (Figures 1c, f), which can be noted by the diffraction contrast in the BF image (Figure 1c) and sharp spots in the SAED pattern (Figure 1f). The NiO nanoparticles show both round and faceted shapes (Figure 1c). Furthermore, the overall morphology and structure information for heat-treated samples can be obtained by larger scale examination, besides the aforementioned individual nanobelt analysis. Figures 2a, b show the completely different morphologies of the samples heated at 500 and 750 °C, respectively. However, both corresponding ringlike SAED patterns (Figures 2c, d) can be indexed as cubic NiO, which is confirmed by the agreement between the experimental results (left parts in Figures 2c, d) and the simulated ones (right parts in Figures 2c, d). Nevertheless, the subtle difference between the two SAED patterns (Figures 2c, d) should not be ignored. The reflections in Figure 2d appear more sharp than those in Figure 2c, indicating the better crystallinity after higher temperature (750 °C) heat treatment. The maintenance of the belt morphology (Figure 2a) after lower temperature (500 °C) heat treatment makes the preferential orientation c of mosaic NiO grains, which leads to the relatively stronger diffraction intensities of {020}c and {022}c in Figure 2c. Besides an immense number of synthesized Ni(SO4)0.3(OH)1.4 nanobelts, a few intermediate products were also fabricated

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Figure 2. Large-scale BF images of products after heat treatment at (a) 500 °C and (b) 750 °C, respectively. (c, d) Corresponding experimental (left) and simulated (right) SAED patterns.

Figure 4. HRTEM images of (a) typical Ni(SO4)0.3(OH)1.4 nanobelts as-synthesized at 180 °C and (b) products heat-treated at 500 °C and (c, d) 750 °C, respectively. The arrows show the long axis of the belt. (e, f, g, h) Enlarged images of the areas marked by white frames in (a, b, c, d) respectively. The insets show corresponding FFT patterns. Figure 3. (a) BF image of the β-Ni(OH)2 sheet observed in assynthesized samples. (b) BF image of agglomerated NiO particles after heat treatment at 750 °C. (c) Corresponding [001]h SAED pattern from the area marked in (a). (d) SAED pattern of an individual NiO particle marked in (b).

during the hydrothermal reaction shown in Figure 3a as sheets. The corresponding SAED pattern reveals that the phase is β-Ni(OH)2 (P3jm1, a ) 3.126 Å, c ) 4.605 Å, JCPDS 14-0117) and the sheet is perpendicular to [001]h (Figures 3a, c), where the subscript “h” denotes hexagonal β-Ni(OH)2. The appearance of β-Ni(OH)2 as a by-product indicates a certain crystallographic relation with Ni(SO4)0.3(OH)1.4, which may be employed to investigate the structural framework of Ni(SO4)0.3(OH)1.4 below. HRTEM observation goes a step further. A typical nanobelt synthesized at 180 °C shows quite smooth edges (Figure 4a) and large lattice spacing (002)m (Figure 4e) characteristic of the monoclinic Ni(SO4)0.3(OH)1.4 phase. The corresponding fast Fourier transformed (FFT) pattern (inset in Figure 4e) shows the same closely spaced spots, which is similar to the SAED

pattern in Figure 1d and corresponds to the aforementioned large lattice spacing (002)m. When the nanobelts are heat-treated at 500 °C, they keep the belt morphology and orientation, but become rough and inhomogeneous with terraces forming (Figure 4b). A closer view reveals that no large lattice spacing exists and the crystallographic structure transforms into cubic NiO showing (002)c and (020)c planes with much finer spacing (Figure 4f). The FFT pattern in the inset of Figure 4f confirms the cubic NiO phase without closely spaced spots. As the heat treatment temperature increases to 750 °C, the belts completely collapse into NiO nanoparticles with different shapes and orientations. Although they have different shapes (round in Figure 4c and faceted in Figure 4d), they possess better crystallinity (Figures 4g, h) than NiO grains in the products heat-treated at 500 °C (Figure 4f). The faceted NiO nanoparticle displays apparent and sharp edges, which can be indexed as {111}c lattice planes (Figures 4d, h). Moreover, the random orientation possessed by the NiO nanoparticles can be clearly revealed (Figures 1c, f; 3b, d; 4c, d, g, h) compared with those

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Figure 5. (a) Low- and (b) high-magnification BF images of an individual Ni(SO4)0.3(OH)1.4 nanobelt showing morphology change after strong electron irradiation damage.

heat-treated at 500 °C (Figures 1b, e; 4b, f), the same as the results shown by large-scale observation (Figure 2). Consequently, it can be concluded that the structural transformation from Ni(SO4)0.3(OH)1.4 to NiO occurs during ex situ heat treatment, and higher heat treatment temperature will help to improve the crystallinity of NiO but destroy the belt morphology. By combining these results with the former investigation of differential thermal and thermal gravimetric analyses, it can be inferred that the breaking and nonuniformity of the nanobelts could result from loss of water molecules adsorbed on the outer surfaces, intercalated water, and most of the sulfate groups step by step.22,23 During 500 °C heat treatment, the Ni(SO4)0.3(OH)1.4 nanobelts lose water molecules, hydroxyls, or sulfate groups, with the layered structures gradually collapsing into cubic NiO and porous beltlike morphology forming. The mosaic microstructure contains NiO grains with poor crystallinity but uniform orientation. The porous morphology shows very large surface with potential applications. Further, the higher heat treatment temperature (750 °C) completely destroys the belt morphology into the agglomerated recrystal NiO nanoparticles with better crystallinity but random orientation. Similar structural transformation from Cd(OH)2 to CdO induced by heat treatment occurs in Cd(OH)2 nanodisks by calcinations.27 3.2. In Situ Electron Irradiation on Ni(SO4)0.3(OH)1.4 Nanobelts. Ni(SO4)0.3(OH)1.4 nanobelts are very sensitive to high-energy electron irradiation which destroys the structure and morphology normally in less than several seconds, and makes observation and determination impossible. Figure 5a shows an individual Ni(SO4)0.3(OH)1.4 nanobelt chosen for in situ electron irradiation experiments, and the enlarged BF image of the irradiated area circled in Figure 5a is shown in Figure 5b. The irradiated part of the nanobelt exhibits rough morphology due to electron irradiation damage, which is quite different from the smooth area with less irradiation damage. The obvious existence of planar defects in the as-prepared sample is also noted here. The SAED patterns of as-prepared Ni(SO4)0.3(OH)1.4 nanobelts under the electron irradiation process demonstrate the sequence of the structural evolution as well (Figure 6). Initially, the closely spaced diffraction spots parallel to the c* direction are clearly observed in the [100]m SAED pattern of Ni(SO4)0.3(OH)1.4 (Figure 6a). Subsequently, they become weaker and weaker (Figure 6b) with the electron irradiation time increasing. Finally, only several strong diffraction spots remain with tiny displacements, and the whole SAED pattern can be reindexed as cubic NiO along the [100]c zone axis (Figure 6c). The corresponding simulated representations (Figures 6d-f), together with the experimental results (Figures 6a-c), clearly illustrate the structural evolution from Ni(SO4)0.3(OH)1.4 to NiO induced by electron irradiation. HRTEM further confirms the transformation from Ni(SO4)0.3(OH)1.4 to NiO during in situ electron irradiation. The

Figure 6. (a-c) Sequence of SAED patterns of nanobelts during the in situ electron irradiation with time elapsing. (d-f) Corresponding schematic representations.

Figure 7. HRTEM images of Ni(SO4)0.3(OH)1.4 nanobelts as-irradiated (a) and postirradiated (b), respectively. The arrows illustrate the long axis of the belt. (c, d) Enlarged images of the areas marked by white frames in (a, b) corresponding to Ni(SO4)0.3(OH)1.4 and NiO, respectively. The insets show FFT patterns related to (c, d).

HRTEM images of nanobelts as-irradiated and postirradiated are shown in Figures 7a, b, together with the enlarged images (Figures 7c, d) of the areas marked by white frames in Figures 7a, b respectively. The apparent structural evolution under

146 J. Phys. Chem. C, Vol. 113, No. 1, 2009 electron irradiation can be recognized in the lattice fringe images. The characteristic (002)m planes with large lattice spacing for Ni(SO4)0.3(OH)1.4 (Figure 7c) fade away gradually with only some winding traces remaining (Figure 7d), which would vanish eventually. The remained orthogonal planes with much finer spacing can be attributed to (020)c and (002)c characteristic of cubic NiO (Figure 7d). The corresponding FFT patterns (insets in Figures 7a, b) also show the gradual vanishing of the closely spaced spots confirming the transformation from Ni(SO4)0.3(OH)1.4 to NiO during electron irradiation. Owing to their extreme sensitivity to electron irradiation, it is difficult, if not completely impossible, to obtain the SAED patterns and HRTEM images of Ni(SO4)0.3(OH)1.4 nanobelts. In the literature,28,29 some reported TEM results for nickel hydroxyl sulfate nanobelts or similar hydrothermal-synthesized compounds are similar to those shown in Figures 6c and 7d, which offer the transformed structure information after strong irradiation damage rather than the as-prepared. In conclusion, both the SAED patterns and the HRTEM images of nanobelts as-irradiated and postirradiated confirm the transformation from Ni(SO4)0.3(OH)1.4 to NiO during in situ electron irradiation. Hu and Ruckenstein reported that LiOH grains could transform into Li2O after electron irradiation.30 They proposed that the facile transformation under electron irradiation could be attributed to the combination of the irradiation-induced distortion of crystal structure and the heating effect of the irradiation, which may be employed to explain our observation during in situ electron irradiation. Surface carbon-coated treatment before TEM observation can reduce electron irradiation damage to some extent by increasing the thermal and electric conductivity. 3.3. Preliminary Structural Framework of Ni(SO4)0.3(OH)1.4. Although the unit-cell parameters of the monoclinic Ni(SO4)0.3(OH)1.4 were determined,21 detailed structural information including atomic sites and space group has not been provided yet. We can discuss and propose a preliminary structural framework of Ni(SO4)0.3(OH)1.4 based on our TEM results. Following the structural evolution from Ni(SO4)0.3(OH)1.4 to NiO, the orientational relationship between Ni(SO4)0.3(OH)1.4 and NiO can be obtained as [100]m//[100]c, (020)m//(022)c, and (008)m//(02j2)c, based on comparing the indexed SAED patterns and HRTEM images during both ex situ heat treatment (Figures 1d-f, 4a, b, e, f) and in situ electron irradiation (Figures 6a-c, 7a-d). The structural model of Ni(SO4)0.3(OH)1.4 can be further constructed by intercalating sulfate and water molecules in the basic β-Ni(OH)2 structure, owing to the fact that intercalation reaction and layer surface grafting are pervasive in the preparation of layered hydroxide salts.20,31,32 Ni(SO4)0.3(OH)1.4 possesses layered structure and basic units of distorted NiO6 octahedra as aforementioned. Based on the known structural model of β-Ni(OH)2,33 the structural relationships of NiO, β-Ni(OH)2, and Ni(SO4)0.3(OH)1.4 can be illustrated by the schematic representations shown in Figure 8. A slightly distorted NiO6 octahedron (blue) is chosen approximately as the basic octahedral unit of NiO. c is parallel to the direction of opposite vertices connection (marked by the dash line) of the selected NiO6 octahedron. (022)c and (02j2)c are normal to two mutually perpendicular edges, respectively, which are both perpendicular to the fixed [100]c. By combining these results with the obtained orientation relationship, a rigid unit cell model of Ni(SO4)0.3(OH)1.4 can be constructed. Then, a preliminary structural framework (yellow) can be obtained after adjustment

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Figure 8. Structural relationships of β-Ni(OH)2, NiO, and Ni(SO4)0.3(OH)1.4, revealing the unit cells of β-Ni(OH)2 (black prism), NiO (blue octahedron), and Ni(SO4)0.3(OH)1.4 (yellow prism).

Figure 9. (a) Structural model of NiO projected along [100]c. (b) Layered structural model of β-Ni(OH)2. (c) Structural framework of Ni(SO4)0.3(OH)1.4 projected along [100]m (the marked cm is the projection of the basic vector c). (d) Layered structural framework of Ni(SO4)0.3(OH)1.4 projected along the same direction as in Figure 8.

using the provided crystallographic parameters21 and appropriate relaxation, where sulfate groups and water molecules are omitted for the sake of convenience. Figures 9a, c illustrate the structural models of NiO and Ni(SO4)0.3(OH)1.4 projected along [100]c and [100]m, respectively, indicating the common units of NiO6 octahedra and the corresponding orientation relationship, [100]m//[100]c, (020)m// (022)c, and (008)m//(02j2)c. Figure 9d shows the layered structure of Ni(SO4)0.3(OH)1.4 formed by intercalating sulfate, water molecules, and other anions between the layers of the basic β-Ni(OH)2 structure (Figure 9b). These intercalated species leads to an increase in the interlayer spacing in Ni(SO4)0.3(OH)1.4 compared with β-Ni(OH)2. It can also be noticed that the SAED patterns of Ni(SO4)0.3(OH)1.4 (Figures 1d and 6a) indicate obvious extinction reflections due to the structural symmetry. According to the reported primitive monoclinic symmetry21 and the observed diffractions of 00l spots with l ) 2n along c* or 0k0 spots with k ) 2n along b*, the possible space group of Ni(SO4)0.3(OH)1.4 may be P21/c, P1c1, P1n1, P2/c, or P21. In addition, the lattice parameter border should be twice as large as the b value ()2.96 Å) provided by the literature at least as aforementioned in

Ni(SO4)0.3(OH)1.4 Nanobelts section 3.1, which has not yet considered in the preliminary model. Therefore, the structural determination of Ni(SO4)0.3(OH)1.4 requires more in-depth investigation based on our results and some information provided by the literature. 21,34 4. Conclusions In summary, the delicate structural features of hydrothermalsynthesized Ni(SO4)0.3(OH)1.4 nanobelts, including crystallographic structure and microstructure such as morphology and defects, are studied by detailed TEM examination. The structural transformation from Ni(SO4)0.3(OH)1.4 to NiO is observed systematically during both ex situ heat treatment and in situ electron irradiation, which provides the detailed structural relationship between the two phases. Furthermore, the preliminary structural framework of Ni(SO4)0.3(OH)1.4 is proposed based on the revealed structural information. The clarification of the structural evolution from Ni(SO4)0.3(OH)1.4 to NiO may be applicable to other similar systems, which is also helpful to large-scale production and/or modification of metal oxides nanoparticles. Acknowledgment. This work was supported by New Century Excellent Talents in University (NCET-07-0640) and Natural Science Foundation for the Outstanding Young Scientists of Hubei Province, China (No. 2005ABB014). References and Notes (1) Goldberger, J.; He, R. R.; Zhang, Y. F.; Lee, S.; Yan, H. Q.; Chol, H. J.; Yang, P. D. Nature 2003, 422, 599–602. (2) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, H. Q. AdV. Mater. 2003, 15, 353–389. (3) Wang, Z. L. AdV. Mater. 2003, 15, 432–436. (4) Hu, J. T.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435–445. (5) Im, S. H.; Lee, Y. T.; Wiley, B.; Xia, Y. N. Angew. Chem., Int. Ed. Engl. 2005, 44, 2154–2157. (6) Leite, E. R.; Vila, C.; Bettini, J.; Longo, E. J. Phys. Chem. B 2006, 110, 18088–18090. (7) Orlandi, M. O.; Ramirez, A. J.; Leite, E. R.; Longo, E. Cryst. Growth Des. 2008, 8, 1067–1072. (8) Bahadur, J.; Sen, D.; Mazumder, S.; Ramanathan, S. J. Solid State Chem. 2008, 181, 1227–1235.

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