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Synthesis, Characterization, and Electrochemical Properties of Cu3V2O7(OH)2 · 2H2O Nanostructures Shaoyan Zhang,* Lijie Ci, and Huiru Liu College of Chemical Engineering, Shijiazhuang UniVersity, Shijiazhuang 050035, People’s Republic of China ReceiVed: February 18, 2009; ReVised Manuscript ReceiVed: April 5, 2009
We report on the shape-controlled synthesis, characterization, and electrochemical properties of Cu3V2O7(OH)2 · 2H2O nanomaterials that were synthesized through a simple and facile solution route without any surfactants or template. It was found that by simply controlling the reaction conditions, Cu3V2O7(OH)2 · 2H2O nanowires, nanoflakes, and nanoparticles can be selectively prepared. Electrochemical measurements revealed that the as-prepared Cu3V2O7(OH)2 · 2H2O nanowires, nanoflakes, and nanoparticles exhibited high discharge capacities and ideal shaped discharge curves. In particular, the Cu3V2O7(OH)2 · 2H2O nanowires showed capacities much higher than those of Cu3V2O7(OH)2 · 2H2O nanoflakes, nanoparticles, and commercial Ag2V4O11 bulk. The correlation between the specific structural features of the nanowires and their superior electrode performance were also discussed. It is anticipated that the novel Cu3V2O7(OH)2 · 2H2O nanowires are promising cathode candidates in the application of primary lithium ion batteries for implantable cardioverter defibrillators (ICDs). Introduction Over the past several years, low-dimensional nanostructured materials such as nanosheets, nanotubes, and nanowires have triggered a worldwide interest because of their unique chemical and physical properties and the wide range of potential applications in nanodevices.1-3 Numerous studies have indicated that the shape, size, and structure of inorganic nanomaterials exert a significant influence on those physical and chemical properties.4,5 For the purpose of property studies and future applications of nanoscale materials, it is doubtless that the primary objective should lie in developing various convenient synthetic strategies to obtain shape-controlled nanostructures. Among various strategies for controlled synthesis, the solution chemical synthesis is an effective method for the chemical synthesis of nanostructural materials with well-controlled shapes, sizes, and structures.6,7 For example, a wide variety of inorganic materials with the desired shape, including metal,8 metal oxide,9 sulfide,10,11 hydrate,12 and other minerals, have been achieved by applying solution chemistry approaches. As important functional inorganic materials, transition metal vanadates have long been studied as potential battery materials for primary or secondary lithium battery applications owing to their layered nature and excellent kinetics. Among them, Ag2V4O11 has been well-characterized and is used commercially as a cathode material in primary lithium batteries.13,14 Both the shape of the discharge curve and the high-rate capability of the Li/Ag2V4O11 system make it ideal as a power source for implantable cardiac defibrillators (ICDs). As the Li/Ag2V4O11 system shows a sloping discharge curve, the state of discharge of the battery can be estimated by just measuring the battery potential, which is a desirable feature in an ICD.15,16 So far, Li/Ag2V4O11 primary batteries are the only type of battery for ICDs.17 However, for use within ICDs, the batteries still need to meet more stringent requirements including high energy density, flexible and lightweight design, and longer life span.13b * To whom correspondence should be addressed. E-mail: zsyedu@ yahoo.com.cn.
To achieve this, alternative cathode active materials are constantly under evaluation. Indeed, recent studies on Ag4V2O6F2,18 β-AgVO3 nanowires,19-22 and Ag2V4O11 nanowires19,23 have displayed enhanced electrochemical activity including the improvement on the discharge capacity and highrate capability. In addition to silver vanadates (SVO), the copper vanadates (CVO) family have the layered structure and multistep reductions during the insertion/intercalation of lithium, which are similar to the character of Ag2V4O11. Recently, the CuV2O6 nanostructures has been successfully synthesized and used as the cathode in primary lithium batteries. During discharge the Cu2+ is reduced to Cu0, and in addition, more than one lithium ion per vanadium can be reacted, giving it a capacity over 500 mAh g-1.24 Thus, the layered copper vanadate materials, which possess high discharge capacity, superior high-rate capability, and safety, greatly expand the range of cathode choices. Volborthite, Cu3V2O7(OH)2 · 2H2O, is a rare copper vanadate mineral.25,26 Just like other transition metal vanadates, Cu3V2O7(OH)2 · 2H2O consists of a layered crystal structure, which may be a potential battery material for primary or secondary lithium batteries owing to their layered structures and excellent kinetics. Although Cu3V2O7(OH)2 · 2H2O has been known since the 18th century, no attempts have been made so far to study the controlled growth and the electrochemical properties of Cu3V2O7(OH)2 · 2H2O nanostructures. In this regard, we report in this paper the synthesis of Cu3V2O7(OH)2 · 2H2O nanostructures with novel shapes and their application as cathode materials in primary lithium batteries. In our work, Cu3V2O7(OH)2 · 2H2O nanostructures have been successfully prepared in high yield via a simple and facile solution route. The formation of different morphologies, including nanowires, nanoflakes, and nanoparticles, was achieved by controlling the reaction parameters. This simple synthetic route, which involves no templates or surfactants and requires no expensive and precise equipment, will offer great opportunities for the scale-up preparation of novel-shaped transition metal vanadate nanostructures. The electrochemical performance of Li/Cu3V2O7(OH)2 · 2H2O primary batteries was systematically
10.1021/jp901490s CCC: $40.75 2009 American Chemical Society Published on Web 04/21/2009
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investigated by cyclic voltammetry and galvanostatic discharge method. The results show that the as-prepared Cu3V2O7(OH)2 · 2H2O nanowires exhibited high discharge capacity, high open circuit voltage, and enhanced kinetics toward lithium intercalation. Furthermore, the Li/Cu3V2O7(OH)2 · 2H2O system shows a sloping discharge curve, and the state of discharge of the battery can be estimated by just measuring the battery potential, making them promising cathode candidates for lithium primary batteries in the application of ICDs. Experimental Section Preparation of Nanoparticles. CuSO4 · 5H2O (0.1498 g) was dissolved into 8 mL of deionized water at room temperature, and 0.0468 g of NH4VO3 was dissolved into another 8 mL of deionized water at 80 °C. Then, the NH4VO3 solution was added slowly to the CuSO4 solution under strong magnetic stirring. A yellow precipitate formed immediately and the pH was ca. 5. After the resulting precursor suspension had been stirred for about 10 min, the final products were collected by centrifugation, washed with deionized water and ethanol, and then vacuum dried at 60 °C for 4 h. Preparation of Nanoflakes. All steps were the same as in the synthesis of Cu3V2O7(OH)2 · 2H2O nanoparticles, except the pH value of the precursor suspension was adjusted to 7-8 with NH3 · H2O. Preparation of Nanowires. The resulting precursor suspension obtained in the synthesis of Cu3V2O7(OH)2 · 2H2O nanoparticles was transferred into a 25 mL Teflon-lined stainless steel autoclave without the adjustion of the pH value. The autoclave was sealed and heated at 180 °C for 20 h. After the reaction, the autoclave was cooled to ambient temperature naturally. A gray-green product was collected by centrifugation, washed with deionized water and ethanol, and then vacuumdried at 60 °C for 4 h. To understand the growth process of the Cu3V2O7(OH)2 · 2H2O nanowires under hydrothermal conditions, the effect of reactiom time on the product was investigated. The reaction time was controlled at 3, 6, and 12 h, respectively. Characterization. Phase structures of the as-prepared samples were characterized by using a Rigaku D/max-2500 X-ray powder diffractometer (XRD) with Cu KR radiation. Surface images were investigated with a Hitachi S-4800 field emission scanning electron microscope (FE-SEM). Electrochemical Measurements. Electrochemical measurements were carried out with use of two-electrode cells with lithium metal as the counter and reference electrode. The working electrode was fabricated by compressing a mixture of the as-prepared Cu3V2O7(OH)2 · 2H2O, conductive material (acetylene black-ATB), and binder (polytetrafluoroethylene (PTFE)) in a weight ratio of active material:ATB:PTFE ) 8:1:1. The electrode was dried at 80 °C for 1 h and cut into a disk (1.0 cm2). The electrolyte solution was 1.0 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio of EC:DMC ) 1:1. The cell assembly was performed in an Ar-filled glovebox. The electrochemical properties were investigated by using a Reference 600 potentiostat analyzer and Land charge-discharge unit at controlled temperatures. The cyclic voltammograms (CV) were measured at a scan rate of 5 mV s-1 and 37 °C. The impedance measurements were carried out under the open-circuit condition in the frequency range from 1 × 106 to 0.1 Hz. The electrode capacity was measured by a galvanostatic discharge method at the current density of 20 mA g-1 to the cutoff voltage of 1.5 V and at a temperature of 37 °C. The capacity was based
Figure 1. XRD patterns of the as-prepared Cu3V2O7(OH)2 · 2H2O (a) nanoparticles, (b) nanoflakes, and (c) nanowires.
on the amount of active material, not including the weight of the additives in the electrode. Results and Discussion Structural Determination. The crystal phase of the products was analyzed by powder X-ray diffraction. Figure 1 shows the typical XRD patterns of the as-prepared Cu3V2O7(OH)2 · 2H2O nanowires, nanoflakes, and nanoparticles. It can be observed that the position of the characteristic peaks of the three products is consistent, whereas the peak intensity is different. The peak intensity of nanowires is much stronger than that of nanoparticles and nanoflakes, indicating its highly crystalline structure obtained at elevated temperatures and pressure via the hydrothermal synthesis route. The peak broadening and the disappearance of some of the peaks for the nanoflakes indicate that the crystalline size of the sample is very fine and that the crystallinity is not as good as that of nanowires and nanoparticles. All the diffraction peaks can be readily indexed to the pure phase of Cu3V2O7(OH)2 · 2H2O with the monoclinic structure [JCPDS-ICDD Card No. 80-1170]. No peaks from other phases have been detected, indicating that the products are of high purity. Morphology Characterization. The morphology of the asprepared Cu3V2O7(OH)2 · 2H2O products was characterized by FE-SEM, which is shown in Figure 2. It can be seen that the morphologies and sizes of Cu3V2O7(OH)2 · 2H2O nanostructures were greatly affected by the preparation conditions. At room temperature and pH 5, the product was composed of nanoparticles with an average diameter of about 50 nm as shown in Figure 2a,b. When the pH value of the precursor solution was adjusted to 8 with NH3 · H2O, a large number of irregularly shaped nanoflakes were observed (Figure 2c). A higher magnification image (Figure 2d) shows that the thickness of the flake is about 30 nm. Such a morphological difference should be caused by the complexing effect of the ammonium ions. When the reaction temperature was increased to 180 °C, numerous nanowires were obtained. It is observed that a large number of nanowires are distributed homogeneously over a wide area indicating that a significant proportion (about 95%) of the product is present in a long, wirelike morphology (Figure 2e). The length of the nanowires is about several micrometers. A higher magnification image (Figure 2f) shows that these nanowires lie close to each other to form a bundle morphology and the diameter of the nanowires is in the range of 70-90 nm. Formation of Nanowires. Up to now, since one-dimensional (1-D) Cu3V2O7(OH)2 · 2H2O nanostructures have rarely been
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Figure 4. FE-SEM images of the products obtained at different hydrothermal reaction times: (a) 3, (b) 6, and (c, d) 12 h.
Figure 2. FE-SEM images of the as-prepared Cu3V2O7(OH)2 · 2H2O (a, b) nanoparticles, (c, d) nanoflakes, and (e, f) nanowires at different magnifications.
Figure 3. XRD patterns of the products obtained at different hydrothermal reaction times: (a) 3, (b) 6, and (c) 12 h.
investigated, the formation mechanism of Cu3V2O7(OH)2 · 2H2O nanowires would be of particular interest. To understand the growth process of the Cu3V2O7(OH)2 · 2H2O nanowires in the solution, we investigated the effect of reaction time on the product. Figure 3 shows the XRD patterns and Figure 4 shows the FE-SEM images of the products obtained at 180 °C for differernt reaction times. Initially, the direct mixing of CuSO4 and NH4VO3 solution immediately led to the formation of a yellow precipitate. All the diffraction peaks of the initial precipitate (Figure 1a) can be indexed to a pure crystalline phase of Cu3V2O7(OH)2 · 2H2O. The relatively broader diffraction peaks suggest the smaller crystallite size for Cu3V2O7(OH)2 · 2H2O formed at the early stage. The FE-SEM image (Figure 2a,b) shows this sample was composed of well-dispersed nanoparticles with a dimater of about 50 nm. After hydrothermal treatment at 180 °C for 3 h, the intensity of the diffraction peaks of the product was increased (Figure 3a) and some rods were observed in addition to the particles (Figure 4a). After a reaction time of 6 h, numerous rods were observed (Figure 4b), and the number of particles gradually decreased, suggesting that the longer rods grow at the expense of smaller particles. Meanwhile, the diffraction peaks (Figure 3b) were considerably narrowed,
suggesting an increase in the crystallite size with further increased reaction time. Interestingly, it is noticed that irregular particles vanished and the rods began to split into nanowires (Figure 4c,d) when the reaction time was prolonged to 12 h. Then the proportion of nanowires in the product increased with increased reaction time, and finally Cu3V2O7(OH)2 · 2H2O nanowires with good crystallinity lie close to each other to form a bundle morphology after 20 h of hydrothermal reaction (Figure 2e,f). In the present synthesis, neither surfactants nor templates were introduced in the reaction system. On the basis of the above experimental results, the formation mechanism for the Cu3V2O7(OH)2 · 2H2O nanowires can be simply depicted as an Ostwald ripening and splitting process. The initial growth mechanism of Cu3V2O7(OH)2 · 2H2O microrods in the solution system is the well-known “Ostwald-ripening process”,27-29 in which the formation of tiny crystalline nuclei in a supersaturated medium occurs at first and then is followed by crystal growth. With the reaction going on, the irregular particles vanish and the longer microrods start to form, suggesting that the longer microrods grow at the expense of smaller particulates due to the energy difference between large particles and smaller particles of a higher solubility based on the Gibbs-Thompson law.19 With the reaction processing, a splitting process occurs, in which the Cu3V2O7(OH)2 · 2H2O microrods split into nanowires. This splitting process is associated with the crystal structure. The basic structure of Cu3V2O7(OH)2 · 2H2O is built up from copper oxide/hydroxide layers, which are held together by the pyrovanadate groups V2O7. The corresponding pillared and layered framework is stacked by water molecules (see the Supporting Information).25,26 Therefore, the interactions between the copper oxide/hydroxide layers are weakened. In a layer structure compound, the chemical bonding between neighboring layers is generally weaker than chemical bonding in the same layers.30-33 Hence, the interaction between the layers in the Cu3V2O7(OH)2 · 2H2O crystalline structure is very weak. Under hydrothermal condition, the high temperature and pressure provide the energy base for the separation of the layers, which results in the splitting process. Such a similar “splitting” process has occurred for the preparation of Bi2S3,34 K2Ti6O13,32 Na2Ti3O7,33 and TiO2 1-D nanostructures.35 It was found from a series of experiments that the reaction temperature has significant effects on the growth of Cu3-
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Figure 5. Typical FE-SEM images of the Cu3V2O7(OH)2 · 2H2O nanostructures obtained at (a) 140 and (b) 160 °C.
Figure 6. FE-SEM images of Cu3V2O7(OH)2 · 2H2O nanostructures obtained by using different copper salts: (a) Cu(NO3)2 · 3H2O, (b) CuCl2 · 2H2O, and (c) Cu(CH3COO)2 · H2O.
V2O7(OH)2 · 2H2O nanowires. The morphology of the product obtained at 140 °C was shown to be a coexistence of short, broken rods and lots of irregular particles (Figure 5a). With an increase in the reaction temperature, the proportion of the 1-D nanostructures in the product increased. After hydrothermal treatment at 160 °C, the 1-D nanostructures became longer, and the particles became fewer (Figure 5b). Above 180 °C, completely 1-D nanostructures were obtained, and the proportion of the 1-D morphology was estimated to be about 95% (Figure 2e). The results indicate that higher temperature is preferable for the anisotropic growth of Cu3V2O7(OH)2 · 2H2O crystal and results in a product with higher aspect ratio and higher crystallinity. Furthermore, the acidity of the copper salts also has effects on the morphology of the product. In our research, there is a tendency that a strongly acidic copper salt is preferable for the anisotropic growth of Cu3V2O7(OH)2 · 2H2O 1-D nanostructures. Parallel experiments were carried out by substituting CuSO4 · 5H2O with other strongly acidic copper salts such as Cu(NO3)2 · 3H2O and CuCl2 · 2H2O. Cu3V2O7(OH)2 · 2H2O 1-D nanostructures were also observed under similar hydrothermal conditions, as shown in Figure 6a,b. However, if weakly acidic copper salt (e.g., Cu(CH3COO)2 · H2O) was used as the starting material, Cu3V2O7(OH)2 · 2H2O 1-D nanostructures were not obtained (Figure 6c). Therefore, we deduce that the acidity of the copper salts also plays a key role in fabricating the 1-D nanostructure of Cu3V2O7(OH)2 · 2H2O. Electrochemical Properties. On the basis of the layered structures, we speculated that Cu3V2O7(OH)2 · 2H2O could be a promising cathode material for lithium ion batteries. It is widely accepted that the electrochemcial properties of the electrode materials depend intimately on their morphologies and sizes. Figure 7 shows the cyclic voltammograms (CVs) of the electrodes made from the as-prepared Cu3V2O7(OH)2 · 2H2O nanowires, nanoflakes, and nanoparticles in the first cycle at a scan rate of 5 mV s-1 and a temperature of 37 °C. It can be seen that, for the nanowire electrode, the corresponding cathodic peaks are relatively stronger than those of the electrode made
Figure 7. CVs of the electrodes made from the as-prepared Cu3V2O7(OH)2 · 2H2O nanowires, nanoflakes, and nanoparticles in the first cycle at a scan rate of 5 mV s-1 and a temperature of 37 °C.
with nanoparticles and nanoflake, although they both present broad peaks. These stronger peaks may indicate that as the active cathode material of primary lithium batteries, the energy density of Cu3V2O7(OH)2 · 2H2O nanowires is greater than that of nanoparticles and nanoflakes. It should also be noted that the corresponding cathodic peak area of the nanowire electrode is much larger than the nanoparticle electrode and the nanoflake electrode during the discharge process, which is an indication of improved discharge capacities. The Nyquist plots at an open-circuit voltage of Cu3V2O7(OH)2 · 2H2O nanowires, nanoflakes, and nanoparticles are shown in Figure 8. The spectra of the three electrodes have a similar shape with a depressed semicircle in the high-frequency region and a straight line with an angle of approximately 45° relative to the real axis in the low-frequency region. The semicircle can be assigned to the charge-transfer resistance associated with the surface properties of insertion materials and the linear portion can be assigned to the solid-state diffusion resistance of lithium ions within the host. As for Cu3V2O7(OH)2 · 2H2O nanowires, their charge-transfer resis-
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Figure 8. Nyquist plots at open-circuit voltage of the electrodes made from the as-prepared Cu3V2O7(OH)2 · 2H2O nanowires, nanoflakes, and nanoparticles before discharge.
Figure 9. Discharge curves of the electrodes made from the as-prepared Cu3V2O7(OH)2 · 2H2O nanowires, nanoflakes, and nanoparticles at the current density of 20 mA g-1 and the temperature of 37 °C.
tance value at high frequencies was lower than that for Cu3V2O7(OH)2 · 2H2O nanoparticles and nanoflakes; this may be interpreted as resulting from their unique 1-D nanostructures, which provide direct 1-D electronic pathways, thus leading to efficient charge transport and lower charge-transfer resistance value.36,37 Figure 9 displays the representative initial discharge curve of the electrodes made from Cu3V2O7(OH)2 · 2H2O nanowires, nanoflakes, and nanoparticles at the current density of 20 mA g-1 and a temperature of 37 °C. The three electrodes exhibit a similar profile for the discharge curves with three sloping potential ranges for the lithium intercalation reaction. However, the open-circuit voltage of the battery made from Cu3V2O7(OH)2 · 2H2O nanowires is about 3.6 V, which is nearly 200 mV higher than that of Cu3V2O7(OH)2 · 2H2O nanoparticles and nanoflakes. To the cutoff voltage of 1.5 V, the discharge capacity of Cu3V2O7(OH)2 · 2H2O nanowires is as high as 530 mAh g-1. In comparison, the discharge capacities of Cu3V2O7(OH)2 · 2H2O nanoflakes and nanoparticles were 501 and 476 mAh g-1, respectively, which are still much higher than that of the previously reported commercial Ag2V4O11 bulk (272 mAh g-1 at 31.5 mA g-1 and 37 °C),38 Ag2V4O11 nanowires,19 and AgVO3 nanostructures.19 Furthermore, the Cu3V2O7(OH)2 · 2H2O nanostructure electrodes demonstrate an attracting feature that the discharge potential declines almost linearly. This kind of discharge curve is very desirable for batteries powering importable cardiac defibrillators because it is convenient to estimate the discharge state of the battery by just measuring its potential. According to the results presented above, the as-prepared Cu3V2O7(OH)2 · 2H2O nanowire electrode exhibited better elec-
Zhang et al. trochemical properties than that of Cu3V2O7(OH)2 · 2H2O nanoparticles and nanoflakes. We believe the significantly improved electrochemical properties of the nanowire electrode may result from the following two effects. One is the fast kinetics related to the unique 1-D nanostructured materials, which offer direct 1-D electronic pathways, thus leading to efficient charge transport and faster electronic kinetics.36,37 Another factor is related to the 3-D network of the interdigitated nanowires in the electrode. The network is able to provide an extra space for the diffusion of electrolyte and reduce the stress caused by the cracking of the structure during the discharge process and, thus, suppress the degradation of the electrode.24,39 In comparison with the commercial Ag2V4O11, Ag2V4O11, AgVO3, and CuV2O6 nanostructures, the Cu3V2O7(OH)2 · 2H2O nanowires exhibited higher discharge capacity, higher open circuit voltage, and a desirable discharge curve, making them promising cathode materials in primary lithium batteries for long-term and highrate applications such as in the field of implantable cardioverter defibrillators (ICDs). Future medical devices, such as heart-assist devices, will require rechargeable systems because the capacity of primary cells cannot provide the power needed for active medical devices.17 As an extension of the work on the primary Li/CVO batteries, the electrochemical properties of the Cu3V2O7(OH)2 · 2H2O nanostrucutres as the cathode material in rechargeable batteries would be of particular interest. Further research on the Li+ intercalation and deintercalation process into the Cu3V2O7(OH)2 · 2H2O nanostructures and the cycling reversibility is underway in our group to facilitate the application of Cu3V2O7(OH)2 · 2H2O as a cathode material as early as possible. Conclusions In summary, we demonstrated the synthesis and application of Cu3V2O7(OH)2 · 2H2O nanomaterials with different morphologies. Cu3V2O7(OH)2 · 2H2O nanowires, nanoflakes, and nanoparticles can be selectively prepared in high yields through a simple and facile solution route. According to comparative experimental results, an Ostwald ripening and splitting process was proposed to elucidate the growth mechanism of the nanowire structure. This simple synthetic route, which involves no templates or surfactants and requires no expensive and precise equipment, will offer great opportunities for the scale-up preparation of other transition metal vanadates with different morphologies. Electrochemical measurements showed that the 1-D nanostructures of Cu3V2O7(OH)2 · 2H2O nanowires are favorable for increasing the discharge capacity and improving the electrode kinetics in contrast with Cu3V2O7(OH)2 · 2H2O nanoparticles and nanoflakes. The Cu3V2O7(OH)2 · 2H2O nanowires exhibited a high discharge capacity of 530 mAh g-1 and a high open circuit voltage of 3.6 V, showing great promise for the application in the field of primary lithium batteries for ICDs. Supporting Information Available: The crystal structure of Cu3V2O7(OH)2 · 2H2O. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897. (b) Sun, Y. G.; Xia, Y. N. Science 2002, 298, 2176. (2) Nalwa, H. S. Handbook of Nanostructured Materials and Nanotechnology; Academic Press: New York, 2000. (3) Burda, C.; Chen, X. B.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (4) Ferna´ndez-Garcı´a, M.; Martı´nez-Arias, A.; Hanson, J. C.; Rodriguez, J. A. Chem. ReV. 2004, 104, 4063.
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