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BaAl2O4:Eu2þ, Dy3þ Nanotube Synthesis by Heating Conversion of Homogeneous Coprecipitates and Afterglow Characteristics Baochang Cheng,* Liting Fang, Zhaodong Zhang, Yanhe Xiao, and Shuijin Lei Institute for Advanced Studying and School of Materials Science and Engineering, Nanchang University, Nanchang 330031, People's Republic of China ABSTRACT: Hexagonal BaAl2O4:Eu2þ, Dy3þ polynary complex nanotubes with long-lasting phosphorescence were obtained through a facile coprecipitation approach followed by a postcalcining reaction in a weak reducing atmosphere. In the case of low annealing temperature, anion vacancies and surface stress can induce lattice contraction due to poor crystallininty; moreover, Eu2þ ions can occupy two different crystallographic Ba2þ sites due to low symmetry, resulting in an appearance of double emission peaks. For the sample annealed at higher temperature, however, Eu2þ ions only occupy substitutedly the Ba2þ sites with lowest energy due to high crystallinity; moreover, as compared to the sample annealed at low temperature, its emission band redshifts as the results of both high crystal symmetry around Eu2þ ions and large average optical path. Additionally, Eu2þ and Dy3þ ions substitute incompletely for Ba2þ sites in nanostructures, leading to the decrease of effective electron trap densities and depths, and therefore tubular nanostructures show fast afterglow decay rate in comparison with the bulk counterpart.
1. INTRODUCTION Since the identification of carbon nanotubes,1 functional selfassembled nanomaterials with well-defined shapes and dimensions are of great current interest.2 Especially for one-dimensional (1D) inorganic tubular nanostructures, they have aroused more extensive attention due to their exceptional properties and potential applications as catalysts, field-emission cathodes, magnetic storage and recording media, bio/gas sensors, solar cells, and electronic components, DNA detection, drug release, and so forth.2a,3 Up to now, various techniques, such as chemical vapor deposition, hydrothermal method, template-assisted approach, and electrospinning, have been put forward to synthesize the tubular nanostructures.4 In particular, the formation of some inorganic nanotubes was generally suggested to occur by a curving and seaming process of lamellar structures formed after heat treatment. In contrast, approaches leading to the formation of multimetal and doped metal oxides by soft chemistry methods are more rarely reported. This is mainly due to the fact that it is generally difficult to match the reactivity of the different metal precursors in solution, which is a prerequisite for the formation of multimetal oxides.5 Therefore, some facile and effective methods are still of importance for the synthesis of polynary complex tubular nanostructures. In the past decades, environmental considerations have promoted an increased demand for the development of radioactivefree luminous pigments because the most handled ones use r 2011 American Chemical Society
radioisotopes such as tritium (3H) and promethium (147Pm). Thus, Eu2þ, Dy3þ-codoped alkaline-earth (Ba, Sr, Ca, Mg) aluminates, as a completely new generation of persistent luminescent phosphors, have attracted extensive attention because they show excellent luminescent properties, such as higher luminescent intensity, higher quantum efficiency and longer-lasting phosphorescence (or afterglow), and higher chemical stability than the traditional ZnS:Cu, Co phosphors.6 These properties lead to a wide application of the materials in many fields such as fieldemission displays, luminous paints in highway, airport, buildings, and escape routes, and optoelectronic devices.7 These phosphors are also used for “cold lighting” that emit no infrared radiation. In particular, nanostructured afterglow phosphors will have tremendous potential applications in biologic and medical science. For example, they can be used for in vitro and in vivo imaging, while no excitation light is needed because they can last for a relatively long time after activation. In addition, they can be used as a light source for photodynamic therapy activation in deep tissue, in which a photosensitizer is activated by afterglow light to produce singlet oxygen for cancer cell destruction.8 Although 4f electrons of Eu2þ are not sensitive to the changes in the crystal field strength due to the shielding function of outer shell, 5d electrons are split easily by these changes. Additionally, the Received: October 12, 2010 Revised: December 10, 2010 Published: January 5, 2011 1708
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The Journal of Physical Chemistry C phosphorescence of Eu2þ in most hosts originates from transitions between the 8S7/2 (4f 7) ground state and the crystal field components of the 4f 65d1 excited-state configuration. Thus, the positions of the emission peaks depend strongly on the nature of Eu2þ surroundings, and Eu2þ ions can emit different visible lights in various crystal fields.9 Therefore, by controlling the crystal structures and sizes of the matrix in which Eu2þ ions reside, visible light emitting with different wavelengths can be expected to be obtained. For alkaline-earth aluminate phosphors, the traditional production process is based on the solid-state reaction. However, it often needs controlled heating at high temperature and long processing time, or some additives as flux, to get a single phase. In particular, this process often results in inhomogeneous products with low surface area and broad particle size distribution. In contrast, the homogeneous coprecipitation approach is an efficient technique for the preparation of the phosphors due to the good mixing of starting materials and relatively low reaction temperature resulting in more homogeneous products.10 In this work, by using the curving and seaming effects of lamellar structures under the assistance of surfactant, the nanotube-like composite precursors can be fabricated simply and effectively through a homogeneous coprecipitation technique, and then polynary complex BaAl2O4:Eu2þ, Dy3þ nanotubes can be obtained by a postannealing conversation. The microstructures and optic properties of the phosphor nanotubes have been investigated in detail.
2. EXPERIMENTAL SECTION Eu2þ and Dy3þ-codoped BaAl2O4 nanotubes, templated by surfactant assemblies, were synthesized through a homogeneous coprecipitation method followed by a postannealing reaction. In a typical procedure, Ba(NO3)2, Al(NO3)3, Eu(NO3)3, Dy(NO3)3, CO(NH2)2, cetyltrimethylammonium bromide (CTAB), and water were mixed at a molar ratio of 1:2:0.04:0.04:9:0.01:80, and simultaneously stirred to obtain a transparent solution. Next, it was transferred into a stainless steel autoclave with a Teflon linear of 50 mL capacity and heated in an oven at 125 °C for 15 h. After the autoclave was air-cooled to room temperature unaided, the resultant white fluffy precipitate was filtered and washed with distilled water and absolute ethanol, and subsequently dried at 100 °C for 2 h. Finally, the coprecipitation precursor was calcined at various temperatures for 3 h to achieve the effective conversion of coprecipitates to hexagonal BaAl2O4 as well as allow the variation of nanocrystal size, and, simultaneously, under a weak reducing atmosphere of flowing 5% H2 and 95% Ar gas to crystallize and to ensure complete reduction of activator ion Eu3þ to Eu2þ. The phase and crystallinity of the synthesized compositions were investigated by powder XRD using Phillips X'Pert X-ray diffractometer with Cu KR radiation. The morphology of the coprecipitation treated samples with the different reaction time and the postannealed products was investigated using a field emission environmental scanning electron microscope (FE-ESEM, FEI Quanta 200F) and a field emission scanning electron microscopy (FE-SEM, JEOL JSM-6700F), respectively. The microstructure was analyzed by transmission electron microscopy (TEM, JEOL 2010), equipped with energy dispersive X-ray spectroscopy (EDS). The Fourier transform infrared (FTIR) spectra of the samples were collected using a Nicolet Magna-750 infrared spectrometer. The excitation and photoluminescence (PL) spectra of products were detected with the use of an Edinburgh FLS920 fluorescence spectrophotometer at room temperature. For the measurement
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Figure 1. XRD patterns of the samples: (a) obtained by a coprecipitation technique; (b-e) postcalcined at 900, 1100, 1200, and 1300 °C, respectively.
of afterglow decay curves, the samples were first irradiated at room temperature for 10 min using a Hg lamp with a wavelength of 365 nm and a power density of 6 mW/cm2, and then the excitation light was cut off and the measurement was automatically carried out by a photometer.
3. RESULTS AND DISCUSSION The phase purity and the evolution of crystal structures were measured by XRD on as-synthesized precursors through a homogeneous coprecipitation method and postannealed samples up to 1300 °C, and the results are given in Figure 1a-d. As can be seen in Figure 1a, the clearly distinguishable sharp peaks of the product, prepared by the soft chemical method, are indicative of high crystallinity. All of the diffraction peaks can be readily indexed to the coexistence of end-centered orthorhombic ammonium aluminum hydroxide carbonate [NH4Al(OH)2CO3] phase (JCPDS no. 76-1923) and orthorhombic barium carbonate (BaCO3) phase (JCPDS no. 71-2394) in the product. After annealing at the temperature of 900 °C, the hexagonal BaAl2O4 phase (JCPDS no. 17-0306) with a P63 space group begins to form, and, furthermore, there still exist some peaks of unreacted BaCO3, as indicated by the XRD patterns shown in Figure 1b. As the heattreatment temperature rises, the diffraction peaks from BaCO3 decrease, and the coprecipitation precursors are almost converted to a pure BaAl2O4 after annealing at 1100 °C. In addition, the width of the Bragg reflections from BaAl2O4 reduces while their corresponding intensity increases, indicating an increase in crystallite size. This also demonstrates that nanostructures are beneficial to the conversation of crystalline phase at relatively low calcination temperature as compared to the traditional solid-state reaction at about 1300 °C. In addition, the lattice constants increase as the calcination temperature increases; for example, a changes from 1.0437 to 1.0466 nm and c changes from 0.8787 to 0.8813 nm as the annealing temperature varies from 1100 to 1300 °C. In the sample annealed at low temperature, the lattice contraction and deformation could be attributed to poor crystallinity, and, furthermore, it can be defined as a function of both the concentration of anion vacancies in the nanocrystal and the intensity of the additional pressure imposed by the surface tension on the crystal.11 In low crystalline nanostructures, therefore, higher densities of vacancy defects and larger specific surface area lead to lattice contraction and deformation. Figure 2 shows various magnification FE-SEM images of the sample annealed at 1200 °C. As can be seen, large quantities of rodlike nanostructures are evident with relatively uniform sizes, and the 1709
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Figure 2. FE-SEM images of the sample postannealed at 1200 °C for 3 h.
diameters are estimated to be around a few tens to 300 nm and the lengths to be several micrometers. In addition, after calcining at relatively high temperature, the sample can still retain rod-like morphology. TEM investigations have provided insight into the microstructure of the products. The bright-field TEM images of the samples annealed at 1100 and 1200 °C are both presented in Figure 3. The representative structures reveal that the shell and core have distinct boundary and no material exists in the core. Therefore, the structure is probably a tube, and the darker region corresponds to the wall of the tube. The nanotubes have outer diameters of 100-300 nm, inner diameters of 40-80 nm, and lengths up to tens of micrometers. Most of the nanotubes are open-ended. However, some close-ended nanotubes were observed occasionally. The selected area electron diffraction (SAED) patterns taken from these randomly oriented nanotubes, as shown in Figure 3c and h, correspond to the samples at 1100 and 1200 °C, respectively. They are both composed of concentric circles, indicating the polycrystalline nature of nanotubes. With the increase of the calcination temperature, moreover, the crystallinity of the samples increases and the diffraction rings become distinguishable. Three diffraction rings of the sample calcined at 1200 °C can be easily identified as (220), (222), and (600) of the hexagonal BaAl2O4 phase by the method of comparative d-spacing. Figure 4c shows further a high-resolution TEM (HRTEM) image of a sidewall portion of a nanotube with a relatively high crystallinity, which was postannealed at 1300 °C. Its corresponding SAED pattern, which can be indexed to [110] zone axis, is also given in Figure 4b. The distinct interplanar spacings are measured to be about 1.044 and 0.905 nm, which are in good agreement with the ideal values of the {110} and {001} planes of the hexagonal BaAl2O4 bulk crystal. In addition, some edge dislocations can be obviously observed within {110} planes, indicating the presence of quantities of defects in host lattices. The chemical composition of individual BaAl2O4:Eu2þ, Dy3þ nanotubes is further determined by the energy dispersive spectroscopy (EDS) with a representative spectrum displayed in Figure 4a. As can be seen, they are mainly composed of Ba, Al, and O elements, and Eu, Dy are not effectively detected due to a relatively low doping concentration and an insufficient resolution of EDS. To understand the probable formation mechanism for the tubelike nanostructures, the FE-ESEM images of the samples synthesized by a coprecipitation reaction for different time were taken and shown in Figure 5. As can be seen, quantities of flake-like nanostructures exist in the sample at the initial reaction stage; however, relatively complete rod-like nanostructures can be obtained after reacting for 10 h. For NH4Al(OH)2CO3, it has the same orthorhombic crystal structure as γ-AlOOH (boehmite) with point group D2hmmm, and space group Cmcm, which is a
Figure 3. Bright-field TEM images of the samples: (a and b) annealed at 1100 °C; (d-g) annealed at 1200 °C. Parts (c) and (h) correspond to the SAED patterns of the samples annealed at 1100 and 1200 °C, respectively.
layer-shape structure.12 In the course of the slow hydrolysis of urea, the produced CO32-, OH-, and NH4þ ions can react with Al3þ, Ba2þ, Eu3þ, and Dy3þ ions to form NH4Al(OH)2CO3, BaCO3, Eu2(CO3)3, and Dy2(CO3)3 coprecipitates, respectively. High densities of Al3þ can nucleate preferentially. Under the assistance of the CTAB surfactant soft template, furthermore, the layer structure remains easily anisotropic nucleation and forms lamellar structure at the initial reaction stage. They then can generate a rolling action and form tubular architectures. Such a slow reaction rate is close to equilibrium reaction conditions, which are favorable for the self-assembly process, as the tubular architectures can be kept during the subsequent coprecipitation reaction. Moreover, the radius of Ba2þ ions (0.134 nm) is very close to the radius of NH4þ ions (0.148 nm), and thus Ba2þ ions can easily stead to NH4þ. Additionally, the omnidirectional rolling of the thin sheet formed in situ, and it would result in the formation of close-ended nanotubes and the coexistence of nanotubes and nanorods. Hexagonal BaAl2O4 belongs to a stuffed tridymite type of structure, which is derived from the structure of SiO2-tridymite. Tridymite is a member of the nepheline family of structures consisting of the corner sharing tetrahedral framework. In BaAl2O4, the framework is built up by AlO4-tetrahedral, and the structure 1710
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Figure 4. (a) EDS spectrum; (c) lattice-resolved HRTEM image of a sidewall portion of a nanotube annealed at 1300 °C, taken along [110] zone axis, and some edge dislocations (arrowed) exist within {110} planes; and (b) the SAED pattern taken from the corresponding nanotube (c).
Figure 5. FE-ESEM images of the samples synthesized by a coprecipitation reaction at different time: (a) 2.5 h; (b) 10 h. The inset corresponds to a local enlargement from (a).
channels parallel to the c-axis are occupied by Ba2þ ions. The projection of hexagonal BaAl2O4 three-dimensional crystal structure is shown in Figure 6a. In addition, Figure 6b also gives the FTIR spectra of BaAl2O4:Eu2þ, Dy3þ nanostructures annealed at 1200 °C. For reference and comparison purpose, the bulk BaAl2O4: Eu2þ, Dy3þ, prepared via a solid-state reaction at 1300 °C for 3 h, was also measured and shown in Figure 6b. As can be seen, in the range of 400-1000 cm-1 the spectra mainly consist of three absorption bands located in the regions of 400-500, 500-700, and 700-1000 cm-1, respectively. They should originate from the vibrational modes of the tetrahedrally coordinated aluminum (AlO4) and can be identified as bending modes of O-Al-O, symmetric stretching modes of Al-O-Al, and asymmetric stretching modes of Al-O-Al, respectively.13 In comparison with its bulk counterpart, in the nanostructured sample the two absorption bands from bending and symmetric stretching modes are both split into double peaks at 440 and 418 cm-1, as well as at 582 and
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636 cm-1, respectively. At relatively low calcining temperatures, the crystallinity is uncomplete, and quantities of defects are present in BaAl2O4 hosts, and thus the two FTIR absorption bands, arising from bending and symmetric stretching modes, are both split into two obvious absorption peaks due to the presence of large asymmetry induced by poor crystallinity in the lattice. The excitation, emission, and afterglow spectra from the ensemble of nanotubes annealed at various temperatures were measured at room temperature and displayed in Figure 7. As can be seen, the broad-band excitation spectra of Eu2þ ions are found from 200 to 400 nm, and, moreover, there exist two apparent absorption peaks located at 280 and 360 nm in the broad band, corresponding to the crystal field splitting of the Eu2þ d-orbital. However, their relative intensity ratio exhibits some variations for the samples annealed at various temperatures due to the difference of Eu2þ surroundings. For the sample postcalcined at 900 °C, its emission spectra not only show the peak with a maximum at 490 nm, but also show a shoulder peaking at about 410 nm. For the sample sintered at 1200 °C, however, it only shows a single broad band peaked at 506 nm; moreover, its wavelength redshifts by about 16 nm as compared to the sample annealed at 900 °C. For the shape of their emission spectra, they display no significant variations with the change of excitation energy in the range of 280-400 nm. The feature of typical abroad emission band in the visible light range indicates that the activator ion Eu is in divalent (Eu2þ, 4f 65df4f 7 green emission) rather than trivalent (Eu3þ, 4f f4f red emission). In BaAl2O4, the 5d levels of the Eu2þ ions are located below the 6PJ state of the 4f 7 configuration, and the abroad-band luminescence is created due to the allowed 4f 65d-4f 7 transition. Moreover, the 5dT4f transition is associated with the change in electric dipole, and the 5d excited state is affected by crystal field effects. Therefore, the emission of Eu2þ is very strongly dependent on the host lattice structure. For these nanostructured phosphors, their crystalline quality can be improved with increase in annealing temperature, and thus the crystal field strength is enhanced, resulting in the redshift of the emission peaks. On the other hand, with an increase in calcination temperature, the crystal size increases, resulting in an increase in average optical path, and this effect can also cause the redshift of emission band. With increasing calcination temperature, therefore, the redshift of the emission spectra can be attributed to the increase both in crystal field strength and in average optical path. Although Eu2þ and Ba2þ possess the same valence, their ionic radii are slightly mismatched (i.e., 0.117 and 0.134 nm, respectively) between them. These factors may affect the efficiency of the Eu2þ luminescence when compared to that from the Ca2þ and Sr2þ aluminates.14 Moreover, there are two Ba2þ sites in the BaAl2O4 structure, and thus the two maxima could correspond to emission from the two sites. The relative intensity of the emission at 410 nm was observed to reduce with increasing calcination temperature. This indicates that one of the Ba2þ sites is preferentially occupied by the Eu2þ ions and that the second site could only be filled in incomplete lattice. According to the crystal structure, the first Ba2þ site (2a) has the multiplicity of two and a site symmetry of C3, while the second one (6c) has six and C1. Both Ba2þ sites have nine-coordination, and the sites are similar in average size [d(Ba-O)ave = 0.286 and 0.287 nm]. However, the lower symmetry site also has shorter Ba-O distances (0.269 nm) corresponding to those typical of Eu2þ-O (2.68).15 The overall symmetry of BaAl2O4 crystal structure is P63 at room temperature. The ideal undistorted structure is described by the P6322 space group. 1711
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Figure 6. (a) Three-dimensional sketch of hexagonal BaAl2O4 crystal structure projected along the c-axis. (b) FTIR spectra of the BaAl2O4:Eu2þ, Dy3þ nanostructure (upper) and the bulk (lower).
Figure 7. PLE spectra (left) and PL and afterglow spectra (right) of BaAl2O4:Eu2þ, Dy3þ annealed at various temperatures. The dot and solid lines correspond to the samples annealed at 900 and 1200 °C, respectively.
Apical oxygen atoms linking layers, which may point either up (U) or down (D), in stuffed tridymites form UDUDUD patterns in each ring. The topologically different arrangement of tetrahedra and/or geometric distortions results in a great number of structures with low symmetry.16 In the nanostructures, additionally, quantities of the antiphase boundaries and twin boundaries also exist, which have local arrangements of the atoms and deviate from that inside a single domain, and thus the local symmetry of the very small nanodomains is most likely orthorhombic or monoclinic.17 As a consequence, Eu can both occupy the two different crystallographic Ba2þ sites in lower symmetry hosts, resulting in the appearance of double emission peaks. Generally, the 6c site can be filled preferentially,14 and hence it is occupied more easily at higher annealing temperature derived from higher crystallinity. The phosphorescence decay curves were recorded at room temperature and plotted in a logarithmic format as shown in Figure 8. Its bulk counterpart afterglow prepared by a solid-state reaction is also measured and displayed in Figure 8 for comparison purpose. It can be seen from the curves that the phosphors show quite long decay time when the powders are efficiently activated by using an Hg lamp for 10 min; for example, after the source lamp is switched off, the phosphorescence of nanostructures annealed at 1200 °C can persist more than 600 s as the emission light intensity is reduced to 0.32 mcd/m2 (i.e., the perception limit of the human eye). Moreover, the luminous intensity decays exponentially, which can be expressed by the following equation at the whole stage: I ¼ At - n
ð1Þ
Figure 8. Afterglow decay curves of BaAl2O4:Eu2þ, Dy3þ excited at 360 nm for 10 min, displayed on a logarithmic scale. (a) Nanostructured sample annealed at 1200 °C; and (b) the bulk prepared by solid-state reaction at 1300 °C.
where I is the luminous intensity at time t after removal of activating light, A is the luminous intensity at t = 1, and n is the decay constant. The longer is the luminous time, the smaller is the n value, usually 1 < n < 2. On the basis of the fit of eq 1, it can be demonstrated that the decay constants of the nanostructure and its bulk counterpart are 1.52 and 1.05, respectively. This indicates that the afterglow decay rate of the nanophosphor is faster than that of its bulk counterpart. Generally, the persistent afterglow luminescence phenomena of BaAl2O4:Eu2þ, Dy3þ can be ascribed to the electron and hole trapped-transported-detrapped process, which has been reported in the literature previously.7a,18 Once the irradiation source was switched off, the trapped holes would be released thermally to the valence band and then migrate to recombine with the excess electrons in the metastable state (Eu1þ)* sites, resulting in the long afterglow. With respect to obtained BaAl2O4:Eu2þ, Dy3þ nanostructures in the present case, its microstructure plays a dominant role in influencing the photoluminescence process. that there exists some difference of the ion radius among Ba2þ, Eu2þ and Dy3þ, Eu2þ and Dy3þ does not occupy completely Ba2þ sites with the lowest energy in the imperfect lattice of nanostructured sample on the basis of above photoluminescence results, especially for the sample calcined at lower temperature. Thus, in contrast to its bulk counterpart, the effective emission sites and the electron trap densities and depths are relatively low in nanostructured phosphors, and, consequently, it shows a faster afterglow decay rate. 1712
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4. CONCLUSIONS In summary, by using a facile homogeneous coprecipitation technique followed by a postannealing reaction, hexagonal BaAl2O4:Eu2þ, Dy3þ polynary complex nanotubes with long-persistent phosphorescence were successfully fabricated. As-obtained precursors through the homogeneous coprecipitation approach are mainly composed of NH4Al(OH)2CO3 and BaCO3, and then can be conversed to hexagonal BaAl2O4 at the annealing temperature above 900 °C. In the sample annealed at relatively low temperature, Eu2þ ions can simultaneously occupy two different crystallographic Ba2þ sites due to an imperfect lattice, resulting in the appearance of double emission peaks, while Eu2þ ions preferentially substitute for the Ba2þ site with lowest energy in the samples calcined at relatively high temperature and hence only show a corresponding single emission peak. Moreover, for the sample annealed at low temperature, the lower crystal symmetry around Eu2þ ions decreases the crystal field strength and the smaller crystal size reduces the average optical path, and therefore its emission peak blueshifts in comparison with the sample annealed at high temperature. Because of the decrease both in luminescent centers and in electronic trap densities and depths, additionally, nanostructured BaAl2O4:Eu2þ, Dy3þ shows a faster afterglow decay rate than its bulk counterpart. These experimental results demonstrate that nanotubular BaAl2O4:Eu2þ, Dy3þ exhibits relatively long afterglow luminescence and initial lightness, suggesting potential applications as micromarkers in future optoelectronic nanodevices and as fluorescent probes in biological fields.
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’ AUTHOR INFORMATION Corresponding Author
*Fax: þ86-791-3969329. E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the Foundation of Jiangxi Educational Committee (GJJ09024), the Natural Science Foundation of Jiangxi Province (00008559), and the Start-up Funds of Nanchang University. ’ REFERENCES (1) Iijima, S. Nature 1991, 354, 56. (2) (a) Wang, J.; Liu, G. D.; Jan, M. R. J. Am. Chem. Soc. 2004, 126, 3010. (b) Lehn, J. M. Angew. Chem., Int. Ed. Engl. 1990, 29, 1304. (3) (a) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, Y. Q. Adv. Mater. 2003, 15, 353. (b) Choi, W. B.; Chung, D. S.; Kang, J. H.; Kim, H. Y.; Jin, Y. W.; Han, I. T.; Lee, Y. H.; Jung, J. E.; Lee, N. S.; Park, G. S.; Kim, J. M. Appl. Phys. Lett. 1999, 75, 3129. (c) Kongkanand, A.; Dominguez, R. M.; Kamat, P. V. Nano Lett. 2007, 7, 676. (d) Keren, K.; Berman, R. S.; Buchstab, E.; Sivan, U.; Braun, E. Science 2003, 302, 1380. (e) Shrestha, N. K.; Macak, J. M.; Schmidt-Stein, F.; Hahn, R.; Mierke, C. T.; Fabry, B.; Schmuki, P. Angew. Chem., Int. Ed. 2009, 48, 969. (f) Chen, J.; Xu, L. N.; Li, W. Y.; Gou, X. L. Adv. Mater. 2005, 17, 582. (4) (a) Wu, C. C.; Wuu, D. S.; Lin, P. R.; Chen, T. N.; Horng, R. H. Cryst. Growth Des. 2009, 9, 4555. (b) Zhang, J.; Sun, L. D.; Liao, C. S.; Yan, C. H. Chem. Commun. 2002, 262. (c) Du, N.; Zhang, H.; Chen, B. D.; Ma, X. Y.; Liu, Z. H.; Wu, J. B.; Yang, D. R. Adv. Mater. 2007, 19, 1641. (d) Zhang, Z. Y.; Li, X. H.; Wang, C. H.; Wei, L. M.; Liu, Y. C.; Shao, C. L. J. Phys. Chem. C 2009, 113, 19397. (5) (a) Pinna, N.; Niederberger, M. Angew. Chem., Int. Ed. 2008, 47, 5292. (b) Karmaoui, M.; Willinger, M. G.; Mafra, L.; Herntrich, T.; Pinna, N. Nanoscale 2009, 1, 360. 1713
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