Synthesis and Characterization of Uniform Spindle-Shaped

Oct 17, 2007 - europium-doped lanthanum phosphate has been developed, which results in the formation of uniform spindle-shaped microarchitectures...
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CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 11 2305–2309

Articles Synthesis and Characterization of Uniform Spindle-Shaped Microarchitectures Self-Assembled from Aligned Single-Crystalline Nanowires of Lanthanum Phosphates Wenbo Bu, Lingxia Zhang, Zile Hua, Hangrong Chen, and Jianlin Shi* State Key Lab of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 200050, P. R. China ReceiVed January 30, 2007; ReVised Manuscript ReceiVed July 17, 2007

ABSTRACT: A new synthetic approach using the Pluronic P123-assisted hydrothermal reaction of lanthanum phosphate and europium-doped lanthanum phosphate has been developed, which results in the formation of uniform spindle-shaped microarchitectures most probably by a self-assembly process. Our results reveal that the obtained spindle-shaped microarchitectures consist of several tens of aligned single-crystalline nanowires with smooth, well-defined facets and highly uniform morphologies. These well-defined spindle-shaped microarchitectures show greatly enhanced photoluminescence in these compounds when compared to their counterparts of disordered arrangements. A possible formation mechanism for these spindle-shaped microarchitectures is presented and discussed.

1. Introduction One-dimensional (1D) nanostructures with nanometer diameters, such as nanowires, nanorods, nanotubes, and nanobelts, have been the focus of extensive research in recent years, mainly because of their unique structural nature and physical/chemical properties, and thus have significant potential for a number of important applications in fabricating the next generation of nanoscale electronic, optoelectronic, and sensing devices.1–3 In particular, the single-crystalline nature of these 1D nanostructures makes them ideal candidates for probing morphologydependent and dimensionality-controlled physical phenomena.4 More recently, one of the research focuses was moving toward the hierarchical self-assembly of 1D nanoscale building blocks into ordered microstructures or complex architectures, which is critical for the success of bottom-up approaches toward integrated, functional nanosystems.5–9 In this regard, one can learn from nature where exquisite structures of highly organized matter, such as diatoms, sea urchins, dendrite spines, and ferns, are routinely self-organized. Although some success has been shown in building hierarchically self-assembled architectures of some systems,10–13 experiments show that it is difficult to achieve controlled organization from 1D nanoscale building units. Hence, finding a controllable synthetic approach to orderly aligned microstructures represents a significant challenge in the field of controllable assembly of nanostructures. On the basis of their unique electronic structures and the numerous well-defined transition modes involving the 4f shell * Author to whom correspondence should be addressed. Tel.: 86-21-52412712. Fax: 86-21-52413122. E-mail: [email protected].

of their ions, lanthanide orthophosphate (LnPO4) micro- and nanostructures, a family of important phosphate compounds, have attracted increasing attention due to their special physical and chemical properties.14–23 Very recently, much attention has been focused on the synthesis and optical properties of lanthanide compound nanostructures and lanthanide-doped nanoparticles,17,18 particularly, the pioneering research by Haase and coworkers on the lanthanide-doped LnPO4 (LnPO4:Ln3+) nanoparticles.14–16 Further along these lines, LnPO4 and LnPO4: Ln3+ single-crystalline nanowires have also been successfully prepared by a conventional hydrothermal process;19–21 however, this method generally suffers from very broad size distributions and nonuniform morphologies. By precisely controlling the growth process of 1D nanostructures developed in our group,22,23 we were able to synthesize well-controlled LnPO4 or lanthanidedoped LnPO4 1D nanostructures with a narrow distribution of diameters and uniform morphology, which are believed to effectively enhance luminescent efficiency. In comparison to these works on LnPO4 or lanthanide-doped LnPO4 1D nanowires or nanorods,19–23 however, few reports can be found concerning the hierarchical self-assembly of LnPO4 1D nanostructures.24 Herein, we report the fabrication of novel uniform spindleshaped microarchitectures self-assembled from single-crystalline lanthanum phosphate nanowires by using a nonionic pluronic amphiphilic triblock copolymer as the structure-directing agent via a facile hydrothermal synthetic method. The possible mechanism leading to spindle-shaped microarchitectures and the optical properties of Eu3+-doped lanthanum phosphates were proposed.

10.1021/cg070104m CCC: $37.00  2007 American Chemical Society Published on Web 10/17/2007

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Figure 2. (a) Representative TEM image and (b) TEM image of an individual LaPO4 single-crystalline nanowire. Right inset: selectedarea electron diffraction (SAED) pattern obtained from the same nanowire. Left inset: A high-resolution TEM image of the nanowire that shows lattice fringes perpendicular to the [001] direction. Figure 1. (a) Low-magnification FE-SEM image of the LaPO4 spindlelike microarchitecture. (b, c) High-magnification FE-SEM images of a single spindlelike microstructure and its selected-area magnified image. (d) XRD pattern of the LaPO4 spindlelike microstructures.

2. Experimental Section All chemicals are analytical-grade reagents and were purchased from Shanghai Chemical Reagent Corp. and used without further purification. In a typical synthesis of uniform spindle-shaped microarchitectures of LaPO4 or LaPO4 doped with 5 mol % europium, 25 mL of Na3PO4 aqueous solution (0.8 M) was added into 25 mL of La(NO3)3 aqueous solution (0.8 M) or a mixed aqueous solution of La(NO3)3 (0.8 M, 23.75 mL) and Eu(NO3)3 (0.8 M, 1.25 mL) under vigorous stirring for 30 min. Then, the pH value of the solution was adjusted to below pH 1.0 using 0.1 M HCl under stirring. After that, 1-4 g of Pluronic P123 (EO20PO70EO20 Mav ) 5800, Aldrich) was added into the resulting suspension under stirring at 35–40 °C, and this was further stirred for 2 h. The above solution was poured into a Teflon-lined autoclave and was heated subsequently to 100 °C for 12 h, and then, the autoclave was allowed to cool to room temperature. Subsequently, the precipitate was filtered, washed with distilled water and absolute ethanol several times, and dried at room temperature for further characterization. X-ray powder diffraction (XRD) patterns were obtained on a Rigaku D/Max X-ray diffractometer with graphite-monochromatized Cu KR radiation. Scanning electron micrographs (SEMs) were taken on a JSM6700F field-emission scanning electron microscope (FE-SEM). Transmission electron microscopy (TEM) images, high-resolution transmission electron microscopic (HRTEM) images, and the selected area electron diffraction (SAED) patterns were recorded on a JEOL200CX microscope with an accelerating voltage of 200 kV. Energy-dispersive X-ray spectroscopy (EDS) was performed with an attached Oxford Link ISIS energy-dispersive spectrometer fixed on a JEM-2010 electron microscope. Photoluminescence (PL) spectra were recorded on a RF-5301PC with Xe lamp at room temperature.

3. Results and Discussion The general morphology and size of the products were examined with a FE-SEM. Figure 1a shows the general morphologies of the as-prepared LaPO4 samples obtained at 100 °C for 12 h with the presence of 4 g of Pluronic P123. Interestingly, the one-dimensional LaPO4 crystalline nanowires self-organized into uniform spindle-shaped assemblies. Notably, almost 100% of the LaPO4 nanowires have self-assembled into the microarchitectures, with architecture diameters of about 1.5 µm at the center and lengths of about 6 µm. Microstructures of other morphologies, such as irregular conglomeration, were not observed. Figure 1b and c show the high-magnification FE-SEM image of a single spindlelike microstructure and its

further magnified image for a selected area. It is clearly demonstrated that the spindle-shaped microarchitecture is composed of several tens of well-aligned homogeneous nanowires, and these nanowires are straight and highly uniform in morphology with an average diameter of 11 nm and a length of up to several micrometers. Apparently, Pluronic P123, which has been generally used as a templating micelle reagent to synthesize mesoporous materials,25 provides a constrained environment during the crystal growth and should be responsible for the oriented microarchitecture of as-made products according to the nematic liquid crystal structure of the surfactant. The crystalline structure and the phase purity of the products were examined by the XRD technique (Figure 1d). The as-prepared sample shows sharp reflections which match well with those assigned to the hexagonal cell of LaPO4 [space group: P6222(180)] with lattice constants comparable to the data in Joint Committee on Powder Diffraction Standards card number 04-0635. No impurity phases can be detected, which indicates that LaPO4 nanowires obtained via our current synthetic method have a pure hexagonal structure. The morphology and microstructure details of these uniform spindle-shaped microarchitectures were further investigated with TEM and HRTEM equipped with SAED. A representative TEM image (Figure 2a and b) shows that the products are composed of well-aligned nanowires, which are much more uniform, especially in diameter, than the previously reported ones.7 The SAED pattern in the lower-left corner of Figure 2b obtained from an individual nanowire confirms that the LaPO4 nanowire is single-crystalline. The sharp diffraction spots could be indexed as the hexagonal phase of LaPO4, with the longitudinal axis of each nanowire being along the [001] direction. Moreover, SAED patterns taken from the different nanowires were found to be identical within experimental accuracy, indicating that all LaPO4 nanowires synthesized via current synthetic methods are of the same hexagonal phase, in agreement with X-ray diffraction results. Its corresponding HRTEM image shown at the top-right corner of Figure 2b further reveals that the nanowires are structurally uniform and highly crystalline without detectable defects or dislocations; this provides further evidence that these nanowires are single-crystalline. The interplanar distance of 0.607 nm observed in this image corresponds to the d-spacing of (100) lattice planes, consistent with the SAED pattern. Both the SAED pattern and the HRTEM image reveal that the preferred growth of aligned LaPO4 single-crystalline nanowires is along the [001] direction. The chemical stoichiometry of the products was investigated with EDS, which gave a molar ratio

Uniform Spindle-Shaped Microarchitectures

Figure 3. FE-SEM images of LaPO4 synthesized under hydrothermal treatment without the addition of P123 (a) and with the presence of different amounts of P123 of 1.0 g (b), 2.0 g (c), and 4.0 g (d). Inset in part a: the corresponding high-magnification FE-SEM image.

of La, P, and O close to the stoichiometric proportions of the formula LaPO4 within experimental error. In the reaction system, it was found that the Pluronic P123, as a structure-directing agent, had a great influence on the morphologies of the obtained samples. In order to investigate the effect of Pluronic P123 on the morphology and the size distribution of the products, a series of parallel experiments were carried out with varied amount of the Pluronic P123 with the other synthetic conditions remaining unchanged. It can be found that the as-prepared samples synthesized without adding Pluronic P123 have irregular shapes composed of randomly oriented nanowires with a much broader size distribution and nonuniform morphology, as shown in Figure 3a. Upon adding 1 g of Pluronic P123, the as-prepared samples are still mainly composed of randomly oriented nanowires with irregular shapes, but some spindleshaped microarchitectures could also be observed (Figure 3b). Further increasing the amount of Pluronic P123 to 2 g results in the formation of a mixture of spindle-shaped microarchitectures and irregularly shaped architectures with randomly oriented nanowires (Figure 3c). As is also well-known, the critical micelle concentration (CMC) of Pluronic P123 in solution is 4.5 wt % at 17 °C.26 When the amount of Pluronic P123 is 2 g, the micelle concentration of Pluronic P123 is about 3.0 wt %, which is below the CMC of Pluronic P123. Thus, a micelle-templated synthesis mechanism is not completely dominant, and spindleshaped microstructures coexist with other irregularly shaped aggregates. When the Pluronic P123 concentration was further increased to 4 g, leading to a micelle concentration higher than the CMC, the product was composed of almost 100% uniform spindle-shaped microarchitectures assembled from LaPO4 singlecrystalline nanowires, as shown in Figure 3d, which indicates that the nematic liquid crystal templating mechanism is dominant in this case. From its TEM image (Figure 2a), it is clearly shown that the uniform spindle-shaped microarchitectures are assembled from LaPO4 single-crystalline nanowires with a much narrower size distribution (10–12 nm). Of particular interest in this Pluronic P123-assisted hydrothermal approach is its role in obtaining a much more uniform morphology, especially in diameter along the entire length, than that previously reported.19–21 Although the reason for the present uniform nanowire morphology is not fully understood, the directing effect of Pluronic P123

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Figure 4. FE-SEM images of LaPO4 synthesized with the addition of 4.0 g of P123 under hydrothermal treatment for different reaction times: (a) 3 h, (b) 5 h, (c) 8 h, and (d) 12 h. Inset in part a: the corresponding high-magnification FE-SEM image.

is undoubtedly significant. This observation is also in agreement with our previous work.22,23 From the above analyses, it can be seen that adding Pluronic P123 leads to the formation of uniform spindle-shaped microarchitectures assembled with LaPO4 single-crystalline nanowires with a narrow size distribution. Therefore, it can be reasonably concluded that Pluronic P123 plays a critical role in constructing the uniform spindle-shaped microarchitectures, which also is similar to the synthesis of the hollow PbWO4 nanospindles.27 According to the previous report,19 the final wire morphology of the as-prepared samples reflects the crystal habits of LaPO4. However, the reason for the self-assembly of these LaPO4 nanowires into highly ordered one-dimensional (1D) nanoscale arrays and finally the spindle-shaped microarchitectures is still unclear, and therefore a preliminary growth model is proposed here to illustrate the formation mechanism of the spindle-shaped microarchitectures. The proposed mechanism mainly comprises three steps in sequence: (1) aggregation of Pluronic P123 molecules into micelles, in parallel with the adsorption of La3+ and HnPO4(n–3) ions onto Pluronic P123 micelle surfaces and the interbonding between La3+ and HnPO4(n–3) in the solution; (2) nucleation and crystal growth of LaPO4 in the constrained environment provided by Pluronic P123 nematic liquid crystals on the mesoscale, which led to the formation of the uniform LaPO4 single-crystalline nanowires; and (3) cooperative assembly and further condensation of the randomly oriented nanowires into uniform spindle-shaped microarchitectures under the direction of Pluronic P123 liquid crystals in solution, the third step as illustrated and evidenced in the TEM images of LaPO4 microarchitectures formed at different hydrothermal reaction times in Figure 4. A schematic illustration for this proposed three-step formation mechanism is presented in Figure 5. Most recently, the photoluminescence of rare-earth-doped lanthanide phosphate 1D nanostructures has been studied, for its potential use as both interconnectors and functional units in future optoelectronic and electroluminescent devices with nanoscale dimensions.19–21 Lanthanum phosphate has been shown to be a useful host structure for other lanthanide ions,

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Figure 5. Schematic illustration of a plausible growth mechanism for the formation of spindle-shaped microarchitectures.

Figure 7. Room-temperature PL spectra (λexc ) 260 nm) of the LaPO4: Eu spindlelike microstructures prepared with the addition of 4.0 g of P123 (solid line) and the LaPO4:Eu samples prepared without P123 (dashed line).

Figure 6. FE-SEM and TEM images of LaPO4:Eu synthesized under hydrothermal treatment with (a, b) and without (c, d) the addition of 4.0 g of P123.

generating phosphorus emissions in the UV–vis region.17,18 Considering the similarity of their crystal structures and lattice constants, the rare-earth-doped LaPO4 spindle-shaped microarchitectures could be prepared by the same hydrothermal crystal growth process as the undoped LaPO4, and the doping alters neither the average structure nor the morphology of the host materials. In this work, 5 mol % Eu3+-doped LaPO4 (formula La0.95Eu0.05PO4) spindle-shaped microarchitectures composed of aligned single-crystalline nanowires were obtained via our current Pluronic P123-assisted hydrothermal synthetic method, as shown in Figure 6a and b, and parallel, irregularly shaped microarchitectures composed of randomly oriented nanowires were also prepared via a conventional hydrothermal process as shown in Figure 6c and d. The two products are denoted as S-form and R-form, respectively. It can be seen that the nanowires in the spindle-shaped microarchitectures are uniform in diameter (10–12 nm in Figure 6a and b), while those in irregularly shaped microarchitectures are widely distributed from 5 to 50 nm in Figure 6c and d. Under excitation at 260 nm, the room-temperature emission spectra from powders of S-form and R-form La0.95Eu0.05PO4 are shown in Figure 7. The spectra show that the luminescent lines of the two samples correspond to transitions between f-electron levels of the europium dopant ions, which proved that europium ions had been doped in LaPO4 nanowires successfully. Among 5D0 — 7FJ (J ) 1, 2, 3, 4) emission lines of Eu3+ shown in Figure 7, the strongest emission originates from the transition of 5D0 to 7F1 rather than 5D0 to 7F2. In terms of Judd-Ofelt theory,28,29 the different radiative transition

behaviors from the 5D0 level to the sublevels of the 7FJ states depend on the symmetry of the local environment of the europium ions. Thus, the present results implies that more Eu3+ ions occupy inversion center sites in our matrix of LaPO4:Eu3+ nanowires because of the nature of the 1D structure. These luminescent properties of Eu3+ in the LaPO4 1D nanostructures are basically in agreement with those previously reported.19 From the emission spectra, it is interesting to note that the La0.95Eu0.05PO4 R-form shows a relatively weak luminescence, whereas the La0.95Eu0.05PO4 S-form displays significantly increased luminescence under the same measurement conditions. This observation leads us to a conclusion that the self-assembly of aligned single-crystalline nanowires into ordered uniform spindle-shaped microarchitectures with a narrow size distribution could enhance the emission efficiency. It has been known that the morphologies, crystalline structure, and size of rare-earth compounds greatly influence the luminescence intensity. Compared to the R-form synthesized without Pluronic P123, S-form La0.95Eu0.05PO4 synthesized with Pluronic P123 has smooth, well-defined facets, and highly uniform morphologies. And especially, in the uniform microarchitectures self-assembled from aligned single-crystalline nanowires, the surface area of the nanowires and therefore the defect concentration at the surfaces are much diminished. Therefore, in accordance with the luminescent process,30 these may lead to significant luminescent enhancement. This explanation for this photoluminescent enhancement has also been given preliminarily in our previous report.22 The greatly enhanced luminescence performance observed in the spindle-shaped microarchitectures is significant and may have significant technological applications in fabricating the next generation of nanoscale optoelectronic devices.

4. Conclusions In summary, novel uniform spindle-shaped microarchitectures self-assembled from single-crystalline nanowires of lanthanum phosphates have been successfully synthesized by a facile, Pluronic P123-assisted hydrothermal approach. The concentration of Pluronic P123 played a key role in the formation of spindle-shaped microarchitectures. The formation mechanism

Uniform Spindle-Shaped Microarchitectures

of the spindle-shaped microarchitectures has been proposed and discussed. In addition, the luminescent property of Eu3+-doped LaPO4 microarchitectures has been demonstrated to be susceptible to the morphology and architecture of the lanthanum phosphates. More detailed work is under progress for building other novel hierarchically self-assembled architectures. It is believed that this copolymer-assisted hydrothermal approach may provide a feasible approach for manipulating various uniform microstructures and building complex architectures with interesting morphologies and properties. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant Nos. 50672115, 50502037) and the National Fundamental Research Program of China (2002CB613300).

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