Fabrication of Lanthanide Phosphate Nanocrystals with Well

Jun 23, 2009 - Synopsis. Lanthanide phosphate nanocrystals with controlled morphologies were fabricated on solid substrates at room temperature by the...
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Fabrication of Lanthanide Phosphate Nanocrystals with Well-Controlled Morphologies by Layer-by-Layer Adsorption and Reaction Method at Room Temperature Xiaokong Liu,† Qifeng Wang,† Zhongmin Gao,‡ Junqi Sun,*,† and Jiacong Shen†

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 8 3707–3713

State Key Laboratory of Supramolecular Structure and Materials and State Key Laboratory of Inorganic Synthesis and PreparatiVe Chemistry, College of Chemistry, Jilin UniVersity, Changchun, P. R. China 130012 ReceiVed April 13, 2009; ReVised Manuscript ReceiVed June 2, 2009

ABSTRACT: Here we report the preparation of crystallized lanthanide phosphate (LnPO4) nanostructures at room temperature on solid substrates by the layer-by-layer adsorption and reaction method comprising repetitive adsorption of lanthanide ions and subsequent reaction with phosphate ions. Taking the fabrication of LaPO4, for instance, the morphology of the nanostructured LaPO4 can be controlled between three-dimensional dandelion-like nanoarchitectures composing of single-crystalline one-dimensional nanoneedles and one-dimensional nanoneedles lying on the substrates by tailoring the concentration of the deposition solutions. This fabrication method can be extended to prepare other kinds of crystallized nanostructures of LnPO4 such as CePO4 and rare-earth-doped luminescent LaPO4. The formation of the single-crystalline nanoneedles under such a mild condition originates from the successive interfacial reaction of lanthanide and phosphate ions and the intrinsic anisotropic growth habit of hexagonal LnPO4.The present study is meaningful in developing new methodologies to fabricate single-crystal nanostructures deposited directly on solid substrates without the requirement of high reaction temperature and tedious procedures. Introduction In recent years, inorganic nanostructures with well-defined shapes and sizes have attracted growing attention because of their unique size- and shape-dependent properties and their widespread potential applications in miniaturized devices of photonics, electronics, magnetics, and sensors and in the exploration of high efficient catalysts.1 Over the past decade, many interesting one-dimensional (1D) nanostructures including nanorods, nanowires, nanobelts, and nanotubes have been fabricated by various approaches such as the template-directed growth methods,2 vapor-liquid-solid (VLS),3 vapor-solid (VS),4 and solution-liquid-solid (SLS) processes,5 solutionphase methods,6 solvothermal/hydrothermal process,7 and so forth. Meanwhile, the synthesis of complex hierarchical threedimensional (3D) micro/nanoarchitectures or the assembly of 1D nanoscale building blocks into 3D complex architectures have been the focus of extensive research because novel functionalities can be derived from the structural complexity.8 Among many kinds of nanostructured materials, the fabrication of lanthanide phosphates (LnPO4) with designed nanostructures has attracted much attention in recent years,9-15 because the pioneering work of Haase and co-workers on the synthesis of lanthanide-doped LnPO4 (LnPO4:Ln3+) nanoparticles.9 LnPO4 is an important family of lanthanide compounds that are broadly used as luminescent or laser materials, catalysts, heat-resistant materials, nuclear waste disposal, proton conductors, versatile biological labels, photon up-conversion materials, magnetic resonance contrast agent and so forth.9,16 Nanostructured LnPO4 with controlled size and dimensions are believed to exhibit novel size- and shape-dependent properties when compared to the bulk ones. For example, Eu3+-doped LaPO4 * To whom correspondence should be addressed. Fax: 0086-431-85193421. Email: [email protected]. † State Key Laboratory of Supramolecular Structure and Materials, Jilin University. ‡ State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University.

nanowires show the highest fluorescence quantum yield and improved radiative transition rate in comparison with their corresponding nanoparticles, microparticles and micorods.10a The optical properties of Eu3+-doped LnPO4 nanocrystals are significantly affected by the morphologies of the nanocrystals.10b Up to now, less attention has been paid to the synthesis of LnPO4 micro/nanoarchitectures with complex 3D structures. As demonstrated by Shi and co-workers very recently, the spindleshaped LnPO4 microarchitectures self-assembled from singlecrystalline nanowires of LnPO4 synthesized by a Pluronic P123assisted hydrothermal approach could exhibit enhanced emission efficiency compared with the irregularly shaped microarchitectures composed of randomly oriented nanowires.11 Therefore, structural control of LnPO4 nanocrystals is important to exploit novel properties of LnPO4 materials. Another issue concerned with the single-crystalline 1D LnPO4 nanomaterials such as nanorods, nanowires and nanocables is that the synthesis usually requires high temperature, sophisticated equipment, and/or the help of template, which is often achieved by hydrothermal or modified hydrothermal methods,11,12 ultrasound irradiation method,13 reactions conducted in an oil bath,14 and the combination of sol-gel process and electrospinning with subsequent high temperature annealing.15 Mann and co-workers synthesized cerium phosphate (CePO4) nanowires at room temperature by taking microemulsion as template. The formation of the crystallized CePO4 nanowires takes a period of 1 month by the surfactant-mediated slow crystallization.17 Yam and coworkers synthesized Tb3+-doped CePO4 nanowires at roomtemperature in the presence of β-cyclodextrin, although the role of β-cyclodextrin is unclear at present.18 Recently, Qian and co-workers developed a solution route to the synthesis of LnPO4 nanowires at room temperature without the help of template, but the growth and crystallization of the LnPO4 nanocrystals also need several tens of days.19 Therefore, further exploration of facile room-temperature methods for the fabrication of LnPO4 nanocrystals with controlled morphologies, without the requirements of tedious procedures and special reaction conditions

10.1021/cg900412n CCC: $40.75  2009 American Chemical Society Published on Web 06/23/2009

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remains a great challenge. Meanthile, the LnPO4 nanocrystals synthesized by the existing methods are always in the form of powder. The fabrication of LaPO4 nanocrystals directly on solid substrate is highly desirable because it will facilitate the fabrication of LnPO4-based nanodevices. Recently, we developed a facile layer-by-layer (LbL) adsorption and reaction method for the preparation of amorphous titanium phosphate ultrathin films by repetitive adsorption of hydrated titanium from aqueous Ti(SO4)2 solution and subsequent reaction with phosphate groups.20 Here in this work, as exemplified by LaPO4 and CePO4, we show that crystallized LnPO4 nanoarchitectures composing of single-crystalline nanonneedles on the surface of solid substrates were successfully fabricated by repetitive adsorption of Ln3+ ions and subsequent reaction with phosphate ions at room temperature without the help of any template. The LnPO4 nanoarchitectures can be tailored from 3D dandelion-like structure to 1D nanoneedles lying on the surface of substrate by simply changing the concentration of dipping solutions of Ln(NO3)3 and phosphates. Meanwhile, rare-earth-doped LaPO4 with strong photoluminescence were easily fabricated by the introduction of suitable amount of rare earth ions into the aqueous La(NO3)3 solution during the LbL adsorption and reaction process. We believe that the present work will provide a novel method to grow wellcontrolled, single-crystalline nanostructures on solid substrates under mild conditions without the requirement of high reaction temperature, sophisticated equipment, and template. Experimental Section Materials. Analytical grade lanthanum nitrate (La(NO3)3 · 6H2O), europium nitrate (Eu(NO3)3 · 6H2O), cerium nitrate (Ce(NO3)3 · 6H2O) and terbium nitrate (Tb(NO3)3 · 6H2O) with a purity of more than 99.0% were purchased from RUIKE State Engineering Research Center of Rare Earth Metallurgy and Functional Materials Co., Ltd., China. Analytical grade sodium hydrogen phosphate dodecahydrate (Na2HPO4 · 12H2O), and sodium dihydrogen phosphate dehydrate (NaH2PO4 · 2H2O) were purchased from Beijing Chemical Reagents Company. Poly(diallyldimethylammonium chloride) (PDDA) aqueous solution with a molecular weight of 100 000-200 000 and poly(methacrylic acid) (PMAA) aqueous solution with a molecular weight of ca. 6500 were purchased from Sigma-Aldrich. Deionized water was used for all the LnPO4 fabrication. The phosphate salt (PS) solution comprises 100 mM or 1 mM phosphate (Na2HPO4 and NaH2PO4) with its pH adjusted by the addition of H2SO4. All chemicals were used as received without further purification. Treatment of the Substrate. A substrate of a silicon or quartz wafer was immersed in a slightly boiled piranha solution (3:1 98% H2SO4: 30% H2O2 mixture) for 20 min and rinsed with copious amounts of water. In this way, the wafer was hydrophilized. Caution: Piranha solution reacts Violently with organic materials and should be handled carefully. Ag-coated quartz crystal microbalance (QCM) resonators were sonicated in ethanol and water and were dried by N2 flow. The freshly cleaned substrate was immersed in aqueous cationic solution of 1 mg mL-1 PDDA for 20 min to obtain a positively charged surface. After water rinsing and N2 drying, such a positively charged substrate was immersed in aqueous polyanion solution of 1 mg mL-1 PMAA for 20 min to deposit a PMAA layer. Two bilayers of PDDA/PMAA films were deposited with PMAA as the outmost layer on the above substrates. In this way, the surface of the substrate was covered with carboxylic acid groups. Fabrication of LnPO4 Nanoarchitectures. Take the fabrication of LaPO4, for instance, the process for the synthesis of nanostructured LnPO4 by the LbL adsorption and reaction method is simple and described as follows: a carboxyl-terminated substrate was first immersed in an aqueous solution of La(NO3)3 for 5 min. The substrate was then transferred to the first phosphate salt solution for 15 s, followed by immersing the substrate into the second phosphate salt solution for 5 min. Next, the substrate was rinsed in two water baths both for 1 min each. Finally, the substrate was dried with N2 flow. In this way, one cycle of LaPO4 was prepared. LaPO4 with large dimensions can be

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Figure 1. QCM frequency decreases (-∆F) of successive deposition of LaPO4. prepared by repeating the above processes in a cyclic fashion. By replacing La(NO3)3 solution with Ce(NO3)3 solution, CePO4 nanoarchitectures can be prepared. Characterization. QCM measurements were taken with a KSV QCM-Z500 using quartz resonators with both sides coated with Ag (F0 ) 9 MHz). SEM observations were carried out on a JEM-6700F field-emission microscope or Hitachi S-5200 field-emission microscope without metal coating on the sample surface. TEM observations were carried out on a Philips Tecnai F20 transmission electron microscope at 200 kV. A 30-cycle dandelion-like LaPO4 or CePO4 nanoarchitectures prepared by the LbL adsorption and reaction method on silicon wafers was scratched off in ethanol and transferred to TEM grids for TEM observation. X-ray diffraction (XRD) patterns were measured on a Rigaku D/Max 2550 diffractometer (with Cuka radiation, λ ) 1.5418 Å) with polished single-crystalline silicon (100) as substrates. The energy-dispersive X-ray spectroscopy (EDX) measurement was conducted on an EDAX Genesis 2000 X-ray microanalysis system attached to an XL30 ESEM FEG scanning electron microscope. Photoluminescence spectra were recorded with a Fluorolog FL3-TCSPC Jobin Yvon spectrophotometer.

Results and Discussion The fabrication process of LaPO4 nanoarchitectures was monitored by QCM measurements. As shown in Figure 1, QCM frequency regularly decreases because of the successive deposition of LaPO4 on the resonator by alternately dipping the substrate into 10 mM La(NO3)3 and 100 mM phosphate salt solution (pH ) 4.0). The frequency decrease for one cycle of LaPO4 deposition was 996.3 ( 202.5 Hz. The QCM measurements confirmed the successful fabrication of LaPO4 by the LbL adsorption and reaction method. The scanning electron microscopy (SEM) images show that the as-prepared 30-cycle LaPO4 has a dandelion-like structure instead of continuous film, which has a lateral dimension of 400 to 500 nm (Figure 2a and b). The dandelion-like LaPO4 is composed of 1D nanoneedles radiating from the center. Some of the dandelion-like LaPO4 get very close to or even overlapped partially with each other. Figure 2c shows the typical X-ray diffraction (XRD) patterns of the as-prepared 30-cycle LaPO4 deposited on a silicon substrate. All the diffraction peaks can be well indexed to a pure hexagonal phase with lattice constants a ) 7.100 Å and c ) 6.494 Å for LaPO4 · 0.5H2O (PCPDF # 46-1439), indicating that dandelion-like LaPO4 crystals are fabricated at room temperature. The chemical composition of the as-prepared 30cycle dandelion-like LaPO4 was investigated with energy dispersive X-ray (EDX) spectroscopy, as shown in Figure 2d. Elements of La, P, O, and Si were detected. The molar ratio of La, P, and O is 0.94:1:5.59, which is in good agreement with the composition confirmed by XRD pattern. The morphology

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Figure 2. Morphology and composition of LaPO4 nanoarthitectures. (a) Top-view SEM image of a 30-cycle LaPO4 prepared with 10 mM La(NO3)3 aqueous solution and 100 mM phosphate salt solution. (b) A higher-magnification SEM image of LaPO4 in a. (c) XRD patterns of a 30-cycle LaPO4 deposited on a silicon wafer. (d) EDX spectrum of a 30-cycle LaPO4. (e) Top-view SEM image of a 20-cycle LaPO4 prepared with 0.10 mM La(NO3)3 aqueous solution and 1 mM phosphate salt solution.

Figure 3. Morphology and composition of CePO4 nanoarchitectures with 30 deposition cycles. (a) Top-view SEM image. (b) XRD patterns of CePO4 deposited on a silicon wafer. (c) EDX spectrum.

of LaPO4 fabricated by the LbL adsorption and reaction method can be easily tuned by simply changing the concentration of aqueous solutions of La(NO3)3 and phosphate salt. Figure 2e shows the top-view SEM image of a 20-cycle LaPO4 fabricated by using aqueous 0.1 mM La(NO3)3 solution and 1 mM phosphate salt solution (pH 4.0). 1D LaPO4 nanoneedles lying on the surface of the substrate are obtained. The as-prepared nanoneedles are about 150-200 nm long and have a diameter of 15-30 nm in the center. By replacing aqueous La(NO3)3 solution with Ce(NO3)3 solution, crystalline CePO4 with dandelion-like structures can also be fabricated by the LbL adsorption and reaction method as indicated in Figure 3, demonstrating the generality of this method to fabricate crystalline LnPO4 nanoarchitectures on solid substrates at room

temperature. The CePO4 is also in hexagonal phase, with a molar ratio of Ce, P, and O being 0.92:1:5.57, as confirmed by XRD and EDX measurements, respectively. The morphology and crystal structure of the dandelion-like LaPO4 and CePO4 were further characterized by transmission electron microscopy (TEM) and high-resolution (HR)TEM. The TEM and HRTEM images of the as-prepared 30-cycle LaPO4 with dandelion-like structures are presented in images a and b in Figure 4, respectively. Figure 4a depicts a typical low magnification TEM image of a single dandelion-like LaPO4, which is composed of 1D nanoneedles with sharp tips and diameters of 10-25 nm and lengths of 100-200 nm. Figure 4b shows a representative HRTEM image taken from an individual nanoneedle of the dandelion-like LaPO4. This image

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Figure 4. Microscopic characterization of LaPO4 and CePO4 nanoarchitectures. (a) Low-magnification TEM image of a 30-cycle LaPO4 nanodandelion. (b) HRTEM image taken from a single LaPO4 nanoneedle of the nanodandelion in (a), Inset: electron diffraction pattern obtained by Fourier transform of the shown area on the nanoneedle. (c) Low-magnification TEM image of a 30-cycle CePO4 nanodandelion. (d) HRTEM image taken from a single CePO4 nanoneedle of the nanodandelion in c; inset, electron diffraction pattern obtained by Fourier transform of the shown area on the nanoneedle.

reveals that the nanoneedle is single-crystalline without visible defects. The calculated d-spacing between two adjacent lattice planes parallel to the long axis is 3.1 Å, corresponding to the theoretical interplanar spacing (3.07 Å) for (200) crystal plane of hexagonal rhabdophane LaPO4. The inset in Figure 4b shows the Fast Fourier transformation (FFT) diffractogram from the HRTEM image described above. The FFT diffractogram pattern can be indexed as a hexagonal LaPO4 single crystal recorded from the [010] zone axis. The axis direction of the LaPO4 nanoneedles could be determined as [001] growth by combining the HRTEM image and the FFT diffractogram pattern. The TEM and HRTEM images of a 30-cycle CePO4 nanostructure are presented in images c and d in Figure 4, respectively. The growth direction of the CePO4 nanoneedles is also along the [001] direction. The growth direction of the LaPO4 and CePO4 nanocrystals fabricated by the LbL adsorption and reaction method at room temperature is in good agreement with their corresponding nanowires/nanorods obtained by hydrothermal or oil bath methods.12a,14 Figure 5 depicts the SEM images of the as-prepared dandelionlike LaPO4 with different deposition cycles. In the early stage of three deposition cycles (Figure 5a), LaPO4 particles with lots of prickles on the surface were obtained. The 3-cycle LaPO4 particles have an average diameter of 177.5 ( 27.1 nm. The surface prickles act as crystal seeds for the further growth of the LaPO4 nanoneedles. Along with the successive adsorption of La3+ ions and subsequent reaction with phosphate groups, the prickles gradually grow longer with the tips in the end while the diameters of the nanoneedles remain almost constant. In this way, 3D dandelion-like LaPO4 developed. The size of the 3D dandelion-like LaPO4 can be tuned by simply changing the deposition cycles of Ln3+ and phosphate ions. It is noticeable that scattered needlelike LaPO4 and their aggregates are observable at the early stage of the LaPO4 nanocrystal growth. With the successive adsorption of La3+ ions and subsequent reaction with phosphate groups, most of the nanoneedles developed into 3D dandelion-like nanostructures. The remarkable point of LbL adsorption and reaction method for the fabrication of LaPO4 and CePO4 nanoarchitectures is the rapid formation of the single-

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crystalline nanoneedles under such a mild condition of room temperature without the guidance of any template or capping agent. The anisotropic growth of the nanocrystal under such a mild condition is mainly governed by its intrinsic structural feature and the growth environment.21 The as-prepared LnPO4 nanocrystals are of hexagonal structure, as demonstrated by XRD and HRTEM. There exist some reports that hexagonal crystal structure is the only stable phase at low temperature for light rare earth phosphates.14 From a thermodynamic perspective, the activation energy for the c axis growth of hexagonal LnPO4 is lower than that of the growth perpendicular to the c axis.12a,14 Therefore, the hexagonal LnPO4 crystal has an intrinsic anisotropic growth habit along the [001] direction. In the hydrothermal fabrication process, an external energy input is needed to dissolve the amorphous LnPO4 particles and make it recrystallized to form single-crystal nanowires/rods via a dissolution-recrystallization process. According to Yu’s opinion, the high temperature and autogenous pressure in a closedreaction environment can promote the motion and circulation of the solute in the reaction solution and thus the cations and anions can be acceleratedly brought to the right position to develop the lattice faces.21 In the LbL adsorption and reaction process, the cations and anions are in two separated solutions. The crystallization of LnPO4 nanocrystal is based on the stepby-step interfacial reaction of the lanthanide and phosphate ions. In each deposition step, the Ln3+/phosphate ions in the solution adsorb and react with the partner ions existed in the outmost surface, followed by crystallization because of the high activity of these ions originated from their ultrathin nature. As the LbL adsorption and reaction process proceeds, single-crystallized LnPO4 nanoneedle grows. At present, we can conclude that the successive interfacial reaction of Ln3+ and phosphate ions and the intrinsic anisotropic growth habit of hexagonal LnPO4 play a crucial role in the rapid fabrication of single-crystalline LnPO4 nanoneedles at room temperature without the guidance of templates. As a comparison, The fabrication of TiPO4 film by the LbL adsorption and reaction method involved the alternate dipping of the substrate into aqueous Ti(SO4)2 and phosphate solutions. It is known that lanthanide ions hydrolyze slightly in aqueous solution under weakly acidic conditions so that they mainly exist in the form of Ln3+.12c,22 Different from lanthanide ions, titanium ions exist in the form of several kinds of hydrated titanium ions in aqueous solution, which can polymerize into titanium oxide chains with different lengths.23 The chainlike titanium oxides adsorb on solid surface to form continuous but disordered layers of titanium oxides. The continuous layers of titanium oxides react with phosphates during the LbL adsorption and reaction process and thus continuous amporphous TiPO4 films were finally obtained. LnPO4 has been shown to be a useful host lattice for rare earth ions to produce phosphors emitting a variety of colors. By replacing La(NO3)3 solution with a mixture solution of La(NO3)3, Ce(NO3)3, and Tb(NO3)3 (concentration: 10 mM, the molar ratio of La(NO3)3:Ce(NO3)3:Tb(NO3)3 ) 0.104:0.132: 0.096) or a mixture solution of La(NO3)3 and Eu(NO3)3 (concentration: 10 mM, the molar ratio of La(NO3)3:Eu(NO3)3 ) 0.8:0.2), Ce3+- and Tb3+-doped LaPO4 (LaPO4:Ce3+:Tb3+) and Eu3+-doped LaPO4 (LaPO4:Eu3+) that emits strong green and red luminescence have been successfully fabricated by the LbL adsorption and reaction method. The as-prepared 30-cycle LaPO4:Ce3+:Tb3+ and LaPO4:Eu3+ has dandelion-like structure (Figure 6a and b), which resembles that of host LaPO4. Figure 6c shows the typical XRD patterns for the as-prepared 30-cycle LaPO4:Ce3+:Tb3+ and LaPO4:Eu3+ deposited on silicon

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Figure 5. SEM images of LaPO4 nanoarchitecture with different deposition cycles: (a) 3, (b) 5, (c) 7, (d) 10, (e) 15, and (f) 30.

Figure 6. Surface morphology and composition of rare earth doped LaPO4 nanoarchitecture with 30 deposition cycles. (a) SEM image of LaPO4: Ce3+:Tb3+. (b) SEM image of LaPO4:Eu3+. (c) XRD patterns of LaPO4:Ce3+:Tb3+ and LaPO4:Eu3+ deposited on silicon wafers. (d) EDX spectra of LaPO4:Ce3+:Tb3+ and LaPO4:Eu3+.

substrates. All the peaks of XRD patterns of each sample in Figure 6c can be readily indexed to a pure hexagonal phase similar with that of the as-prepared host LaPO4. Most of the diffraction peaks in Figure 6c slightly shift to higher angles in comparison with those for the undoped LaPO4 shown in Figure 2c, suggesting that Ce3+, Tb3+, and Eu3+ are really doped into the crystal lattice of the corresponding LaPO4 matrix because CePO4, TbPO4, and EuPO4 all possess smaller d-spacings than LaPO4. The composition of the doped dandelion-like LaPO4 was confirmed by EDX spectroscopy, as shown in Figure 6d. For a 30-cycle LaPO4:Ce3+:Tb3+ deposited on a silicon substrate, elements of La, Ce, Tb, P, O, and Si were detected. The atomic ratio for the detected elements is (La:Ce:Tb):P:O ) (0.27:0.38: 0.39):1:5.68. The total content of the rare earth elements is equal to that of P element. Meanwhile, the atomic ratio for a 30-

cycle LaPO4:Eu3+ is (La: Eu):P:O ) (0.64:0.31):1:5.66. The total content of the rare earth elements is also equal to that of P element. Spectra a and b in Figure 7 are the emission spectra of a 30-cycle LaPO4:Ce3+:Tb3+ and LaPO4:Eu3+ deposited on quartz substrates (with a size of 1.5 × 5.0 cm2), respectively. The photographs of each sample excited by a 16 W UV light at 254 nm are also presented (images c and d in Figure 7). When excited by a UV light at a wavelength of 272 nm, the LaPO4: Ce3+:Tb3+ exhibits strong yellowish-green luminescence due to the transitions between the excited 5D4 state and the 7FJ (J ) 6, 5, 4, 3) ground states of terbium, with the 5D4-7F5 (543 nm) emission as the most prominent group (Figure 7a). The blue emission of the 5D3-7Fk transitions of Tb3+ (between 360 nm and 480 nm) was not detected, which is ascribed to the crossrelaxation between 5D3-5D4 and 7F0-7F6 at the high doping

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Figure 7. Photoluminescence properties of rare earth doped LaPO4 nanoarchitectures with 30 deposition cycles. (a) Photoluminescence spectrum (λexc ) 272 nm) of LaPO4:Ce3+:Tb3+. (b) Photoluminescence spectrum (λexc ) 254 nm) of LaPO4:Eu3+. (c, d) Photograph of (c) LaPO4:Ce3+:Tb3+ and (d) LaPO4:Eu3+ deposited on quartz substrates excited by a 16 W UV light.

concentration of Tb3+.15,24 It is well-known that the luminescence of Tb3+ in the LaPO4:Ce3+:Tb3+ phosphors originates from the energy transfer from the Ce3+ to the Tb3+.9,15,18,24a However, there is no emissions from Ce3+ detected between 300 and 400 nm for the as-prepared LaPO4:Ce3+:Tb3+ nanoarchitectures, suggesting a high energy transfer efficiency from Ce3+ to Tb3+. The high concentration of Tb3+ in the as-prepared LaPO4:Ce3+: Tb3+ nanoarchitectures explains the high energy transfer efficiency from Ce3+ to Tb3+.15,24a Under excitation at 254 nm, the LaPO4:Eu3+ nanoarchitectures exhibit strong orange-red emissions, which correspond to the transition from 5D0 to 7FJ (J ) 1-4) (Figure 7b). Emission from the higher energy levels (5D1, 5D2) of Eu3+ is not detected, possibly because the higher energy levels (5D1, 5D2) and the lowest 5D0 level of Eu3+ can be bridged by the multiphoton relaxation on the basis of the vibration of phosphate groups.12a,14 Different from the rhabdophane-type LaPO4:Eu3+ nanoparticles,10b,25 the emission of the as-prepared dandelion-like LaPO4:Eu3+ from the transition of 5D0 to 7F1 is stronger than that of 5D0 to 7F2, agreeing with the previous reports of the photoluminescence property of 1D Eu3+ doped LaPO4 nanomaterials.11,12a,14 Such a phenomenon is because the transitions of Eu3+ from 5D0 level to 7FJ levels are greatly affected by the local symmetry of Eu3+.26 On the basis of the 1D nature of the nanoneedles in our situation, more Eu3+ ions appear in the inversion center sites in the matrix of LaPO4:Eu3+ nanocrystals, which would enhance the 5D0-7F1 transition.10b,11 The emission of the 5D0-7F4 transitions for the as-prepared dandelion-like LaPO4:Eu3+ at 695 nm is stronger than that at 687 nm, which is different with the previously reported LaPO4:Eu3+ nanowires.11,12a These different radiative transitions might originate from the differences in the crystal field at each lattice site and the energy transfer rate constant between adjacent sites.14 The detailed investigation is undergoing to clarify the luminescence properties of the dandelion-like LaPO4:Ce3+:Tb3+ and LaPO4:Eu3+ nanocrystals fabricated by the LbL adsorption and reaction method. Conclusions In summary, this work describes a facile room-temperature method for preparing crystalline LnPO4 nanostructures with well controlled morphologies by a LbL adsorption and reaction

method. Dandelion-like LnPO4 nanoarchitectures composed of 1D single-crystalline nanoneedles and 1D nanoneedles lying on the surface of substrate were successfully fabricated by tailoring the concentrations of the deposition solutions of Ln3+ and phosphate ions. The aspect ratio of the nananeedles can be finetuned simply by changing the deposition cycles. Dandelionlike Ce3+- and Tb3+-codoped and Eu3+-doped LaPO4 nanoarchitectures emitting strong green and red luminescence, respectively, were also successfully fabricated by replacing La(NO3)3 solution with La(NO3)3 solution mixed with the corresponding rare earth ions. We believe that such a facile method to fabricate nanostructured crystalline LnPO4 directly on solid substrate will facilitate the development of LnPO4based naonodevices. The present study affirmatively confirms that instead of using high reaction temperature, single-crystalline LnPO4 nanoneedles can be synthesized by a successive interfacial reaction process at room temperature. The present study is meaningful in developing new methodologies to grow wellcontrolled single-crystalline nanostructures under mild conditions without the requirement of high reaction temperature and templates. Acknowledgment. This work was supported by the National Basic Research Program (2007CB808003) and the Foundation for the Author of National Excellent Doctoral Dissertation of P. R. China (Grant 200323).

References (1) (a) Alivisatos, A. P. Science 1996, 271, 933. (b) Murray, C. B.; Kagan, C. R. Annu. ReV. Mater., Sci. 2000, 30, 545. (c) Hu, J.; Odom, W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. (d) Murphy, C. J.; Jana, N. R. AdV. Mater. 2002, 14, 80. (e) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (f) Patzke, G. R.; Krumeich, F.; Nesper, R. Angew. Chem., Int. Ed. 2002, 41, 2446. (2) (a) Dai, H.; Wong, E. W.; Lu, Y. Z.; Fan, S.; Lieber, C. M. Nature 1995, 375, 769. (b) Han, W.; Fan, S.; Li, Q.; Hu, Y. Science 1997, 277, 1287. (3) (a) Morales, A. M.; Lieber, C. M. Science 1998, 279, 208. (b) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (4) (a) Yang, P.; Lieber, C. M. Science 1996, 273, 1836. (b) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947.

LnPO4 Nanocrystals by LbL Adsorption (5) Trentler, T. J.; Hickman, K. M.; Goel, S. C.; Viano, A. M.; Gibbons, P. C.; Buhro, W. E. Science 1995, 270, 1791. (6) (a) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 104, 59. (b) Sun, Y.; Gates, B.; Mayers, B.; Xia, Y. Nano Lett. 2002, 2, 165. (7) (a) Heath, J. R.; LeGoues, F. K. Chem. Phys. Lett. 1993, 208, 263. (b) Wang, X.; Li, Y. J. Am. Chem. Soc. 2002, 124, 2880. (8) (a) Milliron, D. J.; Hughes, S. M.; Cui, Y.; Manna, L.; Li, J.; Wang, L. W.; Alivisatos, A. P. Nature 2004, 430, 190. (b) Yuan, J.; Li, W.N.; Gomez, S.; Suib, S. L. J. Am. Chem. Soc. 2005, 127, 14184. (c) Yao, W.-T.; Yu, S.-H.; Liu, S.-J.; Chen, J.-P.; Liu, X.-M.; Li, F.-Q. J. Phys. Chem. B 2006, 110, 11704. (d) Luo, Y.; Li, S.; Ren, Q.; Liu, J.; Xing, L.; Wang, Y.; Yu, Y.; Jia, Z.; Li, J. Cryst. Growth Des. 2007, 7, 87. (e) Baek, Y.; Song, Y.; Yong, K. AdV. Mater. 2006, 18, 3105. (f) Li, Y. B.; Bando, Y.; Golberg, D. Appl. Phys. Lett. 2003, 82, 1962. (g) Shen, G.; Bando, Y.; Lee, C.-J. J. Phys. Chem. B 2005, 109, 10779. (9) (a) Riwotzki, K.; Meyssamy, H.; Schnablegger, H.; Kornowski, A.; Haase, M. Angew. Chem., Int. Ed. 2001, 40, 573. (b) Riwotzki, K.; Meyssamy, H.; Kornowski, A.; Haase, M. J. Phys. Chem. B 2000, 104, 2824. (10) (a) Yu, L.; Song, H.; Lu, S.; Liu, Z.; Yang, L.; Kong, X. J. Phys. Chem. B 2004, 108, 16697. (b) Huo, Z.; Chen, C.; Chu, D.; Li, H.; Li, Y. Chem.sEur. J. 2007, 13, 7708. (11) Bu, W.; Zhang, L.; Hua, Z.; Chen, H.; Shi, J. Cryst. Growth Des. 2007, 7, 2305. (12) (a) Fang, Y.-P.; Xu, A.-W.; Song, R.-Q.; Zhang, H.-X.; You, L.-P.; Yu, J. C.; Liu, H.-Q. J. Am. Chem. Soc. 2003, 125, 16025. (b) Meyssamy, H.; Riwotzki, K.; Kornowski, A.; Naused, S.; Haase, M. AdV. Mater. 1999, 11, 840. (c) Zhang, Y.-W.; Yan, Z.-G.; You, L.-P.; Si, R.; Yan, C.-H. Eur. J. Inorg. Chem. 2003, 4099. (d) Fang, Y.-P.; Xu, A.-W.; Dong, W.-F. Small 2005, 1, 967. (e) Cao, M.; Hu, C.; Wu, Q.; Guo, C.; Qi, Y.; Wang, E. Nanotechnology 2005, 16, 282. (f) Bu, W.; Hua, Z.; Chen, H.; Shi, J. J. Phys. Chem. B 2005, 109, 14461. (13) (a) Brown, S. S.; Im, H.-J.; Rondinone, A. J.; Dai, S. J. Colloid Interface Sci. 2005, 292, 127. (b) Zhu, L.; Liu, X.; Liu, X.; Li, Q.; Li,

Crystal Growth & Design, Vol. 9, No. 8, 2009 3713

(14) (15) (16)

(17) (18) (19) (20) (21) (22) (23) (24)

(25) (26)

J.; Zhang, S.; Jiang, M.; Cao, X. Nanotechnology 2006, 17, 4217. (c) Yu, C.; Yu, M.; Li, C.; Liu, X.; Yang, J.; Yang, P.; Lin, J. J. Solid State Chem. 2009, 182, 339. Wang, X.; Gao, M. J. Mater. Chem. 2006, 16, 1360. Hou, Z.; Wang, L.; Lian, H.; Chai, R.; Zhang, C.; Cheng, Z.; Lin, J. J. Solid State Chem. 2009, 182, 698. (a) Onoda, H.; Nariai, H.; Moriwaki, A.; Makib, H.; Motooka, I. J. Mater. Chem. 2002, 12, 1754. (b) Wang, R.; Pan, W.; Chen, J.; Fang, M.; Cao, Z.; Luo, Y. Mater. Chem. Phys. 2003, 79, 30. (c) OrdonezRegil, E.; Drot, R.; Simoni, E.; Ehrhardt, J. J. Langmuir 2002, 18, 7977. (d) Amezawa, K.; Maekawa, H.; Tomii, Y.; Yamamoto, N. Solid State Ionics 2001, 145, 233. (e) Meiser, F.; Cortez, C.; Caruso, F. Angew. Chem., Int. Ed. 2004, 43, 5954. (f) Stephan, H.; Lehmann, O.; Haase, M.; Gu¨del, H.-U. Angew. Chem., Int. Ed. 2003, 42, 3179. (g) Hifumi, H.; Yamaoka, S.; Tanimoto, A.; Citterio, D.; Suzuki, K. J. Am. Chem. Soc. 2006, 128, 15090. Xing, Y.; Li, M.; Davis, S. A.; Mann, S. J. Phys. Chem. B, 2006, 110, 1111. Li, Q.; Yan, V. W. W. Angew. Chem., Int. Ed. 2007, 46, 3486–3489. Qian, L.; Du, W.; Gong, Q.; Qian, X. Mater. Chem. Phys. 2009, 114, 479. Wang, Q.; Zhong, L.; Sun, J.; Shen, J. Chem. Mater. 2005, 17, 3563. Yu, S.-H.; Liu, B.; Mo, M.-S.; Huang, J.-H.; Liu, X.-M.; Qian, Y.-T. AdV. Funct. Mater. 2003, 13, 639. Baes, C. F. J.; Mesmer, R. E. The Hydrolysis of Cations; John Wiley & Sons: New York, 1976; p 172. (a) Niesen, T. P.; Bill, J.; Aldinger, F. Chem. Mater. 2001, 13, 1552. (b) Ligorio, C.; Work, L. T. Ind. Eng. Chem. 1937, 29, 213. (a) Yu, M.; Lin, J.; Fu, J.; Zhang, H.-J.; Han, Y.-C. J. Mater. Chem. 2003, 13, 1413. (b) Prasad, S. V. G. V. A.; Reddy, M. S.; Kumar, V. R.; Veeraiah, N. J. Lumin. 2007, 127, 637. Buissette, V.; Moreau, M.; Gacoin, T.; Boilot, J.; Chane-Ching, J.Y.; Mercier, T. L. Chem. Mater. 2004, 16, 3767. Judd, B. R. Phys. ReV. 1962, 127, 750.

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