Additive-Mediated Splitting of Lanthanide Orthovanadate Nanocrystals

DOI: 10.1021/cg800207z. Publication Date (Web): September 25, 2008 ... Crystal Growth & Design 2016 16 (9), 5377-5384. Abstract | Full Text HTML | PDF...
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CRYSTAL GROWTH & DESIGN

Additive-Mediated Splitting of Lanthanide Orthovanadate Nanocrystals in Water: Morphological Evolution from Rods to Sheaves and to Spherulites

2008 VOL. 8, NO. 12 4432–4439

Hong Deng,† Chenmin Liu,† Shihe Yang,*,† Si Xiao,‡ Zhang-Kai Zhou,‡ and Qu-Quan Wang‡ Department of Chemistry, The Hong Kong UniVersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China, Department of Physics and Key Laboratory of Acoustic and Photonic Materials and DeVices of Ministry of Education, Wuhan UniVersity, Wuhan 430072, China ReceiVed February 23, 2008; ReVised Manuscript ReceiVed August 6, 2008

ABSTRACT: Ethylenediaminetetraacetic acid (EDTA) is shown to mediate the splitting of LnVO4 (Ln ) Ce and Nd) nanocrystals from rods to sheaves and to spherulites under hydrothermal conditions with remarkable controllability and uniformity. The roles of EDTA are thought to be to (1) chelate the Ln3+ ions in solution, thus decreasing the nanocrystal nucleation rate but increasing the growth rate, and (2) interfere with the nanocrystal growth by capping the nanocrystal surfaces. Upconverted avalanche luminescence from LnVO4 nanoplates, nanowires, and straw-nanosheaves has been demonstrated and compared. We found that the LnVO4 strawsheaves have a relatively more efficient upconversion due to the smaller avalanche excitation threshold caused by the strong crossrelaxation between the bundled nanorods in the sheaves.

1. Introduction Thanks to intensive research efforts during the past two decades, quantum dots (QDs) and nanowires (NWs) of some materials can now be synthesized with reasonable control.1-5 Increasing interest is directed to the fabrication of higher-level, hierarchical nanostructures, which are potentially useful in electronics, optics, catalysis and biomedicine. For example, branched nanostructures can be obtained by accurate control of nanocrystal growth kinetics.6 In nature, unusual structures are often a result of morphogenesis by virtue of organic templates, suggesting an alternative biomimetic approach of morphosynthesis for tailoring nanostructures. Recently, Alivisatos and co-workers reported the synthesis of Bi2S3 sheaf-like nanostructures.7 Whitmire and co-workers also prepared sheaflike nanostructures of iron phosphide.8 Interestingly, the rather different crystal systems (such as orthorhombic for Bi2S3 and hexagonal for Fe2P) could develop into such a seemingly general sheaf-like nanostructure. It is noticed that both syntheses above were performed in hydrophobic organic solvents (such as oleic acid, trioctylphosphane, or trioctylphosphine oxide). For high performance applications in the biological arena, however, NCs must be well dispersed in aqueous media. It is therefore a challenge to develop effective and controllable methodologies for the synthesis of uniform nanostructures of wires, strawsheaves, and spherulites in water. Lanthanide compound nanocrystals (NCs) are currently under vigorous investigation due to their prospective utility in many areas such as optoelectronics, display devices, up-conversion phosphors, and biology.9-12 In particular, lanthanide orthovanadates belong to a group of traditional phosphors widely used in polarizers, electroluminescent devices, and laser host materials. Although there have been various reports on hydrothermal synthesis of nanoscale rare earth orthovanadate materials by adjusting the pH values or by using chelating agents, most of them involved LaVO4 or YVO4 doped with rare earth ions to enhance photoluminescence and little has been studied on * To whom correspondence should be addressed. E-mail: [email protected]. † The Hong Kong University of Science and Technology. ‡ Wuhan University.

higher-order nanostructures of these materials.13 Lanthanide compound NCs with varying morphologies could be used as endogenous fluorophores for two-photon imaging of cancerous tissues,14 in lieu of the traditional organic fluorophores,15 which lack the desired photochemical and photophysical stabilities. Byrappa and co-workers reported on the synthesis of Nd:RVO4 (where R ) Y, Gd) single crystals under mild hydrothermal conditions.16 Most recently, we achieved controlled synthesis of uniform LnVO4 (Ln ) Ce, Nd) NCs with square-plate and H-shaped morphologies in organic solution, and uncovered an unusual phenomenon of upconverted avalanche luminescence in thick films of these NCs.17 Herein, we report the facile, selective, and large-scale preparation of single-crystal LnVO4 (Ln ) Ce, Nd) ultrathin nanowires and sheaf-like and broccolilike nanostructures with high uniformity in aqueous media. Key to the controlled synthesis is the use of ethylenediaminetetraacetic acid (EDTA) to mediate the nanocrystal splittings in aqueous media instead of organic solutions as other researchers commonly used previously. The mediation of crystal splitting using a reagent in water is more relevant to biomineralization because biological systems are housed in aqueous milieu with many amphiphilic molecules. Our systematic studies of the LnVO4 nanostructural evolution in water contribute to the understanding of the rather general fiber-to-spherulite crystallization scheme across different length scales and diverse materials systems. We have further investigated the upconverted avalanche luminescence property in these newly synthesized LnVO4 (Ln ) Ce, Nd) nanomaterials. Because of their peculiar branched structures and novel upconverted avalanche luminescence property, these LnVO4 (Ln ) Ce, Nd) nanomaterials may find applications in optical display, upconversion lasers, catalytic and biological imaging fields.

2. Experimental Section Synthesis of LnVO4 (Ln ) Ce, Nd) Nanostructures. In a typical synthesis, 1-3 mmol of Ln(NO3)3 · 6H2O (Ln ) Ce, Nd) was dissolved in 5 mL of H2O and the aqueous solution was added to a separately prepared solution containing 1-4.5 mmol of EDTA (dissolved in 10 mL of H2O). After stirring of the solution vigorously for 30 min, 1-3 mmol of NaVO4 (dissolved in 15 mL of H2O) and an appropriate

10.1021/cg800207z CCC: $40.75  2008 American Chemical Society Published on Web 09/25/2008

Splitting of Lanthanide Orthovanadate Nanocrystals Table 1. Morphologies of the CeVO4 Samples Obtained with Different Reactant Ratios (see also Figure 4)a Ce(NO3)3

Na3VO4

EDTA

morphology of CeVO4

1 1 1 2 3

1 1 1 2 3

0 1 1.5 3 4.5

nanorods nanowires straw-sheaf straw-sheaf with larger fantails double-head broccoli

a The hydrothermal reactions were at 180 °C and lasted for 24 h. The amounts of the reactants are in unit of mmol.

amount of ammonia were added to adjust the pH to 8-10, and the stirring was continued for 20 min. Afterward, the mixture was sealed again in the 40 mL Teflon-lined stainless steel autoclave and maintained at 180 °C for 24 h. After the autoclave was cooled to room temperature naturally, the turbid suspension inside the autoclave was centrifugally separated and the precipitate was collected. The as-synthesized product was washed several times with anhydrous ethanol and dried at 60 °C for 10 h. Characterization. The as-prepared products were characterized by field emission scanning electron microscopy (SEM), transmission electron microscopy (TEM), and powder X-ray diffraction (XRD) measurements. The morphologies were directly examined by SEM using JEOL JSM-6700F at an accelerating voltage of 5 kV. For TEM observations, the LnVO4 (M ) Ce, Nd) nanostructures were ultrasonically dispersed in ethanol and then dropped onto carbon-coated copper grids. TEM observations were carried out on JEOL 2010F and JEOL 2010 microscopes operating both at 200 kV. Energy-dispersive X-ray spectroscopy (EDS) was attached to the JEOL 2010F. The XRD analyses were performed on a Philips PW-1830 X-ray diffractometer with Cu kR irradiation (λ ) 1.5406 Å) at a scanning speed of 0.025°/s over the 2θ range of 20-70°. Optical Measurements. The excitation source for the photoluminescence of the LnVO4 films was a Ti:Sapphire CW laser (Mira 900, Coherent) with tunable wavelength in the range 700-920 nm. A longwavelength pass filter (LWPF) and a tunable neutral density filter (NDF) were used to filter the short wavelength noise from the laser and adjust the intensity of excitation, respectively. The excitation laser was focused onto the surface of the films (the focus length of the lens is 70 mm). The upconverted luminescence spectra from the LnVO4 films were recorded by a spectrometer (Spectrapro 2500i, Acton) with liquid nitrogen cooled CCD (SPEC-10, Princeton).

3. Results and Discussion X-ray diffraction (XRD) was used to characterize the chemical composition and crystal structure of the as-obtained sheaflike LnVO4 (Ln ) Ce, Nd). All of the XRD reflections could be readily indexed to the tetragonal phase of CeVO4 with lattice constants a ) 0.7399 nm and c ) 0.6496 nm (JCPDS No. 120757), and to the tetragonal phase of NdVO4 with lattice constants a ) 0.7329 nm and c ) 0.4356 nm (JCPDS No. 150769) (see Figure S1 in Supporting Information). Similar XRD patterns of the tetragonal phase were observed for other morphologies of the LnVO4 (Ln ) Ce, Nd) products including nanowires. A typical absorption spectrum of the sheaf-like CeVO4 is presented in Figure S2 (Supporting Information). Samples for the UV-vis absorption experiments were prepared by dispersing the as-synthesized CeVO4 with absolute ethanol in a sonication bath for 10 min to form a translucent solution. There is a strong absorption peak at about 280 nm, which can be ascribed to charge transfer from oxygen ligands to the central vanadium atom in the VO43- ion.13a The size, morphology, and crystal structure of the assynthesized samples obtained at different reaction conditions were investigated by using a number of electron microscopic techniques. We found that EDTA plays a key role in controlling the anisotropic growth of the NCs, which can be clearly seen in Table 1. In general, when EDTA was not used, the reaction of Ce(NO3)3 · 6H2O (1 mmol) and Na3VO4 (1 mmol) at 180 °C

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for 24 h resulted in the formation of CeVO4 nanorods with a diameter of 10-30 nm and a length of 50-200 nm as shown in Figure 1a. The clear lattice fringes in the HRTEM image of a single CeVO4 nanorod (Figure 1b) confirm its high crystallinity. The lattice spacings of 0.32 and 0.37 nm correspond to the (002) and (200) planes of the tetragonal phases of CeVO4, respectively, suggesting the growth direction of [001]. The corresponding fast Fourier transform (FFT) pattern in the inset to Figure 1b also indicates that the nanorod preferentially grows along the c-axis. When a small amount of EDTA (1 mmol) was introduced in the reaction mixture of Ce(NO3)3 · 6H2O (1 mmol) and Na3VO4 (1 mmol) at 180 °C for 24 h, a one-dimensional (1D) structure of CeVO4 could also be obtained but with a much smaller diameter, which are essentially ultrathin nanowires. This is revealed in the transmission electron microscopy (TEM) images in Figure 1c,d. The CeVO4 nanowires are about 50-200 nm long and 5 nm thick, and have a tendency of bundling together. The high-resolution TEM image of a single-crystalline CeVO4 nanowire (Figure 1d) shows that the nanowire is well-crystallized with the (200) fringes running along the nanowire direction and spaced by 0.37 nm, and the (101) fringes can also be observed with a spacing of 0.49 nm. This is consistent with growth of the nanorod along the [001] direction, as indicated by the arrow in Figure 1d. The corresponding fast Fourier transform (FFT) pattern (inset of Figure 1d) also indicates the growth of the nanorod along the [001] direction. The body of the nanowires was subjected to energy-dispersive X-ray spectroscopy (EDS) analysis, confirming the exclusive composition of Ce, V and O except for the Cu signal from the copper TEM grid (see Figure S3 in Supporting Information). When more EDTA (1.5 mmol) was added, however, the reaction between Ce(NO3)3 · 6H2O (1 mmol) and Na3VO4 (1 mmol) at 180 °C for 24 h yielded a very different morphology: nanosheaves of CeVO4. Figure 1e,f shows typical SEM images of the straw-sheaf CeVO4 NCs. The product looks like strawsheaf with two fantails consisting of a bundle of outspread nanorods, which are closely bonded to each other in the middle, so we call it a “straw-sheaf nanocrystal”. The low-magnification SEM image (Figure 1f) shows that the individual straw-sheaf has a length in the range of 0.8-1.5 µm and a middle diameter in the range of 80-120 nm, and the individual nanorods composing the two fantails are about 10-30 nm in diameter. In order to better understand the formation and evolution of CeVO4 nanostructures, further investigation with the addition of more reactants and more EDTA was conducted. It was found that the nanocrystal splitting is increased in magnitude as the concentrations of the reactants and EDTA increase. Figure 2 shows representative SEM, TEM and HRTEM images of the CeVO4 product, which was synthesized with doubled concentrations of Ce(NO3)3 · 6H2O (2 mmol) and Na3VO4 (2 mmol) and an increased concentration of EDTA (3 mmol) at 180 °C for 24 h. The first impression is that the CeVO4 straw-sheaves have opened their dual fantails to a larger extent, but again display a remarkable uniformity (Figure 2a,b). The individual strawsheaves have a length in the range of 0.7-1.3 µm, a fantail diameter of about 300-400 nm, and a middle diameter of about 60-100 nm. This is seen more clearly in the TEM image of an individual straw-sheaf in Figure 2c. The HRTEM image of this straw-sheaf in the central portion (Figure 2d) shows that it is structurally uniform with interplanar spacing about 0.49 and 0.37 nm, which corresponds to the (101) and (200) lattice planes, respectively. The 45° orientation between (101) and (200) lattice plane is also consistent with the FFT patterns, further confirming

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Figure 1. TEM and HRTEM images of CeVO4 nanostructures synthesized with Ce(NO3)3 · 6H2O (1.0 mmol), Na3VO4 (1.0 mmol), and an appropriate amount of EDTA at 180 °C for 24 h. (a, b) No EDTA; (c, d) very little EDTA (1 mmol); (e, f) a little EDTA (1.5 mmol).

the single crystal structure with the growth direction of [001] in the middle of the straw-sheaf. On the other hand, the HRTEM image in the expanding tail region of the straw-sheaf (Figure 2e) also supports the single-crystalline nature of the constituent nanorods. The interlayer distance is calculated to be 0.49 nm, which agrees well with the separation between the (101) lattice planes. Similarly, the 45° orientation between the (101) and (200) lattice plane is also consistent with the FFT patterns, again confirming the [001] growth direction of the tailing nanorods.

Finally, when the concentrations of the reactants were increased further and a large amount of EDTA (4.5 mmol) was added, the CeVO4 product gradually changed to a broccoli-like morphology. Shown in Figure 3 are representative SEM and TEM images of the CeVO4 NCs product, which was synthesized with Ce(NO3)3 · 6H2O (3 mmol), Na3VO4 (3 mmol), and EDTA (4.5 mmol) at 180 °C for 24 h. Clearly, the product takes a spherical nanostructure and is shaped like broccoli with an overall size in the range 1-2 µm (Figure 3a). The higher

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Figure 2. Representative SEM (a and b) and TEM (c, d, and e) images of the CeVO4 sheaves synthesized with a moderate amount of EDTA (3 mmol) and increased concentrations of the reactants (2 mmol). (a, b) General straw-sheaf morphology of CeVO4; (c) individual sheaf-like CeVO4; (d) HRTEM image taken from the middle of a sheaf-like CeVO4 (solid square); (e) HRTEM image taken from the tail of a sheaf-like CeVO4 (open circle).

magnification SEM image in Figure 3b exposes some details of the 3D broccoli-like nanostructure of CeVO4; it is actually built from many nanorods with diameters ranging from 10 to 30 nm. This is more clearly seen in Figure 3c,d, in which the TEM images of a single broccoli-like CeVO4 are presented. A selected area electron diffraction (SAED) pattern taken from the tips of the outspreading CeVO4 nanorods shows that they are well crystallized (Figure 3d). We extended our effort to the synthesis of the rare-earth homologues such as NdVO4. Similar results were obtained (see Figure S4 in Supporting Information). In nature, some minerals are also found to grow into a sheaf-like morphology but normally with a much larger size above a micron.18,19 It is believed that the sheaf structures may be formed by crystal splitting during their growth. In general, crystal splitting often occurs in a crystal with structural anisotropy, which, for example, prefers 1D growth and has relatively small lateral adhesion energy. The tetragonal LnVO4 under investigation here seems to be a good case since without EDTA, 1D growth has already lead to the well-separated LnVO4 nanorods. Kinetically, crystal splitting is associated with fast crystal growth, which depends strongly on the oversaturation of the solution.7 In addition, the presence of additives or impurities could interrupt the crystal growth and result in crystal splitting. In our experiment, EDTA appears to be the key to the formation of

the branched nanostructures of LnVO4. First, EDTA lowers the concentration of Ln3+ by chelation and thus decreases the nucleation rate and increases the growth rate of LnVO4, a scenario favorable to crystal splitting. Second, EDTA could interfere with the growth of the LnVO4 crystals, cap the crystal surfaces, and lower the energy cost for creating new surfaces, thus encouraging the crystal splitting again. This is consistent with our observations. The probable formation mechanism of the nanostructures with various morphologies is proposed in Figure 4, which depicts the structural evolution of LnVO4 under the influence of EDTA (see also Table 1). All of the as-prepared LnVO4 nanostructures crystallize in a zircon-type tetragonal crystal system with a space group of I41/amd as schematically depicted in Figure S5 (see Supporting Information). Since a and b are equivalent axes in the tetragonal system, under appropriate synthetic conditions such as in our case, the intrinsic anisotropy of tetragonal CeVO4 and the more reactive (001) surface resulted in a more rapid growth along the c-axis and thus the formation of the nanorods in the absence of the chelating agent EDTA. The sharp tips of the nanorods might be caused by a devoid of reactant supply at the late stage of the reaction. The addition of a small amount of EDTA (EDTA and [Ce3+] are both 1.0 mmol) has the effect of further confining the 1D growth to ultrathin nanowires perhaps due to the capping of EDTA on the nanowire sidewalls.

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Figure 3. Representative SEM (a and b) and TEM images (c and d) and of the broccoli-like CeVO4 synthesized with high levels of EDTA (4.5 mmol) and even higher concentrations of the reactants (3 mmol). The inset in d shows the corresponding ED pattern.

Figure 4. Schematic representation of EDTA-mediated LnVO4 nanocrystal splitting process. The arrows indicate the direction of increasing amounts of EDTA and/or increasing amounts of Ce(NO3)3 and Na3VO4 (see also Table 1).

However, by increasing the amount of EDTA to 1.5 mmol, the straw-sheaf CeVO4 was obtained. And the dual fantails were opened more and more widely with the increasing concentrations of both the reactants and EDTA. Finally, the broccoli-like spherulites were formed when sufficient amounts of reactants (3 mmol) and EDTA (4.5 mmol) were added. Clearly, a crystal splitting growth mechanism is most likely responsible for the formation of 3D straw-sheaf and broccoli-like nanostructures. As already mentioned above, the crystal-splitting agent of EDTA plays double roles as Ln3+-chelating ligand and surface capping agent. Because the concentration of Ce3+ is inversely proportional to the added amount of EDTA, as more and more EDTA (from 1 to 4.5 mmol) is added, the smaller concentration of Ce3+ results in the formation of a smaller number of nuclei, which is then followed by fast growth, leading to the nanocrystal splitting to form sheaves and spherulites. In addition, due to its multi-hydrophilic nature, EDTA can cap the side walls of the nanorods in the fantails of the star-sheaves, reduce the energy cost of surface generation as a result of the crystal splitting, and allow the tails of the straw-sheaves to fan out. The capping of EDTA on the LnVO4 surfaces was confirmed by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses (see Figure S6 and S7 in Supporting Information). The crystal growth mechanism proposed above is supported by the evolution of the LnVO4 nanostructures as a function of the growth time. A prolonged reaction time (36 h) resulted in more crystal splittings. For example, at fixed initial reactant

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Figure 5. (a-g) Although the double-sheaf morphology dominates the products, more complex morphologies made of three to eight half-sheaves emanating from the common core were also observed.

concentrations and temperature (e.g., Ce(NO3)3 · 6H2O (2 mmol) and Na3VO4 (2 mmol) and EDTA (3 mmol)), as the reaction time increased from 24 to 36 h, the as-synthesized CeVO4 sheaves opened their dual fantails more widely (see Figure S8 in Supporting Information). Besides, the observations of multibranched sheaves, for example, T-shaped, X-shaped, crossshaped sheaves as shown in Figure 5, are also consistent with the proposed mechanism. It is plausible that these multiantennae straw-sheaves all originated from the same core and were induced by multiple-twinning of the NCs in the early nucleation/ growth. First, the branched sheaves are similar in length, suggesting a common starting point instead of branching at a later growth stage. Second, the twinning structure is seen clearly (e.g., Figure 5c). Third, there are only a few fixed angles between the branches, which tally with the tetragonal structure of LnVO4. Finally, the branching is significantly enhanced when the concentrations of EDTA and the reactants are increased because of the increased crystal growth rate, which could arrest the less stable, multiply twinned nuclei by overgrowth. Thick film samples of LnVO4 (Ln ) Ce and Nd) NCs used in the upconversion measurements were prepared by five runs of drop-coating on glass slides using ethanol solutions of LnVO4 NCs (20 mg/mL), and the prepared thick films were dried for 3 h at room temperature. The excitation wavelength for the upconversion was 808 nm. As shown in Figure 6a, thick films of the NdVO4 and CeVO4 NCs exhibit strong upconverted emission centered at 580 and 593 nm, respectively. The upconversion spectra from the LnVO4 remained unchanged when the nanostructures varied from square-nanoplates to nanorods, nanowires and nanosheaves. The excitation power dependence of the upconversion intensity for the thick films of the NdVO4 and CeVO4 NCs shown respectively in Figure 6b,c

shows a sudden increase in the emission intensity of the upconverted photons when the excitation power is larger than a threshold value (Pc), which is usually called avalanche upconversion. Such avalanche processes were documented for several rare earth metal ions such as Pr3+, Ho3+, and Tm3+ doped in certain bulk optical materials and rare earth nanoparticles La0.45Yb0.50Er0.05F3 embedded in thin films,20,21 which were assigned to the cross-relaxation energy transfer between the nearby luminescent centers. Figure 6b,c reveals that the upconversion intensity of the LnVO4 straw-nanosheaves are stronger than those of the nanowires and nanoplates due to the much smaller avalanche threshold excitation power Pc. The Pc values of the NdVO4 straw-nanosheaves, nanowires and nanoplates are 25, 48, and 90 mW, and the corresponding avalanche slopes, ν ) ∂ log IPL/∂ log Pexc, are 4.8, 5.8 and 7.8, respectively. In comparison, the NdVO4 nanoplates reported previously display the largest avalanche threshold Pc but the highest avalanche slope ν as well,16 because of their relatively large size (30 × 30 × 9 nm3) and surface-capping by long-chain surfactants. The most efficient upconversion from the NdVO4 straw-sheaves (with the smallest Pc but also the smallest ν) can be plausibly explained by the strong energy cross relaxation between the massively bundled nanowires separated by small distances within the sheaves. It is interesting that the CeVO4 NCs exhibit a similar intensity-dependent upconversion to that of the NdVO4 NCs, but the avalanche slope ν of the CeVO4 NCs is smaller than that of the NdVO4 NCs, which may be related to the less photon absorption of CeVO4 at the excitation wavelength. Evidently, the precise mechanism of the avalanche upconversion of the LnVO4 NCs needs to be further studied.

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support. This work was partially supported by National Program on Key Science Research (2007CB935300) and NSFC 10534030. Supporting Information Available: XRD patterns of CeVO4 and NdVO4, IR, XPS, UV-vis spectrum of CeVO4, energy-dispersive X-ray spectroscopy (EDS) analysis spectrum of CeVO4 nanowires, TEM images of nanowires and straw-sheaves NdVO4 and SEM images of the sheaf-like CeVO4. This material is available free of charge via the Internet at http://pubs.acs.org.

References

Figure 6. Upconversion of LnVO4 nanocrystal materials excited by ps laser pulses with wavelength of 808 nm. (a) PL spectra of NdVO4 and CeVO4 straw-nanosheaves; (b) and (c) excitation power dependence of the upconversion intensity of the NdVO4 and CeVO4 nanostructures with different morphologies.

4. Conclusion We have presented a simple hydrothermal process for selectively preparing sharp-tipped nanorods, ultrathin nanowires, and three-dimensional sheaf-like and broccoli-like nanostructures of LnVO4 (Ln ) Ce and Nd) with excellent size and morphological uniformity. EDTA is shown to play important roles as a splitting reagent in the formation of well-defined 3D sheaflike and broccoli-like LnVO4 (Ln ) Ce and Nd) nanostructures. A possible morphological evolution mechanism for the formation of LnVO4 (Ln ) Ce and Nd) nanostructures from the nanorods, nanowires to sheaves and broccoli-like spherulites has been proposed by accentuating the chelation roles of EDTA. Upconverted avalanche luminescence from LnVO4 (Ln ) Ce and Nd) nanoplates, nanowires, and straw-sheaves has been demonstrated and compared. We found that the LnVO4 strawsheaves have a relatively more efficient upconversion due to the smaller avalanche excitation threshold caused by the strong cross-relaxation between the bundled nanorods in the sheaves. Our work on controlled morphological evolution in aqueous media shall not only impact on the understanding of biomineralization processes in nature but also aid the development of practicable and cost-effective strategies to tailor materials properties by accurate control of morphologies at the nanoscale dimension. Acknowledgment. We are grateful to the Research Grant Council of Hong Kong (604206) and the Hong Kong University of Science and Technology (RPC06/07.SC03) for financial

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