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Jun 16, 2009 - A straightforward hydrothermal strategy for the controlled synthesis of monodisperse NaLa(MoO4)2 and NaLa(MoO4)2:Eu3+ bipyramid ...
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Oleic Acid/Oleylamine Cooperative-Controlled Crystallization Mechanism for Monodisperse Tetragonal Bipyramid NaLa(MoO4)2 Nanocrystals Wenbo Bu, Zhenxing Chen, Feng Chen, and Jianlin Shi* State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 200050, P. R. China ReceiVed: February 17, 2009; ReVised Manuscript ReceiVed: May 5, 2009

A straightforward hydrothermal strategy for the controlled synthesis of monodisperse NaLa(MoO4)2 and NaLa(MoO4)2:Eu3+ bipyramid nanocrystals is presented using oleic acid/oleylamine as a mixed surfactant. Deprotonated oleic acid was demonstrated to be the decisive structure-directing agent for the bipyramid nanocrystals, and the bipyramid formation efficiency of the oleic acid/oleylamine combination was found to be the highest among the selected surfactant combinations. The roles of oleylamine in the synthesis of monodisperse bipyramid nanocrystals were elucidated. A possible oleic acid/oleylamine cooperative-controlled crystallization mechanism (CCM) for the growth of uniform tetragonal bipyramid NaLa(MoO4)2 nanocrystals is proposed on the basis of our experimental results. The formation of well-defined tetragonal bipyramid morphology is mainly due to the preferential adsorption of deprotonated oleic acid onto {101} and {001}, which changes the surface energy of crystal planes and leads to different growing rates along corresponding directions, while facilitating the formation of bipyramid morphology with exposed {101} faces. Fluorescent investigation revealed that NaLa(MoO4)2:Eu3+ bipyramid nanocrystals possess a dominated hypersensitive red emission 5D0 f 7F2 transition of Eu3+ at 613 nm. 1. Introduction In the past few years, great efforts have been devoted to the synthesis of morphology-controlled colloidal nanocrystals, in that their properties are considerably dependent on composition, phase, shape, size, and size distribution.1-3 Excellent work on shape control synthesis of colloidal nanocrystals has been done by researchers in many countries.4-9 It is of particular interest to synthesize nanocrystals with complex and uniform morphologies by investigating the important parameters for selective growth and ultimate shape determination and to uncover their novel properties. Reports on solution-based techniques for preparing colloidal nanocrystals in various solvents using single or multiple surfactants have shown great success in controlling the size and morphology of nanocrystals.10-12 It is generally accepted that the capping agents could selectively adhere to certain facets of nanocrystals that ensure a slow growth rate and prevent particle agglomeration as well as confer stability to the resulting nanocrystals.13 The crystalline phase structure of nuclei and kinetic growth regulations by structure-directing agents, usually surfactants, are considered to be crucial in the determination of the final morphology of target nanocrystals.14-16 Although a general mechanism of how a single surfactant influences the morphology of certain nanocrystals remains unclear, mixed surfactants have been used in the shapecontrolled synthesis of colloidal nanocrystals.11,12,17 Especially, a combination of oleic acid (cis-9-octadecenoic acid, designated as OA) with oleylamine (cis-1-aminl-9-octadecene, designated as OL) has been well-reported because it has been found to work well with inorganic nanocrystals with varied functions such as magnetic,18,19 optical,20-23 catalytic,24 and so on. However, until now there has been no generally accepted mechanism or experimental evidence for elucidating the “cooperative effect” * To whom correspondence should be addressed. E-mail: jlshi@ sunm.shcnc.ac.cn. Telephone: 86-21-52412712. Fax: 86-21-52413122.

between oleic acid and oleylamine on the size and shape control of target colloidal nanocrystals, and the roles of oleylamine still need further in depth study. To date, techniques using oleic acid and oleylamine to prepare functional colloidal nanocrystals are mostly applied in hot organic solvents.18,21 Reports on using them as mixed crystal growth modifiers under hydrothermal condition and the corresponding investigations about their roles in morphology control can hardly be found. In previous reports, we have demonstrated the successful synthesis of La2(MoO4)3 and La2(MoO4)3:Yb,Tm microarchitectures via a facile hydrothermal process.25,26 In this paper, we report first the simple one-step hydrothermal strategy for monodisperse NaLa(MoO4)2 tetragonal bipyramid nanocrystals using an equimolar combination of oleic acid and oleylamine as a mixed surfactant. Without any time-consuming purification process,27 uniform bipyramid NaLa(MoO4)2 nanocrystals were obtained in high yield (close to 100% with no other types of nanoparticles visible) within just 6 h of hydrothermal treatment at 140 °C. Further, by doping with Eu3+, NaLa(MoO4)2:Eu3+ nanocrystals with uniform tetragonal bipyramid morphology were synthesized for the first time, and their novel optical property was also investigated preliminarily. The influence factors on the growth of tetragonal bipyramid nanocrystals were studied in detail. Roles of oleic acid and oleylamine in the controlled synthesis of uniform bipyramid nanocrystals were demonstrated and discussed. On the basis of our experimental results, a possible oleic acid/oleylamine cooperative-controlled crystallization mechanism (CCM) for the formation of tetragonal bipyramid NaLa(MoO4)2 nanocrystals is proposed. This work might provide some deep insights into the question of why an oleic acid/oleylamine combination always works well in the controlled synthesis of colloidal nanocrystals using solution-based approaches and provide some guidance in the morphology and size controllable synthesis of tungstates,

10.1021/jp901437a CCC: $40.75  2009 American Chemical Society Published on Web 06/16/2009

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carbonates, vanadates, and corresponding ion-doped compounds via a facile hydrothermal strategy with an equimolar combination of oleic acid and oleylamine as a mixed surfactant. The obtained NaLa(MoO4)2:Eu3+ could also find applications as excellent red phosphors for white lighting devices utilizing GaNbased excitation in the near UV. In addition, by doping with rare earth ions such as Tm3+ and Pr3+ or codoped with Er3+ and Ce3+, it is believed that these ion-doped double sodium molybdates with bipyramid morphology could also be fabricated and continue their potential applications as future advanced laser materials. 2. Experimental Section Synthesis of Monodisperse NaLa(MoO4)2 Tetragonal Bipyramid Nanocrystals. The optimal concentration of La3+, MoO42- and a mixed surfactant (total concentration of oleic acid/ oleylamine, designated as [OA + OL]) was [La3+] ) 0.01M, [MoO42-] ) 0.02 M, and [OA + OL] ) 0.4M. In a typical procedure, absolute ethanol (18 mL) was mixed with oleic acid (6 mmol) and oleylamine (6 mmol) under magnetic stirring for 15 min to form homogeneous solution I. Then LaCl3 · 6H2O (106 mg, 0.3 mmol) was dissolved in distilled water (4 mL) and was added slowly into solution I. After that, Na2MoO4 · 2H2O (145.2 mg, 0.6 mmol) in distilled water (4 mL) was added dropwise into solution I under magnetic stirring. The mixture was transferred into a 40 mL Teflon-lined stainless steel autoclave and stirred for 15 min, and then the system was sealed and treated at 140 °C for 6 h. After the reaction was cooled naturally to room temperature, the product was collected at the bottom of the vessel and then washed several times with hexane and ethanol to get rid of excess surfactants (excess means surfactant in the solution that is not bonded to nanoparticles) before being dried in an oven at 60 °C for 4 h. The as-prepared product could be easily redispersed in various nonpolar organic solvents such as hexane, toluene, and chloroform. Synthesis of Monodisperse NaLa(MoO4)2:Eu3+ Tetragonal Bipyramid Nanocrystals. The synthetic procedure of NaLa(MoO4)2:Eu3+ tetragonal bipyramid nanocrystals was the same as that used to synthesize NaLa(MoO4)2 nanocrystals, except that the LaCl3 · 6H2O and EuCl3 · 6H2O solution (0.3 mmol total, molar ratio La3+:Eu3+ ) 9:1) was used as the precursor. Characterization. Powder X-ray diffraction patterns were obtained on a Rigaku D/MAX-2250 V diffractometer with graphite-monochromatized Cu KR radiation. Energy dispersive spectroscopy was taken using an attached Oxford Link ISIS energy dispersive spectrometer fixed onto a JEM-2010 electron microscope operated at 200 kV. Field emission scanning electron microscopy (FESEM) images were obtained from a JEOL JSM6700F microscope. Transmission electron microscopy (TEM) images, high-resolution TEM images, and selected area electron diffraction (SAED) patterns were recorded on a JEOL 200CX microscope with an accelerating voltage of 200 kV. Fourier transform infrared spectroscopy (FTIR) spectra were measured in the range of 400-4000 cm-1 on a Nicolet 7000-C spectrometer, using pressed KBr tablets. Photoluminescence spectra were recorded on a Shimadzu RF-5301 fluorescence spectrophotometer. 3. Results and Discussion NaLa(MoO4)2 is a well-known laser host material.28 Crystals of double sodium molybdates doped with rare earth ions have been studied as laser materials beginning from the mid-1960s.29,30 In the present work, hydrothermal treatment of an amorphous particulate dispersion of LaCl3, Na2MoO4, and a mixed surfac-

Figure 1. XRD patterns of (a) JCPDS 24-1103 and (b) NaLa(MoO4)2 nanocrystals.

tant at 140 °C for 6 h led to the formation of pure phase NaLa(MoO4)2. The XRD pattern shown in Figure 1 reveals that NaLa(MoO4)2 has been formed. All the reflection peaks for NaLa(MoO4)2 are characteristic of a pure tetragonal phase (space group: I41/a). The calculated lattice constants are a ) b ) 5.343 Å and c ) 11.743 Å (JCPDS 24-1103). The morphology of the as-synthesized NaLa(MoO4)2 nanocrystals was examined with field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). It can be seen clearly from panels a and be of Figure 2 that the product is composed of monodisperse nanocrystals with well-defined uniform bipyramid morphology with an axis length of about 100 nm and four hemline lengths of about 120 nm. The bipyramid nanocrystals are in high yield (close to 100% with no other types of nanoparticles visible). The TEM and highresolution TEM (HRTEM) images shown in Figure 2 demonstrate that all of the as-obtained monodisperse nanocrystals are of an identical geometric morphology of bipyramid. From Figure 2c, it is also noted that the as-synthesized tetragonal bipyramid nanocrystals are well-separated from each other and exhibit an interesting two-dimensional ordered arrangement, indicative of regular shapes and narrow size distribution. In addition, an organic layer of capping ligands of about 1.9 nm in thickness on the surfaces of the nanocrystals can be found in the HRTEM image in Figure 2e. A typical HRTEM image shows an interplanar spacing of 2.3 and 1.9 Å, which correspond to the d spacings for the (002) and (011) planes of tetragonal NaLa(MoO4)2, respectively. The corresponding selected area electron diffraction (SAED) pattern at the bottom right corner of Figure 2e reveals a pattern that matches well with the bodycentered tetragonal NaLa(MoO4)2, in accordance with the XRD result. On the basis of these characterizations and the symmetry of the crystal lattice, it is concluded that the as-obtained nanocrystal is enclosed by (011), (01j1), (011j), (01j1j), (101), (1j01), (101j), and (1j01j) facets, which belong to the equivalent {101} plane groups. A schematic diagram of an individual NaLa(MoO4)2 tetragonal bipyramid nanocrystal is shown in Figure 2f. The hydrothermal synthetic method for monodisperse NaLa(MoO4)2 tetragonal bipyramid nanocrystals could be equally applied to the rare earth ion-doped double sodium molybdate, NaLa(MoO4)2:Ln (Ln ) rare earth ions), nanocrystals with an expected bipyramid morphology. In this work, we selectedEu3+ asthedopingionandprepareduniformNaLa(MoO4)2: Eu3+ tetragonal bipyramid nanocrystals successfully. It can be observed from the TEM images in Figure 3a that the as-obtained

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Figure 2. (a, b) FESEM images of NaLa(MoO4)2 nanocrystals synthesized at 140 °C for 6 h in the presence of OA and OL, [La3+] ) 0.01, [MoO42-] ) 0.02, [OA + OL] ) 0.4M, R1 ) 1:2, and R2 ) 40:1. (c, d) TEM images of the same sample. (e) HRTEM image on the tip of the nanocrystal shown in panel d. Inset in panel e is the corresponding SAED pattern. (f) Schematic morphology of an individual NaLa(MoO4)2 nanocrystal.

Figure 3. (a) TEM images of NaLa(MoO4)2:Eu3+ tetragonal bipyramid nanocrystals. (b) Energy dispersive X-ray (EDX) spectrum of a NaLa(MoO4)2: Eu3+ nanocrystal.

NaLa(MoO4)2:Eu3+ nanocrystals retain the uniform tetragonal bipyramid morphology, indicating that the proper doping of Eu3+ might have negligible influence on the size, size distribution, and morphology of the as-synthesized nanocrystals. The energy dispersive X-ray (EDX) spectroscopy was used to determine the composition of the as-obtained nanocrystals, and the results shown in Figure 3b confirm that the element ratios coincide with the chemical formula of NaLa(MoO4)2:Eu3+ (10 atom %), with the La:Eu molar ratio close to 9. Peaks of Cu were caused by the copper grid. Influence Factors for the Growth of NaLa(MoO4)2 Nanobipyramid. It is found that the morphology, size, and crystal phase purity of NaLa(MoO4)2 are mainly affected by synthetic

conditions such as the reaction time, reactant concentration, molar ratio of La3+ to MoO42- (designated as R1 ) [La3+]/ [MoO42-]), and molar ratio of mixed surfactant to La3+ (designated as R2 ) [OA + OL]/[La3+]). Our studies reveal that the morphological evolution of NaLa(MoO4)2 tetragonal bipyramid nanocrystals is time-dependent, and this process could be explained by the classical dissolution-reprecipation mechanism. When two different solutions containing metal ions and precipitators are mixed, a high supersaturation degree will be reached, and amorphous fine particles will form immediately. During the following hydrothermal treatment, we found the formation of tiny crystalline nuclei in supersaturated medium occurred at first and was then

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Figure 4. TEM images of the morphology evolution after hydrothermal treatment for different time periods at 140 °C: (a) 0 h and (b) 1 h, (c) 6 h, and (d) 12 h. Insert in panel b shows high magnification of the same sample.

followed by controlled crystallization and growth of the surfactants. The representative TEM image of the product obtained without any hydrothermal treatment is presented in Figure 4a, and no tetragonal bipyramid particles could be observed, as confirmed by the inset showing the selected area electron diffraction (SAED) pattern of the sample revealing an amorphous state. When the reaction time was prolonged to 1 h, as shown in Figure 4b, small and not well-developed tetragonal bipyramid particles were formed. By further prolonging the reaction time to 6 h, we found regular and well-defined tetragonal bipyramid nanocrystals with larger sizes (Figure 4c). The NaLa(MoO4)2 bipyramid nanocrystals grow bigger and become polydisperse via an Ostwald ripening process (Figure 4d) when the reaction lasted as long as 12 h. Generally speaking, small and uniform nanocrystals imply a strong interaction between the capping agents and nanoparticles. Influences of synthetic parameters such as [La3+], [MoO42-], [OA + OL], R1, and R2 on the final morphologies were also investigated (when using oleic acid and oleylamine as a mixed surfactant, the concentration of either oleic acid or oleylamine was set to be equal). As shown in Figure 5A, the shape of the NaLa(MoO4)2 nanocrystals was poorly developed when halving both concentrations of precursors and mixed surfactant. As the concentration of mixed surfactant decreased to 0.2 M while keeping the concentration of the precursor unchanged, other kinds of particles except bipyramids are present, and the tetragonal bipyramid nanocrystals are no longer monodispersed as shown in Figure 5B, indicating that the lack of mixed surfactant might lead to weaker interaction between surfactants and nanocrystals. In contrast, by halving the concentration of the La3+ to 0.005 M while keeping the concentration of the mixed surfactant constant, bipyramid nanocrystals were also not well-developed, and many inregularly shaped particles are

formed, indicating the lack of enough “monomers” to form the final well-developed bipyramid nanocrystals (Figure 5C). The tetragonal bipyramid nanocrystals could grow larger when doubling the precursor using amounts (Figure 5D). If the ratio of La3+ to MoO42- was tuned to R1 ) 2:3 as shown in panels E and F of Figures 5, polydisperse and irregular bipyramidal shapes were observed. The optimal values of [La3+], [OA + OL], R1, and R2 were found to be [La3+] ) 0.01M, [OA + OL] ) 0.4M, R1 ) 1:2, and R2 ) 40:1. To further validate the cooperative effect of the mixed surfactants on the shape control of NaLa(MoO4)2 nanocrystals, we performed a series of experiments. Table S1 of the Supporting Information lists the synthetic conditions of different samples. Figure S1 of the Supporting Information presents XRD patterns of these products. Except for the case where only oleic acid was used as a crystal growth modifier, pure tetragonal NaLa(MoO4)2 particles were synthesized. However, the corresponding TEM images (Figure 6A-E) show that these products possess different morphologies. Without the addition of any surfactant, irregularly shaped NaLa(MoO4)2 particles of 0.5-1.0 µm in size were obtained (Figure 6A, sample 1,). When oleic acid was used alone, small spherical particles together with long microrods were observed (Figure 6B, sample 2). In the case where only oleylamine was used, the product was found to be irregular clusters (Figure 6C, sample 3). In striking contrast, NaLa(MoO4)2 particles synthesized with equimolar oleic acid and oleylamine possess an amazingly monodisperse tetragonal bipyramid morphology (Figure 6D, sample 4). Compared to the uniform nanocrystals obtained by employing the equimolar oleic acid and oleylamine, it is believed that oleic acid and oleylamine have a great influence on the morphology of the NaLa(MoO4)2 particles, and both of them are important for the formation of the well-defined bipyramid morphology.

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Figure 5. TEM images of NaLa(MoO4)2 nanocrystals synthesized by altering [La3+], [MoO42-], [OA + OL], R1, and R2. (A) [La3+] ) 0.005M, [MoO42-] ) 0.01M, [OA + OL] ) 0.2M, R1 ) 1:2, and R2 ) 40:1 at 140 °C for 6 h. (B) [La3+] ) 0.01M, [MoO42-] ) 0.02, [OA + OL] ) 0.2M, R1 ) 1:2, and R2 ) 20:1 at 140 °C for 6 h. (C) [La3+] ) 0.005M, [MoO42-] ) 0.01M, [OA + OL] ) 0.4M, R1 ) 1:2, and R2 ) 80:1 at 140 °C for 6 h. (D) [La3+] ) 0.02M, [MoO42-] ) 0.04M, [OA + OL] ) 0.4M, R1 ) 1:2, and R2 ) 20:1 at 140 °C for 6 h. (E) [La3+] ) 0.02M, [MoO42-] ) 0.03M, [OA + OL] ) 0.4M, R1 ) 2:3, and R2 ) 20:1 at 140 °C for 8 h. (F) [La3+] ) 0.02M, [MoO42-] ) 0.03M, [OA + OL] ) 0.8M, R1 ) 2:3, and R2 ) 40:1 for 150 °C at 12 h.

Figure 6. TEM images for products synthesized (A) without any surfactant, (B) with [oleic acid] ) 0.2M, (C) with [oleylamine] ) 0.2M, and (D) with [oleic acid/oleylamine] ) 0.4M.

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Figure 7. TEM images for samples synthesized with surfactant combinations of (A) oleic acid/ammonia, (B) oleic acid/dodecylamine, (C) oleic acid/triethanolamine, and (D) oleic acid/oleylamine.

Different combinations of surfactants were utilized to study the role of amines with different chain lengths and bipyramid formation efficiency (Table S2 of the Supporting Information). Figure S2 of the Supporting Information shows the XRD patterns of NaLa(MoO4)2 synthesized with the surfactant combinations of oleic acid/oleylamine, oleic acid/ammonia, oleic acid/dodecylamine, and oleic acid/triethanolamine. Samples obtained with oleic acid/oleylamine and oleic acid/dodecylamine were found to be pure tetragonal phase (space group: I41/a, JCPDS 24-1103). For samples prepared with oleic acid/ triethanolamine and oleic acid/ammonia, additional peaks located at 2θ ) 24.18°and 2θ ) 22.14° were observed, respectively. It can be seen clearly in Figure 7, as compared with the sample prepared with the oleic acid/oleylamine combination (Figure 7D, sample 4) that poorly shaped and polydispersed bipyramids with much lower yield were obtained in samples prepared with the surfactant combinations of oleic acid/ammonia, oleic acid/dodecylamine, and oleic acid/triethanolamine, indicating the highest bipyramid formation efficiency of the oleic acid/oleylamine combination and the important role of oleylamine in the growth of well-defined high-yield bipyramid nanocrystals. Further investigations are needed to uncover the underlying principle of this observation. Surface Characterization of NaLa(MoO4)2. Surfactant adsorption characterization of nanocrystals is important because the organic coating could not only influence the properties of functional nanomaterials23,31 but could also provide vital information about the formation of certain morphologies. HRTEM image displays an almost 1.9 nm thick organic shell coating on the tetragonal bipyramid core (Figure 2e). In order to understand the interaction between surfactants and nanocrystals, we used Fourier transform infrared spectroscopy to analyze this organic shell. Figure 8 presents the FTIR spectra of the samples prepared under different synthetic conditions. Each sample, except sample

Figure 8. FTIR spectra for products synthesized (a) without surfactant, (b) with [oleic acid] ) 0.2M, (C) with [oleylamine] ) 0.2M, and (D) with [oleic acid/oleylamine] ) 0.4M.

1, shows the -CH2 symmetric and asymmetric stretching vibrations at 2850 and 2924 cm-1, respectively, revealing the absorption of the oleyl group on the surface.32,33 Neither the characteristic peak of oleic acid at 1709 cm-1 nor the peaks of oleylamine at 1593 and 3300 cm-1 could be detected in samples 2 and 4, indicating no free oleic acid or oleylamine at the surface.34 A broad peak at 3440 cm-1 is observed in all four spectra, which could not in our opinion simply be assigned to the V(N-H) stretching of the NH2 group35 because it could also be the V(O-H) stretching mode of water. The spectrum of sample 2 indicates the presence of an adsorbed carboxylate group with symmetric and asymmetric modes around 1546 and 1627 cm-1, respectively, while the difference between these two

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characteristic bands is 81 cm-1, revealing a bidentate coordination of oleic acid to the metal atom.36 It has been demonstrated by Shuka in the thermal decomposition synthesis of FePt nanocrystals that isomerization of oleic acid to elaidic acid could occur;37 however, on the basis of the FTIR results here, no isomerization of oleic acid is observed in the present case. In Figure 8c, two bands at 1627 and 1384 cm-1 are assigned to N-H bending and C-N stretching modes, respectively,38 indicating the presence of oleylamine on sample 3. It is noticed that the blue shift of the C-N stretch from that of free amines (1000-1350 cm-1) is probably due to the interaction between N and La through coordination. When NaLa(MoO4)2 nanocrystals were obtained in the presence of oleic acid and oleylamine (sample 4), two strong peaks at 1550 and 1442 cm-1 can be clearly observed (Figure 8d), indicative of a predominantly oleate species. Peaks at 1550 cm-1 are assigned to the bidendate (-COO-M) mode of oleic acid binding, which matches well with the report by Bagaria et al.39 It is noticed in Figure 8d that (1) the difference between the peaks at 1550 and 1442 cm-1 is 108 cm-1, which is significantly larger than that of sample 2 (81 cm-1) and (2) the relative intensity of the -CH2 symmetric and asymmetric stretching vibrations at 2850 and 2924 cm-1 are apparently much higher than that of samples 2 and 3, suggesting much stronger and denser adsorption of carboxylate on the surface of the NaLa(MoO4)2 nanocrystals in sample 2 than in the others. It is a little surprising that no evidence is found for the presence of oleyamine (or -NH2) on the surface of sample 4, and according to Klokkenburg et al. the absence of oleylamine could be probably attributed to the surfactant desorption during the washing step.40 It is believed that carboxylic acid molecules are present as dimers in a nonpolar solvent because of the hydrogen-bonding interaction, and the electron-donating ability of oxygen atoms in the molecules is reduced because of hydrogen bonding.41 Recently, Li et al. reported the controlled synthesis of monodisperse Fe-Mo nanoparticles via the thermal decomposition method using octanoic acid and bis-2-ethylhexylamine as mixed protective agents. Smaller as well as more uniform nanocrystals were obtained by using an equimolar combination of acid and amine than by using them alone or in other ratios, and they believed that such an improvement is due to the formation of carboxylate anions, whose yield could be enhanced with the help of bis-2-ethylhexylamine. However, no experimental evidence was provided to confirm the stronger adsorption of long-chain carboxylic acid, and the role of long-chain carboxylic amine still remained as a logical hypothesis. Combining the FTIR spectra in Figure 8 and previous reports by Klokkenburg40 and Wang,41 we concluded that the added oleylamine could enhance the release of carboxylate anions from oleic acid and guarantee a stronger adsorption of carboxylate anions onto the NaLa(MoO4)2 surfaces, which was clearly revealed by the much more intense -CH2 symmetric and asymmetric stretching vibrations at 2850 and 2924 cm-1 when using the oleic acid/ oleylamine combination than the others as shown in the FTIR results (Figure 8). To the best of our knowledge, this is the first experimental demonstration for the enhanced adsorption of oleic acid in the presence of oleylamine. On the basis of our experimental results and discussions above, it is demonstrated that the shape of NaLa(MoO4)2 nanocrystals depends strongly on the nature and the combination of surfactants used. It is also found that the presence of an equimolar combination of oleic acid and oleylamine is essential for the formation of uniform tetragonal bipyramid NaLa(MoO4)2 nanocrystals. When oleic acid or oleylamine was used alone,

Bu et al. no bipyramid morphology was obtained; however, when samples were synthesized using combinations of oleic acid and other amines than oleylamine, one could only expect poorly shaped bipyramid morphology in low yield. A possible formation mechanism will be discussed in the following section in detail. Possible Formation Mechanism. The general formation process of our uniform NaLa(MoO4)2 tetragonal bipyramid nanocrystals could to a certain extent be understood by the wellknown liquid-solid solution (LSS) phase transfer and separation mechanism proposed by Li.42 There is only one liquid phase (OA-OL-ethanol) at the initial stage, and then the solution phase (ethanol-water) and solid phase (lanthanum-oleate or LaCl3-oleylamine complex38) could form after slowly adding an LaCl3 aqueous solution. When MoO42- was introduced, coprecipitation between La3+ and MoO42- takes place at once. Then at a designated temperature, the system experiences a typical hydrothermal crystallization and ripening process assisted by the mixed surfactant, which could bind selectively to the certain facets of the nanocrystals and provide them with a hydrophobic surface. Phase separation then may occur because of the high density of the NaLa(MoO4)2 nanocrystals and their hydrophobic surfaces, and NaLa(MoO4)2 could be collected at the bottom of the vessel. Although this universal LSS phase transfer and separation mechanism could provide the general reaction procedure of NaLa(MoO4)2 nanocrystals, it could not specifically explain why only the uniform tetragonal bipyramid morphology could be formed and not any other one. It is suggested that the oleic acid/oleylamine mixed surfactant may have subtle control on the growth of tetragonal bipyramid nanocrystals in solution because of a certain unclear cooperative effect that remains to be elucidated. A report by Qi et al. on the shape evolution of gold from nanobelts to nanocombs with cationic/anionic surfactant mixtures (CTAB and SDSn) also suggested a cooperative effect between CTAB and SDSn and considered the mixed surfactant as “binary capping agents” that could function in a synergistic way and change the packing state of particle facets by specific absorption.17 In addition, it is demonstrated by Shukla and Klokkenburg that every oleylamine molecule in solution combines with one oleic acid molecule to form an acid-base complex.34,40 Therefore, in the case of the synthesis of bipyramid NaLa(MoO4)2 nanoccrystals via a hydrothermal strategy with equimolar oleic acid and oleylamine, this acid-base complex might be considered to be a new type of binary capping agent that modifies crystallization. Unfortunately, it is rather difficult to determine how this acid-base complex could selectively bind to specific facets because it is difficult for oleic acid to form carboxylate with oleylamine in the form of an acid-base complex and also bind to the surface of nanocrystals with other metal ions.40 On the basis of our experiments, it is believed that oleylamine plays an important role in morphology formation. However, as mentioned previously, no evidence of oleylamine could be found on the surfaces of bipyramid nanocrystals, and the oleylamine loss cannot be simply attributed to the washing process because the adsorption of oleylamine is demonstrated in sample 3 (where only oleylamine was used, Figure 8c), which was subject to the same washing process as the others. Roles of Oleic Acid and Oleylamine. It is generally accepted that deprotonated oleic acid (carboxylate anions, C17H33COO-) could interact selectively with specific crystal facets strongly and densely because of the high electron-donating ability.41 Our earlier research showed that, without oleylamine, bipyramid particles in microscale could also be obtained in very low yield by using oleic acid/NaOH or an oleic acid/sodium oleate

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combination, demonstrating that the deprotonated oleic acid functions as a bipyramid-determining surfactant (panels A and B of Figure S3 of the Supporting Information). These results clearly demonstrated that mechanisms in the hydrothermal synthesis of the present shape-controlled nanocrystals with oleic acid/oleylamine as cosurfactant are different from high-temperature thermal nonhydrolytic aminolysis (with assistance of the same surfactant combination), where function of oleylamine was thought to be indispensable.18b More importantly, it has been suggested that the deprotonation of oleic acid could be promoted according to the follow equilibrium41

SCHEME 1: Schematic of the Proposed Oleic Acid/ Oleylamine Cooperative-Controlled Crystallization Mechanism (CCM) for the Formation of Monodisperse Bipyramid NaLa(MoO4)2 Nanocrystalsa

C17H33COOH + C18H35NH2 f C17H33COO- + C18H35NH3+

(1)

On the basis of our results above, it is believed that the highest bipyramid formation efficiency by an oleic acid/oleylamine combination might due to the strongest deprotonation of oleic acid promoted by oleylamine among other amines. Therefore, at least triple roles of oleylamine could be seen here: first, enhancing the deprotonation of oleic acid; second, forming an acid-base complex with oleic acid molecules; and third, keeping the pH value stable at 8-9, which is favorable for the formation of NaLa(MoO4)2 of tetragonal phase. When oleic acid was used alone, because of the hydrogen-bonding interaction, the oleic acid presents dominantly in the form of a dimer rather than in carboxylate anions. Therefore, the absence of carboxylate anions leads to a weaker interaction between surfactant and surfaces and, consequently, to the formation of microparticles with nearspherical morphology (Figure 7b). In the case of using oleylamine alone, although oleylamine could bind to the crystal surfaces to a certain extent (confirmed by the FTIR results in Figure 8c), its poor electron-donating ability could only lead to irregular morphologies (Figure 6C). When oleic acid and oleylamine were combined equimolarly, a mixture of carboxylate anions (C17H33COO-), protonated oleylamine (C18H35NH3+), and an acid-base complex (C17H33COO-:C18H35NH3+) could form. The enhanced deprotonation of oleic acid and its highest electron-donating ability guarantee the preferential binding with metal ions on the specific NaLa(MoO4)2 facets. Free oleic acid dimers, oleylamine, and protonated oleylamine as well as an acid-base complex would fail to bind on the crystal surfaces during the selective binding competition due to their weaker electron-donating ability and were washed away during the washing step, which explains the absence of oleylamine and an acid-base complex in the monodisperse tetragonal bipyramid product in FTIR (Figure 8d). Cooperative-Controlled Crystallization Mechanism (CCM). A possible oleic acid and oleylamine cooperative-controlled crystallization mechanism (CCM) is then proposed, on the basis of the above discussions, which considers the crystalline phase of NaLa(MoO4)2 nuclei and kinetic growth regulation as a cooperative effect between oleic acid and oleylamine that is crucial for the determination of the final uniform bipyramid morphology. A schematic illustration of the proposed growth mechanism for bipyramid nanocrystals is in Scheme 1. Before hydrothermal treatment, with the presence of oleylamine (or other amines) in ethanol, enhanced deprotonation of oleic acid into carboxylate anions takes place. Limited ion exchange reactions take place after adding precursors sequentially. Upon adding a LaCl3 aqueous solution, a lanthanide-OA complex, i.e., LaCl3-x(C17H33COO)x, could form via anion exchange. Subsequently, when a Na2MoO4 solution was added, ion

a Oleic acid would deprotonate into carboxylate anions (C17H33COO-), while oleylamine could greatly promote the deprotonation process. During hydrothermal treatment at a designated temperature, the precursor phase of target nanocrystals, i.e., LaNa(MoO4)2-x(C17H33COO)2x, went through a hydrothermal crystallization and ripening process, resulting in the formation of the initial anisotropic carboxylate anion-capped NaLa(MoO4)2 nuclei. The carboxylate anions could be perferentially and selectively adsorbed onto certain faces because of their higher electron-donating ability as well as the anisotropic character of the NaLa(MoO4)2 crystal nuclei, leading to the growth of uniform tetragonal bipyramid morphology with exposed {101} faces.

exchange and substitution among Cl-, C17H33COO-, and MoO42 - as well as ion exchange between La3+ and Na+ take place,43 leading to the precursor phase product, i.e., LaNa(MoO4)2-x(C17H33COO)2x. During hydrothermal treatment, the system goes through a hydrothermal crystallization and ripening process, resulting in the formation of the initial anisotropic carboxylate anions capped NaLa(MoO4)2 nuclei. Deprotonated oleic acid, which possess a higher electrondonating ability than other organic ligands in the system, could preferentially and selectively adsorb on the {101} and {001} faces of NaLa(MoO4)2 nuclei through bidentate coordination to the metal atom because of the higher Na+ and La3+ packing density on the faces of the {101} and {001} plane group than on other faces [based on the cleavage of the NaLa(MoO4)2 crystal shown in Figure 9]. This selective adsorption of deprotonated oleic acid can inhibit the crystalline growth along 〈101〉 and 〈001〉 directions, while the growth rate along 〈100〉 and other equivalent directions is relatively promoted. According to Donnay-Harker rules,44 as to tetragonal structure, the surface energy of {001} faces is higher than that of {101} faces, and the surface energy of {001} faces would be still higher than that of {101} faces even after C17H33COO- adsorption. Therefore, the faster growing rate along the 〈100〉, 〈010〉, and 〈001〉 directions than that along the 〈101〉 direction facilitate the formation of bipyramid morphology with exposed {101} faces (Figure 2). After hydrothermal treatment for a long enough time (more than 6 h), bipyramid-shaped NaLa(MoO4)2 with a layer of capping ligands (bidentately coordinated oleic acid) could be collected at the bottom of the container because of gravity sedimentation and their hydrophobic surfaces. (It should be noted that so far the size or shape of nuclei are still unclear,

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Figure 11. Photoluminescence emission spectra (λex ) 394 nm) of NaLa(MoO4)2:Eu3+ nanocrystals: (a) sample 1, using no surfactant; (b) sample 3, with [oleylamine] ) 0.2M; and (c) sample 4, with [oleic acid/oleylamine] ) 0.4M. Figure 9. (a) Structure model of NaLa(MoO4)2 crystal: (b) (100), (c) (101), and (d) (001) surface cleavage of a NaLa(MoO4)2 crystal.

Figure 10. Photoluminescence excitation spectra (λem ) 613 nm) of NaLa(MoO4)2:Eu3+ nanocrystals: (a) sample 1, using no surfactant; (b) sample 3, with [oleylamine] ) 0.2M; and (c) sample 4, with [oleic acid/oleylamine] ) 0.4M.

and we assume the nuclei is spherical in the proposed schematic illustration, which is similar to assumptions of other researchers.) Optical Properties of NaLa(MoO4)2:Eu3+. Double sodium molybdates, which share scheelite-like structures, show excellent thermal and hydrolytic stability and are considered to be efficient luminescent hosts.45,46 In this paper, by selecting Eu3+ as the doping ion, uniform NaLa(MoO4)2:Eu3+ nanobipyramids were synthesized for the first time, and the photoluminescence property of these NaLa(MoO4)2:Eu3+ nanobipyramids was studied preliminarily. The excitation spectra of samples with different morphologies were performed by monitoring the emission of an Eu3+5D0 f 7F2 transition at 612 nm (Figure 10). The sharp lines in the 360-500 nm range are due to intraconfigurational 4f-4f transitions of Eu3+ in the host lattice, and all three samples show two strong excitation peaks at 394 nm (7F0 f 5L6) and 464 nm (7F0 f 5D2).47 Upon excitation at 394 nm at room temperature, characteristic emission peaks of Eu3+ within the wavelength range from 580 to 620 nm, corresponding

to the transition from 5D0 to 7FJ (J ) 1,2) levels, were observed in the emission spectra (Figure 11). It is well-known that the splitting and intensity patterns of the emission spectrum of an Eu3+-doped nanostructure depend greatly on the strength and symmetry of the local crystal fields. The 5D0 f 7F1 transition is a magnetic dipole transition, which is greatly affected by the local symmetry.48 If more Eu3+ was present in the inversion sites, the emission from the 5D0 f 7F1 would be enhanced; meanwhile, if less Eu3+ was present in the inversion sites, more 5 D0 f 7F2 transition would occur.49 Two emission peaks were observed in the selected samples (Figure 11) dominated by the hypersensitive red emission 5D0 f 7F2 transition of Eu3+ at 613 nm and with a much weaker emission peak located at 592 nm (5D0 f 7F1), indicating less Eu3+ in the inversion sites. It has been demonstrated that optical properties of Eu3+-doped samples depend greatly on the morphology of the host materials;50a therefore, changes in the emission spectrum of an Eu3+ of sample 1, 3, and 4 could also be expected. The similar splitting patterns of the 5D0 f 7F2 transition were observed in luminescence lines of sample 1 and 3, which were caused by the crystal field, indicative of the similarity of the local symmetry of the crystal fields of the Eu3+ site in sample 1 and 3.50b Compared with emission lines of Eu3+ ions in panels a and b of Figure 11, no peak splitting of the 5D0 f 7F2 transition (located at 613 nm) was observed in Figure 11c, indicating a different local symmetry of the Eu3+ sites in NaLa(MoO4)2:Eu3+ with bipyramid morphology (sample 4) from the others (samples 1 and 3).50c The above luminescent properties of Eu3+ in NaLa(MoO4)2 nanoparticles indicate the successful doping of Eu3+ into the host NaLa(MoO4)2 lattice. Further investigation of the optical property is under way. Conclusion In summary, a simple hydrothermal synthesis of monodisperse NaLa(MoO4)2 and NaLa(MoO4)2:Eu3+ bipyramid nanocrystals with highly geometrical symmetry is demonstrated, and a fluorescent investigation revealed that NaLa(MoO4)2:Eu3+ bipyramid nanocrystals possess a dominating hypersensitive red emission 5D0 f 7F2 transition of Eu3+ at 613 nm without peak splitting. The formation of the tetragonal bipyramid morphology was dependent not only on the hydrothermal treatment time, reactant concentration, molar ratio of La3+ to MoO42-, and molar

Crystallization of NaLa(MoO4)2 Nanocrystals ratio of mixed surfactant to La3+ but also strongly on the used combination of surfactants as well. It is demonstrated experimentally that (1) deprotonated oleic acid (carboxylate anion, C17H33COO-) functions as the key bipyramid-determining surfactant in the NaLa(MoO4)2 system and (2) oleylamine molecules could promote the deprotonation of oleic acid into C17H33COO- with the highest efficiency among several amines investigated, which enables the strong and dense selective adsorption of the deprotonated oleic acid onto certain facets of NaLa(MoO4)2 nanocrystals. A possible oleic acid/oleylamine cooperative-controlled crystallization mechanism (CCM) for the formation of monodisperse tetragonal bipyramid NaLa(MoO4)2 nanocrystals is proposed. The formation of uniform tetragonal bipyramid morphology is mainly due to (1) the anisotropic character of NaLa(MoO4)2 crystalline nuclei, which leads to the selective adsorption of deprotonated oleic acid and (2) the cooperative-controlled effect between oleic acid and oleylamine of which the latter facilitates the release and preferential adsorption of carboxylate anions onto {101} and {001} faces because of the higher Na+ and La3+ density, which eventually leads to the formation of uniform tetragonal bipyramid morphology with exposed {101} faces. Acknowledgment. This work is financially supported by the National Natural Science Foundation of China Research (Grant 50672115, 50823007), the Shanghai Rising-Star Program (Grant 07QA14061), and the Shanghai Nanospecial Project (Grant 0852 nm03900). Supporting Information Available: Synthetic procedures of NaLa(MoO4)2 with different surfactant combinations and XRD patterns and TEM images of samples. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Han, J. T.; Huang, Y. H.; Wu, X. J.; Wu, C. L.; Wei, W.; Peng, B.; Huang, W.; Goodenough, J. B. AdV. Mater. 2006, 18, 2145. (2) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (3) Tao, A.; Sinsermsuksakul, P.; Yang, P. D. Angew. Chem., Int. Ed. 2006, 45, 4597. (4) Sun, Y. G.; Xia, Y. N. Science 2002, 298, 2176. (5) Tian, N.; Zhou, Z. Y.; Sun, S. G.; Ding, Y.; Wang, Z. L. Science 2007, 316, 732. (6) Pradhan, N.; Xu, H. F.; Peng, X. G. Nano Lett. 2006, 6, 720. (7) Tao, A. R.; Habas, S.; Yang, P. D. Small 2008, 4, 310. (8) Lee, S. M.; Jun, Y. W.; Cho, S. N.; Cheon, J. J. Am. Chem. Soc. 2002, 124, 11244. (9) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2002, 124, 3343. (10) Park, J.; An, K. J.; Hwang, Y. S.; Park, J. G.; Noh, H. J.; Kim, J. Y.; Park, J. H.; Hwang, N. M.; Hyeon, T. Nat. Mater. 2004, 3, 891. (11) Fan, L.; Guo, R. Cryst. Growth Des. 2008, 8, 2150. (12) Warner, J. H.; Cao, H. Q. Nanotechnology 2008, 19, 5. (13) Yin, Y.; Alivisatos, A. P. Nature 2005, 437, 664. (14) Gong, Q.; Li, G.; Qian, X. F.; Cao, H. L.; Du, W. M.; Ma, X. D. J. Colloid Interface Sci. 2006, 304, 408. (15) Li, C. C.; Shuford, K. L.; Chen, M. H.; Lee, E. J.; Cho, S. O. ACS Nano 2008, 2, 1760. (16) Lee, S. M.; Cho, S. N.; Cheon, J. AdV. Mater. 2003, 15, 441. (17) Zhao, N.; Wei, Y.; Sun, N. J.; Chen, Q. J.; Bai, J. W.; Zhou, L. P.; Qin, Y.; Li, M. X.; Qi, L. M. Langmuir 2008, 24, 991.

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