Polyol-Mediated Synthesis of Hexagonal LaF - American

Feb 20, 2009 - Key Laboratory of Excited State Physics, Changchun Institute of Optics, Fine Mechanics and Physics,. Chinese Academy of Sciences, ...
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CRYSTAL GROWTH & DESIGN

Polyol-Mediated Synthesis of Hexagonal LaF3 Nanoplates Using NaNO3 as a Mineralizer

2009 VOL. 9, NO. 4 1750–1756

Ruifei Qin,†,§ Hongwei Song,*,‡ Guohui Pan,†,§ Xue Bai,‡ Biao Dong,‡ Songhai Xie,| Lina Liu,†,§ Qilin Dai,†,§ Xuesong Qu,†,§ Xinguang Ren,† and Haifeng Zhao† Key Laboratory of Excited State Physics, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, 16 Eastern South-Lake Road, Changchun 130033, P. R. China, State Key Laboratory of Integrated Optoelectronics, College of Electronic Sciences and Engineering, Jilin UniVersity, 2699 Qianjin Street, Changchun 130012, P. R. China, Graduate School of Chinese Academy of Sciences, Beijing 100039, P. R. China, Shanghai Key Laboratory of Molecular Catalysis and InnoVatiVe Materials, Department of Chemistry, Fudan UniVersity, Shanghai 200433, P. R. China ReceiVed July 20, 2008; ReVised Manuscript ReceiVed January 9, 2009

ABSTRACT: Hexagonal LaF3 nanoplates have been successfully prepared via a polyol-mediated route with ethylene glycol (EG) as solvent. NaNO3, which was introduced into the reaction system as a mineralizer, played an important role in the formation process of these nanoplates. The size of the as-prepared nanoplates could be tuned by changing the NaNO3 content and the NH4F content. A possible formation mechanism of these nanoplates has been proposed. The effects of other alkali metal nitrates (LiNO3, KNO3, RbNO3 and CsNO3) and solvents (water, methanol, diethylene glycol, and glycerol) on the products have also been explored. The increased chemical potential caused by the addition of NaNO3 in the reaction system made LaF3 nanoplates formed initially unstable and transform into stable regular hexagonal LaF3 nanoplates with a bigger size by a slow dissolution-recrystallization process. 1. Introduction Recently, considerable attention has been paid to the shapeand size-controlled synthesis of various inorganic nanocrystals (NCs) because of their shape- and size-dependent properties and potential applications in various fields. To date, numerous chemical methods have been used to synthesize different kinds of inorganic NCs with multiple shapes and sizes. In these methods, organic additives and simple ions have often been used as shape modifiers to adjust and control the shape and size of the products.1 These shape modifiers can either promote or inhibit crystal growth through modifying crystal growth dynamically.1 Additionally, a slow crystallization process favors the formation of a regular shape of the products.2 Lanthanum fluoride (LaF3) is an excellent host matrix for luminescent materials because of its low phonon energies and has been used as an extreme pressure and antiwear additive in grease and as solid lubricant under high temperature because of its fairly low hardness, high melting point, and good resistance to thermal and chemical attack.3 Lanthanide-doped LaF3 NCs have received much attention for their wide applications in optics and optoelectronics (e.g., lighting and displays, optical amplifiers, and lasers), microelectronics and especially biological labels and have been prepared via various chemical methods such as modified precipitation,4 polyol,5 and solvothermal methods.6 Plate-built LaF3 cylinders were fabricated through oriented aggregation-based self-assembly of hexagonal LaF3 nanodisks with cavities using a simple hydrothermal process.3 Li et al. have synthesized oil-soluble, monodisperse, hexagonally shaped, and single-crystalline Yb-Er, Yb-Tm, and Yb-Ho codoped LaF3 nanoplates by decomposition of the respective RE(TFA)3 in a multiphase system using a solvother* Corresponding author. E-mail: [email protected]. Phone: 86431-86176320. Fax: 86-431-86176320. † Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences. ‡ College of Electronic Sciences and Engineering, Jilin University. § Graduate School of Chinese Academy of Sciences. | Fudan University.

mal technique.2b By using KBF4 as the fluoride source, LaF3: Eu3+ nanodisks with perfect crystallinity have been successfully synthesized through a facile and fast solution-based method.2c Zhang et al. have fabricated single-crystalline and monodisperse LaF3 triangular and hexagonal nanoplates by thermolysis of La (CF3COO)3 in oleic acid/1-octadecene.7 Liu et al. prepared LaF3: Ce3+,Tb3+ and LaF3:Tb3+ water-soluble nanoparticles using a simple method and demonstrated that these water-soluble scintillation nanoparticles can be potentially used to activate photodynamic therapy for deep cancer treatment.8 The polyol-mediated method, the background of which is the precipitation of a solid while heating suitable precursors in a high-boiling alcohol, has turned out to be a promising method for the preparation of many kinds of nanoscale materials,9 such as metal,10 oxides,11 phosphates,12 and fluorides.13 In this paper, we prepared hexagonal LaF3 nanoplates via this method. NaNO3 was introduced into the reaction system to act as a mineralizer and played a key role in the formation process of these nanoplates. The effects of NaNO3 content, NH4F content, other alkali metal nitrates (LiNO3, KNO3, RbNO3, and CsNO3) and other solvents (water, methanol, diethylene glycol and glycerol) on the products were demonstrated detailedly. A possible formation mechanism of these nanoplates was proposed. 2. Experimental Section Preparation. All chemicals were of analytical grade and were used as received. In a typical synthesis of LaF3 nanoplates, 16 mmol NH4F was dissolved in 20 mL of ethylene glycol (EG). Afterward, a solution of 2 mmol La(NO3)3 · 6H2O and 4mmol NaNO3 in 20 mL of EG was added under vigorous stirring. After stirring for 30 min, the mixed solution was transferred into a Teflon bottle of 50 mL held in a stainless steel autoclave, sealed, and maintained at 180 °C for 48 h. After being cooled to room temperature, the product was collected by centrifugation and washed several times with water and ethanol and finally dried at 80 °C for 12 h in vacuum. The final product was denoted as sample T. Tables S1-3 in the Supporting Information list detailed experimental conditions, crystal structures, and shapes of all the samples studied in this paper.

10.1021/cg800787p CCC: $40.75  2009 American Chemical Society Published on Web 02/20/2009

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Figure 1. Standard data of hexagonal LaF3 (JCPDS 72-1435) as a reference and (a) XRD pattern, (b) EDX spectrum, (c) FE-SEM image and (d) TEM image of sample T. (e, g) TEM images and (f, h) corresponding ED patterns of the nanoplates. (i) HRTEM image of a nanoplate. Characterization. The phase structure and purity of the as-prepared samples were characterized by X-ray power diffraction (XRD) on a Rigaku D/max-rA diffractometer with Cu KR radiation resource (λ ) 1.54078 Å) for 2θ ranging from 20 to 55°. The particle sizes and morphologies of the samples were determined using a field emission scanning electron microscope (FE-SEM, Hitachi S-4800) equipped with an energy dispersive X-ray (EDX) spectrometer and a transmission electron microscope (TEM, JEM 2010) operated at 200 kV. A highresolution TEM (HRTEM) image and electron diffraction (ED) patterns were obtained using the same TEM operated at 200 kV.

3. Results and Discussion Figure 1a shows the XRD pattern of sample T. All diffraction peaks for sample T could be readily indexed to the pure hexagonal phase of LaF3 according to the Joint Committee on Powder Diffraction Standards (JCPDS) file no. 72-1435. Energy-dispersive X-ray (EDX) analysis confirmed that the main elemental components of sample T were La and F (Figure 1b). The Si peak came from the Si substrate and C and O peaks should come from EG adsorbed on the surface of the product. SEM and TEM images showed sample T was composed of well-

dispersed hexagonal nanoplates characterized by being ∼20 nm in average thickness and ∼65 nm in average diameter (sideto-side length) (images c and d in Figure 1). It can be seen that these LaF3 nanoplates were well-defined and the size distribution of them was somewhat broad. Electron diffraction (ED) patterns taken from top/bottom surfaces and side faces of the nanoplates could be indexed as {0001} and {101j0} planes of hexagonal LaF3 structure (space group P6322), respectively (images f and h in Figure 1). The HRTEM image of a nanoplate revealed its highly crystalline nature and showed an interplanar spacing of ∼0.36 nm corresponding to the {112j0} planes (Figure 1i). The dimensions of the samples strongly depended on the contents of NaNO3 and NH4F, which will be discussed in detail below. It should be pointed out that when the effect of a reaction condition was studied, the other reaction conditions were kept same as those for the typical synthesis. Effect of NaNO3 Content. The samples synthesized with NaNO3 contents of 0, 0.5, 1, 2, and 3 mmol were denoted as samples N1-5, respectively. Figure 2a depicts XRD patterns

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Figure 2. (a) XRD patterns of samples synthesized with different NaNO3 contents: (1) sample N1, (2) sample N2, (3) sample N3, (4) sample N4, (5) sample N5. FE-SEM images of these five samples: (b) sample N1, (c) sample N2, (d) sample N3, (e) sample N4, (f) sample N5.

Figure 3. (a) XRD patterns of samples synthesized with different NH4F contents: (1) sample F1, (2) sample F2, (3) sample F3, (4) sample F4, (5) sample F5. FE-SEM images of these five samples: (b) sample F1, (c) sample F2, (d) sample F3, (e) sample F4, (f) sample F5.

of samples N1-5. It can be seen that all five samples crystallized in the hexagonal LaF3 structure. Figure 2b-f shows FE-SEM images of these five samples. As shown in Figure 2b, when no NaNO3 was used, sample N1 was composed of platelike NCs whose shape was irregular and size distribution was broad. However, once NaNO3 was introduced into the reaction system, a significant change took place in the shape of these NCs. When 0.5 mmol NaNO3 was used, sample N2 was composed of regular hexagonal nanoplates and the size distribution of them was much narrower compared with sample N1 synthesized without using NaNO3 (Figure 2c). As can be seen from Figure 2b-f and Figure 1c, the shape of the nanoplates became more and more regular and the size of the nanoplates became bigger and bigger with increasing the NaNO3 content. The average thicknesses of the nanoplates were about 6, 8, 13, 18, and 20 nm and the average diameters were about 28, 35, 40, 50, and 65 nm, corresponding to the NaNO3 contents of 0.5, 1, 2, 3, and 4 mmol, respectively. When the NaNO3 content exceeded 4 mmol, there was no further change in the shape of these NCs. These results demonstrated that NaNO3 played an important role in the formation process of these regular hexagonal LaF3 nanoplates and that the dimensions of these nanoplates could be tuned by changing the NaNO3 content. Effect of NH4F Content. The samples synthesized with the NH4F contents of 6, 11, 21, 26, and 36 mmol were denoted as samples F1-5, respectively. XRD patterns revealed that all the five samples had a hexagonal LaF3 structure and that the crystallinity became better and better with the increase in the NH4F content (Figure 3a). Figure 3b-f shows FE-SEM images of samples F1-5, respectively. When the NH4F content was stoichiometric, i.e., 6 mmol, sample F1 was composed of ∼15 nm NCs whose shape seemed to be spherical (Figure 3b). However, when the NH4F content exceeded 6 mmol, hexagonal nanoplates were formed (Figure 3c-f). In addition, the diameter of the nanoplates increased with the NH4F content, although the thickness of these

nanoplates was about 20 nm all the time. The average diameters of these nanoplates were about 30, 100, 200, and 300 nm corresponding to the NH4F contents of 11, 21, 26, and 36 mmol, respectively. From Figure 3b-f and Figure 1c, it can be concluded that excessive NH4F was necessary for the formation of hexagonal LaF3 nanoplates, and that the shape of these LaF3 nanoplates became more irregular when the NH4F content exceeded a certain value. In principle, crystal morphology is governed by the crystal structure and the growth surroundings.1,14 Although the unitcell symmetry governs the spatial relations between the faces, their selection is mechanistically determined by the relative rates of growth along different crystallographic directions.1,14 In general, faces perpendicular to the fast directions of growth have smaller surface areas, and slow growing faces therefore dominate the final morphology.14 For example, hexagonal crystal seeds have an anisotropic unit-cell structure, which can induce anisotropic growth along crystallographically reactive directions, and thus hexagonal plates or rods are usually obtained depending on the relative growth rates along the 〈0001〉 and 〈101j0〉 directions. It has been well-documented that organic additives and simple ions can adsorb on certain surfaces and restrict the growth along the directions perpendicular to these surfaces. By this way, organic additives and simple ions in a reaction system can remarkably influence the final shape. NaYF4 nanoplates and nanorods with different slendernesses were synthesized by changing the molar ratio of NaF to Y3+ in a water/alcohol/oleic acid system.15 The influence of the molar ratio of NaF to Y3+ on the shape was attributed to the greater capping effect of Fon the {101j0} crystal planes than that on the {0001} planes. Li et al. have fabricated the anisotropic hexagonal rod or prismatic geometries of the β-NaYF4 crystals with different slendernesses by tuning the pH values in the initial reaction solutions in the presence of the organic additive trisodium citrate (Cit3-) with a hydrothermal route.16 They explained that different pH values would bring on different ability of Cit3- to adsorb on certain crystal planes, directly leading to different growth velocities of

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Figure 4. XRD patterns of samples synthesized with different reaction times: (a) sample T1, (b) sample T2, (c) sample T3, (d) sample T4, (e) sample T5, (f) sample T6.

the different crystal planes, resulting in the formation of various geometries of the β-NaYF4 crystals. For NCs with a hexagonal crystal structure and hexagonal plate shape, the surfaces are typically {0001} for top/bottom planes and a family of six energetically equivalent {101j0} side planes ((101j0), (1j010), (01j10), (011j0), (11j00), and (1j100)) on the basis of the known or similar models.16 According to the above experimental results, the effect of the NH4F content on the dimensions of these LaF3 nanoplates can be explained as follows. Excessive F- inevitably capped on crystal surfaces of the obtained LaF3 NCs due to the strong coordination effect between F- and La3+, but the capping effect of F- on the {0001} crystal planes was greater than that on the {101j0} planes, consequently prohibiting the growth along ([0001] directions and driving the growth along six symmetric directions of ([101j0], ([11j00], and ([011j0]. As a result, the diameter of these nanoplates increased with the NH4F content, but the thickness of them was about 20 nm all the time. Formation Process and Mechanism. To study the formation process of these hexagonal LaF3 nanoplates, we carried out timedependent experiments under the reaction conditions same as those for the synthesis of sample T except for the reaction time. The samples synthesized with the reaction times of 0, 1, 2, 8, 16, and 24 h were denoted as samples T1-6, respectively. That the reaction time was 0 h means sample T1 was collected immediately after deposition at room temperature without any thermal treatment. Figure 4 shows XRD patterns of samples T1-6. When the reaction time was 0 or 1 h, the obtained sample crystallized in the hexagonal LaF3 structure (patterns a and b in Figure 4). As the reaction was extended to 2 h, the hexagonal LaF3 structure disappeared and a new phase emerged (Figure 4c). Diffraction peaks shown in Figure 4c could not be indexed to any phase of LaF3 presented in the JCPDS. We speculated that sample T3 was an intermediate product and the new phase was an intermediate phase in the formation process of these hexagonal LaF3 nanoplates. EDX results revealed that sample T3 contained more Na element than samples with the hexagonal LaF3 structure, such as samples T1, T2 and T (Figure 1b and the Supporting Information, Figure S1). Na+ might get into the crystal lattice of sample T3 and lead it to crystallize in the unknown intermediate phase. As shown in Figure 4d, the hexagonal LaF3 phase appeared again when the reaction time was 8 h. Along with a further increase of the reaction time, diffraction peaks of the hexagonal LaF3 phase became stronger and stronger and those of the intermediate phase became weaker and weaker (Figure 4d-f). When the reaction proceeded 24 h,

Figure 5. FE-SEM images of samples synthesized with different reaction times: (a) sample T1, (b) sample T2, (c) sample T3, (d) sample T4, (e) sample T5, (f) sample T6.

the intermediate phase disappeared completely and only the hexagonal LaF3 phase could be detected by XRD (Figure 4f). Figure 5 shows FE-SEM images of samples T1-6. Sample T1 with a reaction time of 0 h consisted of several nanometersized nanoparticles that aggregated severely (Figure 5a). When the reaction time was 1 h, the sample changed from nanoparticles to nanoplates of which the average diameter was ∼17 nm and the average thickness was ∼5 nm (Figure 5b). These nanoplates seemed to be irregular in shape and not uniform in size. Sample T3 with a reaction time of 2 h, namely the intermediate product, was composed of nanoparticles in the size of ∼10 nm. As the reaction was extended to 8 h, besides the dominant nanoparticles, a small quantity of hexagonal nanoplates existed in sample T4 (Figure 5d). On the basis of the XRD results presented in the last paragraph, we can conclude that these hexagonal nanoplates crystallized in the hexagonal LaF3 structure. With further prolonging the reaction time, these nanoparticles as an intermediate product gradually dissolved, and hexagonal nanoplates became bigger and bigger in size and more and more regular in shape (Figure 5d-f). When the reaction time reached 24 h, regular hexagonal nanoplates dominated sample T6 and only a very small quantity of nanoparticles had not yet dissolved. When the reaction proceeded 48 h, regular hexagonal nanoplates were the exclusive product morphology formed (Figure 1c). To shed light on the function of NaNO3 in the formation process of these hexagonal LaF3 nanoplates, control experiments were carried out in which NaNO3 was not used. The samples synthesized in the absence of NaNO3 and with the reaction times of 0, 1, 2, 8, 16, and 24 h were denoted as samples t1-6, respectively. Figure 6a shows XRD patterns of this group of samples. It can be seen that samples t1-6 all adopted a hexagonal LaF3 crystal phase, although the reaction time was different, which differed from the case where NaNO3 was used. Figure 6b-g shows the corresponding particle size and shape variation of this group of samples. It seemed that sample t1 with a reaction time of 0 h contained aggregating nanoparticles

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Figure 7. (a) XRD patterns of samples prepared using other alkali metal nitrates as mineralizers: (1) sample A1, (2) sample A2, (3) sample A3, (4) sample A4. FE-SEM images of these four samples: (b) sample A1, (c) sample A2, (d) sample A3, (e) sample A4.

Figure 6. (a) XRD patterns of samples synthesized in the absence of NaNO3 and with different reaction times: (1) sample t1, (2) sample t2, (3) sample t3, (4) sample t4, (5) sample t5, (6) sample t6. FE-SEM images of these six samples: (b) sample t1, (c) sample t2, (d) sample t3, (e) sample t4, (f) sample t5, (g) sample t6.

(Figure 6b). When the reaction time was equal to or longer than 2 h, the obtained sample was composed of hexagonal nanoplates of which the shape was more irregular, and the thickness of these nanoplates increased with the reaction time (Figures 6c-g). Compared with the case where NaNO3 was used, the shape variation of this group of samples with reaction time was obviously different. On the basis of these time-dependent experiments, a possible formation mechanism of these regular hexagonal LaF3 nanoplates in the presence of NaNO3 is proposed as follows. The liquid precipitation reaction between La(NO3)3 and NH4F in EG at room temperature yielded several nanometer-sized LaF3 nanoparticles that aggregated severely and had the hexagonal crystal structure. These LaF3 nanoparticles were not stable because of their hexagonal crystal structure and transformed into hexagonal LaF3 nanoplates in the first one hour of the reaction. If NaNO3 was not used, these LaF3 nanoplates became thicker and thicker with increasing the reaction time by an Ostwald ripening process in which the larger nanoplates grew on the expense of the smaller ones. If NaNO3 was used, it could act as a mineralizer and increase the chemical potential of the solution. This led these LaF3 nanoplates to be unstable. Via an intermediate product of nanoparticles and by a dissolutionrecrystallization process, these unstable LaF3 nanoplates trans-

formed into stable regular hexagonal nanoplates with a bigger size. From the time-dependent experiments, we can see that the dissolution-recrystallization process was very slow. It has been well-documented that slow nucleation and growth processes can produce high-quality and regular shape NCs.2 We thought this slow dissolution-recrystallization process favored the formation of these regular hexagonal nanoplates. The effects of alkali metal salts on the shape of inorganic NCs have been reported in some literature. Ultralong and singlecrystalline Cd(OH)2 nanowires were fabricated by a hydrothermal method in the presence of alkali metal salts.17 One effect of these salts was thought to increase the chemical potential of the solution, and higher chemical potential conditions would be advantageous for one-dimensional nanostructure growth. Wang et al. controllably synthesized various bismuth ferrite compounds by a hydrothermal method assisted by alkali metal ions (K+, Na+, and Li+), and they believed that alkali metal ions exerted a pronounced influence on the solubility of Bi3+ and Fe3+ hydroxides, which led to the formation of rhombohedral BiFeO3, orthorhombic Bi2Fe4O9, and cubic Bi12(Bi0.5Fe0.5)O19.5, respectively.18 WO3 nanowires were fabricated by a hydrothermal method in the presence of K2SO4, and the authors thought K2SO4 might change the compositions and the corresponding properties of the solution.19 In this work, the function of NaNO3 in the formation process of these hexagonal LaF3 nanoplates is not very clear and needs to be further investigated. Effects of Other Alkali Metal Nitrates and Solvents. As demonstrated above, NaNO3 played an important part in the formation process of these regular hexagonal LaF3 nanoplates via the polyol-mediated route with EG as solvent. In order to examine if regular hexagonal LaF3 nanoplates could be prepared using other alkali metal nitrates and solvents, we did two sets of experiments. In the first set of experiments, LiNO3, KNO3,

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effects of different alkali metal nitrates on the formation process of the products were different; alkali metal ions and solvents might adsorb on certain crystal planes of the products, but different alkali metal ions and solvents had different adsorption effects on these crystal planes. These might be the reasons why different alkali metal nitrates and solvents would lead to the different shapes of the products. It should be noted that it was not NO3- but Na+ that led to these regular hexagonal LaF3 nanoplates. The effects of different alkali metal nitrates on the shapes of products can confirm that point. We have also used NaCl as a mineralizer and obtained similar regular hexagonal LaF3 nanoplates, which can also confirm that point. 4. Conclusions

Figure 8. (a) XRD patterns of samples prepared using water, MeOH, DEG, and Glyc. as solvents: (1) sample S1, (2) sample S2, (3) sample S3, (4) sample S4. FE-SEM images of these four samples: (b) sample S1, (c) sample S2, (d) sample S3, (e) sample S4.

RbNO3, and CsNO3 replaced NaNO3 in the reaction system and the obtained products were denoted as samples A1-4, respectively. All the four samples crystallized in the hexagonal LaF3 structure (Figure 7a). Figure 7b-e shows FE-SEM images of these four samples. When LiNO3 was used, sample A1 was composed of ∼15 nm NCs with a rough sphere shape (Figure 7b). When KNO3 was used, hexagonal nanoplates with an average diameter of about 110 nm and an average thickness of about 8 nm were observed in sample A2 (Figure 7c). When RbNO3 and CsNO3 were used, samples A3 and A4 had a similar shape and they all seemed to contain irregular hexagonal nanoplates with a broad size distribution (images d and e in Figure 7). In the second set of experiments, EG was replaced by water, methanol (MeOH), diethylene glycol (DEG), and glycerol (Glyc.) as solvents and the corresponding products were denoted as samples S1-4, respectively. XRD patterns and FE-SEM images are depicted in Figure 8. When water, MeOH, and Glyc. were used, samples S1, S2, and S4 all adopted the hexagonal LaF3 structure (1, 2, and 4 in Figure 8a). Samples S1 and S2 were composed of nanoparticles and irregular hexagonal nanoplates with a broad size distribution, respectively (images b and c in Figure 8). Sample S4 seemed to contain hexagonal nanoplates with an irregular shape and a broad size distribution (Figure 8e). When DEG was used, the XRD pattern and morphology of sample S3 both were same as those of sample T3 (see Figures 8a3, 8d, 4c, and 5c). These two sets of experiments revealed that only when NaNO3 was used and EG acted as solvent could regular hexagonal LaF3 nanoplates be obtained. Different alkali metal nitrates and solvents resulted in different shapes. In the reaction process, alkali metal nitrates might act as mineralizers, but the

In summary, we have successfully prepared hexagonal LaF3 nanoplates via a polyol-mediated route with EG as solvent. NaNO3 in the reaction system played a key role in the formation process of these nanoplates. Effects of the NaNO3 content and the NH4F content on these nanoplates have been demonstrated detailedly. The diameter of these nanoplates increased with the NH4F content with the thickness kept unchanged, which was ascribed to that the capping effect of F- on the {0001} crystal planes was greater than that on the {10-10} planes. We speculate that NaNO3 acted as a mineralizer and increased the chemical potential of the solution. The increased chemical potential made LaF3 nanoplates formed initially unstable and transform into stable regular hexagonal LaF3 nanoplates with a bigger size by a dissolution-recrystallization process. The hexagonal crystal structure and slow dissolution-recrystallization process led to the regular hexagon shape of these nanoplates. It was also demonstrated that different alkali metal nitrates and solvents would lead to different shapes of the products. These LaF3 nanoplates should have potential applications in optics, optoelectronics, biological detection, and tribology. Furthermore, this study would be suggestive for the controlled growth of inorganic NCs, especially for that using a mineralizer. Acknowledgment. This work is supported by the National Nature Science Foundation of China (Grants 50772042, 10704073, and 10504030) and the 863 project of China (2007AA032314). Supporting Information Available: Summary of detailed experimental conditions, crystal structures and shapes of the products, and EDX spectra of samples T1-3 and S3 (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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