J. Phys. Chem. B 2006, 110, 23247-23254
23247
Selected-Control Hydrothermal Synthesis and Formation Mechanism of Monazite- and Zircon-Type LaVO4 Nanocrystals Weiliu Fan,† Xinyu Song,‡ Yuxiang Bu,‡ Sixiu Sun,*,†,‡ and Xian Zhao*,† State Key Laboratory of Crystal Materials and Department of Chemistry, Shandong UniVersity, Jinan, 250100, People’s Republic of China ReceiVed: July 24, 2006; In Final Form: September 8, 2006
Selective-controlled structure and shape of LaVO4 nanocrystals were successfully synthesized by a simple hydrothermal method without the presence of catalysts or templates. It was found that tuning the pH of the growth solution was a crucial step for the control of the structure transformation, that is, from monoclinic (m-) to tetragonal (t-) phase, and morphology evolution of LaVO4 nanocrystals. Further studies demonstrated that the morphology of the product had a strong dependence on the initial lanthanum sources. In the La(NO3)3 or LaCl3 reaction system, pure t-LaVO4 nanorods with uniform diameters about 10 nm could be obtained. But when using La2(SO4)3 as the lanthanum source, we can get t-LaVO4 nanowiskers with broomlike morphology. The detailed systematic study had shown that a special dissolution-recrystallization transformation mechanism as well as an Ostwald ripening process was responsible for the phase control and anisotropic morphology evolution of the LaVO4 nanocrystals. As a result, the controlled synthesis of m- and t-LaVO4 not only has great theoretical significance in studying the polymorph control and selective synthesis of inorganic materials but also benefits the potential applications based on LaVO4 nanocrystals owing to the unusual luminescent properties induced by structural transformation.
1. Introduction Nanomaterials are of current interest owing to their unique properties and potential applications in catalysis, optoelectronic devices, and so on.1-6 Inherent crystallographic structures of materials play crucial roles in both physical and chemical properties, so phase control of nanocrystals is quite important in preparative chemistry and materials science.7,8 The shape control of nanosized crystals is another important factor for an as-prepared product.2,9-14 The elongate one-dimensional (1D) morphology such as nanorods, nanowires, nanowhiskers, nanobelts, and nanotubes has been studied extensively because of their specific density of electronic state and wide applications as described in some reviews.2,9,15 Studies of phase and shape control of nanocrystals may greatly contribute to the understanding of quantum phenomena and give deep insights into the crystallization mechanism of materials in nanosized scale. However, the challenge of synthetically controlling the phase and shape of nanomaterials has been met with limited success.16-20 Lanthanide orthovanadates are of interest because of their unusual magnetic characteristics and useful luminescent properties.21-24 These materials have been employed as highly efficient laser diode pumped microlasers, as an efficient phosphor, and as very attractive polarizer material. Lanthanide orthovanadates crystallize in two polymorphs, namely, monoclinic (m-) monazite type and tetragonal (t-) zircon type. Generally, with increasing ionic radius, Ln3+ ions show a strong tendency toward monazite-structured orthovanadate because of its higher oxygen coordination number of 9 as compared with * To whom correspondence should be addressed. E-mail: fwl@ sdu.edu.cn. Tel: 86-531-88364879. Fax: 86-531-88564464. † State Key Laboratory of Crystal Materials. ‡ Department of Chemistry.
8 of the zircon one. For this reason LaVO4 chooses the monazite type as the thermodynamically stable state, while the other orthovanadates normally exist in the zircon type.25 LaVO4 is neither a suitable host for luminescent activators26,27 nor a promising catalyst21 owing to its ordinary monoclinic (m-) monazite structure campared to other orthovanadates. On the contrary, t-LaVO4 is expected to possess superior properties and is expected to be a promising phosphor candidate, as revealed by Yan’s research.28 The main challenge lies in the synthesis of zircon type LaVO4, since it is metastable and cannot be obtained by conventional methods. As a result, the selective synthesis of m- and t-LaVO4 not only has great theoretical significance in studying the polymorph conversion/phase transition processes and the structure-dependent properties but also is very important for their potential applications. Generally, thermodynamically stable m-LaVO4 bulk crystals can be obtained by conventional solid-state reaction.29 To obtain t-LaVO4, the metastable phased material, solution processes categorized in “soft chemistry” sometimes work well.25,30,31 Yan and co-workers have generated t-LaVO4 nanorods with La(NO3)3 and Na3VO4 as starting materials by an ethylenediaminetetraacetic acid [EDTA or H4L, where L4- ) (CH2COO)2N(CH2)2N-(CH2COO)24-] mediated hydrothermal method.32 In their case, organic molecules additives play an important role in the polymorph selection of the LaVO4 system. Although good control over nanoparticle structure and shape can be realized in these syntheses, the introduction of additives to the reaction system means a much more complicated process and may bring about an increase of impurity concentration in the final product. Therefore it is required to develop a more facile and reproducible synthetic method of zircon-type LaVO4 that has hopefully high crystallinity. Recently, our group has demonstrated a facile hydrothermal route for the generation of zircon-type LnVO4 (Ln ) La, Nd,
10.1021/jp0646832 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/20/2006
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TABLE 1: LaVO4 Nanocrystals Obtained under Different pH Conditions with La(NO3)3 as the La Source through a Simple Hydrothermal Route (at 180 °C for 48 h) preparative conditions lanthanum source
final pH
temp (°C)
time (h)
morphology
structure
La(NO3)3 La(NO3)3 La(NO3)3 La(NO3)3 La(NO3)3 La(NO3)3
2.5 (no NaOH added) 3.5 (1.6 mL, 1 M NaOH) 4.5 (2.4 mL, 1 M NaOH) 5.5 (2.8 mL, 1 M NaOH) 6.0 (3.0 mL, 1 M NaOH) 6.5 (3.2 mL, 1 M NaOH)
180 180 180 180 180 180
48 48 48 48 48 48
nanoparticles nanoparticles/rods nanorods nanorods nanorods (lower aspect ratio) nanoparticles
monoclinic monoclinic/ tetragonal tetragonal tetragonal tetragonal tetragonal
Sm, Eu, Dy) nanorods without any templates and/or catalysts.33 In this method, LnVO4 seeds were formed by reaction of lanthanide nitrate and sodium m-vanadate in stoichiometric. Nanorods grew over the course of hydrothermal treatment through an Ostwald ripening process by controlling the solution pH. Despite that the capability and feasibility of this method have been successfully illustrated, systematic studies over controllable phase transformation and morphology evolution of both m- and t-LaVO4 nanocrystals are limited, and the growth mechanism of this process is yet to be elucidated. Here we report some progress on this matter: m- and t-LaVO4 nanocrystals with high crystallinity and various morphologies were successfully synthesized by a hydrothermal method without any additives. It was found that reaction parameters governing both the intrinsic crystalline phase (monazite type and/or zircon type of LaVO4) and the growth regime (thermodynamic vs kinetic) are found to be important for the control of the structure transformation (from monoclinic to tetragonal) and morphology evolution of LaVO4 nanocrystals, which include nanoparticles, 1D nanorods, and nanowhiskers. 2. Experimental Section Analytical grade lanthanum nitrate, lanthanum chloride, lanthanum sulfate, and anhydrous sodium m-vanadate (NaVO3) were purchased from Shanghai Chemical Reagent Company and were all used without further purification. 1. Synthesis. The preparation of LaVO4 nanocrystals was carried out by a hydrothermal technique. In a typical procedure, 4 mL of NaVO3 (0.4 M) aqueous solution were added dropwise into 4 mL of La(NO3)3 aqueous solution (0.4 M) at room temperature under constant magnetic stirring; the solution turned yellow immediately after the addition of the NaVO3. The obtained yellow suspension was stirred for about 10 min, andthen 1 M NaOH aqueous solution was added dropwise with stirring to adjust the solution pH to a desirable value. To investigate the pH effect on the structure and morphology of the final product, the amount of added NaOH (1 M) was varied from zero to 3.2 mL; other parameters were kept constant. Then the suspension was stirred for another 10 min. After dilution with deionized water, the resulting yellow suspension was poured into into a 25 mL Teflon-lined stainless autoclave, sealed, and heated at 180 °C for 48 h in a digital type temperature controlled oven, and then allowed to cool to room temperature naturally. The final pH of the growth solution was measured. It was found to vary from 2.5 to 6.5, corresponding to different amount of added NaOH (1 M). The products were recovered by filtration, washed with deionized water and absolute alcohol, and finally dried at 80 °C in air for further characterization. For developing more comprehensive understanding of the structure transformation and morphology evolution of LaVO4 nanocrystals, it is essential to systematically investigate the effect of the lanthanum source on the formation of the nanocrystals. Other La sources, such as LaCl3 and La2(SO4)3 were selected to fulfill this purpose. In these two systems the same hydro-
thermal method was used, and the pH of growth solution was controlled carefully. The results will be discussed elaborately hereinafter. 2. Characterization. The phase purity and crystal structure of the obtained samples were examined by X-ray diffraction (XRD) using a Japan Rigaku D/Max-γA rotation anode X-ray diffractometer equipped with graphite monochromatized Cu KR radiation (λ ) 1.54178 Å), employing a scanning rate of 0.02° s-1 in the 2θ range from 10° to 75°. The operation voltage and current were maintained at 40 kV and 40 mA, respectively. The morphologies and micro- and nanostructure of the as-synthesized LaVO4 products were characterized by transmission electron microscopy (TEM), carried out using JEOL JEM-100 CX¢o` at an accelerating voltage of 100 kV. Further structural characterization was performed on a Philips Tecnai F30 high-resolution field-emission transmission electron microscope (HRTEM) operating at 300 kV. The samples for these measurements were dispersed in absolute ethanol by being vibrated in the ultrasonic pool. Then, the solutions were dropped onto a copper grid coated with amorphous carbon films and dried in air before performance. Fluorescence spectra were recorded on a Hitachi FLS900 spectrophotometer equipped with a 150 W Xe-are lamp at room temperature, and for comparison of the different samples, the emission spectra were measured at a fixed band-pass of 0.2 nm with the same instrument parameter (1.0 nm for excitation split, 1.0 nm for emission split, and 700 V for PMT voltage). 3. Results and Discussion 3.1. pH Effect. To pursue the growth mechanism of LaVO4 nanocrystals, the pH-dependent experiments were carried out to monitor the crystallization process of the products (Table 1). During the experiment, the most interesting phenomenon is that the colors of the as-grown products gradually change from yellowish-green (2.5 e pH < 3.0) to yellow (3.0 e pH < 3.5), then pale yellow (3.5 e pH < 4.0), and finally white powders (final pH g 4.0). This difference of the optical behavior may be related to the variation of the structure and/or shape of the resulting nanocrystals.34 To investigate the reason of the color change, XRD and TEM were employed to analyze the crystalline structure and morphology of the samples. Structure Transformation. The transformation of crystal structure and phase purity of the samples was examined by powder X-ray diffraction (XRD), which as shown in Figure 1. Figure 1a shows a typical XRD pattern of LaVO4 as-prepared with the final pH ) 2.5, that is, with no NaOH added to the reaction system. All the peaks could be readily indexed to a pure monoclinic phase with lattice constants comparable with the values given in JCPDS (25-0427), and no traces of other phases are examined. It is understandable that LaVO4 crystallizes in its thermodynamic state, the monazite structure. The intensity of correlative XRD peaks of monoclinic becomes weaker with the final pH values increased from 2.5 to 3.5. A mixture of monoclinic and tetragonal can be observed from Figure 1b, the peaks at 18°, 24°, 33°, 47°, 59°, and 61° can be easily indexed
Monazite- and Zircon-Type LaVO4 Nanocrystals
J. Phys. Chem. B, Vol. 110, No. 46, 2006 23249
Figure 1. XRD patterns of LaVO4 nanocrystals obtained under different pH conditions with La(NO3)3 as the La source at 180 °C for 48 h: (a) pH ) 2.5; (b) pH ) 3.5, peaks signed with “9” were indexed to m-LaVO4; (c) pH ) 4.5; (d) pH ) 6.0. Figure 3. HRTEM images and corresponding fast two-dimensional Fourier transform (FFT) patterns of the selective-synthesized m- and t-LaVO4 nanocrystals with La(NO3)3 as the La source: (a, b, and c) m-LaVO4 nanocrystals as-obtained at pH ) 2.5 through a hydrothermal route (at 180 °C for 48 h); (d and e) t-LaVO4 nanocrystals as-obtained at pH ) 4.5 through a hydrothermal route (at 180 °C for 48 h).
Figure 2. The corresponding TEM images of LaVO4 nanocrystals obtained under different pH conditions with La(NO3)3 as the La source at 180 °C for 48 h: (a) pH ) 2.5; (b) pH ) 3.5; (c) pH ) 4.5; (d) pH ) 6.0.
to the tetragonal phase of LaVO4 (JCPDS 32-0504). Pure t-LaVO4 could be ultimately obtained at the final pH g 4.5. Figure 1c,d gives the XRD pattern of the product obtained at final pH ) 4.5 and 6.0, respectively, from which we can see that the monoclinic phase of LaVO4 disappeared, and all of the peaks could indexed to a tetragonal structure for bulk LaVO4: space group I41/amd with cell constants a ) 7.50 Å and c ) 6.59 Å (JCPDS 32-0504). Morphology Evolution. As a confirmation of XRD analysis, Figure 2 shows typical TEM images of the LaVO4 nanocrystals corresponding to Figure 1. From the image in Figure 2a, it can be found that the obtained m-LaVO4 at pH ) 2.5 exhibits the forms of small irregularly shaped nanoparticles with an average diameter as 30 nm and weak aggregation. Increasing the amount of NaOH in the reaction mixture leads to the shape of the asobtained nanostructures evolved to 1D by experiencing a nanoparticles and nanorods coexistence period. When there is an increase of the reaction pH to 3.5, it is observed from the TEM image shown in Figure 2b that besides nanoparticles, the sample also consists of large aspect ratio individual nanorods. The length and diameter of the nanorods are not uniform, and most of them do not exceed 50 nm in diameter, with an aspect ratio about 5. The pH-dependent experiments were taken subsequently by increasing the pH to 4.5. Figure 2c indicates
that when the end pH value is 4.5, the obtained pure t-LaVO4 almost exhibits uniform rod shape with an average diameter of about 20 nm. When the final pH value was increased to 6.0, rods of lower aspect ratio were detected as shown in Figure 2d. Upon increasing the pH to a higher value (6.5), the habit of anisotropic growth is destroyed and no rodlike morphology can be detected (data not shown). Therefore, on the basis of the investigation of the XRD patterns and corresponding TEM images, it is said that tuning the pH of the growth solution was a crucial step in the control of the structure and morphology of LaVO4 preparation. The final pH value of 4.5∼5.5 was found optimal in order to get the pure t-LaVO4 nanorods with uniform diameter of about 20 nm. Structure Characterization of LaVO4 Nanocrystals. Further structural analysis of these LaVO4 nanostructures were carried out using high-resolution field-emission TEM (HRTEM) combined with a fast two-dimensional Fourrier transform (FFT) analysis technique. At a high resolution (Figure 3a) fringes are observed spreading uniformly across the individual m-LaVO4 particle (180 °C, 48 h, pH ) 2.5), indicating these nanoparticles are highly crystallized. A lattice-resolved HRTEM image taken from the marked area (as signed in Figure 3a) of the individual nanoparticle is shown in Figure 3b and further reveals its detailed crystallographic structures. The lattice image clearly reveals the (111) lattice planes of m-LaVO4 with a d spacing of about 0.365 nm. The corresponding FFT pattern (Figure 3c) suggests that the nanoparticles grew as single crystals m-LaVO4. Figure 3d shows the HRTEM image of as-prepared t-LaVO4 nanorod (180 °C, 48 h, pH ) 4.5). It shows that t-LaVO4 has an obvious anisotropic growth habit, and the nanorod is structurally uniform, free from defects and dislocations. The lattice spacings of about 0.374 and 0.330 nm correspond to the (200) and (002) plane of tetragonal phase LaVO4, respectively. Figure 3e is a corresponding FFT pattern of the HRTEM image (Figure 3d), which can be indexed to the [010] zone of a tetragonal LaVO4. Further studies of the HRTEM image and FFT pattern demonstrate that the direction of the t-LaVO4 nanorod growth is along the c axis, that is, the [001] direction, as indicated with an arrow in Figure 3d.
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TABLE 2: LaVO4 Nanocrystals Obtained under Varied Hydrothermal Temperature and Time Conditions at pH ) 4.5 with La(NO3)3 as the La Source hydrothermal conditions lanthanum source
pH
temp (°C)
time (h)
structure
morphology
La(NO3)3 La(NO3)3 La(NO3)3 La(NO3)3 La(NO3)3 La(NO3)3 La(NO3)3 La(NO3)3 La(NO3)3 La(NO3)3
4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5
room temp 90 120 150 180 180 180 180 180 180
48 48 48 48 48 6 12 24 48 60
monoclinic monoclinic monoclinic/ tetragonal tetragonal tetragonal monoclinic/ tetragonal tetragonal tetragonal tetragonal tetragonal
nanoparticles nanoparticles/rods nanoparticles/rods nanorods nanorods nanoparticles/rods nanorods nanorods nanorods nanorods
3.2. Influence of Hydrothermal Conditions. Although the pH is the crucial factor which controls the structure transformation and shape evolution of LaVO4 nanocrystals, it seems that the hydrothermal temperature and crystallization time are also the parameters to affect the nanocrystal formation. With fixed pH value (4.5), while other chemical contents were kept constant, the hydrothermal temperature and time were varied in order to investigate their effect on the LaVO4 nanocrystals growth. The results are summarized in Table 2. Figure 4A shows the XRD patterns of the products obtained at different temperature. It is obvious that m-LaVO4 [see Figure 4A (a, b, c)] was the main product at lower temperatures (below 120 °C). This indicates lower temperature is not beneficial for the transformation from m- to t-LaVO4, which mainly originates from the lack of enough energy to speed up the dissolution, renucleation, and crystallization process. A hydrothermal temperature no less than 150 °C was required to obtain pure t-LaVO4 [Figure 4A, (d)]. The effect of reaction time, or so-called crystallizing time, on the phase composition was also investigated (Figure 4B).
Figure 4. XRD patterns of LaVO4 nanocrystals obtained under varied hydrothermal temperature (A) and time (B) conditions at pH ) 4.5 with La(NO3)3 as the La source.
When the product was hydrothermally treated at 180 °C for 6 h, the formation of m-LaVO4 was restrained, and the traces of t-LaVO4 appeared, as can be seen from Figure 4B (a). When the above product was further hydrothermally treated, we can see that m-LaVO4 gradually vanished and t-LaVO4 become dominant [Figure 4B (b)]. With prolonged treatment to more than 24 h, pure t-LaVO4 was obtained, and the crystallinity increased with the increase in duration time, as can be seen from Figure 4B(c and d). The corresponding TEM images of these products are shown in Supporting Information (Figures S1 and S2). The TEM analyses confirmed that the structure transformation is accompanied by a morphological change from irregular particles to 1D rodlike ones. It seemed that the relatively higher reaction temperature (g150 °C) and longer duration time (g 24 h) could provide enough energy required for the activation of specific faces for the anisotropic growth of pure t-LaVO4 nanorods. 3.3. Influence of Lanthanum Sources. Studies that have attempted to characterize the mechanisms controlling the phase and shape of nanocrystals produced through a liquid phase synthesis approach have found the process to be highly dependent on the various chemical species in solution and the thermodynamic stability of the nanoparticle crystalline domains. For developing more comprehensive understanding of the structure transformation and morphology evolution of LaVO4 nanocrystals, it is essential to systematically investigate the effect of the lanthanum source on the formation of the nanocrystals. Hence, LaCl3 and La2(SO4)3 were selected, respectively, as the initial lanthanum sources to replace La(NO3)3 for further investigation. Because the reaction pH value is the key factor for determining the crystal structure and the morphology of the final products, for each lanthanum sources examined, parallel reaction conditions were conducted with pH values varied to examine the effect of pH on crystal structure and morphology. Table 3 shows the polymorphs and morphologies of the asprepared products with different Lanthanum sources. Using LaCl3 as the La Source. When using LaCl3 as the La source, m- and t-LaVO4 can also be selectively synthesized by tuning the pH of growth solution. It is obvious that m-LaVO4 formed at lower pH (pH ) 2.5) without any NaOH (1 M) added to the system (Figure 5a). As the pH was increased to 3.5 with some amount of NaOH (1 M), we can see that m-LaVO4 gradually vanished and t-LaVO4 appeared, as can be seen from Figure 5b. With a further increase of the pH to a higher value (34.5), pure t-LaVO4 was obtained (Figure 5c,d). It is clear and prominent that tuning the pH of growth solution can also drive the transformation of LaVO4 from the monoclinic to the tetragonal phase when using LaCl3 as the La source under hydrothermal treatment. The corresponding samples have been examined by TEM (Figure 6). A similar morphology-developing
Monazite- and Zircon-Type LaVO4 Nanocrystals
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TABLE 3: Structures and Morphologies of the LaVO4 Products Prepared with LaCl3 and La2(SO4)3 as the La Source, Respectively preparative conditions lanthanum source
pH
temp (°C)
time (h)
morphology
structure
LaCl3 LaCl3 LaCl3 LaCl3 La2(SO4)3 La2(SO4)3 La2(SO4)3 La2(SO4)3
2.5 (no NaOH added) 3.5 4.5 6.0 2.0 (no NaOH added) 3.0 4.5 6.5
180 180 180 180 180 180 180 180
48 48 48 48 48 48 48 48
nanoparticles nanoparticles/rods nanorods nanoparticles nanorods nanorods nanowhiskers nanoparticles
monoclinic monoclinic/ tetragonal tetragonal tetragonal tetragonal tetragonal tetragonal tetragonal
process (from monoclinic nanoparticles to tetragonal nanorods and then to tetragonal nanoparticles) has also been observed. On the basis of the experiment results, the pH of growth solution was a key parameter in determining the structure and shape of the products. In addition, the structural and morphological properties of the products were little affected by whether LaCl3 or La(NO3)3 was selected as the La sources. Using La2(SO4)3 as the La Source. LaVO4 also can be synthesized when the La(NO3)3 was replaced by La2(SO4)3, but
Figure 5. XRD patterns of LaVO4 nanocrystals obtained under different pH conditions with LaCl3 as the La source at 180 °C for 48 h: (a) pH ) 2.5; (b) pH ) 3.5, peaks signed with “9” were indexed to m-LaVO4; (c) pH ) 4.5; (d) pH ) 6.0.
Figure 6. TEM images of LaVO4 nanocrystals obtained under different pH conditions with LaCl3 as the La source at 180 °C for 48 h: (a) pH ) 2.5; (b) pH ) 3.5; (c) pH ) 4.5; (d) pH ) 6.0.
the products were pure tetragonal zircon-type despite the final pH value, which was adjusted by the added NaOH (1 M), varied from 2.0 to 7.0. The XRD patterns of the as-obtained products were shown in Figure 7. All the diffraction peaks can be indexed as t-LaVO4 with lattice parameters comparable to that of JCPDS standard card (No. 32-0504); no peaks arising from impurities can be detected. For producing t-LaVO4, the La source of La2(SO4)3 or the SO42- environment was found to be the most effective. As it is in a wider pH range, it works better than La(NO3)3 or LaCl3. When using La2(SO4)3 as the lanthanum source, we found that the morphology of the obtained LaVO4 sample was distinct from that of using La(NO3)3 or LaCl3 as the source. Comparing the TEM images shown in Figure 2 and Figure 6, we noticed that when La(NO3)3 or LaCl3 was used as the La source, the as-synthesized t-LaVO4 nanorods were fairly monodisperse with a uniform diameter throughout its entire length. But when La2(SO4)3 was selected as the La source, the as-obtained t-LaVO4 nanorods are not as uniform as the ones obtained with La(NO3)3 or LaCl3 used as the lanthanum sources; the as-formed t-LaVO4 nanorods are easily glued together (as shown in Figure 8a). As the final pH is increased to 3.0, large c-oriented t-LaVO4 nanorods agglomerated orientationally to form broomlike whisker aggregates (Figure 8b). A higher magnification TEM image (Figure 8c) shows that each bundle is actually formed by lots of t-LaVO4 nanorods branched out radically from the revolutional axis of the rod bundles in a uniform size distribution. The average diameter of a single t-LaVO4 nanorod is about 30 nm and the length of the axis is up to 3.0 µm. The
Figure 7. XRD patterns of LaVO4 nanocrystals obtained under different pH conditions with La2(SO4)3 as the La source at 180 °C for 48 h. (a) pH ) 2.0; (b) pH ) 3.0; (c) pH ) 4.5; (d) pH ) 6.5.
23252 J. Phys. Chem. B, Vol. 110, No. 46, 2006
Fan et al. given in equations as follows:
La(NO3)3 + NaVO3 + H2O h LaVO4 + NaNO3 + 2HNO3 LaCl3 + NaVO3 + H2O h LaVO4 + NaCl + 2HCl La2(SO4)3 + 2NaVO3 + H2O h 2LaVO4 + Na2SO4 + 2H2SO4
Figure 8. TEM images of LaVO4 nanocrystals obtained under different pH conditions with La2(SO4)3 as La source at 180 °C for 48 h: (a) pH ) 2.0; (b) and (c) pH ) 3.0; (d) pH ) 4.5; (e) pH ) 5.5; (f) pH ) 6.5.
morphology difference can be attributed to the effect of anion. The bidentate-specific adsorption of sulfate ions35 to the growing surfaces parallel to the c axis resulted in the formation of nanowhiskers. But nitrate anion and chloride anion are noncomplexing, which has little effect on the size and morphologies of the obtained products. As in the similar phenomenon of the La(NO3)3 or LaCl3 system, when the pH was further increased, the anisotropic growth tendency was also destroyed and the aspect radio of as-obtained nanorods was decreased (Figure 8d,e). When the pH was increased to 6.5, only irregular t-LaVO4 nanoparticles with average diameters of 30 nm can be obtained (Figure 8f). 3.4. The Formation Mechanism of Selective-Synthesis of m- and t-LaVO4 Nanocrystals under Hydrothermal Conditions. From the above experimental results, although the La sources are different, the LaVO4 nanocrystals with controlled structure and shape have been obtained in similar ways. Considering the similarity of their structures and the comparability of the synthetic routes, do they have a common growth process? And what is the original driving force for the structure transformation and morphology evolution in solution at mild hydrothermal conditions? During the crystal growth stage, the delicate balance between the thermodynamic growth and kinetic growth regimes can strongly govern the final structure of the nanocrystals.36,37 When the thermodynamic growth regime is driven by a sufficient supply of thermal energy (KT), the most stable form of nanocrystals is preferred. In contrast, under nonequilibrium kinetic growth conditions, the kinetic growth regimes controlled by changing growth parameters are crucial for the determination of the one-dimensional (1D) nanocrystal geometry. Crystallographic phase transformation in solution usually operates through a dissolution-recrystallization process to minimize the surface energy of the system.38 For our method, NaVO3 was used as the V source, and the whole reaction is
In these reaction systems, the formation of LaVO4 was a process releasing H+, the byproducts being HNO3, HCl, and H2SO4, respectively. These are strong acids, which can dissolve the asobtained vanadate nucleus and thus speed up the dissolution, renucleation, and crystallization process as well as the Ostwald ripening process39-41 through the back reaction shown in the above equations. Additionally, the highly acidic environment may influence the growth rates by protonation of particular surfaces of the nanocrystals. A possible mechanism may be defined as follows: (1) The three lanthanum sources were all in favor of the formation of LaVO4 nanocrystals. The product prepared at lower temperature was in the monazite type LaVO4, because the macroscopically monoclinic phase is more thermodynamically stable than the tetragonal phase (at mild pressures and temperatures). (2) The thermodynamic behavior of small particles differs from that of the bulk material by the free energy term γA the product of the surface (or interface) free energy and the surface (or interfacial) area.42-44 When the surfaces of polymorphs of the same materials possess different interfacial free energies, a change in phase stability can occur in response to both the changes in the surface environment and the particle size. (3) On the basis of the experimental results, tuning the pH of the growth solution was a crucial step in the control of the structure and morphology of LaVO4 preparation. There are a number of roles that the added NaOH may be playing in the synthesis. The main effect is to modulate the thermodynamics and kinetics of nucleation and growth of the system by controlling experimentally the interfacial tension (surface free energy). According to the acid-base surface properties of metal oxides, increasing the pH of precipitation decreases the surface charge density by desorption of protons and consequently increases the interfacial free energies of the system. Thermodynamic colloidal unstability may thus be reached; that is, in the specific reaction circumstance the primarily thermodynamic stable state (m-LaVO4) becomes thermodynamic unstable, resulting in a considerable promoting of the ripening processes. The particles (m-LaVO4) spontaneously grow by an Ostwald ripening process in order to reach a new thermodynamic equilibrium state, where the t-LaVO4 structure is more thermodynamically stable than the m-LaVO4 phase, henceforth making the phase transformation. (4) When NaOH was added to the growth suspension the H+ ions were partially neutralized, which makes the localized [H+] varied on the nanocrystals surface and results in the surface free energies of the various crystallographic planes differing significantly. Obviously, the growth rate of those facets possessing higher free energy would be relatively faster, which affords the possibility of breaking the natural growth habit of crystal and creating additional growth anisotropy. In other words, the added NaOH has the effect of balancing the growth rate of different facets, leading to the smearing restriction of “natural growth habit”. As revealed in the experiments, t-LaVO4 1D nanostructures, such as nanorods/nanowhiskers, can be obtained by tuning the pH of the growth solution.
Monazite- and Zircon-Type LaVO4 Nanocrystals (5) In addition, NaOH is a strong electrolyte, and it may partially neutralize the surface charges of the obtained LaVO4 nanocrystals, preventing them from possible crystallite aggregation. As can be seen in the TEM images, the as-synthesized t-LaVO4 nanorods were fairly monodisperse with a uniform diameter throughout its entire length when using La(NO3)3 or LaCl3 as the La source. Although other unknown factors may exist that influence the selective-synthesis of m- and t-LaVO4 nanocrystals, such as the conventional convection effect of hydrothermal preparations, and so forth, the above mechanism is in good agreement with our experiment results. On the other hand, theoretical calculations based on various theories and methods may be a useful tool to explore and provide more information on the crystal growth. Further work is under way to predict the phase stability of LaVO4 nanocrystals with a thermodynamic model based on first-principle calculations. 3.5. Luminescence Performances Improved by Structural Transformation. The structure of inorganic nanocrystal plays crucial roles in its properties. Despite the fact that vanadates of Y, Gd, and Lu doped with rare earth activators have attracted great interest in regards to luminescent applications,26 LaVO4 is not a suitable host for rare earth activators because of its ordinary monazite structure.26,27 It should be noticed that the tetragonal phased LaVO4 has similar structure to that of YVO4, and it is expected to be a promising phosphor candidate. From a standpoint of materials chemistry and physics, Eu3+ ion activated monazite and zircon type LaVO4 represent interesting systems to test and develop fundamental ideas about synthesis and properties of doped insulators. It may be a good way to prove aforementioned ideas in a pure tetragonal phased LaVO4/ Eu system. The synthesis process of Eu3+ doped m- and t-LaVO4 nanocrystals was similar with that of pure LaVO4 nanocrystals, the only difference is that the La(NO3)3 aqueous solution was replaced by a mixture of La(NO3)3 and Eu(NO3)3 solution with the Eu3+ doping concentration kept at 5 mol %. The structure of the doped LaVO4 nanocrystals is the same as that of the pure LaVO4 nanocrystals prepared at corresponding conditions, and the unit cell volume of 362.1 Å3 corresponds well with the 5 mol % doping level of the smaller Eu3+ compared to that of La3+. This decrease in unit cell volume is a good indication that the doping ions are homogeneously distributed in the host nanoparticles (XRD and TEM results not shown).45 Photoluminescence (PL) spectra of the as prepared samples were recorded by a fluorescence spectrometer at room temperature. Figure 9 shows photoluminescence emission spectra of the as-prepared LaVO4/Eu nanocrystals under the excitation of 320 nm. All of them were normalized to their maximum. The spectra consist of sharp lines ranging from 580 to 720 nm, which are associated with the transitions from the excited 5D0 level to 7F (J ) 1, 2, 3, 4) levels of Eu3+ activators; the most intense J emission is the 5D0 f 7F2 transition located in the range of 600620 nm, corresponding to the red emission, in good accordance with the Judd-Ofelt theory.46 We can see from the spectra that the spectral splitting of zircon and monazite type LaVO4/Eu are quite different owing to the stark effect of different crystal fields. For Eu3+ ions doped phosphors, the structure of the host and the lattice site of Eu3+ ions are very important factors that affect emission efficiency, and the transformation from monazite to the metastable zircon structure resulted in a remarkable improvement of the luminescent properties. The zircon type LaVO4 with a tetragonal crystal structure offers a crystal site with a D194h space group which has very low inversion
J. Phys. Chem. B, Vol. 110, No. 46, 2006 23253
Figure 9. Emission spectra of (a) m-LaVO4/Eu nanoparticles (pH ) 2.5, 180 °C for 48 h) and (b) t-LaVO4/Eu nanoparticles (pH ) 6.0, 180 °C for 48 h) under the excitation of 320 nm at room temperature and Eu3+ doping concentration is kept as 5 mol %.
symmetry,47 and as a result, electric dipole transitions are more allowed, which results in a higher intensity of the electric dipole transitions than those of the monazite one. Figure 9 clearly shows the dominating peak at 617 nm of the 5D0 f 7F2 electric dipole transition. On the basis of these results, the possibility to make nanocrystals with controlled crystal structures becomes very important for tuning the luminescent properties of these kinds of doped-insulators materials. 4. Conclusions In summary, the phase-compositional and morphological controls of the final crystalline LaVO4 were realized via a hydrothermal treatment by tuning the pH of the growth solution. A possible mechanism has been proposed. This low-temperature synthetic route, involving no catalysts or templates and requiring no precise equipment, can be easily adjusted to prepare LaVO4 nanocrystals on a large scale. Simultaneously realizing phase and shape-controlled synthesis of nanocrystals is exciting and challenging in nanochemistry because it demonstrates the ability to tail-aim material on a subtle scale, which is the foundation for practical application. Moreover, the as-synthesized pure t-LaVO4 nanorods may promote both academic interest in lanthanide chemistry and novel applications in nanotechnology. Currently, a systematic study on the luminescent behaviors of the doped zircon-type t-LaVO4 1D nanostructures is under progress. Acknowledgment. We thank Dr. Xuemei Li of Peking University for her help with manuscript revisions and experimental measurements. We also express our hearty thanks to the reviewers for their comments and suggestions. Supporting Information Available: TEM images of the LaVO4 nanocrystals obtained at different hydrothermal temperatures (Figure S1) and reaction times (Figure S2) under pH ) 4.5 with La(NO3)3 as the La source. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Steigerwald, M. L.; Brus, L. E. Acc. Chem. Res. 1990, 23, 183. (2) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226. (3) (a) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (b) Peng, X. G.; Wickham, J.; Alivisatos, A. P. J.
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