Selective synthesis and growth mechanism of CeVO4 nanoparticals via hydrothermal method

JOURNAL OF RARE EARTHS, Vol. 29, No. 2, Feb. 2011, p. 97

Selective synthesis and growth mechanism of CeVO4 nanoparticals via hydrothermal method LIU Fengzhen (刘凤珍)1, SHAO Xin (邵 鑫)1,2, 3, YIN Yibin (尹贻彬)1,3, ZHAO Limin (赵利民)1,3, SUN Qiaozhen (孙巧珍)1, SHAO Zhuwei (邵珠伟)1, LIU Xuehua (刘雪华)1, MENG Xianhua (孟宪华)1 (1. College of Materials Science and Engineering, Liaocheng University, Liaocheng 252059, China; 2. State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China; 3. Renewable Energy & Eco-Materials Engineering Center, Liaocheng University, Liaocheng 252059, China) Received 1 February 2010; revised 22 October 2010

Abstract: Selective-controlled structure and shape of CeVO4 nanocrystals were successfully synthesized via a hydrothermal method from Na3VO4·12H2O and Ce(NO3)3·6H2O. The resulting products were characterized by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM) and energy dispersive spectroscopy (EDS). The influence of hydrothermal temperature, precursor solution concentration on the crystal and morphology of products were further studied. The results showed that the as-synthesized products exhibited pure single-crystal CeVO4 nanoparticles with tetragonal structure. The hydrothermal temperature and precursor solution concentration had important effects on the formation of CeVO4 nanoparticles. Furthermore, the growth mechanism of CeVO4 nanoparticles was explained with Ostwald ripening mechanism. Keywords: CeVO4; hydrothermal method; temperature; precursor solution concentration; Ostwald ripening mechanism; rare earths

Recently, nano-scale materials had attracted considerable interests due to their unique electronic, optical, and magnetic properties which were related with their potential applications. In particular, rare earth elements had been vastly applied in advanced technologies such as luminous materials, fuel cells and photocatalysts because of their particular 4f-5d and 4f-4f electronic transition which were different from other elements[1]. Many rare earth vanadates had been studied because of their outstanding optical, electrical and magnetic properties, and had been widely used in sensors, tribology and heat-resistant materials[2]. Among these vanadates, CeVO4 had diverse potential applications in many fields, such as oxidation catalysts, luminescent materials, gas sensors and electrodes due to its unique catalytic, optical, magnetic and electrical properties[3]. As consequence, the design and synthesis of CeVO4 nanostructures with well-defined size and morphology had attracted much attention[4,5]. To obtain the high quality of vanadate crystals, several methods such as chemical vapor deposition (CVD) method, sol-gel method, plasma chemical vapor deposition (PCVD) method and top-seeded solution growth (TSSG) method had been carried out[6–9]. We reported the systematic synthesis of high-quality CeVO4 nanomaterials through a facile solution-based hydrothermal synthetic pathway. Hydrothermal method, as one of the most promising solution chemical methods, had been widely used to produce various inorganic materials and was known as a quick and useful method with unique morphology and unusual property[10,11]. Herein, this

paper described the formation of tetragonal CeVO4 nanocrystals via hydrothermal method and possible growth mechanism was also discussed.

1 Experimental 1.1 Preparation of CeVO4 nanocrystals All chemicals were analytical-grade reagents and were purchased from Shanghai Chemical Reagent Corp, and used without further purification. The method for fabricating CeVO4 samples with diverse morphologies was as follows. Appropriate amount of Ce(NO3)3·6H2O and EDTA as template dissolved by 2:1 (V/V) ammonia were added to distilled water in a 30.0 ml flask, forming a chelated cerium complex and stirred for several minutes. The pH was adjusted to 9.0 using ammonium hydroxide under stirring subsequently, appropriate amount of stoichiometric Na3VO4·12H2O was added to distilled water in a 30.0 ml flask with vigorous stirring. After substantial stirring, the two optically transparent solutions were mixed. Nucleation occured immediately after mixing, a turbid yellow emulsion was obtained. The mixing solution was stirred for several minutes until it became transparent. Finally, the transparent yellowish solution was transferred into a Teflon-lined stainless-steel autoclave and maintained at different temperatures for 24 h. Naturally as the autoclave cooled down to room temperature, the precipitated powders were separated by centrifugation, washed with deionized water and ethanol for several times. Finally, the

Corresponding author: SHAO Xin (E-mail: [email protected]; Tel.: +86-635-8239863) DOI: 10.1016/S1002-0721(10)60410-3

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CeVO4 nanosized products were dried at room temperature for further characterization. 1.2 Characterizations The phase purity and crystal structure of the obtained samples were examined by X-ray diffraction(XRD) with a Germany Bruker company D8 Advance X-ray diffractometer employing Cu Kα radiation (λ=0.15406 nm) at 40 kV and 30 mA with a scanning rate of 0.04 (°)/s in the 2θ range of 20°–70°. The morphologies and micro-nanostructure of the as-synthesized products were characterized by field emission scanning electron microscopy(FESEM, JSM-6701F), operated at an acceleration voltage of 30.0 kV, energy dispersive spectroscopy (EDS, JSM-6380LV).

2 Results and discussion 2.1

Influences of hydrothermal temperature on the shape of CeVO4 nanocrystals

The composition and phase purity of the products were examined by XRD technique. Fig. 1 shows the XRD patterns of as-prepared CeVO4 samples at different temperatures for 24 h under pH 9.0. The reflection patterns can be readily indexed to that of the pure tetragonal phase of CeVO4 with lattice constants a=0.7399 nm and c=0.6496 nm, which matched well with the literature values (JCPDS No.12-0757). No other impurity phases can be detected, which indicated that tetragonal structure of CeVO4 obtained.

Fig. 1 XRD patterns of CeVO4 nanocrystals (1) 140 ºC; (2) 160 ºC; (3) 180 ºC

Fig. 2 shows the FESEM photographs of the samples prepared with different temperatures showed that the samples exhibited nanorods with diameter of about 50–80 nm at lower temperature and uniform spindle shape at higher temperature. At hydrothermal temperature of 140 ºC, the crystallinity of as-prepared samples was considerably low and the uniformity was not perfect. However, higher temperature favored the formation of thermodynamically stable, well-crystallized and uniform samples. With the rising of temperature, the appearance of samples changed from nanorods to spindle-shaped microstructures gradually. The crystallinity, phase purity and morphological uniformity of products were found to be highly correlative with the hydrothermal temperature. This result was possibly because that hydrothermal temperature of composite materials provided the driving force behind that primarily affected the progress of reaction and crystallization rate, thus affected the quality and morphology of crystals. 2.2 Influences of the precursor solution concentrations on the shape of CeVO4 nanoparticles XRD patterns of as-synthesized CeVO4 samples with different precursor solution concentrations for 24 h at 180 ºC under pH 9.0 are shown in Fig. 3. The reflection patterns can be readily indexed to that of the pure tetragonal phase, which were in good agreement with literature data (JCPDS No.12-0757). The diffraction peaks indicated that the products are well-crystallized, no characteristic peaks are observed for other impurities, indicating that the pure tetragonal structure of CeVO4 can be obtained via our current synthetic method. The general size and morphology of as-obtained CeVO4 samples were examined with FESEM technique. The FESEM images of the samples are illustrated in Fig. 4(a–e). The FESEM photographs of the samples prepared with different precursor solution concentrations showed different morphologies. The samples exhibited nanorods with lengths of up to several micrometers, and diameters of about 30 nm when the Ce3+ concentration was 0.01 mol/L or 0.03 mol/L. Most of the nanorods were straight and uniform along their axial direction. The nanorods were structurally uniform, free from defects and dislocations. When the Ce3+ concentration was increased to 0.05 mol/L, even to 0.2 mol/L, the products

Fig. 2 FESEM patterns of CeVO4 nanocrystals (a) 140 ºC; (b) 160 ºC; (c) 180 ºC

LIU Fengzhen et al., Selective synthesis and growth mechanism of CeVO4 nanoparticals via hydrothermal method

Fig. 3 XRD patterns of CeVO4 nanocrystals (1) 0.01 mol/L; (2) 0.03 mol/L; (3) 0.05 mol/L; (4) 0.2 mol/L; (5) 0.6 mol/L

looked like straw-sheaf with two fantails consisting of a bundle of outspread nanorods, which were closely bonded to each other in the middle, so we called it a “straw-sheaf nanocrystal” Microstructures of other morphologies were not observed. At the Ce3+ concentration of 0.6 mol/L, the CeVO4 products gradually changed to broccoli-like spherulites which were composed of several tens of aligned singlecrystalline nanorods. The precursor solution concentration was found to play a crucial role in achieving uniform morphology of the final products for affecting the aggregation of particles. The body of nanorods was subjected to EDS analysis, confirming the exclusive composition of Ce, V and O except for the Cu signal from the copper FESEM grid in Fig. 4(f). The composition derived from the results indicated an empirical formula of CeVO4.

3 Growth mechanism

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Above experimental results indicated that the process was strongly dependent on the experimental details such as hydrothermal temperature, precursor solution concentration, etc. In our experiment, because our laboratory autoclave can not afford temperature above 200 ºC, to pursue the growth mechanism of CeVO4 nanocrystals, the precursor solution concentration experiments were carried out to monitor the crystallization process of the products. The probable growth mechanism of the nanostructures with various morphologies is proposed in Fig. 5, which depicts the structural evolution of CeVO4 under the influence of precursor solution concentration. With the rising of precursor solution concentration, the morphologies of samples changed from nanorods to straw-sheaf and double-head broccoli gradually, and to broccoli-like spherulites ultimately. Clearly, a crystal splitting growth mechanism was most likely responsible for the formation of 3D straw-sheaf, broccoli-like and broccoli-like spherulites nanostructures[12]. As already mentioned above, they were consisted of a bundle of nanorods, in that case, we think Ostwald ripening was the dominant mechanism of the nanorods growth: the formation of tiny crystalline nuclei in a supersaturated medium occurred at first, and this was followed by crystal growth[13,14]. The larger particles grew at the cost of the small particles; reduction in surface energy was the primary driving force for crystal growth and morphology evolution, due to the difference in solubility between the larger particles and the small particles, according to the well-known Gibbs-Thomson law[7,15,16]. When Ostwald ripening was the dominant mechanism of the growth and morphology evolution, due to the difference in solubility between the larger particles energy was the primary driving force for crystal nanorods growth, the formation of the nanorods must be affected by the character of the starting materials, such as the particle size and chemical activity because the dissolution rate of the material depended on such

Fig. 4 FESEM images of CeVO4 nanocrystals (a–e) and EDS image of the CeVO4 nanocrystals (f) (a) 0.01 mol/L; (b) 0.03 mol/L; (c) 0.05 mol/L; (d) 0.2 mol/L; (e) 0.6 mol/L

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References:

Fig. 5 Schematic representation of the CeVO4 nanocrystals splitting process

characters[7]. During the crystal growth stage, the delicate balance between the thermodynamic and kinetic growth regimes can strongly govern the final structure of the nanocrystals[12]. More and more concentration of Ce3+ resulted in the formation of a number of nuclei, which was then followed by fast growth, leading to the nanocrystal splitting to form sheaves and spherulites. Finally, in this above experiment, our choice of EDTA was mainly based on its appropriately chelating and capping effects, which influenced the growth rate of different facets distinguishingly[13]. The detail study of growth mechanism of the CeVO4 nanocrystals was in progress.

4 Conclusions We demonstrated a simple and mild surfactant-assisted hydrothermal approach for selectively preparing nanorods, straw-sheaf, double-head broccoli and broccoli-like spherulites of CeVO4 nanostructures with excellent size and morphological uniformity. Due to the simplicity of this system and the efficient control over morphologies it has achieved, we supposed that this method may have wide applications in exploring the crystal growth process and provide guidance for the morphology controllable synthesis. The hydrothermal temperature and precursor solution concentration were found to play important roles in determining the morphologies and aspect ratios of the CeVO4 nanocrystals. On the basis of the results presented, the growth mechanism was proposed illustrating the role of hydrothermal temperature and precursor solution concentrationleading to the formation of the CeVO4 nanostructures. Further work is under way to explore the more evidence about the formation mechanism of CeVO4 nanocrystals. Acknowledgments: We greatly acknowledge financial support by the Open Project Program of the State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Acad-

[1] Lucovsky G, Phillips J C. Defects and defect relaxation at internal interfaces between high-k transition metal and rare earth dielectrics and interfacial native oxides in metal oxide semiconductor (MOS) structures. Thin Solid Films, 2005, 486: 200. [2] Duan X, Lieber C M. Nanowires for integrated multicolor nanophotonics. Adv. Mater., 2000, 12: 298. [3] Zhu L, Li Q, Li J Y, Liu X D, Meng J, Cao X Q. Selective synthesis of mesoporous and nanorod CeVO4 without template. J. Nanoparticle Res., 2007, 9: 261. [4] Xia Y N, Yang P D, Sun Y G. One-dimensional nanostructures: Synthesis, characterization, and applications. Adv. Mater., 2003, 15: 353. [5] Chen L M, Liu Y N, Huang K L. Hydrothermal synthesis and characterization of YVO4-based phosphors doped with Eu3+ ion. Mater. Res. Bull., 2006, 41: 158. [6] Uecker R, Wilke H, Schlom D G, Velickov B, Reiche P, Polity A, Bernhagen M, Rossberg M. Spiral formation during Czochralski growth of rare-earth scandates. Cryst. Growth, 2006, 295: 84. [7] Fan W L, Zhao W, You L P, Song X Y, Zhang W M, Yu H Y, Sun S X. A simple method to synthesize single-crystalline lanthanide orthovanadate nanorods. Solid State Chem., 2004, 177: 4399. [8] Wang N, Chen W, Zhang Q F, Dai Y. Synthesis, luminescent, and magnetic properties of LaVO4: Eu nanorods. Mater. Letters, 2008, 62: 109. [9] Huang Q M, Yu J C. Hydrothermal synthesis and structure analysis of indium vanadate (InVO4). Struct. Chem., 2005, 24: 1242. [10] Schmidt M, Ramlau R, Schnelle W. Zum chemischen transport von seltenerdvanadaten(V). Inorg. Chem., 2005, 631: 284. [11] Gu Z J, Zhai T Y, Gao B F, Sheng X H, Wang Y B, Fu H B, Ma Y, Yao J N. Controllable assembly of WO3 nanorods/nanowires into hierarchical nanostructures. Phys. Chem. B., 2006, 110: 23829. [12] Jun Y W, Choi J S, Angew J Cheon. Shape control of semiconductor and metal oxide nanocrystals through nonhydrolytic colloidal routes. Chem. Int. Ed., 2006, 45: 3414. [13] Luo F, Jia C J, Song W. Chelating ligand-mediated crystal growth of cerium orthovanadate. Cryst. Growth Des., 2005, 5: 137. [14] Gates B, Yin Y D, Xia Y N. A solution-phase approach to the synthesis of uniform nanowires of crystalline selenium with lateral dimensions in the range of 10–30 nm. Am. Chem. Soc., 2000, 122: 12582. [15] Yu S H, Biao L, Mo M S, Huang J H, Liu X M, Qian Y T. General synthesis of single-crystal tungstate nanorods/ nanowires: A facile, low-temperature solution approach. Adv. Funct. Mater, 2003, 13: 639. [16] Manna L, Scher E C, Alivisatos A P. Synthesis of soluble and processable rod-, arrow-, teardrop-, and tetrapod-shaped CdSe nanocrystals. J. Am. Chem. Soc., 2000, 122: 12700.