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J. Phys. Chem. B 2006, 110, 58-61
Growth of Single-Crystalline KNbO3 Nanostructures A. Magrez,*,† E. Vasco,‡ J. W. Seo,† C. Dieker,† N. Setter,‡ and L. Forro´ † Laboratoire des Nanostructures et des NouVeaux Mate´ riaux Electroniques, Institut de Physique de la Matie` re Complexe, and Laboratoire de Ce´ ramique, Institut des Mate´ riaux, Ecole Polytechnique Fe´ de´ rale de Lausanne, CH-1015 Lausanne-EPFL, Switzerland ReceiVed: July 11, 2005; In Final Form: October 28, 2005
This communication reports on the growth of highly uniform KNbO3 nanowires exhibiting a narrow diameter distribution around 60 nm and a length-to-width ratio up to 100. The nanowires were prepared by a hydrothermal route, which enables simple, gram-scale production. A systematic study of the synthesized nanowires in terms of the morphological and chemical characteristics was carried out by varying the temperature-pressure conditions and the composition of the starting mixture. The results indicate that highly uniform single-crystalline nanowires form within a narrow window of the ternary phase diagram of KOH-Nb2O5-H2O.
Introduction Nanoscale science based on one-dimensional structures has found a springboard in the discovery of carbon nanotubes. Since then, many different materials have been produced as onedimensional (1D) nanostructures, such as, for example, nanotubes and nanowires. Novel size- and shape-dependent crystal structure1 (special polymorphisms induced by particle size have been reported recently for alkaline niobates2,3) and properties4 of nanoscale materials have been investigated intensively. In particular 1D oxide nanostructures have attracted attention but mainly in binary compounds such as, for example, TiO2, ZnO, VOx, etc. Despite very promising progress in the ability to prepare 1D oxide nanostructures,5 few reports concerning the synthesis of nanowires of functional perovskite oxides are available so far.6,7 This paper presents the preparation of KNbO3 nanowires and the influence of growth conditions on the product characteristics, i.e., particle morphology, purity, and crystalline quality of the synthesized material. Perovskite KNbO3 (KN hereafter) is an attractive oxide for its acousto-optic, electrooptic, nonlinear optical, and piezoelectric properties. KN is employed as a frequency doubling and mixing material and also as optical waveguides and a holographic storage medium. In addition, KN is a promising candidate as a lead-free and biocompatible transducer with tunable piezoelectric response.8 The preparation of such a material as nanowires would drive the development of a novel generation of nanoelectromechanical systems (NEMS) based on nanoscalable components. Recently, KN nanostructured materials have been produced by hydrothermal synthesis9,10 and characterized by means of atomic force microscopy assisted detection of induced piezoelectric vibrations.11 In this paper, we systematically study the hydrothermal route and explore the KOH-Nb2O5-H2O ternary phase diagram. Our results indicate that the morphology of KN nanostructures strongly depend on the temperature-pressure condition and the starting composition. Nanowires with a welldefined structure, a narrow diameter distribution, high aspect * Author to whom correspondence should be addressed. Fax: 0041(21) 693-4470. E-mail:
[email protected]. † Laboratoire des Nanostructures et des Nouveaux Mate ´ riaux Electroniques, Institut de Physique de la Matie`re Complexe. ‡ Laboratoire de Ce ´ ramique, Institut des Mate´riaux.
ratio, and high density have been obtained in a narrow window of the ternary phase diagram. Experimental Section For the synthesis of KN nanostructured materials, hydrothermal treatment is applied, which is a suitable synthetic route for material preparation under mild conditions (low temperature). This method allows a reproducible shape control as well as large-scale synthesis, two aspects very important for the applications mentioned above. In a typical reaction, niobium pentoxide (Fluka) powder is added to distilled water in which potassium hydroxide (Fluka) was dissolved. KOH acts as both the potassium source and the mineralizer. The reactant mixture, which contains KOHNb2O5-H2O with different weight ratios, is subsequently stirred for 2 h. The resulting slurry is poured into the Teflon vessel. Afterward, the autoclave is heated to a temperature ranging from 100 to 225 °C for 6 days, producing a white precipitate. The solid is filtered, washed with distilled water and ethanol, and dried at 120 °C overnight. For the transmission electron microscopy (TEM) study a Philips CM300 microscope was used operating at 300 kV. The TEM sample preparation involved dispersing the synthesized material in isopropyl alcohol by sonication, and a drop of suspension was put on a copper grid covered with holey carbon. Scanning electron microscopy (SEM) micrographs were taken using a Philips XL 30 FEG operated at 30 kV. X-ray powder diffraction experiments were carried out using a Rigaku diffractometer in Bragg-Brentano geometry with monochromatic Cu-KR radiation. The data were collected in the θ-2θ mode. Results and Discussion Ternary Phase Diagram. The synthesis takes place in solution via a dissolution-precipitation process according to the following reactions
3Nb2O5 + 8OH- f Nb6O198- + 4H2O
(1)
Nb6O198- + 34OH- f 6NbO67- + 17H2O
(2)
NbO67- + K+ + 3H2O f KNbO3 + 6OH-
(3)
10.1021/jp053800a CCC: $33.50 © 2006 American Chemical Society Published on Web 12/07/2005
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Figure 1. Schematic illustration of the structural transformations of Nb-containing species along the chemical mechanism of KNbO3 synthesis by hydrothermal treatment. Nb atoms are located in the polyhedrons of oxygen atoms.
As previously reported,12 niobium pentoxide first dissolves into Nb6O198- hexaniobate Lindqvist ion, in which NbO6 octahedrons are sharing edges. This complex transforms afterward, along reaction 2, into single octahedron NbO67- anions, which act further as elementary species for KN perosvkite, with a NbO3- corner-sharing octahedron network. The final step (reaction 3) describes the KN precipitation. The structure of the Nb-containing species is presented in Figure 1. Basically the exploration of the KOH(x)-Nb2O5(y)-H2O(z) ternary phase diagram was restricted to an area enclosed by composition lines corresponding to the limit of solubility of the solid phases or lines deduced from the stoichiometry of reactions (1-3) occurring during the growth process. On one hand, we consider the reaction completed when the product of the synthesis does not contain residual Nb2O5. Therefore for complete consumption of Nb2O5, the composition of the starting mixture should match the y/x e 0.34 [(3MNb2O5)/ [(34 + 8)MKOH) ) 0.34] - 0 e z e 100% condition according to the stoichiometry of reactions 1 and 2. This condition also avoids the growth of undesirable KNb3O8 or K4Nb6O17 phases, which form layered structures.13 These niobates are formed via the following reactions, in the liquid phase, depending on the solution degree of alkalinity
3Nb2O5 + 2OH- f 2Nb3O8- + H2O
(4)
3Nb2O5 + 4OH- f Nb6O174- + 2H2O
(5)
Furthermore, the transformation of Nb2O5 into KNbO3 is completed if the condition y/z e 2.46 - 0 e x e 100%, belonging to reaction 3 stoichiometry, is abided as well. On the other hand, the explored area is limited by the limits of solubility of solids. For KOH, the condition is x/z ) 1 - 0 e y e 100%. The last condition corresponds to the niobium pentoxide axis since its solubility is rather low (x ) z ) 0 - 0 e y e 100%).These lines are plotted into the diagram (weight percentage) of Figure 2, and compositions in agreement with all conditions are contained within the darkest area. Representative SEM images of the products obtained at 200 °C from different compositions are collected in Figure 3. The synthesized products are examined by X-ray powder diffraction (XRPD), and it is found that each sample corresponds to a KN single-phase system. The compositions used in the recent studies9-11 are included to the phase diagram for comparison. A strong dependence of the particle shape on the composition of the starting mixture is observed. In particular, elongated structures are formed for points 3 and 4 in the phase diagram. For KOH content close to the saturation level in water (point 1), the synthesized material reveals anisotropic structures. Nevertheless, these features are substandard nanowires since they exhibit nonuniform width. Their structures can be decribed as the stacking of cubic particles of different sizes that decreases
Figure 2. KOH-Nb2O5-H2O ternary phase diagram. The different conditions to be respected by the starting compositions are presented. The exploration is restricted within the darkest area where all conditions are abided. The dashed triangle represents the position of Figure 3 in the full phase diagram.
from the base toward the tip. Theoretically,14 it was found that this morphology is not an intermediate state of the growth toward nanowires with uniform width but rather toward nanowires where the growth was interrupted by the presence of defects. Our study shows that either the KOH content or the thermal annealing time at 200 °C affects the length and the width of the initial cubic particle as well as the number of the faces from which the nanowires are formed.14 Furthermore, a decrease in the population of defective nanowires compared to the number of nanowires of uniform width was observed as the water content increased. The Liu et al.9,10 process, reproduced by Suyal et al.,11 explored several compositions with a constant Nb2O5 content (∼1 wt %) by modifying the KOH/H2O ratio (Figure 2). A SEM image of point 2 powder confirms their observations. The sample contains mainly isotropic sub-micrometer particles (average size of about 500 nm) with some elongated ones, whose aspect ratios can reach up to 20. Our finding shows that the Nb2O5 amount affects the particle shape as well: When the Nb2O5 proportion is increased from 0.4 to 6.8 wt % (point 3 f 1 f C), the particle aspect ratio decreases down to the isotropic shape level for powders produced by Goh et al.12 In summary, point 3 is very likely the most favorable composition (38.7 wt % of KOH, 0.4 wt % of Nb2O5, and 60.9 wt % of H2O) for growing regular nanowires since regular nanorods and nanowires, exhibiting well-defined edges parallel to low-indexed planes of KN, are produced mostly within these conditions. Temperature and Pressure. The applied temperature and pressure have a strong influence on the solubility of the different species and reaction kinetics. Thus, SEM images of the products obtained from the composition corresponding to point 3 are
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Magrez et al.
Figure 3. Explored area of the KOH-Nb2O5-H2O ternary phase diagram. SEM images of some representative compositions are shown. (The scale bar corresponds to 1 µm.) Samples 1-5 were produced by hydrothermal synthesis at 200 °C for 6 days. The KOH-Nb2O5-H2O ratios in wt % are as follows: (1) 38.1/1.9/60.0, (2) 40.6/0.9/58.5, (3) 38.7/0.4/60.9, (4) 44.4/0.4/55.2, (5) 30.4/0.4/69.2. The compositions of the A-D segment are related to the syntheses in refs 9-11. Points B and C belong to ref 12 compositions. A part of the y/x e 0.34 - 0 e z e 100% condition is also presented as a full line.
Figure 4. Evolution of the morphology and composition of the synthesized products with the bath temperature (T) and hydrostatic pressure (P) in the hydrothermal autoclave. Representative XRPD spectra are inserted (vertical arrows indicate Nb2O5 phase). SEM images are given for each pair of (T,P) values (scale bar corresponds to 2 µm). The orthorhombic-tetragonal phase transition temperature of the KNbO3 bulk phase is included as a reference.
presented in Figure 4 as a function of bath temperature and hydrostatic pressure in the autoclave. First, for a temperature lower than 150 °C, a second phase that contains several micron-sized grains remains after the synthesis. XRPD confirmed the presence of Nb2O5 impurities as unreacted material. Above 150 °C, single-phase KN powder is synthesized. Lattice constants were estimated by refining XRPD patterns assuming the Amm2 orthorhombic space group for all the prepared samples. The so-calculated values (a ) 0.3975(1) nm, b ) 0.5693(2) nm, c ) 0.5717(2) nm) are in good agreement with those reported for distorted KN perovskite structure.15 In addition the sample produced at 150 °C almost exclusively contains nanowires with a regular size and a narrow distribution of diameter around 60 ( 10 nm and an aspect ratio up to 100, as can be seen in Figure 3. These nanowires exhibit morphological characteristics (length, width, aspect ratio, etc.) similar to those reported so far for BaTiO3 nanowires.4,5 Moreover, according to the authors’ knowledge, it is the first time that KN nanowires are produced in gram-scale quantities with such selectivity and such morphological characteristics. From the ideal composition found above, the increase of temperature up to the orthorhombic-tetragonal phase transition temperature16 leads to the production of rough particles. This effect could originate from the enrichment with niobium ionic
species in the liquid phase due to an increase of the solubility limit, tuned by the temperature and the pressure. Consequently, the control of the Nb2O5 dissolution via the bath temperature and pressure plays a key role in modifying the morphology and composition of the final product. Structure Characterization. To study the structural properties of the wires, high-resolution transmission electron microscopy (HRTEM) analysis was performed. Samples synthesized at 150 °C from the optimal mixture composition contain mainly perfect nanowires with a diameter around 60 nm (Figure 4a). As can be seen in Figure 5b, the nanostructures produced are single-crystalline nanowires with edges parallel to a low-indexed plane of KN. The selected area electron diffraction (SAED) pattern, which was obtained from an entire nanowire, was indexed by assuming the lattice constants refined from XRPD measurements. According to this, the nanowire axis runs along the [011] direction of KN referred to the orthorhombic unit cell, and the nanowire edges are parallel to the (010) and (001) planes. Conclusion In summary, regularly sized KN nanowires have been prepared via a hydrothermal route at 150 °C from a 38.7 wt %
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J. Phys. Chem. B, Vol. 110, No. 1, 2006 61
Figure 5. (a) TEM image of KN nanowires synthesized at 150 °C from the optimal mixture composition. (b) HRTEM image and (c) SAED pattern of a KN nanowire. On the basis of the SAED, the zone axis was determined to [100].
KOH, 0.4 wt % Nb2O5, 60.9 wt % H2O mixture. The so-synthesized powder is composed of single-crystalline KN nanowires with an aspect ratio and average diameter of 100 and 60 nm, respectively. These morphological characteristics are suitable and promising for NEMS applications. The influence of the composition of the starting mixture as well as of the temperature-pressure within the hydrothermal autoclave on the product purity and particle shape have clearly been identified. Future works are oriented to the understanding of the growth kinetics of KN nanowires, aiming at a further characterization of the growth mechanism. Nevertheless, these results will allow us to perform similar growth studies on comparable perovskite materials to synthesize other functional perovskite materials as one-dimensional nanostructures. Acknowledgment. This work was carried out within the NCCR Nanoscience program of the Swiss National Science Foundation. The financial support is gratefully acknowledged. Authors thank the Centre Interdisciplinaire de Microscopie Electronique at the EPFL for access to electron microscopes. References and Notes (1) Fernandez-Garcia, M.; Martinez-Arias, A.; Hanson, J. C.; Rodriguez, J. A. Chem. ReV. 2004, 104, 4063.
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