Effect of capping ligands on the synthesis and on the physical properties of the nanoparticles of LiTaO3
Introduction
Nanostructured materials have attracted much interest in the last decade due to their properties, which are both quantitatively and qualitatively different from their bulk counterparts and from the discrete atomic or molecular species from which they are derived [1], [2], [3], [4], [5], [6], [7], [8]. Nanomaterials signify an evolving technology that has the potential to have an impact on an incredibly wide range of industries and markets. There are many novel properties and applications of nanoparticles that have been already demonstrated; from catalysis, environmental remediation, biomedical applications to information displays and electronics [9], [10], [11], [12], [13]. Similarly, nanoferroelectric materials (thin films, particles and composites) have shown new effects or properties [14], [15], [16], [17].
The size effects in ferroelectrics raise many questions like the existence of ferroelectricity, nature of domain structure, domain-wall thickness, transition temperature, etc. The formation of micro- or nano-dispersed ferroelectric phases in ceramics, polymers or glasses with different well-defined geometric structures has opened up a new dimension in engineering and optimization of desired physical properties. To modify the piezoelectric, pyroelectric, dielectric and optical properties of ferroelectric oxides, the nanoparticles of oxides are dispersed either in polymers or in glasses [18], [19], [20], [21], [22], [23].
A wide variety of chemical techniques have been successfully used for synthesizing nanocrystalline ferroelectrics with controlled mean particle size and size distribution. These include sol–gel, co-precipitation, spray pyrolysis, freeze-drying, microemulsion-mediated reactions, hybrid dry–wet processes, etc. [24], [25], [26], [27]. Conventional “dry” powder mixing processes are usually unsuitable because the temperatures involved are often too high, resulting in grain growth and a broad size distribution. Recently, liquid–solid solution has been used to develop nanoparticles of metals, semiconductors, ceramics and ferroelectrics. The sol–gel method is the common method for synthesis of nanoparticles of ferroelectrics [28], [29], [30]. During such a synthesis, the problems of agglomeration, nonuniform shape and size are common. To avoid such problems, capping ligands are used during the synthesis. The importance of capping ligands is to provide suitable synthesis conditions and to fine-tune the sizes and shapes of nanocrystals [24], [31], [32], [33]. Due to differences in electronic and binding properties, capping ligands also influence the optoelectronic and magnetic properties of functional nanocrystalline inorganic materials.
Here we describe the synthesis of lithium tantalate (LiTaO3) ferroelectric nanoparticles using a sol–gel route, and modification of the shapes and sizes of these particles by using anionic lipophilic capping ligands with O-donor atoms (oleic acid), and neutral capping ligands with N-donor atoms (aniline). The sol–gel method has been used because of its advantages in terms of cost, stoichiometry control, processing temperature and homogeneity of the final product.
Section snippets
Experimental details
LiTaO3 nanoparticles were fabricated using a sol–gel process. A typical detailed procedure for synthesis of LiTaO3 sol has been described elsewhere [34], [35]. In brief, lithium ethoxide (99.999% pure), tantalum pentaethoxide (99.999%) and acetic acid (99.9%) were mixed in molar ratio 1:1:10. Absolute ethanol was added into the sol to dissolve the chemicals, and the solution (0.2 mol/l) was stirred at room temperature. All the processes were carried out in a glove box in argon atmosphere.
In the
Results and discussion
In the first experiment, i.e. when the gel was annealed, large, micron-sized crystallites were formed. From XRD (Fig. 1a), it was confirmed that a pure and proper phase of stoichiometric LiTaO3 is formed (reference JCPDS No. 29-0836). It is observed that the peaks are shifted toward the left from the positions (2θ) given in the JCPDS file by an angle varying from 0.23° to 0.42°. This indicates that the grains in the crystallite are in a state of uniform tensile strain. The powder formed from
Conclusions
Spherical, uniformly sized and stable nanoparticles are building blocks of transparent polycrystalline LiTaO3, which is needed for detector and modulator applications. From the above studies, it is clear that spherical, stable LiTaO3 nanoparticles of size 20–40 nm can be formed by the addition of oleic acid into the sol just after its synthesis. The addition of aniline into the sol, just after synthesis, results in nanoparticles of 40–70 nm of irregular shape after heating at 600 °C for 1 h. The
Acknowledgments
We thank Dr. Tapas Ganguli, Mr. Ravi Kumar and Mr. Niyaz Ahamad Madhar, for their support in characterization of the nanopowders. One of us (VKW) wishes to thank the Board of Research in Nuclear Sciences (DAE) for the award of a Raja Ramanna Fellowship. Useful discussions with Dr. V. N. Vaidya are also acknowledged with thanks.
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