Elsevier

Journal of Crystal Growth

Volume 307, Issue 1, 1 September 2007, Pages 185-191
Journal of Crystal Growth

Effect of capping ligands on the synthesis and on the physical properties of the nanoparticles of LiTaO3

https://doi.org/10.1016/j.jcrysgro.2007.05.056Get rights and content

Abstract

This paper reports the synthesis of lithium tantalate (LiTaO3) nanoparticles by a sol–gel process involving the use of capping ligands. When the sol was allowed to form gel and then annealed at 600 °C for 1 h in the presence of oxygen, hexagonal crystallites of size 2–5 μm were formed. But when the sol was heat treated at 600 °C for 1 h in the presence of oxygen before gelation, agglomerated nanoparticles were formed. The addition of aniline before gelation (a neutral capping ligand with N-donor atom) not only restricted gelation, but also helped in the formation of well-separated and irregular nanoparticles of sizes 40–70 nm, which was confirmed from TEM and XRD studies. The anionic-ligand oleic acid (lipophilic ligand with O-donor atoms) restricted gelation by arresting the surface of LiTaO3, and spherical nanoparticles of size 20–40 nm were formed after heating the sol. Comparison of ferroelectric and dielectric properties between bulk particle and spherical nanoparticles (obtained using oleic acid) are reported by characterizing these particles using Raman spectroscopy and differential scanning calorimetry (DSC). The Curie temperature decreases with decreasing the particle size 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.

References (46)

  • H. Gleiter

    Prog. Mater. Sci.

    (1989)
  • Y. Ando et al.

    Mater. Today

    (2004)
  • A.K. Cheetham et al.

    Nano Today

    (2003)
  • B. Rapp

    Mater. Today

    (2004)
  • J. Livage et al.

    Prog. Solid State Chem.

    (1988)
  • Z.G. Wu et al.

    J. Non-cryst. Solids

    (2003)
  • S. Satapathy et al.

    J. Crystal Growth

    (2006)
  • L. Shi

    Solid State Commun.

    (2005)
  • R. Valiev

    Nature

    (2002)
  • A. Hasnaoui et al.

    Science

    (2003)
  • P. Moriarty

    Rep. Prog. Phys.

    (2001)
  • Y.M. Chiang et al.

    J. Electroceram.

    (1997)
  • L.E. Brus

    J. Chem. Phys.

    (1984)
  • J. Karch et al.

    Nature

    (1987)
  • S. Abbet et al.
    (2004)
  • T. Vo-Dinh
    (2004)
  • O. Painter et al.

    Science

    (1999)
  • A. Anliker et al.

    Helv. Phys. Acta

    (1954)
  • W. Kanzig

    Phys. Rev.

    (1955)
  • K. Okazaki et al.

    J. Am. Ceram. Soc.

    (1973)
  • P. Ayyub
    (2004)
  • M. Sadhukhan et al.

    J. Appl. Phys.

    (1999)
  • N.S. Prasad et al.

    J. Mater. Chem.

    (2001)
  • Cited by (0)

    View full text