Sol–gel derived nanocrystalline and mesoporous barium strontium titanate prepared at room temperature

Abstract

Perovskite-type barium strontium titanate (BST) thin films and powders with nanocrystalline and mesoporous structure were prepared by a straightforward particulate sol–gel route at room temperature. The prepared sol had a narrow particle size distribution of about 20 nm. X-ray diffraction (XRD) revealed that phase composition and preferable orientation growth of BST depended upon the annealing temperature. Transmission electron microscope (TEM) images showed that the crystallite size of the powders decreased with increasing annealing temperature from 8 nm at 25 °C down to 5 nm at 800 °C. Field emission scanning electron microscope (FE-SEM) analysis and atomic force microscope (AFM) images revealed that BST thin films had mesoporous and nanocrystalline structure with average grain size of 30 nm at 600 °C. Based on Brunauer–Emmett–Teller (BET) analysis, the synthesized BST showed mesoporous structure containing pores with needle and plate shapes and BET surface area in the range of 49–32 m²/g at 500–800 °C.

Introduction

Ferroelectric thin films have found wide applications in many electronic and electro-optic devices. Among them, barium strontium titanate (BST) is of considerable interest to researchers and engineers due to its high dielectric permittivity, low optical losses and composition dependent Curie temperature. The Curie temperature of BST decreases linearly with increasing amounts of Sr in the BaTiO₃ lattice (Kobayashi & Kobayashi, 1994). This enables the ferroelectric/paraelectric transition temperature to be tailored for specific applications by varying the Sr content. Therefore, depending on Ba:Sr ratio the transition temperature and hence the electrical and optical properties of the solid solution BST show a variation over a broad range (Lahiry & Mansingh, 2008).

The properties of BST material are strongly related to its microstructure, so the homogeneity and uniformity of the material is important. Low-temperature synthesis of nanoscaled BST powders has attracted considerable interest in recent years due to the technological importance of these powders in the fabrication of homogenized ceramics. The powders to produce the homogenized ceramics should be fine and high quality as these particles enable better sintering and the development of uniform microstructure throughout the ceramic component. Furthermore, the fine-grain feature in the resultant ceramic which improves the performance and reliability of physical properties to a large extent has motivated the interest for the synthesis of the ultrafine BST powders. BST is usually synthesized and crystallised by solid-state reaction, in which the carbonates of barium and strontium and titanium oxide are sintered at high temperature around 1200 °C (Sharma, Varadan, & Varadan, 2000). Owing to some limitations of solid-state synthesis, especially in terms of large grain size with uncontrolled and irregular morphologies and its contamination during repeated calcinations process, several alternative chemical methods have been developed, including co-precipitating method (Khollam et al., 2003), organic compound method (Krupanidhi & Peng, 1997), hydrothermal synthesis (Roeder and Slamovich, 1999, Wei and Padture, 2004) and sol–gel process (Li et al., 2007, Zhang et al., 2007). The sol–gel process offers important advantages over other techniques due to its simplicity, low cost, excellent composition control, high homogeneity at the molecular level, lower crystallisation temperature and feasibility of producing thin films on complex shapes when dip coating is used. It has been reported in the literature that crystalline BST films with ferroelectric properties can be prepared only at high processing temperatures (>700 °C). Cheng, Tang, Zhang, Meng, and Chu (2000) reported Ba_0.8Sr_0.2TiO₃ films by sol–gel method from barium acetate, strontium acetate and titanium butoxide system at 750 °C. FE-SEM investigation showed that the films exhibited columnar grains with a diameter of 100–200 nm. Ba_0.8Sr_0.2TiO₃ thin films were produced using barium acetate, strontium acetate and titanium tetra-n-butoxide as starting materials by Tian et al. (2001). XRD measurements showed that the thin films were crystallised above 600 °C and the perovskite phase of thin films formed at 750 °C. Ba_0.8Sr_0.2TiO₃ thin film was reported by a soft solution process using barium carbonate, strontium carbonate and titanium isopropoxide as precursors by Pontes et al. (2002). The film was crystallised at 700 °C containing large grains with a size of about 100 nm. Zhang and Ni (2002) prepared Ba_0.64Sr_0.36TiO₃ thin films by sol–gel method from barium ethylhexanoate, strontium ethylhexanoate and titanium(IV) isopropoxide. The films were crystallised to a perovskite structure after post annealing at 700 °C with crystallite size of 45.8 nm. Sharma, Varadan, and Varadan (2003) reported sol–gel derived Ba_0.65Sr_0.35TiO₃ powder using titanium tetra isopropoxide and 2,4 pentadionate salts of Ba and Sr. It was found that perovskite BST crystallised at 500 °C and a pure BST phase was obtained at 900 °C. Roy, Bhatnagar, Sharma, Manchanda, and Balakrishnan (2004) studied the effect of pre-sintering temperature on the structural properties of Ba_0.5Sr_0.5TiO₃ thin films deposited by sol–gel technique using barium acetate, strontium acetate, titanium isopropoxide, acetic acid, acetyl acetone, 2-methoxyethanol and formamide. It was found that better crystallinity was obtained in case of films pre-sintered at 600 °C. A BST material was prepared by sol–gel process from barium nitrate, strontium nitrate and tetra-n-butyl titanate system by Wang, Yao, and Zhang (2004). The results showed that amorphous solid was formed at 600 °C and the Ba_1−xSr_xTiO₃ phase was formed at 700 °C. Barium strontium titanate (Ba_0.5Sr_0.5TiO₃, BST) thin films containing crystalline seeds of BST nanoparticles were prepared with a complex alkoxide precursor method using barium acetate, strontium acetate and titanium tetraisopropoxide as precursors by Kobayashi, Iizuka, Tanase, and Konno (2005). An unseeded BST film was crystallised at 600 °C (with 60 nm particles), while the seeding with 17 mol%-BST particles promoted crystalline growth of BST and lowered crystallisation temperature of the films to 525 °C. Zhu, Peng, Cheng, and Meng (2006) reported Ba_0.6Sr_0.4TiO₃ thin films by sol–gel technique from barium acetate, strontium acetate, titanium-tetrabutoxide, glacial acetic acid and 2-methoxyethanol system. The crystallised film at 750 °C had average grain size of 50 nm. BST gel was synthesized by a modified sol–gel method using barium diethoxide, strontium isopropoxide and titanium tetraisopropoxide as precursors and methanol and 2-methoxyethanol as solvents by Li et al. (2007). It was found that a temperature of 600 °C was required for complete crystallisation. A modified citrate precursor method, combining the advantages of the nitrate autocombustion method by introducing the nitric acid, was used to synthesize Ba_0.7Sr_0.3TiO₃ powders with an average particle size of 20 nm at 800 °C by Mao, Dong, Zeng, Wang, and Chen (2007). Barium carbonate, strontium carbonate and titanium tetrabutoxide were employed as starting materials. Venkata Saravanan, Sudheendran, Ghanashyam Krishna, James Raju, and Bhatnagar (2007) prepared Ba_0.5Sr_0.5TiO₃ thin films, with crystallite size of 20 nm at 800 °C, by sol–gel technique using titanium(IV) isopropoxide, barium acetate and strontium acetate as starting materials and glacial acetic acid and 2-methoxyethanol as solvents. It was found that complete oxidation and crystallisation occurred at temperature ≥600 °C. BST (Ba_0.7Sr_0.3TiO₃) powder, with average particle diameter of 15 nm at 700 °C, was reported through sol–gel process from barium acetate, strontium acetate and titanium isopropoxide system by Somani and Kalita (2007). Compositionally layered Ba_xSr_1−xTiO₃ (Ba_0.60Sr_0.40TiO₃–Ba_0.75Sr_0.25TiO₃–Ba_0.90Sr_0.10TiO₃) thin film was fabricated via the metal organic solution deposition technique, using barium acetate, strontium acetate and titanium isopropoxide by Cole et al. (2007). The film was crystallised at 750 °C with grain size of 80 nm. Lahiry and Mansingh (2008) prepared barium strontium titanate films by sol–gel method using barium-ethyl-hexanoate, strontium-methyl-hexanoate and titanium(IV)-isopropoxide as precursors, ethanol as solvent and acetylacetone and formamide as additives. The as-deposited film was found to be amorphous and it was crystallised at 700 °C. Singh, Sharma, Sarma, and Phanjoubam (2008) also deposited BST films by sol–gel method using the same precursors as Lahiry and Mansingh (2008). They found that the as-fired films were amorphous, which crystallised to cubic phase after annealing at 550 °C in air for 1 h with crystallite size of 40 nm.

Reduction in the processing temperature is needed in order to enable the instigation of the BST films with other semiconductor devices. In this paper we report, for the first time, BST material with nanocrystalline structure prepared by sol–gel process at room temperature. In addition, the aim of the present work is to synthesis nanocrystalline BST materials, at the low temperature with the minimal heat treatment by employing a suitable aqueous particulate sol–gel route rather than the polymeric sol–gel methods reported previously. This process can be defined as environmentally friendly processing as it uses an aqueous solution. One of the advantages of the present method is the use of an alternative to alkoxides or acetates (i.e., barium and strontium chlorides) as barium and strontium sources to produce a low cost product. Besides controlling the phase structure, composition homogeneity, crystallite size, monodispersity and microstructure, the cost of the product is also an important concern. Therefore, starting with a low cost precursor such as chloride salts rather than alkoxides and acetate salts may reduce the total cost of production.