Colloids and Surfaces A: Physicochemical and Engineering Aspects
Efficient hydroxylation of BaTiO3 nanoparticles by using hydrogen peroxide
Introduction
Barium titanate (BaTiO3) is a typical dielectric material and is used extensively because of its high permittivity and unique ferroelectric, piezoelectric and thermoelectric properties [1]; moreover, it has potential for use in the optical field due to its high refractive index of 2.4 [1], [2]. Since BaTiO3 has a great many potential applications, developing techniques to manipulate its surface chemistry is essential. In general, commercial BaTiO3 nanoparticles are aggregated and show poor dispersion capabilities in most organic media. To improve their dispersion in polymer matrices for the manufacturing of polymer-based nanocomposites [3], [4], surface modification of BaTiO3 is required and is usually achieved by surfactant adsorptions or polymer coatings [5], [6], [7]. However, most of the organic modification agents only physisorb on particles by electrostatic attraction or by van der Waals forces due to the fact that as-received BaTiO3 lacks reactive functional groups; thus, it is difficult to form strong bonding with BaTiO3. Although it has ever been reported that the BaTiO3 powders, especially for the hydrothermally synthesized BaTiO3, contain hydroxyl groups (–OHs) on the particle surface and in the crystal lattice [8], [9], [10], the surface content of –OH is still too low to have a significant effect on improving the surface reactivity with dispersants. Therefore, derivation of functional groups on the particle surface would be the prerequisite for an effective modification of BaTiO3.
In our previous study [11], we proposed a novel idea of using an easy and gentle approach to hydroxylate the BaTiO3 nanoparticles. Many –OHs were found being produced on the BaTiO3 surface as the nanoparticles were pretreated with hydrogen peroxide (H2O2). With hydroxylation, the BaTiO3 nanoparticles can then be easily modified by the commonly used organic surfactant, sodium oleate (SOA), and this significantly improves their dispersion in organic solvents such as tetrahydrofuran (THF), toluene and n-octane. Although most qualitative analyses about the hydroxylation of BaTiO3 by H2O2-treatment have been discussed in the previous study, the result of quantitative analysis has not been discussed yet. Moreover, the chemical behaviors of the derived –OH on the BaTiO3 surface, such as its acid–base property and reactivity with the modification agents, are still unclear.
This investigation is an extended study about the efficient hydroxylation of BaTiO3 using H2O2-treatment, in which the hydroxylation efficiency of the H2O2-treatment is quantified and the acid–base property and chemical selectivity of –OH are further characterized. The hydroxylation efficiency of H2O2-treatment is calculated and determined by the quantitative X-ray photon spectroscopy (XPS) method [12], [13], [14]. A significant increase of OH concentration on the H2O2-treated BaTiO3 is found. The acid–base behavior and chemical selectivity of the –OH are analyzed by XPS and zeta-potentials. They demonstrate that the –OH is primarily derived on the barium (Ba) site of BaTiO3, causing the –OH to be more Bronsted-basic than Bronsted-acidic, and this acid–base property can just affect its chemical selectivity for surface modification agents. In the adsorption study, the surfactant with carrying acidic functional group rather than alkaline group shows more affinity for the H2O2-treated BaTiO3. Besides, the adsorption concentration of the acidic surfactant is much higher on the H2O2-treated BaTiO3 than on the as-received BaTiO3, and the increased adsorption is expected to facilitate the dispersion stability of nanoparticles in solvents.
Section snippets
Raw materials
High-purity BaTiO3 nanoparticles (Sigma–Aldrich, 99%, trace metals basis, USA) have a size distribution of 30–50 nm and an average surface area of 12.11 m2/g with standard deviation of 0.45 m2/g measured by a N2 adsorption (ASAP 2300, Micromeritics, USA) method. An aqueous solution of H2O2 (Showa, 35%, Japan) was used for the hydroxylation of BaTiO3. Surfactants of benzoic acid (Showa, 99.5%, Japan) and benzylamine (Fluka, 98%, Switzerland) were used as the surface modification agents. De-ionized
Hydroxylation efficiency of H2O2-treatment
The effect of H2O2-treatment on the surface chemistry of BaTiO3 nanoparticles is studied by FT-IR, as shown in Fig. 1. For the as-received BaTiO3 (Fig. 1(a)), it shows only two absorption bands, one at 1447 cm−1 and one at 594 cm−1. The band at 1447 cm−1 corresponds to the stretching vibration of –CO32− that comes from the contamination of barium carbonate (BaCO3) in the BaTiO3 [16], and the band at 594 cm−1 represents the Ti–O vibration of BaTiO3 [17]. For the H2O2-treated BaTiO3 nanoparticles (
Conclusions
In our previous investigation, we have showed that the BaTiO3 nanoparticles can be easily and efficiently hydroxylated by H2O2-treatment. In this paper, a more detailed discussion about quantitative analysis of the hydroxylation efficiency and the chemical natures of the –OH on the BaTiO3 surface, including acid–base properties and chemical reactivity, is presented. From the result of quantitative XPS calculations, hydroxylation by H2O2 is found to significantly increase the concentration of
Acknowledgment
The authors are grateful for financial support from the National Science Council of the ROC under Grant No: NSC 98-2221-E-027-033.
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