Morphological Control of Functional TiO2 Films on Magnetic Nanoparticle’s Surface by Utilizing the Boric Acid-Containing Solutions
Authors: Watanabe Yuki , Kishimoto Akira , Ito Takahiro , Kawata Masanobu , Ito Shun , Yokoyama Shun , Takahashi Hideyuki 
Published: February 2024

In recent years, various functional materials are used for electronic devices such as the smartphones and conductive materials. For the further expansion of these material’s usability and usage range, it is important to achieve the highly functionalized material’s manufacturing processes compatible with “green” processes. From this point of view, the development of a sustainable functional material synthesis method that achieves both high performance and reduced the environmental load has been required.
As one of such functional materials, Kishimoto1)et al. reported the synthesis and application of colored magnetic toners powders by using the buffer solution of pH 7-12 as a reaction field. In their methods, metallic iron particles coated with a homogeneous nanometer ranged TiO2 films are synthesized, and their color can be freely controlled by controlling the thickness of their films because of the interference effect.
However, it is also known that not only the homogeneous core-shell materials but also the plate-like or island-like structures can be sometimes formed on the core surface. Needless to say, contamination of these lowering the property of the functional materials. Conversely, if we can elucidate the formation mechanism of these films and can achieve the selectively formation of various structured films, new functional materials can be synthesized.
Therefore, in this study, TiO2 film-coated particles with various morphologies such as plate-like and dot island-like films are tried to synthesized, through the elucidation of the interaction between the surface condition and reaction solution.
Carbonyl iron powder (BASF EM grade, median diameter: 4-5 μm), and Clark& Lubs buffer solution (H3BO3-KCl-NaOHaq, pH 9, 0.4 M) were used. Tetra-isopropoxy titanium (C14H28O4Ti), hydrogen peroxide water, and ammonia water were used as the titanium oxide film raw materials in the subsequent film coating step. The temperature was optimized and film coating was performed so that the peroxotitanium bonds to the substrate surface.
Boric acid buffer solution after suspended the carbonyl iron particles were analyzed by XPS (XPS K-Alpha, Thermo Fisher Scientific, USA), and it showed a tendency to be significantly oxidized as compared to the particles before suspension. Waveform separation of the XPS spectrum of the O1s peak indicated that these peaks consisted by the oxygen atoms in the OH- and Fe2O3 species. ESI-TOF-MS (Bruker Dltonics; microTOFII) analysis results of the solution obtained after contacting the buffer solution and metallic iron particles shows that borate ions tended to decrease after the reaction. Thus, by controlling the relationship between the iron surface, boric acid, and the buffer solution in the solution, surface oxidation condition of the iron substance particles affected the formation of the subsequent TiO2 coating reaction. Details of other results will be present in our presentation.
1) WO/2003/031683, POWDER COATED WITH TITANIA FILM AND METHOD FOR PRODUCTION THEREOF.
2）G. Bhargava et.al, Characterization of the “native”surface thin film on pure polycrystalline iron: A high resolution XPS and TEM study,” Applied Surface Science2007; 253, 4322-4329.