Development of Morphology Control Method of Coating Layer Utilizing the Ligand Interaction between Iron Particle Surface and Buffer Solution
Authors: Asano Yuki , Kishimoto Akira , Ito Takahiro , Kawata Masanobu , Yokoyama Shun , Takahashi Hideyuki 
Published: February 2020

In recent years, with the progress of functional material development technology, our familiar products in electronic devices, such as smartphones and conductive materials, are dramatically improved in performance. In these functional material, various particles which surface were coated by various inorganic films using dry PVD processes, CVD processes and wet alcohol sol-gel processes, to show novel and characteristic properties. Among these coating methods, the inorganic film coating process of an aqueous solution process has been energetically researched, since it is environmental friendly and easiness to industrialize. For example, Kishimoto et,.al reported that uniformly coating method of titanium dioxide film onto iron particle surfaces by using a buffer solution as a mother liquid. Synthesized TiO2 coated iron particles shows the optical interference effects by alternately laminating several films having gaps of refractive indexes, and as a result, the powder color was changed from gray (the color of original iron powder) to blight blue and purple, etc.1

However, the control of the solid-liquid interface in this process is conplicated, and the slight changes in the surface condition of the substrate and/or its composition results in the formation of island-like structure or flat film structure rather than uniform and homogeneous coating. On the contrary, it means that, by the precise control between the solid-liquid interfaces, various coating morphology, for example, nano-dot coating structure using different substances, can be freely synthesized.
Therefore, in our research, we try to develop a method to control the morphology of TiO2 particles formed on the iron particle surface by controlling the interaction with the ligand which is made possible by focusing on the correlation between the iron particle surface and the buffer 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. As for the carbonyl iron powder, those degassed to 6.0 × 10−1 Pa in a vacuum degassing device and those not degassed were used. The carbonyl iron powder and the buffer solution were mixed in equal amounts and heated in a water bath to a solution temperature of 80℃ for 1 hour. Fines were removed from the supernatant after solution standing and the total dissolved iron was measured with ICP-OES (Optima8300, PerkinElmer). In addition, H3BO3-Na3(C3H5O(COO)3,) -Na3PO4・12H2O (pH9)was used for comparison with Clark&Lubs buffer solution. Furthermore, in order to specify a ligand, ESI-TOF-MS analysis was performed using BRUKER micrOTOFII-EST.
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 peroxidated titanium bonds to the substrate surface.
The amount of iron dissolved in the Clark&Lubs buffer solution was 1.7 mg/L, while that for iron particles before degassing was 0.6 mg/L, from the ICP-OES analysis results. In the reaction of iron particles in the buffer solution prepared for comparison, the color of the solution in the case of the iron particles before degassing exhibited a green color, which indicating ferrous citrate formation for about 12 hours, then it was changed into an orange color, which indicating ferric citrate formation. On the other hand, in the case of the iron powder after degassing, the color of the solution were changed into orange. From this results, it was suggested that the form of dissolved iron eluted divalent iron ion, and it changed into trivalent by dissolved oxygen2. From the results of ESI-TOF-MS analysis, multiple isotope peaks of the ligand, which caused the iron particle-buffer solution reaction. These results suggests that the morphology of the coating film can be controlled by regulating the iron complex species. 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)A.J.Francis and C.J.Dodge, Applied and Environmental Microbiology, vol59, No.1, 109-113(1993)