Colloids and Surfaces A: Physicochemical and Engineering Aspects
Immobilization of colloidal particles into sub-100 nm porous structures by electrophoretic methods in aqueous media
Graphical abstract
Introduction
Deposition of suspended particles in aqueous media using electrophoretic deposition (EPD) promotes electrokinetic phenomena such as electrophoresis, water hydrolysis and electro-osmosis [1], [2], [3], [4]. When the electric field implies a bulk liquid that consists of the suspended particles, a significant motion of charged particles in a suspension or electrophoresis is noticed. In the vicinity of two electrodes, deposition of charged particles onto a substrate surface with an opposite charge occurs. Since then, various techniques to deposit particle materials have been invented [5], [6], [7], [8], [9], [10], [11]. However, bubble generation coming from water hydrolysis interrupted the deposition and causes the nm-order particles fail to deposit [5]. Thick film using EPD also contribute to electro-osmosis that could damages deposit layer and the surface when long deposition time was conducted [6]. Therefore, controls of certain EPD parameters are very important in order to ensure the deposit are uninterrupted during the deposition process. For examples to prevent the bubble generation, several EPD techniques has been invented such as alternating current (AC) EPD, pulse DC EPD, low voltage deposition and solvent–aqueous mixtures [5], [6], [7], [8], [9], [10], [12], [13], [14], [15].
On the other hand, the unique porous structure of a substrate is widely employed as not only a container (e.g., for catalysts with a high specific surface area), but also for other applications such as membrane filter and hard template for nano-materials [16], [17], [18], [19]. Electrophoresis approaches using porous substrates based on anodic aluminium oxide (AAO) [20], [21], [22] or graphite [23] had been already reported with using non-aqueous media. In the current work, we demonstrate the immobilization of pre-synthesized nanoparticle (colloidal samples) by EPD technique with conventional-DC and pulse-DC modes onto the porous AAO substrate having pore size of below 100 nm. Several parameters such as pH, applied voltage, and size of colloid were varied in order to evaluate the deposition mechanism. Post-deposition evaluation was also conducted by detachment (removal) of the deposited particles from substrate based on ultrasonic energy.
Section snippets
Preparation of porous substrate (AAO)
Nanoporous of alumina was formed by electrochemical anodizing of pure aluminum (99.99%) with 0.25 mm diameter and 80 mm length using a laboratory-made anodization cell [24], [25]. The plate was used as an anode, and platinum wire with 0.1 mm diameter was used as a cathode. Two electrodes were immerged into 100 ml electrolyte solution, which consisted of 0.6 M phosphoric acid (Wako Pure Chemical Industries, Tokyo) with a distance of 20 mm.
After preliminary experiments using various voltages and times,
Morphology of the AAO
A substrate having arrays with sub-100 nm scale pores used for anode was prepared by anodizing of aluminum using phosphoric acid. Phosphoric acid was used as electrolyte because it can increase pore diameter in comparison with other electrolytes, such as sulfuric acid, oxalic acid, and chromic acid, at an equal applied voltage [24]. Formation of pores during an anodizing process is known to depend on the electrolyte type, applied voltage, concentration and temperature [26]. On the other hand,
Conclusions
Electrophoretic deposition (EPD) of particles onto a sub-100 nm scaled pore arrays made from anodized aluminum has been investigated. By applying voltage lower than the decomposition voltage of water, the number of deposited particles on the surface using constant DC was found to be higher than that of pulse DC due to continuously electric fields attracting particles. The increase in the number of particles deposited on the substrate increased with an increase in pH can be ascribed to the
Acknowledgements
The authors are grateful to Drs. H. Kamiya, M. Iijima, M. Kuwata and M. Tsukada for supports, Dr. K. Kuchitsu for English-editing, Mr. Y. Yamada and M. Gen for assistance in simulation. This study was supported in part by Special Condition Funds for Promoting Science and Technology from the Japan Science and Technology Agency (JST), Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (JSPS: 26420761, 23246132, 23560904) and Grants-in-Aid for Scientific
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