Electrodeposition of Hydrophobic Nickel Composite Containing Surface-Modiﬁed SiO 2 Particles under the Inﬂuence of a Surfactant with an Azobenzene Moiety Nabeen K. Shrestha, Genta Kobayashi, and Tetsuo Saji  Department of Chemistry and Materials Science, Tokyo Institute of Technology, Ohokayama 2-12-1-S1-44, Meguro-ku, Tokyo 152-8552 (Received April 14, 2004; CL-040416) Electrodeposit of nickel with high content of the surface- modiﬁed silica particles by a silane-coupling agent was prepared from the Watts nickel bath containing the silica particles dis- persed with the aid of a surfactant containing an azobenzene moiety. This composite coating plated under optimized condi- tions exhibited the ultra high water repellency property with the contact angle of 156  . Being less sensitive to water, hydrophobic composites are applied in many ﬁelds where the devices and the materials can be hardly used in an aqueous medium. For example, electrodes with hydrophobic composite coating have high oxygen–hydro- gen overpotential and therefore these electrodes can be used in a wide range of potential window to the electrosynthesis of or- ganic compound from an aqueous solution. 1 Similarly, hydro- phobic composite coatings possess high degree of corrosion re- sistance since these coatings are hardly attacked by the aqueous solution of acid, base and salts. Such hydrophobic coatings can be prepared by incorporating the inert and hydrophobic particles into a metallic matrix from the metal plating bath 1 and by the treatment of a metal/silica composite with a silane-coupling agent. 2 The properties of such composite coating depend on the amount of reinforcement and generally, high content of the reinforcements are desirable for many applications. However, the amount of the particle reinforced by the composite plating technique is often limited to few vol %. In our previous experi- ments, we demonstrated that redox-active surfactants with an azobenzene group losses its surface activity upon electrochemi- cal reduction and this phenomenon was applied to the electro- chemical formation of organic thin ﬁlms. 3,4 Further, we demon- strated that this phenomenon can be applied as a device to promote the particle co-deposition of various ceramic particles with nickel. The particle content in this case was considerably higher than that from a nickel bath with the similar type of sur- factant without an azobenzene group. 5,6 SiO 2 is a well-known ceramic, which is used to synthesize a metal matrix composite in order to apply as a protecting coating form corrosion. 7 Recent- ly, we have shown that SiO 2 particles hardly co-deposit with nickel from an aqueous nickel bath owing to its strong hydration energy but it readily deposits to some extent from a non-aqueous nickel-plating bath. 8 Tari et al. 9 had also found that there was al- most no interaction between SiO 2 particles and nickel ions in an aqueous solution. Therefore, they could not co-deposit these par- ticles with nickel from an aqueous nickel-plating bath. In order to overcome this diﬃculty to co-deposit the SiO 2 particles with nickel, they treated these particles with a silane-coupling agent. In the present investigation too, we could hardly deposit the SiO 2 particles into the nickel deposits from Watts bath even using var- ious surfactants. Therefore, in order to promote the co-deposi- tion of the particles, the hydrophilic surface of these particles were modiﬁed into hydrophobic by the treatment with a func- tional short-chain siloxane, viz. oligodimethyl siloxane-,!- diol (OHDMS), which was synthesized according to the method proposed by Kobayashi. 10 100 g of the SiO 2 particles ( ¼ 0:8 mm, Soekawa Chemical Co. Ltd.) was treated with 10 g of the OHDMS in toluene using the method of Terzieva et al. 11 For the composite plating, 10 g dm 3 of these hydrophobic SiO 2 particles was added to the Watts nickel bath (pH 1) contain- ing 300 g dm 3 NiSO 4 . 6H 2 O, 60 g dm 3 NiCl 2 . 6H 2 O, 40 g dm 3 H 3 BO 3 and various amount of azobenzene surfactant (AZTAB, Figure 1). This mixture was stirred with a magnetic stirrer until all the ﬂoated particles were being suspended in the electrolyte. Even in the presence of AZTAB, it took about an hour or more to suspend all the added particles in the bath showing the strong hydrophobic surface property of the parti- cles. This suspension was ultrasonically agitated for 10 min be- fore it was used for the composite plating. A polished and cleaned copper plate was used as the substrate. This substrate was immersed into the nickel bath in a vertical position parallel to a nickel anode plate at a distance of 15 mm and electrodepo- sition was carried out under the current density of 3 A dm 2 for 30 min at the temperature of 50  C. After the deposition, the deposits were cleaned ultrasonically in water for 10 min. The contact angle between a water drop and the coating surface was measured using a contact angle meter (FACE CA-D). The particle content in the coatings was analyzed gravimetrically as described in our previous paper. 6 The Inﬂuence of AZTAB on the co-deposition of surface- modiﬁed hydrophobic SiO 2 particles with nickel is shown in Figure 2. The maximum particle co-deposition of 45 vol %. was achieved when the concentration of AZTAB in the bath was 0.6 g dm 3 . A similar type of co-deposition behavior for oth- er kinds of particles with nickel was observed under the inﬂu- ence of AZTAB. 5,6 Such a large amount of particle co-deposition using other types of surfactant is hardly possible. For example, despite the equivalent surface activity of the n-tetradecyl trime- thylammonium bromide (TDTAB) to that of the AZTAB, 12 the amount of particle co-deposited from a bath containing this sur- factant (TDTAB) was hardly about 10 vol %. This diﬀerence be- tween the above two surfactants for promoting particle co-depo- sition might be due to their diﬀerent redox activity. As explained earlier, 5,6 AZTAB is electrochemically reduced during the dep- osition of nickel. This surfactant after reduction looses its sur- face activity, which leads to deposit the particles on the cathode. C 4 H 9 N N (OC 2 H 4 )-N-(CH 3 ) 3 , Br Figure 1. Molecular structure of AZTAB. 984 Chemistry Letters Vol.33, No.8 (2004) Copyright Ó 2004 The Chemical Society of Japan