Electrodeposition of silane

The electrochemical deposition approach is versatile and applicable to various sol-gel systems, including silanes and metal alkoxides. Most of the research has focused on the electrodeposition of silane-based sol-gel films. The latter are typically electrodeposited from aqueous solutions of the monomers in the presence of ethanol or methanol as a cosolvent The deposition is usually carried out at cathodic potentials, although a few groups [11,14,15] also reported the anodic electrodeposition process. Electrodeposition of non-silicon sol-gel films from... 

Both reactions generate OH ions that increase the interfacial pH in the vicinity of the cathode, which catalyzes the sol-gel process facilitating the film formation. The process is illustrated in Figure 12.2. van Ooij and coworkers [10] showed the enhancement in forming an interfacial layer of —Si—O—metal covalent bond by electrodeposition of silane on aluminum alloys. There are three major advantages of the electrodeposition approach, as clearly pointed out by Mandler and coworkers in their first publication [2] (1) the pH varies only close to the cathode, so the stability of the bulk solution is not affected (2) the deposition process is controllable by electrochemical parameters and (3) the film deposition is restricted to the conducting part of the surface and is controlled by the kinetics of the electrochemical process. Note that some reports refer this approach as electro-assisted deposition, since it is an indirect electrodeposition process where the sol monomer is not directly involved in the electrochemical reactions [15,19,28,29]. 

Sol-gel materials are also known as an excellent matrix for embedding other species due to their tunable physical properties (e.g., flexibility and transparency), high chemical stability, and mild operating conditions. Especially, electrochemical deposition of silane-based sol-gel Aims is usually carried out under mild acidic aqueous solutions at pH 3-6. This allows the co-electrodeposition of silane with nanoparticles [47-50], carbon nanotubes [51-53], metals [54-57], polymers [50,58], enzymes [52,53,59-65], bacteria [66,67], and more. Thus, most of the recent research worlcs also focus on the electrochemical deposition of sol-gel-based composite Aims, with the concern of improving the films performance in corrosion protection, electroanalysis, microextraction, and so on and further broadening the films applications. 

Several approaches have been reported by Hu and coworkers to further improve the anticorrosion performance of electrodeposited silane sol—gel films. They found that adding proper concentration of nitrate ions into the silane deposition solution facilitated the electrodeposition process by reducing nitrate to nitrite, and thus enhanced the corrosion resistance of the films obtained [31]. Co-electrodeposition of silane with silica or titania nanoparticles [47,49], or Ce (N03)s [77], ako improved the films anticorrosion performance. Small content of nanoparticles was foimd to increase the compactness of the electrodeposited silane films. Ce(III) provided a self-healing effect for long-term immersion of the dip-coated Ce(III)-doped bis(triethoxysilylpropyl) tetrasulfide (BTSPS) films in... 

Sol-gel-derived silane-based materials have been proven as a suitable matrix for entrapment of bioactive species, due to biocompatibility of silica and the mild operating conditions. We recall that the solutions used for cathodic electrochemical deposition of silane-based sol-gel films are usually mild acidic (pH 3-6), and the deposition is achieved by electrochemically driving the inter- cial pH near the cathode to mild basic, about pH 8. This environment is favorable for maintaining the activity of biological species such as proteins, enzymes, and bacteria. Many researchers have reported the fabrication of biosensing films by co-electrodeposition of silane with different bioactive substances. The essence of the concept is to entrap bioactive substances within the sol-gel matrix during... 

Herlem, G., O. Segut, and A. Antoniou, 2008. Electrodeposition and characterization of silane thin films from 3-(aminopropyl)triethoxysilane. Surf. Coatings Technol 202 1437-42. 

The mechanism described above is generally accepted, and it is also applicable for the cathodic electrodeposition of non-silicon sol-gel films from their alkox-ide precursors. Mandler and coworkers [2] added phenolphthalein into the silane solution and observed purple color upon applying -1.3V versus Ag/AgCl on the cathode surface. This indicated that the surface pH exceeded 8.2, which could... 

The most reported application of eiectrodeposited silane sol-gel films has been the corrosion protection of metals. The corrosion resistance of the films is usually evaluated by polarization curves and electrochemical impedance spectroscopy (EIS) measured in corrosive media. Mandler and coworkers [73] reported the electrodeposition of TEOS, MTMS, and PhTMS films on aluminum alloys. They found that all the three eiectrodeposited silane films protected the aluminum alloys, and the PhTMS film provided the highest corrosion resistance due to its hydrophobic aromatic ring. Figure 12.20 shows that the eiectrodeposited PhTMS film performs five orders of magnitude higher in impedance modulus... 

The performance of the films can be further improved by co-electrodeposition with other species such as metals, nanoparticles, and carbon nanotubes. Moreover, electrodeposited sol-gel films may also serve as a platform for embedding fiuictional materials. Specifically, the sol-gel system is ideal for immobilizing bioactive species, such as enzymes and bacteria thus, electrodeposition approach allows simple one-step fobrication of biosensors. Electrodeposition also yields selective deposition of silane/CNT antireflective coatings on conductive patterns. Molecularly imprinted films are prepared by co-electrodeposition of the target molecule with sol-gel followed by its removal. 

The effect of deposition potential on the thickness of electrodeposited silane films was first demonstrated by Mandler and coworkers [2]. They found that the thickness of MTMS films increased as the potential of the cathode was made more negative, as shown in Figure 12.4. This could be explained by the enhanced OH generation at more negative potentials due to the electrochemical reduction of O2 and H2O. Similar trend was observed in many later reports, and it affirms the deposition mechanism as suggested in the previous section [21,36]. They also pointed out that much thicker films were electrodeposited on gold than on indium tin oxide (ITO) electrode under the same deposition conditions. This confirms that the deposition is affected by the kinetics of electrochemical OH generation rather than the electrophoretic effect... 

Electrodeposition from sol-gel solutions containing two or more different sol-gel precursors yields hybrid films, as the sol-gel components hydrolyze and condense together forming a cross-linked structure. The hybrid films may be electrodeposited from precursors consisting of either two or more different silanes, or silane with titanium or zirconium alkoxides. 

Electrochemical deposition has been widely applied for fabricating sol-gel-based electrochemical sensors. The porous nature of the electrodeposited silane films, which originates from H2 evolution and in some cases also surfactant, allows... 

A) and the same electrode covered with thiol-functionalized silica films electrodeposited from precursor solutions with different content of MPTMS (molar ratio to total silane) 5%... 

MPTMS content up to 10% (molar ratio to total silane). The electrodeposited pure TEOS film showed no preconcentration behavior. These results indicate that the — SH moiety was essential and drove the accumulation of Hg(II) species. The preconcentration effect depended on the content of —SH groups in the film. Note that when the MPTMS ratio was too high, the electrodeposited films became less porous and therefore inhibited electron transfer on the electrodes. This lead to the decrease in current response for the modified electrodes in both Fe(CN)6 and Hg(II). Furthermore, the authors also demonstrated that the preconcentration effect was sensitive to the hydrophilicity of the films. They found that raising the hydrophobicity of the films by adding even low concentration of MTES (about 5% molar ratio to total silane) in the deposition solution severely deteriorated the electroanalysis performance. Fink and Mandler [79] also electrodeposited MPTMS on cylindrical carbon fiber microelectrodes for electrochemical determination of Hg(II). 

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