Electrodeposition of TEOS

Walcarius and coworkers [36] further investigated the influence of the concentration of precursor on the electrodeposition of TEOS films (Figure 12.6). They found that the thickness of the films increased almost linearly as a function of deposition time when electrodepositing at -1.3 V versus Ag/AgCl from a diluted precursor with 13.6 mM TEOS, and it could be well controlled up to 100 nm. Electrodeposition from 75 mM TEOS solution had similar behavior as... 

The electrochemical deposition of sol-gel hybrid films was carried out in a mixture of at least two different sol-gel precursors. For example, Walcarius and coworkers [19-21,45] reported the co-electrodeposition of TEOS or methyltrie-thoxysilane (MTES) together with APTES or MPTMS for the electroanalysis of Cu(II) and Hg(II) in aqueous media. Liu and Mandler [46] reported the coelectrodeposition of TMOS with Ti(i-OPr)4, where they found that the obtained films had graded structure in composition. These works will be discussed in detail in this section. 

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... 

Figure 12.25 Scheme of the fabrication of amino-functionalized silica-modified gold electrode through self-assembly of MPTMS followed by electrodeposition of TEOS-I-APTES. (Reproduced with permission from Ref. [20].)... 

Electrodeposition of Cd Hgi-xTe films on a titanium [210] and stainless steel substrates [211] from a bath containing CdSO4 and TeO and HgCb was investigated. Film composition and band-gap energy were evaluated. 

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). 

A second, equally powerful means to prepare such materials relies on traditional inorganic polymerization tools, most notably sol-gel polymerization.24 25 A number of excellent reviews have appeared on this subject as well.5,12,17 In sol-gel processing, the functional monomer [i.e., an organoalkoxysilane such as 3-aminopropyltrimethox-ysilane (APTMS)] is combined with the cross-linking agent [i.e., a tetrafunctional alkoxysilane such as tetramethoxysilane (TMOS) or tetraethoxysilane (TEOS)], a catalyst (such as hydrochloric acid or ammonia), and the template molecule. The resultant sol can be left to gel to form a monolith, which can then be dried, sieved, and extensively washed to remove the template. Alternatively, the sol can be spin coated, dip coated, or electrodeposited on a surface to yield a thin film, which can be subsequently washed with a solvent to remove the template and yield the imprinted cavities. 

Te(n, y) "Te reaction. It decays with a half-life of 58 days and is equivalent to the "Sn isotope. Single-line sources have been obtained from matrices of PbTe [43], ZnTe [44], electrodeposited Te/Pt [45], and TeOa [46], but, as described shortly, there are recorded instances of radiation damage affecting the emission line in some cases. The /S-TeOs matrix gives a narrow fine (Ft < 5-5 mm s" ) with a substantial recoil-free fraction (>0-S4 at 80 K and >0-30 at room temperature) [47]. A scattering technique has also been used [40]. 

Microcomposite formation of PPTA film with silica The electrodeposited PPTA film was dried at 100°C to 5 wt% of absorbed water, which was soaked in an ethanol solution of tetraethyl orthosilicate (TEOS) at 60 C for one day. Cross-linking polycondensation of silanol derived from TEOS and water... 

The method is simple to perform. In a typical application, electrodeposition on the Au electrode was carried out in potentiostatic mode, applying -1.2 V. A typical starting solution contained 13.6 mmol TEOS, 20 mL ethanol, 20 mL aqueous solution of O.IM NaN03, and 1 mmol HCl, to which approximately 4.35 mmol CTAB was added under stirring. The authors suggest that H and water reduction (with possible contributions from nitrate reduction) increases the pH in the vicinity of the electrode and stimulates silicate deposition. 

Figure 12.5, that is, the thin-to-thick transformation. The growth of the film slowed down after the film was thicker than about 1200 nm, probably because the thick film blocked the electrode surface, thus reducing the accumulation rate of OH ions. Thick films were obtained by electrodeposition from the solution with 340 mM TEOS precursor even for very short time (about 10 s). 

Electrodeposited sol-gel-based composite films also showed optical applications. Gu and coworkers [85,86] co-electrodeposited Te0 > -Si02 hybrid films from the TEOS-Te(i-PrO)4 niixed precursor for nonlinear optics. Te(IV) was partially reduced during electrodeposition, as characterized by EDX of the obtained films. The as-prepared films had third-order nonlinear susceptibility ix ) of 5.9 X10 to 4.29 X 10 esu, and the films had of 1.551 X 10 esu after posttreatment annealing. Mandler and coworkers co-electrodeposited TMOS with multiwalled carbon nanotubes (MWCNTs) on ITO and silver. The optimized films electrodeposited on ITO showed transparency of about 50% with nonlinear optical properties, and the optimized films electrodeposited on silver had specular reflectance lower than 0.5% in the wavelength range of 400-15 000 nm, which can be used as antireflection coatings. 

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