The subject of sol−gel electrochemistry is introduced, starting with a brief account of milestones in its evolution. Then, the types of sol−gel materials that are useful for electrochemistry are presented, followed by a description of recent advances in the various fields of sol−gel electrochemistry. Modified electrodes, solid electrolytes, electrochromic devices, and corrosion protection coatings are described. Emerging fields such as RuO2 supercapacitors and electrochemical synthesis of sol−gel precursors are also addressed.

Sol−Gel Materials in Electrochemistry

Sol-gel derived films can be deposited on metals to improve their resistance to oxidation and corrosion or to modify their surface properties. However, practical applications are limited by problems intrinsic to sol-gel processing or specific of coating/metal systems. Coatings aimed to improve oxidation and wet corrosion resistance have been the most studied. The results published in the literature show that sol-gel coatings may offer good protection against oxidation. More difficult is to achieve a protection against wet corrosion. An important aspect of the application of the sol-gel method for coating metallic objects is also the deposition technique.

Sol-gel coatings on metals

Controllable synthesis of metastable tetragonal zirconia nanocrystals using citric acid assisted sol–gel method

We report here on the design, synthesis, processing, and dielectric properties of novel cross-linked inorganic/organic hybrid blend (CHB) dielectric films which enable low-voltage organic thin-film transistor (OTFT) operation. CHB thin films (20−43 nm thick) are readily fabricated by spin-coating a zirconium chloride precursor plus an α,ω-disilylalkane cross-linker solution in ambient conditions, followed by curing at low temperatures (∼150 °C). The very smooth CHB dielectrics exhibit excellent insulating properties (leakage current densities ∼10−7 A/cm2), tunable capacitance (95−365 nF/cm2), and high dielectric constants (5.0−10.2). OTFTs fabricated with pentacene as the organic semiconductor function well at low voltages (<−4.0 V). The morphologies and microstructures of representative semiconductor films grown on CHB dielectrics prepared with incrementally varied compositions and processing conditions are investigated and shown to correlate closely with the OTFT response.

Flexible Low-Voltage Organic Thin-Film Transistors Enabled by Low-Temperature, Ambient Solution-Processable Inorganic/Organic Hybrid Gate Dielectrics

Undoped or Y2O3-doped ZrO2 thin films were deposited on self-assembled monolayers (SAMs) with either sulfonate or methyl terminal functionalities on single-crystal silicon substrates. The undoped films were formed by enhanced hydrolysis of zirconium sulfate (Zr(SO4)·4H4O) solutions in the presence of HCl at 70°C. Typically, these films were a mixture of two phases: nanocrystalline tetragonal- (t-) ZrO2 and an amorphous basic zirconium sulfate. However, films with little or no amorphous material could be produced. The mechanism of film formation and the growth kinetics have been explained through a coagulation model involving homogeneous nucleation, particle adhesion, and aggregation onto the substrate. Annealing of these films at 500°C led to complete crystallization to t-ZrO2. Amorphous Y2O3-containing ZrO2 films were prepared from a precursor solution containing zirconium sulfate, yttrium sulfate (Y2(SO4)38·H2O), and urea (NH2CONH2) at pH 2.2–3.0 at 80°C. These films also were fully crystalline after annealing at 500°C.

Synthesis of ZrO2 and Y2O3‐Doped ZrO2 Thin Films Using Self‐Assembled Monolayers

A method for applying zirconia barrier coatings using a sol-gel method is described. The coatings of 8 wt % yttria-stabilized zirconia are applied by spin coating a solution containing zirconium alkoxides and yttrium acetate on to stainless steel substrates. Crystallization of the films was observed for thermal treatments in the range 750 to 1050° C. Excellent adhesion at the interface, due to significant coating-substrate interfacial reactions, indicates that this sol-gel route is a feasible method for applying zirconia coatings.

M. Shane

Papers

Sol-gel synthesis of zirconia barrier coatings