טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentShvartsev Boris
SubjectChemical and Electrochemical Behavior of SiO2
in Room Temperature Ionic Liquids
DepartmentDepartment of Materials Science and Engineering
Supervisor Professor Yair Ein-Eli
Full Thesis textFull thesis text - English Version


Abstract

As the need for healthier, environmentally friendly industrial solvents increases, an ongoing quest for the next electrolyte of choice brought a new class of solutions into the spotlight, as one can find in the Room Temperature Ionic Liquids (RTILs). The RTILs, as a media for 'wet' chemical and electrochemical reactions, possess some uncommon eco-friendly and industrial advantages. Nevertheless, although ionic liquids were studied for more than 30 years, no real large-scale applications may be considered yet. In the work presented here, a unique RTIL media was applied for oxides dissolution reaction. The focus of the research was on silica (SiO2) etching in 1-ethyl-3-methyl-imidazolium oligofluorohydrogenates [EMIm(HF)2.3F]. Initially, in order to allow proper electrochemical measurements in RTIL media, a new type of a Reference electrode was developed, constructed and evaluated. The electrode configuration was based on ferrocene (Fc|Fc) dissolved in EMIm(HF)2.3F /Carbopol 941 gel. A stable potential, at 250±3mV versus a SCE (Calomel) Reference electrode, was measured using a 2.5wt% Carbopol based electrode. Utilizing the constructed electrode allowed different possible electrochemical studies. The dissolution study's methodology was based on Open Circuit Potential (OCP) monitoring of thermally grown SiO2 on Si wafers. The initial research with these samples was conducted applying different volumes of 10%wt. HF aqueous solution. The analysis showed alterations in the HF solution, depending on the thickness of the oxide film. Additionally, in small electrolyte volumes an effect of SiO2 thickness on the dissolution rate was detected. In particular, a higher etch rate of SiO2 was recorded in the smallest solution volume. The detected alterations were explained by a shift in HF activity following SiO2 etching, where spectroscopic NMR and ATR - FTIR analyses of the solutions suggested HF2- accumulation as the main chemical composition change. Similar samples were also applied in dissolution studies in EMIm(HF)2.3F RTIL. The etch rate of SiO2 in this specific ionic liquid media was found to be lower by an order of magnitude than the measured rates in aqueous HF. The spectroscopic studies enabled to determine the dissolution mechanism in oligofluorohydrogenate ionic liquid. A confirmation of both water and SiF62- formation, as reaction products, was established. Moreover, the presence of water was found to influence the anionic ratio in ionic liquid. This was further addressed and the conclusion was that H2F3- disproportionation reaction into H3F4- and HF2-, promoted by the presence of water, was the main anionic change. The final stage of the research presented here addressed the etching reactions of different oxide materials, as generalization of the proposed way of dissolution studies. Based on previously established tools and methodology of investigation, alumina dissolution as part of aluminium activation was studied. It was confirmed that oxide dissolution occurs and a new film is formed on the surface. This surface modification results in a transformation from passive to active behaviour of the metal. The modification was confirmed via the implementation of electrochemical methods and characterization by XPS, AFM and TEM. This newly built-up layer restricts corrosion currents, while enabling high rates of aluminum anodic dissolution.