A study of the oxide/solution interface by capacitance measurements of electrolyte/oxide/semiconductor structures and potentiometric titrations of colloidal oxide suspensions Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/tm70n055n

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  • The oxide/electrolyte solution interface has been studied experimentally by two methods, and a mathematical model for the interface has been developed. The change in interfacial potential with solution composition was determined through the capacitance of electrolyte/oxide/semiconductor (EOS) structures and the charge through potentiometric titrations of colloidal oxide suspensions. The differential capacitance of the EOS structures was determined as a function of applied voltage for several electrolyte solutions with different compositions (pH, ionic strength). The shifts of the capacitance-voltage curves along the voltage axis with changes in solution composition represent directly the corresponding changes in the potential at the oxide/electrolyte interface, ΔΨ₀. A rigorous mathematical model based on the chemical and electrostatic properties of the oxide/electrolyte interface and the physics of the metal/oxide/semiconductor structures have been proposed to explain the behavior of the EOS structures. The following surface hydrolysis reactions were used to describe the chemistry at the oxide/electrolyte interface. XOH + H⁺ = XOH₂⁺K₊ XOH = X0⁻ + H⁺ K₋ An apparent surface ionization constant K[subscript D] is defined as K[subscript D]= (K₊K₋)[superscript 1/2] The Gouy-Chapman model has been used to describe the electrostatics of the interface. The mathematical model has been used to derive an equivalent circuit for the oxide/electrolyte interface, which allows the behavior of the interface to be understood at an intuitive level. The ΔΨ₀/ΔpH values were obtained for the EOS structures with an n-type Si semiconductor and SiO₂ in KNO₃ solutions of pH range 3-7 and ionic strength range 0.01-0.1 M. These AΨ0/ApH values have been explained reasonably well by the proposed model, with a K[subscript D] value of 0.01 and a surface site density N[subscript s] of 8x10¹⁴. The surface excess concentrations of H⁺ on TiO₂ suspended in KNO₃ solutions have been determined by potentiometry. From the intersection of surface excess vs. pH curves, the pH of zero charge of anatase in KNO₃ solutions was found to be 6.4. The surface excess concentration of the electrolyte cation K⁺ was also determined by potentiometry. Comparison of surface excess concentrations of H⁺ and K⁺ and the calculated charge in the diffuse layer indicate that K⁺ and NO₃ are specifically adsorbed on the TiO₂ surface.
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  • 1986
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