Flat-band potential of a semiconductor: using the Mott-Schottky equation

In this experiment, suitable for fourth-year undergraduate and graduate students, we have explored the nature of semiconductor materials through determination of the flat-band potential using the Mott-Schottky equation. Experimentally, a technique was developed for preparing a suitable polycrystalline ZnO electrode for study. Note that a similar approach could be used for other semiconductor electrodes. Electrochemical impedance spectroscopy was then employed to examine the semiconductor-electrolyte, 7 × 10-4 M K3[Fe(CN)6] (1 M KCl), interface as a function of applied voltage. To achieve this a modified Randle's circuit was developed to interpret the impedance data, from which a value for the interfacial capacitance was determined. A Mott-Schottky plot was then constructed that allowed a flat-band potential of -0-316 ± 0.033 V versus SCE to be determined. The number of carriers, ND, was also determined: ND = 2 × 1024 m-3. Both results were comparable to literature data emphasizing the soundness of the technique.