The pH response of the InP/liquid ammonia interface at 223 K: A pure nernstian behavior

Abstract

Liquid ammonia (NH 3 liq.) provides an original electrochemical environment onto semiconductors electrodes (SC). Its particular interest is that water influences can be neglected in opposition to a lot of non-aqueous other solvents. Fundamentals electrochemistries of the SC are related to the energy diagram of the interface. In this paper it is established into NH 3 liq., on both types, on p- and n-InP by flat band potential ( V fb) measurements. The V fb are determined over the whole range of pH that reaches 33 pH units in this non-aqueous solvent. InP exhibits a pure nernstian behavior with a specific 44 mV/pH slope at 223 K. This is particular compare to the “under nernstian” dependency, observed in water onto InP. The actual band positions are related to the initial chemical state of the InP surface, since oxide free surfaces and thin native oxide covered surfaces differ slightly. Reproducible contrasted results from the acid–base equilibrium, which is supported by the V fb according to the pH, for singular interfacial chemistry. This aspect is confirmed by XPS measurements performed before and after InP immersion into NH 3 liq. They establish the perfect stability of the initial chemical composition of the semiconducting surface in contact with NH 3 liq. whatever its pH conditions. NH 3 liq. appears as an inert solvent which is able to create an acid–base equilibrium onto InP surfaces. Each InP surface chemistry support its own linear V fb vs. pH variation. In NH 3 liq., the poor water control on the building of the Helmholtz layer is well shown by the perfect V fb alignment position from the intermediated pH buffered solution obtained from the addition of tetraethyl ammonium hydroxide ((Et) 4N +,OH −) dissolved in water (20%).