(Invited) How Does Oxygen Open Up New Horizons at the Interface III-Vs/Liquid Ammonia?
Authors: Goncalves Anne-Marie , Rakotoarimanana Patie Cendra , Fregnaux Mathieu , Etcheberry Arnaud 
Published: July 2021

Oxygen is involved in many chemical and electrochemical processes due to its abundance and the difficulty to get rid of it. As a strong oxidizing species, the mechanism of oxygen reduction is significant from both a practical and a fundamental point of view. Depending on the nature of the interface, oxygen can be reduced in aqueous solution either via a two electrons reaction to hydrogen peroxide or a four-electron reaction to water. The electrochemical mechanism is indeed complex considering the large number of electrons involved. As a result, many intermediates are detected. Their chemical nature depends also on the solvent used (pH, stability of radicals...). Among non-aqueous solvents (MeCN, dimethylformamide, dimethyl sulfoxide, pyridine, THF...), liquid ammonia (NH3 liq.) has a unique position.
Indeed, in NH3 Liq., the chemical environment is obviously different from water, since their dielectric constants and viscosity coefficients are widely different[1]. Further, at room temperature, ammonia is a very strongly basic solvent (1011 times stronger than water) and a very weak acid (1029 times weaker than water)1. However, there are significant similarities between these two solvents. Both are protic that allows the protonation of oxygen intermediaries. But the strong basicity of NH3 liq avoids the formation of protonated intermediates. Under the atmospheric pressure and low temperature (to –33 ◦C from –78 ◦C), NH3 liq enables to carry out an in-depth study of the oxygen reduction by controlling the pH at many interfaces.
In NH3 liq at the dropping mercury electrode, under negative overvoltage, two successive waves from the oxygen reduction were observed[2]. Under illumination and using p-type III-V semiconductors (GaAs and InP), clarification is given on oxygen reaction mechanism. In buffered neutral pH (pH= 16,5 on NH3 scale) a current doubling effect was shown onto p-GaAs an p-InP under illumination[3]. One step is due to photoelectron capture from the conduction band (CB) and another step results from hole injection into the valence band (VB). The first reduction step was monoelectronic and led to the formation of the radical anion superoxide (O2
-). The second step led to the formation of O2
2-. As soon as NH3 liq becomes acidic (pH = 1, on NH3 scale), the current doubling disappeared and it is replaced by a significant decrease of the current in the dark onto p-GaAs. The reduction of oxygen occurs essentially from holes injection in VB. As a consequence, the slope of the Mott/Schottky plots decrease drastically (in the dark) at n- and p-GaAs. When the sample is removed, scratches are observed on GaAs wafers.
In acidic aqueous solvent, hydrogen peroxide is produced at the interface with III-V[4]. As a corrosive reactant, H2O2, can be also suggested in acidic NH3 Liq. Is it then possible to monitor the formation of oxide on the III-V in NH3 liq.?
[1] J. Jander, Anorganische und allgemeine Chemie in flussigen Ammoniak. Part I, Friedr.;, Vieweg & Sohn, Braunschweig, Germany, 1966.
[2] H. A. Laitinen and C. E. Shoemaker, Polarography of Thallium, Copper, Ammonium and Oxygen, J. Am. Soc., 70, 4975 (1950).
[3] A-M. Gonçalves, C. Mathieu, M. Herlem, and A. Etcheberry, J. Electroanal. Chem., 462, 88 (1998)

[4]J. Jaume, C. Debiemme-Chouvy, J. Vigneron, M. Herlem, E. M. Khoumri, J. L. Sculfort, D. Le Roy, and A. Etcheberry, J. Phys. III Fr., 4, 273 (1994).