One-step wet chemical deposition of NiO from the electrochemical reduction of nitrates in ionic liquid based electrolytes

Abstract

Aprotic PYR14TFSI (1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)) ionic liquid served to develop a new electrochemical route for one-step deposition of NiO from PYR14NO3 reduction (1-butyl-1-methylpyrrolidinium nitrate) in a Ni(TFSI)2 (Nickel (II) bis(trifluoromethanesulfonyl)imide) containing electrolyte. The high solubility of the novel PYR14NO3 salt in PYR14TFSI (>0.1M) in comparison with other oxygenated precursors such as oxygen gas, NaNO3 or KNO3 (i.e. 10–15mM) allows the formulation of a broad variety of electrolytes which opens wide possibilities to tune the physico-chemical properties of NiO films (e.g. morphology: from flat to nanostructured films). Furthermore, electrochemical deposition in an electrolyte containing low water concentration (>30ppm by Karl Fisher titration) served to demonstrate that only a small amount of moisture dramatically affects the electrochemical reduction of NO3−, resulting in OH− generation close to the cathode and subsequent NiO(OH)/Ni(OH)2 deposition, as proved by X-ray diffraction and X-ray photoelectron spectroscopy. This finding highlights the importance of aprotic ionic liquids in developing a general electrochemical route for metal oxide deposition without the formation of metal hydroxide species, thus avoiding the requirement for post-deposition annealing treatments. The versatility of the present deposition route as well as its impact in (opto)electronic devices was pointed out by the successful preparation of nanostructured n–p ZnO/NiO heterojunctions exhibiting rectifying current–voltage characteristics.