The oxygen evolution reaction (OER) in alkaline media was investigated on nanostructured FeO, NiO, and NiFeO (Fe-doped, rocksalt NiO, x = 0.05-0.19) electrocatalysts deposited via microplasma on indium tin oxide. A detailed investigation of film morphology, structure, and chemical surface state using SEM, XRD, and XPS, respectively, was carried out to understand catalytic activity, which was assessed using cyclic voltammetry and chronopotentiometry. Iron was seen to be fully incorporated into the parent rocksalt NiO lattice during microplasma deposition, and overpotentials (η) decreased from 360 mV for NiO to 310 mV for NiFeO at 10 mA cm. Interestingly, overpotential did not change significantly for Fe compositions from 5-19%. The NiFeO films displayed relatively low Tafel slopes of 20-30 mV dec at 0.01-1 mA cm, demonstrating their high activity for (OER). Turn-over-frequency (TOF, i.e., O molecules per Ni atom per s) at η = 350 mV revealed a continuous improvement in activity of the NiO surface with increasing Fe content, where values of 0.07 and 0.48 s were measured for undoped NiO and NiFeO films, respectively. Chronopotentiometry measurements followed by SEM and XPS verified that the as-deposited NiFeO catalysts were mechanically and chemically stable for OER under alkaline conditions. This work highlights that microplasma-based deposition is a general approach to realize conformal coatings of nanostructured, doped oxides with high activity for OER.

A general, substrate-independent method for plasma deposition of nanostructured, crystalline metal oxides is presented. The technique uses a flow-through, micro-hollow cathode plasma discharge (supersonic microplasma jet) with a 'remote' ring anode to deliver a highly-directed flux of growth species to the substrate. A diverse range of nanostructured materials (e.g., CuO, -Fe 2 O 3 , and NiO) can be deposited on any room temperature surface, e.g., conductors, insulators, plastics, fibers, and patterned surfaces, in a conformal fashion. The effects of deposition conditions, substrate type, and patterning on film morphology, nanostructure and surface coverage are highlighted. The synthesis approach presented herein provides a general and tunable method to deposit a variety of functional and hierarchical metal oxide materials on many different surfaces. High surface area, conversion-type CuO electrodes for Li-ion batteries are demonstrated as a proof-of-concept example.The ability to synthesize functional nanoscale materials, as well as to integrate these structures into devices, is fundamental for the development of next-generation micro-and optoelectronic devices, sensors, and energy harvesting and storage technologies [1][2][3][4]. Realization of nanomaterials and multi-scale systems often requires complicated processing steps that may involve a combination of wet chemistry, physical/chemical vapor deposition, vapor-liquid-solid or molecular beam epitaxy, self-and/or directed assembly, lithography, and etching. In addition, both wet and dry conditions, long processing times, high temperatures, vacuum processing, and templates or catalysts can be required. As such, we continually seek to develop general and tunable methods that can easily and rapidly create nanostructured functional materials. For example, atmospheric pressure plasmas [5,6], plasma sprays [7][8][9], and microplasmas [10][11][12][13][14][15][16][17][18][19][20] have shown much promise toward this goal. Extending and adapting such methods in a generic way to different material systems and deposition situations, as well as understanding how plasma operating conditions affect growth processes, is critical for their implementation.