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.

a b s t r a c tMagnetic exchange bias and coercivity of nanogranular NiFe 2 O 4 /NiO thin films, prepared using flowstabilized microplasmas and post-deposition annealing, have been investigated as a function of ferrimagnet/antiferromagnet phase fraction, grain size, and temperature. Exchange bias (EB) and vertical shifts in hysteresis loops observed in the as-deposited and low-T annealed ( r 600°C) films were attributed to exchange coupling between nanocrystalline NiFe 2 O 4 ( $ 8-10 nm) and a structurally-disordered spin glass (SG)-like phase. At higher annealing temperature (850°C), the observed EB was found to arise from exchange coupling between NiFe 2 O 4 and NiO, rather than a SG phase, most likely due to reduction of structurally-disordered interfaces and a substantial increase in NiFe 2 O 4 grain size ( $ 26 nm).

Biphasic, nanostructured NiFe 2 O 4 /NiO films exhibiting exchange bias were synthesized using high-pressure microplasma-based deposition. Organometallic Ni and Fe precursors were dissociated via electron impact in the hollow cathode region of a supersonic plasma jet with oxygen to form a directed flux of Ni and Fe oxide species that were subsequently spray-deposited on SiO 2 /Si substrates to form nanostructured thin films. Exchange bias (H E ) and coercivity (H C ) enhancement were observed for all films containing NiO after field cooling, indicating an exchange interaction between NiFe 2 O 4 and NiO. H E and H C increased with NiO content, which may be explained by a concomitant increase in NiFe 2 O 4 /NiO interfacial density. Large H E (∼250 Oe) and H C (∼700 Oe) were measured at 20 K, and exchange bias persisted at high temperatures (305 K) where an H E and H C of 80 and 400 Oe were observed, respectively. ■ INTRODUCTIONIntimate contact between a ferromagnet (FM) and an antiferromagnet (AFM) can result in exchange bias (EB), a phenomenon where the strong magnetic anisotropy of the AFM causes a shift (H E ) in the magnetic behavior of the softer, FM material after the system is field-cooled through the Neél temperature (T N ) of the AFM. These systems are characterized by pinning of magnetic spins where exchange coupling at the FM/AFM interface imparts supplementary magnetic anisotropy opposing the moment reversal of the FM in moderate to large reverse fields. 1 In addition, increased coercivity (H C ) and opening of hysteresis loops (M R /M S ; remanence/saturation magnetization) are well-known effects of exchange coupling in FM/AFM systems, which are of particular interest for permanent magnet design. 2−4 Exchange bias has also been studied in other inhomogeneous systems, such as those involving ferrimagnets (FiM) and spin glass (SG) phases as well as for applications including data storage (M-RAM), microelectronics (spin and tunneling valves), magnetic sensors (read/write heads), and medicine (drug delivery). 5,6 More recently, there has been renewed interest in exchange biasing of magnetic nanoparticles (d < 20 nm) for applications in ultrahigh density recording 6−9 to surpass the superparamagnetic limit, i.e., the temperature above which ferromagnetism becomes thermally unstable due to a decrease in magnetic anisotropy energy with size.A wealth of fabrication techniques have been explored to synthesize particle-based exchange bias systems, albeit with mixed results. The first and most widely studied method consists of chemically modifying transition metal-based FM nanoparticles using partial oxidation, nitridation, or sulfidization (e.g., Ni−NiO, 10 FeCo−FeCoO, 11 Fe−FeS, 12 Co−CoN 13 ), which can result in exchange bias effects. 14 Issues with these approaches include a limited suite of material combinations and difficulty in independently controlling the (oxide) shell thickness 15 and crystallinity. 16 The resulting "disordered" structures often lead to weak exchange bias and the onset of superparamagn...

We present a hybrid plasma spray deposition technique, based on geometrically-confined, supersonic microplasma jets, which can realize a wide range of metal oxide/sulfide nanoparticles and nanostructured thin film materials (e.g., CuO/CuS, ZnO, SnO2, NiO/NiFe2O4) on virtually any surface. Organometallic precursors are dissociated in a hollow cathode microplasma jet under different reducing/oxidizing atmospheres at high pressure (10-100 torr), creating a directed flux of active metal and oxide species for the subsequent growth of nanostructured films. Interaction of the jet afterglow with the background gas can create additional species (e.g., excited neutrals, radicals, etc.) which participate in film growth. By varying supersonic jet flow characteristics, plasma current, precursor flux, source distance, and deposition time, deposits ranging from isolated nanoparticles to films of fibers, aggregates, nanowires, and dense columns can be realized. The talk will highlight our recent efforts [1-5] in oxide/sulfide nanomaterial synthesis via microplasmas with emphasis on the physics of the jet source, dynamics of the growth process, and applications such as solar cell electrodes, photo(electro)catalysis, and nanogranular films for magnetic exchange bias applications. [1] T. Koh and M.J Gordon, J. Phys. D: App. Phys. 46, 495204 (2013). [2] T. Koh, I. Chiles, and M.J Gordon, Appl. Phys. Lett. 103, 163115 (2013). [3] T. Koh and M.J Gordon, JVST A 31, 061312 (2013). [4] T. Koh and M.J. Gordon, J. Crystal Growth 363, 69 (2012). [5] T. Koh, E. O'Hara, and M.J. Gordon, Nanotechnology 23, 425603 (2012).

Microplasma-based spray deposition was used to create high surface area Ni-Fe and Co oxide nanostructures on a variety of substrates (e.g., ITO, lithographic patterns, fibers) for electrocatalysis and supercapacitor applications. NiO, Fe-doped (rocksalt) NiO, NiFe2O4, and CoxOy films were deposited at 10-100 torr using a flow through, hollow cathode supersonic jet plasma with organometallic precursors. The resulting films were subsequently evaluated for their oxygen evolution reaction (OER) activity and potential use as Faradaic supercapacitors. OER overpotentials were compared for the various Ni-Fe oxides, and Fe-doping of the rocksalt NiO structure at the few % level was seen to lower overpotentials to ~0.3 V at 10 mA/cm2, which compare well with those obtained with more expensive Ru and Ir oxides. Turnover frequency increased to 0.5 1/s with Fe loading at 350 mV overpotential; morphology and surface composition (SEM, XPS) before and after electrochemical testing showed slight restructuring of the catalyst surface and an increase in Ni(OH)2. Electrochemical activity and overpotentials were also very stable, as evidenced by extended chronoamperometry and CV testing. For supercapacitor applications, NiO and CoxOy were directly spray deposited on carbon cloth at room temperature; double layer and Faradaic pseudo-capacitance ranged from 100-300 F/g at 10 mV/s scan rates, with faster plasma jet rastering rates yielding higher surface area and effective capacitance. The talk will highlight the microplasma deposition method and discuss the aforementioned electrochemical results in detail.