Balanced NOx- and Proton Adsorption for Efficient Electrocatalytic NOx- to NH3 Conversion.

Electrocatalytic nitrate (NO3-)/nitrite (NO2-) reduction reaction (eNOx-RR) to ammonia under ambient conditions presents a green and promising alternative to the Haber-Bosch process. Practically available NOx- sources, such as wastewater or plasma-enabled nitrogen oxidation reaction (p-NOR), typically have low NOx- concentrations. Hence, electrocatalyst engineering is important for practical eNOx-RR to obtain both high NH3 Faradaic efficiency (FE) and high yield rate. Herein, we designed balanced NOx- and proton adsorption by properly introducing Cu sites into the Fe/Fe2O3 electrocatalyst. During the eNOx-RR process, the H adsorption is balanced, and the good NOx- affinity is maintained. As a consequence, the designed Cu-Fe/Fe2O3 catalyst exhibits promising performance, with an average NH3 FE of ∼98% and an average NH3 yield rate of 15.66 mg h-1 cm-2 under the low NO3- concentration (32.3 mM) of typical industrial wastewater at an applied potential of -0.6 V versus reversible hydrogen electrode (RHE). With low-power direct current p-NOR generated NOx- (23.5 mM) in KOH electrolyte, the Cu-Fe/Fe2O3 catalyst achieves an FE of ∼99% and a yield rate of 15.1 mg h-1 cm-2 for NH3 production at -0.5 V (vs RHE). The performance achieved in this study exceeds industrialization targets for NH3 production by exploiting two available low-concentration NOx- sources.

Classification tasks using input driven nonlinear magnetization dynamics in spin Hall oscillator.

The inherent nonlinear magnetization dynamics in spintronic devices make them suitable candidates for neuromorphic hardware. Among spintronic devices, spin torque oscillators such as spin transfer torque oscillators and spin Hall oscillators have shown the capability to perform recognition tasks. In this paper, with the help of micromagnetic simulations, we model and demonstrate that the magnetization dynamics of a single spin Hall oscillator can be nonlinearly transformed by harnessing input pulse streams and can be utilized for classification tasks. The spin Hall oscillator utilizes the microwave spectral characteristics of its magnetization dynamics for processing a binary data input. The spectral change due to the nonlinear magnetization dynamics assists in real-time feature extraction and classification of 4-binary digit input patterns. The performance was tested for the classification of the standard MNIST handwritten digit data set and achieved an accuracy of 83.1% in a simple linear regression model. Our results suggest that modulating time-driven input data can generate diverse magnetization dynamics in the spin Hall oscillator that can be suitable for temporal or sequential information processing.

Facet Control of Nickel Nitride Nano-Framework for Efficient Hydrogen Evolution Electrocatalysis via Auxiliary Cooling Assisted Plasma Engineering.

The precise facet modulation of transition metal nitrides (TMNs) has been regarded as an essential issue in boosting electrocatalytic H2 production. Compared to thermal nitridation, the plasma technique serves as a favorable alternative to directly achieve TMNs, but the apparent surface heating effect during plasma treatment inevitably causes the thermally stabilized nitride formation, resulting in the deterioration of the highly reactive facet. To optimize the hydrogen evolution reaction (HER) behavior, an auxiliary cooling assisted plasma system to selectively expose Ni3 N (2-10) with favorable activity by controlling surface heating during plasma nitridation is designed. The resultant nickel nitride (cp-Ni3 N) nano-framework delivers exceptional catalytic performance, evidenced by its low overpotential of 58 and 188 mV at the current density of 10 and 100 mA cm-2 for HER, in stark comparison with that of normal plasma and thermally fabricated Ni3 N. Operando plasma diagnostics along with numerical simulation further confirm the effect of surface heating on typical plasma parameters as well as the Ni3 N nanostructure, indicating the key factor responsible for the high-performance nitride electrocatalyst.

Controlling degeneracy and magnetization switching in an artificial spin ice system of peanut-shaped nanomagnets.

Using extensive numerical simulations, we probe the magnetization switching in a two-dimensional artificial spin ice (ASI) system consisting of peanut-shaped nanomagnets. We also investigated the effect of external magnetic field on the degeneracy of the magnetic states in such a system. The switching field is found to be one order smaller in the proposed ASI system with peanut-shaped nanomagnets as compared to the conventionally used highly-anisotropic nanoisland such as elliptically shaped nanomagnets. The metastable two-in/two-out (Type II) magnetic state is robust at the remanence. We are also able to access the other possible microstate corresponding to Type II magnetic configurations by carefully varying the external magnetic field. It implies that one can control the degeneracy of the magnetic state by an application of suitable magnetic field. Interestingly, the magnetic charge neutrality at the vertex breaks due to the defects induced by removing nanomagnets. In such a case, the system also appears to have one-out/three-in or three-out/one-in (Type III) spin state, reminiscent of magnetic monopole at the vertex. We believe that our study is highly desirable in the context of developing the next-generation spintronics-based devices for future technologies.

Dynamic Color Generation with Electrically Tunable Thin Film Optical Coatings.

Thin film optical coatings have a wide range of industrial applications from displays and lighting to photovoltaic cells. The realization of electrically tunable thin film optical coatings in the visible wavelength range is particularly important to develop energy efficient and dynamic color filters. Here, we experimentally demonstrate dynamic color generation using electrically tunable thin film optical coatings that consist of two different phase change materials (PCMs). The proposed active thin film nanocavity excites the Fano resonance that results from the coupling of a broadband and a narrowband absorber made up of phase change materials. The Fano resonance is then electrically tuned by structural phase switching of PCM layers to demonstrate active color filters covering the entire visible spectrum. In contrast to existing thin film optical coatings, the developed electrically tunable PCM based Fano resonant thin optical coatings have several advantages in tunable displays and active nanophotonic applications.

Challenges and Applications to Operando and In Situ TEM Imaging and Spectroscopic Capabilities in a Cryogenic Temperature Range.

ConspectusIn this Account, we describe the challenges and promising applications of transmission electron microscopy (TEM) imaging and spectroscopy at cryogenic temperatures. Our work focuses on two areas of application: the delay of electron-beam-induced degradation and following low-temperature phenomena in a continuous and variable temperature range. For the former, we present a study of LiMn1.5Ni0.5O4 lithium ion battery cathode material that undergoes electron beam-induced degradation when studied at room temperature by TEM. Cryogenic imaging reveals the true structure of LiMn1.5Ni0.5O4 nanoparticles in their discharged state. Improved stability under electron beam irradiation was confirmed by following the evolution of the O K-edge fine structure by electron energy-loss spectroscopy. Our results demonstrate that the effect of radiation damage on discharged LiMn1.5Ni0.5O4 was previously underestimated and that atomic-resolution imaging at cryogenic temperature has a potential to be generalized to most of the Li-based materials and beyond. For the latter, we present two studies in the imaging of low-temperature phenomena on the local scale, namely, the evolution of ferroelectric and ferromagnetic domains walls, in BaTiO3 and Y3Fe5O12 systems, respectively, in a continuous and variable temperature range. Continuous imaging of the phase transition in BaTiO3, a prototypical ferroelectric system, from the low-temperature orthorhombic phase continuously up to the centrosymmetric high-temperature phase is shown to be possible inside a TEM. Similarly, the propagation of domain walls in Y3Fe5O12, a magnetic insulator, is studied from ∼120 to ∼400 K and combined with the application of a magnetic field and electrical current pulses to mimic the operando conditions as in domain wall memory and logic devices for information technology. Such studies are promising for studying the pinning of the ferroelectric and magnetic domains versus temperature, spin-polarized current, and externally applied magnetic field to better manipulate the domain walls. The capability of combining operando TEM stimuli such as current, voltage, and/or magnetic field with in situ TEM imaging in a continuous cryogenic temperature range will allow the uncovering of fundamental phenomena on the nanometer scale. These studies were made possible using a MEMS-based TEM holder that allowed an electron-transparent sample to be transferred and electrically contacted on a MEMS chip. The six-contact double-tilt holder allows the alignment of the specimen into its zone axis while simultaneously using four electrical contacts to regulate the temperature and two contacts to apply the electrical stimuli, i.e., operando TEM imaging. This Account leads to the demonstration of (i) the high-resolution imaging and spectroscopy of nanoparticles oriented in the desired [110] zone-axis direction at cryogenic temperatures to mitigate the electron beam degradation, (ii) imaging of low-temperature transitions with accurate and continuous control of the temperature that allowed single-frame observation of the presence of both the orthorhombic and tetragonal phases in the BaTiO3 system, and (iii) magnetic domain wall propagation as a function of temperature, magnetic field, and current pulses (100 ns with a 100 kHz repetition rate) in the Y3Fe5O12 system.

Electrically Tunable All-PCM Visible Plasmonics.

The realization of electrically tunable plasmonic resonances in the ultraviolet (UV) to visible spectral band is particularly important for active nanophotonic device applications. However, the plasmonic resonances in the UV to visible wavelength range cannot be tuned due to the lack of tunable plasmonic materials. Here, we experimentally demonstrate tunable plasmonic resonances at visible wavelengths using a chalcogenide semiconductor alloy such as antimony telluride (Sb2Te3), by switching the structural phase of Sb2Te3 from amorphous to crystalline. We demonstrate the excitation of a propagating surface plasmon with a high plasmonic figure of merit in both amorphous and crystalline phases of Sb2Te3 thin films. We show polarization-dependent and -independent plasmonic resonances by fabricating one and two-dimensional periodic nanostructures in Sb2Te3 thin films, respectively. Moreover, we demonstrate electrically tunable plasmonic resonances using a microheater integrated with the Sb2Te3/Si device. The developed electrically tunable Sb2Te3-based plasmonic devices could find applications in the development of active color filters.

Electrically Tunable Singular Phase and Goos-Hänchen Shifts in Phase-Change-Material-Based Thin-Film Coatings as Optical Absorbers.

The change of the phase of light under the evolution of a nanomaterial with time is a promising new research direction. A phenomenon directly related to the sudden phase change of light is the Goos-Hänchen (G-H) shift, which describes the lateral beam displacement of the reflected light from the interface of two media when the angles of incidence are close to the total internal reflection angle or Brewster angle. Here, an innovative design of lithography-free nanophotonic cavities to realize electrically tunable G-H shifts at the singular phase of light in the visible wavelengths is reported. Reversible electrical tuning of phase and G-H shifts is experimentally demonstrated using a microheater integrated optical cavity consisting of a dielectric film on an absorbing substrate through a Joule heating mechanism. In particular, an enhanced G-H shift of 110 times of the operating wavelength at the Brewster angle of the thin-film cavity is reported. More importantly, electrically tunable G-H shifts are demonstrated by exploiting the significant tunable phase change that occurs at the Brewster angles, due to the small temperature-induced refractive index changes of the dielectric film. Realizing efficient electrically tunable G-H shifts with miniaturized heaters will extend the research scope of the G-H shift phenomenon and its applications.

Electro-Ionic Control of Surface Plasmons in Graphene-Layered Heterostructures.

Precise control of light is indispensable to modern optical communication devices especially as the size of such devices approaches the subwavelength scale. Plasmonic devices are suitable for the development of these optical devices due to the extreme field confinement and its ability to be controlled by tuning the carrier density at the metal/dielectric interface. Here, an electro-ionic controlled plasmonic device consisting of Au/graphene/ion-gel is demonstrated as an optical switch, where an external electric field modulates the real part of the electrical conductivity. The graphene layer enhances charge penetration and charge separation at the Au/graphene interface resulting in an increased photoinduced voltage. The ion-gel immobilized on the Au/graphene further enables the electrical tunability of plasmons which modulates the intensity of the reflected laser light. This work paves the way for developing novel plasmonic electro-optic switches for potential applications such as integrated optical devices.

Magnetoimpedance of Epitaxial Y3Fe5O12 (001) Thin Film in Low-Frequency Regime.

The atomically flat interface of the Y3Fe5O12 (YIG) thin film and the Gd3Ga5O12 (GGG) substrate plays a vital role in obtaining the magnetization dynamics of YIG below and above the anisotropy field. Here, magnetoimpedance (MI) is used to investigate the magnetization dynamics in fully epitaxial 45 nm YIG thin films grown on the GGG (001) substrates using a copper strip coil in the MHz-GHz frequency region. The resistance (R) and reactance (X), which are components of impedance (Z), allow us to probe the absorptive and dispersive components of the dynamic permeability, whereas a conventional spectrometer only measures the field derivative of the power absorbed. The distinct excitation modes arising from the resonance in the uniform and dragged magnetization states of YIG are respectively observed above and below the anisotropy field. The magnetodynamics clearly shows the visible dichotomy between two resonant fields below and above the anisotropy field and its motion as a function of the direction of the applied magnetic field. A low value of a damping factor of ∼4.7 - 6.1 × 10-4 is estimated for uniform excitation mode with an anisotropy field of 65 ± 2 Oe. Investigation of below and above anisotropy field-dependent magnetodynamics in the low-frequency mode can be useful in designing the YIG-based resonators, oscillators, filters, and magnonic devices.

Rapid and Stable Plasma Transformation of Polyester Fabrics for Highly Efficient Oil-Water Separation.

Fabrics with special wettability have drawn growing attention in recent years in the area of oil-water separation due to their low cost, good flexibility, and ease of handling. However, an efficient and fast method to enable the required wetting state on fabrics still remains a challenge. In this work, a one-step, rapid, and chemical-free hydrogen plasma treatment is reported to prepare a superhydrophobic and oleophilic polyester fabric. The as-prepared fabrics display a static water contact angle of 153.2° with excellent oil-water separation capability. The mechanism of surface transformation is discussed through chemical analyses, which indicate a significant removal of carboxyl group from the pristine hydrophilic surface. This developed method is envisaged to be used for on-demand large-scale production of materials for emergency oil cleanup through either separation or selective adsorption.

C-Plasma of Hierarchical Graphene Survives SnS Bundles for Ultrastable and High Volumetric Na-Ion Storage.

Tin and its derivatives have provoked tremendous progress of high-capacity sodium-ion anode materials. However, achieving high areal and volumetric capability with maintained long-term stability in a single electrode remains challenging. Here, an elegant and versatile strategy is developed to significantly extend the lifespan and rate capability of tin sulfide nanobelt electrodes while maintaining high areal and volumetric capacities. In this strategy, in situ bundles of robust hierarchical graphene (hG) are grown uniformly on tin sulfide nanobelt networks through a rapid (5 min) carbon-plasma method with sustainable oil as the carbon source and the partially reduced Sn as the catalyst. The nucleation of graphene, CN (with size N ranging from 1 to 24), on the Sn(111) surface is systematically explored using density functional theory calculations. It is demonstrated that this chemical-bonded hG strategy is powerful in enhancing overall electrochemical performance.

Prereduction of Metal Oxides via Carbon Plasma Treatment for Efficient and Stable Electrocatalytic Hydrogen Evolution.

Prereduction of transition metal oxides is a feasible and efficient strategy to enhance their catalytic activity for hydrogen evolution. Unfortunately, the prereduction via the common H2 annealing method is unstable for nanomaterials during the hydrogen evolution process. Here, using NiMoO4 nanowire arrays as the example, it is demonstrated that carbon plasma (C-plasma) treatment can greatly enhance both the catalytic activity and the long-term stability of transition metal oxides for hydrogen evolution. The C-plasma treatment has two functions at the same time: it induces partial surface reduction of the NiMoO4 nanowire to form Ni4 Mo nanoclusters, and simultaneously deposits a thin graphitic carbon shell. As a result, the C-plasma treated NiMoO4 can maintain its array morphology, chemical composition, and catalytic activity during long-term intermittent hydrogen evolution process. This work may pave a new way for simultaneous activation and stabilization of transition metal oxide-based electrocatalysts.

Nitrogen-Plasma-Activated Hierarchical Nickel Nitride Nanocorals for Energy Applications.

Developing transition metal nitrides with unique nanomorphology is important for many energy storage and conversion processes. Here, a facile and novel one-step approach of growing 3D hierarchical nickel nitride (hNi3 N) on Ni foam via nitrogen plasma is reported. Different from most conventional chemical synthesis, the hNi3 N is obtained in much shorter growth duration (≤15 min) without any hazardous or reactive sources and oxide precursors at a moderate reaction zone temperature of ≤450 °C. Among possible multifunctionalities of the obtained nanocoral hNi3 N, herein the performance in reversible lithium ion storage and electrocatalytic oxygen evolution reaction (OER) is demonstrated. The as-obtained hNi3 N delivers a considerable cycling performance and rate stability as a lithium ion battery anode, and its property can be further enhanced by coating the hNi3 N surface with graphene quantum dots. The hNi3 N also serves as an active OER catalyst with high activity and stability. Additionally, on the basis of controlled growth under different nitrogen plasma treatment time, the formation mechanism of the nanocoralline hNi3 N is outlined for further extension to other materials. The results on time- and energy-efficient nitrogen-plasma-based preparation of hNi3 N pave the way for the development of high-performance metal nitride electrodes for energy storage and conversion.

Rapid Synthesis of Cobalt Nitride Nanowires: Highly Efficient and Low-Cost Catalysts for Oxygen Evolution.

Electrochemical splitting of water to produce hydrogen and oxygen is an important process for many energy storage and conversion devices. Developing efficient, durable, low-cost, and earth-abundant electrocatalysts for the oxygen evolution reaction (OER) is of great urgency. To achieve the rapid synthesis of transition-metal nitride nanostructures and improve their electrocatalytic performance, a new strategy has been developed to convert cobalt oxide precursors into cobalt nitride nanowires through N2 radio frequency plasma treatment. This method requires significantly shorter reaction times (about 1 min) at room temperature compared to conventional high-temperature NH3 annealing which requires a few hours. The plasma treatment significantly enhances the OER activity, as evidenced by a low overpotential of 290 mV to reach a current density of 10 mA cm(-2) , a small Tafel slope, and long-term durability in an alkaline electrolyte.

Self-organization of a hybrid nanostructure consisting of a nanoneedle and nanodot.

A special materials system that allows the self-organization of a unique hybrid nanonipple structure is developed. The system consists of a nanoneedle with a small nanodot sitting on top. Such hybrid nanonipples provide building blocks to assemble functional devices with significantly improved performance. The application of the system to high-sensitivity gas sensors is also demonstrated.