Etching-Induced Surface Reconstruction of NiMoO4 for Oxygen Evolution Reaction.

Rational reconstruction of oxygen evolution reaction (OER) pre-catalysts and performance index of OER catalysts are crucial but still challenging for universal water electrolysis. Herein, we develop a double-cation etching strategy to tailor the electronic structure of NiMoO4, where the prepared NiMoO4 nanorods etched by H2O2 reconstruct their surface with abundant cation deficiencies and lattice distortion. Calculation results reveal that the double cation deficiencies can make the upshift of d-band center for Ni atoms and the active sites with better oxygen adsorption capacity. As a result, the optimized sample (NMO-30M) possesses an overpotential of 260 mV at 10 mA cm-2 and excellent long-term durability of 162 h. Importantly, in situ Raman test reveals the rapid formation of high-oxidation-state transition metal hydroxide species, which can further help to improve the catalytic activity of NiMoO4 in OER. This work highlights the influence of surface remodification and shed some light on activating catalysts.

Tailoring Spin State of Perovskite Oxides by Fluorine Atom Doping for Efficient Oxygen Electrocatalysis.

Promoting the initially deficient but economical catalysts to high-performing competitors is important for developing superior catalysts. Unlike traditional nano-morphology construction methods, this work focuses on intrinsic catalytic activity enhancement via heteroatom doping strategies to induce lattice distortion and optimize spin-dependent orbital interaction to alter charge transfer between catalysts and reactants. Experimentally, a series of different concentrations of fluorine-doped lanthanum cobaltate (Fx -LaCoO3 ) exhibiting excellent electrocatalytic activity is synthesized, including a low overpotential of 390 mV at j = 10 mA cm-2 for OER and a large half-wave potential of 0.68 V for ORR. Meanwhile, the assembled rechargeable Zn-air batteries deliver an excellent performance with a large specific capacity of 811 mAh/gZn under 10 mA cm-2 and stability of charge/recharge (120 h). Theoretically, taking advantage of density functional theory calculations, it is found that the prominent OER/ORR performance arises from the spin state transition of Co3+ (Low spin state (LS, t2g 6 eg 0 ) → Intermediate spin state (IS, t2g 5 eg 1 ) and the mediated d-band center upshift by F atom incorporation. This work establishes a novel avenue for designing superior electrocatalysts in perovskite-based oxides by regulating spin states.

Correction: Giant magnetoelectric coupling observed at high frequency in NiFe2O4-BaTiO3 particulate composite.

[This corrects the article DOI: 10.1039/D0RA05782G.].

Surface nitriding to improve the catalytic performance of FeNi3 for the oxygen evolution reaction.

Solving the problem of the slow kinetic process of the oxygen evolution reaction by electrocatalysts has attracted extensive attention. Here, we report an enhancement of the oxygen evolution reaction (OER) electrocatalytic activity via the surface nitriding of FeNi3 nanosheets for the formation of amorphous Fe/Ni-Nx species. The optimized Fe/Ni-Nx@FeNi3 nanosheets exhibit an overpotential of 251 mV to achieve a current density of 10 mA cm-2 and an excellent durability of 210 h. The superior electrocatalytic performance is attributed to the multi-component active sites, where the Fe/Ni-Nx outer layer inhibits metal active site leaching, and the catalyst undergoes dynamic reconfiguration during the OER.

Fe3+ in a tetrahedral position determined the electrocatalytic properties in FeMn2O4.

As an electrocatalyst for the oxygen evolution reaction (OER) for water decomposition purposes, spinel ferrite materials have gained a lot of attention from many researchers. Herein, we document a co-precipitation synthesis of antitypical spinel nanoparticles (FeMn2O4) by post-annealing at different temperatures to enable modulation of the cationic oxidation state and tuning of the conversion degree for efficient and good OER performance. The electrocatalytic activity test shows that the sample annealed at 500 °C has the most optimal catalytic activity with an overpotential of 360 mV at a current density of 10 mA cm-2 and a Tafel slope as low as 105.32 mV dec-1. The formation of FeOOH during in situ OER promotes the catalytic activity of the catalysts. More importantly, according to the results of Brunauer-Emmett-Teller normalization, we demonstrate that the activity of the catalyst is also inseparable from the internal crystal structure. This work broadens the field of research on the electrocatalysis of spinel manganese ferrites.

Significant Change of Metal Cations in Geometric Sites by Magnetic-Field Annealing FeCo2 O4 for Enhanced Oxygen Catalytic Activity.

The application of magnetic fields in the oxygen reduction/evolution reaction (ORR/OER) testing for electrocatalysts has attracted increasing interest, but it is difficult to characterize on-site surface reconstruction. Here, a strategy is developed for annealing-treated FeCo2 O4 nanofibers at a magnetic field of 2500 Oe, named FeCo2 O4 -M, showing a right-shifted half-wave potential of 20 mV for the ORR and a left-shifted overpotential of 60 mV at 10 mV cm-2 for the OER as compared with its counterpart. Magnetic characterizations indicate that FeCo2 O4 -M shows the spin-state transition of cations from a low-spin state to an intermediate-spin state compared with FeCo2 O4 . Mössbauer spectra show that the Fe3+ ion in the octahedral site (0.76) of FeCo2 O4 -M is more than that of FeCo2 O4 (0.71), indicating the effective stimulus of metal cations in geometric sites by magnetic-field annealing. Furthermore, theoretical calculations demonstrate that the d-band centers (εd ) of Co 3d and Fe 3d in the tetrahedral and octahedral sites of the FeCo2 O4 -M nanofibers shift close to the Fermi level, revealing the enhanced mechanism of the ORR/OER activity.

Surface-Electronic-Structure Reconstruction of Perovskite via Double-Cation Gradient Etching for Superior Water Oxidation.

Reconstructing the surface-electronic-structure of catalysts for efficient electrocatalytic activity is crucial but still under intense exploration. Herein, we introduce a double-cation gradient etching technique to manipulate the electronic structure of perovskite LaCoO3. With the gradient dissolution of cations, the surface was reconstructed, and the perovskite/spinel heterostructure V-LCO/Co3O4 (V-LCO refers to LaCoO3 with La and Co vacancies) can be realized. Its surface-electronic-structure is effectively regulated due to the heterogeneous interface effect and abundant vacancies, resulting in a significantly enhanced activity for oxygen evolution reaction (OER). The V-LCO/Co3O4 exhibits low electrochemical activation energy and 2 orders of magnitude higher carrier concentrations (1.36 × 1021 cm-3) compared with LCO (6.03 × 1019 cm-3). Density functional theory (DFT) calculation unveils that the directional reconstruction of surface-electronic-structure enables the d-band center of V-LCO/Co3O4 to a moderate position, endowing perfect adsorption strength for oxo groups and thus promoting the electrocatalytic activity.

Ferromagnetism of two-dimensional transition metal chalcogenides: both theoretical and experimental investigations.

In recent years, with the fast development of integrated circuit electronic devices and technologies, it has become urgent to improve the density of data storage and lower the energy losses of devices. Under these circumstances, two-dimensional (2D) materials, which have a smaller size and lower energy loss compared with bulk materials, are becoming ideal candidates for future spintronic devices. Among them, 2D transition metal chalcogenides (TMCs), which have excellent electronic and optical properties, have attracted great attention from researchers. However, most of them are intrinsically non-magnetic, which severely hinders their further applications in spintronics. Therefore, introducing intrinsic room-temperature ferromagnetism into 2D TMC materials has become an important issue in spintronics. In this work, we review the introduction of intrinsic ferromagnetism into typical 2D TMCs using various strategies, such as defect engineering, doping with transition metal elements, and phase transfer. Additionally, we found that their ferromagnetism could be adjusted via changing the experimental conditions, such as the nucleation temperature, ion irradiation dose, doping amount, and phase ratio. Finally, we provide some insight into prospective solutions for introducing ferromagnetism into 2D TMCs, hoping to shed some light on future spintronics development.

Cr cation-anchored carbon nanosheets: synthesis, paramagnetism and ferromagnetism.

Since the successfully synthesis of monolayer graphene, carbon-based materials have attracted wide and extensive attentions from researches. Due to the excellent transport capacity and conductivity, they are promising to be applied in electronic devices, even substituting the silicon-based electronic devices, optoelectronics and spintronics. Nevertheless, due to the non magnetic feature, many efforts have been devoted to endow carbon materials magnetism to apply them in the spintronic devices fabrication. Herein, a strategy of Cr cation solely anchored on two-dimensional carbon nanosheets by Cr-N bonds is developed, which introduces magnetism in carbon nanosheets. By extended x-ray absorption fine structure characterization, Cr cations are demonstrated to be atomically dispersed with Cr-N3coordination. And after Cr-N3anchored, carbon nanosheets exhibit ferromagnetic features with paramagnetic background. The magnetization varies with Cr content and reaches the maximum (Cr: 2.0%, 0.86 emu g-1) under 3 T at 50 K. The x-ray magnetic circular dichroism and first-principle calculations indicate that the magnetism is caused by the Cr3+component of the anchored Cr cations. This study sets a single cation anchoring carbon as a suitable candidate for future spintronics.

Demonstration of various optical directed logic operations by using an integrated photonic circuit.

Optical directed logic is a novel logic operation scheme that employs electrical signals as operands to control the working states of optical switches to perform the logic operations. In this Letter, we propose and demonstrate an integrated photonic circuit which can implement five different optical logic operations by utilizing two optical modes. The proposed device is fabricated on a silicon-on-insulator substrate by using electron beam lithography and inductively coupled plasma etching processes. The static experimental results show that the fabricated device can implement five different operations correctly-XOR, XNOR, NOR, NOT, and AND-from which we can see that the signal-to-noise ratios are larger than 17.6 dB over the entire C band for all five logic functions. At last, all five logic operations with the speed of 10 Kbps are demonstrated. The proposed device with simple structure, large bandwidth, and versatility would be a promising candidate for information processing in optical mode division multiplexing networks.

Optimized Conductivity and Spin States in N-Doped LaCoO3 for Oxygen Electrocatalysis.

The spin state of antibonding orbital (eg) occupancy in LaCoO3 is recognized as a descriptor for its oxygen electrocatalysis. However, the Co(III) cation in typical LaCoO3 (LCO) favors low spin state, which is mediocre for absorbing oxygen-containing groups involved in oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), thus hindering its further development in electrocatalysis. Herein, both experimental and theoretical results reveal the enhancement of bifunctional electrocatalytic activity in LaCoO3 by N doping. More specifically, electron energy loss spectroscopy and superconducting quantum interference devices magnetic analysis demonstrate that the Co(III) cation in N-doped LaCoO3 (LCON) achieves a moderate eg occupancy (≈1) compared with its low spin state in LaCO3. First-principle calculation results reveal that N dopants play a bifunctional role of tuning the spin-state transition of Co(III) cations and increasing the electrical conductivity of LCO. Thus, the optimized LCON exhibits an OER overpotential of 1.69 V at the current density of 50 mA/cm2 (1.94 V for pristine LCO) and yields an ORR limiting current density of 5.78 mA/cm2 (4.01 mA/cm2 for pristine LCO), which offers a new strategy to simultaneously modulate the magnetic and electronic structures of LCO to further enhance its electrocatalytic activity.

Hydrogen-etched CoS2 to produce a Co9S8@CoS2 heterostructure electrocatalyst for highly efficient oxygen evolution reaction.

There is a pressing requirement for developing high-efficiency non-noble metal electrocatalysts in oxygen evolution reactions (OER), where transition metal sulfides are considered to be promising electrocatalysts for the OER in alkaline medium. Herein, we report the outstanding OER performance of Co9S8@CoS2 heterojunctions synthesized by hydrogen etched CoS2, where the optimized heterojunction shows a low η 50 of 396 mV and a small Tafel slope of 181.61 mV dec-1. The excellent electrocatalytic performance of this heterostructure is attributed to the interface electronic effect. Importantly, the post-stage characterization results indicate that the Co9S8@CoS2 heterostructure exhibits a dynamic reconfiguration during the OER with the formation of CoOOH in situ, and thus exhibits a superior electrocatalytic performance.

Tunable ferromagnetic ordering in phosphorus adsorbed ReS2 nanosheets.

Layered transition metal dichalcogenides (TMDs) are considered as promising materials for electronic, optoelectronic and spintronic devices due to their outstanding properties. Herein, based on rhenium disulfide (ReS2) nanosheets, we realized the intrinsic room temperature ferromagnetism with the adsorption of P adatoms (P-ReS2). Experiments indicate that the saturation magnetization (Ms ) can be tuned by the P ratios, where the maximum Ms can reach up to 0.0174 emu g-1. Besides, density functional theory (DFT) calculation results demonstrate that the strong hybridization between Re d and P p orbitals is the main reason of inducing ferromagnetism in P-ReS2 system. This work provides a novel method to engineer the magnetism of TMDs, endowing them with the possibility of spintronic applications.

Engineering the Nucleophilic Active Oxygen Species in CuTiOx for Efficient Low-Temperature Propene Combustion.

Industrialization has resulted in the rapid increase of volatile organic compound (VOC) emissions, which have caused serious issues to human health and the environment. In this study, an extensive Cu incorporating TiO2 induced nucleophilic oxygen structure was constructed in the CuTiOx catalyst, which exhibited superior low-temperature catalytic activity for C3H6 combustion. Thorough structural, surface characterization and density functional theory (DFT) calculations revealed that the Cu-O-Ti hybridization induced nucleophilic oxygen initiates C3H6 combustion by abstracting the C-H bond. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) results indicated that incorporated copper species acted as the major adsorbent site for the propene molecule. In combination of the DRIFTS and DFT results, the promotion effect of the nucleophilic O on the C-H bond abstraction and CO2 formation pathway was proposed. The surface doping induced nucleophilic oxygen as strong Brønsted basic sites for low-temperature propene combustion exemplified an efficient strategy for rational design of next-generation environmental catalysts.

High efficiency electrocatalyst of LaCr0.5Fe0.5O3 nanoparticles on oxygen-evolution reaction.

Due to the multistep proton-coupled electron transfer, it remains a huge challenge to accelerate the kinetics of oxygen evolution reaction (OER). Here, we demonstrate that perovskite-type LaCr0.5Fe0.5O3 nanoparticles can be used as highly active and stable OER electrocatalysts, where it shows a low overpotential of 390 mV at 10 mA/cm2, a small Tafel slope of 114.4 mV/dec and excellent stability with slight current decrease after 20 h, superior than that of their individual counterparts (LaFeO3 and LaCrO3). This finding confirms that the present hybrid material would be an effective means to electrocatalyst for catalyzing OER.

Tunable Fe3O4 Nanorods for Enhanced Magnetic Hyperthermia Performance.

Magnetic hyperthermia is one of the most promising techniques for treating gynecological cancer, where magnetite (Fe3O4) is the most common nanomaterial used as a magnetic hyperthermia agent. Here, we demonstrate that optimal Fe3O4 nanorods (NRs) can act as a magnetic hyperthermia agent with higher specific absorption rate (SAR), which is mostly attributed to their enhanced surface anisotropy. As a result, Fe3O4 NRs could effectively hinder the growth of gynecological cancer cells in nude mice models, again demonstrating its good magnetic heating properties. These results provide a powerful basis for the development of an ideal magnetic hyperthermia agent with enhanced SAR, thereby effectively treating gynecological cancer in clinical practice.

Interfacial Engineering of NiO/NiCo2O4 Porous Nanofibers as Efficient Bifunctional Catalysts for Rechargeable Zinc-Air Batteries.

To meet the crucial demand of regenerative Zn-air (ZA) batteries, low cost, highly efficient, and durable electrocatalysts for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are needed to replace the noble metal. Herein, porous NiO/NiCo2O4 nanofibers with superior electrocatalytic performance are synthesized by a facile electrospinning strategy with precursor transition metal salts in nonstoichiometric ratio, which confers the heterostructured NiO/NiCo2O4 with abundant interface-related active sites and electronic transmission channels. Density functional calculation results reveal the chemical bonds easily form between NiO and NiCo2O4 to facilitate the charge transfer, while X-ray absorption fine spectroscopy and X-ray photoelectron spectroscopy results demonstrate there are abundant Ni3+ and Co3+ species in NiO/NiCo2O4 due to the interfacial engineering. As a result, the NiO/NiCo2O4 porous nanofibers exhibit highly efficient and durable performances of OER and ORR in KOH solution, including a lower overpotential of 357 mV at 10 mA cm-2 (OER) and half-wave potential of 0.73 V (ORR) than that of the individual. What's more, the NiO/NiCo2O4-based ZA battery displays excellent specific capacities of 814.4 mA h g-1, and good cycling stability of 175 h. Additionally, the flexible ZA battery displays a long cycling life of 14 h and decent flexibility. This work shows that construction of the heterostructure could provide a feasible method to optimize their electrocatalytic performance and make them widely used in power source devices.

Fe-based species anchored on N-doped carbon nanotubes as a bifunctional electrocatalyst for acidic/neutral/alkaline Zn-air batteries.

Exploring efficient and durable bifunctional catalysts in pH-universal media is critical for versatile fuel cells. Herein, Fe-based species anchored on N-doped carbon nanotubes (Fe/Fe3C@N-C) are used for bifunctional oxygen electrocatalysts. The composite electrocatalyst exhibits low potential gaps (ΔE, ΔE = Ej =10 - E 1/2) in a pH-universal environment. The estimated values are about 0.70, 1.07,and 1.10 V in alkaline, neutral, and acidic medias. A neutral Zn-air battery (ZAB) is constructed using an Fe/Fe3C@N-C composite as the air electrode, which exhibits a favorable performance in energy storage with an open-circuit potential (OCP) of 1.42 V and a high power density of 80 mW cm-2. The ZAB also has superior cycling stability with only a 0.5% decay after 1200 charge-discharge cycles at 2 mA cm-2. While the assembled ZAB in acidic media indicates an OCP of 1.40 V, a power density of 23 mW cm-2, and 612 discharge-charge cycles. The ZAB is rechargeable and has a cycling lifespan of 120 h. This work provides potential applications of Fe/Fe3C@N-C as air electrodes for advanced pH-universal media based on ZABs for future energy storage devices.

Bifunctional Oxygen Electrocatalyst of Mesoporous Ni/NiO Nanosheets for Flexible Rechargeable Zn-Air Batteries.

One approach to accelerate the stagnant kinetics of both the oxygen reduction and evolution reactions (ORR/OER) is to develop a rationally designed multiphase nanocomposite, where the functions arising from each of the constituent phases, their interfaces, and the overall structure are properly controlled. Herein, we successfully synthesized an oxygen electrocatalyst consisting of Ni nanoparticles purposely interpenetrated into mesoporous NiO nanosheets (porous Ni/NiO). Benefiting from the contributions of the Ni and NiO phases, the well-established pore channels for charge transport at the interface between the phases, and the enhanced conductivity due to oxygen-deficiency at the pore edges, the porous Ni/NiO nanosheets show a potential of 1.49 V (10 mA cm-2) for the OER and a half-wave potential of 0.76 V for the ORR, outperforming their noble metal counterparts. More significantly, a Zn-air battery employing the porous Ni/NiO nanosheets exhibits an initial charging-discharging voltage gap of 0.83 V (2 mA cm-2), specific capacity of 853 mAh g Zn -1 at 20 mA cm-2, and long-time cycling stability (120 h). In addition, the porous Ni/NiO-based solid-like Zn-air battery shows excellent electrochemical performance and flexibility, illustrating its great potential as a next-generation rechargeable power source for flexible electronics.

Nitrogen-doped RuS2 nanoparticles containing in situ reduced Ru as an efficient electrocatalyst for hydrogen evolution.

The production of hydrogen via water electrolysis brings hope for the realization of hydrogen economy, but there is still a lack of highly efficient and appropriate electrocatalysts for the generation of hydrogen in practical applications. In particular, reasonable construction and feasible preparation strategies are the essential requirements for excellent electrocatalysts. Herein, the heterostructures of N-RuS2/Ru nanoparticles were designed by annealing the RuS2 nanoparticles in ammonia. By introducing a nitrogen dopant and single-phase Ru metal simultaneously, high-efficiency electrocatalytic performance for hydrogen evolution reaction (HER) was implemented, where the electrocatalyst of N-RuS2/Ru exhibited a low onset overpotential of 76 mV and small overpotential of 120 mV at 10 mA cm-2 in an acidic electrolyte. Besides, it displayed a low Tafel slope of 53 mV dec-1, a small interface charge transfer resistance, and long-time stability and durability, suggesting its remarkable properties as a promising HER electrocatalyst candidate.