Topotactically constructed nickel-iron(oxy)hydroxide with abundant in-situ produced high-valent iron species for efficient water oxidation

The low efficiency of oxygen evolution reaction(OER) is regarded as one of the major roadblocks for metal-air batteries and water electrolysis.Herein,a high-performance OER catalyst of NiFe_(0.2)(oxy)hydroxide(NiFe_(0.2)-O_(x)H_(y)) was developed through topotactic transformation of a Prussian blue analogue in an alkaline solution,which exhibits a low overpotential of only 263 mV to reach a current density of 10 mA cm^(-2) and a small Tafel slope of 35 mV dec-1.Ex-situ/operando Raman spectroscopy results indicated that the phase structure of NiFe_(0.2)-O_(x)H_(y) was irreversibly transformed from the type of α-Ni(OH)_(2) to γ-NiOOH with applying an anodic potential,while ex-situ/operando 57Fe Mossbauer spectroscopic studies evidenced the in-situ production of abundant high-valent iron species under OER conditions,which effectively promoted the OER catalysis.Our work elucidates that the amount of high-valent iron species in-situ produced in the NiFe(oxy)hydroxide has a positive correlation with its water oxidation reaction performance,which further deepens the understanding of the mechanism of NiFe-based electrocatalysts.

Exploring the Spatial Control of Topotactic Phase Transitions Using Vertically Oriented Epitaxial Interfaces

Engineering oxygen vacancy formation and distribution is a powerful route for controlling the oxygen sublattice evolution that affects diverse functional behavior.The controlling of the oxygen vacancy formation process is particularly important for inducing topotactic phase transitions that occur by transformation of the oxygen sublattice.Here we demonstrate an epitaxial nanocomposite approach for exploring the spatial control of topotactic phase transition from a pristine perovskite phase to an oxygen vacancy-ordered brownmillerite(BM)phase in a model oxide La_(0.7)Sr_(0.3)MnO_(3)(LSMO).Incorporating a minority phase NiO in LSMO films creates ultrahigh density of vertically aligned epitaxial interfaces that strongly influence the oxygen vacancy formation and distribution in LSMO.Combined structural characterizations reveal strong interactions between NiO and LSMO across the epitaxial interfaces leading to a topotactic phase transition in LSMO accompanied by significant morphology evolution in NiO.Using the NiO nominal ratio as a single control parameter,we obtain intermediate topotactic nanostructures with distinct distribution of the transformed LSMO-BM phase,which enables systematic tuning of magnetic and electrical transport properties.The use of self-assembled heterostructure interfaces by the epitaxial nanocomposite platform enables more versatile design of topotactic phase structures and correlated functionalities that are sensitive to oxygen vacancies.

Preparation of spatially uniform monolayer FeSexTe1−x(0

Spatially uniform high-temperature superconducting films are highly desirable for exploring novel properties and popularizing applications.To improve the uniformity,we fabricate monolayer FeSexTe1−x(0<x≤1)films on SrTiO3(001)by topotactic reaction of monolayer FeTe films with selenium.Using in situ low-temperature scanning tunneling microscopy/spectroscopy,we demonstrate atomic-level uniformity of element distribution and well-defined superconducting gaps of~15 meV in FeSexTe1−x films.In particular,the monolayer FeSe films exhibit fewer line defects and higher superfluid density as evidenced by sharper coherence peaks than those prepared by the co-evaporation method.Our results provide a promising way to optimize sample quality and lay a foundation for studying new physics and drawing reliable conclusions.

A topotactic tailored synthesis of waxberry-like mixed-phase TiO_(2) hollow spheres for dye-sensitized solar cells

The waxberry-like mixed-phase TiO_(2)hollow microstructures (WMTHMs) are controllably prepared via a topotactic synthetic method,involving the synthesis of monodispersed Ca TiO_(2)precursors by a solvothermal method and subsequently transforming them into TiO_(2)through a Na_(2)EDTA-assisted ion-exchange process.The ratio of anatase-rutile is adjustable,and the two phases are connected well with each other.WMTHMs are composed of radially aligned nanorods,speeding up the electron transport.The optimum WMTHMs sample shows a specific surface area of 68.05 m^(2)/g and exhibits an excellent light scattering capacity.The cell based on WMTHMs light scattering layer obtained an optimal efficiency of 9.12%.The improvement of cell efficiency is mainly attributed to the high specific surface area,the efficient light scattering,the appropriate ratio of anatase-rutile,the staggered bandgap structure,and the convenient one-dimensional electron transport channel.

Development of tailored TiO_2 mesocrystals for solar driven photocatalysis

Ordered metal oxides superstructures have attracted much more attention in the fields of fuel generation and environmental purification owing to their unique physiochemical characteristics such as large surface area, fine pore structure, efficient electronic mobility, and good stability. Very recently, TiOmesocrystals（TMCs） having superstructures self-assembled by TiOnanoparticle building blocks, are of considerable interest in current research and application ranging from UV to visible light attributed to their efficient charge separation and superior photocatalytic activity. In this review, we describe the common procedures to prepare unique TMCs and overview of recent developments of TMCs during last 3 years, especially the structure-related or electronic-effected mechanism in photocatalytic reaction. Further, we introduce the characterization and fundamental properties of modified TMCs by the means of single-particle fluorescence microscopy for unraveling the charge transport and photocatalytic properties of individual TMCs and time-resolved diffuse reflectance spectroscopy（TDR） for monitoring the charge transfer dynamics. Finally, various aspects on TMCs are discussed for the future developments of energy and environmental fields.

High current CO_(2)reduction realized by edge/defect-rich bismuth nanosheets

CO_(2)electroreduction has been regarded as an appealing strategy for renewable energy storage.Recently,bismuth(Bi)electrocatalysts have attracted much attention due to their excellent formate selectivity.However,many reported Bi electrocatalysts suffer from low current densities,which are insufficient for industrial applications.To reach the goal of high current CO_(2)reduction to formate,we fabricate Bi nanosheets(NS)with high activity through edge/terrace control and defect engineering strategy.Bi NS with preferential exposure sites are obtained by topotactic transformation,and the processes are clearly monitored by in-situ Raman and ex-situ X-ray diffraction(XRD).Bi NS-1 with a high fraction of edge sites and defect sites exhibits excellent performance,and the current density is up to ca.870 mA·cm^(−2)in the flow cell,far above the industrially applicable level(100 mA·cm^(−2)),with a formate Faradaic efficiency greater than 90%.In-situ Fourier transform infrared(FT-IR)spectra detect*OCHO,and theoretical calculations reveal that the formation energy of*OCHO on edges is lower than that on terraces,while the defects on edges further reduce the free energy changes(ΔG).The differential charge density spatial distributions reveal that the presence of defects on edges causes charge enrichment around the C–H bond,benefiting the stabilization of the*OCHO intermediate,thus remarkably lowering theΔG.

Obtaining tetragonal FeAs layer and superconducting K_(x)Fe_(2)As_(2)by molecular beam epitaxy

Atomic characterization on tetragonal FeAs layer and engineering FeAs superlattices is highly desirable to get deep insight into the multi-band superconductivity in iron-pnictides.We fabricate the tetragonal FeAs layer by topotactic reaction of FeTe films with arsenic and then obtain KxFe_(2)As_(2)upon potassium intercalation using molecular beam epitaxy.The in-situ low-temperature√2×√2scanning tunneling microscopy/spectroscopy investigations demonstrate characteristic reconstruction of the FeAs layer and stripe pattern of KxFe_(2)As_(2),accompanied by the development of a superconducting-like gap.The ex-situ transport measurement with FeTe capping layers shows a superconducting transition with an onset temperature of 10 K.This work provides a promising way to characterize the FeAs layer directly and explore rich emergent physics with epitaxial superlattice design.

Exchange-Biased NiFe_(2)O_(4)/NiO Nanocomposites Derived from NiFe-Layered Double Hydroxides as a Single Precursor

NiFe_(2)O_(4) nanoparticles(<10 nm)embedded in a NiO matrix have been fabricated by calcining the corresponding Ni^(Ⅱ)Fe^(Ⅲ)-layered double hydroxide(LDH)precursors at high temperature(500℃).Compared with the NiFe_(2)O_(4)/NiO nanocomposite obtained by calcination of a precursor prepared by a traditional chemical coprecipitation method,those derived from NiFe-LDH precursors show much higher blocking temperatures(TB)(~380 K).The enhanced magnetic stability can be ascribed to the much stronger interfacial interaction between NiFe_(2)O_(4) and NiO phases due to the topotactic nature of the transformation of the LDH precursor to the NiFe_(2)O_(4)/NiO composite material.Through tuning the Ni^(Ⅱ)/Fe^(Ⅲ) molar ratio of the NiFe-LDH precursor,the NiFe_(2)O_(4) concentration can be precisely controlled,and the TB value as well as the magnetic properties of the final material can also be regulated.This work represents a successful example of the fabrication of ferro(ferri)magnetic(FM)/antiferrimagnetic(AFM)systems with high magnetic stability from LDH precursors.This method is general and may be readily extended to other FM/AFM systems due to the wide range of available LDH precursors.

TiO_2 mesocrystals: Synthesis, formation mechanisms and applications

Mesocrystals, which are assemblies of crystallographically oriented nanocrystals, have received increasing attention due to their unique properties such as high crystallinity, high porosity, oriented subunit alignment, and similarity to highly sophisticated biominerals. However, the controlled synthesis of TiO 2 mesocrystals has not been realized until recently, probably because of the difficulty in accurately controlling the reaction processes that produce TiO 2 crystals. In this review, recent advances in the synthesis and applications of TiO 2 mesocrystals are summarized with particular attention paid to the mechanisms of their formation. Three typical pathways for the preparation of TiO 2 mesocrystals are discussed, namely topotactic transformation, direct synthesis in solution, and growth on supports. The potential applications of TiO 2 mesocrystals in lithium ion batteries, photocatalysis, enzyme immobilization, and antireflection materials are also described.

A multiphase nickel iron sulfide hybrid electrode for highly active oxygen evolution

Development of highly active electrocatalysts for oxygen evolution reaction(OER)is one of the critical issues for water splitting,and most reported catalysts operate at overpotentials above 190 mV.Here we present a multiphase nickel iron sulfide(MPS)hybrid electrode with a hierarchical structure of iron doped NiS and Ni3S2,possessing a benchmark OER activity in alkaline media with a potential as low as 1.33 V(vs.reversible hydrogen electrode)to drive an OER current density of 10 mA cm^-2.The Fe doped NiS,combined with highly conductive disulfide phase on porous Ni foam,is believed to be responsible for the ultrahigh activity.Furthermore,density functional theory simulation reveals that partially oxidized sulfur sites in Fe doped NiS could dramatically lower the energy barrier for the rate-determining elementary reaction,thus contributing to the active oxygen evolution.