Room temperature synthesis of two-dimensional multi layer magnets based on α-Co^(Ⅱ) layered hydroxides

Research on two-dimensional(2D) materials is one of the most active fields in materials science and nanotechnology. Among the members of the 2D family, layered hydroxides(LHs) represent an exceptional case of study due to their unparalleled chemical versatility which allows the modulation of their physicochemical properties at will. Nowadays, LHs based on earth-abundant metals are key materials in the areas of energy storage and conversion, hybrid materials or magnetism. α-Co hydroxides(Simonkolleite-like structures) are promising phases with tuneable electronic and magnetic properties by ligand modification. However, even in the simple case of α-Co^(Ⅱ) hydroxychlorides, the preparation of well-defined large 2D crystals is not straightforward, hindering the development of fundamental studies. Herein, we present the synthesis of 2D hexagonal crystals with outstanding sizethickness relationship(diameter > 5 μm and thickness of 20 ± 7 nm) by a simple homogeneous synthesis taking place at room temperature. In structural terms, no differences are observed between our layered materials and those obtained hydrothermally. However, dynamic susceptibility measurements alert about different arrangements of the magnetic sublattices, which have been rationalized with structural DFT calculations. This work provides an extremely easy bottom-up method to obtain high-quality 2D crystals based on α-CoIIhydroxides,paving the way for the development of fundamental studies and applications.

One step synthesis of high efficiency nickel/mesoporous TiO_2 hybrid catalyst for hydrogen evolution reaction

Nickel-Ti O_2 hybrid catalysts are synthesized by electrodeposition and their catalytic activity with respect to the hydrogen evolution reaction is analyzed.Two types of titanium oxide particles,which are commercial particles of dense Ti O_2 and mesoporous TiO_2 particles synthesized by an aerosol method,are incorporated into the matrix of the nickel catalyst.Both nickel catalysts containing Ti O_2 particles presented higher catalytic activity than the conventional nickel Watts catalyst.Mesoporous TiO_2-modified nickel catalyst showed the highest catalytic activity towards HER in alkaline medium.In addition,this type of nickel catalyst increases its catalytic activity after ageing treatment,which is an indication of an increase in the electro-active area of the electrode.

The effect of rippling on the mechanical properties of graphene

Graphene is the stiffest material known so far but, due to its one-atom thickness, it is also very bendable.Consequently, free-standing graphene exhibit ripples that has major effects on its elastic properties. Here we will summarize three experiments where the influence of rippling is essential to address the results. Firstly, we observed that atomic vacancies lessen the negative thermal expansion coefficient(TEC) of free-standing graphene.We also observed an increase of the Young's modulus with global applied strain and with the introduction of small density defects that we attributed to the decrease of rippling. Here, we will focus on a surprising feature observed in the data: the experiments consistently indicate that only the rippling with wavelengths between 5 and 10 nm influences the mechanics of graphene. The rippling responsible of the negative TEC and anomalous elasticity is thought to be dynamic, i.e. flexural phonons. However, flexural phonons with these wavelengths should have minor effects on the mechanics of graphene, therefore other mechanisms must be considered to address our observations. We propose static ripples as one of the key elements to correctly understand the thermomechanics of graphene and suggest that rippling arises naturally due to a competition of symmetry breaking and anharmonic fluctuations.

Hydrogen evolution kinetics on Ni cathodes modified by spontaneous deposition of Ag or Cu

Nickel modification by spontaneous deposition of transition metals such as Ag and Cu is shown as an economic and simple alternative for the activation of hydrogen evolution reaction（HER） on cathodes in alkaline media. The kinetics of HER is studied on Ni/Ag and Ni/Cu catalysts by cyclic voltammetry and electrochemical impedance spectroscopy（EIS） using a rotating disk electrode（RDE）. Freshly synthesized catalysts, as well as catalysts subjected to a short chronoamperometric ageing procedure, are analyzed and the kinetic and thermodynamic parameters of the HER are obtained. The nickel surface modified with transition metals with an outer shell electronic configuration [xd;（x+1）s;], such as Cu(3d;4s;)and Ag(4d;5s;), shows an improved activity for the HER compared to bare nickel. Furthermore, the Ni/Cu catalyst presents a decreased onset potential. The hydrogen evolution rate, measured as current density at –1.5 V（vs. SCE）, is similar on Ni/Cu and Ni/Ag electrodes.

First-and second-order phase transitions in RE_(6)Co_(2)Ga(RE=Ho,Dy or Gd)cryogenic magnetocaloric materials

Rare-earth(RE)rich intermetallics crystallizing in orthorhombic Ho_(6)Co_(2)Ga-type crystal structure exhibit peculiar magnetic properties that are not widely reported for their magnetic ordering,order of magnetic phase transition,and related magnetocaloric behavior.By tuning the type of RE element in RE_(6)Co_(2)Ga(RE=Ho,Dy or Gd)compounds,metamagnetic anti-to-paramagnetic(AF to PM)phase transitions could be tuned to ferro-to-paramagnetic(FM to PM)phase transitions.Furthermore,the FM ground state for Gd_(6)Co_(2)Ga is confirmed by density functional theory calculations in addition to experimental observations.The field dependence magnetocaloric and Banerjee’s criteria demonstrate that Ho_(6)Co_(2)Ga and Dy_(6)Co_(2)Ga undergo a first-order phase transition in addition to a second-order phase transition,whereas only the latter is observed for Gd_(6)Co_(2)Ga.The two extreme alloys of the series,Ho_(6)Co_(2)Ga and Gd_(6)Co_(2)Ga,show maximum isothermal entropy change(|ΔS_(iso)^(max)(5T)|)of 10.1 and 9.1 J kg^(-1)K^(-1)at 26 and 75 K,close to H_(2)and N_(2)liquefaction,respectively.This outstanding magnetocaloric effect performance makes the RE6 Co_(2)Ga series of potential for cryogenic magnetic refrigeration applications.

There is life after coking for Ir nanocatalyst superlattices

Achieving superior performance of nanoparticle systems is one of the biggest challenges in catalysis.Two major phenomena,occurring during the reactions,hinder the development of the full potential of nanoparticle catalysts:sintering and contamination with carbon containing species,sometimes called coking.Here,we demonstrate that Ir nanocrystals,arranged into periodic networks on hexagonal boron nitride(h-BN)supports,can be restored without sintering after contamination by persistent carbon.This restoration yields the complete removal of carbon from the nanocrystals,which keep their crystalline structure,allowing operation without degradation.These findings,together with the possibility of fine tuning the nanocrystals size,confer this nanoparticle system a great potential as a testbed to extract key information about catalysis-mediated oxidation reactions.For the case of the CO oxidation by O2,reaction of interest in environmental science and green energy production,the existence of chemical processes not observed before in other nanoparticle systems is demonstrated.

重稀土HRENiGa_(2)(HRE=Dy,Ho或Er)化合物的优异低温磁热性能

RENiX_(2)化合物(其中RE为稀土元素,X为p区元素)在低温磁制冷应用中受到高度关注.它们根据元素的不同可以结晶成CeNiSi_(2)型、NdNiGa_(2)型或MgCuAl_(2)型晶体结构,并表现出不同类型的磁有序性从而影响其磁性.MgCuAl_(2)型铝化物由于具有有利的铁磁基态从而表现出比CeNiSi_(2)型硅化物和NdNiGa_(2)型镓化物更大的磁热性能.此外,RENiGa_(2)镓化物根据RE元素的不同可以结晶成NdNiGa_(2)或MgCu Al_(2)型结构.本文中,我们选择重稀土(HRE)元素来探索HRENiGa_(2)(HRE=Dy,Ho或Er)镓化物的微观结构、磁有序和磁热性能.三种化合物均以MgCuAl_(2)型晶体结构结晶,并且随着温度的升高经历了从铁磁到顺磁的二级磁相转变.DyNiGa_(2),HoNiGa_(2)和ErNiGa_(2)化合物的最大等温磁熵变(|ΔSisomax|)值分别为6.2,10.4和11.4 J kg^(-1)K^(-1)(0-5T),这与许多最近报道的低温磁制冷材料性能相当.特别地,HoNiGa_(2)和ErNiGa_(2)化合物(包括它们的复合材料)在氢气液化的温度范围内表现出优异的磁热性能.