SPECTRAL VARIATION AND CHARACTERISTICS OF LOW VALENCY RARE EARTH IONS IN COMPLEX OXIDESⅢ.EFFECT OF THE CRYSTAL ENVIRONMENT AND CHEMICAL FACTORS ON LUMINESCENCE PROPERTIES OF EUROPIUM (Ⅱ) ION

In this paper,the influence of crystal-field on the Luminescence properties of Eu^(2+) in complex oxides are studied theoretically by using purely electrostatic model,the dependence of the 4f^65d levels on Eu-O bond distance is given.Quantum chemistry calculation shows that the splitting extent of 4f^65d energy band in cubic or in octahedral fields will be inversely proportional to R^5,where R is the distance of Eu^(2+) to oxygen ligand.The value of R affects slightly the location of the centre of 4f^65d energy band.According to the exper- imental spectrum data,we have discussed the influence of the host chemical composition,the replaced sites of Eu^(2+) and degree of covalency of Eu-O bond on luminescence properties of Eu^(2+).Some regularity of fluorescence spectrum was observed. In alkali-alkaline earth-phosphates,the splitting extent of 4f^65d band (△E) becomes smaller as the Eu-O bond distance (R) increases.In Na_(3-x)(PO_4)_(1-x)(SO_4)_x and Na_(2-x)CaSi_(1-x)P_xO_4 hosts,d-d emission peak of Eu^(2+) will shift to shorter wavelength with the increase of x's value. The crystal structure data show that Eu^(2+) in K_2Mg_2(SO_4)_3 is affected more strongly by crystal-field and covalancy than in KMgF_3,so K_2Mg_2(SO_4)_3:Eu^(2+) emits blue light (E_(em)~m=400nm) and KMgF_3:Eu^(2+) produces ultraviolet fluorescence.

Insight into sulfur and iron effect of binary nickel-iron sulfide on oxygen evolution reaction

Nickel-iron sulfide has shown attractive activity in electrocatalytic oxygen evolution reaction(OER).However,the effects of low valence sulfur(S^(2−))and metal species on OER in binary nickel-iron sulfide have rarely been systematically studied.Works based on post-catalysis characterization have led to the assumption that the real active species are nickel-iron oxyhydroxide,and that nickel-iron sulfide acts only as a precatalyst.Therefore,to study the role of S,Ni,and Fe for the development of nickel-iron sulfide catalyst is of self-evident importance.Herein,a facile solvothermal method is used to synthesize acetylene black coated with nickel-iron sulfide nanosheets.Electrochemical tests show that the presence of low valence S species makes the catalyst have faster OER kinetics,larger active area,and intermediate active species adsorption area.Therefore,the present study reveals the enhancing effect of low valence sulfur species(S^(2−))on OER in binary nickel-iron sulfide.In situ Raman spectroscopy shows that the generation ofγ-NiOOH intermediate is essential and Fe does not directly participate in the oxygen production.Density functional theory(DFT)calculation shows that Ni-OH deprotonation is a rate-determining step for both binary nickel-iron sulfide and nickel sulfide.The addition of Fe into NiSx lightly increases the charge transfer of Ni atom to O atom,which makes deprotonation easier and thereby improves the OER performance.