Zhiying Guo

General synthesis and definitive structural identification of MN 4 C 4 single-atom catalysts with tunable electrocatalytic activities

AbstractSingle-atom catalysts (SACs) have recently attracted broad research interest as they combine the merits of both homogeneous and heterogeneous catalysts. Rational design and synthesis of SACs are of immense significance but have so far been plagued by the lack of a definitive correlation between structure and catalytic properties. Here, we report a general approach to a series of monodispersed atomic transition metals (for example, Fe, Co, Ni) embedded in nitrogen-doped graphene with a common MN4C4 moiety, identified by systematic X-ray absorption fine structure analyses and direct transmission electron microscopy imaging. The unambiguous structure determination allows density functional theoretical prediction of MN4C4 moieties as efficient oxygen evolution catalysts with activities following the trend Ni > Co > Fe, which is confirmed by electrochemical measurements. Determination of atomistic structure and its correlation with catalytic properties represents a critical step towards the rational design and synthesis of precious or nonprecious SACs with exceptional atom utilization efficiency and catalytic activities.Atomically dispersed metal catalysts are of increasing importance in many catalytic processes, but clear structural identification is challenging. Here, a general synthesis of metal (nickel, iron and cobalt) single-atom catalysts on nitrogen-doped graphene allows the authors to identify a common structure and furthermore correlate structure with electrocatalytic activity.

Anisotropy compensation and spin reorientation in Tb1-xDyx(Fe0.8Co0.2)2

Structure, magnetic properties, magnetostriction and anisotropy compensation of Tb1-xDyx(Fe0.8Co0.2)(2) (0 less than or equal to x less than or equal to 1) have been investigated by means of x-ray diffraction, ac initial susceptibility, SQUID magnetometery, and a standard strain technique. The x-ray diffraction results show that all samples crystallize in a C15 MgCu2-type compound with a Laves phase cubic structure. The lattice parameter as well as the Curie temperature almost linearly decreases with increasing Dy content. The spin reorientation transition occurs at low temperatures when x (111) decreases from 2460 ppm for Tb(Fe0.8Co0.2)(2) to 1900 ppm for Tb0.4Dy0.6(Fe0.8Co0.2)(2) When 0.6 1, the splitting of the (440) peaks almost disappears, which indicates that the composition for anisotropy compensation is shifted to the Tb-rich side compared with Terfenol-D. The polycrystalline magnetostriction exhibits similar behaviour to that of hill. The phase diagram for different spin configurations has been given for Tb1-xDyx(Fe0.8Co0.2)(2) according to the experimental results.

Effect of Mn substitution on structure, magnetic properties and magnetostriction of Sm0.9Pr0.1Fe2

The structure, magnetic properties and magnetostriction of are-melted polycrystalline Sm0.9Pr0.1(Fe1-xMnx)(2) (0 less than or equal to x less than or equal to 0.2) alloys were investigated by microscopy, X-ray diffraction, A.C. initial susceptibility, vibrating sample magnetometer and standard strain gauge techniques. It was found that the matrix consists almost entirely of the MgCu2-type cubic Laves phase. The lattice parameter increases, while the Curie temperatures decrease with increasing x. The spontaneous magnetostriction lambda(111,) polycrystalline saturation magnetostriction lambda(s) and saturation magnetization M-s decrease slowly for x less than or equal to 1.0 and abruptly for x > 1.0 with increasing Mn content. The changes in the magnetization and magnetostriction can be understood on the basis of the decrease of the 3d-sublattice moment.(1) (C) 2000 Elsevier Science S.A. All rights reserved.

Formation of δ phase and its effects on magnetic properties and magnetostriction of Tb0.5Pr0.5(Fe0.4+xCo0.6−x)1.9 (0⩽x⩽0.6)

Structure, magnetic properties and magnetostriction of Tb0.5Pr0.5(Fe0.4+xCo0.6-x)(1.9) (0 less than or equal to x less than or equal to 0.6) alloys are investigated. The existence of the delta phase for 50% Tb and 50% Pr is beneficial to the formation of the MgCu2-type Laves phase. The composition dependence of Curie temperature has the same trend as the Slater-Pauling curve for the moment of Fe-Co alloys. The saturation magnetization M-g and the remanence M-R decrease, whereas the coercivity H-i(c) increases with decreasing Co content, due to the increased amount of the non-cubic PuNi3-type phase. The polycrystalline magnetostriction \lambda parallel to - lambda\ of the alloys at room temperature increases with increasing magnetic field and does not achieve saturation up to 796 kA/m

Revisiting local structural changes in GeO2 glass at high pressure

Despite the great importance in fundamental and industrial fields, understanding structural changes for pressure-induced polyamorphism in network-forming glasses remains a formidable challenge. Here, we revisited the local structural transformations in GeO2 glass up to 54 GPa using x-ray absorption fine structure (XAFS) spectroscopy via a combination diamond anvil cell and polycapillary half-lens. Three polyamorphic transitions can be clearly identified by XAFS structure refinement. First, a progressive increase of the nearest Ge-O distance and bond disorder to a maximum at ~5-16 GPa, in the same pressure region of previously observed tetrahedral-octahedral transformation. Second, a marked decrease of the nearest Ge-O distance at ~16-22.6 GPa but a slight increase at ~22.6-32.7 GPa, with a concomitant decrease of bond disorder. This stage can be related to a second-order-like transition from less dense to dense octahedral glass. Third, another decrease in the nearest Ge-O distance at ~32.7-41.4 GPa but a slight increase up to 54 GPa, synchronized with a gradual increase of bond disorder. This stage provides strong evidence for ultrahigh-pressure polyamorphism with coordination number  >6. Furthermore, cooperative modification is observed in more distant shells. Those results provide a unified local structural picture for elucidating the polyamorphic transitions and densification process in GeO2 glass.

Local structural changes during the disordered substitutional alloy transition in Bi2Te3 by high-pressure XAFS

A2B3-type 3D topological insulators, Bi2Te3 and Sb2Te3, have been reported to transform into disordered substitutional alloys under high pressure. However, γ → δ phase transition and the local structure changes around Bi during the formation of the disordered Bi-Te binary alloy in Bi2Te3 still remain unclear. Here, high-pressure X-ray absorption fine structure (XAFS) combined with high-pressure X-ray diffraction has been used to explore the local structural transformations in the three structural phase transitions of Bi2Te3. The Bi L3-X-ray absorbing near edge structure (XANES) spectra of δ-Bi2Te3 clearly showed that a new absorption feature at energy of about 13 465 eV would emerge during the γ → δ phase transition. Through simulation of the XANES spectra by varying the cluster size, we confirmed that the new absorption peak arises from the medium-range order in bcc structure. The Bi L3-EXAFS results in δ phase reveal that the Bi atoms in the third shell exhibit abnormal elongations with pressure until merging with the shrinking Te shell. Our findings indicate a two-step structural transition of the disordered substitutional alloys via an ordered bcc intermediate phase.A2B3-type 3D topological insulators, Bi2Te3 and Sb2Te3, have been reported to transform into disordered substitutional alloys under high pressure. However, γ → δ phase transition and the local structure changes around Bi during the formation of the disordered Bi-Te binary alloy in Bi2Te3 still remain unclear. Here, high-pressure X-ray absorption fine structure (XAFS) combined with high-pressure X-ray diffraction has been used to explore the local structural transformations in the three structural phase transitions of Bi2Te3. The Bi L3-X-ray absorbing near edge structure (XANES) spectra of δ-Bi2Te3 clearly showed that a new absorption feature at energy of about 13 465 eV would emerge during the γ → δ phase transition. Through simulation of the XANES spectra by varying the cluster size, we confirmed that the new absorption peak arises from the medium-range order in bcc structure. The Bi L3-EXAFS results in δ phase reveal that the Bi atoms in the third shell exhibit abnormal elongations with pressure until m...