Hengjie Liu

Preparation and studies of Ag–TiO2 hybrid nanoparticles of core-shell structure

Abstract Ag–TiO 2 hybrid nanoparticles of core-shell structure were synthesized by an ultraviolet irradiation reduction technique. Phase analysis of the product obtained was carried out by X-ray and electron diffraction patterns. The morphology of the hybrid particles was studied by TEM images. The variation of UV-Vis absorption spectra of the silver colloidal solution with incorporation of TiO 2 was observed. A strong surface-enhance Raman scattering (SERS) and Raman blue shift of TiO 2 was found in Ag–TiO 2 hybrid nanoparticles due to the amalgamation of Ag.

Electronic Structure Reconfiguration toward Pyrite NiS2 via Engineered Heteroatom Defect Boosting Overall Water Splitting

Developing highly active and low-cost heterogeneous catalysts toward overall electrochemical water splitting is extremely desirable but still a challenge. Herein, we report pyrite NiS2 nanosheets doped with vanadium heteroatoms as bifunctional electrode materials for both hydrogen- and oxygen-evolution reaction (HER and OER). Notably, the electronic structure reconfiguration of pyrite NiS2 is observed from typical semiconductive characteristics to metallic characteristics by engineering vanadium (V) displacement defect, which is confirmed by both experimental temperature-dependent resistivity and theoretical density functional theory calculations. Furthermore, elaborate X-ray absorption spectroscopy measurements reveal that electronic structure reconfiguration of NiS2 is rooted in electron transfer from doped V to Ni sites, consequently enabling Ni sites to gain more electrons. The metallic V-doped NiS2 nanosheets exhibit extraordinary electrocatalytic performance with overpotentials of about 290 mV for OER and about 110 mV for HER at 10 mA cm-2 with long-term stability in 1 M KOH solutions, representing one of the best non-noble-metal bifunctional electrocatalysts to date. This work provides insights into electronic structure engineering from well-designed atomic defect metal sulfide.

The preparation, surface modification, and characterization of metallic α-Fe nanoparticles

Abstract α -Fe nanoparticles were prepared by reduction of Fe 2+ using potassium borohydride in a simple ethanol/water system in the presence of surfactant. The in-situ modification of particles was carried out by taking advantage of a modifying solution containing Ni 2+ . The structure and size of the particles were investigated by X-ray diffraction (XRD), X-ray photoelectron spectrum (XPS), transmission electron microscopy (TEM) and electron diffraction (ED). Results showed that the in-situ electrochemical reaction between α-Fe nanoparticles and Ni 2+ resulted in the formation of stable multilayer composite nanostructure. The cores of composite nanostructure were α-Fe.

Single-domain multiferroic BiFeO3 films.

The strong coupling between antiferromagnetism and ferroelectricity at room temperature found in BiFeO3 generates high expectations for the design and development of technological devices with novel functionalities. However, the multi-domain nature of the material tends to nullify the properties of interest and complicates the thorough understanding of the mechanisms that are responsible for those properties. Here we report the realization of a BiFeO3 material in thin film form with single-domain behaviour in both its magnetism and ferroelectricity: the entire film shows its antiferromagnetic axis aligned along the crystallographic b axis and its ferroelectric polarization along the c axis. With this we are able to reveal that the canted ferromagnetic moment due to the Dzyaloshinskii–Moriya interaction is parallel to the a axis. Furthermore, by fabricating a Co/BiFeO3 heterostructure, we demonstrate that the ferromagnetic moment of the Co film does couple directly to the canted moment of BiFeO3.

Hidden lattice instabilities as origin of the conductive interface between insulating LaAlO3 and SrTiO3.

The metallic interface between insulating LaAlO3 and SrTiO3 opens up the field of oxide electronics. With more than a decade of researches on this heterostructure, the origin of the interfacial conductivity, however, remains unsettled. Here we resolve this long-standing puzzle by atomic-scale observation of electron-gas formation for screening hidden lattice instabilities, rejuvenated near the interface by epitaxial strain. Using atomic-resolution imaging and electron spectroscopy, the generally accepted notions of polar catastrophe and cation intermixing for the metallic interface are discounted. Instead, the conductivity onset at the critical thickness of 4-unit cell LaAlO3 on SrTiO3 substrate is accompanied with head-to-head ferroelectric-like polarizations across the interface due to strain-rejuvenated ferroelectric-like instabilities in the materials. The divergent depolarization fields of the head-to-head polarizations cast the interface into an electron reservoir, forming screening electron gas in SrTiO3 with LaAlO3 hosting complementary localized holes. The ferroelectric-like polarizations and electron–hole juxtaposition reveal the cooperative nature of metallic LaAlO3/SrTiO3.

Atomic Iridium Incorporated in Cobalt Hydroxide for Efficient Oxygen Evolution Catalysis in Neutral Electrolyte

Developing highly efficient catalysts for oxygen evolution reaction (OER) in neutral media is extremely crucial for microbial electrolysis cells and electrochemical CO2 reduction. Herein, a facile one-step approach is developed to synthesize a new type of well-dispersed iridium (Ir) incorporated cobalt-based hydroxide nanosheets (nominated as CoIr) for OER. The Ir species as clusters and single atoms are incorporated into the defect-rich hydroxide nanosheets through the formation of rich Co-Ir species, as revealed by systematic synchrotron radiation based X-ray spectroscopic characterizations combining with high-angle annular dark-field scanning transmission electron microscopy measurement. The optimized CoIr with 9.7 wt% Ir content displays highly efficient OER catalytic performance with an overpotential of 373 mV to achieve the current density of 10 mA cm-2 in 1.0 m phosphate buffer solution, significantly outperforming the commercial IrO2 catalysts. Further characterizations toward the catalyst after undergoing OER process indicate that unique Co oxyhydroxide and high valence Ir species with low-coordination structure are formed due to the high oxidation potentials, which authentically contributes to superior OER performance. This work not only provides a state-of-the-art OER catalyst in neutral media but also unravels the root of the excellent performance based on efficient structural identifications.

Growing and Etching MoS2 on Carbon Nanotube Film for Enhanced Electrochemical Performance

In this work we directly synthesized molybdenum disulfide (MoS2) nanosheets on carbon nanotube film (MoS2@CNT) via a two-step chemical vapor deposition method (CVD). By etching the obtained MoS2@CNT into 10% wt HNO3, the morphology of MoS2 decorated on CNT bundles was modulated, resulting in more catalytic active MoS2 edges being exposed for significantly enhanced electrochemical performance. Our results revealed that an 8 h acid etching sample exhibited the best performance for the oxygen evolution reaction, i.e., the current density reached 10 mA/cm2 under 375 mV over-potential, and the tafel slope was as low as 94 mV/dec. The enhanced behavior was mainly originated from the more catalytic sites in MoS2 induced by the acid etching treatment and the higher conductivity from the supporting CNT films. Our study provides a new route to produce two-dimensional layers on CNT films with tunable morphology, and thus may open a window for exploring its promising applications in the fields of catalytic-, electronic-, and electrochemical-related fields.

Well‐Defined Cobalt Catalyst with N‐Doped Carbon Layers Enwrapping: The Correlation between Surface Atomic Structure and Electrocatalytic Property

Admittedly, the surface atomic structure of heterogenous catalysts toward the electrochemical oxygen reduction reaction (ORR) are accepted as the important features that can tune catalytic activity and even catalytic pathway. Herein, a surface engineering strategy to controllably synthesize a carbon-layer-wrapped cobalt-catalyst from 2D cobalt-based metal-organic frameworks is elaborately demonstrated. Combined with synchrotron radiation X-ray photoelectron spectroscopy, the soft X-ray absorption near-edge structure results confirmed that rich covalent interfacial CoNC bonds are efficiently formed between cobalt nanoparticles and wrapped carbon-layers during the polydopamine-assisted pyrolysis process. The X-ray absorption fine structure and corresponding extended X-ray absorption fine structure spectra further reveal that the wrapped cobalt with Co-N coordinations shows distinct surface distortion and atomic environmental change of Co-based active sites. In contrast to the control sample without coating layers, the 800 °C-annealed cobalt catalyst with N-doped carbon layers enwrapping achieves significantly enhanced ORR activity with onset and half-wave potentials of 0.923 and 0.816 V (vs reversible hydrogen electrode), highlighting the important correlation between surface atomic structure and catalytic property.

Ferromagnetism in CVT Grown Tungsten Diselenide Single Crystals with Nickel Doping

Two dimensional (2D) single crystal layered transition materials have had extensive consideration owing to their interesting magnetic properties, originating from their lattices and strong spin-orbit coupling, which make them of vital importance for spintronic applications. Herein, we present synthesis of a highly crystalline tungsten diselenide layered single crystal grown by chemical vapor transport technique and doped with nickel (Ni) to tailor its magnetic properties. The pristine WSe2 single crystal and Ni-doped crystal were characterized and analyzed for magnetic properties using both experimental and computational aspects. It was found that the magnetic behavior of the 2D layered WSe2 crystal changed from diamagnetic to ferromagnetic after Ni-doping at all tested temperatures. Moreover, first principle density functional theory (DFT) calculations further confirmed the origin of room temperature ferromagnetism of Ni-doped WSe2, where the d-orbitals of the doped Ni atom promoted the spin moment and thus largely contributed to the magnetism change in the 2D layered material.

The significant role of covalency in determining the ground state of cobalt phthalocyanines molecule

To shed some light on the metal 3d ground state configuration of cobalt phthalocyanines system, so far in debate, we present an investigation by X-ray absorption spectroscopy (XAS) at Co L2,3 edge and theoretical calculation. The density functional theory calculations reveal highly anisotropic covalent bond between central cobalt ion and nitrogen ligands, with the dominant σ donor accompanied by weak π-back acceptor interaction. Our combined experimental and theoretical study on the Co-L2,3 XAS spectra demonstrate a robust ground state of 2A1g symmetry that is built from 73% 3d7 character and 27% 3d8L¯ (L¯ denotes a ligand hole) components, as the first excited-state with 2Eg symmetry lies about 158 meV higher in energy. The effect of anisotropic and isotropic covalency on the ground state was also calculated and the results indicate that the ground state with 2A1g symmetry is robust in a large range of anisotropic covalent strength while a transition of ground state from 2A1g to 2Eg configuration when is...