Liwen Yang

Upconversion-P25-graphene composite as an advanced sunlight driven photocatalytic hybrid material

Herein, a new nanocomposite consisting of up-conversion (UC) material (YF3:Yb3+,Tm3+), TiO2 (P25) and graphene (GR) has been prepared and shown to be an advanced sunlight activated photocatalyst. During the facile hydrothermal method, the reduction of graphene oxide and loading of YF3:Yb3+,Tm3+ and P25 were achieved simultaneously, and the functionalities of each part were integrated together. The as-prepared ternary UC–P25–GR nanocomposite photocatalyst exhibited great adsorptivity of dyes, a significantly extended light absorption range, efficient charge separation properties and superior durability. Indeed, the photocatalytic activity of this novel ternary nanocomposite under sunlight was improved compared with those of P25–GR nanocomposites and bare P25. Overall, this work could provide new insights into the fabrication of ternary composites as high performance photocatalysts and facilitate their application in environmental protection issues.

Large-scale production of ultrathin topological insulator bismuth telluride nanosheets by a hydrothermal intercalation and exfoliation route

A convenient hydrothermal intercalation/exfoliation method for large-scale manufacturing of bismuth telluride (Bi2Te3) nanosheets is reported here. Lithium cations can be intercalated between the layers of Bi2Te3 using the reducing power of ethylene glycol in the common hydrothermal process, and high quality Bi2Te3 nanosheets with thickness down to only 3–4 nm are obtained by removing lithium in the following exfoliating process. Scanning electron microscopy, transmission electron microscopy and Raman spectrum characterizations confirm that the high yield of Bi2Te3 nanosheets with good quality were successfully achieved and the sizes of the immense nanosheets reached 200 nm width and 1 μm length. This hydrothermal intercalation/exfoliation method is general, as it has been extended to other layered materials, such as Bi2Se3 and MoS2. Our results suggest a simple route for the large-scale production of thin and flat Bi2Te3 nanosheets, which may be beneficial to further electronic and spintronics applications.

Self-assembled FeS2 cubes anchored on reduced graphene oxide as an anode material for lithium ion batteries

A novel composite of reduced graphene oxide (RGO) and FeS2 microparticles self-assembled from small size cubes as a high-performance anode material for lithium-ion batteries (LIBs) has been prepared via a facile one-pot hydrothermal method. The prepared composite shows interconnected networks of reduced graphene oxide sheets and well-dispersed FeS2 microparticles which were composed of small-size cubic FeS2 crystals. The composite not only provides a high contact area between the electrolyte and the electrode, favorable diffusion kinetics for both electrons and lithium ions, but also provides the protection against the volume changes of electroactive FeS2 materials and excellent electrical conductivity of the overall electrode during electrochemical processes as well as an enhanced synergistic effect between cubic FeS2 and RGO. As an anode material for LIBs, it exhibits a very large initial reversible capacity of 1147 mA h g−1 at a current rate of 100 mA h g−1 and maintains 1001.41 mA h g−1 over 60 cycles, which is much higher than that of the theoretical capacity of graphite (372 mA h g−1) and indicates high stability. The results demonstrate that the composite can be a promising candidate for electroactive materials in LIBs.

Size-induced elastic stiffening of ZnO nanostructures: Skin-depth energy pinning

It has long been puzzling regarding the trends and physical origins of the size-effect on the elasticity of ZnO nanostructures. An extension of the atomic “coordination-radius” correlation premise of Pauling and Goldschmidt to energy domain has enabled us to clarify that the elastic modulus is intrinsically proportional to the sum of bond energy per unit volume and that the size-induced elastic stiffening arises from (i) the broken-bond-induced local strain and skin-depth energy pinning and (ii) the tunable fraction of bonds between the undercoordinated atoms, and therefore, the elastic modulus of ZnO nanostructures should increase with the inverse of feature size.

Non-covalent doping of graphitic carbon nitride with ultrathin graphene oxide and molybdenum disulfide nanosheets: An effective binary heterojunction photocatalyst under visible light irradiation

A proof of concept integrating binary p-n heterojunctions into a semiconductor hybrid photocatalyst is demonstrated by non-covalent doping of graphite-like carbon nitride (g-C3N4) with ultrathin GO and MoS2 nanosheets using a facile sonochemical method. In this unique ternary hybrid, the layered MoS2 and GO nanosheets with a large surface area enhance light absorption to generate more photoelectrons. On account of the coupling between MoS2 and GO with g-C3N4, the ternary hybrid possesses binary p-n heterojunctions at the g-C3N4/MoS2 and g-C3N4/GO interfaces. The space charge layers created by the p-n heterojunctions not only enhance photogeneration, but also promote charge separation and transfer of electron-hole pairs. In addition, the ultrathin MoS2 and GO with high mobility act as electron mediators to facilitate separation of photogenerated electron-hole pairs at each p-n heterojunction. As a result, the ternary hybrid photocatalyst exhibits improved photoelectrochemical and photocatalytic activity under visible light irradiation compared to other reference materials. The results provide new insights into the large-scale production of semiconductor photocatalysts.

Raman spectroscopic determination of the length, strength, compressibility, Debye temperature, elasticity, and force constant of the C–C bond in graphene

From the perspective of bond relaxation and bond vibration, we have formulated the Raman phonon relaxation of graphene, under the stimuli of the number-of-layers, the uni-axial strain, the pressure, and the temperature, in terms of the response of the length and strength of the representative bond of the entire specimen to the applied stimuli. Theoretical unification of the measurements clarifies that: (i) the opposite trends of the Raman shifts, which are due to the number-of-layers reduction, of the G-peak shift and arises from the vibration of a pair of atoms, while the D- and the 2D-peak shifts involve the z-neighbor of a specific atom; (ii) the tensile strain-induced phonon softening and phonon-band splitting arise from the asymmetric response of the C(3v) bond geometry to the C(2v) uni-axial bond elongation; (iii) the thermal softening of the phonons originates from bond expansion and weakening; and (iv) the pressure stiffening of the phonons results from bond compression and work hardening. Reproduction of the measurements has led to quantitative information about the referential frequencies from which the Raman frequencies shift as well as the length, energy, force constant, Debye temperature, compressibility and elastic modulus of the C-C bond in graphene, which is of instrumental importance in the understanding of the unusual behavior of graphene.

A Bond-order Theory on the Phonon Scattering by Vacancies in Two-dimensional Materials

We theoretically investigate the phonon scattering by vacancies, including the impacts of missing mass and linkages () and the variation of the force constant of bonds associated with vacancies () by the bond-order-length-strength correlation mechanism. We find that in bulk crystals, the phonon scattering rate due to change of force constant is about three orders of magnitude lower than that due to missing mass and linkages . In contrast to the negligible in bulk materials, in two-dimensional materials can be 3–10 folds larger than . Incorporating this phonon scattering mechanism to the Boltzmann transport equation derives that the thermal conductivity of vacancy defective graphene is severely reduced even for very low vacancy density. High-frequency phonon contribution to thermal conductivity reduces substantially. Our findings are helpful not only to understand the severe suppression of thermal conductivity by vacancies, but also to manipulate thermal conductivity in two-dimensional materials by phononic engineering.

Electrostatic properties of few-layer MoS2 films

Two-dimensional MoS2-based materials are considered to be one of the most attractive materials for next-generation nanoelectronics. The electrostatic properties are important in designing and understanding the performance of MoS2-based devices. By using Kelvin probe force microscopy, we show that few-layer MoS2 sheets exhibit uniform surface potential and charge distributions on their surfaces but have relatively lower surface potentials on the edges, folded areas as well as defect grain boundaries.

Ambipolar charge injection and transport of few-layer topological insulator Bi2Te3 and Bi2Se3 nanoplates

We report the electrostatic properties of few-layer Bi2Te3 and Bi2Se3 nanoplates (NPs) grown on 300u2009nm SiO2/Si substrate. Electrons and holes are locally injected in Bi2Te3 and Bi2Se3 nanoplates by the apex of an atomic force microscope tip. Both carriers are delocalized uniformly over the whole nanoplate. The electrostatic property of topological insulator Bi2Te3 and Bi2Se3 nanoplates after charge injection is characterized by Kelvin probe force microscopy under ambient environment and exhibits an ambipolar surface potential behavior. These results provide insight into the electronic properties of topological insulators at the nanometer scale.

A macroscopic three-dimensional tetrapod-separated graphene-like oxygenated N-doped carbon nanosheet architecture for use in supercapacitors

Macroscopic three-dimensional oxygenated carbon materials with enriched nitrogen (designated as PGOCN) are prepared by a two-step solid-state pyrolysis of a mixture of urea and glucose inside a template framework of melamine sponge in a N2 atmosphere without any functionalizing or crosslinking agents. Characterization by SEM, TEM, XPS and nitrogen adsorption–desorption isotherm measurements reveals that the prepared samples consist of a tetrapod framework embedded with crumpled graphene-like oxygenated N-doped carbon nanosheets, demonstrating a hierarchical porous structure of macro-, meso- and micropores. Considering the hierarchical porous structure combined with the presence of abundant oxygen and nitrogen as well as high electrical conductivity, the application of the PGOCN materials in high-performance supercapacitors is investigated. In three-electrode systems, the PGOCN electrodes show high specific capacitances of 348 F g−1 in acidic electrolytes and 308 F g−1 in alkaline electrolytes at a current density of 1 A g−1, respectively. Remarkably, the PGOCN materials can be directly cut into thin sheets to assemble two-electrode supercapacitor devices without adding binders and conducting additives using 6 M KOH as the electrolyte. The two-electrode supercapacitor device exhibits a high specific capacitance of 220 F g−1 at 0.2 A g−1 and a power density of 1.2 kW kg−1 at an energy density of 3.4 Wh kg−1 as well as outstanding cycling stability after 2000 cycles. The facile and low-cost preparation procedure combined with excellent electrochemical performance indicates that the developed materials have great potential for applications in energy storage devices such as supercapacitors.