Hezhou Liu

Fabrication of ultralight three-dimensional graphene networks with strong electromagnetic wave absorption properties

Thermally reduced graphene networks (TRGN) with low densities, less than 10 mg cm−3, were synthesized by thermal reduction of graphene oxide/poly(vinyl alcohol) networks. To evaluate the electromagnetic wave absorption properties of TRGN, TRGN were nondestructively backfilled with wax via a vacuum-assisted method. The as-prepared TRGN/wax composites, using integrated TRGN as fillers rather than directly dispersing graphene sheets in wax, exhibit better electromagnetic wave absorption capabilities because of the 3D conductive frameworks, which could generate more effective electrical loss in terms of dissipating the induced current in the TRGN/wax composites. Specifically, for the TRGN/wax composite with ∼1 wt% TRGN, the minimum reflection loss reaches −43.5 dB at 12.19 GHz with a thickness of 3.5 mm, and the bandwidth of reflection loss less than −10 dB (90% absorption) can reach up to 7.47 GHz. More importantly, our work provides a promising approach for constructing graphene-based composites with strong electromagnetic wave absorption ability at very low filler loadings.

Evidence for oxygen vacancy or ferroelectric polarization induced switchable diode and photovoltaic effects in BiFeO3 based thin films.

The diode and photovoltaic effects of BiFeO3 and Bi0.9Sr0.1FeO(3-δ) polycrystalline thin films were investigated by poling the films with increased magnitude and alternating direction. It was found that both electromigration of oxygen vacancies and polarization flipping are able to induce switchable diode and photovoltaic effects. For the Bi0.9Sr0.1FeO(3-δ) thin films with high oxygen vacancy concentration, reversibly switchable diode and photovoltaic effects can be observed due to the electromigration of oxygen vacancies under an electric field much lower than its coercive field. However, for the pure BiFeO3 thin films with lower oxygen vacancy concentration, the reversibly switchable diode and photovoltaic effect is hard to detect until the occurrence of polarization flipping. The switchable diode and photovoltaic effects can be explained well using the concepts of Schottky-like barrier-to-Ohmic contacts resulting from the combination of oxygen vacancies and polarization. The sign of photocurrent could be independent of the direction of polarization when the modulation of the energy band induced by oxygen vacancies is large enough to offset that induced by polarization. The photovoltaic effect induced by the electromigration of oxygen vacancies is unstable due to the diffusion of oxygen vacancies or the recombination of oxygen vacancies with hopping electrons. Our work provides deep insights into the nature of diode and photovoltaic effects in ferroelectric films, and will facilitate the advanced design of switchable devices combining spintronic, electronic, and optical functionalities.

The effect of annealing on a 3D SnO2/graphene foam as an advanced lithium-ion battery anode

3D annealed SnO2/graphene sheet foams (ASGFs) are synthesized by in situ self-assembly of graphene sheets prepared by mild chemical reduction. L-ascorbyl acid is used to effectively reduce the SnO2 nanoparticles/graphene oxide colloidal solution and form the 3D conductive graphene networks. The annealing treatment contributes to the formation of the Sn-O-C bonds between the SnO2 nanoparticles and the reduced graphene sheets, which improves the electrochemical performance of the foams. The ASGF has features of typical aerogels: low density (about 19 mg cm−3), smooth surface and porous structure. The ASGF anodes exhibit good specific capacity, excellent cycling stability and superior rate capability. The first reversible specific capacity is as high as 984.2 mAh g−1 at a specific current of 200 mA g−1. Even at the high specific current of 1000 mA g−1 after 150 cycles, the reversible specific capacity of ASGF is still as high as 533.7 mAh g−1, about twice as much as that of SGF (297.6 mAh g−1) after the same test. This synthesis method can be scaled up to prepare other metal oxides particles/ graphene sheet foams for high performance lithium-ion batteries, supercapacitors, and catalysts, etc.

Ionic Conductivity and Air Stability of Al-Doped Li7La3Zr2O12 Sintered in Alumina and Pt Crucibles

Li7La3Zr2O12 (LLZO) is a promising electrolyte material for all-solid-state battery due to its high ionic conductivity and good stability with metallic lithium. In this article, we studied the effect of crucibles on the ionic conductivity and air stability by synthesizing 0.25Al doped LLZO pellets in Pt crucibles and alumina crucibles, respectively. The results show that the composition and microstructure of the pellets play important roles influencing the ionic conductivity, relative density, and air stability. Specifically, the 0.25Al-LLZO pellets sintered in Pt crucibles exhibit a high relative density (∼96%) and high ionic conductivity (4.48 × 10(-4) S cm(-1)). The ionic conductivity maintains 3.6 × 10(-4) S cm(-1) after 3-month air exposure. In contrast, the ionic conductivity of the pellets from alumina crucibles is about 1.81 × 10(-4) S cm(-1) and drops to 2.39 × 10(-5) S cm(-1) 3 months later. The large grains and the reduced grain boundaries in the pellets sintered in Pt crucibles are favorable to obtain high ionic conductivity and good air stability. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy results suggest that the formation of Li2CO3 on the pellet surface is probably another main reason, which is also closely related to the relative density and the amount of grain boundary within the pellets. This work stresses the importance of synthesis parameters, crucibles included, to obtain the LLZO electrolyte with high ionic conductivity and good air stability.

Enhanced photovoltaic effect in BiVO4 semiconductor by incorporation with an ultrathin BiFeO3 ferroelectric layer.

The photovoltaic effect of BiVO4 semiconductor was investigated by incorporating an ultrathin BiFeO3 ferroelectric layer. It is found that the ultrathin ferroelectric layer with strong self-polarization and high carrier density is desirable to enhance the photovoltaic effect and to manipulate the photovoltaic polarity of the semiconductors. The photovoltage increases by 5-fold to 1 V, and the photocurrent density increases by 2-fold to 140 μA/cm(2), in which the photovoltage is the highest compared with the reported values in polycrystalline and epitaxial ferroelectric thin film solar cells. The mechanism for the observed effect is discussed on the basis of a polarization-induced Schottky-like barrier at the BiFeO3/fluorine doped tin oxide interface. Our work provides good guidance for fabrication of cost-effective semiconductor photovoltaic devices with high performance, and this kind of ultrathin ferroelectric film may also have promising applications in copper indium gallium selenide solar cell, dye-sensitized TiO2 solar cell, lighting emitting diode, and other photoelectron related devices.

Electro-active shape memory composites enhanced by flexible carbon nanotube/graphene aerogels

In this manuscript we present a novel, shape memory aerogel/epoxy composite structure composed of a reduced carbon nanotube and graphene compound aerogel as a scaffold and epoxy resin as a matrix. The composite was prepared via a vacuum infusion method and to the best of our knowledge it represents the first instance of a shape memory effect directly driven by an electrical field observable in polymer-infused conductive carbon scaffolds. Furthermore, the composite material obtained displays a high conductivity (i.e., up to 5.2 S m−1). In the manuscript it is shown that the composites high conductivity can be attributed to the built-in 3D network of the thermally reduced graphene and carbon nanotube compound aerogel which displays high conductivity (16 S m−1) coupled with low density (6 mg mL−1). The composite material presented in this work is likely a suitable candidate for applications requiring polymer-infused conductive aerogels such as electromagnetic shielding, actuators and thermal sensors.

Three dimensional Graphene aerogels as binder-less, freestanding, elastic and high-performance electrodes for lithium-ion batteries

In this work it is shown how porous graphene aerogels fabricated by an eco-friendly and simple technological process, could be used as electrodes in lithium- ion batteries. The proposed graphene framework exhibited excellent performance including high reversible capacities, superior cycling stability and rate capability. A significantly lower temperature (75 °C) than the one currently utilized in battery manufacturing was utilized for self-assembly hence providing potential significant savings to the industrial production. After annealing at 600 °C, the formation of Sn-C-O bonds between the SnO2 nanoparticles and the reduced graphene sheets will initiate synergistic effect and improve the electrochemical performance. The XPS patterns revealed the formation of Sn-C-O bonds. Both SEM and TEM imaging of the electrode material showed that the three dimensional network of graphene aerogels and the SnO2 particles were distributed homogeneously on graphene sheets. Finally, the electrochemical properties of the samples as active anode materials for lithium-ion batteries were tested and examined by constant current charge–discharge cycling and the finding fully described in this manuscript.

3D composites of layered MoS2 and graphene nanoribbons for high performance lithium-ion battery anodes

3D composites of layered MoS2 and interconnected graphene nanoribbons (GNRs) are synthesized by a facile one-pot hydrothermal method. During the synthesis process, the GNRs self-assemble into conductive bridges between the MoS2 layers to form 3D structures that exhibit large specific capacity, good cycling stability and rate capability. Specifically, the composites exhibit a specific capacity of 1009.4 mA h g−1 at 200 mA g−1 after 80 cycles of the cycling test. They also exhibit a specific capacity of 606.8 mA h g−1 at 3 A g−1 in the rate capability test. In comparison, the bare MoS2 nanoparticles exhibit a specific capacity of 139.8 mA h g−1 at 200 mA g−1 after 80 cycles of the cycling test and 37.4 mA h g−1 at 3 A g−1 in the rate capability test. The structure, morphology and chemical analysis show that the superiority of the 3D structure is due to the large surface area and abundant mesopores that render a high contact area between the electrode and electrolyte. Moreover, the synergistic effects between the 2D MoS2 layers and the 1D GNRs within the 3D structures enable fast Li ion and electron transportation, inhibit the self-aggregation of MoS2 nanosheets, and accommodate the volume expansion to gain cycling stability and rate capacity.

A green method to prepare TiO2/MWCNT nanocomposites with high photocatalytic activity and insights into the effect of heat treatment on photocatalytic activity

Nanocomposites consisting of well-defined anatase TiO2 nanoparticles with an average diameter of 8 nm and multi-walled carbon nanotubes (MWCNTs) were fabricated by a facile two-step hydrothermal method using water as the main solvent, which is friendly to the environment and totally different from previous methods. The TiO2 nanoparticles were uniformly grafted on the surface of MWCNTs via intimate chemical bonds, which was beneficial for the enhancement of photocatalytic activity. The photocatalytic activities of as-prepared TiO2/MWCNT nanocomposites for degradation of rhodamine B under solar simulator illumination were investigated systemically. It was found that the photocatalytic activity of as-prepared TiO2/MWCNT nanocomposites was 7 times higher than that of pure TiO2 prepared via the same hydrothermal procedure. The enhancement was mainly due to the existence of MWCNTs, which could not only greatly improve the adsorption of rhodamine B, but also retard the recombination of photogenerated electron–hole pairs and absorb more light. The photocatalytic performance was further enhanced after being annealed at 400 °C and 500 °C for 1 h because of the improved crystallinity of anatase TiO2. Interestingly, the photocatalytic activities of samples annealed at 400 °C and 500 °C showed no difference, which was different from previous reports.

Photoelectrochemical response and electronic structure analysis of mono-dispersed cuboid-shaped Bi2Fe4O9 crystals with near-infrared absorption

An n-type mono-dispersed cuboid-shaped Bi2Fe4O9 semiconductor is synthesized via a hydrothermal method in concentrated NaOH solution. It is demonstrated that Bi2Fe4O9 phase is formed by the reaction of Bi25FeO40 crystal and amorphous Fe(OH)3. A doctor-blade technique is employed to determine the orientation of Bi2Fe4O9 cuboids. It is found that the cuboids are preferentially grown along [001] direction with side facets (110) and (10) parallel to it. The UV-visible-near infrared absorption spectrum shows that besides two broad absorption edges in visible spectrum region, remarkable near-infrared absorption is also observed, indicating Bi2Fe4O9 is a promising semiconductor capable of utilizing all solar band energy. Hence, steady and distinct photocurrents are measured to be 0.35 μA cm−2, 7 μA cm−2 and 33 μA cm−2 under near-infrared irradiation, visible-light and simulated sunlight, respectively. First principle calculation is used to reveal the electronic structure of Bi2Fe4O9 and the derived band gap is 1.23 eV, which agrees well with our experimental value of 1.29 eV. The calculation results also show that Bi2Fe4O9 is an indirect bandgap semiconductor which is contrary to previous results. Besides, the extra absorption peak at around 700 nm in the UV-visible-near infrared spectrum should be attributed to the intervalence charge transfer induced by unevenly distributed [FeO6]9− octahedrons and [FeO4]5− tetrahedrons rather than the previously reported d–d transition which is both spin and Laporte forbidden. Our work provides deep insights into the nature of the band structure of Bi2Fe4O9, and will facilitate the design of composite photoanodes that can response to near-infrared light.