Linfeng Fei

Direct TEM observations of growth mechanisms of two-dimensional MoS2 flakes.

A microscopic understanding of the growth mechanism of two-dimensional materials is of particular importance for controllable synthesis of functional nanostructures. Because of the lack of direct and insightful observations, how to control the orientation and the size of two-dimensional material grains is still under debate. Here we discern distinct formation stages for MoS2 flakes from the thermolysis of ammonium thiomolybdates using in situ transmission electron microscopy. In the initial stage (400 °C), vertically aligned MoS2 structures grow in a layer-by-layer mode. With the increasing temperature of up to 780 °C, the orientation of MoS2 structures becomes horizontal. When the growth temperature reaches 850 °C, the crystalline size of MoS2 increases by merging adjacent flakes. Our study shows direct observations of MoS2 growth as the temperature evolves, and sheds light on the controllable orientation and grain size of two-dimensional materials.

Synthesis of bismuth ferrite nanoparticles via a wet chemical route at low temperature

Nanoparticles (NPs) of multiferroic bismuth ferrite (BiFeO3) with narrow size distributions were synthesized via a wet chemical route using bismuth nitrate and iron nitrate as starting materials and excess tartaric acid and citric acid as chelating agent, respectively, followed by thermal treatment. It was found that BiFeO3 NPs crystallized at ∼350°C when using citric acid as chelating agent. Such crystallization temperature is much lower than that of conventional chemical process in which other types of chelating agent are used. BiFeO3 NPs with different sizes distributions show obvious ferromagnetic properties, and the magnetization is increased with reducing the particle size.

Highly entangled carbon nanoflakes on Li3V2(PO4)3 microrods for improved lithium storage performance

An Li3V2(PO4)3/C micro-nano composite is prepared via a facile carbothermal reaction which features porous carbon nanoflakes three-dimensionally entangled on single-crystalline Li3V2(PO4)3 microrods. Such a composite shows improved electrochemical performances, benefiting from its novel hierarchical mesoporous–macroporous structure together with exceptional high tap density.

A rectification-free piezo-supercapacitor with a polyvinylidene fluoride separator and functionalized carbon cloth electrodes

The integration of energy harvesting and energy storage in this device not only enables the conversion of ambient energy into electricity, but also provides a sustainable power source for various electronic devices and systems. It is highly desirable to improve the integration level and minimize unnecessary energy loss in the power-management circuits between energy harvesting and storage devices. In our work, we integrate a PVDF film into a supercapacitor as both the separator and the energy harvester. The double-sides of the polarized PVDF films are coated with H2SO4/poly(vinyl alcohol) (PVA) gel electrolyte. Functionalized carbon cloths are assembled with H2SO4/PVA electrolyte as both anode and cathode, forming a flexible all-solid-state supercapacitor. Externally mechanical impacts establish a piezoelectric potential across the PVDF films, and drive ions in the electrolyte to migrate towards the interface of the supercapacitor electrode, storing the electricity in the form of electrochemical energy. The asymmetric output characteristic of the piezoelectric PVDF film under mechanical impact results in the effective charging of the supercapacitor without any rectification device. The integrated piezo-supercapacitor exhibits a specific capacitance of 357.6 F m−2, a power density of 49.67 mW h m−2, and an energy density of 400 mW m−2. Our hybridized energy harvesting and storage device can be further extended for providing sustainable power source of various types of sensors.

Tailoring Anisotropic Li-Ion Transport Tunnels on Orthogonally Arranged Li-Rich Layered Oxide Nanoplates Toward High-Performance Li-Ion Batteries

High-performance Li-rich layered oxide (LRLO) cathode material is appealing for next-generation Li-ion batteries owing to its high specific capacity (>300 mAh g-1). Despite intense studies in the past decade, the low initial Coulombic efficiency and unsatisfactory cycling stability of LRLO still remain as great challenges for its practical applications. Here, we report a rational design of the orthogonally arranged {010}-oriented LRLO nanoplates with built-in anisotropic Li+ ion transport tunnels. Such a novel structure enables fast Li+ ion intercalation and deintercalation kinetics and enhances structural stability of LRLO. Theoretical calculations and experimental characterizations demonstrate the successful synthesis of target cathode material that delivers an initial discharge capacity as high as 303 mAh g-1 with an initial Coulombic efficiency of 93%. After 200 cycles at 1.0 C rate, an excellent capacity retention of 92% can be attained. Our method reported here opens a door to the development of high-performance Ni-Co-Mn-based cathode materials for high-energy density Li-ion batteries.

Highly Responsive Room-Temperature Hydrogen Sensing of α-MoO3 Nanoribbon Membranes

[001]-Oriented α-MoO3 nanoribbons were synthesized via hydrothermal method at temperature from 120 to 200 °C and following assembled a membrane on interdigital electrodes to form sensors. The sensitivity, response speed, and recovery speed of the sensor improve with the increasing hydrothermal temperature. Among them, the sample obtained at 200 °C exhibits a room-temperature response time of 14.1 s toward 1000 ppm of H2. The nanoribbons also show good selectivity against CO, ethanol, and acetone, as well as high sensitivity to H2 with a concentration as low as 500 ppb. The hydrogen sensing behavior is dependent on the redox reaction between the H2 and chemisorbed oxygen species. Higher hydrothermal temperature creates larger specific surface area and higher Mo(5+) content, leading to increased chemisorbed oxygen species on the nanoribbon surface.

Electrospun Bismuth Ferrite Nanofibers for Potential Applications in Ferroelectric Photovoltaic Devices

Bismuth ferrite (BFO) nanofibers were synthesized via a sol-gel-based electrospinning process followed by thermal treatment. The influences of processing conditions on the final structure of the samples were investigated. Nanofibers prepared under optimized conditions were found to have a perovskite structure with good quality of crystallization and free of impurity phase. Ferroelectric and piezoelectric responses were obtained from individual nanofiber measured on a piezoelectric force microscope. A prototype photovoltaic device using laterally aligned BFO nanofibers and interdigital electrodes was developed and its performance was examined on a standard photovoltaic system. The BFO nanofibers were found to exhibit an excellent ferroelectric photovoltaic property with the photocurrent several times larger than the literature data obtained on BFO thin films.

Controllable in situ synthesis of epsilon manganese dioxide hollow structure/RGO nanocomposites for high-performance supercapacitors

Well-organized epsilon-MnO2 hollow spheres/reduced graphene oxide (MnO2HS/RGO) composites have been successfully constructed via a facile and one-pot synthetic route. The ε-MnO2 hollow spheres with the diameter of ∼500 nm were grown in situ with homogeneous distribution on both sides of graphene oxide (GO) sheets in aqueous suspensions. The formation mechanism of the MnO2HS/RGO composites has been systematically investigated, and a high specific capacitance and good cycling capability were achieved on using the composites as supercapacitors. The galvanostatic charge/discharge curves show a specific capacitance of 471.5 F g(-1) at 0.8 A g(-1). The hollow structures of ε-MnO2 and the crumpled RGO sheets can enhance the electroactive surface area and improve the electrical conductivity, thus further facilitating the charge transport. The MnO2HS/RGO composite exhibits a high capacitance of 272 F g(-1) at 3 A g(-1) (92% retention) even after 1000 cycles. The prominent electrochemical performance might be attributed to the combination of the pseudo-capacitance of the MnO2 nanospheres with a hollow structure and to the good electrical conductivity of the RGO sheets. This work explores a new concept in designing metal oxides/RGO composites as electrode materials.

Commercial Dacron cloth supported Cu(OH) 2 nanobelt arrays for wearable supercapacitors

Wearable supercapacitors have attracted considerable research interest in recent years. However, most of the wearable supercapacitors reported are either in the form of fibers or based on carbon cloth which have to be knitted into commercial cloth for wearable applications. Here we report the growth of Cu(OH)2 nanobelt arrays directly on commercial Dacron cloth which serves as a positive electrode for supercapacitors. The as-prepared electrode has a high specific capacitance of 217 mF·cm−2 at a current density of 0.5 mA·cm−2 with a capacitance retention of 90% at a current density of 2 mA·cm−2 after 3000 charge/discharge cycles. A flexible all-solid-state asymmetrical supercapacitor is fabricated by sandwiching the Dacron cloth supported Cu(OH)2 nanobelt arrays (positive electrode) between two carbon nanofiber matrices (negative electrodes), using KOH-PVA gel as the electrolyte and as the separator. A high areal capacitance of 195.8 mF·cm−2 at a current density of 1 mA·cm−2 can be achieved. The textile supercapacitor exhibits an energy density of 3.6 × 10−2 mWh·cm−2 at a power density of 0.6 mW·cm−2 with a voltage window of 1.2 V. This sandwich type of supercapacitor based on commercial Dacron cloth opens a novel way of integrating supercapacitors into textiles, showing great promise for wearable electronic applications.

Graphene nanocluster decorated niobium oxide nanofibers for visible light photocatalytic applications

In this paper, we report a novel nanocomposite of graphene nanocluster decorated Nb2O5 nanofibers. With the structural modification, the photocatalytically active region of Nb2O5 has been significantly extended from the UV region to the UV-Vis region (from ∼380 nm to ∼800 nm in the adsorption edge); and hence, the visible light photocatalytic performance has been greatly improved. Detailed structural analysis revealed that the enhanced photocatalytic activity of the graphene nanocluster decorated Nb2O5 nanofibers could be mainly attributed to the modification of the bandgaps by the clusters and the unique orientation of graphene layers (nearly perpendicular to the Nb2O5/C interface, which is quite different from classical “core–shell” composites). We believe that the findings suggest a potential candidate for visible-light photocatalytic applications and would inspire further studies.