Junwei Zheng

Tailoring of Molecular Planarity to Reduce Charge Injection Barrier for High‐Performance Small‐Molecule‐Based Ternary Memory Device with Low Threshold Voltage

By introducing a coplanar fluorenone into the center of an azo molecule, the turn-on voltages of the ternary memory devices are significantly decreased to lower than -2 V due to the improved crystallinity and the reduced charge injection barrier. The resulting low-power consumption devices will have great potential applications in high-performance chips for future portable nanoelectronic devices.

Ultrathin Platinum Nanowire Catalysts for Direct CN Coupling of Carbonyls with Aromatic Nitro Compounds under 1 Bar of Hydrogen

Traditionally important in the pharmaceutical, agrochemical, and synthetic dye industries, C-N coupling has proved useful for the preparation of a number of valuable organic compounds. Here, a new method for the direct one-pot reductive C-N coupling from carbonyl and aromatic nitro compounds is described. Employing ultrathin platinum nanowires as the catalyst and hydrogen as the reducing agent, N-alkylamines were achieved in high yields. Debenzylation products were not detected after prolonged reaction times. Time-dependent analysis, ReactIR spectroscopy and DFT calculations revealed that the C-N coupling proceeded through a different mechanism than traditional reductive amination. N-Alkylamines were directly obtained by intermolecular dehydration over platinum nanowires under a hydrogen atmosphere, instead of intramolecular water elimination and imine hydrogenation.

Molecular length adjustment for organic azo-based nonvolatile ternary memory devices

Two conjugated small molecules with different molecular length, DPAPIT and DPAPPD, in which an electron donor dimethylamino moiety and an electron acceptor phthalimide core unit are bridged by another electron-accepting azobenzene block, were designed and synthesized. DPAPIT molecule with longer conjugation length stacked regularly in the solid state and formed uniform nanocrystalline film. The fabricated memory devices with DPAPIT as active material exhibited outstanding nonvolatile ternary memory effect with the current ratio of ∼1u2006:u2006101.7u2006:u2006104 for “0”, “1” and “2” states and all the switching threshold voltages lower than −3 V. In contrast, the shorter molecule DPAPPD showed amorphous microstructure and no obvious conductive switching behavior was observed in the device. The crystallinity and surface roughness of DPAPIT thin films were significantly improved as the annealing temperature increased, lowering the switching threshold voltages which are highly desirable for low-power consumption data-storage devices. It is worth noting that the tristable memory signals of DPAPIT film could also be achieved by using conductive atomic force microscopy with platinum-coated probe, which enables fabrication of nano-scale or even molecular-scale device, a significant progress for the ultra-high density data storage application. Mechanism analysis demonstrated that two charge traps with different depth in the molecular backbone were injected by charge carriers progressively as the external bias increased, resulting in the formation of three distinct conductive states (OFF, ON1 and ON2 states).

Preparation of Pt@Fe2O3 Nanowires and their Catalysis of Selective Oxidation of Olefins and Alcohols

Iron oxide coated platinum nanowires (Pt@Fe(2)O(3)NWs) with a diameter of 2.8 nm have been prepared by the oxygen oxidation of FePt NWs in oleylamine. These cable-like NWs were characterised by transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and X-ray absorption fine structure analysis. These Pt@Fe(2)O(3) NWs were used as non-support heterogeneous catalysts in oxidation of olefins and alcohols. The results revealed that it is an active and highly selective catalyst. Styrene derivatives were tested with molecular oxygen as the sole oxidant, with benzaldehyde successfully obtained from styrene in an absolute yield of 31%, whereas the use of tert-butyl hydroperoxide as the sole oxidant in the oxidation of alcohols led to yields of more than 80% of the corresponding ketone or aldehyde. This unsupported catalyst was found to be more active (TOF=96.5 h(-1)) than other reported Fe(2)O(3) nanoparticle catalysts and could be recycled multiple times without any notable decrease in activity. Our findings will extend the use of such nanomaterial catalysts to new catalytic systems.

Surfactant-assisted synthesis of Fe2O3 nanoparticles and F-doped carbon modification toward an improved Fe3O4@CFx/LiNi0.5Mn1.5O4 battery.

A simple surfactant-assisted reflux method was used in this study for the synthesis of cocklebur-shaped Fe2O3 nanoparticles (NPs). With this strategy, a series of nanostructured Fe2O3 NPs with a size distribution ranging from 20 to 120 nm and a tunable surface area were readily controlled by varying reflux temperature and the type of surfactant. Surfactants such as cetyltrimethylammonium bromide (CTAB), polyvinylpyrrolidone (PVP), poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (F127) and sodium dodecyl benzenesulfonate (SDBS) were used to achieve large-scale synthesis of uniform Fe2O3 NPs with a relatively low cost. A new composite of Fe3O4@CFx was prepared by coating the primary Fe2O3 NPs with a layer of F-doped carbon (CFx) with a one-step carbonization process. The Fe3O4@CFx composite was utilized as the anode in a lithium ion battery and exhibited a high reversible capacity of 900 mAh g(-1) at a current density of 100 mA g(-1) over 100 cycles with 95% capacity retention. In addition, a new Fe3O4@CFx/LiNi(0.5)Mn(1.5)O4 battery with a high energy density of 371 Wh kg(-1) (vs cathode) was successfully assembled, and more than 300 cycles were easily completed with 66.8% capacity retention at 100 mA g(-1). Even cycled at the high temperature of 45 °C, this full cell also exhibited a relatively high capacity of 91.6 mAh g(-1) (vs cathode) at 100 mA g(-1) and retained 54.6% of its reversible capacity over 50 cycles. Introducing CFx chemicals to modify metal oxide anodes and/or any other cathode is of great interest for advanced energy storage and conversion devices.

Preparation of porous and hollow Fe3O4@C spheres as an efficient anode material for a high-performance Li-ion battery

Carbon coated porous nanospheres (p-Fe3O4@C) were successfully synthesized by the pyrolysis treatment at 500 °C of glucose coated porous Fe3O4 spheres and used as an anode material for lithium-ion battery applications. Compared with pure porous Fe3O4 (p-Fe3O4), hollow Fe3O4 (h-Fe3O4) and carbon coated hollow Fe3O4 (h-Fe3O4@C), the as-prepared composite exhibited much better electrochemical performance with a high reversible capacity of 875 mA h g−1 after 50 cycles at a current rate of 0.1 C. Even at a current rate of 1 C (1 C = 928 mA h g−1) and after 300 charge and discharge cycles, the reversible capacity was still as high as about 600 mA h g−1. The improved lithium storage performance was attributed to the presence of an inner pore structure and the carbon layer of the p-Fe3O4@C granting extremely high mechanical support to the whole framework, which could not only prevent the large volume change of Fe3O4, but also improve the electrochemical activity and the stability of the material. The superior electrochemical properties of p-Fe3O4@C nanospheres means they might have promising applications in high performance Li-ion batteries.

A sustainable iron-based sodium ion battery of porous carbon–Fe3O4/Na2FeP2O7 with high performance

A type of porous carbon–Fe3O4 (e.g., PC–Fe3O4) composite with an industrially scalable production was introduced in the sodium ion battery application for the first time. The PC–Fe3O4 composite, consisting of highly dispersed Fe3O4 nanocrystals within the porous carbon with a relatively low weight percent of 45.5 wt%, could efficiently demonstrate high capacities of 225, 168, 127, 103, 98 and 90 mA h g−1 under the current densities of 50, 100, 200, 300, 400 and 500 mA g−1 with a good stability over 400 cycles. The utilization co-efficient of Fe3O4 nanocrystals was proven to be much higher than most of the Fe3O4 nanoparticles reported recently via the study of the capacity contribution of carbon originally. In addition, the robustness of electrode during the charge–discharge was well characterized by ex situ XRD and emission scanning electron microscopy (SEM). More importantly, a new concept of an elemental iron-based sodium ion battery of PC–Fe3O4/Na2FeP2O7 is presented. This is the first example to introduce an element-rich configuration in the sodium ion battery from the viewpoint of sustainability. The full battery demonstrated a superior capacity of 93 mA h g−1, high capacity retention of 93.3% over 100 cycles and work voltage around 2.28 V with the energy density of 203 W h kg−1. Such configuration of an iron-based sodium battery would be highly promising and sustainable owing to its low cost and high stability in grid storage.

Facile synthesis of Pt/Pd nanodendrites for the direct oxidation of methanol

The demand for clean and energy-efficient fuel cell systems requires electrocatalysts with greater activity and stability. Here, we report a facile wet-chemical approach for the synthesis of high-quality three-dimensional (3D) Pt/Pd bimetallic nanodendrites. The simple and unique process developed here used oleylamine as a reducing agent, and hydrogen gas to control the morphology. The as-prepared Pt/Pd nanodendrites were characterized by transmission electron microscopy, x-ray diffraction and energy dispersive x-ray spectroscopy, cyclic voltammetry, linear sweep voltammetry and chronoamperometry. The nanodendrites showed superior electrocatalytic activity (609.565 mA mg(-1) Pt) for the oxidation of methanol compared with Pt nanoparticle electrocatalysts. This method could provide a general approach for the morphology-controlled synthesis of bimetallic Pt-based nanocatalysts, which are promising materials for applications in fuel cells.

One step synthesis of C&N co-doped mesoporous TiO2 with enhanced performance in a lithium-ion battery

C&N co-doped mesoporous TiO2 with excellent performance in lithium-ion batteries was prepared via a one-step synthesis method.

Preparation of a γ‐Fe2O3/Ag Nanowire Coaxial Nanocable for High‐Performance Lithium‐Ion Batteries

In this study, we report the design and synthesis of a silver nanowire-γ-Fe2 O3 coaxial nanocable architecture (Ag NWs@γ-Fe2 O3 nanocable) through mild oxidation of [Fe(CO)5 ] on the surface of silver nanowires followed by a calcination process. After optimization of the structural design, the Ag NWs@γ-Fe2 O3 nanocable could deliver superior lithium storage performance in terms of high reversible capacity, good rate performance, and excellent stability, such as a high reversible capacity of about 890u2005mAu2009hu2009g(-1) after 60 cycles at a current rate of 0.1u2005C (1.0u2005C=1005u2005mAu2009g(-1) ). The reversible capacity remains as high as about 550u2005mAu2009hu2009g(-1) even at a high current rate of 2.0u2005C. This dramatic performance is mainly attributed to the smart coaxial design, which can not only alleviate the large volume change and prevent the aggregation of γ-Fe2 O3 nanoparticles, but also enables good conductivity and thus enhances fast charge transfer. The unique structural features of the Ag NWs@γ-Fe2 O3 nanocable represent a promising anode material in lithium-ion battery applications.