Xiaowei Teng

Effects of surfactants and synthetic conditions on the sizes and self-assembly of monodisperse iron oxide nanoparticles

Monodisperse iron oxide nanoparticles made from the thermal decomposition of iron carbonyl in octyl ether in the presence of oleic and stearic acids have been examined under various reaction conditions. Monodisperse particles with diameters of 3, 5, 10, 16 and 25 nm have been made. Ostwald ripening could be the key reason for making monodisperse nanoparticles with diameters of up to 25 nm, above the largest sizes that have been reported so far for this class of materials. When stearic acid was used as surfactant, the reaction mixtures can reflux at a lower temperature than the reaction using oleic acid, and monodisperse 3 nm Fe2O3 particles can be made. By controlling the temperatures during the drop casting, different superstructures and superlattices can be created. The nanoparticles and their assembly have been characterized by transmission electron microscopy, electron diffraction, powder X-ray diffraction, and X-ray photoemission spectroscopy.

Synthesis of ultrathin palladium and platinum nanowires and a study of their magnetic properties.

noexperimentalconfirmationhasyetbeenreportedforthemagnetismof1Dnanowiresof4dor5dmetals,mainlybecauseofthelimitednumberofsyntheticapproachestomonodisperseultrathinnanowires.Herein we report the synthesis of ultrathin Pd and Ptnanowires by a modified phase-transfer method in whichinorganic palladium nitride or platinum chloride is trans-ferredintoamixtureofoctadecylamine(ODA)andtolueneusinganewtypeofphase-transferagent,namelyn-dodecyl-trimethylammonium bromide (DTAB).

Electronic and Magnetic Properties of Ultrathin Au/Pt Nanowires

We have reported the synthesis of Au(25)Pt(75) and Au(48)Pt(52) alloyed ultrathin nanowires with average widths of less than 3 nm via a wet chemistry approach at room temperature. Using a combination of techniques, including scanning transmission electron microscopy equipped with X-ray energy dispersive spectroscopy, ultraviolet-visible spectroscopy, and X-ray absorption near-edge structure and extended X-ray absorption fine structure spectroscopies, we identified the stoichiometry-dependent heterogeneous crystalline structures, as well as electronic structures with respect to the charge transfer between Pt and Au within both nanowires. In particular, we observed d-charge depletion at the Au site and the d-charge gain at the Pt site in Au(48)Pt(52) nanowires, which accounted for its ferromagnetic magnetic behavior, in contrast to the paramagnetism and diamagnetism appearing respectively in bulk Pt and Au.

Platinum-Tin Oxide Core–Shell Catalysts for Efficient Electro-Oxidation of Ethanol

Platinum-tin (Pt/Sn) binary nanoparticles are active electrocatalysts for the ethanol oxidation reaction (EOR), but inactive for splitting the C-C bond of ethanol to CO2. Here we studied detailed structure properties of Pt/Sn catalysts for the EOR, especially CO2 generation in situ using a CO2 microelectrode. We found that composition and crystalline structure of the tin element played important roles in the CO2 generation: non-alloyed Pt46-(SnO2)54 core-shell particles demonstrated a strong capability for C-C bond breaking of ethanol than pure Pt and intermetallic Pt/Sn, showing 4.1 times higher CO2 peak partial pressure generated from EOR than commercial Pt/C.

Ternary PtSnRh–SnO2 nanoclusters: synthesis and electroactivity for ethanol oxidation fuel cell reaction

Carbon supported ternary Pt52Sn(36−x)Rh12–SnxO2x electrocatalysts with the average diameter of 2.8 ± 0.5 nm were synthesized using a Polyol process followed by thermal treatment. Several techniques including high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) were used to identify the coexistence of homogeneously distributed Pt/Sn/Rh random alloy and non-alloyed SnO2 throughout the catalyst. The Pt52Sn(36−x)Rh12–SnxO2x catalyst showed a superior long-term activity and stability towards ethanol oxidation than the commercial Pt catalyst. Our data of ternary Pt/Sn/Rh catalysts with different chemical compositions and crystalline structures also indicated that the superior performance of Pt52Sn(36−x)Rh12–SnxO2x might result from the electronic effect of the Pt/Sn/Rh random alloy.

Bivalence Mn5O8 with hydroxylated interphase for high-voltage aqueous sodium-ion storage

Aqueous electrochemical energy storage devices have attracted significant attention owing to their high safety, low cost and environmental friendliness. However, their applications have been limited by a narrow potential window (∼1.23 V), beyond which the hydrogen and oxygen evolution reactions occur. Here we report the formation of layered Mn5O8 pseudocapacitor electrode material with a well-ordered hydroxylated interphase. A symmetric full cell using such electrodes demonstrates a stable potential window of 3.0 V in an aqueous electrolyte, as well as high energy and power performance, nearly 100% coulombic efficiency and 85% energy efficiency after 25,000 charge–discharge cycles. The interplay between hydroxylated interphase on the surface and the unique bivalence structure of Mn5O8 suppresses the gas evolution reactions, offers a two-electron charge transfer via Mn2+/Mn4+ redox couple, and provides facile pathway for Na-ion transport via intra-/inter-layer defects of Mn5O8.

Energy transfer between colloidal semiconductor nanocrystals in an optical microcavity

The authors have studied nonradiative energy transfer between semiconductor nanocrystals (NCs) placed in a Fabry-Perot microcavity. The spectrally integrated fluorescence from a monolayer of single sized NCs in the cavity is enhanced by a factor of 4.8 compared to free space. For a monolayer of mixed sized NCs, the acceptor NC fluorescence intensity is enhanced by an additional factor of 2.7 due to energy transfer processes. When the cavity mode is resonant with the acceptor NC fluorescence emission maximum, donor NC emission is completely suppressed, providing a narrow spectral output.

Structural water engaged disordered vanadium oxide nanosheets for high capacity aqueous potassium-ion storage

Aqueous electrochemical energy storage devices using potassium-ions as charge carriers are attractive due to their superior safety, lower cost and excellent transport properties compared to other alkali ions. However, the accommodation of potassium-ions with satisfactory capacity and cyclability is difficult because the large ionic radius of potassium-ions causes structural distortion and instabilities even in layered electrodes. Here we report that water induces structural rearrangements of the vanadium-oxygen octahedra and enhances stability of the highly disordered potassium-intercalated vanadium oxide nanosheets. The vanadium oxide nanosheets engaged by structural water achieves high capacity (183 mAh g−1 in half-cells at a scan rate of 5 mV s−1, corresponding to 0.89 charge per vanadium) and excellent cyclability (62.5 mAh g−1 in full cells after 5,000 cycles at 10 C). The promotional effects of structural water on the disordered vanadium oxide nanosheets will contribute to the exploration of disordered structures from earth-abundant elements for electrochemical energy storage.

Pseudocapacitive Hausmannite Nanoparticles with (101) Facets: Synthesis, Characterization, and Charge‐Transfer Mechanism

Hausmannite Mn3 O4 octahedral nanoparticles of 18.3 ± 7.0 nm with (101) facets have been prepared by an oxygen-mediated growth. The electrochemical properties of the Mn3 O4 particles as pseudocapacitive cathode materials were characterized both in half-cells and in button-cells. The Mn3 O4 nanoparticles exhibited a high mass-specific capacitance of 261 F g(-1), which was calculated from cyclic voltammetry analyses, and a capacitive retention of 78% after 10,000 galvanostatic charge-discharge cycles. The charge-transfer mechanisms of the Mn3 O4 nanoparticles were further studied by using synchrotron-based in situ X-ray absorption near edge spectroscopy and XRD. Both measurements showed concurrently that throughout the potential window of 0-1.2 V (vs. Ag/AgCl), a stable spinel structure of Mn3 O4 remained, and a reversible electrochemical conversion between tetrahedral [Mn(II) O4 ] and octahedral [Mn(III) O6 ] units accounted for the redox activity. Density functional theory calculations further corroborated this mechanism by confirming the enhanced redox stability afforded by the abundant and exposed (101) facets of Mn3 O4 octahedra.

Storage of Potassium Ions in Layered Vanadium Pentoxide Nanofiber Electrodes for Aqueous Pseudocapacitors

Spaced out: This paper investigates potassium-ion storage in vanadium pentoxide nanofibers (VNFs, K0.33 V2 O5 ) with a layered architecture. In situ XRD experiments reveal that the interplanar space of VNF expands/contracts upon extraction/insertion of potassium ions during the redox process.