Chunan Ma

Electric field controlled formation and dissociation of multiwalled carbon nanotube conductive pathways in a polymer melt

Electric field controlled formation and dissociation of multiwalled carbon nanotube (MWCNT) conductive pathways in a polycarbonate (PC) melt are investigated by the dynamic percolation measurement. The results show that field-induced MWCNT alignment causes the decrease in the activation energy of conductive pathway formation. The directional or disordered alignment of MWCNTs in the PC melt results in the transition from a conductor to an insulator as the electric field changes from 500 to 1 V/cm. This electric-controllable directional or disordered alignment technology is promising for the fabrication of low-dimensional conductive materials and applications of voltage-switch devices.

Methanol electro-oxidation on platinum modified tungsten carbides in direct methanol fuel cells: a DFT study

In exploration of low-cost electrocatalysts for direct methanol fuel cells (DMFCs), Pt modified tungsten carbide (WC) materials are found to be great potential candidates for decreasing Pt usage whilst exhibiting satisfactory reactivity. In this work, the mechanisms, onset potentials and activity for electrooxidation of methanol were studied on a series of Pt-modified WC catalysts where the bare W-terminated WC(0001) substrate was employed. In the surface energy calculations of a series of Pt-modified WC models, we found that the feasible structures are mono- and bi-layer Pt-modified WCs. The tri-layer Pt-modified WC model is not thermodynamically stable where the top layer Pt atoms tend to accumulate and form particles or clusters rather than being dispersed as a layer. We further calculated the mechanisms of methanol oxidation on the feasible models via methanol dehydrogenation to CO involving C-H and O-H bonds dissociating subsequently, and further CO oxidation with the C-O bond association. The onset potentials for the oxidation reactions over the Pt-modified WC catalysts were determined thermodynamically by water dissociation to surface OH* species. The activities of these Pt-modified WC catalysts were estimated from the calculated kinetic data. It has been found that the bi-layer Pt-modified WC catalysts may provide a good reactivity and an onset oxidation potential comparable to pure Pt and serve as promising electrocatalysts for DMFCs with a significant decrease in Pt usage.

Tungsten carbide/porous carbon core–shell nanocomposites as a catalyst support for methanol oxidation

Carbon-encapsulated tungsten carbide (WC@C) was prepared by a microwave-assisted synthesis method with resorcinol-formaldehyde resin (RF) as carbon source. WC was encapsulated by porous carbon layer to form core–shell structure which could protect tungsten oxide from occupying the active sites on the surfaces of the WC@C. The characteristics of WC@C composites were determined by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy and Brunauer–Emmet–Teller gas adsorption. Platinum nanoparticles were uniformly distributed on WC@C to synthesize a new electrocatalyst Pt–WC@C. The electro-catalytic performances of prepared Pt–WC@C, commercial Pt/C and PtRu/C toward methanol oxidation were compared by cyclic voltammetry, chronoamperometry and CO stripping test. It was found that Pt–WC@C exhibited higher catalytic activity for methanol oxidation than that of commercial Pt/C and PtRu/C catalysts. Especially, the Pt–WC@C achieved the long-term stability which was attributed to the effective protection by the carbon porous shell structure.

Tungsten carbide-reduced graphene oxide intercalation compound as co-catalyst for methanol oxidation

Abstract Highly dispersed tungsten carbide (WC) nanoparticles (NPs) sandwiched between few-layer reduced graphene oxide (RGO) have been successfully synthesized by using thiourea as an anchoring and inducing reagent. The metatungstate ion, [H 2 W 12 O 40 ] 6− , is assembled on thiourea-modified graphene oxide (GO) by an impregnation method. The WC NPs, with a mean diameter of 1.5 nm, are obtained through a process whereby ammonium metatungstate first turns to WS 2 , which then forms an intercalation compound with RGO before growing, in situ, to WC NPs. The Pt/WC-RGO electrocatalysts are fabricated by a microwave-assisted method. The intimate contacts between Pt, WC, and RGO are confirmed by X-ray diffraction, scanning electron microscope, transmission electron microscope, and Raman spectroscopy. For methanol oxidation, the Pt/WC-RGO electrocatalyst exhibited an electrochemical surface area value of 246.1 m 2 /g Pt and a peak current density of 1364.7 mA/mg Pt, which are, respectively, 3.66 and 4.77 times greater than those of commercial Pt/C electrocatalyst (67.2 m 2 /g Pt, 286.0 mA/mg Pt). The excellent CO-poisoning resistance and long-term stability of the electrocatalyst are also evidenced by CO stripping, chronoamperometry, and accelerated durability testing. Because Pt/WC-RGO has higher catalytic activity compared with that of commercial Pt/C, as a result of its intercalated structure and synergistic effect, less Pt will be required for the same performance, which in turn will reduce the cost of the fuel cell. The present method is facile, efficient, and scalable for mass production of the nanomaterials.

Time-resolved spectroscopy study of the triplet state of 4-diethylaminobenzonitrile (DEABN)

The nonradiative deactivation processes of intramolecular charge transfer (ICT) and intersystem crossing (ISC) and their dependence on solvent polarity for DEABN have been studied using transient absorption (TA) spectroscopy. The TA spectra have been taken in non-polar hexane and polar acetonitrile solvents within the 400–630 nm spectral region at time delays of 2 ps to 6 ns. The results are complemented by nanosecond time-resolved resonance Raman (TR3) spectra of the triplet state of DEABN over the 700 to 2300 cm−1 frequency range. There is no significant difference between the Raman spectra recorded in hexane, acetonitrile and methanol solvents and this implies the triplet state has only one form. Comparison of DEABN triplet Raman spectrum with ab initio DFT geometry and vibrational analysis of DEABN ground state and triplet state TR3 spectra of the closely related compound dimethylaminobenzonitrile (DMABN) indicates the DEABN triplet state is planar or near planar in structure with noticeable biradical character.

Enhanced Electrocatalytic Oxygen Reduction on NiWOx Solid Solution with Induced Oxygen Defects

The continuous solid solution NiWOx is successfully prepared by using precursor W18O49 with plenty of oxygen defects. The NiWOx nanoparticles are characterized by X-ray diffraction, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and X-ray absorption spectroscopy. The crystallographic phase of NiWOx is stable and characterized by the same feature of the parent lattice W18O49 even with various concentrations of dopant Ni which indicates the existence of oxygen defects. The NiWOx nanoparticles could be processed as the appropriate promoter after loading 10 wt % Pt. The Pt/NiWOx displays remarkable response for oxygen reduction reaction in alkaline medium compared with the commercial Pt/C. The analysis of the electrochemistry data shows that the existence of abundant oxygen defects in the solid solution NiWOx is the key factor for the improved ORR catalyst performance. Ni is effective in the catalysts because of its compatibility with W in the solid solution and its active participation in oxygen reduction reaction.

Preparation of the WO 3 /TiO 2 using microwave-heating in ionic liquid and its application in electrocatalysis

A high dispersed WO₃/TiO₂ particles were prepared by using a microwave heating assisted ionic liquid method. The phase structure and morphology of WO₃/TiO₂ particles were characterized by X-ray diffraction and scanning electron microscopy. The result shows that fluffy WO₃ balls were assembled on TiO₂ and the surface structure of WO₃/TiO₂ particle was comprised of numbers of shuttle-like nanoneedles. The different WO₃/TiO₂ particles were obtained by simply changing the concentration of the ionic liquid. Then PtWO₃/TiO₂ catalysts were prepared and its electro-catalytic performances were evaluated by cyclic voltammetry and the CO stripping test. The results revealed that PtWO₃/TiO₂-1.5 catalyst had the better performance for methanol oxidation and CO tolerance which proved that the ionic liquid in this reaction system provided the multiple functions for preparing a promising catalyst for methanol oxidation.

PIRATE - a picosecond time resolved IR spectrometer

PIRATE (picosecond infrared absorption and transient excitation) is a recently developed solid state laser facility offering tunability into the mid-infrared region (down to 1000 cm/sup -1/) for time-resolved infrared (TRIR) spectroscopy. It is used to study the energetic properties, structure and reactivity of short lived (picosecond & femtosecond) intermediates formed during the course of photophysical and photochemical processes. The presentation will briefly describe the PIRATE system and give recent highlights from investigations on solvation and hydrogen bonding within excited molecular states to illustrate the performance of the ps-TRIR spectrometer. This study has now progressed to look at the dynamics of the solvation of excited states and in particular the formation of hydrogen bonds to electronically excited charge transfer states such as for the intramolecular charge transfer reaction of DMABN by monitoring rapid frequency shifts of the cyano stretch of this molecule by TRIR. this work shows the first direct observation of hydrogen bond formation in a molecular excited state.