M. J. Matthews

Optical Properties of MS 2 (M = Mo, W) Inorganic Fullerenelike and Nanotube Material Optical Absorption and Resonance Raman Measurements

The optical properties of inorganic fullerene-like and nanotube MS 2 (M = Mo, W) material are studied through absorption and resonance Raman, and compared to those of the corresponding bulk material. The absorption measurements show that the semiconductivity is preserved. Nevertheless, the positions of the excitons are altered in comparison to the bulk. The Raman spectra of the nanoparticles show a close correspondence to that of the bulk. However, the first-order peaks are broadened and, under resonance conditions, new peaks are observed. The new peaks are assigned to disorder-induced zone edge phonons.

Anode performance of a Li ion battery based on graphitized and B-doped milled mesophase pitch-based carbon fibers

The structures and anode performance of graphitized and boron-doped milled mesophase pitch-based carbon fibers (mMPCFs) have been comparatively studied and the results obtained by X-ray diffraction (XRD), SEM, Raman spectroscopy and electrochemical measurements are discussed. The boron doping at the level of 2.66 at.% (2.4 wt.%) enhances the growth of the crystallite thickness, Lc(002), of the host mMPCF. The B-doped mMPCFs show a strong Raman peak near 1365 cm−1, and a well-defined peak at 1620 cm−1. The E2g2 graphite Raman band at 1580 cm−1 is shifted to 1590 cm−1 due to B-doping. On the basis of the integrated intensity ratio R(ID/IG), it is suggested that the substitutional boron in the mMPCFs is homogeneously distributed within the graphene layer in the fiber form. Boron doping leads to about an 11% increase in charge capacity and also an improved cyclic efficiency. The electrochemical Li intercalation takes place at a higher voltage in boron-doped mMPCFs than in undoped mMPCFs by about 40 mV, presumably because the substitutional boron acts as an electron acceptor in the graphite lattice as well as affecting the exposed dislocation edge-type surface.

Structural characterization of milled mesophase pitch-based carbon fibers

The microstructure of milled mesophase pitch-based carbon fibers (mMPCFs) that have been developed as an anode material for Li ion batteries has been studied as a function of heat treatment temperature (HTT) mainly by Raman spectroscopy, and the results obtained are compared with those by X-ray diffraction (XRD) and scanning electron microscopy (SEM) observations. As a functional material, SEM measurements indicate that these mMPCFs have a characteristic controlled cross-sectional morphology like a bamboo skin structure which is different to that of conventional mesophase pitch-based carbon fibers for composite use. The mMPCFs with HTT>2000°C clearly show the 110 and 112 XRD peaks indicating the development of three-dimensional graphite stacking from a turbostratic structure. The mMPCFs heat-treated at temperatures >2000°C contain larger domain sizes along their longitudinal surface that those along the periphery of the fiber, as demonstrated by microprobe Raman scattering experiments. These mMPCFs could be promising as a functional material for applications, such as to Li ion batteries, but not for composite applications.

Raman spectra of polyparaphenylene‐based carbon prepared at low heat‐treatment temperatures

Raman spectroscopy was used to characterize the structural development of polyparaphenylene (PPP) based carbon at various heat treatment temperatures (THT), with an emphasis on the low THT regime. Experimental data show that PPP‐based carbons undergo a smooth transiiton from a mostly PPP polymer‐type structure for heat treatments below ∼750 °C to a graphitized structure at a heat treatment temperature of 2700 °C. These results give insight into the unusually high specific capacities observed in lithium‐PPP systems which are of particular interest for application to Li ion battery technology.

In situ Raman study of PPP-based disordered carbon as an anode in a Li ion battery

Abstract Electrochemical lithium insertion into polyparaphenylene (PPP)-based disordered carbon was investigated using in situ Raman spectroscopy. The peak position of the high frequency Raman band of 1605 cm −1 indicates characteristically at first no change up to 1.0 V and then the peak downshifts from 1605 cm −1 to 1588 cm −1 , and finally disappears presumably because of the decrease in optical skin depth giving rise to the conductivity change. For the charging process, the opposite change was observed with some degree of irreversibility in the peak position, reflecting the irreversible discharge/charge capacity of the electrode. The change in peak position is somewhat similar to that observed in well-ordered graphite, which reflects that charge transfer occurs between the Li and the defective carbon layers, even in such a low heat treatment temperature material.

Magnetic alignment of mesophase pitch‐based carbon fibers

Mesophase pitch‐based carbon fibers (MPCFs) have recently been developed for use as high performance anode materials in Li ion secondary batteries, having a microscopic as well as macroscopic structure especially suitable for Li storage. Because of the highly anisotropic diamagnetic moment observed between 50 and 310 K in pristine milled MPCF segments, they can easily be oriented parallel to an applied magnetic field, as observed by scanning electron microscopy. A simple model is proposed to explain both the observed alignment of undoped fibers and the suppression of alignment in B‐doped MPCFs for relatively small applied magnetic fields, because of their smaller diamagnetic moment.

STRUCTURAL CHARACTERIZATION OF CARBONS OBTAINED FROM POLYPARAPHENYLENES PREPARED BY THE KOVACIC AND YAMAMOTO METHODS

The structure of polyparaphenylene (PPP)-based carbons prepared by the Kovacic and Yamamoto methods heat-treated at 650–3000 °C have been characterized comparatively by using x-ray diffraction, SEM, TEM, and Raman spectroscopy. Both kinds of carbons indicate not typical but poor graphitizing behavior, especially for the case of PPP Yamamoto samples, and much less for PPP Kovacic samples, by heat treatment up to 3000 °C. The Kovacic-based samples heat-treated at 600–2400 °C have a more developed layer structure than that of Yamamoto-based samples. In contrast, for HTTs (heat-treatment temperature) more than about 2400 °C, PPP Yamamoto-based carbons exhibit a more developed crystallite structure than PPP Kovacic-based carbons. At a given HTT, PPP Kovacic-based carbons have a much more quinoid-like structure and graphene-type structure than PPP Yamamoto-based carbons, as indicated by the carbon yield and Raman scattering measurements. It is suggested that the detailed structure of the starting polymers influences the texture as well as the microstructure of resultant carbons even though both are obtained from the same kinds of precursors. These microstructures also largely influence the anode performance when these carbons are used in Li ion batteries.

Study of the overtones and combination bands in the Raman spectra of polyparaphenylene-based carbons

A detailed study of the second-order Raman spectrum of the polymer polyparaphenylene (PPP) prepared according to the Kovacic method and heat treated at temperatures T HT between 650 and 750 °C is presented. The Raman experiments have been performed with five different laser excitation energies in the visible range between 1.92 and 3.05 eV. Several Raman bands in the region between 2400 and 3400 cm −1 have been detected and assigned to the overtones and combination bands of the two conformations of the PPP polymer (benzenoid and quinoid) that co-exist in our samples. Due to the carbonization process, these bands broaden and decrease in intensity with increasing heat treatment temperature, as is also observed for the corresponding first-order Raman features. The complete absence of these high-frequency Raman bands for PPP with heat treatment temperatures in excess of 750 °C indicates complete transformation of the polymer into a disordered carbon material.

Li Storage behavior in Polyparaphenylene(PPP)-based Disordered Carbon as a Negative Electrode for Li Ion Batteries

Abstract PPP-based carbons prepared by the Kovacic and the Yamamoto methods give rise to slightly different polymer structures. The low temperature forms of PPP-Kovacic-based carbon heat treated around 700°C shows a Li storage capacity (1000mAh/g) which is 3 times larger than that of Yamamoto (280mAh/g) in the 2nd charging process. It is suggested that the texture as well as the microstructure of these carbons, as characterized by SEM, TEM, and Raman observations, affect the anode performance strongly.

Resonant Raman study of polyparaphenylene-based carbons

A resonant Raman study of polyparaphenylene (PPP) prepared by the Kovacic and the Yamamoto methods and heat-treated at temperatures T HT between 650 and 750°C has been performed using different laser excitation energies E laser between 1.92 and 3.05 eV. For samples treated at low T HT , the Raman spectra change with E laser , and this behavior is ascribed to the coexistence of two forms of the PPP polymer (benzenoid and quinoid) as well as a disordered carbon material. For higher T HT samples, only a dispersion of the position of the Raman band as a function of E laser is observed, and this is explained as due to the carbonization of the original polymer. The transition temperature between these two regions of resonance behavior is lower for the Yamamoto-PPP samples than for the Kovacic-PPP samples.