The Effect of Thermal Reduction on the Photoluminescence and Electronic Structures of Graphene Oxides

[[abstract]]Electronic structures of graphene oxide (GO) and hydro-thermally reduced graphene oxides (rGOs)processed at low temperatures (120–1806C) were studied using X-ray absorption near-edge structure XANES), X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS). C K-edge XANES spectra of rGOs reveal that thermal reduction restores C 5 C sp2 bonds and removes some of the oxygen and hydroxyl groups of GO, which initiates the evolution of carbonaceous species. The combination of C K-edge XANES and Ka XES spectra shows that the overlapping p and p* orbitals in rGOs and GO are similar to that of highly ordered pyrolytic graphite (HOPG), which has no band-gap. C Ka RIXS spectra provide evidence that thermal reduction changes the density of states (DOSs) that is generated in the p-region and/or in the gap between the p and p* levels of the GO and rGOs. Two-dimensional C Ka RIXS mapping of the heavy reduction of rGOs further confirms that the residual oxygen and/or oxygen-containing functional groups modify the p and s features, which are dispersed by the photon excitation energy. The dispersion behavior near the K point is approximately linear and differs from the parabolic-like dispersion observed in HOPG.[[notice]]補正完畢[[journaltype]]國外[[incitationindex]]SCI[[ispeerreviewed]]Y[[booktype]]電子版[[countrycodes]]GB

Computational studies for reduced graphene oxide in hydrogen-rich environment

We employ molecular dynamic simulations to study the reduction process of graphene-oxide (GO) in a chemically active environment enriched with hydrogen. We examine the concentration and pressure of hydrogen gas as a function of temperature in which abstraction of oxygen is possible with minimum damage to C-sp$^2$ bonds hence preserving the integrity of the graphene sheet. Through these studies we find chemical pathways that demonstrate beneficiary mechanisms for the quality of graphene including formation of water as well as suppression of carbonyl pair holes in favor of hydroxyl and epoxy formation facilitated by hydrogen gas in the environment.Comment: 9 pages and 9 figures. Animations and movies are available at: http://qmsimulatorgojpc.wordpress.com

Probing the Thermal Deoxygenation of Graphene Oxide using High Resolution In Situ X-Ray based Spectroscopies

Despite the recent developments in Graphene Oxide due to its importance as a host precursor of Graphene, the detailed electronic structure and its evolution during the thermal reduction remain largely unknown, hindering its potential applications. We show that a combination of high resolution in situ X-ray photoemission and X-ray absorption spectroscopies offer a powerful approach to monitor the deoxygenation process and comprehensively evaluate the electronic structure of Graphene Oxide thin films at different stages of the thermal reduction process. It is established that the edge plane carboxyl groups are highly unstable, whereas carbonyl groups are more difficult to remove. The results consistently support the formation of phenol groups through reaction of basal plane epoxide groups with adjacent hydroxyl groups at moderate degrees of thermal activation (~400 {\deg}C). The phenol groups are predominant over carbonyl groups and survive even at a temperature of 1000 {\deg}C. For the first time a drastic increase in the density of states (DOS) near the Fermi level at 600 {\deg}C is observed, suggesting a progressive restoration of aromatic structure in the thermally reduced graphene oxideComment: Pagona Papakonstantinou as Corresponding author, E-mail: [email protected]

Rheological properties of poly(lactic acid) based nanocomposites: Effects of different organoclay modifiers and compatibilizers

Poly(lactic acid) (PLA) nanocomposites containing five types of organically modified, layered silicates and two elastomeric compatibilizers, namely ethylene-glycidyl methacrylate (E-GMA) and ethylene-butyl acrylate-maleic anhydride (E-BA-MAH), were prepared using a twin screw extruder. The morphologies of the nanocomposites were determined by X-ray diffraction (XRD) and transmission electron microscopy (TEM), and the rheological properties of the melts were measured using small-amplitude oscillatory shear. XRD revealed that the addition of E-GMA to the binary nanocomposites resulted in higher compatibility between the organoclay nanoplatelets and the polymer matrix. TEM showed that all of the nanocomposites contained mixed dispersed structures, involving tactoids of various sizes, as well as intercalated and exfoliated organoclay layers. Rheological properties were found to be affected by the differences in the compatibility between the organoclays and the polymer matrix, and by the addition of the compatibilizer. Organoclay types that resulted in high level of dispersion exhibited higher values of complex viscosity compared to that of neat PLA. The addition of E-GMA introduced a solid-like rheological behavior at low frequencies. All of the nanocomposites had similar rheological behavior at high frequencies. (C) 2015 Wiley Periodicals, Inc

Poly(lactic acid)-layered silicate nanocomposites: The effects of modifier and compatibilizer on the morphology and mechanical properties

Poly(lactic acid) (PLA) based nanocomposites were prepared to investigate the effects of types of nanoclays. Five different organically modified nanoclays (Cloisites((R))15A, 25A, and 30B, and Nanofils((R))5 and 8) were used. Two rubbery compatibilizers, ethylene-glycidyl methacrylate (E-GMA) and ethylene-butyl acrylate-maleic anhydride, were used in the nanocomposites as compatibilizer-impact modifier. The degree of clay dispersion, the chemical compatibility between the polymer matrix and the compatibilizers, and changes in the morphology and mechanical properties of the nanocomposites were investigated. The mechanical properties and the morphological studies showed that the interactions between the different compatibilizers and PLA resulted in different structures and properties; such that the dispersion of clay, droplet size of the compatibilizer, and tensile properties were distinctly dependent on the type of the compatibilizer. Compatibility between C25A, C30B, and E-GMA resulted in the best level of dispersion, leading to the highest tensile modulus and toughness among the compositions studied. In the mentioned nanocomposites, a network structure was formed owing to the high reactivity of the epoxide group of GMA towards the PLA end groups resulting in high impact toughness. (c) 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42553

Effects of Mixing Protocols on Impact Modified Poly(lactic acid) Layered Silicate Nanocomposites

Poly(lactic acid)/2 wt % organomodified montmorillonite (PLA/OMMT) was toughened by an ethylene-methyl acrylate-glycidyl methacrylate (E-MA-GMA) rubber. The ternary nanocomposites were prepared by melt compounding in a twin screw extruder using four different addition protocols of the components of the nanocomposite and varying the rubber content in the range of 5-20 wt %. It was found that both clay dispersion and morphology were influenced by the blending method as detected by X-ray diffraction (XRD) and observed by TEM and scanning electron microscopy (SEM). The XRD results, which were also confirmed by TEM observations, demonstrated that the OMMT dispersed better in PLA than in E-MA-GMA. All formulations exhibited intercalated/partially exfoliated structure with the best clay dispersion achieved when the clay was first mixed with PLA before the rubber was added. According to SEM, the blends were immiscible and exhibited fine dispersion of the rubber in the PLA with differences in the mean particle sizes that depended on the addition order. Balanced stiffness-toughness was observed at 10 wt % rubber content in the compounds without significant sacrifice of the strength. High impact toughness was attained when PLA was first mixed with the clay before the rubber was added, and the highest tensile toughness was obtained when PLA was first compounded with the rubber, and then clay was incorporated into the mixture. Thermal characterization by DSC confirmed the immiscibility of the blends, but in general, the thermal parameters and the degree of crystallinity of the PLA were not affected by the preparation procedure. Both the clay and the rubber decreased the crystallization temperature of the PLA by acting as nucleating agents. (c) 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41518

Fermentation and oxygen transfer characteristics in recombinant human growth hormone production by Pichia pastoris in sorbitol batch and methanol fed-batch operation

BACKGROUND: The influence of methanol feed rate on recombinant human growth hormone (rhGH) production by Pichia pastoris hGH-Mut(+) in medium containing sorbitol was investigated at three different specific growth rates (mu), namely, 0.02 (MS-0.02), 0.03 (MS-0.03), and 0.04 (MS-0.04)