Visible-light photocatalytic activity of TiO2−x by heat treatment and plasma-heat treatment

The partial reduction of TiO₂ was attempted by heat treatment and plasma-heat treatment and it was carried out to investigate the photocatalytic characteristics of partially reduced TiO₂ (TiO_2?x) in the visible-light region. As a result, the plasma-heat treatment shows significantly stronger than the heat treatment for the visible-light photocatalytic activity of TiO₂. The red-shifted absorption bands in the visible-light region of TiO_2?x by plasma-heat treatment gave broader than one by heat treatment. The TiO_2?x by heat treatment and plasma-heat treatment was changed white to beige color, and white to navy, respectively.     

Effect of functional groups of carbon nanotubes on the cyclization mechanism of polyacrylonitrile (PAN)

The cyclization mechanism of polyacrylonitrile (PAN) in PAN/functionalized carbon nanotube (CNT) composites was examined. The surface functionalization of CNTs was carried out by using diazonium reagents with 4-substituted aniline. The results strongly suggest that the type of functional groups on the CNTs strongly influences the cyclization mechanism of PAN during the stabilization process. The nitrile of PAN in F–Ph–CNT/PAN composite was cyclized through the free radical reaction during thermal stabilization whereas nitrile of PAN in COOH–Ph–CNT/PAN composite underwent cyclization via the ionic reaction due to the acid groups on the surfaces of the CNTs. The fluoro functional groups on the CNTs can act as effective external initiators for nitrile cyclization in homo PAN, in contrast to acid functional groups. Consequently, a lower cyclization temperature (265 °C) and enthalpy value (688 J/g) of F–Ph–CNT were shown compared to those of homo PAN.     

Effect of dissolved CO2 on the crystallization behavior of linear and branched PLA

In this study, the crystallization behavior of polylactide (PLA) was investigated in the presence of dissolved CO₂ using high-pressure and regular differential scanning calorimeter. The isothermal and non-isothermal melt crystallization results showed that increasing the CO₂ pressure decreased the crystallization half-time. During isothermal and low-cooling-rate non-isothermal crystallization, a very high crystallinity was achieved at 15 bar CO₂ pressure by facilitating more perfect crystal formation with the plasticization effect of CO₂ while limiting the crystal nucleation rate. At higher CO₂ pressures, a larger number of less close-packed crystals were formed due to chain entanglement, and consequently, the final crystallinity was decreased. The non-isothermal results at high cooling rates showed the total crystallinity decreased for all CO₂ contents, because of less time given for crystallization. Also the effects of the CO₂ pressure and the cooling rate on T_c and T_g were investigated.     

An effective oxidation approach for luminescence enhancement in CdS quantum dots by H2O2

The effects of surface passivation on the photoluminescence (PL) properties of CdS nanoparticles oxidized by straightforward H₂O₂ injection were examined. Compared to pristine cadmium sulfide nanocrystals (quantum efficiency ≅ 0.1%), the surface-passivated CdS nanoparticles showed significantly enhanced luminescence properties (quantum efficiency ≅ 20%). The surface passivation by H₂O₂ injection was characterized using X-ray photoelectron spectroscopy, X-ray diffraction, and time-resolved PL. The photoluminescence enhancement is due to the two-order increase in the radiative recombination rate by the sulfate passivation layer.     

A novel negative dielectric constant material based on phosphoric acid doped poly(benzimidazole)

Metamaterials or artificial negative index materials (NIMs) have generated great attention because of their unique electromagnetic properties. The main challenge in current NIM development is creating a homogenous NIM without the need of complex geometric architectures consisting of capacitors and inductors or aggregated fillers, but possessing a tunable plasma frequency. A natural material that can exhibit negative values for permittivity and permeability simultaneously has not been found, or discovered. If one can design a negative dielectric constant material with a tunable plasma frequency of interest, implementing negative permeability into the material or system would be much more readily achievable to create a metamaterial. In this regard, a novel negative dielectric constant material, which is an essential key to creating the NIMs, was developed by doping ions into a polymer, a protonated poly(benzimidazole) (PBI). The doped PBI showed a negative dielectric constant at frequencies of kHz to MHz because of its reduced plasma frequency and an induction effect. As temperature increased, the dielectric spectrum changed from a relaxation to a resonance behavior and exhibited a larger magnitude of negative dielectric constant at a lower frequency. The conductivity of the doped PBI measured as a function of both temperature and frequency followed the same trend as the dielectric constant. With respect to the dielectric constant and the conductivity data, it can be assumed that the origin of the negative dielectric constant is attributed to the resonance behavior of the highly mobile ions at elevated temperatures and high frequencies. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Simple and cost-effective reduction of graphite oxide by sulfuric acid

We report a simple, cost-effective, and environmentally benign process for reducing graphite oxide by treating solely with sulfuric acid. The suggested process consists of a two-step reduction of graphite oxide, first in aqueous sulfuric acid at room temperature and then in concentrated sulfuric acid with refluxing. X-ray diffractometry, X-ray photoelectron spectroscopy, Raman spectroscopy and thermogravimetric analysis demonstrated that the graphite oxide was reduced effectively and was comparable in composition to reduced graphite oxide prepared using previously described methods that rely on toxic and hazardous reducing agents, such as hydrazine, sodium borohydride, or hydrohalic acids.     

Preparation of activated graphene and effect of activation parameters on electrochemical capacitance

Activation parameters such as temperature and the amount of potassium hydroxide (KOH) were varied during the synthesis of activated microwave-exfoliated graphite oxide (a-MEGO) and the effects of these parameters on the specific surface area of a-MEGO and electrochemical capacitance of a-MEGO electrodes were investigated. At 800 °C and a KOH/MEGO mass ratio of 6.5, a maximum specific surface area of 3100 m²/g was obtained and a high specific capacitance of 172 F/g (at 1 A/g constant current and 3.5 V maximal voltage) was measured in a two-electrode cell with a-MEGO electrodes in an organic electrolyte.     

The production of a cellular graphene array by scanning probe lithography and its ability to store electrical charge

A graphene cellular array on an insulating SiO₂ layer was fabricated by scanning probe lithography. The graphene layer was oxidized by an electric field which was applied between the cantilever tip and Si substrate without any electrode directly connected to the graphene layer. When the bias voltage was applied on a cell of patterned graphene through the cantilever tip, charge was accumulated on the cell and preserved for a long time without decay. The accumulated charge and the surface potential were measured by an electrostatic force microscope. The charge retention was measured as a function of time, and the decay time constant was estimated to be ～70 min.     

The Aharonov-Bohm effect in graphene rings with metal mirrors

We measured the Aharonov-Bohm (AB) effect in graphene rings with superconducting-(Al) and normal-metal (Au) mirrors. The mirrors were deposited either on additional stubs connected to the rings in the transverse direction or on the ring bias lines. A significant enhancement of the visible phase coherence was observed in the latter case, in which we observed even the third harmonic of the AB oscillations. The superconductivity of the mirrors appears to be unimportant for the improved coherence in graphene. A large Fermi energy mismatch between graphene and the mirror material is sufficient for this effect. In addition, a transport gap was observed in our graphene structures at the gate voltage close to the Dirac point. The value of the gap can be reproduced by assuming the occurrence of Coulomb blockade effects in graphene.     

Characterization of thermally damaged concrete using a nonlinear ultrasonic method

Thermal damage in concrete usually induces contact-type defects, which result in degradation of the concrete's performance. This paper attempts to visualize the thermal damage in a multiscale, and characterizes the thermally damaged concrete using a nonlinear ultrasonic method. An impact-modulation method is used to obtain nonlinearity parameters, as a quantitative measure of contact-type defects, and shows better sensitivity than phase velocity variation as a linear ultrasonic method for thermally damaged concrete. The measured nonlinearity parameter is compared with the permeable pores, which reflect the occurrence of opening and pores in thermally damaged concrete. Degradation of concrete strength due to thermal damage is also assessed via the measurement.