How photocatalytic activity of the MAO-grown TiO2 nano/micro-porous films is influenced by growth parameters?

Pure titania porous layers consisted of anatase and rutile phases, chemically and structurally suitable for catalytic applications, were grown via micro-arc oxidation (MAO). The effect of applied voltage, process time, and electrolyte concentration on surface structure, chemical composition, and especially photocatalytic activity of the layers was investigated. SEM and AFM studies revealed that pore size and surface roughness of the layers increased with the applied voltage, and the electrolyte concentration. Moreover, the photocatalytic performance of the layers synthesized at medium applied voltages was significantly higher than that of the layers produced at other voltages. About 90% of methylene blue solution was decomposed after 180 min UV-irradiation on the layers produced in an electrolyte with a concentration of 10 g l⁻¹ at the applied voltage of 450 V.     

EMI shielding effectiveness of graphene decorated with graphene quantum dots and silver nanoparticles reinforced PVDF nanocomposites

Graphene decorated with graphene quantum dots (G-D-GQDs) have been successfully synthesized using solvothermal cutting of graphene oxide. The incorporation of G-D-GQDs in polyvinyledene fluoride (PVDF) matrix shows the total EMI shielding effectiveness (SE_T) of 31 dB at 8 GHz. The main mechanism of high EMI shielding effectiveness is reflection and absorption of EM radiation. The high absorption of EM radiation is due to tunneling of electrons from GQDs. Further, decoration of G-D-GQDs with conducting Ag nanoparticles (G-D-GQDsAg) enhances the SE_T value to 43 dB at 8 GHz of PVDF/G-D-GQDsAg nanocomposite, due to increase in electrical conductivity of PVDF/G-D-GQDsAg nanocomposite and enhanced dispersion of G-D-GQDsAg in PVDF matrix. The incorporation of G-D-GQDs and G-D-GQDsAg in PVDF matrix also increases the thermal stability and crystallinity of PVDF. The increase in thermal stability and crystallinity are more for PVDF/G-D-GQDsAg nanocomposite as compare to PVDF/G-D-GQDs nanocomposite, due to better dispersion of G-D-GQDsAg in PVDF matrix. Thus, PVDF/G-D-GQDsAg nanocomposite having high SE_T value can shield 99.9% of electromagnetic radiation in X-band range, which make it suitable for EMI shielding application for consumer electronic equipment’s.     

Theoretical study on the catalytic properties of single-atom catalyst stabilised on silicon-doped graphene sheets

ABSTRACT

The stable configurations, electronic structures and catalytic activities of single-atom metal catalyst anchored silicon-doped graphene sheets (3Si-graphene-M, M?=?Ni and Pd) are investigated by using density functional theory calculations. Firstly, the adsorption stability and electronic property of different gas reactants (O₂, CO, 2CO, CO/O₂) on 3Si-graphene-M substrates are comparably analysed. It is found that the coadsorption of O₂/CO or 2CO molecules is more stable than that of the isolated O₂ or CO molecule. Meanwhile, the adsorbed species on 3Si-graphene-Ni sheet are more stable than those on the 3Si-graphene-Pd sheet. Secondly, the possible CO oxidation reactions on the 3Si-graphene-M are investigated through Eley–Rideal (ER), Langmuir–Hinshelwood (LH) and new termolecular Eley–Rideal (TER) mechanisms. Compared with the LH and TER mechanisms, the interaction between 2CO and O₂ molecules (O₂?+?CO → CO₃, CO₃?+?CO → 2CO₂) through ER reactions (< 0.2?eV) are an energetically more favourable. These results provide important reference for understanding the catalytic mechanism for CO oxidation on graphene-based catalyst.     

Beating oscillation and Fano resonance in the laser assisted electron transmission through graphene δ-function magnetic barriers

We investigate theoretically the transmission of electrons through a pair of δ-function magnetic barriers in graphene in presence of external monochromatic, linearly polarized and CW laser field. The transmission coefficients are calculated in the framework of non-perturbative Floquet theory using the transfer matrix method. It is noted that the usual Fabry–Perot oscillations in transmission through the graphene magnetic barriers with larger inter barrier separation takes the shape of beating oscillations in presence of the external laser field. The laser assisted transmission spectra are also found to exhibit the characteristic Fano resonances (FR) for smaller values of the inter barrier separation. The appearance of the perfect node in the beating oscillation and the asymmetric Fano line shape can be controlled by varying the intensity of the laser field. The above features could provide some useful and potential information about the light - matter interactions and may be utilized in the graphene based optoelectronic device applications.     

Computational study of the formation of aluminum-graphene nanocrystallites

The molecular dynamics method is used to study the formation of the Al/graphene nanocomposite in the structural grains of different size under the action of internal stresses. The behavior of graphene sheets inside an individual structural grain as well as in the process of two Al grains containing graphene are joined is investigated. The motion of graphene films, starting from the middle of the aluminum matrix, ends with their location at the crystallite boundaries. Graphene moves in the Al matrix along closely packed planes. In this case, graphene sheets acquire curvature. An intergrowth of graphene sheets is also observed. A contact between two Al-C nanocrystallites through a graphene interlayer is created. The self-diffusion coefficients of atoms and the partial potential energies increased with decreasing nanocrystallite size. The angular distribution of the nearest geometric neighbors and the distribution of distances to the nearest neighbors are determined using the construction of Voronoi polyhedra.     

Scanning probe microscopy investigations of the electrical properties of chemical vapor deposited graphene grown on a 6H-SiC substrate

Sublimated graphene grown on SiC is an attractive material for scientific investigations. Nevertheless the self limiting process on the Si face and its sensitivity to the surface quality of the SiC substrates may be unfavourable for later microelectronic processes. On the other hand, chemical vapor deposited (CVD) graphene does not posses such disadvantages, so further experimental investigation is needed. In this paper CVD grown graphene on 6H-SiC (0 0 0 1) substrate was investigated using scanning probe microscopy (SPM). Electrical properties of graphene were characterized with the use of: scanning tunnelling microscopy, conductive atomic force microscopy (C-AFM) with locally performed C-AFM current–voltage measurements and Kelvin probe force microscopy (KPFM). Based on the contact potential difference data from the KPFM measurements, the work function of graphene was estimated. We observed conductance variations not only on structural edges, existing surface corrugations or accidental bilayers, but also on a flat graphene surface.     

Thin graphite overlayers: Graphene and alkali metal intercalation

Using LEED and angle resolved photoemission for characterisation we have prepared graphite overlayers with down to monolayer thickness by heating SiC crystals and monitored alkali metal intercalation for the multilayer films. The valence band structure of the monolayer is similar to that calculated for graphene though downshifted by around 0.8 eV and with a small gap at the zone corner. The shift suggests that the transport properties, which are of much present interest, are similar to that of a biased graphene sample. Upon alkali metal deposition the 3D character of the π states is lost and the resulting band structure becomes graphene like. A comparison with data obtained for ex situ prepared intercalation compounds indicates that the graphite film has converted to the stage 1 compounds C₈K or C₈Rb. Advantages with the present preparation method is that the graphite film can be recovered by desorbing small amounts of alkali metal and that the progress of compound formation can be monitored. The energy shifts measured after different deposits indicate that saturation is reached in three steps. Our interpretation is that in the first the alkali atoms are dispersed while the final steps are characterized by the formation of first one and then a second (2 × 2) ordered alkali metal layer adjacent to the uppermost carbon layer.     

Singular orbital magnetism of graphene

The origin of the singular diamagnetic susceptibility at the Dirac point is probed through the study of effects of band-gap opening and spatially varying magnetic field. In the presence of a band gap, the susceptibility is nonzero only inside the band gap and exhibits a discrete jump at the band edges down to zero in the conduction and valence bands. The jump height is understood in terms of the pseudo-spin paramagnetism arising from valley degree of freedom. In spatially varying magnetic field with wave vector q, the susceptibility becomes nonzero only in a finite energy region containing the Dirac point, determined by q. This behavior is understood in terms of electronic states numerically calculated in periodic magnetic field.