EPR study of paramagnetic centers in SiO2:C: Zn nanocomposites obtained by infiltration of fumed silica with luminescent Zn(acac)2 solution

Silica-carbon with zinc (SiO2:C:Zn) nanocomposites obtained via infiltration with aged luminescent zinc acetylacetonate (Zn(acac)2) ethanol solution of two concentrations (1 or 4%) into the fumed silica (SiO2) matrix have been studied using EPR within the temperature range 6…296 K before and after thermal annealing. The EPR spectrum of SiO2:C:Zn nanocomposites consists of three signals with the Lorentzian lineshape corresponding to paramagnetic centers with S = 1/2, which are related to carbon dangling bonds (CDB) (g = 2.0029(3)), silicon dangling bonds (g = 2.0062(3)) and oxygen-centered carbon-related radicals (CRR) (g = 2.0042(3)). A small EPR linewidth (<1 mT) observed for CDB and oxygen-centered CRR allows us to conclude that they are in the sp3-hybridized state. It was found that the temperature dependence of the EPR signal integrated intensity of the CDB and oxygen-centered CRR follows the Curie–Weiss law with a small positive value of the Curie–Weiss constant, which indicates that the weak ferromagnetic exchange interaction takes place in the spin system of CDB and oxygen-centered CRR. It was supposed that the carbon-related centers are clustered in SiO2:C:Zn nanocomposites. We assume that the oxygen-centered CRR in the sp3-hybridized state are associated with luminescent centers in previously reported aged Zn(acac)2/C2H5OH solution.

A Facile and Efficient Bromination of Multi-Walled Carbon Nanotubes

The bromination of multi-walled carbon nanotubes (MWCNT) was performed with vapor bromine in a closed vessel, and they were subjected to intensive stirring with a magnetic stirrer for up to 14 days. The efficiency of bromination was compared depending upon duration. The structure and surface of the crude and purified products were characterized by detailed physicochemical analyses, such as SEM/EDS, TEM, XRD, TGA, Raman, and XPS spectroscopies. The studies confirmed the presence of bromine covalently bound with nanotubes as well as the formation of inclusion MWCNT–Br2 complexes. It was confirmed that Br2 molecules are absorbed on the surface of nanotubes (forming the CNT-Br2 complex), while they can dissociate close to dangling bonds at CNT defect sites with the formation of covalent C−Br bonds. Thus, any covalent attachment of bromine to the graphitic surface achieved around room temperature is likely related to the defects in the MWCNTs. The best results, i.e., the highest amount of attached Br2, were obtained for brominated nanotubes brominated for 10 days, with the content of covalently bound bromine being 0.68 at% (by XPS).

Study of Atomic Hydrogen Concentration in Grain Boundaries of Polycrystalline Diamond Thin Films

This paper describes research focused on investigating the effect of hydrogen (H) atom insertion into the grain boundaries of polycrystalline diamond (PCD) films. This is required in order to understand the key morphological, chemical, physical, and electronic properties of the films. The PCD films were grown using the hot filament chemical vapor deposition (HFCVD) process, with flowing Ar gas mixed with CH4 and H2 gases to control film growth into microcrystalline diamond (MCD, 0.5–3 µm grain sizes), nanocrystalline diamond (NCD, 10–500 nm grain sizes), and ultrananocrystalline diamond (UNCD, 2–5 nm grain sizes) films depending on the Ar/CH4/H2 flow ratios. This study focused on measuring the H atom concentration of the PCD films to determine the effect on the properties indicated above. A simple model is presented, including a hypothesis that the two dangling bonds per unit cell of C atoms serve as the site of hydrogen incorporation. This correlates well with the observed concentration of H atoms in the films. Dangling bonds which are not passivated by hydrogen are postulated to form surface structures which include C double bonds. The Raman peak from these surface structures are the same as observed for transpolyacetyline (TPA). The data reveal that the concentration of H atoms at the grain boundaries is around 1.5 × 1015 atoms/cm2 regardless of grain size. Electrical current measurements, using a conductive atomic force microscopy (CAFM) technique, were performed using an MCD film, showing that the current is concentrated at the grain boundaries. Ultraviolet photo electron spectroscopy (UPS) confirmed that all the PCD films exhibited a metallic behavior. This is to be expected if the nature of grain boundaries is the same regardless of grain size.

Effect of (100) and (001) Hexagonal WO3 Faceting on Isoprene and Acetone Gas Selectivity

The hexagonal WO3 polymorph, h-WO3, has attracted attention due to its interatomic channels, allowing for a greater degree of intercalation compared to other WO3 polymorphs. Our research group has previously demonstrated h-WO3 to be a highly sensitive gas sensing material for a flu biomarker, isoprene. In this work, the gas sensing performance of this polymorph has been further investigated in two distinct configurations of the material produced by different processing routes. The first sample was synthesized using Na2WO4·2H2O and showed (100) faceting. The second sample was synthesized using WCl6 and showed (001) faceting. The gas sensing response of the nanostructured films deposited using the (100) textured h-WO3 sample 1 had a higher response to acetone at 350 °C. The (001) textured h-WO3 sample 2 favored isoprene at 350 °C. The selectivity of the latter to isoprene is explained in terms of the dangling bonds present on the (001) facets. The tungsten and oxygen dangling bonds present on the (001) plane favor the adsorption of the isoprene molecule over that of the acetone molecule due to the oxygen containing dipole present in the acetone molecule.

Modifying Electronic and Elastic Properties of 2-Dimensional [110] Diamond by Nitrogen Substitution

One type of two-dimensional diamonds that are derived from [111] direction, so-called diamane, has been previously shown to be stabilized by N-substitution, where the passivation of dangling bonds is no longer needed. In the present work, we theoretically demonstrated that another type of two-dimensional diamonds derived from [110] direction exhibiting a washboard conformation can also be stabilized by N-substitution. Three structural models of washboard-like carbon nitrides with compositions of C6N2, C5N3, and C4N4 are studied together with the fully hydrogenated washboard-like diamane (C8H4). The result shows that the band gap of this type structure is only open the dangling bonds that are entirely diminished through N-substitution. By increasing the N content, the C11 and C22 are softer and the C33 is stiffer where their bulk modulus are in the same order, which is approximately 550 GPa. When comparing with the hydrogenated phase, the N-substituted phases have higher elastic constants and bulk modulus, suggesting that they are possibly harder than the fully hydrogenated diamane.