Raman spectra of graphene oxide and thermally reduced graphene oxide were analyzed in order to relate spectral parameters with the structural properties. The chemical composition of different graphene oxides was determined by organic elemental analysis, and the microstructure of nanocrystals was analyzed by X-ray diffraction. We find five reported bands (D, D′, G, D″, and D*) in the region between 1000 and 1800 cm–1 in all spectra. The band parameters such as position, intensity ratio, and width have been related with structural properties such as oxygen content, crystallinity, and disorder degree of GO and rGO platelets. An assessment of the validity of the Tuinstra–Koenig and Cuesta models was carried out by using the results obtained from the fit of the first-order spectra of graphene oxide derivatives at five functions: two Gaussian and three pseudo-Voigt peaks.

We study the influence of Nb doping on the TiO 2 anatase-to-rutile phase transition, using combined transmission electron microscopy, Raman spectroscopy, x-ray diffraction and selected area electron diffraction analysis. This approach enabled anatase-to-rutile phase transition hindering to be clearly observed for low Nb-doped TiO 2 samples. Moreover, there was clear grain growth inhibition in the samples containing Nb. The use of high resolution transmission electron microscopy with our samples provides an innovative perspective compared with previous research on this issue. Our analysis shows that niobium is segregated from the anatase structure before and during the phase transformation, leading to the formation of NbO nanoclusters on the surface of the TiO 2 rutile nanoparticles.

No abstract

We present tunneling experiments on Fe͑001͒/MgO͑20 Å͒/FeCo͑001͒ single-crystal epitaxial junctions of high quality grown by sputtering and laser ablation. Tunnel magnetoresistance measurements give 60% at 30 K, to be compared with 13% obtained recently on ͑001͒-oriented Fe/amorphous-Al 2 O 3 /FeCo tunnel junctions. This difference demonstrates that the spin polarization of tunneling electrons is not directly related to the density of states of the free metal surfaceFe͑001͒ in this case-but depends on the actual electronic structure of the entire electrode/barrier system.

SBA‐15 (2D hexagonal structure) and KIT‐6 (3D cubic structure) silica materials are used as templates for the synthesis of two different crystalline mesoporous WO3 replicas usable as NO2 gas sensors. High‐resolution transmission electron microscopy (HRTEM) studies reveal that single‐crystal hexagonal rings set up the atomic morphology of the WO3 KIT‐6 replica, whereas the SBA‐15 replica is composed of randomly oriented nanoparticles. A model capable of explaining the KIT‐6 replica mesostructure is described. A small amount of chromium is added to the WO3 matrix in order to enhance sensor response. It is demonstrated that chromium does not form clusters, but well‐distributed centers. Pure WO3 KIT‐6 replica displays a higher response rate as well as a lower response time to NO2 gas than the SBA‐15 replica. This behavior is explained by taking into account that the KIT‐6 replica has a higher surface area as demonstrated by Brunauer–Emmett–Teller analyses and its mesostructure is fully maintained after the screen‐printing step involved in sensors preparation. The presence of chromium in the material results in a shorter response time and improved sensor response to the lowest NO2 concentrations tested. Electrical differences related to mesostructure are reduced as a result of additive introduction.