148 research outputs found
Multi-walled carbon nanotubes decorated with titanium nanoparticles: synthesis and characterization
No full textA chemical route to decorate MWCNTs by titanium nanoparticles has been developed. MWCNTs, grown by catalytic decomposition of propane, have been used to prepare intercalated K-MWCNTs, which are then reacted with Ti(IV) isopropoxide. The amount of Ti present in the nanostructure is 8.7 wt%. Several techniques have been used for a detailed characterization of MWCNTs decorated by Ti nanoparticles, such as transmission electron microscopy (TEM), selected-area electron diffraction (SAED), high resolution transmission electron microscopy (HREM) and electron energy loss spectroscopy (EELS) as well as Raman spectroscopy and x-ray diffraction (XRD). Titanium is found to be present as alpha-Ti(hcp) dispersed in very small particles, the size of the largest being similar to 2 nm. The Ti-containing phase appears to be fairly homogeneously distributed in the space between the two sides of a MWCNT. No intercalation of Ti between the CNT walls has been detected
A transmission electron microscopy study of CoFe2O4 ferrite nanoparticles in silica aerogel matrix using HREM and STEM imaging and EDX spectroscopy and EELS
No full textMagnetic nanocomposite materials consisting of 5 and 10 wt% CoFe 2O4 nanoparticles in a silica aerogel matrix have been synthesized by the sol-gel method. For the CoFe2O4-10wt% sample, bright-field scanning transmission electron microscopy (BF STEM) and high-resolution transmission electron microscopy (HREM) images showed distinct, rounded CoFe2O4 nanoparticles, with typical diameters of roughly 8 nm. For the CoFe2O4-5wt% sample, BF STEM images and energy dispersive X-ray (EDX) measurements showed CoFe2O4 nanoparticles with diameters of roughly 3 ± 1 nm. EDX measurements indicate that all nanoparticles consist of stoichiometric CoFe2O4, and electron energy-loss spectroscopy measurements from lines crossing nanoparticles in the CoFe2O4-10wt% sample show a uniform composition within nanoparticles, with a precision of at best than ± 0.5 nm in analysis position. BF STEM images obtained for the CoFe2O 4-10wt% sample showed many "needle-like" nanostructures that typically have a length of ? 10 nm and a width of ? 1 nm, and frequently appear to be attached to nanoparticles. These needle-like nanostructures are observed to contain layers with interlayer spacing 0.33 ± 0.1 nm, which could be consistent with Co silicate hydroxide, a known precursor phase in these nanocomposite materials
Structural study of highly porous nanocomposite aerogels
No full textThe structural properties of CoFe2O4-SiO2 highly porous nanocomposite aerogels have been investigated by X-ray Absorption Spectroscopy and Transmission Electron Microscopy techniques. The aerogels are obtained by supercritical drying of composite gels obtained using a two step procedure where fast gelation is achieved using urea in the second step. The formation of CoFe2O4 nanocrystals in the silica matrix begins after calcination at 750 degrees C of the parent aerogel and is complete after calcination at 900 degrees C, while the high porosity of the sample is mostly retained
Cation Distribution and Vacancies in Nickel Cobaltite
No full textSamples of nickel cobaltite, a mixed oxide occurring in the spinel structure which is currently extensively investigated because of its prospective application as ferromagnetic, electrocatalytic, and cost-effective energy storage material were prepared in the form of nanocrystals stabilized in a highly porous silica aerogel and as unsupported nanoparticles. Nickel cobaltite nanocrystals with average size 4 nm are successfully grown for the first time into the silica aerogel provided that a controlled oxidation of the metal precursor phases is carried out, consisting in a reduction under H2 flow followed by mild oxidation in air. The investigation of the average oxidation state of the cations and of their distribution between the sites within the spinel structure, which is commonly described assuming the Ni cations are only located in the octahedral sites, has been carried out by X-ray Absorption Spectroscopy providing evidence for the first time that the unsupported nickel cobaltite sample has a Ni:Co molar ratio higher than the nominal ratio of 1:2 and a larger than expected average overall oxidation state of the cobalt and nickel cations. This is achieved retaining the spinel structure, which accommodates vacancies to counterbalance the variation in oxidation state
Insights into the Structure of Dot@Rod and Dot@Octapod CdSe@CdS Heterostructures
No full textCdSe@CdS dot@rods with diameter around 6 nm and length of either
20, 27, or 30 nm and dot@octapods with pod diameters of ?15 nm and lengths of ?50
nm were investigated by X-ray absorption spectroscopy. These heterostructures are
prepared by seed-mediated routes, where the structure, composition, and morphology of
the CdSe nanocrystals used as a seed play key roles in directing the growth of the second
semiconducting domain. The local structural environment of all the elements in the
CdSe@CdS heterostructures was investigated at the Cd, S, and Se K-edges by taking
advantage of the selectivity of X-ray absorption spectroscopy, and was compared to pure
reference compounds. We found that the structural features of dot@rods are
independent of the size of the rods. These structures can be described as made of a
CdSe dot and a CdS rod, both in the wurtzite phase with a high crystallinity of both the
core and the rod. This result supports the effectiveness of high temperature colloidal
synthesis in promoting the formation of core@shell nanocrystals with very low
defectivity. On the other hand, data on the CdSe@CdS with octapod morphology suggest the occurrence of a core composed of
a CdSe cubic sphalerite phase with eight pods made of CdS wurtzite phase. Our findings are compared to current models
proposed for the design of functional heterostructures with controlled nanoarchitecture
Resistive switching mechanism in TiO<sub>2-x</sub> thin films: an X-ray absorption spectroscopy study
No full textMetal–insulator–metal (MIM) devices based on titanium dioxide thin films exhibit resistive switching behavior (RS); i.e., they have the ability to switch the electrical resistance between high-resistive states (HRS) and low-resistive states (LRS) by application of an appropriate voltage. This behavior makes titanium dioxide thin films extremely valuable for memory applications. The physical mechanism behind RS remains a controversial subject but it has been suggested that it could be interface-type, without accompanying structural changes of the oxide, or filament-type with formation of reduced titanium oxide phases in the film. In this work, X-ray absorption spectroscopy (XAS) at the Ti K-edge (4966 eV) was used to characterize the atomic-scale structure of a nonstoichiometric TiO2–x thin film before and after annealing and for the first time after inclusion in a MIM device based on a Cr/Pt/TiO2–x/Pt stack developed on an oxidized silicon wafer. The advantage of the XAS technique is that is element-specific. Therefore, by tuning the energy to the Ti K-edge absorption, contributions from the Pt, Cr, and Si in the stack are eliminated. In order to investigate the structure of the film after electrical switching, XAS analysis at the Ti K-edge was again performed for the first time on the Cr/Pt/TiO2–x/Pt stack in its virgin state and after switching to LRS by application of an appropriate bias. X-ray absorption near-edge structure (XANES) was employed to assess local coordination and oxidation state of the Ti and extended X-ray absorption fine structure (EXAFS) was used to assess bond distances, coordination numbers, and Debye–Waller factors. XAS analysis revealed that the as-deposited film is amorphous with a distorted local octahedral arrangement around Ti (average Ti–O distance of 1.95 Å and coordination number of 5.2) and has a majority oxidation state of Ti4+ with a slight content of Ti3+. The film remains amorphous upon insertion into the stack structure and after electrical switching but crystallizes as anatase upon annealing at 600 °C. These results do not give any indication of the appearance of conducting filaments upon switching and are more compatible with homogeneous interface mechanisms
NiFe2O4 Nanoparticles Dispersed in an Aerogel Silica Matrix: An X-ray Absorption Study
No full textThe formation of NiFe2O4 nanoparticles dispersed in an aerogel silica matrix was investigated as a function of calcination temperature by X-ray absorption fine structure and X-ray absorption near edge structure at both the Fe and Ni K-edges. In particular, nanocomposite aerogels containing a relative NiFe2O4 amount of 10 wt % and calcined at 450, 750 (1 h and 20 h), and 900 °C were studied. A quantitative determination of the relative occupancy of iron and nickel cations in the octahedral and tetrahedral sites of the spinel structure was obtained. It has been found that nickel ferrite prepared by sol?gel has the classical inverted spinel structure found in bulk materials with nickel(II) cations fully occupying the octahedra sites and iron(III) equally distributed between octahedra and tetrahedra sites
An X-ray absorption spectroscopy study of the NiK edge in NiO-SiO2 nanocomposite materials prepared by the sol-gel method
No full textEXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near edge structure) techniques have been used to study the structural evolution of nanocomposites constituted of nickel oxide nanoparticles embedded into an amorphous silica matrix during their sol-gel preparation. EXAFS data show that no interaction between the metal oxide nanoparticles and the silica network develops since the spectra of all the samples have a structure very similar to that of crystalline NiO. Quantitative information obtained from the fitting of the data using a full multiple scattering calculation shows that the reduction of the EXAFS oscillations in the nanocomposite samples is mainly related to an increase of disorder of the superficial sites while there was no evidence of a significant reduction of coordination numbers due to the size of the crystallites. XANES results support the conclusion that the structure of the nanocomposites is very similar to that of crystalline NiO
X-ray absorption spectroscopy study of TiO<sub>2-x</sub> thin films for memory applications
No full textMetal-insulator-metal (MIM) devices based on titanium dioxide thin films exhibit resistive switching behaviour (RS) i.e. they have the ability to switch the electrical resistance between high-resistive states (HRS) and low-resistive states (LRS) by application of an appropriate voltage. This behaviour makes titanium dioxide thin films extremely valuable for memory applications. The physical mechanism behind RS remains a controversial subject but has been suggested that it could be interface-type, without accompanying structural changes of the oxide, or filament-type with formation of reduced titanium oxide phases in the film. In this work, X-ray absorption spectroscopy (XAS) at the Ti K-edge (4966 eV) was used to characterize the atomic-scale structure of a non-stoichiometric TiO2-x thin film before and after annealing and for the first time after inclusion in a MIM device based on a Cr/Pt/TiO2-x/Pt stack developed on an oxidised silicon wafer. The advantage of the XAS technique is that is element-specific. Therefore, by tuning the energy to the Ti K-edge absorption, contributions from the Pt, Cr and Si in the stack are eliminated. In order to investigate the structure of the film after electrical switching, XAS analysis at the Ti K-edge was again performed for the first time on the Cr/Pt/TiO2-x/Pt stack in its virgin state and after switching to LRS by application of an appropriate bias. X-ray absorption near-edge structure (XANES) was employed to assess local coordination and oxidation state of the Ti and X-ray absorption fine structure (EXAFS) was used to assess bond distances, coordination numbers and Debye-Waller factors. XAS analysis revealed that the as-deposited film is amorphous with a distorted local octahedral arrangement around Ti (average Ti-O distance of 1.95 Å and coordination number of 5.2) and has a majority oxidation state of Ti+4 with a slight content of Ti+3. The film remains amorphous upon insertion into the stack structure and after electrical switching but crystallizes as anatase upon annealing at 600ºC. These results do not give any indication of the appearance of conducting filaments upon switching and are more compatible with homogeneous interface mechanisms
Molecular Dynamics Modelling of Barium Silicate and Barium Fluorozirconate Glasses
No full textAdvancement in science and technology has profoundly depended on new types of glass innovation. The glasses that were studied in this project are binary barium silicate glasses, binary barium fluorozirconate glasses, ZBLAN glasses and ?Eu?^(3+) doped ZBLAN glass (the ZBLAN glasses are based on binary barium fluorozirconate glass). The high atomic number of barium in the barium silicate glasses provides high mass and high electron density providing its applications for heat and X-ray shielding. The phenomena such as phase separation in the barium silicate glass will affect its properties of durability and electrical conductivity. On the other hand, ZBLAN glasses have a broad infrared optical transmission window due to the weaker bonding/interaction of F^- ions. Due to the presence of lanthanum in the composition ZBLAN glass can be easily doped with rare-earth ions such as ?Eu?^(3+) giving it many optical applications such as optical amplifier and fibre lasers.
Hence, it's essential to study the structure of these glasses to understand their properties for applications. This thesis used the classical molecular dynamics modelling technique to study the static atomic structure of glass. Generally, fluoride glasses can be formed by totally replacing oxygen atoms in oxide glasses by fluorine atoms. The oxide silicate glasses are common glasses that follow the Zachriasen rules of glass formation but the fluorozirconate glasses do not and lack fixed structural units.
The structure analysis was performed at short-range order (e.g. coordination number, bond length and bond angle), medium-range order (e.g. network connectivity) and long- range order (e.g. phase separation). The related crystals were also simulated in similar conditions to the glasses to compare their atomic structure. Normally at short-range order glass
structure is similar to its related crystal but the differences between them starts from the position and number of next nearest neighbours and increases thereafter. Additionally, the
new methods such as rotational invariants and grid analysis were used to scrutinise structural units and phase separation respectively.
The model of barium silicate glass shows good agreement with experimental diffraction data. The typical bond length and coordination number for Ba were 2.97 Å and approximately 7 respectively. The model did not show any phase separation at low Ba content and hence for further investigation very large models of alkaline earth silicate glasses were studied to see how Ba, Ca and Mg are distributed in the glass. The grid analysis was used to see the distributions which show homogeneity for Ba and Ca and inhomogeneity for Mg cation.
The structural units of fluorozirconate glasses were carefully studied as they do not follow the Zachriasen glass model. The coordination number for Zr was mixture of 7 and 8. The rotational invariant analysis shows that the structural units of ZrF_n polyhedra for coordination number 7 and 8 were similar to Augmented Triangular Prism and Biaugmented Triangular Prism respectively. However, rotational invariant values for BaF_n polyhedra tend more towards random.
The large complex model of ?Eu?^(3+) doped ZBLAN glass was made as it is studied for optical applications. The initial analysis was to observe whether Zr and Ba has similar structural roles as in binary fluorozirconate glass system which they do. Considering the extra elements in ZBLAN glass, Al behaves like a network former and has octahedra structural units whereas La and Na behave like modifiers. In the glass Eu was uniformly distributed with predominantly coordination number of 8 and does not have well defined structural units
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