Nanosized crystallites

The top-down approach starts with a bulk material and attempts to break it down into nanoscaled materials through physical methods. Hence, most of these techniques are really forms of fabrication rather than synthesis. For nanostructured bulk phases, including powders, the common methods are milling, devitrification of metallic glass, and severe plastic deformation. For nanocrystalline thin films (films with nanosized crystallites), methods include thermal vaporization (under high vacuum), laser ablation, and sputtering (thermal plasma), all of which were... 

The XRD patterns of electrodes fabricated at different temperatures (Fig. 14.6) showed that rutile Sn02 was detected at every temperature. Diffraction lines of electrode prepared at 500° C are broadened, indicating the nanosized crystallites in the surface coating of the electrode. When the annealing temperature increases from 500 to 700°C, the intensity of the peaks increases sharply and it means that the crystal of electrode surface grows more integrated. Better electrocatalytic ability can be achieved. 

Fig. 2a shows the temperature dependence of relative conductivity heating component and any anomalies on the curve are not observed. Such a curve is typical for catalytic MWNTs [10], arc-produced MWNTs [9], carbon nanocomposites [10,11], and for graphite-like nanosize crystallites [12]. In order to find out the influence of other gases on conductivity of catalytic MWNTs we monitored behavior of a(T)/o(300 K) in air-helium environments (50 % helium and 50 % air). 

The size of the zeoKte crystals determines the role of the external surface and the relative importance of diffusion-based shape selectivity in catalysis. So, the external surface of zeoKte Beta, often in the form of nanosized crystallites, is crucial for catalysis. Some topologies like MWW (MCM-22) show structural features that result in the formation of typical half-cages at the external surface. Such confinement effects for molecules can also be the result of delamination of specific structures, viz. ITQ-2 [20]. 

Tin dioxide, an n-type semiconductor with a wide bandgap (3.6 eV at 300 K), has been widely studied as a sensor, a (photo)electrode material and in oxidation reactions for depollution. The performance of tin(iv) oxide is closely linked to structural features, such as nanosized crystallites, surface-to-volume ratio and surface electronic properties. The incentive for carbon-dioxide transformation into value-added products led to examination of the electroreduction of carbon dioxide at different cathodes. It has been recognised that the faradic yield and selectivity to carbon monoxide, methane, methanol, and formic acid rely upon the nature of the cathode and pH. ° Tin(iv) oxide, as cathode, was found to be selective in formate formation at pH = 10.2 with a faradic yield of 67%, whereas copper is selective for methane and ethene, and gold and silver for carbon monoxide. Nano-tin(iv) oxide has been shown to be active and selective in the carboigrlation of methanol to dimethyl carbonate at 150 °C and 20 MPa pressure. The catalyst was recyclable and its activity and selectivity compare with that of soluble organotins (see Section 21.5). 

Nanostructured materials have attracted great interest for many different applications, due to their unusual or enhanced properties compared with bulk materials [9-12]. An example of enhanced property of nanomaterials producing ad-value is the ionic conductivity. Therefore, the investigation of the nanostructured solid conductors, also known as nanoionics, has recently become one of the hottest fields of research. These nanomaterials can be used for advanced energy conversion and storage applications, such as SOFCs. Various synthetic routes, such as thermal evaporation [13], wet chemical processes including coprecipitation [14], the modified sol-gel method [15], the hydrothermal process [11,16], mixing freeze-dried precursors [17], or the combustion [18], have already been developed to produce solid electrolytes composed from nanosized crystallites. 

Our goal was to develop hands-on activities and laboratory experiments that could be adapted for delivery to the different constituent groups (2). Semiconductor quantum dots were chosen as the intellectual focus of the new instructional materials. More specifically, colloidal quantum dots, formed from cadmium selenide (CdSe) and cadmium selenide sulfide (CdSexSi.x) alloys were used to demonstrate the colorful trends that are correlated to the physical size of nanosized crystallites as shown in Figure 1. This visual trend vividly illustrates one of the central themes in nanoscience-/ /7y /ca/properties often depend on... 

Both, Wan et al. and Hutchens et al. prepared HA by using a biomimetic synthesis approach, with a bacterial cellulose hydrogel as template [64, 65]. Their results indicated that spherical HA particles comprised of nanosized crystallites... 

X-ray diffractograms of the samples (Fig. 13.23) presented only a NiAl204 cubic spinel-type structure (JCPDS 10-0339). All samples showed mean crystallite nanosizes. The crystallite size is strongly dependent on the fuel/oxidant ratio, which increases with increasing urea loading. Nanosized crystallites of NiAl204... 

The NiAl (1 1) catalyst showed a high surface area (186 m /g), similar to that reported in the literature, when using an expensive alkoxide precursor and a very long preparation time by the sol-gel method [96, 100, 111, 114]. The NiAl (1 1) presented also unimodal pore size distribution in the range of 3-6 nm and pore volume of 0.18 mL/g, indicating mesoporous materials. On the contrary, the samples with excess of urea, NiAl (2 1), presented lower surface area (10 mVg) and nonporous material crystallite sizes of 13.2 and 16.6 nm, respectively. Nanosized crystallites of NiAl204 were obtained by Han et al. [98, 113] applying the sonochemical method, but with very poor crystallinity. 

Some synthesis parameters have been proved to be decisive for achieving both the chosen final form and nanosized crystallites. The following part will show the impact of the various synthesis parameters on the phase purity, crystal structure and surface areas of the obtained materials. 

All the prepared materials exhibit very high surface area with nanosized crystallites as opposed to compounds prepared by conventional methods. 

The absorption cross-section was found to be higher in the glass ceramic than in the single-crystal. In the nanosized crystallites, some Ln ions are close to the interface with the oxide matrix and are likely to have high oscillator strength [64]. 

The shorter lifetime in the glass ceramic is probably due to the effect of the oxide matrix incorporating the nanoparticles. Indeed, several studies have proved that the oxide glassy matrix interacted with the rare-earth ions situated inside the nanosized crystallites and influenced their spectroscopic properties [65, 66]. Indeed, those Er ions close to the nanocrystallite/glass interface are in distorted sites. As the distortions lower the symmetry, this could result in an increase in the electric dipole transition probability and consequently decrease the radiative lifetime. Moreover, those Er " " ions close to the surface of the crystallites can be sensitive to the presence of oxide ions in their coordination polyhedron, inducing multiphonon nonradiative contribution to the Er " " de-excitation and lowering the lifetime. 

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