Nanocrystalline morphology

Recently reported meso- and macroscale self-assembly approaches conducted, respectively, in the presence of surfactant mesophases [134-136] and colloidal sphere arrays [137] are highly promising for the molecular engineering of novel catalytic mixed metal oxides. These novel methods offer the possibility to control surface and bulk chemistry (e.g. the V oxidation state and P/V ratios), wall nature (i.e. amorphous or nanocrystalline), morphology, pore structures and surface areas of mixed metal oxides. Furthermore, these novel catalysts represent well-defined model systems that are expected to lead to new insights into the nature of the active and selective surface sites and the mechanism of n-butane oxidation. In this section, we describe several promising synthesis approaches to VPO catalysts, such as the self-assembly of mesostructured VPO phases, the synthesis of macroporous VPO phases, intercalation and pillaring of layered VPO phases and other methods. 

The DSSC is the most representative prototype of third-generation PV devices, in which a sensitizer (as light-absorbing material) and a wide band gap semiconductor (with nanocrystalline morphology) are coupled to... 

The nanocrystalline morphology is of crucial importance for the efficient operation of the device. A system based on light harvesting by a molecular absorber attached to a flat semiconductor surface faces two dilemmas. 

It is known that ceria does not react readily with aluminium or boron oxides but Zarur et al were able to deposit up to 25 wt% ceria on boron hexaaluminate (BHA) nanoparticles without forming a separate ceria domain. The ceria deposited on BHA preserved the nanocrystalline morphology up to a calcination temperature of 1300 °G. The GeOg-BHA nanoparticles allowed light-off of a stream of 1 vol% GH4 in air at a low temperature of 400 °G, compared to 540 °G and 690 °G required for Mn- and Go-substituted BHA systems, and attained full GH4 conversion by 600 °G. In addition, various ceria-based materials have been used for VOG combustion with different light-off temperatures as shown in Table 8.2. 

Choi, S. Y. Mamak, M. Coombs, N. Chopra, N. Ozin, G. A. 2004. Thermally stable two-dimensional hexagonal mesoporous nanocrystalline anatase, meso-nc-Ti02 Bulk and crack-free thin film morphologies. Adv. Fund. Mater. 14 335-344. 

The sensitive layers that have been used throughout this book consist of nanocrystalline tin-oxide thick films. The resistance change is the result of a multitude of reactions taking place at the surface and in the bulk. This resistance change depends also on the morphology of the sensitive layer and the contact-electrode geometry. Due... 

Nanocrystalline aluminum can be made in the employed ionic liquid without additives, see Chapter 8. The SEM micrograph of Figure 12.9 shows the surface morphology of a deposited aluminum layer obtained potentiostatically on mild steel at —0.75 V (vs. Al) for 2 h in the upper phase ofthe biphasic mixture [Pyi TfiN M AICI3 at 100 °C. Prior to Al electrodeposition, the electrode was anodically polarized at a potential of 1V (vs. Al) for 2 min. The deposited layer is dense, shining and adherent to the substrate with crystallites in the nanosize regime. 

J. Klabunde, Carbon Dispersion and Morphology in Carbon-Coated Nanocrystalline MgO, Langmuir 19(24), 10426-10433 (2003). 

Hydrothermal reactions typically produce nanometer-sized particles that can be quenched to form a nanoparticle powder or cross-linked to produce nanocrystalline stmctures (Feng and Xu, 2001). Hydrothermal conditions allow for reduction in solubilities of ionic materials and thus more rapid nucleation and increased ion mobility, resulting in faster growth. Via judicious choice of the hydrothermal conditions, a measure of control can be exerted over the size and morphology of the materials. As mentioned earlier, the viscosity and ionic strength of solvents is a function of the temperature and pressure at which the reaction is carried out. Other experimental parameters, such as the precursor material and the pH, have... 

The aerogel-prepared metal oxide nanoparticles constitute a new class of porous inorganic materials because of their unique morphological features such as crystal shape, pore structure, high pore volume, and surface areas. Also, it is possible to load catalytic metals such as Fe or Cu at very high dispersions on these oxide supports and hence the nanocrystalline oxide materials can also function as unusual catalyst supports. Furthermore, these oxides can be tailored for desired Lewis base/Lewis acid strengths by incorporation of thin layers of other oxide materials or by preparation of mixed metal oxides. 

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