Tailored size effects

Overbury et al. [101] elucidated the size effects in a family of rather closely related tailor-made Au-Ti02 (P-25) catalysts (i.e., use of the same support, Au loadings, and impurity levels to minimize effect of non size-related factors), where particle size was varied using variations in heat treatment. It was demonstrated that the TOF of CO oxidation at 298 K changes as a function of particle size. Namely, the TOF for 7.2 wt% catalysts was proportional to while that for 4.5 wt%... 

The demonstration in 1990 that porous silicon could emit efficient tunable visible photoluminescence (PL) at room temperature and attributed to quantum-size effects in crystalline silicon (Canham 1990) has induced considerable worldwide research activities in order to (i) identify the various PL bands and their respective properties and emission mechanisms, (ii) optimize the PL efficiency, (iii) optimize the PL stability, and (iv) tailor the PL spectrum (peak wavelength and FWHM). This chapter reviews briefly the specificities of porous silicon PL measurements, the PL of individual silicon nanocrystals from porous silicon, and the PL of porous silicon layers. 

Potential-Programmed Electropolymerization (PPEP). This technique allows compositional modulation over distances of the order of 100 A. Mesoscopic layered structures with high lateral quality are produced by the PPEP method. An improvement of the flatness and uniformity of the layered structure is achieved by careful choice of an appropriate working electrode (silicon single-crystal wafer), monomer (pyrrole/bithiophene or pyrrole/3-methyl-thiophene) and solvent (propylene carbonate). The PPEP method can produce materials having desirable properties tailored by the quantum size effect [703-705]. 

Because of the capacity to tailor select polymer properties by varying the ratio of two or more components, copolymers have found significant commercial appHcation in several product areas. In fiber-spinning, ie, with copolymers such as nylon-6 in nylon-6,6 or the reverse, where the second component is present in low (<10%) concentration, as well as in other comonomers with nylon-6,6 or nylon-6, the copolymers are often used to control the effect of sphemUtes by decreasing their number and probably their size and the rate of crystallization (190). At higher ratios, the semicrystalline polyamides become optically clear, amorphous polymers which find appHcations in packaging and barrier resins markets (191). 

Lead(II) sulfide occurs widely as the black opaque mineral galena, which is the principal ore of lead. The bulk material has a band gap of 0.41 eV, and it is used as a Pb " ion-selective sensor and IR detector. PbS may become suitable for optoelectronic applications upon tailoring its band gap by alloying with II-VI compounds like ZnS or CdS. Importantly, PbS allows strong size-quantization effects due to a high dielectric constant and small effective mass of electrons and holes. It is considered that its band gap energy should be easily modulated from the bulk value to a few electron volts, solely by changing the material s dimensionality. 

Below the outer membrane is a filter, usually composed of an anionic polymer, e.g. based on salicylate. Its precise composition and dimensions (thickness, pore size, amount and type of plasticizer, fillers, etc.) are optimized in order to tailor the diffusion rates of material crossing the filter from the analyte solution toward the working electrode of the sensor. Ideally, some uncharged molecules, such as H2O2, will traverse the filter so fast that, in effect, the filter is invisible to... 

A promising strategy towards stable and catalyticaUy active metal colloids is their preparation inside the core of micelles formed by amphiphilic block copolymers. This strategy offers a number of advantages (i) micelles represent a nano-structured environment which can be exactly tailored by block copolymer synthesis (ii) polymers act as effective steric stabilizer ]36] (iii) metal leaching might be avoided (iv) micelles allow control over particle size, size distribution and particle solubility [37] and (v) micelles are also supposed to effect catalytic activity and selectivity [38]. 

In summary, the examples given above demonstrate that immobilization of metal salts in a block copolymer micellar system followed by a reduction step is a suitable method to synthesize stable colloids with small particle sizes and narrow size distributions. Moreover, such systems are very interesting for catalytic applications because they offer the possibility of designing tailored catalysts for special demands and can be easily tuned by the choice and combination of different polymer block types and lengths, different types of the metal precursor and of the reduction method used. Additional introduction of further functionalities such as charges or chiral groups could make these catalyst systems even more versatile and effective. 

Notably, the use of heteronuclear surface carbonyl species can lead to the preparation of well-defined supported bimetallic entities that can be used as model catalysts to study the promoter effect of a second metal. The close intimacy achieved between the two metals in the surface carbonyl species is related to the structural characteristics and catalytic properties of the final catalyst In the preparation of supported, tailored, multi-component catalysts, the use of metal carbonyl surface species still deserves to be studied to further explore the exciting field of nano-sized entities in catalysis. 

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