Nanoscale particle structures

In any case, it is interesting to note that catalytic efficacy has been observed with nano- or mesoporous gold sponges [99-101, 145] suggesting that neither a discrete particle nor an oxide support is actually a fundamental requirement for catalysis. An alternative mechanism invokes the nanoscale structural effect noted in Section 7.2.2, and proposes that the catalytic effect of nanoscale gold structures is simply due to the presence of a large proportion of lowly-coordinated surface atoms, which would have their own, local electronic configurations suitable for the reaction to be catalyzed [34, 49,146] A recent and readily available study by Hvolbaek et al. [4] summarizes the support for this alternate view. 

Janssens, T. V. Carlsson, A. Puig-Molina, A. Clausen, B. S., Relation between nanoscale Au particle structure and activity for CO oxidation on supported gold catalysts, J. Catal. 2006, 240(2), 108-113... 

The way in which the iron core in ferritin might build up and the structure of the mineral and its properties have been considered by many researchers over the years and yet there are still many questions that remain to be answered satisfactorily. From one viewpoint the subject belongs in the area of biomineralization, from a different standpoint the nanoscale properties have been of interest, and a third important area of research concerns the health aspects of iron storage and homeostasis. For this latter field the problems of too much or too little are to the fore, with iron overload disease a serious problem in much of Africa and the Middle East while in the Western world iron deficiency is more likely to be a problem. A key aspect to such health problems concerns the response of the organism to local iron levels and is regulated in healthy subjects by an iron response element (IRE) which also seems to involve metalloproteins within the so-called iron response protein. However, this has but little bearing on coordination chemistry aspects of ferritins that we are considering here whereas the chemical questions behind the mineralization processes and the measurement and interpretation of the physical properties of such nanoscale particles are of intense interest. It turns out to be helpful to consider these two aspects in tandem, as one tends to inform the other. 

Chemical properties. Increased surface area increases the chemical activity of a material. For example, a metal in bulk form may not be a catalyst the same metal in nanoscale particles may be an excellent catalyst. Important research measures pH, oxidation and reduction characteristics, and surface properties. An important concern is how nanostructures can change the chemical mechanisms of such key processes as hydrolysis and catalytic responses as well as differing hydrophobic, hydrophilic, or amphipathic surface properties. The atomic structures of high-energy surface sites and various types of defect sites on nanocrystals are needed, as well as their effect on reactivity. An initial priority is to gain exploitable knowledge of the physical chemistry of various nanoparticle surfaces. 

Microemulsions and most surfactants in dilute solutions and dispersions self-assemble into a variety of microstructures spherical or wormlike micelles, swollen micelles, vesicles, and liposomes. Such systems are of biological and technological importance, e.g., in detergency, drug delivery, catalysis, enhanced oil recovery, flammability control, and nanoscale particle production. The macroscopic properties—rheology, surface tension, and conductivity—of these systems depend on their microstructure. As these microstructures are small (1-1000 nm) and sometimes several microstructures can coexist in the same solution, it is difficult to determine their structure. Conventional techniques like radiation scattering, although useful, provide only indirect evidence of microstructures, and the structures deduced are model-dependent. 

Tadaki, T. Koreeda, A. Nakata, Y. Kinoshita, T. Structure of Cu-Au alloy nanoscale particles and the phase transformation. Staf. Rev. and Lett. 1996,3,65-69. 

The drawing shows the crystal structure for bimetallic FePt. Nanoscale particles of this material are magnetic, and hold promise for extremely high density information storage devices. 

A reduction of the required energy could be reached by the incorporation of conductive fillers such as heat conductive ceramics, carbon black and carbon nanotubes [103-105] as these materials allowed a better heat distribution between the heat source and the shape-memory devices. At the same time the incorporation of particles influenced the mechanical properties increased stiffness and recoverable strain levels could be reached by the incorporation of microscale particles [106, 107], while the usage of nanoscale particles enhanced stiffness and recoverable strain levels even more [108, 109]. When nanoscale particles are used to improve the photothermal effect and to enhance the mechanical properties, the molecular structure of the particles has to be considered. An inconsistent behavior in mechanical properties was observed by the reinforcement of polyesterurethanes with carbon nanotubes or carbon black or silicon carbide of similar size [3, 110]. While carbon black reinforced materials showed limited Ri around 25-30%, in carbon-nanotube reinforced polymers shape-recovery stresses increased and R s of almost 100% could be determined [110]. A synergism between the anisotropic carbon nanotubes and the crystallizing polyurethane switching segments was proposed as a possible... 

Figure 15.3 shows micron-scale particles are easier to press than nanoscale particles and a typical consolidated pellet of A1 -I- PTFE pressed to 50% TMD is shown. Compaction can alter particle geometry from spherical to elliptical and create imperfections, cracks, and fractures within the particle structure. Highly compact pellets can exhibit unique combustion behaviors because the reaction mechanism associated with pristine particles is affected when the particle exhibits physical deformations [22]. 

Nanoscale particles can be classified in two ways, i.e. based on (i) structure and (ii) nature of nanoparticles. Structurally, they can be further distinguished as 1-D, 2-D and 3-D nanoparticles where 1-D, 2-D and 3-D indicate (Mie-dimensi(Mial, two-dimensional and three-dimensional nanoparticles. Practically speaking, a... 

Many complex fluids exhibit a yield stress, in which the response is solidlike below a critical stress level and fluidlflce above. Foods and consumer products often have a yield stress, as do some block and graft copolymer melts. Polymer melts with dispersed colloidal or nanoscale particles above a concentration sufficient to form a connected structure are hkely to exhibit a yield stress, and such materials are of growing interest with the development of methods for the dispersal of nanoscale particles hke exfoliated clays and carbon nanotubes. 

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