Single metal nanoparticles properties

Alloy nanoparticles, exhibit different catalytic, magnetic, optical and other chemical and physics properties to those of single metal nanoparticles. They are potentially useful for applications such as chemical and biochemical sensing, catalysis, optoelectronics devices,... 

The continued development of new single-source molecular precursors should lead to increasingly complex mixed-element oxides with novel properties. Continued work with grafting methods will provide access to novel surface structures that may prove useful for catalytic apphcations. Use of molecular precursors for the generation of metal nanoparticles supported on various oxide supports is another area that shows promise. We expect that the thermolytic molecular precursor methods outlined here will contribute significantly to the development of new generations of advanced materials with tailored properties, and that it will continue to provide access to catalytic materials with improved performance. 

Metallic nanoparticles and single-walled carbon nanotubes (SWCNTs) exhibit nanoscale dimensions comparable with the dimensions of redox proteins. This enables the construction of NP-enzyme or SWCNT-enzyme hybrids that combine the unique conductivity features of the nanoelements with the biocatalytic redox properties of the enzymes, to yield wired bioelectrocatalyts with large electrode surface areas. Indeed, substantial advances in nanobiotechnology were achieved by the integration of redox enzymes with nanoelements and the use of the hybrid systems in different bioelectronic devices.35... 

SWCNTs exhibit unique physical and chemical properties that make them very attractive candidates for the production of new materials. Carbon nanotubes are made by wrapping up single sheets of graphite, known as graphene, upon themselves to form hollow, straw-like structures. Traditionally, SWCNTs have been prepared by electric arc-discharge, laser ablation and chemical vapor deposition (CVD) methods these techniques produce significant quantities of impurities, such as amorphous and graphitic forms of carbon and encapsulated catalytic metal nanoparticles. 

However, this assumption is not necessarily justified. Even for a well-faceted nanoparticle there are a number of nonequivalent adsorption sites. For example, in addition to the low-index facets, the palladium nanoparticle exhibits edges and interface sites as well as defects (steps, kinks) that are not present on a Pd(l 1 1) or Pd(lOO) surface. The overall catalytic performance will depend on the contributions of the various sites, and the activities of these sites may differ strongly from each other. Of course, one can argue that stepped/kinked high-index single-crystal surfaces (Fig. 2) would be better models (64,65), but this approach still does not mimic the complex situation on a metal nanoparticle. For example, the diffusion-coupled interplay of molecules adsorbed on different facets of a nanoparticle (66) or the size-dependent electronic structure of a metal nanoparticle cannot be represented by a single crystal with dimensions of centimeters (67). It is also shown below that some properties are merely determined by the finite size or volume of nanoparticles (68). Consequently, the properties of a metal nanoparticle are not simply a superposition of the properties of its individual surface facets. 

Second, apart from single crystals, nanoparticle model catalysts should be employed to better mimic the complex properties of supported metals. Nevertheless, the metal nanoparticles should still exhibit well-defined surface facets to allow more reliable data interpretation and a comparison with single-crystal results. 

Up to now, a large part of the international scientific attention was devoted to the study of metallic nanoparticles (either single particles or in a colloid ensemble) (3, 14). Milling nanometric apertures in a metallic film is an intuitive way to manufacture new nanophotonics devices that are robust and highly reproducible. Although this concept appears very simple, such apertures exhibit attractive physical properties, such as localization of excitation light, strong isolation fi om emission produced by unbound species, and an increase in apparent absorption and emission yield. The simplicity of the structures and their ease of use should further expand their application towards the real-time detection and identification of a small number of molecules. 

These difficulties have stimulated the development of defined model catalysts better suited for fundamental studies (Fig. 15.2). Single crystals are the most well-defined model systems, and studies of their structure and interaction with gas molecules have explained the elementary steps of catalytic reactions, including surface relaxation/reconstruction, adsorbate bonding, structure sensitivity, defect reactivity, surface dynamics, etc. [2, 5-7]. Single crystals were also modified by overlayers of oxides ( inverse catalysts ) [8], metals, alkali, and carbon (Fig. 15.2). However, macroscopic (cm size) single crystals cannot mimic catalyst properties that are related to nanosized metal particles. The structural difference between a single-crystal surface and supported metal nanoparticles ( 1-10 nm in diameter) is typically referred to as a materials gap. Provided that nanoparticles exhibit only low Miller index facets (such as the cuboctahedral particles in Fig. 15.1 and 15.2), and assuming that the support material is inert, one could assume that the catalytic properties of a... 

---