Nanoparticles, metal

Through the study of nanoporous Pd films described in Chap. 4 (Vol. 1), it was demonstrated that the detection limit and response time could be improved in nanoporous structures with increased sirface area and decreased distance for bulk diffusion. Taking into account mentioned above, one can conclude that films from metal nanoparticles and metal nanowires would be ideal structures for fast detection of low gas concentrations. Experiment has shown that this assumption is valid and noble metal nanoparticles can be successfully incorporated into gas sensors. The selection of noble metals such as Au and Pt for gas sensor fabrication is based on their chemical inertness (Dovgolevsky et al. 2009). It was established that, compared to conventional metal oxide chemiresistors, MNP-based devices have the advantage that they can be operated at room temperature or slightly above, which enables easy device integration and low-power operation (Joseph et al. 2008 Saha et al. 2012). 

Korotcenkov, Handbook of Gas Sensor Materials, Integrated Analytical Systems, 

By now, metal NPs of Au, Ag, Pd, Pt, Cu, Co, and Ni ean be easily synthesized, and many different types are commereially available. NPs can be synthesized either by physical methods such as vapor deposition and laser ablation or by chemical methods sueh as metal salt reduction or micelles (De Jongh 1994 Schmid 1994 Braunstein et al. 1999 Brust and Kiely 2002 Matolin et al. 1990 Masala and Seshadri 

Colloids of alloys have been made by the chemical reduction of the appropriate salt mixture in the solution phase. Thus, Ag-Pd and Cu-Pd colloids of varying composition have been prepared by alcohol reduction of mixtures of silver nitrate or copper oxide with palladium oxide (Vasan and Rao 1995). Fe-Pt alloy nanocrystals have been made by thermal decomposition of the Fe and Pt acetylac-etonates in high-boiling organic solvents (Sun et al. 2000). Au-Ag alloy nanocrystals have been made by co-reduction of silver nitrate and chloroauric acid with sodium borohydride (Sandhyarani et al. 2000 He et al. 2002). 

Experimental studies have shown that the above-mentioned features of metal nanoparticles really may be applied in sensor technology, and, by exploiting these nanoscale properties, a highly efficient gas sensor can be designed and fabricated (Shipway et al. 1999 Ahn et al. 2004 Raschke et al. 2004 Drake et al. 2007 Dovgolevsky et al. 2009 Saha et al. 2012). In particular, metal nanoparticles can be incorporated in optical hydrogen and VOCs sensors (Cioffi et al. 2002 Ahn et al. 2004 Filenko et al. 2005). 

The use of self-assembled QDs has also been discussed as the basis of an all-optical storage device in which excitons are optically generated and the electrons and holes are stored separately in coupled QD pairs [133]. By applying an electric field, the electron and hole could be forced to recombine so as to generate a photon which would provide an optical read-out. 

Colloidal QDs have also been used in the development of light-emitting diodes [134, 135], by their incorporation into a thin film of conducting polymer. Moreover, colloidal QDs have also been used in the fabrication of photovoltaic devices [136, 137]. 

Chemically synthesized QDs fluoresce in the visible range with a wavelength that is tunable by the size of the colloids. The possibility to control the onset of absorption 

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Emory S R, Haskins W E and Nie S 1998 Direct observation of size-dependent optical enhancement in single metal nanoparticles J. Am. Chem. Soc. 120 8009-10... 

Wlrile tire Bms fonnula can be used to locate tire spectral position of tire excitonic state, tliere is no equivalent a priori description of the spectral widtli of tliis state. These bandwidtlis have been attributed to a combination of effects, including inlromogeneous broadening arising from size dispersion, optical dephasing from exciton-surface and exciton-phonon scattering, and fast lifetimes resulting from surface localization 1167, 168, 170, 1711. Due to tire complex nature of tliese line shapes, tliere have been few quantitative calculations of absorjDtion spectra. This situation is in contrast witli tliat of metal nanoparticles, where a more quantitative level of prediction is possible. 

The optical properties of metal nanoparticles have traditionally relied on Mie tlieory, a purely classical electromagnetic scattering tlieory for particles witli known dielectrics [172]. For particles whose size is comparable to or larger tlian tire wavelengtli of the incident radiation, tliis calculation is ratlier cumbersome. However, if tire scatterers are smaller tlian -10% of tire wavelengtli, as in nearly all nanocrystals, tire lowest-order tenn of Mie tlieory is sufficient to describe tire absorjDtion and scattering of radiation. In tliis limit, tire absorjDtion is detennined solely by tire frequency-dependent dielectric function of tire metal particles and the dielectric of tire background matrix in which tliey are... 

Klar T ef a/1998 Surface-plasmon resonances in single metallic nanoparticles Phys. Rev. Lett. 80 4249... 

The common underlying principle was shown in Figure 11.2. The electrochemical potential of electrons jl e(=Ep, the Fermi level) in the metal catalyst is fixed at that of the Fermi level of the support.37 This is valid both for electrochemically promoted model catalysts (left) and for seminconducting or ion-conducting-supported metal nanoparticles (right). 

CuNPs) in Fig. 7 shows the monodisperse and uniformly distributed spherical particles of 10+5 nm diameter. The solution containing nanoparticles of silver was found to be transparent and stable for 6 months with no significant change in the surface plasmon and average particle size. However, in the absence of starch, the nanoparticles formed were observed to be immediately aggregated into black precipitate. The hydroxyl groups of the starch polymer act as passivation contacts for the stabilization of the metallic nanoparticles in the aqueous solution. The method can be extended for synthesis of various other metallic and bimetallic particles as well. 

Noble metal nanoparticles dispersed in insulating matrices have attracted the interest of many researchers fromboth applied and theoretical points of view [34]. The incorporation of metallic nanoparticles into easily processable polymer matrices offers a pathway for better exploitation of their characteristic optical, electronic and catalytic properties. On the other hand, the host polymers can influence the growth and spatial arrangement of the nanoparticles during the in situ synthesis, which makes them convenient templates for the preparation of nanoparticles of different morphologies. Furthermore, by selecting the polymer with certain favorable properties such as biocompatibiHty [35], conductivity [36] or photoluminescence [37], it is possible to obtain the nanocomposite materials for various technological purposes. 

Interestingly, when the particle size of metal nanoparticles becomes less than 2 nm, terraces become so small that they carmot anymore support the presence of step-edge site metal atom configurations. This can be observed from Figure 1.15, which shows a cubo-octahedron just large enough to support a step-edge site. 

Controlled cutting and opening of closed carbon systems Direct applications of CNTs (requires 20-100 nm in length) Inner filling and impregnation of CNTs with metal nanoparticles and complexes... 

A second option is to apply the membrane on the particle level (millimeter scale) by coating catalyst particles with a selective layer. As a third option, application at the microlevel (submicrometer scale) is distinguished. This option encompasses, for example, zeolite-coated crystals or active clusters (e.g., metal nanoparticles). Advantages of the latter two ways of application are that there are no sealing issues, it is easy to scale-up, the membrane area is large per unit volume, and, if there is a defect in the membrane, this will have a very limited effect on the overall reactor performance. Because of these advantages, it is believed that using a zeolite... 

Apart from the above described core-shell catalysts, it is also possible to coat active phases other than zeolite crystals, like metal nanoparticles, as demonstrated by van der Puil et al. [46]. More examples of applications on the micro level are given in Section 10.5, where microreactors and sensor apphcations are discussed. 

B. Langmuir-Blodgett Films Containing Metallic Nanoparticles... 

Formation of Metallic Nanoparticles Beneath Langmuir Monolayers... 

Production of Metallic Nanoparticles Inside Langmuir-Blodgett Films... 

Although random and irregular type GaN nanorods have been prepared by using transition metal nanoparticles, such as Ni, Co, and Fe as catalysts and carbon nanotubes as the template, the preparation of controllable regular array of strai t GaN nanorods has not yet been reported. Fabrication of well-ordered nano-structures with high density is very important for the application of nano-structures to practical devices. 

Crystallization of CaCOj with Metal Nanoparticles as Spherical... 

Porter LA, Choi HC, Ribbe AE, Buriak JM (2002) Controlled electroless deposition of noble metal nanoparticle films on Germanium surfaces. Nano Lett 2 1067-1071... 

The primary goal of the researchers has been to produce Q-dots possessing all of the attributes of the Q-dots prepared using liquid-phase synthetic methods (that is adjustability of the nanocrystal identity and diameter and size monodispersity) and also the technological utility of Q-dots prepared by MBE (specifically, the deposition of nanocrystals with a defined orientation and an electrical output contact). It was shown that the E/C-synthesized 5-CuI and CdS Q-dots were indeed epitaxial with narrow size distribution and strong photoluminescence tunable by the particle size. Qne of the advantages of the E/C method is that it can be made size selective. The key point is that the size as well as the size dispersion of product nanoparticles are directed actually by the corresponding properties of the metal nanoparticles therefore the first deposition step assumes special importance. 

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. 

The most intensive development of the nanoparticle area concerns the synthesis of metal particles for applications in physics or in micro/nano-electronics generally. Besides the use of physical techniques such as atom evaporation, synthetic techniques based on salt reduction or compound precipitation (oxides, sulfides, selenides, etc.) have been developed, and associated, in general, to a kinetic control of the reaction using high temperatures, slow addition of reactants, or use of micelles as nanoreactors [15-20]. Organometallic compounds have also previously been used as material precursors in high temperature decomposition processes, for example in chemical vapor deposition [21]. Metal carbonyls have been widely used as precursors of metals either in the gas phase (OMCVD for the deposition of films or nanoparticles) or in solution for the synthesis after thermal treatment [22], UV irradiation or sonolysis [23,24] of fine powders or metal nanoparticles. 

Scheme 1 Illustration of the general synthetic method followed in our group for the synthesis of metal nanoparticles i decomposition of the precimsor, nucleation ii first growth process in ripening or coalescence leading to size and shape controlled objects through addition of stabilizers which prevent the full precipitation of the metal (iv)...

The coordination of ligands at the surface of metal nanoparticles has to influence the reactivity of these particles. However, only a few examples of asymmetric heterogeneous catalysis have been reported, the most popular ones using a platinum cinchonidine system [65,66]. In order to demonstrate the directing effect of asymmetric ligands, we have studied their coordination on ruthenium, palladium, and platinum nanoparticles and the influence of their presence on selected catalytic transformations. 

In summary, we found that Ugands indeed coordinate at the surface of nanoparticles and that they can be firmly or loosely attached to this surface according to their chemical nature. Furthermore, the hgands influence the reactivity of the metal nanoparticles. This is important in catalysis but, as we will see later in this paper, is also important for the control of the growth of metal nanoparticles of defined size and shape. 

The physical properties of metal nanoparticles are very size-dependent. This is clear for their magnetic properties, for which the shape anisotropy term is very important. This is also true for the optical properties of nanoparticles displaying plasmon bands in the visible range (Cu, Ag, Au) and for 111-V... 

All these elements suggest that there is a strong potential for organometalhc chemists to enter this research area concerning the synthesis and properties of metal nanoparticles. This should lead to impressive developments in the field of surface organometalhc chemistry in the future. 

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