Individual Metallic Nanoparticles

The results discussed in Sections 3.1-3.3 turned out as very valuable with respect to the knowledge about the transition from bulk metal to molecule. The most important method, however, to gain direct information from individual metal nanoparticles on their inner electronic life is the tunnelling spectroscopy. The method is based on the single-electron tunnelling (SET) through an intermediate island between two metal electrodes as is indicated in Figure 10. 

Figure 5.25 (a) General scheme of the STM configuration for the investigation of SET on individual metal nanoparticles ... 

Pt Clg], and Cu, respectively, resulting in the individual metal nanoparticles, the size and morphology, and the relative ratio of the crystalline facets of which are unique and different from those of the corresponding nanoparticles electrodeposited conventionally. Some factors controlling the disproportionations and their mechanisms are also discussed. A disproportionation reaction-driven electroless deposition in RTILs is expected as a promising procedure to develop novel nanostructures such as electrocatalysts. 

Thiols and gold surfaces are ideal partners to write any kind of structure in nanometre dimensions. Artificial patterns of appropriately functionalized molecules can be used to bind metal nanoparticles specifically. Figure 25 gives an impression of the individual steps leading to a distinct cluster arrangement. 

The examples are shown in Figure 9.1.10, which gives x-ray diffractograms of three types of physical mixtures of PVP-stabilized Pd, Pt, and Au monometallic nanoparticles, and the corresponding PVP-stabilized bimetallic nanoparticles (53). The diffraction patterns of the physical mixtures are consistent with the sum of two individual patterns, and are clearly different from those of the bimetallic nanoparticles, which have two broader peaks, indicating that several interatomic lengths exist in a single particle. By XRD one can easily understand if the obtained multi-metallic nanoparticles have an alloy structure or are simple physical mixtures of monometallic particles. 

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. 

With respect to the Pd/Al203 model catalysts described below, STM was used to examine the structure of the AI2O3 support and the nucleation and growth of metal deposits (e.g.. References (34,63,73,101,215) and references cited therein), providing information about the size, shape, and height of palladium nanoparticles. In some cases, even atomically resolved images of individual palladium nanoparticles were obtained (206). 

The excitation of the surface plasmon effect also induces strongly enhanced fluorescence properties of gold nanoparticles due to the enhanconent in the radiative rate of the inter-band electronic transitions relative to that in bulk metals. Metal nanoparticles, especially gold nanorods exhibit enhanced two-photon luminescence (TPL) and multi-photon luminescoice (MPL) [7, 8]. Strongly-enhanced TPL has been observed from individual particles [9, 10] and particle solutions [11] under femtosecond NIR laser excitation. This observation raises the possibility of nonlinear optical imaging in the NIR region, where water and biomolecules have... 

The self-assembly technique has attracted much attention since they were observed by Decher in 1991 [49]. Self-assembly is the fundamental principle that provides the precise control of the resulting assemblies and the thickness of an individual layer on the nanometer scale by variation in the bulk concentration of the metal colloids suspension, deposition time, pH, and transport conditions [50]. Recently, the functionalization of metal nanoparticles has opened up new opportunities for the construction of nanostructured self-assembly films to fabricate novel SERS-active Ag substrates. 

The presence of zero-valence palladium and platinum in nanocomposites also has been examined by the absorption spectra. The wide bands of 200-330 nm due to these metal nanoparticles gradually drop down into the long-wavelength range (Fig. lb). These spectra, unlike silver and gold ones, demonstrate the lack of the individual bands for plasmons absorption that may be considered as a consequence of less degree of freedom of electrons in Pd and Pt [5]. 

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