Nanoparticles size-dependence

The method of laser ablation from a metal has been used to prepare nanoparticles dispersed in a solution [202-207]. Recently, colloidal gold nanoparticles in water having an average diameter of 5.5 nm were prepared by laser ablation at 1064 nm (800 mJ/cm ) from a gold metal plate [208]. The final nanoparticle size depends on the laser fluence and the stabilizer concentration (Fig. 17). 

Yoshida et al. recently disclosed an alternative method that allowed them to produce stable suspensions of gold nanoparticles (1-2 nm in diameter) in nematic liquid crystals [315]. They used a simple sputter deposition process, which allowed them to prepare thin liquid crystal films of well-dispersed gold nanoparticles in both 5CB and E47 (available from Merck) with a nanoparticle size depending on the used nematic liquid crystal. Unfortunately, the authors did not provide any details on whether the nanoparticles were capped with a ligand or bare, non-coated particles, which makes it difficult to assess and compare the reported thermal as well as electro-optic data. However, very similar effects were found as a result of nanoparticle doping, including lower nematic-to-isotropic phase transition temperatures compared to the used pure nematics as well as 10% lower threshold voltages at nanoparticle concentrations below 1 wt% [315]. 

Such polymer-immobilized tt-allylic complexes undergoing thermal decomposition can form nanoparticles in polymer [68], during which the thermolysis can be carried out even without a solvent (for example, by heating of the formed metallopolymer films using an infrared lamp). The nanoparticle sizes depend on many factors such as the characteristics of the used dispersant polymer, its molecular mass (the optimal value is M = 100,000), and the nature... 

Redel E, Thomann R, Janiak C (2008) First correlation of nanoparticle size-dependent formation with the ionic liquid anion molecular volume. Inorg Chem 47 14—16... 

Nanoparticle (size dependant tumor interstitium accumulation)... 

Semiconductor-Liquid Junction From Fundamen-tais to Soiar Fuei Generating Structures, Fig. 29 Schematic of a photoelectrocatalytic structure based on Foerster transfer in quantum size nanoparticles size-dependent energy gaps = 2, 3) and Foerster... 

Carlson, C., Hussain, S.M., Schrand, A.M., et al. Unique cellular interaction of silver nanoparticles Size-dependent generation of reactive oxygen species. J. Phys. Chem. B. 112 no. 43 (2008) 13608-13619. 

V.N. Singh, B.R. Mehta, Nanoparticle size-dependent lowering of temperature for phase transition from ln(OH)(3) to ln203. J. Nanosci. Nanotechnol. 5(3), 431-435 (2005)... 

Equation (Ic) accounts for the temperature-dependent and nanoparticle size-dependent interaction effects in the all-polymer nanocomposite. A prefactor (r2/ Rp) is introduced in (Ic) since the number of surface contacts with monomers 2 for each nanoparticle becomes smaller as one increases the nanoparticle radius [14]. This ratio tends toward unity inasmuch Rp— rl as it should, rl being the radius of a repeat unit 1. 

Solvent phase is generally an organic solvent, which can dissolve the polymer completely. It should also be water miscible, i.e., polar so that when the polymer solution is added to the non-solvent, it can rapidly interact, leading to the formation of nanoparticles. Solvent or non-solvent is generally opposite in nature to the polymer. Water is the most common non-solvent used in nanoprecipitation [6]. The choice of organic solvent also depends upon the requirement of the process and the nanoparticles that need to be produced. Additionally, solvent phase is not the only parameter responsible for the final size of the nanoparticles. Since nanoprecipitation is governed by the diffusion of the solvent into the non-solvent, the nanoparticle size depends on the combination of the solvent and non-solvent used. One solvent used in this process is acetone, owing to its polar molecular structure. It can dissolve the polymer and is soluble in water. Other solvents, such as acetonitrile, dimethyl sulfoxide, tetrahydrofuran, methanol, ethanol, isopropanol, dimethyl for-mamide, and ethyl lactate are also used to produce nanoparticles [5, 7, 12, 26]. 

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... 

The definition above is a particularly restrictive description of a nanocrystal, and necessarily limits die focus of diis brief review to studies of nanocrystals which are of relevance to chemical physics. Many nanoparticles, particularly oxides, prepared dirough die sol-gel niediod are not included in diis discussion as dieir internal stmcture is amorjihous and hydrated. Neverdieless, diey are important nanoniaterials several textbooks deal widi dieir syndiesis and properties [4, 5]. The material science community has also contributed to die general area of nanocrystals however, for most of dieir applications it is not necessary to prepare fully isolated nanocrystals widi well defined surface chemistry. A good discussion of die goals and progress can be found in references [6, 7, 8 and 9]. Finally, diere is a rich history in gas-phase chemical physics of die study of clusters and size-dependent evaluations of dieir behaviour. This topic is not addressed here, but covered instead in chapter C1.1, Clusters and nanoscale stmctures, in diis same volume. 

Hyun et al. [345] prepared PbS Q-dots in a suspension and tethered them to Ti02 nanoparticles with a bifunctional thiol-carboxyl linker molecule. Strong size dependence due to quantum confinement was inferred from cyclic voltammetry measurements, for the electron affinity and ionization potential of the attached Q-dots. On the basis of the measured energy levels, the authors claimed that pho-toexcited electrons should transfer efficiently from PbS into T1O2 only for dot diameters below 4.3 nm. Continuous-wave fluorescence spectra and fluorescence transients of the PbS/Ti02 assembly were consistent with electron transfer from small Q-dots. The measured charge transfer time was surprisingly slow ( 100 ns). Implications of this fact for future photovoltaics were discussed, while initial results from as-fabricated sensitized solar cells were presented. 

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... 

Iridium and rhodium nanoparticles have also been studied in the hydrogenation of various aromatic compoimds. In all cases, total conversions were not observed in BMI PF6. TOFs based on mol of cyclohexane formed were 44 h for toluene hydrogenation with Ir (0) and 24 h and 5 h for p-xylene reduction with lr(0) or Rh(0) nanoparticles, respectively. The cis-1,4-dimethylcyclohexane is the major product and the cisitrans ratio depends on the nature of the metal 5 1 for lr(0) and 2 1 for Rh(0). TEM experiments show a mean diameter of 2.3 nm and 2.1 nm for rhodium and iridium particles, respectively. The same nanoparticle size distribution is observed after catalysis (Fig. 4). 

In Section 2 the general features of the electronic structure of supported metal nanoparticles are reviewed from both experimental and theoretical point of view. Section 3 gives an introduction to sample preparation. In Section 4 the size-dependent electronic properties of silver nanoparticles are presented as an illustrative example, while in Section 5 correlation is sought between the electronic structure and the catalytic properties of gold nanoparticles, with special emphasis on substrate-related issues. 

In the previous Sections (2.1-2.3) we summarized the experimental and computational results concerning on the size-dependent electronic structure of nanoparticles supported by more or less inert (carbon or oxide) and strongly interacting (metallic) substrates. In the following sections the (usually qualitative) models will be discussed in detail, which were developed to interpret the observed data. The emphasis will be placed on systems prepared on inert supports, since - as it was described in Section 2.3 - the behavior of metal adatoms or adlayers on metallic substrates can be understood in terms of charge transfer processes. 

The change of the initial state energy of the silver core levels during size reduction clearly suggests that there should be size dependent changes in the valence band DOS of the Ag nanoparticles, which can be visualized by valence band XPS and UPS measurements. 

The environment (e.g. the substrate) of the nanoparticles is a critical experimental parameter, which should be inert with respect to the nanoparticles. In the case of gold the native Si02 covered Si(l 0 0) seems to be an environment without any influence on the valence band of Au nanoparticles. The chemical and catalytic properties which are probably strongly correlated with the electronic structures of different systems, give another possibility to use and check the size dependent properties of nanoparticles. 

Figure 12. Size dependence of the photoemission data for gold nanoparticles at Fermi level (a), (bi), (62), and (c), respectively. Curve Si02 represents the emission from the support only. (Reprinted from Ref. [171], 2002, with permission from Elsevier.)...

Physical properties of metal nanoparticles show characteristic size-dependent changes as their size increases from the limit of isolated atoms towards macroscopic dimensions. 

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