Nanosized spectra

When the silver nanocrystals are organized in a 2D superlattice, the plasmon peak is shifted toward an energy lower than that obtained in solution (Fig. 6). The covered support is washed with hexane, and the nanoparticles are dispersed again in the solvent. The absorption spectrum of the latter solution is similar to that used to cover the support (free particles in hexane). This clearly indicates that the shift in the absorption spectrum of nanosized silver particles is due to their self-organization on the support. The bandwidth of the plasmon peak (1.3 eV) obtained after deposition is larger than that in solution (0.9 eV). This can be attributed to a change in the dielectric constant of the composite medium. Similar behavior is observed for various nanocrystal sizes (from 3 to 8 nm). 

ZnO photocatalyst can also be coupled with other materials in order to improve its chemical and physical properties [183] and photocatalytic activity [184]. Nanosized ZnO was immobilized on aluminum foil for the degradation of phenol [185]. Lanthanum and ZnO were combined to degrade 2,4,6-trichlorophenol [186]. Compared with Ti02 nanomaterial, ZnO nanomaterial generally absorbs a significant amount of the solar spectrum in the visible range therefore, ZnO nanomaterials were combined with Ti02 nanomaterials used as a photocatalyst [187]. 

ESR is known to be a very sensitive tool and can therefore be used in studying structural features of nanosized semiconductor particles doped with paramagnetic metal ions. In many studies vanadium impurities inside the Ti02 matrix or on the particle s surface were used as dopants. Moreover, V4+ ions are very convenient ESR probes since the 51V nuclei have a large magnetic moment leading to informative hyperfine structures (S = 1/2 / = 7/2). At low vanadium concentration, the EPR spectrum has well resolved sharp lines (Fig. 8.10) allowing precise measurement of spin-Hamiltonian parameters. 

Many techniques for the preparation of nanosized materials (sol-gel, thermal treatment of polymeric precursor, electrochemical deposition, atomic layer deposition [ALD], etc.) lead to amorphous or low-crystallinity compounds by quenching of a liquid-state local structure or a very disordered state. At a given temperature, two phenomena can be at the origin of the broadening of the Raman spectrum (1) the loss of periodicity because of the large contribution of surface atoms, and (2) a low crystallinity, that is to say, short-range disorder or bond distortion, hi many cases the exact origin is not obvious and a comparison must be made with TEM. ... 

Another III-V semiconductor was prepared by Li and coworkers [143]. A room temperature sonochemical method for the preparation of GaSb nanoparticles using less hazardous Ga and antimony chloride (SbClj) as the precursors has been described. TEM and SAED results show that the as-prepared solid consists of nanosized GaSb crystals with sizes in the 20-30 run range. The photoacoustic spectrum result reveals that the GaSb nanopartides have a direct band gap of about 1.21 eV. On the basis of the control experiments and the extreme conditions produced by ultrasound, an ultrasound-assisted in situ reduction/combination mechanism has been proposed to explain the reaction. 

Other spectroscopic techniques that have been used with electrochemistry to probe nanoparticles include electronic and vibrational spectroscopies. The spec-troelectrochemistry of nanosized silver particles based on their interaction with planar electrodes has been studied recently [146] using optically transparent thin layer electrodes (OTTLE). Colloidal silver shows a surface plasmon resonance absorption at 400 nm corresponding to 0.15 V vs. Ag/AgCl. This value blue shifts to 392 nm when an Au mesh electrode in the presence of Ag colloid is polarized to —0.6 V (figure 20.12). The absorption spectrum is reported to be quite reproducible and reversible. This indicates that the electron transfer occurs between the colloidal particles and a macroelectrode and vice versa. The kinetics of electron transfer is followed by monitoring the absorbance as a function of time. The use of an OTTLE cell ensures that the absorbance is due to all the particles in the cell between the cell walls and the electrode. The distance over which the silver particles will diffuse has been calculated to be 80 pm in 150 s, using a diffusion coef-... 

The complex anion [HPT C WnC ci]4- has IR bands at 630 and 690 cm"1, assigned as vs, vas (respectively) of the Ti(02) unit.33 The Raman spectra of aqueous sulphuric acid solutions containing titanium indicate the presence of Ti(OI 1)2(804)2(1120)22, 34 The Raman spectrum of a well-characterised TiP207 catalyst has v, of TiOe at 620 cm-1, with v6 at 275/240 cm"1 (together with P03 and P-O-P modes.35 IR and Raman spectroscopy were used to characterise Ti02 nanosized powders formed by TiCl4 laser pyrolysis.36... 

Equations 1 and 5 are fundamental to an understanding of the optical response of nanosized particles, since they directly relate the spectrum of the colloid to the dielectric function of the material. They are valid only for very dilute colloids [5]. For large volume fractions of nanoparticles in glasses, polymers or solutions, dipole coupling becomes important. A more general equation can be derived as follows [6]. 

The band gap of Ti02 does not overlap with the solar spectrum. Dyes that absorb the sunhght must be added to the electrolyte as sensitizers. The photostabiUty of these dyes is a serious problem. The liquid phase is another obstacle and there is now a trend to develop an all solid-state cell. All materials must be optimized, especially the Ti02. So far, nanosized particles have shown the hest performance. ... 

In another attempt to obtain diamond from SiC, no evidence for diamond growth was found by means of XRD and SEM after hydrothermal hydrolysis of P SiC powder at 140 MPa and 800°C [57]. Formation of quartz, a second acicular phase, graphite and maybe nanosized diamond crystals was reported. TEM analysis of whiskers of the second acicular phase revealed lattice spacing of 0.9 nm, a <7-value of 0.56nm and a composition of SiOi 6Co.2- A very sharp band at 1330cm was found in a Raman spectrum and assigned to the silicon oxycarbide, SiOi 6C0.2, rather than to diamond. 

Fillers are typically used to enhance specific properties of polymers, and the polymer/ nanocomposites based on nanoclays have gained attention because of their ability to improve the mechanical, thermal, barrier, and fire-retardant properties of polymers [3]. Nanosized fillers have been introduced in a wide spectrum of applications ranging from providing photocatalyst activation and conductivity to improve melting... 

If metal particles become very small, reaching the nanometer-size scale, a color may occur. This is a typical phenomenon of nanometric metals. Actually, optical absorption may result in the ultraviolet or visible part of the spectrum, and this arises from a surface plasmon resonance. This is due to a collective electron plasma oscillation (plasmon) that is coupled to an external transverse electromagnetic field through the particle surface. It is possible to quantitatively relate the absorption coefficient to the wavelength of the exciting radiation by the Mie theory for spherical inclnsions in a dielectric matrix (34). Far-IR Inminescence is another optical phenomenon frequently observed with nanosized metals (46). 

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