Nanoparticles band bending

One of the most distinguishing features of semiconductor nanoparticles for use in photoelectrocatalysis is the absence of band bending at the semiconductor-electrolyte interface, see Fig. 4.2. In contrast to bulk behavior, for a colloidal semiconductor or a semiconductor comprised of a nanociystalline network in contact with an electrolyte the difference in potentials between the center (r = 0) of the particle and a distance r from the center can be expressed [83] ... 

Fig. 4.12 Diagram illustrating space charge layer formation in microcrystalline and nanocrystalline particles in equilibrium in a semiconductor-electrolyte interface. The nanoparticles are almost completely depleted of charge carriers with negligibly small band bending.

The application of semiconductors as substrates in interfacial supramolecular assemblies has been dominated so far by films consisting of nanoparticles. In an attempt to understand the properties of such particles, and in particular, issues such as light-induced charge separation, their electronic properties will be discussed in some detail. Relevant issues in this respect are quantum effects, the size of the band gap, charge transport and band bending. 

Consequently, negligible band bending takes place over nanoparticle dimensions, and the valence and conduction bands in a semiconductor nanoparticle can be considered as flat for all practical purposes an electron injected into the conduction band of a semiconductor nanoparticle is not subjected to any intrinsic energy gradients driving it to or from the surface. In this respect, the manifestations of size reduction are observed much sooner (at much larger size) for semiconductors than for metals. 

The efforts to enhance efficiency in solar cells through use of metal nanoparticles should provide a general guide for the application of plasmonic concepts to water photoelectrolysis, although differences in device design, such as the presence of an electrolyte, suggest that there may be additional criteria. When metal nanoparticles are located at the semiconductor/electrolyte interface, considerations of stability, Fermi level, and band bending are essential. Secondary considerations may include the influence of catalytic effects and the possible formation of trap states at the metal/semiconductor interface. 

Fig. 4.7. SERS spectrum collected from endosomes in the cell line J774 after incubation with gold nanoparticles. It was identified as SERS spectrum of AMP and/or ATP. Band assignments are based on [86] are indicated. Excitation wavelength <3 x 105W/cm2 at 785nm, collection time Is. Abbreviations cps, counts per second v, stretching mode CO, wagging mode 5, bending mode Rib, ribose Pyr, pyrimidine Im, imidazol...

FTIR spectra of Ti02 nanoparticles exhibiting the transmittance band at 1624 cm present in the anatase phase only was assigned to the bending... 

IR spectroscopy was found to be very useful for studying DND because the surface of DND particles is usually saturated by various functional groups, which are covalently bound to them to form a dense molecular shell around a nanoparticle. Typical features of DND IR spectra can be found in Refs. 37,102-108. Most IR spectra of DND in the literature reveal the presence of water absorption bands, which consist of a broad composite band close to 3600 cm- and a somewhat weaker band, near 1620 cm. The first of them derives from the symmetric and asymmetric stretch vibrations of water molecules, and the other, from the OH bend vibrations. The disagreement with 1645 cm the value usually quoted in the literature, is usually assigned to adsorption effects. 

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