Photoexcitation, colloidal semiconductors

Bavykin, Dmitry V. is a Ph.D. researcher in the Laboratory of photocatalysis on semiconductors at the Boreskov Institute of Catalysis, Novosibirsk, Russia. The title of his PhD thesis (1998) Luminescent and photocatalytic properties of CdS nanocolloids . Area of his interests is the photophysical-photochemical properties of nanosized sulfide semiconductors, including synthesis of particles with definite size and surface properties, their characterisation the study of the photoexcited states dynamics, relaxation in quantum dots by the luminescence and flash photolysis measurements studies of the interfacial charge transfer from colloidal semiconductor particles by the steady state photolysis, luminescence quenching method. 

Interfacial electron transfer across a solid-liquid junction can be driven by photoexcitation of doped semiconductors as single crystals, as polycrystalline masses, as powders, or as colloids. The band structure in semiconductors (281) makes them useful in photoelectrochemical cells. The principles involved in rendering such materials effective redox catalysts have been discussed extensively (282), and will be treated here only briefly. 

Photoexcitation of n-type semiconductors renders the surface highly activated toward electron transfer reactions. Capture of the photogenerated oxidizing equivalent (hole) by an adsorbed oxidizable organic molecule initiates a redox sequence which ultimately produces unique oxidation products. Furthermore, specific one electron routes can be observed on such irradiated surfaces. The irradiated semiconductor employed as a single crystalline electrode, as an amorphous powder, or as an optically transparent colloid, thus acts as both a reaction template and as a directed electron acceptor. Recent examples from our laboratory will be presented to illustrate the control of oxidative cleavage reactions which can be achieved with these heterogeneous photocatalysts. 

The colloidal or powder particle can be composed of either insulating, semiconductive, or conductive molecules. While only semiconductor particles are likely to be photoactive per se (by virtue of the energy gap between the filled valence band and the vacant conduction band), photoactivity of adsorbates can be mediated at the surface of other solids [18] which are often used themselves, or in conjunction with an irradiated semiconductor, as catalytic sites for alteration of kinetics of dark reactions initiated by photoexcitation. 

In this work the authors summarize their own studies of photoprocesses on CdS colloids with particles of various size. In these studies, attention was given precisely to photocatalytic reactions on CdS, the photocatalytic reactions on TiC>2 were considered concurrently with the reported ones. In most cases photocatalytic reactions on semiconductors are the redox reactions. So of special interest was to study the regularities of reactions of interfacial transfer of photoexcited electron by the pulse photolysis and luminescence quenching methods. Many interesting phenomena were found while studying the model photocatalytic reactions by the method of stationary photolysis, i.e., under the conditions of real photocatalysis. 

The rate of the photobleaching relaxation of ultradispersed CdS, and hence the rate of the electron interfacial transfer from CdS to the surrounded media (finally, to protons yielding the hydrogen) appeared to depend on the size of the colloidal particles (see Fig. 2.10). The photobleaching relaxation rate increases as the size of the CdS semiconductor particles decreases. Such behavior may be caused by the increasing of reductive potential of photoexcited electron with decreasing size of semiconductor nanocolloids. In this case, according to the modern concepts of electron interfacial transfer reaction [19], the rate of electron transfer to the surrounded media should increase. 

To study the regularities of photoexcited electron relaxation in the reaction of the electron transfer by the method of flash photolysis in microsecond timescale, we had to change the electron acceptor concentration in a liquid phase. The ability of the acceptor molecules to adsorb at the surface of the semiconductor colloidal particle was found to determine the character of changes in the photobleaching relaxation kinetic curves. 

As already discussed in Sect. 2.2., the bandgap of semiconductor particles increases considerably when their size becomes smaller than about 100 A (Figs. 4 and 5). Accordingly, the position of energy bands is shifted, and it is expected that certain reactions should become possible with quantized particles which do not occur with bulk materials. This has been demonstrated for H2-evolution in 50 A PbSe- and HgSe-colloids, which has not been observed with large particles [181, 182]. An extreme negative shift of the conduction band by about 1.2 eV has been found with 50 A-CdTe-colloids due to their low effective mass. Since COa-reduction to formic add was observed with photoexcited CdTe-colloids, the conduction band must be at < - 1.9 eV, compared to the flatband potential of n-CdTe electrodes of — 0.6 V [181]. 

Silver-overlayer SERRS also has been used to study in situ the redox reactions of methyl viologen (MV) adsorbed on to a p-type InP SC electrode [42]. These experiments are related to earlier time-resolved resonance Raman spectroscopy (TR3S) work on electron transfer reactions at the surfaces of photoexcited semiconductor colloids (TiOa and CdS) involving... 

In the present paper, we report on observation of the pronounced enhancement of photoluminescence of semiconductor nanocrystals near nanostructured metal surfaces which is shown to depend essentially on nanocrystal-metal spacing. Unlike conventional SERS, the surface enhanced PL should exhibit non-monotonous character with distance between emitting dipole (QD) and metal surface (Au colloid). The reason is that at smallest distances when QDs and colloidal particles are in close contact, the QD emission should be damped due to resonant energy transfer (RET) from photoexcited QDs to metal colloidal nanoparticles. Enhancement of photoluminescence (PL) is possibly promoted by surface plasmons excited in the metal. So, at a certain distance the enhanced QD emission would exhibit a maximum. We use a polyelectrolyte multilayers as the most appropriate... 

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