Nanosized colloidal semiconductors are interested as possible photocatalyst. That is why the knowledge of the interfacial electron transfer regularities is useful. It is well known that photostimulated semiconductor photobleaching is related with excess electrons in conductive band of semiconductor. These excess electrons expand the band gap and adsorption spectrum shifts to the UV region. The relaxation of adsorption spectrum of the semiconductor reflects the excited electron relaxation in interfacial electron transfer process.
In present paper the regularities of the photobleaching relaxation of different sized CdS nanocolloids were investigated. Recently we have developed a method of CdS colloids preparation with thioglycerol (Thoi) addition as size adjusting agent. The two types of colloidal particles were prepared using this method: with excess of S2- ions (No1) and with excess of Cd²⁺ ions (No2). It was found that constant rate of photobleaching relaxation depends on the size of colloidal particles for both types of colloids, the increase of particle size result in the decrease of photobleaching relaxation constant rate (Fig. 1).

Fig.1 The kinetics of photobleaching relaxation of different sized CdS particles. CdS was prepared at Cd2+ ions excess, [CdS] = 10-4 M, [SDS] = 2 x10-3 M, [Thio] = 5 x10-3 M

The influence of different electron acceptors addition on photobleaching relaxation kinetic of CdS nanocolloids was investigated. It was appeared that the acceptor molecules ability to adsorb on the surface of CdS colloidal particle determinesmainly the character of changes in kinetic curves after its addition. Moreover this ability is determined by the charges of acceptors ions and charge of CdS surface, which depends on the condition during colloid preparation. The excess of one component during preparation determining the surface charge of the colloidal particles and the ability of colloidal particles to adsorb the ions of the opposite charge.
It was revealed that addition of well adsorb acceptor molecules decreases the initial amplitude of photobleaching and practically it does not change the constant rate of photobleaching relaxation. The addition of acceptor molecules which have small adsorption constant result in an increase of photobleaching relaxation rate constant, the initial amplitude of photobleaching remains unchanged (Fig. 2).

Fig.2 The kinetics of CdS photobleaching relaxation under addition of electron acceptors: a) MV2+ - 0 to 10-7 M, b) PW123- - 0 to 3 x10-7 M CdS was prepared at Cd2+ ions excess, [CdS] = 10-4 M, [SDS] = 2 x10-3 M, [Thio] = 5 x10-3 M

We propose a next explanation of such behavior. In case of well adsorb acceptor molecules, adsorption of all molecules occurs and observed decrease of the initial amplitude of photobleaching is due to the interfacial electron transfer to the adsorb molecules. Subsequent decay of photobleaching is related with interfacial electron transfer to the oxygen in solution which concentration is constant. Therefore, the constant rate of photobleaching relaxation doesn t change. In case of bad adsorb acceptor molecules, observed kinetic of the photobleaching relaxation is connected with interfacial electron transfer to the acceptor molecules in solution. Thus, under addition of such acceptors the effective constant rate of interfacial electron transfer rise.

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