TRANSIENT BLEACHING OF SMALL PBS COLLOIDS - INFLUENCE OF SURFACE-PROPERTIES

Small PbS colloids with a particle diameter of 40 Å were prepared in aqueous solution, and their absorption spectra exhibit several maxima. Injection of electrons into these particles was achieved by using the pulse radiolysis technique. Excess electrons trapped on the surface lead to a blue shift in the absorption edge of colloids. The appearance of this shift depends critically on the method of colloid preparation. PbS and CdS colloids prepared at pH < 6 have long-lived bleaching, which disappears after several seconds. On the other hand, absorption bleaching does not appear after the addition of hydroxide ions to colloidal solutions (pH > 8). The existence of a hydroxide ion on the particle surface most likely removes surface defects on which electrons are trapped. PbS colloids prepared in the presence of 3-mercapto-1,2-propanediol have an unstructured absorption spectrum, which is due to a wide particle size distribution (10-50 Å). The redox potential for 40-Å PbS colloids was determined by an established equilibrium between zwitterionic viologen radicals and colloids. It slightly shifts to the cathodic direction when the number of electrons injected into the particles is increased. © 1990 American Chemical Society.