Photochemistry of colloidal semiconductors 30. Reactions and fluorescence of AgI and AgI — Ag2S colloids

Abstract

AbstractWhen Ag+ ions are precipitated with I− ions in the presence of poly (N‐vinyl pyrrolidone) or poly (vinyl alcohol) the AgI colloid forms slowly (during minutes or hours, depending on conditions), the changes in the absorption spectrum indicate the presence of unusual AgI structures during the first stages of particle growth. Size quantization effects are partly responsible for these changes. — AgI colloids with 10% excess of I− do not photolyse under continuous illumination. However, intermediates can be detected in flash photolysis experiments. In the presence of sulfite, silver is formed, the quantum yield lying in the 10−4 range. Greater yields of silver (10−3 to 10−2) are observed with solutions containing excess Ag+ ions. Less silver is produced in acidic solutions and the silver signal in the flash experiments decays more rapidly. These effects are attributed to the re‐oxidation of small silver clusters formed on the colloidal particles by H+ ions. — AgI colloids have a weak fluorescence close to the threshold of absorption. It is blue‐shifted with decreasing particle size (size quantization effect). The fluorescence is quenched by small amounts of silver deposited on the particles. It is also quenched by an Ag2S deposit. Under these circumstances a new fluorescence having three maxima in the red and infrared appears. This new fluorescence is red‐shifted with increasing size of the silver deposit (size quantization of Ag2S). The fluorescence is explained in terms of electron transfer from AgI (conduction band) to the hole captured at the AgI—Ag2S interface. Size quantization effects are also believed to play a role in the sulfursensitization of photographic plates.