Photosensitized Growth of Silver Nanoparticles under Visible Light Irradiation:  A Mechanistic Investigation

By employing thionine as a sensitizing dye, we have succeeded in inducing silver nanoparticle formation in ethanol/toluene medium. The particle size increases with the duration of photolysis, thus providing a simple experimental parameter to control the particle size. The photoinduced transformations as probed by steady state photolysis and transient absorption spectroscopy provide an insight into the charge-transfer processes leading to the formation of nanoparticles. Under visible light illumination, dye thionine undergoes self-quenching of the excited triplet to produce electron-transfer products in toluene/ethanol (1:1) mixture. The quenching of triplet excited dye by the ground state dye molecules ( k = 4.9 × 10 9 M - 1 s - 1 ) occurs at a much faster rate than the quenching by Ag + ions ( k = 1.30 × 10 6 M - 1 s - 1 ). Semithionine acts as a mild reductant to initiate reduction of silver ions ( k = 7.26 × 10 7 M - 1 s - 1 ) and form silver seeds. These seeds grow with autocatalytic reduction of Ag + ions and by Ostwald ripening to produce larger size particles (10−30 nm diameter).