Nucleation and Growth of Silver Sulfide Nanoparticles

Real time UV−visible absorption measurements of stopped flows of AgNO 3 and (NH 4 ) 2 S are used to study the nucleation and growth of silver sulfide nanoparticles (Ag 2 S NP). Ag 2 S NP in the size range of 2 to 10 nm are formed a few seconds after the flow containing the reactants stops. The absorbance near the band edge of the semiconductor nanoparticle is used to monitor the number of particles formed with time and study the nucleation process. Transmission electron microscopy measurements are used to correlate particle size and the indirect band gap energy, determined from the onset of light absorption. A linear relation is established between Ag 2 S NP particle size and indirect band gap energy. The nucleation and growth process are not well separated in time. The initial nucleation and growth rates are found to increase with initial [AgNO 3 ] o /[(NH 4 ) 2 S] o ratios larger than 1. Silver-rich sulfides are proposed to be involved in the nucleation stage and growth process of Ag 2 S NP. Density functional calculations are consistent with that interpretation: Ag 3 S + is found to have a lower energy than the Ag 2 S and AgSH molecules or the AgS − and Ag 2 SH + ions. The results are discussed in terms of classic nucleation theory and the possible growth mechanisms are discussed.