Composition, particle size, and near-infrared irradiation effects on optical properties of Au-Au₂S nanoparticles

Near-infrared (NIR)-absorbing nanoparticles synthesized by the reduction of tetrachloroauric acid (HAuCl₄) using sodium sulfide (Na₂S) exhibited absorption bands at ∼530 nm and at the NIR region of 650-1100 nm. A detailed study on the structure and microstructure of as,synthesized nanoparticles. was reported previously. The as-synthesized nanoparticles were found to consist of amorphous Au_xS (x = ∼2), mostly well mixed within crystalline Au. In this work, the optical properties were tailored by varying the precursor molar ratios of HAuCl₄ and Na₂S. In addition, a detailed study of composition and particle-size effects on the optical properties was discussed. The change of polarizability by the introduction of S in the form of Au_xS (x = ∼2) had a significant effect on NIR absorption. Also, it was found in this work that exposure of these particles to NIR irradiation using a Nd:YAG laser resulted in loss of the NIR absorption band. Thermal effects generated during NIR irradiation had led to microstructural changes that modified the optical properties of particles.