Structure and microstructure of near infrared-absorbing Au-Au₂S nanoparticles

Near infrared (NIR) absorbing nanoparticles synthesized by the reduction of HAuCl₄ with Na₂S exhibited absorption bands at ∼530 nm, and in the NIR region of 650-1100 nm. The NIR optical properties were not found to be related to the earlier proposed Au₂S-Au core-shell microstructure in previous studies. From a detailed study of the structure and microstructure of as-synthesized particles in this work, S-containing, Au-rich, multiply-twinned nanoparticles were found to exhibit NIR absorption. They consisted of amorphous Au_xS (where x = 2), mostly well mixed within crystalline Au, with a small degree of surface segregation of S. Therefore, NIR absorption was likely due to interfacial effects on particle polarization from the introduction of Au_xS into Au particles, and not the dielectric confinement of plasmons associated with a core-shell microstructure.