Hydrothermal Synthesis of Free-Floating Au2S Nanoparticle Superstructures

We describe the formation of gold sulfide (Au2S) nanoparticle superstructures via hydrothermal synthesis approach at 175 °C using HAuCl4 and Na2S reagents and cetyltrimethylammonium bromide (CTAB) surfactant. Uniform 2−4 nm Au2S nanoparticles were found to assemble into a densely packed lamellar phase structure, and the resulting material displays a cross-sheet-like morphology. The sheets are 125−250 nm in length and can be suspended in solution. XRD, TEM, and EDS characterization of the nanoparticle samples confirmed the composition of the nanoparticles as Au2S. Some isolated faceted gold nanoparticles were also observed. XPS data also support the formation of Au2S nanoparticles. UV−vis absorption spectra showed only absorption features from the Au2S nanoparticles, and an indirect band gap value of 1.77 eV was obtained. Sufficiently high concentrations of Na2S and CTAB in the reaction mixture were concluded to be necessary to promote the growth of Au2S nanoparticles, and reduce the production of gold nanocrystals. Maintenance of the superstructures after boiling in water suggests the absence of CTAB molecules on the particle surfaces. FT-IR, XPS, and NMR results indicate thermolysis of CTAB, and long alkyl chains interact with the Au2S nanoparticles and direct their assembly into a lamellar phase structure.