Unravelling the structural properties of mixed-valence alpha- and beta-AuSe nanostructures using XRD, TEM and XPS

2D materials have received much attention recently as they can potentially offer new routes to electronics and other technologies. Gold selenide is a relatively unexplored 2D material with a complex mixed-valence crystal structure similar to that of Ago. Apart from a 1960s and 1970s reports on the solid state synthesis and crystal structure respectively, little is known about gold selenide. Herein, we report for the first time, the colloidal synthesis of alpha- and beta-AuSe nanostructures by varying the Au:Se ratio and temperature in order to tailor the properties. We probe the structural properties using XRD, TEM, and XPS. The XRD results confirmed the mixed-valence crystal structure where Au1+ and Au3+ co-exist in a unit cell, however, there was co-crystallization of both the alpha- and beta-phase. The TEM showed belt-like nanostructures indicative of the 2D-type material. The Au4f core level spectrum of the XPS, showed only the Au1+ oxidation state, however, using the Se3d core level spectrum, the formation of AuSe (Au1+Au3+Se2) was evident. The colloidal method, nonetheless, resulted in impurities in a form of unreacted selenium and reduced Au-0 due to the reducing capping agent irrespective of the attempted stoichiometric ratios. In an attempt to minimize the impurities and be selective to a single crystal phase, the temperature was varied from 160 degrees C to 280 degrees C. The lower temperature resulted in a single phase of beta-AuSe with some unreacted Se. As the temperature was increased, a mixture of alpha- and beta-AuSe, as well as elemental gold, was obtained.