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

Abstract

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 α- and β-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 α- and β-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 Au0 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 °C to 280 °C. The lower temperature resulted in a single phase of β-AuSe with some unreacted Se. As the temperature was increased, a mixture of α- and β-AuSe, as well as elemental gold, was obtained.