Chemically Driven Crystal Phase Selective Transformation of Covellite CuS Nanocrystals

Abstract

In copper-deficient Cu 2−x S nanocrystals (NCs), it is the crystal phase that most affects their plasmonic properties and cation exchange (CE) reactions. As such, it is highly desirable to advance the phase control of Cu 2−x S NCs. Without an introduction of anions in common copper salts or additional sulfur sources, herein, we utilize covellite CuS nanodisks (NDs) with unique S−S bonds as a "purest" copper precursor to produce other phase Cu 2−x S NDs in the presence of a reducing agent. In particular, the octadecylamine molecules with a functional −NH 2 group and 1octadecene molecules with a functional C�C group reduced the S−S bonds and triggered a covellite-to-cubic digenite Cu 1.8 S transformation. In contrast, the 1-octadecanethiol molecules with a functional −SH group triggered a covellite-to-hexagonal djurleite Cu 1.94 S transformation. The reducing capabilities of alkylamine and alkylthiol molecules are correlated to the heating temperature and their hydrocarbon chain length. During the phase transformation, the external layers along the vertical direction of the NDs were gradually expanded from 2.7 Å of covellite CuS (006) planes to 3.2 Å of digenite Cu 1.8 S (111) planes and 3.4 Å of djurleite Cu 1.94 S (800) planes. In addition, the spontaneous coalescence at the initial stage and subsequent aggregative growth of individual NDs gave rise to an obvious thickness increment of the resulting digenite Cu 1.8 S NDs and djurleite Cu 1.94 S NDs. Therefore, our findings in the present study provide a better comprehension of the phase transformation between different Cu 2−x S NCs and an advanced approach for crystal phase control of the Cu 2−x S NCs, holding great promise in their phase dependent plasmonic applications, and the CE reaction enabled polymorphic syntheses.