Chemically\nDriven Crystal Phase Selective Transformation\nof Covellite CuS Nanocrystals

In copper-deficient Cu<sub>2–<i>x</i></sub>S nanocrystals\n(NCs), it is the crystal phase that most affects their plasmonic properties\nand cation exchange (CE) reactions. As such, it is highly desirable\nto advance the phase control of Cu<sub>2–<i>x</i></sub>S NCs. Without an introduction of anions in common copper salts\nor additional sulfur sources, herein, we utilize covellite CuS nanodisks\n(NDs) with unique S–S bonds as a “purest” copper\nprecursor to produce other phase Cu<sub>2–<i>x</i></sub>S NDs in the presence of a reducing agent. In particular, the\noctadecylamine molecules with a functional −NH<sub>2</sub> group\nand 1-octadecene molecules with a functional CC group reduced\nthe S–S bonds and triggered a covellite-to-cubic digenite Cu<sub>1.8</sub>S transformation. In contrast, the 1-octadecanethiol molecules\nwith a functional −SH group triggered a covellite-to-hexagonal\ndjurleite Cu<sub>1.94</sub>S transformation. The reducing capabilities\nof alkylamine and alkylthiol molecules are correlated to the heating\ntemperature and their hydrocarbon chain length. During the phase transformation,\nthe external layers along the vertical direction of the NDs were gradually\nexpanded from 2.7 Å of covellite CuS (006) planes to 3.2 Å\nof digenite Cu<sub>1.8</sub>S (111) planes and 3.4 Å of djurleite\nCu<sub>1.94</sub>S (800) planes. In addition, the spontaneous coalescence\nat the initial stage and subsequent aggregative growth of individual\nNDs gave rise to an obvious thickness increment of the resulting digenite\nCu<sub>1.8</sub>S NDs and djurleite Cu<sub>1.94</sub>S NDs. Therefore,\nour findings in the present study provide a better comprehension of\nthe phase transformation between different Cu<sub>2–<i>x</i></sub>S NCs and an advanced approach for crystal phase\ncontrol of the Cu<sub>2–<i>x</i></sub>S NCs, holding\ngreat promise in their phase dependent plasmonic applications, and\nthe CE reaction enabled polymorphic syntheses.