Oleylamine‐Assisted Phase‐Selective Synthesis of Cu_2−<i>x</i>S Nanocrystals and the Mechanism of Phase Control

Abstract

Environmentally friendly Cu 2− x S compounds exist in many different mixed phases in nature, while their nanoscale counterparts can be pure phase with interesting localized surface plasmon resonance properties. Because of the complexity of composition and phase, controllable synthesis of Cu 2− x S nanocrystals becomes an important scientific issue in colloidal chemistry. In this work, a hot‐injection method is developed to synthesize Cu 2− x S nanocrystals by injecting a sulfur precursor into a copper precursor using oleylamine and octadecene as solvents. By varying the reaction parameters (temperature, volume ratio of oleylamine/octadecene, molar ratio of Cu/S in the precursors), hexagonal CuS, monoclinic Cu 1.75 S, and rhombohedral Cu 1.8 S, nanocrystals can be selectively synthesized, providing a platform to illustrate the mechanism of crystal phase control. The crystal phase control of Cu 2− x S nanocrystals is oleylamine‐determined by controlling the molar ratio of Cu/S in the reaction precursors as well as the ratio of Cu 2− x S clusters/Cu + in the subsequent reaction. More importantly, temperature plays an important role in varying the molar ratio of Cu/S and Cu 2− x S clusters/Cu + in the reaction system, which significantly influences the crystal phase of the resulting Cu 2− x S nanocrystals. The understanding into crystal control provides a guideline to realize reproducible phase‐selective synthesis and obtain well‐defined high‐quality materials with precise control.