Oleylamine-Assisted Phase-Selective Synthesis of Cu_2-xS Nanocrystals and the Mechanism of Phase Control

Environmentally friendly Cu_2-xS 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-xS nanocrystals becomes an important scientific issue in colloidal chemistry. In this work, a hot-injection method is developed to synthesize Cu_2-xS 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_1.75S, and rhombohedral Cu_1.8S, nanocrystals can be selectively synthesized, providing a platform to illustrate the mechanism of crystal phase control. The crystal phase control of Cu_2-xS nanocrystals is oleylamine-determined by controlling the molar ratio of Cu/S in the reaction precursors as well as the ratio of Cu_2-xS clusters/Cu⁺ in the subsequent reaction. More importantly, temperature plays an important role in varying the molar ratio of Cu/S and Cu_2-xS clusters/Cu⁺ in the reaction system, which significantly influences the crystal phase of the resulting Cu_2-xS 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.