Temperature-Controlled β-Crystal-to-α-Crystal Phase Transition of HgS Nanoparticles

The crystal types of nanoparticles (NPs) are governed by many factors, such as temperature, pH, solvents, surface ligands, etc. In this work, β-HgS and α-HgS NPs coated with captopril (Cap) have been synthesized conveniently in the same raw materials and similar methods just with different temperature at 4 °C and 50 °C, respectively. More interestingly, phase transition of HgS NPs from β-crystal (cubic crystal) to α-crystal (hexagonal crystal) via rising temperature from 4 to 50 °C was confirmed by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). The size of α-HgS NPs grew to 6.7±1.3 nm in the process of phase transition, which was about twice of β-HgS NPs (3.5±1.0 nm). Furthermore, compared with the obvious absorption peak of β-HgS NPs at 688 nm, the absorption peak of α-HgS NPs had a giant blue-shifted to 470 nm in Ultraviolet and visible (UV-vis) spectra. In addition, the β-HgS NPs had a fluorescence emission at 900 nm differing from α-HgS NPs without emission. The α-HgS NPs had three peaks at 215 nm, 251 nm and 318 nm, while the β-HgS NPs had only a distinct peak at 215 nm in circular dichroism (CD) spectra. These results provide a novel insight for the controlling preparation of different crystal types of nanoparticles.