Cation Exchange-Mediated Synthesis of Library of Plasmomagnetic Nanoheterostructures: Transformation of 2-Dimensional-Shaped Fe₇S₈ Nanoplates to Cu–Fe–S-Based Ternary Compound

Cation exchange mediated synthesis of nanoheterostructures (NHSs), core–shell structure or segmented nanostructures are gaining high interest in nanoscience research as the process gives a wide window of opportunity to tune and enhance the photophysical properties of semiconductors. Presence of point defects or layered structure of the material accelerates cation exchange process, which is well established for copper chalcogenide system. Here, we are reporting facile synthesis of high-quality Fe7S8 hexagonal nanoplate by colloidal synthesis route and studied the effect of Cu(I) ion addition by in situ and ex situ condition. The addition and consequent increase of Cu(I) precursor concentration to presynthetic Fe7S8 nanocrystals (ex situ) leads to successive compositional and morphological transformations as follows: Fe7S8 (hexagonal nanoplate) → Fe7S8@Cu5FeS4 (multigrained heterostructure) → Cu1.97S/Cu5FeS4/Cu1.97S (nanodumbbell) → Cu1.97S/Cu5FeS4 (segmented nanohat) → Cu1.97S (nanodisk). Native Fe vacancies in layered crystalline structure of original Fe7S8 drive intercalation and cation exchange processes to stabilize a particular NHS. Different diffusion rates and channels of incoming Cu and outgoing Fe ions result in linear and bent nanodumbbells. The position of strong localized surface plasmon resonance (LSPR) band in NHSs is shifted from visible region to infrared region of the spectrum due to the variation of hole concentration originating from intrinsic Cu vacancy. The LSPR bands in NHS are further tuned by generating more holes through oxidation of Cu(I) → Cu(II) reaction in oxygen atmosphere. Shape anisotropy in NHS gives rise to multiple plasmon modes. Fe7S8 (hexagonal nanoplate) and Cu1.97S/Cu5FeS4/Cu1.97S (nanodumbbell) exhibit features of ferrimagnetism with transition temperature of above 300 and 320 K, respectively. In situ chemical synthesis using same Cu and Fe precursors as that of ex situ process yields CuFeS2 and Cu5FeS4 phases under Cu-deficit and Cu-rich environment. The LSPR resonance bands with high plasmonic sensitivity are also tuned by controlling the hole concentration.