Controlled Growth of a Highly Energetic, Less Electron-Dense Tetrahedral Fe3+ Phase on the Surface of α-Fe2O3 Nanoparticles toward Enhanced Optical, Magnetic, and Supercapacitive Performance

Abstract

Nanoplate to pseudocubic-shaped hematite (α-Fe2O3) nanoparticles along with the gradual engravement of tetrahedral Fe3+ has been synthesized by controlling the precursor (FeCl3 ·6H2O) concentration from 0.005 M (A1) to 0.2 M (A4). The role of the surfactant polyvinylpyrrolidone on the preferential plane ([110] to [012]) growth and highly energetic tetrahedral Fe3+ phase (increases 2.7 times for A4 than A1) nucleation over the hematite surface has been elucidated from the HRTEM images, XPS, and Raman spectroscopy. This tetrahedral phase and the morphology variation facilitate the Fe–Fe interaction significantly, which improves the saturation magnetization from 0.13 emu/g (A1) to 0.37 emu/g (A4). Stronger magnetic d–d coupling in the A4 sample promotes dominant photoluminance (PL) spectra. It also exhibits linear dependence with the excitation wavelength (300–380 nm) that can be exploited in the design of a UV detector. The supercapacitive performance of ethylene glycol supported PEDOT-PSS-α-Fe2O3 composite films as negative electrode materials, in the voltage window of −1.3 to −0.5 V, suggests this material to be a promising candidate for the next-generation charge storage devices. The sample engraved with the richest Fe3+ ions shows the highest specific capacitance of 705.7 F/g in 2 M SO32– electrolytes, and its cyclic stability increases to 111% over 1500 cycles.