A microwave promoted continuous flow approach to self-assembled hierarchical hematite superstructures

Abstract

In this work, a microwave promoted flow (MWPF) system to reproducibly synthesize self-assembled hierarchical hematite superstructures (Hem-SSs) using the sole precursor (Fe(NO₃)₃·9H₂O) and single mode microwave under aqueous conditions was developed. The functional characterisation by XRD, (HR)TEM, XPS, UV-vis and Raman spectroscopy proved that highly crystalline ellipsoid Hem-SSs (∼180 nm × 140 nm) were produced, built from primary hematite nanoparticles, 5–10 nm in size using 0.05 mol L⁻¹ precursor concentration, 1 mL min⁻¹ flow rate and short reaction time (about 6 min). Particles produced via conventional heating (CH) at 120 and 140 °C in the same flow reactor under similar experimental conditions were found to consist of mixtures of goethite and hematite. The effects of precursor concentration (0.1 and 0.2 mol L⁻¹) and flow rate (2 and 5 mL min⁻¹) were further investigated and the synthesis mechanism was also discussed. This novel method opens a window for continuous fabrication of metal or metal oxide nanoparticles/superstructures by a green approach.