Synthesis of iron(III) oxide ultrafine powders by laser-induced vapour-phase reaction

Abstract

Iron(III) oxide ultrafine powders have been synthesized by laser-induced vapour-phase reaction using an IR pulsed CO₂ laser as light source and a mixture of Fe(CO)₅–O₂ as reactant without adding any sensitizes. The optimum experimental conditions were presented. A single laser pulse can initiate an explosive reaction provided that the total Fe(CO)₅–O₂ pressure exceeds 5 kPa and the O₂–Fe(CO)₅ partial pressure ratio is 1. The reaction results in the complete disappearance of iron carbonyl and the energy consumption is very low compared with the conventional chemical process. The maximum quantum efficiency is ca. 10², suggesting that the explosive reaction may proceed according to a chain reaction mechanism. The relationships between the total pressure and the quantum efficiency and laser fluence threshold were both derived. IR, XRD, TEM and XPS were used to characterize the powders. The experimental results showed that the powders consisted of both crystalline γ-Fe₂O₃ and amorphous Fe₂O₃. The former phase is 50–100 nm in diameter while the latter is 5–12 nm in diameter. This is the first time that crystalline γ-Fe₂O₃ prepared in this way has been found to be present as regular polyhedra and not spherical particles.