Mössbauer study of iron oxide nanoparticles produced by laser ablation of metallic iron in water and effects of subsequent laser irradiation

Laser ablation in liquid (LAL) is a very useful, conventional means of producing metal particles. While the wet chemical synthesis of nanoparticles typically requires numerous chemical reagents and complicated handling processes, LAL provides a very simple method of generating nanoparticles while reducing the amount of reagents. Furthermore, so-called naked nanoparticles without coating materials can be obtained using LAL, which provides a facile approach to studying the properties of such materials. Laser-based synthesis and processing have been studied extensively, as has the LAL process itself. Both fragmentation and melting resulted from laser irradiation (LI) of particles suspended in liquid have been found to be important. Laser ablation (LA) of a metal has been shown to produce a plasma vapor that is rapidly quenched by the surrounding solvent to produce particles. In the case that the surrounding solvent is itself decomposed by the plasma vapor, the subsequent reactions can produce particles of various metal compounds. The chemical composition and structure of these nanoparticles can be controlled by tuning the LA conditions and varying the solvent. In addition, in the case that LA is performed in a stagnant solvent, the resulting particles can be said to undergo LI. LI increases the temperature of particles, and the fragmentation and melting of the particles change their chemical composition or their shapes. Using this LAL technique, it is possible to produce metastable materials, and our own group has demonstrated the generation of metastable copper oxide particles (Cu4O3) by LA in water. We have also reported the reaction of iron in organic solvents to produce iron carbide particles. The LA of iron in alcohols gave α-Fe, γ-Fe, Fe3C and amorphous iron carbides. Using this technique in conjunction with a solvent f low allowed separation and collection of the different nanoparticles immediately after production, preventing further photochemical reactions of the material. The effect of LI on iron carbide nanoparticles produced by LA has also been studied, and has been shown to increase the particle size and to change the composition to pure Fe3C. The LA of iron in various liquids has been examined. The formation of α-Fe particles via LA of iron in water has been investigated, with the surfaces of the α-Fe particles protected by surface-stabilizing reagents. The fabrication of FeO nanoparticles based on LA of a pure iron plate in poly (vinylpyrrolidone) solutions has also been reported, during which the particle size was controlled by varying the surfactant concentration. Generally, LA of metallic iron in water without an adequate supply of surfactant produces iron oxide particles. It has been proposed that the LA process generates Fe clusters that react with adjacent H2O molecules to form Fe(OH)2 nanopar ticles, which subsequently decompose to FeO nanoparticles at high temperature and pressure. In other work, iron oxide nanoparticles consisting of a mixture of hematite and magnetite were obtained by LA of metallic iron in water. The size of such iron oxide nanoparticles can evidently be controlled by applying LI, although Mössbauer spectra of the particles were not obtained in previous studies. In the present study, LA of metallic iron in flowing water was performed to produce LA particles that were then further modified by LI in water. These LA and LI processes were analyzed separately to better understand the LAL mechanism.