Recovery of iron/iron oxide nanoparticles from solution: comparison of methods and their effects
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Most methods currently being used to recover Fe0-core/oxide-shell nanoparticles from solutions (including the solvents they are synthesized or stored in) are potentially problematic because they may alter the particle composition (e.g., depositing salts formed from solutes) or leave the particles prone to transformations during subsequent storage and handling (e.g., due to residual moisture). In this study, several methods for recovery of nanoparticles from aqueous solution were studied to determine how they affect the structure and reactivity of the recovered materials. Simple washing of the nanoparticles during vacuum filtration (i.e., “flash drying”) can leave up to ~17 wt% residual moisture. Modeling calculations suggest this moisture is mostly capillary or matric water held between particles and particle aggregates, which can be removed by drying for short periods at relative vapor pressures below 0.9. Flash drying followed by vacuum drying, all under N2, leaves no detectable residue from precipitation of solutes (detectable by X-ray photoelectron spectroscopy, XPS), no significant changes in overall particle composition or structure (determined by transmission electron microscopy, TEM), and negligible residual moisture (by thermogravimetric analysis, TGA). While this improved flash-drying protocol may be the preferred method for recovering nanoparticles for many purposes, we found that Fe0-core/oxide-shell nanoparticles still exhibit gradual aging during storage when characterized electrochemically with voltammetry.
KeywordsRecovery Flash drying Weight loss Colloids Thermogravimetric analysis Transmission electron microscopy X-ray diffraction X-ray photoelectron spectroscopy Linear sweep voltammetry
We acknowledge and thank C.-M. Wang, P. Nachimuthu, M.H. Engelhard, J. Kwak, and C.K. Russell for their assistance with TEM, XRD, XPS, and BET measurements and sample preparation. Samples of nano-Fe0 were donated by the Toda Kogyo Corp. We would also like to thank students Abram J. Ledbetter and Jharana Dhal who conducted some exploratory work on particle recovery during a Nanotechnology course hosted by the Pacific Northwest National Laboratory (PNNL) and the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL). This work was supported by the U.S. Department of Energy (DOE) Division of Chemical Sciences, Geosciences, and Biosciences. Parts of the work were conducted at the EMSL, which is located at PNNL. EMSL is a DOE User Facility operated by Battelle for the DOE Office of Biological and Environmental Research. PNNL is operated for the DOE under Contract DE-AC06-76RLO 1830.
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