Chemical Synthesis and Characterization of Self-Assembled Nanoparticles Prepared in the Presence of Citrate Ions

Iron(II) ions in aqueous solutions containing various concentrations of citrate ions have been reduced to metallic Fe using sodium borohydrate. In comparison to the 10-20 nm diameter Fe particles formed in the absence of citrate, the presence of citrate results in the formation of 100--200 nm spherical (or in some cases cubical) particles self-assembled from much smaller Fe nanoparticles. Structural, chemical, and magnetic measurements indicate that for appropriate citrate/Fe ratios, air stable powders that exhibit a room temperature saturation magnetization of nearly 200 emu/g and a coercivity less than 100 Oe -- even without deoxygenating the reaction solution or an explicit surface passivation step -- can be obtained. Thermal treatments at temperatures between 350 and 450 \r{ }C result in the sintering of the Fe nanoparticles within the larger aggregates while heat treatments at higher temperatures result in the sintering of the aggregates themselves into a continuous porous matrix.