Biophysics Group, Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, UK.
Department of Chemical Engineering, University College London, Torrington Place, London, WC1E 7JE, UK.
Nanoscale. 2019 Apr 4;11(14):6620-6628. doi: 10.1039/c9nr00531e.
Co-precipitation is the most ubiquitous method for forming iron oxide nanoparticles. For a typical co-precipitation synthesis, the pH of a ferrous and/or ferric ion solution is increased via the addition of a base. The latter can be added either slowly (a steady addition over either minutes or hours) or fast (a one-time addition) resulting in an abrupt increase in the pH. However, understanding the mechanism of particle formation is still lacking, which limits the reproducibility of the co-precipitation reaction due to intermediate phases still being present in the final product. In this work, we study in detail a co-precipitation synthesis with an abrupt increase in pH via the addition of sodium carbonate. Fast and reproducible mixing at defined precursor and base solution temperatures was achieved utilising a flow reactor. Transmission electron microscopy, electron diffraction and room temperature 57Fe Mössbauer spectroscopy showed a distinct transition from an amorphous ferrihydrite phase to a mixture of magnetite-maghemite (Fe3O4/γ-Fe2O3). Synchrotron X-ray diffraction revealed the initial formation of crystalline iron hydroxide carbonate (green rust) plates occurring before the Fe3O4/γ-Fe2O3 appeared. The ferrihydrite particles increase in size over time as the proportion of iron hydroxide carbonate plates are re-dissolved into solution, until the ferrihydrite particles crystallise into Fe3O4/γ-Fe2O3.
共沉淀是形成氧化铁纳米粒子最普遍的方法。对于典型的共沉淀合成，通过添加碱来增加亚铁和/或铁离子溶液的 pH 值。后者可以缓慢添加（在几分钟或几小时内缓慢添加）或快速添加（一次性添加），导致 pH 值突然升高。然而，对颗粒形成机制的理解仍然缺乏，这限制了共沉淀反应的重现性，因为最终产物中仍存在中间相。在这项工作中，我们详细研究了通过添加碳酸钠使 pH 值突然升高的共沉淀合成。利用流动反应器实现了在定义的前体和碱溶液温度下快速且可重复的混合。透射电子显微镜、电子衍射和室温 57Fe Mössbauer 光谱显示，从无定形的水铁矿相到磁铁矿-磁赤铁矿（Fe3O4/γ-Fe2O3）的混合物有明显的转变。同步加速器 X 射线衍射表明，在 Fe3O4/γ-Fe2O3 出现之前，首先形成了结晶的铁氢氧化物碳酸盐（绿锈）板。随着时间的推移，铁氢氧化物碳酸盐板的比例重新溶解在溶液中，水铁矿颗粒的尺寸增加，直到水铁矿颗粒结晶成 Fe3O4/γ-Fe2O3。
