The unique properties of magnetite (Fe₃O₄) nanoparticles (MNPs) have promoted a wide range of applications in various fields, but specifying and defining the preparation conditions of MNPs with the desired monodispersed size are still challenges. This paper discusses systematically the influence of synthesis conditions on the structure and size of MNPs in the coprecipitation method without a stabilizer in one system. The ammonium hydroxide (NH₄OH)/iron (III) (Fe³⁺) ratio, iron (III)/iron (II) (Fe²⁺) ratio, initial concentration of hydrochloric acid (HCl), total concentration of iron ions, reaction temperature, aging time and washing times were investigated. The surface of magnetite crystals was easily oxidized at high ammonium hydroxide/iron (III) ratios, and the high pH value of the reaction system can induce a large crystalline size. The hydrodynamic diameter of MNPs was efficiently controlled by the equilibrium between Derjaguin–Landau–Verwey–Overbeek (DLVO) forces and non-DLVO forces. An appropriate initial concentration of hydrochloric acid and a higher reaction temperature can reduce the crystalline size due to the decelerated nucleation rate. The aging time and the total concentration of iron affected the crystalline size positively. Noticeably, the pH value of the MNP dispersion in distilled water would remarkably reduce after each washing by centrifugation due to surface absorption of the hydroxy group. This research provided fundamental support for the reproducible preparation of MNPs in the coprecipitation method.

中文翻译：

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无稳定剂共沉淀法合成条件对纳米磁铁矿结构和尺寸的影响

_{磁铁矿（Fe 3} O ₄ ）纳米粒子（MNP）的独特性质促进了在各个领域的广泛应用，但指定和定义具有所需单分散尺寸的MNP的制备条件仍然是挑战。本文系统地讨论了在一个体系中无稳定剂的共沉淀法中合成条件对MNPs结构和尺寸的影响。氢氧化铵（NH ₄ OH）/铁（III）（Fe ³⁺）比率，铁（III）/铁（II）（Fe ^{2+ ）}）的比例、盐酸（HCl）的初始浓度、铁离子的总浓度、反应温度、陈化时间和洗涤次数进行了研究。在高氢氧化铵/铁(III)比下，磁铁矿晶体的表面很容易被氧化，并且反应体系的高pH值会导致大的晶体尺寸。MNP 的流体动力学直径由 Derjaguin-Landau-Verwey-Overbeek (DLVO) 力和非 DLVO 力之间的平衡有效控制。适当的盐酸初始浓度和较高的反应温度可以由于成核速率减慢而减小晶体尺寸。时效时间和铁的总浓度对晶体尺寸有积极影响。值得注意的是，由于羟基的表面吸收，每次离心洗涤后，MNP 分散在蒸馏水中的 pH 值都会显着降低。该研究为共沉淀法可重复制备MNPs提供了基础支持。