Thermal decomposition is a promising route for the synthesis of magnetic nanoparticles. The simplicity of the synthesis method is counterbalanced by the complex chemistry of the system such as precursor decomposition and surfactant–reducing agent interactions. Control over nanoparticle size is achieved by adjusting the reaction parameters, namely, the precursor concentration. The results, however, are conflicting as both an increase and a decrease in nanoparticle size, as a function of increasing concentration, have been reported. Here, we address the issue of size-controlled synthesis via the precursor concentration. We synthesized iron oxide nanoparticles with sizes from 6 nm to 24 nm with narrow size distributions. We show that the size does not monotonically increase with increasing precursor concentration. After an initial increase, the size reaches a maximum and then shows a decrease with increasing precursor concentration. We argue that the observation of two different size regimes is closely related to the critical role of the amount of surfactant. We confirm the effect of surfactant amount on nucleation and growth and explain the observed trend. Furthermore, we show that the nanoparticles show size-dependent but superior superparamagnetic properties at room temperature.

中文翻译：

---

前驱体浓度对氧化铁纳米颗粒尺寸演变的影响

热分解是合成磁性纳米粒子的有希望的途径。合成方法的简单性被系统的复杂化学平衡，例如前体分解和表面活性剂与还原剂的相互作用。通过调节反应参数，即前体浓度，可以控制纳米颗粒的大小。然而，由于已经报道了纳米颗粒尺寸的增加和减小（作为浓度增加的函数），因此结果是矛盾的。在这里，我们通过前体浓度。我们合成了尺寸范围从6 nm到24 nm的氧化铁纳米颗粒。我们表明，尺寸不会随着前驱物浓度的增加而单调增加。初始增加后，尺寸达到最大值，然后随着前体浓度的增加而减小。我们认为，观察到两种不同尺寸的体系与表面活性剂用量的关键作用密切相关。我们确认了表面活性剂用量对成核和生长的影响，并解释了观察到的趋势。此外，我们表明，纳米粒子在室温下显示出尺寸依赖性但优异的超顺磁性质。