Precision control of ultrasmall metal oxides under continuous flow hydrothermal synthesis

Recently, ultrasmall metal oxide nanoparticles from 1.5 nm in size were synthesized by organic–inorganic reactions under hydrothermal conditions. Precise control of the particle size was achieved by changing the reaction time using a flow reactor. The continuous flow hydrothermal synthesis method is highly promising for exploring new features of metal oxides. In this study, the growth behavior of organically modified ultrasmall metal oxides is studied by varying the process conditions and materials, such as the chain length of the organic modifier, core particle species, metal precursor concentration, and flow dynamics. This study focuses on the initial rapid growth stage, ranging from 40 ms to 10 s, to achieve the precise control of ultrasmall metal oxides (1–5 nm). The results show that varying the core particle compositions and chain length of organic modifiers significantly affect the initial growth rate. Furthermore, higher concentrations and flow rates are advantageous for obtaining smaller particles and are suitable for large-scale synthesis. This study proves the applicability of the continuous flow hydrothermal methodology for the precise synthesis of various ultrasmall metal oxide nanoparticles, while providing deeper understanding of the unconventional fusion growth mechanism of ultrasmall metal oxides.