A mixed-ligand strategy for size-controlled synthesis of hydrophobic ZnO nanocrystals by microfluidic reactor

The intrinsic hydrophilicity of ZnO nanocrystals (NCs) leads to their incompatibility with organic media. Current methods for hydrophobization of ZnO NCs involve complex processes, which hinder their mass production and application. In this work, we propose a mixed-ligand strategy for synthesis of size-controlled hydrophobic ZnO NCs by oxidation of organometallic zinc precursor in the presence of amine and trioctylphosphine oxide (TOPO). Growth kinetic study reveals the dual role of TOPO in accelerating the nucleation and inhibiting the growth of ZnO NCs, demonstrating the importance of TOPO dosage to rapid synthesis of ZnO NCs. Other factors governing the growth kinetic are systematically investigated to optimize the synthesis control for wide range of size tunability. Finally, monodispersed hydrophobic ZnO NCs with the size of 2-13 nm are produced through microfluidic reactor, and one "synthesis run" takes 3-30 min. The hydrophobic ZnO NCs exhibit superior photocatalytic performance than the hydrophilic ones in degradation of Sudan dyes, which is attributed to its strong affinity to the dye molecules. We expected the synthesis strategy will promote the development and applications of hydrophobic ZnO NCs in various fields of photocatalysis, sensors, and optoelectronics.