Generation of ZnO nanoparticles by chemical vapor synthesis using quenching air

With the increasing interest in manufacturing nano-sized metal particles in large quantities for their numerous applications in industry, medicine and daily life as well as nanotoxicity testing, there is a critical demand to improve methods to generate nanoparticles with controllable nano-size and high number concentrations. Here, a stable method for the generation of zinc oxide nanoparticles (ZnO-NPs), especially for the size below 10 nm, by chemical vapor synthesis (CVS) using quenching air was proposed. The present study found that the oxidation step of Zn vapor in the ZnO-NP formation process is the rate-controlling step. Six conditions were tested with various furnace temperatures, carrier gas and quenching air flow rates in a tubular furnace. The experimental results showed that the quenching air flow rates (1-5 L/min) have a great impact on ZnO-NPs generation and characteristics. The geometric mean diameter of synthesized ZnO-NPs or aggregates was decreased from 15.26 to 3.27 nm, the total number concentration was increased from 2.56 × 10⁷ to 4.28 × 10⁷ #/cm³ and the morphology changed from tetrapod-shaped to isometric-shaped when the quenching air flow rate was increased from 1 to 5 L/min at 1 L/min carrier gas flow rate and 500 °C furnace temperature. The diameter and concentration remained stable with only small fluctuations for 5 h at all test conditions, which is very useful for nanotoxicity testing. The size of ZnO-NPs was decreased below 10 nm when the quenching rate was 11,000 K/s or higher at 1 L/min carrier gas flow rate and 500 °C furnace temperature. The experimental yield of ZnO-NPs was found to be 11.93% at 600 °C furnace temperature and less than 1.5% at 500 °C. The low yield of small ZnO-NPs is mainly attributed to the low conversion ratio of Zn vapor by the oxidation reaction, and convection-diffusion deposition loss of Zn vapor, ZnO vapor and small ZnO-NPs, which needs further improvement for future mass production using CVS method. Graphical abstract: [Figure not available: see fulltext.].