Application of microstructured devices for continuous synthesis of tailored platinum nanoparticles

Dispersed colloidal Pt-nanoparticles with mean particle sizes between 1 and 4 nm were prepared continuously in microstructured devices using ethylene glycol as solvent and reducing agent and H2PtCl6 as metal precursor. The Pt-nanoparticles were characterized directly in solution regarding the particle size and particle size distribution by small angle X-ray scattering (SAXS) and after deposition on a grid by TEM. Both methods gave comparable results. The particle size of the Pt-nanoparticles which are formed inside the microstructures can be controlled by the NaOH/Pt ratio. At a high NaOH/Pt-ratio, that means in alkaline medium, the Pt-nanoparticles have a mean particle size lower than 1.7 nm and a narrow size distribution indicating that nanoparticle formation is dominated by nucleation reaction under this condition. At a low NaOH/Pt-ratio, that means in acidic medium, the mean particle size increases to about 3.6 nm whereas also the particle size distribution increases which is mainly attributed to a larger influence of growth reactions on particle formation. Pt-nanoparticles with a similar mean particle size but a clearly narrower particle size distribution were obtained when a two-step process was used. In this process, Pt-seeds were formed in the microreactor in an alkaline medium. After mixing the alkaline Pt-seeds containing solution with a HCl/ethylene glycol solution in a micromixer the now acidic solution flows in a second microreactor where mainly particle growing takes place. The final particle size and its distribution can be influenced by both conditions for seed formation (temperature, flow rate) and particle growth (pH, temperature, flow rate).

► Microstructured devices are suitable tools for continuous preparation of Pt nanoparticles using the polyol process.
► pH value has the strongest influence on particle size.
► Two step process with internal seeds formation was developed.
► Two-step process allows preparation of nanoparticles with tunable particle size and narrow size distribution.
► Small angle X-ray scattering is a suitable tool to follow processes in nanoparticle formation.