Nanoscale mixing during double-flame spray synthesis of heterostructured nanoparticles

The combination of two nanoparticle-producing flame reactors to a double-flame (DF) spray pyrolysis process is an attractive method for the high-temperature gas-phase synthesis of multicompound materials and heterostructures. It allows separate control of particle growth in the individual flames up to the intersection or mixing point where the formation of heterostructures takes place. The effect of mixing of the aerosol streams on the process temperature and product characteristics is investigated based on the example of Pt on TiO₂. Temperatures were determined by Fourier-transform infrared spectroscopy and thermocouple measurements along with computational fluid dynamics, while the degree of mixing was investigated based on surface area, Pt-dispersion measurements, and transmission electron microscopy image analyses. The quadrat method in combination with the variation coefficient was used to quantify the uniformity of the Pt cluster distribution on the TiO₂ support. For high intersection distances of the two flame jets and small intersection angles, nonuniform mixing of the compounds and the formation of large Pt particles are observed. For small intersection distances and large angles, a uniform Pt distribution was achieved. Based on these findings, process design rules were established which can be transferred to other material systems.