Comparison of T-piece and concentric mixing systems for continuous flow synthesis of anatase nanoparticles in supercritical isopropanol/water

T-piece and concentric counter-flow mixing systems are compared in continuous flow supercritical solvothermal synthesis of TiO2 at identical system parameters. The phase pure anatase nanoparticle products were characterized with powder X-ray diffraction (PXRD), transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS), and the particle size, size distribution and absolute crystallinity mapped as a function of temperature, precursor concentration, flow rate and pressure for the two different continuous flow reactors. The particles synthesized with the T-piece geometry are smaller with a narrower size distribution, possibly indicating a more effective mixing, than particles synthesized at the same conditions with concentric counter-flow geometry. In general, an increased synthesis temperature leads to an increase in absolute crystallinity. For the particles synthesized with the concentric reactor geometry crossing of the critical point of the solvent causes a decrease in the particle size and size distribution, and conditions just above the critical temperature are demonstrated to be optimal for continuous solvothermal synthesis of anatase.