One-pot synthesis of nano titanium dioxide in supercritical water

Abstract

<jats:title>Abstract</jats:title><jats:p>Supercritical hydrothermal synthesis of nanosized metal oxides is of great interest for scholars due to its one-pot and readily realizing commercial production process. Hydrothermal synthesis of nanosized TiO<jats:sub>2</jats:sub> in subcritical and supercritical water was investigated with different reaction temperatures (250–450°C), time (3–10 min), precursor species (Ti(SO<jats:sub>4</jats:sub>)<jats:sub>2</jats:sub>, TiCl<jats:sub>4</jats:sub>), and the addition of a surfactant (CH<jats:sub>3</jats:sub>(CH<jats:sub>2</jats:sub>)<jats:sub>5</jats:sub>NH<jats:sub>2</jats:sub>). TiO<jats:sub>2</jats:sub> synthesized under supercritical state has a higher degree of crystallinity when compared with TiO<jats:sub>2</jats:sub> obtained from subcritical state, which is attributed to a higher reaction rate. Extending reaction time from 3 to 10 min leads to the growth of crystal and broader size distribution (36.66 ± 8.5 nm). More uniform products can be obtained when Ti(SO<jats:sub>4</jats:sub>)<jats:sub>2</jats:sub> acting as the precursor compared with TiCl<jats:sub>4</jats:sub> being the precursor. The application of surfactant CH<jats:sub>3</jats:sub>(CH<jats:sub>2</jats:sub>)<jats:sub>5</jats:sub>NH<jats:sub>2</jats:sub> in the synthesis makes chemical bonding between the particle surface and organic ligand. It affects the growth direction of TiO<jats:sub>2</jats:sub> crystal; as a result, rod-like TiO<jats:sub>2</jats:sub> crystal is obtained.</jats:p>

Study on the Synthesis of Nano Zinc Oxide Particles under Supercritical Hydrothermal Conditions

“…The increase in the nucleation rate leads to a decrease in average particle size and crystallite size. On the other hand, according to the Born equation, the reaction rate is inversely proportional to the dielectric constant [10]. Therefore, the extremely low dielectric constant leads to faster hydrolysis, dehydration, nucleation, and growth rates compared to the subcritical region, resulting in a gradual decrease in the average particle size, which also improves the yield of ZnO particles to a certain extent.…”

“…However, due to their high cost, long reaction time, and large product particles, they are not suitable for commercial production of ZnO. The supercritical hydrothermal synthesis method is a novel method used for preparing various kinds of nanomaterials [10][11][12]. To reach supercritical hydrothermal conditions, zinc salt precursor compounds are rapidly heated by directly contacting supercritical water, and zinc oxide nanocrystals are formed during the fast heating period [13].…”