Supercritical hydrothermal synthesis of titanium dioxide nanostructures with controlled phase and morphology

• A supercritical synthesis of controlled TiO2 nanostructures was carried out.
• 3D hierarchical spheres, 2D nanosheets, and 1D nanorods were obtained in 15 min.
• NaOH solution pH and supercritical temperature were two determinant factors.
• A dissolution–nucleation-growth mechanism was proposed.

A novel template- and organic-free synthesis of TiO2 nanostructures with controlled phase and morphology was realized through batch supercritical hydrothermal treatment (400 °C) of titanate nanotubes (TNTs) with H2O2 in NaOH aqueous solution. Well-defined 3D titanate hierarchical spheres (THSs), 2D multilayered titanate nanosheets (TNSs), and 1D monodisperse anatase nanorods (ANRs) exposing (0 1 0) facets were prepared in 15 min by slightly varying the NaOH solution pH. Specifically, the obtained Na/H-THSs (without/with HCl neutralization) exhibited highly porous structures with large specific surface area (109 m2 g−1 and 196 m2 g−1, respectively). Temperature-dependent phase and morphology evolutions of products under subcritical condition (200 and 300 °C) were investigated. The formation of the TiO2 nanostructures from TNTs was proposed mainly following a dissolution–nucleation-growth mechanism, suggesting that both supercritical temperature and NaOH solution pH were determinant factors governing the nucleation and growth process and thus the phase and morphology.

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