Exploiting a synthesis protocol to tailor TiO 2 with a unique morphology and crystal phase has received considerable interest in the energy and environmental fields. We here describe the use of a titanium glycolate precursor in a hydrothermal hydrolysis reaction to engineer TiO 2 nanocrystals with different crystal phases and structures. Anatase TiO 2 nanocrystals with twinned structures were obtained by using a lower amount of NaOH in the hydrolysis system, while brookite TiO 2 nanocrystals were formed when higher amounts of NaOH were employed. The as-synthesized different TiO 2 nanocrystals have a suitable bandgap to harvest photons and a more negative bottom level of the conduction band than the redox potential of H + /H 2 indicating their potential as hydrogen-evolution semiconductor photocatalysts. However, the TiO 2 nanotwins show promoted charge separation efficiency, and thus result in superior photocatalytic H 2 generation activity compared to the anatase and brookite TiO 2 nanocrystals. Our findings provide an effective and versatile solution for the fabrication of TiO 2 -based nanostructures with different phases and morphologies through chemical conversion of powder precursor nanoparticles, which could pave the way to the design of other functional nano-oxides with unique structures.