Abstract Solar-driven water splitting promises a step towards large-scale solar energy storage. However, the bottleneck is always the poor performance of the used photoanodes, particularly in their capability to harvest the visible light. Herein, we report the design of visible-light active anatase titanium dioxide nanocrystals (sub 10 nm in size and with a surface area of ∼99 m 2 /g) using the sol-gel method followed by hydrothermal treatment with H 2 O 2 at relatively low temperature (180 °C). The fabricated nanocrystals demonstrate a band gap of 2.85 eV, with an increased amount of surface defects that overcome the negative effects of bulk defects as revealed by the positron annihilation measurements. These prepared nanocrystals have notable enhancement in solar light harvesting and water splitting efficiency compared to the commercial Degussa P25 counterpart. The photoactivity, structural and electrochemical behavior of the synthesized nanocrystals were investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM), photocurrent measurements, photoluminescence (PL), X-ray photoelectron spectroscopy (XPS), positron annihilation, and Doppler broadening analysis.