Precise synthesis of well-ordered ultrathin nanowire arrays with tunable active surface, though attractive in optoelectronics, remains challenging to date. Herein, well-aligned sub-10 nm TiO2 nanowire arrays with controllable corrugated structure have been synthesized by a unique monomicelle-directed assembly method. The nanowires with an exceptionally small diameter of ∼8 nm abreast grow with an identical adjacent distance of ∼10 nm, forming vertically aligned arrays (∼800 nm thickness) with a large surface area of ∼102 m2 g-1. The corrugated structure consists of bowl-like concave structures (∼5 nm diameter) that are closely arranged along the axis of the ultrathin nanowires. And the diameter of the concave structures can be finely manipulated from ∼2 to 5 nm by simply varying the reaction time. The arrays exhibit excellent charge dynamic properties, leading to a high applied bias photon-to-current efficiency up to 1.4% even at a very low potential of 0.41 VRHE and a superior photocurrent of 1.96 mA cm-2 at 1.23 VRHE. Notably, an underlying mechanism of the hole extraction effect for concave walls is first clarified, demonstrating the exact role of concave walls as the hole collection centers for efficient water splitting.