Growing branches on one-dimensional TiO₂ nanostructures to construct nanotrees is an effective tactic to promote their photoelectrical performance for various applications in photoelectrocatalysis and solar cells. The appropriate choice of the trunk and branch to achieve excellent properties is of general interest. Herein we report a comparative study on TiO₂ nanotrees for photocatalytic (PC) and photoelectrocatalytic (PEC) degradation of phenol in water under UV light illumination. An alkali-hydrothermal technique was adopted to grow anatase TiO₂ arrays of polycrystalline nanobelts and single-crystalline nanowires, separately, on metallic Ti substrates. Using a precursor solution derived from solution combustion synthesis, few-layer TiO₂ nanosheets were then deposited to construct sheet-on-belt (SOB) and sheet-on-wire (SOW) TiO₂ nanotrees. We found that the PEC efficiency is promoted upon constructing the nanotrees; whilst the improvement in the PC activity is relatively insignificant. The length and atomic arrangement of the trunk readily affect the PEC performance. With a fixed branch precipitation duration, the SOW TiO₂ nanotrees with a film thickness of 3 μm exhibited the best PEC capability towards phenol degradation. The PEC reaction rate constant is 0.40 h⁻¹, which is 1.7 times that of the pristine alkali-hydrothermal nanowire film (0.23 h⁻¹). This PEC reaction rate constant is even 47 times that of the pristine nanowire film to assist the PC degradation of phenol in water (0.0085 h⁻¹). The present study suggests that the single-crystalline trunk plays a key role in the photoelectrical performance of TiO₂ nanotrees.