Interfacial Charge Transfer Modulation via Phase Junctions and Defect Control in Spaced TiO2 Nanotubes for Enhanced Photoelectrochemical Water Splitting

Anodic titanium oxide (TiO₂) nanotubes have garnered significant interest as photoelectrodes for photoelectrochemical (PEC) water splitting; however, their intrinsic structural and crystallographic limitations often lead to suboptimal PEC efficiency. Herein, spaced TiO₂ nanotubes (SPNTs) with enhanced intertubular spacing are utilized as photoelectrodes to improve light penetration and harvesting. Amorphous SPNTs are subjected to annealing under various atmospheric and thermal conditions to induce phase transitions, forming anatase, rutile, and anatase–rutile heterojunctions. The highest PEC performance is achieved with SPNTs annealed at 600°C in an argon atmosphere (Ar-600), exhibiting the formation of anatase–rutile heterojunctions and abundant oxygen vacancies (V_O). These features facilitate rapid charge transfer, enhancing PEC activity. Notably, Ar-600 demonstrates a photocurrent density of 0.34 mA cm⁻² at 1.23 V vs. reversible hydrogen electrode (RHE), an incident photon-to-current efficiency of 48% at 360 nm, a charge carrier density of 2.5 × 10¹⁹ cm⁻³, and the lowest charge transfer resistance (R_ct) of 556 Ω. These values represent a 2.1-fold increase in photocurrent and a fourfold reduction in R_ct compared to conventional close-packed TiO₂ nanotubes. Furthermore, the Ar-600 electrode achieves a hydrogen production amount of 105.4 μL cm⁻² after 3 h of PEC operation, highlighting its potential for practical solar-to-hydrogen conversion.