Defect-Engineered TiO₂ microtubes from biological Templates for boosted photocatalytic H₂ evolution

To address the limited specific surface area, insufficient active sites, and rapid recombination of photogenerated charge carriers in conventional TiO₂ particles, this study investigated the effects of different plant-fiber templates on the morphology, pore structure, oxygen vacancies, and photocatalytic H₂ evolution performance of TiO₂ microtubes, with the aim of elucidating the relationships among template characteristics, material structure, and charge-carrier behavior. Hare's-tail grass, reed, cattail, and pampas grass fibers were used as sacrificial hard templates. Four TiO₂ microtube samples, denoted as TiO₂ Lo, TiO₂ Pa, TiO₂ Ct, and TiO₂ Cs, respectively, were prepared through impregnation, hydrolysis, hydrothermal treatment, and calcination at 550 °C. Their phase composition, morphology, pore structure, and oxygen vacancies were characterized by XRD, SEM, N₂ adsorption–desorption, EPR, and XPS. Charge-carrier behavior was investigated using UV–Vis spectroscopy, steady-state and time-resolved PL spectroscopy, EIS, and transient photocurrent measurements. Photocatalytic H₂ evolution activity and cycling stability were evaluated under simulated solar irradiation using an aqueous methanol solution as the sacrificial reagent and 1% Pt as the cocatalyst. The results showed that all four samples consisted of anatase TiO₂ and successfully inherited the tubular structures or surface textures of the corresponding plant fibers, forming TiO₂ microtubes with different mesoporous structures, specific surface areas, and oxygen-vacancy concentrations. The oxygen-vacancy concentration decreased in the order of TiO₂ Cs > TiO₂ Lo > TiO₂ Pa > TiO₂ Ct. TiO₂ Ct exhibited a relatively large specific surface area and a low oxygen-vacancy concentration, together with the lowest steady-state PL intensity, the longest average PL lifetime, lower electrochemical impedance, and a higher transient photocurrent response, indicating more efficient separation and migration of photogenerated charge carriers. The H₂ evolution rates of TiO₂ Lo, TiO₂ Pa, TiO₂ Ct, and TiO₂ Cs were 2.10, 4.62, 7.26, and 5.81 mmol/(g · h), respectively. TiO₂ Ct exhibited the highest photocatalytic activity and maintained relatively stable H₂ evolution performance during a 15 h cycling test. By systematically comparing multiple natural plant-fiber templates using a unified synthesis route, this study provides a new strategy for designing TiO₂ photocatalysts through the synergistic regulation of morphology and defects.