Construction of type Ⅱ ZnO/TiO2 nanotube heterostructures with efficient charge separation via in situ loading onto highly ordered anodic nanotube arrays

The recombination of photogenerated carriers limits the photoelectrochemical (PEC) efficiency of titanium dioxide (TiO 2 ) nanotubes. To address this, we constructed a type Ⅱ ZnO/TiO 2 heterostructure via in situ hydrothermal loading of ZnO nanoparticles onto highly ordered anodic TiO 2 nanotube arrays. A critical interplay between precursor soaking duration (1 day vs. 5 days) and hydrothermal temperature (120 °C vs. 150 °C) was uncovered, that is, prolonged soaking and lower hydrothermal temperatures were beneficial for enhancing photocurrent. A two-step annealing process utilized in this work can enhance precursor infiltration into the nanotube channels, promoting uniform ZnO nucleation and robust interfacial adhesion. The optimized heterostructure electrode achieved a nearly twofold increase in carrier concentration (from 5.56 × 10 18 to 1.14 × 10 19  cm -3 ) along with reduced charge-transfer resistance and enhanced charge separation efficiency compared to unmodified TiO 2 . Electrochemical impedance spectroscopy study revealed that the modified heterostructure electrode exhibits lower surface charge-transfer resistance and higher bottom barrier-layer resistance than the unmodified electrode, thereby facilitating efficient interfacial charge transfer and suppressing dark current. Using glucose as a probe, the modified electrode delivered a higher photocurrent density and a faster response than the unmodified one. This work provides an alternative approach to optimizing semiconductor heterojunctions, and the improved process demonstrates promising potential for glucose sensing applications.