Modulation of Bipolar Ultraviolet Current in TiO₂ Nanofilms for Switching Logic Devices via Ti Valence State Control

Recently, the application of titanium dioxide (TiO₂) in the context of the photoelectrochemical photocurrent switching (PEPS) effect has been extensively explored, offering significant potential for TiO₂ materials in areas such as logic gates, biosensing, and communications. Ti ions exist in multiple oxidation states, with each state exhibiting different photoelectrochemical activities, playing a crucial role in regulating the PEPS effect. However, research in this area remains relatively scarce. In this study, we utilized a thermal annealing method to modulate the oxidation states of Ti ions in TiO₂ nanofilms and investigated their respective PEPS effects. No bipolar behavior of the photocurrent was observed in untreated or low-temperature annealed amorphous TiO₂ thin nanofilms, whereas clear bipolar behavior was evident in the high-temperature annealed rutile TiO₂. This phenomenon was primarily attributed to the high activity of Ti³⁺ ions introduced by the phase transition, enabling photogenerated electrons to overcome the semiconductor–electrolyte potential barrier and participate in the reduction reaction within the solution. Furthermore, our research revealed a remarkable phenomenon where the potential barrier between high-temperature annealed rutile TiO₂ nanofilms and the electrolyte is influenced by the wavelength of the incident light source, leading to a reversal in current polarity under 254 and 365 nm illumination. This effect was a result of the accumulation of photogenerated electrons at the semiconductor/electrolyte interface, creating an opposing built-in electric field that lowered the potential barrier between the semiconductor and electrolyte. Finally, we constructed externally biased tunable Boolean logic gates based on rutile TiO₂ nanofilms, utilizing varying wavelengths of solar-blind ultraviolet light as input sources. This innovative approach offers a pathway toward achieving the multifunctional integration of optoelectronic devices in the post-Moore era.