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