Epitaxial Self-Assembled Vertically Aligned Nanocomposite Photoelectrodes with Tunable Ambipolar Response

Photocurrent polarity switching in semiconductor photoelectrodes enables the development of optoelectronic switches and logic devices driven by illumination and applied potential. Here, a new class of ambipolar photoelectrodes that display controlled photoelectrochemical photocurrent switching (PEPS) behavior, based on epitaxial self-assembled vertically aligned NiTiO₃–TiO₂ nanocomposite thin films synthesized via pulsed laser deposition (PLD) is reported. Phase-pure NiTiO₃ and TiO₂ films are grown under optimized deposition conditions, while mixed-phase nanocomposites are engineered by adjusting the Ni/Ti ratio of the ablated targets. Structural characterization confirms the formation of vertically aligned epitaxial rutile TiO₂ nanocolumns within the NiTiO₃ matrix at high Ni/Ti ratios, and NiTiO₃ nanocolumns within the TiO₂ matrix at low Ni/Ti ratios. Photoelectrochemical (PEC) measurements reveal ambipolar behavior with a composition-dependent crossover potential between anodic and cathodic photocurrents tunable over a range of 800 mV. Phase-pure NiTiO₃ exhibits intrinsic photocurrent switching, whereas TiO₂ maintains a stable photoanodic response. The introduction of NiTiO₃ into TiO₂ substantially disrupts photoanodic performance, while TiO₂ incorporation into NiTiO₃ enhances photoanodic photocurrent without significantly affecting photocathodic activity. These findings establish vertically aligned nanocomposites (VANs) as a new platform for switchable photoelectrodes and provide new insights into the role of mixed-phase oxide structures in PEC logic devices and optoelectronic switching applications.