From Photoanodes to HER Catalysts: Electrochemical Strategies for Multifunctional TiO₂-Based Composites

Titanium dioxide (TiO₂) remains one of the most attractive semiconductor platforms for water splitting due to its abundance, chemical stability, low cost, and compatibility with electrochemical modification strategies. However, its wide band gap and limited visible light activity require modification to improve charge carrier separation, extend optical response, and introduce additional catalytic functionality. This study summarizes a series of electrochemical approaches for engineering TiO₂-based hybrid composite films for photoelectrochemical (PEC) water splitting and alkaline hydrogen evolution reaction (HER).

TiO₂ films were primarily synthesized by plasma electrolytic oxidation (PEO), enabling the formation of porous oxide frameworks with tunable morphology and structure. Incorporation of copper species during synthesis produced TiO₂/Cu_xO heterostructures with modified optical properties and lower band gap values, resulting in enhanced PEC performance. Further development involved electrodeposition of Cu into through-oxide TiO₂ channels, followed by controlled annealing into Cu₂O, CuO or mixed oxide phases. This architecture promoted TiO₂-CuO interfacial contact and enabled unusual photocurrent switching behavior, where the photocurrent could be controlled by applied potential. Visible light sensitivity and improved charge transport highlighted the importance of conductive Cu pathways and oxide heterojunction formation. Beyond photoanode design, the same TiO₂/Cu architecture was adapted for alkaline HER. Galvanic displacement of ultralow Rh loadings onto Cu/TiO₂ produced Rh@Cu nanostructures embedded within the porous matrix. The optimized electrode, containing only 0.28 wt% Rh, achieved 10 mA cm^-2 at 58 mV in 1 M KOH, with a Tafel slope of 40 mV dec^-1, demonstrating acidic-like HER activity under alkaline conditions. Ongoing work with Ag@Cu/TiO₂ further explores plasmonically assisted visible light activity and photocurrent switching.

Overall, this study demonstrates electrochemical synthesis as a versatile platform for designing multifunctional TiO₂-based composites for PEC and electrocatalytic water splitting.