Probing the Defect-Driven Tunable Photo(electro)catalytic Water-Splitting Behavior of Pulsed-Laser-Deposited Titania

With the aim of offering advanced and selective catalysis, a series of defect-rich titania (TiO_2.40, TiO_1.81, TiO_1.74, TiO_1.72, and TiO_1.54) are prepared via scalable, precise pulsed laser deposition technique. Their catalytic performance is compared to stoichiometric Degussa P25 TiO₂. On mere decreasing O/Ti ratio, native titania turns from a photoelectrocatalyst to electrocatalyst for improved water splitting. At a stoichiometric composition of TiO_1.81, titania acts as an absolute photoanode for an oxygen evolution reaction and generates a photocurrent of 0.62 mA cm^–2 at 0 V versus reversible hydrogen electrode under AM1.5 simulated solar illumination while acting as a poor electrocatalyst with high onset potential of 650 mV for a hydrogen evolution reaction. On increasing the oxygen vacancies in titania, relatively higher electrocatalytic hydrogen evolution is observed for defect-rich TiO_1.54 and affords a current density of 10 mA cm^–2 at just an overpotential of 610 mV, despite its negligible photoelectrocatalytic activity. Since defect concentration in titania is mainly responsible for the trade-off between electrocatalytic and photoelectrocatalytic water-splitting behavior, systematic attempts have also been made to understand the interplay between defects and catalysis of titania. The different intrinsic characteristics of defect-rich titania ranging from microscopic structural evolution (X-ray diffraction and microscopic imaging) to chemical speciation (X-ray photoelectron spectroscopy, electron paramagnetic resonance, and ultraviolet–visible spectroscopy) to kinetics of electro/photoelectrochemical water splitting [intensity-modulated photocurrent/photovoltage spectroscopy, electrochemical impedance spectroscopy, open-circuit photovoltage decay, depth of trap states (DOS) measurement] are extensively studied in this work. Unequivocally, the higher photoelectrocatalytic water-splitting activity of TiO_1.81 is due to its large microstrain (1.9%) associated optimum defect-induced lattice distortion index (0.006), which facilitates high charge transfer efficiency (82%) with a low recombination rate constant (1.98 s^–1) of photogenerated electron–hole pair by favorable DOS. On the other hand, high density of oxygen vacancy in TiO_1.54 induces a magnanimous distortion index (0.035) in TiO₆ octahedra with a small microstrain of 0.4%, which provides a high donor density (10.2 × 10¹⁸ cm^–3) and favors efficient electrocatalytic water-splitting activity. Overall, this work highlights the overlooked and unexploited facets of defect engineering.