With
the aim of offering advanced and selective catalysis, a series
of defect-rich titania (TiO2.40, TiO1.81, TiO1.74, TiO1.72, and TiO1.54) 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 TiO1.81, 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 TiO1.54 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 TiO1.81 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 TiO1.54 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.
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