Enhanced photoelectrocatalytic water splitting at hierarchical Gd³⁺:TiO₂ nanostructures through amplifying light reception and surface states passivation

The influence of rare earth gadolinium (Gd³⁺) ion doping on optical and photoelectrochemical properties of TiO₂ is studied. The hierarchical clump-type TiO₂ nanostructure was fabricated using poly-vinyl acetate as soft-template. The optical absorbance quantity of TiO₂ was strikingly promoted at bandgap energy region (380 nm) by Gd³⁺ doping, as well as it extend a wide absorbance in visible wavelength region (400 - 800 nm) elucidating the sub-bandgap formation. As a result, Gd³⁺:TiO₂ exhibits high photocurrent density than undoped TiO₂ in photoelectrocatalytic experiments. Another plausible reason for enhancing the photocurrent density at Gd³⁺:TiO₂ was analyzed through electrochemical impedance spectroscopy. The underlying mechanism of surface states controlled charge transfer at TiO₂/electrolyte interfaces affected the photoelectrocatalytic hydrogen fuel generation, and compete with Gd³⁺ ion doping through bottlenecking of photoelectrons trapping at surface states. The improved charge separation (e^-/h⁺) at Gd³⁺:TiO₂ result effective photoelectron collection and thus yield 180 % higher hydrogen gas (~ 2.34 mL.h^-1.cm^-2 ) generation compare to pristine TiO₂ (1.28mL.h^-1.cm^-2) under UV light irradiation. The improved optical and charge transfer characteristics of hierarchical TiO₂ by Gd³⁺ ions can be implemented to wide range of other metal oxide based photocatalytic fuel generation.