Enhanced Photoelectrocatalytic Water Splitting at Hierarchical Gd³⁺:TiO₂Nanostructures through Amplifying Light Reception and Surface States Passivation

Abstract

The influence of rare earth gadolinium (Gd 3+ ) ion doping on optical and photoelectrochemical properties of TiO 2 is studied. The hierarchical clump-type TiO 2 nanostructure was fabricated using poly-vinyl acetate as soft-template. The optical absorbance quantity of TiO 2 was strikingly promoted at bandgap energy region (380 nm) by Gd 3+ doping, as well as it extend a wide absorbance in visible wavelength region (400 -800 nm) elucidating the sub-bandgap formation. As a result, Gd 3+ :TiO 2 exhibits high photocurrent density than undoped TiO 2 in photoelectrocatalytic experiments. Another plausible reason for enhancing the photocurrent density at Gd 3+ :TiO 2 was analyzed through electrochemical impedance spectroscopy. The underlying mechanism of surface states controlled charge transfer at TiO 2 /electrolyte interfaces affected the photoelectrocatalytic hydrogen fuel generation, and compete with Gd 3+ ion doping through bottlenecking of photoelectrons trapping at surface states. The improved charge separation (e − /h + ) at Gd 3+ :TiO 2 result effective photoelectron collection and thus yield 180 % higher hydrogen gas (∼ 2.34 mL.h −1 .cm −2 ) generation compare to pristine TiO 2 (1.28 mL.h −1 .cm −2 ) under UV light irradiation. The improved optical and charge transfer characteristics of hierarchical TiO 2 by Gd 3+ ions can be implemented to wide range of other metal oxide based photocatalytic fuel generation. The photoelectrocatalytic (PEC) water oxidation phenomenon configured with light and semiconductor opens new pathways in cost-effective fuel generation (hydrogen and oxygen) using water as feed stock.1 Merits counting the zero-emission of harmful CO 2 into environment, 2 possibility of beneficial simultaneous process of pollutant treatment, 3,4 and/or biomass reformation toward hydrogen fuel generation foster PEC water oxidation as an emerging technology in green energy fuels. Nanostructured titanium dioxide (TiO 2 ) is one of the most studied candidates in PEC applications including fuel generation, 5-7 organic pollutant degradation, 8,9 photoelectrochemical biosensors, 10,11 photocatalytic self-cleaning coatings 12,13 and antimicrobial coatings.14 The clean fuel generation from photoelectrolysis of water using TiO 2 mesoporous electrodes received much attention, despite its bandgap (3.2 eV) lies in UV region, due to the appreciable chemical stability and environmentally begin nature. One of the approaches to promote PEC performance of TiO 2 in solar to fuel generation is extending the visible light activity of TiO 2 by bandgap modification through doping with metal ions (Fe,15 21 or inducing defects in crystallite lattice. 22,23 However the doping material perhaps shift the TiO 2 valence band more negative than the water oxidation potential 1.2 V vs RHE, which in turn might affect the PEC water oxidation rates. 24The lanthanide (Ln) rare earth ion (RE) doping at TiO 2 matrix is an alternative approach to enhance the light reception at TiO 2 . Furthermore, RE ions doping can alter the bandgap structure o...

Multifunctional Gadolinium‐Doped Mesoporous TiO2 Nanobeads: Photoluminescence, Enhanced Spin Relaxation, and Reactive Oxygen Species Photogeneration, Beneficial for Cancer Diagnosis and Treatment

“…There are strong differences in the chemical properties of Gd 3+ and Ti 4+ , and the mismatch in the ionic radii (180 ± 6 and 68 pm, respectively) makes it increasingly difficult to substitute Ti 4+ by Gd 3+ within the anatase lattice without perturbing the structure. Depending on the concentration, Gd 3+ ions can then adopt different environments when incorporated within titania, where Gd is in low concentration will likely enter the titania lattice, which is the reason for the increased crystal size . It has also been reported that when the lanthanide content increases, most dopant cations will be incorporated close to the semiconductor crystallite surface in a glass‐like environment, inhibiting the host material crystallization process …”

“…The enhancement of photocatalytic activity after Gd‐doping can be assigned to the higher adsorption, improved charge‐transfer efficiency, and the prevention of electron–hole recombination . To further explore how the electronic changes in the Gd‐doped TiO 2 affect the absorption and possibly improved charge transfer and electron–hole recombination, DFT calculations were performed and analyzed for undoped and Gd‐doped anatase TiO 2 …”

“…Depending on the concentration, Gd 3+ ions can then adopt different environments when incorporated within titania, where Gd is in low concentration will likely enter the titania lattice, which is the reason for the increased crystal size. [36] It has also been reported that when the lanthanide content increases, most dopant cations will be incorporated close to the semiconductor crystallite surface in a glasslike environ ment, inhibiting the host material crystallization process. [37] For higher concentrations, sufficient Gd and oxygen con tent can then result in a beneficial environment for the crea tion of new crystalline phases with RE participation.…”

“…[1] To further explore how the electronic changes in the Gddoped TiO 2 affect the absorption and possibly improved charge transfer and electron-hole recombination, DFT calculations were performed and analyzed for undoped and Gddoped anatase TiO 2 . [36,55]…”

Lanthanides Effects on TiO2 Photocatalysts

“…Moreover, the lanthanides adsorbed on the surface may act as traps to water molecule and -OH (hydroxyl anion) groups, which increase their density on the photocatalyst surface and can promote more intense formation of hydroxyl radicals. Another important feature related to the use of these compounds is that they serve as a Lewis base, which could concentrate the contaminants dispersed in aqueous medium on the semiconductor surface enhancing the electron transfer for the direct degradation of the contaminant or increasing the probability of interaction between the molecules and the radicals formed [11][12][13][14][15].…”