Enhancement of Photo-Oxidation Activities Depending on Structural Distortion of Fe-Doped TiO2 Nanoparticles

Abstract

To design a high-performance photocatalytic system with TiO2, it is necessary to reduce the bandgap and enhance the absorption efficiency. The reduction of the bandgap to the visible range was investigated with reference to the surface distortion of anatase TiO2 nanoparticles induced by varying Fe doping concentrations. Fe-doped TiO2 nanoparticles (Fe@TiO2) were synthesized by a hydrothermal method and analyzed by various surface analysis techniques such as transmission electron microscopy, Raman spectroscopy, X-ray diffraction, scanning transmission X-ray microscopy, and high-resolution photoemission spectroscopy. We observed that Fe doping over 5 wt.% gave rise to a distorted structure, i.e., Fe2Ti3O9, indicating numerous Ti(3+) and oxygen-vacancy sites. The Ti(3+) sites act as electron trap sites to deliver the electron to O2 as well as introduce the dopant level inside the bandgap, resulting in a significant increase in the photocatalytic oxidation reaction of thiol (-SH) of 2-aminothiophenol to sulfonic acid (-SO3H) under ultraviolet and visible light illumination.

Keywords: 2-Aminothiophenol; Bandgap narrowing; Distorted TiO2; Photo-oxidation.

Fig. 1

TEM images. a 1, b 3, and c 5 wt.%, and corresponding high-resolution images (a′), (b′), and (c′). White arrows indicate the location of inset diffraction patterns

Fig. 2

XRD and corresponding Raman spectra show that the crystal structures and chemical states vary with the amount of Fe dopant. a, d 1 wt.% Fe@TiO₂. b, e 3 wt.% Fe@TiO₂. c, f 5 wt.% Fe@TiO₂

Fig. 3

XAS spectra of a Fe L-edge, b Ti L-edge, and c O K-edge of 1 wt.% Fe@TiO₂ (black line), 3 wt.% Fe@TiO2 (red line), and 5 wt.% Fe@TiO2 (blue line). Insets are STXM images of each spectrum

Fig. 4

HRPES results for Fe 2p, Ti 2p, and O 1s core level spectra of Fe@TiO₂ nanoparticles with various doping levels. a, d, g Core level spectra from 1 wt.% Fe@TiO₂. b, e, h Those from 3 wt.% Fe@TiO₂. c, f, i Those from 5 wt.% Fe@TiO₂. HRPES results corresponding to the S 2p core level spectrum obtained after photocatalytic oxidation of 2-aminothiophenol on j 1, k 3, and l 5 wt.% Fe@TiO₂ nanoparticles

Fig. 5

Plots of the ratios of S3 to S1, indicating photocatalytic activity by oxidation of 2-ATP as a function of molecular exposure under a 365-nm UV light and b 540-nm visible light

Fig. 6

a Valence band spectra and b the Fermi-edge obtained at 0, 1, 3, and 5 wt.% of Fe-doped TiO₂ nanoparticles (marked) from the photon energy of 80 eV

Scheme 1

Schematic diagram for charge transfer with reaction mechanism