Defect-Induced Yellow Color in Nb-Doped TiO₂ and Its Impact on Visible-Light Photocatalysis

Abstract

Doping TiO 2 photocatalysts with foreign ions has been deemed an effective method to enhance visible light absorption, and thus increase their photocatalytic performance. Herein, we report that Nb-doped TiO 2 porous microspheres prepared by ultrasonic spray pyrolysis of peroxide precursor solution are yellow colored, and the yellow coloration becomes increasingly conspicuous with increasing Nb dopant concentration.Comprehensive spectral analyses show that both surface peroxo species and bulk Ti 3+ are introduced into TiO 2 microsphere samples together by charge compensation with Nb 5+ dopant, and are responsible for the coloration of TiO 2 . The Nb-doped microspheres show higher photocatalytic rates than undoped TiO 2 for the degradation of gaseous acetaldehyde under visible irradiation, but slower rates under ultraviolet light. Moreover, the photocatalytic mineralization rates of acetaldehyde to CO 2 are lowered with Nb-doping under both visible and UV irradiation. Correlation between the results of surface photovoltage spectroscopic (SPS) characterizations and photocatalytic tests suggests that surface peroxo states are relevant to the visible light stimulated charge separation and photocatalytic reactions, albeit holes trapped in these states have lower reactivity than in valence band. On the other hand, the enhanced photoluminescence in the near-infrared region, reduced SPS response in the UV region as well as photochromic phenomena during photocatalytic process indicated that Ti 3+ defects serve as charge carrier recombination centers and display adverse effect to photocatalytic activity of Nb-doped TiO 2 , especially under UV irradiation.

The Role of Nb Addition in TiO2 Nanoparticles: Phase Transition and Photocatalytic Properties

“…The authors investigated the optical properties of the TiO 2 anatase phase, observing a slight reduction with the addition of Nb ions, that is, 3.25 eV ( x = 0.0) to 3.18 eV ( x = 8.3 mol.%) . Similar results have been reported for Ti 1− x Nb i O 2 solid solutions prepared by different routes …”

“…[47] Similar results have been reported for Ti 1Àx Nb i O 2 solid solutions prepared by different routes. [15,17,23,57,81] Figure 6(A,B) show the evolution in the degradation of methylene blue (MB) and rhodamine B (RhB) dyes for the selected pristine (x ¼ 0.0) and Nb-doped (x ¼ 0.037) TiO 2 samples, respectively, annealed at 400 and 900 C.…”

Recent Advances in Niobium-Based Materials for Photocatalytic Solar Fuel Production

“…For TiO 2 , the defect state provided by the doping is located upon the conduction band minimum and contributes electrons to the unoccupied Ti 3d orbital without introducing additional states in the bandgap. However, in order to satisfy the charge compensation, the substitution can be accompanied by the incorporation of other defects as oxygen vacancies or eVen Ti 3+ defects below the conductive band, which lead to a visible range absorption [61]. In Table 2, examples of Nb-doped semiconductors for H 2 eVolution are listed.…”

“…Another Niobium has been used also as dopant, as it can improve the photocatalytic activities of anatase TiO 2 [61,[99][100][101], by changing the crystal structure, electrical properties, and absorption characteristics of TiO 2 . It was proposed that Nb would dope into TiO 2 lattice and decrease the band gap energy of the photocatalyst, and thereby Nb substitution on the Ti site creates a Nb 5+ defect state located upon the conduction band minimum.…”

Surface Segregation of Niobium and Tantalum in Titanium Dioxide. Overview

“…Kong et al have revealed that niobium incorporation results in reduction in the grain size of TiO 2 .…”