Characterisation of oxygen defects and nitrogen impurities in TiO2 photocatalysts using variable-temperature X-ray powder diffraction

Abstract

TiO2-based powder materials have been widely studied as efficient photocatalysts for water splitting due to their low cost, photo-responsivity, earthly abundance, chemical and thermal stability, etc. In particular, the recent breakthrough of nitrogen-doped TiO2, which enhances the presence of structural defects and dopant impurities at elevated temperatures, exhibits an impressive visible-light absorption for photocatalytic activity. Although their electronic and optical properties have been extensively studied, the structure-activity relationship and photocatalytic mechanism remain ambiguous. Herein, we report an in-depth structural study of rutile, anatase and mixed phases (commercial P25) with and without nitrogen-doping by variable-temperature synchrotron X-ray powder diffraction. We report that an unusual anisotropic thermal expansion of the anatase phase can reveal the intimate relationship between sub-surface oxygen vacancies, nitrogen-doping level and photocatalytic activity. For highly doped anatase, a new cubic titanium oxynitride phase is also identified which provides important information on the fundamental shift in absorption wavelength, leading to excellent photocatalysis using visible light.

Comparative visible-light driven selective oxidation to aldehydes of phenylmethanol (benzyl alcohol) and 4-pyridinylmethanol (4-pyridinecarbinol) on N-TiO2 and some commercial TiO2 samples

“…However, broad the signal is, it is possible to observe a larger peak at g = 2.0045 and weaker peaks at g = 2.022 and g = 1.985, each at a distance of 32 G from the main one. This often-reported EPR triplet is seemingly a feature of nitrogen impurities in TiO 2 [1,11,18], and corresponds to a paramagnetic species N i • from interstitial nitrogen (N i ) chemically bound to a lattice [11,56]. The N i localized energy state is located a few tenths of eV above the valence band [57] and is identified as the active centre of visible light photoactivity.…”

Catalytic Composite Systems Based on N-Doped TiO2/Polymeric Materials for Visible-Light-Driven Pollutant Degradation: A Mini Review

“…It is well-known that the incorporation of nitrogen into the TiO2 framework occurs in two ways: substitutional and interstitial. In a more detailed sense, substitutional doping implies the replacement of oxygen with nitrogen element [32], whereas interstitial doping is based on the addition of a nitrogen element into the TiO2 lattice [31], as reported in Figure 2. Irie et al [33] argued that oxygen sites substituted by a nitrogen atom induce the generation of new electron-occupied energy levels above the valence band (VB) of TiO2.…”

A Review of MOFs and Their Composites‐Based Photocatalysts: Synthesis and Applications

“…[ 1 ] Hence, developing highly efficient photocatalysts is quite important in the field of photocatalysis. Up to now, many different types of functional nanomaterials, including metal oxides, [ 2–5 ] metal phosphides, [ 6–8 ] metal sulfides [ 9–12 ] as well as carbon‐based composites [ 13–15 ] have been synthesized and utilized as the photocatalysts for energy conversion systems. However, photocatalysts based on these materials still face some challenges, such as the low electronic conductivity and poor apparent quantum efficiency of the metal oxides‐based photocatalysts, [ 1 ] rapid recombination of the charge carries of the metal phosphides‐based photocatalysts, [ 6 ] the poor chemical stability of the metal sulfides‐based photocatalysts, [ 10 ] poor cycling stability of the carbon materials‐based photocatalysts.…”