Effect of Annealing Temperature on the Photocatalytic Activity of TiO2 Thin Films

Magnetic titanium dioxide based nanocomposites have been gaining increasingly high attention in sustainable environmental protection fields; these materials can integrate the advantages of magnetic recovery and the superior photocatalysis performance of titanium dioxide. However, direct contact between crystalline photoactive titanium dioxide and magnetic iron oxide gives rise to lower photoreactivity than pure titanium dioxide for the pollutant degradation. To overcome the challenge, a number of researches have been done and significant process has been made in this area in recent years. This review primarily focuses on the properties of magnetic particles, structural models, preparation methods and techniques to improve photocatalytic performance of magnetic titanium dioxide based photocatalysts, and aims to provide a systematic overview of the current knowledge of these subjects. Notably, the recently discovered technique coupling graphene into magnetic iron oxide–titanium dioxide system is discussed, which opens a promising future for preparation of magnetic semiconductors with excellent properties. Current challenges and prospects are also proposed at the end of this article. In particular, the examination of corresponding deactivation or regeneration mechanisms should be investigated further, from the point of view of practical applications.

Magnetic titanium dioxide based nanomaterials: synthesis, characteristics, and photocatalytic application in pollutant degradation

Visible light photocatalytic performance of La-Fe co-doped SrTiO3 perovskite powder

Synthesis and characterization of Fe-doped β-Bi2O3 porous microspheres with enhanced visible light photocatalytic activity

The present work describes the effects of water on Fe-doped nanoparticulate CeO2, produced by flame spray pyrolysis, which is a critical environmental issue because CeO2 is not stable in typical atmospheric conditions. It is hygroscopic and absorbs ~29 wt % water in the bulk when exposed to water vapor but, more importantly, it forms a hydrated and passivating surface layer when immersed in liquid water. In the latter case, CeO2 initially undergoes direct and/or reductive dissolution, followed by the establishment of a passivating layer calculated to consist of ~69 mol % solid CeO2·2H2O and ~30 mol % gelled Ce(OH)4. Under static flow conditions, a saturated boundary layer also forms but, under turbulent flow conditions, this is removed. While the passivating hydrated surface layer, which is coherent probably owing to the continuous Ce(OH)4 gel, would be expected to eliminate the photoactivity, this does not occur. This apparent anomaly is explained by the calculation of (a) the thermodynamic stability diagrams for Ce and Fe; (b) the speciation diagrams for the Ce4+-H2O, Ce3+-H2O, Fe3+-H2O, and Fe2+-H2O systems; and (c) the Pourbaix diagrams for the Ce-H2O and Fe-H2O systems. Furthermore, consideration of the probable effects of the localized chemical and redox equilibria owing to the establishment of a very low pH (<0) at the liquid-solid interface also is important to the interpretation of the phenomena. These factors highlight the critical importance of the establishment of the passivating surface layer and its role in photocatalysis. A model for the mechanism of photocatalysis by the CeO2 component of the hydrated phase CeO2·2H2O is proposed, explaining the observation of the retention of photocatalysis following the apparent alteration of the surface of CeO2 upon hydration. The model involves the generation of charge carriers at the outer surface of the hydrated surface layer, followed by the formation of radicals, which decompose organic species that have diffused through the boundary layer, if present.

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Aqueous and Surface Chemistries of Photocatalytic Fe-Doped CeO2 Nanoparticles

Effect of Fe doping on TiO2 films prepared by spin coating

Essentially fully dense titania thin films were spin coated on fused quartz substrates under identical conditions and subjected to annealing over the range 750°–900°C. The films were of a consistent ~400 nm thickness. The anatase → rutile phase transformation temperature was between 750°C and 800°C, with first-order kinetics; annealing at 900°C yielded single-phase rutile. Silicon contamination from the fused quartz substrate was considered to be critical since it suppressed both titania grain growth (maintaining constant grain size) and the phase transformation (occurring at an unusually high temperature); its presence also was considered to be responsible for the formation of lattice defects, which decreased the transmittances and the band gaps.

Effect of annealing temperature on titania thin films prepared by spin coating

Thin films of TiO2 doped with Fe and Mn were deposited on F-doped SnO2-coated glass by spin coating. Dopant concentrations of 3–7 wt% (metal basis) were used. The structural, chemical, and optical characteristics of the films were investigated. Laser Raman microspectroscopy and glancing angle X-ray diffraction data showed that the films consisted of the anatase polymorph of TiO2. X-ray photoelectron spectroscopy data indicated the presence of Fe3+, Mn4+, and Mn3+ in the doped films, as predicted by calculated thermodynamic stability diagrams, and the occurrence of atomic disorder and associated structural distortion. Ultraviolet–visible spectrophotometry data showed that the optical indirect band gap of the films decreased significantly with increasing dopant levels, from 3.36 eV (undoped) to 2.95 eV (7 wt% Fe) and 2.90 eV (7 wt% Mn). These improvements are attributed to single (Fe) or multiple (Mn) shallow electron/hole trapping sites associated with the dopant ions.

Fe-doped and Mn-doped titanium dioxide thin films

Mn-doped titania thin films prepared by spin coating

Thin films of TiO 2 doped with Mn were deposited on F-doped SnO 2 -coated glass using spin coating. The concentration of the dopant was in the range 0–7 wt% Mn (metal basis). The films were examined in terms of the structural, chemical, and optical properties. Glancing angle X-ray diffraction data show that the films consisted of the anatase polymorph of TiO 2 , without any contaminant phases. The X-ray photoelectron spectroscopy data indicate the presence of Mn 3+ and Mn 4+ in the doped films as well as atomic disorder and associated structural distortion. Ultraviolet–visible spectrophotometry data show that the optical indirect band gap of the films decreased significantly with increasing manganese doping, from 3.32 eV for the undoped composition to 2.90 eV for that doped with 7 wt% Mn.

C.Y.W. Lin

Papers

Effect of Fe doping on TiO2 films prepared by spin coating

Effect of annealing temperature on titania thin films prepared by spin coating

Fe-doped and Mn-doped titanium dioxide thin films

Mn-doped titania thin films prepared by spin coating

Multivalent Mn-doped TiO2 thin films