Heterophase Polymorph of TiO2 (Anatase, Rutile, Brookite, TiO2 (B)) for Efficient Photocatalyst: Fabrication and Activity

TiO₂ exists naturally in three crystalline forms: Anatase, rutile, brookite, and TiO₂ (B). These polymorphs exhibit different properties and consequently different photocatalytic performances. This paper aims to clarify the differences between titanium dioxide polymorphs, and the differences in homophase, biphase, and triphase properties in various photocatalytic applications. However, homophase TiO₂ has various disadvantages such as high recombination rates and low adsorption capacity. Meanwhile, TiO₂ heterophase can effectively stimulate electron transfer from one phase to another causing superior photocatalytic performance. Various studies have reported the biphase of polymorph TiO₂ such as anatase/rutile, anatase/brookite, rutile/brookite, and anatase/TiO₂ (B). In addition, this paper also presents the triphase of the TiO₂ polymorph. This review is mainly focused on information regarding the heterophase of the TiO₂ polymorph, fabrication of heterophase synthesis, and its application as a photocatalyst.

Fast organic degradation on Ti- and Bi-based photocatalysts via co-deposited Pt and Ni3(PO4)2 nanoparticles

Environmental remediation via semiconductor (SC) photocatalysis has attracted great attention over the past three decades. However, prospect for large scale application is still under debate, basically due to the bottleneck of fast charge recombination and/or slow surface reaction. Herein we report a universal solution of speeding up organic degradation simply via co-deposited Pt and nickel phosphate (NiP). Several representative SCs have been examined, including TiO₂ (anatase, rutile, and brookite) under a 320 nm light, and Bi-based SC (BiVO₄, Bi₂WO₆, and Bi₂MoO₆) under a 420 nm light. In all cases, the rates of phenol degradation in aqueous solution always varied not only in the order of NiP/Pt/SC > Pt/SC > NiP/SC > SC, but also NiP/Pt/SC > (Pt/SC + NiP/SC + SC). Meanwhile, hydroquinone and benzoquinone were produced as the main intermediates, but their concentration was much lower than that of phenol decreased, especially for NiP-containing sample. The solid was characterized with several techniques, including photoluminescence and (photo)electrochemical measurement. It is proposed that Pt and NiP act as co-catalysts for O₂ reduction and phenol oxidation, respectively. Such electron and hole transfer promote each other, additionally improving the efficiency of charge separation, and further increasing the rates of surface reactions. This work highlights the necessity of a versatile co-catalyst in SC photocatalysis.

Different effects of fluoride and phosphate anions on TiO2 photocatalysis (rutile)

At the same amounts adsorbed on Pt/rutile, fluoride was approximately 3 times more active than phosphate. A radical mechanism is proposed.

Trace Amount CoFe2O4 Anchored on a TiO2 Photocatalyst Efficiently Catalyzing O2 Reduction and Phenol Oxidation

Semiconducting TiO₂ is the most studied photocatalyst for organic oxidation by O₂. To accelerate the reaction, a cocatalyst for O₂ reduction or for organic oxidation is often used, but the bifunctional one is rare. Herein we report a spinel CoFe₂O₄ (CF) efficiently catalyzing O₂ reduction and phenol oxidation on TiO₂ in aqueous suspensions at pH 3-11. The composite materials (CF/TiO₂) were made by depositing 0-5 wt % CF onto TiO₂ through a hydrothermal method. Solid characterization showed that CF nanoparticles (5 nm) homogeneously distributed in CF/TiO₂, whereas the TiO₂ phase remained unchanged in crystal structure and crystallite size. For phenol oxidation under UV light, CF was nearly not active, but 0.01 wt % CF/TiO₂ was more active than TiO₂, by approximately a factor of 3.6. Such trend in activity among the catalysts was also observed from the photocatalytic reduction of O₂ to H₂O₂, from the electrochemical reduction of O₂, and from the photoelectrochemical oxidation of phenol and H₂O. An open-circuit potential and photoluminescence measurement suggest that there is an interfacial electron transfer from TiO₂ to CF, followed by O₂ reduction. Accordingly, a possible mechanism is proposed, involving CF catalysis for O₂ reduction and phenol oxidation. Then the mutual promotion between electron and hole transfer results in great enhancement in the efficiency of charge separation and hence in the rate of chemical reaction. Because spinel compounds have rich composition and unique structures, they are worthy of being further investigated as cocatalysts of a semiconductor photocatalysis.

TiO2–SiO2 Janus particles with highly enhanced photocatalytic activity

TiO₂–SiO₂ Janus particles, prepared by a Pickering emulsion method and calcined at 450 °C, due to their unique morphology and electronic structure showed highly enhanced photocatalytic activity.