Junqing Yan

Understanding the effect of surface/bulk defects on the photocatalytic activity of TiO2: anatase versus rutile

The sole effect of surface/bulk defects of TiO2 samples on their photocatalytic activity was investigated. Nano-sized anatase and rutile TiO2 were prepared by hydrothermal method and their surface/bulk defects were adjusted simply by calcination at different temperatures, i.e. 400-700 °C. High temperature calcinations induced the growth of crystalline sizes and a decrease in the surface areas, while the crystalline phase and the exposed facets were kept unchanged during calcination, as indicated by the characterization results from XRD, Raman, nitrogen adsorption-desorption, TEM and UV-Vis spectra. The existence of surface/bulk defects in calcined TiO2 samples was confirmed by photoluminescence and XPS spectra, and the surface/bulk defect ratio was quantitatively analyzed according to positron annihilation results. The photocatalytic activity of calcined TiO2 samples was evaluated in the photocatalytic reforming of methanol and the photocatalytic oxidation of α-phenethyl alcohol. Based on the characterization and catalytic results, a direct correlation between the surface specific photocatalytic activity and the surface/bulk defect density ratio could be drawn for both anatase TiO2 and rutile TiO2. The surface defects of TiO2, i.e. oxygen vacancy clusters, could promote the separation of electron-hole pairs under irradiation, and therefore, enhance the activity during photocatalytic reaction.

Sub-10 nm rutile titanium dioxide nanoparticles for efficient visible-light-driven photocatalytic hydrogen production

Titanium dioxide is a promising photocatalyst for water splitting, but it suffers from low visible light activity due to its wide band gap. Doping can narrow the band gap of titanium dioxide; however, new charge-carrier recombination centres may be introduced. Here we report the design of sub-10 nm rutile titanium dioxide nanoparticles, with an increased amount of surface/sub-surface defects to overcome the negative effects from bulk defects. Abundant defects can not only shift the top of the valence band of rutile titanium dioxide upwards for band-gap narrowing but also promote charge-carrier separation. The role of titanium(III) is to enhance, rather than initiate, the visible-light-driven water splitting. The sub-10 nm rutile nanoparticles exhibit the state-of-the-art activity among titanium dioxide-based semiconductors for visible-light-driven water splitting and the concept of ultra-small nanoparticles with abundant defects may be extended to the design of other robust semiconductor photocatalysts.

Tungsten oxide single crystal nanosheets for enhanced multichannel solar light harvesting.

Substoichiometric tungsten oxide single-crystal nanosheets are successfully prepared via the exfoliation of layered tungstic acid and subsequent introduction of oxygen vacancies. The combination of different strategies, i.e., 2D-structure construction, the introduction of surface oxygen vacancies, and the creation of localized surface plasmon resonance can promote the light-harvesting performance of tungsten oxide through accumulative and synergistic effects.

Synergetic promotion of the photocatalytic activity of TiO2 by gold deposition under UV-visible light irradiation.

Under UV-visible light irradiation, multiple synergetic promotion effects on the photocatalytic activity of TiO2 by gold deposition can be observed, and plasmonic Au/TiO2 shows great potential for photocatalytic solar conversion.

Facile synthesis of an iron doped rutile TiO2 photocatalyst for enhanced visible-light-driven water oxidation

A facile fast hydrolysis route to a three-dimensional flow-like iron doped rutile TiO2 nanostructure is developed. With iron species doping into both the bulk phase and the surface, the bandgap narrowing of rutile TiO2 is realized and the dissociative adsorption of water on the surface is promoted, which accordingly lead to greatly enhanced activity in visible-light-driven water oxidation.

Ultrafine metal nanoparticles loaded on TiO2 nanorods: Synthesis strategy and photocatalytic activity

Ultrafine noble metal nanoparticles (Pt, Pd, or Au) co-catalyst loaded on the surface of rutile and brookite TiO2 were prepared via a simple photo-deposition strategy under high vacuum conditions. The properties of the prepared samples were determined by different characterization techniques, including X-ray diffraction, transmission electron microscopy, diffuse reflectance ultraviolet–visible spectroscopy, and photoluminescence spectroscopy. The photocatalytic performance of the samples was evaluated by monitoring the reforming of methanol. Co-catalyst loading greatly improved the photocatalytic activity of TiO2. Specifically, Pt-TiO2 displayed the highest photocatalytic activity among all samples studied, followed by Pd-TiO2 and then Au-TiO2. Furthermore, this photocatalytic behavior was not influenced by the intrinsic nature of the TiO2 semiconductor photocatalyst. Similar photocatalytic activity trends were achieved with both sets of noble metal-loaded photocatalysts prepared using rutile and brookite TiO2 as supports. By examining the physicochemical and photocatalytic properties, the factors controlling the photocatalytic activity of the noble metal-loaded TiO2 samples were discussed in detail.

Polyoxometalate-Based Metal–Organic Frameworks as Visible-Light-Induced Photocatalysts

Two stable 3D polyoxometalate-based metal-organic frameworks (PMOFs), [CuI12(trz)8(H2O)2][α-SiW12O40]·2H2O (1) and [CuI12(trz)8Cl][α-PW12O40] (2) (Htrz = 1- H-1,2,4-triazole) based on Keggin-type POMs were successfully obtained and fully characterized. The basic building units of the two PMOFs are [CuI12(trz)8], but polyoxoanion (POA) template effect leads to different structures and properties: 1 represents an interesting example that [α-SiW12O40]4- locate in the nine-membered Cu-trz rings through Cu···O weak interactions to form a 3D framework, whereas 2 shows a 3D structure constructed from 2D bilayer cationic network [CuI12(trz)8Cl]3+ and [α-PW12O40]3- lying in the adjacent layers via Cu···O weak interactions. PMOF 1 as unusual visible-light photocatalyst exhibit significantly enhanced photocatalytic activity under visible-light and excellent stability during the photocatalysis process for recovering and recycling, as well as photocatalytic hydrogen evolution activity.

Hydrothermal synthesis and photocatalytic properties of tantalum pentoxide nanorods

Abstract Tantalum pentoxide (Ta 2 O 5 ) nanorods were hydrothermally synthesized using polyethylene glycol (PEG) as a guiding agent. The nanorods were characterized by X-ray diffraction, scanning and transmission electron microscopies, and diffuse reflectance ultraviolet-visible and photoluminescence spectroscopies. The effects of crystallization duration and Ta 2 O 5 /Sr(OH) 2 ratio on the product morphology were investigated, and a growth mechanism was proposed. Phase-pure Ta 2 O 5 nanorods with controlled morphology were formed in the presence of PEG and Sr(OH) 2 , which was necessary to form the nanorods. Sr(OH) 2 induced the surface dissolution and re-growth of Ta 2 O 5 . PEG induced the anisotropic growth of Ta 2 O 5 by acting as a capping agent. The products were used to photocatalytically degrade rhodamine B under ultraviolet irradiation. The catalytic activity directly correlated with the length-diameter ratio of the Ta 2 O 5 nanorods. A maximum apparent reaction rate constant of 0.156 min −1 was obtained. The Ta 2 O 5 nanorods were stable during photocatalytic reaction and could be recycled several times without loss of activity.