First-Transition Metal Oxocomplex–Surface-Modified Titanium(IV) Oxide for Solar Environmental Purification

The ongoing global energy and environmental issues warrant the development of environmental catalysts for decomposing pollutants in water and air by utilizing solar energy named as “solar environmental catalysts.” This chapter describes the recent studies on a novel class of solar environmental catalysts consisting of TiO2 and molecular-scale first-row transition metal oxide clusters (or metal oxocomplexes) on the surface (MOs/TiO2). The TiO2 surface modification with the oxocomplexes by the chemisorption–calcination cycle (CCC) technique presents a novel band engineering for fine-tuning the band energy. The unique physicochemical and electronic properties of MOs/TiO2 give rise to the outstanding photocatalytic activity for the decomposition of organic pollutants. The combination with the rapidly growing technique for preparation of TiO2 nanostructures allows us to expect further improvement of the performances and the wide application to the solar chemical transformation for producing useful substances

Molecular metal oxide cluster-surface modified titanium (IV) dioxide photocatalysts

The surface modification of TiO2 with molecular sized metal oxide clusters has recently been shown to be a promising approach for providing TiO2 with visible-light activity and/or improved UV activity. This short review summarizes the effects of the surface modification of TiO2 with the oxides of iron and tin selected from d- and p-blocks, respectively, on the photocatalytic activity. Fe(acac)(3) and [Sn(acac)(2)]Cl-2 chemisorption on the TiO2 surface occurs by ligand-exchange and ion-exchange, respectively. Taking advantage of the strong adsorption, we formed extremely small metal oxide clusters on TiO2 by the chemisorption-calcination cycle (CCC) technique with their loading amount strictly controlled. The iron oxide surface modification of P-25 (anatase/rutile = 4: 1, w/w, Degussa) gives rise to a high level of visible-light activity and a concomitant increase in the UV-light activity for the degradation of model organic pollutants. On the other hand, only the UV-light activity is increased by the tin oxide surface modification of ST-01 (anatase, Ishihara Sangyo). This striking difference can be rationalized on the basis of the material characterization and DFT calculations, which show that FeOx surface modification of rutile leads to visible-light activity, while SnO2-modified anatase enhances only the UV-light activity. We propose the mechanisms behind the FeOx and SnO2 surface modification, where the surface-to-bulk and bulk-to-surface interfacial electron transfer are taken into account in the former and the latter, respectively.Research Front (Open Access

Tin oxide-surface modified anatase titanium(IV) dioxide with enhanced UV-light photocatalytic activity

[Sn(acac)(2)]Cl-2 is chemisorbed on the surfaces of anatase TiO2 via ion-exchange between the complex ions and H+ released from the surface Ti-OH groups without liberation of the acetylacetonate ligand (Sn(acac)(2)/TiO2). The post-heating at 873 K in air forms tin oxide species on the TiO2 surface in a highly dispersed state on a molecular scale ((SnO2)(m)/TiO2). A low level of this p block metal oxide surface modification (similar to 0.007 Sn ions nm(-2)) accelerates the UV-light-activities for the liquid- and gas-phase reactions, whereas in contrast to the surface modification with d block metal oxides such as FeOx and NiO, no visible-light response is induced. Electrochemical measurements and first principles density functional theory (DFT) calculations for (SnO2)(m)/TiO2 model clusters (m = 1, 2) indicate that the bulk (TiO2)-to-surface interfacial electron transfer (BS-IET) enhances charge separation and the following electron transfer to O-2 to increase the photocatalytic activity

Origin of the visible-light response of nickel(II) oxide cluster surface modified titanium(IV) dioxide

A number of NiO clusters have been formed on TiO2 (anatase/rutile = 4/1 w/w, P-25, Degussa) in a highly dispersed state (NiO/TiO2) by the chemisorption-calcination cycle technique. The NiO/TiO2 causes high visible-light activities for the degradations of 2-naphthol and p-cresol exceeding those of FeOx/TiO2 (Tada et al. Angew. Chem., Int. Ed. 2011, 50, 3501-3505). The main purpose of this study is to clarify the origin at an electronic level by the density functional simulation for NiO, Ni2O2, Ni3O3, and Ni4O4 clusters supported on TiO2 rutile (110) and anatase (001) surfaces. The clusters adsorb strongly on both rutile and anatase with adsorption energies ranging from -3.18 to -6.15 eV, creating new interfacial bonds between the clusters and both surfaces. On rutile, intermetallic Ni-Ti bonds facilitate stronger binding compared with anatase. The electronic structure shows that the top of the valence bands (VBs) of rutile and anatase arises from electronic states on the NiO cluster. On the other hand, the conduction band of rutile is from the Ti 3d states, whereas NiO cluster levels are generated near the conduction band minimum of anatase. This is in contrast to the SnO2/rutile TiO2 system, where the density of states near the conduction band minimum increases with the VB unmodified. In the NiO/TiO2 system, the band gaps of both rutile and anatase are narrowed by up to 0.8 eV compared with pristine TiO2, which pushes the photoactivity into the visible region. In view of the calculated electronic structure, we have attributed the enhanced photocataltyic activity both to the charge separation due to the excitation from the Ni 3d surface sub-band to the TiO2 conduction band and the action of the NiO species as a mediator for the electron transfer from the TiO2 conduction band to O-2

Loading effect in copper(II) oxide cluster-surface-modified titanium(IV) oxide on visible- and UV-light activities

Cu(acac)2 is chemisorbed on TiO2 particles [P-25 (anatase/rutile = 4/1 w/w), Degussa] via coordination by surface Ti–OH groups without elimination of the acac ligand. Post-heating of the Cu(acac)2-adsorbed TiO2 at 773 K yields molecular scale copper(II) oxide clusters on the surface (CuO/TiO2). The copper loading amount (Γ/Cu ions nm–2) is controlled in a wide range by the Cu(acac)2 concentration and the chemisorption–calcination cycle number. Valence band (VB) X-ray photoelectron and photoluminescence spectroscopy indicated that the VB maximum of TiO2 rises up with increasing Γ, while vacant midgap levels are generated. The surface modification gives rise to visible-light activity and concomitant significant increase in UV-light activity for the degradation of 2-naphthol and p-cresol. Prolonging irradiation time leads to the decomposition to CO2, which increases in proportion to irradiation time. The photocatalytic activity strongly depends on the loading, Γ, with an optimum value of Γ for the photocatalytic activity. Electrochemical measurements suggest that the surface CuO clusters promote the reduction of adsorbed O2. First principles density functional theory simulations clearly show that, at Γ 1, the VB maximum rises and the unoccupied Cu 3d levels move to the conduction band minimum of TiO2. These results suggest that visible-light excitation of CuO/TiO2 causes the bulk-to-surface interfacial electron transfer at low coverage and the surface-to-bulk interfacial electron transfer at high coverage. We conclude that the surface CuO clusters enhance the separation of photogenerated charge carriers by the interfacial electron transfer and the subsequent reduction of adsorbed O2 to achieve the compatibility of high levels of visible and UV-light activities

Photocatalytic activities of tin(IV) oxide surface-modified titanium(IV) dioxide show a strong sensitivity to the TiO 2 crystal form

Surface modification of rutile TiO2 with extremely small SnO2 clusters gives rise to a great increase in its UV light activity for degradation of model organic water pollutants, while the effect is much smaller for anatase TiO2. This crystal form sensitivity is rationalized in terms of the difference in the electronic modification of TiO2 through the interfacial Sn−O−Ti bonds. The increase in the density of states near the conduction band minimum of rutile by hybridization with the SnO2 cluster levels intensifies the light absorption, but this is not seen with modified anatase. The electronic transition from the valence band to the conduction band causes the bulk-to-surface interfacial electron transfer to enhance charge separation. Further, electrons relaxed to the conduction minimum are smoothly transferred to O2 due to the action of the SnO2 species as an electron transfer promoter

On Subject Aux Inversion

0. Subject Aux Inversion (henceforth SAI) is a rule which inverts the order of subject and auxiliary. For example, it converts a sentence John would be happy into the corresponding interrogative Would John be happy? ..

Application of in situ surface-enhanced Raman spectroscopy (SERS) to the study of citrate oxidation on silica-supported silver nanoparticles

Surface-enhanced Raman spectroscopy (SERS) was used to characterize citrate anions adsorbed on nanometer-sized particles of Ag supported on SiO 2 . The magnitude of the surface-enhancement effect was determined to be $ 3 Â 10 2 on the as-prepared samples of Ag/SiO 2 . Upon heating in air above 373 K, the citrate anions undergo oxidation to uni-and bidentate carbonate species and then decomposition to CO 2 and adsorbed O atoms. In the SERS of Ag/SiO 2 , a very strong enhancement of the ðC ¼ OÞ signal for the bidentate CO 3 species was observed for temperatures between 398 and 448 K, which is accompanied by an increase in the UV-vis absorbance of the sample at the frequency of the laser line used for Raman spectroscopy. This phenomenon is attributed to an increase in the surface-enhancement effect caused by clustering of the Ag nanoparticles as they sinter at elevated temperatures. The present investigation shows that the proper interpretation of in situ SERS spectra requires an understanding of the changes occurring in the UV-vis spectrum of the sample

Linguistics

Contains table of contents for Section 4, an introduction and abstracts for five doctoral dissertations

Bifunctionality of Rh3+ Modifier on TiO2 and Working Mechanism of Rh3+/TiO2 Photocatalyst under Irradiation of Visible Light

A rhodium(III) ion (Rh3+)-modified TiO2 (Rh3+/TiO2) photocatalyst, prepared by a simple adsorption method and exhibiting high levels of photocatalytic activity in degradation of organic compounds, was investigated by using X-ray absorption fine structure (XAFS) measurements, (photo)electrochemical measurements, double-beam photoacoustic (DB-PA) spectroscopic measurements, and photoluminescence measurements. Based on the results, the features of the Rh3+ modifier and the working mechanism of the Rh3+/TiO2 photocatalyst are discussed. XAFS measurements revealed that the Rh3+ species were highly dispersed and almost atomically isolated on TiO2. The (photo)electrochemical measurements, DB-PA spectroscopic measurements, and photoluminescence showed a unique bifunction of the Rh3+ modifier as a promoter for O2 reductions and an electron injector to the conduction band of TiO2 for response to visible light. The reasons for the Rh3+/TiO2 photocatalyst exhibiting higher levels of photocatalytic activity than those of TiO2 photocatalysts modified with other metal ions are also discussed on the basis of obtained results