Cu(acac) 2 is chemisorbed on TiO 2 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 TiO 2 at 773 K yields molecular scale copper(II) oxide clusters on the surface (CuO/TiO 2 ). 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 TiO 2 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 CO 2 , 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 O 2 . First principles density functional theory simulations clearly show that, at Γ < 1, unoccupied Cu 3d levels are generated in the midgap region, and at Γ > 1, the VB maximum rises and the unoccupied Cu 3d levels move to the conduction band minimum of TiO 2 . These results suggest that visible-light excitation of CuO/TiO 2 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 O 2 to achieve the compatibility of high levels of visible and UV-light activities.
Type of publicationArticle (peer-reviewed) AbstractThis progress review describes our work on the design of new TiO2 based photocatalysts. The key concept is the formation of composite structures through the modification of anatase and rutile TiO2 with molecular-sized nanoclusters of metals oxides. Our density functional theory (DFT) level simulations have been compared with experimental work synthesizing and characterizing surface modified TiO2. We use DFT to show that nanoclusters of metal oxides such as TiO2, SnO/SnO2, PbO/PbO2, ZnO and CuO are stable when adsorbed at rutile and anatase surfaces and can lead to a significant red shift in the absorption edge which will induce visible light absorption; this is the first requirement for a useful photocatalyst. We determine the origin of the red shift and the fate of excited electrons and holes. For p-block metal oxides the oxidation state of Sn and Pb can be used to modify the magnitude of the red shift and its mechanism. We describe comparisons of recent experimental studies of surface modified TiO2 that validate our DFT simulations. These nanocluster-modified TiO2 structures 2 form the basis of a new class of photocatalysts which will be useful in oxidation reactions and with a correct choice of nanocluster modified can be applied to other reactions.
The development of environmental catalysts is an urgent subject to be tackled by scientists and engineers all over the world due to the borderless nature of environmental pollution. We named the catalyst enabling the decomposition of the pollutants by effectively utilizing the solar energy from ultraviolet to infrared as “solar environmental catalysts”. This Feature Article reviews the recent studies on a novel class of solar environmental catalysts consisting of TiO2 and molecular scale oxides of 3d metals and for comparison d10 (Sn) on the surface (MOs/TiO2). The TiO2 surface modification with MO clusters by the chemisorption–calcination cycle (CCC) technique presents novel band engineering for finely tuning the top of the valence band, while the unique physicochemical and electronic properties of MOs/TiO2 give rise to the outstanding photo- and thermocatalytic activities for the decomposition of organic pollutants. In the first part following the Introduction, the CCC technique for forming extremely small MO clusters on TiO2, the structures, physicochemical properties, and electronic structures of MOs/TiO2 are described. The second part deals with their thermo- and photocatalytic activities for the degradation of model organic pollutants and the essential action mechanisms of the MO clusters. The combination of experiments and first-principles density functional theory simulations shows that Co2O3/TiO2 can be a prototype of the solar environmental catalyst with high levels of photo(UV and visible)- and thermocatalytic activities.
Access to the full text of the published version may require a subscription. surface. We find that all three clusters adsorb strongly with adsorption energies ranging from -3 eV to -4.5 eV. The more stable adsorption structures show a larger number of new Ti-O bonds formed between the cluster and the surface. These new bonds increase the coordination of cluster Ti and O as well as surface oxygen, so that each has more neighbours. The electronic structure shows that the top of the valence band is made up of cluster derived states, while the conduction band is made 15 up of Ti 3d states from the surface, resulting in a reduction of the effective band gap and spatial separation of electrons and holes after photon absorption, which shows their potential utility in photocatalysis. To examine reactivity, we study the formation of oxygen vacancies in the clustersupport system. The most stable oxygen vacancy sites on the cluster and show formation energies that are significantly lower than in bulk TiO 2 , demonstrating the usefulness of this composite 20 system for redox catalysis. Rights © The Royal Society of Chemistry 2011. This is the Accepted
Access to the full text of the published version may require a subscription. states, whereas NiO cluster levels are generated near the conduction band minimum 22 of anatase. This is in contrast to the SnO 2 /rutile TiO 2 system, where the density of states near the conduction band minimum 23 increases with the VB unmodified. In the NiO/TiO 2 system, the band gaps of both rutile and anatase are narrowed by up to 0.8 24 eV compared with pristine TiO 2 , which pushes the photoactivity into the visible region. In view of the calculated electronic 25 structure, we have attributed the enhanced photocataltyic activity both to the charge separation due to the excitation from the Ni 26 3d surface sub-band to the TiO 2 conduction band and the action of the NiO species as a mediator for the electron transfer from 27 the TiO 2 conduction band to O 2 . Rights
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