Loading Effect in Copper(II) Oxide Cluster-Surface-Modified Titanium(IV) Oxide on Visible- and UV-Light Activities

Abstract

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.

Design of Novel Visible Light Active Photocatalyst Materials: Surface Modified TiO2

“…Some results from experimental work in refs [77,78] on CuO-and FeOx-modified TiO2 were briefly described to highlight the experimental activities on these materials and herein we expand and discuss the correspondence between the experimental and simulation results.…”

“…In this regard, Figure 4 presents some key spectroscopic results from this experimental work [77,78] , showing the effect of CuO modification on the electronic structure of TiO2. (a), (b) and (c) Reproduced with permission from [78] , 2013, American Chemical Society (d) and (e) Reproduced with permission from [77] , 2011, Wiley Thus, the work from the groups of Hashimoto and Tada show that the strategy of modifying TiO2 with metal oxide nanostructures has potential to meet some of the outlined requirements for a visible light active photocatalyst, while addressing the issues encountered by TiO2 doping or by instability in, e.g. CdS-modified TiO2.…”

“…Our experimental collaborators in the group of Prof. Tada have presented studies of TiO2 modified with CuO [78] and FeOx [77] nanoclusters; a short summary of some findings was presented in Section 3. We have used our DFT simulation to examine the effect of these nanocluster modifications on the key properties for their use as a photocatalyst.…”

“…[89][90][91] and will be discussed in Section 4. Reproduced from [78] , 2013, American Chemical Society For modification of rutile with a CuO nanocluster a new, empty Cu 3d state is present in the VB-CB gap, which arises from the presence of a Cu 2+ oxidation state, with a 3d 9 electronic configuration. We would therefore expect excitation of an electron from the TiO2 derived valence band into the empty Cu 3d states below the TiO2 conduction band edge, shifting light absorption to lower energy, consistent with the interpretation of Hashimoto and co-workers [74] .…”

A new bimetallic plasmonic photocatalyst consisting of gold(core)-copper(shell) nanoparticle and titanium(IV) oxide support

“…Visible-light photocatalysis of CuO/TiO 2 has been reported for the degradation of model organic pollutants, 33,34 which can stem from the photo-induced interfacial electron transfer between the surface CuO levels and TiO 2 . 33 Surprisingly, Au@Cu/TiO 2 exhibits a much higher photocatalytic activity than Au/TiO 2 and CuO/TiO 2 . Table I summarizes the results on the photocatalytic oxidation of the benzylamine derivatives.…”