The light fandango: Anatase and anatase–rutile composite TiO2 has been modified electronically with highly dispersed surface iron oxide species. The chemisorption–calcination cycle technique was used with [Fe(acac)3] as a precursor (acac=acetylacetonate), leading to pronounced visible‐light activity and a concomitant increase in activity under illumination with UV light (cb=conduction band, vb=valence band).

Heteronanojunction systems consisting of narrow gap semiconductors represented by metal sulfides and TiO(2) are highly expected as visible-light-active photocatalysts and the key materials for various photoelectrochemical devices. The common central issue is increasing efficiency of the light-induced interfacial electron transfer from the metal sulfide quantum dots (QDs) to TiO(2). We have newly developed simple and versatile low-temperature photodeposition techniques for directly coupling metal sulfide QDs and TiO(2) by taking advantage of its photocatalysis and the photoinduced surface superhydrophilicity. This critical review summarizes the recent developments in the photodeposition techniques and their unique characteristics. Subsequently to the Introduction, a theoretical view of the interfacial electron transfer is presented to obtain the guidelines for the design of the heteronanojunction systems. Then, the itemized description is given for the photodepositions of several kinds of metal sulfides on TiO(2) followed by the summary of the features of the photodeposition technique. Finally, the applications of the resulting heteronanojunction systems to the photocatalysts and QD-sensitized solar cells are described, and the excellent performances are discussed by relating with the features of the photodeposition technique (87 references).

Fe(acac)3 is chemisorbed on the surfaces of anatase TiO2 via partial ligand exchange between the acetylacetonate and surface Ti−OH groups [Fe(acac)2/TiO2]. The postheating at 773 K in air forms iron oxide species on the TiO2 surface in a highly dispersed state at a molecular level ((FeO x ) m /TiO2). As a result of the iron oxide surface modification, the band gap of TiO2 decreases, while the absorption due to the d−d transition clearly observed for the usual impregnation samples is very weak. (FeO x ) m /TiO2 gives rise to a noticeable visible light activity concomitantly with a significant increase in UV light activity, whereas Fe(acac)2/TiO2 hardly responds to visible light. Valence-band X-ray photoelectron spectra of (FeO x ) m /TiO2 showed that the band gap narrowing results from the rise in the valence band top with surface modification. Also, photoluminescence spectroscopy indicated that the surface iron oxide species rapidly capture the excited electrons in the conduction band of TiO2 to suppress recombination via surface oxygen vacancy levels. Furthermore, the surface iron oxide species act as excellent mediators for electron transfer from TiO2 to O2.

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.

A key material for artificial photosynthesis including water splitting is heteronanostructured (HNS) photocatalysts. The photocatalytic activity depends on the geometry and dimension, and the quality of junctions between the components. Here we present a half-cut Au(core)-CdS(shell) (HC-Au@CdS) nanoegg as a new HNS plasmonic photocatalyst for water splitting. UV-light irradiation of Au nanoparticle (NP)-loaded ZnO (Au/ZnO) at 50 °C induces the selective deposition of hexagonal CdS on the Au surface of Au/ZnO with an epitaxial (EPI) relation of CdS{0001}/Au{111}. The subsequent selective dissolution of the ZnO support at room temperature yields HC-Au@CdS with the Au NP size and EPI junction (#) retained. Red-light irradiation (λ = 640 nm) of HC-Au@#CdS gives rise to continuous stoichiometric water splitting with an unprecedentedly high external quantum yield of 0.24%.