Hiroaki Tada

Titanium(IV) Dioxide Surface‐Modified with Iron Oxide as a Visible Light Photocatalyst

TiO2 has three polymorphic forms: anatase, rutile, and brookite. Anatase usually has the highest photocatalytic activity under illumination of UV light; the activity can further be improved by coupling with rutile. The development of a general method for endowing commercial anatase and anatase–rutile composite TiO2 with visible-light response and concomitantly increasing their UV-light activity should dramatically expand their viability. To this end, doping of various transition metals and anions has been extensively studied. In particular iron, which is harmless and abundant in nature is an ideal candidate; however, the positive doping effect is only limited to TiO2 particles smaller than 10 nm in diameter. This limit mainly arises because the doping generates impurity and/or vacancy levels in the bulk, which act as the recombination centers. As an alternative, Kisch et al. have devised the photosensitization of TiO2 by surface modification with platinum(IV) chloride. This approach is attractive in that the visible-light response can be induced by the simple procedure without introduction of the impurity/ vacancy levels. Recently, the research groups of Ohno and Hashimoto have shown that the surface modification of rutile TiO2 with Fe 3+ by the impregnation method leads to high visible-light activities for the decomposition of model organic pollutants. However, the effect is small for anatase TiO2. On the other hand, we have developed the chemisorption–calcination cycle (CCC) technique, in which metal complexes are adsorbed by chemical bonds and the organic (ligand) part is oxidized by post-heating to prepare metal oxide clusters and ultrathin films at a molecular scale. Herein we show that the surface modification of two kinds of TiO2 particles (see the Experimental Section) with highly dispersed iron oxides by the CCC technique ((FeOx)m/TiO2) gives rise to a high level of visible-light-induced activity and greatly heightens the activity under UV-light irradiation. [Fe(acac)3] was adsorbed on the TiO2 surface by a partial ligand exchange between the acetylacetonate and surface OH groups [Equation (1)]

Photocatalytic activity of rutile-anatase coupled TiO2 particles prepared by a dissolution-reprecipitation method.

Rutile TiO(2) particles were partly dissolved into aqueous solutions of H(2)SO(4), and the Ti(4+) ions were reprecipitated by adding NH(3) aq. Rutile-anatase coupled TiO(2) particles were prepared by heating the solid recovered after centrifugation of the suspension. The content of anatase (c(A), wt%) could be controlled arbitrarily by changing the dissolved amount of rutile. The photocatalytic activity for the gas-phase oxidation of acetaldehyde was evaluated. The first-order rate constant, k, strongly depended on both c(A) and heating temperature (T(c)), increasing with an increase in T(c) at T(c)</=800 degrees C. The sample with c(A)=0.5 wt% prepared at T(c)=800 degrees C exhibited a much higher level of activity (k=0.94 h(-1)m(-2)) than those of pure rutile (k=0.08 h(-1)m(-2)) and anatase (k=0.34 h(-1)m(-2)). This remarkable enhancing effect was explained mainly in terms of photoinduced interfacial electron transfer from anatase to rutile.

Facile synthesis and catalytic activity of MoS2/TiO2 by a photodeposition-based technique and its oxidized derivative MoO3/TiO2 with a unique photochromism

UV-light irradiation to TiO(2) in an aqueous ethanol solution of (NH(4))(2)MoS(4) under deaerated conditions has yielded molybdenum(IV) sulfide nanoparticles on a TiO(2) surface (MoS(2)/TiO(2)) to be transformed into molybdenum(VI) oxide species highly dispersed at a molecular level by a subsequent heating at 773K in air (m-MoO(3)/TiO(2)). In HCOOH aqueous solutions, the MoS(2)/TiO(2) system exhibits a high level of photocatalytic activity for H(2) generation, while the m-MoO(3)/TiO(2) system shows unique photochromism.

Photodeposition of Ag2S quantum dots and application to photoelectrochemical cells for hydrogen production under simulated sunlight.

UV light irradiation of TiO(2) (λ > 320 nm) in a mixed solution of AgNO(3) and S(8) has led to the formation of Ag(2)S quantum dots (QDs) on TiO(2), while Ag nanoparticles (NPs) are photodeposited without S(8). Photoelectrochemical measurements indicated that the Ag(2)S photodeposition proceeds via the preferential reduction of Ag(+) ions to Ag(0), followed by the chemical reaction with S(8). The application of this in situ photodeposition technique to mesoporous (mp) TiO(2) nanocrystalline films coated on fluorine-doped SnO(2) (FTO) electrodes enables formation of Ag(2)S QDs (Ag(2)S/mp-TiO(2)/FTO). Ag(2)S/mp-TiO(2)/FTO has the interband transition absorption in the whole visible region, while in the spectrum of Ag/mp-TiO(2)/FTO, a localized surface plasmon resonance absorption of Ag NPs is present centered at 490 nm. Ag(2)S QD-sensitized photoelectrochemical cells using the Ag(2)S/mp-TiO(2)/FTO and Ag/mp-TiO(2)/FTO photoanodes were fabricated. Under illumination of one sun, the Ag(2)S photoanode cell yielded H(2) at a rate of 0.8 mL·h(-1) with a total conversion efficiency of 0.29%, whereas the Ag/mp-TiO(2)/FTO photoanode is inactive.

Acceleration of Oxidations and Retardation of Reductions in Photocatalysis of a TiO2 / SnO2 Bilayer‐Type Catalyst

An anatase TiO 2 film (75 ± 5 nm thick) was formed on SnO 2 film (580 ± 80 nm thick) coated soda-lime glass by a sol-gel method (TiO 2 /SnO 2 /glass). Depth profiles of the compositions determined hy X-ray photoelectron spectroscopy confirmed that the amounts of Na + and F - ions are negligibly small throughout the TiO 2 film. The activities for four kinds of photocatalytic reactions (v) were examined under no external applied potential in comparison with those of a TiO 2 /quartz standard sample (ν 0 ), The TiO 2 /SnO 2 /glass bilayer-type photocatalyst showed remarkably higher activities for oxidations (ν/ν 0 = 3.0 ± 0.2), whereas the activities for reductions are lower (ν/ν 0 = 0.62 ± 0.03). These results could be explained by efficient charge separation via the interfacial electron transfer from TiO 2 to SnO 2 .

High Photocatalytic Activity of F-Doped TiO2 Film on Glass

The photoreactivity of sol-gel (SG)-TiO2 films was enhanced significantly with addition of a small amount of trifluoroacetic acid (TFA) into the starting solution. The promoting effect was confirmed in two separate experiments of the methylsiloxane monolayer oxidation and the photocurrent measurements. The results of film characterization indicated that the absorption coefficient for ultraviolet light (λ < 360 nm) increases due to film densification and the crystallinity is improved upon F doping. The improvement in the photoreactivity was ultimately attributed to the increase in the rate at which the photogenerated charge carriers reach the surface in the photostationary state.

Visible‐Light‐Induced Electron Transport from Small to Large Nanoparticles in Bimodal Gold Nanoparticle‐Loaded Titanium(IV) Oxide

A key to realizing the sustainable society is to develop highly active photocatalysts for selective organic synthesis effectively using sunlight as the energy source. Recently, metal-oxide-supported gold nanoparticles (NPs) have emerged as a new type of visible-light photocatalysts driven by the excitation of localized surface plasmon resonance of Au NPs. Here we show that visible-light irradiation (λ>430 nm) of TiO2 -supported Au NPs with a bimodal size distribution (BM-Au/TiO2 ) gives rise to the long-range (>40 nm) electron transport from about 14 small (ca. 2 nm) Au NPs to one large (ca. 9 nm) Au NP through the conduction band of TiO2 . As a result of the enhancement of charge separation, BM-Au/TiO2 exhibits a high level of visible-light activity for the one-step synthesis of azobenzenes from nitrobenzenes at 25 °C with a yield greater than 95 % and a selectivity greater than 99 %, whereas unimodal Au/TiO2 (UM-Au/TiO2 ) is photocatalytically inactive.

Patterning Effect of a Sol-Gel TiO2 Overlayer on the Photocatalytic Activity of a TiO2/SnO2 Bilayer-Type Photocatalyst

TiO2 films with a thickness of 75 ± 5 nm (anatase) were formed on SnO2-film (580 ± 80 nm) coated soda-lime glass substrates (SnO2/SL-glass) by a sol-gel method. Although the photocatalytic activity for CH3CHO oxidation (λex > 300 nm) significantly exceeded that of a standard TiO2/quartz sample, it decayed with illumination time (t) at t > 0.75 h. Stripes of anatase TiO2 films of 40 nm in thickness and 1 mm in width were prepared on the SnO2/SL-glass substrate in a 1-mm pitch by photolysis of an organically modified sol-gel film. The TiO2 patterning further increased the photocatalytic activity by a factor of 4.1 as compared to the non-patterned sample, and it was also maintained at 0 < t < 2 h. The flat band potentials of the TiO2 and SnO2 films are determined to be −0.34 and +0.07 V (vs. SHE), respectively, at pH = 7 by the Mott-Schottky plots. On the basis of the results, the outstanding patterning effects could be rationalized in terms of the vectorial charge separation at the interface between TiO2 and SnO2.

Visible-light-driven copper acetylacetonate decomposition by BiVO4.

Visible-light irradiation to monoclinic scheelite BiVO(4) (m-BiVO(4)) in a solution of copper acetylacetonate (Cu(acac)(2)) has led to its decomposition and Cu recovery. The photonic efficiency at λ = 440 ± 15 nm reaches 3.4%, exceeding the value for the TiO(2)-photocatalyzed reaction at λ = 355 ± 23 nm (2.0%). The adsorption isotherm and the light intensity-dependence of the decomposition rate indicate high adsorptivity of m-BiVO(4) for Cu(acac)(2) or its sufficient supply to the surface reaction sites, which mainly contributes to the high photocatalytic activity. Electrochemical measurements using cyclic voltammetry suggest that the reaction proceeds via the oxidative degradation of the ligand followed by the reduction of the resulting Cu(2+) ions. Under aerobic conditions, the Cu(2+) ions mediate the electron transfer from the conduction band of m-BiVO(4) to O(2) to complete the catalytic cycle.

Heterosupramolecular photocatalysis: oxidation of organic compounds in nanospaces between surfactant bilayers formed on TiO2Electronic supplementary information available: further characterization data and experimental details. See http://www.rsc.org/suppdata/cc/b2/b204593a/

A heterosupramolecular photocatalyst consisting of TiO2 and a cationic surfactant bilayer formed on the surface has achieved a very high level of activity in the oxidation of 2-naphthol through the cooperation of the inorganic and organic components.