Musashi Fujishima

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)]

Highly proton-conductive copper coordination polymer, H2dtoaCu (H2dtoa=dithiooxamide anion)

Abstract Room-temperature proton conductivity, coordination geometry, and pore-diameter distribution of the title Cu(II) coordination polymer have been investigated by AC conductivity, EXAFS, and N2 adsorption isotherm measurements. The AC proton conductivity (σp) obtained from Cole–Cole plot analysis under relative humidity of 75% and 300 K exhibits a considerable high value of 10 −6 S cm −1 as a room-temperature σp. The mechanism of proton conduction seems to be similar to that of a proton-exchange membrane, Nafion, which contains much cluster of water molecules in the porous space of the polymer. The present Cu coordination polymer was revealed to possess porous space of about 6 A, which includes much water molecules more than 10H2O per unit cell. The dimeric Cu(II) square-planar coordination geometry was confirmed by EXAFS analysis using synchrotron radiation source at SPring-8.

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.

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 TiO(2)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)/TiO(2)). The post-heating at 873 K in air forms tin oxide species on the TiO(2) surface in a highly dispersed state on a molecular scale ((SnO(2))(m)/TiO(2)). A low level of this p block metal oxide surface modification (~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 FeO(x) and NiO, no visible-light response is induced. Electrochemical measurements and first principles density functional theory (DFT) calculations for (SnO(2))(m)/TiO(2) model clusters (m = 1, 2) indicate that the bulk (TiO(2))-to-surface interfacial electron transfer (BS-IET) enhances charge separation and the following electron transfer to O(2) to increase the photocatalytic activity.

Interfacial chemical bonding effect on the photocatalytic activity of TiO2-SiO2 nanocoupling systems.

TiO(2) nanoparticles (NPs) were deposited on the surfaces of SiO(2) microspheres with a mesoporous structure prepared by a hydrolysis-controlled sol-gel technique. The TiO(2) NPs were firmly combined on the surfaces of SiO(2) microspheres through the interfacial Si-O-Ti bonds. The coupling causes the bandgap widening up to 3.37 eV, enhancing the photocatalytic activity for the decomposition of acetaldehyde under illumination of UV-light (330 < λ < 400 nm). Density functional theory calculations for model clusters suggested that the observed results are derived from the lowering in the valence band edge energy with the interfacial bond formation.

Photodeposition of copper sulphide nanocrystals on titanium(IV) oxide nanorods and their application in smart windows

Rutile TiO2 nanorods (NRs) have been synthesized by a low-temperature route, using polyethylene glycol as a multi-functional agent for restricting particle growth, controlling particle shape, promoting crystallization, and dispersing particles. CuS nanocrystals were then photodeposited on the surface of the TiO2 NRs at 298 K (CuS–TiO2 NRs). CuS–TiO2 NRs exhibit strong absorption due to the interband transitions and the localized surface plasmon resonance in the visible-near infrared region. A dipole particle suspension (DPS) cell using a polydimethylsiloxane suspension of CuS–TiO2 NRs has been fabricated. This all-inorganic DPS cell exhibits a large solar transmittance change by applying an electric voltage. Application of this new type of DPS cell to smart windows would greatly help to reduce energy consumption by air conditioning systems in buildings and automobiles.

Coverage control of CdSe quantum dots in the photodeposition on TiO2 for the photoelectrochemical solar hydrogen generation.

CdSe quantum dots (QDs) have successfully been formed on the TiO2 surface by the photodeposition of Se QDs and their subsequent transformation into CdSe QDs (CdSe/TiO2) (Fujishima et al., 2014). The addition of mercaptoacetic acid (MAA) in the second step of the two-step photodeposition process significantly decreases the CdSe particle size and the contact angle against the TiO2 surface to increase the TiO2-surface coverage by CdSe QDs with the particle size distribution sharpened. X-ray photoelectron and Raman spectroscopy measurements indicated that MAA is densely chemisorbed on the surface of CdSe QDs through CdS bond, whereas sparsely adsorbed on the TiO2 surface. Photoelectrochemical (PEC) cells using CdSe/TiO2 as the photoanode for hydrogen (H2) generation from aqueous sulfide solution were fabricated. The rate of H2 generation strongly depends on the concentration of MAA (C) added in the photoanode preparation, and the photoanode prepared at C=0.04mM affords a maximum solar-to-hydrogen conversion efficiency of 0.028%.

Red-Light-Driven Water Splitting by Au(Core)–CdS(Shell) Half-Cut Nanoegg with Heteroepitaxial Junction

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 (λex = 640 nm) of HC-Au@#CdS gives rise to continuous stoichiometric water splitting with an unprecedentedly high external quantum yield of 0.24%.

Lead selenide–Titanium dioxide heteronanojunction formation by photocatalytic current doubling-induced two-step photodeposition technique

PbSe quantum dots (QDs) were formed on TiO2 by a two-step photodeposition technique. At the first step, UV-light irradiation of TiO2 in an ethanol solution of H2SeO3 yields Se QDs on the TiO2 surface in a highly dispersed state (Se/TiO2). At the second step, UV-light irradiation of Se/TiO2 in an ethanol solution of Pb(ClO4)2 transforms Se QDs into several tens of nanometer-sized cubic deposits identified as PbSe (PbSe/TiO2) by X-ray diffraction, electronic absorption measurements and X-ray photoelectron spectroscopy. Photochronopotentiometry measurements suggested that the PbSe QDs are formed on TiO2 via the Pb(2+) ion-assisted reduction of Se particles.