Hiromasa Tokudome

Photoelectrochemical deoxyribonucleic acid sensing on a nanostructured TiO2 electrode

A nanostructured TiO2 electrode chemisorbed with probe deoxyribonucleic acid (DNA) can photoelectrochemically detect a dye-labeled target DNA molecule. After the hybridization between the probe and target DNA molecules, light irradiation generates electrons in the dye molecules, and these electrons are injected into the TiO2 electrode. The resulting photocurrent can be measured and corresponds to the concentration of target DNA. This sensor can quantitatively detect target DNA at lower than nanomolar concentrations. In addition, by utilizing two different dyes, different DNA sequences can be detected on the TiO2 electrode.

Electron field emission from TiO2 nanotube arrays synthesized by hydrothermal reaction

Conductive TiO2 nanotube arrays were grown on metal Ti substrates by hydrothermal reaction and subsequent postannealing in vacuum. The nanotubes were vertically grown and adhered well to the substrates. The crystal structure of the postannealed TiO2 nanotubes was identified to be oxygen-defective anatase. The nanotube arrays exhibited efficient electron field emission even at room temperature with rather low turn-on fields ∼280V per electrode distance of 100μm. The emission current density exceeded 0.15mA∕cm2 at an extraction voltage of 800V. The emission current was reproducible and stable in the lower voltage (<800V) region.

Super-hydrophilic and transparent thin films of TiO2 nanotube arrays by a hydrothermal reaction

Titanate nanotube arrays were grown on sapphire substrates by sputtering deposition of titanium films and a subsequent hydrothermal treatment in an aqueous NaOH solution. Post annealing in air transformed the crystals from a titanate structure to an anatase structure. Titanate and anatase nanotube arrays were optically transparent and adhered well to the substrates. Both titanate and anatase nanotube films exhibited high-hydrophilic states with water contact angles below 5° even after dark storage for more than three months due to their surface nanostructures. UV illumination caused super-hydrophilic states with 0° water contact angles, i.e. band-gap excitations decreased the initial water contact angles in dark conditions (∼3.5°) to 0°. The hydrophilicizing rate of the anatase nanotubes was faster than that of the titanate nanotubes because the anatase nanotubes had a higher crystallinity and more absorbed photons.

Facile preparation of stable aqueous titania sols for fabrication of highly active TiO2 photocatalyst films

Highly stable aqueous titania sols were prepared via a facile process using titanium tetraisopropoxide in the co-existence of acetylacetone and acetic acid. Their co-existence efficiently suppressed the hydrolysis and condensation reaction of titanium tetraisopropoxide even in water, retaining the diameter of titania colloidal particles lower than 10 nm (average diameter d = ca. 4 nm). The titania sols possessed significantly high stability for more than 1 year and could be easily coated on various substrates such as quartz glass via a simple spin-coating method, forming highly homogeneous and transparent films. The calcinations of the coated films in the air at 600 °C produced densely packed anatase TiO2 particles; the diameter was retained below 50 nm even after the calcinations at 900 °C without phase transition to rutile, maintaining the fairly good transparency of the films. On the other hand, the TiO2 particles prepared on a quartz substrate from other precursors, such as titanium–peroxo-citrate complexes, were found to transform from anatase into the rutile phase at above 700 °C with a significant increase in the particle size up to ca. 200 nm. The TiO2 films prepared from the present aqueous titania sols exhibited higher activity for photo-induced surface superhydrophilicity under UV light irradiation than those prepared from other precursors.

Visible-Light-Driven Superhydrophilicity by Interfacial Charge Transfer between Metal Ions and Metal Oxide Nanostructures

Single-crystalline rutile nanorods were synthesized by a facile acid treatment on titanate nanotubes. These rutile nanorods could be highly dispersed in water to form a stable colloidal solution. Cu(2+) ions were grafted onto these rutile nanorods, and the Cu(2+)-grafted nanorods could absorb visible light by the interfacial charge transfer between the valence band of rutile TiO(2) and surface-modified Cu(2+) ions. Transparent thin films of Cu(2+)-grafted nanorods were coated on substrates by a facile spin-coating method. These films exhibited superhydrophilic conversion under visible light irradiation.

Titanate nanotube thin films via alternate layer deposition

Layer-by-layer growth of titanate nanotubes on glass substrates was achieved by alternate layer deposition using an aqueous solution of colloidal titanate nanotubes and that of a polycation. Even a single layer thin film of titanate nanotube shows high photoinduced hydrophilicity.

Highly hydrophilic conversion on oriented TiO2 thin films synthesized by a facile spin-coating method

Single crystalline rectangular shape nanorods with an anatase or rutile TiO2 were synthesized by a soft-chemical process. These nanorods were highly dispersed in aqueous solutions and their surfaces have well-defined crystal faces, i.e., anatase (100) and rutile (110) faces. Highly oriented thin films with either an anatase and rutile phase were fabricated by facile spin coating of these colloidal solutions of nanorods without an annealing procedure. The surfaces of these films exhibited highly hydrophilic conversion under white fluorescent light bulb illumination.

Facile water-based preparation of Rh-doped SrTiO3 nanoparticles for efficient photocatalytic H2 evolution under visible light irradiation

Fine particles of Rh doped SrTiO3 (SrTiO3:Rh) were prepared via a newly developed facile water-based process; stable aqueous precursor solutions were prepared by simple mixing of stable aqueous titania sol with Sr and Rh salts in the presence of an acrylic emulsion, then dried and finally calcined in air at 900–1050 °C. The SrTiO3:Rh particles prepared at 1000 °C were smaller than 50 nm in diameter and exhibited much higher efficiency for H2 evolution from methanol aqueous solution under visible light (e.g. 13.2% of quantum yield at 420 nm) than those prepared by conventional solid state reaction (∼5%). On the other hand, the SrTiO3:Rh particles prepared from the aqueous titania sol without the acrylic emulsion or from other aqueous titania precursors with the acrylic emulsion were found to have an increased particle size up to 100 nm, and exhibited lower photocatalytic activity, indicating that the combination of the aqueous titania sol and the acrylic emulsion effectively suppresses the particle growth, and consequently yields SrTiO3:Rh particles having a small particle size and high crystallinity, both of which are necessary to achieve highly efficient photocatalysis.

Preparation of fine particles of sheelite-monoclinic phase BiVO4via an aqueous chelating method for efficient photocatalytic oxygen evolution under visible-light irradiation

In this paper, we introduce a new synthesis method to prepare fine particles of BiVO4 with a scheelite-monoclinic (s-m) phase, which is known as the most favorable crystal phase for photocatalytic water oxidation (O2 evolution) under visible light irradiation, based on a coordination chemistry approach in water. Stable aqueous solutions that contain both Bi3+ and V5+ complexes were prepared by simply mixing two aqueous solutions in which each cation was stabilized with an appropriate chelating agent. The use of chelating agents (glycolic acid (gly), L(+)-tartaric acid (tart), citric acid (cit), or ethylenediamine tetraacetic acid (edta)) was effective to form stable V5+ complexes from NH4VO3. On the other hand, only the use of two equivalents of edta with Bi(NO3)3·5H2O was effective to stabilize the Bi3+ complex in water, while the use of other ligands resulted in precipitations. Evaporation of the aqueous solution containing the stable Bi3+ and V5+ complexes and subsequent calcination in air at 500 °C yielded s-m BiVO4 particles smaller than 300 nm, which were much smaller than BiVO4 particles prepared via conventional solid-state reactions (1–10 μm). In particular, the BiVO4 particles that were prepared with the tart ligand for V5+ stabilization possessed the smallest size (∼80 nm) and exhibited the highest photocatalytic activity for O2 evolution from an aqueous solution containing an electron acceptor (Ag+ or Fe3+) under visible-light irradiation. These results strongly suggested that the tart ligand effectively suppresses particle growth during the crystallization process and thereby affords small BiVO4 particles with high crystallinity, both of which are necessary to achieve highly efficient photocatalysis.

Synthesis and Applications of Titanium Oxide Nanotube Thin Films

Layer-by-layer or vertically aligned TiO2 nanotube thin films were fabricated by using hydrothermally grown titanate nanotubes. These films were optically transparent and exhibited various functions. Layer-by-layer growth of TiO2 nanotubes on glass substrates was achieved by alternate layer deposition using an aqueous solution of colloidal titanate nanotubes and that of a polycation. These films exhibited photoinduced hydrophilic conversion, low-reflectivity, and significant electrochromism, owing to their unique one dimensional open-pore nanostructure. In addition, transparent thin films of vertically aligned TiO2 nanotube arrays were grown by a hydrothermal treatment of metal Ti thin film on glass substrates. These nanotube arrays were well adhered to the substrates and exhibited super-hydrophilicity even under the dark condition and the efficient electron field emission.