Sayuri Okunaka

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.

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.

Structure-controlled porous films of nanoparticulate Rh-doped SrTiO3 photocatalyst toward efficient H2 evolution under visible light irradiation

Porous films of Rh-doped SrTiO3 (SrTiO3:Rh) were prepared on glass substrates as a visible light-responsive photocatalyst panel that can generate H2 efficiently under visible light irradiation. The films were prepared by simple screen-printing using the pastes of the fine particles (ca. 50 nm) of SrTiO3:Rh (WH-particles), which were prepared via a facile water-based process, followed by calcination at 500 °C. The use of WH-particles having relatively narrow size distribution allowed us to prepare homogeneous films with controlled thickness ranging from 1 to 10 μm, along with enough mechanical strength. On the other hand, the use of SrTiO3:Rh particles (SS-particles, ca. 300 nm) prepared by a conventional solid state reaction method resulted in inhomogeneous surfaces and exposure of the glass substrates when the thickness was below 3 μm. The SrTiO3:Rh films (WH-films) consisting of the fine particles exhibited much higher efficiency for H2 evolution from methanol aqueous solution under visible light than those prepared from the SS-particles (SS-films) at each thickness. The H2 evolution rate on WH-films increased to reach maximum value at 10 μm (3.2 μmol h−1) while those on SS-films saturated at a thinner thickness of 5 μm (0.7 μmol h−1), and was confirmed to be higher than those on the suspended system with the same amounts of WH-particles. Furthermore, the addition (10–20 wt%) of the large particles to the WH-films further increased the H2 evolution rate, probably due to the light scattering with large SS-particles, by which the neighboring small WH-particles in the films can effectively absorb the scattered light and generate more H2.

Green fabrication of nanoporous BiVO4 films on ITO substrates for photoelectrochemical water-oxidation

A new green method was developed to prepare nanoporous BiVO4 films on ITO substrates for photoelectrochemical (PEC) water-oxidation under visible light irradiation. The films can be prepared by simple drop-casting of a stable aqueous solution of Bi3+ and V5+ complexes with tartaric acid and ethylenediaminetetraacetic acid, followed by drying and calcination in air. Thanks to these ligands, the aqueous precursor solution is remarkably stable over a wide range of pH (pH 4–9). The BiVO4 films on ITO substrates possess a 3D-network structure comprised of nanoparticles with a scheelite–monoclinic phase and a diameter of ca. <100 nm, after calcination at 450–500 °C for 1 h. The PEC performance clearly depended on the film thickness that can be controlled by coating times, and calcination conditions (temperature and time). The CoPi-loaded BiVO4 electrodes exhibited relatively high performance for PEC water oxidation (ABPE of 0.35% at 0.8 V vs. RHE) under simulated sunlight irradiation.