Although various nanostructured materials have been reported in the last few decades, [1,2] research in the field of nanotechnology has shifted toward high performance and new functions based on these materials. In catalysis, a key factor to enhance catalytic performance is to design nanoparticle catalysts on nanostructural support materials. [3][4][5] In particular, site-selective deposition of cocatalysts on host materials is important for efficient charge separation. [6,7] Herein, we focus on a tungsten trioxide (WO 3 ) photocatalyst because it is one of the best candidates for visible-light-driven photocatalysis. [8,9] Although WO 3 was once considered to be inactive due to its low conduction-band level and low reduction power of electrons to reduce absorbed oxygen molecules, [10] several groups have recently reported that efficient cocatalysts, such as Pt, [11] tungsten carbide (WC), [12] CuO, [13] Cu(II) ion, [14] enhance the visible-light activity of WO 3 by multielectron reduction. The previous report indicated that the position of the cocatalyst on WO 3 greatly influenced the photocatalytic property.[15] Herein, we successfully synthesized thin films of aligned WO 3 nanotrees using a simple hydrothermal reaction of metal tungsten, and found that a photocatalytic reduction reaction selectively deposits palladium (Pd) metal nanoparticles, which act as cocatalysts, onto the WO 3 nanotrees. Moreover, the wavelength of the light irradiated controlled the height position of the Pd nanoparticles and, consequently, super-hydrophilic thin films were produced based on siteselective Pd-modified WO 3 nanotrees.We have focused on hydrothermal synthesis because this process can be economically applied to large-area synthesis. Although several types of unique WO 3 nanostructures have been produced by wet chemical synthesis, [16,17] aligned thin films of these nanostructures have yet to be reported. Herein, aligned WO 3 nanotree films were directly grown on a metal tungsten substrate in a one-step hydrothermal synthesis. A metal tungsten plate with a thin oxidized layer was hydrothermally treated in a Teflon-lined autoclave in an aqueous solution of oxalic acid (H 2 C 2 O 4 ), rubidium sulfate (Rb 2 SO 4 ), and nitric acid (HNO 3 ). A thin oxidized layer formed by preannealing led to good adhesion between the WO 3 nanotree films and the substrates. The hydrothermal-reaction temperature was kept at 150 8C for 30 h, and the subsequent annealing was performed in air at 500 8C for 30 min to reduce oxygen defects in WO 3 crystal. Figure 1 shows typical scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of a WO 3 nanotree thin film. Numerous WO 3 nanotrees, which were composed of ''trunks'' and ''branches,'' were uniformly grown on a metal W substrate. The trunks presented a diameter and length of about 100 nm and 2 mm, respectively, while the branches presented values below 50 and 500 nm, respectively. The branches were grown along the hexagonal-symmetry axis on the side faces of the trunks....

Solid solutions of titanium diboride–tungsten diboride (TiB2–WB2) were synthesized by induction‐field‐activated combustion synthesis (IFACS) using elemental reactants. In sharp contrast to conventional methods, solid solutions could be formed by the IFACS method within a very short time, ∼2 min. Solutions with compositions ranging from 40–60 mol% WB2 were synthesized with a stoichiometric ratio (Ti + W)/B =½; however, samples with excess boron were also made to counter the loss of boron by evaporation. The dependence of the lattice constants of the resulting solid solutions on composition was determined. The “a” parameter decreased only slightly with an increase in the WB2 content, whereas the “c” parameter exhibited a significant decrease over the range 40–60 mol% WB2. Solid‐solution powders formed by the IFACS method were subsequently sintered in a spark plasma sintering (SPS) apparatus. After 10 min at 1800°C, the samples densified to relative density 86%. XRD analysis showed the presence of only the solid‐solution phase.

The synthesis of dense nanometric composites of TiN-TiB 2 by mechanical and field activation was investigated. Powder mixtures of Ti, BN, and B were mechanically activated through ball milling. Some powders were milled to reduce crystallite size but to avoid initiating a reaction. In other cases powders were milled and allowed to partially react. All these were subsequently reacted in a spark plasma synthesis (SPS) apparatus. The products were composites with equimolar nitride and boride components with relative densities ranging from 90.1% to 97.2%. Crystallite size analyses using the XRD treatments of Williamson-Hall and Halder-Wagner gave crystallite sizes for the TiN and TiB 2 components in the range 38.5-62.5 and 31.2-58.8 nm, respectively. Vickers microhardness measurements (at 2 N force) on the dense samples gave values ranging from 14.8 to 21.8 GPa and fracture toughness determinations (at 20 N) resulted in values ranging from 3.32 to 6.50 MPa⅐m 1/2 .

Surface wettability conversion of WO3 was investigated through electrochemical or photochemical reaction on a polycrystalline WO3 electrode. The initial water contact angle for the WO3 electrode was about 15°, and it was slightly reduced to 9° by cathodic polarization in water under darkness. In contrast, when the WO3 electrode was irradiated with visible light during anodic polarization, the water contact angles were drastically decreased to below 3°. The wettability conversions under aerobic or humid atmospheres were also evaluated, and the presence of oxygen and water molecules was important for the hydrophilic conversion of WO3. On the basis of these results, we discuss a possible mechanism for the surface wettability conversion of WO3. We found that photogenerated holes are indispensable in achieving superhydrophilic conversion on the surface of WO3, whereas the reduction in WO3 by electrons could only generate a slight hydrophilic change.

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