Facile synthesis of single-crystalline MnO2 nanowire arrays with high photothermal catalytic activity.

A simple process has been developed to form single-crystalline ß-MnO2 nanowire arrays (NWAs) with a large surface area of 125 m2 g-1 on a glass plate working as a highly active three dimensional (3D) photothermal catalyst under the illumination of near infrared light due to the efficient light harvesting and heat confinement near the reaction field in addition to the large surface area.

Monodisperse manganese oxide nanoparticles: Synthesis, characterization, and chemical reactivity.

Highly monodisperse amorphous manganese oxide (MnOx) nanospheres with diameter of ca. 300nm have been obtained from ammonia aqueous solution of KMnO4 at room temperature. The amorphous MnOx nanospheres successfully converted to monodisperse K-OMS-2 (cryptomelane) and K-OMS-2/Mn2O3 nanoraspberries through calcination process at 600 and 800°C, respectively. Analyzing the structure of such amorphous MnOx has been a challenge because fewer reports are available to examine amorphous structure. Thus, shape, crystallinity, and structure of the amorphous and crystalline MnOx nanostructures were characterized in detail by X-ray diffraction (XRD), thermogravimetry/differential thermal analysis (TG/DTA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray Photoelectron Spectroscopy (XPS), and energy dispersive spectroscopy (EDS). We discussed a plausible formation mechanism of amorphous MnOx nanospheres based on the investigations. The obtained MnOx nanostructures have been demonstrated to possess oxidative degradation ability of Rhodamine B (RhB) under acidic aqueous condition without any additives such as chemical oxidizing agents and UV and/or visible light irradiation. RhB degradation rate of amorphous MnOx nanospheres was about one hundred times faster than that of K-OMS-2 nanoraspberries.

Light-reducible dissipative nanostructures formed at the solid-liquid interface.

Dissipative structures are macroscopic or even larger ordered structures that emerge under conditions far from thermodynamic equilibrium. In contrast, molecular self-assembly has been investigated near at the thermodynamic equilibrium, which provides basically smaller, nano-to-micron sized structures. In terms of the formation principles, there exists an essential gap between the dissipative structures and molecular self-assemblies. To fill this gap, molecular self-assembly of light-reducible organic-inorganic ion pairs was investigated under far-from-equilibrium conditions. When solid films of tetraalkylammonium hexafluorophosphate were immersed in aqueous Au(OH)4(-) and immediately photoirradiated, gold nanowires are formed at the solid-aqueous interface. On the other hand, such nanowires were not formed when the photoirradiation was conducted for the specimens after a prolonged immersion period of 60 min. These observations indicate spontaneous growth of dissipative nanofibrous self-assemblies consisting of light-reducible ion pairs [tetraalkylammonium ion][Au(OH)4(-)] at the interface and their photoreduction to give developed nanowires. These nanowires are not available by the photoreduction of Au(OH)4(-) ions under conditions near at the thermodynamic equilibrium. A picture for the dissipative nanostructures is obtained: the formation of amphiphilic light-reducible nanowire structures is based on the static self-assembly near at the thermodynamic equilibrium, whereas their spontaneous, anisotropic growth from the interface to the aqueous phase is directed by dynamic, dissipative self-assembly phenomena under the far-from-equilibrium conditions. Thus, the both elements of dissipative self-assembly (dynamic) and static molecular self-assembly fuse together at the nanoscale, which is an essential feature of the dissipative nanostructures.

Synthesis of TiO2 nanocoral structures in ever-changing aqueous reaction systems.

A far-from-equilibrium strategy is developed to synthesize coral-like nanostructures of TiO(2) on a variety of surfaces. TiO(2) nanocoral structures consist of anatase base film and rutile nanowire layers, and they are continuously formed on substrates immersed in aqueous TiOSO(4)-H(2)O(2). The sequential deposition of TiO(2) starts with hydrolysis and condensation reactions of titanium peroxocomplexes in the aqueous phase, resulting in deposition of amorphous film. The film serves as adhesive interface on which succeeding growth of rutile nanowires to occur. This initial deposition reaction is accompanied by shift in pH of the reaction media, which is favorable condition for the growth of rutile nanocrystals. During the growth of rutile nanocoral layers, the amorphous base films are transformed to anatase phase. These sequential deposition reactions occur at temperatures as low as 80 °C, and the mild synthetic condition allows the use of a wide range of substrates such as ITO (indium tin oxide), glass, and even organic polymer films. The thickness of nanocoral layer is controllable by repeating the growth reaction of rutile nanocorals. TiO(2) nanocorals show photocatalytic activity as demonstrated by site-specific reduction of Ag(I) ions, which proceeds preferentially on the rutile nanowire layer. The rutile nanowire layer also shows photocatalytic decomposition of acetaldehyde, which is promoted upon increase of the thickness of the nanowire layer. The use of temporally transforming reaction media allows the formation of biphasic TiO(2) nanocoral structures, and the concept of nonequilibrium synthetic approach would be widely applicable to developing structurally graded inorganic nanointerfaces.

Dense aqueous colloidal gold nanoparticles prepared from highly concentrated precursor solution.

Gold nanoparticles were fabricated by reduction of highly concentrated Au(III) ions (200 mM) with casein proteins from milk. The gold nanoparticles were converted to nanoparticle-powders after washing and subsequent vacuum drying without aggregation. The nanoparticle-powders completely re-dispersed in aqueous solution, and stable colloidal gold nanoparticles were obtained. UV-vis extinction spectra and dynamic light scattering (DLS) measurements revealed that large assemblies (size, ca. 3 µm) and subaggregates (size, <0.5 µm) composed of gold nanoparticle-casein protein chain-Au(III) ion were dynamically formed and disintegrated over the course of the growth of the gold nanoparticles. Fourier transform infrared (FT-IR) spectra indicated conformational changes of casein proteins induced by the interaction of casein protein-Au(III) ion and -gold nanoparticle. Finally, rapid, one-pot, and highly concentrated synthetic procedures of gold and silver nanoparticle powders protected by casein (mean diameters below 10 nm) were successfully developed using 3-amino-1-propanol aqueous solutions as reaction media. Dense colloidal gold (40 g L(-1)) and silver (22 g L(-1)) nanoparticle aqueous solutions were obtained by re-dispersing the metal nanoparticle powders.

Nanocrystal bilayer for tandem catalysis.

Supported catalysts are widely used in industry and can be optimized by tuning the composition and interface of the metal nanoparticles and oxide supports. Rational design of metal-metal oxide interfaces in nanostructured catalysts is critical to achieve better reaction activities and selectivities. We introduce here a new class of nanocrystal tandem catalysts that have multiple metal-metal oxide interfaces for the catalysis of sequential reactions. We utilized a nanocrystal bilayer structure formed by assembling platinum and cerium oxide nanocube monolayers of less than 10 nm on a silica substrate. The two distinct metal-metal oxide interfaces, CeO(2)-Pt and Pt-SiO(2), can be used to catalyse two distinct sequential reactions. The CeO(2)-Pt interface catalysed methanol decomposition to produce CO and H(2), which were subsequently used for ethylene hydroformylation catalysed by the nearby Pt-SiO(2) interface. Consequently, propanal was produced selectively from methanol and ethylene on the nanocrystal bilayer tandem catalyst. This new concept of nanocrystal tandem catalysis represents a powerful approach towards designing high-performance, multifunctional nanostructured catalysts.

One-pot room-temperature synthesis of single-crystalline gold nanocorolla in water.

A room-temperature nanocarving strategy is developed for the fabrication of complex gold nanoplates having corolla- and propeller-like architectures. It is based on the simultaneous growth and etching of gold nanoplates in aqueous solution, which occur in the course of photoreduction of Au(OH)(4)(-) ions. The presence of bromide ion, poly(vinylpyrrolidone) (PVP), and molecular oxygen is indispensable, where bromide ions play multiple roles. First, they promote formation of nanoplate structures by forming adlayers on the fcc(111) surface. Second, they facilitate oxidative dissolution of gold nanocrystals by converting the oxidized Au(I) species to soluble AuBr(2)(-) ions, which lead to the formation of ultrathin nanocrevasses. PVP also stabilizes the nucleation of gold nanoplates. Although the overall reactions proceed in one-pot, the crystal growth and etching show interplay and occur with different kinetics due to changes in the concentration of Au(OH)(4)(-) and other species with time. Corolla- or propeller-like gold nanoplates formed under these conditions are single-crystalline, as indicated by selected area electron diffraction patterns and the observation of moire fringes. The morphology of corolla- or propeller-like gold nanoplates is controllable depending on the concentration of bromide ion and PVP in the aqueous mixture. On the basis of these results, a preliminary mechanism is proposed which involves the concurrent crystal growth and oxidative etching on the surface of nanocrystals.

Ultrafast photosynthetic reduction of elemental sulfur by Au nanoparticle-loaded TiO2.

Nanometer-sized gold particles with varying mean size from 3.2 to 12.2 nm were loaded on the surfaces of TiO2 particles in a highly dispersed state with the loading amount maintained constant (0.46 +/- 0.02 mass %) using the deposition-precipitation method. Light irradiation (lambda(ex) > 300 nm) to a deaerated ethanol TiO2 particle suspension containing elemental sulfur (S8) led to the energetically uphill reduction of S8 to H2S. It has been found that this reaction is dramatically enhanced with such a low level of Au loading on TiO2 and that the zero-order rate constant of reaction increases with decreasing mean size of Au nanoparticles (d). The effects of reaction parameters (substrate concentration, light intensity, temperature) on the rate of reaction were studied to infer the essential reaction mechanism. Further, a kinetic analysis has led to a conclusion that the increase in the rate of reaction with decreasing d results from the improvement of the charge separation efficiency.

Photoinduced dissolution and redeposition of Au nanoparticles supported on TiO2.

Au particles (mean size ca. 3 nm) supported on TiO(2) particles were irradiated by UV light (>300 nm) in aqueous solutions at 278 K. Photo-induced dissolution of Au nanoparticles followed by redeposition occurred in aqueous solutions containing halogen ions. The dissolution of Au nanoparticles yielded a Au(III) complex with a halogen ion; subsequent reduction of the Au(III) complex caused precipitation of larger Au particles on TiO(2).

Photoinduced desorption of sulfur from gold nanoparticles loaded on metal oxide surfaces.

We now report photoinduced sulfur desorption from the surfaces of Au nanoparticles loaded on metal oxides. This reaction occurs in water at ordinary temperature and pressure. Nanometer-sized Au particles have been formed on the surfaces of various metal oxides by deposition-precipitation (Au/oxides). Elemental sulfur (S8) is selectively adsorbed on the Au nanoparticle surfaces of Au/oxides in an atomic state at a coverage of (theta) 1/3. Irradiation (lambdaex > 300 nm) of the sulfur adsorbed Au/anatase TiO2 in water has led to reductive desorption of the sulfurs at room temperature. Electrochemical measurements using Au/oxides indicate that the driving force for this reaction is the photoinduced upward shift of Fermi energy of the metal oxide-supported Au nanoprticles. This study will open up a novel and wide application of heterogeneous photocatalysis for thermal catalysts.