Motonari Adachi

Formation of Titania Nanotubes and Applications for Dye-Sensitized Solar Cells

Highly efficient dye-sensitized solar cells were produced using single-crystalline TiO 2 nanotubes as a thin-film semiconductor because of the very high electron transfer through single-crystalline TiO 2 nanotubes when compared to that through nanoporous TiO 2 films composed of nanoparticles. The dye-sensitized solar cells with single-crystalline TiO 2 nanotubes showed more than double the short-circuit current density than those made of titania nanoparticles Degussa P-25 in the thin-film thickness region. Titania nanotubes were synthesized using molecular assemblies composed of surfactant molecules, i.e., laurylamine hydrochloride, and titanium alkoxide, i.e., tetraisopropylorthotitanate modified with acetylacetone, as a template. They have outer and inner diameters of about 10 and 5 nm, respectively, a length in the range from 30 nm to several hundred nanometers, and have a single-crystalline structure of anatase, as confirmed on lattice images observed by high-resolution transmission electron microscopy. The light to electricity conversion of the titania nanotube cells was around 5%. They also showed the highest photocatalytic activity when compared to the commercially available nanocrystalline titania.

Surfactant-Mediated Fabrication of Silica Nanotubes

Hollow nanotubes are attracting a great deal of attention in both fundamental and industrial studies. They have novel properties, and could be used to study the physical and chemical properties of molecules confined in their inner and outer spaces. They also have potential applications in fields such as electronics, optics, advanced catalysis, and energy storage/conversion, and could be designed to mimic biological channels. Thus, methods must be developed to fabricate hollow nanotubes and modify the properties by filling and coating the tubes for particular applications. Inorganic hollow tubes that have been fabricated so far include those composed of carbon, boron nitride, silica, and vanadium oxide. Apart from vanadium oxide, these inorganic nanotubes have been synthesized under high temperature reaction conditions. For example, carbon nanotubes are produced by arc-discharge evaporation of carbon. On the other hand, recent advances in molecular biology have shown us that nature uses molecular self-assembly to construct microstructures of biomaterials. The bio-inspired method is another important route to the fabrication of nanotubes. The Mobil research group has synthesized ordered nanotubes in condensed forms, i.e., mesoporous materials (the so-called M41S family) using surfactant assemblies as the template. Since then, a variety of mesoporous materials have been synthesized. These materials have condensed forms of unit cylindrical structure. However, single nanotubes or bundles of a few tubes have not yet been synthesized. Nakamura and Matsui obtained silica-gel tubes by the sol±gel method in the absence of template. Lin and Mou synthesized hollow tubes whose wall has the MCM41 structure. Their diameters are all micrometer-size, i.e., much larger than nano-size. The single nanotube is a building unit for fabricating a more complicated hierarchical structure. Also, single nanotubes can be converted to a composite structure by filling the inner cavity with functional molecules and also coating the outside of the tube. Thus, the modification of single nanotubes has great potential for deriving novel properties, which cannot be expected for condensed materials such as the M41S family. Here, we describe the sol±gel method for fabricating single silica nanotubes or bundles of a few tubes by the surfactant-mediated template mechanism and the method for controlling the geometry of the tubes. The strategy of fabricating single nanotubes is as follows: Surfactants such as alkyl ammonium salts self-organize into micelles of various shapes in the equilibrium state. The factor that determines the shape is the packing index, P = Vc/(Slc): [21,22] For spheres, P < 1/3; for cylinders, 1/3 < P < 1/2; and for bilayers, 1/2 < P < 1. Here, Vc and lc represent the volume and effective length, respectively, of the hydrocarbon chain attached to a surfactant polar head, whose area is S. In the sol±gel process under acidic conditions, silicon alkoxide is first hydrolyzed, and then the condensation reaction proceeds to yield silica polymer. When the hydrolysis reaction is fast compared with the condensation reaction, combined molecules composed of the surfactant and hydrolyzed alkoxide are formed. They have an amphiphilic nature and self-organize into cylindrical aggregates if the P value for the combined molecules becomes 1/3 to 1/2. Thereafter, the condensation reaction slowly proceeds on the aggregate surface, resulting in the formation of single cylindrical aggregates covered by silica. The resultant aggregates are converted to nanotubes by calcination. The essential points in the formation of nanotubes are as follows: 1) The combined molecules self-organize into cylindrical assemblies in a quasi-equilibrium state. 2) Deformation of the generated aggregates does not occur during the condensation reaction, i.e., no mismatch of the array occurs between the surfactant molecules and silica units on the aggregate surface during the condensation reaction. We selected a laurylamine hydrochloride (LAHC)/tetraethoxysilane (TEOS) system. The experimental procedure is as follows: TEOS was added to 0.1 M LAHC aqueous solution (pH 4.5), and the reaction was started in a stirred cell at 313 K. The TEOS-to-LAHC molar ratio was adjusted to 4±12. TEOS does not dissolve in water to yield an emulsified solution in the early stages of the reaction. After 2±3 h, TEOS completely dissolved in the aqueous solution due to hydrolysis, and the solution became transparent. After 13±14 h, the solution turned into a homogeneous gel state.

Synthesis of Morphology-Controlled Titania Nanocrystals and Application for Dye-Sensitized Solar Cells

Development of renewable energy resources in the near future is an urgent issue. One attractive strategy is the development of dye-sensitized solar cells (DSSCs); they are extremely promising, because they are made of low-cost materials and do not need elaborate apparatus to manufacture. Titania is the most promising material for the electrode of DSSCs, and then morphological control and carrier transport optimization are the key properties needed in titanium oxide materials for DSSCs. We review the formation procedures and characteristics of titanium oxide nanocrystalline products, which exhibit various morphological shapes in nanometer scale, i. e., nanotubes, nanorods, nanowires and nanosheets, and their arrays. We also present new findings in our laboratory on the formation of titania nanorods and network structures of single-crystal-like titania nanowires as well as their application for DSSCs. In order to evaluate the electrical properties of DSSCs with electrodes composed of various nanoscale titania materials, measurement procedures for electron transport processes in DSSCs are also reviewed, together with our results in electrochemical impedance spectroscopy to determine various parameters concerning about electron transport.

Morphology Control of Anatase TiO2 by Surfactant-assisted Hydrothermal Method*

By hydrolysing titanium isopropoxide in a long hydrocarbon chain surfactant-containing solution, TiO2 fine particles with a diversity of well-defined morphologies was synthesized in this study by a hydrothermal route. The structural change during the formation process was monitored by scanning electron microscopy, transmission electron microscopy and X-ray diffraction analysis. TiO2 with various morphologies such as particle, sheet, rod, tube and flower-like shape was obtained by carefully controlling the preparation conditions. The experimental results show that the pH value is crucial for shape control of the produced TiO2 because it can change the charge state of the surfactant in the solution and the adsorption potential of the surfactant on the TiO2 surface. The shape evolvement of anatase TiO2 was elucidated by quenching the reaction at different stage and the formation mechanism of different shaped TiO2 was suggested.

Bioaffinity separation of trypsin using trypsin inhibitor immobilized in reverse micelles composed of a nonionic surfactant.

Trypsin inhibitor was converted to hydrophobic states by covalently combining cholesteryl groups using an acylation reaction, and was immobilized in reverse micelles composed of a nonionic surfactant. Using this reverse micellar phase containing trypsin inhibitor as an affinity ligand, trypsin was selectively separated with high recoveries from a mixture of several kinds of contaminating proteins by forward and backward extraction. No loss of activity of the recovered trypsin was observed through these operations. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 53: 406-408, 1997.

Preparation of nanoporous MgO using gel as structure-direct template

MgO with various nanoscaled pore diameters were formed from magnesium nitrate adsorbed into the gel templates that are synthesized from hydroxyl ethyl methacrylate (HEMA) and ethylene glycol dimethacrylate (EGDMA) where the EGDMA worked as the cross-linker. The effects of molar ratio [HEMA]/[EGDMA] in the gel template on the porous structure of MgO formed in the template were investigated. The results indicated that the pore diameter of MgO could be varied by the [HEMA]/[EGDMA] molar ratio which directly affects the templating structure of gel. The smaller pore size and distribution were obtained with the increased molar ratio. The mechanism of the reaction was simply speculated.

Mechanism of protein solubilization in sodium bis(2-ethylhexyl) sulfosuccinate water-in-oil microemulsion

Abstract We selected α-chymotrypsinogen A (CTN) as a model protein, and the mechanism for protein solubilization in a sodium bis(2-ethylhexyl) sulfosuccinate (AOT) water-in-oil microemulsion was investigated using the two-phase transfer method. The extraction of CTN by the microemulsion is composed of two processes; a fast extraction and a subsequent slow back-extraction process. The adsorption of AOT onto the surface of CTN through the electrostatic interaction between CTN cations and AOT anions converts CTN from a hydrophilic to a hydrophobic state. This adsorption is responsible for the fast extraction process. The adsorption of AOT due to a hydrophobic interaction with CTN in turn makes the CTN-surface hydrophilic. The slow back-extraction process is attributed to the adsorption due to the latter interaction. The presence of two adsorption modes is ascertained by cation-exchange and hydrophobic-interaction chromatography, and spectroscopic measurements. In the fast extraction stage, CTN is extracted to the microemulsion accompanied with large amounts of AOT and water when the CTN-to-AOT mole ratio is comparatively large, suggesting the formation of large clusters composed of many AOT, water and CTN molecules. With a decrease in the ratio, the large cluster is divided into finer aggregates. The distribution of CTN between the microemulsion and the aqueous phase was examined by altering the salinity and organic solvent species of the microemulsion phase in the fast extraction stage. Both effects of the salinity and the solvent species were ascribed to the size effect of the microemulsion droplets. The weak interaction between the microemulsion droplet surface and the tails of the AOT chian adsorbed on the protein plays an essential role in the solubilization of the guest proteins. Since the energy of the droplet deformation required for uptake of the guest protein exceeds this weak interaction, the droplet has the ability to recognize the size of the guest molecules.

Selective separation of trypsin from pancreatin using bioaffinity in reverse micellar system composed of a nonionic surfactant.

Selective separation of trypsin from a mixture involving many kinds of contaminating proteins, i.e., pancreatin, was achieved using trypsin inhibitor immobilized in the reverse micelles, which were composed of a nonionic surfactant, tetra-oxyethylene monodecylether. To determine the efficient operations throughout the whole separation process we examined the operating conditions, which affect the immobilization efficiency of trypsin inhibitor and also the forward and backward extractions of trypsin. Fifty percent of the recovery of trypsin from pancreatin was realized with no loss of activity of the recovered trypsin.

Bioaffinity separation of chymotrypsinogen using antigen-antibody reaction in reverse micellar system composed of a nonionic surfactant

Selective separation of chymotrypsinogen using anti-chymotrypsinogen-antibodies as affinity ligands was realized in reverse micellar system composed of a nonionic surfactant tetra-oxyethylenemonodecylether. Antibodies as affinity ligands were immobilized in reverse micelles by combining cholesteryl groups covalently. Selective separation of proteins using bioaffinity ligands was extended to antigen-antibody reaction system, which enables us to choose any kind of target proteins.

Evaluation of TiO2 Nanoparticle Thin Films Prepared by Electrophoresis Deposition

The absence of cracks and a high optical transparency are critical factors for obtaining high performance when TiO2 thin films are used as photocatalysts and as the cathode material in dye-sensitized solar cells. Synthesized TiO2 nanoparticles were deposited by constant-current electrophoresis in ethanol. TiO2 nanoparticle thin films deposited at a low current density had no apparent cracks and a high optical transparency. Small TiO2 nanoparticles deposited are thought to be transported at low current densities. This enables TiO2 nanoparticle chains to form by the oriented attachment mechanism and thereby increases the electron diffusion length.