Yasushi Umemura

Layered structure of hybrid films of an alkylammonium cation and a clay mineral as prepared by the Langmuir–Blodgett method

Hybrid monolayers of a clay mineral and an amphiphilic alkylammonium (octadecylammonium) cation have been prepared by spreading the alkylammonium cation onto surfaces of clay suspensions. π–A isotherm curves at various clay concentrations indicate that the floating monolayers of the amphiphilic alkylammonium cation are hybridized with clay platelets at an air–clay suspension interface, and that the density of the alkylammonium cation in the hybrid monolayer is dependent on the concentration of the clay in the suspension. The hybrid monolayers were deposited on glass surfaces by horizontal dipping to form hybrid multilayers. The p-polarized infrared external reflection spectra of the hybrid multilayers show that the alkyl chains of the alkylammonium cations in the film are almost perpendicular to the film surface. According to the X-ray diffraction data of the multilayers, the thickness of a unit layer (one clay layer and one alkylammonium cation layer) decreases with the increase in the clay concentration in the suspension used for the film preparation. A model of a layered structure of the hybrid multilayer has been proposed.

Formation of hybrid monolayers of alkylammonium cations and a clay mineral at an air-water interface: clay as an inorganic stabilizer for water-soluble amphiphiles

Abstract Smectite-type clay particles dispersed in an aqueous subphase stabilize a monolayer of water-soluble alkylammonium cations at an air–water interface. When a chloroform solution of the alkylammonium salt is spread onto the clay dispersion, the ammonium cations are adsorbed on the clay particles electrostatically at the interface to form a floating hybrid monolayer. Atomic force microscopy (AFM) observation of the film confirms hybridization of the ammonium cations with the clay particles.

Photoreduction of Prussian Blue Intercalated into Titania Nanosheet Ultrathin Films

Ultrathin films composed of titania nanosheets and Prussian Blue were prepared by using the modified Langmuir-Blodgett technique. The ultrathin films could be deposited in a layer-by-layer manner with lamellar structure. Upon band gap excitation of titania nanosheets, the generated electrons were injected into ferromagnetic Prussian Blue, resulting in reduction to paramagnetic Prussian White. It was revealed that the reaction could proceed only in the presence of hole-scavenging interlayer water molecules.

Surface potential studies on adsorption processes of clay nanosheets onto a floating molecular film of an amphiphilic alkylammonium cation.

A floating molecular film was formed when a chloroform solution of dimethyldioctadecylammonium bromide (DMDOA(+)Br(-)) was spread on an aqueous dispersion of a clay mineral (sodium montmorillonite). At a low concentration (<50 ppm: ppm = mg dm(-3)), a clay mineral was exfoliated into negatively charged layers (denoted by clay nanosheets). Clay nanosheets in a dispersion were adsorbed onto a floating film because of electrostatic interactions. At various clay concentrations (0-50 ppm), surface potential was measured as a function of time to obtain the quantitative information about the adsorption of clay nanosheets on a condensed floating film of DMDOA(+) ions. It was concluded that the adsorption equilibrium obeyed the Langmuir adsorption isotherm, which was supported by the atomic force microscopy (AFM) observation. The rate constants of adsorption and desorption processes were determined by the fitting analyses based on the Langmuir type kinetics. Interestingly, the delay of the adsorption was observed in the early stage indicating that clay nanosheets were removed from the surface region through the repulsion by a counteranion (Br(-)). This explanation was supported by the model simulation using the forward difference method.

Photomagnetic Langmuir–Blodgett films consisting of azobenzene and Prussian Blue: Correlation between the film structure and the photomagnetic efficiency

Abstract A photomagnetic organized thin film consisting of an amphiphilic azobenzene, a smectite clay, and Prussian Blue has been prepared by utilizing the Langmuir–Blodgett (LB) method and an ion-exchange reaction. The magnetic LB films were revealed to possess an organized layered structure, and reversible photoisomerization of the azobenzene was observed even in the LB films. Upon photoillumination at 2 K, reversible changes in the magnetization were realized with the value of ca. 11%. This photoswitching in the magnetization is due to changes in the electrostatic field induced by photoisomerization of the azobenzene. A large photoinduced change in the magnetization would be owing to the highly ordered structure of the LB films. In order to verify the correlation between the film structure and photoswitching efficiency, the LB films have been fabricated in other condition. As results, it is confirmed that the packing state of the azobenzene plays an important and crucial role in accomplishment of the highly efficient photomagnetic switching.

Fabrication of Three-Layer-Component Organoclay Hybrid Films with Reverse Deposition Orders by a Modified Langmuir-Schaefer Technique and Their Pyroelectric Currents Measured by a Noncontact Method.

In an aqueous clay mineral (montmorillonite) dispersion at a low concentration, isolated clay nanosheets with negative charges were suspended. When a solution of amphiphilic octadecylammonium chloride (ODAH(+)Cl(-)) was spread on an air-dispersion interface, the clay nanosheets were adsorbed on the ODAH(+) cations at the interface to form a stable ultrathin floating film. The floating film was transferred onto a substrate by the Schaefer method, and then the film was immersed in a [Ru(dpp)3]Cl2 (dpp = 4,7-diphenyl-1,10-phenanthroline) solution. The Ru(II) complex cations were adsorbed on the film surface because the film surface possessed a cation-exchange ability. The layers of ODAH(+), clay nanosheets, and [Ru(dpp)3](2+) were deposited in this order. By repeating these procedures, three-layer-component films were fabricated (OCR films). In a similar way, three-layer-component films in which the layers of [Ru(dpp)3](2+), clay nanosheets, and ODAH(+) were deposited in the reverse order (RCO films) were prepared by spreading a [Ru(dpp)3](ClO4)2 solution and immersing the films in an ODAH(+)Cl(-) solution. Both OCR and RCO films were characterized by surface pressure-molecular area (π-A) curve measurements, IR and visible spectroscopy, and the XRD method. The OCR and RCO film systems possessed nearly the same properties in the densities of ODAH(+) and [Ru(dpp)3](2+) and the tilt angle of the Ru(II) complex cation, although the layer distance for the RCO film was a little longer than that for the OCR film and the layered structure for the RCO film was less ordered than that for the OCR film. Pyroelectric currents for the films were measured by a noncontact method using an (241)Am radioactive electrode. When the films were heated, the pyroelectric currents were observed and the current directions for the OCR and RCO films were different. This was clear evidence that the layer order in the OCR film was reverse of that in the RCO film.

Structure-distortion-induced photomagnetic effect in azobenzene/polyoxometalate Langmuir–Blodgett films

We have prepared photomagnetic Langmuir-Blodgett films composed of an amphiphilic azobenzene and a magnetic polyoxometalate. The obtained film possesses a well-organized layered structure where polyoxometalate anions are sandwiched by azobenzene cations in a single-layered manner. Reversible photoisomerization of azobenzene was achieved even at low temperature, accompanying intensity changes in the d-d transition of polyoxometalate anions. The photomagnetic effect was observed reversibly upon alternate UV and visible light irradiation. Based on polarized spectroscopy, the observed photomagnetic effect is ascribed to the structure-distortion of polyoxometalate layers.

Clay Minerals as Natural Nanosheets

Clay mineral particles consist of nanosized layers which are negatively charged due to isomorphous substitution. Exchangeable cations in the interlayer space compensate this negative charge and are exchangeable by almost any cationic species: inorganic, organic, polymeric. These properties allow for the synthesis of nanosized films with known techniques: casting, spin coating, layer-by-layer assemblage, Langmuir–Blodgett. The molecules in the interlayer space of the films adopt specific organizations, depending on their size and the charge density of the clay minerals. The molecule–clay mineral surface interaction suppresses molecular aggregation in the interlayer space. This opens the way to formation of functional films. The functionality of these films depends on the functionality of the molecule, the molecular organization in the interlayer space, and the organization of the clay mineral layers in the films.

Evidence for distortion of [FeL3]2+(L = en, amp, bpy, phen) in a Y-type zeolite

Mossbauer spectra of [Fe(bpy)3]2+ and [Fe(phen)3]2+ formed in the supercage of a Y-zeolite revealed structural distortions due to steric interactions of the complex ions with the internal surface of the zeolite, suggesting that the threefold axis of the complex ions may be parallel to the axis of the supercage.

Formation of artifically controlled zirconium–hafnium multilayers on solid surface by use of metal–ligand bonds

Films of alternately layered zirconium and hafnium phosphonates have been prepared on a silicon substrate by immersing a silicon water, modified with an anchoring agent [OHSiMe2–(CH2)3PO3H21], into aqueous solutions in the following order: ZrOCl2, H2O3P(CH2)10PO3H2(DBPA), HfOCl2, DBPA, ZrOCl2.