Electron Transfer through Clay Monolayer Films Fabricated by the Langmuir−Blodgett Technique
Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan.Langmuir (Impact Factor: 4.46). 12/2006; 22(23):9591-7. DOI: 10.1021/la061668f
Hybrid films composed of amphiphilic molecules and clay particles were constructed by the modified Langmuir-Blodgett (LB) method. Clays used were sodium montmorillonite (denoted as mont) and synthetic smectite containing Co(II) ions in the octahedral sites (denoted as Co). Two kinds of amphiphilic molecules were used-[Ru(dC(18)bpy)(phen)2](ClO4)2 (dC(18)bpy = 4,4'-dioctadecyl-2,2'-bipyridyl and phen = 1,10-phenanthroline) (denoted as Ru) and octadecylammonium choloride (ODAH+Cl- or denoted as ODAH). Three kinds of hybrid films (denoted as Ru-mont, Ru-Co, and ODAH-Co films) were prepared by spreading an amphiphilic molecule onto an aqueous suspension of a clay. Atomic force microscopy (AFM) analyses of the films deposited on silicon wafers indicated that closely packed films were obtained at 20 ppm for all the above three cases. Cyclic voltammetry (CV) was measured on an ITO electrode modified with a hybrid film or a monolayer film of pure Ru(II) complex salt (denoted as Ru film). The Ru(II) complexes incorporated in the Ru-mont film lost their redox activity, indicating that montmorillonite layers acted as a barrier against electron transfer. In contrast, the same complexes in the Ru-Co film were electrochemically active with the simultaneous appearance of the redox peaks due to the Co(II)/Co(III) (or Co(II)/Co(IV)) couple. The results implied that electron transfer through cobalt clay layers was possible via mediation by Co(II) ions in a clay sheet. For an aqueous solution containing nitrite ions (NO2-) at pH 3.0, a large catalytic oxidation current was observed for both the electrodes modified with the Ru-mont and Ru-Co films. The results were interpreted in terms of the mechanisms that the charge separation of an incorporated Ru(II) complex took place to produce a pair of a Ru(III) complex and an electron and that the generated Ru(III) complex was reduced by a nitrite ion before it recombined with the electron.
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ABSTRACT: A thin film of trimethylammonium-exchanged perovskite-type niobate ((CH3)3NHSr2Nb3O10) was prepared by casting its exfoliated aqueous dispersion onto a glass substrate. The electric conductivity of a film (3−9 μm thick) was measured in vacuum under the illumination of light (280450 nm). Photocurrent raised and decayed slowly in the time range of 103 seconds in response to the on-and-off of an incident light. Atmospheric oxygen accelerated the decay rate of current in the dark. In the absence of oxygen, the decay curve obeyed the equation of stretched exponential relaxation (ip = ip(0) exp(−(t/τ)β). From the dependence of the photocurrent on various parameters such as film thickness, light wavelength, and temperature, the observed persistent phenomena were interpreted according to the following mechanisms: (1) a free electron was photogenerated under the illumination of light most effectively when photon energy corresponded to the edge of a band gap transition; (2) the generated electrons were trapped at surface oxygen vacancies before they became conductive; and (3) a persistent current appeared through the multistep trapping−detrapping processes under the gradient of electric field. It was suggested that vacancies were produced by the elimination of lattice oxygen atoms as dianions during the acid treatment of original niobate (KSr2Nb3O10). As far as we know, the present finding is the first example of persistent photocurrents in inorganic thin films.
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ABSTRACT: A heterojunction photodiode was fabricated by forming two contact regions on a glass substrate: one side was a cast film of perovskite-type niobate [(CH3)3NHSr2Nb3O10] as a n-type photosemiconductor and the other side a cast film of Zn-saponite (Na0.96[Si7.18Al0.64]Zn6.20O20 (OH)2) as a p-type semiconductor under oxygen atmosphere. Diode-type current--voltage characteristics were obtained under the illumination of light (340 nm) and oxygen atmosphere (1 atm) at 25--100 ℃. The interfacial structure was studied by means of focused ion-beam and transmission electron microscopy techniques, confirming the contact of the two different nanosheets on a nanometer scale. The results are discussed on the basis of the nanosheet band structures.
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