Electron−hole recombination kinetics was observed in NaTaO3 photocatalysts doped with Ca, Sr, Ba, and La using time-resolved infrared absorption. The recombination rate was compared with the ultraviolet light-derived H2 production rate in the water splitting reaction to estimate the electron-to-H2 conversion efficiency. The conversion efficiency was sensitive to the nanometer-scale topography of the photocatalyst surface. The particularly high efficiency on the nondoped and 0.5 mol % Sr-doped photocatalysts was related to the flat (100) crystalline surfaces exposed on the photocatalyst particles. The mobility of holes was suggested to be restricted by doping on the basis of a kinetic simulation of the recombination rate.

•We succeeded to observe Cs+ adsorption on the surface of nanosized montmorillonite by frequency modulation atomic force microscopy (FM-AFM).•Cs+ make lines along the [110] direction on the (001) face of montmorillonite.•The Cs+ distribution is independent on the charge distribution in the silicate layer.•Differences of the amount and the distribution of adsorbed cation between on montmorillonite surface and on muscovite surface were demonstrated.Cation exchange of clay mineral is typically analyzed without microscopic study of the clay surfaces. In order to reveal the distribution of exchangeable cations at the clay surface, we performed in situ atomic-scale observations of the surface changes in Na-rich montmorillonite due to exchange with Cs cations using frequency modulation atomic force microscopy (FM-AFM). Lines of protrusion were observed on the surface in aqueous CsCl solution. The amount of Cs of the montmorillonite particles analyzed by energy dispersive X-ray spectrometry was consistent with the ratio of the number of linear protrusions to all protrusions in the FM-AFM images. The results showed that the protrusions represent adsorbed Cs cations. The images indicated that Cs cations at the surface were immobile, and their occupancy remained constant at 10% of the cation sites at the surface with different immersion times in the CsCl solution. This suggests that the mobility and the number of Cs cations at the surface are controlled by the permanent charge of montmorillonite; however, the Cs distribution at the surface is independent of the charge distribution of the inner silicate layer. Our atomic-scale observations demonstrate that surface cations are distributed in different ways in montmorillonite and mica.Download high-res image (106KB)Download full-size image

•Individual CH3 side-chains of iPP were resolved in lamella by FM-AFM observation.•Molecular chains in lamellae were identified on a (110) facet of α-form iPP.•Crystalline lamellae embedded in amorphous domains were identified.•Fragmentation of crystalline lamellae occurred by extensive stretching.Isotactic polypropylene sheets were uniaxially stretched and observed with a frequency-modulation atomic force microscope operated in phenyloctane liquid. Crystalline lamellae were seen in fibrils with their axis parallel to the stretched direction. Individual CH3 side-chains of three-fold helices were identified in the lamellae. Fragmentation of the lamellae was induced by further stretching. The real-space features observed with the microscope were successfully compared with X-ray scattering results obtained in a synchrotron radiation facility.

In this research, sodium tantalate (NaTaO3) photocatalyst was doped with Sr2+ cations via solid-state or hydrothermal reactions. NaTaO3–SrSr1/3Ta2/3O3 solid solutions that had been doped through the solid-state reaction with a Sr-rich shell covering a Sr-poor core appeared. The lattice mismatch at the heteroepitaxial core–shell interface induced surface reconstruction with regularly separated steps. The steady-state population of electrons excited with Hg–Xe lamp irradiation increased by up to 180 times in the solid-solution photocatalysts in which A sites and B sites of the perovskite-structured lattice were doped with Sr2+. On the other hand, A sites were selectively doped with the same cations through the hydrothermal reaction. Surface reconstruction and an enhanced electron population were absent in the A-site-doped photocatalysts. Compatible results were obtained by doping with Ca or Ba instead of Sr. Tuning of doping sites was essential to restricting recombination in NaTaO3 photocatalysts doped with the alkaline-earth metals.Keywords: doping; perovskite structure; photocatalysis; reaction kinetics; recombination; solar hydrogen; solid solution; surface reconstruction

Sodium tantalate (NaTaO3) photocatalysts doped with Sr2+ produce core–shell-structured NaTaO3–SrSr1/3Ta2/3O3 solid solutions able to split water efficiently, when prepared via the solid-state method. In this study, the photocatalysts were chemically etched to examine the different roles of the core and shell with respect to the recombination of electrons and holes. Under excitation by Hg–Xe lamp irradiation, the steady-state population of electrons in the core–shell-structured photocatalyst with a bulk Sr concentration of 5 mol % increased by 130 times relative to that of the undoped photocatalyst. During etching for the first 10 min, the shell detached from the top of the core, and the electron population in the uncovered core further increased by 40%. This population enhancement indicates that electrons are excited in the core and recombined in the shell. Etching up to 480 min resulted in the reduction of the electron population. To interpret the population reduction in this stage of etching, a Sr concentration gradient that controls the electron population in the core is proposed.

•Frequency-modulation AFM (FM-AFM) was applied for imaging mercaptohexanol on gold.•Sub-nanometer topography of assembly was observed in water.•Water structured on the assembly was visualized.Mercaptohexanol is adsorbed on gold (1 1 1) and probed in an aqueous KCl solution by frequency-modulation atomic force microscopy (FM-AFM). Two different features, dot-like and rod-like protrusions, appear in the surface topography. Laterally periodic modulation of the solution density is recognized in the tip-surface force distribution cross-sectional to the surface. A possible assembly of mercaptohexanol, in which two mercaptohexanol molecules are laid down in a rectangular (3×23) unit cell, is proposed to explain the experimental results.

In this study, SrTiO3 photocatalysts doped with Rh and Sb are prepared by hydrothermal synthesis. Doping Rh alone causes optical absorption centered at 580 and 420 nm, attributed to Rh4+ and Rh3+. By adding Sb, the oxidation state of Rh shifts to be 3+ and the absorption at 580 nm disappears. The doped and nondoped photocatalysts exhibit transient infrared absorption at 3000–1000 cm–1 due to excited electrons, when pumped by 355 or 532 nm light pulses. The absorbance decay is observed in a vacuum as a function of microsecond time delays to deduce the relative rate of electron–hole recombination. The active role of Rh4+ as a recombination center is evidenced by the photocatalyst doped with Rh alone having the fastest absorbance decay. The decay retards in the presence of Sb to show the limited role of Rh3+ in recombination. Steady-state H2 production in an aqueous methanol solution is examined in the presence of Pt cocatalyst on the doped SrTiO3 photocatalysts. By comparing the H2 production rate with the recombination rate, a common efficiency of electron-to-H2 conversion is suggested, regardless of different dopant compositions. On the basis of the O2 production rate observed in a AgNO3 solution, the hole-to-O2 conversion efficiency is suggested to be sensitive to the dopant compositions.

The competitive adsorption of long-chain (C18 and C24) carboxylic acids versus n-decanol on graphite was investigated using frequency-modulation atomic force microscopy. A long-range-ordered monolayer of the solute (stearic acid or lignoceric acid) developed in saturated decanol solution, whereas an ordered decanol monolayer was deposited from dilute solutions. The piconewton-order tip–surface force was observed in solutions as a function of the vertical and lateral coordinates, together with the topography of the monolayers. The tip–surface force was periodically modulated, which was interpreted with a solution structure commensurate with the ordered assembly of adsorbed monolayers. These results show that the solution structure at the interface was controlled by the competitively adsorbed species and thus was sensitive to the composition of the bulk solution.

The structure of liquid water and 2-propanol on the (104) surface of calcite (CaCO3) was probed by frequency-modulation atomic force microscopy. The microscope tip scanned each liquid to record the tip–surface force perturbed by the liquid structure at the interface. In water, the force distribution on planes cross-sectional to the surface presents a 0.5-nm-thick checkerboard-like pattern matching the corrugated topography of the calcite surface. This provides evidence that the local water density was laterally and vertically modulated. With 2-propanol, a laterally uniform, vertically layered structure was found between the first laterally structured layer and the bulk liquid. These results are consistent with the density distributions of water and ethanol proposed in earlier X-ray and simulation studies.

Acetone adsorption on oxidized and reduced rutile TiO2(110) surfaces was examined in a vacuum with a scanning tunneling microscope. A vacuum-annealed TiO2 surface was oxidized with O2 and then exposed to acetone vapor at room temperature. The acetone−oxygen complex proposed in early studies appeared in constant-current topography. We assigned the complex to η2-2,2-propanediolates by comparing its topography with that of coadsorbed acetate. When the vacuum-annealed surface was directly exposed to acetone, mobile η1-acetone was observed on Ti-atom rows.

The charge transfer from a nanometer-sized transition metal particle to a catalyst support is thought to affect reactions over the metal surface. We propose the application of Kelvin probe force microscope, which is an extension of the atomic force microscope, to observe the charge transfer particle-by-particle. Our recent results on Na adatoms, Cl adatoms, Pt adatoms and particles, and Ni particles evaporated on TiO2(110) are reviewed.

A Raman-based, nonlinear optical spectroscopy is a promising method for observing vibrational modes localized at buried interfaces. The principles of Raman excitation and interface-selective detection of coherent vibrations are described. Applications to air–liquid, liquid–liquid, air–solid, liquid–solid, and solid–solid interfaces are reviewed.

A Raman-based, nonlinear optical spectroscopy is a promising method for observing vibrational modes localized at buried interfaces. The principles of Raman excitation and interface-selective detection of coherent vibrations are described. Applications to air–liquid, liquid–liquid, air–solid, liquid–solid, and solid–solid interfaces are reviewed.