Atomic-scale topography of rutile TiO2(110) in aqueous solutions: A study involving frequency-modulation atomic force microscopy.

The interfaces between metal oxides and liquids represent the next frontier in the study of oxide chemistry. In this work, (110)-oriented rutile TiO2 wafers were annealed in oxidative atmospheres and immersed in aqueous KCl solutions of pH 3, 6, and 11. Topographic imaging of the TiO2 wafers was carried out in solution via atomic force microscopy using the frequency-modulation force detection technique. Crystalline terraces of 100 nm in width were observed with no sign of solution-induced etching. In a pH-6 solution, ridges parallel to the [001] axis with trenches in between were observed and assigned to the rows of oxygen anions protruding from the surface plane to the solution. Individual anions were further resolved in the ridges, revealing atomic-size protrusions located on the (1 × 1) meshes of the (110) truncation. The topography in an acidic solution (pH 3) was similar to that observed in a neutral solution and could be interpreted as protruding oxygen anions covered partially by protons. In a basic solution with pH 11, qualitatively different features were observed; atomic-size swellings formed a p(2 × 1) superstructure covering the surface, which was hypothesized to be Ti-OH- on five-fold coordinated Ti cations in the surface plane. These results show the feasibility of advanced atomic force microscopy for probing metal-oxide surfaces submerged in liquids.

Phonon mode of TiO2 coupled with the electron transfer from N3 dye.

Low frequency vibrational spectra of submonolayer N3 dye (Ru(4,4(')-dicarboxy-2,2(')-bipyridine)2(NCS)2) adsorbed on TiO2 (110) were reported by using fourth-order coherent Raman spectroscopy, which is interface-sensitive vibrational spectroscopy. Most of the peaks observed in the experiment were at the same frequency as that of Raman and infrared spectra of the dye and TiO2. Two interfacial modes at 640 and 100 cm(-1) and one resonantly enhanced phonon at 146 cm(-1) appeared in addition to the pure TiO2 and N3 spectra. Adsorption of N3 dye on TiO2 contributed to the enhancement of 100 and 146 cm(-1) mode. The results not only reported interfacial low-frequency vibrations of TiO2 (110) with N3 dye adsorption but also suggested the coupling between the surface vibrations of TiO2 and charge transfer between N3 dye and TiO2 on the surface.

Atomic Force Microscopy Imaging of Crystalline Sucrose in Alcohols.

Imaging nanometer- or molecule-scale topography has been achieved by dynamic atomic force microscopy (AFM) when a solid object of interest is damaged by vacuum exposure or electron irradiation. Imaging in a liquid offers a means to remove contaminations from the surface scanned using the microscope tip when the object is soluble to the surrounding liquid, typically water. In the present study, we attempted to take topographic images of crystalline sucrose. A problem arose due to the high solubility of this compound to water. Cantilever oscillation could not be excited in the saturated, viscous aqueous solution. By using n-hexanol instead of water, the solubility in the solvent and thus viscosity of the solution were reduced sufficiently to excite cantilever oscillation. Single-height steps and sucrose molecules were recognized in the images and thereby recorded on the (001)-oriented facets of sucrose crystals. Furthermore, two-dimensional distribution of liquid-induced force pushing or pulling the tip was mapped on planes perpendicular to the hexanol-sucrose interface. Observed uneven force distributions indicated liquid hexanol structured on the corrugated surface of sucrose. The viscosity tuning demonstrated here, which is not limited to hexanol instead of water, extends the range of liquid-solid interfaces to be probed by dynamic AFM.

Optically excited near-surface phonons of TiO(2) (110) observed by fourth-order coherent Raman spectroscopy.

We observed the fourth-order and third-order optical responses in the time domain on a TiO(2) (110) surface covered with trimethyl acetates. Coherent vibrations assignable to near-surface phonon modes were present at 179, 191, 359, 440, 507, 609, and 823 cm(-1) in the fourth-order responses. The amplitude and phase of each mode were determined with different azimuths and polarizations of pump and probe light pulses. Vibrational assignments and possible mechanisms to excite the vibrations were discussed.

Nanometer-Scale Distribution of a Lubricant Modifier on Iron Films: A Frequency-Modulation Atomic Force Microscopy Study Combined with a Friction Test.

Liquid lubricants used in mechanical applications are low-vapor-pressure hydrocarbons modified with a small quantity of polar compounds. The polar modifiers adsorbed on the surface of sliding solids dominate the friction properties when the sliding surfaces are in close proximity. However, a few methods are available for the characterization of the adsorbed modifiers of a nanometer-scale thickness. In this study, we applied frequency-modulation atomic force microscopy to evaluate the vertical and lateral density distributions of the adsorbed modifier in a real lubricant, namely, poly-α-olefin (PAO) modified with an orthophosphoric acid oleyl ester. The liquid-induced force on the probing tip was mapped on a plane that was perpendicular to the lubricant-iron interface with a force sensitivity on the order of 10 pN. The PAO in the absence of the ester modifier was directly exposed to the film, which produced a few liquid layers parallel to the film surface with layer-to-layer distances of 0.6-0.7 nm. A monomolecular layer of the modifier was intermittently adsorbed with increasing ester concentration in the bulk lubricant, with complete coverage seen at 20 ppm. The C18H35 chains of the oleyl esters fluctuating in the lubricant produced a repulsive force on the tip, which monotonically decayed with the tip-to-surface distance. The dynamic friction coefficient of sliding steel-lubricant-steel interfaces, which was separately determined using a friction tester, was compared with the force map determined on the iron film immersed in the corresponding lubricant. The complete monomolecular layer of the ester modifier on the static lubricant-iron boundary is a requirement for achieving smooth and stable friction at the sliding interface.

Probe microscope observation of platinum atoms deposited on the TiO2(110)-(1 x 1) surface.

Titanium dioxide (TiO2) (110) surfaces with Pt adatoms were examined using a noncontact atomic force microscope (NC-AFM) and a Kelvin probe force microscope (KPFM). Topographic images with NC-AFM identify Pt atoms adsorbed at three different sites. These sites are on the Ti atom rows, on the O atom rows, and in O atom vacancies. Most Pt adatoms were observed on Ti atom rows. Successively recorded images show that the Pt adatoms on Ti atom rows (adatoms A) and O atom rows (adatoms C) are mobile while the adatoms in the O atom vacancies (adatoms B) are not. Adatoms A and adatoms B were identified in KPFM images. However, adatoms C were not visualized in KPFM images because they moved quickly or were swept out by the tip. The KPFM measurements reveal that the work function on adatoms A are lower than that on the surrounding (1 x 1) surface by 0.24 eV whereas adatoms B reduced the work function by 0.26 eV. The work function decrease is interpreted with an electric dipole moment directed toward the vacuum, as a result of electron transfer from the adatoms to the TiO2 substrate. In an O atom vacancy, the adatom B is in contact with two Ti atoms and therefore the electron transfer can be enhanced.

STM observation of a ruthenium dye adsorbed on a TiO2(110) surface.

Individual Ru(4,4'-dicarboxy-2,2'-bipyridine)2(NCS)2 molecules, commonly known as N3, adsorbed on a TiO2 surface were visualized with a scanning tunneling microscope (STM) operated in ultrahigh vacuum. A TiO2(110)-(1 x 1) crystal was taken out from the vacuum vessel and immersed into an acetonitrile solution of N3. A monolayer of pivalate ((CH3)3CCOO-) ions was used to protect the (1 x 1) surface from contamination during the wetting process of the N3 adsorption. The N3 molecules adsorbed on the flat terraces protruded by 0.65 nm from the pivalate monolayer. The image height difference of the admolecules could be understood with the assumption that the N3 molecules anchor to the TiO2 surface via two carboxyl groups. The number density of the N3 molecules on the steps was higher than that on the terraces. The poorly coordinated Ti atoms exposed at the step edges form preferential sites where the carboxyl groups can approach, due to a lower steric obstacle or because the structure of the adsorbed N3 molecules suffers less distortion.

Local work function of Pt clusters vacuum-deposited on a TiO2 surface.

Surface topography and work function maps were simultaneously obtained for Pt-evaporated titanium dioxide (TiO(2)) surfaces by using a Kelvin probe force microscope (KPFM). Platinum clusters with diameters of 2-3 nm and heights of 0.2-0.4 nm were obtained on rutile TiO(2)(110)-(1 x 1) surfaces. The work function on the Pt clusters was smaller than that on the surrounding TiO(2) surface. With the assumption that the work function was perturbed by electric dipole moments created at the Pt-TiO(2) interface, the work function decrease indicates that dipole moments were created at the interface and directed toward the vacuum. Such dipole moments can be formed by electron transfer from the originally neutral Pt atoms to the Ti cations exposed on the (1 x 1) surface. A simple model is constructed by assuming a uniform dipole moment per unit interface area. Using this model, the size-dependent perturbation of the work function can be interpreted. The electrostatic potential is more perturbed above the Pt clusters with a larger interface area since the number density of dipole moments is equal to that of the Ti cations and is uniform. A similar correlation between the work function decrease and interface area was observed for the clusters formed on terraces and on step edges. The work function maps showed no peculiar contribution for Ti atoms exposed at the step edges. Vacuum annealing caused a considerable change in the work function on the clusters. The work function was decreased on some clusters relative to the TiO(2) substrate, while it increased on the other clusters. The atomistic structure of the interface may be modified upon annealing, thus perturbing the electron transfer across the interface.

Photochemical reaction of trimethyl acetate on Pt/TiO2(110).

Nanometer-sized Pt particles were prepared on an atomically flat surface of rutile TiO(2). Trimethyl acetate (TMA) adsorbed on the Pt-modified surface was photochemically decomposed under ultraviolet light irradiation in a vacuum. Residing TMA anions were imaged by a scanning tunneling microscope to deduce the local rate of decomposition. Increasing the number density of Pt particles led to an enhancement of the initial reaction rate. The degree of this enhancement did not depend on the distance from the Pt particles.

Microscopic techniques bridging between nanoscale and microscale with an atomically sharpened tip - field ion microscopy/scanning probe microscopy/ scanning electron microscopy.

Over a hundred years an atomistic point of view has been indispensable to explore fascinating properties of various materials and to develop novel functional materials. High-resolution microscopies, rapidly developed during the period, have taken central roles in promoting materials science and related techniques to observe and analyze the materials. As microscopies with the capability of atom-imaging, field ion microscopy (FIM), scanning tunneling microscopy (STM), atomic force microscopy (AFM) and transmission electron microscopy (TEM) can be cited, which have been highly evaluated as methods to ultimately bring forward the viewpoint of reductionism in materials science. On one hand, there have been difficulties to derive useful and practical information on large (micro) scale unique properties of materials using these excellent microscopies and to directly advance the engineering for practical materials. To make bridges over the gap between an atomic scale and an industrial engineering scale, we have to develop emergence science step-by-step as a discipline having hierarchical structures for future prospects by combining nanoscale and microscale techniques; as promising ways, the combined microscopic instruments covering the scale gap and the extremely sophisticated methods for sample preparation seem to be required. In addition, it is noted that spectroscopic and theoretical methods should implement the emergence science.Fundamentally, the function of microscope is to determine the spatial positions of a finite piece of material, that is, ultimately individual atoms, at an extremely high resolution with a high stability. To define and control the atomic positions, the STM and AFM as scanning probe microscopy (SPM) have successfully demonstrated their power; the technological heart of SPM lies in an atomically sharpened tip, which can be observed by FIM and TEM. For emergence science we would like to set sail using the tip as a base. Meanwhile, it is significant to extend a model sample prepared for the microscopies towards a microscale sample while keeping the intrinsic properties found by the microscopies.In this study we present our trial of developing microscopic combined instruments among FIM, field emission microscopy (FEM), STM, AFM and scanning electron microscopy (SEM), in which we prepared and characterized the tips for the SPM, and in addition, the sample preparation to take a correlation between nanoscale and microscale properties of functional materials. Recently, we developed a simple sample preparation method of a rutile single crystal TiO2 covered with an epitaxially-grown monolayer of SiO2 by annealing the crystals in a furnace at high temperatures in air; the crystal samples were placed into a quartz container in the furnace [1]. The vapor of SiO evaporated from the quartz container were adsorbed on the crystal while the crystal surfaces being fully oxidized in air. The SiO2-TiO2 composite systems are promising to protect catalytic TiO2 performance; the photo-catalytic activity is kept by coating with hard and stable SiO2 layers and to extend the lifetime of water super-hydrophilicity even in dark, though understanding of their properties is insufficient due to the lack of techniques to fabricate a well-characterized system on a nanoscale to conduct control experiments. The SiO2 overlayers were observed by low energy electron diffraction (LEED) in vacuum and frequency-modulation (FM) AFM in water [1,2], and water contact angles (WCA) were measured [2]. Although the WCA measurement seems a classic characterization, this method possesses a high potential to make a bridge by controlling the environmental conditions. We will discuss the details.

Kelvin probe force microscopy study of a Pt/TiO2 catalyst model placed in an atmospheric pressure of N2 environment.

A catalyst model comprising platinum nanoparticles deposited on a TiO(2)(110) wafer was prepared in a vacuum, transferred in air, and characterized with a Kelvin probe force microscope placed in a N(2) environment. The topography and local work function of individual nanoparticles were observed with single-nanometer resolution in the N(2) environment of one atmosphere pressure. Some nanoparticle presented positive shifts of work function relative to that of the TiO(2) surface, while the others showed negative shifts. This finding suggests heterogeneous properties of the nanoparticles exposed to air and then N(2). The ability of the advanced microscope was demonstrated in observing the work function of metal nanoparticles on a metal oxide support even in the presence of vapor environments.

Scanning tunneling microscopy study of black dye and deoxycholic acid adsorbed on a rutile TiO2(110).

Trithiocyanato(4,4',4''-tricarboxy-2,2':6',2''-terpyridine)ruthenium(II), "black dye", was adsorbed on a rutile TiO(2)(110) surface and imaged by an ultrahigh vacuum scanning tunneling microscope. The TiO(2)(110)-(1 x 1) surface was prepared in a vacuum, covered with pivalate monolayer, and immersed in acetonitrile containing black dye. Black dyes exchanging preadsorbed pivalates were observed on the surface as protrusions with lateral dimensions from 2 to 10 nm. Protrusions with a minimum lateral dimension of 2 nm were assigned to single, isolated black dyes, and larger protrusions were attributed to aggregated dyes. When deoxycholic acid was added to the dye solution, the number ratio of the single dyes to the aggregated dyes increased, while adsorbed deoxycholic acid was not observed.