Charge State of Au Atoms on an Oxidized Rutile TiO2(110) Surface by AFM/KPFM at 78 K.

The charge state of noble metal atoms on a semiconductor surface is an important factor in surface catalysis. In this study, Au atoms were deposited on the rutile TiO2(110) surface to characterize its charge properties using atomic force microscopy with Kelvin probe force microscopy at 78 K. Au single atoms, dimers, and trimers at different sites on the surface were investigated. Positively charged Au atoms were verified at oxygen sites, while negatively charged Au atoms were found near oxygen vacancy sites. Furthermore, the charge states of small Au nanoclusters were clarified. Understanding the charge states of Au atoms is significant for identifying their efficient catalytic effects in surface catalysis.

Tip-activated single-atom catalysis: CO oxidation on Au adatom on oxidized rutile TiO2 surface.

Single-atom catalysis of carbon monoxide oxidation on metal-oxide surfaces is crucial for greenhouse recycling, automotive catalysis, and beyond, but reports of the atomic-scale mechanism are still scarce. Here, using scanning probe microscopy, we show that charging single gold atoms on oxidized rutile titanium dioxide surface, both positively and negatively, considerably promotes adsorption of carbon monoxide. No carbon monoxide adsorption is observed on neutral gold atoms. Two different carbon monoxide adsorption geometries on gold atoms are identified. We demonstrate full control over the redox state of adsorbed gold single atoms, carbon monoxide adsorption geometry, and carbon monoxide adsorption/desorption by the atomic force microscopy tip. On charged gold atoms, we activate Eley-Rideal oxidation reaction between carbon monoxide and a neighboring oxygen adatom by the tip. Our results provide unprecedented insights into carbon monoxide adsorption and suggest that the gold dual activity for carbon monoxide oxidation after electron or hole attachment is also the key ingredient in photocatalysis under realistic conditions.

Single hydrogen atom manipulation for reversible deprotonation of water on a rutile TiO2 (110) surface.

The discovery of hydrogen atoms on the TiO2 surface is crucial for many practical applications, including photocatalytic water splitting. Electronically activating interfacial hydrogen atoms on the TiO2 surface is a common way to control their reactivity. Modulating the potential landscape is another way, but dedicated studies for such an activation are limited. Here we show the single hydrogen atom manipulation, and on-surface facilitated water deprotonation process on a rutile TiO2 (110) surface using low temperature atomic force microscopy and Kelvin probe force spectroscopy. The configuration of the hydrogen atom is manipulated on this surface step by step using the local field. Furthermore, we quantify the force needed to relocate the hydrogen atom on this surface using force spectroscopy and density functional theory. Reliable control of hydrogen atoms provides a new mechanistic insight of the water molecules on a metal oxide surface.

Atomic Scale Three-Dimensional Au Nanocluster on a Rutile TiO2 (110) Surface Resolved by Atomic Force Microscopy.

The atomic structure of a three-dimensional Au nanocluster on a TiO2 surface is important for various studies such as photocatalysis and chemical reactions on metal oxide surfaces. However, accessing the atomic structure of a metal nanocluster supported on a metal oxide surface is still challenging possibly because of its small size, complexity, and flexible structure. Here, we report a detuning imaging mode combined with an atomically sharp tip to address these challenges. Using this method, for the first time, we resolve a three-dimensional Au nanocluster on a rutile TiO2 (110) surface. We found that the Au nanocluster was continuously adsorbed at the upper and lower terraces. Our results open up a new methodology for studying subnanometer clusters at various surfaces at an atomic scale.

Elucidating the charge state of an Au nanocluster on the oxidized/reduced rutile TiO2 (110) surface using non-contact atomic force microscopy and Kelvin probe force microscopy.

The charge state of Au nanoclusters on oxidized/reduced rutile TiO2 (110) surfaces were investigated by a combination of non-contact atomic force microscopy and Kelvin probe force microscopy at 78 K under ultra-high vacuum. We found that the Au nanoclusters supported on oxidized/reduced surfaces had a relatively positive/negative charge state, respectively, compared with the substrate. In addition, the distance dependence of LCPD verified the contrast observed in the KPFM images. The physical background of charge transfer observation can be explained by the model of charge attachment/detachment from multiple oxygen vacancies/adatoms surrounding Au nanoclusters. These results suggest that the electronic properties of the Au nanoclusters are dramatically influenced by the condition of the support used.

Imaging the surface potential at the steps on the rutile TiO2(110) surface by Kelvin probe force microscopy.

Although step structures have generally been considered to be active sites, their role on a TiO2 surface in catalytic reactions is poorly understood. In this study, we measured the contact potential difference around the steps on a rutile TiO2(110)-(1 × 1) surface with O2 exposure using Kelvin probe force microscopy. A drop in contact potential difference was observed at the steps, indicating that the work function locally decreased. Moreover, for the first time, we found that the drop in contact potential difference at a <1-11> step was larger than that at a <001> step. We propose a model for interpreting the surface potential at the steps by combining the upward dipole moment, in analogy to the Smoluchowski effect, and the local dipole moment of surface atoms. This local change in surface potential provides insight into the important role of the steps in the catalytic reaction.

Tip-Induced Control of Charge and Molecular Bonding of Oxygen Atoms on the Rutile TiO2 (110) Surface with Atomic Force Microscopy.

We study a low-temperature on-surface reversible chemical reaction of oxygen atoms to molecules in ultrahigh vacuum on the semiconducting rutile TiO2(110)-(1 × 1) surface. The reaction is activated by charge transfer from two sources, natural surface/subsurface polarons and experimental Kelvin probe force spectroscopy as a tool for electronic charge manipulation with single electron precision. We demonstrate a complete control over the oxygen species not attainable previously, allowing us to deliberately discriminate in favor of charge or bond manipulation, using either direct charge injection/removal through the tip-oxygen adatom junction or indirectly via polarons. Comparing our ab initio calculations with experiment, we speculate that we may have also manipulated the spin on the oxygens, allowing us to deal with the singlet/triplet complexities associated with the oxygen molecule formation. We show that the manipulation outcome is fully governed by three experimental parameters, vertical and lateral tip positions and the bias voltage.

Measurement and Manipulation of the Charge State of an Adsorbed Oxygen Adatom on the Rutile TiO2(110)-1×1 Surface by nc-AFM and KPFM.

For the first time, the charge states of adsorbed oxygen adatoms on the rutile TiO2(110)-1×1 surface are successfully measured and deliberately manipulated by a combination of noncontact atomic force microscopy and Kelvin probe force microscopy at 78 K under ultrahigh vacuum and interpreted by extensive density functional theory modeling. Several kinds of single and double oxygen adatom species are clearly distinguished and assigned to three different charge states: Oad-/2Oad-, Oad2-/2Oad2-, and Oad--Oad2-, i.e., formal charges of either one or two electrons per atom. Because of the strong atomic-scale image contrast, these states are clearly resolved. The observations are supported by measurements of the short-range force and local contact potential difference as a function of the tip-sample distance as well as simulations. Comparison with the simulations suggests subatomic resolution by allowing us to resolve the rotated oxygen p orbitals. In addition, we manage to reversibly switch the charge states of the oxygen adatoms between the Oad- and Oad2- states, both individually and next to another oxygen, by modulating the frequency shift at constant positive voltage during both charging and discharging processes, i.e., by the tip-induced electric field of one orientation. This work provides a novel route for the investigation of the charge state of the adsorbates and opens up novel prospects for studying transition-metal-oxide-based catalytic reactions.

Direct observation of atomic step edges on the rutile TiO2(110)-(1 × 1) surface using atomic force microscopy.

Clarifying the atomic configuration of step edges on a rutile TiO2 surface is crucial for understanding its fundamental reactivity, and the direct observation of atomic step edges is still a challenge. AFM is a powerful tool for investigating surface structures with true atomic resolution, and it provides the opportunity to resolve the real structure of step edges with improved techniques. In this work, we successfully imaged the atomic configuration of 001 and 1-11 step edges on the surface of rutile TiO2(110)-(1 × 1), and we present the direct observation of oxygen vacancies along the 1-11 step edges, indicating that one 1-11 step edge site corresponds to one oxygen vacancy using AFM. We also made use of the simultaneous AFM/STM measurements to explore the electronic structure of step edges, which enhanced the evidence of oxygen vacancies existing along the 1-11 step edges and further demonstrated that the 001 step edge was terminated by an O row. The effect of the reduced 1-11 step edges was explored by probing the O2 adsorption and the nucleation behavior of gold clusters. It was found that oxygen vacancies along the 1-11 step edges could contribute to O2 dissociative adsorption and there was no obvious difference compared with the oxygen vacancies on the flat terrace. The reduced step edge and terrace likewise acted as nucleation and growth sites for gold atoms/nanoparticles, in line with previous reports. The present study provides a complete characterization of the atomic configuration of the step edges on the TiO2(110) surface and plays an important role in investigating the surface chemistry of metal oxides.

Stable contrast mode on TiO2(110) surface with metal-coated tips using AFM.

We investigated a method to obtain a stable contrast mode on the TiO2(110) surface. The stable contrast rate is approximately 95% with a W-coated Si cantilever, which demonstrates that a stable tip apex plays an important role to obtain the real geometry of the surface during atomic force microscopy measurement. Information related to surface structure and tunnelling current on the TiO2(110) surface can be obtained by the W-coated Si cantilever. It is possible to investigate the electronic structure and surface potential on the TiO2(110) surface with atomic resolution. In particular, the proposed method could be widely applied to investigate the catalytic activity and the mechanism of a catalytic reaction by a metal-coated tip in the future.

Chemical Synthesis of the Epimeric (23R)- and (23S)-Fluoro Derivatives of Bile Acids via Horner-Wadsworth-Emmons Reaction.

A method for the synthesis of two (23R)- and (23S)-epimeric pairs of 23-fluoro-3α,7α,12α-trihydroxy-5ß-cholan-24-oic acid and 23-fluoro-3α,7α-dihydroxy-5ß-cholan-24-oic acid is described. The key intermediates, 23,24-dinor-22-aldehyde peracetates were prepared from cholic and chenodeoxycholic acids via the 24-nor-22-ene, 24-nor-22ξ,23-epoxy, and 23,24-dinor-22-aldehyde derivatives. The Horner-Wadsworth-Emmons reaction of the 23,24-dinor-22-aldehydes using triethyl 2-fluoro-2-phosphonoacetate in the presence of LiCl and 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU), and subsequent hydrogenation of the resulting 23ξ-fluoro-22-ene ethyl esters, followed by hydrolysis, gave a mixture of the epimeric (23R)- and (23S)-fluorinated bile acids which were resolved efficiently by preparative RP-HPLC. The stereochemical configuration of the fluorine atom at C-23 in the newly synthesized compounds was confirmed directly by the X-ray crystallographic data. The (1)H and (13)C NMR spectral differences between the (23R)- and (23S)-epimers were also discussed.

Biliary bile acids in birds of the Cotingidae family: taurine-conjugated (24R,25R)-3α,7α,24-trihydroxy-5ß-cholestan-27-oic acid and two epimers (25R and 25S) of 3α,7α-dihydroxy-5ß-cholestan-27-oic acid.

Three C(27) bile acids were found to be major biliary bile acids in the capuchinbird (Perissocephalus tricolor) and bare-throated bellbird (Procnias nudicollis), both members of the Cotingidae family of the order Passeriformes. The individual bile acids were isolated by preparative RP-HPLC, and their structures were established by RP-HPLC, LC/ESI-MS/MS and NMR as well as by a comparison of their chromatographic properties with those of authentic reference standards of their 12α-hydroxy derivatives. The most abundant bile acid present in the capuchinbird bile was the taurine conjugate of C(27) (24R,25R)-3α,7α,24-trihydroxy-5ß-cholestan-27-oic acid, a diastereomer not previously identified as a natural bile acid. The four diastereomers of taurine-conjugated (24ξ,25ξ)-3α,7α,24-trihydroxy-5ß-cholestan-27-oic acid could be distinguished by NMR and were resolved by RP-HPLC. The RRT of the diastereomers (with taurocholic acid as 1.0) were found to be increased in the following order: (24R,25R)<(24S,25R)<(24S,25S)<(24R,25S). Two epimers (25R and 25S) of C(27) 3α,7α-dihydroxy-5ß-cholestan-27-oic acid were also present (as the taurine conjugates) in both bird species. Epimers of the two compounds could be distinguished by their NMR spectra and resolved by RP-HPLC with the (25S)-epimer eluting before the (25R)-epimer. Characterization of the taurine-conjugated (24R,25R)-3α,7α,24-trihydroxy-5ß-cholestan-27-oic acid and two epimers (25R and 25S) of 3α,7α-dihydroxy-5ß-cholestan-27-oic acid should facilitate their detection in peroxisomal disease and inborn errors of bile acid biosynthesis.

Chemical synthesis of the 17-propanamide derivatives of stereoisomeric Δ14-17α- and 17ß-estradiols: potential 17ß-hydroxysteroid dehydrogenase inhibitors.

The 17-propanamide derivatives of diastereomeric Δ(14)-17α- and 17ß-estradiols, the potential candidates of a 17ß-hydroxysteroid dehydrogenase (17ß-HSD) inhibitor, were synthesized in 11 steps from estrone. The principal reactions employed involved in (1) conversion of estrone to the corresponding Δ(14)-estrone, (2) Grignard reaction of Δ(14)-estrone with allylmagnesium bromide followed by regioselective hydroboration of the resulting stereoisomeric 17ξ-allyl-Δ(14)-17ξ-ols with 9-borabicyclo[3.3.1]nonane (9-BBN), and (3) direct amidation of the 17ξ-O-/17ξ-C-spiro-γ-lactones with NH(3) under positive pressure of H(2).

Observation of beat oscillation generation by coupled waves associated with parametric decay during radio frequency wave heating of a spherical tokamak plasma.

We present an observation of beat oscillation generation by coupled modes associated with parametric decay instability (PDI) during radio frequency (rf) wave heating experiments on the Tokyo Spherical Tokamak-2. Nearly identical PDI spectra, which are characterized by the coexistence of the rf pump wave, the lower-sideband wave, and the low-frequency oscillation in the ion-cyclotron range of frequency, are observed at various locations in the edge plasma. A bispectral power analysis was used to experimentally discriminate beat oscillation from the resonant mode for the first time. The pump and lower-sideband waves have resonant mode components, while the low-frequency oscillation is exclusively excited by nonlinear coupling of the pump and lower-sideband waves. Newly discovered nonlocal transport channels in spectral space and in real space via PDI are described.

Chemical synthesis of (22E)-3alpha,6alpha,7alpha,12alpha-Tetrahydroxy-5beta-chol-22-en-24-oic acid and its N-acylamidated conjugates with glycine or taurine: precursors of the [22,23-(3)H] labelled tracers.

(22E)-3alpha,6alpha,7alpha,12alpha-Tetrahydroxy-5beta-chol-22-en-24-oic acid and its N-acylamidated conjugates with glycine or taurine were synthesized from cholic acid. The key reactions employed are: 1) degradation of the side chain in intermediary C(24) 3alpha,6alpha,7alpha,12alpha-tetrahydroxylated bile acid to the corresponding C(22) 23,24-dinor-aldehyde, followed by Wittig reaction with methyl (triphenylphosphoranylidene)acetate and 2) N-acylamidation of the unconjugated tetrahydroxy-Delta(22)-5beta-cholenoic acid with glycine (or taurine) in the presence of diethylphosphorocyanide and triethylamine as coupling reagents.