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Solution-based organometallic nanoclusters are unique nanoscale precursors due to the ability to precisely control their size, shape, structure, and assembly. The interaction of extreme ultraviolet...

Ruthenium dioxide is used extensively as a co-catalyst in a wide range of photocatalytic water splitting systems. As such, understanding how it promotes the evolution of hydrogen and oxygen is of high importance. In this study, we employ scanning tunneling microscopy to study the adsorption and reaction of hydrogen on a model RuO2(110) surface. We find that at low coverages H2 dissociates heterolytically to hydride‑hydroxyl pairs even at 5 K. The onset temperature for the low coverage H2 dissociation is surprising as it is not expected based on prior temperature programmed desorption studies. At higher coverages, H2 chemisorbs molecularly on bare ruthenium sites that are neighboring the hydride‑hydroxyl pairs. Upon annealing, the chemisorbed H2 disappears by 120 K, primarily due to H2 desorption from the surface. Further annealing between 120 and 295 K leads to the conversion of hydride species to bridging hydroxyls. The bridging hydroxyls ultimately recombine between 450 and 600 K to form water, yielding clean RuO2(110) with oxygen vacancies.

Sodium bismuthate is a commercially available, inexpensive, non-toxic and very potent inorganic oxidant and photocatalyst. It is one of the important reagents for oxidative separation of Am3+ from the chemically similar lanthanide ions, for its recovery or safe disposal from reprocessed nuclear fuel. While the structure of NaBiO3 has been described from powder and neutron diffraction; the structure of NaBiO3·XH2O, the manufactured form of sodium bismuthate, is currently unknown. Herein, we describe the structure of NaBiO3·XH2O (X = 3) using pair distribution function (PDF) analysis of X-ray total scattering data. In our proposed structure model, NaBiO3·3H2O is similar to NaBiO3, but with turbostratic disorder in the stacking direction of the alternating Bi-O and Na-O layers. We propose locations for the lattice water, and its role in creating turbostratic disorder. We also used PDF to describe the structural evolution of sodium bismuthate upon exposure to nitric acid, the conditions employed in for nuclear fuel reprocessing. We supported the proposed model for pristine NaBiO3·3H2O and its acidified derivatives by a variety of techniques including thermogravimetry, powder X-ray diffraction (PXRD), energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). By employing both surface and bulk techniques, we hypothesize that the bismuth reduced to Bi3+ upon aqueous acid exposure remains in the lattice, rather than completely dissolving and/or depositing on the surface, as prior suggested. Using pretreated acidified sodium bismuthate samples, we delineated the effects of acid strength vs. bismuthate structure/composition on Ce3+ to Ce4+ oxidation efficacy.

Thermal diffusion of CO adsorbed on rutile TiO2(110) was studied in the 20–110 K range using photon-stimulated desorption (PSD), temperature-programmed desorption (TPD), and scanning tunneling microscopy. During UV irradiation, CO desorbs from certain photoactive sites (e.g., oxygen vacancies). This phenomenon was exploited to study CO thermal diffusion in three steps: first, empty these sites during a first irradiation cycle, then replenish them with CO during annealing, and finally probe the active site occupancy in the second PSD cycle. The PSD and TPD experiments show that the CO diffusion rate correlates with the CO adsorption energy—stronger binding corresponds to slower diffusion. Increasing the CO coverage from 0.06 to 0.44 monolayer (ML) or hydroxylation of the surface decreases the CO binding and increases the CO diffusion rate. Relative to the reduced surface, the CO adsorption energy increases and the diffusion decreases on the oxidized surface. The CO diffusion kinetics can be modeled satisfa...

The adsorption and photochemistry of CO on rutile TiO2(110) are studied with scanning tunneling microscopy (STM), temperature-programmed desorption, and angle-resolved photon-stimulated desorption (PSD) at low temperatures. Site occupancies, when weighted by the concentration of each kind of adsorption site on the reduced surface, show that the adsorption probability is the highest for the bridging oxygen vacancies (VO). The probability distribution for the different adsorption sites corresponds to very small differences in CO adsorption energies (

The influence of MnOx overlayers/nanoclusters deposited on the Pd(111) single-crystal model catalyst surface on the catalytic dehydrogenation of double-deuterated formic acid (FA, DCOOD) was studied under ultrahigh vacuum conditions via temperature-programmed desorption and X-ray photoelectron spectroscopy techniques. A significant boost in D-2 generation was observed in the catalytic FA dehydrogenation on MnOx/Pd(111) as compared to that of a clean Pd(111) model catalyst, demonstrating the cooperative interaction between Pd(111) and MnOx sites. Maximum FA conversion was observed at a submonolayer MnOx surface coverage of 0.25 ML (monolayer) on Pd(111), whereas D-2 formation was found to be suppressed when the Pd(111) surface was entirely covered with relatively thick (15 ML) MnOx overlayers. A direct correlation between increasing relative abundance of oxidized Mn surface states (i.e., Mn2+, Mn3+, and Mn4+) and increasing catalytic FA dehydrogenation was observed. Different modes of promotion of FA dehydrogenation via MnOx (i.e., catalytic promotion versus noncatalytic/stoichiometric promotion) were discussed as a function of the differences in the model catalyst preparation and the extent of oxidation of the MnOx overlayer.

Understanding adsorbed water and its dissociation to surface hydroxyls on oxide surfaces is key to unraveling many physical and chemical processes, yet the barrier for its deprotonation has never been measured. In this study, we present direct evidence for water dissociation equilibrium on rutile-TiO2(110) by combining supersonic molecular beam, scanning tunneling microscopy (STM), and ab initio molecular dynamics. We measure the deprotonation/protonation barriers of 0.36 eV and find that molecularly bound water is preferred over the surface-bound hydroxyls by only 0.035 eV. We demonstrate that long-range electrostatic fields emanating from the oxide lead to steering and reorientation of the molecules approaching the surface, activating the O–H bonds and inducing deprotonation. The developed methodology for studying metastable reaction intermediates prepared with a high-energy molecular beam in the STM can be readily extended to other systems to clarify a wide range of important bond activation processes.

Understanding the reactivity of H2 is of critical importance in controlling and optimizing many heterogeneous catalytic processes, particularly in cases where its adsorption on the catalyst surface is rate-limiting. In this work, we examine the temperature-dependent adsorption of H2/D2 on the clean RuO2(110) surface using the King and Wells molecular beam approach, temperature-programmed desorption (TPD), and scanning tunneling microscopy (STM). We show that the adsorption probability of H2/D2 on this surface is highly temperature-dependent, decreasing from ∼0.4 below 25 K to0.01 at 300 K. Both STM and TPD reveal that adsorption (molecular or dissociative) is severely limited once the temperature exceeds the trailing edge temperature of the H2 TPD state (∼150 K). The presence of coadsorbed water or oxygen does not appear to alter this situation. Previous literature reports of extensive RuO2(110) surface hydroxylation from H2/D2 exposures at 300 K may instead be the result of background contamination brought about by chamber backfilling.

The coordination of H2 to a metal center via polarization of its σ bond electron density, known as a Kubas complex, is the means by which H2 chemisorbs at Ru(4+) sites on the rutile RuO2(110) surface. This distortion of electron density off an interatomic axis is often described as a 'banana-bond.' We show that the Ru-H2 banana-bond can be destabilized and split using visible light. Photodesorption of H2 (or D2) is evident by mass spectrometry and scanning tunneling microscopy. From time-dependent density functional theory, the key optical excitation splitting the Ru-H2 complex involves an interband transition in RuO2 which effectively diminishes its Lewis acidity, thereby weakening the Kubas complex. Such excitations are not expected to affect adsorbates on RuO2 given its metallic properties. Therefore, this common thermal cocatalyst employed in photocatalysis is, itself, photoactive.

I briefly describe my joint efforts and experiences with M.G.Nakhodkin in the field of scanning tunneling microscopy (STM), including a construction of home-built microscopes, the application of this technique in various scientific endeаvours, fruitful and enlightening discussions, and the collaboration on the international scale with M.G.Nakhodkin and members of his scientific group. Our co-operation was focused on the novel aspects of the preparation and the conditioning of STM probes, coupling the STM junction with laser irradiation, and STM-based nanolithography., Я коротко наводжу нашу спiльну з М.Г. Находкiним роботу в областi растрової тунельної мiкроскопiї (РТМ), включаючи створення мiкроскопiв i застосування цiєї технiки в рiзних наукових завданнях, кориснi пояснювальнi обговорення, i спiвпрацю мiжнародного масштабу з М.Г. Находкiним i членами його наукової групи. Нашу спiвпрацю було сфокусовано на нових аспектах виготовлення та досягнення необхiдних параметрiв РТМ-датчикiв; на впливi випромiнюванням лазера на область, що дослiджується за допомогою РТМ; i нанолiтографiї на основi РТМ.

Advances in extreme ultraviolet (EUV) photolithography require the development of next-generation resists that allow high-volume nanomanufacturing with a single nanometer patterning resolution. Organotin-based photoresists have demonstrated nanopatterning with high resolution, high sensitivity, and low-line edge roughness. However, very little is known regarding the detailed reaction mechanisms that lead to radiation-induced solubility transitions. In this study, we investigate the interaction of soft X-ray radiation with organotin clusters to better understand radiation-induced chemistries associated with EUV lithography. Butyltin Keggin clusters (β-NaSn13) were used as a model organotin photoresist, and characterization was performed using ambient-pressure X-ray photoelectron spectroscopy. The changes in relative atomic concentrations and associated chemical states in β-NaSn13 resists were evaluated after exposure to radiation for a range of ambient conditions and photon energies. A significant reductio...

Metal oxide nanomaterials have shown promise for use as EUV resists. Recently, significant efforts have focused on tinoxo clusters that have high absorption coefficient Sn centers and radiation sensitive organic ligands. In our studies, we have investigated a β-Keggin butyl-Sn cluster (β-NaSn13), which is charge-neutral and allows studying radiation induced chemistries without interference from counterions. We have used ambient pressure X-ray photoelectron spectroscopy (APXPS) to investigate the contrast properties of the β-NaSn13 in ultrahigh vacuum (UHV) and in the presence of ambient oxygen. These contrast studies indicate that ambient oxygen reduces the dose requirements for the solubility transition of the β-NaSn13 photoresists. APXPS spectra collected before and after the solubility transition shows that ambient oxygen causes a greater loss of butyl ligands from the samples and the formation of more tin oxide for larger doses, suggesting the presence of reactive oxygen species. APXPS was also used to study processes during the post exposure bake, where we compared the differences in film chemistries in ambient oxygen or in UHV. There were only very small differences in the APXPS spectra before exposure and after exposure and the post exposure bake. However, ambient oxygen resulted in some changes for unexposed regions during the post exposure bake; there was a greater ratio of tin oxide to other oxygen species (alkoxy ligands, hydroxyls) for samples annealed in oxygen. These results have significance for EUV and e-beam lithography processing parameters, as well as implications for cluster design and ligand chemistries.

The influence of reactant coverage on photochemical activity was explored using scanning tunneling microscopy (STM) and ultraviolet photoelectron spectroscopy (UPS). We observed diminished reactivity of carboxylate species (trimethyl acetate, TMA) on TiO2(110) as a function of increasing coverage. This effect was not linked to intermolecular interactions of TMA but to the accumulation of the coadsorbed bridging hydroxyls (HOb) deposited during (thermal) dissociative adsorption of the parent, trimethylacetic acid (TMAA). Confirmation of the hindering influence of HOb groups was obtained by the observation that HOb species originated from H2O dissociation at O-vacancy sites have a similar hindering effect on TMA photochemistry. Though HOb’s are photoinactive on TiO2(110) under ultrahigh vacuum conditions, UPS results show that these sites trap photoexcited electrons, which in turn likely (electrostatically) attract and neutralize photoexcited holes, thus suppressing the hole-mediated photoreactivity of TMA....

Despite the importance of RuO2 in photocatalytic water splitting and catalysis in general, the interactions of water with even its most stable (110) surface are not well understood. In this study we employ a combination of high-resolution scanning tunneling microscopy imaging with density functional theory based ab initio molecular dynamics, and we follow the formation and binding of linear water clusters on coordinatively unsaturated ruthenium rows. We find that clusters of all sizes (dimers, trimers, tetramers, extended chains) are stabilized by donating one proton per every two water molecules to the surface bridge bonded oxygen sites, in contrast with water monomers that do not show a significant propensity for dissociation. The clusters with odd number of water molecules are less stable than the clusters with even number and are generally not observed under thermal equilibrium. For all clusters with even numbers, the dissociated dimers represent the fundamental building blocks with strong intradimer ...

The formation and coupling of methylene upon dissociation of formaldehyde on reduced TiO2(110) are studied using variable temperature scanning tunneling microscopy (STM). In agreement with prior studies, formaldehyde preferably adsorbs on the bridging-bonded oxygen vacancy (VO) defect site. VO-bound formaldehyde couples with Ti-bound formaldehyde forming a diolate species, which appears as the majority species on the surface at 300 K. Here, STM images directly visualize a low-temperature coupling reaction channel. Two VO-bound formaldehyde molecules can couple and form Ti-bound species, which desorbs above ∼215 K. This coupling reaction heals both VO sites indicating the formation and the desorption of ethylene. We also directly observed the diffusion of methylene groups to nearby empty VO sites upon dissociation of the C–O bond in VO-bound formaldehyde, which suggests that the ethylene formation occurs via coupling of the methylene groups. Statistical analysis shows that the sum of visible reaction produ...

Direct imaging of site-specific reactions of individual molecules as a function of temperature has been a long-sought goal in molecular science. Here, we report the direct visualization of molecular coupling of formaldehyde on reduced rutile TiO2(110) surfaces as we track the same set of molecules using scanning tunneling microscopy (STM) when the temperature is increased from 75 to 170 K. Our previous study showed that formaldehyde preferably adsorbs at bridging-bonded oxygen (Ob) vacancy (VO) defect site. Herein, images from the same area as the temperature is increased show that VO-bound formaldehyde couples with Ti-bound formaldehyde forming a diolate intermediate. Consequently, exposure of formaldehyde at room temperature leads to diolate as the majority species on the surface, and no VO-bound formaldehyde is observed. The diolate species are the key reaction intermediates in the formation of ethylene reported in previous ensemble-averaged studies.

The adsorption and photochemistry of CO on rutile TiO

Surface reactions of formaldehyde with reduced TiO2(110) surfaces have been studied using variable-temperature scanning tunneling microscopy (STM) and density functional theory (DFT). STM images taken from a same area at various temperatures clearly show that formaldehyde preferentially adsorbs on the bridge-bonded oxygen (Ob) vacancy (VO) defect sites. Bias-dependent STM images show that the STM features corresponding to both the Ti-bound CH2O and the VO-bound CH2O are positioned between the Ob row and the Ti row. While the VO-bound formaldehyde rotates at 95 K, the Ti-bound CH2O does not. The VO-bound CH2O starts to diffuse along the Ob row as –CH2– at ~170 K and starts to diffuse along the Ti row as an intact molecule at ~215 K. However, the stabilities and the configurations of the Ti-bound and VO-bound formaldehyde calculated using DFT are not in line with the experimental results. The discrepancy between the experiment and theory indicates the presence of a complex charge distribution related to the surface defects.

We have examined the chemical and photochemical properties of molecular oxygen on the (110) surface of rutile TiO2 at 100 K using electron energy loss spectroscopy (EELS), photon stimulated desorption (PSD), and scanning tunneling microscopy (STM). Oxygen chemisorbs on the TiO2(110) surface at 100 K through charge transfer from surface Ti3+ sites. The charge-transfer process is evident in EELS by a decrease in the intensity of the Ti3+ d-to-d transition at ∼0.9 eV and formation of a new loss at ∼2.8 eV. On the basis of comparisons with the available homogeneous and heterogeneous literature for complexed/adsorbed O2, the species responsible for the 2.8 eV peak can be assigned to a surface peroxo (O22–) state of O2. This species was identified as the active form of adsorbed O2 on TiO2(110) for PSD. The adsorption site of this peroxo species was assigned to that of a regular five-coordinated Ti4+ (Ti5c) site based on comparisons between the UV exposure-dependent behavior of O2 in STM, PSD, and EELS data. Ass...

Understanding how water binds and dissociates on surfaces has broad implications in a vast range of physical and chemical processes. The relative stability of molecularly and dissociatively bound water has been debated for decades on many oxide surfaces, but it has never been successfully measured. Our study describes unique instrumentation, direct measurements, and a state-of-the-art computation and theory approach that yield a detailed kinetic and dynamic description of water deprotonation equilibrium on TiO2(110), a prototypical surface commonly used in mechanistic studies of photocatalytic water splitting. This unique study demonstrates that the molecularly bound water on TiO2(110) is preferred over the surface-bound hydroxyls by only 0.035 eV.

Photostimulated reactions of single O

Adsorption, binding, and diffusion of CO2 molecules on model rutile TiO2(110)-1×1 surfaces were investigated experimentally using scanning tunneling microscopy, infrared reflection adsorption spectroscopy (IRAS), molecular beam scattering, and temperature programmed desorption and theoretically via dispersion corrected density functional theory and ab initio molecular dynamics. In accord with previous studies, bridging oxygen (Ob) vacancies (VO’s) are found to be the most stable binding sites. Additional CO2 adsorbs on 5-coordinated Ti sites (Ti5c) with the initial small fraction stabilized by CO2 adsorbed on VO sites. The Ti5c-bound CO2 is found to be highly mobile at 50 K at coverages of up to 1/2 monolayer (ML). Theoretical studies show that the CO2 diffusion on Ti5c rows proceeds via a rotation-tumbling mechanism with extremely low barrier of 0.06 eV. The Ti5c-bound CO2 molecules are found to bind preferentially to a single Ti5c with the O═C═O axis tilted away from the surface normal. The binding ener...

The growth of TiO2 anatase films on Nb‐doped SrTiO3(001) molecular beam epitaxy has been studied in-situ by scanning tunneling microscopy. We show that the initial growth follows the Stranski–Krastanov mode, where islands form on top of a wetting layer consisting of two monolayers (ML) of TiO2. The epitaxial islands subsequently nucleate and coalesce into large commonly oriented crystallites. The reconstruction observed by reflection high-energy electron diffraction (RHEED) is shown to result from the coexistence of individual (4 × 1) and (1 × 4) reconstructions present on different crystallite surfaces. The anatase grows in units of bilayers, resulting in a step height of 2 ML (~ 0.5 nm). This result explains the fact that the measured period of the RHEED specular-beam intensity oscillations corresponds to the time required for deposition of 2 ML. Ar ion sputtering and UHV annealing results in a transformation to coexisting (4 × 1) and (1 × 4) reconstructed terraces on individual crystallites, as commonly observed by ex-situ STM studies.

Combined scanning tunneling microscopy, temperature programmed desorption, photo stimulated desorption, and density functional theory studies have probed the formation and reactivity of highly-hydroxylated rutile TiO(2)(110) surfaces, which were prepared via a novel, photochemical route using trimethyl acetic acid (TMAA) dissociative adsorption and subsequent photolysis at 300 K. Deprotonation of TMAA molecules upon adsorption produces both surface bridging hydroxyls (OH(b)) and bidentate trimethyl acetate (TMA) species with a saturation coverage of nearly 0.5 monolayers (ML). Ultra-violet light irradiation selectively removes TMA species, producing a highly-hydroxylated surface with up to ~0.5 ML OH(b) coverage. At high coverages, the OH(b) species typically occupy second-nearest neighbor sites along the bridging oxygen row locally forming linear (2 × 1) structures of different lengths, although the surface is less ordered on a long scale. The annealing of the highly-hydroxylated surface leads to hydroxyl recombination and H(2)O desorption with ~100% yield, thus ruling out the diffusion of H into the bulk that has been suggested in the literature. In agreement with experimental data, theoretical results show that the recombinative H(2)O desorption is preferred over both H bulk diffusion and H(2) desorption processes.

Photostimulated reactions of single O2 molecules on reduced TiO2(110) surfaces were directly observed at an atomic level with high-resolution scanning tunneling microscopy at 50 K. Two distinct reactions of O2 desorption and dissociation occur at different active sites of terminal Ti atoms and bridging O vacancies, respectively. Two reaction channels follow very different kinetics. While hole-mediated O2 desorption is promptly and fully completed, electron-mediated O2 dissociation is much slower and is quenched above some critical O2 coverage. Evidently, the O2 photochemistry on TiO2(110) is quite more complex than thought previously. Density functional theory calculations indicate that both coordination and charge state of an O2 molecule chemisorbed at the specific site largely determine a particular reaction pathway.

Although the last decade in electron microscopy has seen tremendous gains in image resolution, new challenges in the field have come to the forefront. First, new ultra-sensitive instruments bring about unprecedented environmental specifications and facility needs for their optimal use. Second, in the quest for higher spatial resolutions, the importance of developing and sharing crucial expertise—from sample preparation to scientific vision—has perhaps been deemphasized. Finally, for imaging to accelerate discoveries related to large scientific and societal problems, in situ capabilities that replicate real-world process conditions are often required to deliver necessary information. This decade, these are among the hurdles leaders in the field are striving to overcome.

Combined analysis employing in-situ X-rays photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and ex-situ X-ray diffraction (XRD) has demonstrated the feasibility to deposit single-phase barium oxide overlayers by a direct sublimation of BaO material from an effusion cell. Furthermore, in situ scanning tunneling microscopy (STM) has confirmed that the BaO grows as nanoclusters with dimensions down to ~2 nm.

Molecularly chemisorbed O2 species were directly imaged on reduced TiO2(110) at 50 K with high-resolution scanning tunneling microscopy (STM). Two different O2 adsorption channels, one at bridging oxygen vacancies (VO) and another at 5-fold coordinated terminal titanium atoms (Ti5c), have been identified. While O2 species at the Ti5c site appears as a single protrusion centered on the Ti5c row, the O2 at VO manifests itself by a disappearance of the VO feature. It is found that the STM tip can easily dissociate O2 species, unless extremely low magnitude of the tunneling parameters are used. The O2 molecules chemisorbed at low temperatures at these two distinct sites are the most likely precursors for the two O2 dissociation channels, observed at temperatures above 150 and 230 K at the VO and Ti5c sites, respectively.

A combination of scanning tunneling microscopy and density functional theory has been used to investigate the interactions between water molecules and terminal hydroxyls (OHt’s) adsorbed on the TiO2(110) surface at 300 K. We show that OHt’s have a significant effect on the water reactivity. Two distinctive reaction pathways are unraveled depending on the whether H2O and OHt are on the same or adjacent Ti rows. The underlying reaction mechanisms involve proton transfer from H2O to OHt leading to the formation of new H2O molecules, accompanied by O scrambling and along- or across-row apparent motion of OHt’s.

The technological importance of TiO 2 has led to a broad effort aimed at understanding the elementary steps that underlie catalytic and photocatalytic reactions. The most stable surface, rutile TiO 2 (1 1 0), in particular, has became a prototypical model for fundamental studies of TiO 2 . In this critical review we have selected oxygen, water, and alcohols to evaluate recent progress relevant for applications in the areas of water splitting and oxidation of organic contaminants. We first focus on the characterization of defects and the distribution of excess charge that results from their formation. The subsequent section concentrates on the role of individual surface sites and the effect of available charge in the adsorption processes. The discussion of adsorbate dynamics follows, providing models for intrinsic and extrinsic diffusion processes as well as rotational dynamics of anchored alkoxy species. The final section summarizes our current understanding of TiO 2 (1 1 0) catalyzed reactions between water, oxygen, and their dissociation products.

O2 chemisorption on reduced, rutile TiO2(110) with various concentrations of oxygen vacancies (Ov) and bridging hydroxyls (OHb) is investigated with scanning tunneling microscopy, temperature-programmed desorption, and electron-stimulated desorption. On the annealed surface, two oxygen molecules can be chemisorbed per Ov. The same amount of O2 chemisorbs on surfaces where each Ov is converted to two OHb’s by exposure to water (i.e., 1 O2 per OHb). Surfaces with few or no Ov’s or OHb’s can be created by exposing the hydroxylated surface to O2 at room temperature, and the amount of O2 that chemisorbs on these surfaces at low temperatures is only ∼20% of the amount on the annealed (reduced) surface. In contrast, the amount of chemisorbed O2 increases by more than a factor of 2 when the OHb concentration is enhanced—without changing the concentration of subsurface Ti interstitials. The results indicate that the reactivity of TiO2(110) is primarily controlled by the amount of electron-donating surface species ...

We have used high-energy resolution X-ray photoelectron spectroscopy to measure valence band offsets at the epitaxial anatase TiO 2 (0 0 1)/ n -SrTiO 3 (0 0 1) heterojunction prepared by molecular beam epitaxy. The valence band offsets range between −0.06 ± 0.05 and +0.16 ± 0.05 eV for anatase thicknesses between 1 and 8 monolayers and three different methods of substrate surface preparation, with no systematic dependence on film thickness. The conduction band offset (CBO) varies over a comparable range by virtue of the fact that anatase and SrTiO 3 exhibit the same bandgap (∼3.2 eV). In contrast, density functional theory predicts the VBO to be +0.55 eV. The lack of agreement between theory and experiment suggests that either some unknown factor in the interface structure or composition excluded from the modeling is influencing the band offset, or that density functional theory cannot accurately calculate band offsets in these oxide materials. The small experimental band offsets have important implications for the use of this interface for fundamental investigations of surface photocatalysis. Neither electrons nor holes are likely to become trapped in the substrate and thus be unable to participate in surface photocatalytic processes.

The reactions of molecular oxygen with bridging hydroxyl groups (OHb’s) formed by H2O dissociation on bridging oxygen vacancies (VO’s) of TiO2(110) are studied at low and high OHb coverages as a function of the O2 exposure, using scanning tunneling microscopy, temperature programmed desorption, and electron stimulated desorption techniques. In agreement with prior studies, oxygen adatoms (Oa), hydroperoxyls (HO2), and terminal hydroxyls (OHt) are observed as intermediates of the reactions with O2 ultimately leading to H2O as a product. Here, we show that water plays an important role in the room-temperature reactions of O2 with both partially and fully hydroxylated TiO2(110). On partially hydroxylated surfaces, water is found to be involved in the reaction cycle that leads to the consumption of Oa and VO sites thus resulting in a practically Oa- and VO-free surface. In these reactions, water is observed to participate in multiple ways—as a reactant, product, and catalyst. On fully hydroxylated TiO2(110), ...

We report the results of a combined experimental and theoretical investigation of the reaction of molecular oxygen with a partially hydroxylated TiO2(110) surface. The consecutive steps of both primary and secondary site-specific reactions have been tracked with high-resolution scanning tunneling microscopy (STM). We have directly imaged stable, adsorbed hydroperoxyl (HO2) species, which is believed to be a key intermediate in many heterogeneous photochemical processes but generally metastable and “elusive” until now. We also found terminal hydroxyl groups, which are another critical but never previously directly observed intermediates. Conclusive evidence that O2 reacts spontaneously with a single bridging OH group as an initial reaction step is provided. The experimental results are supported by density functional theory (DFT) calculations that have determined the energies and configurations of these species. Reported observations provide a base for a consistent description of the elementary reaction st...

Surfaces of titanium dioxide in both rutile and anatase polymorphs have attracted significant attention in catalysis and photochemistry. The (110) orientation of rutile, and to a lesser extent other rutile orientations, have been studied on an atomic scale, yielding information on surface structure and chemical reactivity. In contrast, the thermal and photochemistry of well-defined, single-crystal anatase surfaces has not been investigated, largely because of the metastable nature of anatase, as well as the lack of availability of high-quality surfaces. Here we describe a study of the adsorption and photoreactivity of an organic adlayer, trimethyl acetate (TMA), on structurally excellent anatase (001) epitaxial thin films grown by oxygen plasma assisted molecular beam epitaxy (OPAMBE). High-resolution scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and photodesorption spectrometry have been used to study the chemisorption and ultraviolet (UV) light-induced photodecomposition o...

A gallium focused ion beam has been used to create discrete pits on a SrTiO 3 (100) surface with the idea that these pits will serve as the nucleation sites for subsequent Cu 2 O nanodot growth. The concentration of gallium within these pits has been analysed using a high-resolution Auger system immediately after pit formation and also following wet chemical etching and thermal annealing of the surface. The geometry of the pits has been determined following etching and annealing using atomic force microscopy (AFM). Growth of Cu 2 O nanodots on the patterned surfaces has been performed for different processing and Ga dose conditions. Growth of Cu 2 O nanodots within the pits is the primary mode of dot formation. In several samples, dot growth within pits appears to occur by a two-step process with pits filling before initiation of a second, distinct phase of nanodot growth above the plane of the original SrTiO 3 surface.

Several technical modifications related to the fabrication and ultra-high vacuum (UHV) treatments of the scanning tunneling microscope (STM) tips have been implemented to improve a reliability of the tip preparation for high-resolution STM. Widely used electrochemical etching drop-off technique has been further refined to enable a reproducible fabrication of the tips with a radius ⩽3 nm. For tip cleaning by a controllable UHV annealing, simple and flexible setup has been developed. Proper W tip preparation has been demonstrated via an imaging of the TiO2 (1 1 0) surface atomic structure.

The combined experimental and theoretical study of intrinsic hydrogen diffusion on bridge-bonded oxygen (BBO) rows of TiO 2(110) is presented. Sequences of isothermal scanning tunneling microscopy images demonstrate a complex behavior of hydrogen formed by water dissociation on BBO vacancies. Different diffusion rates are observed for the two hydrogens in the original geminate OH pair suggesting the presence of a long-lived polaronic state. For the case of separated hydroxyls, both theory and experiment yield comparable temperature-dependent diffusion rates. Density functional theory calculations show that there are two comparable low energy diffusion pathways for hydrogen motion along the BBO from one BBO to its neighbor, one by a direct hop and the other by an intermediate minimum at a terrace O. The values of kinetic parameters (prefactors and diffusion barriers) determined experimentally and theoretically are significantly different and indicate the presence of a more complex diffusion mechanism. We speculate that the hydrogen diffusion proceeds via a two-step mechanism: the initial diffusion of localized charge, followed by the diffusion of hydrogen. Both experiment and theory show the presence of repulsive OH-OH interactions.

Tracking the same region of the reduced TiO2(110) surface by scanning tunneling microscopy before and after oxygen exposure at room temperature (RT) confirms that O2 molecules dissociate only at the bridging oxygen vacancies, with one O atom healing a vacancy and other O atom bonding at the neighboring Ti site as an adatom. The majority of O adatoms (∼81%) are found separated from the original vacancy positions by up to two lattice constants along the [001] direction. Since at RT the thermal diffusivity of O adatoms has been found to be rather small, with an experimentally estimated activation energy of ∼1.1 eV, we conclude that the observed lateral distribution of the oxygen adatoms is attained through a nonthermal, transient mobility during the course of O2 dissociation. Unlike for other known cases of the dissociation of diatomic molecules where both “hot” adatoms accommodate at equivalent sites, in the studied system, the oxygen atoms filling the vacancies are locked into the bridging oxygen rows, and...

The ability to synthesis well-ordered two-dimensional materials under ultra-high vacuum and directly characterize them by other techniques in-situ can greatly advance our current understanding on their physical and chemical properties. In this paper, we demonstrate that iso-oriented {\alpha}-MoO3 films with as low as single monolayer thickness can be reproducibly grown on SrTiO3(001) (STO) substrates by molecular beam epitaxy ( (010)MoO3 || (001)STO, [100]MoO3 || [100]STO or [010]STO) through a self-limiting process. While one in-plane lattice parameter of the MoO3 is very close to that of the SrTiO3 (aMoO3 = 3.96 {\AA}, aSTO = 3.905 {\AA}), the lattice mismatch along other direction is large (~5%, cMoO3 = 3.70 {\AA}), which leads to relaxation as clearly observed from the splitting of streaks in reflection high-energy electron diffraction (RHEED) patterns. A narrow range in the growth temperature is found to be optimal for the growth of monolayer {\alpha}-MoO3 films. Increasing deposition time will not lead to further increase in thickness, which is explained by a balance between deposition and thermal desorption due to the weak van der Waals force between {\alpha}-MoO3 layers. Lowering growth temperature after the initial iso-oriented {\alpha}-MoO3 monolayer leads to thicker {\alpha}-MoO3(010) films with excellent crystallinity., Comment: One supporting figure is attached at the end of the manuscript

Thin film of silicon germanium (Si1-xGex) with tailored composition was grown on Si (100) substrate at 650oC in an ultrahigh vacuum molecular beam epitaxy system. The nominal x-value is ranged from 0 to 0.14. The quality of the film was investigated by Rutherford backscattering spectrometry (RBS) in random and channeling geometries, glancing angle x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM), energy dispersive x-ray spectroscopy (EDX), and atomic force microscopy (AFM). RBS/Channeling measurements indicate that the strain associated with lattice mismatch is compressive in the film. Both RBS and EDX analyses indicate the compositional graded incorporation of Ge in the film with x ranging from 0 to 0.14. The film shows island growth with each island centering around an interface dislocation.

Trimethylacetic acid (TMAA) adsorption evolution on the rutile TiO2(110) surface from submonolayer to saturation coverages was examined at the atomic level by scanning tunneling microscopy using the same area analysis approach. Upon TMAA deprotonation, no evidence of terminal OH group formation has been found. It has been suggested that uncommon geometry associated with detached hydrogen atom takes place instead, with proton bonding to pair bridging oxygen atoms. Such a configuration is likely to be stabilized by adjacent adsorbed TMA groups and, in turn, be a factor in the formation of TMA (2 × 1) reconstruction at saturation coverage. It also shown that the surface defects do not have a major impact on TMAA adsorption.

RuO2 has proven to be indispensable as a co-catalyst in numerous systems designed for photocatalytic water splitting. In this study, we have carried out a detailed mechanistic study of water behavior on the most stable RuO2 face, RuO2(110), by employing variable-temperature scanning tunneling microscopy and density functional theory calculations. We show that water monomers adsorb molecularly on Ru sites, become mobile above 238 K, diffuse along the Ru rows, and form water dimers. The onset for dimer diffusion is observed at ∼277 K, indicating a significantly higher diffusion barrier than that for monomers. More importantly, we find that water dimers deprotonate readily to form Ru-bound H3O2 and bridging OH species. The observed behavior is compared and contrasted with that observed for water on isostructural rutile TiO2(110).

Many $\mathrm{Ti}{\mathrm{O}}_{2}$ applications (e.g., in heterogeneous catalysis) involve contact with ambient atmosphere and/or water. The resulting hydroxylation can significantly alter its surface properties. While the behavior of a single, isolated OH species on the model metal oxide surface of rutile $\mathrm{Ti}{\mathrm{O}}_{2}(110)$ is relatively well understood, much less is known regarding highly hydroxylated surfaces and/or whether $\mathrm{Ti}{\mathrm{O}}_{2}(110)$ could be fully hydroxylated under ultrahigh vacuum conditions. Here we report the in situ formation of a well-ordered, fully hydroxylated $\mathrm{Ti}{\mathrm{O}}_{2}(110)\ensuremath{-}(1\ifmmode\times\else\texttimes\fi{}1)$ surface using an enhanced photochemical approach, the key parts of which are predosing of water and multistep dissociative adsorption and subsequent photolysis of the carboxylic (trimethyl acetic) acid. Combining scanning tunneling microscopy, ultraviolet photoelectron spectroscopy, and density functional theory results, we show that the attained ``super OH'' surface is also fully reduced, as a result of the photochemical charging of electron traps associated with the OH groups.

X-ray photoelectron spectroscopy analysis during the oxygen plasma assisted molecular beam epitaxy, combined with atomic force microscopy, scanning Auger microscopy, and theoretical simulation studies have been used to evaluate the mechanism of single-phase Cu₂O nanodot formation on the SrTiO₃(100) surface. Formation of pure crystalline Cu₂O nanodots occurs rather in a narrow growth parameter window, outside which a coexistence of the multiple phases has been observed. Cuprous oxide nanodots on the SrTiO₃(100) substrate follow a growth mechanism which differs significantly from the growth modes observed for the majority of semiconductor quantum dots. Growth starts without wetting layer formation with appearance of well-ordered truncated square-based nanodots at submonolayer coverages. At the initial stages of growth, the nanodot size is only weakly changes with coverage and exponentially scales with temperature. After reaching a critical, temperature dependent dot density (~ 10¹³ cm-² for 760 K growth temperature), growth of mid-sized nanoclusters starts through coalescence, which is eventually followed by large dome-shaped cluster formation at higher coverages. The coexistence of the different types of the clusters at high coverages results in a multi-modal distribution of sizes and shapes.

By using a combination of scanning tunneling microscopy (STM), density functional theory (DFT), and secondary-ion mass spectroscopy (SIMS), we explored the interplay and relative impact of surface versus subsurface defects on the surface chemistry of rutile TiO2 . STM results show that surface O vacancies (VO ) are virtually absent in the vicinity of positively charged subsurface point defects. This observation is consistent with DFT calculations of the impact of subsurface defect proximity on VO formation energy. To monitor the influence of such lateral anticorrelation on surface redox chemistry, a test reaction of the dissociative adsorption of O2 was employed and was observed to be suppressed around them. DFT results attribute this to a perceived absence of intrinsic (Ti), and likely extrinsic interstitials in the nearest subsurface layer beneath inhibited areas. We also postulate that the entire nearest subsurface region could be devoid of any charged point defects, whereas prevalent surface defects (VO ) are largely responsible for mediation of the redox chemistry at the reduced TiO2 (110).

Step fluctuations on a multi-component surface of Al/Si(1 1 1)-(√3×√3) were studied using variable-temperature STM at temperatures 770–1020 K. The reduced step diffusivity, b 2 / a , has been found to vary from 0.45 A at 770 K up to 1.00 A at 1020 K. Its temperature dependence follows the expected exponential law, from which the effective energy of kink formation has been determined to be 0.214 eV. Above 870 K, temporal effects complicate the analysis of the spatial pair correlation function. It is demonstrated that consistent results can be obtained in this case by employing surfaces quenched to room temperature.

STM-assisted nanostructure formation using precursor molecules at negative sample bias is controlled by the applied electric field. Sharp nanostructures are produced from the Cu I (hfac) (vtms) molecule on the Si(111)-(7×7) surface at 300 K. The growth process involves the field-induced surface diffusion from regions outside the growing nanostructure, which acts to supply molecules to the nanostructure growth region under the tip. Molecular decomposition occurs above a critical electric field under the tip. The growth process starts when the electric field at the sample surface exceeds a critical level of ∼0.2 V A −1 . Simulations of the growth mechanism, which involves both the radial diffusion of the precursor species from outer sample surface regions and their activation underneath the tip, give quantitative agreement with the experimental data.

The scanning tunneling microscope in a near-field emission mode has been employed to create nanostructures using the hexafluoroacetylacetonate Cu (I) vinyltrimethylsilane precursor molecule on the Si(111) surface at 300 K. Two distinctive mechanisms controlling the nanostructure formation have been delineated. The first process involves excitation of the molecule by the applied electric field, and the field induced surface diffusion acts to supply molecules to the nanostructure growth region under the tip. The second mechanism involves the dissociation of the molecule by an electron attachment process. The generated nanostructure topology is quite different for each excitation mechanism. Narrow cone-like structures are produced by the electric field while broad structures of lower height are produced by the electron attachment process. Both mechanisms operate simultaneously in the low bias voltage regime (V