Low-temperature dissociation of CO2 molecules on vicinal Cu surfaces.

The reaction of carbon dioxide on the vicinal Cu surfaces at low temperatures was investigated by infrared reflection absorption spectroscopy, scanning tunneling microscopy, X-ray photoelectron spectroscopy, and quadrupole mass spectrometry. Dissociation of CO2 molecules into CO on the Cu(997) and Cu(977) surfaces was observed at temperatures between 80 K and 90 K, whereas it did not occur on Cu(111) under a similar condition. CO and physisorbed CO2 were the main adsorbates during the reaction. In contrast, the amount of atomic oxygen on the surface was small. The dissociation of CO2 was promoted by the small amount of oxygen produced by the CO2 dissociation on the Cu surfaces. This leads to the induction period in the CO2 reaction; the initial reaction rate on the clean Cu surfaces was low, and the coadsorbed oxygen enhanced the dissociation reactivity of CO2. Mass analysis of desorption species during the reaction revealed that the observed CO formation on the vicinal Cu surface is mainly caused by an oxygen-exchange reaction with residual CO in an ultra-high vacuum chamber.

Hydrogen-induced Sulfur Vacancies on the MoS2 Basal Plane Studied by Ambient Pressure XPS and DFT Calculations.

Sulfur vacancy on an MoS2 basal plane plays a crucial role in device performance and catalytic activity; thus, an understanding of the electronic states of sulfur vacancies is still an important issue. We investigate the electronic states on an MoS2 basal plane by ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) and density functional theory calculations while heating the system in hydrogen. The AP-XPS results show a decrease in the intensity ratio of S 2p to Mo 3d, indicating that sulfur vacancies are formed. Furthermore, low-energy components are observed in Mo 3d and S 2p spectra. To understand the changes in the electronic states induced by sulfur vacancy formation at the atomic scale, we calculate the core-level binding energies for the model vacancy surfaces. The calculated shifts for Mo 3d and S 2p with the formation of sulfur vacancy are consistent with the experimentally observed binding energy shifts. Mulliken charge analysis indicates that this is caused by an increase in the electronic density associated with the Mo and S atoms around the sulfur vacancy as compared to the pristine surface. The present investigation provides a guideline for sulfur vacancy engineering.

Elucidation of the atomic-scale processes of dissociative adsorption and spillover of hydrogen on the single atom alloy catalyst Pd/Cu(111).

Hydrogen spillover is a crucial process in the selective hydrogenation reactions on Pd/Cu single atom alloy catalysts. In this study, we report the atomic-scale perspective of these processes on the single atom alloy catalyst Pd/Cu(111) based on the experimental and theoretical results, including infrared reflection absorption spectroscopy (IRAS), temperature programmed desorption (TPD), high-resolution X-ray photoelectron spectroscopy (HR-XPS), and density functional theory (DFT) calculations for core-level excitation. The hydrogen spillover onto Cu(111) was successfully observed in real time using time-resolved IRAS measurements at 80 K. The chemical shifts of Pd 3d5/2 indicate that H2 is dissociated and adsorbed at the Pd site. In addition, a "two-step" chemical shift of the Pd 3d5/2 binding energy was observed, indicating two types of hydrogen adsorption states at the Pd site. The proposed mechanism of the hydrogen dissociation and spillover processes is as follows: (i) a hydrogen molecule is dissociated at a Pd site, and the hydrogen atoms are adsorbed on the Pd site; (ii) the number of hydrogen atoms on the Pd site increases up to three; and (iii) the hydrogen atoms will spill over onto the Cu surface.

The roles of step-site and zinc in surface chemistry of formic acid on clean and Zn-modified Cu(111) and Cu(997) surfaces studied by HR-XPS, TPD, and IRAS.

The adsorption, desorption, and decomposition of formic acid (HCOOH) on Cu(111), Cu(997), Zn-Cu(111), and Zn-Cu(997) were systematically studied by high-resolution x-ray photoelectron spectroscopy, temperature programmed desorption, and infrared reflection absorption spectroscopy. On the clean Cu(111) surface, 13% of formic acid molecules adsorbed at 83 K were dissociated to form bidentate formate species by heating at 300 K; however, on the Zn-Cu(111) surface, only 4% of adsorbed HCOOH molecules were dissociated into the bidentate formate species. On the contrary, 13% of adsorbed HCOOH molecules were already dissociated into monodentate formate species on Cu(997) even at 83 K and 17% of adsorbed formic acid molecules were transformed to bidentate formate species by heating at 300 K, indicating that the stepped Cu surface has higher reactivity for HCOOH dissociation at low temperature. On the Zn-Cu(997) surface, 20% of formic acid became bidentate formate species in contrast to the case with Zn-Cu(111). Thus, the Zn deposited Cu step surface shows special activity for adsorption and dissociation of formic acid. The desorption peak maxima of the formate decomposition products (CO2 and H2) on Zn-Cu(997) were shifted to higher temperatures than those on Cu(997). Zn on Cu surfaces plays an important role in the stabilization of formate species, which probably leads to the decrease in the activation barrier for hydrogenation on the Zn-Cu alloyed surface.

Formation of BN-covered silicene on ZrB2/Si(111) by adsorption of NO and thermal processes.

We have investigated the adsorption and thermal reaction processes of NO with silicene spontaneously formed on the ZrB2/Si(111) substrate using synchrotron radiation x-ray photoelectron spectroscopy (XPS) and density-functional theory calculations. NO is dissociatively adsorbed on the silicene surface at 300 K. An atomic nitrogen is bonded to three Si atoms most probably by a substitutional adsorption with a Si atom of silicene (N≡Si3). An atomic oxygen is inserted between two Si atoms of the silicene (Si-O-Si). With increasing NO exposure, the two-dimensional honeycomb silicene structure gets destroyed, judging from the decay of typical Si 2p spectra for silicene. After a large amount of NO exposure, the oxidation state of Si becomes Si4+ predominantly, and the intensity of the XPS peaks of the ZrB2 substrate decreases, indicating that complicated silicon oxinitride species have developed three-dimensionally. By heating above 900 K, the oxide species start to desorb from the surface, but nitrogen-bonded species still exist. After flashing at 1053 K, no oxygen species is observed on the surface; SiN species are temporally formed as a metastable species and BN species also start to develop. In addition, the silicene structure is restored on the ZrB2/Si(111) substrate. After prolonged heating at 1053 K, most of nitrogen atoms are bonded to B atoms to form a BN layer at the topmost surface. Thus, BN-covered silicene is formed on the ZrB2/Si(111) substrate by the adsorption of NO at 300 K and prolonged heating at 1053 K.

Enhancement of CO2 adsorption on oxygen-functionalized epitaxial graphene surface under near-ambient conditions.

The functionalization of graphene is important in practical applications of graphene, such as in catalysts. However, the experimental study of the interactions of adsorbed molecules with functionalized graphene is difficult under ambient conditions at which catalysts are operated. Here, the adsorption of CO2 on an oxygen-functionalized epitaxial graphene surface was studied under near-ambient conditions using ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). The oxygen-functionalization of graphene is achieved in situ by the photo-induced dissociation of CO2 with X-rays on graphene in a CO2 gas atmosphere. The oxygen species on the graphene surface is identified as the epoxy group by XPS binding energies and thermal stability. Under near-ambient conditions of 1.6 mbar CO2 gas pressure and 175 K sample temperature, CO2 molecules are not adsorbed on the pristine graphene, but are adsorbed on the oxygen-functionalized graphene surface. The increase in the adsorption energy of CO2 on the oxygen-functionalized graphene surface is supported by first-principles calculations with the van der Waals density functional (vdW-DF) method. The adsorption of CO2 on the oxygen-functionalized graphene surface is enhanced by both the electrostatic interactions between the CO2 and the epoxy group and the vdW interactions between the CO2 and graphene. The detailed understanding of the interaction between CO2 and the oxygen-functionalized graphene surface obtained in this study may assist in developing guidelines for designing novel graphene-based catalysts.

CO2 adsorption on the copper surfaces: van der Waals density functional and TPD studies.

We investigated the adsorption of CO2 on the flat, stepped, and kinked copper surfaces from density functional theory calculations as well as the temperature programmed desorption and X-ray photoelectron spectroscopy. Several exchange-correlation functionals have been considered to characterize CO2 adsorption on the copper surfaces. We used the van der Waals density functionals (vdW-DFs), i.e., the original vdW-DF (vdW-DF1), optB86b-vdW, and rev-vdW-DF2, as well as the Perdew-Burke-Ernzerhof (PBE) with dispersion correction (PBE-D2). We have found that vdW-DF1 and rev-vdW-DF2 functionals slightly underestimate the adsorption energy, while PBE-D2 and optB86b-vdW functionals give better agreement with the experimental estimation for CO2 on Cu(111). The calculated CO2 adsorption energies on the flat, stepped, and kinked Cu surfaces are 20-27 kJ/mol, which are compatible with the general notion of physisorbed species on solid surfaces. Our results provide a useful insight into appropriate vdW functionals for further investigation of related CO2 activation on Cu surfaces such as methanol synthesis and higher alcohol production.

Observation of Fano line shapes in infrared vibrational spectra of CO2 adsorbed on Cu(997) and Cu(111).

Adsorption states of carbon dioxide on the Cu(997) and Cu(111) surfaces were investigated by infrared reflection absorption spectroscopy, temperature programmed desorption, and X-ray photoelectron spectroscopy. CO2 molecules are physisorbed on the Cu(997) surface at temperatures below 70 K; neither chemisorption nor dissociation of CO2 occurs on the Cu(997) surface at this low temperature. However, the vibrational spectra of adsorbed CO2 depend significantly on the substrate temperature and coverage. IR spectra of CO2 vibrational modes at 70 K show asymmetric Fano line shapes, while only normal absorption bands are observed when CO2 is adsorbed at 20 K. Fano line shapes are also observed for CO2 on Cu(111) at 85 K. The observation of Fano effect indicates the coupling between the electronic continuum states of the Cu surface and the internal vibrational modes of CO2 even in such physisorbed system.

Quantitative analysis of desorption and decomposition kinetics of formic acid on Cu(111): The importance of hydrogen bonding between adsorbed species.

Quantitative analysis of desorption and decomposition kinetics of formic acid (HCOOH) on Cu(111) was performed by temperature programmed desorption (TPD), X-ray photoelectron spectroscopy, and time-resolved infrared reflection absorption spectroscopy. The activation energy for desorption is estimated to be 53-75 kJ/mol by the threshold TPD method as a function of coverage. Vibrational spectra of the first layer HCOOH at 155.3 K show that adsorbed molecules form a polymeric structure via the hydrogen bonding network. Adsorbed HCOOH molecules are dissociated gradually into monodentate formate species. The activation energy for the dissociation into monodentate formate species is estimated to be 65.0 kJ/mol at a submonolayer coverage (0.26 molecules/surface Cu atom). The hydrogen bonding between adsorbed HCOOH species plays an important role in the stabilization of HCOOH on Cu(111). The monodentate formate species are stabilized at higher coverages, because of the lack of vacant sites for the bidentate formation.

Energy level alignment of cyclohexane on Rh(111) surfaces: the importance of interfacial dipole and final-state screening.

Adsorption states and electronic structure of cyclohexane on clean and hydrogen-saturated Rh(111) surfaces were investigated by scanning tunneling microscopy and photoelectron spectroscopy. Monolayer cyclohexane molecules form an ordered superstructure on the clean Rh(111) surface. The energy level alignment of adsorbed cyclohexane depends on each adsorption site; molecular orbitals of adsorbed cyclohexane on the atop site have lower binding energies than those on the other sites. In contrast, it becomes insensitive to adsorption sites on the hydrogen-saturated Rh(111) surface. By preadsorption of hydrogen, all cyclohexane molecular orbitals are uniformly shifted to lower binding energy compared to those on the clean Rh(111) surface. The observed energy level alignment of cyclohexane on the Rh(111) surfaces is determined by the vacuum level shift and the final-state screening effects.

Kinetic and geometric isotope effects originating from different adsorption potential energy surfaces: cyclohexane on Rh(111).

Novel isotope effects were observed in desorption kinetics and adsorption geometry of cyclohexane on Rh(111) by the use of infrared reflection absorption spectroscopy, temperature programmed desorption, photoelectron spectroscopy, and spot-profile-analysis low energy electron diffraction. The desorption energy of deuterated cyclohexane (C(6)D(12)) is lower than that of C(6)H(12). In addition, the work function change by adsorbed C(6)D(12) is smaller than that by adsorbed C(6)H(12). These results indicate that C(6)D(12) has a shallower adsorption potential than C(6)H(12) (vertical geometric isotope effect). The lateral geometric isotope effect was also observed in the two-dimensional cyclohexane superstructures as a result of the different repulsive interaction between interfacial dipoles. The observed isotope effects should be ascribed to the quantum nature of hydrogen involved in the C-H···metal interaction.

Two-dimensional superstructures and softened C-H stretching vibrations of cyclohexane on Rh(111): effects of preadsorbed hydrogen.

Adsorption structures and interaction of cyclohexane molecules on the clean and hydrogen-preadsorbed Rh(111) surfaces were investigated using scanning tunneling microscopy, spot-profile-analysis low-energy electron diffraction, temperature-programmed desorption, and infrared reflection absorption spectroscopy (IRAS). Various ordered structures of adsorbed cyclohexane were observed as a function of hydrogen and cyclohexane coverages. When the fractional coverage (θ(H)) of preadsorbed hydrogen was below 0.8, four different commensurate or higher-order commensurate superstructures were found as a function of θ(H); whereas more densely packed incommensurate overlayers became dominant at higher θ(H). IRAS measurements showed sharp softened C-H vibrational peaks at 20 K, which originate from the electronic interaction between adsorbed cyclohexane and the Rh surface. The multiple softened C-H stretching peaks in each phase are due to the variation in the adsorption distance from the substrate. At high hydrogen coverages they became attenuated in intensity and eventually diminished at θ(H) = 1. The gradual disappearance of the soft mode correlates well with the structural phase transition from commensurate structures to incommensurate structures with increasing hydrogen coverage. The superstructure of adsorbed cyclohexane is controlled by the delicate balance between adsorbate-adsorbate and adsorbate-substrate interactions which are affected by preadsorbed hydrogen.

Dewetting growth of crystalline water ice on a hydrogen saturated Rh(111) surface at 135 K.

We investigated the water (D(2)O) adsorption at 135 K on a hydrogen pre-adsorbed Rh(111) surface using temperature programmed desorption and infrared reflection absorption spectroscopy (IRAS) in ultrahigh vacuum. With increasing the hydrogen coverage, the desorption temperature of water decreases. At the saturation coverage of hydrogen, dewetting growth of water ice was observed: large three-dimensional ice grains are formed. The activation energy of water desorption from the hydrogen-saturated Rh(111) surface is estimated to be 51 kJ/mol. The initial sticking probability of water decreases from 0.46 on the clean surface to 0.35 on the hydrogen-saturated surface. In IRAS measurements, D-down species were not observed on the hydrogen saturated surface. The present experimental results clearly show that a hydrophilic Rh(111) clean surface changes into a hydrophobic surface as a result of hydrogen adsorption.

The growth process of first water layer and crystalline ice on the Rh(111) surface.

The adsorption states and growth process of the first layer and multilayer of water (D(2)O) on Rh(111) above 135 K were investigated using infrared reflection absorption spectroscopy (IRAS), temperature programed desorption, spot-profile-analysis low-energy electron diffraction, and scanning tunneling microscopy (STM). At the initial stage, water molecules form commensurate ( radical3x radical3)R30 degrees islands, whose size is limited for several hexagonal units; the average diameter is approximately 2.5 nm. This two-dimensional (2D) island includes D-down species, and free OD species exist at the island edge. With increasing coverage, the D-up species starts to appear in IRAS. At higher coverages, the 2D islands are connected in STM images. By the titration of Xe adsorption we estimated that the D-down domain occupies about 55% on Rh(111) at the saturation coverage. Further adsorption of water molecules forms three-dimensional ice crystallites on the first water layer; thus, the growth mode of crystalline water layers on Rh(111) is a Stranski-Krastanov type. We have found that an ice crystallite starts to grow on D-down domains and the D-down species do not reorient upon the formation of a crystalline ice.

Adsorption and reaction of NO on the clean and nitrogen modified Rh(111) surfaces.

The adsorption states and thermal reactions of NO on the clean and nitrogen modified Rh(111) surfaces were investigated between 20 and 150 K using infrared reflection adsorption spectroscopy (IRAS) and temperature programmed desorption. On the clean surface, singleton species at atop and hollow sites were observed at 1816 and 1479 cm(-1), respectively. Using time-resolved IRAS, the activation energy and pre-exponential factor of the site change from atop to hollow sites on Rh(111) were estimated to be 117 meV and 1.7x10(10) s(-1), respectively. On the saturated monolayer, physisorbed NO dimers were formed. In the second layer, they were adsorbed with the N-N bond nearly parallel to the surface. In the multilayer formed at 20 K, the NO dimers were randomly oriented. On the nitrogen modified Rh(111) surface, a new adsorption state of chemisorbed monomer was observed as well as atop and hollow species. Physisorbed NO dimers were a precursor to N(2)O formation on the nitrogen modified Rh(111) surface. In the N(2)O formation reaction, three kinds of N(2)O species were identified. The first species desorbed from the surface immediately after the formation reaction, which is a reaction-limited process. The second species was physisorbed on the surface and desorbed at 86 K, which is a desorption-limited process. The third species was chemisorbed on the surface and decomposed above 100 K.

Coverage-dependent sticking probability and desorption kinetics of water molecules on Rh(111).

The adsorption and desorption kinetics of water molecules on Rh(111) were investigated using temperature programed desorption (TPD). Water molecules on Rh(111) show coverage-dependent sticking probability; the initial sticking probability is estimated to be 0.46. In the desorption process, a dilute gaslike phase and two-dimensional islands of water coexist on the surface. Based on the model proposed by Kreuzer and Payne [Surf. Sci.200, L433 (1988)], the apparent fractional-order TPD spectra can be interpreted as first-order desorption from the coexistence of two phases on which the sticking probabilities are different. Based on this, the previous estimation of pre-exponential factors assuming half-order desorption [A. Beniya et al., J. Chem. Phys.125, 054717 (2006)] should be revised.

A miniature effusion cell for the vacuum deposition of organic solids with low vapor pressures in surface science studies.

A miniature effusion cell for the vacuum deposition of volatile organic solids with low vapor pressures has been fabricated and used in surface science studies. The effusion cell is designed to operate at up to 200 degrees C under ultrahigh vacuum (UHV) conditions. The size of this cell is so small that it is attached to the top of a transfer rod and can be introduced from a subchamber into a main UHV chamber (retrievable). In addition, the small heat capacity of this cell means rapid heating and cooling rates. The advantages of this evaporator are its simplicity of design and ease of fabrication, assembly, and operation.

Water adsorption on Rh(111) at 20 K: from monomer to bulk amorphous ice.

The adsorption of water (D(2)O) molecules on Rh(111) at 20 K was investigated using infrared reflection absorption spectroscopy (IRAS). At the initial stage of adsorption, water molecules exist as monomers on Rh(111). With increasing water coverage, monomers aggregate into dimers, larger clusters (n = 3-6), and two-dimensional (2D) islands. Further exposure of water molecules leads to the formation of three-dimensional (3D) water islands and finally to a bulk amorphous ice layer. Upon heating, the monomer and dimer species thermally migrate on the surface and aggregate to form larger clusters and 2D islands. Based on the temperature dependence of OD stretching peaks, we succeeded in distinguishing water molecules inside 2D islands from those at the edge of 2D islands. From the comparison with the previous vibrational spectra of water clusters on other metal surfaces, we conclude that the number of water molecules at the edge of 2D islands is comparable with that of water molecules inside 2D islands on the Rh(111) surface at 20 K. This indicates that the surface migration of water molecules on Rh(111) is hindered as compared with the cases on Pt(111) and Ni(111) and thus the size of 2D islands on Rh(111) is relatively small.

Chemically homogeneous and thermally reversible oxidation of epitaxial graphene.

With its exceptional charge mobility, graphene holds great promise for applications in next-generation electronics. In an effort to tailor its properties and interfacial characteristics, the chemical functionalization of graphene is being actively pursued. The oxidation of graphene via the Hummers method is most widely used in current studies, although the chemical inhomogeneity and irreversibility of the resulting graphene oxide compromises its use in high-performance devices. Here, we present an alternative approach for oxidizing epitaxial graphene using atomic oxygen in ultrahigh vacuum. Atomic-resolution characterization with scanning tunnelling microscopy is quantitatively compared to density functional theory, showing that ultrahigh-vacuum oxidization results in uniform epoxy functionalization. Furthermore, this oxidation is shown to be fully reversible at temperatures as low as 260 °C using scanning tunnelling microscopy and spectroscopic techniques. In this manner, ultrahigh-vacuum oxidation overcomes the limitations of Hummers-method graphene oxide, thus creating new opportunities for the study and application of chemically functionalized graphene.

Real-space observation of local anisotropic correlation between buckled dimers on Si(100) induced by a bidentate adsorbed molecule.

The anisotropic correlation between buckled dimers on Si(100) was investigated by scanning tunneling microscopy. A bidentate ligand molecule was used to pin two neighboring dimers at 300 K. The chemically pinned dimer induces antiferromagnetic interaction along the dimer rows. Observed results agree well with Monte-Carlo simulations semi-quantitatively.