Configurational Transitions of Benzene and Pyridine Adsorbed on Pt{111} and Cu{110} Surfaces: An Infrared Study

Electron stimulated desorption from condensed benzene

The electron induced dissociation of condensed benzene (C6H6) in thin films deposited on a Pt substrate is investigated by electron stimulated desorption (ESD) of anions and cations. The desorbed yields are recorded as a function of incident electron energy in the range of 10 to 950 eV for a fixed film thickness of 2 monolayers (ML) and for a fixed energy of 950 eV, as well as a function of film thickness from 0.5 to 8 monolayers (ML) for anions, and from 0.5 to 12ML for cations. Both energy and thickness dependencies are discussed in terms of the three main mechanisms yielding positively and/or negatively charged fragments: dissociative electron attachment (DEA), dipolar dissociation (DD) and dissociative ionization (DI) processes. At the probed energies, DD is the major mechanism, while DEA is predominantly induced by secondary electrons from the Pt substrate. Desorption of the parent positive ion is strongly suppressed. Similar qualitative behaviours are observed for the energy dependence of both anion and cation ESD yields, while some discrepancies exist in the thickness dependence, including a very significant systematic magnitude difference found between such ions formation. An estimation of the effective DD cross-section including the desorption probability is obtained. Feasible mechanisms behind the observed energy and thickness dependences for anion and cation yields are proposed. These results highlight the need for further investigations to better understand the underlying processes of electron induced dissociation in condensed matter.

Surface Sensitive Infrared Spectroelectrochemistry using Palladium Electrodeposited on ITO-Modified Internal Reflection Elements

Palladium nanoparticles have been electrodeposited on the surfaces of conductive indium tin oxide (ITO) modified silicon internal reflection elements. The resulting films are shown to be excellent platforms for attenuated...

Development of Highly Sensitive Raman Spectroscopy for Subnano and Single-Atom Detection

Direct detection and characterisation of small materials are fundamental challenges in analytical chemistry. A particle composed of dozens of metallic atoms, a so-called subnano-particle (SNP), and a single-atom catalyst (SAC) are ultimate analysis targets in terms of size, and the topic is now attracting increasing attention as innovative frontier materials in catalysis science. However, characterisation techniques for the SNP and SAC adsorbed on substrates requires sophisticated and large-scale analytical facilities. Here we demonstrate the development of an ultrasensitive, laboratory-scale, vibrational spectroscopic technique to characterise SNPs and SACs. The fine design of nano-spatial local enhancement fields generated by the introduction of anisotropic stellate-shaped signal amplifiers expands the accessibility of small targets on substrates into evanescent electromagnetic fields, achieving not only the detection of isolated small targets but also revealing the effects of intermolecular/interatomic interactions within the subnano configuration under actual experimental conditions. Such a development of “in situ subnano spectroscopy” will facilitate a comprehensive understanding of subnano and SAC science.

Silica-supported, narrowly distributed, subnanometric Pt-Zn particles from single sites with high propane dehydrogenation performance

The development of highly productive, selective and stable propane dehydrogenation catalysts for propene production is strategic due to the increasing need for propene and the availability of shale gas, an abundant source of light alkanes. In that context, the combination of surface organometallic chemistry (SOMC) and a thermolytic molecular precursor (TMP) approach is used to prepare bimetallic subnanometric and narrowly distributed Pt-Zn alloyed particles supported on silica via grafting of a Pt precursor on surface OH groups present in a Zn single-site containing material followed by a H₂ reduction treatment. This material, that exhibits a Zn to Pt molar ratio of 3 : 2 in the form of alloyed Pt-Zn particles with a 0.2 to 0.4 fraction of the overall Zn amount remaining as Zn^II sites on the silica surface, catalyzes propane dehydrogenation (PDH) with high productivity (703 g_C3H6 g_Pt ^-1 h^-1 to 375 g_C3H6 g_Pt ^-1 h^-1) and very low deactivation rates (k _d = 0.027 h^-1) over 30 h at high WHSV (75 h^-1). This study demonstrates how SOMC can provide access to highly efficient and tailored catalysts through the stepwise introduction of specific elements via grafting to generate small, homogeneously and narrowly distributed supported alloyed nanoparticles at controlled interfaces.

Diruthenium complexes having a partially hydrogenated bipyridine ligand: plausible mechanism for the dehydrogenative coupling of pyridines at a diruthenium site

The reactions of the diruthenium tetrahydrido complex, Cp*Ru(μ-H)4RuCp* (1) (Cp* = η5-C5Me5), with pyridines were investigated in relation to the dehydrogenative coupling of 4-substituted pyridines. Complex 1 reacted with γ-picoline to yield the bis(μ-pyridyl) complex, {Cp*Ru(μ-H)(μ-4-MeC5H3)}2 (2a), with the elimination of dihydrogen. Complex 2a immediately reacted with the liberated dihydrogen to yield μ-η2-dihydrobipyridine (dhbpy) complex 4avia C-C bond formation between the two pyridyl groups, in which one of the pyridine rings underwent partial hydrogenation. The X-ray structure of 4a shows that the dhbpy moiety adopts a μ-η2 coordination mode at the Ru2 site. Complex 4a was reversibly converted to 5avia the elimination of dihydrogen in which the dhbpy moiety adopts a μ-η2:η2 mode. Although 5a was coordinatively saturated, 5a readily reacted with tBuNC to yield 6a. This was owing to the ability of the dhbpy ligand changing its coordination mode between the μ-η2:η2 and μ-η2 modes. This also causes the dehydrogenation from the dhbpy ligand to yield μ-η2:η2-bipyridine complex 7a at 140 °C. However, 7a was not shown to be an intermediate of the catalysis. The reaction of 1 with 1,10-phenanthroline afforded μ-η2-phenanthroline complex 8 containing two hydrides, which can be a model compound for the bipyridine elimination from the Ru2 site. Dynamic NMR studies suggested that 8 was isomerised to an unsaturated μ-N-heterocyclic carbene (NHC) complex. The unsaturated nature of the μ-NHC complex is likely responsible for the uptake of the third pyridine molecule to turn over the catalytic cycle.

Flexible NO2 -Functionalized N-Heterocyclic Carbene Monolayers on Au (111) Surface

The formation of flexible self-assembled monolayers (SAMs) in which an external trigger modifies the geometry of surface-anchored molecules is essential for the development of functional materials with tunable properties. In this work, it is demonstrated that NO₂ -functionalized N-heterocyclic carbene molecules (NHCs), which were anchored on Au (111) surface, change their orientation from tilted into flat-lying position following trigger-induced reduction of their nitro groups. DFT calculations identified that the energetic driving force for reorientation was the lower steric hindrance and stronger interactions between the chemically reduced NHCs and the Au surface. The trigger-induced changes in the NHCs' anchoring geometry and chemical functionality modified the work function and the hydrophobicity of the NHC-decorated Au surface, demonstrating the impact of a chemically tunable NHC-based SAM on the properties of the metal surface.

Adsorption of Mono- and Divalent 4-(Dimethylamino)pyridines on Gold Surfaces: Studies by Surface-Enhanced Raman Scattering and Density Functional Theory

The adsorption thermodynamics of 4-(dimethylamino)pyridine (DMAP) and its five divalent derivatives di-DMAP- n (2 ≤ n ≤ 6) with gradually increasing methylene-spacer lengths n binding to planar gold surfaces has been studied by surface-enhanced Raman spectroscopy (SERS) and density functional theory (DFT). SERS intensities of the totally symmetrical breathing mode of the pyridine ring at approximately 1007 cm^-1 are used to monitor the surface coverage of the DMAP and di-DMAP- n ligands on gold surfaces at different concentrations. The equilibrium constant as a measure of the binding affinity is obtained from these measurements by using a modified Langmuir isotherm. Due to multivalent binding to the gold substrate, a characteristic enhancement of the binding affinity of di-DMAP- n compared to the monovalent DMAP is observed for all divalent species. First principles calculations of the di-DMAP- n ligands on an ideal Au(111) surface model as well as step terrace models have been performed to understand the adsorption structures and the multivalent binding enhancements. Furthermore, Raman spectra of the adsorbed molecules have been studied by first principles calculations to correlate the binding affinities to experimentally determined adsorption constants. The joint experimental and theoretical investigation of an oscillatory behavior of the binding affinity as a function of the methylene-spacer length in mono- and divalent 4-(dimethylamino)pyridines reveals that the molecular architecture plays an important role for the structure-function interplay of multivalently bound adsorbates.

Ultrafast dynamics of the dipole moment reversal in a polar organic monolayer

Pyridine layers on Cu(110) possess a strong electric field due to the large dipole of adsorbed pyridine. This electric field is visible as an enhanced sum frequency response from both the copper surface electrons and the aromatic C-H stretch of pyridine via a third order susceptibility. In response to a visible pump pulse, both surface electron and C-H stretch sum frequency signals are reduced on a subpicosecond time scale. In addition, the relative phase between the two signals changes over a few hundred femtoseconds, which indicates a change in the electronic structure of the adsorbate. We explain the transients as a consequence of the previously observed pyridine dipole field reversal when the pump pulse excites electrons into the pyridine π^* orbital. The pyridine anions in the pyridine layer cause a large-scale structural change which alters the pyridine-copper bond, reflected in the altered sum frequency response.

Direct observation of the conformational transitions of single pyridine molecules on a Ag(110) surface induced by long-range repulsive intermolecular interactions

The transition between two conformations of pyridine molecules adsorbed on a Ag(110) surface at 13 K was investigated by performing single-molecule manipulation at a very low coverage and the track-imaging of pyridines for various surface coverages using a variable low-temperature scanning tunneling microscope. A single tilted conformer was converted to an upright conformer when another coadsorbed tilted pyridine molecule approached to within ∼2 nm. The conversion probability depends on the molecular separation. The tilted conformers that are prevalent at a very low coverage were converted to upright conformers with an increasing surface coverage. The minimum molecular separation before this transition is induced was determined to be 2.2 nm using molecular track-imaging and statistical analysis of the pyridine separation as a function of the molecular coverage. The conformation transition was attributed to substrate-mediated long-range repulsive interactions between the pyridine molecules, which are produced by charge redistribution that occurs upon pyridine adsorption on the silver surface.

Surface Enhanced Infrared Studies of 4-Methoxypyridine Adsorption on Gold Film Electrodes

This work uses electrochemical surface sensitive vibrational spectroscopy to characterize the adsorption of a known metal nanoparticle stabilizer and growth director, 4-methoxypyridine (MOP). Surface enhanced infrared absorption spectroscopy (SEIRAS) is employed to study the adsorption of 4-methoxypyridine on gold films. Experiments are performed under electrochemical control and in different electrolyte acidities to identify both the extent of protonation of the adsorbed species as well as its orientation with respect to the electrode surface. No evidence of adsorbed conjugated acid is found even when the electrolyte pH is considerably lower than the pKa. Through an analysis of the transition dipole moments, determined from DFT calculations, the SEIRA spectra support an adsorption configuration through the ring nitrogen which is particularly dominant in neutral pH conditions. Adsorption is dependent on both the electrical state of the Au film electrode as well as the presence of ions in the electrolyte that compete for adsorption sites at positive potentials. Combined differential capacitance measurements and spectroscopic data demonstrate that both a horizontal adsorption geometry and a vertical adsorption phase can be induced, with the former being found on negatively charged surfaces in acidic media and the latter over a wide range of polarizations in neutral solutions.

Infrared Signature of the Cation-π Interaction between Calcite and Aromatic Hydrocarbons

The cation-π interaction is proposed as an important mechanism for the adsorption of aromatic hydrocarbons having non-zero quadrupole moments by mineral surfaces. Direct evidence supporting such a mechanism is, however, limited. Using the model mineral calcite, we probe the cation-π interaction with adsorbed benzene, toluene, and ethylbenzene (BTE) molecules using attenuated total reflectance Fourier transform infrared spectroscopy. We show that the presence of calcite increases the energy required to excite the synchronized bending of aromatic C-H bonds of BTE molecules. The unique conformation of this vibrational mode indicates that the planar aromatic rings of BTE molecules are constrained in a tilted face-down position by the cation-π interaction, as further confirmed by density functional theory calculations. Our results suggest that the shift of the excitation energy of the aromatic C-H bending may be used as an infrared signature for the cation-π interaction occurring on mineral surfaces.

Interactions between alkanes and aromatic molecules: a rotational study of pyridine-methane

The rotational spectrum of the adduct pyridine-methane shows that methane links to an aromatic molecule apparently through a C-H···π weak hydrogen bond. The shape and the internal dynamics behaviour of this complex are very similar to that of the van der Waals complexes involving aromatic molecules with rare gases.

Photolysis and thermolysis of pyridyl carbonyl azide monolayers on single-crystal platinum

The photochemical and thermal reactivity of a number of acyl azide-substituted pyridine compounds, namely nicotinyl azide, isonicotinyl azide, picolinyl azide and dinicotinyl azide with investigated as saturated monolayers on a single-crystal Pt(111) surface in an ultrahigh vacuum chamber. Multilayers of the substrates exhibited a maximum rate of desorption at 270 K, above which, stable saturated monolayers formed as characterized by reflection-absorption infrared spectroscopy by observation of C=O and N3 bands at 1700 cm(-1), and 2100 and 1300 cm(-1) respectively. The monolayers were stable up to 400 K. Photolysis of the monolayer (or heating above 400 K) results in the formation of the respective isocyanate intermediate after loss of nitrogen as evidenced by the appearance of a new infrared band at 2260 cm(-1) with concomitant loss of the azide bands. The resulting isocyanate saturated monolayer is stable in absence of nucleophiles, but can be quenched with appropriate nucleophiles.

Size-dependent valence and conduction band-edge energies of semiconductor nanocrystals

Through the use of photoelectron spectroscopy in air (PESA), we investigate the size-dependent valence and conduction band-edge energies of CdSe, CdTe, PbS, and PbSe semiconductor quantum dots (QDs). The results are compared to those of previous studies, based on differing experimental methods, and to theoretical calculations based on k·p theory and state-of-the-art atomistic semiempirical pseudopotential modeling. To accurately map out the energy level landscapes of QDs as a function of size, the QDs must be passivated by comparable surface chemistries. This is highlighted by studying the effect of surface chemistry on the valence band-edge energy in an ensemble of 4.7 nm CdSe QDs. An energy level shift as large as 0.35 eV is observed for this system through modification of surface chemistry alone. This shift is significantly larger than the size-dependent valence band-edge shift that is observed when comparable surface chemistries are used.

Chemoselective photochemical surface reaction — Ketene versus carbene reactivity from the photolysis of saturated monolayers of pyridyl diazoesters on single-crystal Pt

Irradiation of saturated monolayers of 3- and 4-substituted pyridyl diazoacetates on single-crystal Pt surfaces leads to either the corresponding reactive carbene or stable ketene intermediate with the chemoselectivity determined by the position of the photoreactive substituent on the pyridyl ring, which ultimately directs the available interactions with neighboring substrates.

Potassium-benzene interactions on Pt(111) studied by metastable atom electron spectroscopy

Electron emission spectra obtained by thermal collisions of He(∗)(2(3)S) metastable atoms with C(6)H(6)/Pt(111), C(6)H(6)/K/Pt(111), and K/C(6)H(6)/Pt(111) were measured in the temperature range of 50-200 K to elucidate the adsorption/aggregation states, thermal stabilities of pure and binary films, and local electronic properties at the organic-metal interface. For C(6)H(6)/Pt(111), the He(∗)(2(3)S) atoms de-excite on the chemisorbed overlayer predominantly via resonance ionization followed by Auger neutralization and partly via Penning ionization (PI) yielding weak emission just below the Fermi level (E(F)). We assigned this emission to the C(6)H(6) π-derived states delocalized over the Pt 5d bands on the basis of recent density functional calculations. During the layer-by-layer growth, the C(6)H(6)-derived bands via PI reveal a characteristic shift caused by the final-state effect (hole response at the topmost layer). C(6)H(6) molecules chemisorb weakly on the bimetallic Pt(111) (θ(K)=0.1) and physisorb on the K multilayer. In both cases, the sum rule was found to be valid between the K 4s and C(6)H(6)-derived bands. The band intensity versus exposure plot indicates that the C(6)H(6) film grows on the K multilayer by the Volmer-Weber mechanism (island growth), reflecting the weak K-C(6)H(6) interactions. In case of K/C(6)H(6)/Pt(111), the K atoms are trapped on the topmost C(6)H(6) layer at 65 K, forming particlelike clusters. The surface plasmon satellite was identified for the first time and the loss energy increases with increasing cluster size. The K clusters are unstable above ∼100 K due to thermal migration into the C(6)H(6) film. When the cluster coverage is low, the K 4s band extends below and above E(F) of the Pt substrate and the anomaly is discussed in terms of vacuum level bending around the cluster.

Rational design of two-dimensional nanoscale networks by electrostatic interactions at surfaces

The self-assembly of aromatic carboxylic acids and cesium adatoms on a Cu(100) surface at room temperature has been investigated by scanning tunneling microscopy and X-ray photoelectron spectroscopy. The highly ordered molecular nanostructures are comprised of a central ionic coupling motif between the anionic carboxylate moieties and Cs cations that generate distinctive chiral arrangements of the network structures. The primary electrostatic interaction results in highly flexible bond lengths and geometries. The adsorbate-substrate coupling is found to be important for the determination of the structures. With the use of rod-like carboxylic linker molecules, the dimension of the porous networks can be tuned through the variation of the aromatic backbone length.

High-resolution electron spectroscopy and sigma/pi structures of M(pyridine) and M+(pyridine) (M = Li, Ca, and Sc) complexes

Metal-pyridine (metal = Li, Ca, and Sc) complexes are produced in laser-vaporization molecular beams and studied by pulsed-field-ionization zero-electron-kinetic-energy (ZEKE) spectroscopy and theoretical calculations. Both sigma and pi structures are considered for the three complexes by theory, and preferred structures are determined by the combination of the ZEKE spectra and calculations. The Li and Ca complexes prefer a sigma bonding mode, whereas the Sc complex favors a pi mode. Adiabatic ionization energies and metal-ligand vibrational frequencies are determined from the ZEKE spectra. Metal-ligand bond dissociation energies of the neutral complexes are obtained from a thermodynamic cycle. The ionization energies follow the trend of Li-pyridine (32,460 cm(-1)) < Ca-pyridine (39,043 cm(-1)) < Sc-pyridine (42,816 cm(-1)), whereas the bond energies are in the order of Ca-pyridine (27.0 kJ mol(-1)) < Li-pyridine (49.1 kJ mol(-1)) < Sc-pyridine (110.6 kJ mol(-1)). The different bonding modes between the main group metals and transition element are discussed in terms of Sc 3d orbital involvement. The bond energy differences between the Li and Ca metals are explained by the number of valence s electrons and the size of the metal atoms.

Application of surface-enhanced infrared absorption spectroscopy to investigate pyridine adsorption on platinum-group electrodes

In situ surface-enhanced infrared absorption spectroscopy (SEIRAS) was applied to investigate adsorption configurations of pyridine (Py) on platinum, palladium, ruthenium, and rhodium nanoparticle film electrodes. The results reveal that alpha-pyridyl species predominantly form on Pt electrodes by assuming an edge-on configuration with its ring N and alpha-C atoms bonding to the Pt surface, while on Ru and Rh electrodes pyridine molecules essentially remain intact by adopting a slightly edge-tilted configuration through bonding with its N lone pair electrons. Py adsorption on a Pd electrode may lie in between the above two cases; both alpha-pyridyl species and edge-tilted intact pyridine could be significantly present. Further comparison of the typical adsorption configurations on the above four electrodes with those on Ag, Au, Cu, Cd, and Ni film electrodes suggests that valence electrons and the periodic row of metals may play an important role in determining the adsorption configuration.

Pulsed-field ionization electron spectroscopy and molecular structures of copper-(pyridine)n (n = 1, 2) complexes

Cu-(pyridine)n (n = 1, 2) complexes are prepared in a pulsed laser ablation cluster source and identified using laser photoionization time-of-flight mass spectrometry. High-resolution electron spectra of these complexes are obtained using pulsed-field ionization zero electron kinetic energy (ZEKE) photoelectron spectroscopy. Metal-pyridine and pyridine-based vibrational modes are identified by comparing the ZEKE spectra with previous spectroscopic studies of isolated pyridine, pyridine adsorbed on metal surfaces, and other Cu complexes. Ground electronic states and molecular structures are determined by comparing the ZEKE spectra with ab initio and multidimensional Franck-Condon factor calculations. Metal-pyridine bond energies of the neutral complexes are derived from the measured ionization energies and thermochemical relations. The mono-ligand complex has C2v symmetry in both the neutral and ionized forms, whereas the di-ligand complex has an eclipsed pyridine configuration with D2h and C2 symmetries for the ion and neutral species, respectively. Although both the mono- and di-pyridine Cu complexes are formed by Cu binding to nitrogen atoms, important binding differences are found between these two complexes.Key words: pulsed-field ionization, ZEKE, photoelectron, ab initio, copper-pyridine complexes.[Traduit par la Rédaction]

Interaction of benzene with amorphous solid water adsorbed on polycrystalline Ag

The interaction of benzene with polycrystalline Ag and amorphous solid water (D(2)O) deposited thereupon at 124 K was investigated. Metastable impact electron spectroscopy, Reflection-absorption infrared spectroscopy, and temperature programmed desorption were utilized to obtain information on the electronic structure and the relative contribution to the bonding properties of the aromatic molecules among themselves and with D(2)O. On Ag, the benzene molecular plane is oriented parallel to the surface in the first layer. The second layer is tilted with respect to the first one. A total work function decrease of 0.8 eV takes place during the buildup of the first two layers. On amorphous solid water, the orientational distribution of the benzene molecular planes is initially peaked at an angle parallel to the water surface. During the completion of the first adlayer a coverage-induced reorientation takes place, inducing a tilt of the benzene molecules of the first adlayer. Still larger benzene exposures appear to lead to the formation of three-dimensional benzene clusters. Films produced by codepositing benzene and D(2)O or by postdepositing D(2)O layers on benzene films display "volcano like" benzene desorption during ice crystallization.

A new design strategy for dispersion stabilization of Ni particles based on the surface acid and base properties of Ni particles

A dispersion technology for Ni particles suspended in a non-aqueous medium based on the quantitative evaluation of surface acid-base properties of Ni particles is described. A quantitative analysis of surface acid-base properties of Ni particles was performed using non-aqueous titration. Dimethylamino ethanol and acetic acid were used as probe molecules to detect surface acid-base amounts of Ni particles. The dispersion system was designed on the basis of the amounts of surface acid-base sites on the Ni particle surface. Rheological behavior and agglomerate particle size data demonstrate that the dispersion stability of the designed Ni suspension is markedly improved, as expected. Therefore, the design strategy to improve the dispersion stability of Ni particles was successful. This strategy is expected to be applicable to dispersion systems of other particles suspended in a non-aqueous medium.