Microcalorimetry of oxygen adsorption on fcc Co{110}

The coverage dependent heats of adsorption and sticking probabilities for oxygen on fcc Co{110} have been measured at 300 K using single crystal adsorption calorimetry (SCAC). Initial adsorption is consistent with dissociative chemisorption at low coverage followed by oxide formation above 0.6 ML coverage. The initial heat of adsorption of 633 kJ mol(-1) is similar to heat values calorimetrically measured on other ferromagnetic metal surfaces, such as nickel and iron. As the coverage increases, the heat of adsorption and sticking probability drop very rapidly up to the onset of oxidation. As already observed for other oxygen-metal surface systems, strong lateral adatom repulsions are responsible for the transition from the chemisorption regime to oxide film formation at higher coverage. The heat of oxide formation at the onset is 475 kJ mol(-1), which is consistent with the formation of CoO crystallites. The oxide film formation is discussed in terms of nucleation and island growth, and the Mott-Cabrera mechanisms, the latter being evidenced by the relatively constant heat of adsorption and sticking probability in contrast to the nickel and iron oxidation cases.     

Heat of adsorption of naphthalene on Pt(111) measured by adsorption calorimetry

The heat of adsorption of naphthalene on Pt(111) at 300 K was measured with single-crystal adsorption calorimetry. The heat of adsorption on the ideal, defect-free surface is estimated to be (300 - 34 - 199(2)) kJ/mol. From this, a C-Pt bond energy for aromatic hydrocarbons on Pt(111) of approximately 30 kJ/mol is estimated, consistent with earlier results for benzene on Pt(111). There is higher heat of adsorption at very low coverage, attributed to step sites where the adsorption heat is >/=330 kJ/mol. Saturation coverage, = 1 ML, corresponds to 1.55 x 10(14) molecules/cm(2). Sticking probability measurements of naphthalene on Pt(111) give a high initial value of 1.0 and a Kisliuk-type coverage dependence that implies precursor-mediated sticking. The ratio of the hopping rate to the desorption rate of this precursor is approximately 51. Naphthalene adsorbs transiently on top of chemisorbed naphthalene molecules with a heat of adsorption of 83-87 kJ/mol.     

A density functional study of NO adsorption and decomposition on Ni(211) and Pd(211) surfaces

The adsorption and decomposition of NO have been investigated by using density functional theory method at the generalized gradient approximation level. We have performed calculations on adsorption energies and structures of NO on Ni(211) and Pd(211) surfaces with full-geometry optimization and compared them with the experimental data. The most favorite adsorption on both surfaces occurs at the bridge site parallel to step edge (sb), while the energy difference from the second favorite site of a threefold hollow site near step edge is less than 0.1 eV. Decomposition pathways have been investigated with transition state search. The decomposition pathway, where NO leans toward the step, is most probable for both surfaces. The overall activation energy for decomposition is 0.39 and 1.26 eV for Ni(211) and Pd(211), respectively. The present results clearly show that the NO molecules on Pd(211) are less activated than those on Ni(211). We have studied also reorganization of NO on Pd(211) at higher coverages up to 1/3 ML (monolayer) [three NO molecules in a (3 x 1) unit cell]. The site occupation is not in a sequential manner as the NO coverage is increased, and a reorganization of NO adsorbates occurs (the NO molecule at sb becomes tilting up at higher coverage), which can interpret the experimental data of Yates and co-workers very well.     

Energetics of cyclohexene adsorption and reaction on Pt(111) by low-temperature microcalorimetry

The heat of adsorption and sticking probability of cyclohexene on Pt(111) were measured as a function of coverage using single-crystal adsorption calorimetry in the temperature range from 100 to 300 K. At 100 K, cyclohexene adsorbs as intact di-sigma bonded cyclohexene on Pt(111), and the heat of adsorption is well described by a second-order polynomial (130 - 47 theta - 1250 theta(2)) kJ/mol, yielding a standard enthalpy of formation of di-sigma bonded cyclohexene on Pt(111) at low coverages of -135 kJ/mol and a C-Pt sigma bond strength of 205 kJ/mol. At 281 K, cyclohexene dehydrogenates upon adsorption, forming adsorbed 2-cyclohexenyl (c-C6H(9,a)) and adsorbed hydrogen, and the heat of adsorption is well described by another second-order polynomial (174 - 700 theta + 761 theta(2)) kJ/mol. This yields a standard enthalpy of formation of adsorbed 2-cyclohexenyl on Pt(111) at a low coverage of -143 kJ/mol. At coverages below 0.10 ML, the sticking probability of cyclohexene on Pt(111) is close to unity (>0.95), independent of temperature.     

Effect of coverage and defects on the adsorption of propanethiol on Au(111) surface: a theoretical study

Periodic density functional calculations have been carried out to investigate both the thiol adsorption on Au(111) surface and the reaction mechanism for the formation of the self-assembled monolayers, taking propanethiol as a representative example. The effect of coverage and surface defects (adatoms and vacancies) has been analyzed. It is found that the most stable physisorption (undissociated) site is an adatom site, whereas the chemisorption site for the thiol is a vacancy site or protrusion consisting of a pair of adatoms, followed by one adatom site. The results point out that the thiolate self-assembled monolayer adsorption process occurs preferentially on step edges.     

氢原子在Ni(111)次表面和Ni(211)、(533)台阶面上的吸附与振动

应用原子与表面簇合物相互作用的五参数Morse势(5-MP)方法对氢原子在Ni(111)表面和次表面以及Ni(211), (533)台阶面进行了系统研究, 得到了氢原子在上述各面的吸附位、吸附几何、结合能和本征振动频率. 计算结果表明, 在Ni(111)面上, 氢原子优先吸附在三重位, 随着覆盖度的增加会吸附在次表面八面体位和四面体位. Ni(211), (533)的最优先吸附位都是四重位, 当氢原子的覆盖度增大时占据(111)平台的三重吸附位. 靠近台阶面的吸附位受台阶和平台高度的影响很大. 此外, 我们计算了氢原子在各表面的不同吸附位的扩散势垒, 获得氢原子在各表面的最低能量扩散通道.     

Adsorption energy, growth mode, and sticking probability of Ca on poly(methyl methacrylate) surfaces with and without electron damage

The adsorption of Ca atoms on pristine and electron-irradiated poly(methyl methacrylate) (PMMA) surfaces at 300 K has been studied by adsorption microcalorimetry, atomic beam/surface scattering, and low-energy He+ ion scattering spectroscopy (ISS). On pristine PMMA, the initial sticking probability of Ca is 0.5, increasing quickly with Ca coverage. Below 0.5 ML, the heat of adsorption is 730-780 kJ/mol, much higher than Ca's sublimation energy (178 kJ/mol). The Ca here is invisible to ISS, which is attributed to Ca binding to ester groups below the CH3/CH2-terminated PMMA surface. The adsorption energy increases with coverage, suggesting attractions between neighboring Ca-ester complexes. Above 0.5 ML, Ca starts to grow as three-dimensional (3D) Ca clusters on top of the surface, which dominate growth after 2 ML. It is proposed that each Ca reacts with two esters to form the Ca carboxylate of PMMA, because this reaction's heat would be close to that observed. The total amount of Ca that binds to subsurface sites is estimated from the integral heat of adsorption to involve 4-6 layers of ester groups. Exposing the PMMA surface to electrons increases Ca's initial sticking probability but lowers its adsorption energy. This is attributed to electron-induced defects acting as nucleation sites for 3D Ca islands, whose growth now competes kinetically with Ca diffusing to subsurface esters. Consequently, only two layers of subsurface esters get populated at saturation. The heat eventually reaches Ca's bulk heat of sublimation on all PMMA surfaces, where pure, bulk-like Ca thin films form.     

The sticking probability for H2 on some transition metals at a hydrogen pressure of 1 bar

The sticking probability for hydrogen on films of Co, Ni, Cu, Ru, Rh, Pd, Ir, and Pt supported on graphite has been measured at a hydrogen pressure of 1 bar in the temperature range 40-200 degrees C. The sticking probability is found to increase in the order Ni, Co, Ir, Pd, Pt, Rh, and Ru at temperatures below 150 degrees C, whereas at higher temperatures, the sticking probability for Pd is higher than for Pt. The sticking probability for Cu is below the detection limit of the measurement. The measured sticking probabilities are slightly lower than those obtained at high hydrogen coverage under ultrahigh vacuum conditions. This could be a consequence of the higher hydrogen pressure used here. The apparent desorption energies extracted from the steady-state desorption rate are found to agree reasonably well with published values for the heat of adsorption at high coverage. However, the sticking probability is not related in a simple way to published values for the heat of adsorption at low coverage, with Ru and Rh giving exceptionally high values for the sticking probability. It is suggested that this is due to the presence of adsorption sites with very low desorption energy on Ru and Rh.     

Calcium adsorption on MgO(100): energetics, structure, and role of defects

The adsorption of Ca on the MgO(100) surface at 300 K has been studied using microcalorimetry, in combination with LEED, AES, ISS, work function, sticking probability measurements, and density functional theory (DFT) calculations. The MgO(100) thin films (approximately 4 nm thick) were grown epitaxially on a 1 microm thick Mo(100) single-crystal. The sticking probability of Ca on MgO(100) at 300 K is unity. On the basis of AES and ISS measurements, it was determined that Ca grows mainly as 3D particles on the MgO(100) surface with a density of approximately 1 x 10(12) islands/cm2. Ca adsorbs initially at defect sites with a very high heat of adsorption (approximately 410 kJ/mol). DFT calculations attribute this high initial heat to Ca binding to kink sites (376 kJ/mol), step sites (205 kJ/mol), and lower concentrations of stronger binding sites. The heat of adsorption decreases rapidly with coverage, reaching a minimum of 162 kJ/mol at approximately 0.3 ML, where Ca is mainly adding to small 3D Ca clusters. Afterward, it increases to the value of bulk Ca heat of sublimation (178 kJ/mol) at approximately 1.2 ML, attributed to the increase in stability with increasing Ca particle size. A 1.0 eV decrease of the work function with Ca coverage from 0 to 0.3 ML indicates that Ca adsorbed at defects is cationic, in agreement with calculations showing that Ca donates electron density to the MgO. Light ion sputtering of the MgO(100) surface generates point defects, but these do not change the heat of adsorption versus coverage, implying that they do not nucleate Ca particles. Oxygen vacancies are a likely candidate; DFT calculations show that F and F+ center vacancies bind Ca more weakly than terrace sites. More extensive sputtering creates extended defects (such as steps and kinks) that adsorb Ca with heats of adsorption up to approximately 400 kJ/mol, similar to that at the intrinsic defect sites.     

Molecular hydrogen adsorption at surface adatoms

Using electron-energy-loss spectroscopy, we have measured preferential adsorption of molecular hydrogen at Cu and Au adatoms deposited on a cold Cu(100) surface. We show, with particular attention to the D(2)-Au system, that the molecules adsorb at the adatoms, with an enhanced binding energy. The adsorption state is not of chemisorption character, the D(2) rotational and internal vibrational transition energies are close to the corresponding gas phase values, a characteristic property of a physisorbed state. A revealing signature of the D(2)-Au interaction is an induced dipole activity of the rotational transition, which discriminates molecules adsorbed at the adatoms from those adsorbed on the bare substrate surface. The average number of molecules per Au atom depends on the Au coverage and increases at lower coverages, for example, at 4% of an adatom monolayer, there are approximately six D(2) molecules per Au adatom. In this limit, Au monomers prevail, and a cluster of six D(2) around a single Au adatom appears to be an optimal dense two-dimensional configuration.     

Adsorption dynamics of water on Pt{110}-(1 x 2)

The dynamics of H(2)O adsorption on Pt{110}-(1 x 2) is studied using supersonic molecular beam and temperature programed desorption techniques. The sticking probabilities are measured using the King and Wells method at a surface temperature of 165 K. The absolute initial sticking probability s(0) of H(2)O is 0.54+/-0.03 for an incident kinetic energy of 27 kJmol. However, an unusual molecular beam flux dependence on s(0) is also found. At low water coverage (theta<1), the sticking probability is independent of coverage due either to diffusion in an extrinsic precursor state formed above bilayer islands or to incorporation into the islands. We define theta=1 as the water coverage when the dissociative sticking probability of D(2) on a surface predosed with water has dropped to zero. The slow falling H(2)O sticking probability at theta>1 results from compression of the bilayer and the formation of multilayers. Temperature programed desorption of water shows fractional order kinetics consistent with hydrogen-bonded islands on the surface. A remarkable dependence of the initial sticking probability on the translational (1-27 kJ/mol) and internal energies of water is observed: s(0) is found to be essentially a step function of translational energy, increasing fivefold at a threshold energy of 5 kJ/mol. The threshold migrates to higher energies with increasing nozzle temperature (300-700 K). We conclude that both rotational state and rotational alignment of the water molecules in the seeded supersonic expansion are implicated in dictating the adsorption process.     

DFT investigation of CO adsorption on Pt(211) and Pt(311) surfaces from low to high coverage

Adsorption of CO on Pt(211) and Pt(311) surfaces has been investigated by the density functional theory (DFT) method (periodic DMol3) with full geometry optimization. Adsorption energies, structures, and C-O stretching vibrational frequencies are studied by considering multiple possible adsorption sites and comparing them with the experimental data. The calculated C-O stretching frequencies agree well with the experimental ones, and precise determination of adsorption sites can be carried out. For Pt(211), CO adsorbs at the atop site on the step edge at low coverage, but CO adsorbs at the atop and bridge sites simultaneously on both the step edge and the terrace with further increasing CO coverage. The present results interpret the reflection adsorption infrared (RAIR) spectra of Brown and co-workers very well from low to high coverage. For Pt(311), CO adsorbs also at the atop site on the step edge at low coverage. The lifting of reconstruction by CO adsorption occurs also for Pt(311), whereas the energy gain for lifting the reconstruction of the Pt(311) surface is smaller than that for Pt(110). The largest difference between the stepped Pt(211)/Pt(311) and Pt(110) surfaces is the occupation on the edge sites at higher coverage. For the stepped surfaces, the bridge site begins to be occupied at higher coverage, whereas the atop site is always occupied for the Pt(110) surface.     

An equilibrium ab initio atomistic thermodynamics study of chlorine adsorption on the Cu(001) surface

The effect of chlorine (Cl) chemisorption on the energetics and atomic structure of the Cu(001) surface over a wide range of chlorine pressures and temperatures has been studied using equilibrium ab initio atomistic thermodynamics to elucidate the formation of cuprous chloride (CuCl) as part of the Deacon reaction on copper metal. The calculated surface free energies show that the 1/2 monolayer (ML) c(2 × 2)-Cl phase with chlorine atoms adsorbed at the hollow sites is the most stable structure for a wide range of Cl chemical potential, in agreement with experimental observations. It is also found that at very low pressure and exposure, but elevated temperature, the 1/9 ML and 1/4 ML phases become the most stable. By contrast, a high coverage of Cl does not lead to thermodynamically stable geometries. The subsurface adsorption of Cl atoms, however, dramatically increases the stability of the 1 ML and 2 ML adsorption configurations providing a possible pathway for the formation of the bulk-chloride surface phases in the kinetic regime.     

Dynamics of analyte binding onto a metallophthalocyanine: NO/FePc

The gas-surface reaction dynamics of NO impinging on an iron(II) phthalocyanine (FePc) monolayer were investigated using King and Wells sticking measurements. The initial sticking probability was measured as a function of both incident molecular beam energy (0.09-0.4 eV) and surface temperature (100-300 K). NO adsorption onto FePc saturates at 3% of a monolayer for all incident beam energies and surface temperatures, suggesting that the final chemisorption site is confined to the Fe metal centers. At low surface temperature and low incident beam energy, the initial sticking probability is 40% and decreases linearly with increasing beam energy and surface temperature. The results are consistent with the NO molecule sticking onto the FePc molecules via physisorption to the aromatics followed by diffusion to the Fe metal center, or precursor-mediated chemisorption. The adsorption mechanism of NO onto FePc was confirmed by control studies of NO sticking onto metal-free H2Pc, inert Au111, and reactive Al111.     

Chemisorption of CO2 on nickel surfaces

CO2 chemisorption on the Ni(111), Ni(100), and Ni(110) surfaces was investigated at the level of density functional theory. It was found that the ability of CO2 chemisorption is in the order of Ni(110) > Ni(100) > Ni(111). CO2 has exothermic chemisorption on Ni(110) and endothermic chemisorption on Ni(111), while it is thermally neutral on Ni(100). It is also found that there is no significant lateral interaction between the adsorbed CO2 at 1/4 monolayer (ML) coverage, while there is stronger repulsive interaction at 1/2 ML. On all surfaces, the chemisorbed CO2 is partially negatively charged, indicating the enhanced electron transfer, and the stronger the electron transfer, the stronger the C=O bond elongation. The bonding nature of the adsorbed CO2 on nickel surfaces has been analyzed. The thermodynamics of CO2 dissociative chemisorption, compared with CO and O adsorption, has been discussed, and the thermodynamic preference is in the sequence Ni(100) > Ni(111) > Ni(110).     

Coverage-dependent formation of chiral ethylthiolate-Au complexes on Au(111)

We studied the coverage-dependent self-assembly of the flat-lying phase of ethylthiolate on Au(111). At low coverage, we observed the formation of short stripes of chiral Au-(SC(2)H(5))(2) complexes that arrange in a disordered phase. The latter grow partly at the expense of the native Au(111) surface reconstruction, which is fully lifted for a coverage of ～0.60 ML. We found that the lift of the reconstruction and evaporation from step edges are competing adatom sources. Close to saturation coverage (0.70 to 0.75 ML), large, well-ordered domains with a (8 × √3)rectangular superstructure formed. Alternation of chirality was found in adjacent stripes as already reported for other short alkanethiolates. We suggest that, because of a simple geometrical consideration, the chirality should, on the contrary, be preserved in the stripe phase of longer alkanethiolates.     

Formation of gold-methanethiyl self-assembled monolayers

The energetics of formation of thiyl-gold self-assembled monolayers is investigated using density-functional theory simulations. It is found that the chemisorption of dimethyl disulfide on the reconstructed Au(111) (22 x radical3) surface is most favored at the fcc reconstruction stripe, with initial physisorption leading to disulfide dissociation, adatom/vacancy-pair formation, and then, at a coverage of 7.8% sulfur atoms per gold atom, surface reconstruction lifting. At higher coverages, monolayer formation proceeds similarly on the unreconstructed surface, leading to surface pitting. Formation of the analogous adatom/vacancy-pair bound dissociated adsorbate complex on exposure of the clean unreconstructed surface to methanethiol is shown to be endothermic, however.     

Interaction of oxygen with TiN(001): N<-->O exchange and oxidation process

This work presents a detailed experimental and theoretical study of the oxidation of TiN(001) using a combination of synchrotron-based photoemission and density functional theory (DFT). Experimentally, the adsorption of O2 on TiN(001) was investigated at temperatures between 250 and 450 K. At the lowest temperature, there was chemisorption of oxygen (O(2,gas)-->2O(ads)) without significant surface oxidation. In contrast, at 450 K the amount of O2 adsorbed increased continuously, there was no evidence for an oxygen saturation coverage, a clear signal in the Ti 2p core level spectra denoted the presence of TiOx species, and desorption of both N2 and NO was detected. The DFT calculations show that the adsorption/dissociation of O2 is highly exothermic on a TiN(001) substrate and is carried out mainly by the Ti centers. A high oxygen coverage (larger than 0.5 ML) may induce some structural reconstructions of the surface. The exchange of a surface N atom by an O adatom is a highly endothermic process (DeltaE=2.84 eV). However, the overall oxidation of the surface layer is thermodynamically favored due to the energy released by the dissociative adsorption of O2 and the formation of N2 or NO. Both experimental and theoretical results lead to conclude that a TiN+mO2 -->TiOx + NO reaction is an important exit channel for nitrogen in the oxidation process.     

DFT study of gas-phase adsorption of benzotriazole on Cu(111), Cu(100), Cu(110), and low coordinated defects thereon

The adsorption of benzotriazole--an outstanding corrosion inhibitor for copper--on Cu(111), Cu(100), Cu(110), and low coordinated defects thereon has been studied and characterized using density functional theory (DFT) calculations. We find that benzotriazole can either chemisorb in an upright geometry or physisorb with the molecular plane being nearly parallel to the surface. While the magnitude of chemisorption energy increases as passing from densely packed Cu(111) to more open surfaces and low coordinated defects, the physisorption energy is instead rather similar on all three low Miller index surfaces. It is pointed out that due to a large dipole moment of benzotriazole the dipole-dipole interactions are rather important. For perpendicular chemisorption modes the lateral repulsion is very long ranged, extending up to the nearest-neighbor distance of about 60 bohrs, whereas for parallel adsorption modes the lateral interactions are far less pronounced and the molecules experience a weak attraction at distances ?25 bohrs. The chemisorption energies were therefore extrapolated to zero coverage by a recently developed scheme and the resulting values are -0.60, -0.73, and -0.92 eV for Cu(111), Cu(100), and Cu(110), respectively, whereas the zero-coverage physisorption energy is about -0.7 eV irrespective of the surface plane. While the more densely packed surfaces are not reactive enough to interact with the molecular π-system, the reactivity of Cu(110) appears to be at the onset of such interaction, resulting in a very stable parallel adsorption structure with an adsorption energy of -1.3 eV that is ascribed as an apparent chemisorption+physisorption mode.     

Dissociation mechanism of 2-propanol on a Si(111)-(7x7) surface studied by scanning tunneling microscopy

Adsorption of 2-propanol, (CH3)2CHOH, on a Si(111)-7x7 surface was studied by scanning tunneling microscopy. (CH3)2CHOH adsorbs equally on the faulted and unfaulted half unit cells by forming Si-OCH(CH3)2 and Si-H on an adatom and rest atom pair. Si-OCH(CH3)2 is consecutively increased in each half unit cell, and the adsorption is saturated when every half unit cell has three Si-OCH(CH3)2, which corresponds to 0.5 of the adatom coverage. The sticking probability for the dissociation of (CH3)2CHOH is independent of the adatom coverage from 0 to 0.4, but it depends on coverage at higher than 0.4. By counting the darkened adatoms, Si-OCH(CH3)2 on the center adatom (m) and that on the corner adatom (n), it was found the m/n ratio is ca. 4 for the first dissociation of (CH3)2CHOH in virgin half unit cell, but it becomes ca. 1.9 and 1.8 when two and three Si-OCH(CH3)2 are contained in a half unit cell. This result reveals that the dissociation probability of (CH3)2CHOH at the adatom-rest atom pair site is influenced by the nearest Si-OCH(CH3)2 in the half unit cell.