Chemisorption Of Chlorine Research Articles

Study on Mechanisms of Photon-Induced Material Removal on Silicon at Atomic and Close-to-Atomic Scale

This paper presents a new approach for material removal on silicon at atomic and close-to-atomic scale assisted by photons. The corresponding mechanisms are also investigated. The proposed approach consists of two sequential steps: surface modification and photon irradiation. The back bonds of silicon atoms are first weakened by the chemisorption of chlorine and then broken by photon energy, leading to the desorption of chlorinated silicon. The mechanisms of photon-induced desorption of chlorinated silicon, i.e., SiCl2 and SiCl, are explained by two models: the Menzel–Gomer–Redhead (MGR) and Antoniewicz models. The desorption probability associated with the two models is numerically calculated by solving the Liouville–von Neumann equations for open quantum systems. The calculation accuracy is verified by comparison with the results in literatures in the case of the NO/Pt (111) system. The calculation method is then applied to the cases of SiCl2/Si and SiCl/Si systems. The results show that the value of desorption probability first increases dramatically and then saturates to a stable value within hundreds of femtoseconds after excitation. The desorption probability shows a super-linear dependence on the lifetime of excited states.

DFT study of chlorine adsorption on bimetallic surfaces - Case study of Pd3M and Pt3M alloy surfaces

Chlorine adsorption on Pd3M and Pt3M alloy surfaces (M=Fe, Co, Ni, Rh, Pd, Pt) with ideal bulk surface composition and Pt and Pd-skin type surfaces was analyzed by means of spin-polarized periodic Density Functional Theory calculations. The Pd-based surfaces tended to adsorb chlorine more strongly than the Pt-based surfaces. On bimetallic surfaces, Cl preferred to adsorb on the high-coordination sites. The ratio of surface stabilities, that is the preference of non-segregated surfaces or the skin-type ones, can be inverted under chlorine chemisorption conditions. The relative surface stability was linked to catalytic activity of Pt3Ni surfaces towards the oxygen reduction reaction in chloride-containing solutions. The charge transfer from metallic substrate to the Cl adatom was found to become more pronounced when the position of the solute M in the Periodic Table of Elements moved upward and left. However, the degree of bond ionicity did not exceed 20% in any case. In order to contribute to the comprehension of the adsorption trends, the Cl adsorption energy was correlated to the charge transfer parameters and to the electronic structure of the investigated bimetallic surfaces.

Parameter estimation by Density Functional Theory for a lattice-gas model of Br and Cl chemisorption on Ag (1 0 0)

We study Bromine and Chlorine chemisorption on a Ag (1 0 0) surface, using a lattice-gas model and the quantum-mechanical Density Functional Theory (DFT) method. In this model the Br and Cl ions adsorb at the fourfold hollow sites of the Ag (1 0 0) surface, which can be represented by a square lattice of adsorption sites. Five different coverages were used for each kind of adsorbate. For each adsorbate and coverage, we obtained the minimum-energy configuration, its energy, and its charge distribution. From these data we calculated dipole moments, lateral interaction energies, and binding energies. Our results show that for Br the lateral interactions obtained by fitting to the adsorption energies obtained from the DFT calculation are consistent with long-range dipole–dipole lateral interactions obtained using the dipole moments calculated from the DFT charge distribution. For Cl we found that, while the long-range dipole–dipole lateral interactions are important, short-range attractive interactions are also present. Our results are overall consistent with parameter estimates previously obtained by fitting room-temperature Monte Carlo simulations to electrochemical adsorption isotherms [I. Abou Hamad, et al., J. Electroanal. Chem. 554 (2003) 211; Electrochim. Acta 50 (2005) 5518].

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.

Chlorine chemisorption on Cu(0 0 1) by surface X-ray diffraction: Geometry and substrate relaxation

We report on the precise location of Cl atoms chemisorbed on a Cu(0 0 1) surface and the interlayer relaxations of the metal surface. Previous studies have shown that chlorine dissociates on Cu(0 0 1) to form a c(2 × 2) chemisorbed layer with Cl atoms occupying four-fold hollow sites. A Cu–Cl interlayer spacing of 1.60 Å and a slightly expanded Cu–Cu first interlayer spacing of 1.85 Å (1.807 Å for bulk Cu) was determined by LEED. The resulting Cu–Cl bond length, 2.41 Å, is very similar to the SEXAFS value of 2.37 Å. Contradictory results were obtained by angle-resolved photoemission extended fine structure: while confirming the Cu–Cl interlayer spacing of 1.60 Å, no first Cu–Cu interlayer relaxation has been observed. On the other hand, a small corrugation of the second Cu layer was pointed out. We carried out a detailed structural determination of the Cu(0 0 1)– c(2 × 2)-Cl system using surface X-ray diffraction technique with synchrotron radiation. We find a Cu–Cl interlayer spacing of 1.584(5) Å and confirm the expansion of the first Cu–Cu interlayer, with an average spacing of 1.840(5) Å. In addition, we observe a small corrugation of the second Cu layer, with Cu atoms just below Cl atoms more tightly bound to the surface layer, and even a second Cu–Cu interlayer expansion.

Intercooled Double-Bed Reactor for LTWGS Reaction with Catalyst Poisoning by Chlorine: Inlet Temperatures for the Maximization of the Production

An intercooled double-bed reactor for industrial Low Temperature WGS reaction, subject to catalyst deactivation by irreversible chlorine chemisorption is modelled and optimised.Decline of the catalyst activity due to poisoning by chlorine decreases the reactor CO outlet conversion. After some time of operation, the lowest targeted CO conversion is reached, and further operation is inefficient. The reactor Efficient Production (EP) is calculated as the total amount of CO converted until the CO outlet conversion falls below the permissible minimum value, namely 0.85, which occurs at the operation time called the Catalyst Lifetime (CLT).Deactivation kinetics was assumed to be first order to chlorine partial pressure, and both the chlorine adsorption and deactivation rate constants were considered independent of temperature. For computation, spatio-temporal discretization of the reactor was adopted, and maximum CLT of 26 kh has been assumed.As the operation temperature is determining the overall reactor performance, two inlet temperature strategies for both reactors have been employed to maximise the EP: (a) isothermal and (b) time-trajectory.For a base case, optimum inlet isothermal temperatures for the first and second reactors of 483 and 458 K were found, respectively, at which 972.0 kmol of CO were converted within the CLT of 25.584 kh, being the EP of 958.6 kmol of CO. The maximum EP of 971.0 kmol was achieved for 483 and 462.3 K, for the first and second reactor, respectively. Varying the inlet temperature for both reactors along the time, higher production and EP were achieved.In comparison to the EP obtained for the single-bed reactor, the intercooled double-bed reactor allows to achieve higher EP, for both strategies.Also a sensitivity study to other operational variables, such as steam to gas ratio and total inlet flow, was carried out, in order to investigate their influence on optimal inlet temperatures.

Chemisorption of atomic chlorine on metal surfaces and the interpretation of the induced work function changes

The interaction of Cl on different metal surfaces has been studied by density functional theory. The surface is represented by periodic slab models. It is shown that the change on the surface work function induced by the presence of the adsorbed halide can be either positive or negative, depending on the surface metal. While this has already been pointed out for some particular cases such as Cl on Cu(0 0 1) or N on W(1 0 0), we show that this effect may also appear in the case of having the same adsorbate on different metal surfaces. Therefore, one may conclude that the use of the work function change to extract information about the net charge on the adsorbate should be regarded with extreme caution.

Chlorine-induced modifications in the electronic structure of Ag surfaces: ametastable deexcitation spectroscopy and photoemission comparativestudy

Surface-sensitive spectroscopic techniques, namely metastable deexcitationspectroscopy (MDS) and ultraviolet photoemission (UPS), have been applied toinvestigate the effects of chlorine chemisorption on the electronic properties(surface density of states and charge density) of Ag(100), Ag(110) and Ag(111)surfaces. Initial stages of chemisorption, up to the formation of a saturated Cloverlayer, have been examined. In particular, MDS permitted us to observe atlow Cl gas exposure a progressive depletion of the Ag (5s) charge due totransfer and bonding with Cl atoms. From both MDS and UPS it waspossible to observe the development of Cl (3p) bonding and anti-bondingstates, the amount of their splitting increasing with coverage. Differencesbetween chemisorption at the three surfaces have been noticed and theyhave been justified in terms of the different adatom packing and possibleformation of small AgCl clusters (especially for the Ag(111) surface).

Anisotropic Initial Stage of Laser-Induced Chlorine Reaction on Si(111) Observed by Surface-Sensitive Optical Methods

We investigate chlorine chemisorption and the initial stage of laser-induced reaction on Si(111) using second-harmonic generation (SHG) and reflectance difference spectroscopy (RDS). SHG signals that originate from direct bulk transition, generated by 1.6 and 1.7 eV probe light, show good threefold symmetry reflecting the symmetry of bulk Si, and SHG signals that originate from the surface, generated by 2.0 eV probe light, show threefold symmetry that intensifies in the [211] direction after chlorine chemisorption. The difference in reflectivity between the [211] direction and the [011] direction in the initial stage of the reaction is observed. In both clean Si surface and chlorinated Si surface, the RDS signal gradually changes with the progress of etching reaction, and the anisotropic initial stage of the reaction that depends on the direction of the atomic steps is observed.

Dissociative chemisorption of molecular chlorine on Si(100) – a first principles study

The reaction mechanism for the dissociative adsorption of molecular chlorine Cl 2 on Si(100) has been studied by density functional theory calculations. The direct activated channel with one end of Cl 2 bonded to a surface Si atom to form a terminal adsorbate dominates the channels studied. The energy barrier for the subsequent complete dissociation from such an adsorbate depends on the adsorption site, and the ordering of the energy barriers can account for the ordering of the statistical weights observed in STM experiments. Our results also fill in the details of the reaction mechanisms as previously probed by a molecular beam surface scattering experiment.

Oxidation of carbons in the presence of chlorine

The oxidation reaction of carbon black, sucrose carbon and graphite in the presence of chlorine was studied by thermal analysis (TGA and DTA). The oxidation rate was shown to depend upon the characteristics of each carbon. Heating in chlorine caused different degrees of mass increase in each of the three carbons, with two reaction zones due to physisorption and chemisorption of chlorine on the carbon surface. Burning of the carbons in oxygen gave the following reactivity order: carbon black>sucrose carbon>graphite. Burning of the carbons in the presence of chlorine showed its inhibiting effect, being strongest in carbon black. Oxidation in absence of chlorine started at 525, 560 and 660°C for carbon black, sucrose carbon and graphite, respectively. When chlorine was present in the gaseous phase, oxidation started at 650, 590 and 770°C. Therefore, the reactivity order in the presence of oxygen was sucrose carbon>carbon black>graphite.

Surface etching of InP(100) by chlorine

The cleaning and etching of the InP(100) surface by chlorine gas is investigated using synchrotron-radiation photoemission spectroscopy. A clean InP surface with a 4×2 configuration is obtained by ion sputtering or chlorine etching, followed by annealing to 650 K. The clean surface obtained by chlorine etching and annealing is indium-rich with the surface indium atoms showing metallic characterics. The chemisorption of chlorine leads to the formation of various InCl x ( x=1–3) and PCl species on the InP surface at 110 K and their corresponding chemical shifts are assigned. The chlorination of the InP surface causes surface band bending by about 0.36 eV at the saturation coverage. Argon-ion sputtering enhances the surface reactivity so that the sputtered surface can be chlorinated to a higher extent than the clean surface.

Influence of surface defects on chlorine chemisorption on Si(100)-(2 × 1)

The influence on chlorine chemisorption of surface defects created by low-fluence Ar + sputtering of the Si(100)-(2 × 1) surface has been studied. A distinctly different type of Cl bonding is observed on defect sites compared to Cl bonding on Si Si dimers, as judged by electron-stimulated desorption ion angular distribution (ESDIAD) measurements. On the ordered Si(100) surface only terminally bonded Si Cl species are observed (producing four off-normal Cl + beams); on the disordered Si(100) surface an additional Cl + beam emitted in the normal direction is present at 120 K. This Cl + beam is interpreted as a bridge-bonded Cl species chemisorbed inside of the dimer vacancy defects. In SiCl 2 thermal desorption, a new low-temperature desorption channel is observed on disordered Si(100) surfaces, indicating that defect sites enhance the rate of surface etching. In the range of Ar + fluences studied (0.2–5 monolayers) the Cl saturation coverage increases by as much as 30%, over that observed on non-defective Si(100)-(2 × 1).

Chemisorption of chlorine on GaAs(100) surfaces studied by high-resolution electron energy-loss spectroscopy

The adsorption of chlorine on GaAs(100)-c(8 × 2) surfaces was investigated by high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED). Chlorine molecules were released from an electrochemical AgAgCl-graphite cell. For the clean c(8 × 2)-reconstructed surface, besides the well-known Fuchs-Kliewer phonon of GaAs at 35.8 meV, two further surface phonons are observed at 7.9 and 11.5 meV. They can be characterized by comparison with the volume phonon dispersion curves of GaAs in the high-symmetry directions of the Brillouin zone. After chlorine exposure, the XPS measurements show an etching process which removes gallium atoms from the surface. With HREELS three adsorbate-induced loss features at 18, 43 and 46 meV are found. Comparing the loss energies with vibrational energies of AsClBr 2, GaCl and GaCl 2 suggests the attribution of the loss features to the GaCl valence vibration (46 meV) and the GaCl 2 scissor mode (18 meV) for small chlorine exposures, and to the AsCl stretching vibration (43 meV) in the high exposure regime when etching occurs.

Chlorine chemisorption and reaction on the Si(111) surface and its characterization using second-harmonic generation

The electronic states of the Cl-chemisorbed Si(111) surface and the basic characteristics of laser-induced surface reaction of the Si(111)/Cl system have been studied using second-harmonic generation (SHG). The SHG spectrum from the Cl-chemisorbed Si(111) surface showed resonance peaks involving the chlorine-induced electronic states at 1.6 and 2 eV. The laser-induced surface reaction of the Si(111)/Cl system was strongly affected by the interference between the pump and probe beams.

Atomic Sulfur and Chlorine Interaction with PdnClusters (n= 1−6): A Density Functional Study

Sulfur and chlorine change the activity and selectivity of palladium-based hydrogenation catalysts, but the origins of these effects at the microscopic level remain elusive. The chemisorption of atomic sulfur and chlorine on small palladium clusters has been studied with the hybrid Hartree−Fock/density functional method (B3LYP functional). Full geometry optimizations have been performed, allowing the occurrence of low-symmetry structures, in order to investigate the size dependence of atomic adsorption geometry and energy. The latter properties vary in a monotonous way as the number of palladium atoms increase and seem to rapidly converge toward limiting values. Sulfur was found to adsorb on the higher coordination sites, with a Pd−S bond distance of 2.3 Å, whereas chlorine prefers the 2-fold sites and the Pd−Cl bond is longer (2.5 Å). An adsorption energy of about 25−30 kcal/mol per Pd−X bond was estimated for both atoms. The topological analysis of the electron localization function (ELF) revealed that the bond between sulfur or chlorine adatoms and palladium is not of the shared-electrons type.

An ab initio Hartree-Fock/density functional study of the cluster simulated Si(111)7 × 7:Cl adsorption system

The first-principles all-electron Hartree-Fock/density functional method contained within the Gaussian92/DFT package has been employed to study the chemisorption of atomic chlorine onto small clusters modelling the adatom and restatom sites of the Si(111)7 × 7 surface. Both the equilibrium configurations and the binding energies of the various SiCl x complexes (1 ≤ x ≤ 3) produced by the adsorption of up to six chlorine atoms in the vicinity of an adatom and restatom site, have been determined. As progressively more chlorine atoms are adsorbed near these chemisorption sites, the surface topology is found to reconstruct to yield stable configurations for SiCl, SiCl 2 and SiCl 3. A chlorine bonded adatom is observed to move from its original three-fold T 4 site (one chlorine atom) to an adjacent bridge site (two and three chlorine atoms), and then on-top of a neighbouring first layer silicon atom (more than three chlorine atoms). Similar behaviour is exhibited by the restatom except that movement towards the on-top position occurs when three chlorine atoms are chemisorbed, rather than four as in the adatom case. The lowest binding energies are determined to be 0.74 eV for SiCl 3 desorption from a restatom site and 0.80 eV for the removal of SiCl 2 from an adatom site. Our results also suggest that a relatively low fluence of chlorine should produce a stable chlorosylil layer consisting of SiCl on the restatom sites and SiCl 3 on the adatom sites, and yielding a saturation coverage of 1.35 mL. Increased chlorine exposure should result in further uptake at the restatom sites to produce SiCl 3 complexes at both chemisorption sites and a maximum saturation coverage of 1.84 ML. Both of these predictions appear consistent with experiment.

Atomic chemisorption of chlorine on Ag(110) studied by high-resolution electron energy loss spectroscopy

We have studied atomic chemisorption at room temperature of chlorine on Ag(110) using high-resolution electron energy loss spectroscopy (HREELS), supplemented by XPS and LEED. The ClAg vibration energy (around 25 meV) and the line-width of this loss peak show well resolved variations with both chlorine coverage and substrate temperature T. The observed shift with T is related to the anharmonicity of the potential. Based on the Morse potential we derive an anharmonicity parameter x a = 6.2 × 10 −2 for the (2 × 1)Cl-overlayer. This indicates that the anharmonicity is enhanced by about a factor of two as compared to the bulk. In contrast, we find x a < 0.2 × 10 −2 for c(4 × 2)Cl. By comparison to other data we conclude that the (2 × 1)Cl-phase is a simple overlayer, with no significant reconstruction of the topmost substrate layer.

Chemical modification of diamond surfaces using a chlorinated surface as an intermediate state

Chemical modification of diamond powder surfaces was performed using chlorine chemisorption as an intermediate. We found OH, NH, and CF groups on the diamond surface. Chemisorbed species on the diamond powder surface were characterized by diffuse reflectance IR Fourier-transform spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed desorption spectroscopy. Hydrogen chemisorbed on the diamond surface is abstracted by chlorine above 250 °C, followed by the chemisorption of chlorine. Chlorine desorbs thermally from the diamond surface at approximately 300 °C. Hydroxyl groups are produced by treatment of the chlorinated diamond with water vapor at room temperature. Amino groups are produced by treatment of chlorinated diamond with ammonia at 425 °C. CF groups are produced by treatment of chlorinated diamond with CHF 3 at 600 °C. Chlorine can easily chemisorb onto the diamond surface and easily desorb from the diamond surface. Using the chlorinated surface as an intermediate state, it is possible further to modify chemically the diamond surface.

Gas-sensitive resistors: surface interaction of chlorine with semiconducting oxides

The electrical response of WO3 and FeNbO4 to the presence of small concentrations of Cl2 in air, in the presence and absence of water vapour, is reported. The response of WO3 was very large and fast; that of FeNbO4 was smaller and slower. The electrical conductivity of the niobate could be remarkably decreased by making it slightly niobium-deficient, but this had little effect on the chlorine response. A simple, surface-trap-limited model describes reasonably well the variation of electrical resistance with gas concentration. A competitive, dissociative chemisorption of chlorine and oxygen to form the surface-trap states Oads– and Clads– along with OHads–(from water vapour) is postulated and the enthalpy of displacement of surface oxygen by chlorine derived from the temperature dependence of response: ΔHads=(–152 ± 28) kJ mol–1 on FeNbO4. The different behaviour of the different materials is rationalised in terms of differing trap-state energies for Oads–, Clads– and OHads–.