Temperature-programmed Desorption Spectra Research Articles

Surface reactions of TiCl4 and Al(CH3)3 on GaAs(100) during the first half-cycle of atomic layer deposition

GaAs(100) was exposed to pulses of trimethylaluminum (TMA, Al(CH3)3) and titanium tetrachloride (TiCl4) to mimic the first half-cycle of atomic layer deposition (ALD). Both precursors removed the 9.0±1.6Å-thick mixed oxide consisting primarily of As2O3 with a small Ga2O component that was left on the surface after aqueous HF treatment and vacuum annealing. In its place, TMA deposited an Al2O3 layer, but TiCl4 exposure left Cl atoms adsorbed to an elemental As layer. This suggests that oxygen was removed by the formation of a volatile oxychloride species. A small TiO2 coverage of approximately 0.04 monolayer remained on the surface for deposition temperatures of 89°C to 135°C, but no TiO2 was present from 170°C to 230°C. The adsorbed Cl layer chemically passivated the surface at these temperatures and blocked TiO2 deposition even after 50 full ALD cycles of TiCl4 and water vapor. The Cl and As layers desorbed simultaneously at higher temperature producing peaks in the temperature programmed desorption spectrum in the range 237–297°C. This allowed TiO2 deposition at 300°C in single TiCl4 pulse experiments. On the native oxide-covered surface where there was a higher proportional Ga oxide composition, TiCl4 exposure deposited TiO2.

The effect of boron incorporation on the structure and properties of glassy carbon

Boron was introduced into glassy carbon by chemical modification and by irradiating the precursor polymer with boron ions. Using chemical modification, a polymer with uniform distribution of boron in the bulk was obtained, whereas irradiation with B 3+ produced a polymer with boron located in a narrow region under the surface. Following modification with boron, the samples were carbonized at 1273 K and examined using X-ray diffraction, Raman spectroscopy, temperature programmed desorption (TPD) and by hardness measurements. Structural analysis results suggest that boron is mostly substitutionally bonded in the glassy carbon structure regardless of the method used to introduce boron into the polymer precursor. Formation of crosslinks between glassy carbon ribbons during carbonization as a consequence of the recombination of ion beam induced changes in the polymer structure and a high concentration of boron in the subsurface layer is proposed to account for an increase of glassy carbon hardness. An increase in the total amount of surface oxygen groups was observed in the TPD spectra of both boron-modified samples due to an increase in the number of active sites. Results show that the nature of these sites is influenced by the initial distribution of boron in the polymer precursor.

Effect of subsurface boron on photoluminescence from silicon nanocrystals

Silicon (Si) nanocrystals (NCs) less than 5 nm in diameter are grown on SiO 2 surfaces using hot wire chemical vapor deposition in an ultrahigh vacuum chamber and the dangling bonds are passivated using atomic deuterium. The passivated NCs are subsequently exposed to BD x radicals formed by dissociating deuterated diborane (B 2D 6) over a hot tungsten filament and photoluminescence quenching is observed. Temperature programmed desorption spectra reveal the presence of additional D 2 desorption peaks beyond those found for surfaces that have only been passivated by atomic deuterium. The additional peaks appear at lower temperatures and this can be attributed to deuterium desorption from surface Si atoms bonded to subsurface boron atoms. The subsurface boron likely enhances nonradiative Auger recombination leading to photoluminescence quenching.

Crystallization Kinetics and Excess Free Energy of H2O and D2O Nanoscale Films of Amorphous Solid Water

Temperature-programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS) are used to investigate the crystallization kinetics and measure the excess free energy of metastable amorphous solid water films (ASW) of H(2)O and D(2)O grown using molecular beams. The desorption rates from the amorphous and crystalline phases of ASW are distinct, and as such, crystallization manifests can be observed in the TPD spectrum. The crystallization kinetics were studied by varying the TPD heating rate from 0.001 to 3 K/s. A coupled desorption-crystallization kinetic model accurately simulates the desorption spectra and accurately predicts the observed temperature shifts in the crystallization. Isothermal crystallization studies using RAIRS are in agreement with the TPD results. Furthermore, highly sensitive measurements of the desorption rates were used to determine the excess free energy of ASW near 150 K. The excess entropy obtained from these data is consistent with there being a thermodynamic continuity between ASW and supercooled liquid water.

Changes in the morphology of interstellar ice analogues after hydrogen atom exposure

The morphology of water ice in the interstellar medium is still an open question. Although accretion of gaseous water could not be the only possible origin of the observed icy mantles covering dust grains in cold molecular clouds, it is well known that water accreted from the gas phase on surfaces kept at 10 K forms ice films that exhibit a very high porosity. It is also known that in the dark clouds H(2) formation occurs on the icy surface of dust grains and that part of the energy (4.48 eV) released when adsorbed atoms react to form H(2) is deposited in the ice. The experimental study described in the present work focuses on how relevant changes of the ice morphology result from atomic hydrogen exposure and subsequent recombination. Using the temperature-programmed desorption (TPD) technique and a method of inversion analysis of TPD spectra, we show that there is an exponential decrease in the porosity of the amorphous water ice sample following D-atom irradiation. This decrease is inversely proportional to the thickness of the ice and has a value of ϕ(0) = 2 × 10(16) D-atoms cm(-2) per layer of H(2)O. We also use a model which confirms that the binding sites on the porous ice are destroyed regardless of their energy depth, and that the reduction of the porosity corresponds in fact to a reduction of the effective area. This reduction appears to be compatible with the fraction of D(2) formation energy transferred to the porous ice network. Under interstellar conditions, this effect is likely to be efficient and, together with other compaction processes, provides a good argument to believe that interstellar ice is amorphous and non-porous.

NOx reduction performance of fresh and aged Fe-zeolites prepared by CVD: Effects of zeolite structure and Si/Al2 ratio

A series of Fe/zeolites with different pore structures (MFI, BEA, FER, LTL, MOR) and Si/Al2 ratios (6–240) were prepared by chemical vapor deposition (CVD). The activities for selective catalytic reduction by ammonia (NH3-SCR) of fresh and hydrothermally aged (700°C) samples were measured in order to determine the effect of zeolite species on reactivity and hydrothermal stability. SCR activity decreased with increasing Si/Al2 ratios for MFI (Si/Al2=40, 73 and 90) and MOR (Si/Al2=20, 30 and 240) samples. The SCR activity of fresh samples was dependent on the type of zeolite used as follows: BEA>MFI>FER>LTL>MOR. The order could not be explained by the amount of Fe loading or by specific surface area (SSA), although activity was well correlated with the higher temperature (HT) peak in temperature-programmed desorption (TPD) spectra of NO2 and with NO oxidation conversion under NH3-free conditions. The HT peak indicates the number of active Fe sites. Therefore, the major factor affecting SCR activity is the amount of active Fe available to catalyze the oxidation of NO. After hydrothermal aging, the activity of all samples decreased and the order of activity by zeolite type was: MFI>BEA>FER>LTL>MOR. Activity retention ratios estimated from conversions for fresh and aged samples were independent of the number of acid sites and SSA. In contrast, the retention ratios were correlated with the crystal size of the parent H-zeolites.

Formation of sulfite-like species on Cr 2O 3 after SO 2 chemisorption

The adsorption of sulfur dioxide (SO 2) on polycrystalline Cr 2 O 3 was experimentally investigated using temperature-programmed desorption (TPD). The chemisorption of SO 2 on the (0001) surface was also studied using theoretical methods. Different adsorption geometries were explored for SO 2 adsorption on the α-Cr 2 O 3 (0001) surface. Two similar adsorption configurations were found to be the most stable with chemisorption energies of − 3.09 and − 2.79 eV/molecule. In both calculated stable adsorption configurations the appearance of sulfite-like species is predicted on the (0001) surface after adsorption. It is important to emphasize that these results are predicted only within the DFT + U framework. Under these conditions and despite great efforts, no stable sulfate-like geometry was found on this surface. The TPD spectrum exhibit a desorption peak at T p ≈ 870 °C with a heating rate of β ≈ 0.12 °C/s. The desorption energy calculated by the analysis given by Redhead and Adams, assuming the rate of desorption is given by a Polanyi–Wigner equation, is ≈ − 3.12 eV. This value is in good agreement with the predicted one using DFT + U calculations. To our knowledge, this is the first theoretical study of SO 2 adsorption on the Cr 2 O 3 (0001) surface.

Retention and release mechanisms of deuterium implanted into beryllium

The fraction of deuterium (D) that is retained upon irradiation of beryllium (Be) as well as the temperatures at which implanted D is released are of importance for the international fusion experiment ITER, where Be will be used as an armor material. The influence of single parameters on retention and release is investigated in laboratory experiments performed under well defined conditions with the aim to identify dominant underlying mechanisms and thus be able to predict the behavior of the Be wall in ITER. Recent progress in the quantification of retained fractions and release temperatures as well as in the understanding of the governing mechanisms is presented. The retained fraction upon implantation of D at 1 keV into Be ( 1 1 2 ¯ 0 ) to fluences far below the saturation threshold of 10 21 m −2 is almost 95%, the remaining 5% being attributed to reflection at the surface. At these low fluences, no dependence of the retained fractions on implantation energy is observed. At fluences of the order of 10 21 m −2 and higher, saturation of the irradiated material affects the retention, leading to lower retained fractions. Furthermore, at these fluences the retained fractions decrease with decreasing implantation energies. Differences in the retained fractions from implanted Be ( 1 1 2 ¯ 0 ) and polycrystalline Be are explained by anisotropic diffusion of interstitials during implantation, leading to an amount of surviving D–trap complexes that depends on surface-orientation. Temperature-programmed desorption (TPD) spectra are recorded after implantation of fluences of the order of 10 19 m −2 at various energies and simulated by means of a newly developed code based on coupled reaction–diffusion systems (CRDS). The asymmetric shape of the TPD peaks is reproduced by introducing a local D accumulation process into the model.

Hydrogen storage properties of MgH 2 mechanically milled with α and β SiC

To understand the influence of various crystallographic phases on hydrogen storage properties, ball milling of MgH 2 with hexagonal (α) and cubic (β) SiC have been performed. Structural characterization of all samples has been done by X-ray diffraction (XRD) analysis, particle size analysis and scanning electron microscopy (SEM). Investigation of hydrogen desorption properties of prepared nanocomposites has been done using temperature programmed desorption (TPD) technique. Despite the results of structural and morphological characterization of obtained nanocomposites are very similar, TPD spectra show significant differences regarding existence of intermediate temperature peak. In the sample milled with hexagonal SiC this peak originates both from H 2 and H 2O, while in the sample milled with cubic phase it only comes from H 2O. Both samples exhibit low temperature H 2 peak at 385 K.

Formation, Characterization, and Reactivity of Adsorbed Oxygen on BaO/Pt(111)

The formation of adsorbed O (Oad) species and their reactivities in CO oxidation on BaO/Pt(111) (at two BaO coverages) were studied with temperature programmed desorption (TPD), infrared reflection absorption (IRA), and X-ray photoelectron (XP) spectroscopies. In neither of these two systems was the Pt(111) surface completely covered with BaO. On the system with lower BaO coverage (∼45% of the Pt(111) surface is covered by BaO), two different Oad species form following the adsorption of O2 at 300 K: O adsorbed on the BaO-free Pt(111) sites (OPt) and at the Pt−BaO interface (Oint). On the system with higher BaO coverage (∼60% of the Pt(111) surface is covered by BaO), two types of Oint are seen at the Pt−BaO interface. The desorption of OPt from the BaO-free portion of the Pt(111) surface gives an O2 desorption peak with a maximum desorption rate at ∼690 K. Migration of Oint to the Pt(111) sites and their recombinative desorption give two explosive desorption features at ∼760 and ∼790 K in the TPD spectrum...

Adsorptive desulfurization of propylene derived from bio-ethanol

We studied adsorption characteristics of a series of LTA zeolite as an adsorbent for desulfurization of propylene, that was produced from bioethanol by ethanol-to-olefin (ETO) conversion. A breakthrough curve (BTC) for adsorption of methanethiol, as one of the sulfur impurities of propylene produced from bioethanol, in the presence of propylene was measured using a fixed-bed column packed with the LTA zeolite. The BTC revealed that the effect of the competitive adsorption of propylene on the LTA zeolite strongly depended on a cation species exchanged in the micropores of the zeolite. Among the cation species examined in this study, bivalent cation of zinc (Zn2+) was proved to be the most effective one to increase the amount of methanethiol adsorbed on the LTA zeolite under the presence of propylene. The specific interaction of methanethiol with the LTA zeolite exchanged with Zn2+ was confirmed by the measurement of a temperature-programmed desorption (TPD) spectrum of methanethiol.

Diffusion-promoted-desorption mechanism for D 2 desorption from Si(100) surfaces

Temperature-programmed-desorption (TPD) spectra and isothermal desorption rates of D 2 molecules from a Si(100) surface have been calculated to reproduce experimental β 1, A -TPD spectra and isothermal desorption rate curves. In the diffusion-promoted-desorption (DPD) mechanism, hydrogen desorption from the Si(100) (2 × 1) surfaces takes place via D atom diffusion from doubly-occupied Si dimers (DODs) to their adjacent unoccupied Si dimers (UODs). Taking a clustering interaction among DODs into consideration, coverages θ DU of desorption sites consisting of a pair of a DOD and UOD are evaluated by a Monte Carlo (MC) method. The TPD spectra for the β 1, A peak are obtained by numerically integrating the desorption rate equation R = ν A exp(− E d, A / k B T) θ DU, where ν A is the pre-exponential factor and E d, A is the desorption barrier. The TPD spectra calculated for E d, A = 1. 6 eV and ν A = 2.7 × 10 9 /s are found to be in good agreement with the experimental TPD data for a wide coverage range from 0.01 to 0.74 ML. Namely, the deviation from first-order kinetics observed in the coverage dependent TPD spectra as well as in the isothermal desorption rate curves can be reproduced by the model simulations. This success in reproducing both the experimental TPD data and the very low desorption barrier validates the proposed DPD mechanism.

ABSENCE OF CO DISSOCIATION ON Mo(110): TPD AND DFT STUDY

The problem of the CO dissociation on Mo (110) has been addressed by means of temperature-programmed desorption (TPD) and density-functional (DFT) calculations. The TPD spectra show a first-order CO desorption, which indicates the desorption from a "virgin" state, not a recombinative form of desorption. The height of the potential barrier for the dissociation (2.75 eV), estimated from DFT calculations, substantially exceeds the energy of CO chemisorption (2.1 eV), which makes the thermally induced CO dissociation on Mo improbable. Monte Carlo simulations of TPD spectra, performed using estimated chemisorption energies, are in good agreement with experiment and demonstrate that the two-peak shape of the spectra can be explained without involving the CO dissociation. Thus, the results of the present study finally refute the concept of a dissociative form of CO adsorption on Mo surfaces.

Adsorption studies of trichloroethylene (TCE) on MgO(100)/Mo(100)

Employing ultraviolet photoelectron spectroscopy (UPS, He I), the more surface sensitive metastable impact electron spectroscopy (MIES) and temperature programmed desorption (TPD) measurements of the adsorption properties of the pollutant trichloroethylene (TCE) on thin MgO(100) films, grown on a Mo(100) single crystal, have been investigated. From TPD spectra of different coverages it is concluded that TCE interacts only weakly with MgO, which is attributed to physisorption. For increasing coverages a change from one peak to two peaks in the TPD spectra, one at higher, the second at lower temperatures with respect to the single peak is detected. Additionally, the observation of a local minimum for the work function (WF) for both MIES and UPS spectra is presented. Such a local minimum has been reported previously for the adsorption of metals with outer s valence electrons on transition metal substrates and adsorption of metals with outer s valence electrons on metal oxide films. Herein, we present the first WF minimum observed for a system of organic molecules adsorbed on an insulating surface. Two different models are discussed in order to understand the presented results.

Influence of the surface chemistry of multi-walled carbon nanotubes on their activity as ozonation catalysts

Multi-walled carbon nanotubes (MWCNTs) with different surface chemical properties were prepared by oxidative treatments with HNO 3, H 2O 2 and O 2 to introduce oxygen-containing surface groups and by thermal treatments for their selective removal. The texture and surface chemistry of the MWCNTs were characterized by nitrogen adsorption, temperature programmed desorption (TPD) and pH at the point of zero charge. A deconvolution procedure of the TPD spectra was used to quantify the oxygenated surface groups. These materials were used as catalysts for ozone decomposition, and for the ozonation of oxalic and oxamic acids. Generally, all these catalytic processes are favoured by carbon nanotubes with low acidic character. MWCNTs were shown to exhibit higher activity for the ozonation of oxalic and oxamic acids, compared to activated carbon. Successive experimental runs of oxalic acid removal carried out with a selected MWCNT sample show that the catalyst suffers some deactivation as a result of the introduction of oxygenated groups on the surface. Therefore, the effect of the surface chemistry is mainly observed for the fresh catalysts.

Changes of hydrogen storage properties of MgH 2 induced by boron ion irradiation

MgH 2 powder samples have been irradiated with 45 keV B 3+ ions with different ion fluencies ranging from 10 12 to 10 16 ions/cm 2. Irradiation effects have been estimated by SRIM calculations. To characterize induced modifications and its influence on the hydrogen desorption behavior of MgH 2, X-ray diffraction (XRD) analysis, particle size analysis and temperature programmed desorption (TPD) have been used. Changes of TPD spectra with irradiation conditions suggest that there are several mechanisms involved in desorption process which depend on defect concentration and their interaction and ordering. It has been demonstrated that the changes in near-surface area play the crucial role in hydrogen desorption kinetics. The results also confirm that there is possibility to control the thermodynamic parameters by controlling vacancies concentration in the systems.

Electric field induced oscillations in the catalytic water production on rhodium: A theoretical analysis

Methods are developed to describe catalytic reactions on multi-facetted surfaces in high electric fields in the conditions of field emission microscopy. These methods are applied to the hydrogen–oxygen–rhodium system for which a mean-field kinetic model is established. This model is shown to reproduce not only the nonequilibrium regimes of bistability and oscillations and their nanopatterns, but also the temperature-programmed desorption spectra of hydrogen and oxygen on rhodium, as well as the equilibrium phase diagram of oxygen on rhodium with the transition to the trilayer surface oxide ORhO. The dependence of the kinetic constants on the surface orientation of the facets is taken into account by expanding them in kubic harmonics suitable for the fcc rhodium crystal. The electric field modifies the gas pressure as well as the activation energies of the different kinetic processes. The tip of the field emission microscope is shown to behave as a nanoreactor.

Breaking through the glass ceiling: The correlation between the self-diffusivity in and krypton permeation through deeply supercooled liquid nanoscale methanol films

Molecular beam techniques, temperature-programmed desorption (TPD), and reflection absorption infrared spectroscopy (RAIRS) are used to explore the relationship between krypton permeation through and the self-diffusivity of supercooled liquid methanol at temperatures (100-115 K) near the glass transition temperature, T(g) (103 K). Layered films, consisting of CH(3)OH and CD(3)OH, are deposited on top of a monolayer of Kr on a graphene covered Pt(111) substrate at 25 K. Concurrent Kr TPD and RAIRS spectra are acquired during the heating of the composite film to temperatures above T(g). The CO vibrational stretch is sensitive to the local molecular environment and is used to determine the supercooled liquid diffusivity from the intermixing of the isotopic layers. We find that the Kr permeation and the diffusivity of the supercooled liquid are directly and quantitatively correlated. These results validate the rare-gas permeation technique as a tool for probing the diffusivity of supercooled liquids.

Chemistry of Silicon Nanocrystal Surfaces Exposed to Ammonia

Silicon nanocrystals (NCs) 8−10 nm in diameter are grown on SiO2 surfaces in an ultrahigh-vacuum chamber using hot wire chemical vapor deposition. These NCs are subjected to varying exposures of deuterated ammonia (ND3). The surface chemistry of Si NCs is studied using X-ray photoelectron (XP) spectroscopy and temperature-programmed desorption (TPD). The dissociative adsorption of ND3 on Si NCs results in the formation of ND2 species prior to TPD, and Si3N (nitride) formation is observed after TPD. D2 desorption is observed only from the monodeuteride species at 780 K. In separate experiments, a hot tungsten filament is used to predissociate ND3 before adsorption on the NC surface. XP spectra reveal that ND2 species form initially, and as the dose is increased, ND species dominate. After TPD, a SixNy species is observed. D2 desorption is observed from the mono-, di-, and trideuteride species when ND3 is predissociated. Irrespective of the technique used for dosing ammonia, TPD spectra do not contain any N...

A technique for extending the precision and the range of temperature programmed desorption toward extremely low coverages

In this paper, an improvement of the temperature programmed desorption (TPD) technique is introduced, which facilitates fully automated sampling of TPD spectra with excellent reproducibility, especially useful for extremely low coverages. By averaging many sampled TPD spectra, the range of the TPD technique can be extended toward lower coverages, as well as the quality of the spectra can be improved. This allows for easy extraction of information about the adsorbate-surface bond. A state of the art TPD apparatus with a two chamber setup and a high quality quadrupole mass spectrometer was extended by automated components. These are an automated gas dosing system, ensuring precise dosing of gas, combined with a motor driven sample manipulation unit and a liquid nitrogen cryostat with automatic refilling. In addition all components were controlled by a computer. A large number of TPD cycles could be sampled without the need of interaction of an operator. Here, it is shown for up to more than 400 TPD cycles. This opens a wide range of new interesting applications for the TPD technique, especially in the limit of zero coverage. Here, basic experiments on well known adsorbate systems are shown to view the ability and limit of this approach.