Surface Reaction Products Research Articles

Optical emission spectroscopy as a tool for studying, optimizing, and monitoring plasma-assisted atomic layer deposition processes

In this note it is demonstrated that optical emission spectroscopy (OES) is an easy-to-implement and valuable tool to study, optimize, and monitor thin film growth by plasma-assisted atomic layer deposition (ALD). The species in the plasma can be identified through the analysis of the light emitted by the plasma. OES provides therefore information on the reactant species delivered to the surface by the plasma but it also yields unique insight into the surface reaction products and, as a consequence, on the reaction mechanisms of the deposition process. Time-resolved measurements reveal information about the amount of precursor dosing and length of plasma exposure needed to saturate the self-limiting half reactions, which is useful for the optimization of the ALD process. Furthermore, time-resolved OES can also be used as an easy-to-implement process monitoring tool for plasma-assisted ALD processes on production equipment; for example, to monitor reactor wall conditions or to detect process faults in real time.

Role of microporosity and surface chemistry in adsorption of 4,6-dimethyldibenzothiophene on polymer-derived activated carbons

Two carbon samples derived from poly(4-styrenesulfonic acid-co-maleic acid) based polymer by carbonization between 700 and 800 °C were oxidized to two different levels of surface acidity. The surfaces of resulting adsorbents were characterized by potentiometric titration, adsorption of nitrogen, FTIR, SEM/EDAX and thermal analysis. The materials were used as adsorbents of 4,6-dimethyldibenzothiophene (4,6-DMDBT) from hexadecane with initial concentration of sulfur between 10–150 ppmw. Although it was found that pores with diameter less than 10 Å govern the amount of 4,6-DMDBT adsorbed, that amount is enhanced when acidic groups are present in the larger pores owing to the contributions of specific interactions. Surface chemistry plays an important role in reactive adsorption and deposition of the products of surface reactions in the pore system.

Effect of Y on the bio-corrosion behavior of extruded Mg–Zn–Mn alloy in Hank's solution

The bio-corrosion properties of Mg–Zn–Mn alloys with and without Y in Hank's solution at 37 °C were investigated by using electrochemical test and electrochemical impedance spectra (EIS). The results of open circuit potential (OCP) and polarization tests indicated that Y could reduce the cathodic current density. A passivative stage appeared in the Tafel curve of the Y containing magnesium alloy, indicating that a passivative film was formed on the surface of the Y containing magnesium alloy. EIS results showed that the Y containing alloy had higher charge transfer resistance and film resistance, but lower double layer capacity than the alloy without the Y element. The surface reaction product identification by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) showed that the surface corrosion products were hydroxide and phosphate (Mg 3Ca 3(PO 4) 4) for Mg–Zn–Mn alloy and phosphate (MgNaPO 4) for the Y containing Mg–Zn–Mn alloys. The XPS results also showed that a Y 2O 3 protective film was formed on the surface of the Y containing magnesium alloy which contributed mainly to the low cathodic current density and the high resistance.

Kinetic Isolation of Reaction Intermediates on Ice Surfaces. Precursor States of SO2 Hydrolysis

We studied the interaction of SO2 with ice films at temperatures above 80 K, with emphasis on the examination of the precursor states of SO2 hydrolysis, or SO2 surface complexes. Cs+ reactive ion scattering (RIS) and low energy sputtering (LES) techniques were used to examine the surface reaction products, in conjunction with temperature-programmed desorption (TPD) to monitor the desorbing species. The study indicated that the reaction of SO2 with the ice surface occurred through several distinct intermediate states, including a solvated SO2 species, a DSO2 species, and a strongly ionic molecular species, and these intermediates could be isolated on the ice surface due to kinetic trapping.

Semiquantitative Study of the EDC/NHS Activation of Acid Terminal Groups at Modified Porous Silicon Surfaces

Infrared spectroscopy is used to investigate the transformation of carboxyl-terminated alkyl chains immobilized on a surface into succinimidyl ester-terminated chains by reaction with an aqueous solution of N-ethyl-N'-(3-(dimethylamino)propyl)carbodiimide (EDC) and N-hydroxysuccinimide (NHS). The acid chains are covalently grafted at the surface of hydrogenated porous silicon whose large specific surface area allows for assessing the activation yield in a semiquantitative way by infrared (IR) spectroscopy and detecting trace amounts of surface products and/or reaction products of small IR cross section. In this way, we rationalize the different reaction paths and optimize the reaction conditions to obtain as pure as possible succinimidyl ester-terminated surfaces. A diagram mapping the surface composition after activation was constructed by systematically varying the solution composition. Results are accounted for by NHS surface adsorption and a kinetic competition between the various EDC-induced surface reactions.

Silica‐Supported Zirconium Complexes and their Polyoligosilsesquioxane Analogues in the Transesterification of Acrylates: Part 1. Synthesis and Characterization

AbstractVarious silica‐supported acetylacetonate and alkoxy zirconium(IV) complexes have been prepared and characterized by quantitative chemical measurements of the surface reaction products, quantitative surface microanalysis of the surface complexes, in situ infrared spectroscopy, CP‐MAS 13C NMR spectroscopy and EXAFS. The complex (SiO)Zr(acac)3 (acac=acetylacetonate ligand) (1) can be obtained by reaction of zirconium tetraacetylacetonate [Zr(acac)4] with a silica surface previously dehydroxylated at 500 °C. The complexes (SiO)3Zr(acac) (2) and (SiO)3Zr(O‐n‐Bu) (n‐Bu=butyl ligand) (3) can be synthesized by reaction of (SiO)3ZrH with, respectively, acetylacetone and n‐butanol at room temperature. The spectroscopic data, including EXAFS spectroscopy, confirm that in compound 1 the zirconium is linked to the surface by only one SiOZr bond whereas in the case of compounds 2 and 3 the zirconium is linked to 3 surface oxygen atoms which are sigma bonded. EXAFS data indicate also that the acetylacetonate ligands behave as chelating ligands leading to a hepta‐coordination around the zirconium atom in 1 and a penta‐coordination in 2. In order to provide a molecular analogue of 1, the synthesis of the following polyoligosilsesquioxane derivative (c‐C5H9)7Si8O12(CH3)2Zr(acac)3 (1′) was achieved. The compound 1′ is obtained by reacting (c‐C5H9)7Si8O11(CH3)2(OH), 4, with an equimolecular amount of Zr(acac)4. In the same manner, syntheses of complexes (c‐C5H9)7Si7O12Zr(acac) (2′) and of (c‐C5H9)7Si7O12Zr(O‐n‐Bu) (3′) were achieved by reaction of the unmodified trisilanol, (c‐C5H9)7Si7O9(OH)3, with respectively Zr(acac)4 and Zr(O‐n‐Bu)4 at 60 °C in tetrahydrofuran. Compounds 1′, 2′ and 3′ can be considered as good models of 1, 2 and 3 since their spectroscopic properties are comparable with those of the surface complexes. The synthetic results obtained will permit us to study the catalytic properties of these surface complexes and of their molecular analogues with the ultimate goal of delineating clear structure‐activity relationships.

Desulfurization of air at high and low H 2S concentrations

Several commercially available coconut-based activated carbons were tested as adsorbents of hydrogen sulfide (H 2S) at high and low H 2S concentrations. These carbons were compared to an experimental coconut-based carbon modified by impregnation with strongly basic compounds and an oxidant. The breakthrough curves for H 2S at 3000 ppm and 10 ppm were measured at room temperature. The effects of humidity and water content on H 2S removal were investigated. The carbon samples were characterized before and after H 2S adsorption using XRF, sorption of nitrogen, thermal analysis in combination with mass spectrometry (TGA–MS) and thermal desorption–gas chromatography–mass spectrometry (TD–GC–MS), along with pH measurements. These methods were used to track the activated carbon surface chemical changes upon H 2S adsorption/reactive adsorption. The amount of H 2S adsorbed on all activated carbons was found to be dependent on the amount of surface basic groups. TGA–MS and TD–GC–MS analysis revealed H 2S is adsorbed via reactive adsorption mechanisms. Metal sulfates, elemental sulfur, carbon sulfide, oxides of sulfur and dihydrogen disulfide are the products of surface reactions.

Effects of surface chemistry on the reactive adsorption of hydrogen cyanide on activated carbons

Two activated carbons of different origins were modified by heating at 950 °C either with or without previous urea impregnation. The treatment causes changes in surface chemistry and porosity. The materials obtained were used as adsorbents for hydrogen cyanide in dry air at ambient conditions. The samples before and after adsorption were characterized using nitrogen adsorption, potentiometric titration, elemental analysis and thermal analysis. On selected samples extraction was carried out to identify surface reaction products soluble in alcohols. The results indicated differences in the amount adsorbed and the products of surface reactions on specific surface features. The presence of nitrogen incorporated in the carbon matrix leads to an enhanced performance owing to the basic environment and the ability of the surface to activate oxygen. These lead to complex surface reactions in which the derivatives of hydrogen cyanide form oxamide, and are incorporated in the carbon matrix, or are deposited as the bulky insoluble polymers of paracyanogen on the surface. The reactions mainly occur in pores with sizes between 10 and 20 Å where the functional groups can be present and HCN, or its derivatives, and water can take part in the reactions.

Mechanism and dynamics of the reaction of XeF2 with fluorinated Si(100): Possible role of gas phase dissociation of a surface reaction product in plasmaless etching

Xenon difluoride is observed to react with Si-Si sigma-dimer and sigma-lattice bonds of Si(100)2 x 1 at 150 K by single and two atom abstraction at F coverages above 1 ML. As in the limit of zero F coverage, a measurable fraction of the scattered, gas phase product of single atom abstraction, XeF, is sufficiently internally excited to dissociate into F and Xe atoms before detection. Using the XeF internal energy and orientation distributions determined in the limit of zero coverage, the laws of conservation of momentum, energy, and mass are applied to the measured F velocity and angular distributions at higher coverage to simulate the Xe atom velocity and angular distributions and their intensities at higher coverage. The simulation predicts the observed Xe atom velocity and angular distributions at high coverage reasonably well, largely because the exothermicity channeled to XeF remains approximately constant as the coverage increases. This constancy is an opportune consequence of the trade-off between the attractiveness of the potential energy surface as the coverage is increased and the dynamics of the XeF product along the potential surface. The energy, momentum, and mass conservation analysis is also used to distinguish between Xe atoms that arise from XeF gas phase dissociation and Xe atoms that are produced by two atom abstraction. This distinction enables the calculation of percentages of the single and two atom abstraction pathways, as well as the percentages of the two pathways available to the Xe atom produced by two atom abstraction, inelastic scattering, and desorption. Finally, the simulation reveals that between 9% and 12% of F atoms produced by gas phase dissociation of XeF are scattered back toward the surface. These F atoms likely react readily with Si to form the higher fluorides that ultimately lead to etching. Gas phase dissociation of the scattered product of a surface reaction is a novel mechanism to explain the unique reactivity of XeF(2) to etch Si in the absence of a plasma.

Surface-Induced Dissociation and Chemical Reactions of C 2D 4+ on Stainless Steel, Carbon (HOPG), and Two Different Diamond Surfaces

Surface-induced interactions of the projectile ion C(2)D(4)(+) with room-temperature (hydrocarbon covered) stainless steel, carbon highly oriented pyrolytic graphite (HOPG), and two different types of diamond surfaces (O-terminated and H-terminated) were investigated over the range of incident energies from a few eV up to 50 eV. The relative abundance of the product ions in dependence on the incident energy of the projectile ion [collision-energy resolved mass spectra, (CERMS) curves] was determined. The product ion mass spectra contained ions resulting from direct dissociation of the projectile ions, from chemical reactions with the hydrocarbons on the surface, and (to a small extent) from sputtering of the surface material. Sputtering of the surface layer by low-energy Ar(+) ions (5-400 eV) indicated the presence of hydrocarbons on all studied surfaces. The CERMS curves of the product ions were analyzed to obtain both CERMS curves for the products of direct surface-induced dissociation of the projectile ion and CERMS curves of products of surface reactions. From the former, the fraction of energy converted in the surface collision into the internal excitation of the projectile ion was estimated as 10% of the incident energy. The internal energy of the surface-excited projectile ions was very similar for all studied surfaces. The H-terminated room-temperature diamond surface differed from the other surfaces only in the fraction of product ions formed in H-atom transfer surface reactions (45% of all product ions formed versus 70% on the other surfaces).

Biocorrosion behavior of magnesium alloy in different simulated fluids for biomedical application

The biocorrosion behavior of a magnesium alloy in two simulated body solutions, Hank's solution and simulated blood plasma (SBP) solution was investigated by electrochemical and weight loss testing for biomedical application. The solution volume/surface area (SV/SA) ratio was changed to reveal the effect of immersion condition on the biocorrosion behavior. A same tendency was observed in the corrosion rate of magnesium alloy in all testing conditions: a high corrosion rate at the initial stage, and rapid decrease in the first 2–3 days followed by a stable corrosion rate in the following stage. A higher corrosion rate was observed in Hank's solution than in SBP solution due to high Cl −, low Ca 2+ and PO 4 3− concentration in Hank's solution. However, no difference in the surface reaction product was observed between the samples immersed in Hank's solution and in SBP solution. It was found that the SV/SA ratio significantly affected the corrosion rate of magnesium alloy. Low ratio resulted in a high pH, which resisted the corrosion. But when the ratio was high enough, 6.7 for example, the influence was negligible. By changing the ratio, the biocorrosion behavior of magnesium implant in different implantation sites can be simulated, for example, low ratio for the case of in muscle and high ratio for the case of in marrow cavity. It is suggested that selection of the simulated solution and the SV/SA ratio would be very necessary to simulate different the in-vivo biodegradation behavior of magnesium in different implantation environment.

Surface species studied by angle-resolved product desorption: Emission mechanism and spatial distribution

Recent progress and future developments in angle-resolved measurements of desorbing surface reaction products are reviewed for the analysis of surface species. Spatial and energy distributions of desorbing products with hyperthermal energy deliver the most direct structural information of the transition state including active intermediates and product formation sites. Knowledge of the desorption mechanism is requisite to achieve structural information since the method is directly linked to the chemical process. Structural analyses are exemplified in photoinduced O 2 desorption and CO oxidation, and compared with those from thermal N 2O decomposition and CO oxidation as well as their applications to steady-state catalytic processes.

Chemical Erosion of Refractory-Metal Nozzle Inserts in Solid-Propellant Rocket Motors

An integrated theoretical/numerical framework is established and validated to study the chemical erosion of refractory-metal (tungsten, rhenium, and molybdenum) nozzle inserts in solid-rocket-motor environments, with a primary focus on tungsten. The formulation takes into account multicomponent thermofluid dynamics in the gas phase, heterogeneous reactions at the surface, energy transport in the solid phase, and nozzle material properties. Typical combustion species of nonmetallized ammonium-perchlorate/hydroxyl-terminated-polybutadiene propellants at practical motor operating conditions are considered. The erosion rates calculated by employing three different sets of chemical kinetics data available in the literature for the tungsten-steam reaction have been compared. The effect of considering either of two different tungsten oxides, WO2 or WO3, as the final product of surface reactions is also investigated. The predicted erosion rates compare well with experimental data. The oxidizing species of H2O proved more detrimental than CO2 in dictating the tungsten nozzle erosion. The material recessionrateiscontrolledbyheterogeneouschemicalkineticsbecausethediffusionlimitisnotreached.Theerosion rate increases with increasing chamber pressure, mainly due to higher convective heat transfer and enhanced heterogeneous surface reactions. The tungsten nozzle erodes much more slowly than graphite, but at a rate comparable with that of rhenium. The molybdenum nozzle exhibits the least erosion for flame temperatures lower than 2860 K. Its low melting temperature (2896 K), however, restricts applications for propellants with high flame temperatures.

Surface chemistry reactions of α-terpineol [(R)-2-(4-methyl-3-cyclohexenyl)isopropanol] with ozone and air on a glass and a vinyl tile

The surface-phase reaction products of alpha-terpineol [(R)-2-(4-methyl-3-cyclohexenyl)isopropanol] with ozone (O(3)), air or nitrogen (N(2)) on both a glass and vinyl flooring tile were investigated using the recently published FLEC Automation and Control System (FACS). The FACS was used to deliver O(3) (100 ppb), air or N(2) to the surface at a specified flow rate (300 ml/min) and relative humidity (50%) after application of a 1.6%alpha-terpineol solution in methanol. Oxidation products were detected using the derivatization agents: O-(2,3,4,5,6-pentafluorobenzyl) hydroxylamine and N,O-bis(trimethysilyl)trifluoroacetamide. The positively identified reaction products were glyoxal, methylglyoxal and 4-oxopentanal. The proposed oxidation products based on previously published VOC/O(3) reaction mechanisms were: 4-methylcyclohex-3-en-1-one, 6-hydroxyhept-en-2-one, 3-(1-hydroxy-1-methylethyl)-6-methylcyclohex-2-en-1-one) and one surface-enhanced reaction product: 5-(1-hydroxy-1-methylethyl)-2-methylcyclohex-2-en-1-one. Though similar products were observed in gas-phase alpha-terpineol/O(3) reactions, the ratio of the reaction products were different suggesting stabilization of larger molecular weight species by the surface. Emission profiles of these oxidation products over 72 h are also reported. Volatile organic compounds (VOCs) can interact with indoor initiators [such as hydroxyl radicals (OH*), ozone and nitrate radicals (NO(3)*)] to form a number of oxygenated by-products in the gas-phase. However, when VOCs are applied to or are present on the surface, heterogeneous chemistry with indoor initiators can also occur. The surface can influence the reaction mechanism to produce new surface reaction products. The work, described here, shows the interaction of alpha-terpineol (major component of pine oil) with ozone and air on both glass and vinyl flooring. These results demonstrated emissions of oxygenated organic compounds as a result of reaction and that further investigations of this chemistry are required to accurately estimate indoor occupant exposures.

Femtosecond real-time probing of transition state dynamics in a surface photoreaction: Methyl desorption from CH3I on MgO(100)

A novel experimental approach to the investigation of surface adsorbate reaction dynamics is presented. The direct time-resolved monitoring of the surface reaction transition state and product formation dynamics were accomplished via pump-probe mass spectrometry. As an example, methyl iodide molecules adsorbed at submonolayer coverage on an ultrathin magnesia film on Mo(100) were photoexcited to the A-band by ultrafast laser pulse irradiation. Employing time-delayed multiphoton ionization the dynamics of the dissociative methyl iodide transition state and of the emerging methyl photoproduct could be detected with femtosecond resolution. The reaction times deduced from the temporal evolution of the methyl ion mass signal indicate a strong interaction of the methyl fragment with the substrate surface prior to desorption.

Reaction mechanisms during plasma-assisted atomic layer deposition of metal oxides: A case study for Al2O3

Plasma-assisted atomic layer deposition (ALD) of metal oxide films is increasingly gaining interest, however, the underlying reaction mechanisms have rarely been addressed. In this work, a case study is presented for the plasma-assisted ALD process of Al2O3 based on Al(CH3)3 dosing and O2 plasma exposure. A complementary set of time-resolved in situ diagnostics was employed, including spectroscopic ellipsometry, quartz crystal microbalance, mass spectrometry, and optical emission spectroscopy. The saturation of the Al(CH3)3 adsorption reactions was investigated, as well as the reaction products created during both the precursor dosing and the plasma exposure step. The generality of the observations was cross-checked on a second commercial ALD reactor. The main observations are as follows: (i) during the precursor dosing, the Al(CH3)3 predominantly binds bifunctionally to the surface at 70°C through a reaction in which H is abstracted from the surface and CH4 is released into the gas phase; (ii) during the plasma exposure, O radicals in the plasma are consumed at the surface by combustionlike reactions with the surface −CH3 ligands, producing mainly H2O, CO2, and CO; (iii) small gas phase densities of CH4 and higher hydrocarbons (C2Hx) are also present during the O2 plasma exposure step indicating complementary surface reactions including a secondary thermal ALD-like reaction by the H2O produced at the surface; (iv) the plasma and its optical emission are strongly affected by the surface reaction products released in the plasma. In the latter respect, optical emission spectroscopy proved to be a valuable tool to study the surface reaction products during the plasma exposure as well as the saturation of the surface reactions. The implications of the experimental observations are addressed and it is discussed that the reaction mechanisms are generic for plasma-assisted ALD processes based on metal organic precursors and O2 plasma as oxidant source.

Structural study of Fe(II) adsorption on hematite [formula omitted

The structure of α - Fe 2 O 3 ( 1 1 ¯ 02 ) reacted with Fe(II) under anoxic conditions was studied using crystal truncation rod (CTR) diffraction. The CTR results show the crystalline termination of α - Fe 2 O 3 ( 1 1 ¯ 02 ) is modified due to adsorption of Fe(II) at crystallographic lattice sites. In addition, the binding sites for adsorbed Fe are similar for all studied conditions: reaction for 2 h at pH 5.0, for 34 d at pH 5.0, and for 5.5 h at pH 7.0. The occupancy of adsorbed Fe increases with both reaction time and pH, which is consistent with typical cation adsorption behavior on iron (hydr)oxide surfaces. The metal–oxygen bond lengths of the (ordered) surface Fe atoms are characteristic of Fe(III), which provides indirect evidence for oxidation of adsorbed Fe(II) and is consistent with recent studies indicating that Fe(III)-hydroxides are effective oxidants for dissolved ferrous iron. Grazing-incidence X-ray diffraction measurements indicate that no crystalline surface reaction products formed during the course of Fe(II) reaction. Overall, the structural characterization of the Fe(II) adsorption reaction results in an enhanced understanding of how reduced iron affects the structure, stability and reactivity of hematite.

Tantalum Nitride Atomic Layer Deposition Using (tert-Butylimido)tris(diethylamido)tantalum and Hydrazine

Tantalum nitride atomic layer deposition (ALD) was performed using sequential exposures of (tert-butylimido)tris(diethylamido) tantalum (TBTDET) and either hydrazine or ammonia . X-ray reflectivity studies demonstrated that hydrazine consistently yielded films with larger growth rates and higher densities than the films grown using ammonia. The ALD growth rate of using hydrazine was higher than the growth rate obtained using ammonia over the temperature range from . Quartz crystal microbalance measurements were also used to confirm the ALD growth rates and to verify the self-limiting behavior of each reactant. Hydrazine yielded a density of at that was significantly greater than the highest density of obtained at with ammonia. In situ resistivity measurements on the ALD films grown using hydrazine at obtained resistivities of at and after annealing to . The surface species and reaction products during the sequential exposures of TBTDET and hydrazine at were examined using Fourier transform infrared spectroscopy and quadrupole mass spectrometry. These studies allowed a growth mechanism to be proposed for ALD using TBTDET and hydrazine.

Nanografting for Surface Physical Chemistry

This article reveals the enabling aspects of nanografting (an atomic force microscopy-based lithography technique) in surface physical chemistry. First, we characterize self-assembled monolayers and multilayers using nanografting to place unknown molecules into a matrix with known structure or vice versa. The availability of an internal standard in situ allows the unknown structures to be imaged and quantified. The same approaches are applied to reveal the orientation and packing of biomolecules (ligands, DNA, and proteins) upon immobilization on surfaces. Second, nanografting enables systematic investigations of size-dependent mechanics at the nanometer scale by producing a series of designed nanostructures and measuring their Young's modulus in situ. Third, one can investigate systematically the influence of ligand local structure on biorecognition and protein immobilization by precisely engineering ligand nanostructures. Finally, we also demonstrate the regulation of the surface reaction mechanism, kinetics, and products via nanografting.

Role of Microporosity and Nitrogen Functionality on the Surface of Activated Carbon in the Process of Desulfurization of Digester Gas

Microporous coconut-shell based activated carbon was modified by heating with urea or melamine at 450 and 950 °C. The materials were characterized using adsorption of nitrogen, thermal analysis, elemental analysis, and potentiometric titration to obtain detailed information about surface chemistry and texture. Then they were used as adsorbents of hydrogen sulfide from digester gas where CO2 and CH4 are present in predominant quantities. The experiments were run in dry conditions and with preadsorbed water on the surface. The results revealed the importance of water, which, by its adsorption in small pores, eliminated the adsorption of methane, and thus caused the accessibility of adsorption centers for H2S. That water also helps in H2S dissociation. The introduction of nitrogen groups, which provide basicity needed for hydrogen sulfide dissociation, enhanced significantly the removal process. Since oxygen is not supplied to the system and only that oxygen which is chemisorbed on the surface can be used, elemental sulfur is a predominant product of surface reaction. The importance of nitrogen is in providing basicity, which is not deactivated by the presence of CO2 in the challenge gas.