Molecular Desorption Research Articles

Chemical reactivity on gas-phase metal clusters driven by blackbody infrared radiation.

We report the observation of chemical reactions in gas-phase Rh(n)(N2O)m(+) complexes driven by absorption of blackbody radiation. The experiments are performed under collision-free conditions in a Fourier transform ion cyclotron resonance mass spectrometer. Mid-infrared absorption by the molecularly adsorbed N2O moieties promotes a small fraction of the cluster distribution sufficiently to drive the N2O decomposition reaction, leading to the production of cluster oxides and the release of molecular nitrogen. N2O decomposition competes with molecular desorption and the branching ratios for the two processes show marked size effects, reflecting variations in the relative barriers. The rate of decay is shown to scale approximately linearly with the number of infrared chromophores. The experimental findings are interpreted in terms of calculated infrared absorption rates assuming a sudden-death limit.

Thermal chemistry of the Cu-KI5 atomic layer deposition precursor on a copper surface

The thermal chemistry of a Cu(I) ketoiminate complex, Cu-KI5, resulting from the modification of the known Air Products CupraSelect® copper CVD precursor Cu(hfac)(tmvs) designed to tether the two ligands via an isopropoxide linker, was studied under ultrahigh vacuum on a Cu(110) single-crystal surface by using a combination of temperature programmed desorption (TPD) and x-ray photoelectron spectroscopy. Adsorption at low temperatures was determined to take place via the displacement of the vinyl ligand by the surface. Molecular desorption was seen at 210 K, and the evolution of Cu(II)-KI52 was established to take place at 280 K, presumably from a disproportionation reaction that also leads to the deposition of Cu(0). Other sets of desorption products were seen at 150, 250, and 430 K, all containing copper atoms and small organic moieties with molecular masses below 100 amu. The latter TPD peak in particular indicates significant fragmentation of the ligands, likely at the C–N bond that holds the vinylsilane-isopropoxide moiety tethered to the ketoimine fragment, and possibly also at the union between the vinylsilane and the alkoxide linker. The 430 K temperature measured for this chemistry may set an upper limit for clean Cu film deposition, but since reactivity on the surface was also found to be inhibited at higher surface coverages, it may be delayed to higher temperatures under atomic layer deposition conditions.

Nanoporosity-Enhanced Catalysis on Subwavelength Au Nanoparticles: a Plasmon-Enhanced Spectroscopic Study

Using surface-enhanced Raman spectroscopy (SERS) to monitor catalytic reactions in real time on Au nanocatalysts has been a significant challenge because plasmon-enhanced spectroscopies require the utilization of subwavelength Au nanostructures as substrates while heterogeneous catalysis requires small Au nanoparticles in the sub-5 nm size regime. Here, we show that subwavelength Au nanoparticles with nanoscale surface porosity represent a unique bifunctional nanostructure that serves as both high-performance SERS substrates and efficient surface catalysts, allowing one to unravel the kinetics and pathways of surface-catalyzed reactions with unprecedented sensitivity and detail through time-resolved plasmon-enhanced spectroscopic measurements. The origin of the nanoporosity-enhanced catalytic activity can be interpreted as a consequence of high abundance of undercoordinated surface atoms at the steps and kinks on the highly curved surfaces of Au porous nanoparticles. By measuring SERS signals from the monolayer molecules preadsorbed on the surfaces of Au porous nanoparticles, we have been able to gain quantitative new insights into the intrinsic kinetics and mechanisms of Au-catalyzed hydrogenation of aromatic nitro compounds with minimal complication introduced by the molecular diffusion, adsorption, and desorption.

Hydrophobic polydimethylsiloxane (PDMS) coating of mesoporous silica and its use as a preconcentrating agent of gas analytes.

Mesoporous silica with mean pore size of ∼14 nm was coated by polydimethylsiloxane (PDMS) using a thermal deposition method. We showed that the inner walls of pores larger than ∼8 nm can be coated by thin layers of PDMS, and the surfaces consisting of PDMS-coated silica were superhydrophobic, with water contact angles close to 170°. We used the PDMS-coated silica as adsorbents of various gas-phase chemical warfare agent (CWA) simulants. PDMS-coated silica allowed molecular desorption of various CWA simulants even after exposure under highly humid conditions and, therefore, is applicable as an agent for the preconcentration of gas-phase analytes to enhance the sensitivities of various sensors.

The dissociation-induced displacement of chemisorbed O2 by mobile O atoms and the autocatalytic recombination of O due to chain fragmentation on Ag(110)

The interplay between thermal desorption of chemisorbed dioxygen and its dissociation was studied with temperature programmed methods. Analysis of the kinetics of molecular desorption and the fraction of adsorbed molecules which dissociate is consistent with a model in which oxygen atoms released by the dissociation event induce desorption of the molecular species. These unequilibrated atoms exhibit a mean free path relative to the chemisorbed dioxygen of 1.8nm prior to thermalization with the surface, displacing chemisorbed dioxygen within their reach. Each dissociation event leads to desorption of two oxygen molecules if the space between chemisorbed molecules approaches the minimum of 0.58nm. This condition can be achieved experimentally by saturating the population of chemisorbed dioxygen (0.33 ML O2) at 90–100K. Oxygen adatoms recombine near 580K from the reconstructed (n×1)-O adlayer with kinetics dictated by progressive fragmentation of the O(AgO)m rows. This behavior gives rise to autocatalytic recombination kinetics of oxygen adatoms which produces both an acceleration of rate at constant temperature and unusual recombination kinetics in temperature programmed desorption.

Imaging molecular adsorption and desorption dynamics on graphene using terahertz emission spectroscopy

Being an atomically thin material, graphene is known to be extremely susceptible to its environment, including defects and phonons in the substrate on which it is placed as well as gas molecules that surround it. Thus, any device design using graphene has to take into consideration all surrounding components, and device performance needs to be evaluated in terms of environmental influence. However, no methods have been established to date to readily measure the density and distribution of external perturbations in a quantitative and non-destructive manner. Here, we present a rapid and non-contact method for visualizing the distribution of molecular adsorbates on graphene semi-quantitatively using terahertz time-domain spectroscopy and imaging. We found that the waveform of terahertz bursts emitted from graphene-coated InP sensitively changes with the type of atmospheric gas, laser irradiation time, and ultraviolet light illumination. The terahertz waveform change is explained through band structure modifications in the InP surface depletion layer due to the presence of localized electric dipoles induced by adsorbed oxygen. These results demonstrate that terahertz emission serves as a local probe for monitoring adsorption and desorption processes on graphene films and devices, suggesting a novel two-dimensional sensor for detecting local chemical reactions.

Optically induced oxygen desorption from graphene measured using femtosecond two-pulse correlation

Recently, there has been a great deal of interest in the effect of atmospheric gases on the properties of graphene. We investigate the electrical and optical response of graphene-based field effect transistors that have been exposed to high purity oxygen gas using a combination of ultrafast two-pulse correlation (to give high temporal resolution) and low-frequency transport measurements (to monitor the photoinduced changes in the Fermi level). By measuring the Fermi level shifts, we are only sensitive to the oxygen atoms that interact directly with the surface. We compare our results to predictions of the empirical friction model for molecular desorption. We show the time scale of the relaxation associated with oxygen desorption to be $\ensuremath{\sim}100$ fs, suggesting the desorption proceeds through hot electron generation in the graphene rather than heating of the lattice through hot phonon generation.

Hydrogen Surface Reactions and Adsorption Studied on Y2O3, YSZ, and ZrO2.

The surface reactivity of Y2O3, YSZ, and ZrO2 polycrystalline powder samples toward H2 has been comparatively studied by a pool of complementary experimental techniques, comprising volumetric methods (temperature-programmed volumetric adsorption/oxidation and thermal desorption spectrometry), spectroscopic techniques (in situ electric impedance and in situ Fourier-transform infrared spectroscopy), and eventually structural characterization methods (X-ray diffraction and scanning electron microscopy). Reduction has been observed on all three oxides to most likely follow a surface or near-surface-limited mechanism involving removal of surface OH-groups and associated formation of water without formation of a significant number of anionic oxygen vacancies. Partly reversible adsorption of H2 was proven on the basis of molecular H2 desorption. Dictated by the specific hydrophilicity of the oxide, readsorption of water eventually takes place. The inference of this surface-restricted mechanism is further corroborated by the fact that no bulk structural and/or morphological changes were observed upon reduction even at the highest reduction temperatures (1173 K). We anticipate relevant implications for the use of especially YSZ in fuel cell research, since in particular the chemical state and structure of the surface under typical reducing high-temperature conditions affects the operation of the entire cell.

Development of mass spectrometry by high energy focused heavy ion beam: MeV SIMS with 8 MeV Cl7+ beam

Particle induced X-ray emission (PIXE) at microprobe of Jožef Stefan Institute is used to measure two-dimensional quantitative elemental maps of biological tissue. To improve chemical and biological understanding of the processes in vivo, supplementary information about chemical bonding and/or molecular distributions could be obtained by heavy-ion induced molecular desorption and a corresponding mass spectroscopy with Time-Of-Flight (TOF) mass spectrometer. As the method combines the use of heavy focused ions in MeV energy range and TOF Secondary Ion Mass Spectrometry, it is denoted as MeV SIMS.At Jožef Stefan Institute, we constructed a linear TOF spectrometer and mount it to our multipurpose nuclear microprobe. A beam of 8MeV 35Cl7+ could be focused to a diameter of better than 3μm×3μm and pulsed by electrostatic deflection at the high-energy side of accelerator. TOF mass spectrometer incorporates an 1m long drift tube and a double stack microchannel plate (MCP) as a stop detector positioned at the end of the drift path. Secondary ions are focused at MCP using electrostatic cylindrical einzel lens.Time of flight spectra are currently acquired with a single-hit time-to-digital converter. Pulsed ion beam produces a shower of secondary ions that are ejected from positively biased target and accelerated towards MCP. We start our time measurement simultaneously with the start of the beam pulse. Signal of the first ion hitting MCP is used to stop the time measurement. Standard pulses proportional to the time of flight are produced with time to analog converter (TAC) and fed into analog-to-digital converter to obtain a time histogram. To enable efficient detection of desorbed fragments with higher molecular masses, which are of particular interest, we recently implemented a state-of art Field Programmable Gate Array (FPGA)-based multi-hit TOF acquisition. To test the system we used focused 8MeV 35Cl7+ ion beam with pulse length of 180ns. Mass resolution of measured SIMS spectra, dominantly determined by the duration of the beam pulse, is in good agreement with resolution estimated from pulse length. With improved high-voltage switching ability that will enable beam pulses with duration of 50ns, a mass resolution of better than 500 is anticipated.

A first-principles study of methyl lactate adsorption on the chiral Cu (643) surface

We used dispersion-corrected density function theory (DFT) to investigate the enantiospecific adsorption of R- and S-methyl lactate on the chiral Cu (643)R surface. Initial study of methyl lactate adsorbed on the Cu (111) surface revealed that the most strongly bound states are associated with interaction of the hydroxyl and alkoxide group with the surface. Using dispersion-corrected DFT-derived pre-factors and desorption energies within the Redhead analysis predicts peak temperatures that are in relatively good agreement with experimental values for molecular methyl lactate desorption from both the Cu (111) and Cu (643)R surface. The global minimum of S-methyl lactate is more firmly bound by 9.5kJ/mol over its enantiomer on the Cu (643)R surface, with a peak temperature difference of 25K versus an experimental value of 12K.

Plasma-induced photoresponse in few-layer graphene

A device consisting of a few layers of graphene (FLG) sheets was exposed to atmospheric plasma, resulting in the generation of significant number of defects, oxygen absorption, and doping. The plasma-induced electrical transformation and photoconducting properties of pristine FLG and plasma-irradiated FLG (p-FLG) were compared under visible and ultraviolet (UV) light illumination. The visible light photoresponsivity of p-FLG was 0.47AW−1 at 535nm, comparatively higher than that observed for pristine FLG (10mAW−1); this result was attributed to the formation of defect midgap states band by plasma irradiation. Photoinduced molecular desorption causes the responsivity of the higher energy (UV) photons. Our results suggest that plasma irradiation is a simple, novel way to tailor the optoelectronic properties of graphene layers.

Effect of molecular desorption on the electronic properties of self-assembled polarizable molecular monolayers

We investigated the interfacial electronic properties of self-assembled monolayers (SAM)-modified Au metal surface at elevated temperatures. We observed that the work functions of the Au metal surfaces modified with SAMs changed differently under elevated-temperature conditions based on the type of SAMs categorized by three different features based on chemical anchoring group, molecular backbone structure, and the direction of the dipole moment. The temperature-dependent work function of the SAM-modified Au metal could be explained in terms of the molecular binding energy and the thermal stability of the SAMs, which were investigated with thermal desorption spectroscopic measurements and were explained with molecular modeling. Our study will aid in understanding the electronic properties at the interface between SAMs and metals in organic electronic devices if an annealing treatment is applied.

Adsorption of maleic anhydride on Pt(111)

The surface chemistry of maleic anhydride (MA) has been studied on Pt(111) with temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations. Adsorption of MA takes place at 170K forming multilayers. We have studied the behavior of distinct carbon and oxygen species of MA depending on the surface temperature. MA-TPD indicates three main desorption temperatures; at 240K, approximately 60% of total MA on the surface shows molecular desorption. At high temperatures (360 and 550K) MA shows dissociative decomposition with production of C2H2, CO and CO2 fragments. A plausible decomposition pathway of MA on the Pt(111) surface is discussed. DFT calculations provide details of the adsorption geometry.

Thermal annealing as an easy tool for the controlled arrangement of gold nanoparticles in block-copolymer thin films

Thermal annealing was used for the bottom-up fabrication of morphologically controlled gold–block-copolymer (Au–BC) nanocomposites. Three different blends formed by polystyrene (PS) homopolymer and PS-coated gold nanoparticles (PSSH@Au) were used as modifiers of asymmetric polystyrene-b-polymethylmethacrylate (PS-b-PMMA): PS26/PS26SH@Au, PS75/PS75SH@Au and PS167/PS167SH@Au (where the subscripts refer to the number of styrene monomeric units). The results indicated that all three blends used as modifiers (PSn/PSnSH@Au) were successfully located in the PS phase during thermally induced BC self-assembly for a composition range from 5 to 43 wt% without macro-phase separation. The PSnSH@Au moiety experienced molecular desorption, nanocrystal core coalescence and partial molecular re-encapsulation processes during thermal annealing, leading to sphere-like gold NPs with a larger average size (without exceeding an interdomain distance). Ligand chain length regulated the degree of coalescence and re-encapsulation, defining ultimate core size. Furthermore, proper combination of chain length and composition enabled tuning of NP partitioning and arrangement on different length scales through thermally activated cooperative assembly processes. These results have not only significant impact for establishing thermal processing as a useful tool for the precise control of NP size and distribution, but also much broader implications for many nanoparticle-based technologies.

Low-Fluence Laser Induced Fragmentation and Desorption of 3,4,9,10-Perylenetetracarboxylic Dianhydride (PTCDA) Thin Film

Laser-induced fragmentation and desorption of fragments of PTCDA films vacuum-deposited on GaAs (100) substrate has been studied by time-of-flight (TOF) mass spectroscopy. The main effect caused by pulsed laser light irradiation (pulse duration: 10 ns, photon energy: 2.34 eV and laser fluence ranging from 0.5 to 7 mJ/cm2) is PTCDA molecular fragmentation and desorption of the fragments formed, whereas no desorption of intact PTCDA molecule was detected. Fragments formed are perylene core C20H8, its half C10H4, carbon dioxide, carbon monoxide and atomic oxygen. All desorbing fragments have essentially different kinetic energy. The mechanism of photoinduced molecular fragmentation and desorption is discussed.

Predicting the chemical stability of monatomic chains

A simple model for evaluating the thermal atomic transfer rates in nanosystems (Lin Z.-Z. et al., EPL, 94 (2011) 40002) was developed to predict the chemical reaction rates of nanosystems with small gas molecules. The accuracy of the model was verified by MD simulations for molecular adsorption and desorption on a monatomic chain. By the prediction, a monatomic carbon chain should survive for 1.2 × 102 years in the ambient of 1 atm O2 at room temperature, and it is very invulnerable to N2, H2O, NO2, CO and CO2, while a monatomic gold chain quickly ruptures in vacuum. It is worth noting that since the model can be easily applied via common ab initio calculations, it could be widely used in the prediction of chemical stability of nanosystems.

Molecular Adsorption and Desorption Behavior on Silicon Surface in a Complex Ambient Atmosphere Containing Vapors of Diethylphthalate, Acetic Acid and Water

Using a complex ambient atmosphere containing vapors of an organic compound, acid and water, the molecular adsorption and desorption behavior on a silicon surface was studied using the in-situ measurement of a quartz crystal microbalance linked to the rate theory. Because the behavior of diethylphthalate (DEP) could be reproduced assuming a single-component system, acetic acid (ACA) and DEP are concluded to separately exist in the water film and at the water film surface, respectively. This conclusion was obtained from both the adsorption and desorption behaviors. The process developed in this study is useful for determining the layer in which chemical compounds are present.

Alcohol and Thiol Adsorption on (Oxy)hydroxide and Carbon Surfaces: Molecular Dynamics Simulation and Desorption Experiments

Classical molecular dynamics simulations were used to investigate the interaction of methyl, ethyl, and n-propyl alcohols and thiols with the hydroxylated basal surfaces of aluminum hydroxide and iron oxyhydroxide, as well as a model graphite surface. Comparisons were made to concurrently run desorption experiments investigating the interaction of methyl, ethyl, and n-propyl alcohols with aluminum hydroxide and activated carbon. The metal (oxy)hydroxide surfaces represent the basal surfaces of the neutral end-member layered double hydroxides gibbsite and lepidocrocite, respectively, while the graphite surface is a simplified model of the pore walls in activated carbon used in the chemisorption experiment. Adsorption enthalpies obtained from simulations at infinite adsorbate dilution show that adsorption is greatly favored on the hydroxylated surfaces compared to the graphite surface, with the ethyl molecules adsorbing most favorably. Heats of desorption calculated from chemisorption experiments show the same increased interaction strength for the alcohols on the aluminum hydroxide surface compared with activated carbon, with the most favorable interaction being ethanol with the aluminum hydroxide surface. In general, simulations show that alcohols adsorb more strongly than thiols on the hydroxylated surfaces, while the reverse is true on the graphite surface. The structure of adsorbed monolayers was obtained from simulations of a liquidlike layer above each surface. As expected, monolayer surface densities decreased with increasing molecule size. The hydroxylated surfaces were found to be amphoteric with respect to both alcohol and thiol adsorption, and primary adsorption sites facilitate hydrogen bonding between the adsorbate and several surface hydroxyl groups. Alcohols and thiols adsorb at much larger distances to the graphite surface, resulting in the smaller adsorption enthalpies.

Thermal Analysis of Magnesium/Perfluoropolyether Pyrolants

AbstractExothermic reactions between metals and fluorinated polymers are found in a variety of energetic materials, including reactive binder systems and the Magnesium‐Teflon‐Viton incendiary composition. This paper describes the reactions between a high molecular weight perfluoropolyether, Fomblin Y 140/13, and magnesium in a variety of morphologies including μm‐scale powders and nano‐scale layered films. Using Temperature Programmed Desorption and Temperature Programmed Reaction we have found that the magnesium‐perfluoropolyether interaction is characterized by: (1) competition between Fomblin decomposition and desorption, and (2) magnesium passivation by the formation of magnesium fluoride. Differential Scanning Calorimetry measurements establish a lower‐bound estimate of the specific reaction energy of 9.2 kJ g−1. High molecular weight Fomblin (6500 amu) undergoes a competitive reaction/desorption process with desorption occurring at 550 K and decomposition at 610 K. Decomposition becomes more favorable relative to desorption for higher heating rates and thicker films. Perfluoropolyethers produce several characteristic ions in the 70 eV election ionization mass spectra, with the CF3+ ion being the most abundant ion observed during both the molecular desorption and decomposition. Larger fragment ions with masses of 235 and 285 amu are observed in relatively high concentrations during desorption and low concentrations during decomposition. The reaction between magnesium and Fomblin begins at 400 K, producing CF3+, CO+, and C2F5+ in the electron ionization mass spectrum. We propose that these reactions form a passivating layer of magnesium fluoride that protects the remaining metal as it approaches the magnesium melting point. Most of the reaction takes place at 800 K and above when the magnesium fluoride film ruptures.

Optimizing Adsorption of Co(II) and Ni(II) by 13× Molecular Sieves Using Response Surface Methodology

Batch experiments were carried out to investigate the adsorption characteristics of Co(II) and Ni(II) by 13× molecular sieves which are hydrothermally synthetic byproducts accompanied with preparation of potassium carbonate from insoluble potash ores. The response surface methodology technique was utilized to optimize the process conditions. The combined effects of the major parameters including pH, initial concentration of metal ions, and temperature on the adsorption were investigated using central composite design. The analysis of variance of the quadratic model suggested that the predicted values were in good agreement with experimental data. The optimum conditions were found to be: initial concentration of metal ions, 20 mg/L; temperature 40 °C for both the metals; and pH, 6.29 and 8.0 for Co(II) and Ni(II), respectively. Scanning electron microscope and X-ray diffraction analyses testified to the obvious change of the surface morphology and the presence of metal on the sorbent after adsorption. The results from the sequential adsorption–desorption cycles showed that 13× molecular sieves adsorbent held good desorption and reusability.