Study of adsorption and decomposition of H2O on Ge(100)

The adsorption and decomposition of water on Ge(100) have been investigated using real-time scanning tunneling microscopy (STM) and density-functional theory (DFT) calculations. The STM results revealed two distinct adsorption features of H2O on Ge(100) corresponding to molecular adsorption and H-OH dissociative adsorption. In the molecular adsorption geometry, H2O molecules are bound to the surface via Ge-O dative bonds between the O atom of H2O and the electrophilic down atom of the Ge dimer. In the dissociative adsorption geometry, the H2O molecule dissociates into H and OH, which bind covalently to a Ge-Ge dimer on Ge(100) in an H-Ge-Ge-OH configuration. The DFT calculations showed that the dissociative adsorption geometry is more stable than the molecular adsorption geometry. This finding is consistent with the STM results, which showed that the dissociative product becomes dominant as the H2O coverage is increased. The simulated STM images agreed very well with the experimental images. In the real-time STM experiments, we also observed a structural transformation of the H2O molecule from the molecular adsorption to the dissociative adsorption geometry.     

金属-有机配合物分子在Au(111)表面的吸附行为

用STM对含氧桥的金属-有机配合物[Cu₂(μ-O)(dptap)4(NO₃)₂]分子在Au(111)表面的吸附行为进行了研究. STM结果表明, 该分子同时存在非解离吸附和解离吸附, 大部分分子在Au(111)面形成有规则的排列, 少量分子发生解离吸附, 并形成(√3×√3)R30°Cu原子吸附结构. 探讨了两种吸附现象共存的起因.     

Magnetic graphene oxide as adsorbent for the determination of polycyclic aromatic hydrocarbon metabolites in human urine

Detection of monohydroxy polycyclic aromatic hydrocarbons metabolites in urine is an advisable and valid method to assess human environmental exposure to polycyclic aromatic hydrocarbons. In this work, novel Fe₃O₄/graphene oxide composites were prepared and their application in the magnetic solid‐phase extraction of monohydroxy polycyclic aromatic hydrocarbons in urine was investigated by coupling with liquid chromatography and mass spectrometry. In the hybrid material, superparamagnetic Fe₃O₄ nanoparticles provide fast separation to simplify the analytical process and graphene oxide provides a large functional surface for the adsorption. The prepared magnetic nanocomposites were characterized by X‐ray diffraction, scanning electron microscopy, transmission electron microscopy, and vibrating sample magnetometry. The experimental conditions were optimized systematically. Under the optimal conditions, the recoveries of these compounds were in the range of 98.3–125.2%, the relative standard deviations ranged between 6.8 and 15.5%, and the limits of detection were in the range of 0.01–0.15 ng/mL. The simple, quick, and affordable method was successfully used in the analysis of human urinary monohydroxy polycyclic aromatic hydrocarbons in two different cities. The results indicated that the monohydroxy polycyclic aromatic hydrocarbons level in human urine can provide useful information for environmental exposure to polycyclic aromatic hydrocarbons.     

Adsorption of Large Organic Molecules on Clean and Hydroxylated Rutile TiO2(110) Surfaces

Behavior of large organic molecules equipped with spacer groups (Violet Landers, VL) on the TiO₂(110)‐(1×1) surfaces is investigated by means of high‐resolution scanning tunneling microscopy (STM). Two distinct adsorption geometries are observed. We demonstrate that the molecule adsorption morphology can be alternated by well‐controlled STM tip‐induced manipulation. It is used to probe the mobility of molecules and reveals locking in one of the analyzed adsorption sites, thus allow to enhance or reduce the mobility along the [001] direction. Field induced hydrogen desorption is used to perform lateral STM manipulation on a hydroxyl‐free surface, which provides insight into the influence of surface hydroxyl groups on the molecule behavior. The ability to image with submolecular resolution both the central board and the spacer groups of the VL molecule is demonstrated.     

Scanning tunneling microscope imaging of (CH3S)2 on Cu(111)

Scanning tunneling microscope (STM) images of isolated molecules of dimethyl disulfide, (CH(3)S)(2), adsorbed on the Cu(111) surface were successfully obtained at a sample temperature of 4.7 K. A (CH(3)S)(2) molecule appears as an elliptic protrusion in the STM images. From density functional theory calculation, it was suggested that the bright part in the protrusion corresponds to the molecular orbital which is widely spread around H atoms in each CH(3) group in the (CH(3)S)(2) molecule. The STM images revealed that the molecules have a total of six equivalent adsorption orientations on Cu(111), which are given by the combination of three equivalent adsorption sites and two conformational isomers for each adsorption site.     

Current-voltage characteristics of a homologous series of polycyclic aromatic hydrocarbons

A novel alkyl-substituted polycyclic aromatic hydrocarbon (PAH) with D(2h) symmetry and 78 carbon atoms in the aromatic core (C78) was synthesized, thereby completing a homologous series of soluble PAH compounds with increasing size of the aromatic pi system (42, 60, and 78 carbon atoms). The optical band gaps were determined by UV/Vis and fluorescence spectroscopy in solution. Scanning tunneling microscopy (STM) and spectroscopy (STS) revealed diode-like current versus voltage (I-V) characteristics through individual aromatic cores in monolayers at the interface between the solution and the basal plane of graphite. The asymmetry of the current-voltage (I-V) characteristics increases with the increasing size of the aromatic core, and the concomitantly decreasing HOMO-LUMO gap. This is attributed to resonant tunneling through the HOMO of the adsorbed molecule, and an asymmetric position of the molecular species in the tunnel junction. Consistently, submolecularly resolved STM images at negative substrate bias are in good agreement with the calculated pattern for the electron densities of the HOMOs. The analysis provides the basis for tailoring rectification with a single molecule in an STM junction.     

An ONIOM and DFT study of water adsorption on rutile TiO2 (110) cluster

Density functional theory (DFT) calculations performed at ONIOM DFT B3LYP/6‐31G**‐MD/UFF level are employed to study molecular and dissociative water adsorption on rutile TiO₂ (110) surface represented by partially relaxed Ti₂₅O₃₇ ONIOM cluster. DFT calculations indicate that dissociative water adsorption is not favorable because of high activation barrier (23.2 kcal/mol). The adsorption energy and vibration frequency of both molecularly and dissociatively adsorbed water molecule on rutile TiO₂ (110) surface compare well with the values reported in the literature. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Interaction of O2, CO and CO2 with Co films

Metastable impact electron spectroscopy (MIES), ultraviolet photoelectron spectroscopy (UPS(HeI)) and x‐ray photoelectron spectroscopy (XPS) were applied to study the interaction of O₂, CO and CO₂ with Co films at room temperature. The films were produced on Si(100) surfaces under the in situ control of MIES, UPS and scanning tunnelling microscopy (STM). For O₂, dissociative adsorption takes place initially and then incorporation of oxygen starts at exposures of ～5 L. Comparison of the MIES and UPS spectra with those published for CoO shows that near‐stoichiometric CoO films can be obtained by co‐deposition of Co and O₂. The CO is adsorbed molecularly up to a maximum coverage of ～0.6 monolayer, with the C‐end pointing towards the surface. The CO₂ adsorption is dissociative, resulting in the formation of Co–CO bonds at the surface. The resulting oxygen atoms are mostly incorporated into the Co layer. For all studied molecules the interaction with Co is similar to that with Ni. Copyright © 2005 John Wiley & Sons, Ltd.     

Modeling formation of molecules in the interstellar medium by radical reactions with polycyclic aromatic hydrocarbons

Adsorptions of CH°₂, CH°₃, NH°₂, and OH° radicals and molecule formation on a partially hydrogenated surface of a polycyclic aromatic hydrocarbon (PAH) (C₂₄H₂₇⁺) were modeled. It was found that radical adsorptions are feasible with important modifications of surface bond strengths and bond distances. Adsorbed hydrogen may diffuse due to adsorbate‐surface interactions. Formations of CH₄, NH₃, H₂O, CH₃NH₂, and CH₃OH were studied by Eley‐Rideal (ER) and Langmuir‐Hishelwood (LH) mechanisms. Potential energetic surfaces were performed for both mechanisms and the ER presents lower reaction energy barriers than the LH one, in all cases. Parametric quantum program (CATIVIC) was employed and comparisons with DFT results were performed. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem 110:2560–2572, 2010     

Using mass defect plots as a discovery tool to identify novel fluoropolymer thermal decomposition products

Fire events involving halogenated materials, such as plastics and electronics, produce complex mixtures that include unidentified toxic and environmentally persistent contaminants. Ultrahigh‐resolution mass spectrometry and mass defect filtering can facilitate compound identification within these complex mixtures. In this study, thermal decomposition products of polychlorotrifluoroethylene (PCTFE, [–CClF–CF₂–]_n), a common commercial polymer, were analyzed by Fourier transform ion cyclotron resonance mass spectrometry. Using the mass defect plot as a guide, novel PCTFE thermal decomposition products were identified, including 29 perhalogenated carboxylic acid (PXCA, X = Cl,F) congener classes and 21 chlorine/fluorine substituted polycyclic aromatic hydrocarbon (X‐PAH, X = Cl,F) congener classes. This study showcases the complexity of fluoropolymer thermal decomposition and the potential of mass defect filtering to characterize complex environmental samples. Copyright © 2014 John Wiley & Sons, Ltd.     

The reactive chemisorption of alkyl iodides at Cu(110) and Ag(111) surfaces: a combined STM and XPS study

The chemisorption of methyl and phenyl iodide has been studied at Cu(110) and Ag(111) surfaces at 290 K with STM and XPS. At both surfaces dissociative adsorption of both molecules leads to chemisorbed iodine, with the STM showing c(2 x 2) and (square root 3 x square root 3)R30 structures at the Cu(110) and Ag(111) surfaces, respectively. At the Cu(110) surface a comparison of coexisting c(2 x 2) I(a) and p(2 x 1) O(a) domains shows the iodine adatoms to be chemisorbed in hollow sites with evidence at low coverage for diffusion in the (110) direction. In the case of methyl iodide no carbon adsorption is observed at either the silver or the copper surfaces, but chemisorbed phenyl groups are imaged at the Cu(110) surface after exposure to phenyl iodide. The STM images show the phenyl groups as bright features approximately 0.7 nm in diameter and 0.11 nm above the iodine adlayer, reaching a maximum surface concentration after approximately 6 Langmuir exposure. However, the phenyl coverage decreases with subsequent exposures to PhI and is negligible by approximately 1000 L exposure, consistent with the formation and desorption of biphenyl. The adsorbed phenyls are located above hollow sites in the substrate, they are stabilized at the top and bottom of step edges and in paired chains (1.1 nm apart) on the terraces with a regular interphenyl spacing within the chains of 1.0 nm in the (110) direction. The interphenyl ring spacing and diffusion of individual phenyls from within the chains shows that the chains do not consist of biphenyl species but may be a precursor to their formation. Although the XPS data shows carbon present at the Ag(111) surface after exposure to PhI, no features attributable to phenyl groups were observed by STM.     

DFT Simulations of Water Adsorption and Activation on Low‐Index α‐Ga2O3 Surfaces

Density functional theory (DFT) calculations are used to explore water adsorption and activation on different α‐Ga₂O₃ surfaces, namely (001), (100), (110), and (012). The geometries and binding energies of molecular and dissociative adsorption are studied as a function of coverage. The simulations reveal that dissociative water adsorption on all the studied low‐index surfaces are thermodynamically favorable. Analysis of surface energies suggests that the most preferentially exposed surface is (012). The contribution of surface relaxation to the respective surface energies is significant. Calculations of electron local density of states indicate that the electron‐energy band gaps for the four investigated surfaces appears to be less related to the difference in coordinative unsaturation of the surface atoms, but rather to changes in the ionicity of the surface chemical bonds. The electrochemical computation is used to investigate the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) on α‐Ga₂O₃ surfaces. Our results indicate that the (100) and (110) surfaces, which have low stability, are the most favorable ones for HER and OER, respectively.     

Loading controlled magnetic carbon dots for microwave‐assisted solid‐phase extraction: Preparation,extraction evaluation and applications in environmental aqueous samples

An adsorbent of carbon dot@poly(glycidyl methacrylate)@Fe₃O₄ nanoparticles has been developed for the microwave‐assisted magnetic solid‐phase extraction of polycyclic aromatic hydrocarbons in environmental aqueous samples prior to high‐performance liquid chromatography with UV/visible spectroscopy detection. Poly(glycidyl methacrylate) was synthesized by atom transfer radical polymerization. The chain length and amount of carbon dots attached on them can be easily controlled through changing polymerization conditions, which contributes to tunable extraction performance. The successful fabrication of the nano‐adsorbent was confirmed by transmission electronic microscopy, Fourier transform infrared spectroscopy, Raman spectroscopy and vibrating sample magnetometry. The extraction performance of the adsorbent was evaluated by using polycyclic aromatic hydrocarbons as model analytes. The key factors influencing the extraction, such as microwave power, adsorption time, desorption time and desorption solvents were investigated in detail. Under the optimal conditions, the microwave‐assisted method afforded magnetic solid‐phase extraction with short extraction time, wide dynamic linear range (0.02–200 μg/L), good linearity (R² ≥ 98.57%) and low detection limits (20–90 ng/L) for model analytes. The adsorbent was successfully applied for analyzing polycyclic aromatic hydrocarbons in environmental aqueous samples and the recoveries were in the range of 86.0–124.2%. Thus, the proposed method is a promising candidate for fast and reliable preconcentration of trace polycyclic aromatic hydrocarbons in real water samples.     

Theoretical study on the adsorption mechanism of iodine molecule on platinum surface in dye-sensitized solar cells

By means of density functional theory calculations, the adsorption process of I₂ at Pt (111) surface in dye-sensitized solar cells (DSSCs) has been investigated. The obtained adsorption energies and stable structures depending on the adsorption sites of the Pt surface are in good agreement with experimental values. Our results show that the dissociative chemisorption and the non-dissociative chemisorption are competitive for the adsorption of I₂ on the Pt surface, and the dissociative pathway is more favored in energy. This study is expected to enrich the understanding on the origin of the excellent heterogeneous catalytic performance of Pt for triiodide reduction and the complex iodine chemistry in DSSCs. Understanding of this adsorption mechanism is helpful for rational screening for redox couple and the Pt-free alternative counter electrode materials.     

Determination of trace hydroxyl polycyclic aromatic hydrocarbons in urine using graphene oxide incorporated monolith solid‐phase extraction coupled with LC‐MS/MS

The biomonitoring of hydroxy polycyclic aromatic hydrocarbons in urine, as a direct way to access multiple exposures to polycyclic aromatic hydrocarbons, has raised great concerns due to their increasing hazardous health effects on humans. Solid‐phase extraction is an effective and useful technique to preconcentrate trace analytes from biological samples. Here, we report a novel solid‐phase extraction method using a graphene oxide incorporated monolithic syringe for the determination of six hydroxy polycyclic aromatic hydrocarbons in urine coupled with liquid chromatography‐tandem mass spectrometry. The effect of graphene oxide amount, washing solvent, eluting solvent, and its volume on the extraction performance were investigated. The fabricated monoliths gave higher adsorption efficiency and capacity than the neat polymer monolith and commercial C₁₈ sorbent. Under the optimum conditions, the developed method provided the detection limits (S/N = 3) of 0.02–0.1 ng/mL and the linear ranges of 0.1–1500 ng/mL for six analytes in urine sample. The recoveries at three spiked levels ranged from 77.5 to 97.1%. Besides, the intra column‐to‐column (n = 3) and inter batch‐to‐batch (n = 3) precisions were ≤ 9.8%. The developed method was successfully applied for the determination of hydroxy polycyclic aromatic hydrocarbons in urine samples of coke oven workers.     

Dissociative adsorption of molecular hydrogen onto Pt<Subscript>6</Subscript> and Pt<Subscript>19</Subscript> platinum clusters located on the tin dioxide surface: Quantum-chemical modeling

It is suggested that, for the operation of platinum catalysts based on tin dioxide in air hydrogen fuel cells, hydrogen spillover (migration) leading to a change in the electron and proton contributions of the catalyst conductivity is of crucial importance. The hydrogen adsorption, dissociation, and migration in the platinum-tin dioxide-hydrogen system surface have been modeled by the density functional theory method within the generalized gradient approximation (GGA) under periodic conditions using a projector-augmented plane-wave (PAW) basis set with a pseudopotential. It has been demonstrated that the adsorption energy of a hydrogen molecule onto a platinum cluster increases from 1.6 to 2.4 eV as the distance to the SnO₂ substrate decreases. The calculated Pt-H bond length for adsorbed structures is 1.58–1.78 Å. The computer modeling has demonstrated that: (1) the hydrogen adsorption energy on clusters is higher than on the perfect platinum surface; (2) dissociative chemisorption onto Pt _n clusters can occur without a barrier and depends on the adsorption site and the cluster structure; (3) the adsorption energy of hydrogen onto the SnO₂ surface is higher than the adsorption energy onto the platinum cluster surface: (4) multiple H₂ dissociation on the tin dioxide surface occurs with a barrier; (5) the dissociation adsorption of hydrogen molecules onto the platinum cluster surface followed by atom migration (spillover) is energetically favorable.     

Influence of the Number of Anchoring Groups on the Electronic and Mechanical Properties of Benzene‐, Anthracene‐ and Pentacene‐Based Molecular Devices

One of the central issues of molecular electronics (ME) is the study of the molecule–metal electrode contacts, and their implications for the conductivity, charge‐transport mechanism, and mechanical stability. In fact, stochastic on/off switching (blinking) reported in STM experiments is a major problem of single‐molecule devices, and challenges the stability and reliability of these systems. Surprisingly, the ambiguous STM results all originate from devices that bind to the metallic electrode through a one‐atom connection. In the present work, DFT is employed to study and compare the properties of a set of simple acenes that bind to metallic electrodes with an increasing number of connections, in order to determine whether the increasing numbers of anchoring groups have a direct repercussion on the stability of these systems. The conductivities of the three polycyclic aromatic hydrocarbons are calculated, as well as their transmission spectra and current profiles. The thermal and mechanical stability of these systems is studied by pulling and pushing the metal–molecule connection. The results show that molecules with more than one connection per electrode exhibit greater electrical efficiency and current stability.     

Application of sunflower stalk‐carbon nitride nanosheets as a green sorbent in the solid‐phase extraction of polycyclic aromatic hydrocarbons followed by high‐performance liquid chromatography

A green biocomposite of sunflower stalks and graphitic carbon nitride nanosheets has been applied as a solid‐phase extraction adsorbent for sample preparation of five polycyclic aromatic hydrocarbons in different solutions using high‐performance liquid chromatography with ultraviolet detection. Before the modification, sunflower stalks exhibited relatively low adsorption to the polycyclic aromatic hydrocarbons extraction. The modified sunflower stalks showed increased adsorption to the analytes extraction due to the increase in surface and existence of a π–π interaction between the analytes and graphitic carbon nitride nanosheets on the surface. Under the optimal conditions, the limits of detection and quantification for five polycyclic aromatic hydrocarbons compounds could reach 0.4–32 and 1.2–95 ng/L, respectively. The method accuracy was evaluated using recovery measurements in spiked real samples and good recoveries from 71 to 115% with relative standard deviations of <10% have been achieved. The developed method was successfully applied for polycyclic aromatic hydrocarbons determination in various samples—well water, tap water, soil, vegetable, and barbequed meat (kebab)—with analytes contents ranging from 0.065 to 13.3 μg/L. The prepared green composite as a new sorbent has some advantages including ease of preparation, low cost, and good reusability.     

Tight Molecular Recognition of Benzo[a]pyrene by a High‐Affinity Antibody

Benzo[a ]pyrene, which is produced during the incomplete combustion of organic material, is an abundant noxious pollutant because of its carcinogenic metabolic degradation products. The high‐affinity (K _D≈3 nm ) monoclonal antibody 22F12 allows facile bioanalytical quantification of benzo[a ]pyrene even in complex matrices. We report the functional and X‐ray crystallographic analysis of 22F12 in complex with 3‐hydroxybenzo[a ]pyrene after cloning of the V‐genes and production as a recombinant Fab fragment. The polycyclic aromatic hydrocarbon is bound in a deep pocket between the light and heavy chains, surrounded mainly by aromatic and aliphatic amino acid side chains. Interestingly, the hapten–antibody interface is less densely packed than expected and reveals polar, H‐bond‐like interactions with the polycyclic aromatic π‐electron system, which may allow the antibody to maintain a large, predominantly hydrophobic binding site in an aqueous environment while providing sufficient complementarity to its ligand.     

Step‐Mediated Anisotropic Adsorption and Condensation of tert‐Butylamine on Cu(111)

Scanning tunneling microscopy (STM) combined with density functional theory (DFT) calculations were applied in studying the anisotropic adsorption and condensation of tert‐butylamine (t‐BA) molecules in the vicinity of the steps on the Cu(111) surface. The preferential adsorption at the upper step edges and uneven distribution of t‐BA in the vicinity of the steps illustrate the asymmetric electronic structure of the surface steps. Our observation demonstrates that the adsorption and diffusion of a polar molecule would be significantly mediated by steps on metal surfaces due to the molecule–step interaction and the intermolecular interactions.