Growth and Redox Properties of Boron on Al(111): Competing Affinities in the Case of Honeycomb AlB2

The complexity of the geometric and electronic structure of boron allotropes is associated with the multicentric bonding character and the consequent B polymorphism. When growth is limited to two-dimensions (2D), the structural and electronic confinement yields the borophenes family, where the interaction with the templating substrate actually determines the stability of inequivalent boron phases. We report here a detailed study of the growth of the honeycomb AlB2 phase on Al(111), followed by an investigation of its oxidation and reduction properties. By means of a combined experimental and theoretical approach, we show that the structure of the B/Al interface is affected by the complex interplay between B, Al, and common reactive agents like oxygen and hydrogen. While kinetic effects associated with diffusion and strain release influence the AlB2 growth in vacuo, Al, B, O, and H chemical affinities determine its redox behavior. Reduction with atomic hydrogen involves the B layer and yields an ordered honeycomb borophane H/AlB2 phase. Instead, oxidation takes place at the Al interface, giving origin to buried and 1D surface aluminum oxide phases.

From borophene polymorphs towards a single honeycomb borophane phase: reduction of hexagonal boron layers on Al(111)

The recent interest in characterizing 2D boron polymorphs has led to claims of the first stabilization of a honeycomb phase with conical Dirac-like electron dispersion. However, the synthesis of chemically stable, single, and homogeneous 2D boron phases still represents a significant experimental challenge. This is ascribed to the intrinsic boron electronic configuration that, at variance with carbon, leads to the formation of multi-center covalent bonds. External charge compensation by substrate-induced doping can steer the geometry of the layer, both in the buckling and in the density of B vacancies, like in the case of the recently achieved stabilization of honeycomb boron layers on Al(111). The price to pay is however a strong boron-support interaction, resulting in general in a limiting kinetic hindrance with respect to the synthesis of homogenous single phases. In the specific case of Al(111) an AlB2 layer is known to form at the surface, quite far from a desirable quasi-freestanding borophene monolayer and at variance with graphene, which can be easily synthesized in an almost freestanding configuration e.g. on Ir(111). We provide here evidence for the (reversible) formation of well-ordered honeycomb borophane upon hydrogenation of the honeycomb boron phase on Al(111).

The Oxygen Evolution Reaction Drives Passivity Breakdown for Ni-Cr-Mo Alloys

Corrosion is the main factor limiting the lifetime of metallic materials, and a fundamental understanding of the governing mechanism and surface processes is difficult to achieve since the thin oxide films at the metal-liquid interface governing passivity are notoriously challenging to study. In this work, a combination of synchrotron-based techniques and electrochemical methods is used to investigate the passive film breakdown of a Ni-Cr-Mo alloy, which is used in many industrial applications. This alloy is found to be active toward oxygen evolution reaction (OER), and the OER onset coincides with the loss of passivity and severe metal dissolution. The OER mechanism involves the oxidation of Mo4+ sites in the oxide film to Mo6+ that can be dissolved, which results in passivity breakdown. This is fundamentally different from typical transpassive breakdown of Cr-containing alloys where Cr6+ is postulated to be dissolved at high anodic potentials, which is not observed here. At high current densities, OER also leads to acidification of the solution near the surface, further triggering metal dissolution. The OER plays an important role in the mechanism of passivity breakdown of Ni-Cr-Mo alloys due to their catalytic activity, and this effect needs to be considered when studying the corrosion of catalytically active alloys.

In Situ Observation of C-C Coupling and Step Poisoning During the Growth of Hydrocarbon Chains on Ni(111)

The synthesis of high-value fuels and plastics starting from small hydrocarbon molecules plays a central role in the current transition towards renewable energy. However, the detailed mechanisms driving the growth of hydrocarbon chains remain to a large extent unknown. Here we investigated the formation of hydrocarbon chains resulting from acetylene polymerization on a Ni(111) model catalyst surface. Exploiting X-ray photoelectron spectroscopy up to near-ambient pressures, the intermediate species and reaction products have been identified. Complementary in situ scanning tunneling microscopy observations shed light onto the C-C coupling mechanism. While the step edges of the metal catalyst are commonly assumed to be the active sites for the C-C coupling, we showed that the polymerization occurs instead on the flat terraces of the metallic surface.

Graphene as an Adsorption Template for Studying Double Bond Activation in Catalysis

Hydrogenated graphene (H-Gr) is an extensively studied system not only because of its capabilities as a simplified model system for hydrocarbon chemistry but also because hydrogenation is a compelling method for Gr functionalization. However, knowledge of how H-Gr interacts with molecules at higher pressures and ambient conditions is lacking. Here we present experimental and theoretical evidence that room temperature O₂ exposure at millibar pressures leads to preferential removal of H dimers on H-functionalized graphene, leaving H clusters on the surface. Our density functional theory (DFT) analysis shows that the removal of H dimers is the result of water or hydrogen peroxide formation. For water formation, we show that the two H atoms in the dimer motif attack one end of the physisorbed O₂ molecule. Moreover, by comparing the reaction pathways in a vacuum with the ones on free-standing graphene and on the graphene/Ir(111) system, we find that the main role of graphene is to arrange the H atoms in geometrical positions, which facilitates the activation of the O=O double bond.

Gas Pulse-X-Ray Probe Ambient Pressure Photoelectron Spectroscopy with Submillisecond Time Resolution

A setup capable of conducting gas pulse-X-ray probe ambient pressure photoelectron spectroscopy with high time resolution is presented. The setup makes use of a fast valve that creates gas pulses with an internal pressure in the mbar range and a rising edge of few hundreds of microseconds. A gated detector based on a fast camera is synchronized with the valve operation to measure X-ray photoemission spectra with up to 20 μs time resolution. The setup is characterized in several experiments in which the N2 gas is pulsed either into vacuum or a constant flow of another gas. The observed width of the pulse rising edge is 80 μs, and the maximum internal pulse pressure is ∼1 mbar. The CO oxidation reaction over Pt (111) was used to demonstrate the capability of the setup to correlate the gas phase composition with that of the surface during transient supply of CO gas into an O2 stream. Thus, formation of both chemisorbed and oxide oxygen species was observed prior to CO gas perturbation. Also, the data indicated that both the Langmuir-Hinshelwood and Mars-van-Krevelen mechanisms play an important role in the oxidation of carbon monoxide under ambient conditions.

HIPPIE: a new platform for ambient-pressure X-ray photoelectron spectroscopy at the MAX IV Laboratory

HIPPIE is a soft X-ray beamline on the 3 GeV electron storage ring of the MAX IV Laboratory, equipped with a novel ambient-pressure X-ray photoelectron spectroscopy (APXPS) instrument. The endstation is dedicated to performing in situ and operando X-ray photoelectron spectroscopy experiments in the presence of a controlled gaseous atmosphere at pressures up to 30 mbar [1 mbar = 100 Pa] as well as under ultra-high-vacuum conditions. The photon energy range is 250 to 2200 eV in planar polarization and with photon fluxes >10¹² photons s^-1 (500 mA ring current) at a resolving power of greater than 10000 and up to a maximum of 32000. The endstation currently provides two sample environments: a catalysis cell and an electrochemical/liquid cell. The former allows APXPS measurements of solid samples in the presence of a gaseous atmosphere (with a mixture of up to eight gases and a vapour of a liquid) and simultaneous analysis of the inlet/outlet gas composition by online mass spectrometry. The latter is a more versatile setup primarily designed for APXPS at the solid-liquid (dip-and-pull setup) or liquid-gas (liquid microjet) interfaces under full electrochemical control, and it can also be used as an open port for ad hoc-designed non-standard APXPS experiments with different sample environments. The catalysis cell can be further equipped with an IR reflection-absorption spectrometer, allowing for simultaneous APXPS and IR spectroscopy of the samples. The endstation is set up to easily accommodate further sample environments.

Highlighting the Dynamics of Graphene Protection toward the Oxidation of Copper Under Operando Conditions

We performed spatially resolved near-ambient-pressure photoemission spectromicroscopy on graphene-coated copper in operando under oxidation conditions in an oxygen atmosphere (0.1 mbar). We investigated regions with bare copper and areas covered with mono- and bi-layer graphene flakes, in isobaric and isothermal experiments. The key method in this work is the combination of spatial and chemical resolution of the scanning photoemission microscope operating in a near-ambient-pressure environment, thus allowing us to overcome both the material and pressure gap typical of standard ultrahigh-vacuum X-ray photoelectron spectroscopy (XPS) and to observe in operando the protection mechanism of graphene toward copper oxidation. The ability to perform spatially resolved XPS and imaging at high pressure allows for the first time a unique characterization of the oxidation phenomenon by means of photoelectron spectromicroscopy, pushing the limits of this technique from fundamental studies to real materials under working conditions. Although bare Cu oxidizes naturally at room temperature, our results demonstrate that such a graphene coating acts as an effective barrier to prevent copper oxidation at high temperatures (over 300 °C), until oxygen intercalation beneath graphene starts from boundaries and defects. We also show that bilayer flakes can protect at even higher temperatures. The protected metallic substrate, therefore, does not suffer corrosion, preserving its metallic characteristic, making this coating appealing for any application in an aggressive atmospheric environment at high temperatures.

Wet chemistry route for the decoration of carbon nanotubes with iron oxide nanoparticles for gas sensing

In this work, we investigated the parameters for decorating multiwalled carbon nanotubes with iron oxide nanoparticles using a new, inexpensive approach based on wet chemistry. The effect of process parameters such as the solvent used, the amount of iron salt or the calcination time on the morphology, decoration density and nanocluster size were studied. With the proposed approach, the decoration density can be adjusted by selecting the appropriate ratio of carbon nanotubes/iron salt, while nanoparticle size can be modulated by controlling the calcination period. Pristine and iron-decorated carbon nanotubes were deposited on silicon substrates to investigate their gas sensing properties. It was found that loading with iron oxide nanoparticles substantially ameliorated the response towards nitrogen dioxide.

Metal-free catalysis based on nitrogen-doped carbon nanomaterials: a photoelectron spectroscopy point of view

In this review, we discuss the use of doped carbon nanomaterials in catalysis, a subject that is currently intensively studied. The availability of carbon nanotubes since the 1990's and of graphene ten years later prompted the development of novel nanotechnologies. We review this topic linking fundamental surface science to the field of catalysis giving a timely picture of the state of the art. The main scientific questions that material scientists have addressed in the last decades are described, in particular the enduring debate on the role of the different nitrogen functionalities in the catalytic activity of nitrogen-doped carbon nanotubes and graphene.

Probing the interaction between 2,2'-bithiophene-5-carboxylic acid and TiO2 by photoelectron spectroscopy: A joint experimental and theoretical study

The interaction between 2,2'-bithiophene-5-carboxylic acid (PT2) sublimed under ultra-high vacuum conditions and anatase (101) and rutile (110) TiO₂ single crystal surfaces is investigated by studying the electronic spectral density near the Fermi level with synchrotron-based spectroscopy. The experimental results are compared to density functional theory calculations of the isolated PT2 molecule and of the molecule adsorbed on an anatase TiO₂ (101) cluster. The relative concentrations of Ti, C, and S atoms indicate that the adsorbed molecule remains intact upon deposition, which is typical of a Stranski-Krastanov growth mode. The analysis of the O1s spectrum suggests a predominant bidentate geometry of the adsorption with both rutile and anatase surfaces, as supported by previous theoretical simulations. It is also theoretically and experimentally demonstrated that the PT2 adsorption causes the appearance of new electronic states in the gap near the TiO₂ valence band. A pinning effect of the LUMO level of the dye is also theoretically predicted.

Fluorination of vertically aligned carbon nanotubes: from CF4 plasma chemistry to surface functionalization

The surface chemistry of plasma fluorinated vertically aligned carbon nanotubes (vCNT) is correlated to the CF4 plasma chemical composition. The results obtained via FTIR and mass spectrometry are combined with the XPS and Raman analysis of the sample surface showing the dependence on different plasma parameters (power, time and distance from the plasma region) on the resulting fluorination. Photoemission and absorption spectroscopies are used to investigate the evolution of the electronic properties as a function of the fluorine content at the vCNT surface. The samples suffer a limited ageing effect, with a small loss of fluorine functionalities after two weeks in ambient conditions.

Probing plasma fluorinated graphene via spectromicroscopy

The graphene fluorination using CF₄ and SF₆ plasma is investigated by combining spectroscopy and microscopy techniques.

Spectromicroscopy of C60 and azafullerene C59N: Identifying surface adsorbed water

C60 fullerene crystals may serve as important catalysts for interstellar organic chemistry. To explore this possibility, the electronic structures of free-standing powders of C60 and (C59N)2 azafullerenes are characterized using X-ray microscopy with near-edge X-ray adsorption fine structure (NEXAFS) spectroscopy, closely coupled with density functional theory (DFT) calculations. This is supported with X-ray photoelectron spectroscopy (XPS) measurements and associated core-level shift DFT calculations. We compare the oxygen 1s spectra from oxygen impurities in C60 and C59N, and calculate a range of possible oxidized and hydroxylated structures and associated formation barriers. These results allow us to propose a model for the oxygen present in these samples, notably the importance of water surface adsorption and possible ice formation. Water adsorption on C60 crystal surfaces may prove important for astrobiological studies of interstellar amino acid formation.

Exfoliation of graphite and graphite oxide in water by chlorin e6

An ultrasonic process for the exfoliation of graphite and graphite oxide in water was devised for the production of chlorine e₆ nanohybrids with remarkable potential applications in energy and biomedicine.

Graphene-Induced Magnetic Anisotropy of a Two-Dimensional Iron Phthalocyanine Network

A single layer of flat-lying iron phthalocyanine (FePc) molecules assembled on graphene grown on Ir(111) preserves the magnetic moment, as deduced by X-ray magnetic circular dichroism from the Fe L2,3 edges. Furthermore, the FePc molecules in contact with the graphene buffer layer exhibit an enhancement of the magnetic anisotropy, with emergence of an in-plane easy magnetic axis, reflected by an increased orbital moment of the FePc molecules in contact with the C atoms in the graphene sheet. The origin of the increased magnetic anisotropy is discussed, considering the absence of electronic state hybridization, and the breaking of symmetry upon FePc adsorption on graphene.

Nitrogen segregation in nanocarbons

We explore the behaviour of nitrogen doping in carbon nanomaterials, notably graphene, nanotubes, and carbon thin films. This is initially via a brief review of the literature, followed by a series of atomistic density functional calculations. We show that at low concentrations, substitutional nitrogen doping in the sp(2)-C graphenic basal plane is favoured, however once the nitrogen concentration reaches a critical threshold there is a transition towards the formation of the more thermodynamically-favoured nitrogen terminated 'zigzag' type edges. These can occur either via formation of finite patches (polycyclic aromatic azacarbons), strips of sp(2) carbon with zigzag nitrogen edges, or internal nitrogen-terminated hole edges within graphenic planes. This transition to edge formation is especially favoured when the nitrogen can be partially functionalised with, e.g. hydrogen. By comparison with available literature results, notably from electron energy loss spectroscopy and X-ray spectroscopy, the current results suggest that much of the nitrogen believed to be incorporated into carbon nanoobjects is instead likely to be present terminating the edges of carbonaceous impurities attached to nanoobject's surface. By comparison to nitrogen-doped tetrahedrally amorphous carbon, we suggest that this transition at around 10-20% nitrogen concentration and above towards sp(2) coordination via internal nitrogen-terminated edge formation may be a general property of nitrogen-doped carbon materials.

Sonication-induced effects on carbon nanofibres in composite materials

The preparation, characterization and filling of carbon nanofibre–gellan gum composite materials is presented.

Plasma fluorination of vertically aligned carbon nanotubes: functionalization and thermal stability

Grafting of fluorine species on carbon nanostructures has attracted interest due to the effective modification of physical and chemical properties of the starting materials. Various techniques have been employed to achieve a controlled fluorination yield; however, the effect of contaminants is rarely discussed, although they are often present. In the present work, the fluorination of vertically aligned multiwalled carbon nanotubes was performed using plasma treatment in a magnetron sputtering chamber with fluorine diluted in an argon atmosphere with an Ar/F2 ratio of 95:5. The effect of heavily diluted fluorine in the precursor gas mixture is investigated by evaluating the modifications in the nanotube structure and the electronic properties upon plasma treatment. The existence of oxygen-based grafted species is associated with background oxygen species present in the plasma chamber in addition to fluorine. The thermal stability and desorption process of the fluorine species grafted on the carbon nanotubes during the fluorine plasma treatment were evaluated by combining different spectroscopic techniques.

Gas sensing with gold-decorated vertically aligned carbon nanotubes

Vertically aligned carbon nanotubes of different lengths (150, 300, 500 µm) synthesized by thermal chemical vapor deposition and decorated with gold nanoparticles were investigated as gas sensitive materials for detecting nitrogen dioxide (NO2) at room temperature. Gold nanoparticles of about 6 nm in diameter were sputtered on the top surface of the carbon nanotube forests to enhance the sensitivity to the pollutant gas. We showed that the sensing response to nitrogen dioxide depends on the nanotube length. The optimum was found to be 300 µm for getting the higher response. When the background humidity level was changed from dry to 50% relative humidity, an increase in the response to NO2 was observed for all the sensors, regardless of the nanotube length.

Energetics and hierarchical interactions of metal-phthalocyanines adsorbed on graphene/Ir(111)

The adsorption of metal-phthalocyanine (MPc) layers (M = Fe, Co, Cu) assembled on graphene/Ir(111) is studied by means of temperature-programmed X-ray photoemission spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS). The balance between interaction forces among the organometallic molecules and the underlying graphene gives rise to flat-lying molecular layers, weakly interacting with the underlying graphene. Further MPc layers pile up face-on onto the first layer, up to a few nanometers thickness, as deduced by NEXAFS. The FePc, CoPc, and CuPc multilayers present comparable desorption temperatures, compatible with molecule-molecule interactions dominated by van der Waals forces between the π-conjugated macrocycles. The MPc single layers desorb from graphene/Ir at higher temperatures. The CuPc single layer desorbs at lower temperature than the FePc and CoPc single layers, suggesting a higher adsorption energy of the FePc and CoPc single layers on graphene/Ir with respect to CuPc, with increasing molecule-substrate interaction in the order E(CuPc) < E(FePc) ~ E(CoPc).

Nonenzymatic ligation of an RNA oligonucleotide analyzed by atomic force microscopy

The products of ligation reaction of a 24 nucleotides long PolyA RNA adsorbed on mica were observed by atomic force microscopy. The occurrence of oligonucleotides at different degrees of polymerization has been quantitatively studied before and after ligation reaction. The microscopy images at the nanoscale show that nonenzymatic ligation of pristine RNA monomers results in the formation of supramolecular aggregates, with prevalence of dimers and tetramers. Analytical conditions were defined allowing the identification, the quantitative evaluation, and their distribution after ligation reaction, also providing an estimate of the degree of hydration of the objects. Such investigation is of particular biological relevance and provides the simplest yet model system for direct investigation of RNA reactions by advanced microscopy.

Application of the Roche Amplicor Mycobacterium tuberculosis (PCR) test to specimens other than respiratory secretions

The ability of the Roche AMPLICOR Mycobacterium tuberculosis (MTB) test to detect M. tuberculosis in specimens other than respiratory secretions was evaluated. A total of 249 specimens from 219 patients were tested. Of these, 12 specimens grew isolates of the M. tuberculosis complex and four grew isolates of the M. avium complex. The AMPLICOR MTB test was positive for 10 of the 12 specimens which grew M. tuberculosis and for three specimens which were culture negative. Two of the latter specimens were from patients with a clinical diagnosis of tuberculosis and with multiple sputum specimens which grew M. tuberculosis. Four specimens grew M. avium complex isolates, and all yielded negative AMPLICOR MTB test results. The sensitivity, specificity, and positive and negative predictive values for the AMPLICOR MTB test were 85.7%, 99.5%, 92.3%, and 99.1%, respectively. Our data indicate that the AMPLICOR MTB test will permit the rapid detection of M. tuberculosis in specimens other than respiratory secretions.

Rapid diagnosis of pulmonary tuberculosis by using Roche AMPLICOR Mycobacterium tuberculosis PCR test

A rapid PCR-based test for the diagnosis of pulmonary tuberculosis, the Roche AMPLICOR Mycobacterium tuberculosis test (AMPLICOR MTB), was evaluated. Results from AMPLICOR MTB were compared with culture results and the final clinical diagnosis for each patient. A total of 985 specimens from 372 patients were tested. When AMPLICOR MTB results were compared with resolved results, i.e., a specimen grew M. tuberculosis or was obtained from a patient with a clinical diagnosis of tuberculosis, the sensitivity, specificity, positive predictive value, and negative predictive value for the AMPLICOR MTB test were 66.7, 99.6, 91.7, and 97.7%, respectively. These results were comparable to those obtained from culture. Test results were available approximately 6.5 h after specimen receipt in the laboratory. Our data demonstrate that AMPLICOR MTB will provide rapid, valuable information for the diagnosis and control of tuberculosis.

Collaborative feasibility study of a biphasic system (Roche Septi-Chek AFB) for rapid detection and isolation of mycobacteria

A study to delineate the feasibility of a biphasic-culture approach for detection and isolation of mycobacteria from clinical specimens except blood was conducted in four medical centers. The biphasic system (Septi-Chek AFB, Roche Diagnostic Systems, Nutley, N.J.) was compared with conventional mycobacterial isolation media and the BACTEC system. Septi-Chek AFB showed the highest degree of mycobacterial recovery. In addition, Septi-Chek AFB consistently shortened the time required for recovery of mycobacteria from clinical specimens and supported the growth of small inoculum numbers of stock cultures of 14 mycobacterial species. The study indicates the feasibility and potential advantages of the biphasic approach for detection and isolation of mycobacteria.