How magnetism emerges in low-dimensional materials such as transition metal dichalcogenides at the monolayer limit is still an open question. Herein, we present a comprehensive study of the magnetic ...properties of single-crystal and monolayer VSe2, both experimentally and ab initio. Magnetometry, X-ray magnetic circular dichrosim (XMCD), and ab initio calculations demonstrate that the charge density wave in bulk stoichiometric VSe2.0 causes a structural distortion with a strong reduction in the density of states at the Fermi level, prompting the system toward a nonmagnetic state but on the verge of a ferromagnetic instability. In the monolayer limit, the structural rearrangement induces a Peierls distortion with the opening of an energy gap at the Fermi level and the absence of magnetic order. Control experiments on defect-induced VSe2−δ single crystals show a breakdown of magnetism, discarding vacancies as a possible origin of magnetic order in VSe2.
In spite of the fact that the formic acid oxidation reaction on electrode surfaces has been extensively investigated, a detailed mechanism explaining all the available experimental evidence on ...platinum has not been yet described. Herein, using a combined experimental and computational approach, the key elements in the mechanism of the formic acid oxidation reaction on platinum have been completely elucidated, not only for the direct path, through an active intermediate, but also for the CO formation route. The experimental results suggest that the direct oxidation path on platinum takes place in the presence of bidentate adsorbed formate. However, the results reported here provide evidence that this species is not the active intermediate. Monodentate adsorbed formate, whose evolution to the much more favorable bidentate form would be hindered by the presence of neighboring adsorbates, has been found to be the true active intermediate. Moreover, it is found that adsorbed formic acid would have a higher acid constant than in solution, which suggests that adsorbed formate can be originated not only from solution formate but also from formic acid. The CO formation path on platinum can proceed, also from monodentate adsorbed formate, through a dehydrogenation process toward the surface, during which the adsorbate transitions from a Pt-O adsorption mode to a Pt-C one, to form carboxylate. From this last configuration, the C-OH bond is cleaved, on the surface, yielding adsorbed CO and OH. The results and mechanisms reported here provide the best explanation for the whole of the experimental evidence available to date about this reaction, including pH, surface structure and electrode potential effects.
The key elements in the mechanism of the formic acid oxidation reaction on platinum have been completely elucidated, not only for the direct path through an active intermediate, but also for the CO formation route.
Aims. We present the novel InterStellar Astrochemistry Chamber (ISAC), designed for studying solids (ice mantles, organics, and silicates) in interstellar and circumstellar environments: ...characterizing their physico-chemical properties and monitoring their evolution as caused by (i) vacuum-UV irradiation; (ii) cosmic ray irradiation; and (iii) thermal processing. Experimental study of thermal and photodesorption of the CO ice reported here simulates the freeze-out and desorption of CO on grains, providing new information on these processes. Methods. ISAC is an UHV set-up, with base pressure down to P = 2.5 × 10-11 mbar, where an ice layer is deposited at 7 K and can be UV-irradiated. The evolution of the solid sample was monitored by in situ transmittance FTIR spectroscopy, while the volatile species were monitored by QMS. Results. The UHV conditions of ISAC allow experiments under extremely clean conditions. Transmittance FTIR spectroscopy coupled to QMS proved to be ideal for in situ monitoring of ice processes that include radiation and thermal annealing. Thermal desorption of CO starting at 15 K, induced by the release of H2 from the CO ice, was observed. We measured the photodesorption yield of CO ice per incident photon at 7, 8, and 15 K, respectively yielding 6.4 ± 0.5 × 10-2, 5.4 ± 0.5 × 10-2, and 3.5 ± 0.5 × 10-2 CO molecules photon (7.3–10.5 eV)-1. Our value of the photodesorption yield of CO ice at 15 K is about one order of magnitude higher than the previous estimate. We confirmed that the photodesorption yield is constant during irradiation and independent of the ice thickness. Only below ~ 5 monolayers ice thickness the photodesorption rate decreases, which suggests that only the UV photons absorbed in the top 5 monolayers led to photodesorption. The measured CO photodesorption quantum yield at 7 K per absorbed photon in the top 5 monolayers is 3.4 molecules photon-1. Conclusions. Experimental values were used as input for a simple model of a quiescent cloud interior. Photodesorption seems to explain the observations of CO in the gas phase for densities below 3–7 × 104 cm-3. For the same density of a cloud, 3 × 104 cm-3, thermal desorption of CO is not triggered until T = 14.5 K. This has important implications for CO ice mantle build up in dark clouds.
The co-adsorption of CO and OH on two Pt stepped surfaces vicinal to the (111) orientation has been evaluated by means of density functional theory (DFT) calculations. Focusing on Pt(533) and ...Pt(221), which contain (100) and (111)-steps, respectively, we find that (111)-steps should be more reactive towards CO oxidation than surfaces containing (100)-steps. The DFT results are compared with electrochemical experiments on the CO adsorption and oxidation on these vicinal surfaces.
The electrooxidation of CO on stepped Pt(111) surfaces has been investigated both experimentally and theoretically, revealing a high reactivity for surface containing (111)-steps.
Abstract
Intumescent paints’ behaviour is investigated for protecting wood pre-treated with phosphate or a silicate-based flame retardant. Additionally, the ageing effects of intumescent paint under ...several conditions are investigated. The cone calorimeter is applied, measuring the reaction to heat responses and the char development. Pre-treatment showed an improved fire performance of samples with intumescent coatings. The effects depend on the moisture contents and the homogeneity of the pre-treatments. Also, the experiments showed differences in fire resistance depending on the ageing method. UV exposure ageing retained the heat-insulating properties, while samples immersed in acid suffered a drastic reduction of their fire resistance. The latter ageing test increased the char’s pore size resulting in a reduced expansion factor and reduced thermal insulation performance of the intumescent coating.
The electronic structure origins of interfacial losses in hematite photoanodes and the cause of the literature-reported order-of-magnitude photocurrent increase upon short high-temperature annealing ...are investigated. Synchrotron-based soft X-ray absorption spectroscopy is used to probe the unoccupied states at and near the interface between hematite (α-Fe2O3) and fluorine-doped tin oxide (FTO). Oxygen K-edge and iron L-edge absorption spectra indicate that the interfacial interaction reduces the degree of p–d hybridization and alters the crystal field in α-Fe2O3. The interface is found to be associated with a distribution of unoccupied oxygen p-hybridized states located below the lowest unoccupied iron 3d states in α-Fe2O3 (just below the conduction band minimum), which are eliminated with high-temperature processing. These data facilitate future efforts to engineer favorable interfacial compositions and associated electrochemical potential gradients within photoanodes, which are required to efficiently separate charge carriers in operating photoelectrochemical systems such as solar cells and photocatalytic devices.
A set of density-functional theory based tools is employed to elucidate the influence of chemical and surface-induced changes on the core level shifts of X-ray photoelectron spectroscopy experiments. ...The capabilities of our tools are demonstrated by analyzing the origin of an unpredicted component in the N 1s core level spectra of metal phthalocyanine molecules (in particular ZnPc) adsorbed on Cu(110). We address surface induced effects, such as splitting of the lowest unoccupied molecular orbital or local electrostatic effects, demonstrating that these cannot account for the huge core level shift measured experimentally. Our calculations also show that, when adsorbed at low temperatures, these molecules might capture hydrogen atoms from the surface, giving rise to hydrogenated molecular species and, consequently, to an extra component in the molecular core level spectra. Only upon annealing, and subsequent hydrogen release, would the molecules recover their nominal structural and electronic properties.
Formic acid electrooxidation on Tl modified Pt single crystal electrodes has been carried out in sulfuric acid media. Voltammetric experiments demonstrated that Pt(100) modified by Tl displays a ...significant enhancement towards formic acid oxidation both lowering the oxidation onset potential and increasing the maximum current density in the positive going-sweep. A similar behavior has been observed in Pt(s)(100)x(111) stepped surfaces. On the other hand, for Pt(111) surfaces, the incorporation of Tl also induced a shift of the oxidation onset to lower potential values, a diminution of the hysteresis between the positive and negative going-sweep and also an increase of the oxidation currents. These results have been corroborated using in situ FTIR experiments, where the CO adsorption band disappears completely when Pt(100) and its vicinal surfaces are modified by Tl adatom, leading to the formation of CO2 at lower overvoltages.
We present a unique remodeling of flat copper terraces induced by organic molecules. In particular, we show how metal–phthalocyanines (MPc, M = Zn and Cu) tailor the formation of regular arrays of Cu ...nanostripes on its (110) surface. The MPc mediated reshaping of Cu(110) terraces is found to involve a massive reorganization of Cu adatoms, at variance with the conventional changes of metal reconstructions upon molecular adsorption observed so far. By combining experimental and theoretical surface science techniques, we reveal the key role played by the metal atom at the molecular center in reshaping the terrace structure. Moreover, we demonstrate that extra Cu adatoms surrounding the molecules stabilize the molecular decoration of the Cu nanostripes. The massive surface reshaping is thus due to molecular mediated unidirectional blocking of adatom surface diffusion followed by adatom capture and accumulation.
Molecular bilayers made up of a donor–acceptor blend in contact with the metal and capped with a single-component layer show a tunable energy level alignment at both metal–organic and organic–organic ...interfaces. To ensure sharp heteromolecular interfaces, a contact layer is formed by a stable blend of pentacene (PEN) and perfluorinated copper phthalocyanine (F16CuPc) on Au(111) and Ag(111) and of perfluoropentacene (PFP) and copper phthalocyanine (CuPc) on Ag(111). Core-level and valence band photoemission reveal that, upon capping with pure F16CuPc, CuPc, and PEN, the electronic states of both contact and capping layers “realign” with respect to the monolayer and the multilayer references. The sign of the shift depends clearly on whether the capping layer is donor-like (PEN, CuPc) or acceptor-like (F16CuPc). As revealed by NEXAFS, the shift in electronic levels of the contact layer upon capping leads to a spectral density variation across the Fermi edge (E F); i.e., it induces molecule/metal charge transfer.
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