Adsorption of cytosine and aza derivatives of cytidine on Au single crystal surfaces

The adsorption of cytosine on the Au(111) and Au(110) surfaces has been studied using both aqueous deposition and evaporation in vacuum to prepare the samples. Soft X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure spectroscopy (NEXAFS) were used to determine the electronic structure and orientation of the adsorbates. In addition, three derivatives of cytosine, 6-azacytosine, 6-azacytidine and 5- azacytidine, were studied. Monolayer films of the latter three samples were adsorbed on Au(111) from aqueous solution, and the nature of bonding was determined. Spectra have been interpreted in the light of published calculations of free cytosine molecules and new ab initio calculations of the other compounds. Surface core level shifts of Au 4f imply that all of these compounds are chemisorbed. Cytosine adsorbs as a single tautomer, but in two chemical states with different surface-molecule bonding. For deposition in vacuum, a flat-lying molecular state bonded through the N(3) atom of the pyrimidine ring dominates, but a second state is also present. For deposition from solution, the second state dominates, with the molecular plane no longer parallel to the surface. This state also bonds through the N(3) atom, but in addition interacts with the surface via the amino group. Two tautomers of 6-azacytosine were observed, and they and 6-azacytidine adsorb with similar geometries, chemically bonding via the azacytosine ring. The ribose ring does not appear to perturb the adsorption of azacytidine compared with azacytosine. The azacytosine ring is nearly but not perfectly parallel to the surface, like 5-azacytidine, which adsorbs as an imino tautomer. ...Comment: 40 pages, 3 tables and 8 figure

Compression Stress-Induced Internal Magnetic Field in Bulky TiO2 Photoanodes for Enhancing Charge-Carrier Dynamics

Enhancing charge-carrier dynamics is imperative to achieve efficient photoelectrodes for practical photoelectrochemical devices. However, a convincing explanation and answer for the important question which has thus far been absent relates to the precise mechanism of charge-carrier generation by solar light in photoelectrodes. Herein, to exclude the interference of complex multi-components and nanostructuring, we fabricate bulky TiO2 photoanodes through physical vapor deposition. Integrating photoelectrochemical measurements and in situ characterizations, the photoinduced holes and electrons are transiently stored and promptly transported around the oxygen-bridge bonds and 5-coordinated Ti atoms to form polarons on the boundaries of TiO2 grains, respectively. Most importantly, we also find that compressive stress-induced internal magnetic field can drastically enhance the charge-carrier dynamics for the TiO2 photoanode, including directional separation and transport of charge carriers and an increase of surface polarons. As a result, bulky TiO2 photoanode with high compressive stress displays a high charge-separation efficiency and an excellent charge-injection efficiency, leading to 2 orders of magnitude higher photocurrent than that produced by a classic TiO2 photoanode. This work not only provides a fundamental understanding of the charge-carrier dynamics of the photoelectrodes but also provides a new paradigm for designing efficient photoelectrodes and controlling the dynamics of charge carriers

Photoelectrochemical N2-to-NH3 Fixation with High Efficiency and Rates via Optimized Si-Based System at Positive Potential versus Li0/+

As a widely used commodity chemical, ammonia is critical for producing nitrogen-containing fertilizers and serving as the promising zero-carbon energy carrier. Photoelectrochemical nitrogen reduction reaction (PEC NRR) can provide a solar-powered green and sustainable route for synthesis of ammonia (NH3). Herein, an optimum PEC system is reported with an Si-based hierarchically-structured PdCu/TiO2/Si photocathode and well-thought-out trifluoroethanol as the proton source for lithium-mediated PEC NRR, achieving a record high NH3 yield of 43.09 µg cm−2 h−1 and an excellent faradaic efficiency of 46.15% under 0.12 MPa O2 and 3.88 MPa N2 at 0.07 V versus lithium(0/+) redox couple (vs Li0/+). PEC measurements coupled with operando characterization reveal that the PdCu/TiO2/Si photocathode under N2 pressures facilitate the reduction of N2 to form lithium nitride (Li3N), which reacts with active protons to produce NH3 while releasing the Li+ to reinitiate the cycle of the PEC NRR. The Li-mediated PEC NRR process is further enhanced by introducing small amount of O2 or CO2 under pressure by accelerating the decomposition of Li3N. For the first time, this work provides mechanistic understanding of the lithium-mediated PEC NRR process and opens new avenues for efficient solar-powered green conversion of N2-to-NH3

Nitridation of InP(1 0 0) surface studied by synchrotron radiation

The nitridation of InP(1 0 0) surfaces has been studied using synchrotron radiation photoemission. The samples were chemically cleaned and then ion bombarded, which cleaned the surface and also induced the formation of metallic indium droplets. The nitridation with a Glow Discharge Cell (GDS) produced indium nitride by reaction with these indium clusters. We used the In 4d and P 2p core levels to monitor the chemical state of the surface and the coverage of the species present. We observed the creation of In-N and P-N bonds while the In-In metallic bonds decrease which confirm the reaction between indium clusters and nitrogen species. A theoretical model based on stacked layers allows us to assert that almost two monolayers of indium nitride are produced. The effect of annealing on the nitridated layers at 450 $^\circ$C has also been analysed. It appears that this system is stable up to this temperature, well above the congruent evaporation temperature (370 $^\circ$C) of clean InP(1 0 0): no increase of metallic indium bonds due to decomposition of the substrate is detected as shown in previous works [L. Bideux, Y. Ould-Metidji, B. Gruzza, V. Matolin, Surf. Interface Anal. 34 (2002) 712] studying the InP(1 0 0) surfaces

Probing the Roughness of Porphyrin Thin Films with X-ray Photoelectron Spectroscopy

Thin-film growth of molecular systems is of interest for many applications, such as for instance organic electronics. In this study, we demonstrate how X-ray photoelectron spectroscopy (XPS) can be used to study the growth behavior of such molecular systems. In XPS, coverages are often calculated assuming a uniform thickness across a surface. This results in an error for rough films, and the magnitude of this error depends on the kinetic energy of the photoelectrons analyzed. We have used this kinetic-energy dependency to estimate the roughnesses of thin porphyrin films grown on rutile TiO2(110). We used two different molecules: cobalt (II) monocarboxyphenyl-10,15,20-triphenylporphyrin (CoMCTPP), with carboxylic-acid anchor groups, and cobalt (II) tetraphenylporphyrin (CoTPP), without anchor groups. We find CoMCTPP to grow as rough films at room temperature across the studied coverage range, whereas for CoTPP the first two layers remain smooth and even; depositing additional CoTPP results in rough films. Although, XPS is not a common technique for measuring roughness, it is fast and provides information of both roughness and thickness in one measurement.Fil: Kataev, Elmar. Universitat Erlangen-Nuremberg; AlemaniaFil: Wechsler, Daniel. Universitat Erlangen-Nuremberg; AlemaniaFil: Williams, Federico José. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Química Inorgánica, Analítica y Química Física; ArgentinaFil: Köbl, Julia. Universitat Erlangen-Nuremberg; AlemaniaFil: Tsud, Natalia. Karlova Univerzita (cuni); República ChecaFil: Franchi, Stefano. Istituto di Struttura della Materia; Italia. Consiglio Nazionale delle Ricerche; ItaliaFil: Steinruck, Hans Peter. Universitat Erlangen-Nuremberg; AlemaniaFil: Lytken, Ole. Universitat Erlangen-Nuremberg; Alemani

Structural investigation of As-Se chalcogenide thin films with different compositions: formation, characterization and peculiarities of volume and near-surface nanolayers

As₂₀Se₈₀, As₄₀Se₆₀ and As₅₀Se₅₀ films were studied by Raman spectroscopy in order to examine the local- and medium-range order of the structure. In addition, X-ray photoelectron, Raman and surface enhanced Raman spectroscopy were used to characterize the structural peculiarities at the top surface of As-Se nanolayers. Raman investigations reveal the dominance of the As₂Se₃ and As₄Se₄ molecules in the volume of the As₄₀Se₆₀ and As₅₀Se₅₀ films and significant contribution of Se in the structure of the As₂₀Se₈₀ film. The composition and local structure of the surfaces were determined by curve fitting of the experimental X-ray photoelectron As 3d and Se 3d core level spectra. A significant Se-enrichment was found at the near-surface layers in comparison with the composition of deeper layers which is confirmed by the dominance of As-3Se structural units in all compositions. This enrichment was also observed by surface enhanced Raman spectroscopy. Processes of arsenic oxidation and desorption of the oxidized products are impacting the structure of the surface layers of As₂₀Se₈₀, As₄₀Se₆₀ and As₅₀Se₅₀ films

Decomposition of Methanol on Mixed CuO–CuWO₄ Surfaces

Mixed CuO­(2 × 1)–CuWO₄ layers on a Cu(110) surface have been prepared by the on-surface reaction of the CuO(2 × 1) surface oxide with adsorbed (WO₃)₃ clusters. The adsorption and decomposition of methanol on these well-defined CuO–CuWO₄ surfaces has been followed by high-resolution X-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS), and temperature-programmed desorption (TPD) to assess the molecular surface species and their concentration, while the state of the surface oxide phases before and after methanol decomposition has been characterized by scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and XPS. Surface methoxy species form the primary methanol decomposition products, which desorb partly by recombination as methanol at 200–300 K or decompose into CH_<i>x</i> and possibly CO. The most reactive surfaces are mixed CuO–CuWO₄ phase, with CuWO₄ coverages 0.5–0.8 monolayer, thus pointing at the importance of oxide phase boundary sites. In a minority reaction channel, a small amount of formaldehyde is detected on the CuWO₄ surface. The CuWO₄ oxide phase becomes modified as a result of reduction and a morphology transition triggered by the methanol decomposition, but the pristine surface state can be recovered by a postoxidation treatment with oxygen

SOFT X-RAY SPECTROSCOPY OF GLYCYL-GLYCINE ADSORBED ON Cu(110) SURFACE

Author Institution: Sincrotrone Trieste, Basovizza (Trieste), Italy; Institute of Physics, Prague, Czech Republic; Charles University, Prague, Czech Republic; CNR-Institute of Chemical Physical Processes, Pisa, ItalyStudies of the interaction between organic compounds and surfaces are motivated by their application as bio sensors, and their relevance to biocompatibility of implants and the origin of life. In the present work interaction of the simplest peptide, glycyl-glycine, with the Cu surface has been studied. Multilayer, monolayer and sub-monolayer films of this dipeptide on the clean and oxygen modified Cu(110) surface were prepared by thermal evaporation in high vacuum. The techniques used were soft X-ray photoelectron spectroscopy, near edge X-ray absorption fine structure spectroscopy and density functional theory calculations. By comparing the experimental and theoretical spectra, detailed models of the electronic structure and adsorption geometry for each coverage have been proposed, which are in good agreement with the theoretical calculations. The carboxylic acid group of glycyl-glycine loses hydrogen and the molecule is coordinated via the carboxylate oxygen atoms to the surface. At low coverage the amino group bonds to the surface via a hydrogen atom, while at higher coverage the bonding is via the nitrogen lone pair. The peptide group is not involved in the bonding to the surface