•M (M = Co, Cu, Ag, Au) surface free layered Pd–M bimetallic films were prepared.•Increasing d-band center shifts in the following order: Pd, Pd–Co, –Cu, –Ag, and –Au.•Pd–M bimetallic films show better activity than Pd toward formic acid oxidation.•The relationship between d-band center shift and activity exhibit a volcano shape.•Pd–Cu film displays the highest electrochemical activity toward formic acid oxidation.Pd films of about 4.5 ± 0.5 nm were deposited on different transition metals (M = Co, Cu, Ag, and Au) by vapor deposition, and characterized by X-ray photoelectron spectroscopy (XPS), cyclic voltammetric (CV), and chronoamperometric (CA) tests. The Tafel method was used to determine the rate constant parameters, k0. The d-band center of the Pd–M samples exhibits a shift away from the Fermi level in the increasing order of Pd–Co, –Cu, –Ag, and –Au films compared to that of the bulk Pd film. As the position of the d-band center downshifts to a critical value, a weaker bond between Pd and formate (HCOO) is expected for the formic acid oxidation resulting in an improved electrochemical activity of the Pd surface for the Pd–Co and –Cu bimetallic films. As a critical value of the d-band center is reached formate is more difficult to adsorb on the Pd surface and its electrochemical activity toward formic acid oxidation decreases (e.g., Pd–Ag and –Au bimetallic films). Among the layered Pd–M films, the Pd–Cu shows the highest electrochemical activity toward the formic acid oxidation, which indicates that it has the optimum Pd–HCOO bond strength.

Vapor deposition method is used to prepare layered Pd–Cu bimetallic films, with different thickness of Pd over a Cu film supported on glassy carbon. X-ray photoelectron spectroscopy (XPS) measurements of core level binding energy (BE) and valence band region are used to investigate the contribution of charge transfer resulting from the bonding of these two dissimilar metals in a layered structure. As this layered bimetallic film is annealed at increasing temperature from 298 to 650 K, atomic inter-diffusion occurs to form an alloy. We differentiate this interfacial charge transfer effect between these two dissimilar metals from that of surface alloying resulting from the annealing effect. Cyclic voltammetry (CV) tests combined with XPS confirm that (1) the surface is free of Cu atoms for Pd films with thicknesses between 2.0 and 7.0 ± 0.3 nm at room temperature and (2) Pd–Cu inter-diffusion propagates to the surface and the formation of surface alloys take place at temperature greater than 350 K. For these Pd thicknesses, the difference in BE shifts of Cu 2p and Pd 3d peaks between layered and alloyed structures are +0.16 eV and −0.17 eV, respectively. This difference in BE shifts allows for a clear distinction between these two bimetallic structures and the tailoring of the optimum configuration that enhances catalytic activity toward formic acid. Additionally, angle resolved XPS data as a function of temperature and XPS depth profiling at room temperature provide insight about the inter-diffusion length. Furthermore, CV measurements in H2SO4 and formic acid confirm that for a Pd–Cu bimetallic surface with a 3.8 ± 0.3 nm Pd film over Cu shows significantly improved activity toward formic acid oxidation compared to bulk Pd at 298 K and after being annealed at temperature of 350 K.

Vapor deposition method is used to prepare layered Pd–Cu bimetallic model catalytic surfaces, with different thickness of Pd, supported on glassy carbon. Charge transfer from Pd to Cu produces a positive binding energy (BE) shift for the overlayer Pd 3d and an opposite shift for Cu 2p as measured by X-ray photoelectron spectroscopy (XPS). Similarly, the d-band center moves away from the Fermi level as the thickness of Pd decreased for these bimetallic materials. This electronic perturbation of the Pd–Cu systems results in a drastic increase in current density and an improved stability for formic acid electro-oxidation. Our findings indicate that layered Pd–Cu bimetallic surfaces with Pd layers thicker than 1 nm do not form a surface alloy at room temperature and are better electrocatalytic materials than Bulk Pd and could be used to enhance direct formic acid fuel cell (DFAFC) performance.Graphical abstractHighlights► Pd–Cu bimetallic model surface was prepared by vapor deposition method. ► XPS and d-band center values showed that electrons transfer from Pd to Cu layer. ► Both activity and stability of Pd–Cu for formic acid electro-oxidation were tested. ► Pd–Cu surface showed improved electrochemical performances than pure Pd surface.

A new organic compound (tertmethoxy di-triphenylamine di-thiophene-benzothiadiazole; DTBT-DTPA-TMeO) recently synthesized displays two strong absorption peaks in the visible range of the solar spectrum. Thermal behavior of this new donor–acceptor–donor (D–A–D) oligomer is investigated by differential scanning calorimetry and thermogravimetric analysis. The decomposition temperature is found to be at about 400 °C and a smectic mesophase transition at about 115 °C (crystal liquid–crystal transition). Solution and solution processed films onto ITO and ITO/MoO3 are characterized by UV–Vis, AFM and XPS-UPS measurements. An optical band gap of 1.85 eV is reported from the UV–Vis data and agrees well with the theoretical value of 1.99 eV. AFM analysis of spin coated films shows smooth uniform coverage free of pin holes. Work functions and HOMO energy values are determined from UPS measurements and allow the construction of an energy band diagram. The hole mobility for a un-annealed single diode device is measured at about 10−5 cm2/Vs.Highlights► New donor–acceptor–donor oligomer as donor material. ► Thermal stability and good quality film morphology allow for optimized charge mobility. ► Optical and electronic properties make this oligomer a promising light harvesting material for solar cell applications. ► Single carrier type diodes allows for initial measurements of hole mobility (μ0).

Nanocomposite matrices of silver/poly(3-hexylthiophene) (P3HT) were prepared in ultrahigh vacuum through vapor-phase codeposition. Change in microstructure, chemical nature, and electronic properties with increasing filler (Ag) content were investigated using in situ XPS and UPS, and ambient AFM. At least two chemical binding states occur between Ag nanoparticles and sulfur in P3HT at the immediate contact layer, but no evidence of interaction between Ag and carbon (in P3HT) was found. AFM images reveal a change in Ag nanoparticles size with concentration which modifies the microstructure and the average roughness of the surface. Under codeposition, P3HT largely retains its conjugated structures, which is evidenced by the similar XPS and UPS spectra to those of P3HT films deposited on other substrates. We demonstrate here that the magnitude of the barrier height for hole injection (εvF) and the position of the highest occupied band edge (HOB) with respect to the Fermi level of Ag may be controlled and changed by adjusting the metal (Ag) content in the composite. Furthermore, UPS reveals distinct features related to the C 2p (σ states) in the 5−12 eV regions, indicating the presence of ordered P3HT, which is different from solution processed films.Keywords: nanocomposites; photoelectron spectroscopy; polythiophene; silver; vapor phase codeposition

X-ray diffraction (XRD), X-ray photoemission (XPS) as well as ultraviolet photoemission (UPS) spectroscopy experiments on MoO2 powders were carried out to examine the bulk, the core level energies, and the electronic structure of MoO2 samples that were employed as catalysts for the partial oxidation of isooctane. Five fresh 0.5-g MoO2 samples were exposed for 0, 0.5, 9, 20, and 43 h to identical reforming environments and their spent samples were analyzed using the techniques mentioned above. Our results indicate the rapid appearance of an intermediate Mo phase with a binding energy of 228.5 eV and whose concentration progressively increases with time. The oxidation state for this new phase was graphically estimated to approximately +2.6 and assigned to the compound Mo2O3, which forms on the catalyst surface as a result of its exposure to the reforming environment. The electronic structure probed by UPS reveals two bands, one at 1.62 eV and another at 0.55 eV below the Fermi level, that decrease with the increasing time on stream. These results correlate very well with the drop in the catalytic performance of MoO2 for the partial oxidation of isooctane and with the decline in the concentration of dioxide (Mo4+) detected not only on the catalyst surface, but also in the bulk structure, as confirmed by our XRD analysis.

Secondary field emission scanning electron microscopy (FE SEM), atomic force microscopy (AFM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to investigate native near-isogenic soft and hard wheat kernels and their roller milled flours. FE SEM images of flat-polished interior endosperm indicated distinct differences between soft and hard wheats with less internal continuity in the soft wheat, whereas individual starch granules were much less evident in the hard kernel due to a more continuous matrix. AFM images revealed two different microstructures. The interior of the hard kernel had a granular texture with distinct individual spheroid features of 10–50 nm while the images obtained for the soft kernel revealed less distinct small grains and more larger features, possibly micro-structural features of starch granules. Raman spectra resolved identical distinct frequencies for both kernel types with slightly different intensities between types. Finally, the chemical surface compositions of flour for these two types of kernels obtained by XPS provided subtle insight into the differences between soft and hard wheat kernels. These combined advanced microscopic and spectroscopic analyses provide additional insight into the differences between the soft and hard wheat kernels.