Enhancing the Oxygen Electroreduction Activity through Electron Tunnelling: CoO_<i>x</i> Ultrathin Films on Pd(100)

Electron transfer is the most crucial step in several electrochemical reactions; therefore, finding alternative ways for its control represents a huge step toward the design of advanced electrocatalytic materials. We demonstrate that the electrons from an oxide-buried metal interface can be efficiently exploited in electrochemical reactions. This is proven by studying the electrochemical activity of <i>model systems</i> constituted by cobalt oxide ultrathin (<2 nm) films epitaxially grown on Pd(100). Metal/metal oxide interfacial hybridization and electron tunnelling from the metal substrate through the oxide endow CoO_<i>x</i> ultrathin films with exceptional electrochemical activity and improved poison tolerance. In situ XPS and Raman measurements indicate that during the oxygen reduction reaction, CoO is transformed into CoOOH, whereas Co₃O₄ is stable. These results demonstrate that the in situ study of ultrathin films on single crystals is a powerful method for the identification of materials active phase and of novel phenomena such as electron tunnelling

Fluorine- and Niobium-Doped TiO₂: Chemical and Spectroscopic Properties of Polycrystalline n‑Type-Doped Anatase

Doping titanium dioxide (anatase) with elements carrying an extra electron, such as Nb and F, or with their mixtures leads to n-type materials showing peculiar properties with respect to the pristine oxide. Niobium and fluorine are present in the lattice in the form of Nb⁵⁺ and F^– ions (detected by XPS), and the extra electrons carried by the dopants are stabilized on titanium ions, which become EPR-visible as Ti³⁺ ions homogeneously dispersed in the bulk of the crystals. Under such conditions, the optical band gap transition is slightly red-shifted (by a few tenths of an electronvolt) for all samples containing fluorine, and the Fermi level lies, depending on the material, at the boundary or even in the lower region of the conduction band. The typical Ti³⁺(I) centers generated by valence induction are responsible for the already reported conductivity properties of the system. The presence of these centers also influences the process of electron injection in the solid, favoring the dilution of additional reduced centers in the bulk, thereby leading to a homogeneously reduced material with optoelectronic properties differing from those of reduced anatase

Atomic Structure and Special Reactivity Toward Methanol Oxidation of Vanadia Nanoclusters on TiO₂(110)

We have grown highly controlled VO_<i>x</i> nanoclusters on rutile TiO₂(110). The combination of photoemission and photoelectron diffraction techniques based on synchrotron radiation with DFT calculations has allowed identifying these nanostructures as exotic V₄O₆ nanoclusters, which hold vanadyl groups, even if vanadium oxidation state is formally +3. Our theoretical investigation also indicates that on the surface of titania, vanadia mononuclear species, with oxidation states ranging from +2 to +4, can be strongly stabilized by aggregation into tetramers that are characterized by a charge transfer to the titania substrate and a consequent decrease of the electron density in the vanadium 3d levels. We then performed temperature programmed desorption experiments using methanol as probe molecule to understand the impact of these unusual electronic and structural properties on the chemical reactivity, obtaining that the V₄O₆ nanoclusters can selectively convert methanol to formaldehyde at an unprecedented low temperature (300 K)

Microscopic View on a Chemical Vapor Deposition Route to Boron-Doped Graphene Nanostructures

Single layer boron-doped graphene layers have been grown on polycrystalline copper foils by chemical vapor deposition using methane and diborane as carbon and boron sources, respectively. Any attempt to deposit doped layers in one-step has been fruitless, the reason being the formation of very reactive boron species as a consequence of diborane decomposition on the Cu surface, which leads to disordered nonstoichiometric carbides. However, a two-step procedure has been optimized: as a first step, the surface is seeded with pure graphene islands, while the boron source is activated only in a second stage. In this case, the nonstochiometric boron carbides formed on the bare copper areas between preseeded graphene patches can be exploited to easily release boron, which diffuses from the peripheral areas inward of graphene islands. The effective substitutional doping (of the order of about 1%) has been demonstrated by Raman and photoemission experiments. The electronic properties of doped layers have been characterized by spatially resolved photoemission band mapping carried out on single domain graphene flakes using a photon beam with a spot size of 1 μm. The whole set of experiments allow us to clarify that boron is effective at promoting the anchoring carbon species on the surface. Taking the cue from this basic understanding, it is possible to envisage new strategies for the design of complex 2D graphene nanostructures with a spatially modulated doping

Activation Energy Paths for Graphene Nucleation and Growth on Cu

The synthesis of wafer-scale single crystal graphene remains a challenge toward the utilization of its intrinsic properties in electronics. Until now, the large-area chemical vapor deposition of graphene has yielded a polycrystalline material, where grain boundaries are detrimental to its electrical properties. Here, we study the physicochemical mechanisms underlying the nucleation and growth kinetics of graphene on copper, providing new insights necessary for the engineering synthesis of wafer-scale single crystals. Graphene arises from the crystallization of a supersaturated fraction of carbon-adatom species, and its nucleation density is the result of competition between the mobility of the carbon-adatom species and their desorption rate. As the energetics of these phenomena varies with temperature, the nucleation activation energies can span over a wide range (1–3 eV) leading to a rational prediction of the individual nuclei size and density distribution. The growth-limiting step was found to be the attachment of carbon-adatom species to the graphene edges, which was independent of the Cu crystalline orientation

Single and Multiple Doping in Graphene Quantum Dots: Unraveling the Origin of Selectivity in the Oxygen Reduction Reaction

Singly and multiply doped graphene oxide quantum dots have been synthesized by a simple electrochemical method using water as solvent. The obtained materials have been characterized by photoemission spectroscopy and scanning tunneling microscopy, in order to get a detailed picture of their chemical and structural properties. The electrochemical activity toward the oxygen reduction reaction of the doped graphene oxide quantum dots has been investigated by cyclic voltammetry and rotating disk electrode measurements, showing a clear decrease of the overpotential as a function of the dopant according to the sequence: N ∼ B > B,N. Moreover, assisted by density functional calculations of the Gibbs free energy associated with every electron transfer, we demonstrate that the selectivity of the reaction is controlled by the oxidation states of the dopants: as-prepared graphene oxide quantum dots follow a two-electron reduction path that leads to the formation of hydrogen peroxide, whereas after the reduction with NaBH_4, the same materials favor a four-electron reduction of oxygen to water

Substrate Grain-Dependent Chemistry of Carburized Planar Anodic TiO₂ on Polycrystalline Ti

Mixtures or composites of titania and carbon have gained considerable research interest as innovative catalyst supports for low- and intermediate-temperature proton-exchange membrane fuel cells. For applications in electrocatalysis, variations in the local physicochemical properties of the employed materials can have significant effects on their behavior as catalyst supports. To assess microscopic heterogeneities in composition, structure, and morphology, a microscopic multitechnique approach is required. In this work, compact anodic TiO₂ films on planar polycrystalline Ti substrates are converted into carbon/titania composites or multiphase titanium oxycarbides through carbothermal treatment in an acetylene/argon atmosphere in a flow reactor. The local chemical composition, structure, and morphology of the converted films are studied with scanning photoelectron microscopy, micro-Raman spectroscopy, and scanning electron microscopy and are related with the crystallographic orientations of the Ti substrate grains by means of electron backscatter diffraction. Different annealing temperatures, ranging from 550 to 850 °C, are found to yield different substrate grain-dependent chemical compositions, structures, and morphologies. The present study reveals individual time scales for the carbothermal conversion and subsequent surface re-oxidation on substrate grains of a given orientation. Furthermore, it demonstrates the power of a microscopic multitechnique approach for studying polycrystalline heterogeneous materials for electrocatalytic applications

Fast One-Pot Synthesis of MoS₂/Crumpled Graphene p–n Nanonjunctions for Enhanced Photoelectrochemical Hydrogen Production

Aerosol processing enables the preparation of hierarchical graphene nanocomposites with special crumpled morphology in high yield and in a short time. Using modular insertion of suitable precursors in the starting solution, it is possible to synthesize different types of graphene-based materials ranging from heteroatom-doped graphene nanoballs to hierarchical nanohybrids made up by nitrogen-doped crumpled graphene nanosacks that wrap finely dispersed MoS₂ nanoparticles. These materials are carefully investigated by microscopic (SEM, standard and HR TEM), diffraction (grazing incidence X-ray diffraction (GIXRD)) and spectroscopic (high resolution photoemission, Raman and UV−visible spectroscopy) techniques, evidencing that nitrogen dopants provide anchoring sites for MoS₂ nanoparticles, whereas crumpling of graphene sheets drastically limits aggregation. The activity of these materials is tested toward the photoelectrochemical production of hydrogen, obtaining that N-doped graphene/MoS₂ nanohybrids are seven times more efficient with respect to single MoS₂ because of the formation of local p–n MoS₂/N-doped graphene nanojunctions, which allow an efficient charge carrier separation

Unraveling the Structural and Electronic Properties at the WSe₂–Graphene Interface for a Rational Design of van der Waals Heterostructures

WSe₂ thin films grown by chemical vapor deposition on graphene on SiC(0001) are investigated using photoelectron spectromicroscopy and electron diffraction. By tuning of the growth conditions, micrometer-sized single or multilayer WSe₂ crystalline islands preferentially aligned with the main crystallographic directions of the substrate are obtained. Our experiments suggest that the WSe₂ islands nucleate from defective WSe_<i>x</i> seeds embedded in the support. We explore the electronic properties of prototypical van der Waals heterostructures by performing μ-angle resolved photoemission spectroscopy on WSe₂ islands of varying thickness (mono- and bilayer) supported on single layer, bilayer, and trilayer graphene. The experiments are substantiated by DFT calculations indicating that the interaction between WSe₂ and graphene is weak and the electronic properties of the resulting heterostructures are unaffected by the thickness of the supporting graphene layer or by the crystallographic orientation. Yet the WSe₂–graphene distance and the WSe₂/WSe₂ interlayer separation strongly influence the electronic band alignment at the high symmetry points of the Brillouin zone. The values of technology relevant quantities such as splitting of spin polarized bands and effective mass of electrons at band valleys are extracted from experimental angle resolved spectra. These findings establish further strategies for tuning the morphology and electronic properties of artificially fabricated van der Waals heterostructures that may be used in the fields of nanoelectronics and valleytronics

Electrochemical Behavior of TiO_<i>x</i>C_<i>y</i> as Catalyst Support for Direct Ethanol Fuel Cells at Intermediate Temperature: From Planar Systems to Powders

To achieve complete oxidation of ethanol (EOR) to CO₂, higher operating temperatures (often called intermediate-<i>T</i>, 150–200 °C) and appropriate catalysts are required. We examine here titanium oxycarbide (hereafter TiO_<i>x</i>C_<i>y</i>) as a possible alternative to standard carbon-based supports to enhance the stability of the catalyst/support assembly at intermediate-<i>T</i>. To test this material as electrocatalyst support, a systematic study of its behavior under electrochemical conditions was carried out. To have a clear description of the chemical changes of TiO_<i>x</i>C_<i>y</i> induced by electrochemical polarization of the material, a special setup that allows the combination of X-ray photoelectron spectroscopy and electrochemical measurements was used. Subsequently, an electrochemical study was carried out on TiO_<i>x</i>C_<i>y</i> powders, both at room temperature and at 150 °C. The present study has revealed that TiO_<i>x</i>C_<i>y</i> is a sufficiently conductive material whose surface is passivated by a TiO₂ film under working conditions, which prevents the full oxidation of the TiO_<i>x</i>C_<i>y</i> and can thus be considered a stable electrode material for EOR working conditions. This result has also been confirmed through density functional theory (DFT) calculations on a simplified model system. Furthermore, it has been experimentally observed that ethanol molecules adsorb on the TiO_<i>x</i>C_<i>y</i> surface, inhibiting its oxidation. This result has been confirmed by using in situ Fourier transform infrared spectroscopy (FTIRS). The adsorption of ethanol is expected to favor the EOR in the presence of suitable catalyst nanoparticles supported on TiO_<i>x</i>C_<i>y</i>