Sb2Se3(100): A strongly anisotropic surface

Interest in antimony selenide (Sb2Se3) has been constantly growing in the recent past, especially for its promising properties for applications in the field of solar energy technologies. Surprisingly, the surface properties of this material, which is built from van der Waals stacked one-dimensional (1D) ribbons, have not been studied in detail yet. Here we demonstrate that Sb2Se3 crystals cleave along the (100) planes. The resulting surface shows a pronounced 1D structure, reflecting the stacking of ribbons in the bulk crystal. The cleaving process leads to the formation of slightly tilted surface domains, with the tilt angles oriented invariably along the ribbon directions, suggesting a strong anisotropy of the internal friction forces. Our angle-resolved photoemission data reveal that the 1D character of the crystalline structure of this material is also reflected in its electronic band structure

Water-induced modifications of GaP(100) and InP(100) surfaces studied by photoelectron spectroscopy and reflection anisotropy spectroscopy

In this work, we investigate the initial interaction of water and oxygen with different surface reconstructions of GaP(100) applying photoelectron spectroscopy, low-energy electron diffraction, and reflection anisotropy spectroscopy. Surfaces were prepared by metal-organic vapour phase epitaxy, transferred to ultra-high vacuum, and exposed to oxygen or water vapour at room temperature. The (2 4) reconstructed, Ga-rich surface is more sensitive and reactive to adsorption, bearing a less ordered surface reconstruction upon exposure and indicating a mixture of dissociative and molecular water adsorption. The p(2 2)=c(4 2) P-rich surface, on the other hand, is less reactive, but shows a new surface symmetry after water adsorption. Correlating findings of photoelectron spectroscopy with reflection anisotropy spectroscopy could pave the way towards optical in-situ monitoring of electrochemical surface modifications with reflection anisotropy spectroscopy

Functionalization of Ultrathin Alumina Films of Atomically Precise Thickness with Dye Molecules as Model Systems for Photoelectrodes

Dye-sensitized solar cells and photoelectrochemical cells are promising technologies for the future energy sector, and ultrathin insulating layers can improve their performance significantly. Here a suitable well defined model system is established for surface science to study the dye/insulator interface and first prototypical experiments are performed toachieve a better understanding of adsorption structures of dyes and of occurring charge transfer processes.A crystalline atomic bilayer of alumina can be grown on the NiAl(110) surface and is an established well understood substrate for catalysis studies studies under ultra-high vacuum (UHV) conditions. To enable studies with variable tunneling barriers two methods are explored to increase the thickness of this thin film: The high-temperature oxidationof the clean NiAl(110) surface and the continued oxidation of the crystalline alumina bilayer. The latter balances the relevant thermodynamic processes in a more favorable manner and enables the production of thicker well-ordered films, as can be deducted from low-energy electron diffractograms. The film growth is monitored using quantitativeX-ray photoelectron spectroscopy (XPS) and the topographic structure is of the films is studied using low-energy energy diffraction (LEED) and X-ray photoelectron diffraction (XPD). LEED indicates that the large unit cell of the alumina surface oxide is maintained in the observed thickness range of up to 1.5nm and combined with XPD suggests the formation of sub-nanometer sized g-Al2O3(111) nuclei in this unit cell. The electronic band structure of the thin films is mapped with angle-resolved photoemission and does not change drastically during the film growth. In a next step these alumina films are functionalized with metalorganic dye molecules. The ultrathin alumina films on NiAl(110) are not stable in atmosphere and the NiAl continues to oxidize in an uncontrolled manner, which obstacles the functionalization with dye molecules from solution as a self-assembled monolayer (SAM). A vacuum chamber wasdeveloped to overcome this limitation and to deposit SAMs without taking the substrate out on air. The uncontrolled oxidation of the substrate can be reduced with this method to less then one atomic layer. The deposited SAM acts as a capping and passivates the dye/insulator/metal-heterostructure against air. Five different dye molecules are depositedon ultrathin alumina/NiAl(110) with this setup. Two prototypical ruthenium dyes and three rhenium dyes are characterized with XPS and ultraviolet photoelectron spectroscopy (UPS) to elucidate their electronic structure and to compare the molecular density of these molecules. Further light is shed on the counterion coadsorption of one rhenium and one ruthenium dye. Ultrafast time-resolved core-level spectroscopy of rhenium 4f electrons in adye/insulator/metal-heterostructure is used to observe the oxidation of the metal center upon optical excitation due to the metal-to-ligand charge transfer. An oscillating appearance of the oxidized species is observed. It is found that ultrafast time-resolved corelevel spectroscopy is a promising technique with the potential to shine light on the chargecarrier dynamics in such heterostructures and into the intramolecular charge transfer

Atomically dispersed hybrid nickel-iridium sites for photoelectrocatalysis

Atomically dispersed supported catalysts can maximize atom efficiency and minimize cost. In spite of much progress in gas-phase catalysis, applying such catalysts in the field of renewable energy coupled with electrochemistry remains a challenge due to their limited durability in electrolyte. Here, we report a robust and atomically dispersed hybrid catalyst formed in situ on a hematite semiconductor support during photoelectrochemical oxygen evolution by electrostatic adsorption of soluble monomeric [Ir(OH)6]2− coupled to positively charged NiOx sites. The alkali-stable [Ir(OH)6]2− features synergistically enhanced activity toward water oxidation through NiOx that acts as a “movable bridge” of charge transfer from the hematite surface to the single iridium center. This hybrid catalyst sustains high performance and stability in alkaline electrolyte for >80 h of operation. Our findings provide a promising path for soluble catalysts that are weakly and reversibly bound to semiconductorsupported hole-accumulation inorganic materials under catalytic reaction conditions as hybrid active sites for photoelectrocatalysis

The impact of metalation on adsorption geometry, electronic level alignment and UV-stability of organic macrocycles on TiO2(110)

Metal complexes of the tetradentate bipyridine based macrocycle pyrphyrin (Pyr) have recently shown promise as water reduction catalysts in homogeneous photochemical water splitting reactions. In this study, the adsorption and metalation of pyrphyrin on stoichiometric TiO2(110) is investigated in ultrahigh vacuum by means of scanning tunneling microscopy, photoelectron spectroscopy, low-energy electron diffraction, and density functional theory. In a joint experimental and computational effort, the local adsorption geometry at low coverage, the long-range molecular ordering at higher coverage and the electronic structure have been determined for both the bare ligand and the cobalt-metalated Pyr molecule on TiO2. The energy level alignment of CoPyr/TiO2 supports electron injection into TiO2 upon photoexcitation of the CoPyr complex and thus renders it a potential sensitizer dye. Importantly, Co-incorporation is found to stabilize the Pyr molecule against photo-induced degradation, while the bare ligand is decomposed rapidly under continuous UV-irradiation. This interesting phenomenon is discussed in terms of additional de-excitation channels for electronically highly excited molecular states

Atomically dispersed hybrid nickel-iridium sites for photoelectrocatalysis

Atomically dispersed supported catalysts can maximize atom efficiency and minimize cost. In spite of much progress in gas-phase catalysis, applying such catalysts in the field of renewable energy coupled with electrochemistry remains a challenge due to their limited durability in electrolyte. Here, we report a robust and atomically dispersed hybrid catalyst formed in situ on a hematite semiconductor support during photoelectrochemical oxygen evolution by electrostatic adsorption of soluble monomeric [Ir(OH)6]2− coupled to positively charged NiOx sites. The alkali-stable [Ir(OH)6]2− features synergistically enhanced activity toward water oxidation through NiOx that acts as a “movable bridge” of charge transfer from the hematite surface to the single iridium center. This hybrid catalyst sustains high performance and stability in alkaline electrolyte for >80 h of operation. Our findings provide a promising path for soluble catalysts that are weakly and reversibly bound to semiconductor-supported hole-accumulation inorganic materials under catalytic reaction conditions as hybrid active sites for photoelectrocatalysis

Functionalization and passivation of ultrathin alumina films of defined sub-nanometer thickness with self-assembled monolayers

Instability of ultrathin surface oxides on alloys under environmental conditions can limit the opportunities for applications of these systems when the thickness control of the insulating oxide film is crucial for device performance. A procedure is developed to directly deposit self-assembled monolayers (SAM) from solvent onto substrates prepared under ultra-high vacuum conditions without exposure to air. As an example, rhenium photosensitizers functionalized with carboxyl linker groups are attached to ultrathin alumina grown on NiAl(1 1 0). The thickness change of the oxide layer during the SAM deposition is quantified by x-ray photoelectron spectroscopy and can be drastically reduced to one atomic layer. The SAM acts as a capping layer, stabilizing the oxide thin film under environmental conditions. Ultraviolet photoelectron spectroscopy elucidates the band alignment in the resulting heterostructure. The method for molecule attachment presented in this manuscript can be extended to a broad class of molecules vulnerable to pyrolysis upon evaporation and presents an elegant method for attaching molecular layers on solid substrates that are sensitive to air

Comparative Study of the Different Anchoring of Organometallic Dyes on Ultrathin Alumina

Significant improvements in incident photon-to-current efficiencies can be obtained by covering inorganic semiconductors with ultrathin alumina films and sensitizing them with adsorbed dye molecules. The anchoring mode of the latter to the substrate affects the charge transport between the dye and the electrode via tunneling, and consequently, the device efficiency. In this work, we employ X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS) for a comparative study of the adsorption of three rhenium and two ruthenium organometallic dyes, one of each being ionic, with different anchoring modes on single-crystalline ultrathin alumina films. Molecular monolayers were prepared by self-assembly from solution. Quantitative XPS analysis reveals higher surface densities for the Re dyes. Nearly stoichiometric coadsorption of counterions is observed for the ionic dyes. Density functional theory (DFT) calculations for the Re dyes show that the most stable adsorption configurations exhibit the expected bonding via the dedicated anchoring groups (carboxyls or methylphosphonic acid), with an additional sulfur–aluminum bond for the dyes containing a thiocyanate ligand. The alignment of the occupied molecular levels with respect to the alumina valence band maximum, obtained for these geometries, follows the experimental trend in the UPS data and places the lowest unoccupied molecular orbitals (LUMOs) close to the Fermi level of the systems, far inside the alumina band gap. Dynamical charge screening is found to be important for this type of system when comparing UPS and DFT results. This work provides a general guideline for the systematic characterization of related molecules on surfaces