Growth and electronic properties of Co-doped Mn3O4 thin films: a combined experimental and theoretical investigation.

In this study, we have prepared and investigated the electronic properties of a new and promising cobalt doped Mn3O4 oxide surface by site-selective and element-sensitive X-ray-absorption (XAS) and photoemission spectroscopy (XPS and resonant PES) combined with density functional theory (DFT) calculations. The crystallinity of both pristine and Co doped thin films was ensured by low energy electron diffraction measurements, in which similar diffraction patterns were obtained for both films suggesting the inclusion of the dopant species within the crystalline Mn3O4 thin film. According to our combined experimental data and theoretical calculation results, XAS measurements and replace energy calculations could identify Co impurities adopting preferentially a 2+ oxidation state substituting a Mn2+ cation on tetrahedral sites (80%) as well as the Mn3+ on octahedral sites (20%). Direct evidence of these findings could be found by comparing the pristine Mn3O4 electron absorption and photoemission spectral features with those of the doped ones. For instance, the formation of oxygen vacancies related to the formation of Co2+ in an octahedral site could be directly observed. Remarkably, the valence band spectrum of Co-Mn3O4 thin films presents additional spectral features close to the Fermi edge that can be directly attributed to Co states when compared to the PDOS obtained by the DFT calculations. It is noteworthy that the formation and stabilization of these Co dopant species in the host Mn3O4 surface could potentially affect and obviously modulate its capability for adsorption of molecular species and transfer of electrons, which makes the cobalt doped Mn3O4 surface potentially promising for catalytic applications.

Interaction of water and carbon monoxide with MnO(001) thin films on Au(111).

Atomically defined MnO(001) thin films were grown on an Au(111) substrate, and their interaction with water (D2O) was investigated by infrared reflection absorption spectroscopy (IRAS) and thermal desorption spectroscopy (TDS). Carbon monoxide adsorption experiments were performed to probe surface atoms and defects on oxide films. Next, water interaction was investigated from which an associative binding pathway and a dissociative binding pathway were revealed, where the water molecules adsorb at terraces and water dissociation takes place at oxygen vacancies mediated by nearby Mn2+ sites. The IRAS data are supported by TDS experiments, which also manifest the importance of defects in the adsorption characteristics of MnO.

Structure and registry of the silica bilayer film on Ru(0001) as viewed by LEED and DFT.

Silica bilayers are stable on various metal substrates, including Ru(0001) that is used for the present study. In a systematic attempt to elucidate the detailed structure of the silica bilayer film and its registry to the metal substrate, we performed a low energy electron diffraction (I/V-LEED) study. The experimental work is accompanied by detailed calculations on the stability, orientation and dynamic properties of the bilayer at room temperature. It was determined, that the film shows a certain structural diversity within the unit cell of the metal substrate, which depends on the oxygen content at the metal-bilayer interface. In connection with the experimental I/V-LEED study, it became apparent, that a high-quality structure determination is only possible if several structural motifs are taken into account by superimposing bilayer structures with varying registry to the oxygen covered substrate. This result is conceptually in line with the recently observed statistical registry in layered 2D-compound materials.

MnO2-Ir Nanowires: Combining Ultrasmall Nanoparticle Sizes, O-Vacancies, and Low Noble-Metal Loading with Improved Activities towards the Oxygen Reduction Reaction.

Although clean energy generation utilizing the Oxygen Reduction Reaction (ORR) can be considered a promising strategy, this approach remains challenging by the dependence on high loadings of noble metals, mainly Platinum (Pt). Therefore, efforts have been directed to develop new and efficient electrocatalysts that could decrease the Pt content (e.g., by nanotechnology tools or alloying) or replace them completely in these systems. The present investigation shows that high catalytic activity can be reached towards the ORR by employing 1.8 ± 0.7 nm Ir nanoparticles (NPs) deposited onto MnO2 nanowires surface under low Ir loadings (1.2 wt.%). Interestingly, we observed that the MnO2-Ir nanohybrid presented high catalytic activity for the ORR close to commercial Pt/C (20.0 wt.% of Pt), indicating that it could obtain efficient performance using a simple synthetic procedure. The MnO2-Ir electrocatalyst also showed improved stability relative to commercial Pt/C, in which only a slight activity loss was observed after 50 reaction cycles. Considering our findings, the superior performance delivered by the MnO2-Ir nanohybrid may be related to (i) the significant concentration of reduced Mn3+ species, leading to increased concentration of oxygen vacancies at its surface; (ii) the presence of strong metal-support interactions (SMSI), in which the electronic effect between MnOx and Ir may enhance the ORR process; and (iii) the unique structure comprised by Ir ultrasmall sizes at the nanowire surface that enable the exposure of high energy surface/facets, high surface-to-volume ratios, and their uniform dispersion.

Semiconductor-Based Microswimmers: Attention to Detail Matters.

Colloidal active matter is known for its sensitivity to external conditions; for example, the swimming speeds depend strongly on substrates, fuel concentration, and in the case of light-driven colloids, the illumination. While these points are regularly considered, the nanoscopic material properties of the motor bodies are often barely mentioned, but they are highly influential in the case of photocatalysts. In order to demonstrate the influence of subtle differences in chemical composition and interfacing between the different material compounds, we designed a system based on colloidal titania spheres asymmetrized by different nanoscale cobalt oxide species. We examine how the material properties and combinations lead to highly specific catalytic activity and cross-relate the subtle differences to the typical active behaviors of these complex materials.

Cu@TiO2 Janus microswimmers with a versatile motion mechanism.

We report novel metal-capped TiO2 photochemically-active colloids endowed with a 'hybrid drive': directional motion is achieved in water upon UV illumination, as well as in dilute peroxide solutions upon illumination with UV or visible light. From the different behaviours of nearby particles, we infer that distinct reaction pathways affect the local composition and flow of the solution.

Autocatalytic growth of ZnO nanorods from flat Au(111)-supported ZnO films.

Physical vapour deposition of ZnO on an Au(111) support has been investigated as a function of the oxygen chemical potential by means of scanning tunnelling microscopy and luminescence spectroscopy. Whereas a layer-by-layer growth of ZnO is revealed in oxygen excess, formation of oxide nanorods with large height-to-diameter ratio prevails at lower oxygen chemical potentials. We ascribe the formation of 3D nanostructures in the latter case to traces of Au atoms on the surface that promote trapping and dissociation of the incoming oxygen molecules. The Au residuals, acting as catalyst for the oxide formation, are indeed found on top of the ZnO nanorods.

Donor characteristics of transition-metal-doped oxides: Cr-doped MgO versus Mo-doped CaO.

The ability of Mo (Cr) impurities in a CaO (MgO) matrix to act as charge donors to adsorbed gold has been investigated by means of scanning tunneling microscopy and density functional theory. Whereas CaO(Mo) features a robust donor characteristic, as deduced from a charge-transfer-driven crossover in the Au particles' geometry in the presence of dopants, MgO(Cr) is electrically inactive. The superior performance of the CaO(Mo) system is explained by the ability of the Mo ions to evolve from a +2 oxidation state in ideal CaO to a +5 state by transferring up to three electrons to the Au adislands. Cr ions in MgO, on the other hand, are stable only in the +2 and +3 charge states and can provide a single electron at best. Since this electron is likely to be captured by cationic vacancies or morphological defects in the real oxide, no charge transfer to Au particles takes place in this case. On the basis of our findings, we have developed general rules on how to optimize the electron donor characteristics of doped oxide materials.

Direct immobilization of avidin protein on AFM tip functionalized by acrylic acid vapor at RF plasma.

The atomic force microscopy (AFM) has been used as a force sensor to measure unbinding forces of single bound complexes in the nanonewton and piconewton range. Force spectroscopy measurements can be applied to study both intermolecular and intramolecular interactions of complex biological and synthetic macromolecules. Although the AFM has been extensively used as a nano force sensor, the commercially available cantilever is limited to silicon and silicon nitride. Those materials reduce the adhesion sensitivity with specific surface and/or molecule. Here, we functionalized the AFM tip with carboxylic groups by applying acrylic acid (AA) vapor at radio frequency plasma treatment at 100 W for 5 min. This method provides a remarkable sensitivity enhancement on the functional group interaction specificity. The functionalized tip was characterized by scanning electron microscopy. The electron beam high resolution images have not shown significant tip sharpness modification. Silicon wafers (1 0 0)-no treated and functionalized by AA plasma treatment-were characterized by Auger electron spectroscopy to elucidate the silicon surface sputtering and demonstrate functionalization. The Fourier transform-infrared spectroscopy spectrum shows a high absorbance of avidin protein over the silicon surface functionalized by AA plasma treatment.We carried out force spectroscopy assay to measure the unbinding force between the well-established pair biotin-avidin. At pulling speed of 2 µm/s, we measured the unbinding force of 106 ± 23 pN, which is in good agreement with the literature, demonstrating the effectiveness of the tip functionalization by AA plasma treatment in biological studies.

Oxidation of Au by surface OH: nucleation and electronic structure of gold on hydroxylated MgO(001).

The nucleation and electronic structure of vapor-deposited Au on hydroxylated MgO(001) surfaces has been investigated under ultrahigh vacuum conditions. Hydroxylated MgO(001) surfaces with two different hydroxyl coverages, 0.4 and 1 monolayer, respectively, were prepared by exposure to water (D(2)O) at room temperature. Scanning tunneling microscopy experiments show significantly higher gold particle densities and smaller particle sizes on the hydroxylated MgO surface as compared to gold deposited on clean MgO(001). Infrared spectroscopy and X-ray photoelectron spectroscopy experiments were performed to reveal details about the initial nucleation of gold. Gold atoms are found to chemically interact with a specific type of hydroxyl groups on the MgO surface, leading to the formation of oxidized gold particles. The enhanced adhesion of Au particles, which is due to the formation of strong Au-O interfacial bonds, is responsible for the observed higher stability of small Au clusters toward thermal sintering on hydroxylated MgO surfaces. The results are compared to similar studies on Au/TiO(2)(110) model systems and powder samples prepared by the deposition-precipitation route.

Raman study of ion-induced defects in N-layer graphene.

Raman scattering is used to study the effect of low energy (90 eV) Ar(+) ion bombardment in graphene samples as a function of the number of layers N. The evolution of the intensity ratio between the G band (1585 cm(-1)) and the disorder-induced D band (1345 cm(-1)) with ion fluence is determined for mono-, bi-, tri- and ∼50-layer graphene samples, providing a spectroscopy-based method to study the penetration of these low energy Ar(+) ions in AB Bernal stacked graphite, and how they affect the graphene sheets. The results clearly depend on the number of layers. We also analyze the evolution of the overall integrated Raman intensity and the integrated intensity for disorder-induced versus Raman-allowed peaks.