Highly dispersed molybdenum oxide supported on mesoporous silica SBA-15 has been prepared by anion exchange resulting in a series of catalysts with changing Mo densities (0.2-2.5 Mo atoms nm(-2) ). X-ray absorption, UV/Vis, Raman, and IR spectroscopy indicate that doubly anchored tetrahedral dioxo MoO4 units are the major surface species at all loadings. Higher reducibility at loadings close to the monolayer measured by temperature-programmed reduction and a steep increase in the catalytic activity observed in metathesis of propene and oxidative dehydrogenation of propane at 8 % of Mo loading are attributed to frustration of Mo oxide surface species and lateral interactions. Based on DFT calculations, NEXAFS spectra at the O-K-edge at high Mo loadings are explained by distorted MoO4 complexes. Limited availability of anchor silanol groups at high loadings forces the MoO4 groups to form more strained configurations. The occurrence of strain is linked to the increase in reactivity.

p-Type surface conductivity is a uniquely important property of hydrogen-terminated diamond surfaces. In this work, we report similar surface-dominated electrical properties in silicon nanowires (SiNWs). Significantly, we demonstrate tunable and reversible transition of p(+)-p-i-n-n(+) conductance in nominally intrinsic SiNWs via changing surface conditions, in sharp contrast to the only p-type conduction observed on diamond surfaces. On the basis of Si band energies and the electrochemical potentials of the ambient (pH value)-determined adsorbed aqueous layer, we propose an electron-transfer-dominated surface doping model, which can satisfactorily explain both diamond and silicon surface conductivity. The totality of our observations suggests that nanomaterials can be described as a core-shell structure due to their large surface-to-volume ratio. Consequently, controlling the surface or shell in the core-shell model represents a universal way to tune the properties of nanostructures, such as via surface-transfer doping, and is crucial for the development of nanostructure-based devices.

Energy band alignment plays an important role in heterojunction thin-film solar cells. In this work, we report the application of ternary Cd x Zn 1−x S buffer layers in antimony selenide (Sb 2 Se 3 ) thin-film solar cells. The results of our study revealed that the Cd/Zn element ratios not only affected the optical band gap of Cd x Zn 1−x S buffers but also modified the band alignment at the junction interface. A Sb 2 Se 3 solar cell with an optimal conduction-band offset value (0.34 eV) exhibited an efficiency of 6.71%, which represents a relative 32.1% enhancement as compared to the reference CdS/Sb 2 Se 3 solar cell. The results further indicated that a "spike"-like band structure suppressed the recombination rate at the interface and hence increased the device open-circuit voltage and fill factor. Electrochemical impedance spectroscopy analysis exhibited that the Cd x Zn 1−x S/Sb 2 Se 3 solar cell had higher recombination resistance and longer carrier lifetime than the CdS/Sb 2 Se 3 device. KEYWORDS: Sb 2 Se 3 , band alignment, Cd x Zn 1−x S/Sb 2 Se 3 heterojunction, substrate configuration, electrochemical impedance spectroscopy

Experiments using noncontact atomic force microscopy (NC-AFM) with CO-molecule-functionalized tips have distinctly imaged chemical structures within conjugated molecules. Here we describe a detailed model based on an ab initio approach of the interaction force between the AFM tip and the sample molecule that yields atomicscale images, which agree very well with the experimental images we considered. The key ingredient of our model is to explicitly include the effect on the image due to the tilt of the CO molecule at the tip apex resulting from the lateral force exerted by the sample. On the basis of this model, we specifically discuss the distortion seen in AFM images. As reported very recently, the distortion in AFM images originates from an intrinsic effect, namely, different extents of π-electron orbitals, as well as from an extrinsic effect, specifically CO tilt. We find that intrinsic distortion is scanning height dependent, attributing to the integrated electron density in the tip−sample overlapping region moving away from (the vertical projection of) the atom or bond positions. This intrinsic distortion is dominant in AFM images, although the atomic positions could be displaced even more by the extrinsic distortion due to CO tilt.

Oxygen core excitations in different molecular molybdena silica models are evaluated using density-functional theory (DFT). These results can be compared with in situ X-ray absorption fine structure (NEXAFS) measurements near the O K-edge of molybdena model catalysts supported on SBA-15 silica, used for exploratory catalytic activity studies. The comparison allows an analysis of structural details of the molybdena species. The silica support is found to contribute to the NEXAFS spectrum in an energy range well above that of the molybdena units, allowing a clear separation between the corresponding contributions. Different types of oxygen species, O(1) in terminal MdO bonds, O(2) in interphase Mo—O—Si bridges and in Mo—O—Mo linkages, as well as O(2) in terminal Mo—O—H groups can be distinguished in the theoretical spectra of the molybdena species with molybdenum in tetrahedral (dioxo species), pentahedral (monooxo species), and octahedral coordination. The experimental NEXAFS spectra exhibit a pronounced double-peak structure in the O 1s to Mo 4dO 2p excitation range of 529 536 eV. Comparison with the present theoretical data gives clear indications that dioxo molybdena species with tetrahedral MoO4 units can explain the experimental spectrum. This does not fully exclude species with other Mo coordination, like pentahedral. However, the latter are believed to exist in the present samples in much smaller amounts. The experimental NEXAFS spectrum for the supported molybdena species differs substantially from that for MoO3 bulk material with octahedral MoO6 units where the observed asymmetric peak structure is also reproduced by the calculations

Chemical etching method shows potential for large-scale and low-cost processed MXenes but incorporates surface terminations such as F&OH that probably deteriorate the lithium storage characteristics. Herein, we propose that tailoring appropriate surface functionalization and the intrinsic electrical properties can dramatically enhance the lithium storage capability of Ti3C2T x (T stands for F, OH, and O) MXene materials. By carefully controlling the annealing process, the Ti3C2T x films  possess fewer F&OH elements so that with higher conductivity, they are still freestanding and flexible. Density functional theory computations of the low-F&OH-containing Ti3C2T x show low-ion-diffusion barrier values of 0.34–0.43 eV and a significant increase of Li adsorption energy by 6–30 times compared to those of high-F&OH-containing Ti3C2T x , suggesting high Li-ion storage and transfer capabilities can be achieved in low-F&OH-containing Ti3C2T x MXenes. Dramatically, the heteroatom-controlled Ti3C2T x MXene films show reversible capacities of 221 mAh g–1 for lithium-ion batteries at a current density of 0.1 C (1 C = 320 mAh g–1), which is the highest up to now for pure Ti-based MXenes. These results demonstrate the importance of surface chemistry and electronic structure of MXene in the lithium storage capability, which provides valuable information on designing high-performance MXene-based materials for energy storage, optoelectronic, thermoelectric, and magnetic applications.

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Aufgesetztes Verhalten: Das Verändern der Oberfläche von Siliciumnanodrähten (SiNWs) durch unterschiedliche Endgruppen und/oder Adsorbate kann die elektrischen Eigenschaften der SiNWs beeinflussen. Durch eine solche zerstörungsfreie Modifizierung lässt sich die Leitfähigkeit von SiNWs für potenzielle Anwendungen anpassen.