Band bending at heterovalent interfaces: hard X-ray photoelectron spectroscopy of GaP/Si(0 0 1) heterostructures. - In: Applied surface science, Bd. 565 (2021), 150514
https://doi.org/10.1016/j.apsusc.2021.150514
Atomic surface control of Ge(100) in MOCVD reactors coated with (Ga)As residuals. - In: Applied surface science, Bd. 565 (2021), 150513
Heteroepitaxy of planar, low-defect III-V semiconductor layers on Ge(100) requires a single-domain substrate surface, where dimer rows are aligned in parallel on atomically well-ordered terraces, which are separated by steps of even numbered atomic height. The presence of Ga and As in the sample ambience crucially impacts the preparation of such Ge(100) surfaces. Ga and As are commonly omnipresent, when applying metalorganic chemical vapor deposition (MOCVD), either directly supplied by precursors, in the form of MOCVD reactor residuals, or both. We study the impact of the growth conditions on the Ge(100) surface formation in situ, in dependence on the reactor pre-conditioning, the type of As supply, and/or temperature, utilizing surface-sensitive reflection anisotropy spectroscopy. We benchmark the in situ spectra to in system X-ray photoelectron spectroscopy, low energy electron diffraction and scanning tunneling microscopy. We find that interaction of tertiarybutylarsine (TBAs) with a coating of the inner MOCVD reactor walls by GaAs residuals favors desorption of As from reactor parts resulting in As-dimers on the Ge(100) surface, which are rotated by 90˚ compared to preparation routes employing TBAs in Ga-free ambience. The optical in situ control enables precise adjustment and switching between distinct Ge(100) surface reconstructions for subsequent III-V heteroepitaxy.
https://doi.org/10.1016/j.apsusc.2021.150513
Electric-field control of a single-atom polar bond. - In: Physical review letters, ISSN 1079-7114, Bd. 126 (2021), 21, 216801, insges. 6 S.
We expose the polar covalent bond between a single Au atom terminating the apex of an atomic force microscope tip and a C atom of graphene on SiC(0001) to an external electric field. For one field orientation, the Au-C bond is strong enough to sustain the mechanical load of partially detached graphene, while for the opposite orientation, the bond breaks easily. Calculations based on density-functional theory and nonequilibrium Green's function methods support the experimental observations by unveiling bond forces that reflect the polar character of the bond. Field-induced charge transfer between the atomic orbitals modifies the polarity of the different electronegative reaction partners and the Au-C bond strength.
https://doi.org/10.1103/PhysRevLett.126.216801
Electrical conductivity adjustment for interface capacitive-like storage in sodium-ion battery. - In: Advanced functional materials, ISSN 1616-3028, Bd. 31 (2021), 24, 2101081, insges. 11 S.
Sodium-ion battery (SIB) is significant for grid-scale energy storage. However, a large radius of Na ions raises the difficulties of ion intercalation, hindering the electrochemical performance during fast charge/discharge. Conventional strategies to promote rate performance focus on the optimization of ion diffusion. Improving interface capacitive-like storage by tuning the electrical conductivity of electrodes is also expected to combine the features of the high energy density of batteries and the high power density of capacitors. Inspired by this concept, an oxide-metal sandwich 3D-ordered macroporous architecture (3DOM) stands out as a superior anode candidate for high-rate SIBs. Taking Ni-TiO2 sandwich 3DOM as a proof-of-concept, anatase TiO2 delivers a reversible capacity of 233.3 mAh g^-1 in half-cells and 210.1 mAh g^-1 in full-cells after 100 cycles at 50 mA g^-1. At the high charge/discharge rate of 5000 mA g^-1, 104.4 mAh g^-1 in half-cells and 68 mAh g^-1 in full-cells can also be obtained with satisfying stability. In-depth analysis of electrochemical kinetics evidence that the dominated interface capacitive-like storage enables ultrafast uptaking and releasing of Na-ions. This understanding between electrical conductivity and rate performance of SIBs is expected to guild future design to realize effective energy storage.
https://doi.org/10.1002/adfm.202101081
Molecular dynamics and proton hyperpolarization via synthetic and crude oil porphyrin complexes in solid and solution states. - In: Langmuir, ISSN 1520-5827, Bd. 37 (2021), 22, S. 6783-6791
The use of vanadyl porphyrins either in synthetic compounds or naturally occurring in asphaltenes is investigated as a source of proton hyperpolarization via dynamic nuclear polarization (DNP) in nuclear magnetic resonance (NMR) experiments. The features of dynamics and location of the vanadyl VO2+ complex in aggregates within the oil asphaltene molecules are studied by means of DNP, electron paramagnetic resonance (EPR), and NMR field cycling relaxometry. Both the solid effect and Overhauser DNP were observed for the asphaltene solution in benzene, as well as in the solution and solid states for synthetic compounds. By comparison with a solution of synthetic vanadyl porphyrins, it is shown that vanadyl porphyrins in asphaltene aggregates are localized outside of the interface of the asphaltene aggregates and more exposed to the maltene molecules than free carbon-centered radicals associated with the core of asphaltene molecules. The perceptible contribution of scalar interaction is observed in solutions for both synthetic and asphaltene vanadyl porphyrins.
https://doi.org/10.1021/acs.langmuir.1c00882
Tunning of templated CuWO4 nanorods arrays thickness to improve photoanode water splitting. - In: Molecules, ISSN 1420-3049, Bd. 26 (2021), 10, 2900, insges. 14 S.
Im Titel ist "4" tiefgestellt
The fabrication of the photoanode of the n-type CuWO4 nanorod arrays was successfully carried out through electrochemical deposition using anodic aluminum oxide (AAO) control templates and for the first time produced distinct gaps between the nanorod arrays. The effectiveness and efficiency of the resulting deposition was shown by the performance of the photoelectrochemical (PEC) procedure with a current density of 1.02 mA cm^-2 with irradiation using standard AM 1.5G solar simulator and electron changed radiation of 0.72% with a bias potential of 0.71 V (vs. Ag/AgCl). The gap between each nanorod indicated an optimization of the electrolyte penetration on the interface, which resulted in the expansion of the current density as much as 0.5 × 1024 cm^-3 with a flat band potential of 0.14 V vs. Ag/AgCl and also a peak quantum efficiency of wavelength 410 nm. Thus, also indicating the gaps between the nanorod arrays is a promising structure to optimize the performance of the PEC water splitting procedure as a sustainable energy source.
https://doi.org/10.3390/molecules26102900
High-performance quasi-solid-state Na-air battery via gel cathode by confining moisture. - In: Advanced functional materials, ISSN 1616-3028, Bd. 31 (2021), 22, 2011151, insges. 9 S.
https://doi.org/10.1002/adfm.202011151
Insight into nickel-cobalt oxysulfide nanowires as advanced anode for sodium-ion capacitors. - In: Advanced energy materials, ISSN 1614-6840, Bd. 11 (2021), 18, 2100408, insges. 9 S.
Transition metal oxides have a great potential in sodium-ion capacitors (SICs) due to their pronouncedly higher capacity and low cost. However, their poor conductivity and fragile structure hinder their development. Herein, core-shell-like nickel-cobalt oxysulfide (NCOS) nanowires are synthesized and demonstrated as an advanced SICs anode. The bimetallic oxysulfide with multiple cation valence can promote the sodium ion adsorption and redox reaction, massive defects enable accommodation of the volume change in the sodiation/desodiation process, meanwhile the core-shell-like structure provides abundant channels for fast transfer of sodium ions, thereby synergistically making the NCOS electrode exhibit a high reversible sodium ion storage capacity (1468.5 mAh g^-1 at 0.1 A g^-1) and an excellent cyclability (90.5% capacity retention after 1000 cycles). The in-situ X-ray diffraction analysis unravels the insertion and conversion mechanism for sodium storage in NCOS, and the enhanced capability of NCOS is further verified by the kinetic analysis and theoretical calculations. Finally, SICs consisting of the NCOS anode and a boron-nitrogen co-doped carbon nanotubes cathode deliver an energy density of 205.7 Wh kg^-1, a power density of 22.5 kW kg^-1, and an outstanding cycling lifespan. These results indicate an efficient strategy in designing a high-performance anode for sodium storage based on bimetallic dianion compounds.
https://doi.org/10.1002/aenm.202100408
Molecular dynamics in ionic liquid/radical systems. - In: The journal of physical chemistry, ISSN 1520-5207, Bd. 125 (2021), 18, S. 4850-4862
Molecular dynamics of the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide (Emim-Tf2N) with either of the four organic stable radicals, TEMPO, 4-benzoyloxy-TEMPO, BDPA, and DPPH, is studied by using Nuclear Magnetic Resonance (NMR) and Dynamic Nuclear Polarization (DNP). In complex fluids at ambient temperature, NMR signal enhancement by DNP is frequently obtained by a combination of several mechanisms, where the Overhauser effect and solid effect are the most common. Understanding the interactions of free radicals with ionic liquid molecules is of particular significance due to their complex dynamics in these systems, influencing the properties of the ion-radical interaction. A combined analysis of EPR, DNP, and NMR relaxation dispersion is carried out for cations and anions containing, respectively, the NMR active nuclei 1H or 19F. Depending on the size and the chemical properties of the radical, different interaction processes are distinguished, namely, the Overhauser effect and solid effect, driven by dominating dipolar or scalar interactions. The resulting NMR relaxation dispersion is decomposed into rotational and translational contributions, allowing the identification of the corresponding correlation times of motion and interactions. The influence of electron relaxation time and electron-nuclear spin hyperfine coupling is discussed.
https://doi.org/10.1021/acs.jpcb.1c02118
Recent advances in ferromagnetic metal sulfides and selenides as anodes for sodium- and potassium-ion batteries. - In: Journal of materials chemistry, ISSN 2050-7496, Bd. 9 (2021), 15, S. 9506-9534
In next-generation rechargeable batteries, sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) have been considered as attractive alternatives to lithium-ion batteries due to their cost competitiveness. Anodes with complicated electrochemical mechanisms determine the performance and safety of battery systems to a large degree. Among a wide range of anode materials for SIBs and PIBs, ferromagnetic metal (Fe, Co, and Ni) sulfides and selenides have captured prominent attention by virtue of their high theoretical capacities, suitable potentials, and relatively low price. Although some breakthrough results have been achieved, a few intrinsic issues stemming from the materials themselves need to be further explored and studied, especially in the field of PIBs, an emerging research interest. Herein, in this review, we highlight the pioneering investigation of typical ferromagnetic metal sulfides and selenides for application as anodes in SIBs and PIBs and overview their recent research progress. Meanwhile, the preparation methods, structural characteristics, charge storage mechanisms, and electrochemical properties are outlined. Finally, the present challenges and research perspectives are discussed.
https://doi.org/10.1039/D1TA00831E