Fluorite CeO 2 doped with group IV elements is studied within the density functional theory ( DFT ) and DFT + U framework. Concentration‐dependent formation energies are calculated for Ce 1− x Z x O 2 (Z = C, Si , Ge , Sn , Pb , Ti , Zr , Hf ) with 0 ≤ x ≤ 0.25 and a roughly decreasing trend with ionic radius is observed. The influence of the valence and near valence electronic configuration is discussed, indicating the importance of filled d and f shells near the Fermi level for all properties investigated. A clearly different behavior of group IVa and IVb dopants is observed: the former are more suitable for surface modifications and the latter are more suitable for bulk modifications. For the entire set of group IV dopants, there exists an inverse relation between the change, due to doping, of the bulk modulus, and the thermal expansion coefficients. Hirshfeld‐I atomic charges show that charge‐transfer effects due to doping are limited to the nearest‐neighbor oxygen atoms.
The structural characterization of complex crystalline materials can be simplified by closely comparing theoretical and experimental diffraction patterns.
Pt deposited on a Ge(001) surface spontaneously forms nanowire arrays. These nanowires are thermodynamically stable and can be hundreds of atoms long. The nanowires only occur on a reconstructed Pt-Ge-surface where they fill the troughs between the dimer rows on the surface. This unique connection between the nanowires and the underlying substrate make a thorough understanding of the latter necessary for understanding the growth of the nanowires. In this paper we study possible surface reconstructions containing 0.25 and 0.5 of a monolayer of Pt. Comparison of calculated scanning tunneling microscope (STM) images to experimental STM images of the surface reconstruction reveal that the Pt atoms are located in the top layer, creating a structure with rows of alternating Pt-Ge and Ge-Ge dimers in a $c(4\ifmmode\times\else\texttimes\fi{}2)$ arrangement. Our results also show that Pt atoms in the second or third layer cannot be responsible for the experimentally observed STM images.
We study formation of the nanowires formed after deposition of Pt on a Ge(001) surface. The nanowires form spontaneously after high temperature annealing. They are thermodynamically stable, only one atom wide and up to a few hundred atoms long. Ab initio density functional theory calculations are performed to identify possible structures of the Pt-Ge (001) surface with nanowires on top. A large number of structures is studied. With nanowires that are formed out of Pt or Ge dimers or mixed Pt-Ge dimers. By comparing simulated scanning tunneling microscopy images with experimental ones we model the formation of the nanowires and identify the geometries of the different phases in the formation process. We find that the formation of nanowires on a Pt-Ge(001) surface is a complex process based on increasing the Pt density in the top layers of the Ge(001) surface. Most remarkably we find the nanowires to consist of germanium dimers placed in troughs lined by mixed Pt-Ge dimer rows.
Sodium‐ion batteries are alternatives for lithium‐ion batteries in applications where cost‐effectiveness is of primary concern, such as stationary energy storage. The stability of sodium‐ion batteries is limited by the current generation of electrolytes, particularly at higher temperatures. Therefore, the search for an electrolyte which is stable at these temperatures is of utmost importance. Here, such electrolytes are introduced in the form of nonflammable deep eutectic solvents (DESs), consisting of sodium bis(trifluoromethane)sulfonimide (NaTFSI) dissolved in N ‐methyl acetamide (NMA). Increasing the NaTFSI concentration replaces NMA—NMA hydrogen bonds with strong ionic interactions between NMA, Na + , and TFSI − . These interactions lower NMA's highest occupied molecular orbital (HOMO) energy level compared with that of TFSI − , leading to an increased anodic stability (up to ≈4.65 V versus Na + /Na). (Na 3 V 2 (PO 4 ) 2 F 3 /carbon nanotube [CNT])/(Na 2+ x Ti 4 O 9 /C) full cells show 97.0% capacity retention after 250 cycles at 0.2 C and 55 °C. This is considerably higher than for (Na 3 V 2 (PO 4 ) 2 F 3 /CNT)/(Na 2+ x Ti 4 O 9 /C) full cells containing a conventional electrolyte. According to the electrochemical impedance analysis, the improved electrochemical stability is linked to the formation of more robust surface films at the electrode/electrolyte interface. The improved durability and safety highlight that DESs can be viable electrolyte alternatives for sodium‐ion batteries.
Metal–organic frameworks (MOFs) have gained much interest due to their intrinsic tunable nature. In this work, we study how linker functionalization modifies the electronic structure of the host MOF, more specifically, the MIL-47(V)-R (R = −F, −Cl, −Br, −OH, −CH3, −CF3, and −OCH3). It is shown that the presence of a functional group leads to a splitting of the π orbital on the linker. Moreover, the upward shift of the split-off π-band correlates well with the electron-withdrawing/donating nature of the functional groups. For halide functional groups the presence of lone-pair back-donation is corroborated by calculated Hirshfeld-I charges. In the case of the ferromagnetic configuration of the host MIL-47(V+IV) material a half-metal to insulator transition is noted for the −Br, −OCH3, and −OH functional groups, while for the antiferromagnetic configuration only the hydroxy group results in an effective reduction of the band gap.
We report the structural and electrochemical properties of P2-type Na 0.67 Mn 1-x Cu x O 2 (where x=0.20 - 0.50) via various techniques. X-ray diffraction (XRD) reveals a reduction of the unit cell volume upon substitution of Cu elucidated through detailed Rietveld analysis. The cyclic voltammetry (CV) behaviors are also affected by the Cu substitution indicating new redox reactions stemming from Cu presence. Galvanostatic cycling measurements at room temperature show that when x=0.35 in P2-type Na 0.67 Mn 1-x Cu x O 2 cathode, the best electrochemical performance is obtained. Na-ion diffusion rate is found to be strongly dependent upon the environmental temperature. Changes in the valence state and the local structures of P2-type Na 0.67 Mn 1-x Cu x O 2 during the charge/discharge are elucidated through the Operando X-ray absorption spectroscopy (XAS) technique.
