Characteristics of Lithium Ions and Superoxide Anions in EMI-TFSI and Dimethyl Sulfoxide

To clarify the microscopic effects of solvents on the formation of the Li⁺-O₂^– process of a Li–O₂ battery, we studied the kinetics and thermodynamics of these ions in dimethyl sulfoxide (DMSO) and 1-ethyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide (EMI-TFSI) using classical molecular dynamics simulation. The force field for ions–solvents interactions was parametrized by force matching first-principles calculations. Despite the solvation energies of the ions are similar in both solvents, their mobility is much higher in DMSO. The free-energy profiles also confirm that the formation and decomposition rates of Li⁺-O₂^– pairs are greater in DMSO than in EMI-TFSI. Our atomistic simulations point out that the strong structuring of EMI-TFSI around the ions is responsible for these differences, and it explains why the LiO₂ clusters formed in DMSO during the battery discharge are larger than those in EMI-TFSI. Understanding the origin of such properties is crucial to aid the optimization of electrolytes for Li–O₂ batteries

Imaging the Evolution of <i>d</i> States at a Strontium Titanate Surface

Oxide electronics is a promising alternative to the conventional silicon-based semiconductor technology, owing to the rich functionalities of oxide thin films and heterostructures. In contrast to the silicon surface, however, the electronic structure of the SrTiO₃ surface, the most important substrate for oxide thin films growth, is not yet completely understood. Here we report on the electronic states of a reconstructed (001) surface of SrTiO₃ determined in real space, with scanning tunneling microscopy/spectroscopy and density functional theory calculations. We found a remarkable energy dependence of the spectroscopic image: Theoretical analysis reveals that symmetry breaking at the surface lifts the degeneracy in the <i>t</i>_2<i>g</i> state (<i>d</i>_<i>xy</i>, <i>d</i>_<i>yz</i>, and <i>d</i>_<i>zx</i>) of Ti 3<i>d</i> orbitals, whose anisotropic spatial distribution leads to a sharp transition in the spectroscopic image as a function of energy. The knowledge obtained here could be used to gain further insights into emergent phenomena at the surfaces and interfaces with SrTiO₃

Self-Assembly Strategy for Fabricating Connected Graphene Nanoribbons

We use self-assembly to fabricate and to connect precise graphene nanoribbons end to end. Combining scanning tunneling microscopy, Raman spectroscopy, and density functional theory, we characterize the chemical and electronic aspects of the interconnections between ribbons. We demonstrate how the substrate effects of our self-assembly can be exploited to fabricate graphene structures connected to desired electrodes

Electronic Structure and Photoelectrochemical Properties of an Ir-Doped SrTiO₃ Photocatalyst

The effect of iridium valence in Ir:SrTiO₃ on the electronic structure and the photocatalytic activity in a water splitting reaction was studied. Epitaxial thin film photoelectrodes were grown with controlled Ir valence and used to measure the electrochemical efficiency of Ir:SrTiO₃. The positions of the in-gap Ir impurity levels were determined by optical and X-ray photoelectron spectroscopies. Comparison with ab initio calculations was used to assign the observed electronic states to either Ir⁴⁺ or Ir³⁺ dopants in SrTiO₃. The measurements show that Ir³⁺:SrTiO₃ forms a single midgap impurity state that is strongly localized, completely quenching the photoelectrochemical response. An anodic photoresponse was seen in Ir⁴⁺:SrTiO₃ under visible-light illumination up to a wavelength of 600 nm (hν = 2.0 eV). Ir⁴⁺:SrTiO₃ contains an in-gap state close to the top of the SrTiO₃ valence band. The performance of Ir⁴⁺:SrTiO₃ in electrochemical reactions is compared with cathodic Rh³⁺:SrTiO₃, clearly illustrating the importance of strong dopant d-electron hybridization with the oxygen 2p valence band of SrTiO₃ for improving the energy conversion efficiency in SrTiO₃-based photocatalysts

Bottom-Up Graphene-Nanoribbon Fabrication Reveals Chiral Edges and Enantioselectivity

We produce precise chiral-edge graphene nanoribbons on Cu{111} using self-assembly and surface-directed chemical reactions. We show that, using specific properties of the substrate, we can change the edge conformation of the nanoribbons, segregate their adsorption chiralities, and restrict their growth directions at low surface coverage. By elucidating the molecular-assembly mechanism, we demonstrate that our method constitutes an alternative bottom-up strategy toward synthesizing defect-free zigzag-edge graphene nanoribbons

Elucidation of Rh-Induced In-Gap States of Rh:SrTiO₃ Visible-Light-Driven Photocatalyst by Soft X‑ray Spectroscopy and First-Principles Calculations

The occupied and unoccupied in-gap electronic states of a Rh-doped SrTiO₃ photocatalyst were investigated by X-ray emission spectroscopy and X-ray absorption spectroscopy for different Rh impurity valence states and doping levels. An unoccupied midgap Rh⁴⁺ acceptor state was found 1.5 eV below the SrTiO₃ conduction band minimum. Both Rh⁴⁺ and Rh³⁺ dopants were found to have an occupied donor level close to the valence band maximum of SrTiO₃. The density of states obtained from first-principles calculations show that all observed spectral features can be assigned to electronic states of substitutional Rh at the Ti site and that Rh:SrTiO₃ is an unusual titanate compound with a characteristic p-type electronic structure. The Rh doping results in a large decrease of the bandgap energy, making Rh:SrTiO₃ an attractive material for use as a visible-light-driven H₂-evolving photocatalyst in a solar water splitting reaction