Organization of Alkane Amines on a Gold Surface: Structure, Surface Dipole, and Electron Transfer

Surface molecular self-assembly is a fast advancing field with broad applications in molecular electronics, sensing and advanced materials. Although a large number of practical systems utilize alkanethiols, there is increasing interest in alkylamine self-assembled monolayers (SAMs). In this article, the molecular and electronic structure of alkylamine SAMs on Au surfaces was studied. It was found that amine-terminated alkanes self-assemble, forming a compact layer with the amine headgroup interacting directly with the Au surface and the hydrocarbon backbone tilted by around 30° with respect to the surface normal. The dense layers formed substantially decrease electron tunneling across the metal/solution interface and form a dipole layer with positive charges residing at the monolayer/vacuum interface

Molecular and Electronic Structure of Self-Assembled Monolayers Containing Ruthenium(II) Complexes on Gold Surfaces

Ru­(II) bipyridyl complexes were covalently bonded to self-assembled monolayers (SAM) on Au surfaces. Their molecular and electronic structure was studied by means of polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), photoelectron spectroscopies, scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. We found that attaching the Ru complex to the SAM does not cause great modifications to its molecular structure, which retains the alkyl chain 30 deg tilted with respect to the surface normal. Furthermore, the Ru center is located 20 Å away from the metal surface, i.e., at a sufficient distance to prevent direct electronic interaction with the substrate. Indeed the electronic structure of the Ru complex is similar to that of the free molecule with a HOMO molecular orbital mainly based on the Ru center located 2.1 eV below the Fermi edge and the LUMO molecular orbital based on the bipyridine groups located 1 eV above the Fermi level

Self-Assembled Monolayers of NH₂‑Terminated Thiolates: Order, p<i>K</i>_a, and Specific Adsorption

Self-assembled monolayers (SAMs) of amino-terminated alkanethiols on Au were characterized by a combination of electrochemical (LSV, CV, and EIS) and spectroscopic (XPS and SER) techniques. Clear correlations were obtained between the apparent surface p<i>K</i>_a values determined by impedimetric titrations and order parameters such as the content of trans conformers in the SAMs. These results contrast with previous studies that exhibit dispersions of up to 6 pH units in the reported p<i>K</i>_a values. In addition, we determined that inorganic and organic phosphate species bind specifically to these SAMs mediating adsorption and heterogeneous electron transfer of positively charged macromolecules such as cytochrome <i>c</i>

Structure, Dynamics, and Phase Behavior of Water in TiO₂ Nanopores

Mesoporous titania is a highly studied material due to its energy and environment-related applications, which depend on its tailored surface and electronic properties. Understanding the behavior of water in titania pores is a central issue for practical purposes in photocatalysis, solar cells, bone implants, or optical sensors. In particular, the mechanisms of capillary condensation of water in titania mesopores and the organization and mobility of water as a function of pore filling fraction are not yet known. In this work, molecular dynamics simulations of water confined in TiO₂-rutile pores of diameters 1.3, 2.8, and 5.1 nm were carried out at various water contents. Water density and diffusion coefficients were obtained as a function of the distance from the surface. The proximity to the interface affects density and diffusivity within a distance of around 10 Å from the walls, beyond which all properties tend to converge. The densities of the confined liquid in the 2.8 and the 5.1 nm pores decrease, respectively, 7% and 4% with respect to bulk water. This decrease causes the water translational mobility in the center of the 2.8 nm pore to be appreciably larger than in bulk. Capillary condensation takes place in equilibrium for a filling of 71% in the 2.8 nm pore and in conditions of high supersaturation in the 5.1 nm pore, at a filling of 65%. In the former case, the surface density increases uniformly with filling until condensation, whereas in the larger nanopore, a cluster of water molecules develops on a localized spot on the surface for fillings just below the transition. No phase transition is detected in the smaller pore. For all the systems studied, the first monolayer of water is strongly immobilized on the interface, thus reducing the accessible or effective diameter of the pore by around 0.6 nm. As a consequence, the behavior of water in these pores turns out to be comparable to its behavior in less hydrophilic pores of smaller size

Electrochemical and Diffusional Investigation of Na₂Fe^IIPO₄F Fluorophosphate Sodium Insertion Material Obtained from Fe^III Precursor

Sodium iron fluorophosphate (Na₂Fe^IIPO₄F) was synthesized by economic solvothermal combustion technique using Fe^III precursors, developing one-step carbon-coated homogeneous product. Synchrotron diffraction and Mössbauer spectroscopy revealed the formation of single-phase product assuming an orthorhombic structure (s.g. <i>Pbcn</i>) with Fe^II species. This Fe^III precursor derived Na₂Fe^IIPO₄F exhibited reversible Na⁺ (de)­intercalation with discharge capacity of 100 mAh/g at a rate of C/10 involving flat Fe^III/Fe^II redox plateaus located at 2.92 and 3.05 V (vs Na/Na⁺). It delivered good cycling stability and rate kinetics at room temperature. The stability of Na₂FePO₄F cathode was further verified by electrochemical impedance spectroscopy at different stages of galvanostatic analysis. Bond valence site energy (BVSE) calculations revealed the existence of 2-dimensional Na⁺ percolation pathways in the <i>a–c</i> plane with a moderate migration barrier of 0.6 eV. Combustion synthesized Na₂Fe^IIPO₄F forms an economically viable sodium battery material. Although the capacity of this cathode is relatively low, this study continues systematic work, which attempts to broaden the scope of reversible sodium insertion materials

Improving Energy Density and Structural Stability of Manganese Oxide Cathodes for Na-Ion Batteries by Structural Lithium Substitution

We report excellent cycling performance for P2–Na_0.6Li_0.2Mn_0.8O₂, an auspicious cathode material for sodium-ion batteries. This material, which contains mainly Mn⁴⁺, exhibits a long voltage plateau on the first charge, similar to that of high-capacity lithium and manganese-rich metal oxides. Electrochemical measurements, X-ray diffraction, and elemental analysis of the cycled electrodes suggest an activation process that includes the extraction of lithium from the material. The “activated” material delivers a stable, high specific capacity up to ∼190 mAh/g after 100 cycles in the voltage window between 4.6–2.0 V versus Na/Na⁺. DFT calculations locate the energy states of oxygen atoms near the Fermi level, suggesting the possible contribution of oxide ions to the redox process. The addition of Li to the lattice improves structural stability compared to many previously reported sodiated transition-metal oxide electrode materials, by inhibiting the detrimental structural transformation ubiquitously observed with sodium manganese oxides during cycling. This research demonstrates the prospect of intercalation materials for Na-ion battery technology that are active based on both cationic and anionic redox moieties