Effect of Surface Chemistry on the Electrical Double Layer in a Long-Chain Ionic Liquid.

Room temperature ionic liquids (ILs) can create a strong accumulation of charges at solid interfaces by forming a very thin and dense electrical double layer (EDL). The structure of this EDL has important consequences in numerous applications involving ILs, for example, in supercapacitors, sensors, and lubricants, by impacting the interfacial capacitance, the charge carrier density of semiconductors, as well as the frictional properties of the interfaces. We have studied the interfacial structure of a long chain imidazolium-based IL (1-octyl-3-methylimidazolium dicyanamide) on several substrates: mica, silica, silicon, and molybdenum disulfide (MoS2), using atomic force microscopy (AFM) experiments and molecular dynamics (MD) simulations. We have observed 3 types of interfacial structures for the same IL, depending on the chemistry of the substrate and the water content, showing that the EDL structure is not an intrinsic property of the IL. We evidenced that at a low water content, neutral and apolar (thus hydrophobic) substrates promote a thin layer structure, where the ions are oriented parallel to the substrate and cations and anions are mixed in each layer. In contrast, a strongly charged (thus hydrophilic) substrate yields an extended structuration into several bilayers, while a heterogeneous layering with loose bilayer regions was observed on an intermediate polar and weakly charged substrate and on an apolar one at a high bulk water content. In the latter case, water contamination favors the formation of bilayer patches by promoting the segregation of the long chain IL into polar and apolar domains.

Oxidation of copper nanowire based transparent electrodes in ambient conditions and their stabilization by encapsulation: application to transparent film heaters.

Whereas the integration of silver nanowires in functional devices has reached a fair level of maturity, the integration of copper nanowires still remains difficult, mainly due to the intrinsic instability of copper nanowires in ambient conditions. In this paper, copper nanowire based transparent electrodes with good performances (33 Ω sq-1 associated with 88% transparency) were obtained, and their degradation in different conditions was monitored, in particular by electrical measurements, transmission electron microscopy, x-ray photoelectron spectrometry and Auger electron spectroscopy. Several routes to stabilize the random networks of copper nanowires were evaluated. Encapsulation through laminated barrier film with optical clear adhesive and atmospheric pressure spatial atomic layer deposition were found to be efficient and were used for the fabrication of transparent film heaters.

Cu/Cu2 O Electrodes and CO2 Reduction to Formic Acid: Effects of Organic Additives on Surface Morphology and Activity.

Copper/copper oxide (Cu/Cu2 O) electrodes are known to display interesting electrocatalytic performances for the reduction of CO2 , and thus, deserve further investigation for optimization. Here, we show that the addition of nitrogen-based organic additives greatly improves the activity of these electrodes (higher current densities, greater selectivity, and higher faradaic yields). The best effector is found to be tetramethyl cyclam. For example, electrolysis at -2.0 V versus Fc+ /Fc in CO2 -saturated DMF/H2 O (99:1, v/v) in the presence of this effector results in formic acid with almost 90 % faradaic yield. SEM and XPS analysis of the electrode surface reveals that the organic additive promotes the formation of active Cu0 nanoparticles from Cu2 O during electrolysis. This simple approach provides a straightforward strategy toward the optimization of Cu/Cu2 O electrodes.

Evidence for Charge Transfer at the Interface between Hybrid Phosphomolybdate and Epitaxial Graphene.

The interfacing of polyoxometalates and graphene can be considered to be an innovative way to generate hybrid structures that take advantage of the properties of both components. Polyoxometalates are redox-sensitive and photosensitive compounds with high temperature stability (up to 400 °C for some), showing tunable properties depending on the metal incorporated inside the complex. Graphene has a unique electronic band structure combined with good material properties for electrical and optical applications. The spontaneous, rather than electrochemical, functionalization of epitaxial graphene on SiC with Keggin phosphomolybdate derivative TBA3[PMo11O39{Sn(C6H4)C≡C(C6H4)N2}] (named K(Mo)Sn[N2(+)]) bearing a phenyl diazonium unit is investigated. Graphene decoration is evidenced by means of AFM, Raman, XPS, and cyclic voltammetry, indicating a successful immobilization of the polyoxomolybdate. The covalent bonding of the polyoxometalate to the graphene substrate can be deduced from the appearance of a D band in the Raman spectra and from the loss of mobility in the electrical conduction. High-resolution XPS spectra reveal an electron transfer from the graphene to the Mo complex. The comparison of charge-carrier density measurements before and after grafting supports the p-type doping effect, which is further evidenced by work function UPS measurements.

Ultrafast photocarrier dynamics in nanocrystalline ZnOxNy thin films.

We examined the ultrafast dynamics of photocarriers in nanocrystalline ZnOxNy thin films as a function of compositional variation using femtosecond differential transmittance spectroscopy. The relaxation dynamics of photogenerated carriers and electronic structures are strongly dependent on nitrogen concentration. Photocarriers of ZnOxNy films relax on two different time scales. Ultrafast relaxation over several picoseconds is observed for all chemical compositions. However, ZnO and oxygen-rich phases show slow relaxation (longer than several nanoseconds), whereas photocarriers of films with high nitrogen concentrations relax completely on subnanosecond time scales. These relaxation features may provide a persistent photocurrent-free and prompt photoresponsivity for ZnOxNy with high nitrogen concentrations, as opposed to ZnO for display applications.

Toward the Improvement of Silicon-Based Composite Electrodes via an In-Situ Si@C-Graphene Composite Synthesis for Li-Ion Battery Applications.

Using Si as anode materials for Li-ion batteries remain challenging due to its morphological evolution and SEI modification upon cycling. The present work aims at developing a composite consisting of carbon-coated Si nanoparticles (Si@C NPs) intimately embedded in a three-dimensional (3D) graphene hydrogel (GHG) architecture to stabilize Si inside LiB electrodes. Instead of simply mixing both components, the novelty of the synthesis procedure lies in the in situ hydrothermal process, which was shown to successfully yield graphene oxide reduction, 3D graphene assembly production, and homogeneous distribution of Si@C NPs in the GHG matrix. Electrochemical characterizations in half-cells, on electrodes not containing additional conductive additive, revealed the importance of the protective C shell to achieve high specific capacity (up to 2200 mAh.g-1), along with good stability (200 cycles with an average Ceff > 99%). These performances are far superior to that of electrodes made with non-C-coated Si NPs or prepared by mixing both components. These observations highlight the synergetic effects of C shell on Si NPs, and of the single-step in situ preparation that enables the yield of a Si@C-GHG hybrid composite with physicochemical, structural, and morphological properties promoting sample conductivity and Li-ion diffusion pathways.

ZIF-8 thin films by a vapor-phase process: limits to growth.

Metal-organic frameworks are a class of porous materials that show promising properties in the field of microelectronics. To reach industrial use of these materials, gas phase techniques are often preferred and were recently introduced. However, the thicknesses achieved are not sufficient, limiting further development. In this work, an improved gas phase process allowing ZIF-8 layer formation of several hundreds of nm using cyclic ligand/water exposures is described. Then, by a combination of in-depth surface analyses and molecular dynamics simulations, the presence and role of hydroxyl defects in the ZIF-8 layer to reach this thickness are established. At the same time, this study unveils an inherent limit of the method: thickness growth is consubstantial with defect repairing upon the crystallites ripening; such defect repairing eventually leads to the decrease of the pore window below the diffusion radius of the incoming linker, thus apparently capping the maximum MOF thickness observable for this type of material topology through this growth method.

Lithium-Rich Rock Salt Type Sulfides-Selenides (Li2TiSexS3-x): High Energy Cathode Materials for Lithium-Ion Batteries.

Lithium-rich disordered rocksalt Li2TiS3 offers large discharge capacities (>350 mAh·g−1) and can be considered a promising cathode material for high-energy lithium-ion battery applications. However, the quick fading of the specific capacity results in a poor cycle life of the system, especially when liquid electrolyte-based batteries are used. Our efforts to solve the cycling stability problem resulted in the discovery of new high-energy selenium-substituted materials (Li2TiSexS3−x), which were prepared using a wet mechanochemistry process. X-ray diffraction analysis confirmed that all compositions were obtained in cation-disordered rocksalt phase and that the lattice parameters were expanded by selenium substitution. Substituted materials delivered large reversible capacities, with smaller average potentials, and their cycling stability was superior compared to Li2TiS3 upon cycling at a rate of C/10 between 3.0−1.6 V vs. Li+/Li.

Cobalt oxide polymorph growth on electrostatic self-assembled nanoparticle arrays for dually tunable nano-textures.

We report on a method for surface nano-texturing on a plastic substrate. Nano-objects with a silica nanoparticle core and a textured cobalt oxide crown are created with selectable density on the plastic substrate. The resulting dual morphology is easily tuned over large areas, either by changing the parameters directing nanoparticle deposition through electrostatic self-arrangement for nano-object density control, or the parameter directing cobalt oxide deposition for shape control. The entire process takes place at room temperature, with no chemicals harmful to the plastic substrate. The ready modulation of the dual morphology is used to control the wettability properties of the plastic film, which is covered by nano-objects.

Corrigendum: Dispersion and Stabilization of Exfoliated Graphene in Ionic Liquids.

[This corrects the article DOI: 10.3389/fchem.2019.00223.].

Interfacial complexation driven three-dimensional assembly of cationic phosphorus dendrimers and graphene oxide sheets.

High content nitrogen, sulfur and phosphorus heteroatoms assembled in tree-like dendrimers (DG n ) are confined within the galleries of two-dimensional graphene oxide (GO). The presence of the ternary diethyl-N-ethyl-ammonium groups on the dendrimer peripheries ensures the exfoliation of graphene sheets thereby affording interfacially bridged, three-dimensional heteroatom-enriched graphene-based hybrid nanostructures (DG n -GO). Dendrimer generation (from 1 to 4) that reflects the bulkiness of these conceived nano-trees impacts increasingly the degree of dispersion-exfoliation and sheet desordering. The long-term stability of these aqueous suspensions associated with their handling flexibility allows uniform accommodation of the resulting hybrid materials as flame-retardants in bioplastics.

Influence of the Core/Shell Structure of Indium Phosphide Based Quantum Dots on Their Photostability and Cytotoxicity.

With the goal to improve their photostability, InP-based QDs are passivated with three types of inorganic shells, namely (i) a gradient ZnSexS1-x shell, (ii) an additional ZnS shell on top of the gradient shell with two different thicknesses (core/shell/shell, CSS), (iii) an alumina coating on top of ZnS. All three systems have photoluminescence quantum yields (PLQY) > 50% and similar PL decay times (64-67 ns). To assess their photostability they are incorporated into a transparent poly (methyl methacrylate) (PMMA) matrix and exposed to continuous irradiation with simulated sunlight in a climate chamber. The alumina coated core/shell system exhibits the highest stability in terms of PLQY retention as well as the lowest shift of the PL maximum and lowest increase of the PL linewidth, followed by the CSS QDs and finally the gradient shell system. By means of XPS studies we identify the degradation of the ZnS outer layer and concomitant oxidation of the emissive InZnP core as the main origins of degradation in the gradient structure. These modifications do not occur in the case of the alumina-capped sample, which exhibits excellent chemical stability. The gradient shell and CSS systems could be transferred to the aqueous phase using surface ligand exchange with penicillamine. Cytotoxicity studies on human primary keratinocytes revealed that exposure for 24 h to 6.25-100 nM of QDs did not affect cell viability. However, a trend toward reduced cell proliferation is observed for higher concentrations of gradient shell and CSS QDs with a thin ZnS shell, while CSS QDs with a thicker ZnS shell do not exhibit any impact.

Dispersion and Stabilization of Exfoliated Graphene in Ionic Liquids.

The liquid-phase exfoliation of graphite is one of the most promising methods to increase production and commercial availability of graphene. Because ionic liquids can be easily obtained with chosen molecular structures and tuneable physicochemical properties, they can be use as media to optimize the exfoliation of graphite. The understanding of the interactions involved between graphite and various chemical functions in the solvent ions will be helpful to find liquids capable of dissociating and stabilizing important quantities of large graphene layers. After a step of sonication, as a mechanical precursor, samples of suspended exfoliated graphene in different ionic liquids have been characterized experimentally in terms of flake size, number of layers, total concentration and purity of the exfoliated material. Nine different ionic liquids based on imidazolium, pyrrolidinium and ammonium cations and on bis(trifluoromethylsulfonyl)imide, triflate, dicyanamide, tricyanomethanide, and methyl sulfate anions have been tested. UV-vis, Raman and X-ray photoelectron in addition to high resolution transmission electron and atomic force microscopy have been selected to characterize suspended exfoliated graphene in ionic liquids. The number of layers in the flakes exfoliated, the size and concentration depend of the structure of the ionic liquid selected. In order to obtain large flake sizes, ionic liquids with bis(trifluoromethylsulfonyl)imide anions and a cation with an alkyl chain of medium length should be selected. Smaller cation and anion favors the exfoliation of graphene. The exfoliation caused the formation of C-H bonds and the oxidation of the graphitic surface.

Nitridation-driven conductive Li4Ti5O12 for lithium ion batteries.

To modify oxide structure and introduce a thin conductive film on Li4Ti5O12, thermal nitridation was adopted for the first time. NH3 decomposes surface Li4Ti5O12 to conductive TiN at high temperature, and surprisingly, it also modifies the surface structure in a way to accommodate the single phase Li insertion and extraction. The electrochemically induced Li4+deltaTi5O12 with a TiN coating layer shows great electrochemical properties at high current densities.

Tailoring electronic structures of carbon nanotubes by solvent with electron-donating and -withdrawing groups.

Various electron-donating and -withdrawing groups in aromatic and aliphatic backbones of solvent have been introduced to tailor the electronic structures of single-walled carbon nanotubes (SWCNTs). In the case of solvent with a withdrawing group, electrons were extracted mainly from metallic SWCNTs, whereas small charge transfer was also observed in semiconducting SWCNTs. On the other hand, in the case of solvent with a donating group, electrons were donated to both metallic and semiconducting SWCNTs. This effect was less prominent in solvent with an aliphatic backbone than that with an aromatic backbone. The strong correlation between the sheet resistance and electronic structures of nanotubes is further discussed in conjunction with a modulation of Schottky barrier height.

Selective oxidation on metallic carbon nanotubes by halogen oxoanions.

Chlorine oxoanions with the chlorine atom at different oxidation states were introduced in an attempt to systematically tailor the electronic structures of single-walled carbon nanotubes (SWCNTs). The degree of selective oxidation was controlled systematically by the different oxidation state of the chlorine oxoanion. Selective suppression of the metallic SWCNTs with a minimal effect on the semiconducting SWCNTs was observed at a high oxidation state. The adsorption behavior and charge transfer at a low oxidation state were in contrast to that observed at a high oxidation state. Density functional calculations demonstrated the chemisorption of chloro oxoanions at the low oxidation state and their physisorption at high oxidation states. These results concurred with the experimental observations from X-ray photoelectron spectroscopy. The sheet resistance of the SWCNT film decreased significantly at high oxidation states, which was explained in terms of a p-doping phenomenon that is controlled by the oxidation state.

Fermi level engineering of single-walled carbon nanotubes by AuCl3 doping.

We investigated the modulation of optical properties of single-walled carbon nanotubes (SWCNTs) by AuCl 3 doping. The van Hove singularity transitions (E 11 (S), E 22 (S), E 11 (M)) in absorption spectroscopy disappeared gradually with an increasing doping concentration and a new peak appeared at a high doping concentration. The work function was downshifted up to 0.42 eV by a strong charge transfer from the SWCNTs to AuCl 3 by a high level of p-doping. We propose that this large work function shift forces the Fermi level of the SWCNTs to be located deep in the valence band, i.e., highly degenerate, creating empty van Hove singularity states, and hence the work function shift invokes a new asymmetric transition in the absorption spectroscopy from a deeper level to newly generated empty states.

Heteroleptic, two-coordinate [M(NHC){N(SiMe3)2}] (M = Co, Fe) complexes: synthesis, reactivity and magnetism rationalized by an unexpected metal oxidation state.

The linear, two-coordinate and isostructural heteroleptic [M(IPr){N(SiMe3)2}] (IPr = 1,3-bis(diisopropylphenyl)-imidazol-2-ylidene), formally MI complexes (M = Co, 3; Fe, 4) were obtained by the reduction of [M(IPr)Cl{N(SiMe3)2}] with KC8, or [Co(IPr){N(SiMe3)2}2] with mes*PH2, mes* = 2,4,6-tBu3C6H2. The magnetism of 3 and 4 implies CoII and FeII centres coupled to one ligand-delocalized electron, in line with XPS and XANES data; the ac susceptibility of 4 detected a pronounced frequency dependence due to slow magnetization relaxation. Reduction of [Fe(IPr)Cl{N(SiMe3)2}] with excess KC8 in toluene gave the heteronuclear 'inverse-sandwich' Fe-K complex 7, featuring η6-toluene sandwiched between one Fe0 and one K+ centre.

High mobility and high stability glassy metal-oxynitride materials and devices.

In thin film technology, future semiconductor and display products with high performance, high density, large area, and ultra high definition with three-dimensional functionalities require high performance thin film transistors (TFTs) with high stability. Zinc oxynitride, a composite of zinc oxide and zinc nitride, has been conceded as a strong substitute to conventional semiconductor film such as silicon and indium gallium zinc oxide due to high mobility value. However, zinc oxynitride has been suffered from poor reproducibility due to relatively low binding energy of nitrogen with zinc, resulting in the instability of composition and its device performance. Here we performed post argon plasma process on zinc oxynitride film, forming nano-crystalline structure in stable amorphous matrix which hampers the reaction of oxygen with zinc. Therefore, material properties and device performance of zinc oxynitride are greatly enhanced, exhibiting robust compositional stability even exposure to air, uniform phase, high electron mobility, negligible fast transient charging and low noise characteristics. Furthermore, We expect high mobility and high stability zinc oxynitride customized by plasma process to be applicable to a broad range of semiconductor and display devices.

New Method for Super Hydrophobic Treatment of Gas Diffusion Layers for Proton Exchange Membrane Fuel Cells Using Electrochemical Reduction of Diazonium Salts.

The purpose of this article is to report a new method for the surface functionalization of commercially available gas diffusion layers (GDLs) by the electrochemical reduction of diazonium salt containing hydrophobic functional groups. The method results in superhydrophobic GDLs, over a large area, without pore blocking. An X-ray photoelectron spectroscopy study based on core level spectra and chemical mapping has demonstrated the successful grafting route, resulting in a homogeneous distribution of the covalently bonded hydrophobic molecules on the surface of the GDL fibers. The result was corroborated by contact angle measurement, showing similar hydrophobicity between the grafted and PTFE-modified GDLs. The electrochemically modified GDLs were tested in proton exchange membrane fuel cells under automotive, wet, and dry conditions and demonstrated improved performance over traditional GDLs.