Charge Propagation Research Articles

Development and kinetic characterization of hierarchical bioelectrocatalytic system utilizing a redox mediator, functionalized carbon nanotubes and an enzyme for glucose oxidation

A multifunctional bioelectrocatalytic material that permits shifting the onset potential for the oxidation of glucose to the value starting below −0.2V (vs. Ag/AgCl) is described. It includes carbon nanotubes modified with 4‑(pyrrole‑1‑yl) benzoic acid, which introduces carboxyl groups that play an important role in enzyme (glucose oxidase) immobilization and improve the stability of the system. Glucose oxidase has been immobilized by a chemical crosslinking method, where bovine serum albumin and glutaraldehyde are used as a blocking and a crosslinking agent, respectively. Here N-methylphenazonium methyl sulfate has been considered as co-catalyst and a component of the redox mediating system facilitating the effective flow of electrons from redox centers of the enzyme to the electrode surface at fairly negative potentials. The mediator has been confined to the system by the electrostatic attraction with the sulfonate groups of Nafion. Dynamics of charge propagation in the proposed hybrid bioelectrocatalytic films have been assessed from chronocoulometric measurements. The combination of carbon nanotubes, glucose oxidase and N-methylphenazonium methyl sulfate produces a system that is capable of effective oxidation of glucose in 0.1moldm−3 phosphate buffer solution at pH7.0. It can be concluded on the basis of various biochemical and electroanalytical approaches that the system is well-behaved, stable, and redox facile. The Michaelis-Menten constant and the maximum current density (measured in the glucose-containing solutions) are 18mmoldm−3 and 0.32mAcm−2, respectively. By referring to fundamentals of electrochemical kinetic analysis, the rate constant in heterogeneous units, 5∗10−5cms−1, has been determined. The calibration plot has a linear response range for glucose concentrations up to ca. 10–12mmoldm−3 (sensitivity, 12.1μAmmol−1dm3cm−2). An observation of importance to the biofuel cell technology is the appearance of the glucose oxidation voltammetric peak at fairly negative potentials, namely ca. −0.05V (vs. Ag/AgCl).

Direct investigation of charge transfer in neurons by electrostatic force microscopy

Charge transfer plays fundamental roles in information transmission in cells, especially in neurons. To date, direct observation of charge propagation in neurons at nanometer level has not been achieved yet. Herein, a combined charge injection and Electrostatic Force Microscopy (EFM) detection approach is applied to directly study charge propagation and distribution at nanometer resolution in spines and synapses of hippocampal neurons. Charge density, charge mobility and membrane potential in neural signal transmission process through the spines of axons and dendrites of hippocampal neurons were investigated quantitatively. Postsynaptic densities (PSD) in spines of axons and dendrites were revealed and studied. The methods and results from present work provide insights into physiological activities and processes related with electrical properties in nervous system and other biological samples.

Determination of Lee‐Tarver Model Parameters of JO‐11C Explosive

AbstractIn order to determine the Lee‐Tarver model parameters of JO‐11C when the density is 1.707 g/cm3, the detonation propagation of cylindrical charge with different diameters is modelling by AUTODYN software. The output performance parameters such as critical dimension, detonation velocity, detonation pressure and reaction time of different charge diameters through simulation are consistent with the experimental results by fixing and adjusting material model parameters of the JO‐11C explosive .The results show that the critical dimension is between 0.4 mm and 0.5 mm, the detonation velocity and pressure increase with the charge diameter increasing and the rule of reaction time is opposite. According to the determined parameters of the Lee‐Tarver model, the detonation pressure attenuation of JO‐11C charge with different diameter bearing on different steel gap were simulated and compared with the test. The simulation results are in good agreement with the experimental results when charge diameter is greater than 2 mm, which the error is less than 10 %. In smaller charge diameter, the error is also not more than 20 %.

Charge Propagation in Redox-Conducting Materials: Diagnosis of Mechanisms and Dynamics with Microelectrochemical Devices

Historically, electrochemistry in the absence of a bulk liquid phase has focused on the problems of energy storage and production. Recently, it has become possible to apply conventional electrochemical methods to characterize solid-state type redox electrolytes with respect to analytical characterization of materials, measurement of electron transfer rates, and determination of analytes using amperometric sensing. The characterization of materials by solid-state voltammetry primarily has focused on mixed-valence, ionically conducting compounds and on diverse organic ionomers hosting identifiable molecular (redox) sites. We are going to address practical issues related to the principles and analytical aspects of electrochemical studies (typically done at room temperature) of solid, rigid or semi-rigid (nonfluid) systems in the absence of a liquid solution phase. Representative examples include redox and conducting organic polymers, melts and solid solutions of redox centers in solid ionic conductors, mixed-valence polynuclear inorganic materials, transition metal salts, oxides, and zeolites. The emphasis is on the elements of dynamics for the efficient delivery of charge and on reactivity of the ‘redox conducting’ materials. The effective (apparent) diffusional mechanism is critical to the success of most analytical measurements in solid-state. The potential materials for solid-state electrochemical measurements are expected to contain three-dimensionally distributed highly concentrated redox centers between which fast electron self-exchange (hopping) is feasible. These redox centers are fixed and, although they may have short range mobility about an equilibrium position, they classically are macroscopically immobile. The applicable materials also must host mobile counter-ions that are capable of providing charge balance during electron transfers, thereby serving the same purpose as the supporting electrolytes in conventional electrochemistry. The population of these ions must be sufficient to support diffusive mass transport of electrons and to minimize ohmic effects. We will describe typical electrochemical cells and experimental tactics to overcome the relatively slow dynamics of transport in solid (non-fluid) systems. Application of microdimensional electrodes leads to an improvement in the quality of solid-state electrochemical data and provides new diagnostic and analytical possibilities. Basic concepts of mechanistic studies, which are relevant to the development of novel analytical methods, together with trends towards possible future applications will also be addressed.

Phenomena at Porous Carbon Electrode/Saturated Electrolyte Interface in Electrochemical Capacitors

Development of electrochemical capacitors focuses today on the improvement of their energy density (or specific energy) and cyclability. The energy density might be improved by capacitance enhancement and operating voltage increase. To some extent, the relation between capacitance and voltage and their impact on the energy output reflects an interfacial character of charge storage mechanisms in capacitors; capacitance is intimately linked with the electrode material whereas the operating voltage is governed by the electrolyte applied. Of course, they cannot be considered separately, since the final performance is always a combination of various factors (e.g., electrode porosity – electrolyte viscosity, wettability, etc.). In case of capacitance-related issues, it appears that activated carbons with their well-developed surface area and suitable porosity, recently reached the limit of purely capacitive storage and further progress in this field require another insight into the electrostatic charge accumulation. That approach enforces intensive research on the novel architectures of the electrode materials. On the other hand, the electrolyte-related issues create an interesting pathway for investigations aiming at maximum voltage increase and energy density enhancement. Undoubtedly, organic media (based on acetonitrile or propylene carbonate) and ionic liquids are the optimal electrolytic solutions in terms of electrochemical stability. However, their impact on the environment and user safety is quite often questioned. Water-based electrolytes seem to be an interesting alternative, but the major objection against their commercialization concerns their low electrochemical stability governed by water decomposition voltage. Although the neutral electrolytes might demonstrate a significant decomposition overpotential once applied to porous carbon electrodes and thus provide the operating voltage up to 1.8 V, the performance of water-based capacitors is still not satisfactory enough for a broad application. Since the significant disadvantage of water-based solutions is attributed to the solvent decomposition, in our recent study, we decided to decrease the amount of water in aqueous electrolytes to the minimal level. Following the approach presented by the Belanger group, we have formulated the series of electrolytes with water: salt ratio up to 50% (either by mass, volume or molarity). Since the primary study has been done on LiTFSI salt, in our study, we have focused on the conventional inorganic salts, based on the well-known anions like NO3 - or SO4 2-. It appeared that these formulations demonstrated various correlations with concentration, conductivity and decomposition potentials. Moreover, their viscoelastic properties perfectly correlated with electrochemical performance, mostly with charge propagation. Surprisingly, the role of cation appeared to be crucial, essentially for capacitance values. The paper will discuss the correlation between the pore size distribution of carbon electrodes, electrolyte viscosity, and capacitor performance. Elucidation of the energy/power characteristics as well as cyclability at various temperatures will give a novel insight into ‘water’-based electrolytes. Spectroelectrochemical measurements will support the physicochemical data (e.g., operando Raman spectroscopy), unraveling (or trying to unravel) the mysteries at carbon electrode/highly concentrated electrolyte interface. Acknowledgements European Research Council is acknowledged for financial support within the Starting Grant project (GA 759603) under European Unions’ Horizon 2020 research and innovation programme.

Enhancement of Electrocatalytic Reduction of Carbon Dioxide at Electrodes Modified with Highly-Acidic Mixed-Metal-Oxide Films

Electroreduction of carbon dioxide to simple organic fuels and chemicals is a topic of growing scientific and technological interest. The reaction provides means for both reducing emissions of CO2 into atmosphere and storing renewable energy. The presentation will address low-temperature CO2-conversion processes based on electrocatalytic and photoelectrochemical approaches. Among important issues are choice of the catalytic or semiconducting materials, their morphology and operating conditions including temperature, solvent, electrolyte, pH etc. There is a need to improve the reaction dynamics and selectivity toward specific products. In practical electrolysis cells, the CO2-reduction (at cathode) is accompanied by water oxidation (at anode or photoanode). Given the fact that the CO2 molecule is very stable, its electroreduction processes are characterized by large overpotentials. It is often postulated that, during electroreduction, the rate limiting step is the protonation of the adsorbed CO product to form the CHO adsorbate. In this respect, the proton availability and its mobility at the photo(electro)chemical interface has to be addressed. On the other hand, competition between such parallel processes as hydrogen evolution and carbon dioxide reduction has also to be considered. Recently, mixed oxide systems stabilized through Zr-O-W bonds have been demonstrated to be very attractive acid catalysts exhibiting high catalytic activities and good stabilities in many demanding industrial reactions. The mixed WO3/ZrO2 systems are characterized by fast charge (electron, proton) propagation during the system’s redox transitions. By dispersing metallic Cu electrocatalytic nanoparticles over, under or within, such active WO3/ZrO2 matrices, the electrocatalytic activities of the respective systems toward the reduction of carbon dioxide have been significantly enhanced even at decreased loadings in acid media. The enhancement effects should be attributed to features of the mixed metal oxide support such as porosity and high population of hydroxyl groups (due to presence of ZrO2), high Broensted acidity of sites formed on mixed WO3/ZrO2, fast electron transfers coupled to unimpeded proton displacements (e.g. in HxWO3), as well as strong metal-support interactions between nanosized metals (Cu) and the metal (W,Zr) oxo species. Application of the hybrid system composed of both above-mentioned oxides resulted in high current densities originating from electroreduction of carbon dioxide mostly to methanol (CH3OH) as demonstrated upon identification of final products.

Amino-functionalized silica anchored to multiwall carbon nanotubes as hybrid electrode material for supercapacitors

Silica particles (SiO2) have been prepared by using sol-gel synthesis technique and are amino functionalized by treating with (3-Aminopropyl)triethoxysilane followed by composite formation using acid-functionalized multiwall carbon nanotubes (MWCNTs). The synthesized materials are characterized using morphological and structural techniques like SEM, TEM, XRD, TGA, and FTIR. The MWCNTs with large active surface area and good conductivity present several sites for the growth of spherical SiO2 particles of ∼200 nm diameter. The composite electrode is studied for supercapacitor application between −0.2 to +0.8 V and the electrochemical studies of sample electrodes have been done using cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance analysis. The specific capacitance, power density and energy density have been compared at various current rates. The MWCNTs electrode exhibit max specific capacitance of 128.4 F g−1 with a power density of 1000 W kg−1 and energy density of 17.8 Wh kg−1 at 2 A g−1 compared to 11.8 F g−1, 1.64 Wh kg−1 and 1000 W kg−1, respectively for SiO2/MWCNTs electrode at the same current rate. High charge propagation is observed for MWCNTs electrode due to conducting and mesoporous nature along with good resilience. Apparently, the MWCNTs electrode displays lower internal resistance with higher capacitive performance compared to SiO2/MWCNTs composite electrode. It has been observed that the power density of electrodes is strongly dependent on current density and reaches up to 1000 W kg−1 for SiO2/MWCNTs composite as current density is increased up to 2 A g−1.

(Invited) Interfacial Functionalization of Carbon Nanostructures: From Effective Charge Propagation and Storage to Enhancement of Electrocatalytic and Bioelectrocatalytic Properties

Of particular interest to the preparation of advanced materials is synthesis and characterization of carbon nanostructures (e.g. nanotubes) and noble metal nanoparticles, their stabilization (e.g. through self-assembly), as well as organization into two-dimensional arrays and controlled fabrication (e.g. through the sequential attraction) into three-dimensional network films. They can form nanosized materials with well-defined composition, structure and thickness. The interfaces can be also highly functionalized, and they can exhibit specific catalytic or unique electronic, charge storage, optical and sensing properties. We explore here the ability of inorganic structures to stabilize and derivatize metal and carbon nanostructures. Among inorganic systems, polyoxometallates of molybdenum and tungsten are attractive since they can not only adsorb irreversibly on solid surfaces but also exhibit reversible stepwise multielectron transfer reactions. The concept of the layer-by-layer formation of hybrid (organic-inorganic) assemblies composed of anionic polyoxometallate-protected carbon nanotubes (or metal nanoparticles) and ultra-thin films of positively charged conducting polymers (e.g. such as polyaniline or PEDOT) will be described and discussed here. The resulting novel composite materials have been fabricated as thin or moderately thick (µm level) films on electrode surfaces. As evidenced from STM and scanning electron microscopy, their morphology is still granular but the structure is fairly dense. Further, they are characterized by fast dynamics of charge propagation. Obviously, this research is of importance to the construction of effectively operating charge storage devices (capacitors), charge mediators (e.g. in bioelectrochemistry), molecular electronic systems and electrocatalysis. In the latter case, polyoxometallates can also be applied to stabilize and link Pt-Ru, Pt-Sn and various alloyed Pt-based nanoparticles. It is apparent from diagnostic cyclic voltammetric, rotating disk voltammetric and chronoamperometric measurements that such systems exhibit attractive properties towards electroreduction of oxygen or oxidation of alcohols (ethanol, methanol). Here, it is possible that addition of polytungstate or molybdate clusters to ruthenium or tin hydroxo species at the catalytic interface results in activating effect on dispersed platinum particles. An alternate explanation may involve a possibility of electronic effects and/or different morphologies of the catalytic films in the presence and absence of polyoxometallate.

Highly-Acidic Mixed-Metal-Oxides (WO3-ZrO2) As Active Supports for Dispersed Metal Centers: Enhancement of Electrocatalytic Reduction of Oxygen and Carbon Dioxide

Application of metal oxides as active matrices in electrocatalysis is particular importance. The hydrous behavior, which favors proton mobility and affects overall reactivity, reflects not only the oxide’s chemical properties but its texture and morphology as well. What is of importance to electrochemical science and technology, certain nonstoichiometric mixed-valence oxides could exhibit pseudo-metallic conductivity and possess appreciable catalytic activity. Recently, mixed oxide systems stabilized through Zr-O-W bonds have been demonstrated to be very attractive acid catalysts exhibiting high catalytic activities and good stabilities in many demanding industrial reactions. For electrocatalytic applications, mixed-valent tungsten(VI,V) oxide and zirconium(IV) oxide have been sequentially deposited and integrated through voltammetric potential cycling to form sub-microstructured films on glassy carbon electrode. The mixed WO3/ZrO2 systems are characterized by fast charge (electron, proton) propagation during the system’s redox transitions. By dispersing metallic Pt or Cu electrocatalytic nanoparticles over such active WO3/ZrO2 supports, the electrocatalytic activities of the respective systems toward the reduction of oxygen or carbon dioxide have been enhanced even at decreased loadings in acid media. The enhancement effects should be attributed to features of the mixed metal oxide support such as porosity and high population of hydroxyl groups (due to presence of ZrO2), high Broensted acidity of sites formed on mixed WO3/ZrO2, fast electron transfers coupled to unimpeded proton displacements (e.g. in HxWO3), as well as strong metal-support interactions between nanosized metals (Pt or Cu) and the metal (W, Zr) oxo species. The fact that WO3/ZrO2 nanostructures are in immediate contact with the metallic catalytic sites leads to the specific interactions (via the surface hydroxyl groups) with the reaction intermediates (H2O2 or CO adsorbates). Further mechanistic studies involving rotating ring disk voltammetry and gas-diffusion electrode measurements are planned in addition to spectroscopic and XPS measurements. The mixed-metal systems will also be considered together with nanostructured carbon carriers such as reduced graphene oxide, carbon nanotubes or carbon black.

Nanowire-Templated Three-Dimensional out-of-Plane Fuzzy Graphene As Thin Film Supercapacitor

Supercapacitors have gained popularity as future energy storage solutions due to their simple architecture, long cycling life, pulse power supply, and high dynamic charge propagation.1 They are generally categorized as Electrical Double Layer Capacitors (EDLCs) or pseudo-capacitors.2 EDLCs store energy as electrostatic charge on the electrodes’ surfaces; their efficiency is thus limited by the electrochemically active surface area available to the electrolyte.3 Whereas, pseudo-capacitors utilize fast and reversible faradaic processes due to electro-active species localized on the electrode surface to enhance current density.1, 4 Concentration of electro-active charged species and electro-chemical efficiency governs the efficiency of these supercapacitors. Recently, carbon-based materials (such as CNTs, graphene foams, graphene aerogels) have received research interest as promising candidates for supercapacitors due to their high electrochemical surface area, high electrical conductivity and accessible surface active sites for modification with electro-active species.5 Graphene has emerged as an ideal EDLC electrode material due to its superior electrical properties and high theoretical surface area.6 Current graphene electrodes are either pinned to a two-dimensional (2D) surface (such as graphene foams) or are tethered to a 2D substrate (such as carbon nanowalls). Graphene based hybrid-nanomaterials that leverage graphene’s outstanding surface-to-volume ratio coupled with pseudo-capacitive species for high efficiency supercapacitors have not been demonstrated to date. Here we demonstrate that direct synthesis of three-dimensional (3D) out-of-plane fuzzy graphene (3DFG) on Si nanowire (SiNW) mesh template (NT-3DFG) provides an ideal surface for functionalization with electro-active species to develop EDLC and pseudo-capacitive supercapacitors. We report fine control over 3DFG flake size and edge density by varying the partial pressure of CH4 precursor, plasma enhanced chemical vapor deposition (PECVD) synthesis process time, and PECVD synthesis temperature (Figure 1.A and B).7 Through electron microscopy (SEM and TEM) and Raman spectroscopy a distinct increase in single-to-few layer out-of-plane graphene edges is observed. High density of graphene edges presents high concentration of active sites for surface functionalization through surface doping, such as that through treatment with HNO3. Post treatment, the electrical conductivity of NT-3DFG is observed to be as high as 2400 S m-1. Leveraging on graphene’s outstanding surface-to-volume ratio, we have developed a material capable of attaining electrochemically active surface area of ca. 1017 m2 g-1. The introduction of electro-active species on this high surface area material renders specific capacitance as high as 87 ± 8 F g-1 (Figure 1.C). This value is further enhanced by decorating the template with metal oxide nanoparticles. The reported novel graphene based hybrid-nanomaterial shows great potential to be utilized in next-generation energy storage solutions. Figure 1. Characterization of NT-3DFG. (A) SEM image of NT-3DFG synthesized with increasing partial pressure of CH4 and increasing PECVD process time. (B) Effect of increasing PECVD process temperature on NT-3DFG synthesized under 25 mTorr partial pressure of CH4 for 10 min. (C) Cyclic voltammetry characterization of NT-3DFG before and after HNO3 treatment. (I) Representative cyclic voltammograms presenting double layer capacitance of NT-3DFG. Blue, yellow and red denote Au working electrode, NT-3DFG synthesized under 25 mTorr partial pressure of CH4 for 30 min and NT-3DFG synthesized under 25 mTorr partial pressure of CH4 for 90 min, respectively. Dashed lines represent respective samples after 2-hour HNO3 treatment. (II) Current density as a function of scan rate. Blue, yellow and red denote Au working electrode, NT-3DFG synthesized under 25 mTorr partial pressure of CH4 for 30 min and NT-3DFG synthesized under 25 mTorr partial pressure of CH4 for 90 min, respectively. Closed and open square represent samples before and after 2-hour HNO3 treatment, respectively.

Entanglement contour perspective for “strong area-law violation” in a disordered long-range hopping model

We numerically investigate the link between the delocalization-localization transition and entanglement in a disordered long-range hopping model of spinless fermions by studying various static and dynamical quantities. This includes the inverse participation ratio, level-statistics, entanglement entropy and number fluctuations in the subsystem along with quench and wave-packet dynamics. Finite systems show delocalized, quasi-localized and localized phases. The delocalized phase shows strong area-law violation whereas the (quasi)localized phase adheres to (for large subsystems) the strict area law. The idea of `entanglement contour' nicely explains the violation of area-law and its relationship with `fluctuation contour' reveals a signature at the transition point. The relationship between entanglement entropy and number fluctuations in the subsystem also carries signatures for the transition in the model. Results from Aubry-Andre-Harper model are compared in this context. The propagation of charge and entanglement are contrasted by studying quench and wavepacket dynamics at the single-particle and many-particle levels.

Development of a UiO-Type Thin Film Electrocatalysis Platform with Redox-Active Linkers.

Metal-organic frameworks (MOFs) as electrocatalysis scaffolds are appealing due to the large concentration of catalytic units that can be assembled in three dimensions. To harness the full potential of these materials, charge transport to the redox catalysts within the MOF has to be ensured. Herein, we report the first electroactive MOF with the UiO/PIZOF topology (Zr(dcphOH-NDI)), i.e., one of the most widely used MOFs for catalyst incorporation, by using redox-active naphthalene diimide-based linkers (dcphOH-NDI). Hydroxyl groups were included on the dcphOH-NDI linker to facilitate proton transport through the material. Potentiometric titrations of Zr(dcphOH-NDI) show the proton-responsive behavior via the -OH groups on the linkers and the bridging Zr-μ3-OH of the secondary building units with pKa values of 6.10 and 3.45, respectively. When grown directly onto transparent conductive fluorine-doped tin oxide (FTO), 1 μm thin films of Zr(dcphOH-NDI)@FTO could be achieved. Zr(dcphOH-NDI)@FTO displays reversible electrochromic behavior as a result of the sequential one-electron reductions of the redox-active NDI linkers. Importantly, 97% of the NDI sites are electrochemically active at applied potentials. Charge propagation through the thin film proceeds through a linker-to-linker hopping mechanism that is charge-balanced by electrolyte transport, giving rise to cyclic voltammograms of the thin films that show characteristics of a diffusion-controlled process. The equivalent diffusion coefficient, De, that contains contributions from both phenomena was measured directly by UV/vis spectroelectrochemistry. Using KPF6 as electrolyte, De was determined to be De(KPF6) = (5.4 ± 1.1) × 10-11 cm2 s-1, while an increase in countercation size to n-Bu4N+ led to a significant decrease of De by about 1 order of magnitude (De(n-Bu4NPF6) = (4.0 ± 2.5) × 10-12 cm2 s-1).

Electronic Transport in Single-Stranded DNA Molecule Related to Huntington’s Disease

We report a numerical analysis of the electronic transport in single chain DNA molecule consisting of 182 nucleotides. The DNA chains studied were extracted from a segment of the human chromosome 4p16.3, which were modified by expansion of CAG (cytosine-adenine-guanine) triplet repeats to mimics Huntington’s disease. The mutated DNA chains were connected between two platinum electrodes to analyze the relationship between charge propagation in the molecule and Huntington’s disease. The computations were performed within a tight-binding model, together with a transfer matrix technique, to investigate the current-voltage (I–V) of 23 types of DNA sequence and compare them with the distributions of the related CAG repeat numbers with the disease. All DNA sequences studied have a characteristic behavior of a semiconductor. In addition, the results showed a direct correlation between the current-voltage curves and the distributions of the CAG repeat numbers, suggesting possible applications in the development of DNA-based biosensors for molecular diagnostics.

Ultrafast modification of the polarity at LaAlO3/SrTiO3 interfaces

Oxide growth with semiconductor-like accuracy has led to atomically precise thin films and interfaces that exhibit a plethora of phases and functionalities not found in the oxide bulk material. This yielded spectacular discoveries such as the conducting, magnetic or even superconducting LaAlO$_3$/SrTiO$_3$ interfaces separating two prototypical insulating perovskite materials. All these investigations, however, consider the static state at the interface, although studies on fast oxide interface dynamics would introduce a powerful degree of freedom to understanding the nature of the LaAlO$_3$/SrTiO$_3$ interface state. Here we show that the polarization state at the LaAlO$_3$/SrTiO$_3$ interface can be optically enhanced or attenuated within picoseconds. Our observations are explained by a model based on charge propagation effects in the interfacial vicinity and transient polarization buildup at the interface.

Theoretical modeling of charge transport in triphenylamine-benzimidazole based organic solids for their application as host-materials in phosphorescent OLEDs.

The dynamic disorder and electric field effects on charge transport in triphenylamine–benzimidazole based molecular solids have been investigated using electronic structure calculations, molecular dynamics and Monte-Carlo simulations. During the charge propagation, the energy loss of the carrier in each hopping step is monitored by Monte-Carlo simulation. We derive a survival probability correlated momentum–energy distribution for drift-diffusion analysis and we demonstrate the dispersion initiated charge trapping mechanism which is indeed ideal for light emission efficiency via recombination. In the present model, the proposed carrier drift energy–current density expression and Shockley diode current density equation are used to study the current density–voltage characteristics; accordingly the ideality factor (∼1.8–2.0) dictates the deviation of Einstein's classical diffusion–mobility relation (where the ideality factor is unity). The dual mechanism of electric field assisted site energy gap on coherent-like transport and the electric field stretched dispersion on recombination are observed in tris(3′-(1-phenyl-1H-benzimidazole-2-yl)biphenyl-4-yl)amine (TBBI) and tris(4′-(1-phenyl-1H-benzimidazole-2-yl)biphenyl-4-yl)amine (TIBN) molecular systems, which can be used as host materials in organic light emitting diodes (OLEDs). We find the transport going from coherent to incoherent, due to the conversion mechanism of dynamic to static disorder. This can also be a controlled by applied electric field. By adjusting the applied electric field, film thickness and changing the π-stacked molecular aggregation via substitutions, one can fix the dispersive parameter and accordingly calculate the charge transport properties to design efficient host-materials for photovoltaic and light emitting diode devices.

Binder-Free Hybrid Titanium-Niobium Oxide/Carbon Nanofiber Mats for Lithium-Ion Battery Electrodes.

Free-standing, binder-free, titanium-niobium oxide/carbon hybrid nanofibers are prepared for Li-ion battery applications. A one-pot synthesis offers a significant reduction of processing steps and avoids the use of environmentally unfriendly binder materials, making the approach highly sustainable. Tetragonal Nb2 O5 /C and monoclinic Ti2 Nb10 O29 /C hybrid nanofibers synthesized at 1000 °C displayed the highest electrochemical performance, with capacity values of 243 and 267 mAh g-1 , respectively, normalized to the electrode mass. At 5 A g-1 , the Nb2 O5 /C and Ti2 Nb10 O29 /C hybrid fibers maintained 78 % and 53 % of the initial capacity, respectively. The higher rate performance and stability of tetragonal Nb2 O5 compared to that of monoclinic Ti2 Nb10 O29 is related to the low energy barriers for Li+ transport in its crystal structure, with no phase transformation. The improved rate performance resulted from the excellent charge propagation in the continuous nanofiber network.

Redox Processes in Solid‐State Uranyl (Oxy)hydroxide Minerals

AbstractThe correlation between the structure of a series of uranyl minerals of the type [(UO2)8O2(OH)12] and M[(UO2)xOy(OH)z] (M=Na, x=4, y=2, z=5; M=K2, x=6, y=4, z=6; M=Ca, x=6, y=4, z=6), and their electrochemical properties have been investigated by using solid‐state cyclic voltammetry. Solid‐state electrochemical investigations at near‐neutral pH values reveal that all minerals undergo redox reactions in the potential range from −0.72 to −0.91 V vs. Ag/AgCl. This process is assigned to the U(VI)/U(V) redox couple and a further reduction, observed between −0.95 and −1.20 V, is associated with the U(V)/U(IV) reduction. The nature of the reduced products has been probed by using vibrational spectroscopy and SEM−EDX, and was found to be UO2+x. The mechanism of reduction has been elucidated and shown to be dominated by two independent one‐electron transfer reactions with disproportionation playing at most a minor role. The rate of charge propagation through the solid deposits, characterized by the homogeneous charge‐transport diffusion coefficient, DCT, is much lower in these minerals than structurally related systems with ion, rather than electron, transfer being rate limiting. Overall, the insights into the nature and rate of redox conversion suggest that the uranyl minerals formed by the oxidative corrosion of spent nuclear fuels are not electrochemically innocent.

Modeling the frequency-dependent detective quantum efficiency of photon-counting x-ray detectors.

To find a simple model for the frequency-dependent detective quantum efficiency (DQE) of photon-counting detectors in the low flux limit. Formula for the spatial cross-talk, the noise power spectrum and the DQE of a photon-counting detector working at a given threshold are derived. Parameters are probabilities for types of events like single counts in the central pixel, double counts in the central pixel and a neighboring pixel or single count in a neighboring pixel only. These probabilities can be derived in a simple model by extensive use of Monte Carlo techniques: The Monte Carlo x-ray propagation program MOCASSIM is used to simulate the energy deposition from the x-rays in the detector material. A simple charge cloud model using Gaussian clouds of fixed width is used for the propagation of the electric charge generated by the primary interactions. Both stages are combined in a Monte Carlo simulation randomizing the location of impact which finally produces the required probabilities. The parameters of the charge cloud model are fitted to the spectral response to a polychromatic spectrum measured with our prototype detector. Based on the Monte Carlo model, the DQE of photon-counting detectors as a function of spatial frequency is calculated for various pixel sizes, photon energies, and thresholds. The frequency-dependent DQE of a photon-counting detector in the low flux limit can be described with an equationcontaining only a small set of probabilities as input. Estimates for the probabilities can be derived from a simple model of the detector physics.

Electrochemical capacitor materials based on carbon and luminophors doped with lanthanide ions

The described research is focused on the hybrid, bi-functional composite materials dedicated to the electrochemical capacitor electrodes. The novel material exhibits both luminescent and capacitive properties. The fabrication process of semi-products and the final composite is described. The structure and homogeneity of luminophors are confirmed with the XRD analysis. The morphology of materials is also determined by TEM and SEM images. The detailed spectroscopic characterization includes excitation and emission spectra, luminescence decay curves, emission lifetimes, CIE chromaticity indexes. The electrochemical studies of composite electrodes carried out by cyclic voltammetry and impedance spectroscopy exhibit good charge propagation. For the first time, inorganic luminophors containing doped LaF3 and GdVO4 have been successfully used for electrochemical capacitor. It is the primary stage to design a new generation of light emitting capacitors utilizing more stable inorganic luminophors than organic-based ones.

Polaron Disproportionation Charge Transport in a Conducting Redox Polymer

Herein we report a mechanistic study of the charge transport in poly-3-((2,5-hydroquinone)vinyl)-1H-pyrrole by conductance measurements at various temperatures performed in situ during doping of the polypyrrole backbone in contact with an aqueous electrolyte. Charge transport was found to occur by electron hopping with associated electron transfer activation energies in the range of 0.08–0.2 eV. In situ electron paramagnetic resonance experiments indicated polarons as the dominant charge carriers and the charge transport was found to follow a second-order dependence with respect to the number of accumulated charges. Based on the findings, we present a polaron comproportionation/disproportionation model for electron conduction in poly-3-((2,5-hydroquinone)vinyl)-1H-pyrrole, thus, providing a complement to existing models for charge propagation in conducting polymers.