Flavin adenine dinucleotide (FAD) is a cofactor for many enzymes, but also an informative redox active surface probe for electrode materials such as carbon nanotubes (CNTs) and nitrogen-doped CNTs (N-CNTs). FAD spontaneously adsorbs onto the surface of CNTs and N-CNTs, displaying Langmuir adsorption characteristics. The Langmuir adsorption model provides a means of calculating the electroactive surface area (ESA), the equilibrium constant for the adsorption and desorption processes (K), and the Gibbs free energy of adsorption (ΔG°). Traditional ESA measurements based on the diffusional flux of a redox active molecule to the electrode surface underestimate the ESA of porous materials because pores are not penetrated. Techniques such as gas adsortion (BET) overestimate the ESA because it includes both electroactive and inactive areas. The ESA determined by extrapolation of the Langmuir adsorption model with the electroactive surface probe FAD will penetrate pores and only include electroactive areas. The redox activity of adsorbed FAD also displays a strong dependency on pH, which provides a means of determining the pKa of the surface confined species. The pKa of FAD decreases as the nitrogen content in the CNTs increases, suggesting a decreased hydrophobicity of the N-CNT surface. FAD desorption at N-CNTs slowly transforms the main FAD surface redox reaction with E1/2 at −0.84 V into two new, reversible, surface confined redox reactions with E1/2 at −0.65 and −0.76 V (vs Hg/Hg2SO4), respectively (1.0 M sodium phosphate buffer pH = 6.75). This is the first time these redox reactions have been observed. The new surface confined redox reactions were not observed during FAD desorption from nondoped CNTs.

Urea electrooxidation has attracted considerable interest as an alternative anodic reaction in the electrochemical generation of hydrogen due to both the lower electrochemical potential required to drive the reaction and also the possibility of eliminating a potentially harmful substance from wastewater during hydrogen fuel production. Nickel and nickel-containing oxides have shown activities comparable to those of precious-metal catalysts for the electrooxidation of urea in alkaline conditions. Herein, we investigate the use of nanostructured LaNiO3 perovskite supported on Vulcan carbon XC-72 as an electrocatalyst. This catalyst exhibits an exceptionally high mass activity of ca. 371 mA mgox–1 and specific activity of 2.25 A mg–1 cmox–2 for the electrooxidation of urea in 1 M KOH, demonstrating the potential applications of Ni-based perovskites for direct urea fuel cells and low-energy hydrogen production. While LaNiO3 is shown to be stable at low overpotentials, through in-depth mechanistic studies the catalyst surface was observed to restructure and there was apparent CO2 poisoning of the LaNiO3 upon extended cycling, a result that may be extended to other Ni-based systems.Keywords: electrocatalyst; nickel; oxidation; perovskite; urea

Transition-metal phosphates (TMPs) are potential materials for large-scale applications of lithium ion batteries (LIBs). Yet, high-voltage TMP cathodes have not met commercial success due to ill understood failure mechanisms. In this article we studied the surface chemistry of Li3V2(PO4)3 composite electrodes using X-ray photoelectron spectroscopy (XPS) post-electrochemical cycling in a stable electrochemical window of 3.0–4.2 V vs Li/Li+ and in the wider window of 3.0–4.8 V vs Li/Li+ where a dramatic fade in capacity is noted. In addition, we performed aging experiments in LiPF6 EC/DEC electrolyte with no electrochemical bias applied to investigate a possible spontaneous solid electrolyte interphase (SEI) formation as has been described for lithium transition-metal oxide (LixMyOz) electrodes. An SEI was found on the Li3V2(PO4)3 composite electrodes cycled in both potential windows and after aging with similar chemical compositions including ethers, alkoxides, esters, carboxylates, and carbonates as well as decomposed salt products. Analogous experiments were performed on the individual constituents of the composite electrode (active material, binder, and carbon additive). It was determined that the carbon additive and not Li3V2(PO4)3 formed an SEI both spontaneously and electrochemically. Therefore, the carbon additive and its properties are crucial in the formation of the SEI on TMP cathodes for LIBs which directly affect its lithium intercalation performance.

Fluoroethylene carbonate (FEC) has become a standard electrolyte additive for use with silicon negative electrodes, but how FEC affects solid electrolyte interphase (SEI) formation on the silicon anode’s surface is still not well understood. Herein, SEI formed from LiPF6-based carbonate electrolytes, with and without FEC, were investigated on 50 nm thick amorphous silicon thin film electrodes to understand the role of FEC on silicon electrode surface reactions. In contrast to previous work, anhydrous and anoxic techniques were used to prevent air and moisture contamination of prepared SEI films. This allowed for accurate characterization of the SEI structure and composition by X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry depth profiling. These results show that FEC reduction leads to fluoride ion and LiF formation, consistent with previous computational and experimental results. Surprisingly, we also find that these species decrease lithium-ion solubility and increase the reactivity of the silicon surface. We conclude that the effectiveness of FEC at improving the Coulombic efficiency and capacity retention is due to fluoride ion formation from reduction of the electrolyte, which leads to the chemical attack of any silicon-oxide surface passivation layers and the formation of a kinetically stable SEI comprising predominately lithium fluoride and lithium oxide.

The electrochemical behavior of hydrogen peroxide (H2O2) at carbon nanotubes (CNTs) and nitrogen-doped carbon nanotubes (N-CNTs) was investigated over a wide potential window. At CNTs, H2O2 will be oxidized or reduced at large overpotentials, with a large potential region between these two processes where electrochemical activity is negligible. At N-CNTs, the overpotential for both H2O2 oxidation and reduction is significantly reduced; however, the reduction current from H2O2, especially at low overpotentials, is attributed to increased oxygen reduction rather than the direct reduction of H2O2, due to a fast chemical disproportionation of H2O2 at the N-CNT surface. Additionally, N-CNTs do not display separation between observable oxidation and reduction currents from H2O2. Overall, the analytical sensitivity of N-CNTs to H2O2, either by oxidation or reduction, is considerably higher than CNTs, and obtained at significantly lower overpotentials. N-CNTs display an anodic sensitivity and limit of detection of 830 mA M–1 cm–2 and 0.5 μM at 0.05 V, and a cathodic sensitivity and limit of detection of 270 mA M–1 cm–2 and 10 μM at −0.25 V (V vs Hg/Hg2SO4). N-CNTs are also a superior platform for the creation of bioelectrodes from the spontaneous adsorption of enzyme, compared to CNTs. Glucose oxidase (GOx) was allowed to adsorb onto N-CNTs, producing a bioelectrode with a sensitivity and limit of detection to glucose of 80 mA M–1 cm–2 and 7 μM after only 30 s of adsorption time from a 81.3 μM GOx solution.

An amperometric glucose biosensor based on immobilization of glucose oxidase on nitrogen-doped carbon nanotubes (N-CNTs) was successfully developed for the determination of silver ions. Upon exposure to glucose, a steady-state enzymatic turnover rate was detected through amperometric oxidation of the H2O2 byproduct, directly related to the concentration of glucose in solution. Inhibition of the steady-state enzymatic glucose oxidase reaction by heavy metals ions such as Ag+, produced a quantitative decrease in the steady-state rate, subsequently creating an ultrasensitive metal ion biosensor through enzymatic inhibition. The Ag+ biosensor displayed a sensitivity of 2.00 × 108 ± 0.06 M–1, a limit of detection (σ = 3) of 0.19 ± 0.04 ppb, a linear range of 20–200 nM, and sample recovery at 101 ± 2%, all acquired at a low-operating potential of 0.05 V (vs Hg/Hg2SO4). Interestingly, the biosensor does not display a loss in sensitivity with continued use due to the % inhibition based detection scheme: loss of enzyme (from continued use) does not influence the % inhibition, only the overall current associated with the activity loss. The heavy metals Cu2+ and Co2+ were also detected using the enzyme biosensor but found to be much less inhibitory, with sensitivities of 1.45 × 106 ± 0.05 M–1 and 2.69 × 103 ± 0.07 M–1, respectively. The mode of GOx inhibition was examined for both Ag+ and Cu2+ using Dixon and Cornish-Bowden plots, where a strong correlation was observed between the inhibition constants and the biosensor sensitivity.

Opaque and transparent carbon ultramicro- to nanoelectrode arrays were made using a previously reported facile versatile fabrication method (Duay et al. Anal. Chem. 2014, 86, 11528). First, opaque carbon ultramicroelectrode arrays (CUAs) were characterized for their analytical response to hydrogen peroxide (H2O2) oxidation using cyclic voltammetry. The alumina blocking layer was found to contribute to the noise and thus had undesirable effects on the array’s limit of detection (LOD) for H2O2 at fast scan rates. Nonetheless, at slower scan rates (ν ≤ 250 mV s–1), the LODs for H2O2 for both opaque (O-CUAs) and transparent arrays (T-CUAs) were found to be lower than previously reported levels for array-based UMEs. LODs as low as 35 nM H2O2 are obtained for T-CUA at a 2.5 mV s–1 scan rate. Furthermore, the transparent arrays were analyzed for their spectroelectrochemical response during the oxidation/reduction of ferrocenemethanol. Results showed very good correlation between the optical and electrochemical response for ferrocenemethanol at a UV wavelength of 254 nm. Thus, these electrodes allow for the in situ mechanistic and kinetic characterization of heterogeneous electrochemical and intermediate homogeneous chemical reactions with high electroanalytical sensitivity, low detection limits, and wide dynamic range.

•Controlled modification of epitaxial graphene is achieved by using diaryliodonium salts.•The grafting density can be reproducibly tuned from 4 × 1013 up to 3 × 1014 molecules cm−2.•The grafting of nitrophenyl molecules occurs preferentially at basal planes.•Most of the grafts present a threefold-symmetry.Owing to its high conductivity, graphene holds promise as an electrode for energy devices such as batteries and photovoltaics. However, to this end, the work function and doping levels in graphene need to be precisely tuned. One promising route for modifying graphene’s electronic properties is via controlled covalent electrochemical grafting of molecules. We show that by employing diaryliodonium salts instead of the commonly used diazonium salts, spontaneous functionalization is avoided. This allows for precise tuning of the grafting density. By employing bis(4-nitrophenyl)iodonium(III) tetrafluoroborate (DNP) salt calibration curves, the surface functionalization density (coverage) of glassy carbon was controlled using cyclic voltammetry in varying salt concentrations. These electro-grafting conditions and calibration curves translated directly over to modifying single layer epitaxial graphene substrates (grown on insulating 6H-SiC (0 0 0 1)). In addition to quantifying the functionalization densities using electrochemical methods, samples with low grafting densities were characterized by low-temperature scanning tunneling microscopy (LT-STM). We show that the use of buffer-layer free graphene substrates is required for clear observation of the nitrophenyl modifications. Atomically-resolved STM images of single site modifications were obtained, showing no preferential grafting at defect sites or SiC step edges as supposed previously in the literature. Most of the grafts exhibit threefold symmetry, but occasional extended modifications (larger than 4 nm) were observed as well.

Optoelectronic applications often rely on indium tin oxide (ITO) as a transparent electrode material. Improvements in the performance of such devices as photovoltaics and light-emitting diodes often requires robust, controllable modification of the ITO surface to enhance interfacial charge transfer properties. In this work, modifier films were deposited onto ITO by the electrochemical reduction of di(4-nitrophenyl) iodonium tetrafluoroborate (DNP), allowing for control over surface functionalization. The surface coverage could be tuned from submonolayer to multilayer coverage by either varying the DNP concentration or the number of cyclic voltammetry (CV) grafting scans. Modification of ITO with 0.8 mM DNP resulted in near-monolayer surface coverage (4.95 × 1014 molecules/cm2). X-ray photoelectron spectroscopy (XPS) analysis confirmed the presence of 4-nitrophenyl (NO2Ph) moieties on the ITO surface through the detection of a NO2 nitrogen signal at 405.6 eV after grafting. Further XPS evidence suggests that the NO2Ph radicals do not bond to the surface indium or tin sites, consistent with modification occurring either through bonding to surface hydroxyl groups or through strong physisorption on ITO. CV in the presence of an electroactive probe and electrochemical impedance spectroscopy (EIS) were used to investigate the electronic effects that modification via DNP has on ITO. Even at submonolayer coverage, the insulating organic films can reduce the current response to ferrocene oxidation and reduction by more than 25% and increase the charge transfer resistance by a factor of 10.

An integrated microfluidic/magnetophoretic methodology was developed for improving signal response time and detection limits for the chronoamperometric observation of discrete nanoparticle/electrode interactions by electrocatalytic amplification. The strategy relied on Pt-decorated iron oxide nanoparticles which exhibit both superparamagnetism and electrocatalytic activity for the oxidation of hydrazine. A wet chemical synthetic approach succeeded in the controlled growth of Pt on the surface of FeO/Fe3O4 core/shell nanocubes, resulting in highly uniform Pt-decorated iron oxide hybrid nanoparticles with good dispersibility in water. The unique mechanism of hybrid nanoparticle formation was investigated by electron microscopy and spectroscopic analysis of isolated nanoparticle intermediates and final products. Discrete hybrid nanoparticle collision events were detected in the presence of hydrazine, an electrochemical indicator probe, using a gold microband electrode integrated into a microfluidic channel. In contrast with related systems, the experimental nanoparticle/electrode collision rate correlates more closely with simple theoretical approximations, primarily due to the accuracy of the nanoparticle tracking analysis method used to quantify nanoparticle concentrations and diffusion coefficients. Further modification of the microfluidic device was made by applying a tightly focused magnetic field to the detection volume to attract the magnetic nanoprobes to the microband working electrode, thereby resulting in a 6-fold increase to the relative frequency of chronoamperometric signals corresponding to discrete nanoparticle impact events.Keywords: electrocatalytic amplification; hybrid nanoparticles; magnetophoresis; microfluidics; multifunctional nanoparticles; nanoparticle collisions;

We investigate the source of Raman background signal commonly misidentified as fluorescence in nonaqueous electrolytes via a variety of spectroscopies (Raman, fluorescence, NMR) and find evidence of hydrogen-bonding interactions. This hydrogen bonding gives rise to broadband anharmonic vibrational modes and suggests that anions play an important and underappreciated role in the structure of nonaqueous electrolytes. Controlling electrolyte structure has important applications in advancing in operando spectroscopy measurements as well as understanding the stability of high concentration electrolytes for next-generation electrochemical energy storage devices.

Titanium dioxide has been identified as a prospective anode material for use in lithium ion batteries. The higher lithiation potential of TiO2 versus other common anodes is more electrochemically compatible with most organic electrolytes, thus leading to reduced solid electrolyte interphase formation and overall more stable battery systems. However, in this study TiO2 has exhibited poor cycling stability with common electrolytes containing lithium hexafluorophosphate (LiPF6) salt. Combined electrochemical and spectroscopic analyses have revealed this to be due to the onset of reversible fluorination of the anode and chemical conversion of TiO2 to TiOF2 during lithiation. Comparison of electrochemical cycling of atomic-layer-deposited TiO2 anodes using LiPF6 with and without a hydrofluoric acid scavenger, tributylamine, to cycling using a nonfluorinated lithium perchlorate (LiClO4) salt indicates that the in situ formation of hydrofluoric acid in the electrolyte from decomposition of the PF6– anion alters the lithiation electrochemistry. X-ray photoelectron spectroscopy (XPS) analysis and time-of-flight secondary-ion mass spectrometry (ToF SIMS) depth profiling measurements confirmed the presence of TiF2 and TiOF surface and bulk phases, respectively, in the electrode upon lithiation.

Nitrogen-doped carbon nanotubes (N-CNTs) have been shown to be electrocatalytic toward the oxidation of dihydronicotinamide adenine dinucleotide (NADH), the reduced form of the coenzyme necessary for enzymatic turnover in NAD+-dependent dehydrogenases. The observed oxidation potential of the electrocatalyst, however, still shows a significant overpotential, suggesting that even for effective electrocatalysts, electrooxidation may be kinetically controlled. We demonstrate using the Koutecky–Levich rotating disk electrode technique that the observed electron transfer rate constant (kobs) is a function of potential over a wide potential window; however, kobs could only be accurately measured for a portion of that window for the electrocatalytic N-CNTs. More importantly, electrochemically measured enzyme kinetics, acquired after adsorption of glucose dehydrogenase onto the N-CNTs, are never independent of potential, even when the electron transfer rate constant is too fast to measure by the rotating disk technique. Thus, electrochemically obtained kinetics (e.g., KMapp and Vmax) are actually measuring the electrochemical kinetics of NADH oxidation at the electrode surface, rather than the spontaneous and potential-independent enzymatic turnover.Keywords: carbon nanotubes; dehydrogenases; dihydronicotinamide adenine dinucleotide; enzyme kinetics; Michaelis−Menten kinetics; NADH; nitrogen-doped carbon nanotubes

We present a series of perovskite electrocatalysts that are highly active for both the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in an aqueous alkaline electrolyte. Lanthanum-based perovskites containing different transition metal active sites (LaBO3, B = Ni, Ni0.75Fe0.25, Co, Mn) are synthesized by a general colloidal method, yielding phase pure catalysts of homogeneous morphology and surface area (8–14 m2/g). Each perovskite’s ability to catalyze the OER and ORR is examined using thin film rotating disk electrochemistry (RDE). LaCoO3 supported on nitrogen-doped carbon is shown to be ∼3 times more active for the OER than high-surface-area IrO2. Furthermore, LaCoO3 is demonstrated to be highly bifunctional by having a lower total overpotential between the OER and ORR (ΔE = 1.00 V) than Pt (ΔE = 1.16) and Ru (ΔE = 1.01). The OER and ORR pathways are perturbed by the introduction of peroxide disproportionation functionality via support interactions and selective doping of the catalyst. LaNi0.75Fe0.25O3’s ability to disproportionate peroxide is hypothesized to be responsible for the ∼50% improvement over LaNiO3 in catalytic activity toward the ORR, despite similar electronic structure. These results allow us to examine the pathways for OER and ORR in context of support interactions, transition metal redox processes, and catalytic bifunctionality.

TiO2(B) exhibits unique electrochemical lithiation behavior where two closely spaced reduction and oxidation peaks, referred to as “double peaks,” are observed for 3-D forms of bulk and nanostructured TiO2(B) while a single broad redox peak is observed for 2-D nanosheet architectures. In this study, we have used a combination of transmission electron microscopy (TEM), powder X-ray diffraction (XRD), Raman spectroscopy and cyclic voltammetry on TiO2(B) nanosheets as well as a series of thermally annealed nanosheets to map the morphological and phase transformation pathways that help clarify the structure-dependent lithiation behavior. We found that the double peak redox behavior only arises once a three dimensional nanocrystalline structure of TiO2(B) exists by observing nanoparticle growth on the TiO2(B) nanosheet surface via TEM at temperatures above 150 °C. This morphological transformation was also verified by Raman spectroscopy. The appearance of low-energy torsional modes at temperatures above 150 °C which are not observed in 2-D morphologies of TiO2(B) agrees well with TEM evidence of 3-D nanoparticle formation. Using scan rate dependent cyclic voltammetry we also verified that all lithiation behavior associated with TiO2(B) (either nanosheet or nanoparticle) is due primarily to a surface redox (pseudocapacitive) mechanism. The thermal annealing study also shows the phase transformation of surface nucleated TiO2(B) nanoparticles to anatase nanoparticles at temperatures above 200 °C. These studies clearly show how nano-morphological control can influence electrochemical lithiation behavior and help identify a possible mechanism to explain the double peak phenomenon observed for TiO2(B).

Nonaqueous solvents in modern battery technologies undergo electroreduction at negative electrodes, leading to the formation of a solid–electrolyte interphase (SEI). The mechanisms and reactions leading to a stable SEI on silicon electrodes in lithium-ion batteries are still poorly understood. This lack of understanding inhibits the rational design of electrolyte additives, active material coatings, and the prediction of Li-ion battery life in general. We prepared SEI with a common nonaqueous solvent (LiPF6 in PC and in EC/DEC 1:1 by wt %) on silicon oxide and etched silicon (001) surfaces in various states of lithiation to understand the role of surface chemistry on the SEI formation mechanism and SEI structure. Anhydrous and anoxic techniques were used to prevent air and moisture contamination of prepared SEI films, allowing for more accurate characterization of SEI chemical stratification and composition by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) depth profiling. Additionally, multivariate statistical methods were used to better understand TOF-SIMS depth profiling studies. We conclude that the absence of native-oxide layer on silicon has a significant impact on the formation, composition, structure, and thickness of the SEI.Keywords: lithium-ion batteries; PCA; SEI; solid−electrolyte interphase; TOF-SIMS; XPS

Here, we report a potentiometric method for detecting single platinum nanoparticles (Pt NPs) by measuring a change in open-circuit potential (OCP) instead of the current during single Pt NP collisions with the mercury-modified Pt ultramicroelectrode (Hg/Pt UME). Similar to the current–time (i–t) response reported previously at Hg/Pt UMEs, the OCP–time (v–t) response consists of repeated potential transient signals that return to the background level. This is because Hg poisons the Pt NP after collision with the Hg/Pt UME due to amalgamation and results in deactivation of the redox reaction. For individual Pt NP collisions the amplitude of the OCP signal reaches a maximum and decays to the background level at a slower rate compared to the comparable i–t response. Due to this, OCP events are broader and more symmetrical in shape compared to i–t “spikes.” The collision frequency of Pt NPs derived from v–t plots (0.007 to 0.020 pM–1 s–1) is in good agreement with the value derived from i–t plots recorded at Hg/Pt UMEs (0.016 to 0.024 pM–1 s–1) under similar conditions and was found to scale linearly with Pt NP concentration. Similar to the current response, the amplitude of the OCP response increased with the NP’s size. However, unlike the change in current in a i–t response, the change in OCP in a v–t response observed during single Pt NP collisions with Hg/Pt UME is larger than the estimated change in OCP based on the theory. Therefore, the Pt NP sizes derived from the v–t response did not correlate with the TEM-derived Pt NP sizes. In spite of these results the potentiometric method has great value for electroanalysis because of its significant advantages over the amperometric method such as a simpler apparatus and higher sensitivity.Keywords: mercury-modified Pt UMEs; mixed potential concept; open-circuit potential; single nanoparticle collisions

Electrodeposition of selenium from 1-propyl-1-methylpiperidinium bis(trifluoromethylsulfonyl)imide is reported. In situ UV–vis spectroelectrochemistry was used to investigate the reduction of diethyl selenite to form elemental selenium thin films from an ionic liquid–acetonitrile medium. Three reduction peaks of diethyl selenite were observed via cyclic voltammetry and are attributed to the stepwise reduction of the selenium precursor adsorbed on the electrode. The electrodeposition mechanism is influenced by both potential and time. Electrodeposition at −1.7 V vs Pt QRE resulted in the deposition of elemental selenium nanoparticles that with time coalesced to form a continuous film. At reduction potentials more negative than −1.7 V the morphology of the deposit changed significantly due to the reduction of elemental Se to Se2–. In addition, p-type photoconductivity of the films was observed during the spectroelectrochemical measurements. X-ray diffraction and Raman spectroscopy confirmed that the deposited selenium films were amorphous. X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy confirm the films consisted of pure selenium with minor residual contamination from the precursor and ionic liquid.

We report on the effect of convection on electrochemically active collisions between individual Pt nanoparticles (PtNPs) and Hg and Au electrodes. Compared to standard electrochemical cells utilizing Hg and Au ultramicroelectrodes (UMEs) used in previous studies of electrocatalytic amplification, microelectrochemical devices offer two major advantages. First, the PtNP limit of detection (0.084 pM) is ∼8 times lower than the lowest concentration measured using UMEs. Second, convection enhances the mass transfer of PtNPs to the electrode surface, which enhances the collision frequency from ∼0.02 pM–1 s–1 on UMEs to ∼0.07 pM–1 s–1 in microelectrochemical devices. We also show that the size of PtNPs can be measured in flowing systems using data from collision experiments and then validate this finding using multiphysics simulations.

Electric vehicles and grid storage devices have potentialto become feasible alternatives to current technology, but only if scientists can develop energy storage materials that offer high capacity and high rate capabilities. Chemists have studied anatase, rutile, brookite and TiO2(B) (bronze) in both bulk and nanostructured forms as potential Li-ion battery anodes. In most cases, the specific capacity and rate of lithiation and delithiation increases as the materials are nanostructured. Scientists have explained these enhancements in terms of higher surface areas, shorter Li+ diffusion paths and different surface energies for nanostructured materials allowing for more facile lithiation and delithiation. Of the most studied polymorphs, nanostructured TiO2(B) has the highest capacity with promising high rate capabilities. TiO2(B) is able to accommodate 1 Li+ per Ti, giving a capacity of 335 mAh/g for nanotubular and nanoparticulate TiO2(B). The TiO2(B) polymorph, discovered in 1980 by Marchand and co-workers, has been the focus of many recent studies regarding high power and high capacity anode materials with potential applications for electric vehicles and grid storage. This is due to the material’s stability over multiple cycles, safer lithiation potential relative to graphite, reasonable capacity, high rate capability, nontoxicity, and low cost (Bruce, P. G.; Scrosati, B.; Tarascon, J.-M. Nanomaterials for Rechargeable Lithium Batteries. Angew. Chem., Int. Ed.2008, 47, 2930–2946). One of the most interesting properties of TiO2(B) is that both bulk and nanostructured forms lithiate and delithiate through a surface redox or pseudocapacitive charging mechanism, giving rise to stable high rate charge/discharge capabilities in the case of nanostructured TiO2(B). When other polymorphs of TiO2 are nanostructured, they still mainly intercalate lithium through a bulk diffusion-controlled mechanism. TiO2(B) has a unique open crystal structure and low energy Li+ pathways from surface to subsurface sites, which many chemists believe to contribute to the pseudocapacitive charging.Several disadvantages exist as well. TiO2(B), and titania in general, suffers from poor electronic and ionic conductivity. Nanostructured TiO2(B) also exhibits significant irreversible capacity loss (ICL) upon first discharge (lithiation). Nanostructuring TiO2(B) can help alleviate problems with poor ionic conductivity by shortening lithium diffusion pathways. Unfortunately, this also increases the likelihood of severe first discharge ICL due to reactive Ti–OH and Ti–O surface sites that can cause unwanted electrolyte degradation and irreversible trapping of Li+. Nanostructuring also results in lowered volumetric energy density, which could be a considerable problem for mobile applications. We will also discuss these problems and proposed solutions.Scientists have synthesized TiO2(B) in a variety of nanostructures including nanowires, nanotubes, nanoparticles, mesoporous-ordered nanostructures, and nanosheets. Many of these structures exhibit enhanced Li+ diffusion kinetics and increased specific capacities compared to bulk material, and thus warrant investigation on how nanostructuring influences lithiation behavior. This Account will focus on these influences from both experimental and theoretical perspectives. We will discuss the surface charging mechanism that gives rise to the increased lithiation and delithiation kinetics for TiO2(B), along with the influence of dimensional confinement of the nanoarchitectures, and how nanostructuring can change the lithiation mechanism considerably.

The adsorption of flavin adenine dinucleotide (FAD) and glucose oxidase (GOx) onto carbon nanotube (CNT) and nitrogen-doped CNT (N-CNT) electrodes was investigated and found to obey Langmuir adsorption isotherm characteristics. The amount adsorbed and adsorption maximum are dependent on exposure time, the concentration of adsorbate, and the ionic strength of the solution. The formal potentials measured for FAD and GOx are identical, indicating that the observed electroactivity is from FAD, the redox reaction center of GOx. When glucose is added to GOx adsorbed onto CNT/N-CNT electrodes, direct electron transfer (DET) from enzyme-active FAD is not observed. However, efficient mediated electron transfer (MET) occurs if an appropriate electron mediator is placed in solution, or the natural electron mediator oxygen is used, indicating that GOx is adsorbed and active on CNT/N-CNT electrodes. The observed surface-confined redox reaction at both CNT and N-CNT electrodes is from FAD that either specifically adsorbs from solution or adsorbs from the holoprotein subsequently inactivating the enzyme. The splitting of cathodic and anodic peak potentials as a function of scan rate provides a way to measure the heterogeneous electron-transfer rate constant (ks) using Laviron’s method. However, the measured ks was found to be under ohmic control, not under the kinetic control of an electron-transfer reaction, suggesting that ks for FAD on CNTs is faster than the measured value of 7.6 s–1.

Nitrogen-doped carbon nanotubes (N-CNTs) substantially lower the overpotential necessary for dihydronicotinamide adenine dinucleotide (NADH) oxidation compared to nondoped CNTs or traditional carbon electrodes such as glassy carbon (GC). We observe a 370 mV shift in the peak potential (Ep) from GC to CNTs and another 170 mV shift from CNTs to 7.4 atom % N-CNTs in a sodium phosphate buffer solution (pH 7.0) with 2.0 mM NADH (scan rate 10 mV/s). The sensitivity of 7.4 atom % N-CNTs to NADH was measured at 0.30 ± 0.04 A M–1 cm–2, with a limit of detection at 1.1 ± 0.3 μM and a linear range of 70 ± 10 μM poised at a low potential of −0.32 V (vs Hg/Hg2SO4). NADH fouling, known to occur to the electrode surface during NADH oxidation, was investigated by measuring both the change in Ep and the resulting loss of electrode sensitivity. NADH degradation, known to occur in phosphate buffer, was characterized by absorbance at 340 nm and correlated with the loss of NADH electroactivity. N-CNTs are further demonstrated to be an effective platform for dehydrogenase-based biosensing by allowing glucose dehydrogenase to spontaneously adsorb onto the N-CNT surface and measuring the resulting electrode’s sensitivity to glucose. The glucose biosensor had a sensitivity of 0.032 ± 0.003 A M–1 cm–2, a limit of detection at 6 ± 1 μM, and a linear range of 440 ± 50 μM.

We demonstrate that the reduction of p-nitrophenol to p-aminophenol by NaBH4 is catalyzed by both monometallic and bimetallic nanoparticles (NPs). We also demonstrate a straightforward and precise method for the synthesis of bimetallic nanoparticles using poly(amido)amine dendrimers. The resulting dendrimer encapsulated nanoparticles (DENs) are monodisperse, and the size distribution does not vary with different elemental combinations. Random alloys of Pt/Cu, Pd/Cu, Pd/Au, Pt/Au, and Au/Cu DENs were synthesized and evaluated as catalysts for p-nitrophenol reduction. These combinations are chosen in order to selectively tune the binding energy of the p-nitrophenol adsorbate to the nanoparticle surface. Following the Brønsted–Evans–Polanyi (BEP) relation, we show that the binding energy can reasonably predict the reaction rates of p-nitrophenol reduction. We demonstrate that the measured reaction rate constants of the bimetallic DENs is not always a simple average of the properties of the constituent metals. In particular, DENs containing metals with similar lattice constants produce a binding energy close to the average of the two constituents, whereas DENs containing metals with a lattice mismatch show a bimodal distribution of binding energies. Overall, in this work we present a uniform method for synthesizing pure and bimetallic DENs and demonstrate that their catalytic properties are dependent on the adsorbate’s binding energy.

Monodisperse palladium nanoparticles consisting of 10–200 atoms were prepared using poly(amido)amine (PAMAM) dendrimer templates and were evaluated as catalysts using the model reduction of para-nitrophenol. The use of dendrimer templates allows for fine control of the average number of atoms per nanoparticle and systematic investigation of the effect of size on the catalytic activity of nanoparticles less than 2 nm in diameter. The palladium dendrimer-encapsulated nanoparticles (DENs) were found to be highly active for the hydrogenation of para-nitrophenol to para-aminophenol, with surface area-normalized rate constants ranging from 0.87 to 1.65 L s–1 m–2 (which is greater than any previously reported system). A near linear dependence of the observed rate constant on the synthetic Pd2+:dendrimer ratio was observed, suggesting that, within the size regime studied, most of the atoms lie on the surface of the nanoparticle and contribute to the catalytic activity. Interestingly, for Pd clusters containing between 10 and 50 atoms, the rate constant normalized on a per atom basis shows little variability, supporting the idea that all atoms lie on the surface of these clusters. However, for particles containing between 50 and 200 atoms, a decrease in per-atom activity is observed with increasing particle size, suggesting that in this size regime some atoms are located in the catalytically inactive core. Additionally, the generation of the PAMAM dendrimer template was found to have a significant effect on the observed rate constant due to steric crowding at the periphery (whereas the choice of an amine- or hydroxyl-termination on the dendrimer periphery did not).

Monoclinic α-Li3V2(PO4)3 has a complex 3-D metal phosphate framework that provides mobility for all three lithium ions, giving it the highest gravimetric capacity (197 mAh/g) of all the transition-metal phosphates. Along with its high gravimetric capacity, its thermal and electrochemical stability make it of great interest as a cathode material for lithium-ion energy storage devices. Raman spectroscopy has proven to be a unique analytical tool for studying electrode materials of lithium-ion batteries due to its ability to probe structural changes at the level of chemical bonds. In this work, the calculated Raman spectrum of α-Li3V2(PO4)3 provided by density functional theory is presented along with symmetry assignments for all of the calculated and observed modes through Raman microscopy. Furthermore, the phase stability of microcrystalline α-Li3V2(PO4)3 was studied as a function of irradiation power density. Follow-up thermal studies confirm that two structural phase transitions, β and γ, occur at elevated temperatures or high irradiation power density before degradation to α-LiVOPO4 under an oxygen-rich atmosphere. Calculated and experimentally determined Raman modes for α-Li3V2(PO4)3 are in good agreement. It is also noted that careful consideration of the irradiation power density employed must be taken into account to prevent misinterpretation of Raman spectral features.

Perovskites are of great interest as replacements for precious metals and oxides used in bifunctional air electrodes involving the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Herein, we report the synthesis and activity of a phase-pure nanocrystal perovskite catalyst that is highly active for the OER and ORR. The OER mass activity of LaNiO3, synthesized by the calcination of a rapidly dried nanoparticle dispersion and supported on nitrogen-doped carbon, is demonstrated to be nearly 3-fold that of 6 nm IrO2 and exhibits no hysteresis during oxygen evolution. Moreover, strong OER/ORR bifunctionality is shown by the low total overpotential (1.02 V) between the reactions, on par or better than that of noble metal catalysts such as Pt (1.16 V) and Ir (0.92 V). These results are examined in the context of surface hydroxylation, and a new OER cycle is proposed that unifies theory and the unique surface properties of LaNiO3.Keywords: bifunctional; catalyst; metal−air battery; oxygen evolution; oxygen reduction; perovskite; water oxidation;

Flavin adenine dinucleotide (FAD) is a cofactor for many enzymes, but also an informative redox active surface probe for electrode materials such as carbon nanotubes (CNTs) and nitrogen-doped CNTs (N-CNTs). FAD spontaneously adsorbs onto the surface of CNTs and N-CNTs, displaying Langmuir adsorption characteristics. The Langmuir adsorption model provides a means of calculating the electroactive surface area (ESA), the equilibrium constant for the adsorption and desorption processes (K), and the Gibbs free energy of adsorption (ΔG°). Traditional ESA measurements based on the diffusional flux of a redox active molecule to the electrode surface underestimate the ESA of porous materials because pores are not penetrated. Techniques such as gas adsortion (BET) overestimate the ESA because it includes both electroactive and inactive areas. The ESA determined by extrapolation of the Langmuir adsorption model with the electroactive surface probe FAD will penetrate pores and only include electroactive areas. The redox activity of adsorbed FAD also displays a strong dependency on pH, which provides a means of determining the pKa of the surface confined species. The pKa of FAD decreases as the nitrogen content in the CNTs increases, suggesting a decreased hydrophobicity of the N-CNT surface. FAD desorption at N-CNTs slowly transforms the main FAD surface redox reaction with E1/2 at −0.84 V into two new, reversible, surface confined redox reactions with E1/2 at −0.65 and −0.76 V (vs Hg/Hg2SO4), respectively (1.0 M sodium phosphate buffer pH = 6.75). This is the first time these redox reactions have been observed. The new surface confined redox reactions were not observed during FAD desorption from nondoped CNTs.

Here we describe a very simple, reliable, low-cost electrochemical approach to detect single nanoparticles (NPs) and evaluate NP size distributions and catalytic activity in a fast and reproducible manner. Single NPs are detected through an increase in current caused by electrocatalytic oxidation of N2H4 at the surface of the NP when it contacts a Hg-modified Pt ultramicroelectrode (Hg/Pt UME). Once the NP contacts the Hg/Pt UME, Hg poisons the Pt NP, deactivating the N2H4 oxidation reaction. Hence, the current response is a “spike” that decays to the background current level rather than a stepwise “staircase” response as previously described for a Au UME. The use of Hg as an electrode material has several quantitative advantages including suppression of the background current by 2 orders of magnitude over a Au UME, increased signal-to-noise ratio for detection of individual collisions, precise integration of current transients to determine charge passed and NP size, reduction of surface-induced NP aggregation and electrode fouling processes, and reproducible and renewable electrodes for routine detection of catalytic NPs. The NP collision frequency was found to scale linearly with the NP concentration (0.016 to 0.024 pM–1 s–1). NP size distributions of 4–24 nm as determined from the current–time transients correlated well with theory and TEM-derived size distributions.

The reaction of Fe(N(SiMe3)2)3 with PH3 in THF at 100 °C gives amorphous FeP2 in high yield. As an anode material in a Li ion battery, this material shows remarkable performance toward electrochemical lithiation/delithation, with gravimetric discharge and charge capacities of 1258 and 766 mA h g–1, respectively, translating to 61% reversibility on the first cycle and a discharge capacity of 906 mA h g–1 after 10 cycles. This translates to 66% retention of the theoretical full conversion capacity of FeP2 (1365 mA h g–1).

The ability to design and characterize uniform, bimetallic alloy nanoparticles, where the less active metal enhances the activity of the more active metal, would be of broad interest in catalysis. Herein, we demonstrate that simultaneous reduction of Ag and Pd precursors provides uniform, Ag-rich AgPd alloy nanoparticles (∼5 nm) with high activities for the oxygen reduction reaction (ORR) in alkaline media. The particles are crystalline and uniformly alloyed, as shown by X-ray diffraction and probe corrected scanning transmission electron microscopy. The ORR mass activity per total metal was 60% higher for the AgPd2 alloy relative to pure Pd. The mass activities were 2.7 and 3.2 times higher for Ag9Pd (340 mA/mgmetal) and Ag4Pd (598 mA/mgmetal), respectively, than those expected for a linear combination of mass activities of Ag (60 mA/mgAg) and Pd (799 mA/mgPd) particles, based on rotating disk voltammetry. Moreover, these synergy factors reached 5-fold on a Pd mass basis. For silver-rich alloys (Ag≥4Pd), the particle surface is shown to contain single Pd atoms surrounded by Ag from cyclic voltammetry and CO stripping measurements. This morphology is favorable for the high activity through a combination of modified electronic structure, as shown by XPS, and ensemble effects, which facilitate the steps of oxygen bond breaking and desorption for the ORR. This concept of tuning the heteroatomic interactions on the surface of small nanoparticles with low concentrations of precious metals for high synergy in catalytic activity may be expected to be applicable to a wide variety of nanoalloys.

Nanocomposites composed of MnO2 and graphitic disordered mesoporous carbon (MnO2/C) were synthesized for high total specific capacitance and redox pseudocapacitance (CMnO2) at high scan rates up to 200 mV s−1. High resolution transmission electron microscopy (HRTEM) with energy dispersive X-ray spectroscopy (EDX) demonstrated that MnO2 nanodomains were highly dispersed throughout the mesoporous carbon structure. According to HRTEM and X-ray diffraction (XRD), the MnO2 domains are shown to be primarily amorphous and less than 5 nm in size. For these composites in aqueous 1 M Na2SO4 electrolyte, CMnO2 reached 500 F/gMnO2 at 2 mV s−1 for 8.8 wt% MnO2. A capacitance fade of only 20% over a 100-fold change in scan rate was observed for a high loading of 35 wt% MnO2 with a CMnO2 of 310 F/gMnO2 at the highest scan rate of 200 mV s−1. The high electronic conductivity of the graphitic 3D disordered mesoporous carbon support in conjunction with the thin MnO2 nanodomains facilitate rapid electron and ion transport offering the potential of improved high power density energy storage pseudocapacitors.

Ru and RuxNi30 dendrimer encapsulated nanoparticles (DENs) were synthesized using a redox-displacement method. DEN catalytic activity for the reduction of p-nitrophenol was evaluated and found to be dependent on the ratio of metals present.

Spectrophotometric titration and a binding isotherm were used to accurately assess the loading capacity of generation four polyamido(amine) (PAMAM) dendrimer templates with terminal alcohol groups (G4-OH). Preparation of bimetallic G4-OH dendrimer-encapsulated metal nanoclusters (DENs) necessitates knowledge of the precise metal-ion binding capacity. The binding of metal ions such as Pt2+ and Pd2+ has proven difficult to assess via UV–vis spectroscopy because the absorbance shifts associated with metal-ion binding within the dendrimer template are masked by the absorbance of the PAMAM dendrimer itself. In contrast, the binding of Cu2+ to G4-OH PAMAM dendrimer results in a strong, distinct absorption band at 300 nm, making UV–vis spectrophotometric titration with copper straightforward. Here we use copper binding as a means to assess the number of binding sites remaining within the PAMAM G4-OH dendrimer after the complexation of a specified molar excess of Pd2+ or Pt2+. In addition, we use a binding isotherm to mathematically estimate the loading capacity of the dendrimer in each case. The loading capacities for M2+ in the G4-OH dendrimer were found to be ∼16 for copper alone, ∼21 for copper combined with palladium, and ∼25 for copper combined with platinum.

This study investigates electrogenerated graphitic oxides (EGO) on the surface of carbon optically transparent electrodes (C-OTEs) using a combined UV–vis spectroelectrochemical approach. By monitoring the π–π* aromatic carbon transition for reduced GO (270 nm) and GO (230 nm), we observe the growth of GO in KCl upon applying oxidizing potentials. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (TOF-SIMS) are used to confirm sample composition and location of salt ions within the electrode. Formation of EGO stable enough to be observed by UV–vis is found to be unique to alkali chloride supporting electrolytes due to formation of a solid–electrolyte interphase (SEI) which incorporates the alkali cation to stabilize the negatively charged oxygen functional groups while the presence of chloride anion acts as a passivation agent that protects the electrode surface from dissolution. The spectroelectrochemical approach highlights the detection and study of EGO that cannot be detected by electrochemical measurements. Specifically, the amount of EGO observed by UV–vis scales with increasing cation size (Li+, Na+, K+) despite all the cations showing identical electrochemical response.

Interfacial interactions between sub-4 nm metal alloy nanoparticles and carbon supports, although not well understood at the atomic level, may be expected to have a profound influence on catalytic properties. Pd3Pt2 alloy particles comprised of a disordered surface layer over a corrugated crystalline core are shown to exhibit strong interfacial interactions with a ∼20–50 nm spherical carbon support, as characterized by probe aberration corrected scanning transmission electron microscopy (pcSTEM). The disordered shells were formed from defects introduced by Pd during arrested growth synthesis of the alloy nanoparticles. The chemical and morphological changes in the catalyst, before and after cyclic stability testing (1000 cycles, 0.5–1.2 V), were probed with cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and pcSTEM. The strong metal–support interaction, along with the uniform alloy structure raised the mass activity by a factor of 1.8 versus pure Pt. The metal–support interactions also mitigated nanoparticle coalescence, dissolution, and ripening, resulting in only a 20% loss in mass activity (versus 60% for pure Pt on carbon) after the cyclic stability test. The design of alloy structure, guided by insight from atomic scale pcSTEM, for enhanced catalytic activity and stability, resulting from strong wetting with a deformable disordered shell, has the potential to be a general paradigm for improving catalytic performance.

Here, we present the Li+ insertion behavior of mesoporous ordered TiO2(B) nanoparticles (meso-TiO2(B)). Using presynthesized 4 nm TiO2(B) nanoparticles as building blocks and a commercially available ethylene glycol-propylene glycol block copolymer (P123) as a structure-directing agent, we were able to produce mesoporous structures of high-purity TiO2(B) with nanocrystallinity and mesopore channels ranging from 10 to 20 nm in diameter. We compared the Li+ insertion properties of nontemplated TiO2(B) nanoparticles (nano-TiO2(B)) to meso-TiO2(B) via voltammetry and galvanostatic cycling and found significant increases in overall Li+ insertion capacity for the latter. While nano-TiO2(B) and meso-TiO2(B) both show surface charging (pseudocapacitive) Li+ insertion behavior, meso-TiO2(B) exhibits a higher overall capacity especially at high charge rates. We attribute this effect to higher electrode/electrolyte contact area as well as the improved electron and ion transport in meso-TiO2(B). In this study, we have demonstrated the influence of both nanostructuring and mesoporosity on Li+ insertion behavior by rationally controlling the overall architecture of the TiO2(B) materials.

This study investigates pyrolyzed photoresist film (PPF)-based carbon optically transparent electrodes (C-OTEs) for use in electrogenerated chemiluminescence (ECL) studies. Oxidative–reductive ECL is obtained with a well-characterized ECL system, C8S3 J-aggregates with 2-(dibutylamino)ethanol (DBAE) as coreactant. Simultaneous cyclic voltammograms (CVs) and ECL transients are obtained for three thicknesses of PPFs and compared to nontransparent glassy carbon (GC) and the conventional transparent electrode indium tin oxide (ITO) in both front face and transmission electrode cell geometries. Despite positive potential shifts in oxidation and ECL peaks, attributed to the internal resistance of the PPFs that result from their nanoscale thickness, the PPFs display similar ECL activity to GC, including the low oxidation potential (LOP) observed for amine coreactants on hydrophobic electrodes. Reductive–oxidative ECL was obtained using the well-studied ECL luminophore Ru(bpy)32+, where the C-OTEs outperformed ITO because of electrochemical instability of ITO at very negative potentials. The C-OTEs are promising electrodes for ECL applications because they yield higher ECL than ITO in both oxidative–reductive and reductive–oxidative ECL modes, are more stable in alkaline solutions, display a wide potential window of stability, and have tunable transparency for more efficient detection of ECL.

Achieving synergy between inexpensive metals and metal oxides is a key challenge for the development of highly active, economical catalysts. We report the synthesis and characterization of a highly active oxygen reduction reaction (ORR) catalyst composed of Ag particles (3 nm) in intimate contact with thin (∼1 nm) MnOx domains on Vulcan carbon (VC) as shown via electron microscopy. A new electroless co-deposition scheme, whereby MnO4– ions are reduced by carbon, formed nanosized MnOx reduction centers for Ag nanoparticle deposition. A bifunctional mechanism for ORR is proposed, in which the HO2– intermediate is formed electrochemically and is regenerated via disproportionation into OH– and O2. A 3× mass activity enhancement is observed for Ag-MnOx/VC (125 mA/mgAg+MnOx) over the linear combination of pure component activities using rotating disk voltammetry. The Ag-MnOx/VC mass activity is comparable to commercial Pd/VC (111 mA/mgPd) and Pt/VC (136 mA/mgPt). Furthermore, the number of electrons transferred for ORR reaches 3.5 for Ag-MnOx, higher than for MnOx (2.8) and close to the full four-electron ORR. The synergy can be rationalized by ensemble effects, where Ag and MnOx domains facilitate the formation and disproportionation of HO2–, respectively, and ligand effects from the unique electronic interaction at the Ag-MnOx interface.

Since the potential for alloying lithium with silicon is outside the window of stability of common commercial electrolytes, silicon surfaces form an amorphous solid electrolyte interphase (SEI) under applied potential, which hampers silicon's performance as a lithium-ion battery anode. We have investigated the composition, distribution, and ambient stability of the SEI formed on undoped silicon (001) wafers configured as model electrodes in three different electrochemical conditions using a reduced oxidation interface for transporting air-sensitive samples from a glovebox to an ultra-high-vacuum chamber for X-ray photoelectron spectroscopy (XPS) analysis. Variable potential cycling and step experiments included linear sweep voltammetry (LSV), cyclic voltammetry (CV), and chronoamperometry (CA). CV and LSV experiments on silicon electrodes scanned from open-circuit voltage to lithiation (3–0.01 V vs Li/Li+) showed a suppression of carbonate-containing species relative to CA experiments (potential step for 300 s at 0.01 V vs Li/Li+) in anoxic XPS measurements. When silicon electrodes were exposed to ambient air, SEI layers reacted through both fluorination and combustion processes to produce different SEI product distributions than those prepared under anoxic conditions.

The lithium insertion behavior of nanoparticle (3-D) and nanosheet (2-D) architectures of TiO2(B) is quite different, as observed by differential capacity plots derived from galvanostatic charging/discharge experiments. DFT+U calculations show unique lithiation mechanisms for the different nanoarchitectures. For TiO2(B) nanoparticles, A2 sites near equatorial TiO6 octahedra are filled first, followed by A1 sites near axial TiO6 octahedra. No open-channel C site filling is observed in the voltage range studied. Conversely, TiO2(B) nanosheets incrementally fill C sites, followed by A2 and A1. DFT+U calculations suggest that the different lithiation mechanisms are related to the elongated geometry of the nanosheet along the a-axis that reduces Li+–Li+ interactions between C and A2 sites. The calculated lithiation potentials and degree of filling agree qualitatively with the experimentally observed differential capacity plots.Keywords: 2-D nanostructures; anodes; DFT+U; lithium ion batteries; TiO2(B);

Vanadium oxide (V2O5) is a multifaceted material possessing desirable redox properties, including accessibility to multiple valence states, which make it attractive as a cathode for lithium ion batteries and microbatteries. Studies show that performance of this electrode material is dependent on the electrolyte employed and that solid electrolyte interphase (SEI) layer formation is responsible for the fade in capacity with multiple cycling. Nanostructured V2O5 thin films synthesized through reactive ballistic deposition (RBD) were studied with electrochemical methods, ex situ Raman and ex situ XPS in two widely used electrolytes: LiClO4/propylene carbonate (PC) and LiPF6/diethyl carbonate (DEC) + ethylene carbonate (EC). Films cycled in LiPF6/DEC+EC experienced a 32% greater capacity fade between the first and second lithiathion/delithiation cycles than those cycled in LiClO4/PC, due to a redox-induced change in the surface morphology and composition and an irreversible transformation into an amorphous state as monitored by ex situ Raman. From X-ray photoelectron spectroscopy (XPS), it was shown that V2O5 cycled in LiPF6/DEC+EC contained a high atomic concentration percentage of fluoride (16.18%) in comparison with V2O5 electrodes cycled in LiClO4/PC (3.94%). No significant amounts of carbonates, oxalates, or oxyfluorophosphates typically associated with SEI formation were found when V2O5 was cycled in either electrolyte. The results obtained suggest instead that HF, formed upon water contamination of the electrolyte, reacts with V2O5 through a self-catalyzed process both at open circuit and under applied potential. The formation of vanadium oxyfluorides causes active mass loss and severe capacity fade upon discharging/charging.

Nitrogen-doped carbon nanotubes (N-CNTs) provide a simple, robust, and unique platform for biosensing. Their catalytic activity toward the oxygen reduction reaction (ORR) and subsequent hydrogen peroxide (H2O2) disproportionation creates a sensitive electrochemical response to enzymatically generated H2O2 on the N-CNT surface, eliminating the need for additional peroxidases or electron-transfer mediators. Glassy carbon electrodes were modified with 7.4 atom % N-CNTs, lactate oxidase (LOx), and a tetrabutylammonium bromide (TBABr)-modified Nafion binder. The resulting amperometric l-lactate biosensors displayed a sensitivity of 0.040 ± 0.002 A M–1 cm–2, a low operating potential of −0.23 V (vs Hg/Hg2SO4), a repeatability of 1.6% relative standard deviation (RSD) for 200 μM samples of lactate, a fabrication reproducibility of 5.0% (RSD), a limit of detection of 4.1 ± 1.6 μM, and a linear range of 14–325 μM. Additionally, over a 90 day period, the repeatability for 200 μM samples of lactate remained below 3.4% (RSD). Direct electron transfer was observed between the LOx redox-active center and the N-CNTs with the electroactive surface coverage determined to be 0.27 nmol cm–2.

This study investigates superradiant organic dye J-aggregates as a potential new class of aqueous luminophores for electrogenerated chemiluminescence (ECL). Simultaneous cyclic voltammograms (CVs) and ECL transients are obtained from the self-assembled double-walled tubular J-aggregates formed from the amphiphilic cyanine dye 3,3′-bis(2-sulfopropyl)-5,5′,6,6′-tetrachloro-1,1′-dioctylbenzimidacarbocyanine (C8S3) immobilized on glassy carbon electrodes in the presence of the oxidative−reductive coreactant 2-(dibutylamino)ethanol (DBAE). ECL is produced by both the direct oxidation of DBAE at the electrode and the catalytic oxidation of DBAE by the C8S3 J-aggregates. Optimization studies of the DBAE concentration and pH of the electrolyte show the most intense ECL signal was obtained with ∼17 mM DBAE as coreactant (saturated solution in 1 M KNO3) at pH 12.85, an effect of DBAE solubility and pKa. The overlaid ECL spectrum and the fluorescence spectrum were in good agreement, confirming that the ECL emission is associated with the singlet exciton delocalized on the tubular C8S3 J-aggregates. Amphiphilic J-aggregates are promising new systems for ECL applications because of their unique characteristics such as accessible redox chemistry in the aqueous potential window, increased fluorescence emission, and narrow emission lines.

MnO2–mesoporous carbon hybrid nanocomposites were synthesized to achieve high values of redox pseudocapacitance at scan rates of 100 mV s−1. High-resolution transmission electron microscopy (HRTEM) along with energy dispersive X-ray spectroscopy (EDX) demonstrated that ∼1 nm thick MnO2 nanodomains, resembling a conformal coating, were uniformly distributed throughout the mesoporous carbon structure. HRTEM and X-ray diffraction (XRD) showed formation of MnO2 nanocrystals with lattice planes corresponding to birnessite. The electrochemical redox pseudocapacitance of these composite materials in aqueous 1 M Na2SO4 electrolyte containing as little as 2 wt% MnO2 exhibited a high gravimetric MnO2 pseudocapacitance (CMnO2) of 560 F gMnO2−1. Even for 30 wt% MnO2, a high CMnO2 of 137 F gMnO2−1 was observed at 100 mV s−1. Sodium ion diffusion coefficients on the order of 10−9 to 10−10 cm2 s−1 were measured using chronoamperometry. The controlled growth and conformal coating of redox-active MnO2–mesoporous carbon composites offer the potential for achieving high power energy storage with low cost materials.

Generalized ellipsometry and quartz crystal nanogravimetry are combined to determine adsorption isotherms and changes in the optical properties of biaxial TiO2 thin films by monitoring changes in the Mueller matrix. Individual Mueller matrix elements, corresponding to a variety of polarization states, exhibit dramatically different sensitivities to the adsorption of toluene. While some elements are sensitive to structural anisotropy and orientation, others report uniquely on the refractive index. The fast (na) optical axis reflects the greatest change in refractive index due to the adsorption, leading to a decrease from Δn800nm = 0.4 to 0.1. This change is discussed in terms of the Bragg−Pippard (B−P) effective medium approximation, which is shown to accurately describe changes in optical behavior in response to adsorption. The integration of generalized ellipsometry with quartz crystal nanogravimetry establishes a highly sensitive technique for acquiring adsorption isotherms and for chemical optical sensing of structurally anisotropic thin films.Keywords (keywords): anisotropy; ellipsometric porosimetry; glancing angle deposition; Mueller matrix; reactive ballistic deposition; titanium dioxide;

Au nanocrystals stabilized by dodecanethiol were deposited into 100−150 nm thick TiO2 films with evenly spaced perpendicular nanopillars and mesochannels on the order of 10 nm supported on conducting ITO/glass electrodes. Electrophoretic deposition was used to enhance nanocrystal deposition within the mesoporous TiO2 film. X-ray photoelectron spectroscopy (XPS), scanning electron microscopy with energy-dispersive X-ray (EDX), UV−vis spectroscopy, variable-angle spectroscopic ellipsometry (VASE), and scanning surface potential microscopy (SSPM) were used to characterize the resulting Au nanocrystal/TiO2 composites. Au nanocrystal loadings reached 21 wt % and were not kinetically limited at 10 min, relative to depositions performed for 20 h. Both VASE measurements of the anisotropy of the imaginary refractive index, k, and X-ray photoelectron spectroscopy (XPS) depth profiling studies indicate that Au nanocrystals are dispersed within the vertically aligned mesopores and distributed throughout the film. The mean penetration depth of a single nanocrystal penetrating inside the film is described with a model in terms of the electric field and a local deposition rate constant, which is influenced by ligand binding and architecture on the nanocrystal surface.

Ru and RuxNi30 dendrimer encapsulated nanoparticles (DENs) were synthesized using a redox-displacement method. DEN catalytic activity for the reduction of p-nitrophenol was evaluated and found to be dependent on the ratio of metals present.

MnO2–mesoporous carbon hybrid nanocomposites were synthesized to achieve high values of redox pseudocapacitance at scan rates of 100 mV s−1. High-resolution transmission electron microscopy (HRTEM) along with energy dispersive X-ray spectroscopy (EDX) demonstrated that ∼1 nm thick MnO2 nanodomains, resembling a conformal coating, were uniformly distributed throughout the mesoporous carbon structure. HRTEM and X-ray diffraction (XRD) showed formation of MnO2 nanocrystals with lattice planes corresponding to birnessite. The electrochemical redox pseudocapacitance of these composite materials in aqueous 1 M Na2SO4 electrolyte containing as little as 2 wt% MnO2 exhibited a high gravimetric MnO2 pseudocapacitance (CMnO2) of 560 F gMnO2−1. Even for 30 wt% MnO2, a high CMnO2 of 137 F gMnO2−1 was observed at 100 mV s−1. Sodium ion diffusion coefficients on the order of 10−9 to 10−10 cm2 s−1 were measured using chronoamperometry. The controlled growth and conformal coating of redox-active MnO2–mesoporous carbon composites offer the potential for achieving high power energy storage with low cost materials.

TiO2(B) exhibits unique electrochemical lithiation behavior where two closely spaced reduction and oxidation peaks, referred to as “double peaks,” are observed for 3-D forms of bulk and nanostructured TiO2(B) while a single broad redox peak is observed for 2-D nanosheet architectures. In this study, we have used a combination of transmission electron microscopy (TEM), powder X-ray diffraction (XRD), Raman spectroscopy and cyclic voltammetry on TiO2(B) nanosheets as well as a series of thermally annealed nanosheets to map the morphological and phase transformation pathways that help clarify the structure-dependent lithiation behavior. We found that the double peak redox behavior only arises once a three dimensional nanocrystalline structure of TiO2(B) exists by observing nanoparticle growth on the TiO2(B) nanosheet surface via TEM at temperatures above 150 °C. This morphological transformation was also verified by Raman spectroscopy. The appearance of low-energy torsional modes at temperatures above 150 °C which are not observed in 2-D morphologies of TiO2(B) agrees well with TEM evidence of 3-D nanoparticle formation. Using scan rate dependent cyclic voltammetry we also verified that all lithiation behavior associated with TiO2(B) (either nanosheet or nanoparticle) is due primarily to a surface redox (pseudocapacitive) mechanism. The thermal annealing study also shows the phase transformation of surface nucleated TiO2(B) nanoparticles to anatase nanoparticles at temperatures above 200 °C. These studies clearly show how nano-morphological control can influence electrochemical lithiation behavior and help identify a possible mechanism to explain the double peak phenomenon observed for TiO2(B).