Electrochemical vapor deposition of semiconductors from gas phase with a solid membrane cell.

We demonstrate the feasibility of semiconductor deposition via the electrochemical reduction of gaseous precursors by the use of an anhydrous proton-conducting membrane, the solid acid CsHSO4, at 165 °C. This membrane electrode assembly was operated within the oxidation of hydrogen on a porous Pt anode and the deposition of Si or Ge under bias at the cathode from chloride-based gaseous precursors; SiCl4 and GeCl4 in an Ar flow with a reduction potential over -1.0 V (vs RHE).

Nano-size layered manganese-calcium oxide as an efficient and biomimetic catalyst for water oxidation under acidic conditions: comparable to platinum.

Inspired by Nature's catalyst, a nano-size layered manganese-calcium oxide showed a low overvoltage for water oxidation in acidic solutions, which is comparable to platinum.

Open circuit (mixed) potential changes upon contact between different inert electrodes-size and kinetic effects.

We investigate the principle of the open circuit potential (OCP) change upon a particle collision event based on mixed potential theory and confirmed by a mimic experiment in which we studied the changes in the OCP when two different electrodes (Pt and Au) are brought into contact in a solution that contains some irreversible redox couples. A micrometer-sized Au ultramicroelectrode, when connected in parallel to a Pt micro- or nanoelectrode, showed clearly measurable OCP changes whose magnitude matches well with that predicted by a simplified mixed potential theory for a pair of different electrode materials. On the basis of the study, each electrode establishes a different mixed potential involving two or more half reactions that have different heterogeneous electron transfer kinetics at different electrodes and the OCP changes are very sensitive to the relative ratio of the rate constant of the individual half reaction at different materials.

Observation of single metal nanoparticle collisions by open circuit (mixed) potential changes at an ultramicroelectrode.

Single nanoparticle (NP) collisions were successfully observed by a potentiometric measurement. The open circuit potential (OCP) of a measuring Au ultramicroelectrode (UME) changes when Pt NPs collide with the UME in a hydrazine solution. The OCP change is related to the redox processes, the concentration of particles, particle size, and electrode size. Compared with the amperometric technique, this approach has several advantages: higher sensitivity, simpler apparatus, fewer problems with NP decomposition, and contamination.

Synthesis and characterization of a p-type boron arsenide photoelectrode.

A p-type boron arsenide photoelectrode was prepared from a material consisting of a thin layer of boron arsenide on a boron substrate. The structure of the material was identified using X-ray diffraction and scanning electron microscopy, and the surface composition was determined by means of X-ray photoelectron spectroscopy. The electrode was found to be photoactive under both visible light and UV-vis irradiation and displayed a photocurrent of ~0.1 mA/cm(2) under UV-vis irradiation at an applied potential of -0.25 V vs Ag/AgCl. Mott-Schottky plots for this boron arsenide electrode displayed an estimated flat-band potential near the onset photopotential. The estimated indirect band gap, as determined from incident photon-to-electron conversion efficiency plots, is 1.46 ± 0.02 eV.

Stochastic electrochemistry with electrocatalytic nanoparticles at inert ultramicroelectrodes--theory and experiments.

Collisions of several kinds of metal or metal oxide single nanoparticles (NPs) with a less catalytic electrode surface have been observed through amplification of the current by electrocatalysis. Two general types of current response, a current staircase or a current blip (or spike) are seen with particle collisions. The current responses were caused by random individual events as a function of time rather than the usual continuous current caused by an ensemble of a large number of events. The treatment of stochastic electrochemistry like single NP collisions is different from the usual model for ensemble-based electrochemical behaviour. Models for the observed responses are discussed, including simulations, and the frequency of the steps or blips investigated for several systems experimentally.

Observing iridium oxide (IrO(x)) single nanoparticle collisions at ultramicroelectrodes.

We describe the electrochemical detection of single iridium oxide nanoparticle (IrO(x) NP) collisions on a NaBH(4)-treated Pt ultramicroelectrode (UME). We observe single NP events through the enhanced current by electrocatalytic water oxidation, when IrO(x) contacts the electrode and transiently sticks to it. The overall current transient consists of repeated current spikes that return to the background level, superimposed on a current decay, rather than the staircase response seen where an NP sticks on the UME. Here each event produces a unique current spike (or "blip"). The frequency of the spikes was directly proportional to the particle concentration, and the peak current increased with the applied potential. The observed current is very sensitive to the material and surface state of the measuring electrode; a NaBH(4)-treated Pt UME was important in obtaining reproducible results.

Electrogenerated chemiluminescence of partially oxidized highly oriented pyrolytic graphite surfaces and of graphene oxide nanoparticles.

Electrogenerated chemiluminescence (ECL) can be obtained from an electrochemically oxidized highly oriented pyrolytic graphite electrode or a graphene oxide suspension in an aqueous solution containing 0.1 M NaClO(4), phosphate buffer (PBS) (pH = 7.0) and 13 mM tri-n-propylamine. Single ECL events from individual graphene oxide nanosheets can be observed. The observation of ECL in such systems may lead to the development of a new class of carbon-based nanomaterials for potentially highly sensitive analytical applications.

Electrochemistry of oxygen in concentrated NaOH solutions: solubility, diffusion coefficients, and superoxide formation.

The diffusion coefficient (D(O(2))), solubility (C(O(2))), and electrochemical behavior of oxygen reduction were investigated by cyclic voltammetry and transient amperometry on a Pt ultramicroelectrode in aqueous solutions containing various concentrations of NaOH (1-12 M). The results show that both D(O(2)) and C(O(2)) decrease as the solution viscosity (eta) increases significantly with increasing concentration of NaOH. The Stokes-Einstein relationship (D(O(2)) vs 1/eta) is followed, yielding a radius for the O(2) molecule, 2.8 A, that does not change over the concentration range of NaOH studied. From results reported previously for C(O(2)) in more dilute NaOH solutions and the new results here, the number of electrons, n, involved in the first step of the oxygen reduction reaction (ORR) was found to change with the concentration of NaOH, from n = 2 at low NaOH concentrations (1-2 M) to n = 1 at high concentrations (>6 M), in line with the changing water activity. Scanning electrochemical microscopy (SECM) was employed as a sensitive tool to investigate the electrochemical behavior of the product of the ORR in 10 M NaOH solution, O(2)(*-), as a function of potential. The SECM approach curves depend on the substrate bias and state of the Pt substrate surface, and the apparent rate constant for the redox couple of O(2)/O(2)(*-) is determined to be about 2.6 x 10(-4) cm/s in this solution.

The Kv channel blocker 4-aminopyridine enhances Ag+ uptake: a scanning electrochemical microscopy study of single living cells.

We report that silver ion (Ag(+)) uptake is enhanced by 4-aminopyridine (4-AP), a well known voltage-sensitive potassium ion channel (K(v)) blocker. Both bacterial (Escherichia coli) and mammalian (3T3 fibroblast) cells were used as model systems. Ag(+) uptake was monitored with a scanning electrochemical microscope with an amperometric Ag(+) ion-selective electrode (Ag(+)-ISE) and the respiration rates of E. coli cells were measured by oxygen reduction at an ultramicroelectrode. The results showed that not only the amount but also the rate of silver uptake by the cells increased significantly when 4-AP was added to the solution. For fibroblasts, the Ag(+) uptake rate was 4.8 x 10(7) ions per cell per sec without 4-AP compared with 1.0 x 10(8) ions per cell per sec with 0.2 mM 4-AP. For E. coli cells, the uptake rate was 1.5 x 10(4) ions per cell per sec without 4-AP vs. 3.5 x 10(4) ions per cell per sec with 0.5 mM 4-AP and 5.9 x 10(4) ions per cell per sec with 1 mM 4-AP. Thus, 4-AP might be useful where silver is used as antimicrobial agent to speed its uptake.

Electrogenerated chemiluminescence of single conjugated polymer nanoparticles.

We demonstrate a novel and powerful method to study electrogenerated chemiluminescence (ECL) of single nanoparticles (NPs) (r = 25 +/- 15 nm) of a conjugated polymer, F8BT, on an ITO electrode in the presence of a co-reactant, such as tri-n-propylamine (TPrAH) in acetonitrile solution. The results reveal that the maximum formation rate of ECL of individual NPs is achieved after a long "build-up" time (10-40 s after pulse application). The high number of detected ECL photons from individual NPs (1500 photons during 100 s) highlights the potential of this technique as a very sensitive analytical method. Additionally, TPrAH acts as a very efficient protecting agent against irreversible electrochemical processes occurring in F8BT, as found in photoluminescence studies. This protection mechanism probably involves the neutralization of holes at the particle surface via electron transfer by both TPrAH and TPrA radical (TPrA*).

Spontaneous formation and electrogenerated chemiluminescence of tris(bipyridine) Ru(II) derivative nanobelts.

The single crystalline nanobelts were successfully fabricated with an ionic compound by a simple reprecipitation method. The compound used is the water-insoluble derivative of tris(bipyridine) Ru(II), [Ru(bpy)2(4,4'-(CH3(CH2)14COO)2-bpy)](ClO4)2. The prepared nanobelts show an enhanced fluorescence emission and relatively strong electrogenerated chemiluminescence (ECL), that have potential analytical applications. More interesting, ECL of a single nanobelt deposited on an ultramicroelectrode was observed. The observation of ECL in such nanostructures leads to the development of a new class of ECL systems that may prove useful for a variety of purposes.

Observing single nanoparticle collisions by electrogenerated chemiluminescence amplification.

We demonstrate a novel method of observing single particle collision events with electrogenerated chemiluminescence (ECL). A single event is characterized by the enhancement of ECL intensity during the collision of an individual platinum nanoparticle (Pt NP) on an indium tin oxide electrode, which catalyzes the oxidation of Ru(bpy)3(2+) and a coreactant, for example, tri- n-propylamine (TPrA), present in the solution. Every collision produces a unique photon spike whose amplitude and frequency can be correlated with the size and concentration of the Pt NPs. A large amplification of ECL intensity can occur by choosing an appropriate measuring electrode and using high concentrations of Ru(bpy)3(2+) and the coreactant.

Scanning electrochemical microscopy.

This review describes work done in scanning electrochemical microscopy (SECM) since 2000 with an emphasis on new applications and important trends, such as nanometer-sized tips. SECM has been adapted to investigate charge transport across liquid/liquid interfaces and to probe charge transport in thin films and membranes. It has been used in biological systems like single cells to study ion transport in channels, as well as cellular and enzyme activity. It is also a powerful and useful tool for the evaluation of the electrocatalytic activities of different materials for useful reactions, such as oxygen reduction and hydrogen oxidation. SECM has also been used as an electrochemical tool for studies of the local properties and reactivity of a wide variety of materials, including metals, insulators, and semiconductors. Finally, SECM has been combined with several other nonelectrochemical techniques, such as atomic force microscopy, to enhance and complement the information available from SECM alone.

Current transients in single nanoparticle collision events.

Electrochemical hydrazine oxidation and proton reduction occur at a significantly higher rate at Pt than at Au or C electrodes. Thus, the collision and adhesion of a Pt particle on a less active Au or C electrode leads to a large current amplification by electrocatalysis at single nanoparticles (NPs). At low particle concentrations, the collision of Pt NPs was characterized by current transients composed of individual current profiles that rapidly attained a steady state, signaling single NP collisions. The characteristic steady-state current was used to estimate the particle size. The fluctuation in collision frequency with time indicates that the collision of NPs at the detector electrodes occurs in a statistically random manner, with the average frequency a function of particle concentration and diffusion coefficient. A longer term current decay in single current transients, as opposed to the expected steady-state behavior, was more pronounced for proton reduction than for hydrazine oxidation, revealing microscopic details of the nature of the particle interaction with the detector electrode and the kinetics of electrocatalysis at single NPs. The study of single NP collisions allows one to screen particle size distributions and estimate NP concentrations and diffusion coefficients.

Charging and discharging of single conjugated-polymer nanoparticles.

Despite intense, long-term interest in organic semiconductors from both an applied and fundamental perspective, key aspects of the electronic properties of these materials remain poorly defined. A particularly challenging problem is the molecular nature of positive charge carriers, that is, holes or oxidized species in organics. Here, the unique ability of single-molecule spectroelectrochemistry (SMS-EC) to unravel complex electrochemical process in heterogeneous media is used to study the oxidation of nanoparticles of the conjugated polymer poly(9,9-dioctylfluorene-co-benzothiadiazole). A reversible hole-injection charging process has been observed that occurs primarily by initial injection of shallow (untrapped) holes, but soon after the injection, a small fraction of the holes becomes deeply trapped. Good agreement between experimental data and simulations strongly supports the presence of deep traps in the studied nanoparticles and highlights the ability of SMS-EC to study energetics and dynamics of deep traps in organic materials at the nanoscale.

Scanning electrochemical microscopy. 58. Application of a micropipet-supported ITIES tip to detect Ag+ and study its effect on fibroblast cells.

We report the use of a micropipet-supported ITIES (interface between two immiscible electrolyte solutions, also called a liquid/liquid (L/L) or water/oil (W/O) interface) as a scanning electrochemical microscopy (SECM) tip to detect silver ion and explore Ag+ toxicity in living cells. A 1,2-dichloroethane solution containing a commercially available calixarene-based Ag+ ionophore (IV) was injected into a micrometer-size glass pipet to construct an Ag+-selective SECM tip. The local Ag+ concentration, down to the micromolar level, in the vicinity of living fibroblast cells, was monitored by SECM approach curves and through imaging of the uptake and efflux of Ag+ by living fibroblast cells in real time. The results show that several stages of interaction between Ag+ and fibroblast cells exist. Since a number of biological processes of cells are involved with non-redox-active ions, the work presented here provides a new way to explore cell metabolism, drug delivery, and toxicity assessment by SECM.

Facile electrochemical characterization of core/shell nanoparticles. Ag core/Ag(2)O shell structures.

We report in this paper a facile approach for the formation and electrochemical characterization of silver-silver oxide core-shell nanoparticles (NPs). Thus, thermal treatment at temperatures between 200 and 360 degrees C of Ag NP, in the gas phase or in an organic solvent, has been used to achieve the formation Ag@Ag2O NP. The evidence of formation of such a core-shell structure was obtained by cyclic voltammetry using a Nafion modified electrode (where Nafion containing carbon particles is used as the matrix to encapsulate the core-shell NP). Initial positive scans measure free Ag. Initial negative scans measure Ag2O, with the following positive scan, compared to the initial one, providing a measure of "trapped" or core Ag. The results presented demonstrate the utility of this approach in characterizing core-shell structures, like Ag@Ag2O, which could be extended to other core-shell forms, such as bimetallic core-shell NP.

Single-molecule spectroelectrochemistry (SMS-EC).

We introduce single-molecule spectroelectrochemistry (SMS-EC), a powerful new technique for studying electrochemical kinetics in highly heterogeneous systems. This technique uses fluorescence single-molecule spectroscopy to indirectly measure electrochemical kinetics one molecule at a time, offering for the first time the distribution of key electrochemical variables, such as the half-wave potential, E1/2, not just the ensemble averages. In SMS-EC, the potential of the working electrode of an electrochemical cell is linearly scanned while simultaneously measuring the florescence intensity, Ifl(t), of individual single molecules as a function of time in a wide-field microscope. SMS-EC is used herein to study the oxidation at an indium tin oxide (ITO) electrode of single molecules of the organic conjugated polymer F8BT. The results reveal both excited singlet state and ground state oxidation of F8BT. The latter process occurs over a narrow distribution of single-molecule half-wave potential values, indicating a relatively uniform electrochemical potential at the electrode.