Olena Yurchenko

Electrochemically Induced Reversible and Irreversible Coupling of Triarylamines

The electrochemical coupling and dimerization behavior of the low molecular compounds triphenylamine (TPA) and 9-phenylcarbazole (PHC) in comparison to tri-p-tolylamine (p-TTA) with para-blocked methyl groups has been investigated in detail. In contrast to the unsubstituted radical cations of TPA and PHC, the radical cations of p-TTA are stable in the radical cation state and do not undergo any further coupling reactions. However, we found that the dicationic state of p-TTA does undergo two different competitive reaction pathways: (1) an irreversible intramolecular coupling reaction which leads to phenylcarbazole moieties and (2) a reversible intermolecular dimerization leading to charged σ-dimers. The σ-dimers become decomposed upon discharging at low potentials (E(pc) = -0.97 V vs Fc/Fc(+)) so that the starting monomer p-TTA is partially regenerated. In particular, the reversible dimerization reaction has not been described in literature so far. Polymeric systems containing para-methyl blocked triarylamines in the side chain exhibit similar coupling behavior upon electrochemical doping.

Soft-Etch Mesoporous Hole-Conducting Block Copolymer Templates

We present a mesoporous hole-conducting polymer film resulting from spontaneous block copolymer self-assembly based on a simple spin-coating protocol. A diblock copolymer consisting of a triphenylamine side group polymer and a poly(d,l-lactide) block (PSTPA-b-PLA) is shown to microphase separate to form ordered 13 nm cylindrical PLA microdomains embedded in the semiconducting PSTPA matrix. Partially ordered and film-spanning PLA domains could be identified in films immediately after spin coating from toluene solutions on conducting substrates. Selective mild etching of the minority PLA domains (in weak aqueous base) leads to a mesoporous hole-conducting polymer matrix. The pore structure is replicated electrochemically in platinum, demonstrating the viability of this approach to producing nano-organized heterojunction structures in thin films.

Nanomolar detection of methylparaben by a cost-effective hemoglobin-based biosensor

This work describes the development of a new biosensor for methylparaben determination using electrocatalytic properties of hemoglobin in the presence of hydrogen peroxide. The voltammetric oxidation of methylparaben by the proposed biosensor in phosphate buffer (pH=7.0), a physiological pH, was studied and it was confirmed that methylparaben undergoes a one electron-one proton reaction in a diffusion-controlled process. The biosensor was fabricated by carbon paste electrode modified with hemoglobin and multiwalled carbon nanotube. Based on the excellent electrochemical properties of the modified electrode, a sensitive voltammetric method was used for determination of methylparaben within a linear range from 0.1 to 13μmolL(-1) and detection limit of 25nmolL(-1). The developed biosensor possessed accurate and rapid response to methylparaben and showed good sensitivity, stability, and repeatability. Finally, the applicability of the proposed biosensor was verified by methylparaben evaluation in various real samples.

Rational design of morphological characteristics to determine the optimal hierarchical nanostructures in heterogeneous catalysis

This work draws attention to the optimal hierarchical nanostructure morphology and the morphological characteristics that lead to a rational design of heterogeneous nanocatalysts, especially for reactions that exhibit sluggish kinetics. A simplified methanol oxidation on two types of hierarchical nanostructures, external and internal, is reported. A complex system of asymmetric geometries was simplified by mapping 3 D geometries into 2 D models by using a mass transport approach. The macropore size was the most comprehensive characteristic to evaluate the specific activity and current density of hierarchical nanostructures. The optimal current densities for both types of nanostructures were achieved in macropore size ranges of 3.2–4.5 and 1.9–3.2 μm, respectively. The optimal mass activity of the internal nanostructures was achieved in the porosity range of 40–50 %, whereas that of the external hierarchical nanostructures was achieved at high porosity values. In comparison to internal hierarchical nanostructures, external hierarchical nanostructures tend to be cost‐effective catalysts that have a high catalytic activity.

Modified Graphene as Electrocatalyst towards Oxygen Reduction Reaction for Fuel Cells

This paper reports modified graphene-based materials as metal-free electrocatalysts for oxygen reduction reaction (ORR) with outstanding electrocatalytic activity in alkaline conditions. Nitrogen-doped graphene samples are synthesized by a novel procedure. The defect density in the structure of the prepared materials is investigated by Raman spectroscopy. Further structural characterization by X-ray photoelectron spectroscopy reveals the successful nitrogen doping of graphene. The electrochemical characterization of graphene and nitrogen-doped graphene in 0.1 M KOH solution demonstrates the materials electrocatalytic activity towards ORR. For graphene an onset potential of – 0.175 V vs. Ag/AgCl reference electrode is determined, while for nitrogen-doped graphene the determined onset potential is – 0.160 V. Thus, the electrocatalytic activity of nitrogen-doped graphene towards ORR is enhanced which can be ascribed to the effect of nitrogen doping.

Effect of the aromatic precursor flow rate on the morphology and properties of carbon nanostructures in plasma enhanced chemical vapor deposition

Understanding the effects of the synthesis parameters on the morphology and electrochemical properties of nanocarbon layers is a key step in the development of application-tailored nanostructures. In this paper we used an aromatic carbon as a new kind of precursor for the synthesis of carbon based nanostructures by plasma enhanced chemical vapor deposition (PECVD). Complex precursor molecules enable a new degree of influence over the atomic structure of PECVD synthesized carbons. Here, we report on the synthesis and characterization of the nanostructures resulting from varied flow rates of p-xylene used as carbon precursor. By changing the flow rate of the precursor, three different morphologies with graphitic character were synthesized. The resulting structures were carbon nanofibers (CNF), freestanding carbon nanowalls (fCNW) and interconnected carbon nanowalls (iCNW), formed at flow rates of 3 ml h−1, between 1 and 3 ml h−1 and less than 1 ml h−1, respectively. Structural characterization by transmission electron microscopy and Raman spectroscopy indicate a lower defect density for the CNF in comparison to the CNW nanostructures. The electrochemical characterization of the oxygen reduction reaction onset potential and effective surface area feature a significantly higher onset at around −171 mV and an electrochemically active surface area of 0.76 μm−1 for the iCNW compared to −196 mV, 0.61 μm−1 and 0.22 μm−1 for the fCNW and CNF, respectively. The similarities in defect density and differences in activity observed for the iCNW and fCNF suggest that the kind of the defects determines the electrochemical properties. Thus, the iCNW was identified as the most appropriate morphology for further investigations.

Control over fuel cell performance through modulation of pore accessibility: investigation and modeling of carbon nanotubes effects on oxygen reduction at N-graphene-based nanocomposite

The lack of performance of graphene-based electrocatalysts for oxygen reduction (ORR) is a major concern for fuel cells which can be mastered using nanocomposites. This work is highlighted by the optimization of nitrogen(N)-doped graphene/carbon nanotubes (CNTs) nanocomposites ORR performance examined by galvanostatic measurements in realistically approached glucose half-cells. Obtained results mark an essential step for the development of nanocarbon-based cathodes, as we specifically evaluate the electrode performance under real fuel cell conditions. The 2D simulations exclusively represent an important approach for understanding the catalytic efficiency of the nanocomposite with unique structure. The kinetics features extracted from simulations are consistent with the experimentally determined kinetics. The morphology analysis reveals a 3D porous structure. The results demonstrate that the incorporation of CNTs implements mesoscale channels for improved mass transport and leads to efficient 4-electron transfer and enhanced overall catalytic activity in pH-neutral media. The nanocomposite shows increased specific surface area of 142 m2 g-1, positively shifted ORR onset potential of 67 mV and higher open circuit potential of 268 mV versus Ag/AgCl compared to N-graphene (11 m2 g-1, -17, 220 mV). The findings are supported by 2D simulations giving qualitative evidence to the significant role of CNTs for achieving better accessibility of pores, i.e. enabling improved transfer of oxygen and OH-, and providing more reaction sites in the nanocomposite. The nanocomposite demonstrates better ORR performance than constituent components regarding potential application in miniaturized single-compartment glucose-based fuel cells.

Bimetallic nanoparticles for optimizing CMOS integrated SnO 2 gas sensor devices

We present gas sensor devices based on ultrathin SnO2 films, which are integrated on CMOS fabricated micro-hotplate (μhp) chips. Bimetallic nanoparticles (NPs) such as PdAu, PtAu, and PdPt have been synthesized for optimizing the sensing performance of these sensors. We demonstrate that functionalization of nanocrystalline SnO2 gas sensing films with PdAu-NPs leads to a strongly improved sensitivity to the toxic gas carbon monoxide (CO) while the cross sensitivity to humidity is almost completely suppressed. We conclude that specific functionalization of CMOS integrated SnO2 thin film gas sensors with different types of NPs is a powerful strategy towards sensor arrays capable for distinguishing several target gases. Such CMOS integrated arrays are highly promising candidates for realizing smart multi-parameter sensing devices for the consumer market.

CMOS integrated tin dioxide gas sensors functionalized with bimetallic nanoparticles for improved carbon monoxide detection

In this work, we present the integration of functionalized tin dioxide gas sensors on CMOS fabricated microhotplate chips. Spray pyrolysis was used to deposit the gas sensitive films, with a thickness of 50 nm, on CMOS microhotplates. The SnO2 thin films were functionalized with noble bimetallic nanoparticles — PdAu — by inkjet printing and the influence of the nanoparticles on the sensor performance was evaluated. The functionalization of the CMOS integrated SnO2 sensors with PdAu nanoparticles lead to an almost three times higher sensor response towards carbon monoxide compared to the bare SnO2 thin film. The CMOS microhotplate chips are also applicable for 3D-integration of different gas sensing systems based on through-silicon-via technology. Building devices for daily life applications is possible with such 3D-integrated nanosensors.

A novel study of the kinetics of external hierarchical nanostructures in methanol fuel cell

For the purpose of a direct methanol fuel cell (DMFC), this research investigates the kinetics of methanol oxidation in a porous layer consisting of external hierarchical nanostructures through 2D COMSOL simulation. Three different lengths of nanowires (L) were considered in simulations. The investigation showed that specific activity was reversely proportional to nanowires length and roughness factor (Rf). However, the current density increased by increasing Rf. Although the current density in case of L = 200 nm and 500 nm was identical with respect to Rf, there was a slight deviation when L = 1000 nm due to the changing from kinetic to diffusion controlled regime, which was identified by investigation of Thiele modulus. The catalytic efficiency for L = 1000 nm dropped to 50% at Rf = 140, whereas the high efficiency with no mass-transport limitation was achieved by shorter nanowires. Therefore, increasing Rf within the simulation range resulted in increasing the total catalytic activity but simultaneously decreasing the specific activity because of the decrease in pore accessibility and catalytic efficiency of nanostructures.