Peter S. Toth

Electron Transfer Kinetics on Mono- and Multilayer Graphene

Understanding of the electrochemical properties of graphene, especially the electron transfer kinetics of a redox reaction between the graphene surface and a molecule, in comparison to graphite or other carbon-based materials, is essential for its potential in energy conversion and storage to be realized. Here we use voltammetric determination of the electron transfer rate for three redox mediators, ferricyanide, hexaammineruthenium, and hexachloroiridate (Fe(CN)(6)(3-), Ru(NH3)(6)(3+), and IrCl(6)(2-), respectively), to measure the reactivity of graphene samples prepared by mechanical exfoliation of natural graphite. Electron transfer rates are measured for varied number of graphene layers (1 to ca. 1000 layers) using microscopic droplets. The basal planes of mono- and multilayer graphene, supported on an insulating Si/SiO(2) substrate, exhibit significant electron transfer activity and changes in kinetics are observed for all three mediators. No significant trend in kinetics with flake thickness is discernible for each mediator; however, a large variation in kinetics is observed across the basal plane of the same flakes, indicating that local surface conditions affect the electrochemical performance. This is confirmed by in situ graphite exfoliation, which reveals significant deterioration of initially, near-reversible kinetics for Ru(NH3)(6)(3+) when comparing the atmosphere-aged and freshly exfoliated graphite surfaces.

Photoelectrochemistry of Pristine Mono- and Few-Layer MoS2.

Two-dimensional crystals are promising building blocks for the new generation of energy materials due to their low volume, high surface area, and high transparency. Electrochemical behavior of these crystals determines their performance in applications such as energy storage/conversion, sensing, and catalysis. Nevertheless, the electrochemistry of an isolated monolayer of molybdenum disulfide, which is one of the most promising semiconducting crystals, has not been achieved to date. We report here on photoelectrochemical properties of pristine monolayer and few-layer basal plane MoS2, namely the electron transfer kinetics and electric double-layer capacitance, supported by an extensive physical and chemical characterization. This enables a comparative qualitative correlation among the electrochemical data, MoS2 structure, and external illumination, although the absolute magnitudes of the electron transfer and capacitance are specific to the redox mediator and electrolyte used in these measurements ([Ru(NH3)6](3+/2+) and LiCl, respectively). Our work shows a strong dependence of the electrochemical properties on the number of MoS2 layers and illumination intensity and proves that an effective interlayer charge transport occurs in bulk MoS2. This highlights the exciting opportunities for tuning of the electrochemical performance of MoS2 through modification of its structure, external environment, and illumination.

Electrochemistry in a drop: a study of the electrochemical behaviour of mechanically exfoliated graphene on photoresist coated silicon substrate

A micro apparatus for electrochemical studies on individual high quality graphene flakes is presented. A microinjection-micromanipulator system has been employed to deposit droplets of aqueous solutions containing redox-active species directly on selected micro-scale areas of mechanically exfoliated graphene layers on polymer coated silicon wafers. This approach allows the clear distinction between the electrochemical activity of pristine basal planes and the edges (defects) or steps to be measured. Voltammetric measurements were performed in a two-electrode configuration, and the standard heterogeneous electron transfer rate (k°) for reduction of hexachloroiridate (IrCl62−) was estimated. The kinetics of electron transfer were evaluated for several types of graphene: mono, bi, and few layer basal planes, and the k° was estimated for an edge/step between two few layer graphene flakes. As a comparison, the kinetic behaviour of graphite basal planes was measured for the deposited aqueous droplets. The appearance of ruptures on the graphene monolayer was observed after deposition of the aqueous solution for the case of graphene on a bare silicon/silicon oxide substrate.

Enhanced Photoelectrochemical Performance of Cuprous Oxide/Graphene Nanohybrids

Combination of an oxide semiconductor with a highly conductive nanocarbon framework (such as graphene or carbon nanotubes) is an attractive avenue to assemble efficient photoelectrodes for solar fuel generation. To fully exploit the possible synergies of the hybrid formation, however, precise knowledge of these systems is required to allow rational design and morphological engineering. In this paper, we present the controlled electrochemical deposition of nanocrystalline p-Cu2O on the surface of different graphene substrates. The developed synthetic protocol allowed tuning of the morphological features of the hybrids as deduced from electron microscopy. (Photo)electrochemical measurements (including photovoltammetry, electrochemical impedance spectroscopy, photocurrent transient analysis) demonstrated better performance for the 2D graphene containing photoelectrodes, compared to the bare Cu2O films, the enhanced performance being rooted in suppressed charge carrier recombination. To elucidate the precise role of graphene, comparative studies were performed with carbon nanotube (CNT) films and 3D graphene foams. These studies revealed, after allowing for the effect of increased surface area, that the 3D graphene substrate outperformed the other two nanocarbons. Its interconnected structure facilitated effective charge separation and transport, leading to better harvesting of the generated photoelectrons. These hybrid assemblies are shown to be potentially attractive candidates in photoelectrochemical energy conversion schemes, namely CO2 reduction.

Asymmetric MoS2/Graphene/Metal Sandwiches: Preparation, Characterization, and Application

The polarizable organic/water interface is used to construct MoS2 /graphene nanocomposites, and various asymmetrically dual-decorated graphene sandwiches are synthesized. High-resolution transmission electron microscopy and 3D electron tomography confirm their structure. These dual-decorated graphene-based hybrids show excellent hydrogen evolution activity and promising capacitance performance.

Electron transfer kinetics on natural crystals of MoS2 and graphite

Here, we evaluate the electrochemical performance of sparsely studied natural crystals of molybdenite and graphite, which have increasingly been used for fabrication of next generation monolayer molybdenum disulphide and graphene energy storage devices. Heterogeneous electron transfer kinetics of several redox mediators, including Fe(CN)6(3-/4-), Ru(NH3)6(3+/2+) and IrCl6(2-/3-) are determined using voltammetry in a micro-droplet cell. The kinetics on both materials are studied as a function of surface defectiveness, surface ageing, applied potential and illumination. We find that the basal planes of both natural MoS2 and graphite show significant electroactivity, but a large decrease in electron transfer kinetics is observed on atmosphere-aged surfaces in comparison to in situ freshly cleaved surfaces of both materials. This is attributed to surface oxidation and adsorption of airborne contaminants at the surface exposed to an ambient environment. In contrast to semimetallic graphite, the electrode kinetics on semiconducting MoS2 are strongly dependent on the surface illumination and applied potential. Furthermore, while visibly present defects/cracks do not significantly affect the response of graphite, the kinetics on MoS2 systematically accelerate with small increase in disorder. These findings have direct implications for use of MoS2 and graphene/graphite as electrode materials in electrochemistry-related applications.

Exfoliation of natural van der Waals heterostructures to a single unit cell thickness

Weak interlayer interactions in van der Waals crystals facilitate their mechanical exfoliation to monolayer and few-layer two-dimensional materials, which often exhibit striking physical phenomena absent in their bulk form. Here we utilize mechanical exfoliation to produce a two-dimensional form of a mineral franckeite and show that the phase segregation of chemical species into discrete layers at the sub-nanometre scale facilitates franckeites layered structure and basal cleavage down to a single unit cell thickness. This behaviour is likely to be common in a wider family of complex minerals and could be exploited for a single-step synthesis of van der Waals heterostructures, as an alternative to artificial stacking of individual two-dimensional crystals. We demonstrate p-type electrical conductivity and remarkable electrochemical properties of the exfoliated crystals, showing promise for a range of applications, and use the density functional theory calculations of franckeites electronic band structure to rationalize the experimental results.Weak interlayer van der Waals interactions in bulk crystals facilitate their mechanical exfoliation to monolayer and few-layer two-dimensional (2D) materials, which exhibit striking physical phenomena absent in their bulk form. Here we study a 2D form of a mineral franckeite and show that phase segregation into discrete layers at the sub-nanometre scale facilitates its layered structure and basal cleavage. This behaviour is likely to be common in a wider family of complex crystals and could be exploited for a single-step synthesis of van der Waals heterostructures, as an alternative to stacking of 2D materials. Mechanical exfoliation allowed us to produce crystals down to a single unit cell thickness and rationalise its basal cleavage by atomic-resolution scanning transmission electron microscopy (STEM). We demonstrate p-type electrical conductivity and remarkable electrochemical properties in exfoliated crystals, which shows promise for energy storage applications.

On the unexpected cation exchange behavior, caused by covalent bond formation between PEDOT and Cl- ions: Extending the conception for the polymer-dopant interactions

The ionic motion in connection with the redox transformation of poly(3,4-ethylenedioxythiophene) (PEDOT) conjugated polymer have been studied by both experimental-electrochemical (electrochemical quartz crystal nanobalance, EQCN) and spectroscopic (infrared spectroscopy, IR-ATR)-and theoretical methods. The observations have been completed by direct, semiquantitative analytical data, provided by energy dispersive X-ray (EDX) microanalysis. The EQCN results suggested an anomalous behavior, since only cationic movements have been observed for films deposited from chloride solutions. Chloride ions were proved to be immobile also when bulky tetrabutylammonium (Bu(4)N(+)) cations were substituted with even larger (hexadecyltrimethylammonium) cations. Since PEDOT films synthesized in the presence of other spherical, not too large anions-such as perchlorate and tetrafluoroborate-endowed mixed ion exchange behavior together with the Bu(4)N(+) cation, the possibility of a special interaction between chloride and the polymeric chain has been assumed. Semiempirical and DFT calculations indicated that chloride ions interact with the α carbon atoms of the thiophene rings of the oxidized EDOT oligomers, creating sp(3) type perturbations in the polymer chain. FTIR-ATR spectra evidenced the appearance of C-Cl bonds. Elementary analysis, performed by EDX spectroscopy with eight polymer samples at different doping levels clearly showed the permanent presence of constant amount of chlorine, independently of the oxidation state of the PEDOT layer. Finally, the presented observations call attention to the fact that unique dopant-polymer interactions during the electrochemical polymerization are of prime importance, being able to rule over conventions for the charge compensation of conjugated polymers, often solely based on steric parameters.

Mechanical stability of substrate-bound graphene in contact with aqueous solutions

We report on the damage caused to mechanically exfoliated monolayer graphene, bound to silicon dioxide substrate, upon contact with liquids. This phenomenon is of significant importance for a wide range of applications where monolayer graphene sheets are used with liquids, especially as an electrode material in electrochemical applications such as energy storage and conversion. Liquid-induced damage to SiO2-bound graphene was previously observed with a range of solvents. A recently developed microdroplet system, used for a detailed examination of this behaviour, reveals that few-layer graphene flakes down to a bi-layer are stable with respect to aqueous electrolyte droplet formation, but the stability of these droplets is significantly reduced on monolayer graphene and irreversible rupture of the underlying graphene flake occurs. This damage, which we attribute to the presence of nanoscale defects and high adhesion between the graphene and the substrate, seems specific to plasma-cleaned SiO2 substrates and is not observed on flakes transferred to other substrates. Furthermore, the introduction of impurities, in the form of both polymer residues and native impurities between the flake and the SiO2 substrate, significantly enhance graphenes immunity to external strain as shown by optical microscopy, atomic force microscopy, and Raman spectroscopy.

Interfacial doping of carbon nanotubes at the polarisable organic/water interface: a liquid/liquid pseudo-capacitor

The electrochemical reactivity of single-walled carbon nanotube (SWCNT) films, assembled at a polarisable organic/water interface, has been probed using model redox species. Electrons generated by the oxidation of organic 1,1′-dimethylferrocene (DMFc) to DMFc+ can be transferred through the assembled SWCNT layer and reduce aqueous ferricyanide (Fe(CN)63−) to ferrocyanide (Fe(CN)64−), with a doping interaction observed. Several electrochemical techniques, including cyclic voltammetry and electrochemical impedance spectroscopy (EIS), were employed to confirm that the model redox couples dope/charge the SWCNTs. In situ Raman spectro-electrochemistry was also applied to verify the charge transfer processes occurring at the assembled SWCNT films and confirm that the doping effect of the carbon nanotubes is initiated by electrochemical reactions. This doping interaction indicated that the adsorbed SWCNT films can act as a pseudo-capacitor, showing a high area-normalised capacitance. The deeper understanding of the electrochemical properties of SWCNTs, gained from this study, will help determine the performance of this material for practical applications.