Stephen D. Worrall

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.

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.

Facile fabrication of metal–organic framework HKUST-1-based rewritable data storage devices

Metal–organic framework (MOF) HKUST-1 coated Cu electrodes synthesised by a quick and scalable anodic dissolution process and assembled into a sealed, symmetrical, two electrode coin cell configuration are shown to demonstrate redox-based data storage behaviour. The observed behaviour is confirmed to be definitively due to the HKUST-1 itself as opposed to the underlying Cu/CuO electrode surface by demonstration of the same effect using HKUST-1/PVDF composite coated carbon fibre electrodes. This data storage behaviour is attributed to the immobilisation of a fixed number of electrically accessible Cu cations within the porous HKUST-1 framework that are in the vicinity of the electrode surfaces and their ability to undergo facile Cu2+/+ redox interconversion as a function of the potential applied across the electrodes. This proposed redox-based mechanism for the observed data storage effect has never previously been reported for MOF-based devices. Optimisation of electrolyte composition, electrolyte concentration and electrode separation results in “on”/”off” current densities of the order of 1 mA cm−2, an “on” : “off” ratio of ∼5, “on”/”off” states stable to at least 10 consecutive reads, rewritability that persists over 6000 cycles, and the storage of data that can still be read hours after writing. The resulting data storage devices are more stable to successive reading and can be rewritten many more times than other previously reported pure MOF-based devices.

Desalination and Nanofiltration through Functionalized Laminar MoS2 Membranes

Laminar membranes of two-dimensional materials are excellent candidates for applications in water filtration due to the formation of nanocapillaries between individual crystals that can exhibit a molecular and ionic sieving effect, while allowing high water flux. This approach has been exemplified previously with graphene oxide, however these membranes suffer from swelling when exposed to liquid water, leading to low salt rejection and reducing their applicability for desalination applications. Here, we demonstrate that by producing thin (∼5 μm) laminar membranes of exfoliated molybdenum disulfide (MoS2) in a straightforward and scalable process, followed by a simple chemical functionalization step, we can efficiently reject ∼99% of the ions commonly found in seawater, while maintaining water fluxes significantly higher (∼5 times) than those reported for graphene oxide membranes. These functionalized MoS2 membranes exhibit excellent long-term stability with no swelling and consequent decrease in ion rejection, when immersed in water for periods exceeding 6 months. Similar stability is observed when exposed to organic solvents, indicating that they are ideal for a variety of technologically important filtration applications.

Anodic dissolution growth of metal–organic framework HKUST-1 monitored via in situ electrochemical atomic force microscopy

In situ electrochemical atomic force microscopy (ec-AFM) is utilised for the first time to probe the initial stages of metal–organic framework (MOF) coating growth via anodic dissolution. Using the example of the Cu MOF HKUST-1, real time surface analysis is obtained that supports and verifies many of the reaction steps in a previously proposed mechanism for this type of coating growth. No evidence is observed however for the presence or formation of Cu2O, which has previously been suggested to be both key for the formation of the coating and a potential explanation for the anomalously high adhesion strength of coatings obtained via this methodology. Supporting in situ electrochemical Raman spectroscopy also fails to detect the presence of any significant amount of Cu2O before or during the coatings growth process.