Martin Rosillo-Lopez

Ice Nucleation Properties of Oxidized Carbon Nanomaterials.

Heterogeneous ice nucleation is an important process in many fields, particularly atmospheric science, but is still poorly understood. All known inorganic ice nucleating particles are relatively large in size and tend to be hydrophilic. Hence it is not obvious that carbon nanomaterials should nucleate ice. However, in this paper we show that four different readily water-dispersible carbon nanomaterials are capable of nucleating ice. The tested materials were carboxylated graphene nanoflakes, graphene oxide, oxidized single walled carbon nanotubes and oxidized multiwalled carbon nanotubes. The carboxylated graphene nanoflakes have a diameter of ∼30 nm and are among the smallest entities observed so far to nucleate ice. Overall, carbon nanotubes were found to nucleate ice more efficiently than flat graphene species, and less oxidized materials nucleated ice more efficiently than more oxidized species. These well-defined carbon nanomaterials may pave the way to bridging the gap between experimental and computational studies of ice nucleation.

Selectively-sized graphene-based nanopores for in-situ single molecule sensing

The use of nanopore biosensors is set to be extremely important in developing precise single molecule detectors and providing highly sensitive advanced analysis of biological molecules. The precise tailoring of nanopore size is a significant step toward achieving this, as it would allow for a nanopore to be tuned to a corresponding analyte. The work presented here details a methodology for selectively opening nanopores in real-time. The tunable nanopores on a quartz nanopipette platform are fabricated using the electroetching of a graphene-based membrane constructed from individual graphene nanoflakes (ø ∼30 nm). The device design allows for in situ opening of the graphene membrane, from fully closed to fully opened (ø ∼25 nm), a feature that has yet to be reported in the literature. The translocation of DNA is studied as the pore size is varied, allowing for subfeatures of DNA to be detected with slower DNA translocations at smaller pore sizes, and the ability to observe trends as the pore is opened. This approach opens the door to creating a device that can be target to detect specific analytes.

Formation and chemistry of carboxylic anhydrides at the graphene edge

Using graphene for a wide range of applications often requires delicate chemical processing and functionalisation. Here we first report the synthesis of graphene nanoflakes with carboxylic acid groups at the graphene edges from multi-wall carbon nanotube materials. Using this material we then show that highly reactive carboxylic anhydride groups exist in dynamic equilibrium with the carboxylic acids in aqueous dispersion. Heating in vacuum shifts the equilibrium as about 80% of the carboxylic acid groups transform to the anhydrides. These new insights into graphene chemistry enable us to develop a simple, straight-forward chemical functionalisation protocol for graphene making use of the anhydrides. The graphene nanoflakes were found to react readily with amine nucleophiles in aqueous dispersion to yield the corresponding amides. The new protocol allows us to alter the zetapotential of the graphene nanoflakes and to decorate the graphene edges with gold nanoparticles. Due to its simplicity, we expect this procedure to find wide-spread use in the chemical functionalisation of graphene and to enable new graphene-based applications.

Detailed crystallographic analysis of the ice VI to ice XV hydrogen ordering phase transition

The D2O ice VI to ice XV hydrogen ordering phase transition at ambient pressure is investigated in detail with neutron diffraction. The lattice constants are found to be sensitive indicators for hydrogen ordering. The a and b lattice constants contract whereas a pronounced expansion in c is found upon hydrogen ordering. Overall, the hydrogen ordering transition goes along with a small increase in volume, which explains why the phase transition is more difficult to observe upon cooling under pressure. Slow-cooling ice VI at 1.4 GPa gives essentially fully hydrogen-disordered ice VI. Consistent with earlier studies, the ice XV obtained after slow-cooling at ambient pressure is best described with P-1 space group symmetry. Using a new modelling approach, we achieve the atomistic reconstruction of a supercell structure that is consistent with the average partially ordered structure derived from Rietveld refinements. This shows that C-type networks are most prevalent in ice XV, but other structural motifs outside of the classifications of the fully hydrogen-ordered networks are identified as well. The recently proposed Pmmn structural model for ice XV is found to be incompatible with our diffraction data, and we argue that only structural models that are capable of describing full hydrogen order should be used.

Highly efficient heavy-metal extraction from water with carboxylated graphene nanoflakes

Heavy metals such a lead or cadmium have a wide range of detrimental and devastating effects on human health. It is therefore of paramount importance to efficiently remove heavy metals from industrial wastewater streams as well as drinking water. Carbon materials, including graphene and graphene oxide (GO), have recently been advocated as efficient sorption materials for heavy metals. We show that highly carboxylated graphene nanoflakes (cx-GNF) outperform nano-graphene oxide (nGO) as well as traditional GO with respect to extracting Fe2+, Cu2+, Fe3+, Cd2+ and Pb2+ cations from water. The sorption capacity for Pb2+, for example, is more than six times greater for the cx-GNF compared to GO which is attributed to the efficient formation of lead carboxylates as well as strong cation–π interactions. The large numbers of carboxylic acid groups as well as the intact graphenic regions of the cx-GNF are therefore responsible for the strong binding of the heavy metal cations. Remarkably, the performance of the as-made cx-GNF can easily compete with previously reported carbon materials that have undergone additional chemical-functionalisation procedures for the purpose of heavy-metal extraction. Furthermore, the recyclability of the cx-GNF material with respect to Pb2+ loading is demonstrated as well as the outstanding performance for Pb2+ extraction in the presence of excess Ca2+ or Mg2+ cations which are often present under environmental conditions. Out of all the graphene materials, the cx-GNF therefore show the greatest potential for future application in heavy-metal extraction processes.

Assembly of graphene nanoflake–quantum dot hybrids in aqueous solution and their performance in light-harvesting applications

Graphene nanoflakes and CdSe/ZnS quantum dots were covalently linked in environmentally friendly aqueous solution. Raman spectroscopy and photoluminescence studies, both in solution and on surfaces at the single nanohybrid level, showed evidence of charge transfer between the two nanostructures. The nanohybrids were further incorporated into solar cell devices, demonstrating their potential as light harvesting assemblies.