New salts of graphite,C12+HF2− & C24+SiF5− and the treshold for the oxidative intercalation of graphite

Third row transition metal hexafluorides (MF₆) for which the electron affinity exceeds 130 kcal/mole (M = Os, Ir, Pt) have been found to intercalate graphite with electron oxidation of the host lattice, whereas those with inferior electron affinities (M = W, Re) do not intercalate¹. This behavior can be rationalized on kinetic or thermodynamic grounds; arguing for the latter, a simple Born-Haber cycle may be used which suggests an electron affinity threshold of 120–130 kcal/mole for the MF₆ intercalation reaction. For the general case of intercalation reactions by metal fluorides (with or without added fluorine), wherein the graphite lattice is oxidized, the threshold is determined by the free energy of the half-reaction which produces the intercalating fluoro-anion. The lattice energy of the graphite salt must also be taken into account when comparing free energy thresholds for large (e.g., MF₆) and small (e.g. HF₂^?) intercalating species.We have evaluated the free energy of formation of a number of fluoro-anions from the heats of formation and lattice energies of salts which contain them. These studies indicate a threshold free energy of ca. 110 kcal/mole for graphite intercalation. Two ‘borderline’ second stage compounds, C₂₄⁺SiF₅^? and C₁₂⁺HF₂^?, have been synthesized.     

Individual single-walled nanotubes and hydrogels made by oxidative exfoliation of carbon nanotube ropes

Single-walled carbon nanotubes were oxidized by a technique previously developed for the oxidation of graphite to graphite oxide (GO). This process involves treatment with concentrated H(2)SO(4) containing (NH(4))(2)S(2)O(8) and P(2)O(5), followed by H(2)SO(4) and KMnO(4). Oxidation results in complete exfoliation of nanotube ropes to yield individual oxidized tubes that are 40-500 nm long. The C:O:H atomic ratio of vacuum-dried oxidized nanotubes is approximately 2.7:1.0:1.2. XPS and IR spectra show evidence for surface O-H, C=O, and COOH groups. The oxidized nanotubes slowly form viscous hydrogels at unusually low concentration (>or=0.3 wt %), and this behavior is attributed to the formation of a hydrogen-bonded nanotube network. The oxidized tubes bind readily to amine-coated surfaces, on which they adsorb as smooth and dense monolayer films. Thin films of the oxidized nanotubes show ohmic current-voltage behavior, with resistivities in the range of 0.2-0.5 Omega-cm.     

Motility of catalytic nanoparticles through self-generated forces

Small-scale synthetic motors capable of generating their own motive forces by exploiting the chemical free energy of their environment represent an important step in developing practical nanomachines. Catalytic particles are capable of generating concentration and other gradients that can be used to self-propel small objects. However, the autonomous movement of catalytic nanoparticles by self-generated forces is a relatively unexplored area in colloid and interfacial chemistry. This paper explores the potential of catalytically self-generated forces for propulsion of small objects through fluids.     

A high-throughput optical screening method for the optimization of colloidal water oxidation catalysts

A high-throughput method has been developed for screening and optimization of colloidal water oxidation catalysts. The catalysts are irradiated in parallel by visible light from an overhead projector in solutions containing tris(2,2'-bipyridyl)ruthenium(II) (Ru(bpy)(3)(2+)) and persulfate. The array of reaction solutions is held in a 96-well plate, and absorbance readings are taken intermittently using a bioassay plate reader. The absorbance at 430 nm is indicative of the amount of Ru(bpy)(3)(2+) remaining in solution. The best catalysts give the most persistent absorbance, because the oxygen evolution reaction is kinetically competitive with decomposition of Ru(bpy)(3)(3+). Reagent concentrations were varied using a factorial design-of-experiment approach in order to optimize reaction conditions for a IrO(2).xH(2)O colloidal catalyst. A higher colloid concentration, a lower Ru(bpy)(3)(2+) concentration, and a higher pH buffer doubled the number of turnovers relative to the original conditions. Metal oxide colloids consisting of IrO(2).xH(2)O doped with varying amounts of Pt, Ru, and Os were made using a parallel microwave synthesis technique and were tested both by the parallel screening method and by direct measurement of oxygen evolution. The correlation between the two methods was good, with Ir-Pt-Os oxide compositions showing the highest activity. The effect of adding small amounts of Pt and Os to IrO(2).xH(2)O appears to be predominantly to reduce the particle size of the colloids.     

Photosensitized production of doubly reduced methylviologen followed by highly efficient methylviologen radical formation using self-assembling ruthenium(II) complexes

Supramolecular fibers of Ru(II) complexes act as efficient photosensitizers, photochemically generating methylviologen radical (MV.+) and then converting it quantitatively to doubly reduced methylviologen, (MV0), in the presence of triethanolamine.     

Exfoliation of layered rutile and perovskite tungstates

The layered trirutile phases HMWO6 (M = Nb, Ta) and the layered perovskite H2W2O7 (synthesized by acid leaching of Bi2W2O9) were exfoliated into nanoscale colloids by reaction with quaternary ammonium hydroxides.     

Combined experimental and theoretical DFT study of molecular nanowires negative differential resistance and interaction with gold clusters

Electric-field-assisted assembly has been used to place rod-shaped metal nanowires containing 4-[[2-nitro-4-(phenylethynyl) phenyl] ethynyl] benzenthiol molecules onto lithographically defined metal pads. These junctions exhibited negative differential resistance. The quantum chemical approach was used to compare the properties of Au-bonded 4-[[2-nitro-4-(phenylethynyl) phenyl] ethynyl] benzenthiol molecule and a molecule that does not exhibit the negative differential resistance, Au-bonded 4-[[4-(phenylethynyl) phenyl] ethynyl] benzenthiol. The influence of the static electric field and charge variation were modelled for both systems.     

Standing wave enhancement of red absorbance and photocurrent in dye-sensitized titanium dioxide photoelectrodes coupled to photonic crystals

The light harvesting efficiency of dye-sensitized photoelectrodes was enhanced by coupling a TiO(2) photonic crystal layer to a conventional film of TiO(2) nanoparticles. In addition to acting as a dielectric mirror, the inverse opal photonic crystal caused a significant change in dye absorbance which depended on the position of the stop band. Absorbance was suppressed at wavelengths shorter than the stop band maximum and was enhanced at longer wavelengths. This effect arises from the slow group velocity of light in the vicinity of the stop band, and the consequent localization of light intensity in the voids (to the blue) or in the dye-sensitized TiO(2) (to the red) portions of the photonic crystal. By coupling a photonic crystal to a film of TiO(2) nanoparticles, the short circuit photocurrent efficiency across the visible spectrum (400-750 nm) could be increased by about 26%, relative to an ordinary dye-sensitized nanocrystalline TiO(2) photoelectrode.