Electrochemistry of redox-active molecules confined within narrow carbon nanotubes

Confinement of molecules within nanocontainers can be a powerful tool for controlling the states of guest-molecules, tuning properties of host-nanocontainers and triggering the emergence of synergistic properties within the host-guest systems. Among nanocontainers, single-walled carbon nanotubes - atomically thin cylinders of carbon, with typical diameters below 2 nm and lengths reaching macroscopic dimensions - are ideal hosts for a variety of materials, including inorganic crystals, and organic, inorganic and organometallic molecules. The extremely high aspect ratio of carbon nanotubes is complemented by their functional properties, such as exceptionally high electrical conductivity and thermal, chemical and electrochemical stability, making carbon nanotubes ideal connectors between guest-molecules and macroscopic electrodes. The idea of harnessing nanotubes both as nanocontainers and nanoelectrodes has led to the incorporation of redox-active species entrapped within nanotube cavities where the host-nanotubes may serve as conduits of electrons to/from the guest-molecules, whilst restricting the molecular positions, orientations, and local environment around the redox centres. This review gives a contemporary overview of the status of molecular redox chemistry within ultra-narrow carbon nanotubes (nanotubes with diameters approaching molecular dimensions) highlighting the opportunities, pitfalls, and gaps in understanding of electrochemistry in confinement, including the role of nanotube diameter, size and shape of guest-molecules, type of electrolyte, solvent and other experimental conditions.

One-step synthesis of visible light CO2 reduction photocatalyst from carbon nanotubes encapsulating iodine molecules

We describe the synthesis and visible-light CO2 photoreduction catalytic properties of a three-component composite consisting of AgI, AgIO3, and single-walled carbon nanotubes (SWCNTs). The catalyst is synthesized by immersing SWCNTs encapsulating iodine molecules in AgNO3 aqueous solution, during which neutral iodine (I2) molecules encapsulated in SWCNTs transform disproportionately to I5+ (AgIO3) and I- (AgI), as revealed from the characterization of the composite by Raman spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. In addition, photoirradiation experiments using a solar-simulator (AM1.5G) showed that the obtained three-component composite works as a CO2 photoreduction catalyst under visible light despite the wide band gap of AgIO3, suggesting possible transfer of the visible light-excited electron from AgI via SWCNTs.

Constraint spaces in carbon materials

Nano-sized pores in carbon materials are recently known to give certain constraints to the encapsulated materials by keeping them inside, accompanied with some changes in their structure, morphology, stability, etc. Consequently, nano-sized pores endow the constrained materials with improved performances in comparison with those prepared by conventional processes. These pores may be called "constraint spaces" in carbon materials. Here, we review the experimental results related to these constraint spaces by classifying as nanochannels in carbon nanotubes, nanopores and nanochannels in various porous carbons, and the spaces created by carbon coating.

Electrochemical Reactions of Iodine Molecules Encapsulated in Single-Walled Carbon Nanotubes

We prepared iodine molecules encapsulated in single-walled carbon nanotubes (I@SWCNTs) by electro-oxidation of iodide ions with empty SWCNT electrode. Li-ion battery electrode properties of I@SWCNTs were investigated. It was found that the I@SWCNT sample can catch and release Li ions reversibly. We performed Raman measurements to reveal the Li-ion storage mechanism of I@SWCNT. It is plausible that chemical reactions of I₂ from/into LiI in SWCNTs occur during Li-ion charging/discharging of I@SWCNT. We also prepared the CsI@SWCNT sample to verify that alkali metal ions can be extracted from alkali metal halide in SWCNTs. The extraction of cesium ions from CsI@SWCNT was confirmed by Raman measurements. It was also found that I@SWCNT can work as a Li-ion battery electrode in solid electrolyte as well.

Quinone molecules encapsulated in SWCNTs for low-temperature Na ion batteries

We have performed Li and Na ion charge-discharge experiments of 9,10-phenanthrene quinone (PhQ) molecules encapsulated in single-walled carbon nanotubes (SWCNTs) with mean tube diameters of 1.5 and 2.5 nm at room temperature and also at low temperatures. The Na ion reversible capacity of PhQ encapsulated in the larger diameter SWCNTs, measured at a low temperature of 0 °C, remained as high as that measured at room temperature (RT), while the capacity of PhQ in the smaller diameter SWCNTs at 0 °C was about a half of that at RT. The diameter dependence of the capacity should be attributed to the difference in the interactions between the encapsulated PhQ molecules and the host SWCNTs, which was revealed by Raman peak profile analysis. Charge-transfer reaction from metallic tubes to PhQ molecules encapsulated in the smaller diameter SWCNTs was detected by Raman measurements. The electrostatic interaction between charged SWCNTs and PhQ molecules, induced by the charge-transfer reaction, would partly contribute to the stabilization of PhQ molecules in the smaller diameter SWCNTs, while only van der Waals interaction stabilizes PhQ molecules in the larger diameter SWCNTs. The difference in stability was confirmed by thermogravimetric, x-ray photoelectron spectroscopy, and Raman measurements. Charge-discharge curves of PhQ encapsulated in SWCNTs were also discussed based on the stability difference.

Solubility origin at the nanoscale: enthalpic and entropic contributions in polar and nonpolar environments

Nanostructures are known to be poorly soluble, irrespective of their elemental composition, shape, electronic structure, dipole moment, hydrophobicity/hydrophilicity and the employed solvent. The methods of colloid chemistry allow for preparing suspensions - metastable systems, the stabilities of which differ greatly from one another - but not real solutions. A systematic investigation of the solubility origin at the nanoscale is hereby reported in terms of its fundamental constituents: enthalpy and entropy. Slightly different one-dimensional solutes - narrow carbon nanotubes (CNTs) of different lengths - were considered in hydrophilic (water) and hydrophobic (benzene) environments. We decompose the process of solvation into the solid → gas transition (sublimation) and the gas → liquid transition (condensation). Sublimation is a thermodynamically unfavorable process under room conditions, while the condensation transition depends on the solvent-solute interactions (enthalpic contribution). Unlike solvation of small molecules, solvation of the nanostructures results in a significant alteration of entropy. This alteration is proportional to the linear dimensions of the nanostructure. If the solvent exhibits peculiar solvent-solvent interactions (such as hydrogen bonding in water), solvation is entropically forbidden, irrespective of the solute nature and its nanoscale dimensions. In the case of the hydrophobic solvent (benzene), the condensation transition can be both enthalpically and entropically favorable. The free energy of solvation is in direct proportion to the CNT length. While highlighting principal difficulties in solvating nanostructures, this paper discusses an optimal choice of solvents for solutes exhibiting hydrophobic and hydrophilic interactions with their environments. Our results allow us to predict the solvation of an arbitrary nanostructure using its small, about 2 nm, atomistic model.

The dispersion, solubilization and stabilization in “solution” of single-walled carbon nanotubes

Methods for the solubilization and dispersion of single-walled carbon nanotubes in water and organic solvents by physical and chemical methods have been reviewed.

Electrochemical lithium-ion storage properties of quinone molecules encapsulated in single-walled carbon nanotubes

We investigated the electrochemical lithium-ion storage properties of 9,10-anthraquinone (AQ) and 9,10-phenanthrenequinone (PhQ) molecules encapsulated in the inner hollow core of single-walled carbon nanotubes (SWCNTs).

Methods for Dispersion of Carbon Nanotubes in Water and Common Solvents

Contemporary methods for dispersion of carbon nanotubes in water and non-aqueous media are discussed. Main attention is paid to ultrasonic, plasma techniques and other physical techniques, as well as to the use of surfactants, functionalizing and debundling agents of distinct nature (elemental substances, metal and organic salts, mineral and organic acids, oxides, inorganic and organic peroxides, organic sulfonates, polymers, dyes, natural products, biomolecules, and coordination compounds).