Doping of Carbon Nanotubes with Encapsulated Phosphorus Chains.

Single-walled carbon nanotubes (SWCNTs) are a perfect host for the formation of one-dimensional phosphorus structures and to obtain hybrid materials with a large P-C ratio. This work presents a procedure for high-yield phosphorus filling of commercial Tuball SWCNTs and efficient removal of phosphorus deposits from the external nanotube surface. We probed white and red phosphorus as precursors, varied the synthesis temperature and the ampoule shape, and tested three solvents for sample purification. High-resolution transmission electron microscopy and Raman spectroscopy indicated crystallization of interior phosphorus in a form resembling fibrous red phosphorus. An aqueous sodium hydroxide solution allowed removing the majority of external phosphorus particles. Thermogravimetric analysis of the product determined ∼23 wt % (∼10 atom %) of phosphorus, and the X-ray photoelectron spectroscopy (XPS) data showed that ca. 80% of it is in the form of elemental phosphorus. Externally purified SWCNTs filled with phosphorus were used to study the interaction between the components. Raman spectroscopy and core-level XPS revealed p-type SWCNT doping. Valence-band XPS data and density functional theory calculations confirmed the transfer of the SWCNT electron density to the encapsulated phosphorus.

Single-Walled Carbon Nanotubes with Red Phosphorus in Lithium-Ion Batteries: Effect of Surface and Encapsulated Phosphorus.

Single-walled carbon nanotubes (SWCNTs) with their high surface area, electrical conductivity, mechanical strength and elasticity are an ideal component for the development of composite electrode materials for batteries. Red phosphorus has a very high theoretical capacity with respect to lithium, but has poor conductivity and expends considerably as a result of the reaction with lithium ions. In this work, we compare the electrochemical performance of commercial SWCNTs with red phosphorus deposited on the outer surface of nanotubes and/or encapsulated in internal channels of nanotubes in lithium-ion batteries. External phosphorus, condensed from vapors, is easily oxidized upon contact with the environment and only the un-oxidized phosphorus cores participate in electrochemical reactions. The support of the SWCNT network ensures a stable long-term cycling for these phosphorus particles. The tubular space inside the SWCNTs stimulate the formation of chain phosphorus structures. The chains reversibly interact with lithium ions and provide a specific capacity of 1545 mAh·g-1 (calculated on the mass of phosphorus in the sample) at a current density of 0.1 A·g-1. As compared to the sample containing external phosphorus, SWCNTs with encapsulated phosphorus demonstrate higher reaction rates and a slight loss of initial capacity (~7%) on the 1000th cycle at 5 A·g-1.

Hydrothermal Activation of Porous Nitrogen-Doped Carbon Materials for Electrochemical Capacitors and Sodium-Ion Batteries.

Highly porous nitrogen-doped carbon nanomaterials have distinct advantages in energy storage and conversion technologies. In the present work, hydrothermal treatments in water or ammonia solution were used for modification of mesoporous nitrogen-doped graphitic carbon, synthesized by deposition of acetonitrile vapors on the pyrolysis products of calcium tartrate. Morphology, composition, and textural characteristics of the original and activated materials were studied by transmission electron microscopy, X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, infrared spectroscopy, and nitrogen gas adsorption method. Both treatments resulted in a slight increase in specific surface area and volume of micropores and small mesopores due to the etching of carbon surface. Compared to the solely aqueous medium, activation with ammonia led to stronger destruction of the graphitic shells, the formation of larger micropores (1.4 nm vs 0.6 nm), a higher concentration of carbonyl groups, and the addition of nitrogen-containing groups. The tests of nitrogen-doped carbon materials as electrodes in 1M H2SO4 electrolyte and sodium-ion batteries showed improvement of electrochemical performance after hydrothermal treatments especially when ammonia was used. The activation method developed in this work is hopeful to open up a new route of designing porous nitrogen-doped carbon materials for electrochemical applications.