Chemically Attached Single-Wall Carbon Nanotubes on Polycrystalline Platinum Surfaces: Probed as Anode for Lithium Intercalation
Rosario-Castro, Belinda I.
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High-surface-area of single-wall carbon nanotubes (SWNTs) were tested for lithium intercalation for possible use in lithium storage system in order to move toward smaller, lighter and more efficient Li rechargeable batteries. Recently, research on Li rechargeable batteries have been focus on the development of nanobatteries. Nanoscale batteries can be used in a huge variety of commercial and scientific appliances. In an attempt to develop a SWNTs electrode that can possibly be used as anode for Li ion batteries, a combination of self-assembly and chemical derivatization was used. The first step in the development of a SWNTs assembly, was the deposition of a 4- aminothiophenol (4-ATP) monolayer on a platinum surface. Electrochemical characterization served to find the optimal conditions to get a good packing quality of the 4- ATP assembly. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to obtain structural information of the SWNT-modified Pt electrodes. Surface chemical analysis using reflection-absorption infrared (RAIR), Raman, and X-ray photoelectron (XPS) spectroscopic results were used to ascertain the chemical nature of the adsorption of 4-ATP molecules to the Pt surface. These studies indicate that the 4-ATP molecules are attached to the platinum surface through the sulfur, forming S-Pt bonds and giving an amino-terminated self-assembled monolayer. Self-assembly of 4-ATP on platinum electrodes certainly serves as the base for the attachment of carboxyl-functionalized SWNTs, by means of amide bonds formation. As-received SWNTs were purified in order to obtain their optimal performance, and get rid of possible interference species in the Li ion intercalation. The removal of Fe catalysts and amorphous carbon was performed combining an acid reflux and gas phase oxidation methods. The purified SWNTs were characterized by thermogravimetric analysis (TGA), transmission electron microscopy (TEM), and SEM. The iron content was decreased from 5.29% to 0.60% by weight, along with an increase in degradation temperature from 487 ºC for as-received SWNTs to 650 ºC for purified SWNTs. The purified SWNTs, which are too long tangled for convenient use in the development of nanodevices, were cut into nanometric length pieces by chemical oxidation with a mixture of strong acids. TEM analysis showed that the chemical oxidation produces SWNTs pieces with lengths under 500 nm. These oxidized SWNTs were also characterized by SEM and TGA. Additionally, chemical oxidation of SWNTs produces open ends on carbon nanotubes, which are functionalized with carboxylic acid groups. Infrared (IR) spectroscopy and XPS were used to study SWNT surface composition transformations during purification and oxidation steps. Raman spectroscopy was used to study changes in SWNTs diameter distribution caused by the purification and oxidation processes. A preferential decomposition of smaller diameter nanotubes occurred during the purification and oxidation processes. The second step in the development of nanoscale SWNTs electrodes is the attachment of SWNTs to a 4-ATP-modified Pt surface through a condensation reaction. The rich chemistry available at chemically oxidized SWNTs was used to derivatize the aminoterminated 4-ATP self-assembled monolayer (SAM) deposited over a Pt electrode surface. The attachment of carboxyl-SWNTs (oxidized SWNTs) on platinum electrodes was performed by reacting them with the amino-terminated SAM with the aid of dicyclohexyl carbodiimide as condensing agent. The characterization of SWNTs attached over modified Pt electrodes was performed using RAIR, XPS, Raman spectroscopy, SEM, and atomic force microscopy (AFM). Both XPS and RAIR spectra suggest amide bond formation between the 4-ATP SAM amino groups on Pt electrodes and the carboxyl groups of SWNTs. This constitutes the building driving force of the SWNTs assemblies. SEM and AFM images were obtained to study the structure of the SWNTs assemblies. These studies revealed that SWNTs are homogeneously distributed with a mixture of film thickness throughout the surfaces. Cyclic voltammetries (CVs) in sulfuric acid demonstrate that attachment of SWNTs on Pt is markedly stable, even after 30 cycles. CV in ruthenium-hexamine and sulfuric acid implies that SWNTs assembly is similar to a closely packed array of microelectrodes. The reliability of SWNTs assemblies as Li anodes for batteries was probed by galvanostatic charge-discharge (CD) experiments. The electrochemical lithium intercalation on attached SWNTs provides an insight on the amount of lithium ions that can be intercalated into the nanotubes. High resolution X-ray photoelectron spectroscopy (HRXPS), X-ray diffraction (XRD), SEM, and RAIR spectroscopy were performed to study the composition of the solid electrolyte interface formed at lithiated SWNTs electrodes. Lithium intercalation in attached SWNTs was expected to be higher than in graphite because insertion occurs not only within the nanotubes, but into central cavity of open ended nanotubes. Lithium diffusion into the inner cores of the open SWNTs is the key process to obtain anodes with higher capacity (amount of Li that can be intercalated reversibly). CVs and CD curves were performed to study the electrochemical response of SWNTs electrodes. The electrochemical intercalation for SWNTs occurs through multiple chemical processes, characteristic of oxidized SWNTs and carbon materials. The reversible capacity for Li intercalation was investigated in order to show that SWNTs assembly intercalation is higher than in graphite, and thus are feasible anodes for the development of nanoscale batteries.