(529c) Multiwalled Carbon Nanotube Functionalization and Characterization: Evidence for Lactone and Ester Formation

The presence and density of surface functions on carbon materials greatly affect their utility in adsorption processes (e.g., for heavy metals or organics), in catalysis (e.g., as supports or in metal-free catalysis), and as carbon-fiber composites (e.g., increasing mechanical strength). Oxygen-based surface functions are the most commonly studied and introduced through treatment in strong oxidizing agents or exposure to plasma, which are frequently used to functionalize carbon materials. Treatment with plasma is a quick and environmentally-benign process for introducing oxygen-based surface functions on carbon materials depending on the source composition; however, the precise identity of the surface functions that form is still unclear despite significant efforts in this area. The most commonly implicated moieties on carbon surfaces are ethers (C-O-C), alcohols (-OH), carboxylic acids (-COOH), and esters or lactones (O=C-O). Commonly implemented spectroscopic techniques (e.g., NMR, XPS, IR) are limited due to weak signal from low density of functions or lack chemical specificity and resolution to distinguish among plausible functions. Here, we present a reactive-titration method for identifying and quantifying the density of -COOH and O=C-O surface functions on O₂ plasma treated multiwalled carbon nanotubes (MWCNT), which provides a methodology for characterizing these materials that will allow researchers to better-understand how the presence, proximity, and density of these functions can impact the physicochemical properties of a material.

The surface of MWCNTs were functionalized by treatment with O₂ plasma in a capacitively-coupled plasma chamber (300 W, 220 mTorr, 25 cm^-3min^-1, 0 – 4 min; March CS-1701, Nordson March). The elemental abundance of oxygen was evaluated by X-ray photoelectron spectroscopy, which shows that the oxygen content of MWCNT increases after plasma treatment (oxygen peak fraction from 17% to 24 %). Peak fitting of C1s features in the XP spectra show that the abundance of C-O and O=C-O increase slightly (5.5% to 6.3% and 4.3% to 5.0%, respectively), while the density of C=O moieties decreases (3.3% to 2.6%) after plasma treatment (4 min). Unfortunately, the identity of the chemical functions that contribute to these XP features cannot be assigned clearly by XPS alone. Therefore, we have developed potentiometric titration protocols for doing so.

Chemical reactions induced via titrations (e.g., protonation, hydrolysis) with sodium hydroxide can yield important information regarding the nature of the functional group (e.g., pKa).^1-2 Potentiometric titrations (0.01 M NaOH as titrant) of plasma-treated MWCNT (pt-MWCNT) show a single gaussian feature for acidic function (0.54 mmol g^-1) with a pKa of 6.3, while only small amount of Bronsted acid sites (0.07 mmol g^-1) are observed for untreated MWCNT between 3 – 11 pH units. Upon re-acidification (0.01 M HCl) and sequential titration with NaOH, the acidic function detected on pt-MWCNT does not reappear during titration, which suggests that this surface function irreversibly reacted with NaOH and is not a -COOH moiety. More likely, this feature is related to a lactone or ester group that undergoes saponification upon titration with NaOH. The pKa of common -COOH groups (3 – 6) should be observable after acidification in sequential titration, which suggests that the partially oxidized MWCNT surface acts as an electron-withdrawing functional group vicinal to the -COOH and gives a lower pKa for the -COOH (and one below the pH titration window in water, i.e., 3 – 11). To test this hypothesis, we used ethyl pyruvate (an α-ketoester) as a probe molecule, which showed a single feature at pKa of 9.2 upon initial titration, with no features observed in repeated titrations. The same trend of ethyl pyruvate and pt-MWCNT in titration suggests that treatment of MWCNT in O₂ plasma may produce α-ketoester surface functions, which can undergo saponification to produce alcohols and α-ketoacids out of pH window of potentiometric titration. We conclude that oxygen plasma treatments introduce lactone or ester functions onto MWCNT, and their close vicinity to electron withdrawn groups decreases the pKa of -COOH that form by hydrolysis.

1. Contescu, C.; Jagiello, J.; Schwarz, J. A., Heterogeneity of proton binding sites at the oxide/solution interface. Langmuir 1993, 9 (7), 1754-1765.
2. Contescu, A.; Contescu, C.; Putyera, K.; Schwarz, J. A., Surface acidity of carbons characterized by their continuous pK distribution and Boehm titration. Carbon 1997, 35 (1), 83-94.