Controlling and Quantifying Oxygen Functionalities on Hydrothermally

ARTICLE pubs.acs.org/JPCC

Controlling and Quantifying Oxygen Functionalities on Hydrothermally and Thermally Treated Single-Wall Carbon Nanotubes George E. Romanos,† Vlassis Likodimos,† Rita R. N. Marques,‡ Theodore A. Steriotis,† Sergios K. Papageorgiou,† Joaquim L. Faria,‡ Jos
e L. Figueiredo,‡ Adri
an M. T. Silva,*,‡ and Polycarpos Falaras*,† † ‡

Institute of Physical Chemistry, NCSR Demokritos, 153 10, Aghia Paraskevi Attikis, Athens, Greece Laborat
orio de Cat
alise e Materiais (LCM), Laborat
orio Associado LSRE/LCM, Departamento de Engenharia Química, Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal

bS Supporting Information ABSTRACT: The effects of hydrothermal and thermal treatments on surface oxygen functionalization of single-wall carbon nanotubes (SWNTs) were quantitatively investigated by means of water adsorption/desorption, temperature-programmed desorption (TPD), Raman spectroscopy, thermogravimetric analysis, and nitrogen porosimetry. SWNTs hydrothermally treated under mild acidic conditions were compared to highly purified reference materials heavily functionalized under aggressive reflux conditions. Water adsorption/desorption and TPD analysis were successfully combined to determine the nature, concentration, thermal stability, and acidic strength of the oxygen functional groups on the SWNT’s surface. These results were correlated to Raman spectroscopy data that allowed identifying the marked evolution of the defect-activated phonon modes of SWNTs. The concomitant charge transfer effects were differentiated through the distinct variation of both first- and second-order Raman modes as a function of the amount and acidity of the surface oxygen groups as well as the SWNT’s chirality. In addition, analytical investigations on thermally treated SWNTs in mild oxidative (in air) and pyrolytic conditions (under Ar) confirmed the formation of amorphous carbon that depends primarily on the acidification process, although a significant fraction of functional groups remains attached on the SWNTs’ walls rather than on carboxylated carbonaceous fragments. Quenched solid density functional theory (QSDFT) analysis of the bimodal pore size distribution of the functionalized SWNTs revealed pronounced variations of the underlying microporous and mesoporous structure, associated with the diverse effects of the packing between SWNT bundles and the closer aggregation of individual carbon nanotubes upon surface oxidation and thermal treatment. The SWNTs functionalization procedure can be effectively controlled and quantified, and the optimum conditions can be defined in relation to the desired physicochemical properties and pore structure characteristics for specific applications.

1. INTRODUCTION The unique mechanical, optical, electronic and thermal properties of carbon nanotubes (CNTs) have been the impetus of intensive research toward a variety of advanced applications within diverse areas ranging from electronics and energy storage to biology and environmental engineering.1,2 Single-wall CNTs (SWNTs) formed by rolling a single graphene sheet into a seamless cylinder with diameter of ∼1 nm, represent the distinct class of CNT materials where the effects of low dimensionality have been amply verified.3 Despite the marked progress in the production and purification of SWNTs, effective functionalization of their atomically smooth surfaces4,5 remains a major challenge for their practical utilization in advanced composites1 as well as sensors, high-flux membranes, and sorbents. Oxidation in the liquid phase is the most prevalent processing step for the purification of as-grown SWNTs6
8 as well as for their surface functionalization through the covalent attachment r 2011 American Chemical Society

of hydrophilic oxygen-containing groups. These functional groups improve the CNTs’ solubility and simultaneously act as anchoring sites for further chemical derivatization (amidation or esterification)5 and binding with other technologically relevant materials such as TiO2 for the realization of innovative photoelectrodes9,10 or composite photocatalysts with enhanced visible light activity.11
13 Among various oxidants, nitric acid has been the most common agent for the oxidation of CNTs.8 The underlying reaction mechanism related to the selective removal of small diameter metallic SWNTs and most importantly the resulting modification of the structural and electronic properties of SWNTs determined by the subtle interplay of various effects, i.e., oxidation damage, material’s Received: January 16, 2011 Revised: March 31, 2011 Published: April 13, 2011 8534

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The Journal of Physical Chemistry C loss, tube shortening and charge transfer, as well as the establishment of quantitative structure
property relationships has been the subject of intensive investigations.14
24 Particular attention has been recently drawn to the preferential formation of oxygen functionalities on carboxylated carbonaceous fragments (CCFs) produced by the SWNT’s consumption during oxidation.20
23 Detachment of the physisorbed CCFs from the CNT walls after base washing may then leave intact the supporting SWNTs and severely hinder their integration in composite materials. Moreover, the large variety of nowadays SWNT production techniques (arc discharge, laser ablation, thermal or plasma-assisted chemical vapor deposition (CVD), and high-pressure catalytic decomposition of carbon monoxide (HiPCO)) generates the necessity for supplementary information on the physicochemical characteristics of the starting material (i.e., surface chemistry and aggregation properties) in order to proceed with the investigation of the functionalization processes and the evaluation of their efficiency. Various experimental techniques have been exploited to probe the functional groups on the surface of oxidized SWNTs, the most prominent being acid
base titration, X-ray photoelectron spectroscopy, infrared and Raman vibrational spectroscopies, temperature-programmed desorption (TPD) and thermogravimetric analysis (TGA) together with electron microscopy,25,26 though accurate identification of the nature and the amount of oxygen functionalities by single spectroscopic or microscopic means remains a rather intricate task. In particular, resonance Raman spectroscopy has been established as a powerful tool for the characterization of SWNTs unveiling unique information on their electronic and vibrational properties, hardly accessible by other techniques,27 while recent studies rendered it an accurate method for the quantitative analysis of stacking order, finite crystallite size, and defect structure of nanographitic materials.28 However, despite the widespread use of Raman spectroscopy for the investigation of functionalized SWNTs through the activation of several defect-induced Raman modes29,30 and the high sensitivity of the first order phonon modes of SWNTs to charge transfer,17,31 its application has been essentially hampered by the lack of complementary experimental data on the defect density as well as on the modifications of the porosity and texture of the bundled structure of most commercially available SWNTs.32 The effect of different functionalization procedures on the pore structural characteristics of SWNT bundles can be primarily investigated by means of the standard liquid nitrogen (LN2) adsorption isotherms. The bimodal pore texture of SWNTs consists of micropores that mainly comprise the interstitial tube space in each bundle, the internal space of the SWNTs and the defects existing on the rolled graphene layers, whereas mesopores represent the larger gaps originating from the loose aggregation of the bundles.33
35 Several modern simulation techniques such as the grand canonical Monte Carlo (GCMC) and the nonlocal density functional theory (NLDFT), adapted to the graphitic carbon material, are currently applied to resolve the microporosity of SWNTs.33
39 The mesopore structure is usually evaluated with the classical BJH (Barrett
Joyner
Halenda method, pore distribution data derived from Brunauer
Emmett
Teller (BET) nitrogen adsorption isotherms) analysis for slit like pores. However, despite the confirmed accuracy of these methods, it is difficult to end up with definite conclusions on the physicochemical origin of the pore structure alterations that may occur during the functionalization of SWNTs bundles. Very recently, a nitric acid (HNO3) based soft hydrothermal method was developed to finely tailor the surface chemistry of pristine SWNTs through the thermally activated introduction of

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oxygen functional groups.40 TPD analysis coupled with mass spectrometry (TPD-MS) was employed to identify the nature and quantify the concentration of different oxygenated functionalities, which was analytically correlated with the HNO3 concentration. The controlled functionalization prevents considerable structural damage of the SWNTs architecture and CCFs formation that otherwise would require the application of further, chemical consuming, purification steps.22,23 In this work, a systematic investigation of oxygen functionalization on nitric acid hydrothermally treated SWNTs was performed by combining TPD-MS with water adsorption/ desorption, TGA, and Raman spectroscopy to quantitatively identify the nature, density, acidic strength, and the underlying charge transfer effects of oxygen functional groups on the SWNT surface. Furthermore, the effect of surface oxidation on the microporous and mesoporous structure of the SWNTs bundles was thoroughly studied using quenched solid density functional theory (QSDFT) calculations. The functionalization efficiency of the soft hydrothermal treatment was compared to that of a commercial reference material (Carbon Solutions, Inc.),41 with a high functionalization degree and a large number of extremely active carboxylic acid groups after aggressive (in boiling nitric acid) reflux. Mild oxidative and pyrolytic treatments in air and argon atmosphere, respectively, at different temperatures were further applied on the fully functionalized and pristine samples in order to determine the extent of amorphous carbon generation and the relative distribution of functional groups and CCFs on the SWNTs.

2. EXPERIMENTAL SECTION SWNTs (NTP) produced by CVD were purchased from Shenzen Nanotechnologies Co. Ltd. (China) with length in the range of 5
15 μm and mean diameter 90%) and a narrow distribution of diameters around 1.4 nm, while they are in a heavily functionalized state (∼6 wt % of carboxylic acid groups) after oxidative treatment in concentrated HNO3.8,41 Thermal analysis studies were conducted on a Setaram SETSYS Evolution 16/18, TGA/DSC analyzer. Argon treatment was performed with a ramp rate of 5 °C/min up to the temperature of 500 °C and an isothermal step of 30 min. For the air treatment, the ramp rate was maintained at 2.5 °C/min up to the temperature of 300 °C followed by an isothermal step of 30 min. 8535

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The Journal of Physical Chemistry C Liquid nitrogen (LN2) isotherms were measured on a Autosorb-1 MP (Quantachrome) porosimeter at 77 K. Prior to each measurement, the samples were degassed under high vacuum (10
5 mbar) for 24 h at the outgassing stations of the instrument. The maximum degassing temperature was 180 °C to ensure that no loss of the surface functional groups occurs. Water vapor adsorption
desorption measurements were conducted on a microbalance (CI Electronics, UK). The mass of the samples and counterweight pans, the hooks, the counterweight material, and the hang chains of the microbalance assembly were on the order of 100
300 mg/item and were defined with an accuracy of (0.1%. The materials were appropriately selected to induce a symmetrical configuration to the balance setup in order to minimize buoyancy effects. The microbalance had a 0.1 μg stable resolution. Before each measurement, the samples were degassed under high vacuum (10
5mbar) at 180 °C. TPD analysis was carried out using an AMI-200 Catalyst Characterization Instrument (Altamira Instruments) equipped with a quadrupole mass spectrometer (Ametek, Mod. Dymaxion). The sample (0.1 g) was placed in a U-shaped quartz tube and heated at 5 °C/min in an electrical furnace under a constant flow of 25 cm3 min
1 (STP) of helium. The concentrations of released CO and CO2 were determined using the respective calibrations performed at the end of each analysis. Raman measurements were performed in backscattering configuration using a Renishaw inVia Reflex microscope with an Arþ ion laser (λ = 514.5 nm, E = 2.41 eV) and a high power nearinfrared (NIR) diode laser (λ = 785 nm, E = 1.58 eV) as excitation sources. The laser light was focused on the samples using a long working distance (8 mm) 50 (NA = 0.55) objective of a Leica DMLM microscope at power density lower than 0.05 mW/μm2 for both laser lines, to avoid sample heating. The spectra were averaged over 5
10 randomly different spots for each sample, while the frequency shifts were calibrated by an internal Si reference. Spectral deconvolution was carried out by nonlinear least-squares fitting of the Raman peaks to a mixture of Lorentzian and Gaussian lineshapes. Pore size distributions (PSDs) were obtained after using the QSDFT N2-carbon equilibrium transition kernel at 77 K of Quantachrome’s Library.42 Although this model is based on slit pore geometry, it was considered as the most appropriate for the present case for the following reasons: (a) all experimental N2 adsorption isotherms at 77 K reveal hysteresis loops of H3 type,43 which is typical for layered materials/slit pore geometries, (b) in contrast to all other NLDFT (or GCMC) models that assume structureless homogeneous graphitic pore walls, the QSDFT method takes into account the effects of surface heterogeneity, which is typical for our functionalized samples, and (c) the QSDFT method eliminates artificial gaps and minimizes artificial peaks in the PSDs, a typical problem encountered in NLDFT calculations.

3. RESULTS AND DISCUSSION 3.1. Identification of Surface Functionalities by Water Adsorption/Desorption. The markedly different concentration

of oxygen functional groups in the CSI and NTP samples is directly evidenced by their water vapor adsorption/desorption isotherms shown in Figure 1, respectively, for the as-received CSI sample (CSI, Figure 1a) and for both pristine (NTP, Figure 1c) and 0.30 M HNO3-functionalized NTP (NTP 0.3 M, Figure 1b). According to the manufacturer specifications, the as-received

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Figure 1. Water vapor adsorption/desorption isotherms for the (a) fully functionalized CSI, (b) NTP treated with 0.30 M of HNO3, and (c) pristine NTP samples. The corresponding isotherms after air (300 °C) and Ar (500 °C) treatment are also included for each sample. Full symbols: adsorption; open symbols: desorption.

CSI sample, produced by EA, possesses ∼6 wt % carboxylic acid groups, whereas the NTP sample, produced by CVD, has a rather low degree of oxygen functionalities (as confirmed below by TPD-MS analysis). The water adsorption/desorption isotherms obtained for the NTP samples suggest that the surface chemistry of the pristine sample (Figure 1c) was truly modified by the HNO3 hydrothermal treatment (Figure 1b). Regarding the fully functionalized CSI samples, the most pronounced features are the enhanced hysteresis between the water adsorption/desorption branches and the significant amount of water (1250 μmol/g) that remains adsorbed even after keeping the sample for 24 h under high vacuum (10
5 mbar) conditions (Figure 1a). These are indicative of the specific interaction between the oxygen-containing, acidic, surface groups and water molecules. Apart from that, the filling of the micropores and the adsorption on nonacidic external surface sites could also influence the overall water adsorption capacity of SWNTs. However, as verified by means of water immersion calorimetry,44 the ΔH (kJ/mol) values corresponding to the aforementioned interactions are minor compared to those of the water molecules with oxygenated groups such as carboxyls, lactones, quinones, and carboxylic anhydrides. Assuming a 1:1 correspondence between the remaining adsorbed water molecules and the acidic surface groups, their concentration is estimated to be 1250 μmol/g, in very good agreement with the carboxylic acid 8536

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The Journal of Physical Chemistry C groups’ content of 1333 μmol/g, provided in the CSI specifications (∼6 wt %). On the other hand, there is still a question of whether the amount of water adsorbed by the CSI sample is due to its high degree of surface activation or due to the filling of very small pores (micropores) with water molecules, which can be promoted by the presence of hydrophilic surface functional groups.45
47 Results of adsorption experiments with other probe molecules are presented in the Supporting Information (Figures S1 and S2 and corresponding text) and conclude that the contribution of micropores in the water adsorption of the CSI sample is negligible. Thereby, it can be inferred that water adsorption on the functionalized SWNTs is highly specific, depending primarily on the degree of functionalization and can be thus used as a tool to quantify and possibly qualify oxygen surface functionalities on SWNTs. In particular, the amount of water adsorbed upon completion of the monolayer can be related to the total content of the acidic surface groups, whereas the amount of water remaining adsorbed after evacuation can be associated with the most acidic ones among the different oxygen groups on the SWNT surface. 3.2. Control and Quantification of Surface Functionalities on Hydrothermally Functionalized NTP Samples. To provide further evidence and validate the above argument, we explored the water adsorption/desorption isotherms on the hydrothermally treated NTP samples at different HNO3 concentrations (0.03, 0.10, 0.20, and 0.30 M). The surface chemistry of SWNTs can be finely controlled by using this HNO3 hydrothermal treatment, and the results were analyzed in relation to the TPD-MS data.40 In fact, TPD-MS analysis allowed the identification of the oxygen functional groups (Figure 2a) that are created on the SWNT’s surface upon hydrothermal nitric acid oxidation, because it is known that these groups are released as CO and CO2 at different temperatures.48,49 As an example, Figure 2b,c shows the TPD spectra for the NTP sample that was treated under the highest HNO3 concentration (0.30 M) and their respective deconvolution to obtain the distinct peaks from the decomposition of acidic carboxylic groups, carboxylic anhydrides, lactones, phenols, and carbonyl/quinones. The main groups identified in the CO2 spectra were carboxylic acids and lactones, while the most abundant functional groups determined from the CO spectra were phenols, indicating a high degree of hydroxylation upon hydrothermal HNO3 oxidation. The total concentrations of oxygen-containing functional groups for all samples, as well as the concentrations determined for individual groups, are shown in Table 1. The corresponding water adsorption/desorption isotherms of the NTP samples as a function of the HNO3 concentration are presented in Figure 3, while Table 1 allows comparison of the obtained results with the concentrations of the different surface groups derived by TPD-MS. Among the different oxygen functional groups introduced on the surface of SWNTs, carboxylic acids are of the highest acidity (pKa