Understanding the Influence of N-Doping on the CO2 Adsorption

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Understanding the Influence of N-Doping on the CO2 Adsorption Characteristics in Carbon Nanomaterials Deepu J. Babu, Michael Bruns, Reinhard Schneider, Dagmar Gerthsen, and Joerg J. Schneider J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11686 • Publication Date (Web): 22 Dec 2016 Downloaded from http://pubs.acs.org on December 27, 2016

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The Journal of Physical Chemistry

Understanding the Influence of N-Doping on the CO2 Adsorption Characteristics in Carbon Nanomaterials Deepu J. Babua, Michael Brunsb, Reinhard Schneiderc, Dagmar Gerthsenc and Jörg J. Schneidera,* a

Fachbereich Chemie, Eduard-Zintl-Institut für Anorganische und Physikalische Chemie,

Alarich-Weiss-Strasse 12, Technische Universität Darmstadt, 64287 Darmstadt, Germany b

Institute for Applied Materials (IAM-ESS) and Karlsruhe Nano Micro Facility (KNMF),

Hermann-von-Helmholtz-Platz 1, Karlsruhe Institute of Technology (KIT), 76344 EggensteinLeopoldshafen, Germany c

Laboratory for Electron Microscopy (LEM), Karlsruhe Institute of Technology (KIT), Engesserstrasse 7, 76131, Karlsruhe, Germany * Corresponding author: Fax: +49 6151 16 3470.  E-mail address: [email protected] (J.J. Schneider).

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ABSTRACT Compared to metal organic frameworks and zeolites, carbon based materials are particularly interesting for gas capture applications due to their better stability against moisture and corrosive flue gases. Increasing the accessible surface area and incorporation of heteroatoms are generally the two different strategies put forward to further enhance the adsorption characteristics of carbon materials. The influence of nitrogen incorporation on the gas adsorption characteristics, especially CO2 adsorption, is however a controversial research topic with conflicting conclusions reported by various studies. In the present work, using vertically aligned carbon nanotubes (VACNTs), we investigate for the first time the influence of N-doping on the CO2 adsorption characteristics of carbon materials. The present study aims to shed additional light on the parameters known so far as well as those which are currently not considered but are additionally responsible for gas adsorption in functionalized carbon materials. To this end vertically aligned carbon nanotubes (VACNTs) are ideal carbon model structures for gas adsorption studies as they have a well-defined, reproducible mesoporous pore structure with a chemically homogeneous surface. Thus the interfering influence of micropores, which are generally present in carbon based adsorbents, is avoided making a straightforward interpretation of the exact influence of Ndoping possible for the first time. N-incorporation in the form of pyridinic or pyrrolic/pyridonic groups is achieved by using a N2 rf plasma treatment. The presence of these nitrogen functionalities is found to have a beneficial influence on the CO2 adsorption characteristics over a wide range of pressure (0 – 36 bar). The nature of interaction is determined by calculating the isosteric heat of adsorption and finally by comparing the adsorption characteristics of asprepared and N-doped VACNTs with oxygen functionalized VACNTs, the importance of determining the oxygen functional groups in the adsorbent is presented.


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1. Introduction

Nitrogen doped carbon materials have generated considerable interest in the field of energy storage, catalysis, field emission devices, gas sensors and adsorption.1–5 Though the beneficial role of N-incorporation in carbon materials for electrocatalysis is well-established,6–9 many conflicting studies are reported on the role of N-functionalities on gas adsorption, especially CO2 adsorption. Most of the early studies reported an enhanced CO2 adsorption on N-doped carbon materials.10–27 Many different mechanisms like Lewis acid - Lewis base interaction,9,11,13,28 Hbonding15 and quadrupolar interaction29,30 were proposed to explain the observed increase in CO2 adsorption. However some of the later studies found no correlation between the presence of nitrogen functionalities and the CO2 adsorption behavior.28,31–34 Sevilla et al.32 studied the CO2 adsorption characteristics of N-free and N-doped carbon microspheres and concluded that the presence of nitrogen functionalities did not influence the CO2 adsorption properties. Similar conclusions were also drawn by Adeniran et al.34 by investigating the CO2 adsorption on a series of carbons with closely matched porosity but varying nitrogen content. Both these works identified the volume of the micropores with a size of ~ 0.7 nm as the critical parameter for CO2 adsorption. By using molecular simulations, Kumar et al.28 investigated CO2 adsorption characteristics of N-doped graphitic and disordered carbon and found no beneficial influence of N-functionalities on CO2 adsorption. However, equally intriguing are the adsorption studies by Hao et al.11 and Tascón et al.26 In order to isolate and identify the individual contributions of Ncontaining surface groups and microporosity for CO2 adsorption, Hao et al.11 treated the microporous carbon monoliths with HCl acid to neutralize the basic nitrogen sites. The porosity remained unchanged but a 41 % decrease in CO2 uptake was observed. Tascón et al.26 investigated the influence of porous texture and surface chemistry on CO2 adsorption and found


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the N-content to be the dominant factor governing CO2 adsorption at 25˚C. While at lower temperatures (0˚C), the ultramicropore volume was shown to be the most influential factor. Thus the controversy over the role of nitrogen is far from settled and points to the need for more studies on well-defined systems. It is interesting to find that irrespective of the differences in findings on the role of N doping, the majority of the studies agree up on the critical influence of the presence of pores of size ~ 0.7 - 0.8 nm on CO2 adsorption (only the extent of its influence on CO2 adsorption is debated). Still most of the studies investigating the influence of N-doping on carbon materials were carried out on microporous materials which invariably have ultramicropores to varying extent, complicating the interpretation of the observed adsorption behavior. The influence of nitrogen functionalities can be better isolated and identified by investigating CO2 adsorption on mesoporous carbons with a well-defined pore structure and surface chemistry. As the amount adsorbed on the mesopores will be low at atmospheric pressure, such a study should also measure adsorption over a larger pressure range. However most of the reported CO2 adsorption studies on mesoporous carbons are reported only up to a pressure of 1 bar.14,21,33,35 An in-depth analysis about the influence of nitrogen incorporation is missing in those studies which have reported adsorption over a wider pressure range.36 Addressing all of these afore mentioned issues which appear in some way conflicting in their outcome is a major concern of our work reported herein. It is conducted with the goal to shed additional light on the parameters known so far and those which have been currently not considered but are equally important for gas adsorption in functionalized carbon materials. To this end vertically aligned carbon nanotubes (VACNTs) are ideal carbon model structures for gas adsorption studies as they have a well-defined, reproducible mesoporous pore structure with a


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chemically homogeneous surface. In our previous works, using a combined experimental and theoretical approach we have investigated CO2 adsorption on such vertically aligned carbon nanotubes and have demonstrated its feasibility as an ideal model carbon structure.37,38 It was shown that as-prepared vertically aligned carbon nanotubes (diameter ~ 8 nm) are devoid of any surface functional groups and the CO2 adsorption capacity increases almost linearly with pressure. It was also found that the addition of oxygen functional groups, mainly hydroxyl and carboxyl, led to considerable improvement in the adsorption capacity especially at near ambient pressures.39 To the best of our knowledge, there are no reported studies of CO2 adsorption on Ndoped CNTs. It should be noted that N-incorporation by polymer infiltration is not considered in this work as this approach typically leads to pore blockage and changes in the porosity. In general, N-incorporation in carbon nanotubes can be achieved by in-situ or ex-situ approaches. Though in-situ N-doping results in an uniform distribution of nitrogen functionalities along the length of the CNT, the incorporation of the N-functionalities during the CNT growth stage leads to the formation of typical bamboo type morphology.40–42 Such a compartmentalization of the CNT endohedral sites (inside of CNT), considerably reduces the accessible surface area and negatively impacts the adsorption characteristics. Hence the actual influence of the nitrogen incorporation cannot be evaluated. Among the various ex-situ approaches like annealing in NH343,44 or N2H4 vapor,45 plasma treatment is a simple and a versatile technique for grafting nitrogen functionalities on a CNT surface.46–50 The functionalization process itself is rapid and does not involve multiple washing/filtration cycles or toxic solvents. Moreover, it retains the structural integrity and does not destroy the vertical alignment of the CNTs.


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In the present study nitrogen incorporation on VACNTs is carried out by a N2 rf plasma treatment. The changes in surface chemistry and porous texture brought about by the plasma treatment is examined and the influence of nitrogen doping on CO2 adsorption is studied by measuring adsorption isotherms up to a pressure of 40 bar. To determine the nature of interaction, heat of adsorption is calculated and finally adsorption characteristics of N-doped VACNTs are compared with oxygen plasma functionalized VACNTs. 2. Experimental 2.1. Sample preparation Vertically aligned carbon nanotubes were prepared by a water assisted chemical vapor deposition method. Briefly, 13 nm of Al and 1.4 nm of Fe were deposited on a Si/SiO2 substrate by thermal evaporation and sputtering respectively. The substrate with the catalyst deposited was then transferred to a CVD oven and heated to 850˚C in a reducing atmosphere. The synthesis was carried out for 15 min using ethene as the carbon source in the presence of ppm quantities of water. N2 plasma functionalization was carried out in a radio frequency (rf 13.56 MHz) plasma setup (Femto, Diener electronic GmbH, Germany) with a power rating of 300 W (max. rf power limited to 200 W). The parallel plate setup is fitted with two mass flow controllers (MKS instruments) having a maximum flow rate of 20 sccm, for precise delivery of the desired gas. The distance between the plate is fixed at 55 mm and the plasma chamber has a dimension of 280 x 100 x 100 mm3 which is evacuated using a rotary pump. The power transfer from the generator to the discharge chamber is optimized by an automatic impedance matching network. The samples were generally degassed to pressures less than 0.2 mbar before introducing the desired gas. The rf generator was switched on only after achieving a stable chamber pressure at


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the set flow rate. N2 gas of high purity 99.999 % was used for all experiments and the plasma treatment was carried out at 20 % maximum power at a chamber pressure of 0.9 mbar.

2.2. Characterization SEM measurements were carried out on a Philips XL30 FEG, HRSEM. Raman spectra were recorded using a LabRam high resolution microscope (Horiba Jobin Yvon, model HR 800) with an argon excitation source (514.5 nm). Conventional transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were conducted on an FEI Titan3 80-300 microscope. On the optical axis below the lower pole piece of the objective lens, the microscope is equipped with a so-called Cs image corrector (CEOS GmbH) which is used to minimize the spherical aberration and other lens aberrations like, e.g., coma, higher-order astigmatism, and star aberration.51 Images were taken by means of a 2K CCD camera (Gatan UltraScan 1000 P). Image recording was done by the Digital Micrograph (Gatan) software at exposure times between 0.5 s for HRTEM and some seconds for conventional TEM. Moreover, this TITAN microscope is equipped with an imaging energy filter of the type Gatan Tridiem model 865 ER, which has also a 2K UltraScan CCD camera as detector and allows to perform electron energy loss spectroscopy (EELS) as well as energy-filtered TEM (EFTEM). EFTEM images were obtained via the threewindow method52 with typical recording times of several ten seconds per individual image. For drift correction and further image processing like background extrapolation and subtraction the Digital Micrograph program was used too. EELS spectra were recorded in the scanning TEM mode (STEM) with an electron probe of approximately 1 nm in diameter. All TEM/EELS experiments were done at 80 kV accelerating voltage in order to decrease damage of the CNT


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samples by high-energy electrons. Owing to the Cs image corrector, in HRTEM imaging structures as small as about 0.15 nm can be resolved even at 80 kV. XPS measurements were performed using a K-Alpha XPS spectrometer (ThermoFisher Scientific, East Grinstead, UK). Data acquisition and processing using the Thermo Avantage software is described elsewhere.53 All samples were analyzed using a microfocused, monochromated Al Kα X-ray source (30-400 µm spot size). The K-Alpha charge compensation system was employed during analysis, using electrons of 8 eV energy and low-energy argon ions to prevent any localized charge build-up. The spectra were fitted with one or more Voigt profiles (BE uncertainty: +/- 0.2 eV). The analyzer transmission function, Scofield sensitivity factors54 and effective attenuation lengths (EALs) for photoelectrons were applied for quantification. EALs were calculated using the standard TPP-2M formalism.55 All spectra were referenced to the C 1s peak of graphite at 284.4 eV binding energy controlled by means of the well-known photoelectron peaks of metallic Cu, Ag, and Au. N2 adsorption (77 K) measurements were carried out on a Quantachrome NOVA3000e system. The samples were heated in vacuum overnight at 150˚C. Surface area was calculated by using multi-point BET method. Pore size distributions were determined by using non-local density functional theory (NLDFT) kernel assuming a slit/cylindrical pore, available in the software NOVAWin10. The t-plot analysis was carried out using the carbon black method available in the NOVAWin10 software applied to a relative pressure range of 0.2 ≤ P/P0 ≤ 0.5.

2.3. CO2 adsorption measurements Ambient pressure CO2 adsorption measurements were carried out in a modified TG setup (TG209F1 Iris, Netzsch GmbH). About 8 mg of the sample was taken in an alumina crucible and


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heated to 300˚C in argon atmosphere. The adsorption step was carried out at 35˚C under 50 sccm of CO2 for an hour more details of which could be found elsewhere.38 A reference measurement without the sample was performed under identical conditions to account for the buoyancy corrections. High pressure adsorption measurement were carried out in a home-built volumetric setup. About 75 mg of sample was used for the measurement and the isotherms were measured at 20˚C, 25˚C and 30˚C. A water bath was used for maintaining the isothermal conditions, more details of the volumetric setup and the measurement can be found elsewhere.56

3. Results and discussion The as-prepared vertically aligned carbon nanotubes have an average height of about 600 µm and display high degree of vertical alignment as shown in Figure 1a. As seen from the HRTEM image (Figure 1b), the as-prepared VACNTs are usually double or multi walled (no. of walls ≤ 6) with an internal diameter of about 6 - 8 nm and are free from catalyst particles.38,39,57


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Figure 1: a) High magnification SEM image and b) high resolution TEM image of the asprepared vertically aligned carbon nanotubes. The as-prepared VACNTs display a high degree of vertical alignment and are usually double or multiwalled (no. of walls ≤ 6).

Raman spectrum of as-prepared VACNTs (Figure 2a) exhibit peaks corresponding to the Dband (~1340 cm-1), G-band (~1570 cm-1) and 2D band (~2670 cm-1), typical for CNTs. The intensity ratio of D-band to G-band can be considered as a measure of disorder/defects in sp2 carbon systems. For the as-prepared VACNTs, the ID/IG ratio is found to be ~ 0.8. According to the revised IUPAC classification,58 with the observed capillary condensation accompanied by the hysteresis at the high relative pressures, the N2 adsorption isotherm at 77 K for the as-prepared VACNTs (Figure 2b) resembles a type-IV(a) isotherm. The BET specific surface area of the asprepared VACNTs is found to be 499 m2g-1. NLDFT pore size distribution analysis of asprepared VACNTs, shown in the inset of Figure 2b, reveals a broad peak at around 5 nm, roughly corresponding to the internal diameter of CNT observed from the TEM images. The


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pore size distribution analysis also indicates the presence of pores smaller than 5 nm close to the lower confidence limit of 2.12 nm specified by the software. Therefore, the t-plot analysis was carried out to determine the presence of micropores. The t-plot analysis (Table 1) however, did not indicate any micropores and as known in such cases the external surface area is found to be similar to the BET specific surface area. It is well known that surface chemistry of the adsorbent significantly influences its adsorption characteristics. Activated carbons usually have a significant amount of functional groups e.g. hydroxyl/carboxyl, originally present in the parent material or formed during the synthesis process. For an adsorbent like activated carbon with a highly complex unordered pore system, mapping the distribution of these functional groups along the pores is impractical. However, this information is crucial for understanding the adsorption phenomenon, as it is the combination of pore geometry and surface chemistry that defines an adsorption process. In this regard vertically aligned carbon nanotubes with its defined pore geometry and surface chemistry are ideal model structures for understanding adsorption process in carbon materials. This is clearly evident from the XPS survey spectrum of the as-prepared VACNTs (Figure 2c), which shows the sole presence of carbon and some traces of oxygen. The presence of oxygen is believed to be due to the atmospheric handling of the sample. This is confirmed by the sputter depth profile measurements (Figure 2d) which shows that the oxygen functionalities present are indeed surface bound as the O 1s atomic concentration drops to near zero in less than 5 s of sputtering.


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Figure 2: a) Comparison of the Raman spectrum of as-prepared VACNTs and N-doped VACNTs. b) N2 adsorption isotherm at 77 K of as-prepared and N-doped VACNTs. Inset shows the pore size distribution of as-prepared and N-doped VACNTs. c) XPS survey spectrum of as-prepared VACNTs, d) Sputter depth profile measurement of as-prepared VACNTs. Inset shows the initial zoomed-in part of the O 1s sputter depth profile.

In order to incorporate nitrogen, the as-prepared VACNTs were treated with N2 rf plasma. It is known that nitrogen dissociation efficiency in an rf plasma is low compared with microwave


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activated plasma.59 One way of improving the dissociation efficiency is by the addition of an easily dissociable gas like argon.60–62 However, the addition of argon leads to considerable etching of the CNTs, leading to the creation of defects on the CNT surface. These defects are known to be high energy sites promoting adsorption.63,64 Therefore, the addition of argon gas was not an option. An alternative is to increase the plasma treatment time. Since the weight loss occurring in N2 rf plasma treatment was found to be negligible, VACNTs were subjected to 60 min of plasma functionalization. As shown in Figure 3a, significant changes in surface morphology were observed after the plasma treatment. The aggregation of the surface of CNTs after plasma treatment is a known phenomenon.57,65,66 High resolution TEM investigation (Figure 3b) indicated no apparent changes to the CNT structure due to the prolonged plasma exposure. Even after subjecting to 60 min of plasma treatment, no major damages to the tubular structure are observed.

Table 1: Textural characteristics of as-prepared and N-doped VACNTs t-plot method Sample

BET specific surface area (m2g-1)

micropore vol. (cm3g-1)

External surface area (m2g-1)

as-prepared VACNTs

499

0

499

N-doped VACNTs

447

0

447

Raman spectroscopy of the N-incorporated VACNTs (Figure 2a) however revealed significant differences. The ID/IG ratio displayed a sharp increase from 0.8 to 1.43 after plasma treatment. It is known that D-band is not a Raman allowed mode for perfect sp2 carbon materials but is activated due to symmetry breaking defects.67,68 The incorporation of the nitrogen


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Figure 3:a) SEM image of VACNTs after 60 min of N2 plasma treatment, b) HRTEM image of 60 min N2 plasma treated CNTs.

functionalities especially pyridinic type reportedly diminishes the high symmetry of hexagonal sp2 structure leading to an increase in the D-band intensity compared to the G-band.69,70 The creation of defects due to the bombardments of high energy particles on the CNT surface in the plasma is another reason for the observed sharp increase in the D-band. This however, did not lead to the creation of additional porosity as revealed by the N2 adsorption measurements (Figure 2b). No additional micropores were introduced as a result of the plasma treatment and the pore size distribution remained essentially similar. A slight decrease in the volume of the pores around 5-6 nm is observed and is reflected in a slightly decreased BET specific surface area of 447 m2g-1. The t-plot analysis (Table 1) confirmed the absence of generation of micropores and similar to the as-prepared VACNTs, the observed specific surface area is entirely due to the mesopores.


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XPS measurements were carried out on the N2 plasma treated VACNTs to determine the extent and type of nitrogen groups incorporated. A prominent N 1s peak observed in the survey spectrum (Figure 4a) confirms the presence of nitrogen on the VACNT surface. Since the binding energy difference between carbon-oxygen and carbon-nitrogen groups is negligible, the peak assignment of the C 1s spectrum (Figure 4b) is complicated. Nevertheless, a characteristic major hump in the C 1s spectrum indicates significant functionalization by either nitrogen or oxygen groups. To obtain a more detailed picture, the high resolution N 1s spectrum of N2 plasma treated VACNTs was recorded. As shown in Figure 4c, the N 1s spectrum obtained is deconvoluted into 2 peaks at 399.2 eV and 400.5 eV which could be assigned to pyridinic and pyridonic/pyrrolic type, respectively.12,22,23,33,43,71 Since the binding energy difference between pyridonic type and pyrrolic type is negligible, the two groups cannot be differentiated by XPS measurement.12 Although a high oxygen concentration can be seen in the XPS survey spectrum, the absence of any peaks in the N 1s spectrum at B.E. > 402 eV indicates the absence of N-O species.

Table 2: XPS quantitative analysis of N-doped VACNTs N (at. %) Sample

C (at. %)

O (at.%)

N-doped VACNTs

68.79

15.68

pyridinic (399.2 eV) 5.93

pyridonic/pyrrolic (400.5 eV) 9.59


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Figure 4: XPS measurements on N2 plasma functionalized VACNTs, a) Survey spectrum, b) high resolution C 1s spectrum, c) high resolution N 1s spectrum, d) sputter depth profile measurement. Inset shows the schematic representation of different possible types of Nincorporation in an sp2 carbon framework.

In the inset of Figure 4d, the different possible types of N-incorporation in CNT is shown schematically. It is interesting to note that there are no substitutional type nitrogen functionalities present and the presence of pyridinic and pyrrolic/pyridonic type, explains the sharp increase in the D-band observed in the Raman spectrum. Quantitative XPS analysis indicated a total


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nitrogen content of 15.52 at. % on the surface with pyridinic and pyridonic/pyrrolic group constituting 5.93 and 9.59 at. % respectively (Table 2). To ascertain the uniformity of distribution of N-incorporation along the length of the CNT, sputter depth profile measurements were carried out. As shown in Figure 4d, the nitrogen content is found to decrease rapidly away from the surface. Within about 50 s of sputtering, the N-concentration decreased from 15.52 at.% to less than 7 at.% and thereafter the concentration remained more or less constant at around 5 6 at.%. This value is close to the reported N-doping values (4 - 6 at. %) on carbon materials.10,24,28 The O-atomic concentration decreased more drastically compared to N 1s and within 50 s of sputtering, the O-at. % decreased from ~ 15.5 at. % at the surface to less than 2.5 at. % as shown in Figure 4d. After 150 s of sputter etching, only about 1 at. % of oxygen is detected. Thus, it is clear that the higher oxygen concentration is limited to the surface and in the bulk only less than 1 at. % oxygen exist. On the other hand, a significant nitrogen concentration of 5.5 at. % is observed even after 150 s of etching, indicating a more uniform N-incorporation. This is consistent with the EELS measurement of N-doped VACNTs. From the EELS spectrum of N-doped VACNTs shown in Figure 5a, a prominent C-K edge (threshold energy of 283 eV) and a very weak signal at 400 eV energy loss corresponding to the N-K edge can be identified. Other works on N-doping on nanoporous carbon materials also reported a similar weak N-K signal.72 The contribution of the N-K signal to the entire EEL spectrum is extremely small due to two reasons: on the one hand the C-K ionization edge dominate the spectrum since it is just in front of the N-K edge and has a higher ionization cross-section. Secondly, because the local content of nitrogen is low, the height of the N-K signal is also low. The peak at about 320 eV belongs to the C-K edge, and hence is no characteristic feature of a different chemical element.


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Figure 5: EELS measurement on N-doped VACNTs. a) EELS spectrum of N-doped VACNTs showing C-K edge (ionization energy of 283 eV) and some small signal at 400 eV due to the N-K edge, b) TEM image of N-doped VACNTs and corresponding EFTEM maps of c) carbon and d) nitrogen.

But, this maximum can be observed for many carbon materials.73,74 The peak intensity seems to depend on


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the structural order of the specific carbon material under investigation. Often, the maximum around 320 eV of the C-K edge is assigned to the extended energy-loss fine structure (EXELFS) which gives information about the nearest neighbors to carbon. The energy-filtered TEM images of Figure 5d demonstrate the uniform distribution of nitrogen on the VACNTs CO2 adsorption studies Ambient pressure CO2 adsorption measurements carried out on N-doped VACNTs at 35˚C indicated an almost double fold increase in the adsorption capacity as shown in Figure 6a. Under similar conditions, as-prepared VACNTs exhibit an adsorption capacity of 5.6 mg g-1 39 while N-doped VACNTs is found to adsorb 12.4 mg g-1. To determine the regenerability of the adsorbent, the measurement was repeated on the same sample for a second cycle and identical results were obtained. Ambient pressure CO2 adsorption studies were also carried out on VACNTs subjected to 10 min of plasma treatment. Keeping all other plasma parameters constant, however decreasing the plasma treatment time from 60 min to 10 min resulted in a decrease in the total nitrogen content from 15.52 at. % to about 7.87 at. % on the VACNT surface (see figure S1 in supporting information for more details). The decrease in the total nitrogen content also resulted in a decrease in the CO2 adsorption capacity. As shown in Figure 6a, VACNT subjected to 10 min of N2 plasma treatment exhibited an adsorption capacity of 7.9 mg/g, lower than VACNT treated for 60 min but higher than as-prepared VACNT. In our previous studies,37,38 we have reported that the intertube distance significantly influences the CO2 adsorption. For a dense CNT structure obtained by liquid induced densification, the maximum adsorption capacity under similar conditions was found to be about 8.8 mg g-1.38 It should be noted that the agglomeration of the VACNTs produced by the plasma treatment (Figure 3a) did not lead to a volume shrinkage or densification as observed in the


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case for liquid induced densified CNT structures. In the absence of any additional porosity or reduced intertube distance/diameter of CNTs, the observed double fold increase in adsorption capacity for N-doped VACNTs at ambient conditions is attributed to the enhanced interaction brought about by the N-incorporation. To investigate the influence of N-doping over a wider pressure range, adsorption measurements were carried out on 60 min plasma functionalized VACNT up to a pressure close to 36 bar and at three different temperatures, 20˚C, 25˚C and 30˚C. As shown in Figure 6b the adsorption capacity is found to decrease with an increase in temperature indicating a physisorption adsorption mechanism (A magnified view of the adsorption capacity in the low pressure region (0.5 - 2.5 bar) is given in figure S2 in the supporting information). At the highest studied pressure of about 36 bar, N-doped VACNTs is found to have an adsorption capacity of 8 mmol g-1 at 25˚C. This value is greater than the reported adsorption capacity of other adsorbents like zeolites,75 ordered mesoporous carbon nitride,76 N-doped ordered mesoporous carbon,36 MCM-4177 or carbon nanohorns56 but is lower than that of MOFs.78


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Figure 6: a) Ambient pressure CO2 adsorption on 60 min and 10 min plasma treated VACNTs at 35˚C. For comparison adsorption capacity of as-prepared VACNTs [* ref. 39] is also given. b) High pressure CO2 adsorption isotherms of N-doped VACNTs (60 min) at 20, 25 and 30˚C. c) Heat of adsorption of CO2 on N-doped VACNTs. d) Comparison of high pressure CO2 adsorption characteristics of N-doped VACNTs with as-prepared VACNTs and O2 plasma functionalized VACNTs [* ref. 39].


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To determine the nature of interaction between the adsorbate CO2 molecule and the N-doped VACNT adsorbent, isosteric heat of adsorption was calculated according to the method of Czepiriski et al.79 and Sun et al.80 Briefly the isotherms obtained at the three different temperatures were fitted using a virial type equation79 given by:

1 () = + + () where n is the amount adsorbed at pressure P and temperature T. ai and bi are empirical parameters. From these parameters the isosteric heat of adsorption was calculated using the relation:

= −

where R is the universal gas constant. The heat of adsorption thus obtained is plotted as a function of loading in Figure 6c. At 0.5 mmol g-1 of loading, the heat of adsorption is found to be 19.5 kJ mol-1 clearly indicating a physisorption process. The low heat of adsorption value is also a clear indication of the absence of other type of nitrogen functional groups like amine, which typically lead to CO2 chemisorption with a high heat of adsorption. It is interesting to compare the adsorption characteristics of as-prepared VACNTs, oxygen plasma functionalized CNTs and N-doped CNTs (Figure 6d). Compared with as-prepared VACNTs, N-doped VACNTs exhibit an enhanced CO2 adsorption capacity over the entire pressure regime investigated. It should be noted that though the specific surface area of N-doped VACNTs (447 m2 g-1) is lower than that of as-prepared VACNTs (499 m2 g-1), an enhanced adsorption is observed at all pressures. This clearly suggests that CO2 adsorption on carbon materials is not directly proportional to its surface area and the incorporation of N-functionalities have a positive influence on the CO2 adsorption characteristics.


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A comparison of N-doped VACNTs with oxygen plasma functionalized VACNTs provide valuable insights on the CO2 adsorption characteristics. From our previous studies on O2 plasma functionalization, it is known that the O2 plasma treatment can lead to an increase in the adsorption capacity in the low-mid pressure (0 - 40 bar) regime. However, in an O2 plasma, significant etching of CNT occurs and consequently an increase in specific surface area was observed after the O2 plasma treatment. Therefore the increase in the observed adsorption behavior for oxygen plasma treated VACNT is due to the combined effect of changes in porosity as well as addition of functional groups. However for N2 plasma treated VACNT, no changes in porosity or increase in specific surface area is observed. Hence the observed increase in the adsorption capacity for N-doped VACNT can be attributed to the incorporation of nitrogen functionalities rather than any general plasma induced effects. Another interesting point is the role of oxygen functionalities on CO2 adsorption. From our multiple adsorption studies on VACNT treated with O2 plasma, the adsorption capacity was found to increase with the oxygen concentration. This was even more clear when we observed an increase in the CO2 adsorption capacity with CO2 plasma treated VACNT which are known to induce fewer structural defects but results in almost the same extent of O2 functionalization as that of O2 plasma.57 The adsorption scenario thus becomes more complicated when nitrogen is incorporated onto a carbon surface which already have a certain percentage of oxygen functionalities. Hence the influence of nitrogen doping is more evident only when nitrogen is incorporated on a pristine carbon surface devoid of other functional groups. This correlates well with our previous finding of CO2 adsorption on graphene oxide (known to have a large concentration of oxygen functionalities). We did not observe any increase in adsorption capacity after the incorporation of nitrogen functionalities.81 This points towards the importance of the


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need for a detailed characterization of not only the porosity but also the surface characteristics of such a nanomaterial. However, most of the N-doping studies on various carbon materials, neglect the presence of oxygen functionalities. This might be one of the reasons for the discrepancies observed in the literature regarding the influence of N-doping.

Conclusions In conclusion, using vertically aligned carbon nanotubes (VACNTs) with well-defined porosity and surface chemistry, the controversial role of N-functionalities on the CO2 adsorption behavior in carbon materials were investigated. The mesoporous nature of these VACNT structures helped in an analysis of the role that nitrogen atom incorporation plays in nanostructured carbon materials without the interfering influence of micropores. In this regard VACNTs are a material to study different adsorption effects in porous carbons in a most ideal manner. Nitrogen in the form of pyridinic and pyrrolic/pyridonic functionalities were incorporated on to the vertically aligned carbon nanotube surface by N2 rf plasma treatment. The porosity measurements confirmed that the plasma treatment did not lead to the creation of micropores or any additional porosity. CO2 adsorption measurements on these N-doped VACNT confirmed the beneficial role of nitrogen incorporation at near ambient conditions as well as at high pressures (~ 36 bar). The CO2 adsorption capacity was found to be not directly related to the specific surface area of the adsorbent. The isosteric heat of adsorption value of 19.5 kJ mol-1 confirmed the reversible CO2 binding nature typically associated with such nitrogen functionalities. Finally, by comparing the adsorption characteristics of N-doped VACNTs with oxygen plasma functionalized VACNTs, the importance of investigating adsorption over a wider


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pressure range and the significance of taking into account the presence of oxygen functional groups were presented.

Associated Content Supporting information available Acknowledgements The authors would like to acknowledge Sandeep Yadav (TU Darmstadt) for Raman measurements and Dietmar Klank (Quantachrome Instruments) for discussion regarding the tplot method. XPS and EELS measurement were carried out with the support of the Karlsruhe Nano Micro Facility (KNMF) at KIT. DJB and JJS acknowledge the ongoing funding from the DFG SPP 1570 program.

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Understanding the Influence of N-Doping on the CO2 Adsorption

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