Towards a fine-tuning of surface chemistry in aligned carbon nanotubes induced by nitrogen plasma discharge post-treatment: a combined microscopic and spectroscopic study

Abstract

The modification of the surface chemistry of individual nanotubes inside vertically aligned carbon nanotube (VACNT) arrays is a key challenge in developing successful active materials for many electronic, photonic and catalytic applications. To this end, an efficient, clean, non-destructive and adjustable process for nitrogen functionalization and doping of the VACNTs in a controlled manner is highly required. In this study, the results of a systematic study using plasma discharge for this purpose by varying the N₂/H₂ gas volume ratio, discharge exposure time, and the radio-frequency (r.f.) power are presented. This process resulted in the generation of a few defects induced deliberately by the nitrogen radicals into the graphitic framework, mainly as in; amine, amide, pyridinic, pyrrolic, graphitic-type nitrogen. The parameters of plasma discharge were adjusted in a way that the densities and the relative ratios of nitrogen-containing functional groups can be selectively controlled. Evidence for the induced structural and chemical bonding changes, and the formation of different functional groups on the surface of the VACNTs are examined as a function of nitrogen content, using proper combination of analytical methods of high-resolution transmission electron microscopy (HRTEM), Raman, X-ray photoelectron (XPS) and Fourier transform infrared (FTIR) spectroscopy. At the relatively low level of r.f. power and discharge nitrogen flow rate in the feed, a preservation of the graphitic framework of the VACNTs has been demonstrated. The current study, therefore, sets the stage for selective control of the densities and the relative ratios of different nitrogen-containing acidic and basic surface functional groups on or within the nanotube framework under defined process parameters without causing a dramatic loss of graphitic structures.

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1. Introduction

Since the first reported synthesis and observation of carbon-based nano-tubular structure by Iijima in 1991,¹ the sp²-hybridized or graphitic nanocarbons (GNCs) have attracted a great interest due to their unique structural perfection and chemical inertness. These types of nanocarbons can form one- and two-dimensional structures, i.e. carbon nanotubes (CNTs) and graphene or carbon nanowalls, respectively.² However, in many instances, GNCs often raise difficulties during the post-processing, e.g. handling, purification, sorting, dispersing, and mixing. Moreover, for some potential applications, there is a need to tune the physical, chemical and electronic properties of the GNCs based on the specific requirements.³

To this end, heteroatom doping or functionalization of CNTs can considerably modify the electronic and optical properties by means of incorporating dopants, due to the quantum confinement effects and the curvature of the cylindrical surfaces of nanotubes, which can be utilized in nano-electronics and photonics.⁴ Selective controlling of the incorporation of defects or foreign atoms results in tremendous technological implications, and still drives many experimental and theoretical investigations focused on this topic.

It is commonly known that nitrogen doping (N-doping) of CNTs creates electron donor states in the conduction band near the Fermi level.⁵ Many recent research reports have demonstrated that N-doping chemically activates the surface of CNTs, changes their solubility, improves their electrical, thermal and magnetic properties,^6,7 and enhances electrocatalytic activity with respect to the oxygen reduction reaction (ORR) in metal–air batteries⁸ and fuel cells as well.^9,10 Besides nitrogen-doped CNTs (N-CNTs) have potential applications in the synthesis of three-dimensional porous architectures,¹¹ supercapacitor systems,¹² in bio-sensing,¹³ as vapor or gas-sensing elements,¹⁴ in the enhancement of lithium storage capability,¹⁵ as nanocomposites,¹⁶ or as field emission sources.¹⁷ Owing to the greater electron affinity of nitrogen in comparison with carbon, the direct substitution of carbons by nitrogen atoms in the graphitic framework could facilitate the generation of n-type materials.¹⁸ An overview of different possibilities that incorporate nitrogen atoms in CNTs is given in Fig. 1.

Fig. 1 Graphical representation of proposed general structures for the modified graphitic framework induced by nitrogen (a) and oxygen (b) incorporations as well as surface functionalization, blue ball: nitrogen, red ball: oxygen. Note: in-plane nitrogen incorporation leads typically to N-doping.

Doping as well as functionalization typically requires a rearrangement of neighboring carbon atoms in the GNC framework.¹⁹ Generally, such an arrangement can induce different kinds of defect creation (dangling bonds, substitutional, interstitial, single and multiple vacancies) which are integral to the modification of the properties of GNCs.

So far the majority of studies have concentrated on an in situ doping of CNTs directly during their growth in the gas phase via a chemical vapour deposition (CVD).²⁰ Nevertheless, this technique brings some disadvantages, e.g. it results in apparently uneven distribution of dopants, changes the nanotube morphology towards a bamboo-like structure,²¹ and ultimately gives potential rise to a to higher wall defect concentrations, which renders the resulting CNTs mechanically weaker.^6,22

In the most reported post-synthesis approach, a wet- or dry-based synthesis process can be employed as an alternative route to functionalization and doping. In both a wet-²³ and vapor-based process,²⁴ the doping elements can be implemented into already existing defect sites in graphitic framework, such as pentagon–heptagon defects (Stone–Wales).²⁵ This type of process is often performed in a solution and it requires rather harsh conditions due to multiple bond-breaking events necessary for the functionalization or introduction of the dopants. It typically requires multistep reaction sequences, which are often hard to control with respect to the outcome of the doping process.²⁶ On the other side, intensive or long sonication processes are required to disperse CNTs in organic solvents, which could also cause a severe damage to the graphitic framework of the CNTs and may even initiate cutting into shorter tube fragments.²⁷ Furthermore, the amount of elemental doping has to be well-controlled in order to maintain the characteristic properties of CNTs, e.g. electronic and mechanical properties.²⁸

Along this direction a modification of the CNT properties in a “dry” process, e.g. the ball-milling of CNTs in reactive atmospheres,²⁹ has also recently been proposed. This technique, on the other hand, has also some drawbacks such as the inhomogeneous distribution of doping elements along the nanotube. Other approaches apply high temperatures (over 1250 °C),³⁰ ion implantation³¹ or plasma discharge.^32,33 With respect to the latter, precursor gasses can be directly introduced into the plasma chamber which allows incorporating oxygen,^16,34 fluorine³⁵ or nitrogen^13,32,33 in the graphitic framework of CNTs. Typically, free radicals play a key role in such doping process. Radio-frequency (r.f.) or microwave plasma discharge sources have been applied for the formation of atomic, radical or other unstable species.³⁶ It is well known that the degree and type of sidewall functionalization vs. graphitic framework incorporation depend upon the discharge parameters, such as the level of applied power, gas compositions and exposure time.^34,35 Additionally, r.f. plasma discharge has been demonstrated by many studies to be the most efficient technique for incorporating nitrogen atoms into the graphitic framework.³⁷ Yet, an in-depth study is still necessary to investigate the effect of discharge parameters on the surface morphology of graphitic layer.

The main goal of this study is thus to fine-tune the surface chemistry of the VACNTs. To this end, a systematic study has been carried out using N₂/H₂ gas plasma discharge as an efficient, clean, non-destructive and adjustable process for functionalization and N-doping of the VACNTs. The selective control of the densities and the relative ratios of different nitrogen-containing functional groups are monitored using detailed and combined microscopic and spectroscopic techniques. The influence of varying process parameters, such as r.f. power, discharge exposure time and partial nitrogen flow rate in the feed on the chemical bonding changes is additionally discussed and correlated to each type of nitrogen-containing functional groups.

2. Experimental

2.1. Surface modification of the VACNTs by r.f. plasma discharge

The VACNT arrays were synthesized using a catalyst driven, water assisted thermal CVD, as described elsewhere.³⁸ In short, ethylene 100 sccm (standard cubic centimeters per minute) in the presence of argon (600 sccm) was pyrolyzed at 850 °C for 15 minutes. In a typical growth experiment, 400 sccm H₂ and 200 ppm of H₂O were applied.

The VACNT samples were subjected to a plasma discharge treatment from the top side, using low-pressure parallel-plate plasma system working at a capacitively coupled r.f. of 13.56 MHz, and with a power up to 200 W (FEMTO, Diener Electronic, Ebhausen, Germany). The plasma discharge chamber was evacuated to a base pressure less than 0.1 mbar and kept for few minutes to reduce the effect of residual air on the sample. The desired flow of high purity nitrogen and hydrogen to the system was established through electronic mass flow controllers (MKS Instruments), keeping the amount of total admitted gas at 20 sccm. The working pressure was allowed to increase and the r.f. generator was switched after achieving a constant working pressure. The optimized power transfer to the plasma discharge was maintained with the help of an automatic impedance matching network. The incident power (P_i) and reflected power (P_r) were measured and the impedance was adjusted until the P_r was very low (i.e. less than 0.02 W).

After the plasma reaction was terminated to quench any reactive surface groups, first argon, then air was fed into the plasma chamber until the system returned to atmospheric pressure. Subsequently, the chamber was opened. Unless otherwise noted, all plasma discharges post-treatments are summarized in Table 1.

Table 1 Process parameters of plasma discharge at about 50 °C

Experiment	N2/H2 flow rate (sccm)	Working pressure (mbar)	r.f. power (W)	Post-treatment duration (min)
5N	05/15	0.70–0.76	30	5, 60
10N	10/10	0.84–0.92	30	60
15N	15/05	1.10	30, 60	60
20N	20/0	1.50–1.80	30	60

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2.2. Methods of analysis

Micro-Raman spectra were recorded for the pristine and as-treated VACNTs in a backscattering mode using LabRam HR 800 micro-Raman spectrometer (Horiba Jobin Yvon, Bensheim, Germany) equipped with an air-cooled, non-polarized green Ar⁺ laser (λ = 514.5 nm). The excitation line has its own interference filter (to filter out the plasma emission) and a Raman notch filter (for laser-light rejection). The measurements were performed with a spectrometer grating of 1800 g mm⁻¹ and a confocal microscope (long-working-distance objective, magnification ×50; numerical aperture (NA) 0.5; focused laser spot diameter about. 2 μm). The laser power (20 mW) was attenuated on the sample in the range of 2 mW to 20 μW by using neutral density (ND) filters. Each Raman spectrum was collected between 200–3500 cm⁻¹ with a peak position resolution of less than 1 cm⁻¹ and the spatial resolution of about 1 μm. D and G-band positions, their full width at half maximum (FWHM) and integrated intensities were obtained by applying Lorentzian peak fitting using OriginPro (OriginLab, Northampton, MA, USA) for the representative Raman spectra for each sample.

The morphology of the VACNT samples was examined using a high-resolution transmission electron microscopy (TEM, FEI CM 20 ST) operated at accelerating voltage of 200 kV. The VACNT samples were dispersed in ethanol with assistance of ultrasonication for few minutes and then one droplet of the dispersion was transferred to a carbon-coated copper TEM grid (PLANO, Wetzlar).

The elemental analysis and the chemical composition of the as-treated VACNTs were carried out by using X-ray photoelectron spectroscopy (XPS). The photoelectron spectra were collected using a PHI VersaProbe 5000 spectrometer equipped with monochromatic Al Kα source (hν = 1486.6 eV) and using the electron escape angle of 45°. The photoelectron survey spectra were acquired with an analyzer pass energy of 93.9 eV in the 0–1000 eV binding energy (BE) range to determine elemental composition. The C(1s), O(1s) and (N1s) high-resolution core level photoelectron spectra were collected using a 23.5 eV pass energy with the energy resolution of the spectrometer ≤0.7 eV measured by a reference experiment using the full width at half-maximum intensity (FWHM) of the Ag3d_5/2 photoelectron line of a cleaned (carbon-free) Ag foil. The base pressure in the measurement chamber was about 10⁻⁹ mbar and the diameter of the X-ray illuminated sample's area was 200 μm. All the core-level BEs were referred to the Fermi level of a cleaned Ag foil. The background under the high-resolution photoelectron spectra was subtracted using a Shirley-type function. The photoelectron peak positions and areas were obtained by applying a weighted least-square fitting of a mixed Gaussian–Lorentzian product function to the experimental data using XPSPEAK 4.1 software, taking into account a Doniach–Sunjic high BE tail for the C(1s) photoelectron line.³⁹ During the deconvolution, all the fitted peaks were self-consistent and reproducible using 1.2 eV as a maximum limit value of FWHM, allowing quantitative comparisons.

Fourier transform infrared (FTIR) spectra were recorded for the as-treated VACNT samples with a Nicolet Magna 760 FT-IR spectrometer operating in the attenuated total reflectance (ATR) mode; 32 scans were taken with a resolution of 8 cm⁻¹. The use of ATR-IR was in order to establish a straightforward qualitative method for measuring CNT samples. Furthermore, since the difference in refractive index between the carbon sample and germanium (Ge) crystal is large (i.e. n = 4.0) compared to commonly used diamond or ZnSe crystal (n = 2.42), the ATR unit was equipped with the Ge which casts a more shallow depth of beam penetration compared to diamond.⁴⁰

3. Results and discussion

3.1. Raman spectroscopy

Raman spectroscopic technique is a powerful tool to analyze CNTs. In typical micro-Raman studies using a visible laser source, the Raman-scattering bands mainly consist of two most prominent characteristic modes. The first mode is associated with sp²-hybridized carbon, first-order Raman, corresponding to the aromatic stretching vibration in the graphitic planes (so-called E_2g mode or G-band), which is clearly observed at about 1582 cm⁻¹ and the second one is related to disorder mode (so-called A_1g breathing mode or D-band) at about 1350 cm⁻¹.⁴¹ The additional weaker band at 1610 cm⁻¹, referred to as D′-band, corresponds to a maximum in the phonon density of states and normally exhibits a similar trend to that of the D-band.⁴² As is well-known, the graphitic state of the carbon can only be confirmed by the presence of the G mode.⁴³

For an efficient nitrogen doping, the effect of variation in plasma discharge parameters on the graphitization degree of the as-treated VACNTs in comparison with the as-grown VACNT microstructures will be correlated with the results obtained from Raman spectroscopy.

Influence of r.f. power level. It is quite obvious that the level of r.f. power should be limited in order to avoid any possibility of the excessive VACNT etching and deterioration. Indeed, the Raman spectra (Fig. 2a) show that a loss of the CNT graphitic structure mounts up with the increasing level of r.f. power. The ratio between the integrated intensity of the D-band to that for the G-band can be correlated with a number of defects existing within the nanotube frameworks.^34,35

Fig. 2 (a) Raman spectra of the as-treated VACNTs for the sample named 15N with a variation of the level of r.f. power at constant discharge duration of 60 min, (b) the correlation between Raman-derived degree of disorder and graphitization degree, (c) the correlation between G-band position and FWHM for D-band.

The Raman-derived degree of disorder which can be obtained from the integrated peak intensity ratio, I_D/(I_D + I_G), escalated from 0.42 for the pristine VACNTs to 0.47 and 0.56 for 30 and 60 W of employed r.f. power, respectively (Fig. 2b). In general, this indicates an increase of the degree of bond disorder in the graphitic framework corresponding to a reduction in the size or number of planar coordinated sp²-hybridized carbon centers.⁴⁴ This is clearly supported by the emergence of D′-band (Fig. 2b), which becomes strongly Raman active with the increasing r.f. power, and reflects the presence of some lattice defect inside the VACNT wall. On the other side, in the as-treated VACNTs, the surface ordering of carbon atoms is an indicative parameter for graphitization which is reflected by the FWHM of the G-band (W_G),⁴⁵ and it typically remains constant upon plasma discharge. By contrast, there is a decrease of about 7.8 cm⁻¹ (Fig. 2c) for the FWHM of the D-band (W_D) for the VACNT sample treated at 60 W r.f. power compared to one treated at 30 W under other similar plasma discharge conditions.⁴² This may be indicative of the fact that at low r.f. power, one can only remove some impurities, e.g. amorphous carbons from the VACNTs due to a moderate plasma discharge, which is unlike the relatively high r.f. power that could result in a severe etching or damage to the graphitic framework. Additionally, the W_G/W_D ratio of the treated samples at 30 W reveals similar to that of the pristine VACNTs, which is shifted by about 0.1 for the sample treated at 60 W. The latter represents rather a decrease in the graphitization degree compared to the pristine VACNTs. This could be understood, by increasing the incorporation of nitrogen in the graphitic framework of the VACNTs, which would be expected from an increase in the number of generated free radicals.^17,46 Still, downshifts of about 7.0 to 4.5 cm⁻¹ of the G-band positions are found (Fig. 2c) for the samples treated at 30 and 60 W, respectively, which are apparently uncorrelated to the r.f. power employed. Nevertheless, those could be understood in a way that at 30 W, plasma discharge treatment generates a limited number of defect sites and a possible N-doping can be generated. Nonetheless, the size or number of sp²-hybridized carbon centers is still kept, which can easily be correlated with a broadening of the D-band (i.e. high W_D). This is in contrast with the post-treatment at the r.f. power of 60 W, i.e. reducing of sp²-hybridized carbon centers with an increase in the amount of bond disorder and defects.

In summary, the result under 30 W r.f. power confirms that the moderate plasma discharge processes rather promote limited structural defects and lead to acceptable N-doping concentrations, without substantially affecting the size of graphitic domains in the as-treated VACNTs.

Influence of plasma discharge exposure time. In order to investigate the effect of plasma discharge duration, the r.f. power was kept at the optimum value of 30 W. Fig. 3a presents the normalized Raman spectra of the as-treated VACNTs and clearly shows the emergence of D′-band, which becomes strongly Raman active (Fig. 3a) by increasing the plasma discharge exposure time. In addition, a detailed peak analysis in Fig. 3b reflects that the I_D/(I_D + I_G) value shifts from 0.42 for the pristine VACNTs to 0.44 and to 0.43 for 5 and 60 minutes, respectively.

Fig. 3 (a) Raman spectra of the as-treated VACNTs with variation of plasma discharge duration for the sample named 5N at the constant r.f. power of 30 W, (b) the correlation between Raman-derived degree of disorder and graphitization degree, (c) the correlation between G-band position and FWHM for D-band.

This again indicates an increase in bond disorder in the nanotube framework, which manifests itself in a decrease in the size or number of sp²-hybridized carbon centers. Nevertheless, a strong decrease in the first five minutes of plasma discharge treatment again might be due to the initial removal of some amounts of amorphous carbon being present in the pristine VACNTs. After that, the nitrogen doping effect commences and leads to an increase in N-doping content. This interpretation is supported with the fact that W_D increases by about 11.4 cm⁻¹ upon plasma discharge for 5 minutes, compared to the pristine VACNTs (Fig. 3c). The W_G/W_D ratio, however, decreases only by about 0.02 for plasma discharge treatment of 60 minutes, but it shifts significantly by about 0.1 that is a fivefold increase for the VACNT sample treated only for 5 minutes. The latter case represents an apparent increase in graphitization degree compared with the pristine VACNTs, unlike to the sample treated for 60 minutes due to an efficient incorporating of nitrogen atoms into nanotube frameworks, as will be discussed in XPS analysis section. A significant downshift of about 8.5 cm⁻¹ was also obtained for the G-band positions in the samples treated for 60 minutes (Fig. 3c). This can be attributed to the existence of more bond disorder or defects which provides indirect evidence suggesting an increase of N-doping content and can be confirmed by the narrowing of D-band (low W_D).

Influence of discharge N₂ flow rate. In order to investigate the effect of discharge nitrogen flow rate and detect possible influences on the amount of nitrogen functionalization during plasma discharge post-treatment, the r.f. power and the discharge duration were kept at optimum values of 30 W and 60 minutes, respectively. Moreover, Fig. 4a shows the normalized Raman spectra of the as-treated VACNTs, which can be apparently correlated. The peak analysis in Fig. 4b, shows that the I_D/(I_D + I_G) value shifts from 0.42 for the pristine VACNTs to 0.43, 0.44, 0.47, and 0.48 for 5N, 10N, 15N and 20N, respectively. In general, this could reflect an increase of the degree of bond disorder in the as-treated VACNTs, correlating with a decrease in the size or number of sp²-hybridized carbon centers and it is supported by the emergence of D′-band, which becomes strongly Raman active with the increase of discharge nitrogen flow rate in the feed (Fig. 4a). Additionally, the W_G/W_D ratio is mainly constant for the 5N, 10N and 15N samples and similar to that of the pristine VACNTs.

Fig. 4 (a) Raman spectra of the as-treated VACNTs with a variation of discharge N₂/H₂ flow rate at the constant r.f. power of 30 W and discharge duration of 60 min, (b) the correlation between Raman-derived degree of disorder and graphitization degree, (c) the correlation between G-band position and FWHM for D-band.

On the other hand, there is an upshift by about 0.08 for the 20N sample, which could reflect a little decrease in graphitization degree. This will be discussed later intensively in the XPS section. In addition, Raman downshift up to 9 cm⁻¹ for the G-band positions in all the as-treated VACNT samples (Fig. 4c) suggests generation of defects or incorporation of nitrogen atoms within the nanotube framework. Interestingly, the higher W_D value (i.e. a broadening of D-band) for 10N and 15N samples provides an indirect evidence of the increasing number or size of the sp²-hybridized carbon centers compared to other samples, which can suggest an acceptable N-doping content without affecting the graphitization degree significantly.

Taken as a whole, the Raman spectroscopic investigations suggest that the parameters of plasma discharge treatment have been optimized by using discharge nitrogen flow rate between 10 to 15 sccm at 30 W for 60 minutes (see Table 1).

3.2. Surface morphological investigation

The electron microscopic measurements on the as-treated VACNTs have been performed in order to verify the structural change and the surface morphology of the as-treated nanotubes in comparison with the previous result of the VACNTs on the pristine nanotubes.⁴⁷

HRTEM images of an individual nanotube of the as-treated VACNT before and after thermal post-treatment at 550 °C in an air atmosphere (Fig. 5) clearly depict a decrease in non-conjugated organic compounds. This can probably be ascribed to the partial etching and gasification of amorphous carbons through plasma discharge (Fig. 5a) and thermal post-treatment (Fig. 5b), respectively. This is due to an apparent result of a moderate post-treatment, and no further purification is required. Interestingly, the presence of a little distortion in the linearity of the nanotube structure along with a few corrugated fringes and round cap morphologies on the as-treated nanotube clearly indicate the incorporation of nitrogen atoms.⁴⁸

Fig. 5 HRTEM images of the as-treated VACNTs (15N sample) before (a and b) and after (c and d) heat-treated at 550 °C under air for 15 minutes.

3.3. XPS surface characterization

The changes in the elemental composition and chemical bonding state of the carbon atoms induced by N-doping of the graphitic framework were analyzed by XPS.

The XPS survey spectra of the as-treated VACNTs (Fig. 6a) demonstrate the presence of carbon, nitrogen and oxygen with the BEs centered at about 284.7 eV, 400.0 eV and 532.0 eV for C(1s), N(1s) and O(1s) core-level electrons, respectively.⁴⁹ No impurities in the samples have been detected. It is widely recognized that the presence of oxygen-containing functional groups accompanies the nitrogen functionalization. This is either due to passivation of physical defects by oxygen during ambient air exposure of the VACNT surfaces, or probably related to the residual air in the plasma discharge chamber.⁵⁰

Fig. 6 (a) X-ray photoelectron survey spectra of the as-treated VACNT samples, (b) the nitrogen to oxygen atomic ratio as estimated from the high-resolution O(1s) and N(1s) spectral envelopes.

As could be expected, the atomic surface concentrations of nitrogen and oxygen increase in the as-treated VACNTs with the increase of nitrogen flow rate in the feed during plasma discharge functionalization, as seen in Fig. 6b and the inset table. However, there is a slight increase of the N/O atomic ratio at 10 sccm nitrogen flow rate, which can be considered as an optimum condition for the efficient discharge process with high content of nitrogen atomic radicals.

The high-resolution C(1s), N(1s) and O(1s) XPS spectral envelopes of the as-treated VACNT as a function of different nitrogen contents were plotted and fitted into their components. In general, the interpretation of XPS data is not straightforward due to controversy in the literature.⁵¹ Yet, Table 2 summarizes the most important documented XPS data of characteristic BEs (in eV, ±0.2) and their assignments for various nitrogen and oxygen moieties in some carbon-based materials. As the nitrogen content increases, the C(1s) XPS spectral envelopes are becoming more asymmetric and slightly broader towards higher BEs (Fig. 7a) which is a sign of nitrogen incorporation into the graphitic lattice. The C(1s) XPS spectral envelopes were fitted into six main components; C1, C2, C3, C4, C5 and C6. Fig. 7b presents the area percentages of fitted peaks as a function of nitrogen content. Thus, the most intense C(1s) photoelectron line at about 284.5–284.7 eV, denoted as C1 in Table 2, is assigned to the carbon bond with the sp²-hybridized character as in graphite.^{24,49,52–54} As the nitrogen content is raised to 12.39 at%, the peak area of the C1 component is slightly increased, which is correlated with an increase of the graphitic behavior observed in the as-treated VACNTs.

Table 2 Set of characteristic BEs (in eV, ±0.2) and their assignments in some reported carbon-based materials

Peak	Binding energies (eV)
C(1s)	C1	C2	C3	C4	C5	C6
	284.1–284.7	285.0–285.7	285.9–286.9	287.0–288.0	288.4–288.8	289.1–290.4
		C sp^3 ^52–54	sp^3 C–N	C–*C=O		O–C=O
			sp C≡N^52,55	N–C=O	N–C(N)=O	N–O–C=O
	C sp^2	sp^2 C=N	C–OH	epoxy		O=C–O–C=O
	Graphite^24,49,52–54	Pyridine^53	C*–O–C=O	O–C–O	O=C–N–C=O^52	N–C(O)=O
			C–O–C^24,47,50,52,54	N–C–O^24,49,52,54,56		O–C(O)=O^52

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O(1s)	O1	O2	O3	O4	O5	O6
	530.7–531.1	531.3–532.3	532.5–532.9	533.1–533.5	533.6–534.0	534.7–535.3
	quinone^24,51,57	C–C=O	OH	epoxy	*C–C=O
					*O–C(O*)=O
		N–C=O	C–C(O2)–O	C–O	O=C–O*–C=O	C–O–NO^*2^52
		O–C=O*	C–NO2^52	C–O–C	N–C(O*)=O
		N–C(N)=O		O–C–C^52,54,57	C–O*–NO2 ^52
		O=C–N–C=O^52

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N(1s)	N1	N2	N3	N4	N5
	398.3–399.3	399.7–400.2	400.1–400.9	401.1–401.8	402.0–406.1
	sp^2 C–N	amine	sp^3 C–N
			pyrrolic	substitutional (graphitic)	pyridine-N-oxide
		amide	pyridone
		pyrrolidone	imide	quaternary N^51–53,60	NOx^9,51–53,58–60
	pyridinic-N^22,51,52,58–60	urea N^51,52,61	carbamate N^51,52,58–60

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Fig. 7 (a) High-resolution C(1s) photoelectron scans for the as-treated VACNTs and the fitted peaks, (b) the percentages of peak areas as a function of discharge nitrogen content.

This result is consistent with the previous Raman spectroscopy measurements, indicating that the graphitic framework of the VACNTs is preserved for 15N sample in comparison with 20N sample. On the other side, there is a decrease in the peak areas associated with C2 and C3 components, at about 285.2–285.3 and 286.0–286.2 eV BEs. The C2 component is usually attributed to the carbons in sp³-hybridization^52–54 and/or sp²-hybridized bonding of carbon with nitrogen moieties as in pyridine,⁵⁵ while C3 component refers either to sp³- and sp-hybridized bonding of carbon connected to nitrogen,^52,55 or to oxygen moieties including ether or alcohol groups.^{24,49,52,54,56} Interestingly, the C4 component at about 287.1–287.5 eV shows a very slight increase in the peak areas over the entire range of nitrogen content attributed to carbonyl moieties, e.g. in ketone, amide and lactam functional groups.^{24,49,52,54,56} In addition, the peak areas of the C5 component at about 288.4–288.7 eV are increased gradually and can correspond to either carbamide (urea) or imide functional groups.⁵² The increase of nitrogen content at the expense of hydrogen intake in the plasma discharge chamber makes all reactions take place under a relatively low reducing atmosphere, i.e. N-doping of the VACNTs occurs at more oxidative conditions as the nitrogen content is augmented to about 23.35 at%, (Fig. 6a). This generates an intense peak, denoted by C6 in Table 2, at 289.6 which is mainly assigned to carbamate (urethate) functional groups,⁵² as will be discussed later in the ATR-FTIR section. Further, resonance shake-up satellite peak caused by π–π* transitions located above 290.5 eV, refers to the presence of aromatic structures.¹⁰ This confirms that the optimized plasma discharge parameters preserve to some extent the graphitization degree of the VACNTs. Moreover, some aromatic nitrogen-containing structures could also give rise to this peak.

The O(1s) XPS spectral envelopes in Fig. 8a were fitted into six main components; O1, O2, O3, O4, O5 and O6, as denoted in Table 2.

Fig. 8 (a) High-resolution O(1s) photoelectron scans for the as-treated VACNTs and the fitted peaks, (b) the percentages of peak areas as a function of discharge nitrogen content.

These may be related to different bonding constitutions of carbon to oxygen moieties corresponding to quinine,^24,51,57 carbonyl, e.g. in ketone, amide and carbamide,⁵² hydroxyl and/or nitro (NO₂),⁵² ether and/or epoxy,^52,54,57 carboxyl and/or carbamate,⁵² and nitrate (NO₃) functional groups,⁵² respectively.

Fig. 8b presents the area percentages of fitted peaks as a function of nitrogen content. As the nitrogen content increases, the oxygen content associated with the O4 component rises gradually. On the other side, there is an increase of the peak areas as the nitrogen content approaches to about 12.39 at%, for both the O1 and O2 components at the expense of the O5 and O3 components, respectively. This is due to the chemical conversion of hydroxyl and carboxyl moieties to quinine and amide/carbamide functional groups in the presence of more oxygen and nitrogen radicals (only 5 sccm H₂). Additionally, at the highest nitrogen content in this study (23.35 at%), the new component O6 was emerged at the higher BE of about 535.0 eV, along with the increase of both O3 and O5 components. This is due to relatively strong oxidative conditions, which results in a generation of more nitrogen oxides, (e.g. NO₂, NO₃) and carbamate functional groups, respectively.

To gain a more detailed insight into the surface chemical bonding states of nitrogen-containing functional groups, the N(1s) XPS spectral envelopes as a function of nitrogen content are plotted in Fig. 9a. Generally, the N1s photoelectron emission demonstrates a shift to higher BEs with the increase of nitrogen content in the graphitic framework of the as-treated VACNT samples. The N(1s) XPS spectral envelopes were fitted into five main components; N1, N2, N3, N4 and N5. Fig. 9b presents the area percentages of fitted peaks as a function of nitrogen content. The N(1s) XPS component at about 398.5–399.1 eV, denoted by N1 in Table 2, is associated with pyridinic-type nitrogen (Fig. 1), which occurs using exciting defects or at the edge sites.^{22,51–53,58–60} The N2 component at about 399.4–400.0 eV indicates to primarily the presence of amide or amine-like nitrogen,^51,52,61 whereas pyrrolic, carbamate or imide-like nitrogen contributes to the N3 component with BEs centered at about 400.2–400.9 eV.^{51,52,58–60} In addition, the N4 at about 401.1–401.8 eV is attributed to substitutional nitrogen (so-called graphitic and/or quaternary nitrogen) as shown in Fig. 1.^{51,52,58–60}

It is worth noting that only at high nitrogen content of about 23.35 at% (i.e. for 20N sample), can the weak N1s XPS component at 402.8 eV be merged compared to other samples with the lower nitrogen content. This component, referred to as N5 in Table 2, can be associated with either pyridine-N-oxide or different nitrogen oxide functional groups.^{9,51–53,58–60}

Fig. 9 (a) High-resolution N(1s) photoelectron scans for the as-treated VACNTs and the fitted peaks, (b) the percentages of peak areas as a function of discharge nitrogen content.

Interestingly, from the relative atomic content in the inset table of Fig. 6b, one can conclude that the augmented content of graphitic (N4) and pyrrolic (N3) nitrogen in the as-treated VACNTs is generally observed demonstrating the same trend with increasing nitrogen content. Additionally, the increase of the amount of pyrrolic-N (N3) obtained at 12.39 at% nitrogen content exhibits a trend inversely correlated with that of pyridinic-N (N1). This is supposed to be mainly due to a significant fraction of bond disorder within the nanotube framework induced by an increase in nitrogen content. On the other side, the amount of graphitic-N (N4) at 10.18 at% nitrogen content shows a significant increase at the expense of the dangling bonds (N2) and the defects (mainly vacancies) created during the plasma discharge. Besides, the rearrangement between the neighboring carbon and incorporated nitrogen atoms in the VACNTs creates a nitrogen mainly with sp³-hybridized character.²⁵

Taking also the results above obtained from the C(1s) XPS study into consideration, one can conclude that they verify the efficient incorporation of nitrogen atoms into the graphitic framework.

3.4. Analysis of surface functionalization by ATR-FTIR

High-resolution XPS analyses have revealed the presence of several types of nitrogen and oxygen moieties, such as those in amines, amide, imines, as well as hydroxyl, ether, carboxyl, carbonyl etc. However, the C(1s), N(1s) and O(1s) XPS spectral envelopes cannot be fully resolved and could lead to misleading and ambiguous results in terms of the calculated distribution of functional groups. To overcome the limitations inherent in XPS peak fitting, the chemical characterization of the N-VACNTs has to be strengthened further by an additional analytical method, such as ATR-FTIR spectroscopy. This method can provide molecular information that is complementary to the XPS data, particularly regarding the organic functional groups. Fig. 10 illustrates the FTIR spectra of the pristine and as-treated-VACNTs samples. The characteristic IR absorption bands that could be used in identifying various nitrogen and oxygen moieties show broad spectra of the as-treated VACNT samples that present signals with some spectral overlapping.

Fig. 10 ATR-FTIR spectra of the as-treated VACNT samples compared to the pristine VACNTs.

Nevertheless, the presence of amides can easily be detected due to the presence of a relatively high-intensity spectral peak around 1665 and 1560 cm⁻¹ corresponding to amides I and amide II, respectively.⁶² Moreover, the broad IR absorption region of the spectrum, which extends from 1450 to 1600 cm⁻¹ for all the as-treated and pristine VACNTs samples, is commensurate with the preservation of their graphitic structure.

In general, it is possible to identify six distinct absorption bands around 3250, 2910, 1665, 1560, 1450, 1165 and 760–860 cm⁻¹. These bands can be assigned to N–H stretching vibrations in amines or amides, sp³-hybridized C–H stretching vibrations, overlap stretching vibrations of C=O and C=N moieties in amides I (e.g. carbamide) and imines, respectively, overlap stretching vibrations C=C/C=N stretching vibrations in aromatic systems and N–H bending vibrations in amines, sp³-hybridized C–H bending vibrations in alkanes (i.e. amorphous carbon), overlap stretching vibrations of C–N and C–O moieties, and out-of-plane bending vibrations of both N–H and aromatic ring C–H.^62,63 It is worth noting that in the 20N sample, a broad absorption band centered at 1710 cm⁻¹ has been observed to give rise to overlap stretching vibrations of both C=O and N–O moieties. Taking into consideration the above results obtained from the XPS study, these overlap vibrations indicate clearly the presence of carbamate formed and nitrogen oxide functional groups, respectively. The formation of carbamates normally occurs by reaction of isocyanates with alcohols. Finally, the presence of N–O moieties is also evidenced by the emergence of weak band absorption around 1370 cm⁻¹, which is mainly attributed to the nitrate functional groups.⁶²

4. Conclusions

This work investigates an efficient, clean, non-destructive and adjustable process of modifying the surface chemistry of the VACNTs by functionalization and doping in a controlled manner. A systematic study has been performed by means of r.f. plasma discharge with varying process parameters, e.g., N₂/H₂ gas volume ratio in the feed, r.f. power and discharge exposure time. The r.f. power was intentionally kept in the low range in order to minimize the possible deterioration of the graphitic framework.

A combined spectroscopic and microscopic investigation was carried out to evaluate the structural disorder of the as-treated VACNTs as a function of nitrogen flow rate. The C(1s) XPS results demonstrate that as nitrogen content increases up to 12.39 at%, i.e. in the presence of both nitrogen and hydrogen in the feed inside the plasma discharge chamber, the pure sp²-hybridized carbons in the VACNTs increase. This occurred along with the formation of sp³-hybridized carbons as well as some of C–N and C–O moieties that become more Raman active and reflected by the emergence of D′-band. At the high nitrogen flow rate and absence of hydrogen in the feed, a formation of carbamate, nitrogen oxide and pyridinic oxide on the surface of the VACNTs were confirmed by XPS and ATR-FTIR analysis as well. Interestingly, the textural characteristics using HRTEM investigation indicated some corrugated fringes and round cap morphologies on the as-treated nanotube with a significant decrease in non-conjugated organic compounds (i.e. amorphous carbon).

Moreover, the detailed XPS analysis revealed that the density of some functional groups with acidic properties, mainly pyrrolic and graphitic-type N, escalates as the nitrogen content in the as-treated VACNTs increases, along with the undeliberate augmentation of the acidic oxygen-containing functional groups. The decrease in the densities of the pyridinic and amine-type nitrogen-containing functional groups cannot be explained as a simple creation of edges terminated by nitrogen atoms, but rather as a rearrangement of the neighboring carbon atoms that can take place within the nanotube framework. On the contrary, under low nitrogen content, i.e. at reducing atmosphere, the density of nitrogen-containing functional groups with basic properties, mainly amine and pyridinic-type N, increases.

Last but not least, the present work contends to lay the groundwork for selective control of the densities and the relative ratios of different acidic and basic surface functional groups on the surface of the as-treated VACNTs or within the nanotube framework. The active N-CNTs are highly demanded in different potential applications, e.g. in gas adsorption, terahertz time-domain spectroscopy, gas sensing and catalysis.

Acknowledgements

The LOEWE-Research Cluster “Sensors towards Terahertz STT” has funded this work through the state of Hesse. The scientific support from Prof. Jörg J. Schneider at TU Darmstadt by allowing use his lab infrastructure is highly acknowledged. The author thanks Dr Jörg Engstler for performing the TEM measurements. The author is also grateful to Mr Deepu Babu, Mr Sandeep Yadav and Dr Gennady Cherkashinin at TU Darmstadt for their assistance in synthesizing the pristine VACNT samples as well as Raman and XPS measurements, respectively. Special thanks go to Prof. Christian Hess at TU Darmstadt for fruitful discussions.

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