nanomaterials

Article One-Step Plasma Synthesis of Nitrogen-Doped Carbon Nanomesh

Alenka Vesel 1,* , Rok Zaplotnik 1 , Gregor Primc 1 , Luka Pirker 2 and Miran Mozetiˇc 1

1 Department of Surface Engineering, Jozef Stefan Institute, Jamova Cesta 39, 1000 Ljubljana, Slovenia; [email protected] (R.Z.); [email protected] (G.P.); [email protected] (M.M.) 2 Department of Condensed Matter Physics, Jozef Stefan Institute, Jamova Cesta 39, 1000 Ljubljana, Slovenia; [email protected] * Correspondence: [email protected]

Abstract: A one-step method for plasma synthesis of nitrogen-doped carbon nanomesh is presented. The method involves a molten polymer, which is a source of carbon, and inductively coupled nitrogen plasma, which is a source of highly reactive nitrogen species. The method enables the deposition of the nanocarbon layer at a rate of almost 0.1 µm/s. The deposited nanocarbon is in the form of randomly oriented multilayer graphene nanosheets or nanoflakes with a thickness of several nm and an area of the order of 1000 nm2. The concentration of chemically bonded nitrogen on the surface of the film increases with deposition time and saturates at approximately 15 at.%. Initially, the oxygen concentration is up to approximately 10 at.% but decreases with treatment time and finally saturates at approximately 2 at.%. Nitrogen is bonded in various configurations, including graphitic, pyridinic, and pyrrolic nitrogen.



Keywords: plasma synthesis; one-step procedure; nitrogen doping; carbon nanowalls


Citation: Vesel, A.; Zaplotnik, R.; Primc, G.; Pirker, L.; Mozetiˇc,M. One-Step Plasma Synthesis of 1. Introduction Nitrogen-Doped Carbon Nanomesh. Nitrogen-doped graphene-like nanostructures (hereafter: N-graphene) have attracted Nanomaterials 2021, 11, 837. https:// significant scientific attention because of their good electronic properties [1–4] which allows doi.org/10.3390/nano11040837 their possible application in electrochemical devices such as fuel cells, super-batteries, and supercapacitors [5–7]. As early as 2011, Zhang and Xia provided a feasible explanation Academic Editor: Giuliana Faggio for the superior properties of such materials in fuel cells [8]. They provided theoretical backgrounds considering the asymmetry spin density and atomic charge density, making Received: 3 March 2021 it possible for N-graphene to show high electrocatalytic activities for the oxygen reduction Accepted: 22 March 2021 reaction [8]. Published: 25 March 2021 Several techniques for the synthesis of N-graphene have been reported [9–15]. The most straightforward one is thermal annealing of thin films containing both carbon and Publisher’s Note: MDPI stays neutral nitrogen, usually accompanied by hydrogen. The film is heated to a temperature high with regard to jurisdictional claims in enough to enable thermal degradation, in particular de-hydrogenation, and thus the published maps and institutional affil- formation of a layer rich in carbon and nitrogen. Such a technique was reported by Kwon iations. et al. [16]. A 20 nm thick film of polypyrrole was deposited onto an activated copper substrate and heated to 900 ◦C to obtain a graphene-like material with approximately 3 at.% nitrogen, as revealed from X-ray photoelectron spectroscopy (XPS) survey spectra. A somewhat more complicated technique was adopted by Lin et al. [17]. They prepared Copyright: © 2021 by the authors. a composite of graphene oxide and polypyrrole, which was annealed at 800 ◦C to obtain Licensee MDPI, Basel, Switzerland. N-graphene of approximately 2–3 at.% nitrogen and a porous structure with micro-and This article is an open access article mesopores of high catalytic activity. distributed under the terms and Porous structures with dense N-graphene can also be synthesized at lower tempera- conditions of the Creative Commons Attribution (CC BY) license (https:// tures. An established technique for synthesizing nanomaterials is hydrothermal treatment. creativecommons.org/licenses/by/ Sun et al. [18] used graphene oxide and urea to synthesize N-graphene of a large surface 4.0/).

Nanomaterials 2021, 11, 837. https://doi.org/10.3390/nano11040837 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2021, 11, 837 2 of 16

and with an excellent capacitive performance at 180 ◦C. The nitrogen content was as large as 11 at.% using the hydrothermal technique. An alternative to such classical techniques is the application of highly non-equilibrium gaseous plasma. Jeong et al. [19] synthesized high-performance ultracapacitors by perform- ing the reduction of graphene oxide in hydrogen plasma, followed by further treatment in the nitrogen plasma. The final treatment step was annealing at 300 ◦C to remove residual functional groups. A similar technique with graphene oxide as the raw material and application of a gas mixture containing hydrogen and nitrogen or ammonia was used by Zhang et al. [20] and Kumar et al. [21]. An overview of plasma-based techniques for the synthesis of N-graphene was published recently in [22]. The methods mentioned above enable the synthesis of high-quality N-graphene, but the applicability is limited because of rather long treatment times. Mass application of such materials is possible only when using a technique that enables rapid deposition, preferable in a continuous mode. Traditional plasma techniques rely on introducing a carbon- and a nitrogen-containing gas into the plasma reactor simultaneously [14]. The carbon-containing gas could be methane and any other light hydrocarbon or a monomer containing nitrogen. The source of nitrogen could be N2 or NH3. The precursors are partially decomposed upon plasma conditions, and the radicals condense on a surface facing plasma to form any coating from thin films of hydrogenated carbon [23,24] to highly- oriented carbon nanowalls [25,26]. The growth rate, however, is limited by the scavenger effect: the dominant product is gaseous hydrogen cyanide [27]. The growth rate may be increased by increasing the pressure in the plasma reactor; however, a rather large density of precursors in gaseous plasma causes the formation of dust particles in the gas phase, which is often detrimental to the quality of the deposits [28]. The upper limitations may be avoided by using an alternative source of carbon in the plasma reactor. Graphite as a raw material seems to be a promising source for synthesizing N-graphene nanostructures; however, it will not be etched at sufficient rates upon treatment with nitrogen plasma at a reasonable temperature to assure for rapid deposition of thin films. A substitute for graphite could be aromatic polymers. Such polymers decompose at elevated temperatures forming conjugated aromatic rings [29]. The aromatic rings interact with reactive nitrogen species and enter the plasma where they radicalize and represent building blocks for N-doped graphene nanomesh. In the present paper, we introduce a novel direct one-step method for fast synthesis of N-doped carbon nanomesh using nitrogen plasma as a nitrogen source and a polymer material as a source of carbon precursor.

2. Materials and Methods 2.1. Plasma Synthesis The experimental set-up used for the deposition of N-doped carbon nanomesh is shown schematically in Figure1. The plasma reactor employs an inductively coupled radiofrequency (RF) discharge to sustain the rather dense and uniform plasma in nitrogen. A coil with six turns was mounted on the discharge tube, which was pumped with a rotary pump with a nominal pumping speed of 80 m3/h. The base pressure was approximately 1 Pa. Titanium foil was used as a substrate for the deposition of N-doped carbon nanomesh. Ti substrates of dimensions 8 mm × 8 mm × 0.1 mm were placed into a glass discharge tube together with a sample of polymer polyethylene terephthalate (PET), as shown in Figure1. The dimensions of the PET foil were 20 mm × 20 mm × 0.25 mm. N2 gas of commercial purity 99.999% was used as a nitrogen source, and the PET polymer was a source of carbon. Nitrogen pressure in the discharge tube was set to 15 Pa. The plasma was ignited and sustained at the forward power of the RF generator of 500 W. At these conditions, nitrogen plasma was sustained in the H-mode, where the absorbed power was high. Ti substrate was heated in the plasma because of exothermic heterogeneous surface reactions and reached a temperature of approximately 800 ◦C after several seconds. The deposition time was varied from 10 to 120 s. Nanomaterials 2021, 11, x FOR PEER REVIEW 3 of 17

Nanomaterials 2021, 11, 837 Ti substrate was heated in the plasma because of exothermic heterogeneous surface3 reac- of 16 tions and reached a temperature of approximately 800 °C after several seconds. The dep- osition time was varied from 10 to 120 s.

Figure 1. Experimental set-up. Figure 1. Experimental set-up.

TheThe N-atomN-atom densitydensity inin nitrogennitrogen plasmaplasma waswas measuredmeasured withwith aa catalyticcatalytic probe,probe, andand itit waswas approximatelyapproximately 0.4 ×× 10102020 mm−3.− Plasma3. Plasma was was also also characterized characterized by optical by optical emission emission spec- spectroscopytroscopy (OES) (OES) using using an anAvaSpec-3648 AvaSpec-3648 Fibe Fiberr Optic Optic Spectrometer Spectrometer (Avantes, (Avantes, Apeldoorn, TheThe Netherlands).Netherlands). TheThe spectrometerspectrometer resolutionresolution waswas 0.50.5 nmnm inin thethe rangerange ofofwavelengths wavelengths betweenbetween 200200 toto 11001100 nm.nm. TheThe integrationintegration timetime waswas 11 ms.ms.

2.2.2.2. CharacterizationCharacterization ofof thethe SamplesSamples 2.2.1.2.2.1. X-rayX-ray PhotoelectronPhotoelectron SpectroscopySpectroscopy (XPS)(XPS) XPSXPS characterizationcharacterization waswas performedperformed byby usingusing anan XPSXPS (TFA XPSXPS PhysicalPhysical Electronics,Electronics, α Münich,Münich, Germany).Germany). The The samples samples were were excited excited with with monochromatic monochromatic Al Al K Kα1,21,2 radiationradiation at at1486.6 1486.6 eV eV over over an an area area with with a adiameter diameter of of 400 400 µm.µm. Photoelectrons Photoelectrons were were detected detected with with a ◦ ahemispherical hemispherical analyzer analyzer positioned positioned at at an an angle angle of of 45° 45 withwith respect to the normalnormal ofof thethe samplesample surface.surface. SurveySurvey spectraspectra werewere measuredmeasured toto determinedetermine thethe nitrogennitrogen contentcontent inin thethe samples.samples. High-resolutionHigh-resolution spectraspectra ofof C1sC1s andand N1sN1s werewere alsoalso measured.measured. TheThe C1sC1s andand N1sN1s spectraspectra werewere measuredmeasured atat aa pass-energypass-energy ofof 23.523.5 eVeV usingusing anan energyenergy stepstep ofof 0.10.1 eV.eV. AugerAuger CKLLCKLL spectrumspectrum (1190–1250 (1190–1250 eV) eV) was was also also measured measured with with a pass-energya pass-energy of 117.4of 117.4 eV usingeV using an energy step of 0.25 eV. The measured spectra were analyzed using MultiPak v8.1c software an energy step of 0.25 eV. The measured spectra were analyzed using MultiPak v8.1c soft- (Ulvac-Phi Inc., Kanagawa, Japan, 2006) from Physical Electronics, which was supplied ware (Ulvac-Phi Inc., Kanagawa, Japan, 2006) from Physical Electronics, which was sup- with the spectrometer. Shirley background subtraction was used. No flood gun for charge plied with the spectrometer. Shirley background subtraction was used. No flood gun for neutralization was needed. The binding energy was corrected by taking sp2 carbon for a charge neutralization was needed. The binding energy was corrected by taking sp2 carbon reference at 284.4 eV. The sp2 carbon was fitted with an asymmetric function determined for a reference at 284.4 eV. The sp2 carbon was fitted with an asymmetric function deter- on highly-oriented pyrolytic graphite (HOPG) reference. mined on highly-oriented pyrolytic graphite (HOPG) reference. 2.2.2. Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) 2.2.2. Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) Time-of-flight secondary ion mass spectrometry (ToF-SIMS) analyses of the sample depositedTime-of-flight at 60 s of secondary plasma exposure ion mass were spectrom performedetry (ToF-SIMS) using a ToF-SIMS analyses 5of instrumentthe sample (ION-TOF,deposited Münster,at 60 s of Germany). plasma exposure The instrument were performed was equipped using with a ToF-SIMS a bismuth 5 liquid instrument metal ion(ION-TOF, gun with Münster, a kinetic Germany). energy of 30The keV. instrume The analysesnt was wereequipped performed with a in bismuth an ultra-high liquid vacuummetal ion of gun approximately with a kinetic 10 −energy7 Pa. The of 30 SIMS keV. spectra The analyses were measured were performed by scanning in an an ultra- ion beamhigh vacuum of Bi3+ clusters. of approximately The diameter 10−7 Pa. of theThe beam SIMS wasspectra 1 µm, were and measured it was scanned by scanning over anan areaion beam of 500 of× Bi5003+ clusters.µm2. The The negative diameter secondary of the beam ion masswas 1 spectra µm, and were it was measured scanned to over obtain an thearea characteristic of 500 × 500 µm mass2. The fragments. negative Imaging secondary of ion the mass surface spectra was performedwere measured on an to area obtain of 20the× characteristic20 µm2. Depth mass profiling fragments. was alsoImaging performed of the usingsurface Cs was+ ions performed with an energyon an area of 2 of keV 20 to reveal nitrogen distribution in the deposit.

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2.2.3. Raman Spectroscopy Raman spectroscopy was performed using a WITec Alpha 300 RS scanning confocal Raman microscope (WITec GmbH, Lise-Meitner, Germany) in backscattered geometry with a polarized Nd:YAG laser operating at the 532 nm wavelength and a He:Ne laser operating at 633 nm wavelength. The laser beam was focused through a 100×/0.9 microscope objective. The power of the laser was approximately 1 mW.

2.2.4. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) Scanning electron microscopy (SEM) micrographs were acquired using a Schottky field-emission electron microscope Jeol JSM-7600F (Jeol Ltd., Tokyo, Japan). It is a semi in-lens SEM with the adoption of a high-power optics irradiation system that delivers high- resolution, high-speed, and high-accuracy element analysis. SEM analyses were performed without any coating of the samples because they are conductive enough to prevent any charge accumulation during the imaging. A sample was placed onto a stainless steel holder and fixed with adhesive carbon tape. The primary electron beam was rastered on the surface of an area of approximately 6 µm2, and the image was obtained from secondary electrons. The electron kinetic energy was 5 keV and the sample’s working distance (from the pole piece) was 4.5 mm. Transmission electron microscopy (TEM) images were acquired using a transmission electron microscope JEM-2100 (Jeol Ltd., Tokyo, Japan). The sample was scratched directly to the lacey carbon-supported copper grid. Detailed structural investigations of the sample were performed using a conventional 200 kV TEM equipped with a LaB6 electron source.

3. Results and Discussion 3.1. Synthesis Mechanism of N-Doped Carbon Nanomesh Nitrogen plasma at our experimental conditions was rich with nitrogen atoms, ions, as well as with N2 molecules in various rotational and vibrational states. Because of the large density of reactive species and a high vibrational temperature, which is typical for N2 plasma [30], it caused heating and melting of the polymer. The reactive nitrogen species interacted chemically with the molten polymer, forming various molecules containing carbon, nitrogen, and hydrogen. The molecules desorbed from the surface of the molten polymer and entered the gas phase, where they were partially decomposed and ionized upon plasma conditions. Nitrogen-containing carbon fragments condensed on the substrate surface, forming a deposit of carbon nanomesh rich with nitrogen. Such condensation is typical for plasma-enhanced chemical vapor deposition (PECVD) and is explained by a high sticking coefficient of plasma radicals on a surface of a material facing the plasma. A moderately high substrate temperature prevented hydrogenation of the deposited film. The substrate temperature was approximately 800 ◦C in our case. Because of the low thermal capacity of the titanium substrate and high power absorption in plasma in the H-mode, both the substrate and the polymer sample were heated to elevated temperatures within a few seconds after turning on the discharge. Reactive plasma species during deposition of N-doped carbon nanomesh were moni- tored by OES. Figure2 shows a comparison of the OES spectra of N 2 plasma without and with the polymer sample. For plasma without the sample (Figure2a), nitrogen atomic lines, as well as nitrogen molecular bands arising from the 1st and 2nd N2 positive systems, were 3 3 3 + observed. These transitions arise from B Πg and C Πu states and terminate in A ∑u and 3 3 B Πg states, respectively. The potential energy of the basic vibration state for the B Πg and 3 C Πu levels is at approximately 7.4 and 11.1 eV, respectively. The nitrogen molecules are found at a high vibrational temperature in low-pressure plasmas [31], so the spectrum in Figure2a contains numerous features corresponding to transitions from various vibrational states to a lower electronic level of numerous vibrational states. Furthermore, the rotational temperature is not negligible in inductively coupled plasma in the H-mode, which adds to the complexity of the spectrum in Figure2a. Nevertheless, the nitrogen-plasma spectrum qualitatively indicates a good excitation of the states of rather high potential energy. Such Nanomaterials 2021, 11, x FOR PEER REVIEW 5 of 17

molecules are found at a high vibrational temperature in low-pressure plasmas [31], so the spectrum in Figure 2a contains numerous features corresponding to transitions from various vibrational states to a lower electronic level of numerous vibrational states. Fur- Nanomaterials 2021, 11, 837 5 of 16 thermore, the rotational temperature is not negligible in inductively coupled plasma in the H-mode, which adds to the complexity of the spectrum in Figure 2a. Nevertheless, the nitrogen-plasma spectrum qualitatively indicates a good excitation of the states of rather highstates potential are chemically energy. reactive. Such states Furthermore, are chemic clearlyally reactive. distinguished Furthermore, N-atomic clearly lines distin- in the guishednear infrared N-atomic (IR) partlines of in the the spectrum near infrared also indicate (IR) part a significantof the spectrum dissociation also indicate fraction. a Thesig- nificantOES spectrum dissociation during fraction. the carbon The OES nanomesh spectrum deposition during the is shown carbon in nanomesh Figure2b. deposition Although isN-atomic shown in lines Figure and 2b. the Although N2 positive N-atomic systems lines are still and observed, the N2 positive the spectrum systems is are now still domi- ob- served,nated by the the spectrum CN band, is Cnow2 Swan dominated band, as by well the as CN with band, oxygen C2 Swan and hydrogen band, as well atomic as lineswith oxygen(Hα,Hβ and). These hydrogen spectral atomic features lines indicate (Hα, H theβ). partialThese spectral degradation features of the indicate molecules the formedpartial degradationon the surface of of the the molecules molten polymer formed upon on the plasma surface exposure. of the molten The OES, polymer unfortunately, upon plasma does exposure.not allow forThe observing OES, unfortunately, larger molecules. does not Still, allo it givesw for anobserving insight into larger the molecules. plasma chemistry Still, it givesupon an the insight interaction into the of reactive plasma nitrogenchemistry species upon withthe interaction the molten of polymer. reactive Thenitrogen intensive spe- ciesband with arising the frommolten CN polymer. is expected The because intensive HCN band is the arising simplest from molecule CN is expected that is formed because as HCNa consequence is the simplest of the molecule etching ofthat the is organicformed materialas a consequence with nitrogen of the plasma.etching of Furthermore, the organic materialthe triple with bond nitrogen C≡N is plasma. among theFurthermore, strongest chemical the triple bonds bond at C 887≡N is kJ/mol among (9.2 the eV), strongest thus it chemicalis unlikely bonds to break at 887 the bondkJ/mol even (9.2 upon eV), plasmathus it conditions.is unlikely Moreto break interesting the bond is theeven absence upon plasmaof the atomic conditions. carbon More line (whichinteresting should is the appear absence at 248 of the nm) atomic [32] and carbon a rather line intensive(which should Swan appearband, which at 248 is nm) a consequence [32] and a rather of radiative intensive relaxation Swan ofband, C2 dimers. which is The a consequence absence of the of C-line radi- ativeindicates relaxation incomplete of C2 dimers. radicalization The absence of the nitratedof the C-line aromatic indicates hydrocarbons, incomplete which radicalization are likely ofto the be formednitrated upon aromatic nitrogen hydrocarbons, interaction which with hydrocarbons are likely to be at formed high temperatures upon nitrogen [33 ].inter- The actionSwan bandwith hydrocarbons indicates that at a fractionhigh temperatures of the molecules [33]. The formed Swan upon band etching indicates of thethat molten a frac- tionpolymers of the withmolecules nitrogen formed plasma upon is decomposedetching of the to molten the C 2polymersdimers. Therefore, with nitrogen the plasma chemistry enables the partial decomposition of etching products in the gas phase and is decomposed to the C2 dimers. Therefore, the plasma chemistry enables the partial de- compositioncondensation of of etching such products products on in the the substrate gas phase surface. and condensation The substrate’s of such high temperatureproducts on theassures substrate at least surface. partial The dehydrogenization substrate’s high temperature and thus the assures growth at ofleast a film partial consisting dehydro- of genizationN-doped graphene. and thus the growth of a film consisting of N-doped graphene.

(a) (b)

Figure 2. Optical emission spectroscopyspectroscopy (OES)(OES) spectrum spectrum of of nitrogen nitrogen plasma: plasma: (a ()a without) without any any sample, sample, and and (b )(b during) during carbon car- bonnanomesh nanomesh deposition. deposition.

3.2.3.2. Surface Surface Characterization Characterization of of N-Doped N-Doped Carbon Carbon Nanomesh TheThe N-doped N-doped carbon carbon na nanomeshnomesh was deposited at various treatment times between 10 andand 120 120 s. Figure Figure 33 showsshows SEMSEM imagesimages ofof N-dopedN-doped carboncarbon nanomeshnanomesh samples.samples. WeWe cancan seesee alreadyalready that 10 s of exposure to N 2 plasmaplasma was was enough enough to to form form a dense mesh of nanoflakes nanoflakes (Figure(Figure 33a).a). TheThe morphologymorphology ofof nanoflakesnanoflakes diddid notnot changechange muchmuch tilltill approximatelyapproximately 6060 ss ofof plasma deposition deposition time time (Figure (Figure 3b,c).3b,c). At At longer longer deposition deposition times times (Figure (Figure 3d,e),3d,e), changes changes in in surface morphology were observed. Figure3d,e reveal something resembling the clustering of the deposits leading to the formation of gaps of a width of approximately 100 nm. Clustering is even more pronounced after long treatment (Figure3f,g), where some agglomerates are observed. Such agglomerates may be detrimental to the porosity of the deposited material; therefore, it is regarded as an unwanted effect. The agglomeration may appear because of increased surface temperature. Namely, the discharge parameters were independent of the treatment time. The film consisting of N-doped carbon nanomesh has Nanomaterials 2021, 11, x FOR PEER REVIEW 6 of 17

surface morphology were observed. Figure 3d,e reveal something resembling the cluster- ing of the deposits leading to the formation of gaps of a width of approximately 100 nm. Clustering is even more pronounced after long treatment (Figure 3f,g), where some ag- Nanomaterials 2021, 11, 837 glomerates are observed. Such agglomerates may be detrimental to the porosity 6of of the 16 deposited material; therefore, it is regarded as an unwanted effect. The agglomeration may appear because of increased surface temperature. Namely, the discharge parameters were independent of the treatment time. The film consisting of N-doped carbon nanomesh poorhas poor thermal thermal conductivity conductivity because because of the of high the porosity;high porosity; therefore, therefore, the surface the surface temperature temper- mayature increase may increase with increasing with increasing thickness thickness and thus and treatment thus treatment time. time. The thicknessThe thickness of the of depositedthe deposited film offilm N-doped of N-doped carbon carbon nanomesh nano wasmesh measured was measured after several after several treatment treatment times. µ µ Attimes. 20 s ofAt treatment, 20 s of treatment, it was just it was below just 1 belowm, and 1 µm, it increased and it increased to approximately to approximately 3.5 m for 3.5 a minute of treatment time, as revealed from Figure3h. The thickness of the deposit was µm for a minute of treatment time, as revealed from Figure 3h. The thickness of the de- about 5 µm for the longest deposition time of 120 s. Poor increase of the film thickness posit was about 5 µm for the longest deposition time of 120 s. Poor increase of the film between 60 and 120 s of treatment may be a consequence of the agglomeration. thickness between 60 and 120 s of treatment may be a consequence of the agglomeration.

Figure 3. (a–g) Scanning electron microscopy (SEM) images of N-doped carbon nanomesh deposited at various deposition times, and (h) cross-section of the deposited formed at 60 s of treatment.

Additionally to SEM characterization, TEM analyses were also performed to confirm the structure of the carbon nanomesh. For convenience, an example of a TEM image is shown in Figure4 for the samples treated for 60 (Figure4a) and 105 s (Figure4b). Graphene layers can be observed in Figure4 for both treatments. The distance between the NanomaterialsNanomaterials 2021 2021, ,11 11, ,x x FOR FOR PEER PEER REVIEW REVIEW 77 of of 17 17

FigureFigure 3. 3. ( (aa––gg)) Scanning Scanning electron electron microscopy microscopy (SEM) (SEM) im imagesages of of N-doped N-doped carbon carbon nanomesh nanomesh depos- depos- itedited at at various various deposition deposition times, times, and and ( (hh)) cross-section cross-section of of the the deposited deposited formed formed at at 60 60 s s of of treatment. treatment.

Nanomaterials 2021, 11, 837 AdditionallyAdditionally to to SEM SEM characterization, characterization, TEM TEM analyses analyses were were also also performed performed to to confirm confirm7 of 16 thethe structurestructure ofof thethe carboncarbon nanomesh.nanomesh. ForFor coconvenience,nvenience, anan exampleexample ofof aa TEMTEM imageimage isis shownshown inin FigureFigure 44 forfor thethe samplessamples treatedtreated forfor 6060 (Figure(Figure 4a)4a) andand 105105 ss (Figure(Figure 4b).4b). Gra-Gra- phene layers can be observed in Figure 4 for both treatments. The distance between the neighboringphene layers layerscan be of observed graphene in was Figure estimated 4 for both to be treatments. approximately The distance 0.36 nm, between which is the in neighboring layers of graphene was estimated to be approximately 0.36 nm, which is in agreementneighboring with layers other of reportsgraphene [34 was,35]. estimate Despited the to factbe approximately that the SEM image0.36 nm, of thewhich sample is in agreement with other reports [34,35]. Despite the fact that the SEM image of the sample inagreement Figure3f with already other shows reports signs [34,35]. of agglomeration, Despite the fact the that graphene-like the SEM image structure of the was sample still in Figure 3f already shows signs of agglomeration, the graphene-like structure was still preserved.in Figure 3f already shows signs of agglomeration, the graphene-like structure was still preserved.preserved.

Figure 4. Transmission electron microscopy (TEM) image of N-doped graphene showing graphene layers: (a) 60 s of treat- FigureFigure 4.4. TransmissionTransmission electron electron microscopy microscopy (TEM) (TEM) image image of ofN-doped N-doped graphene graphene showing showing graphene graphene layers: layers: (a) 60 (a s) of 60 treat- s of ment, and (b) 105 s of treatment. treatment,ment, and and(b) 105 (b) 105s of streatment. of treatment.

SamplesSamplesSamples were werewere further furtherfurther characterized characterizedcharacterized by byby XPS XPSXPS to to determine determine their their chemical chemical composition. composition. FigureFigureFigure 5 55 shows showsshows an anan example exampleexample of ofof the thethe XPS XPSXPS survey surveysurvey spectrum spectrumspectrum of ofof the thethe sample samplesample deposited depositeddeposited at atat 105 105105 s ss treatmenttreatmenttreatment time, time, showing showingshowing its its surface surfacesurface elemental elementalelemental composition. composition. In In addition additionaddition to toto carbon, carbon,carbon, nitrogen nitrogennitrogen isisis clearly clearlyclearly visible visiblevisible as asas well wellwell as asas some somesome oxygen. oxygen.oxygen. The TheThe detailed detaileddetailed surface surfacesurface composition compositioncomposition of of the thethe N-doped N-dopedN-doped carboncarboncarbon nanomesh nanomesh deposited deposited deposited at at at various various various treatmen treatmen treatmenttt times times times is is shown shown is shown in in Table Table in Table 1. 1. Nitrogen Nitrogen1. Nitrogen con- con- centrationconcentrationcentration in in the the in samples samples the samples increased increased increased with with withplasma plasma plasma treatment treatment treatment time. time. time.AtAt low low At deposition deposition low deposition times, times, nitrogentimes,nitrogen nitrogen concentrationconcentration concentration waswas approximatelyapproximately was approximately 11 atat.%,.%, and 1and at.%, forfor and thethe longest forlongest the longest depositiondeposition deposition time,time, itit increasedtime,increased it increased toto almostalmost to 1515 almost at.%.at.%. For 15For at.%. oxygenoxygen For concconc oxygenentration,entration, concentration, thethe oppositeopposite the trendtrend opposite waswas trend observed;observed; was i.e.,observed;i.e., itit decreaseddecreased i.e., it with decreasedwith increasingincreasing with deposition increasingdeposition time.depositiontime. TheThe resultsresults time. in Thein TableTable results 11 thusthus in Table confirmconfirm1 thus thethe successfulconfirmsuccessful the enrichment enrichment successful of of enrichment carbon carbon nanomesh nanomesh of carbon with with nanomesh nitrogen nitrogen with with increasing increasing nitrogen withtreatment treatment increasing time, time, whereastreatmentwhereas thethe time, oxygenoxygen whereas remainedremained the oxygen withinwithin remained thethe tolerable tolerable within concentration.concentration. the tolerable concentration.

Figure 5. X-ray photoelectron spectroscopy (XPS) survey spectrum of N-doped carbon nanomesh sample with deposition time of 105 s.

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Table 1. Surface composition of as-deposited N-doped carbon nanomesh versus deposition time.

Time C N O N/C O/C (s) (at.%) (at.%) (at.%) (%) (%) 10 91.4 0.8 7.9 0.8 8.6 20 90.6 2.2 7.2 2.4 7.9 30 90.2 1.3 8.5 1.5 9.4 45 91.6 1.4 7.1 1.5 7.7 60 93.8 3.0 3.2 3.2 3.4 75 89.9 8.3 1.8 9.3 2.0 90 88.2 10.2 1.6 11.6 1.8 105 85.6 12.7 1.7 14.8 2.0 120 85.7 12.5 1.7 14.6 2.0

A significant concentration of oxygen on the surface of N-doped graphene was ob- served by all authors, including [11,19,36–38], and was explained by a higher susceptibility of the N-doped graphene to functionalization with oxygen [39]. A feasible explanation for our observation, i.e., decreasing O-concentration and increasing N-concentration, could be in thermal degradation of the PET material. Thermal cleavage of the ester bonds in the PET backbone results in the formation of water vapor and carbon dioxide, leaving the surface rich in conjugated aromatic rings as shown by Holland et al. [29] and Turnbull et al. [40]. The initial stage of plasma treatment, therefore, involves the desorption of gases rich in oxygen (H2O and CO2) which radicalize to highly oxidizing OH and O radicals upon plasma conditions. The initial stage of deposition is therefore governed by excessive oxidizing radicals, which cause significant oxidation of the carbon nanomesh. As the plasma treatment prolongs, the supply of oxidizing radicals is suppressed, so the deposits assume the composition of only 2 at.% of oxygen after about a minute of plasma treatment. Simultaneously, the concentration of nitrogen in the deposits increased because they oc- cupy the binding sites at the edge defects of the graphene structure. Another explanation for decreasing O-concentration with treatment time may also be thermal effects. Kundu et al. [41] have investigated thermal stability of oxygen functional groups on carbon nanos- tructures and found decomposition of carboxylic groups already at 300 ◦C. The authors also found that oxygen concentration decreased from 10.7 to 4.3 at.% when the temperature was increased from 300 to 720 ◦C. Figure6 shows the evolution of high-resolution carbon C1s and nitrogen N1s spectra with deposition time. Figure6a reveals peaks at approximately 289 eV for the lowest treatment times (30 and 45 s), which are not pronounced at longer treatment times. The peak at this binding energy is characteristic of highly oxidized carbon, for example, the O=C-O group. The rather large concentration of oxygen at these short treatment times favors the formation of such a carbon-oxygen functional group with high oxygen content. The absence of the peak at longer treatment times could be explained either by the shortage of oxygen in the gas phase or thermal degradation, as explained in [41]. While the carbon C1s spectra in Figure6a were rather similar, large differences were observed for the nitrogen N1s spectra shown in Figure6b, indicating that the ratio between various nitrogen configurations was changing with treatment time. We can observe two trends. Up to 60 s of treatment, the N1s peaks at high-binding energies were dominant, whereas, at treatment times above 75 s, the peaks at lower binding energy prevailed. It has been reported that nitrogen in graphene usually appears in three typical configurations: pyridinic, pyrrolic, and quaternary (graphitic) nitrogen [12,39,42,43]. Nanomaterials 2021, 11, x FOR PEER REVIEW 9 of 17

Nanomaterials 2021, 11, 837 9 of 16 Nanomaterials 2021, 11, x FOR PEER REVIEW 9 of 17

Figure 6. Evolution of: (a) high-resolution C1s, and (b) N1s spectra of N-doped carbon nanomesh versus deposition time.

While the carbon C1s spectra in Figure 6a were rather similar, large differences were observed for the nitrogen N1s spectra shown in Figure 6b, indicating that the ratio be- tween various nitrogen configurations was changing with treatment time. We can observe two trends. Up to 60 s of treatment, the N1s peaks at high-binding energies were domi-

nant, whereas, at treatment times above 75 s, the peaks at lower binding energy prevailed. FigureFigure 6. 6.Evolution Evolution of: of: ((aa)) high-resolutionhigh-resolutionIt has been reported C1s,C1s, andand that (b) N1s nitrogen spectra in of of graphe N-doped N-dopedne carbon carbonusually na nanomesh nomeshappears versus versusin three deposition deposition typical time.configura- time. tions: pyridinic, pyrrolic, and quaternary (graphitic) nitrogen [12,39,42,43]. WhileInIn FigureFigure the7 carbon7a,ba,b are are C1s examples examples spectra of ofin N1s FigureN1s spectra spec 6atra we with withre rather subcomponents subcomponents similar, large showing showing differences nitrogen nitrogen were observedconfigurationsconfigurations for the forfor nitrogen 6060 andand 9090 N1s ss of spectra treatment, shown respectively.respectively. in Figure TheThe6b, spectraindicatingspectra werewere that fittedfitted the withwithratio four fourbe- tweenpeaks,peaks, various positionedpositioned nitrogen at 398.6,398.6, configurations 400.3,400.3, 401.6,401.6, was andan chd 403.5anging403.5 eV,eV, with whichwhich treatment werewere assigned time.assigned We tocanto pyridinicpyridinic observe twonitrogen,nitrogen, trends. pyrrolicpyrrolic Up to nitrogen, 60nitrogen, s of treatment, graphitic graphitic the nitrogen, nitrog N1s peaksen, and and oxidized at oxidizedhigh-binding nitrogen nitrogen energies groups, groups, respectively.were respec-domi- nant,Whiletively. whereas, graphiticWhile graphitic at nitrogen treatment nitrogen concentration times concentration above 75 did s, notthe did peaks change not atchangenoticeably lower noticeably binding with energy deposition with depositionprevailed. time, Itremarkabletime, has beenremarkable reported variation variation that was nitrogen observed was observed in for graphe pyrrolic forne pyrrolicusually and pyridinic appearsand pyridinic nitrogen in three withnitrogen typical treatment configura-with treat- time. tions:Atment low time.pyridinic, deposition At low pyrrolic timesdeposition when, and timesquaternary nitrogen when concentration nitrogen(graphitic) concentration nitrogen was low, [12,39,42,43]. pyrrolic was low, nitrogen pyrrolic prevailed nitrogen (Figure7a). In contrast, at high deposition times, where a high nitrogen concentration in the prevailedIn Figure (Figure 7a,b 7a). are In examples contrast, ofat highN1s specdepotrasition with times, subcomponents where a high showing nitrogen nitrogen concen- sample was measured, pyridinic nitrogen became dominant (Figure7b). Pyrrolic nitrogen configurationstration in the sample for 60 andwas 90measured, s of treatment, pyridinic respectively. nitrogen became The spectra dominant were (Figurefitted with 7b). fourPyr- is bonded to a hydrogen atom, while pyridinic is free from hydrogen. The evolution of both peaks,rolic nitrogen positioned is bonded at 398.6, to 400.3,a hydrogen 401.6, atom,and 403.5 while eV, pyridinic which wereis free assigned from hydrogen. to pyridinic The configurations, as observed from XPS results, is sound with the reports of other authors nitrogen,evolution pyrrolicof both configurations,nitrogen, graphitic as observed nitrogen, from and XPS oxidized results, nitrogen is sound groups,with the respec-reports mentioned in the Introduction regarding the thermal effects: prolonged treatment times tively.of other While authors graphitic mentioned nitrogen in the concentration Introduction did regarding not change the noticeably thermal effects: with deposition prolonged cause de-hydrogenation because of the higher surface temperature. time,treatment remarkable times cause variation de-hydrogenation was observed beca for pyrrolicuse of the and higher pyridinic surface nitrogen temperature. with treat- ment time. At low deposition times when nitrogen concentration was low, pyrrolic nitrogen 2100 prevailed (Figure 7a). In contrast, at high3400 deposition times, where a high nitrogen concen- tration in the sample was measured, pyridinic nitrogen became dominant (Figure 7b). Pyr- 3100 pyridinic N 2000 rolic nitrogenpyrrolic N is bonded to a hydrogen atom, while pyridinic is free from hydrogen. The evolution of both configurations, as observed2800 from XPS results,pyrrolic is N sound with the reports 1900 pyridinic N of other authors mentioned in the Introduction regarding the thermal effects: prolonged 2500 graphitic N graphitic N 1800 treatment times cause de-hydrogenation because of the higher surface temperature. 2200 2100 pyridinic oxide 3400 pyridinic oxide 1700 Intensity(counts/s) Intensity (counts/s) Intensity 1900 3100 pyridinic N 2000 pyrrolic N 1600 1600 2800 pyrrolic N 1900 pyridinic N 1500 1300 408 406 404 402 400 398 396 394 2500 408 406 graphitic404 N402 400 398 396 394 graphitic N (b) (a) 1800 Binding energy (eV) Binding energy (eV) 2200 Figurepyridinic 7. High-resolution oxide nitrogen spectra N1s for N-doped carbon samplepyridinic treated oxide for (a) 60 s, and (b) 90 s. 1700Figure 7. High-resolution nitrogen spectra N1s for N-doped carbon sample treated for (a) 60 s, and (b) 90 s. Intensity(counts/s) Intensity (counts/s) Intensity 1900

1600 In addition to XPS high-resolution1600 spectra, the first-derivatives of Auger CKLL spectra acquired with the XPS instrument were also examined and are shown in Figure8. The 1500 distance between the minimum and1300 maximum is known as the “D-parameter”. It was 408 406 404 402 400 398 396 394 408 406 404 402 2400 3983 396 394 reported that the D-parameter depends(b) on the ratio between sp and sp carbon configu- (a) Binding energy (eV) Binding energy (eV) rations, being larger for sp2 than for sp3 [44,45]. According to the literature, the reported Figure 7. High-resolutionD-parameter nitrogen spectra was D N1s≈ 21 for and N-doped 14 eV carbon for graphite sample and treated diamond, for (a) 60 respectively s, and (b) 90 [ 44s. ,45]. How- ever, we should also note that this approach may be uncertain because the determination of the exact position of the minimum and maximum in the Auger CKLL spectrum is not completely reliable. Nevertheless, in our case, we used reference materials such as HOPG and a diamond powder and obtained values of D ≈ 19.4 and 13.7 eV, respectively. For N-

Nanomaterials 2021, 11, x FOR PEER REVIEW 10 of 17

In addition to XPS high-resolution spectra, the first-derivatives of Auger CKLL spec- tra acquired with the XPS instrument were also examined and are shown in Figure 8. The distance between the minimum and maximum is known as the “D-parameter”. It was reported that the D-parameter depends on the ratio between sp2 and sp3 carbon configu- rations, being larger for sp2 than for sp3 [44,45]. According to the literature, the reported D-parameter was D ≈ 21 and 14 eV for graphite and diamond, respectively [44,45]. How- ever, we should also note that this approach may be uncertain because the determination Nanomaterials 2021, 11, 837 of the exact position of the minimum and maximum in the Auger CKLL spectrum10 is ofnot 16 completely reliable. Nevertheless, in our case, we used reference materials such as HOPG and a diamond powder and obtained values of D ≈ 19.4 and 13.7 eV, respectively. For N- doped carbon nanomesh,nanomesh, thethe D-parameterD-parameter for for short short deposition deposition times times up up to to 60 60 s wass was similar simi- larto HOPG, to HOPG, i.e., i.e., D ≈ D18.8, ≈ 18.8, whereas whereas for longerfor longer deposition deposition times, times, it decreased it decreased to approximately to approxi- matelyD ≈ 16.8. D ≈ This 16.8. observation This observation indicates indicates a decrease a decrease of sp2 contentof sp2 content with increasing with increasing deposition dep- ositiontimes. Bytimes. considering By considering SEM images SEM images of Figure of Figure3, one may3, one conclude may conclude that the that agglomerates the agglom- eratesobserved observed in Figure in Figure3f,g are 3f,g rich are in sprich3. in sp3.

Figure 8. TheThe first first derivative derivative of of Auger Auger CKLL CKLL spectra spectra of of N-doped N-doped carbon carbon nanomesh nanomesh deposited deposited at at various times and of highly-orientedhighly-oriented pyrolytic graphite (HOPG)(HOPG) andand diamonddiamond referencereference samples.samples.

To further investigate investigate the the nature nature of of carbon carbon configuration, configuration, an an attempt attempt was was made made to fit to thefit thecarbon carbon C1s C1sspectra spectra shown shown in Figure in Figure 6a. Because6a. Because the asymmetric the asymmetric shape shapeline is linecharac- is teristiccharacteristic of graphene of graphene [43,46,47], [43, 46the,47 spectra], the spectra were fitted were with fitted the with asymmetric the asymmetric peak previ- peak 2 ouslypreviously acquired acquired using usingthe HOPG the HOPG reference. reference. It is known It is knownthat the thatmain the sp2 main carbon sp peakcarbon for graphenepeak for graphene domains domainsis positioned is positioned at 284.4 eV at 284.4[48,49]. eV The [48 ,spectrum49]. The spectrum of graphene of graphene also dis- π π playsalso displays several severalpeaks caused peaks causedby energy by energylosses because losses because of π-π of* excitation,- * excitation, which which are posi- are tionedpositioned on the on high the highbinding binding energy energy side and side ex andtend extend several several eV from eV approximately from approximately 289 eV 289 eV onward [50]. In our case, these π-π* features were fitted with one broad peak. onward [50]. In our case, these π-π* features were fitted with one broad peak. Defects Defects induced to the graphene structure during doping cause a change in the line shape induced to the graphene structure during doping cause a change in the line shape on3 both on both high- and low-binding energy sides. As commonly adopted in literature,3 sp C is high- and low-binding energy sides. As commonly adopted2 in literature, sp C is posi- positioned up to about 1 eV at higher binding energy than2 sp . It was also reported that at tioned up to about 1 eV at higher binding energy than sp . It was also reported that2 at the the same time, another peak Cdis corresponding to the introduction of defects in sp carbon same time, another peak Cdis corresponding to the introduction of defects in sp2 carbon could be observed at lower binding energies of approximately 284.0 eV [11,47,50,51]. In could be observed at lower binding energies of approximately 284.0 eV [11,47,50,51]. In the range between 285 and 289 eV, peaks arising from carbon-nitrogen and carbon-oxygen the range between 285 and 289 eV, peaks arising from carbon-nitrogen and carbon-oxygen bonds appear and partially overlap. For N-doped samples, peaks corresponding to sp2 bonds appear and partially overlap. For N-doped samples, peaks corresponding to sp2 Nanomaterials 2021, 11, C=Nx FOR PEER and spREVIEW3 C-N should appear at approximately 285.5–286.3 eV and 286.9–288.3 eV, 11 of 17 C=N and sp3 C-N should appear at approximately 285.5–286.3 eV and 286.9–288.3 eV, re- respectively [13,48,52–54]. By considering all these facts, the C1s peak was fitted with spectively [13,48,52–54]. By considering all these facts,2 the C1s peak was3 fitted with2 six six subpeaks, which correspond to Cdis (peak 1), sp C (peak 2), sp C and sp C-N subpeaks, which correspond to Cdis (peak 1), sp2 C (peak 2), sp3 C and sp2 C-N (peak 3), C- (peak 3), C-O,O, and and sp sp33 C-NC-N (peak (peak 4), 4), C=O and and (peak(peak 5), 5), O=C-O O=C-O (peak (peak 6), 6), and and π-π* excitation. excitation.Two Two examples examples are are shown shown in in Figure Figure 9.9. Figure Figure 9a9a shows shows an an example example of of the the C1s peak for C1s peak forshort short deposition deposition times, times, whereas whereas Figure Figure 9b9 bfor for long long deposition deposition times. times. The The results for all

results forsamples all samples obtained obtained from from fitting fitting as shown as shown in Figure in Figure 9 are9 summarized are summarized in Figure in 10. Figure Figure 10. Figure10 shows 10 shows the evolution the evolution of functional of functional groups groups with withdeposition deposition time. time. Here Herewe should also we shouldnote also notethat overlapping that overlapping of the of O=C-O the O=C-O peak peakat ~289 at eV ~289 with eV the with π- theπ* peakπ-π* brings peak some un- brings somecertainties uncertainties in determining in determining the corresponding the corresponding concentrations. concentrations. In Figure In Figure 10, we 10 can, observe 2 we can observea decreasing a decreasing concentration concentration of sp of2 carbon sp carbon with withdeposition deposition time, time, which which is in agreement is in with agreement withthe results the results of the of D-parameter the D-parameter shown shown in Figure in Figure 8. We8. We can can also also observe observe the the increasing in- 3 increasing intensitytensity of of peaks peaks related related to to sp sp3 andand nitrogen nitrogen content. content. The The detailed detailed analyses analyses of ofthe C1s peak, therefore, support the conclusion brought on the basis of the Auger CKLL spectra. Here, the overlapping of the oxygen and nitrogen-containing functional groups should be stressed again. For example, the functionality marked as “C-O and sp3 C-N” is likely to be almost free from oxygen at high deposition times.

Figure 9. Selected C1s spectra of as-deposited N-doped carbon nanomesh samples. The deposition times were: (a) 30 s, and (b) 120 s.

Figure 10. Evolution of chemical structure with deposition time.

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O, andO, and sp3 spC-N3 C-N (peak (peak 4), 4),C=O C=O and and (peak (peak 5), 5),O=C-O O=C-O (peak (peak 6), 6),and and π-π π*- πexcitation.* excitation. TwoTwo examples examples are are shown shown in Figurein Figure 9. Figure9. Figure 9a shows9a shows an anexample example of theof the C1s C1s peak peak for for shortshort deposition deposition times, times, whereas whereas Figure Figure 9b 9bfor forlong long deposition deposition times. times. The The results results for forall all samplessamples obtained obtained from from fitting fitting as shownas shown in Figurein Figure 9 are 9 are summarized summarized in Figurein Figure 10. 10.Figure Figure 10 shows10 shows the the evolution evolution of functionalof functional groups groups with with deposition deposition time. time. Here Here we weshould should also also notenote that that overlapping overlapping of theof the O=C-O O=C-O peak peak at ~289at ~289 eV eVwith with the the π-π π*- peakπ* peak brings brings some some un- un- certaintiescertainties in determining in determining the the corresponding corresponding concentrations. concentrations. In FigureIn Figure 10, 10,we wecan can observe observe Nanomaterials 2021, 11, 837 11 of 16 a decreasinga decreasing concentration concentration of sp of2 spcarbon2 carbon with with deposition deposition time, time, which which is in is agreement in agreement with with thethe results results of theof the D-parameter D-parameter shown shown in Figurein Figure 8. We 8. We can can also also observe observe the the increasing increasing in- in- tensitytensity of peaks of peaks related related to sp to3 spand3 and nitrogen nitrogen content. content. The The detailed detailed analyses analyses of the of the C1s C1s peak, peak, therefore,thetherefore, C1s peak, support support therefore, the the conclusion support conclusion thebrought brought conclusion on onthe broughtthe basis basis of on theof thethe Auger basisAuger CKLL of CKLL the spectra. Auger spectra. CKLLHere, Here, thespectra.the overlapping overlapping Here, the of overlapping theof the oxygen oxygen of and the and oxygennitrogen-c nitrogen-c andontaining nitrogen-containingontaining functional functional groups functional groups should should groups be be stressedshouldstressed beagain. stressed again. For For example, again. example, For the example, the functionality functionality the functionality marked marked as “C-O markedas “C-O and asand sp “C-O3 spC-N”3 C-N” and is splikely is3 likelyC-N” to be to is be almostlikelyalmost tofree be free from almost from oxygen free oxygen from at high at oxygen high deposition deposition at high times. deposition times. times.

FigureFigureFigure 9. Selected 9. Selected C1s C1s spectraspectra spectra ofof as-deposited as-deposited of as-deposited N-doped N-doped N-doped carbon carbon carbon nanomesh nanomesh nanomesh samples. samples. samples. The The deposition The deposition deposition times times were:times were:( awere:) 30(a) s, 30(a and) s,30 s, and (b) 120 s. (b)and 120 s.(b) 120 s.

FigureFigureFigure 10. 10. Evolution10.Evolution Evolution of of chemical chemicalof chemical structure structure structure with with with deposition deposition deposition time. time. time.

ToF-SIMS measurements support the XPS results. Figure 11a shows the SIMS spectrum of negative ions of the selected sample treated for 60 s. This sample was chosen because of the highest content of the pyrrolic nitrogen configuration and because of the uniform deposit with a lack of any agglomeration, as revealed by SEM (Figure3c). Figure 11a reveals characteristic peaks belonging to carbon-containing fragments from graphene (C−, − − − − − − − C2 ,C3 ,C4 ) as well as fragments containing nitrogen (CN ,C2N ,C3N ,C5N ) and oxygen (CNO−). Depth profiling was also performed to investigate the distribution of nitrogen within the sample. The depth profiles are shown in Figure 11b. The variation of

characteristic fragments is shown versus sputtering time. The sputtering rate was roughly estimated at 0.2 nm/s. We can see that nitrogen is quite uniformly distributed in the sample − because the signal is rather stable, and only a minor decrease in the intensity of the C3N is observed. However, such a decrease can also be an artifact because of the matrix effects typical for the SIMS technique. Nanomaterials 2021, 11, x FOR PEER REVIEW 12 of 17

ToF-SIMS measurements support the XPS results. Figure 11a shows the SIMS spec- trum of negative ions of the selected sample treated for 60 s. This sample was chosen be- cause of the highest content of the pyrrolic nitrogen configuration and because of the uni- form deposit with a lack of any agglomeration, as revealed by SEM (Figure 3c). Figure 11a reveals characteristic peaks belonging to carbon-containing fragments from graphene (C−, C2−, C3−, C4−) as well as fragments containing nitrogen (CN−, C2N−, C3N−, C5N−) and oxygen (CNO−). Depth profiling was also performed to investigate the distribution of nitrogen within the sample. The depth profiles are shown in Figure 11b. The variation of charac- teristic fragments is shown versus sputtering time. The sputtering rate was roughly esti- mated at 0.2 nm/s. We can see that nitrogen is quite uniformly distributed in the sample − Nanomaterials 2021, 11, 837 because the signal is rather stable, and only a minor decrease in the intensity of the12 ofC3 16N is observed. However, such a decrease can also be an artifact because of the matrix effects typical for the SIMS technique.

FigureFigure 11. 11.( a()a Secondary) Secondary ion ion mass mass spectrometry spectrometry (SIMS) (SIMS) negative negative ion ion spectrum spectrum of of the the N-doped N-doped carbon carbon nanomesh nanomesh sample sample (deposition(deposition time time 60 60 s) s) and and (b ()b SIMS) SIMS depth depth profile profile of of this this sample. sample.

Furthermore,Furthermore, carboncarbon isis known as a a material material with with a avery very low low sputtering sputtering yield. yield. In Inad- addition,dition, surface surface topography topography has has a significant a significant effe effectct on onthe thesputtering sputtering yield yield because because of vari- of variousous alignments alignments of ofnanowalls nanowalls an and,d, thus, thus, different different incidence incidence an anglesgles of impinging ionsions [55[55].]. ItIt can can also also be be expected expected that that the the sputtering sputtering yield yield may may change change during during the the sputtering sputtering of of the the sample.sample. AllAll these effects, therefore, therefore, cause cause diffic difficultiesulties in in the the interpretation interpretation of of the the SIMS SIMS re- results.sults. Some Some variation variation of of the the signal signal observed observed just just at at the the surface surface after after starting starting the the etching etching is isprobably probably because because of of surface-induced surface-induced effects. effects. Additionally, Additionally, we we show show in inFigure Figure 12 12 two two di- dimensionalmensional SIMS SIMS mapping mapping of of the the surface of thethe N-dopedN-doped carboncarbon nanomesh.nanomesh. FigureFigure 12 12aa − − − − − − illustratesillustrates the the distribution distribution of theof the carbon carbon fragments fragments (sum (sum of C of, CH C−, CH,C2−, ,CC2−2,H C2H,C−,3 C,C3−, 4C4,−, − CC4H4H−),), whereas FigureFigure 1212bb showsshows thethe distributiondistribution ofof the the nitrogen-containing nitrogen-containing fragments fragments − − − − − (sum(sum of of CN CN,C−, C2N2N−, CNOCNO−,,C C33NN−, C,C5N5N−). This). This is isopposite opposite to to the the results results of LuoLuo etet al.,al., who who foundfound non-uniform non-uniform nitrogen nitrogen distribution distribution for for the the sample sample synthesized synthesized by by CVD CVD in in NH NH3/He3/He Nanomaterials 2021, 11, x FOR PEER REVIEW 13 of 17 combinedcombined with with C C2H2H44/H/H2 gas [56], [56], thethe surfacesurface inin ourour casecase is is very very homogenous homogenous indicating indicating uniformuniform nitrogen nitrogen distribution. distribution.

20 2 ∙10 20 ∙102

3.4 16 2.4 16

3.2 2.2 12 12 3.0

2.0 8 8 2.8

1.8 2.6 4 4 2.4 1.6

0 0 μm 0 10 20 μm 0 10 20 13 Sum of: C-, CH-, C2-, C2H-, C3-, C4-, C4H- Sum of: CN-, CN-, C2N-, CNO-, C3N-, C5N- (a) (b)

FigureFigure 12. 12.2D 2D SIMSSIMS imageimage of of N-dopedN-doped carbon carbon nanomesh nanomesh surface: surface: ( a(a)) distribution distribution of of carbon carbon fragments, fragments, and and ( b(b)) distribution distribution ofof nitrogen-containing nitrogen-containing fragments. fragments. The The deposition deposition time time was was 60 60 s. s.

RamanRaman spectra of of selected selected samples samples are are shown shown in inFigure Figure 13a. 13 Thea. Themain main D and D G and bands G bandsare ploted are ploted in more in detail more detailin Figure in Figure 13b. As 13 ab. reference, As a reference, we also we show also showin Figure in Figure 14 an 14ex- anample example of pure of pureundoped undoped carbon carbon nanomesh nanomesh deposited deposited at similar at similar plasma plasma conditions. conditions. The TheRaman Raman spectrum spectrum of the of reference the reference undoped undoped carbon carbon nanomesh nanomesh (Figure (Figure14a) shows 14a) a showstypical aG typical band, characteristic G band, characteristic for crystalline for crystalline graphite (sp graphite2 carbon (sp domains),2 carbon and domains), a D band, and char- a D acteristic for disordered structure, i.e., graphene edges. The G band peak is accompanied by a shoulder corresponding to D` band, as shown in detail in Figure 14b. The D and D` bands are associated with the presence of the edges of graphene nanowalls and defects and thus also with more nanocrystalline structures [57]. In Figure 14a, we can also observe their combination modes and second-order bands: D + D’’, 2D, D + D’, and 2D’.

Figure 13. (a) Comparison of Raman spectra of N-doped carbon nanomesh samples, (b) a detail showing a D’ band.

Nanomaterials 2021, 11, x FOR PEER REVIEW 13 of 17

20 2 ∙10 20 ∙102

3.4 16 2.4 16

3.2 2.2 12 12 3.0

2.0 8 8 2.8

1.8 2.6 4 4 2.4 1.6

0 0 μm 0 10 20 μm 0 10 20 13 Sum of: C-, CH-, C2-, C2H-, C3-, C4-, C4H- Sum of: CN-, CN-, C2N-, CNO-, C3N-, C5N- (a) (b) Figure 12. 2D SIMS image of N-doped carbon nanomesh surface: (a) distribution of carbon fragments, and (b) distribution of nitrogen-containing fragments. The deposition time was 60 s.

Raman spectra of selected samples are shown in Figure 13a. The main D and G bands Nanomaterials 2021, 11, 837 are ploted in more detail in Figure 13b. As a reference, we also show in Figure 14 13an of ex- 16 ample of pure undoped carbon nanomesh deposited at similar plasma conditions. The Raman spectrum of the reference undoped carbon nanomesh (Figure 14a) shows a typical band,G band, characteristic characteristic for for disordered crystalline structure, graphite i.e.,(sp2 graphenecarbon domains), edges. Theand Ga D band band, peak char- is accompaniedacteristic for disordered by a shoulder structure, corresponding i.e., graphene to D‘ edges. band, The as shown G band in peak detail is inaccompanied Figure 14b. Theby a D shoulder and D‘ bandscorresponding are associated to D` withband, the as presenceshown in of detail the edges in Figure of graphene 14b. The nanowalls D and D` andbands defects are associated and thus with also withthe presence more nanocrystalline of the edges of structures graphene [ 57nanowalls]. In Figure and 14 defectsa, we canand alsothus observealso with their more combination nanocrystalline modes struct andures second-order [57]. In Figure bands: 14a, Dwe + can D”, also 2D, observe D + D’, andtheir 2D’. combination modes and second-order bands: D + D’’, 2D, D + D’, and 2D’.

Nanomaterials 2021, 11, x FOR PEER REVIEW 14 of 17

FigureFigure 13.13. ((aa)) ComparisonComparison ofof RamanRaman spectraspectra ofof N-dopedN-doped carboncarbon nanomeshnanomesh samples,samples, ((bb)) aa detaildetail showingshowing aa D’D’ band.band.

FigureFigure 14.14. ((aa)) RamanRaman spectraspectra ofof aa referencereference undopedundoped carboncarbon nanomesh,nanomesh, ((bb)) a a detail detail showing showing a a D’ D’ band. band.

TheThe RamanRaman spectra spectra of of N-doped N-doped carbon carbon nanomesh nanomesh (Figure (Figure 13) considerably13) considerably differ differ from Figurefrom Figure 14. In 14. Figure In Figure 13 we 13 can we observe can observe a significant a significant broadening broadening of all of peaks. all peaks. Furthermore, Further- themore, intensity the intensity of the 2D of bandthe 2D is muchband is lower much for lower the case for ofthe N-doped case of thanN-doped pristine than materials. pristine Thematerials. D’ band The cannot D` band be cannot distinguished be distinguis anymorehed fromanymore the Gfrom band, the exceptG band, for except the sample for the treatedsample fortreated 60 s for (Figure 60 s (Figure 13b). The 13b). spectrum The spectrum in Figure in Figure 13 is 13 similar is similar to those to those observed observed by Manojkumarby Manojkumar et al. et [al.58 ][58] for for the the case case of nitrogenof nitrogen implanted implanted vertical vertical graphene graphene nanowalls. nanowalls. TheThe broadening of Raman Raman spectra spectra can can be be expl explainedained by by amorphization, amorphization, as asreported reported by + byLucchese Lucchese et al. et [59]. al. [ 59Lucchese]. Lucchese [59] expo [59]sed exposed graphene graphene to various to various doses dosesof Ar+ ofions Ar andions in- andvestigated investigated the evolution the evolution of Raman of spectra. Raman spectra.They found They that found the intensity that the of intensity the disordered of the 14 −2 disorderedD band increased D band with increased an increasing with an dose; increasing however, dose; at a however, dose of 10 at14 acm dose−2, the of intensity 10 cm of, thethe intensityD band decreased of the D band and decreasedsignificantly and broadened, significantly which broadened, was explained which wasby amorphiza- explained bytion amorphization and partial sputtering and partial of sputteringthe sample ofand the thus sample more and structural thus more disorder. structural From disorder. Figure From13 we Figurecan thus 13 conclude we can thus that concludeour N-doped that carbon our N-doped nanomesh carbon has a nanomesh higher degree has aof higher amor- degreephization of amorphizationthan undoped thancarbon undoped nanomesh. carbon nanomesh. In Figure 13 we can also notice that the curve for the sample treated for 60 s is the narrowest. This can be explained by the time evolution of the carbon nanomesh growth. As revealed by SEM, after the initial formation of carbon nanomesh, the most uniform deposit is formed at 60 s of treatment before agglomerates begin to appear with the further increase of treatment time

4. Conclusions A method for direct plasma synthesis of N-doped carbon nanomesh was presented. The N-doped carbon nanomesh was deposited on Ti substrates by a PECVD method using nitrogen plasma and molten polymer as a carbon source. Nitrogen radicals reacted with the polymer, causing its melting and desorption of nitrogen-containing carbon fragments on the Ti substrate. This effect enabled the formation of a layer of N-doped carbon na- nomesh on the substrate. The deposition rate depended on the evolution of the film, but on average, it was approximately 3 µm/min. The nitrogen concentration in the deposit increased with deposition time from approximately 1 to almost 15 at.%. However, as shown by various characterization techniques including SEM, TEM, Raman, and XPS, the quality of the deposited N-doped carbon nanomesh was better at a deposition time of 60 s as long as the merit is the uniform surface morphology and high sp2/sp3 ratio. If the nitrogen content is the merit, longer deposition time is better; however, one must bear in mind that the sp2/sp3 ratio is lower. With longer treatment time the predominant nitrogen configuration also changed from pyrrolic to pyridinic and some agglomerates were formed. The concentration of graphitic nitrogen was more or less independent of treat- ment time.

Nanomaterials 2021, 11, 837 14 of 16

In Figure 13 we can also notice that the curve for the sample treated for 60 s is the narrowest. This can be explained by the time evolution of the carbon nanomesh growth. As revealed by SEM, after the initial formation of carbon nanomesh, the most uniform deposit is formed at 60 s of treatment before agglomerates begin to appear with the further increase of treatment time

4. Conclusions A method for direct plasma synthesis of N-doped carbon nanomesh was presented. The N-doped carbon nanomesh was deposited on Ti substrates by a PECVD method using nitrogen plasma and molten polymer as a carbon source. Nitrogen radicals reacted with the polymer, causing its melting and desorption of nitrogen-containing carbon fragments on the Ti substrate. This effect enabled the formation of a layer of N-doped carbon nanomesh on the substrate. The deposition rate depended on the evolution of the film, but on average, it was approximately 3 µm/min. The nitrogen concentration in the deposit increased with deposition time from approximately 1 to almost 15 at.%. However, as shown by various characterization techniques including SEM, TEM, Raman, and XPS, the quality of the deposited N-doped carbon nanomesh was better at a deposition time of 60 s as long as the merit is the uniform surface morphology and high sp2/sp3 ratio. If the nitrogen content is the merit, longer deposition time is better; however, one must bear in mind that the sp2/sp3 ratio is lower. With longer treatment time the predominant nitrogen configuration also changed from pyrrolic to pyridinic and some agglomerates were formed. The concentration of graphitic nitrogen was more or less independent of treatment time.

Author Contributions: Conceptualization, R.Z. and G.P.; methodology, R.Z.; validation, R.Z., G.P., and A.V.; formal analysis, G.P., and L.P. investigation, R.Z. and A.V.; data curation, R.Z., G.P., and A.V.; writing—original draft preparation, A.V.; writing—review and editing, M.M.; visualization, R.Z.; supervision, M.M.; project administration, A.V.; funding acquisition, A.V. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Slovenian Research Agency—project No. L2-1834 (Carbon nanowalls for future supercapacitors), core fund P2-0082 (Thin-film structures and plasma surface engineering), and P1-0099 (Physics of soft matter, surfaces, and nanostructures). The research was also supported by a bilateral project to co-fund international bilateral scientific cooperation with the USA (Project No. BI-US/18-20-023, Advanced catalysts based on multilayered vertically oriented graphene nanostructures). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Informed consent was obtained from all subjects involved in the study. Data Availability Statement: Data is contained within the article. Acknowledgments: We thank Sandra Drev for TEM analysis and Jernej Ekar for SIMS analysis. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

References 1. Joucken, F.; Henrard, L.; Lagoute, J. Electronic properties of chemically doped graphene. Phys. Rev. Mater. 2019, 3, 110301. [CrossRef] 2. Tison, Y.; Lagoute, J.; Repain, V.; Chacon, C.; Girard, Y.; Rousset, S.; Joucken, F.; Sharma, D.; Henrard, L.; Amara, H.; et al. Electronic Interaction between Nitrogen Atoms in Doped Graphene. ACS Nano 2015, 9, 670–678. [CrossRef][PubMed] 3. Joucken, F.; Tison, Y.; le Fèvre, P.; Tejeda, A.; Taleb-Ibrahimi, A.; Conrad, E.; Repain, V.; Chacon, C.; Bellec, A.; Girard, Y.; et al. Charge transfer and electronic doping in nitrogen-doped graphene. Sci. Rep. 2015, 5, 14564. [CrossRef] 4. Joucken, F.; Tison, Y.; Lagoute, J.; Dumont, J.; Cabosart, D.; Zheng, B.; Repain, V.; Chacon, C.; Girard, Y.; Botello-Méndez, A.R.; et al. Localized state and charge transfer in nitrogen-doped graphene. Phys. Rev. B 2012, 85, 161408. [CrossRef] 5. Hu, M.; Lv, Q.; Lv, R. Controllable synthesis of nitrogen-doped graphene oxide by tablet-sintering for efficient lithi-um/sodium- ion storage. ES Energ. Environ. 2019, 3, 45–54. [CrossRef] Nanomaterials 2021, 11, 837 15 of 16

6. Wang, X.; Zeng, X.; Cao, D. Biomass-derived nitrogen-doped porous carbons (NPC) and NPC/ polyaniline composites as high performance supercapacitor materials. Eng. Sci. 2018, 1, 55–63. [CrossRef] 7. Lv, Y.; Zhu, L.; Xu, H.; Yang, L.; Liu, Z.; Cheng, D.; Cao, X.; Yun, J.; Cao, D. Core/shell Template-derived Co, N-doped Carbon Bifunctional Electrocatalysts for Rechargeable Zn-air Battery. Eng. Sci. 2019, 7, 26–37. [CrossRef] 8. Zhang, L.; Xia, Z. Mechanisms of Oxygen Reduction Reaction on Nitrogen-Doped Graphene for Fuel Cells. J. Phys. Chem. C 2011, 115, 11170–11176. [CrossRef] 9. Zhang, C.; Xie, Y.; Deng, H.; Zhang, C.; Su, J.-W.; Lin, J. Nitrogen Doped Coal with High Electrocatalytic Activity for Oxygen Reduction Reaction. Eng. Sci. 2019, 8, 39–45. [CrossRef] 10. Yu, H.; Yang, L.; Cheng, D.; Cao, D. Zeolitic-imidazolate framework (ZIF)@ZnCo-ZIF core-shell template derived Co, N-doped carbon catalysts for oxygen reduction reaction. Eng. Sci. 2018, 3, 54–61. [CrossRef] 11. Evlashin, S.A.; Maksimov, Y.M.; Dyakonov, P.V.; Pilevsky, A.A.; Maslakov, K.I.; Mankelevich, Y.A.; Voronina, E.N.; Vavilov, S.V.; Pavlov, A.A.; Zenova, E.V.; et al. N-Doped Carbon NanoWalls for Power Sources. Sci. Rep. 2019, 9, 6716. [CrossRef][PubMed] 12. Bundaleska, N.; Henriques, J.; Abrashev, M.; Rego, A.M.B.D.; Ferraria, A.M.; Almeida, A.; Dias, F.M.; Valcheva, E.; Arnaudov, B.; Upadhyay, K.K.; et al. Large-scale synthesis of free-standing N-doped graphene using microwave plasma. Sci. Rep. 2018, 8, 12595. [CrossRef][PubMed] 13. Wei, D.; Liu, Y.; Wang, Y.; Zhang, H.; Huang, L.; Yu, G. Synthesis of N-Doped Graphene by Chemical Vapor Deposition and Its Electrical Properties. Nano Lett. 2009, 9, 1752–1758. [CrossRef][PubMed] 14. Xu, H.; Ma, L.; Jin, Z. Nitrogen-doped graphene: Synthesis, characterizations and energy applications. J. Energy Chem. 2018, 27, 146–160. [CrossRef] 15. Wang, H.; Maiyalagan, T.; Wang, X. Review on Recent Progress in Nitrogen-Doped Graphene: Synthesis, Characterization, and Its Potential Applications. ACS Catal. 2012, 2, 781–794. [CrossRef] 16. Kwon, O.S.; Park, S.J.; Hong, J.-Y.; Han, A.-R.; Lee, J.S.; Lee, J.S.; Oh, J.H.; Jang, J. Flexible FET-Type VEGF Aptasensor Based on Nitrogen-Doped Graphene Converted from Conducting Polymer. ACS Nano 2012, 6, 1486–1493. [CrossRef] 17. Lin, Z.; Waller, G.H.; Liu, Y.; Liu, M.; Wong, C.-P. 3D Nitrogen-doped graphene prepared by pyrolysis of graphene oxide with polypyrrole for electrocatalysis of oxygen reduction reaction. Nano Energy 2013, 2, 241–248. [CrossRef] 18. Sun, L.; Wang, L.; Tian, C.; Tan, T.; Xie, Y.; Shi, K.; Li, M.; Fu, H. Nitrogen-doped graphene with high nitrogen level via a one-step hydrothermal reaction of graphene oxide with urea for superior capacitive energy storage. RSC Adv. 2012, 2, 4498–4506. [CrossRef] 19. Jeong, H.M.; Lee, J.W.; Shin, W.H.; Choi, Y.J.; Shin, H.J.; Kang, J.K.; Choi, J.W. Nitrogen-Doped Graphene for High-Performance Ultracapacitors and the Importance of Nitrogen-Doped Sites at Basal Planes. Nano Lett. 2011, 11, 2472–2477. [CrossRef] 20. Zhang, L.; Qing, X.; Chen, Z.; Wang, J.; Yang, G.; Qian, Y.; Liu, D.; Chen, C.; Wang, L.; Lei, W. All pseudocapacitive nitrogen-doped reduced graphene oxide and polyaniline nanowire network for high-performance flexible on-chip en-ergy storage. ACS Appl. Energy Mater. 2020, 3, 6845–6852. [CrossRef] 21. Kumar, N.A.; Nolan, H.; McEvoy, N.; Rezvani, E.; Doyle, R.L.; Lyons, M.E.G.; Duesberg, G.S. Plasma-assisted simul-taneous reduction and nitrogen doping of graphene oxide nanosheets. J. Mater. Chem. A 2013, 1, 4431–4435. [CrossRef] 22. Vesel, A.; Zaplotnik, R.; Primc, G.; Mozetiˇc,M. A review of strategies for the synthesis of N-doped graphene-like ma-terials. Nanomaterials 2020, 10, 2286. [CrossRef][PubMed] 23. Lai, L.-H.; Shiue, S.-T. Effects of acetylene/ammonia mixtures on the properties of carbon films prepared by thermal chemical vapor deposition. Surf. Coat. Technol. 2013, 215, 161–169. [CrossRef] 24. Prioli, R.; Zanette, S.I.; Caride, A.O.; Franceschini, D.F. Atomic force microscopy of amorphous hydrogenated carbon–nitrogen films deposited by radio-frequency-plasma decomposition of methane–ammonia gas mixtures. J. Vacuum Sci. Technol. A 1996, 14, 2351–2355. [CrossRef] 25. Fu, Y.B.; Chen, Q.; Xu, W.C. Effects of ammonia on the growth of carbon nanotubesby plasma synthesis. Beijing Ligong Daxue Xuebao/Trans. Beijing Inst. Technol. 2009, 29, 146–148. 26. Bell, M.S.; Teo, K.B.K.; Lacerda, R.G.; Milne, W.I.; Hash, D.B.; Meyyappan, M. Carbon nanotubes by plasma-enhanced chemical vapor deposition. Pure Appl. Chem. 2006, 78, 1117–1125. [CrossRef] 27. Guo, Z.; Yi, Y.; Wang, L.; Yan, J.; Guo, H. Pt/TS-1 Catalyst Promoted C–N Coupling Reaction in CH4–NH3 Plasma for HCN Synthesis at Low Temperature. ACS Catal. 2018, 8, 10219–10224. [CrossRef] 28. Denysenko, I.B.; von Wahl, E.; Mikikian, M.; Berndt, J.; Ivko, S.; Kersten, H.; Kovacevic, E.; Azarenkov, N. Plasma properties as function of time in Ar/C2H2 dust-forming plasma. J. Phys. D Appl. Phys. 2019, 53, 135203. [CrossRef] 29. Holland, B.; Hay, J. The thermal degradation of PET and analogous polyesters measured by thermal analysis–Fourier transform infrared spectroscopy. Polymer 2002, 43, 1835–1847. [CrossRef] 30. Sirse, N.; Harvey, C.; Gaman, C.; Ellingboe, A.R. Investigation of plasma uniformity, rotational and vibrational tem-perature in a 162 MHz multi-electrode capacitive discharge. J. Phys. D Appl. Phys. 2020, 53, 335203. [CrossRef] 31. Fantz, U.; Briefi, S.; Rauner, D.; Wünderlich, D. Quantification of the VUV radiation in low pressure hydrogen and nitrogen plasmas. Plasma Sources Sci. Technol. 2016, 25, 045006. [CrossRef] 32. Jamroz, P.; Zyrnicki, W. Optical emission spectroscopy study for nitrogen–acetylene–argon and nitrogen–acetylene–helium 100 kHz and dc discharges. Vacuum 2010, 84, 940–946. [CrossRef] Nanomaterials 2021, 11, 837 16 of 16

33. Borillo, G.C.; Tadano, Y.S.; Godoi, A.F.L.; Pauliquevis, T.; Sarmiento, H.; Rempel, D.; Yamamoto, C.I.; Marchi, M.R.; Potgieter- Vermaak, S.; Godoi, R.H. Polycyclic Aromatic Hydrocarbons (PAHs) and nitrated analogs associated to particulate matter emission from a Euro V-SCR engine fuelled with diesel/biodiesel blends. Sci. Total. Environ. 2018, 644, 675–682. [CrossRef] [PubMed] 34. Jiang, N.; Wang, H.X.; Zhang, H.; Sasaoka, H.; Nishimura, K. Characterization and surface modification of carbon nanowalls. J. Mater. Chem. 2010, 20, 5070–5073. [CrossRef] 35. Sakulsermsuk, S.; Singjai, P.; Chaiwong, C. Influence of plasma process on the nitrogen configuration in graphene. Diam. Relat. Mater. 2016, 70, 211–218. [CrossRef] 36. McClure, J.P.; Thornton, J.D.; Jiang, R.Z.; Chu, D.; Cuomo, J.J.; Fedkiw, P.S. Oxygen reduction on metal-free nitro-gen-doped carbon nanowall electrodes. J. Electrochem. Soc. 2012, 159, F733–F742. [CrossRef] 37. Dias, A.; Bundaleski, N.; Tatarova, E.; Dias, F.M.; Abrashev, M.; Cvelbar, U.; Teodoro, O.M.N.D.; Henriques, J. Pro-duction of N-graphene by microwave N2-Ar plasma. J. Phys. D Appl. Phys. 2016, 49, 055307. [CrossRef] 38. Singh, G.; Sutar, D.S.; Botcha, V.D.; Narayanam, P.K.; Talwar, S.S.; Srinivasa, R.S.; Major, S.S. Study of simultaneous reduction and nitrogen doping of graphene oxide Langmuir–Blodgett monolayer sheets by ammonia plasma treatment. Nanotechnology 2013, 24, 355704. [CrossRef] 39. Boas, C.R.S.V.; Focassio, B.; Marinho, E.; Larrude, D.G.; Salvadori, M.C.; Leão, C.R.; dos Santos, D.J. Characterization of nitrogen doped graphene bilayers synthesized by fast, low temperature microwave plasma-enhanced chemical vapour deposition. Sci. Rep. 2019, 9, 13715. [CrossRef] 40. Turnbull, L.; Liggat, J.; Macdonald, W. Thermal degradation chemistry of poly(ethylene naphthalate)—A study by thermal volatilisation analysis. Polym. Degrad. Stab. 2013, 98, 2244–2258. [CrossRef] 41. Kundu, S.; Wang, Y.; Xia, W.; Muhler, M. Thermal Stability and Reducibility of Oxygen-Containing Functional Groups on Multiwalled Carbon Nanotube Surfaces: A Quantitative High-Resolution XPS and TPD/TPR Study. J. Phys. Chem. C 2008, 112, 16869–16878. [CrossRef] 42. Schiros, T.; Nordlund, D.; Pálová, L.; Prezzi, D.; Zhao, L.; Kim, K.S.; Wurstbauer, U.; Gutiérrez, C.; Delongchamp, D.; Jaye, C.; et al. Connecting Dopant Bond Type with Electronic Structure in N-Doped Graphene. Nano Lett. 2012, 12, 4025–4031. [CrossRef] 43. Bertóti, I.; Mohai, M.; László, K. Surface modification of graphene and graphite by nitrogen plasma: Determination of chemical state alterations and assignments by quantitative X-ray photoelectron spectroscopy. Carbon 2015, 84, 185–196. [CrossRef] 44. Mezzi, A.; Kaciulis, S. Surface investigation of carbon films: From diamond to graphite. Surf. Interface Anal. 2010, 42, 1082–1084. [CrossRef] 45. Kaciulis, S.; Mezzi, A.; Calvani, P.; Trucchi, D.M. Electron spectroscopy of the main allotropes of carbon. Surf. Interface Anal. 2014, 46, 966–969. [CrossRef] 46. Kovtun, A.; Jones, D.; Dell’Elce, S.; Treossi, E.; Liscio, A.; Palermo, V. Accurate chemical analysis of oxygenated gra-phene-based materials using X-ray photoelectron spectroscopy. Carbon 2019, 143, 268–275. [CrossRef] 47. Blume, R.; Rosenthal, D.; Tessonnier, J.-P.; Li, H.; Knop-Gericke, A.; Schlögl, R. Characterizing Graphitic Carbon with X-ray Photoelectron Spectroscopy: A Step-by-Step Approach. ChemCatChem 2015, 7, 2871–2881. [CrossRef] 48. Susi, T.; Pichler, T.; Ayala, P. X-ray photoelectron spectroscopy of graphitic carbon nanomaterials doped with het-eroatoms. Beilstein J. Nanotechnol. 2015, 6, 177–192. [CrossRef] 49. Leiro, J.; Heinonen, M.; Laiho, T.; Batirev, I. Core-level XPS spectra of fullerene, highly oriented pyrolitic graphite, and glassy carbon. J. Electron. Spectrosc. Relat. Phenom. 2003, 128, 205–213. [CrossRef] 50. Maddi, C.; Bourquard, F.; Barnier, V.; Avila, J.; Asensio, M.-C.; Tite, T.; Donnet, C.; Garrelie, F. Nano-Architecture of nitrogen-doped graphene films synthesized from a solid CN source. Sci. Rep. 2018, 8, 1–13. [CrossRef][PubMed] 51. Fujimoto, A.; Yamada, Y.; Koinuma, M.; Sato, S. Origins of sp3C peaks in C1s X-ray Photoelectron Spectra of Carbon Materials. Anal. Chem. 2016, 88, 6110–6114. [CrossRef][PubMed] 52. Cho, H.J.; Kondo, H.; Ishikawa, K.; Sekine, M.; Hiramatsu, M.; Hori, M. Effects of nitrogen plasma post-treatment on electrical conduction of carbon nanowalls. Jpn. J. Appl. Phys. 2014, 53, 40307. [CrossRef] 53. Jang, J.W.; Lee, C.E.; Lyu, S.C.; Lee, T.J.; Lee, C.J. Structural study of nitrogen-doping effects in bamboo-shaped mul-tiwalled carbon nanotubes. Appl. Phys. Lett. 2004, 84, 2877–2879. [CrossRef] 54. Yanilmaz, A.; Tomak, A.; Akbali, B.; Bacaksiz, C.; Ozceri, E.; Ari, O.; Senger, R.T.; Selamet, Y.; Zareie, H.M. Nitrogen doping for facile and effective modification of graphene surfaces. RSC Adv. 2017, 7, 28383–28392. [CrossRef] 55. Tartz, M.; Neumann, H. Sputter Yields of Carbon Materials under Xenon Ion Incidence. Plasma Process. Polym. 2007, 4, S633–S636. [CrossRef] 56. Luo, Z.; Lim, S.; Tian, Z.; Shang, J.; Lai, L.; Macdonald, B.J.; Fu, C.; Shen, Z.; Yu, T.; Lin, J. Pyridinic N doped graphene: Synthesis, electronic structure, and electrocatalytic property. J. Mater. Chem. 2011, 21, 8038–8044. [CrossRef] 57. Hiramatsu, M.; Hori, M. Carbon Nanowalls: Synthesis and Emerging Applications; Springer: Wien, Austria, 2010. [CrossRef] 58. Manojkumar, P.A.; Krishna, N.G.; Mangamma, G.; Albert, S.K. Understanding the structural and chemical changes in vertical graphene nanowalls upon plasma nitrogen ion implantation. Phys. Chem. Chem. Phys. 2019, 21, 10773–10783. [CrossRef] [PubMed] 59. Lucchese, M.; Stavale, F.; Ferreira, E.M.; Vilani, C.; Moutinho, M.; Capaz, R.B.; Achete, C.; Jorio, A. Quantifying ion-induced defects and Raman relaxation length in graphene. Carbon 2010, 48, 1592–1597. [CrossRef]