Synthesis and Characterization of Nitrogen-Doped Carbon Nanotubes Derived from G-C3N4

materials

Article Synthesis and Characterization of Nitrogen-doped Carbon Nanotubes Derived from g-C3N4

Klaudia Ma´slana , Ryszard J. Kale ´nczuk,Beata Zieli ´nska* and Ewa Mijowska Nanomaterials Physicochemistry Department, Faculty of Chemical Technology and Engineering, West Pomeranian University of Technology, Piastow Av. 45, 70-311 Szczecin, Poland; [email protected] (K.M.); [email protected] (R.J.K.); [email protected] (E.M.) * Correspondence: [email protected]

Received: 4 February 2020; Accepted: 13 March 2020; Published: 17 March 2020

Abstract: Here, nitrogen-doped carbon nanotubes (CNT-N) were synthesized using exfoliated graphitic carbon nitride functionalized with nickel oxides (ex-g-C3N4-NixOy). CNT-N were produced at 900 ◦C in two steps: (1) ex-g-C3N4-NixOy reduction with hydrogen and (2) ethylene assisted chemical vapor deposition (CVD). The detailed characterization of the produced materials was performed via atomic force microscopy (AFM), transmission electron microscopy (TEM), Raman spectroscopy, X-ray diﬀraction (XRD) and thermogravimetric analysis (TGA). The possible mechanism of nanotubes formation is also proposed.

Keywords: graphitic carbon nitride; nickel oxides; nitrogen-doped carbon nanotubes; CVD process

1. Introduction The discovery of diverse nanocarbon allotropes and its nanocomposites has inspired scientists for a range of potential applications. Here, for many years, carbon nanotubes have received huge scientific interest due to their unusual features, such as unique atomic structure, high surface, remarkable mechanical, electrical and thermal properties. The modification of carbon nanotubes (CNTs) by attaching functional groups to the surface and doping with heteroatoms or metal/metal oxide nanoparticles is the most promising strategy with respect to their potential application. For example, the doping of CNTs with nitrogen makes defects that alter the chemical properties of CNTs and creates a path to applications. It is well known that the properties of solid materials are attributed to surface morphology. Thus, materials have to be modified prior to applications to improve their specific properties [1–6]. The surface modification of materials can be done by two main paths: (i) by attaching functional groups [7–9] and (ii) by introducing heteroatoms (e.g., nitrogen, boron, phosphorus, sulfur) in the molecular structure of materials [10–12]. From all of the heteroatoms, nitrogen is the one of the most investigated because it is sustainable for carbon replacing due to the fact that N and C atoms possess similar atomic radius and are chemically relatively easy to replace [13,14]. Furthermore, nitrogen possesses one electron more than carbon, which is desirable for electrochemical reactions such as the oxygen evolution reaction (OER) [15]. Nitrogen doping may also affect electron conductivity, electron-donor properties and chemical stability of the host material due to the modification of the electronic and crystal structure [16]. Moreover, the incorporation of nitrogen into the structure of molecules modifies their geometric properties, which results in an increase in the number of active sites and improves the interaction between the carbon structure and absorbate [17]. Nitrogen-doped carbon nanotubes have found application in many various fields, such as electrochemistry, energy storage, CO2 adsorption, Li-ion batteries and others [18–22]. In this context, Wu et al. [23] demonstrated that N-doped CNT enhanced the energy storage capacity and improved

Materials 2020, 13, 1349; doi:10.3390/ma13061349 www.mdpi.com/journal/materials Materials 2020, 13, 1349 2 of 12 properties of produced batteries. Ariharan et al. [17] revealed that nitrogen-doped carbon nanotubes showed a higher hydrogen storage capacity of ~ 2 wt% total adsorption, which is higher compared to other nitrogen-enriched carbon nanostructures. Another team, Liu et al. [24], reported that nitrogen-doped carbon nanotubes and graphitic carbon nitride nanocomposites exhibit an excellent 1 1 ability to photocatalytic hydrogen evolution rate of 1208 µmol g− h− . In addition to applications related to electrochemistry, nitrogen-doped carbon nanotubes have been used for production sensors [25] or organic pollutant removal [26]. Various preparation methods have been employed to obtain nitrogen-doped carbon nanotubes. The most widely used method of preparing nitrogen-doped carbon nanotubes is chemical vapor deposition [27,28]. One of the most signiﬁcant advantages of this method is its simplicity, which is the reason for its widespread use. However, N-doped CNT are obtained by other methods as well, such as in situ synthesis and post nitridation treatment [29], catalytic pyrolysis of dimethylformamide [30] and plasma treatment [31]. It is well known that ethylene plays the most important role during the chemical vapor deposition process. It is the main source of carbon, which in this process is necessary for the growth of carbon nanotubes. During the production of single-walled carbon nanotubes and multi-walled carbon nanotubes, hydrocarbons such as ethylene, acetylene, benzene or methane have been decomposed into single carbon atoms or linear dimers/trimers. This step takes place on the surface of metallic catalysts and results in CNT growth [32,33]. The process usually takes place in the temperature range of 500–1000 ◦C[34]. In this work, we present a novel route to prepare nitrogen-doped carbon nanotubes (CNT-N) from graphitic carbon nitride functionalized with nickel oxides (ex-g-C3N4-NixOy) by the chemical vapor deposition (CVD) method. We also propose the mechanism of reshaping of the ﬂake-like structure into the tubular form.

2. Details Experimental

2.1. Materials and Procedures Melamine (C H N , 99.0%) and nickel (II) acetate tetrahydrate (Ni(OCOCH ) 4H O, 99.0%) 3 6 6 3 2· 2 ≥ were purchased from Sigma Aldrich (St. Louis, MI, USA).

2.1.1. Synthesis of Exfoliated Graphitic Carbon Nitride

Bulk graphitic carbon nitride (g-C3N4) was synthesized via thermal polycondensation of melamine. The process was carried out in muﬄe furnace at 550 ◦C for 2 h with a heating rate of 2 ◦C/min in air atmosphere. Next, the obtained yellow product was exfoliated by sonication using Tip-ultra-sonicator (500 W, Sonics VC505, Sonics & Materials Inc., Newtown, USA). Here, a certain amount of the bulk g-C3N4 was mixed with isopropyl alcohol and sonicated for 24 h. The ﬁnal powdered product was obtained by evaporation of the solvent at 50 ◦C. The exfoliated sample was denoted as ex-g-C3N4.

2.1.2. Synthesis of Functionalized Exfoliated Graphitic Carbon Nitride with Nickel Oxides In a typical synthesis of the procedure, ex-g-C N and Ni(OCOCH ) 4H O in a weight ratio of 3 4 3 2· 2 1:1 were mixed in isopropyl alcohol by sonication for 3 h. Then, the obtained dispersion was stirred until the solvent evaporated. Finally, the product was annealed at 440 ◦C for 10 min. with a heating rate of 6 ◦C/min in a vacuum.

2.1.3. Synthesis of Nitrogen-doped Carbon Nanotubes

Exfoliated graphitic carbon nitride functionalized with nickel oxides (ex-g-C3N4-NixOy) was placed in the ceramic boat and introduced into the tubular furnace. The furnace was heated under ﬂowing nitrogen up to 900 ◦C. When the temperature was reached, hydrogen was introduced (100 sccm) for 3 h. Afterward, the hydrogen supply was cut oﬀ and ethylene was introduced to the furnace for Materials 2020, 13, 1349 3 of 12

Materials 2020, 13, x FOR PEER REVIEW 3 of 12 10 min. Subsequently, the furnace was cooled down to room temperature. The as-obtained product wasfor denoted 10 min. as Subsequently, CNT-N. the furnace was cooled down to room temperature. The as-obtained product was denoted as CNT-N. 2.2. Characterization 2.2. Characterization The morphology of the obtained samples was investigated by transmission electron microscopy with energyThe morphology dispersive X-rayof the (EDX)obtained spectroscopy samples was (Tecnai investigated F20-based by transmission at 200 kV electron accelerating microscopy voltage, Thermowith Fisherenergy Scientific,dispersive Waltham,X-ray (EDX) MA, spectroscopy USA) and scanning(Tecnai F20-based electron microscopyat 200 kV accelerating (TESCAN, voltage, VEGA 3, acquiredThermo in Fisher the 30 Scientific, kV acceleration Waltham, voltage, MA, USA) TESCAN, and scanning Brno, Czech electron Republic). microscopy Selected (TESCAN, area electronVEGA diffraction3, acquired (SAED) in the patterns 30 kV acceleration were obtained voltage, using TESC TEMAN, microscope Brno, Czech (Thermo Republic). Fisher Selected Scientific, area electron Waltham, MA,diffraction USA). The (SAED) AFM patterns study waswere performed obtained using using TEM an atomic microscope force microscope(Thermo Fisher (Nanoscope Scientific, V MultimodeWaltham, 8, MA, Bruker USA). AXS, The Mannheim,AFM study was Germany). performed X-ray using di anff ractionatomic force (XRD) microscope patterns (Nanoscope were carried outV using Multimode an Empyrean 8, Bruker (Malvern AXS, Ma Panalytical,nnheim, Germany). Malvern, X-ray UK) diffractio diffractometern (XRD) usingpatterns CuK wereα radiation. carried out using an Empyrean (Malvern Panalytical, Malvern, UK) diffractometer using CuKα radiation. Thermogravimetric analysis was carried out using TA-Q600 SDT TA Instrument (TA Instrument, Thermogravimetric analysis was carried out using TA-Q600 SDT TA Instrument (TA Instrument, New Castle, DE, USA) under the air and argon atmosphere at ramping rate of 10 C/min from room New Castle, DE, USA) under the air and argon atmosphere at ramping rate of 10 °C/min◦ from room temperature to 900 C. Raman spectra were determined on inVia Raman Microscope (RENISHAW, temperature to 900◦ °C. Raman spectra were determined on inVia Raman Microscope (RENISHAW, New Mills Wotton-under-Edge, UK) with an excitation wavelength of 785 nm. New Mills Wotton-under-Edge, UK) with an excitation wavelength of 785 nm. 3. Results and Discussion 3. Results and Discussion 3.1. Microscopic Analysis 3.1. Microscopic Analysis The morphologies of g-C3N4, ex-g-C3N4 and ex-g-C3N4-NixOy were studied by transmission The morphologies of g-C3N4, ex-g-C3N4 and ex-g-C3N4-NixOy were studied by transmission electron microscopy. The TEM images of the samples are presented in Figure1. TEM images of electron microscopy. The TEM images of the samples are presented in Figure 1. TEM images of g- g-C3N4 and ex-g-C3N4 (Figure1A,B) show a visible di fference in the thickness of these two samples. C3N4 and ex-g-C3N4 (Figure 1A,B) show a visible difference in the thickness of these two samples. Figure1C and D exhibit that Ni xOy nanoparticles were successfully deposited on the flakes of ex-g-C N Figure 1C and D exhibit that NixOy nanoparticles were successfully deposited on the flakes of ex-g-3 4 (ex-g-C N -Ni O ). The average diameter of Ni O particles is ~32 nm. C3N34 (ex-g-C4 x 3Ny4-NixOy). The average diameter xof Niy xOy particles is ~32 nm.

Figure 1. Transmission electron microscopy (TEM) images of (A) g-C3N4,(B) ex-g-C3N4 and (C,D) ex-g-C N -Ni O . 3 4 x y Materials 2020, 13, x FOR PEER REVIEW 4 of 12

Figure 1. Transmission electron microscopy (TEM) images of (A) g-C3N4, (B) ex-g-C3N4 and (C,D) ex- Materials 2020, 13, 1349 4 of 12 g-C3N4-NixOy.

Additionally, the topography of the exfoliated g-C3N4 was also examined via atomic force Additionally, the topography of the exfoliated g-C3N4 was also examined via atomic force microscopy (Figure 2). The obtained data indicate that the thickness of ex-g-C3N4 is ~ 4–5 nm, which microscopy (Figure2). The obtained data indicate that the thickness of ex-g-C 3N4 is ~ 4–5 nm, which meansmeans that that this this sample sample is is composed composed ofof ~10–13 layers. layers. According According to to the the fact fact that that bulk bulk graphitic graphitic carbon carbon nitride is composed of ~100–170 layers, we can conclude that the exfoliation step was efficient. Based nitride is composed of ~100–170 layers, we can conclude that the exfoliation step was eﬃcient. Based on AFM images, the number of ex-g-C3N4 layers is reduced by more than 30 times in comparison to on AFM images, the number of ex-g-C3N4 layers is reduced by more than 30 times in comparison to the pristine g-C3N4, which is supported by TEM. the pristine g-C3N4, which is supported by TEM.

(A) (B)

FigureFigure 2. 2.AFM AFM imagesimages of ex-g-C33NN44 (A(A) )and and corresponding corresponding high high profile proﬁle (B). (B ).

FigureFigure3 shows3 shows SEM SEM images images ofof g-Cg-C3N3N4,4 ex-g-C, ex-g-C3N34N and4 and the thesample sample produced produced in the in process the process of ofhydrogen hydrogen reduction reduction of ofex-g-C ex-g-C3N43-NiN4x-NiOy andxOy nextand CVD next CVDin ethylene in ethylene in the presence in the presence of reduced of reducedex-g- ex-g-CC3N4-Ni3N4xO-Niy (CNT-N).xOy (CNT-N). g-C3N g-C4 and3N 4ex-g-Cand ex-g-C3N4 (Figure3N4 (Figure 3A,B) exhibited3A,B) exhibited aggregated aggregated morphology morphology with withirregular irregular multiple-layered multiple-layered structures. structures. The Themorphology morphology of CNT-N of CNT-N (Figure (Figure 3C,D)3C,D) is quite is quite different di ﬀerent from that of g-C3N4 and ex-g-C3N4. This sample is composed of nanotubular structures. Moreover, from that of g-C3N4 and ex-g-C3N4. This sample is composed of nanotubular structures. Moreover, nickelnickel residues residues which which served served asas catalystscatalysts in nano nanotubetube growth growth were were observed observed inside inside the the tubes tubes (green (green arrowsarrows in in Figure Figure3D). 3D). The The obtained obtained nanotubes nanotubes tendtend to agglomerate into into larger larger clusters. clusters. The The reference reference experiment without ethylene was also performed (Figure 4). This confirmed that ethylene is crucial experiment without ethylene was also performed (Figure4). This conﬁrmed that ethylene is crucial in in the process of nitrogen-doped carbon nanotube formation. the process of nitrogen-doped carbon nanotube formation. Materials 2020, 13, x FOR PEER REVIEW 5 of 12

FigureFigure 3. Scanning3. Scanning electron electron microscopy microscopy (SEM) (SEM) images images of of (A ()A g-C) g-C3N3N4,(4,B (B) ex-g-C) ex-g-C3N3N44 andand ((CC,,DD)) CNT-N.CNT- N.

Figure 4. SEM (A,B) and TEM (C) images of g-C3N4-NixOy treated with hydrogen at 900 °C.

To determine the role of ethylene during tube formation, an additional experiment was performed. In this step, g-C3N4-NixOy was heated to 900 °C and then treated with hydrogen for 3 h. In this case, the sample had not been exposed to ethylene. SEM and TEM images of the obtained product are presented in Figure 4. Here, tubular forms did not evolve. Additionally, metal particles agglomerated into larger clusters. A further experiment with g-C3N4-NixOy treated only with ethylene (no hydrogen step) at 900 °C was conducted (Figure 5). It revealed that the obtained sample contained mostly onion-like particles. Some of these structures are formed on the surface of metal particles. Detailed TEM investigations revealed a few irregular, deformed tubular structures.

MaterialsMaterials 20202020,, 1313,, xx FORFOR PEERPEER REVIEWREVIEW 55 ofof 1212

MaterialsFigureFigure2020 ,3.3.13 ScanningScanning, 1349 electronelectron microscopymicroscopy ((SEM)SEM) imagesimages ofof ((AA)) g-Cg-C33NN44,, ((BB)) ex-g-Cex-g-C33NN44 andand ((CC,,DD)) CNT-CNT-5 of 12 N.N.

FigureFigure 4.4.4. SEMSEM ((AA,B,B)) andand TEMTEM ((CC)) imagesimages ofofof g-Cg-C33NN44-Ni-NixxOOOyyy treatedtreatedtreated withwith with hydrogenhydrogen hydrogen atat at 900900 900 °C.°C.◦C.

ToTo determinedeterminedetermine the thethe role rolerole of ethyleneofof ethyleneethylene during duringduring tube formation, tutubebe formation,formation, an additional anan additionaladditional experiment experimentexperiment was performed. waswas performed.Inperformed. this step, InIn g-C thisthis3N 4step,step,-Nix Og-Cg-Cy was33NN44-Ni-Ni heatedxxOOyy waswas to 900 heatedheated◦C and toto 900900 then °C°C treated andand thenthen with treatedtreated hydrogen withwith for hydrogenhydrogen 3 h. In this forfor case, 33 h.h. IntheIn thisthis sample case,case, had thethe notsamplesample been hadhad exposed notnot beenbeen to ethylene. exposedexposed toto SEM ethylene.ethylene. and TEM SEMSEM images andand TEMTEM of the imagesimages obtained ofof thethe product obtainedobtained are productpresentedproduct areare in presentedpresented Figure4. Here, inin FigureFigure tubular 4.4. Here,Here, forms tubulartubular did not formsforms evolve. diddid Additionally, notnot evolve.evolve. metalAdditionally,Additionally, particles metalmetal agglomerated particlesparticles agglomeratedintoagglomerated larger clusters. intointo larger larger A further clusters.clusters. experiment A A further further with experimentexperiment g-C3N4-Ni withwithxOy g-Cg-Ctreated33NN44-Ni-Ni onlyxxOOy withy treatedtreated ethylene onlyonly withwith (no hydrogen ethyleneethylene (nostep)(no hydrogenhydrogen at 900 ◦ step)Cstep) was at at conducted 900900 °C°C waswas (Figure conductedconducted5). (FigureIt(Figure revealed 5). 5). ItIt that revealedrevealed the obtained thatthat thethe obtainedobtained sample contained samplesample containedcontained mostly mostlyonion-likemostly onion-likeonion-like particles. particles.particles. Some of SomeSome these ofof structures thesethese structuresstructures are formed areare onformedformed the surface onon thethe of surfacesurface metal of particles.of metalmetal particles.particles. Detailed DetailedTEMDetailed investigations TEMTEM investigationsinvestigations revealed revealed arevealed few irregular, aa fefeww irregular,irregular, deformed deformeddeformed tubular structures. tubulartubular structures.structures.

Figure 5. TEM (A,B) images of g-C3N4-NixOy treated with ethylene at 900 ◦C.

In order to study the morphology and elemental composition of CNT-N in greater detail, TEM, STEM and EDS analyses were performed (Figure6). TEM images (Figure6A,B) confirmed that the nanotubes have a diameter in the range of 200–300 nm. The elemental mapping (Figure6D,E), carried out for the area marked as a white square on the STEM image (Figure6C), revealed that carbon and nitrogen are the only elements present in the sample. Moreover, both components are homogeneously distributed in nanotubes. Furthermore, the line scan along the direction denoted by the red line in the STEM image (Figure6G) was carried out and the obtained profiles are presented in Figure6F. This analysis additionally confirms the chemical composition of CNT-N nanotubes. Materials 2020, 13, x FOR PEER REVIEW 6 of 12

Figure 5. TEM images of g-C3N4-NixOy treated with ethylene at 900 °C.

In order to study the morphology and elemental composition of CNT-N in greater detail, TEM, STEM and EDS analyses were performed (Figure 6). TEM images (Figure 6A,B) confirmed that the nanotubes have a diameter in the range of 200–300 nm. The elemental mapping (Figure 6D,E), carried out for the area marked as a white square on the STEM image (Figure 6C), revealed that carbon and nitrogen are the only elements present in the sample. Moreover, both components are homogeneously distributed in nanotubes. Furthermore, the line scan along the direction denoted by theMaterials red line2020 in, 13 the, 1349 STEM image (Figure 6G) was carried out and the obtained profiles are presented6 of 12 in Figure 6F. This analysis additionally confirms the chemical composition of CNT-N nanotubes.

FigureFigure 6. 6.TEMTEM images images of of CNT-N ((AA,,BB),), STEMSTEM image image of of CNT-N CNT-N (C )(C with) with corresponding corresponding mapping mapping image imageof nitrogen of nitrogen (D) and (D) carbonand carbon (E), and(E), lineand profileline profile (F) of (F CNT_N) of CNT_N with with corresponding corresponding STEM STEM image image (G). (G). Figure7 presents HR-TEM and SAED images for g-C 3N4, ex-g-C3N4, ex-g-C3N4-NixOy and CNT-N.Figure For 7 presents g-C3N4 and HR-TEM ex-g-C and3N4 crystalSAED images fingers arefor notg-C observed,3N4, ex-g-C which3N4, ex-g-C indicates3N4-Ni thatxO thosey and products CNT- N.are For amorphous, g-C3N4 and whichex-g-C3 isN confirmed4 crystal fingers by SEAD are not analysis observed, [35]. which ex-g-C indicates3N4_Ni xthatOy andthose CNT_N products exhibit are amorphous,SAED patterns which typical is confirmed for a polycrystalline by SEAD analysis structure. [35]. ex-g-C ex-g-C3N3N44-Ni_NixOxOyy showsand CNT_N a few diexhibitffraction SAED rings patternswith discrete typical reflections for a polycrystalline corresponding structure. to the di ex-g-Cfferent3N orientations4-NixOy shows of the a few single diffraction phase. The rings following with discreterings have reflections been characterized: corresponding (002) to the corresponding different orientations to nickel particles, of the single (202) tophase. Ni2O The3, (110) following to NiO 2 ringsand have (110) been to Ni. characterized: The pattern of (002) copper corresponding (from TEM grid) to nickel can also particles, be observed (202) andto Ni is2O marked3, (110) withto NiO (220).2 This analysis is in full agreement with the XRD analysis, which proved the presence of different nickel compounds in the sample. CNT-N exhibits three clear rings. The first two rings (111) and (200) are typical for nickel nanoparticles. Similar to the previous sample, the rings from copper can be detected. Materials 2020, 13, x FOR PEER REVIEW 7 of 12

and (110) to Ni. The pattern of copper (from TEM grid) can also be observed and is marked with (220). This analysis is in full agreement with the XRD analysis, which proved the presence of different nickel compounds in the sample. CNT-N exhibits three clear rings. The first two rings (111) and (200) Materialsare typical2020, 13 for, 1349 nickel nanoparticles. Similar to the previous sample, the rings from copper can7 of be 12 detected.

3 4 3 4 3 4 x y FigureFigure 7. 7.HR-TEM HR-TEM (A (–AD–D) and) and SAED SAED (E (–EH–)H images) images of of g-C g-C3N4N(A (A,E,),E), ex-g-C ex-g-C3NN4 ( B(B,F,F),), ex-g-C ex-g-C3NN4-Ni-NixOOy (C(,CG,)G and) and CNT-N CNT-N (D (,DH,H). ).

3.2.3.2. Spectroscopic, Spectroscopic, Crystal Crystal and and Thermogravimetric Thermogravimetric Studies Studies

XRDXRD patterns patterns ofof g-Cg-C3N44,, ex-g-C ex-g-C3N3N4, 4ex-g-C, ex-g-C3N43-NiN4x-NiOy xandOy CNT-Nand CNT-N samples samples are shown are shownin Figure in Figure8. The8. peaks The peaks located located at 13.1° at 13.1 and◦ 27.56°and 27.56 can ◦becan observed be observed for three for threesamples samples (g-C3 (g-CN4, ex-g-C3N4, ex-g-C3N4, ex-g-3N4, ex-g-CC3N4-Ni3N4x-NiOy) xandOy) they and are they attributed are attributed to the topresence the presence of graphitic of graphitic carbon nitride. carbon Peaks nitride. at 13.1° Peaks and at 13.127.56°◦ and correspond 27.56◦ correspond to the (100) to the and (100) (002) and crystal (002)line crystalline phase, respectively phase, respectively (JCPDS 87–1526). (JCPDS 87–1526). Nickel- Nickel-functionalizedfunctionalized graphitic graphitic carbon carbon nitride nitride(ex-g-C (ex-g-C3N4-Nix3ONy)4 -NiexhibitsxOy) the exhibits presence the presenceof different of diformsfferent of θ formsa nickel of acompound. nickel compound. The diffraction The di peakffraction at 2 peak44.8° atand 2θ 58.7°44.8 is◦ andrelated 58.7 to◦ theis related nickel (III) to the oxide nickel (JCPDS (III) oxide14–0481), (JCPDS while 14–0481), two peaks while at ~39.4° two peaks and ~41.8° at ~39.4 belong◦ and to ~41.8 nickel◦ belong (II) oxide to nickel(JCPDS (II) 85–1977) oxide (JCPDS[36,37]. 85–1977)During [the36,37 last]. Duringstage of the CNT-N last stage synthesis, of CNT-N nickel synthesis, oxide is nickelreduced oxide to the is reduced metallic toform the metallicof nickel form and ofcatalyzes nickel and the catalyzes growth theof nanotubes. growth of nanotubes.This is conf Thisirmed is by confirmed the disappearance by the disappearance of the peaks of from the peaks NiO fromand NiO Ni2O and3 and Ni the2O3 appearanceand the appearance of signals of characteristic signals characteristic for the metallic for the form metallic of nickel form oflocated nickel at located ~44.5° atand ~44.5 ~51.9°◦ and (JCPDS ~51.9◦ (JCPDS 87–0712) 87–0712) [38]. Moreover, [38]. Moreover, the diffraction the diffraction peak peak at 26.2° at 26.2 observed◦ observed for forCNT-N CNT-N is isassigned assigned to to the the (002) (002) plane plane of of graphitic graphitic carbon. carbon. Materials 2020, 13, x FOR PEER REVIEW 8 of 12

FigureFigure 8.8. XRD patterns of g-C33N4,, ex-g-C ex-g-C33NN44, ,ex-g-C ex-g-C3N3N4-Ni4-NixOxyO andy and CNT-N. CNT-N.

Thermogravimetric analysis was carried out in order to determine the thermal properties of the samples. Figure 9 represents the TG (a) and DTG (b) curves of g-C3N4, ex-g-C3N4, ex-g-C3N4-NixOy and CNT-N heat-treated under air flow. Here, the exfoliation process does not affect the thermal stability of graphitic carbon nitride. For both g-C3N4 and ex-g-C3N4, the degradation process starts at about 500 °C and continues until complete decomposition. Functionalization with nickel oxides (ex- g-C3N4-NixOy) significantly decreased the decomposition temperature to 350 °C. The residue mass after the analysis is ~28.7 wt% and this value is assigned to the amount of nickel compounds in the sample. Two steps can be distinguished on the TGA curve obtained for the CNT-N sample. The first one begins at a temperature of 468 °C and is attributed to the removal of more defected carbon structures. The second one, from a temperature of 565 °C, is related to the degradation of carbon nanotubes. The residual mass is ~24.8 wt% which is the nickel compounds content in the sample.

Figure 9. Thermogravimetric a) and derivative thermogravimetric curves b) of g-C3N4, ex-g-C3N4, ex-

g-C3N4-NixOy and CNT-N conducted in air.

Additionally, TGA analysis performed in argon atmosphere was conducted. Thermogravimetric (TG) (Figure 9a) and derivative thermogravimetric (DTG) (Figure 9b) curves of g-C3N4, ex-g-C3N4, ex-g-C3N4-NixOy and CNT-N heat-treated under argon flow are presented in Figure 10. g-C3N4 and ex-g-C3N4 are stable up to ~500 °C, either in air or argon atmosphere (see Figures 9 and 10). This means that both g-C3N4 and ex-g-C3N4 can endure high temperatures, even in an oxidizing atmosphere. The weight loss at above 500 °C is not due to the oxidation by O2, but to the direct thermal decomposition of g-C3N4 itself [39]. Moreover, the residual mass of ~3 wt% is observed in an inert atmosphere. ex-g-C3N4-NixOy exhibited different thermal stability in air and argon atmosphere,

Materials 2020, 13, x FOR PEER REVIEW 8 of 12

Materials 2020, 13, 1349 8 of 12 Figure 8. XRD patterns of g-C3N4, ex-g-C3N4, ex-g-C3N4-NixOy and CNT-N.

ThermogravimetricThermogravimetric analysis analysis was was carried carried out out in in or orderder to to determine determine the the thermal thermal properties properties of of the the samples.samples. Figure Figure 9 representsrepresents thethe TGTG (a)(a) and and DTG DTG (b) (b) curves curves of of g-C g-C3N3N4,4, ex-g-C ex-g-C33NN44,, ex-g-Cex-g-C33NN44-NixOOyy andand CNT-N CNT-N heat-treated heat-treated under under air air flow. flow. Here, Here, the the ex exfoliationfoliation process process does does not not affect affect the the thermal thermal stabilitystability of of graphitic graphitic carb carbonon nitride. nitride. For For both both g-C g-C3N3N4 and4 and ex-g-C ex-g-C3N34N, the4, the degradation degradation process process starts starts at aboutat about 500 500°C and◦C and continues continues until until complete complete decomposi decomposition.tion. Functionalization Functionalization with withnickel nickel oxides oxides (ex- g-C(ex-g-C3N4-NixOy)3N4-NixOy) significantly significantly decreased decreased the decompositi the decompositionon temperature temperature to 350 to°C. 350 The◦C. residue The residue mass aftermass the after analysis the analysis is ~28.7 is wt% ~28.7 and wt% this and value this valueis assigned is assigned to the to amount the amount of nickel of nickel compounds compounds in the in sample.the sample. Two Twosteps steps can be can distinguished be distinguished on the on TG theA TGAcurve curve obtained obtained for the for CNT-N the CNT-N sample. sample. The first The onefirst begins one begins at a attemperature a temperature of 468 of 468°C ◦andC and is a isttributed attributed to tothe the removal removal of of more more defected defected carbon carbon structures.structures. The The second second one, one, from from a atemperature temperature of of 565 565 °C,◦C, is is related related to to the the degradation degradation of of carbon carbon nanotubes.nanotubes. The The residual residual mass mass is is ~24.8 ~24.8 wt% wt% which which is is the the nickel nickel compounds compounds content content in in the the sample. sample.

Figure 9. Thermogravimetric (a) and derivative thermogravimetric curves (b) of g-C N , ex-g-C N , Figure 9. Thermogravimetric a) and derivative thermogravimetric curves b) of g-C3N43, ex-g-C4 3N4,3 ex-4 ex-g-C N -Ni O and CNT-N conducted in air. g-C3N4-Ni3 4xOy andx y CNT-N conducted in air. Additionally, TGA analysis performed in argon atmosphere was conducted. Thermogravimetric Additionally, TGA analysis performed in argon atmosphere was conducted. Thermogravimetric (TG) (Figure9a) and derivative thermogravimetric (DTG) (Figure9b) curves of g-C 3N4, ex-g-C3N4, (TG) (Figure 9a) and derivative thermogravimetric (DTG) (Figure 9b) curves of g-C3N4, ex-g-C3N4, ex-g-C3N4-NixOy and CNT-N heat-treated under argon flow are presented in Figure 10. g-C3N4 ex-g-C3N4-NixOy and CNT-N heat-treated under argon flow are presented in Figure 10. g-C3N4 and and ex-g-C3N4 are stable up to ~500 ◦C, either in air or argon atmosphere (see Figures9 and 10). ex-g-C3N4 are stable up to ~500 °C, either in air or argon atmosphere (see Figures 9 and 10). This This means that both g-C3N4 and ex-g-C3N4 can endure high temperatures, even in an oxidizing means that both g-C3N4 and ex-g-C3N4 can endure high temperatures, even in an oxidizing atmosphere. The weight loss at above 500 ◦C is not due to the oxidation by O2, but to the direct atmosphere. The weight loss at above 500 °C is not due to the oxidation by O2, but to the direct thermal decomposition of g-C3N4 itself [39]. Moreover, the residual mass of ~3 wt% is observed in an thermalMaterials 2020decomposition, 13, x FOR PEER of REVIEW g-C3N4 itself [39]. Moreover, the residual mass of ~3 wt% is observed9 in of an 12 inert atmosphere. ex-g-C3N4-NixOy exhibited different thermal stability in air and argon atmosphere, inert atmosphere. ex-g-C3N4-NixOy exhibited different thermal stability in air and argon atmosphere, which was higher in Ar flow. flow. Furthermore, the residual mass after analysis is ~34 wt% (~24.8 wt% wt% in air). The The obtained obtained CNT-N CNT-N is stable in argon atmosphere.

Figure 10. Thermogravimetric (a) and derivative thermogravimetric (b) curves of g-C N , ex-g-C N , Figure 10. Thermogravimetric a) and derivative thermogravimetric b) curves of g-C33N4, ex-g-C3N44, ex-g-C N -NixOy and CNT-N conducted in Ar. ex-g-C33N44-NixOy and CNT-N conducted in Ar.

Raman spectroscopy was introduced to provide information about the chemical structures of g- C3N4, ex-g-C3N4 and CNT-N (Figure 11). Raman spectra of g-C3N4 and ex-g-C3N4 contain several bands observed in the range of 700–1630 cm−1 which are attributed to graphitic carbon nitride [40– 42]. Additionally, the vibrations at 753, 977, 1120, 1156, 1236 and 1314 cm−1 are assigned to the stretching vibration of aromatic C-N heterocycle characteristic to melem. [43,44] The peaks at 700– 1000 cm−1 are related to the different types of ring breathing modes of s-triazine. The Raman response of the tubular sample exhibits D (1318 cm−1), G (1580 cm−1) and 2D (2700 cm−1) bands what confirm the formation of graphitic carbon structure in the sample [45,46].

Figure 11. Raman spectra of g-C3N4, ex-g-C3N4 and CNT-N.

3.3. Mechanism of the CNT-N Formation All the performed experiments clearly indicate that the metallic nickel serves as an active catalyst for the formation of the nanotubes. To verify this, the additional experiments in which g-C3N4-NixOy were treated with hydrogen or ethylene at 900 °C were performed (see Figures 4 and 5). The tubular

Materials 2020, 13, x FOR PEER REVIEW 9 of 12 which was higher in Ar flow. Furthermore, the residual mass after analysis is ~34 wt% (~24.8 wt% in air). The obtained CNT-N is stable in argon atmosphere.

MaterialsFigure2020 ,10.13, Thermogravimetric 1349 a) and derivative thermogravimetric b) curves of g-C3N4, ex-g-C3N4,9 of 12

ex-g-C3N4-NixOy and CNT-N conducted in Ar.

Raman spectroscopy spectroscopy was was introduced introduced to to provide provide in informationformation about about the the chemical chemical structures structures of g- of g-C N , ex-g-C N and CNT-N (Figure 11). Raman spectra of g-C N and ex-g-C N contain several C3N34, ex-g-C4 3N34 and4 CNT-N (Figure 11). Raman spectra of g-C33N44 and ex-g-C33N44 contain several 1 bands observed inin thethe rangerange of of 700–1630 700–1630 cm cm− −1which which are are attributed attributed to to graphitic graphitic carbon carbon nitride nitride [40 [40––42]. 1 42].Additionally, Additionally, the vibrations the vibrations at 753, at 977, 753, 1120, 977, 1156, 1120, 1236 1156, and 1236 1314 and cm −1314are assignedcm−1 are toassigned the stretching to the 1 stretchingvibration of vibration aromatic of C-N aromatic heterocycle C-N heterocycle characteristic characteristic to melem. [43 to,44 melem.] The peaks [43,44] at The 700–1000 peaks cm at− 700–are 1000related cm to−1 are the related diﬀerent to types the different of ring breathingtypes of ring modes breathing of s-triazine. modes The of s-triazine. Raman response The Raman of the response tubular 1 1 1 ofsample the tubular exhibits sample D (1318 exhibits cm− ), D G (1318 (1580 cm−1),) G and (1580 2D (2700cm−1) cmand− 2D) bands (2700 what cm−1 conﬁrm) bands what the formation confirm theof graphitic formation carbon of graphitic structure carbon in the structure sample [in45 the,46]. sample [45,46].

FigureFigure 11.11. Raman spectraspectra ofof g-Cg-C33N4,, ex-g-C ex-g-C33NN44 andand CNT-N. CNT-N.

3.3. Mechanism Mechanism of of the the CNT-N CNT-N Formation All the the performed performed experiments experiments clearly clearly indicate indicate that thatthe metallic the metallic nickel nickel serves servesas an active as an catalyst active forcatalyst the formation for the formation of the nanotubes. of the nanotubes. To verify this, To the verify additional this, the experiments additional in experiments which g-C3N in4-Ni whichxOy wereg-C3N treated4-NixO withy were hydrogen treated with or ethylene hydrogen at or900 ethylene °C were at performed 900 ◦C were (see performed Figures 4 (see and Figures 5). The4 tubular and5). The tubular forms did not evolve when g-C3N4-NixOy was treated only with H2. Moreover, treating of g-C3N4-NixOy only with ethylene (no hydrogen step) at 900 ◦C results in the formation of cocktail of carbon onion-like structures or short irregular tube-like structures. Therefore, the key step was the reduction of nickel oxides to metallic nickel in the hydrogen/ethylene flow. This was also confirmed by XRD studies (see Figure4). In the next step, carbon and nitrogen from g-C 3N4 diffused into nickel particles and when the oversaturation was reached these elements started to grow in the form of tubes. The elemental and the structural properties of the tubes were confirmed in EDX analysis and the Raman response of the sample. The whole process of the tubes formation is presented schematically in Figure 12. Materials 2020, 13, x FOR PEER REVIEW 10 of 12

forms did not evolve when g-C3N4-NixOy was treated only with H2. Moreover, treating of g-C3N4- NixOy only with ethylene (no hydrogen step) at 900 °C results in the formation of cocktail of carbon onion-like structures or short irregular tube-like structures. Therefore, the key step was the reduction of nickel oxides to metallic nickel in the hydrogen/ethylene flow. This was also confirmed by XRD studies (see Figure 4). In the next step, carbon and nitrogen from g-C3N4 diffused into nickel particles and when the oversaturation was reached these elements started to grow in the form of tubes. The elemental and the structural properties of the tubes were confirmed in EDX analysis and the Raman Materialsresponse2020 of, 13the, 1349 sample. The whole process of the tubes formation is presented schematically in 10Figure of 12 12.

FigureFigure 12.12. SchematicSchematic diagramdiagram ofof N-dopedN-doped carboncarbon nanotubesnanotubes synthesis.synthesis.

4.4. ConclusionsConclusions InIn summary,summary, wewe presentpresent aa simplesimple andand eeffectiveffective methodmethod forfor thethe synthesissynthesis ofof N-dopedN-doped carboncarbon nanotubesnanotubes withwith aa diameterdiameter inin thethe rangerange ofof 200–300200–300 nm.nm. CNT-N werewere producedproduced inin thethe reactionreaction ofof chemicalchemical vaporvapor depositiondeposition inin thethe presencepresence ofof ethyleneethylene flow.flow. Here,Here, exfoliatedexfoliated graphiticgraphitic carboncarbon nitridenitride functionalizedfunctionalized withwith nickelnickel oxidesoxides hashas beenbeen usedused asas aa startingstarting materialmaterial inin CNT-NCNT-N formation. formation. ThisThis materialmaterial playsplays aa doubledouble role:role: (1)(1) nickelnickel waswas aa catalystcatalyst inin nanotubenanotube growthgrowth andand (2)(2) graphiticgraphitic carboncarbon nitridenitride servedserved asas supportsupport forfor nickelnickel nanoparticlesnanoparticles andand asas carboncarbon/nitrogen/nitrogen source.source. TheThe resultsresults alsoalso proveprove thatthat ethyleneethylene playsplays aa crucialcrucial rolerole inin thethe formationformation ofof tubes.tubes. OnOn oneone side,side, itit carboreducedcarboreduced thethe nickelnickel oxideoxide (II) to metallic metallic nickel nickel which which was was an an active active phase phase in in the the growth growth of ofthe the nanotubes, nanotubes, and, and, on onthe the other other hand, hand, it was it was a source a source of carbon of carbon in the in thefinal final sample. sample. The Theobtained obtained N-doped N-doped nanotubes nanotubes may mayshow show promising promising performance performance in a in variety a variety of ofapplications applications such such as as catalysis, catalysis, lithium-ion lithium-ion batteries, supercapacitorssupercapacitors andand soso on.on. Author Contributions: Conceptualization, E.M. and B.Z.; Funding acquisition, E.M.; Investigation, K.M. and B.Z.;Author writing—original Contributions: draft . Conceptualization, preparation, K.M., E.M. B.Z. and and B.Z.; E.M., Funding writing—review acquisition, and E.M.; editing, Investigation, K.M., B.Z. andK.M. E.M.; and supervision,B.Z.; writing—original E.M. and R.J.K. draft Allpreparatio authorsn, have K.M, read B.Z. and and agreed E.M., towriting—review the published versionand editing, of the K.M, manuscript. B.Z. and E.M; supervision, E.M and R.J.K. Funding: This study was funded by National Science Centre, Poland (grant number 2015/19/B/ST8/00648). ConflictsFunding: ofThis Interest: study wasThe authorsfunded by declare National that theyScience have Ce nontre, conflict Poland of (grant interest. number 2015/19/B/ST8/00648). Conflicts of Interest: The authors declare that they have no conflict of interest. References References 1. Mozetiˇc,M. Surface Modification to Improve Properties of Materials. Materials 2019, 12, 441. [CrossRef] 1. [MozetiPubMedč, ]M. Surface Modification to Improve Properties of Materials. Materials 2019, 12, 441. 2.2. Kalia,Kalia, S.;S.; Boufi, Boufi, S.; S.; Celli, Celli, A. NanofibrillatedA. Nanofibrillated cellulose: cellulose: Surface Surface modification modification and potential and potential applications. applications.Colloid Polym.Colloid Sci.Polym.2014 Sci., 292 2014, 5–31., 292 [,CrossRef 5–31. ] 3.3. Figueiredo,Figueiredo, J.L.;J.L.; Pereira,Pereira, M.F.R.;M.F.R.; Freitas,Freitas, M.M.A.;M.M.A.; ÓÓrfão,rfão, J.J.M.J.J.M. ModificationModification ofof thethe surfacesurface chemistrychemistry ofof activatedactivated carbons.carbons. Carbon 19991999,, 3737,, 1379–1389.1379–1389. [CrossRef] 4. Kukulka, W.; Cendrowski, K.; Mijowska, E. Electrochemical performance of MOF-5 derived carbon nanocomposites with 1D, 2D and 3D carbon structures. Electrochem. Acta 2019, 307, 582–594. [CrossRef] 5. Zhou, Q.; Li, L.; Xin, Z.; Yu, Y.; Wang, L.; Zhang, W. Visible light response and heterostructure of composite CdS@ZnS–ZnO to enhance its photocatalytic activity. J. Alloys Compd. 2020, 813, 152190. [CrossRef]

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