Polymer-Derived Nitrogen-Doped Carbon Nanosheet Cluster and Its Application for Water Puriﬁcation

Article Polymer-Derived Nitrogen-Doped Carbon Nanosheet Cluster and Its Application for Water Puriﬁcation

Xiaoyan Yu 1, Ting Zheng 1,2 and Srikanth Pilla 1,2,3,4,*

1 Department of Automotive Engineering, Clemson University, 4 Research Drive, Greenville, SC 29607, USA; [email protected] (X.Y.); [email protected] (T.Z.) 2 Clemson Composites Center, Clemson University, Greenville, SC 29607, USA 3 Department of Materials Science and Engineering, Clemson University, Clemson, SC 29602, USA 4 Department of Mechanical Engineering, Clemson University, Clemson, SC 29602, USA * Correspondence: [email protected]

Abstract: A series of nitrogen-doped carbons (NCs) were prepared by the pyrolysis (300–900 ◦C) of crystalline polyazomethine (PAM) synthesized via a facile condensation reaction in methanol solvent. The controlled solvent evaporation resulted in PAM crystals in the form of nanosheet clusters with a sheet thickness of ~50 nm. Such architecture was maintained after pyrolysis, obtaining porous CNs of high speciﬁc surface areas of up to 700 m2/g. The resulting NCs were used as absorbents to remove aromatic Rhodamine B from water. The NC that pyrolyzed at 750 ◦C exhibited the highest adsorption capacity (0.025 mg/mg), which is attributed to its high surface area and surface condition.

Keywords: nitrogen doping; carbon; water puriﬁcation; polymer-derived carbon

Citation: Yu, X.; Zheng, T.; Pilla, S. 1. Introduction Polymer-Derived Nitrogen-Doped Carbon Nanosheet Cluster and Its Porous carbons (PCs) are important materials that have been used throughout human Application for Water Purification. history. Due to their large surface area, good conductivity, excellent chemical stability, and Macromol 2021, 1, 84–93. https:// abundant availability, PC materials have been widely used as absorbents, energy storage doi.org/10.3390/macromol1020007 materials, catalyst supports, etc. [1,2]. Historically, PCs have been obtained easily by pyrolyzing forestry and agricultural products. PCs produced by this top-down process have Academic Editors: Ana predetermined structures that are inherited from their naturally synthesized lignocellulose María Díez-Pascual and Luc Avérous framework [3]. On the other hand, bottom-up routes which build PCs from molecules provide advantages of pre-designable architecture, controllable synthesis, and tunable Received: 17 December 2020 functionality; therefore, it is an attractive option to create PCs with tailor-made structural Accepted: 17 March 2021 and functional development. Among various bottom-up routes, including chemical va- Published: 2 April 2021 por deposition (CVD) [4], the templating method [5], and the hydrothermal method [6], polymer-derived carbons are the most attractive one, owing to their high yield and low Publisher’s Note: MDPI stays neutral cost. In this method, PCs are created by pyrolyzing the synthesized polymeric precursors with regard to jurisdictional claims in (e.g., phenol, resorcinol, ethylene glycol, formaldehyde etc. [7,8]), endowing us with the published maps and institutional affil- ability to alter the properties of PCs through synthesizing different precursors. iations. Besides the traditional polymeric precursors mentioned above, a special type of polymer termed covalent organic frameworks (COFs) has drawn great attention in preparing PCs. COFs are crystalline porous polymer frameworks with well-defined high-order structures. This special architecture is attributed to the “self-templated” polymerization, where Copyright: © 2021 by the authors. the initial molecular structure provides specific sites to guide the subsequent monomers Licensee MDPI, Basel, Switzerland. into place, letting the polymer chain extend over a 2D plane or 3D frame with a highly This article is an open access article aligned arrangement [9]. COFs possess the advantages of having a large surface area, high distributed under the terms and porosity, tunable composition, and designable dimension of molecular skeleton and pores, conditions of the Creative Commons all of which are essential parameters to design and build PCs in the bottom-up manner. Attribution (CC BY) license (https:// π creativecommons.org/licenses/by/ To synthesize COFs, the building blocks, mainly the monomers with -backbones and 4.0/). rigid conformations, are connected through boronate-ester, imine, C=C, and other linkages

Macromol 2021, 1, 84–93. https://doi.org/10.3390/macromol1020007 https://www.mdpi.com/journal/macromol Macromol 2021, 1 85

under mild conditions [9], thus resulting in a facile method to prepare COFs as precursors for PC production. COFs have been reported as a PC precursor in many recent studies, exhibiting a high surface area, high porosity, and hetero-element doping in versatile applications such as absorbents [10], membrane capacitive deionization [11], electrodes of batteries [1], among others. In these studies, COFs were synthesized via a condensation reaction between geo- metrically symmetric building blocks with multiple reaction sites. The insoluble crosslinked COFs precipitated from the liquid phase in the form of irregular porous resins and yielded PCs of a similar architecture [1,10,11]. We therefore hypothesized that, if the COF separation process can be conducted in a more controlled manner (than precipitation), it could allow the slow stacking and organization of COFs molecules to yield COFs with long-range ordering and a well-developed 2D architecture. These resultant 2D COFs can be further converted to 2D carbons, which are reminiscent of graphene; the carbon has excellent properties but is difficult to produce. In pursuit of the facile creation of 2D nanocarbon, we report a method to synthesize poly(azomethine)-based (PAM-based) 2D COFs via the condensation of p-phenylenediamine and terephthalaldehyde. Unlike any other previously reported PAM-based COFs, in this method the as-synthesized PAMs molecule self-assembled into a COF with a 2D architecture upon the controlled solvent evaporation. The product from the high-efficiency synthesis and controlled self-assembly process resembled a porous graphitic structure with individual graphenic planar sheets. Using a facile annealing method, the COF was further converted to 2D carbon with nitrogen-doped graphitic structures and a tunable surface area (up to ~700 m2/g). We also demonstrate the utility of the COF-derived PC for water purification. While this report confined the application to water purification, we expect the material to find use in various other areas, including electronics or optoelectronics, catalysis, photovoltaics, etc., leading to the greater utilization of PAM-based COFs.

2. Experimental Section 2.1. Materials All the reagents used in this study including terephthalaldehyde (Acros Organics, 98%), p-phenylenediamine (Alfa Aesar, 97%), methanol (VWR chemicals BDH, 99.8+%), and Rhodamine B (Acros Organics, 98+%) were commercially available and used without further puriﬁcation Deionized water was used in all experiments.

2.2. Synthesis of Polyazomethine P-phenylenediamine (0.3 mol) and terephthalaldehyde (0.3 mol) were dissolved in 150 mL of methanol individually and mixed together in a glass beaker. The mixture was then placed in the fume hood to let the reaction complete and the methanol evaporate. After all the methanol evaporated, the solid product was collected and washed on a filter paper with 200 mL of methanol. The product was then subjected to the annealing at 300, 450, 600, ◦ ◦ 750, and 900 C for 2 h (at the heating/cooling rate of 10 C/min) under a N2 flow in a furnace. The final carbonaceous products were named NC-300, NC-450, NC-600, NC-750, and NC-900 according to their carbonization temperature and stored in dry conditions.

2.3. Material Characterization Attenuated Total Reﬂectance Fourier Transform-Infrared (ATR-FTIR) Spectroscopy was performed on a Thermo-Nicolet Magna 550 FTIR spectrometer. Thermogravimetric analysis (TGA) was performed using a TGA Q5000 instrument. A ~5.0 mg specimen was ◦ ◦ heated from room temperature to 1000 C at 10 C/min in the N2 atmosphere. Scanning Electron Microscopy (SEM) (Hitachi, S-4800) was used to obtain the surface morphology of PAM and NCs at an accelerating voltage of 5.0 kV. The as-prepared PAM, NC-300, and NC-450 were coated with a 2 nm gold layer before imaging in order to obtain clear SEM images, while other CN samples were imaged without the Au coating due to their sufﬁcient conductivity. Transmission Electron Microscopy (TEM) (Hitachi, HD-9500) was used to Macromol 2021, 1 86

measure pristine PAM and NC-750 at voltages from 200 to 500 KV. All the specimens were imbedded in epoxy resin ﬁrst and then cut into 80-nm-thick slides for observation. Both TEM images and live fast Fourier transform (FFT) of the TEM images were collected. X-ray Diffraction (XRD) measurements were carried out using a Rigaku Ultima III X- ray diffractometer equipped with a primary monochromator and Cu Kα source. The XRD patterns were collected with steps of 0.05◦ in the 2θ angular range from 10◦ to 50◦. Raman spectroscopy were performed on a Jobin Yvon Horiba—LabRam confocal Raman spectrometer using the He-Ne laser with an emission at 632 nm. The spectra were collected from 100 to 300 nm. X-ray Photoelectron Spectroscopy (XPS) was performed with a Kratos Axis Ultra XPS spectrometer. The surface areas of PAM and CNs were determined using a Quantachrom Autosorb 1Q apparatus. The nitrogen adsorption isotherms were collected at −196 ◦C. The samples were degassed at 100 ◦C overnight before the adsorption measurement. The speciﬁc surface areas were calculated using the Brunauer–Emmett– Teller (BET) equation (P/Po = 0.05–0.20).

2.4. Water Purification Capacity Measurement Adsorption experiments were carried out at room temperature to investigate the adsorption behaviors of Rhodamine B (RhB) towards the PAM and CNs. The absorbent—i.e., the PAM or NCs—was added to the 0.132 mg/mL RhB solution to the final concentration of 1, 2, 3, or 6 mg/mL, followed by stirring (200 rpm) at room temperature for 60 min. The free RhB in the solution was monitored using an Ultraviolet-Visible spectrophotometer (UV-Vis) (VWR UV-6300PC). Specifically, every 10 min an aliquot of the mixture was collected and centrifuged. Then, the absorbance of the supernatant was measured. The adsorption capacities of the adsorbents were calculated according to the Equation (1):

A − A C Puriﬁcation capacity of NC = 0 t × R (1) A0 CNR

where A0 and At represent the UV-Vis absorbance values at the beginning and at t min. CR and CNR are the initial concentrations of RhB and CN, respectively.

3. Results As shown in Figure1, the PAM could be facilely prepared via the condensation reaction between the dialdehyde and diamine in the methanol. The FTIR spectra of the product are shown in Figure2, conﬁrming the formation of imine. The pronounced band at ~1612 cm−1 in the spectrum corresponds to CH=N stretch in azomethine, which is evidence of imine being synthesized by the reaction. The smaller band at ~1691 cm−1 can be assigned to the unreacted aldehydes and chain ends of synthesized PAM. The peak at ~2730 cm−1 corresponds to the aldehyde stretching; the peak at ~3350 cm−1 is attributed to the secondary amine stretching. These results indicate a large amount of residual aldehyde and amine groups existing in PAM, implying the relatively low degree of polymerization of PAM [12]. The low molecular PAM was dissolvable in methanol, resulting in a translucent Macromol 2021, 1, FOR PEER REVIEWsolution in the color of orange (Figure1B). The oligomers further self-assemble to layered 4

structures during the evaporation of methanol, obtaining ﬂask-like crystals.

. Figure 1. A B Figure 1. ((A)) The The condensation condensation reaction reaction and and (B () a) picture a picture of ofthe the as-prepared as-prepared PAM PAM in methanol in methanol solution.solution.

Figure 2. FT-IR spectrum of PAM.

TGA was carried out to investigate the thermal stability of PAM (Figure 3). Two weight loss stages were observed in the TGA curve. The first stage of 6.4 wt% loss occurred in the range of 100 to 300 °C with a maximum mass loss rate at ~214 °C. This weight loss could be attributed to the release of the newly produced water by the condensation between the remaining dialdehyde and diamine at an elevated temperature. This result is consistent with that for the imine-linked amorphous and porous polymer reported earlier [13,14], suggesting that the condensation polymerization can further proceed at a high temperature (e.g., 214 °C). The second stage occurred between ~440 and ~540 °C, with a mass loss of ~55 wt%. This mass reduction is ascribed to the carbonization of PAM in N2, indicating the temperature range of carbonization [15].

Figure 3. TGA and DTG curve for PAM.

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. . Macromol 2021, 1 Figure 1. (A) The condensation reaction and (B) a picture of the as-prepared PAM in methanol 87 solution.Figure 1. (A) The condensation reaction and (B) a picture of the as-prepared PAM in methanol solution.

Figure 2. FT-IR spectrum of PAM. FigureFigure 2. 2.FT-IR FT-IR spectrum spectrum of of PAM. PAM.

TGATGA waswas carried carried out out to investigateto investigate the thermalthe thermal stability stability of PAM of (FigurePAM (Figure3). Two 3). weight Two weightTGA loss stageswas carried were observedout to investigate in the TG theA curve.thermal The stability first stage of PAM of 6.4 (Figure wt% loss 3). oc-Two lossweight stages loss were stages observed were in observed the TGA in curve. the TG TheA ﬁrstcurve. stage The of first 6.4 wt% stage loss of occurred6.4 wt% inloss the oc- rangecurred of in 100 the to range 300 ◦ofC 100 with to a300 maximum °C with a mass maximum loss rate mass at ~214 loss rate◦C. Thisat ~214 weight °C. This loss weight could losscurred could in be the attributed range of 100 to theto 300 release °C with of th a emaximum newly produced mass loss water rate atby ~214 the °C.condensation This weight beloss attributed could be to theattributed release to of the the release newly producedof the newly water produced by the condensation water by the between condensation the remainingbetween the dialdehyde remaining anddialdehyde diamine and at an diamine elevated at an temperature. elevated temperature. This result isThis consistent result is between the remaining dialdehyde and diamine at an elevated temperature. This result is withconsistent that for with the that imine-linked for the imine-linked amorphous amorphous and porous and polymer porous polymer reported reported earlier [ 13earlier,14], consistent with that for the imine-linked amorphous and porous polymer reported earlier suggesting[13,14], suggesting that the condensationthat the condensation polymerization polymerization can further can proceed further at a proceed high temperature at a high [13,14], suggesting that the condensation polymerization can further proceed at a high (e.g.,temperature 214 ◦C). (e.g., The second214 °C). stage The occurredsecond stag betweene occurred ~440 between and ~540 ~440◦C, and with ~540 a mass °C, losswith of a temperature (e.g., 214 °C). The second stage occurred between ~440 and ~540 °C, with a ~55mass wt%. loss This of ~55 mass wt%. reduction This mass is ascribed reduction to theis ascribed carbonization to the ofcarbonization PAM in N2, of indicating PAM in theN2, mass loss of ~55 wt%. This mass reduction is ascribed to the carbonization of PAM in N2, temperatureindicating the range temperature of carbonization range of [carbonization15]. [15]. indicating the temperature range of carbonization [15].

FigureFigure 3.3. TGA andand DTGDTG curvecurve forfor PAM.PAM. Figure 3. TGA and DTG curve for PAM. The morphology of the synthesized PAM and NCs derived at different temperatures is shown in Figure4. The PAM shows a ﬂake-like morphology and a porous architecture, which is distinct from that of other PAM-derived porous carbons in the morphologies of needlelike [12], disk-like [16], or random aggregates [13]. The formation of such highly ordered structures is due to the dynamic covalent chemistry controlled condensation reaction of p-phenylenediamine and terephthalaldehyde as well as the conjugation of the linear PAM molecule chains during evaporation [17]. The as-synthesized PAM exhibited a typical crystalline structure, with a thickness of sheets of less than 50 nm (Figure4). After annealing, the polymer crystalline structure changed (Figure5A,B), while the nanosheet Macromol 2021, 1, FOR PEER REVIEW 5

The morphology of the synthesized PAM and NCs derived at different temperatures is shown in Figure 4. The PAM shows a flake-like morphology and a porous architecture, which is distinct from that of other PAM-derived porous carbons in the morphologies of needlelike [12], disk-like [16], or random aggregates [13]. The formation of such highly ordered structures is due to the dynamic covalent chemistry controlled condensation reaction of p-phenylenediamine and terephthalaldehyde as well as the conjugation of the Macromol 2021, 1 linear PAM molecule chains during evaporation [17]. The as-synthesized PAM exhibited88 a typical crystalline structure, with a thickness of sheets of less than 50 nm (Figure 4). After annealing, the polymer crystalline structure changed (Figure 5A,B), while the nanosheet clustercluster remained. remained. As As presented presented in thethe SEMSEM images, images, the the NC NC sheets sheets are are of of ~50 ~50 nm nm in thickness,in thickness,possessing possessing large large surface surface areas. areas. Additionally, Additionally, holes and holes defects and appeareddefects appeared in the nanosheet in the nanosheetafter carbonization after carbonization (white arrows (white in arrows Figure 4in), Figure further 4), increasing further increasing their surface their area. surface The area.formation The formation of defects of results defects from results the from high-temperature the high-temperature annealing annealing activation, activation, which is whichevident is evident in sample in sample NC-750 NC-750 (dash circle (dash in circ Figurele in4 Figure). When 4). the When carbonization the carbonization temperature tem- ◦ peratureexceeds exceeds 750 C, the750 surface°C, the surface defects defects were exacerbated, were exacerbated, accompanied accompanied by the by shrinkage the shrink- and agecollapse and collapse of the nanosheet of the nanosheet clusters, thereby clusters, reducing thereby the reducing overall surfacethe overall area. surface These resultsarea. Theseare consistent results are with consistent the decreased with the carbon decrease yieldsd carbon at high yields carbonization at high carbonization temperatures, tem- as peratures,observed as in observed the TGA measurementsin the TGA measurements (Figure3). (Figure 3).

Polymer NC- 300 NC- 450 Macromol 2021, 1, FOR PEER REVIEW 6

peak at 2θ = 44°, implying its high10 um graphite percentage10 um [19]. Moreover, the high-resolution TEM image and FFT result show that the PAM has a highly regulated structure (see spots indicated by pink arrows in Figure 5), further confirming the crystallinity of the PAM-based COF. In addition, lattice fringes appeared in the NC-750 TEM image (white- circled region in Figure 4), suggesting the formation of nanostructures. These results are 500 nm 500 nm in agreement with the XRD analysis, demonstrating the structural transition from crystalline PAM to graphitic carbon. NC- 600The Raman spectraNC- 750 of PAM and NC-NC-750 900 are shown in Figure 6. NC-750 exhibits an evident D band (at 1337 cm−1) and G band (at 1584 cm−1), corresponding to the structure of sp3-hybridized disordered carbon and sp2-hybridized graphitic carbon, respectively. The Raman pattern suggests the formation of graphitic carbon after high-temperature an- 10 um 10 um 10 um nealing. The pronounced D band in NC-750 indicates the presence of defect sites [20,21], which are associated with defects caused by nitrogen atoms in the graphitic structure [22]. The 2D band in NC-750 at 2700 cm−1 suggests the graphitic sp2 materials and implies the existence of single-layer graphene-like structure in the materials. The Raman bands of PAM at 1570 500cm nm−1 and 1622 cm−1500 are nm attributed to the 500 nm stretching vibrations of the benzene

ring and C=N, respectively. Meanwhile, the band at 1197 cm−1 corresponds to the C-C of FigureFigurethe benzene 4. 4. SEMSEM images imagesring [23]. of of PAM PAM The crystal crystalspectra and and of CNs. CNs.PAM White White further a arrowsrrows confirm and circles the imine indicate formed thethe surfacesurface by the defectsde- con- fectsofdensation CNs. of CNs. reaction [24].

The XRD patterns of PAM and CNs are presented in Figure 5. Four peaks were observed in both PAM and NC-300 at 2θ of 14.4°, 20.3°, 23.7°, and 29.0°, corresponding to (002), (110), (200), and (210) diffractions, respectively [18]. The sharp peaks suggest the PAM has a high crystallinity, which is associated with molecule self-assembly during methanol evaporation. The typical peaks observed in PAM were retained in NC-300. Meanwhile, for NC-450, NC-600, NC-750, and NC-900, a broad peak centered at 2θ of 24° and a narrow peak centered at 2θ of 44° were observed, which are attributed to (002) and (100) diffractions of a layered graphitic structure. The loss of sharp diffraction signals indicates the transition from crystalline polymer to an amorphous random framework during the pyrolysis. These two bands indicate the formation of graphitic structures in NC- 450, NC-600, NC-750, and NC-900. Compared to other CNs, NC-900 exhibited the highest

FigureFigure 5. 5.( (AA,B,B)) XRD XRD and and of of PAM PAM and and NCs. NCs. (C (C,D,D) The) The high-resolution high-resolution TEM TEM image image and and FFT FFT results results of PAMof PAM and and NC750. NC750.

Macromol 2021, 1, FOR PEER REVIEW 6

peak at 2θ = 44°, implying its high graphite percentage [19]. Moreover, the high-resolution TEM image and FFT result show that the PAM has a highly regulated structure (see spots indicated by pink arrows in Figure 5), further confirming the crystallinity of the PAM-based COF. In addition, lattice fringes appeared in the NC-750 TEM image (white- circled region in Figure 4), suggesting the formation of nanostructures. These results are in agreement with the XRD analysis, demonstrating the structural transition from crystalline PAM to graphitic carbon. The Raman spectra of PAM and NC-750 are shown in Figure 6. NC-750 exhibits an Macromol 2021, 1 evident D band (at 1337 cm−1) and G band (at 1584 cm−1), corresponding to the structure89 of sp3-hybridized disordered carbon and sp2-hybridized graphitic carbon, respectively. The Raman pattern suggests the formation of graphitic carbon after high-temperature an- The XRD patterns of PAM and CNs are presented in Figure5. Four peaks were nealing. The pronounced D band in NC-750 indicates the presence of defect sites [20,21], observed in both PAM and NC-300 at 2θ of 14.4◦, 20.3◦, 23.7◦, and 29.0◦, corresponding which are associated with defects caused by nitrogen atoms in the graphitic structure [22]. to (002), (110), (200), and (210) diffractions, respectively [18]. The sharp peaks suggest The 2D band in NC-750 at 2700 cm−1 suggests the graphitic sp2 materials and implies the the PAM has a high crystallinity, which is associated with molecule self-assembly during existence of single-layer graphene-like structure in the materials. The Raman bands of methanol evaporation. The typical peaks observed in PAM were retained in NC-300. PAM at 1570 cm−1 and 1622 cm−1 are attributed to the stretching vibrations of the benzene Meanwhile, for NC-450, NC-600, NC-750, and NC-900, a broad peak centered at 2θ of ring and C=N, respectively. Meanwhile, the band at 1197 cm−1 corresponds to the C-C of 24◦ and a narrow peak centered at 2θ of 44◦ were observed, which are attributed to (002) andthe benzene (100) diffractions ring [23]. of The a layered spectra graphitic of PAM structure.further confirm The loss the of imine sharp formed diffraction by the signals con- indicatesdensation the reaction transition [24]. from crystalline polymer to an amorphous random framework during the pyrolysis. These two bands indicate the formation of graphitic structures in NC- 450, NC-600, NC-750, and NC-900. Compared to other CNs, NC-900 exhibited the highest peak at 2θ = 44◦, implying its high graphite percentage [19]. Moreover, the high-resolution TEM image and FFT result show that the PAM has a highly regulated structure (see spots indicated by pink arrows in Figure5), further conﬁrming the crystallinity of the PAM-based COF. In addition, lattice fringes appeared in the NC-750 TEM image (white-circled region in Figure4), suggesting the formation of nanostructures. These results are in agreement with the XRD analysis, demonstrating the structural transition from crystalline PAM to graphitic carbon. The Raman spectra of PAM and NC-750 are shown in Figure6. NC-750 exhibits an evident D band (at 1337 cm−1) and G band (at 1584 cm−1), corresponding to the structure of sp3-hybridized disordered carbon and sp2-hybridized graphitic carbon, respectively. The Raman pattern suggests the formation of graphitic carbon after high-temperature annealing. The pronounced D band in NC-750 indicates the presence of defect sites [20,21], which are associated with defects caused by nitrogen atoms in the graphitic structure [22]. The 2D band in NC-750 at 2700 cm−1 suggests the graphitic sp2 materials and implies the existence of single-layer graphene-like structure in the materials. The Raman bands of PAM at 1570 cm−1 and 1622 cm−1 are attributed to the stretching vibrations of the benzene ring and C=N, respectively. Meanwhile, the band at 1197 cm−1 corresponds to the C-C ofFigure the benzene5. (A,B) XRD ring and [23 of]. PAM The spectraand NCs. of (C PAM,D) The further high-resolution conﬁrm TEM the imine image formedand FFT byresults the condensationof PAM and NC750. reaction [24].

Figure 6. Raman spectra of PAM and NC-750.

The existence of various functionalities is evident in the XPS spectrum (Figure 7). The strong C1s signal at 284.6 eV (Figure 7A) is associated with the sp2-hybridized carbon, suggesting that carbon elements were dominantly arranged in the graphitic structure. The broad peak at 288.5 eV is the overlaid C-N and C-O peaks, suggesting that the nitrogen or oxygen elements were doped into the graphitic structure as defects. The peak centered at 286.8 eV indicates the presence of C=N, probably related to the amide synthesized in the Macromol 2021, 1 condensation reaction. The deconvolution of the O1s spectrum (Figure 7B) gives90 three components (centered at 530.6, 532.1, and 533.7 eV), which correspond to the N-C=O, C=O, and C-O entities, respectively. It has been reported that oxygen-containing functional C=O, and C-O entities, respectively. It has been reported that oxygen-containing functional groups at the carbon surface play a critical role in the adsorption capacity of porous car- groups at the carbon surface play a critical role in the adsorption capacity of porous carbons bonsbecause because they they promote promote carbon carbon wettability wettability and also regulateand also the regulate formation the of waterformation clusters of at water clustersthe carbon at the surface carbon [25 surface,26]. The [25,26]. N1s peaks The centered N1s peaks at 398.5, centered 400.6, at and 398.5, 402.9 400.6, eV (Figure and 7402.9C) eV (Figurecorrespond 7C) correspond to the -N=C-, to -N+H-,the -N=C-, and =N+H--N+H-, components. and =N+H- The components. XPS spectra The conﬁrm XPS the spectra confirmC-N conjugated the C-N conjugated structure in structure the graphitic in the structure graphitic of NCs. structure of NCs.

FigureFigure 7. The 7. The deconvoluted deconvoluted XPS XPS spectra spectra of of ( (AA)) the the C1s,C1s, ((BB)) N1s, N1s, and and (C ()C O1s) O1s signal signal of NC-750. of NC-750.

To evaluate the water purification capacity of NCs, the rhodamine B (RhB) was chosen To evaluate the water purification capacity of NCs, the rhodamine B (RhB) was cho- as the model contaminant, and its aqueous solution was treated with CNs of different sentypes as the and model amounts. contaminant, The levels and of residual its aqueous RhB insolution the solution was treated were then with determined CNs of different by typesmeasuring and amounts. the absorbance The levels of the of treatedresidual RhB Rh solution.B in the solution The maximum were then absorbance determined was by measuringobserved the at 554 absorbance nm (Figure of8A), the which treated is proportional RhB solution. to the The concentration maximum of absorbance the remaining was ob- servedRhB at in the554 solution.nm (Figure The 8A), RhB solutionwhich is after proportional the NC-750 to treatment the concentration showed the of lowest the remaining peak RhBintensity, in the solution. implying The it had RhB the solution highest purification after the NC-750 capacity treatment of NC-750 showed among CNs. the lowest On the peak intensity,contrary, implying NC-300 gaveit had the the highest highest peak purification height, suggesting capacity it of had NC-750 the lowest among adsorption CNs. On the contrary,capacity. NC-300 Furthermore, gave the the high continuousest peak increase height, in suggesting the adsorption it had capacity the lowest (from PAM adsorption to CN-750) reflects the evolution of the material during the carbonization. NC-300 presented a capacity. Furthermore, the continuous increase in the adsorption capacity (from PAM to similar dye absorption profile to that of PAM (Figure8A), implying the lower impacts of the CN-750)300 ◦C reflects heat treatment the evolution on the adsorption of the material properties during of the the polymer. carbonization. This result NC-300 is consistent presented a similarwith the dye XRD absorption results (Figure profile5A), to inthat which of PAM the typical(Figure PAM 8A), crystalline implying structurethe lower was impacts of theretained 300 °C in NC-300.heat treatment On the other on hand,the adsorption higher carbonization properties temperatures of the polymer. have significant This result is consistentimpacts with on the the absorption XRD results properties (Figure of 5A), CNs. in The which water the purification typical PAM capacity crystalline of CNs struc- ◦ tureincreased was retained dramatically in NC-300. when On the the carbonization other hand, temperature higher carbonization exceeded 300 temperaturesC, achieved a have ◦ significantmaximum impacts at 750 C,onand the reducedabsorption slightly properties in NC-900 of (Figure CNs. 8TheB,D). water The RhB purification absorption capacity is of CNsrelated increased to the surface dramatically area (SA) and when the dye-carbonthe carbonization interaction temperature at the surface, exceeded both of which 300 °C, are associated with the pyrolysis temperature. The NC SA measured by the BET method achieved a maximum at 750 °C, and reduced slightly in NC-900 (Figure 8B,D). The RhB clearly demonstrated the correlation between high SA and high carbonization temperature absorption(Figure8C,D). is related Pyrolysis to the at surface 300 ◦C area leads (SA) to a and decrease the dye-carbon in SA, which interaction might be relatedat the surface, to boththe of collapse which are of smallassociated pores causedwith the by pyrolysis polymer temperature. melting. Pyrolysis The NC above SA 600 measured◦C led to by the BETan method SA of 675–700 clearly m demonstrated2/g, which is considerably the correlati higheron between compared high with SA that and of high other carboniza- PAM- tionderived temperature NCs without (Figure further 8C,D). activation Pyrolysis [13 at]. 300 Therefore, °C leads the to lowera decrease purification in SA, capacitywhich might be relatedobserved to in the NC-600 collapse and of NC-900 small could pores be ca ascribedused by to polymer their surface melting. conditions, Pyrolysis which above are 600 °C ledunfavorable to an SA for of RhB-NC675–700 adsorption. m2/g, which It is is conceived considerably that aromatic higher compared RhB was adsorbed with that on theof other π π PAM-derivedgraphene-like NCs carbons without surface further through activation intensive [1-3].stacking Therefore, interaction the lower and electrostaticpurification ca- interaction (graphene is negatively charged and RhB is positively charged) [27]. The pacity observed in NC-600 and NC-900 could be ascribed to their surface conditions, CN-600 possessed a low adsorption capability in spite of its high SA, probably implying the undeveloped graphene structure processed at low temperature (600 ◦C). Figure8C demonstrates absorption kinetics of the CN. It is noted that curves of CN-750 were not presented, regardless of its high purification capacity. This is because CN-750 absorbs RhB very fast, reaching a ~100% RhB removal at a low CN-750 loading within a short period (e.g., 1 mg/mL within 30 min and 2 mg/mL within 20 min), yielding overlapped curves Macromol 2021, 1, FOR PEER REVIEW 8

which are unfavorable for RhB-NC adsorption. It is conceived that aromatic RhB was adsorbed on the graphene-like carbons surface through intensive π-π stacking interaction and electrostatic interaction (graphene is negatively charged and RhB is positively charged) [27]. The CN-600 possessed a low adsorption capability in spite of its high SA, probably implying the undeveloped graphene structure processed at low temperature (600 °C). Figure 8C demonstrates absorption kinetics of the CN. It is noted that curves of Macromol 2021, 1 CN-750 were not presented, regardless of its high purification capacity. This91 is because CN-750 absorbs RhB very fast, reaching a ~100% RhB removal at a low CN-750 loading within a short period (e.g., 1 mg/mL within 30 min and 2 mg/mL within 20 min), yielding that are unclearoverlapped to demonstrate. curves Instead,that are theunclear curves to ofdemonst CN-600rate. manifested Instead, the curves absorption of CN-600 mani- behavior morefested clearly, the suggesting absorption that behavior the puriﬁcation more clearly, rates suggesting decreased that after the 20 minpurification and rates de- became stablecreased gradually after after 20 30min min and for became all different stable concentrations.gradually after 30 Both min the for 3 all and different 6 mg concentra- NC-600 can removetions. Both all the the contaminant 3 and 6 mg NC-600 within can 60 min, remove resulting all the incontaminant the colorless within solution 60 min, resulting seen in Figure8inC. the colorless solution seen in Figure 8C.

Figure 8. (AFigure) The UV-visible 8. (A) The UV-visiblespectra, (B) spectra, water purification (B) water puriﬁcation capacity, (C capacity,) absorption (C) absorptioncurves, and curves, (D) BET and surface (D) area of PAM and NCs.BET surface area of PAM and NCs.

4. Conclusions4. Conclusions We have demonstratedWe have an demonstrated easy one-step methodan easy to synthesizeone-step method polyazomethine to synthesize nanosheet polyazomethine clusters via a condensationnanosheet clusters reaction via a followedcondensation by solvent reaction evaporation. followed by solvent The hierarchical evaporation. The hier- structure couldarchical be retained structure after could pyrolysis, be retained during after which pyrolysis, the crystalline during which polymer the crystalline was polymer converted to nitrogen-dopedwas converted to carbon nitrogen-doped nanosheets carbon (NCs). na Thenosheets pyrolysis (NCs). temperature The pyrolysis was temperature found to be a criticalwas found parameter to be a critical for the parameter properties for of thethe CNs.properties CNs treatedof the CNs. at 600–900 CNs treated◦C at 600–900 possessed large°C surface possessed areas large of upsurface to 700 areas m2/g, of up assembled to 700 m2 by/g, assembled ~50 nm-thick by ~50 nanosheets. nm-thick nanosheets. Pyrolysis alsoPyrolysis created holes also created and defects holes and on the defects sheets, on th whiche sheets, further which increased further increased surface surface area area of CNs. Theof CNs. resulting The resulting CNs were CNs composed were composed mainly of mainly graphic of carbonsgraphic withcarbons nitrogen with nitrogen and and oxygen dopingoxygen that doping was that introduced was introduced by the by polymer the polymer synthesis synthesis and carbonization.and carbonization. The as- The as-preparedprepared porous porous CNs were CNs usedwere toused absorb to absorb aromatic aromatic dye indye the in water.the water. The The CNs CNs exhibited exhibited a gooda good adsorption adsorption capacity. capacity. CN-750 CN-750 displayed displayed the the highest highest adsorption adsorption capacity capacity (0.025 mg), (0.025 mg), attributed to its high surface area, graphitic nature, and surface condition. This study highlights the bottom-up synthesis of 2D carbon nanosheets through self-templated assembly. This method enables molecular architectural design, controllable synthesis, and tunable functionality, therefore providing potential applications not only in water puriﬁcation but also where next-generation carbon materials with a high surface area, open pores, precision molecular design, and long-range regularity could be used for, e.g., energy storage, gas separation, catalysis, and electronic devices. Macromol 2021, 1 92

Author Contributions: Conceptualization, X.Y. and S.P.; methodology, X.Y.; validation, X.Y., and T.Z.; formal analysis, X.Y. and T.Z.; writing—original draft preparation, X.Y.; writing—review and editing, T.Z.; supervision, S.P.; project administration, S.P.; funding acquisition, S.P. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Animal Coproducts Research and Education Center consortium and the office of the Vice-President for Research at Clemson University, Robert Patrick Jenkins Professorship, and Dean’s Faculty Fellow Professorship. T.Z. would like to acknowledge the financial support provided by the Cooper-Standard Postdoctoral Fellowship. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: The authors would like to acknowledge Kimberly Ivey for her support in TGA and FTIR analysis. Conflicts of Interest: The authors declare no conflict of interest.

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