Functional Carbon Materials Derived Through Hypergolic Reactions at Ambient Conditions

nanomaterials

Article Functional Carbon Materials Derived through Hypergolic Reactions at Ambient Conditions

Nikolaos Chalmpes 1, Georgios Asimakopoulos 1, Konstantinos Spyrou 1, Konstantinos C. Vasilopoulos 1 , Athanasios B. Bourlinos 2,*, Dimitrios Moschovas 1, Apostolos Avgeropoulos 1 , Michael A. Karakassides 1 and Dimitrios Gournis 1,*

1 Department of Materials Science & Engineering, University of Ioannina, 45110 Ioannina, Greece; [email protected] (N.C.); [email protected] (G.A.); [email protected] (K.S.); [email protected] (K.C.V.); [email protected] (D.M.); [email protected] (A.A.); [email protected] (M.A.K.) 2 Physics Department, University of Ioannina, 45110 Ioannina, Greece * Correspondence: [email protected] (A.B.B.); [email protected] (D.G.); Tel.: +30-265-100-7141 (D.G.)

Received: 13 February 2020; Accepted: 16 March 2020; Published: 20 March 2020

Abstract: Carbon formation from organic precursors is an energy-consuming process that often requires the heating of a precursor in an oven at elevated temperature. In this paper, we present a conceptually diﬀerent synthesis pathway for functional carbon materials based on hypergolic mixtures, i.e., mixtures that spontaneously ignite at ambient conditions once its ingredients contact each other. The reactions involved in such mixtures are highly exothermic, giving-oﬀ sizeable amounts of energy; hence, no any external heat source is required for carbonization, thus making the whole process more energy-liberating than energy-consuming. The hypergolic mixtures described here contain a combustible organic solid, such as nitrile rubber or a hydrazide derivative, and fuming nitric acid (100% HNO3) as a strong oxidizer. In the case of the nitrile rubber, carbon nanosheets are obtained, whereas in the case of the hydrazide derivative, photoluminescent carbon dots are formed. We also demonstrate that the energy released from these hypergolic reactions can serve as a heat source for the thermal conversion of certain triazine-based precursors into graphitic carbon nitride. Finally, certain aspects of the derived functional carbons in waste removal are also discussed.

Keywords: hypergolic reactions; fuming nitric acid; carbon nanosheets; graphitic carbon nitride; photoluminescent carbon dots

1. Introduction Hypergolic mixtures consist of two diﬀerent substances that spontaneously ignite upon coming into contact at ambient conditions. One substance plays the role of a strong oxidizer (fuming HNO3, N2O4,H2O2, Cl2), whereas the other one the role of the combustible fuel (aniline, nitrile rubber, hydrazine derivatives, ionic liquids, acetylene) [1–6]. Such reactions are often used in chemistry demonstrations to show the impressive power of chemical energy, as well as ways to exploit it in real life applications (e.g., rocket propellants and fuels). The ﬂames, smokes, sound, and motion produced by these reactions create a stunning result that captivates the senses of an observer. Interestingly, many of these reactions produce carbon, if organic fuels are used; surprisingly, in the literature, only little emphasis is given on this important aspect of hypergolic reactions [6]. Taking into consideration the central role of carbon in materials science and synthesis, hypergolic reactions can certainly provide an energy-saving and thermodynamically favoured alternative towards the direct synthesis of carbon materials at ambient conditions. In this respect, no external source of heat is required to carbonize the organic precursor, such as energy-consuming ovens operating at elevated temperature. As a matter of

Nanomaterials 2020, 10, 566; doi:10.3390/nano10030566 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, x FOR PEER REVIEW 2 of 16

Nanomaterials 2020, 10, 566 2 of 13 is required to carbonize the organic precursor, such as energy-consuming ovens operating at elevated temperature. As a matter of fact, hypergolic reactions release sizeable amounts of energy fact,that could hypergolic be further reactions exploited release in sizeable the production amounts of of useful energy work that could(mechanical, be further electrical, exploited chemical, in the productionetc.) [7]. In addition, of useful such work reactions (mechanical, are kinetica electrical,lly chemical,favoured, etc.)thus [allowing7]. In addition, rapid product such reactions formation are kineticallyat room temperature favoured, thusand atmospheric allowing rapid pressure product (e.g., formation within few at room seconds). temperature The main and purpose atmospheric of this pressurework is to (e.g., highlight within the few importance seconds). Theof hypergolic main purpose reactions of this in workthe area is toof highlightcarbon materials the importance synthesis. of hypergolicTo this aim, reactions we provide in the examples area of carbon of hypergolic materials reactions synthesis. that To thisdirectly aim, lead we provideto the formation examples of hypergolicdifferent types reactions of functional that directly carbon lead tomaterials the formation at ambient of diff erentconditions. types ofFunctional functional carbon carbon materials atinclude ambient carbon conditions. nanosheets Functional [8], grap carbonhitic carbon materials nitride include [9], and carbon phot nanosheetsoluminescent [8], carbon graphitic dots carbon [10]. nitrideIn one [case,9], and we photoluminescent show that the reaction carbon of dots disposab [10]. Inle one nitrile case, gloves we show (acrylonitrile that the reaction butadiene of disposable rubber) nitrilewith fuming gloves nitric (acrylonitrile acid (100% butadiene HNO3) rubber)results in with carbon fuming nanosheets, nitric acid thus (100% offering HNO a3) way results to transform in carbon nanosheets,this waste plastic thus o intoffering a valuable a way to product. transform Moreover, this waste the plastic heat intoproduced a valuable fromproduct. the ignition Moreover, of the theglove heat can produced be further from utilized the ignition for the of thepreparation glove can of be other further important utilized forcarbon the preparation materials, ofsuch other as importantgraphitic carbon carbon nitride materials, by suchin-situ as graphiticheating of carbon a suitable nitride molecular by in-situ precursor. heating of In a suitableanother molecular case, the precursor.Girard’s reagent In another T case,(a hydrazide the Girard’s derivative) reagent T (a is hydrazide treated derivative)with fuming is treated nitric with acid fuming to afford nitric acidaqueous-dispersible to afford aqueous-dispersible photoluminescent photoluminescent carbon dots. carbonThe choice dots. of The Girard’s choice reagent of Girard’s T as reagent hypergolic T as hypergolicfuel is novel fuel in is the novel literature in the literature and was and merely was merelybased basedon the on fact the that fact hydrazine that hydrazine derivatives derivatives are arecommon common ingredients ingredients in inhypergolic hypergolic mixtures. mixtures. The The molecular molecular structure structure of of each each combustible fuel (nitrile(nitrile rubberrubber andand Girard’sGirard’s reagent reagent T) T) is is shown shown in in Scheme Scheme1. 1. Lastly, Lastly, we we show show practical practical applications applications of theof the derived derived functional functional carbons carbons in wastein waste removal, removal, such such as hexavalentas hexavalent chromium chromium and and organic organic dyes. dyes.

Scheme 1. Molecular structures of the disposable nitrile glove (left) and Girard’s reagent T (right). Scheme 1. Molecular structures of the disposable nitrile glove (left) and Girard’s reagent T (right). 2. Materials and Methods 2. Materials and Methods All procedures were performed in a safety hood with a ceramic tile bench. For the synthesis of carbonAll nanosheets, procedures a were medium performed size disposable in a safety nitrile hood glove with (ca. a ceramic4 g, DURAGLOVE tile bench.® For, Penang, the synthesis Malaysia) of wascarbon placed nanosheets, inside a largea medium test tube size (diameter: disposable 3 cm; nitrile length: glove 20 cm),(ca. followed4 g, DURAGLOVE by the quick®, additionPenang, Malaysia) was placed inside a large test tube (diameter: 3 cm; length: 20 cm), followed by the quick of 10 mL fuming HNO3 (100%, Sigma–Aldrich, St. Louis, MO, USA). Upon contact, nitric acid and nitrileaddition glove of 10 were mL spontaneously fuming HNO ignited,3 (100%, releasing Sigma–Aldrich, brown gases St. Louis, (nitrogen MO, oxides) USA). andUpon an contact, intense flame.nitric Afteracid and reaction nitrile completion, glove were carbon spontaneously foam was ignited, formed releasing (minor residues brown ofgases unreacted (nitrogen glove oxides) were and simply an cutintense out). flame. The foam After was reaction crashed completion, into a fine powdercarbon foam and copiously was formed washed (minor with residues dimethylformamide of unreacted (DMF),glove were tetrahydrofuran simply cut out). (THF), The a saturatedfoam was ethylenediaminetetraaceticcrashed into a fine powder acid(EDTA) and copiously aqueous washed solution with dimethylformamide (DMF), tetrahydrofuran (THF), a saturated ethylenediaminetetraacetic (EDTA: chelating agent), water, and finally with acetone prior to drying at 80 ◦C for a day. The obtained acid(EDTA) aqueous solution (EDTA: chelating agent), water, and finally with acetone prior to powderNanomaterials is thereafter 2020, 10, x FOR denoted PEER asREVIEW C-GLOVE (yield: 5%). The overall procedure is visualized in Figure3 of 116. drying at 80 °C for a day. The obtained powder is thereafter denoted as C-GLOVE (yield: 5%). The overall procedure is visualized in Figure 1.

FigureFigure 1.1. FumingFuming nitricnitric acidacid causescauses ignitionignition ofof nitrilenitrile glove,glove, leadingleading toto thethe formationformation ofof carboncarbon foamfoam (far-right(far-right photo).photo). Crashing and purification puriﬁcation of thethe foam gives a ﬁnefine powderpowder composedcomposed ofof carboncarbon nanosheetsnanosheets (far-right(far-right inset).inset).

For the synthesis of carbon dots, 1.5 g Girard’s reagent T (Sigma–Aldrich, St. Louis, MO, USA) was charged in a small test tube (diameter: 1.5 cm; length: 15 cm), followed by the dropwise addition of 1.5 mL fuming nitric acid (100 %, Sigma–Aldrich, St. Louis, MO, USA). The hydrazide derivative was instantly ignited upon contact with the strong oxidizer releasing brown fumes and an intense flame (Figure 2). Simultaneously, a brown residue was formed on the walls of the tube (Figure 2), which was carefully collected and dissolved in a few mL of water. The aqueous solution was filtered off and then placed in a dialysis membrane (molecular weight cut-off MWCO: 14,000) for 48 h—dialysis against water (water was sporadically changed from time to time during dialysis). Finally, the dialyzed solution was filtered off to afford a clear yellowish dispersion of carbon dots in water (yield: 1%).

Figure 2. Dropwise addition of fuming nitric acid into Girard’s reagent T ignites the hydrazide derivative, thus resulting in a water-soluble brown residue on the walls of the test tube (see inset).

Powder X-ray diffraction (XRD) was performed using background-free Si wafers and Cu Ka radiation from a Bruker Advance D8 diffractometer (Bruker, Billerica, MA, USA). Raman spectra were recorded with a RM 1000 Renishaw micro-Raman system (Renishaw, Old Town, UK) using a laser excitation line at 532 nm (1 mW). Attenuated total reflection infrared spectroscopy (ATR-IR) measurements were performed using a Jasco IRT-5000 microscope coupled with a FT/IR-4100 spectrometer (Jasco, Easton, MD, USA). The ZnSe prism of the ATR objective had a 250 μm area in contact with the sample. Background was subtracted and the baseline was corrected for all spectra. Ultraviolet-Visible (UV-Vis) spectra were measured in quartz cuvettes with a UV2401 (PC)-Shimadzu spectrophotometer (Shimadzu, Kyoto, Japan). Fluorescence measurements were carried out using a luminescence spectrofluorometer Jasco-8300 (Tokyo, Japan) using a 1 cm path length quartz cuvette. Slit widths with a nominal band pass of 5 nm were used for both excitation

Nanomaterials 2020, 10, x FOR PEER REVIEW 3 of 16

Figure 1. Fuming nitric acid causes ignition of nitrile glove, leading to the formation of carbon foam (far-right photo). Crashing and purification of the foam gives a fine powder composed of carbon Nanomaterials 2020, 10, 566 3 of 13 nanosheets (far-right inset).

For the synthesis of carbon dots, 1.5 g Girard’s reagent T (Sigma–Aldrich, St. Louis, MO, USA) was charged in a small test tube (diameter:(diameter: 1.5 cm; length: 15 cm), followed followed by the dropwise addition of 1.5 mL fuming nitric acid (100 %,%, Sigma–Aldrich,Sigma–Aldrich, St. Louis, MO, USA). The hydrazide derivative was instantly ignited upon contact with the strong oxidizer releasing brown fumes and an intense flameflame (Figure 22).). Simultaneously,Simultaneously, aa brownbrown residueresidue waswas formedformed onon thethe wallswalls ofof thethe tubetube (Figure(Figure2 2),), which waswas carefullycarefully collectedcollected and and dissolved dissolved in in a a few few mL mL of of water. water. The The aqueous aqueous solution solution was was filtered filtered off andoff and then then placed placed in a dialysis in a dialysis membrane membrane (molecular (molecular weight cut-oweightff MWCO: cut-off 14,000)MWCO: for 14,000) 48 h—dialysis for 48 againsth—dialysis water against (water waswater sporadically (water was changed sporadically from time changed to time from during time dialysis). to time Finally, during the dialysis). dialyzed solutionFinally, the was dialyzed filtered osolutionff to afford was a filtered clear yellowish off to afford dispersion a clear ofyellowish carbon dotsdispersion in water of (yield:carbon 1%). dots in water (yield: 1%).

Figure 2. DropwiseDropwise addition addition of of fuming fuming nitric nitric acid in intoto Girard’s reagent T ignites the hydrazide derivative,derivative, thus resulting in a water-soluble brownbrown residueresidue onon thethe wallswalls ofof thethe testtest tubetube (see(see inset).inset).

Powder X-ray didiffractionffraction (XRD) was performed using background-free Si wafers and Cu Ka radiation from a Bruker Advance D8 didiffractometffractometerer (Bruker, Billerica, MA, USA). Raman spectra were recorded with a RM 1000 Renishaw micro-Ramanmicro-Raman system (Renishaw, Old Town, UK) using a laser excitation line at 532 nm (1 mW). Attenuated total reflection reflection infrared spectroscopy (ATR-IR) (ATR-IR) measurements were performed using a Jasco IRT-5000 microscope coupled with a FTFT/IR-4100/IR-4100 spectrometer (Jasco, Easton, MD, USA). The The ZnSe ZnSe prism prism of of the the ATR ATR objective had a 250 μµm area in contact with the sample. Background Background was was subtracted subtracted and and the the baseline baseline was was corrected corrected for for all all spectra. spectra. Ultraviolet-Visible (UV-Vis)(UV-Vis) spectra spectra were were measured measured in quartz in cuvettes quartz with cuvettes a UV2401 with (PC)-Shimadzu a UV2401 spectrophotometer(PC)-Shimadzu spectrophotometer (Shimadzu, Kyoto, (Shimadzu, Japan). Fluorescence Kyoto, Japan). measurements Fluorescence were measurements carried out using were a luminescencecarried out using spectrofluorometer a luminescence Jasco-8300spectrofluoromet (Tokyo,er Japan) Jasco-8300 using (Tokyo, a 1 cm pathJapan) length using quartz a 1 cm cuvette. path Slitlength widths quartz with cuvette. a nominal Slit widths band with pass aof nominal 5 nm wereband usedpass of for 5 bothnm were excitation used for and both emission excitation ray. The fluorescence emission spectra were recorded from 350 to 650 nm after excitation at different wavelengths, with a scan speed of 100 nm min 1. All emission measurements were taken at room · − temperature and for every scanned sample, a baseline was recorded and subtracted from the spectrum. X-ray photoelectron spectroscopy (XPS) measurements were performed in an ultra-high vacuum at a base pressure of 4 10 10 mbar with a SPECS GmbHspectrometer equipped with a monochromatic × − Mg Kα source (hv = 1253.6 eV) and a Phoibos-100hemispherical analyser (Berlin, Germany). The N2 adsorption-desorption isotherms were measured at 77 K on a Sorptomatic 1990, Thermo Finnigan porosimeter (Thermo Finnigan LLC, San Jose, CA, USA). All samples were outgassed at 150 ◦C for 20 h under vacuum before measurements. Specific surface areas were determined with the Brunauer-Emmett-Teller (BET) method. Atomic force microscopy (AFM) images were collected in tapping mode with a Bruker Multimode 3D Nanoscope (Ted Pella Inc., Redding, CA, USA) using a microfabricated silicon cantilever type TAP-300G, with a tip radius of <10 nm and a force constant of Nanomaterials 2020, 10, 566 4 of 13 approximately 20–75 N m 1. The transmission electron microscopy (TEM) study of samples deposited · − on carbon coated copper grids (CF300-CU-UL, carbon square mesh, CU, 300 mesh from Electron Microscopy Science) was performed using the instrument JEM HR-2100, JEOL Ltd., Tokyo, Japan operated at 200 kV in bright-field mode.

3. Results and Discussion

3.1. Carbon Nanosheets

The XRD pattern of C-GLOVE exhibited a very broad reflection with d002 value of 3.7–4.1 Å (Figure3, top). This value is higher than the interlayer distance of crystalline graphite (3.34 Å), thus signalling the formation of amorphous carbon [11]. C-GLOVE also contained several other inorganic phases originating from the nitrile glove. Hence, we could additionally identify the presence of clay (layered silicate) and titania fillers, the latter being added in the glove for mechanical reinforcement and protection against thermal or light degradation [12,13]. Titanium was also qualitatively detected in the glove by X-ray fluorescence (XRF) analysis. Based on thermal gravimetric analysis (TGA) in air, the fillers account for nearly 15% of the C-GLOVE’s composition. It is expected that these residual fillers could further enhance the material’s potentiality in absorption/removal processes or/and photocatalysis. Attempts to remove these fillers by HF (48%) and concentrated HCl (35%) resulted in complete etching of clay but not of titania, especially rutile (Figure3 top, inset). Nanomaterials 2020, 10, x FOR PEER REVIEW 5 of 16

FigureFigure 3. X-ray 3. X-ray diffraction diffraction (XRD) (XRD) pattern pattern (top ()top and) and Raman Raman spectrum spectrum (bottom (bottom) of) of C-GLOVE. C-GLOVE. The The XRD patternXRD of thepattern nitrile of glovethe nitrile is also glove presented is also presented for comparison for comparison (blue line). (blue The line). XRD The pattern XRD ofpattern the acid-etched of the C-GLOVEacid-etched is given C-GLOVE as inset. is given as inset.

SimilarlySimilarly with with XRD, XRD, Raman Raman spectroscopy spectroscopy also also pinpointed pinpointed the the formation formation of of amorphous amorphous carbon carbon [11], 1 −1 1 −1 showing[11], showing broad D broad (1373 D cm (1373− ) cm and) Gand (1590 G (1590 cm −cm) bands) bands with with aa relatively high high intensity intensity ratio ratio of of ID/IG I=D/I0.7G = [0.714 ][14] (Figure (Figure3, bottom).3, bottom). AFM AFM study study of of the the samplesample revealed revealed the the presence presence of ofthin thin nanosheets nanosheets with an average thickness of 2 to 2.5 nm and micron-sized lateral dimensions (Figure 4, top). The morphology and size of the sheets were confirmed by TEM as well (Figure 4, bottom).

Nanomaterials 2020, 10, 566 5 of 13 with an average thickness of 2 to 2.5 nm and micron-sized lateral dimensions (Figure4, top). TheNanomaterials morphology 2020,and 10, x FOR size PEER of the REVIEW sheets were conﬁrmed by TEM as well (Figure4, bottom). 6 of 16

Figure 4. Top: Atomic force microscopy (AFM) images of cross-sectional analysis of selected carbon nanosheetsFigure 4. inTop C-GLOVE.: Atomic forceBottom microscopy: Representative (AFM) images transmission of cross-sectional electron analysis microscopy of selected (TEM) carbon images of nanosheets in C-GLOVE. Bottom: representative transmission electron microscopy (TEM) images of the sheets. the sheets. The XPS survey spectra of C-GLOVE (Figure5, bottom) demonstrated the presence of C, O, and N The XPS survey spectra of C-GLOVE (Figure 5, bottom) demonstrated the presence of C, O, and in their composition (the observed silicon peaks are due to the Si substrate used for the measurements). N in their composition (the observed silicon peaks are due to the Si substrate used for the Themeasurements). high resolution The C1s high XPS resolution spectra revealed C1s XPS thespectra diff erentrevealed types the ofdifferent carbon types chemical of carbon bonds chemical (C–O, C=O, C–O–C,bonds C–N, (C–O, C–C) C=O, presentC–O–C, inC–N, the C–C) samples present along in th withe samples their along percentages with their (Figure percentages5, left). (Figure After 5, fitting on theleft). high After resolution fitting on N1s the XPShigh spectra,resolution we N1s additionally XPS spectra, observed we additionally pyridinic, observed pyrrolic, pyridinic, and graphitic nitrogenpyrrolic, species and (Figuregraphitic5, right)nitrogen [7]. species The C /N(Figur ratioe was5, right) estimated [7]. The approximately C/N ratio was C/N estimated= 15.2. approximatelyIn one application, C/N = 15.2. the efficiency of C-GLOVE for adsorption of hexavalent chromium ions from aqueous solutions was investigated by conducting sorption kinetic experiments [15]. Figure6 shows the effectiveness of the material in Cr (VI) removal at initial concentrations equal to 6 mg/L as a function of reaction time for two different pH values (5.5 and 3) at 25 ◦C. The analogous effectiveness of the well-known ordered mesoporous carbon (CMK-3) measured under same conditions is also shown for comparison [15]. Accordingly, the chromium removal efficiencies of C-GLOVE and CMK-3 after 24 h reaction time at pH 5.5 were determined to be 15% and 12% respectively. On the other hand, at pH 3, the corresponding removal efficiencies reached 50% and 92% for C-GLOVE and CMK-3 respectively. Worth noting, the kinetic data for both carbons were well interpreted by a pseudo-second-order model [15]. Hence, the plots of 1/Cr(VI) versus time (not shown) produced linear plots with correlation coefficients (R2) higher than 0.98, indicating that the rate of chromium adsorption by the sorbents fitted well the pseudo second-order model. Based on these plots, the adsorption rate for C-GLOVE was estimated at 1.26 mg L 1 h 1 versus 2.7 mg L 1 h 1 for CMK-3 [15]. These results can be rationalized · − · − · − · − by the much higher specific surface area of CMK-3 compared to C-GLOVE (1650 m2/g versus 10 m2/g), which in turns favours a greater adsorption.

Nanomaterials 2020, 10, 566 6 of 13 Nanomaterials 2020, 10, x FOR PEER REVIEW 7 of 16

Nanomaterials 2020, 10, x FOR PEER REVIEW 8 of 16 Nanomaterials 2020, 10, x FOR PEER REVIEW 8 of 16

Figure 5. C1s photoelectron spectrum (left), N1s photoelectron spectrum (right), and X-ray photoelectron NanomaterialsFigure 5. 2020C1s, 10photoelectron, x FOR PEER REVIEW spectrum (left), N1s photoelectron spectrum (right), and X-ray8 of 16 spectroscopy (XPS) (bottom) survey of C-GLOVE. photoelectron spectroscopy (XPS) (bottom) survey of C-GLOVE.

In one application, the efficiency of C-GLOVE for adsorption of hexavalent chromium ions from aqueous solutions was investigated by conducting sorption kinetic experiments [15]. Figure 6 shows the effectiveness of the material in Cr (VI) removal at initial concentrations equal to 6 mg/L as a function of reaction time for two different pH values (5.5 and 3) at 25 °C. The analogous effectiveness of the well-known ordered mesoporous carbon (CMK-3) measured under same conditions is also shown for comparison [15]. Accordingly, the chromium removal efficiencies of C-GLOVE and CMK-3 after 24 h reaction time at pH 5.5 were determined to be 15% and 12% respectively. On the other hand, at pH 3, the corresponding removal efficiencies reached 50% and 92% for C-GLOVE and CMK-3 respectively. Worth noting, the kinetic data for both carbons were well interpreted by a pseudo-second-order model [15]. Hence, the plots of 1/Cr(VI) versus time (not shown) produced linear plots with correlation coefficients (R2) higher than 0.98, indicating that the rate of chromium adsorption by the sorbents fitted well the pseudo second-order model. Based on these plots, the adsorption rate for C-GLOVE was estimated at 1.26 mg·L−1·h−1 versus 2.7 mg·L−1·h−1 for CMK-3 [15]. These results can be rationalized by the much higher specific surface area of CMK-3 compared to C-GLOVE (1650 m2/g versus 10 m2/g), which in turns favours a greater adsorption.

Figure 6. Effect of initial pH value (5.5 and 3) on the Cr (VI) removal efficiency by C-GLOVE Figure 6. Effect of initial pH value (5.5 and 3) on the Cr (VI) removal efficiency by C-GLOVE ( ) and CMK-3 ( ). FigureFigure 6. E ﬀ6.ect Effect of initialof initial pH pH value value (5.5 (5.5 andand 3)3) on the the Cr Cr (VI) (VI) removal removal efficiency eﬃciency by C-GLOVE by C-GLOVE ( ) ( ) andand CMK-3 CMK-3( )( (and CMK-3 ). ). ( ). In another application, the heat produced from the reaction of nitrile glove with fuming nitric In another application,In another application, the heat produced the heat producedfrom the fromreaction the reactionof nitrile of glovenitrile withglove fumingwith fumingacid nitric was nitric exploited for the rapid thermal transformation of a suitable molecular precursor into acid was exploitedacid was for exploited the rapid for thermalthe rapid transformation thermal transformation of a suitable of a suitable molecular molecular precursor precursorgraphitic into into carbon nitride. To this aim, the middle finger glove tip from a medium size nitrile glove graphitic carbongraphitic nitride. carbon To thisnitride. aim, To the this middle aim, the finger middle glove finger tip glove from tip a frommedium a medium size nitrile sizewas nitrile glove cut glove out (mass of ca. 0.4 g) and stuffed with 1 g of the triazine derivative dichloroisocyanuric acid, was cut out (masswas cut of outca. 0.4(mass g) andof ca. stuffed 0.4 g) and with stuffed 1 g of with the 1 triazine g of the derivativetriazine derivative dichloroisocyanuric dichloroisocyanuricsodium acid, acid, salt hydrate (sodium troclosene or pool chlorine, Ezidesaqua, 98%). In general, triazines are sodium salt hydratesodium salt(sodium hydrate troclosene (sodium trocloseneor pool chlorine, or pool chlorine,Ezidesaqua, Ezidesaqua, 98%). In 98%). general, In general, triazinesconsidered triazines are are well-suited precursors towards graphitic carbon nitride by heat [9]. The stuffed tip was considered well-suitedconsidered precursorswell-suited precursorstowards graphitic towards graphiticcarbon nitride carbon bynitride heat by [9]. heat The [9]. stuffed The stuffed hermiticallytip was tip was folded and placed at the bottom of an alumina crucible (diameter: 4 cm; height: 5.5 cm). hermitically folded and placed at the bottom of an alumina crucible (diameter: 4 cm; height: 5.5 cm). hermitically folded and placed at the bottom of an alumina crucible (diameter: 4 cm; height: 5.5To cm).the crucible, 1 mL of fuming nitric acid was quickly added. The ignition of the nitrile wrap To the crucible, 1 mL of fuming nitric acid was quickly added. The ignition of the nitrile wrap To the crucible, 1 mL of fuming nitric acid was quickly added. The ignition of the nitrileprovided wrap the necessary heat for the thermal transformation of the enclosed white-coloured triazine provided the necessary heat for the thermal transformation of the enclosed white-coloured triazine provided the derivativenecessary into heat yellow for the graphitic thermal carbon transfor nitride,mation thus of theacting enclosed as a sort white-coloured of a “heating stove”.derivativetriazine After into yellow graphitic carbon nitride, thus acting as a sort of a “heating stove”. After derivative intocooling, yellow the graphitic inner product carbon was nitride,mechanically thus separateacting asd from a sort the ofouter a “heating carbon debris stove”. incooling, theAfter form the of inner product was mechanically separated from the outer carbon debris in the form of cooling, the inneryellow product chunks. wasThe mechanicallychunks were crashed separate intod afrom fine thepowder outer and carbon washed debris several in timesthe yellowform with of warm chunks. The chunks were crashed into a fine powder and washed several times with warm yellow chunks.water The and chunks acetone were prior crashed to drying into ata fineroom powder temperature. and washed The overall several proc timesedure withis visualizedwater warm and in acetone prior to drying at room temperature. The overall procedure is visualized in water and acetoneFigure 7.prior to drying at room temperature. The overall procedure is visualizedFigure in 7. Figure 7.

Nanomaterials 2020, 10, 566 7 of 13

In another application, the heat produced from the reaction of nitrile glove with fuming nitric acid was exploited for the rapid thermal transformation of a suitable molecular precursor into graphitic carbon nitride. To this aim, the middle finger glove tip from a medium size nitrile glove was cut out (mass of ca. 0.4 g) and stuffed with 1 g of the triazine derivative dichloroisocyanuric acid, sodium salt hydrate (sodium troclosene or pool chlorine, Ezidesaqua, 98%). In general, triazines are considered well-suited precursors towards graphitic carbon nitride by heat [9]. The stuffed tip was hermitically folded and placed at the bottom of an alumina crucible (diameter: 4 cm; height: 5.5 cm). To the crucible, 1 mL of fuming nitric acid was quickly added. The ignition of the nitrile wrap provided the necessary heat for the thermal transformation of the enclosed white-coloured triazine derivative into yellow graphitic carbon nitride, thus acting as a sort of a “heating stove”. After cooling, the inner product was mechanically separated from the outer carbon debris in the form of yellow chunks. The chunks were crashed into a fine powder and washed several times with warm water and acetone prior to drying at roomNanomaterials temperature. 2020, 10, x The FOR overall PEER REVIEW procedure is visualized in Figure7. 9 of 16

Figure 7. Top left to right bottom: the middle finger glove tip from a medium size nitrile glove was cut outFigure and stuffed7. Top left with to granular right bottom dichloroisocyanuric: the middle finger acid, glove sodium tip saltfrom hydrate a medium (top size inset nitrile structure). gloveThe was stuffedcut out tip and was stuffed hermitically with granular folded dichloroisocyanuric and placed at the bottom acid, sodium of an alumina salt hydrate crucible. (top Afterinset structure). the quick additionThe stuffed of fuming tip was nitric hermitically acid, the nitrile folded wrap and was placed ignited, at the providing bottom theof an necessary alumina heat crucible. for the After thermal the transformationquick addition of of the fuming enclosed nitric triazine acid, derivativethe nitrile intowrap graphitic was ignited, carbon providing nitride. The the innernecessary product heat was for mechanicallythe thermal transformation separated from of the the outer enclosed carbon triazine debris inderivative the form into of yellow graphitic chunks carbon (glass nitride. vial at The far-right inner bottom).product Purificationwas mechanically of the chunks separated afforded from yellow the oute graphiticr carbon carbon debris nitride in the powder form of (bottom yellow inset). chunks (glass vial at far-right bottom). Purification of the chunks afforded yellow graphitic carbon nitride Thepowder XRD (bottom pattern inset). of the yellow powder (Figure8, top) showed a strong (002) di ﬀraction peak corresponding to an interlayer spacing of ca. 3.2 Å, the latter being consistent with graphitic carbon nitrideThe [16 ,XRD17]. Thepattern weak of shoulder the yellow centered powder near (Figure 2θ = 14 8,◦ top)is attributed showed toa strong the (100) (002) in-plane diffraction diﬀraction peak peakcorresponding of the layered to an solid. interlayer The ATR-IR spacing spectrum of ca. 3.2 of theÅ, the sample latter (Figure being8 ,consistent middle) exhibited with graphitic broad bandscarbon 1 1 1 fromnitride pending [16,17]. –NH The2 groups weak atshoulder 3352 cm −centeredand 3200 near cm −2θ, while= 14° the is bandattributed at 800 cmto −thewas (100) assigned in-plane to 1 triazinediffraction rings peak [16,17 of]. Thethe layered absorption solid. peaks The observed ATR-IR in spectrum the range of 1200–1600 the sample cm− (Figurecorrespond 8, middle) to the 1 stretchingexhibited modebroad ofband C=Ns from/C-N heterocyclespending –NH [172 groups]. Lastly, at the3352 peak cm− at1 and 2180 3200 cm− cmwas−1, while ascribed the toband pending at 800 cm−1 was assigned to triazine rings [16,17]. The absorption peaks observed in the range 1200–1600 cm−1 correspond to the stretching mode of C=N/C-N heterocycles [17]. Lastly, the peak at 2180 cm−1 was ascribed to pending –CN triple bonds [18]. The UV-Vis spectrum of the material recorded as a fine solid suspension in ethanol (Figure 8, bottom) displayed a strong absorption centered at 400 nm (violet part of the visible spectrum), thus explaining the yellow colour of the semiconducting solid due to its intrinsic band-gap energy [19]. AFM study revealed the presence of thick plates (ca. 20 nm) with multilayered texture near the edges, the latter being typical of layered materials (Figure 9).

Nanomaterials 2020, 10, 566 8 of 13

–CN triple bonds [18]. The UV-Vis spectrum of the material recorded as a ﬁne solid suspension in ethanol (Figure8, bottom) displayed a strong absorption centered at 400 nm (violet part of the visible spectrum), thus explaining the yellow colour of the semiconducting solid due to its intrinsic band-gap energy [19]. AFM study revealed the presence of thick plates (ca. 20 nm) with multilayered texture nearNanomaterials the edges, 2020, the10, x latter FOR PEER being REVIEW typical of layered materials (Figure9). 10 of 16

Figure 8. XRD pattern (top), Attenuated total reﬂectance-infrared spectroscopy (ATR-IR) spectrum (Figuremiddle 8.) andXRD ultraviolet-visible pattern (top), Attenuated (UV-Vis) spectrumtotal reflectance-infrar (bottom); ﬁneed solid spectroscopy suspension (ATR-IR) in ethanol spectrum of the as-received(middle) and graphitic ultraviolet-visible carbon nitride. (UV-Vis) spectrum (bottom); fine solid suspension in ethanol) of the as-received graphitic carbon nitride.

NanomaterialsNanomaterials2020 2020,,10 10,, 566 x FOR PEER REVIEW 119 ofof 1316

Figure 9. AFM images of cross-sectional analysis and 3D morphologies of graphitic carbon nitride Figure 9. AFM images of cross-sectional analysis and 3D morphologies of graphitic carbon nitride showing the presence of thick multi-layered plates. showing the presence of thick multi-layered plates.

In this same token, another interesting heat source is the aniline-HNO3 hypergolic system [4] used In this same token, another interesting heat source is the aniline-HNO3 hypergolic system [4] here for the carbonization of coffee grains [20,21]. The aniline-HNO3 pair instantly ignites towards the formationused here offor complex the carbonization yet soluble productsof coffee that, grains upon [20,21]. washing The with aniline-HNO water and3 acetone,pair instantly leave behindignites minortowards carbon the formation residue. Briefly, of complex 0.6 g ofyet instant soluble co productsffee grains that, was upon wetted washing with 1 mLwith aniline water inand atest acetone, tube toleave from behind a thick minor paste. carbon At this residue. stage, no Briefly, reaction 0.6 between g of instant aniline coffee and thegrains coff eewas grains wetted was with observed 1 mL (e.g.,aniline heating in a test or colortube to change). from a thick Upon paste. the quick At this addition stage, ofno 1reaction mL of fuming between nitric aniline acid, and the the mixture coffee ignitedgrains was with observed short delay (e.g., to produceheating or flu colorffy carbon change). on theUpon walls the and quick the addition rim of the of test1 mL tube. of fuming Apparently, nitric theacid, energy the mixture released ignited from thewith spontaneous short delay ignition to produce of the fluffy hypergolic carbon mixtureon the walls provided and the necessaryrim of the heattest tube. for the Apparently, carbonization the ofenergy the co releasedffee grains. from The the productspontaneous was scratchedignition of o theff, washed hypergolic thoroughly mixture provided the necessary heat for the carbonization of the coffee grains. The product was scratched off, with water and acetone, and finally dried at 80 ◦C. An amorphous, lightweight carbon powder was obtained,washed thoroughly which was with able towater decolorize and acetone, dye-contaminated and finally dried water at (Figure 80 °C. 10An). amorphous, lightweight carbon powder was obtained, which was able to decolorize dye-contaminated water (Figure 10).

Nanomaterials 2020, 10, 566 10 of 13 Nanomaterials 2020, 10, x FOR PEER REVIEW 12 of 16

Figure 10. Top: The vial on the left shows the volume of the lightweight carbon versus that of commercial Figure 10. Top: the vial on the left shows the volume of the lightweight carbon versus that of charcoal (right vial). Both vials contained 180 mg of the material. Bottom: Addition of the lightweight commercial charcoal (right vial). Both vials contained 180 mg of the material. Bottom: addition of the carbon to an aqueous solution of the organic dye methylene blue and subsequent filtration resulted in lightweight carbon to an aqueous solution of the organic dye methylene blue and subsequent decolorization of the aqueous phase. filtration resulted in decolorization of the aqueous phase.

The N2 adsorption-desorption isotherms of the coffee-derived carbon are shown in Figure 11. The sampleThe N2 exhibitedadsorption-desorption a mixed I and IIisotherms type corresponding of the coff toee-derived a microporous carbon structure are shown with in an Figure indefinite 11. Themulti-layer sample formationexhibited aftera mixed completion I and ofII thetype monolayer. corresponding The BET to methoda microporous was applied structure for calculation with an indefinite multi-layer formation after completion of the monolayer. The BET2 method1 was applied for of specific surface area; the material’s SBET was calculated as ~288 m g− . From the pore size · 2 −1 calculationdistribution of obtained specific usingsurface DFT area; simulation, the material’s the sample SBET was possessed calculated mainly as ~288 micropores m ·g . From with the average pore size distribution obtained using DFT simulation, the sample possessed3 1mainly micropores with size, ~1.9 nm. Finally, the micropore volume was found to be ~0.12 cm g− and the cumulative pore average size, ~1.93 nm.1 Finally, the micropore volume was found to· be ~0.12 cm3·g−1 and the volume ~0.19 cm g− . cumulative pore volume· ~0.19 cm3·g−1.

Nanomaterials 2020, 10, x FOR PEER REVIEW 13 of 16

Nanomaterials 2020, 10, 566 11 of 13 Nanomaterials 2020, 10, x FOR PEER REVIEW 13 of 16

Figure 11. N2 adsorption-desorption isotherms and pore size distribution (inset) for the coffee-derived carbon.

Figure 11. N2 adsorption-desorption isotherms and pore size distribution (inset) for the coffee-derived carbon. 3.2. PhotoluminescentFigure 11. N2 adsorption-desorption Carbon Dots isotherms and pore size distribution (inset) for the coffee-derived carbon. 3.2. PhotoluminescentThe ignition of Carbonnitrile Dotsglove by fuming nitric acid is considered a classic demonstration 3.2.experiment PhotoluminescentThe ignition in the of area nitrile Carbon of hypergolic glove Dots by fuming reactions nitric that acid, as is shown considered here, a resulted classic demonstration in carbon nanosheets. experiment In anin theeffort area to ofdemonstrate hypergolic the reactions generic that, character as shown of the here, concept resulted towards in carbon the synthesis nanosheets. of other In an types effort of to The ignition of nitrile glove by fuming nitric acid is considered a classic demonstration functionaldemonstrate carbon the generic materials, character we also of present the concept here the towards formation the synthesis of photoluminescent of other types carbon of functional dots by experiment in the area of hypergolic reactions that, as shown here, resulted in carbon nanosheets. In thecarbon ignition materials, of Girard’s we also reagent present T here with the fuming formation nitric of acid. photoluminescent To the best of carbonour knowledge, dots by the the ignition Girard’s of an effort to demonstrate the generic character of the concept towards the synthesis of other types of reagentGirard’s T-HNO reagent3 Thypergolic with fuming system nitric is acid. reported To the for best the of first our ti knowledge,me in the literature. the Girard’s The reagentreaction T-HNO led to a3 functional carbon materials, we also present here the formation of photoluminescent carbon dots by hydrophilichypergolic system brown is residue reported that for theafter first dialysis time in agai thenst literature. water gave The reactionan aqueou leds to dispersion a hydrophilic of carbon brown the ignition of Girard’s reagent T with fuming nitric acid. To the best of our knowledge, the Girard’s dotsresidue with that a spherical after dialysis morphology against waterand mean gave size an aqueousof 5 nm based dispersion on AFM of carbon(Figure dots 12). withThe hydrophilic a spherical reagent T-HNO3 hypergolic system is reported for the first time in the literature. The reaction led to a naturemorphology of the and dots mean is ascribed size of 5 nmto basedthe quaternary on AFM (Figure ammonium 12). The type hydrophilic of structure nature of ofthe the starting dots is hydrophilic brown residue that after dialysis against water gave an aqueous dispersion of carbon hydrazideascribed to derivative the quaternary [22]. ammonium type of structure of the starting hydrazide derivative [22]. dots with a spherical morphology and mean size of 5 nm based on AFM (Figure 12). The hydrophilic nature of the dots is ascribed to the quaternary ammonium type of structure of the starting hydrazide derivative [22].

Figure 12. Topography images of carbon dots (section-analysis,(section-analysis, 3D morphology, and depth-analysis histograms are included).

FigureInterestingly, 12. Topography the dispersed images ofdots carbon fluoresced fluoresced dots (section-a in th theenalysis, visible 3D region, morphology, as as shown shown and depth-analysisin in Figure Figure 1313 (left). (left). Withhistograms respectrespect toto are thethe included). fluorescencefluorescence spectraspectra (Figure(Figure 1313,, right),right), whenwhen thethe excitationexcitation wavelengthwavelength varied from 350 to 600 nm, the spectra red shifted and the fluorescence intensity decreased gradually. Such Interestingly, the dispersed dots fluoresced in the visible region, as shown in Figure 13 (left). With respect to the fluorescence spectra (Figure 13, right), when the excitation wavelength varied

Nanomaterials 2020, 10, x FOR PEER REVIEW 14 of 16 Nanomaterials 2020, 10, 566 12 of 13

from 350 to 600 nm, the spectra red shifted and the fluorescence intensity decreased gradually. Such excitation-dependedexcitation-depended emissionemission behaviourbehaviour is consistent with photoluminescentphotoluminescent carbon dots [10,22,23]. [10,22,23]. TheThe ﬂuorescencefluorescence quantumquantum yieldyield waswas roughlyroughly estimatedestimated toto bebe 1–2%.1–2%.

Figure 13. Left: aqueous dispersion of carbon dots under natural and UV light. Right: fluorescence Figure 13. Left: aqueous dispersion of carbon dots under natural and UV light. Right: fluorescence emission spectra of the dots at different excitation wavelengths (λex as inset). emission spectra of the dots at different excitation wavelengths (λex as inset). 4. Conclusions 4. Conclusions This work shows a new synthesis method for functional carbon materials (carbon nanosheets, graphiticThis carbonwork shows nitride, a and new photoluminescent synthesis method carbon for fu dots)nctional based carbon on hypergolic materials mixtures (carbon consistingnanosheets, of agraphitic combustible carbon organic nitride, solid andand fuming photoluminescent nitric acid as strongcarbon oxidizer. dots) based In allinstances, on hypergolic the carbon mixtures phase formationconsisting wasof a fast,combustible spontaneous, organic and solid exothermic and fuming at ambient nitric conditions,acid as strong making oxidizer. hypergolic In all instances, mixtures anthe interesting carbon phase new formation source of was carbon fast, nanomaterial. spontaneous, and Some exothermic practical usesat ambient of the conditions, derived functional making carbonshypergolic included mixtures here an are interesting hexavalent new chromium source removalof carbon and nanomaterial. decolorization Some of water-soluble practical uses organic of the dyes.derived These functional findings carbons build uponinclud previoused here are results hexavalent from our chromium group on removal carbon materialsand decolorization obtained atof ambientwater-soluble conditions organic from dyes. pyrophoric These findings lithium dialkylamides build upon previous salts (e.g., results carbon from nanosheets) our group or hypergolicon carbon acetylene-chlorinematerials obtained mixtures at ambient (e.g., conditions synthetic graphite)from pyro [6phoric,7]. Some lithium additional dialkylamides paradigms salts for future(e.g., carbon study includenanosheets) hypergolic or hypergolic mixtures containing acetylene-chlorine a combustible mixtures organic (e.g., compound synthetic and sodiumgraphite) peroxide [6,7]. (source Some ofadditional highly concentrated paradigms for H future2O2), and study manganese include hype heptoxidergolic mixtures or chromyl containing chloride a combustible as strong oxidizers organic (seecompoundBretherick’s and Handbooksodium peroxide of Reactive (source Chemical of highly Hazards concentrated). H2O2), and manganese heptoxide or chromyl chloride as strong oxidizers (see Bretherick’s Handbook of Reactive Chemical Hazards). Author Contributions: Conceptualization, A.B.B. and D.G.; formal analysis, N.C., G.A., K.S., D.M., A.A., K.C.V. andAuthor M.A.K. Contributions: All authors haveConceptualization, read and agreed A.B.B. to the and published D.G.; formal version analysis, of the N.C., manuscript. G.A., K.S., D.M., A.A., K.C.V. and M.A.K. All authors have read and agreed to the published version of the manuscript. Funding: We acknowledge the support of the project “National Infrastructure in Nanotechnology, Advanced MaterialsFunding: We and acknowledge Micro-/Nanoelectronics” the support of (MIS the 5002772),project ‘‘Nation implementedal Infrastructure under thein Na Actionnotechnology, “Reinforcement Advanced of theMaterials Research and andMicro-/Nanoelectronics” Innovation Infrastructure”, (MIS 5002772), funded impl byemented the Operational under the Programme Action ‘‘Reinforcement “Competitiveness, of the Entrepreneurship and Innovation” (NSRF 2014-2020) and co-financed by Greece and the European Union Research and Innovation Infrastructure”, funded by the Operational Programme ‘‘Competitiveness, (European Regional Development Fund). This research was also co-financed by Greece and the European Union (EuropeanEntrepreneurship Social Fund- and ESF)Innovation” through (NSRF the Operational 2014-2020) Programme and co-financed “Human by Resources Greece and Development, the European Education Union and(European Lifelong Regional Learning” Development in the context Fund). of the This project research “Strengthening was also co-financed Human Resources by Greece Research and the Potential European via DoctorateUnion (European Research” Social (MIS-5000432), Fund- ESF) implemented through the byOper theational State Scholarships Programme Foundation‘‘Human Resources (IKY). Development, Acknowledgments:Education and LifelongThe authorsLearning” greatly in the acknowledge context of Ch.the Papachristodoulouproject ‘‘Strengthening for the Human XRD measurements.Resources Research Potential via Doctorate Research” (MIS-5000432), implemented by the State Scholarships Foundation (ΙΚY). Conflicts of Interest: The authors declare no conflict of interest. Acknowledgements: The authors greatly acknowledge Ch. Papachristodoulou for the XRD measurements.

ReferencesConflicts of Interest: The authors declare no conflict of interest. 1. Hypergolic propellant. Available online: https://en.wikipedia.org/wiki/Hypergolic_propellant (accessed on 28 December 2019). 2. Silva, G.d.; lha, K. Hypergolic systems: A review in patents. J. Aerosp. Technol. Manage. 2012, 4, 407–412. [CrossRef] 3. Schneider, S.; Hawkins, T.; Rosander, M.; Vaghjiani, G.; Chambreau, S.; Drake, G. Ionic liquids as hypergolic fuels. Energy Fuels 2008, 22, 2871–2872. [CrossRef]

Nanomaterials 2020, 10, 566 13 of 13

4. Making test tube liquid rockets. Available online: https://www.youtube.com/watch?v=OszX18NLtrY&t=520s (accessed on 4 October 2018). 5. Fuming Nitric Acid vs. Lab Gloves. Available online: https://www.youtube.com/watch?v=aBVdGGml6bU (accessed on 5 April 2014). 6. Chalmpes, N.; Spyrou, K.; Bourlinos, A.B.; Moschovas, D.; Avgeropoulos, A.; Karakassides, M.A.; Gournis, D. Synthesis of highly crystalline graphite from spontaneous ignition of in situ derived acetylene and chlorine at ambient conditions. Molecules 2020, 25, 297. [CrossRef][PubMed] 7. Baikousi, M.; Chalmpes, N.; Spyrou, K.; Bourlinos, A.B.; Avgeropoulos, A.; Gournis, D.; Karakassides, M.A. Direct production of carbon nanosheets by self-ignition of pyrophoric lithium dialkylamides in air. Mater. Lett. 2019, 254, 58–61. [CrossRef] 8. Fan, H.; Shen, W. Carbon nanosheets: Synthesis and application. ChemSusChem 2015, 8, 2004–2027. [CrossRef] [PubMed] 9. Thomas, A.; Fischer, A.; Goettmann, F.; Antonietti, M.; Müller, J.-O.; Schlögl, R.; Carlsson, J.M. Graphitic carbon nitride materials: Variation of structure and morphology and their use as metal-free catalysts. J. Mater. Chem. 2008, 18, 4893–4908. [CrossRef] 10. Baker, S.N.; Baker, G.A. Luminescent carbon nanodots: Emergent nanolights. Angew. Chem. Int. Ed. 2010, 49, 6726–6744. [CrossRef][PubMed] 11. Roh, J.-S. Structural study of the activated carbon fiber using laser raman spectroscopy. Carbon Lett. 2008, 9, 127–130. [CrossRef] 12. Thomas, P.; Thomas, S.; George, G.; Thomas, S.; Joseph, K. Impact of filler geometry and surface chemistry on the degree of reinforcement and thermal stability of nitrile rubber nanocomposites. J. Polym. Res. 2011, 18, 2367–2378. [CrossRef] 13. Datta, J.; Kosiorek, P.; Włoch, M. Effect of high loading of titanium dioxide particles on the morphology, mechanical and thermo-mechanical properties of the natural rubber-based composites. Iran. Polym. J. 2016, 25, 1021–1035. [CrossRef] 14. Choudhury, S.; Zeiger, M.; Massuti-Ballester, P.; Fleischmann, S.; Formanek, P.; Borchardt, L.; Presser, V. Carbon onion–sulfur hybrid cathodes for lithium–sulfur batteries. Sustain. Energy Fuels 2017, 1, 84–94. [CrossRef] 15. Baikousi, M.; Bourlinos, A.B.; Douvalis, A.; Bakas, T.; Anagnostopoulos, D.F.; Tuˇcek,J.; Šafáˇrová, K.; Zboril, R.;

Karakassides, M.A. Synthesis and characterization of γ-Fe2O3/carbon hybrids and their application in removal of hexavalent chromium ions from aqueous solutions. Langmuir 2012, 28, 3918–3930. [CrossRef][PubMed] 16. Guo, Q.; Xie, Y.; Wang, X.; Lv, S.; Hou, T.; Liu, X. Characterization of well-crystallized graphitic carbon nitride nanocrystallites via a benzene-thermal route at low temperatures. Chem. Phys. Lett. 2003, 380, 84–87. [CrossRef] 17. Xu, N.; Wang, Y.; Rong, M.; Ye, Z.; Deng, Z.; Chen, X. Facile preparation and applications of graphitic carbon nitride coating in solid-phase microextraction. J. Chromatogr. A 2014, 1364, 53–58. [CrossRef][PubMed] 18. Lu, Q.; Deng, J.; Hou, Y.; Wang, H.; Li, H.; Zhang, Y. One-step electrochemical synthesis of ultrathin graphitic carbon nitride nanosheets and their application to the detection of uric acid. Chem. Commun. 2015, 51, 12251–12253. [CrossRef][PubMed] 19. Wang, X.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J.M.; Domen, K.; Antonietti, M. A metal-free polymeric photocatalyst for hydrogen production from water under visible light. Nat. Mater. 2009, 8, 76–80. [CrossRef][PubMed] 20. Wen, X.; Liu, H.; Zhang, L.; Zhang, J.; Fu, C.; Shi, X.; Chen, X.; Mijowska, E.; Chen, M.-J.; Wang, D.-Y. Large-scale converting waste coffee grounds into functional carbon materials as high-efficient adsorbent for organic dyes. Bioresour. Technol. 2019, 272, 92–98. [CrossRef][PubMed] 21. Gao, G.; Cheong, L.-Z.; Wang, D.; Shen, C. Pyrolytic carbon derived from spent coffee grounds as anode for sodium-ion batteries. Carbon Resour. Convers. 2018, 1, 104–108. [CrossRef] 22. Bourlinos, A.B.; Zboˇril,R.; Petr, J.; Bakandritsos, A.; Krysmann, M.; Giannelis, E.P. Luminescent surface quaternized carbon dots. Chem. Mater. 2012, 24, 6–8. [CrossRef] 23. Essner, J.B.; Kist, J.A.; Polo-Parada, L.; Baker, G.A. Artifacts and errors associated with the ubiquitous presence of fluorescent impurities in carbon nanodots. Chem. Mater. 2018, 30, 1878–1887. [CrossRef]

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).