Journal of C Carbon Research

Article Hypergolic Materials Synthesis through Reaction of Fuming with Certain Cyclopentadienyl Compounds

Nikolaos Chalmpes 1 , Athanasios B. Bourlinos 2,*, Veronika Šedajová 3 , VojtˇechKupka 3, 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] (D.M.); [email protected] (A.A.); [email protected] (M.A.K.) 2 Physics Department, University of Ioannina, 45110 Ioannina, Greece 3 Regional Centre of Advanced Technologies and Materials, Faculty of Science, Palacky University in Olomouc, Slechtitelu 27, 779 00 Olomouc, Czech Republic; [email protected] (V.Š.); [email protected] (V.K.) * Correspondence: [email protected] (A.B.B.); [email protected] (D.G.); Tel.: +30-26510-07141 (D.G.)

 Received: 17 August 2020; Accepted: 2 October 2020; Published: 5 October 2020 

Abstract: Recently we have shown the importance of hypergolic reactions in carbon materials synthesis. However, hypergolic reactions could be certainly expanded beyond carbon synthesis, offering a general preparative pathway towards a larger variety of materials. Cyclopentadienyls are one of the most common ligands in organometallic chemistry that react hypergolicly on contact with strong oxidizers. By also considering the plethora of cyclopentadienyl compounds existing today, herein we demonstrate the potential of such compounds in hypergolic materials synthesis in general (carbon or inorganic). In a first example, we show that cyclopentadienyllithium reacts hypergolicly with fuming nitric acid to produce carbon. In a second one, we show that and also react hypergolicly with the concentrated acid to afford magnetic inorganic materials, such as γ-Fe2O3 and metallic Co, respectively. The present results further emphasize the importance and universal character of hypergolic reactions in materials science synthesis, as an interesting new alternative to other existing and well-established preparative methods.

Keywords: materials synthesis; hypergolic reactions; cyclopentadienyl compounds; fuming nitric acid

1. Introduction In hypergolic reactions, two chemical reagents ignite spontaneously upon contact to liberate energy and gases in a non-explosive manner. Although hypergolic reactions find application in rocket fuels and propellants, their role in materials processing still remains largely unexplored. It is only lately that hypergolics has been introduced as a new synthesis tool in materials science, particularly for the fast and spontaneous preparation of carbon materials at ambient conditions in an energy liberating manner [1–5]. For this purpose, an organic compound serving as the carbon source and a strong oxidizer were spontaneously ignited upon contact at room temperature and atmospheric pressure to afford carbon without the need of any external source of heat (e.g., baking oven, hydrothermal treatment, or chemical vapor deposition). In this respect, hypergolic synthesis should be regarded as an energy-liberating rather than an energy-consuming process. Typical paradigms developed from our group within this context include carbon nanosheets directly derived from self-ignitable lithium dialkylamides salts [1], highly crystalline graphite through

C 2020, 6, 61; doi:10.3390/c6040061 www.mdpi.com/journal/carbon C 2020, 6, 61 2 of 12 spontaneous ignition of in situ derived acetylene–chlorine hypergolic mixture [2], carbon nanosheets or carbon dots from hypergolic mixtures based on nitrile rubber or Girard’s reagent T and fuming nitric acid as a strong oxidizer [3], carbon nanosheets or fullerols from hypergolic pairs based on coffee grains or C60 and sodium peroxide as a strong oxidizer [4], and finally, carbon soot from the exothermic reaction between ferrocene and liquid bromine at ambient conditions [5]. In all instances, the reactions were immediate and exothermic at ambient conditions, with the released energy being further exploited in some cases for the production of useful work (chemical, thermoelectric, photovoltaic, etc.). A classic guide to organic compounds that are hypergolic on contact with strong oxidizers is Bretherick’s Handbook of Reactive Chemical Hazards. By scrutinizing the vast number of chemicals listed there, we came across the -fuming nitric acid hypergolic pair (e.g., cyclopentadiene ignites upon contact with concentrated nitric acid [6]). Motivated by this information, we set out to study the carbon residue produced from the hypergolic reaction of cyclopentadienyllithium with fuming nitric acid, the former acting as the carbon source. It turned out that the produced carbon was amorphous and oxidized, mostly adopting discoidal morphology. Interestingly, ferrocene and cobaltocene bearing cyclopentadienyl rings in their structure also reacted hypergolicly with fuming nitric acid, resulting instead in magnetic nanopowders as the major phase (γ-Fe2O3, metallic Co). Therefore, cyclopentadienyl compounds, due to a large variety, are potentially interesting starting reagents for the hypergolic synthesis of a wider range of materials at ambient conditions in an energy-liberating manner (i.e., exothermically). In a broader sense, hypergolics offer a new synthesis scheme in materials science that is complementary to other established preparative methods, such as solid state, sol-gel, wet chemistry, precipitation, hydrothermal, pyrolysis, flame spray pyrolysis, chemical vapor deposition, etc. At its current form, the present method serves as a proof of concept in materials science. Certain advantages of hypergolic reactions include the fast rate, spontaneity, and exothermic character at ambient conditions. In addition, hypergolics under the materials science perspective provides a useful way to deal with waste from rocket propellants. On the other side, although such reactions are pretty safe to run at a small scale in the lab, nevertheless, safety issues should be seriously taken into consideration upon scaling up the method (e.g., pilot set-ups closely resembling rocket fuel engines should be rather devised). In a parallelism relation, flame-spray pyrolysis is another example of hazardous technique involving hot flames and flammable hydrocarbons; however, the method is now widely used in labs and industries for the large-scale production of nanomaterials. This was only possible due to systematic technical upgrades over time towards the advancement of safe and sophisticated apparatus. In another point, the reaction yields (typically 2–15%), though acceptable, are lower than those of classic preparative methods, such as precipitation, sol-gel, or flame-spray pyrolysis. But this mainly has to do with the carbon or inorganic content of the precursors used in the hypergolic reactions. The study or discovery of new hypergolic pairs that will provide even higher yields remains a challenge.

2. Materials and Methods Synthesis was conducted in a fume hood with ceramic tile bench. A glass test tube (diameter: 1.5 cm; length: 15 cm) was charged with 0.5 g cyclopentadienyllithium (97% C5H5Li, Sigma–Aldrich, St. Louis, MO, USA) followed by the slow, dropwise addition of 0.5 mL fuming nitric acid (100%, Sigma–Aldrich, St. Louis, MO, USA). The cyclopentadienyl salt and fuming nitric acid ignited instantly upon contact to from a carbon residue within the test tube. The residue was collected and washed thoroughly with de-ionized water ( 5.5 10 5 S m 1), acetone ( 99% Merck KGaA, ≈ × − − ≥ Darmstadt, Germany), and tetrahydrofuran (THF, 99% Merck KGaA, Darmstadt, Germany) prior ≥ to drying at 80 ◦C. A fine carbon powder was obtained at a yield of 2%, compared to the weight of 2 1 the cyclopentadienyl compound used for reaction (N2 specific surface area: 225 m g− ). The whole process is illustrated in Figure1. C 2020, 6, x FOR PEER REVIEW 3 of 13 C 2020, 6, 61 3 of 12 C 2020, 6, x FOR PEER REVIEW 3 of 13

Figure 1. The hypergolic reaction of cyclopentadienyllithium with fuming nitric acid produced a carbonFigure 1.fine1. TheThe powder: hypergolic hypergolic (a) The reaction reaction test tube of of cyclopentadienyllithium containedcyclopentadienyllithium the salt before with nitricwith fuming fumingacid nitricaddition nitric acid withacid produced aproduced pipette. a carbon (ab – dfinecarbon) Dropwise powder: fine powder: (additiona) The test(a )of The tube the test contained acid tube triggered contained the salt igni before thetion, salt nitric givingbefore acid nitricoff addition an acid intense with addition a burst pipette. with of (flame.ab –pipette.d) Dropwise (e) (bThe– residueadditiond) Dropwise within of the addition the acid tube triggered of was the collected acid ignition, triggered and giving washed igni otion,ff toan give intensegiving a fine off burst blackan ofintense powder. flame. burst (e) Theof flame. residue (e) within The theresidue tube within was collected the tube and was washed collected to an gived washed a fine black to give powder. a fine black powder. Similarly to the cyclopentadienyllithium salt, ferrocene (≥ 98%, Sigma–Aldrich, St. Louis, MO, USA)Similarly and cobaltocene to the cyclopentadienyllithium (98%, Sigma–Aldrich, salt, salt,St. Loferrocene ferroceneuis, MO, ( (≥ USA)98%,98%, Sigma–Aldrich,also Sigma–Aldrich, reacted hypergolicly St. St. Louis, Louis, MO, with MO, ≥ fumingUSA)USA) andand nitric cobaltocene cobaltocene acid as above (98%, (98%, Sigma–Aldrich,to Sigma–Aldrich, afford magnetic St. St. Louis, nanopowders, Louis, MO, MO, USA) USA) such also also reactedas γ -Fereacted2 hypergoliclyO3 and hypergolicly metallic with Co, fumingwith at a yieldnitricfuming of acid nitric10% as abovecomparedacid as to above afford to theto magnetic affordweights magnetic nanopowders, of the nanopowders,cyclopentadienyl such as suchγ-Fe compounds 2asO 3γand-Fe2O metallic3 andused metallic for Co, reaction at aCo, yield at (N a of 2 specific10%yield compared of surface 10% compared toareas: the weights 36 forto theγ of-Fe weights the2O3 cyclopentadienyl and of28 them2 gcyclopentadienyl−1 for compoundsCo). Characteristically, usedcompounds for reaction the used ignition (N for2 specific reaction of ferrocene surface (N2 2 1 2 −1 byareas:specific fuming 36 surface for nitricγ-Fe areas: acid2O3 is36and illustrated for 28 γ-Fe m 2Og −3in and forFigure 28 Co). m 2. g Characteristically,Due for to Co). a small Characteristically, yield, the reactions ignition the of were ignition ferrocene repeated of ferrocene by fumingseveral timesnitricby fuming acidin order is nitric illustrated to acid collect is in illustrated enough Figure2 .material Duein Figure to a for small 2. charDue yield,acterizations. to a reactionssmall yield, wereIn addition,reactions repeated wereall several presented repeated times reactions several in order weretotimes collect completedin order enough to within materialcollect lessenough for than characterizations. material a minute, for thuschar Inacterizations. allowing addition, a all rapid In presented addition, product reactions all formation presented were at completedreactions ambient conditions.withinwere completed less thanLastly, a within minute, there wasless thus athan allowingsudden a minute, release a rapid thus of product a allowingsizeable formation amounta rapid at of ambientproduct energy conditions.formation upon direct at Lastly, contactambient there of thewasconditions. reagents a sudden Lastly, at release ambient there of wasaconditio sizeable a suddenns amount (i.e., release energy-releasing of of energy a sizeable upon amount directprocess). contact of energyThe ofheat theupon produced reagents direct contact at from ambient ofthe reactionsconditionsthe reagents provided (i.e., at energy-releasing ambient the necessaryconditio process).ns energy (i.e., Theenergy-releasing needed heat produced for the process). fromcarbonization the reactionsThe heat of providedtheproduced cyclopentadienyl the from necessary the compoundsenergyreactions needed provided (e.g., for in the situthe carbonization pyrolysis).necessary energy of the cyclopentadienylneeded for the carbonization compounds (e.g., of the in situ cyclopentadienyl pyrolysis). compounds (e.g., in situ pyrolysis).

Figure 2. (a–d) Dropwise addition of fuming nitric acid into a test tube containing ferrocene powder Figure 2. (a–d) Dropwise addition addition of of fuming fuming nitric nitric acid acid into into a atest test tube tube containing containing ferrocene ferrocene powder powder resulted in the hypergolic ignition of the cyclopentadienyl compound by the acid (e.g., intense burst resultedresulted inin thethe hypergolichypergolic ignitionignition ofof thethe cyclopentadienylcyclopentadienyl compoundcompound byby thethe acidacid (e.g.,(e.g., intenseintense burstburstof offlame) flame) and and the the subsequent subsequent formation formation of γof-Fe γ-FeO2Oinside3 inside the the tube. tube. The The magnetic magnetic solid solid was was recovered recovered by of flame) and the subsequent formation of γ-Fe2 23O3 inside the tube. The magnetic solid was recovered byscratchingby scratchingscratching off offand and washing washing the the dark dark brown brownbrown residue residueresidue from from from inside inside inside the the the tube. tube. tube. X-ray diffraction (XRD) was conducted on background-free Si wafers using Cu Kα radiation from X-rayX-ray diffractiondiffraction (XRD) was conducted onon background-freebackground-free SiSi wafers wafers using using Cu Cu K Kαα radiation radiation a Bruker Advance D8 diffractometer (Bruker, Billerica, MA, USA). Attenuated total reflection infrared fromfrom aa BrukerBruker AdvanceAdvance D8 diffractometer (Bruker,(Bruker, Billerica,Billerica, MA, MA, USA). USA). Attenuated Attenuated total total reflection reflection spectroscopy (ATR-IR) measurements were performed using a Jasco IRT-5000 microscope coupled infraredinfrared spectroscopyspectroscopy (ATR-IR) measurements werewere performedperformed using using a a Jasco Jasco IRT-5000 IRT-5000 microscope microscope coupledwithcoupled a FT withwith/IR-4100 aa FT/IR-4100FT/IR-4100 spectrometer spectrometer (Jasco, Easton, (Jasco,(Jasco, MD, Easton,Easton, USA). MD, TheMD, ZnSe USA).USA). prism TheThe ofZnSe ZnSe the prism ATRprism objective of of the the ATR hadATR a µ objective250objectivem area hadhad in aa contact 250 μm with area the in sample. contact The withwith background thethe sample.sample. was TheThe subtracted backgroundbackground and thewas was baseline subtracted subtracted was and correctedand the the baselineforbaseline all spectra. waswas correctedcorrected Raman spectra for all werespectra. obtained RamanRaman on specspec a DXRtratra Raman werewere obtainedobtained microscope on on a (Thermoa DXR DXR Raman Raman Scientific microscope microscope Waltham, C 2020, 6, x FOR PEER REVIEW 4 of 13 C 2020, 6, 61 4 of 12 (Thermo Scientific Waltham, MA, USA) using a laser excitation line at 455 nm, 2 mW. X-ray MA,photoelectron USA) using spectroscopy a laser excitation (XPS) line was at 455performed nm, 2 mW. on X-raya PHI photoelectron VersaProbe II spectroscopy (Physical Electronics, (XPS) was performedChanhassen, on MN, a PHI USA) VersaProbe spectrometer II (Physical using Electronics, Al Kα source Chanhassen, (15 kV, MN,50 W). USA) Deconvolution spectrometer of using spectra Al Kwasα source made (15 using kV, 50the W). MultiPak Deconvolution software of spectrapackage was (Ulvac-PHI, made using Inc., the MultiPakMiami, FL, software USA). packageThe N2 adsorption-desorption isotherms were measured at 77 K on a volumetric gas adsorption analyzer (Ulvac-PHI, Inc., Miami, FL, USA). The N2 adsorption-desorption isotherms were measured at 77 K on (3Flex, Micromeritics, Norcross, GA, USA). The carbon, γ-Fe2O3 and Co samples were outgassed at a volumetric gas adsorption analyzer (3Flex, Micromeritics, Norcross, GA, USA). The carbon, γ-Fe2O3 130 °C for 12 h under high vacuum (10−4 Pa) before measurements. Specific4 surface areas were and Co samples were outgassed at 130 ◦C for 12 h under high vacuum (10− Pa) before measurements. Specificdetermined surface with areas the wereBrunauer–Emmett–Teller determined with the Brunauer–Emmett–Teller(BET) method for all three (BET) samples. method Atomic for all threeforce samples.microscopy Atomic (AFM) force images microscopy were (AFM)obtained images in the were tapping obtained mode in thewith tapping a Bruker mode Multimode with a Bruker 3D MultimodeNanoscope 3D(Ted Nanoscope Pella Inc., (Ted Redding, Pella Inc., CA, Redding, USA) CA, using USA) a usingmicrofabricated a microfabricated silicon silicon cantilever cantilever type TAP-300G, with a tip radius of < 10 nm and a force constant of approximately 20–75 N m−1 The1 type TAP-300G, with a tip radius of < 10 nm and a force constant of approximately 20–75 N m− Thetransmission transmission electron electron microscopy microscopy (TEM) (TEM) study study wa wass performed performed using using the the instrument instrument JEM JEM HR-2100, (JEOL(JEOL Ltd.,Ltd., Tokyo,Tokyo, Japan)Japan) operatedoperated atat 200200 kVkV inin bright-fieldbright-field mode.mode. SEMSEM imagesimages werewere obtainedobtained usingusing aa JEOLJEOL JSM-6510 LV LV SEM SEM Microscope Microscope (Ltd., (Ltd., Tokyo, Tokyo, Japan) Japan) equipped equipped with with an an X-Act X-Act EDS-detector EDS-detector by byOxford Oxford Instruments Instruments (Abingdon, (Abingdon, Oxfordshire, Oxfordshire, UK, UK, an an acceleration acceleration voltage voltage of of 20 20 kV kV was applied). MagneticMagnetic measurementsmeasurements were were carried carried out at out room at temperature room temperature using a Vibrating using Samplea Vibrating Magnetometer Sample (VSM)Magnetometer Lakeshore (VSM) model Lakeshore 7300 (Westerville, model 7300 OH (Westerville, USA). OH USA).

3.3. Results and Discussion

3.1.3.1. Cyclopentadienyllithium-HNO33 hypergolic Pair

TheThe carboncarbon obtained obtained from from the cyclopentadienyllithium-HNOthe cyclopentadienyllithium-HNO3 hypergolic3 hypergolic pair was pair easily was identified easily byidentified the XRD by and the Raman XRD techniques.and Raman Thetechniques. XRD pattern The ofXRD the productpattern of exhibited the product a very exhibited broad reflection a very centeredbroad reflection at d002 centered= 4.0 Å (Figureat d002 =3 ,4.0 top), Å (Figure signaling 3, top), the formationsignaling the of formation highly disordered of highly carbon disordered [ 7]. Incarbon this case,[7]. In the this lack case, of the periodicity lack of periodicity between layersbetween could layers be could ascribed be ascribed to the rapid to the release rapid release of gases of duringgases during reaction reaction that pushes that pushes the carbon the carbon layers layers apart fromapart each from other each (e.g.,other exfoliation). (e.g., exfoliation). At the At same the 1 time,same Ramantime, Raman spectroscopy spectroscopy (Figure (Figure3, bottom) 3, botto gavem) characteristic gave characteristic D (1354) D and (1354) G (1582 and G cm (1582− ) bands cm−1) 3 2 3 2 withbands intensity with intensity ratio (I Dratio/IG~1) (ID typical/IG~1) typical of amorphous of amorphous carbon carbon containing containing both sp both/sp spdomains/sp domains [7]. [7].

FigureFigure 3.3. XRD pattern (top)(top) andand RamanRaman spectrumspectrum (bottom)(bottom) ofof thethe carboncarbon sample.sample. C 2020, 6, x FOR PEER REVIEW 5 of 13

The ATR infrared spectrum of the carbon solid displayed several characteristic absorptions bands (Figure 4). The broad band at 1200 cm−1 was due to C–O/C–OH stretching, the band at 1580 cm−1 was ascribed to the presence of C=C/C=N bonds, the weak shoulder at 1700 cm−1 was assigned to carbonyl groups C=O, and finally, the weak peaks near 3000 cm−1 to residual C–H moieties. The presence of oxygen-containing groups pinpointed oxidation of the carbon surface, in line with the XPS results discussed in the next paragraph. Furthermore, the IR-active C=C stretching vibration was quite close to the Raman-active G band of the solid as a result of symmetry breaking by lattice defects, nitrogen doping, and/or surface oxidation (e.g., the so-called “doing Raman with IR”) [8]. Lastly, residual hydrogens normally resulted from the organic salt precursor by ignition. The XPS survey spectrum of the sample showed predominantly carbon (81%) and, to a lesser

Cextent,2020, 6, oxygen 61 (16.1%) and nitrogen (2.9%) (Figure 5a). Note that no trace of lithium was detected5 of 12in the region between 55–57 eV, which is characteristic of the element. Oxygen was due to ignition in open air (e.g., air-oxidation), while nitrogen due to the nitric acid used for reaction (e.g., N-doping by HNOThe ATR3) [9]. infrared According spectrum to this of thereference, carbon solidnitric displayedacid served several as the characteristic nitrogen source absorptions through bands the 1 1 (Figurerelease4 of). Thereactive broad nitrogen band at species 1200 cm that− wasbind due to th toe C–Ocarbon/C–OH surface stretching, by the aim the bandof heat at produced 1580 cm− fromwas 1 ascribedthe exothermic to the presence reaction. of Deconvolution C=C/C=N bonds, of thetheweak high-resolution shoulder at 1700C1s spectrum cm− was (Figure assigned 5b) to carbonylrevealed 1 groupsdifferent C =carbonO, and bonding finally, the states weak (C=C, peaks C–C, near C–O/C– 3000 cmN,− toC=O, residual and satellite) C–H moieties. [10,11]. The Therefore, presence the of oxygen-containingsample consisted of groups sp2 and pinpointed sp3 carbon oxidation configurations of the carbon with surface,an intense in linepresence with theof C–O XPS results(which discussedoverlapped in with the next the paragraph.C–N bond region) Furthermore, and C=O the IR-activebonds, in Cagreement=C stretching with vibration the atomic was composition quite close toof thethe Raman-active sample. The presence G band of of the oxygen-containing solid as a result of groups, symmetry which breaking may act by as lattice surface defects, binding nitrogen sites, doping,and the andrelatively/or surface good oxidation specific (e.g.,surface the area so-called of the “doing sample Raman are attractive with IR”) structural [8]. Lastly, features residual in hydrogenscarbon-based normally adsorption/r resultedemoval from thetechnologies organic salt (e.g., precursor carbon byfilters). ignition.

FigureFigure 4.4. ATRATR infraredinfrared spectrumspectrum ofof thethe carboncarbon solid.solid.

The XPS survey spectrum of the sample showed predominantly carbon (81%) and, to a lesser extent, oxygen (16.1%) and nitrogen (2.9%) (Figure5a). Note that no trace of lithium was detected in the region between 55–57 eV, which is characteristic of the element. Oxygen was due to ignition in open air (e.g., air-oxidation), while nitrogen due to the nitric acid used for reaction (e.g., N-doping by HNO3)[9]. According to this reference, nitric acid served as the nitrogen source through the release of reactive nitrogen species that bind to the carbon surface by the aim of heat produced from the exothermic reaction. Deconvolution of the high-resolution C1s spectrum (Figure5b) revealed di fferent carbon bonding states (C=C, C–C, C–O/C–N, C=O, and satellite) [10,11]. Therefore, the sample consisted of sp2 and sp3 carbon configurations with an intense presence of C–O (which overlapped with the C–N bond region) and C=O bonds, in agreement with the atomic composition of the sample. The presence of oxygen-containing groups, which may act as surface binding sites, and the relatively good specific surface area of the sample are attractive structural features in carbon-based adsorption/removal technologies (e.g., carbon filters). An AFM study of the sample revealed two types of morphologies, namely carbon nanosheets and discoidal nanocarbon [12] (Figure6a–c). The micron-sized thin nanosheets appeared with an average thickness of ca. 2.5 nm according to section analysis (Figure6a). On the other hand, the submicron-sized discoidal carbon exhibited a bimodal thickness distribution with thicknesses of ca. 1 (Figure6b) and ca. 5 nm (Figure6c) (e.g., the presence of thinner and thicker discs). The discoidal particles appeared flat as evidenced by their 3D morphology (Figure6d). The appearance frequency of the discoidal morphology in the sample was up to 92% (Figure6e), thereby largely dominating C 2020, 6, 61 6 of 12 the sum. It is worth noting that nearly 77% of the discs appeared to have similar thickness with graphene (1 nm). Hence, we can safely conclude that the good specific surface area of the sample stems in part from the highly exfoliated state and nanosized thickness of the discs. The disc-like morphology of the sample was additionally confirmed by SEM and TEM microscopies, showing both individual andC 2020 aggregated, 6, x FOR PEER nanodiscs REVIEW of submicron lateral dimensions (Figure7). 6 of 13

FigureFigure 5.5. ((aa)) XPS XPS survey survey spectrum spectrum of the of carbon the carbon sample sample with the with respective the respective atomic composition. atomic composition. (b) C 2020(b,High-resolution 6),High-resolution x FOR PEER REVIEW XPS XPS C1s C1senvelope envelope with withassigned assigned deconvoluted deconvoluted components. components. 7 of 13

An AFM study of the sample revealed two types of morphologies, namely carbon nanosheets and discoidal nanocarbon [12] (Figure 6a–c). The micron-sized thin nanosheets appeared with an average thickness of ca. 2.5 nm according to section analysis (Figure 6a). On the other hand, the submicron-sized discoidal carbon exhibited a bimodal thickness distribution with thicknesses of ca. 1 (Figure 6b) and ca. 5 nm (Figure 6c) (e.g., the presence of thinner and thicker discs). The discoidal particles appeared flat as evidenced by their 3D morphology (Figure 6d). The appearance frequency of the discoidal morphology in the sample was up to 92% (Figure 6e), thereby largely dominating the sum. It is worth noting that nearly 77% of the discs appeared to have similar thickness with graphene (1 nm). Hence, we can safely conclude that the good specific surface area of the sample stems in part from the highly exfoliated state and nanosized thickness of the discs. The disc-like morphology of the sample was additionally confirmed by SEM and TEM microscopies, showing both individual and aggregated nanodiscs of submicron lateral dimensions (Figure 7).

FigureFigure 6.6. ((aa––cc)) AFMAFM imagesimages ofof cross-sectionalcross-sectional analysisanalysis ofof thethe carboncarbon samplesample (nanosheets(nanosheets andand discs).discs). ((dd)) 3D3D morphologymorphology ofof aa flatflat disc.disc. ((ee)) HistogramHistogram ofof frequencyfrequency raterate ofof thethe didifferentfferent morphologiesmorphologies alongalong withwith theirtheir thicknesses.thicknesses. C 2020, 6, 61 7 of 12 C 2020, 6, x FOR PEER REVIEW 8 of 13

FigureFigure 7. 7.SEMSEM (a (,ab,b) )and and TEM TEM ((cc,,dd)) imagesimages of of carbon carbon discs. discs.

3.2. Ferrocene- and Cobaltocene-HNO3 Hypergolic Pairs 3.2. Ferrocene- and Cobaltocene-HNO3 Hypergolic Pairs Another interesting class of cyclopentadienyl compounds are , such as ferrocene Another interesting class of cyclopentadienyl compounds are metallocenes, such as ferrocene and cobaltocene. In the present study, the pure compounds reacted hypergolicly on contact with and cobaltocene. In the present study, the pure compounds reacted hypergolicly on contact with fuming nitric acid to give magnetic γ-Fe2O3 and Co nanoparticles as the major phase. Figure8 displays fuming nitric acid to give magnetic γ-Fe2O3 and Co nanoparticles as the major phase. Figure 8 the indexed XRD patterns of the ferrocene-derived γ-Fe2O3 and cobaltocene-derived Co magnetic displaysphases. the Even indexed though XRD magnetite patterns and γof-Fe the2O3 ferrocene-derivedhave similar XRD patterns, γ-Fe2O nevertheless,3 and cobaltocene-derived Fe3O4 formation Co magneticrequires phases. anaerobic Even conditions though in magnetite order to avoid and air-oxidation γ-Fe2O3 have into similarγ-Fe2O3 .XRD In this patterns, respect, ignitionnevertheless, in Fe3Oair4 formation under strongly requires oxidizing anaerobic conditions conditions rather favors in order the formation to avoid ofair-oxidation the more stable intoγ-Fe γ-Fe2O32Ophase.3. In this respect,Furthermore, ignition somein air residualunder strongly metallic Fe oxidizing/Fe3C phases conditions were also rather detected favors due tothe possible formation carbothermal of the more stablereduction γ-Fe2O3 of phase. the oxide. Furthermore, Turning to some the cobalt residual case, tracesmetallic of cobaltFe/Fe oxides3C phases were were additionally also detected observed due to possibleas a carbothermal result of metal reduction oxidation. of Generally, the oxide. ferrocene Turning isto a the well-known cobalt case, precursor traces of to cobalt magnetic oxides iron were oxides [13,14]; similarly, cobaltocene is a well-known precursor to metallic cobalt [15]. additionally observed as a result of metal oxidation. Generally, ferrocene is a well-known precursor to magnetic iron oxides [13,14]; similarly, cobaltocene is a well-known precursor to metallic cobalt [15].

CC2020 2020, 6, ,6 61, x FOR PEER REVIEW 89 ofof 1213

Figure 8. XRD patterns of (top) the as-derived γ-Fe2O3 and (bottom) Co magnetic powders. The Figure 8. XRD patterns of (top) the as-derived γ-Fe2O3 and (bottom) Co magnetic powders. The insets showinsets the show strong the attractionstrong attraction of the magnetic of the magnet solidsic by solids Nd-magnet. by Nd-magnet.

TheThe transition transition metals metals in in ferrocene ferrocene and and cobaltocene cobaltocene should should normally normally catalyze catalyze carbon carbon combustion combustion duringduring ignitionignition byby HNOHNO3;3; however,however, thethe samples samples did did contain contain residual residual carbon. carbon. Indeed,Indeed, thermalthermal gravimetricgravimetric analysis analysis in thein the air showedair showed clear weightclear we lossesight oflosses 30–35% of due30–35% to combustion due to combustion of amorphous of carbonamorphous near 300 carbon◦C. Practically, near 300 this°C. meansPractically, that mildthis heatmeans treatment that mild of the heat samples treatment between of the 300–400 samples◦C inbetween the air 300 should–400 e°Cffectively in the air remove should most effectively of the remove amorphous most carbon of the withoutamorphous severely carbon a ffwithoutecting theseverely structure affecting of the the magnetic structure phases of the (e.g., magneticγ-Fe2 Ophases3 is stable (e.g., up γ-Fe to2O 4003 is◦ C,stable whereas up to Co400 oxidation °C, whereas in airCo commences oxidation in> 400air ◦commencesC). The AFM > study400 °C). of theThe magnetic AFM study solids of revealedthe magnetic the presence solids revealed of spherical the nanoparticlespresence of spherical with an average nanoparticles size of 8–11with nman average for γ-Fe 2sizeO3 and of 8–11 19–21 nm nm for for γ metallic-Fe2O3 and Co (Figure19–21 nm9a,b). for Thesemetallic values Co (Figure were in 9a,b). a good These agreement values withwere thosein a good calculated agreement from XRDwith andthose the calculated Scherrer from equation. XRD Theand round the Scherrer shape of equation. the nanoparticles The round was shape additionally of the nanoparticles confirmed by was the corresponding additionally confirmed 3D morphology by the imagescorresponding of the nanoparticles 3D morphology (Figure images9c,d). of the nanoparticles (Figure 9c,d). C 2020, 6, 61 9 of 12 C 2020, 6, x FOR PEER REVIEW 10 of 13

FigureFigure 9.9. Cross-sectionalCross-sectional analysis analysis (a (,ab,)b and) and 3D 3D morphology morphology (c,d ()c ,ofd) the of themagnetic magnetic nanoparticles. nanoparticles. (a,c)

(γa,-Fec) γ2O-Fe3 and2O3 (andb,d)( bCo.,d) Co.

TheThe correspondingcorresponding magnetic magnetic powders powders were were strongly strongly attracted attracted by by the the Nd-magnet Nd-magnet (insets, (insets, Figure Figure8). Figure8). Figure 10 shows 10 shows the magnetizationthe magnetization curves curves of the of ferrocene- the ferrocene- and cobaltocene-derived and cobaltocene-derived magnetic magnetic solids atsolids room at temperature. room temperature. The nanoparticles The nanoparticles did not exhibit did not hysteresis exhibit (e.g.,hysteresis almost (e.g., negligible almost coercivity). negligible 1 Oncoercivity). the other On hand, the other the saturation hand, the magnetizationsaturation magnetization values were values found were to be found 71 emu to be g− 71for emuγ-Fe g−21 Ofor3 1 1 andγ-Fe 702O emu3 and g −70for emu Co. g These−1 for Co. values These were values lower were than thoselower of than the bulkthose phases of the ( γbulk-Fe2 Ophases3: 83 emu (γ-Fe g−2O;3 Co:: 83 1 162emu emu g−1;g Co:− ) due162 emu to the g− quantum1) due to sizethe quantum effect. Overall, size effect. the magnetization Overall, the magnetization data confirmed data the nanosizedconfirmed dimensionsthe nanosized of thedimensions obtained of particles. the obtained particles. Likewise, cobaltocene, nickelocene (99% Acros Organics™, Morris County, NJ, USA) afforded magnetic nanoparticles after ignition with fuming nitric acid. Moreover, the method was useful for the synthesis of alloy nanoparticles as well. For this purpose, equimolar amounts of cobaltocene and nickelocene were ground in the presence of acetone in a mortar and pestle to obtain a homogenous powder after drying. Acetone is a good solvent for both metallocenes, thus allowing molecular blending of the precursors. Following, the mixture was ignited by fuming nitric acid to afford CoNi magnetic nanoparticles displaying the characteristic (111), (200), and (220) reflections of the alloy [16]. C 2020, 6, 61 10 of 12

Lastly, chromocene (95% Sigma–Aldrich, St. Louis, MO, USA) reacted similarly as above, to give instead ceramic Cr2O3. The corresponding XRD patterns of all solids are given in Figure 11. Therefore, depending on the cyclopentadienyl precursor many more materials can be foreseen and explored throughC 2020, 6, x hypergolics FOR PEER REVIEW in the near future. 11 of 13

Figure 10. Magnetization curves of the ferrocene-(top red loop) and cobaltocene-derived (bottom blue Figure 10. Magnetization curves of the ferrocene- (top red loop) and cobaltocene-derived (bottom loop) magnetic solids at room temperature. blue loop) magnetic solids at room temperature.

Likewise, cobaltocene, nickelocene (99% Acros Organics™, Morris County, NJ, USA) afforded magnetic nickel nanoparticles after ignition with fuming nitric acid. Moreover, the method was useful for the synthesis of alloy nanoparticles as well. For this purpose, equimolar amounts of cobaltocene and nickelocene were ground in the presence of acetone in a mortar and pestle to obtain a homogenous powder after drying. Acetone is a good solvent for both metallocenes, thus allowing molecular blending of the precursors. Following, the mixture was ignited by fuming nitric acid to afford CoNi magnetic nanoparticles displaying the characteristic (111), (200), and (220) reflections of the alloy [16]. Lastly, chromocene (95% Sigma–Aldrich, St. Louis, MO, USA) reacted similarly as C 2020, 6, x FOR PEER REVIEW 12 of 13

above, to give instead ceramic Cr2O3. The corresponding XRD patterns of all solids are given in CFigure2020, 6 ,11. 61 Therefore, depending on the cyclopentadienyl precursor many more materials can11 of be 12 foreseen and explored through hypergolics in the near future.

Figure 11. XRD patterns of the as-derived Ni, CoNi alloy, andand CrCr22O3 nanoparticles. 4. Conclusions 4. Conclusions Cyclopentadienyl compounds are versatile reagents in the hypergolic synthesis of different types of Cyclopentadienyl compounds are versatile reagents in the hypergolic synthesis of different nanomaterials, such as carbon and inorganic. Indicatively,the hypergolic reaction of cyclopentadienyllithium types of nanomaterials, such as carbon and inorganic. Indicatively, the hypergolic reaction of with fuming nitric acid gave a carbon residue mostly composed of discoidal particles, whereas analogous cyclopentadienyllithium with fuming nitric acid gave a carbon residue mostly composed of reactions with ferrocene and cobaltocene afforded γ-Fe2O3 and Co magnetic nanoparticles, respectively. discoidal particles, whereas analogous reactions with ferrocene and cobaltocene afforded γ-Fe2O3 In all instances, the reactions were fast, spontaneous, and exothermic at ambient conditions, resulting and Co magnetic nanoparticles, respectively. In all instances, the reactions were fast, spontaneous, in rapid product formation with fairly good yields. Based on these grounds, hypergolics merits further and exothermic at ambient conditions, resulting in rapid product formation with fairly good yields. attention in materials science as a new preparative method of a broader class of nanomaterials. Based on these grounds, hypergolics merits further attention in materials science as a new Authorpreparative Contributions: method ofConceptualization, a broader class experiments,of nanomaterials. and writing A.B.B. and D.G.; formal analysis, experiments, and writing N.C., V.Š., V.K., D.M., A.A., and M.A.K. All authors have read and agreed to the published version of Author Contributions: Conceptualization, experiments, and writing A.B.B. and D.G.; formal analysis, the manuscript. experiments, and writing N.C., V.Š., V.K., D.M., A.A., and M.A.K. All authors have read and agreed to the Funding:published Weversion acknowledge of the manuscript. support of this work by the project “National Infrastructure in Nanotechnology, Advanced Materials and Micro-/Nanoelectronics” (MIS-5002772) which was implemented under the action “ReinforcementFunding: We acknowledge of the Research support and of Innovationthis work by Infrastructure”, the project ‘‘National funded Infrastructure by the Operational in Nanotechnology, Programme “Competitiveness,Advanced Materials Entrepreneurship and Micro-/Nanoelectronics” and Innovation” (MIS-5002772) (NSRF which 2014-2020), was implemented and co-financed under bythe Greeceaction and the European Union (European Regional Development Fund). This research was also co-financed by ‘‘Reinforcement of the Research and Innovation Infrastructure”, funded by the Operational Programme Greece and the European Union (European Social Fund- ESF) through the Operational Programme “Human Resources‘‘Competitiveness, Development, Entrepre Educationneurship and and Lifelong Innovation” Learning” (NSRF in the2014-2020) context,of and the co-financed project “Strengthening by Greece and Human the ResourcesEuropean Union Research (European Potential Regional via Doctorate Development Research” Fund). (MIS-5000432), This research implemented was also co-financed by the State by Scholarships Greece and Foundationthe European (IKY). Union V.Š. (European acknowledges Soci theal Fund- assistance ESF) provided through bythe the Operat Researchional Infrastructure Programme NanoEnviCz‘‘Human Resources II from MEYSDevelopment, CZ (LM2018124) Education and and the Lifelong support Learning” from the in Internal the context Student of the Grant project Agency ‘‘Strengthening of the Palack Humaný University Resources in Olomouc,Research Potential Czech Republic via Doctorate (IGA_PrF_2020_022). Research” (MIS-5000432), V.Š. and V.K. implemented also acknowledge by the the State financial Scholarships support Foundation from MEYS CZ(ΙΚY). under V.Š. the acknowledges project CZ.02.1.01 the assistance/0.0/0.0/16_019 provided/0000754. by the Research Infrastructure NanoEnviCz II from MEYS CZ Conflicts(LM2018124) of Interest: and the Thesupport authors from declare the Internal no conflict Student of interest. Grant Agency of the Palacký University in Olomouc, Czech Republic (IGA_PrF_2020_022). V.Š. and V.K. also acknowledge the financial support from MEYS CZ under the project CZ.02.1.01/0.0/0.0/16_019/0000754.

Conflicts of Interest: The authors declare no conflict of interest. C 2020, 6, 61 12 of 12

References

1. 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] 2. 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] 3. Chalmpes, N.; Asimakopoulos, G.; Spyrou, K.; Vasilopoulos, K.C.; Bourlinos, A.B.; Moschovas, D.; Avgeropoulos, A.; Karakassides, M.A.; Gournis, D. Functional carbon materials derived through hypergolic reactions at ambient conditions. Nanomaterials 2020, 10, 566. [CrossRef][PubMed] 4. Chalmpes, N.; Spyrou, K.; Vasilopoulos, K.C.; Bourlinos, A.B.; Moschovas, D.; Avgeropoulos, A.; Gioti, C.; Karakassides, M.A.; Gournis, D. Hypergolics in carbon nanomaterials synthesis: New paradigms and perspectives. Molecules 2020, 25, 2207. [CrossRef][PubMed] 5. Chalmpes, N.; Tantis, I.; Bakandritsos, A.; Bourlinos, A.B.; Karakassides, M.A.; Gournis, D. Rapid carbon formation from spontaneous reaction of ferrocene and liquid bromine at ambient conditions. Nanomaterials 2020, 10, 1564. [CrossRef][PubMed] 6. Trent, C.H.; Zucrow, M.J. The hypergolic reaction of dicyclopentadiene with white fuming nitric acid. J. Am. Rocket Soc. 1951, 21, 129–131. [CrossRef] 7. Roh, J.-S. Structural study of the activated carbon fiber using laser Raman spectroscopy. Carbon Lett. 2008, 9, 127–130. [CrossRef] 8. Ferrari, A.C.; Rodil, S.E.; Robertson, J. Interpretation of infrared and Raman spectra of amorphous carbon nitrides. Phys. Rev. B 2003, 67, 155306. [CrossRef] 9. D’Arsié, L.; Esconjauregui, S.; Weatherup, R.S.; Wu, X.; Arter, W.E.; Sugime, H.; Cepek, C.; Robertson, J. Stable, efficient p-type doping of graphene by nitric acid. RSC Adv. 2016, 6, 113185–113192. [CrossRef] 10. Bakandritsos, A.; Pykal, M.; Bło´nski,P.; Jakubec, P.; Chronopoulos, D.D.; Poláková, K.; Georgakilas, V.; Cˇ épe, K.; Tomanec, O.; Ranc, V.; et al. Cyanographene and graphene acid: Emerging derivatives enabling high-yield and selective functionalization of graphene. ACS Nano 2017, 11, 2982–2991. [CrossRef][PubMed] 11. Zoppellaro, G.; Bakandritsos, A.; Tuˇcek,J.; Bło´nski,P.; Susi, T.; Lazar, P.; Bad’ura, Z.; Steklý, T.; Opletalová, A.; Otyepka, M.; et al. Microwave energy drives “on–off–on” spin-switch behavior in nitrogen-doped graphene. Adv. Mater. 2019, 31, 1902587. [CrossRef][PubMed] 12. Zygouri, P.; Tsoufis, T.; Kouloumpis, A.; Patila, M.; Potsi, G.; Sevastos, A.A.; Sideratou, Z.; Katsaros, F.; Charalambopoulou, G.; Stamatis, H.; et al. Synthesis, characterization and assessment of hydrophilic oxidized carbon nanodiscs in bio-related applications. RSC Adv. 2018, 8, 122–131. [CrossRef] 13. Amara, D.; Margel, S. Solventless thermal decomposition of ferrocene as a new approach for the synthesis of porous superparamagnetic and ferromagnetic composite microspheres of narrow size distribution. J. Mater. Chem. 2011, 21, 15764–15772. [CrossRef] 14. Amara, D.; Margel, S. Synthesis and characterization of elemental iron and iron oxide nano/microcomposite particles by thermal decomposition of ferrocene. Nanotechnol. Rev. 2013, 2, 333. [CrossRef] 15. Dormans, G.J.M.; Meekes, G.J.B.M.; Staring, E.G.J. OMCVD of cobalt and cobalt silicide. J. Cryst. Growth 1991, 114, 364–372. [CrossRef] 16. Wei, X.-W.; Zhou, X.-M.; Wu, K.-L.; Chen, Y. 3-D flower-like NiCo alloy nano/microstructures grown by a surfactant-assisted solvothermal process. CrystEngComm 2011, 13, 1328–1332. [CrossRef]

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