AN INVESTIGATION ON THE OPTICAL PROPERTIES OF CARBON BLACK,FULLERITE, AND OTHER CARBONACEOUS MATERIALS IN RELATION TO THE SPECTRUM OF INTERSTELLAR EXTINCTION OF LIGHT

ABSTRACT

In this work, the optical properties in the UV-VIS of three carbon blacks having specific surface area from 145 to 7?m²/g and produced with furnace or thermal processes have been studied. The results have been compared to the optical properties (in the same spectral range) of C₆₀ fullerene photopolymer, to fullerite i.e., the carbon soot containing fullerenes and to a sample of carbonaceous matter containing carbyne. The scope of the work was to verify if any of the carbonaceous matter studied was able to match the interstellar extinction spectrum which shows a “bump” at 217.5?nm and which was originally attributed to interstellar graphitic particles. None of the materials studied has shown a peak at 217.5?nm. All the carbonaceous materials studied show maxima of absorption from 252 to 267?nm with the exclusion of C₆₀ photopolymer which has a completely peculiar and different spectrum from all other materials with three maxima at 271, 389 and 510?nm. All the carbon materials studied do not match the 217?nm peak, hence, cannot be considered the carrier of the interstellar “bump” but may be present in some circumstellar shells of late type stars, where the peaks at 240–250?nm have been recorded and attributed to a more ordered and partially graphitized carbonaceous matter. In the discussion, the matrix effect, the particle size, and the clumping of the material studied have been considered as cause of the shift of the peak to longer wavelength.

The carbon black samples have also been extracted with pentane or ethanol and the polycyclic aromatic hydrocarbons (PAH) identified have been discussed in the frame of recent works and in relation to the role that they play in the interstellar medium as potential carriers of the diffuse interstellar bands (DIBS).     

GENERATION OF HIGHER FULLERENES FROM LASER ABLATION OF CARBYNE AND C60 PHOTOPOLYMER ASTROCHEMICAL IMPLICATIONS

Laser ablation of targets of carbonaceous matter containing carbyne nanodomains (the sp hybridised carbon chains) or targets of C₆₀ photopolymer produced carbon clusters which have been detected by FT-ICR (Fourier-Transform Ion Cyclotron Resonance) mass spectrometer. When the carbonaceous matter containing carbyne has been employed as laser target, no C₆₀ has been generated but only fullerene cages from C₇₄ up to C₁₂₄. Larger cages were also obtained but with odd number. Starting from C₆₀ photopolymer, laser ablation regenerates free C₆₀ and creates a sequence of C₆₀ superior homologues all possessing even number and each member of the series is separated from the preceding and the following member by the loss or by the addition respectively of a C₂ unit. Fullerenes up to C₁₆₂ have been recorded. The implications about the presence of free C₆₀ fullerene in the interstellar and circumstellar space, its formation from carbyne chains and its stability towards its photopolymerization tendency and its regeneration from the photopolymer together with its superior homologues have been discussed thoroughly in the present paper.     

A STUDY ON THE THERMAL STABILITY OF THE PHOTOPOLYMER, THE OZOPOLYMER, AND THE PHOTOCHLORINATED DERIVATIVE OF C60 FULLERENE

The thermal stability of the C₆₀ photopolymer, the C₆₀ ozo-polymer, and photochlorinated C₆₀ was studied by thermogravimetric analysis (TGA) ultraviolet-visible (UV-vis) and TGA-differential thermal analysis techniques up to 950°C in comparison to graphite and pure C₆₀. The ozopolymer was found to be the least stable material followed by C₆₀Cl_x. The resulting residual carbonaceous matter formed by the decomposition of the photopolymer and the ozopolymer has been studied by Fourier transform-infrared (FTIR) spectroscopy and has been found to be completely comparable to carbon black. The thermal decomposition of the C₆₀ photopolymer prepared in solution yields negligible amounts of C₆₀. The main product is carbon black.     

He ION BOMBARDMENT OF C60 FULLERENE: AN FT-IR AND RAMAN STUDY

C₆₀ fullerene films have been bombarded with He⁺ ions at 30 keV at room temperature in vacuum. The structural changes undergone by C₆₀ have been followed by both FT-IR and Raman spectroscopy. Raman spectroscopy was the most useful tool for this scope. It has been clearly discovered that at low radiation dose C₆₀ forms oligomers but at higher radiation doses it is converted into an amorphous carbonaceous matter. The implications of these results on the possible survival of C₆₀ fullerene in the interstellar space have been discussed briefly in connection with the previous results on the effects of various types of electromagnetic radiation over C₆₀.     

Fullerane, the Hydrogenated C60 Fullerene: Properties and Astrochemical Considerations

Hydrogenated C₆₀ fullerene, C₆₀H₃₆ was prepared in different solvents using Zn/HCl as reducing agents. The structure of C₆₀H₃₆ was confirmed both by electronic and FT-IR spectroscopy and the purity of the reaction product was checked by HPLC analysis. It has been confirmed that C₆₀H₃₆ is not stable in air, especially in presence of light which enhances the oxidation. The oxidation of C₆₀H₃₆ was studied by FT-IR spectroscopy and by differential scanning calorimetry (DSC) in air; the formation of hydroxyl groups on the fullerene cage and ketonic groups (involving cage breakdown) have been detected. Furthermore, the action of O₃ on C₆₀H₃₆ was investigated and it has been found that O₃ exerts practically the same effect of air but causing an enhanced cage breakdown. The thermal stability of C₆₀H₃₆ was checked by a thermogravimetric analysis (TGA) coupled with a differential thermal analysis (DTA) under N₂ flow. The vaporization of C₆₀H₃₆ occurs at very high temperature: the DTA trace has shown an endothermic peak at 540°C (at a heating rate of 20°C/min). C₆₀H₃₆ shows an electronic absorption spectrum with a maximum at about 217 nm and it is able to match both in position and in half width the peak at 217.5 nm observed in the spectrum of the interstellar extinction of light which was attributed to hydrogenated, radiation processed and thermally annealed carbon dust. Similarly, the absorption spectrum of C₆₀H₃₆ is able to match several infrared emission bands (called UIBs) detected from certain astrophysical objects like the protoplanetary nebulae (PPNe). It is proposed that hydrogenated fullerenes can be used as model compounds in the laboratory simulation studies of interstellar carbon dust.     

THE REACTION OF C60F20 WITH ANTHRACENE: FORMATION OF AN OXIDISED-ANTHRACENE 1 : 1 COMPLEX

The reaction of C₆₀F₂₀ (“Saturnene”) with anthracene yields a white 1 : 1 cycloadduct (1) in which two oxygen atoms have inserted into the anthracene framework due to fullerene-catalysed oxidation; the ¹⁹F NMR spectrum of the product shows evidence of through-cage homoconjugation.     

CO-DEPOSITION OF FULLERENES AND SULFUR FROM DILUTED SOLUTION

A new method to synthesize fullerene and sulfur compounds has been developed. Using this method, C₆₀S₁₆ and C₇₀S₁₆ compounds were grown from dilute fullerene and sulfur toluene solution. Their atomic structures were analyzed by x-ray diffraction with the single crystal. The C₆₀S₁₆ crystal is C-centered monoclinic structure of a=2.0874 nm, b=2.1139 nm, c=1.05690 nm and β=111.93°, and the C₇₀S₁₆ has a primitive monoclinic, P2₁/c, with lattice parameters of a=1.5271 nm, b=1.49971 nm, c=2.18024 nm and β=109.791°. In this compound, the structure of fullerenes is maintained and sulfur atoms form S₈ rings placed around the fullerenes.     

GENERATION OF HIGHER FULLERENES FROM LASER ABLATION OF CARBYNE AND C60 PHOTOPOLYMER ASTROCHEMICAL IMPLICATIONS

ABSTRACT

Laser ablation of targets of carbonaceous matter containing carbyne nanodomains (the sp hybridised carbon chains) or targets of C₆₀ photopolymer produced carbon clusters which have been detected by FT-ICR (Fourier-Transform Ion Cyclotron Resonance) mass spectrometer. When the carbonaceous matter containing carbyne has been employed as laser target, no C₆₀ has been generated but only fullerene cages from C₇₄ up to C₁₂₄. Larger cages were also obtained but with odd number. Starting from C₆₀ photopolymer, laser ablation regenerates free C₆₀ and creates a sequence of C₆₀ superior homologues all possessing even number and each member of the series is separated from the preceding and the following member by the loss or by the addition respectively of a C₂ unit. Fullerenes up to C₁₆₂ have been recorded. The implications about the presence of free C₆₀ fullerene in the interstellar and circumstellar space, its formation from carbyne chains and its stability towards its photopolymerization tendency and its regeneration from the photopolymer together with its superior homologues have been discussed thoroughly in the present paper.     

MOLECULAR AND CRYSTAL STRUCTURE OF THE ADDUCTS OF C60F18 WITH AROMATIC HYDROCARBONS

The solubility of C₆₀F₁₈ in aromatic hydrocarbons, benzene, toluene, and xylenes (0.48-1.23 mg/mL), and decomposition enthalpies for the 1 : 1 and 1 : 2 complexes (31-70 kJ/mole) have been determined. The C₆₀F₁₈ molecule has near perfect C_3v symmetry, and the x-ray single-crystal structures of the C₆₀F₁₈ L complexes (L=hexamethylbenzene, o-, m-, p-xylene, and bromobenzene) are compared in terms of 12 types of C-C and four types of C-F bonds. Analysis of the packing modes in the crystals shows an influence of the size and polarity of the aromatic hydrocarbon molecule.     

A STUDY ON THE THERMAL STABILITY TO 1000°C OF VARIOUS CARBON ALLOTROPES AND CARBONACEOUS MATTER BOTH UNDER NITROGEN AND IN AIR

The thermal behavior of graphite, C₆₀ fullerene, fullerene black (carbon soot containing fullerenes), extracted fullerene black and diamond has been analyzed to 1000°C by TGA-DTA (thermogravimetric analysis and differential thermal analysis) under a nitrogen flow at a heating rate of 20°C/min. Very small weight losses have been recorded in the case of graphite and diamond. Furthermore no diamond graphitization has been observed. The sublimation of pure C₆₀ and the fullerene fraction of fullerene black (both pristine and extracted) has been observed and discussed.

The combustion reaction in air flow of graphite, C₆₀ and C₇₀ fullerenes, fullerene black (both unextracted and extracted), carbon nanotubes and diamond has been studied by TGA-DTA at a heating rate of 20°C/min. C₇₀ fullerene and fullerene black have been found to be the most reactive carbon materials with O₂. The role played by C₇₀ in the degradation of fullerites has been discussed. Among the carbon materials examined, the best resistance to O₂ attack has been shown by diamond and carbon nanotubes. The behavior of graphite is intermediate between diamond and fullerene blacks. The behavior of C₆₀ fullerene appears closer to that of graphite although it appears to be more reactive with O₂. Samples of graphite and carbon blacks N375 and N234 have been studied by TGA-DTA in air flow before and after a radiation treatment with neutrons or γ radiation. The effect of the radiation damage in the combustion reaction of these carbon materials has been discussed.     

A STUDY ON THE THERMAL STABILITY OF THE PHOTOPOLYMER,THE OZOPOLYMER,AND THE PHOTOCHLORINATED DERIVATIVE OF C60 FULLERENE

The thermal stability of the C₆₀ photopolymer, the C₆₀ ozo-polymer, and photochlorinated C₆₀ was studied by thermogravimetric analysis (TGA) ultraviolet-visible (UV-vis) and TGA-differential thermal analysis techniques up to 950°C in comparison to graphite and pure C₆₀. The ozopolymer was found to be the least stable material followed by C₆₀Cl _x . The resulting residual carbonaceous matter formed by the decomposition of the photopolymer and the ozopolymer has been studied by Fourier transform-infrared (FTIR) spectroscopy and has been found to be completely comparable to carbon black. The thermal decomposition of the C₆₀ photopolymer prepared in solution yields negligible amounts of C₆₀. The main product is carbon black.     

ON THE ABSORPTION SPECTRUM OF C60 IN THE VISIBLE SPECTRAL REGION

A complete (720-380 nm), strong visible absorption spectrum (OD₅₉₈=1.6) for C₆₀ in a decalin/methylcyclohexane inert matrix at 77 K is reported for the first time. The structure of this spectrum allows one to construct an acceptable approximation to the gas-phase spectrum for this substance at a low temperature. The 0-0 component of the S₀→S₁ transition in the gas phase at a low temperature was estimated to be at about 635 nm.     

On the Action of γ Radiation on Solid C60 and C70 Fullerenes: A Comparison with Graphite Irradiation

C₆₀ and C₇₀ fullerene have been treated in sealed flasks under Ar with γ radiation using radiation dosages ranging from 10 to 1000 kGy. The treated samples studied by electronic and FT-IR spectroscopy have not shown any evidence about fullerenes decomposition or radiopolymerization. However, through Raman spectroscopy it was possible to observe that γ radiation induces C₆₀ dimerization and trimerization. It has additionally been discovered that γ-treated C₆₀ (oligomerized) can be easily photopolymerized in the solid state by post-irradiation with laser light at 514 nm while this phenomenon has not been observed by using laser light at 782 nm and considerably higher laser power. Previously to this study, C₆₀ photopolymerization was known to occur only by using ultraviolet light.

For comparison also graphite was irradiated with 1000 kGy of γ radiation. Irradiated graphite shows considerably changes in its Raman spectrum, showing the formation of glassy carbon domains, perhaps carbon onions. The l_d bandshift to 1310 cm^-1 could be interpreted in terms of formation of hexagonal diamond.     

SOME IMPLICATIONS OF THE RADIATION-TREATMENT OF GRAPHITE AND CARBON BLACK

Graphite and carbon black N234 radiation damaged with γ radiation or with neutron bombardment have been studied with Raman spectroscopy. The radiation damaging results completely evident in the case of graphite with the development of the I_D band, but it is less evident in the case of an already disordered material like carbon black. The radiation damage caused by γ radiation appears comparable to that caused by neutrons, at least for the radiation dose used. Moreover, in both cases there are evidences that the radiation-induced defects appear in carbon material under the form of fullerene-like sites (onion-like carbon and carbon nanotubes) as well as under the form of hexagonal diamond and hence sp³ hybridized carbon.

Neutron damaged carbon black once treated with laser light at 782 nm shows a featureless Raman spectrum with a maximum at 2287 cm^-1 suggesting that neutron bombardment followed by laser light annealing causes the formation of carbyne (polyyne) domains.

The radiation treatment of graphite, carbon black and even amorphous precipitated silica enhances in a spectacular way their natural rubber adsorption power. This has been measured through the “bound rubber phenomenon” which is the irreversible rubber grafting reaction on filler surface. This enhancement has been attributed to the increased concentration of “defective” sites on filler surface induced by radiation treatment which improves the rubberfiller interaction. Another macroscopic consequence of this increased interaction can be manifested in SBR-based or in natural rubber based vulcanizates filled with radiation pre-treated carbon blacks. In both cases a dramatic improvement in the reinforcing effect as measured by stress-strain curve can be observed.     

The Solubility of C60 Fullerene in Long Chain Fatty Acids Esters

A series of fatty acid esters of glycerol as linseed, sunflower, soybean and olive oils have been tested as C₆₀ fullerene solvents together with a mixture of methyl ester fatty acids derived from brassica oilseeds and used as a biofuel known as “biodiesel.” All the oils evaluated are effective solvents of C₆₀. The solubility of C₆₀ in the selected vegetable oils has been determined spectrophotometrically. The C₆₀ solubility in vegetable oils may pave the way for easier application of C₆₀ fullerene in medicinal chemistry and in additive chemistry for varnishes and fuels. It has been found that C₆₀ is not only soluble in the fatty acid esters but is also reactive with them under mild conditions, giving addition products easily recognized by a characteristic absorption band at 435 nm. The addition reaction mechanism has been discussed.     

ON THE REACTIVITY OF C60 FULLERENE WITH DIENE RUBBER MACRORADICALS. I. THE CASE OF NATURAL AND SYNTHETIC CIS-1,4-POLYISOPRENE UNDER ANAEROBIC AND THERMOOXIDATIVE DEGRADATION CONDITIONS

C₆₀ fullerene has been studied as thermal stabilizer and as antioxidant of both natural rubber (cis-1,4-polyisoprene) and synthetic cis-1,4-polyisoprene. The study has been conducted respectively under nitrogen flow and under air flow by simultaneous thermogravimetric analysis and differential thermal analysis (TGA-DTA) on rubber samples containing known quantities of fullerene in comparison to a “blank” of pure rubber. The results show that C₆₀ fullerene (in absence of oxygen) is a thermal stabilizer of cis-1,4-polyisoprene because it reacts with the polyisoprene macroradicals formed by the thermally-induced chain scission reaction slowing down the degradation reaction. Conversely, under thermo-oxidative degradation conditions (in air flow) fullerene C₆₀ acts as an antioxidant for cis-1,4-polyisoprene, provided that the heating rate of the samples is slow (5°/min). At higher heating rates (20°C/min) C₆₀ does not show any antioxidant effect.     

UNSTABLE PRODUCTS FROM THE OZONATION OF C60 IN SOLVENTS

The ozonation of a solution of C₆₀ in toluene results in the formation of several unstable compounds C₆₀X and C₆₀Y[1-5] which decay completely at room temperature within about 1 h. The products are oxides C₆₀O[6,6], C₆₀O₂[I], C₆₀O₂[II], and possibly an isomer of C₆₀O₃. The transformations are not due to oxidation by atmospheric oxygen but are spontaneous. The ozonation of a solution of C₆₀O[6,6] in toluene results in the formation of two unstable compounds C₆₀Z[1-2] which also decay completely to C₆₀O₂[I] and C₆₀O₂[II]. It is suggested that all unstable “parents” are ozonides.     

A Model Compound Study About Carbon Black and Diene Rubber Interaction: The Reactivity of C60 Fullerene with Squalene

The reaction between squalene and C₆₀ fullerene was studied by electronic and FT-IR spectroscopy as well as by thermal analysis (TGA and DTA). This study was conducted to simulate in vitro, with model compounds, the interaction occuring during mixing between dienic rubber and carbon black. Squalene was used as model compound for dienic rubber and C₆₀ fullerene as model compound for carbon black since fullerene-like sites have been recently identified on carbon black surface.

The experimental results show that 2,5 molecules of C₆₀ become chemically bound to each squalene molecule under thermo-oxidative conditions simulating part of the mixing cycle between rubber and carbon black.

The implications of this result involve the explanation of the phenomenon known as “bound rubber”, which is the amount of chemisorbed polymer on filler surface after mixing, as well as the reinforcement effects observed by filling rubber with carbon black and the mechanical hysteresis of a rubber compound.     

Imaging Fullerene C60 with Atomic Resolution Using a Scanning Tunnelling Microscope

C₆₀ was purified and imaged utilizing scanning tunnelling microscopy (STM) in a constant current mode. By fixing the Fullerenes on the substrate (“frozen state” - no movement or rotation), direct imaging of C₆₀ with atomic resolution was possible, showing one pentagon and one hexagon carbon ring of C₆₀.     

ISOLATION AND CHARACTERISATION OF C60(CF3)2

From the products of reactions of [60]fullerene with either K₂PtF₆ at 470 °C or AgF at 520 °C, we have isolated C₆₀(CF₃)₂, the simplest trifluoromethylfullerene, which gives a single ¹⁹F NMR line at -69.5 ppm. The HPLC retention time is less than that of C₆₀F₂ confirming the trend observed for other fluoro- vs. trifluoromethylfullerenes namely that the latter elute more rapidly. Other trifluoromethyl- containing species, C₆₀(CF₃)₄O, C₆₀F₅CF₃, C₆₀(CF₃)₄H₂, C₆₀(CF₃)₆H₂, and C₆₀(CF₃H)₃ were detected in the product mixture.