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Comparative Study of Condensation Routes for Formation of Nanoand Microsized Carbon Forms in Hydrocarbon, Fluorocarbon, and Fluoro-Hydrocarbon Systems at High Pressures and Temperatures Valery N Khabashesku, Valery A. Davydov, and Vyacheslav N. Agafonov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10805 • Publication Date (Web): 06 Dec 2016 Downloaded from http://pubs.acs.org on December 8, 2016

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The Journal of Physical Chemistry

Comparative Study of Condensation Routes for Formation of Nanoand Microsized Carbon Forms in Hydrocarbon, Fluorocarbon, and Fluoro-Hydrocarbon Systems at High Pressures and Temperatures Valery A. Davydova, Viatcheslav Agafonov b, Valery N. Khabashesku *c,d a L.F.Vereshchagin

Institute for High Pressure Physics of the RAS, Troitsk, Moscow, Russia UMR CNRS-7347, Université F. Rabelais, Tours , France c Department of Chemical and Biomolecular Engineering, University of Houston, USA d Present address: Center for Technology Innovation, Baker Hughes Inc., Houston, TX, USA

b GREMAN,

ABSTRACT Thermal transformations of binary mixtures of hydrocarbons and fluorocarbons under high pressures demonstrate significantly lower initiation temperature thresholds for all major transformation stages, including carbonization, graphitization and diamond formation, than the transformations of pure hydrocarbon and fluorocarbon components. In addition, along with the formation of micron-size diamond fraction, typical for transformation products of pure hydrocarbons, massive formation of nanosize diamond fraction has been observed in the products of conversion of mixtures. In order to gain understanding of the nature of simultaneous formation of micro- and nanosize fractions of diamond in the binary mixtures of hydrocarbon and fluorocarbon compounds, a comparative studies of the attributes of thermal transformations of pure hydrocarbon, fluorocarbons and their homogeneous and heterogeneous mixtures has been carried out under pressure of 8 GPa and temperatures up to 1500°С. Naphthalene (C10H8) and octafluoronaphthalene (C10F8), two hydrocarbon and fluorocarbon structural analogs, have been used as model compounds. The massive formation of micron- and nano-size fractions of diamond in the products of high pressure-high temperature treatment of homogeneous С10Н8С10F8 mixtures has been explained by lowering of the initiation temperature threshold for diamond formation due to synergistic effect of fluorine and hydrogen on the transformations of С10Н8 and С10F8 in the mixture, which resulted in conservation of significant amounts of onionlike nanoparticles of 5-15 nm size along with submicron size particles of graphitic carbon in the carbonization products of C10F8 at temperatures ~ 900-1100 °С. These two size types of carbon particles act as precursors for formation of micro- and nanosize diamond fractions. A substantial effect of gas-phase transport of carbon on the formation of solid products under high pressure in the studied systems has been considered in the discussion of mechanisms of formation of different diamond fractions. The reported results potentially open a new direction for metal catalyst-free synthesis of nano/micro-size fractions of pure and doped diamonds.

* Corresponding author. E-mail: [email protected] (Valery Khabashesku) 1 ACS Paragon Plus Environment


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1. INTRODUCTION Studies of high pressure and high temperature induced transformations of naphthalene (С10Н8), graphite fluoride (CF1.1) and their homogeneous binary mixtures have uncovered significant differences in p,T parameters, morphology and fraction size composition of the products of main transformation stages in the systems comprised of pure hydrocarbon, fluorocarbon compounds and their homogeneous binary mixtures.1 It was noted a significant decline of the initiation temperatures for the processes of carbonization, graphitization and diamond formation under conditions of binary mixtures with respect to individual compounds under the same pressures. It was particularly shown that temperature threshold for initiation of diamond formation from a homogeneous С10Н8-CF1.1 mixture at 8.0 GPa was 900ºC, which is about 400º below the starting temperature of diamond formation from naphthalene or other aromatic hydrocarbons (about 1300ºC at 8 GPa).2 Another distinctive feature of the thermal transformations of binary mixtures of С10Н8CF1.1 under high pressures has been a massive formation of nanosize diamond fraction appearing in the products along with the micron-size diamond fraction that has typically been formed in the processes of thermal transformation of naphthalene and other hydrocarbons.2 These findings, along with the observed decline in the temperature of diamond formation from binary mixtures, became of special attention since from practical point of view they could open new opportunities for controlled synthesis of different size fractions of high purity and doped diamonds, particularly, nanodiamonds with N-V, Si-V luminescent centers.3 These doped nanodiamonds are currently considered as the most promising candidates for single-photon emitters in quantum physics and biological applications. According to1, a substantial difference in the p,T parameters of transformations of С10Н8 and CF1.1, can be attributed to a synergistic effect of fluorine and hydrogen on various stages of the transformation of individual compounds under conditions of binary mixture. It is believed that the synergistic effect is due to the mutual influence of the mobile gas and fluid-like fluorinecontaining decomposition products of CF1.1 on the processes of transformations of naphthalene, and gas and fluid-like hydrogen-containing products of the decomposition of С10Н8 on the conversion of CF1.1. This suggests that the nature of synergistic effect can be related to the changes of the mechanisms and activation energy values of the transformations of naphthalene and highly fluorinated graphite under conditions of binary mixtures. Under these conditions, a “purely” thermoactivated cleavages of hydrocarbon C-H and fluorocarbon C-F bonds in the initial stages of transformation of hydrocarbon and fluorocarbon compounds, are assisted by the thermochemical channels of the reactions. The latter are facilitated by the fluorine-hydrogen 2 ACS Paragon Plus Environment

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interactions leading to formation of hydrogen fluoride molecules which are capable of significant modification of different stages of thermal transformation of carbon-containing systems. However, the nature of the simultaneous formation of micro- and nanocarbon fractions in such systems remains debatable. The goal of this work was to carry out a more detailed study in order to gain knowledge on size-specific attributes of this process. Studies of thermal transformations of naphthalene and other polycyclic aromatic compounds under high pressures showed that at pressure of 8 GPa and temperatures of 12001300 °C the particles of highly ordered graphite of 3-30 micron size are formed as the main products. At temperatures higher than 1300 °C, these particles act as precursors for the formation of diamond monocrystals with 5-40 micron size.2 It is believed that diamond is formed mainly through a hydrogen “catalyzed” reactions which proceed by hydrogenation of edge atoms of graphite, transition of these atoms from sp2 to sp3 hybridization state, and formation of hydrogenated intermediate layer that secures the transformation of graphitic carbon into diamond. The mechanism of such transformations was proposed by Lambrecht et al.4 This mechanism provides an explanation of how a yet insignificant amount of hydrogen remaining in the system at final stages of carbonization of hydrocarbons can be capable of facilitating a massive transformation of micron sized graphite particles into a micron sized diamond monocrystals. The size distribution function for diamond monocrystals resembles in this case a Gaussian function with one distinct maximum. Such shape of particle size distribution function usually confirms the existence of unified mechanisms for both the formation of seeds and growth of particles of new phase.5 Polydispersive morphology of the product in this case has been explained by a nonsimultaneous formation of seeds of different particles. Qualitative dissimilarity of size distribution functions for diamond products formed by transformations of pure hydrocarbon and mixed hydrocarbon-fluorocarbon systems can be enabled by the difference in mechanisms of formation of nano- and micron-sized diamond fractions in the case of binary mixtures. They in turn can be related to a dissimilar structures and sizes of particles of intermediate carbon states formed in the thermal transformations of naphthalene and its binary С10Н8 - CF1.1 mixture and serving as direct precursors for diamond formation in the studied systems. Size variations of carbon particles may be caused by differences in temperatures of their formation at 8 GPa (~1300°C in case of hydrocarbon, and ~900°C in case of fluorine-hydrogencarbon system) and dissimilarities in the structure and chemical nature of parent hydrocarbon and fluorocarbon compounds, which can lead to a diversity of intermediate carbon states formed during the carbonization processes of these compounds. As a result, the differences in fractional composition of precursor states may be caused by both the "temperature" and "chemical 3 ACS Paragon Plus Environment


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structure" factors. Evaluation of the effect of these factors on the formation of different size diamond fractions has been the goal of this study. A more detailed consideration was devoted to the effect of "chemical" factor on the fractional composition of the products of transformations of binary mixtures of hydrocarbon and fluorocarbon compounds. From the general considerations, it is evident that even under the same treatment conditions of a binary mixture, due to significant differences in the structure and chemical nature of starting hydrocarbon and fluorocarbon compounds, the processes of their carbonization and graphitization may proceed through different intermediate states. This will result in formation of different carbon products, serving as precursors for the synthesis of diamond. However, the analysis of a possible "genetic" connection between the dimensional, structural and morphological features of a particular product of thermal transformations of binary mixtures with the hydrocarbon or fluorocarbon constituent of the initial mixture has not previously been carried out. In order to identify the degree of influence of "chemical" nature of starting hydrocarbon and fluorocarbon compounds on fractional composition of the transformation products and minimize a possible contribution from structural differences in the initial hydrocarbon and fluorocarbon compounds into the specifics of their transformation reactions, two structural analogues, naphthalene (C10H8) and octafluoronaphthalene (C10F8), have been selected as model compounds for this study.

2. EXPERIMENTAL METHODS From methodology point of view, this work is a comparative study of different highpressure states resulting from transformations of naphthalene, octafluoronaphthalene and their homogeneous and "heterogeneous" mixtures under the same pressure, temperature and time of isothermal exposure. Naphthalene (Chemapol) and octafluoronaphthalene (Alfa Aesar), containing less than 0.5 wt. % of impurities, have been used. Samples of pure C10H8 and C10F8 in the form of tablets (5 mm diameter, 4 mm height) were obtained by cold-pressing of starting materials. Homogeneous C10H8-C10F8 mixtures with different ratios of components have been prepared by careful grinding of the respective batches of starting materials in an agate mortar. This step was followed by cold pressing to obtain the tablets from homogeneous mixtures having the same dimensions as the samples from pure compounds. Studies of the transformations of so-called heterogeneous mixtures C10H8-C10F8 have been carried out after preparing a bilayer compacts composed of two separate tablets of naphthalene and octafluoronaphthalene each with the diameter of 5 mm and 2 mm height. 4 ACS Paragon Plus Environment

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The prepared samples were placed into graphite cell, which also served as a heater for the high-pressure "toroid"- type apparatus6, and then processed at a pressure of 8 GPa and variable temperatures up to 1500°C and isothermal exposure time for 20 seconds. Details of the experimental procedure have been described previously.1 Use of graphite as a cell material means that the systems under study are not closed from a thermodynamic point of view and can exchange the gas components of the transformations with the environment. TGA analysis of the products of thermal transformations of naphthalene and octafluoronaphthalene showed that in the studied systems a significant mass loss took place at temperatures above ~ 400 °C. At 8 GPa in particular, the maximum mass loss values have been obtained at temperatures above ~ 800 °C, comprising ~ 25 wt. % for naphthalene, and ~ 70 wt.% for octafluoronaphthalene. Samples of high-pressure states, resulting from thermobaric treatment of precursor materials, were initially quenched under pressure, then maintained under normal conditions and characterized by X-ray diffraction, Raman spectroscopy, scanning and transmission electron microscopy. Х-ray diffraction patterns of powder samples have been recorded on an INEL CPS 120 diffractometer using Co Kα1 radiation source. Microscopy characterization of samples was carried out on SEM Ultra plus (Carl Zeiss), TEM JEOL JEM-1230 and JEM-2100F microscopes.

3. RESULTS AND DISCUSSION Fig. 1 shows the X-ray diffraction patterns of pristine naphthalene, octafluoronaphthalene and products of their treatment under pressure of 8 GPa at variable temperatures. Fig. 1 also presents the XRD data for the samples obtained through treatment at 8 GPa of homogeneous mixtures С10Н8-С10F8 at 1:1 mass ratio in the 500-1100°С temperature range.

G

D

G

o

G

1000 C

1100oC

D

G

1400oC

(111)D

1300oC

Intensity, a.u.

1000oC

1000oC

D 800oC

600oC

800o C (100)G (101) G

800oC

(002) G

600oC

500oC

600oC o

400 C 400oC 500o C

C10H8 -C10 F8

C10H8 Pristine

(10)

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C10F8 Pristine

10 15 20 25 30 35 40 45 50 55 60

10 15 20 25 30 35 40 45 50 55 60

10 15 20 25 30 35 40 45 50 55 60

2Θ, degree

2Θ, degree

2Θ, degree

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Fig. 1. XRD patterns of pristine C10H8, C10F8 and products of their treatment as well as homogeneous binary mixture С10Н8-С10F8 under pressure of 8.0 GPa and different temperatures (G – graphite, D – diamond).

XRD data clearly indicate that active stages of carbonization of octafluoronaphthalene under pressure of 8 GPa occur at lower temperatures than for naphthalene which under shorttime treatments remains stable up to ~ 600°C. According to Fig. 1, two broad peaks near 30.5 and 50.5° appear in the X-ray pattern of the products of C10F8 treatment already at temperature of 500°С. These peaks belong to a three-dimensional (002) and two-dimensional (10) reflexes from turbostratic carbon and witness the formation of packing of small graphene planes lacking a three-dimensional ordering.7, 8 In case of naphthalene, similar states of carbon are formed at temperatures of 700-800°С. Further increase of temperature results in the graphitization, leading to increase of linear dimensions and three-dimensional ordering of graphene layers. On the X-ray diffractograms, this is shown by the narrowing and intensity increase of main (002) reflex, appearance of distinct modulation of two-dimensional (10) reflex and its subsequent splitting in two three-dimensional (100) and (101) reflexes of graphite. In case of octafluoronaphthalene, the formation of graphite has been noted at temperature of ~ 900°С, while for naphthalene - above 1100°С. These data also indicate that unlike naphthalene, the X-ray patterns of samples produced from octafluoronaphthalene at temperatures of 600-800°С exhibit an elevated background level in the small angle range. Usually, this fact signals for a presence of substantial share of nonordered or weakly ordered carbon states in the sample. The results of microscopy studies of the products of transformations of naphthalene (Fig. 2) and octafluoronaphthalene (Fig. 3) confirm the X-ray diffraction data.

a

b

Fig. 2. SEM image of the product of transformation of naphthalene at 8 GPa and temperatures of 800°С (a) and 1000°С (b). The top inset shows the product with x50000 magnifications. 6 ACS Paragon Plus Environment

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SEM images of the product of transformation of naphthalene, produced at 8 GPa and 800°С (Fig. 2a), show that this product consists from layers of rather uniform flaky carbon particles having an average size of ~ 60 nm. At 1000 °С, these nanoparticles undergo convertion into a micron-size particles of graphitic carbon (Fig. 2b). Mechanisms of thermally induced carbonization of polycyclic aromatic compounds and graphitization of their carbon residues under subatmospheric and elevated (up to 250 MPa) autogenic pressures have been studied in detail by now.9-17 According to current views9, 11, the primary act of the carbonization process of polycyclic aromatic compounds is the disproportionation reaction of two molecules resulting in the formation of a free radical intermediate and a hydroaromatic molecule, respectively. In case of naphthalene, the disproportionation reaction can be presented by the following equation: 2С10Н8 → С10Н6••+С10Н10 Formation of a free radical and hydroaromatic molecules defines two directions of further evolution of the system. The first one is the condensation of free-radical molecules into a large planar aromatic molecules containing from 10 up to 20 hexagonal carbon rings called mesogenes. Under influence of van der Waals’ forces, these mesogenic molecules can form a small 2-5 layered packings, known as basic structural units (BSU) for the design of anisotropic mesophase microdomens which represent a disk-shaped phases with either smectite columnar or nematic internal structure.12-15 At treatment temperatures of 1500-2000ºC, the mesogenic molecules from neighboring basic structural units (BSU) undergo coalescence. This results in formation of layered packings of continuous carbon layers which in the course of annealing at temperatures above 2000 ºC and healing of various defects become gradually transformed into a perfect graphite structure.12 Simultaneously with the formation of BSU from large mesogenic molecules, a thermochemical destruction of hydroaromatic molecules takes place. This is accompanied by formation of molecular hydrogen and volatile low-molecular hydrocarbons, including methane and acetylene9, and solid carbon residues, formed during the condensation of small hydrocarbon radicals produced by the destruction of hydroaromatic molecules. In open systems, a part of the most volatile products of carbonization can exit the reaction zone. As the result, the composition of solid products of carbonization of hydrocarbon molecules becomes less uniform. Therefore, these products can contain beside the graphite-like particles, typical products of carbonization of primary free radical molecules, yet the other allotropic modifications of carbon. Studies of the processes of carbonization of polycyclic aromatic compounds under high pressures (up to 10 GPa) in open systems, experimentally realized in the high pressure chamber 7 ACS Paragon Plus Environment


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of “Toroid” type, show that thermal transformation under high pressures proceed through the same stages as under a subatmospheric pressures.2 They all involve the disproportionation stage and are accompanied with the formation of solid and volatile products of carbonization. However, exposure of material to high pressure leads to increase of the density, lowering of the activation energy for the formation of primary clusters of solid forms of carbon and increase of the rate of their formation under the same treatment temperature. This results in a significant decline of the formation temperature of different carbon states serving as intermediates for formation of graphite. Moreover, the sequence of thermal transformations of hydrocarbons (carbonization–graphitization) under high pressures is supplemented by one more stage – formation of diamond. Given the comments made above and the supporting X-ray and electron microscopy data, we can conclude that the main products of transformation of naphthalene at 8 GPa and 800°C represent dense packing of microdomains of turbostratic carbon tending to adopt the shape of nematic phase. Increase of treatment temperature to 1000 °C facilitates the aggregation of microdomains in the system and leads to formation of micron-sized particles of fairly wellordered (although not yet perfect) graphite as a main product. SEM images of the products of transformation of octafluoronaphthalene at 8.0 GPа and 800 °С are shown in Fig. 3. The survey SEM image of the sample (Fig. 3a) shows that unlike the naphthalene, the product of transformation of octafluoronaphthalene is not homogeneous and presents a mixture of graphitic submicron size (M) and non graphitic nanosize (N) fractions of carbon. More detailed images of these fractions (Fig. 3b-d) show that the nanosize fraction consists of aggregates of individual particles with the size of about 10 nm (Fig. 3b). The submicron size fraction presents particles of graphitic carbon (Fig. 3c, d) with the size of 0.2-1.0 micrometers, many of which exhibit a polygonal shape.

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a

b

M N

N c

d

M Fig. 3. SEM images of the products of transformation of octafluoronaphthalene at 8 GPa and 800 °С. a) Mixture of graphitic submicron-size (M) and non graphitic nano-size (N) fractions of carbon, b) aggregates of individual particles of about 10 nm size, c) mixture of M and N fractions, d) individual submicron size particles of graphitic carbon.

The possibility of formation of various forms of nanoscale carbon along with the micronsized ones during the thermobaric conversion of hydrocarbons, naphthalene in particular, has been noted previously.18 However, the nanoscale states of carbon comprised in that case only a small share of the transformation products by not exceeding just a few weight percents. As the temperature increased, the share of nanoscale fraction in the products of transformation of octafluoronaphthalene decreased gradually and at temperatures above 1200°C, a micron-sized particles of perfect graphite became a dominant fraction in the reaction products (Fig. 4). However, even in this case, two different size fractions of graphite can be clearly detected (Fig. 4), a coarse crystalline one with the particle size of 5-10 micrometers and a fine crystalline with the particle size of ~ 1 micrometer.



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Fig. 4. SEM image of fine and course crystalline fractions of graphite produced by treatment of octafluoronaphthalene at 8 GPa and 1200 °С. These results indicate that at elevated temperatures the processes of recrystallization of nanoscale states of carbon indeed occur in the system. From one hand, these processes lead to considerable, especially lateral, growth of micron-sized particles of graphitic carbon observed at 800°C; from the other hand, they are accompanied by formation of fine crystalline graphite particles. General view of the carbon material obtained at 1200°C suggests that the transfer of carbon atoms through the gas transport processes can play a significant role in the recrystallization of products produced from octafluoronaphthalene. Studies of high pressure-high temperature-induced transformations of pure carbon nanosystems, carried out earlier on polyhedral and onion-like carbon nanoparticles19, showed that due to low diffusion mobility of carbon atoms at high pressures in the temperature range of ~ 800-1600 °C the processes of solid-phase restructuring of nanoparticles in pure carbon system under the same temperatures are limited to a single carbon particle and do not affect the recrystallization of the whole system. The latter process, however, can be initiated by introduction of even small quantities of hydrogen-containing components into the system. Thus, at pressure of 8 GPa the processes of cumulative recrystallization of polyhedral carbon 10 ACS Paragon Plus Environment

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nanoparticles (PCN) are noted to occur in binary mixtures of PCN with naphthalene (at ~1 wt.% hydrogen content) already at ~ 900 °C. At 1000°C, cumulative recrystallization of PCN with 3080 nm sizes is being followed by formation of micron-size particles of graphitic carbon. At 1200 °C, the crystals of diamond and perfect graphite, sized up to 20 microns, become the main products of transformation of binary mixture. These data strongly suggest that the processes of gas-phase transport of carbon atoms in the form of light hydrocarbon compounds play significant role in cumulative recrystallization of carbon nanosystem when a hydrocarbon component is introduced. Besides that, these data provide a clear evidence for “catalytic” role of hydrogen in the processes of transformation of graphitic materials into diamond. The efficient event of cumulative recrystallization of transformation products of octafluoronaphthalene shows that the processes of gas-phase transport of carbon atoms can be enabled not only by volatile hydrocarbons but also by fluorocarbon compounds. However, absence of diamond in the products of transformation of pure fluorocarbon materials gives an indication that fluorine atoms do not provide the same “catalytic” effect on transformations of graphitic materials into diamond as hydrogen atoms. In a pure fluorocarbon system, “hydrogen catalytic” mechanism of conversion of graphitic materials into diamond 4 is probably not realized. This is likely due to significant difference in atomic radii of hydrogen and fluorine which cause such a notable deformation of the structure of transition layer that replacing the hydrogen atoms by fluorine makes the formation of diamond thermodynamically unfavorable. From the point of view of physico-chemical evolution of solid state5, formation of nanoand micron-size carbon materials in the processes of carbonization of hydrocarbon systems under high pressures represent different stages of condensation route to formation of solid material taking place at its condensation from oversaturated vapour, solution and vapour-fluid media. The uniqueness of the transformation in this case is the condensation of matter which is not comprised of pure carbon but from hydrocarbon, fluorocarbon or mixed fluorine-hydrogencarbon media. According to theory of physico-chemical evolution of solid state in closed systems, the properties of matter undergo transformation along a common evolution route consisting of the following sequence of stages: creation of primary cluster, formation from it of critical seed of particular material modification, growth of seeds, the aggregation of seeds, and subsequent structural ordering and maturation of micron-size particles. At the end of this route, the material emerges in the form of large crystals having perfect shape and equilibrium habitus.5 Thus, the existence of material in the nanoscale state during the process of formation of new phase is limited to initial stages of evolution of the system preceding the aggregation of particles. The necessary conditions for accumulation of nanosize material in this case are the high rate of 11 ACS Paragon Plus Environment


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particles formation and the lack of opportunity for their enlargement until the oversaturation is removed. The possibility of simultaneous formation of several metastable phases along with one stable phase under conditions of oversaturated environment represents the uniqueness of the formation of the material in the solid state along the condensation route. In case of carboncontaining systems, a particularly large variety of possible metastable states can exist. Graphite, diamond, fullerenes, spherical and polyhedral onion-like partucles, nanoribbons, and different formes of amorphous carbon are representative of these states.18 It is important to note that according to size dependent phase diagram of carbon, the relative stabilities of different allotropic forms of carbon in the nanosize region change continuously with the size increase. In particular, according to size dependent d-T (size-temperature) diagram of carbon at P=0 GPa, proposed in work20 and considering four nano-size forms of carbon (fullerenes, onions, nanosize diamond and graphite particles), each nanocarbon form has its own size region of stability on the diagram. The sequence of size stability of these carbon clusters at temperature of 0 K is the following: fullerenes (0-2.3 nm), onions (2.3-2.7 nm), diamond (2.7-13.4 nm), graphite (> 13.4 nm). Increase of temperature causes a drastic change of this size region only for diamondgraphite phase transformation for which the critical size of diamond nanoparticle initiating its transition into graphite reduces from 13.4 nm at 0 K to 4.3 nm at 1500 K. The aforementioned data give evidence for possibility of direct formation of nanodiamond in the process of carbonization of carbon-containing systems. However, according to the same data, the formed nanodiamond particles may not necessarily become seeds for growth of micron size diamonds, undergoing conversion into graphite in the course of particle growth. Under conditions of considerable oversaturation of the system it is possible that the condensation process can result in the simultaneous formation of all noted allotropes of carbon. A classic condensation route scheme suggests that the processes of formation and growth of new phase from oversaturated media end up in transition of the system from oversaturated state into a state of undersaturation of the media relatively to the reaction product. In case of simultaneous formation of several allotropic modifications, difference in the extent of undersaturation for each of them becomes a driving force for disappearance (dissolution/evaporation) of various metastable modifications and transfer of matter to the particles of the phase with lowest solubility. The drive of the dispersed system to the lowest surface energy state also facilitates the processes of collective recrystallization which lead to disappearance of the smaller and disordered forms of matter and expansion of the larger and more ordered ones.


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In every particular system the evolution stages can be realized differently, sometimes they either overlap or entirely degenerate. In open systems, stage by stage evolution of particles can also be violated. However, the overlapping of stages does not lead to violation of the main evolution route in all systems. In the present work, the processes of carbonization, graphitization and formation of diamond proceed under conditions of partially closed system allowing the release of the most volatile components of carbonization products from the reaction zone of high pressure chamber. Nevertheless, even in this case, all major stages of evolution route for formation of solid compounds can be observed in all studied systems. Comparative analysis from the theory of physico-chemical evolution point of view has led to conclusion that the materials, produced from C10H8 and C10F8 at 8.0 GPa and 800 °C, represent a submicron and nanosize states of carbon formed during the same stages of evolution route which precedes the stage of particle aggregation. However, in case of transformation of C10F8, the presence of two different size fractions of carbon particles has been observed. This indicates that under given treatment parameters, the processes of recrystallization of metastable states formed during the initial stages of carbon condensation under conditions of high extent of oversaturation of the media become incomplete in the fluorocarbon system, unlike the hydrocarbon one. Such specifics of transformation of hydrocarbon and fluorocarbon systems explains the observed difference in fractional and size composition of carbon states formed in these systems under treatment temperatures of 800-1100°С. X-ray data presented in Fig. 1 show that the temperature threshold for the formation of diamond in the system based on pure naphthalene is ~ 1300 °C. In case of octafluoronaphthalene, formation of diamond has not been observed up to the temperature of 1500 °С. In this regard, high temperature conversion of C10F8 is similar to the conversion of a highly fluorinated graphite CF1.1 at 8 GPa1, where diamond formation in the same temperature range has also not been observed. In case of C10H8-C10F8 binary mixture, significant decline of the temperature thresholds of the main stages of transformations in comparison with pure C10H8 and C10F8 has been observed. According to Fig. 1, formation of graphite in the system, composed of homogeneous C10H8-C10F8 mixture, has been observed already at 800°C, while formation of diamond - at 1000°C. At temperatures higher than 1100°С, diamond becomes a major product of transformation of homogeneous mixture. Microphotograph of cleaved surface of the sample, obtained by treatment of the homogeneous C10H8 -C10F8 mixture at 8.0 GPa and 1100°C (Fig. 5a), shows that the formation of diamond occurs with virtually 100% yield. Absence of visible trace of graphite-like material on the cleavage surface of the sample also confirms a virtually 13 ACS Paragon Plus Environment


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complete conversion of the products of carbonization of both naphthalene and octafluoronaphthalene into diamond under conditions of homogeneous mixtures. SEM images of this sample (Fig. 5b) show the nanosize diamonds, observed in the form of shapeless agglomerates of nanoparticles, along with the micron-size fraction of diamond crystals with the sizes ranging from a few to 20 microns. The appearance of significant share of nanoscale diamond fraction in the reaction products of homogeneous mixture C10H8 -C10F8 (similar to the case of the high pressures-high temperature induced transformations of homogeneous mixtures C10H8-CF1.1 1) shows that this seems to be common for binary mixtures of hydrocarbon and fluorocarbon compounds, between starting hydrocarbon and fluorocarbon components of the mixture.

a

b

Fig. 5. Optical micrograph (a) and SEM image of the central zone of diamond sample obtained by treatment of homogeneous mixture of C10H8-C10F8 at 8.0 GPa and 1100 ºC (b). Given that under processing conditions of homogeneous binary mixtures it is difficult to trace a clear relationship between any conversion product and particular component of original binary mixture, this work was focused on investigation of the transformations of samples of "heterogeneous" mixtures C10H8-C10F8, which could provide a distinct relashionship of observed carbon products to any initial component of the mixture. Fig. 6 presents a microphotograph of the cleavage surface of the contact zone of the sample of heterogeneous mixture C10H8-C10F8, initially assembled into a two-layer compact and then treated at 8 GPa and 1000 °C, i.e. at the temperature that corresponds to the threshold of the beginning of the formation of diamond in a homogeneous mixture C10H8-C10F8. The photograph clearly demonstrates a visual difference between the products of transformation of hydrocarbon and fluorocarbon components of the mixture formed under conditions of treatment of 14 ACS Paragon Plus Environment

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heterogeneous mixture. Fig. 6 shows that the material, formed from naphthalene, possesses a substantially high reflectivity indicating the presence of a rather large particles having a perfect surface structure. Carbon material, formed from octafluoronaphthalene, has a matte surface which is indicative of substantial content of smaller grain particles with a non-perfect surface and weak reflectivity.

Fig. 6. Optical microphotograph of the cleavage surface of contact zone (left). Idealized sketch of the sample (right) produced by treatment of a two-layer heterogeneous mixture C10H8-C10F8 at 8 GPa and 1000 °С. The products formed from naphthalene and octafluoronaphthalene are denoted by C10H8 and C10F8, respectively. SEM images of carbon material formed from naphthalene in the contact layer region are shown in Fig. 7.

a

b

Fig. 7. SEM images of the products of transformation of naphthalene in the contact layer region of the sample produced by treatment of a two-layer heterogeneous mixture C10H8-C10F8 at 8 GPa and 1000 °С: micron size diamonds (in the center) (a), micron size graphite (b).



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They show that micron size fractions of diamond and graphite are the main products of transformation of naphthalene. The content of diamond crystals observed in a relatively narrow region directly adjacent to the layers’ boundary is relatively small (Fig. 7a), and does not exceed 15 wt. % of carbon residue produced from naphthalene. Graphite is a major product of this transformation (Fig. 7b). SEM images of carbon residue formed from octafluoronaphthalene at 1000°С in the contact layer region are shown in Fig. 8.


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Fig. 8. SEM images of the products of transformation of octafluoronaphthalene in the contact layer region of the sample produced by treatment of a two-layer heterogeneous mixture C10H8C10F8 at 8 GPa and 1000 °С: general view (a) and zoomed images of selected areas with M (micron- sized) and N (nano-sized) fractions (b-f). Analysis of these images clearly shows that in the contact layer region two types of carbon states are present, submicron-sized (M), and nano-sized (N). They are qualitatively similar to carbon structures observed in the products of transformation of pure octafluoronaphthalene at 8 GPa and 800-1000 °C (Fig. 3). Submicron-sized fraction (Fig. 8c,d) presents a collection of individual flakes of graphitic material with the sizes of up to ~ 1µm. It should be noted that many particles exhibit on these images a polygonal shape. Nanoscale carbon material fraction (Fig. 8e, f) consists of a plenty of small carbon nanoparticles with the sizes of ~ 5-15 nm. SEM images of these nanoparticles resemble agglomerates of nanodiamonds. However, the HRTEM images of this material (Fig. 9) indicate that the latter is a combination of onion-like and polyhedral nanoparticles made up of several (2-5) closed graphene layers. The internal cavities of these nanoparticles contain both a disordered and lowordered carbon.



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Fig. 9. HRTEM image of nano-size fraction of the products of octafluoronaphthalene transformation in the contact layer region of the sample produced by treatment of a two-layer heterogeneous mixture C10H8-C10F8 at 8 GPa and 1000°С. Analysis of SEM images of micron sized particles of graphitic material in the contact zone helps to reveal the features of their formation under experimental conditions studied. Thus, Fig. 10 clearly shows that the particles of graphitic material are mainly constructed from a dense packings of submicron mesophase domains in the shape of disk-like nematic phase, and are not built directly from the nanoscale particles of carbon fraction also observed in Fig. 10.

Fig. 10. A detailed SEM image of the conversion products of octafluoronaphthalene in the contact layer region of the sample obtained by treatment of a bilayer heterogeneous mixture C10H8-C10F8 at 8 GPa and 1000 °C. After increasing the processing temperature of a two-layer compact of heterogeneous mixture to 1100 °C, virtually all carbon residue produced from naphthalene becomes converted into diamond. SEM images of this diamond product (Fig. 11) show that major ingredient of the conversion product consists of micron sized fraction of diamond. However, in the contact zone the sample also contains a small aggregates of nanoscale diamond (Fig. 11 c,d) formed as the result of transformation of onion-like nanoparticles happened to appear in the contact zone of the sample.


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Fig. 11. SEM images of diamond material produced from naphthalene through treatment of a two-layer samples of heterogeneous mixture C10H8-C10F8 at 8 GPa and 1100 °C (Nnanodiamond). SEM images of the conversion product of octafluoronaphthalene in the contact zone of a two-layer sample of heterogeneous mixture treated at 8 GPa and 1100 °C are shown in Fig. 12. They indicate that in this case a major reaction product consists of large graphite particles with up to 10 micrometer sizes. Besides graphite, the product material also exhibits individual inclusions of micron-sized and nano-size diamonds. The shown images reveal interesting features of transformation of carbon residues of octafluoronaphthalene under conditions of heterogeneous mixture. The most notable is the sharp decrease of the content of nanosize fraction in the sample to marginal quantities. Evidently, the undersaturation of the media, taking place at temperature of 1100 °С, leads to virtually complete recrystallization of nanosize onion-like fraction. In the course of recrystallization only an insignificant part of nanoparticles becomes transformed into nanodiamonds. A major mass is consumed by additional growth of large micron size particles of graphite and diamond. Gasphase transport is likely to play significant role in the carbon transfer during this process.



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According to Fig. 12, gas-phase transport of carbon atoms enable the lateral growth of graphite particles. As seen from Fig. 12a-c, the boundaries of graphite particles are largely formed by embedding a thin leaflets of carbon-containing clusters of submicron size.

Fig. 12. SEM images of the products of transformation of octafluoronaphthalene obtained by treatment of a two-layer sample of heterogeneous mixture C10H8-C10F8 at 8 GPa and 1100 °C. General view of the product containing micro-diamond (MD) and graphite crystals (a). Leaflets oriented predominantly perpendicular to the side surface of the graphite (b). Area showing the nanodiamonds (ND) (c). Graphite crystals with the nanodiamonds on the surface (d).

These leaflets are oriented predominantly perpendicular to the side surface of the growing particle. Formation of such leaflets is apparently due to a light mobile fractions of different carbonaceous components resulting from decomposition of the starting hydrocarbon and fluorocarbon compounds which can become incorporated into a growing graphene layers through the gas phase–surface reactions. SEM images shown in Fig. 13 indicate that under conditions of our experiments this growth mechanism is possible not only for the case of graphite, but also diamond. Fig. 13a presents an image of growing face of the diamond crystal where the steps of growth are clearly seen. Zoomed image of the growth step (Fig. 13b) shows that these steps are being followed by insertion of individual leaflets into the step edge of the same carbon-containing clusters. In this 20 ACS Paragon Plus Environment

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case, the inserting leaflets are oriented parallel to the edge of the growing step and perpendicular to the plane of the underlying layer.

Fig. 13. SEM image of diamond, produced from carbonization products of octafluoronaphthalene in a two-layer sample of C10H8-C10F8 mixture treated at 8 GPa and 1100 °C (a). Zoomed image at 100000x magnification (b).

Analysis of the products of thermal transformations of heterogeneous mixtures C10H8C10F8 shows that unlike homogeneous binary mixtures, full transformation of carbonization products of both components of initial binary mixture into diamond does not occur. Only carbonization product of naphthalene undergoes a full conversion into diamond while the product of carbonization of octafluoronaphthalene remains in the form of graphite. This result shows that under conditions of heterogeneous mixtures, a gas-fluid fluorine-hydrogen media comprising the entire volume of reaction zone has not been formed in the system. It is likely due to a quick release of hydrogen and other volatile hydrogen-containing components of the decomposition of naphthalene, capable of “catalyzing” the processes of transformation of various carbonaceous products of decomposition of C10H8 and C10F8 into diamond. The obtained results indicate that formation of micro- and nanosize fractions of diamond in the studied range of pressures and temperatures occur only in a hydrogen-containing systems, pure hydrocarbons and their mixtures with fluorocarbon compounds. In this case, micron size diamonds are formed from micron size particles of graphite and graphitic carbon by the mechanism of hydrogen “catalyzed” transformation into diamond structure, as proposed by Lambrecht et al.4 Formation of nanosize diamonds in the products of transformations of binary mixtures of hydrocarbon and fluorocarbon compounds occurs on basis of nanosize particles of carbon, arising from transformation of fluorocarbon component of the mixture, most likely by direct conversion into diamond under hydrogen-containing media at pressure of 8 GPa. The possibility 21 ACS Paragon Plus Environment


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of transformation of nanosize packings of graphene layers into diamond under hydrogenation conditions was studied in detail in works.21, 22 It is known23,24 that the activation barrier for graphite-diamond transition at the level of macrosize states of carbon is exceptionally high (~0.4 eV/atom), therefore implementation of direct transition requires high pressures and temperatures. However, according to modeling calculations carried out in21, 22, the nanosize particles consisting of 2-6 layered packings of graphene can be converted into diamond in absence of activation energy under conditions of hydrogenation of outer layers of the packing. Given that the structures of model objects considered in works21, 22 are close to the structures of layered packings of graphene planes observed in nanoparticles formed during carbonization of fluorocarbon compounds, it is possible to consider the mechanism of diamond formation, proposed by authors21, 22, also as the main mechanism of nanodiamond formation under conditions of pressure and temperature induced transformations of binary mixtures of hydrocarbon and fluorocarbon compounds. It should be noted that the reduced initiation temperatures for diamond formation in binary systems lead to a significantly higher content of hydrogen in the direct precursor states for diamond formation than in the case of pure naphthalene. For this reason, the degree of hydrogenation of intermediary carbon states near the temperature threshold for diamond formation turns out to be higher than for pure naphthalene. Consequently, the amorphous carbon states, being present in the products of treatment of binary mixtures at these temperatures, can be similar to the states of so-called dense amorphous hydrogenated carbon (a-C:H). According to the data from work25, this type of material presents a favorable media for spontaneous volume nucleation of diamond at temperatures of 800-900 °С under conditions of high energy impacts. At the same time, the data showing a significant role of gas transport reactions in the processes of thermobaric transformations of hydrocarbon and fluorocarbon compounds under pressure, do not exclude a CVD mechanisms of diamond growth under the conditions studied in present work. SEM images of micron size crystals of diamond (Figs. 11c and 13) show 3 types of surfaces of produced diamonds: 1 – absolutely smooth and free of any surface steps, 2 – a steplike, representing a superposition of several atomically flat terraces (Fig. 11c), 3 – a rough one (Fig. 13). According to growth models, developed for homoepitaxial diamond synthesis by chemical vapor deposition26, three noted types of diamond surfaces can be related to three different options of homoepitaxial growth of diamond: 1 – lateral growth, 2 – 2D island growth, 3 – 3D growth. The predominance of one or the other mechanism of homoepitaxial growth by CVD synthesis method is determined by the content of carbon in the composition of growth gas 22 ACS Paragon Plus Environment

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mixtures. At low concentration of methane in the gas mixture (CH4/(H2+CH4) ~ 0.005-0.025% ), the surface of diamond grows perfectly flat without any atomic steps. At intermediate concentration of methane ( CH4/(H2+CH4) ~ 0.05-0.25% ), formation of equilateral-triangular islands and/or layered step-like terraces on the surface of diamond is observed. At high concentration of methane in the growth mixture (CH4/(H2+CH4) > 0.25% ), a rough surface of diamond is formed.26 There are obvious differences between the HPHT and CVD methods of diamond synthesis, which are mostly related to different synthesis pressures and composition of growth mixtures. Nevertheless, the similarity of the surface morphologies of diamond samples observed in the experiments on homoepitaxial growth of diamond by CVD method with the samples obtained in present work by HPHT method from C10H8 and binary mixtures С10Н8-С10F8, allows to accept the possibility of CVD mechanisms in the homoepitaxial growth of diamond at maturation stages of micron size diamond particles in our experiments as well. It is interesting to note that, as in our case, introduction of fluorine-containing components into gas mixtures in the CVD method results in significant decline of temperature (up to 600 °C) of diamond synthesis as compared to traditional hydrogen-hydrocarbon mixtures.11,27 Perhaps, this can be explained by differences in the thermal effects of surface chemical reactions leading to formation of C-C bonds between hydrocarbon and fluorocarbon radicals, СН3● and CHF2●, respectively. According to data from work27, the formation of C-C bonds by reaction between neighboring CH3 groups with the release of H2, is endothermic and requires an elevated temperature to occur. On contrary, the formation of C-C bonds via reaction between neighboring CHF2 groups with the release of HF is exothermic and can proceed at lower temperatures. It is quite likely that the similar effects also facilitate the reduction of temperature of diamond synthesis under high static pressures in the systems comprised of binary mixtures of hydrocarbon and fluorocarbon compounds as compared to pure hydrocarbons.

4. CONCLUSION In order to gain understanding of the nature of simultaneous formation of nano- and micronsize diamond fractions in the products of thermobaric transformations of homogeneous binary mixtures of hydrocarbon and fluorocarbon compounds, we have carried out a comparative study of thermal transformations of their representative structural analogs, naphthalene and octafluoronaphthalene, as well as their homogeneous and heterogeneous mixtures, under pressure of 8 GPa and variable temperatures up to 1500°С. Analysis of the obtained results, carried out on basis of solid state evolution theory, helped to understand the specifics of condensation routes for formation of solid states of carbon at pressure of 8 GPa and various 23 ACS Paragon Plus Environment


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temperatures in pure hydrocarbon, fluorocarbon and mixed fluorine-hydrogen-carbon systems. According to our experimental data, the reason for simultaneous massive formation of nano- and micronsize diamond fractions in the products of thermobaric treatment of homogeneous binary mixtures С10Н8-С10F8 is a considerable reduction of temperature threshold for initiation of diamond formation under conditions of binary mixture relatively to pure naphthalene. This event enables a simultaneous generation of two types of carbon precursors for diamond formation in the system, a nano- (non graphitic) and micron size (graphitic) ones. The micronsize diamond fractions are formed directly from graphitic precursors, while the nanosize fraction of diamond is formed from a 2-5 layered onion-like carbon nanoparticles of 5-15 nm size. Qualitative difference in sizes of these two types of precursors lead to different mechanisms of diamond formation. It was found that despite known high reactivity of both fluorine and hydrogen atoms in the transformation of carbon in the graphitic materials from sp2 into sp3 hybridization state, formation of diamond in the products of thermobaric transformations under the studied range of pressures and temperatures can be observed only under conditions of hydrogen-containing systems. The results of this work show that thermal transformations of studied systems under pressure of 8 GPa do not occur exclusively in the solid phase: important role in the transformation processes under studied range of temperatures is played by gas-phase or fluidphase transport of carbon and surface chemical reactions.

ACKNOWLEGEMENT The work was supported by the Russian Foundation for Basic Research (Grant № 15-03-04490).

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