Mechanism of Transformation of Ferrocene into Carbon-Encapsulated

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Mechanism of Transformation of Ferrocene into CarbonEncapsulated Iron Carbide Nanoparticles at High Pressures and Temperatures Arseniy O. Baskakov,*,† Igor S. Lyubutin,† Sergey S. Starchikov,† Valery A. Davydov,‡ Ludmila F. Kulikova,‡ Tolganay B. Egorova,§ and Vyacheslav N. Agafonov∥ †

Shubnikov Institute of Crystallography of FSRC “Crystallography and Photonics” RAS, Moscow 119333, Russia L.F. Vereshchagin Institute for High Pressure Physics of RAS, Troitsk, Moscow 108840, Russia § Chemistry Faculty, Lomonosov Moscow State University, Moscow 119991, Russia ∥ GREMAN, UMR CNRS 7347, Francois Rabelais University, 37200 Tours, France

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‡

ABSTRACT: A mechanism was established for the formation of nanosized iron carbide particles encapsulated in carbon shells via the processes of ferrocene thermal conversions at high pressures. At a pressure of 8.0 GPa, products of ferrocene decomposition were studied as a function of temperature by X-ray diffraction, Raman and Mö ssbauer spectroscopy, scanning and transmission electron microscopy. It was shown that the mechanism of formation of the carbonencapsulated iron carbide nanoparticles at high pressures and temperatures differs qualitatively from the known mechanism of their formation in the gas-phase processes of laser pyrolysis or photolysis of ferrocene. At high pressures and temperatures, the formation of iron carbide nanoparticles occurs not due to the primary growth of pure iron particles and the subsequent dissolution of carbon in iron. Nanoparticles are formed due to the direct fusion of iron−carbon clusters, which are formed at intermediate stages of ferrocene thermal destruction. Then, obtained amorphous iron carbides Fe1−xCx with a high carbon content start to crystallize. Two crystalline carbon-encapsulated forms of iron carbide (Fe7C3 and Fe3C) are the main products of crystallization of the amorphous Fe1−xCx depending on the temperature of the ferrocene treatment.

1. INTRODUCTION

The mechanism of gas-phase photolysis of ferrocene was considered in a series of studies.4−7 Processes of thermally induced fragmentation of ferrocene molecules were analyzed both experimentally (with the transformation of ferrocene under the conditions of a gas mixture with argon under the influence of an ArF excimer laser) and theoretically by molecular dynamics and density functional theory. According to the results obtained, high-temperature degradation of ferrocene proceeds in several stages. During the first stage in the ferrocene molecules, there is a breakdown of C−H, then C−C, and finally Fe−C bonds.4,6 Condensation of the obtained atomic−molecular gas mixture containing atoms of Fe, C, and H and various molecular fragments, such as CH, C2, and C3, proceeds with the formation, first, of pure iron nanoparticles.4 In this case, pure iron nanoparticles can occur in the form of both α-Fe and γ-Fe. The characteristic size of αFe nanoparticles is ∼13 nm, and that of γ-Fe ∼5 nm. According to refs 4 and 7, the formation of iron nanoparticles occurs mainly because of the addition of individual iron atoms to the emerging particle. The role of coalescence processes in the growth of particles is assumed to be negligible. Carbon fragments of decomposition deposited on the surface of iron

Recent studies of ferrocene Fe(C5H5)2 transformations induced by high pressure and temperature1−3 have shown that they open a new route for the synthesis of various nanoscale iron carbide nanoparticles encapsulated in carbon shells. Previously, the synthesis of magnetic nanostructures, based on iron carbides coated with carbon shells, by laser pyrolysis methods in gas-phase mixtures of iron pentacarbonyl Fe(CO)5 with ethylene C2H4 and ultraviolet photolysis of ferrocene in an argon atmosphere were investigated in detail.4−9 The FexCy@C nanostructures can also be obtained by an arc-discharge method between carbon electrodes containing metal inserts,10 during the carbonization of binary mixtures of tetramethylbenzene with ferrocene under autogenous pressure in an autoclave,11 with a detonation method using explosive mixtures with various combinations of carbon and metal-containing materials,12 by means of flame spray synthesis based on thermal transformations of mixtures of organometallic compounds with acetylene,13 and by laser ablation of iron in organic solvents.14 Despite the obvious methodological diversity of these synthesis methods, they all represent the same condensation approach to the production of nanoscale states of matter from the perspective of the theory of physical and chemical evolution of solid matter.15 © XXXX American Chemical Society

Received: September 18, 2018

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DOI: 10.1021/acs.inorgchem.8b02660 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

scanning electron microscope (Ultra-Plus Gemini, Zeiss) and transmission electron microscopes (JEM 1230 and JEOL 2100). The 57Fe Mössbauer spectra of the samples were recorded at room temperature with a standard spectrometer (MS-1104Em). The γ-ray source 57Co(Rh) was at room temperature, and a metal α-Fe standard absorber was used for calibration. Computer analysis of the spectra was performed using the program SpectrRelax,19 which allows restoration of the distribution of hyperfine parameters with the modification of the fitting models. The values of Mö ssbauer parameters depend on the sizes of nanoparticles, and the model fit of the spectra based on the distribution of hyperfine parameters is more appropriate in the case of a distribution in geometric sizes of the particles.

nanoparticles located in the zone of laser radiation can dissolve in liquid metal droplets, leading to the formation of solutions of carbon in iron. During the cooling of nanoparticles, the solubility of carbon in iron decreases and excess carbon leaves the volume of the particle moving to the surface, thus forming the carbon coating of nanoparticles. It should be noted that according to refs 4−7 the formation of encapsulated particles of iron carbides Fe3C@C or/and Fe7C3@C, which are the main products of ferrocene thermal transformations at high pressures, was not observed in the ferrocene photolysis processes. This fact indicates a qualitative difference in the mechanisms of gas-phase photolytical transformations of ferrocene and its thermal transformations at high pressures. Core−shell nanocomposites of iron carbides in carbon shells now attract the attention of researchers because of the wide possibility of their use. The low toxicity and distinctive magnetic properties of such carbon-encapsulated magnetic nanoparticles allow us to consider them as innovative magnetically controlled nanoplatforms for many prospective biomedical applications, for example, in drug and gene delivery systems, detection of diseases, cancer therapy, rapid toxic cleaning, biochemical sensing, and magnetic resonance imaging.2,16 The use of similar nanocomposites in lithiumion batteries can lead to an increase in their capacity.17 It is also worth noting that core−shell nanocomposites based on magnetic cores (typically based on iron compounds) with a carbon shell are considered as very convenient agents for the purification of impurities in aqueous solutions.18 The need to develop methods for producing materials of this kind with desired properties requires a detailed study of the mechanisms of their formation in various types of transformations. In this work, a study of the mechanism of decomposition of ferrocene molecules at high pressures and high temperatures was performed. We applied Mössbauer and Raman spectroscopy, powder X-ray diffraction, and scanning and transmission electron microscopies to determine and understand the mechanism and stages of substance transformations over a wide range of synthesis temperatures.

3. RESULTS 3.1. X-ray Diffraction. The XRD patterns of the initial ferrocene and products of its treatment at 8 GPa and different temperatures are shown in Figure 1.

2. EXPERIMENTAL SECTION

Figure 1. XRD patterns of the ferrocene and products of its treatment at a pressure of 8 GPa and different temperatures. Peak G corresponds to the (002) reflection of graphite.

Ferrocene Fe(C5H5)2 (Aldrich, impurity contents of 20fold. It follows that when ferrocene decomposes to Fe7C3, a large amount of free carbon is released. This again confirms the carbon composition of these flake-type shape objects in Figure 3d (bottom inserts). Further details about the structural evolution of the encapsulated nanoparticles depending on the temperature of ferrocene processing appear in the TEM and HRTEM images shown in Figure 4. The TEM image of the material obtained at 8.0 GPa and 800 °C (Figure 4a) shows the initial stages of formation of carbide nanoparticles with the size of approximately 2−8 nm. Nanoparticles are located in the matrix of amorphous carbon. It is important that according to the HRTEM image (Figure 4a), the resulting carbide nanoparticles do not yet have a pronounced carbon shell. At a treatment temperature of 1100 °C, the size of the carbide nanoparticles increases to 10−20 nm (Figure 4b). Nanoparticles are still located in the matrix of amorphous carbon, which is seen as a gray background in Figure 4b. In this case, all nanoparticles acquire a carbon envelope that is approximately 4−6 nm in thickness. The HRTEM image of such nanoparticles reveals a rather complex structure of the carbon shell (Figure 4c). It consists of a double-layered shell of amorphous carbon immediately adjacent to the carbide core and somewhat more ordered outer carbon layers, in which fragments of flat and curved two- to five-layer graphene packages of different lengths were identified (Figure 4c). An onion-like layered structure can be seen in the outer carbon layers (Figure 4c). Carbide nanoparticles formed at 1500 °C have dimensions from several tens to several hundred nanometers (Figure 4d). In this case, the increase in the particle size occurs mainly due to the increase in the dimensions of the carbide core, while the thickness of the outer carbon shell remains at approximately 4−6 nm. Samples no longer contain an amorphous carbon

Figure 4. TEM and HRTEM images of the products of ferrocene treatment at 8 GPa and (a) 800, (b and c) 1100, and (d) 1500 °C.

micrometers, become the main products of ferrocene conversion (see the top and bottom insets in Figure 3d). In addition, an increase in the dimensions of carbide-containing particles is accompanied by a change in the particle’s habit, which transforms from spherical to polyhedral (the top inset in Figure 3d). Meanwhile, as shown below, the presence of Fe7C3 and Fe3C phases in our samples was proven by XRD and Mössbauer spectroscopy data. In the initial ferrocene, the ratio of the number of Fe atoms to C (Fe/C) is 1/10, whereas in Fe7C3 D


Article

Inorganic Chemistry

temperature, the chemical bonds between iron and carbon rings in ferrocene begin to break down, and a mixture of ferrocene phases with an amorphous carbon−iron compound is formed. However, Mössbauer data indicate that only a part of ferrocene is transformed into an intermediate amorphous state under given conditions of synthesis. It is noteworthy that this phase mixture does not exhibit magnetic properties even at a temperature of 90 K. With a further increase in the treatment temperature to 900 °C, ferrocene is completely transformed into a new amorphous paramagnetic phase (Figure 5). This is confirmed by XRD data, which show that in this temperature range the destruction of ferrocene actually occurs, but good crystalline phases are not observed in Mössbauer spectrum of this sample measured at 5 K, which demonstrates a broadened magnetic sextet (Figure 6)

matrix, which was transformed into graphite particles (see Figure 3d). 3.4. Mössbauer Spectroscopy. Room-temperature Mössbauer spectra of the samples revealed a pronounced change in the shape and parameters of the spectra depending on the temperature of the ferrocene treatment (Figure 5).

Figure 6. Low-temperature Mössbauer spectra of the Fe−C samples obtained at 900 and 1600 °C. Solid lines are the calculated spectra obtained by the SpectrRelax program. Tm is the measurement temperature. Figure 5. Room-temperature Mössbauer spectra of products of the ferrocene treatment at 8 GPa and different temperatures. Solid lines are the calculated spectra obtained by SpectrRelax program.

indicating a magnetic ordering of this phase at low temperatures. The same magnetic component was also found at 10 K in the spectrum of the sample treated at 1600 °C (see the lowintensity outer lines in Figure 6). This supports the presence of the amorphous magnetic phase in the entire range of treatment temperatures from 900 to 1600 °C. In the samples obtained at temperatures above 1000 °C, the room-temperature Mössbauer spectra reveal the appearance of a new phase of iron carbide with magnetically split lines (Figure 5). This is the crystalline phase of Fe7C3. In addition, one more crystalline magnetic phase of cementite Fe3C was identified in the Mössbauer spectra of the samples obtained at 1200−1600 °C. The magnetic components with narrow lines indicate the formation of magnetic iron carbides. With the treatment temperature increasing, the content of the amorphous phase of carbide Fe1−xCx decreases; however, a small amount of the amorphous phase remains even at the highest Ttr of 1600 °C (Figure 5). According to XRD data (Figure 1), two phases of iron carbides Fe7C3 and Fe3C are present in the sample obtained at 1500 °C. Mössbauer spectroscopy can reliably identify both coexisting phases. There are 13 iron atoms in the hexagonal unit cell of the Fe7C3 compound, which are located in three possible structural sites. In the Fe3C compound, the iron ions

For the samples obtained at treatment temperatures of ≤600 °C, the Mössbauer spectra do not differ from the spectrum of the original ferrocene Fe(C5H5)2. The spectra show a quadrupole doublet indicating a paramagnetic state of iron ions (Figure 5). The hyperfine parameters of the spectra, isomer shift δ and quadrupole splitting Δ, are 0.44 and 2.36 mm/s, respectively. This indicates that the local environment of iron remains unchanged at these treatment temperatures. The Mössbauer spectra show no magnetic splitting, indicating a nonmagnetic state of iron atoms at room temperature. In the samples obtained at 800 °C, a new paramagnetic phase with broadened lines appears with a δ of 0.34 mm/s and a Δ of 0.25 mm/s; meanwhile, the Mössbauer component of the ferrocene phase Fe(C5H5)2 also remains. Generally, homogeneously broadened lines of the Mössbauer spectrum are characteristic of the amorphous state of iron compounds.14,30 Hyperfine parameters of this component indicate that this phase is similar to the amorphous paramagnetic iron carbides Fe1−xCx, supersaturated with carbon, which have already been detected in some studies.14,31 Apparently, at this E


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Inorganic Chemistry

Figure 7. (a) Room-temperature Mössbauer spectrum of the products of ferrocene treatment at 8.0 GPa and 1400 °C. Solid lines are the calculated subspectra corresponding to several magnetic components (sextets) and one paramagnetic doublet. (b) Distributed values of the magnetic hyperfine field Hhf at iron nuclei corresponding to the Fe3C magnetic phase are shown by the P(Hhf) function. Two peaks in the P(Hhf) distribution correspond to iron nuclei in the non-equivalent sites of the crystal structure of Fe3C. Dashed lines are Gaussian approximations.

observed above 700 °C. Ferrocene decomposition is accompanied by the formation of an amorphous paramagnetic state of iron carbide with a high carbon content, Fe1−xCx. At a processing temperature of ∼1000 °C, the system develops solid-phase crystallization of the amorphous carbide phase, leading to the formation of a crystalline phase of iron carbide Fe7C3, which becomes the main component of the conversion products at 1300 °C. However, according to the Mössbauer data, even at a treatment temperature of ∼1200 °C, the crystallization of the amorphous iron carbide occurs with the formation of not only the Fe7C3 carbide but also another crystalline phase of Fe3C. This phase could not be uniquely identified in the products of ferrocene conversion at these temperatures by X-ray diffraction. At a treatment temperature of 1600 °C, only one crystalline phase of iron carbide Fe3C is present. In the samples obtained at 1500−1600 °C, the amorphous phase of iron carbide Fe1−xCx also remains at a level of ∼10%. It is interesting to note the change in the isomer shift δ values in the samples during the transformation of ferrocene into iron carbides. As shown in Table 1, the δ value varies from

occupy two sites that have similar structural parameters; therefore, the Mössbauer parameters are also similar.8 The Mössbauer spectra of the samples containing both Fe7C3 and Fe3C phases were fitted with the model of several magnetic components (sextets) and one paramagnetic doublet. We found that the Mössbauer spectrum of the Fe7C3 magnetic phase can be approximated by three subspectra (Figure 7a) according to three possible non-equivalent structural sites of iron in the hexagonal structure of Fe7C3 (space group P63mc).8,32,33 In the case of Fe3C, the broadened magnetic Mössbauer spectrum was approximated by distributed values of the magnetic hyperfine field Hhf at iron nuclei P(Hhf). An example of the fit shown in Figure 7b indicates a narrow field distribution P(Hhf), which is in accordance with the expected similar structural parameters of iron in two crystal sites of this compound.8 From the quantitative analysis of Mössbauer spectra, the relative content of ferrocene, amorphous (Fe1−xCx), and crystalline (Fe7C3 and Fe3C) carbide phases in the products of ferrocene conversion was calculated at different treatment temperatures (Figure 8). According to the data obtained, an active decomposition of ferrocene at short isothermal holding times used in the work is

Table 1. Values of Isomer Shifts δ in the Products of Ferrocene Decomposition into Iron Carbides Obtained from Experimental Mössbauer Spectra at Room Temperature δ (mm/s)

ferrocene Fe(C5H5)2

Fe1−xCx

Fe7C3a

Fe3C

0.44

0.34

0.26−0.34

0.19

The indicated range of the δ values corresponds to iron atoms in different local crystal sites of the Fe7C3 structure. a

0.44 mm/s in the initial ferrocene to 0.19 mm/s in Fe3C. Such a decrease in δ with a decrease in the amount of carbon in iron carbides has already been mentioned in the literature.34 This confirms the assumption that the paramagnetic intermediate phase is amorphous carbide Fe1−xCx that is supersaturated with carbon.35,36 It should be recalled here that the value of the shift δ in metallic iron (α-Fe) is 0 mm/s. It is important that the formation of pure iron nanoparticles in the processes of thermal ferrocene conversions at high pressures is not observed in the entire investigated temperature range up to 1600 °C.

Figure 8. Relative iron content in the form of ferrocene and its decomposition products at 8 GPa and different treatment temperatures. F


Article

Inorganic Chemistry

4. DISCUSSION The key question of the mechanism of thermal conversion of ferrocene at high pressures (into encapsulated iron carbide nanoparticles) is whether a thermal decomposition of ferrocene passes through the stage of formation of nanosized particles of pure iron that is characteristic of gas-phase pyrolytic or photolytic transformations of organometallic compounds under the action of a laser or an electric arc. XRD and Mössbauer data obtained in this study indicate that the formation of pure iron nanoparticles is not observed in the pressure−temperature-induced ferrocene conversions in the entire investigated range of pressures and temperatures. This fact suggests that the formation of iron carbides in products of ferrocene conversion at high pressures and temperatures occurs not due to the diffusion of carbon atoms deposited on the surface of pure α-Fe or γ-Fe nanoparticles, as in the gas-phase pyrolysis of organometallic systems at 2500−3000 °C. This process occurs through the direct condensation of iron−carbon clusters, the formation of nanoparticles of amorphous carbide phase Fe1−xCx, and their subsequent crystallization. The possibility of such a mechanism for the formation of carbon-encapsulated nanoparticles Fe7C3@C and Fe3C@C is explained by the lower temperatures of ferrocene conversions and the increased density of matter in the reaction zone at high pressures in comparison with the temperatures and density of matter that occur during photolytic transformations under the action of laser irradiation. Taking into account the fact that ferrocene photolysis is performed at approximately 2500−3000 °C and thermal transformations under pressure at 1300 °C into graphite particles with a high degree of crystallographic perfection. With an increase in the processing temperature, sizes of iron carbide nanoparticles increase from several nanometers to several tens and then hundreds of nanometers. At a treatment temperature of ∼1600 °C, the Fe7C3 phase is completely transformed into the Fe3C phase. Thus, the mechanism of formation of the carbonencapsulated particles of iron carbides Fe7C3@C and Fe3C@ C from crystalline ferrocene under high pressure and temperature conditions is associated with the breakdown of the ferrocene structure and its transition to the paramagnetic, amorphous, carbon-saturated Fe1−xCx phase and its further crystallization into the Fe7C3 and Fe3C phases. This process is G


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Inorganic Chemistry

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accompanied by a decrease in the carbon concentration in the iron-containing phase and the release of carbon in its free form and in the form of a shell of core−shell nanoparticles. Our results open one more direction for obtaining various size fractions of amorphous and crystalline phases of iron carbide nanoparticles encapsulated in carbon shells.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +7-906-748-1368. ORCID

Arseniy O. Baskakov: 0000-0002-5868-6354 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Russian Science Foundation (Project 14-12-00848-P) in part of Mössbauer and Raman spectroscopy studies and data analysis and by the Ministry of Science and Higher Education within the State assignment FSRC “Crystallography and Photonics” RAS in part of electron microscopy analysis. V.A.D. and L.F.K. thank the Russian Foundation for Basic Research (Grant 18-03-00936) for financial support in part of sample preparation and XRD measurements.

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REFERENCES

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Mechanism of Transformation of Ferrocene into Carbon-Encapsulated

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