Synthesis of unique metal oxide/carbon composites via sealed-tube pyrolysis of metal acetylacetonates and the mechanism of their formation

Abstract

Owing to their favorable thermal characteristics and (relative) non-toxicity, metal acetylacetonates are often employed as precursors for material synthesis in chemical vapor deposition (CVD) and atomic layer deposition (ALD). Iron and manganese acetylacetonates are separately pyrolysed at high temperature in a sealed tube, in inert ambient, giving rise to unique MnO/C and Fe₃O₄/C composites, respectively. We present a detailed report on the synthesis of such carbon composites, which also comprise a small proportion of the respective metal, a result of the reducing conditions in the tube. In contrast with the conventional low-pressure CVD process, product formation in sealed tube pyrolysis (STP) takes place in a closed system, at high pressure. In the STP-formed composites of the present work, the carbon is obtained both as amorphous powder and as micron-sized solid carbon spheres. As the duration of the STP process was increased, transformation of the initially formed carbon into a CNT-like fiber structure occurs at the high pressure in the STP chamber, although low pressures are typically required for the CVD of CNTs. An attempt is made to understand the STP process and the resultant product morphology based on the thermal characteristics of the precursor metal complex. Electrochemical measurements have been carried out on the carbonaceous powder composite, which reveal its excellent capacitive behavior; but the specific capacitance is limited by the solid-sphere morphology of some of the carbon in the composite.

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Introduction

Composites of carbon with metal oxides offer prospects for synthesizing new, improved functional materials, as they would exploit the unique properties of both of their components. Such composites have promising applications in energy storage (batteries and supercapacitors), catalysis, and as adsorbents; the addition of carbon overcomes the inherent poor electronic conductivity of most metal oxides, while enhancing the specific surface area of the material.^1–10 Various forms of carbon, such as activated carbon, mesoporous carbon, carbon nanotubes, graphene, carbon fiber, and carbon spheres have been employed to obtain metal oxide-carbon composites.^9,11–18

Typically, composites in powder or thin film form are obtained through a multi-step process involving separate precursors for individual components followed by mixing, or a single-step process with different starting materials.^4,12,19–27 For example, one of the fabrication methods begins with the infiltration of carbon fiber paper with polymer resorcinol and subsequent pyrolysis at high temperatures to obtain a highly porous carbon structure, which is subsequently loaded with FeO_x or MnO_x through electrochemical deposition to form electrode material for capacitors.² In another process for the fabrication of composite capacitor electrodes, Co₃O₄ nanowires were deposited onto a graphene foam formed from graphene deposited separately by CVD.²⁷

We have reported recently that, in contrast with multi-step methods ordinarily required to form metal oxide–carbon composites, inert-ambient CVD using metal–organic complexes of Mn and Fe as precursors can be employed to obtain coatings of MnO/C and Fe₃O₄/C, which are carbon composites of the respective oxides.^28,29

One class of metal–organic complexes are metal acetylacetonates (often referred to as acac's), which are versatile starting materials for the synthesis of metal oxides by a variety of techniques: sol–gel, polyol, CVD/ALD.^30–38 Metal acetylacetonates can be good starting materials for the synthesis of metal oxide/carbon composite materials, since each molecule of a metal acac contains direct metal–oxygen bonds and hydrocarbon moieties, which together constitute the “elements” needed for a metal oxide–carbon composite. Such metal acac's were employed in obtaining the MnO/C and Fe₃O₄/C composite coatings cited above.

However, low-pressure CVD is not suitable for the preparation of powder material. We have therefore employed sealed-tube pyrolysis (STP) of metal acetylacetonates in inert ambient in an attempt to obtain metal oxide–carbon composites in the powder form. The STP process conducted in inert ambient also provides a contrast with the inert-ambient, low-pressure CVD process. CVD is a steady-state process that takes place in an open system at an elevated temperature, STP, by definition, occurs in a closed system, in which one can expect chemical reactions to occur at a high pressure (of several bars) caused by the elevated temperature needed for pyrolysis. Further, while being held at the elevated temperature for prolonged periods, the system would be at equilibrium, in contrast with the steady state prevailing in the CVD process. Further, it becomes a possibility that the duration of STP affects the products formed in the process. (One may also expect that the products eventually found would be affected by the rate of cool-down to room temperature.)

Herein we report the synthesis of carbon composites of oxides of manganese and iron by a simple, one-step STP process by the respective “acac” complex as the precursor, and without the aid of any catalysts or any heat treatment. We show that composites formed this way are homogeneous on a macroscopic scale, with interesting variations in the morphology of the carbon formed. We propose a mechanism to explain the formation of the observed micrometer-sized solid carbon spheres in the STP product of ferric acetylacetonate, Fe(acac)₃. Electrochemical measurements show that the resulting iron oxide–carbon composite has excellent capacitive characteristics, though the specific capacitance is limited by the formation of solid carbon spheres, in which only the carbon on the surface of the spheres contributes to capacitance.

Experimental

Pyrolysis experiments were carried out separately with Fe(acac)₃ and Mn(acac)₂ in a quartz tube of 1.5 cm in diameter and ∼15 cm in length. The middle part of each tube was thinned to form a small neck, which could be easily cut and sealed. The tube was evacuated and backfilled with ultra-high purity (UHP) nitrogen. About 0.3 g precursor powder was introduced into the tube, followed by evacuation and backfilling with UHP nitrogen. The tube was cut and sealed in the middle. The sealed-tube was heated to the desired temperature and held at that temperature for a chosen duration, typically one hour, after which the tube was allowed to cool naturally to room temperature. A range of temperatures from 500–700 °C was used for pyrolysis. After cooling to room temperature, the tube appeared completely black. (The seal has to be broken with care, as the pressure inside the tube would be high because of decomposition taking place at elevated temperatures.) The inner walls of the broken tube were scraped carefully to collect the product of the STP process.

Characterization

X-ray diffraction (XRD) patterns of films were recorded using a powder diffractometer (Panalytical, Cu-K_α radiation). Scanning electron microscopy (SEM) data were obtained in a field emission scanning electron microscope (FESEM, Zeiss Ultra55). Transmission electron microscopy (TEM, TECNAI F-30) was used to analyze the finer details of the morphology and the crystallinity of the powder material. Raman spectra were obtained using an argon ion laser of wavelength 514 nm (Horiba Jobin-Yvon LabRAM HR 100 spectrometer).

The as-synthesized powder was examined for its performance as electrode material for capacitors, using a 3-electrode cell assembly. The working electrode material was prepared by mixing, by weight, 70% of the STP product, 20% carbon black as conducting material, and 10% PVDF as binder. The mixture was ground well and a thick slurry was prepared using few drops of N-methyl pyrrolidone. The slurry was coated on both sides of a cleaned “flag-type” SS316 substrate. This coated structure was used as the working electrode in a 3-electrode cell assembly, with a platinum foil counter electrode, and Ag/AgCl as reference electrode. Electrochemical measurements were carried out with aqueous 0.1 M Na₂SO₄ and KOH as electrolytes (separately). (Measurements were made on an Eco Chemie Autolab PGSTAT100 potentiostat equipped with the FRA2 frequency response analyzer module and GPES/FRA software.)

Results and discussion

STP carried out for both Mn(acac)₂ and Fe(acac)₃ over the entire range of pyrolysis temperatures resulted in black powder. The powder XRD patterns reveal the formation of polycrystalline MnO (JCPDS card number-07-0230) and Fe₃O₄ (magnetite, JCPDS card number-19-0629) from Mn(acac)₂ and Fe(acac)₃, respectively (Fig. 1(a) and (b)).

Fig. 1 XRD pattern for STP products of (a) Mn(acac)₂ and (b) Fe(acac)₃ synthesized at various temperatures together with the standard (JCPDS) powder patterns for MnO and Fe₃O₄.

MnO crystallite size (Scherrer formula) varied from 13–21 nm and Fe₃O₄ crystallite size varied from 50–70 nm. (Details in ESI Table 1†).

Raman spectra collected from the powders show the presence of carbon (Fig. 2). The first-order Raman spectrum consisted an envelope of peaks centered at G (1597 cm⁻¹) and D (1352 cm⁻¹), typical of amorphous graphitic carbon. The second-order feature G′ was absent.

Fig. 2 Typical Raman spectrum for STP products of Fe(acac)₃ and Mn(acac)₂.

Structural disorder in the carbon produced by the STP process results in many Raman-active vibrational modes that are forbidden in the case of graphite.^39,40 The observed spectrum could be deconvoluted into five peaks (Fig. 2) at the vibrational frequencies 1257, 1357, 1491, 1591, 1611 cm⁻¹ among which, apart from the 1591 cm⁻¹ peak that corresponds to the E_2g vibrational motion of the sp² carbon network, the other peaks arise from structural disorders. It is important to note that the various product species formed during pyrolysis are retained at high pressure in the sealed-tube while elemental carbon is being formed and are therefore present together in it. Therefore, carbon obtained from STP could contain defects such as organic moieties, functional groups, edges of graphene units, C–C and C=C stretching vibrations of polyene-like structures, which subsequently result in extra peaks in the Raman spectrum.

XPS spectra collected on the STP powder samples exhibit characteristic peaks for MnO and Fe₃O₄ and are in good agreement with literature data (Fig. 3(a) and (b)).^41–43 Interestingly, the STP samples from Mn(acac)₂ and Fe(acac)₃ showed some metallic content (Mn and Fe), as indicated by XPS spectra.

Fig. 3 XPS spectrum of (a) MnO/C and (b) Fe₃O₄/C, confirming the formation of MnO and Fe₃O₄.

The SEM micrograph for MnO/C is shown in Fig. 4(a). The sample consists of carbon spheres several microns in diameter and carbon in non-spherical form mixed with metal oxide (Fig. 4); the spheres and the shapeless powder co-exist as separate entities. The spheres are solid and consist of only carbon, as inferred from the images and EDAX of broken spheres (Fig. 4(c)). TEM analysis of the STP products shows (Fig. 4(b)) MnO nanocrystals are present in a carbon-rich matrix. Individual units of unique structure that could be observed (Fig. 4(a)) were made of MnO crystals enveloped in amorphous carbon. When the duration of pyrolysis (“holding time”) was one hour, the formation of carbon spheres was observed at temperatures above 550 °C. The diameter of spheres ranges from 2–6 μm, which did not vary much with the temperature of pyrolysis.

Fig. 4 (a) SEM micrograph of MnO/C powder obtained by STP of Mn(acac)₂ – the solid spheres are made entirely of carbon, whereas the particle aggregates are composed of metal oxide and carbon (b) TEM micrograph and the corresponding electron diffraction pattern confirming the presence of MnO (c) SEM micrograph of a broken sphere and corresponding EDAX, confirming that the spheres are solid and made entirely of carbon.

Similarly, STP product of Fe(acac)₃ is composite of Fe₃O₄ and elemental carbon (Fig. 5), which includes solid, all-carbon spheres. Fig. 5(b) shows the bright-field TEM images of the Fe₃O₄/C sample prepared at 650 °C. Due to their inherent magnetism, the well-faceted nanocrystals were found self-assembled into chain-like aggregates. Most of the nanocrystals are found to be octahedral in shape, indicative of faster growth along the 〈100〉 direction of the cubic structure. Such shape-controlled nanoparticle geometry is mostly achieved using capping agents, templates, etc.^44,45 By contrast, in the present synthesis method, nanocrystals of magnetite with specific shapes were formed without any growth-directing agent, and in a matrix that is carbon-rich. This is presumably due to growth under equilibrium conditions prevailing at high pressure within the sealed tube. The small size of the crystals (Fe₃O₄ and MnO) is presumably due to the limited mobility of the growth species in the carbon-rich matrix.

Fig. 5 (a) SEM micrographs of Fe₃O₄/C powders obtained from the STP of Fe(acac)₃ wherein the solid spheres are made exclusively of carbon; particle aggregates are composed of metal oxide and carbon (b) TEM micrographs of crystalline powders obtained at 650 °C inset showing typical SAED pattern of the individual crystallite (c) TEM micrograph of fibre-like structures observed in STP conducted above 650 °C.

As indicated by Raman spectra, the spheres are made of non-crystalline carbon in both cases. TEM analysis also affirms the presence of amorphous carbon in the powder material (other than spherical structures, which are too thick for TEM analysis) intermixed with metal oxide crystallites, resulting in a unique composite. Furthermore, thermal analysis data (ESI S1 and S2†) complement results from Raman analysis and microscopy, and show that carbon present in MnO/C and Fe₃O₄/C samples is in different chemical environments. As a result, during TGA, carbon in the samples is oxidized over a range of temperatures. The carbon content in the samples prepared above 600 °C was estimated to be ∼43 at%, whereas pyrolysis at a lower temperature resulted in a smaller carbon content.

As noted, the carbon spheres that are a part of both the MnO/C and Fe₃O₄/C samples measure several microns in diameter, and are solid. They are therefore similar to the carbon spheres formed by the inert-ambient STP of a polymer, as reported by Pol et al.⁴⁶

Carbon was also spotted occasionally in a fibre-like structure (Fig. 5(c)) in Fe₃O₄/C samples prepared above 650 °C, the proportion of which was small and was not apparent from SEM. The fibers were non-crystalline, unlike carbon nanotubes, while they enclosed iron oxide particles at their tip, as confirmed by HRTEM analysis.

Effect of reaction duration on sealed-tube pyrolysis

STP was carried out in inert ambient at 700 °C following the procedure described in the previous section, but for durations longer than one hour.

In case of Fe(acac)₃, a part of the powder resulting from pyrolysis was converted to CNT-like structure (Fig. 6), with carbon spheres also present. Eventhough cognizable changes occurred in the microstructure of the product, no distinguishable changes were recorded in XRD and Raman data (ESI S3†). The formation of CNT-like structures was much more evident in the samples pyrolysed for 24 hours, and present in large clusters, as evidenced by SEM and TEM (Fig. 6(b) and (c)). The nature of carbon produced changes with the duration of the STP process, and the presence of elemental iron may be involved in this process, as the “fibers” formed consist of particles at their tip. CNT formation from hydrocarbon precursors in CVD, in the presence of hydrogen, at low pressures is well known. The formation of carbon fibres in an inert sealed-tube environment, where pressures up to 5 bar build up, is in contrast with the conditions generally believed to be with necessary for the formation of CNT-like structures.^47,48 It is noteworthy that CNT-like structures have been obtained from a metal complex, not hydrocarbons, through sealed-tube pyrolysis in inert ambient; this is somewhat analogous to the formation of CNTs during low-pressure CVD in inert ambient, with Fe(acac)₃ as the precursor.²⁹

Fig. 6 SEM micrographs of STP products obtained at 700 °C for a holding time of (a) 5 hours (b) 24 hours (c) TEM micrograph and SAED pattern of CNT-like structures formed at 700 °C when the holding time was 24 hours.

The nature of the carbon formed in sealed-tube pyrolysis is not static, and appears to evolve as a function of the duration of STP. However, in contrast with Fe(acac)₃, no discernable changes were observed in the microstructure, XRD pattern, and Raman spectrum of the product when reaction duration was increased in the STP of Mn(acac)₂.

Insight into the STP process

Fe(acac)₃ and Mn(acac)₂ have been used as starting materials in the solvothermal process, and the direct decomposition of these complexes has been utilized in CVD. However, the microstructure obtained from STP, particularly the formation of carbon spheres, is the result of unique nucleation and growth processes that take place. The STP growth process may be understood by observing the microstructure of the resulting products, together with the thermal analysis data of the metal complex from which the products arise.

It is proposed here that the formation and subsequent development of the resulting composite is affected by the thermal characteristic of the (precursor) metal complex. The TG-DTA of Fe(acac)₃ (Fig. 7) indicates melting just prior to a large weight loss, as evidenced by the sharp endotherm at about 180 °C. This is followed by decomposition, as indicated by the endotherm at about 225 °C and the broad exotherm that spans ∼230 °C to ∼300 °C, leaving a residue of nearly 21 wt%, corresponding to the formation of (involatile) iron oxide.

Fig. 7 Schematic of the STP mechanism along with a typical TG-DTA graph in nitrogen ambient for M(acac)_n (M = metal) employed in present STP process.

It is surmised that, during decomposition in the inert ambient of the sealed-tube, the hydrocarbon groups break away from the metal in the complex. Hence, M − O moieties are detached from the carbon-containing group, leading to the separate formation of metal oxide species. The high pressure caused by the inert gas at high temperature becomes even higher because of the gaseous products of pyrolysis. Such high pressures (probably exceeding 5 bar) may lead to the condensation of the carbonaceous products of pyrolysis, with “droplets” of this condensate filling the volume of the tube at the elevated pyrolysis temperatures. The evidence for condensation and formation of droplets from the condensate is the perfectly spherical carbon entities that result upon cooling, as also the “flow” of the dense condensate which smears the entire inner wall of the quartz tube and adheres strongly to it. The average size of the spherical entities formed, about 3 μm, is characteristic of atomization that occurs, say, in spray pyrolysis. As the carbonaceous liquid is everywhere in the sealed-tube that is placed horizontally in the furnace, the inner wall of the tube gets coated by the viscous condensate, leading to an adherent cake-like formation.

The spheres that are seen by SEM on the surface of the product (or the top layer of the scraped sample) arise from the droplets that are the last to condense during the (natural) cool-down process.

Such a process can explain the observation of copious numbers of perfectly spherical solid carbon entities of elemental carbon present in the STP product of Fe(acac)₃. Where the metal–oxygen moiety is separated from the hydrocarbon entity due to decomposition prior to vapourisation, the spheres formed can be expected to be carbonaceous, without metal or oxygen content. This is indeed so in the STP product of Fe(acac)₃. The formation of Fe₃O₄ in the oxygen-free STP ambient, rather than Fe_1−xO, the lowest oxide, can be understood as being due to the disproportionation of Fe_1−xO into Fe₃O₄ and Fe in the carbonaceous ambient at elevated temperatures.⁴⁹ This is consistent with experiment, as reported above.

Thermal analysis data show (ESI S4†) that, just as Fe(acac)₃ does, the complex Mn(acac)₂ appears to melt congruently at about 280 °C, accompanied by significant weight loss and followed by decomposition, leaving a substantial residue, corresponding an oxide of manganese. Thus, as per the hypothesis outlined above, solid carbon spheres are formed, during the cool down, from the droplets of the “melt” formed by carbonaceous moieties that are detached from the “acac” molecule through decomposition. In the inert gas environment and in the carbonaceous matrix, no oxide higher than MnO can form and any crystals of MnO formed cannot grow, due to the limited mobility of the growth species in a carbon-rich matrix; hence the observed nanometric size of the crystals of MnO. The average crystallite size of the oxide Fe₃O₄ obtained from Fe(acac)₃ (50–70 nm) is significantly greater than that of the oxide from Mn(acac)₂ (13–21 nm). This is probably related to the disproportionation of Fe_1−xO involved in the formation of Fe₃O₄ and Fe, as noted above.

Electrochemical studies

Given that composites of various transition metal oxides and carbon have been investigated extensively as potential electrode materials in supercapacitors, electrochemical measurements were made on the MnO/C and Fe₃O₄/C composite powders in 0.1 M KOH and Na₂SO₄.^2,9,10,50 It is noteworthy that, compared to other oxides of manganese (MnO₂ and Mn₃O₄), MnO has been less investigated as electrode material in supercapacitors.^2,9,10

The powders displays near-rectangular cyclic voltammograms, indicating excellent capacitive behaviour. The constant-current charge–discharge curves exhibited linear voltage–time characteristics. MnO/C samples showed a maximum capacitance of 95 F g⁻¹ in KOH and Fe₃O₄/C 55 F g⁻¹ (ESI S5†).

Though specific capacitance obtained in present work was comparable to values reported in the literature, higher performance has been observed for Fe₃O₄/C composites in some cases.^10,50,51 Specific capacitance is dependent on the surface area of the electrode accessible to the electrolyte. Even though a large amount of carbon is present in the STP sample, the surface area accessible to the electrolyte is reduced by the presence of solid carbon spheres. Attempts made to enhance specific capacitance are detailed in ESI (S6 and S7†).

Conclusions

Inert-ambient sealed-tube pyrolysis is a simple route to the synthesis of carbon composites of metal oxides from the corresponding metal acetylacetonates, since their molecular structure contains carbon as well as direct metal–oxygen bonds. Thus, in contrast with the known methods, STP provides a single-step route to the formation of a metal oxide/carbon composite in powder form. STP of Mn(aacac)₂ and Fe(acac)₃ leads, respectively, to the formation of the composites MnO/C and Fe₃O₄/C, with the proportion of carbon close to that in the molecular structure (because an unknown amount of carbon is lost when gaseous hydrocarbon species escape when the seal is broken). Elemental Mn and Fe, a result of the reducing ambient, are present in the composites in small proportion detected by XPS but not by powder XRD. The carbon is of the amorphous graphitic kind, with much structural disorder that is attributable to the complex chemical environment prevailing at elevated temperature and pressure. In addition to being in the form of shapeless powder, carbon is found to be present also as solid, micrometer-sized spheres. Further, the nature of the carbon is found to depend on the duration of STP, with lengthy pyrolysis of Fe(acac)₃ leading to the formation of CNT-like, amorphous structures. This is a new finding, as previous work has led to the inference that the formation of CNTs by a chemical process requires low pressures. Presumably, the presence of elemental iron formed in the inert, reducing ambient in the sealed tube leads to the formation of the observed structures, despite the high pressure prevailing. A mechanism, which takes account of the inert ambient and the thermal properties of the metal acetylacetonate, and hypotheses that a melt is formed during the STP process, is proposed to explain the observed formation of solid carbon spheres, together with the oxide and the metal. Electrochemical measurements show that the MnO/C and Fe₃O₄/C composites have excellent capacitive characteristics, but that the specific capacitance attainable is limited by the small specific surface area of the carbon spheres that form a significant part of the composite.

Acknowledgements

The authors acknowledge gratefully the Centre for Nano Science and Engineering (CeNSE) and the Advanced Facility for Microscopy and Microanalysis (AFMM) of the Indian Institute of Science, for supporting analyses by XPS, SEM, and TEM reported in this study. The authors thank N. Munichandraiah for advice on electrochemical measurements. The work was supported by a grant under the NPMASS, Govt. of India.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra08455a

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