N Valle
email: valle@lippmann.lu

M Martinez Celis

J Lacaze

I O Thorbjornsson

B Johannesson

J T Thorgrimsson

A SIMS and TEM investigation of the microstructure of wear-resistant ductile cast iron

Keywords: microstructure, ductile cast iron, chromium carbides, TEM, nano-analysis SIMS

The benefit of the presence of Cr containing inserts inside ductile iron on the tribological properties of these materials has been recently demonstrated. In order to understand these improved properties, a secondary ion mass spectrometry (SIMS) and transmission electron microscopy investigation of these wear-resistant ductile cast irons has been performed. The results indicate that the principal change in the microstructure induced by inserts occurs in the inserts and in their surroundings where the formation of carbides with different sizes and compositions is observed: M 3 C, M 7 C 3 and M 23 C 6 (M stands for Fe and/or Cr). These are not pure binary carbides since among the different elements studied by SIMS, Cr, Fe, Mn and, to a lesser extent, Ni are soluble in these carbides.

Introduction

Ductile irons are essentially ternary Fe-C-Si alloys in which the concentrations of carbon and silicon vary typically in the range of 3.5-3.9% and 1.8-2.8%, respectively. In ductile iron, graphite is in the form of spherical nodules thanks to the addition to the melt of tiny amounts of spheroidising elements such as magnesium. These nodules provide an enhanced ductility to the material. The majority of the castings produced with nodular cast irons have a pearlitic, a ferritic or a ferritic-pearlitic matrix in the as-cast state. [START_REF] Davis | Cast irons[END_REF][START_REF] Labrecque | Ductile iron[END_REF] Ductile irons are used in demanding applications for which the load and wear are usually localized at the surface of parts. In order to improve their wear resistance, a successful surface hardening can be obtained by the introduction of chromium containing steel inserts in the mould of parts to be cast with ductile iron. [START_REF] Thorbjörnsson | international patent application under the PCT treaty[END_REF] The basic idea, already mentioned by Qian et al., [START_REF] Qian | [END_REF] is to form hard carbides by using stainless steel inserts as a source of chromium and the ductile iron as a source of carbon. The tribological properties of reinforced ductile irons can be further improved when subjected to austempering treatment. [5] In order to understand this local reinforcement, the microstructure, the composition of carbides and the distribution of elements in the wear-resistant ductile iron were characterized using transmission electron microscopy (TEM) and secondary ion mass spectrometry (SIMS).

Materials and methods

Materials

The reinforced ductile iron studied has been elaborated by following the steps described in the patented process detailed in reference [ [START_REF] Thorbjörnsson | international patent application under the PCT treaty[END_REF] , see also [6] ]. The inserts were made of stainless steel, i.e. Fe-Ni-Cr alloy containing about 16-20 wt% Cr, 8 wt% Ni and small amounts of manganese and silicon.

Cylinder-shaped inserts were appropriately located in the mould cavity where the improved wear resistance is required for the final part. Their thickness and size were, respectively, in the ranges of 1-2 mm and 5-10 cm. The melt used in the present study is an unalloyed standard nodular cast iron (C: 3.7-3.9 wt.%, Si: 2.7-2.8 wt.%, Mn: 0.12-0.16 wt. %, Mg: 0.03 wt%, Cu: 276-426 ppm, Ni: 268-345 ppm, P: 219-265 ppm, S: 109-110 ppm, Mo < 10 ppm). The casting conditions were adapted to guarantee a partial dissolution of the inserts before solidification of the cast iron. A sample machined out from the casting has been analyzed in its as-cast state and another one after austempering. The austempering treatment consists of a two-stage process: a heating to and holding at 900 C for 60 min followed by a quench to 300 C and holding at that temperature for 75 min before aircooling.

Analytical techniques

SIMS experiments were carried out with a CAMECA NanoSIMS 50 instrument to investigate chromium carbides from the core of the insert to the bulk of the ductile iron. The coaxial optics of the NanoSIMS imposes an opposite polarity for primary ions (Cs + or O -from the ion source) and secondary ions (from the sample). While the use of Cs + primary ion favours the emission of carbon as a negative ion, the use of a O -primary ion allows to enhance ion emission of positive chromium ion Cr + . A compromise was found by studying the chromium carbides through the measurements of the cluster CrC -with a caesium primary source at an energy of 16 keV. The different CrC -images, with a size of 256Â256 pixels, were acquired simultaneously to those of C -and Si -thanks to a multicollection system by using a focused Cs + primary ion beam (few pA, probe size %200 nm) scanned across an area of a few tens of micrometers in size. A presputtering of the surface was performed before any measurement in order to minimize the surface contamination (mainly carbon and oxygen). The acquisition times per image were around 5-10 min. The mass resolution defined as the ratio M/ΔM was around 3000.

To complement the investigation of chromium carbides, the imaging of positive ions (Mn + , Cr + and Ni + ) was performed on a modified CAMECA IMS 6f equipped with a gallium liquid metal ion gun operating at an energy of 25.5 keV. Contrary to NanoSIMS analyses, the analyses were performed in monocollection mode. They were assisted with oxygen flooding (10 À6 mbar) for a higher sensitivity. The primary beam with an intensity of around 150 pA was focused down to 500 nm. The mass resolution was around 300. The counting times for Cr + , Ni + and Mn + were 20 s.

For SIMS measurements, each specimen studied contained at least one stainless steel insert. Rectangular parallelepiped-shaped samples were cut in the direction parallel to the length of the insert in order to image the distribution of carbides in the region all around the inserts. They were polished with SiC paper up to grade 1000 and then with diamond pastes (7, 3 and 1 mm) before being embedded in a Wood's alloy in a copper ring of 8 mm in diameter. The samples were then ultrasonically cleaned in alcohol.

At the same time, microstructure observations and electron diffraction for phase identification were carried out by TEM with a LEO 922 OMEGA operating at 200 kV. Thin foils were prepared by mechanical polishing disc specimens of 3 mm in diameter down to 100 mm. Then, a precision dimple grinder instrument was used to reduce the thickness of the disc down to 50 mm at the thinnest part of the dimple. For the last step of thinning, ion milling was preferred to electropolishing because of the large differences between the phases present in the sample. The angles of the two ion beams in the final stage of milling were set at 6 for best results.

Results

The reinforced ductile iron consists of stainless steel inserts fixed inside the cast iron as illustrated in Fig. 1. A rapid observation under optical microscope highlights three well-marked areas for each specimen: the Cr containing steel inserts, a region surrounding the inserts called the transition zone and further away from the inserts, the ductile iron.

During casting, the partial dissolution of the external part of the insert produces a flux of elements, mainly chromium, from the insert to the liquid cast iron. At the same time, carbon present in the melt diffuses in the opposite direction into the solid insert. This results in a gradual modification of the microstructure in the region surrounding the inserts. [7] The resulting transition zone is characterized by a composition and a microstructure which differ from those of the insert and the ductile iron. Depending on the casting conditions and the size of the inserts, the transition zone can extend to 2 mm.

The insert and the transition zone have been investigated by SIMS. The CrC -maps, placed end to end from the core of the insert up to a distance of about 200 mm from its border, reveal the existence of chromium carbides in different shapes and sizes (Fig. 2).

The insert

The carbides, the most enriched in chromium, identified by TEM as M 23 C 6 , have been found furthest within the insert (over 80 mm) where the matrix is composed of austenite. [7] While the biggest ones are visible on the SIMS maps (fig. 2), others with 50 nm in size were only observable by TEM. Most of them are intragranular and are next to some scarce big lamellar carbides (up to 20 mm in size, top right-hand side in Fig. 2). Nearer the interface, a second type of chromium carbide appears over a region covering around 50 mm. The associated electron diffraction pattern corresponds to M 7 C 3 carbides (Fig. 3). These precipitates are ring shaped with a uniform composition in chromium and carbon. As revealed by the SIMS maps, they also contain manganese while nickel (not shown here) and silicon are rejected in the core outside of the ring (Fig. 3). The region containing M 7 C 3 carbides can be as thick as 50 mm with an increase of carbides density as one moves towards the border of the insert.

The formation of a third type of carbide identified as M 3 C (Fig. 4) takes place inside grains and at grain boundaries in the most external part of the insert, which is characterized by an important depletion of chromium content. This region can extend up to 10 mm and is adjacent to the transition zone. The interface between these two areas is clearly visible for the as-cast sample on the C -ion map where the pearlite exhibits a lamellar mixture of ferrite (grey contrast) and cementite (white contrast) whereas the C -image of the matrix that consists in martensite is much less contrasted. The transition martensite / pearlite corresponds to the interface insert / transition zone.

No significant changes in microstructure were observed in the insert after austempering treatment.

The transition zone

The transition zone of the as-cast sample is composed of carbides, pearlite, ferrite and graphite nodules, smaller near the insert than in the bulk of the cast iron and without the ferrite shell existing in ductile iron. The nodules density increases up to the bulk of cast iron. The proportion of the different phases graphite, carbides, pearlite and ferrite determined at a distance of 100 mm and 1300 mm from the edge of the insert change, respectively, from 2.5 to 10%; 10.9 to 1%; 86.6 to 30.8% and 0 to 58.2%. The transition zone is composed of three distinct regions. The first one, adjacent to the interface, consists of a band of pearlite whose thickness can reach up to 100 mm. Further away from the interface, the presence of two types of carbides, M 3 C and M 7 C 3 , has been confirmed by electron diffraction (Fig. 2, Fig. 5 and [7] ). The second region of the transition zone, spreading to 80 mm, contains both types of carbides whereas only M 3 C is present in the third region covering the largest part of the transition zone (~1800 mm). Inside a few M 7 C 3 precipitates, a ferrite islet (black contrast in Fig. 5-a) containing small non-identified carbides (50 nm in size, Fig. 5-b) can remain. The M 7 C 3 carbides present a uniform composition in Cr and Mn, and they are free from Si and could contain some traces of Ni (Fig. 5).

After austempering treatment, the matrix is transformed into ausferrite, and some M 7 C 3 type carbides are partially dissolved in the region near the inserts (bottom right side in Fig. 2).

Conclusion

The reason for the improved wear properties of reinforced ductile cast iron [5] is found in the modification of the microstructure induced by the introduction of stainless steel inserts in the mould before casting. The local hardening is due to the formation of hard carbides in the insert and in its surroundings. Although the nano-analysis SIMS allows, thanks to the acquisition of CrC -maps, to give evidence of the presence of carbides, their identification requires electron diffraction pattern.

Three different types of carbides have been found in reinforced ductile iron: M 3 C, M 7 C 3 , M 23 C 6 . They are present together in the insert, while only M 3 C and M 7 C 3 coexist in the transition zone. They are not pure binary carbides since among the different elements studied by SIMS, Cr, Fe, Mn and, to a lesser extent, Ni are soluble in these carbides. The solubility of these elements depends on the type of carbide. [8,9]

Figure 1 .

 1 Figure 1. Description of the different regions of the reinforced ductile iron.

Figure 2 .

 2 Figure 2. 12 C 52 Cr -ion images of an as-cast sample (left) and after austempering (right). White contrast represents highest intensities of the element, while black contrast represents lowest intensities.

Figure 3 .

 3 Figure 3. Round M 7 C 3 carbides in the insert. At the top: TEM micrograph and diffraction pattern, zone axis [3 2 0]. At the bottom: SIMS maps of C -, Si -, Cr + and Mn + . The images have been obtained in different areas for negative and positive mode.

Figure 4 .

 4 Figure 4. Microstructure in the transition zone, next to the insert, for the as-cast sample. At the top: C -ion maps. At the bottom: TEM micrograph. The black arrows indicate the position of the interface insert / transition zone. Pearlite, martensite and M 3 C carbides have been identified by TEM (the electron diffraction patterns are not shown here).

Acknowledgements

The authors wish to thank E. Lentzen, P. Grysan and B. El Adib for their helpful contribution to this work. This work was financially supported by the Fonds National de la Recherche du Luxembourg (DIWEAR project, INTER -ERA-Net MATERA programme).


Igor P Asanov

Tatyana I Asanova

Lyubov G Bulusheva

Elena V Shlyakhova

Alexander V Okotrub

Emmanuel Flahaut

Thermal Decomposition of Co-Doped Calcium Tartrate and Use of the Products for Catalytic Chemical Vapor Deposition Synthesis of Carbon Nanotubes

This work was partially

' INTRODUCTION

Carbon nanotubes (CNTs) present a class of one-dimensional carbon cages characterized by a variety of number of the constituting graphitic layers, outer diameter, defectness, morphology, etc. Depending on the structural and physicochemical properties, each kind of CNT could find its own application. 1 For example, single-wall CNTs are considered as elements of nanoelectronics and optical devices, highly defective CNTs can be used as effective sorbents and supports, and few-wall CNTs are attractive as reinforcement additives in composites. The structure of CNTs can be tuned, varying the synthesis parameters. A major processing route toward the CNTs in mass production is catalytic chemical vapor deposition (CCVD), where carbon-containing molecules decompose at a catalyst surface. 2 Catalyst, which commonly consists of a compound of one or two transition metals, 3 is a key parameter in the CCVD due to a rigorous correlation between the diameter of CNTs and size of metallic particles. 4,5 Despite numerous works devoted to the study of CNT morphology as a function of catalyst composition and structure, 6À10 the dependences are still not very clear. Researchers believe that understanding mechanisms of CNT growth, particularly the role of chemical oxidation state of catalyst in the CCVD process, should allow the production of CNTs with the required characteristics, depending on the desired application. 11 One of the important problems hindering the control of the size and crystallinity of catalyst is an agglomeration of metallic nanoparticles at elevated temperatures of synthesis. 12 To restrict the nanoparticle enlargement, the metal is deposited on a substrate, though the catalystÀsubstrate interaction also exerts an influence on particle coalescence and thus on the nanotube growth. 13À16 For substrate material, inorganic compounds with high specific surface area such as Al 2 O 3 , SiO 2 , MgO, CaO, and CaCO 3 are used. 17À21 The latter three compounds have an advantage over the other substrates because alkaline-earth metal oxide can be easily removed from the synthesis product by dissolution with a weak acid treatment. 22,23 An often-used method for the formation of catalytic nanoparticles is the impregnation of powder substrate by transition metal salt solution. 24 Activation of the catalyst arises from the heating process followed by reduction. For CaCO 3 substrate, all these pretreatment steps occur directly in the reactor, 25 which simplifies the CCVD process.

We have suggested that doped salts containing both alkalineearth metal and transition metal could be suitable precursors for supported catalysts for the CNT formation and checked the idea on the example of calcium tartrate. 26 The pure and doped ABSTRACT: Thermal decomposition of Co-doped calcium tartrate in an inert atmosphere or air was studied using thermogravimetric analysis and X-ray absorption fine structure (XAFS) spectroscopy. It was shown that the powder substance containing 4 at.% of cobalt completely decomposes within 650À730 °C, depending on the environment, and the formation of Co clusters does not proceed before 470 °C. The products of decomposition were characterized by transmission electron microscopy, XAFS, and X-ray photoelectron spectroscopy. Surfaceoxidized Co metal nanoparticles as large as ∼5.6 ( 1.2 nm were found to form in an inert atmosphere, while the annealing in air led to a wide distribution of diameters of the nanoparticles, with the largest nanoparticles (30À50 nm) mainly present as a Co 3 O 4 phase. It was found that the former nanoparticles catalyze the growth of CNTs from alcohol while a reducing atmosphere is required for activation of the latter nanoparticles. We propose the scheme of formation of CaO-supported catalyst from Co-doped tartrate, depending on the thermal decomposition conditions. calcium tartrate crystals owing to their electrical properties are employed in transducers and many linear and nonlinear mechanical devices. 27,28 They can be also used as catalysts for asymmetric synthesis and as chiral intermediates. 29 The structure of doped calcium tartrate crystals was studied by Torres et al., 30 who showed that ions of a transition metal occupy interstitial sites in the host lattice. Thermal stability of pure calcium tartrate or with impurities depends on the crystal perfectness, and the conversion to calcium oxide is completed between 420 °C and 807 °C, 31,32 which is close to the CCVD operation temperature. On the basis of these data, we hoped that the decomposition of calcium tartrate doped with transition metal may result in monodisperse catalyst nanoparticles supported over CaO. Our investigations indicated that the limiting concentration of transition metal in calcium tartrate lattice is ∼4 at.%, and after thermal decomposition in the inert atmosphere, the average size of supported catalyst nanoparticles is ∼6 nm. 33 Feeding the ethanol to the CCVD reactor just after precursor decomposition in an inert atmosphere gave thin multiwall CNTs with narrow outer diameter distribution determined by the size of the Co nanoparticles. However, the mechanism of the formation of metallic nanoparticles catalyzing the CNT growth remained unclear.

Here, using the example of Co-doped calcium tartrate, we undertake a study of changing the oxidation state of transition metal with thermal decomposition of the salt using in situ X-ray absorption fine structure (XAFS) measurements additionally to the methods of high-resolution transmission electron microscopy (HRTEM), thermogravimetry (TG), and X-ray photoelectron spectroscopy (XPS). The products obtained in air or inert atmosphere are examined as catalytic systems for growth of CNTs from a specific feedstock. Suggestions about how the conditions of doped calcium tartrate decomposition influence the structure of CNTs are made.

' EXPERIMENTAL SECTION

Synthesis and Decomposition of Calcium Tartrate Doped with 4 at.% Co. Co-doped calcium tartrate was synthesized by the procedure described elsewhere. 33 Co acetate and Ca chloride were used as metal sources. The sodium tartrate solution was added to the solution containing cobalt and calcium ions (the theoretical ratio of Co to Ca was 1:9 CNT Synthesis. CNT synthesis was carried out in a horizontal quartz tube. The reactor chamber was pumped out and heated up to 800 °C. Then Ar flow (200 mL/min) was supplied, and a boat loaded with the Co-doped calcium tartrate was moved to the hot zone of the reactor and heated for 10 min to obtain syst1. The duration of the precursor decomposition in the reactor as well as temperature of synthesis of few-walled CNTs was optimized in ref 33. CNT growth was performed under ethanol vapor pressure of ∼65 mbar for 60 min. At the end of the synthesis, the carbon source feed was stopped and the reactor was cooled to room temperature in an inert gas flow. The product obtained was a black, friable substance labeled as cnt1. The synthesis of CNTs using methane as a carbon source was performed according to the method previously reported for double-walled CNT synthesis. 34 The boat loaded with syst2 was inserted in the reactor chamber which was then heated up to 1000 °C at the rate of 5 °C/min in the H 2 ÀCH 4 (18 mol % CH 4 ) flow. The reactor was cooled to room temperature at the same rate. The synthesis product obtained from methane using the catalytic system syst2 is denoted as cnt2.

Characterization Techniques. Thermogravimetric (TG) decomposition patterns of Co-doped calcium tartrate were measured in He flow (150 mL/min) or in air on a Q-1000 derivatograph. The heating rate was 10 °C/min. The yield of CNTs was estimated by means of scanning electron microscopy (SEM) on a JEOL JSM-6700 F microscope. TEM characterization was performed on a JEM-2010EX transmission electron microscope. Fourier analysis of TEM images was used to estimate the interplanar distances and identify crystal structures. Energy dispersed X-ray (EDX) analysis was performed on a spectrometer (Phoenix) with a Si(Li) detector at an energy resolution of ∼130 eV and a probe size of 10 nm. Room-temperature Raman spectra were measured using the Ar laser lines at 488 and 514.5 nm and HeNe laser line at 633 nm on a Triplemate spectrometer.

The in situ high temperature Co K-edge XAFS spectra of the Co-doped calcium tartrate as well as ex situ spectra of the samples were measured at the 7C1 beamline PLS POSTECH (Pohang Light Source, Republic of Korea) with a ring electron current of 120À170 mA at 2.0 GeV. A Si (111) double-crystal monochromator with an energy resolution of ΔE/E = 2 Â 10 À4 was used. The ex situ XAFS spectra were recorded in a fluorescence mode while in situ XAFS spectra were in transmission mode. In the in situ XAFS experiment, the sample was heated to the required temperature for 10 min and then stabilized for the next 10 min. Temperatures of 200 °C, 370 °C, 470 °C in air or 200°, 400 °C in He atmosphere were chosen as corresponding to the thermal decomposition stages. IFEFFIT computer program 35,36 was employed for analysis of XAFS data.

XPS measurements were carried out on a Specslab PHOIBOS 150 spectrometer using Al Kα 1,2 (1486.6 eV) X-ray radiation in vacuum (1.3 Â 10 À7 Pa) at room temperature. The binding energies were measured relative to the C 1s level (285.0 eV) from surface hydrocarbon contaminations.

' RESULTS

Study of Co-Doped Calcium Tartrate and Products of Its Thermal Decomposition. Thermogravimetric Analysis. TG curves for the Co-doped calcium tartrate in He atmosphere and air are compared in Figure 1. In both cases it is possible to denote four stages of the thermal decomposition of the salt that agree with data for undoped calcium tartrate. 32 Because the concentration of Co atoms in the host lattice is about 4 at.% according to the chemical analysis, the main weight loss of the samples can be assigned to the calcium tartrate decomposition described by the reactions mentioned below. In the first two stages we observe almost no difference between the decomposition processes in the different environments. The weight loss occurring in the region 60À206 °C is attributed to the crystallization water removal (27 wt % theoretically) according to the reaction The difference in the final temperatures of the tartrate decomposition in He atmosphere and air (∼650 and 730 °C) is probably related to the larger agglomeration of CaCO 3 particles in air conditions. TEM Study of Thermal Decomposition Products. A typical TEM image of the product of Co-doped calcium tartrate decomposition in an inert atmosphere (syst1) is presented in Figure 2a. According to EDX data (Figure S2 in Supporting Information), the dark brown powder is a nanocomposite of CaO and Co nanoparticles (the dark inclusions in the image). Statistical analysis of TEM images revealed that the size of Co nanoparticles is uniform and ranges between 1.3 and 8.9 nm, with an average value of 5.6 nm. TEM study of the syst2 sample showed that the size distribution of the nanoparticles formed in the CaO matrix varies over quite a wide range from 30 to 180 nm (Figure 2b). To identify the structure of the nanoparticles, the interplanar distances were determined by Fourier transform (FT) analysis of HRTEM images. It was found that the nanoparticles in the syst1 sample are mainly composed of metallic Co and a small portion of CoO. In the syst2 sample the nanoparticles with a composition of Co 3 O 4 are the most abundant, while a few CoO and Co nanoparticles are also detected. The presence of Co nanoparticles in the nanocomposites is due to the CaO matrix, which can protect metallic nanoparticles from oxidation in air.

XAFS Study. Co K-edge XANES spectra of the Co-doped calcium tartrate and products of its thermal decomposition in He or air are shown in Figure 3 together with the spectra of reference compounds Co foil, CoO, and Co 3 O 4 . The spectrum of the initial tartrate shows the high intensity of the main peak after the absorption edge, the so-called "white line", that is typical for CoO, Co 3 O 4 , and LiCoO 2 . The energy position of the edge is similar to that of CoO. FT EXAFS spectrum shows a single peak at ∼1.6 Å (Figure S3 in Supporting Information), which most probably corresponds to an oxygen coordination shell. Indeed, according to the data obtained from the fitting procedure this shell consists of six oxygen atoms and has a radius of ∼2.09 Å (Table 1). This radius is also characteristic for CoO (2.13 Å). The obtained radius and coordination number of CoÀO shell indicates that cobalt atoms are located in vacant interstitial sites in calcium tartrate lattice. Otherwise, if Co atoms had been substituted with Ca atoms, the average radius and coordination number would have been ∼2.4 Å and 8, respectively, similar to that of the CaÀO shell in calcium tartrate. 37 Co K-edge XANES spectra of the Co-doped calcium tartrate heated in situ up to temperatures corresponding to stages 1À3 in the process of thermal decomposition show the insignificant differences between the samples measured in air and the inert atmosphere (Figure 3). When the temperature is increased, the intensity of the "white line" decreases. This probably arises from the decrease in the first coordination number of Co. The estimations of EXAFS parameters for the products in the third stage (400 °C in inert atmosphere and 470 °C in air) are also in favor of this assumption. CoÀO coordination numbers were estimated to be 3.2 (3) and 3.8 (4) for the product obtained in air and the inert atmosphere, respectively, while the radius of the shell is approximately the same for both products ∼1.98 (1) Å. In all the spectra the pre-edge peak at 7710 eV and the energy positions of the Co K-edge are constant over the measured temperature range. Thus, the oxidation state of Co in the sample equals +2, and it is not changed in the products of thermal decomposition in different environments up to 470 °C. Note that the spectra measured in air at 370 °C and 470 °C and in He atmosphere at 400 °C have gently sloping absorption edges, and they are considerably different from those measured at the temperature (200 °C) characteristics of the first stage of decomposition of Co-doped calcium tartrate. A gently sloping edge is typical for samples having an absorbing atom in several oxidation states or interacting with different types of atoms (for example CoÀO and CoÀC).

Co K-edge XAFS spectra show that complete thermal decomposition of doped tartrate in the inert atmosphere and air yields the different resultant products syst1 and syst2 (Figure 3). In addition to the low-intensity "white line", these spectra have a feature typical for metallic cobalt, indicating that the resultant products consist of metallic cobalt and cobalt oxides. The spectrum of syst2 shows a shift to the higher energy compared to that of syst1. The difference in the shape of the "white line" can also be observed in the spectra of syst1 and syst2. These distinctions can originate from nanoparticles with several composite structures or several phases, and their different ratio in the sample. Three peaks detected in the FT EXAFS spectrum of syst1 and syst2 samples in a range from 1.0 to 3.3 Å (Figure S3) are due to CoÀO (∼1.4 Å) and two CoÀCo interactions, one of which originated from Co metal (2.1 Å) and the other one from cobalt oxide (∼2.7À2.9 Å). The FT peaks of syst2 were well described using a combination of two structures, metallic Co and Co 3 O 4 , with a ratio of 2.3:1. The obtained EXAFS parameters are collected in Table 1. However, the best model to fit the experimental FT of syst1 was not found. It mostly deals with the presence of several structures/phases (more than two) in the sample. In this case many EXAFS parameters should be included in the fitting. This approach can improve the agreement between the theoretical and experimental spectra but makes EXAFS parameters unreasonable. The low value of the CoÀO coordination number for syst2 could result from a thin surface layer of cobalt oxide on the metallic Co nanoparticles. FT peaks in the range of 3.3À5.4 Å may be attributed to long-distance CoÀCo coordination shells.

XPS Study. The XPS survey spectrum of the Co-doped calcium tartrate (part a of Figure S4 in Supporting Information) detected the signals of carbon, oxygen, calcium, and cobalt. The concentration of Ca at the sample surface (without considering hydrogen atoms) calculated from the XPS spectrum is 7 at.%; the atomic ratio Co/Ca is 0.04. Two peaks at 286.7 and 288.7 eV in the C 1s spectrum of the sample correspond to carbon atoms bonded with hydroxyl groups and carbon atoms in carboxyl groups. Oxygen atoms of these groups contribute to the O 1s spectrum components located at 533.2 and 531.8 eV with an intensity ratio of 1:2 (part b of Figure S4 in Supporting Information). The Co 2p 3/2 line is centered at 780.6 eV and has a broad, intensive satellite line at a distance of 4 eV from the higher binding energy (Figure 4). The spinÀorbital splitting For the syst1 and syst2 samples, XPS revealed the presence of calcium carbonates and a 2-fold increase in concentration of calcium on the surface compared to that of initial tartrate (Figure S5 in Supporting Information). In addition, the surface concentration of carbon atoms at 285 eV is higher in the syst1 sample than in the syst2 one, which could be due to incomplete burning of carbon in syst1 in agreement with the TG result. The surface Co concentration in syst1 is unchanged compared to that of the initial tartrate while it is three times higher in the syst2 sample. The binding energies of Co2p 3/2 levels are 781.0 and 780.4 eV for syst1 and syst2, respectively, and intensity of the satellite considerably decreases and shifts to higher energy as compared to that in the XPS spectrum of the initial tartrate (Figure 4). These data indicate the formation of Co 3 O 4 on the surface of the samples. 38 It is worth noting that in the syst1 spectrum the Co2p 3/2 peak is shifted to higher binding energy and the satellite intensity is higher than that in the syst2 spectrum, which could be related to covering of the cobalt nanoparticles by oxides that are intermediate between CoO and Co 3 O 4 . This is consistent with TEM and XAFS conclusions about the incomplete oxidation of Co in syst1. Note that metallic cobalt was not detected in syst1 and syst2 while the photoelectrons with the used kinetic energy probed the samples with a penetration depth of ∼3 nm.

Study of Products of CCVD Synthesis. Structural Characterization. SEM and TEM images of the products of CCVD synthesis resulting from the decomposition of ethanol vapors or methane using catalytic systems syst1 or syst2 are presented in Figure 5. According to TEM and XPS data after inserting the Co-doped calcium tartrate in the reaction zone of CVD reactor and its decomposition in Ar atmosphere at 800 °C for 10 min, surface-oxidized Co nanoparticles were formed in the CaO matrix (syst1 sample). Decomposition of ethanol vapors on these nanoparticles produced CNTs (Figure 5a,b). TEM study of the cnt1 sample showed formation of multiwall CNTs with an outer diameter ranging from 4 to 21 nm and a mean value of ∼10 nm (Figure 5b). The measurement of the metallic particles present in the carbon material detected that their average diameter increased to 6.7 nm, which is indicative of the aggregation process at 800 °C during the CNT growth. In the case of ethanol decomposition over the syst2 sample, where the catalytically active atoms are mainly as Co 3 O 4 , formation of a very limited number of CNTs among the agglomerates of CaO was observed (part a of Figure S6 in Supporting Information).

Thermal decomposition of the Co-doped calcium tartrate immediately under the conditions of the reduction atmosphere (CH 4 ÀH 2 ) produced an extremely limited number of CNTs (part b of Figure S6). The size of metallic particles formed during the in situ decomposition of tartrate in the reactor is more likely to be insufficient to nucleate CNTs, and decomposition of methane mainly yields amorphous carbon. Alternatively, using syst2 under the same conditions resulted in the formation of CNTs (Figure 5c) in an amount comparable with the decomposition of ethanol over syst1 (Figure 5a). TEM analysis revealed a large variation of CNTs in diameter, and the occurrence of a double-walled CNT with ∼4 nm outer diameter, in addition to the thick ∼60 nm CNTs (Figure 5d). Most CNTs in sample cnt2 are ∼20 nm in diameter.

The structural peculiarities of the samples cnt1 and cnt2 derived from the TEM analysis were supported by Raman spectroscopy. An increase in the number of walls in the CNTs composing the cnt2 sample resulted in better graphitization of the layers and narrowing of the D mode resonance 39 as compared to the spectrum of cnt1 sample (Figure S7 in Supporting Information). The average outer diameter of CNTs was estimated from the frequency of the G mode, which has been shown to decrease with the multiwall CNT diameter increasing. 40 The values 1594 cm À1 for cnt1 and 1587 cm À1 for cnt2 correspond to CNTs with the diameters less than 10 nm in the former sample and in the range 10À20 nm in the latter one. The peaks detected in the radial breathing mode (RBM) region of the Raman spectrum of the cnt2 sample could correspond to single-walled CNTs or the inner tubes of double-walled CNTs. Diameters of these nanotubes were calculated using the equation from ref 41 to be within 1.2À1.8 nm. A ratio of different carbon species in the cnt2 sample was evaluated from the TG data (Figure S9 in Supporting Information). The major part of the sample (∼85%) is multiwall CNTs burning around 630 °C, 8 the quantity of double-walled and single-walled CNTs destroyed within 430À520 °C42,43 is about 8%, and the rest of the carbon likely corresponds to amorphous species.

XAFS and XPS Investigation of CCVD Products. Co K-edge XANES spectra of samples cnt1 and cnt2 and Co foil as a reference compound are compared in Figure 6. The spectra of the produced samples are similar in appearance and notably different from that of metallic Co, particularly, the first peak after the absorption edge is much better resolved in energy in the spectra of samples cnt1 and cnt2. Similar Co K-edge XAFS spectra have been observed for bimetallic nanoparticles AgCo 44 and PdCo, 45 where Co replaced the Ag/Pd position in a face-centered cubic (fcc) structure. It is well-known that the structure of metallic Co has three different forms: hexagonal hcp (α-phase), cubic fcc (β-phase), and primitive cubic (ε-phase). 46À48 The β-phase commonly occurs in nanoparticles with a size less than 20 nm 49 and in bulk at temperatures higher than 422 °C. FT EXAFS of the samples cnt1 and cnt2 (Figure S10 in Supporting Information) show all peaks typical for Co foil although the difference in the amplitude of the peaks is clearly seen. To determine the local atomic parameters of Co in the samples, the fitting in the range 1.2À5.1 Å was carried out with the βand α-phase structures as initial models. The best model was found to be the β-structure. The obtained local atomic parameters of Co in the samples cnt1 and cnt2 are listed in Table 1. The smaller effective coordination number is obtained for cnt1 (∼11), for which FT peaks also show the smallest amplitude. All of these are results of a size effect. Thus, the average particle size is smaller for cnt1, which agrees well with TEM results.

XPS Co 2p 3/2 spectra of cnt1 and cnt2 are compared in Figure 7. A major peak at ∼778.5 eV corresponds to atoms of metallic Co nanoparticles. 50,51 A shoulder at 780.5 eV is associated with Co oxides. The integral intensity of this shoulder is higher in the spectrum of cnt2, which indicates a higher surface concentration of Co oxide. Thus, regardless of the initial state of metal atoms in the catalytic systems syst1 and syst2, the products of CCVD synthesis (cnt1 and cnt2) mainly consist of metallic Co nanoparticles and some amount of Co oxides. The presence of the latter species in CCVD products probably is related to the surface oxidation of non-carbon-encapsulated metallic nanoparticles under the conditions of laboratory storage.

' DISCUSSION

Experimental data presented above allowed us to propose a scheme of transformations of the Co-doped calcium tartrate under heating in inert or oxidizing atmosphere and of the obtained mixed metal oxide systems during the process of CCVD synthesis of CNTs using a specific feedstock (Figure 8).

XAFS data showed that the doping of calcium tartrate with Co atoms results in the formation of an interstitial solid solution. Therefore, Co atoms are separated from each other and uniformly distributed in the interstices of the calcium tartrate lattice. On the basis of this structural model, it might be expected that under heating, Co atoms are able to move easily and form metallic nanoparticles. However, according to the The size distribution of the Co nanoparticles in the decomposition products strongly depends on the heating atmosphere. In syst1 the nanoparticle size varies from 1.3 to 19 nm, while in syst2 the nanoparticles are much larger and reach 30À180 nm. Perhaps the wide range of the particle sizes obtained by the thermal oxidation of the Co-doped calcium tartrate is also related to the different cobalt compositions in syst2.

According to XPS, the surface of syst1 is enriched in carbon as compared to that of syst2. From TG data, the carbon excess in the product of the decomposition of Co-doped calcium tartrate in the inert atmosphere may be due to a deficiency in oxygen in the tartrate lattice to completely burn carbon. We suggest that the excess carbon forms a shell around nanoparticle, which prevents an uncontrolled cobalt agglomeration, observed in syst2. The effective growth of CNTs from ethanol using syst1 in the CCVD process can be attributed to the formation of a metallic nanoparticle surface during the burning of the carbon shell. In the absence of oxygen in the feedstock, only a small amount of CNTs is observed in the synthesis product. To remove oxygen from nanoparticles in syst2, a reducing atmosphere is required. This is evidenced by the formation of CNTs from CH 4 ÀH 2 and their almost complete absence in the sample where alcohol, containing a small amount of active hydrogen, was used as the carbon source.

After CCVD syntheses of CNTs in both samples obtained by using the catalytic systems syst1 or syst2, the metallic Co nanoparticles have an fcc structure (β-phase). According to XPS data, the nanoparticles' surface is coated with CoO, the relative amount of CoO being much higher in the case of the use of syst2 for CNT growth. The presence of CoO in CCVD products could be due to an interaction of Co with oxygen from calcium tartrate. 52 It is possible if the catalytic nanoparticles have a strong bond with the CaO matrix. The possibility must not be ruled out that CoO covers Co nanoparticles located inside the nanotubes. In this case it would be necessary to further investigate the role of cobalt oxide in the process of CNT growth.

TEM images of the products of thermal decomposition of Codoped calcium tartrate and CNTs formed with the use of these products showed a correlation between the metallic nanoparticles size and the nanotubes diameter. However, in case of CCVD synthesis using syst2 containing numerous large metallic nanoparticles (30À180 nm), small-diameter CNTs, particularly, double-walled nanotubes and probably single-walled nanotubes, were obtained. The reason for this could lie in the reduction process of cobalt oxides. The reduction of a large nanoparticle can run nonuniformly. Because in our case the carbon source is supplied to the surface of the oxidized nanoparticle simultaneously with hydrogen, the growth of small-diameter CNTs on the reduced area of the metal nanoparticles can be expected. It is also possible that the small-diameter CNTs could result from catalysis by small-diameter Co nanoparticles formed in situ in addition to the large-diameter nanoparticles already present.

' CONCLUSIONS

A study of the thermal decomposition of Co-doped calcium tartrate in an inert atmosphere and air has been performed using TG analysis and in situ XAFS experiments. The XAFS investigations showed that Co atoms are isolated from each other and occupy interstitial sites in the calcium tartrate lattice. Formation of metallic Co nanoparticles occurs during the last stage of thermal decomposition of the tartrate above 500 °C independently on the environment. The nanoparticles have narrow distribution in diameter when decomposition proceeds in an inert atmosphere and the reason for this according to TG and XPS data is the incomplete burning of carbon contained in the tartrate. Abundant carbon forms a shell around Co nanoparticles and thereby prevents uncontrolled agglomeration of the nanoparticles. To remove this shell for activation of the metallic catalyst, the presence of oxygen-containing compounds is necessary in the CVD process. Therefore, from ethanol, CNTs are formed while from methane, CNTs do not practically grow. On the contrary, with the use of the thermal decomposition product obtained in air, CNT growth occurs only in a reducing atmosphere. After CCVD synthesis, an fcc structure of Co in the nanoparticles is formed which does not depend on the cobalt state in the catalytic system. The results from this study reveal that varying the conditions of thermal decomposition of calcium tartrate doped with transition metal allow the change in size and chemical state of forming metallic nanoparticles. These nanoparticles distributed in CaO can be used as catalysts for the CCVD growth of different kinds of CNTs. We showed that catalysts can produce very defective few-walled CNTs, multiwall CNTs with high crystallinity of graphitic layers, and even double-walled and probably singlewalled CNTs. A specific characteristic of the CCVD products is the nonbundling of CNTs, and such dispersive material could find application as fillers in various composites. Furthermore, we believe that alkali-metal tartrates (Ca, Mg, Ba, and their mixtures) doped with transition metals (Ni, Co, Fe, and their mixtures) will decompose, following a mechanism determined for the Co-doped calcium tartrate that would open possibilities for creation of new materials, particularly, carbon-encapsulated transition metal nanoparticles or supported catalysts for CNT growth.

' ASSOCIATED CONTENT b S Supporting Information. Figure of TG, DTG, and DTA curves, HRTEM image and EDX analysis of the product of thermal decomposition of Co-doped calcium tartrate, Fourier transform of the experimental k 2 χ(k) Co K-edge EXAFS and fits for the initial Co-doped calcium tartrate and products of its decomposition, figure of XPS spectra, figure of SEM images, Raman spectra, TG, DTG, and DTA analysis, and Fourier transform of the experimental k 2 χ(k) Co K-edge EXAFS and fit for the CCVD products of CH 4 or C 2 H 5 OH decomposition over the catalytic systems syst1 and syst2. This material is available free of charge via the Internet at http://pubs.acs.org.

  CaC 4 H 4 O 6 3 xH 2 O f ÀxH 2 O CaC 4 H 4 O 6 . The water stoichiometry in doped calcium tartrate estimated from the TG data is 4. The second stage at 246À322 °C is assigned to the transformation of the tartrate into oxalate (23 wt % theoretically) by the reactionCaC 4 H 4 O 6 f À2H 2 O, À 2C CaC 2 O 4 . The main distinctions between the measured curves are obtained in the third stage, in the range 372À487 °C. Here it is assumed that the transformation of oxalate into carbonate proceeds according to CaC 2 O 4 f ÀCO CaCO 3 .The weight loss in He atmosphere and air is 6 wt % and 11 wt %, respectively. We assign the larger weight loss and the exothermal effect observed in differential thermal analysis (DTA) (part b of FigureS1in Supporting Information) with tartrate decomposition in air to the reaction C + O 2 f CO 2 . The delayed weight loss in the case of the third stage of the decomposition in He atmosphere can be attributed to a slow burning of the carbon-containing phase in the presence of oxygen traces in the inert gas (purity 99.995). The last stage is associated with the conversion of carbonate to the oxide according to CaCO 3 f ÀCO 2 CaO (17 wt % theoretically).

Figure 1 .

 1 Figure 1. Thermal gravimetric (TG) curves of decomposition of Codoped calcium tartrate in air and He atmosphere. The numbers indicate the stage of decomposition.

Figure 2 .

 2 Figure 2. TEM images of products of decomposition of Co-doped calcium tartrate in inert (a) and air (b) atmosphere. The inserts show high-resolution images of nanoparticles.

Figure 3 .

 3 Figure 3. Co K-edge XANES of Co-doped calcium tartrate, heated in air at 200 °C, 370 °C, and 470 °C or in He at 200 °C and 400 °C and products of final decomposition of the tartrate in inert atmosphere (syst1) or in air (syst2) in comparison with the spectra of reference samples Co foil, CoO, and Co 3 O 4 .

Figure 4 .

 4 Figure 4. XPS spectra of the Co 2p 3/2 level for Co-doped calcium tartrate and products of its decomposition in Ar atmosphere (syst1) or in air (syst2).

Figure 5 .

 5 Figure 5. SEM (a) and HRTEM (b) image of sample produced by C 2 H 5 OH decomposition over syst1 at 800 °C. SEM (c) and HRTEM (d) image of sample produced by CH 4 decomposition over syst2 at 1000 °C.

Figure 6 .

 6 Figure 6. Co K-edge XANES of CCVD products of C 2 H 5 OH decomposition over syst1 (cnt1) and CH 4 decomposition over syst2 (cnt2) in comparison with the spectrum of Co foil.

Figure 7 .

 7 Figure 7. XPS spectra of Co 2p 3/2 level for CNT samples synthesized by C 2 H 5 OH decomposition over syst1 (cnt1) and CH 4 decomposition over syst2 (cnt2).

Figure 8 .

 8 Figure 8. Scheme of transformation of Co-doped calcium tartrate under heating in inert atmosphere or in air and of the CaO-supported metallic nanoparticles in the CCVD process utilizing specific feedstock (ethanol or CH 4 + H 2 ).

  Ca 1Àx Co x C 4 H 4 O 6 + O 2 f CaO + Co x O y + CO 2 + H 2 O. The product of this synthesis is labeled syst2.

	). The precipitation of doped calcium
	tartrate was a result of the following reaction: Na 2 C 4 H 4 O 6 +
	CaCl 2 + Co(CH 3 COO) 2 3 4H 2 O f Ca 1Àx Co x C 4 H 4 O 6 3 4H 2 OV + NaCl + NaCH 3 COO. The precipitate was filtered and repeatedly
	washed with deionized water and then dried in air at ambient
	conditions. The obtained pink fine powder was decomposed in
	an argon atmosphere or in air. It was expected that the first method
	should lead to nanosized Co particles distributed in the CaO
	matrix in agreement with the reaction Ca 1Àx Co x C 4 H 4 O 6 f
	CaO + Co + CO 2 + H 2 O. The product obtained by this method
	is labeled syst1. The formation of syst1 was performed within the
	CCVD reactor just before the CNT synthesis. The other catalytic
	system was prepared by decomposition of doped tartrate in air at
	800 °C for 30 min. In this case, the formation of a mixture of
	Co and Ca oxides was expected as a result of the following reaction:

Table 1 .

 1 EXAFS Parameters for Co-Doped Calcium Tartrate, Product of Its Thermal Decomposition in Air (syst2) and Products of CCVD Synthesis of CNTs Using syst1 and syst2 Samples As the Catalytic Systems: R, Interatomic Distance; σ 2 , DebyeÀWaller Factor; N, Coordination Number; R-Factor, Quality of the Fit Co 2p 3/2À1/2 is 14.7 eV. Such a Co 2p spectrum indicates that cobalt atoms are in the Co (+2) charge state in an octahedral oxygen environment. These results agree well with XAFS data.

		coordination shell	R, Å	σ 2 , Å 2	N	notes	R-factor, %
	tartrate	CoÀO	2.088 (9)	0.0055 (6)	6.2 (2)		1
	syst2	CoÀO	1.882 (4)	0.004 (3)	3 (1)	O from Co 3 O 4	2
		CoÀCo	3.333 (7)	0.012 (6)	8 (4)	Co from Co 3 O 4	2
		CoÀCo	2.518 (9)	0.005 (1)	4 (1)	Co metal	2
	cnt1	CoÀCo	2.494 (4)	0.0074 (6)	11.0 (5)	Co metal	1
		CoÀCo	3.526 (6)	0.0108 (9)	5.5 (2)	Co metal	1
		CoÀCo	4.319(8)	0.011 (1)	22.1 (9)	Co metal	1
		CoÀCo	4.987 (9)	0.011 (1)	11.0 (5)	Co metal	1
	cnt2	CoÀCo	2.504 (4)	0.0069 (4)	11.8 (5)	Co metal	1
		CoÀCo	3.541(6)	0.0088 (4)	4.7 (2)	Co metal	1
		CoÀCo	4.336 (7)	0.009 (1)	23.5 (9)	Co metal	1
		CoÀCo	5.008 (8)	0.009 (1)	11.8 (5)	Co metal	1

supported by the French Embassy in Russia. We are grateful to Dr. V.