- Email: [email protected]
In situ TEM investigations of reactions of Ni, Fe and Fe–Ni alloy particles and their oxides with amorphous carbon
CARBON
4 7 ( 2 0 0 9 ) 8 5 6 –8 6 5
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
In situ TEM investigations of reactions of Ni, Fe and Fe–Ni alloy particles and their oxides with amorphous carbon R. Anton* Institut fu¨r Angewandte Physik, Universita¨t Hamburg, Jungiusstrasse 11, D-20355 Hamburg, Germany
A R T I C L E I N F O
A B S T R A C T
Article history:
Ni, Fe and Ni–Fe alloy particles were vapour deposited on thin films of amorphous carbon
Received 22 July 2008
(a-C) inside a specially equipped transmission electron microscope, and reactions with the
Accepted 20 November 2008
substrate were observed at elevated temperatures. The influence of oxidation of the parti-
Available online 3 December 2008
cles was also investigated. In contrast to Ni, which was found in earlier work to graphitise the carbon at above 600 C without bulk carbide being involved, pure Fe reacted with the a-C support at about 500 C to Fe3C, which graphitised the carbon similar to Ni, starting at about 600 C. No carbide was formed from oxidised Fe particles. FeO decomposes above 500 C, higher oxides (Fe3O4, Fe2O3) only above 750 C. The remaining Fe particles graphitised the carbon support directly. Alloy particles with composition Ni80:Fe20 (permalloy) graphitised a-C in the same way as pure Ni, without any phase separation. Annealing of a mixed phase of finely dispersed Ni–Fe-oxide or deposition of Ni–Fe under oxygen at above 300 C resulted in nucleation of three-dimensional crystallites of virtually pure Ni, which graphitised the carbon, while the remaining phase of small particles was converted to inactive Ni–ferrite, NiFe2O4. 2008 Elsevier Ltd. All rights reserved.
1.
Introduction
Catalytic reactions of transition metals with carbon have long been known, and the interest has recently revived, for example with regard to the formation of carbon nanotubes and fibres or encapsulation by carbon layers [1–8]. While in many experiments the carbon is supplied from the gas phase, we have investigated the catalytic graphitisation of thin films of amorphous carbon (a-C) by supported metal particles [9–11]. As has been described in detail in our earlier work for the case of Ni, the reaction starts with encapsulation of the particle by graphite layers followed by outflow of the metal and spreading on the substrate, which is concomitantly graphitised. Similar reactions also occur with other transition metals. In the case of Fe, graphitisation of carbon has been observed by other authors to proceed via the formation of Fe–carbide, whereas ambiguous results were found for Ni [7]. We did
not find indications for the existence of intermediate Ni–carbide in our in situ TEM experiments [10,11]. In the following, more detailed investigations on reactions of Fe and of a Ni–Fe alloy with carbon are presented. In particular, the role of oxidation of all three phases, Ni, Fe and Ni–Fe is addressed. This has so far not systematically been investigated. While it is known that Ni-oxide decomposes prior to the graphitisation reaction, without formation of intermediate carbide, it will be shown that oxidation of Fe prevents the formation of Fe–carbide, and, after decomposition of the oxide, virtually pure Fe graphitises the carbon directly, although only at somewhat higher temperatures.
2.
Experimental
The experiments were performed in a modified transmission electron microscope (Philips EM400), which was equipped
* Fax: +49 40 42838 7095. E-mail address: [email protected] 0008-6223/$ - see front matter 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2008.11.038
CARBON
4 7 (2 0 0 9) 8 5 6–86 5
with an additional ultra high vacuum chamber at the specimen stage of the microscope. More details have been described elsewhere [12]. The chamber was differentially pumped with a turbomolecular pump with magnetic bearings, which did not compromise the specified point resolution of 0.3 nm. The base pressure was in the mid 10 9 mbar range. Thin film substrates of amorphous carbon (a-C) with 20 nm thickness were prepared by vapour deposition of carbon on mica, floating off, and capture on TEM grids of molybdenum or gold. These were mounted in a heatable sample holder of the microscope, which contained a crucible of molybdenum, at the edge of which a Pt/Pt–Rh thermocouple was spot welded. The temperature reading was calibrated by melting experiments with tin and bismuth. In the microscope, the samples were outgassed at 600 C for several hours prior to the deposition and reaction experiments. The metals were evaporated from rods by electron bombardment and the flux was measured by collecting simultaneously formed ionised species. Permalloy particles were produced by evaporation from an alloy rod with composition Ni80:Fe20. Generally, the working pressure was about 1 · 10 8 mbar, but some depositions were performed under a partial pressure of oxygen of 5 · 10 6 mbar in order to obtain oxide particles. Substrate temperatures during deposition were in the range from 250 to 500 C. Immediately after deposition, the temperature was increased in steps up to 800 C, depending on the type and progress of the reaction. Temperature steps were typically 25 C each, so that the specimen stabilised (temperature and drift) within less than a minute. Sequences of images were recorded during depositions and reactions via a CCD camera in a computer via a firewire interface at a frame rate of about 10/s. By using an image intensifier, the electron illumination was reduced to about 0.1 A/cm2, at the routine operating voltage of 100 kV. Some annealing experiments were also performed in another microscope (Philips CM12), where images were recorded on plates. In any case, no indications of electron beam artefacts were found. This was verified by
857
comparing irradiated areas with non-irradiated areas during and after the reactions. Electron diffraction patterns were quantitatively analysed by calibration with reflexions from graphite eventually evolving during reactions and/or from additionally deposited MgO crystallites. High-resolution TEM images were obtained in another instrument (Philips CM300). Energy dispersive X-ray analysis (EDX) was employed in the CM12 in order to detect the local distribution of the metal and oxide phases on the substrate before and after the reactions.
3.
Results
3.1.
Ni and NiO
The graphitisation of a-C by Ni had been investigated in earlier work [9–11]. The details will not be repeated here. In summary, deposition of Ni on a-C at temperatures between 200 and 400 C in a vacuum of some 10 8 mbar yield three-dimensional, isolated metal particles. Upon heating, these start to react with the substrate at about 600 C. Initially, the particles are encapsulated by graphene layers, which nucleate at the metal surface and metal–graphene interface, respectively. This graphite shell is rather stable, and while thickening inwards, the metal core is eventually driven out, and spreads on the substrate, which is thereby graphitised at the surface. Measurements of the propagation speeds at various temperatures yielded the activation energy of 1.8 eV, in close agreement with a theoretical estimate of 1.6 eV [8,11]. Exposure of the specimen to a partial pressure of oxygen of 5 · 10 6 mbar has been found in earlier work to have no influence on the graphitisation reaction [9]. Likewise, initial oxidation of Ni does not play a role, as the oxide decomposes already at about 300 C, as was verified in recent experiments by X-ray spectroscopy (EDX) and electron diffraction. Decomposition of small oxide particles causes compacting, and an increase of contrast. At above 400 C, the remaining particles
Fig. 1 – TEM bright field and diffraction images taken after the reaction of NiO particles on a-C at 650 C for 10 min. Ni had been deposited under oxygen at 250 C, which yielded finely dispersed particles of NiO. These were reduced to f.c.c. Ni during heating to above 350 C. Further heating to above 600 C led to large scale coalescence, and the Ni crystallites reacted with the a-C substrate, starting with encapsulation with a graphite shell, followed by partial outflow and spreading on the substrate (arrows), which is then graphitised, too. Note spotty rings from f.c.c. Ni and continuous rings from graphite in the diffraction image.
858
CARBON
4 7 ( 2 0 0 9 ) 8 5 6 –8 6 5
of pure Ni coalesce, and above 600 C, larger aggregates start to react with the carbon substrate as was described above. A medium stage of this reaction is illustrated in Fig. 1.
3.2.
Fe
A typical reaction of Fe particles with the carbon support is illustrated in Fig. 2. Deposition of Fe on a-C at 300 C in a vacuum of some 10 8 mbar yielded finely dispersed particles with b.c.c. structure (not shown). Heating up to 600 C resulted in coalescence to larger, more isolated crystallites with mean size of about 20 nm. At around 700 C, some particles became active in spreading on the substrate, thereby growing by incorporation of neighbouring particles (see Fig. 2a). Areas
Fig. 2 – TEM bright field and selected area diffraction images taken during the reaction of Fe particles on a-C. (a, b) At 700 C, Fe particles are encapsulated by thin layers of graphite, and react to Fe3C, which spreads on the substrate and aggregates by coalescence with other particles (scattered spots in the diffraction image, some are marked with arrows), thereby graphitising the substrate. (c, d) After further heating to 725 C for 10 min, large patches of Fe3C have formed (an example of a diffraction pattern from such a particle is shown in d), and the substrate surface is overall graphitised. (e) Perspective view of the sample tilted by about 24. The arrows mark some graphite shells, which had been abandoned by their originating Fe particle (see text).
left free by the particles reveal their ‘‘footprints’’, which appear to be remnants of graphite shells, as well as mottled contrast, which indicates graphitisation of the substrate. At this stage, the diffraction image shows, besides rings from b.c.c. Fe particles, rings from graphite, as well as some scattered spots from Fe–carbide, Fe3C. Dark field images confirmed that these spots originate from the spreading particles. Further heating increased the speed of reaction, until all Fe particles were incorporated in large flat aggregates of carbide, and the substrate surface was overall graphitised (Fig. 2c). The carbide aggregates were mostly single crystals, but occurred in different orientations. The selected area diffraction image in Fig. 2d is just an example. The perspective view of the sample being tilted in the TEM (Fig. 2e) shows the rugged morphology of the graphitised substrate, as well as graphite shells, which formed on the particle surfaces before spreading. This is also shown in the high-resolution TEM image in Fig. 3. An iron particle is partly encapsulated by graphene layers (with spacing 0.335 nm), but part of the metal has moved away. The lattice spacing of the metal core was measured to 0.2030 ± 0.0005 nm. This value would correspond to both, planes of Fe(1 1 0) with spacing 0.2029 nm and of Fe3C(0 2 2) with spacing 0.2025 nm. In any case, the images shown in Fig. 2 strongly suggest that spreading only occurs after encapsulation and reaction to carbide. Deposition of Fe on a-C at 450 C resulted in isolated, three-dimensional particles of Fe3C, which were produced by direct reaction with the carbon support. This is illustrated in Fig. 4. Row (a) shows the state just after deposition. It is interesting to note that aggregation of Fe atoms into particles during deposition must have occurred before formation of carbide, as no traces of any reaction, e.g. graphite, could be
Fig. 3 – HRTEM image of a Fe particle partly encapsulated in graphene layers. Part of the metal has flown away, apparently along the channel, which is indicated by an arrow. Lattice spacings of 0.335 nm correspond to graphite, while spacings of 0.203 nm could originate from both b.c.c. Fe and cubic F3C (see text).
CARBON
4 7 (2 0 0 9) 8 5 6–86 5
859
seen in the bright field image (see arrows in Fig. 4c, left). This again indicates that the reaction of the original Fe particles is preceded by encapsulation, as was the case for deposition at lower temperatures.
3.3.
Fe-oxide
Partly oxidised Fe particles were obtained by short exposure of freshly deposited Fe to air during transfer to another TEM. Bright field imaging and diffraction revealed a thin layer of FeO at the surface of the particles, which can be seen in Fig. 5a. Correspondingly, X-ray spectra (EDX) showed a small peak of oxygen. Heating to 600 C caused coarsening of the metal particles and concomitant decomposition of the oxide.
Fig. 4 – TEM bright field and diffraction images recorded during the reaction of Fe–carbide particles with the a-C support film. (a) Deposition of Fe on a-C at 450 C yielded particles of Fe3C, as was verified by the corresponding diffraction pattern. (b) Same area after 10 min at 600 C: some particles have started to spread on the substrate, leaving traces of graphite behind. (c) After 10 min at 650 C, moving carbide aggregates have coalesced, and the whole substrate surface is graphitised (see rings marked in the diffraction image, not all are marked). In the bright field image, some remnants of graphite shells, which originate from the original particles, are marked by arrows.
detected on the still amorphous support film between the particles at this stage. This means that at 450 C, diffusion of Fe atoms on a-C is much faster than incorporation into carbide. On the other hand, the reaction of the particles to carbide is fast enough that virtually no pure Fe could be detected in the diffraction pattern. Rather, all rings could be attributed to Fe3C. During heating to 600 C, some particles started to react further with the substrate by spreading away and leaving traces of graphitisation behind, as is indicated by the mottled contrast (row b). Further heating to 650 C increased the reaction speed, and, while moving carbide aggregates coalesced, the substrate surface was overall graphitised, which is also apparent by the corresponding strong diffraction rings (row c). Some remnants of graphite shells, originating from initial encapsulation of the original particles can be
Fig. 5 – TEM bright field and diffraction images recorded during the reaction of slightly oxidised Fe with the a-C support film. Row a: Fe deposited on a-C at 410 C after exposure to air for about 10 min. Arrows in the bright field image indicate oxidation of the particle edges. The diffraction pattern shows weak rings from FeO, besides the dominant rings from b.c.c. Fe. Row b: after heating to 600 C for 15 min, the oxide has disappeared, and the remaining Fe particles coalesce. Row c: after 1 h at 700 C, Fe aggregates have graphitised the substrate surface. Arrows in the bright field image mark graphite shells around particles. The diffraction image shows rings from graphite in addition to the spotty rings from Fe.
860
CARBON
4 7 ( 2 0 0 9 ) 8 5 6 –8 6 5
The remaining virtually pure b.c.c. Fe particles coalesced into large, flat, irregularly shaped crystallites, which started to spread and graphitise the substrate only at about 700 C, without forming carbide, as is apparent from the diffraction image in Fig. 5c. Oxygen could no longer be detected by EDX at this stage. Extended exposure of freshly deposited Fe particles to air resulted in a mixture of FeO and Fe3O4, which slowly decomposed during heating to above 600 C, while at 700 C, the reaction of the remaining metal with the substrate proceeded similar to the case of only slight oxidation. Bulk Fe-oxide particles turned out to be rather stable, and decomposition and reaction with carbon was only observed to start at about 800 C. A typical reaction sequence is illustrated in Fig. 6. Fe was deposited on a-C at 350 C under oxygen at 5 · 10 6 mbar, yielding finely dispersed particles of Feoxide. The diffraction image exhibited diffuse rings, which could be attributed to various oxide phases. The sample was then transferred to another TEM (equipped with EDX), in which the annealing experiment was performed. Heating to 700 C caused some coarsening by coalescence, and the formation of a network of small oxide particles of mainly Fe3O4 with interspersed larger crystallites of both Fe3O4, but
mostly cubic c-Fe2O3. This stage is shown in Fig. 6a–d. Unambiguous identification is generally difficult because of the similarity of the diffraction patterns, but the presence of the (2 0 0) reflexion is indicative for Fe2O3. Representative EDX spectra are shown in Fig. 7. Quantitative analysis revealed oxygen contents of around 55–57 atomic percent for the smaller particles, which would roughly correspond to Fe3O4, and up to 60 at%for the large crystallites, which would indicate stoichiometric Fe2O3. Definite identification, however, is questionable, due to error limits of the measurements of about 1– 2 at%. Heating to 800 C was needed for slow decomposition of the oxide and reaction of the remaining virtually pure Fe with the carbon support, which then proceeded similar to the cases of only slight initial oxidation described at the beginning of this section. It seems that this reaction started with those large crystallites not fully oxidised to Fe2O3.
3.4.
Ni–Fe alloy
Alloy particles were deposited on a-C by electron beam evaporation from a rod of ‘‘permalloy’’ with composition Ni80:Fe20. At substrate temperatures above 300 C, this re-
Fig. 6 – (a) Bright field image of Fe-oxide on a-C after heating to 700 C for 15 min. (b) Corresponding diffraction pattern from an area about twice as large as the field of view in (a), showing mainly the rings of Fe3O4, some of which are marked and indexed. (c) Selected area diffraction pattern of an individual large crystallite similar to that shown in (a) at centre, here of cubic c-Fe2O3. Note the presence of the (2 0 0) reflexion. (d) SAD pattern of another large crystallite, probably of Fe3O4, as the (2 0 0) reflexion is missing. Additional spots are from other particles in the direct vicinity.
CARBON
4 7 (2 0 0 9) 8 5 6–86 5
Fig. 7 – EDX spectra of Fe-oxide on a-C after annealing at 700 C. (a) Area with only small grains of Fe3O4 as shown in Fig. 6a, recorded with spot size of 200 nm. (b) Single crystallite of Fe2O3 like that shown in Fig. 6a, centre, recorded with a spot size of 50 nm. Note the relatively increased oxygen peak. The carbon peak is strongly reduced by absorption in the thick particle.
sulted in isolated, three-dimensional crystallites with f.c.c. structure. Electron diffraction analysis revealed a lattice constant of 0.357 nm, which is virtually the same as of pure Ni of 0.352 nm, taking into account the error limits of the measurement. EDX analyses of individual particles revealed the nominal composition and virtually no dispersion within the error margin of 1–2 atomic percent. Upon heating to 600 C and above, the reaction with the carbon substrate proceeded in a rather similar way as with pure Ni, although initial encapsulation of the particles by graphene layers seemed to be less perfect. Original Ni–Fe particles deposited on a-C at 500 C, as well as selected stages of the reaction at 650 C are shown in Fig. 8. In this example, only part of the original alloy particle has flown out of the graphite shell and spread on the substrate, while the rest remained trapped. A representative EDX spectrum of the spread part is shown in Fig. 9. Quantitative analysis yielded about the same composition as of the original particles, as well as of eventual remnants of the alloy left in the graphite shell, as seen in Fig. 8. No indications for segregation of the metals or carbide formation could either be detected by electron diffraction. It is interesting to compare the reactivity with that of pure Ni, which had been ana-
861
Fig. 8 – Top row, left: NiFe crystallites deposited at 500 C on a-C; right: selected area electron diffraction images after deposition (upper part) and after reaction at 650 C (bottom part). Note rings from graphite in the lower image as indicated. Beneath: four selected stages during the reaction with the support at 650 C. Figures at top right corners indicate time lapse in seconds. Note the strong contrast within the graphite shell, which indicates that a certain amount of alloy remained trapped. All are video images.
lysed in our earlier work [11]. From the video images, the speed of propagation of the reaction front of 0.5 nm was measured. This is somewhat smaller, but not too far from the value of about 0.85 nm/s of pure Ni at the same temperature, taking into account the error margins of the measurements. A rather peculiar behaviour of nucleation and reaction was observed with Ni80:Fe20 depositions under oxygen. Deposition at 250 C resulted in a finely dispersed phase of a mixed Ni–Fe-oxide. A definite identification by electron diffraction was not possible due to the diffuse ring pattern. During annealing up to 500 C (after deposition and oxygen admission was stopped), three-dimensional crystallites nucleated out of this phase, which consisted of virtually pure Ni, as detected by EDX. This is illustrated in Fig. 10. In electron diffraction, the scattered crystallites revealed f.c.c. structure and the lattice constant of Ni (0.352 nm), while the rings from the surrounding oxide were still diffuse. However, EDX analysis resulted in a composition corresponding to Ni–ferrite, NiFe2O4
862
CARBON
4 7 ( 2 0 0 9 ) 8 5 6 –8 6 5
Fig. 9 – EDX spectrum of spread NiFe on a-C after reaction at 650 C corresponding to a composition Ni79Fe21 (spot size 100 nm, accumulation time 200 s). Similar results were obtained for eventual remnants of alloy left in graphite shells, as well as for the original particles, as shown in Fig. 8. The Au M peak originates from scattering effects from the Au support grid.
(see Fig. 13). Apparently, the Ni crystallites were growing by partial decomposition of the Fe–Ni-oxide and selective incor-
poration of Ni diffusing from the neighbourhood. During deposition of Ni80:Fe20 at 360 C under oxygen, a finely dispersed phase of oxide plus scattered thee dimensional crystallites already nucleated simultaneously, and the behaviour during annealing at up to 500 C was the same as above. Upon heating further to 670 C, the larger crystallites started to react with the a-C substrate, as is typical for pure Ni. Selected video images taken during such reaction are shown in Fig. 11. After being partially encapsulated by graphite, the Ni escapes and, during spreading, graphitises the a-C substrates and also incorporates the metal from the surrounding oxide, which concomitantly decomposes. Fig. 12a shows a still image taken on plate after such a reaction in another, but similar experiment, with the sample at room temperature, in which areas of local EDX analyses are indicated. These revealed only Ni in the remaining metal cores in the centre of the reaction scene, but some Fe with contents of up to about 10 at% in the spread alloy. No oxygen was detected in these phases, only on the surrounding small particles, which assumed a composition corresponding to NiFe2O4, as mentioned above. In fact, selected area electron diffraction images ( Fig. 12b) showed rings from particles of Ni–ferrite and Ni–Fe alloy, and from graphite. It is worth noting that the overall metallic contents, averaged over large areas, still correspond to the original composition Ni80:Fe20. Only the content of oxygen is reduced during the reaction.
Fig. 10 – Selected video images recorded during annealing at 510 C of a Ni(Fe)-crystallite embedded in small particles of mixed Fe–Ni-oxide. Time lapse in b is 6 min, in c plus 4 min. Growth occurs by accumulation of material from the surrounding. Arrows in c mark coalescence with adjacent small particles. Note the rhombic dark contrast in b and c, which is typical for twinning in cubic crystallites.
Fig. 11 – Selected video images of the reaction of Ni crystallites, embedded in Ni–Fe-oxide, with the a-C support film at 670 C. Field of view is 380 nm · 280 nm. The reaction starts with encapsulation of the Ni crystallite by graphene layers, followed by partial outflow and spreading on the substrate, which is then graphitised. At the reaction front, the surrounding oxide decomposes while the metal is integrated in the spreading alloy (see also Fig. 12 and text).
CARBON
4 7 (2 0 0 9) 8 5 6–86 5
863
Fig. 12 – (a) This image was taken on plate after the reaction at 670 C of neighbouring Ni(Fe) particles in a Ni–Fe-oxide deposit on a-C. Numbered circles mark areas of EDX measurements shown in Fig. 13. The reaction started from two large crystallites (like that shown in Fig. 10) located at positions #1 and #2 by encapsulation in graphene layers and subsequent outflow. During spreading, the substrate was graphitised (note mottled contrast in the area left behind), and the surrounding small particles of mixed Fe(Ni)-oxide (see EDX spectra at positions 5–7) were integrated with concomitant decomposition of the oxide. The active, spreading aggregates contain some Fe but no oxygen. The remaining cores #1 and #2 are virtually pure Ni. (b) Selected area diffraction pattern selected from a similar experiment, but with somewhat larger ferrite particles. The rings from graphite are continuous, while the different abundances and strengths of reflexions on the rings from the Ni–ferrite particles, and from the spread Ni–Fe alloy aggregates indicate their different number densities and sizes.
Fig. 13 – EDX spectra after deposition of Ni–Fe-oxide on a-C at 360 C (top left) and after reaction at 670 C. Top right: the remnant particle in the centre of the reaction area (#1 in Fig. 12a) is virtually pure Ni. Bottom left: the spreading metal (#3 in Fig. 12a) contains some Fe, but no oxygen. Bottom right: the composition of the small particles in the surroundings (#5, 6, and 7 in Fig. 12a) corresponds to Ni–ferrite, NiFe2O4.
864
4.
CARBON
4 7 ( 2 0 0 9 ) 8 5 6 –8 6 5
Discussion and conclusion
Our comparative experiments with particles of pure Ni and Fe supported on a-C have revealed some similarities, but also striking differences in their reactions with carbon. Most noteworthy are their different tendencies to form carbide. Ni–carbide has been reported to decompose at above 430 C in vacuum [13]. Thus, it does not play a role neither during deposition of Ni particles on a-C at temperatures above 450 C nor during reactions at above 600 C. In the case of Fe, carbide was formed in our experiments either during deposition on a-C at or above 450 C or during the reaction of low temperature deposits of virtually pure Fe at above 600 C, and appeared to be stable up to at least 750 C. The observation of direct nucleation of large and isolated iron carbide particles on a-C at deposition temperatures as low as 450 C indicates that diffusion of Fe atoms on the a-C surface is much more effective than incorporation into the substrate at the site of impingement from the vapour beam. In addition, it appears very unlikely that carbide molecules or clusters would diffuse on the substrate. In fact, no traces of such reactions were found. Rather, the carbide must have formed during nucleation by simultaneous reaction with the a-C support. Interestingly, both Fe–carbide and virtually pure Fe were found to graphitise the carbon support under certain conditions. Generally, the reaction starts at about 600 C with diffusion of carbon atoms from the substrate either through the particle or on its surface, or both, and nucleation of a graphene layer on the surface. Subsequent layers nucleate at the metal–graphene interface, thus forming a graphite shell, which thickens inwards, thereby driving out the metal core. This has been observed with Ni particles in our earlier work cited above, and also here with Ni80:Fe20 in the case of permalloy. However, when starting from pure Fe, encapsulation is accompanied with the formation of carbide, and the graphite shell appears to be much less complete, so that the carbide flows away more easily than in the case of Ni. This may be one reason, why Ni is often deemed as the better catalyst for growing carbon nanotubes and fibres. The presence of oxygen during production of the particles and/or during the reaction with the carbon support affects the reaction paths of the metals differently. Virtually no effect on the graphitisation reaction was found for Ni, even when starting from NiO, as the oxide already decomposes at around 350 C. This corresponds to our earlier work, in which we have not seen any change of the high-temperature reactivity of Ni in the presence of oxygen at a residual gas pressure of 5 · 10 6 mbar [9]. In contrast, oxidation of Fe particles was found to strongly affect the reactivity with a-C, depending on the oxide phase. FeO was found to decompose at above 500 C, and the remaining metal coalesced into large crystallites of virtually pure b.c.c. Fe, which started to graphitise the a-C support at above 650 C, similar to the case of Fe3C, but somewhat slower. The reactivity of more strongly oxidised Fe deposits appeared to decrease with increasing amounts of bulk Fe3O4 and c-Fe2O3, while these phases slowly decomposed only well above 700 C. Graphitisation of the a-C support was only observed at 800 C, without intermediate formation of carbide.
The reason, why initial oxidation of Fe apparently prevents the formation of carbide, is not quite clear yet. If it were residual oxygen, as may be suspected, the amount would be below the detection limit of EDX. It is interesting to note that other authors have reported that iron oxide particles supported on a-C transformed to cementite upon heating in the range from 600 to 800 C, which then graphitised the carbon [14]. It may well be that this could be caused by differing experimental conditions, perhaps of the vacuum environment. The peculiar behaviour of oxidised Ni80:Fe20 deposits is probably the result of the different tendencies of the metals to form oxides. Apparently, the stronger binding of oxygen to iron than to nickel allows the Ni to segregate and nucleate as large crystallites, until the remaining phase assumes the composition and structure of Ni–ferrite, NiFe2O4, which is reported to form only above 450 C, but being stable up to at least 750 C [15]. Segregation of Ni into well separated crystallites implies diffusion of Ni atoms over long distances, up to several tens of nanometres at above 500 C, quite similar as during vapour deposition of Ni at such temperatures. At above 600 C, only the Ni crystallites graphitise the substrate surface. Upon spreading, the surrounding ferrite is destroyed by uptake of the Fe content and decomposition of the oxide. This is interesting for two reasons. First, it may seem to stay in contradiction to the initial segregation of Ni. However, segregation had started from a mixed oxide, and ferrite had not yet formed. Second, decomposition of the ferrite by excess Ni seems to be initiated by the concomitant reaction with the aC support, although the details are not quite clear yet. As a conclusion, the combination of in situ TEM experiments with electron diffraction and X-ray spectroscopy, although mostly not at an atomic level of resolution, have revealed interesting insight into differences of reaction mechanisms of Ni and Fe with a-C, which may also help in understanding other types of reactions of these materials, like growth of nanofibres or nanotubes.
R E F E R E N C E S
[1] Marsh H, Warburton AP. Catalysis of graphitization. J Appl Chem 1970;20:133–41. [2] Derbyshire FJ, Presland AEB, Trimm DL. The formation of graphite films by precipitation of carbon from nickel foils. Carbon 1972;10:114–5. [3] Baker RTK, Harris PS. The formation of filamentous carbon. In: Walker PL, Thrower PA, editors. Chemistry and physics of carbon. New York: Marcel Dekker; 1978. p. 83–165. [4] Lamber R, Jaeger N, Schultz-Ekloff G. Electron microscopy study of the interaction of Ni, Pd and Pt with carbon. Surf Sci 1988;197:402–14. [5] Jiao J, Seraphin S, Wang X, Withers JC. Preparation and properties of ferromagnetic carbon-coated Fe, Co, and Ni nanoparticles. J Appl Phys 1996;80:103–8. [6] De Jong KP, Geus JW. Carbon nanofibres: catalytic synthesis and applications. Catal Rev Sci Eng 2000;42:481–510. [7] Bokhonov B, Korchigan M. The formation of graphite encapsulated metal nanoparticles during mechanical activation and annealing of soot with iron and nickel. J Alloy and Compd 2002;333:308–20.
CARBON
4 7 (2 0 0 9) 8 5 6–86 5
[8] Helveg S, Lo´pes-Cartes C, Sehestad J, Hansen PL, Clausen BS, Rostrup-NielsenR J, Abild-Pedersen F, Noerskov JK. Atomic scale imaging of carbon nanofibre growth. Nature 2004;427:426–9. [9] Anton R, Reetz O, Schmidt AA. In situ TEM investigation of processes catalyzed by PdNi alloy particles on carbon substrates in the presence and absence of oxygen. J Catal 1994;149:474–85. [10] Anton R. In situ transmission electron microscopy study of the growth of Ni nanoparticles on amorphous carbon and of the graphitization of the support in the presence of hydrogen. J Mater Res 2005;20:1837–43. [11] Anton R. On the reaction kinetics of Ni with amorphous carbon. Carbon 2008;46:656–62.
865
[12] Anton R, Reetz O, Schmidt AA. Performance of turbomolecular pumps in an extended TEM specimen chamber equipped for in situ vapor deposition experiments. Ultramicroscopy 1992;41:303–16. [13] Nakagura S. Study of metallic carbides by electron diffraction Part 1. Formation and decomposition of nickel carbide. J Phys Soc Japan 1957;12:482. [14] Takai Y, Kida T. In situ high-resolution TEM observations of graphite formation using iron oxide nanoparticles. 16th Intern Microsc Congress, Sapporo 2006, Private communication. [15] Ceylan A, Oczan S, Ni C, Shah SI. Solid State Reaction Synthesis of NiFe2O4 nanoparticles. J Magn Magn Mater 2008;320:857–63.
4 7 ( 2 0 0 9 ) 8 5 6 –8 6 5
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
In situ TEM investigations of reactions of Ni, Fe and Fe–Ni alloy particles and their oxides with amorphous carbon R. Anton* Institut fu¨r Angewandte Physik, Universita¨t Hamburg, Jungiusstrasse 11, D-20355 Hamburg, Germany
A R T I C L E I N F O
A B S T R A C T
Article history:
Ni, Fe and Ni–Fe alloy particles were vapour deposited on thin films of amorphous carbon
Received 22 July 2008
(a-C) inside a specially equipped transmission electron microscope, and reactions with the
Accepted 20 November 2008
substrate were observed at elevated temperatures. The influence of oxidation of the parti-
Available online 3 December 2008
cles was also investigated. In contrast to Ni, which was found in earlier work to graphitise the carbon at above 600 C without bulk carbide being involved, pure Fe reacted with the a-C support at about 500 C to Fe3C, which graphitised the carbon similar to Ni, starting at about 600 C. No carbide was formed from oxidised Fe particles. FeO decomposes above 500 C, higher oxides (Fe3O4, Fe2O3) only above 750 C. The remaining Fe particles graphitised the carbon support directly. Alloy particles with composition Ni80:Fe20 (permalloy) graphitised a-C in the same way as pure Ni, without any phase separation. Annealing of a mixed phase of finely dispersed Ni–Fe-oxide or deposition of Ni–Fe under oxygen at above 300 C resulted in nucleation of three-dimensional crystallites of virtually pure Ni, which graphitised the carbon, while the remaining phase of small particles was converted to inactive Ni–ferrite, NiFe2O4. 2008 Elsevier Ltd. All rights reserved.
1.
Introduction
Catalytic reactions of transition metals with carbon have long been known, and the interest has recently revived, for example with regard to the formation of carbon nanotubes and fibres or encapsulation by carbon layers [1–8]. While in many experiments the carbon is supplied from the gas phase, we have investigated the catalytic graphitisation of thin films of amorphous carbon (a-C) by supported metal particles [9–11]. As has been described in detail in our earlier work for the case of Ni, the reaction starts with encapsulation of the particle by graphite layers followed by outflow of the metal and spreading on the substrate, which is concomitantly graphitised. Similar reactions also occur with other transition metals. In the case of Fe, graphitisation of carbon has been observed by other authors to proceed via the formation of Fe–carbide, whereas ambiguous results were found for Ni [7]. We did
not find indications for the existence of intermediate Ni–carbide in our in situ TEM experiments [10,11]. In the following, more detailed investigations on reactions of Fe and of a Ni–Fe alloy with carbon are presented. In particular, the role of oxidation of all three phases, Ni, Fe and Ni–Fe is addressed. This has so far not systematically been investigated. While it is known that Ni-oxide decomposes prior to the graphitisation reaction, without formation of intermediate carbide, it will be shown that oxidation of Fe prevents the formation of Fe–carbide, and, after decomposition of the oxide, virtually pure Fe graphitises the carbon directly, although only at somewhat higher temperatures.
2.
Experimental
The experiments were performed in a modified transmission electron microscope (Philips EM400), which was equipped
* Fax: +49 40 42838 7095. E-mail address: [email protected] 0008-6223/$ - see front matter 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2008.11.038
CARBON
4 7 (2 0 0 9) 8 5 6–86 5
with an additional ultra high vacuum chamber at the specimen stage of the microscope. More details have been described elsewhere [12]. The chamber was differentially pumped with a turbomolecular pump with magnetic bearings, which did not compromise the specified point resolution of 0.3 nm. The base pressure was in the mid 10 9 mbar range. Thin film substrates of amorphous carbon (a-C) with 20 nm thickness were prepared by vapour deposition of carbon on mica, floating off, and capture on TEM grids of molybdenum or gold. These were mounted in a heatable sample holder of the microscope, which contained a crucible of molybdenum, at the edge of which a Pt/Pt–Rh thermocouple was spot welded. The temperature reading was calibrated by melting experiments with tin and bismuth. In the microscope, the samples were outgassed at 600 C for several hours prior to the deposition and reaction experiments. The metals were evaporated from rods by electron bombardment and the flux was measured by collecting simultaneously formed ionised species. Permalloy particles were produced by evaporation from an alloy rod with composition Ni80:Fe20. Generally, the working pressure was about 1 · 10 8 mbar, but some depositions were performed under a partial pressure of oxygen of 5 · 10 6 mbar in order to obtain oxide particles. Substrate temperatures during deposition were in the range from 250 to 500 C. Immediately after deposition, the temperature was increased in steps up to 800 C, depending on the type and progress of the reaction. Temperature steps were typically 25 C each, so that the specimen stabilised (temperature and drift) within less than a minute. Sequences of images were recorded during depositions and reactions via a CCD camera in a computer via a firewire interface at a frame rate of about 10/s. By using an image intensifier, the electron illumination was reduced to about 0.1 A/cm2, at the routine operating voltage of 100 kV. Some annealing experiments were also performed in another microscope (Philips CM12), where images were recorded on plates. In any case, no indications of electron beam artefacts were found. This was verified by
857
comparing irradiated areas with non-irradiated areas during and after the reactions. Electron diffraction patterns were quantitatively analysed by calibration with reflexions from graphite eventually evolving during reactions and/or from additionally deposited MgO crystallites. High-resolution TEM images were obtained in another instrument (Philips CM300). Energy dispersive X-ray analysis (EDX) was employed in the CM12 in order to detect the local distribution of the metal and oxide phases on the substrate before and after the reactions.
3.
Results
3.1.
Ni and NiO
The graphitisation of a-C by Ni had been investigated in earlier work [9–11]. The details will not be repeated here. In summary, deposition of Ni on a-C at temperatures between 200 and 400 C in a vacuum of some 10 8 mbar yield three-dimensional, isolated metal particles. Upon heating, these start to react with the substrate at about 600 C. Initially, the particles are encapsulated by graphene layers, which nucleate at the metal surface and metal–graphene interface, respectively. This graphite shell is rather stable, and while thickening inwards, the metal core is eventually driven out, and spreads on the substrate, which is thereby graphitised at the surface. Measurements of the propagation speeds at various temperatures yielded the activation energy of 1.8 eV, in close agreement with a theoretical estimate of 1.6 eV [8,11]. Exposure of the specimen to a partial pressure of oxygen of 5 · 10 6 mbar has been found in earlier work to have no influence on the graphitisation reaction [9]. Likewise, initial oxidation of Ni does not play a role, as the oxide decomposes already at about 300 C, as was verified in recent experiments by X-ray spectroscopy (EDX) and electron diffraction. Decomposition of small oxide particles causes compacting, and an increase of contrast. At above 400 C, the remaining particles
Fig. 1 – TEM bright field and diffraction images taken after the reaction of NiO particles on a-C at 650 C for 10 min. Ni had been deposited under oxygen at 250 C, which yielded finely dispersed particles of NiO. These were reduced to f.c.c. Ni during heating to above 350 C. Further heating to above 600 C led to large scale coalescence, and the Ni crystallites reacted with the a-C substrate, starting with encapsulation with a graphite shell, followed by partial outflow and spreading on the substrate (arrows), which is then graphitised, too. Note spotty rings from f.c.c. Ni and continuous rings from graphite in the diffraction image.
858
CARBON
4 7 ( 2 0 0 9 ) 8 5 6 –8 6 5
of pure Ni coalesce, and above 600 C, larger aggregates start to react with the carbon substrate as was described above. A medium stage of this reaction is illustrated in Fig. 1.
3.2.
Fe
A typical reaction of Fe particles with the carbon support is illustrated in Fig. 2. Deposition of Fe on a-C at 300 C in a vacuum of some 10 8 mbar yielded finely dispersed particles with b.c.c. structure (not shown). Heating up to 600 C resulted in coalescence to larger, more isolated crystallites with mean size of about 20 nm. At around 700 C, some particles became active in spreading on the substrate, thereby growing by incorporation of neighbouring particles (see Fig. 2a). Areas
Fig. 2 – TEM bright field and selected area diffraction images taken during the reaction of Fe particles on a-C. (a, b) At 700 C, Fe particles are encapsulated by thin layers of graphite, and react to Fe3C, which spreads on the substrate and aggregates by coalescence with other particles (scattered spots in the diffraction image, some are marked with arrows), thereby graphitising the substrate. (c, d) After further heating to 725 C for 10 min, large patches of Fe3C have formed (an example of a diffraction pattern from such a particle is shown in d), and the substrate surface is overall graphitised. (e) Perspective view of the sample tilted by about 24. The arrows mark some graphite shells, which had been abandoned by their originating Fe particle (see text).
left free by the particles reveal their ‘‘footprints’’, which appear to be remnants of graphite shells, as well as mottled contrast, which indicates graphitisation of the substrate. At this stage, the diffraction image shows, besides rings from b.c.c. Fe particles, rings from graphite, as well as some scattered spots from Fe–carbide, Fe3C. Dark field images confirmed that these spots originate from the spreading particles. Further heating increased the speed of reaction, until all Fe particles were incorporated in large flat aggregates of carbide, and the substrate surface was overall graphitised (Fig. 2c). The carbide aggregates were mostly single crystals, but occurred in different orientations. The selected area diffraction image in Fig. 2d is just an example. The perspective view of the sample being tilted in the TEM (Fig. 2e) shows the rugged morphology of the graphitised substrate, as well as graphite shells, which formed on the particle surfaces before spreading. This is also shown in the high-resolution TEM image in Fig. 3. An iron particle is partly encapsulated by graphene layers (with spacing 0.335 nm), but part of the metal has moved away. The lattice spacing of the metal core was measured to 0.2030 ± 0.0005 nm. This value would correspond to both, planes of Fe(1 1 0) with spacing 0.2029 nm and of Fe3C(0 2 2) with spacing 0.2025 nm. In any case, the images shown in Fig. 2 strongly suggest that spreading only occurs after encapsulation and reaction to carbide. Deposition of Fe on a-C at 450 C resulted in isolated, three-dimensional particles of Fe3C, which were produced by direct reaction with the carbon support. This is illustrated in Fig. 4. Row (a) shows the state just after deposition. It is interesting to note that aggregation of Fe atoms into particles during deposition must have occurred before formation of carbide, as no traces of any reaction, e.g. graphite, could be
Fig. 3 – HRTEM image of a Fe particle partly encapsulated in graphene layers. Part of the metal has flown away, apparently along the channel, which is indicated by an arrow. Lattice spacings of 0.335 nm correspond to graphite, while spacings of 0.203 nm could originate from both b.c.c. Fe and cubic F3C (see text).
CARBON
4 7 (2 0 0 9) 8 5 6–86 5
859
seen in the bright field image (see arrows in Fig. 4c, left). This again indicates that the reaction of the original Fe particles is preceded by encapsulation, as was the case for deposition at lower temperatures.
3.3.
Fe-oxide
Partly oxidised Fe particles were obtained by short exposure of freshly deposited Fe to air during transfer to another TEM. Bright field imaging and diffraction revealed a thin layer of FeO at the surface of the particles, which can be seen in Fig. 5a. Correspondingly, X-ray spectra (EDX) showed a small peak of oxygen. Heating to 600 C caused coarsening of the metal particles and concomitant decomposition of the oxide.
Fig. 4 – TEM bright field and diffraction images recorded during the reaction of Fe–carbide particles with the a-C support film. (a) Deposition of Fe on a-C at 450 C yielded particles of Fe3C, as was verified by the corresponding diffraction pattern. (b) Same area after 10 min at 600 C: some particles have started to spread on the substrate, leaving traces of graphite behind. (c) After 10 min at 650 C, moving carbide aggregates have coalesced, and the whole substrate surface is graphitised (see rings marked in the diffraction image, not all are marked). In the bright field image, some remnants of graphite shells, which originate from the original particles, are marked by arrows.
detected on the still amorphous support film between the particles at this stage. This means that at 450 C, diffusion of Fe atoms on a-C is much faster than incorporation into carbide. On the other hand, the reaction of the particles to carbide is fast enough that virtually no pure Fe could be detected in the diffraction pattern. Rather, all rings could be attributed to Fe3C. During heating to 600 C, some particles started to react further with the substrate by spreading away and leaving traces of graphitisation behind, as is indicated by the mottled contrast (row b). Further heating to 650 C increased the reaction speed, and, while moving carbide aggregates coalesced, the substrate surface was overall graphitised, which is also apparent by the corresponding strong diffraction rings (row c). Some remnants of graphite shells, originating from initial encapsulation of the original particles can be
Fig. 5 – TEM bright field and diffraction images recorded during the reaction of slightly oxidised Fe with the a-C support film. Row a: Fe deposited on a-C at 410 C after exposure to air for about 10 min. Arrows in the bright field image indicate oxidation of the particle edges. The diffraction pattern shows weak rings from FeO, besides the dominant rings from b.c.c. Fe. Row b: after heating to 600 C for 15 min, the oxide has disappeared, and the remaining Fe particles coalesce. Row c: after 1 h at 700 C, Fe aggregates have graphitised the substrate surface. Arrows in the bright field image mark graphite shells around particles. The diffraction image shows rings from graphite in addition to the spotty rings from Fe.
860
CARBON
4 7 ( 2 0 0 9 ) 8 5 6 –8 6 5
The remaining virtually pure b.c.c. Fe particles coalesced into large, flat, irregularly shaped crystallites, which started to spread and graphitise the substrate only at about 700 C, without forming carbide, as is apparent from the diffraction image in Fig. 5c. Oxygen could no longer be detected by EDX at this stage. Extended exposure of freshly deposited Fe particles to air resulted in a mixture of FeO and Fe3O4, which slowly decomposed during heating to above 600 C, while at 700 C, the reaction of the remaining metal with the substrate proceeded similar to the case of only slight oxidation. Bulk Fe-oxide particles turned out to be rather stable, and decomposition and reaction with carbon was only observed to start at about 800 C. A typical reaction sequence is illustrated in Fig. 6. Fe was deposited on a-C at 350 C under oxygen at 5 · 10 6 mbar, yielding finely dispersed particles of Feoxide. The diffraction image exhibited diffuse rings, which could be attributed to various oxide phases. The sample was then transferred to another TEM (equipped with EDX), in which the annealing experiment was performed. Heating to 700 C caused some coarsening by coalescence, and the formation of a network of small oxide particles of mainly Fe3O4 with interspersed larger crystallites of both Fe3O4, but
mostly cubic c-Fe2O3. This stage is shown in Fig. 6a–d. Unambiguous identification is generally difficult because of the similarity of the diffraction patterns, but the presence of the (2 0 0) reflexion is indicative for Fe2O3. Representative EDX spectra are shown in Fig. 7. Quantitative analysis revealed oxygen contents of around 55–57 atomic percent for the smaller particles, which would roughly correspond to Fe3O4, and up to 60 at%for the large crystallites, which would indicate stoichiometric Fe2O3. Definite identification, however, is questionable, due to error limits of the measurements of about 1– 2 at%. Heating to 800 C was needed for slow decomposition of the oxide and reaction of the remaining virtually pure Fe with the carbon support, which then proceeded similar to the cases of only slight initial oxidation described at the beginning of this section. It seems that this reaction started with those large crystallites not fully oxidised to Fe2O3.
3.4.
Ni–Fe alloy
Alloy particles were deposited on a-C by electron beam evaporation from a rod of ‘‘permalloy’’ with composition Ni80:Fe20. At substrate temperatures above 300 C, this re-
Fig. 6 – (a) Bright field image of Fe-oxide on a-C after heating to 700 C for 15 min. (b) Corresponding diffraction pattern from an area about twice as large as the field of view in (a), showing mainly the rings of Fe3O4, some of which are marked and indexed. (c) Selected area diffraction pattern of an individual large crystallite similar to that shown in (a) at centre, here of cubic c-Fe2O3. Note the presence of the (2 0 0) reflexion. (d) SAD pattern of another large crystallite, probably of Fe3O4, as the (2 0 0) reflexion is missing. Additional spots are from other particles in the direct vicinity.
CARBON
4 7 (2 0 0 9) 8 5 6–86 5
Fig. 7 – EDX spectra of Fe-oxide on a-C after annealing at 700 C. (a) Area with only small grains of Fe3O4 as shown in Fig. 6a, recorded with spot size of 200 nm. (b) Single crystallite of Fe2O3 like that shown in Fig. 6a, centre, recorded with a spot size of 50 nm. Note the relatively increased oxygen peak. The carbon peak is strongly reduced by absorption in the thick particle.
sulted in isolated, three-dimensional crystallites with f.c.c. structure. Electron diffraction analysis revealed a lattice constant of 0.357 nm, which is virtually the same as of pure Ni of 0.352 nm, taking into account the error limits of the measurement. EDX analyses of individual particles revealed the nominal composition and virtually no dispersion within the error margin of 1–2 atomic percent. Upon heating to 600 C and above, the reaction with the carbon substrate proceeded in a rather similar way as with pure Ni, although initial encapsulation of the particles by graphene layers seemed to be less perfect. Original Ni–Fe particles deposited on a-C at 500 C, as well as selected stages of the reaction at 650 C are shown in Fig. 8. In this example, only part of the original alloy particle has flown out of the graphite shell and spread on the substrate, while the rest remained trapped. A representative EDX spectrum of the spread part is shown in Fig. 9. Quantitative analysis yielded about the same composition as of the original particles, as well as of eventual remnants of the alloy left in the graphite shell, as seen in Fig. 8. No indications for segregation of the metals or carbide formation could either be detected by electron diffraction. It is interesting to compare the reactivity with that of pure Ni, which had been ana-
861
Fig. 8 – Top row, left: NiFe crystallites deposited at 500 C on a-C; right: selected area electron diffraction images after deposition (upper part) and after reaction at 650 C (bottom part). Note rings from graphite in the lower image as indicated. Beneath: four selected stages during the reaction with the support at 650 C. Figures at top right corners indicate time lapse in seconds. Note the strong contrast within the graphite shell, which indicates that a certain amount of alloy remained trapped. All are video images.
lysed in our earlier work [11]. From the video images, the speed of propagation of the reaction front of 0.5 nm was measured. This is somewhat smaller, but not too far from the value of about 0.85 nm/s of pure Ni at the same temperature, taking into account the error margins of the measurements. A rather peculiar behaviour of nucleation and reaction was observed with Ni80:Fe20 depositions under oxygen. Deposition at 250 C resulted in a finely dispersed phase of a mixed Ni–Fe-oxide. A definite identification by electron diffraction was not possible due to the diffuse ring pattern. During annealing up to 500 C (after deposition and oxygen admission was stopped), three-dimensional crystallites nucleated out of this phase, which consisted of virtually pure Ni, as detected by EDX. This is illustrated in Fig. 10. In electron diffraction, the scattered crystallites revealed f.c.c. structure and the lattice constant of Ni (0.352 nm), while the rings from the surrounding oxide were still diffuse. However, EDX analysis resulted in a composition corresponding to Ni–ferrite, NiFe2O4
862
CARBON
4 7 ( 2 0 0 9 ) 8 5 6 –8 6 5
Fig. 9 – EDX spectrum of spread NiFe on a-C after reaction at 650 C corresponding to a composition Ni79Fe21 (spot size 100 nm, accumulation time 200 s). Similar results were obtained for eventual remnants of alloy left in graphite shells, as well as for the original particles, as shown in Fig. 8. The Au M peak originates from scattering effects from the Au support grid.
(see Fig. 13). Apparently, the Ni crystallites were growing by partial decomposition of the Fe–Ni-oxide and selective incor-
poration of Ni diffusing from the neighbourhood. During deposition of Ni80:Fe20 at 360 C under oxygen, a finely dispersed phase of oxide plus scattered thee dimensional crystallites already nucleated simultaneously, and the behaviour during annealing at up to 500 C was the same as above. Upon heating further to 670 C, the larger crystallites started to react with the a-C substrate, as is typical for pure Ni. Selected video images taken during such reaction are shown in Fig. 11. After being partially encapsulated by graphite, the Ni escapes and, during spreading, graphitises the a-C substrates and also incorporates the metal from the surrounding oxide, which concomitantly decomposes. Fig. 12a shows a still image taken on plate after such a reaction in another, but similar experiment, with the sample at room temperature, in which areas of local EDX analyses are indicated. These revealed only Ni in the remaining metal cores in the centre of the reaction scene, but some Fe with contents of up to about 10 at% in the spread alloy. No oxygen was detected in these phases, only on the surrounding small particles, which assumed a composition corresponding to NiFe2O4, as mentioned above. In fact, selected area electron diffraction images ( Fig. 12b) showed rings from particles of Ni–ferrite and Ni–Fe alloy, and from graphite. It is worth noting that the overall metallic contents, averaged over large areas, still correspond to the original composition Ni80:Fe20. Only the content of oxygen is reduced during the reaction.
Fig. 10 – Selected video images recorded during annealing at 510 C of a Ni(Fe)-crystallite embedded in small particles of mixed Fe–Ni-oxide. Time lapse in b is 6 min, in c plus 4 min. Growth occurs by accumulation of material from the surrounding. Arrows in c mark coalescence with adjacent small particles. Note the rhombic dark contrast in b and c, which is typical for twinning in cubic crystallites.
Fig. 11 – Selected video images of the reaction of Ni crystallites, embedded in Ni–Fe-oxide, with the a-C support film at 670 C. Field of view is 380 nm · 280 nm. The reaction starts with encapsulation of the Ni crystallite by graphene layers, followed by partial outflow and spreading on the substrate, which is then graphitised. At the reaction front, the surrounding oxide decomposes while the metal is integrated in the spreading alloy (see also Fig. 12 and text).
CARBON
4 7 (2 0 0 9) 8 5 6–86 5
863
Fig. 12 – (a) This image was taken on plate after the reaction at 670 C of neighbouring Ni(Fe) particles in a Ni–Fe-oxide deposit on a-C. Numbered circles mark areas of EDX measurements shown in Fig. 13. The reaction started from two large crystallites (like that shown in Fig. 10) located at positions #1 and #2 by encapsulation in graphene layers and subsequent outflow. During spreading, the substrate was graphitised (note mottled contrast in the area left behind), and the surrounding small particles of mixed Fe(Ni)-oxide (see EDX spectra at positions 5–7) were integrated with concomitant decomposition of the oxide. The active, spreading aggregates contain some Fe but no oxygen. The remaining cores #1 and #2 are virtually pure Ni. (b) Selected area diffraction pattern selected from a similar experiment, but with somewhat larger ferrite particles. The rings from graphite are continuous, while the different abundances and strengths of reflexions on the rings from the Ni–ferrite particles, and from the spread Ni–Fe alloy aggregates indicate their different number densities and sizes.
Fig. 13 – EDX spectra after deposition of Ni–Fe-oxide on a-C at 360 C (top left) and after reaction at 670 C. Top right: the remnant particle in the centre of the reaction area (#1 in Fig. 12a) is virtually pure Ni. Bottom left: the spreading metal (#3 in Fig. 12a) contains some Fe, but no oxygen. Bottom right: the composition of the small particles in the surroundings (#5, 6, and 7 in Fig. 12a) corresponds to Ni–ferrite, NiFe2O4.
864
4.
CARBON
4 7 ( 2 0 0 9 ) 8 5 6 –8 6 5
Discussion and conclusion
Our comparative experiments with particles of pure Ni and Fe supported on a-C have revealed some similarities, but also striking differences in their reactions with carbon. Most noteworthy are their different tendencies to form carbide. Ni–carbide has been reported to decompose at above 430 C in vacuum [13]. Thus, it does not play a role neither during deposition of Ni particles on a-C at temperatures above 450 C nor during reactions at above 600 C. In the case of Fe, carbide was formed in our experiments either during deposition on a-C at or above 450 C or during the reaction of low temperature deposits of virtually pure Fe at above 600 C, and appeared to be stable up to at least 750 C. The observation of direct nucleation of large and isolated iron carbide particles on a-C at deposition temperatures as low as 450 C indicates that diffusion of Fe atoms on the a-C surface is much more effective than incorporation into the substrate at the site of impingement from the vapour beam. In addition, it appears very unlikely that carbide molecules or clusters would diffuse on the substrate. In fact, no traces of such reactions were found. Rather, the carbide must have formed during nucleation by simultaneous reaction with the a-C support. Interestingly, both Fe–carbide and virtually pure Fe were found to graphitise the carbon support under certain conditions. Generally, the reaction starts at about 600 C with diffusion of carbon atoms from the substrate either through the particle or on its surface, or both, and nucleation of a graphene layer on the surface. Subsequent layers nucleate at the metal–graphene interface, thus forming a graphite shell, which thickens inwards, thereby driving out the metal core. This has been observed with Ni particles in our earlier work cited above, and also here with Ni80:Fe20 in the case of permalloy. However, when starting from pure Fe, encapsulation is accompanied with the formation of carbide, and the graphite shell appears to be much less complete, so that the carbide flows away more easily than in the case of Ni. This may be one reason, why Ni is often deemed as the better catalyst for growing carbon nanotubes and fibres. The presence of oxygen during production of the particles and/or during the reaction with the carbon support affects the reaction paths of the metals differently. Virtually no effect on the graphitisation reaction was found for Ni, even when starting from NiO, as the oxide already decomposes at around 350 C. This corresponds to our earlier work, in which we have not seen any change of the high-temperature reactivity of Ni in the presence of oxygen at a residual gas pressure of 5 · 10 6 mbar [9]. In contrast, oxidation of Fe particles was found to strongly affect the reactivity with a-C, depending on the oxide phase. FeO was found to decompose at above 500 C, and the remaining metal coalesced into large crystallites of virtually pure b.c.c. Fe, which started to graphitise the a-C support at above 650 C, similar to the case of Fe3C, but somewhat slower. The reactivity of more strongly oxidised Fe deposits appeared to decrease with increasing amounts of bulk Fe3O4 and c-Fe2O3, while these phases slowly decomposed only well above 700 C. Graphitisation of the a-C support was only observed at 800 C, without intermediate formation of carbide.
The reason, why initial oxidation of Fe apparently prevents the formation of carbide, is not quite clear yet. If it were residual oxygen, as may be suspected, the amount would be below the detection limit of EDX. It is interesting to note that other authors have reported that iron oxide particles supported on a-C transformed to cementite upon heating in the range from 600 to 800 C, which then graphitised the carbon [14]. It may well be that this could be caused by differing experimental conditions, perhaps of the vacuum environment. The peculiar behaviour of oxidised Ni80:Fe20 deposits is probably the result of the different tendencies of the metals to form oxides. Apparently, the stronger binding of oxygen to iron than to nickel allows the Ni to segregate and nucleate as large crystallites, until the remaining phase assumes the composition and structure of Ni–ferrite, NiFe2O4, which is reported to form only above 450 C, but being stable up to at least 750 C [15]. Segregation of Ni into well separated crystallites implies diffusion of Ni atoms over long distances, up to several tens of nanometres at above 500 C, quite similar as during vapour deposition of Ni at such temperatures. At above 600 C, only the Ni crystallites graphitise the substrate surface. Upon spreading, the surrounding ferrite is destroyed by uptake of the Fe content and decomposition of the oxide. This is interesting for two reasons. First, it may seem to stay in contradiction to the initial segregation of Ni. However, segregation had started from a mixed oxide, and ferrite had not yet formed. Second, decomposition of the ferrite by excess Ni seems to be initiated by the concomitant reaction with the aC support, although the details are not quite clear yet. As a conclusion, the combination of in situ TEM experiments with electron diffraction and X-ray spectroscopy, although mostly not at an atomic level of resolution, have revealed interesting insight into differences of reaction mechanisms of Ni and Fe with a-C, which may also help in understanding other types of reactions of these materials, like growth of nanofibres or nanotubes.
R E F E R E N C E S
[1] Marsh H, Warburton AP. Catalysis of graphitization. J Appl Chem 1970;20:133–41. [2] Derbyshire FJ, Presland AEB, Trimm DL. The formation of graphite films by precipitation of carbon from nickel foils. Carbon 1972;10:114–5. [3] Baker RTK, Harris PS. The formation of filamentous carbon. In: Walker PL, Thrower PA, editors. Chemistry and physics of carbon. New York: Marcel Dekker; 1978. p. 83–165. [4] Lamber R, Jaeger N, Schultz-Ekloff G. Electron microscopy study of the interaction of Ni, Pd and Pt with carbon. Surf Sci 1988;197:402–14. [5] Jiao J, Seraphin S, Wang X, Withers JC. Preparation and properties of ferromagnetic carbon-coated Fe, Co, and Ni nanoparticles. J Appl Phys 1996;80:103–8. [6] De Jong KP, Geus JW. Carbon nanofibres: catalytic synthesis and applications. Catal Rev Sci Eng 2000;42:481–510. [7] Bokhonov B, Korchigan M. The formation of graphite encapsulated metal nanoparticles during mechanical activation and annealing of soot with iron and nickel. J Alloy and Compd 2002;333:308–20.
CARBON
4 7 (2 0 0 9) 8 5 6–86 5
[8] Helveg S, Lo´pes-Cartes C, Sehestad J, Hansen PL, Clausen BS, Rostrup-NielsenR J, Abild-Pedersen F, Noerskov JK. Atomic scale imaging of carbon nanofibre growth. Nature 2004;427:426–9. [9] Anton R, Reetz O, Schmidt AA. In situ TEM investigation of processes catalyzed by PdNi alloy particles on carbon substrates in the presence and absence of oxygen. J Catal 1994;149:474–85. [10] Anton R. In situ transmission electron microscopy study of the growth of Ni nanoparticles on amorphous carbon and of the graphitization of the support in the presence of hydrogen. J Mater Res 2005;20:1837–43. [11] Anton R. On the reaction kinetics of Ni with amorphous carbon. Carbon 2008;46:656–62.
865
[12] Anton R, Reetz O, Schmidt AA. Performance of turbomolecular pumps in an extended TEM specimen chamber equipped for in situ vapor deposition experiments. Ultramicroscopy 1992;41:303–16. [13] Nakagura S. Study of metallic carbides by electron diffraction Part 1. Formation and decomposition of nickel carbide. J Phys Soc Japan 1957;12:482. [14] Takai Y, Kida T. In situ high-resolution TEM observations of graphite formation using iron oxide nanoparticles. 16th Intern Microsc Congress, Sapporo 2006, Private communication. [15] Ceylan A, Oczan S, Ni C, Shah SI. Solid State Reaction Synthesis of NiFe2O4 nanoparticles. J Magn Magn Mater 2008;320:857–63.