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Observation of local changes of “carbon-to-metal ratio” in the growth mechanism of carbon nanostructures grown from FePd-based and Fe3C catalysts by pyrolysis of ferrocene and dichlorocyclooctadiene-palladium mixtures: the crucial role of Cl

One of the major challenges in the field of carbon nanomaterials consists of understanding their growth mechanism dynamics. Recent reports have shown preliminary steady state chemical vapour deposition (CVD) experiments for the encapsulation of FePd-based alloys inside carbon-based nanostructures. However, very little is known about their growth mechanism dynamics. Here we investigate the possible presence of local changes of “carbon-to-metal ratio” in the growth mechanism of carbon nanostructures grown from FePd-based and Fe3C catalyst-particles by steady state CVD and viscous-boundary-layer chemical vapour synthesis (CVS) experiments involving the pyrolysis of ferrocene/dichlorocyclooctadiene-palladium mixtures and of ferrocene/dichlorobenzene mixtures. In the first case, we demonstrate the observation of an unusual growth mechanism in which a direct transition from a spherically elongated carbon-onion-like (CNOs-like) morphology (low carbon-to-metal ratio condition) to a carbon fiber-like morphology (high carbon-to-metal ratio condition) is present within the same carbon structure. We attribute such transitions to the variation of the Cl-radicals concentration during the pyrolysis process, which implies a variation in the local “carbon to metal ratio” parameter. The reported mechanism is then compared in detail with that of Fe3C filled CNTs obtained by CVD of ferrocene/dichlorobenzene mixtures and with those reported in the literature for other transition metal catalyst systems. In contrast, when a viscous boundary layer is created between the pyrolysed ferrocene/dichlorocyclooctadiene-palladium precursors and a rough surface, radial CNT structures filled with large quantities of both FePd3 and Fe3C crystals are found as major reaction products. The presence of filled-CNOs within the radial structures-core (resulting from the homogeneous nucleation of particles in the viscous boundary layer) suggests that fluctuations in the local “carbon-to-metal ratio” are also present in this type of mechanism. The morphological, cross-sectional and structural properties of the obtained carbon structures are analyzed in detail by SEM, TEM, STEM, HRTEM, ED and X-ray diffraction.

In particular, considering previous works on the encapsulation of metal-particles inside CNTs by arc-discharge processes, 6-8 the work by Elliott et al. had a great signicance since it attempted to create a general qualitative carbon nanostructures growth map on the basis of the cumulative results in literature. This important growth map is based on two main growth parameters: the local-path carbon-to-metal ratio (LCM) and the global carbon-to-metal ratio (GCM). LCM refers to metal particles having the same local conditions along the same spatial pathway, while the GCM determines the weighting between different LCMs. 6 Considering the growth map of Elliott et al. 6 it is possible to assume that the formation of CNOs is favourable only with slow cooling rates and low carbon supply or in conditions where the diffusion carbon ux (diffusion toward the particle surface) is dominant. Therefore two types of carbon supply can be identied: type 1 expulsion (from the particle core to the external surface) and type 2 deposition (from the external surface to the particle core). 6 For slow growth processes the nal product was found to depend on the difference between the particle-graphite interface energy and the vacuum surface energies g, and the energy to bend the graphite sheets. 6 In particular the formation of CNOs has been associated to growth-conditions with low g interaction. 6 Thus, according with this map, a low value of (local) carbon to metal ratio results in a high metal nanoparticle count and consequently rapid consumption of carbon feedstock which tends to diffuse into the nucleated particles. As a consequence the formation of a metastable metal carbide can be expected. The decomposition of such carbide phase would then drive the ow of carbon within the particle core toward the particlesurface where the formation of graphitic layers will take place. The most likely product for this kind of growth conditions is therefore a metal nanocrystal encapsulated in a graphitic shell. 6 Differently a higher value of carbon to metal ratio would result in the formation of carbon nanotube structures due to the higher value of local carbon supply. The highest carbon to metal ratio regime will then result in the formation of urchin-like nanotube structures, induced by a fast nucleation process or spring like-structures and nanotube structures. 6 It seems therefore clear that the (local) carbon to metal ratio parameter controls the formation mechanism of single carbonnanostructures products and that, following the reported map, no hybrid CNTs/CNOs structures have been yet observed.
Further reports of the growth mechanism and lling process of metal-lled CNTs structures were also given in 1999 by Demoncy et al. Indeed it was shown that the presence of sulfur in arc discharge methods has a key role in promoting the lling rates of numerous type of metals such as chromium, nickel and germanium within the CNTs core. 7,8 A nucleation and growth mechanism consisting of 4 main steps was suggested as follows: a rst step involving the growth of CNTs from a carbon-rich metal particle deposited on the cathode. A second step in which sulfur promotes the carbon precipitation and is then trapped by the metallic material. A third step in which the Senriched liquid will then ow inside the CNTs core and a fourth step involving the solidication of the metal-lling material. 7,8 Recently CVD methods have largely replaced arc-production approaches for the achievement of single growth products. These methods can indeed guarantee LCM control by careful reactor design and careful control of process parameters. [9][10][11][12][13][14][15][16] However, in CVD processes the lling rate of the obtained carbon nanostructures is generally strongly dependent on the stoichiometry of the used metallocene precursor; thus the global carbon to metal ratio (GCM) is xed by the stoichiometry of the used metallocene molecules (for steady state conditions). The results of these experiments generally yield CNTs partially lled with metal or interstitial-alloy particles, rather than long nanowires. Specically in these experiments the nucleation and growth of the lled CNTs structures is generally described by a vapor liquid solid (VLS) mechanism as follows: [24][25][26][27] (1) Exothermic catalytic decomposition of hydrocarbons at the surface of the transition metal-carbon nanoparticles at elevated temperature.
(2) The carbon species diffuse into the metal nanoparticle until the solubility limit is reached.
(4) Due to carbide instability, the carbon precipitates on the particles surface and crystallizes into energetically stable graphitic carbon (endothermic process).
(5) Alternatively, a high carbon ux at the nanoparticle surface can also result in graphitic shell formation.
(6) The temperature gradient created by the exo-and endothermal processes would then drive the growth of the carbon nanostructure.
These growth-mechanism features are generally accepted for the description of CNTs growth. However several issues related to the control of the evaporation process of the used precursors still remain and can strongly limit the control of the LCM parameter. Indeed recent reports have shown that for CVD experiments performed in presence of dichlorobenzene/ ferrocene mixtures 28-37 the presence of in-homogeneities in the pyrolysed metallocene vapour can lead to a variation in the morphology of the grown lms in conditions of high pyrolysis temperatures. [28][29][30][31] The presence of such in-homogeneities has been attributed to the physical time-scale necessary for reaching the desired temperature of the preheater used for the precursorevaporation. Within this time-scale, the inhomogeneous evaporation rate of the precursor (i.e. not all the grains of the metallocene reach the melting status simultaneously) will translate in a not homogeneous pyrolysed vapour. Furthermore, due to the presence of Cl radicals which spontaneously form CCl 4 species, the use of extremely low ow rates may change the carbon to metal ratio-value leading to the formation of an CNOs-product. 30 The presence of Cl radicals in a CVD reaction is therefore expected to change for a limited time the LCM parameter.
Interestingly, another type of synthesis method known as "viscous boundary layer chemical vapour synthesis" which has recently attracted important attention, tends to exploit local pathway uctuations by deliberately creating a viscous boundary layer between a rough surface and a laminar metallocene/Ar ow. Such viscous boundary layer has been reported to induce (1) the homogeneous nucleation of spherical metal particles in the pyrolysed metallocene vapour and (2) the subsequent growth of radial CNTs morphologies characterized by exceptional metal lling rates. [17][18][19][20] In addition, in these type of experiments the metallocene evaporation temperature has been reported to play a crucial role in selecting the growth of specic CNTs or CNOs morphologies. 20 So far these studies have been mainly performed for the specic case of Fe-, Ni-or Cocontaining metallocenes. [17][18][19][20] Interestingly recent reports have shown preliminary steady state CVD experiments for the encapsulation of FePd-based alloys inside carbon-based nanostructures. However, very little is known about their growth mechanism dynamics. [21][22][23] Indeed the formation of unusual CNTs morphologies has been reported as result of low vapour ow-rate (steady state ow) pyrolysis of ferrocene/ dichlorocyclooctadiene-palladium mixtures.
In this work we investigate in higher detail the growth mechanism of carbon nanostructures obtained by pyrolysis of Fe-and Pd-containing metallocene-like precursor in steady state CVD and viscous boundary layer CVS methods. In particular we report the observation of an unusual growth mechanism in which a direct transition from a CNOs morphology to a carbon ber-like morphology is observed within the same carbon structure. This is found in conditions of low vapour owrate (steady state ow) CVD involving pyrolysis of ferrocene/ dichlorocyclooctadiene-palladium mixtures. The cross sectional morphology, structure and growth mechanism of the obtained structures are investigated in detail by transmission electron microscopy (TEM), high resolution TEM (HRTEM) and electron diffraction (ED). The observed growth mechanism is then compared with of Fe 3 C lled CNTs produced by CVD of ferrocene/dichlorobenzene mixtures. In contrast, for experiments performed in the presence of a viscous boundary layer between the pyrolysed metallocene-like precursors and the used rough surface of the substrate, radial CNTs structures lled with FePd 3 and Fe 3 C crystals are found as major product in the reaction. This latter result conrms the existence of the radial structure growth mechanism also in presence of Fe and Pd species. However much lower lling rates with respect to those previously observed by Boi et al. in the case of single ferrocene or mixed metallocenes-pyrolysis are found due to the low solubility of carbon in FePd 3 alloys. The morphological, crosssectional and structural properties of the obtained radial structure are analyzed in detail by SEM, TEM, scanning TEM (STEM), HRTEM and X-ray diffraction.

Experimental
The used CVD/CVS system consisted of one pre-heater zone, one reaction zone and a quartz tube reactor. The pre-heater and reaction zone furnaces are mounted in a rail-system. This allows to perform a fast cooling (quench) by sliding the reaction zone furnace along the rail system aer each reaction. Note that two methods of precursor evaporation have been used. In the method-1, the precursors are placed in proximity of the substrate used for the product-deposition and evaporated aer sliding the main-reaction zone furnace along the rail system (see ESI Fig. 1 †). This allows a fast evaporation process.
Instead in the method-2 (ESI Fig. 2 †) the reactants are evaporated from the preheater-zone. Due to the very low evaporation temperature used in some of the experiments, an inhomogeneous evaporation rate of the precursor (i.e. not all the grains of ferrocene reach the melting status simultaneously) is expected. Two schematics of the used CVD reactors can be found in ESI Fig. 1 and 2. † See Fig. 3

and 4 † ESI for table
showing the molar ratios of the used reactants. The following experimental conditions were used: Hybrid CNO/CNF-like structures: 90 mg of dichlorocyclooctadiene-palladium (0.00032 mol) and 90 mg of ferrocene (0.00048 mol) were evaporated at 300 C (method-1) and pyrolyzed at 1050 C (molar ratio of the reactants: 1.53475). An Ar ow rate of 10-12 ml min À1 was used. The used quartz tube reactor consisted of the following dimensions: a quartz tube with a length of 1.5 m, an outer diameter of 22 mm and a wall thickness of 2.5 mm. Note that in this type of experiments no preheater was used; instead the reactants were placed in proximity of a Si/SiO 2 (111) substrate used for the productdeposition at a distance of approximately 3 cm. The reaction time was of approximately 10 min.
Fe 3 C lled CNTs type 1 : 0.05 ml of dichlorobenzene (0.00042 mol) and 60 mg of ferrocene (0.00032 mol) were evaporated at 300-350 C (method-1) and pyrolysed at 990 C (molar ratio of the reactants: 0.76010). An Ar ow rate of 10-12 ml min À1 was used. The used quartz tube reactor consisted of the following dimensions: a quartz tube with a length of 1.5 m, an outer diameter of 22 mm and a wall thickness of 2.5 mm. A Si/SiO 2 substrate (111) was placed in the reaction zone for the deposition of the growth product. The reaction time was of approximately 10 min. Note that in this type of experiments no preheater was used; instead the reactants were placed in proximity of a Si/SiO 2 substrate used for the product-deposition at a distance of approximately 3 cm. In this region of the reactor a temperature of 300-350 C was measured with a thermocouple. Fe 3 C lled CNTs type 2 : 0.15 ml of dichlorobenzene (0.00127 mol) and 1.5 g of ferrocene (0.00806 mol) were evaporated at 70 C (method-2) and pyrolysed at 990 C (molar ratio of the reactants: 6.33423). An Ar ow rate of 5 ml min À1 was used. The reaction time was of 1 h. The used quartz tube reactor consisted of the following dimensions: a quartz tube with a length of 1.5 m, an inner diameter of 44 mm and a wall thickness of 3 mm. A Si/SiO 2 substrate (111) was placed in the reaction zone for the deposition of the growth product.
Radial CNTs structures led with FePd 3 : 1.1 g of ferrocene (0.00591 mol) and 0.3 g of dichlorocyclooctadiene-palladium (0.00105 mol) were evaporated at the temperature of approximately 200 C (method-2) and pyrolysed at the temperature of 990 C (molar ratio of the reactants: 5.62742). An Ar ow rate of 11 ml min À1 was used. The reaction time was of 1 h. The used quartz tube reactor consisted of the following dimensions: a quartz tube with a length of 1.5 m, an inner diameter of 44 mm and a wall thickness of 3 mm. In this latter case the radial structures are grown in the viscous boundary layer created by introducing a rough surface (roughness in the order of 100 micrometres) in the deposition zone of the reactor. A disappearance of the radial structure morphology was observed when high evaporation temperatures of approximately 350-500 C were used. Instead relatively low evaporation temperatures favoured the formation of the radial structures.
The characterization and growth mechanism investigation was performed with the following technique: TEM, HRTEM, (selective area) ED (SAED) with a 200 kV American FEI Tecnai G 2 F20. For these analyses the as grown structures were rst dispersed in samples bottles with ethanol. The TEM samples were then prepared by depositing a small quantity of the as dispersed material on the top of a TEM grid comprising a holey carbon lm and a copper grid-frame. Note that no ultrasonication method was used for the dispersion. SEM analyses were performed with a JSM-7500F at 5-20 kV. XRD analyses were performed with an Empyrean Panalytical diffractometer (Cu Ka with l ¼ 0.154 nm). The characterization of the raw data extracted from the as measured XRD pattern was then performed by conversion of the .xrdml les into a .gsa format for Rietveld renement analyses. The Rietveld method, which uses the least-squares approach to match a theoretical line prole to the diffractogram, was used to identify and estimate the relative abundances of the encapsulated phases from the area enclosed by the diffraction peaks. This method allowed also for the extraction of the unit-cell parameters of the crystal phases encapsulated inside the grown carbon nanostructures.

Results and discussion
The morphology of the unusual carbon structures obtained with the rst approach was rstly revealed by SEM micrographs. As shown in Fig. 1 these ber-like structures (CNFs) exhibit an unusual urchin-like morphology and appear to grow in all the areas of the substrate. In particular Fig. 1A shows the morphology of these structures from a top view, while Fig. 1B shows the morphology with a higher detail. Further crosssectional analyses were then considered with the use of TEM.
As shown in Fig. 2A these analyses revealed the presence of a cross-sectional morphology characterized by two main regions. A base region comprising unusual elongated approximately spherical CNOs-like particles which can be hollow or can encapsulate FePd-based alloys (see red arrow), and an intermediate region with a ber-like structure which terminates with a capping-like closure (see inset). The observation of a direct transition from an CNOs-like morphology into a ber-like morphology (CNF) is very unusual and to the best of our knowledge has not been reported in previous literature reports. Note that, as shown in Fig. 2A with the magenta arrow and in Fig. 2B with an higher detail the transition from an CNOs-like morphology into a CNF-like morphology appears to be linked to a change of shape of the CNOs-like particles shells and volume. This observation is also conrmed by TEM micrographs of the transition area taken with a higher detail (see magenta arrow in Fig. 2C).
Considering the growth mechanism of carbon nanostructures described in the qualitative map of Elliott et al., 6 it is possible to notice that none of the described morphologies has similarity with that shown in Fig. 2. The formation of CNOs-and CNTs-like structures require in principle different values of LCM. Indeed the CNOs morphology requires low values of local carbon to metal ratio, while elongated CNTs-like structures require high values of local carbon to metal ratio. The growth of the hybrid structure in Fig. 2 could be therefore explained by considering the presence of a local variation in the carbon to metal ratio parameter during the growth.
Such variation could be associated to the variation of Clconcentration in the pyrolysed vapour, which is expected to remove carbon-species by forming chlorinated carbon clusters (CCl 4 ). [30][31][32][33][34][35][36][37] In order to verify this interpretation further experiments were performed by using ferrocene and dichlorobenzene as precursors with experimental conditions similar to those reported by Boi et al. and Guo et al. 28,30 Interestingly, as shown in Fig. 3 (see Experimental section, for Fe 3 C lled CNTs type 1, method-1 of evaporation) the two types of morphologies, namely fully lled CNTs and lled CNOs-like structures could be observed.
However, differently with respect to the hybridnanostructure shown in Fig. 1 and 2, in this case the CNTs and CNOs-like structures are found in the form of separate This interpretation is conrmed by the HRTEM analyses in Fig. 3B and C which show the presence of a preferred orientation of the encapsulated Fe 3 C crystal with the 010 axis of the crystal pointing perpendicularly with respect to the CNT-walls.
This interpretation is conrmed also by Fourier transform investigation of the lled CNT, as shown in Fig. 4, which reveal the preferred crystal growth with direction perpendicular to the 010 crystal axis. The observed reections are indexed as follows: the cyan circles correspond to the 010 reection of Fe 3 C (lattice spacing of 0.67 nm) while the magenta circles correspond to the 121 reection of Fe 3 C (lattice spacing of 0.24 nm). The observation of both CNTs and CNOs products could be considered also in this case a consequence of the presence of Cl-radicals in the CVD system. The formation of CNOs is indeed expected only in the rst stages of precursor pyrolysis, when a very high concentration of Cl-species is present due to the high evaporation temperature of the precursors and a low carbon-to-metal ratio is probable. The carbon-to-metal ratio is then expected to change again once the Cl-radicals are consumed by the formation of CCl 4 species.
In order to conrm the interpretation above, further experiments were then considered in conditions of higher dichlorobenzene quantities of 0.15 ml, very low evaporation temperature of 70 C and very low vapour ow rates of 5 ml min À1 (see Experimental section, for Fe 3 C lled CNTs type 2, method-2 of evaporation). Indeed previous reports have shown that in these experimental conditions a crust of onions is present on the top  of the obtained CNTs lms. 30 In agreement with the observations reported in literature, we also nd that in these experimental conditions a crust of CNOs is present on the top of the grown CNTs buckypaper-lm (see Fig. 5 and 6 for SEM and TEM images of the CNOs crust).
The observation of such large quantity of CNOs can be associated to the presence of a low carbon to metal ratio conditions in the rst stages of the CVD reaction, due to the presence of high quantities of Cl-radicals and low quantities of ferrocene. However, note that due to the low evaporation temperature used in this type of experimental conditions, the presence of Cl radicals is expected for timescales much longer.
These observations can justify the variation in the quantity of obtained CNOs with the evaporation temperature and Cl concentration in the vapour. Also, it is important to notice that in both these type of experiments, the observed carbon nanostructures are found in a separate morphology and don't form any hybrid structure.   These ndings appear therefore to suggest two important key factors which can control the (local) carbon-to-metal ratio parameter described by Elliott et al.: 6 (i) the concentration of Clcontaining precursor which controls the concentration of Cl radicals in the pyrolysed vapour and (ii) the used catalyst (and therefore the variation of C-solubility parameters for the chosen catalyst 22 ). [38][39][40][41][42][43] Indeed for the specic case of Fe 3 C, the morphologies of the obtained nanostructures is comparable to those described in the map of Elliott et al. 6 Instead such comparison is not valid for the FePd-based catalyst case. This difference can be attributed also to the different carbon solubility in the two types of catalysts. In view of these observations, further analyses of the hybrid structures produced with FePdbased catalysts were considered.
Additional examples of the cross-sectional morphologies of the unusual CNFs structures shown in Fig. 2 are shown in Fig. 7-10. In Fig. 7A-C the CNOs-like structures observed in the base of the analysed CNFs are shown with increasing level of detail. Note that the majority of the CNOs structures are found to be catalyst-free and arranged with a surprising close-packing structure and a wide diameter of approximately 350-400 nm. A high detail of a CNOs-like structure lled with a FePd-based crystal is also shown in Fig. 7C (dark region). Another example of these type of nanostructures is shown in Fig. 8. Interestingly, as shown in Fig. 8A and B by the red and cyan arrow also in this case the presence of closepacked CNOs-like particles at the base of the CNF-structure were found. Examples of lled CNOs are also indicated with the magenta arrow in Fig. 8B. The lattice spacing observed in the CNOs shell structure is instead shown in Fig. 8C. The Fourier transform in the inset clearly shows the presence of a 002 reection from graphitic carbon. A similar nanostructure is also shown in Fig. 9A and B. Also in this case the observed CNOs-like structures were found to be lled and unlled. The HRTEM investigations conrmed not only the presence of a graphitic like arrangement of the CNOs shells, but also the presence of lattice spacing compatible with those of a FePd 3 phase (Fig. 9C). Such observation is also conrmed by further selective area ED analyses, as shown in Fig. 9D. Indeed ED measurements revealed the presence of a 002 graphitic reection (blue arrow) corresponding to a lattice spacing of 0.356 nm and the presence of a lattice spacing of 0.197 nm which can be associated to the 200 reections of FePd 3 with space group Fm 3m. Further examples of these types of structures are also shown in Fig. 10. The red arrows in Fig. 10B and C indicate the CNOslike particles at the base of the CNF. The morphology of the CNF is shown with a high detail in Fig. 10D.
From the analyses above, it seems therefore clear that the FePd-based catalyst-system leads to different reaction dynamics and does not follow directly the growth mechanism predicted by Elliott et al. 6 In addition, the observed morphology is much different with respect to that reported in recent reports on the encapsulation of FePd 3 crystals inside CNTs. 21 Such difference can be associated to the different carbon solubility (very low if compared to that in Fe) present in the FePd 3 phase and to the different quantity of dichloro-cyclooctadiene palladium employed in the reactions and therefore to the different concentration of Cl radicals. 21  In order to further investigate the properties of this system, additional experiments on carbon nanostructures grown from FePd-based catalyst alloys were considered in presence of a viscous boundary layer. As mentioned above, viscous boundary layer CVS is a novel technique which allows (for the specic case of Fe-based catalyst) the growth of radial CNTs structures with exceptional lling rates by creating uctuations in the (local) carbon to metal ratio. Up to now this technique has been used for the fabrication of radial structures lled with a-Fe, Fe 3 C, g-Fe, FeNi and FeCo. [17][18][19][20] However no reports have demonstrated up to now the dynamics of the viscous boundary layer in presence of an ironand palladium-rich pyrolysed vapour.   The result of these experiments is shown in the SEM analyses of Fig. 11 and 12 and in the TEM analyses of Fig. 13. Interestingly, these measurements revealed that in presence of dichloro-cyclooctadiene-palladium as precursor together with ferrocene, radial CNTs structures are still obtained as dominant product of the reaction. Such experimental results can be associated to the much lower dichloro-cyclooctadienepalladium concentrations (ferrocene/dichloro-cyclooctadienepalladium molar ratio ¼ 5.62742) which is xed by both the precursor quantity and evaporation temperature and implies a relatively low concentration of Cl-radicals in the CVS system. Indeed high concentration of Cl are expected to inhibit the homogeneous nucleation mechanism of CNOs and therefore prevent the growth of radial structures. Note that a complete disappearance of the radial structure morphology was found in conditions of very high evaporation temperatures of the dichloro-cyclooctadiene-palladium (approximately 350-500 C) precursor due to the much higher Cl-concentrations achievable in those conditions. As shown in Fig. 13A by the red arrows and in Fig. 13B in scanning TEM mode, the radial structures obtained in conditions of low evaporation temperature (approximately 200 C) are characterized by a core constituted by an agglomeration of CNOs. These characteristics can be clearly visualized in Fig. 13B in STEM mode, thanks to the atomic contrast (bright areas). This result is in agreement with those reported in previous literature reports on viscous boundary layer created by rough surfaces, where the obtained product consisted of radial structures lled with ferromagnetic a-Fe, Fe 3 C, FeNi or FeCo and characterized by an agglomeration of CNOs as central core. [17][18][19][20] The CNTs growing from the radial structure core are shown in Fig. 13C and 14 (magenta arrow). Note that the CNTs are found to grow with a high alignment. A higher detail of the central core of the radial structures previously show in Fig. 13A and B is also shown in Fig. 14. The orange arrow indicates a typical high-magnication example of CNO comprised in the radial structure core.
An example of aligned CNTs grown from the radial structure is then shown in Fig. 15. Note that the aligned CNTs exhibit a small diameter in the order of 50 nm. An example of FePd based particle encapsulated within the CNT core is indicated by the red star in Fig. 15. Owing to the high thickness of the radial structures, HRTEM or ED analyses could not be successfully performed in these structures. Further evidence of the structural arrangement of the encapsulated FePd-based phases was then sought by XRD. A typical XRD pattern of the as grown powder containing radial structures is shown in Fig. 16. Interestingly, in agreement with previous experiments performed with the only ferrocene, 17 also in this case two main carbon peaks could be found in the region of 26 degrees 2q. The rst peak indicated with the magenta star is associated to the contribution of the graphitic walls of the CNTs comprised in the radial structures (002 reection). Instead the second peak indicated with the red star could be assigned to the contribution of the graphitic carbon in the CNOs comprised in the radial structures core. Further analyses at higher values of 2q degrees revealed the presence of several metal and alloys phases within the sample. Note that the presence of FePd-based (FePd 3 ) phases was conrmed by the Fig. 11 SEM micrographs showing the morphology of the as grown radial CNTs-structures filled with FePd 3 . As shown by the red arrow these structures are found to be clustered-together in large clusters. The cyan arrow indicates an example of isolated radial structure. 3m. In addition other Fe-based phases such as Fe 3 C, g-Fe and a-Fe were found. Further conrmation of this interpretation was sought by Rietveld renement analyses. Interestingly, as shown in Fig. 17 these analyses conrmed the interpretation above and allowed for the extraction of the following metal/alloy phase abundances: 24.3% of FePd 3 (unit cell parameters of a ¼ b ¼ c: 0.385 nm and a ¼ b ¼ c: 0.381 nm), 46.1% of Fe 3 C with space group Pnma (unit cell parameters a: 0.510 nm, b: 0.675 nm, c: 0.453 nm), 17.8% of a-Fe with space group Im 3m (unit cell parameters of a ¼ b ¼ c: 0.287 nm) and 11.8% of g-Fe with space group Fm 3m (unit cell parameters a ¼ b ¼ c: 0.360 nm). In particular the observation of two types of unit cell parameters for the specic case of FePd 3 suggests a possible role of the pressure imposed by CNTs or CNOs walls on the encapsulated crystals.
These analyses were further conrmed by the observation of a remarkable difference between the cross-sectional   morphology of the CNTs encapsulating Fe-based crystals and the morphology of the CNTs lled with FePd-based phases. A typical example of Fe-lled CNT is shown in Fig. 18. Note that in this case a much larger diameter and a high CNTs-wall thickness is found.

Conclusion
In conclusion we have investigated the possible presence of local changes of "carbon-to-metal ratio" in the growth mechanism of carbon nanostructures grown from FePd-based and Fe 3 C catalyst in conditions of steady state CVD and viscousboundary-layer CVS experiments by pyrolysis of ferrocene and dichlorocyclooctadiene-palladium mixtures. We have demonstrated the observation of an unusual growth mechanism in which a direct transition from a CNOs morphology to a carbon ber-like morphology is observed within the same carbon structure. This is found in conditions of low vapour ow-rates (in steady state ow) CVD and can be attributed to the concentration of Cl radicals in the pyrolysing vapour. The observed growth mechanism has been compared in detail with that reported for Fe 3 C lled CNTs in previous reports on CVD of ferrocene/dichlorobenzene mixtures and with those reported in literature for other transition metal catalyst systems. In contrast, for experiments performed in the presence of a viscous boundary layer between the pyrolysed precursors and the used rough surface of the substrate a different result is found. Indeed radial CNTs structures lled with large quantities of FePd 3 and Fe 3 C crystals are found as major product in this type of reaction. This latter result conrms the existence of the radial structure growth mechanism (involving the homogeneous nucleation of particles in a viscous boundary layer) also in presence of FePdbased catalyst species but in conditions of very low Cl concentration induced by the lower evaporation temperature. The morphological, cross-sectional and structural properties of the obtained carbon structures have been analyzed in detail by SEM, TEM, STEM, HRTEM, ED and X-ray diffraction.