Polymorphic Transformation of Iron-Phthalocyanine and the Effect on

Mar 8, 2008 - Organometallic compounds such as phthalocyanine are useful precursors for carbon nanotube formation by pyrolysis because they can supply...
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J. Phys. Chem. C 2008, 112, 5339-5347

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Polymorphic Transformation of Iron-Phthalocyanine and the Effect on Carbon Nanotube Synthesis Adriyan S. Milev,*,† Nguyen Tran,† G. S. Kamali Kannangara,† Michael A. Wilson,‡ and Isak Avramov§ School of Natural Sciences, UniVersity of Western Sydney, Penrith South DC 1797, Australia, CSIRO, Petroleum, North Ryde NSW 2113, Australia, and Institute of Physical Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria ReceiVed: NoVember 15, 2007; In Final Form: January 10, 2008

Organometallic compounds such as phthalocyanine are useful precursors for carbon nanotube formation by pyrolysis because they can supply both carbon and metal catalysts needed for synthesis. Prior milling of iron-phthalocyanine (FePc, FeC32H16N8) is investigated and shown to affect the sublimation temperature of the precursor and the nanotube diameter. Without prior milling, a sublimation temperature of 600-650 °C is necessary to produce a sufficient amount of vapors prior to pyrolysis. At that temperature, there is also some decomposition. Milled FePc sublimates at the highest rate at 400-450 °C, where no decomposition occurs. The lower temperature shift of the maximum of the sublimation rate appears to be due to changes in polymorphs upon milling. Carbon K-edge near-edge X-ray absorption fine structure, infrared spectroscopies, and X-ray diffraction analysis show that packing of the phenyl subunits of FePc is modified upon milling and an R-like polymorph is produced. Upon heating, the milled material undergoes polymorphic transformation to a mixture of R and β forms and a third unidentified phase. Above 550 °C, this mixture transforms entirely to the β polymorph. During pyrolysis of the FePc vapors at 900 °C, multiwalled carbon nanotubes (MWCNT) with different diameters are produced between milled and non-milled samples. Transmission electron microscopy shows the average diameter of the MWCNTs produced from the non-milled and milled FePc precursor is about 40-100 nm and 15-50 nm, respectively. It is suggested that the decrease in nanotube diameter caused by the milling of the precursor is due to presence of higher concentrations of un-decomposed FePc molecules with fixed C/Fe atomic ratio in the gas-phase prior to pyrolysis. These results show the importance on the choice of materials for CNT synthesis since small changes in the structure of precursors affect nanotube formation kinetics.

1. Introduction Phthalocyanine (Pc) is a planar heterocyclic molecule of about 1.3 nm diameter having four fused together phenyl and pyrrole subunits, which are linked to each other through aza bridges (Figure 1A). The Pc macromolecule is able to coordinate various metal cations (Fe, Ni, Co, etc.) in its center with the four central nitrogens belonging to the pyrrolic subunits. With iron, this is called iron phthalocyanine (FeC32H16N8, FePc), which is useful in carbon nanotube synthesis. In this compound, two polymorphs (R metastable and β stable) can occur because of the slight differences in the π-π electronic interaction between the neighbor molecules in the lattice.1-4 The main difference between these two polymorphs is the tilt angle of the molecular plane with respect to the stacking b-axes (Figure 1B). In the R-polymorph, the tilt angle is 25-30° with respect to the normal to the molecular ring, whereas in the β-polymorphs the angle is 45-49°.5-8 Iron phthalocyanine has been successfully converted to CNTs using chemical vapor deposition. Sublimation at 600-650 °C * To whom correspondence should be addressed. School of Natural Sciences, University of Western Sydney, Building LZ, Parramatta Campus, Locked Bag 1797, Penrith South DC 1797, Australia. Tel: +61 2 9685 9945. Fax: +61 2 9685 9915. E-mail: [email protected]. † University of Western Sydney. ‡ CSIRO. § Bulgarian Academy of Sciences.

and pyrolysis at 800-1000 °C gives good yields of multiwall nanotubes, but other structures are also formed.9-19 These include tubular structures having diameters up to 100 nm, and bamboo or conical structures. By employing ball-milling treatment of the FePc precursor prior to pyrolysis, Chen and coworkers20,21 have reported a significant improvement of the structural quality and yield of the CNT product. They have also shown that ball milling reduces the onset of sublimation of the FePc precursor from about 600 to 200 °C; however, there is no detail of mechanism. The rationale of the acceleration of the sublimation process by milling has not been studied and understood. It is not known if milling changes only the crystal structure facilitating the sublimation of whole FePc molecules, or if milling damages the macromolecule bonds creating more volatile molecular fragments. For this reason, a detailed characterization of the structure of the milled FePc is necessary and has been carried out in this work. The main focus is on the microstructure changes upon milling and sublimation of the FePc precursor and their possible affect upon the carbon nanotube morphology. This paper builds upon other work performed by our research group examining the change in the level of disorder of graphite during milling,22 and also the effect of milling on the shortrange order of Fe, Ni, and Co phthalocyanines.23

10.1021/jp710923f CCC: $40.75 © 2008 American Chemical Society Published on Web 03/08/2008

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Figure 1. (A) Structure of a of metal-phthalocyanine molecule. Carbon-to-metal ratio of 32 (B) Stable β-polymorph, (C) Metastable R-polymorph.

2. Experimental Section 2.1. Sample Preparation. Iron-phthalocyanine (FePc) powders (Sigma-Aldrich, >90% purity) were first dried at 125 °C at low pressure (∼10-2 Pa) for 48 h and then were ball-milled using a Pulverizette 6 (Fritzsch) planetary mill. To prevent possible iron contaminations from the milling tools, container and balls made from zirconia ceramic (ZrO2) were used. Fivegram samples were loaded into an 80-mL container with 300 balls with diameters of 5 mm. The closed container was flushed with ultrapure argon gas. The milling was carried out for 100 h at 400 rpm in argon under atmospheric pressure. For the MWCNT synthesis, the milled and un-milled FePc precursors were sublimated at two different temperatures and the vapors subsequently pyrolyzed at the temperatures of up to 900 °C. 2.2. Sample Characterization. 2.2.1. Infrared Spectroscopy. The IR spectra in the 4000-400 cm-1 wavenumber region were recorded on a Bruker Vertex 70 spectrometer (∼0.5 mg sample in 200 mg KBr, 128 scans, 2 cm-1 resolution). Peak fitting in the wavenumber interval of 745-695 cm-1 was employed. The peaks in that wavenumber interval were (1) baseline corrected, (2) normalized to two, and (3) fitted by mixed LorentzianGaussian peak shapes using Levenberg-Marquardt minimization algorithm as implemented in Bruker OPUS 6 software. 2.2.2. X-ray Diffraction. The samples for X-ray diffraction analysis were prepared by evaporating acetone phthalocyanine suspension on a background-free silicon single-crystal sampleholder. The diffraction patterns were acquired by a Philips PW 1825/20 powder diffractometer (Cu KR, 40 kV, 30 mA, 3-35°, 0.02° step, 10 s/step). The average size of the coherently diffracting domains (D) were derived from full width at halfmaximum (fwhm) values by the Scherrer equation:24 D ) Kλ/ βcosθ, where β ) 180/(π fwhm), λ is X-ray wavelength, K ) 0.89, and θ is diffraction angle. After background separation and KR2 stripping, the diffraction lines fwhm values were determined by fitting of Voigt function. The instrumental broadening was also determined by fitting of Voigt function to line profiles of a standard silicon specimen described in “Standard Reference Material” SRM 640c. 2.2.3. NEXAFS. Carbon K-edge near edge X-ray Absorption Fine structure NEXAFS spectra were acquired at the wide range beamline (BL24A, energy approximately 10-1500 eV) at the National Synchrotron Radiation Research Centre (NSRRC) in Hsinchu, Taiwan. Prior to the measurements, samples were heated in the ultrahigh vacuum analysis chamber (base pressure approximately 10-9 mbar) for 24 h. The spectra were acquired with the surface sensitive, total electron yield (about 1-5 nm sampling depth for energies 99.999% purity) flowing at 25 mL/min during measurements. Activation energies (∆Ea) in the temperature range of 100550 °C were determined using a Netzsch Phoenix 204 F1 differential scanning calorimeter (DSC). The FePc samples with weights ranging from 7 to 10 mg were loaded in PtRh pans covered with pierced lids. The DSC data were collected at scanning rates of 1, 5, 10, 25, and 50°/min in ultrahigh purity argon (>99.999% purity) flowing at 25 mL/min. The DSC measurements were used to calculate activation energies (∆Ea) of transformation below 500 °C. The Ea values were determined according to eq 1.

ln(q) ) -

∆Ea 1 + const. R Tp

(1)

where, q (K‚s-1) is the heating rate, Tp (K) shifts with the heating rate, ∆Ea is activation energy, R (8.3 J/molK) is the gas constant.27 The logarithm of the heating rate ln(q) was plotted over the reciprocal temperature of the maximum temperatures of DSC curves (1/Tp). The slope of the yielded straight line is proportional to the activation energy ∆Ea. 3. Results 3.1. Spectra 3.1.1. Near Edge X-ray Absorption Fine Structure (NEXAFS). Figure 2, parts A and B, compares normalized electron yield (EY) and fluorescence yield (FY) carbon K-edge NEXAFS spectra for non-milled and milled FePc. The presence of several peaks and shoulders in these spectra is attributed to two distinct chemical environments for carbon atoms in the phenyl (CdC) and the pyrrole (CdN) subunits (Figure 1A).28-31 According to Koch and co-workers28 and Mauerer and co-workers,32 the C1s f π* transitions of nonequivalent carbon atoms (CdC, CdN) of the pyrrole subunits give rise to overlapping peaks at 286.2 and 286.8 eV, respectively, whereas the C1s f π* transitions of the carbon atoms of phenyl result in a peak at 285 eV. Carbon is electrondeficient due to the electron withdrawing nature of N in CdN bond. Consequently, the C1s f π* transition in the CdN bond appears at higher energies than the C1s f π* transition in the CdC bond. The existence of a weak peak at about 283.6 eV in

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Figure 2. Normalized carbon K-edge NEXAFS spectra of Fe-phthalocyanine collected in (A) Electron yield (EY) mode. (B) Fluorescence yield (FY) mode.

Figure 3. Normalized IR spectra of non-milled and milled FePc in the range of: (A) 1625-1000 cm-1, (B) 800-675 cm-1.

lithium phthalocyanine has been previously related to C1s f π* transitions;29 however, it has not been assigned to any specific structure. The features in the 287.0 to 289.8 eV interval have been assigned to overlapped π*+σ* excitations, which also includes contributions from the C 1s f σ* (C-H) excitations from the phenyl subunits.33,34 At energies higher than 290 eV, the electronic transitions into the unoccupied σ* states within the phenyl and pyrrole subunits occur.30 [In Figure 2, the peak labeled with (*) symbol at 299.2 eV is due to surface contamination of the synchrotron optics but was used as an internal standard.] Upon milling, the surface-sensitive, EY spectrum reveals that the intensities of the π* excitations are lower, while those assigned to the excitations to σ* are much stronger and broader than that of the non-milled sample. The bulk sensitive, FY spectra, however, show virtually no difference between nonmilled and milled phthalocyanine (Figure 2B). The comparison of the FY with the respective EY spectra reveals two major differences: (1) the peak at 293.5 and the shoulder at about 296 eV could not be observed in FY spectra; (2) the weak shoulder at 285 eV (EY) is a strong and well-defined peak at 285.1 eV (FY). It seems that these discrepancies are related to the different origin of the signals acquired by these acquisition modes. It may well be, because the photons (FY) interact much weaker with the sample and the photons’ escape depth is about 10-50× deeper25 than that of the electrons (EY). Thus, the combined EY and FY NEXAFS spectra suggest that milling modified the alignment of the outer molecules of the FePc aggregates while the molecules within bulk of the particles remained unchanged. 3.1.2. Infrared Spectroscopy. More details of the molecular modifications during milling have been examined using IR spectroscopy. Figure 3A compares normalized IR spectra of non-

milled and milled Pc in the 1625 - 1000 cm-1 interval. The difference between the vibration spectra of the two samples above 1300 cm-1 is limited to small intensity variations of the resonances at 1495, 1467, 1377, and 1305 cm-1. These peaks originate from the strong coupling of phenyl CdC, pyrrole Cd N, and aza bridge C-NdC modes of the Pc skeletal vibrations.35-41 The resonances below 1300 cm-1 are primarily due to the in-plane deformation vibrations of C-H moieties of the phenyl subunits. While these show some intensity differences and the peak at 1084 cm-1 also shows a slight broadening of its low-wavenumber shoulder, no peak shifts were observed. Therefore, the close similarity of the IR spectra indicates that the skeletal bond structure of the FePc is unaffected by the milling Figure 3B shows the region of 800-675 cm-1, where the most significant spectral differences between the two samples are observed. The resonances in this spectral region are due to the out-of-plane C-H deformations of the phenyl rings (732 and 781 cm-1), and from the CdC in-plane ring deformations (755 cm-1).35-41 Upon milling, the latter shifts to 753 cm-1, while the resonances 732 cm-1 and 781 cm-1 shift by 7 cm-1 to 725 and 774 cm-1, respectively. Stymne and co-workers,35,36 and Sharp and Lardon37 reported that the vibrations in the region below 800 cm-1 are sensitive to short-range interactions between neighbor Pc molecules can be used to differentiate between various Pc polymorphs. The stable β-polymorph gives rise to the vibrations at about 732 cm-1, while the metastable R-polymorph produces a resonance at 725 cm-1.35-38 Therefore, the IR data indicate the milling causes structural modifications that resemble the polymorphic transformations of phthalocyanine. 3.1.3. X-ray Diffraction. The diffraction pattern of the nonmilled FePc shown in Figure 4A is typical of the monoclinic FePc β-polymorph (ICDD 14-0926). The XRD pattern of the

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Figure 4. X-ray diffraction patterns of non-milled and milled iron phthalocyanine. (A) Non-milled FePc β-polymorph (ICDD 14-0926). (B) Milled FePc demonstrating diffraction features of R-polymorph (ICDD 22-1771). The insert shows pattern B magnified five times.

milled FePc is dominated by a peak at 6.9°, two weak peaks at about 9.9, 15.7, and a diffuse scattering halo in 24-28° interval (Figure 4B). Fitting shows that the peaks at 6.9, 9.9, and 15.7° match very well with the two-theta positions of the strongest reflections (200, -202 and -402) of the metastable R polymorph of FePc (ICDD 22-1771). It is perhaps also true for the diffuse halo in 24-28° interval, which appears at the maxima of the strong (011) and (-211) reflections of R polymorph (Figure 4B, insert). However, the poor intensity of these reflections indicates that the long-range periodicity is lost, except along the [h00] direction. The average size of the coherently diffracting domains, D, along the [h00] direction was estimated from the full width at half-maximum (fwhm) values of the respective (h00) peaks according to Scherrer equation.24 The D-value determined from the (100) reflection of the non-milled FePc showed and average size of the coherently diffraction domains is 41 nm. Upon milling, the D-value determined from the (200) reflection decreased to 11 nm. As the unit cell length along the a-axis of R-polymorph is 2.55 nm (ICDD 22-1771), the size of the ordered domains in the [h00] direction for the milled FePc is less than five unit cells length. To this end, the NEXAFS spectroscopy and the skeletal IR vibrations indicate that most of the bonds within FePc molecules remain intact upon milling. The low wavenumber IR region and XRD indicate that milled FePc exhibits spectral and some diffraction features of R-polymorph. However, the weak and broad (hkl) reflections and the low-wavenumber shift of the outof-plane C-H deformations suggest weakened interactions between the peripheral phenyl subunits of neighboring molecules in the lateral plane directions. 3.2. Thermal Properties. 3.2.1. Simultaneous DSC/TGA Analysis (STA). Figure 5A shows the mass-loss (TG) and differential scanning calorimetric (DSC) traces of non-milled and milled FePc measured in argon flowing at 25 mL/min with a heating rate of 10°/min to 1200 °C. Both non-milled and milled FePc undergo four weight-loss steps in the temperature intervals given in Table 1. However, the mass-losses below 500 °C are about 2.9× more for milled than that of non-milled sample. The simultaneously measured heat-flow effects (DSC traces) of the non-milled FePc shows that the first mass-loss step (150-

Figure 5. (A) Simultaneous DSC-TGA traces of non-milled and milled FePc collected by using STA Jupiter 449C at 10°/min. A second set of samples was two times heated to 500 °C. The second TG runs are horizontal lines between 150 and 500 °C (B) DSC traces of the nonmilled and milled FePc collected by Phoenix 204 at 10°/min. The effect of the thermal history of heat-flow effects is also shown by the second runs to 550 °C.

TABLE 1: Mass-Losses of Milled and Non-milled FePc Heated at 10°/min in Argon at 25 mL/mina mass loss (wt %) sample

150500 °C

500700 °C

700850 °C

8501200 °C

total

non-milled milled

7.4 21.7

8.7 9.4

16.7 12.9

4.1 2.1

37.0 46.0

a

The samples were placed in PtRd crucibles with pierced lids.

500 °C) is associated with a weak endothermic effect at 430 °C, while the milled sample shows three endothermic effects: two very weak at 220, 354 °C and a stronger at 431 °C. In the temperature interval of 500-700 °C, both samples show several consequent endothermic events at 605, 615, and 640 °C (non-milled) and 614, 640, and 655 °C (milled). Above 700 °C, the milled and non-milled samples produce two exothermic peaks at about 770 and 920 °C, which for the milled samples are weaker and broader. At 1138 °C, both samples show endothermic effects not related to mass-loss steps, which indicates that this event could be a physical process such as melting.

Polymorphic Transformation of Iron-Phthalocyanine

Figure 6. Activation energies of transformation. T1 at 364 (at 10°/ min) and T2 at 434 °C (at 10°/min). Straight line corresponds to the non-milled, while the dashed line to the milled FePc.

The effect of the thermal history of non-milled and milled samples was investigated by heating of the samples to 500 °C at 10°/min followed by cooling to 150 °C. At subsequent reheating to 500 °C, the total mass losses were less than 0.3 wt %, and no heat flow effects were detected for either sample (Figure 5 A). 3.2.2. DSC Measurements. The heat flow effects were also studied by a Phoenix 201 F1 DSC analyzer, which below 600 °C is more sensitive than the STA DSC/TG analyzer. At the heating rate of 10°/min, the DSC trace of the milled sample shows endothermic effects at 220, 360, and 434 °C (Figure 5B). Between about 250 and 330 °C, a broad exothermic effect

J. Phys. Chem. C, Vol. 112, No. 14, 2008 5343 cantered at about 296 °C suggests a gradual energy release during heating. Non-milled FePc shows same type thermal effects at similar temperatures but with much lower intensity (Figure 5B). It seems that the thermal effects below 500 °C correspond to similar events but for non-milled sample, the heatflow effects are very weak to be detected by the simultaneous DSC/TG analyzer. The thermal history study did not show any heat flow effects during the second heating to 550 °C. 3.2.3. ActiVation Energy of Transformation. The samples were heated at heating rates (q) of 1, 5, 10, 25, and 50°/min to 550 °C in argon using PtRh crucibles with pierced lids. The sublimation temperatures of non-milled and milled FePcs shifted to higher temperatures when the heating rate (q) was increased from 1 to 50°/min. According to Kissinger, the shift of the temperatures is related to the activation energy of the process (eq 1 in the Experimental Section).27 From the three endothermic effects below 550 °C, however, only those at 364 °C and at 434 °C (at 10°/min heating rate) were strong enough at each heating rate for the activation energy analysis. For convenience, these endothermic effects are termed peak T1 and peak T2, respectively. Logarithm (q) vs 1000/T plots constructed for the five heating rates based on the DSC data were used to calculate the values of the activation energies (∆Ea) of the processes associated with the peak T1 and peak T2. Figure 6 shows the activation energies of transformation associated with the peaks at 364 and 434 °C. It is seen that activation energies of transformation of milled FePc at T1 and T2 are close 193 ( 15 kJ/mol and 208 ( 10 kJ/mol, respectively. The transformation at T2 of the non-milled FePc also shows similar activation energy (228 ( 4 kJ/mol); however, the activation energy related to the process at T1 is significantly higher 287 ( 10 kJ/mol. The T1 peak associated with the thermal effect was quite broad, which indicates that more than one process may contribute to the observed heat absorption at T1.

Figure 7. (A, B) Normalized IR spectra of milled FePc heated from 200 to 500 °C at 10°/min in argon, then cooled to ambient temperature. (C) Sample heated to 550 and 650 °C. (D) Magnified 3100-2880 cm-1 spectral region of milled FePc heated to 550 and 650 °C.

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Figure 8. X-ray diffraction patterns of milled FePc heated to (A) 400 and 450 °C, (B) 550 and 650 °C. For comparison, the XRD pattern of the non-milled FePc heated to 550 °C is also shown. β-polymorph ICDD 14-0926, R polymorph ICDD 22-1771.

3.3. Characterization of Nonvolatile Products. The transformations observed by the DSC and DSC/TGA analyses were further studied by IR and X-ray diffraction analyses. Samples heated to temperatures varying from 200 to 650 °C in argon and when the temperature was reached, the sample was immediately cooled to ambient temperature. 3.3.1. IR Spectroscopy. The monitored wavenumbers in the 800-700 cm-1 region are associated with the out-of-plane C-H bending vibrations of the four peripheral phenyl rings of Pc molecule. Since FePc is a planar molecule, these deformation modes will be influenced by the orientation of the adjacent molecules that determines the polymorphic form. All samples, with the exception of that heated to 650 °C, demonstrated typical spectral features of the FePc molecule. Differences were observed only in the polymorph sensitive 800-680 cm-1 region (Figure 7A-C). The spectra of the samples heated to 200, 300, and 360 °C show minor differences related to the bandwidth of the 720-740 cm-1 contour. The bandwidth of the sample heated to 300 °C decreases compared to the sample heated to 200 °C. During heating to 360 °C; however, it slightly increases and then again decreases at higher temperatures. Compared to the milled FePc (Figure 3 B), which demonstrated broad peaks centered at 726, 753, and 774 cm-1, heating to 360 °C shifted these peaks to 727, 754, and 778 cm-1, respectively, while the peak at 774 shifted to 778 cm-1. After heating to 400 °C, the peak at 727 cm-1 was split into two components centered at 733 cm-1 and at 729 cm-1. A weak shoulder at about 720722 cm-1 was also observed. At 450 °C, the peaks at 733 and 729 cm-1 become narrower and more resolved, whereas the relative intensity of the shoulder at 722 cm-1 decreased. Upon heating to 500 °C, the intensity of the peak at 722 further decreased. The same is observed for the peak at 729 cm-1 that is seen as a weak low wavenumber asymmetry of the strong

Figure 9. Normalized IR spectra of condensed FePc vapors produced during isothermal heating of milled FePc at 450 and at 650 °C for 1 h in argon. (A) Magnified C-H stretching region, (B) Skeletal region, (C) C-H out-of-plane deformation region.

peak at 733 cm-1. Similar behavior is followed by the broad peak centered at 774 cm-1: with the temperature, it splits into two better-solved peaks at 780 cm-1 and 772 cm-1 and then the intensity of the latter peak gradually decreases. Therefore, the IR data identifies the evolution of β, R and intermediate (I) polymorphs that can be assigned to the peaks at 733, 729, and 722 cm-1, respectively. Heating to 650 °C produces two strong peaks cantered at 738 and 722 cm-1, while the peak at 780 cm-1 is not present (Figure 7C). The high-wavenumber region from 2880 to 3100 cm-1 contains bands for C-H stretching modes typical of aliphatic CH3 and CH2 groups (Figure 7D). For comparison, the IR spectrum of the sample heated to 550 °C is given. The latter shows strong C-H stretches due to aromatic hydrocarbons and much weaker C-H nonaromatic hydrocarbon stretches in the wavenumber. The appearance of CH3 and CH2 moieties and the lack of C-H stretches above 3000 cm-1 is indicative of a break-up of the phenyl rings and formation of nonaromatic hydrocarbons.

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Figure 10. TEM micrographs of CNT produced during pyrolysis at 900 °C. (A) Non-milled FePc sublimated at 600-650 °C. (B) Milled FePc sublimated at 450-500 °C.

3.3.2. X-ray Diffraction. Figure 8, parts A and B, show the X-ray diffraction patterns of the milled FePc samples heated to 400, 450, 500, 550, and 650 °C. The simultaneous presence of β- and R-polymorphs at 400 °C is supported by the splitting of the peak in the two-theta range of 6.5 to 7.5°, where the strongest (100) and (200) reflections of the β- and R-polymorphs appear. The sample heated to 450 °C shows only slight lower-angle broadening of the XRD peak at 7.5° indicating that the relative amount of R-polymorph decreases with the temperature. While the comparison of the diffraction patterns of the nonmilled and milled FePcs heated to 550 °C shows the typical reflections of the β-polymorphs only, they exhibit different reflections intensities (Figure 8 B). The non-milled FePc sample heated to 550 °C showed similar (100)/(-102) intensity ratio as is in non-milled FePc precursor. The milled and heated to 550 °C FePc, however, shows low intensity of the (-102) reflection as compared to (100) reflection, which on the other hand is stronger in the pattern of the non-milled FePc. This indicates that the growing crystals of the β polymorph follow the residual molecular orientation along the (200) reflections present in the milled FePc. This is not surprising because a lower amount of energy is necessary for single molecules to form the nuclei of the new phase with preferential growth along the existing molecular orientation. A change of the crystallization direction might require a major structure transformation and therefore more energy would be required.42 Samples heated to 650 °C show no diffraction pattern, which indicates that above 550 °C decomposition of the phthalocyanine takes place. The DSC, TGA, and IR data noted above would suggest the endothermic effect at 364 °C is associated with further disordering of the FePc molecules from the state reached during milling and this is accompanied by a considerable mass-loss. The activation energy of transformation at ∼364 °C must be therefore associated with two processes: (1) rearrangement of the Pc molecules prior to β-polymorphic conversion and (2) sublimation. The new β-polymorph is not formed unless the sample had been heated to a temperature above 360 °C, where it coexists with the R and I states. At increasing heating temperature, the R and I states progressively disappear and only the β polymorph remains in the sample heated to 550 °C. This is also confirmed by the decrease and disappearance of the (200) reflection in the XRD patterns. Therefore, the observed masslosses seem to be related to sublimation of less stable R and I states during heating. 3.4. Characterization of Volatile Products. Milled FePc sample were isothermally heated for 1 h at 450, 500, 550, and

650 °C in 25 mL/min argon current. The volatile products were condensed on a stainless steel substrate and the material was examined by IR spectroscopy. Figure 9 compares the IR spectra of the samples condensed during heating at 450 and at 650 °C only because the IR spectra of the condensate products derived form the milled FePc isothermally heated at 500 and 550 °C were the same as those produced during heating at 450 °C. These three samples exhibited all of the spectral features of the precursor FePc. The comparison between the samples heated to 450 and 650 °C revealed minor differences in the skeletal and in-plane deformation vibrations region (1625-1000 cm-1, Figure 9B). However, the appearance of strong aliphatic C-H stretches (3150-2880 cm-1, Figure 9A) vibrations and almost no aromatic C-H stretches suggest a near complete breakdown of the phenyl subunits. Figure 9C indicates that the condensate derived from the samples heated at 450 °C has the IR spectroscopic features of β- and R-polymorph, whereas the condensate collected from the samples heated at 650 °C has no defined polymorphic structure. The characterization of the condensed vapors therefore shows that the chosen temperature of sublimation may change the bond structure of the sublimated FePc molecules. Condensed vapors produced at 450-550 °C have the same bond structure as the FePc precursor. Increasing the temperature of sublimation to 650 °C partially decomposes the FePc, which after condensation demonstrates presence of significant amount of nonaromatic hydrocarbons. 4. Discussion The spectroscopic and diffraction data have provided strong evidence that milling changes the mutual orientation of the FePc molecules, while the bond structure within the individual molecules remains intact. The lack of strong (0hk) reflections indicates that the milling changes the angle between each molecule and the column axis in an unsystematic way. The lower two-theta shift of the strongest XRD (h00) reflection and the increase of the respective fwhm value, show that the decrease of the lattice period along the a-axis is associated with an increase of the intermolecular distance during milling. Therefore, milling causes a destruction of crystalline network of the β phase. A disordered state with short-range order similar to that of the R-crystals is formed. This increases the energy of the system. The thermal history data indicate that the sublimation mechanisms from milled and non-milled FePc are similar. The sublimation is related to the existence of loosely bound FePc

5346 J. Phys. Chem. C, Vol. 112, No. 14, 2008 molecules that are present in both samples. The IR spectrum of non-milled FePc shows a low-wavenumber broadening of the peak at 733 cm-1, which suggests that the precursor FePc contains some molecules that exhibit R-like features. The X-ray diffraction line broadening analysis shows an average size of the coherently diffracting domains (ordered regions) is 41 nm along the a-axis. In other words, the ordered regions are separated by regions of less or no order containing loosely bound randomly oriented molecules, which can sublimate upon heating. This explains the behavior on milling. In the milled FePc, the relative amount of the disordered region is significantly increased, and hence, the amount of material sublimated during heating. Heating to 650 °C causes two parallel processes, both of which expend the loosely bound FePc molecules; (1) the sublimation increases with the temperature, that leads to a significant mass-loss below 450 °C (2) at temperatures higher than 400-450 °C loosely bound FePc are consumed by the developing β-polymorph, as seen from the narrowing of the X-ray diffraction peaks and the high-wavenumber shift of the out-of-plane C-H deformation bands. The transmission electron micrographs of CNT produced by sublimation followed by pyrolysis at 900 °C from non-milled and milled FePc are shown in Figure 10, parts A and B. The difference is the sublimation temperature, which for the milled sample was 400-450 °C, whereas for the non-milled FePc by necessity the temperature was 600-650 °C. The CNTs produced from non-milled FePc have irregular shaped walls and diameters ranging from about 40 to 100 nm (Figure 10A), unlike the CNTs produced from milled FePc sublimated at 400-450 °C, which show straighter walls and diameters, in some cases, down to about 15 nm (Figure 10B). As noted above, at about 600650 °C, decomposition of the FePc molecules takes place prior to pyrolysis, and therefore, the vapors contain molecular fragments with lower than stoichiometric C/Fe ratio. If sublimation is carried out at about 400-450 °C, which is possible for the milled FePc, the vapors prior to pyrolysis consist mostly of whole molecules with the stoichiometric C/Fe ratio, which may be preferential for growing thinner/longer nanotubes. 5. Conclusions 1. The milling of iron-phthalocyanines at 400 rpm for 100 h in zirconia ceramic container modifies the three-dimensional packing order. The molecular crystal undergoes transformation that resembles stable f metastable (β f R) transformation followed by disappearance of the long-range periodicity. 2. The milling induces no detectable level of molecular damage. Milling decreases the cohesive forces between the FePc molecules and increases the energy potential by changing the molecular mutual orientation so that they can be sublimated without decomposition at temperatures below 550 °C in argon. The sublimation process is impeded by the crystallization and formation of the β-polymorph. Once formed, the β-polymorph is very stable and does not sublimate but decomposes if heated above 600-650 °C, producing volatile and nonvolatile molecular fragments. 3. The milling assists the sublimation of whole FePc molecules at temperatures below the decomposition temperature so that vapors containing molecules with fixed C/Fe ratio of 32 prior to formation of CNTs are produced. The presence of fragments rather than whole FePc molecules changes the C/Fe ratios prior to pyrolysis and CNT nucleation and growth and hence the nature of the products. 4. These results show the importance on the choice of prior preparation of materials for CNT synthesis since small changes

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