The effect of the chemical environment on the kinetics of carbon phases formation at high-pressure and high-temperature

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1 The effect of the chemical environment on the kinetics of carbon phases formation at high-pressure and high-temperature A.E.L. Villanueva 1,2, K.S. Trentin 1,3, J.A.H. da Jornada 1,4, N.M. Balzaretti 1, A.S. Pereira 1,5 1 Instituto de Física, UFRGS, , Porto Alegre, RS, Brazil 2 DACIS-FIIS, UNAS, Tingo María, Peru 3 PGCIMAT, UFRGS, , Porto Alegre, RS, Brazil 4 Instituto Nacional de Metrologia, Normalização e Qualidade Industrial, , Duque de Caxias, RJ, Brazil 5 Escola de Engenharia, UFRGS, , Porto Alegre, RS, Brazil <altair@if.ufrgs.br> The objective of this work was to investigate the effect of the chemical environment on the formation of carbon phases during the thermal decomposition of sucrose (C 12 H 22 O 11 ) under high pressure. The samples were processed at 7.7 GPa in the temperature range from 900 C to 1700 C in a toroidal-type high pressure apparatus, during 15 min, inside the thermodynamic region of stability of the diamond phase. The samples consisted of pure sucrose, sucrose diluted in water, and mixtures of sucrose + diamond powder, and diluted sucrose + NiCl 2 :6H 2 O. The reaction products were analyzed by micro-raman spectroscopy and by X-ray diffraction. The thermal decomposition of sucrose under pressure yielded the formation of graphite whose crystallinity depended on the chemical environment. The dilution of sucrose in water hindered the graphitization process. The addition of metallic ions in the chemical environment did not induce the formation of diamond. The presence of pyrolised sucrose enhanced the annealing of defects on micrometer sized diamonds promoted by the thermal treatment at high pressure. I. Introduction The reactions induced on organic compounds at high pressures and high temperatures (HPHT) have been investigated since the first experimental evidences of diamond formation from carbonaceous materials in the 1950 s (Bovenkerk, 1959). A rather comprehensive overview of the main results obtained so far is presented by Davydov et al. (Davydov, 2004). The actual role of the molecular structure and the H/C ratio of the starting organic material on the phase formation during the HPHT processing is still an open question and controversial results are presented in the literature. Some authors (Wentorf Jr., 1965; Onodera, 2000) have found that aromatic compounds produce graphite even when processed in conditions where diamond is the stable phase of carbon. On the other hand, Davydov et al. (Davydov, 2004) have not found any significant influence of the starting structure of polycyclic aromatic hydrocarbons on the conditions for diamond formation. According to them, the decisive factor for the diamond synthesis would be a good design of the HPHT reaction cell, which should be permeable to hydrogen, in order to allow an extensive decomposition of the starting organic compound. Another aspect which seems to be decisive for diamond formation from various carbon-containing compounds at HPHT is the production, in an intermediate stage, of a mixture of a C-O-H fluid and graphite (Yamaoka, 2002). Yamaoka and co-workers have proposed that after long processing times, if such mixture is kept confined, it would promote diamond formation by a dissolution-precipitation reaction. It is important to remark that these

2 authors have used sealed metallic capsules, made of Ta, Mo or Pt, which are known as potentially catalytic for the graphite/diamond transition. All these results indicate that the kinetics of formation of carbonaceous phases can be very dependent of the chemical environment at HPHT. In order to have a better understanding of such effect, we started a study about the phases obtained by thermal decomposition of sucrose (C 12 H 22 O 11 ) in different environments under HPHT conditions. In this paper we report the preliminary results obtained for samples of pure sucrose, sucrose diluted in water, and mixtures of sucrose + diamond powder, and diluted sucrose + NiCl 2 :6H 2 O. The decomposition of pure sucrose at HPHT is expected to produce a mixture of carbon and water at supercritical conditions. The other compositions were processed to investigate the effect on the kinetics of phase formation at HPHT of the carbon dilution and of the presence of a stable phase or a possible metallic catalyst in the carbon+water solution. II. Experimental The experiments were carried out in a toroidal-type high pressure apparatus. The sample volume was cylindrical, 3 mm in diameter and 2 mm height, confined by a hbn capsule and a graphite cylinder, which acts as an electric heating furnace. Initially the sample was submitted to 7.7 GPa, then the temperature was increased up to the desired value (from 900 to 1700 o C) and maintained for 15 min. After that, the temperature was quenched down to room temperature and, finally, the pressure was released. In order to investigate the effect of the chemical environment on the carbon phase formation under HPHT, we selected the following set of samples containing sucrose: (a) pure sucrose; (b) sucrose (10 wt%) + water (90 wt%); (c) sucrose (10 wt%) + micrometer sized diamond powder (90 wt%), and (d) sucrose (5 wt%) + water (90 wt%) + NiCl 2 :6H 2 O (5 wt%), sucrose (22 wt%) + water (13 wt%) + NiCl 2 :6H 2 O (65 wt%). To avoid leakage, the watercontaining samples were frozen inside Cu containers before processing. The carbon solubility into copper is extremely low and this metal is not a good catalyst for diamond synthesis. The samples which were not diluted in water were processed inside a NaCl container. After processing, the samples were analyzed by a Raman microprobe, using a He-Ne laser (632.8 nm) as the excitation source. The samples which were not diluted in water were also analysed by X-ray diffraction (XRD), using a Siemens D500 diffractometer with monochromated Cu-Kα radiation. In order to improve the quality of the diffraction patterns from the small cylinders obtained after HPHT processing, the sample was placed over a monocrystalline silicon wafer. III. Results and Discussion Raman Spectroscopy Figures 1 and 2 show the Raman spectra for the pure sucrose and the mixture of sucrose (10 wt%) + water (90 wt%), processed at 7.7 GPa, during 15 min and different temperatures. For both samples the results indicate only the formation of graphite (sp 2 carbon phase). The D (~ 1330 cm -1 ) and the G (~1580 cm -1 ) peaks are observed for all the processing temperatures, but the D peak (~1620 cm -1 ) is only clearly identified for temperatures higher than 1000 o C. As can be seen from figure 3, for pure sucrose the width of the G peak slightly decreases with the increase of the processing temperature. This variation is much more significant for sucrose diluted in water. The Raman peaks of this sample processed at 1000 C show much larger widths than those observed for pure sucrose processed at 900 C. At higher temperatures the peaks widths are similar for both samples. This result indicates that the dilution of sucrose into water modifies the kinetics of phase formation (specially

3 graphite crystallization) at temperatures around 1000 C, even for a relatively short processing time (15 min) o C Intensity o C o C 900 o C Raman shift (cm -1 ) Figure 1: Raman spectra of pure sucrose processed during 15 min at 7.7 GPa, and temperatures in the range of 900 o C to 1700 o C o C Intensity (a.u.) o C 1000 o C Raman shift (cm -1 ) Figure 2: Raman spectra of sucrose + water processed during 15 min at 7.7 GPa, and temperatures of 1000 o C, 1200 o C and 1500 o C.

4 Sucrose Diluted sucrose FWHM G peak (cm -1 ) T ( o C) Figure 3: Temperature dependence of the FWHM of the G peak for pure and diluted sucrose after HPHT processing. For pure sucrose, during the Raman measurements the sample was randomly displaced on a motorized table. For the diluted sample, due to the very small amount of sample, the measurements were performed with the sample fixed at place. In this case, different positions were measured for each sample, corresponding to the several data points presented in the plot. The Raman spectra (figure 4) for the samples of diluted sucrose + NiCl 2 :6H 2 O processed at about 1500 C also show the production of graphite and there is no conclusive evidence of formation of other carbon phase. The Raman spectrum for the low diluted mixture [sucrose (22 wt%) + water (13 wt%) + NiCl 2 :6H 2 O (65 wt%)] is very similar to those obtained for pure and diluted sucrose processed at the same temperature. For the highly diluted mixture [sucrose (5 wt%) + water (90 wt%) + NiCl 2 :6H 2 O (5 wt%)], the spectrum is typical of a poorly crystallized graphite. The G peak is wide (FWHM 80 cm -1 ) and weak and the width of the D peak (~80 cm -1 ) is very large compared to the values obtained for other samples processed at the same temperature (~40 cm -1 ). In addition to the usual D and G peaks, there is also a weak peak around 1140 cm -1. A shoulder on the D peak, located at this region, has been already related to nanodiamond formation, but there is some controversy about this subject in the literature (Nemanich, 1988). The addition of NiCl 2 in the environment generated by the thermal decomposition of sucrose under HPHT hindered the graphitization process in the highly diluted mixture. However, this effect seems to be very similar to that observed by the simple dilution of sucrose in water.

5 2000 5% sucrose + 5% NiCl2 + 90% water 1500 diluted sucrose Intensity (a.u.) % sucrose + 65% NiCl2 + 13% water 500 sucrose Raman shift (cm -1 ) Figure 4: Raman spectra of samples processed at 1500 C and 7.7 GPa. X-ray diffraction No indication of formation of a new crystalline phase was observed for the sample of pure sucrose processed at 900 C. However, for the samples processed at higher temperatures the 002 diffraction peak of graphite was clearly identified. The results summarized in Table I, show that, with the increase of the processing temperature, there is a narrowing of the 002 peak together with a systematic displacement for higher values of 2θ (decreasing of d 002 ). This kind of effect has been associated in the literature to a better stacking of successive carbon layers and to the annealing of turbostratic defects (A. Onodera, 1990). For the sample processed at 1700 C, the 002 peak position corresponds to the interplanar distance (d 002 =3.356Å) observed for natural graphite crystals. For the mixture of sucrose and diamond powder processed at 900 C the X-ray diffraction pattern showed only the presence of diamond. In agreement with the results obtained for pure sucrose, the 002 peak of graphite was also observed for samples processed at higher temperatures. However, this peak was well defined only for the sample treated at 1700 C. As can be seen from Table 1, there is no systematic decrease of the d 002 value, neither a significant narrowing of the 002 peak with the increase of the processing temperatures. The XRD analysis of the sucrose+diamond mixture also showed a large decrease of the width of all the diamond diffraction peaks at higher processing temperatures. That can be seen from Table I, where the values for the 311 peak are shown for the sucrose+diamond mixture before (pristine sample) and after HPHT processing. The peak width (0.14 ) obtained

6 for the sample treated at 1700 C corresponds to the estimated instrumental contribution for the peak widening. The values obtained for a reference sample (diamond powder processed at the same conditions) are also shown in Table 1 for comparison. A great improvement in the crystallinity of the diamond phase was promoted by the presence of the products of the sucrose pyrolisis. We believe this is mainly related to the annealing of structural defects which seems to be enhanced in an environment with carbon and water at HPHT. Table 1: Results of x-ray diffraction analysis for the samples processed at 7.7 GPa and different temperatures. Sample T ( o C) sucrose sucrose+diamond diamond Graphite d 002 (Ǻ) FWHM d 002 (deg) Diamond d 311 (A) FHWM d 311 (deg) pristine IV. Conclusions The Raman and the XRD results show that the thermal decomposition of sucrose at 7.7 GPa yields the nucleation of graphite. In the case of pure sucrose, the larger concentration of carbon atoms seems to favour the growth of graphene layers even at 900 o C, as indicated by the small width of the G peak in the Raman spectra (Figures 1 and 3). For higher temperatures the XRD results indicate that these layers are stacked with a low level of defects. The graphitization also occurs for water diluted sucrose, but the narrowing of the G peak was only observed for higher temperatures. For both pure and diluted sucrose, it is possible to identify the D Raman peak at ~1620 cm -1. This satellite peak is probably related to turbostratic defects after the stacked graphene planes grow above a critical size. Despite the recent results of Davydov et al. (Davidov, 2004), we did not find any evidence of diamond formation from sucrose, processed at the same range of HPHT

7 conditions and encapsulated in a NaCl container, which should be able to allow the extensive decomposition of the starting organic compound. Also the presence of a possible metallic catalyst in the carbon+water solution did not induce the formation of diamond, at least for the relatively short processing time investigated (15 min). The only change observed was a hindering of the graphite formation, which could have the same origin of that observed by the simple dilution of sucrose in water. The thermal decomposition of sucrose at HPHT strongly improved the process of deffect annealing for micrometric diamond grains. This shows that there is no intrinsic kinectic limitation for the growth of a sp 3 carbon phase in the environment produced by the thermal decomposition of sucrose at HPHT. In conclusion, our results are in agreement with the observation of Onodera and coworkers (Onodera, 2000) that the graphitization process seems to be inevitable for the beginning stages of the thermal decomposition of organic materials in the presence of supercritical water, even at HPHT conditions where diamond is the stable phase of carbon. This was still observed when NiCl 2 was added in the solution processed at HPHT. However, the kinectics of graphite formation is strongly affected by the relative concentration of carbon in the solution and by the presence of diamond. Acknowledgments This work was partially supported by PRONEX/CNPQ and CAPES. References Bovenkerk H. P., Bundy F. P., Hall H. T., Strong H. M., Wentorf Jr. R. H. Nature 184, (1959). Davydov V. A., Rakhmanina A. V., Agafonov V., Narynbetov B., Boudou J. P. Carbon 42, (2004). Nemanich R.J., Glass J.T., Lucovsky G., Schroder R.E. J. Vac. Sci. Technol. A 6, (1988). Onodera A., Suito K.. Synthesis of diamond from carbonaceous materials. In: M. H. Maghanani, W. J. Nellis, M. F. Nicol (Eds.) Science and technology of high pressure. Proceedings of AIRAPT-17. Hyderabad, India. University Press, 2000, Yamaoka S., Kumar M. D. S., Kanda H., Akaishi M. Diamond Relat. Mater. 11, (2002). Yamaoka S., Kumar M. D. S., Kanda H., Akaishi M. Diamond Relat. Mater. 11, (2002). Wentorf Jr. R. H. Carbonaceous materials at high pressure and temperatures 69, (1965).