Chinese Journal of Chemistry, 2005, 23, 1027—1029 Full Paper

* E-mail: wenbin@dlut.edu.cn Received August 5, 2004; revised March 15, 2005; accepted April 20, 2005. Project supported by the National Natural Science Foundation of China (Nos. 50402025, 50234020, 50274018) and the Young Teacher’s Foundation

of Dalian University of Technology (No. 893202).

© 2005 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Transformation from n-Diamond to sp3-Banding Carbon

WEN, Bina,*(温斌) LI, Ting-Jub(李廷举) DONG, Chuanga(董闯) ZHANG, Xing-Guob(张兴国) YAO, Shanb(姚山) CAO, Zhi-Qiangb(曹志强)

WANG, De-Hea(王德和) JI, Shou-Huaa(季首华) JIN, Jun-Zeb(金俊泽) a Department of Materials Engineering, Dalian University of Technology, Dalian, Liaoning 116023, China

b Laboratory of Special Processing of Raw Materials, Dalian University of Technology, Dalian, Liaoning 116023, China

With admixture of n-diamond and diamond-like carbon powders (DLC) as carbon source, transparent wafers have been synthesized by hydrothermal process at 100 ℃ and atmosphere pressure. Scanning electron microscopy, X-ray diffraction, Raman spectroscopy, transmission electron microscopy, electron-probe microanalysis and Fourier-transform infrared spectrometer were used to analyze those transparent wafers. These results indicated that the transparent wafers were amorphous sp3-banding carbon wafer, and that the wafers were not aggregate of DLC from the carbon source but a new kind of reaction product by hydrothermal treatment.

Keywords hydrothermal process, n-diamond, diamond-like carbon

Introduction

Deducing from geological evidence, De Vries sug-gested that diamonds capably be synthesized by hydro-thermal process in 1987.1-3 In 1992, Yamaoka et al. ob-tained grown diamond crystal at 2200 ℃ and 7.7 GPa in the presence of graphite and water.4 In 1994, Gogotsi et al. reported that silicon carbide could be coated with sp3-banding carbon film of nanometer to micrometer thickness by hydrothermal treatment at 300—800 ℃.5,6 In 1995, Roy et al. used hydrothermal pyrolysis of hy-drocarbons and halogenated hydrocarbons to grow dia-mond on diamond seeds at 100—500 MPa and 800 ℃.7,2 In 1997, Zhao and Roy8 reported that aggregates, tens of micrometers in size, of small diamond crystal could be grown on diamond seeds under a hydrothermal environment of a mixture of glassy carbon, water and metal (usually pure nickel) at 140 MPa and 800 ℃. The studies mentioned above imply that diamond materials could be synthesized by hydrothermal process at a lower temperature and pressure on condition that the carbon source and metal catalyzer were selected correctly.

In 2001, Konyashin, Jarkov and their coworkers validated a new kind of carbon allotrope, which was a metallic form of carbon with face-centered cubic struc-ture, with a lattice constant of 0.3594 nm.9,10 In addition, this new allotrope has been reported in Refs. 9, 10, 14—19. The n-diamond was synthesized accidentally by various processes such as radio frequency plasma-aided decomposition of hydrocarbon,11 plasma-assisted chemical vapour deposition using diluted hydrocar-bons,12 transformation of graphite under shock com-

pression,13 transformation of C60 films under shock compression,14 transformation of graphite at high pres-sure and temperature,15 plasma-chemical synthesis with the aid of a carbon plasma jet,10 treatment of a diamond surface in hydrogen plasma9 annealing of silica wafers embedded with carbon atoms,16,17 etc.

The output of n-diamond was lower by the process mentioned above. Our recent work indicated that n-diamond powders could be produced largely by the method of catalyzed carbon black in a high magnetic field.18 The stability of n-diamond was studied recently, indicating that n-diamond was a metastable phase, and it can decompose at room temperature slowly.19

In this work, a unique experimental procedure was designed to transform the n-diamond at atmospheric pressure, i.e. hydrothermal experiment was done at a temperature of 100 ℃ , and finally amorphous sp3- banding carbon wafers were synthesized.

Experimental

Admixture of n-diamond and diamond-like carbon (DLC) was prepared as described in Ref. 18. An admix-ture of carbon black N231 powders and colloidal Fe(OH)3 (the reaction product of FeCl3 solution and NaOH solution) was compressed into an open stainless steel tank of 100-mL capacity. The mass ratio of carbon to iron in the mixture was 10∶1. The tank was main-tained at 300 ℃ for 100 min and sealed in air. In a high magnetic field of 10 T, the tank was maintained at 1100 ℃ for 100 min and then cooled to room temperature in

1028 Chin. J. Chem., 2005, Vol. 23, No. 8 WEN et al.

© 2005 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

the furnace. The products were washed with 6 mol/L H2SO4, rinsed with distilled water and dried in an oven at 110 ℃. The final products were analyzed in our pre-venient work in detail,18 and mainly consisted of mix-ture of n-diamond and DLC.

In this experiment, the admixture of n-diamond and DLC was processed hydrothermally by boiling distilled water at atmosphere pressure, and at the same time the admixture of n-diamond, DLC and water was stirred with a muddler until the admixture was dried. Ulti-mately, a few transparent wafers were synthesized in the admixture. Those transparent wafer samples were de-tected with scanning electron microscopy (SEM, JSM-5600LV), X-ray diffraction (XRD, XRD-6000), Raman spectroscopy (Spex 1403), transmission electron microscopy (TEM, JEM-100CXⅡ), Fourier-transform infrared spectrometer (FTIR, 20DXB FTIR) and elec-tron-probe microanalysis (EPMA, EPMA-1600). And the quantitative EPMA indicated that the content of the carbon element in the transparent wafers was 99.9 wt%.

Results and discussion

The scanning electron microscope image of a trans-parent wafer is shown in Figure 1, and the wafer is of about millimeter scale in width, and micron scale in thickness.

Figure 1 Scanning electron microscopy image of the wafer.

The phase composition of the wafers was examined using the Cu Kα radiation (wavelength 0.154 nm). The XRD pattern of the wafers shown in Figure 2 indicated that the transparent wafers were of an amorphous phase. To further support the XRD results as well as to clarify the phase nature of the amorphous carbon, TEM and Raman spectroscopy were used to characterize the wa-fer.

Figure 3a shows a bright field image of triturating of the transparent wafers. The corresponding electron dif-fraction pattern (Figure 3b) of triturating of the trans-parent wafers reveals one broad ring, typical of an amorphous phase. Figure 3c displays the selected area electron diffraction (SAED) pattern of nano DLC from catalyzed carbon black in a high magnetic field in Ref. 18, this SAED pattern has two broad rings and is dif-ferent from the electron diffraction pattern of triturating of the transparent wafers. These results indicated that the wafers were not aggregate of DLC from the carbon source but a new kind of reaction product by hydro-

thermal treatment. The TEM examination further vali-dated the XRD results.

Figure 2 X-ray diffraction pattern of the wafers.

Figure 3 Transmission electron microscopy bright field image of wafer trituration (a), corresponding electron diffraction pattern of the wafer triturating (b) and the SAED pattern of nano DLC from catalyzed carbon black in a high magnetic field in Ref. 18 (c).

A micro-Raman analysis was performed with the wafers using the 514.5 nm radiation of Ar-ion laser. The Raman spectrum (Figure 4) exhibits an intense broad peak at 1332 cm－1, indicating that the transparent wa-

Hydrothermal process Chin. J. Chem., 2005 Vol. 23 No. 8 1029

© 2005 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

fers were sp3-banding carbon.20-24 The broads peak at 1587 cm－1 shows the existence of some sp2-banding carbon in the wafers. Because the Raman scattering ef-ficiency for sp2-banding carbon is 50 times larger than the efficiency for the sp3-banding carbon, the transpar-ent wafers actually contain more of the sp3-banding form of carbon than what may be inferred from the spectra in Figure 4.25

Figure 4 Raman spectrum of the wafers.

Figure 5 illustrates the FTIR absorption spec- tra of the transparent wafers in the wavenumber range 400—4000 cm－1, and the FTIR spectra of the samples in KBr pellets were collected. KBr pellet spectra treated in the same condition as the sample pellet were used as the background ones in order to subtract the contamina-tion of water adsorbed by KBr. The spectra of the sam-ple were the same as those of nano diamonds reported by Jiang et al.26 The absorption bands of the transparent wafers in the spectra mainly correspond to two groups, one is related to the vibration of the sp3-banding carbon framework (2955, 2855, 1405 and 1095 cm－1), and the other is to the vibration of water (3460 cm－1).27

Figure 5 FTIR absorption spectra of the transparent wafers.

Conclusion

With admixture of n-diamond and DLC as carbon source, millimeter scale transparent wafers have been

synthesized by hydrothermal process at 100 ℃ and atmosphere pressure, and the transparent wafers were sp3-banding carbon wafers.

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(E0408052 ZHAO, X. J.; FAN, Y. Y.)