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Chemical modification of diamond films using photolysis of perfluoroazooctane
Diamond and Related Materials 13 (2004) 1084–1087
Chemical modification of diamond films using photolysis of perfluoroazooctane T. Nakamura*, K. Tsugawa, M. Ishihara, T. Ohana, A. Tanaka, Y. Koga National Institute of Advanced Industrial Science and Technology, Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
Abstract Photolysis of perfluoroazooctane with diamond films led the chemical modification of the surface to introduce perfluorooctyl functional group, confirming by means of FT-IR, XPS, Raman and TOF-SIMS measurements. The diamond films modified with fluorine moiety showed improvement of reduction of surface energy evaluated by contact angle 1188 to water, compared with pristine diamond film. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Diamond film; Adsorption; Surface characterization; Surface energy
1. Introduction Diamond is a material, which has been widely investigated because of its various unique properties such as electrical, thermal and mechanical properties w1x. Chemical modifications of the diamond surfaces have been expected to lead improvement of its original behaviors w2–4x. We can find recent studies on introduction of organic functional groups such as benzoyloxy w5x, DNA w6,7x and alkyl substituents w8x on the surface of diamond films and powders terminated with hydrogen or oxygen by using photo- and thermochemical methods. However, perfluoroalkyl-containing organic compounds have attracted much attention in the field of medicinal chemistry and material science because of their unique properties derived from the presence of fluorine atoms, namely biological activities and watery oil repellent properties w9x. Introductions of substituents containing fluorine atom would result in improvement of the behavior of diamond surfaces, which is enhanced lubricity and stability under extreme conditions w10x. To date, fluorination of diamond surfaces has been studied by use of F2 gas w11x, CF4 plasma w12x and X-ray irradiation w13x. These methods would make problems about handling of the reactions and make it necessary to use special vessels. Previously we reported that *Corresponding author. Tel.: q81-29-861-9307 fax: q81-29-8515425. E-mail address: [email protected] (T. Nakamura).
photolysis of perfluoroazooctane (1) gave perfluorooctyl radicals effectively in solutions under mild conditions w14x for perfluoroalkylation of the surface of diamond powders w15x. In this article, we report on a useful method for chemical modification of diamond films with perfluoroalkyl substituent by using photolysis of 1 under mild condition. The behaviors of diamond surface modified with fluorine moiety were investigated by means of measurement of contact angle to water. 2. Experimental Diamond films on Si substrate (film thickness: 5–10 mm, grain size: 5–7 mm) were purchased from Sumitomo Electric Industries Co., Ltd. as polycrystalline films grown by CVD method. Perfluoroazooctane (1) was prepared according to the literature w16x. Perfluorohexane and n-hexane were obtained from Lancaster Synthesis Inc. and from Kanto Chemical Co., Inc., respectively, and used without purification. Azo compound 1 (3.7 mg) in perfluorohexane (4 ml) was irradiated with a 60 W low-pressure mercury lamp (Eikosha EL-S-SQ-60) at room temperature for 4 h in the presence of diamond films (8=8-mm size) and an argon atmosphere with stirring. After washing with three 2 ml portions of perfluorohexane and with 2 ml hexane to remove unreacted starting material, the samples were analyzed by Fourier transform infrared spectroscopy using reflection absorption spectroscopy (RAS
0925-9635/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2003.11.010
T. Nakamura et al. / Diamond and Related Materials 13 (2004) 1084–1087
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Scheme 1. Reaction mechanism of perfluorooctylation of H-terminated diamond films.
Fig. 1. FT-IR spectra of (a) untreated diamond films, (b) diamond films after treatment of perfluorooctylation.
FT-IR), X-ray photoelectron spectroscopy (XPS), timeof-flight secondary ionization mass spectrometer (TOFSIMS) and Raman spectroscopy. FT-IR spectra were recorded with a JASCO FTyIR680 Plus spectrometer using RAS attachment. XPS was measured by a PHI ESCA model5800 using an aluminum Ka radiation without sputtering the surface of diamond films before measurements in order to avoid any changes of chemical structure on the surface with Ar bombardment. The fluorinated sample was analyzed by TOF-SIMS (Physical Electronics TFS-2000). Primary ion bombardment was done with a 15 kV Gaq ions. The width of each ion pulse is 12 ns. Water contact angle measurements were carried out with an Elma G1-1000. Raman spectra were measured with a JASCO Laser Raman Spectrometer NRS-2100 using 514.5 nm line of an argon ion laser with power of 100 mW. Scanning electron microscopy (SEM) was used to examine surface morphology before and after irradiation. Samples were mounted on an Al stub using conducting carbon paste before examination in a Hitachi S-5000 model. 3. Results and discussion Perfluorohexane solution of azo compound 1 prepared according to the literature w16x, was irradiated with a low-pressure mercury lamp at room temperature for 4 h in the presence of as-grown CVD diamond films under an argon atmosphere. After washing with perfluorohexane and hexane to remove unreacted starting material, the samples were analyzed by RAS FT-IR, XPS, TOFSIMS and Raman spectroscopy. Fig. 1 shows RAS FT-IR spectra of the diamond films before and after treatment of irradiation with 1. The
Fig. 2. XPS spectra of diamond films (a) before and (b) after irradiation with perfluoroazooctane (1). The inset is carbon 1s spectra of fluorinated diamond films.
sample after the photoreaction indicates observations of a new peak at 1142 cmy1 with C–F stretching bands and of disappearance of C–H stretching vibration of 2920 and 2851 cmy1 deriving from pristine diamond film. It is known that the surface of as-grown CVD diamond films has hydrogen termination w17,18x. These results suggest that perfluorooctyl radical generated by photolysis of 1 with elimination of nitrogen molecule, abstracted hydrogen atom on diamond surface to produce C8F17H, and then another C8F17 radical reacted with remained carbon radical of diamond surface to give perfluorooctyl substituent on the surface (Scheme 1). As shown in Fig. 2, XPS spectra of the diamond films were measured before and after treatment of perfluorooctylation, showing that a new peak at 687.9 eV of fluorine 1s appeared after irradiation. In the carbon 1s region, a new peak at 290.9 eV with higher binding energies ascribed to carbon atom bound to fluorine atom was observed by accompanying with diamond C1s peak at 286.4 eV. In order to reveal the formation of perfluorooctyl functionality on diamond surface, the fluorinated mate-
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T. Nakamura et al. / Diamond and Related Materials 13 (2004) 1084–1087
Fig. 3. Raman spectra of (a) pristine diamond films, (b) diamond films after treatment of perfluorooctylation.
rial was analyzed with Raman spectroscopy. As shown in Fig. 3, a broadened peak at 1337 cmy1 was observed with red shift (4 cmy1), compared with that of untreated diamond films at 1333 cmy1. These results show that diamond peak was shifted and became broad due to the introduction of fluorine-containing functional group, and that the chemical modification of diamond films did not influence its diamond structure after the photoreaction. Moreover, the presence of the fluorinated surface on diamond film was characterized with a TOF-SIMS measurement for further confirmation. The positive
Fig. 4. TOF-SIMS spectrum of fluorinated diamond films.
spectrum shown in Fig. 4 exhibited peaks of myz 12 q q (Cq), 31 (CFq), 50 (CFq 2 ), 69 (CF3 ), 100 (C2F4 ), q q q q 119 (C2F5 ), 131 (C3F5 ), 169 (C3F7 ), 193 (C5F7 ), q q q 307 (C5Fq 13), 326 (C5F14), 345 (C5F15), 357 (C6F15), q q 376 (C6F16), 395 (C6F17), which were characteristic fragments from perfluoroalkyl group. Introduction of fluorine moiety would be expected the decrease in surface energy. Surface properties were investigated with measurements of contact angles of fluorinated diamond film to water. As shown in Fig. 5a and b, contact angle of C8F17 modified diamond film showed 1188, whereas untreated diamond film did 818.
Fig. 5. Water contact angle of (a) pristine and (b) fluorinated diamond films. SEM images of (c) untreated and (d) diamond films after treatment of perfluorooctylation.
T. Nakamura et al. / Diamond and Related Materials 13 (2004) 1084–1087
It was found that fluorine modified diamond surface provided water repulsion behavior, comparable to polytetrafluoroethylene (PTFE) whose contact angle is 1038 w19x. It was known that the value of contact angle depends on surface morphology together with surface chemical structure. In order to investigate influence of surface morphology of C8F17 modified diamond surface, SEM images of pristine and treated diamond films were observed. As shown in Fig. 5c and d, surface morphologies of diamond films did not change before and after modification with C8F17 moiety. These results indicate that this water repulsion behavior of fluorinated diamond surface was derived from introduction of C8F17 moiety. 4. Conclusion Photolysis of azo compound 1 with diamond films led a chemical modification of the surface to form perfluorooctyl functional groups. The introduction of the perfluoroalkyl substituents was confirmed by FT-IR, Raman, XPS and TOF-SIMS measurements. The fluorinated film shows improvement of water repulsion behavior compared with pristine diamond film. References w1x J. Wei, J.T. Yates Jr, Crit. Rev. Surf. Chem. 5 (1995) 73. w2x J.B. Miller, D.W. Brown, Langmuir 12 (1996) 5809.
1087
w3x T. Ando, M. NishitaniGamo, R.E. Rawles, K. Yamamoto, M. Kamo, Y. Sato, Diamond Relat. Mater. 5 (1996) 1136. w4x Y. Ikeda, T. Saito, K. Kusakabe, S. Morooka, H. Maeda, Y. Taniguchi, et al., Diamond Relat. Mater. 7 (1998) 830. w5x S. Ida, T. Tsubota, O. Hirabayashi, M. Nagata, Y. Matsumoto, A. Fujishima, Diamond Relat. Mater. 12 (2003) 601. w6x T. Knickerbocker, T. Strother, M.P. Schwartz, J.N. Russell, J. Butler, L.M. Smith, et al., Langmuir 19 (2003) 1938. w7x T.N. Rao, T.A. Ivini, C. Terashima, B.V. Sarada, A. Fujishima, New Diam. Front. Carbon Technol. 13 (2003) 79. w8x T. Strother, T Knickerbocker, J.N. Russell, J.E. Butler, L.M. Smith, R.J. Hamers, Langmuir 18 (2002) 968. w9x R.E. Banks, Organofluorine Chemicals and their Industrial Applications, Ellis Horwood, London, 1979. w10x P.A. Molian, B. Janvrin, A.M. Molian, Wear 165 (1993) 133. w11x C.P. Kealey, T.M. Klapotke, D.W. McComb, M.I. Robertson, J.M. Winfield, J. Mater. Chem. 11 (2001) 879. w12x P.A. Molian, B. Janvrin, A.M. Molian, J. Mater. Sci. 30 (1995) 4751. w13x V.S. Smentkowski, J.T. Yates, Science 271 (1996) 193. w14x T. Nakamura, A. Yabe, J. Chem. Soc. Chem. Commun. (1995) 2027. w15x T. Nakamura, M. Ishihara, T. Ohana, Y. Koga, Chem. Commun. (2003) 900. w16x W.J. Chambers, C.W. Tullock, D.D. Coffman, J. Am. Chem. Soc. 84 (1962) 2337. w17x C.J. Chu, M.P. D’Evelyn, R.H. Hauge, J.L. Margrave, J. Mater. Res. 5 (1990) 2405. w18x F.G. Celii, P.E. Patterson, H.-T. Wang, J.E. Butler, Appl. Phys. Lett. 52 (1988) 2043. w19x Y. Inoue, Y. Yoshimura, Y. Ikeda, A. Kohno, Coll. Suf. B 19 (2000) 257.
Chemical modification of diamond films using photolysis of perfluoroazooctane T. Nakamura*, K. Tsugawa, M. Ishihara, T. Ohana, A. Tanaka, Y. Koga National Institute of Advanced Industrial Science and Technology, Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
Abstract Photolysis of perfluoroazooctane with diamond films led the chemical modification of the surface to introduce perfluorooctyl functional group, confirming by means of FT-IR, XPS, Raman and TOF-SIMS measurements. The diamond films modified with fluorine moiety showed improvement of reduction of surface energy evaluated by contact angle 1188 to water, compared with pristine diamond film. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Diamond film; Adsorption; Surface characterization; Surface energy
1. Introduction Diamond is a material, which has been widely investigated because of its various unique properties such as electrical, thermal and mechanical properties w1x. Chemical modifications of the diamond surfaces have been expected to lead improvement of its original behaviors w2–4x. We can find recent studies on introduction of organic functional groups such as benzoyloxy w5x, DNA w6,7x and alkyl substituents w8x on the surface of diamond films and powders terminated with hydrogen or oxygen by using photo- and thermochemical methods. However, perfluoroalkyl-containing organic compounds have attracted much attention in the field of medicinal chemistry and material science because of their unique properties derived from the presence of fluorine atoms, namely biological activities and watery oil repellent properties w9x. Introductions of substituents containing fluorine atom would result in improvement of the behavior of diamond surfaces, which is enhanced lubricity and stability under extreme conditions w10x. To date, fluorination of diamond surfaces has been studied by use of F2 gas w11x, CF4 plasma w12x and X-ray irradiation w13x. These methods would make problems about handling of the reactions and make it necessary to use special vessels. Previously we reported that *Corresponding author. Tel.: q81-29-861-9307 fax: q81-29-8515425. E-mail address: [email protected] (T. Nakamura).
photolysis of perfluoroazooctane (1) gave perfluorooctyl radicals effectively in solutions under mild conditions w14x for perfluoroalkylation of the surface of diamond powders w15x. In this article, we report on a useful method for chemical modification of diamond films with perfluoroalkyl substituent by using photolysis of 1 under mild condition. The behaviors of diamond surface modified with fluorine moiety were investigated by means of measurement of contact angle to water. 2. Experimental Diamond films on Si substrate (film thickness: 5–10 mm, grain size: 5–7 mm) were purchased from Sumitomo Electric Industries Co., Ltd. as polycrystalline films grown by CVD method. Perfluoroazooctane (1) was prepared according to the literature w16x. Perfluorohexane and n-hexane were obtained from Lancaster Synthesis Inc. and from Kanto Chemical Co., Inc., respectively, and used without purification. Azo compound 1 (3.7 mg) in perfluorohexane (4 ml) was irradiated with a 60 W low-pressure mercury lamp (Eikosha EL-S-SQ-60) at room temperature for 4 h in the presence of diamond films (8=8-mm size) and an argon atmosphere with stirring. After washing with three 2 ml portions of perfluorohexane and with 2 ml hexane to remove unreacted starting material, the samples were analyzed by Fourier transform infrared spectroscopy using reflection absorption spectroscopy (RAS
0925-9635/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2003.11.010
T. Nakamura et al. / Diamond and Related Materials 13 (2004) 1084–1087
1085
Scheme 1. Reaction mechanism of perfluorooctylation of H-terminated diamond films.
Fig. 1. FT-IR spectra of (a) untreated diamond films, (b) diamond films after treatment of perfluorooctylation.
FT-IR), X-ray photoelectron spectroscopy (XPS), timeof-flight secondary ionization mass spectrometer (TOFSIMS) and Raman spectroscopy. FT-IR spectra were recorded with a JASCO FTyIR680 Plus spectrometer using RAS attachment. XPS was measured by a PHI ESCA model5800 using an aluminum Ka radiation without sputtering the surface of diamond films before measurements in order to avoid any changes of chemical structure on the surface with Ar bombardment. The fluorinated sample was analyzed by TOF-SIMS (Physical Electronics TFS-2000). Primary ion bombardment was done with a 15 kV Gaq ions. The width of each ion pulse is 12 ns. Water contact angle measurements were carried out with an Elma G1-1000. Raman spectra were measured with a JASCO Laser Raman Spectrometer NRS-2100 using 514.5 nm line of an argon ion laser with power of 100 mW. Scanning electron microscopy (SEM) was used to examine surface morphology before and after irradiation. Samples were mounted on an Al stub using conducting carbon paste before examination in a Hitachi S-5000 model. 3. Results and discussion Perfluorohexane solution of azo compound 1 prepared according to the literature w16x, was irradiated with a low-pressure mercury lamp at room temperature for 4 h in the presence of as-grown CVD diamond films under an argon atmosphere. After washing with perfluorohexane and hexane to remove unreacted starting material, the samples were analyzed by RAS FT-IR, XPS, TOFSIMS and Raman spectroscopy. Fig. 1 shows RAS FT-IR spectra of the diamond films before and after treatment of irradiation with 1. The
Fig. 2. XPS spectra of diamond films (a) before and (b) after irradiation with perfluoroazooctane (1). The inset is carbon 1s spectra of fluorinated diamond films.
sample after the photoreaction indicates observations of a new peak at 1142 cmy1 with C–F stretching bands and of disappearance of C–H stretching vibration of 2920 and 2851 cmy1 deriving from pristine diamond film. It is known that the surface of as-grown CVD diamond films has hydrogen termination w17,18x. These results suggest that perfluorooctyl radical generated by photolysis of 1 with elimination of nitrogen molecule, abstracted hydrogen atom on diamond surface to produce C8F17H, and then another C8F17 radical reacted with remained carbon radical of diamond surface to give perfluorooctyl substituent on the surface (Scheme 1). As shown in Fig. 2, XPS spectra of the diamond films were measured before and after treatment of perfluorooctylation, showing that a new peak at 687.9 eV of fluorine 1s appeared after irradiation. In the carbon 1s region, a new peak at 290.9 eV with higher binding energies ascribed to carbon atom bound to fluorine atom was observed by accompanying with diamond C1s peak at 286.4 eV. In order to reveal the formation of perfluorooctyl functionality on diamond surface, the fluorinated mate-
1086
T. Nakamura et al. / Diamond and Related Materials 13 (2004) 1084–1087
Fig. 3. Raman spectra of (a) pristine diamond films, (b) diamond films after treatment of perfluorooctylation.
rial was analyzed with Raman spectroscopy. As shown in Fig. 3, a broadened peak at 1337 cmy1 was observed with red shift (4 cmy1), compared with that of untreated diamond films at 1333 cmy1. These results show that diamond peak was shifted and became broad due to the introduction of fluorine-containing functional group, and that the chemical modification of diamond films did not influence its diamond structure after the photoreaction. Moreover, the presence of the fluorinated surface on diamond film was characterized with a TOF-SIMS measurement for further confirmation. The positive
Fig. 4. TOF-SIMS spectrum of fluorinated diamond films.
spectrum shown in Fig. 4 exhibited peaks of myz 12 q q (Cq), 31 (CFq), 50 (CFq 2 ), 69 (CF3 ), 100 (C2F4 ), q q q q 119 (C2F5 ), 131 (C3F5 ), 169 (C3F7 ), 193 (C5F7 ), q q q 307 (C5Fq 13), 326 (C5F14), 345 (C5F15), 357 (C6F15), q q 376 (C6F16), 395 (C6F17), which were characteristic fragments from perfluoroalkyl group. Introduction of fluorine moiety would be expected the decrease in surface energy. Surface properties were investigated with measurements of contact angles of fluorinated diamond film to water. As shown in Fig. 5a and b, contact angle of C8F17 modified diamond film showed 1188, whereas untreated diamond film did 818.
Fig. 5. Water contact angle of (a) pristine and (b) fluorinated diamond films. SEM images of (c) untreated and (d) diamond films after treatment of perfluorooctylation.
T. Nakamura et al. / Diamond and Related Materials 13 (2004) 1084–1087
It was found that fluorine modified diamond surface provided water repulsion behavior, comparable to polytetrafluoroethylene (PTFE) whose contact angle is 1038 w19x. It was known that the value of contact angle depends on surface morphology together with surface chemical structure. In order to investigate influence of surface morphology of C8F17 modified diamond surface, SEM images of pristine and treated diamond films were observed. As shown in Fig. 5c and d, surface morphologies of diamond films did not change before and after modification with C8F17 moiety. These results indicate that this water repulsion behavior of fluorinated diamond surface was derived from introduction of C8F17 moiety. 4. Conclusion Photolysis of azo compound 1 with diamond films led a chemical modification of the surface to form perfluorooctyl functional groups. The introduction of the perfluoroalkyl substituents was confirmed by FT-IR, Raman, XPS and TOF-SIMS measurements. The fluorinated film shows improvement of water repulsion behavior compared with pristine diamond film. References w1x J. Wei, J.T. Yates Jr, Crit. Rev. Surf. Chem. 5 (1995) 73. w2x J.B. Miller, D.W. Brown, Langmuir 12 (1996) 5809.
1087
w3x T. Ando, M. NishitaniGamo, R.E. Rawles, K. Yamamoto, M. Kamo, Y. Sato, Diamond Relat. Mater. 5 (1996) 1136. w4x Y. Ikeda, T. Saito, K. Kusakabe, S. Morooka, H. Maeda, Y. Taniguchi, et al., Diamond Relat. Mater. 7 (1998) 830. w5x S. Ida, T. Tsubota, O. Hirabayashi, M. Nagata, Y. Matsumoto, A. Fujishima, Diamond Relat. Mater. 12 (2003) 601. w6x T. Knickerbocker, T. Strother, M.P. Schwartz, J.N. Russell, J. Butler, L.M. Smith, et al., Langmuir 19 (2003) 1938. w7x T.N. Rao, T.A. Ivini, C. Terashima, B.V. Sarada, A. Fujishima, New Diam. Front. Carbon Technol. 13 (2003) 79. w8x T. Strother, T Knickerbocker, J.N. Russell, J.E. Butler, L.M. Smith, R.J. Hamers, Langmuir 18 (2002) 968. w9x R.E. Banks, Organofluorine Chemicals and their Industrial Applications, Ellis Horwood, London, 1979. w10x P.A. Molian, B. Janvrin, A.M. Molian, Wear 165 (1993) 133. w11x C.P. Kealey, T.M. Klapotke, D.W. McComb, M.I. Robertson, J.M. Winfield, J. Mater. Chem. 11 (2001) 879. w12x P.A. Molian, B. Janvrin, A.M. Molian, J. Mater. Sci. 30 (1995) 4751. w13x V.S. Smentkowski, J.T. Yates, Science 271 (1996) 193. w14x T. Nakamura, A. Yabe, J. Chem. Soc. Chem. Commun. (1995) 2027. w15x T. Nakamura, M. Ishihara, T. Ohana, Y. Koga, Chem. Commun. (2003) 900. w16x W.J. Chambers, C.W. Tullock, D.D. Coffman, J. Am. Chem. Soc. 84 (1962) 2337. w17x C.J. Chu, M.P. D’Evelyn, R.H. Hauge, J.L. Margrave, J. Mater. Res. 5 (1990) 2405. w18x F.G. Celii, P.E. Patterson, H.-T. Wang, J.E. Butler, Appl. Phys. Lett. 52 (1988) 2043. w19x Y. Inoue, Y. Yoshimura, Y. Ikeda, A. Kohno, Coll. Suf. B 19 (2000) 257.