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Photochemical Modification of Diamond Films: Introduction of Perfluorooctyl Functional Groups on Their Surface Takako Nakamura,* Masahiro Suzuki, Masatou Ishihara, Tsuguyori Ohana, Akihiro Tanaka, and Yoshinori Koga National Institute of Advanced Industrial Science and Technology, Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Received January 8, 2004. In Final Form: April 27, 2004 Photolysis of perfluoroazooctane with diamond films led the chemical modification of the surface to introduce perfluorooctyl functional groups, confirmed by means of Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and time-of-flight secondary ion mass spectrometry measurements. Diamond films modified with fluorine moieties showed improved frictional property and reduction of surface energy, as evaluated by contact angle to water, compared with a pristine diamond film. The contact angle and friction coefficient of chemically modified diamond film are 118° and 0.1, respectively. The results of the value of the contact angle depending on irradiation times are consistent with those of the F/C ratio of fluorinated diamond films by monitoring with XPS.
Introduction Diamond is a material that has been widely investigated because of its various unique properties such as electrical, thermal, and mechanical properties.1 Chemical modifications of the diamond surfaces have been expected to lead to improvement of its native properties.2-4 We can find recent studies on introduction of organic functional groups such as benzoyloxy,5 DNA,6,7 and alkyl subsitutents8 on the surface of diamond films and powders terminated with hydrogen or oxygen by using photo- and thermochemical methods. On the other hand, 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 water/oil repellent properties.9 Introduction of substituents containing fluorine atoms would result in improvement of the behavior of diamond surfaces, which is enhanced lubricity and stability under extreme conditions.10 To date, fluorination of diamond surfaces has been studied by use of F2 gas,11 CF4 plasma,12 X-ray irradiation,13 and perfluoroalkyl * To whom correspondence may be addressed. Tel: +81-29-8619307. Fax: +81-29-851-5425. E-mail:
[email protected]. (1) Wei, J.; Yates, Jr., J. T. Crit. Rev. Surf. Chem. 1995, 5, 73. (2) Miller, J. B.; Brown, D. W. Langmuir 1996, 12, 5809. (3) Ando, T.; NishitaniGamo, M.; Rawles, R. E.; Yamamoto, K.; Kamo, M.; Sato, Y. Diamond Relat. Mater. 1996, 5, 1136. (4) Ikeda, Y.; Saito, T.; Kusakabe, K.; Morooka, S.; Maeda, H.; Taniguchi, Y. Fujiwara, Y. Diamond Relat. Mater. 1998, 7, 830. (5) Ida, S.; Tsubota, T.; Hirabayashi, O.; Nagata, M.; Matsumoto, Y.; Fujishima, A. Diamond Relat. Mater. 2003, 12, 601. (6) Knickerbocker, T.; Strother, T.; Schwartz, M. P.; Russell, J. N.; Butler, J.; Smith, L. M.; Hamers, R. J. Langmuir 2003, 19, 1938. (7) Rao, T. N.; Ivini, T. A.; Terashima, C.; Sarada, B. V.; Fujishima, A. New Diamond Front. Carbon Technol. 2003, 13, 79. (8) Strother, T.; Knickerbocker, T.; Russell, J. N.; Butler, J. E.; Smith, L. M.; Hamers, R. J. Langmuir 2002, 18, 968. (9) Banks, R. E. Organofluorine Chemicals and their Industrial Applications; Ellis Horwood: London, 1979. (10) Molian, P. A.; Janvrin, B.; Molian, A. M. Wear 1993, 165, 133. (11) Kealey, C. P.; Klapotke, T. M.; McComb, D. W.; Robertson, M. I.; Winfield, J. M. J. Mater. Chem. 2001, 11, 879. (12) Molian, P. A.; Janvrin, B.; Molian, A. M. J. Mater. Sci. 1995, 30, 4751. (13) Smentkowski, V. S.; Yates, J. T. Science 1996, 271, 193.
iodide under low temperatures.14 These methods make handling of the reactions difficult and make it necessary to use special vessels. Previously we reported that photolysis of perfluoroazooctane (1) gave perfluorooctyl radicals effectively in solutions under mild conditions15 for perfluoroalkylation of the surface of diamond powders.16 In this paper, we report on a useful method for chemical modification of diamond films with perfluoroalkyl substituents by using photolysis of 1 under mild conditions. The behavior of diamond surfaces modified with fluorine moieties was investigated by means of measurement of contact angles to water and by tribological tests. Experimental Section Samples and Reagents. Diamond films on Si substrate were purchased from Sumitomo Electric Industries Co., Ltd., as polycrystalline films grown by the chemical vapor deposition (CVD) method. Perfluoroazooctane (1) was prepared according to the literature.17 Perfluorohexane and n-hexane were obtained from Lancaster Synthesis, Inc., and from Kanto Chemical Co., Inc., respectively, and used without purification. Perfluorooctylation of Diamond Surfaces. 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 in the presence of diamond films (8 × 8 mm size) and an argon atmosphere with stirring. After being washed with three 2-mL portions of perfluorohexane and with 2 mL of hexane to remove unreacted starting material, the samples were analyzed by Fourier transform infrared spectroscopy using reflection absorption spectroscopy (RAS FT-IR), X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ionization mass spectrometer (TOF-SIMS), and Raman spectroscopy. The irradiation time dependence of perfluorooctylation on diamond film surfaces was investigated by monitoring with XPS. Measurements. FT-IR spectra were recorded with a JASCO FT/IR-680 Plus spectrometer using RAS attachment. XPS was measured by a PHI ESCA, model 5800, using an aluminum KR (14) Kim, C. S.; Mowrey, R. C.; Butler, J. E.; Russell, J. N. J. Phys. Chem. B 1998, 102, 9290. (15) Nakamura, T.; Yabe, A. J. Chem. Soc. Chem. Commun. 1995, 2027. (16) Nakamura, T.; Ishihara, M.; Ohana, T.; Koga, Y. Chem. Commun. 2003, 900. (17) Chambers, W. J.; Tullock, C. W.; Coffman, D. D. J. Am. Chem. Soc. 1962, 84, 2337.
10.1021/la0499362 CCC: $27.50 © 2004 American Chemical Society Published on Web 05/29/2004
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Figure 1. FT-IR spectra of (a) untreated diamond films and (b) diamond films after treatment of perfluorooctylation.
Figure 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.
radiation. The fluorinated sample was analyzed by TOF-SIMS (Physical Electronics TFS-2000). Primary ion bombardment was done with 15 kV Ga+ ions. The width of each ion pulse is 12 ns. Water contact angle measurements were carried out with an Elma G-1-1000. Sliding tests were performed with a reciprocating sliding tester in air at 55% humidity and temperature of 25 °C. Stainless steel balls (SUS440C) were used as opposing specimens against diamond films. Raman spectra were measured with a JASCO laser Raman spectrometer, NRS-2100, using the 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 model S-5000.
Scheme 1. Chemical Modification on the Surface of a Diamond Film with Perfluorooctyl Functionality
Results and Discussion
and then another C8F17 radical reacted with a remaining carbon radical on the diamond surface to give perfluorooctyl substituent on the surface (Scheme 1). As shown in Figure 2, XPS spectra of the diamond films were measured before and after perfluorooctylation, showing that new peaks at 687.9 and 800-900 eV, fluorine 1s and KLL Auger, respectively, appeared after irradiation accompanied with C 1s and O 1s peaks. In the carbon 1s region, a new peak at 290.9 eV with higher binding energies ascribed to a carbon atom bound to fluorine atom was observed by accompanying the diamond C 1s peak at 286.4 eV. To investigate the influence of the formation of perfluorooctyl functionality on a diamond surface, the fluorinated material was analyzed by Raman spectroscopy. As shown in Figure 1 of Supporting Information, a characteristic diamond peak was observed at 1337 cm-1, which was shifted and broadened compared with the pristine diamond film (1333 cm-1), with a new small peak at 1620 cm-1 ascribed to a defect peak in the case of fluorinated material. Vouagner et al. reported that UV laser irradiation to diamond films caused surface structural transformations correlating to stress vibrations in diamond with Raman peak shift and broadening.27 In this reaction, the diamond characteristic peak was shifted and broadened in a similar manner as previous report, and the defect peak appeared due to covalent functionalization at grain boundaries. Moreover, the presence of the fluorinated surface on a diamond film was characterized with a TOF-SIMS mea-
Preparation of Perfluorooctylated Diamond Film. Perfluorohexane solution of azo compound 1, prepared according to the literature,17 was irradiated with a lowpressure mercury lamp at room temperature in the presence of as-grown CVD diamond films under an argon atmosphere. After being washed with perfluorohexane and hexane to remove unreacted starting material, the samples were analyzed by RAS FT-IR, XPS, TOF-SIMS, and Raman spectroscopy. Characterization of Chemically Modified Diamond Films. Figure 1 shows RAS FT-IR spectra of the diamond films before and after treatment of irradiation with 1. The sample after the photoreaction indicates observations of a new peak at 1142 cm-1 with C-F stretching band and of disappearance of C-H stretching vibration of 2920 and 2851 cm-1 deriving from pristine diamond film. It is known that the surface of as-grown CVD diamond films has hydrogen termination.18-26 These results suggest that a perfluorooctyl radical, generated by photolysis of 1 with elimination of the nitrogen molecule, abstracted a hydrogen atom on diamond surface to produce C8F17H, (18) Chu, C. J.; D′Evelyn, M. P.; Hauge, R. H.; Margrave, J. L. J. Mater. Res. 1990, 5, 2405 (19) Celii, F. G.; Patterson, P. E.; Wang, H.-T.; Butler, J. E. Appl. Phys. Lett. 1988, 52, 2043. (20) Martin, L. R.; Hill, M. W. Appl. Phys. Lett. 1989, 55, 2248. (21) Celii, F. G.; Butler, J. E. Appl. Phys. Lett. 1989, 54, 1031. (22) Celii, F. G.; Pattersson, P. E.; Wang, H.-T.; Nelson, H. H.; Butler, J. E. AlP Conf. Proc. 1989, 191, 747. (23) Harris, S. J.; Martin, L. R. J. Mater. Res. 1990, 5, 2313. (24) Harris, S. J.; Weiner, A. M. J. Appl. Phys. 1990, 67, 6520. (25) Harris, S. J. Appl. Phys. Lett. 1990, 56, 2298. (26) Martin, L. R.; Hill, M. W. Mater Sci. Lett. 1990, 9, 621.
(27) Vouagner, D.; Champagnon, B.; LeBrusq, J.; Show, Y.; GirardeauMontaut, J. P. Appl. Surf. Sci. 2003, 208, 352.
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Figure 3. Water contact angle of (a) pristine and (b) fluorinated diamond films. SEM images of (c) untreated and (d) diamond films after treatment of perfluorooctylation.
surement for further confirmation. The positive spectrum shown in Figure 2 of Supporting Information exhibited peaks of m/z 12 (C+), 31 (CF+), 50 (CF2+), 69 (CF3+), 100 (C2F4+), 119 (C2F5+), 131 (C3F5+), 169 (C3F7+), 193 (C5F7+), and 243 (C6F9+), which were characteristic fragments from the perfluoroalkyl group. Surface Properties of Diamond Films with C8F17 Functionality. Introduction of fluorine moieties would be expected to decrease the diamond surface energy. Surface properties were investigated with measurements of contact angles of fluorinated diamond film to water. As shown in parts a and b of Figure 3, the contact angle of a C8F17 modified diamond film showed 118°, whereas an untreated diamond film showed 81°. It was found that a fluorine-modified diamond surface provided water repulsion behavior, comparable to poly(tetrafluoroethylene) (PTFE) whose contact angle is 103°.28 It was known that the value of the contact angle depends on surface morphology together with surface chemical structure. To investigate influence of surface morphology of C8F17 on a modified diamond surface, SEM images of pristine and treated diamond films were observed. As shown in parts c and d of Figure 3, 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 a C8F17 moiety. Furthermore, these surface properties depend on a F/C area ratio of XPS in Figure 4. Irradiation time dependence of the formation of a perfluorooctyl moiety on a diamond surface was investigated by monitoring with XPS. The diamond C 1s peak at 284.6 eV binding energy was used as an internal standard. Figure 4 shows the ratio of the area of a F 1s signal to that of the total signal as a function of irradiation time. It was found that due to the formation of C8F17 moiety on a diamond surface, the value of the fluorine atom increased with increase of irradiation time for 4 h and then decreased with longer irradiation time. We reported previously that 185 nm wavelength irradiation was effective for giving C8F17 radical from photolysis of azo compound 1.29 Longer irradiation times would lead to decomposition of C8F17 groups on the surface on account of the use of short wavelength light to result in dissociation of a C-C bond from the C8F17 moiety. The tendency of changes of contact angle as function of irradiation time is similar with that of F/C ratio. Diamond has good tribological properties, namely, low friction and high wear resistance.30,31 It is expected that (28) Inoue, Y.; Yoshimura, Y.; Ikeda, Y.; Kohno, A. Colloids Surf., B 2000, 19, 257. (29) Nakamura, T.; Yabe, A. Chem. Lett. 1995, 533.
Figure 4. Irradiation time dependence of F/C area ratio from XPS measurements and of contact angle of functionalized diamond films.
Figure 5. Friction coefficients of (a) pristine and (b) fluorinated diamond films.
chemical modification of diamond surface with fluorine functionalities would result in improvement of its friction property. To investigate mechanical properties of diamond film with C8F17 moieties, friction tests were used, showing that the friction coefficient of the chemically modified diamond film is 0.1 and that of pristine diamond film is 0.2 at a load of 9.8 mN (Figure 5). It shows that the introduction of a C8F17 functional group also provides change of friction property at low load. Conclusions Chemical modification of diamond film surface with perfluorooctyl functional groups was investigated by photolysis of perfluoroazooctane (1) upon 185 nm irradiation. The introduction of the perfluoroalkyl substituents was confirmed by FT-IR, XPS, and TOF-SIMS measurements. The comparison of Raman spectra between pristine and perfluorooctylated diamond films shows no significant (30) Bouden, F. P.; Tabor, D. The Friction and Lubrication of Solids Part 2; Clarendon: Oxford, 1954. (31) Gardos, M. N.; Soriano, B. L. J. Mater. Res. 1990, 5, 2599.
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changes, revealing the maintainance of diamond structure after the photoreaction. In this reaction, perfluorooctyl radical generated by photolysis of 1 with elimination of nitrogen molecule, abstracted hydrogen atom on diamond surface, and then the reaction of another C8F17 radical reacted with remaining carbon radicals of the diamond surface to give perfluorooctyl substituent on the surface. The fluorinated film shows water repulsion and low friction coefficient property, which are the contact angle 118° and friction coefficient 0.1. The results of the value of the
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contact angle depending on irradiation time are consistent with those of F/C ratio of fluorinated diamond film by monitoring with XPS. Supporting Information Available: Raman spectra of pristine and modified diamond films and TOF-SIMS spectrum of fluorinated diamond films. These materials are available free of charge via the Internet at http://pubs.acs.org. LA0499362
