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Adsorption behaviors of methanol, ethanol, n-butanol, n-hexanol and n-octanol on mica surface studied by atomic force microscopy
Thin Solid Films 458 (2004) 197–202
Adsorption behaviors of methanol, ethanol, n-butanol, n-hexanol and n-octanol on mica surface studied by atomic force microscopy Li Wang, Yonghai Song, Bailin Zhang, Erkang Wang* State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun Jilin 130022, PR China Received 22 May 2003; received in revised form 22 November 2003; accepted 5 January 2004
Abstract The adsorption behavior of methanol, ethanol, n-butanol, n-hexanol and n-octanol on mica surface was investigated by atomic force microscopy. All these alcohols have formed homogeneous films with different characteristics. Upright standing bilayer structure was formed on methanol adsorbed mica surface. For ethanol, bilayer structure and monolayer one were simultaneously formed, while for n-butanol and n-hexanol, rough films were observed. What was formed for n-octanol? Close-packed flat film was observed on n-octanol adsorbed mica substrate, the film was assumed to be a tilted monolayer. The possible adsorption model for each alcohol molecule was proposed according to its adsorption behavior. 䊚 2004 Elsevier B.V. All rights reserved. Keywords: Atomic force microscopy; Adsorption; Methanol; Ethanol; n-Butanol; n-Hexanol; n-Octanol; Mica
1. Introduction Organic monolayers in molecular thickness play an important role in many interfacial phenomena, including wetting, lubrication, adhesion, and molecular and biological recognition w1x. Research on the formation of organic monolayers by self-assembly (including physisorption or chemical bonding) on metal and other substrates has experienced exponential growth in the past decade w2x. Many phenomena of interest for a wide range of fields are related to the adsorption of alkanol on surfaces in ambient conditions. Processes such as oxidation, lubrication and catalysis are just a few examples. The study of the adsorption process of alkanols on a nanometric scale is important for the fundamental understanding of these processes w3x. The adsorption behaviour of long-chain alkanes and alkyl derivatives on the basal plane of graphite studied by scanning tunnelling microscopy (STM) has been of considerable interest as a model for molecular adsorption w4–17x. These measurements have shown that alkanes and alkanols formed densely packed monolayers with *Corresponding author. Tel.: q86-4315262003; fax: q864315689711. E-mail address: [email protected] (E. Wang).
their carbon skeleton planes parallel to the surface, and the molecular axes are perpendicular to the lamellar boundaries. STM studies have been further applied to the physisorbed hydrocarbon monolayers on the basal planes of MoSe2, MoS2 w18,19x and gold substrate w20– 23x. Tapping䉸 mode atomic force microscopy (AFM) is the most adequate method for soft and fragile adsorbed films of small molecules and soft samples w24–27x. Recently, different methods related to AFM have also been applied to study the adsorption of small molecules such as water molecules w28–32x, the structured water layer on mica w33x, the formation and shape of liquid drops w34x, the wetting behavior of liquid crystal and metal films w35x, the various aggregate shapes for surfactant on graphite, mica, silica and gold surfaces w36–43x, and the self-organization of physisorbed secondary alcohol molecules on a graphite surface w44x. However, studies concerning alcohol monolayers adsorbed on mica surface had not been studied extensively. It is well known that the surface of freshly cleaved mica is hydrophilic and will readily adsorb water molecules and short chain alkanol molecules when exposed to their environments. These thin films can vary from one to several molecular monolayers and are
0040-6090/04/$ - see front matter 䊚 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.01.043
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thought to play a key role in atmospheric and environmental chemistry. The interaction and adhesive forces between a mica plate and SiO2 surfaces in water– alcohol mixture and their correlation with the microstructure of the layer adsorbed on the interface were investigated using AFM w45x. The adsorption behaviors of methanol on substrates of different surface properties (mica, highly oriented pyrolytic graphite and indium– tin oxide glass) were studied in our previous work w46x, and images with different features were observed on these substrates. The growth kinetics of self-assembled monolayers formed by exposing mica to octanol solution has been studied by AFM and Fourier-transform infrared spectroscopy w47x, and the integral flat monlayer was formed after 24 h adsorption and growth. However, no systematic study on the adsorption behavior of alcohol molecules with different lengths on mica surface has been reported. In this study, the adsorption behavior of methanol, ethanol, n-butanol, nhexanol and n-octanol on mica surface was investigated by tapping䉸 mode AFM. And the possible adsorption model of each alcohol molecule was proposed, respectively. 2. Experimental details 2.1. Chemicals Methanol, ethanol, 1-butanol, 1-hexanol and 1-octanol of analytical-reagent grade were purchased from Beijing Chemical Reagent Factory (China) and purified by redistillation.
Fig. 1. Typical AFM image (scan area: 1.0=1.0 mm2, vertical scale: 2 nm) of methanol bilayer adsorbed mica surface and the section analysis of the line traversing the image. The depth of the pits was estimated as 0.85 nm.
2.2. Sample preparation
minimize the force of interaction between the tip and the surface. The scan rate was 1.0 Hz (256=256 pixels), and integral and proportional gains were both set as 0.18. All images represent typical film features from repeated scans performed on different samples with different tips. The thickness of the film is estimated by section analysis of defects in the film on mica. Root mean square (RMS) roughness analysis was used to evaluate the mean height of the film.
Freshly cleaved mica was immersed into different alcohol solutions for 24 h, which is long enough for all species to reach equilibrium. Then the sample was dried in desiccator before each test. To minimize the influence of evaporation, drying time was controlled. For methanol and ethanol, drying time was approximately 30–40 min. For butanol and hexanol, drying time was elongated to 1–2 h. And the drying time was 3–4 h for octanol.
3. Results and discussions 2.3. AFM measurement AFM images were obtained with a Nanoscope IIIa (Digital Instruments, Santa Barbara, CA) in tapping䉸 mode at room temperature under ambient conditions. Silicon cantilevers were purchased from Veeco Instruments Inc. (125 mm nominal length, resonance frequency of approx. 300 kHz). Signal amplitude from the free resonance of the tip was set at 2.000 V, amplitude set point during scanning was varied in the range of 1.400– 1.600 V. In general, the set point was kept as high as possible without losing connection with the surface to
Fig. 1 shows a typical AFM image of mica surface in which formation of a structured methanol layer can be observed. The RMS roughness of 0.16 nm measured over an area of 2.0=2.0 mm2 indicates that the film was very smooth, while some pits with uniform size (30–40 nm in diameter) were unevenly distributed in the film. The depth of the pits as estimated by section analysis is approximately 0.85 nm. It corresponds to the length of two methanol molecules (the length of the methanol molecule is approximately 0.45 nm as calculated from van der Waals radii, and computational tools
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cess, bilayer and monolayer structure were all formed on mica surface. The thickness of the methanol and ethanol films nearly being the double of the length of the alcohol molecules, thus these molecules are assumed to adsorb vertically on mica surface in bilayer structure w45x. As freshly cleaved mica surface is hydrophilic, thus it is possible to assume that alcohol molecules are adsorbed vertically on the mica surface through the hydroxyl groups and alkylchains directed towards the alcohol solution in the first layer w49x. In the second layer, the hydrophobic tail of the alcohol molecules are facing the hydrophobic tail of the first layer leaving the hydroxy group pointing outward in the bilayer structure as illustrated in Scheme 1a–b. And the formed bilayer was most stable from the view of thermodynamics. In AFM images of n-butanol film adsorbed mica surface (as shown in Fig. 3), wormlike pattern emerges instead of the flat surface observed for methanol and ethanol films. The RMS roughness measured over an area of 2.0=2.0 mm2 was 0.33 nm. The bumps in the image were approximately 0.55–0.95 nm high and 50– 70 nm wide. The lengths of the long and short chains
Fig. 2. Typical AFM image (scan area: 1.0=1.0 mm2, vertical scale: 2.5 nm) of ethanol adsorbed mica surface and the section analysis of the line traversing the image. The depth of the pits was estimated as 1.0 nm.
based on molecular mechanics w48x was adopted to clarify the optimizing models of the alkanol molecule. The same method was also used to calculate the length of other alcohol molecules studied in the following). By in-situ AFM, Kanda et al. w45x have concluded that in water–alcohol (methanol, ethanol and n-propanol) mixtures, when the weight fraction of alcohol is sufficiently large ()0.99), a stable layer of alcohol is formed with vertical orientation of molecules on the substrate. According to their conclusions and our AFM results, we assume that methanol has adsorbed homogeneously onto mica and formed an upright standing bilayer structure. For ethanol, the adsorption behavior was quite similar to that of methanol. As shown in Fig. 2, a flat layer of ethanol with some defects was also observed by AFM. From the section analysis, the thickness of the film was estimated to be 1.0 nm, which corresponds to a length of two ethanol molecules (the length of the ethanol molecule is 0.55 nm, calculated as mentioned above). Therefore, we propose that ethanol molecules also form a bilayer structure on mica surface. Within these defects, some small terraces of 0.5 nm in height corresponding to a monolayer structure of ethanol molecules were also observed, indicating that during ethanol adsorption pro-
Scheme 1. Proposed models for methanol (a) and ethanol (b) bilayers adsorbed on mica surfaces. The molecules are upright stand on the surface.
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increasing length of the carbon-chain, the inter-molecular interactions enhanced, thus the molecules were arranged more compact and the film was more stable and order. For n-octanol, the adsorption behavior was very different from those alkanols mentioned above. Flat film with few defects was observed on the mica surface, as shown in Fig. 5. The RMS roughness measured over an area of 2.0=2.0 mm2 was 0.23 nm. The thickness of the film was estimated to be 0.80 nm, which is shorter than the length of an n-octanol molecule (approx. 1.2 nm, calculated as mentioned above). Accordingly, the n-octanol molecules were proposed to adsorb on mica surface with the hydrocarbon chain has a 408 bevel with respect to the mica surface and form a compact monolayer. The growth kinetics of n-octanol on mica surface has been discussed in detail in our previous work w47x. From these AFM results, the thicknesses of n-butanol, n-hexanol and n-octanol molecular films were found to be smaller than the length of these molecules and bigger than their short axis. These molecules were thus proposed to adsorb onto mica surface in monolayer with tilted orientation. It was very different from the bilayer structures formed for methanol and ethanol. We think it
Fig. 3. Typical AFM image (scan area: 0.9=0.9 mm2, vertical scale: 2 nm) of butanol adsorbed mica surface.
of the butanol molecule are approximately 0.83 nm and 0.35 nm (calculated as mentioned above), respectively. Thus, the formed layer should not be an upright standing bilayer of butanol molecules or flat lying butanol layers. On the basis of XRD-measurements, Weiss et al. w50x suggested that the alkyl chains might lie flat or in a tilted upright position to the mica surface depending on the charge density of the substrate and the length of the chains. On the basis of our experimental results, we propose that butanol molecules adsorbed on mica surface with tilted or vertical orientation. The features observed for the n-hexanol films were similar to that of n-butanol, as shown in Fig. 4. nHexanol molecules adsorbed on mica surface in the form of some discontinuous bumps. These bumps were estimated to be 0.3–0.5 nm high and 20–50 nm wide. The heights of these bumps were much shorter than the length of an upright standing hexanol molecule (approx. 1.08 nm, calculated as mentioned above). Accordingly, the hexanol molecules seem to adsorb on the mica surface with tilted orientation. The RMS roughness measured over an area of 2.0=2.0 mm2 was 0.19 nm, indicating that the hexanol films were smoother than that of butanol. It might be due to the fact that with
Fig. 4. Typical AFM image (scan area: 1.0=1.0 mm2, vertical scale: 1 nm) of hexanol adsorbed mica surface.
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Fig. 5. Typical AFM image (scan area: 1.0=1.0 mm2, vertical scale: 2 nm) of the octanol adsorbed mica surface and the section analysis of the line traversing the image. The depth of the two layers was estimated as 0.8 nm.
is due to the fact that the bilayer structure could stabilize the film with lowest energy, while it is difficult for nbutanol, n-hexanol and n-octanol to form a bilayer structure, thus these alcohol molecules may exist mainly in monolayer structure. With the increasing length of the carbon-chain, the forces between molecule and molecule and molecule and substrate were increased. As a result the stability of molecular layers would be enhanced for a longer chain alkanol w50x. Thus the structure of n-octanol film was much more stable and ordered than that of n-butanol and n-hexanol. This was illustrated in Scheme 2a–c, which was corresponding to the experimental results depicted in Figs. 3–5 for nbutanol, n-hexanol and n-octanol, respectively. The influence of sample preparation on the adsorption process was also studied. If the drying time was elongated to one day or longer (depends on the specific species), the homogeneous structure disappeared from the substrate. The influence of adsorption time did not discussed here. In the present work, the main aim is to observe the adsorption behavior of alkanols with different carbon-chains under long enough adsorption time.
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Scheme 2. (a–c) Proposed models showing the evolution of alkanols adsorption with increased carbon-chains. The molecules are tilted on the mica.
4. Conclusions In summary, tapping䉸 mode AFM was applied to study the adsorption behavior of alcohols with different chain length, i.e. methanol, ethanol, n-butanol, n-hexanol and n-octanol. AFM results show that methanol and ethanol adsorbed on mica with bilayer structures, while n-butanol, n-hexanol and n-octanol in monolayer structures. The orientation of the first two molecules was vertical to the mica surface, while the last three molecules were tiled on the surface, only when the length of molecule was long enough that ordered compact film structure could be obtained. The adsorption of these simple linear saturated hydrocarbon chains on the basal plane of mica surface provides a model for molecular adsorption. The information may improve our knowledge of the mechanisms underlying various interfacial processes, and could help in the development of the theoretical models for dynamic simulations of molecule adsorption.
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Acknowledgments This work was supported by the National Natural Science Foundation of China (no. 20275037 and no. 29975028). References w1x J.P. Rabe, S. Buchholz, Science 253 (1991) 424. w2x A. Ulman, An Introduction to Ultrathin Organic Films, Academic Press, San Diego, CA, 1991. w3x M.A. Osman, M. Ernst, B.H. Meier, U.W. Suter, J. Phys. Chem. B 106 (2002) 653. w4x A.J. Groszek, Nature 196 (1962) 531. w5x A.J. Groszek, Nature 204 (1962) 680. w6x M. Liphard, P. Glanz, G. Pilarski, G.H. Findenegg, Prog. Colloid Polym. Sci. 67 (1980) 131. w7x U. Bien-Vogelsang, G.H. Findenegg, Colloid Surf. 21 (1986) 469. w8x G.H. Findenegg, M. Liphard, Carbon 25 (1987) 119. w9x G.C. McGonigal, R.H. Bernhardt, D.J. Thomson, Appl. Phys. Lett. 57 (1990) 28. w10x G.C. McGonigal, R.H. Bernhardt, Y.H. Yeo, D.J. Thomson, J. Vac. Sci. Technol. B 9 (1991) 1107. w11x G. Watel, F. Thibaudau, J. Cousty, Surf. Sci. Lett. 281 (1993) L297. w12x S. Buchholz, J.P. Rabe, Angew. Chem. Int. Edit. 31 (1992) 189. w13x C.L. Claypool, F. Faglioni, W.A. Goddard III, H.B. Gray, N.S. Lewis, R.A. Marcus, J. Phys. Chem. B 101 (1997) 5978. w14x A. Askadskaya, J.P. Rabe, Phys. Rev. Lett. 69 (1992) 1395. w15x H. Fang, L.C. Giancarlo, G.W. Flynn, J. Phys. Chem. B 102 (1998) 7421. w16x R. Hentschke, B.L. Schurmann, J.P. Rabe, J. Chem. Phys. 96 (1992) 6213. w17x K.W. Herwig, B. Matthies, H. Taub, Phys. Rev. Lett. 75 (1995) 3154. w18x S. Cincotti, J.P. Rabe, Appl. Phys. Lett. 62 (1993) 3531. w19x L.C. Giancarlo, H. Fang, S.M. Rubin, A.A. Bront, G.W. Flynn, J. Phys. Chem. B 102 (1998) 10255. w20x T.K. Xia, U. Landman, Science 261 (1993) 1310. w21x T.K. Xia, U. Landman, Phys. Rev. B 48 (1993) 11313. w22x K. Uosaki, R. Yamada, J. Am. Chem. Soc. 121 (1999) 4090. w23x Z.X. Xie, X. Xu, J. Tang, B.W. Mao, Chem. Phys. Lett. 323 (2000) 209. w24x S. Gao, L.F. Chi, S. Lenhert, B. Anczykowski, C.M. Niemeyer, M. Adler, H. Fuchs, Chem. Phys. Chem. 1 (2001) 384.
w25x W.H. Han, S.M. Lindsay, M. Dlakic, R.E. Harrington, Nature 386 (1997) 563. w26x R.B. Cary, S.R. Peterson, J.T. Wang, D.G. Bear, E.M. Bradbury, D.J. Chen, Proc. Natl. Acad. Sci. USA 94 (1997) 4267. w27x C. Rivetti, M. Guthold, C. Bustamante, J. Mol. Biol. 264 (1996) 919. w28x M. Luna, J. Colchero, A. Gil, J. Gomez-Herrero, A.M. Baro, Appl. Surf. Sci. 157 (2000) 393. w29x J. Hu, X.D. Xiao, M. Salmeron, Appl. Phys. Lett. 67 (1995) 476. w30x M. Luna, F. Rieutord, N.A. Melman, Q. Dai, M. Salmeron, J. Phys. Chem. A 102 (1998) 6793. w31x A. Gil, J. Colchero, M. Luna, J. Gomez-Herrero, A.M. Baro, Langmuir 16 (2000) 5086. w32x R.K. Workman, S. Manne, Langmuir 18 (2002) 661. w33x C. Spagnoli, K. Loos, A. Ulman, M.K. Cowman, J. Am. Chem. Soc. 125 (2003) 7124. w34x T. Pompe, A. Fery, S. Herminghaus, Langmuir 14 (1998) 2585. w35x S. Herminghaus, K. Jacobs, K. Mecke, J. Bischof, A. Fery, M. Ibn-Elhaj, S. Schlagowski, Science 282 (1998) 916. w36x B.Y. Li, M. Fujii, K. Fukada, T. Kato, T. Seimiya, Thin solid Films 312 (1998) 20. w37x S. Manne, J.P. Cleveland, H.E. Gaub, G.D. Stucky, P.K. Hansma, Langmuir 10 (1994) 4409. w38x S. Manne, H.E. Gaub, Science 270 (1995) 1480. w39x I.A. Aksay, M. Trau, S. Manne, I. Honma, N. Yao, L. Zhou, P. Fenter, P.M. Eisenberger, S.M. Gruner, Science 273 (1996) 892. w40x E.J. Wanless, W.A. Ducker, J. Phys. Chem. 100 (1996) 3207. w41x W.A. Ducker, L.M. Grant, J. Phys. Chem. 100 (1996) 11507. w42x W.A. Ducker, E.J. Wanless, Langmuir 12 (1996) 5915. w43x J.F. Liu, G. Min, W.A. Ducker, Langmuir 17 (2001) 4895. w44x C. Le Poulennec, J. Cousty, Z.X. Xie, C. Mioskowski, Surf. Sci. 448 (2000) 93. w45x Y. Kanda, T. Nakamura, K. Higashitani, Colloid Surf. A 139 (1998) 55. w46x L. Wang, Y. Song, A. Wu, Z. Li, B. Zhang, E. Wang, Appl. Surf. Sci. 199 (2002) 67. w47x L. Wang, J. Jiang, Y. Song, B. Zhang, E. Wang, Langmuir 19 (2002) 4953. w48x R.L. Andrew, Molecular Modeling Principles and Applications, Addison Wesley Longman, 1996, Chap. 3. w49x C.J. Brinker, G.W. Scherer, Sol–Gel Science, Academic Press, New York, 1990. w50x A. Weiss, Angew. Chem. Int. Edit. 2 (1963) 134.
Adsorption behaviors of methanol, ethanol, n-butanol, n-hexanol and n-octanol on mica surface studied by atomic force microscopy Li Wang, Yonghai Song, Bailin Zhang, Erkang Wang* State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun Jilin 130022, PR China Received 22 May 2003; received in revised form 22 November 2003; accepted 5 January 2004
Abstract The adsorption behavior of methanol, ethanol, n-butanol, n-hexanol and n-octanol on mica surface was investigated by atomic force microscopy. All these alcohols have formed homogeneous films with different characteristics. Upright standing bilayer structure was formed on methanol adsorbed mica surface. For ethanol, bilayer structure and monolayer one were simultaneously formed, while for n-butanol and n-hexanol, rough films were observed. What was formed for n-octanol? Close-packed flat film was observed on n-octanol adsorbed mica substrate, the film was assumed to be a tilted monolayer. The possible adsorption model for each alcohol molecule was proposed according to its adsorption behavior. 䊚 2004 Elsevier B.V. All rights reserved. Keywords: Atomic force microscopy; Adsorption; Methanol; Ethanol; n-Butanol; n-Hexanol; n-Octanol; Mica
1. Introduction Organic monolayers in molecular thickness play an important role in many interfacial phenomena, including wetting, lubrication, adhesion, and molecular and biological recognition w1x. Research on the formation of organic monolayers by self-assembly (including physisorption or chemical bonding) on metal and other substrates has experienced exponential growth in the past decade w2x. Many phenomena of interest for a wide range of fields are related to the adsorption of alkanol on surfaces in ambient conditions. Processes such as oxidation, lubrication and catalysis are just a few examples. The study of the adsorption process of alkanols on a nanometric scale is important for the fundamental understanding of these processes w3x. The adsorption behaviour of long-chain alkanes and alkyl derivatives on the basal plane of graphite studied by scanning tunnelling microscopy (STM) has been of considerable interest as a model for molecular adsorption w4–17x. These measurements have shown that alkanes and alkanols formed densely packed monolayers with *Corresponding author. Tel.: q86-4315262003; fax: q864315689711. E-mail address: [email protected] (E. Wang).
their carbon skeleton planes parallel to the surface, and the molecular axes are perpendicular to the lamellar boundaries. STM studies have been further applied to the physisorbed hydrocarbon monolayers on the basal planes of MoSe2, MoS2 w18,19x and gold substrate w20– 23x. Tapping䉸 mode atomic force microscopy (AFM) is the most adequate method for soft and fragile adsorbed films of small molecules and soft samples w24–27x. Recently, different methods related to AFM have also been applied to study the adsorption of small molecules such as water molecules w28–32x, the structured water layer on mica w33x, the formation and shape of liquid drops w34x, the wetting behavior of liquid crystal and metal films w35x, the various aggregate shapes for surfactant on graphite, mica, silica and gold surfaces w36–43x, and the self-organization of physisorbed secondary alcohol molecules on a graphite surface w44x. However, studies concerning alcohol monolayers adsorbed on mica surface had not been studied extensively. It is well known that the surface of freshly cleaved mica is hydrophilic and will readily adsorb water molecules and short chain alkanol molecules when exposed to their environments. These thin films can vary from one to several molecular monolayers and are
0040-6090/04/$ - see front matter 䊚 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.01.043
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thought to play a key role in atmospheric and environmental chemistry. The interaction and adhesive forces between a mica plate and SiO2 surfaces in water– alcohol mixture and their correlation with the microstructure of the layer adsorbed on the interface were investigated using AFM w45x. The adsorption behaviors of methanol on substrates of different surface properties (mica, highly oriented pyrolytic graphite and indium– tin oxide glass) were studied in our previous work w46x, and images with different features were observed on these substrates. The growth kinetics of self-assembled monolayers formed by exposing mica to octanol solution has been studied by AFM and Fourier-transform infrared spectroscopy w47x, and the integral flat monlayer was formed after 24 h adsorption and growth. However, no systematic study on the adsorption behavior of alcohol molecules with different lengths on mica surface has been reported. In this study, the adsorption behavior of methanol, ethanol, n-butanol, nhexanol and n-octanol on mica surface was investigated by tapping䉸 mode AFM. And the possible adsorption model of each alcohol molecule was proposed, respectively. 2. Experimental details 2.1. Chemicals Methanol, ethanol, 1-butanol, 1-hexanol and 1-octanol of analytical-reagent grade were purchased from Beijing Chemical Reagent Factory (China) and purified by redistillation.
Fig. 1. Typical AFM image (scan area: 1.0=1.0 mm2, vertical scale: 2 nm) of methanol bilayer adsorbed mica surface and the section analysis of the line traversing the image. The depth of the pits was estimated as 0.85 nm.
2.2. Sample preparation
minimize the force of interaction between the tip and the surface. The scan rate was 1.0 Hz (256=256 pixels), and integral and proportional gains were both set as 0.18. All images represent typical film features from repeated scans performed on different samples with different tips. The thickness of the film is estimated by section analysis of defects in the film on mica. Root mean square (RMS) roughness analysis was used to evaluate the mean height of the film.
Freshly cleaved mica was immersed into different alcohol solutions for 24 h, which is long enough for all species to reach equilibrium. Then the sample was dried in desiccator before each test. To minimize the influence of evaporation, drying time was controlled. For methanol and ethanol, drying time was approximately 30–40 min. For butanol and hexanol, drying time was elongated to 1–2 h. And the drying time was 3–4 h for octanol.
3. Results and discussions 2.3. AFM measurement AFM images were obtained with a Nanoscope IIIa (Digital Instruments, Santa Barbara, CA) in tapping䉸 mode at room temperature under ambient conditions. Silicon cantilevers were purchased from Veeco Instruments Inc. (125 mm nominal length, resonance frequency of approx. 300 kHz). Signal amplitude from the free resonance of the tip was set at 2.000 V, amplitude set point during scanning was varied in the range of 1.400– 1.600 V. In general, the set point was kept as high as possible without losing connection with the surface to
Fig. 1 shows a typical AFM image of mica surface in which formation of a structured methanol layer can be observed. The RMS roughness of 0.16 nm measured over an area of 2.0=2.0 mm2 indicates that the film was very smooth, while some pits with uniform size (30–40 nm in diameter) were unevenly distributed in the film. The depth of the pits as estimated by section analysis is approximately 0.85 nm. It corresponds to the length of two methanol molecules (the length of the methanol molecule is approximately 0.45 nm as calculated from van der Waals radii, and computational tools
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cess, bilayer and monolayer structure were all formed on mica surface. The thickness of the methanol and ethanol films nearly being the double of the length of the alcohol molecules, thus these molecules are assumed to adsorb vertically on mica surface in bilayer structure w45x. As freshly cleaved mica surface is hydrophilic, thus it is possible to assume that alcohol molecules are adsorbed vertically on the mica surface through the hydroxyl groups and alkylchains directed towards the alcohol solution in the first layer w49x. In the second layer, the hydrophobic tail of the alcohol molecules are facing the hydrophobic tail of the first layer leaving the hydroxy group pointing outward in the bilayer structure as illustrated in Scheme 1a–b. And the formed bilayer was most stable from the view of thermodynamics. In AFM images of n-butanol film adsorbed mica surface (as shown in Fig. 3), wormlike pattern emerges instead of the flat surface observed for methanol and ethanol films. The RMS roughness measured over an area of 2.0=2.0 mm2 was 0.33 nm. The bumps in the image were approximately 0.55–0.95 nm high and 50– 70 nm wide. The lengths of the long and short chains
Fig. 2. Typical AFM image (scan area: 1.0=1.0 mm2, vertical scale: 2.5 nm) of ethanol adsorbed mica surface and the section analysis of the line traversing the image. The depth of the pits was estimated as 1.0 nm.
based on molecular mechanics w48x was adopted to clarify the optimizing models of the alkanol molecule. The same method was also used to calculate the length of other alcohol molecules studied in the following). By in-situ AFM, Kanda et al. w45x have concluded that in water–alcohol (methanol, ethanol and n-propanol) mixtures, when the weight fraction of alcohol is sufficiently large ()0.99), a stable layer of alcohol is formed with vertical orientation of molecules on the substrate. According to their conclusions and our AFM results, we assume that methanol has adsorbed homogeneously onto mica and formed an upright standing bilayer structure. For ethanol, the adsorption behavior was quite similar to that of methanol. As shown in Fig. 2, a flat layer of ethanol with some defects was also observed by AFM. From the section analysis, the thickness of the film was estimated to be 1.0 nm, which corresponds to a length of two ethanol molecules (the length of the ethanol molecule is 0.55 nm, calculated as mentioned above). Therefore, we propose that ethanol molecules also form a bilayer structure on mica surface. Within these defects, some small terraces of 0.5 nm in height corresponding to a monolayer structure of ethanol molecules were also observed, indicating that during ethanol adsorption pro-
Scheme 1. Proposed models for methanol (a) and ethanol (b) bilayers adsorbed on mica surfaces. The molecules are upright stand on the surface.
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increasing length of the carbon-chain, the inter-molecular interactions enhanced, thus the molecules were arranged more compact and the film was more stable and order. For n-octanol, the adsorption behavior was very different from those alkanols mentioned above. Flat film with few defects was observed on the mica surface, as shown in Fig. 5. The RMS roughness measured over an area of 2.0=2.0 mm2 was 0.23 nm. The thickness of the film was estimated to be 0.80 nm, which is shorter than the length of an n-octanol molecule (approx. 1.2 nm, calculated as mentioned above). Accordingly, the n-octanol molecules were proposed to adsorb on mica surface with the hydrocarbon chain has a 408 bevel with respect to the mica surface and form a compact monolayer. The growth kinetics of n-octanol on mica surface has been discussed in detail in our previous work w47x. From these AFM results, the thicknesses of n-butanol, n-hexanol and n-octanol molecular films were found to be smaller than the length of these molecules and bigger than their short axis. These molecules were thus proposed to adsorb onto mica surface in monolayer with tilted orientation. It was very different from the bilayer structures formed for methanol and ethanol. We think it
Fig. 3. Typical AFM image (scan area: 0.9=0.9 mm2, vertical scale: 2 nm) of butanol adsorbed mica surface.
of the butanol molecule are approximately 0.83 nm and 0.35 nm (calculated as mentioned above), respectively. Thus, the formed layer should not be an upright standing bilayer of butanol molecules or flat lying butanol layers. On the basis of XRD-measurements, Weiss et al. w50x suggested that the alkyl chains might lie flat or in a tilted upright position to the mica surface depending on the charge density of the substrate and the length of the chains. On the basis of our experimental results, we propose that butanol molecules adsorbed on mica surface with tilted or vertical orientation. The features observed for the n-hexanol films were similar to that of n-butanol, as shown in Fig. 4. nHexanol molecules adsorbed on mica surface in the form of some discontinuous bumps. These bumps were estimated to be 0.3–0.5 nm high and 20–50 nm wide. The heights of these bumps were much shorter than the length of an upright standing hexanol molecule (approx. 1.08 nm, calculated as mentioned above). Accordingly, the hexanol molecules seem to adsorb on the mica surface with tilted orientation. The RMS roughness measured over an area of 2.0=2.0 mm2 was 0.19 nm, indicating that the hexanol films were smoother than that of butanol. It might be due to the fact that with
Fig. 4. Typical AFM image (scan area: 1.0=1.0 mm2, vertical scale: 1 nm) of hexanol adsorbed mica surface.
L. Wang et al. / Thin Solid Films 458 (2004) 197–202
Fig. 5. Typical AFM image (scan area: 1.0=1.0 mm2, vertical scale: 2 nm) of the octanol adsorbed mica surface and the section analysis of the line traversing the image. The depth of the two layers was estimated as 0.8 nm.
is due to the fact that the bilayer structure could stabilize the film with lowest energy, while it is difficult for nbutanol, n-hexanol and n-octanol to form a bilayer structure, thus these alcohol molecules may exist mainly in monolayer structure. With the increasing length of the carbon-chain, the forces between molecule and molecule and molecule and substrate were increased. As a result the stability of molecular layers would be enhanced for a longer chain alkanol w50x. Thus the structure of n-octanol film was much more stable and ordered than that of n-butanol and n-hexanol. This was illustrated in Scheme 2a–c, which was corresponding to the experimental results depicted in Figs. 3–5 for nbutanol, n-hexanol and n-octanol, respectively. The influence of sample preparation on the adsorption process was also studied. If the drying time was elongated to one day or longer (depends on the specific species), the homogeneous structure disappeared from the substrate. The influence of adsorption time did not discussed here. In the present work, the main aim is to observe the adsorption behavior of alkanols with different carbon-chains under long enough adsorption time.
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Scheme 2. (a–c) Proposed models showing the evolution of alkanols adsorption with increased carbon-chains. The molecules are tilted on the mica.
4. Conclusions In summary, tapping䉸 mode AFM was applied to study the adsorption behavior of alcohols with different chain length, i.e. methanol, ethanol, n-butanol, n-hexanol and n-octanol. AFM results show that methanol and ethanol adsorbed on mica with bilayer structures, while n-butanol, n-hexanol and n-octanol in monolayer structures. The orientation of the first two molecules was vertical to the mica surface, while the last three molecules were tiled on the surface, only when the length of molecule was long enough that ordered compact film structure could be obtained. The adsorption of these simple linear saturated hydrocarbon chains on the basal plane of mica surface provides a model for molecular adsorption. The information may improve our knowledge of the mechanisms underlying various interfacial processes, and could help in the development of the theoretical models for dynamic simulations of molecule adsorption.
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