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Preparation and characterization of doped polyaniline films
Thin Solid Films 350 (1999) 5±9
Letter
Preparation and characterization of doped polyaniline ®lms L.H. Huo a, b, L.X. Cao a, D.M. Wang a, H.N. Cui a, G.F. Zeng a, S.Q. Xi a,* a
Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People's Republic of China b College of Chemistry and Chemical Engineering, Heilongjiang University, Harbin 150080, People's Republic of China Received 6 October 1998; received in revised form 7 April 1999; accepted 15 April 1999
Abstract Stable monolayer of the polyaniline doped with camphor sulfonic acid at the air-water interface has been obtained, of which multilayers have been successfully deposited by Langmuir±Blodgett technique onto CaF2 substrate. The limiting mean molecular area and collapse pressure are found to be 0.294 nm 2 and 41 mN/m, respectively. The multilayers were characterized by IR and UV-Vis-NIR spectroscopies. X-ray small-angle diffraction data show that the multilayer was periodic layer structure with the layer spacing of 1.60 nm. The comparisons are also made with characterization of the casting ®lm. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Monolayer; Langmuir±Blodgett ®lm; Casting ®lm; Doped polyaniline; Structural characterization
1. Introduction Polyaniline (PANI), especially doped polyaniline, has been attracted much attention in the past ten years owing to its ease of preparation, good environmental stability and high conductivity among conducting polymers. It was just started to study the preparation and properties of thin ®lms, particularly those of the ordered multilayer ultrathin ®lms in recent years, for their potential application as molecular or supramolecular devices. Langmuir±Blodgett technique is now the main employed means to obtain ordered organic ultrathin ®lms [1,2]. But the requirement of amphiphilicity and processibility of the material in preparation of LB ®lm limited fabrication of PANI LB ®lm. Currently, three approaches have been used to overcome these problems, such as the substitution of long alkyl chain in benzoid and quinoid rings [3], preparation of mixed LB ®lm with amphiphilic molecule [4] and the choice of suitable solvents (including N-methylpyrrolidone (NMP) and the mixed solvent of NMP-CHCl3) [5] or suitable subphase conditions [6]. Stable monolayer and its multilayer ®lms can also be obtained. However, the preparation of the polyaniline thin ®lm doped with functional acids by LB technique is not presented till now. In this paper, the surface pressure-area isotherm of camphor sulfonic doped polyaniline monolayer and its transfer to CaF2 substrate were studied. The struc* Corresponding author. Tel.: 186-431-568-2801; fax: 186-431-5685653. E-mail address: [email protected] (S.Q. Xi)
tural characterizations by IR, UV-Vis-NIR and X-ray smallangle diffraction equipment have also been carried out. The comparisons are made with characterization of the casting ®lm. 2. Experimental section Camphor sulfonic acid doped polyaniline (PANI-CSA) was provided by Geng [7]. The doped ratio was 2:1 of two CSA molecules in one PANI repeating unit. Fig. 1 shows the chemical structure of PANI, CSA and PANICSA molecules. A mixture of m-cresol-chloroform (1:9) was used as the spreading solvent. They are all A.R. grade. The concentration of PANI-CSA solution was 1:0 £ 1023 g/ml. The substrates used in this work were all CaF2 that had been pretreated by ultrasonic cleaning, sequentially with CHCl3, C2H5OH and H2O for 20 min. The casting ®lm was prepared by adding the PANI-CSA solution on the substrate dropwisely. Surface pressure-mean molecular area isotherm determination and multilayer LB ®lm deposition were carried out using KSV-5000 system. Deionized and double distilled water (pH 6.5) was used as the subphase. Mean molecular area has been calculated on the basis of a repeating unit including a benzoid unit, a quinoid unit, an imine unit and two camphor sulfonic acid molecules (formular weight 826, see Fig. 1c). The LB multilayer ®lms for characterization were all deposited as Y-type with 23 layers, under constant surface pressure of 25 mN/m at 20.5 ^ 0.18C. The dipping speed was 10 mm/min. The different deposition modes are depicted in Fig. 2.
0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0040-609 0(99)00293-X
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L.H. Huo et al. / Thin Solid Films 350 (1999) 5±9
ray small-angle diffraction pattern was obtained using a Rigaku D/Max-diffractometer with Cu Ka radiation (l 0:15418 nm). The incident X-ray beam was normal to the surface of the PANI-CSA ®lm. The generator was operated at 50 kV and 150 mA, and the X-ray was monochromated by graphite ®lter.
3. Results and discussion 3.1. Surface pressure±Area isotherm and multilayer ®lm deposition Fig. 3a shows the surface pressure-mean molecular area (p ±A) isotherm of the PANI-CSA monolayer at the
Fig. 1. The chemical structure of PANI-CSA molecule.
IR spectra were obtained from Bio-Rad FTS-135 at the resolution of 4 cm 21. Perkin±Elmer Lambda 9 spectrophotometer was used to take UV-Vis-NIR absorption spectra. X-
Fig. 2. Different deposition modes of LB ®lm: (a)X-type, (b) Y-type, (c) Ztype.
Fig. 3. Surface pressure-mean molecular area(p ±A) isotherm of PANICSA monolayer; (a) p ±A isotherm at 20.58C, (b)p ±A isotherm at different temperature.
L.H. Huo et al. / Thin Solid Films 350 (1999) 5±9
subphase temperature of 20:5 ^ 0:18C. It can be seen that the investigated material could form good expanded monolayer on the pure water surface. It is because the doped camphor sulfonic acid with bulky geometry is helpful for the polyaniline chains to expand in the solvent and on the air-water interface. From the p ±A curve, it can be obtained that the collapse pressure of the monolayer is about 41 mN/ m and the mean molecular area for each repeating unit is around 0.294 nm 2. The latter is larger than those of the emeraldine base of PANI monolayer (0.08 nm 2 [6], 0.20 nm 2 [5]) and a little larger than that of 2-octadecoxy substituted PANI (0.23±0.27 nm 2 [3]). We consider that the main reason of the above values in a large range is the exist of the camphor sulfonic acid dopant. The second reason is the differences of the preparation conditions of monolayer. For example, the different solvents have different spreading ability when they are used to spread the polyaniline molecules. It also in¯uences the formation of the monolayer and calculation of the mean molecular area. From Fig. 3b, changes can be seen in the p ±A isotherms of the PANI-CSA monolayer obtained at different subphase temperature. The two p ±A isotherms give similar curve shape, but the limiting mean molecular area obtained at 10.28C is larger than that at 20.58C. It is considered to be related to the rigidity of the PANI chains at air-water interface [6]: At low temperature, the rigidity of PANI chains is comparatively strong, it is hard to be compressed to close packing. The subphase temperature also in¯uenced the multilayer ®lm transfer onto the substrate. When the multilayer was attempted to transfer onto CaF2 substrate at 10.28C with the dipping speed of 5 mm/min, the transfer ratio was found to be in the range of 1.8±4.1 and it could only be transferred by Z-type deposition mode. The multilayer LB ®lm with good transfer ratio could not be prepared even changing the dipping speed. But when raising the subphase temperature to 20.58C, the Y-type multilayer ®lm can be obtained with the transfer ratio of 0.85±0.95 at the dipping speed of 10 mm/min. The research results of the multilayer transfer onto different substrates indicate that the choice of suitable substrate is also important for this kind of polymer. In our research conditions, the substrates with good transfer ratios are in the following order: SiO2±CaF2>glass>Si.
Fig. 4. IR spectra of PANI-CSA ®lms.
shift to 1570 and 1490 cm 21 in LB ®lm, respectively. The other peaks occurred red shift due to the close packing of molecules. Especially the characteristic peak of the doped PANI at 1148 cm 21 shifts 23 cm 21 to lower frequency. This change is considered to be derived from the ordered organization. It is con®rmed by the IR spectral changes (see Fig. 5) of the same LB ®lm during heating treatment at different temperature for 1 h. From Fig. 5, it can be seen that the intensities of all the absorption bands in the region of 2000±900 cm 21 are decreased with the heating temperature raised. The original absorption band at 1125 cm 21 gradually shifts to higher frequency with the ordered extent of LB ®lm destroyed deeply. After the ®lm was heated at 1508C for 1 h, this characteristic peak shifts to 1139 cm 21 while the other absorption bands are not observed so distinct position shift. And the peak shape of 1139 cm 21 is very similar with that of 1148 cm 21 in casting ®lm. Except to the blue-shift of absorption bands, the half-peak widths are also broadened with the temperature raised. For example, the half-peak width of band at 1300 cm 21 are 28.9, 30.0, 31.6, 32.6
3.2. Infrared spectra The IR absorption bands of PANI and doped PANI molecules have been assigned by Jing [8,9]. Fig. 4 is the IR spectra of the PANI-CSA ®lms. As comparison with the casting ®lm, the LB ®lm has more narrow and sharp peaks indicating the PANI-CSA molecular orientation ordered in LB ®lm. The intensity and position of the absorption band changed obviously. The CvC stretching vibration of benzoid and quinoid units became stronger. The relative absorption band at 1566 and 1482 cm 21 in casting ®lm
7
Fig. 5. IR spectra of LB ®lm at different temperature.
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L.H. Huo et al. / Thin Solid Films 350 (1999) 5±9
cm 21 when the heating temperature was raised from 25 to 1508C (see Fig. 5). These results are all due to the change from ordered ®lm to disordered one. 1300 and 1006 cm 21 bands are assigned to the CZN stretching and CZH in-plane bending vibrations, respectively. The increase of their intensities probably involved in the molecule close arrangement. In addition, there is a new small peak at 1372 cm 21 that is assigned to the CZN stretching vibration in quinoid unit of PANI intrinsic state [9] in LB ®lm. It indicated that the doped PANI molecule occurred partially dedoped. That is to be said that some CSA molecules were ripped out of PANI chains during the compressive process of PANICSA monolayer. 3.3. UV-Vis-NIR spectra Fig. 6 is the UV-Vis-NIR spectra of PANI-CSA ®lms. There are a strong absorption band at 445 nm and a small wide band at 860 nm in the casting ®lm (see Fig. 6a). These are the characteristic bands of the doped polyaniline [10,11], assigned to the lattice absorption of the polaron [12]. The weak shoulder absorption band at 330 nm is the p ±p * electron transition of the conjugate aromatic ring. In addition, there is a continuously increasing `free-carrier tail' in the near infrared region, similar to the typical absorption of metals or polyacetylene [13]. The exist of this absorption band is considered to be related to the conductivity of PANI, and also indicates that the PANI chain is in the expanded state. After the PANI chain was orderly oriented by LB technique (see Fig. 6b), the absorption bands at 330 and 445 nm occurred a little red shift, and the intensity of 330 nm absorption peak increased while that of 445 nm absorption peak decreased. There is a new broad and ¯at absorption band at , 680 nm which is considered to be the electron transition of quinoid unit [14,15]. This gives further evidence of partial CSA molecular dedoped during the close arrangement process of PANI-CSA molecule. This result is consistent with that of IR spectra. In the same time, the polaron absorption band at 860 nm shifts to , 960 nm. The increasing of this carrier tail absorption after
Fig. 6. UV-Vis-NIR spectra of PANI-CSA ®lms: (a) casting ®lm, (b) LB ®lm.
Fig. 7. X-ray small-angle diffraction pattern of PANI-CSA LB ®lm.
1000 nm is similar to that in casting ®lm, showing that the PANI chain in LB ®lm is still in the expanded state. 3.4. Structural characterization for the PANI-CSA LB ®lm The X-ray small-angle diffraction pro®le (Fig. 7) of the PANI-CSA LB ®lm exhibited a clear Bragg peak at 1.3828 while that of the casting ®lm gave no peaks. It indicated that the LB ®lm has a well-de®ned layer structure along the LB ®lm stacking direction, while the casting ®lm is in disordered state. This result provides further evidence to support those of IR and UV-Vis-NIR spectra. The observed layer spacing of PANI-CSA multilayer LB ®lm which is normal to the substrate direction is 6.39 nm. According to the bond length data [16,17] and model of PANI-CSA in m-cresol solvent [18], the distance between the two CSA molecules that situated on the two sides of PANI backbone, respectively (see Fig. 1c), is about 1.5± 1.9 nm. This value is very close to one quarter of the observed value, 1.6 nm. It is said that the repeated periodicity of PANI-CSA LB ®lm is composed of four PANI-CSA chains, which arranged alternately in the ®lm. The layered
Fig. 8. The possible structure of the PANI-CSA LB ®lm.
L.H. Huo et al. / Thin Solid Films 350 (1999) 5±9
structure is suggested to organize as that the PANI chains and CSA molecules lie on the ®lm surface in which the CSA molecules act as spacers between PANI chains. The resident m-cresol molecules may bond to nitrogen atom of PANI chain [18] like CSA molecules through hydrogen bond. They have no effect on the arrangement of PANI-CSA molecules in the LB ®lm. The reasonable layer spacing of this LB ®lm is 1.6 nm. The schematic picture for the possible structure of the ideal PANI-CSA LB ®lm is depicted in Fig. 8 (the related molecular structure see Fig. 1c). 4. Conclusions Stable monolayer of the polyaniline doped with camphor sulfonic acid at the air-water interface has been obtained, and multilayers of which have been successfully deposited by Langmuir±Blodgett technique onto CaF2 substrate. Characterization results of IR and UV-Vis-NIR spectra indicate that the LB ®lm has the more ordered layer structure. X-ray small-angle diffraction data give a layered structure with the layer spacing of 1.60 nm. Acknowledgements We'd like to thank Prof. X.B. Jing, Dr. Y.H. Geng and Z.C. Sun of our institute for their providing the investigated sample and helpful discussion on the spectra analyses. This work was supported by the National Natural Science Foundation of China.
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References [1] G. Roberts, Langmuir±Blodgett Films, Plenum Press, New York, 1990 pp. 93±123. [2] A. Ulman, An Introduction to Ultrathin Organic Films From Langmuir±Blodgett to Self-Assembly, Academic Press, London, 1991 pp. 146±149. [3] M. Ando, Y. Watanabe, T. Iyoda, K. Honda, T. Shimidzu, Thin Solid Films 179 (1989) 225. [4] A. Dhanabalan, R.B. Dabke, S.N. Datta, N. Prasanth, Kumar, S. Major, S.S. Talwar, Thin Solid Films 295 (1997) 255. [5] N.E. Agbor, M.C. Petty, A.P. Monkman, M. Harris, Synth. Met. 55-57 (1993) 3789. [6] A. Dhanabalan, R.B. Dabke, N. Prasanth, Kumar, S.S. Talwar, S. Major, R. Lal, A.Q. Contractor, Langmuir 13 (1997) 4395. [7] Y.H. Geng, Ph.D. Thesis, Changchun Institute of Applied Chemistry, 1996. [8] X.B. Jing, J.S. Tang, Y. Wang, L.C. Lei, B.C. Wang, F.S. Wang, Science in China B 33 (1990) 787. [9] J.S. Tang, X.B. Jing, B.C. Wang, F.S. Wang, Synth. Met. 24 (1988) 231. [10] Y. Cao, P. Smith, A.J. Heeger, Synth. Met. 32 (1989) 263. [11] W.S. Santos, A.G. MacDiarmid, Polym. 34 (1993) 1833. [12] Y.N. Xia, J.M. Wiesinger, A.G. MacDiarmid, Chem. Mater. 7 (1995) 443. [13] A.P. Monkman, P. Adamas, Synth. Met. 41±43 (1991) 627. [14] M. Ohira, T. Sakai, M. Takeuchi, Y. Kobayashi, Synth. Met. 18 (1987) 347. [15] Y. Furukawa, T. Hara, Y. Yyodo, I. Harada, Synth. Met. 16 (1986) 189. [16] M.C. Dos, S. antos, J.L. Bredas, Synth. Met. 29 (1989) E321. [17] R.C. Weast, M.J. Astle, CRC Handbook of Chemistry and Physics, 63rd ed., CRC Press, Boca Raton, FL, 1982. Ä sterholm, P.J. Passiniemi, [18] O.T. Ikkala, L.O. Pietila, L. Ahjopalo, H. O J. Chem. Phys. 103 (1995) 9855.
Letter
Preparation and characterization of doped polyaniline ®lms L.H. Huo a, b, L.X. Cao a, D.M. Wang a, H.N. Cui a, G.F. Zeng a, S.Q. Xi a,* a
Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People's Republic of China b College of Chemistry and Chemical Engineering, Heilongjiang University, Harbin 150080, People's Republic of China Received 6 October 1998; received in revised form 7 April 1999; accepted 15 April 1999
Abstract Stable monolayer of the polyaniline doped with camphor sulfonic acid at the air-water interface has been obtained, of which multilayers have been successfully deposited by Langmuir±Blodgett technique onto CaF2 substrate. The limiting mean molecular area and collapse pressure are found to be 0.294 nm 2 and 41 mN/m, respectively. The multilayers were characterized by IR and UV-Vis-NIR spectroscopies. X-ray small-angle diffraction data show that the multilayer was periodic layer structure with the layer spacing of 1.60 nm. The comparisons are also made with characterization of the casting ®lm. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Monolayer; Langmuir±Blodgett ®lm; Casting ®lm; Doped polyaniline; Structural characterization
1. Introduction Polyaniline (PANI), especially doped polyaniline, has been attracted much attention in the past ten years owing to its ease of preparation, good environmental stability and high conductivity among conducting polymers. It was just started to study the preparation and properties of thin ®lms, particularly those of the ordered multilayer ultrathin ®lms in recent years, for their potential application as molecular or supramolecular devices. Langmuir±Blodgett technique is now the main employed means to obtain ordered organic ultrathin ®lms [1,2]. But the requirement of amphiphilicity and processibility of the material in preparation of LB ®lm limited fabrication of PANI LB ®lm. Currently, three approaches have been used to overcome these problems, such as the substitution of long alkyl chain in benzoid and quinoid rings [3], preparation of mixed LB ®lm with amphiphilic molecule [4] and the choice of suitable solvents (including N-methylpyrrolidone (NMP) and the mixed solvent of NMP-CHCl3) [5] or suitable subphase conditions [6]. Stable monolayer and its multilayer ®lms can also be obtained. However, the preparation of the polyaniline thin ®lm doped with functional acids by LB technique is not presented till now. In this paper, the surface pressure-area isotherm of camphor sulfonic doped polyaniline monolayer and its transfer to CaF2 substrate were studied. The struc* Corresponding author. Tel.: 186-431-568-2801; fax: 186-431-5685653. E-mail address: [email protected] (S.Q. Xi)
tural characterizations by IR, UV-Vis-NIR and X-ray smallangle diffraction equipment have also been carried out. The comparisons are made with characterization of the casting ®lm. 2. Experimental section Camphor sulfonic acid doped polyaniline (PANI-CSA) was provided by Geng [7]. The doped ratio was 2:1 of two CSA molecules in one PANI repeating unit. Fig. 1 shows the chemical structure of PANI, CSA and PANICSA molecules. A mixture of m-cresol-chloroform (1:9) was used as the spreading solvent. They are all A.R. grade. The concentration of PANI-CSA solution was 1:0 £ 1023 g/ml. The substrates used in this work were all CaF2 that had been pretreated by ultrasonic cleaning, sequentially with CHCl3, C2H5OH and H2O for 20 min. The casting ®lm was prepared by adding the PANI-CSA solution on the substrate dropwisely. Surface pressure-mean molecular area isotherm determination and multilayer LB ®lm deposition were carried out using KSV-5000 system. Deionized and double distilled water (pH 6.5) was used as the subphase. Mean molecular area has been calculated on the basis of a repeating unit including a benzoid unit, a quinoid unit, an imine unit and two camphor sulfonic acid molecules (formular weight 826, see Fig. 1c). The LB multilayer ®lms for characterization were all deposited as Y-type with 23 layers, under constant surface pressure of 25 mN/m at 20.5 ^ 0.18C. The dipping speed was 10 mm/min. The different deposition modes are depicted in Fig. 2.
0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0040-609 0(99)00293-X
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L.H. Huo et al. / Thin Solid Films 350 (1999) 5±9
ray small-angle diffraction pattern was obtained using a Rigaku D/Max-diffractometer with Cu Ka radiation (l 0:15418 nm). The incident X-ray beam was normal to the surface of the PANI-CSA ®lm. The generator was operated at 50 kV and 150 mA, and the X-ray was monochromated by graphite ®lter.
3. Results and discussion 3.1. Surface pressure±Area isotherm and multilayer ®lm deposition Fig. 3a shows the surface pressure-mean molecular area (p ±A) isotherm of the PANI-CSA monolayer at the
Fig. 1. The chemical structure of PANI-CSA molecule.
IR spectra were obtained from Bio-Rad FTS-135 at the resolution of 4 cm 21. Perkin±Elmer Lambda 9 spectrophotometer was used to take UV-Vis-NIR absorption spectra. X-
Fig. 2. Different deposition modes of LB ®lm: (a)X-type, (b) Y-type, (c) Ztype.
Fig. 3. Surface pressure-mean molecular area(p ±A) isotherm of PANICSA monolayer; (a) p ±A isotherm at 20.58C, (b)p ±A isotherm at different temperature.
L.H. Huo et al. / Thin Solid Films 350 (1999) 5±9
subphase temperature of 20:5 ^ 0:18C. It can be seen that the investigated material could form good expanded monolayer on the pure water surface. It is because the doped camphor sulfonic acid with bulky geometry is helpful for the polyaniline chains to expand in the solvent and on the air-water interface. From the p ±A curve, it can be obtained that the collapse pressure of the monolayer is about 41 mN/ m and the mean molecular area for each repeating unit is around 0.294 nm 2. The latter is larger than those of the emeraldine base of PANI monolayer (0.08 nm 2 [6], 0.20 nm 2 [5]) and a little larger than that of 2-octadecoxy substituted PANI (0.23±0.27 nm 2 [3]). We consider that the main reason of the above values in a large range is the exist of the camphor sulfonic acid dopant. The second reason is the differences of the preparation conditions of monolayer. For example, the different solvents have different spreading ability when they are used to spread the polyaniline molecules. It also in¯uences the formation of the monolayer and calculation of the mean molecular area. From Fig. 3b, changes can be seen in the p ±A isotherms of the PANI-CSA monolayer obtained at different subphase temperature. The two p ±A isotherms give similar curve shape, but the limiting mean molecular area obtained at 10.28C is larger than that at 20.58C. It is considered to be related to the rigidity of the PANI chains at air-water interface [6]: At low temperature, the rigidity of PANI chains is comparatively strong, it is hard to be compressed to close packing. The subphase temperature also in¯uenced the multilayer ®lm transfer onto the substrate. When the multilayer was attempted to transfer onto CaF2 substrate at 10.28C with the dipping speed of 5 mm/min, the transfer ratio was found to be in the range of 1.8±4.1 and it could only be transferred by Z-type deposition mode. The multilayer LB ®lm with good transfer ratio could not be prepared even changing the dipping speed. But when raising the subphase temperature to 20.58C, the Y-type multilayer ®lm can be obtained with the transfer ratio of 0.85±0.95 at the dipping speed of 10 mm/min. The research results of the multilayer transfer onto different substrates indicate that the choice of suitable substrate is also important for this kind of polymer. In our research conditions, the substrates with good transfer ratios are in the following order: SiO2±CaF2>glass>Si.
Fig. 4. IR spectra of PANI-CSA ®lms.
shift to 1570 and 1490 cm 21 in LB ®lm, respectively. The other peaks occurred red shift due to the close packing of molecules. Especially the characteristic peak of the doped PANI at 1148 cm 21 shifts 23 cm 21 to lower frequency. This change is considered to be derived from the ordered organization. It is con®rmed by the IR spectral changes (see Fig. 5) of the same LB ®lm during heating treatment at different temperature for 1 h. From Fig. 5, it can be seen that the intensities of all the absorption bands in the region of 2000±900 cm 21 are decreased with the heating temperature raised. The original absorption band at 1125 cm 21 gradually shifts to higher frequency with the ordered extent of LB ®lm destroyed deeply. After the ®lm was heated at 1508C for 1 h, this characteristic peak shifts to 1139 cm 21 while the other absorption bands are not observed so distinct position shift. And the peak shape of 1139 cm 21 is very similar with that of 1148 cm 21 in casting ®lm. Except to the blue-shift of absorption bands, the half-peak widths are also broadened with the temperature raised. For example, the half-peak width of band at 1300 cm 21 are 28.9, 30.0, 31.6, 32.6
3.2. Infrared spectra The IR absorption bands of PANI and doped PANI molecules have been assigned by Jing [8,9]. Fig. 4 is the IR spectra of the PANI-CSA ®lms. As comparison with the casting ®lm, the LB ®lm has more narrow and sharp peaks indicating the PANI-CSA molecular orientation ordered in LB ®lm. The intensity and position of the absorption band changed obviously. The CvC stretching vibration of benzoid and quinoid units became stronger. The relative absorption band at 1566 and 1482 cm 21 in casting ®lm
7
Fig. 5. IR spectra of LB ®lm at different temperature.
8
L.H. Huo et al. / Thin Solid Films 350 (1999) 5±9
cm 21 when the heating temperature was raised from 25 to 1508C (see Fig. 5). These results are all due to the change from ordered ®lm to disordered one. 1300 and 1006 cm 21 bands are assigned to the CZN stretching and CZH in-plane bending vibrations, respectively. The increase of their intensities probably involved in the molecule close arrangement. In addition, there is a new small peak at 1372 cm 21 that is assigned to the CZN stretching vibration in quinoid unit of PANI intrinsic state [9] in LB ®lm. It indicated that the doped PANI molecule occurred partially dedoped. That is to be said that some CSA molecules were ripped out of PANI chains during the compressive process of PANICSA monolayer. 3.3. UV-Vis-NIR spectra Fig. 6 is the UV-Vis-NIR spectra of PANI-CSA ®lms. There are a strong absorption band at 445 nm and a small wide band at 860 nm in the casting ®lm (see Fig. 6a). These are the characteristic bands of the doped polyaniline [10,11], assigned to the lattice absorption of the polaron [12]. The weak shoulder absorption band at 330 nm is the p ±p * electron transition of the conjugate aromatic ring. In addition, there is a continuously increasing `free-carrier tail' in the near infrared region, similar to the typical absorption of metals or polyacetylene [13]. The exist of this absorption band is considered to be related to the conductivity of PANI, and also indicates that the PANI chain is in the expanded state. After the PANI chain was orderly oriented by LB technique (see Fig. 6b), the absorption bands at 330 and 445 nm occurred a little red shift, and the intensity of 330 nm absorption peak increased while that of 445 nm absorption peak decreased. There is a new broad and ¯at absorption band at , 680 nm which is considered to be the electron transition of quinoid unit [14,15]. This gives further evidence of partial CSA molecular dedoped during the close arrangement process of PANI-CSA molecule. This result is consistent with that of IR spectra. In the same time, the polaron absorption band at 860 nm shifts to , 960 nm. The increasing of this carrier tail absorption after
Fig. 6. UV-Vis-NIR spectra of PANI-CSA ®lms: (a) casting ®lm, (b) LB ®lm.
Fig. 7. X-ray small-angle diffraction pattern of PANI-CSA LB ®lm.
1000 nm is similar to that in casting ®lm, showing that the PANI chain in LB ®lm is still in the expanded state. 3.4. Structural characterization for the PANI-CSA LB ®lm The X-ray small-angle diffraction pro®le (Fig. 7) of the PANI-CSA LB ®lm exhibited a clear Bragg peak at 1.3828 while that of the casting ®lm gave no peaks. It indicated that the LB ®lm has a well-de®ned layer structure along the LB ®lm stacking direction, while the casting ®lm is in disordered state. This result provides further evidence to support those of IR and UV-Vis-NIR spectra. The observed layer spacing of PANI-CSA multilayer LB ®lm which is normal to the substrate direction is 6.39 nm. According to the bond length data [16,17] and model of PANI-CSA in m-cresol solvent [18], the distance between the two CSA molecules that situated on the two sides of PANI backbone, respectively (see Fig. 1c), is about 1.5± 1.9 nm. This value is very close to one quarter of the observed value, 1.6 nm. It is said that the repeated periodicity of PANI-CSA LB ®lm is composed of four PANI-CSA chains, which arranged alternately in the ®lm. The layered
Fig. 8. The possible structure of the PANI-CSA LB ®lm.
L.H. Huo et al. / Thin Solid Films 350 (1999) 5±9
structure is suggested to organize as that the PANI chains and CSA molecules lie on the ®lm surface in which the CSA molecules act as spacers between PANI chains. The resident m-cresol molecules may bond to nitrogen atom of PANI chain [18] like CSA molecules through hydrogen bond. They have no effect on the arrangement of PANI-CSA molecules in the LB ®lm. The reasonable layer spacing of this LB ®lm is 1.6 nm. The schematic picture for the possible structure of the ideal PANI-CSA LB ®lm is depicted in Fig. 8 (the related molecular structure see Fig. 1c). 4. Conclusions Stable monolayer of the polyaniline doped with camphor sulfonic acid at the air-water interface has been obtained, and multilayers of which have been successfully deposited by Langmuir±Blodgett technique onto CaF2 substrate. Characterization results of IR and UV-Vis-NIR spectra indicate that the LB ®lm has the more ordered layer structure. X-ray small-angle diffraction data give a layered structure with the layer spacing of 1.60 nm. Acknowledgements We'd like to thank Prof. X.B. Jing, Dr. Y.H. Geng and Z.C. Sun of our institute for their providing the investigated sample and helpful discussion on the spectra analyses. This work was supported by the National Natural Science Foundation of China.
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