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J. Phys. Chem. B 2003, 107, 2748-2751
Surface Reactions on Polymer Thin Films Studied by Surface-Enhanced Raman Scattering Dong-Shan Zhou, Ning Xu, Liang Li, Gendin Ji, and Gi Xue* The State Key Laboratory of Functional Polymer Materials of Adsorption and Separation at Nankai, Department of Polymer Science and Engineering, Nanjing UniVersity, Nanjing 210093, People’s Republic of China ReceiVed: February 5, 2002; In Final Form: December 20, 2002
The laser-induced chemical reactions on the surface of a polymer film were investigated by surface-enhanced Raman scattering (SERS) using chemically deposited silver as the SERS activating layer on the surface of the film. The silver-catalyzed pyrolysis at the outermost surface of the polyacrylonitrile film under laser radiations with different wavelengths was investigated by this SERS technique. The extremely low carbonization temperature at room temperature under laser implies several responsible mechanisms.
Introduction Nowadays there is much interest in thin polymer films because of their importance in science and technoloy.1-6 Some microstructuring studies of molecularly thin polymer layers by photolithography such as the solid-state photocatalytic reaction for fabricating polymeric patterns, microfabrication of polymer devices for automated sample delivery of peptides, pulsed-laser deposition for organic electroluminescent device applications, fabrication from multilayered materials, and so forth7-10 have been reported. Among the instrumental methods for surface studies, surfaceenhanced Raman scattering (SERS) is a unique technique resulting in intense Raman scattering signals from molecules adjacent to a rough silver substrate.11 The enhancement decreases rapidly as a function of distance from the molecular surface, making SERS particularly surface-sensitive.12-15 In recent years, the SERS technique has been used to probe single molecular chains and single nanoparticles.16,17 In this study, we focus on the photocatalytic reaction for fabricating polymeric patterns on thin polymer films. Using surface-enhanced Raman scattering produced by a chemically deposited silver overcoating, we report our experimental results on surface chemical and physical structures of a polyacrylonitrile (PAN) thin film and provide a new understanding of molecular structure at the outermost surface after irradiation by the laser source. Polyacrylonitrile (PAN) is well known to undergo structural changes in the presence of oxygen at high temperatures.18 Recent attention has focused on surface or interfacial reactions between PAN and metals. An ex situ infrared reflection-absorption study of PAN films obtained by electropolymerization on a nickel substrate showed that cyclization had been completed after 24 h at 200 °C, reticulation had been completed at 300 °C, and dehydrogenation and denitrogenation started well above 300 °C.19 An XPS and IR study of PAN films on aluminum and coppers surfaces illustrated that a doubly coupled conjugated system was formed after pyrolysis on Al at 300 °C or on Cu at 200 °C for 2 h.20 These temperatures are respectively about 100 and 200 °C lower than those previously reported for bulk PAN.21 Another report revealed that the polymer may undergo facile photodegradation on a smooth silver surface upon ultraviolet * Corresponding author. E-mail:
[email protected] Fax: 86-25-3317761.
irradiation at wavelengths in the region from 250-400 nm. Below 266 nm, the silver-backed PAN turned yellowish brown and increased in electrical conductivity, suggesting the presence of a chromophore system of the type -(CdN)-n.22 We report here our surface-enhanced Raman scattering (SERS) studies of the structural changes of PAN beneath the chemically deposited silver layer. Since the electrical conductivity of the doubly coupled conjugated system can be increased by about 10 orders of magnitude upon doping,23,24 knowledge of such structural changes on metal surfaces should help not only in the preparation of electrically conductive polymeric modifiers on metal electrodes but also in the understanding of the mechanisms of the conduction. Experimental Section All reagents were purchased from Aldrich Chemical Corporation and were reagent grade. PAN (molecular weight 9.0 × 104) was dissolved in dimethylformamide to make solutions with concentrations of 0.08, 0.2, and 3 wt %. Thin films were prepared by spin coating the dilute solutions at 2000 rpm for 40 s on clean optical glass substrates, and their thickness was controlled in the range of 100 ( 40 nm, as measured by an ellipsometer. Thick films were prepared by spin coating the concentrated solutions. The silver overcoating layer was prepared by depositing silver particles by Tollen’s test method onto the polymer film for investigation.15 In a 10 mL beaker was a piece of 10 × 10 mm2 test film. A silver-ammonia complex (5 mL, 0.05 M) and formaldehyde (2 mL) were mixed in the beaker. A few seconds later, the solution turned gray and black. Meanwhile, the silver ions were reduced to form colloidal particles and were deposited onto the test film to form an overcoating layer at room temperature. After withdrawing, the tested film covered with silver overcoatings was washed with distilled water and dried. Then the specimen was ready for Raman studies. The diameter of the deposited silver particles was a few tens of nanometers, as measured by EM. The schematic diagram for the backscattering geometry for the SERS study of the PAN film with silver particles as an overcoating layer is shown in Figure 1. Laser radiation passed through the silver overcoating layer and interacted with the silver/polymer interface, and the scattered radiation was collected from the silver overcoating layer.
10.1021/jp025607b CCC: $25.00 © 2003 American Chemical Society Published on Web 03/05/2003
Surface Reactions on Polymer Thin Films
J. Phys. Chem. B, Vol. 107, No. 12, 2003 2749
Figure 1. Schematic diagram of the backscattering geometry for the SERS measurement for the polymer film under the chemically deposited silver overcoating layer.
Figure 3. (A) Conventional FT Raman spectrum of a polyacrylonitrile thin film with a thickness of 100( 40 nm. (B) SERS spectrum of a polyacrylonitrile thin film under a chemically deposited silver layer. All of the Raman spectra were recorded with a 1065-nm laser line with a power of 200 mW.
Figure 2. (A) Conventional FT Raman spectrum of polyacrylonitrile film bulk with a thickness of 10 µm. (B) Conventional FT Raman spectrum of a polyacrylonitrile thin film with a thickness of 100 ( 40 nm. (C) SERS spectra for a silver overcoating layer on the same polyacrylonitrile thin film as in B. All of the Raman spectra were recorded with a 1065-nm laser line with a power of 30 mW.
Raman spectra were mainly recorded with a Bruker IFS 100 Fourier transform Raman spectrometer equipped with an aircooled Nd:YAG laser source (1064 nm), with an output power of 30-200 mW and a Ge detector cooled by liquid nitrogen. Raman spectra were also recorded on a SPEX-1403 spectrometer using an argon ion laser as the exciting source. Results and Discussion Recently, we developed an effective SERS technique for characterizing the surface structure of films without disturbing the morphology of the samples.6,15 The silver colloids were directly deposited onto the surface of the film to form a SERSactive overcoating layer. The incident laser radiation penetrated through the silver overcoating layer and reached the interface of the silver/sample. Then the scattered radiation that contained information about the film’s surface was collected and analyzed.6,15 (See Figure 1.) Figure 2 shows the Raman spectra of PAN that were collected with a FT Raman spectrometer using an air-cooled Nd:YAG laser source (1064 nm). The incident power on the sample was 30 mW. Figure 2A is a normal FT Raman spectrum collected from a thick film (10 µm). For comparison, the spectrum of the thin PAN film (100 ( 40 nm) on a glass substrate without a silver overcoating layer is listed in Figure 2B, which shows no Raman signals for PAN. However, a SERS spectrum for PAN of the thin film under the silver overcoating is readily
Figure 4. (A) Conventional FT Raman spectrum of a polyacrylonitrile film with a thickness of 2 µm. (B) SERS spectrum of polyacrylonitrile film with thickness of 2 µm under a chemically deposited silver layer. All of the Raman spectra were recorded with a 1065-nm laser line with a power of 200 mW.
obtained, as shown in Figure 2C, which illustrates several typical Raman signals at 2941, 2245, and 1455 cm-1 for PAN.25 Spectra in Figures 3 and 4 were collected from a PAN thin film (100 ( 40 nm) and a thicker film (2 µm) with a FT Raman spectrometer when the laser power that reached the samples was increased to 200 mW, respectively. Both normal Raman spectra in Figures 3A and 4A show small signals for PAN since the films were too thin to be detected by the conventional Raman scattering technique even when the incident laser power was increased. SERS spectra of Figures 3B and 4B were readily obtained using silver particles as the overcoating layers. One can see a dramatic difference between the Raman spectra of PAN measured under lower laser power (Figure 2) and the SERS spectrum of the thin film recorded at higher laser power (Figure 3B). The band at 2245 cm-1 in Figure 4 that is attributable to the stretching vibration of the C-N side groups becomes rather weak in the SERS spectrum of Figure 3B, whereas new bands appearing at 1608, 1393, 1130, and 1005
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SCHEME 1: Pyrolysis Mechanism of Polyacrylonitrile under a Silver Overcoating with a 1065-nm Laser Line with a Power of 200 mW
cm-1 in Figure 3B are absent in the normal Raman spectrum, indicating that PAN has undergone structural changes under higher laser power. It should be emphasized that no Raman shifts attributable to the solvent were detected, therefore eliminating the possibility of any undesirable solvent-induced side reactions. The SERS spectrum is typical of aromatic ring vibrations and correlates well with the FTIR spectra of thermally degraded PAN.25 The bands at 1608 cm-l can be assigned to -(CdC)or -(CdN)- conjugation ring-stretching vibrations, implying that the adsorbed PAN has been turned into a fused ring structure. This is consistent with the great reduction in the signal intensity of the 2245-cm-l band. The bands at 1130 and 1005 cm-l can be ascribed to in-plane ring C-H deformation frequencies. Because of their strong intensity, it is reasonable to propose that the aromatic ring structure is sticking up to the metal surface via its N atoms or at least that the fused rings are tilted off of the surface as long as the electromagnetic theory on the SERS selection rule is applied.26 It has generally been accepted that adsorption of PAN on metal surfaces necessitates the bonding of cyano groups to metal atoms.20,27 Such a favorable configuration and/or orientation at polymer/metal interfaces facilitate the cyclization under the laser at room temperature as the laser power was increased to 200 mW. The reason for the metal-induced cyclization at the interface may thus be attributed to the back-donation of electrons from the metal to the CN system and the accompanied reduction of the CtN triple-bond energy. Similar surface geometry effects on the surface-induced reactions have been found in the case of spontaneous polymerization.20,27 The pyrolysis mechanism is shown in Scheme 1. The spectrum in Figure 4B shows Raman signals of the pyrolysized PAN at 1608 and 1005 cm-l and of the nonreacted polymer at 2245 and 1041 cm-1, indicating that only the PAN chains at the outermost surface were pyrolized whereas the bulk PAN remains unchanged under laser radiation. The pyrolysis of PAN under the laser decreases rapidly as a function of distance from the molecular surface. Figure 5B shows the SERS spectrum of the PAN thin film under the silver overcoatings when the incident laser excitation was from the 514.5-nm line from an argon ion laser and the incident power was 200 mW. The spectrum reveals that PAN beneath Ag particles has further been converted to a graphitelike structure under laser excitation with a higher frequency. This spectrum is exactly identical to the reported SERS spectra obtained by evaporating silver on carbon fibers or graphite fibers28 as well as the ordinary Raman spectra of graphite or PAN-based graphite fiber.29 The intensity ratio R ) I(1585)/I(1370) and the width of the 1370-cm-1 band are sensitive to both the
Figure 5. (A) Conventional Raman spectrum of a polyacrilonitile thick film (10 µm). (B) SERS spectrum of a polyacrylonitrile thin film (100 ( 40 nm). All of the Raman spectra were recorded with a 514.5-nm laser line with a power of 200 mW.
Figure 6. (A) Conventional Raman spectrum of a polyacrylonitrile film with a thickness of 2 µm. (B) SERS spectrum of a polyacrylonitrile film with a thickness of 2 µm under a chemically deposited silver layer. All of the Raman spectra were recorded with a 514.5-nm laser line with a power of 200 mW.
degree of graphitization and the orientation of 2D-ordered graphite planes.30 In contrast, a bulk PAN film (10 µm) without silver overcoatings underwent only partial cyclization after it had been treated in the same environment (Figure 5A), let alone graphitization. Figure 6 illustrates a SERS spectrum of PAN with a thickness of 2 µm, which shows Raman signals for both the carbonized PAN near 1585 and 1375 cm-1 and the unreacted PAN at 2243 cm-1. It is clear that the graphitization of PAN happens only at the outermost surface layer under Ag particles. Our SERS technique is a unique method resulting in intense Raman scattering signals from molecules adjacent to the overcoated silver layer. The enhancement decreases rapidly as a function of distance from the upper surface, making SERS particularly surface-sensitive for a polymer film.31
Surface Reactions on Polymer Thin Films A full graphitization of PAN usually needs a temperature of 2000 °C. On Ni, Al, and Cu surfaces, dehydrogenation did not start until 300 °C. The extremely low carbonization temperature at room temperature under the laser implies several responsible mechanisms. The catalytic action of the metal is of course one possible explanation for the observed lowering. Silver in particular is known to function as a catalyst for the thermal dehydrogenation of hydrocarbon compounds.32 Another mechanism for the lowering of the transition temperatures may originate from the inducement of laser radiation of higher frequency.33 Conclusions We have also performed SERS measurements with the chemically deposited silver overcoating layer on samples of current interest such as polymer thin films with nanometer thickness. The results of this work show that our SERS sampling technique is particularly effective for the study of the structure at the outermost surface. The extremely low pyrolysis and carbonization temperature at room temperature implies several responsible mechanisms. The catalytic action of the overcoated silver is of course one possible explanation for the observed lowering. Another mechanism for the lowering of the transition temperatures may originate from the inducement of laser radiation. Future work will certainly be attractive in the field concerned, although this is merely the first report on tentatively developing the technique as an analytical tool for the characterization of the outermost surface structure of films. Acknowledgment. We are indebted to support by the National Natural Science Foundation of China, (Nos. 29974014, 90103036, and 50133010) and to support by the State Key Laboratory of Polymer Physics at Changchun Applied Chemistry Institute. References and Notes (1) Frank, C. W.; Rao, V.; Despotopoulou, M. M.; Pease, R. F. W.; Hinsherg, W. D.; Miller, R. D.; Rabolt, J. F. Science (Washington, D.C.) 1996, 273, 912. (2) de Gennes, P. G. Eur. Phys. J. E 2000, 2, 201. (3) Pitard, E.; Bouchaud, J. P. Eur. Phys. J. E 2001, 5, 133. (4) Vormoor, O. Eur. Phys. J. E 2001, 5, 551.
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