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Fabrication and mechanism study of CuO layers on double surfaces of polyimide substrate using surface modification
Composites Science and Technology 72 (2012) 1020–1026
Contents lists available at SciVerse ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Fabrication and mechanism study of CuO layers on double surfaces of polyimide substrate using surface modification Jiayu Zhan a,b, Guofeng Tian a, Shengli Qi a, Zhanpeng Wu a, Dezhen Wu a,⇑, Riguang Jin a a b
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China Beijing Building Materials Academy of Science Research/Solid Waste Resources Utilization and Energy Saving Building Materials State Key Laboratory, Beijing 100041, PR China
a r t i c l e
i n f o
Article history: Received 29 April 2011 Received in revised form 13 March 2012 Accepted 17 March 2012 Available online 23 March 2012 Keywords: A. Polymer–matrix composites A. Nano particles B. Surface treatments Mechanism
a b s t r a c t Formation process and mechanism of continuous CuO layers on double surfaces of polyimide films were studied. The composite films were prepared using the facile surface modification and ion exchange technique. By alkaline-induced chemical modification and ion-exchange reaction, Cu2+ ions were incorporated into the surface of polyimide substrate. Thermal treatment in ambient atmosphere resulted in the formation of CuO particles that further agglomerated on the film surface and produced well-defined CuO thin layers on the double surfaces of polyimide films. The changes in the chemical structure, surface morphology, crystalline state and the surface roughness with the increase of ambient temperature were investigated. It was interesting to find that the conversion of metallic copper and low valence sub-oxide Cu2O to high valence oxide CuO was observed in the thermal treatment process. The agglomeration mechanism for the CuO particles was proposed and proved by three steps, which illustrated that copper-catalyzed and oxygen-assisted decomposition of the polyimide overlayer resulted in the agglomeration of CuO particles. The final composite films retained the thermal stability of the pure polyimide. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Over recent years, aromatic polyimide has been considered to be one of the most important substrates of composite materials, because of its excellent mechanical properties, thermal stability and chemical resistance [1–4]. The fabrication of metal or metal oxide nanoparticles onto polyimide surfaces has attracted much attention due to the combination of both the outstanding properties of polyimide and the unique functions of nanoparticles. Inorganic nanoparticles, such as Ag [5–7], Cu [8–10], Pd [11], Ni [12], Au [13], NiO [14], Fe2O3 [15], SnO2 [16,17], ZnO [17,18] and Co3O4 [19], have been incorporated onto polyimide surface respectively to endow polyimide with conductive, reflective, catalytic, semiconductive or magnetic properties. For example, the copper metallization of polyimide substrate is important in the fabrication of microelectronic devices including large-scale integrated circuits (LSI) and printed circuit boards (PCB) [9]. Silver metallized polyimide films with excellent reflective and conductive properties are widely attractive in the microelectronics and aerospace industries [6]. The incorporation of semiconductive metal oxide, such as SnO2, Co3O4, on polyimide surface provides potential applications in catalysis, gas sensing, conducting electrodes, as well as in lithium batteries [16,19].
⇑ Corresponding author. Tel./fax: +86 10 64421693. E-mail address: [email protected] (D. Wu). 0266-3538/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compscitech.2012.03.014
Conventional approaches for the fabrication of polyimide composite films mainly focused on external deposition process which directly deposited metal phase onto polyimide substrate or further oxidized the deposited metal to metal oxide [20–22]. A well-established inorganic layer could be formed on the film surface. However, this process always leaded to poor adhesive property between inorganic layer and polymeric layer. In the 1990s, Southward and coworkers [23] developed an in situ method, which involved dissolving proper metal salts or complex into polyimide precursor solution. Thermal treatment converted metal salt or complex to corresponding metal or metal oxide nanoparticles which further aggregated on the film surface. This method provided an outstanding adhesion at polymer-inorganic interface. Surface-silvered polyimide films fabricated with this technique can obtain a maximum reflectivity more than 97% and the surface resistivity less than 0.1 X/square. However, the metal complex is usually unstable and quite expensive. Furthermore, polyimide substrate suffered from a serious degradation during thermal treatment [24,25]. Recently, surface modification and ion-exchange technique provided the probability of combining the advantages of both external deposition process and in situ synthesis process. An alkaline induced surface hydrolysis was performed on polyimide film surfaces to obtain ion exchangeable carboxylate groups. The thickness of the modified layer can be controlled by the alkali treatment conditions such as concentration, time and temperature. Metal ions were incorporated into the modified layer via ion exchange reaction.
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The corresponding metal or metal oxide particles were formed by reduction, decomposition or oxidization. With this method, polyimide films coated with NiO [14], Co3O4 [19], Ag [6,7] or Cu [9,10] layers have been prepared. Much attention was paid on the formation process of inorganic layers on polyimide film surface, but the aggregation mechanism of inorganic nanoparticles has rarely been studied. As a member of transition metal oxide family, CuO in nanometer-scale dimension exhibits advanced functions in catalyst, semiconductor, gas sensor, and metallurgy [26–29]. Here, we report the fabrication of CuO layer on double surfaces of polyimide film via surface modification and ion-exchange technique. To our knowledge, CuO layers contained polyimide films prepared in this technique have not been reported. Furthermore, a mechanism for the aggregation of CuO particles has been proposed according to the experimental results. The mechanism is scientifically proved by a three-step experiment, confirming to be a copper-catalyzed decomposition of polymer overlayer assisted by oxygen. The thermal stability and adhesion property for the composite film have been extensively investigated.
accelerating voltage of 20 kV. All the samples were coated with a ca. 5 nm of platinum layer prior to measurement. Cross-sectional morphology was observed using a Hitachi H800 transmission electron microscope (TEM) at an accelerating voltage at 200 kV. The samples were sectioned into slices using an ultramicrotome with a diamond knife. These thin sections were mounted onto the carbon-coated TEM copper grids for observation. X-ray photoelectron spectroscopy (XPS) measurement was carried out on an ESCALAB 250 spectrometer (Thermo Electron Corporation) with a monochromatic Al Ka X-ray source and a magnetic lens system that yields high spatial resolution and high sensitivity. The pressure in the analysis chamber was maintained at 2 1010 Torr or lower during each measurement. Atomic force microscopy (AFM) observations were carried out using a Nanoscope IIIa AFM system (Digital Instruments Inc., USA) operating in tapping mode. A scanning area of 2 lm 2 lm was examined in air at room temperature. The arithmetic mean
Polyimide film film 2. Experimental parts 2.1. Materials Commercial 80 lm thick pyromellitic dianhydride–oxydianiline (PMDA–ODA)-type polyimide films were purchased from Liyang Huajing Ltd., Jiangsu province, China. Films were cleaned using ethanol solution under ultrasonication for 15 min prior to use. Potassium hydroxide (KOH) (analytically pure) and copper(II) nitrate (Cu(NO3)23H2O) (analytically pure) were purchased from Sinopharm Chemical Reagent Co., Ltd., and used without further purification.
1)KOH treatment 2) Ion exchange
-COO-Cu2+ -OOC -
3)Thermal treatment
Polyimide film CuO CuO layers layers
2.2. Preparation of composite films The procedure to prepare the CuO layer on the polyimide surface was similar to those of the surface silvered polyimide films previously reported [6]. Briefly, commercial polyimide films were initially immersed into a 2 M KOH aqueous solution at 15 °C for 13 h to perform alkaline-induced hydrolysis of film surface, and then washed with deionized water. The surface modified films were next immersed into a 0.4 M Cu(NO3)2 aqueous solution at 15 °C for 1 h to load copper ions into the modified region via ion exchange. After being rinsed thoroughly with copious amount of deionized water, the copper(II)-contained films were thermally treated in an forced-air oven at ambient atmosphere to the target temperature. The cure cycles involved heating over 1 h to a temperature of 135 °C and then holding for 1 h, followed by heating to 350 °C over 2 h, and then keeping the temperature constant at 350 °C.
Scheme 1. Schematic illustration for the preparation of polyimide/CuO composite films.
2.3. Characterization Attenuated total reflection-Fourier transform infrared (ATRFTIR) spectra of the PI films were recorded using a Nicolet Nexus670 IR spectrometer with an ATR attachment. X-ray diffraction (XRD) patterns were performed on an X-ray diffractometer (D/Max2500VB2+/PC, Rigaku, Japan) at a scanning rate of 0.18° per second, in the 30–90° region. The X-ray beam was generated by a Cu Ka radiation source (k = 0.154056 nm), using a tube voltage of 40 kV and a current of 200 mA. Surface morphology was recorded on a Hitachi S-4700 fieldemission scanning electron microscope (FE-SEM) operating at an
Fig. 1. TEM image for the copper ions contained film thermally treated at 350 °C for 7 h.
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of the surface roughness (Ra) was calculated from the roughness profile determined by AFM. Thermal gravimetric analysis (TGA) was conducted on a TGA Q50 thermogravimetric analyser (TA instruments). The temperature of the sample gradually increased from 40 °C to 800 °C at a heating rate of 10 °C/min.
Fig. 2. ATR-FTIR spectra for the films (a) pure PI; (b) pure PI after treated by KOH for 9 h, and (c) the copper ions contained films after thermal treated at 350 °C for 7 h.
3. Results and discussion 3.1. Surface modification process Previous investigations [6,9,10,14] of the fabrication of polyimide-based surface metallization films conclude that polyimide can be hydrolyzed by strong alkalis to cleave its imide ring yielding amide bonds and ion-exchangeable metal salts of carboxylic acid. Based on this fact, we propose the ideal synthetic protocol for the formation of CuO layer on double surface of polyimide film, which has been schematically presented in Scheme 1. Here we perform the hydrolysis of polyimide on the film surface by KOH aqueous solutions. The thickness of modified layer is controllable by adjusting the concentration of alkaline aqueous solution, hydroxylation time and temperature. Potassium ions bound to the carboxylic anions exchange with copper(II) ions in the copper(II) nitrate aqueous solution. Theoretically, the amount of divalent Cu2+ ions incorporated into the modified layer is half that of K+ ions initially. After Cu2+ ions incorporated into the modified layer of polyimide film, the process of thermal treatment was performed, in which the hydrolyzed polyimide surface was re-imidized coupled with the formation of CuO layer under ambient circumstance. For clarity, the morphology of final polyimide film containing CuO layer prepared following the above steps was exhibited by TEM result in Fig. 1. For the sample treated by 3 M KOH aqueous solution for 13 h, 0.4 M Cu(NO3)2 for 1 h, and followed by thermal treatment at 350 °C for 7 h, we can observe that the final composite film had a continuous CuO layer on the film surface with the thickness corresponding to about 520 nm. A distinguishable interface between the CuO layer and polyimide substrate can be clearly ob-
Fig. 3. SEM images for the copper ions contained composite films thermally treated at (a) 238 °C; (b) 350 °C for 0 h; (c) 350 °C for 1 h; (d) 350 °C for 5 h; and (e) 350 °C for 7 h.
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served. Concretely, the formation process of CuO particles and their agglomeration will be discussed below. The surface modification process was tracked by ATR-FTIR characterization. Fig. 2 shows the chemical structure change of the pure polyimide films before and after alkaline induced hydrolysis by KOH and subsequent thermal treatment. The strong absorbance peaks of pure polyimide at 1710, 1775, and 1364 cm1 represent the symmetric carbonyl stretching, asymmetric carbonyl stretching, and imide ring CANAC stretching, respectively. After the film were treated by KOH, no imide ring related bands are observed and amide bands at 1660 (amide I; carbonyl stretching) and 1544 cm1 (amide II; coupling of CAN stretch and NAH deforma-
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tion) are visible (Fig. 2b). The appearance of an absorbance peak at 1366 cm1 and the broad band near 1600 cm1 in Fig. 1b indicate that K+ are present in the form of potassium polycarboxylate salts in the modified layer. Upon applying the thermal treatment, the absorbance peaks arising from imide ring bands at 1710, 1775, and 1364 cm1 are visible as shown in Fig. 2c. This indicates that the re-imidization reaction of the surface modified layer proceeds. The unchangeable peak of benzene ring C@C stretching at 1494 cm1 is generally used as the internal standard for determining the degree of imidization. The calculation is performed by comparing the ratio of the intensity of the absorbance at 1710 cm1 to that at 1494 cm1 (A1710/A1494). For the pure polyimide film (Fig. 2a), the peak height 0.448 of imide rings at 1710 cm1 divided by the peak height 0.449 of the peak at 1494 cm1 was 0.998, which was regarded as the standard for a degree of imidization of 100%. After the copper ions contained films thermally treated at 350 °C for 7 h, the relative degree of imidization reached to 98.9% indicating that the already opened imide ring has almost totally reformed under this experimental condition. 3.2. Effect of temperature on particle growth SEM micrographs and AFM images, reflecting the variation of the surface topographies of the produced polyimide/CuO films at different thermal treatment stage, are shown in Figs. 3 and 4, respectively. The film surface is relatively smooth without the detection of any particles for the sample subjected to a thermal treatment of 238 °C (Fig. 3a). The mean roughness value (Ra) of the film surface is 2.295 nm. Correspondingly, the X-ray diffraction pattern in Fig. 5 for the film subjected to a thermal treatment of 238 °C exhibits no reflections expected for the copper oxide. After the temperature is raised to 350 °C, the film surface is homogeneously covered with small particles and has a mean roughness of 2.970 nm. In contrast, XRD patterns give very distinct peaks characteristic of the crystalline state at this time and temperature. It is interesting to find that the diffractions peaks at this temperature correspond to the reflection of CuO mix with Cu2O and Cu. Upon further thermal treatment to 350 °C for 1 h, the SEM image
Fig. 4. AFM images of the copper ions contained composite films thermally treated at (a) 238 °C; (b) 350 °C for 0 h; and (c) 350 °C for 1 h; surface roughness: (a) Ra = 2.295 nm, (b) Ra = 2.970 nm, and (c) Ra = 4.828 nm.
Fig. 5. XRD patterns for the films thermal treated at (a) 238 °C; (b) 350 °C for 0 h; (c) 350 °C for 1 h; (d) 350 °C for 5 h; and (e) 350 °C for 7 h.
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gives very distinct change. A large amount of particles aggregate at the film surfaces and an increase in the dimension of particles can be observed with the thermal treatment time prolonging. AFM 3D images in Fig. 4c show that the dimension of CuO clusters is large enough to contact with each other and form continuous layers. The Ra value of the film surface increases to 4.828 nm. However, it is strange that the peaks corresponded to Cu2O and Cu in the XRD patterns are all disappeared, indicating that the Cu2O and Cu have totally converted to CuO at this point. From the above results, we suggested that the formation process of CuO particles included the reactions (Eqs (1)–(4)) as follows. At the lower temperature, copper(II) ions were reduced to metallic copper initiated by thermal treatment (Reaction (1)) and subsequently the unstable metallic copper was oxidized to Cu2O or CuO under high temperature ambient circumstance as depicted in Reactions (2) and (3). Subsequently, the Cu sub-oxide was further oxidized to high valence oxide (Reaction (4)).
Cu2þ þ 2e ! Cu
ð1Þ
4Cu þ O2 ! 2Cu2 O
ð2Þ
2Cu þ O2 ! 2CuO
ð3Þ
2Cu2 O þ O2 ! 4CuO
ð4Þ
in this experiment. After the copper(II) ions contained film being reduced by polyol method, the thickness of Cu nanoparticles contained layer reached to the value of 6 lm [30], which is much more larger than the thickness of CuO layer formed in this experiment (Fig. 1). In order to clarify the agglomeration mechanism for the CuO clusters, polyimide/copper composite film prepared by polyol method was further thermally treated under three different conditions: (1) vacuum, (2) insufficient oxygen, and (3) air circumstance. Figs. 6 and 7 suggested the thickness variation of particles contained layer and the crystallite change, respectively. For the film subjected to the thermal treatment under vacuum, the thickness
3.3. Mechanism of CuO layer formation Along with the thermal treatment process, a significant agglomeration of formed CuO clusters on the double surfaces of polyimide film was observed in the previous results. However, the agglomeration process and mechanism for CuO particles formation is confusing. Therefore, we performed several thermal treatment steps to track this process. We have utilized the same commercial polyimide films to prepare polyimide/Cu composite films by polyol method [30]. The experimental conditions for alkaline-induced hydrolysis and ion exchange copper(II) ions are the same as that
Fig. 7. XRD patterns for the ion-exchanged films reduced by ethylene glycol and then thermal treated in (a) vacuum, (b) insufficient oxygen, and (c) air at 350 °C for 3 h.
Fig. 6. Cross-sectional TEM images for the ion-exchanged films reduced by ethylene glycol and then thermal treated in (a) vacuum, (b) insufficient oxygen, and (c) air at 350 °C for 3 h.
J. Zhan et al. / Composites Science and Technology 72 (2012) 1020–1026
Fig. 8. XPS wide scan and O 1s core-level spectra of copper ions contained polyimide films cured at (a) 238 °C and (b) 350 °C for 1 h.
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of particles contained layer decreased to 5 lm because of the further imidization and release of H2O molecules. In addition, the particles dispersed uniformly and kept the crystallite of metallic copper as shown in Figs. 6a and 7a. Upon the treatment under insufficient oxygen condition, cross-sectional TEM image exhibits an obvious concentration gradient of particles. More particles concentrate on the near-substrate side instead of near-surface side and the corresponding layer thickness decreased to 3.2 lm. The XRD pattern in Fig. 7b indicates the total conversion of metallic copper to Cu2O crystallite. If the film is directly treated under air circumstance which is the same as the preparation condition of polyimide/CuO composite films in the present experiment, a layer of only 529 nm in thickness was formed on the film surface as shown in Fig. 6c. Both the particles morphology and the layer thickness are consistent with the result in Fig. 1. Therefore, we conclude that the formation of CuO layer is a result of copper-catalyzed decomposition of polymer overlayer assisted by oxygen other than the migration of CuO from the near-substrate side to the near-surface side. XPS analysis was also performed on the composite films to further clarify the oxygen-assisted decomposition on polyimide. Fig. 8 displays the respective survey scan and O 1s core-level spectra for the copper ions contained polyimide films thermally treated at (a) 238 °C and (b) 350 °C for 1 h. As for the film treated at 238 °C, the binding state of O 1s core-level electrons can be resolved to three contributions, characterization by the binding energies at 531.8 eV for the C@O species, at 533.7 eV for the CAOAC species, and at 530.0 eV for the CuAO species. A very faint intensity of CuAO signal suggested a small amount of CuO formed on the film surface at this temperature. After the film subjected to 350 °C for 1 h, intense Cu signals were detected in survey scan spectrum. The shape of O 1s also exhibited significant variation. A sharp increase in the peak intensity at 530.0 eV which was associated with the oxygen atom in the CuAO bonds indicated that the aggregation of CuO substance occurred on the film surface. 3.4. Film properties
Fig. 9. TG analyses of (1) pure polyimide film and (2) copper ions contained composite films thermally treated at 350 °C for 7 h.
Thermal stability of the copper ions contained polyimide films after thermally treated at 350 °C for 7 h was evaluated by thermogravimetric analysis (Fig. 9-2). For clarity, the profile of pure polyimide film was also recorded for comparison (Fig. 9-1). The decomposition temperature for the polyimide/CuO composite film
Fig. 10. SEM micrographs for the surfaces of the films: (a) before and (b) after the adhesion test.
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was 593.5 °C at the point of 10% weight loss when tested under nitrogen atmosphere. This temperature was a little lower than the decomposition temperature of pure polyimide film (603.8 °C). The result indicated that the formation of CuO layer on the polyimide surface did not make pronounced effect on the thermal stability of the polyimide film. The adhesion between CuO layer and polyimide substrate was a primary concern for applications. Mechanical debonding studies were performed on the polyimide/CuO composite film using commercial transparent adhesive tape. The film was fixed on the on a substrate and tested using adhesive tape by sticking it on the film surface and then peeling it off. SEM analysis was carried out on the film surface before and after the adhesion test as a qualitative evaluation of bonding strength between CuO particles and the polyimide substrate. After the peeling test (Fig. 10b), it cannot be ignored that some CuO agglomerates were drawn away from the film surface. However, the overall morphologies before and after the peeling test were not distinctly altered. Continuous CuO layers were still maintained. This result indicated that the adhesion strength at the CuO–polyimide interface was quite acceptable for the films prepared by surface modification and ion exchange technique. The strong adhesion was attributed to the mechanical interlocking effect as reported in the literature [7,23,31]. 4. Conclusions We have demonstrated the fabrication process of CuO layers on double surfaces of polyimide substrate involving the alkalineinduced surface hydrolysis, using KOH, ion exchange reaction between K+ and Cu2+, and thermal treatment in ambient circumstance. CuO thin layers with the thickness of ca.520 nm were observed. The dimension of CuO particles and surface roughness increased with the thermal treatment time prolonging. Metallic copper and Cu2O crystallite was observed during the conversion process from Cu2+ to CuO. The agglomeration mechanism for the CuO particles on the film surface was proved to be the coppercatalyzed and oxygen-assisted decomposition of the polyimide overlayer. The final composite film basically maintained the thermal stability of pristine polyimide. The adhesion at the CuO–polyimide interface was quite acceptable for many practical applications. Acknowledgements This work was supported by the National Natural Science Foundation of China (Project No. 51071015), the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT, IRT0706), and the Polymer Chemistry and Physics of Beijing Municipal Education Commission (BMEC, No. XK100100640). References [1] Yen CT, Chen WC. Effects of molecular structures on the near-infrared optical properties of polyimide derivatives and their corresponding optical waveguides. Macromolecules 2003;36:3315–9. [2] Yang CP, Chen RS. Organosoluble polyimides and copolyimides based on 1,1bis[4-(4-aminophenoxy)phenyl]-1-phenylethane and aromatic dianhydrides. J Polym Sci A Polym Chem 2000;38:2082–90. [3] Kreuz JA, Edman JR. Polyimide films. Adv Mater 1998;10:1229–32. [4] Magaraphan R, Lilayuthalert W, Sirivat A, Schwank JW. Preparation, structure, properties and thermal behavior of rigid-rod polyimide/montmorillonite nanocomposites. Compos Sci Technol 2001;61:1253–64.
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Contents lists available at SciVerse ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Fabrication and mechanism study of CuO layers on double surfaces of polyimide substrate using surface modification Jiayu Zhan a,b, Guofeng Tian a, Shengli Qi a, Zhanpeng Wu a, Dezhen Wu a,⇑, Riguang Jin a a b
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China Beijing Building Materials Academy of Science Research/Solid Waste Resources Utilization and Energy Saving Building Materials State Key Laboratory, Beijing 100041, PR China
a r t i c l e
i n f o
Article history: Received 29 April 2011 Received in revised form 13 March 2012 Accepted 17 March 2012 Available online 23 March 2012 Keywords: A. Polymer–matrix composites A. Nano particles B. Surface treatments Mechanism
a b s t r a c t Formation process and mechanism of continuous CuO layers on double surfaces of polyimide films were studied. The composite films were prepared using the facile surface modification and ion exchange technique. By alkaline-induced chemical modification and ion-exchange reaction, Cu2+ ions were incorporated into the surface of polyimide substrate. Thermal treatment in ambient atmosphere resulted in the formation of CuO particles that further agglomerated on the film surface and produced well-defined CuO thin layers on the double surfaces of polyimide films. The changes in the chemical structure, surface morphology, crystalline state and the surface roughness with the increase of ambient temperature were investigated. It was interesting to find that the conversion of metallic copper and low valence sub-oxide Cu2O to high valence oxide CuO was observed in the thermal treatment process. The agglomeration mechanism for the CuO particles was proposed and proved by three steps, which illustrated that copper-catalyzed and oxygen-assisted decomposition of the polyimide overlayer resulted in the agglomeration of CuO particles. The final composite films retained the thermal stability of the pure polyimide. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Over recent years, aromatic polyimide has been considered to be one of the most important substrates of composite materials, because of its excellent mechanical properties, thermal stability and chemical resistance [1–4]. The fabrication of metal or metal oxide nanoparticles onto polyimide surfaces has attracted much attention due to the combination of both the outstanding properties of polyimide and the unique functions of nanoparticles. Inorganic nanoparticles, such as Ag [5–7], Cu [8–10], Pd [11], Ni [12], Au [13], NiO [14], Fe2O3 [15], SnO2 [16,17], ZnO [17,18] and Co3O4 [19], have been incorporated onto polyimide surface respectively to endow polyimide with conductive, reflective, catalytic, semiconductive or magnetic properties. For example, the copper metallization of polyimide substrate is important in the fabrication of microelectronic devices including large-scale integrated circuits (LSI) and printed circuit boards (PCB) [9]. Silver metallized polyimide films with excellent reflective and conductive properties are widely attractive in the microelectronics and aerospace industries [6]. The incorporation of semiconductive metal oxide, such as SnO2, Co3O4, on polyimide surface provides potential applications in catalysis, gas sensing, conducting electrodes, as well as in lithium batteries [16,19].
⇑ Corresponding author. Tel./fax: +86 10 64421693. E-mail address: [email protected] (D. Wu). 0266-3538/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compscitech.2012.03.014
Conventional approaches for the fabrication of polyimide composite films mainly focused on external deposition process which directly deposited metal phase onto polyimide substrate or further oxidized the deposited metal to metal oxide [20–22]. A well-established inorganic layer could be formed on the film surface. However, this process always leaded to poor adhesive property between inorganic layer and polymeric layer. In the 1990s, Southward and coworkers [23] developed an in situ method, which involved dissolving proper metal salts or complex into polyimide precursor solution. Thermal treatment converted metal salt or complex to corresponding metal or metal oxide nanoparticles which further aggregated on the film surface. This method provided an outstanding adhesion at polymer-inorganic interface. Surface-silvered polyimide films fabricated with this technique can obtain a maximum reflectivity more than 97% and the surface resistivity less than 0.1 X/square. However, the metal complex is usually unstable and quite expensive. Furthermore, polyimide substrate suffered from a serious degradation during thermal treatment [24,25]. Recently, surface modification and ion-exchange technique provided the probability of combining the advantages of both external deposition process and in situ synthesis process. An alkaline induced surface hydrolysis was performed on polyimide film surfaces to obtain ion exchangeable carboxylate groups. The thickness of the modified layer can be controlled by the alkali treatment conditions such as concentration, time and temperature. Metal ions were incorporated into the modified layer via ion exchange reaction.
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The corresponding metal or metal oxide particles were formed by reduction, decomposition or oxidization. With this method, polyimide films coated with NiO [14], Co3O4 [19], Ag [6,7] or Cu [9,10] layers have been prepared. Much attention was paid on the formation process of inorganic layers on polyimide film surface, but the aggregation mechanism of inorganic nanoparticles has rarely been studied. As a member of transition metal oxide family, CuO in nanometer-scale dimension exhibits advanced functions in catalyst, semiconductor, gas sensor, and metallurgy [26–29]. Here, we report the fabrication of CuO layer on double surfaces of polyimide film via surface modification and ion-exchange technique. To our knowledge, CuO layers contained polyimide films prepared in this technique have not been reported. Furthermore, a mechanism for the aggregation of CuO particles has been proposed according to the experimental results. The mechanism is scientifically proved by a three-step experiment, confirming to be a copper-catalyzed decomposition of polymer overlayer assisted by oxygen. The thermal stability and adhesion property for the composite film have been extensively investigated.
accelerating voltage of 20 kV. All the samples were coated with a ca. 5 nm of platinum layer prior to measurement. Cross-sectional morphology was observed using a Hitachi H800 transmission electron microscope (TEM) at an accelerating voltage at 200 kV. The samples were sectioned into slices using an ultramicrotome with a diamond knife. These thin sections were mounted onto the carbon-coated TEM copper grids for observation. X-ray photoelectron spectroscopy (XPS) measurement was carried out on an ESCALAB 250 spectrometer (Thermo Electron Corporation) with a monochromatic Al Ka X-ray source and a magnetic lens system that yields high spatial resolution and high sensitivity. The pressure in the analysis chamber was maintained at 2 1010 Torr or lower during each measurement. Atomic force microscopy (AFM) observations were carried out using a Nanoscope IIIa AFM system (Digital Instruments Inc., USA) operating in tapping mode. A scanning area of 2 lm 2 lm was examined in air at room temperature. The arithmetic mean
Polyimide film film 2. Experimental parts 2.1. Materials Commercial 80 lm thick pyromellitic dianhydride–oxydianiline (PMDA–ODA)-type polyimide films were purchased from Liyang Huajing Ltd., Jiangsu province, China. Films were cleaned using ethanol solution under ultrasonication for 15 min prior to use. Potassium hydroxide (KOH) (analytically pure) and copper(II) nitrate (Cu(NO3)23H2O) (analytically pure) were purchased from Sinopharm Chemical Reagent Co., Ltd., and used without further purification.
1)KOH treatment 2) Ion exchange
-COO-Cu2+ -OOC -
3)Thermal treatment
Polyimide film CuO CuO layers layers
2.2. Preparation of composite films The procedure to prepare the CuO layer on the polyimide surface was similar to those of the surface silvered polyimide films previously reported [6]. Briefly, commercial polyimide films were initially immersed into a 2 M KOH aqueous solution at 15 °C for 13 h to perform alkaline-induced hydrolysis of film surface, and then washed with deionized water. The surface modified films were next immersed into a 0.4 M Cu(NO3)2 aqueous solution at 15 °C for 1 h to load copper ions into the modified region via ion exchange. After being rinsed thoroughly with copious amount of deionized water, the copper(II)-contained films were thermally treated in an forced-air oven at ambient atmosphere to the target temperature. The cure cycles involved heating over 1 h to a temperature of 135 °C and then holding for 1 h, followed by heating to 350 °C over 2 h, and then keeping the temperature constant at 350 °C.
Scheme 1. Schematic illustration for the preparation of polyimide/CuO composite films.
2.3. Characterization Attenuated total reflection-Fourier transform infrared (ATRFTIR) spectra of the PI films were recorded using a Nicolet Nexus670 IR spectrometer with an ATR attachment. X-ray diffraction (XRD) patterns were performed on an X-ray diffractometer (D/Max2500VB2+/PC, Rigaku, Japan) at a scanning rate of 0.18° per second, in the 30–90° region. The X-ray beam was generated by a Cu Ka radiation source (k = 0.154056 nm), using a tube voltage of 40 kV and a current of 200 mA. Surface morphology was recorded on a Hitachi S-4700 fieldemission scanning electron microscope (FE-SEM) operating at an
Fig. 1. TEM image for the copper ions contained film thermally treated at 350 °C for 7 h.
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of the surface roughness (Ra) was calculated from the roughness profile determined by AFM. Thermal gravimetric analysis (TGA) was conducted on a TGA Q50 thermogravimetric analyser (TA instruments). The temperature of the sample gradually increased from 40 °C to 800 °C at a heating rate of 10 °C/min.
Fig. 2. ATR-FTIR spectra for the films (a) pure PI; (b) pure PI after treated by KOH for 9 h, and (c) the copper ions contained films after thermal treated at 350 °C for 7 h.
3. Results and discussion 3.1. Surface modification process Previous investigations [6,9,10,14] of the fabrication of polyimide-based surface metallization films conclude that polyimide can be hydrolyzed by strong alkalis to cleave its imide ring yielding amide bonds and ion-exchangeable metal salts of carboxylic acid. Based on this fact, we propose the ideal synthetic protocol for the formation of CuO layer on double surface of polyimide film, which has been schematically presented in Scheme 1. Here we perform the hydrolysis of polyimide on the film surface by KOH aqueous solutions. The thickness of modified layer is controllable by adjusting the concentration of alkaline aqueous solution, hydroxylation time and temperature. Potassium ions bound to the carboxylic anions exchange with copper(II) ions in the copper(II) nitrate aqueous solution. Theoretically, the amount of divalent Cu2+ ions incorporated into the modified layer is half that of K+ ions initially. After Cu2+ ions incorporated into the modified layer of polyimide film, the process of thermal treatment was performed, in which the hydrolyzed polyimide surface was re-imidized coupled with the formation of CuO layer under ambient circumstance. For clarity, the morphology of final polyimide film containing CuO layer prepared following the above steps was exhibited by TEM result in Fig. 1. For the sample treated by 3 M KOH aqueous solution for 13 h, 0.4 M Cu(NO3)2 for 1 h, and followed by thermal treatment at 350 °C for 7 h, we can observe that the final composite film had a continuous CuO layer on the film surface with the thickness corresponding to about 520 nm. A distinguishable interface between the CuO layer and polyimide substrate can be clearly ob-
Fig. 3. SEM images for the copper ions contained composite films thermally treated at (a) 238 °C; (b) 350 °C for 0 h; (c) 350 °C for 1 h; (d) 350 °C for 5 h; and (e) 350 °C for 7 h.
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served. Concretely, the formation process of CuO particles and their agglomeration will be discussed below. The surface modification process was tracked by ATR-FTIR characterization. Fig. 2 shows the chemical structure change of the pure polyimide films before and after alkaline induced hydrolysis by KOH and subsequent thermal treatment. The strong absorbance peaks of pure polyimide at 1710, 1775, and 1364 cm1 represent the symmetric carbonyl stretching, asymmetric carbonyl stretching, and imide ring CANAC stretching, respectively. After the film were treated by KOH, no imide ring related bands are observed and amide bands at 1660 (amide I; carbonyl stretching) and 1544 cm1 (amide II; coupling of CAN stretch and NAH deforma-
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tion) are visible (Fig. 2b). The appearance of an absorbance peak at 1366 cm1 and the broad band near 1600 cm1 in Fig. 1b indicate that K+ are present in the form of potassium polycarboxylate salts in the modified layer. Upon applying the thermal treatment, the absorbance peaks arising from imide ring bands at 1710, 1775, and 1364 cm1 are visible as shown in Fig. 2c. This indicates that the re-imidization reaction of the surface modified layer proceeds. The unchangeable peak of benzene ring C@C stretching at 1494 cm1 is generally used as the internal standard for determining the degree of imidization. The calculation is performed by comparing the ratio of the intensity of the absorbance at 1710 cm1 to that at 1494 cm1 (A1710/A1494). For the pure polyimide film (Fig. 2a), the peak height 0.448 of imide rings at 1710 cm1 divided by the peak height 0.449 of the peak at 1494 cm1 was 0.998, which was regarded as the standard for a degree of imidization of 100%. After the copper ions contained films thermally treated at 350 °C for 7 h, the relative degree of imidization reached to 98.9% indicating that the already opened imide ring has almost totally reformed under this experimental condition. 3.2. Effect of temperature on particle growth SEM micrographs and AFM images, reflecting the variation of the surface topographies of the produced polyimide/CuO films at different thermal treatment stage, are shown in Figs. 3 and 4, respectively. The film surface is relatively smooth without the detection of any particles for the sample subjected to a thermal treatment of 238 °C (Fig. 3a). The mean roughness value (Ra) of the film surface is 2.295 nm. Correspondingly, the X-ray diffraction pattern in Fig. 5 for the film subjected to a thermal treatment of 238 °C exhibits no reflections expected for the copper oxide. After the temperature is raised to 350 °C, the film surface is homogeneously covered with small particles and has a mean roughness of 2.970 nm. In contrast, XRD patterns give very distinct peaks characteristic of the crystalline state at this time and temperature. It is interesting to find that the diffractions peaks at this temperature correspond to the reflection of CuO mix with Cu2O and Cu. Upon further thermal treatment to 350 °C for 1 h, the SEM image
Fig. 4. AFM images of the copper ions contained composite films thermally treated at (a) 238 °C; (b) 350 °C for 0 h; and (c) 350 °C for 1 h; surface roughness: (a) Ra = 2.295 nm, (b) Ra = 2.970 nm, and (c) Ra = 4.828 nm.
Fig. 5. XRD patterns for the films thermal treated at (a) 238 °C; (b) 350 °C for 0 h; (c) 350 °C for 1 h; (d) 350 °C for 5 h; and (e) 350 °C for 7 h.
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gives very distinct change. A large amount of particles aggregate at the film surfaces and an increase in the dimension of particles can be observed with the thermal treatment time prolonging. AFM 3D images in Fig. 4c show that the dimension of CuO clusters is large enough to contact with each other and form continuous layers. The Ra value of the film surface increases to 4.828 nm. However, it is strange that the peaks corresponded to Cu2O and Cu in the XRD patterns are all disappeared, indicating that the Cu2O and Cu have totally converted to CuO at this point. From the above results, we suggested that the formation process of CuO particles included the reactions (Eqs (1)–(4)) as follows. At the lower temperature, copper(II) ions were reduced to metallic copper initiated by thermal treatment (Reaction (1)) and subsequently the unstable metallic copper was oxidized to Cu2O or CuO under high temperature ambient circumstance as depicted in Reactions (2) and (3). Subsequently, the Cu sub-oxide was further oxidized to high valence oxide (Reaction (4)).
Cu2þ þ 2e ! Cu
ð1Þ
4Cu þ O2 ! 2Cu2 O
ð2Þ
2Cu þ O2 ! 2CuO
ð3Þ
2Cu2 O þ O2 ! 4CuO
ð4Þ
in this experiment. After the copper(II) ions contained film being reduced by polyol method, the thickness of Cu nanoparticles contained layer reached to the value of 6 lm [30], which is much more larger than the thickness of CuO layer formed in this experiment (Fig. 1). In order to clarify the agglomeration mechanism for the CuO clusters, polyimide/copper composite film prepared by polyol method was further thermally treated under three different conditions: (1) vacuum, (2) insufficient oxygen, and (3) air circumstance. Figs. 6 and 7 suggested the thickness variation of particles contained layer and the crystallite change, respectively. For the film subjected to the thermal treatment under vacuum, the thickness
3.3. Mechanism of CuO layer formation Along with the thermal treatment process, a significant agglomeration of formed CuO clusters on the double surfaces of polyimide film was observed in the previous results. However, the agglomeration process and mechanism for CuO particles formation is confusing. Therefore, we performed several thermal treatment steps to track this process. We have utilized the same commercial polyimide films to prepare polyimide/Cu composite films by polyol method [30]. The experimental conditions for alkaline-induced hydrolysis and ion exchange copper(II) ions are the same as that
Fig. 7. XRD patterns for the ion-exchanged films reduced by ethylene glycol and then thermal treated in (a) vacuum, (b) insufficient oxygen, and (c) air at 350 °C for 3 h.
Fig. 6. Cross-sectional TEM images for the ion-exchanged films reduced by ethylene glycol and then thermal treated in (a) vacuum, (b) insufficient oxygen, and (c) air at 350 °C for 3 h.
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Fig. 8. XPS wide scan and O 1s core-level spectra of copper ions contained polyimide films cured at (a) 238 °C and (b) 350 °C for 1 h.
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of particles contained layer decreased to 5 lm because of the further imidization and release of H2O molecules. In addition, the particles dispersed uniformly and kept the crystallite of metallic copper as shown in Figs. 6a and 7a. Upon the treatment under insufficient oxygen condition, cross-sectional TEM image exhibits an obvious concentration gradient of particles. More particles concentrate on the near-substrate side instead of near-surface side and the corresponding layer thickness decreased to 3.2 lm. The XRD pattern in Fig. 7b indicates the total conversion of metallic copper to Cu2O crystallite. If the film is directly treated under air circumstance which is the same as the preparation condition of polyimide/CuO composite films in the present experiment, a layer of only 529 nm in thickness was formed on the film surface as shown in Fig. 6c. Both the particles morphology and the layer thickness are consistent with the result in Fig. 1. Therefore, we conclude that the formation of CuO layer is a result of copper-catalyzed decomposition of polymer overlayer assisted by oxygen other than the migration of CuO from the near-substrate side to the near-surface side. XPS analysis was also performed on the composite films to further clarify the oxygen-assisted decomposition on polyimide. Fig. 8 displays the respective survey scan and O 1s core-level spectra for the copper ions contained polyimide films thermally treated at (a) 238 °C and (b) 350 °C for 1 h. As for the film treated at 238 °C, the binding state of O 1s core-level electrons can be resolved to three contributions, characterization by the binding energies at 531.8 eV for the C@O species, at 533.7 eV for the CAOAC species, and at 530.0 eV for the CuAO species. A very faint intensity of CuAO signal suggested a small amount of CuO formed on the film surface at this temperature. After the film subjected to 350 °C for 1 h, intense Cu signals were detected in survey scan spectrum. The shape of O 1s also exhibited significant variation. A sharp increase in the peak intensity at 530.0 eV which was associated with the oxygen atom in the CuAO bonds indicated that the aggregation of CuO substance occurred on the film surface. 3.4. Film properties
Fig. 9. TG analyses of (1) pure polyimide film and (2) copper ions contained composite films thermally treated at 350 °C for 7 h.
Thermal stability of the copper ions contained polyimide films after thermally treated at 350 °C for 7 h was evaluated by thermogravimetric analysis (Fig. 9-2). For clarity, the profile of pure polyimide film was also recorded for comparison (Fig. 9-1). The decomposition temperature for the polyimide/CuO composite film
Fig. 10. SEM micrographs for the surfaces of the films: (a) before and (b) after the adhesion test.
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was 593.5 °C at the point of 10% weight loss when tested under nitrogen atmosphere. This temperature was a little lower than the decomposition temperature of pure polyimide film (603.8 °C). The result indicated that the formation of CuO layer on the polyimide surface did not make pronounced effect on the thermal stability of the polyimide film. The adhesion between CuO layer and polyimide substrate was a primary concern for applications. Mechanical debonding studies were performed on the polyimide/CuO composite film using commercial transparent adhesive tape. The film was fixed on the on a substrate and tested using adhesive tape by sticking it on the film surface and then peeling it off. SEM analysis was carried out on the film surface before and after the adhesion test as a qualitative evaluation of bonding strength between CuO particles and the polyimide substrate. After the peeling test (Fig. 10b), it cannot be ignored that some CuO agglomerates were drawn away from the film surface. However, the overall morphologies before and after the peeling test were not distinctly altered. Continuous CuO layers were still maintained. This result indicated that the adhesion strength at the CuO–polyimide interface was quite acceptable for the films prepared by surface modification and ion exchange technique. The strong adhesion was attributed to the mechanical interlocking effect as reported in the literature [7,23,31]. 4. Conclusions We have demonstrated the fabrication process of CuO layers on double surfaces of polyimide substrate involving the alkalineinduced surface hydrolysis, using KOH, ion exchange reaction between K+ and Cu2+, and thermal treatment in ambient circumstance. CuO thin layers with the thickness of ca.520 nm were observed. The dimension of CuO particles and surface roughness increased with the thermal treatment time prolonging. Metallic copper and Cu2O crystallite was observed during the conversion process from Cu2+ to CuO. The agglomeration mechanism for the CuO particles on the film surface was proved to be the coppercatalyzed and oxygen-assisted decomposition of the polyimide overlayer. The final composite film basically maintained the thermal stability of pristine polyimide. The adhesion at the CuO–polyimide interface was quite acceptable for many practical applications. Acknowledgements This work was supported by the National Natural Science Foundation of China (Project No. 51071015), the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT, IRT0706), and the Polymer Chemistry and Physics of Beijing Municipal Education Commission (BMEC, No. XK100100640). References [1] Yen CT, Chen WC. Effects of molecular structures on the near-infrared optical properties of polyimide derivatives and their corresponding optical waveguides. Macromolecules 2003;36:3315–9. [2] Yang CP, Chen RS. Organosoluble polyimides and copolyimides based on 1,1bis[4-(4-aminophenoxy)phenyl]-1-phenylethane and aromatic dianhydrides. J Polym Sci A Polym Chem 2000;38:2082–90. [3] Kreuz JA, Edman JR. Polyimide films. Adv Mater 1998;10:1229–32. [4] Magaraphan R, Lilayuthalert W, Sirivat A, Schwank JW. Preparation, structure, properties and thermal behavior of rigid-rod polyimide/montmorillonite nanocomposites. Compos Sci Technol 2001;61:1253–64.
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