Preparation of Polyimide / Zinc Oxide Nanocomposite Films via an Ion-Exchange Technique and Their Photoluminescence Properties

Polyimide (PI) composite films with ZnO nanoparticles embedded in the surface layer were prepared by alkali hydrolyzation following ion exchange in Zn(NO3)2 solution and thermal treatment of the zinc ion-doped PI films in air atmosphere. The effect of alkali treatment, ion exchange, and thermal treatment conditions was investigated in relation to the amount of zinc atomic loading, morphology, photoluminescence (PL), and thermal properties of the PI/ZnO composite films using ICP, XPS, FE-SEM, TEM, Raman microscope, TGA, and DSC. ZnO nanoparticles were formed slowly and dispersed uniformly in the surface-modified layers of PI films with an average diameter of 20 nm. The PL spectra of all the PI/ZnO nanocomposite films obtained at 350◦C/7 h possessed a weak ultraviolet emission peak and a broad and strong visible emission band. The PI/ZnO nanocomposite films maintained the excellent thermal property of the host PI films.


Introduction
Nanoparticles and/or nanocrystals of metal oxide semiconductor material have been extensively studied in the past decade due to their unique properties and wide application in diverse areas.Many research groups have focused on dispersing metal oxide nanoparticles into polymer matrix, as the hybrid nanocomposites not only inherit the functionalities of semiconductor nanoparticles but also possess advantages of polymers such as flexibility, film integrity, and conformity.Among metal oxide materials, ZnO is a multifunctional n-type semiconductor with prominent physical and chemical properties such as wide band gap (3.4 eV), large exciton binding energy (60 meV), good chemical stability, and low dielectric constant.It is of particular interest in many applications including antireflection coating, transparent electrodes in solar cells, gas sensors, ultraviolet light-emitting diodes, piezoelectric devices, and surface acoustic wave devices [1][2][3][4][5].Combining ZnO nanoparticles with a polymer should enhance their optical properties such as fluorescence, transparence and radiation durability.The polymer/ZnO composites have been produced with many different matrices such as poly (vinylpyrrolidone) [6], poly(methyl methacrylate) [7], poly (hydroxyethyl methacrylate) [8], poly (ethylene glycol) [9], low density polyethylene [10], poly(ethylene oxide) [11], Nylone-6 [12], polyurethane [13], and polyimide (PI) [14][15][16][17][18][19][20][21][22][23].Among the various polymers, PI is a promising matrix which has attracted a lot of interest for photonic applications because of its conjugate π-electron system.Moreover, PI possesses excellent dielectric properties, outstanding thermal and chemical stability, and low coefficients of thermal expansion [24][25][26].These attributes make the PI/ZnO composites have potential applications in optoelectronics, light-emitting diodes, photonics, and flatpanel display industries.
The most widely used method to prepare the ZnO nanoparticles dispersed in polymer matrix is directly incorporating ZnO nanoparticles or zinc salts or complex into polymer precursor solution via bulk polymerization of monomers and then thermal curing of the composites [6][7][8][9][10][11][12][13][20][21][22].However, these processes often lead to inhomogeneous agglomeration of the ZnO nanoparticles in polymer matrix.

Bare PI
Surface-modified PI Zn 2+ -doped PI To obtain a sharp size distribution, ZnO nanoparticles need to be modified by surfactant or polymer, making these processes very complex.Kim and co-workers [22,23] developed a new method to fabricate ZnO nanoparticles in very thin PI film by spin-coating the PI precursor, poly (amic acid) (PAA), on thin Zn metal film and then thermal-treating the hybrids during which Zn thin film reacted with PAA to form ZnO nanoparticles which were distributed within the PI thin layer.However, the size and distribution of the ZnO nanoparticles and the composite film thickness are difficult to control.
Recently, several groups [24][25][26][27][28], including ours [24,25], developed an ion-exchange technique to prepare metal particles embedded in polyimide film.This technique utilizes a simple alkali treatment-based surface modification of the PI film to form carboxylic acid groups in the modified layer through imide ring-cleavage reactions, as well as incorporation of metal ions to form metal salts of PAA via ion-exchange reactions in metal salt solution.Final thermal, hydrogen, or reductant-induced reduction of metal ions leads to the formation of metal nanoparticles embedded in PI film surface layers.This method allows control of the morphologies and properties of the composite films by adjusting the initial alkali treatment (time, concentration, and temperature), subsequent ion-exchange conditions (time, concentration, and temperature), and final reduction conditions.This ion-exchange technique could be extended to synthesizing transitional metal oxide nanoparticles dispersed in PI film surface layers when the thermal treatment process of transitional metal ion-doped precursor film is conducted in air atmosphere.Very recently, based on the ion-exchange technique, we reported the Co 3 O 4 metal oxide layers to a flexible PI substrate.Such technique resulted in Co 3 O 4 layers with Co 3 O 4 average diameter of 15∼45 nm formed on both surfaces of the PI films [29].In this study, due to ZnO endowed with above-mentioned many potential applications, we extend this technique to fabricating PI/ZnO films.Different content of ZnO nanoparticles embedded PI/ZnO films was prepared via the ion-exchange technique.The variation in surface morphologies and photoluminescence (PL) properties of the resulted composite films during thermal treatment was investigated.In addition, the effect of initial zinc ion loading on the morphologies, PL properties, and thermal properties of the final composite films were also studied.

Experimental
2.1.Materials.Pyromelltic dianhydrides oxydianiline(PMDA-ODA)-based polyimide films with thickness of 50 μm were obtained from Liyang Huajing Electronic Material Co., Ltd., Jiangsu province (China).The PI films were rinsed by ultrasonic cleaning for 10 min in deionized water and dried in ambient environment prior to use.Zinc nitrate hexahydrate (Zn(NO 3 ) 2 •6H 2 O, ≥99%) was purchased from Sinopharm Chemical Reagent Co., Ltd (China).Potassium hydroxide (KOH) was obtained from Beijing Chemicals Works.All chemicals were analytical grade and used without further purification.

Preparation of PI/ZnO Nanocomposite Films.
The procedures for preparing PI/ZnO nanocomposite films are illustrated in Figure 1.First, PI films were treated by 5 M aqueous KOH solution at 50 • C for several minutes to hydrolyze the film surfaces.After removal from the base, they were rinsed with copious amount of deionized water and were then blown dry with air.Then, the surface-modified PI films were immersed into aqueous Zn(NO 3 ) 2 solution at room temperature for certain time to incorporate zinc ions into the films through an ion-exchange reaction between potassium and zinc ions.After ion exchange, the films were rinsed by deionized water to remove the surface residual Zn(NO 3 ) 2 solution and dried in ambient atmosphere.Finally, the precursor films were reimidized by slowly heating to 350 • C within 3 h in a forced air oven and were kept at 350 • C for several hours.The alkali treatment time, concentration of aqueous Zn(NO 3 ) 2 solution, and ion-exchange time are listed in Table 1.

Characterization.
The amount of potassium and zinc ions incorporated into the PI films was estimated by a Seiko Instruments SPS 8000 inductively coupled plasma (ICP) atomic emission spectrometer.The doped potassium and zinc ions in the films were extracted by immersing the films into a dilute (5 vol %) aqueous nitric acid solution.
The X-ray photoelectron spectra (XPS) were recorded on an ESCALAB 250 spectrometer (Thermo Electron), using Al Kα X-ray as the excitation source.The spectra were collected at a takeoff angle of 45 • .The pressure in the analysis chamber was maintained at 2×10 −10 mbar or lower during each measurement.To compensate for surface charging effects, all binding energies (BEs) were referenced to the C 1s hydrocarbon peak at 284.6 eV.In peak synthesis, the full width at half maximum (FWHM) of Gaussian peaks was maintained constant for all components in a particular spectrum.Surface morphologies were observed using a fieldemission scanning electron microscope (FE-SEM) (SEM-4700, Hitachi) at an accelerating voltage of 20 kV after samples were coated with ca. 5 nm of palladium-gold alloy.The cross-sectional morphologies of the films were investigated by transmission electron microscopy (TEM, Hitachi H-800) operating at 200 kV.The samples were cut perpendicular to the film surface using an ultramicrotome.Then, the thin sections floating on a water bath were mounted onto the carbon-coated TEM copper grids for analysis.
The room temperature photoluminescence spectra were obtained on a Renishaw RM-1000 Confocal Raman microscope using 325 nm (He-Cd laser source) as the excitation wavelength.
Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analysis were performed using a thermal analyses system (NETZSCH STAR 449C) at a heating rate of 10 • C/min.

Consideration and Characterization of the Zinc Ion-Doped Precursor
Films.Many investigations reported in the literature have defined the chemical mechanism of polyimide surface reactions when polyimide is modified by aqueous alkaline solution, which consists in the hydrolytic cleavage of the imide groups contained in the repeating unit to form potassium (or sodium) salts of carboxyl acid groups and amide bonds [26][27][28].The zinc ions (Zn 2+ ) can be incorporated via subsequent ion-exchange process by immersing the surface-modified films into aqueous Zn(NO 3 ) 2 solution to form poly(amic acid) zinc salt.Table 1 shows the potassium and zinc ion loadings per unit area of the PI films under various alkali treatment and ion-exchange conditions.When the PI films were hydrolyzed in 5 M aqueous KOH solution at 50 • C for 5 min and ion-exchanged in aqueous Zn(NO 3 ) 2 solution at room temperature for 5 min, the zinc ion loadings increased from 465 nmol/cm 2 to 803 nmol/cm 2 with increasing the concentration of aqueous Zn(NO 3 ) 2 solution from 0.01 M to 0.40 M. When the PI film was hydrolyzed in 5 M aqueous KOH solution at 50 • C for 30 min and ion-exchanged in 0.40 M aqueous Zn(NO 3 ) 2 solution at room temperature for 60 min, the zinc ion loading was increased to 3960 nmol/cm 2 .Theoretically, zinc ions are stoichiometrically incorporated into the modified layer through an ion-exchange reaction with potassium ions in a 1 : 2 molar ratio.The metal ion loading is affected by many different factors such as the thickness of modified layer [26,28] and ion-exchange conditions (temperature, time, and concentration).Therefore, the total number of loaded zinc ions could be controlled by simply varying the initial modified layer thickness of PI film or by varying the concentration of zinc salt solution while maintaining the ionexchange time in very short time.Ikeda et al. [26,28] have reported that when the PI films were surface-treated in 5 M aqueous KOH solution at 50 • C for 5 min and ion-exchanged in 0.05 M CuSO 4 solution at room temperature for 5 min, potassium ions in modified layer could be completely replaced by copper ions.However, our results show that about 29% of potassium ions were remained in the PI films under the same alkali hydrolyzation and ion-exchange conditions.Even for the film that was ion exchanged in 0.40 M aqueous Zn(NO 3 ) 2 solution for 60 min, about 6% potassium ions were remained.These results indicate that the ion-exchange reaction between potassium and zinc ions may be a relatively slow process compared with that between potassium and other metal ions such as copper ions. 2 shows the color changes of PI film during the whole preparation process.The bare PI film exhibits light yellow color (Figure 2(a)).After alkali treatment and following ion exchange in Zn(NO 3 ) 2 solution, only slight difference in film color was observed compared with that of the bare PI film (Figures 2(b) and 2(c)).However, the film color changed dramatically upon treatment temperature and time during thermal-treating the zinc ion-doped PI film in air atmosphere.Typically, in the initial 3 h heating process, the film color gradually became darker with increasing thermal treatment temperature.When the temperature reached 350 • C, the PI film displayed dark brown color as seen in Figure 2(d).Further extending thermal treatment time at 350 • C, the film color gradually returned to light yellow again.We ascribe the PI film color evolution during thermal treatment to the following two possible reasons.First, ZnO particles generally show white appearance.But when the ZnO nanoparticles are in very small size, they may present different colors as it has been recognized that the color of inorganic nanoparticles depends much on the size, shape and the refractive index of the particles, and the mass concentration of particle material [30,31].The ZnO nanoparticles may have already been formed in the surface-modified layers of PI film when temperature reached 350 • C. Owing to the very small size, these newly formed ZnO nanoparticles make the film exhibit dark brown color.Further prolonging the thermal treatment time at 350 • C, the small ZnO nanoparticles gradually grow larger or aggregate together, leading to the fading of dark brown color of the PI film.Besides, the ZnO nanoparticles in the surface layers of PI film may catalyst the degradation of surrounding PAA or PI during thermal treatment.The metal oxidecatalyzed oxidative degradation of PI has been observed in many studies on PI nanocomposites containing metal oxides such as iron oxide, cobalt oxide, titania, and zinc oxide [20,29,32,33], which resulted in decrease of thermal stability of the hybrid films.Due to the nanosize effect, the newly formed ZnO nanoparticles have stronger catalytic activity.The decomposition products of PAA or PI such as the residual carbon on the film surface would make the film exhibit dark color.With extending thermal treatment time at high temperature, ZnO nanoparticles grow larger and their catalyst effect was greatly weakened.In the meantime, the residual decomposition products of PAA or PI on the film surface formed on the initial thermal treatment stages were gradually volatized.Therefore, the film color slowly returned to yellow again.Figure 3 shows SEM images of the zinc ion-doped PI films thermally treated at 350 • C for different hours.As can be observed, no particles could be detected on the PI films surface even after 3 h thermal treatment time at 350 • C. It is believed that the formed ZnO nanoparticles are too small to be detected, and most of them are embedded in the matrix of the surface-modified layer of PI film.As the treatment time at 350 • C was increased to 5 h, very small nanoparticles or clusters were observed on the PI film surface along with the fading of dark brown color to bright.Further extending thermal treatment time at 350 • C to 7 h, the small clusters grew to larger nanoparticles (about 15 nm) which uniformly aggregate on the PI film surface.The surface atomic concentration of the zinc ion-doped PI films before and after thermal treatment at 350 • C for 7 h extracted from the XPS measurement is listed in Table 2.The surface zinc concentration of the PI films before thermal treatment was only 2.2 at%.However, it increased to 13.2 at% after thermal treatment, while the carbon concentration decreased about 28%.Therefore, the zinc concentration increase was due to the relatively large amount of weight loss of carbon as a form of CO and CO 2 gas during long period of thermal treatment at 350 • C [33,34].The color evolvements and surface morphology changes of the PI composite films during thermal treatment indicate that the growth and aggregation of ZnO nanoparticles is a relatively slower process.Figure 4 depicts the XPS spectra for the Zn 2p and O 1s core levels of the zinc ion-doped PI film and that thermally treated at 350 • C for 7 h.For the zinc ion-doped PI film, the Zn 2p XPS spectrum consists of the main 2p 3/2 and 2p 1/2 spin-orbit components at 1022.3 and 1045.5 eV, respectively.The O 1s core level of the zinc ion-doped PI film can be curve-fitted with two peak components, having BEs at 533.05 eV for the ether species and 531.8 eV for the carboxyl species [35,36].Thermal treatment of the zinc ion-doped PI film in ambient atmosphere resulted in the formation of one additional oxygen species at the BE of 530.5 eV.This peak component was attributable to the Zn-O species in zinc oxide [1].The Zn 2p spectrum also consists of the main 2p 3/2 and 2p 1/2 spin-orbit components at 1022.2 and 1045.2 eV, respectively.These features are identical to those already reported for related ZnO systems [1].Thus, by comparing the Zn 2p and O 1s XPS core level spectra of the zinc ion-doped PI films before and after thermal treatment, it can be concluded that ZnO was formed on the PI films surface after thermal treatment at 350 • C for 7 h.The attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectrum of the film obtained by heat treatment of the zinc ion-exchanged sample in ambient air is very similar to that of the bare PI film (the results were not shown here), indicating complete reimidization of the modified layer due to the elimination of water molecules from carboxyl groups and amide bonds to form heterocyclic imide rings.

Formation of the PI/ZnO Composite Films. Figure
Figure 5 shows the TEM images of the PI/ZnO nanocomposite films with different initial zinc ion loadings in the same alkali-treated layers.As all the PI films in Figure 5 were hydrolyzed in 5 M aqueous KOH solution for 5 min at 50 • C, all the modified layer thicknesses of the PI films should be the same.However, after zinc ion incorporation and thermal treatment at 350 • C for 7 h, the surface composite layer of the films decreased from 700 nm to 90 nm in thickness with increasing the initial zinc ion loadings from 465 to 803 nmol/cm 2 .It is believed that the thickness decrease was due to the weight loss of carbon during thermal treatment in air, and the TGA-DSC results of the zinc ion-doped PI film can account for this.
Figure 6 shows the DSC and TGA curves of the zinc iondoped (3960 nmol/cm 2 ) PI film obtained in air atmosphere and argon atmosphere.In argon atmosphere, there was no pronounced thermal effect in DSC curve, and the weight loss of the film was very slowly progressed until 570 • C, rapidly progressed between 570 and 650 • C, then became plateau until 800 • C, showing weight loss of only 40%.On the other hand, in air atmosphere, two exothermic peaks appear in the temperature ranges of 330∼500 • C and 520∼ 700 • C for the zinc ion-doped PI film (Figure 6(a)-1).The exothermic peak at 330∼500 • C with a weight loss of about 10% is suggested to arise from the oxidative decomposition of carbon in the film catalyzed by the newly formed ZnO nanoparticles.During the formation of ZnO nanoparticles in the surface layer, the weight loss of film was accelerated due to the catalytic effect on the oxidative decomposition of carbon.As increasing the zinc ion loading, considerable amount of catalytic sites increased and resulted in more active degradation of surrounding PI film.During the time, the unstable ZnO nanoparticles with high surface free energy are readily to aggregate together.Thus, the surface composite layer of the final obtained PI/ZnO nanocomposite films becomes thinner with increasing the initial zinc ion loadings.As the ZnO nanoparticles grow larger or aggregate together Journal of Nanomaterials with surrounding nanoparticles, their catalytic effect will be greatly weakened.Therefore, the oxidative degradation of PI catalyzed by the newly formed ZnO nanoparticles only occurred in a limited temperature range as can be observed in Figures 6(a)-1 and 6(b)-1.When the temperature reached over 520 • C, significant weight loss was progressed up to 700 • C due to the oxidative degradation of PI matrix.
3.3.PL Characteristics of ZnO-Modified PI Films.Figure 7 shows the room temperature PL spectra for the bare PI film and the PI film with 3960 nmol/cm 2 initial zinc ion loading after thermal treatment at 350 • C for different hours.For the bare PI film, there was only one broad visible emission bond centered at 586 nm, while the PL spectra for the zinc ion-doped PI films after thermal treatment at 350 • C for 3 h and 5 h were much similar to that of the bare PI film.However, when the PI composite film was thermally treated at 350 • C for 7 h, an ultraviolet (UV) emission peak emerged at 367 nm.According to the literature, the UV emission peak at 367 nm was originated from the excitonic recombination corresponding to the near band edge (NBE) emission of wide band gap ZnO [2][3][4][5].This indicates that thermal treatment of the composite films should be carried out for at least 7 h at 350 • C in order to obtain effective PL properties.With increasing thermal treatment time at 350 • C, the visible emission peak gradually shifted to lower wavelength.It was known that ZnO nanoparticles usually exhibit two emission peaks when excited with UV light.One is exhibited at around 370 nm, and the other is near 525 nm (the so-called green luminescence band) whose mechanism seems much more complicated [3].The blue shifting of the visible emission peaks by long-time thermal treatment may be due to the synergy of ZnO particles and PI matrix.Besides, another emission peak at 672 nm appeared in the PL spectrum of the PI/ZnO composite film obtained at 350 • C/7 h.The emission at ∼672 nm is frequently observed in the ZnO materials which exhibits oxygen-rich nature, that is, the stoichiometric ratio Zn : O is (1 − x) : 1 (x > 0) [5,37,38].It is suggested that the emission peak at 672 nm observed in the PI/ZnO nanocomposite film might also be the consequence of excess oxygen as the film was thermally treated in air atmosphere.
The PL spectra of all the PI/ZnO composite films with 465∼803 nmol/cm 2 initial zinc ion loadings obtained at 350 • C/7 h possess a weak UV emission peak and a broad and strong visible emission peak centered at 374 nm and 608 nm, respectively.Figure 8 shows the PL spectra in wavelength of 330∼450 nm of the PI/ZnO nanocomposite films with 611, 803, and 3960 nmol/cm 2 initial zinc ion loadings.An increase in peak density with increasing initial zinc ion loadings was observed.It was also found that the UV emission of the PI/ZnO nanocomposite film has a slight blue shift.This blue shift is probably caused by the size change of ZnO nanoparticles [39] and the high concentration aggregates of ZnO nanoparticles in PI film surface layers [21] as shown in Figure 5(f).zinc ion loading.During elevating temperature in TGA-DSC measurement, these small ZnO nanoparticles may accelerate the oxidative decomposition of surrounding carbon.However, the weight loss resulted from this exothermic effect is negligible as can be seen in Figure 9(b).

Conclusions
Different content of ZnO nanoparticles embedded PI/ZnO films can be prepared by KOH hydrolyzation, ion exchange in aqueous Zn(NO 3 ) 2 solution, and thermal treatment of the zinc ion-doped PI films in air atmosphere.The zinc ion loadings in the surface-treated layer of PI films after same alkali treatment were enhanced with increasing the concentration of aqueous Zn(NO 3 ) 2 solution while maintaining ion-exchange time at 5 min.However, satisfying ion exchange can be performed by extending alkali treatment and ion-exchange time.ZnO nanoparticles of about 10∼ 15 nm were homogenously dispersed in the PI film surface layers.The formation and growth of ZnO nanoparticles during thermal treatment was a relative slower process, which was accompanied by the oxidative decomposition of surrounding PI matrix in the modified layer catalyzed by the ZnO nanoparticles.The room temperature PL spectrum of the PI/ZnO composite films showed only one broad visible band when they were thermally treated at 350 • C for less than 7 h, while showed a weak UV emission band and a broad and strong visible band when the film was thermal-treated at 350 • C for more than 7 h.The intensity of the UV emission peak of the PI/ZnO composite films obtained at 350 • C/7 h increased with enhancing the concentration of ZnO.The composite film kept the excellent thermal properties of the host PI film.

Figure 1 :
Figure 1: Schematic illustration of the preparation process for PI/ZnO films: (i) surface modification of PI films in aqueous KOH solution, (ii) incorporation of zinc ions via an ion-exchange reaction, and (iii) thermal-treatment-induced reimidization and formation of ZnO.

Figure 4 :
Figure 4: XPS spectra for the Zn 2p and O 1s core levels of (a, b) the Zn 2+ -doped film and (c, d) the PI composite film after thermal treatment at 350 • C for 7 h.

Figure 9 :
Figure 9: (a) TGA and (b) DSC curves of the bare PI film and the PI/ZnO composite films obtained at 350 • C/7 h.

Table 1 :
Potassium and zinc ions loadings in the PI films after KOH hydrolysis and ion exchange in aqueous Zn(NO 3 ) 2 solution.

Table 2 :
Surface atomic concentration of the Zn 2+ -doped PI precursor film and the PI/ZnO film obtained at 350 • C/7 h with initial zinc ion loadings of 803 nmol/cm 2 .