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polyethyleneimine layer-by-layer assembled film for enhanced hydrogen barrier property
Composites Part B 92 (2016) 252e258
Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Preparation of graphene oxide/polyethyleneimine layer-by-layer assembled film for enhanced hydrogen barrier property Lili Zhao a, d, Hongyu Zhang a, Nam Hoon Kim b, David Hui c, Joong Hee Lee b, *, Qi Li e, Haixiang Sun a, Peng Li a, * a
State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China University of Petroleum, Huadong, Qingdao 266580, China Advanced Materials Research Institute for BIN Convergence Technology, Department of BIN Convergence Technology, Chonbuk National University, Jeonju, Jeonbuk 561-756, South Korea c Department of Mechanical Engineering, University of New Orleans, New Orleans, LA 70148, USA d State Key Laboratory of Bioactive Seaweed Substances, Qingdao Bright Moon Seaweed Group Co Ltd, Qingdao 266400, China e Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 November 2015 Received in revised form 19 January 2016 Accepted 16 February 2016 Available online 24 February 2016
A super hydrogen barrier film was prepared via layer-by-layer self-assembly of graphene oxide (GO) and polyethyleneimine (PEI), and the effects of pH of the PEI solution on the performance of LBL films were studied in detail. Results show that the pH value of PEI solution has a significant influence not only on the adsorbed amount of self-assembly materials but also the stacking morphology of GO sheets. When the pH values of GO suspension and PEI solution are identical, the self-assembled film shows a superior hydrogen gas barrier performance under pH of 3.5. When the pH value of GO suspension was fixed and that of PEI solution was varied, the adsorbed amount of the film was increased and reached a maximum value when the pH of PEI solution was 12, and the film prepared under this condition had a 12.7% increase on thickness and a 55.5% decrease on its hydrogen transmission rate (H2TR). © 2016 Elsevier Ltd. All rights reserved.
Keywords: A. Layered structures B. Microstructure D. Surface analysis E. Assembly
1. Introduction Among the burgeoning energy sources, hydrogen energy has become one of the most practical replacements for fossil fuels due to it being environment-friendly and possessing high efficiency characteristics as a secondary energy source. However, the technical bottleneck for large-scale application of hydrogen is in the question of how to safely, economically and efficiently store hydrogen gas which has not been solved [1]. Hydrogen has a high diffusion rate in the air and is also flammable and combustible, which makes it a hazardous gas. In order to enhance the pressure tolerance of the containers, materials with improved hydrogen gas barrier property are still necessary. In 2008, Bunch et al. [2] reported that even the smallest helium molecules cannot pass through a single-layered graphene, proving that graphene has an excellent gas barrier property. The graphene oxide (GO) is obtained through the oxidation and exfoliation process of graphite, and the oxygen-containing groups [3] makes the GO sheet easily exfoliated
* Corresponding authors. E-mail addresses: [email protected] (J.H. Lee), [email protected] (P. Li). http://dx.doi.org/10.1016/j.compositesb.2016.02.037 1359-8368/© 2016 Elsevier Ltd. All rights reserved.
and dispersed in water. The obtained GO sheet can self-assemble into a 2D thin film on a variety of substrates with nanoscale thickness [4]. The GO films can be prepared via numerous methods such as LangmuireBlodgett assembly electrophoresis, immersion coating, spin coating, spray coating, vacuum filtration and chemical vapor deposition [5e11]. The GO thin films have been widely used in the fields of gas separation, gas barrier, water treatment [12e16]. Except for the mentioned methods, layer-by-layer self-assembly (LBL) through electrostatic interaction has been widely reported since it is a facile route and the structure of the thin film can be precisely controlled on a nanoscale level. The GO composite material prepared by LBL method has more excellent gas barrier performance than that of the conventional filler composite due to its high exfoliation degree and orientation. The results of our research group also indicate that LBL films prepared by using polyethyleneimine (PEI) and GO on the polyethylene terephthalate (PET) film surface show an excellent hydrogen gas barrier performance [17]. Yu et al. [18] and Yang et al. [13] also reported the excellent oxygen gas barrier property of GO/PEI LBL composite film prepared via electrostatic interaction.
L. Zhao et al. / Composites Part B 92 (2016) 252e258
of PEI solution on the adsorbed amount and assembly morphology of composite films.
Table 1 The charge density of 0.5% PEI solution with different pH values. pH
2
3.5
5
7
8
253
9.5
12 13.5
Charge density (mmol g1) 12.02 11.92 8.77 5.96 2.89 0.058 0
0
2. Experiment 2.1. Materials
It's known that the charge density of the polyelectrolyte affects the adsorption efficiency of polyelectrolyte on the self-assembled thin film, while the ionization degree of the polyelectrolyte is greatly affected by the pH of its solution. Some literature reported that the thickness [19] and adsorption morphology [20,21] of a self-assembled film could be changed by adjusting the solution's pH value. Hasan et al. [22] described the microstructure of GO thin film prepared through electrostatic interaction at different pH values. Chen et al. [23] studied changes of nanostructure and oxygen barrier properties of self-assembled GO/PEI film prepared with GO dispersion of different pH values. All results indicate that the variation of the pH has valuable impact on barrier performance of GO/PEI LBL films. However, in the literature [23], only the self-assembled morphology of GO sheets after changing the pH of GO suspension was studied, the effects of the pH of PEI solution on the stretched configuration of adsorbed GO sheet during the alternating immersion process were ignored. Though GO plays a more important role in gas barrier performance of GO/ PEI film, pH of PEI solution not only affects its adsorption but also may affect the GO morphology on the surface of assembled film when the film is immersed in PEI solution with a different pH value. Therefore, it is necessary to investigate the effects of pH value of PEI solution on the hydrogen gas barrier performance of GO/PEI film. In the present study, the PEI solution and GO suspension with the same pH were used for self-assembly at first, which can exclude the effects of pH of PEI solution on GO. Subsequently, under the condition of fixed pH value of GO suspension, the pH value of PEI solution was varied for the self-assembly process. The H2TR of (GO/PEI)10 films prepared at different pH was measured, and the growth process and morphology variation of these films were observed as well. The comparison between these two experimental results was used to analyze the effects of pH values
Table 2 Composition of the chemical groups of GO. Material
GO
GO (oxidation degree > 95%) in powder form was purchased from the Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, China. Pure PEI (MW ¼ 10,000 g mol1) was purchased from Aladdin Co., Ltd. (Shanghai, China). The dipping concentration of polyelectrolytes was 0.5 wt% for PEI solution, 0.05 wt% for GO suspension. The pH of the GO suspension and PEI solution was controlled by adding hydrochloric acid and NaOH solution, and the charge density of PEI solutions was measured by colloid titration method [24]. Pure industrial polyester film, PET (type 6020; thickness ¼ 160 mm), was purchased from Yuxiang Electronic Material Co., Ltd. (Shanghai, China). The PET film was pretreated with the same method as we had reported early [17]. 2.2. Multilayer membrane preparation LBL self-assembly films were prepared on PET substrate and Si wafers respectively with the same procedure reported earlier [17], and Si wafers were pretreated with a piranha solution (70% H2SO4 and 30% H2O2) for 1 h under 90 C. The pretreated substrate was dipped into a PEI solution for 20 min. After rinsing with pure water and drying on a spin coater, the PET film was dipped into a negatively charged solution of GO for 20 min following by rinsing and drying again. One deposition cycle involved two different layers, that is, one bilayer. This process was repeated to prepare polyelectrolytes films with different numbers of bilayers [17]. 2.3. Characterization of GO X-ray photoelectron spectroscopy (Thermo escalab 250Xi, Thermo electron, American) was used to characterize the chemical structures of GO. The thickness, size and number of layers of GO sheet were analyzed by atomic force microscopy (Bruker Multimode 8, Germany). The zeta-potential of the GO suspensions was measured by dynamic light scattering (Zetasizer Nano ZS90, Malvern, UK). 2.4. Test and characterization of films
Chemical group composition (%) CeC/C]C
CeOH
C]O
CeO (epoxy)
37.4
10.2
20.1
32.3
The film thickness of the self-assembled membranes was measured using an Alpha-SE Ellipsometer (EC-400 and M-2000V, J.
Fig. 1. AFM image of GO.
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with different pH values was measured with zeta potential instrument, as shown in Fig. 2. It can be seen that GO has a significant ionization within the pH range of 6e10. When pH is higher than 10 or lower than 5, the ionized state of GO becomes very weak.
3.2. GO/PEI LbL films prepared under same pH value
Fig. 2. Zeta potential of GO suspension at different pH values.
A. Woollam Co. Inc., Lincoln, NE, USA). The surface morphologies and the surface roughness were imaged with a multimode scanning probe microscope (MultiMode® 8, Bruker Corp., Billerica, MA, USA), operated in tapping mode. The hydrogen gas barrier performances were measured with a pressure permeation instrument (Labthink Instruments Co., Ltd., Shandong, China).
On the basis of maintaining the same pH values between PEI solution and GO suspension, the pH values of solution and suspension were simultaneously changed. The hydrogen transmission rate (H2TR) of (GO/PEI)10 LBL film was measured, as shown in Fig. 3. Within the range of 3 < pH < 5.5, the H2TR is relatively small, and it reached the minimum value when the pH was 3.5. Fig. 4 demonstrates the thickness of GO/PEI LBL films increases at a near-linear behavior with increasing of the number of bilayers, indicating the adsorbed GO and PEI amount was near identical in each layer of film. After linear fitting, it was found that the growth rate of film at pH ¼ 3.5 is 3.45 nm per bilayer (dipping in both PEI solution and GO dispersion once) which is bigger than that of film prepared under the other two pH values. Comparison between the abovementioned H2TR results and the thickness of GO/PEI films (Fig. 4) at different pH values indicates that the change of film thickness shows an identical trend with the change of H2TR. However, the
3. Results and discussion 3.1. Characterization of PEI and GO sheets The branched PEI contains a large amount of primary and secondary amine groups, with pKa values of 4.5 and 6.7, respectively [25]. The measured charge density of PEI at different pH values is shown in Table 1. It can be seen from Table 1 that PEI has a significant ionization within the pH range of 2e8. The ionization degree decreases with an increase of the pH value. When pH is higher than 9.5, the ionized state of PEI is too weak to be measured. XPS results show the contents of oxygen-containing functional group as follows: CeOH 10.2%, COeC 32.3%, C]O 20.1% (see Table 2). Fig. 1 illustrates an AFM image and height profile of GO. As seen in the figure, the size and thickness of GO sheet are 2000 nm and 1 nm, respectively. The electrical potential of GO dispersion
Fig. 3. Hydrogen transmission rate of (GO/PEI)10 film at different pH values.
Fig. 4. (a) Thickness of (GO/PEI)10 film and (b) thicknesses of the film with increasing the number of layers at different pH values.
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255
Fig. 5. AFM images and height-profiles of (GO/PEI)10 film prepared at different pH values: (a) and (e), pH ¼ 2; (b) and (f), pH ¼ 3.5; (c) and (g), pH ¼ 5.5; (d) and (h), pH ¼ 7.
variation of thickness is slightly greater than the variation of H2TR. For example, the H2TR of thin film at pH ¼ 2 and pH ¼ 7 is both about 1150 cm3/m2 24 h 0.1 MPa while the film thickness at pH ¼ 2 is 20% thicker than that of pH ¼ 7. Therefore, it can be concluded that H2TR is not only related to the thickness of composite film, but also the stacking morphology of the self-assembled film. The surface morphologies of the (GO/PEI)10 films at different pH values were characterized by AFM, as shown in Fig. 5. The GO sheets adsorbed on the film have uniform distribution and their curves fluctuation in the section height map are relatively small under pH values of 5.5 and 3.5; more adsorption defects of GO sheets were observed on the film and their curves fluctuation in the section height map are relatively big under pH values of 2 and 7, which indicates the adsorbed GO sheets wrinkle significantly under pH values of 2 and 7. This result corresponds to the measured thickness result which clearly shows that a uniform and dense LBL film has good hydrogen gas barrier performance. It can thus be seen that the effects of pH on self-assembled films are composed of two
Fig. 6. Hydrogen transmission rate of (GO/PEI)10 film at different pH value of PEI solution.
Fig. 7. (a) Thickness of (GO/PEI)10 film and (b) thicknesses of the film with increasing the number of layers at different pH values of PEI solution.
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Fig. 8. AFM phase images of (GO/PEI)10 film at different pH value of PEI solution: (a) pH 7 and (b) pH 12.
aspects based on the adsorption amount and the morphology of the film. The reasons may be as follows: the driving forces of selfassembly process are electrostatic force and hydrogen bond. Ionized PEI and GO will be adsorbed each other by electrostatic force and non-ionized PEI and GO will be assembled by hydrogen bond between amino groups of PEI and carboxyl group or hydroxyl groups of GO. Meanwhile, the variation in charge density results in the difference in the stretched state of PEI and GO. When the pH value is less than 3.5, the charge density of GO is
small, but the ionization degree of PEI solution is too high. Therefore, the electrostatic force between PEI and GO is weak and the repulsive force between PEI molecular is strong. Therefore, the formed PEI layer will be sparse with some vacant positions and the assembled film will be not very uniform and dense. When the pH value is greater than 3.5, the ionization degree of GO is so high that the GO sheets easily repulse each other, resulting in folded-shape stacking of GO sheets that can't uniformly tile to form good gas barrier layer [23]. When the pH value equals to 3.5, the GO ionization degree is moderate with adequately stretched
Fig. 9. AFM images and height-profiles of (GO/PEI)10 film prepared at different pH values of PEI solution: (a) and (d), pH ¼ 2; (b) and (e), pH ¼ 7; (c) and (f), pH ¼ 12.
L. Zhao et al. / Composites Part B 92 (2016) 252e258
257
Fig. 10. The self-assembly mechanism of GO suspension with pH ¼ 3.5 and PEI solution with pH ¼ 12.
sheets and PEI has a proper ionization degree, resulting in good mutual adsorption. 3.3. GO/PEI LBL film prepared under different pH of PEI and GO When the pH of GO dispersion was fixed and the pH of PEI solution was changed, during the alternating immersion adsorption process, ionization degree of the adsorbed GO sheets on the surface of the thin film will change due to the effect of the PEI's pH. It results in variation of the adsorbed amount and micro-structure of GO, hydrogen barrier performances of the film can be effectively altered. Fig. 6 shows the change of H2RT of (GO/PEI)10 films with the pH value of PEI solution. When the pH value increased, the H2TR of film decreased. When the pH value of PEI solution was 12, the H2TR of the film reached its minimum value, 145 cm3/m2 24 h 0.1 MPa. When the pH of PEI solution was higher than 12, the H2TR of film increased. This can be mainly attributed to the obviously increased NaOH content in the PEI solution inhibited the ionization of GO, and then reduced the adsorption force in self-assembly process. Fig. 7 shows thickness of the film increased linearly with the number of film bilayers. When the pH value of PEI solution was 12, the growth rate of film's thickness reached its maximum, 4.24 nm per bilayer. The comparison between above H2TR results and thicknesses of (GO/PEI)10 films in Fig. 7 shows that the variation trend of thickness was identical in nature with the variation of H2TR. The AFM phase images of films in Fig. 8 showed the GO sheets uniformly stretched on the film and the whole area showed a single phase, without severe trenches or holes when the pH values of PEI were 7 and 12. Comparing the AFM height-profile of films prepared at three pH values (Fig. 9), it can be found that the height fluctuation of the composite film gradually increased with the increasing of pH value. 3.4. Comparisons between the results above Comparing the results shown in Figs. 3, 4 and Figs. 6, 7, it indicates that the pH value of PEI solution clearly affects the selfassembly adsorption of GO sheets. For example, when the pH values of GO suspension was fixed and pH of PEI solution was 12, the thickness of (GO/PEI)10 film was 12.7% more and its H2TR was 55.5% lower than those of film prepared at the pH values of PEI solution and GO suspension which were both at 3.5. Possible explanations for this phenomenon are analyzed as follow: when the pH of GO suspension was 3.5, GO was in a weakly ionized state (Fig. 7), while the ionization degree of PEI was related
to the pH of solution (Table 1): (1) when the pH value of PEI is lower than 8, the charge density of PEI was too large that obvious repulsive forces were formed between molecular chains, which made it easy to form holes and defects during self-assembly process; (2) when the pH value is higher than 9.5, the PEI was in a weakly ionized state while the GO was also in a weakly ionized state. In this case, the charge density of PEI and GO was too small to be favorable for the self-assembly process. However, during the alternating immersion process, the different pH values between GO and PEI alter the charge density of the other component, thus promoting the self-assembly process. Fig. 10 shows the example of self-assembly process with GO suspension pH of 3.5 and PEI solution pH of 12. Firstly, the film adsorbed PEI; then the adsorbed PEI molecular chains presented a coiled shape due to the low adsorbed amount as the ionization degree of PEI was low. However, when they occurred at the environmental pH of 3.5 (the pH of GO suspension), the ionization degree of PEI was strengthened, which enhanced its ability to absorb GO sheets; meanwhile, the increased charge density can stretch the PEI molecular chain and the smaller adsorbed amount of PEI won't result in strong repulsion between molecular chains, the adsorption of GO can therefore be enhanced with less vacant position. In addition, since the ionization degree of GO was relatively low, the GO sheet can be added up to a moderate amount while still possessing a good stretched state. When the GO sheets were immersed into the PEI solution with pH value of 12, the ionization degree of GO sheets was still small (Fig. 2), as well as PEI. In this condition, hydrogen bond will accelerate the assembling of both as well as the adsorption of PEI. 4. Conclusions In the present paper, the effects of pH value of PEI solution on the self-assembly of composite film in GO/PEI system were investigated. The following conditions were studied: (1) the pH values of PEI solution and GO were held at the same value, (2) The PEI's pH was varied and the pH of GO was fixed. The H2TR of film was measured and the growth process and microstructure of film were observed. When the pH value of PEI solution was the same as the GO, the adsorbed amount reached its maximum at pH ¼ 3.5, with a film thickness growth rate of 3.45 nm for each bilayer and had the most uniform film structure. As the pH value of PEI solution changed, the adsorbed amount of GO/PEI composite film increased further. When the pH ¼ 12, the film thickness growth rate reached its maximum (4.24 nm for each bilayer) and the GO sheets maintained its tiled stretching and uniform stacking. Comparison of the
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(GO/PEI)10 films prepared under two different conditions indicated that the thickness of self-assembled film prepared at the PEI's pH of 12 was 12.7% higher than that of film prepared when both values were held at 3.5; meanwhile, the H2TR reduced by 55.5%, indicating obvious improvement on the hydrogen barrier performance. It can be seen that in the LBL self-assembly process of GO/PEI system, the thickness and microstructure of composite films can be further regulated by adjusting the pH values of PEI during the preparation process of hydrogen barrier films, which can efficiently improve the hydrogen barrier properties of composite films. Acknowledgments The study was supported by the National Natural Science Foundation of China (Grant Nos. 51203186 and 21406268), the Converging Research Center Program funded by the Korean Ministry of Education (2014M3C1A8048834), and the Fundamental Research Funds for the Central Universities.
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[9] Yamaguchi H, Eda G, Mattevi C, Kim HK, Chhowalla M. Highly uniform 300mm wafer-scale deposition of single and multilayered chemically derived graphene thin films. ACS Nano 2010;4(1):524e8. [10] Putz KW, Compton OC, Segar C, An Z, Nguyen SBT, Brinson LC. Evolution of order during vacuum-assisted self-assembly of graphene oxide paper and associated polymer nanocomposites. ACS Nano 2011;5(8):6601e9. [11] Kim KS, Zhao Y, Jang H, Lee SY, Kim JM, Kim KS, et al. Large-scale pattern growth of graphene films for stretchable transparent electrodes. Nature 2009;457:706e10. [12] Shen J, Liu GP, Huang K, Jin WQ, Lee KR, Xu NP. Membranes with fast and selective gas-transport channels of laminar graphene oxide for efficient CO2 capture. Angew Chem Int Ed 2015;54(2):578e82. [13] Yang YH, Bolling L, Priolo MA, Grunlan JC. Graphene: super gas barrier and selectivity of graphene oxideepolymer multilayer thin films. Adv Mater 2013;25(4):503e8. [14] Vickery JL, Patil AJ, Mann S. Fabrication of grapheneepolymer nanocomposites with higher-order three-dimensional architectures. Adv Mater 2009;21(21):2180e4. [15] Ramanathan T, Abdala AA, Stankovich S, Dikin DA, Herrera-Alonso M, Piner RD, et al. Functionalized graphene sheets for polymer nanocomposites. Nat Nanotechnol 2008;3:327e31. [16] Goh K, Setiawan L, Wei L, Rongmel S, Fane AG, Wang B, et al. Graphene oxide as effective selective barriers on a hollow fiber membrane for water treatment process. J Membr Sci 2015;474:244e53. [17] Zhao LL, Sun HX, Kim NH, Lee JH, Kong Y, Li P. Hydrogen gas barrier property of polyelectrolyte/GO layer-by-layer films. J Appl Polym Sci 2015;132(20): 41973. [18] Yu L, Lim YS, Han JH, Kim K, Kin JY, Choi SY, et al. A graphene oxide oxygen barrier film deposited via a self-assembly coating method. Synth Met 2012;162(7e8):710. [19] Shiratori SS, Rubner MF. pH-dependent thickness behavior of sequentially adsorbed layers of weak polyelectrolytes. Macromolecules 2000;33(11): 4213e9. [20] Mendelsohn JD, Barrett CJ, Chan VV, Pal AJ, Mayes AM, Rubner MF. Fabrication of microporous thin films from polyelectrolyte multilayers. Langmuir 2000;16(11):5017e23. [21] Mendelsohn JD, Yang SY, Hiller JA, Hochbaum AI, Rubner MF. Rational design of cytophilic and cytophobic polyelectrolyte multilayer thin films. Biomacromolecules 2003;4(1):96e106. [22] Hasan SA, Rigueur J, Harl RR, Krejci AJ, Gonzalo I, Rogers BR, et al. Transferable graphene oxide films with tunable microstructures. ACS Nano 2010;4(12): 7367e72. [23] Chen JT, Fu YJ, An QF, Lo SC, Huang SH, Hung WS, et al. Tuning nanostructure of graphene oxide/polyelectrolyte LbL assemblies by controlling pH of GO suspension to fabricate transparent and super gas barrier films. Nanoscale 2013;5(19):9081e8. [24] Chen FS. Colloid titration method to determine the charge of polymer. China Pulp Pap 1999;2:30e4. [25] Willner I, Eichen Y, Frank AJ, Fox MA. Photoinduced electron-transfer processes using organized redox-functionalized bipyridiniumepolyethyleneimineeTiO2 colloids and particulate assemblies. J Phys Chem 1993;97(28):7264e71.
Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Preparation of graphene oxide/polyethyleneimine layer-by-layer assembled film for enhanced hydrogen barrier property Lili Zhao a, d, Hongyu Zhang a, Nam Hoon Kim b, David Hui c, Joong Hee Lee b, *, Qi Li e, Haixiang Sun a, Peng Li a, * a
State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China University of Petroleum, Huadong, Qingdao 266580, China Advanced Materials Research Institute for BIN Convergence Technology, Department of BIN Convergence Technology, Chonbuk National University, Jeonju, Jeonbuk 561-756, South Korea c Department of Mechanical Engineering, University of New Orleans, New Orleans, LA 70148, USA d State Key Laboratory of Bioactive Seaweed Substances, Qingdao Bright Moon Seaweed Group Co Ltd, Qingdao 266400, China e Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 November 2015 Received in revised form 19 January 2016 Accepted 16 February 2016 Available online 24 February 2016
A super hydrogen barrier film was prepared via layer-by-layer self-assembly of graphene oxide (GO) and polyethyleneimine (PEI), and the effects of pH of the PEI solution on the performance of LBL films were studied in detail. Results show that the pH value of PEI solution has a significant influence not only on the adsorbed amount of self-assembly materials but also the stacking morphology of GO sheets. When the pH values of GO suspension and PEI solution are identical, the self-assembled film shows a superior hydrogen gas barrier performance under pH of 3.5. When the pH value of GO suspension was fixed and that of PEI solution was varied, the adsorbed amount of the film was increased and reached a maximum value when the pH of PEI solution was 12, and the film prepared under this condition had a 12.7% increase on thickness and a 55.5% decrease on its hydrogen transmission rate (H2TR). © 2016 Elsevier Ltd. All rights reserved.
Keywords: A. Layered structures B. Microstructure D. Surface analysis E. Assembly
1. Introduction Among the burgeoning energy sources, hydrogen energy has become one of the most practical replacements for fossil fuels due to it being environment-friendly and possessing high efficiency characteristics as a secondary energy source. However, the technical bottleneck for large-scale application of hydrogen is in the question of how to safely, economically and efficiently store hydrogen gas which has not been solved [1]. Hydrogen has a high diffusion rate in the air and is also flammable and combustible, which makes it a hazardous gas. In order to enhance the pressure tolerance of the containers, materials with improved hydrogen gas barrier property are still necessary. In 2008, Bunch et al. [2] reported that even the smallest helium molecules cannot pass through a single-layered graphene, proving that graphene has an excellent gas barrier property. The graphene oxide (GO) is obtained through the oxidation and exfoliation process of graphite, and the oxygen-containing groups [3] makes the GO sheet easily exfoliated
* Corresponding authors. E-mail addresses: [email protected] (J.H. Lee), [email protected] (P. Li). http://dx.doi.org/10.1016/j.compositesb.2016.02.037 1359-8368/© 2016 Elsevier Ltd. All rights reserved.
and dispersed in water. The obtained GO sheet can self-assemble into a 2D thin film on a variety of substrates with nanoscale thickness [4]. The GO films can be prepared via numerous methods such as LangmuireBlodgett assembly electrophoresis, immersion coating, spin coating, spray coating, vacuum filtration and chemical vapor deposition [5e11]. The GO thin films have been widely used in the fields of gas separation, gas barrier, water treatment [12e16]. Except for the mentioned methods, layer-by-layer self-assembly (LBL) through electrostatic interaction has been widely reported since it is a facile route and the structure of the thin film can be precisely controlled on a nanoscale level. The GO composite material prepared by LBL method has more excellent gas barrier performance than that of the conventional filler composite due to its high exfoliation degree and orientation. The results of our research group also indicate that LBL films prepared by using polyethyleneimine (PEI) and GO on the polyethylene terephthalate (PET) film surface show an excellent hydrogen gas barrier performance [17]. Yu et al. [18] and Yang et al. [13] also reported the excellent oxygen gas barrier property of GO/PEI LBL composite film prepared via electrostatic interaction.
L. Zhao et al. / Composites Part B 92 (2016) 252e258
of PEI solution on the adsorbed amount and assembly morphology of composite films.
Table 1 The charge density of 0.5% PEI solution with different pH values. pH
2
3.5
5
7
8
253
9.5
12 13.5
Charge density (mmol g1) 12.02 11.92 8.77 5.96 2.89 0.058 0
0
2. Experiment 2.1. Materials
It's known that the charge density of the polyelectrolyte affects the adsorption efficiency of polyelectrolyte on the self-assembled thin film, while the ionization degree of the polyelectrolyte is greatly affected by the pH of its solution. Some literature reported that the thickness [19] and adsorption morphology [20,21] of a self-assembled film could be changed by adjusting the solution's pH value. Hasan et al. [22] described the microstructure of GO thin film prepared through electrostatic interaction at different pH values. Chen et al. [23] studied changes of nanostructure and oxygen barrier properties of self-assembled GO/PEI film prepared with GO dispersion of different pH values. All results indicate that the variation of the pH has valuable impact on barrier performance of GO/PEI LBL films. However, in the literature [23], only the self-assembled morphology of GO sheets after changing the pH of GO suspension was studied, the effects of the pH of PEI solution on the stretched configuration of adsorbed GO sheet during the alternating immersion process were ignored. Though GO plays a more important role in gas barrier performance of GO/ PEI film, pH of PEI solution not only affects its adsorption but also may affect the GO morphology on the surface of assembled film when the film is immersed in PEI solution with a different pH value. Therefore, it is necessary to investigate the effects of pH value of PEI solution on the hydrogen gas barrier performance of GO/PEI film. In the present study, the PEI solution and GO suspension with the same pH were used for self-assembly at first, which can exclude the effects of pH of PEI solution on GO. Subsequently, under the condition of fixed pH value of GO suspension, the pH value of PEI solution was varied for the self-assembly process. The H2TR of (GO/PEI)10 films prepared at different pH was measured, and the growth process and morphology variation of these films were observed as well. The comparison between these two experimental results was used to analyze the effects of pH values
Table 2 Composition of the chemical groups of GO. Material
GO
GO (oxidation degree > 95%) in powder form was purchased from the Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, China. Pure PEI (MW ¼ 10,000 g mol1) was purchased from Aladdin Co., Ltd. (Shanghai, China). The dipping concentration of polyelectrolytes was 0.5 wt% for PEI solution, 0.05 wt% for GO suspension. The pH of the GO suspension and PEI solution was controlled by adding hydrochloric acid and NaOH solution, and the charge density of PEI solutions was measured by colloid titration method [24]. Pure industrial polyester film, PET (type 6020; thickness ¼ 160 mm), was purchased from Yuxiang Electronic Material Co., Ltd. (Shanghai, China). The PET film was pretreated with the same method as we had reported early [17]. 2.2. Multilayer membrane preparation LBL self-assembly films were prepared on PET substrate and Si wafers respectively with the same procedure reported earlier [17], and Si wafers were pretreated with a piranha solution (70% H2SO4 and 30% H2O2) for 1 h under 90 C. The pretreated substrate was dipped into a PEI solution for 20 min. After rinsing with pure water and drying on a spin coater, the PET film was dipped into a negatively charged solution of GO for 20 min following by rinsing and drying again. One deposition cycle involved two different layers, that is, one bilayer. This process was repeated to prepare polyelectrolytes films with different numbers of bilayers [17]. 2.3. Characterization of GO X-ray photoelectron spectroscopy (Thermo escalab 250Xi, Thermo electron, American) was used to characterize the chemical structures of GO. The thickness, size and number of layers of GO sheet were analyzed by atomic force microscopy (Bruker Multimode 8, Germany). The zeta-potential of the GO suspensions was measured by dynamic light scattering (Zetasizer Nano ZS90, Malvern, UK). 2.4. Test and characterization of films
Chemical group composition (%) CeC/C]C
CeOH
C]O
CeO (epoxy)
37.4
10.2
20.1
32.3
The film thickness of the self-assembled membranes was measured using an Alpha-SE Ellipsometer (EC-400 and M-2000V, J.
Fig. 1. AFM image of GO.
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with different pH values was measured with zeta potential instrument, as shown in Fig. 2. It can be seen that GO has a significant ionization within the pH range of 6e10. When pH is higher than 10 or lower than 5, the ionized state of GO becomes very weak.
3.2. GO/PEI LbL films prepared under same pH value
Fig. 2. Zeta potential of GO suspension at different pH values.
A. Woollam Co. Inc., Lincoln, NE, USA). The surface morphologies and the surface roughness were imaged with a multimode scanning probe microscope (MultiMode® 8, Bruker Corp., Billerica, MA, USA), operated in tapping mode. The hydrogen gas barrier performances were measured with a pressure permeation instrument (Labthink Instruments Co., Ltd., Shandong, China).
On the basis of maintaining the same pH values between PEI solution and GO suspension, the pH values of solution and suspension were simultaneously changed. The hydrogen transmission rate (H2TR) of (GO/PEI)10 LBL film was measured, as shown in Fig. 3. Within the range of 3 < pH < 5.5, the H2TR is relatively small, and it reached the minimum value when the pH was 3.5. Fig. 4 demonstrates the thickness of GO/PEI LBL films increases at a near-linear behavior with increasing of the number of bilayers, indicating the adsorbed GO and PEI amount was near identical in each layer of film. After linear fitting, it was found that the growth rate of film at pH ¼ 3.5 is 3.45 nm per bilayer (dipping in both PEI solution and GO dispersion once) which is bigger than that of film prepared under the other two pH values. Comparison between the abovementioned H2TR results and the thickness of GO/PEI films (Fig. 4) at different pH values indicates that the change of film thickness shows an identical trend with the change of H2TR. However, the
3. Results and discussion 3.1. Characterization of PEI and GO sheets The branched PEI contains a large amount of primary and secondary amine groups, with pKa values of 4.5 and 6.7, respectively [25]. The measured charge density of PEI at different pH values is shown in Table 1. It can be seen from Table 1 that PEI has a significant ionization within the pH range of 2e8. The ionization degree decreases with an increase of the pH value. When pH is higher than 9.5, the ionized state of PEI is too weak to be measured. XPS results show the contents of oxygen-containing functional group as follows: CeOH 10.2%, COeC 32.3%, C]O 20.1% (see Table 2). Fig. 1 illustrates an AFM image and height profile of GO. As seen in the figure, the size and thickness of GO sheet are 2000 nm and 1 nm, respectively. The electrical potential of GO dispersion
Fig. 3. Hydrogen transmission rate of (GO/PEI)10 film at different pH values.
Fig. 4. (a) Thickness of (GO/PEI)10 film and (b) thicknesses of the film with increasing the number of layers at different pH values.
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Fig. 5. AFM images and height-profiles of (GO/PEI)10 film prepared at different pH values: (a) and (e), pH ¼ 2; (b) and (f), pH ¼ 3.5; (c) and (g), pH ¼ 5.5; (d) and (h), pH ¼ 7.
variation of thickness is slightly greater than the variation of H2TR. For example, the H2TR of thin film at pH ¼ 2 and pH ¼ 7 is both about 1150 cm3/m2 24 h 0.1 MPa while the film thickness at pH ¼ 2 is 20% thicker than that of pH ¼ 7. Therefore, it can be concluded that H2TR is not only related to the thickness of composite film, but also the stacking morphology of the self-assembled film. The surface morphologies of the (GO/PEI)10 films at different pH values were characterized by AFM, as shown in Fig. 5. The GO sheets adsorbed on the film have uniform distribution and their curves fluctuation in the section height map are relatively small under pH values of 5.5 and 3.5; more adsorption defects of GO sheets were observed on the film and their curves fluctuation in the section height map are relatively big under pH values of 2 and 7, which indicates the adsorbed GO sheets wrinkle significantly under pH values of 2 and 7. This result corresponds to the measured thickness result which clearly shows that a uniform and dense LBL film has good hydrogen gas barrier performance. It can thus be seen that the effects of pH on self-assembled films are composed of two
Fig. 6. Hydrogen transmission rate of (GO/PEI)10 film at different pH value of PEI solution.
Fig. 7. (a) Thickness of (GO/PEI)10 film and (b) thicknesses of the film with increasing the number of layers at different pH values of PEI solution.
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Fig. 8. AFM phase images of (GO/PEI)10 film at different pH value of PEI solution: (a) pH 7 and (b) pH 12.
aspects based on the adsorption amount and the morphology of the film. The reasons may be as follows: the driving forces of selfassembly process are electrostatic force and hydrogen bond. Ionized PEI and GO will be adsorbed each other by electrostatic force and non-ionized PEI and GO will be assembled by hydrogen bond between amino groups of PEI and carboxyl group or hydroxyl groups of GO. Meanwhile, the variation in charge density results in the difference in the stretched state of PEI and GO. When the pH value is less than 3.5, the charge density of GO is
small, but the ionization degree of PEI solution is too high. Therefore, the electrostatic force between PEI and GO is weak and the repulsive force between PEI molecular is strong. Therefore, the formed PEI layer will be sparse with some vacant positions and the assembled film will be not very uniform and dense. When the pH value is greater than 3.5, the ionization degree of GO is so high that the GO sheets easily repulse each other, resulting in folded-shape stacking of GO sheets that can't uniformly tile to form good gas barrier layer [23]. When the pH value equals to 3.5, the GO ionization degree is moderate with adequately stretched
Fig. 9. AFM images and height-profiles of (GO/PEI)10 film prepared at different pH values of PEI solution: (a) and (d), pH ¼ 2; (b) and (e), pH ¼ 7; (c) and (f), pH ¼ 12.
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Fig. 10. The self-assembly mechanism of GO suspension with pH ¼ 3.5 and PEI solution with pH ¼ 12.
sheets and PEI has a proper ionization degree, resulting in good mutual adsorption. 3.3. GO/PEI LBL film prepared under different pH of PEI and GO When the pH of GO dispersion was fixed and the pH of PEI solution was changed, during the alternating immersion adsorption process, ionization degree of the adsorbed GO sheets on the surface of the thin film will change due to the effect of the PEI's pH. It results in variation of the adsorbed amount and micro-structure of GO, hydrogen barrier performances of the film can be effectively altered. Fig. 6 shows the change of H2RT of (GO/PEI)10 films with the pH value of PEI solution. When the pH value increased, the H2TR of film decreased. When the pH value of PEI solution was 12, the H2TR of the film reached its minimum value, 145 cm3/m2 24 h 0.1 MPa. When the pH of PEI solution was higher than 12, the H2TR of film increased. This can be mainly attributed to the obviously increased NaOH content in the PEI solution inhibited the ionization of GO, and then reduced the adsorption force in self-assembly process. Fig. 7 shows thickness of the film increased linearly with the number of film bilayers. When the pH value of PEI solution was 12, the growth rate of film's thickness reached its maximum, 4.24 nm per bilayer. The comparison between above H2TR results and thicknesses of (GO/PEI)10 films in Fig. 7 shows that the variation trend of thickness was identical in nature with the variation of H2TR. The AFM phase images of films in Fig. 8 showed the GO sheets uniformly stretched on the film and the whole area showed a single phase, without severe trenches or holes when the pH values of PEI were 7 and 12. Comparing the AFM height-profile of films prepared at three pH values (Fig. 9), it can be found that the height fluctuation of the composite film gradually increased with the increasing of pH value. 3.4. Comparisons between the results above Comparing the results shown in Figs. 3, 4 and Figs. 6, 7, it indicates that the pH value of PEI solution clearly affects the selfassembly adsorption of GO sheets. For example, when the pH values of GO suspension was fixed and pH of PEI solution was 12, the thickness of (GO/PEI)10 film was 12.7% more and its H2TR was 55.5% lower than those of film prepared at the pH values of PEI solution and GO suspension which were both at 3.5. Possible explanations for this phenomenon are analyzed as follow: when the pH of GO suspension was 3.5, GO was in a weakly ionized state (Fig. 7), while the ionization degree of PEI was related
to the pH of solution (Table 1): (1) when the pH value of PEI is lower than 8, the charge density of PEI was too large that obvious repulsive forces were formed between molecular chains, which made it easy to form holes and defects during self-assembly process; (2) when the pH value is higher than 9.5, the PEI was in a weakly ionized state while the GO was also in a weakly ionized state. In this case, the charge density of PEI and GO was too small to be favorable for the self-assembly process. However, during the alternating immersion process, the different pH values between GO and PEI alter the charge density of the other component, thus promoting the self-assembly process. Fig. 10 shows the example of self-assembly process with GO suspension pH of 3.5 and PEI solution pH of 12. Firstly, the film adsorbed PEI; then the adsorbed PEI molecular chains presented a coiled shape due to the low adsorbed amount as the ionization degree of PEI was low. However, when they occurred at the environmental pH of 3.5 (the pH of GO suspension), the ionization degree of PEI was strengthened, which enhanced its ability to absorb GO sheets; meanwhile, the increased charge density can stretch the PEI molecular chain and the smaller adsorbed amount of PEI won't result in strong repulsion between molecular chains, the adsorption of GO can therefore be enhanced with less vacant position. In addition, since the ionization degree of GO was relatively low, the GO sheet can be added up to a moderate amount while still possessing a good stretched state. When the GO sheets were immersed into the PEI solution with pH value of 12, the ionization degree of GO sheets was still small (Fig. 2), as well as PEI. In this condition, hydrogen bond will accelerate the assembling of both as well as the adsorption of PEI. 4. Conclusions In the present paper, the effects of pH value of PEI solution on the self-assembly of composite film in GO/PEI system were investigated. The following conditions were studied: (1) the pH values of PEI solution and GO were held at the same value, (2) The PEI's pH was varied and the pH of GO was fixed. The H2TR of film was measured and the growth process and microstructure of film were observed. When the pH value of PEI solution was the same as the GO, the adsorbed amount reached its maximum at pH ¼ 3.5, with a film thickness growth rate of 3.45 nm for each bilayer and had the most uniform film structure. As the pH value of PEI solution changed, the adsorbed amount of GO/PEI composite film increased further. When the pH ¼ 12, the film thickness growth rate reached its maximum (4.24 nm for each bilayer) and the GO sheets maintained its tiled stretching and uniform stacking. Comparison of the
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(GO/PEI)10 films prepared under two different conditions indicated that the thickness of self-assembled film prepared at the PEI's pH of 12 was 12.7% higher than that of film prepared when both values were held at 3.5; meanwhile, the H2TR reduced by 55.5%, indicating obvious improvement on the hydrogen barrier performance. It can be seen that in the LBL self-assembly process of GO/PEI system, the thickness and microstructure of composite films can be further regulated by adjusting the pH values of PEI during the preparation process of hydrogen barrier films, which can efficiently improve the hydrogen barrier properties of composite films. Acknowledgments The study was supported by the National Natural Science Foundation of China (Grant Nos. 51203186 and 21406268), the Converging Research Center Program funded by the Korean Ministry of Education (2014M3C1A8048834), and the Fundamental Research Funds for the Central Universities.
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