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Synthesis and properties of iridescent Zn-containing anodic aluminum oxide films
Thin Solid Films 586 (2015) 8–12
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Synthesis and properties of iridescent Zn-containing anodic aluminum oxide films Xiaoxuan Jia, Huiyuan Sun ⁎, Lihu Liu ⁎, Xue Hou, Huiyuan Liu College of Physics Science & Information Engineering, Hebei Normal University, Shijiazhuang 050024, PR China Key Laboratory of Advanced Films of Hebei Province, Shijiazhuang 050024, PR China
a r t i c l e
i n f o
Article history: Received 8 December 2014 Received in revised form 8 March 2015 Accepted 31 March 2015 Available online 18 April 2015 Keywords: Structural color Porous materials Nanowires Optical property
a b s t r a c t A simple method of fabricating Zn-containing anodic aluminum oxide films for multifunctional anticounterfeit technology is reported. The resulting membranes were characterized with UV–vis illumination studies, natural light illumination color experiments, and electron microscopy analysis. Deposition of Zn in the nanopore region can enhance the color saturation of the thin alumina film with different colors dramatically. Both the anodization time and etching time have great influence on the structural color. The mechanisms for the emergence of this phenomenon are discussed and theoretical analysis further demonstrates the experimental results. © 2015 Elsevier B.V. All rights reserved.
1. Introduction It has been found that structural color exists commonly in the biological world [1–4]. Owing to the many useful characters [5,6], structural color may have a wide range of applications in environment-monitor, interior decoration, liquid sensors and anti-counterfeiting. Many researchers have attempted to replicate the biological structures, they have focused efforts on finding and preparing ordered nanostructure materials with bright structural color in the visible light range. In recent years, PAA membrane [7], a nanostructure material with highly ordered nanopore arrays, has been studied extensively. For instance, Diggle et al. [8] had reported that a PAA thin film supported on an Al substrate can produce bright color on reflection in the visible light range. It is worth noting that the films they achieved show a low saturated structural color due to the high reflectivity for visible light of the Al substrate. Xu et al. [9] introduced a technique to enhance the saturation of the interference color by removing the Al substrate. Zhao et al. [10] found that depositing carbon in the PAA nanopores can improve the color saturation of PAA film supported on Al substrate greatly. Zhang et al. [11] devised color tunable PAA thin films embedded with Ni nanowires, and an enhanced color saturation was achieved in this system without getting rid of the Al substrate. In this paper, we report a more economical and more efficient preparation technology of PAA thin film with brilliant structural colors by means of embedding metal Zn in the pore of it. In the process, the oxidation time is about 10 to 14 min only, which shortens materially the ⁎ Corresponding authors. Tel.: +86 18333162601; fax: +86 311 86268314. E-mail address: [email protected] (H. Sun).
http://dx.doi.org/10.1016/j.tsf.2015.03.077 0040-6090/© 2015 Elsevier B.V. All rights reserved.
traditional two-step anodic oxidation process [12–14]. In addition, compared with oxalic acid and sulfuric acid, PAA films produced in phosphoric acid [15–20] have irregular pores and more branched arrays. It is therefore reasonable that the quasi-ordered microstructure might scatter the light reflected from the PAA/Al interface more effectively than the regular arrays ones, and thereby leading to more highly saturated color. More importantly, deposition of Zn in the nanopore region can further enhance the color saturation of the film, and the mechanism of it will be analyzed in detail. The detailed fabrication process and optical properties of the Zn@PAA films we produced are given. 2. Experiment 2.1. Preparation of PAA/Al templates PAA thin films were prepared by one-step anodization procedure. However, there are several steps that had been completed before implementing the anodization. Firstly, high-purity aluminum foils (99.999%) were annealed at 673 K for 4 h in argon. Secondly, the annealed aluminum foils were tailored to be disks with a diameter of 20 mm. Then, the processed aluminum disks were electropolished with a voltage of 30 V for 5 min to smooth the surface, and the mixed solution of ethanol and perchloric acid (1:4) was used as the polishing solution. Subsequently, aluminum disks were cleaned and degreased by ultrasonication in acetone, anhydrous ethanol, and deionized water. Finally, the one-step anodization process was carried out at room temperature. A 45 g/L phosphoric acid solution was used as electrolyte, the applied direct current voltage was kept at 30 V. In particular, the anodization time was varied from 10 to 14 min.
X. Jia et al. / Thin Solid Films 586 (2015) 8–12
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Fig. 1. SEM images for the top view of the samples (a) the PAA thin film, (b) the Zn@PAA nanocomposite film and the cross-sections of the samples, (c) the PAA thin film, and (d) the Zn@ PAA nanocomposite film.
2.2. Deposition of Zn into the PAA/Al template Concerning the characteristics of the two different deposition methods dc electrodeposition and ac electrodeposition, ac deposition has been adopted in this work because it can obtain a more uniform deposition inside the pores. In our cases, the electrodeposition process was carried out in a mixture of 80 g/L ZnSO4 · 7H2O and 20 g/L H3BO3 under an ac voltage of 15 V (50 Hz) for 1.5 min. What is important to point out is that the pH value of the electrolyte needs to be controlled at 2.5 for it determines whether the film deposited uniformly successfully or not. In the ac deposition process, graphite plate and the as-prepared template with aluminum substrate (PAA/Al templates) are used as counter electrode and working electrode, respectively.
2.3. Characterization of PAA/Al templates and Zn@PAA/Al nanocomposite films In this manuscript, we report on the effects of electrodepositing Zn into PAA/Al on the optical properties of the composite film. Optical
digital camera (Canon IXUS 9515) was used to record the color of PAA/Al templates and Zn@PAA/Al nanocomposite films. The crystal structure and morphology of PAA/Al templates and Zn@PAA/Al nanocomposite films were characterized by X-ray diffraction (XRD) with Cu Kα radiation, and field emission scanning electron microscopy (FE-SEM, Hitachi S-4800), respectively. The chemical composition of the samples was determined by an energy dispersive X-ray spectrometry (EDS). Optical properties of the samples were measured via the UV– vis spectrophotometer (Hitachi U-3010) equipped with an integrating sphere and BaSO4 was used as a reference.
3. Results and discussion Fig. 1(a) shows a SEM image of the PAA template anodized for 13 min. The average pore diameter and interpore distance are estimated as 40 nm and 80 nm, respectively. It can be seen that the distribution of the pores is not uniform as those in films formed in oxalic acid solution [21]. The thickness, d, of PAA template is about 400 nm, as shown in the inset of Fig. 1(a). The surface morphology of the Zn@PAA nanocomposite film is
Fig. 2. EDS spectra of the Zn@PAA nanocomposite film with Al substrate.
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X. Jia et al. / Thin Solid Films 586 (2015) 8–12
shown in Fig. 1(b). We find that the average diameter of the pores is about 50 nm, which is slightly lager than the ones of the PAA templates. This can be attributed to the partially dissolved of PAA template when ac deposition is carried out in the acid solution (pH = 2.5). From Fig. 1(d) we can see that Zn nanoparticles were deposited into the pores of PAA template and formed Zn@PAA composite film. The average length of the Zn nanowires was measured to be 78 nm. The composition and crystallographic structure of the nanocomposite films were studied by EDS and XRD means. Fig. 2 depicts a typical energy dispersion spectrum of the porous anodic alumina embedded Zn nanocomposite films with Al substrate, which provides evidence for the presence of Zn and the absence of other elemental impurities. The XRD measurements demonstrate an hcp crystalline nature of the sample due to the Zn (100) and (101) planes as shown in Fig. 3. In order to clarify how the deposition of Zn to affect the color saturation of the samples, Fig. 4 gives the digital camera pictures of PAA/Al templates anodized for 13 min and PAA film deposited with Zn. From the comparison result, we can see that the color saturation of PAA is lower and it is much improved by electrodepositing with Zn. It reveals that Zn nanolayer formed in the PAA template has strong shielding action to the reflected light. The detailed explanations will be given as follows. The mechanisms of this observation can be interpreted by constructive interference between the light reflected from the air/PAA interface and the emergent light after reflection on the PAA/Al interference, as shown in Fig. 5. It can be observed from this sketch map that during the incident light passes through this nanocomposite film and changes to be emergent light, both reflection and refraction occur. Therefore, we attempt at an analysis of this mechanism and find it according to the Bragg–Snell formula [22,23]: 1 λ 2nA dA cosθ þ 2nB dB cosθ ¼ m þ 2 where nA and nB are the refractive index of the PAA film and Zn@PAA film; and λ is the wavelength of incident light. Correspondingly, dA is used to represent the thickness of the PAA film and dB is used to represent the thickness of Zn@PAA nanocomposite film layer, m is the order of interference, and θ is the refraction angle. The effective refractive indices of the PAA film and the Zn@PAA nanocomposite films can be calculated using effective medium approximations given by Maxwell–Garnett [24].
ε eff
ε eff
2εAl2 O3 þ εvoid þ 2P εvoid −ε Al2 O3 ¼ εAl2 O3 2ε Al2 O3 þ εvoid −P εvoid −ε Al2 O3 2n2Al2 O3 þ n2Zn þ 2P n2Zn −n2Al2 O3 2 ¼ nAl2 O3 2n2Al O þ n2Zn −P n2Zn −n2Al O 2
3
2
3
Fig. 3. The XRD diffraction pattern of the Zn@PAA nanocomposite film without Al substrate.
Fig. 5. Optical processes of a light beam incident on the Zn@PAA nanocomposite film.
where εAl203 is the dielectric constant of alumina, εeff is the effective dielectric constant and εvoid is the medium inside the pores, respectively. The dielectric constant and the refractive index are related by ε = n2. The porosity of the PAA film is given by P, which can be given for hexagonal structure by: 2π r 2 P ¼ pffiffiffi 3 Dint where Dint is the interpore spacing and r is the radius of the pores. Nielsch et al. reported that self-ordered PAA films obtained by electrochemical anodization have a porosity of 0.1 [25]. However, in our experiment, the radius of the pores is about 25 nm, and the interpore spacing is about 80 nm, so we can get the porosity of 0.354. Then the effective refraction index of our films can be calculated to be nA = 1.486 (PAA) and nB = 1.659. Obviously, the refractive index of the Al substrate, PAA film and Zn@PAA film obeys the relationship: nAl b nB N nA. As nAl b nB N nA, half-wave loss should be considered. And when light vertically incident upon parallel medium layers, i.e. θ = 0°, the Bragg– Snell formula can be written as: 2nA dA þ 2nB dB ¼
1 mþ λ: 2
Ultimately, according to the obtained data and the simplified Bragg– Snell formula, we can obtain the maximum reflected wavelength, which is various with different parameters, as summarized on Table 1. The refractive index of the PAA film is about 1.486, which will be increased after depositing Zn in the pores of the film. So an enhancement of the
Fig. 4. Digital camera pictures of (a) PAA/Al template anodized for 13 min and (b) PAA film deposited with Zn.
X. Jia et al. / Thin Solid Films 586 (2015) 8–12
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Table 1 Comparison of the theoretical and experiment values of wavelengths of maximum reflectance. Anodization time (min)
dA (nm)
dB (nm)
λT (nm)
λE (nm)
10 11 12 13 14
130 150 160 180 200
78 78 78 78 78
431 470 489 529 568
442 475 494 519 560
color saturation can be found due to the weaker reflected light caused by the increased refractive index. Fig. 6(a) shows the color of samples with different anodization time and Fig. 6(b) is the UV–vis reflection spectra of those samples. It can be seen that the interference band with the maximum reflectance in the visible region shows a red shift with increasing anodization time. This indicates that wavelength of maximum reflection increases with increasing film thickness in accord with Bragg's equation. The calculated wavelengths listed in Table 1 correspond well with the experimental results indicated with vertical arrows at long-wave band. According to the difference in color saturation before and after deposition, some patterns can be recorded on PAA thin films as shown in Fig. 7. Photographs of patterns recorded on the nanocomposite films taken in natural light at nearly normal incidence. It is worth mentioning that such films have friability-resistant feature, this is well beneficial to the applications. Here, we show the procedure of fabricating the pattern as follows. First, the desired pattern was painted with colorless ink on
Fig. 7. Optical photographs of the Zn@PAA films support by Al substrate at different incident angles θ. The images were taken at the angles θ of 0° and 60°.
the top surface of the PAA template. Then, Zn was deposited in the pores of the PAA template. Finally, the ink was removed with acetone leaving the patterns. Obviously, it can be seen from Fig. 7 that the color also changes with the viewing angle, as is expected with Bragg– Snell formula. 4. Conclusions Using one-step anodic oxidation method, a series of PAA/Al films were prepared in phosphoric acid electrolyte at room temperature. Optical photographs show that the PAA films have highly saturated color. Then, using the PAA/Al films as template, Zn@PAA nanocomposite films with structural color were synthesized by an ac electrodeposition method. By depositing metal element Zn in the pores of PAA, the effective refractive index of PAA composite film increases. We believe that the deposition of Zn can weaken the light reflected from the PAA/Al interface, and thereby lead to more highly saturated color. UV–vis reflectance spectrum measurement results indicate that the maximum reflected wavelength is in good agreement with the observed color. Acknowledgments This work is supported by the Natural Science Foundation of Hebei Province (Grant Nos. A2012205038, A2015202343). References
Fig. 6. (a) Photographs of Zn@PAA films were taken in natural light at nearly normal incidence. The nanocomposite films with different anodization time from 10 min to 14 min and (b) UV–vis reflection spectra of the nanocomposite films.
[1] M. Kolle, M.S. Pedro, R.J.S. Maik, H. Fumin, V. Pete, M. Sumeet, Mimicking the colourful wing scale structure of the Papilio blumei butterfly, Nat. Nanotechnol. 5 (1997) 511. [2] H. Noh, S.F. Liew, V. Saranathan, S.G.J. Mochrie, R.O. Prum, E.R. Dufresne, How noniridescent colors are generated by quasi-ordered structures of bird feathers, Adv. Mater. 22 (2010) 2871. [3] M. Rassart, P. Simonis, A. Bay, O. Deparis, J.P. Vigneron, Scale coloration change following water absorption in the beetle Hoplia coerulea (Coleoptera), Phys. Rev. E 80 (2009) 031910. [4] A.R. Parker, V.L. Welch, D. Driver, N. Martini, Structural colour: opal analogue discovered in a weevil, Nature 426 (2003) 786. [5] D.W. Thompson, P.G. Snyder, L. Castro, L. Yan, P. Kaipa, J.A. Woollam, Optical characterization of porous alumina from vacuum ultraviolet to midinfrared, J. Appl. Phys. 97 (2005) 113511. [6] S. Kinoshita, S. Yoshioka, J. Miyazaki, Physics of structural colors, Rep. Prog. Phys. 71 (2008) 076401. [7] H.M. Chen, C.F. Hsin, R.S. Liu, S.F. Hu, C.Y. Huang, Controlling optical properties of aluminum oxide using electrochemical deposition, J. Electrochem. Soc. 154 (2007) K11. [8] J.W. Diggle, T.C. Downie, C.W. Goulding, Anodic oxide films on aluminum, Chem. Rev. 69 (1969) 365. [9] X.L. Zhao, G.W. Meng, Q.L. Xu, F.M. Han, Q. Huang, Color fine-tuning of CNTs@AAO composite thin films via isotropically etching porous AAO before CNT growth and color modification by water infusion, Adv. Mater. 22 (2010) 2637. [10] Q. Xu, Y.H. Yang, J.J. Gu, Z.Y. Li, H.Y. Sun, Influence of Al substrate on the optical properties of porous anodic alumina films, Mater. Lett. 74 (2012) 137. [11] J.J. Zhang, Z.Y. Li, Z.J. Zhang, T.S. Wu, H.Y. Sun, Optical and magnetic properties of porous anodic alumina/Ni nanocomposite films, J. Appl. Phys. 113 (2013) 244305.
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[12] X.F. Zhu, Y. Song, D.L. Yu, C.S. Zhang, W. Yao, A novel nanostructure fabricated by an improved two-step anodizing technology, Electrochem. Commun. 29 (2013) 71. [13] H. Masuda, K. Fukuda, Ordered metal nanohole arrays made by a two-step replication of honeycomb structures of anodic alumina, Science 268 (1995) 1466. [14] P. Erdogan, B. Yuksel, Y. Birol, Effect of chemical etching on the morphology of anodic aluminum oxides in the two-step anodization process, Appl. Surf. Sci. 258 (2012) 4544. [15] Q. Xu, H.Y. Sun, Y.H. Yang, L.H. Liu, Z.Y. Li, Optical properties and color generation mechanism of porous anodic alumina films, Appl. Surf. Sci. 258 (2011) 1826. [16] J.J. Zhang, Z.Y. Li, H.M. Zhang, X. Hou, H.Y. Sun, Optical and magnetic properties of porous anodic alumina films embedded with Co nanowires, Chin. Phys. B 22 (2013) 087805. [17] Z.J. Zhang, J.J. Zhang, X. Hou, T.S. Wu, H.Y. Sun, Iridescent thin films of porous anodic aluminum oxide with embedded silver nanowires, Thin Solid Films 558 (2014) 344. [18] J.J. Zhang, X. Hou, L.H. Liu, H.Y. Sun, Optical and magnetic properties of PAA@Fe nanocomposite films, AIP Adv. 3 (2013) 072116. [19] Q. Xu, W.J. Ye, S.Z. Feng, H.Y. Sun, Synthesis and properties of iridescent cocontaining anodic aluminum oxide films, Dyes Pigments 111 (2014) 185.
[20] X.F. Zhu, L. Liu, Y. Song, H.B. Jia, H.D. Yu, X.M. Xiao, X.L. Yang, Oxygen bubble mould effect: serrated nanopore formation and porous alumina growth, Monatsh. Chem. 139 (2008) 999. [21] S. Liu, S.G. Tang, H.H. Zhou, C.P. Fu, Z.Y. Huang, H.Y. Liu, Y.F. Kuang, Fabrication of AAO films with controllable nanopore size by changing electrolytes and electrolytic parameters, J. Solid State Electrochem. 17 (2013) 1931. [22] S.G. Romanov, M. Bardosova, D.E. Whitehead, I.M. Povey, M. Pemble, C.M. Sotomayor Torres, Erasing diffraction orders: opal versus Langmuir–Blodgett colloidal crystals, Appl. Phys. Lett. 90 (2007) 133101. [23] S. Kinoshita, S. Yoshioka, Structural colors in nature: the role of regularity and irregularity in the structure, ChemPhysChem 6 (2005) 1442. [24] G.D. Sulka, K. Hnida, Distributed Bragg reflector based on porous anodic alumina fabricated by pulse anodization, Nanotechnology 23 (2012) 075303. [25] K. Nielsch, J. Choi, K. Schwirn, R.B. Wehrspohn, U. Gösele, Self-ordering regimes of porous alumina: the 10% porosity rule, Nano Lett. 2 (2002) 677.
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Synthesis and properties of iridescent Zn-containing anodic aluminum oxide films Xiaoxuan Jia, Huiyuan Sun ⁎, Lihu Liu ⁎, Xue Hou, Huiyuan Liu College of Physics Science & Information Engineering, Hebei Normal University, Shijiazhuang 050024, PR China Key Laboratory of Advanced Films of Hebei Province, Shijiazhuang 050024, PR China
a r t i c l e
i n f o
Article history: Received 8 December 2014 Received in revised form 8 March 2015 Accepted 31 March 2015 Available online 18 April 2015 Keywords: Structural color Porous materials Nanowires Optical property
a b s t r a c t A simple method of fabricating Zn-containing anodic aluminum oxide films for multifunctional anticounterfeit technology is reported. The resulting membranes were characterized with UV–vis illumination studies, natural light illumination color experiments, and electron microscopy analysis. Deposition of Zn in the nanopore region can enhance the color saturation of the thin alumina film with different colors dramatically. Both the anodization time and etching time have great influence on the structural color. The mechanisms for the emergence of this phenomenon are discussed and theoretical analysis further demonstrates the experimental results. © 2015 Elsevier B.V. All rights reserved.
1. Introduction It has been found that structural color exists commonly in the biological world [1–4]. Owing to the many useful characters [5,6], structural color may have a wide range of applications in environment-monitor, interior decoration, liquid sensors and anti-counterfeiting. Many researchers have attempted to replicate the biological structures, they have focused efforts on finding and preparing ordered nanostructure materials with bright structural color in the visible light range. In recent years, PAA membrane [7], a nanostructure material with highly ordered nanopore arrays, has been studied extensively. For instance, Diggle et al. [8] had reported that a PAA thin film supported on an Al substrate can produce bright color on reflection in the visible light range. It is worth noting that the films they achieved show a low saturated structural color due to the high reflectivity for visible light of the Al substrate. Xu et al. [9] introduced a technique to enhance the saturation of the interference color by removing the Al substrate. Zhao et al. [10] found that depositing carbon in the PAA nanopores can improve the color saturation of PAA film supported on Al substrate greatly. Zhang et al. [11] devised color tunable PAA thin films embedded with Ni nanowires, and an enhanced color saturation was achieved in this system without getting rid of the Al substrate. In this paper, we report a more economical and more efficient preparation technology of PAA thin film with brilliant structural colors by means of embedding metal Zn in the pore of it. In the process, the oxidation time is about 10 to 14 min only, which shortens materially the ⁎ Corresponding authors. Tel.: +86 18333162601; fax: +86 311 86268314. E-mail address: [email protected] (H. Sun).
http://dx.doi.org/10.1016/j.tsf.2015.03.077 0040-6090/© 2015 Elsevier B.V. All rights reserved.
traditional two-step anodic oxidation process [12–14]. In addition, compared with oxalic acid and sulfuric acid, PAA films produced in phosphoric acid [15–20] have irregular pores and more branched arrays. It is therefore reasonable that the quasi-ordered microstructure might scatter the light reflected from the PAA/Al interface more effectively than the regular arrays ones, and thereby leading to more highly saturated color. More importantly, deposition of Zn in the nanopore region can further enhance the color saturation of the film, and the mechanism of it will be analyzed in detail. The detailed fabrication process and optical properties of the Zn@PAA films we produced are given. 2. Experiment 2.1. Preparation of PAA/Al templates PAA thin films were prepared by one-step anodization procedure. However, there are several steps that had been completed before implementing the anodization. Firstly, high-purity aluminum foils (99.999%) were annealed at 673 K for 4 h in argon. Secondly, the annealed aluminum foils were tailored to be disks with a diameter of 20 mm. Then, the processed aluminum disks were electropolished with a voltage of 30 V for 5 min to smooth the surface, and the mixed solution of ethanol and perchloric acid (1:4) was used as the polishing solution. Subsequently, aluminum disks were cleaned and degreased by ultrasonication in acetone, anhydrous ethanol, and deionized water. Finally, the one-step anodization process was carried out at room temperature. A 45 g/L phosphoric acid solution was used as electrolyte, the applied direct current voltage was kept at 30 V. In particular, the anodization time was varied from 10 to 14 min.
X. Jia et al. / Thin Solid Films 586 (2015) 8–12
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Fig. 1. SEM images for the top view of the samples (a) the PAA thin film, (b) the Zn@PAA nanocomposite film and the cross-sections of the samples, (c) the PAA thin film, and (d) the Zn@ PAA nanocomposite film.
2.2. Deposition of Zn into the PAA/Al template Concerning the characteristics of the two different deposition methods dc electrodeposition and ac electrodeposition, ac deposition has been adopted in this work because it can obtain a more uniform deposition inside the pores. In our cases, the electrodeposition process was carried out in a mixture of 80 g/L ZnSO4 · 7H2O and 20 g/L H3BO3 under an ac voltage of 15 V (50 Hz) for 1.5 min. What is important to point out is that the pH value of the electrolyte needs to be controlled at 2.5 for it determines whether the film deposited uniformly successfully or not. In the ac deposition process, graphite plate and the as-prepared template with aluminum substrate (PAA/Al templates) are used as counter electrode and working electrode, respectively.
2.3. Characterization of PAA/Al templates and Zn@PAA/Al nanocomposite films In this manuscript, we report on the effects of electrodepositing Zn into PAA/Al on the optical properties of the composite film. Optical
digital camera (Canon IXUS 9515) was used to record the color of PAA/Al templates and Zn@PAA/Al nanocomposite films. The crystal structure and morphology of PAA/Al templates and Zn@PAA/Al nanocomposite films were characterized by X-ray diffraction (XRD) with Cu Kα radiation, and field emission scanning electron microscopy (FE-SEM, Hitachi S-4800), respectively. The chemical composition of the samples was determined by an energy dispersive X-ray spectrometry (EDS). Optical properties of the samples were measured via the UV– vis spectrophotometer (Hitachi U-3010) equipped with an integrating sphere and BaSO4 was used as a reference.
3. Results and discussion Fig. 1(a) shows a SEM image of the PAA template anodized for 13 min. The average pore diameter and interpore distance are estimated as 40 nm and 80 nm, respectively. It can be seen that the distribution of the pores is not uniform as those in films formed in oxalic acid solution [21]. The thickness, d, of PAA template is about 400 nm, as shown in the inset of Fig. 1(a). The surface morphology of the Zn@PAA nanocomposite film is
Fig. 2. EDS spectra of the Zn@PAA nanocomposite film with Al substrate.
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X. Jia et al. / Thin Solid Films 586 (2015) 8–12
shown in Fig. 1(b). We find that the average diameter of the pores is about 50 nm, which is slightly lager than the ones of the PAA templates. This can be attributed to the partially dissolved of PAA template when ac deposition is carried out in the acid solution (pH = 2.5). From Fig. 1(d) we can see that Zn nanoparticles were deposited into the pores of PAA template and formed Zn@PAA composite film. The average length of the Zn nanowires was measured to be 78 nm. The composition and crystallographic structure of the nanocomposite films were studied by EDS and XRD means. Fig. 2 depicts a typical energy dispersion spectrum of the porous anodic alumina embedded Zn nanocomposite films with Al substrate, which provides evidence for the presence of Zn and the absence of other elemental impurities. The XRD measurements demonstrate an hcp crystalline nature of the sample due to the Zn (100) and (101) planes as shown in Fig. 3. In order to clarify how the deposition of Zn to affect the color saturation of the samples, Fig. 4 gives the digital camera pictures of PAA/Al templates anodized for 13 min and PAA film deposited with Zn. From the comparison result, we can see that the color saturation of PAA is lower and it is much improved by electrodepositing with Zn. It reveals that Zn nanolayer formed in the PAA template has strong shielding action to the reflected light. The detailed explanations will be given as follows. The mechanisms of this observation can be interpreted by constructive interference between the light reflected from the air/PAA interface and the emergent light after reflection on the PAA/Al interference, as shown in Fig. 5. It can be observed from this sketch map that during the incident light passes through this nanocomposite film and changes to be emergent light, both reflection and refraction occur. Therefore, we attempt at an analysis of this mechanism and find it according to the Bragg–Snell formula [22,23]: 1 λ 2nA dA cosθ þ 2nB dB cosθ ¼ m þ 2 where nA and nB are the refractive index of the PAA film and Zn@PAA film; and λ is the wavelength of incident light. Correspondingly, dA is used to represent the thickness of the PAA film and dB is used to represent the thickness of Zn@PAA nanocomposite film layer, m is the order of interference, and θ is the refraction angle. The effective refractive indices of the PAA film and the Zn@PAA nanocomposite films can be calculated using effective medium approximations given by Maxwell–Garnett [24].
ε eff
ε eff
2εAl2 O3 þ εvoid þ 2P εvoid −ε Al2 O3 ¼ εAl2 O3 2ε Al2 O3 þ εvoid −P εvoid −ε Al2 O3 2n2Al2 O3 þ n2Zn þ 2P n2Zn −n2Al2 O3 2 ¼ nAl2 O3 2n2Al O þ n2Zn −P n2Zn −n2Al O 2
3
2
3
Fig. 3. The XRD diffraction pattern of the Zn@PAA nanocomposite film without Al substrate.
Fig. 5. Optical processes of a light beam incident on the Zn@PAA nanocomposite film.
where εAl203 is the dielectric constant of alumina, εeff is the effective dielectric constant and εvoid is the medium inside the pores, respectively. The dielectric constant and the refractive index are related by ε = n2. The porosity of the PAA film is given by P, which can be given for hexagonal structure by: 2π r 2 P ¼ pffiffiffi 3 Dint where Dint is the interpore spacing and r is the radius of the pores. Nielsch et al. reported that self-ordered PAA films obtained by electrochemical anodization have a porosity of 0.1 [25]. However, in our experiment, the radius of the pores is about 25 nm, and the interpore spacing is about 80 nm, so we can get the porosity of 0.354. Then the effective refraction index of our films can be calculated to be nA = 1.486 (PAA) and nB = 1.659. Obviously, the refractive index of the Al substrate, PAA film and Zn@PAA film obeys the relationship: nAl b nB N nA. As nAl b nB N nA, half-wave loss should be considered. And when light vertically incident upon parallel medium layers, i.e. θ = 0°, the Bragg– Snell formula can be written as: 2nA dA þ 2nB dB ¼
1 mþ λ: 2
Ultimately, according to the obtained data and the simplified Bragg– Snell formula, we can obtain the maximum reflected wavelength, which is various with different parameters, as summarized on Table 1. The refractive index of the PAA film is about 1.486, which will be increased after depositing Zn in the pores of the film. So an enhancement of the
Fig. 4. Digital camera pictures of (a) PAA/Al template anodized for 13 min and (b) PAA film deposited with Zn.
X. Jia et al. / Thin Solid Films 586 (2015) 8–12
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Table 1 Comparison of the theoretical and experiment values of wavelengths of maximum reflectance. Anodization time (min)
dA (nm)
dB (nm)
λT (nm)
λE (nm)
10 11 12 13 14
130 150 160 180 200
78 78 78 78 78
431 470 489 529 568
442 475 494 519 560
color saturation can be found due to the weaker reflected light caused by the increased refractive index. Fig. 6(a) shows the color of samples with different anodization time and Fig. 6(b) is the UV–vis reflection spectra of those samples. It can be seen that the interference band with the maximum reflectance in the visible region shows a red shift with increasing anodization time. This indicates that wavelength of maximum reflection increases with increasing film thickness in accord with Bragg's equation. The calculated wavelengths listed in Table 1 correspond well with the experimental results indicated with vertical arrows at long-wave band. According to the difference in color saturation before and after deposition, some patterns can be recorded on PAA thin films as shown in Fig. 7. Photographs of patterns recorded on the nanocomposite films taken in natural light at nearly normal incidence. It is worth mentioning that such films have friability-resistant feature, this is well beneficial to the applications. Here, we show the procedure of fabricating the pattern as follows. First, the desired pattern was painted with colorless ink on
Fig. 7. Optical photographs of the Zn@PAA films support by Al substrate at different incident angles θ. The images were taken at the angles θ of 0° and 60°.
the top surface of the PAA template. Then, Zn was deposited in the pores of the PAA template. Finally, the ink was removed with acetone leaving the patterns. Obviously, it can be seen from Fig. 7 that the color also changes with the viewing angle, as is expected with Bragg– Snell formula. 4. Conclusions Using one-step anodic oxidation method, a series of PAA/Al films were prepared in phosphoric acid electrolyte at room temperature. Optical photographs show that the PAA films have highly saturated color. Then, using the PAA/Al films as template, Zn@PAA nanocomposite films with structural color were synthesized by an ac electrodeposition method. By depositing metal element Zn in the pores of PAA, the effective refractive index of PAA composite film increases. We believe that the deposition of Zn can weaken the light reflected from the PAA/Al interface, and thereby lead to more highly saturated color. UV–vis reflectance spectrum measurement results indicate that the maximum reflected wavelength is in good agreement with the observed color. Acknowledgments This work is supported by the Natural Science Foundation of Hebei Province (Grant Nos. A2012205038, A2015202343). References
Fig. 6. (a) Photographs of Zn@PAA films were taken in natural light at nearly normal incidence. The nanocomposite films with different anodization time from 10 min to 14 min and (b) UV–vis reflection spectra of the nanocomposite films.
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