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The effect of the voltage waveform on the microstructure and optical properties of porous anodic alumina photonic crystals
Optical Materials xxx (xxxx) xxx
Contents lists available at ScienceDirect
Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
The effect of the voltage waveform on the microstructure and optical properties of porous anodic alumina photonic crystals Shiyuan Zhang a, Qin Xu a, *, Shunzhen Feng b, Chunxin Sun a, Qi Peng a, Tian Lan a a b
Department of Applied Physics, Hebei University of Technology, Tianjin, 300401, China Department of Physics and Electric Information Engineering, Shijiazhuang University, Shijiazhuang, 050035, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Porous anodic alumina Photonic crystal Structural colour
Porous anodic alumina photonic crystals (PAA-PhCs) with brilliant colours were successfully fabricated by pulse anodization of aluminium in an oxalic acid electrolyte. The influence of the voltage waveform on the micro structure of the PAA-PhCs was investigated by adjusting the anodization period, anodization amplitude, and anodization offset. Here, we report a simplified method for compensation of the photonic band gap (PBG) drift using AC waveforms instead of the periodic length-controlled method. Scanning electron microscopy (SEM) results showed that the layer thickness of the PAA-PhC prepared by an AC pulse was gradually reduced from 360 nm to 290 nm, which was completely different from a previous report showing that the layer thickness of a sample prepared by a DC pulse remained unchanged. In addition, the effect of chemical corrosion in a phosphoric acid solution on the microstructures of the PAA-PhCs was analysed by comparative experiments. PBG drift caused by minor defects in the nanostructure during the anodization process could also be compensated for by adjusting the anodizing time and corrosion time. Our approach may help to broaden the potential applications of and provide important guidance for photonic crystal display and new devices.
1. Introduction The bright colours of many substances and living organisms in nature are not produced by pigments but by the interaction of light with their nanostructures [1–4]. For example, photonic crystals (PhCs), such as opal and the scabbard wings of some beetles, are composed of nano structures with periodic dielectric coefficients, ε, and disallowed wavelength bands called photonic band gaps (PBGs) [5,6]. PhCs can present different colours when their PBGs are in the visible region of the spectrum. It is widely known that one-dimensional (1D) layered PAA-PhCs can be fabricated by applying a periodic voltage or current to aluminium foil. Then, by chemical etching, nanochannels and nano pores can connect to each other to form a three-dimensional (3D) PAA-PhC structure, and the PBG can be adjusted concurrently [7]. Wang et al. prepared a series of 3D PAA-PhCs showing different structural colours by periodic voltage anodization in an H2C2O4 solution and corrosion in a H3PO4 solution [8,9]. Guo et al. prepared a PAA-PhC with a complete PBG by utilizing a periodically controlled current method [10]. By applying a fixed-period waveform, a PAA-PhC was prepared that had a periodic layered structure in which each layer had the same thickness d [11,12]. During the electrolytic oxidation process, the
corrosion effect of the electrolyte on the PAA was not uniform from top to bottom, which led to the porosity decreasing layer by layer; that is, the effective refractive index neff corresponding to each layer gradually increased [13], according to the Maxwell-Garnett theory. Cumulatively, a redshift in the upper layer band will cause the PBG to drift or widen. To compensate for the above defects, Liu et al. reported a strategy to obtain a complete PBG by linearly reducing the periodic pulse width of the current during the anodization process, which could keep neff con stant by successively reducing the d layer by layer [13]. The sample showed a brilliant colour directly on the aluminium substrate. However, the controllable current source device was very expensive and required more than one instrument to be used simultaneously, which was not easy to operate. Considering that a reverse voltage can accelerate the corrosion of the barrier layer by the electrolyte and help to increase the pore size, in this experiment, we attempt to use an AC pulse, that is, to use the reverse voltage to decrease the PBG drift. Herein, detailed studies of the effect of different voltage waveforms (DC or AC, and si nusoidal or sawtooth pulses) on the PAA-PhC nanostructures and optical properties are reported.
* Corresponding author. E-mail address: [email protected] (Q. Xu). https://doi.org/10.1016/j.optmat.2019.109488 Received 24 August 2019; Received in revised form 12 October 2019; Accepted 27 October 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Shiyuan Zhang, Optical Materials, https://doi.org/10.1016/j.optmat.2019.109488
S. Zhang et al.
Optical Materials xxx (xxxx) xxx
Fig. 1. Anodization profiles and UV–Vis transmission spectra of PAA-PhCs, anodization period T ¼ 10 min; (a) DC saw-tooth waves (5 V–42 V); and (b) AC saw-tooth waves ( 1.6 V–42 V).
2. Experimental
voltage reverses, and the chemical equations are as follows,
PAA-PhCs were prepared by a process similar to that reported pre viously [14]. Pulse anodization was first performed using a function signal generator (Rigol DG-4062) to generate a series of periodic oscil latory voltage signals, with different profiles (saw-tooth and sinusoidal), amplitudes, and periods. The signals were then amplified using a voltage amplifier (Pintek HA-305). The PAA-PhCs were prepared in a 0.3 M oxalic acid solution at room temperature. The aluminium substrates on the back of the PAA-PhCs were removed in a saturated CuCl2 solution. Finally, the PAA-PhCs were characterized by field-emission scanning electron microscopy (FE-SEM, Hitachi S-570). The current density dur ing the pulse voltage anodizing was recorded with a computer-controlled Keithley 2450 Sourcemeter. The transmission spectra were recorded on a UV–Vis spectrophotometer (UV–Vis, Hitachi UV-3010).
Al2 O3 þ 6H þ ¼ 2Al3þ þ 3H2 O
(1)
2H þ þ 2e ¼ H2 ↑
(2)
The reverse voltage method is usually used to thin or remove the PAA template barrier layer [18]. Here, reversing the voltage throughout the anodization process accelerates the corrosion of the PAA pores and the barrier layer, and leads to an increase in the pore diameter, such that the porosity difference between the periodic layers is greater. Wang et al. [18] reported that the greater the difference in ε between the periodic layers, the easier it is to achieve a complete PBG, and the greater the porosity difference between the corresponding layers. Therefore, PAA-PhCs were prepared by using an AC pulse anodization process. 3.1. Effect of a saw-tooth waveform on the nanostructure and optical properties of PAA-PhCs
3. Results and discussion
For comparison, two samples were prepared by the DC and AC sawtooth pulse anodization approaches. In our experiments, the PAA-PhCs were produced by means of saw-tooth waves with an anodization period of Tp ¼ 10 min (tup:tdown ¼ 4:1) and an anodization time of S ¼ 24 h. The anodization profiles and UV–Vis transmission spectra of
We present here a new anodization process, termed AC pulse anod ization. Unlike conventional DC pulse anodization processes, in which high-purity aluminium sheets are anodized as the anode [15–17], for AC pulse anodization, the aluminium sheet changes to the cathode as the 2
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Fig. 2. SEM images of the PAA-PhC prepared by the AC saw-tooth pulse anodization process ( 1.6 V–42 V), all images are of the same film measured at different positions. (a) top cross section; (b) middle cross section; (c) bottom cross section; (d) surface; and (e) back side.
the PAA-PhCs are shown in Fig. 1. Fig. 1a shows that the spectrum has a peak at approximately 512 nm and a weak transmission region from 548 nm to 649 nm, which clearly represents a relatively large PBG drift. In Fig. 1b, the spectrum shows an obvious transmission peak at approximately 445 nm, in good agreement with the colour seen in the inset of Fig. 1b. The PBG drift of the PAA-PhC formed by AC pulse anodization had been improved to a certain extent. To investigate the influence of the reverse voltage on the nano structures of the PAA-PhCs, characteristic SEM images of the sample fabricated by the AC saw-tooth pulse anodization process ( 1.6 V–42 V) are shown in Fig. 2. These images clearly shows that the period length d of the PAA-PhC decreases from 360 nm to 290 nm, which is different from the results of the same period length d for the sample formed by the DC saw-tooth pulse anodization process [13]. This indicates that the reason for the reverse voltage improving the PBG drift can be attributed mainly to a decrease of the period length d. It is worth mentioning that the effect of the reverse voltage is similar to that of the period length control method [13], as we have previously mentioned. The reason for the decrease of the periodic thickness d of each layer in the above samples was analysed as indicated below. We believe that the oxide layer is thinner and has a smaller resistance at the initial stage of the electrolytic reaction; thus, the generated alumina layer has a larger periodic thickness; when the voltage is reversed, the reverse current is larger and the dissolution effect is larger. Thus, the porosity between the main channels (low pore density layers) and the branch channels (high pore density layers) varies greatly, which corresponds to the upper portion of the oxide layer. As the reaction progresses, the oxide layer gradually thickens, and the reverse current caused by the reverse voltage gradually decreases. At this time, the dissolution effect decreases and corresponds to the middle of the oxide layer. In the later stage of the electrolytic reaction, when the oxide layer reaches a certain thickness, it has a higher resistance, thus the generated alumina layer has a smaller periodic thickness. Meanwhile, the reverse current is approximately zero due to the small reverse voltage value and high resistance, so the reverse voltage portion of the AC waveform has almost no effect, and the oxide layer is completely generated by the direct voltage. Moreover, the electrolyte is weakly corrosive to the lower layer, thus, the porosity between the main channels and the branch channels varies slightly, which corresponds to the lower portion of the oxide layer. Since the SEM microstructures of the upper, middle and lower regions of the same AC pulsed sample are slightly different, we take the middle region of the sample for scanning. To study the effect of the anodization period T on the nanostructures of PAA-PhCs prepared by the AC saw-tooth pulse anodization process
( 1.6 V–42 V) [19], the anodization period T was varied from 8 min to 15 min. Fig. 3 shows the anodization profiles and the corresponding UV–Vis transmission spectra of the PAA-PhCs. The optical photographs of the PAA-PhCs are shown in the insets of Fig. 3. The top photograph is the as-prepared sample, and the corresponding bottom photograph is the sample after etching in a 6 wt% phosphoric acid solution at 35 � C. As shown in Fig. 3a, before corrosion, the sample prepared with a voltage period of 8 min has high overall transmission, and three peaks appeared in the red region from 610 nm to 690 nm. After corrosion, the total transmission of the sample decreases slightly, and the blueshift of the reflection band is in the region from 550 nm to 670 nm. In addition, the transmission is approximately 20% in the orange band from approximately 606 nm–621 nm. In Fig. 3b, before corrosion, the sample prepared with a voltage period of 10 min has a first reflection peak at approximately 445 nm with a low transmission of 20% and two smaller peaks at approximately 502 nm and 529 nm with transmissions of approximately 50%. In addition, the overall transmission is high, and the sample appears blue. After corrosion, the overall transmission of the sample declines, and the first peak had a large blueshift to the region between 411 nm and 427 nm with a lower transmission of approxi mately 15%; furthermore, the other two smaller peaks become less obvious, and the sample appears indigo. In Fig. 3c, before corrosion, the sample prepared with a voltage period of 15 min has two high-intensity reflection bands. The red band has two peaks at 723 nm and 733 nm, and the blue band has one peak at 489 nm; therefore, the sample appears to have a mixed colour, cyan. After corrosion, the transmission decreases in general, and there is a blueshift of the two transmission bands. The red band has two new peaks, one at 635 nm and the other between 672 nm and 684 nm. The blue band has one new peak at 439 nm and another peak at 461 nm; thus, the sample appears to have a mixed colour, magenta. In summary, we find that, for the non-corroded series of samples, the main peak has a blue shift as the voltage period increases from 8 min to 10 min. This is mainly caused by the reduction of average layer thickness resulted from the sharp increase of the corresponding reverse current period. When the voltage period is increased to 15 min, the transmission peak shifts slightly, and the sample shows two transmission peaks in the red and blue-violet regions of the visible wavelength range. We specu late that the positive and negative currents reach a critical equilibrium state during aluminum anodizing, leading to the change of PAA-PhCs structure. After corrosion in phosphoric acid, the transmission of all the samples shows a decreasing trend. Generally, the thicker the PAA grown in the electrolyte, the greater the deviation of the effective refractive index neff between the upper and 3
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Fig. 3. Anodization profiles and UV–Vis transmission spectra of the PAA-PhCs prepared with different anodization periods. (a) 8 min; (b) 10 min; and (c) 15 min.
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Fig. 4. (a) AC sawtooth voltage wave ( 0.2 V–42 V), total electrolysis time S ¼ 8 h, etched for 30 min. (b) SEM characterization image. (c) UV–Vis transmission spectrum of the sample.
Fig. 5. (a) AC sinusoidal voltage wave ( 15 V–42 V), T ¼ 5 min, S ¼ 12 h. (b) UV–Vis transmission spectra after etching for 0 min, 10 min, and 20 min.
Fig. 6. SEM characterization image (etched for 20 min). 5
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Fig. 7. Pulse anodization profile of PAA-PhCs prepared by the AC sinusoidal pulse anodization process ( 15 V–42 V).
lower layers is, in addition to the larger shift of the band gap. Therefore, it is necessary to reduce the time of anodization to reduce the number of periodic layers in the PhCs, which can compensate for the PBG drift. Based on this, we used the AC sawtooth wave shown in Fig. 4a, short ened the anodization time to 8 h and etched the sample in phosphoric acid for 30 min. SEM characterization and the UV–Vis transmission spectrum were measured and are shown in Fig. 4b–c. After corrosion, the pore size increases, and the channel walls decrease and become slightly fractured, similar to discontinuous layer opening, as indicated by the arrows. In addition, the region between contiguous branches is corroded through, and a clear vertical pore (channel) appears as a 3D-shaped PAA-PhC, as indicated by the dotted lines. In addition, the UV–Vis transmission curve shows a single peak with a transmission rate of 13%, and the half-peak width was approxi mately 30 nm, which shows that for the sample prepared with an AC, the decrease in the top-down thickness caused by the reverse voltage and the increase in the effective refractive index neff caused by uneven corrosion of the electrolyte offset each other, thus effectively decreasing the PBG drift.
approximately 32% in the red-orange band from 615 nm to 635 nm. The sample is yellow-green. To summarize, with increasing corrosion time, the high-intensity reflection band of the samples appears to blueshift and the trans mission decreases, similar to the transmission change of the samples prepared by the AC sawtooth wave. A reasonable explanation is given below. The surface and cross section of the sample (etched for 20 min) were characterized by SEM and are shown in Fig. 6. We found that irregular fragmentation occurs on the sample surface and top layer after longterm corrosion. As indicated by the arrows and dashed boxes in Fig. 6, fractured segments cracked and separated from each other. The average pore size increases, and the transverse channels show a 3D-networked structure [21], especially near the top layer. Through analysis of these images, we believe that the scattered surface and the irregular internal fragmented structure strongly scatter the incident light, which results in a decrease in the average transmission of the sample. In addition, the scattered structure increases the PBG drift; thus, the reflection bands in the spectra (Fig. 5) are wider and gradually split. It is known that the growth of anodic film is decided by current density. Pulse anodization profile of PAA-PhCs prepared in the first 20 cycles is shown in Fig. 7. It can be seen that the average reverse current density in each period decreases with time during aluminum anodizing [22]. This is well consistent with the above analysis in section 3.1. In summary, chemical corrosion can effectively regulate the position of the PBG within a certain range. Generally, the time of anodization and chemical corrosion should be strictly controlled. Using AC pulse method can avoid the traditional acid etching process, and obtain PAA-PhCs with photonic band gaps directly. While the samples prepared with a DC pulse, the effective refractive index neff of a PAA-PhC can be adjusted by means of phosphoric acid corrosion. Based on this, we utilized two sets of DC sinusoidal waves to prepare samples, as shown in Fig. 8. Contrasting the two samples in Fig. 8, Sample (b) achieves thinner layers by utilizing a shorter voltage period. In addition, after phosphoric acid corrosion, Sample (b) has larger pores and channels in the vertical direction (dotted lines), which appear in the 3D structure. Fracturing easily occurs in the vicinity of the through channels, with disconnection (arrowheads) between the layers. As shown by the UV–Vis transmission spectra, for Sample (a), there are two obvious transmission peak bands
3.2. The effect of a sinusoidal waveform on the nanostructures and optical properties of PAA-PhCs A sinusoidal voltage wave, for which the parameters are illustrated in Fig. 5a, was constructed to further verify the above conclusions [20]. UV–Vis transmission spectra were measured before and after etching in a 6 wt% phosphoric acid solution at 35 � C and are illustrated in Fig. 5b. As shown in Fig. 5, light in the red band from 650 nm to 700 nm for the uncorroded sample was reflected, with a transmission at 666 nm of approximately 20%, resulting in a red colour and having a small PBG drift. After corroding for 10 min, a blueshift in the spectrum occurs, and the average transmission decreases. Additionally, the high-intensity reflection band, centred around the orange region at 590 nm–650 nm, broadens and has a transmission of approximately 33% (610 nm). After corroding for 20 min, the spectrum continues to blueshift and the average transmission continues to decrease. The high-intensity reflec tion band broadens and splits into two bands: one peak at 570 nm with a transmission of approximately 20% over the yellow-green band from 525 nm to 580 nm, and the other peak at 630 nm with a transmission of 6
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Fig. 8. DC sinusoidal voltage waves and UV–Vis transmission spectra. (a) 5 V–42 V, T ¼ 10 min, S ¼ 24 h, etched for 0 min; and (b) 20 V–50 V, T ¼ 5 min, S ¼ 2 h, etched for 15 min.
in the visible light region, located near 505 nm and 734 nm, which in dicates that the PBG formed by the sample was not complete. For Sample (b), only one extreme peak appears located at 456 nm with a narrow half-peak width, while the transmission of the other wavelengths is greater than 80%, which shows that the PBG structure of the sample is relatively complete. In addition, due to the longer anodization time, the overall transmission of Sample (a) is much lower than that of Sample (b).
4. Conclusions In conclusion, SEM characterization shows that the layer thickness of PAA-PhCs prepared with an AC pulse decreases gradually, which helps to decrease the PBG drift to a certain extent. Different from the previous reports, using AC pulse method can avoid the traditional acid etching process. So the AC pulse method can be used to directly obtain PAA-PhCs with photonic band gaps. Additionally, controlling the anodization time 7
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and corrosion time can further solve the problem. In addition, after corrosion in phosphoric acid, the average transmission of all samples shows a downward trend. Though analysis, we believe that this might be caused by channel fracturing and separation, which are due to the excessive corrosion.
[6] W. Lee, A. Chan, M.A. Bevan, J.A. Lewis, P.V. Braun, Nanoparticle-mediated epitaxial assembly of colloidal crystals on patterned substrates, Langmuir 20 (2004) 5262–5270. [7] E. Yablonovitch, Photonic band-gap structures, J. Opt. Soc. Am. B: Opt. Phys. 10 (1993) 283–285. [8] B. Wang, G.T. Fei, M. Wang, M.G. Kong, L.D. Zhang, Preparation of photonic crystals made of air pores in anodic alumina, Nanotechnology 18 (2007), 365601. [9] G.L. Shang, G.T. Fei, Y. Zhang, Y. Peng, Fano resonance in anodic aluminum oxide based photonic crystals, Sci. Rep. 4 (2014), 3601. [10] D.L. Guo, L.X. Fan, F.H. Wang, S.Y. Huang, Porous anodic aluminum oxide bragg stacks as chemical sensors, J. Phys. Chem. C 112 (2008) 17952–17956. [11] Y. Chen, A. Santos, D. Ho, Y. Wang, T. Kumeria, J. Li, C. Wang, D. Losic, On the generation of interferometric colors in high purity and technical grade aluminum: an alternative green process for metal finishing industry, Electrochim. Acta 174 (2015) 672–681. [12] Y. Chen, A. Santos, Y. Wang, T. Kumeria, Interferometric nanoporous anodic alumina photonic coatings for optical sensing, Nanoscale 7 (2015) 7770–7779. [13] Y. Liu, Y. Chang, Z. Ling, X. Hu, Y. Li, Structural coloring of aluminum, Electrochem. Commun. 13 (2011) 1336–1339. [14] Q. Xu, H.M. Ma, Y.J. Zhang, R.S. Li, H.Y. Sun, Synthesis of iridescent Ni-containing anodic aluminum oxide films by anodization in oxalic acid, Opt. Mater. 52 (2016) 107–110. [15] W. Lee, R. Scholz, U. G€ osele, A continuous process for structurally well-defined Al2O3 nanotubes based on pulse anodization of aluminium, Nano Lett. 8 (2008) 2155–2160. [16] P. Yan, G.T. Fei, G.L. Shang, B. Wu, Fabrication of one-dimensional alumina photonic crystals with a narrow band gap and their application to high-sensitivity sensors, J. Mater. Chem. C 1 (2013) 1659–1664. [17] Z.Y. Ling, S.S. Cheng, X. Hu, Y. Li, Optical transmission spectra of anodic aluminum oxide membranes with a dual layer-by-layer structure, Chin. Phys. Lett. 26 (2009) 133–135. [18] J.B. Wang, X.Z. Zhou, Q.F. Liu, D.S. Xue, Magnetic texture in iron nanowire arrays, Nanotechnology 15 (2004) 485–489. [19] W. Lee, J.C. Kim, Highly ordered porous alumina with tailor-made pore structures fabricated by pulse anodization, Nanotechnology 21 (2010), 485304. [20] A. Santos, J.H. Yoo, C.V. Rohatgi, T. Kumeria, Realisation and advanced engineering of true optical rugate filters based on nanoporous anodic alumina by sinusoidal pulse anodisation, Nanoscale 8 (2015) 1360–1373. [21] J. Martín, M. Martíngonz� alez, J.F. Fern� andez, O. Caballerocalero, Ordered threedimensional interconnected nanoarchitectures in anodic porous alumina, Nat. Commun. 5 (2014) 5130. [22] X. Hu, Z.Y. Ling, X.H. He, S.S. Chen, Controlling transmission spectra of photonic crystals under electrochemical oxidization of aluminum, J. Electrochem. Soc. 156 (2009) C176–C179.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work is supported by the National Natural Science Foundation of China, China (Grant No. 51702083); the Youth Talent Support Pro gram of Hebei Province, China; the Natural Science Foundation of Hebei Province, China (Grant Nos. A2015202343 and A2019202190); the Research Project of Hebei Science and Technology Department, China (Grant No. 15211043), and Student’s Platform for Innovation and Entrepreneurship Training Program of Hebei Province, China (Grant No. 201710080288). References [1] J.P. Vigneron, J.-F. Colomer, M. Rassart, A.L. Ingram, V. Lousse, Structural origin of the colored reflections from the black-billed magpie feathers, Phys. Rev. E. 73 (2006), 021914. [2] S. Yoshioka, Y. Shimizu, S. Kinoshita, B. Matsuhana, Structural color of a lycaenid butterfly: analysis of an aperiodic multilayer structure, Bioinspiration Biomim. 8 (2013), 045001. [3] B.T. Tang, C. Wu, T. Lin, S. Zhang, Heat-resistant PMMA photonic crystal films with bright structural color, Dyes Pigments 99 (2013) 1022–1028. [4] B.T. Tang, X.X. Zheng, T. Lin, S. Zhang, Hydrophobic structural color films with bright color and tunable stop-bands, Dyes Pigments 104 (2014) 146–150. [5] M. Hok, C.T. Chan, C.M. Soukoulis, Existence of a photonic gap in periodic dielectric structure, Phys. Rev. Lett. 65 (1990) 3152–3155.
8
Contents lists available at ScienceDirect
Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
The effect of the voltage waveform on the microstructure and optical properties of porous anodic alumina photonic crystals Shiyuan Zhang a, Qin Xu a, *, Shunzhen Feng b, Chunxin Sun a, Qi Peng a, Tian Lan a a b
Department of Applied Physics, Hebei University of Technology, Tianjin, 300401, China Department of Physics and Electric Information Engineering, Shijiazhuang University, Shijiazhuang, 050035, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Porous anodic alumina Photonic crystal Structural colour
Porous anodic alumina photonic crystals (PAA-PhCs) with brilliant colours were successfully fabricated by pulse anodization of aluminium in an oxalic acid electrolyte. The influence of the voltage waveform on the micro structure of the PAA-PhCs was investigated by adjusting the anodization period, anodization amplitude, and anodization offset. Here, we report a simplified method for compensation of the photonic band gap (PBG) drift using AC waveforms instead of the periodic length-controlled method. Scanning electron microscopy (SEM) results showed that the layer thickness of the PAA-PhC prepared by an AC pulse was gradually reduced from 360 nm to 290 nm, which was completely different from a previous report showing that the layer thickness of a sample prepared by a DC pulse remained unchanged. In addition, the effect of chemical corrosion in a phosphoric acid solution on the microstructures of the PAA-PhCs was analysed by comparative experiments. PBG drift caused by minor defects in the nanostructure during the anodization process could also be compensated for by adjusting the anodizing time and corrosion time. Our approach may help to broaden the potential applications of and provide important guidance for photonic crystal display and new devices.
1. Introduction The bright colours of many substances and living organisms in nature are not produced by pigments but by the interaction of light with their nanostructures [1–4]. For example, photonic crystals (PhCs), such as opal and the scabbard wings of some beetles, are composed of nano structures with periodic dielectric coefficients, ε, and disallowed wavelength bands called photonic band gaps (PBGs) [5,6]. PhCs can present different colours when their PBGs are in the visible region of the spectrum. It is widely known that one-dimensional (1D) layered PAA-PhCs can be fabricated by applying a periodic voltage or current to aluminium foil. Then, by chemical etching, nanochannels and nano pores can connect to each other to form a three-dimensional (3D) PAA-PhC structure, and the PBG can be adjusted concurrently [7]. Wang et al. prepared a series of 3D PAA-PhCs showing different structural colours by periodic voltage anodization in an H2C2O4 solution and corrosion in a H3PO4 solution [8,9]. Guo et al. prepared a PAA-PhC with a complete PBG by utilizing a periodically controlled current method [10]. By applying a fixed-period waveform, a PAA-PhC was prepared that had a periodic layered structure in which each layer had the same thickness d [11,12]. During the electrolytic oxidation process, the
corrosion effect of the electrolyte on the PAA was not uniform from top to bottom, which led to the porosity decreasing layer by layer; that is, the effective refractive index neff corresponding to each layer gradually increased [13], according to the Maxwell-Garnett theory. Cumulatively, a redshift in the upper layer band will cause the PBG to drift or widen. To compensate for the above defects, Liu et al. reported a strategy to obtain a complete PBG by linearly reducing the periodic pulse width of the current during the anodization process, which could keep neff con stant by successively reducing the d layer by layer [13]. The sample showed a brilliant colour directly on the aluminium substrate. However, the controllable current source device was very expensive and required more than one instrument to be used simultaneously, which was not easy to operate. Considering that a reverse voltage can accelerate the corrosion of the barrier layer by the electrolyte and help to increase the pore size, in this experiment, we attempt to use an AC pulse, that is, to use the reverse voltage to decrease the PBG drift. Herein, detailed studies of the effect of different voltage waveforms (DC or AC, and si nusoidal or sawtooth pulses) on the PAA-PhC nanostructures and optical properties are reported.
* Corresponding author. E-mail address: [email protected] (Q. Xu). https://doi.org/10.1016/j.optmat.2019.109488 Received 24 August 2019; Received in revised form 12 October 2019; Accepted 27 October 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Shiyuan Zhang, Optical Materials, https://doi.org/10.1016/j.optmat.2019.109488
S. Zhang et al.
Optical Materials xxx (xxxx) xxx
Fig. 1. Anodization profiles and UV–Vis transmission spectra of PAA-PhCs, anodization period T ¼ 10 min; (a) DC saw-tooth waves (5 V–42 V); and (b) AC saw-tooth waves ( 1.6 V–42 V).
2. Experimental
voltage reverses, and the chemical equations are as follows,
PAA-PhCs were prepared by a process similar to that reported pre viously [14]. Pulse anodization was first performed using a function signal generator (Rigol DG-4062) to generate a series of periodic oscil latory voltage signals, with different profiles (saw-tooth and sinusoidal), amplitudes, and periods. The signals were then amplified using a voltage amplifier (Pintek HA-305). The PAA-PhCs were prepared in a 0.3 M oxalic acid solution at room temperature. The aluminium substrates on the back of the PAA-PhCs were removed in a saturated CuCl2 solution. Finally, the PAA-PhCs were characterized by field-emission scanning electron microscopy (FE-SEM, Hitachi S-570). The current density dur ing the pulse voltage anodizing was recorded with a computer-controlled Keithley 2450 Sourcemeter. The transmission spectra were recorded on a UV–Vis spectrophotometer (UV–Vis, Hitachi UV-3010).
Al2 O3 þ 6H þ ¼ 2Al3þ þ 3H2 O
(1)
2H þ þ 2e ¼ H2 ↑
(2)
The reverse voltage method is usually used to thin or remove the PAA template barrier layer [18]. Here, reversing the voltage throughout the anodization process accelerates the corrosion of the PAA pores and the barrier layer, and leads to an increase in the pore diameter, such that the porosity difference between the periodic layers is greater. Wang et al. [18] reported that the greater the difference in ε between the periodic layers, the easier it is to achieve a complete PBG, and the greater the porosity difference between the corresponding layers. Therefore, PAA-PhCs were prepared by using an AC pulse anodization process. 3.1. Effect of a saw-tooth waveform on the nanostructure and optical properties of PAA-PhCs
3. Results and discussion
For comparison, two samples were prepared by the DC and AC sawtooth pulse anodization approaches. In our experiments, the PAA-PhCs were produced by means of saw-tooth waves with an anodization period of Tp ¼ 10 min (tup:tdown ¼ 4:1) and an anodization time of S ¼ 24 h. The anodization profiles and UV–Vis transmission spectra of
We present here a new anodization process, termed AC pulse anod ization. Unlike conventional DC pulse anodization processes, in which high-purity aluminium sheets are anodized as the anode [15–17], for AC pulse anodization, the aluminium sheet changes to the cathode as the 2
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Fig. 2. SEM images of the PAA-PhC prepared by the AC saw-tooth pulse anodization process ( 1.6 V–42 V), all images are of the same film measured at different positions. (a) top cross section; (b) middle cross section; (c) bottom cross section; (d) surface; and (e) back side.
the PAA-PhCs are shown in Fig. 1. Fig. 1a shows that the spectrum has a peak at approximately 512 nm and a weak transmission region from 548 nm to 649 nm, which clearly represents a relatively large PBG drift. In Fig. 1b, the spectrum shows an obvious transmission peak at approximately 445 nm, in good agreement with the colour seen in the inset of Fig. 1b. The PBG drift of the PAA-PhC formed by AC pulse anodization had been improved to a certain extent. To investigate the influence of the reverse voltage on the nano structures of the PAA-PhCs, characteristic SEM images of the sample fabricated by the AC saw-tooth pulse anodization process ( 1.6 V–42 V) are shown in Fig. 2. These images clearly shows that the period length d of the PAA-PhC decreases from 360 nm to 290 nm, which is different from the results of the same period length d for the sample formed by the DC saw-tooth pulse anodization process [13]. This indicates that the reason for the reverse voltage improving the PBG drift can be attributed mainly to a decrease of the period length d. It is worth mentioning that the effect of the reverse voltage is similar to that of the period length control method [13], as we have previously mentioned. The reason for the decrease of the periodic thickness d of each layer in the above samples was analysed as indicated below. We believe that the oxide layer is thinner and has a smaller resistance at the initial stage of the electrolytic reaction; thus, the generated alumina layer has a larger periodic thickness; when the voltage is reversed, the reverse current is larger and the dissolution effect is larger. Thus, the porosity between the main channels (low pore density layers) and the branch channels (high pore density layers) varies greatly, which corresponds to the upper portion of the oxide layer. As the reaction progresses, the oxide layer gradually thickens, and the reverse current caused by the reverse voltage gradually decreases. At this time, the dissolution effect decreases and corresponds to the middle of the oxide layer. In the later stage of the electrolytic reaction, when the oxide layer reaches a certain thickness, it has a higher resistance, thus the generated alumina layer has a smaller periodic thickness. Meanwhile, the reverse current is approximately zero due to the small reverse voltage value and high resistance, so the reverse voltage portion of the AC waveform has almost no effect, and the oxide layer is completely generated by the direct voltage. Moreover, the electrolyte is weakly corrosive to the lower layer, thus, the porosity between the main channels and the branch channels varies slightly, which corresponds to the lower portion of the oxide layer. Since the SEM microstructures of the upper, middle and lower regions of the same AC pulsed sample are slightly different, we take the middle region of the sample for scanning. To study the effect of the anodization period T on the nanostructures of PAA-PhCs prepared by the AC saw-tooth pulse anodization process
( 1.6 V–42 V) [19], the anodization period T was varied from 8 min to 15 min. Fig. 3 shows the anodization profiles and the corresponding UV–Vis transmission spectra of the PAA-PhCs. The optical photographs of the PAA-PhCs are shown in the insets of Fig. 3. The top photograph is the as-prepared sample, and the corresponding bottom photograph is the sample after etching in a 6 wt% phosphoric acid solution at 35 � C. As shown in Fig. 3a, before corrosion, the sample prepared with a voltage period of 8 min has high overall transmission, and three peaks appeared in the red region from 610 nm to 690 nm. After corrosion, the total transmission of the sample decreases slightly, and the blueshift of the reflection band is in the region from 550 nm to 670 nm. In addition, the transmission is approximately 20% in the orange band from approximately 606 nm–621 nm. In Fig. 3b, before corrosion, the sample prepared with a voltage period of 10 min has a first reflection peak at approximately 445 nm with a low transmission of 20% and two smaller peaks at approximately 502 nm and 529 nm with transmissions of approximately 50%. In addition, the overall transmission is high, and the sample appears blue. After corrosion, the overall transmission of the sample declines, and the first peak had a large blueshift to the region between 411 nm and 427 nm with a lower transmission of approxi mately 15%; furthermore, the other two smaller peaks become less obvious, and the sample appears indigo. In Fig. 3c, before corrosion, the sample prepared with a voltage period of 15 min has two high-intensity reflection bands. The red band has two peaks at 723 nm and 733 nm, and the blue band has one peak at 489 nm; therefore, the sample appears to have a mixed colour, cyan. After corrosion, the transmission decreases in general, and there is a blueshift of the two transmission bands. The red band has two new peaks, one at 635 nm and the other between 672 nm and 684 nm. The blue band has one new peak at 439 nm and another peak at 461 nm; thus, the sample appears to have a mixed colour, magenta. In summary, we find that, for the non-corroded series of samples, the main peak has a blue shift as the voltage period increases from 8 min to 10 min. This is mainly caused by the reduction of average layer thickness resulted from the sharp increase of the corresponding reverse current period. When the voltage period is increased to 15 min, the transmission peak shifts slightly, and the sample shows two transmission peaks in the red and blue-violet regions of the visible wavelength range. We specu late that the positive and negative currents reach a critical equilibrium state during aluminum anodizing, leading to the change of PAA-PhCs structure. After corrosion in phosphoric acid, the transmission of all the samples shows a decreasing trend. Generally, the thicker the PAA grown in the electrolyte, the greater the deviation of the effective refractive index neff between the upper and 3
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Fig. 3. Anodization profiles and UV–Vis transmission spectra of the PAA-PhCs prepared with different anodization periods. (a) 8 min; (b) 10 min; and (c) 15 min.
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Fig. 4. (a) AC sawtooth voltage wave ( 0.2 V–42 V), total electrolysis time S ¼ 8 h, etched for 30 min. (b) SEM characterization image. (c) UV–Vis transmission spectrum of the sample.
Fig. 5. (a) AC sinusoidal voltage wave ( 15 V–42 V), T ¼ 5 min, S ¼ 12 h. (b) UV–Vis transmission spectra after etching for 0 min, 10 min, and 20 min.
Fig. 6. SEM characterization image (etched for 20 min). 5
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Fig. 7. Pulse anodization profile of PAA-PhCs prepared by the AC sinusoidal pulse anodization process ( 15 V–42 V).
lower layers is, in addition to the larger shift of the band gap. Therefore, it is necessary to reduce the time of anodization to reduce the number of periodic layers in the PhCs, which can compensate for the PBG drift. Based on this, we used the AC sawtooth wave shown in Fig. 4a, short ened the anodization time to 8 h and etched the sample in phosphoric acid for 30 min. SEM characterization and the UV–Vis transmission spectrum were measured and are shown in Fig. 4b–c. After corrosion, the pore size increases, and the channel walls decrease and become slightly fractured, similar to discontinuous layer opening, as indicated by the arrows. In addition, the region between contiguous branches is corroded through, and a clear vertical pore (channel) appears as a 3D-shaped PAA-PhC, as indicated by the dotted lines. In addition, the UV–Vis transmission curve shows a single peak with a transmission rate of 13%, and the half-peak width was approxi mately 30 nm, which shows that for the sample prepared with an AC, the decrease in the top-down thickness caused by the reverse voltage and the increase in the effective refractive index neff caused by uneven corrosion of the electrolyte offset each other, thus effectively decreasing the PBG drift.
approximately 32% in the red-orange band from 615 nm to 635 nm. The sample is yellow-green. To summarize, with increasing corrosion time, the high-intensity reflection band of the samples appears to blueshift and the trans mission decreases, similar to the transmission change of the samples prepared by the AC sawtooth wave. A reasonable explanation is given below. The surface and cross section of the sample (etched for 20 min) were characterized by SEM and are shown in Fig. 6. We found that irregular fragmentation occurs on the sample surface and top layer after longterm corrosion. As indicated by the arrows and dashed boxes in Fig. 6, fractured segments cracked and separated from each other. The average pore size increases, and the transverse channels show a 3D-networked structure [21], especially near the top layer. Through analysis of these images, we believe that the scattered surface and the irregular internal fragmented structure strongly scatter the incident light, which results in a decrease in the average transmission of the sample. In addition, the scattered structure increases the PBG drift; thus, the reflection bands in the spectra (Fig. 5) are wider and gradually split. It is known that the growth of anodic film is decided by current density. Pulse anodization profile of PAA-PhCs prepared in the first 20 cycles is shown in Fig. 7. It can be seen that the average reverse current density in each period decreases with time during aluminum anodizing [22]. This is well consistent with the above analysis in section 3.1. In summary, chemical corrosion can effectively regulate the position of the PBG within a certain range. Generally, the time of anodization and chemical corrosion should be strictly controlled. Using AC pulse method can avoid the traditional acid etching process, and obtain PAA-PhCs with photonic band gaps directly. While the samples prepared with a DC pulse, the effective refractive index neff of a PAA-PhC can be adjusted by means of phosphoric acid corrosion. Based on this, we utilized two sets of DC sinusoidal waves to prepare samples, as shown in Fig. 8. Contrasting the two samples in Fig. 8, Sample (b) achieves thinner layers by utilizing a shorter voltage period. In addition, after phosphoric acid corrosion, Sample (b) has larger pores and channels in the vertical direction (dotted lines), which appear in the 3D structure. Fracturing easily occurs in the vicinity of the through channels, with disconnection (arrowheads) between the layers. As shown by the UV–Vis transmission spectra, for Sample (a), there are two obvious transmission peak bands
3.2. The effect of a sinusoidal waveform on the nanostructures and optical properties of PAA-PhCs A sinusoidal voltage wave, for which the parameters are illustrated in Fig. 5a, was constructed to further verify the above conclusions [20]. UV–Vis transmission spectra were measured before and after etching in a 6 wt% phosphoric acid solution at 35 � C and are illustrated in Fig. 5b. As shown in Fig. 5, light in the red band from 650 nm to 700 nm for the uncorroded sample was reflected, with a transmission at 666 nm of approximately 20%, resulting in a red colour and having a small PBG drift. After corroding for 10 min, a blueshift in the spectrum occurs, and the average transmission decreases. Additionally, the high-intensity reflection band, centred around the orange region at 590 nm–650 nm, broadens and has a transmission of approximately 33% (610 nm). After corroding for 20 min, the spectrum continues to blueshift and the average transmission continues to decrease. The high-intensity reflec tion band broadens and splits into two bands: one peak at 570 nm with a transmission of approximately 20% over the yellow-green band from 525 nm to 580 nm, and the other peak at 630 nm with a transmission of 6
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Fig. 8. DC sinusoidal voltage waves and UV–Vis transmission spectra. (a) 5 V–42 V, T ¼ 10 min, S ¼ 24 h, etched for 0 min; and (b) 20 V–50 V, T ¼ 5 min, S ¼ 2 h, etched for 15 min.
in the visible light region, located near 505 nm and 734 nm, which in dicates that the PBG formed by the sample was not complete. For Sample (b), only one extreme peak appears located at 456 nm with a narrow half-peak width, while the transmission of the other wavelengths is greater than 80%, which shows that the PBG structure of the sample is relatively complete. In addition, due to the longer anodization time, the overall transmission of Sample (a) is much lower than that of Sample (b).
4. Conclusions In conclusion, SEM characterization shows that the layer thickness of PAA-PhCs prepared with an AC pulse decreases gradually, which helps to decrease the PBG drift to a certain extent. Different from the previous reports, using AC pulse method can avoid the traditional acid etching process. So the AC pulse method can be used to directly obtain PAA-PhCs with photonic band gaps. Additionally, controlling the anodization time 7
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and corrosion time can further solve the problem. In addition, after corrosion in phosphoric acid, the average transmission of all samples shows a downward trend. Though analysis, we believe that this might be caused by channel fracturing and separation, which are due to the excessive corrosion.
[6] W. Lee, A. Chan, M.A. Bevan, J.A. Lewis, P.V. Braun, Nanoparticle-mediated epitaxial assembly of colloidal crystals on patterned substrates, Langmuir 20 (2004) 5262–5270. [7] E. Yablonovitch, Photonic band-gap structures, J. Opt. Soc. Am. B: Opt. Phys. 10 (1993) 283–285. [8] B. Wang, G.T. Fei, M. Wang, M.G. Kong, L.D. Zhang, Preparation of photonic crystals made of air pores in anodic alumina, Nanotechnology 18 (2007), 365601. [9] G.L. Shang, G.T. Fei, Y. Zhang, Y. Peng, Fano resonance in anodic aluminum oxide based photonic crystals, Sci. Rep. 4 (2014), 3601. [10] D.L. Guo, L.X. Fan, F.H. Wang, S.Y. Huang, Porous anodic aluminum oxide bragg stacks as chemical sensors, J. Phys. Chem. C 112 (2008) 17952–17956. [11] Y. Chen, A. Santos, D. Ho, Y. Wang, T. Kumeria, J. Li, C. Wang, D. Losic, On the generation of interferometric colors in high purity and technical grade aluminum: an alternative green process for metal finishing industry, Electrochim. Acta 174 (2015) 672–681. [12] Y. Chen, A. Santos, Y. Wang, T. Kumeria, Interferometric nanoporous anodic alumina photonic coatings for optical sensing, Nanoscale 7 (2015) 7770–7779. [13] Y. Liu, Y. Chang, Z. Ling, X. Hu, Y. Li, Structural coloring of aluminum, Electrochem. Commun. 13 (2011) 1336–1339. [14] Q. Xu, H.M. Ma, Y.J. Zhang, R.S. Li, H.Y. Sun, Synthesis of iridescent Ni-containing anodic aluminum oxide films by anodization in oxalic acid, Opt. Mater. 52 (2016) 107–110. [15] W. Lee, R. Scholz, U. G€ osele, A continuous process for structurally well-defined Al2O3 nanotubes based on pulse anodization of aluminium, Nano Lett. 8 (2008) 2155–2160. [16] P. Yan, G.T. Fei, G.L. Shang, B. Wu, Fabrication of one-dimensional alumina photonic crystals with a narrow band gap and their application to high-sensitivity sensors, J. Mater. Chem. C 1 (2013) 1659–1664. [17] Z.Y. Ling, S.S. Cheng, X. Hu, Y. Li, Optical transmission spectra of anodic aluminum oxide membranes with a dual layer-by-layer structure, Chin. Phys. Lett. 26 (2009) 133–135. [18] J.B. Wang, X.Z. Zhou, Q.F. Liu, D.S. Xue, Magnetic texture in iron nanowire arrays, Nanotechnology 15 (2004) 485–489. [19] W. Lee, J.C. Kim, Highly ordered porous alumina with tailor-made pore structures fabricated by pulse anodization, Nanotechnology 21 (2010), 485304. [20] A. Santos, J.H. Yoo, C.V. Rohatgi, T. Kumeria, Realisation and advanced engineering of true optical rugate filters based on nanoporous anodic alumina by sinusoidal pulse anodisation, Nanoscale 8 (2015) 1360–1373. [21] J. Martín, M. Martíngonz� alez, J.F. Fern� andez, O. Caballerocalero, Ordered threedimensional interconnected nanoarchitectures in anodic porous alumina, Nat. Commun. 5 (2014) 5130. [22] X. Hu, Z.Y. Ling, X.H. He, S.S. Chen, Controlling transmission spectra of photonic crystals under electrochemical oxidization of aluminum, J. Electrochem. Soc. 156 (2009) C176–C179.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work is supported by the National Natural Science Foundation of China, China (Grant No. 51702083); the Youth Talent Support Pro gram of Hebei Province, China; the Natural Science Foundation of Hebei Province, China (Grant Nos. A2015202343 and A2019202190); the Research Project of Hebei Science and Technology Department, China (Grant No. 15211043), and Student’s Platform for Innovation and Entrepreneurship Training Program of Hebei Province, China (Grant No. 201710080288). References [1] J.P. Vigneron, J.-F. Colomer, M. Rassart, A.L. Ingram, V. Lousse, Structural origin of the colored reflections from the black-billed magpie feathers, Phys. Rev. E. 73 (2006), 021914. [2] S. Yoshioka, Y. Shimizu, S. Kinoshita, B. Matsuhana, Structural color of a lycaenid butterfly: analysis of an aperiodic multilayer structure, Bioinspiration Biomim. 8 (2013), 045001. [3] B.T. Tang, C. Wu, T. Lin, S. Zhang, Heat-resistant PMMA photonic crystal films with bright structural color, Dyes Pigments 99 (2013) 1022–1028. [4] B.T. Tang, X.X. Zheng, T. Lin, S. Zhang, Hydrophobic structural color films with bright color and tunable stop-bands, Dyes Pigments 104 (2014) 146–150. [5] M. Hok, C.T. Chan, C.M. Soukoulis, Existence of a photonic gap in periodic dielectric structure, Phys. Rev. Lett. 65 (1990) 3152–3155.
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