Synthesis and optical properties of iridescent porous anodic alumina thin films

Article abstract of DOI:10.1149/2.016201jes

Highly ordered porous anodic alumina thin films with tunable structural colors covering the entire visible region have been fabricated using chemical etching. The relationship of the structural color to the anodization time and etching time in a phosphoric acid solution, as well as the angle of incidence of illuminating light are discussed. Multicolor patterns were obtained by an organics-assisted process. A theoretical study of the changes in structural color due to various factors is consistent with the experimental results. These films show promise in many areas including decoration, display and anti-counterfeiting applications.

Full text of DOI:10.1149/2.016201jes

Journal of The Electrochemical Society, 159 (1) C25-C28 (2012)
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013-4651/2012/159(1)/C25/4/$28.00 © The Electrochemical Society
Synthesis and Optical Properties of Iridescent Porous Anodic
Alumina Thin Films
z
Qin Xu, Yu-Hua Yang, Li-Hu Liu, Jian-Jun Gu, Jing-Jin Liu, Zi-Yue Li, and Hui-Yuan Sun
College of Physics Science & Information Engineering, Hebei Normal University, and Key Laboratory of Advanced
Films of Hebei Province, Shijiazhuang 050016, China
Highly ordered porous anodic alumina thin films with tunable structural colors covering the entire visible region have been fabricated
using chemical etching. The relationship of the structural color to the anodization time and etching time in a phosphoric acid solution,
as well as the angle of incidence of illuminating light are discussed. Multicolor patterns were obtained by an organics-assisted process.
A theoretical study of the changes in structural color due to various factors is consistent with the experimental results. These films
show promise in many areas including decoration, display and anti-counterfeiting applications.
©
2011 The Electrochemical Society. [DOI: 10.1149/2.016201jes] All rights reserved.
Manuscript submitted July 21, 2011; revised manuscript received September 6, 2011. Published December 6, 2011.
Color production in nature is usually based on either structural
color or pigmentation. Structural colors in insects and bird feathers
have long attracted the attention of biologists due to their many useful
characters including iridescence, high reflectivity and polarization. In
recent years, a large amount of experimental work, especially on the
microscopic and submicroscopic levels, has been undertaken to clarify
the relationship between the brilliant colors and the microstructures
1
–4
involved. These extensive studies have now progressed to the point
where there are numerous applications in many industrial fields such
as painting, automobiles, cosmetics, display technologies and textiles.
Recently, this research has revealed that even very simple nat-
ural structures can have surprising multiple functions,5,6 and many
Figure 1. FE-SEM images of a PAA thin film anodized for 100 s, after removal
of the Al substrate. (a) surface image and (b) cross section image.
7–12
researchers have attempted to replicate biological structures.
For
13
instance, Lezec et al. reported the fabrication of submicron, periodic
dimple arrays on reflective silicon surfaces that may find a wide range
of applications where low-reflectivity surfaces are required. Diggle
◦
0
.3 M oxalic acid at 5 C for 6 hours, the alumina film was chemically
removed by immersing it in a mixture of phosphoric acid (6 wt %) and
◦
14
chromic acid (4 wt %) at 40 C for 8 hours. Subsequently, a second an-
et al. have reported that a thin porous anodic alumina (PAA) thin
film supported on an Al substrate produces, on reflection, a bright color
in the visible light range due to the interference of the reflected light.
Such a surface with unique reflectance properties could be useful for
odization was conducted for a few minutes under the same conditions
as in the first step. Part of the sample was then etched in a saturated
CuCl solution that removed the remaining aluminum substrate and
2
15
left a thin alumina film.
decoration and for anti-counterfeiting. Zhao et al. constructed a PAA
film embedded with carbon nanotubes (CNTs@PAA) on an Al sub-
strate and found that infusion with water resulted in a significant color
change. Such films might be used as water sensors. Finally, Wang
The thin PAA films were characterized by field-emission scanning
electron microscopy (FE-SEM, Hitachi S-570), and by an optical digi-
tal camera (Sony Dsc-T20) with a rough black cloth as the background.
The morphology was observed using a scanning probe microscope
16
et al. reported that brilliant carbon-coated PAA thin films on an
Al substrate were useful not only for weather-resistant decorative pur-
poses, but also showed promise as effective broadband optical limiters
for nanosecond laser pulses.
(
SPM, Veeco Nanoscope IV). UV-Vis diffuse reflectance spectra were
recorded on a Hitachi UV-3010 spectrophotometer equipped with an
integrating sphere. BaSO was used as a reference.
4
Figure 1 shows FE-SEM images of the surface and cross section
of a typical PAA thin film formed after a second anodization for
It is worth noting that all the above films were supported on an
Al substrate, which limits the applications to some extent. Moreover,
since Al has high reflectivity for visible light, the Al substrate will
tend to lead to structural colors that are far from being saturated. In
this connection, CNTs@PAA composite films display brilliant colors
due to the fact that the embedded carbon efficiently screens the re-
flected light from the Al substrate. For many applications, however,
the inclusion of carbon nanotubes is a complexity to be avoided, and
it is desirable to discuss the structural colors of PAA thin films in the
absence of the Al substrate. However, to date, there have been no re-
ported attempts to produce highly saturated colors using films of this
type. In the present paper, we report the fabrication of PAA thin films
with brilliant structural colors by means of electrochemical oxidation
of Al and etching in phosphoric acid. We also discuss the dependence
of the optical properties on the parameters of the fabrication as well
as the manner of viewing the films.
1
00 s. (See Experimental section). As shown in Figure 1a, a highly
ordered nanopore array arranged in a close-packed hexagonal pattern
can be clearly observed. The ordered pore arrays have identical pore
diameters of about 25 nm, and an interpore spacing of approximately
1
25 nm. Figure 1b shows the cross section image from which the
thicknesses of the PAA thin film can be directly measured to be
about 200 nm. From these data, the pore growth rate of the PAA thin
film during anodization can be estimated to be about 2 nm s . The
PAA films discussed in this article were prepared by adjusting the
anodization time from 100 s to 160 s. The corresponding thicknesses
of the films are shown in Table I, where t is anodized time, d the
thickness of the film, and λ the reflected wavelength.
−
1
Table I. The character of PAA thin films.
In this paper, high-purity aluminum foils (99.999%) were annealed
◦
at 400 C for 2 hours in an argon atmosphere, and then were electropol-
t (s)
100
110
120
140
150
160
4 2 5
ished in a 1:4 (volume) mixture of HClO and C H OH for 5 minutes.
After a first anodization, carried out at a constant voltage of 45 V in
d (nm)
λ (nm)
200
419
220
461
240
502
280
586
300
628
320
670
z
E-mail: huiyuansun@126.com
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Journal of The Electrochemical Society, 159 (1) C25-C28 (2012)
Figure 2 shows the colors of the films anodized for times ranging
from 100 s to 160 s observed in natural light at nearly normal inci-
dence, illustrating how the color gradually changes from violet to red
as the anodization time is increased by 60 s. The structural color pre-
dominantly stems from the interference between light reflected from
the top and bottom surfaces of the film. For free standing films, the
phase of the light reflected from either the top or the lower surface of
the film undergoes an addition phase change corresponding to a path
difference of half a wavelength. Bragg’s equation can then be written:
Figure 2. Colors of PAA thin films anodized from 100 s to 160 s.
ꢀ
ꢁ
1
2
nd cos θ = m + 2
λ
[1]
where n is the refractive index of the film at a wavelength of λ, d the
film thickness, θ the refraction angle, m the order of the interference,
and λ the wavelength outside the film, respectively.
In our experiments, the effective refractive index was calculated
17
to be about 1.57 according to Maxwell-Garnett theory. The order of
the interference can be estimated through film thickness and the corre-
sponding maximum wavelength obtained by the experiment, here the
order of the interference was one. So we can calculate the maximum
reflected wavelength of the samples as shown in Table I, which is in
good accordance with the observed color.
UV-Vis reflection spectra of PAA thin films with different thick-
nesses, all viewed at nearly normal incidence, are shown in Figure 3. 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 I correspond well with the ex-
perimental results indicated with vertical arrows at long-wave band.
However, the calculated value is less than the experimental result at
1
6
Figure 3. UV-Vis reflectance spectra of PAA thin films with different thick-
nesses and anodization times between 100 s to 160 s. The films show interfer-
ence colors varying from purple to red.
short-wave band due to the increasing of the refractive index. For
the film anodized for 160s, the reflection spectra appears the second
peak besides the first peak at the visible range, and the wavelength of
the second order of the interference still obeys the Bragg’s equation.
The color not only changes with the anodization time, but also
varies with the angle of incidence of the light used to view the film
as shown in Figure 4a. It can be seen that the color of the PAA film
changes from green to violet as the angle of incidence changes from
◦
◦
0
to 40 . Modulation of the reflectance spectra depending on the
change of the incident angle is shown in Figure 4b. The maxima
calculated using Bragg’s equation are at 523 nm, 515 nm, 492 nm,
4
53 nm, and 401 nm, which is good agreement with the experimental
results indicated with vertical arrows. This demonstrates that the above
change is also consistent with Bragg’s equation.
The pore growth rate of the PAA in the anodization is very fast,
so it would be difficult to tune the color of the PAA thin film in a
well-controlled manner. In order to obtain precise color tuning of the
PAA thin films, it is necessary to reduce the thicknesses of the PAA
films slowly. It has been reported that controllable precise color tuning
of CNTs@PAA composite thin films can be achieved via phospho-
ric acid etching which not only reduces the film thickness, but also
15
increases the diameter of the pores. Likewise in our experiment PAA
thin films exhibiting different colors can be obtained by etching with
phosphoric acid at room temperature. The PAA films were floated on
a 6 wt % phosphoric acid aqueous solution surface to make sure that
only the back surface of PAA films came into contact with the solu-
tion. Photographs of PAA films anodized for 175 s but with different
phosphoric acid etching times from 0 to 90 min are shown in Figure 5.
Figure 4. Photographs (a) and UV-Vis reflectance spectra (b) of the PAA
Figure 5. Photographs of the PAA thin film anodized for 175 s with different
etching times from 0 to 90 min.
film anodized for 125 s as observed using white light incident from different
◦
◦
incident angles from 0 to 40 .
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Journal of The Electrochemical Society, 159 (1) C25-C28 (2012)
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Figure 6. For the PAA thin film anodized for 175 s, (a) back side image with
etching time of 90 min, and (b) cross section image with an etching time of
5 min.
Figure 8. The patterns of the PAA thin films deposited with Co exhibit dif-
ferent colors.
5
It can be seen that the color changes from violet to blue over this
time. It is important to note that initially, the color changes slowly, but
subsequently changes more quickly. The initial slow color change for
PAA films etched for shorter than 55 min can be attributed mainly to
overall thinning of the film, while the faster color changes observed
for etching times longer than 55 min is due to both thinning of the
film and a decrease in the refractive index due an increase in the pore
face of different color by a SPM, the results are shown in Figure 7d.
It illustrates the change of the thickness across two areas of different
color. These films have not been subjected to etching in phosphoric
acid.
It is known that PAA templates can be used for depositing other
metal material using electrochemical method. If the color still keeps
after depositing metal, this will make up for the shortage of fragile.
Therefore, we have fabricated Co@PAA nanocomposite films, and
found that such colored films can be obtained by changing the de-
posited condition. At present, we have obtained some initial results in
this area, as shown in Figure 8. In addition, such films have excellent
magnetic properties combined with color. This suggests that the above
nanocomposite materials may find application in multifunction anti-
counterfeiting, in which the technology not only employs the optical
properties, but also the functions of magnetic patterning. This tech-
nology would expand the range of potential applications of colored
nancomposite films. Further experiments are under way to investigate
nanocomposite magnetic materials.
In summary, the microstructure characteristics and optical prop-
erties of PAA thin films have been analyzed. The FE-SEM results
confirmed that the structure has an ordered hexagonal pattern. Pre-
cise color tuning of the PAA thin films could be achieved by etching
the films in phosphoric acid solution. It was found that the max-
ima of the reflected wavelengths in the UV-Vis reflectance spec-
tra were consistent with the observed colors and with theoretical
analysis.
15
diameters.
Further characterization of the above results was carried out by
FE-SEM imagery. From Figure 6a, it may be observed that the PAA
thin film which was etched for 90 min already presents an orderly
array of holes that traverse the entire thickness of the film. The av-
erage pore diameter has increased to 60 nm after 90 min of phos-
phoric acid etching. Moreover, the porosity increase gives rise to
a reduction in the refractive index.18 The cross section of a PAA
film anodized for 175 s after phosphoric acid etching for 55 min is
shown in Figure 6b, illustrating the extent to which the pore diame-
ter increases. The thickness of the film was approximately 295 nm,
showing that the rate of etching in the phosphoric acid solution was
−
1
about 1 nm min , which is much lower than the rate of growth during
anodization.
Such colored PAA thin films are useful for decoration and anti-
counterfeiting technology, since a wide variety of patterns can be
recorded on thin PAA films. A PAA thin film with a distinct pat-
tern formed by anodizing different areas for different times is shown
in the Figure 7a–7c, which clearly indicates that multicolor films
can be fabricated via anodizing different areas of the aluminum foil.
We obtained the pattern by covering part of the top surface of the
PAA thin film with an ink pen before removal of the Al substrate,
anodizing the uncovered part for a few seconds, and then repeat-
ing the above process, finally removing the ink with acetone. It can
be seen that the pattern presents different colors across the visible
spectrum. From the above discussion we know that different color
corresponds to different thickness. The thickness of covered ink part
is less than the uncovered part. Therefore, we characterized the inter-
Acknowledgments
This work is supported by the Natural Science Foundation of Hebei
Province (Grant No. A2009000254), and the Ph.D. fund from Hebei
Normal University (Grant No. L2006B10; No. L2009Y03). The au-
thors wish to thank Dr. Norm Davison for helpful discussion.
Figure 7. (a) Photograph of the PAA thin film with differently anodized areas, viewed in natural light. The anodization time was 10 s for each area after the purple
area anodized for 100 s. Patterns of a butterfly (b) and a flower (c). (d) Three dimensional topographic image of the barrier layer side of the part of Figure 7a in
the white rectangle area, exhibiting different thicknesses.
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Journal of The Electrochemical Society, 159 (1) C25-C28 (2012)
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