galvanoluminescence of oxide
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Galvanoluminescence of oxide films formed byanodization of aluminum in phosphoric acid
S. Stojadinovic *, Lj. Zekovic, I. Belca, B. Kasalica
Faculty of Physics, Belgrade University, Studentski trg 12-16, 11000 Belgrade, Serbia and Montenegro
Received 13 February 2004; received in revised form 24 February 2004; accepted 25 February 2004
Published online:
Abstract
The presented results of our galvanoluminescence measurements of porous oxide films formed by anodization of aluminum in
phosphoric acid show strong influence of the surface pretreatment of samples and concentration of impurities on galvanolumi-
nescence intensity. We have also presented the influence of anodic conditions (current density, temperature of electrolyte and
electrolyte concentration) on GL intensity. For the first time, spectral measurements of galvanoluminescence for early stage of film
growth (barrier type of films) as well as for thick porous film are performed. We have obtained identical spectra with two spectral
peaks at about 425 and 595 nm.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Galvanoluminescence; Porous anodic films; Phosphoric acid; Spectra
1. Introduction
The galvanoluminescence (GL) is a common name for
light appearing at one of the electrodes in an electrolytic
solution during anodization. This phenomenon has been
known since as early as 1898, when it was discovered by
Braun [1]. It has been investigated by many authors but
explanations of some aspects of this phenomenon are
still incomplete. The interest in GL has taken a new di-
mension in recent years in light of new applications of
perfectly arrayed pores of porous alumina for templates
in nanotechnology [2,3], as photonic crystals, etc. [4,5].
These arrays, with well determined parameters (pore
diameter, interpore distance, etc.), can be obtained by
electrolytic process in various electrolytes.
From the papers of many authors, we can conclude
that the nature of galvanoluminescence is not the same
in organic and inorganic electrolytes. Shimizu and Taj-
ima [6] attributed GL to impact excitation of lumines-
cent centra (LC). They believe that in the case of organic
electrolytes carboxylate ions act as luminescent centra.
In another paper [7], GL in inorganic electrolytes is
correlated to the existence of ‘‘flaws’’ in the oxide film,
generated by impurities from the surface. There are ar-
ticles showing that GL is generated by selected impuri-
ties (activators) (Mn, Eu, etc.) [8,9]. In spite of
numerous articles about GL in various electrolytes (both
inorganic and organic), there is a lack of data on GL
spectra obtained in completely defined sample pre-
treatment and anodizing conditions (annealing temper-
ature, surface cleaning, electrolyte temperature and
concentration, anodizing voltage and current density).
The same applies for GL from oxide films obtained by
aluminum anodization in phosphoric acid. There are
only few articles considering the GL spectral intensity in
this electrolyte [8,9]. Therefore, the aim of this work is to
present the results of our spectral GL measurements
during the aluminum anodization in H3PO4 in strictly
defined experimental conditions as well as the influence
of pretreatment and anodization parameters on GL.
* Corresponding author. Tel.: +381-11-630152; fax: +381-11-
3282619.
E-mail address: [email protected] (S. Stojadinovic).
1388-2481/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.elecom.2004.02.016
Electrochemistry Communications 6 (2004) 427–431
www.elsevier.com/locate/elecom
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2. Experimental
Anodic oxide films were formed on an aluminum
sample of dimensions 25 mmÂ10 mm 0.12 mm. The
aluminum anodization was carried out in a vessel with
flat glass windows. The electrolyte was thermostated at a
certain temperature. The electrolyte circulated throughthe chamber–reservoir system, and the control temper-
ature sensor was situated immediately by the sample.
The optical and detection system consisted of a large-
aperture achromatic lens, an optical monochromator of
a rather high luminosity (Zeiss SPM-2) and a very sen-
sitive cooled (at approximately )40 °C) photomultiplier
(RCA J1034 CA). The optical-detection system was
calibrated using a standard tungsten strip lamp (Osram
Wi – 17G). The intensity of galvanoluminescence, the
voltage of anodization and the temperature of electro-
lyte were recorded by 20-channel digital PC-controlled
multimeter HP 34970A.
Two types of aluminum samples were used: high
purity (99.999%) aluminum samples (sample A) and
99.99% purity aluminum samples (sample B). The alu-
minum samples were annealed for five hours at various
temperatures (150, 250, 350 and 450 °C) and then slowly
cooled. The surface of aluminum was prepared for an-
odization in three ways: (a) electropolished in HClO4/
C2H5OH solution (1:4 by volume) following the proce-
dure given by Tajima et al. [10], rinsed with ethanol and
dried; (b) chemically cleaned in the bath consisting of 20
g/l chromium trioxide and 35 ml/l concentrated phos-
phoric acid at 80 °C for 5 min followed by rinsing in
distilled water and dried; (c) just degreased in ethanol byusing ultrasonic cleaner.
For anodization of aluminum samples we have used
0.1 M water solution of H3PO4. The electrolyte was
prepared by use of double distillated, deionized water
and PA grade phosphoric acid ( J.T. Baker production).
Anodizing was carried out at different current densities
in the 2.5–10 mA/cm2 range and different temperatures
in the 10–30 °C range. The temperature of the electrolyte
was maintained during the anodization to within 0.1 °C.
All GL measurements were carried out at 560 nm
wavelength. GL spectra were recorded in an early stage
of anodization and in a steady state regime for thick
porous film by using two recording methods. In the
early stage of anodization, only a single datum was ta-
ken per sample corresponding to the maximum of GL
intensity for a certain wavelength. The recording meth-
od in the steady state anodizing regime was explained in
our previous article [11].
3. Results
Fig. 1 shows typical voltage vs. time and GL intensity
vs. time characteristics for two types of samples (A and
B) during anodization in 0.1 M phosphoric acid. Even
single measurement for both samples indicates a great
difference in GL intensity. Both samples generate much
higher GL intensity in early stage of anodization (for-
mation of barrier layer of film). In steady state regime
(thick porous film), GL almost completely disappears
for the sample A, while for the sample B, GL reaches a
constant value (following anodization voltage). Fur-
thermore, maximum of GL intensity for the sample A is
shifted comparing to voltage maximum which corre-
sponds to the beginning of pore formation [12]. This
shift is negligible for the sample B.
We assume that in this type of electrolyte, ‘‘flaws’’ in
oxide film are responsible for GL. ‘‘Flaws’’ is general
term for microfissures, cracks, local regions of different
compositions and impurities, etc. [13]. Differences in GL
results for the A and B samples, point to impurities both
surface and internal as a possible main source of lumi-
nescence. We understand internal impurities as impuri-
ties present in the metal and their concentration is
governed by aluminum purity (declared by manufac-
turer). But, we think that the surface impurities are
impressed in samples’ surface during the rolling process
in the manufacturer’s factory and that their concentra-
0 200 400 6000
50
100
150
2000.1M H
3PO
4
j=5mA/cm2
tel
=21oC
GL i n t en s i t
y [ ar b . uni t ]
v o l t a g
e [ V ]
time [s]
0.0
0.5
1.0
1.5
GL(t)
U(t)
0 100 200 300 400 500 6000
50
100
150
200 GL i n t e
n s i t y [ ar b . uni t ]
v o l t a g e [ V
]
time [s]
0
1
2
3
4
GL(t)
U(t)
0.1M H3PO
4
j=5mA/cm2
tel
=21oC
(A)
(B)
Fig. 1. Voltage–time and GL–time dependence for sample A and
sample B.
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tion is independent of the metal purity. This assumption
is supported by further results.
3.1. Influence of surface pretreatment
We found that surface pretreatment has a significant
role in GL intensity during anodization in phosphoricacid primarily for high purity aluminum. Fig. 2 shows
the effect of surface pretreatment on both A and B
samples. Degreased samples produce the highest GL
intensity, chemically cleaned lower and electropolished
samples the lowest GL intensity. This result is similar to
the results that Shimizu [7] obtained in ammonium bo-
rate, also inorganic electrolyte, and it also indicates that
‘‘flaws’’ are a possible source of GL. As there is mutual
relation between the surface state and the concentration
of ‘‘flaws’’ in the oxide film [13], we can attribute the
absence of GL in A type electropolished samples to
completely removed surface impurities. The maintaining
of GL for thick porous film in B type aluminum can be
attributed to a ‘‘high’’ concentration of impurities (in
the sense of ‘‘luminescence purity’’) [8].
Another pretreatment factor that affects GL intensity
is the temperature of annealing. This influence is rep-
resented in Fig. 3. The higher GL intensity is obtained
for higher temperature of annealing. This statement isvalid for A type samples but there is almost no influence
on B type samples. We can also explain such a result
with the fact that concentration of ‘‘flaws’’ is governed
with the state of samples surface. Annealing at different
temperatures has different influence on the state of
samples surface, the number of defects, the crystal grains
and their orientation, in other words, on concentration
of flaws [14]. For type B samples, the concentration of
impurities inside of oxide layer is high enough and that
is why there is a low influence of annealing on the
concentration of ‘‘flaws’’.
3.2. Influence of anodization conditions
According to van Geel et al. [15], GL intensity is
proportional to current density for constant thickness
(thickness of a barrier part is determined by anodization
voltage) and that is in agreement with results in Fig. 4,0 100 200 300
0.0
0.5
1.0
1.5
2.0
3
2
1
0.1M H3PO
4
j=5mA/cm2
tel
=21oC
1 - electropolished
2 - chemically cleaned
3 - degreased
G L i n t e n s i t y [ a
r b . u n i t ]
time [s]
0 200 400 6000.0
0.5
1.0
1.5
2.0
2.5
3 2
1
0.1M H3PO
4
j=5mA/cm2
tel
=21oC
1 - electropolished
2 - chemically cleaned
3 - degreased
G L i n t e n s i t y [ a r b .
u n
i t s ]
time [s]
(A)
(B)
Fig. 2. Effect of the surface pretreatment on GL for sample A and
sample B.
0 50 100 1500.0
0.5
1.0
1.5
2.0
4
5
3
2
1
A 0.1M H3PO
4
j=5mA/cm2
tel
=19oC
1 - 450oC
2 - 350oC
3 - 250oC
4 - 150oC
5 - 25oC
G L i n t e n s i t y [ a r b . u n i t ]
voltage [V]
Fig. 3. Effect of temperature of annealing on GL for sample A.
0 50 100 150 2000
1
2
3
4
3
2
1
A
1 - j=10 mA/cm2
2 - j=5 mA/cm2
3 - j=2.5 mA/cm2
0.1 M H3PO
4
tel
= 19oC
G L i n t e n s i t y [ a r b . u n
i t ]
voltage [V]
Fig. 4. GL intensity for various current density (t el ¼ 19 °C) for sample
A.
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e.g., higher current density results in higher GL intensity
for the same voltage.
Our results show strong dependency of GL intensityupon electrolyte temperature. Lower temperature
(Fig. 5) results in higher GL intensity and this depen-
dence is the same as GL of porous oxide films in organic
electrolytes [16] and completely different from GL in
barrier oxide films in inorganic electrolytes [10].
There are very few articles about the influence of
electrolyte concentration on GL intensity. In our mea-
surements (Fig. 6), higher concentration of phosphoric
acid produces lower GL intensity.
It is not clear from numerous articles and contra-
dictory results how the temperature and electrolyte
concentration affect GL intensity while other anodiza-
tion parameters are fixed or controlled. This result
should be clarified in future experiments.
3.3. GL spectra
The results of our spectral measurements are shown
in Fig. 7. Two peaks, at about 425 and 595 nm, can be
resolved there. The spectrum taken in the early stage of
anodization (formation of barrier layer of films) has thesame shape as the spectrum recorded for thick porous
film. That implies the same mechanism of GL in both
stages of film growth and for both types of Al (A and B).
We can compare this spectra to Ganley’s [8], where two
peaks can be perceived, about 430 nm and about 600
nm. Relative ratio of those peaks vary with concentra-
tion of impurities and type of excitation. The problem
with Ganley’s results is that there is no data about
electrolyte temperature, sample pretreatment and spec-
tra are measured in constant voltage regime. On the
other hand, spectra obtained in H3PO4 differ from
spectra obtained in our previous measurement (two
peaks at 450 and 490 nm) in oxalic acid and that implies
there are two different mechanisms or two different types
of luminescent centra responsible for GL in these two
electrolytes. This is in agreement with Shimizu’s as-
sumption about different types of luminescence in or-
ganic and inorganic electrolytes [10].
4. Conclusion
We have presented the characterization of the GL
phenomenon during anodization of aluminum in H3PO4
0 50 100 150 2000.0
0.5
1.0
1.5
2.0
2.5
3
2
1A1 - 12
oC
2 - 19oC
3 - 25oC
0.1 M H3PO
4
j=5mA/cm2
G L i n t e n s
i t y [ a r b . u
n i t ]
voltage [V]
Fig. 5. GL intensity for various temperature of electrolyte ( j ¼ 5 mA/
cm2) for sample A.
0 50 100 150 2000
1
2
3
4
5 43
2
1
A j=5mA/cm2
tel
=14oC
1 - 0.05M
2 - 0.1M
3 - 0.25M4 - 0.5M
5 - 1M
G L i n t e n s i t y [ a r b .
u n i t ]
voltage [V]
Fig. 6. GL intensity for various concentration of electrolyte ( j ¼ 5 mA/
cm2, t el ¼ 14 °C) for sample A.
400 450 500 550 600 650 7000
20
40
60
80
100
0.1M H3PO
4
j=5 mA/cm2
tel
=20oC
R e l a t i v e G L
i n t e n s i t y [ % ]
wavelenght [nm]
400 450 500 550 600 650 7000
20
40
60
80
100
0.1M H3PO
4
j=5 mA/cm2
tel
=20oC
R e l a t i v e G L i n t e n s i t y [ % ]
wavelenght [nm]
(A)
(B)
Fig. 7. Galvanoluminescence spectra, normalized at 595 nm and cor-
rected for spectral sensitivity of the measurement system, for sample A
and sample B.
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for various pretreatment and anodizing conditions.
Comparative GL measurements of two types of Al (high
purity 99.999% and 99.99%) as well as the same spectra
(two peaks at 425 and 595 nm) for these two types of
measurements indicate the same source and mechanism
of GL. Higher intensity of GL in B type Al is result of
higher concentration of impurities in this sample. Theinfluence of the surface pretreatment of high purity
aluminum indicates surface generation of ‘‘flaws’’ which
are responsible for GL in this case. The concentration of
flaws is minimal for electropolished samples.
Higher GL intensity for higher current density is
in agreement with van Geel’s results and empirical
formula.
Higher electrolyte temperature and higher electrolyte
concentration result in lower GL intensity and this
phenomenon should be resolved by further research.
Acknowledgements
The authors would like to express their appreciation
to Serbian Ministry of Science and Technology for fi-
nancial support.
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