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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http://mail%20to:%[email protected]/



2. Experimental

Anodic oxide films were formed on an aluminum

sample of dimensions 25 mmÂ10 mmÂ 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.

428 S. Stojadinovic et al. / Electrochemistry Communications 6 (2004) 427–431



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.

S. Stojadinovic et al. / Electrochemistry Communications 6 (2004) 427–431 429



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.

430 S. Stojadinovic et al. / Electrochemistry Communications 6 (2004) 427–431



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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Galvanoluminescence of Oxide