Electrochemical preparation of lead-doped amorphous Se films and underpotential deposition of lead onto these films

Article abstract of DOI:10.1016/S0039-6028(03)00218-8

The process of the underpotential deposition (UPD) of Pb adatoms (Pb_ad) onto Se was used to produce nanocomposite films consisting of amorphous Se and nanosized PbSe clusters distributed throughout the film bulk. It was found that doping lead into Se films modifies their optical and photoelectrochemical properties and increases the efficiency of the charge transfer both in the film bulk and through the semiconductor | electrolyte interface. Introducing lead into the bulk of Se films significantly promotes the process of Pb_ad UPD onto Se surface. The underpotentially deposited Pb_ad interact chemically with Se surface atoms, resulting in the formation of a PbSe monolayer. The PbSe formed can be identified by the anodic peak corresponding to its electrochemical oxidation.

Full text of DOI:10.1016/S0039-6028(03)00218-8

Surface Science 532–535 (2003) 1092–1097
www.elsevier.com/locate/susc
Electrochemical preparation of lead-doped amorphous Se
films and underpotential deposition of lead onto these films
a,
Dmitry K. Ivanov , Nikolay P. Osipovich , Sergey K. Poznyak ,
b
b
*
a
Eugene A. Streltsov
a
Chemical Department, Belarusian State University, Leningradskaya st. 14, Minsk 220050, Belarus
Institute of Physico-Chemical Problems, Belarusian State University, Leningradskaya st. 14, Minsk 220050, Belarus
b
Abstract
The process of the underpotential deposition (UPD) of Pb adatoms (Pbad) onto Se was used to produce nano-
composite films consisting of amorphous Se and nanosized PbSe clusters distributed throughout the film bulk. It was
found that doping lead into Se films modifies their optical and photoelectrochemical properties and increases the ef-
ficiency of the charge transfer both in the film bulk and through the semiconductor j electrolyte interface. Introducing
lead into the bulk of Se films significantly promotes the process of Pbad UPD onto Se surface. The underpotentially
deposited Pbad interact chemically with Se surface atoms, resulting in the formation of a PbSe monolayer. The PbSe
formed can be identified by the anodic peak corresponding to its electrochemical oxidation.
Ó 2003 Elsevier Science B.V. All rights reserved.
Keywords: Electrochemical methods; Chalcogens; Lead; Amorphous thin films; Adatoms; Semiconductor–electrolyte interfaces;
Semiconductor–semiconductor thin film structures
1
. Introduction
peculiarities of the electron properties of nano-
layers, which differ from those of bulk objects.
We have recently reported on the possibility of
the photoinduced UPD of Pbad onto polycrystal-
line Se electrodes with p-type conductivity [15]. In
the dark, the UPD of Pbad onto Se surface does
not occur because of the extended space charge
region (SCR) being formed, which hinders the
charge transport through the Se j electrolyte inter-
face. On illumination, electron–hole pairs photo-
generated in Se are separated by the field in the
SCR. Photoelectrones move towards the electrode
surface and reduce the Pb ions, resulting in the
photoinduced UPD of Pbad on selenium. The in-
teraction of the Pb adatoms with Se matrix can
result in the formation of PbSe clusters at the
surface. Once the Se surface has been modified by
The study of the underpotential deposition of
metals onto the surface of semiconductor elec-
trodes is of interest for electrodeposition of tex-
tured [1] and epitaxial films [2–8], electrosynthesis
of metal chalcogenides [9–12] and superlattice
structures [13,14]. In the past decade, an additional
interest in the underpotential deposition (UPD)
processes has been stimulated by the progress in
studying the physics and chemistry of nanosized
objects and has been associated with revealing the
2þ
*
Corresponding author. Tel.: +375-182-239400; fax: +375-
72-264-696.
E-mail address: elchem@bsu.by (D.K. Ivanov).
1
0
039-6028/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0039-6028(03)00218-8

D.K. Ivanov et al. / Surface Science 532–535 (2003) 1092–1097
1093
nanosized PbSe clusters promoting the charge
transfer through the semiconductor j electrolyte
interface, the UPD process becomes possible [15].
In the present work, amorphous Se films with
lead doped in their bulk owing to the UPD process
were prepared. Their structure as well as the op-
tical, electro- and photoelectrochemical properties
were studied, and the effect of lead doping on the
electrochemical activity of Se in the Pb UPD
process was examined and discussed.
window, a platinum counter-electrode and a sat-
urated AgjAgCljKClðsatÞ electrode as the reference
one (+0.201 V vs. SHE). All potentials were con-
trolled by a conventional potentiostat with a pro-
grammer and determined with respect to this
reference electrode. Photocurrent spectra were re-
corded using a set-up equipped with a high in-
tensity grating monochromator, a 1000 W Xe
lamp and a slowly rotating light chopper (0.3 Hz).
The photocurrent spectra were corrected for the
spectral intensity distribution at the monochro-
mator output.
2
. Experimental
Optical spectra were measured using a Specord
M40 UV-Vis spectrometer (Carl Zeiss, Jena).
AFM images were obtained with a FemtoScan-
2
.1. Film preparation
001 microscope (Advanced Technologies Center,
Amorphous selenium films and Se films doped
MGU, Moscow) operated in the constant force
mode (1.5–5 nN) under ambient conditions. X-ray
diffraction analysis of the films was performed on a
with lead were formed electrochemically on the
surface of Au foil electrodes. For optical mea-
surements, those films were deposited onto con-
ducting ITOfilms on a glass sheet. Amorphous Se
was deposited potentiostatically ()0.3 V) in elec-
a
HZG-4A diffractometer (Cu-K radiation, Ni fil-
ter). The Pb content in the films was determined
after the film dissolution by AES method using a
Spectroflame Modula atomic emission spectro-
meter.
All chemicals used were of analytical grade or
of the highest purity available. The solutions were
prepared using doubly distilled water.
trolyte containing 0.02 M SeO + 0.1 M HNO3
(
2
T ¼ 20 °C) for 30 min under illumination with a
À2
1
50 W halogen lamp (J ¼ 40 mW cm ).
Lead-doped Se films (Se(Pb)) were produced
potentiostatically in the dark in a similar solution
containing Pb(NO . The Pb/Se ratio in the films
was varied by changing the Pb concentration in
3 2
)
2þ
À5
À3
solution from 2 Â 10
to 2 Â 10
M and by
3. Results and discussion
changing the electrode potential from )0.1 to )0.3
V. The thickness of the Se and Se(Pb) films esti-
mated from their elemental analysis with allow-
ance made for the density of amorphous Se (4.28
g cm ) was found to be about 0.1 lm.
PbSe thin films used for comparison with the
3.1. Electrochemical formation of Pb-doped amor-
phous Se films
À3
During the cathodic deposition of Se films in
the dark under potentiostatic control (potentials
from 0 to )0.4 V were applied) in 0.02 M
Se(Pb) ones were deposited onto an Au electrode
)
from 1 M Pb(NO
3
2
+ 0.001 SeO
2
+ 0.1 M HNO
3
2 3
SeO + 0.1 M HNO solution, the current drops
solution under potentiostatic polarization at E ¼
À0:3 V for 30 min. These films are polycrystalline
and have the composition: 49 Æ 1 at.% Pb and
sharply with time (by 90% in 30 s). The film
growth virtually stops after 5 min deposition. This
effect is related to the fact that Se has a low p-type
conductivity and cuts off the cathodic current. The
thickness of the films deposited under those con-
ditions did not exceed 10 nm.
Adding Pb ions into the electrolyte eliminates
the problems associated with the current cut-off
and makes it possible to deposit films with the
thickness up to several micrometers. We carried
5
1 Æ 1 at.% Se.
2
.2. Apparatus and chemicals
2þ
Electrochemical and photoelectrochemical mea-
surements were carried out in a standard three-
electrode cell equipped with an optical quality

1094
D.K. Ivanov et al. / Surface Science 532–535 (2003) 1092–1097
out the film deposition at potentials more positive
2þ
than the Pb /Pb reversible potential. Under these
conditions, selenium is deposited with an over-
voltage in the regime of the limiting diffusion
current, and lead is co-deposited due to the UPD
process on Se [9]. The potential region from )0.1
to )0.3 V is suitable for the deposition of both Pb
adatoms and Se monolayers on the Au electrode
surface. It is important to note that early in the
process of Se(Pb) film growth, the Pb UPD occurs
both at the Au surface and at the Se monolayer
surface, because this process is possible on the Se
monolayer unlike the case of the bulk Se films [10].
Further co-deposition of Pb and Se proceeds at
the surface of the Pb-doped Se films.
Fig. 1. X-ray diffraction patterns for the Pb-doped Se films
1 at.% Pb (1), 8 at.% Pb (2) and 11 at.% Pb (3)) electrode-
posited at an Au substrate.
(
The Pb content in the deposited Se(Pb) films
varies in the range from 1 to 30 at.% and increases
2
þ
with Pb concentration in solution or when the
electrode potential is shifted to more negative
values.
3.2. Structure and morphology of Se and Se(Pb)
films
X-ray diffraction analysis showed that the
undoped Se film and Se(Pb) films with the Pb
content <5 at.% are amorphous. At higher Pb
content, XRD peaks of the PbSe phase appear
(
Fig. 1). These peaks are strongly broadened,
which is indicative of a high-disperse state of PbSe.
The average size of PbSe crystallites estimated
from the XRD peaksÕ half-width is 2–4 nm.
AFM images of the film surface demonstrate
that introducing lead into the Se film results in the
appearance of a distinct globular structure. The
specific surface area of the Se(Pb) films increases
with increasing the Pb content. The average size
of the globules for the Se(Pb) films (8 at.% Pb) is
Fig. 2. (a) Absorption (––) and photocurrent quantum yield
(- - -) spectra of the undoped selenium film (1, 3) and Pb-doped
Se film (1 at.% Pb (2, 4) and 3 at.% Pb) (5). Photocurrent
2
00–300 nm.
3
spectra were measured in 0.1 M HNO solution at a potential of
3.3. Optical, electro- and photoelectrochemical
properties of Se and Se(Pb) films
)0.2 V. (b) Long-wavelength edge of the photocurrent spectra
of the undoped (1) and Pb-doped (1 at.% Pb) (2) selenium films.
Fig. 2(a) shows spectral dependences of the
absorption coefficient (a) for the undoped Se films
and Se films doped with 1 and 3 at.% Pb. The
spectrum of Se has a wide region of the exponen-
tial rise of a in the photon energy range from 1.7 to
2.2 eV (so-called Urbach tail), which is character-
istic of amorphous semiconductors.

D.K. Ivanov et al. / Surface Science 532–535 (2003) 1092–1097
1095
Introducing lead into the Se films results in a
significant rise of the absorption in the long-
wavelength region of the spectrum, where amor-
phous Se absorbs negligibly. Furthermore, some
shift of the Urbach edge (by 0.1 eV) to lower en-
ergies is observed (Fig. 2(a)). Such changes in the
optical properties of Se films on the lead doping
can be due to a number of reasons. At low doping
levels, impurity atoms can cause an increase in the
localized electron state density at the tails of the Se
band edges, widen these tails into the band gap
and, respectively, shift the absorption edge to
lower energies. At high doping levels, the absorp-
tion of PbSe nanoparticles formed in the Se matrix
can contribute significantly to the long-wavelength
region of the spectrum.
bulk, which give rise to the recombination losses
and, respectively, to the photocurrent drop. The
fact that the spectrum of the photoelectrochemical
activity of the Pb-doped electrodes is widened up
to 1100 nm can be associated with inducing the
electron transfers from these intra-bandgap states
into the Se conduction band.
A sharp rise in the efficiency of the dark ca-
thodic and anodic processes observed for the
Se(Pb) electrodes in comparison to that for the Se
ones can be also explained by the appearance of
the intra-bandgap states, which efficiently ex-
change charge carriers with the valence band of Se.
Therefore, the dark cathodic current associated
þ
with the H reduction and the reduction of Se to
H Se appears at more positive potentials ()0.2 V)
2
The lead doping also has a pronounced effect on
the action spectra of the films. Fig. 2(a) presents
spectral dependences of the photocurrent quantum
yield (Y ) of the undoped and Pb-doped Se film
for the Se(Pb) electrodes as compared with that for
the Se ones ()0.3 V) (Fig. 3(a)). On the following
potential scan in the positive direction, an anodic
2
current related to the H Se oxidation is observed
electrodes recorded in 0.1 M HNO
3
solution. It
at the Se(Pb) electrodes and is absent at the Se
ones. The efficiency of the cathodic processes in
solutions containing different redox species, for
should be noted that the long-wavelength edge of
the photocurrent spectra is significantly shifted to
higher energies as compared with that of the ab-
sorption spectra. A similar effect has been reported
previously when comparing the absorption and
photoconductivity spectra of sputtered Se films
and assigned by a number of researchers [16,17] to
the exciton absorption at the long-wavelength
edge, which does not contribute to the action
spectra. Doping lead in the Se films results in a
significant decrease of the photocurrent quantum
yield in the wide spectral region up to 550 nm,
while a marked sub-bandgap photoresponce,
manifesting itself as a photocurrent tail at k > 600
nm, appears for the Se(Pb) films in contrast to the
undoped ones (Fig. 2(b)). We have previously
shown that the Pb adatoms at the Se surface are
efficient recombination centers for the photogen-
erated charge carriers [15]. The formation of Pb–
Se chemical bonds has been found to result in the
appearance of a spectrum of electron surface
states, which are located near the valence band
edge and are capable of an efficient electron ex-
change with this band. The lead doping can result
in the formation of a rather high density of such
intra-bandgap states throughout the whole film
3
À
4À
example Fe(CN) /Fe(CN)
6
redox pair, also
6
Fig. 3. Voltammograms of the undoped (1, 3) and Pb-doped
1 at.% Pb) Se film electrodes (2, 4). Electrolyte: (a) 0.1 M
(
HNO
scan rate: 0.02 V s
3
; (b) 0.02 M K
À1
3
[Fe(CN)
6
] + 0.05 K
4
[Fe(CN)
6
]. Potential
.

1096
D.K. Ivanov et al. / Surface Science 532–535 (2003) 1092–1097
increases drastically at the Se(Pb) electrodes (Fig.
3
1
of the peak C to more positive potentials, indi-
(b)).
cating that the energy states of the Pbad at the
Se(Pb) surface change significantly when cycling.
3.4. Lead underpotential deposition onto Pb-doped
Se films
1 1
The steady-state position of the peaks C and A
after the repeated potential cycling coincides with
the position of similar peaks of a PbSe electrode
(Fig. 4(a), dashed line). This fact allows us to as-
sume that a PbSe layer is formed on the Se(Pb) film
surface on the potential cycling. We have previ-
ously shown that the conversion of a Pbad mono-
layer into PbSe on the Se surface is due to its
chemical oxidation by Se and to the partial diffu-
sion of the Pbad into the Se matrix [15]. The com-
Cyclic voltammograms of Se(Pb) electrodes
2þ
with a Pb content of 1 at.% in Pb -containing
solution are shown in Fig. 4(a). It should be noted
that the Pb UPD is feasible in the dark at the
Se(Pb) electrodes in contrast to the Se ones and
results in the appearance of a cathodic peak C
)
1
at
0.31 V (Fig. 4(a), curve 1). A related anodic peak
A
1
associated with the Pbad oxidation is observed
1
plete coincidence of the peak C for the Se(Pb) films
at 0.27 V. Repeated potential cycling between )0.4
and 0.4 V gives rise to a significant shift (by 0.25 V)
with that for the PbSe electrodes indicates that the
PbSe layer is continuous. The formation of a dis-
continuous layer is less possible because in that
case several peaks corresponding to different en-
ergy states of Pbad (Pbad on Se and Pbad on PbSe)
would have been observed in the cathodic vol-
tammograms.
The PbSe layer formed on the Se(Pb) surface
can be readily identified electrochemically by the
relevant oxidation peak A2 observed when the
potential is swept in the positive direction (Fig.
4(b)). The charge corresponding to this peak in-
creases as the PbSe accumulates and finally
À2
reaches a limiting value of 500 lC cm . This value
indicates that the amount of PbSe formed on the
Se(Pb) surface is rather small and can be estimated
as several monolayers.
4. Conclusions
The use of the lead UPD onto selenium during
the Se and Pb electrochemical co-deposition makes
it possible to produce nanocomposite films con-
sisting of the amorphous Se and nanosized PbSe
clusters distributed throughout the film bulk. The
lead doping strongly modifies the optical and
photoelectrochemical properties of the Se films
and significantly increases the efficiency of the
charge transfer both in the film bulk and through
the semiconductor j electrolyte interface. Owing to
this modification, the Pb UPD onto the Se(Pb)
surface in the dark, which does not occur on the
Fig. 4. (a) Cyclic voltammograms of Pb-doped Se film elec-
trode (1 at.% Pb) (––) and PbSe film electrode (- - -) in 0.05 M
Pb(NO
potential scan. (b) Anodic voltammograms of the Pb-doped
1 at.% Pb) Se film measured under the same conditions after 5,
3 2 3
) + 0.1 M HNO solution for the first, second and 50th
(
1
0, and 50 scans of potential between )0.4 and 0.4 V. Potential
À1
scan rate: 0.02 V s
.

D.K. Ivanov et al. / Surface Science 532–535 (2003) 1092–1097
1097
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