Electrodeposition of selenium from 1-butyl-1-methylpyrrolidinium trifluoromethylsulfonate

Article abstract of DOI:10.1016/j.electacta.2011.10.064

Electrodeposition of Se films from air and water stable 1-butyl-1-methylpyrrolidinium trifluoromethylsulfonate ionic liquid was studied in open air conditions. The electrochemical behavior of H₂SeO ₃ on gold and copper substrates was investigated in ionic liquid-water mixtures, and the influence of the deposition parameters such as time and bath temperature on the crystallinity of the film was examined. For the gold substrate, a reddish deposit containing amorphous, hexagonal and rhombohedral Se phases was identified at room temperature, while a grayish film of hexagonal and rhombohedral phases was formed at higher temperatures. The reddish Se film was found to exhibit a smoother surface with lower reflectivity in comparison to the grayish one. The band gap of the reddish film was close to that of pure amorphous red Se reported in literature, while the band gap of the grayish film was close to the pure hexagonal film. Photoelectron spectroscopy and energy dispersive X-ray spectroscopy show that both films consist of pure Se with only slight surface contaminations by remnants from the electrodeposition. In the case of a copper substrate, the electrodeposition of Se was accompanied by the formation of copper-selenide due to the reactivity of copper in H ₂SeO₃.

Full text of DOI:10.1016/j.electacta.2011.10.064

Electrochimica Acta 59 (2012) 228–236
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electrodeposition of selenium from 1-butyl-1-methylpyrrolidinium
trifluoromethylsulfonate
A. Abdel Aala,b,c,∗, F. Voigtsa, D. Chakarovb, F. Endresa,∗∗
a Institute of Particle Technology, Clausthal University of Technology, 38678 Clausthal-Zellerfeld, Germany
b Chalmers University of Technology, Department of Applied Physics, SE-412 96 Göteborg, Sweden
c Central Metallurgical Research & Development Institute, CMRDI, P.O. 87 Helwan, Cairo, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 August 2011
Received in revised form 20 October 2011
Accepted 21 October 2011
Available online 28 October 2011
Electrodeposition of Se films from air and water stable 1-butyl-1-methylpyrrolidinium trifluoromethyl-
sulfonate ionic liquid was studied in open air conditions. The electrochemical behavior of H2SeO3 on
gold and copper substrates was investigated in ionic liquid–water mixtures, and the influence of the
deposition parameters such as time and bath temperature on the crystallinity of the film was examined.
For the gold substrate, a reddish deposit containing amorphous, hexagonal and rhombohedral Se phases
was identified at room temperature, while a grayish film of hexagonal and rhombohedral phases was
formed at higher temperatures. The reddish Se film was found to exhibit a smoother surface with lower
reflectivity in comparison to the grayish one. The band gap of the reddish film was close to that of pure
amorphous red Se reported in literature, while the band gap of the grayish film was close to the pure
hexagonal film. Photoelectron spectroscopy and energy dispersive X-ray spectroscopy show that both
films consist of pure Se with only slight surface contaminations by remnants from the electrodeposi-
tion. In the case of a copper substrate, the electrodeposition of Se was accompanied by the formation of
copper–selenide due to the reactivity of copper in H2SeO3.
Keywords:
Selenium
Electrodeposition
Ionic liquids
Copper–selenide
© 2011 Elsevier Ltd. All rights reserved.
1. Introduction
is no clear distinction between amorphous and vitreous states [6,7].
Consequently, the formation of insulating allotropes can only be
Due to their good semiconducting properties and light sensitiv-
ity, selenium thin films attract considerable attention. Similarly,
compound semiconductors containing selenium (i.e. InSe, CdSe
or CuInSe2) are of interest for optoelectric applications, includ-
ing advanced solar cells, IR detectors and solid-state lasers [1].
Selenium appears in different phases. The amorphous phase
(red, brown and black) exhibits low electronic conductivity. The
crystalline phases, include several solid allotropes, namely, the
rhombohedral one, the ␣, ␤ and ␥ monoclinic ones and the trig-
onal (hexagonal) one. Hexagonal Se is gray and it is the densest and
the most stable of all phases [2]. Black Se consists of large polymeric
rings having up to 1000 atoms per ring [3], the red amorphous
Se has a deformed chain structure, and upon heat treatment it
transforms to the stable hexagonal phase [4]. The monoclinic form
is thermodynamically metastable and spontaneously converts to
the trigonal one at higher temperature [5]. Selenium generally can
adopt any of the allotropic modifications in its solid state and there
avoided by careful control of electrodeposition conditions (e.g.,
temperature and current density), furthermore, a careful selection
of substrate materials is required [8].
In literature, a large variety of techniques is described for the
preparation of Se thin films such as photodeposition, radiofre-
quency sputtering and vacuum deposition [9–11]. However, a low
cost method, such as electrodeposition, would be beneficial for
large-area thin film production. Usually, the electrodeposition of
selenium from conventional aqueous solutions at low tempera-
ture leads to the formation of amorphous red selenium, which is
an insulator. Graham et al. reported that amorphous Se layers with
thicknesses up to 500 nm can be plated using acidic (pH 0.7–0.9)
or slightly alkaline (pH 7.5–8.0) electrolytes in the temperature
range of 20–40 ◦C [12]. However, they noticed that colloidal Se is
formed near the cathode surface due to the plating process. The
particles aggregate and increase in number and size with time
causing defects in the deposited Se film in the form of particle
inclusions. Uzoh and Aksu avoided such defects by addition of
an anti-coagulation agent, like 1,4-bis(3-aminopropyl) piperazine,
which inhibits the growth of Se particles and promotes the dispersal
of such Se particles during the plating process of Cu–Se [13]. Hip-
pel could successfully deposit gray Se using an acidic electrolyte
composed of saturated selenium dioxide in 9 molar H2SO4 at a
∗
Corresponding author. Tel.: +49 5323722494; fax: +49 5323722460.
Corresponding author.
E-mail addresses: ibrahim.alsayed@tu-clausthal.de (A.A. Aal),
∗∗
frank.endres@tu-clausthal.de (F. Endres).
0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.10.064

A.A. Aal et al. / Electrochimica Acta 59 (2012) 228–236
229
temperature of 100 ◦C [14]. Such harsh operating conditions for the
deposition of gray Se can hardly be applied to a mass production
process.
(Princeton Applied Research) controlled by the PowerCV and Pow-
erStep software. All experiments were performed under a hood in
open air conditions. After deposition, the samples were rinsed with
iso-propanol to ensure removal of the ionic liquid and subsequently
dried under vacuum at room temperature for 2 h.
A high-resolution field emission scanning electron microscope
(Carl Zeiss DSM 982 Gemini) was utilized to investigate the surface
morphology of the electrolysis product. The X-ray diffractogram of
the electrolysis product was acquired by a Siemens D-5000 diffrac-
tometer with CoK␣ radiation.
Reddish and grayish films deposited at a room temperature and
a temperature of 110 ◦C were imaged by tapping-mode atomic force
microscopy (Veeco/DI Dimension 3000, USA) in air using NSC15
cantilevers (MikroMasch). Images were plane-fit and analyzed
using the Scanning Probe Image Processor (SPIP, version 3.0.0.9,
Image Metrology Inc., Denmark). Because the surface roughness
is a function of the thickness of the films, the same thickness of
films (500 nm) were fabricated in order to compare the surface
roughness.
Alternatively, our group suggested the utilization of ionic liq-
uids for Se deposition due to their wide electrochemical window
and extremely low vapour pressures, which allow deposition at
temperatures above 100 ◦C. Our previous studies showed that gray
Se can easily be deposited at temperatures of more than 100 ◦C,
which is practically impossible in aqueous galvanic baths [15]. The
only shortcoming is that the employed SeCl4 and the whole bath
have to be handled under dry box conditions, which would compli-
cate a technical process. The most recent research by Steichen and
Dale focused on the influence of precursor salt on the electrochem-
ical behavior, as well as the phase, morphology and crystallinity
of the Se deposits from 1-ethyl-3-methylimidazolium tetrafluo-
roborate/chloride at temperatures >100 ◦C [16]. They found that
applying different precursors (SeCl4 and SeO2) leads to the deposi-
tion of amorphous and high aspect ratio single-crystalline trigonal
Se nanorods films.
In the present paper, we demonstrate that the deposition of
crystalline Se can be realized at higher temperatures (70–110 ◦C)
from the air and water stable 1-butyl-1-methylpyrrolidinium tri-
fluoromethylsulfonate (Py1,4[CF3SO3]). The electrodeposition of Se
on gold and copper electrodes was studied in water containing ionic
liquids in open air conditions. Water is essential for the deposi-
tion of Se from H2SeO3, furthermore it enhances the solubility of
the Se precursor. Since Py1,4[CF3SO3] does not react with water,
electroplating baths based on air and water stable ionic liquids
containing water might be a promising way for the deposition of
Se in open air conditions. We would like to mention that there
is an effect of cations on the Se deposits which will be reported
elsewhere.
The reflectivity of reddish and grayish films was measured using
Cary 5000 Spectrophotometer with external reflectance acces-
sories (DRA 2500 Integrating Sphere) at room temperature. The
samples where mounted in the centermount position [17].
X-ray photoelectron spectroscopy (XPS) was performed in
an ultrahigh vacuum (UHV) apparatus with a base pressure of
1 × 10−10 hPa using a hemispherical analyzer (Omicron EA 125)
in combination with a non-monochromatic X-ray source (Omi-
cron DAR 400). During XPS, X-ray photons hit the surface under
an angle of 45◦ to the surface normal. For all measurements pre-
sented here, the Mg K␣ line with photon energy of 1253.6 eV was
used. Electrons were recorded by the hemispherical analyser with
a calculated analyser resolution of 0.83 eV for detail spectra and
2.07 eV for survey spectra. The analyser was mounted at an angle
of 45◦ to the surface normal, analysing a spot of about 1.5 mm in
diameter. All XPS spectra were displayed as a function of electron
binding energy with respect to the Fermi level.
For quantitative XPS analysis, background substraction after
Shirley was employed [18]. Photoelectron peak areas were cal-
culated via mathematical fitting with Gauss-type profiles using
OriginPro 7G including the PFM fitting module, which uses
Levenberg-Marquardt algorithms to achieve the best agreement
possible between experimental data and fit. Photoelectric cross-
sections calculated by Scofield [19] and inelastic mean free paths
calculated by Seah and Dench [20] as well as the transmission func-
tion of our hemispherical analyzer are taken into account when
calculating stoichiometry. The composition of the samples was
given in relation to the deposited layer, i.e. contributions from the
substrate are not included in the calculation.
2. Experimental
H2SeO3 powder (Sigma–Aldrich, 99.99%) was used as selenium
source. The ionic liquid 1-butyl-1-methylpyrrolidinium trifluo-
romethylsulfonate (IOLITEC, commercial quality), employed as
electrolyte, was slightly yellowish from a Li free synthesis with Cl−
<100 ppm and butylpyrrolidin <5000 ppm. Other chemicals used
in this work were obtained in analytical grade and used without
further purification. The dissolution of H2SeO3 in Py1,4[CF3SO3]
electrolyte was examined in pure ionic liquid and its mixture with
water (1–5 vol%). It was found that addition of 5 vol% water enables
the dissolution of H2SeO3.
The cyclic voltammetry and electrodeposition experiments
were carried out in open air conditions, using a three-electrode
cell setup. Platinum was used as counter electrode and a quasi-
reference electrode, respectively. We have to mention that the Pt
quasi-reference electrode is definitely not optimal, as during the
massive electrolysis the reference electrode potential can shift by
100–200 mV. However, there is no other reference electrode which
does definitely not react with the Se precursor under these condi-
tions. Gold substrates from Arrandee (200–300 nm thick gold films
deposited on chromium-covered borosilicate glass), and Cu sheets
(99.99%) were used as working electrodes, respectively. Directly
before use, the gold substrates were very carefully heated in a
hydrogen flame to red glow while the copper sheet electrodes were
polished, etched by 5% HNO3 and sonicated in iso-propanol for
10 min. An electrochemical cell made of polytetrafluoroethylene
(Teflon©) was clamped over a Teflon covered Viton© o-ring onto
the substrate, thus yielding an exposed surface area of 0.6 cm2.
Prior to use, all parts in contact with the solution were thor-
oughly cleaned in a mixture of 50/50 vol% H2SO4/H2O2 followed by
refluxing in bidistilled water. All electrochemical measurements
were performed using a VersaStat 263A Potentiostat/Galvanostat
For all XPS experiments, the sample was mounted on a molyb-
denum holder and introduced into the UHV by means of a sample
transfer system. No additional pretreatment is performed except
stated otherwise.
Where stated, the samples were cleaned by etching with argon
ions from an Omicron ISE 5 ion source. For this process, Ar+
ions with 2 keV kinetic energy were used, which results in tar-
get currents of 10 ␮A. For a single sputter cleaning cycle, the Ar+
bombardment was sustained for 300 s.
3. Results and discussion
3.1. Films on gold substrate
3.1.1. Electrodeposition at room temperature
The current–potential curves of gold in pure Py1,4[CF3SO3], its
mixture with 5 vol% H2O and Py1,4[CF3SO3]/5 vol%H2O with 0.16 M
H2SeO3 are presented in Fig. 1. For the cyclic voltammogram of

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A.A. Aal et al. / Electrochimica Acta 59 (2012) 228–236
(a)
(b)
(c)
C1
C2
-3
-2
-1
0
1
E / V vs. Pt-quasi ref
Fig. 1. Cyclic voltammograms of (a) Py1,4[CF3SO3], (b) Py1,4[CF3SO3]/5 vol%H2O and
(c) Py1,4[CF3SO3]/5 vol%H2O with 0.16 M H2SeO3 on gold at room temperature, scan
rate: 10 mV/s, Pt as a quasi-reference electrode.
H2SeO3 (0.16 M) in Py1,4[CF3SO3]/5 vol%H2O, two cathodic peaks
(C1, C2) are obtained and at C1 the deposition of a reddish film,
presumably red Se, is observed.
According to recent studies by Lai et al., for aqueous solutions,
the first peak C1 might be associated to the electroreduction of
H2SeO3 to Se0 onto the gold substrate through a four-electron step
as shown by Eq. (1) [21]. The second peak C2 might be related to
the six-electron reduction of H2SeO3 to Se2− species, according to
Eq. (2). This is also supported by the area under the peak (charge)
involved in peak C2 which is much higher than that involved in
peak C1. The product Se2− is subsequently subjected to a conpro-
portionation reaction with H2SeO3 in solutions (Eq. (3)), leading to
the chemical formation of Se0.
Fig. 2. SEM of Se film deposited at −1.2 V on gold after (a) 10 min, (b) 30 min, and
(c) 120 min at room temperature.
H2SeO3 + 4H+ + 4e− = Se + 3H2O
H2SeO3 + 6H+ + 6e− = H2Se + 3H2O
H2SeO3 + 2H2Se = 3Se + 3H2O
(1)
(2)
(3)
However, a film deposited in 120 min was loosely adhering and
peeled off during rinsing.
The phase structure of the deposited Se films was charac-
terized by X-Ray diffraction and is presented in Fig. 3. The
XRD exhibits different sharp peaks confirming the crystalline
nature of the as-deposited films. The XRD pattern also exhibits
The scan rate dependence of the peak currents in our exper-
iments is not straightforward and we assume that kinetic
phenomena lead to the peak shape in the CV. In [22], we discussed
that solvation layers of ionic liquids might influence electrochem-
ical reactions. Thus, it is also possible that the reduction of H2SeO3
to Se occurs differently to the mechanism reported for aqueous
solutions.
The surface morphology of Se films deposited at C1 onto the
Au substrate was investigated at different deposition times (10, 30,
120 min). SEM images showed that the surface coverage increases
with increasing the deposition time from 10 to 30 min (Fig. 2).
a
peak at 2ꢀ of 40.3◦ corresponding to the gold substrate.
The comparison of the observed peaks at 2ꢀ of 27.3, 31.7,
34.6 and 36.6◦ with the standard data proves that the red-
dish deposited Se is a mixture of textured rhombohedral (JCPDS
32-0992) and hexagonal (JCPDS 42-1425) crystal phases. The
reddish color is related to the formation of amorphous red Se
having no lattice (Bragg peaks), and cannot be visible in XRD
pattern.

A.A. Aal et al. / Electrochimica Acta 59 (2012) 228–236
231
Au
Au
(c)
(b)
(a)
JCPDS 42-1425
JCPDS 32-0992
12
18
24
30
36
42
JCPDS 42-1425
JCPDS 32-0992
o
Fig. 5. X-ray diffraction analysis of Se film deposited at −1.2 V for 60 min on gold
from Py1,4[CF3SO3]/5 vol%H2O mixture with 0.16 M H2SeO3 at temperatures of (a)
70 ◦C, (b) 90 ◦C and (c) 110 ◦C.
12
18
24
30
36
42
o
Fig. 3. XRD pattern of Se film deposited at −1.2 V for 120 min on gold at room
temperature.
obtained at C2. In the reverse scan, a small anodic peak (A1) was
observed which seems to be due to the partial oxidation of the sur-
face Se species deposited at C2. Crossing of the reverse anodic scan
with the cathodic one is a hint for nuclei formation on the elec-
trode surface [23]. The peak C3 is probably due to the reduction of
Se0 to Se2− species which are soluble in the electrodeposition bath.
At temperatures of 90 and 110 ◦C, the CV measurements exhibited
similar trends as in the case of lower temperatures (Fig. 4b and c).
As higher temperatures are known to favor the reaction rate due
to higher diffusion rate and less viscosity, an increase of the area
under the peaks was observed.
3.1.2. Electrodeposition at higher temperatures
The effect of bath temperature on the Se film deposition and
its morphology was also investigated. The electrochemical behav-
ior of H2SeO3 in Py1,4[CF3SO3]/5 vol%H2O at temperatures of 70, 90
and 110 ◦C was studied. Fig. 4 shows the cyclic voltammogram of
Py1,4[CF3SO3]/5 vol%H2O and H2SeO3 (0.16 M) at 70, 90 and 110 ◦C
on gold. Similar to the CV at room temperature, two cathodic peaks
(C1 and C2) are observed. However, this time a grayish deposit was
The phase structure and the chemical composition of the grayish
deposited films were characterized by XRD and EDX, respectively.
The analysis of the obtained XRD pattern for the grayish film
deposited at 70, 90 and 110 ◦C confirmed that the observed peaks at
2ꢀ of 27.3, 31.7, 34.6 and 36.6◦ can be ascribed to the gray hexagonal
A1
(a)
C1
C2
˚
phase of Se (JCPDS 42-1425) with lattice parameters a = 4.36 A and
˚
c = 4.95 A and the rhombohedral phase (JCPDS 32-0992) (Fig. 5a).
The spectra also revealed that increasing of temperature from 90
to 110 ◦C leads to sharper XRD peaks confirming the formation of
well-crystallized Se films (Fig. 5b and c). The EDX analysis indicated
that the products were pure Se without any detectable impurities.
The SEM analysis of the grayish film deposited at 70 ◦C showed
different, quite interesting structures. Fig. 6a–e presents images of
Se deposited during 10 min. The deposit is composed of regular
micrometer-sized spheres with smooth surfaces and a diameter
of ∼500 nm (Fig. 6a). Surprisingly, a careful observation revealed
that on the surface of some microspheres an outer shell seemed to
grow (Fig. 6b and c). In some areas, a hole on the sphere appeared,
and thus the inner core became visible (Fig. 6d). On other areas, Se
micro-rods were observed to grow from the spheres, either from
one side or from both sides (Fig. 6e). With increasing the deposition
time to 60 min, solely spheres with a diameter of ∼250 nm were
observed (Fig. 6f). We can currently not explain why such a different
morphology is observed. It is possible that the shell consists of a
different phases leading to a limited number of nucleation sites,
maybe further influenced by the varying bath composition during
deposition.
A1
C3
C3
(b)
C2
C1
A1
(c)
C2
-2
C1
C3
The morphology of the deposited films at temperatures of 90 and
110 ◦C is shown in Fig. 7. It is clear that both films show a similar
surface structure at low and high magnification, where the deposit
consists of crystalline particles. Such similarity in morphologies
might indicate that at temperature ≥90 ◦C, the nucleation and
crystal-growth mechanism are apparently temperature indepen-
dent. Further studies are required to achieve detailed information
-3
-1
0
1
E / V vs. Pt-quasi ref
Fig. 4. Cyclic voltammogram of Py1,4[CF3SO3]/5 vol%H2O mixture with 0.16 M
H2SeO3 on gold at temperatures of (a)70 ◦C, (b) 90 ◦C and (c) 110 ◦C scan rate:
10 mV/s, Pt as a quasi-reference electrode.

232
A.A. Aal et al. / Electrochimica Acta 59 (2012) 228–236
Fig. 6. Different magnifications of SEM images of the Se microstructure deposited at −1.2 V and a temperature of 70 ◦C on gold for 10 min (a–e) and 60 min (f).
on the nucleation process as well as a better control of growth
kinetics, orientation and morphology.
near infrared regions. It is clear that the films have different absorp-
tion edges, where the reddish Se exhibited higher reflectance (less
absorption and scattering) than that of the corresponding grayish
one. The increased absorbance of the grayish film is thought to be
due to a roughening effect and higher surface area (Fig. 8).
3.2. Characterization of Se deposits
The reddish Se film has a reflectance minimum (absorption max-
imum) at 2.30 eV whereas the grayish film shows a broad peak in
the range of 1.89 to 2.37 eV. Taking into account that the grayish
film is composed of rhombohedral phase besides the main hexago-
nal one, the obtained results are close to that measured by Prosser
and Henisch for pure red (amorphous) and gray (hexagonal) Se
[24]. They reported two reflectance peaks for hexagonal Se: the
long wavelength peak at 1.85 eV and the short wave length peak
at 2.4 eV. Moreover, Stuke et al. has also reported two peaks for
hexagonal Se at 1.95 and 2.25 eV [25]. Gates et al. reported that
the lowest energy transition (i.e. grayish film in our results) from
the bonding states could be attributed mostly to interchain inter-
actions by calculating the electronic charge distributions, while,
the higher energy transitions (i.e. reddish film) were determined
to arise from covalent bonds within the selenium chain [26]. Spec-
troscopic absorption studies by Bogomolov et al. on isolated helical
chains of Se atoms further confirmed that the strong absorption
at ∼2.2 eV came from the intermolecular transition between the
spiraling chains of selenium [27].
3.2.1. Surface topography
Surface topography of Se films deposited at room and
higher temperature (110 ◦C) was examined using atomic force
microscopy. Fig. 8 presents the typical AFM image of a 5 × 5 ␮m2
area of deposited reddish and grayish films at a vertical z-scale of
72 and 922 nm, respectively. As shown from images, the reddish
Se film has smoother surface with root main square roughness of
∼8 nm, peak-to-valley height of ∼58 nm, whereas the grayish Se
film showed rough surface with an average root main square rough-
ness of ∼85 nm and peak-to-valley height of ∼560 nm. The coarse
particles of grayish film are formed due to the increased deposition
rate at higher temperature.
3.2.2. Optical properties
The purpose of these measurements is to investigate the optical
properties of reddish Se thin film deposited at room temperature
comparing with the grayish one obtained at 110 ◦C. Since the films
were opaque, the reflectance measurement was the proper test.
Fig. 9 shows the diffuse reflectance (R,%) spectra in the visible and

A.A. Aal et al. / Electrochimica Acta 59 (2012) 228–236
233
Fig. 7. Different magnifications of SEM images of the Se microstructure deposited at −1.2 V on gold at temperatures of 90 ◦C (a,b) and 110 ◦C (c,d).
The diffuse reflectance spectra were translated into the absorp-
tion spectra by the Kubelka–Munk’s equation that is described as
follows [28,29]:
Eg were determined by extrapolating the linear region of the plots
to zero absorption (˛hv = 0). The optical band gap of the deposited
reddish film from the inset Fig. 9 is ∼2.04 eV which is closed to these
results reported for amorphous Se (2.05 eV) by Zienab Mandouh
[32] and 2.09 eV by Bindu et al. [33]. The value of band gap of the
grayish film is found to be 1.71 eV in agreement with a previously
reported one for the pure hexagonal gray Se of 1.85 eV by Yan et al.
[34] and 1.8 eV by Daveln [35].
(1 − R)2
a =
(4)
2R
In Eq. (4), ˛ and R represent the absorption coefficient and reflec-
tivity, respectively. This conversion is available to estimate the band
gap from powder diffuse reflection spectra. [30]. The optical band
gap energy, was calculated on the basis of recorded optical spectra
using the well-known relation:
3.2.3. XPS analysis
Fig. 10 shows XPS results from the reddish Se film as introduced
into the UHV. Emission from the deposited selenium layer and the
gold substrate as well as from several remnants due to the pro-
duction process can be detected on the surface. The contributions
from oxygen (6 at% of the deposited layer) and fluorine (2 at%) can
be removed completely by a short sputter process, (see the second
scan). The carbon fraction in the layer decreases from 28 to 10 at%
during the sputter cleaning process. The peak at 784 eV is attributed
a hv = C(ahv − Eg)n
(5)
where C is constant, hꢁ the photon energy, while n depends on the
nature of the transition. For direct transitions n = 1/2 or 3/2 while
for indirect ones n = 2 or 3, depending whether they are allowed or
forbidden, respectively. The best fit of (˛hꢁ)/n versus the photon
energy, in the case of both reddish and grayish Se thin films, was
obtained for n = 1/2 [31]. The values of the direct optical band gap
Fig. 8. AFM images of Se films deposited at −1.2 V on gold (a) at room temperature and (b) at 110 ◦C.

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A.A. Aal et al. / Electrochimica Acta 59 (2012) 228–236
100
75
50
25
0
XPS
grayish film
Mg Kα
grayish film, sputtered
Na 1s
Se 3d
Se 3p
imp.
Au4f
Se 3s
Au4s
Au4p
1/2
Au4d
5/2
Au4p
3/2
Au4d
3/2
F 1s
O 1s
C 1s
C KLL
O KLL
F KLL
0
200
400
600
800
1000
1
2
3
4
5
binding energy / eV
photon energy / eV
Fig. 11. XPS spectra of grayish Se film deposited at −1.2 V on gold as introduced
Fig. 9. Reflectivity of reddish and grayish Se films deposited at −1.2 V on gold.
into the UHV and sputtered.
to not completely removed impurities of barium or cobalt (<1 at%
in both cases).
binding energy side (green Gaussians). This is due to emission from
Se 3d orbitals in a different chemical environment (core level shift).
The shift amounts to 2.3 eV for the reddish film and 2.5 eV for the
grayish Se. That is too small to originate from a SeO2, which exhibits
a chemical shift of 4.4 eV [36]. It may be due to a surface layer of
oxidized Seı+ with 0< ı < 4.
The comparison of the peak areas reveals that more than 93%
of the reddish film in this sample is in the metallic state Se0, while
only less than 7% is in the oxidized state Seı+. For the grayish film,
the analysis reveals that the fraction of Seı+ in that sample is about
4 at%.
The detection of carbon, oxygen and fluorine with XPS is not a
contradiction to the EDX result, but is caused by the much higher
surface sensitivity of XPS. The fact that a short sputter process can
remove these remnants suggests that it is a very thin surface layer
only. Fig. 11 shows results from grayish Se in the same manner
as Fig. 10. Like on reddish Se, remnants from the production pro-
cess can be detected. The fraction of oxygen (8 at% of the deposited
layer), fluorine (3 at%) and sodium (<1 at%) can be sputtered away
easily. The carbon contribution is reduced from 25 to 4 at% during
sputtering.
Fig. 12 shows a chemical shift analysis performed on detail spec-
tra from the Se 3d region for the reddish film and the grayish Se.
For this analysis, results from the sputtered samples are used. The
experimental data is shown as black dots while the Gaussians used
for the fitting procedure are drawn as blue and green lines. The bold
red line represents the sum of all Gaussians. The function used for
Shirley background subtraction is shown as the solid black line at
the bottom.
3.3. Films on copper substrate
As Se is one component in the CuInSe2 compound semicon-
ductor, we also tested the deposition of Se on copper. Fig. 13
presents the CV of Py1,4[CF3SO3]/5 vol%H2O and its mixture with
H2SeO3 (0.16 M) at room temperature onto copper. In contrast
to gold under comparable conditions, the voltammetric curve
showed three cathodic peaks at −0.5, −1.2 and −1.9 V besides an
anodic process. The peaks C2 and C3 might be attributed to the
The main double peak is due to emission from the metallic Se
3d5/2 and Se 3d3/2 orbitals (blue Gaussians). In addition to these
contributions additional intensity can be detected on the high
XPS
Mg K
grayish film
mathematical fit
reddish film
mathematical fit
Se +
Se +
60
Se0
56
Se0
56
binding energy / eV
0
200
400
600
800
1000
52
60
52
binding energy / eV
binding energy / eV
Fig. 10. XPS spectra of reddish Se film deposited at −1.2 V on gold as introduced
Fig. 12. XPS detail spectra from the Se 3d region of the sputtered reddish Se film
(left) and sputtered grayish Se film (right).
into the UHV and sputtered.

A.A. Aal et al. / Electrochimica Acta 59 (2012) 228–236
235
(a)
(a)
(b)
C1
C3
C2
(c)
JCPDS 20-1020
24
28
32
36
40
44
48
52
o
(b)
C1
C2
C3
-3
-2
-1
0
1
E / V vs. Pt-quasi ref
Fig.
13. Cyclic
voltammogram
of
Py1,4[CF3SO3]/5 vol%H2O
(a),
Py1,4[CF3SO3]/5 vol%H2O with 0.16 M H2SeO3 on copper at
a
room tempera-
ture (b) and a temperature of 70 ◦C (c), scan rate: 10 mV/s, Pt as a quasi-reference
electrode.
JCPDS 071-0044
JCPDS 32-0992
reduction of selenium ions as suggested in Eqs. (1)–(3). The peak
C1 seems to be correlated to the formation of Cu–Se compounds.
The voltammetric features of the Cu/Se system in aqueous solutions
were studied by Mishra and Rajeshwar [37]. The data on the binary
system were consistent with the facile formation of a Cu2−xSe solid
phase with x varying from 0 to 1. Additionally, Skyllas-Kazacos and
Miller concluded from their hydrodynamic voltammetry data on
Cu disk electrodes that a reaction between Cu and Se leads to the
postulated formation of Cu2Se [38]. Pottier and Maurin have con-
firmed the formation of Cu2Se via X-ray diffractometry [39]. On the
other hand, Ueno et al. found no evidence for Cu2Se formation, but
instead reported X-ray diffraction evidence for the Cu3Se2 phase
[40].
24
28
32
36
40
44
48
52
o
Fig. 14. XRD of Se film deposited at −1.2 V for 30 min on copper (a) at room tem-
perature and (b) at a temperature of 70 ◦C.
The EDX analysis in our experiments indicated that the blackish
film deposited at C2 consists of Se (24 at%) and Cu (6 at%), con-
taminated by adsorbed ionic liquid components as C, F, S and N.
However, the phase structure of CuSe (JCPDS 20-1020) was identi-
fied as illustrated by XRD peaks at 2ꢀ of 31.3 and 46.1◦ (Fig. 14a) in
good agreement with Villalvilla and Velasco who found that CuSe
is the most likely compound formed by reaction of Cu with H2SeO3
at room temperature [41]. From the thermodynamic point of view,
CuSe is the most stable compound at room temperature on the
phase diagram of the Cu–Se binary system [42].
For comparison, a typical CV was obtained for the copper
substrate in Py1,4[CF3SO3]/5 vol% H2O with H2SeO3 (0.16 M) at a
temperature of 70 ◦C (Fig. 13c). In this case, the deposited film
obtained at C2 was gray. Although the current peaks (C2, C3) well
agree with those ascribed to the four-electron and six-electron
reduction of Se(IV) on gold, XRD peaks at 2ꢀ of 31.3, 36.3, 48.1 and
52.1◦, in Fig. 14b, clearly indicated the presence of cubic Cu1.8Se
phase with the berzelianite structure (JCPDS 71-0044) besides the
rhombohedral Se phase (JCPDS 32-0992). The EDX spectrum con-
firms that the deposited film is composed of Cu (62.8 at%) and
Se (36.1 at%) with an atomic ratio of 1.74 that is close to the
Fig. 15. EDX analysis and SEM image of Se film deposited at −1.2 V for 30 min on
copper at temperature of 70 ◦C.

236
A.A. Aal et al. / Electrochimica Acta 59 (2012) 228–236
stoichometry of Cu1.8Se (Fig. 15). Consequently, selenium is
deposited at low electrode potentials and the copper electrode is
subject to a compound formation with Se. It is known that Hg, Ag
and Cu electrodes can react with Se [43–45]:
Clausthal and V. Gusak, M. Zäch, and C. Langhammer from Chalmers
University.
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Financial support by the Alexander von Humboldt Foundation
is gratefully acknowledged. The authors thankfully acknowledge
experimental assistance by W. Gruber and C. Lehmann from the TU