Electrodeposition and characterization of manganese-bismuth system from chloride based acidic bath

Article abstract of DOI:10.1016/j.electacta.2010.09.107

In this work, Mn-Bi system has been successfully electrodeposited on Cu/Si substrates from aqueous ammonium chloride containing electrolyte. A thermodynamic study of the electrolysis bath highlights the possible formation of manganese complexes (MnCl+,MnCl2,MnCl3-) and bismuth complexes (BiCl2+,BiCl2+,BiCl3,BiCl4-,BiCl52-,BiCl63-). The mechanism process involving Mn and Bi electrodeposition is investigated by cyclic voltammetry. Mn-Bi films can be grown under potentiostatic control. The physico-chemical, morphological and structural characterizations of the deposits were carried out by scanning electron microscopy (SEM) and X-rays analysis (XRD). The results reveal a granular surface quality and a heterogeneous chemical composition of the films. The energy dispersive spectroscopy (EDS) analysis reveals the presence of manganese and bismuth peaks whose relative intensities vary according to the imposed potential and the position on the sample. The X-rays diffraction analysis showed three different crystalline phases: α-manganese (body centred cubic system), γ-manganese (body centred tetragonal system) and bismuth (rhombohedral system).

Full text of DOI:10.1016/j.electacta.2010.09.107

Electrochimica Acta 56 (2011) 1275–1282
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electrodeposition and characterization of manganese–bismuth system from
chloride based acidic bath
B. Benfeddaa,∗, N. Benbrahima, A. Kadria, E. Chainetb, F. Charlotc, S. Coindeauc
a Laboratoire de Physique et Chimie des Matériaux (LPCM), Université M. Mammeri de Tizi-Ouzou, Algeria
b Laboratoire d’Electrochimie et de Physico-chimie des Matériaux et des Interfaces (LEPMI), ENSEEG, 1130 rue de la piscine, B.P. 75, 38402 Saint Martin d’Hères, Grenoble, France
c C.M.T.C., Grenoble I.N.P. Bât Phelma Campus, Domaine Universitaire, B.P. 75, 1260 rue de la piscine, 38402 Saint Martin d’Hères, France
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work, Mn–Bi system has been successfully electrodeposited on Cu/Si substrates from aqueous
ammonium chloride containing electrolyte. A thermodynamic study of the electrolysis bath highlights
the possible formation of manganese complexes (MnCl+, MnCl2, MnCl3−) and bismuth complexes
Received 2 August 2010
Received in revised form
24 September 2010
Accepted 26 September 2010
Available online 23 October 2010
(BiCl2+
, ,
BiCl+2, BiCl3, BiCl4−, BiCl52− BiCl63−). The mechanism process involving Mn and Bi electrode-
position is investigated by cyclic voltammetry. Mn–Bi films can be grown under potentiostatic control.
The physico-chemical, morphological and structural characterizations of the deposits were carried out
by scanning electron microscopy (SEM) and X-rays analysis (XRD). The results reveal a granular surface
quality and a heterogeneous chemical composition of the films. The energy dispersive spectroscopy (EDS)
analysis reveals the presence of manganese and bismuth peaks whose relative intensities vary according
to the imposed potential and the position on the sample. The X-rays diffraction analysis showed three
different crystalline phases: ␣-manganese (body centred cubic system), ␥-manganese (body centred
tetragonal system) and bismuth (rhombohedral system).
Keywords:
Mn–Bi
Electrodeposition
Cyclic voltammetry
SEM
XRD
© 2010 Elsevier Ltd. All rights reserved.
1. Introduction
tant to surround conditions to elaborate near stoichiometric MnBi
films.
The development of magnetic nanomaterials took a consider-
able attention in several research tasks in recent years due to their
potential applications in high density recording [1–7]. Currently,
the efforts made in this field were based primarily on making
new materials able to meet increasing power needs. Indeed, many
metal systems were studied (FePt, CoPt, MnBi) [8–10]. The devel-
opment of the MnBi alloy can emerge and make a significant place
within the scientific community considering the crossing inter-
est of this type of material in the magneto-optical recording field
[11,12]. In fact, it is a hard magnetic material with a strong magnetic
anisotropy parallel to the c-axis at low temperature in its hexagonal
phase. This makes it a very good candidate for the perpendicular
recording [13,14], in particular when the material is fabricated in
the nanowires form. Several works related to fabrication of this
material by physical techniques exist in the literature [15–18] but
in our knowledge, no paper related to the electrodeposition method
was reported.
The electrodeposition of the MnBi alloy is a very delicate process,
considering the complexity of the manganese deposition kinetic
[19–23].
In the present paper, a preliminary study of the conditions of
the Mn–Bi system electrodeposited in a chloride bath in the pres-
ence of ammonium chloride is described. In this respect, cyclic
voltammetry has been used to determine the nature of the vari-
ous reactional stages occurred during the electrodeposition of Mn
as a function of ammonium chloride concentration, and during
the electrodeposition of Bi. To identify the electroactive species,
a thermodynamic study of the electrolysis bath was discussed. The
physico-chemical, morphological and structural characterizations
were also performed.
2. Experimental
A vitreous carbon rotating disk electrode (RDE) (0.2 cm2) and
either Cu/Si substrate were used as working electrodes in a classi-
cal three electrodes electrochemical cell. The cell was fitted with
a thermostatically controlled water jacket. A platinum wire, of
1 cm2 area, was used as the counter electrode. All the potentials
were measured with respect to a saturated calomel reference elec-
trode (SCE) (E = −0.242 V vs SHE). All measurements were made
at 25 0.5 ◦C in a chloride bath under nitrogen gas bubbling. The
The aim of this work is the electrodeposition of the Mn–Bi sys-
tem in thin layer. This stage seems important to control the film
morphology, composition and structure. In particular, it is impor-
∗
Corresponding author.
E-mail address: baya pg@yahoo.fr (B. Benfedda).
0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.09.107

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B. Benfedda et al. / Electrochimica Acta 56 (2011) 1275–1282
Table 1
chemical composition of the Mn–Bi electrolysis bath was MnCl2
4H2O (0.4 mol dm−3), BiCl3 (10−3 mol dm−3) and different concen-
trations of NH4Cl. A preliminary study was conducted as a function
of the pH to avoid bismuth precipitation. The adjusted value is 2.26.
The solution was prepared immediately prior to each experiment
using deionised water and analytical grade reagents (Aldrich).
The electrochemical measurements were performed using an
EG&G 273A potentiostat/galvanostat controlled by a microcom-
puter via GPIB interface operated by M352 EG&G software. The
deposited Mn–Bi layers were characterized by energy dispersive
X-ray spectroscopy (EDS), X-ray Diffraction (XRD), and scanning
electron microscopy SEM (secondary electron (SE) and backscat-
tered secondary electron (BSE) imaging).
Repartition of Mn (II) and Bi (III) complexes in the MnBi used bath (pCl = −0.58).
(a) Manganese chlorocomplexes repartition (mol %)
Mn2+
0.4
6.16
37.16
56.252
MnCl+
MnCl2−
MnCl3
(b) Bismuth chlorocomplexes repartition (mol %)
Bi3+
414.22 × 10−11
5.86 × 10−6
2.8 × 10−4
8.47 × 10−2
1.615
BiCl2+
+
BiCl2
BiCl3−
BiCl4
2−
3−
BiCl5
BiCl6
24.4529
73.8465
3. Results and discussions
The equilibrium potential calculated, by the Nernst equation, for
each chloride concentration (taking into account the ionic force of
the solution) is in the range:
3.1. Thermodynamic study
Manganese is the least noble metal obtained by electrolysis in
aqueous solution. Its electrodeposition is very delicate and requires
some precautions. Moreover, Mn can exist under several states of
oxidation. So, to maintain the Mn2+ ions in suspension in the solu-
tion their complexation using Cl− as ligand proves to be essential.
The thermodynamic study presented below was carried out for
a solution containing the metal ions Mn2+. In the presence of Cl−,
several manganese complexes such as MnCl+, MnCl2 and MnCl3−
are susceptible to be formed. These last are respectively character-
−1.468
V
vs SCE ≤ Eeq ≤ −1.495
V vs SCE
Several bismuth complexes are also−susceptible to be formed in a
chloride bath: BiCl2+, BiCl+, BiCl3, BiCl , BiCl2−, and BiCl36− (Fig. 2).
2
4
5
These complexes have been respectively characterized by their
formation cumulative constants ˇ1 = 102.4, ˇ2 = 103.5, ˇ3 = 105.4
ˇ4 = 106.1, ˇ5 = 106.7, ˇ6 = 106.6 according to [24].
,
For weak values of pCl, bismuth is essentially present as BiCl63−
complex species whereas for higher values it is rather stable in
the form of metallic ion Bi3+. For intermediate pCl, several com-
plexes species of bismuth are present such as BiCl2+, BiCl3, BiCl4−,
ized by their formation cumulative constants ˇ1 = 100.6, ˇ2 = 100.8
,
ˇ3 = 100.4 [24]. The distribution diagram of the various complexes
and not complexed species is illustrated in Fig. 1.
and BiCl25−
.
It can be seen that Mn may exist in several complexes form
as a function of pCl. Indeed, for pCl ≤ −2, Mn2+ is mainly in the
MnCl−3 form. For weak chloride concentrations pCl ≤ −2, Mn2+ is
the dominating species. In the range of (−2 < pCl < 2), the inter-
mediate species MnCl+ and MnCl2 are also present with MnCl−3 .
The pCl relating to the present study is located in the range
−0.25 ≤ pCl ≤ −0.76. These values were obtained by taking into
account the chloride concentration in the different electrolysis
baths. In this case MnCl−3 is the dominating species in the bath
(Table 1). Thus, we can assume that the manganese reduction may
occur as follow:
According to pCl = −0.58 (used value for the Mn–Bi bath), BiCl63−
is the dominating species (Table 1). So the deposition process may
be presented by the following reaction:
BiCl63− + 3e− ↔ Bi + 6Cl− EB0iCl
= 0.176
V vs SHE
3−
/Bi
6
The equilibrium potential calculated by the Nernst equation,
taking into account the ionic force of the bath is then:
EeqBiCl
= −0.202
V vs SCE
3−
/Bi
6
MnCl−3 + 2e− ↔ Mn + 3Cl− (EM0 nCl
= −1.197
V
vs SHE)
It should be noted that for the pH 2.26 (the used bath), the equi-
librium potential of hydrogen evolution according to the following
−
/Mn
3
Fig. 1. Distribution of the Mn (II) complexes in a chloride bath.
Fig. 2. Distribution of the Bi (III) complexes in a chloride bath.

B. Benfedda et al. / Electrochimica Acta 56 (2011) 1275–1282
1277
Fig. 4. Cyclic voltammograms for Bi deposition onto vitreous carbon RDE in
10−3 mol dm−3 BiCl3+3; 3 mol dm−3 NH4Cl; pH 2, 26; Vb = 20 mV/s; ω = 250 rpm.
Effect of the cathodic limit potential (a) E = −0.5 V vs SCE: (b) E = −1 V vs SCE, (c)
E = −1.2 V vs SCE.
reaction:
H+ + e− ↔ (1/2)H2 is Eeq = −0.377
V vs SCE
In the light of this thermodynamic study, the evolution of hydro-
gen from the acidic bath and water decomposition which occurs
around −1 V vs SCE will take place before the manganese reduction;
this may reduce the current efficiency of the manganese deposit.
3.2. Electrochemical study
3.2.1. Electrodeposition of manganese
In order to control the manganese electrodeposition kinetic, the
effect of two factors on the evolution of the shape of the voltam-
mograms is considered, namely the cathodic limit potential and the
NH4Cl concentration.
3.2.1.1. Effect of the cathodic limit potential. The electrochemical
reactions taking place at the vitreous carbon RDE in the chloride
based bath were first analysed in the absence of ammonium chlo-
ride.
Different voltammograms are reported for various cathodic
limit potential (Fig. 3a). Starting from the rest potential (open cir-
cuit potential: EOCP = − 0.8 V vs SCE) in the negative direction scan,
all the voltammograms exhibit a current onset between 0.8 V vs
SCE and −1.3 V vs SCE. This may be explained in terms of proton
discharge reaction, as can be seen on the insert of Fig. 3a recorded
in the blank electrolyte. From −1.3 V vs SCE, a weak deposition
plateau followed by a strong increase in the cathodic current at
−1.58 V vs SCE is observed, this is due to both manganese deposi-
tion and water decomposition which is important in this range of
potential as predicted in the thermodynamic studies.
Fig. 3. (A) Cyclic voltammograms for Mn electrodeposition onto vitreous car-
bon RDE in 0.4 mol dm−3 MnCl2; 0 mol dm−3 of NH4Cl; pH 2.26; Vb = 20 mV/s;
ω = 250 rpm. Effect of the cathodic limit potential (a) E = −1.65 V vs SCE, (b)
E = −1.70 V vs SCE, (c) E = −1.75 V vs SCE, (d) E = −1.80 V vs SCE. Insert: Blank elec-
trolyte. (B) Cyclic voltammograms for Mn deposition onto vitreous carbon RDE in
0.4 mol dm−3 MnCl2; pH 2.26; Vb = 20 mV/s; ω = 250 rpm. Effect of NH4Cl concentra-
different cathodic limit potentials: E = −1.65 V vs SCE, E = −1.70 V vs SCE, E = −1.75 V
vs SCE, E = −1.80 V vs SCE and vs different NH4Cl concentrations: (a) 1 mol dm−3
,
(b) 3 mol dm−3, (c) 4 mol dm−3, (d) 5 mol dm−3. Blank electrolyte voltammograms
vs cathodic limit potential, Vb = 20 mV/S, ω = 250 rpm, pH 2.26, 0 mol dm−3 NH4Cl,
(a) E = −1.65 V vs SCE, (b) −1.7 V vs SCE, (c) E = −1.75 V vs SCE, (d) E = −1.8 V vs SCE.
Blank electrolyte voltammograms vs NH4Cl concentration: Vb = 20 mV/s, vitreous
tion (a) 0 mol dm−3, (b) 1 mol dm−3, (c) 2 mol dm−3, (d) 3 mol dm−3, (e) 4 mol dm−3
(f) 5 mol dm−3
Insert: Blank electrolyte. (C) Manganese current efficiency vs
,
carbon RDE, ω = 250 rpm, pH 2.26, E = −1.7 V vs SCE. (a) 0 mol dm−3, (b) 3 mol dm−3
,
(c) 5 mol dm−3
.
.

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In the reverse scan, the current loop noticed between the for-
ward and the reverse scan results from the nucleation process on
vitreous carbon RDE. In fact, an over potential is required to cause
the nucleation of Mn on vitreous carbon [25]. Anodic current peak
appears at −1.44 V vs SCE. It presents two principal structures,
the peak A1 located at −1.35 V vs SCE and the peak A2 at −1.1
V vs SCE, their intensities and positions evolve according to the
cathodic limit potential. These are attributed to the dissolution of
manganese (peak A1) and probably its oxides and/or hydroxides
(peak A2) especially in this range of cathodic potential. The forma-
tion of manganese hydroxide has already been reported in other
works [19–21].
3.2.1.2. Effect of NH4Cl concentration. Series of experimentation are
carried out to highlight the contribution of NH4Cl in the kinetic
process of Mn deposition. For this purpose, the experimental pro-
cedure consists of a 0.5 mol dm−3 addition of this additive. The
Fig. 3b shows only the voltammograms obtained for 0, 1, 2, 3, 4
and 5 mol dm−3 of NH4Cl.
Fig. 5. (a) Linear potential sweep curves for Bi3+ reduction on vitreous carbon RDE
in a chloride bath (10−3 mol dm−3 BiCl3, 3 mol dm−3 NH4Cl, pH 2.26, Vb = 5 mV/s,(a)
400 rpm,(b) 600 rpm, (c) 800 rpm,(d) 1200 rpm, (e) 1400 rpm. (b) Linear poten-
tial sweep curves for Bi3+ reduction on vitreous carbon RDE in a chloride bath
(10−3 mol dm−3 BiCl3, 3 mol dm−3 NH4Cl, pH 2.26, Vb = 20 mV/s, (a) 400 rpm, (b)
600 rpm, (c) 800 rpm, (d) 1200 rpm, (e) 1400 rpm. (c) ilim vs (ω1/2).
Fig. 6. (a) Cyclic voltammograms for Mn–Bi system deposition on vitreous carbon
RDE in 0.4 mol dm−3 MnCl2 4H2O, 10−3 mol dm−3 BiCl3, 3 mol dm−3 NH4Cl; pH 2.26;
Vb = 20 mV/s; ω = 250 rpm at different cathodic limit potentials: (a) E = −1.65 V vs
SCE, (b) −1.7 V vs SCE, (c) E = −1.75 V vs SCE, (d) E = −1.8 V vs SCE. (b) Current–time
transients for Mn–Bi films deposited on copper substrate in 0.4 mol dm−3 MnCl2
4H2O, 10−3 mol dm−3 BiCl3, 3 mol dm−3 NH4Cl; pH 2.26; (a) E = −1.6 V vs SCE, (b)
−1.65 V vs SCE, (c) E = −1.75 V vs SCE, (d) E = −1.8 V vs SCE.

B. Benfedda et al. / Electrochimica Acta 56 (2011) 1275–1282
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It can be noticed that the shape of the voltammograms is
strongly dependent upon NH4Cl concentration. Indeed, as in Fig. 3a
plots, in the negative scan a weak deposition plateau appears in
the vicinity of −1.3 V vs SCE followed by a strong cathodic cur-
rent up to −1.58 V vs SCE. It can also be noticed that the current
density increases with increasing the NH4Cl concentration, this is
attributed to improvement of bath conductivity and also probably
to the discharge of the ammonium proton ions according to the
following reaction:
to the deposit improvement, so then NH4Cl must play in more the
role of a pH buffer.
The use of ammonium salts, as a buffering agent on the pH, in
the electrodeposition of manganese has been reported earlier in
the literature [26–28]. It was reported that this additive can prevent
precipitation of manganese hydroxides during electrodeposition as
the pH increases. But when added with small quantities the pH
decreases (the buffering effect is not efficient) and when added in
excess the current efficiency decreases.
According to our experimental results, it is confirmed that the
NH4Cl improves the Mn deposition but at higher concentration (up
to 4 mol dm−3) hydrogen evolution is dominating.
NH4Cl ↔ NH+ + Cl−
4
NH+4 + H2O → H+ + NH3 + H2O
H+ + 1e− → (1/2)H2
The current efficiency of manganese reduction has been
estimated by calculating the total charge from different voltammo-
grams recorded at different cathodic limit potentials and different
NH4Cl concentrations, assuming that the anodic peak correspond
to the amount of the deposited manganese and the cathodic cur-
rent to both manganese and hydrogen evolution. It is found that
the best current efficiency (22%) is obtained for 3 mol dm−3, while
for 5 mol dm−3 the current efficiency decreases (14%) (Fig. 3c). This
is foreseeable as the hydrogen evolution became very important
(insert Fig. 3b). It must be noted that the current efficiency cal-
culated with this method is very low compared to that calculated
by other authors using the weight gain of the sample [19]. In this
method, we under estimate the real efficiency as the manganese
deposit is not completely dissolved at the end of the cyclic voltam-
metry [20].
This is confirmed by the blank electrolyte voltammograms dis-
played on the insert of Fig. 3b. We note also in this figure a
displacement of the hydrogen current density to less cathodic
potential with increasing the NH4Cl concentration, this effect is
more pronounced for 5 mol dm−3 value.
In the reverse scan, towards the anodic direction, one observe
a strong over potential from −1.57 V vs SCE up to −1.40 V vs SCE,
this is allotted, as in the Fig. 3a to the typical nucleation process on
the working electrode. Further, in the anodic part of the voltammo-
grams, a high anodic peak is observed at −1.44 V vs SCE, its intensity
changes with the NH4Cl concentration, indeed it increases up to
4 mol dm−3, and decreases for higher concentration.
It is important to note that the shape of this peak is quite dif-
ferent from the one observed in Fig. 3a for the same cathodic limit
potential, where different structures were observed. In the pres-
ence of NH4Cl, the anodic peak has just one structure located at
−1.35 V vs SCE. This peak is then attributed to the dissolution of
manganese deposit in the ammonium chloride bath without the
presence of other species (hydroxides, etc.).
The intermediate concentration (3 mol dm−3) seems then to be a
good compromise to enhance the Mn deposits in our experimental
conditions.
The experimental equilibrium potential, as measured from the
voltammograms does not have a significant variation with the
NH4Cl concentration. The values are located in the range ( − 1.44
V vs SCE to − 1.45 V vs SCE). It is very close to the one calcu-
lated in the thermodynamic study (Eeq ∼ −1.47 V vs SCE). The light
shift observed in the experimental value may be due to additional
current like hydrogen evolution.
If the role of NH4Cl was only to complex with chlorides, a modi-
fication of the interfacial pH could occur, thus it will not contribute
Fig. 7. SE images obtained in SEM of the electrodeposited Mn–Bi films in a chloride bath (0.4 mol dm−3 MnCl2 4H2O, 10−3 mol dm−3 BiCl3, 3 mol dm−3 NH4Cl) on Cu/Si at
various deposition potentials, (a) −1.6 V vs SCE, (b) −1.65 V vs SCE, (c) −1.75 V vs SCE, (d) −1.8 V vs SCE, corresponding to the following deposition charge: (a) Q = −6.7 C, (b)
Q = −11.17 C, (c) Q = −19.14 C, (d) Q = −26.73 C.

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B. Benfedda et al. / Electrochimica Acta 56 (2011) 1275–1282
3.2.2. Electrodeposition of bismuth
For better illustrating the kinetic of the bismuth deposit, we car-
ried out a linear sweep voltammograms as a function of the speed
ω of the RDE, at a cathodic limit potential of −0.55 V vs SCE and two
different sweep rates 5 mV s−1 and 20 mV s−1 (Fig. 5a and b). The
voltammograms show a region where the current is approximately
constant. This is the limiting current plateau where the Bi3+ reduc-
tion is under complete mass-transport control. When the potential
is made more negative (up to −0.35 V vs SCE), a secondary cathode
reaction, hydrogen evolution, takes place in addition to the Bi3+
reduction. For the Levich plots, the limiting current values were
taken from the cross of the vertical line at −0.3 V vs SCE in each
current potential curve. The experimental points (Fig. 5c) are on
straight lines through the origin, this indicates that the process of
bismuth ions reduction was under diffusion control. It can also be
The electrochemical reactions taking place in the chloride bath
with the presence of bismuth cations were also studied on vitreous
carbon RDE. Fig. 4 plots show the cyclic voltammograms obtained
at several cathodic limit potential.
Starting from the rest potential ( − 0.2 V vs SCE), in the direction
of cathodic scan, a plateau of weak current, corresponding to bis-
muth reduction is observed at small potentials. Up to −1.05 V vs
SCE, one observes current fluctuations generated by the process of
hydrogen evolution. In the reverse scan, the dissolution peak of Bi is
observed up to −0.16 V vs SCE. Its intensity increases as a function
of the cathodic limit potential, in other words as the increase of the
deposit. No displacement in the peak position is observed, which
means that there is no change in the deposit composition.
Fig. 8. BSE images obtained in a SEM and corresponding EDX spectra of the Mn–Bi films electrodeposited on Cu/Si in 0.4 mol dm−3 MnCl2 4H2O, 10−3 mol dm−3 BiCl3,
3 mol dm−3 NH4Cl, at two different deposition potentials: (a) −1.6 V vs SCE, (b) −1.8 V/vs SCE.

B. Benfedda et al. / Electrochimica Acta 56 (2011) 1275–1282
1281
seen that the increase of the scanning rate did not have a significant
effect on the Levich plot, this indicates that the current is at or near
the steady state.
3.2.3. Electrodeposition of the Mn–Bi system
Manganese and bismuth reduction kinetics have been studied
simultaneously in a solution containing Mn2+ and Bi3+ under the
same conditions, the ammonium chloride concentration is fixed
at 3 mol dm−3 according to the last study. On the voltammograms
displayed in Fig. 6a a plateau of weak current corresponding to bis-
muth reduction is observed when scanning towards small cathodic
potential, starting from −0.2 V vs SCE. As the cathodic potential
increases (E < 1.57 V vs SCE), a significant current corresponding to
manganese deposition and hydrogen evolution is observed. In the
reverse scan two anodic peaks are observed. Peak A, the more sig-
nificant, observed at −1.44 V vs SCE is attributed to manganese
dissolution whereas the peak B at −0.2 V vs SCE is attributed to
bismuth dissolution. The relative intensity of the two dissolution
peaks is related to the species concentration in the electrolysis
bath.
3.3. Physicochemical and morphological characterization
According to the electrochemical study of Mn–Bi deposition,
different samples are elaborated on (Cu/Si) electrode, at various
potentials corresponding to different current densities (Fig. 6b).
The SEM images of the deposited films are shown in Fig. 7 (SE
images). They illustrate that the deposit is granular, it consists of
a large shaped of small crystallites which grain size is less than
5 ␮m. The number and the coalescence of these particles increase
with increasing the imposed potential.
The deposit composition is heterogeneous according to the BSE
images and the corresponding EDS spectra (Fig. 8). It reveals that
the deposit presents separate grains of manganese and bismuth.
The relative intensities of each element peak vary with the position
on the sample and the applied potential.
Several cases have been observed according to the deposition
potential (E): in the Fig. 8a (E = −1.6 V vs SCE), some large Mn grains
(dark area) and small Bi grains (white area) have been observed. The
Cu substrate is not completely recovered by the deposit. This fact
is clearly seen in the EDS spectra where mainly Cu is detected with
minor element of Mn and Bi.
In Fig. 8b (E = −1.8 V vs SCE), we have always some large Mn
grains (spectrum 1) with a thin isolated Bi grains (spectrum 2).
The chemical analysis is difficult due to the fact that the
volume of analysis is around 1 ␮m3 and the deposit is heteroge-
neous.
Fig. 9. (a) X-ray diffraction pattern (Cu K␣ radiation) of Mn–Bi electrodeposited on
Cu/Si at various deposition potentials in (0.4 mol dm−3 MnCl2 4H2O, 10−3 mol dm−3
BiCl3, 3 mol dm−3 NH4Cl, (a) Cu/Si substrate, (b) −1.6 V vs SCE, (c) −1.65 V vs SCE, (d)
−1.75 V vs SCE, (e) −1.8 V vs SCE. (b) Zoom of the X-ray diffraction pattern between
2Â = 35◦ and 2Â = 45◦.
peak intensity increases with increasing potential whereas the peak
related to bismuth decreases. These results are in agreement with
those obtained in the electrochemical study.
3.4. X-rays analysis
Fig. 9a shows the XRD pattern of the Mn–Bi films obtained at
various deposition potentials. We note peaks related to copper
and silicon which is used as a substrate. Concerning the deposit,
the spectra obtained reveal two different manganese phases ␣-Mn
(body centred cubic system) and ␥-Mn (body centred tetragonal
system), this is in agreement with results obtained from the lit-
erature [20]. XRD spectrum reveals the presence of characteristic
peaks of bismuth in its rhombohedral system with a predomi-
nant [0 1 2] texture, in agreement with others works [29,30]. A
small amount of manganese oxide is also detected in XRD spec-
tra (Fig. 9b), it was probably formed later on the surface of the
deposit.
4. Conclusions
The electrodeposition of Mn–Bi system from acidic chloride bath
was studied. It is found that the electrodeposition of Mn–Bi is pos-
sible in this bath. It is important to note that, the presence of an
optimal concentration of the ammonium chloride as additive is a
decisive parameter to obtain manganese deposit with a good effi-
ciency, to improve the electrolysis bath conductivity and also to
inhibit the formation of manganese hydroxide.
The electrodeposition of manganese occurs at more cathodic
potentials than bismuth, thus the BSE observations and its cor-
responding EDS analysis show that the Mn–Bi deposits have a
heterogeneous composition. It consists of separate grains of man-
ganese and bismuth. Further the crystal structure analysed by XRD
indicates the presence of two manganese phases, ␣-manganese
It is important to note that the intensity of the peak related to
each element notably evolves according to the deposition poten-
tial. So, at more cathodic potential, the manganese corresponding

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B. Benfedda et al. / Electrochimica Acta 56 (2011) 1275–1282
body centred cubic system and ␥-manganese body centred tetrago-
nal system. We have also observed characteristic peaks of bismuth
in its rhombohedral system.
Annealing of the samples is required to induce interdiffusion of
the two elements (Mn and Bi) and to form homogenous MnxBi1−x
alloy. Actually these experiments are in progress.
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