Effect of some experimental conditions on the electroplated Mn-Bi film composition
Salem Boudinar, Nassima Benbrahim, Baya Benfedda, Aziz Kadri
Laboratoire de Physique et Chimie des Matériaux (LPCM) Université Mouloud MAMMERI de Tizi-Ouzou
Tizi-Ouzou, Algérie boudinarphy@yahoo.fr
Eric CHAINET
LEPMI, Université Grenoble Alpes, F-38000 Grenoble France
CNRS, LEPMI, F-38000 Grenoble Grenoble, France
Abstract—In this study, the effect of some experimental parameters on the electrodeposition of Mn-Bi thin film from a mixed sulfate-nitrate bath was investigated under direct potentiostatic mode based on the previous published paper. The plating bath is characterized by the presence of some additives like boric acid and ammonium sulfate. Cyclic voltammety is used to the electrochemical analysis, the chemical composition and the morphology of the films obtained at different experimental conditions were examined by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The deposits have a cauliflower appearance with a heterogeneous distribution of manganese and bismuth. The Mn and Bi content in the deposit estimated by the EDS analysis revealed that the film composition depend strongly on the applied potential and the pH bath but no significant variation is obtained with varying the deposition time.
Keywords—Mn-Bi; Electrodeposition; Cyclic voltammety;
EDS; MEB
I. INTRODUCTION
The most study reported in the literature for the elaboration of the MnBi system is investigated using the physical methods, such as molecular beam epitaxial (MBE) [1,2] and vacuum evaporation [3,4]. But the electrochemical method is not sufficiently used for the development of this system. In this context and recently the electrochemical method was also explored for the electrodeposition of the Mn-Bi system. The problem often encountered is to find the suitable electrolytic bath for electrodepositing simultaneously the two elements in a good efficiency. In this case, there are just four paper published speaking about the possibility of electroplating the two elements Mn and Bi in a chloride bath [5], a mixed sulfate-nitrate bath [6,7] and very recently from a chloride bath [8]. This lack of articles is probably due the very low equilibrium potential value of the Mn2+/Mn couple in aqueous solution (EMn2+/Mn=-1.18V/SHE) compared to that of Bi3+/Bi (E Bi3+/Bi= 0.3V/SHE). And also to the complexicity of the manganese electrodeposition kinetics, in fact reduction of Mn2+ to Mn is always accompanied by hydrogen evolution, which makes it difficult to clearly detect the manganese reduction process. The manganese deposit is also easily oxidized [9].
In this work, we propose to study the electrodeposition of Mn-Bi system from a mixed sulfate-nitrate in thin films on a
copper substrate with a near stoichiometric composition and the effect of some experimental conditions on the composition Mn-Bi films.
II. MATERIAL
Electrochemical experiments were performed in a classical three-electrode glass cell. The working electrode was a gold rotating disk electrode (RDE) with an area of 0.038 cm2 and a copper substrate with an exposed area of 0.28 cm2. The counter electrode was a platinum wire. Saturated calomel electrode (SCE) was used as reference electrode.
All electrochemical experiments were carried with EG&G 273A potentiostat/galvanostat controlled with Power-suite software. The electrolytic baths consisted of 0.4 M MnSO4 1H2O, 10-3 M Bi (NO3)3 5H2O, 0.5 M H3BO3. (NH4)2SO4 (2.5 M) was used as a complexing agent to avoid the Mn and Bi precipitation and as an electrolyte support to increase the conductivity of the bath. The electrolytes were freshly prepared with de-ionized water. pH was adjusted to 2.3 by addition of H2SO4. Before the experiments, solutions were deaerated with nitrogen for 30 min. Deposition was realized at room temperature (22±1°C).
The electrochemical characterization was performed using the cyclic voltammetry (CV). The Mn-Bi deposits were characterized by scanning electron microscopy SEM (secondary electron (SE) and back scattered electron (BSE) imaging) and energy dispersive spectroscopy (EDS).
III. RESULTANDDISCUSSION III.1.Electrochemical characterization
The study of the electrodeposition kinetics of the Mn-Bi system in a mixed sulfate-nitrate bath containing both Mn2+
and Bi3+ ions posed some problems related to the difference of the two ions solubility. Consequently a study of the electrochemical kinetics of Mn-Bi as a function of the pH and the ammonium sulfate concentration is necessary to avoid Mn and Bi precipitation. From this study it was found that the optimal pH value required to avoid this precipitation is 2.3 and the optimal ammonium sulfate concentration is 2.5M.
The Figure (1) shows the cyclic voltammetry of Mn-Bi deposition and dissolution on the gold rotating electrode. The cathodic limit potential was fixed at-1.8 V.vs.SCE.
Fig.1. Cyclic voltammetry study of Mn-Bi co-deposition.Vb= 20 mV.S-1
Starting from the open circuit potential (E=0.18V.vs.SCE) and scanning in the cathodic potential direction, a weak current plateau corresponding to the discharge of Bi3+ ions on the gold electrode is observed. A slight increase in the current density is observed at -0.9V.vs.SCE, this corresponds to the hydrogen evolution at this range of potential with bismuth reduction. A further increase in the current density is observed at - 1.65V.vs.SCE, it is attributed to the discharge of both Mn2+
and Bi3+ with hydrogen reduction.
In the reverse scan two anodic peaks are observed, a dissolution peak (A) observed at -1.42V.vs.SCE is attributed to the Mn dissolution. This dissolution peak has a negative current density value because the cathodic reduction of H+ and H2O are still occurring there; so the dissolution of small amount of Mn will not bring the current back to positive value. Another peak (B) is observed at 0.01 V.vs.SCE attributed to the bismuth dissolution.
The study of the electrodeposition kinetics of Mn-Bi system as a function of the pH and the ammonium sulfate concentration by cyclic vollammetry was used to estimate the current efficiency of Mn-Bi electrodeposition. In this method the current efficiency of manganese-bismuth electrodeposition has been estimated by calculating the total charge from different voltammograms recorded at different pH and ammonium sulfate concentrations, assuming that the anodic peak correspond to the amount of the deposited Mn-Bi and the cathodic current to both Mn, Bi and hydrogen evolution.
Globally, we can note that the all curves (Fig.2) show the same evolution of current efficiency as a function of the pH and ammonium sulfate concentration. It was found that the current efficiency increase with the two parameters.
Fig.2. Current efficiency of Mn-Bi electrodeposition as a function of pH bath and ammonium sulfate concentration.
The best current efficiency is obtained for 2.5 mol/L (6.91%), while for 2 mol/L the current efficiency decreases (6.61%) (Fig.2). This observation can be attributed to the hydrogen evolution became very important. But the current efficiency increase from 3.2% at pH=1.8 to 7 % at pH=2.3 and becomes relatively constant.
In this method, we under estimate the real efficiency because the Mn-Bi deposit is not completely dissolved at the end of the cyclic voltammetry
III.2.SEM characterization
According to the electrochemical study, thin films of Mn-Bi system have been electrodeposited on cooper substrate at different duration time and different pH.
Fig.3. SEM images of the Mn-Bi electrodeposited on copper substrate at different duration time (insert image (correpoding BSE images)).
(a) 150s, (b) 300s, (c) 400s, (d) 600s, E=-1.8V/SCE.
Fig.4. SEM images of the Mn-Bi electrodeposited on copper substrate at different pH (insert image (correpoding BSE images)). t=300s, E=-1.8V/SCE.
(a) pH=1.8, (b) pH=2, (c) pH=2.5, (d) pH=2.9.
The SEM images of the deposit obtained at different duration time and pH bath are shown in Fig (3,4) respectively. As we can see that the deposit has a cauliflower morphology and a heterogeneous appearance, with a dispersed grains of bismuth (white area) and a more continuous manganese deposit (dark area), this is related to the difference between the equilibrium potential of the two redox couple and to the strong immiscibility of the two elements. This observation is confirmed by the BSE imaging shown in the insert image.
III.3.EDS characterization
In order to study the effect of some experimental conditions such as the pH bath, potential and the deposition time on the Mn and Bi contents, the energy dispersive spectroscopy is used. The Fig.5 (a,b,c) shows the variation of Mn and Bi at% contents on the deposit at different experimental conditions.
The global compositional analysis obtained at different deposition potential (Fig 5(a)) indicates that Bi predominate at lower potential (-1.7V/SCE). As the deposition potential increases the Bi content decreases while the Mn content increases up to a deposition potential of -1.75 V/SCE where the film composition becomes relatively constant (63% Mn and 37%Bi), this observation present a good agreement with the electrochemical study. In other hand the contents of Mn-Bi films depends strongly on the pH bath (Fig 5(b)), in the fact le bismuth predominate at tower pH, so the manganese content increase with increasing the pH bath.
But with increasing the deposition time (Fig 5(c)) no appreciable variation in the Mn and Bi content is observed, at higher time, the deposits became dark and poorer in quality.
Fig.5. Variation of Mn and Bi at% contents on the deposit at different experimental condition, (a) potential t = 300 s, (b) time E=-1.8V.vs.SCE, (c)
pH bath, E=-1.8V.vs.SCE.
(a)
(b)
(c)
IV. CONCLUSION
In this study, the effect of some experimental parameters on the electrodepostion of Mn-Bi thin films was presented.
The electrochemical study of the Mn-Bi electrodeposition is used to determine the potential region for the deposition and dissolution of manganese-bismuth system.
The current efficiency of Mn-Bi deposition is very low.
The morphological characterization of the deposit by scanning electron microscopy with SE and BSE mode shows a heterogeneous surface with a dispersed bismuth grains and a more continuous manganese deposit. This appearance is attributed to the difference between the equilibrium potential of the two redox couple and to the strong immiscibility of the two elements.
The EDS analysis indicates that the content of Mn and Bi in the deposit depends enormously of the pH bath and the applied potential. The equiatomic composition of Mn-Bi system can be obtained for the control of pH bath and applied potential.
In the future, it is interesting to test some additives to complex the Mn-Bi system.
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