Electrodeposition of Cu-Zn-Sn coating in citrate medium

The electrodeposition of Cu-Zn-Sn (CZT) coating at ambient temperature was investigated. The bath consists of metal salts SnSO4, ZnSO4,7H2O and CuSO4,5H2O and sodium citrate (NaC6H5Na3O7,2H2O) as a complexing agent. For precipitation, the pH is maintained at 5. The reducing of copper, tin and zinc through Cu2HCit, Sncit and ZnHcit complexes respectively are confirmed by the presence of three cathodic peaks on the voltammograms realized on steel and ITO glass substrate. X-ray diffraction patterns revealed peaks corresponding to the phases: Cu-Zn cubic, Cu-Sn hexagonal and β-Sn tetragonal. The deposition rate is 35 μm/h. SEM observation and EDAX analysis showed that the coating consists of a uniform CZT layer of which composition is 55% copper, 20% zinc and 25% tin at -1.5V. A preliminary study showed a remarkable improvement in the corrosion resistance of CZT coated steel in comparison with bare steel.

Furthermore, tin, zinc and copper are the main parts that constitute the layers used in photovoltaic cells when combined with sulfur or selenium to form quaternary deposits (Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe )) [21][22][23] .
Kesterite Cu2ZnSnS4 (CZTS) thin films are attracting much interest as an alternative system to Cu(In, Ga)Se2 (CIGS) and CdTe thin films because In, Ga and Te are rare and expensive elements and the other reason is the toxicity of cadmium [24][25][26][27][28][29] . We also note that CZTS thin films offer excellent properties such as good mechanical properties and good absorption coefficient 28 . Besides, CZTS film contains Zn and Sn that are naturally abundant, very cheap materials to replace In and Ga. Beside, CZTS thin film has the same tetragonal structure as that of CIGS 29,30 .
The major problem of co-deposition of metals is to bring their reduction potential closer. The addition of complexants in baths is a simple and effective solution. Thus, various complexing agents have been used in the case of the alloy CZT: tartrate 2 , gluconate 10 , citrate 4,13,20 and trisodium nitrilotriacetic 31 . In a previous Sn-Zn alloy electrodeposition study, we have chosen citrate to co-deposit tin and zinc because of the stability of the bath at a near-neutral pH in order to make industrial application easy 32 .
We have opted for the same complexant in the case of the CZT ternary alloy coating electrodeposition. We have used the thermodynamic model of Kazimierczak H. et al. to determine the optimal pH range to obtain a stable electrolyte 4 .
The CZT coating was carried out on copper and common steel substrates for the electrochemical study then on ITO glass in order to fabricate CZT precursor layers for a possible application in photovoltaic cells. On the other hand, morphology, composition, crystallographic study and corrosion resistance of CZT deposits were characterized by SEM, EDX, DRX and electrochemical impedance.

Experimental details
The electrolysis cell is a double-walled Pyrex cylinder with a volume of 100 ml. It is equipped with a Teflon lid with 5 apertures. Three of them allow the passage of electrodes necessary for electrochemical measurements: the working electrode, the counter electrode or auxiliary electrode and the reference electrode. The other two allow purging of the dissolved oxygen by nitrogen bubbling and temperature control. We have used steel, copper and ITO substrates with a surface area of 1cm 2 as working electrode, Pt plate as the counter while SCESat as the reference electrode to which potentials will be referred in the following.
Before the immersion test, the steel and copper substrates were abraded using emery paper up to 1200 grade, cleaned with ethanol, etched in 10% dilute sulfuric acid, washed with distilled water and dried finally. In the case of ITO glass, substrates were cleaned ultra-sonically in ethanol and dried before electrodeposition of CZT coating.
The electrolytes used are composed of tin sulfate, zinc sulfate, copper sulfate and sodium citrate as a complexing agent. The concentrations are given in Table 1. The pH was adjusted at 5 by sulfuric acid, and the temperature is set at 20 ± 2°C. This formulation was chosen after some preliminary tests on the chemical stability in order to avoid the precipitation of the hydroxides.
The electrochemical measurements were carried out using Voltalab PGZ 100® (Potentiostat/Galvanostat) monitored by Voltamaster 4. The potential range was performed in between 0.5V and -2V with a scan rate of 10mV/s and 25mV/s. The impedance studies were carried out in a frequency range of 100 kHz to 10mHz with the amplitude of ± 10mV. X-ray diffraction was performed by a PANalytical X'PERT3 POWDER diffractometer with Cu Kα1 radiation.
The Ecorr corrosion potential and the Icorr corrosion current was performed using nonlinear regression using the Origin 6 software. The morphology of the coatings was analyzed by the FEI Quanta 200 scanning electron microscope. The chemical composition of the CZT deposit elements was determined by EDX. The mass of the deposit was determined by an analytical balance with an accuracy of 0.1mg.

Baths stability
The major problem in the electroplating of CZT ternary alloys is the preparation of a stable electrolyte. Preventing the precipitation of tin (II), zinc (II) and copper (II) hydroxides in these baths is essential.
Thus, the addition of citrate in electrodeposition baths is of great interest because it forms aqueous and electroactive complexes with Sn (II), Zn (II) and Cu (II). We used the H. Kazimierczak We observed a cathodic current from -1.05V related to the process of hydrogen evolution via the reduction of the protonated form H2cit 2according to the reaction: An identical result was obtained on steel in a previous study 32 . Fig.5 shows the voltammogram realized in the solution containing cupric ions and citrate. A wave of reduction appearing at -0.1V would be attributed to the reduction of Cu2HCit2 3cupric complex according to the reaction:

Citrate-cupric ion bath
A slight plateau of cathodic current density is observed due to the diffusion of cupric ions and then a rapid increase in the current density related to the evolution process of hydrogen. This potential is shifted by 100 mV compared to the study carried out on steel 32 . A plateau of cathodic current density attributed to the diffusion of stannous ions and a process of evolution of hydrogen from the protonated form H2cit 2are observed. Fig.7 shows the voltammogram obtained in a solution containing citrate and zinc ions. A cathodic current appears around -1V linked to the process of hydrogen evolution via the reduction of the protonated form H2cit 2-. Then, we observe a decrease in the current density from -1.6V related to the reduction of hydrogenated Zn-citrate complexes:

Citrate-zinc ion bath
ZnHcit -+ H + + 2e -→ Zn + H2cit 2-  Fig.8 shows the voltammogram obtained in the electrolyte containing citrate and cupric and stannous ions. It presents three reduction waves of Cu2HCit2 3-, Sncit 2and H2Cit 2that appear at potentials practically similar to those obtained in the baths containing respectively citrate-cupric ions, citratestannous ions and citrate. On the other hand, we notice that the intensity of the reduction peak of the stannous ions is high. Indeed, the X-ray diffraction patterns of a deposit made at a potential -0.9V show two phases tetragonal β-Sn and Cu6Sn5 (Fig.9) .   Fig.10 shows the voltammogram realized in a citrate solution containing copper and zinc ions. Copper reduction begins at a potential of -0.1V as expected.

Citrate-cupric ion bath and zinc ions
The cathodic current around -1V would be related to the process of hydrogen evolution via the reduction of the protonated form H2Cit 2-. Then, we see a substantial increase in the current density due to the zinc electrodeposition via the hydrogenated Zn-citrate complexes.   We observe in both cases four cathodic waves corresponding respectively to the reduction of the species Cu2HCit2 3-, Sncit 2-, H2Cit 2and ZnHcitappearing at the same potentials as for the electrolytes containing each metal ion with citrate. Indeed, the deposit is ternary and consists of copper, tin and zinc. The X-ray diffraction patterns of CZT coating electrodeposited at a current density of -22.5mA/cm² (-1.5V) on steel show three phases: tetragonal β-Sn and Cu6Sn5 and Cu18.20 Zn33.80 (Fig.14). During the reverse scan, we observe three anodic waves corresponding to the oxidation of zinc, tin and copper as observed by authors 20 .

The deposition efficiency Rd = Δ / mth
The values of mass variation, thickness, rate and deposition efficiency are summarized in Table 2. We note that the deposition rate is relatively high, and the deposition efficiency Rd is less than 1. This is attributed to the fact that part of the applied current is consumed by the reduction of hydrogen. The same procedure was carried out in the case of a coating electrodeposited on ITO glass; the deposition rate is twice slower. Fig.15 shows SEM image of CZT coating obtained at E = -1.5V (I = -22.5mA/cm 2 ) on steel. We observe granular and adherent coating without cracks, but it presents some pores and irregularities. It consists of 55 at% copper, 25 at% tin and 20 at% zinc (Fig.16).

Corrosion resistance
The corrosion resistance of the CZT coating electrodeposited at -22.5mA/cm² in NaCl (3%) was characterized by the determination of the Icorr corrosion current and the Ecorr corrosion potential using polarization curves and electrochemical impedance polarization resistance. Fig.17 shows the polarization curves for steel and CZT coated steel with a thickness of 8.7μm. The electrochemical parameters are reported in Table 3. We note that the Ecorr corrosion potential of CZT coated steel is much more anodic than that of bare steel and that Icorr corrosion current significantly decreases in the case of the ternary coating.   Table 4. We note that the polarization resistance Rp is significantly higher for CZT coated steel. Thus, the polarization and electrochemical impedance measurements show that the CZT coating provides excellent protection against corrosion.

Conclusion
In this study, the deposit of CZT was developed at ambient temperature on a steel and ITO substrates with citrate as a complexing agent. At a potential of -1.5V (I=-22.5mA.cm -²), the deposition rate of the coating is 34.8 μm/h on steel whereas on ITO glass is twice slower. Electrochemical investigations showed that copper, tin and zinc were reduced via Cu-citrate (Cu2HCit2 3-), Sn-citrate (Sncit² -) and Zn-citrate (ZnHcit -) complexes, respectively. The SEM observation revealed a granular and adherent coating without cracks, but some pores and irregularities were observed. The coating composition at -1,05V consists of 55 at% copper, 25 at% tin and 20 at% zinc according to EDAX analysis. The X-ray diffraction patterns of CZT coating electrodeposited at a current density of -22.5 mA/cm² (-1.5V) on steel show three phases: tetragonal β-Sn and Cu6Sn5 and Cu18.20 Zn33.80. There is also a remarkable improvement in the corrosion resistance of the steel coated by the CZT deposit compared to bare steel. Indeed, the polarization resistance Rp measured by electrochemical impedance is significantly higher for the coated steel.