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Studying the kinetics of electrode reactions on copper, silver and gold in acid thiourea-citrate electrolytes
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 6 (2019) 141–149
www.materialstoday.com/proceedings
3rd ISE SSRSEU 2018
Studying the kinetics of electrode reactions on copper, silver and gold in acid thiourea-citrate electrolytes O. Smirnovaa*, А. Brovina, A. Pilipenkoa, Yu. Zhelavskaa Department of technical electrochemistry, National Technical University «Kharkiv Polytechnic Institute» Kyrpychova str., 2, Kharkiv, 61002, Ukraine
Abstract The kinetics of electrode reactions that take place on copper, silver and gold electrode in acid thiourea-citrate electrolytes has been studied. It is established that the anodic dissolution of metals is accompanied by the formation of stable thiourea metal complexes of a cation-type complex. The citric acid provides the acidic pH-value of the solutions and contributes to the active dissolution of anodes. The ionization rate is limited by the diffusion of charged particles into the electrolyte volume. The cathode reduction of metals obeys the mechanisms of mixed kinetics that includes the diffusion and the preceding chemical complexes dissociation reaction. The charge of the complex ions of copper, silver and gold is +1. Therefore, their dissolution at the anode and reduction at the cathode occurs with the maximum possible electrochemical equivalent. © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of 3rd ISE Satellite Student Regional Symposium on Electrochemistry in Ukraine. Keywords: complex compounds; polarization relationship; reaction overpotential; limiting stage; mixed kinetics
1. Introduction An advance of electronic, electrotechnical and jewelry industries demands more quality functional and protective-decorative coatings. Therefore, galvanic engineering faces the tasks of developing new productive and environmentally friendly technologies of coatings deposition. The main ways of their improvement are the development of new electrolytes and the optimization of coating deposition modes.
* Corresponding author. Tel.: +38-066-489-3775 E-mail address: [email protected] 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of 3rd ISE Satellite Student Regional Symposium on Electrochemistry in Ukraine.
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O. Smirnova et al./ Materials Today: Proceedings 6 (2019) 141–149
Copper, silver and gold coatings are widely used for electroplating. Cyanide electrolytes are widely used for industrial purposes. These electrolytes are toxic and are not-easy-to prepare and use. These also require appropriate arrangements for the waste solution disposal and the waste water neutralization. Today, the electrolytes are developed that enables simultaneous solution of the problems of ecological safety and the production of coatings with prescribed characteristics. Acid thiourea-citrate electrolytes that can oust cyanide electrolytes have great prospects. At the first stage of the development of new electrolytes intended for the coating deposition we investigate the kinetics of anodic and cathodic reactions in the given electrolytes. 2. Reference Data Analysis and Problem Definition The use of thiourea for copper, silver and gold technologies was considered for the crude ore enrichment processes [1–3]. The electrolytes were developed containing the thiourea and citric acid in the form of additions to improve the quality of coatings or the buffer properties of solutions. An ample amount of data arrays was obtained for the deposition kinetics, structure and the properties of copper coatings produced from sulfate, nitrate, pyrophosphate, citrate, tartrate-sulphamate and other electrolytes. The papers [4, 5] give consideration to the influence that the added thiourea has on the structure of electrodeposited copper in copper baths. It hampers the sulfate electrolyte reduction process of copper ions providing the formation of fine crystalline copper coatings. The addition of metal citrates to copper electrolytes provides the formation of compact copper depositions [6, 7]. Silver plating electrolytes and gold plating electrolytes are known to be based on the potassium hexacyanoferrate (II). These can be attributed to conditionally cyanide-free electrolytes, because metal ions are situated in the cyanide complex and KCN can be accumulated in the solution in operation. The sulfate electrolytes of silver plating and gold plating are also known. Despite their relative ecological safety these failed to find wide application due to their complicated operation. The paper [8] describes the use of thiourea electrolytes for the silver deposition as an alternative for cyanide electrolytes. The thiourea forms various complexes with Ag (I), the composition of which depends on the ratio of ligand to silver in the solution [9, 10]. The thiourea affects the smoothness and uniformity of silver coatings [11, 12]. The acid electrolyte of silver plating was applied for in the patent [13]. It contains silver nitrate, sulfamic acid and thiourea. The electrolyte was recommended for the application of finish coatings when producing printed circuit boards. The patent [14] suggests the electrolyte that contains silver nitride, thiourea and sodium chloride and cetyltrimethylammonium bromide intended for the stainless steel silver plating. The data are available on the development of acid chloride-sulfate electrolyte for the gold-plating [15]. This electrolyte is recommended for the deposition of gold coatings onto corrosion-resistant alloys containing chrome and nickel. The paper [16] describes the mercaptotriazole-based electrolyte for the gold deposition. The papers [17, 18] note that thiourea undergoes decomposition during electrolysis in neutral solutions, and the bath fails to operate. The paper [19] describes the peculiarities of the use of thiourea electrolytes to produce nanostructured gold deposits. Thus, the electrolytic deposition of copper, silver and gold from the cyanide-free electrolytes faces many problems related to the drawbacks of those electrolytes and their limited use. The development of new electrolytes still remains to be a vital problem whose solution will provide its wide application for different branches of the industry. 3. Objective and Problems of the Investigation The objective of the research was to study the kinetics of electrode processes that occur on copper, silver and gold in acid thiourea-citrate electrolytes. To achieve the goal the following tasks were set up: – studying the influence of electrolyte composition on the behavior of copper, silver and gold in the conditions of electrode polarization in thiourea and citric acid-base solutions; – defining the nature of a limiting stage and the kinetic characteristics of metal ionization reactions and those of the discharge of their complex ions.
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4. Methods of Experimental Investigations Working electrolyte solutions were prepared using the distilled water-based reagents. The electrolyte solution composition included the following components: metal salts-copper citrate (II) Cu2C6H4O7 · 2.5H2O, silver nitrate (I) AgNO3, gold chloride (III) AuCl3 and also organic ligands, in particular thiourea CS(NH2)2 and citric acid C6H8O7. The experiment was carried out in the temperature-controlled cell YCE-2 with the electrolyte volume of 100 cm3 in the temperature range of 15 to 60 °C. The electrodes with the surface area of 1cm2 made of copper (the MM grade), silver (the Sl 999.9) and gold (the Gl 999.9) were used as working electrodes. The electrode surface was prepared prior to the experiment using the standard technique. Platinum (the Pl 99.9 grade) with the surface area of 2 cm2 was used as an auxiliary electrode. The potentials were measured and compared with the chloride-silver saturated reference electrode EVL-1M1. All the potential values were recalculated in terms of hydrogen scale. The pH value was defined using the pH-673M device. The volt-ampere characteristics of electrode processes were obtained using the pulse potentiostat PI-50-1.1 equipped with the PR-8 programmer. A change in the current or voltage was recorded by the self-recorder LKD-4. The registered current was related to the geometric surface of test specimen. 5. Electrode Process Kinetics Research Data It is known that the metals of copper subgroup (Cu, Ag, Au) form stable compounds with thiourea. These compounds look like a cation with the charge +1 (for copper, silver and gold) and +2 (for copper). These compounds are the most stable in the acid medium with pH = 1–4. Therefore, to study the kinetics of electrode reactions on the given metals we selected thiourea solutions with the citric acid. It was established experimentally that these systems are rather stable at the temperatures of 18 to 60 °C. When thiourea is in abundance, the ions of Cu+, Ag+ and Au+ are totally bonded forming the complexes with CS(NH2)2 molecule. Depending on the ligand concentration, these complexes have different coordination numbers. The complexes with the citric acid have lower stability in comparison to those of the given metals with thiourea. The paper [20] gives the complexes formulas and their instability constants. To study the kinetics of anodic and cathodic reactions the volt-amperometric measurements were taken with the subsequent graphic processing of obtained data. Fig. 1 gives anodic polarization relationships taken for copper, silver and gold in the given solutions. Thiourea and citric acid concentrations had an equivalent molar ratio. Mass concentration of metal salts in all the solutions was the same, i. e. 1 g·dm–3 in terms of metal.
Fig. 1. Anodic polarization relationships obtained for copper subgroup metals in the solutions containing 0.1M CS(NH2)2 + 0.1M C6H8O7 at t = 25 °C and υs = 1·10–2 V·s–1.
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The curves show the anodic polarization of each electrode results in an increased current density forming the regions of the active dissolution of given metals. The achieved current maximum for the first ascending branches of curves characterizes the limiting anode current density. An increase in the current after a maximum is achieved, is explained by the further metal oxidation with the formation of ions of a higher charge Cu2+, Au3+ and electrochemical oxidation CS(NH2)2. To define the type of control for the process of anodic dissolution of the metals, the appropriate initial sections of polarization curves in Fig. 1 were reorganized in the coordinates of reaction overpotential relationship as the function of current density logarithm. Fig. 2 shows the type of these relationships that correspond to different types of the kinetics. Fig. 2 shows that a maximum linearity of these relationships is manifested in mixed kinetics coordinates. It means that the anode reaction rate is limited simultaneously by the electrochemical stage and the diffusion stage. Extrapolation of the initial section of the relationships (fig. 2, a) allowed us to define the slope angle of relationships and accordingly the coefficients a and b of the Tafel equation for copper, silver and gold. The exchange current density j0, a number of electrons participating in the reaction and the charge transfer coefficient β were also determined. The kinetic characteristics of electrochemical reactions and calculated constants of the instability K of metal compounds in these solutions are given in Table 1. Table 1. Kinetic characteristics of the ionization of Cu, Ag and Au in the solutions containing 0.1M CS(NH2)2 + 0.1M C6H8O7 and the calculated constants of instability of metal compounds containing thiourea. Metal
a (V)
b (V)
j0 (А·см–2)
β
z
Кi
Cu Ag Au
1.59 1.36 1.00
0.48 0.38 0.25
4.3·10–3.3 3.0·10–3.6 1.5·10–4.0
0.12 0.15 0.24
1 1 1
1.3·10–13.8 2.1·10–15.06 4.0·10–25.15
Fig. 2. The dependence of anodic overpotential on the current density in the semilogarithmic coordinates of electrochemical (a), diffusion (b) and mixed kinetics (c).
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Fig. 3 shows cathode polarization potentiodynamic relationships obtained for copper, silver and gold in acid thiourea-citrate electrolytes. A mass concentration of metal salts is similar in all the solutions and is equal to 5 g·dm–3 in terms of metal. To provide the stability of complexes, the thiourea concentration in the solution was increased in proportion to an increase in the metal concentration. Fig. 3 shows that the polarization of copper, silver and gold electrodes results in the cathode reduction of copper, silver and gold, accordingly. The cathode polarization is characterized by an increase in the current density reaching the maxima (limiting current densities). Only one reaction takes place in this case, i.e. the discharge of complex cations of the metals. Then the limiting current plateau is formed. A further biasing of the cathode potential gives the two reactions – the limiting current metal deposition and the hydrogen release. The processing of polarization curves Fig. 3 in the semilogarithmic coordinates shows that the absolute linearity of reaction overpotential as a function of the current density is developed in none of the coordinates at the cathode reduction of copper, silver and gold (Fig. 4). However, the highest linearity of the three options is observed in mixed kinetics coordinates. It means that the electrode reaction is limited both by the diffusion stage of discharging particles and directly by the discharge transfer. The contribution to the cumulative overpotential of chemical stage, in particular dissociation of the complex cations of metals is not excluded. Exchange current density values j0, a number of electrons participating and the charge transfer coefficient were also determined. The kinetic characteristics of electrochemical reactions and the calculated instability constants of the complex ions of metals in these solutions are given in Table 2. Table 2. The kinetic characteristics of the discharge reaction of complex ions of copper, silver and gold in the solutions containing 0.5M CS(NH2)2 + 0.1M C6H8O7 and the calculated instability constants of the complexes of these metals with thiourea. Metal
-a (V)
-b (V)
j0 (А·см–2)
α
z
Кi
Cu Ag Au
0.59 0.47 0.50
0.15 0.12 0.10
2.6·10–3.9 1.1·10–4.1 3.2·10–5.0
0.39 0.51 0.59
1 1 1
6.4·10–13.05 5.4·10–13.39 2.0·10–22.5
A comparison of equilibrium electrode potentials recorded for copper, silver and gold shows that these potentials depend to a great extent on the concentration of metals and thiourea, and to a lesser extent on the concentration of citric acid (Fig. 5). The concentration of thiourea as the main ligand affects the overpotential and the reaction rate. First of all citric acid is essential because of its acidic value of pH that is important for the stability of electrolytes. In the anode process citric acid provides a uniform dissolution of the metals and prevents their salt passivation [21]. Fig. 6 shows the influence of the concentration of thiourea on the limiting current density at the anodic dissolution of copper, silver and gold in the solutions with the same concentration of citric acid in logarithmic coordinates. The data approximation allowed us to determine the order of chemical reaction for the reagent, i.e. thiourea.
Fig. 3. Cathode polarization relationships obtained for the metals of copper sub-group in the solutions containing 0.1M CS(NH2)2 + 0.1M C6H8O7 at t = 25 °C and υs = 1·10–2 V·s–1.
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Fig. 4. The dependence of the overpotential of cathode reaction on the current density in the semilogarithmic coordinates of electrochemical (a), diffusione (b) and mixed kinetics (c).
Fig. 5. Dependence of the equilibrium potentials recorded on Cu, Ag, and Au - electrodes on the concentration of CS(NH2)2 in the solutions without C6H8O7 and with 0.1M C6H8O7.
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Fig. 6. Logarithmic dependence of the limiting current density for the anodic dissolution of metals on the concentration of thiourea in the electrolytes containing 0.1M C6H8O7 + 0.1–1M CS(NH2)2.
The thiourea-calculated orders of reactions for copper, silver and gold are close to 1. It means that the ionization of these metals takes place at the first stage of anodic process with the loss of one electron and adjunction of one molecule of thiourea. The next stage of the process is the diffusion of the formed complex cations of metal in depth of the solution and the adjunction of the ligand molecule to the compound. A number of thiourea molecules coordinated by the ion of metal, i.e. the complexing compound depends on the concentration of thiourea in the electrolyte solution. The orders of thiourea reactions for the cathodic process of the reduction of copper, silver and gold is also approximately equal to 1. It means that the discharge of the ions of these metals occurs with the loss of one molecule of the ligand. To determine the limiting stage of cathode process, the Semerano criterion or the criterion of velocity was used. Fig. 7 gives logarithmic dependences of the limiting current density on the potential scanning rate. Fig. 7 shows that such connection for the relationships 1 and 2 is linear and it is not ascending to a zero. The relationship 3 in Fig. 7 is linear and crosses the origin of coordinates. A character of the relationship 3 shows that the electrode reaction is irreversible and the rate of electrochemical stage is lower than that of the diffusion stage. Calculated values of the Semerano criterion are as follows: for copper – 0.3 and for silver – 0.2. For gold the Semerano criterion is equal to 0.5. This value of the velocity criterion is peculiar for the processes that are limited by the charge transfer rate, diffusion and the chemical reaction kinetics, and it excludes catalytic and adsorption processes.
Fig. 7. Logarithmic relationship of the limiting current density as the function of potential scanning rate for cathode polarization curves obtained in the acid thiourea-citrate electrolyte.
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6. Discussion of the Research Data of the Kinetics of Electrode Processes Based on the obtained experimental data we can assume that the dissolution of metals of the copper sub-group proceeds adhering to the following mechanism. Initial stages of the process are the metal ionization and the interaction of their singly charged ions with one molecule of thiourea. Later on one more or even several molecules of thiourea are added with an increase in the coordination number of complex compound. Then the complex cations of metal interact with citric acid anions with the association of well-dissolved complexes. For example, Cu0 – e– + CS(NH2)2 → [Cu(SC(NH2)2)]+ [Cu(SC(NH2)2)]+ + CS(NH2)2 → [Cu(SC(NH2)2)2]+ [Cu(SC(NH2)2)2]+ + C6H7O7– = [Cu(SC(NH2)2)2][(C6H7O7)]
(1) (2) (3)
Ag0 – e– + CS(NH2)2 → [Ag(SC(NH2)2)]+ [Ag(SC(NH2)2)]+ + 2CS(NH2)2 → [Ag(SC(NH2)2)3]+ [Ag(SC(NH2)2)3]+ + C6H7O7– = [Ag(SC(NH2)2)3][(C6H7O7)]
(4) (5) (6)
Au0 – e– + CS(NH2)2 → [Au(SC(NH2)2)]+ [Au(SC(NH2)2)]+ + CS(NH2)2 → [Au(SC(NH2)2)2]+ [Au(SC(NH2)2)2]+ + C6H7O7– = [Au(SC(NH2)2)2][(C6H7O7)]
(7) (8) (9)
The cathode reduction of copper, silver and gold in acid thiourea-citrate electrolytes proceeds according to the sequence mechanism inverse to the anode process. The preliminary stage is the dissociation stage of complex compounds followed by the ion discharge [Cu(SC(NH2)2)]+, [Ag(SC(NH2)2)]+ and [Au(SC(NH2)2)]+ with the attachment of one electron and the cathode deposit crystallization. Also, the influence of such factors as the concentration of metals in solution, temperature and mixing of electrolyte on the rate of electrode reactions was studied in the paper. It is established that the temperature and mixing of the electrolyte actively influence the anodic dissolution of copper and silver. This indicates diffusion control of the process [21]. The potential scanning rate actually has no influence on the anodic dissolution of gold. It is indicative of the control of electrochemical and chemical stages. The content of metal salt in the solution and its temperature are more essential for the cathode reduction of copper [22], silver and gold. The stirring during the deposition of gold coatings is not of great importance, which is indicative of the retardation of cathode reaction at the phase of chemical and electrochemical stages. This difference in the metals kinetics can be explained by a higher stability of the thiourea complex of gold in comparison with the analogous compounds for copper and silver. 7. Conclusions Acid thiourea-citrate solutions with pH = 1–4 can be used for the creation of new efficient and environmentally sound electrolytes for copper plating, silver plating and gold plating. These electrolytes are prepared using simple metal salts, thiourea and citric acid. Thiourea forms fairly stable complex cation-type compounds with metal ions. Citric acid functions as a buffer reagent, providing the electrolyte stability. Active dissolution of metals is due to the joint presence in the solution of thiourea and citric acid. The anode process proceeds without the passivation of anodes and oxidation of organic substances on them, which is of great importance for the stable electrolyte operation. The kinetics of electrode reactions on copper, silver and gold in acid thiourea-citrate electrolytes obeys the laws of mixed kinetics. However, diffusion control is more typical for the anodic process, and for the cathodic process, the retardation at the stage of the previous chemical reaction (dissociation of complex) is more characteristic. The charge of complex ions of copper, silver and gold in thiourea-citrate electrolytes is equal to +1. Their reduction on the cathode occurs with maximum possible electrochemical equivalent that provides the cathode process energy efficiency. The metal deposition rate can be increased due to an increase in the metal concentration in solution and the electrolyte heating (before 40–50 °C).
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References [1] V. I. Radomskaja, R.M. Radomskij, and L. M. Pavlova, Georesursy, 5 (2013) 22–27 [2] L. F. Kozin, Himija i tehnologija blagorodnyh metallov – zolota, serebra. T. 1. Problemy i perspektivy. – Kiev: TOV «NPP Interservis», 2014 [3] D. Jerdjenjechimjeg, and D. Dorzh, Vestnik Burjatskogo gosudarstvennogo universiteta, 3 (2012) 91–94 [4] V. S. Donepudi, R. Venkatachalapathy, P. O. Ozemoyah, C. S. Johnson, and J. Prakash, Electrochem. Solid State Lett., 4 (2001), C13 [5] N. Tantavichet, and M. D. Pritzker, Transactions of the IMF, 84 (2006), 36–46 [6] F. I. Lizama-Tzec, L. Canché-Canul, and G. Oskam, Electrochim. Acta, 56 (2011), 9391–9396 [7] W.-G. Wu, F.-Z. Yang, M.-H. Luo, Z.-O. Tian, and S.-M. Zhou, Acta Phys.-Chim. Sin., 26 (2010), 2625–2632 [8] E. Gómez, J. García-Torres, and E. Vallés, J. Electroanal. Chem., 594 (2006), 89–95 [9] P. M. Heinrichs, J. H. Ackerman, and G. E. Maciel, J. Am. Chem. Soc., 99 (1977), 2544–2548 [10] S. Ahmad, A. A. Isab, and H. P. Perzanowski, Transition. Met. Chem., 27 (2002), 782–785 [11] O. Azzaroni, P. L. Schilardi, R. C. Salvarezza, and A. J. Arvia, Langmuir, 15 (1999), 1508–1514 [12] B. Reents, W. Plieth, V. A. Macagno, and G. I. Lacconi, J. Electroanal. Chem., 453 (1998), 121–127 [13] Patent RU 2536127 S2, MPK C 25 D 3/46/ (RU). Kislyj jelektrolit dlja serebrenija [14] A. S. 679649 SSSR, MPK S 25 D 3/46/. Jelektrolit serebrenija [15] Patent UA № 31325, MPK S 25 D 3/00/. Elektrolit zolotinnja [16] S. Dimitrijević, M. Rajčić-Vujasinović, S. Alagić, V. Grekulović, and V. Trujić, Electrochim. Acta, 104 (2013), 330–336 [17] V. K. Varentsov, and V. I. Varentsova, Chemistry and Sustainable Development, 3 (2004), 293–301 [18] S. Kulandaisamy, J. P. Rethinaraj, P. Adaikkalam, G. N. Srinivasan, and M. Raghavan, JOM, 55 (2003), 35–38 [19] L. F. Kozin, A. K. Bogdanova, and N. F. Zaharchenko, Nanosystemy, nanomaterialy, nanotehnologii', 9 (2011), 207–218 [20] O. L. Smirnova, Ju. L. Kutenko, and E. S. Lazarenko, Visnyk NTU ”HPI”, 40 (2013), 155–161 [21] O. Smirnova, A. Pilipenko, H. Pancheva, A. Zhelavskyi, and K. Rutkovska, Eastern-european journal of enterprise technologies, 1 (2018), 47–51 [22] Patent UA № 121564, MPK S 25 D 3/38. Elektrolit midnennja
ScienceDirect Materials Today: Proceedings 6 (2019) 141–149
www.materialstoday.com/proceedings
3rd ISE SSRSEU 2018
Studying the kinetics of electrode reactions on copper, silver and gold in acid thiourea-citrate electrolytes O. Smirnovaa*, А. Brovina, A. Pilipenkoa, Yu. Zhelavskaa Department of technical electrochemistry, National Technical University «Kharkiv Polytechnic Institute» Kyrpychova str., 2, Kharkiv, 61002, Ukraine
Abstract The kinetics of electrode reactions that take place on copper, silver and gold electrode in acid thiourea-citrate electrolytes has been studied. It is established that the anodic dissolution of metals is accompanied by the formation of stable thiourea metal complexes of a cation-type complex. The citric acid provides the acidic pH-value of the solutions and contributes to the active dissolution of anodes. The ionization rate is limited by the diffusion of charged particles into the electrolyte volume. The cathode reduction of metals obeys the mechanisms of mixed kinetics that includes the diffusion and the preceding chemical complexes dissociation reaction. The charge of the complex ions of copper, silver and gold is +1. Therefore, their dissolution at the anode and reduction at the cathode occurs with the maximum possible electrochemical equivalent. © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of 3rd ISE Satellite Student Regional Symposium on Electrochemistry in Ukraine. Keywords: complex compounds; polarization relationship; reaction overpotential; limiting stage; mixed kinetics
1. Introduction An advance of electronic, electrotechnical and jewelry industries demands more quality functional and protective-decorative coatings. Therefore, galvanic engineering faces the tasks of developing new productive and environmentally friendly technologies of coatings deposition. The main ways of their improvement are the development of new electrolytes and the optimization of coating deposition modes.
* Corresponding author. Tel.: +38-066-489-3775 E-mail address: [email protected] 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of 3rd ISE Satellite Student Regional Symposium on Electrochemistry in Ukraine.
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O. Smirnova et al./ Materials Today: Proceedings 6 (2019) 141–149
Copper, silver and gold coatings are widely used for electroplating. Cyanide electrolytes are widely used for industrial purposes. These electrolytes are toxic and are not-easy-to prepare and use. These also require appropriate arrangements for the waste solution disposal and the waste water neutralization. Today, the electrolytes are developed that enables simultaneous solution of the problems of ecological safety and the production of coatings with prescribed characteristics. Acid thiourea-citrate electrolytes that can oust cyanide electrolytes have great prospects. At the first stage of the development of new electrolytes intended for the coating deposition we investigate the kinetics of anodic and cathodic reactions in the given electrolytes. 2. Reference Data Analysis and Problem Definition The use of thiourea for copper, silver and gold technologies was considered for the crude ore enrichment processes [1–3]. The electrolytes were developed containing the thiourea and citric acid in the form of additions to improve the quality of coatings or the buffer properties of solutions. An ample amount of data arrays was obtained for the deposition kinetics, structure and the properties of copper coatings produced from sulfate, nitrate, pyrophosphate, citrate, tartrate-sulphamate and other electrolytes. The papers [4, 5] give consideration to the influence that the added thiourea has on the structure of electrodeposited copper in copper baths. It hampers the sulfate electrolyte reduction process of copper ions providing the formation of fine crystalline copper coatings. The addition of metal citrates to copper electrolytes provides the formation of compact copper depositions [6, 7]. Silver plating electrolytes and gold plating electrolytes are known to be based on the potassium hexacyanoferrate (II). These can be attributed to conditionally cyanide-free electrolytes, because metal ions are situated in the cyanide complex and KCN can be accumulated in the solution in operation. The sulfate electrolytes of silver plating and gold plating are also known. Despite their relative ecological safety these failed to find wide application due to their complicated operation. The paper [8] describes the use of thiourea electrolytes for the silver deposition as an alternative for cyanide electrolytes. The thiourea forms various complexes with Ag (I), the composition of which depends on the ratio of ligand to silver in the solution [9, 10]. The thiourea affects the smoothness and uniformity of silver coatings [11, 12]. The acid electrolyte of silver plating was applied for in the patent [13]. It contains silver nitrate, sulfamic acid and thiourea. The electrolyte was recommended for the application of finish coatings when producing printed circuit boards. The patent [14] suggests the electrolyte that contains silver nitride, thiourea and sodium chloride and cetyltrimethylammonium bromide intended for the stainless steel silver plating. The data are available on the development of acid chloride-sulfate electrolyte for the gold-plating [15]. This electrolyte is recommended for the deposition of gold coatings onto corrosion-resistant alloys containing chrome and nickel. The paper [16] describes the mercaptotriazole-based electrolyte for the gold deposition. The papers [17, 18] note that thiourea undergoes decomposition during electrolysis in neutral solutions, and the bath fails to operate. The paper [19] describes the peculiarities of the use of thiourea electrolytes to produce nanostructured gold deposits. Thus, the electrolytic deposition of copper, silver and gold from the cyanide-free electrolytes faces many problems related to the drawbacks of those electrolytes and their limited use. The development of new electrolytes still remains to be a vital problem whose solution will provide its wide application for different branches of the industry. 3. Objective and Problems of the Investigation The objective of the research was to study the kinetics of electrode processes that occur on copper, silver and gold in acid thiourea-citrate electrolytes. To achieve the goal the following tasks were set up: – studying the influence of electrolyte composition on the behavior of copper, silver and gold in the conditions of electrode polarization in thiourea and citric acid-base solutions; – defining the nature of a limiting stage and the kinetic characteristics of metal ionization reactions and those of the discharge of their complex ions.
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4. Methods of Experimental Investigations Working electrolyte solutions were prepared using the distilled water-based reagents. The electrolyte solution composition included the following components: metal salts-copper citrate (II) Cu2C6H4O7 · 2.5H2O, silver nitrate (I) AgNO3, gold chloride (III) AuCl3 and also organic ligands, in particular thiourea CS(NH2)2 and citric acid C6H8O7. The experiment was carried out in the temperature-controlled cell YCE-2 with the electrolyte volume of 100 cm3 in the temperature range of 15 to 60 °C. The electrodes with the surface area of 1cm2 made of copper (the MM grade), silver (the Sl 999.9) and gold (the Gl 999.9) were used as working electrodes. The electrode surface was prepared prior to the experiment using the standard technique. Platinum (the Pl 99.9 grade) with the surface area of 2 cm2 was used as an auxiliary electrode. The potentials were measured and compared with the chloride-silver saturated reference electrode EVL-1M1. All the potential values were recalculated in terms of hydrogen scale. The pH value was defined using the pH-673M device. The volt-ampere characteristics of electrode processes were obtained using the pulse potentiostat PI-50-1.1 equipped with the PR-8 programmer. A change in the current or voltage was recorded by the self-recorder LKD-4. The registered current was related to the geometric surface of test specimen. 5. Electrode Process Kinetics Research Data It is known that the metals of copper subgroup (Cu, Ag, Au) form stable compounds with thiourea. These compounds look like a cation with the charge +1 (for copper, silver and gold) and +2 (for copper). These compounds are the most stable in the acid medium with pH = 1–4. Therefore, to study the kinetics of electrode reactions on the given metals we selected thiourea solutions with the citric acid. It was established experimentally that these systems are rather stable at the temperatures of 18 to 60 °C. When thiourea is in abundance, the ions of Cu+, Ag+ and Au+ are totally bonded forming the complexes with CS(NH2)2 molecule. Depending on the ligand concentration, these complexes have different coordination numbers. The complexes with the citric acid have lower stability in comparison to those of the given metals with thiourea. The paper [20] gives the complexes formulas and their instability constants. To study the kinetics of anodic and cathodic reactions the volt-amperometric measurements were taken with the subsequent graphic processing of obtained data. Fig. 1 gives anodic polarization relationships taken for copper, silver and gold in the given solutions. Thiourea and citric acid concentrations had an equivalent molar ratio. Mass concentration of metal salts in all the solutions was the same, i. e. 1 g·dm–3 in terms of metal.
Fig. 1. Anodic polarization relationships obtained for copper subgroup metals in the solutions containing 0.1M CS(NH2)2 + 0.1M C6H8O7 at t = 25 °C and υs = 1·10–2 V·s–1.
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The curves show the anodic polarization of each electrode results in an increased current density forming the regions of the active dissolution of given metals. The achieved current maximum for the first ascending branches of curves characterizes the limiting anode current density. An increase in the current after a maximum is achieved, is explained by the further metal oxidation with the formation of ions of a higher charge Cu2+, Au3+ and electrochemical oxidation CS(NH2)2. To define the type of control for the process of anodic dissolution of the metals, the appropriate initial sections of polarization curves in Fig. 1 were reorganized in the coordinates of reaction overpotential relationship as the function of current density logarithm. Fig. 2 shows the type of these relationships that correspond to different types of the kinetics. Fig. 2 shows that a maximum linearity of these relationships is manifested in mixed kinetics coordinates. It means that the anode reaction rate is limited simultaneously by the electrochemical stage and the diffusion stage. Extrapolation of the initial section of the relationships (fig. 2, a) allowed us to define the slope angle of relationships and accordingly the coefficients a and b of the Tafel equation for copper, silver and gold. The exchange current density j0, a number of electrons participating in the reaction and the charge transfer coefficient β were also determined. The kinetic characteristics of electrochemical reactions and calculated constants of the instability K of metal compounds in these solutions are given in Table 1. Table 1. Kinetic characteristics of the ionization of Cu, Ag and Au in the solutions containing 0.1M CS(NH2)2 + 0.1M C6H8O7 and the calculated constants of instability of metal compounds containing thiourea. Metal
a (V)
b (V)
j0 (А·см–2)
β
z
Кi
Cu Ag Au
1.59 1.36 1.00
0.48 0.38 0.25
4.3·10–3.3 3.0·10–3.6 1.5·10–4.0
0.12 0.15 0.24
1 1 1
1.3·10–13.8 2.1·10–15.06 4.0·10–25.15
Fig. 2. The dependence of anodic overpotential on the current density in the semilogarithmic coordinates of electrochemical (a), diffusion (b) and mixed kinetics (c).
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Fig. 3 shows cathode polarization potentiodynamic relationships obtained for copper, silver and gold in acid thiourea-citrate electrolytes. A mass concentration of metal salts is similar in all the solutions and is equal to 5 g·dm–3 in terms of metal. To provide the stability of complexes, the thiourea concentration in the solution was increased in proportion to an increase in the metal concentration. Fig. 3 shows that the polarization of copper, silver and gold electrodes results in the cathode reduction of copper, silver and gold, accordingly. The cathode polarization is characterized by an increase in the current density reaching the maxima (limiting current densities). Only one reaction takes place in this case, i.e. the discharge of complex cations of the metals. Then the limiting current plateau is formed. A further biasing of the cathode potential gives the two reactions – the limiting current metal deposition and the hydrogen release. The processing of polarization curves Fig. 3 in the semilogarithmic coordinates shows that the absolute linearity of reaction overpotential as a function of the current density is developed in none of the coordinates at the cathode reduction of copper, silver and gold (Fig. 4). However, the highest linearity of the three options is observed in mixed kinetics coordinates. It means that the electrode reaction is limited both by the diffusion stage of discharging particles and directly by the discharge transfer. The contribution to the cumulative overpotential of chemical stage, in particular dissociation of the complex cations of metals is not excluded. Exchange current density values j0, a number of electrons participating and the charge transfer coefficient were also determined. The kinetic characteristics of electrochemical reactions and the calculated instability constants of the complex ions of metals in these solutions are given in Table 2. Table 2. The kinetic characteristics of the discharge reaction of complex ions of copper, silver and gold in the solutions containing 0.5M CS(NH2)2 + 0.1M C6H8O7 and the calculated instability constants of the complexes of these metals with thiourea. Metal
-a (V)
-b (V)
j0 (А·см–2)
α
z
Кi
Cu Ag Au
0.59 0.47 0.50
0.15 0.12 0.10
2.6·10–3.9 1.1·10–4.1 3.2·10–5.0
0.39 0.51 0.59
1 1 1
6.4·10–13.05 5.4·10–13.39 2.0·10–22.5
A comparison of equilibrium electrode potentials recorded for copper, silver and gold shows that these potentials depend to a great extent on the concentration of metals and thiourea, and to a lesser extent on the concentration of citric acid (Fig. 5). The concentration of thiourea as the main ligand affects the overpotential and the reaction rate. First of all citric acid is essential because of its acidic value of pH that is important for the stability of electrolytes. In the anode process citric acid provides a uniform dissolution of the metals and prevents their salt passivation [21]. Fig. 6 shows the influence of the concentration of thiourea on the limiting current density at the anodic dissolution of copper, silver and gold in the solutions with the same concentration of citric acid in logarithmic coordinates. The data approximation allowed us to determine the order of chemical reaction for the reagent, i.e. thiourea.
Fig. 3. Cathode polarization relationships obtained for the metals of copper sub-group in the solutions containing 0.1M CS(NH2)2 + 0.1M C6H8O7 at t = 25 °C and υs = 1·10–2 V·s–1.
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Fig. 4. The dependence of the overpotential of cathode reaction on the current density in the semilogarithmic coordinates of electrochemical (a), diffusione (b) and mixed kinetics (c).
Fig. 5. Dependence of the equilibrium potentials recorded on Cu, Ag, and Au - electrodes on the concentration of CS(NH2)2 in the solutions without C6H8O7 and with 0.1M C6H8O7.
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Fig. 6. Logarithmic dependence of the limiting current density for the anodic dissolution of metals on the concentration of thiourea in the electrolytes containing 0.1M C6H8O7 + 0.1–1M CS(NH2)2.
The thiourea-calculated orders of reactions for copper, silver and gold are close to 1. It means that the ionization of these metals takes place at the first stage of anodic process with the loss of one electron and adjunction of one molecule of thiourea. The next stage of the process is the diffusion of the formed complex cations of metal in depth of the solution and the adjunction of the ligand molecule to the compound. A number of thiourea molecules coordinated by the ion of metal, i.e. the complexing compound depends on the concentration of thiourea in the electrolyte solution. The orders of thiourea reactions for the cathodic process of the reduction of copper, silver and gold is also approximately equal to 1. It means that the discharge of the ions of these metals occurs with the loss of one molecule of the ligand. To determine the limiting stage of cathode process, the Semerano criterion or the criterion of velocity was used. Fig. 7 gives logarithmic dependences of the limiting current density on the potential scanning rate. Fig. 7 shows that such connection for the relationships 1 and 2 is linear and it is not ascending to a zero. The relationship 3 in Fig. 7 is linear and crosses the origin of coordinates. A character of the relationship 3 shows that the electrode reaction is irreversible and the rate of electrochemical stage is lower than that of the diffusion stage. Calculated values of the Semerano criterion are as follows: for copper – 0.3 and for silver – 0.2. For gold the Semerano criterion is equal to 0.5. This value of the velocity criterion is peculiar for the processes that are limited by the charge transfer rate, diffusion and the chemical reaction kinetics, and it excludes catalytic and adsorption processes.
Fig. 7. Logarithmic relationship of the limiting current density as the function of potential scanning rate for cathode polarization curves obtained in the acid thiourea-citrate electrolyte.
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6. Discussion of the Research Data of the Kinetics of Electrode Processes Based on the obtained experimental data we can assume that the dissolution of metals of the copper sub-group proceeds adhering to the following mechanism. Initial stages of the process are the metal ionization and the interaction of their singly charged ions with one molecule of thiourea. Later on one more or even several molecules of thiourea are added with an increase in the coordination number of complex compound. Then the complex cations of metal interact with citric acid anions with the association of well-dissolved complexes. For example, Cu0 – e– + CS(NH2)2 → [Cu(SC(NH2)2)]+ [Cu(SC(NH2)2)]+ + CS(NH2)2 → [Cu(SC(NH2)2)2]+ [Cu(SC(NH2)2)2]+ + C6H7O7– = [Cu(SC(NH2)2)2][(C6H7O7)]
(1) (2) (3)
Ag0 – e– + CS(NH2)2 → [Ag(SC(NH2)2)]+ [Ag(SC(NH2)2)]+ + 2CS(NH2)2 → [Ag(SC(NH2)2)3]+ [Ag(SC(NH2)2)3]+ + C6H7O7– = [Ag(SC(NH2)2)3][(C6H7O7)]
(4) (5) (6)
Au0 – e– + CS(NH2)2 → [Au(SC(NH2)2)]+ [Au(SC(NH2)2)]+ + CS(NH2)2 → [Au(SC(NH2)2)2]+ [Au(SC(NH2)2)2]+ + C6H7O7– = [Au(SC(NH2)2)2][(C6H7O7)]
(7) (8) (9)
The cathode reduction of copper, silver and gold in acid thiourea-citrate electrolytes proceeds according to the sequence mechanism inverse to the anode process. The preliminary stage is the dissociation stage of complex compounds followed by the ion discharge [Cu(SC(NH2)2)]+, [Ag(SC(NH2)2)]+ and [Au(SC(NH2)2)]+ with the attachment of one electron and the cathode deposit crystallization. Also, the influence of such factors as the concentration of metals in solution, temperature and mixing of electrolyte on the rate of electrode reactions was studied in the paper. It is established that the temperature and mixing of the electrolyte actively influence the anodic dissolution of copper and silver. This indicates diffusion control of the process [21]. The potential scanning rate actually has no influence on the anodic dissolution of gold. It is indicative of the control of electrochemical and chemical stages. The content of metal salt in the solution and its temperature are more essential for the cathode reduction of copper [22], silver and gold. The stirring during the deposition of gold coatings is not of great importance, which is indicative of the retardation of cathode reaction at the phase of chemical and electrochemical stages. This difference in the metals kinetics can be explained by a higher stability of the thiourea complex of gold in comparison with the analogous compounds for copper and silver. 7. Conclusions Acid thiourea-citrate solutions with pH = 1–4 can be used for the creation of new efficient and environmentally sound electrolytes for copper plating, silver plating and gold plating. These electrolytes are prepared using simple metal salts, thiourea and citric acid. Thiourea forms fairly stable complex cation-type compounds with metal ions. Citric acid functions as a buffer reagent, providing the electrolyte stability. Active dissolution of metals is due to the joint presence in the solution of thiourea and citric acid. The anode process proceeds without the passivation of anodes and oxidation of organic substances on them, which is of great importance for the stable electrolyte operation. The kinetics of electrode reactions on copper, silver and gold in acid thiourea-citrate electrolytes obeys the laws of mixed kinetics. However, diffusion control is more typical for the anodic process, and for the cathodic process, the retardation at the stage of the previous chemical reaction (dissociation of complex) is more characteristic. The charge of complex ions of copper, silver and gold in thiourea-citrate electrolytes is equal to +1. Their reduction on the cathode occurs with maximum possible electrochemical equivalent that provides the cathode process energy efficiency. The metal deposition rate can be increased due to an increase in the metal concentration in solution and the electrolyte heating (before 40–50 °C).
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