CHEMIJA. 2006. Vol. 17. No. 2–3. P. 11–15 © Lietuvos mokslų akademija,Bi electrodeposition 2006 on Pt in acidic medium. 2. Hydrodynamic voltammetry 11 © Lietuvos mokslų akademijos leidykla, 2006

Bi electrodeposition on Pt in acidic medium 2. Hydrodynamic voltammetry

Ignas Valsiūnas*, A Pt rotating disc was used to provide some kinetic information concerning the electrodeposition of bismuth from Bi3+ acidic perchlorate solution 1 M

Laima Gudavičiūtė, HClO4 + 0.05 M Bi(ClO4)3 at 20 °C. The electrodeposition of Bi from this solution may be interpreted in terms of an irreversible stepwise discharge of Vidmantas Kapočius and Bi3+ ions proceeding either through three successive one-electron steps with the transfer of the first electron as the rate-limiting step or via two successive Antanas Steponavičius steps with the transfer of two electrons in the first stage as the rate-limiting step. From the experimental data the diffusion coefficient D for Bi3+ ion was Institute of Chemistry, calculated and was found to be 4.9 · 10-6 cm2 s-1. A. Goštauto 9, LT-01108 Vilnius, Lithuania Key words: bismuth, electrodeposition, perchlorate solutions, Pt rotating disc electrode, rate-determining steps

INTRODUCTION technique was therefore employed in order to elucidate the further information on the mechanism of Bi Although bismuth, as a semimetal, exhibits some unusual electrochemical deposition on a polycrystalline Pt thermal, electrical and magnetic properties that make its (Pt(poly)) electrode. actual and potential applications [1, 2] to be rather wide, its electrochemical reduction from Bi3+ solutions has not EXPERIMENTAL been studied very extensively. Several electrochemical studies have been devoted to the investigation of current- Details on the preparation of perchlorate Bi3+ solution, potential (i/E) characteristics [3–7]. In most investigations on the electrochemical cell, instrumentation and the pre- of Bi deposition only electrolysis in an unstirred Bi3+ paration of a Pt RDE (1 cm2 in area) have been repor- solution under a potential control has been employed, ted previously [7]. The working solution containing 0.05 and these cannot give sufficient information on the M Bi(ClO4)3 + 1 M HClO4 was used for electrochemi- kinetics and mechanism of the process. Such statement cal measurements. The potential of Pt RDE (E) was can be supported by the results of our recent work [7] scanned slowly (5 mV s-1) during rotation. All poten- in which we have shown that the experimental tials in the paper are quoted versus the standard hydro- relationships between the characteristics of cyclic gen electrode (SHE). voltammograms (CVs) and the conditions of electrolysis, including the Bi3+ concentration (c), potential sweep rate THEORETICAL ANALYSIS (v) and the number of cycles (n) during a successive cycling do not completely satisfy the diagnostic criteria For a bare rotating disc electrode, the current measured for a reversible or a totally irreversible electrochemical under non-limiting conditions is related to the kinetic reaction predicted by the cyclic voltammetry (CV) theory and diffusion-controlled currents by the Levich–Kou- [8, 9]. On the basis of these results, it has been suggested tecky (K–L) equation [11]: [7] that charge transfer during the Bi deposition onto a 1/i = 1/i + 1/Bw1/2, (1) Pt electrode should be a more complex process consisting kin of several separate steps. where ikin is the kinetically controlled current in the To the best of our knowledge, there is only one absence of the mass transfer effects and B is the inver- work [10] in which the voltammetric data, under se of the K-L slope at each potential. In the diffusion- hydrodynamically-defined conditions, have been reported controlled region, the limiting current ilim is described for the electrodeposition of Bi. by the following Levich (L) equation [11] : In the present study, which is a continuation of our i = 0.62nFAD2/3cν-1/6w1/2, (2) previous work [7], a rotating disc electrode (RDE) lim where n, D, A, n, c and w are the number of the * Corresponding author. E-mail: [email protected] exchanged electrons, diffusion coefficient (cm2 s-1), 12 Ignas Valsiūnas, Laima Gudavičiūtė, Vidmantas Kapočius and Antanas Steponavičius apparent area of RDE (cm2), kinematic viscosity (cm2 (the symmetry factor) of the rds, A is the electrode s-1), bulk concentration of a reactant (mol cm-3) and surface area. RDE rotation rate (rad s-1), respectively. The kinetic The overall current is current is described by the following equation [11] : -i = n(i –i ) = i – i ,(11) fm bm f b ikin = nFAkf c, (3) where i denotes the overall reaction net current which where k is the potential-dependent forward rate constant f is negative for the reduction reaction, i is the back- and the other variables are as described above. bm ward reaction current of the rds, i and i represent the It is generally agreed [11–13] that if more than one f b forward and backward reaction current of the overall electron is transferred during an electrode reaction, the reaction, respectively. mechanism of such process is usually analyzed using Then, following the considerations in the literature, the following simplifying assumptions: (i) only one elec- e.g. [14], the overall current can be represented as fol- tron is transferred in an elementary step; (ii) one of the lows: elementary steps is much slower than the others and, therefore, is a rate-determining step (rds), the elemen- α η α η -i = i0{exp[(m + c)(F /RT)]-exp[-(n – m - a)(F / tary processes preceding the rds are at quasi-equilib- RT)]} = i [exp(α Fη/RT) – exp(-α Fη/RT)], (12) rium. Then, considering the reduction reaction invol- 0 c,app a,app ving the transfer of n electrons, where i0 is the exchange current at the equilibrium po- tential E , α and a are the cathodic and anodic trans- → eq c a O + ne R (4) α α α fer coefficients of the rds, c,app = m + c and a,app = α the sequence of steps can be described as follows n – m – a are the apparent transfer coefficients, res- α α (see, e. g. [14]): pectively. Obviously, there is c,app + a,app = n and α /α = (m + α )/(n – m – α ), while α + α =1, O + e → A (5a) c,app a,app c a c a 1 which only holds if the same step remains rate-control- A + e → A m steps are in equilibrium (5b) 1 2 ling over the wide range of the η involved [13]. The ………….. stoichiometric number ν* here is assumed to be 1. In A + e → A (5c) m-1 m α α most systems c or a turns out to lie between 0.3 and → Am + e Am+1 rds (5d) 0.7, and it can be approximated to 0.5 in the absence of actual measurements. A + e → A (5e) m+1 m+2 In the case of the overall electrode reaction studied …………… (n – m – 1) steps are in equilibrium 3+ → → Bi + 3e Bi (13) An-1 + e R (5f) α α α α The concentration of A1 at the surface of the elec- n = 3, c,app/ a,app = (m + c)/(3 – m – a). Then, if trode (c1) can be determined from the Nernst equation: the transfer of the 1st electron is the rds, m = 0, and 0 α α α α α E = E1 + (RT/F)ln(c0/c1) (6) c,app/ a,app = c/(3 – a). c is predicted to be equal to α c1 = K1c0exp(-FE/RT), (7) c,app. If the transfer of the 2nd electron is the rds, then 0 α α α α α where E1 is the formal electrode potential of the reac- m = 1, c,app = 1 + c and ac,app/ a,app = (1 + c)/(2 – a). tion of the 1st electron transfer, the constant If the transfer of the 3rd electron is the rds, then m = 0 α α α α α α K1 = exp(FE1 /RT). 2, c,app = 2 + c and c,app/ a,app = (2 + c)/(1 – a). Similarly, For multistep electrode reactions, in the absence of mass-transport limitations and at high overpotentials, η, c = K c exp(-FE/RT) = K K c exp(-2FE/RT) (8) 2 2 1 2 1 0 η particularly, at high c, the cathodic Tafel slope bc = δη δ α cm = Kmcm-1exp(-FE/RT) = Kfc0exp(-mFE/RT), (9) c/ log|i| = 2.303RT/ c,appF. Then, for the overall elec- trode reaction (13) and taking ν* = 1, T = 293K, the where the constant K = exp(FE 0/RT), i = 1, 2,…m, m i following experimental values of b can be predicted: m c m 116, ca. 39 and ca. 23 mV dec-1 for the transfer of the and the constant K = ∏ K = exp[(F/RT) 0]. f i ∑ E i 1st, 2nd and 3rd electron as the rds, respectively. i = 1 = i 1 In general (see, e.g. [13]), most electrode processes, The forward reaction current of the rds, i , can be fm especially those with more than one charge transfer expressed as follows [14]: event, may also involve adsorption and desorption of

0 primary reactants, intermediates and ultimate products. i = 0 c exp[–(α F/RT)(E- E + )] = fm FAk m+1 m c m 1 On the other hand, in general, for such complex elec- α trochemical systems, there is a certain difficulty in a = 0 (K ) c K c exp[-(m + α )(FE/RT)], (10) FAk m+1 m+1 f 0 c making distinction between the overall n value of the electrode reaction (4) and the number of electrons of 0 where k m+1 is the standard reaction rate constant at the rds, usually designed as z because the latter may 0 + α E m 1 of the rds, c is the cathodic transfer coefficient not be readily apparent. Bi electrodeposition on Pt in acidic medium. 2. Hydrodynamic voltammetry 13

A multielectron electrode reaction may also occur The limiting currents for the Bi deposition plotted 1/2 via a number of routes including sequential and paral- against w are shown in Fig. 2. ilim depends linearly lel pathways that in complex kinetics can also be ana- on w1/2 and shows a zero intercept indicating the limit- lyzed individually in terms of elementary electrochemi- ing diffusion control, as predicted by the L Eq. (2). δ δ 1/2 ν cal or chemical steps. It is quite obvious that at the From the experimental slope ilim/ w for n = 3, = given E in sequential reactions, the step with the smal- 0.01 cm2 s-1 and c = 5 · 10-5 mol cm-3, the diffusion 3+ -6 ler rate coefficient kf is the rds, whereas in parallel coefficient D for Bi ion was found to be 4.9 · 10 paths the observed i will be determined by the path cm2 s-1. It can be noted that this magnitude of D is in -6 2 -1 with a larger value of kf [12, 13]. reasonable agreement with D = 5.7 · 10 cm s measured earlier with the Pt RDE in the Bi3+ perchlo- RESULTS AND DISCUSSION rate solution [10], but it differs more significantly from the value of (8 ± 1) 10-5 cm2 s-1 obtained from the RDE experiments were performed to obtain the kinetic chronoamperometric data in the solution 0.05 M 3+ information on the process of the discharge of Bi ion. Bi(NO3)3 + 5.8 M HNO3 [5]. This process on the Pt RDE is characterized by a sole Further, the currents recorded with the Pt RDE for wave (Fig. 1). It was found that the determination of the non-limiting conditions, in particular for the poten- the ilim presented no difficulty up to ca. 1200 rpm or tial on the ascending portion of the voltammogram in 127 rad s-1. At higher w values of the Pt RDE used the range + 0.130 to + 0.080 V, were used to construct 1/2 here, the experimental determination of ilim becomes mo- the plot i vs. w . As one can see from Fig. 3, in all re difficult because ilim is no longer well defined. cases for this E window, i practically linearly depends In the experiments, the currents were analyzed both on w1/2 with non-zero intercepts. Therefore, it can be in the limiting conditions using a Levich plot ilim vs. suggested that although rather clear indications of the w1/2 and in the non-limiting conditions applying a plot i presence of a mixed control under the conditions of the vs. w1/2 and also a Levich–Koutecky plot i-1 vs. w-1/2. Bi electrodeposition are revealed, the diffusion-control- led component is still of first importance in the expe- rimentally registered total i. 0.00 Applying the K–L equation (1) for the determina-

tion of the kinetically controlled current ikin, the straight lines i-1 vs. w-1/2 were obtained in the E range +0.13 to -2 -0.02 1 +0.08 V (Fig. 4). The slope of these plots, δ|i-1|/δw-1/2, 2 was found to depend slightly on E. In the meantime,

i, Acm 3 we cannot clearly explain the reasons for such a devia- -0.04 4 tion from the parallelism mentioned above. Formally 5 speaking, such a circumstance might be considered as a certain indication that the reaction order in respect of 6 -0.06 the bulk c of Bi3+ somewhat differs from the first re- 7 action order. On the other hand, it should be noted that -0.1 0.0 0.1 0.2 this feature of the plots i-1 vs. w-1/2 can arise not only from a variation in the reaction order, but also from E, V other reasons. In order to elucidate the reason for this Fig. 1. Cathodic current vs. potential curves at different Pt RDE feature, some additional experiments are needed. rotation speed (w) in 0.05 M Bi(ClO4)3 + 1 M HClO4 solution. The experimental values of the parameter B derived w: 1 – 14.1, 2 – 25.1, 3 – 39.3, 4 – 56.5, 5 – 76.9, 6 – 100.5 from the data presented in Fig. 4 are 5.23 · 10-3 to and 7 – 127.2 rad s-1. Potential sweep rate 5 mV s-1 6.31 · 10-3 A cm-2 s1/2 which compare favourably with the value of 6.34 · 10-3 A cm-2 s1/2, calculated using the -0.06 Schmidt number corrected Newman’s expression for B [15]:

-2 ν1/2 ν -2/3 ν -1/3 -0.04 B = nF c{0.621( /D) [1 + 0.298( /D) + 0.145(ν/D)-2/3]} (14)

, A cm with c = 5 · 10-5 mol cm-3, ν = 0.01 cm2 s-1, D = 5.7 · lim i -0.02 10-6 cm2 s-1 [10] and n = 3. From the intersections of the K–L plots (Fig. 4),

0.00 the values of ikin were determined and used to construct 01234 the mass-transport corrected Tafel plot for the case of 0.5 -1 0.5 w , (rad s ) the discharge of Bi3+ at the Pt RDE (Fig. 5). The slope -1 Fig. 2. Levich limiting current vs. square root of rotation of this plot was obtained to be bc,1 = 62 mV dec for speed plot obtained from the data in Fig. 1 the E range +130 to +80 mV. 14 Ignas Valsiūnas, Laima Gudavičiūtė, Vidmantas Kapočius and Antanas Steponavičius

E, V case where the overall reaction (13) occurs in three suc- 0.13 cessive one-electron steps with the transfer of the first 0.04 0.12 α electron as the rds and with c = 0.72. The other mag- 0.11 -1 0.10 nitude of the cathodic Tafel slope, bc,1 = 62 mV dec , is -2 0.09 not the one that is normally expected for this mechanism. 0.03 0.08 On the other hand, if one assumes the rate cons-

i, A cm tants kf of the transfer of the first and the second elec- 0.02 trons to be close, i.e., the overall electrode reaction (13) is assumed to proceed in two steps, the first step involving two-electron transfer as the rds, and the se- 0.01 cond step with one-electron transfer, both of the expe- 46810 rimental values of bc mentioned above can be applied α 0.5 -1 0.5 in the calculation of the kinetic parameters cz and ac. w , (rad s ) α α In the same E region, bc,1 and bc,2 give cz = 0.94, c = α α Fig. 3. Current vs. square root of rotation speed plots obtain- 0.47 and cz = 0.72, c = 0.36, respectively. ed from the data in Fig. 1 at various E Using the experimental facts concerning the electro- de process under study, the following mechanisms can 80 formally be suggested:

1) Bi3+ + e →rds Bi(II)(aq) + e → Bi(I)(aq) + → 0 60 + e Bi (16)

or –1 ) 3+ rds 0 –2 2) Bi + 2e → Bi(I)(aq) + e → Bi . (17) 40 E, V Considering the nature of the Bi species in perchlo- 0.13

, (A cm , (A rate solutions, it should be noted that in strongly acidic

–1 0.12 3+ I media (in our case, 1 M HClO ) the aqua Bi prevails 0.11 4 0.10 (>97%) and that hydrolyzed forms such as Bi (OH) 6+ 6 12 20 0.09 0.08 -0.6 ) 0 -2 -0.9 0.0 0.1 0.2 0.3 -0.5 -1 -0.5

w , (rad s ) , A cm -1.2 kin b = 0.0625 V Fig. 4. Koutecky-Levich plots i-1 vs. w-1/2 obtained from the c log(i data in Fig. 1 at various E -1.5

According to Cheh and Sard [16], another form of 0.14 0.12 0.10 0.08 the Volmer–Tafel equation, when the anodic portion of E, V the equation is neglected is: Fig. 5. Cathodic Tafel plot obtained from the data in Fig. 4 α 0 E = [-RT/(1 – cnF)]ln[i/i0(1 – i/ilim)] + E , (15) where E0 is the equilibrium potential. A corresponding -0.8 )] plot of the log[i ilim/(ilim – i)] vs. E at various w is c presented in Fig. 6. The slope of the approximating - i -1 -1.0

≈ lim straight line was found to be bc,2 80 mV dec .

It is unclear what leads to this difference between -1 )/(i )/(i w, rad s

the experimental values of the Tafel slope b and b . lim c,1 c,2 -1.2 25.12

Whether or not the overall electrode reaction (13) pro- i c 56.52 ceeds through the one-electron steps and which step is 76.93 the rds is still to be elucidated because both values of -1.4 100.48 α log[(i the Tafel slope differ from those obtained with c = 0.5 and z = 1 and sometimes quoted as “typical” Tafel 0.14 0.12 0.10 0.08 slopes [17]. E, V

According to theoretical analysis, the experimental va- Fig. 6. Plot of log[i · ilim/(ilim – i)] vs. E constructed according -1 lue of bc,2, ca. 80 mV dec , may be expected only in the to the equation (15) Bi electrodeposition on Pt in acidic medium. 2. Hydrodynamic voltammetry 15 and BiOH2+exist only to a very small extent [18]. As far 6. M. S. Martin-González, A. L. Prieto, R. Gronsky, T. Sands as we know, the literature data on the chemistry of Bi and A. M. Stacy, J. Electrochem. Soc., 149, C546 (2002). intermediates such as Bi(II)(aq) and Bi(I)(aq) appear to 7. I. Valsiūnas, L. Gudavičiūtė and A. Steponavičius, Che- be rather scarce, especially the data related to the Bi(II) mija, 16 (3-4), 21 (2005). species. The presence of the Bi(I) species has been no- 8. R. S. Nicholson and I. Shain, Analyt. Chem., 36, 706 ted in some earlier works (see, e.g. [2, 19, 20]). It is (1964). noteworthy that the electrode reaction of Bi3+ reduction 9. R. Greef, R. Peat, L. M. Peter, D. Pletcher and J. Robin- to Bi+ and finally from Bi+ to Bi, has been considered son, Instrumental Methods in Electrochemistry, Chapt. 6, when the deposition of Bi from an aqueous nitrate so- John Wiley & Sons, New York–Chichester–Brisbane– lution onto different substrates was studied using the cyc- Toronto (1985). lic voltammetry, XRD and SEM techniques [2]. 10. S. H. Cadle and St. Bruckenstein, Analyt. Chem., 44, 1993 In any case, presently it seems to be rather complica- (1972). ted to discriminate between these two suggested mecha- 11. A. J. Bard and L. R. Faulkner, Electrochemical Methods. Fun- nisms of Bi deposition onto Pt from an acidic perchlorate damentals and Applications, Chapt. 8, John Wiley & Sons, solution. The first mechanism is commonly accepted to be New York–Chichester–Brisbane–Toronto–Singapore (1980). sufficiently suitable for most multielectron systems becau- 12. I. Rubinstein, Physical Electrochemistry (Ed. I. Rubin- se of its rather detailed theoretical background (see, stein), Chapt. 1, Marcel Dekker Inc., New York–Basel– e.g.,[12–14]). Analogous mechanism has already been pro- Hong Kong (1995). posed for some systems, for instance, for the As deposi- 13. E. J. Calvo, Comprehensive Chemical Kinetics (Eds. tion on Au(111) substrate in a phosphate buffer [14]. The C. H. Bamford and R. G. Compton), Vol. 26, Electrode second mechanism involves a stage of the transfer of two Kinetics. Principles and Methodology, Chapt. 1, Elsevier, electrons simultaneously, as the rds. Such mechanism is Amsterdam–Oxford–New York–Tokyo (1986). considered more rarely at present [12, 13]. 14. Z. Jia, A. O. Simm, X. Dai and R. G. Compton, J. Elec- troanal. Chem., 587, 247 (2006). CONCLUSIONS 15. J. Newman, J. Phys. Chem., 70, 1327 (1966). 16. Y. H. Cheh and R. Sard, J. Electrochem. Soc., 118, 1737 The electrodeposition process of bismuth onto polycrystal- (1971). line Pt rotating disc electrode from 1 M HClO4 + 0.05 M 17. V. V. Losev, Modern Aspects of Electrochemistry (Eds. ° Bi(ClO4)3 solution at 20 C was found to be an irreversible J. O’M. Bockris and B. E. Conway), Vol. 7, Chapt. 5, electrode reaction. On the basis of the analysis of the Tafel Butterworths, London (1972). plots constructed using the Pt RDE data, two possible re- 18. A. Alin, Acta Chem. Scand., 11, 1445 (1957). action mechanisms were proposed. According to the first 19. B. Lovreček, I. Mekjavić and M. Metikoš-Huković, Stan- mechanism, the electrode reaction proceeds through three dard Potentials in Aqueous Solution (Eds. A. J. Bard, R. continuous one-electron steps with the transfer of the first Parsons and J. Jordan), Chapt. 5, Marcel Dekker Inc., electron as the rate-determining step, and other two steps New York–Basel (1985). are in equilibrium. The second mechanism involves a si- 20. J. Emsley, The Elements, Clarendon Press, Oxford (1991). multaneous transfer of the first and the second electrons as the rds. It is rather problematic to decide which of these Ignas Valsiūnas, Laima Gudavičiūtė, proposed mechanisms is more suitable. The value of the Vidmantas Kapočius, Antanas Steponavičius diffusion coefficient for Bi3+ ion calculated from the depen- dence of the cathodic limiting current on the rotation speed Bi ELEKTROLITINIS NUSODINIMAS ANT Pt of RDE was found to be 4.9 · 10-6 cm2 s-1. RŪGŠČIOSE TERPĖSE 2. HIDRODINAMINĖ VOLTAMPEROMETRIJA Received 20 June 2006 Accepted 07 July 2006 Santrauka Pt sukamas disko elektrodas panaudotas gauti informaciją apie References Bi elektrolitinio nusodinimo iš rūgštaus perchloratinio tirpalo 1 o M HClO4 + 0,05 M Bi(ClO4)3, kai temperatūra 20 C, procesą. 1. K. Liu, C. L. Chien, P. C. Searson and K. Yu-Zhang, Bi elektrolitinis nusodinimas iš šio tirpalo gali būti interpretuo- Appl. Physics Letters, 73, 1436 (1998). jamas taikant stadijinio Bi3+ jonų išsikrovimo modelio šitokius 2. Sh. Jiang, Y.-H. Huang, F. Luo, N. Du and Ch.-H. Yan, variantus: (i) išsikrovimų per tris vienelektronines stadijas su Inorg. Chem. Commun., 6, 781 (2003). pirmojo elektrono pernešimu kaip reakcijos greitį limituojančia 3. M. Takahashi, Y. Oda, T. Ogino and Sh. Furuta, J. Elec- stadija, arba (ii) per dvi stadijas, kai pirmųjų dviejų elektronų trochem. Soc., 140, 2550 (1993). pernešimas laikytinas reakcijos greitį limituojančia stadija. Re- 4. J. P. Ziegler, Solar Energy Materials & Solar Cells, 56, miantis gautais eksperimentiniais duomenimis, apskaičiuotas Bi3+ 477 (1999). jonų difuzijos koeficientas D, kuris lygus 4.9 · 10-6 cm2 s-1. 5. C. O’Reilly, G. Hinds and J. M. D. Coey, J. Electro- chem. Soc., 148, C674 (2001).