CC BY
28УДК 544.651.22: 544.653.3 В.Л. Красиков
Vladimir L. Krasikov1
роль промежуточных
частиц
электрохимического восстановления кобальта в образовании
кислородсодержащих примесеи
ОАО Российский институт радионавигации и времени, пл. Растрелли, д. 2, Санкт-Петербург,.191124, Россия e-mail: kras257@km.ru
Рассмотрены закономерности катодного восстановления ионов кобальта при различных условиях. Показано, что если восстановление ионов Co(II) протекает стадийно, то при осаждении поликристаллического покрытия при относительно невысоких плотностях тока на границе соприкосновения растущих кристаллов могут возникать условия, при которых происходит задержка переноса второго электрона к промежуточной частице Co(I), вследствие чего вместо её восстановления до адатома металла может происходить её химическое или электрохимическое окисление с образованием кислородсодержащих соединений, включающихся в состав катодного покрытия. Показано также, что адсорбированные промежуточные частицы Co(I) могут химически взаимодействовать с ПАВ, находящимися в электролите, а также приводить к образованию водорода по необычному механизму, отличному от механизмов Фольмера-Тафеля и Фольмера-Гейровского.
Ключевые слова: кобальт, катодное восстановление, промежуточные частицы
Cathodic reduction of iron-group metals, cobalt in particular, is usually accompanied by hydrogen evolution, which is thought to proceed as a parallel independent process by Volmer-Tafel or Volmer-Heyrovsky mechanism on the most active sites of the metal surface [1-3]. Cobalt current efficiency can reach 95-98 % [4-5], but never equals to 100 %, even when deposited from metal-concentrated and pH-neutral electrolytes. Current efficiency - current density (Dk) curves for cobalt and other electronegative metals is usually of extreme character [5-9]. Current efficiency lowering, when Dk is high, is related to arising metal ions diffusion limitations and increasing relative rate of water molecule discharge that can lead to cathode area alkalization and with the formation of insoluble metal hydroxide compounds included in cathodic coating [10-28].
Metal current efficiency lowering, when Dk is low (0.1-1 A/dm2), can be explained on the basis of partial polarization curves of cobalt ions reduction and water molecule discharge on cobalt surface and also may be caused by increasing of current ratio going into hydrogen evolution. However, in electrolytes with significant buffer capacity when cathodic current density is about 0.1-1 A/dm2 and cobalt current efficiency is 85-95 %, absolute rate of hydrogen evolution is low, the change in pH value of the near
the role of
electrochemical
cobalt reduction
intermediates
in the formation of
oxygen-containing
admixtures
JSC Russian Institute of Radionavigation and Time, pl. Ras-trelli, 2, St. Petersburg, 191124, Russia, e-mail: kras257@km.ru
The article deals with the cobalt ions cathode reduction regularities occurred under various conditions. It is shown, that if Co(II) ions reduction proceeds in two steps under relatively low current density, forming polycrystalline deposit, there may occur special conditions at the boundary of growing crystals that may lead to delay of the second electron transfer to Co(I) intermediate particle. It results in its chemical or electrochemical oxidation formation of oxygen-containing compounds, incorporated into cathodic coating, instead of its reduction to the metal adatom. The adsorbed Co(I) intermediates may chemically interact with surface-active substances and other components of electrolyte, as well as result the hydrogen formation by means of an unconventional mechanism which is different from Volmer-Tafel and Volmer-Heyrovsky mechanisms.
Keywords: cobalt, electrochemical deposition, intermediates.
electrode layer is neglectable, which was confirmed by experiment with micro glass electrode [29-31] and cannot lead to insoluble compounds formation. Under these conditions the only side reaction is water molecule discharge and hydrogen evolution that can be partially adsorbed by the metal as an admixture. However accurate component analysis of polycrystalline cobalt coatings deposited from various electrolytes, when Dk is low, shows that the basic admixture in the metal is not hydrogen, but oxygen, lumped on the metal grain boundaries [32]. This contradiction cannot be explained with cobalt hydroxide occlusion because the conditions for their formation do not arise. Hence, the other process is a reason for oxygen occurrence in cobalt as well as for its structure imperfection, lowered corrosion resistance and change in many other physical and chemical properties in comparison with metallurgical (melted) cobalt, but not a parallel electrochemical hydrogen evolution reaction.
This paper set a task to analyze the reasons that could lead to oxygen-containing admixtures occurrence in cobalt electrodeposits when current density is relatively low, absolute rate of parallel hydrogen evolution process is not high and virtually there is no near-electrode layer alkaliza-tion.
1 Vladimir L. Krasikov, Ph.D. (Chem.), deputy chief of department, e-mail: kras257@km.ru Красиков Владимир Леонидович, канд. хим. наук, зам. начальника отд., e-mail: kras257@km.ru
Received August, 31 2015
Дата поступления 31 августа 2015 года
Results and discussion
The multiple studies results analysis for the process of cobalt nucleation on a foreign substrate, such as on platinum [10, 33-34], doped silicon [35-36], glassy carbon [25, 37-40], gold [13, 41-44], stainless steel [45-46] and a range of other materials shows that in the initial stage of electrocrystallization all the current goes into metal nuclei formation and just only after a while a parallel hydrogen evolution process begins. This fact allows to suggest that the breach of "normal" path for metal ions reduction to crystalline metal takes place during the process of polycrystalline cobalt layer growth and begins at the moment of separate nuclei coalescence.
Kinetics studies for iron-group metals ions Me(II) reduction from simple and complex electrolytes show that the process goes in two steps with the first electron slow transfer [2, 3, 18, 47-54] (excluding reduction from strongly acidic electrolytes [55-57]) through the stage of MeOHads intermediate formation. In particular, we showed [47-48] that nickel and cobalt cathodic reduction from complex pyrophosphate electrolytes is limited with the transfer of the first electron to the metal hydropyrophosphate evolved in the near-electrode layer as a result of fast preceding hydrolysis of dipyrophosphate complex that is predominant in solution.
[Me(P2Oz) 2]6- + OH" - [Me(OH)(P2O7)]3" + (P2O7)]4- (1)
[Me(OH)(P2O7)f + e- - MeOHads + (P2O7)]4- (2)
In many cases the first electron transfer reaction can be presented as follows:
Me(OH)R(n-1}- + e- - MeOHads + Rn- (3),
where Rn- is polybasic acid anion. After evolving, MeOHads intermediate particle accepts the second electron and reduces to metal at the second electrochemical stage:
MeOHads + e- - Me0 + OH- (4)
The transfer of the second electron is fast, therefore there is no MeOHads intermediates accumulation on the cathodic surface. It is notable that many researchers assume OH'-ions participation in iron-group metals cathodic reduction in order to explain the observed regularities. However, despite hundreds of publications, there is just a small number of papers where reaction order of metal reduction on hydroxyl ions is determined, as well as it was performed, for example, in [47-48].
Iron-group metals anodic dissolution and their corrosion also begins with MeOHads intermediate formation [58-69]. Besides, for an iron electrode it is specified [70-72] that hydrogen cathodic reduction in acidic solutions proceeds through the formation of MeOH intermediate arising as a result of fast irreversible reaction of iron-water interacting:
Fe + H2O - FeOHads + Hads (5).
However it should be noted that another viewpoints prevail concerning the mechanism of hydrogen evolution reaction on iron [73]. In papers [62-63] it is experimentally shown that when freshly formed iron or nickel surface is affected by water, the oxidation of iron and formation of MeOHads intermediate takes place very quickly, in less than one millisecond and then, being thermodynamically instable, MeOHads intermediate transforms, usually with increase in its oxidation state.
Thus on the basis of experimental data, Me(II) ions cathodic reduction, anodic oxidation and corrosion of iron-group metals, as well as, in certain terms, cathodic hydrogen evolution proceed through the stage of Me(I) intermediate formation, apparently in the form of hydrated MeOHads particle. After formation, depending on the electrode process direction, that is on the electrode potential, Me(I) intermediate can either
be oxidized or reduced. Me(I) intermediate formation results from kinetic investigations, though no reliable experimental proofs of Me(I) intermediate existence for iron-group metals have been taken, as it was done for copper with the help of ring-disk electrode [74-75] or by the method of cyclic voltammetry [76-77], which is conditioned with Me(I) intermediate extreme instability and its strong adsorption on the electrode surface. In aqueous medium Fe(I), Ni(I) and Co(I) compounds can exist persistently only in the form of some stable complexes with several organic ligands [78-80].
Analyzing all the totality of experimental data on reduction kinetics, chemical composition, properties and electrolytic cobalt structure, we can explain oxygen-containing compounds occurrence in cobalt deposits at the metal grain boundaries, lowered corrosion resistance and less than 100% current efficiency of polycrystalline layer deposition as follows:
c
Figure 1. Plan of oxygen-containing admixtures formation in the process of cobalt crystals growth. Explanations in the text.
At the initial moment after switching on current, metal nuclei with definite or random orientation are formed on the cathodic surface [81-84] (Figure 1a). All the current goes into metal ions Me(II) reduction to metal [10, 35-38]. On the surface of each separate cobalt microcrystal in statu nascendi there is a very dynamic "layer" of adsorbed particles CoOHads, which is actually a source for the metal growth (in Figure 1 CoOHads "layer" is shown schematically as a dashed line). On reaching certain size the nuclei come into contact, some CoOHads particles are found at the boundaries of two touching cobalt crystals and belong simultaneously to two adsorption layers (Figure 1b). Owing to nonequipotentiality of the metal surface in microscale and screening effect [82-84], one may suppose that at point N of two touching crystals (Figure 1b) the local electrode potential becomes less negative than on the rest of open flat surface of growing crystals, so the activation energy of CoOHads intermediates reduction to metal adatom increases. At the boundaries of contacting crystals there may arise special conditions under which the rate of CoOHads
intermediates reduction to metal becomes lower than the rate of their formation. It leads to Co(I) particles accumulation. As for as CoOHads intermediates are thermodynamically instable, they transform electrochemically as follows [58-69]:
gard to reaction orders found in [48] by electrolyte components, cobalt reduction rate is equal to:
CoOHads + H2O - Co(OH)2 + H+ + e-CoOHads - Co(OH)+ + e-CoOHads - CoO + H+ + e-CoOHads + OH- - Co(OH)2 + e-CoOHads + OH- - CoO + H2O + e-
(6)
(7)
(8) (9)
(10)
Taking into account the data from papers [7072] and the fact that Me(I) is a strong reducing agent, one cannot exclude the possibility of chemical interaction between intermediate and water
CoOHads + H2O - Co(OH)2 + % H2
(11)
The result of every process (6-11) is a formation of stable cobalt (II) oxide or hydroxide. The peculiarity of the process (11) is the fact that its rate does not depend on the electrode potential as well as the fact that reactions (3) and (11) together lead to hydrogen formation by an unusual mechanism, different from Volmer-Tafel and Volmer-Heyrovsky mechanisms, through the stage of Me(I) intermediate formation. Besides owing to chemical process (11) proceeding, cobalt current efficiency cannot reach 100%, while during the anodic electrochemical processes (6-10) the electrons, previously expended for cathodic reaction (3), formally return to the cathode, increasing current efficiency.
Oxidation processes proceeding simultaneously with cobalt cathodic reduction is stimulated with the more positive electrode potential at point N, than on the rest of the surface. As a result, CoOHads intermediates, being formed under a specified cathodic potential, find themselves at the point of less cathodic potential and so their future reduction becomes doubtful. While the thickness of the metal layer is increasing, point N is going farther from the initial position, and at the cobalt grains boundaries there remains a thin layer of oxide (or hydroxide) incorporated into metal (Figure 1c). The same phenomenon can be observed when the growth steps are forming on the faces of crystals. The current, going into CoOHads intermediates formation according to the reaction (3) and further cobalt oxide or hydroxide formation according to the reactions (6-11), is small and does not influence significantly upon the kinetic parameters measured. Volume and weight of substance in Co(OH)2 film are negligibly small but the influence of reactions (6-11) upon epy electrolytic cobalt structure and properties is very significant.
All mentioned above can be illustrated with potential curves (Figure 2). The curve ABCDH corresponds with metal ion reduction to adatom, and points A, C and H comply with stable energy state of Me(II), Me(I) and Me0 respectively. Point B corresponds with activated complex Me(II)* formation with Gibbs topmost energy and special atom configuration, U1 is the first electron transfer activation energy (reaction (3)), U2 is Co(I) intermediate reduction to metal adatom activation energy. The rate of total cobalt ions reduction process k is determined with the value of potential barrier U1 = U0 + a1FE, where U0 is the first electron transfer activation energy at zero potential E0, measured respective to the chosen reference electrode, a - the first electron transfer coefficient:
h ~ К\aCo(OH)R
(1 - в)е
RT
( 12 )
Discharging particles activity aco(OH)R depends on electrolyte type and the character of preliminary chemical reaction. In particular, for a pyrophosphate electrolyte with re-
I = Ka
a
CoR-, R OH
'аИ(\-в)е
a{FE RT
(13)
where 0 is the degree of surface coverage with intermediates and side reaction products. On the flat surface of growing crystals 0 value is close to zero.
Figure 2. Potential curves of reactive particles energy transition.
Explanations in the text.
In the gap between two crystals, where cathodic potential is lowered by means of screening effect, the energy state of Me(I) intermediate, to a first approximation, corresponds with point Cx, and activation energy of Me(I) particle reduction to metal adatom is determined with the difference of Gibbs energy between Cx and D levels. However, most probably that Me(I) particle configuration at point N (Figure 1) is different from that on the rest of the metal flat surface (another hydration shell structure, another charge distribution [85]) and corresponds with point Cy. Between Cx and Cy positions there is a potential barrier either (is not shown in Figure 2). Further Me(I) particle transformation proceeds with another reaction path, along the Y axis. Overcoming U3 potential barrier, Me(I) particle transforms into the end product, namely cobalt oxide (or hydroxide). At the point B there takes place the bifurcation of the discharging particle path.
In the gap between metal grains, 0 value approaches to one, the rate of reaction (3) goes down to zero, resulting in "empty" gaps between grains, resembling cracks. As it was shown by scanning electron microscopy the gaps between metal grains after washing and drying the cathodic coating may actually be empty but during the cobalt electrodeposition process they are full of side reaction products.
The papers usually consider parallel reactions proceeding at the same electrode potential, but in this case the possibility of parallel reactions proceeding is determined with the potential difference on the different microsites of the growing metal surface. In view of the presented model the role of the electrolyte's brighteners, surfactants, levelling components and other active substances becomes clear in the structure formation and the depositing metal chemical composition. For example, it is known that if saccharin is present in the electrolyte for depositing Ni-Co alloy, the oxygen-containing phase at the metal grain boundaries does not form [86], instead of it saccharin's disintegration products - sulfur and carbon [87] - can be found there. Based on this fact one can
conclude that in the process of the metal layer growth there is a chemical interaction between adsorbed saccharin and adsorbed MeOHads particles. This process is apparently a main source of sulfur in the coating, while saccharin direct electrochemical reduction on the cathode material is of minor importance [88-89]. Sulfur-containing compounds behavior in hy-drometallurgical processes of cobalt electrodeposition can be explained analogously [9]. Side reactions products, whatever small in quantity, can play a decisive role in the cathodic coating structure, texture and morphology formation, as well as change significantly its mechanical, corrosion, electrical and magnetic properties [3, 32, 55, 89-105].
As far as oxygen-containing admixtures presence deteriorates many coating properties, it is desirable to prevent their formation. Admixtures formation mechanism, presented with chemical equations (3) and (6-11) and Figure 1, shows that it is hardly possible to exclude admixtures formation in electrodeposition under direct current. To obtain much better coatings it is necessary to employ non-stationary electrolysis, setting short cathodic current pulses during which only a very thin layer of metal (with thickness of several monolayers) is deposited, and then pauses sufficient for intermediate particles relaxation. This conclusion has been confirmed, in particular, in articles [106-109].
According to papers [55-57] data, there are no Co(I) intermediate formation in the process of cobalt electrodeposition from strongly acid solutions: it is known that in solutions with pH value not greater than 2, Co(II) ions reduce with simultaneous transfer of two electrons, Tafel slope of partial polarisation curves is close to 60 mV. Being deposited under these conditions cobalt has face-centered cubic lattice (P-cobalt) [32, 110-113]. At the same time, cobalt is reduced in two steps in pH>5 electrolytes with the slow first electron transfer [2, 3, 39, 48, 54] through the stage of Co(I) intermediate formation. Under these conditions, cobalt crystalline modification is quite different: mainly a-Co with hexagonal close-packed lattice is deposited [32, 110-113]. In our opinion, the change in cobalt reduction mechanism and simultaneous change in crystalline cobalt structure is one of the basic reasons for complicated dependence of cobalt and its alloys properties on pH, described for example in [114-115].
Besides, if there are surface-active substances in the electrolyte, for example saccharin or thiourea, that can interact with Me(I) intermediates, then, other conditions being equal, the observed surface-active substances influence will be various depending on pH value. In electrolytes with pH 5 and greater, Me(II) ions reduction proceeds in two steps and Me(I) adsorbed particles interact with surface-active substances, forming various minor products and certain structure and texture of the coating. In strongly acid solutions (pH<2) there is no such interaction because of Me(I) particles absence. The difference in surface-active substances influence in the electrolytes at various pH values complicates the interpretation of cobalt and its alloys elec-trodeposition regularities, especially in transition region with pH from 2 to 5. So detailed information about metal ions reduction mechanism allows to solve both theoretical and complex applied problems.
In conclusion, we should note that the role of metal ions reduction intermediates Me(I)ads is very diverse and is not exhausted with the processes considered here.
Conclusions
On the example of cobalt cathodic deposition at relatively low current densities we have viewed the mechanism of oxygen-containing admixtures formation and their incorporation into the coating. It is shown that a probable reason for admixtures formation is the metal surface nonequipotentiality, local cathodic potential lowering at the boundary of growing cobalt crystals and corresponding rise in reduction activation energy of Me(I) intermediates to metal.
The assumption was made, that intermediates can chemically interreact with the electrolyte components forming
various minor products, significantly influencing on the coating properties and the electrodeposition process parameters.
The author expresses deep gratitude to professor A.N.Podobaev (Moscow State University of Mechanical Engineering) and professor V.V.Kondratiev (Saint Petersburg State University) for their precious advice and comments during the work on the text.
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