Surface morphology of electrolytic deposits of vanadium, cobalt, and manganese oxides

Article abstract of DOI:10.1134/S1070427206090102

The surface morphology of V₂O₅, Co₂O ₃, and MnO₂ electroplated under different electrolysis conditions was studied. The nature of the electrocrystallization of these materials was suggested on the bas

Full text of DOI:10.1134/S1070427206090102

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2006, Vol. 79, No. 9, pp. 1443 1446. Pleiades Publishing, Inc., 2006.
Original Russian Text V.M. Nagirnyi, R.D. Apostolova, E.M. Shembel’, 2006, published in Zhurnal Prikladnoi Khimii, 2006, Vol. 79, No. 9, pp. 1459
1
462.
APPLIED ELECTROCHEMISTRY
AND CORROSION PROTECTION OF METALS
Surface Morphology of Electrolytic Deposits of Vanadium,
Cobalt, and Manganese Oxides
V. M. Nagirnyi, R. D. Apostolova, and E. M. Shembel’
Ukrainian State University of Chemical Engineering, Dnepropetrovsk, Ukraine
Received November 10, 2005; in final form, May 2006
Abstract The surface morphology of V O , Co O , and MnO electroplated under different electrolysis
2
5
2
3
2
conditions was studied. The nature of the electrocrystallization of these materials was suggested on the basis
of the analysis results.
DOI: 10.1134/S1070427206090102
Electrolytic deposition of compact coatings and
a crystallization character is possible at continuous
degeneration of crystals in the step close to the nuclea-
tion and may be favored by the high sorption capacity
of V O [8], which is responsible for the overgrowth
dispersed deposits of vanadium and cobalt oxides and
of two-phase systems based on manganese dioxide
onto anode in a quantitative yield from aqueous solu-
tions of appropriate salts was considered previously
from the scientific and technological standpoints
2
5
of the deposited oxide molecules with atomic and
molecular oxygen and with water molecules. The ad-
sorption layer forming in the process impedes the
three-dimensional growth of the crystalline nucleus
because of hindered access of discharging VO2
ions toward the active sites on the anode surface and
retarded attachment of the forming oxide molecules to
the nucleus.
[
1 5]. These oxide compounds are promising materi-
als of lithium batteries. They have high electro-
chemical activity and a satisfactory tendency to cy-
cling at stable and reasonably high reversible capacity
+
[
6, 7]. In the above studies, the electrosynthesis of
the above materials was not considered in the aspect
of the structural characteristics of the deposits ob-
tained and the related phase formation processes. Here
this problem was considered by analyzing the surface
When sodium ions are added to solution no. 1
(
solution no. 2), the character of the V O crystalliza-
2 5
morphology of electrolytic V O , Co O , and MnO
deposits.
2
5
2
3
2
Composition of solutions and electrolysis parameters for
deposition of oxide compounds at 80 85 C
The deposits were prepared in the form of compact
coatings 1.5 3.5 m thick on smooth plates made of
Solu-
tion
no.
I ,
mA cm
a
Oxide
Solution, M
pH
2
12Cr18Ni9Ti steel (surface finishing grade 8). The
surface morphology of the deposits was studied by
atomic-force microscopy on a IIIa nanoscope (Digital
Instruments). The compositions of the solutions for
preparing deposits and the electrolysis parameters are
given in the table.
V O
1
2
VOSO4 0.25
VOSO4 0.25,
Na SO4 0.15
1.8 2.0 7.5 10
1.8 2.0 7.5 10
2
5
2
3
4
NH VO
6.0 6.5 3.0 3.5
5.5 6.0 3.5 4.0
4
3sat
NH VO
4
3sat
The surface morphology of the deposits studied is
shown in Fig. 1. All the deposits have a crystalline
CoSO4 0.025,
Co O
5
6
CoSO4 7H O 0.07
4.5 5.0 10 15
CoSO4 7H O 0.025 4.5 5.0 8.0 10
2
3
2
2
structure. The exception is vanadium oxide V O5
2
prepared from solution no. 1 of oxovanadium sul-
Cr (SO ) 6H O 0.04
2
4 3
2
fate, with fine-grained, nearly amorphous structure
MnO2
7
8
MnSO4 5H O 0.45
2.0 2.5 10 12.5
MnSO4 5H O 0.20, 2.0 2.5 8.5 10
2
(Fig. 1a). In this case, a pronounced relief formed
2
by accumulation of randomly projecting comb-like
aggregates is observed on the V O surface. Such
Cr (SO ) 6H O 0.40
2
4 3
2
2
5
1443

1444
NAGIRNYI et al.
(b)
(
a)
(c)
m
m
m
(
d)
(e)
(
f)
m
m
m
(
g)
(h)
m
Fig. 1. Surface morphology of electrolytic deposits of V O , Co O , and MnO : (a) V O formed from oxovanadium sulfate
m
2
5
2
3
2
2 5
+
solutions; (b) the same as in (a), in the presence of Na ions; (c) V O deposited from ammonium metavanadate solutions;
2
5
2
+
(
d) the same as in (c), in the presence of Co ions; (e) Co O deposited from CoSO solution; (f) MnO deposited from sulfate
2 3 4 2
3
+
3+
solution; (g) the same as in (e), in the presence of Cr ions; (h) the same as in (f), in the presence of Cr ions in higher
concentration.
tion and the structure of the anodic deposit change
markedly (Fig. 1). The deposit has a crystalline struc-
ture consisting of densely packed crystals growing
layer-by-layer in the form of well-defined steps. In
this case, sodium atoms may participate in the forma-
tion of the energy barrier making the sorption activity
of V O molecules lower, which creates favorable
conditions for the ordered crystal growth. The reaction
of sodium ions with vanadium oxide forming on the
anode yields a compound belonging to the class of
2
5
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 79 No. 9 2006

SURFACE MORPHOLOGY OF ELECTROLYTIC DEPOSITS
1445
vanadium bronzes of the composition Na V O (x = by jointly deposited Co O molecules. The presence
x
2
5
2 3
0
.23 0.28), which is confirmed by X-ray analysis
of Co O in these deposits was confirmed by X-ray
2 3
of the deposits [9]. In this case, the formation of
the deposit decelerates considerably, probably because
diffraction analysis [3]. This makes it possible to
consider these deposits as an interstitial solid solution
of Co O in V O . Probably, the globular structure
of the deposits is formed owing to the fact that growth
of V O crystals is decelerated with adsorbing Co O
2 3
of the hindered discharge of VO2+ ions due to a quali-
tative change in the energy state of the support [2].
2
3
2 5
2
5
Large-block crystalline deposits are formed on
the anode from ammonium vanadate solution no. 3,
with steps of crystal growth clearly edged and the
sequentially superposed stacks having a smooth facet
surface (Fig. 1c). The stacks have nearly the same
thickness and are arranged so that their frontal facets
are shifted at a nearly equal distance from each other.
Such a structure suggests fairly fast development of
primary two-dimensional crystalline aggregates grow-
ing in one plane, with the volume crystal growth by
the sequential deposition of the molecular layers. This
mechanism can be realized when the concentration
conditions and the direction of growth of the step are
strongly coordinated. In our case, this may be facili-
tated by the joint action of two mechanisms of the
V O formation:
molecules and they are deposited between the growth
steps at the limited supply of Co2 ions toward the
anode surface.
+
The Co O deposits electroplated from solution
2
3
no. 5 also tend to form spheroidal crystalline struc-
tures (Fig. 1d). This may be due to the fact that the
strict geometry of the step growing at the frontal face
of the crystal is broken at the limited and nonuniform
supply of oxide molecules toward its active points.
In this case, a certain role may be played by the
structure of Co O molecules, whose rhombic struc-
2
3
ture is likely to facilitate their packing in aggregation.
The MnO2 deposits electroplated from solu-
tion no. 7 have a lamellar structure (Fig. 1d). Each
lamella is a multilayer dense stack with projecting
growth steps. Some of these superimposed steps form
coarser crystalline grains. The size distribution of the
lamellas over the deposit surface is fairly uniform.
This structure suggests a specific mechanism of the
phase formation, largely dependent on the structure of
2
5
coagulation
2
HVO3
V O + H O
2 5 2
and electrochemical mechanisms:
VO2+ + H O V O + 6H+ [3].
the oxide lattice. This lattice belongs to the -MnO2
modification having a complex polymolecular rhombic
2
2e
2
2 5
structure. Therefore, it may be suggested that the
The coagulation mechanism, which is mainly due
to the electrostatic interaction, involves no energy
consumption and promotes the crystal growth owing
to direct formation of molecular crystalline aggregates
on the anode surface. Their further growth is evidently
provided via both the first and the second mechan-
isms. In this case, dark orange or brownish deposits
nucleation and growth of MnO crystals and the for-
2
mation of the three-dimensional structure of the
crystal lattice occur simultaneously. A two-dimen-
sional nucleus, or layer, forming in the process is
actually a three-dimensional formation with the height
equal to the corresponding lattice parameter. If the
crystal growth is a diffusion-controlled process, the
limit of the oxide molecules formed electrochemically
in the adjacent region of the solution will be rapidly
exhausted, which will restrict the growth of molecular
layers in the stack.
are formed, unlike light orange crystalline V O5
2
deposits formed by the traditional thermochemical
method, which confirms the above suggestion.
In the presence of Co2+ ions (solution no. 4), the
surface morphology of the V O deposit electroplated
2
5
The introduction into the Co O3 and MnO2
from ammonium vanadate solution changes essential-
ly (Fig. 1d). Break of the large-block structure of crys-
talline grains with simultaneous decrease in their size
and transformation into the qualitatively new globular
structure is responsible for this result. In the process,
the deposits acquire a dark gray color. However, fine-
ly grained dark orange deposits are observed along the
grain boundaries. These formations probably originate
from the fact that, in the step close to the onset of
nucleation, the growth of V O crystals is suppressed
2
electroplating solutions (solutions nos. 5, 7) of chro-
mium ions (solutions nos. 6, 8, respectively), results
in an essential change in the phase composition and
crystal structure of the deposits. The crystalline grains
become considerably finer, and the surface mor-
phology changes. The Co O deposits plated in the
2
3
presence of Cr3 ions consist of rather uniformly dis-
tributed fine aggregates in the form of layered petal-
shaped stacks, with growth steps projecting on the end
+
2
5
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 79 No. 9 2006

1446
NAGIRNYI et al.
faces (Fig. 1g). The crystalline grains have random
crystallographic orientation. The formation of such a
structure is likely due to the blocking of the frontal
Thus, the electrocrystallization of the oxide com-
pounds under consideration is determined by the elec-
trolysis conditions, on the one hand, and by the crys-
tal lattice structure, on the other hand. This is clearly
seen in the case of electrolytic V O , whose crystalli-
zation depends directly on the composition of a solu-
tion and electrolysis conditions. The correlation with
the oxide crystal system is reflected in essentially
different crystal structures of the corresponding elec-
trolytic deposits.
boundary of a growing stack with Cr O molecules.
2
3
As a result, its development is inhibited as the con-
centration of Co O molecules in the growth zone
2
5
2
3
decreases.
Increasing the concentration of the Cr3+ ions in the
solution results in a marked change in the crystal
structure of the deposits (cf. Figs. 1g and 1h, solu-
tion nos. 8 and 6). The deposits consist of fine close-
packed pillared crystalline grains forming a fairly
uniform surface. Apparently, the formation of this
structure is due to a strong effect of jointly deposited
CONCLUSIONS
(1) The morphology of the surface of electrolytic
Cr O molecules on the formation and growth of
2
3
V O , Co O , and MnO deposits was analyzed.
2
5
2
3
2
nascent MnO crystals. The Cr O molecules confine
2
2
3
The probable mechanisms responsible for the deposi-
tion and formation of the corresponding crystalline
structures were suggested. It was assumed that a com-
paratively low overvoltage in formation of the oxides
considered is due to the three-dimensional molecular
structure facilitating crystal nucleation through the
intermediate oxide radicals in the course of interaction
with adsorbed oxygen.
the development of nuclei in layers in the longitudinal
direction by blocking their frontal faces in the initial
steps of the formation. This creates the favorable con-
ditions for their sequential superposition and for the
growth of pillared crystals in the direction normal to
the electrode surface.
The analysis of the surface morphology of elec-
trolytic V O , Co O , and MnO deposits showed
2
5
2
3
2
(2) The formation of crystalline structures of the
deposits depends on the electrolysis conditions and
crystal lattice structure of the oxide.
that the electrocrystallization of these oxides has
a number of common features determining their crys-
tal structure and the energy balance in their formation.
For example, according to the voltammetric character-
istics of the deposits [1 4], the corresponding anodic
polarization, determined from difference between the
potential of the onset of the oxide deposition and the
standard electrode potential, lies within 0.07 0.15 V,
decreasing in the order Na V O > Co O > V O >
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2
5
2
3
2 5
MnO . This implies that the given oxide systems are
2
deposited at a moderate overvoltage of the electrode
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2
5
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6
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2
2 5
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2
3
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Co2+ + (M O)
CoO+ + (M O)
e
CoO+ + M,
Co O + M.
9
2
2
3
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 79 No. 9 2006