Science of Sintering, 39 (2007) 273-279
________________________________________________________________________
doi: 10.2298/SOS0703273S
UDK 662.785:661.871
Phase Composition and Crystallinity Degree of Nanostructured
Products of Anode Oxidation of Manganese(II) Ions Doped by
Ions of Lithium and Cobalt(II)
G. Sokolsky1*, N. Ivanova2, S. Ivanov1, T.Tomila3, Ye. Boldyrev2
1
National Aviation University, Cosmonaut Komarov Avenue 1, 04058 Kiev 58,
Ukraine
2
Institute of General and Inorganic Chemistry of Ukrainian National Academy of
Science, Palladin Avenue 32-34, 252680 Kiev 142, Ukraine
3
Institute for Materials Sciences Problems of Ukrainian National Academy
Krzyzanovsky Str. 3, 252142 Kiev 180, Ukraine
Abstract:
The influence of dopant ions on the phase composition and structure ordering of
electrodeposited nanoxides of manganese was investigated. Additives of dopant ions of Li and
Co(II) and fluorine-containing electrolytes have been applied for preparation of samples by
anode electrodeposition. The samples have been studied by methods of chemical analysis,
XRD, TEM, Fourier spectroscopy etc. TEM observation showed that the presence of dopant
ions of lithium and cobalt in electrolyte decreased crystallinity of samples, influenced the
length and perfection of nanorods, and made structure more defective. The diametres of the
nanorods ranged from 10 to 20 nm.
Keywords: Mn, Li, Co, Nanostructures, Phase composition.
Introduction
Еlectrochemical synthesis from fluoride containing electrolytes has many advantages
comparеd with the state-of-the-art electrolytic methods including the realisation of higher
rates of electrodeposition. Fluoride containing electrolytes enable production of electrolysis
products with a significant concentration of defects by changing the fluoride-ligand
concentration in the electrolyte [1,2]. The concentration of the latter ion influences the ratio of
electrochemical stage of the process to chemical dissolution stage. In accordance with the
bifunctional electrochemical system (BES) model developed by the authors of this work a
process of electrodeposition proceeds through the formation of a film of intermediate products
of varying composition [1]. Changing the concentration of fluoride, the nonstoichiometry of
the product can be varied as well as its electrochemical and catalytic activity.
It was shown recently that electrochemical activity of manganese dioxide of different
origin in alkaline electrolytes correlates well with concentration of defects (Mn3+/Mn4+ ratio,
hydroxide group content in the bulk), EPR signal width etc [3,4]. Electrolytic samples of
manganese dioxide obtained from fluoride containing electrolytes without additives form
_____________________________
*)
Corresponding author: [email protected]
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predominately nonstoichiometric nanorods of γ-polymorph of MnO2 and their diameter ranges
from 5 to 25 nm and the length can achieve 300-400 nm [5]. They work as one of the best
cathode materials for power sources at low loadings in aqueous and nonaqueous
electrolytes [1,2,6].
We suppose addition of dopant ions at electrochemical synthesis to be the next step of
adjustment of phase and chemical composition, and also defects in electrodeposited samples.
The purpose of this paper is to investigate the phase composition, crystallinity degree, and
other physico-chemical properties of nanostructured manganese dioxide electrodeposited
from fluoride containing electrolytes and doped by ions of lithium and cobalt (II).
Experimental
The products of anode oxidation of manganese(II) ions in the presence of additives of
lithium and cobalt ions were synthesised using the electrochemical method from fluoride
containing electrolytes [2]. The concentration ranges for dopants used in the electrolyte were
chosen on the basis of the solubility of the corresponding compounds and some preliminary
results of chemical analysis.
Electrodeposition of the doped manganese dioxide was studied on a Pt anode in
0.5 M HF containing 0.7 M MnSO4. For doping with cations, the corresponding salt was
introduced by the addition of 0.025 — 0.15 mole·L-1 LiOH. A constant current density range
was 1—6 A·dm-2 at room temperature. A plate of steel of 1CH18N10T grade served as a
cathode. Deposits were mechanically removed from the anode and rinsed with distilled water
onto a vacuum filter, dried in air without heating.
Some samples were prepared using trinary electrolytes by ions of metals. They
contained besides manganese sulfate and hydrofluoric acid HF additives of cobalt sulfate and
lithium hydroxide. The dopant ions were introduced by the addition of 0.03—1 mole·L-1 of
CoSO4 and 0.1—4 mole·L-1 LiOH. The concentration of manganese ions was also varied in
the range 0.1—0.7 mole·L-1.
X-ray powder diffraction (XRD) analyses of the products were carried out on a
DRON-3, DRON-4 X-ray diffractometers with Cu Kα radiation (λ= 0.15406 nm). The
multiphase phase composition and grain size of the samples was investigated by a computer
program "Powder Cell for Windows v 2.4" (http://www.iucr.ac.uk) The Fourier Transmission
Infrared (FTIR) spectra were taken on a FSM-1201 Fourier spectrometer, using standard KBr
pellet methods. Transmission Electron Microscopy (TEM) images were obtained at 100 kV
with a JEM-100CX II electron microscope. were well ground before being dispersed in
acetone under ultrasonic conditions. Fine particles were placed on a carbon-coated Cu grid for
TEM measurements. Thermogravimetric Analysis (TGA) data were collected with a
derivatograph of Paulic-Paulic-Erdey (Hungary). The temperature was increased from
ambient to 900oC at a rate of 10oC/min. The total content of cobalt, lithium, and manganese in
the doped samples was determined by the method of atom absorption spectroscopy.
Results and Discussion
Disordered and semi-amorphous oxides of manganese were prepared by the anode
electrodeposition from fluoride containing electrolytes of manganese sulfate in the presence
of additives of cobalt(II) and lithium ions. Our choice of fluoride containing electrolytes was
by no means accidental. As mentioned above, one can control the composition and properties
of oxide compounds and produce them at high rates. Namely the fluoride containing
complexes in electrolytes of manganese, such as [Mn(H2O)6(SO4)2(NH4)F]2-, exhibit higher
G.Sokolsky et al./Science of Sintering, 39 (2007) 272-279
275
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mobility at electrodeposition of manganese dioxide [7]. Moreover, the presence of fluoride
increases markedly the content of incorporated cobalt at anode deposition of lead dioxide [8].
Authors supposed that F-containing complexes, probably bearing a partial negative charge,
such as [Co(OH)xFy ](2-x-y ), are responsible for an increase of cobalt surface concentration due
to more favourable adsorption. The maximal content of incorporated cobalt and lithium in our
experiments was observed for minimal manganese and maximal cobalt contents in an
electrolyte (see above). It made up about 2—3% and 0.03—0.06%, respectively. The
dependence of cobalt content in the electrodeposited samples on the composition of
manganese and cobalt in an electrolyte is summarized in Fig. 1.
3
C o, %
2
60
5
20
10
M n , g . -1
L
15
20
.
Co ,
40
g L -1
1
25
Fig. 1 The dependence of the content of cobalt (by weight, %) in electrodeposited samples on
the composition of Mn2+ and Co2+ in the electrolyte.
The total manganese content is gradually decreased when the content of dopants grows
in electrolyte and samples, correspondingly. The amount of physically sorbed water is
increased simultaneously. Thermogravimetric analysis of the sample with maximal content of
dopant ions of cobalt and lithium used in this work showed that the content of physically
sorbed water makes up about 20%. The endothermic heat effect of its loss is observed at
120 oC. The exothermic heat effect at 320 oC characterises a phase transition to more regular
polymorph of β-manganese dioxide. The endoeffect of transformation MnO2—Mn2O3 is
observed at lower temperature (478oC) in comparison with more regular undoped samples of
manganese dioxide (520-560oC) [9].
A program for processing powder patterns “Powder Cell for Windows v. 2.4” is used
in this work to relate the intensities to the angle 2θ by Rietveld’s method. On account of
similarity of the MnO2 polymorphs we used the program for the phase analysis of the
samples. The method of trial and error was utilized to select the phase composition, and then
this program was employed to derive a theoretical X-ray pattern, which coincides fairly well
with the experimental one.
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Lithium as a dopant stabilises the more opened α-polymorph of manganese dioxide
with the structure type of hollandite (I4/m space group). This structure consists of double
chains of MnO6 octahedra that are linked by shared vertices forming channels where in our
case ions of lithium occupy the blank space inside of channels. The growth of the lithium
content causes expansion of the α-MnO2 unit cell [10].
It can be stated that all samples obtained in this work have complex phase
composition as well as any electrodeposited γ-MnO2 sample produced by a state-of-the-art
method. It is commonly thought [11] that the latter polymorph of manganese dioxide is the
structure of microtwinning and intergrowth of ramsdellite and pyrolusite structure types of
manganese dioxide. Therefore, ramsdellite and pyrolusite are the limiting cases of a series of
compositions that are known as γ- MnO2.
The sample obtained from binary by ions of metal electrolytes with a maximal content
of lithium (0.15 mole.L-1) showed the following approximate phase composition: 55-65% of
α-, 20-30% of γ-, 10-15% of β-, and 5-10% of ε-polymorphs of manganese dioxide.
Incorporation of cobalt affects the phase composition of electrodeposited manganese dioxide
samples similarly to other transition metal copper. The typical multiphase sample doped by
Cu2+ (0.02 mole.L-1) showed the presence of 80% γ-MnO2, 15% α-MnO2, and 5% ε-MnO2
[12].
In accordance with the model proposed recently by Ruetschi [13], it is now widely
believed that γ-manganese dioxide contains cation vacancies in the crystallographic structure
in addition to defects of microtwinning and intergrowth. We suppose that these defects are
positions where incorporation of the foreign species such as ions of copper and cobalt in
manganese dioxide occurs. Such conclusion conforms to the results reported earlier by
Velichenko [8].
The cases of low and heavy doping can be distinguished. At first, the presence of the
dopant ion is able to stabilise structure ordering of the electrodeposition product in
comparison with the undoped semiamorphous samples. The further growth of concentration
of dopant ions in an electrolyte exerts the negative influence on the degree of crystallinity and
stoichiometry of electrodeposited samples. These doped samples of manganese dioxide are XRay amorphous or simply amorphous. For the case of so-called heavy doping XRD analysis
and electron diffraction could not detect the phase composition of most disordered samples
excepting electron diffraction patterns of surface states of manganese monoxide (MnO).
200 nm
200 nm
Fig. 2 TEM images of the manganese dioxide sample maximally doped by Li (a); manganese
dioxide sample with maximal content of Co and Li in this work (b).
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The simultaneous presence of ions of Li and Co(II) in fluoride containing electrolytes
and growth of their concentrations exhibit the following peculiarities: decrease of
crystallinity, the length and perfection of nanorods of the main phase of manganese dioxide
samples and X-ray amorphous behaviour. The TEM-images in Fig. 2 show a typical region
of the doped samples at high magnification. Comparison of TEM images confirmed an
assumption concerning the change of samples dispersity and crystallinity at doping. The
needle-like crystallites of manganese dioxide sample doped only by ions of lithium
[0.15 mole·L-1 LiOH] (Fig. 2 a) loose their perfection and decrease their size at maximal
content of additives of cobalt and lithium (Fig. 2 b). The results of TEM investigations
showed that such products consist of nanostructured crystallites of 5-10 nm diameter and
length to width ratio below 50.
It should be pointed out that isostructural to manganese dioxide modifications of
oxyhydroxides of octahedrally coordinated manganese Mn3+ exist — groutite, α-MnOOH,
and manganite, γ-MnOOH [14]. Edge-sharing single chains of manganite are derivatives of
rutile structure of β-MnO2, whereas edge-sharing octahedral double chains of α-MnOOH,
groutite are also known for MnO2, ramsdellite. In addition, the existence of isostructural
phases of dioxide and oxyhydroxide of manganese explains the ability of the former to
support high defects concentrations, i.e. the presence of Mn3+ ions in the host sites of Mn4+ in
the lattice and therefore the simplicity of formation of amorphous state.
6
5
Transmission (Arbitrary units)
4
3
2
1
400
600
800
-1
ν , cm
Fig. 3 FTIR spectra of electrodeposited
samples: 1 — electrodeposited from
fluorine
containing
electrolytes
undoped sample of α-polymorph of
MnO2; 2-5 doped by Li and Co samples
(see data in Figure 1 of black colour ); 6
— γ-polymorph of MnO2 sample of
EMD-2 brand obtained from sulfate
electrolytes.
It is known that Fourier transform infrared (FTIR) spectroscopy is of great value for
structural characterisation of oxides used as electrode materials in rechargeable lithium
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batteries [15]. As shown in [16] for mineralogical analysis of manganese oxides, infrared
spectroscopy is often a necessary alternative and generally a useful supplement to X-ray
diffraction study. It is sensitive to amorphous components and those with short-range order as
well as to materials with long-range order.
The main task of FTIR investigation presented in this work was to study the shortrange order in semiamorphous and amorphous heavy doped samples of manganese dioxide.
The series of electrochemically prepared samples (curves 2—5 in Fig. 3) had a gradual
increase of concentrations of dopant ions of lithium and cobalt (see caption in Fig. 3). All
samples were X-ray amorphous excepting sample 2 that showed the presence of traces of αMnO2. Gradual change of behaviour of these samples in FTIR spectra region 400—1000 cm-1
can be seen, where the main contributions are attributed to the stretching mode of the MnO6
octahedra.
At least two types of spectra are distinguished in this Figure.. In accordance with them
and XRD data additional manganese dioxides samples were chosen for comparison (curves 1
and 6). They are electrolytic manganese dioxides of α- and γ-polymorphs. Curve 1 represents
electrolytic γ-manganese dioxide of the EMD-2 brand synthesized in Georgia by OST-6-2234-76 and curve 6 corresponds to undoped and more ordered manganese dioxide of hollandite
structure type obtained from fluoride containing electrolytes.
The behaviour of infrared spectra of samples heavily doped with lithium and cobalt
confirmed our results of XRD and TEM investigation. As shown above, the presence of
lithium in electrolyte favours formation of more opened α-polymorph of manganese dioxide.
This situation is observed upon curves 2 and partially 3. However, the spectrum of curves 4
and 5 is more close to γ-manganese dioxide EMD-2.
It is well known that the absence of thin structure of FTIR spectrum signifies the low
crystallinity of samples. In our opinion, the fact of similarity of curves 4—6 probably
indicates increasing influence of cobalt on the phase formation process. As proposed above,
its incorporation in manganese dioxide can occur at defect sites of γ-manganese dioxide
framework.
Finally, manganese dioxides can be classified according to the nature of the
polymerization of MnO6 units and the number of MnO6 octahedral chains between two basal
layers to form tunnel (Tm,n) openings. For instance, the T1,n group includes the pyrolusite βMnO2 (T1,1) and the ramsdellite R-MnO2 (T1,2); the T2,n group includes hollandite (T2,2),
romanechite(T2,3) etc. As reported earlier by Turner and co-workers [17], the possibility of
simultaneous intergrowth of more opened polymorphs like hollandite into γ-manganese
dioxide exists at atomic level.
Conclusions
In summary, disordered and semi-amorphous oxides of manganese have been prepared
by anode electrodeposition from fluoride containing electrolytes of manganese sulfate in the
presence of additives of cobalt(II) and lithium ions. Their further potential as electrode
materials in power sources and as catalysts is currently under investigation.
It is shown by XRD method and Fourier spectroscopy that low and heavy doping can be
distinguished. Lithium as a dopant stabilises more opened α-polymorph of manganese
dioxide with the structure type of hollandite (I4/m space group) where ions of lithium occupy
the blank space of structure channels. The growth of the lithium content causes the expansion
of the unit cell of α-MnO2. Based on different phase compositions and the peaks shifts of Xray diffraction patterns in samples doped by cobalt and lithium it is concluded that
incorporation of the foreign species such as ions of cobalt in manganese dioxide occurs at
defect sites.
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The heavier doping leads to the formation of X-ray amorphous products. The results of
chemical analysis, TEM and FTIR-spectroscopy investigations of such products showed that
these samples belong to disordered nanostructured dioxide of manganese consisting of
nanorods of 5-10 nm diameter and length to width ratio below 50. Its peculiarity is probably
simultaneous intergrowth of fragments of tunnels of various sizes into host structure of γmanganese dioxide at atomic level.
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Садржај: Проучен је утицај допантских јона на фазни састав и структурну
организацију нано-оксида мангана добијеног електродепозицијом. Као адитиви
коришћени су допантски јони Li и Co(II). Електролити који су садржали флуор су
примењени на припрему узорака анодном електродепозицијом. Узорци су проучени
методама хемијске анализе, рентгенске дифракције , ТЕМ, Фуријер спектроскопије и
сл. ТЕМ посматрања су показала да је присуство допантских јона литијума и кобалта
у електролиту смањило кристалиничност узорака, имало утицаја на дужину и
квалитет наношипке и структура је постајала дефективнија. Пречници наношипки су
били од 10 до 20 нм.
Кључне речи: Mn, Li, Co, наноструктуре, фазни састав.