Phase Composition and Crystallinity Degree of Nanostructured Products of Anode Oxidation of Manganese ( II ) Ions Doped by Ions of Lithium and Cobalt ( II )

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.


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 (Mn 3+ /Mn 4+ 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 predominately nonstoichiometric nanorods of γ-polymorph of MnO 2 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 MnSO 4 .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 CoSO 4 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 900 o C at a rate of 10 o C/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(H 2 O) 6 (SO 4 ) 2 (NH 4 )F] 2-, exhibit higher 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) x F y ] (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. .

C o , %
C o , g L -1 M n , g L - 1  .
Fig. 1 The dependence of the content of cobalt (by weight, %) in electrodeposited samples on the composition of Mn 2+ and Co 2+ 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 o C. The exothermic heat effect at 320 o C characterises a phase transition to more regular polymorph of β-manganese dioxide.The endoeffect of transformation MnO 2 -Mn 2 O 3 is observed at lower temperature (478 o C) in comparison with more regular undoped samples of manganese dioxide (520-560 o C) [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 MnO 2 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.
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 MnO 6 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-MnO 2 unit cell [10].
It can be stated that all samples obtained in this work have complex phase composition as well as any electrodeposited γ-MnO 2 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-MnO 2 .
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 X-Ray 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).

nm nm
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 Mn 3+ exist -groutite, α-MnOOH, and manganite, γ-MnOOH [14].Edge-sharing single chains of manganite are derivatives of rutile structure of β-MnO 2 , whereas edge-sharing octahedral double chains of α-MnOOH, groutite are also known for MnO 2 , 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 Mn 3+ ions in the host sites of Mn 4+ in the lattice and therefore the simplicity of formation of amorphous state.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 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 α-MnO 2 .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 MnO 6 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 αand γ-polymorphs.Curve 1 represents electrolytic γ-manganese dioxide of the EMD-2 brand synthesized in Georgia by OST-6-22-34-76 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 T 1,n group includes the pyrolusite β-MnO2 (T1,1) and the ramsdellite R-MnO2 (T1,2); the T 2,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-MnO 2 .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.
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.

Fig. 2
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).