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Topic reserved by: Sofia Rantala

Introduction


MnObelongs to transition metal oxides and it can be called manganese dioxide or manganese(IV)oxide. MnOis an inorganic compound. Oxidation number of Mn is +IV in MnO2. Manganese can form also some other oxides such as MnO, Mn3O4 and Mn2O3

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E.R. Stobbe, B.A. de Boer, J.W. Geus, The reduction and oxidation behavior of manganese oxides, Catalysis today1999, 47, 161-167 (https://doi.org/10.1016/S0920-5861(98)00296-X).

. Oxidation number is different in these manganese oxides. MnO2 has various polymorphs which are for example λ-MnO2, β-MnO2, ε-MnO2 and γ-MnO2

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C. Kim, Z. Akase, L. Zhang, A. Heuer, A. Newman, P. Hughes, The structure and ordering of ε-MnO2Journal of solid state chemistry2006, 179, 753-774 (https://doi.org/10.1016/j.jssc.2005.11.042).

. The crystal structure of MnO2 depends on what kind of MnO2 polymorph there is. Polymorphs of MnO2 have different kind of crystal structures. β-MnO2 can be also called pyrolusite which is a mineral and it can be found from the nature

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N. Curetti, D. Bernasconi, P. Benna, G. Fiore, A. Pavese, High-temperature ramsdellite-pyrolusite transformation kinetics, Physics and chemistry of minerals2021, 48, 43 (https://doi.org/10.1007/s00269-021-01166-2).

. Pyrolusite belongs to the most stable polymorph which MnO2 has

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. ε-MnO2 polymorph is also mineral like β-MnObut it is different mineral than pyrolusite. ε-MnO2 is a mineral which is called akhtenskite. 
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In addition, MnO2 has one more polymorph which is R-MnO2 and it is also a mineral. It has different crystal structure than the other minerals and it is called ramsdellite. 
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 MnOcan be used in many applications like in batteries and brick industry

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N.N. Greenwood, A. Earnshaw, Chemistry of the elements, Elsevier, Oxford, 1997.

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Table 1. Physical properties of MnO2.

Physical properties of MnO2
Molar mass86,94 g/mol

Density of β-MnO2

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D. Emerson, P. Schmidt, Pyrolusitic supergene manganese oxides: inductive properties, EM conductivity and magnetic susceptibility, Preview, 2018, 2018, 42-50 https://doi.org/10.1071/PVv2018n197p42).

4,8-5 g/cm3

Appearance

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L. Khandare, S. Terdale, Gold nanoparticles decorated MnO2 nanowires for high performance supercapacitor, Applied surface science2017, 418, 22-29 (https://doi.org/10.1016/j.apsusc.2016.12.036).

Black, brown

Melting point

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Z. Mobley, S. Jackson, G. George, Z. Luo, Chemical Microanalysis of SnO2-MnO2 nano fibers in an electron probe microanalyzer, Microsc. Microanal., 2018, 24, 2052 (https://doi.org/10.1017/S1431927618010747)

535 °C



Synthesis

There are several synthesis methods which can be used to synthesize MnO2. Synthesis methods which can be used for example are sol-gel synthesis approach, hydrothermal synthesis method and flame synthesis method

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Y. Kumar, S. Chopra, A. Gupta, Y. Kamar, S.J. Uke, S.P. Mardikar, Low temperature synthesis of MnO2 nanostructure for supercapacitor application, Materials science for energy technologies, 2020, 3, 566-574 (https://doi.org/10.1016/j.mset.2020.06.002).

. In many cases there is used hydrothermal method to synthesize MnO2. There are couple of examples how MnO2 can be synthesized when there is used hydrothermal method. The synthesis method differs slightly depending on which polymorph you want to get as a product. 

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F. Cheng, J. Chao, W. Song, C. Li, H. Ma, J. Chen, P. Shen, Facile controlled synthesis of MnO2 nanostructure of novel shapes and their application in batteries, Inorg. Chem., 2006, 45, 2038-2044 (https://doi.org/10.1021/ic051715b).


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S. Birgisson, D. Saha, B. Iversen, Formation mechanisms of nanocrystalline MnO2 polymorphs under hydrothermal conditions, Cryst. Growth Des., 2018, 18, 827-838 (https://doi.org/10.1021/acs.cgd.7b01304).

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Y. Jiang, L. Yuan, X. Wang, W. Zhang, J. Liu, X. Wu, K. Huang, Y. Li, Z. Liu, S. Feng, Jahn-Teller disproportionation induced exfoliation of unit cell scale ε-MnO2, Angewandte chemie, 2020, 59, 22659-22666 (https://doi.org/10.1002/anie.202010246).

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F. Yang, S. Cheng, C. Shi, F. Xue, X. Zhang, S. Ju, W. Xing, A facile synthesis of hexagonal spinel λ-MnO2 ion-sieves for highly selective Li+ adsorption, Processes, 2018, 6, 59 (https://doi.org/10.3390/pr6050059).


One example of hydrothermal synthesis reaction for β-MnO2:

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Mathinline
2K_{(aq)}^++2MnO_{4(aq)}^-+3Mn_{(aq)}^{2+}+3SO_{4(aq)}^{2-}+2H_2 O_{(l)}→5MnO_{2(s)}+2K_{(aq)}^++SO_{4(aq)}^{2-}+4H_{(aq)}^++2SO_{4(aq)}^{2-}        (1)


One example of hydrothermal synthesis reaction for γ-MnO2: 

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Mathinline
MnSO_4+(NH_4 )_2 S_2 O_8+2H_2 O→γ-MnO_2+(NH_4 )_2 SO_4+2H_2 SO_4            
(2)


One example of exfoliation synthesis reaction for ε-MnO2:

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4La_{0,5} Sr_{0,5} MnO_3+12H^+→3ε-MnO_2+2La^{3+}+2Sr^{2+}+Mn^{2+}+6H_2 O          
(3)   


λ-MnOcan be synthesized so that first there has to been done hydrothermal treatment so that the product which we get is LiMn2O4. After the hydrothermal treatment there then happens adsorption-desorption mechanism so that then we get the final product which is λ-MnO2. From the LiMn2Owe get then λ-MnOpolymorph.

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Crystal Structure

In the figures 1, 2, 3 and 4 can be seen crystal structures of β-MnO2, ε-MnO2, λ-MnO2 and γ-MnO2 polymorphs. In the table 2 can be seen crystal structure information of β-MnO2, ε-MnO2, λ-MnO2 and γ-MnO2 polymorphs. There can be seen from the figures 1, 2, 3, 4 and table 2 that there are four possible crystal systems which are tetragonal, hexagonal, cubic and orthorhombic. There can be also seen from the figures 1, 2, 3 and 4 that although the crystal systems are different in the β-MnO2, ε-MnO2, λ-MnO2 and γ-MnO2 polymorphs, the coordination polyhedron is same in β-MnO2, ε-MnO2, λ-MnO2 and γ-MnO2 polymorphs. The coordination polyhedron is octahedron. There can be seen that each manganese atom is bound to six oxygen atoms.      

Figure 1: Manganese is color purple and oxygen is color red. On the left there is crystal structures of β-MnO2. On the right there is crystal structure of β-MnO2 but there are also polyhedrons. There can be seen the coordination polyhedron from the right side of the figure. The figure is visualized with Vesta. Data from

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. (Figure: Sofia Rantala)

  

Figure 2: Manganese is color purple/white and oxygen is color red. On the left there is crystal structures of ε-MnO2. On the right there is crystal structure of ε-MnO2 but there are also polyhedrons. There can be seen the coordination polyhedron from the right side of the figure. The figure is visualized with Vesta. Data from 

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. (Figure: Sofia Rantala)

Figure 3: Manganese is color purple and oxygen is color red. On the left there is crystal structures of λ-MnO2. On the right there is crystal structure of λ-MnO2 but there are also polyhedrons. There can be seen the coordination polyhedron from the right side of the figure. The figure is visualized with Vesta. Data from

Single cite
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B. Vadlamani, K. An, M. Jagannathan, K.S. Chandran, An in-situ electrochemical cell for neutron diffraction studies of phase transitions in small volume of Li-ion batteries, Journal of electrochemical society, 2014, 161, A1731-A1741 (https://doi.org/10.1149/2.0951410jes).

. (Figure: Sofia Rantala)


Figure 4: Manganese is color purple and oxygen is color red. On the left there is crystal structures of γ-MnO2. On the right there is crystal structure of γ-MnO2 but there are also polyhedrons. There can be seen the coordination polyhedron from the right side of the figure. The figure is visualized with Vesta. Data from

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C. Fong, B.J. Kennedy, M.M. Elcombe, A powder neutron diffraction study of λ and γ manganese dioxide and of LiMn2O4Zeitschrift fuel kristallographie, 1994, 209, 941-945 (https://doi.org/10.1524/zkri.1994.209.12.941).

. (Figure: Sofia Rantala)


The crystal structure depends on the temperature. λ-MnOand γ-MnOhave different temperatures when the λ-MnOand γ-MnObecomes β-MnO2. λ-MnOis the first one which becomes β-MnO2 because it needs the lowest temperature to become β-MnO2. The temperature where λ-MnObecomes β-MnO2 is 240 °C. The next polymorph of MnOwhich becomes β-MnOis γ-MnO2. This happens when the temperature is 400 °C. When the temperature is above 500 °C then happens so that β-MnObecomes Mn2O3. λ-MnOhas structure which is called defect spinel structure. γ-MnOhas structure which is called intergrowth structure.

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T. Hatakeyama, N. Okamoto, T. Ichitsubo, Thermal stability of MnOpolymorphs, Journal of solid state chemistry, 2022, 305, 122683, (https://doi.org/10.1016/j.jssc.2021.122683).

ε-MnO2 doesn't become β-MnObefore it would then become Mn2O3. ε-MnOstarts to become Mn2Oand Mn3O4 when the temperature is 500 °C

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Table 2. Crystal structures of β-MnO2, ε-MnO2, λ-MnO2 and γ-MnO2 polymorphs. 

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Crystal structure of β-MnO(pyrolusite)
Crystal systemTetragonal rutile structure
Space groupP42/mnm
Unit cell parametersa = b= 4,403 Å

c = 2,874 Å

α = β = γ = 90°
ICSD code112710
Crystal structure of ε-MnO2
Crystal systemHexagonal symmetry
Space groupP63/mmc
Unit cell parametersa = b = 2,801 Å

c = 4,448 Å

α = β = 90°

γ = 120°
ICSD code29561
Crystal structure of λ-MnO2
Crystal systemCubic symmetry
Space groupFd-3m
Unit cell parametersa = b = c = 8,060 Å

α = β = γ = 90°
ICSD code193445
Crystal structure of γ-MnO2
Crystal systemOrthorhombic
Space groupPnam
Unit cell parametersa = 9,323 Å

b = 4,453 Å

c = 2,848 Å

α = β = γ = 90°
ICSD code78331













Characterization

MnO2 can be characterized with different characterization methods. MnO2 can be characterized for example with Raman spectroscopy, IR spectroscopy and X-ray powder diffraction (XRD). Here can be seen Raman spectrum of pyrolusite in the figure 5. There can be seen two bigger peaks from the Raman spectrum in the figure 5. There is one peak between 500-600 Raman shift cm-1 and one other peak which is between 600-700 Raman shiftcm-1. There can be also seen XRD patterns for the all four different polymorphs of MnO2 in the figures 6 and 7. From the figures 6 and 7 there can be seen that the XRD patterns little differ from each other depending which polymorph of MnOthere is. 

Raman spectrum

Figure 5: Raman spectrum of pyrolusite when the wavelength of the laser is 532 nm. Source: University of Arizona Mineral Museum 11514, RRUFF ID: R050361

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The RRUFF Database, Pyrolusite. Data available: https://rruff.info/pyrolusite/display=default/R050361 [17.3.2023]

. (License: Public domain)

X-ray powder diffraction


Figure 6: Here can be seen XRD patterns of β-MnO2 on the left and ε-MnO2 on the right. The figure is visualized with Vesta. Data from

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. (Figure: Sofia Rantala)


Figure 7: Here can be seen XRD patterns of λ-MnO2 on the left and γ-MnO2 on the right. The figure is visualized with Vesta. Data from

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. (Figure: Sofia Rantala)

Applications

Batteries are one of the applications where MnO2 can be used. MnOis that kind of material which can be used in primary as well as in the secondary batteries. MnOcan be utilized so that there is MxMnOand the M can be Na or Li. This belongs to the ion battery applications. 

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Usually γ-MnOis the polymorph of MnO2 which is utilized in battery applications because it is polymorph which is electrochemically active. In addition, ε-MnOpolymorph could be one option for the battery applications because when there is compared γ-MnOand ε-MnOthere can be noticed that they have similar electrochemical activity. 
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References

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