1. Hydrothermal Growth of Two-Dimensional SrMnO3 Matthew A
Hydrothermal Growth of Two-Dimensional SrMnO3 Matthew A. Bloodgood, Nikita Deshpande and Tina T. Salguero* Department of Chemistry, University of Georgia
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Introduction
Manganite materials have received strong interest for their highly desirable properties ideal for future applications on the nano-scale, including magnetic sensors, ultrafast computing and magnetic memory. Magnetoresistance, specifically, is a change in the electrical resistance in the presence of a magnetic field, important for small scale magnetic sensors. Some manganite materials exhibit a very large change in resistivity and are called colossal magnetoresistors.1
Manganites can be synthesized through various methods, including melt flux, hydrothermal, and solid state reactions.2,3,4 Solid state reactions are an effective synthetic route; however, manganite morphology control is difficult. Hydrothermal syntheses offer a facile, water-based route to creating nanoscale manganites while enabling morphological control via reaction parameters.
Synthesis and Characterization of SrMnO3
The synthetic process begins with the creation
of MnO2 nanosheet precursors by two methods:
Fig. 6. SrMnO3 patterns for varying Mn:Sr ratios (2:1, 4:3, and 1:1) at 200 °C for 10 h. The patterns match to SrMnO3 JCPDS , but also show a SrCO3 impurity.
Solid state synthesis: Mn2O3, K2CO3, 800 °C, 30 h
Requires a 10-day acid leach in 1 M HCl for K+ exchange before being redispersed with tetrabutylammonium (TBA) hydroxide.
Low temperature, solution-based synthesis: KMnO4, EtOAc, H2O, 85 °C, 72 h
Requires solvent removal through rotary evaporation; the MnO2 powder is then isolated by membrane filtration.
Fig. 1. Magnetic moment reorganization in the presence of an external magnetic field. B
Fig. 3. Transmission electron microscopy (TEM) images of solid state (a) and low-temperature (b) MnO2.
Fig. 7. An overnight acid treatment with 1 M acetic acid was effective in removing the SrCO3 (*) impurity; SrMnO3, however, remains after acid treatment.
Fig. 4. X-ray diffraction (XRD) patterns for MnO2 precursors. Shift in the solid state sample is due to residual TBA+.
SrMnO3 nanosheets prepared via hydrothermal reaction: MnO2 (0.35 mmol), Sr(NO3)2, and NaOH (42 mmol) are mixed in 30 mL of water and then stirred for 15 min before being placed into a Teflon-lined autoclave.
Fig. 8. TEM images of SrMnO3 at varied Mn:Sr ratios. Image (a) utilized the solid state MnO2 in a 2:1 ratio; images (b) - (d) utilized the low temperature MnO2 precursor in 2:1, 4:3 and 1:1 ratios, respectively.
Table 1. Reaction results for varying starting materials and conditions.
Research Objectives
Synthesize 2-dimensional SrMnO3 using a MnO2 nanosheet precursor through a facile method.
Characterize the synthesized materials for their composition, morphology, and properties. Mn
Source
Ratio
Temp. (°C)
Time
(h)
Results
Data
Low-Temp. MnO2
1:1
200
5, 10, 20
5 h: Thin SrMnO3 nanobelts, SrMnO3 crystals, unreacted MnO2
10 h: Some thin SrMnO3 nanobelts and rods, unreacted MnO2
20 h: Large SrMnO3 belts, SrMnO3 crystals
Fig 8 b
250 10
Thick SrMnO3 nanobelts, bipyramidal SrMnO3 crystals
4:3
Bipyramidal SrMnO3 crystals, some SrMnO3 nanosheets
Fig 8 c
Few SrMnO3 nanobelts, SrCO3 np, bulk
2:1
Large SrMnO3 nanobelts and sheets, some unreacted MnO2
Fic 8 b
Small, thin SrMnO3 nanobelts, some unreacted MnO2
10:1
Unreacted MnO2, MnO2 with sharp edges
Solid State
Some small SrMnO3 platelets, bulk, unreacted MnO2
SrMnO3 nanobelts, little unreacted MnO2
Fig 8 a
KMnO4
Crumpled MnO2
Bulk MnO2
Rutile MnO2
MnO2 (np)
Some SrMnO3 nanosheets, unreacted MnO2
Material Structures
Manganites are found with a perovskite structure and general formula ABO3. Perovskite structures can exhibit polymorphism depending on the A and B atoms in the material. Two examples are shown in Fig. 2.
SrMnO3 is most commonly found as the hexagonal 4H-SrMnO3 polymorph. In this structure, MnO6 octahedra (blue) are organized into dimers, surrounded by Sr atoms (green). The orientation of MnO6 octahedra, along with any charge differences between Mn atoms, leads to unique properties, magnetoresistance in particular.
Fig. 9. Atomic force microscopy (AFM) of a SrMnO3 [2:1, 200 C, 10 h] sample indicates an average height of less than 4 nm. This height corresponds to a nanosheet containing 6 to 8 layers.
Fig. 2. (a) Cubic (ideal) perovskite structure. (b) Hexagonal, 4H perovskite structure.
Fig. 5. XRD patterns for SrMnO3 products from different MnO2 precursors. These reactions were carried out at 200 °C for 10 h with a Mn:Sr ratio of 2:1.
References
Rodriguez-Martinez, L. M., Attfield, J. P., Phys. Rev. B. 1996, 54 (22) 622–625
Zhang, X., et al., Crys. Growth Des. 2011, 11 (7),
Gonzalez-Jimenez, I. N., et al., Crys. Growth Des. 2015, 15 (5),
Rezlescu, N., et al., Digest J. of Nanomater. Biostruc. 2013, 8 (2), 8
Acknowledgements
Department of Energy
Office of Basic Energy Sciences
Award DE-SC