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Understanding hydrothermal transformation from Mn2O3 particles to Na0.55Mn2O4·1.5H2O nanosheets, nanobelts and single crystalline ultra-long Na4Mn9O18 nanowires

Abstract

Manganese oxides are one of the most valuable materials for batteries, fuel cells and catalysis. Herein, we report the change in morphology and phase of as-synthesized Mn2O3 by inserting Na+ ions. In particular, Mn2O3 nanoparticles were first transformed to 2 nm thin Na0.55Mn2O4·1.5H2O nanosheets and nanobelts via hydrothermal exfoliation and Na cation intercalation and finally to sub-mm ultra-long single crystalline Na4Mn9O18 nanowires. This paper reports the morphology and phase-dependent magnetic and catalytic (CO oxidation) properties of the as-synthesized nanostructured Na intercalated Mn-based materials.

Introduction

Manganese (Mn) oxides are indispensable materials in many applications, particularly in batteries, fuel cells, supercapacitors and catalysts1,2,3,4,5,6,7,8,9,10. Several attempts have been made to increase the efficiency of Mn materials (MnO2, Mn2O3 and Mn3O4) in the aforementioned applications. Tailoring the morphology has been a major approach and a range of morphologies have been reported, including wires/rods (1-D) and plates/sheets (2-D)4,11,12,13,14,15,16,17,18,19,20,21,22,23,24. Single-unit cell thick Mn3O4 sheets were synthesized by a solution method using Mn(NO3)2 and aminoethanol, which has shown a coercivity of 5.8 kOe at 5 K21. Tan et al. controlled the Mn3O4 morphology in the shape of nanowires, nanorods and nanoparticles by varying the relative amounts of cosolvents (CH3CN and water) using Mn(AC)3 precursor and reported a large coercivity, HC = 10.7 kOe at 5 K, for the nanowires22. Liu et al. prepared single-layer MnO2 nanosheets via a simple one-step reaction of KMnO4 and sodium dodecyl sulfate (SDS), where SDS acted as the precursor of dodecanol (a reducer) and a sheet-structure agent23. A graphene oxide–template method was used to synthesize the MnO2 nanosheets with a high surface area of 157 m2/g and good capacitance (>1017 F/g) and rate capability (>244 F/g)24. For applications to batteries, the insertion/deinsertion behaviors of alkali ions (Li and Na) over Mn oxides25,26,27,28. and their synthesis/characterization have been studied29,30,31,32. Spinel LiMn2O4 has attracted the most interest as a cathode martial because of its thermal stability and high performance2,7,14,25,33,34. Zhang et al. prepared LiMn2O4 polyhedrons (with 200–1000 nm sizes) by a solid-state reaction using Mn3O4 nanowires and LiOH·H2O at 750 °C for 6 hr and achieved a discharge capacity of 115 mAh/g14. As potential alternative to Li-ion batteries, Na-inserted Mn materials have attracted considerable interest owing to their lower cost (and high abundance) and similar physicochemical properties (e.g., redox potential and intercalation behavior)29,30,31,35,36,37. Recently, orthorhombic Na4Mn9O18 (referred to as Na0.44MnO2) has attracted a great deal of interest as a cathode material for Na-ion rechargeable batteries32,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56. Several methods have been used to synthesize the material, including sol-gel/high- temperature calcinations32,42,43,52, solid-state reaction27,39, thermal-conversion of a precursor41,polymer-pyrolysis45 and hydrothermal method52. Hosono et al. used a hydrothermal method (Teflon-lined autoclave at 205 °C for 2 days) using Mn3O4 powder in a 5.0 M NaOH solution and obtained single-crystalline Na0.44MnO2 nanowires with superior capacity of 120 mAh/g and high charge-discharge cyclability52. In these cases, the efficiency of the material was shown to be dependent on the surface area and morphology; hence, an understanding of the change in morphology during Na (or Li and K) ion-insertion is very important. Liu et al. prepared Na0.44MnO2 nanorods with recipes of MnSO4, KMnO4 and NaOH solutions by a hydothermal method56. Le et al. reported a change in morphology (from nanosheets to nanowires) and crystal structure (from Mn2O3 to birnessite and Na0.44MnO2) after the hydrothermal reaction of Mn2O3 powder in a 5.0 M NaOH solution48. Although many studies have reported the electrochemical properties of Na-inserted MnOx materials32,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56, this study examined the undiscovered Na-insertion and morphological behaviors of Mn2O3 nanoparticles during a hydrothermal reaction process.

This paper reports a facile process to control the morphology and phase of alkali metal intercalated Mn oxides using a simple hydrothermal technique. Three different alkali metals (Li, Na and K) were intercalated into the Mn2O3 powder (particles) to nanosheets, nanobelts and nanowires. In particular, quantum-thick Na0.55Mn2O4·1.5H2O nanosheets, nanobelts and single crystalline ultra-long Na4Mn9O18 nanowires were produced by inserting Na with different concentrations and reaction durations. The magnetic and catalytic (CO oxidation) properties of the as-synthesized Mn oxides are reported in detail. In addition to the new findings of the morphological behaviors (by Na-insertion)/detailed characterization and magnetic properties, the laser-induced Na-deinsertion behavior was also examined by Raman spectroscopy. The present study provides several new insights into the development of alkali metal ion intercalated Mn materials.

Results and Discussion

Figure 1 presents powder XRD patterns and scanning electron microscopy (SEM) images of the starting materials (Mn3O4 and Mn2O3) and the synthesized Na-intercalated Mn oxides by varying the reaction conditions. The insets in the SEM images in Fig. 1 also show photographs of the powder, indicating the change in color of the sample from black (for Mn2O3) to brown (for Na4Mn9O18), as the hydrothermal reaction time was increased. The XRD patterns (□) of the initial starting material synthesized by a hydrothermal method at 120 °C for 12 hrs revealed tetragonal Mn3O4. Upon annealing at 750 °C for 4 hrs, the crystal structure of tetragonal Mn3O4 (■) changed to cubic phase (la-3) Mn2O3 (JCPDS 1-071-0636). A hydrothermal reaction was then performed with the Mn2O3 nanoparticles (NPs) dispersed in 1.0 and 10 M NaOH solutions at 200 °C for different durations. With increasing hydrothermal reaction time in a 10 M NaOH solution, new XRD peaks (Δ) appeared at 12.5° and 25.1° 2θ and their intensity increased. The 2θ position of these two new peaks were in good agreement with the (001) and (002) planes of monoclinic (C2/m) Na0.55Mn2O4·1.5H2O (JCPDS 43-1456). At the same time, the intensity of the XRD peaks (■) of cubic phase Mn2O3 decreased gradually. On the other hand, for the sample prepared by treating Mn2O3 NPs hydrothermally in a 1.0 M NaOH solution at 200 °C, the intensity of these new peaks (Δ) did not increase significantly, even though the reaction was performed for 3 weeks, which was attributed to the lack of Na+ ions. On the other hand, in the 10 M NaOH solution, these two diffraction peaks (Δ) for Na0.55Mn2O4·1.5H2O showed significant intensities upon a reaction for less than 3 days. Upon the reaction for 1 week, the XRD peaks corresponding to the cubic phase Mn2O3 were disappeared completely. Interestingly, several new diffraction peaks (ο) appeared. With further increases in the reaction time to 1~3 weeks, the two peaks (Δ) for Na0.55Mn2O4·1.5H2O at 12.5° and 25.1° 2θ decreased in intensity. After a reaction for 3 weeks, the newly appeared peaks (ο) were mainly present, which matched orthorhombic (Pbam) Na4Mn9O18 (JCPDS 27-0750) (Supporting Information Fig. S1 and S2)32,42,46. This suggests a complete change in the crystal phase of Na0.55Mn2O4·1.5H2O to Na4Mn9O18 with increasing hydrothermal reaction duration to 3 weeks in 10 M NaOH at 200 °C. The high purity Na4Mn9O18 nanowires were finally obtained after the intermediate mixture; a mixture of Na0.55Mn2O4•1.5H2O and Na4Mn9O18 followed by a mixture of Na0.55Mn2O4•1.5H2O and Mn2O3. High purity Na0.55Mn2O4•1.5H2O nanosheets were not observed in the hydrothermal method.

Figure 1
figure1

XRD patterns of the starting materials (Mn3O4 and Mn2O3) and the synthesized materials according to the reaction time in the 1.0 and 10 M NaOH solution.

The insets show the corresponding SEM images (left) and Rietveld refinement powder XRD patterns of a mixed phase sample (top right). The additional Figures are provided in the Supporting Information (Figs S1, S2 and S3a, S3b) to understand the change in the crystal phase with varying reaction conditions. The reaction time was written on the right of the corresponding XRD.

Rietveld analysis was performed for a sample with mixed crystal phases (Na0.55Mn2O4·1.5H2O:Na4Mn9O18 = 22.7%:77.3%). The inset in Fig. 1 shows the observed and Rietveld refinement XRD patterns (see Supporting Information, Fig. S3). The crystal structures were fully refined and the detailed structural parameters are provided in the Supporting Information Fig. S3, Tables S1 and S2.

The SEM and TEM/HRTEM images of the corresponding samples were examined to further understand the recrystallization mechanism of Mn2O3 NPs in a NaOH solution under hydrothermal conditions at 200 °C for the specified duration. Figure 2 shows SEM images of the starting materials (Mn3O4 and Mn2O3) and the synthesized materials prepared by a hydrothermal method in 1.0 M NaOH, LiOH and KOH solutions for 24 hrs. The starting Mn3O4 and Mn2O3 showed particle morphologies with different sizes. On the other hand, after a hydrothermal reaction (1.0 M NaOH) at 200 °C, the surface morphology had changed entirely to ultrathin nanosheets. Under LiOH and KOH solution conditions, the surface morphologies were also changed to nanosheets, but were thicker than those prepared in the NaOH solution. Supporting Information, Fig. S4 provides additional SEM images of the nanosheets obtained by Na, Li and K intercalation. The SEM images and the XRD patterns (Fig. 1) indicate that the sheet morphology originated from the monoclinic Na0.55Mn2O4·1.5H2O phase, which was formed by the exfoliation of Mn2O3 upon Na and H2O concomitant intercalation. On the other hand, the presence of a Mn2O3 phase for the samples prepared in a short duration (<3 weeks in 1 M NaOH or <3 days in 10 M NaOH) was attributed to the incomplete conversion of Mn2O3 present primarily in the core part of the powder, whereas the surface consisted mainly of ultra-thin nanosheets (Fig. S5,SI). TEM, HRTEM images and electron diffraction patterns were also obtained for the ultrathin nanosheets, as shown in Fig. 2. The TEM image (top right, Fig. 2) supports the nanosheet morphology shown in the SEM images. High resolution TEM (HRTEM) (bottom right, Fig. 2) revealed the continuous lattice, indicating the crystalline nature of the nanosheets with a lattice spacing of 0.25 nm, corresponding to the (200) plane of monoclinic Na0.55Mn2O4·1.5H2O48. The selected area electron diffraction (SAED) patterns of the distinct spots on the rings shown as an inset of the HRTEM image further confirmed the crystalline nature of these nanosheets. More TEM and HRTEM images were provided in the Supporting Information, Fig. S5. For comparison, Ma et al. employed a similar hydrothermal (170 °C for 12 hrs to 1 week) method using Mn2O3 powder in a 10 M NaOH solution57. On the other hand, they reported Na+-ion free birnessite-related layered MnO2 nanobelts (5–15 nm width), which is inconsistent with the present study.

Figure 2
figure2

SEM images of Mn3O4, Mn2O3 and the synthesized materials in 1.0 M NaOH, LiOH and KOH solutions.

TEM and HRTEM images of the nanosheets synthesized in 1.0 M NaOH solution. The inset shows the SAED pattern of the nanosheets.

To measure the accurate thickness of the ultrathin nanosheets discussed above, a more skillful technique was employed, as described in Fig. 3. The nanosheets were first sandwiched between epoxy supported by disks, as illustrated in the Figure. Various treatment steps such as bonding, slicing, disk cutting and ion milling, were then performed to make a suitable TEM specimen. The thickness of the TEM specimen was finally less than 5 μm. TEM, HRTEM and high-angle annular dark field (HAADF) images were taken, which clearly showed the edge of the nanosheets. Mn in the nanosheets edge was also confirmed by an EDX profile (Supporting Information, Fig. S6). The HRTEM image showed lattice fringes with neighboring distances of 0.25 nm, corresponding to the (200) plane of monoclinic Na0.55Mn2O4·1.5H2O as mentioned above. The thickness of the nanosheet edge was measured to be 2 nm, which is close to the unit cell thickness (also see Supporting Information, Fig. S7).

Figure 3
figure3

TEM sample preparation procedures (top), HRTEM image of the edge of nanosheets (bottom left).

The inset shows the illustrated crystal planes. HAADF image (bottom right).

Because the crystal phase of Mn2O3 was not completely changed using 1.0 M NaOH, the NaOH concentration was increased to 10.0 M and a hydrothermal reaction was performed for various reaction durations. The morphologies and microstructures of the samples obtained by the hydrothermal treatment of Mn2O3 in 10 M NaOH for 20 h at 200 °C were examined further by SEM and TEM/HRTEM, as shown in Figs 4 and 5. The Mn2O3 particles initially changed to nanosheets and nanobelts with a few nanowires (or nanothreads) for a reaction duration of less than 1 week (Supporting Information, Fig. S8), whereas the Mn2O3 nanoparticles were still present in the synthesized samples. This was supported by the corresponding XRD patterns (Fig. 1). As the reaction time increased, the nanobelts evolved slowly to ultra-long nanowires. Mixed morphologies were observed in the SEM images (Supporting Information, Fig. S9). For the corresponding XRD results (Fig. 1), the XRD patterns (∆) of Na0.55Mn2O4·1.5H2O were diminished slowly and those (ο) of Na4Mn9O18 were remarkable. Upon the reaction for 3 weeks, the SEM image in Fig. 5 showed mostly ultra-long (sub-mm) nanowires (also see Supporting Information, Fig. S10). The corresponding optical microscopy images showed that the black color of the Mn2O3 (with particle morphology) changed to a brown color as the crystal phase changed to Na0.55Mn2O4·1.5H2O and Na4Mn9O18 (Supporting Information, Fig. S11). The morphology appeared like nanofibers for the final Na-intercalated Mn product. HRTEM images of the nanobelts showed a clear lattice spacing of 0.23 nm, corresponding to the (200) plane of monoclinic Na0.55Mn2O4·1.5H2O (Fig. 4). This was also observed for the ultrathin nanosheets (Figs 2 and 3), suggesting a similar growth direction of nanosheets and nanobelts. The SAED pattern confirmed the single crystal nature of the Na0.55Mn2O4·1.5H2O nanobelts. Supporting Information, Fig. S12 shows the corresponding simulated diffraction patterns. A structure projection model in Fig. 4 displays the corresponding [001] planes of Na0.55Mn2O4·1.5H2O. Figure 5 shows representative SEM, TEM and HRTEM images of the Na4Mn9O18 nanowires obtained using 10 M NaOH at 200 °C for 3 weeks. The HRTEM image shows a lattice spacing of 0.442 nm for the nanowires, which is in accordance with the (200) plane of orthorhombic Na4Mn9O1832. The spot SAED pattern confirms the single crystal structure of these nanowires. The wire grew along the [100] direction. Figure 6 shows the corresponding structure projections and crystal models of the Na-intercalated samples. In the case of the Na0.55Mn2O4·1.5H2O nanobelts, H2O and Na cations were concomitantly intercalated between the skeletons of Mn-O sheets. For the ab plane structure of the Na4Mn9O18 nanowires, Na was embedded in the Mn-O tunnel frame, which is consistent with the MnO5 square pyramids and MnO6 octahedra58. The Na cations are situated in two different sites (with a unique tunnel structure) and the c-axis is the charge-discharge paths of Na cation diffusion27,32,44. The SAED and simulated patterns of the starting material, i.e. Mn2O3, are provided in the Supporting Information, Fig. S13.

Figure 4
figure4

SEM (left column), low-magnification TEM and HRTEM images of Na0.55Mn2O4·1.5H2O nanobelts.

SAED and a model of the corresponding crystal planes are shown on the lower right.

Figure 5
figure5

SEM (left column), low-magnification TEM and HRTEM images of Na4Mn9O18 nanowires.

SAED and the model of the corresponding crystal planes are shown on the lower right.

Figure 6
figure6

Structure projections and crystal models of Na0.55Mn2O4·1.5H2O (top) and Na4Mn9O18 (bottom).

The change in crystal phase was further confirmed by FT-IR spectroscopy (Supporting Information, Fig. S14). The characteristics of the Mn-O vibrational peaks were observed between 500 and 800 cm−1 for all samples13. No OH stretching bands at approximately 3400 cm−1 was observed for the starting material, i.e. Mn2O3 powder. Upon the formation of Na0.55Mn2O4·1.5H2O, strong OH stretching bands were observed at 3430 and 3350 cm−1. On the other hand, the FTIR peaks became weaker and broader for the Na4Mn9O18 nanowires (Fig. 5 and Fig. S10). The much weaker broad band at 3400 cm−1 for Na4Mn9O18 was attributed to the adsorbed H2O (and OH) species.

Figure 7 shows the Raman spectra of the Na4Mn9O18 nanowires measured with different laser powers (0.004 mW to 2.7 mW). At a low laser power (<0.012 mW), no obvious signal was observed. With increasing laser power to 0.19 mW, the Raman signals became clear at 637.9 cm−1 and a shoulder was observed at 561.8 cm−1 (see Supporting Information, Fig. S15). Upon further increases in the laser power to 2.7 mW, a strong fluorescence signal was observed (also see Supporting Information, Fig. S16) and the peak at 637.9 cm−1 was decreased significantly. Upon reducing the laser power to 0.19 mW, critically different Raman signals were obtained (Supporting Information, Fig. S15). This suggests that the crystal phase of Na4Mn9O18 had changed irreversibly to Mn2O3 by the high power laser irradiation. The laser light induces the de-insertion of Na cations in the structure, which requires further study. The newly obtained Raman spectrum shows peaks at 312.7, 374.3 and 656.8 cm−1, which match the bulk Mn2O317. Similar Raman spectral profiles and behaviors were also observed for the Na0.55Mn2O4·1.5H2O sample (Supporting Information, Fig. S15, S16 and S17).

Figure 7
figure7

Raman spectra of the Na4Mn9O18 nanowires with increasing laser power.

The inset shows an image of the analyzed area.

X-ray photoelectron spectroscopy (XPS) was used to examine the chemical states of Na4Mn9O18 nanowires and compared with those of the starting material, i.e., hydrothermally synthesized Mn2O3 powders, as displayed in Fig. 8. A typical survey XPS scan of Mn2O3 showed Mn, O and impurity carbon signals, whereas that of Na4Mn9O18 showed additional Na as well as Mn, O and C (Supporting Information, Fig. S18). The distinct peaks at ~653.8 and ~642.1 eV (Fig. 8, top left) were assigned to the Mn 2p1/2 and Mn 2p3/2 XPS peaks, respectively, with a spin-orbit energy splitting of 11.7 eV49. The Mn 2p XPS peaks for Na4Mn9O18 were shifted slightly to a lower binding energy, confirming the Na insertion and reduction of the oxidation state of Mn59,60. The O 1s XPS spectra showed two broad peaks at 532.0 and 529.7 eV (Fig. 8, top right) due to the absorbed surface oxygen (e.g., OH, H2O and O2) species and lattice oxygen atoms of the Mn oxides, respectively13. The Na 1s XPS and Na KLL Auger peaks for Na4Mn9O18 (Fig. 8, bottom panel) were observed at 1070.7 and 494.2 eV, respectively49.

Figure 8
figure8

High resolution Mn 2p, O1s, N 1s and Na KLL photoelectron spectra of the Mn2O3 particles and Na4Mn9O18 nanowires.

The magnetic properties of the Na4Mn9O18 nanowires were examined by SQUID. Figure 9 presents zero-field-cooling (ZFC) and field-cooling (FC) magnetization curves measured at an applied field of H = 100 Oe (0.1 kOe) over the temperature range of 5−300 K. The top inset in Fig. 9 shows the magnetization (M−H) curves measured at various temperatures from 5 K to 300 K and magnetic fields from −50 to 50 kOe. An ideal linear plot (with no hysteresis loop) of magnetization was obtained with an applied magnetic field at temperatures between 300 K and 50 K, indicating the paramagnetic and antiferromagnetic properties of the Na4Mn9O18 nanowires. The M−H curves showed no saturation magnetism in the external fields up to 50 kOe. A magnetization of 2.19 emu g−1 was measured at 50 kOe and 300 K. The mass magnetic susceptibility of the nanowires at 300 K was 4.39 × 10−5 emu·g−1·Oe−1. This increased with decreasing temperature and was determined to be 5.58 × 10−5 emu·g−1·Oe−1 at 50 K. Interestingly, a magnetic hysteresis loop was clearly observed at 5 K (Supporting Information, Fig. S19), suggesting typical ferromagnetic behavior. On the other hand, the M−H curve showed no saturation, indicating antiferromagnetic property. The residual magnetism (or remanence) and coercive force were measured to be 0.136 emu·g−1 and 475 Oe, respectively. A coercivity of 10.7 kOe at 5 K was reported for the Mn3O4 nanowires22. For single unit cell thickness Mn3O4 nanosheets, Huang et al. observed paramagnetic and ferromagnetic (with a coercivity of 5.8 kOe) behaviors at room temperature and 5K, respectively21. The FC magnetization curve increased with decreasing temperature. On the other hand, the ZFC curve was increased slowly with decreasing temperature to 25 K and decreased below that temperature. The ZFC curve showed a maximum at 25 K. This suggests a clear transition from paramagnetic to ferromagnetic at a temperature below 25 K. The FC and ZFC curves showed no overlap at all temperatures up to 300 K.

Figure 9
figure9

Mass-normalized FC and ZFC curves of Na4Mn9O18 nanowires from 5 to 300 K in H = 100 Oe.

The inset show the magnetization (M−H) curves measured at various temperatures.

The surface resistance of Na4Mn9O8 nanowires was measured as a function of temperature (Supporting Information, Fig. S20). The resistance of 12.5 MΩ at room temperature decreased linearly to 1.0 MΩ with increasing temperature to 200 °C. For the Mn3O4 (in Fig. 2) and Mn2O3 powder samples, the surface resistance could not be measured because of the high resistance.

The CO oxidation activities (Supporting Information, Fig. S21) of Mn3O4 (in Fig. 2), Mn2O3 (in Fig. 2) and Na0.55Mn2O4·1.5H2O nanosheets (or Mn2O3@Na0.55Mn2O4·1.5H2O core-shell structures; sample prepared with NaOH solution in Fig. 2) was tested for catalytic applications, such as CO oxidation using low cost materials13. In the first CO oxidation runs, the CO oxidation onsets were observed in the order of Mn2O3 (200 °C) < Na0.55Mn2O4·1.5H2O (250 °C) < Mn3O4 (280 °C). The T10% (the temperature at 10% CO conversion) for Mn2O3, Mn3O4 and Na0.55Mn2O4·1.5H2O was observed at 240 °C, 280 °C and 320 °C, respectively. In the second runs, the order was the same as the onset temperatures of 180 °C (Mn2O3), 260 °C (Na0.55Mn2O4·1.5H2O) and 300 °C (Mn3O4). The T10% for Mn2O3, Mn3O4 and Na0.55Mn2O4·1.5H2O was observed at 230 °C, 320 °C and 365 °C, respectively. Only the Mn2O3 nanoparticles showed an increase in CO oxidation activity in the second run. The Na-insertion into Mn2O3 (forming Na0.55Mn2O4·1.5H2O nanosheets on the surface) showed no synergistic effect for CO oxidation. Ji et al. prepared α- Mn2O3 nanowires (by a molten salt method), Mn2O3 nanoparticles and mixed Mn2O3/Na2Mn8O16 (a ratio of 9/1) samples and tested their CO oxidation activities13. They reported that α- Mn2O3 nanowires (T10% ≈ 180 °C) showed much catalytic activity than the others (T10% ≈ 220 °C) and Na2Mn8O16 did not relate to their high catalytic activity. Their conclusions are in good agreement with the present study.

Conclusion

Na-ion intercalation into Mn2O3 was initially transformed into ultra-thin monoclinic Na0.55Mn2O4·1.5H2O nanosheets and nanobelts. The nanobelts were then evolved to single crystalline ultra-long orthorhombic Na4Mn9O18 nanowires with a (Na-ion mobile) tunnel structure. This synthesis process was extended further to other alkali metals (Li and K) using a simple hydrothermal method in a Mn2O3–dispersed alkali hydroxide (LiOH, NaOH and KOH) solution. SEM and TEM confirm the transformation of the morphology. XRD and HRTEM were used to examine the crystal phase change and microstructure. Detailed crystal structural parameters were obtained by Rietveld refinement analysis. XPS confirmed the presence of inserted Na cation. Moreover, high power laser irradiation readily induces the irreversible Na-deinsertion behavior from Na4Mn9O18 to Mn2O3, as confirmed by Raman spectroscopy. The Na4Mn9O8 nanowires exhibited ferromagnetic behavior at temperatures below 25 K and paramagnetic behavior at above that temperature. The surface resistance of Na4Mn9O8 nanowires was 12.5 MΩ at room temperature and decreased linearly to 1.0 MΩ with increasing temperature to 200 °C. The CO oxidation activity (T10% = 230 °C) of the Mn2O3 nanoparticles was substantially decreased after Na-intercalation. The very detailed transformation mechanism and the new fundamental characterization provide new insights into the development of alkali metal cation intercalated Mn oxides.

Methods

Material synthesis

Mn3O4 was synthesized by a hydrothermal method, as described below. Briefly, 10 mL of 0.1 M Mn(II) nitrate tetrahydrate (Sigma-Aldrich. >97.0%) was mixed with 10 mL of deionized water (18.2 MΩ cm resistivity) in a Teflon jar (120 mL capacity) and 1.0 mL of an ammonia solution was then added to obtain the precipitates. The reaction jar was capped tightly and placed in an oven (120 °C) for 12 hours, after which the oven was cooled naturally to room temperature. The brown precipitate was collected after washing with deionized water followed by ethanol and then dried in an air convection oven (80 °C). Bulk Mn2O3 was obtained by the post-annealing of Mn3O4 at 750 °C for 4 hrs. To synthesize the Na(or Li and K)-intercalated Mn materials, the Mn2O3 (~25 mg) was dispersed in a 20.0 mL 1.0 M (or 10 M) NaOH (or LiOH and KOH) solution. The solution in a Teflon-lined stainless autoclave was placed at 200 °C for a reaction time, which was varied from 12 hrs to 3 weeks. After a specified time (12 hrs, 1 day, 3 days, 1, 2 and 3 weeks were selected to show in the present article), the oven was stopped and cooled naturally to room temperature and the powder product was collected by centrifuging. The powder was finally washed and dried for further characterization. Although the slow reaction process took time and patience (and somewhat industrially impractical) we employed the slow process to disclose new findings and to carefully examine change in morphology which has never been reported for Mn oxide material.

Material characterization

The surface morphology of the synthesized powder samples was examined by field emission scanning electron microscopy (FE-SEM, Hitachi SE-4800). High resolution transmission electron microscopy (HRTEM) and the electron diffraction patterns were obtained using a FEI Tecnai G2 F20 at an operating voltage of 200 kV. The powder X-ray diffraction (XRD) patterns were obtained using a PANalytical X’Pert Pro MPD diffractometer operated at 40 kV and 30 mA using Cu Kα radiation. The Rietveld refinement was performed using the TOPAS software program (ver. 4.2, Bruker 2005). Further details are described elsewhere61. The Fourier-transform infrared (FT-IR) spectroscopy was performed using a Thermo Scientific Nicolet iS10 spectrometer in ATR (attenuated total reflectance) mode. The X-ray photoelectron spectra were obtained using a Thermoscientific K-alpha X-ray photoelectron spectrometer with a monochromated Al Kα X-ray source, a pass energy of 20.0 eV and an analyzed spot size of 400 μm. Confocal Raman microscopy (PRISM, NOST Co., South Korea) was conducted to take the Raman spectra for the powder samples at a laser wavelength of 532 nm and a 100 ×, 0.9NA microscope objective. The laser intensity was varied from 0.004 mW to 2.7 mW. All the Raman spectra were referenced to the Raman spectrum of cyclohexane. The magnetic properties of the Na4Mn9O18 nanowires were examined using a MPM5-XL-7 superconducting quantum interference device (SQUID) magnetometer (Quantum Design, Inc.) at various temperatures.

CO oxidation and surface resistance tests

The CO oxidation experiments were performed on a continuous flow quartz U-tube reactor with a 10 mg sample. A mixed gas (1% CO and 2.5% O2 in N2 balance) was introduced into the reactor at a flow rate of 40 mL/min. The temperature heating rate was fixed to 20 °C/min. The reaction gas products were analyzed using a SRS RGA200 quadrupole mass spectrometer. The surface resistance of the pelletized sample was measured using a home-built four-probe resistance measurement instrument.

Additional Information

How to cite this article: Park, Y. et al. Understanding hydrothermal transformation from Mn2O3 particles to Na0.55Mn2O4·1.5H2O nanosheets, nanobelts and single crystalline ultra-long Na4Mn9O18 nanowires. Sci. Rep. 5, 18275; doi: 10.1038/srep18275 (2015).

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Acknowledgements

This work was financially supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (NRF-2014R1A1A2055923) and the Science and Education Research Board (SERB), Department of Science and Technology, New Delhi through the grant SB/S1/IC-15/2013. The authors gratefully acknowledge the Raman measurements by NOST Co., Ltd.

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Y.S. designed the main experimental concepts and prepared the manuscript. D.P. analyzed HRTEM data and prepared the manuscript. Y.P. mainly performed the material synthesis. S.W.L. contributed to structural analysis. K.H. Kim performed magnetic measurements and analysis. B.K.M. performed the thickness measurement. A.K.N. performed HRTEM measurements.

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Park, Y., Woo Lee, S., Kim, K. et al. Understanding hydrothermal transformation from Mn2O3 particles to Na0.55Mn2O4·1.5H2O nanosheets, nanobelts and single crystalline ultra-long Na4Mn9O18 nanowires. Sci Rep 5, 18275 (2015). https://doi.org/10.1038/srep18275

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