Manganese oxides synthesized via microwave-assisted hydrothermal method: phase evolution and structure refinement

doi:10.21203/rs.3.rs-131869/v1

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Manganese oxides synthesized via microwave-assisted hydrothermal method: phase evolution and structure refinement

https://doi.org/10.21203/rs.3.rs-131869/v1

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Manganese oxides were synthesized during 40 min at 140 ºC via Microwave-Assisted Hydrothermal (MAH) method and treated at different temperatures in order to evaluate the phase evolution using structure refinement (Rietveld method). The samples obtained were heat treated at temperatures defined by means of thermal analysis (160 ºC, 480 ºC, 715 ºC, 870 ºC, 920 ºC and 1150 ºC) and analyzed by XRD, XRF, FTIR spectroscopy, Raman scattering and SEM. Structural characterizations allowed to identify five distinct phases: α-MnO₂, Mn₃O₄, Mn₅O₈, Na₂Mn₅O₁₀ and Na₄Mn₉O₁₈ with weight percentages dependent on the heat treatment. The hausmannite structure (average crystallite size ranging from 28.9 nm to 99.1 nm) is present in all samples and go through various oxidation and reduction processes from 160 ºC to 1150 ºC without any major variation in the lattice parameters. The results presented enables a better interpretation of the thermal and structural characteristics of manganese oxides synthesized via MAH.

Thermodynamics and statistical mechanics

Ceramics

Manganese Oxides

Hausmannite

Microwave-Assisted Hydrothermal

Rietveld Refinement

Mn5O8

Manganese oxides (Mn_1 − xO, MnO₂, Mn₂O₃, Mn₃O₄, Mn₅O₈, etc.) present remarkable technological importance due to their diverse crystalline structures, many of them constituted by tunnels, that is a direct consequence of the varied oxidation states presented by Mn (2 + to 7+), which give them important applications, such as in energy storage devices, fuel cells components and supercapacitor optimization [1–5].

Mn₃O₄ (hausmannite), for example, has a spinel-like structure with a unit cell consisting of 32 oxygen atoms and 24 manganese atoms, the latter having di- and trivalent cationic states (with Mn²⁺ ions forming the tetrahedral clusters and the ions Mn³⁺ forming the octahedral clusters) [6], a particular configuration that allows this material to be used in electrochemical processes [7] and in heterogeneous photocatalysis [8]. Based on these applications, several studies report the use of hausmannite with different crystalline systems in order to photodegrade dyes such as Alizarin Yellow, Methylene Blue and Methyl Orange [9–11]. Hausmannite is also extensively used in electrochemical energy storage devices, mainly in Electrical Double-Layer Capacitors (EDLCs), replacing cobalt oxides that are more toxic and less abundant. In addition, the production of manganese oxides in their bulk form its 20 times cheaper than the production of cobalt oxides [12, 13].

Another notable stoichiometry of manganese oxides is represented by Mn₅O₈, where the Mn cations present the states Mn²⁺ and Mn⁴⁺, in addition to forming lamellar structures, such as birnessite (Mn₂O₄) [14]. Because it is a metastable structure, only recently this oxide has been used, mainly as a catalyst in denitration processes [15]. The particular mixture of valence states, with its antiferromagnetic characteristic, allows Mn₅O₈ to be used in hard disk sensors and devices based on magnetic thin films [16].

Usually, Mn₂O_3, Mn₃O₄, Mn₅O₈, among others structures, are obtained through thermal decomposition of MnO₂, particularly α-MnO₂, where the temperatures for the various Mn oxidations depends on the characteristics of the precursor used, such as its average particle size [17, 18]. The use of thermal decomposition for manganese oxides synthesis allows to obtain a large amount of material at a reasonably low cost, however, the powder produced has a high variation in particle size, generally aggregated and without morphology control, which results in a ceramic with low density and anisotropic properties [19]. Therefore, it becomes necessary to explore other synthesis routes that allow greater control of the material's microstructure and morphology. For hausmannite, for example, several methods can be used to control the morphology of the final product, such as chemical reduction, co-precipitation, auto-combustion, sol-gel, solid state reaction, carburization or through the conventional hydrothermal method [20–25]. Additionally, the Microwave-Assisted Hydrothermal (MAH) method is an alternative way to synthesize manganese oxides, making it possible to obtain a final material with high crystallinity, reasonable control of particle size and morphology, in addition to be environmentally friendly (the synthesis medium is not organic) and energetically viable (short synthesis times at low temperatures) [26].

This technique has gained notoriety in recent decades and is now widely used in the synthesis of advanced ceramics [27, 28]. Particularly for manganese oxides, it is likely that the first synthesis using the MAH method occurred in 2006, where Apte et al. obtained the α-MnO₂ and Mn₃O₄ phases [29]. Subsequent researches has shown that, by controlling temperature and precursors during MAH synthesis, it is possible to obtain different phases and morphologies for manganese oxide, as reported by Yu et al. on the synthesis of clew-like ε-MnO₂ [30] and by Li et al. on the synthesis of flower-like and nanotubes of α-MnO₂ [31]. In addition, the MAH method also makes it possible to control the amount of hausmannite nanocrystals on the surface of composites with Reduced Graphene Oxide (RGO), materials used for the development of supercapacitors [32].

Despite the better understanding of the influence that the synthesis parameters have on the characteristics of manganese oxides produced by MAH method, there are only a few studies that focus on the phase evolution of these materials from subsequent heat treatments. It was not possible to find articles that deal with the structure refinement for manganese oxides obtained by this route. Therefore, this work has as main objective the study of the MAH synthesis of manganese oxide and its crystalline phases, through the Rietveld refinement, aiming to search for a correlation between the synthesis/sintering parameters with the phase evolution after specific heat treatments.

For the MAH synthesis of manganese oxide, 50 mL (0.5 M) of MnCl₂.4H₂O (99%, Alphatec) and 40 mL (5.5 M) of NaOH (98%, Synth) solutions were prepared using distilled water (~ 2.5 µS/cm) as the reaction medium. The solutions were mixed using a magnetic stirrer for 5 min, in a Teflon® vessel with maximum capacity of 100 mL, where deionized water was added until the volume of the vessel was completed. The vessel was placed in a sealed autoclave installed inside an adapted domestic microwave oven (2.45 GHz) with a fixed power of 1.0 kW and a temperature control system.

The heating rate adopted was 100 ºC/min with a synthesis time of 40 min at 140 ºC and a maximum pressure of 1.0 bar. After the MAH synthesis, the sample was several times washed with distilled water until the solution reached neutral pH and then the supernatant was discarded and the precipitate remained in an kiln (80 ºC, 12 hs). The resulting brownish-colored powder was de-agglomerated in an agate mortar (sample MnO). Another synthesis was performed using these same parameters in order to evaluate the reproducibility of the synthesis method.

The synthesized manganese oxide (~ 12.3 mg) was submitted to thermal analysis (SDT Q-600, TA Instruments), using alumina crucibles, a heating rate of 10 ºC/min, an equilibrium temperature of 30 ºC, synthetic air atmosphere with 100 mL/min flow and maximum temperature of 1200 ºC. The weight loss values and the maximum and/or minimum positions of the thermal processes were determined from the equipment software (Universal Analysis 2000). Then, according to the identified reactions in the thermal analysis, the MnO sample was heat treated in a low-temperature oven (EDG 3000), using alumina crucibles, with a heating rate of 10 ºC/min at the following temperatures: 160 ºC, 480 ºC, 715 ºC, 870 ºC, 920 ºC and 1150 ºC, during 1 h. These samples were denominated as MnO 160 ºC, MnO 480 ºC, MnO 715 ºC, MnO 870 ºC, MnO 920 ºC and MnO 1150 ºC, respectively.

The crystalline phases of the samples, before and after the heat treatment, were determined through an X-Ray Diffractometer - XRD - (XRD-6000, Shimadzu) at room temperature, using Cu K_α1 (λ = 1.5406 Å) and Cu K_α2 (λ = 1.5444 Å) radiation, divergence and reception slits of 1º, in continuous scanning mode (2º/min), 40 kV, 30 mA and 2θ angular range from 10º to 80º. The diffraction patterns were identified using the Powder Diffraction Files (PDF) of the JCPDS-ICDD database (Joint Committee on Powder Diffraction Standards – International Center for Diffraction Data). An estimate of the average crystallite sizes of the analyzed samples was performed using the Scherrer equation, with background subtraction, K_α2 stripping and a shape factor of 0.9.

For quantitative results on the percentages of the phases, structure refinement was performed (Rietveld method), using the GSAS software (General Structure Analysis System, available by A. C. Larson and R. B. von Dreele) [33]. This method uses the best approximation between the calculated and observed diffractograms to readjust the crystalline structure so that it is closest to the real one (best fitting approach). Specifically for the Rietveld refinement, divergence and reception slits of 0.5º, scanning speed of 0.2º/min and angular range 2θ from 20º to 110º were adopted. Crystallography Information Framework (CIF) files from the Crystallography Open Database were also used as refinement control files.

Approximate values of the atomic percentages of the samples were obtained using an X-Ray Fluorescence spectrometer - XRF - (EDX7000, Shimadzu). A Rh cathode was used as the primary source of radiation. The scanning adopted covered characteristic energies ranging from Na to U, in qualitative-quantitative mode, at room temperature and vacuum. Biaxally oriented polyester substrates of poly(ethylene terephthalate) (boPET, Mylar®) were used and an area of approximately 80 mm² was analyzed. The sample morphologies (Au metallization) were observed using a Scanning Electron Microscope - SEM - (EVO LS 15, Zeiss).

The MnO samples were also subjected to Raman scattering via a spectrometer (inVia, Renishaw), equipped with Leica microscope, a 1800 lines/mm grid and CCD detector with scanning from 300 to 5000 cm^− 1, 100 scans and excitation laser at 633 nm (He-Ne source). Portions of the samples were also mixed with KBr (99%, Sigma-Aldrich) in a 1:100 ratio (MnO/KBr) and uniaxially pressed (80 kN for 2 min, resulting in a 1.3 cm diameter and 3 mm thick pellets) to perform the FTIR characterization (Fourier-Transform Infrared Spectroscopy - Tensor 27, Bruker), in the range of 250 to 750 cm^− 1, with spectral resolution of 4 cm^− 1 and 128 scans.

The XRD patterns of the MnO sample (Fig. 1) shows the presence of two phases, tetragonal Mn₃O₄ (hausmannite) (JCPDS 89-4837), with well-defined peaks, indicating high crystallinity and tetragonal α-MnO₂ (JCPDS 72-1982), which it presents low and wide peaks, indicating low crystallinity.

Apte et al. [29], using manganese nitrate, ethanolamine and ethylenediamine, obtained tetragonal phase of hausmannite with high crystallinity, even in short microwave irradiation times (1–5 min), however Li et al., using KMnO₄ and HCl as precursors, obtained birnessite-type MnO₂ and tetragnonal α-MnO₂ structures, with 25 min of synthesis time and 100 ºC and 140 ºC, respectively, both with low crystallinity [31]. Comparing these results with the obtained phases in this work, it is very important to mention the role of precursors and synthesis parameters during the use of the MAH method in the preparation of advanced ceramics. Both in the work of Apte et al. [29] as in the synthesis of the MnO sample, the importance of a hydrothermal solution rich in OH⁻ groups is highlighted, which usually favor the construction of the crystalline network of various ceramic oxides [34, 35], and in this case, favor the crystallization of the hausmannite structure.

When produced by the ionic liquid method, Mn₃O₄, as in this work, also has a small MnO₂ impurity, suggesting that the synthesis environment where there is a high concentration of hydroxyls is adequate to stabilize manganese ions and promote the nucleation of Mn₃O₄ but can result in spurious phases [6, 36]. After the precursors dissociation and the sodium chloride and manganese hydroxide precipitation, partial oxidation of Mn (Mn²⁺ to Mn³⁺) occurs, with the interaction with hydroxyls, resulting in formation of Mn₃O₄ structure. It is assumed that the formation of a small portion of α-MnO₂ is the result of a charge imbalance promoted by the insertion of Na⁺ ions (from NaOH mineralizer) into the interstices of the synthesized material, since hausmannite has a reversible intercalation capacity for alkali metal ions [37–39]. These chemical reactions involved in the construction of the Mn₃O₄ crystalline network can be summarized as follows:

MnCl₂.4H₂O_(s) → Mn²⁺_(aq) + 2Cl⁻_(aq) + 4H₂O_(l) (dissociation – aqueous medium)

NaOH_(s) → Na⁺_(aq) + OH⁻_(aq) (dissociation – aqueous medium)

Na⁺_(aq) + Cl⁻_(aq) → NaCl_(s) (precipitation)

Mn²⁺_(aq) + 2OH⁻_(aq) → Mn(OH)_2(s) (precipitation)

3Mn(OH)_2(s) + 2OH⁻_(aq) → Mn₃O_4(s) + 4H₂O_(l) + 2e⁻ (dehydration + partial oxidation of Mn)

It is known that phase transformations in relation to the temperature variation in manganese oxides depends on the used precursors, stoichiometry, particle size and the morphology of the synthesized materials [6]. The transition temperature from the Mn₃O₄ phase to Mn₅O₈ metastable phase, for example, has a range of up to 130 ºC (from 350 ºC to 480 ºC), depending mainly on the used precursors and the particle size of the treated material [10, 40, 41]. Therefore, to evaluate these phase transformations specifically for the manganese oxide synthesized via MAH, the MnO sample was subjected to thermal analysis.

The thermal analysis up to 1200 ºC (thermogravimetry and differential scanning calorimetry) of the MnO sample is shown in Fig. 2. Two endothermic reactions are observed up to approximately 117 ºC, accompanied by a weight loss of 1.34%, which are associated with the dessorption of molecules on the sample surface, usually water molecules, a common phenomenon that occurs in this type of oxide [42].

The relatively intense exothermic reaction at 204 ºC and the low intensity endothermic reaction at approximately 355 ºC are most likely related to the reduction and oxidation processes of both α-MnO₂ and Mn₃O₄, respectively [43]. Thermal oxidation processes are usually accompanied by weight losses, resulting from the interaction of the treated sample with the furnace atmosphere, this weight loss in the 117 ºC-454 ºC range was approximately 1.60% and may also be related to the desorption of hydroxyls still present on the particle surface and the loss of structural water [44]. The characteristic weight gain (0.15%) between 454 ºC and 524 ºC can be related to the manganese oxide reduction, particularly during the transformation of Mn₅O₈ to Mn₂O₃ and Mn₃O₄, where there is a total reduction of Mn⁴⁺ ions to Mn³⁺ [45]. The existence of the Mn₅O₈ metastable phase in this temperature range can be confirmed through the XRD patterns of the MnO 480 ºC sample (Fig. 3 (c)).

Then, between 524 ºC and 715 ºC, the reactions indicate the conversion of Mn₂O₃ phase to Mn₃O₄ (peaked at 657 ºC) and O₂ release, resulting in a considerable weight loss, around 2.08%. Finally, still in Fig. 2, from 715 ºC to 1200 ºC, several low intensity reactions are noticed, most likely related to the movement of ions such as Na⁺ in the hausmannite network. Of these low intensity reactions, only the peaks around 975 ºC stand out, where there is a new conversion of the Mn₃O₄ to Mn₂O₃ [46], it is more liked that in this work this temperature stands belows 975 ºC (from 870 ºC), and is represented by a slow reaction, since there is no prominent peak of 700 ºC up to 1000 ºC. In addition, two reactions stand out in the range 715 ºC-1200 ºC, one around 1052 ºC [45], characteristic of the second conversion from the Mn₂O₃ to Mn₃O₄ phase, which in this case is represented by a rapid endothermic reaction and another around 1175 ºC, characteristic of the transformation from the tetragonal Mn₃O₄ to cubic Mn₃O₄ [47]. In this same range, there is a weight loss of approximately 2.31%, also related to the release of O₂. Considerations regarding the thermal analysis of the MnO sample are summarized in Table 1.

Table 1

– Thermal phenomena for the MnO sample in the range of 30 ºC to 1200 ºC.
Temperature Range (ºC)	Thermal Phenomena	Phase Transformation
30–117	Desorption of molecules, such as water, from the surface of α-MnO₂ and Mn₃O₄ particles.	-
117–454	Reduction of α-MnO₂ (204 ºC) and oxidation of Mn₃O₄ (355 ºC). Structural water removal. Hydroxyl desorption.	α-MnO₂ → Mn₃O₄ Mn₃O₄ → Mn₅O₈
454–524	Reduction of Mn₅O₈.	Mn₅O₈ → Mn₂O₃ Mn₅O₈ → Mn₃O₄
534–715	Oxidation of Mn₂O₃ (657 ºC). O₂ release.	Mn₂O₃ → Mn₃O₄
715–1200	Ionic movement in the Mn₃O₄ network. Reduction of Mn₃O₄. Oxidation of Mn₂O₃ (1052 ºC). Transformation of the tetragonal Mn₃O₄ phase to cubic Mn₃O₄ (1175 ºC). O₂ release.	Mn₃O₄ ↔ Mn₂O₃ Tetragonal Mn₃O₄ → Cubic Mn₃O₄

Figure 3 shows the XRD patterns of MnO sample compared with the diffraction patterns of the samples treated at 160 ºC, 480 ºC, 715 ºC, 870 ºC, 920 ºC and 1150 ºC, for 1 h. These temperatures were set to analyze the sample structure right after a weight loss range indicated by the thermogram. As expected, for MnO 160 ºC sample (Fig. 3 (b)), α-MnO₂ and Mn₃O₄ phases are still present, however, the peaks located at 19º and 25º referring to α-MnO₂ phase are less intense and broader compared to MnO sample, indicating long-range disorder and/or smaller particle size of α-MnO₂ phase around 160 ºC. According to Fig. 2, its oxidation (α-MnO₂ → Mn₃O₄) will only occur at 204 ºC. The diffractogram of MnO 480 ºC sample indicates the existence of two distinct phases, Mn₃O₄, well crystallized and monoclinic Mn₅O₈ (JCPDS 39-1218), with low crystallinity. Also according to the thermal analysis, the oxidation process Mn₃O₄ → Mn₅O₈ starts at approximately 355 ºC, however, at 480 ºC these two phases coexist, due to the consequent reduction of metastable Mn₅O₈, which starts around 454 ºC [45].

From MnO 715 ºC sample, the only stoichiometry of the manganese oxide present is hausmannite, although there are several phase transformations at intermediate temperatures, which means that the selected temperatures (except in 480 ºC) coincide with the stability temperatures of Mn₃O₄. It is interesting to observe the appearance of the monoclinic Na₂Mn₅O₁₀ phase (JCPDS 27–749), verified from the peaks around 17º, 19º, 30º and 37º, in MnO 715 ºC sample (Fig. 3 (d)), and orthorhombic Na₄Mn₉O₁₈ phase (JCPDS 27–750), verified from the peak around 38º, in the samples MnO 870 ºC and MnO 920 ºC (Fig. 3 (e) and (f)). The presence of these manganese oxides with sodium is a clear indication that, even before heat treatments, Na⁺ ions are inserted in the some sites of the synthesized material network. As previously mentioned, the existence of these doped ions probably resulted in the unbalance of charges that allowed the formation of the residual α-MnO₂ right after the MAH synthesis, as well as in low intensity endothermic reactions in the 715 ºC-1200 ºC range, which most likely are related to the crystallization of Na₂Mn₅O₁₀ and Na₄Mn₉O₁₈ phases which, in MnO 1150 ºC sample (Fig. 3 (g)), no longer exist – that is the only sample that presents a single crystalline phase represented by Mn₃O₄, still tetragonal.

Table 2 shows the results of the semi-quantitative chemical analysis via XRF spectometry, performed in a vacuum chamber in qualitative-quantitative mode, of untreated and thermally treated MnO samples. The atomic percentages of the samples vary from 98.36% to 98.76% (mean value of 98.50%) for Mn and from 1.24% to 1.79% (mean value of 1.50%) for Na, these values are within the equipment error (+/- 0.5%). These results are consistent with the observation of Na₂Mn₅O₁₀ and Na₄Mn₉O₁₈ phases in the diffractograms and are sufficient to confirm the presence of interstitial Na in the samples where there is no crystallization of the manganese and sodium oxides. As mentioned earlier, hausmannite works as an alkali metal ion intercalation compound, so it is understandable that, from the MAH solution rich in Na⁺, there is insertion of this ion in the network of the synthesized material. The most used electrolyte in Mn₃O₄-based capacitive systems, are aqueous solutions of Na₂SO₄, several studies report the formation of Na_xMn_yO_δ species from these solutions and these oxides are responsible for the pseudocapacitive behavior of Mn₃O₄ [37, 38, 48].

In addition, Fig. 4 shows the XRD patterns of the manganese oxide replicated samples, the results are essentially the same, indicating reproducibility of the synthesis method.

Table 2

– XRF elementary analysis of MnO, MnO 160 ºC, MnO 480 ºC, MnO 715 ºC, MnO 870 ºC, MnO 920 ºC and MnO 1150 ºC samples.
Sample	Atomic Percentage (at%)
Sample	Mn	Na
MnO	98.39	1.61
MnO 160 ºC	98.56	1.44
MnO 480 ºC	98.64	1.36
MnO 715 ºC	98.48	1.52
MnO 870 ºC	98.30	1.70
MnO 920 ºC	98.36	1.64
MnO 1150 ºC	98.76	1.24
Mean Value	98.50	1.50

Table 3 shows the average crystallite sizes, calculated from the most intense peak of each identified phase, for MnO samples and their replicates. For both sets of samples, it is possible to observe that α-MnO₂ phase is the one with the lowest values for the average sizes (mean value of 20.6 nm) and Mn₃O₄ phase is the one with the largest variations for these values in relation to heat treatment, ranging from 28.9 nm to 99.1 nm. The crystallites for Na₄Mn₉O₁₈ phase are slightly larger than those presented by Na₂Mn₅O₁₀ phase (mean values of 28.2 nm and 51.8 nm, respectively), which may be associated with the higher theoretical volume of Na₄Mn₉O₁₈ phase. Furthermore, for comparison, Rani et al. [12] report the average crystallite sizes of 28.3 nm and 56.6 nm for the Mn₃O₄ phase synthesized by co-precipitation and sol-gel, respectively. Liu et al. [32] obtained Mn₃O₄/RGO nanocomposites by MAH method with reduced crystallite size (around 18.4 nm).

Table 3

– Average crystallite sizes of MnO, MnO 160 ºC, MnO 480 ºC, MnO 715 ºC, MnO 870 ºC, MnO 920 ºC and MnO 1150 ºC samples and its respective replicas.
Sample	Average Crystallite Sizes (nm)
Sample	α-MnO₂	Mn₃O₄	Mn₅O₈	Na₂Mn₅O₁₀	Na₄Mn₉O₁₈
MnO	20.9	80.5	-	-	-
MnO 160 ºC	19.6	67.2	-	-	-
MnO 480 ºC	-	28.9	32.1	-	-
MnO 715 ºC	-	77.5	-	26.2	-
MnO 870 ºC	-	99.1	-	-	48.6
MnO 920 ºC	-	43.5	-	-	57.2
MnO 1150 ºC	-	46.5	-	-	-
Sample (Replica)	Average Crystallite Sizes (nm)
Sample (Replica)	α-MnO₂	Mn₃O₄	Mn₅O₈	Na₂Mn₅O₁₀	Na₄Mn₉O₁₈
MnO	21.3	72.5	-	-	-
MnO 160 ºC	20.5	70.7	-	-	-
MnO 480 ºC	-	31.9	33.3	-	-
MnO 715 ºC	-	72.5	-	30.2	-
MnO 870 ºC	-	84.2	-	-	48.2
MnO 920 ºC	-	44.6	-	-	53.4
MnO 1150 ºC	-	47.8	-	-	-

The main vibrational modes for MnO samples, determined by FTIR characterization, are shown in Fig. 5, in the range 250–750 cm^− 1. Four broad bands are initially observed for all samples, indicating overlapping of vibrational modes and possible symmetry breaks [49–53] in the MnO_x clusters which may be related to the presence of Na in these materials, as evidenced by XRF (Table 2). A wide band around 300 cm^− 1 can be attributed to external vibrations caused by translational movement from the MnO₆ cluster [31, 53]. It is also possible to notice the characteristic vibrational coupling mode of the Mn–O stretch at the tetrahedral and octahedral sites of Mn₃O₄ around 370 cm^− 1 [12, 49], suggesting that all samples have the hausmannite phase, as shown in the diffractograms. A clear vibrational separation in this band (372 cm^− 1 and 381 cm^− 1) is observed for MnO 715 ºC, MnO 870 ºC and MnO 920 ºC samples, which are the same samples that presents the sodium-manganese oxide phase crystallization. It is likely that the orderly presence of Na⁺ around the MnO₄ and MnO₆ clusters results in the Mn²⁺ and Mn³⁺ ions displacement, resulting in the appearance of the new band.

The absorption band centered at 470 cm^− 1 is characteristic of the stretching vibrations of the Mn³⁺–O bonds in the octahedral sites of the Mn₃O₄ phase [54]. When there is an excess of vacancies in this structure, Mn³⁺ to Mn⁴⁺ oxidation usually occurs and this causes this band to move to highest wavenumbers, characterizing the Mn⁴⁺–O bond [54]. It is possible to observe this displacement for the MnO 480 ºC sample, where the band displace to 482 cm^− 1, however, this is the result of the Mn⁴⁺ ions in Mn₅O₈ phase, formed from the heat treatment.

A wide band centered at approximately 598 cm^− 1 (Mn–O bending of the tetrahedral site together with distortion vibration of Mn–O in the octahedral site [55]), presents another remarkable separation (595 cm^− 1 and 600 cm^− 1) only for the samples MnO 715 ºC, MnO 870 ºC and MnO 920 ºC, one more indicative of the distortion of MnO₄ and MnO₆ clusters, resulting from the presence of Na. It is worth mentioning that, although the monoclinic Na₂Mn₅O₁₀ and orthorhombic Na₄Mn₉O₁₈ are known and currently explored for use in ionic sodium batteries [1, 51], it was not possible to find articles that deal with the specific vibrational modes for these materials. Additionally, it is important to note that the decrease in the absorption bands is related to the lower crystallinity/quality of the material [50], this occurs mainly in the MnO 1150 ºC sample, in agreement with the less intense diffraction patterns for this sample observed in Fig. 3 (g).

To complement the considerations made about the vibrational modes identified, Fig. 6 shows the Raman scattering for the MnO samples in the ranges 300–1200 cm^− 1 and 300–5000 cm^− 1 (Fig. 6 (a) and (b), respectively). It is possible to observe that, in all the analyzed ranges, there is an evident decrease in the background in relation to the heat treatment temperature that which supposedly increases the size of the particles with increasing temperature. This is assumed to be related to luminescent emissions, which are highly dependent on particle size [56, 57]. J. Wang et al. [58] report two main absorption regions for hausmanite nanoparticles, a larger one around 450 nm and a smaller one around 650 nm, so it is consistent to assume that there is a partial absorption of the excitation laser used in Raman scattering (633 nm) and this is evident from the observed luminescent emissions (wide bands in the infrared region, from 300 cm^− 1 to 5000 cm^− 1). This same absorption of the excitation laser and consequent emission in the infrared region was observed by C. B. Azzoni et al. [59] for Mn₅O₈ powder.

According to Fig. 6 it is possible to assume that with the increase of temperature, there is particle growth and consequent decrease in the luminescent emission. Therefore, the vibrational modes for most samples are overlapped by these emissions, which makes it difficult to interpret the results properly. In addition, MnO 480 ºC sample has two prominent luminescent emission intervals, around 2150 cm^− 1 and 3750 cm^− 1, which can be attributed to Mn₅O₈ and Mn₃O₄ which together are only present in this sample.

The hausmannite vibrational modes are only evident in the spectra of the MnO 920 ºC and MnO 1150 ºC samples (Fig. 6 (a)), in this samples the fluorescence emissions are not able to overlap the vibrational modes in low wavenumbers, due to the larger particle sizes which suppress emissions. The 319 cm^− 1 and 373 cm^− 1 bands are assigned to the T_2g vibrational mode of tetragonal Mn₃O₄ [12] and 647 cm^− 1 (MnO 920 ºC) and 657 cm^− 1 (MnO 1150 ºC) bands can be assigned to A_1g mode, referring to Mn–O bonds (stretching) of the hausmannite divalent Mn ions with tetrahedral coordination [57]. The widening of this band and the consequent displacement to smaller wavenumbers from the MnO 1150 ºC sample to the MnO 920 ºC, is probably linked to the smaller particle size and the presence of the Na₄Mn₉O₁₈ phase (Fig. 3) in the MnO 920 ºC sample.

To estimate the percentage and network parameters of the identified phases, structure refinement of the samples synthesized via MAH and treated was carried out using the Rietveld method. Their respective diffractograms are shown in Figs. 7 and 8 and the results are summarized in Table 4. Tetragonal Mn₃O₄ (CIF 1514115) was identified for all temperatures, its portion in the MnO and MnO 160 ºC samples approaches 100% (despite the existence of the tetragonal α-MnO₂ phase in these samples, it was not taken into account in the refinement due to wide and low intensity peaks that lead to divergency), decreases to 5.53% in 480 ºC, due to the transformation of hausmannite in the metastable phase (monoclinic Mn₅O₈ - CIF 1514100) and, in the following temperatures, remains above 89%, ending in 100% for MnO 1150 ºC sample.

It is interesting to point out the percentages of the monoclinic Na₂Mn₅O₁₀ (CIF 1528293) and orthorhombic Na₄Mn₉O₁₈ (CIF 2017971) phases apparent only in the samples MnO 715 ºC, MnO 870 ºC and MnO 920 ºC, these values vary from 1.14% of Na₂Mn₅O₁₀ (MnO 715 ºC sample), to 9.78% and 10.64% of Na₄Mn₉O₁₈ in the MnO 870 ºC and MnO 920 ºC samples, respectively.

Table 4

– Phase percentages, lattice parameters and convergence parameters of MnO, MnO 160 ºC, MnO 480 ºC, MnO 715 ºC, MnO 870 ºC, MnO 920 ºC and MnO 1150 ºC samples.
Sample	Phase Percentage and Lattice Parameters							R_WP (%)	R_EXP (%)	χ²
MnO	Tetragonal Mn₃O₄ (~ 100 wt%)*							3.121	1.766	1.330
	a,b (Å)		c (Å)			V (Å³)
	5.7698		9.4521			314.67
MnO 160 ºC	Tetragonal Mn₃O₄ (~ 100 wt%)*							3.516	2.587	1.833
	a,b (Å)		c (Å)			V (Å³)
	5.77033		9.4654			315.17
MnO 480 ºC	Tetragonal Mn₃O₄ (5.53 wt%)			Monoclinic Mn₅O₈ (94.47 wt%)				10.053	4.274	5.533
	a,b (Å)	c (Å)	V (Å³)	a (Å)	b (Å)	c (Å)	V (Å³)
	5.7602	9.4885	314.83	10.4530	5.7560	4.8734	276.13
MnO 715 ºC	Tetragonal Mn₃O₄ (98.86 wt%)			Monoclinic Na₂Mn₅O₁₀ (1.14 wt%)				4.388	2.601	2.827
	a,b (Å)	c (Å)	V (Å³)	a (Å)	b (Å)	c (Å)	V (Å³)
	5.7626	9.4675	314.40	8.9605	11.0579	2.8053	277.96
MnO 870 ºC	Tetragonal Mn₃O₄ (90.22 wt%)			Orthorhombic Na₄Mn₉O₁₈ (9.78 wt%)				3.723	2.563	2.110
	a,b (Å)	c (Å)	V (Å³)	a (Å)	b (Å)	c (Å)	V (Å³)
	5.7633	9.4684	314.50	9.0986	26.1584	2.8254	672.47
MnO 920 ºC	Tetragonal Mn₃O₄ (89.36 wt%)			Orthorhombic Na₄Mn₉O₁₈ (10.64 wt%)				3.865	2.558	2.283
	a,b (Å)	c (Å)	V (Å³)	a (Å)	b (Å)	c (Å)	V (Å³)
	5.7631	9.4677	314.45	9.1049	26.1970	2.8255	673.94
MnO 1150 ºC	Tetragonal Mn₃O₄ (100 wt%)							3.249	2.674	1.476
	a,b (Å)		c (Å)			V (Å³)
	5.7648		9.4693			314.70

* The α-MnO₂ phase was not considered in the refinement due to its wide and low intensity peaks, with no possibility of convergence if it is taken into account.

Despite the atomic percentage of Na being around 1.50% in the studied samples, the high percentage observed for Na₄Mn₉O₁₈ phase is related to the high volume of the Na₄Mn₉O₁₈ unit cell (which ranges from 672.47 Å³ to 673.94 Å³) in relation to the volume of Mn₃O₄ (which ranges from 314.40 Å³ to 315.17 Å³). In addition, the refinements showed great convengente parameters (R_WP, R_EXP) and goodness of fit (χ²): 1.330 (MnO), 1.833 (MnO 160 ºC), 5.533 (MnO 480 ºC), 2.827 (MnO 715 ºC), 2.110 (MnO 870 ºC), 2.283 ( MnO 920 ºC) and 1,476 (MnO 1150 ºC), indicating a good approximation of the observed results in comparison with those calculated.

Finally, to assess the influence of the synthesis method and thermal treatment on the morphology of the material produced, the samples were characterized by SEM (Fig. 9). According to the synthesis method and the precursors used, the same compound can present different morphologies, therefore, it is important to analyze the microscopy of samples synthesized by MAH. The samples MnO and MnO 160 ºC presented many particles with well-defined edges, some rods with a triangular section, with many particles with different morphologies and uniform size (Fig. 9 (a) and (b)). Such microscopies suggest the crystalline hausmannite, according to Rani et al., Liu et al. and Yang et al. [12, 32, 60]. The MnO 480 ºC sample also showed particles with well-defined edges, but more agglomerated, which can characterize the Mn₅O₈ phase (Fig. 9 (c)), as suggested by Gao et al. and Aghazadeh et al. [13, 14].

The MnO 715 ºC sample (Fig. 9 (d)) presents particles with different morphologies and uniform size. One of these morphologies is characterized by the presence of particles in the form of needles or rods, which suggest the formation of Na₂Mn₅O₁₀, according to Liu et al. and Tsuda et al. [1, 61]. Both authors suggest the formation of romanechite with sodium. The samples MnO 870 ºC and MnO 920 ºC are similar, they show bars with hexagonal base, particles with different morphologies of different sizes, agglomerates and spheroidal particles (Fig. 9 (e) and (f)). Such spheroidal formations, according to Ta et al. [51], suggest the Na₄Mn₉O₁₈ phase, agreeing with the diffractograms presented.

The MnO 1150 ºC sample has particles with smooth surfaces. The morphology and size were neither defined nor uniform, although some particles have an octahedral shape with the chamfered corners (Fig. 9 (g)). For this temperature, it can be observed that the particles greatly increase their sizes in relation to the other samples which agrees with the assumptions made for the luminescent emissions observed from the Raman scattering (Fig. 6): the increase in the heat treatment temperature results in an increase in the particle size, which reduces the observed luminescent emission.

The Microwave-Assisted Hydrothermal (MAH) method proved to be effective in the synthesis of manganese oxide, mainly with the hausmannite phase (Mn₃O₄), in a simplified, reproductive and fast way compared to other synthesis methods found in the literature. It was possible to observe a cyclical evolution of the hausmannite structure from room temperature to 1150 ºC where, in the initial temperatures (room and 160 ºC), the tetragonal Mn₃O₄ phase with traces of α-MnO₂ was identified. Around 480 ºC, there was almost total transformation of the Mn₃O₄ phase (5.53%) into Mn₅O₈ (94.47%). At subsequent temperatures, it is possible to notice that the synthesized material acted as a Na intercalation compound, because at 715 ºC, there was crystallization of Na₂Mn₅O₁₀ phase (1.14%), still with the presence of hausmannite (98.86%) and at 870 ºC/920 ºC, the crystallization of the Na₄Mn₉O₁₈ phase (9.78%/10.64%, together with 90.22% and 89.36% of Mn₃O₄, respectively), to finally culminate in the single-phase sample (100% Mn₃O₄), treated at 1150 ºC. From Raman scattering it was possible to observe that there is luminescent emission (at 633 nm excitation) mainly for samples treated at lower temperatures, with smaller particle sizes. The results presented allowed a better interpretation of the chemical, thermal and structural characteristics of manganese oxide samples synthesized via MAH.

Acknowledgements

This work was supported by FAPESP/CEPID [2013/07296-2] and CNPq [573636/2008-7].

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Manganese oxides synthesized via microwave-assisted hydrothermal method: phase evolution and structure refinement

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