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

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 renement (Rietveld method). The samples obtained were heat treated at temperatures dened 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 ve distinct phases: α-MnO 2 , Mn 3 O 4 , Mn 5 O 8 , Na 2 Mn 5 O 10 and Na 4 Mn 9 O 18 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.

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][2][3][4][5].
Mn 3 O 4 (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 Mn 2+ ions forming the tetrahedral clusters and the ions Mn 3+ forming the octahedral clusters) [6], a particular con guration 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][10][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 Mn 5 O 8 , where the Mn cations present the states Mn 2+ and Mn 4+ , in addition to forming lamellar structures, such as birnessite (Mn 2 O 4 ) [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 Mn 5 O 8 to be used in hard disk sensors and devices based on magnetic thin lms [16].
Usually, Mn 2 O 3, Mn 3 O 4 , Mn 5 O 8 , among others structures, are obtained through thermal decomposition of MnO 2 , particularly α-MnO 2 , 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 nal product, such as chemical reduction, co-precipitation, auto-combustion, sol-gel, solid state reaction, carburization or through the conventional hydrothermal method [20][21][22][23][24][25]. Additionally, the Microwave-Assisted Hydrothermal (MAH) method is an alternative way to synthesize manganese oxides, making it possible to obtain a nal 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 rst synthesis using the MAH method occurred in 2006, where Apte et al. obtained the α-MnO 2 and Mn 3 O 4 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-MnO 2 [30] and by Li et al. on the synthesis of ower-like and nanotubes of α-MnO 2 [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 in uence 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 nd articles that deal with the structure re nement 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 re nement, aiming to search for a correlation between the synthesis/sintering parameters with the phase evolution after speci c heat treatments.

Materials And Methods
For the MAH synthesis of manganese oxide, 50 mL (0.5 M) of MnCl 2 .4H 2 O (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 Te on® 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 xed 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 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 identi ed 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 re nement 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 tting approach). Speci cally for the Rietveld re nement, 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) les from the Crystallography Open Database were also used as re nement control les.
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 mm 2 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.
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 KMnO 4 and HCl as precursors, obtained birnessite-type MnO 2 and tetragnonal α-MnO 2 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, Mn 3 O 4 , as in this work, also has a small MnO 2 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 Mn 3 O 4 but can result in spurious phases [6,36]. After the precursors dissociation and the sodium chloride and manganese hydroxide precipitation, partial oxidation of Mn (Mn 2+ to Mn 3+ ) occurs, with the interaction with hydroxyls, resulting in formation of Mn 3 O 4 structure. It is assumed that the formation of a small portion of α-MnO 2 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][38][39]. These chemical reactions involved in the construction of the Mn 3 O 4 crystalline network can be summarized as follows: 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 Mn 3 O 4 phase to Mn 5 O 8 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 speci cally 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-MnO 2 and Mn 3 O 4 , 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].  [45]. The existence of the Mn 5 O 8 metastable phase in this temperature range can be con rmed through the XRD patterns of the MnO 480 ºC sample (Fig. 3 (c)).  [47]. In this same range, there is a weight loss of approximately 2.31%, also related to the release of O 2 .
Considerations regarding the thermal analysis of the MnO sample are summarized in Table 1.  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-MnO 2 and Mn 3 O 4 phases are still present, however, the peaks located at 19º and 25º referring to α-MnO 2 phase are less intense and broader compared to MnO sample, indicating long-range disorder and/or smaller particle size of α-MnO 2 phase around 160 ºC. According to Fig. 2 (Fig. 3 (d)), and orthorhombic Na 4 Mn 9 O 18 phase (JCPDS 27-750), veri ed 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-MnO 2 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 Na 2 Mn 5 O 10 and Na 4 Mn 9 O 18 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 Mn 3 O 4 , still tetragonal.  [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.   and orthorhombic Na 4 Mn 9 O 18 are known and currently explored for use in ionic sodium batteries [1,51], it was not possible to nd articles that deal with the speci c 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 identi ed, 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 Mn 5 O 8 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 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 Na 4 Mn 9 O 18 phase (Fig. 3) in the MnO 920 ºC sample.
To estimate the percentage and network parameters of the identi ed phases, structure re nement of the samples synthesized via MAH and treated was carried out using the Rietveld method.  Finally, to assess the in uence 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-de ned 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-de ned edges, but more agglomerated, which can characterize the Mn 5 O 8 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 Na 2 Mn 5 O 10 , 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 Na 4 Mn 9 O 18 phase, agreeing with the diffractograms presented.
The MnO 1150 ºC sample has particles with smooth surfaces. The morphology and size were neither de ned 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.

Conclusions
The Microwave-Assisted Hydrothermal (MAH) method proved to be effective in the synthesis of manganese oxide, mainly with the hausmannite phase (Mn 3 O 4 ), in a simpli ed, 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 Mn 3 O 4 phase with traces of α-MnO 2 was identi ed. Around 480 ºC, there was almost total transformation of the 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.