Polymorphism and Optical-vibration Properties of MnV2O6ânH2O (n

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Polymorphism and Optical-vibration Properties of MnV2O6·nH2O (n = 0, 2, 4) Prepared by Microwave Irradiation Jéssica I. Viegas, Roberto L. Moreira, and Anderson Dias Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 24 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

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Crystal Growth & Design

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Polymorphism and Optical-vibration Properties of MnV2O6∙nH2O (n = 0, 2, 4) Prepared by Microwave Irradiation

Jéssica I. Viegas,† Roberto L. Moreira‡ and Anderson Dias*,†

†Departamento

de Química, ICEx, Universidade Federal de Minas Gerais, Belo Horizonte-

MG, 30123-970, Brazil ‡Departamento

de Física, ICEx, Universidade Federal de Minas Gerais, Belo Horizonte-MG,

30123-970, Brazil

ACS Paragon Plus Environment

Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT: Polymorphism and optical-vibration properties of manganese vanadates were investigated as a function of the microwave irradiation conditions. Single-phase, impurity free materials were prepared and described as monoclinic MnV2O6∙4H2O (C2/c, #15), orthorhombic MnV2O6∙2H2O (Pnma, #62), and monoclinic MnV2O6 (C2/m, #12). The structural, morphological and optical-vibration properties of the hydrated and anhydrous phases were investigated by X-ray diffraction, transmission electron microscopy and Raman spectroscopy. Vibrational studies for these materials are reported for the first time. The chosen synthesis methodology showed to be a less time and energy consuming process compared to previous studies. The results of this work complement the knowledge about manganese vanadates and improve the possibilities of future applications for these materials as catalysts in several processes.

Keywords: Polymorphs, Manganese vanadates, Phonon modes, Microwave irradiation, Catalysts.


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1.

INTRODUCTION

Manganese vanadates have attracted the interest of the scientific community aiming to expand the industrial uses of these ceramic materials. Due to their photocatalytic and electrochemical properties, these materials allow several applications. Recently, a p-type MnV2O6 was reported as a small bandgap semiconductor (1.45 eV) capable of oxidizing and reducing water to produce oxygen and hydrogen under visible light radiation.1 Previous studies also reported the photodegradation of methylene blue under visible light illumination using the monoclinic phase of MnV2O6.2,3 In addition, several authors reported the application of vanadium manganese oxides as anodes and cathodes in rechargeable lithiumion batteries, showing good charge/discharge capacity.4-10 Ni et al.11 have summarized the key development of manganese vanadates for anode materials in Li-ion batteries, with emphasis toward their practical application in commercial Li-ion cells. According to these authors, future research in MnV2O6 materials can be implemented toward two aspects: i) deep study on the charge/discharge mechanism of MnV2O6 anode materials for batteries, with the aid of advanced technologies, such as Raman spectroscopy and high-resolution electron microscopies; ii) expanding the applications of MnV2O6 by fabricating freestanding electrodes, exploring the utilization of MnV2O6 in Na- and K-ion batteries, and searching for new advanced MnV2O6 materials, which may have potential application in energy storage. It is well-known that the performance of a given device is directly related to the knowledge of the fundamental issues concerning the materials employed on their building. Particularly, their crystalline structures and morphological features must be deeply explored to better understand the mechanisms involved during operation. For example, Beltrán et al.12 and Song et al.13 reported extensive theoretical and experimental research aiming to contribute to increasing the fundamental knowledge of metal vanadates; in particular, the authors collected information related to their structural, electronic, and vibrational properties,



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including a microscopic interpretation of the variation of the relative stability of the crystal phases for different polymorphs. Transition-metal vanadates belong to the brannerite (ThTi2O6) structure with general formula AB2O6.14 Divalent metal vanadates AV2O6 (A = Mg, Mn, Co, Cu, Zn, and B = V+5) have been classified as monoclinic C2/m, although C2 and Cm symmetries can also be found in this family.14,15 Previous studies showed that the brannerite structure is converted into the orthorhombic (Pbcn) columbite (FeNb2O6) structure when the material is synthesized at high temperature or pressure.16,17 The monoclinic to orthorhombic transition involves a change in vanadium coordination and a rearrangement of the octahedra.15,18 The structure of MnV2O6 is a typical representative of brannerite-type AV2O6 compounds. At mild conditions, it is obtained in a monoclinic structure with space group C2/m, while at extreme conditions of pressure and temperature, it is obtained in an orthorhombic phase with Pbcn space group.19 Manganese vanadates can also be found in the hydrated forms MnV2O6∙nH2O (n = 1, 2, 4).6,20-23 Similarly to the anhydrous phase, previous publications indicate that the hydrated phases are promising battery materials.6,20,22 Several methods have been developed to synthesize anhydrous and hydrated phases of manganese vanadates. The chemical and physical properties of these materials are closely related to their structures and morphologies, which are an outcome of the synthetic pathway.8 Previous reports show that manganese vanadates phases can be obtained by solid-state reaction,18,22,24,25 conventional hydrothermal synthesis,3,7,8,26,27 coprecipitation,23,28,29 polymer gelatin method30 and microwave radiation.20,31 It is generally accepted that the hydrothermal method presents some advantages over these processes listed above in obtaining pure crystalline phases without additional heat treatment, spending less time and energy. Moreover, in the last decades, it was observed that the application of microwave irradiation into the hydrothermal reactors is a viable alternative to reduce temperature and time conditions.20 According to the literature, the microwave irradiation leads to a rapid


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dissolution of the precipitated materials and to homogeneous nucleation.31 For the conventional synthesis of MnV2O6 materials, the literature is relatively broad. The experimental conditions are diverse, with temperatures ranging from 70°C to 200°C, and times from 4 h to 8 days. Zhang et al.6 and Khan et al.32 studied the hydrothermal synthesis of MnV2O6.2H2O at 70-120°C, for times up to 16 h, while many authors used a fixed temperature of 180°C and very extended times from 12 h to 8 days to produce anhydrous MnV2O6.3,8,9,27,33-39 For higher processing temperatures, Zhang et al.7 and Inagaki et al.40 have produced anhydrous phases at 200°C for times up to 10 h. Parhi et al.20 reported, for the first time, the synthesis of MnV2O6∙H2O assisted by microwave energy using a domestic microwave oven, which does not allow a rigid control of temperature and pressure. In view of the lack of a detailed investigation on the use of the microwave irradiation for the synthesis of manganese vanadates, besides the fact that the conventional hydrothermal conditions employed very extended times to produce the desired composition, we decided to employ this processing route to obtain different polymorphs at optimized conditions of temperature and time. Therefore, in this work, we report the synthesis of crystalline, phase-pure hydrated and anhydrous compounds MnV2O6∙4H2O, MnV2O6∙2H2O and MnV2O6 using a microwave reactor with rigorous control of temperature and pressure. In order to broad the understanding of the fundamental issues of these polymorphs aiming to a better performance in their final applications, their structural, morphological and optical-vibration properties were investigated by X-ray diffraction, thermal analysis, scanning and transmission electron microscopies, and Raman spectroscopy. In this respect, at the best of our knowledge, the optical-vibration properties of manganese vanadates were not yet investigated. The goal is to establish the spectral signatures (fingerprints) for all the prepared polymorphs, which can help the theoretical researchers to develop optimized devices.



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2.

EXPERIMENTAL SECTION

Manganese vanadates were obtained by coprecipitation and microwave irradiation using stoichiometric amounts of Mn(NO3)2∙4H2O and NH4VO3 (Sigma-Aldrich, purity >99.9%) as starting materials. Manganese nitrate was firstly dissolved in deionized water and added to ammonium vanadate solution, prepared previously at 80°C under vigorous magnetic stirring. The obtained solution was kept under stirring at 80°C for 10 min. The resultant solution (50 mL) was put in double-walled digestion vessels (100 mL of capacity) with an inner line and cover made of Teflon Tetrafluormethaxil (TFM) and an outer high strength vessel shell made of Polyether ether ketone (PEEK). The microwave irradiation was conducted using a Milestone BatchSYNTH equipment (2.45 GHz). The temperatures applied were optimized as 110, 150 and 200°C under autogenous pressure, which correspond to about 1.5, 5 and 15 atm, respectively. The irradiation times ranged from 5 min to 60 min, for heating rates of about 20°C/min. After cooling down to the room temperature, the products were rinsed several times with deionized water/ethanol and dried at 60°C, for 24 h. Another experiment was conducted using conventional hydrothermal synthesis in a stainless-steel reactor. The solution was prepared following the same coprecipitation procedure previously described (350 mL) and heated under at 10°C/min up to the processing temperature (250°C), for 7 days. After synthesis, the powder was rinsed and dried, as presented above. The structural investigation of the samples was performed by X-ray diffraction (XRD) by a PANalytical-EMPYREAN diffractometer with CuKα radiation. (λ=1.541874 Å), highresolution monochromator and Ni filter. The experiments were registered in the angular range of 10-90°2θ, by applying a voltage of 45 kV and a current of 40 mA. TG measurements were carried out in a Shimadzu Thermal Analyzer 51-H, operating from room temperature to 500°C under nitrogen atmosphere. Around 20-30 mg of each sample was heated at 10°C/min


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in order to confirm the composition of the anhydrous and hydrated manganese vanadate phases. The morphologies and crystalline features of the samples were evaluated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM images were obtained on loose powders prepared after ultrasound dispersion in acetone (15 min), fixed on freshly cleaved Si wafers in carbon-coated adhesive tapes, followed by sputter coating with a 2 nm thick Au-Pd layer in order to prevent charge-up during electron-beam exposure. The samples were examined in a Quanta FEG 3D FEI equipment at 15 kV and a beam diameter of around 20-30 nm, under high vacuum conditions. TEM images were taken under 200 kV using a Tecnai G2-20 transmission microscope. The samples were dispersed in isopropanol and submitted to ultrasound (15 min) prior to being placed in holey carbon-copper grids (#300 mesh). High-resolution TEM (HRTEM), selected-area electron diffraction (SAED) and electron energy loss spectroscopy (EELS) were also employed. The optical-vibration properties were investigated using a Horiba LABRAM-HR spectrometer with the 632.8 nm line of a helium-neon laser as excitation source (effective power of 6 mW at the surface of each sample), diffraction gratings of 600 and 1800 grooves/mm, Peltier-cooled charge coupled device (CCD) detector, confocal Olympus microscope (100× objective), and experimental resolution of typically 1 cm-1 for 10 accumulations of 30 s on loose powders.

3.

RESULTS AND DISCUSSIONS

3.1.

Manganese vanadate tetrahydrate: MnV2O6.4H2O Figure 1a shows the XRD pattern for the coprecipitated sample at room temperature.

MnV2O6∙4H2O phase was obtained, which belongs to the monoclinic space group C2/c (#15) and was indexed by the JCPDS #01-086-0720 card. The calculated lattice parameters were



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a=13.13 Å, b=10.10 Å, c=6.98 Å, and β=111.64º (V=860.4 Å3), in agreement with the literature (see Table S1 in Supporting Information).21 The structure of MnV2O6.4H2O is made up by the linkage of MnO4(OH2)2 octahedra and parallel metavanadate (VO3)nn- chains comprising distorted edge-sharing VO5 square pyramids in the [001] direction, with water molecules situated in the tunnels thus formed.23 The insets in Figure 1a present HRTEM images, which show the crystalline nature of this sample evidenced by well defined interplanar spacings of the order 2.9 Å, 3.2 Å and 3.9 Å. These values correspond to the (311), (002) and (220) planes of the monoclinic C2/c space group, respectively, also indicated in the XRD pattern for comparison. Figure 1b shows a low-magnification TEM image and SAED pattern for the highly crystalline coprecipitated rod-like materials, with lengths in the micrometer range and nanometer-sized widths and thicknesses. Also, Figure 1b presents the energy-loss spectral information from the sample (EELS), where the L-edge lines for V and Mn are displayed, besides the K-edge line for O, showing the chemical purity of the samples.


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(b)

VL OK

MnL



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Figure 1. (a) XRD pattern for the coprecipitated MnV2O6∙4H2O with characteristic Miller indexes, according to the JCPDS #01-086-0720 card. Inset: HRTEM images and interplanar spacings marked according to the XRD planes. (b) Morphological features of the same sample in a low-magnification TEM image and SAED pattern, which indicate its crystalline nature. The EELS spectrum exhibits the L-edge lines for V and Mn, as well as the K-edge line for O, which confirms the chemical purity of the samples.

Following, the synthesis of MnV2O6∙4H2O was conducted under microwave irradiation conditions at 110ºC, for times ranging from 5 to 60 min. Phase-pure samples were obtained for all the experimental times, and were indexed by the JCPDS card #01-086-0720. TG analyses were carried out to confirm the composition of the hydrated polymorph prepared. Figure S1 (Supporting Information) shows that the thermal decomposition of a typical MnV2O6.4H2O occurs in two well-defined steps at about 60-130°C and 150-210°C, as also verified by Wu et al.22 The observed mass losses were around 11.1% and 10.8%, respectively, which correspond to the loss of two water molecules at each step. Figure 2 shows the XRD patterns and Miller indexes for the sample prepared for 60 min (top). Lattice parameters were calculated for this monoclinic space group and the results are presented in Table S1 (Supporting Information), which are in good agreement with the literature for this tetrahydrated phase (Table S1 also presents the lattice parameters exhibited by the reference card #01-086-0720). The very high intensities for the planes (110) at 11.3°2 could suggest a possible preferential growth in this crystallographic direction. In this sense, SEM and TEM/HRTEM investigations were carried out in order to check this hypothesis. Figure S2 (Supporting Information) presents SEM images for the polymorphs prepared at 110°C, for 5-60 min. As it can be seen, rod-like morphology was


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confirmed for all times, which tend to grow by the “oriented attachment” mechanism, as previously observed by Dias et al.41-45 Figure 3 shows TEM/HRTEM images for the MnV2O6∙4H2O prepared at 110°C, for 60 min, where rod-like morphology (micrometer-sized in length) is observed in a low-magnification TEM image, as also verified previously for the coprecipitated sample. However, widths and thicknesses are much smaller than the observed for the coprecipitated vanadate, which are now in the order of dozens of nanometers. The good crystallinity was then verified by selected area electron diffraction (SAED, bottom inset), where the observed well-defined spots confirm the single-crystalline nature of the obtained samples The upper inset exhibits the HRTEM image with characteristic (110) planes for monoclinic C2/c space group. Differently from the coprecipitated sample, these planes were easily identified in the rod-like materials showed by the low-magnification image (Figure 3), which indicates that the microwave synthesis could be effective in producing materials with preferential growth directions. This result was recently observed by Dias et al.,41-45 who verified that the ac electrical fields during microwave irradiation can lead to an anisotropic growth of materials.



110°C (242) (422) (-314) (-424) (-551) (-134)

(042) (-513) (151) (-442)

(130) (400) (002) (311) (-131) (-402)(-312) (022) (420)

(200) (-111) (002) (111) (021) (220) (-311)

(110)

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Intensity (arb. units)

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2Theta (°) Figure 2. XRD patterns for MnV2O6∙4H2O samples prepared by microwave irradiation at 110ºC, for 5, 10, 20 and 60 min. The red lines show the standard patterns of the JCPDS #01086-0720 card. Miller indexes are presented for the material obtained for 60 min (top).


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Figure 3. TEM/HRTEM images for the MnV2O6∙4H2O sample obtained by microwave irradiation at 110°C, for 60 min. The low-magnification image shows the micrometer-sized rod-like materials with nanostructured widths and thickness, while the bottom inset exhibits the SAED pattern confirming the high crystallinity of the sample. Upper inset presents a HRTEM image with interplanar spacings of the order of 7.3 Å, which correspond to the (110) planes of the monoclinic space group.

Concerning the optical-vibration properties of manganese vanadates, the Raman



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scattering is presented in this work for the first time. The lack of such information in the current literature was the main motivation for the investigation of the phonon modes and their relationship with the crystal structure of the manganese vanadates (polymorphs) investigated here. Figure 4 presents Raman spectroscopic results for the MnV2O6.4H2O samples prepared by microwave irradiation. A complex band structure was observed, which is dominated by a series of modes in the wavenumber regions 150-450 cm-1 and 750-1000 cm-1. The similarity between the spectra is in good agreement with the results from XRD, as expected. However, different band intensities are identified between samples, which could be an indication of disorder for materials prepared in shorter times. A better knowledge of the vibrational features for these hydrated phases can only be fully accessed by fitting procedures, and theoretical analysis for identifying the phonon modes.41-45 Using the site symmetry and the site occupation factor of each atom of MnV2O6.4H2O, group-theoretical tools46 were used to foreseen the Raman and infrared activities at the Brillouin-zone center ( ), for the monoclinic C2/c space group. For this structure, Mn ions are in the 4d Wyckoff sites (Ci symmetry), while all the V, O and H ions are in the 8f Wyckoff sites, with C1 symmetry. The group-theory calculations are summarized in Table S2 (Supporting Information), where 60 Raman modes (30Ag+30Bg) and 63 infrared ones (32Au+31Bu) can be seen, as predicted for this monoclinic structure. Since two inequivalent oxygen atoms and four hydrogen atoms are related to the hydration water (Table S2), which represent 18Ag+18Bg modes, the remaining 24 Raman-active modes (12Ag+12Bg) are expected for MnV2O6.4H2O in the low frequency region (lattice modes, below 1000 cm-1). In view of that, the unpolarized Raman spectrum of MnV2O6.4H2O prepared at 110°C, for 60 min, was analyzed by peak fitting procedures. Figures 5a-c show the fitted spectra (red lines) for the experimental data (open squares). The Lorentzian lines used for the adjustments are also presented (green curves), in three spectral regions, in order to facilitate the visualization. The obtained characteristics (frequencies and


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full-width at half-maxima, fwhm) of the depicted Raman modes of the hydrated phase are given in Table S5. Twenty phonon modes were determined by this procedure in the wavenumber region below 1000 cm-1, in good agreement with the 24 predicted bands for this frequency region. The four missing modes are probably overlapped (Ag+Bg) modes, which cannot be resolved in the Raman spectra of powder samples.

110°C

Raman intensity (arb. units)

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Wavenumber (cm ) Figure 4. Raman spectra for MnV2O6.4H2O synthesized by microwave irradiation at 110°C, for 5, 10, 20 and 60 min.



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Figure 5. Micro-Raman spectra for the MnV2O6.4H2O prepared by microwave irradiation at 110°C, for 60 min. The experimental data (open squares) were adjusted by a sum (red curve) of Lorentzian lines (green curves) in three different wavenumber regions: (a) 50-225 cm-1; (b) 225-600 cm-1; and (c) 600-1000 cm-1.

3.2.

Manganese vanadate dihydrate: MnV2O6.2H2O The microwave preparation of MnV2O6∙2H2O was conducted at 150°C, for processing

times in the range 5-60 min. Figure 6 presents the XRD results, where it can be observed the existence of both dihydrate and tetrahydrate manganese metavanadates. This heterogeneous material continues to exist for longer processing times, but with decreasing amounts of tetrahydrate vanadates up to 60 min, where only the phase-pure dihydrate compound can now be noticed. According to Liao et al.,23 although their chemical formulae differ only in water contents, these hydrated compounds are not interconvertable and undergo independent dehydration processes. MnV2O6∙2H2O was then obtained at 150°C, for 60 min, and was indexed by the orthorhombic Pnma space group (JCPDS #01-086-0721).6 This phase contains MnO4(OH2)2 octahedra and parallel (VO3)nn- polyvanadate chains built up from corner-sharing VO4 tetrahedra.23 As in the structure of MnV2O6.4H2O, these chains are linked altogether through MnO4(OH2)2 octahedra. Thus, a channel network is formed in the [100] direction. From the XRD analysis, the calculated lattice parameters for this sample were a=5.61 Å, b=10.94 Å e c=12.50 Å (V=768.9 Å3), in good agreement with the literature (Table S1 also presents the lattice parameters exhibited by the reference card #01-086-0721).6 TG analyses were made to confirm the composition of the MnV2O6.2H2O. Figure S1 (Supporting Information) shows that the thermal decomposition of the sample prepared at 150°C, for 60 min, occurs in a well-defined step at about 200-290°C. The observed mass losses were around 12.0%, which correspond to the loss of two water molecules, as expected.



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Figure S3 (Supporting Information) shows SEM images for the polymorphs prepared at 150°C under microwave irradiation. It seems that a mixture of morphologies could be possible, i.e., rod-like (see Fig. S3b) and needle-like bundles (see Fig. S3c) occur simultaneously, which is in agreement with the dual phase materials verified for irradiation times shorter than 60 min. Fig. S3d shows a predominance of highly interconnected rod-like materials, corresponding to the phase-pure hydrated polymorph (n=2), previously confirmed by XRD and TG measurements. Figure 7 exhibits TEM/HRTEM images for the MnV2O6.2H2O sample obtained at 150°C, for 60 min. The low-magnification image shows micrometer-sized rod-like materials, as also verified by the tetrahydrate vanadate and discussed before. In this image, it was observed the so-called “oriented attachment” that occurred during synthesis, which is a very common process in microwave irradiated materials.44 SAED pattern (top inset) indicates the good crystallinity of the sample, while the HRTEM image (bottom inset) exhibits a characteristic interplanar spacing of 4.0 Å,

(172) (156)

(125) (026) (215) (321) (154) (206) (235) (304) (071) (260)

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(051) (134) (204)

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(102) (022) (031) (122)

(020)

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(101)

corresponding to the (022) planes of the orthorhombic Pnma structure.


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Figure 6. XRD patterns for manganese vanadates synthesized by microwave irradiation at 150ºC, for 5, 10, 20 and 60 min. The red and blue lines show the standard patterns of MnV2O6∙4H2O and MnV2O6∙2H2O, respectively. The Miller indexes for the phase-pure MnV2O6∙2H2O sample are presented.

Figure 7. TEM images for the MnV2O6∙2H2O samples synthesized at 150 °C. Top inset shows the SAED pattern, while the bottom inset exhibits a HRTEM image with the (022) planes of the orthorhombic Pnma space group.



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Figure 8 presents the Raman spectroscopic results for all the experiments conducted under microwave irradiation conditions towards the preparation of phase-pure MnV2O6.2H2O samples. Now, differently of the XRD technique, Raman scattering is useful to detect local surroundings in terms of the crystalline phase below the laser spot. Then, the results displayed in Figure 8 are a mixture of compounds also evidenced by XRD (Figure 6). For example, the sample obtained at 150°C for 5 min (Figure 8) shows a spectrum quite similar to those obtained for the MnV2O6.4H2O phase (Figure 4), as discussed in the later section. On the other hand, the experimental Raman data obtained for the sample prepared at 150°C for 10 min shows that additional bands are present. Particularly, a narrow, intense band at around 930 cm-1 begins to appear and became more intense for the sample prepared for 20 min. For this sample, the characteristic modes of the tetrahydrated vanadates are less intense. Finally, the spectrum for the material synthesized for 60 min shows a set of modes that can are considered as a fingerprint of the MnV2O6.2H2O. These results corroborate the XRD data for a mixture of dihydrate and tetrahydrate vanadates for times shorter than 60 min. In view of that, as also verified by XRD, the microwave irradiation was able to prepare a phase-pure MnV2O6.2H2O only after 60 min at 150°C. Then, this sample was studied in detail by Raman spectroscopy. A complex band structure was observed, in which different sets of less intense modes below 800 cm-1 and a few stronger bands in the wavenumber region 800-1000 cm-1 are present. Fitting procedures were then applied, as discussed before, now for this orthorhombic material. Using the site symmetry and the site occupation factor of each atom of MnV2O6.2H2O, group-theoretical tools46 were used to foreseen the Raman and infrared activities at the Brillouin-zone center ( ), for the orthorhombic Pnma space group. For this structure, Mn ions are in the 4c Wyckoff sites (CS symmetry), while the V and three O ions


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are in the 8d Wyckoff sites, with C1 symmetry. Two oxygen atoms (4c Wyckoff sites) are related to hydration water, besides three hydrogen atoms (4c and 8d Wyckoff sites). Table S3 (Supporting Information) summarizes the group-theory calculations, where can be seen that 90 Raman modes (25Ag+20B1g+25B2g+20B3g) and 67 infrared ones (24B1u+19B2u+24B3u) are predicted for this orthorhombic structure. Since two oxygen atoms and three hydrogen atoms are related to the hydration water (Table S2), which represent 11Ag+7B1g+11B2g+7B3g modes, 54 Raman-active lattice modes (14Ag+13B1g+14B2g+13B3g) are expected for MnV2O6.2H2O in the low frequency region (below 1000 cm-1). In view of that, the unpolarized Raman spectrum of MnV2O6.2H2O prepared at 150°C, for 60 min, was analyzed by peak fitting procedures. Figures 9a-c show the fitted spectra (red lines) for the experimental data (open triangles). The Lorentzian lines used for the adjustments are also presented (green curves), in the three chosen spectral regions, in order to facilitate the visualization. The obtained characteristics (frequencies and full-width at half-maxima, fwhm) of the depicted Raman modes of the hydrated phase are given in Table S5. Twenty-six phonon modes were determined by this procedure in the wavenumber region below 1000 cm1,

a lower number than 54 bands expected for this frequency region, which can also be

explained by quasi-accidental degeneracy of the unresolved modes of different symmetries.



22

150°C


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Figure 8. Raman spectra for the manganese vanadates synthesized by microwave irradiation at 150°C, for 5, 10, 20 and 60 min.


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Figure 9. Micro-Raman spectra for the MnV2O6.2H2O prepared by microwave irradiation at 150°C, for 60 min. The experimental data (open triangles) were adjusted by a sum (red curve) of Lorentzian lines (green curves) in three different wavenumber regions: (a) 50-300 cm-1; (b) 300-750 cm-1; and (c) 750-1000 cm-1.

3.3.

Manganese vanadate (anhydrous): MnV2O6 The microwave irradiation was applied at 200°C to prepare the anhydrous phase

MnV2O6 phase, which appears after just 5 min of reaction (Figure 10). The monoclinic structure C2/m (#12)20 indexes this material, according to the JCPDS #01-072-1837 card. The lattice parameters were calculated from the XRD data of Figure 10 for all samples and the results are displayed in Table S1 (Supporting Information). For example, the sample prepared at 200°C for 60 min presents the values a=9.46 Å, b=3.53 Å, c=6.78 Å e β=112.80º (V= 208.7 Å3), in good agreement with the literature. TG analyses made on the anhydrous samples confirm the expected composition for this polymorph, as showed in Figure S1 (Supporting Information). No significant mass loss was detected for these samples, in accordance to the XRD results. Figure S4 (Supporting Information) shows SEM images for


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the samples prepared at 200°C, which exhibit bundles of needle-like materials suggesting the same growth mechanism discussed above. Figure 11 shows TEM/HRTEM images for the MnV2O6 samples prepared by microwave irradiation at 200°C, for 60 min. It is also formed by micrometer-sized rod-like materials (low-magnification central image) with very good crystallinity (SAED pattern, top inset) and interplanar spacings of the order of 3.3 Å and 2.8 Å, corresponding to the (110) and (111) planes of the monoclinic C2/m space group (bottom

(-222) (510) (-601) (-513)

(113)

(020)

(-403)

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(-112)

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(-201)

inset).


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2Theta (°) Figure 10. XRD patterns for anhydrous manganese vanadates synthesized at 200ºC, for 5, 10, 20 and 60 min. The blue lines show the standard pattern of MnV2O6 (JCPDS #01-072-1837). Miller indexes are also presented for the sample prepared for 60 min.



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Figure 11. Anhydrous MnV2O6 prepared by microwave irradiation at 200°C, for 60 min. Low-magnification TEM image (central) shows the morphology of the sample, while the top inset exhibits the SAED pattern for this highly crystalline material. Bottom inset shows an HRTEM image with interplanar spacings corresponding to the (110) and (111) planes of the monoclinic C2/m space group.

Figure 12 presents the Raman spectra for all the MnV2O6 samples, which are very similar to those observed for the monoclinic C2/m phases of ZnV2O6 and MgV2O6 reported


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by Tang et al.15,17 and Busca et al.47 Fitting procedures were then applied for the sample prepared at 200°C, for 60 min. The site symmetry and the site occupation factor of each atom of MnV2O6 were investigated to explore the Raman and infrared activities at the Brillouinzone center ( ) for this anhydrous vanadate. For the monoclinic C2/m structure, Mn ions are in the 2a Wyckoff sites (C2h symmetry), while V and three O ions are in the 4i Wyckoff sites, with CS symmetry. Table S4 (Supporting Information) summarizes the group-theory calculations, where can be seen that 12 Raman modes (8Ag+4Bg) and 12 infrared ones (4Au+8Bu) are predicted for this monoclinic structure. The unpolarized Raman spectrum was then analyzed by peak fitting procedures. Figures 13a-b show the fitted spectra (red lines) for the experimental data (open circles). The Lorentzian lines used for the adjustments are also presented (green curves), in two spectral regions, in order to facilitate the visualization. The obtained characteristics (frequencies and full-width at half-maxima, fwhm) of the depicted Raman modes of the hydrated phase are given in Table S5. Twelve phonon modes were determined by this procedure in the wavenumber region below 1000 cm-1, in perfect agreement with the group-theory calculations.



28

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Wavenumber (cm ) Figure 12. Raman spectra for MnV2O6 synthesized by microwave irradiation at 200°C, for 5, 10, 20 and 60 min.


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Figure 13. Micro-Raman spectra for the MnV2O6 obtained by microwave irradiation at 200°C, for 60 min. The experimental data (open circles) were adjusted by a sum (red curve) of Lorentzian lines (green curves) in two different wavenumber regions: (a) 100-600 cm-1 and (b) 600-1000 cm-1.

Finally, in order to investigate the influence of the hydrothermal conditions on the morphological features and crystalline structure of manganese vanadates, a sample was



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prepared in conventional reactors at 250ºC, for 7 days. XRD, SEM, TEM/HRTEM and Raman scattering were employed to compare the results with those obtained under microwave-assisted reactions. Figure 14 presents the XRD and Raman data for the sample prepared in conventional hydrothermal conditions, and the results showed that the manganese vanadates are very similar to those prepared under microwave irradiation (see Figures 10 and 12 for comparison). Figure S4d (Supporting Information) shows a SEM image for the polymorph prepared in a Parr reactor at 250°C, for 7 days, which exhibit a very different morphology if compared with the observed under microwave irradiation. In order to better explore this topic, Figure 15 exhibits the TEM/HRTEM results for this MnV2O6 sample. The morphology of the polymorph (top left inset) shows nanometer-sized particles with some agglomeration. SAED pattern (top right inset) confirmed the good crystallinity of the samples, while HRTEM images exhibits characteristic interplanar spacings for the planes (001) and (-201) of the monoclinic C2/m structure. One could expect that the higher temperature and longer times could produce big crystals, as verified in manganese molybdates in a recent report.41 However, for our manganese vanadates, the microwaveassisted reactors are more effective to grow larger crystals than the conventional hydrothermal processing.


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Figure 14. (a) XRD pattern and (b) Raman spectrum of the MnV2O6 synthesized by conventional hydrothermal reactors at 250°C for 7 days. The blue line in (a) shows the standard pattern (JCPDS #01-072-1837) for the monoclinic C2/m phase.



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Figure 15. MnV2O6 obtained at 250°C, for 7 days, in a conventional hydrothermal reactor. Top left inset shows the morphology in a low-magnification image. Top right inset exhibits the SAED pattern, in which defined spots confirms the high crystallinity of the sample. HRTEM images present interplanar spacings of 6.1 Å and 4.4 Å, which correspond to the crystallographic planes (001) and (-201), respectively.

Comparing to the synthetic pathway applied in other studies,7,19,20 the microwave irradiation has shown to be more time and energy efficient and a more precise method to obtain the desired phases. For instance, Zhang et al.,7 using a conventional hydrothermal synthesis, spent 6 h to obtain phase-pure MnV2O6; also, Hneda et al.19 prepared pure monoclinic MnV2O6 at 725°C, for 48 h. In contrast, in the present work, the same phase-pure


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material was obtained within 60 min using microwave irradiation, which represents a less energy consuming process. Regarding to the reaction parameters, the methodology applied in this work allows an effective control of temperature and pressure during synthesis, since the reaction occurs inside the monitored vessels. Parhi et al.20 performed the reaction in a domestic microwave oven, shortening the reaction time; however, it is not possible to control the reaction parameters into a domestic equipment. In summary, the microwave irradiation employed in this work allowed us to prepare different phase-pure manganese vanadate polymorphs, which provided the first determination of the Raman vibrational modes of tetrahydrated, dihydrate and anhydrous structures of this material. A final remark must be done regarding the very recent publication of Beltrán et al.48 on the related compound ZnV2O6. These authors made a systematic investigation using firstprinciple calculations (DFT level) on the stability, geometry, and electronic properties of the metastable ZnV2O6 polymorphs through high-pressure synthesis. By using different computational procedures, they rationalized the results in terms of both local polyhedral, and the corresponding structural changes can be associated to Zn-O and V-O bond distances of ZnO6 octahedra and VO6 octahedra, and the relative compressibility of Zn−O and V−O bonds of each polymorph. It was verified that the monoclinic brannerite structure (C2/m) transforms to the columbite Pbcn orthorhombic structure at 5.0 GPa. Also, the ThTi2O6-type polymorph (C2/c) becomes more stable than the orthorhombic structure above 8, 13 and 15 GPa, depending upon the functional employed in the calculations. Thus, the polymorphic transformations observed for the zinc vanadates investigated by Beltran et al.48 show a similar structural sequence (i.e., between the same space groups) to that verified by the manganese vanadates studied here. From anhydrous to bi- and tetrahydrated samples, the structural sequence observed was C2/m  Pnma  C2/c. In this sense, we are stimulated to



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propose that the structural water present in our system plays a role similar to pressure in those ZnV2O6 samples.

4.

CONCLUSIONS

Nanostructured polymorphs of the system MnV2O6.nH2O (n = 0, 2, 4) were prepared under different microwave irradiation conditions. The pure anhydrous and hydrated phases were identified and characterized according to structural and optical-vibration properties. Monoclinic MnV2O6∙4H2O materials belonging to the space group C2/c was obtained as a single-phase by the coprecipitation of Mn(NO3)2∙4H2O and NH4VO3. The same impurity free phase was obtained under microwave irradiation for times as short as 5 min (110ºC), as verified by X-ray diffraction and transmission electron microscopy. The orthorhombic MnV2O6∙2H2O phase, space group Pnma, was identified as phase-pure material at 150ºC after 60 min of irradiation. The single-phase monoclinic MnV2O6, belonging to the space group C2/m, was obtained after microwave irradiation at 200ºC and also by conventional hydrothermal processing at 250ºC, for 7 days, as shown by XRD and TEM analysis. The described methodology provides a less energy and time-consuming pathway to obtain the desired pure phases of anhydrous and hydrated manganese vanadates. Optical-vibration properties were presented for first time and the results were compared to similar materials, such as ZnV2O6 and MgV2O6. This work adds to the previous knowledge related to the structural characteristics of manganese vanadates and draws attention to the vibrational behavior of these polymorphs, which are fundamental for future applications.

ASSOCIATED CONTENT


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This material is available free of charge via the Internet at http:pubs.acs.org. The Supporting Information is available free of charge on the ACS Publications website at DOI: ??? Supporting Information: Lattice parameters (Table S1), TG analyses (Figure S1), SEM images (Figures S2-S4), site-group analyses (Tables S2-S4) and Raman phonon modes (Table S5) for the MnV2O6 polymorphs investigated in this work (PDF)

AUTHOR INFORMATION Corresponding Author *(A.D.) E-mail: [email protected]. ORCID Jéssica Ivone Viegas: 0000-0002-4865-5052 Roberto Luiz Moreira: 0000-0001-6820-0269 Anderson Dias: 0000-0001-7413-1087 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors acknowledge the financial support from CAPES, CNPq, FINEP and FAPEMIG. Thanks are also due to the Center of Microscopy at the Universidade Federal de Minas Gerais (UFMG) for providing equipment and technical support for the experiments involving electron microscopy; the LMODRX/UFOP for the XRD data; and the Laboratory of



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Sustainable Valorization, Upgrade and Recycling of Solid Biowastes of the Department of Mechanical Engineering at UFMG for the thermogravimetric (TG) analyses.


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FOR TABLE OF CONTENTS USE ONLY

MANUSCRIPT TITLE: Polymorphism and Optical-vibration Properties of MnV2O6∙nH2O (n = 0, 2, 4) Prepared by Microwave Irradiation AUTHOR LIST: Jéssica I. Viegas, Roberto L. Moreira and Anderson Dias TOC GRAPHIC

SYNOPSIS: Microwave reactors were employed to investigate the polymorphism in manganese vanadates, as a function of the irradiation conditions of temperature and time. Different phases were prepared and their optical-vibration properties were investigated in detail beside group-theoretical calculations.


Polymorphism and Optical-vibration Properties of MnV2O6ânH2O (n

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