Investigation of Polymorphism and Vibrational Properties of MnMoO4

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Investigation of Polymorphism and Vibrational Properties of MnMoO4 Microcrystals Prepared by Hydrothermal Process Guilherme M. Martins, Pamela O. Coelho, Kisla PF Siqueira, Roberto L. Moreira, and Anderson Dias Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00102 • Publication Date (Web): 15 Mar 2018 Downloaded from http://pubs.acs.org on March 16, 2018

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Crystal Growth & Design

1

Investigation of Polymorphism and Vibrational Properties of MnMoO4 Microcrystals Prepared by Hydrothermal Process

Guilherme M. Martins,† Pâmela O. Coelho,† Kisla P. F. Siqueira†, Roberto L. Moreira‡ and Anderson Dias*,†

†

Departamento de Química, ICEB, Universidade Federal de Ouro Preto, 35400-000,

Ouro Preto MG, Brazil

‡

Departamento de Física, ICEx, Universidade Federal de Minas Gerais, C.P. 702,

30123-970, Belo Horizonte MG, 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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2 ABSTRACT: MnMoO4 microcrystals were synthesized by hydrothermal methods and their structural, morphological and vibrational properties were investigated. Conventional reactors and microwave-heated hydrothermal vessels were used to synthesize micrometer-sized MnMoO4 polymorphs at different temperature and time conditions. MnMoO4.H2O crystals were obtained in both reactors at temperatures below 200°C, for times up to 24 h. These microcrystals belong to the triclinic ܲ1ത space group and their 26 characteristic Raman bands were identified. Monoclinic MnMoO4 polymorph belonging to the P2/c structure was synthesized at temperatures higher then 200°C only into conventional hydrothermal reactors. Polarized micro-Raman analyses have evidenced and allowed the assignment of 18 phonon modes predicted by group-theory calculations for this polymorph. Another MnMoO4 polymorph, within the monoclinic C2/m space group, was obtained by two processing routes: (i) heating the P2/c microcrystals at 600°C; or (ii) heating the MnMoO4.H2O phase at 250°C. For this monoclinic phase, 33 Ramanactive bands were identified and assigned, in very good agreement with grouptheoretical calculations, which predict 36 modes for the C2/m polymorph. Well-faceted, highly crystalline microcrystals were clearly observed by scanning and transmission electron microscopies, in perfect agreement with XRD and Raman spectroscopic analysis. Finally, the appearing of characteristic phonon modes related to molybdenum in octahedral coordination suggests an incipient crystallization of a new, unreported αMnMoO4 polymorph, at least in a short-range degree.


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3 1.

INTRODUCTION

Transition-metal molybdates include a wide range of scientifically and technologically noteworthy compounds investigated since the 1960s.1 These materials are chemically planned and further synthesized by using different methods depending on the final goal: organic-inorganic hybrids, composites, single crystals or ceramics.2-5 A huge diversity of physical and chemical properties can be obtained depending on the choice of the transition metal.6-12 For the manganese molybdates (MnMoO4), a vast set of works have investigated in the last 50 years many of their intrinsic and extrinsic properties aiming to enlarge the industrial applications. For example, the magnetic and multiferroic properties were recently investigated in phase-pure as well as in solid solutions with tungstates.13-16 Nowadays, the use of MnMoO4 as supercapacitors in both chemical and electrochemical devices emerges as a challenge in comparison with standard materials in new technological applications.4,17-37 Other investigations report on the use of manganese molybdates in very broad catalytic uses, including hydrogen generation and many oxidative processes.38-43 Concerning the synthetic routes, there are many reports on the preparation of MnMoO4. These materials have been produced by solid-state ceramic route,26,28 combustion synthesis,42 precipitation,45 sonochemical,25,37,46 microemulsion-based method,47,48 ultra-low temperature,5 solvent-free microwave,49 and hydrothermal technology.19,35,36,50-57 In this respect, the hydrothermal technology appears as the most powerful synthesis methodology to produce single-phase, crystalline materials.58 This technology make use of solubilization/precipitation phenomena shared with redox and acid-base chemistry, which allow the assessment to new hybrid inorganic/organic compounds. For example, compared to solid-state synthesis, which are governed by the nature of the product, solution-based methods allow different approaches to the



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4 production of relevant materials. The main advantages of the hydrothermal processing include the application of green reagents/solvents, mild pressures and temperatures. Also, the hydrothermal technology works with closed reactors, which facilitate any separation, charging and cycling/recycling; presents high deposition rates; and account with the lowest one total energy consumption among the green chemistry-based methods.58 Recently, improvements in the hydrothermal technology were accomplished by the introduction of microwaves into the reactors to produce materials more rapidly.59 Among the advantages over conventional autoclave heating, we emphasize its rapid heating up to the crystallization temperature, its fast supersaturation by the rapid dissolution of precipitated hydroxides, which leads to a homogeneous nucleation in lower temperatures and shorter times.58,59 In order to attain optimized properties and performance discussed above, it was demonstrated that the knowledge and control over the crystallographic phases are mandatory.13-16,21,30,54 MnMoO4 exhibits a complex set of crystal structures, including hydrated phases, which are not fully dominated under hydrothermal conditions.19,35,36,5057

For example, a previous paper from Ding et al.54 explores the mechanisms for the

hydrothermal synthesis of MnMoO4 hydrates. However, microwave-assisted procedures were not yet deeply evaluated by the literature.53 Thus, the present work deals with the specific investigation on the synthesis of MnMoO4 microcrystals by conventional and microwave-assisted hydrothermal processing. It is our goal to compare these two processing routes in order to understand the mechanisms involved on the synthesis of microcrystals belonging to different crystalline structures. In this sense, all polymorphs obtained were investigated in terms of their morphologies, crystalline phases and phonon modes by X-ray diffraction, scanning and transmission electron microscopies, and Raman spectroscopy. For this last technique, special polarized light configurations


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5 were employed to identify and assign the phonon modes, in order to propose fingerprints for each polymorph, which certainly can be useful in designing their final properties.

2.

EXPERIMENTAL SECTION

MnMoO4 microcrystals were obtained by using MnCl2.4H2O, H2MoO4 and NaOH (>99%) as starting materials. First, H2MoO4 and NaOH were dissolved in deionized water and reacted at room temperature in order to produce Na2MoO4. Then, the resulting solution was mixed with manganese chloride under vigorous stirring. For conventional hydrothermal synthesis (CH), a Parr autoclave (stainless steel, Model 4913) equipped with turbine-type impellers was employed. The hydrothermal solution occupied about a half of the total volume and was heated under stirring at 10°C/min up to the processing temperature (110-250°C), for 24 h. After synthesis, the powders were rinsed with deionized water (18.2 Ω.cm) to remove any remaining subproducts and dried at 70°C. Additional long-time experiments were conducted at 110°C and 250°C, for 7 days. For the investigations regarding polymorphic transformations in hydrothermally synthesized microcrystals, heat treatments in the temperature range 250600°C were conducted in air in conventional oven, for 2 h. Microwave-assisted hydrothermal (MW) syntheses were conducted in a BatchSYNTH reactors operating at 2.45 GHz (Milestone Inc.). The hydrothermal solutions (50 mL) were introduced in double-walled reactors of 100 mL of capacity. Both inner line and cover are made of teflon tetrafluormethaxil, while the outer highstrength shell is made of polyether ether keton. Following, the vessels were mounted in a high-strength rotor body and secured with a calibrated torque wrench. The equipment



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6 operates at 1600 W, with automatic pressure and temperature full control. Experimental conditions include a heating time of 2 min up to the processing temperature (150°C), for times between 10 and 120 min (final conditions were 150±1°C and 4.6±0.2 bar). After synthesis, the crystals were washed with deionized water and dried at 80°C. X-ray diffraction technique (XRD) was used to investigate the structural properties. A high resolution Shimadzu D-6000 diffractometer was employed, operating at 40 kV, 20 mA, and using FeKα radiation (λ = 0.1936 nm). Experimental conditions were 10–60°2θ, with 15 s for each step of 0.02°2θ, and the results were converted to CuKα radiation for data manipulation. Scanning and transmission electron microscopies were carried out to investigate the morphology and structure of the microcrystals. A Quanta 200 (FEI) field-emission scanning electron microscope (FESEM) was employed to analyze the morphology of the manganese molybdates under 30 kV. A Tecnai G2-20 (FEI) transmission microscope (TEM) was used to evaluate the morphologies and crystalline aspects of the crystals under 200 kV. The microcrystals were dispersed in isopropanol and submitted to ultrasound (15 min) prior to be placed in holey carboncopper grids. High-resolution TEM (HRTEM) and selected-area electron diffraction (SAED) were also employed under 200 kV. Micro-Raman spectra were measured in back-scattering configuration in a Horiba LABRAM-HR spectrometer equipped with 600/1800 grooves/mm diffraction gratings and edge filters. A He-Ne laser (632.8 nm with nominal power of 18 mW) was used as exciting line, while a Peltier-cooled charge coupled device (CCD) was the detector of the scattered light. In a general way, it was employed accumulation times of 10 s (20 ×) for a spectral resolution better than 1 cm-1. All obtained spectra were mathematically corrected for the Bose-Einstein thermal factor.60 Polarized Raman spectra were carried out in all microcrystals by using appropriate interference and edge filters, polarizers and half-wave plates.


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7 3.

RESULTS AND DISCUSSIONS

3.1.

Manganese molybdate monohydrate: MnMoO4.H2O The structural evolution of manganese molybdates under varying synthesis

temperature and time was investigated by XRD and Raman spectroscopy. Figure 1a exhibits the XRD patterns for the crystals synthesized by CH (blue lines) at 110°C (for 24 h and 7 days) and 150°C (for 24 h), and for crystals synthesized by MW (red lines) at 150°C, for times between 10 and 120 min. The results showed that MnMoO4.H2O microcrystals were obtained for all the hydrothermal conditions tested. Few nonindexed peaks are marked with asterisks in Figure 1a, which could probably due to impurities. All patterns were indexed by the ICDD (International Committee of Diffraction Data) card #01-78-0220, in agreement with previous publications.41 This crystalline phase is triclinic, belonging to the space group ܲ1ത (#2, Ci1 ), being isostructural with the analogous compounds CoMoO4.nH2O61 and NiMoO4.nH2O.62 All samples were considered as single-phase crystals and their lattice parameters were then obtained (Table 1), which are in perfect agreement with those expected for the hydrated phase (last line in Table 1 presents the lattice parameters exhibited by the reference card #01-78-0220). The higher intensities for the planes (-110) and (002)/(110) at 18.85°2Θ and 26.22°2Θ, respectively, could suggest a preferential growth in that directions. In this respect, Mi et al.47 synthesized hydrated phases and observed similar results, concluding that the direction (110) was adopted as preferential growth direction for these triclinic crystals.



(0-23) (202) (0-31) (1-32) (-1-23) (-321) (2-31)

CH 110°C/7 d (002) (1-22)(110) (-202) (003)

(-110) (-101) (1-11) (-111) (101) (002) (-1-11) (110) (0-12) (-120)(-210) (0-21) (200) (-1-12)

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y t i s n e t n I

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MW 150°C/30 min MW 150°C/20 min MW 150°C/10 min

*

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MnMoO4.H2O #01-078-0220

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)


1000

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9 Figure 1. (a) XRD and (b) Raman data for the manganese molybdate monohydrates obtained by CH (blue lines) and MW (red lines) synthesis. CH experiments were conducted at 110°C (24 h and 7 days) and 150°C (24 h), while MW experiments occurred at 150°C, for 10, 20, 30 and 120 min. Asterisks represent non-indexed peaks.

Table 1. Lattice parameters for all the hydrothermally synthesized MnMoO4.H2O crystals. The values presented by the reference card ICDD #01-078-0220 are also shown for comparison purposes. Sample MW 150°C/10 min MW 150°C/20 min MW 150°C/30 min MW 150°C/120 min CH 110°C/24 h CH 150°C/ 24 h CH 110°C/7 days #01-078-0220

a(Å) 5.755 5.781 5.731 5.759 5.743 5.644 5.716 5.776

b(Å) 5.920 5.988 5.989 5.991 5.988 5.915 5.940 5.964

c(Å) 6.986 6.948 6.940 6.897 6.973 7.020 6.974 6.992

α(º) 100.18 99.93 100.67 99.70 100.23 103.19 99.81 100.32

β(º) 96.34 95.19 95.86 95.70 96.39 96.47 95.09 95.56

γ(º) 107.26 107.11 106.49 107.70 106.59 106.22 106.87 106.81

V(Å3) 220.34 223.85 221.40 220.55 222.77 215.14 220.84 223.99

Figure 1b presents Raman spectroscopic results for all the MnMoO4.H2O crystals. A complex band structure was observed, which is dominated by a series of modes in the wavenumber regions 300-400 cm-1 and 800-1000 cm-1. The similarity between all spectra is in good agreement with the results from XRD, as expected. Nevertheless, a better knowledge of the vibrational features for these hydrated phases can only be fully accessed by fitting procedures, as previously done by our research group.63-65 Using the site symmetry and the site occupation factor (s.o.f.) of each atom of MnMoO4.H2O given in refs. 61 and 62, group-theoretical tools66 were used to foreseen the Raman and infrared activities at the Brillouin-zone center ( Γ), for the triclinic ܲ1ത space group. For this structure, all the Mn, Mo and O ions are in the 2i Wyckoff sites, with C1 symmetry. Table S1 (Supporting Information) summarizes the ACS Paragon Plus Environment


10 group-theory calculations, where it can be see that 42 Ag Raman modes and 39 Au infrared ones are foreseen for this triclinic structure. Since the last two oxygen atoms are related to the hydration water (Table S1), which represent 6 Ag modes, 36 Ramanactive bands are expected for MnMoO4.H2O in the low frequency region (below 1000 cm-1). In view of that, the unpolarized Raman spectrum of MnMoO4.H2O was analyzed by peak fitting procedures. Figures 2a-d show the fitted spectra (red lines) for the experimental data of the sample synthesized by CH at 250°C, for 7 days (black dots). The Lorentzian lines used in the adjustment them are also presented (green curves), in four 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 2. Twenty-six phonon modes were determined by this procedure in the wavenumber region below 1000 cm-1, a number lower than the 36 bands predicted for this frequency region.

(a)


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Figure 2. Raman spectra of triclinic MnMoO4.H2O crystals synthesized by CH at 250°C, for 7 days: (a) 50-160 cm-1; (b) 160-480 cm-1; (c) 480-720 cm-1; (d) 720-1000 cm-1. Black dots represent the experimental data, solid red lines the adjusted curves, and the green lines show the individual Lorentz-like modes.

Table 2. Obtained Raman modes for triclinic MnMoO4.H2O crystals obtained by CH at 250°C, for 7 days, in the low-frequency region (below 1000 cm-1).

Band Frequency FWHM # (cm-1) (cm-1) 1 73.1 6 2 82.5 5 3 85.8 7 4 102.9 4 5 107.0 7 6 119.0 11 7 123.8 8 8 139.6 16 9 181.8 25 10 202.0 12 11 238.8 10 12 251.7 12 13 299.5 28

Band Frequency FWHM # (cm-1) (cm-1) 14 333.5 13 15 353.7 17 16 370.3 22 17 514.0 5 18 535.6 25 19 563.1 22 20 586.9 18 21 624.4 33 22 656.4 26 23 799.4 19 24 822.6 17 25 867.4 8 26 930.9 21


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13 The morphological, chemical and structural behaviors of all samples were examined by FESEM. Figures 3a-d present a sequence of typical FESEM images (secondary electrons) for crystals synthesized at different hydrothermal conditions. As it can be observed, the morphology changed depending upon the method employed: crystals produced in conventional reactors are constituted by rounded-shaped crystals (Figures 3c-d), while the MW processing produced micrometer-sized rods (Figures 3ab). It is important to emphasize the effect of the microwave heating on the morphological properties of MnMoO4.H2O crystals, increasing their sizes substantially if compared with the crystals obtained by conventional heating. The results show that the MW synthesis introduces a high anisotropy (aspect ratio) and a growing mechanism previously observed by Mi et al.47 Also, each micrometer-sized rod has an uniform shape along its entire length (Figure 3b), indicating that the growth anisotropy is preserved throughout the process. As it can be seen in Figure 3b, the crystals show wellfaceted crystal ends along the c-axis without branching. Dias et al.63-65 have observed similar results on other compounds obtained under microwave processing. It is believed that under applied field conditions, the solvent (water) becomes superheated and the homogeneous nucleation occurs instantaneously. The rapid nucleation generates very small crystals, which rotate freely according to the applied electromagnetic fields in the microwave reactor. In the sequence, the nanostructured crystals would grow via a “cementing mechanism”, which encompasses precise, crystallographically controlled crystal growth of the primary individual nucleos.66



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14

(a)

(c)

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Figure 3. FESEM images for the MnMoO4.H2O crystals obtained under MW at 150°C, for (a) 10 min and (b) 120 min; and under CH conditions at 110°C, for (c) 24 h and (d) 7 days.

Figure 4 shows TEM images obtained for the MW synthesized samples at 150°C, for 120 min, discussed above. For these samples (see Figure 3b), a high anisotropy was observed, which lead us to investigate them more deeply by using transmission electron microscopy with electron diffraction facilities. The morphology of the samples was again verified: well-faceted, micrometer-sized rods with large aspect ratio (Figure 4, upper inset). High-resolution images showed interplanar spacings of the


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15 order of 4.7 Å, which can be related to the (-110) planes of the triclinic ܲ1ത space group. The preferential growth through the plane (-110) agrees well with results showed by XRD. The good crystallinity was then verified by selected area electron diffraction (bottom inset), where the observed well-defined spots confirm the single-crystalline nature of the obtained micro-crystals.

Figure 4. HRTEM images and SAED patterns for the MnMoO4.H2O crystals hydrothermally produced in microwave reactors at 150°C for 120 min. The upper inset exhibits the morphology in a low magnification TEM image, while the HRTEM image shows the interplanar spacing for the (-110) planes of the triclinic ܲ1ത structure. The



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16 SAED patterns are also shown (bottom inset), evidencing the single-crystalline nature of the microcrystals.

3.2.

Monoclinic w-MnMoO4 (P2/c) MnMoO4.H2O microcrystals submitted to higher hydrothermal temperatures

experienced a remarkable polymorphic transformation. Above 200°C, in conventional hydrothermal reactors, the hydrated phase transformed into a monoclinic MnMoO4. Figure 5 shows the XRD data (Figure 5a) and Raman spectra (Figure 5b) for the crystals obtained at 200°C and 250°C, for 24 h and 7 days. Single-phase crystals were obtained and were indexed by the ICDD card #01-78-0221, in agreement with previous works.68 4 This polymorph belongs to the space group P2/c (#13, C 2h ), being isostructural with the

wolframite-type (w) analogous.13,54 The lattice parameters for the w-MnMoO4 crystals were calculated and are in perfect agreement with those expected for this monoclinic phase (Table 3). According to the reference data, the higher intensity peaks occurs for the planes (-111) and (111) at 29.96°2Θ and 30.28°2Θ, respectively. However, the results showed that the plane (100) at 18.40°2Θ are the most intense, which could suggest a preferential growth in that direction. Besides, the strong intensity of the plane (200) corroborates this assumption. For the w-MnMoO4 obtained for 7 days at 250°C, additional peaks are observed at 32.93°2Θ and 38.97°2Θ, which correspond to the planes (-222) and (-203) of the monoclinic phase C2/m (#12). This result will be discussed later in the next section.


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(330) (-322) (-142) (400) (223)

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18 Figure 5. (a) XRD and (b) Raman data for the manganese molybdates synthesized by CH method at 200°C (24 h) and 250°C (24 h and 7 days). Peaks are indexed by the monoclinic P2/c space group (ICDD #01-78-0221). XRD peaks coming from the monoclinic phase C2/m are also indexed (in red) by the ICDD card #01-72-0285.

Table 3. Lattice parameters for the CH synthesized w-MnMoO4 crystals. The values presented by the reference card ICDD #01-078-0221 are also shown for comparison purposes. Sample 200°C/24 h 250°C/24 h 250°C/7 days #01-078-0221

a (Å) 4.815 4.810 4.818 4.818

b (Å) 5.57 5.756 5.759 5.759

c (Å) 4.978 4.969 4.964 4.965

β (º) 90.91 90.91 90.89 90.82

V (Å3) 137.98 137.54 137.72 137.75

Figure 5b presents the Raman spectra for the w-MnMoO4 crystals. As it can be seen, the vibrational bands are equally distributed along the wavenumber range 50-900 cm-1, for all samples, which is in accordance with the results from XRD. For these crystals, the site symmetry and the s.o.f. of the MnMoO4 atoms are given in refs. 62-64. Manganese ions are in 2f Wyckoff sites (C2 symmetry), while Mo ions are in 2e Wyckoff positions (C2 symmetry); oxygen ions are 4g positions (C1 symmetry). Grouptheoretical tools66 were then used to calculate the first-order Raman and infrared modes (at the Γ-point), for the monoclinic P2/c structure. The calculations are showed in Table S2 (Supporting Information), where we can see that, for this compound, 18 Raman modes (8Ag + 10Bg) and 15 infrared ones (7Au + 8Bu) are expected. Following, Raman spectra were experimentally obtained under special polarized configurations for the wMnMoO4 crystals synthesized at 250°C. Figure 6 shows the polarized Raman data for the sample synthesized by CH at 250°C, for 7 days, in special crystal positions, which


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19 demonstrate clearly the that the Raman symmetries can be resolved by polarization. Therefore, it is possible to find crystal regions were Ag modes are enhanced (in parallelpolarized light) or weakened (in crossed-polarized light), because the base functions of this irreducible representation within the C2 h point group are xx, yy, zz, and xy, while those for the Bg modes are xz and yz. Thus, based upon the relative intensities of bands in parallel configuration (favoring the 8 expected Ag modes) or in crossed one (favoring the 10Bg ones), a symmetry assignment of all 18 gerade modes of w-MnMoO4 microcrystals is also presented in Table 4. These assignments are based on those conducted by Iliev et al.68 and Dias et al.69,70 for the MnWO4 analogous.

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Figure 6. Polarized micro-Raman data of w-MnMoO4 crystals obtained at 250°C. Parallel-polarized spectra (black curves) and cross-polarized one (red one) favor, respectively, the Ag and Bg modes.



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20 Table 4. Complete set of Raman vibrational modes for the monoclinic w-MnMoO4 sample, including a tentative symmetry assignment of the modes based on their relative intensities in parallel (//) or in perpendicular (⊥) scattering geometries.

Band Wavenumber FWHM Band Wavenumber FWHM Assignment Assignment (#) (cm-1) (cm-1) (#) (cm-1) (cm-1) 1 96.6 8 Bg 10 339.8 28 Ag 2 127.9 12 Ag 11 366.2 19 Bg 3 151.8 10 Bg 12 385.2 21 Ag 4 180.6 14 Bg 13 493.6 16 Bg 5 191.2 18 Bg 14 516.5 28 Ag 6 212.5 22 Ag 15 653.5 18 Bg 7 250.3 16 Bg 16 682.6 20 Ag 8 269.6 25 Ag 17 764.7 22 Bg 9 312.9 28 Bg 18 845.2 40 Ag

The morphological, chemical and structural behaviors of all samples were examined by FESEM and HRTEM. Figure 7 present typical FESEM images (secondary electrons) for w-MnMoO4 crystals obtained at 250°C, for 24 h and 7 days under CH conditions. The morphology exhibited by the crystals is similar to those obtained for the hydrated phases produced under microwaves and discussed above. Micrometer-sized rods were produced with a large aspect ratio and uniform shape along its length, again demonstrating that the growth anisotropy was preserved during synthesis. For longer processing times (see Figure 3b), the crystals grew significantly along all axes, and well-faceted crystal ends can be observed. Figure 8 presents TEM images for the phasepure monoclinic w-MnMoO4 crystals produced at 250°C, for 7 days. The lowmagnification image (Figure 8b) shows that microcrystals with well-defined morphology were produced, while HRTEM images (Figures 8a and 8c) show different crystallographic planes attesting the high quality of the crystals obtained. Interplanar spacing were determined to be about 4.8 Å and 2.5 Å, corresponding to the (100) and (002) planes of the monoclinic P2/c space group. The preferential growth through the


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21 plane (100) agrees well with results observed by XRD and discussed above. Figure 8d exhibits the SAED patterns for the same crystal, in which well-defined spots observed corroborate its single-crystalline nature.

(a)

(b)

Figure 7. FESEM images for w-MnMoO4 crystals synthesized by CH processing at 250°C, for (a) 24 h and (b) 7 days.



Page 22 of 46

22

(a)

(b)

Å (c)

Å (d)

Figure 8. HRTEM images and SAED patterns for the MnMoO4 crystals hydrothermally produced in conventional reactors at 250°C for 7 days. (a) and (c) are HRTEM images showing the interplanar spacing corresponding to the (100) and (002) planes of the P2/c space group; (b) is a low-magnification TEM image; (d) exhibits the SAED patterns evidencing the single-crystalline nature of the crystals.

3.3.

Monoclinic β-MnMoO4 (C2/m) Now, starting from the w-MnMoO4 crystals synthesized by CH method at

250°C, for 7 days, heat treatments were conducted in the temperature range 300-600°C. The results of XRD are presented in Figure 9 and can be interpreted as follows: wMnMoO4 (P2/c) remained stable until 400°C, in whose temperature β-MnMoO4 (C2/m) begin to appear; at 500°C, practically all w-MnMoO4 have disappeared; however, only at 600°C, single-phase β-MnMoO4 polymorphs were obtained. Previous works have determined transition temperatures around 500°C for this transformation, as also


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23 observed in the present work. It is important to note that w-MnMoO4 and β-MnMoO4 present molybdenum atoms in a six (octahedral) and four (tetrahedral) coordination, respectively.13,21,35,44,71,72 Similarly, the Raman spectra exhibited in Figure 10 seems to predict the same general behavior for this polymorphic transformation. In order to get a more extensive understanding, the Raman data were plotted in three different wavenumber regions (Figures 10a-c). Starting from 300°C, the vibrational bands of the P2/c polymorph tend to broaden; besides, frequency downshifts occur up to 550°C. At this temperature and above, the bands from the P2/c polymorph abruptly disappear and new modes from the C2/m polymorph emerge in all the wavenumber regions. The transformation between these two polymorphs ends at 600°C, where only the monoclinic C2/m phase is present, in perfect agreement with XRD data.



24

600°C

550°C

500°C

)

s t i n u . b r a

400°C

(

y t i s n e t n I

300°C

CH 250°C/7 days

MnMoO4 P2/C #01-078-0221

15

20

25 30 ()

°

10

MnMoO4 C2/m #01-072-0285

a t e h T 2

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

Page 24 of 46

35

40

Figure 9. XRD data for the β-MnMoO4 produced by CH reactors at 250°C, for 7 days, and its thermal behavior in the temperature range 300-600°C. The polymorphic transformation is showed between the monoclinic space groups P2/c and C2/m. Peaks are indexed by the ICDD cards #01-78-0221 (blue) and #01-72-0285 (red).


Page 25 of 46

25

(a)

C2/m 600

)

s t i n u . b r a

550


(

500

400

300

P2/c

250

120 150 180 210 240 270 300 1 -

(

m c

90

r e b m u n e v a W

)

(b)

)

s t i n u . b r a

(


350

400

450

550

1 -

(

500

m c

300

r e b m u n e v a W

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


)


600


26

(c)

)

s t i n u . b r a

(


660

720

780

900

960

1 -

(

840

m c

600

r e b m u n e v a W

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

Page 26 of 46

)

Figure 10. Micro-Raman spectra for the β-MnMoO4 obtained by CH synthesis at 250°C, for 7 days, and its thermal behavior in the temperature range 300-600°C (indicated in the Figure). The polymorphic transformation from the monoclinic P2/c space group to the C2/m ones are showed in different wavenumber regions: (a) 70-300 cm-1; (b) 300-600 cm-1; (c) 600-960 cm-1.

For the β-MnMoO4, the site symmetry and the s.o.f. of each atom are given in the refs. 66 and 67. Manganese, molybdenum and oxygens are distributed in the Wyckoff sites 4g, 4h, 4i, and 8j (symmetries C2, Cs and C1). Group-theoretical tools66 were then used to foreseen the Raman- and infrared- active modes at the Brillouin-zone center, for the monoclinic C2/m structure. The calculations are showed in Table S3 (Supporting Information), where it can be seen that 36 Raman (19Ag + 17Bg) modes and


Page 27 of 46

27 33 infrared (14Au + 19Bu) ones are predicted for this polymorph. In the sequence, polarized Raman spectra were obtained under special configurations for the β-MnMoO4 crystals heated at 600°C. As discussed in the previous sections, the Raman spectra were then treated by peak fitting procedures. Figure 11 (divided in two frequency regions: 40-450 cm-1 and 450-1000 cm-1) exhibits the polarized Raman spectra in suitable configurations in order to show the enhancement of Ag or Bg modes in parallelpolarized or in cross-polarized light. By using these procedures, it was possible to find 33 modes of the 36 predicted ones for β-MnMoO4 microcrystals (18 Ag + 15 Bg). The results are summarized in Table 5. These assignments agree well with those reported by Saleem et al.72 and Kanesaka et al.,73 also expanding their findings. The inset in Figure 11b shows a FESEM image for the microcrystal heat treated at 600°C, for 2 h.


(a)

Ag

Ag

Bg Bg Ag Bg

// Bg Bg

T

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


50

Ag

Ag Ag Bg

100

150

Bg

Bg

Ag

Bg

Ag B Ag Ag g Ag

200

250

300

350 -1

Wavenumber (cm )


400

450


28


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

Page 28 of 46

(b)

Ag

Bg

Ag

Ag

Bg Ag Bg

500

Ag

Ag

600

Bg

700

Bg A g

800

900

1000

-1

Wavenumber (cm ) Figure 11. Unpolarized (black line) and polarized Raman spectra of β-MnMoO4 crystals obtained at 600°C, for 2 h. The parallel-polarized spectrum (mainly Ag modes) is represented by the blue curves (//), while the cross-polarized one (Bg modes) is shown in red (┴). (a) 50-450 cm-1 and (b) 450-1000 cm-1. Inset in (b): FESEM image for the microcrystal heat treated at 600°C, for 2 h.

Table 5. Raman phonons for the monoclinic β-MnMoO4 polymorph obtained at 600°C, for 2 h, and their assignments according to their relative intensities in parallel (//) or in perpendicular (⊥) scattering configurations. Band Wavenumber FWHM Band Wavenumber FWHM Assignment Assignment (#) (#) (cm-1) (cm-1) (cm-1) (cm-1) 1 75.6 9 Ag 18 358.8 18 Ag 2 87.6 7 Ag 19 362.4 22 Bg 3 101.3 6 Bg 20 370.7 22 Bg 4 121.6 12 Bg 21 404.7 24 Bg 5 132.5 11 Ag 22 543.4 18 Bg 6 156.5 10 Ag 23 588.9 19 Ag 7 175.8 14 Bg 24 628.0 25 Ag 8 206.2 13 Ag 25 659.6 33 Ag 9 228.2 16 Bg 26 711.4 32 Bg 10 241.1 13 Ag 27 748.2 25 Bg


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29 11 12 13 14 15 16 17

3.4.

257.6 271.4 283.3 300.8 326.6 333.9 347.7

Triclinic

18 19 21 18 16 22 23

Ag Ag Bg Ag Bg Bg Ag

(MnMoO4.H2O)

28 29 30 31 32 33

to

monoclinic

791.1 827.7 854.3 887.1 936.8 946.6

24 21 18 31 26 16

(β β -MnMoO4)

Ag Ag Bg Bg Ag Ag

polymorphic

transformation It was observed in the present work that hydrated phases were obtained in all MW conditions. By using CH reactors, these hydrated phases were obtained up to 200°C, followed by a monoclinic MnMoO4 (P2/c) phase. When this polymorph is submitted to heat treatments, a new monoclinic phase is observed at 600°C, belonging to the C2/m space group. Besides, another polymorphic transformation was noted from the hydrated phase (triclinic ܲ1ത) directly to the monoclinic C2/m one (note that both polymorphs present molybdenum atoms in tetrahedral coordination). This result was previously obtained;13,21,35,44 however, the processing condition in which this finding was attained deserves consideration. Experiments showed that crystals synthesized under MW (150°C/120 min) or by CH (150°C/24 h) methods transform directly into MnMoO4 crystals (C2/m), after thermal treatment at 250°C, for 2 h. Figure 12a presents the XRD for these experiments, which show that it was possible to obtain β-MnMoO4 directly from triclinic MnMoO4.H2O in milder temperatures than those employed in the current literature.13,21,35,44 Also, the crystal behavior under heat treatment is different from that in aqueous solutions, since hydrated phases were converted in monoclinic P2/c crystals, when in hydrothermal solutions above 200°C (see Figures 9 and 10). Figure 12b shows the Raman spectra for the same polymorphs, which agree well with the results from XRD, as expected. However, an additional band was observed in ACS Paragon Plus Environment


Page 30 of 46

30 crystals whose origin was the CH process (in blue), and was not observed in samples obtained under MW (red line). The spectrum for the β-MnMoO4 is the red one, which was discussed in section 3.3 (above). In that case, the C2/m polymorph was obtained by heating the P2/c one at 600°C. This new Raman mode, at 662 cm-1, reveals that this sample is not a phase-pure β-MnMoO4 (this band is not present in P2/c polymorphs). Previous reports showed that this band could be due to α-polymorphs, as verified by αCoMoO4 and α-NiMoO4 around 700 cm-1.72 In this phase, the coordination of molybdenum atom is six (octahedral), which is absolutely new for MnMoO4, since all works for this compound reported only a four-coordinated ion.13,21,35,44,71,72 This result could be interpreted as an evidence of incipient crystallization, in a short range degree, of an α-polymorph, with molybdenum ions in octahedral coordination, for the manganese molybdates, which was not observed by XRD in the present work. Similar results were observed by Siqueira and Dias70 in microwave synthesized transition-metal tungstates, which incipient crystallization was investigated by TEM and micro-Raman spectroscopy.


Page 31 of 46

(-113)

(330)

(202)

(400)

(-222) (-312)

(-202) (130) (-131) (112)

(110)

(200) (-201)

)

s t i n u . b r a

(021) (201)

(a)

(-112) (002)

(220)

31

CH

(

y t i s n e t n I

MW

MnMoO4 C2/m #01-072-0285

20

25

( )

30

°

15

a t e h T 2

10

35

(b)

)

s t i n u . b r a

(


MW

CH

1 -

(

m c

120 240 360 480 600 720 840 960 r e b m u n e v a W

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


)


40


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32 Figure 12. (a) XRD and (b) Raman spectra of the resulting β-MnMoO4 crystals after direct heat treatment at 250°C, for 2 h, of MnMoO4.H2O synthesized by MW (red lines) and CH (blue lines) at 150°C, for 120 min and 24 h, respectively.

Finally, the procedures and results obtained in the present work can be summarized in Figure 13, as follows: after preparation of the hydrothermal solutions, experiments were carried out by using CH and MW reactors. The results showed that MnMoO4.H2O microcrystals were produced under MW in all experimental conditions employed, while CH experiments produced hydrated phases and wolframite-type P2/c crystals. The thermal behavior was investigated for these two set of samples. The hydrated phases were converted to β-MnMoO4 at 250°C, for 2h, independently of their origin (CH or MW). This temperature is significantly lower than the temperature reported by the literature (around 500°C) for this polymorphic transformation. Both crystalline phases (triclinic ܲ1ത and monoclinic C2/m) contain molybdenum atoms in a tetrahedral coordination with oxygen ions. Wolframite-type crystals obtained by CH synthesis at 250°C were completely converted to β-MnMoO4 only by heating at 600°C, for 2 h. These results summarize relevant contributions on the understanding of the mechanisms and polymorphism phenomena involved during hydrothermal processing of manganese molybdates.


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33

Hydrothermal solutions Room temperature

Microwavehydrothermal

Conventional hydrothermal

150°C/10-120 min

110-250°C/24 h ,7 d

MnMoO4.H2O

MnMoO4.H2O

Triclinic P-1

Triclinic P-1

Heat treatment 250°C/2 h

β-MnMoO4 Monoclinic C2/m

w-MnMoO4 Monoclinic P2/c

Heat treatment 600°C/2 h

β-MnMoO4 Monoclinic C2/m

Figure 13. Scheme for the hydrothermal syntheses of manganese molybdates and respective polymorphs obtained in the present work.



Page 34 of 46

34 4.

CONCLUSIONS The hydrothermal technology was applied to explore the polymorphism and

vibrational properties of manganese molybdates. Microcrystals were obtained under different CH and MW conditions of temperature and time. Triclinic MnMoO4.H2O polymorphs (ܲ1ത) were obtained under microwave in all conditions of temperature and time, as well as in conventional hydrothermal reactors at temperatures below 200°C. Raman spectroscopic analysis showed that 26 Ag modes among the 36 Ag predicted phonons by group-theory calculations could be identified. Monoclinic wolframite-type crystals (P2/c) were obtained only under conventional hydrothermal experiments above 200°C. Their vibrational properties evidenced all the 18 Raman-active modes predicted, which could be fully identified and assigned to their symmetries (8Ag + 10Bg). The thermal behavior of the hydrated phases revealed that they were converted to βMnMoO4 (C2/m space group) at 250°C, for 2h, independently of their origin (conventional or microwave-assisted processing route). This temperature observed for the polymorphic transformation is significantly lower than that reported by the current literature, which is around 500°C. Additionally, wolframite-type crystals (P2/c space group) synthesized by CH synthesis at 250°C were completely converted to β-MnMoO4 by heat treatments at 600°C, for 2 h. The phonon features of the Raman modes of this phase were also well characterized (33 of the 36 predicted modes were depicted and their symmetries unveiled). The results obtained in the present work constitute important contributions on the understanding of the mechanisms and polymorphism phenomena involved during hydrothermal processing of manganese molybdates.

ASSOCIATED CONTENT


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35 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: Site group analyses for all the MnMoO4 polymorphs investigated in this work (Tables S1-S3) (PDF)

AUTHOR INFORMATION Corresponding Author *(A.D.) E-mail: [email protected]; [email protected]. ORCID Guilherme Mendes Martins: 0000-0001-7620-6708 Pâmela Oliveira Coelho: 0000-0003-1561-5852 Kisla Prislen Félix Siqueira: 0000-0002-6344-3103 Roberto Luiz Moreira: 0000-0001-6820-0269 Anderson Dias: 0000-0001-7413-1087 Notes The authors declare no competing financial interest.



Page 36 of 46

36 ACKNOWLEDGEMENTS The authors acknowledge the financial support from CNPq, CAPES, FINEP and FAPEMIG. The Center of Microscopy at the Universidade Federal de Minas Gerais is acknowledged for providing equipment and technical support for the experiments involving electron microscopy.


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37 REFERENCES

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46 FOR TABLE OF CONTENTS USE ONLY

MANUSCRIPT TITLE: Investigation of Polymorphism and Vibrational Properties of MnMoO4 Microcrystals Obtained by Hydrothermal Technology AUTHOR LIST: Guilherme M. Martins, Pâmela O. Coelho, Kisla P. F. Siqueira, Roberto L. Moreira and Anderson Dias

TOC GRAPHIC

SYNOPSIS Conventional and microwave-assisted hydrothermal reactors were employed to investigate the polymorphism in manganese molybdates. Well-faceted, highly crystalline microcrystals, belonging to different triclinic and monoclinic space groups, were obtained at different conditions of temperature and time.


Investigation of Polymorphism and Vibrational Properties of MnMoO4

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