Thermal destruction of giant polyoxometalate nanoclusters: A vibrational spectroscopy study

Inorganica Chimica Acta 489 (2019) 287–300

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Research paper

Thermal destruction of giant polyoxometalate nanoclusters: A vibrational spectroscopy study

T

K.V. Grzhegorzhevskiia, , P.S. Zelenovskiya,b, O.V. Koryakovac, A.A. Ostroushkoa ⁎

a

Institute of Natural Sciences and Mathematics, Ural Federal University named after the B.N. Yeltsin, 620002 Ekaterinburg, Russia Department of Chemistry & CICECO−Aveiro Institute of Materials, University of Aveiro, 3810-193 Aveiro, Portugal c Institute of Organic Synthesis Ural Branch of RAS, 620990 Ekaterinburg, Russia b

ABSTRACT

Thermal destruction of giant polyoxometalate (POM) high-symmetry nanoclusters (toroidal Mo138 and Keplerates Mo132 and Mo72Fe30) was in situ investigated by Raman and IR spectroscopy. The key spectral features of POM integrity and destruction products were determined. Polarization Raman measurements of Keplerate nanoclusters carried out for the first time allowed us to determine the full set of totally symmetric modes and improve the attribution of spectral bands. The obtained results are general for all types of POMs and can be used to control the POM’s integrity during the synthesis of new nanoclusters’ based hybrid materials.

1. Introduction Giant polyoxometalates (POM) are a modern class of nanoscaled macroanions consisted of large number of atoms of various transitional metals (Mo, Fe, V, W, Cr, Cu etc.) inside oxygen polyhedrons connected through the common edges and corners. Being the result of the selfassembly process, nanocluster POMs can form different high-symmetry shapes: toroidal [1–5] (Mo138, Mo154 etc., so-called, wheel-like, Fig. 1a), Keplerate [6,7] (Mo132 and Mo72Fe30, so-called, fullerene-like, Fig. 1b, c), super-cluster [8] (Mo368, also called, “nano-hedgehog” or lemon-like) and basket-like nanocluster [9] produced by the unlatching of the corresponding Keplerates. Hydrophilic, highly reactive and mosaic surface of toroidal and Keplerate polyoxomolybdate nanoclusters (Fig. 1), providing different types of the intermolecular interactions (such as electrostatic, van-der-Waals and hydrogen bonding), allows their functionalization by various atoms and molecules [10–16]. That is why they are considered as the one of the most promising nanoclusters for production of new multifunctional inorganic-organic hybrid materials, for various applications: selective ion-transport membranes [10], magnetic hollow nanoparticles [11], nanoreactors [12], photosensitizers [13,14], potential drug delivery systems [15], contrast agent with complex of Gd(III) [16] etc. The structure of giant polyoxomolybdates depends on many factors including the temperature. Heating of nanoclusters accompanied the synthesis of different hybrid materials can lead to POM decomposition, if it exceeds a certain threshold temperature [17]. Therefore, the control of POMs’ structural integrity plays crucial role both for nanoclusters production and for their applications in the material science.

⁎

Earlier, the thermogravimetric analysis (TGA), differential thermal analysis (DTA) and mass spectrometry (MS) detection of a gas phase allowed to determine the main final destruction products of POMs (MoO3 for Mo138 [18] and Mo132 [17], and Fe2(MoO4)3 for Mo72Fe30 [19]) and to establish the common stages of POM decomposition [17,20,21]. There are: (1) endothermic stepwise loosing of weakly bonded water (100–200 °C); (2) detaching of structural water molecules (200–500 °C), and (3) removing of organic components as carbon dioxide (250–500 °C) with either endo- or exothermic effects [17,20,21]. Regardless the temperature of water losing correlates with changing of POM structure, it does not coincide with POM destruction temperature. The latter was determined by electron paramagnetic resonance (EPR) [18,21]. The abrupt changes of g-factor for Mo138 and Mo72Fe30 nanoclusters observed at 57–77 °C and 67 °C, respectively, were attributed to the beginning of POMs structural transformations. Thus, the obvious controversy between TGA-DTA-MS and EPR data does not clarify the details of POMs decomposition. Moreover, the study of temperature dependence of Mo132 using the Extended X-Ray Absorption Fine Structure (EXAFS) technique allowed to determine that the dominating factor responsible for Mo132 structure stability is the rigidity of the MoVMoV bridges connecting the {Mo6} building blocks [22]. However, this did not shed light on the temperature threshold of Keplerate destruction. IR and Raman spectroscopy seems to be more convenient experimental methods for control the structural integrity of nanoclusters, than a sophisticated EXAFS technique. Vibrational spectroscopies are quite fast, structure sensitive and non-destructive. Obviously, their usage requires deep knowledge of POM’s vibrational properties and

Corresponding author at: Institute of Natural Sciences and Mathematics, Ural Federal University, 620002, 19 Mira street, Ekaterinburg, Russia. E-mail address: [email protected] (K.V. Grzhegorzhevskii).

https://doi.org/10.1016/j.ica.2019.01.016 Received 1 October 2018; Received in revised form 14 January 2019; Accepted 16 January 2019 Available online 21 January 2019 0020-1693/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. The polyhedron representation of giant POM nanoclusters: (a) toroidal Mo138 and (b) Keplerate Mo132 and (c) Keplerate Mo72Fe30. Yellow octahedra in (c) correspond to FeO6 units.

temperature variations of the spectra. However, the attention of researchers was mainly focused at several strongest vibrational bands only, whereas other less strong, but important features were left out [1,6,7,23,24]. Therefore it is not surprising that few investigations of POMs thermal stability carried out up to now [18,20,25] demonstrate contradictory results. So, the heating threshold of 350 °C for structural stability of toroidal nanocluster Mo138 established by IR data [25] is 150 °C higher than the temperature of MoO3 formation determined by X-ray diffraction (XRD) [18]. Moreover, the studies of IR and Raman variation during the heating of Keplerates nanoclusters Mo132 and Mo72Fe30 still were not performed. All of that makes the investigations of POMs temperature decomposition an important fundamental and applied task for modern solid state chemistry. In this work we focused our attention on three main types of POMs: toroidal Mo138 and Keplerates Mo132 and Mo72Fe30 (Fig. 1). The crystal structure of these POMs is described in details in Section S1 of Supporting Information. We jointly used polarized Raman and IR measurements to improve the attribution of vibrational bands for all nanoclusters, studied thermal decomposition of POMs under the isothermal and in situ annealing and determined the most reliable spectral features for analysis of POMs integrity and their destruction products. Since Mo138 and Mo132 clusters have similar composition but different structure, whereas Mo132 and Mo72Fe30 possess similar structure, but different composition, the obtained results have a sufficient credibility and can be applied to wide range of POMs.

High absorbance of toroidal POM Mo138 in 640–800 nm range led to fast overheating of the sample and nanoclusters destruction, therefore the solid-state laser with excitation wavelength 488 nm was used. Laser beam was focused at the sample surface by 10 × objective with NA = 0.2. Scattered light was collected by the same objective in backscattering geometry and passed through the appropriate edgefilter. The spectrum was detected by Andor Newton CCD camera with 1600 × 200 pixels matrix thermoelectrically cooled down to −60 °C. A 50 μm diameter multimode optical fiber was used as a confocal pinhole. The microscope was equipped by the rotating polarizer-analyzer system for measuring the polarized Raman spectra. Addition characterization was performed for Mo72Fe30 sample using the X-ray photoelectron electron spectrometer K-Alpha+ (Thermo Fisher Scientific, USA) with monochromatic Al Kα radiation (1486 eV). The survey spectrum and corresponding zoomed region are presented in the Supporting information (Fig. S1), where one may see the absence of signal of sodium atoms, which could be contained into the inner cavity of Mo72Fe30 instead of the {Mo2O7(H2O)}2{H2Mo2O8(H2O)}fragment, as that was described in some literature. 2.1. Thermal destruction study Study of the thermal decomposition of nanoclusters was carried out in two ways: (1) ex situ monitoring after consecutive isothermal annealing, and (2) in situ observation under the laser beam annealing. High-quality ex situ spectra were used for detail analysis of the vibrational bands, whereas spectra recorded during in situ annealing allowed us to monitor the kinetics of POMs destruction. The consecutive isothermal annealing consisted in exposition of Mo138, Mo132 and Mo72Fe30 crystals at different conditions: 100 °C – 90 min (first stage), 150 °C – 90 min (second stage) and 200 °C – 110 min (final stage). After cooling the crystals down to room temperature at each stage, IR and Raman spectra of nanoclusters were measured. Raman spectra were obtained only for the first and second stages, since after the third stage nanoclusters quickly destructed under laser beam. Raman spectra were measured with low laser power and long acquisition time. The in situ annealing under the laser beam was performed during simultaneous measuring of Raman spectra. Two modes were realized: spectra registration during gradual laser power increasing; and sequential spectra registration at a constant laser power. The first mode was used to Mo132 and Mo138, and the second – for Mo72Fe30, because of relatively low laser power was enough to initiate quick destruction of the iron-molybdenum POM. The assignment of spectrum, obtained at in situ experiment, to certain temperature was carried out by the comparison with Raman spectra obtained for samples, which were exposed under isothermal conditions.

2. Experimental details Three types of giant POMs were studied in this work [1,6,7,24]: V Mo138 = (NH4)32[MoVI 110Mo28O416H6(H2O)58(CH3CO2)6]·(∼250H2O); V Mo132 = (NH4)42[MoVI 72Mo60O372(CH3COO)30(H2O)72]·(∼300H2 O)·(∼10 CH3COONH4); III Mo72Fe30 = [MoVI 72Fe30O252(CH3COO)12{Mo2O7(H2O)}2{H2Mo2 O8(H2O)}(H2O)91]∙150H2O. Crystalline powders of these POMs were synthesized in accordance with previously reported methods [1,6,7]. All chemicals were analytical reagent grade. Structure verification of the obtained POMs was carried out by the comparison of IR and Raman spectra with reference data from literature [1,6,7]. The spectra are presented in Figs. 2–4. IR spectra of POMs crystalline powders were measured at room temperature using FT-IR spectrometer Nicolet 6700 (Thermo Scientific) in both diffuse reflectance absorption (DRA) mode and in KBr pellet, in the range 4000–400 cm−1 with spectral resolution 2 cm−1. Raman spectra of POMs single crystals were measured using the confocal Raman microscope Alpha 300 AR (WiTec GmbH) at low power of laser beam in order to prevent uncontrollable destruction of the nanoclusters. Raman spectra of Keplerate POMs Mo132 and Mo72Fe30 were obtained using He-Ne laser with excitation wavelength 633 nm.

288

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Fig. 2. IR spectrum in DRA mode from powder (top) and Raman spectra from single crystal with 488 nm laser (bottom) of toroidal POM nanocluster – Mo138. Insert in dotted frame demonstrates the magnified fragment of compound band near 1000 cm−1.

400 cm−1. Conventional IR spectrometers usually cut the LFR, therefore the information about crystal lattice vibrations can be obtained primarily by Raman spectroscopy. The existing interpretations of IR and Raman spectra of pure giant POMs (Mo72Fe30, Mo132, Mo138) are not completed yet and several unassigned bands and ambiguous attributions still exist [27]. To clarify the controversial points in bands attribution [27] and to determine the vibrations of substructural building units the polarized Raman spectra of Keplerates Mo132 and Mo72Fe30 were measured (Figs. 5, 6 and Figs. S2, S3 in Supporting Information), and depolarization ratios for the observed spectral bands were determined. Depolarization ratio (ρ) of a Raman band is defined as ratio of its intensity measured with crossed

3. Results and discussion 3.1. Polarized Raman study of Keplerate POMs Vibrational spectra of POM crystals as well as other molecular crystals can be divided into two separate regions [26]: 1) low frequency region (LFR, below 400 cm−1), containing lines corresponding to crystal lattice vibrations, and 2) high frequency region (HFR, above 400 cm−1), containing lines corresponding to vibrations of single molecules (individual nanoclusters in our case) or molecular fragments in accordance with their local symmetry group. In case of heavy molecules, the lower edge of HFR in Raman spectrum can lie below

Fig. 3. IR spectrum in DRA mode from powder (top) and Raman spectra from single crystal with 633 nm laser (bottom) of Keplerate POM nanocluster – Mo132. 289

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Fig. 4. IR spectrum in DRA mode from powder (top) and Raman spectra from single crystal with 633 nm laser (bottom) of Keplerate POM nanocluster – Mo72Fe30.

Fig. 5. Polarized Raman spectra of Mo132 (HH – parallel; HV – perpendicular) obtained with constant laser (633 nm) power. Insert demonstrates optical image of the oriented crystal of POM.

Fig. 6. Polarized Raman spectra of Mo72Fe30 (HH – parallel; HV – perpendicular) obtained with constant laser (633 nm) power. Insert demonstrates optical image of the oriented crystal of POM.

polarizers to the intensity measured with parallel polarizers. As predicted from Rayleigh theory, for totally symmetric vibrations the value of ρ lies between 0 and 3/4 (such vibration bands are totally or partially polarized), whereas for ρ > 3/4, the vibrations possess an asymmetric type and are totally depolarized. Due to the icosahedral symmetry of Keplerate POMs, only one totally symmetric Ag-mode and one fivefold degenerated Hg-mode can be observed in Raman spectra. Calculated values of depolarization ratio are listed in Table S1 (Supporting Information). For all Raman bands of Mo132 the values of ρ are less than 3/4 (Table S1 Supporting Information), thus these bands correspond to the totally symmetric vibrations. Two strongest bands at 875 and 376 cm−1 demonstrate the smallest values of depolarization ratio and could be attributed to Ag and Hg mode respectively, that is in line with previous theoretical suggestions [6,27]. Additional information about the nanocluster skeleton motions corresponding to Ag and Hg modes can be obtained by calculating the length of Mo-O bond (R) and bond order (s) using empirical expressions [28,29]:

R=

1 R ln and s = 2.073 32895 1.882

6

The obtained values are presented in Table S2 (Supplementary Information). The bond length and bond order for Ag mode are 1.75 and 1.56, respectively, whereas for Hg mode they are 2.16 and 0.44, respectively. Comparison of these values with published crystallographic data [7,30] allows attributing Ag and Hg modes to certain atomic vibrations. Since the higher bond order typically corresponds to shorter bond length and vice versa, Ag mode should correspond to the stretching vibrations of the O-Mo-O bonds including terminal oxygens (-OH groups, -O−), whereas Hg-mode can be assigned to stretching vibrations of (Mo-Oμ-Mo) bonds including the bridge oxygens (μ2-O and μ3-O), so-called breathing vibrations. These results are in contrast to previous interpretation [27], where Hg-vibration was attributed to band ca. 870 cm−1. However, large reduced mass of nanocluster fragments containing μ3-O atoms should lead to lower wavenumber value of this vibration (in accordance to known formula of harmonic oscillator frequency: = k / m , where k is a bond force constant, and m is reduced mass). Therefore, the spectral band at 290

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376 cm−1 should be attributed to Hg-mode, the skeleton breathing vibrations. Ag mode also can be considered as breathing vibration, but involving terminal Mo-O bonds. The suggested assignment is also confirmed by the comparison of Mo132 Raman spectra measured in aqueous solution and in solid state (see Fig. S4 Supporting Information). A 30% smaller intensity (as well as noticeably smaller frequency) of 875 cm−1 band observed in the solid state with respect to the solution and almost no changes for 316 cm−1 band demonstrate active participation of Ag mode in interaction between solvent and POM surface, obviously via the terminal oxygens. The Raman spectrum of Mo72Fe30 (also belonged to point group Ih) is more complicated than that of Mo132 due to the presence in the inner cavity of nanocluster a {Mo2} = {Mo2O7(H2O)}2{H2Mo2O8(H2O)} fragment [7] leading to symmetry reduction and appearing of new vibration bands. At the beginning of the Mo72Fe30 study, there was a discussion concerning that the sodium cations may be present in the POM’s inner cavity rather than disordered {Mo2} groups. However, XPS data recorded using the sample investigated (see Fig. S1 Supporting Information) by us in this report do not indicate the presence of any sodium. Thus, we procced from the statement that the {Mo2} groups contribute to the observed vibrational signatures of the Mo72Fe30, instead of the sodium cations. Leaning on the determined depolarization ratios, calculated bond orders (see Tab. S1 and Tab. S2 Supporting Information), and crystallographic data [7], we attributed spectral band at 452 cm−1 to Hg mode, and composite band at 952 cm−1 to Ag mode. Lower intensity of Hg mode in Raman spectrum of Mo72Fe30 than in Mo132 is probably due to the different values of Raman tensor after the replacement of bridged bonds Mo-O-Mo to Fe-O-Mo. Since the metal atoms have different electronegativity values (electronegativity of MoVI and MoV are 2.35 and 2.27 respectively, FeIII is 1.96 [31]) such replacement leads to bond polarization. At the same time, high frequency shift of Hg and Ag modes in Raman spectrum of Mo72Fe30 is caused by decreasing the reduced mass of bridged fragment (from 12 kg−1 for Mo-O-Mo fragment to 11 kg−1 for Fe-O-Mo). A composite nature of Ag band is apparently caused by the presence of different nonequivalent position of {Mo2O7(H2O)}2{H2Mo2O8(H2O)} fragment and contribution of substructural build units vibration. The performed polarization measurements showed that low frequency shoulder at 909 cm−1, overlapped with Ag band, possesses higher depolarization ratio (0.36) as compared with neighboring bands 952, 969 cm−1 (0.05 for both). This points to the belonging of 909 cm−1 band to terminal Mo-O stretching in a {Mo2O7(H2O)}2{H2Mo2O8(H2O)} fragment, since the stretching vibrations in this region are commonly attributed to vibrations of terminal groups (O-Mo-O) or (Mo=O) in molybdates [32,33] (see Raman spectra of simpler POMs (NH4)6Mo7O24 and Na2MoO4 in Fig. S5 Supporting Information). Moreover, small contribution of vibrations of {Mo2O7(H2O)}2{H2Mo2O8(H2O)} fragment into the common Raman spectrum of Mo72Fe30, allows supposing that weak band at 586 cm−1 with quite high depolarization ratio (1.17) is also attributed to this fragment. Beside all abovementioned spectral bands, several substructural bands were revealed in Raman spectra of Keplerate POMs that were not attributed earlier. These bands are located at 717, 842, 945 and 969 cm−1. Band at 717 cm−1 is observed in the spectrum of Mo72Fe30 and is absent in the spectrum of Mo132 (in contrast to 717 cm−1 band, the band at 710 cm−1 presented in spectrum of Mo132 possesses totally symmetric type and is a combination of 376 cm−1 and 316 cm−1 bands). In order to determine the nature of this band we analyzed Raman spectra of isostructural low-molecular compounds [34] Fe2(MoO4)3 and Cr2(MoO4)3. The ferric molybdate was chosen because there are no bridged bonds (Fe-O-Fe), and all oxygen polyhedrons of iron (III) are connected with neighboring tetrahedral of molybdenum (VI) forming bridged bonds of (Fe-O-Mo)-type [35]. One of the main

bands in ferric molybdate spectrum is located at 781 cm−1 and is absent in chromium molybdate. Therefore, this band spectrum can be assigned to stretching vibration of (Fe-O-Mo) bond. That allowed us to assign band at 717 cm−1 to the motion of Fe-containing substructural units, such as (Fe-O-Mo) bridged bonds. As a result of reduced mass increasing (we mean the comparison of the Mo72Fe30 and the Fe2(MoO4)3 reduce mass) and/or coordination of acetate group to iron-oxygen polyhedrons in the POM inner cavity [7], in the Raman spectrum of Mo72Fe30 the (Fe-O-Mo) stretching is shifted to 717 cm−1. Additional confirmation of this assignment is the appearance of very strong band at 749 cm−1 in IR-spectrum. It is in good correlation with large intrinsic dipole moment of (Fe-O-Mo) bond that leads to not so high intensity of this band in Raman spectrum. Important to note, that the characteristic band at 1000 cm−1 corresponding to Fe(OH) groups in low-molecular compounds [34,36] is not observed in the POM Raman spectrum. Another substructural vibration mode at 842 cm−1 remains at the same position in Raman spectra of both Keplerates in spite of the replacement of {Mo2} in Mo132 to Fe(III) oxygen polyhedrons in Mo72Fe30. This allowed us to assign this band to fragment {Mo6} = {MoO6}5{MoO7}, which is common for both Keplerates (see Fig. S6 in Supporting Information). Moreover, the bond order of 1.45 calculated for this band from Raman spectrum [28,29] (see Table S2 in Supporting Information) and analysis of crystallographic data [6] point out that this band includes vibrations of terminal bonds in {Mo6} unit. The values of ρ for this band determined for Mo132 and Mo72Fe30 from polarized Raman spectra are 0.35 and 0.63, respectively. This is somewhat higher than that of Ag and Hg modes, but it still allows attributing this band to totally symmetric modes. However, only two totally symmetric modes (Ag and Hg) are allowed in the Raman spectrum for Ih point group of Keplerates. This contradiction can be resolved if we take into account the local symmetry C5h of {Mo6} substructural unit that permits their own totally symmetric mode in Raman spectrum. Thus, the band at 842 cm−1 can be attributed to totally symmetric mode A' of {Mo6} unit. Other substructural bands, located at 945 cm−1 in the Raman spectrum of Mo132 and at 969 cm−1 in the spectrum of Mo72Fe30, possess high bond order (1.76 and 1.86) and low depolarization ratio (0.20 and 0.05). Moreover, the position of 945 cm−1 band shifts at 955 cm−1 in aqueous solution that is typical for terminal groups. Therefore, as in the previous case, these bands correspond to the totally symmetric local modes and should be included in the stretching vibrations of terminal Mo-O bonds [32,33] in nanocluster’s skeleton fragments. Thus, the carried out polarization Raman measurements allowed us to summarize all data into the common interpretation Table 1. The polarization measurements for low symmetry Mo138 were not performed because of complicated Raman spectra. However, the spectral band corresponding to terminal bonds vibration in {Mo6} unit should also presence in the Raman spectrum of toroidal nanocluster although at slightly different position with respect to that in Keplerates’ spectra. This vibration can be assigned to a wide band with maximum at 820 cm−1 (Fig. 2), whereas its widening can be attributed to defect structure of this toroidal POM. 3.2. Nanoclusters’ thermal decomposition Suggested interpretation of IR spectra of POMs based on the performed analysis of the reference data array is presented in Table 2. It is worth noting, that only spectra obtained by the same mode (DRA, or in attenuated total reflectance mode (ATR), or in KBr pellet) can be directly compared at different temperatures. Figs. S7 – S9 (Supporting Information) demonstrate the IR spectra of POMs obtained in the KBr pellets. One can see the shift of several POMs’ bands with respect to their positions in spectra measured in DRA mode (Figs. 2–4). The reason of this shift is change of solvation arrangement in dry KBr as 291

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Table 1 Assignment of several bands in Raman spectra of POMs. Assignment

Band position, cm−1

Reference data

Mo138

Mo132

Mo72Fe30

as(Mo=O)

961 s

945 m,

969 s (sh)

ν(O-Mo-O)terminal

820 s

875 s, 842 m (sh)

ν(Mo-O-Fein-bridge)

−

−

952vs, 909 m (sh), 842 m 717 m

νas(Mo-O-Mo) νs (Mo-O-Mo)

717 s, 653 s 532 m, 487vs, 439 m 372w, 322 m 239 s, 217 s −

− 376 s

− 586vw, 518 m, 452 m

H. Tian et al. [34], M. Rapposch et al. [35], H. Zajac et al. [37] I. Wachs [32], D. Chen [38] I. Wachs [32], H. Hu and I. E. Wachs [33]

316 m 215w 710w ≈ 376 s + 316 m (Δ = 18 cm−1); 1250w ≈ 875 s + 376 s (Δ = 1 cm−1)

370 m 240 m −

H. Hu and I. E. Wachs [33], D. Chen et al. [39] H. Hu and I. E. Wachs [33], D. Chen et al. [39] −

νs,

δ(Mo=O) δ(O-Me-O) Combination band

A. Müller [1], I. Wachs [32], H. Hu and I. E. Wachs [33] I. Wachs [32]

Shorthand notation: vs – very strong; s – strong; m – medium; w – weak; vw – very weak; sh - shoulder; Δ – error.

compared to normal atmosphere humidity. Despite of slightly different band positions, the below-discussed trends are the same.

nanocluster above this temperature. Final destruction products of Mo72Fe30 are ferric molybdate [19] and molybdenum oxide, since 30 Fe-atoms in each Mo72Fe30 nanocluster can decay into maximum 15 molecules of Fe2(MoO4)3. That is confirmed by Raman spectra (Fig. 8b) mainly consisting of Fe2(MoO4)3 bands (at 988, 969, 935, 824, 783, 351 cm−1 [34]) and with small amount of MoO3 bands (at ca.819, 994 cm−1 and, most clearly, by the number of bands placed below 300 cm−1 [39], for more details see Fig. S5 Supporting Information). For deeper understanding the Mo72Fe30 destruction mechanism, in situ measurements of Raman spectra were performed. The dynamic changes of Raman spectra during POM destruction under the laser beam are shown on Fig. 8a, where the abrupt transition between the

3.2.1. Keplerates Mo72Fe30 and Mo132 Mo72Fe30: We found that the iron-molybdenum POM Mo72Fe30 possesses the lowest thermal stability among all studied nanoclusters. Raman spectra measured using 633 nm laser after isothermal annealing are shown in Fig. 7. Heating up to 100 °C leads to appearing a new intensive band at 768 cm−1. Moreover, the 952 cm−1 band shifts to 958 cm−1, intensive shoulder at 969 cm−1 disappears, and other bands below 800 cm−1 decrease their intensity and significantly widen. All of these unambiguously indicate the significant decomposition of the Table 2 Assignment of several bands in IR spectra of POMs. Assignment

Band position, cm−1

Reference data

Mo138

Mo132

Mo72Fe30

ν(OH···H)

3550 s/3407 s

3470 s

ν(NH4+) ν(CeH asymmetric/symmetric for: CH3–, –CH2- and –CH = groups) ν(C]O) for ammonium acetate δ(H2O)

3207 s 3056 s/ 2845 m − 1617 m 1573w

3215 s 3050 s/2855 m 1711w 1614 m 1549 s

region 3500–3000 s, very wide band with maxima at ∼ 3340, ∼3165 − region 3000 – 2800 m, wide band with maximum at ∼ 3030 − 1613 m 1532 s

δ(NH4+)

1416 m

1418 s

−

νs(COO–) coordinated to metal

1348w (sh)

1350 w (sh)

1411 s

δ(COO )/ or δ(CeH) at CH3-group of acetate ion

−

1337w

1342w

δi.p.(C-CH3), in plane rocking for structural acetate groups and for surface adsorbed molecules of ammonium acetate (only for Mo132) ν(Fe-OH) ν(Mo=O) ν(Mo-O-Mo)/or ν(O-Mo-O) ν([Mo/Fe]-O-Mo) ν(Mo-μ2O-Mo) or ν(Mo-μ3O-Mo)

1123w

1105vw/1050vw

−

− 984 s, 967vs 957vs/ 898 s − 789 s, 743 s, 633 s 554 s − 490 m, 425w, 406w

− 968 s 938 s/854 m − 795 s, 724 s, 634 m 573 m, 512 m − 472 m, 451w, 439w, 419w

1045w, 1032w 965 s/ 945 s, 849 s 749vs 624 m

M. I. Ivanovskaya et al. [45] G. Guzman et al. [46] W. Dong and B. Dunn [47] − −

547 s, 435w −

G. Guzman et al. [46] G. Guzman et al. [46] −

νas(COO–) coordinated to metal

–

δ(O-Mo-O) δ([Mo/Fe]-O-Mo δ(Mo-μ2O-Mo) or δ(Mo-μ3O-Mo)

L. J. Bellamy [40] L. J. Bellamy [40] L. J. Bellamy [40] L. J. Bellamy [40] L. J. Bellamy [40] L. J. Bellamy [40], R. E. Kagarise [41], E. Spinner [42]

E. L. Wagner and D. F. Horning [43] L. J. Bellamy [40], R. E. Kagarise [41], E. Spinner [42] L. J. Bellamy [40]/ or E. Spinner [42] A. Müller et al.26 , E. Spinner [42], K. Ito and H. J. Bernstein [44]

Shorthand notation: vs – very strong; s – strong; m – medium; w – weak; vw – very weak; sh – shoulder; μ2O/μ3O – bridged oxygen atom connected with two or three molybdenum respectively. 292

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Fig. 7. Temperature variation of IR (a) and Raman (b) spectra of Mo72Fe30 obtained after annealing at isothermal conditions.

initial POM structure and the ferric molybdate phase-nuclei is observed. Summarizing isothermal and in situ spectra measurement, we defined that the nanocluster skeleton bands, which are the most sensitive to beginning of the POM decomposition, are 452, ca. 952 and 969 cm−1. As expected, all these bands have a totally symmetric type (see Table S1 in Supporting Information). Since the spectral bands corresponding to destruction products’ overlap with Ag-mode at 952 cm−1 of the nanocluster (Fig. 8a), in practical applications for control the nanocluster integrity the usage of bands at 452 and 969 cm−1 is more convenient. Finally, one of the main sign of POM’s decomposition is appearance of new strong wide band at 768 cm−1 corresponding to ferric molybdate [34].

Thus, Fig. 7 shows that Mo72Fe30 does not save integrity above 100 °C. However, the preservation of Mo72Fe30 structure under boiling water condition in the presence of excess of acetic acid was established in research [48] using IR data only. In our opinion, the proof of POM structure integrity requires using both IR and Raman spectroscopies. Furthermore, in agreement with symmetry selection rules, IR spectrum should not be analyzed partially – only complex approach including band set and spectrum shape analysis allows the identification of the structure type. Probably, the presented IR spectrum in reference [48] points out to coexisting of Mo72Fe30 structural motive and, also, destruction products. The same situation, probably, is realized in two other studies [19,49] dedicated to developing a new fast self-assembly 293

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Fig. 8. (a) Variation of Raman spectra of Mo72Fe30 during in situ destruction under the laser beam (633 nm), and (b) Raman spectra of Mo72Fe30 and its destruction products.

in range 1045–1032 cm−1 attributed to stretching vibrations of Fe-OH bonds become sharper, and new wide relatively intensive bands at 1147 cm−1 (also corresponded to Fe-OH bonds [45]) and at 1705 cm−1 (characteristic for C]O∙∙∙H bond in carboxyl group [40]) appear. Further annealing at 150 °C leads to loss of characteristic shape of the POM spectrum in the region 1000–400 cm−1, and the structure of Fe-OH units (as follows from the spectral region 1000–1200 cm−1) significantly changes. Annealing at 200 °C leads to appearance of the band at 1668 cm−1 corresponding to additional type of C]O group involved into strong hydrogen bonding. A new wide band with maximum at 1136 cm−1 can be attributed to stretching of Fe-OH group in different kinds of intermediate destruction products. Band at 547 shifts to 573 cm−1. Finally, as was reported earlier [21], the significant loss of water from nanocluster happens during the Mo72Fe30 heating (see Fig. S10 in Supporting Information).

method for Mo72Fe30 which is in contrast with well-known two-stepped synthesis of this POM based on the Mo132 [7]. The carried analysis of variation of IR spectra during POM’s thermal decomposition is well correlated with Raman data (Fig. 7). Annealing of Mo72Fe30 at 100 °C during 90 min leads to significant broadening of low frequency bands, widening 547 cm−1 band and decreasing of intensity of bands 749, 965/945 cm−1 and 624 cm−1. Probably, the widening of all low frequency bands (below 1000 cm−1) is a result of POM partial decomposition leading to formation of different disconnected structural fragments having the same set of bond types. The apparent shifting of the band at 749 cm−1 to 771 cm−1 is a result of spectrum envelope distortion by the wide band with maximum at ca. 816 cm−1 due to appearance of the destruction products. This band contributes to growth of envelope intensity near the band at 849 cm−1 and also leads to slight seeming shifting of its position to 852 cm−1. In addition, bands 294

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Fig. 9. Temperature variation of IR (a) and Raman (b) spectra of Mo132 obtained after annealing at isothermal conditions.

From practical point of view the positions of bands at 749 and 547 cm−1, and the presence of band 624 cm−1 can be used for characterization of POM’s structure decomposition. The disappearing of 749 cm−1 band and appearing of 816 cm−1 one are clear signs of POM full destruction. Mo132: In contrast to Mo72Fe30, Mo132 possesses the most stable structure among all discussed nanoclusters. Annealing at 100 °C during 90 min did not lead to significant changes in neither IR nor Raman spectra (Fig. 9). Raman spectra of Mo132 annealed at 150 °C possess minor changes (Fig. 9), thus demonstrating the preservation of the Keplerate structure

motive. In situ annealing of Mo132 shows fast (in first 30–50 s of laser irradiation) transformation between POM’s motive to nuclei of MoO3 phase that corresponds to isothermal heating above 150 °C (Fig. 10a). During the destruction, the intensity of the totally symmetric bands Ag and Hg (at 875 and 376 cm−1, respectively) drastically decreased, whereas the intensity of band at 842 cm−1 did not change significantly. Thus for Mo132, as well as for Mo72Fe30, totally symmetric bands possess the highest sensitivity to POM decomposition. When intensities of bands at 875 and 842 cm−1 are close to each other, then the motive of Keplerate structure exists only. The minor changes of band at 842 cm−1 at early stages of decomposition for both Keplerates also confirms the 295

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Fig. 10. (a) Variation of Raman spectra of Mo132 during in situ destruction under the laser beam, and (b) Raman spectra of Mo132 and its destruction product MoO3.

suggested attribution of this band to the vibrational motion of {Mo6}. IR spectrum measured after Mo132 isothermal heating up to 100 °C is shown in Fig. 9a. The obvious change with respect to initial composition is significant intensity decreasing of bands at 634, 512, 472 cm−1. The intensity ratio I(968 cm−1)/I(795 cm−1) growth is caused by the appearing of destruction products band at 974 cm−1. New bands appeared at 1757 and 1255 cm−1 are assigned to C]O stretching and to rocking in plane vibrations of C-CH3 fragments, respectively (for more details about the stretching vibration of the carboxylate group in the nanoclusters see Section S2 in Supporting Information). Besides, the intensity of the existing bands 1712 and 1026/1047 cm−1, which are also attributed to acetate ions (Table 2), increased with heating. The joint presence of signals of C]O and C-CH3 groups, belonged to initial (before annealing) and to current (after annealing) structure, points out the co-existing of two types of acetic ions with different coordination types during POM’s annealing. Isothermal annealing at 150 °C resulted in widening of the Keplerate skeleton’s characteristic bands similar to

those for Mo72Fe30. This probably points to active decomposition process of Mo132 into the POM motive at this stage. Furthermore, the peak at 1130 cm−1 and the diversity of bending modes of C-CH3 fragment were observed in spectral range of 1000–1300 cm−1. The appearance of new intensive bands at 1648 cm−1 and 1130 cm−1, attributed to NH2 group bending and rocking mode of NeH bond in NH2 respectively [50], indicates the presence of acetamide molecules. The additional 1670 cm−1 band, which is typical for acetamide and originate from the mixed nature of stretching vibrations of hydrogen bonded C]O group, CeN stretching or NH2 bending vibrations[50], appears after annealing under 200 °C at slightly shifted position 1687 cm−1. Appearance of acetamide as thermal destruction product of Mo132 is in line with previous assumptions made on the results of TGA/DTA-MS study in gas phase, where the fragments of ionized acetamide and acetonitrile molecules were observed [17]. In our work the registered bands of acetamide are shifted with respect to the reference data [17] probably due to acetamide adsorption on the inorganic destruction 296

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Fig. 11. Temperature variation of IR (a) and Raman (b) spectra of Mo138 obtained after annealing at isothermal conditions.

products of POM. Furthermore, during the heating from 100 °C to 200 °C, significant loss of weak-bonded and structural water molecules was observed (see Fig. S11 in Supporting Information) that is also in agreement with TGA/DTA-MS data [17]. After 200 °C several new bands at 3430 and 3350 cm−1 appear, which can be assigned to different types of structural water in decomposition products or to stretching of NeH bound in acetamide [50]. At this temperature, no traces of Keplerate Mo132 integrity were found in the spectra. Thus, the most obvious sign of Mo132 decomposition is disappearing of bands at 634, 512 and 472 cm−1.

Mo72Fe30 and Mo132. For Keplerate Mo132 the complete decomposition can be achieved at temperatures above 150 °C, whereas for iron-molybdenum POM this is close to 100 °C. It was expected that lacunary nanocluster Mo138 does not possess the high thermal stability. However, the Mo138 was chosen for our study because it is a potential candidate for wide-range catalytic applications due to the larger concentration of MoV/MoVI-centers per POM as compared to non-defective toroidal clusters Mo154 or Mo176. It is worth nothing, that first-order Raman spectra of initial Mo138 measured here by 488 nm laser (Fig. 2) differs from those in published data (bands at 798vs, 531 s, 465 s, 322 s, 215 s cm−1 [1]), where the Hyper-Raman spectrum, obtained with laser 1064 nm providing the intervalence charge transfer of molybdenum (MoV → MoVI) [23], was

3.2.2. Toroidal POM – Mo138 Thermal stability of the toroidal POM is placed between those of 297

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stretching vibrations of bridged bonds Mo-O (band at 633 cm−1) and bending vibrations of Mo-O-Mo bonds (band at 554 cm−1). These bands can be used to control the integrity of Mo138 structure. Observation of full set of characteristic bands of Mo138 in the spectrum at this temperature (100 °C) indicates the preserving the POM structure motive. After annealing at 200 °C the formation of acetamide was revealed by the presence of characteristic bands at 1683 and 1640 cm−1 [50] (Fig. 11a). The appearing of acetamide during the Mo138 destruction is well correlating with TGA/DTA-MS data obtained in previous research [18]. Similar to Keplerate Mo132, heating up to 150 °C of Mo138 leads to appearance of new signals from C]O and C-CH3 groups in addition to signals of existing acetate ions. This also indicates the formation of two types of acetate ligands in Mo138 structure, which can possess different coordination. The significant loss of weak-bonded and structural water molecules was observed for Mo138 during the heating from 100 °C to 200 °C. This is typical for all considered POM [17,20,21] (Figs. S10-S12 in Supporting Information). All Raman and IR spectral bands characteristic for POMs integrity are collected in Table 3. Based on the above-discussed results and early performed TGA-DTA-MS study [17,20,21] we may draw up a conclusion concerning the common mechanism of giant nanocluster POM thermal decomposition. For Mo132, Mo72Fe30 and Mo138 crystals, the limit of thermal stability of their structures is no higher than the 150 °C, as it was shown above. At this temperature range (25–150 °C), the POM decomposition is presented by the main process - endothermic stepwise loosing of weakly bonded water (100–200 °C) (see Introduction). During the further heating of the sample, we may emphasize the amorphization stage of the initial destruction products and next stage of new phases’ nucleation that was observed in our in situ Raman experiments. Thus, the primary destruction stage, which is general for studied giant POMs, is initiated by the dehydration process.

Fig. 12. Variation of Raman spectra of Mo138 during in situ destruction under the laser beam (488 nm).

presented. The measured first-order Raman spectrum of Mo138 contains the abovementioned Hyper-Raman bands at slightly shifted positions (820 s, 532 m, 487vs, 322 m, 217 s cm−1), additional bands not described in reference [1] (439 m, 372w, 239 s), which were also observed in Hyper-Raman spectrum for toroidal POM [23], and new bands at 961 s, 717 s, 653 s cm−1. The latter new bands are in good correlation with IR-spectrum and can be assigned to stretching of terminal oxygen (Mo = O – 961 s cm−1) and of the bridge bonds (MoO-Mo – 717 s and 653 s cm−1). Raman spectra measured after isothermal annealing of Mo138 are shown in Fig. 11. Relative intensity of the strongest band at 487 cm−1 decreases during the POM heating, but this band still exists at 150 °C after significant decomposition of POM as follows from the temperature variation of IR spectra (Fig. 11a). In situ Raman measurements (Fig. 12) show similar behavior for bands at 217/239 and 961 cm−1. Consequently, the most temperature sensitive Raman bands of Mo138 are 532, 439, 372 and 322 cm−1. In accordance with earlier published data [18], the final destruction product of Mo138, molybdenum (VI) oxide, is the same as for Mo132 (Fig. 12). In IR spectra, bands corresponding to toroidal structure of Mo138 can still be observed after heating up to 100 °C, whereas after annealing at 150 °C the only residual skeleton bands of Mo138 are observed (Fig. 11a). Heating of Mo138 up to 100 °C leads to essential decreasing of intensities of complex band at 984–957 cm−1 and bands at 633 and 554 cm−1. First band is attributed to stretching vibrations of terminal Mo = O or O-Mo-O/Mo-O-Mo bonds (Tab. 2), whereas the others – to

4. Conclusions Joint usage of IR and Raman spectroscopy including polarized measurements allowed us to improve the attribution of vibration bands and to reveal Ag and Hg modes and substructural build units’ vibrations common for all studied nanoclusters: Mo132, Mo138 and Mo72Fe30. For Keplerates Mo132 and Mo72Fe30 the Hg-mode breathing vibrations including μ3-O atoms are placed at 376 and 452 cm−1 respectively. The second type breathing vibrations, which correspond to Ag-mode and include the terminal oxygens, are placed at 875 and 952 cm−1 for Mo132 and Mo72Fe30 respectively. The vibration appearing at 842 cm−1 for Keplerates and at slightly shifted position 820 cm−1 for toroidal POM is attributed to common structural block {Mo6}. The Raman signals from fragment {Mo2O7(H2O)}2{H2Mo2O8(H2O)} located in the inner cavity of iron-molybdenum POM were indicated at 586 and 909 cm−1 with high values of depolarisation ratio as compared to Keplerate skeleton vibrations. This attribution allowed us to perform detailed study of the nanoclusters thermal destruction under isothermal and in situ annealing, and to establish main spectral features of POMs’ decomposition. In the Raman spectra of Keplerates the most sensitive bands to structural integrity were assigned to totally symmetric mode. Determination of the destruction products of POMs Mo138 and Mo132

Table 3 The manifestation of POMs’ destruction in IR and Raman spectra. Disappearing IR bands, cm−1

Appearing IR bands, cm−1

Disappearing Raman bands, cm−1

Appearing Raman bands, cm−1

Mo72Fe30 Mo132

749, 624, 547(shf573), 634, 512, 472

1147, ca.1705–1710 1648 and 1687 (coexisting)

Mo138

984/967/957 (complex band), 633, 554

1725(sharp), 1640, 1683

452*, ca.952*, 969* 875* and 376* (as compared with band at 842) 322, 372, 439, 532

768 The bands of Mo132 or Mo138 and destruction product – MoO3 do not appear at a time

*Totally symmetric mode; shf – shift to. 298

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was performed and the formation of acetamide molecules was revealed that confirms the assumption made in previous TGA-DTA-MS studies. The comparison of performed vibrational analysis with TGA-DTA-MS data allowed us to establish the dehydration process as the primary stage for thermal decomposition of all-studied nanocluster. That obtained information can be used to control the nanocluster integrity or their isostructural analogues, and to structurally study the POM-based hybrid materials using vibrational spectroscopy as a fast and sensitive technique.

[13]

[14]

[15]

Conflicts of interest

[16]

There are no conflicts to declare. Acknowledgements

[17]

This work was performed in frames of State Task by the RF Ministry of Education and Science (project no. 4.6653.2017/8.9), and Competitiveness Enhancement Program of Ural Federal University (founded by the Government of the Russian Federation Act 211, contract № 02.A03.21.0006). We express our gratitude to “Russian Foundation for Basic Research” which supported these researches (project number 16-33-00570). The equipment of Ural Centre for Shared Use “Modern nanotechnology” Ural federal university was used.

[18]

[19]

[20]

Appendix A. Supplementary data

[21]

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ica.2019.01.016. Notes and references

[22]

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