Communication pubs.acs.org/IC
Isolation of Blackberry-Shaped Nanoparticles of a Giant {Mo72Fe30} Cluster and Their Transformation to a Crystalline Nanoferric Molybdate Raju Mekala,†,‡ Sabbani Supriya,*,‡ and Samar K. Das*,† †
School of Chemistry, University of Hyderabad, P.O. Central University, Hyderabad 500046, India School of Physical Sciences, Jawaharlal Nehru University, New Mehrauli Road, New Delhi 110067, India
‡
S Supporting Information *
logical sciences, materials science, and even mathematics.4−17 The stability of this compound has extensively been studied in its solid state18 as well as in its solution.19,20 An amorphous form of this cluster-containing compound was recently reported by the pioneering group of Müller and co-workers.21 The synthetic procedure21 for this amorphous {Mo72Fe30} compound is quite comparable to that of ferrimolybdite, synthesized by Kerr et al., who also isolated a yellow amorphous substance in the course of synthesizing a yellow coating mineral (ferrimolybdite) frequently found on ores of molybdenum.1 What is this yellow amorphous substance in Kerr et al.’s mineral synthesis, a water suspension of which results in the formation of ferrimolybdite upon longstanding or heating? In the report, we succeeded in answering this by achieving an instantaneous aqueous synthesis of a {Mo72Fe30}-containing compound as an amorphous substance. We have explored the nanomorphology of this amorphous substance by demonstrating the formation of blackberry-like nanoparticles, formed by the aggregation of {Mo72Fe30} cluster units, which further aggregate to a macroporous material. We have also studied the thermal stability of this nanomaterial to understand the formation of synthetic ferrimolybdite relevant to the evolution of ferrimolybdite mineral in nature. When an aqueous solution of sodium molybdate, acidified with acetic acid, was treated with ferric chloride, an instantaneous yellow precipitation was observed in good yield [see the Supporting Information (SI) for its detailed synthesis]. The IR spectrum of this substance is comparable to that of crystalline [Mo 72 Fe 30 O 252 (CH 3 COO) 12 {Mo 2 O 7 (H 2 O)} 2 {H 2 Mo 2 O 8 (H2O)}(H2O)91]·150 H2O (see Figure S1 in the SI for the IR spectrum). However, this compound is not crystalline, as reflected by the amorphous nature of its powder X-ray diffraction (PXRD) pattern (see the SI). When this yellow amorphous substance is subjected to field-emission scanning electron microscopy (FESEM) measurement, the pertinent image shows aggregated nanosized particles (Figure 2a,b) having sizes in the range of 30−100 nm. This observation does have significant relevance with recent past work of Liu et al. on assembling giant {Mo72Fe30} clusters in an aqueous solution to blackberry-shaped nanovesicles.22 More specifically, in an aqueous solution, a {Mo72Fe30} cluster acts as a weak nanosized inorganic acid that can be deprotonated to different extents, depending on the pH of the concerned aqueous solution and the
ABSTRACT: When an aqueous solution of sodium molybdate is added to an aqueous solution of ferric chloride, acidified with acetic acid, a giant {Mo72Fe30} cluster is instantaneously formed as the amorphous substance Na 2 [Mo 72 Fe 30 O 252 (CH 3 COO) 4 (OH) 16 (H2O)108]·180 H2O (1). Compound 1 consists of aggregated nanovesicles of {Mo72Fe30} clusters, as confirmed by field-emission scanning electron microscopy and transmission electron microscopy images of 1. An aqueous suspension of 1 upon moderate heating results in the formation of crystalline nanoferric molybdate, which gives insight into understanding the formation of a yellow coating mineral, ferrimolybdite, frequently found on the ores of molybdenum. n 1963, an original research paper entitled “The nature and synthesis of ferrimolybdite” was published by Kerr et al.1 This work turns out to be very important today as far as modern inorganic research on giant metal-oxide-based spherical clusters is concerned. [Mo72Fe30O252(CH3COO)12{Mo2O7(H2O)}2{H2Mo2O8(H2O)}(H2O)91] is such a giant metal-oxide-based spherical cluster in which 12 pentagonal [(Mo)Mo5O21(H2O)6]6− units are linked to each other by 30 {FeIIIO5(H2O)}3+ linkers (Figure 1). The concerned compound
I
Figure 1. Polyhedral and stick representation of the {Mo72Fe30} cluster.
[Mo 72 Fe 30 O 252 (CH 3 COO) 12 {Mo 2 O 7 (H 2 O)} 2 {H 2 Mo 2 O 8 (H2O)}(H2O)91]·150H2O was isolated as a crystalline material more than a decade ago.2 The importance of this particular compound is reflected in the fact that more than 80 research articles based on only this compound have been published to date.3 This compound has attracted immense interest and attention because of its importance in diverse disciplines, e.g., chemical science, molecular physics, magnetochemistry, bio© XXXX American Chemical Society
Received: September 21, 2016
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DOI: 10.1021/acs.inorgchem.6b02292 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
presence of a {Mo72Fe30} icosahedral cluster in amorphous 1 was confirmed by comparing the Raman spectrum of the present amorphous substance 1 with the characteristic Raman spectra of crystalline [Mo 72 Fe 30 O 252 (CH 3 COO) 12 {Mo 2 O 7 (H 2 O)} 2 {H2Mo2O8(H2O)}(H2O)91]·150H2O (rhombohedral crystals: discrete {Mo72Fe30} clusters)2 and crystalline [H4Mo72Fe30O254(CH 3COO) 10{Mo2 O7(H2O)}{H2 Mo2 O8(H2O)} 3(H 2O) 87 ]· 80H2O (platelike crystals: {Mo72Fe30} clusters link to twodimensional layers),23 as shown Figure S2a in the SI. The most intense line at ∼950 cm−1 (Ag-type vibration) in the Raman spectra (Figure S2a in the SI) of all three compounds is due to the totally symmetric ν(MoO) in-phase breathing-type vibration of the {Mo72Fe30} cluster.22 It is already well-established that each {Mo72Fe30} cluster unit is formed by 12 [(Mo)Mo5O21(H2O)6]6− pentagons and 30 {FeIIIO5(H2O)}3+ linkers.2 Their combination creates 18 positive (+) charges, which demand 18 negative (−) charges for charge compensation. From analyses, we found four acetate anions and two sodium cations (repeated molybdenum analyses correspond to 72 molybdenum ions per formula unit). After these sodium and acetate analyses were taken into consideration, there is a shortage of 16 negative charges. We thus took 16 OH− anions per formula unit to compensate for the charge balance. This indicates that some of the iron-coordinated water molecules of the nascent {Mo 72 Fe 30 } cluster, just formed in an instantaneous aqueous synthesis, get deprotonated, resulting in an increase of the charge density on the cluster surface, causing aggregation to nanovesicles. Generating this charge density on the cluster surface by deprotonation of iron-coordinated water molecules is essential for the {Mo72Fe30} cluster units to selfassemble in an aqueous solution into blackberry-like supramolecular structures with countercations placed between cluster units, as demonstrated by Liu and their co-workers,22 who dealt with a dilute aqueous solution (0.5−1.0 mg/mL) of the crystalline {Mo72Fe30}-cluster-containing compound. In contrast, we are dealing with an aqueous synthesis of very high concentrations of the reactants (the addition of 2.09 g of FeCl3· 6H2O dissolved in 5 mL of water to 25 mL of an aqueous solution of 3.0 g of Na2MoO4·2H2O acidified with 25 mL of 100% acetic acid results in the formation of 2.5 g of 1 instantaneously), whereby the anticipated concentration of the {Mo72Fe30}cluster-containing compound is 50 mg/mL (it is not possible to determine the concentration of the soluble {Mo72Fe30} cluster in this case because once the cluster is formed in the solution, it precipitates instantaneously as an amorphous 1; thus, we calculated this value of 50 mg/mL from the yield of 1). This high concentration (50 mg/mL) plays an important role in the isolation of aggregated nanoblackberries (compound 1). Liu demonstrated an unusually slow (several months) self-assembly of {Mo72Fe30} clusters in a very dilute aqueous solution (0.185 mg/mL) of crystalline [Mo72Fe30O252(CH3COO)12{Mo2O7(H 2 O)} 2 {H 2 Mo 2 O 8 (H 2 O)}(H 2 O) 91 ]·150H 2 O in forming vesicles.24 When this concentration is relatively larger (0.5−1.0 mg/mL),22 the formation of blackberries is much faster. Thus, there is a direct relationship between the concentration of the {Mo72Fe30} cluster entities and the rate of its aggregation into supramolecular vesicles: the higher the concentration of the discrete cluster, the faster its aggregation, as shown by Liu and his group over the years.22,24,25 Because we are in the synthesis front (in contrast to the solubility limitation of the crystalline {Mo72Fe30} compound), we could take the large concentration of reactants corresponding to the formation of 50 mg/mL of {Mo72Fe30}-cluster-containing compound 1, which is 50−100
Figure 2. (a and b) FESEM and (c) TEM images of amorphous {Mo72Fe30} compound 1. Its corresponding SAED is shown in the inset. (d) HRTEM image of {Mo72Fe30} vesicles in compound 1.
cluster surface charge densities, to form blackberry-type nanovesicles of diverse sizes.22 In the present study, when an aqueous solution of sodium molybdate is acidified with acetic acid, the pH reaches to ∼3.0. The subsequent addition of ferric chloride to this solution causes immediate precipitation of an amorphous substance, and the pH of the reaction mixture/ suspension (after precipitation) drops to ∼2.0. This lowering of the pH in the solid−solution suspension, after the formation of an amorphous substance, can be explained in two ways: (i) the addition of FeCl3·6H2O to an aqueous solution may cause the solution to be acidic (because it generates HCl), and (ii) the {Mo72Fe30} clusters, formed immediately after the addition of FeCl3·6H2O, may be deprotonated to some extent to form supramolecular structures (blackberry-type nanovesicles), resulting in H3O+ ions. The second probability is more likely because repeated analyses of this amorphous substance recommend its formula as Na2[Mo72Fe30O252(CH3COO)4(OH)16(H2O)108]· 180 H2O (1) having 16 OH− groups. The amorphous nature of this compound is confirmed not only from PXRD studies (Figure S3a in the SI) but also from high-resolution transmission electron microscopy (HRTEM) studies (Figures S7 and S8 in the SI), which do not show any kind of clear strips (Figure 2). The relevant selected-area electron diffraction (SAED) of assynthesized compound 1 does not show any kind of diffraction (inset, Figure 2c). To our surprise, this substance contains only four acetate ligands per formula unit (we have done repeated CHN analysis), even though we have used a sufficient amount of acetic acid in the synthesis (see experimental section S1 in the SI). Likewise, we could not ignore the presence of a small percentage of sodium (0.25%), which we obtained by multiple measurements. The following results (Table 1) of elemental analysis and thermogravimetric analysis (TGA) studies are in agreement with the formula for 1 of the title compound. The Table 1. Elemental Analyses and Water of Hydration
calcd obsd
C%
Na % (ICP)
Fe % (ICP)
Mo % (ICP)
H2O % (TGA)
0.52 0.47
0.25 0.25
9.13 9.15
37.63 36.65
16.65 16.70 B
DOI: 10.1021/acs.inorgchem.6b02292 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry times more than that (0.5−1.0 mg/mL) used by Liu et al.22 Because of this large concentration, the aggregation tendency of the {Mo72Fe30} cluster, formed in solution, is so high that each cluster accommodates only four acetate ligands (despite the presence of a large excess of acetic acid in the relevant synthesis) to form the positively charged hypothetical cationic cluster [Mo72Fe30O252(CH3COO)4 (H2O)124]14+, which can be deprotonated to [Mo72Fe30O252(CH3COO)4(OH)16(H2O)108]2− instantaneously, which aggregates to blackberries in the form of amorphous 1. Coordination of more acetate ligands would not facilitate deprotonation as well as aggregation. Aggregation, followed by amorphous isolation, is so instantaneous that the concerned system does not get sufficient time to make an order arrangement of the constituent {Mo72Fe30} clusters, i.e., a crystalline product. In the present work, we observe 2-fold aggregations: first, an assembly of {Mo72Fe30} clusters to form nanoblackberries with particle sizes ranging from 30 to 100 nm and then agglomeration of these nanoparticles resulting in the formation of a macroporous nanomaterial having open pores on the surface (Figure 2a,b; see also Figure S4 in the SI). The surface porosity of these aggregated nanoparticles is characterized by a (N2) gas adsorption study (Figure S12 in the SI). The aggregation of nanoblackberries (Figure 2a,b) prompted us to measure the ζ potential of this isolated nanomaterial in its sonicated aqueous suspension, and it was found to be −17.6 mV (Figures S16 and S17 in the SI), indicating negative surface charges of the nanoblackberries. Thus, these nanovesicles having negative surface charges undergo second-generation aggregation in water to form even bigger supramolecular assemblies (with a hydrodynamic radius of ∼300 nm), as found in the DLS studies (Figure S18 in the SI). An aqueous suspension of compound 1 (amorphous) upon heating at 70 °C for 36 h yields the crystalline nanorosettes of ferric molydate, as shown in Figure 3 (right). When the same
long-standing or moderate heating transformed to a crystalline ferric molybdate, as confirmed by PXRD studies.1 Thus, our present work indicates that the amorphous compound obtained by Kerr et al. in 1963 was nothing but the amorphous substance 1 of the present study. In nature, iron is present mostly in a 3+ oxidation state, oxophilic molybdenum is available as molybdate, and acetic acid is excreted by the bacteria acetobacter genus and Clostridium acetobutylicum; these acetic acid bacteria are found not only in foodstuffs and water but also universally in soil. Therefore, there is no wonder that an amorphous substance consisting of aggregated giant {Mo72Fe30} clusters can be formed on the surface of molybdenum ores in nature and with time this amorphous substance of Keplerates transforms into a ferrimolybdite mineral. The amorphous substance of giant {Mo72Fe30} clusters can be described as a kinetically controlled product, and the crystalline nanoferricmolybdate can be called a thermodynamically controlled product. In summary, the present study on Müller’s Keplerate not only unravels the nanomorphology and porous nature of the instantaneously isolated amorphous substance but also offers insight into understanding the formation of a natural mineral (ferrimolybdite) frequently found in molybdenum ores. Currently, we are working on this amorphous nanomaterial in terms of its surface charge to understand the aggregation of nanoblackberries (30−100 nm diameter) to a supramolecular structure (a hydrodynamic radius of around 300 nm: a preliminary result) of blackberries.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02292. Experimental details, FESEM and TEM images, IR spectrum, PXRD pattern, SAED studies (TEM), TGA and gas adsorption plots, ζ-potential plots, and DLS plots including ICP and EDAX elemental analysis plots (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Samar K. Das: 0000-0002-9536-6579 Notes
The authors declare no competing financial interest.
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Figure 3. FESEM images of nanoiron molybdate obtained from amorphous substance 1: (left) platelike morphology obtained at 100 °C; (right) rosette-like morphology obtained at 70 °C.
ACKNOWLEDGMENTS S.K.D. thanks Professor Achim Müller for his support of work on his Keplerate systems. We thank SERB, DST, Government of India, for financial support (Project SB/S1/IC-34/2013). S.S. acknowledges SERB, DST, Government of India, for financial support (Project SB/EMEQ- 090/2014). S.S. also is thankful for a grant from UPE-II at Jawaharlal Nehru University sponsored by the UGC. We are thankful to Professor Tushar Jana and Raju Kesinenu for providing us with the DLS data. Finally, we are grateful to Dr. S. Raghunandan, BHEL, Corporate R&D, Hyderabad, India, for the ζ-potential data. We are also thankful for a UPE-II grant at the University of Hyderabad.
suspension is refluxed (100 °C) for 36 h, platelike morphology of nanoferric molybdate is obtained, as shown in Figure 3 (left), as was obtained from the crystalline {Mo72Fe30} compound.20 We repeated this experiment of obtaining nanorosettes (at 70 °C) and nanoplates (at 100 °C) several times (section 9, Figures S19 and S20 in the SI). This transformation of a amorphous {Mo72Fe30} compound to a crystalline ferric molybdate does have a strong connection with Kerr et al.’s original work1 describing the laboratory synthesis of a yellow coating mineral (ferrimolybdite), often found on molybdenum ores in nature. In their laboratory synthesis, Kerr et al.1 first obtained a yellow amorphous compound from an aqueous reaction mixture of sodium molybdate, ferric chloride, and acetic acid, which upon
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REFERENCES
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DOI: 10.1021/acs.inorgchem.6b02292 Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.inorgchem.6b02292 Inorg. Chem. XXXX, XXX, XXX−XXX
