Hexagonal MIn2S4 (M = Mn, Fe, Co): Formation and Phase Transition

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Hexagonal MIn2S4 (M = Mn, Fe, Co): Formation and Phase Transition Yong-Fang Shi,†,‡ Yue Wang,† and Li-Ming Wu*,†,‡ †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’s Republic of China ‡ Key Laboratory of Design and Assembly of Functional Nanostructures, Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’s Republic of China S Supporting Information *

ABSTRACT: Bulk MIn2S4 (M = Mn, Fe, Co) only have a thermodynamically stable cubic phase known to date. In this paper, new hexagonal phases (MnIn2S4, FeIn2S4, CoIn2S4) are first discovered as nano petals of microsphere flowers by a solventless method. As the anneal temperature increases, the hexagonal to cubic phase transformation for MnIn2S4 and FeIn2S4 has been observed. The theoretical analyses imply that such a phase transition can be explained by the competition between the proportions of the surface energy and the bulkphase energy to the total energy. These results also explain clearly why the novel hexagonal MIn2S4 (M = Mn, Fe, Co) phase is only stable as nanometer-scale structures.

1. INTRODUCTION

packed sulfur ions and the distribution of the cations. The layer structure of all these polytypes is essentially based on a repeated sequence of S−Intet−S−Inoct−S−Zntet−S along the c-axis (Figure 1, left). Besides, the hexagonal ZnIn2S4 forms several polytypes that differ in the stacking sequence and the cation distributions.10 The solventless method is a facile approach to prepare binary chalcogenides (such as Bi2S3,11 Cu2S,12−14 CuS,15 PbS16) and metallic elements (e.g., Ag,17 Bi18), but its application on ternary chalcogenides has not been explored yet. Previous studies reveal that the morphology of the product is controlled by the structure of the crystalline precursor.19 Thus, by adjusting/selecting the crystal structure of the precursor, one can obtain a nano product with expected morphology.20 Here, we further consider, when selecting a precursor, instead of a solid state crystalline precursor, one can select a solution dissolving different types of metal salts with the designed ratio. Such a solution can be regarded as a “pre-mixing at the molecular level” of the heterometallic precursors. After the evaporation of the solvent, the resulted precursor will be fired to generate the product, during which the right stoichiometry and the good mixing of the precursor will benefit the formation and growth of the product from the solid state chemistry point of view. Guided by this new thought, MIn2S4 (M = Mn, Fe, Co) phases adopting the hexagonal ZnIn2S4-type structure have been discovered as nanolayers constructing self-supporting flower-like microspheres by the pyrolysis of the well-mixed M(OTC)2 and In(OTC)3 (OTC = dioctyldithiocarbamate1,15)

Semiconductor thioindiates MIn2S4 (M = transition metals) have attracted considerable attention because of their interesting properties, such as water reduction and dye degradation,1−5 and magnetic, optical, and dielectric properties.6−8 Up to now, MIn2S4 compounds are known as cubic phase constructed by close packed sulfur atoms with M atoms tetrahedrally coordinated and In atoms octahedrally coordinated by S atoms (Figure 1, right).9 The only exception is ZnIn2S4 showing two isomeric structures (cubic and hexagonal phases). Hexagonal ZnIn2S4 forms several different polytypic structures due to the different stacking sequence of the close

Received: July 16, 2013 Revised: September 4, 2013

Figure 1. Crystal structures of hexagonal and cubic ZnIn2S4. © XXXX American Chemical Society

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precursors at 180−380 °C. This should be the first report on the hexagonal phase of this composite. The hexagonal to cubic phase transition as well as several key experimental parameters are discussed, and the intrinsic formation reason has also been studied with the aid of VASP analyses.

composition, size, and morphology of the as-synthesized nanoproducts, respectively. The XRD patterns were collected with the aid of a PANalytical X′Pert Pro diffractometer or a Rigaku DMAX 2500 diffractometer with Cu Kα radiation at room temperature. The XRD patterns of hexagonal MIn2S4 (M = Mn, Fe, Co) were simulated by using Diamond 3.0 and CCDC Mercury 1.4.2 programs. EDX analyses were performed on a carbon-film-coated Cu grid with the aid of a JEM 2010 transmission electron microscope equipped with an Oxford INCA spectrometer. The elemental chemical analyses were performed by Vario EL III (Elementar Co.). The ICP analyses were performed on the Ultima 2 inductively coupled plasma optimal emission spectrometer. The IR spectra were measured by a Nicolet Magna 750 FT-IR spectrophotometer in the range of 800−480 cm−1. Samples were ground with KBr and pressed into transparent pellets for the IR spectra measurement. The DTA−TG measurements were performed by using a NETZSCH STA449C instrument under N2 flow in the temperature range of 30−600 °C with a heating rate of 10 °C/min. 2.5. Computational Details. The total and surface energies of MIn2S4 (M = Mn, Fe, Co) were calculated by the Vienna ab initio simulation package (VASP)21 with plane waves as a basis set. The calculations were performed within the projector augmented wave (PAW) method 22 and the generalized gradient approximation (GGA)23 expressed by Perdew−Burke−Ernzerhof (PBE)24 functional. A plane wave cutoff energy of 350 eV was used. The lattice parameters of bulk-models were taken from experimental results on bulk samples with the internal coordinates fully optimized. The cubic (111)- and hexagonal (001)-surface-models were constructed with about 7−9 layers of atoms cleaved perpendicular to the corresponding directions before a vacuum slab of 12.00 Å in thickness was built on the surface. The first two layers of atoms in the surface models were optimized with internal coordinates of atoms fixed. The Monkhorst−Pack scheme25 with a 8 × 8 × 8 mesh for cubic phase and 8 × 8 × 4 for hexagonal phase were employed, while a 4 × 4 × 1 mesh for cubic (111)-surface-model and 7 × 7 × 7 for hexagonal (001)surface-model were employed, respectively. The detailed data

2. EXPERIMENTAL AND THEORETICAL METHOD 2.1. Chemicals. Manganese acetate tetrahydrate (Mn(CH3COO)2·4H2O), ferrous chloride hexahydrate (FeCl2· 6H2O), cobalt acetate tetrahydrate (Co(CH3COO)2·4H2O), indium nitrate hydrate (In(NO3)3·4.5H2O), carbon disulfide (CS2), potassium hydrate (KOH), ethanol (C2H5OH), chloroform (CHCl3), dichloromethane (CH2Cl2), and acetone (CH3COCH3) were purchased from Shanghai Chemical Co. [CH3(CH2)7]2NH (Alfa, A.R.) was used. Ar gas (99.99%) was purchased from Fuzhou Xinhang Gas Co. All reactants are used as received. 2.2. Synthesis of M(OTC)2 (M = Mn, Fe, Co), and In(OTC)3 Precursors. Precursors were prepared according to the reactions shown in Scheme 1. Below 0 °C, CS2 had reacted Scheme 1. Reactions to Generate MIn2S4

with [CH3(CH2)7]2NH and KOH to produce enough dithiocarbamate salt in ethanol (Reaction 1). Then, an ethanol solution with excess In(NO3)3·4.5H2O was added to generate white suspension In[S2CN(C8H17)2]3 (Reaction 2). After stirring for 5 min, an ethanol solution with an appropriate amount of M(CH3COO)2·4H2O (M = Mn, Co) or FeCl2· 6H2O with a mole ratio of M2+:In3+ = 1:2.5 was added and stirred for another 2 h to ensure the complete reaction to form In(OTC)3 and M(OTC)2 precursors (Reaction 3) and make them mix evenly. A solid was obtained after the rotary evaporation of ethanol and then dissolved in CHCl3 and filtrated. Then, this CHCl3-filtration was rotary evaporated to generate the M(OTC)2/In(OTC)3 precursor, a soil-like solid. Finally, the precursors were washed by acetone. The precursor colors are different: dark purple for Mn-, dark brown for Fe-, and green black for Co-precursor. 2.3. Synthesis of the MIn2S4. The precursor was transferred into a glass tube, then put into a long silica tube (6 cm × 1 m), capped with stops with outlets on both ends, purged with Ar for 5 min first, and heated at 220−280 °C for 1.5−3 h in argon gas (Reaction 4). After heating, the resulted solid was dispersed in CHCl3, then reprecipitated with ethanol to remove the possible byproducts, and then washed with CH2Cl2 and ethanol−water in turn by centrifugation several times to ensure the organics were removed completely and dried in air. Hence no C/H/N has been detected in the final products by the elemental analysis. The MnIn2S4 product is yellow; FeIn2S4 and CoIn2S4 samples are black. 2.4. Characterization. X-ray powder diffraction (XRD), energy dispersive X-ray spectrum (EDX), inductively coupled plasma (ICP) spectra, and scanning electron microscopy images (SEM) were used to characterize the structure,

Table 1. Surface Energy of Cubic and Hexagonal MIn2S4 (M = Mn, Fe, Co) sample

(111) surface energy of the cubic phase (Esurf‑c/10−4 kJ/m2)

(001) surface energy of the hexagonal phase (Esurf‑h/10−4 kJ/m2)

MnIn2S4 FeIn2S4 CoIn2S4

15.57 14.27 14.03

0.04 0.16 0.21

are listed in Tables 1 and 2, and the surface energy is achieved through formula 1. Table 2. Total Energy of Cubic and Hexagonal MIn2S4 (M = Mn, Fe, Co)a

a

B

sample

cubic (Etot‑c/eV)

hexagonal (Etot‑h/eV)

MnIn2S4 FeIn2S4 CoIn2S4

−69.43 −66.26 −63.00

−68.41 −65.30 −62.45

The calculation of Etol is in consideration of magnetism. dx.doi.org/10.1021/jp407067d | J. Phys. Chem. C XXXX, XXX, XXX−XXX

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synthesized MIn2S4 (M = Mn, Fe, Co) samples do not match with the cubic phase reported in ref 26 or the JCPDS database but are similar to that of hexagonal ZnIn2S4.27 Then, the diffraction patterns are simulated with a hexagonal P63mc space group and a = 3.82−3.89(2) Å, c = 25.60(4)−26.40(4) Å following a modified process (Supporting Information).28 The simulated and experimental patterns match well, indicating the as-synthesized samples are hexagonal phase. The stoichiometry and composition of the as-synthesized samples are confirmed by both energy dispersive spectra (EDS, Figure S2) and inductively coupled plasma spectra (ICP, Table S2). Experimental results reveal that loading excess In3+ is necessary, which has an important influence on the phase formation and the morphology development. The SEM images shown in Figure 3a reveal the morphology of marigold-like microspheres with sizes of 3−6 μm. These spheres are built up by interconnected nanolayers (Figure 3b). On going from Mn to Co, the thickness of such petals is about 25 nm. These agree with the observations on CdSe nanocrystals that requires an optimal size to achieve metastability.29 However, the spacing between petals slightly changes. For example, the petals on the FeIn2S4 microspheres are about 200−500 nm apart (Figure 3c), which is wider than that on the CoIn2S4 microspheres (100−200 nm, Figure 3d). Regarding the size dispersion, CoIn2S4 and FeIn2S4 microspheres show the widest and the narrowest size distribution, respectively. It is very interesting that all of our products grow in flower shapes, but why? Our previous work (ZnIn2S4 Nano/ Micropeony)1 suggests that the capping effect of OTC may play an important role for formation of the flower-like morphology. Accordingly, the flower-like morphology of MIn2S4 is thought to be the result of the equilibrium of the anisotropic lamella-growth controlled by the capping reagent

(1)

Esurf is the surface energy, Etot is the total energy, Ecell is the energy of per unit cell of the bulk MIn2S4, “n” refers to the number of unit cell involved in the established model, and S is the surface area of the model.

3. RESULTS AND DISCUSSION 3.1. Structures and Morphologies of MIn2S4 (M = Mn, Fe, Co). As shown in Figure 2, the XRD patterns of the as-

Figure 2. Experimental and simulated XRD patterns of samples synthesized at different temperatures: (a) MnIn2S4, 280 °C; (b) FeIn2S4, 220 °C; (c) CoIn2S4, 280 °C.

Figure 3. SEM images of hexagonal MIn2S4 synthesized at different temperatures: (a) MnIn2S4, 280 °C; (b) magnified image of rectangle area in a; (c) FeIn2S4, 220 °C; (d) CoIn2S4, 280 °C. C

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Figure 4. Middle infrared spectra (the wave range of 800−450 cm−1) of compounds: (a) hexagonal FeIn2S4; (b) cubic FeIn2S4.

Figure 5. (Left) TG-DTA curve for heating the precursors of Fe(OTC)2/In(OTC)3 from 30 to 600 °C under N2 flow. (Right) XRD patterns of FeIn2S4 synthesized at: (a) 220 °C; (b) 250 °C; (c) 280 °C; (d) 310 °C; (e) 330 °C.

5a−c, right); hexagonal and cubic phases coexist around 280 °C (Figure 5c, right), and the cubic phase concentration increases as the temperature increase from 280 to 330 °C (Figure 5c−e, right); at 330 °C, the hexagonal phase nearly disappears, and pure cubic phase is produced (matches well with JCPDS No. 80-0608) indicating the completeness of the phase transition (Figure 5e, right). (3) The third endothermic peak on the TGDTA curve is at about 338 °C, above which the weight loss percentage is constantly 87% (very close to the theoretical proportion of the organic byproduct component, 85%). This means the organic byproducts have volatilized entirely at such a temperature, and no more weight loss is observed at higher temperature. The XRD and TG-DTA analyses (Figure 5) suggest that the hexagonal-to-cubic phase transition of FeIn2S4 starts around 250 °C and completes about 330 °C. The phase-temperature dependence has been illustrated in Figure 6. But the phase transformation does not go simultaneously with a morphology change according to the SEM observations. At 200 °C, hexagonal FeIn2S4 is a stacking-plate accumulation (Figure S3a). At 220 °C, hexagonal FeIn2S4 forms beautiful flower-like spheres (Figure S3b). From 250 to 280 °C, although the hexagonal phase has transformed to the cubic phase (Figure 6), the major morphology is still flower-like microsphere with a small degree of collapse indicated by the appearance of some fragments (Figure S3c). At 310 °C, the cubic phase undergoes overwhelming sphere collapse and the intergrowth and agglomerate of the flakes (Figure S3d). Together with the TG-DTA analyses, it is possible that the maintenance of the microspheres of the cubic phase at 280 °C is related to the

OTC and the isotropic dentrite-growth controlled by the dynamic Ostwald ripening process. 3.2. Different IR Spectra between Hexagonal and Cubic FeIn2S4. Hexagonal FeIn2S4 (Figure 4a) and ZnIn2S4 exhibit similar IR spectra.30 The broad bands in the range of 800−450 cm−1 are assigned as the combination and overtone transitions of the lower frequency fundamental modes. These broad bands of FeIn2S4 (Figure 4a) shifted about 30 cm−1 to the higher frequency than in ZnIn2S4, which is probably because of the mass difference between Zn and Fe. Differently, the cubic FeIn2S4 shows totally different IR spectrum in the same range 800−450 cm−1 (Figure 4b, almost no absorption). The IR data also confirm that the as-synthesized FeIn2S4 is hexagonal phase. 3.3. Phase-Temperature Dependence of FeIn2S4. The TG-DTA curves recorded on the soil-like Fe(OTC)2/In(OTC)3 precursor shown in Figure 5 (left) suggest several processes: (1) from 30 to 150 °C, the endothermic process corresponds to the melting of the precursor to a viscous melt. The first endothermic peak at about 147 °C may relate to the cleavage of the C−S bonds in the precursor and the formation of the hexagonal FeIn2S4 phase according to the previous studies. These observations agree with the XRD results indicating the hexagonal FeIn2S4 phase has already formed at 220 °C (Figure 5a, right). (2) The second endothermic peak at about 240 °C may correspond to the hexagonal-to-cubic phase transition accompanying with the loss of the organic byproduct. Such a hexagonal-to-cubic phase transition is confirmed by XRD studies. In detail, from 220−280 °C, the hexagonal phase concentration decreases as the temperature increases (Figure D

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pure hexagonal MnIn2S4 phases are obtained in the temperature range of 240−280 °C for MnIn2S4; 180−220 °C for FeIn2S4; and 280−310 °C for CoIn2S4. Above 280 or 220 °C, hexagonal MnIn2S4 or FeIn2S4 transforms to cubic phase. A mixture of hexagonal and cubic MnIn2S4 is obtained from 310 to 390 °C. Hexagonal FeIn2S4 completely transforms into cubic phase at 330 °C. Hexagonal CoIn2S4 decomposes to an unknown phase above 330 °C. 3.5. Different Morphology-Temperature Dependence of MIn2S4 (M = Mn, Fe, Co). Different from FeIn2S4 (Figure S3), MnIn2S4 and CoIn2S4 maintain their microsphere morphology better at the high temperature (Figures S7, 8). The TG-DTA curves of MnIn2S4 and CoIn2S4 show the first endothermic peaks locating around 152 and 132 °C, and the weight loss starting around 200 °C (Figure S4). CoIn2S4 starts the C−S cleavage at 132 °C, about 20° lower than those of MIn2S4 (M = Fe, Mn). Thus, CoIn2S4 has the longest temperature interval between the first endothermic peak and the starting of the weight loss, which may allow a relative long time for CoIn2S4 to develop the flower-like microsphere; as a result, the as-synthesized CoIn2S4 microspheres are the most uniform ones among the three MIn2S4 studied.

Figure 6. Phase-temperature dependence of MIn2S4 (M = Mn, Fe, Co). Black hexagon: hexagonal phase; black square: cubic phase; hexagon with a cross: mixture of hexagonal and cubic phases; hexagon with an X-shape: mixture of hexagonal and unknown phases.

existence of the in situ organic byproducts (Figure S3c). After their thorough evaporation above 330 °C, the flower-like microsphere collapses completely. 3.4. Possible Reason of the Hexagonal-to-Cubic Phase Transition. Why does the hexagonal-to-cubic phase transition happen (Figure 6)? May this relate to the different thermodynamic energies between the bulk solid and the nanocrystal? As we know, for a substance, the total energy of includes surface energy and bulk-phase energy that can be denoted as Etot = Esurf + Ebulk. When a substance is of nanometer-scale, its Esurf may take a large proportion of Etot, which should therefore play an important role to determine the formation. Taking MIn2S4 (M = Mn, Fe, Co) as an example, the bulk cubic phase is stabler than the bulk hexagonal phase, because Ebulk‑c is significantly lower than Ebulk‑h (Tables 1 and 2). However, the cubic (111) surface energy (Esurf‑c > 14 × 10−4 kJ/m2) is significantly higher than the hexagonal (001) surface energy (Esurf‑h < 0.3 × 10−4 kJ/m2). This means if a very thin slab is under consideration, the hexagonal slab may be thermodynamically stabler than the cubic slab instead. On the other hand, increasing the thickness of a nano slab increases the Ebulk proportion to the total energy; consequently, the nano hexagonal phase will transform into the thermodynamically stabler bulk cubic phase as the reaction temperature is above some certain point. These VASP energy analyses also agree with the layered structural motif of the hexagonal phase. The layered hexagonal structural (Figure S1) feature suggests a weak van der Waals force between the neighboring layers. Thus the neighboring layers in the hexagonal structure should be easily split. Consistently, the hex-(001) surface energy, Esurf‑h, is very small, and thus a thin (001) slab should be stable. These discussions agree with our experimental observations that all of the hexagonal MIn2S4 nanolayers are about 25 nm in thickness below 280 °C (Figure 3). Similar as binary CdSe nanoparticles,29 in the case of ternary MIn2S4 nanolayers, we presume there is also an optimal size (thickness) to achieve metastability. Whereas the three-dimensional network of the cubic MIn2S4 is constructed by strong covalent bonds with a very high (111) surface energy, and the cleavage of the network along any direction is equally difficult, layered morphology may be hard to obtain. MnIn2S4 and CoIn2S4 flower-like microspheres undergo the similar thermal but different phase transition processes (Figure S4−6). The phase-temperature dependence of MIn2S4 illustrated in Figure 6 shows that the

4. CONCLUSION In summary, for the first time, hexagonal MIn2S4 (M = Mn, Fe, Co) nanolayers constructing the flower-like microspheres have been discovered by a solventless method. Their characteristic XRD patterns and IR spectra confirm the existence. The hexagonal-to-cubic phase transitions of MnIn2S4 and FeIn2S4 and the decomposition of hexagonal CoIn2S4 have been established as a function of temperature. The VASP analyses reveal that the formation of the metastable hexagonal phase is limited by the competition between the low (001) surface energy (Esurf‑h) and the bulk energy (Ebulk‑h) (Etot = Esurf + Ebulk). When the material is of nanoscale, the surface energy dominates the total energy; consequently, the hexagonal MIn2S4 phase only stabilizes as a nanostructure. This work provides a facile approach for the syntheses of metastable MIn2S4 phases, which may shed useful light on the further study on the related systems. As the important first step, successful synthesis will make it possible to explore new phase exhibiting new structural chemistry and properties.

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ASSOCIATED CONTENT

S Supporting Information *

Details of the simulation of XRD patterns, ICP data, XRD, TGDTA, and SEM. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (+86)-591-8370-4774. Tel.: (+86)-591-8370-5401. Present Address

Y. Wang: Chongqing University of Education, Nanan District, Chongqing 400065, People’s Republic of China. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the 973 program (project 2010CB933501), National Natural Science Foundation of E

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China (projects 20973175, 21233009, and 21103190), and the NSF of Fujian Province (2011J05036).

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