Journal of Alloys and Compounds 617 (2014) 93–101

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Journal of Alloys and Compounds

journal homepage: www.elsevier .com/locate / ja lcom

Hydrothermal synthesis, characterization, and magnetic propertiesof cubic MnSe2/Se nanocomposites material

http://dx.doi.org/10.1016/j.jallcom.2014.08.0130925-8388/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +98 361 591 2383; fax: +98 361 555 2930.E-mail address: salavati@kashanu.ac.ir (M. Salavati-Niasari).

Azam Sobhani a, Masoud Salavati-Niasari b,⇑a Department of Chemistry, Kosar University of Bojnord, Bojnord, Islamic Republic of Iranb Institute of Nano Science and Nano Technology, University of Kashan, Kashan P.O. Box 87317-51167, Islamic Republic of Iran

a r t i c l e i n f o a b s t r a c t

Article history:Received 26 June 2014Received in revised form 27 July 2014Accepted 1 August 2014Available online 9 August 2014

Keywords:MnSe2

NanocompositesNanorodsHydrothermalSurfactantHydrazine

The cubic MnSe2/Se nanocomposites were produced under hydrothermal condition, by reduction of SeCl4

to Se and Se2�, and reaction of the reduced selenium with Mn2+ ion during the next step, in the presenceof different surfactants using hydrazine as reductant. The main factors affecting the morphology, the par-ticle size and the phase of the products, such as surfactant, reductant and its amount, reaction temper-ature and time were studied. The pure Se or a mixture of Se and MnSe2 nanorods were obtained in thepresence of different surfactants and small amounts of hydrazine. The cubic MnSe2/Se nanocompositeswere formed at 120 �C for 12 h or longer periods of time, in the presence of polyethylene glycol (PEG)and large amounts of hydrazine. The size of the as-prepared cubes decreases with increasing the reactiontime. With increasing temperature of reaction from 120 �C to 180 �C, the morphology of the productschanges from cubes to the mixture of nanorods and nanoparticles.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Transition metal chalcogenides and dichalcogenides havedrawn considerable attention because of their important optical,electrical, and transport properties [1–9]. Hence, investigationson the synthesis and modification of the nanosized MnSe2 haveattracted tremendous attentions [10,11]. The MnSe2 belongs tothe family of transition metal chalcogenides, an important classof semiconductors with numerous applications including photo-voltaics and thermoelectrics [12]. The MnSe2 has been shown tobe a promising material for use as a solar absorber and continuedstudy of this material is recommended [12]. This rarely encoun-tered solid is structurally similar to iron pyrite, which is a NaCl-likearrangement of M and X2 groups with the axes of the X2 groupsparallel to the various body diagonals [13,14]. The MnSe2 exhibitsinteresting electronic, magnetic and transport properties and havefound several applications in the field of materials science [15–18].These properties show a considerable increase in the MnSe2/Senanocomposites compared to the MnSe2 nanostructures. The aimof making the composite nanomaterials in this study is to improvethe properties and applications of the MnSe2.

A number of methods for the synthesis of manganese selenideshave been explored, such as hydrothermal [19–22], solvothermal[23], reaction elemental Mn and Se [24], and so on. Recently, thehydrothermal synthesis has proved to be a useful method to pro-duce semiconductors [25]. For example MnSe2 nanorods andmicrocrystals have been synthesized via the hydrothermal process[26]. Here, we report a facile hydrothermal method to prepare thecubic MnSe2/Se nanocomposites at low temperature. This work hasprovided a simple and effective method to control the composition,phase structure, and morphology of the metal selenides in theaqueous solution, which will be important for inorganic synthesismethodology and further applications of the selenides.

2. Experimental

2.1. Materials and experiments

All the chemicals used in our experiments were of analytical grade, and pur-chased from Merck and used as received without further purification. A Teflon-linedstainless steel autoclave with 150 ml capacity was used for the synthesis. PowderX-ray diffraction (XRD) patterns were collected from a diffractometer of PhilipsCompany with X0PertPro monochromatized Cu Ka radiation (k = 1.54 Å). Micro-scopic morphology of products was visualized by a LEO 1455VP scanning electronmicroscope (SEM). Transmission electron microscopy (TEM) images were obtainedon a JEM-2100 with an accelerating voltage of 60–200 kV equipped with a high res-olution CCD camera. X-ray energy dispersive spectroscopy (EDS) analysis with20 kV accelerated voltage was done. The magnetic measurement was carried outin a vibrating sample magnetometer (VSM) (BHV-55, Riken, Japan) at roomtemperature.

Fig. 1. SEM images of samples prepared at 120 �C for 12 h: (a) in the absent of surfactant (sample No. 1) and in the presence of: (b) PVA (sample No. 2), (c) SDBS (sample No.3), (d) SDS (sample No. 4), (e) PEG4000 (sample No. 5), (f) PEG600 (sample No. 6), (g) CTAB (sample No. 7).

Table 1The reaction conditions of samples prepared from MnCl2�4H2O and SeCl4 in this work.

Sample No. Surfactant Time (h) T (�C) Reductant agent Morphology Size Product

1 � 12 120 N2H4�H2O (2.5 ml) Nanorods Diameter: 150 nm�1 lm �2 PVA 12 120 N2H4�H2O (2.5 ml) Nanorods Diameter: 100–500 nm MnSe2 + Se3 SDBS 12 120 N2H4�H2O (2.5 ml) Nanorods Diameter: 150–950 nm Se4 SDS 12 120 N2H4�H2O (2.5 ml) Nanorods Diameter: 100–400 nm Se5 PEG4000 12 120 N2H4�H2O (2.5 ml) Nanorods Diameter: 250 nm�1.8 lm MnSe2 + Se6 PEG600 12 120 N2H4�H2O (2.5 ml) Nanorods Diameter: 200–700 nm MnSe2 + Se7 CTAB 12 120 N2H4�H2O (2.5 ml) Nanorods Diameter: 200–600 nm MnSe2 + Se8 SDS 12 90 N2H4�H2O (2.5 ml) Nanorods Diameter: 130–500 nm �9 SDS 12 180 N2H4�H2O (2.5 ml) Aggregated nanoparticles Micrometer �

10 SDBS 12 120 N2H4�H2O (5 ml) Aggregated microstructures Micrometer �11 PEG4000 12 120 N2H4�H2O (15 ml) Microcubes 5–23 lm MnSe2 + Se12 PEG4000 12 180 N2H4�H2O (15 ml) Nanoparticles + rods Micrometer MnSe + Se13 PEG4000 24 120 N2H4�H2O (20 ml) Microcubes 5.5–14 lm MnSe2 + Se14 PEG6000 48 120 N2H4�H2O (15 ml) Microcubes 8–10 lm MnSe2 + Se15 PEG6000 12 120 KBH4 Nanorods Diameter: 125–650 nm, length: 1–7 lm Se16 PEG600 24 120 N2H4�H2O (5 ml) Nanorods + microcubes Rods: 0.7–1.2 lm, cubes: 7–15 lm MnSe2 + Se17 PEG600 24 120 N2H4�H2O (8 ml) Microcubes 3–10 lm MnSe2 + Se18 PEG600 24 120 N2H4�H2O (15 ml) Microcubes 1.5–7 lm MnSe2 + Se

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Fig. 2. XRD patterns of samples prepared at 120 �C for 12 h in the presence of: (a)PVA (sample No. 2), (b) SDBS (sample No. 3), (c) SDS (sample No. 4), (d) PEG4000(sample No. 5), (e) PEG600 (sample No. 6), (f) CTAB (sample No. 7).

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2.2. Synthesis of MnSe2/Se nanocomposites

In a typical experiment for the synthesis of MnSe2/Se nanocomposites,MnCl2�4H2O and surfactant were dissolved in 40 ml distilled water. After stirringthe solution for 15 min, SeCl4 was dissolved in 20 ml of distilled water and addedinto the solution under strong magnetic stirring at room temperature. Then hydra-zine was added drop-wise. The solution was added to the autoclave and maintainedat 120 �C for 12 h. The autoclave was cooled to room temperature on its own, thegray precipitate was separated by centrifugation and transferred into 10 ml of citricacid (1 M) solution to remove the by-product Mn(OH)2. Then the products werewashed with distilled water and anhydrous ethanol several times, and dried undervacuum at 60 �C for 4 h. The growth mechanism and the detailed reactions ofmechanism have been described in Ref. [11].

3. Result and discussion

Fig. 1a–g illustrates SEM images of the products synthesized at120 �C for 12 h in the presence of 2.5 ml hydrazine and differentsurfactants. From figure, it can be observed that the nanorods arethe products obtained in the presence of all surfactants. Withchange at the surfactant, morphology of the samples remainsnearly constant and only size and agglomeration of the nanorodschange a little. Fig. 1a shows the SEM image of the productobtained in the absent of surfactant (sample No. 1). The formationof the nanorods with a high degree of agglomeration is seen in thisfigure. Fig. 1c–g shows that the nanorods with large diameters arealso obtained in the presence of SDBS (sample No. 3), PEG4000(sample No. 5), PEG600 (sample No. 6) and CTAB (sample No. 7).When PVA is used as surfactant (sample No. 2), the morphologyof the products is nanorods with a smaller diameter (Fig. 1b).Fig. 1d shows that the nanorods obtained in the presence of SDS(sample No. 4) array with smaller and same diameters (seeTable 1).

The crystal structure and composition of the as-prepared prod-ucts were determined by XRD. The XRD patterns of samples No.2–7 are depicted in Fig. 2. As shown in Fig. 2a, d–f when PVA,PEG4000, PEG600 and CTAB are used as surfactant and the amountof hydrazine is 2.5 ml, the samples are found to be a mixture of Seand MnSe2 phases. In Fig. 2, the peaks which have been indexedwith star indicate the cubic phase of MnSe2 with JCPDS card No.73-1525. According to Fig. 2b and c, the products obtained in thepresence of SDBS (sample No. 3) and SDS (sample No. 4) are pureSe with hexagonal crystal lattice and lattice constants a = b =0.436 nm and c = 0.495 nm.

In this study, the effect of reaction temperature on the morphol-ogy of samples in the presence of SDS was investigated. Three tem-peratures were applied and other conditions were constant. TheSEM image of the product obtained in the presence of SDS at120 �C showed formation of the nanorods with small and samediameters (Fig. 1d). Fig. 3a and b shows the SEM images of theproducts obtained at two different temperatures including 90 �Cand 180 �C. When the reaction temperature is decreased from120 �C to 90 �C in sample No. 8 (Fig. 3a), nanorods with largerdiameters are obtained. With increasing temperature to 180 �C insample No. 9, aggregated nanoparticles and bulk structures areformed as shown in Fig. 3b. Also, we investigated the effect ofthe reaction temperature in the presence of PEG4000 that will beexplained in continuous.

In other side, effect of the amount of hydrazine on the morphol-ogy, composition and structure of products in the presence ofSDBS, PEG600 and PEG4000 was investigated. During the reactionprocess, the amount of hydrazine is important for the formation ofmanganese selenides. According to Fig. 3c, aggregated microstruc-tures are formed in the presence of SDBS and 5 ml hydrazine. Withchange at the surfactant from SDBS to PEG600 or PEG4000 andincrease of the amount of hydrazine added to the reaction mixture,we obtained purer product with more favored morphology. TheXRD patterns and SEM images of the products synthesized in the

presence of PEG600 and different amounts of hydrazine (2.5, 5, 8and 15 ml) show that the products obtained in the presence of15 ml hydrazine are purer with more favored morphology andsmaller particle sizes [11]. The nanorods are obtained in the pres-ence of 2.5 ml hydrazine. These nanorods have been coated withnanoparticles. With increasing the amount of hydrazine to 5 ml,the as-prepared samples have two morphologies: rods and cubes.The surface of each cube has been coated with thousands of nano-rods and nanospheres. The microrods disappear when 8 ml hydra-zine is used, while there are still cubic microcrystals. The cubicmicrocrystals exist in the presence of 15 ml hydrazine. The size

Fig. 3. SEM images of samples prepared in the presence of: (a) SDS and 2.5 ml hydrazine at 90 �C for 12 h (sample No. 8), (b) SDS and 2.5 ml hydrazine at 180 �C for 12 h(sample No. 9), (c) SDBS and 5 ml hydrazine at 120 �C for 12 h (sample No. 10).

Fig. 4. SEM images of samples prepared in the presence of PEG4000 and: (a, b) 15 ml hydrazine at 120 �C for 12 h (sample No. 11), (c, d) 15 ml hydrazine at 180 �C for 12 h(sample No. 12), (e, f) 20 ml hydrazine at 120 �C for 24 h (sample No. 13).

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of the as-prepared cubes decreases with increasing the amount ofthe hydrazine [11].

Scheme 1 shows a schematic diagram of the formation processof the cubic MnSe2/Se nanocomposites. We have proposed agrowth mechanism of these structures, in which the individualnanoparticles become unstable and aggregate into three-dimen-sional cubes. It has been suggested that the formation of the nano-particles undergoes two dynamic processes [27,28]. The firstprocess is nucleation, and the nuclei grow to form the nanoparti-

cles. The sequential second process is aggregation of the nanopar-ticles to form the cubic structures. For the formation of the MnSe2

nanoparticles, nucleation takes place immediately after mixing theprecursors. The nucleation rate for the samples can reach a highvalue in a short time when amount of hydrazine is large. Therefore,the nanoparticles are aggregated and cubic structures are formed.

The SEM images of the as-prepared samples in the presence ofhydrazine and PEG 4000 (samples No. 11–13) have been shownin Fig. 4. Fig. 4a, b, e and f shows that microcubes are products

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obtained at 120 �C, in the presence of 15 ml hydrazine for 12 h(sample No. 11) and also 20 ml hydrazine for 24 h (sample No.13). These cubes have been coated with nanorods and their edgesizes are 5–23 lm and 5.5–14 lm in samples No. 11 and 13,respectively. In our experiments, the microcubes are not obtainedby increasing the temperature of the reaction. From Fig. 4c and d, itis found that the product obtained at 180 �C (sample No. 12) ismixture of rod-like structures and agglomerated nanoparticles.Based on the classical crystal growth theory, nucleation stageand growth process are involved in the preparation of crystals[29–31]. Increasing of the reaction temperature always results inan increase of the rate of reaction; therefore at relatively high tem-peratures the reaction can be completed within a short time andmore nuclei formed before the growth process, which causes theformation of nanoparticles and nanorods. The higher rate of reac-tion at 180 �C does not allow ample time for the particles toagglomerate and grow into more regular cubic shapes.

Fig. 5 shows XRD patterns of MnSe2/Se and MnSe/Se nanocom-posites synthesized at different temperatures, in presence ofPEG4000 and different amounts of hydrazine. As shown in Fig. 5aand c, in the presence of large amounts of hydrazine (15 or20 ml) when temperature is 120 �C, the samples are found to beMnSe2/Se nanocomposites. With increasing the temperature to

Fig. 5. XRD patterns of samples prepared in the presence of PEG4000 and: (a) 15 ml hy(sample No. 12), (c) 20 ml hydrazine at 120 �C for 24 h (sample No. 13).

180 �C, the peaks of MnSe2 disappear and the characteristic peaksof cubic MnSe appear after 12 h. However an amount of hexagonalSe still exists in the product, as shown in Fig. 5b. The high reactiontemperatures lead to a preference for the formation of a thermody-namically stable phase. The obtained MnSe2 will transform intoMnSe, which is the thermodynamic stable phase when the temper-ature or time is increased [10,11]. However, with increasing time ofreaction from 12 h to 24 h at 120 �C, MnSe2 is our unique product.Therefore, 24 h is not long enough for the formation of MnSe, alsohigher temperature from 120 �C is need for this target. From Fig. 5,it can be observed that with increasing the temperature (from120 �C to 180 �C) or time (from 12 h to 24 h), the crystallinity ofthe nanocomposites is increased.

In continuation, the effect of reductant type on the morphology,crystal structure and composition of products in the presence ofPEG6000 was investigated. The SEM images and XRD patterns ofsamples prepared at 120 �C, in the presence of 15 ml hydrazine(sample No. 14) and KBH4 (sample No. 15) are depicted in Fig. 6.The SEM images of the sample No. 14 are shown in Fig. 6a. It canbe observed that cubic MnSe2/Se nanocomposites that have beencoated by thousands of nanoparticles and nanorods are productobtained in the presence of PEG6000 and 15 ml hydrazine. Thecubes have edge size in the range of 8–10 lm. The presence of

drazine at 120 �C for 12 h (sample No. 11), (b) 15 ml hydrazine at 180 �C for 12 h

Fig. 6. SEM images and XRD patterns of samples prepared in the presence of PEG6000 and: (a, b) 15 ml hydrazine at 120 �C for 48 h (sample No. 14), (c, d) KBH4 at 120 �C for12 h (sample No. 15).

Fig. 7. TEM images of MnSe2/Se nanocomposites: (a–c) sample No. 11, (d) sample No. 13, (e) sample No. 18.

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Fig. 8. EDS patterns of samples prepared in the presence of: (a) SDBS (sample No. 10), (b) PEG4000 (sample No. 11), (c) PEG6000 (sample No. 14).

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nanoparticles and nanorods on the surface of cubes can increasethe specific surface area of the sample and is in favor of somepotential applications such as chemical supports. In Fig. 6b, thepeaks have been indexed with red1 color and . indicate the cubicphase of MnSe2 (JCPDS card No. 73-1525) and the hexagonal phaseof Se (JCPDS card No. 42-1425), respectively. Thus the cubic MnSe2/Se nanocomposites that have been coated by nanorods are formed at120 �C for 12 h or longer periods of time, in the presence of PEG(PEG600, PEG4000 and PEG6000) and large amounts of hydrazine.The SEM image of the as-prepared sample in the presence ofPEG6000 and KBH4 (sample No. 15) has been shown in Fig. 6c. In thisfigure, nanorods with diameters of 125–650 nm and lengths of 1–7 lm can be observed. Hydrazine is relatively slower in the reduc-tion rate compared to hydroborates. Our group attributes the slower

1 For interpretation of color in Fig. 6, the reader is referred to the web version ofthis article.

action of hydrazine as the reason for the cubic MnSe2/Se nanocom-posites obtain. The slower rate of reduction allows ample time forthe particles to agglomerate and grow into more regular shapes.Fig. 6d indicates a pure Se phase with JCPDS card No. 06-0362 isformed in the presence of KBH4.

The TEM images were taken for further elucidate the size andmorphology of the nanocomposites and are shown in Fig. 7. Theimages show the formation of nanorods and nanoparticles on thesurface of the cubes in samples No. 11, 13 and 18. The TEM imagesof the as-prepared sample in the presence of PEG4000 and 15 mlhydrazine at 120 �C (sample No. 11) show that nanorods and nano-particles formed on the surface of cubes have uniform sizes(Fig. 7a–c). The nanoparticles with the diameters of 7–70 nm andthe nanorods with the diameters of 7–50 nm and lengths of70–700 nm can be observed in this figure. However the nanorodsand nanoparticles still exist on the surface of cubes when theamount of hydrazine and time are increased (sample No. 13), as

Fig. 9. M–H hysteresis at 300 K for: (a) sample No. 11 and (b) sample No. 14.

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shown in Fig. 7d. From Fig. 7e, the nanorods with the diameters of20–150 nm and the lengths between 300 nm and 1.2 lm areobserved on the surface of cubes, in the as-prepared sample inthe presence of PEG600 and 15 ml hydrazine (sample No. 18).

The EDS technique was employed to investigate the chemicalcomposition and purity of as-synthesized products. The EDS pat-terns in Fig. 8 confirm the presence of Mn and Se in samples No.10, 11 and 14. So, the samples can be called MnSe2/Senanocomposites.

Scheme 1. Schematic diagram of the formation

The MnS2, MnSe2 and MnTe2 show antiferromagnetic ordering[32]. Hastings et al. performed magnetic susceptibility measure-ments down to 76 K and neutron diffraction experiments at 4.2 K[33]. The susceptibility measurement gives leff = 5.93 lB forMnSe2, corresponding to a S = 5/2 spin state. The Curie-Weiss tem-perature is hp = 483 K. The MnSe2 magnetic structure as given byHastings et al. can not exist in the immediate vicinity of a sec-ond-order transition to the paramagnetic state [33]. Dimmock sug-gested a sinusoidally modulated structure close to TN [34]. Fromneutron-diffraction measurements, Plumier and Sougi have shownthe existence of a first-order transition at Tt = 48.5 K from the pre-viously reported structure to two independent sinusoidally modu-lated magnetic structures of comparable amplitudes andpropagation vectors along [35]. Plot of magnetization versusapplied field (M–H) at 300 K for samples No. 11 and 14 are shownin Fig. 9. The plots show antiferromagnetic behavior in both sam-ples. The magnetic properties of nanomaterials are believed to behighly dependent on the material structure, size and shape of thegrain, crystallinity, magnetization and applied field direction, lat-tice spacing, chemical composition, temperature, defect concentra-tion, atomic order, impurities and so on.

4. Conclusion

In the present work, we design a controlled synthetic method inaqueous solution, which can conveniently produce high-qualityMnSe2/Se nanocomposites at low temperature (120 �C). The MnCl2-

�4H2O, SeCl4 and hydrazine are used as reactants in this hydrother-mal method. The effects of preparation parameters such as: kind ofsurfactant, kind of reductant and its amount, reaction time andtemperature on morphology, particle size, phase and purity ofthe products are investigated. The high yield, simple reaction appa-ratus, and low reaction temperature give this novel method a goodin future applications.

Acknowledgments

Authors are grateful to the council of University of Kashan forproviding financial support to undertake this work by Grant No(159271/166).

process of cubic MnSe2/Se nanocomposites.

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