hydrothermal synthesis of the ternary metal chalcogenide ZnIn2S4

Materials Letters 65 (2011) 2537–2540

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Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t

Ionothermal/hydrothermal synthesis of the ternary metal chalcogenide ZnIn2S4 Cuixia Li a, b, Honghua Li a, Lijun Han a, Chunshan Li a,⁎, Suojiang Zhang a,⁎⁎ a b

State Key Lab of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, China Graduate University of Chinese Academy of Sciences, Beijing, 100049, China

a r t i c l e

i n f o

Article history: Received 15 March 2011 Accepted 12 May 2011 Available online 15 May 2011 Keywords: Ionic liquid [Bmim][BF4] Ternary metal chalcogenide ZnIn2S4 Semiconductor Microstructure

a b s t r a c t The ternary metal chalcogenide ZnIn2S4 was synthesized through the ionothermal/hydrothermal method using the ionic liquid [Bmim][BF4]. The crystal structure and surface chemical state of the product were confirmed by X-ray diffractometry and X-ray photoelectron spectroscopy. A small amount of water was essential for the obtaining of yellow ZnIn2S4 product. ZnIn2S4 with a hexagonal phase was obtained in a wide range of [Bmim][BF4]:water ratios of 9.3 ml:(0.1–5 ml). The size and morphology of the synthesized samples were strongly affected by the aid of [Bmim][BF4], the amount of water, the zinc source, reaction temperature, and reaction time. The ZnIn2S4 showed different visible light absorption ranges when the water content of the reaction mixture was varied. © 2011 Elsevier B.V. All rights reserved.

1. Introduction

2. Experimental section

The ternary metal chalcogenide, ZnIn2S4, conventionally synthesized via hydrothermal [1] or solvothermal methods [2,3], is an important semiconducting material of the A IIB III2X IV4 family. The synthesis of ZnIn2S4 microspheres [4,5], porous microspheres, hierarchically porous submicrospheres [6], hollow microspheres, nanotubes, nanoribbons [7], and nano/micropeony [8] using the hydrothermal or solvothermal methods have been reported. Ionic liquids are considered as attractive reaction media for the synthesis of inorganic materials because of their unique properties, such as a wide liquid range, high thermal stability, no vapor pressure, especially the low interface tension and the associated high nucleation rate [9]. The synthesis of metal chalcogenides using ionic liquids is a promising approach. Several binary sulfides, such as CdS, ZnS, CuS, Cu2S, PbS, MoS2, CoS, Bi2S3, and Sb2S3, have been synthesized using, or in the presence of, ionic liquids [10–24]. Typical ionic liquids used in reactions are [Bmim]Cl, [Bmim][BF4], [Bmim] [PF6], [Emim][EtSO4], [Bmim][MeSO4], and [Bmim][SCN], etc. However, the synthesis of ternary metal chalcogenides using ionic liquids has not yet been reported. In the present work, we report the synthesis of the ternary metal chalcogenide ZnIn2S4 in [Bmim][BF4]-water binary emulsions. The effects of [Bmim][BF4], water content, zinc source, temperature, and reaction time on the crystal structure, size, shape, and optical properties of the sample were examined.

2.1. Synthesis The raw materials were 1 mmol of Zn(CH3COO)2·H2O, 2.01 mmol of InCl3·4H2O, 8 mmol of C2H5NS, [Bmim][BF4] and water. The raw materials were added into a 23 ml Teflon autoclave. After stirring at room temperature for more than 1 h, the autoclave was sealed and maintained at 160 °C for 20 h, and then air cooled to room temperature.

Relative intensity

a. 0 ml H 2O b. 0.1 ml H2O c. 0.5 ml H2O

(001) (003) (011)

d. 1 ml H2O

(110)

e. 5 ml H2O f. 16 ml H 2O

10

20

30

40

50

60

70

2θ (degree) ⁎ Corresponding author. Tel./fax: + 86 10 82547800. ⁎⁎ Corresponding author. Tel./fax: + 86 10 82627080. E-mail addresses: [email protected] (C. Li), [email protected] (S. Zhang). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.05.052

Fig. 1. XRD patterns of the products synthesized at 160 °C for 20 h under a series of [Bmim][BF4]-water ratios. (a)–(e) 9.3 ml of BmimBF4; (f) no [Bmim][BF4].

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Relative intensity

(SEM) and diffuse reflection spectroscopy (DRS). SEM was performed on a JEOL JSM-6700F field-emission scanning electron microscope. DRS was obtained on a Shimadzu UV-2550 UV–vis near infrared spectrophotometer, using pure BaSO4 pellet as the reference. a. 140 °C ,20 h b. 150 °C ,20 h

3. Results and discussion

c. 160 °C , 5 h d. 160 °C ,10 h

3.1. Crystal structure

e. 160 °C ,20 h f. 160 °C ,20 h,ZnCl2 g. 160 °C ,60 h h. 180 °C ,20 h 10

20

30

40

50

60

70

2θ (degree) Fig. 2. XRD patterns of the products synthesized at different temperature and time under [Bmim][BF4]-water ratio of 9.3 ml/1 ml.

The precipitate obtained was filtered and washed several times with water and ethanol, and then dried at 70 °C.

2.2. Characterization The crystal structure of the product was analyzed by X-ray diffraction (XRD) and X-ray photoelectron spectra (XPS). XRD patterns were collected on a X'Pert Pro MPD X-ray diffractometer (PANalytical), using Cu Kα radiation (λ = 0.15405 nm) at scanning angles 2θ from 5 to 90°. XPS was conducted on a PHI Quantera SXM spectrometer (ULVAC-PH INC) with a monochromated Al Kα X-ray source operated at 2 kV. The binding energies were calibrated with the C1s peak of the adventitious carbon at 284.8 eV. The morphology and optical property were examined by scanning electron microscopy

1019.0

Zn2p

Fig. 1 shows the XRD patterns of the products synthesized at various [Bmim][BF4]:water ratios. Previous studies have shown that water is essential for the synthesis of ZnIn2S4. A light brown material was the main reaction product when pure [Bmim][BF4] ionic liquid was the reaction medium. Yellow ZnIn2S4 product with a hexagonal phase (PDF No. 01-089-3962) was obtained in a wide range of [Bmim] [BF4]:water ratios of 9.3 ml:(0.1–5 ml). The crystallinity was quite low when 0.1 ml water was used. The position of the (003) peak slightly shifted to a lower angle with decreasing water content, which results in the increasing of d(003) space. The intensity of the (001) peak in the XRD pattern is particularly strong (Fig. 1b–e) in the presence of [Bmim][BF4], this indicates the preferential growth orientation of ZnIn2S4 was along this direction. The crystallite size of the (011) plane was 21, 26, 27, 28, and 58 nm in the presence of 0.1, 0.5, 1, 5, and 16 ml of water, respectively. As shown in Fig. 2, other reaction conditions, such as the zinc source, reaction time, and temperature, had little effect on the crystal structure and peak intensity. In the presence of water, In 3+ and Zn 2+ reacted with H2S released by TAA, and gave rise to complexes of the tetrahedral In–S4 and Zn–S4 and the octahedral In–S6 species. These metal sulfur species further combined in situ, and form a thermodynamically stable hexagonal ZnIn2S4 phase after the high temperature crystallization process [7]. The increasing of d(003) space means that [Bmim][BF4] would insert into the layered crystal structure and expand the interlayer space along the c-axis of the ZnIn2S4 products during the hydrothermal

441.8

In3d 449.2

1041.8

1050

1040

1030

1020

455

Binding energy

445

440

Binding energy (eV) 529

158.4

S2p

170

450

O1s

165

160

Binding energy (eV)

155

540

535

530

525

Binding energy (eV)

Fig. 3. XPS spectra of the product synthesized at 160 °C for 20 h under [Bmim][BF4]-water ratio of 9.3 ml/1 ml.

C. Li et al. / Materials Letters 65 (2011) 2537–2540

Absorbance (a.u.)

synthetic process. In summary, [Bmim][BF4] plays a similar role with CTAB and other organic solvents in controlling the crystal structure of ZnIn2S4 [4,7]. The surface elemental composition and valence state of ZnIn2S4 confirmed by XPS were shown in Fig. 3. The Zn 2p peaks at 1041.8 and 1019.0 eV indicate the presence of zinc ions, and the In 3d peaks at 449.2 and 441.8 eV suggest the presence of trivalent indium ions. Both Zn 2p and In 3d gave rise to two peaks which were due to spin orbit split. The S 2p peak at 158.4 eV shows the presence of sulfur ions. The presence of the O 1s peak may correspond to the partial oxidation and oxygen adsorption on the surface.

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c. 0.5 ml H2O d. 1 ml H2O e. 5 ml H2O f. 16 ml H2O

a. 0 ml H2O b. 0.1 ml H2O

3.2. Morphology SEM images of the corresponding products are shown in Fig. 4. The size and morphology of the synthesized samples were strongly affected by the aid of [Bmim][BF4], the water content, reaction temperature, reaction time, and the type of zinc source when the ionic liquid-water binary emulsions were used as reaction media. Based on the experimental results, a possible formation mechanism of the ZnIn2S4 with different morphology and micro-structures is proposed. The synthesis could be performed and the products obtained were spherical in shape when 9.3 ml of [Bmim][BF4] and equal to or lower

a. 0.1 ml H2O

300

f. 160 °C, 60 h

1 µm

600

700

Fig. 5. Diffuse reflection spectra of the products synthesized at 160 °C for 20 h under a series of [Bmim][BF4]-water ratios. (a)–(e) 9.3 ml of [Bmim][BF4]; (f) no [Bmim][BF4].

than 1 ml of water were used, as shown in Fig. 4a–c. The reaction proceeds in ions solvents which are much different from molecular solvents. In the synthesis process the ionic liquids act both as solvents

2 µm

d. 5 ml H2O

2 µm

500

Wavelength (nm)

b. 0.5 ml H2O

2 µm

400

c. 1 ml H2O

1 µm e. 16 ml H2O

2 µm g. 180 °C, 20 h

1 µm

1 µm h. ZnCl2, 160 °C, 20 h

2 µm

Fig. 4. SEM images of the products. (a)–(d) 160 °C for 20 h, 9.3 ml of [Bmim][BF4]; (e) 160 °C for 20 h, no [Bmim][BF4]; (f)–(h) 9.3 ml of [Bmim][BF4], 1 ml of H2O.

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and stabilizers. Water was mainly coordinated with In 3+ and Zn 2+ in the form of [In(H2O)6] 3+, [In(H2O)4] 3+ and [Zn(H2O)4] 2+. The freshly generated metal chalcogenide nuclei were coated by the ionic liquid. The low interfacial tension of [Bmim][BF4] leads to high nucleation rates, thus enabling the generation of small ZnIn2S4 spherical nanoparticles. Ref. [14] demonstrated the same ionic liquid effect in the synthesis of binary sulfide ZnS and CdS. As shown in Fig. 4d, microspheres and three-dimensional flowerlike structures (1 to 5 μm diameter), which were formed by self-assembly of numerous nanosheets, were obtained in the medium with 9.3 ml of [Bmim][BF4] and 5 ml of water. When the above solution was used as a reaction medium, [Bmim][BF4] forms aggregates above the critical aggregation concentration and a large number of vesicles with a nano or micro size are formed [11,27]. This phenomenon was observed with TEM by Ref. [11] and illustrated by Ref. [27]. The hydrophobic [Bmim] + will be inside the vesicles, and the hydrophilic [BF4] − groups will be outside the vesicles. The tetrahedral [In(TAA)4]3+, [Zn(TAA)4] 2+, and the octahedral [In(TAA)6] 3+ complex in the diluted ionic liquid solution will be associated with [BF4] − groups. Then ZnIn2S4 will start to crystallize on the surface of these vesicles. The growth tendency of hexagonal ZnIn2S4 into nanosheets depends on its intrinsic layered feature as in the hydrothermal process. Therefore, the microspheres and three-dimensional flowerlike structures composed of numerous nanosheets can be constructed. Due to the high interfacial tension of water, freshly formed nanosheets have a tendency to randomly aggregate until they become stable. For this reason, ZnIn2S4 with irregular small nanosheets were formed in pure water during hydrothermal process (Fig. 4e). When the other reaction conditions changed (such as the reaction time was extended to 60 h, or the reaction temperature was increased to 180 °C, or zinc chloride was used as the zinc source instead of zinc acetate), there was a coexistent of spherical nanoparticles and nanosheets (30–40 nm thickness) in the medium of 9.3 ml of [Bmim] [BF4] and 1 ml of water (Fig. 4f–h). The appearance of nanosheets was probably due to the changed generation rate of H2S by the TAA in the condition of prolonged time, increased temperature and lowered pH value with zinc chloride as the raw material. 3.3. Optical properties As shown in Fig. 5, with the exception of the sample synthesized in pure [Bmim][BF4], all products had a steep absorption edge in the visible region. This indicates that the relevant band gap was due to the intrinsic transition of ZnIn2S4 rather than the transition from impurity levels. The band gaps calculated from the DRS spectrum [25] were 2.97, 2.95, 2.82, 2.75, 2.71, and 2.73 eV in the presence of 9.3 ml of [Bmim][BF4] mixed with 0, 0.1, 0.5, 1, 5, and 16 ml of water, respectively. All of them are wider than that of the bulk ZnIn2S4 (2.3 eV). This result, which was particularly similar to that of Ref. [8,26], can be attributed to the particle size and morphology effect.

4. Conclusions Ternary metal chalcogenide ZnIn2S4 exhibiting a hexagonal phase was synthesized in [Bmim][BF4]-water binary emulsions using the ionothermal/hydrothermal method. A small amount of water was essential to the synthesis of ZnIn2S4. The position of the (003) peak slightly shifted to a lower angle with decreasing water content, and there was a preferential growth orientation along the (001) direction of ZnIn2S4 in the presence of [Bmim][BF4]. By controlling the reaction conditions, ZnIn2S4 spherical nanoparticles, microspheres, or threedimensional flower-like structures could be selectively prepared. The band gap of obtained ZnIn2S4 can also be widened in the presence of [Bmim][BF4].

Acknowledgment This work was supported by Solar Energy Initiative of the Knowledge Innovation Program of the Chinese Academy of Sciences under Grant No. KGCX2-YW-393.

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