Size-Tunable Hydrothermal Synthesis of SnS2 Nanocrystals with High

ARTICLE pubs.acs.org/est

Size-Tunable Hydrothermal Synthesis of SnS2 Nanocrystals with High Performance in Visible Light-Driven Photocatalytic Reduction of Aqueous Cr(VI) Yong Cai Zhang,*,† Jing Li,† Ming Zhang,† and Dionysios D. Dionysiou‡ †

Key Laboratory of Environmental Material and Environmental Engineering of Jiangsu Province, College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, China ‡ Environmental Engineering and Science Program, 705 Engineering Research Center, University of Cincinnati, Cincinnati, Ohio 45221-0012, United States

bS Supporting Information ABSTRACT: SnS2 nanocrystals with adjustable sizes were synthesized via a hydrothermal method from the aqueous solution of common and inexpensive SnCl4 3 5H2O, thioacetamide and citric acid, simply by varying the reaction temperature and reaction time. The structures, Brunauer
Emmett
Teller (BET) specific surface areas and optical properties of the resultant SnS2 nanocrystals were characterized by X-ray diffraction, transmission electron microscopy, N2 adsorption/desorption isotherms, and UV
vis diffuse reflectance spectra. Besides, their photocatalytic properties were tested for the reduction of aqueous Cr(VI) under visible light (λ > 420 nm) irradiation. It was found that the photocatalytic activities of SnS2 nanocrystals in aqueous suspension depended on their synthesis conditions. The product synthesized under suitable hydrothermal conditions (for example, at 150 °C for 12 h) not only showed high visible light-driven photocatalytic activity in the reduction of aqueous Cr(VI), but also showed good photocatalytic stability. Our photocatalytic results suggested that SnS2 nanocrystals are a promising photocatalyst in the efficient utilization of solar energy for the treatment of Cr(VI)-containing wastewater.

’ INTRODUCTION Cr(VI) is a frequent contaminant in the wastewaters arising from industrial processes such as leather tanning, paint making, electroplating and chromate production, etc. It is toxic to most organisms, and has been classified as carcinogenic and mutagenic.1
10 Due to its high toxicity and high mobility in water, Cr(VI) has been included in the list of priority pollutants and its concentration in drinking water has been regulated by many countries. For instance, the allowable limit of Cr(VI) in drinking water in China is 0.05 mg/L. Therefore, how to economically and efficiently treat the Cr(VI)-containing wastewaters has attracted intense interest from both academic and industrial societies.1
10 A common method of treating Cr(VI) in water is to convert it into Cr(III), which is considered as a nontoxic and essential trace metal in human nutrition.1
10 Furthermore, Cr(III) can be precipitated as Cr(OH)3 in neutral or alkaline solutions (KspQ(Cr(OH)3) = 6.3  10
31) and removed as a solid waste.1
10 However, the conventional chemical reduction methods require massive use of reducing agents such as ferrous sulfate, sodium hydrogensulfite, sodium pyrosulfite, hydrazine hydrate, or sulfur dioxide, etc. which are not cost-effective. Compared with the conventional chemical reduction methods, the semiconductor photocatalytic reduction of aqueous Cr(VI) has some obvious advantages, such as simple r 2011 American Chemical Society

operation, ambient conditions, low cost, high efficiency, reusability, direct use of infinite, clean and safe natural solar energy, and no use and no release of other unwanted chemicals.1
10 Consequently, the semiconductor photocatalytic reduction method is widely regarded as a promising way in treating aqueous Cr(VI).1
10 TiO2 is undoubtedly the most studied semiconductor photocatalyst so far, by virtue of its low cost, high activity for many photocatalytic reactions, excellent chemical and photochemical stability, and good biocompatibility.1
10 However, due to its wide band gap (3.2 eV), TiO2 cannot be activated by the visible light, which accounts for about 46% of the total solar energy.11 In order to make full use of solar energy, it is desirable to develop new visible light-responsive semiconductor photocatalysts. The semiconducting metal sulfides usually have light-absorbing capabilities in the visible and short-wavelength near-infrared regions, which enable them to work as a class of promising sensitizers for wide band gap semiconductors or visible lightdriven photocatalysts.12
17 Among them, CdS, which has a band Received: June 14, 2011 Accepted: October 4, 2011 Revised: September 7, 2011 Published: October 04, 2011 9324

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Table 1. Abbreviated Names and Properties of the SnS2 Products Synthesized under Different Reaction Conditionsa BET SA (m2/g)

Eg (eV)

DAA (%)

k (min
1)

103.8

2.32

4.6

0.0129

87.6

2.30

5.8

0.0198

14
34 (T); 14 (S)

82.4

2.27

10.4

0.0394

18

15
35 (T);16 (S)

81.3

2.27

9.9

0.0301

150

24

15
41 (T); 17 (S)

76.6

2.25

8.0

0.0205

170

12

21
42 (T); 21 (S)

73.1

2.22

9.3

0.0252

name

T (°C)

t (h)

SnS2-(a)

130

12

SnS2-(b)

150

6

10
24 (T); 10 (S)

SnS2-(c)

150

12

SnS2-(d)

150

SnS2-(e) SnS2-(f)

size (nm) 7
17 (T); 7 (S)

a

SA = surface area; DAA = dark adsorption amount for Cr(VI); k = the photocatalytic reaction rate constants obtained using the pseudo-first-order model; T and S denote the sizes derived from TEM and Scherrer formula, respectively.

gap of about 2.4 eV, is currently the focus of significant attention.13
15 However, CdS itself is detrimental to human health and the environment due to its high toxicity. SnS2 is a CdI2-tpye layered semiconductor with a band gap of about 2.2 eV,16 which is a little smaller than that of CdS. It is innocuous, chemically stable in acid or neutral aqueous solution, and so has the potential to be an efficient visible light-driven photocatalyst.12,16,17 Although SnS2 is superior to CdS in terms of lower toxicity and wider spectral response (or higher photocatalytic activity), there is still no report about its use as photocatalyst in the reduction of aqueous Cr(VI) by far. The hydrothermal method is a versatile wet chemical process that has been widely used to prepare semiconductor nanomaterials.17
19 It not only enables the preparation of highly crystalline products at low temperatures (generally below 200 °C), but also has the ability of controlling the morphology and size of the resultant products.17
19 Herein, we propose a simple and practical hydrothermal route for the size-tunable synthesis of SnS2 nanocrystals at 130
170 °C for 6
24 h, using common and inexpensive SnCl4 3 5H2O, thioacetamide and citric acid as the reactants and water as the solvent. The structures, BET specific surface areas and optical properties of the resultant SnS2 nanocrystals are characterized by X-ray diffraction, transmission electron microscopy, N2 adsorption/desorption isotherms and UV
vis diffuse reflectance spectra, and their photocatalytic properties are evaluated in aqueous suspension for the reduction of Cr(VI) under visible light (λ > 420 nm) irradiation.

’ EXPERIMENTAL SSECTION All the reagents used were of analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. Synthesis of SnS2 Nanocrystals. Forty mL of aqueous solution containing 5 mmol SnCl4 3 5H2O and equimolar citric acid was first prepared in Teflon-lined stainless steel autoclaves of 50 mL capacity, and subsequently 10 mmol thioacetamide was added to the autoclaves with stirring. The autoclaves were sealed and heated at 130
170 °C for 6
24 h, then cooled naturally to room temperature. The as-formed yellow precipitates were filtered, washed with deionized water, and dried in vacuum at 100 °C for 4 h. For the convenience of description, the SnS2 products synthesized under different reaction conditions were hereinafter named as “SnS2-(a)”, “SnS2-(b)”, and “SnS2-(c)”, etc., as shown in Table 1. Characterization. X-ray diffraction (XRD) patterns of the obtained products were recorded on a German Bruker AXS D8 ADVANCE X-ray diffractometer under the conditions of

Figure 1. XRD patterns of SnS2-(a
f) and SnS2-AP (the product collected after the fifth reuse cycle of SnS2-(c) in photocatalysis).

generator voltage = 40 kV; generator current = 200 mA; divergence slit = 1.0 mm; scan speed = 6.0 degree/min; Cu Kα (λ = 1.5406 Å); graphite diffracted-beam monochromator; dynamic scintillation counter detector; and polyethylene holder. Transmission electron microscopy (TEM) images were taken on a Holland Philips Tecnai-12 transmission electron microscopy. BET surface areas were measured on an American Micromeritics Instrument Corporation TriStar II 3020 surface area and porosity analyzer. UV
vis diffuse reflectance spectra were measured on a Japan Shimadzu UV-3101PC ultraviolet
visible-near-infrared spectrophotometer, using BaSO4 as reference. High-resolution transmission electron microscopy (HRTEM) images were taken on an American FEI Tecnai G2 F30 S-TWIN fieldemission transmission electron microscopy. X-ray photoelectron spectroscopy (XPS) measurements were conducted on an American Thermo-VG Scientific ESCALAB 250 XPS system with Al Kα radiation as the exciting source, where the binding energies were calibrated by referencing C 1s peak (284.6 eV) to reduce the sample charging effect. Photocatalytic Tests. The photocatalytic experiments were carried out in a homemade photochemical reactor (Supporting Information Figure S1), which includes mainly four parts: light source system including a 250 W Xe lamp, (λ > 420 nm) cutoff filters and cooling attachments such as electric fan; reactor (two-layer Pyrex glass bottles of 400 mL capacity, the space between the two layers is filled with circulating water to cool the reactor); magnetic stirrer; and temperature controller. In each experiment, the distance between the Xe lamp and the reactor was set to be 5.8 cm, and the reaction temperature was 20 °C. Prior to illumination, 300 mL of 50 mg/L K2Cr2O7 aqueous solution containing 0.3 g of photocatalyst was magnetically stirred in the dark for 1 h. During illumination, about 4 mL of suspension was taken from the reactor at a scheduled interval 9325

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Figure 2. TEM images of SnS2-(a
f).

and centrifuged to separate the photocatalyst. The Cr(VI) content in the supernatant solution was determined colorimetrically at 540 nm using the diphenylcarbazide method with a detection limit of 0.005 mg/L.20 The measured absorbance intensities at different illumination times were transformed to the reduction ratio of Cr(VI), which is calculated using the following expression: reduction ratio of CrðVIÞ ¼ ðA0
At Þ=A0  100% where A0 and At are the absorbance intensities when illuminated for 0 (that is, just after the dark adsorption) and t min, respectively.

’ RESULTS AND DISCUSSION Structural Characterization. The XRD patterns of the assynthesized SnS2-(a
f) are shown in Figure 1. The high background intensities of the XRD patterns may suggest the

existence of amorphous materials in SnS2-(a
f). All the products displayed only the characteristic XRD peaks of hexagonal phase SnS 2 (JCPDS card no. 23-677). Nevertheless, when the reaction temperature or the reaction time was increased (while the other reaction conditions remained the same), the XRD peaks of the resultant SnS2 products generally became stronger and sharper, suggesting the increase of their crystal sizes. Using the well-known Scherrer formula based on the half-widths of (001) peak in their XRD patterns, the crystal sizes of SnS2-(a
f) were calculated and presented in Table 1. The particle sizes of SnS2-(a
f) were further determined from their TEM images (Figure 2), and the obtained results are given in Table 1. As can be seen from Figure 2(a
f) and Table 1, all the as-synthesized SnS2 products comprised nanoparticles. Although there was some difference between the particle sizes derived from TEM and XRD (Scherrer formula) due to the shape (such as nanoparticles or nanoplates) and aggregation effects of the 9326

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Figure 4. UV
vis diffuse reflectance spectra of SnS2-(a
f). Figure 3. N2 adsorption/desorption isotherms of SnS2-(a
f).

as-synthesized SnS2 nanoparticles, the size-changing trends inferred from these two methods are in agreement: the sizes of SnS2 nanoparticles generally increased with the increase of the reaction temperature and reaction time, while the other synthesis conditions were kept constant. These phenomena may be explained by the effects of the reaction temperature and reaction time on the nucleation and crystal growth processes of SnS2 under the hydrothermal conditions, as shown in the following section of Formation mechanism. Formation Mechanism. The current hydrothermal synthesis of SnS2 nanocrystals was carried out using SnCl4 3 5H2O, thioacetamide and citric acid as the reactants and water as the solvent. The Sn4+ cations released from the dissociation of SnCl4 3 5H2O in water (Reaction 1) will combine with citric acid (CA) to form the [Sn(CA)]4+ complex (Reaction 2).21 The formation and dissociation reactions of [Sn(CA)]4+ are reversible (Reaction 2). Meanwhile, thioacetamide (CH3CSNH2) hydrolyzes in the aqueous solution to yield H2S (Reaction 3), which further dissociates to generate S2
anions (Reaction 4 and 5).22 The dissociation/hydrolysis reactions of [Sn(CA)]4+ and CH3CSNH2 can proceed faster at higher temperatures.21,22 SnCl4 3 5H2 O ¼ Sn4þ þ 4Cl
þ 5H2 O

ð1Þ

Sn4þ þ CA h ½SnðCAÞ4þ

ð2Þ

CH3 CSNH2 þ H2 O ¼ CH3 CONH2 þ H2 S

ð3Þ

H2 S þ H2 O h H3 Oþ þ HS


ð4Þ

HS
þ H2 O h H3 Oþ þ S2


ð5Þ

4+

2


The released Sn and S ions will combine to form SnS2 precipitate once the ionic product of Sn4+ and S2
exceeds the solubility product of SnS2, as expressed by Reaction 6: Sn4þ þ 2S2
¼ SnS2 V

ð6Þ

But, [Sn(CA)]4+ and CH3CSNH2 tend to release Sn4+ and S2
ions gradually, rather than entirely at once, through the Reactions 2
5. So, the nucleation rate of SnS2 is controlled by the releasing rates of Sn4+ and S2
ions, and the number of SnS2 crystal nucleus formed in the initial stage (for example, during the temperature rise from room temperature to 130 °C) is limited. Accordingly, there are still large amounts of unreacted Sn4+ and S2
ions in the reaction solution in the forms of [Sn(CA)]4+ and CH3CSNH2, respectively. When the reaction temperature is increased or the reaction time is prolonged, the

Figure 5. Plots of (F(R∞)hν)2 versus (hν) for obtaining the band gaps of SnS2-(a
f).

successively released Sn4+ and S2
ions will further combine with each other to form SnS2. Because the energy of nucleation required in the heterogeneous phases is lower than that in the homogeneous phase,23,24 the later nucleation of SnS2 should be mainly on the surface of preformed SnS2 crystal nucleus, making contribution to the crystal growth process of SnS2. In addition, the increase of temperature and time also helps to the crystal growth of SnS2 via the Ostwald ripening process, where larger crystals grow at the expense of smaller ones.25,26 Thus, the size of SnS2 nanocrystals becomes bigger with the increase of the reaction temperature and reaction time. BET Surface Area. Figure 3 shows the N2 adsorption/ desorption isotherms of SnS2-(a
f). All the products displayed type IV isotherms with hysteresis loops at relative pressure (P/P0) between 0.4 and 1.0, indicative of their mesoporous feature.27
31 The mesoporous structures of SnS2-(a
f) were considered to be formed from the aggregation of their primary particles.27
31 The values of the BET specific surface areas of SnS2-(a
f) are listed in Table 1, which exhibited the order of SnS2-(a) > SnS2-(b) > SnS2-(c) > SnS2-(d) > SnS2-(e) > SnS2-(f). Optical Characterization. The UV
vis diffuse reflectance spectra of SnS2-(a
f) were measured and converted into the absorption spectra (Figure 4) using the Kubelka
Munk function:32
35 FðR ∞ Þ ¼ ð1
R ∞ Þ2 =2R ∞ ¼ α=S R ∞ ¼ R sample =R BaSO4 where F(R∞), R, α and S are the Kubelka
Munk function, reflectance, absorption coefficient and scattering coefficient, respectively. According to a previous study,17 the band gaps 9327

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Figure 6. Photocatalytic activities of SnS2-(a
f) and P25 TiO2 in the reduction of aqueous Cr(VI) under visible light (λ > 420 nm) irradiation.

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Figure 7. Photocatalytic performances of SnS2-(c) in the first five reuse cycles.

(Eg) of SnS2-(a
f) were determined based on the theory of optical absorption for direct band gap semiconductors: αhν ¼ Bðhν
Eg Þ1=2 where hν and B are discrete photon energy and a constant related to the material, respectively. The value of α can be calculated from the diffuse reflectance data using the Kubelka
Munk function. But, for the diffused reflectance spectra, the Kubelka
Munk function can be used instead of α for estimating the optical absorption edge energy.32
35 So, the curves of (F(R∞)hν)2 versus (hν) for SnS2-(a
f) are plotted in Figure 5. By extrapolating the linear portion of the (F(R∞)hν)2 versus (hν) curves to F(R∞) = 0, the Eg values of SnS2-(a
f) were estimated to be 2.22
2.32 eV (Table 1). Photocatalytic Tests. Photocatalytic Activities. Figure 6 shows the photocatalytic activities of SnS2-(a
f) and P25 TiO2 in the reduction of aqueous Cr(VI) under visible light (λ > 420 nm) irradiation. As can be seen from Figure 6, in the presence of P25 TiO2 or in the absence of any photocatalyst, the reduction of Cr(VI) hardly occurred under visible light (λ > 420 nm) irradiation for 120 min. Instead, the reduction of Cr(VI) proceeded quite rapidly in the presence of SnS2-(a
f). Nevertheless, the photocatalytic activities of SnS2-(a
f) differed greatly, indicating that the synthesis conditions of SnS2 nanocrystals played an important role in their photocatalytic activities. The photocatalytic activities of SnS2-(a
f) followed the order of SnS2-(c) > SnS2-(d) > SnS2-(f) > SnS2-(e) > SnS2-(b) > SnS2-(a). For instance, when irradiated for 120 min, the reduction ratios of Cr(VI) over SnS2-(c), SnS2-(d), SnS2-(f), SnS2-(e), SnS2-(b), and SnS2-(a) were 99.6%, 97.5%, 96.0%, 91.2%, 90.4%, and 77.9%, respectively. Moreover, the photocatalytic reaction rate constants (k) in the presence of SnS2-(c), SnS2-(d), SnS2-(f), SnS2-(e), SnS2-(b), and SnS2-(a) were in turn 0.0394, 0.0301, 0.0252, 0.0205, 0.0198, and 0.0129 min
1 (Supporting Information Figure S2 and Table 1), obtained using the pseudo-first-order model as expressed by36
41 lnðC0 =CÞ ¼ kt The difference in the photocatalytic activities of SnS2-(a
f) was most likely a result of the combined action of many factors, such as particle size, specific surface area, adsorption capacity for Cr(VI), band gap, morphology, composition, crystallinity, crystal defects, and dispersibility, etc. Since almost all of the aforementioned factors were strongly coupled, it was difficult to characterize the specific function and influence of a single parameter in the

Figure 8. HRTEM image of SnS2-AP. The fringe interval of 0.316 nm in this image is consistent with the interplanar spacing of (100) crystal planes of hexagonal phase SnS2.

photocatalytic activity of SnS2 nanocrystals. But, there was a direct correlation between the dark adsorption amounts for Cr(VI) and the photocatalytic activities of SnS2-(a
f) (that is, both the photocatalytic activities and the dark adsorption amounts for Cr(VI) of SnS2-(a
f) followed the same order of SnS2-(c) > SnS2-(d) > SnS2-(f) > SnS2-(e) > SnS2-(b) > SnS2-(a), as shown in Table 1), suggesting that the adsorption capacities for Cr(VI) of SnS2 nanocrystals should play a predominant role in their photocatalytic activities. Because the photocatalytic reactions are commonly believed to occur on the surface of the photocatalyst, the larger adsorption amounts of Cr(VI) onto SnS2 nanocrystals may contribute to the faster reduction rate of Cr(VI).41,42 Photocatalytic Stability. Since the stability of sulfide photocatalysts has always been a concern, it is important to investigate the stability and reusability of the as-synthesized SnS2 nanocrystals in photocatalytic reduction of aqueous Cr(VI). So, in the current work, SnS2-(c) was recycled for five times in the same photocatalytic reactions. After each reuse cycle which lasted for 120 min, the photocatalyst was separated from the aqueous suspension by filtration, washed with 1 mol/L HNO3 aqueous solution (to reduce the amount of greenish Cr(OH)3 deposited on the surface of SnS2-(c)) and deionized water, dried in vacuum at 100 °C for 4 h, and weighed for the next reuse cycle. Taking into account the mass loss of photocatalyst during each reuse cycle, the fourth reuse cycle must be conducted twice in order to accumulate enough sample for the fifth reuse cycle, the third reuse cycle must be conducted twice in order to accumulate enough sample for the fourth reuse cycle, and so on. Figure 7 shows the photocatalytic performance of SnS2-(c) in the first five 9328

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Figure 9. XPS spectra of SnS2-AP and SnS2-(c).

reuse cycles. Apparently, the photocatalytic activity of SnS2-(c) deteriorated with the increase in the number of reuse cycle, but only very slightly. Even in the fifth reuse cycle of SnS2-(c), the reduction ratio of Cr(VI) can still reach 97% after visible light irradiation for 120 min. The product collected after the fifth reuse cycle of SnS2-(c) in photocatalysis (which was hereinafter called SnS2-AP for the convenience of description) was further characterized by means of XRD, HRTEM, and XPS. Both the XRD pattern (Figure 1 SnS2-AP) and the HRTEM image (Figure 8) of SnS2-AP demonstrated that it was still hexagonal phase SnS2. The survey XPS spectrum of SnS2-AP (Figure 9) showed the presence of Sn and S components, as well as Cr, C, and O contaminants. From the high resolution XPS spectra of Sn 3d and S 2p core levels (Figure 9), it can be seen that the binding energies of Sn 3d and S 2p of SnS2-AP were nearly the same as those of SnS2-(c), for instance, the binding energies of Sn 3d5/2 and S 2p3/2 of SnS2-AP and SnS2-(c) were 486.61 and 486.65 eV, 161.68, and 161.74 eV, respectively. Furthermore, the binding energies of Sn 3d5/2 and S 2p3/2 of SnS2-AP and SnS2-(c) were all consistent with the reference data of Sn4+ and S2
in SnS2.11,43,44 Besides, the binding energy of Cr 2p3/2 was observed at 577.36 eV (Figure 9), which corresponded to Cr(III) in Cr(OH)3.45,46 The formation of Cr(OH)3 on the surface of SnS2-AP can be due to the hydrolysis-precipitation of Cr(III) cations, which were generated from the photocatalytic reduction of adsorbed Cr(VI). Unfortunately, the deposition of Cr(OH)3 on the surface of SnS2-(c) was likely to occupy some photocatalytic active sites of the latter, and accordingly decreased slightly the photocatalytic activity of SnS2-(c) during its reuse.46

’ ASSOCIATED CONTENT

bS

Supporting Information. Figure S1 and Figure S2. This material is available free of charge via the Internet at http://pubs. acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: 086 0514 87962581; fax: 086 0514 87975244; e-mail: [email protected].

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