Synthesis of a hierarchical SnS2 nanostructure for efficient adsorption of Rhodamine B dye

Accepted Manuscript Regular Article Synthesis of a hierarchical SnS2 nanostructure for efficient adsorption of Rhodamine B dye Shuyu Wang, Bin Yang, Yuping Liu PII: DOI: Reference:

S0021-9797(17)30822-6 http://dx.doi.org/10.1016/j.jcis.2017.07.053 YJCIS 22582

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

27 April 2017 10 July 2017 16 July 2017

Please cite this article as: S. Wang, B. Yang, Y. Liu, Synthesis of a hierarchical SnS2 nanostructure for efficient adsorption of Rhodamine B dye, Journal of Colloid and Interface Science (2017), doi: http://dx.doi.org/10.1016/ j.jcis.2017.07.053

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Synthesis of a hierarchical SnS2 nanostructure for efficient adsorption of Rhodamine B dye

Shuyu Wanga,b, Bin Yangc, Yuping Liua,b, a

b

Research Center for Analytical Sciences, College of Chemistry, Nankai University,

Tianjin Key Laboratory of Biosensing and Molecular Recognition, Tianjin 300071, China c

Shenyang Branch, Shimadzu (China) Co., Ltd, China

Corresponding address: College of Chemistry, Nankai University, Tianjin, 300071, P. R. China Tel: +86-22-23503034 Fax: +86-22-23499394 E-mail: [email protected]



Corresponding author. 1

Abstract Using Sn2+ instead of Sn4+ as Sn source, H2O2 as oxidizer, and L-cysteine as sulphur source and structure-directing agent, hierarchical SnS2 nanostructure built with the thickness of 10 nm nanosheets was successfully synthesized by a green hydrothermal process. The morphology and structure of the products were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) with energy-dispersive X-ray spectrometer (EDS) and Elemental mapping, fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). The as-prepared nanostructured SnS2 nanomaterials were tested for adsorption of Rhodamine B (RhB) from aqueous solution. The effects of contact time, adsorbent dosage and initial concentration were investigated in detail. The adsorption kinetic data were well fitted to the pseudo-second-order model and the equilibrium adsorption data were well described by the Langmuir isotherm model, with a maximum capacity of 200.0 mg g-1. This study provides a novel approach to obtain metal sulfide nanostructures using low-valence metal salts.

Key words: SnS2; L-Cysteine; H2O2; Adsorption behavior; Rhodamine B

2

1. Introduction In recent years, more attention have been paid to severe organic water pollution caused by the discharge of dye solutions from textiles, paints, leathers and paper industries [1-5]. The dye-containing effluence is highly toxic, and it may cause serious environmental issues and healthy problems of human being [3,6,7]. Therefore, it is necessary to remove dyes from aqueous solution. Several physical [2,8], chemical [9,10] and biological decolorization [11] methods have been applied to remove dyes from wastewater. In fact, many dyes are chemically stable for their application and not easy to be degraded by biological decolorization method [3]. Therefore, the degradations via photocatalysis [10,12-14] and chemical oxidation [15] and adsorption [16-18] are main ways to remove the dyes from aqueous solution. Generally, the degradations are to break down the complex molecular structure of organic dyes. Nevertheless, dyes may break down to yield toxic, harmful and carcinogens compounds if their degradation are inadequate. Compared with the degradations, adsorption technique is a simple and green physical method to separate dyes from waste water through enriching process [8,19-23]. In the past decade, a series of natural adsorbents, including activated carbon materials [16], clay [17] and silicon nanomaterials [18], have been developed for the removal of dyes from wastewater. However, the above natural adsorbents have some disadvantages, such as nonresistance to acid, small surface area and relatively long adsorption contact time [3,24]. Thus, researchers showed more devotion to the synthetic adsorbents. Besides the 3

typical resin-type adsorbents [25], metal oxide-type adsorbents attracted more interest in the treatment of dye wastewater due to their significant adsorption capacities [20,26]. For example, NiO-SiO2 hollow microspheres were excellent adsorbents for Congo red wastewater, with a maximum adsorption capacity of 204.1 mg g -1 [20]. Hierarchical WO3 hydrates synthesized by an ion exchange method presented excellent adsorption performance for methylene blue organic dye from wastewater, which resulted from high specific surface area and the negative charge functional groups on surface, such as W=O and O-H [26]. Recently, metal sulfides were used to be adsorbents [27-29]. Li et al. [27] synthesized MoS2 ultrathin nanosheets through a hydrothermal process and their maximum adsorption capacity for selective removal of Methylene blue from aqueous solution reached up to 146.43 mg g-1. And uniform 3D flower-like MoS2 nanostructures were applied as an adsorbent in Rhodamine B (RhB) aqueous solution with a maximum adsorption capacity of 49.2 mg g-1 [28]. Wu et al. [29] reported that SnS2 nanosheet-based microstructures used as photocatalysts demonstrated good adsorption performance for RhB during the adsorption–desorption equilibrium in the dark. Based on the high specific surface areas, specific structure and abundant oxygen-containing functional groups of graphene oxide, Lu et al. [19] synthesized SnS2/reduced graphene oxide flake-on-sheet nanocomposites via in situ reduction of graphene oxide by Sn2+ and used them for RhB dye removal with the maximum adsorption capacity of 49.2 mg g-1. SnS2 is nonpoisonous, chemically stable in acidic and neutral aqueous solutions, and therefore has potential to be a promising adsorbent. Well-defined SnS2 4

nanostructures with various morphologies have been successfully synthesized by many methods including chemical vapor deposition [30,31], polyol refluxing process [32], hydrothermal [33,34] and solvothermal [35,36] treatments. However, some of the methods mentioned above involved high toxic organic solvent, such as toluene [31] and acetone [34]. They may cause the secondary pollutions. So it is of great importance to search for new friendly synthetic strategies to obtain SnS2 adsorbent materials. As an important amino acid, L-cysteine with sulphur-based structure was widely used as sulphur source and structure-directing agent for the preparation of nanostructured sulphides [37-40]. Using Sn4+ and L-cysteine as starting materials, SnS2/SnO2 composites were synthesized through a microwave-assisted method [39] and three-dimensional hierarchical SnS2 spheres were prepared by a one-pot hydrothermal reaction, respectively [40]. In this paper, utilizing L-cysteine as sulphur source and structure-directing agent and Sn2+ instead of Sn4+ as Sn source, we have successfully achieved nanostructured SnS2 in the presence of H2O2 under hydrothermal condition. Afterwards, we evaluated the adsorption behavior of the obtained samples for the removal of RhB from aqueous solutions. The effects of contact time, adsorbent dosage and initial concentration were investigated, and adsorption isotherms and kinetics were also systematically analyzed.

2. Experimental 2.1 Materials Chemicals including tin sulfate (SnSO4), hydrogen Peroxide (w/% = 30.0%,

5

H2O2), L-cysteine and Rhodamine B (RhB) were purchased from Aladdin Industrial Corporation. All of the chemical reagents used in this experiment were of analytical grade and used without further purification. Deionized (DI) water was used in all experiments. The hierarchical SnS2 nanostructure was synthesized by a simple one-pot hydrothermal treatment. In a typical synthesis, 4 mmol SnSO4 and 12 mmol L-cysteine were first dissolved in 90 mL of deionized water, and then 0.3 g H2O2 was added to the solution. After stirring for half an hour, the above solution was transferred into Teflon-lined autoclave, sealed and maintained at 180 C for 12 h, and then cooled to room temperature naturally. The resulting precipitation was centrifuged and washed several times alternately with deionized water and ethanol to remove possible residues, and then dried at 80 C overnight. The prepared sample was named as S1. In addition, the contrast experiment without H2O2 was carried out under the same condition. The obtained sample was named as S2. Then the collected S2 product was all dispersed into 90 mL aqueous solution containing 0.3 g H2O2, followed by further hydrothermal treatment at 180 C for 12 h. The final product obtained from secondary hydrothermal treatment was named as S3.

2.2 Characterization X-ray power diffraction (XRD) patterns were obtained on a Rigaku D/Max Ф diffractometer with a graphite monochromatic Cu Kα radiation (λ=0.15418 nm).

Scanning electron microscopy (SEM, HITACHI S-4800) and transmission electron

6

microscopy (TEM, JEM-2100F) were conducted to identify the size and morphology. The chemical composition of the prepared nanostructure was measured by EDS (energy dispersive X-ray spectroscopy) performed in TEM. Infrared spectra were recorded on Nicolet Magna-560 Fourier transform infrared spectrometer (FT-IR) in the region of 400-4000 cm-1, X-ray photoelectron spectroscopy (XPS) was performed on a Kratos Axis Ultra DLD spectrometer equipped with a monochromatic Al Kα X-ray source (hγ = 1486.6 eV). N2 adsorption–desorption isotherms of the samples were

acquired using Quantachrome NOVA 2000e apparatus at 77 K. Pore volumes were determined from the data at p/p0 = 0.99, the specific surface areas calculated with Brunauer-Emmett-teller (BET) equation, and pore-size distributions determined by BJH method according to the adsorption branch of the isotherms.

2.3 Adsorption experiments The adsorption of dyes from aqueous solution was carried out at room temperature (about 25 C). Typically, 2 mg of the obtained adsorbent was added to 25 mL aqueous solution containing a certain concentration of Rhodamine (RhB). The resulting mixture was then stirred for 12 h to reach the adsorption-desorption equilibrium. The aqueous solutions was centrifuged at 12000 rpm for 5 min to remove the adsorbent and then monitored by a UV-2550. The residual concentration of RhB (Ci) was determined by monitoring the change in the main absorbance centered at 553 nm. The equilibration adsorption capacity Qe (mg g-1) and the adsorbent amount Qt (mg g-1) at time t (min) were calculated by the following equations:

7

Qe = (C0-Ce) V/m Qt = (C0-Ct) V/m where C0, Ce, and Ct (mg L-1) are the initial, equilibrated and t-time concentrations of RhB aqueous solution, respectively; V (L) and m (g) are the volume of solution and the amount of adsorbent, respectively. In addition, the effects of contact time, adsorbent dosage and initial concentration were systematically investigated.

3. Results and discussion 3.1 Analysis of structure and morphology To examine the crystallinity and crystal phase, the obtained samples were characterized by XRD (Fig. 1). As shown in Fig. 1a for S1 prepared with H2O2 from one-pot hydrothermal treatment, all the sharp diffraction peaks can be readily indexed to a pure hexagonal phase of SnS2 (JCPDS Card Feil No.23-0677). Apparently, no any diffraction peaks of other phase were detected, indicating its high crystallinity. Meanwhile, the blank sample S2 obtained without H2O2 is the mixture of hexagonal SnS2 (JCPDS Card Feil No.23-0677) and orthorhombic SnS (JCPDS Card Feil No.39-0354) (Fig. 1b). Herein, H2O2 as an oxidizer causes a phase transformation from SnS to SnS2 for S1. However，as for S3 obtained from further treated S2 with H2O2, its XRD pattern indicates that hexagonal SnS2 (JCPDS Card Feil No.23-0677) , orthorhombic SnS (JCPDS Card Feil No.39-0354) and tetragonal SnO2 (JCPDS Card Feil No.41-1445) coexist (Fig. 1c). The addition of H2O2 causes part of SnS phase to

8

transform into SnO2 phase and SnS2 phase. It could be concluded that the one-pot hydrothermal reaction is more complete, compared with secondary hydrothermal treatment.

Fig. 1. XRD patterns of the prepared samples. (a) S1; (b) S2; (c) S3.

The general morphology of as-prepared samples was investigated by SEM. A large number of uniform nanostructured spheres with an average diameter of 3-4 m can be observed from the SEM image of S1 (Fig. 2a). It is obviously seen from the high-resolution image（Fig. 2b）that the hierarchical SnS2 consists of thin layers with a thickness of ~10 nm and these thin layers are curling up and regularly arranged. S2 also exhibits the nanostructured spheric morphology (Fig. 2c). The obtained microspheres are made up of nanolayers with a thickness of ~13 nm (Fig. 2d). These nanolayers are slightly thicker and arranged randomly compared with those of S1. From the SEM images of S3 (Fig. 2e and f), irregular nanosheets are only observed. It might be ascribed to the damage of nanostructured spheric morphology resulting from 9

the secondary hydrothermal treatment. In conclusion, the morphology of S1 obtained from one-pot hydrothermal treatment is more uniform and regular.

Fig. 2. SEM images of the obtained samples. (a, b) S1, the inset is a magnified image of nanosheets; (c, d) S2 and (e, f) S3.

To determine the detailed microstructure and crystal quality, S1 was examined by transmission electron microscopy (TEM). From the representative TEM image of S1 (Fig. 3a), an intact hierarchical nanostructure composed of thin layers is clearly observed, which is in good agreement with the SEM results of S1. The high resolution TEM image of S1 (Fig. 3b) displays clear and continuous lattice fringes and the typical interplanar distance of the lattice fringes is measured to be 0.295 nm, 10

corresponding well to the (002) crystalline planes of a hexagonal SnS2 [41]. In addition, selected area electron diffraction pattern (SEAD) (Fig. 3c) further reveals the single crystalline characteristic of S1. TEM-EDS elemental mapping analysis and EDS spectrum were used to further characterize the composition of S1. The EDS elemental maps of Sn and S (Fig. 3d and e) show that Sn and S are evenly distributed on the thin layers. Except the C and Cu peaks from TEM grid, only the elements Sn and S are detected in the EDS pattern (Fig. 3f), indicating that the pure product has been successfully synthesized.

Fig. 3. TEM characterization of S1. (a) TEM image; (b) HRTEM image; (c) SAED pattern; EDS elemental maps of (d) Sn and (e) S; (f) EDS spectrum.

11

3.2 FT-IR analysis FT-IR analysis was applied to investigate the surface composition of S1 with and without RhB. As shown in Fig. 4, the bands located at around 3430 and 1480 cm-1 are attributed to the stretching and bending vibration of –OH of physisorption water molecule [42], and the peak at 3430 cm-1 in Fig. 4b is stronger than that in Fig. 4a. The adsorption band at 1451 cm-1 may belong to the presence of the benzene ring fragments [43]. The peak of asymmetric C-H vibration of benzene ring is around 2923 cm-1 [19], while the adsorption band at 1615 cm-1 corresponds to the aromatic C=C bonds [43]. The adsorption bands at about 1240 and 1150 cm-1 are characteristic peaks of RhB. Hence, RhB molecules were adsorbed on the surface of SnS 2.

Fig. 4. FT-IR spectra of (a) S1, (b) S1 after RhB adsorption and (c) RhB.

3.3 XPS spectra The surface chemical composition of S1 was further confirmed by X-ray photoelectron spectroscopy（XPS）analysis. As the reference at 284.6 eV, C 1s is 12

standardized for specimen charging of the binding energies obtained in the XPS analysis. Fig. 5 presents XPS spectra taken from Sn and S regions of S1. The two strong peaks at the binding energies of 486.2 eV and 494.6 eV (Fig. 5a) can be assigned to Sn 3d5/2 and Sn 3d3/2, respectively, which is characteristic of Sn4+ in SnS2. The gap between Sn 3d5/2 and Sn 3d3/2 level is 8.40 eV, which is approximately the same as in the standard spectrum of Sn [44]. In Fig. 5b, each S 2p region spectrum consists of a 2p3/2-2p1/2 doublet with a 1.2 eV splitting own to spin-orbit coupling. The peaks at 161.7 eV and 163.1 eV can be assigned to S 2p 3/2. They are associated with SnS2 and the sulfide just as the residual L-cysteine respectively, which is in agreement with the reported data [45, 46].

Fig. 5. XPS region spectra of S1: (a) Sn 3d and (b) S 2p.

3.4 Adsorptive performance From an economical standpoint of design of wastewater treatment system, equilibrium time is considered as an important parameter. Thus, effect of contact time for RhB adsorption onto the obtained samples was studied. Fig. 6 shows the effect of 13

contact time on adsorption of 100 mL 5 mg L-1 RhB aqueous solution containing 40 mg samples. As shown in Fig. 6, rapid adsorption happened in the first 5 min for S2

Fig. 6. Effect of contact time for RhB adsorption onto the obtained samples. (a) S1, (b) S2 and (c) S3.

and S3, but in the first 15 min for S1. This phenomenon was attributed to the abundant availability of active sites on the sample surface and the electrostatic attraction between cationic dye RhB and negative charged sample surface [47]. Subsequently, these sites were occupied by RhB molecule and the electrostatic repulsion between the RhB molecule existing in the aqueous solution and the RhB molecule adsorbed on the surface of samples increased gradually [48], which made the adsorption less efficient. Finally, the adsorption equilibrium was established. On the whole, the hierarchical S1 nanostructure possesses faster adsorption rate and higher adsorption capacity than S2 and S3. It is generally accepted that the hierarchical structure can efficiently improve the diffusion of dye molecules and offer 14

more excellent transport paths, and large surface area can provide abundant adsorption sites [49]. According to the N2 adsorption-desorption isotherms (Fig. S1 and S2), the pore structural parameters were summarized in Table. 1. Table. 1 demonstrates that S1 has higher BET surface area and larger pore volume compared with S2 and S3. Thus, it might be believed that this good adsorption performance of S1 might be ascribed to synergetic effects among large BET surface area, well-defined hierarchical structure and the adsorption between S2- and cationic dyes. Table. 1. Structure parameters of S1, S2 and S3. Sample

BET-specific surface area (m2/g)

Pore volume (cm3/g)

Pore diametera (nm)

S1 S2 S3

48.6 16.7 26.2

0.40 0.12 0.22

32.76 29.77 32.87

a

Calculated from the adsorption branch of the isotherm.

Considering the above analysis of microstructure and adsorption efficiency, well-defined hierarchical S1 was selected for follow-up adsorption evaluation. During an adsorption process, external mass transfer in bulk liquid phase, boundary layer diffusion and intraparticle mass transfer are three approaches. In order to investigate the mechanism and rate-controlling step, pseudo-first-order and pseudo-second-order kinetic models are adopted to examine the overall adsorption process. The pseudo-first-order and pseudo-second-order kinetic models can be expressed as the following equations:

ln(Qe-Qt) = lnQe-k1t t/Qt = 1/k2Qe2+t/Qe where Qe and Qt (mg g-1) are the dye adsorption capacity at equilibrium and at time t 15

(min).

The

parameter

k1

and

k2

represent

the

pseudo-first-order

and

pseudo-second-order rate constant for the kinetic model respectively. The parameters k1 and Qe for the pseudo-first-order kinetic model could be determined from the slope and intercept of the plots of log(Qe-Qt) versus t. And for the pseudo-second-order

Fig. 7. Kinetic adsorption data plots for RhB: (a) pseudo-first-order kinetics plot ln(Qe-Qt) vs.t and (b) pseudo-second-order kinetics plot t/Qt vs.t.

kinetic model the Qe and k2 values can be obtained from the slopes and intercepts of plots of t/Qt versus t which are illustrated in Fig. 7. The correlation coefficients (r 2) of linear plots at different concentration is 0.825 for pseudo-first-order and is 0.998 for pseudo-second-order. Therefore, the adsorption of RhB onto SnS2 nanomaterials is due to a chemical adsorption. The effect of adsorbent dosage on adsorption of RhB was investigated by changing the amount of S1 in 30 mg L-1 RhB aqueous solution. As shown in Fig. 8, it reveals that equilibration adsorption capacity decreased from 150.9 to 48.40 mg g-1 with the increase of adsorbent dosage from 2 mg to 12 mg. When the absorbent 16

dosage is more than 4 mg, equilibration adsorption capacity decreased significantly. In addition, the residual concentration of RhB (Ce) in the aqueous solution also decreased. It means the removal percentage of RhB increased，which might be attributed to the greater surface area and the availability of more adsorption sites [50].

Fig. 8. Effect of adsorbent dosage on adsorption of RhB.

Initial concentration has a great influence on the adsorption of RhB, as shown in Fig. 9. The experimental results demonstrate that the equilibration adsorption capacity increases from 58.90 to 192.8 mg g-1 when initial RhB concentration ranged from 5 to 100 mg L-1. The variation of initial dye concentration provides the necessary driving force to overcome the mass transfer of RhB between the aqueous and solid phases. Thus, the interaction was enhanced between dye and SnS2 with increasing in initial concentration.

17

Fig. 9. Effect of initial concentration on the adsorption of RhB.

Adsorption isotherm models have been proposed to explain adsorption equilibrium and expressed the interactive behavior between the adsorbents and adsorbates. The understanding of adsorption isotherms is required for design of adsorption processes and analysis of the adsorption data. To describe the adsorption capacities and affinity of SnS2, the Langmuir isotherm model and Freundlich model were used to analyze the adsorption data. The Langmuir adsorption is based on the assumption of monolayer adsorption on homogeneous adsorbent surface, where all the sorption sites are identical and energetically equivalent, and there is no interaction between the adsorbed molecules. It can be expressed as follows:

Ce/Qe=Ce/Qm+1/kLQm

where Ce is the concentration of RhB at equilibrium (mg L-1), Qe is the amount of dye adsorbed by the SnS2 at equilibrium (mg g-1), Qm is the theoretical maximum adsorption capacity corresponding to monolayer coverage (mg g-1), and kL is the 18

Langmuir isotherm model constant (L mg-1). The liner form was shown in Fig. 10a, suggesting that the adsorption behavior was well fitted to the Langmuir model. From

Fig. 10. (a) Langmuir adsorption isotherm and (b) Freundlich adsorption isotherm data plot for RhB on S1 at room temperature.

the slope and intercept, the value of Qm and kL can be calculated to be 200.0 mg g-1 and 0.2381 L mg-1. Another common model Freundlich isotherm is also used to match the experiment data. It is applicable to the adsorption on heterogeneous surfaces with interaction between adsorbed molecules. The equation is given as follows:

lnQe=lnkF + (1/n) lnCe

Where kF (L mg-1) is the Freundlich adsorption equilibrium constant and indicates the adsorption capacity. And n is another characteristic coefficient relating to adsorption intensity. They were determined from the intercept and slope of the fitting plot. The value of 1/n is 0.3997, which is smaller than 1, representing that the adsorption process is favourable. The parameters are listed in Table 2. The correlation coefficient 19

of Langmuir model is better than that of Freundlich model, so the equilibrium data were well described by the Langmuir isotherm model.

Table. 2. The value of the parameters about adsorption isotherm models. Isotherm Paramenters Value

Langmuir Qm(mg/g) 200.0

Freundlich kL 0.2381

2

r 0.9930

1/n 0.3997

kF 35.64

r2 0.9061

The maximum absorption capacity of the hierarchical SnS2 nanostructure towards RhB in this work with some different adsorbents reported previously are compared in Table 3. It can be seen that the removal ability of RhB onto the present nanomaterial is lower than Gg-cl-P(AA-co-AAm)/Fe3O4 nanocomposite but much higher than other adsorbents listed. Therefore, corresponding to the environmentally friendly design aspect, the hierarchical SnS2 nanostructure is a promising adsorbent material for RhB in wastewater.

Table. 3. Comparison of the adsorption capacities on various adsorbrnts for RhB. Adsorbents

Adsorption capacity (mg g-1)

Ref.

S1 SnS2/rGO MoS2 Fe3O4/rGO Gg-cl-P(AA-co-AAm)/Fe3O4 nanocomposite Modifying Fe3O4 nanoparticles with humic acid W18O49 nanowire WO3 Iron-pillared bentonite Jute stick powder Kaolinite

200 94.07 49.2 142.86 654.87 161.8 120 64 98.62 87.7 46.08

This work [19] [28] [51] [52] [53] [54] [55] [56] [57] [58]

20

4. Conclusion In summary, hierarchical SnS2 nanostructure self-assembled by thin nanosheets was successfully synthesized by Sn2+ and L-Cysteine through hydrothermal method under H2O2 assisted. The experimental results show that the prepared SnS2 nanostructure is an excellent adsorbent for the removal of RhB from aqueous solution. The isotherm study indicates that adsorption data fit well with pseudo-second-order model and the maximum uptake capacity of SnS2 for RhB is 200.0 mg g-1. The obtained SnS2 nanostructure might be used as a low-cost and relatively efficient adsorbent for removal of RhB from aqueous solution. This work provides a green synthetic route for metal sulfide nanostructures.

Acknowledgements This work was financially supported by the Natural Science Foundation of Tianjin (17JCYBJC17100), which is gratefully acknowledged.

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