Abstract: SnS 2 nanomaterials with different morphologies were synthesized by hydrothermal method usingdifferent surfactants and different sulfur sources. The influence of reaction condition on morphology and propertywas discussed. The structure and composition of the as-prepared SnS 2 nanomaterials were characterized by X-raydiffraction (XRD), scanning electron microscope (SEM) and Brunauer-Emmett-Teller (BET) surface area analysis.The photocatalytic performance of the as-synthesized SnS 2 was evaluated by catalytic degradation of Rhodamine B(RhB). The results show that the surfactant and sulfur source play an important role in the structure andmorphology of SnS 2 . When the molar ratio of Sn 4+ to Surfactant is 1∶1, the samples are all pure hexagonal phaseSnS 2 . The obtained SnS 2 nanoplates employing sodium citrate as surfactant and thiourea as sulfur source show thebest photocatalytic performance and the larger BET surface area.

Key words: hydrothermal method; SnS 2 ; photocatalysis; nanomaterials

Metal sulfides have attracted great interestnowadays because of their narrow band gapscompared with corresponding oxides. They could be good candidates for photovoltaic materials,photocatalysts, and water treatment especially whenthey are hierarchical structures [1-3] . So far, there havebeen many methods for preparing metal sulfides withdiverse morphologies [4-11] . Among all these methods,surfactant-assisted hydrothermal method is simple andwidely used in the preparation of nanomaterials.

SnS 2 has a CdI 2 -related crystal structure, which isconsisted of two layers of hexagonal close packedsulfur anions with sandwiched tin cations octahedrallycoordinated by six nearest neighbor sulfur atoms. Theadjacent sulfur layers are bound by weak van derWaals interactions [12] . It has been used as a semicon-ductor with a bandgap of 2.18~2.44 eV [13-15] , and hasgood stability in acid and neutral aqueous solution aswell as certain oxidative and thermal stability in air,which makes it a promising visible light-sensitivephotocatalyst [16] . At the same time, for a semiconductor,SnS 2 has also been used as Li-ion battery anodematerials[17-19] , opto-electronic devices [20-21] ,sensors[22] ,etc. Nowadays, SnS 2 nanomaterials have beenprepared in the form of nanowires [23] , nanoparticles [24] ,nanoplates [25] , nanosheets [26] and nanoflowers [27] , respec-tively. To the best of our knowledge, the photocataliticproperty of such nanomaterials with different morphol-ogies has not been compared with each other so far.

In this work, SnS 2 nanomaterials with differentmorphologies have been prepared via simplehydrothermal method with different surfactants andsurfur resources. The preparation conditions wereinvestigated in detail. The photocatalytic performanceof the SnS 2 with different morphologies was evaluatedby degrading Rhodamine B (RhB).

1 Experimental

1.1 ReagentsStannic chloride pentahydrate, thiourea, thioace-tamide, sodium citrate, L-arginin, glycine and absoluteethanol (all in A.R. grade) were purchased fromBeijing Chemical Company and were used withoutfurther purification.

1.2 Synthesis of SnS 2 by thioureaIn a typical experiment, 1 mmol SnCl 4 · 5H 2 O and2 mmol thiourea were dissolved in 30 mL of deionizedwater, and then 2 mmol/1 mmol surfactant (sodiumcitrate, L-arginine, glycine) was added under magneticMetal sulfides have attracted great interestnowadays because of their narrow band gapscompared with corresponding oxides. They could be good candidates for photovoltaic materials,photocatalysts, and water treatment especially whenthey are hierarchical structures [1-3] . So far, there have stirring. The above solutions were transferred to a 50mL Teflon-lined stainless steel autoclave and heatedin an electronic oven at 150 ℃ for 24 h. All thesamples were washed several times using deionizedwater and absolute ethanol, respectively, and dried inan oven at 60 ℃ overnight for further characterization.

1.3 Synthesis of SnS 2 by thioacetamide (TAA)In a typical experiment, 1 mmol SnCl 4 · 5H 2 O, 2mmol thioacetamide and 1 mL concentrated hydro-chloric acid were dissolved in 20 mL of deionizedwater, and then 1 mmol surfactant (sodium citrate, L-arginine, glycine) was added under magnetic stirring.The above solution was transferred to a 50 mL Teflon-lined stainless steel autoclave and heated in anelectronic oven at 120 ℃ for about 12 h. All thesamples were washed several times using deionizedwater and absolute ethanol, respectively, and dried inan oven at 60 ℃ overnight for further characterization.In order to study the effect of surfactants, the blankexperiment was also performed.

1.4 CharacterizationThe crystalline structure of the samples werecharacterized by powder X-ray diffraction (XRD)employing a scanning rate of 0.125° · s -1 in a 2θ rangeof 10° to 90°, using a Rigaku D/max-RA X-ray diffra-ctometer equipped with monochromatized Cu Kαradiation (λ=0.154 18 nm). The morphology and sizeof the samples were inspected by a field emissionscanning electron microscope (FE-SEM, XL-30, FEICompany). BET surface areas were measured with theV-Sorb 2800P surface area and porosity analyzer. Thephotocatalytic properties were analyzed by the UV-1240 spectrophotometer.

1.5 Photocatalytic testsThe photocatalytic property of the products wasevaluated by degrading aqueous RhB (Rhodamine B)under ultraviolet light irradiation. Prior toillumination, 100 mL of RhB aqueous solutionscontaining different samples (50 mg) were ultrasonicdispersed in the dark for 10 min to achieveadsorption/desorption equilibrium between thephotocatalysts and RhB. At given time intervals, 6.0mL of the suspension was removed for analysis after centrifugation. The sample powders were thenseparated by centrifuging and the aqueous RhBsolutions were analyzed. The experiments were carriedout at room temperature in air. The concentration ofthe aqueous RhB solutions was determined bymonitoring the height of the maximum of theabsorbance in the UV-Visible spectra by an UV-1240spectrophotometer.

2 Results and discussion

2.1 XRD analysisThe XRD patterns of the as-prepared SnS 2 bythiourea with different molar ratios of Sn to Surfactantare shown in Fig.1. As can be seen from the XRDpatterns, high crystallinity can be obtained for thedifferent surfactant systems. From Fig.1(a~c) it can beseen that when the molar ratio of Sn 4+ to surfactant is1∶2, the products are not in accordance with any purehexagonal SnS 2 . Some peaks of SnO 2 appear in Fig.1ato Fig.1c. When the surfactant is sodium citrate (a), itis almost all the peaks of SnO 2 . When the molar ratioof Sn 4+ to surfactant is 1∶1, the sharp patterns indicatethat the products are well crystallized and all thepeaks can be indexed to a pure hexagonal SnS 2 (PDFNo.83-1705) from Fig.1d to Fig.1g. We can thusconclude that the dosage of surfactants plays animportant role in the formation of the pure phase.The XRD patterns of the as-prepared SnS 2 by thioacetamide are shown in Fig.2. All the identifiedpeaks of the XRD patterns can be unambiguouslyassigned to hexagonal SnS 2 (PDF No.23-0677). Whenthe molar ratio of Sn 4+ to surfactant is 1∶1, purehexagonal SnS 2 could be obtained.Amino acids have hydrolysis equilibrium insolution, and sodium citrate is a weak acid-strong-alkaline salt. When the dosage of the surfactantincreases, there are more OH-in the solution, thenSn 4 + prefers to combine with OH-to form Sn(OH) 4which is then oxidized to SnO 2 rather than to combinewith S 2- to form SnS 2 . When the dosage of the surfactantdecreases, there are few OH - in the solution, then Sn 4+will combine with S 2- to form SnS 2 .

2.2 SEM analysisFrom Fig.3(a~d), one can see that differentmorphologies are obtained by different surfactantswith the same sulfur source. Fig.3a exhibits overallan anisotropic lamellar morphology. The origin for theformation of flake-like morphology may be stronglyrelated to the intrinsic anisotropic nature of hexagonalphase SnS 2 , which is a CdI 2 -related crystal structure.The length of these individual nanoflakes is 400~500nm, the thickness is 20 nm. Fig.3b displays that thesamples are made up of nanoplates. The length of theindividual nanoplate is 400~600 nm, the thickness is40 nm. As shown in Fig.3c the samples are mainlyhexagonal nanoplates and the small parts ofnanoplates with interpenetrating growth order areconnected each other to form large flowerlike architectures. The diameter of flowerlike architectureis 5 μm. When the surfactant is changed into glycine,the morphology is converted into the structure ofoctahedrons in Fig.3d, and it has a uniformdistribution, the edge of octahedrons is 200 nm or so.From the above results, the surfactants have a certaininfluence on the formation of morphology.The morphology is different by changing thesulfur source with the same surfactant from Fig.3(d,e).The thioacetamide and thiourea could both behydrolyzed in solution, but their hydrolyzing rates aredifferent. The ability of supplying electron of NH 2 inthiourea is better than that of CH 3 in thioacetamide,so the hydrolyzing rate of thiourea is lower thanthioacetamide. Thus the nucleation of thioacetamide isthat of faster and the morphology is more uniform.The representative EDS spectrum (Fig.3f) showsthat the products consist of Si, Au, Sn and Selements. The Si and Au elements come from thetesting process, so all the samples are consisted of Snand S. Combined with the XRD analysis, it couldinfer that the samples are hexagonal SnS 2 .

2.3 Photocatalytic properties of the SnS 2nanomaterialsThe absorption intensity is transformed into the degradation ratio, which can be defined as:Degradation ratio=(A 0 -A t )/A 0 ×100%where A 0 and A t are the absorbance intensity of RhBaqueous solution before illumination (t=0 min) andafter illumination for t min, respectively.Fig.4 shows the degradation of aqueous RhB inthe presence of the SnS 2 nanomaterials with differentmorphologies under Ultravoilet light irradiation. Totalconcentration of RhB is simply determined by themaximum peaks of the RhB located at 553 nm. After UV light irradiation for 90 min, the degradationpercentage exhibits the following order: 92.25%(b) >91.57%(e) > 80.25%(a) > 79.51%(c) > 61.15%(d).The results indicate that the photocatalytic perfor-mance varies with samples of different surfactants anddifferent sulfur sources, and the hexagonal SnS 2nanoplates employing sodium citrate as the surfactantand thiourea as the sulfur source display the bestphotocatalytic property.

2.4 BET surface areaThe values of the BET specific surface areas ofthe samples are listed in Table 1, which exhibit thefollowing order: 57 m 2 · g -1 (b) > 54 m 2 · g -1 (e) > 40 m 2 ·g -1 (a)> 36 m 2 · g -1 (c) >26 m 2 · g -1 (d). It is noteworthythat the photocatalytic degradation rate is related toBET specific surface area. Therefore surface area isthe main influence factor on the photocatalyticproperty. The reason may be that higher surface areaprovides more photocatalytic active centers, whichresult in higher photocatalytic degradation rate. At thesame time, the morphology and the crystallization alsoplay important roles in photocatalytic degradation.

3 ConclusionsIn summary, flake-like, flower-like andoctahedral structures of SnS 2 nanomaterials have beenprepared with surfactant-assisted hydrothermalmethod. The pure hexagonal phase of SnS 2 could beobtained when the molar ratio of Sn 4+ to surfactant is1∶1. The BET surface area affects the photocatalyticperformance of the samples, and the hexagonal SnS 2nanoplates employing sodium citrate as the surfactantand thiourea as the sulfur source display the bestphotocatalytic property.

Acknowledgments: This work was supported by theNational Natural Science Foundation of P. R. China (NSFC)(Grant No.51072026) and the Development of Science and Technology Plan Projects of Jilin province (Grant No.20130206002GX).

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