Facile Synthesis and Assemblies of Flowerlike SnS2 and In3+-Doped SnS2: HierarchicalStructures and Their Enhanced Photocatalytic Property

Yongqian Lei,†,‡ Shuyan Song,†,‡ Weiqiang Fan,†,‡ Yan Xing,†,‡ and Hongjie Zhang*,†

State Key Laboratory of Rare Earth Resource Utilizations, Changchun Institute of Applied Chemistry,Chinese Academy of Sciences, Changchun 130022, Jilin, China, and Graduate School of the Chinese Academyof Sciences, Beijing 100039, P. R. China

ReceiVed: September 9, 2008; ReVised Manuscript ReceiVed: NoVember 16, 2008

Novel flowerlike SnS2 and In3+-doped SnS2 hierarchical structures have been successfully synthesized by asimple hydrothermal route using biomolecular L-cysteine-assisted methods. The L-cysteine plays an importantrole both as assistant and as sulfur source. Experiments with various parameters indicate that the pH valueshave a strong effect on the morphology of the assembly. Based on the experiments, a growth mechanicalprocess was proposed. The synthetic samples were characterized by XRD, SEM, TEM (HRTEM), BETmeasurement, TGA, and XPS in detail. Further investigation of the photocatalytic degradation of three differentdyes, methylene blue, methylene green, and ethyl violet, indicate that both samples have high photocatalyticactivity, and the doped In3+ has enhanced the photoactivity of SnS2.

Introduction

In the past decades, ordered nanostructures, assemblies usingsemiconductor nanoparticles, nanorods, nanobelts, and nanosheets,as building blocks, have attracted great interest because of theircollected physics and chemistry properties.1 It is an importantprocess for the fabrication of functional electronic and photonicnanodevices using these building blocks. In this field, remarkableprogress has been made for the controllable synthesis of complexinorganic materials.2 However, it remains a big challenge tofreely operate the material at the micro/nanolevel using a facilemethod. Among all the semiconductors, the metal chalcogenideshave driven extensive research interest when they were preparedat the micro/nanolevel.3 Metal sulfides have narrow band gapsand band at relatively negative potentials compared withcorresponding oxides, which can be good candidates forphotovoltaic materials, photocatalysts, and water treatmentespecially when they have hierarchical structures.4 Nowadaysdiverse morphologies such as fullerene-like nanoparticles,nanobelts, nanotubes, and nanoflakes have been synthesized byvarious methods including chemical vapor deposition, electro-chemical deposition, molecular beam epitaxy, template-assistedsolvothermal, hydrothermal, and solvothermal treatments.5

However, there are still very few reports on a general facile,low-energy spent route. Of all the methods, biomolecule-assistedsynthesis methods have became a new and promising focus inthe preparation of various micro/nanomaterials. Biomoleculesplay an important role in morphology control as shown in ourprevious work and are also environmently friendly ways topractice green chemistry.6 Recently, Xie et al. synthesized Bi2S3

and PbS hierarchical architectures by biomolecular L-cysteine-assisted methods.7 Inspired by their work, it is interesting toinvestigate the preparation and assembly of other sulfidenanomaterials by L-cysteine-assisted biological approaches. It

is also necessary to fabricate facile, rapid synthesis methods ofmicro/nano hierarchical materials to meet the potential applica-tion fields.

As one of the important IV-VI semiconductors, SnS2 isknown for its strong anisotropy of optical properties andpotential applications in efficient solar cell materials as well aselectrical switching.8 In our previous work, SnS2 nanosheetswere synthesized by the surfactant-assisted hydrothermal route,which represented excellent gas sensitivity.9 In this work, wereported a facile one-pot hydrothermal route synthesis offlowerlike SnS2 and In3+-doped SnS2 hierarchical structures andinvestigated the structure and morphology with comprehensivetechniques including electron microscopy (SEM/TEM), X-raydiffraction (XRD), XPS, TGA, and nitrogen adsorption char-acteriztion. By monitoring the structure evolution with variousexperimental parameters during the reaction process withelectron microscopy, we proposed a formation mechanism ofthe hierarchical flowerlike structure assembly process. Further-more, it is known that composition affected their properties.Therefore, another aim of this work is to investigate theevolution of photocatalytic activity because the serious envi-ronmental issues associated with industrial pollution drive usto search for new catalytic materials for contamination treatment.It is also a hot topic for the direct solar-to-chemical conversion/degradation of organic pollutants by semiconductor-basedcatalysts in environmental science.10

Experimental Section

All the reagents were of analytical grade and were purchasedand used as received without further purification. In a typicalexperiment, 0.35 g (1 mmol) SnCl4 ·5H2O and 0.25 g (about2.1 mmol) L-cysteine were dissolved in 30 mL of deionizedwater, and then the mixture underwent ultrasonic treatment fora few minutes to become a transparent solution. The abovesolution was transferred to a 50 mL Teflon-lined autoclave andheated under an electronic oven at 160 °C for about 24 h. TheIn3+-doped SnS2 sample was obtained when 0.4 mL of 0.05 MInCl3 (2%) solution was added before hydrothermal treatment.The samples were washed several times using deionized water

* To whom correspondence should be addressed. Telephone: +86-431-85262127. Fax: +86-431-85698041. E-mail: hongjie@ciac.jl.cn.

† Changchun Institute of Applied Chemistry, Chinese Academy ofSciences.

‡ Graduate School of the Chinese Academy of Sciences.

J. Phys. Chem. C 2009, 113, 1280–12851280

10.1021/jp8079974 CCC: $40.75 2009 American Chemical SocietyPublished on Web 01/06/2009

and ethanol, respectively, and dried in a vacuum oven at 60 °Covernight for further characterization.

Characterization

X-ray Diffraction and Electron Microscopy Characteriza-tion. The crystalline and phase purity of the products wereexamined by powder XRD. Measurements were performed ona Rigaku X-ray diffractometer with Cu KR radiation; theaccelerating voltage and applied current were 40 kV and 40mA, respectively. The size, general morphology, and structureof the as-synthesized samples were characterized using field-emission scanning electron microscopy (FEI XL30 EFSEM) atan accelerating voltage of 15 kV and a Hitachi 8100 transmis-sion electron microscope (TEM) at an operating voltage of 200kV.

Surface Properties and TGA Characterization. A ThermoESCALAB 250 X-ray photoelectron spectroscope (XPS) equippedwith a standard and monochromatic source (Al KR hν ) 1486.6eV) was employed for surface analysis. TGA measurementswere performed on a Perkin–Elmer Thermal Analysis PyrisDiamond TG/DTA. The samples were heated under a N2 gasstream from 40 to 700 °C at a heat rate of 10 ° C/min.

Nitrogen Adsorption and Desorption. The specific areasof the powders were determined by using a Micromeritics ASAP2020 specific area and porosity analyzer using the method ofBrunauer–Emmett–Teller (BET). All the samples were degassedat 373 K overnight before BET measurements.

Photocatalytic Activity Test. The photocatalytic measure-ments were carried out in an aqueous solution at ambienttemperature and performed on a three-necked column container,which was equipped with a glass pipe inset. Briefly, a 10 mgsample was suspended in a 300 mL aqueous solution of 0.025mM dye. The cylindrical quartz pipe was surrounded by acirculating water jacket to cool the system. Prior to irradiation,the suspension was magnetically stirred in the dark for 10 minto establish an adsorption/desorption equilibrium. The sampleswere evaluated by degradation of dyes in an aqueous solutionunder a 300 W mercury lamp. The cooling water was placedaround the quartz lamp and pipe wall to take the heat away.The concentration of the dyes during the degradation wasmonitored by collecting solutions in step time using a UV-visspectrometer (TU-1901). Simultaneously, the collected solutionwas filtered to remove the residue before UV-vis characteriza-tion.

Results and Discussion

The phase purity of the prepared samples was characterizedby powder X-ray diffraction (XRD). Figure 1 shows the XRDpatterns of the products, and the sharp pattern indicates thatthe products were well crystallized. All the peaks in the XRDpattern can be readily indexed to a pure hexagonal phase ofSnS2 with lattice constants a ) 3.649 Å and c ) 5.899 Å, whichare in good agreement with the reported values (JCPDS cardNo. 22-0951). Interestingly, the (001) diffraction peak showedthe strongest intensity in the pattern. These observations mayindicate that their (001) planes tend to be preferentially orientedparallel to the surface of the supporting substrate. Comparedwith the XRD patterns of the two samples, the introduction of2% In3+ did not destroy the hexagonal phase structure.

The typical overall SEM image shows that the samples areuniform flowerlike spheres with an average diameter of 1.5 µm(Figure 2a). A magnified view shows that these individualflower-shaped spheres consist of large number of uniformnanoplate architectures as building blocks. These quantities of

nanoplates with interpenetrating growth order are connected toeach other to build flowerlike architectures. With long-timeultrasonic treatment, a part of the individual flowerlike sphereis observed. The deep black TEM image indicates that the nano-plates are closely packed across the flower center. The individualflowerlike structure is further characterized by high-resolutionTEM (HRTEM). As shown in Figure 3, the distortion of the

Figure 1. XRD patterns of the samples (a) SnS2 and (b) In3+-dopedSnS2.

Figure 2. SEM images of the synthesized SnS2: (a) overview, (b)enlarged and (c) TEM image of the samples, and (d) an individualnanoplate. SEM image of the synthesized In3+-doped SnS2: (e)overview, (f) enlarged and (g) TEM image of the samples, and (h) anindividual nanoplate.

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nanoplate is clearly observed. The magnified image shows theclear lattice stripe (Figure 3b) of the nanoplate. This imagereveals that the interplanar distance of the lattice fringes is 0.295nm, which is consistent with the (002) planes of a hexagonalphase SnS2. The fast Fourier transform (FFT) pattern displaysa bright spot of the lattice fringes in Figure 3c, which shows aSnS2 hexagonal phase diffraction pattern.

Interestingly, when 2% In3+ was introduced in the syntheticSnS2 sample, the color of the dried sample changed from buffto reseda. Simultaneously, the assembled building block of thenanoplates seems to be frizzy, and their surface tends to besmooth (Figure 2e). Compared with the pure sample, theflowerlike spheres tend to connect to each other (see enlargedFigure 2f). The difference between the two samples can alsobe seen in the TEM images (Figure 2g and 2h). Unlike the close-packed SnS2 hierarchical structure, the clear contrast of the darkcenter and bright edge indicate that the crimped nanoplates grewoutward. A close view of the individual flower also shows theinner configuration. The building blocks of the nanoplates wereobservably thin and crimped.

The surface electronic states and the chemical compositionof the doped sample were further confirmed by XPS analysis.Figure 4a shows the high-resolution S 2p spectra. The weakshoulder peak may be due to the residual organic sulfur reactant.The two strong peaks of Figure 4b can be indexed to the Sn3d3/2 and Sn 3d5/2, respectively. To precisely investigate the Inelectronic state, the sample was examined in situ with electron

beam treatment for 6 min to remove the surface area. It can beseen that the In 3d peaks are remarkably enhanced (red line).The maximum peak of In 3d5/2 at 444.5 eV can be attributed tothe In-S binding energy (BE). The small offset of the peaks atthe surface (black line) may be attributed to the partial oxidation.Figure 4d shows the overall spectrum of the doped sample; theweak C1s and O1s peaks come from adsorbed reactant andgaseous molecules in the atmosphere.

The normalized TGA analysis curves (Figure 5) indicate thatthe two samples are decomposed at about 430 °C. The slowdecline of the curve before 400 °C may be attributed to theadsorbed water in the atmosphere and the residual organicmolecules on their large surface area. The clearly differentweight percentages of the final products are ascribed to thedecreased percentage weight loss of the doped In atom. Thesamples at different pH values were also tested (see Figure S4).It can be seen that the curves of samples at pH 3 and 7 aresimilar. The remarkable decrease before 400 °C of the sampleat pH 10 may be attributed to hydroxyl at basic conditions. Allthe samples show maximum weight loss at about 430 °C, whichindicates their stability toward the pH value.

Nitrogen gas adsorption-desorption isotherms and Barrett-Joyner-Halenda (BJH) methods characterization further verifiedthe adsorption ability of the hierarchical flowerlike SnS2 (Figure6) and In3+-doped SnS2 (Figure S1). The type-IV isotherm witha hysteresis loop in the range of 0.4-1.0 P/P0 is in accordancewith the assembled platelike structure11 and similar to theflowerlike In2S3 hierarchical nanostructure.12 The quantitativecalculation shows that the BET surface area of as-prepared SnS2

Figure 3. (a) TEM images of the individual SnS2 nanoflower. (b)HRTEM image and (c) corresponding FFT pattern.

Figure 4. XPS spectra of the doped SnS2 sample: (a-c) high-resolutionspectra of S 2p, Sn 3d, and In 3d of the sample without surface treatment(black line) and with surface treatment (red line). (d) Survey of theXPS spectrum.

Figure 5. TGA curve of the samples: (a) In3+-doped SnS2 and (b)SnS2.

Figure 6. Nitrogen adsorption-desorption isotherm of the preparedSnS2 sample and pore volume distribution curve (inset).

1282 J. Phys. Chem. C, Vol. 113, No. 4, 2009 Lei et al.

and In3+-doped SnS2 flowerlike architectures are 95 and 106m2/g, respectively. This result is higher than the reportedflowerlike Fe2O3 (40 m2/g),13 In2S3 (78 m2/g),12 MnO2 (60.4 m2/g),14 CeO2 (34 m2/g),15 and Ni(OH)2 (68.03 m2/g)16 with similarmorphology. The increased surface area may be attributed tothe small assembled hierarchical building blocks. This type ofhierarchical 3D architecture with high surface area is beneficialfor the applications as catalyst or for water treatment.

The Influence of pH Value. We further investigated theinfluence of experimental parameter variation on the assembledmorphology. It is well-known that amino acids have hydrolysisequilibrium in solution. The cysteine solution presents acidityowing to the stronger dissociation of the carboxyl group thanthe amino group (-NH3

+).17 The pH value is about 3 when thereactants are mixed naturally. While the solution was adjustedto pH ) 7 by ammonia solution, the obtained sample turned togray. Furthermore, only deep gray colloid was obtained underthe same conditions when the pH value was 10. The detailedcorresponding morphologies of the samples are shown in Figure7.

When the pH value was adjusted to 7, the obtained productswere near uniform spheres 1-2 µm in diameter (Figure 7a).The detailed individual image (inset) indicated that thesemicrospheres were composed of small crimp nanosheets withclose-packed assembly. When the pH value was changed to 10,only crimped nanosheets were obtained at the same reactionconditions. This morphology variation tendency was also foundin the synthesis of In-3+ doped SnS2. The products were crimpedrose flowers (as shown in Figure 7c) when the pH value wasset at 7. The detailed flower (Figure 7c inset) indicated that thenanosheet across the axis formed the flower architecture. Whenthe pH increased higher, only crimped colloid was obtained.

The Influence of Various S Sources. During the metal sulfursynthesis, various S sources were used such as S powder,thiourea Na2S, and thioacetamide. In our reaction system, theseS sources were also used to study the morphology of theassembly. As shown on the Figure 8, four different morphologieswere obtained with Na2S, thiourea (Tu), thiosemicarbazide, andthioacetamide (TTA) instead of L-cysteine. When Na2S wasadded to the SnCl4 solution, yellow precipitate generated rapidlyowing to the high concentration of S2- quickly supplied, andthe final morphology was nanoparticles (Figure 8a). Withthiourea and TTA, the morphology changed to irregular nano-plates. Compared with those S sources, L-cysteine as an ordinaryamino acid biomolecule has three functional groups, -NH2,-COOH, and -SH, which have a strong tendency to coordinatewith inorganic cations and metals.9 This precursor complex

decomposed to form the SnS2 nuclei in a supersaturated mediumat the initial stage. Followed by a general crystal growth kineticat the expense of the small crystals, the hexagonal plate formedin advance with the inner hexagonal phase. The presence ofthese groups has made cysteine a commonly used self-assemblyreagent in the preparation of 3D hierarchical structure. It actsboth as S source and template inducer agent in the solutionduring the hydrothermal route.

It is believed that the physical and chemical properties ofthe solvent can influence the solubility, reactivity, and diffusionbehavior of the reagents and the intermediate.18 In our reactionsystem, pH-dependent experiments indicate that the morphologyof the samples is sensitive to the pH value. It seems that highbasicity is beneficial for the formation of crimped nanosheets.Although the exact assembly process is unclear, a possiblegrowth mechanical process can be illustrated in Scheme 1 basedon above experiments. Here, we present evidence for theformation mechanism of the assembled objects from nanoplatesto two types of the flower-shaped morphologies directly capturedby electron microscopy. The two major morphologies can beassumed in which there are two steps in the generation processof the flowerlike structures: First, flat and smooth 2D nanosheetsare formed based on the traditional nuclei formation growththeory.19 Second, with the random connection there are two typesof connected nanosheets: one is edge to edge; the other is angleto angle. Under basic conditions, the nanosheet growth tends

Figure 7. SEM images of the samples at different pH values: SnS2

(a) pH 7.6 with detailed image (inset); (b) pH 10, In3+-doped SnS2; (c)pH 7.3 with detailed image (inset); (d) pH 9.7. Figure 8. SEM images of SnS2 samples with different S sources: (a)

Na2S, (b) thiourea (Tu), (c) thiosemicarbazide, and (d) thioacetamide(TTA).

SCHEME 1: Possible Growth Mechanical Scheme of theAssembly Flowerlike Structure

Flowerlike SnS2 and In3+-Doped SnS2 J. Phys. Chem. C, Vol. 113, No. 4, 2009 1283

to be the former. With acidic conditions, the growth tends tobe the latter. Interestingly, in the former type, two kinds ofhelixes we called left-hand and right-hand are observed duringthe assembly process. This phenomenon may be due to thedifferent growth direction of the border. Finally, the two kindsof helical nanosheets continue to grow across the axis and radialdirection and form the flowerlike architecture. In the other type,the angle to angle nanosheets grow under two directions evenlymatched. With the continued growing, the cross-linked nanosheetsare eventually formed. This type of assembly was also observedon the Co(OH)2 and fullerene structure.20

Photocatalytic Property. We further compared the photo-catalytic activity of the two samples by analyzing the photo-degradation of three different dyes under UV-vis light irra-diation. Methylene blue (MB), methylene green (MG), and ethylviolet (EV) were used as probes of the prepared samples. Theinitial dye concentration is 0.025 mM. The photocatalyticperformance of the two samples was estimated from thevariation of the color in the reaction system by its visible lightabsorption intensity. Total concentrations of all dyes were simplydetermined by the maximum peaks of the MB, MG, and EV,which were located at 664, 653, and 595 nm, respectively.Figure 9 shows the typical photodegradation MB curve of theprepared SnS2 sample. The intensity of major absorption peaksat 664 nm decreases step by step. In order to compare thephotocatalytic performance, the Degussa TiO2 photocatalystcommerce P25 was used as the reference. As shown on theconcentration charge curve, the blank sample, without anycatalyst, shows liner decrease (Figure 9a). With both our samples(Figure 9c and 9d) and P25 (Figure 9b), the dye concentrationdecreases remarkably in the initial 10 min. The concentrationdecays to half of the initial for about 20 min. Only after 1 h ofirradiation, the conversion is close to 90% for both samplesand 80% for P25, which suggests the excellent photocatalyticactivity of the prepared samples. This typical concentration curveobeys the pseudo-first-order kinetics law according to thereported photodegradation MB experiment.21 Obviously, thephotocatalytic performance of In3+-doped SnS2 is enhancedcompared with the pure SnS2 sample. Other dyes were alsotested under the same conditions, and similar results wereobtained. The corresponding adsorption and concentration decaycurve (Figure S2-S3) shows the same enhanced photocatalyticperformance. Based on these results, it can be seen that thedopant can improve the photocatalytic ability of SnS2.

We also did the photocatalytic experiments under visible lightirradiation. However, these photocatalysts are inactive in thevisible region. The absorption spectrum of the doped sample(Figure S5) shows a weak blue shift, and little enhancement ofabsorption in the UV region. However, the detailed mechanismis still unclear, and there are no related reports so far. For thephotocatalysts, even though the principle and activity-controlling

factors of the photomineralization process in semiconductorbased photocatalysts have been discussed previously,9,22 manyaspects of the function of inorganic photocatalysts are stillunclear, such as the detailed mechananism reduction andoxidation on the semiconductor surface, electrons and holestransferring in and out of the catalyst, and the effect of variablematerial preparations and surface impurities on the catalyticactivity of semiconductors.23 Considering our prepared samples,the enhancement may be attributed to the introduced dopantion result in the generation of holes which efficiently suppressthe photocorrosion and improve the photocatalytic activity ofsulfide photocatalysts.24 On the other hand, the large value ofsurface to volume ratio can increase the number of active surfacesites where the photogenerated electron-hole pairs are able toinduce more hydroxyl and superoxide radicals to react withabsorbed molecules.25 As for our prepared two samples, SnS2

(BET 95 m2/g) and In3+-doped SnS2 (BET 106 m2/g), thehierarchical structure is beneficial to quicken the rate ofinterfacial charge transfer and inhibit high rate of charge carrierrecombination.26

Conclusion

In summary, 3D flowerlike SnS2 and In3+-doped SnS2

hierarchical structures were successfully synthesized through asimple one-pot hydrothermal route. Electronic microscopy,XRD, and XPS characterization confirmed their structure. TGAand BET characterization indicated that the samples have goodthermal stability and large surface area, which ensured theirpotential application. A systematic approach was applied to thefundamental factors’ (various S sources, pH values) effect onthe synthesis, assembly, and morphology. During our experi-ments, L-cysteine played an important role both as S sourceand template agent. Interestingly, there are two types ofassembly when the pH value changed. These two kinds ofassembly finally lead to two types of flowerlike structures, anda possible growth mechanical was discussed. The experimentsof photocatalytic degradation of three dyes indicated that bothof our prepared samples have high photocatalytic activity, andthe doped In3+ can improve the photocatalytic activity. Thisfacile preparation method and assembly reported here areexpected to be utilized in fabrications of unique hierarchicalstructures of various semiconductor functional materials withadvanced properties.

Acknowledgment. The authors are grateful for the financialaid from the National Natural Science Foundation of China(Grant Nos. 20631040 and 20771099) and the MOST of China(Grant Nos. 2006CB601103 and 2006DFA42610)..

SupportingInformationAvailable: N2adsorption-desorptionisotherms of the doped sample, photocatalytic decomposedcurves of MG and EV, TGA curves of the samples at differentpH values, and absorption spectra of the samples. Thesematerials are free of charge via the Internet at http://pubs.acs.org.

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