Well-defined Sb 2 S 3 nanostructures: citric acid-assisted synthesis, electrochemical hydrogen storage properties

Cryst. Res. Technol. 48, No. 8, 566–573 (2013) / DOI 10.1002/crat.201300151

Well-defined Sb2S3 nanostructures: citric acid-assisted synthesis,electrochemical hydrogen storage properties†

Chunshuang Yan, Gang Chen*, Rencheng Jin, Xian Zou, Haiming Xu, and Chade Lv

Department of Chemistry, Harbin Institute of Technology, Harbin 150001, P. R. China

Received 21 May 2013, revised 27 June 2013, accepted 31 July 2013Published online 21 August 2013

Key words antimony sulfide, hierarchical nanostructure, solvothermal synthesis, anisotropic growth.

Well-defined (three-dimensional) 3-D dandelion-like Sb2S3 nanostructures consisted of numerous nanorodshave been achieved via a facile citric acid-assisted solvothermal process. The as-prepared products were char-acterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmissionelectron microscopy (TEM) and high-resolution TEM (HRTEM), respectively. The influence factors of theformation of the hierarchical Sb2S3 nanostructures are discussed in details based on FESEM characterizations.By simply controlling the quantity of citric acid, the nucleation and growth process can be readily tuned, whichbrings the different morphologies and nanostructures of the final products. On the basis of a series of contrastiveexperiments, the aggregation-based process and anisotropic growth mechanism are reasonably proposed to un-derstand the formation mechanism of Sb2S3 hierarchical architectures with distinctive morphologies includingnanorods, and dandelion-like nanostructures. Charge-discharge curves of the obtained Sb2S3 nanostructureswere measured to investigate their electrochemical hydrogen storage behaviors. It revealed that the morphologyplayed a key role on the hydrogen storage capacity of Sb2S3 nanostructure. The dandelion-like Sb2S3 nanos-tructures exhibited higher hydrogen storage capacity (108 mAh g−1) than that of Sb2S3 nanorods (95 mAh g−1)at room temperature.

C© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction

In recent decades, semiconductor nanomaterials have been the leading actors on the stage of the scientificcommunity due to their novel characteristics including electrical, optical, mechanical and chemical properties[1,2]. These magical properties make them widely used in catalysts, optoelectronics, biological industry, sensors,etc [3–7]. Numerous efforts have been focused on controlling the sizes, shapes, and phases of semiconductornanocrystals [8,9]. Up to now, lots of unique semiconductor architectures have been synthesized by a variety ofmethods, including sol-gel process, reverse micelle method, electrochemical deposition, hydrothermal treatment,et al. [10,11]. However, the shape controlled synthesis of the materials for novel properties still remains a greatchallenge in both experimental design and fundamental theory.

Sb2S3, Group V-VI binary semiconductor compound, [17] has been received a great deal of attention dueto their applications in television cameras with photoconducting targets, fast ion conductors, electrochemicalhydrogen storage and IR-spectroscopy [12–14]. Recently, For example, Sb2S3 nanocrystals with feather-like,rod-like, flower-like, tube-like and straw-bundled-like morphologies were synthesized by using solvothermalmethod [15], microwave-assisted solute [16] and refluxing treatment [17,18]. Yang et al. have been prepared theSb2S3 nanotubes with chain-like structures by chemical vapor transport method [19]. One-dimensional (1D) Sb2S3

wires with straw-tied-like architectures had been successfully synthesized through the one-pot hydrothermalmethod [20]. Zhang and co-workers reported that uniform shuttle-like Sb2S3 nanorod-bundles were synthesizedvia a polyvinylpyrrolidone (PVP) assisted solvothermal approach under alkaline condition [21]. However, to thebest of our knowledge, less work has focused on a one-pot preparation of 3-D Sb2S3 nanostructures in solution [22].And to exploit a facile solution route for fabricating 3-D Sb2S3 complex nanoarchitectures still remains a great

∗Corresponding author: e-mail: [email protected]†Electronic Supplementary Information (ESI) available: [The XRD pattern of the as-prepared samples at different conditions]

C© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Cryst. Res. Technol. 48, No. 8 (2013) 567

challenge. In this paper, 1-D and 3-D Sb2S3 structures were synthesized by a citric acid assisted solvothermalprocess. As a kind of soft template, citric acid plays an important role in this experiment. By just tuning the qualityof citric acid, Sb2S3 nanostructures from 1-D to 3-D (from nanorods to hierarchical dandelion-like architecturescomposed of nanorods) have been successfully fabricated. Furthermore, the electrochemical hydrogen storageproperties of Sb2S3 nanocrystals with various morphologies have been detected. The hierarchical dandelion-likeSb2S3 showed higher electrochemical hydrogen storage property than those of Sb2S3 nanorods.

2 Experimental section

2.1 Materials and synthesis All reactants and solvents are of analytical grade and are used without furtherpurification. In the typical experiment, 0.135 g antimony potassium tartrate, 0.021 g sulfur powder and 0–0.6 gcitric acid were dispersed in 20 mL of ethylene glycol and 8 mL of water under vigorous stirring for 40 min.Afterwards, the mixed solution was transferred into a Teflon-lined stainless-steel autoclave with a capacity of40 mL. Then, 2 mL of hydrazine hydrate (N2H4·H2O, 80%) was added into the autoclave. Then the autoclavewas sealed and put into the oven, maintained at 180 ◦C for 8 h. After the solvothermal treatment, the autoclavewas taken out and cooled to room temperature naturally. The dark grey precipitates were collected, washed withdistilled water and ethanol and dried in the vacuum at 60 ◦C for 12 h.

2.2 Characterization The X-ray diffraction pattern of the products was collected on a Rigaku-D/MAX-2550PC diffractometer using Cu Kα radiation. The morphology and the size of the as-prepared samples weredetected on field emission scanning electron microscope (FESEM, FEI Quanta 200F), and transmission electronmicroscope (TEM, FEI Tecnai G2 S-Twin).

2.3 Electrochemical measurements The electrochemical hydrogen storage was performed on the Landbattery system (CT2001A) following the similar method reported in the literature [23]. The electrode was formedby directly pressing the as-synthetized samples to a sheet of nickel foam under 20 MPa pressure (12 mm in lengthand 1 mm in thickness). Electrochemical measurements were performed in a three-electrode cell in 6 M KOH atroom temperature. The Sb2S3 nanostructures electrodes were used as the working electrode, Ni(OH)2/NiOOHas the counter electrode, and Hg/HgO as the reference electrode. The working electrodes were charged for 4 h ata current density of 50 mA g−1 and then discharged at the same current density after a rest of 1 min.

3 Results and discussion

3.1. Characterization of Sb2S3 nanorods and dandelion-like structure X-ray diffraction (XRD) analy-sis was used to determine the structure of the sample. The XRD pattern of the samples synthesized (adding 0.2 gof citric acid) through solvothermal process under 180 ◦C for 8 h is shown in figure 1a. All peaks can be wellindexed to an orthorhombic structure of Sb2S3 (JCPDS:42–1393). No peaks of impurities such as SbOCl andSb2O3 can be detected, indicating the high purity of the as-prepared Sb2S3 samples. The morphology and size ofthe as-synthesized Sb2S3 are examined by a field-emission scanning electron microscope. The low magnificationSEM images (figure 1b–c) demonstrate that the representative products are consisted of a large quantity of 3Ddandelion-like Sb2S3 crystals. Careful observation indicates that the system architectures are composed of manybranches which are exactly defined as nanorods. The three-dimensional dandelion-like structure is congregatedrandomly by numerous nanorods. The higher magnification image presents that the length of the Sb2S3 nanorodsis as large as 20–40 μm, and the typical diameters are in the range of 150–300 nm.

The morphology and microstructure of dandelion-like Sb2S3 structures were further investigated by usingTEM and HRTEM (figure 2a–b). The TEM image shown in figure 2a confirms that the length and diameter ofthese nanorods are in accordance with the SEM results (figure 1d). It is worth noting that some dandelion-likestructures can be transformed into disperse nanorods after ultrasonic treatment because of the weak interactionbetween nanorods. HRTEM result (figure 2b) points out that the nanorods are well crystallized and grow along the[001] direction. The obvious crystal lattice fringes with an average neighboring distance of 0.794 nm correspondto the (110) plane of orthorhombic Sb2S3.

It was worthwhile to mention that the proportion of solvent has an effect on the formation of the Sb2S3

nanostructures [24–26]. The proportion of solvent is changed while keeping other conditions constant. Theacquired nanostructures had completely different morphologies. Figure 3a shows the overall SEM image ofthe sample obtained with volume ratio of 20 : 8 (water to ethylene glycol), it can be seen that a majority of

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568 C. Yan et al.: Well-defined Sb2S3 nanostructures

Fig. 1 (a) XRD pattern of dandelion-like Sb2S3 prepared by a solvothermal reaction in a mixed solvent with0.2 g of citric acid; (b, c) Low magnification SEM image of Sb2S3 dandelion-like microspheres; (d) Highmagnification SEM image of Sb2S3 dandelion-like microspheres.

Fig. 2 (a) Typical TEM image of the dandelion-like Sb2S3; (b) HRTEM image of the edge area of thedandelion-like microsphere shown in (a).

as-prepared Sb2S3 obviously exhibit the polyhedron-like morphology with obvious polygonal cross-sections, butit is not uniform. Occasionally, some nanorods are observed in the products. Increasing the volume of ethyleneglycol to 12 ml, the as-prepared Sb2S3 is composed of many nanorods. However, the size distribution of thenanorods is wide. When 16 ml of the ethylene glycol is introduced into the reaction system, the obtained sampleshows the root hair-shaped, which is very slim and is like the roots spreaded from a plant. These needlelike roothairs are actually many slender nanorods. When water and glycol are adjusted to the optimal ratio (8 : 20), theuniform dandelion-like Sb2S3 can be synthesized, which is described in figure 1b–d. Continuing to increase theamount of ethylene glycol, that is, trace water is added to the hydrothermal system. From figure 3d, it can be

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Fig. 3 SEM images of samples obtained in the different proportion of a mixed solvent at 180 ◦C for 8 h.(a) water: glycol = 20:8; (b) water: glycol = 16:12; (c) water: glycol = 12:16; (d) water: glycol = 0:28.

Fig. 4 SEM images of samples obtained at different reaction temperature (a) 120 ◦C; (b) 200 ◦C.

seen that the as-prepared Sb2S3 sample shows the spine-like morphology. Occasionally, a few microspheres canbe found in the sample.

In the meantime, we found that the morphology of the products was extremely sensitive to hydrothermaltemperature. The SEM images of products prepared at different temperatures for 8 h were shown in figure 4.When the hydrothermal temperature is 120 ◦C, the feature of the sample is obviously different from the dandelion-shaped Sb2S3. At lower reaction temperature, the slow nucleation-growth rate may lead to the different growthdirections. As can be seen in figure 4a, the obtained product is composed of numerous irregular nanoparticles.However, at relatively high reaction temperatures (200◦C), the rate of nucleation and growth will increase. Thus,the rod-like Sb2S3 forms.

3.2. Growth process and formation mechanism of Sb2S3 nanostructures To explore growth processand formation mechanism of Sb2S3 nanostructures, a comparative experiment with different amount of citric



Fig. 5 SEM images of the samples obtained by a solvothermal reaction in a mixed solvent at 180 ◦C for 8 hfor different concentrations of citric acid. (a) 0 g; (b) 0.4 g; (c) 0.6 g.

acid was carried out. Figure S1 confirm that samples prepared are of orthorhombic phase Sb2S3 after comparisonwith the standard pattern from JCDPS:42–1393. Figure 5a shows that the rod-like Sb2S3 nanostructures canbe fabricated without citric acid. When 0.2 g of citric acid is added to reaction system, the 3D dandelion-like Sb2S3 is obtained (figure 1a). However, when the quantity of citric acid is regulated to 0.4 g with otherconditions unchanged, the obtained sample becomes inhomogenous compared with one obtained by adding0.2 g of citric acid (figure 5b). The morphology of product becomes more inhomogenous and dandelion-shapedSb2S3 breaks when the quantity of citric acid is increased to 0.6 g (figure 5c). From the above SEM observations,we can conclude that the citric acid plays crucial role in the formation of dandelion-like morphology. Aswe known, antimony trisulfide has an orthorhombic crystal structure with strong covalent bonds along the ccrystallographic axis and weaker vander Waals bonding between the chains of the layered structure. Therefore,the materials with this crystal structure prefer to grow along one direction and facilitate the formation of one-dimensional nanostructures. When no citric acid was introduced in the reaction solution, rod-like Sb2S3 canbe obtained. Citric acid, containing three carbonyl groups (-COO-) and two hydroxyl groups (-OH), can beused as a surfactant in the fabrication of inorganic materials. Because of the hydrogen bond and electrostaticeffects of citric acid, the generated Sb2S3 nuclei can be adsorbed by the citric acid [27]. And then these nucleiwere quickly built and spontaneously aggregated together with the assistance of citric acid. It should mentionthat there were numerous small protuberances on the surface of the aggregations, which could provide manyhigh-energy sites for nanocrystals growing. Due to the anisotropic crystal structure of Sb2S3, Sb2S3 will growalong one direction. Thus, dandelion-like Sb2S3 composed of nanorods is obtained. Further increases the amountof citric acid, the obtained nuclei can be protected by the redundant citric acid in the solution. This will impedethe growth of Sb2S3, which leads to the nonuniform morphology of the final product. The possible formationmechanism is illustrated in figure 6. On the basis of the above discussion, the multi-carbonyl group (-COO-) andhydroxyl group (-OH-) are responsible for the morphology of Sb2S3 nanostructures through the hydrogen bond.In order to further validate the inference, we employ glucose and sodium tartrate to replace citric acid. XRD data(figure S2) corroborated the orthorhombic phase synthesis of Sb2S3. Figure 7a–b show the overall SEM images ofthe samples obtained by taking glucose and sodium tartrate as the surfactant with the other conditions remaining



Fig. 6 (a) Schematic illustration of a proposed mechanism for the formation of Sb2S3 hierarchicalnanostructures.

Fig. 7 (a) SEM images of the samples obtained by taking glucose as the surfactant with the other conditionsremaining unchanged; (b) SEM images of the samples obtained by taking sodium tartrate as the surfactant withthe other conditions remaining unchanged.

unchanged, respectively. Similar dandelion-like Sb2S3 was obtained when citric acid was substituted by glucoseand sodium tartrate, confirming the effects of multi-carbonyl group (-COO-) and hydroxyl group (-OH-).

3.3. Electrochemical properties Layered sulfides nanomaterial (Ni2S3, MoS2, TiS2, Bi2S3, etc.) havebeen reported as new negative electrode materials for nickel-metal hydride (Ni–MH) batteries with higherdischarge capacity comparing with the traditional materials [28]. As a kind of layered sulfide, Sb2S3 has goodelectrochemical hydrogen storage property. Therefore, the electrochemical hydrogen storage properties of theobtained samples including nanorods and 3-D dandelion-like Sb2S3 as negative electrode material of nickel-metal hydride (Ni-MH) battery have been investigated. Figure 8a displays the charge and discharge curvesof the products at room temperature. For the Sb2S3 nanorods, one obvious plateau of potential appeared at20 mAh g−1 can be observed and no other plateau is perceptible, which indicates that only one hydrogenadsorption site exists in the product. In the corresponding discharging process, a total discharge capacity of95 mAh g−1 is obtained. When the electrode material was replaced by dandelion-like Sb2S3 (figure 8b), similarcharging curve with one plateau of potential can be observed and the discharge capacity reaches to 108 mAh g−1.The value of discharge capacity is higher than that for the Sb2S3 nanorods. Li et al. and our research grouphave proposed that the BET surface areas of the products have much effect on the discharging capacity [29,30].



Fig. 8 Charge-discharge curves of different morphologies of Sb2S3. Current density: 50 mA g−1. (a) nanorods;(b) dandelion-like.

Fig. 9 Typical nitrogen adsorption-desorption isotherm of the dandelion-like Sb2S3.

Therefore, we conjecture the Sb2S3 nanorods have lower discharge capacity due to its lower BET surfaceareas. Although further investigation is necessary to elucidate the hydrogen absorption-desorption mechanismof metal-sulfide nanodandelions, the investigations of electrochemical hydrogen storage of this sulfide will helpus to understand other sulfide compounds and thus inspire us to prepare the nanostructures with the higherBET surface areas of these series of compounds to explore their electrochemical properties. We expect thatthey can appropriate the higher discharge capacity to themselves. To make the speculation more convictive,the nanostructure of Sb2S3 was investigated by nitrogen adsorption/desorption measurements, subsequently.It revealed a typical type IV isotherm in figure 9. The BET specific surface area of dandelion-like Sb2S3

calculated from nitrogen adsorption is about 6.9054 m2·g−1. The surface area of Sb2S3 nanorods calculated is4.9287 m2·g−1. The BET surface area between the two samples are not so eminent. But, the discharge capacitybetween the two samples are also inapparent. Therefore, It’s possible that the BET surface areas of the productsmake a contribution to the discharging capacity. In addition, the products’ morphologies have exerted a noticeableinfluence on the capacity of electro-chemical hydrogen storage. The nanopores from the dandelion-like Sb2S3

nanostructure can provide more hydrogen adsorption sites. The nanopores are also advantageous to the releaseof hydrogen, therefore, the dandelion-like Sb2S3 nanostructure exhibited higher hydrogen storage capacity.



4 Conclusions

In summary, Sb2S3 nanostructure with well-controlled morphologies, including nanorods, and dandelion-likestructure self-assembly from nanorods were successfully fabricated via a solvothermal reaction using citric acidas surfactant. The effect of reaction conditions including the quality of citric acid, reaction temperature andvolume ratio of the mixed solvents were investigated in detail. Based on a series of contrastive experimentalresults, a different coordination effect between metal salts and polybasic carboxylic acid or polyol was proposed toexplain the formation of the Sb2S3 nanorods and 3D dandelion-like structures. The morphologies of the sampleshave much effect on the electrochemical properties, the dandelion-like Sb2S3 nanostructure exhibited higherhydrogen storage capacity (108 mAh g−1) than that of Sb2S3 nanorods (95 mAh g−1) at room temperature due toits high BET surface areas (6.9054 m2·g−1). Consequently, the present work gives inspiration to the controllablesynthesis of other semiconductors with high BET surface areas.

Acknowledgements The authors are grateful for financial support from the National Natural Science Foundation of China(Project no. 20871036, 21071036), Province Natural Science Foundation of Hei-longjiang Province (ZD201011), ScientificResearch Inno-vation Foundation of Harbin Institute of Technology (Project no. GFCQ98332122) and Development Programfor Outstanding Young Teachers in Harbin Institute of Technology (HITQNJS. 2009. 001).

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Well-defined Sb 2 S 3 nanostructures: citric acid-assisted synthesis, electrochemical hydrogen storage properties