Tartaric Acid Assisted Growth of Sb2S3 Nanorods by a Simple WetChemical Method
Jyotiranjan Ota and Suneel Kumar Srivastava*
Inorganic Materials and Nanocomposites Laboratory, Department of Chemistry, Indian Institute ofTechnology, Kharagpur, India
ReceiVed August 18, 2006; ReVised Manuscript ReceiVed October 26, 2006
ABSTRACT: Good quality nanorods of Sb2S3 have been synthesized by a simple wet chemical method under refluxing conditions.In this, tartaric acid has been successfully used as a complexing agent to grow these single-crystalline nanorods at a comparativelymuch lower temperature (115°C) than reported earlier (180-200°C). X-ray diffraction (XRD) and electron diffraction (ED) studiesshowed that the rods correspond to the pure orthorhombic phase of Sb2S3, the phase purity of which is further confirmed by energydispersive X-ray (EDX) and X-ray photoelectron spectroscopy (XPS). A possible growth mechanism has been discussed on thebasis of a series of transmission electron microscopy (TEM) studies of the product obtained at different durations. The effect ofconcentration of tartaric acid on the formation of Sb2S3 and its morphology has also been discussed. The morphology of the finalproduct remained same for different sulfur sources used, though there is small change in dimension. The band gap was found to be1.56 eV, suitable for photovoltaic applications.
Introduction
The unique and interesting properties of the materials in thenanoregime have attracted researchers to put forward their bestefforts to develop the materials for future applications.1,2
Specifically, the one-dimensional nanostructures, nanotubes,-rods, -belts, -wires, etc., owing to their functional advantagesover other forms of the materials, are more promising in thisconcern.3,4 The binary chalcogenides of V-VI (A 2
VB3VI) are
very useful semiconductor materials having applications inthermoelectric and optoelectronic devices.5 Moreover, they findapplications in IR spectroscopy, paints, photo-emitting diodes,microwave switches, etc.6 The band gap of a material determinesits applicability as an optoelectronic material; therefore, thetailoring of the band gap is very helpful. The band gap of theSb2S3 varies between 1.5 and 2.2 eV due to changes incrystallinity, size, and shape in the nanoregime.7 Therefore,various methods have been employed to synthesize this materialon the nanoscale, especially in one dimension. It is a highlyanisotropic material with a layered structure that crystallizes inpurely orthorhombic phase.8 It has a ribbon-like polymericstructure in which SbS3 and SSb3 layers form interlockedpyramids, which makes this material anisotropic and helps inconfined growth. So far, the different research groups haveemployed hydrothermal,9 sonochemical,10 single-source decom-position,11 and polyol-assisted routes12 to synthesize this materialin either rod, ribbon, or wire form. Chen et al. used Sb2S3
powder to grow nanotubes of the same by a thermal evaporationmethod.13 However, till now the developments regarding thesynthesis of this material are very little, especially related tothe growth using simple chemical methods. In the recent past,much research has been focused on using templates forunidirectional growth of different materials. Among all othergrowth-directing agents, the use of a complexing agent toconfine the growth in the desired direction remains an interestingaspect. Bi- or multidentate amines (ethylene diamine, triethanolamine),14 glycols (ethylene glycol),15 acids (such as ethylene-diaminetetraacetic acid (EDTA),16 citric acid, and acetic acid),17
etc. have been used as complexing agents in synthesizing
nanorods or tubes. These polyfunctional ligands coordinate withthe inorganic ions to form complexes and also act as a softtemplate in the growth process. However, in the case of thegroup V-VI semiconductors, the use of the acidic ligands orcomplexing agents has not been widely studied, though thereare some recent reports on synthesis of Bi2S3 nanorods usingeither bismuth citrate18 or EDTA.19 Deng et al. preparednanorods of Bi2Te3 by a controlled oriented attachment usingan EDTA complex as a template.20 The same group alsoobtained chainlike crystal structures of Bi2Te3, as well as Ag,in a hydrothermal process using EDTA or polyvinyl pyrrolidone(PVP).21 On the other hand, there have been a number of studiesdone regarding the deposition of thin films of various semi-conductors using complexing agents.22 Bhosale et al. havereported the deposition of thin films of Sb2S3 using tartaric acidas a complexing agent and also studied the effect of concentra-tion of the same.23 However, not many attempts have been madeso far to exploit the use of tartaric acid for the growth of thenanorods in wet chemical methods. There is only one such reporton formation of nanorod bundles, where tartaric acid has beenused to completely dissolve SbCl3 in water.24 Moreover, mostof the synthetic methods for the Sb2S3 nanorods under solvo-thermal conditions have been carried out in the temperaturerange of 180-200 °C. Hence, obtaining these nanorods atrelatively much lower temperature, that too under refluxingconditions, is valuable from a chemist’s prospective and hasremained always a challenge.
Our group has been studying the possibilities of using varioussoft templates for synthesis of group V-VI semiconductors andpreviously reported the fabrication of Bi2S3 nanotubes througha micelle template assisted route.25 In the present work, we havesuccessfully used tartaric acid to synthesize nanorods of Sb2S3
through a simple soft chemical route. Potassium thiocyanatehas been used as a sulfur source for the first time in this work.The effect of concentration of the complexing agent has beenstudied carefully, and a possible reaction as well as growthmechanism for the formation of Sb2S3 is proposed.
Experimental Section
All the chemical reagents used in this work were of analytical gradeand used without further purification. The reaction was carried out in
* To whom correspondence should be addressed. E-mail: [email protected]. Fax: 91-3222-755303. Tel: 91-3222-283334.
a 100 mL round-bottom flask where 1.5 g (10 mmol) of tartaric acid(TA) was added to 50 mL of water followed by 0.625 g (2.7 mmol) ofSbCl3 with constant stirring till a clear solution was obtained. To this,0.5 g (5 mmol) of potassium thiocyanate (KSCN) was added, and thewhole solution was refluxed at 115°C for 24 h. Finally, the dark brownprecipitate was filtered and washed with water and ethanol. The resultantpowder was dried under vacuum at 60°C for 4 h and characterized.The same reaction was repeated using 5 and 0 mmol of TA, all otherconditions remaining same.
The phase analysis of the products were performed on a Philips PW-1710 X-ray diffractrometer (40 kV, 20 mA) using Cu KR radiation (λ) 0.154 18 nm) at a scan rate of 0.05°/s in the range 10-70°. X-rayphotoelectron spectroscopy (XPS) was carried out on ESCALab MKIIX-ray photoelectron spectrometer, using non-monochromatized Mg KRX-ray as the excitation source. The morphology of the sample wasstudied by scanning electron microscopy (SEM) using a JEOL JSM-5800 at an accerating voltage of 20 kV. Transmission electronmicroscopy (TEM) images, high-resolution TEM (HRTEM) images,and selected area electron diffraction (SAED) pictures were recordedon a JEOL 2100 electron microscope operating at 200 kV. Energydispersive X-ray (EDX) analysis of the samples was carried out on anOXFORD INCA instrument attached to the transmission electronmicroscope in the scanning range of 0 to 20 kV to find out the chemicalcomposition. A Perkin-Elmer Lambda20 spectrophotometer was usedto get the absorption spectra of the sample using Ba2SO4 as a referencematerial.
Results and Discussion
Figure 1 shows the X-ray diffraction (XRD) pattern of theproduct obtained under reflux conditions and is scanned for thepresence of all possible phases. It shows the presence of sharppeaks, which could be indexed on the basis of pure orthorhombicphase of the Sb2S3 (JCPDS 42-1393). Absence of any otherpeak due to impurities indicates the purity of the product. Thecomposition and purity of the product was further checked byXPS analysis, where high-resolution spectra of the Sb 3d andS 2p are obtained using C 1s as the reference at 284.6 eV. Figure2a represents the full/wide scan spectra of the sample where noimpurities could be detected, thereby further confirming thepurity of the product. The high-resolution spectra of the Sb 3din Figure 2b show the presence of two peaks for Sb3d5/2 andSb 3d3/2 at 530.4 and 539.7 eV, respectively. It may beinteresting to mention that these peaks are characteristics of theSb3+ oxidation state confirming that the antimony is not in the+5 state. Figure 2c represents the high-resolution spectra ofthe S 2p with the peak at 162.2 eV. The peak positions forboth Sb and S also matched very well with those in theliterature.26 The ratio of Sb to S is found to be is 1:1.46 fromthe quantification of area of the respective peaks, which is veryclose to 1:1.5, an ideal value for the stoichiometry corresponding
to the Sb2S3. Hence, XPS also proves the stoichiometry andpurity of the sample within some experimental error.
Morphology of the product was studied by SEM analysis.Figure 3 represents the typical SEM micrograph of the sample,where rodlike morphology can be seen with a diameter in thenanometer range and length varying in some micrometers. Thesample was characterized further by TEM to obtain moreinformation on the structure and morphology and is shown inthe Figure 4. The typical TEM image in Figure 4a shows abunch of nanorods having diameters in the range of 50-150nm and length of a few micrometers. The purity of a singlenanorod was examined by EDX analysis, and the spectrum isgiven in Figure 4b. In the spectra, peaks for Sb, S, and Cu couldbe discerned. The peak for Cu is due to the copper grid overwhich sample has been mounted. The quantitative analysis gavethe ratio of Sb/S as nearly 2:3 (Sb 41.25%; S 58.75%), furtherconfirming the purity of the nanorods. One single nanorod ofdiameter around 50 nm and length∼400 nm has been focusedand is shown in Figure 4c. The SAED pattern of the demarcatedportion of the nanorod is also shown in the inset. The parallely
Figure 1. XRD profile of the product indexed according to JCPDSfile 42-1393.
Figure 2. XPS spectra of the product: (a) low resolution; (b) high-resolution spectrum of Sb 3d; (c) high-resolution spectrum of S 2p.
344 Crystal Growth & Design, Vol. 7, No. 2, 2007 Ota and Srivastava
arranged bright spots confirm that the nanorods are singlecrystalline in nature, and these spots correspond to the orthor-hombic phase of Sb2S3. A HRTEM image of the tip portion ofa nanorod showing the clear fringes is displayed in Figure 4d.The interlayer spacing of 0.358 nm corresponds to the (130)plane of the Sb2S3. This suggests that growth occurs perpen-dicular to this plane along the tip of the nanorod, that is, towardthe (001) direction. This matches well with the general orienta-tion of growth in one-dimensional nanostructures3b and can beexplained on the basis of a typical crystal structure of Sb2S3 asshown in Figure 5. It consists of chainlike (Sb4S6)n moietiesrunning parallel to the 001 axis that contain two types of Sband three types of S atoms.27 Out of the three types of sulfuratoms, two are formally trivalent and one is divalent. Withinthe chain, the divalent sulfur and one trivalent sulfur areconnected to antimony by strong covalent bonds. However, thethird sulfur is connected to the antimony of the second parallelyrunning chain by weaker van der Waals bonds that areresponsible for the cleavage of the crystal. Thus, the cleavageoccurs parallel to thec-axis (001) in the 010 plane, where only
van der Waals bonds are to be ruptured. As a result, Sb2S3 breakseasily along thec-axis, and this results in formation of a one-dimensional structure, either as nanowires or nanorods.
In order to understand the growth mechanism for theformation of the nanorods, the products obtained at differentreaction durations have been analyzed through TEM images.Figure 6a represents a typical image of the product formed after1 h and indicates the presence of agglomerated particles in thenanorange. Figure 6b represents the TEM image of the productobtained after 3 h. It clearly suggests that increase in reactionduration from 1 to 3 h induces a rodlike morphology, alongwith the particles in the product. The product obtained after 6h contains a majority of the nanorods of very narrow dimensionamidst of some particles, as is evident from Figure 6c. TEMimages of the product recorded after 12 h shows the presenceof only rodlike morphologies. The EDX analysis of the particlespresent in the sample obtained at both 1 and 3 h durationsuggests a composition of the reaction product as SbxOySz. The
Figure 3. SEM image of the product obtained, showing a large numberof nanorods.
Figure 4. TEM images of the Sb2S3 nanorods: (a) image at lowmagnification; (b) EDX spectrum of a single nanorod (inset shows thenanorod selected for the experiment); (c) TEM image of a singlenanorod (inset is the SAED pattern obtained from the area demarcatedon the nanorod); (d) HRTEM image of tip of a nanorod.
Figure 5. Crystal structure of Sb2S3 projected on the (010) plane. TheSb-S covalent bonds are shown by solid lines and the weak van derWaals bonds by dotted lines. A possible cleavage site is demarcatedby the red line.
Figure 6. TEM image of the products obtained at different reactiondurations: (a) after 1 h, showing agglomerated nanoparticles; (b) after3 h, showing presence of some rods along with the particles; (c) after6 h, majority of the product being the nanorods with some particles;(d) only nanorods can be seen after 12 h.
formation of a similar intermediate during preparation of Sb2S3
under hydrothermal conditions has also been reported by Yu etal.9 According to them, SbxOySz itself is unstable and gives riseto Sb2S3 when subjected to suitable reaction conditions. In ourcase, it is the reaction duration that facilitates the decompositionof SbxOySz to form Sb2S3 nanoparticles, which serve as nucleiin their subsequent growth as nanorods. The tartaric acid in thereaction mixture, which acts as a capping agent, is adsorbed onthe surface of these nuclei and controls the overall growthkinetics. The growth of the nuclei is restricted only to specificfacets due to hindrance of the capping agent, and driven by theanisotropic crystal structure, this results in the growth of theproduct along thec-axis in the direction of the 001 plane. Theformation of nanoparticles and nanorods continued simulta-neously with increase in the duration for some time. Therefore,a mixture of particles and rods are obtained in the product upto 6 h. However, with increase in the reaction duration, onlynanorods are found at 12 h, and these are subsequentlycrystallized to give well crystalline nanorods after 24 h.
In the present work, we have also studied the effect ofconcentration of the complexing agent (tartaric acid) on thephase purity and the final morphology of the product. Figure7a,b,c shows XRD patterns of the products obtained for 0, 5,and 10 mmol of tartaric acid, respectively. It is observed fromFigure 7a that the product obtained without using complexingagent consists of more intense peaks that are mainly due toSb4O5Cl2 (JCPDS 70-1102), though a few peaks for Sb2S3
appeared. In Figure 7b, it can be seen that the peaks for Sb2S3
are developed though not fully crystallized. It also contains afew less intense peaks for Sb4O5Cl2. Interestingly, on increasingthe concentration of the complexing agent to 10 mmol, well-crystallized peaks only for the pure Sb2S3 are observed. Thesestudies clearly demonstrate that the formation of Sb2S3 is largelydependent on the concentration of the complexing agent. Acomplex of the Sb3+ with the tetradentate acid is formed whenSbCl3 is added to the aqueous solution of the tartaric acid.28
Release of Sb3+ ions from this complex takes place in acontrolled manner, and the ions react with the sulfur ion obtainedfrom KSCN to give an intermediate like SbxOySz, whichsubsequently forms Sb2S3. In the absence of complexing agent,SbCl3 is completely hydrolyzed to give Sb4O5Cl2, and perhaps
the reaction conditions are not favorable for this to react withKSCN and form Sb2S3. Therefore, tartaric acid-Sb complexacts as a precursor for the source of Sb3+ and controls thereaction for the formation of Sb2S3 in rodlike morphology.Figure 8a is a typical SEM image of the product obtained forthe reaction with 5 mmol of tartaric acid. It shows some rodlikemorphology, along with the flakelike microstructures. Absenceof proper concentration of the complexing agent might be thecause for this. However, the reaction is optimized for 10 mmolof the complexing agent leading to the formation of onlynanorods as a final product. When same reaction was carriedout in autoclave in the temperature range of 120-160 °C for24 h, Sb2S3 was obtained. The morphology was rodlike butmuch bigger in dimension. The typical SEM micrograph inFigure 8b shows that the rods are 2-4 µm wide and their lengthis around 15-20 µm.
The same reaction was repeated using either thiourea orthioacetamide as an alternative source of sulfur while keepingall the other conditions the same. In the case of thioacetamide,the nanorods are almost comparable in dimension to thoseobtained earlier using potassium thiocyanate. However, whenthiourea is used as the sulfur source, the yield is comparativelyless, though the morphology remained more or less identical.The decrease of yield in the case of thiourea can be explainedon the basis of the thermodynamic stability of the respectiveintermediates formed during the reaction progress. The reactionof thiourea and thioacetamide in acidic medium has been widely
Figure 7. XRD pattern of the product obtained for different concentra-tions of tartaric acid (TA): (a) product obtained without any TA added;(b) for 5 mmol of TA; (c) for 10 mmol of TA.
Figure 8. SEM image of (a) the product obtained for 5 mmol of theTA when the reaction was carried out under refluxing conditions and(b) the product obtained in the reaction being carried out in autoclaveat 120-160 °C for 10 mmol of TA used.
346 Crystal Growth & Design, Vol. 7, No. 2, 2007 Ota and Srivastava
studied for the deposition of thin films of different metalsulfides.22,29According to Rajpure et al.,29ain an acidic mediumthioacetamide forms an intermediate (CH3CSH2NH+) thatsubsequently gives H2S, which acts as the sulfur source.Thioacetamide and thiourea have a similar type of structure withonly a methyl group substituted by amine in case of the latter.Therefore, a similar type of intermediate, that is, (NH2CSH2-NH+), is also expected for thiourea. But the latter one (NH2-CSH2NH+) is thermodynamically more stable due to resonancebetween the lone pair on NH2 group and the positive chargedeveloped on S. As a result, the subsequent progress of thereaction is retarded in the case of thiourea, which results in alow yield of Sb2S3 compared with the thioacetamide.
Sb2S3 is very important from an optoelectronic point of viewdue to its comparatively higher band gap among the othersemiconducting materials of the same group. Thus, the opticalproperties of the nanorods have been studied by absorptionspectroscopy, and the corresponding UV-visible absorptionspectrum of the nanorods is shown in the Figure 9. The bandgap is calculated from this by extrapolating the linear part ofcurve of (Rhν)2 with respect tohν (inset) on the latter axis andis found to be 1.56 eV. This is quite comparable to the valuesreported for nanoribbons and nanorods of comparable dimen-sions.9,30 It is also noted that the nanorods in our dimensionrange do not show a quantum confinement effect, a fact alsoestablished by these workers. This may be attributed to the lowerBohr’s radius for this material. However, the band gap is suitablefor the material to be used for photovoltaic conversion.
Conclusion
In summary, Sb2S3 nanorods have been successfully synthe-sized at relatively much lower temperature (115°C) underrefluxing conditions using tartaric acid as a complexing agent.Tartaric acid acts as both a coordinating agent to make the SbCl3
soluble in aqueous medium and a capping agent for one-dimensional growth. The phase purity of the nanorods has beenconfirmed by help of XRD, XPS, and EDX studies. The effectof concentration of the complexing agent on formation of Sb2S3
nanorods has also been shown by XRD, suggesting that an
optimum concentration of it is necessary for complete formationof the Sb2S3. No change in the morphology of the product isobserved even by changing the sulfur source, though theirdimensions differed slightly from one another. The band gapof the nanorods is found to be 1.56 eV, which is suitable forphotovoltaic conversion.
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Figure 9. UV-visible spectrum obtained from Sb2S3 nanorods. Insetis the plot of (Rhν)2 with respect tohν.
