Morpholine-4-Carbodithioate Se and Te Complex as Single Source

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Morpholine-4-Carbodithioate Se and Te Complex as Single Source Precursor for Synthesis of Se and Te with Diverse Morphologies Published as part of a virtual special issue on Structural Chemistry in India: Emerging Themes Ashish K. Sahoo and Suneel K. Srivastava* Inorganic Materials and Nanocomposite Laboratory, Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India

bS Supporting Information ABSTRACT: In the present work, we report growth of Se and Te of the diverse morphologies by the solvothermal decomposition of the respective morpholine-4carbodithioate (MCDT) complexes. The nano particles and one-dimensional rods of trigonal Se and Te are formed when the reaction is carried out for the duration of 24 h at 100 and 140
C in presence of 10% water in tetrahydrofurane (THF)
water mixture. When disodium salt of ethylenediaminetetraacetic acid (EDTA) is used as template, flowery morphology of Se is formed at lower temperature (100
C), whereas the formation of smooth micrometer sized rods with high aspect ratio takes place at high temperature (140
C). On the other hand, when cetyl trimethylammnium bromide (CTAB), Se with the particle type of morphology along with few large micro rods is formed even at 140
C, where in the case of Te, the product consists of the micro particles and microrods at 100 and 140
C respectively. It is also observed that the Te-complex forms nanorods at 140
C in pure aqueous medium, along with presence of particle morphology; where as mixed water-organic solvent of appropriate dielectric constant is required for the growth of microrods of Se from Se-complex at the same temperature. The effects of temperature and volume percentage of components of solvent mixture in understanding the evolution of morphology of Se and Te have also been studied by X-ray diffraction (XRD), infrared spectroscopy (IR), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), Raman and photo luminescence (PL) studies.

’ INTRODUCTION The morphological control, physical and chemical properties of semiconducting micro/nanomaterals is strongly influenced by their dimensionality. As a result, one-dimensional (1D) nanosized Se and Te nanostructures, such as nanotubes,1
4 nanorods,5,6 nanowires,7,8 nanobelts,9,10 and nanoribbons,11,12 have gained a considerable amount of attention in the past several decades due to their unique properties. Selenium is a nontoxic semiconductor having a bandgap of 1.7 eV and finds applications in photocells, electrical rectifiers, photographic exposure meters, xerography, glass industries, catalysts, thermoconductivity, and high piezoelectric, high photoconductivity, nonlinear optical responses, etc.13 Tellurium on the contrary is a toxic and narrow bandgap (0.35 eV) semiconductor finding its applications in photoconductivity, nonlinear optical responses, thermoelectric, and optoelectronic devices, as a holographic recording material, an infrared photoconductive detector and for nonlinear infrared optics etc.14 Different synthetic techniques have been used for the preparation of 1D Se and Te nanostructures using various reducing agents.15
22 Previously, our group synthesized high purity single crystalline trigonal selenium wires and scrolled nanotubes using new reducing agents such as sulphurous acid15 and sodium sulphide,16 r 2011 American Chemical Society

respectively, under hydrothermal conditions at low temperature. Tang and his co-workers17 successfully reported that the decomposition of Na2SeSO3 in the presence of H2O2 as a reducing agent formed single crystalline Se microtubes. L-Cystine has been used as a reducing agent as well as soft template to control the growth of Se nanorods.18 Zhu et al.23 used the microwave polyol approach in synthesizing Se nanowires and nanorods from SeO2. Wang et al.24 prepared Se nanorod bundles using sodium selenosulphite as a selenium source and poly(vinyl alcohol) as polymer. Fan et al.25 synthesized Se microrods and microtubes by using Se powder and NaOH as starting materials under hydrothermal conditions. A considerable amount of research has also been focused on growth of nanowires,21 nanorods,20
22,26 nanotubes,20 flowers,27 and microrods28 of tellurium. Most of these synthetic methods involve sulphurous acid,15 sodium sulphide,16 hydrogen peroxide,17 L-cystine,18 biomolecules (such as sugars and amino acids),26 sodium gluconate,19 and cellulose9 formamide,20 sodium borohydride,21,29 ascorbic acid22 as reducing agents. Diethyldithiocarbamato tellurium(IV) and [(CH3)4N]4Ge4Se10 Received: November 24, 2010 Revised: March 18, 2011 Published: March 22, 2011 1597

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Crystal Growth & Design have also been used as single source precursors for the formation of tellurium with flower-like morphology and selenium nanowires by Wang et al.27 and Gautam et al.,29 respectively. However, it is desirable to develop more simple routes for the synthesis of Se and Te of diverse morphologies from the same single source precursors under different experimental conditions, which could improve the performance of the existing devices and also act as a good template for the preparation of important functional materials, such as Cu2Se, Ag2Se, Bi2Se3, CdSe, CdTe, PbTe, and Nb3Te4.30
35 Morpholine-4-carbodithioate is a ligand that forms yellow complexes with Se and Te at pH of 5.0
6.5.36,37 However, neither of these complexes has been utilized for the growth of elemental Se and Te with different morphologies. Therefore, in the present work, we focused on the solvothermal growth of trigonal Se and Te of diverse morphologies in the absence of template if any at 100
140
C using their morpholine-4-carbodithioate complex as single source precursors in a water
THF mixture. In order to study the effect of templating agents on growth and morphology of Se and Te, experiments have been conducted using disodium ethylenediamine tetraaceticacid (EDTA) and cetyltrimethylammoniumbromide (CTAB) as soft templates. It is anticipated that all these synthetic routes may provide a new chemical approach in designing the syntheses of 1D Se and Te nanostructures preferably at low temperature without involving any reducing agents, which is still a challenge.

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Figure 1. XRD patterns of the products obtained by the solvothermal decomposition of morpholine-4-carbodithioate complex of selenium in 10% water in THF in the presence of EDTA at (a) 100
C, (b) 120
C, and (c) 140
C.

’ EXPERIMENTAL SECTION All the reagents were of analytical grade and used without further purification. Morpholine and sodium tellurite were procured from Sigma Aldrich, Germany. Disodium salt of ethylenediammine tetraaceticacid (EDTA), tetrahydrofuran (THF), sodium hydroxide, diethyl ether and hydrochloric acid were obtained from E.Merc, Mumbai, India. Selenium dioxide and N-cetryl-N,N,N-trimethyl ammonium bromide (CTAB) were purchased from Loba Chemie Pvt. Ltd. Carbon disulfide was purchased from Nice Chemicals Pvt. Ltd. India.

Preparation of Morpholine-4-carbodithioate Complex of Selenium and Tellurium. Morpholine-4-carbodithioate (MCDT) ligand and its Se and Te complexes were prepared by the method as reported by Srivastava and others.36 According to this, 1 mL of morpholine was dissolved in 5 mL of diethyl ether in a beaker and the mixture was placed on the ice-bath and stirred for 5
10 min. Subsequently, 1.5 mL of carbon disulfide was added dropwise to the above solution, the vigorous exothermic reaction took place and the solution turned to white. On continued stirring for another 30 min, this white colored solution gradually transformed to white pasty mass of morpholine-4carbodithioate, which was filtered and dried at room temperature in air. This morpholine-4-carbodithioate ligand was used for the preparation of the corresponding complexes of Se and Te.36 For this purpose, sodium salt of morpholine-4-carbodithioate was prepared in a beaker by dissolving 0.15 g of morpholine-4-carbodithioate in the minimum volume of 3  10
4 M NaOH. In another beaker, 0.2 g of selenium dioxide was dissolved in 10 mL of water and slowly added to the above solution. Within a few minutes, a yellow colored solution was formed, which was made acidic by adding 1 M HCl until the pH of the resulting solution attained the value of ∼5 followed by the stirring continued for 1 h. This led to the formation of a huge quantity of yellow precipitate of morpholine-4-carbodithioate complex of selenium, which was filtered and dried in air.

Preparation of Selenium and Tellurium from Corresponding Morpholine-4-carbodithioate Complex of Selenium and Tellurium. Morpholine-4-carbodithioate complex of selenium was

Figure 2. XRD patterns of the products obtained by the solvothermal decomposition of morpholine-4-dithiocarbonate complex of tellurium in 10% water in THF at (a) 100
C, (b) 120
C, and (c) 140
C. dissolved in water as well as in varying volume % (10, 30, 50, 70, and 100) mixture of water in THF. In a separate container, a stock solution was prepared by dissolving 0.5 g of EDTA in 20 mL of distilled water and used to prepare water/THF solutions of the desired compositions. Subsequently, 70 mL of the solutions of different compositions were transferred to quartz linear and placed inside the high pressure autoclave and reaction was carried out at 100, 120, and 140
C for 24 h. The same set of reactions was carried out in the presence of 1.8 g of CTAB as a soft template in the place of EDTA. The reactions were also conducted in the absence of EDTA or CTAB while maintaining other identical conditions. A similar set of reactions was carried out by using sodium tellurite (0.2 g) in place of SeO2 keeping other experimental conditions the same.

’ CHARACTERIZATION TECHNIQUES The powder X-ray diffraction patterns of the hydrothermal products were recorded on an X’Pert Pro PANalytical instrument using Cu KR radiation in the range of 2θ = 10
70
with a 1598

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Figure 4. SEM images of selenium in 10% water in THF containing EDTA as template and prepared at (a) 100
C, (b) 120
C and (c) 140
C.

range of 20 keV. IR studies were done on a Perkin-Elmer instrument. The thermal analysis was performed with a Perkin-Elmer instrument in the range of 50
400
C with a heating rate of 5
min
1. Raman and photoluminescence analysis were carried out with a TRIAX550 single monochromator equipped with a notch filter and a CCD detector and Perkin-Elmer instrument PRECISELY LS 55 luminescence spectra having Xe source. Figure 3. SEM image of selenium prepared at 140
C at varied volume percentage THF for (a) 30%, (b) 50%, (c) 70%, and (d) 90% THF in THF
water mixture.

scanning rate of 0.5
per minute. The morphology of the samples was studied by a scanning electron microscope (SEM) on an instrument Carl Zeiss instrument at an accelerating voltage of 20 kV. A JEOL instrument (JSM-5800) transmission electron microscope operating at 200 kV was used to obtain digitally acquired images on a Gatan multipole charge coupled device (CCD) camera for the sample dispersed in ethanol and placed on carbon-coated Cu grid and selected area electron diffraction (SAED) analysis was also performed. Energy dispersive X-ray (EDX) analysis of the sample was carried out on Oxford instrument INCA attached to the SEM in the scanning

’ RESULTS AND DISCUSSION IR spectra of the morpholine-4-carbodithioate ligand and its complex with Se and Te (as given in Supporting Information, Figure S1) shows the presence of vibration modes observed at about 986 and 1452 cm
1 in the ligand corresponding to C
S and C
N stretching modes respectively. These peaks are shifted to higher wave numbers in the case of its complex with Se and Te. The peaks at about 1120 cm
1 in either case corresponds to the stretching mode of ether linkage of the ligand. The mode at 1260 cm
1 refers to the stretching mode of the C
N bond, in the morpholine moiety, whereas the peak at 1430 cm
1 corresponds to the C
H bending vibrations. It is noted that the position of these IR peaks remain virtually unaltered due to the 1599

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Figure 5. (a) HRTEM image of the rod and (b) SAED pattern of a portion of the Se rod prepared at 140
C in THF 10% water in THF containing EDTA.

large separation of C
O and C
H groups from the coordinating centers. Thermogravimetric (TG) analysis of the morpholine-4-carbodithioate complex of Se and Te was done in nitrogen atmosphere and the results are shown in Supporting Information, Figure S2. The initial weight loss of about 40% (calculated 44%) in TG in the temperature range of 165
220
C in Se complex is due to simultaneous loss of two MCDT ligands. However, the loss of the first MCDT ligand is not detected in the TG plot, which may be due to fast elimination of two ligands in the above temperature range. Subsequently, the product, that is, MCDT complex of Se associated with two ligands, undergoes decomposition to form SeS2, the presence of which is also confirmed from the corresponding weight (39%) in TG (calculated 37%). It is accompanied by the further loss in weight forming residue of about 14% due to the formation of elemental Se (calculated 10.8%). In the case of Te complex, the weight loss of 75% (calculated 75.4%) at about 270
C is observed, which corresponds to the formation of TeS2. TeS2 so formed is found to be stable up to about 365
C and undergoes the decomposition at a higher temperature to residue of 22 wt % (calculated 16.4%) corresponding to Te. All the above steps are in accordance with the earlier report,36 although a slight variation at the temperature of decomposition is observed. Figures 1 and 2 show the diffraction patterns of the products obtained by the solvothermal decomposition of morpholine-4dithiocarbonate complex of selenium and tellurium dissolved in 10% water in THF in the presence of and in the absence of EDTA respectively at 100, 120, and 140
C. It is noted that the diffractogram of the product matched well with standard trigonal

Figure 6. SEM images of selenium in 10% water in THF containing CTAB and prepared for a duration of 24 h at (a) 100
C, (b) 120
C, (c) 140
C, and (d) for 40 h at 140
C.

selenium (JCPDS card no. 01-0853) and tellurium (JCPDS card no.79-0736), respectively. No additional peaks are observed in the diffraction patterns corresponding to any other possible phases due to the impurities. The diffraction patterns of Se powder at 140
C suggest the abnormally high intensity of the peak corresponding to the Æ100æ plane indicating the preferential 1D growth along Æ001æ. X-ray diffraction (XRD) studies also confirmed the formation of Se and Te at 100, 120, and 140
C with varied concentration of 30, 50, 70% water in THF and for 100% water. However, our experimental observations suggest 1600

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Figure 8. (a) HRTEM image and (b) SAED pattern of a portion of the tellurium rod obtained at 140
C in 10% water in THF.

Figure 7. SEM image of tellurium in the presence of 10% water in THF prepared at (a) 100
C, (b) 120
C, and (c) 140
C.

that the product (Se or Te) is not formed at all under above reaction conditions or even at raised temperature and duration (160
C/36 h) when THF is used as only medium. Han et al.38 also observed a similar phenomenon that the decomposition of AuCl4
takes place in formamide medium rather than water or alcohols. It is anticipated that the ligand to central atom bond in the complex is easily dissociated in the presence of the solvents with high dielectric constant value. However, the presence of water as the reaction medium weakens the coordination bonds between ligand to selenium and forms the product even at lower temperatures. It is well established that that crystallization takes place provided that the nucleation rate is lower than the growth rate.39 In addition, the formation of a crystal requires a reversible pathway between the building blocks on the solid surface and those in the liquid phase.40 These conditions allow the building blocks to easily adopt the appropriate positions necessary for developing the long-range-ordered, crystalline lattice.40 The formation of the crystals with homogeneous composition and uniform morphology requires a supply of these building blocks at a well controlled rate.40 Therefore, it is anticipated that that the morphology of the decomposition product of MCDT complex of Se and Te could be varied by allowing the reaction to proceed in different volume percentage ratio of the constituent solvent mixture (THF and

water), at different temperatures, in the presence and absence of templating agents. The SEM image of the product obtained at 140
C from Se complex in the presence of 100% water without using any template shows the formation of irregular shaped particles of Se, while the formation of rods is rarely observed. When the mixture of water and THF is taken as solvent, an increasing tendency of 1D growth is observed with an increasing % of THF in water and is maximum for 90% THF as evident from the SEM images in Figure 3. Thus, it is conceived that in a solvent like water having a high dielectric constant (78.3), the irreversible generation of the nucleating species due to the fast breaking of the coordination bonds in the ligand to Se complex is unfavorable for the 1D growth of Se. On the contrary, when the volume % of THF in THF and water mixture is increased from 0, 30, 50, 70, and 90, the dielectric constant decreases from 78.3, 64.9, 54.6, 35.4, and 17.9, respectively,41 which gradually tends to maintain the reversible generation of nucleating species, whereby 1D rods of a high aspect ratio is achieved. It has already been mentioned earlier that the optimum control of the nucleation step is achieved at 90 vol % of THF. On the other hand, no reaction takes place in 100% THF medium due to a very low value of dielectric constant (6.7), which is not enough to allow the fragmentation of ligandSe bonds of the complex at even up to160
C. Temperature also makes an important contribution in the growth of the seeds produced by the decomposition of the Se complex of MCDT in the presence of the water
THF mixture. In all probability, the MCDT complex of Se reversibly releases nucleating seeds, which are responsible for the growth of Se crystals. At 100 and 120
C, the concentrations of such seeds are relatively low, and hence the product remains mostly in an 1601

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Figure 9. SEM image of tellurium obtained at (a) 100
C, (b) 120
C, and (c) 140
C in 100% water.

underdeveloped state (particle and microrods of a lower aspect ratio). At 140
C, both the complexes are expected to produce a higher concentration of nucleating seeds, and as a result, the products tend to acquire 1D morphology due to the high anisotropic tendency of Se. When the same reaction is carried out in the presence of EDTA and mixed solvent (10% water and in 90% THF) at the three different temperatures viz. 100, 120, and 140
C, a drastic change in morphology is observed. SEM images in Figure 4 clearly show that the Se acquires a typical flowery morphology at 100
C (Figure 4a). It is also worth mentioning here that the complex of Se with EDTA is not yet reported. Thus, in all possibilities, EDTA acts as soft template42 and helps in the growth toward the evolution of the flower type morphology of the product. Interestingly, in the absence of EDTA, the morphology of the product at the same temperature (100
C) is found to be irregular (Figure S5, Supporting Information) When the temperature is increased to 120
C, the product consists of flowery morphology as well as micrometer-sized rods of selenium. As the temperature is further increased to 140
C, the morphology is completely transformed to microrods having diameters of about 700
800 nm and lengths of several tens of micrometers. It can also be seen from SEM images that the rods

Figure 10. (a) Low magnification, (b) high magnification images, (c) SAED pattern of a portion of nanorod of tellurium prepared at 140
C.

gain a uniform surface with a high aspect ratio compared to that obtained at lower temperatures. This suggests that in all probability, flowery selenium starts breaking to generate microrods when the temperature is increased from 100 to 120
C, and this transformation is complete at 140
C. It is to be noted that the reports are available describing evolution of rods from flower type morphology of the materials.43 Figure 5a shows the HRTEM image of the Se powder sample prepared at 140
C in the presence of the EDTA and 10% water in THF which confirms the formation of the smooth surface of the selenium microrods. The lattice fringe separation of the rods is found to be 0.37 nm, which is in agreement with the d value of Æ100æ planes as obtained from the X-ray diffractogram. The SAED patterns in Figure 5b shows well aligned parallel spots indicating the single crystalline nature of the product. 1602

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Scheme 1. Schematic Representation Depicting the Reaction Pathways Towards Formation of Diverse Morphologies of Selenium and Tellurium through Solvothermal Decomposition of Respective Morpholine-4-carbodithioate Complexes

Figure 6 shows the SEM images of the selenium prepared by solvothermal decomposition of Se complex of MCDT at 100, 120, and 140
C in the presence of CTAB as a soft template and 10% water in THF for 24 h. It is clearly seen that the microparticles of irregular shape are formed at 100
C. When the temperature is increased to 120 and 140
C, the formation of the small fraction of micrometer-sized rods, the growth being better at 140
C, along with the microparticles of irregular shape is observed. This clearly suggests that CTAB is not suitable in our case for the 1D growth of Se. This may be attributed to the tendency of CTAB to form spherical micelles in pure water,44 which in all likelihood, hinders 1D growth of Se. As a result, a higher concentration of particle morphology is observed even at 140
C. It may be noted that, when the duration of the reaction is increased to 40 h, at 140
C, uniformly distributed microrods of low aspect ratio are observed. Possibly, the seeds of Se initially formed have enough time to coalesce together to form 1D morphology owing to the anisotropic tendency of Se that surpasses the effect of spherical micelles of CTAB in solution. Figure 7 represents the SEM images of the product obtained from morpholine-4-carbodithioate complex of Te dissolved in 90% THF in a water mixture at 100, 120 and 140
C. It clearly demonstrates that the small particles of tellurium are formed at 100
C and which are transformed to microsized rods at 120
C. The transformation of tellurium microrods is almost complete at 140
C, with a relatively much smoother surface and high aspect ratio. It appears that the complex at this temperature breaks completely to give a sufficiently high concentration of tellurium which aids in Ostwald’s ripening process which is consequently stabilized by the THF molecules45 so as to form discrete nanorods. It is also noted that unlike Se, the microrods of Te are formed

with a lower aspect ratio and rough surface, when EDTA is used as a template (Supporting Information, Figure S6). When the same reaction is carried out in the presence of CTAB, Te adopts a particle type of morphology at lower temperatures (100 and 120
C) (Figure S7, Supporting Information) and uniformly distributed Te rods of a low aspect ratio at 140
C. Figure 8a shows the HRTEM image of Te powder obtained at 140
C by the decomposition of Te complex of morpholine-4carbodithioate in THF and 10% water mixture, indicating equispaced lattice fringes having a separation of about 0.38 nm, which matches well with the lattice separation of Æ100æ planes as calculated from XRD pattern. The SAED pattern (Figure 8b) of a single area of the rod indicates the single crystalline nature of the product. When the same reaction is carried out in THF even at a temperature as high as 165
C, tellurium is not precipitated at all. On the other hand, when the reaction carried out in the presence of 100% water, at lower (100
C) and medium (120
C) temperatures, only Te nanoparticles are formed (Figure 9a,b). As the temperature increased further to 140
C, evolution of nanorods along with the presence of nanoparticle morphology is observed (Figure 9c). The diameter of the nanorods was found to be about 15 nm and length nearly 100 nm as observed in TEM images in Figure 10a. Also the SAED pattern in Figure 10c indicates the single crystalline nature of the rods. The morphology of the sample, at this point, remains more or less unchanged even at the higher temperature (170 or 180
C). The formation of 1D growth of Te in water medium unlike that observed in the case of selenium could be explained on the basis of respective bonding in their crystal lattices. Both selenium and tellurium have six outer electrons (s2p4). Two of these electrons are paired in the s-orbital, two in one of the three p-orbitals, and the 1603

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Figure 11. Raman spectra of (a) selenium and (b) tellurium.

Figure 12. Photoluminescence spectra of (a) selenium and (b) tellurium.

remaining two are available for covalent bonding in half-filled p-orbitals.46 The structure of both selenium and tellurium consists of a spiral chain of atoms with three atoms per turn and corresponding atoms in each chain forms a hexagonal network, although the highest symmetry axis is 3-fold and is chosen as the c-axis.47 Though the bonding between the Se (and Te) atoms on the helical chain is covalent, it is a mixture of electronic and van der Waals types46 and covalent among the chains for Se and Te, respectively.6 Thus, tellurium leads to a greater anisotropy due to the presence of such strong covalent bonding among the helical chains, leading to its growth along longitudinal orientation.6 Hence, it is anticipated that tellurium leads to the formation of 1D growth, in spite of the fast generation of nuclei in water medium. Scheme 1 shows the evolution of diverse morphology of the solvothermal decomposition product of morpholine-4-carbodithioate complex of selenium and tellurium at varied temperatures in 10% water in THF and 100% water in the presence and absence of EDTA and CTAB. Such variation in the morphologies of these semiconducting nanostructures may lead the variation in the properties, for example, optical, photoluminescence, catalytic, photocatalytic, sensing, field emission, etc., similar to the other reported semiconducting materials of diverse morphologies.43,47
54 Figure 11a,b shows Raman spectra of Se (micro rods, flowers) and Te (particles and nanorods). The presence of peaks at around 130 and 233 cm
1 in (a) corresponds to the characteristic fundamental stretching vibration of polymeric helical chain in microrods of the t-Se.15 In addition, two peaks at 211 and

264 cm
1 are also observed in the Raman spectra of Se microflowers, which could be attributed to the 2A2 due to the atoms in neighboring trigonal chains of selenium55 and A1 bond because of the stretching vibrations of Se8 molecule,56 respectively. Raman spectra of Te nanorods in Figure 11b show the presence of two peaks at about 118 and 134 cm
1 corresponding to the characteristic E bond and A1 bond-stretching stretching.22 Interestingly, no shift in peak positions is observed for the particle type of morphology of tellurium. The photoluminescence spectra of selenium (microrods, flowers) and tellurium (particles and nanorods) were obtained at a excitation wavelength of 630 and 365 nm respectively and findings are displayed in Figure 12. A broad emission peak at around 755 nm is observed in the case of selenium microrods. The photoluminescence spectra of tellurium shows the presence of three broad peaks with their shoulders at 488, 525, and 579 nm. It is also noted that the positions of all the emission maxima for selenium as well as tellurium remains more or less unaltered due to the insensitivity to the size as well as morphology of the sample.22 Figure 12b shows that the intensity of Te nanorods is relatively more, possibly due to its better crystallinity compared to its particle type morphology.57 TGA of the synthesized selenium microrods (Figure S10, Supporting Information) shows a weight loss of 2
3% up to the temperature of 330
C which is probably due to evaporation of volatile matters. After that, it shows a sharp decrease in weight loss up to 500
C, and above this temperature, the selenium is completely evaporated. This is attributed to vaporization of 1604

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Crystal Growth & Design selenium under N2 flow condition.13 DTA of synthesized selenium microrods recorded in the range of ambient to 300
C shows the presence of an endothermic peak at around 212
C due to melting of the Se. TGA curve for tellurium (Figure S11, Supporting Information) shows about 5% weight loss up to the temperature of 345
C, which is possibly due to the presence of a trace amount of TeS2 which is converted to elemental Te at this temperature. It is observed that the weight loss remains more or less constant up to 435
C and followed by a rapid weight loss above this temperature due to the rapid evaporation of the Te similar to that of Se as discussed earlier. Differential thermal analysis (DTA) curve shows presence of an endothermic peak at 435
C and which corresponds to the melting of the element.

’ CONCLUSION Trigonal selenium and tellurium have been successfully synthesized in diverse morphologies from solvothermal decomposition of respective morpholine-4-carbodithioate complexes in the temperature range 100
140
C at varied volume % of a water
THF mixture. It is observed that the synthesized product (both Se and Te) in the absence of templating agent (EDTA and CTAB) at lower temperatures (100
120
C) possesses irregular growth with particle type morphology, whereas the optimum 1D growth at 140
C is observed for 10% water in THF. On the other hand, the presence of EDTA induces the formation of flower morphology at 100
C followed by microrods at 140
C, suggesting that 1D morphology is evolved due to the breaking of the flowery morphology. However, in the presence of CTAB Se has a greater tendency toward formation of particle morphology, even at 140
C. Under identical conditions, the formation of Te microrods with a rough surface with a lower aspect ratio is observed in the presence of EDTA, but monodispersed particle and uniform micrometer-sized rods of low aspect ratio are formed with CTAB at 100 and 140
C, respectively. On the contrary, morpholine-4-carbodithioate complex of tellurium is found to generate nanorods of tellurium along with particle morphology in 100% water medium at 140
C. The complexes, as well as products (Se and Te), have been well characterized through thermal analysis, XRD, SEM, TEM, Raman, and PL studies. ’ ASSOCIATED CONTENT

bS

Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; tel.: (þ91-3222) 283334; fax: (þ91-3222) 255303.

’ ACKNOWLEDGMENT S.K.S. dedicates this piece of work to his esteemed teachers, Late Professor T. N. Srivastava and Professor P. C. Srivastava, Department of Chemistry, University of Lucknow, for their inspiration. Authors are also thankful to Prof. S. Ram of Material Science Centre and Prof. A. Roy of Department of Physics, Indian Institute of Technology, Kharagpur, India, for their help with the measurements of PL and Raman spectra of the samples, respectively.

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