Accepted Manuscript Solid state synthesis and spectral investigations of nanostructure SnS2 G. Kiruthigaa, C. Manoharan, C. Raju, J. Jayabharathi, S. Dhanapandian PII: DOI: Reference:

S1386-1425(14)00499-5 http://dx.doi.org/10.1016/j.saa.2014.03.088 SAA 11921

To appear in:

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Received Date: Revised Date: Accepted Date:

9 January 2014 28 February 2014 15 March 2014

Please cite this article as: G. Kiruthigaa, C. Manoharan, C. Raju, J. Jayabharathi, S. Dhanapandian, Solid state synthesis and spectral investigations of nanostructure SnS2, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa.2014.03.088

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Solid state synthesis and spectral investigations of nanostructure SnS2 G.Kiruthigaa a,C.Manoharan a,*, C.Raju b, J.Jayabharathi c, S.Dhanapandian a a b

Department of Physics, Annamalai university, Annamalai nagar 608002, Tamilnadu, India

Chemistry wing of DDE, Annamalai university, Annamalai nagar 608002, Tamilnadu, India c

Department of Chemistry, Annamalai university, Annamalai nagar 608002, Tamilnadu, India

Abstract Nanometer sized SnS2 particles were synthesised by solid state reaction between tin chloride and thiourea in air at 150-350°C. The structural, morphological and optical properties were characterized by using X-ray diffraction (XRD), FT-IR, FT-Raman, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), field emission scanning electron microscopy (FE-SEM), photoluminescence (PL) and UV-Vis spectra. The X-ray diffraction (XRD) pattern of the product was indexed to the hexagonal phase of SnS2. Crystallite size, microstrain and dislocation density were evaluated from the XRD data. EDS analysis indicated that the elemental ratio was similar to tin disulphide (SnS2). The blue shift in the absorption edge was observed from the UV-Vis spectrum. The Photoluminescence spectra showed two emission peaks corresponding to blue and red emission. Keywords: SnS2, Solid state reaction, Optical band gap, Photoluminescence *Corresponding author. Tel.: +91 9443787811 E-mail address: [email protected]

1. Introduction In recent years, metal sulphide semiconductors have attracted much attention due to their physical and chemical properties which are related to their phase, size, surface morphology and crystal defects. Among the semiconductors, tin sulphide (SnS2) has received a great deal of interest owing to its optical and electronic properties [1], which are potentially useful for technological applications. SnS2 has a layered CdI2-type structure with a band gap of 2-3 eV [2]. It is an n-type semiconductor and chemically stable in neutral aqueous solutions, which makes it a promising visible light sensitive photo-catalysts. Such a tunable band gap and good light absorbing nature makes it a potential candidate for solar cells and opto-electronic devices [3] and also has been used as a semiconductor [4]. Besides, SnS2 can also be used as an anode material for lithium ion batteries [5], pigment [6], gas sensor [7], photoconductor [8] and photoluminescence material [9]. Moreover the constituent elements are less toxic and abundant in nature leading to the development of devices that are environmentally safe. Various methods have been employed to prepare SnS2 nanoparticles including solvothermal process [10], hydrothermal process [11], mechanochemical route [12], solid state reactions [13], pyrolysis of organotin precursor [14], and tetrabutyltin precursor [15]. However, new simple method still needs to be developed due to the complexity and expensiveness of some of these methods. The solid state reaction is most widely used to prepare phosphors, ceramics and nanomaterials usually at higher temperatures. Hence, solid state reaction at low temperature is an attractive option for the preparation of nanomaterials in which the solvent and high temperature heating are not involved in the process.

There is no systematic report on solid state synthesis of nanostructure SnS2 using SnCl4.5H2O as a precursor. In this work, the structural and optical properties of nanostructure SnS2 synthesized via the solid state reaction between SnCl4.5H2O and thiourea in air at 150-350°C for 2h are reported. 2. Experimental details All the chemical reagents used were of analytically pure. In a typical synthesis of SnS2, 0.01 M of SnCl4.5H2O and 0.02M of thiourea were first mixed and ground thoroughly with a mortar then transferred into a crucible with lid. The crucible containing the reactant was heated in a furnace for 2h at different temperature (T=150-350°C). After 2h of reaction time the reaction mixture was allowed to cool to room temperature naturally. The resultant products were refluxed with de-ionised water for several times, to remove the impurities. The product was dried in hot air oven at 80°C, and finally yellow SnS2 products were obtained. The phase purity of SnS2 was examined by X-ray diffraction (XRD) using SHIMADZU-6000 X-ray diffractometer with CuKα radiation (λ=1.5406Å) at room temperature. The FT-IR spectrum was recorded by SHIMADZU-8400 spectrometer using KBr pellet technique. The Raman spectrum was recorded using a BRUKER RFS 27 Raman spectrometer. The morphology and composition of the product was investigated by SEM and EDS analysis with a JEOL JSM 6390. The FE-SEM images were taken by HITACHI S-4700. An optical absorption study was carried out in the range 200-1200nm using SHIMADZU 1650 spectrophotometer. The PL spectrum was recorded on a fluorescence spectrophotometer (VARIAN) with an excitation wavelength of 300nm at room temperature. A Photocatalytic property of the product was tested by the degradation of red reactive 120 (RR 120) dye under UV-A light irradiation. For the degradation by UV-A light (365 nm), a Heber Multilamp-

photoreactor HML MP 88 was used. This model consists of 8W medium pressure mercury vapour lamps set in parallel and emitting 365 nm wavelength. 3. Results and discussion 3.1 Structural analysis The XRD patterns of the SnS2 at various reaction temperatures are shown in Fig.1. The spectra of the samples obtained at temperatures in the range 150-250°C exhibit strong peaks at around 28°, 32° and 50° corresponds to the (100), (011), (110) plane of SnS2 phase. The increased full width at half maximum (FWHM) of the above observed peaks indicates the characteristic of nanoparticles with a low crystallinity. When the temperature is increased from 250° to 300°C, the diffraction peaks at 15°, 28.3°, 32.1°, 41.9°, 50.3° corresponds to the (001), (100), (011), (102), (110) plane of SnS2 with hexagonal crystal structure (JCPDS card no. 892028) are intensified. The decrease in FWHM of the peak obtained at 300°C indicates the increase of crystalline nature. It is also evident from the Table 1 that the crystallite size increases with increase in temperature. When the reaction is carried out at higher temperature of 350°C, remarkable changes in the XRD pattern is observed. The remarkable changes are due to the presence of mixture of phases, SnS2 with a preferential orientation along (011) direction and SnS with (210), (411), (512) and (522) orientations (JCPDS card no.752115). The crystallite size is evaluated from the Scherer formula [16]

(1)

where θ is the Bragg’s angle and β is the full width at half maximum of the peaks and λ is the X-ray wave length. The micro strain (ε) is calculated using the relation [17]

ε=

(2)

The dislocation density (δ) is calculated using the formula [18]

δ=

(3)

The structural parameters like crystallite size, lattice constant, dislocation density and microstrain of SnS2 are displayed in Table 1. The crystallite size increases from 1.64 to 6.59 nm with increase in temperature up to 350°C. The crystal defect parameters like microstrain and dislocation density show a decreasing trend with increase in crystallite size.

FT-IR spectroscopy was used to analyse the chemical composition of the as-synthesized SnS2 nanoparticles and is shown in Fig.2. The strong infrared absorption band at 300-450 cm-1 is characteristic of binary metal sulphide. The absorption bands observed in the spectrum at around 320 cm-1 and 660 cm-1 are due to the formation of Sn-S bond [19].The presence of SnS bond confirms the synthesized nanoparticle is SnS2.

The Raman spectrum of the SnS2 nanoparticle is shown in Fig.3, which exhibit an intense peak at about 313 cm-1, corresponding to A1g mode of SnS2 [20]. The A1g peak position of SnS2 nanoparticles shifts to lower frequency by four wavenumbers in comparison with that of bulk (317 cm-1). The absence of Raman peak located at 210 cm-1 of the first-order Eg mode is due to the nano size effect [21]. The broadening of the peak indicates the nanocrystalline nature of the as-obtained product.

3.2 Surface morphology and compositional analysis The SEM study reveals the surface morphology of the SnS2 nanoparticle. Fig.4a shows the SEM image of the SnS2 nanoparticles with well defined sphere-like morphology and densely packed possessing compact texture. The elemental composition of the SnS2

nanoparticles prepared at 300°C are shown in Fig.4b, which confirms the presence of Sn and S in near stoichiometric ratio (1:2). The atomic percentage of the products is given in Table 2. It can be noticed that the absence of any other precursor elements in EDS spectrum reveals that the prepared SnS2 nanoparticles are free from impurities and contaminants. Fig.4 (c and d) shows the low and high magnification of FESEM images. The FESEM image of the SnS2 shows the aggregation of nanosphere and exhibits the flower like structure.

3.3 Optical studies Fig. 5 shows the absorption spectra of the SnS2 synthesised at various temperatures. A very strong absorption peak at around 295 nm is due to the phase of tin disulphide. A shift of the peak position towards the lower wavelength is obtained with increasing temperature. The absorption co-efficient is calculated using the formula

α=

(4)

where A is the absorbance and l is the path length. The value of optical band gap is determined from the absorption spectra using the Tauc relation αhυ =A(hυ- Eg) n

(5)

where α is the absorption co-efficient, A is the constant having separate value for different transitions, hυ is the photon energy, Eg is the band gap energy, the value of n depends upon the transition. The values of n for allowed direct, allowed indirect, forbidden direct, forbidden indirect transition are 1/2, 2, 3/2 and 3 respectively. The band gap energies are found to be a negative number for n =2, 3/2 and 3, so the relationship fitting to the SnS2 is n=1/2, which confirms the SnS2 nanoparticles are allowed direct transition. Fig.6 shows the curves of (αhυ)2 versus hυ for SnS2 nanoparticles prepared at different temperature. The energy band gaps are obtained by extrapolating the straight line

portion of the graph to the X-axis. The evaluated energy band gaps from the plots are 3.52 eV, 3.45 eV, 3.44 eV, 3.41 eV and 3.36 eV for the temperature 150°C, 200°C, 250°C, 300°C and 350°C respectively. The observed energy band gap values (3.36- 3.52 eV) are higher than that of the value for bulk SnS2 (2.44 eV), indicates the blue shift. The blue shift can be attributed to the small size of the SnS2 nanostructure. The observed results are in good agreement with the reports of SnS2 [22]. Moreover, the larger band gap of the sample prepared at 300°C (3.41eV) is expected to facilitate electron injection from photo-excited dye molecules in dye sensitized solar cells [23]. The observed band gap value decreases with increase of crystallite size is in agreement with the report [24]. The photoluminescence spectrum of newly synthesised SnS2 nanoparticles at 300˚C with an excitation wavelength of 300 nm is shown in Fig.7. The spectrum exhibits two emission peaks at around 426 and 725 nm. The luminescence peak at 426 nm corresponds to recombination of the bound excitons and explanation for the origin of the latter broad peak may be from the inner deep level emission [24]. This deep level arises due to the stoichiometric variation in SnS2 nanoparticles. 3.4 Photocatalytic activity The photocatalytic activity of SnS2 is evaluated by measuring the degradation of reactive red 120 (RR 120) dye in an aqueous solution under UV –A light (365 nm) irradiation. Fig.8. shows the obvious changes in the absorbance spectra (200 to 800 nm) of Reactive red 120 with SnS2 (T=300°C) photocatalysis at different time intervals under UV-A light irradiation. The absorbance at the maximum absorption peak (512 nm) corresponds to the n-π* transition of the azo and hydrazone forms, which is due to the colour of azo dyes. The absorbance at 200-400 nm is attributed to the π-π* transition of benzene rings, representing the aromatic content of azo dyes, and its decrease is due to the degradation of

aromatic part of the dye. As the reaction time increased, the intensity of the peaks decreased with respect to time and the full spectrum scanning model distorted apparently after 3 hours. It designated that the main chromophores and aromatic part in the original dye solution were shattered in the presence of SnS2 under simulated UV-A light irradiation. UV-Vis spectra are analysed to point towards that the dye can be degraded successfully by SnS2 under UV-A light irradiation. Moreover, these spectra also reveal that the intermediates do not absorb the analytical wavelengths. Conclusions The SnS2 nanoparticles have been synthesized by solid state reaction and the hexagonal crystal structure with preferred orientation along (011) direction of SnS2 was proved by XRD. The FESEM image showed densely packed nanosphere with flower like morphology. The optical band gap energy calculated from the optical spectrum of the SnS2 at 300°C was 3.41eV. Due to the nano size, the energy band gaps exhibit a significant blue shift. The photoluminescence spectra of the SnS2 nanoparticles exhibited blue and red emission at around 426 nm and 725 nm. The nanostructured SnS2 with a large band gap (3.41eV) and a PL emission at 426 nm and 725 nm have potential application in solar cells, optoelectronic devices and dye sensitized solar cells. The photocatalytic result suggested that the SnS2 nanoparticles should be effectively used as photocatalyst in the degradation of RR 120 dye. Acknowledgement The authors wish to thank Dr.M.Shanthi, Dept. of Chemistry, Annamalai University for providing the facility to carry out photocatalytic activity.

References [1] C. Julien, M. Eddreif, I. Samaras, M. Balkanski, Mater.Sci.Eng.B 15 (1992) 70-72. [2] S. Polarz, B. Smarsly, C.Goltner, M.Antonietti , Adv. Mater. 12 (2000) 1503-1507. [3] G. Domingo, R.S.Itoga, C.R.Cannewurf, Phys. Rev. 14 (1966) 536-541. [4] Y.T. Lin, J.B. Shi, Y.C. Chen, C.J. Chen, P.Feng , Nanoscale. Res. Lett. 4 (2009) 694– 698. [5] T.J.Kim, C.Kim, D.Son, M.Choi, B.Park, J.Power Sources 167 (2007) 529-535. [6] P.W.Shen, J.T.Wang, Dictionary of Compounds, Shanghai; Shanghai Lexicographical Publishing House, 2002. [7] W.Shi, L.Huo,H.Wang, H.Zhang, J.Yang,P.Wei, Nanotechnol. 17 (2006) 2918-2924. [8] T.Shibata, Y. Muranushi, T. Miura, T.Kishi, J.Phys. Chem. Solid 51 (1990) 12971300. [9] C.Wang, K.Tang, Q.Yang, Y.Qian, Chem. Phys. Lett. 357 (2002) 371-375. [10] Y.Q.Lei, S.Y.Song, W.Q.Fan, Y.Xing, H.J.Zhang, J.Phys.Chem.C 113 (2009) 12801285. [11] H. Mukaibo, A.Yoshizawa, T.Momma, T.Osaka, J.Power Sources 60 (2003) 119-121. [12] P.Balaz, T.Ohtani, Z.Bastl, E.Boldizarova, J.Solid State Chem. 144(1) (1999) 1-7. [13] I.P.Parkin, A.T.Rowley, Polyhedron 12 (1993) 2961-2964. [14] P. Boudjouka, J. Seidlera, B. R. Jarabeka, G. Griera, B. E.Verya, R. L. Jarabeka, G. J. McCarthy, Mater. Res. Bulln. 34 (1999) 2327–2332.

[15] Q.Yang, K.Tang, C.Wang, D.Zhang, Y.Qian, J. Solid State Chem. 164 (2002) 106109. [16] C.N.J. Wagner, Local Arrangement for X-ray Diffraction, eds. J. B. Cohen and J. E. Hilliard (Gordon and Breach), New York, 1966. [17] P.P. Hankare, P.A. Chate, D.J. Sathe, P.A. Chavan, V.M. Bhuse, J. Mater. Sci. Mater. Electron. 20 (2009) 374-379. [18] P.Kathirvel, D.Manoharan, S.M.Mohan, S.Kumar, J.Optoelectr. Biomed.Mater. 1 (2009) 25-33. [19] K.Nakamoto, IR Spectra of Inorganic and Coordination Compounds, 2 nd Ed., Wiley Eastern, New York, 1970. [20] X.L.Gou, J.Chen, P.W.Shen, Mater. Chem. Phys. 93 (2005) 557-566. [21] H.Xiao, Y.C.Zhang, Mater. Chem. Phys.112 (2008) 742-744. [22] A.Chakrabarti, J.Lu, A.M.Mcnamara, L.M.Kuta, S.M.Stanley, Z.L.Xiao, J.A. Maguire, N.S. Hosmane, Inorganica Chimica Acta 374 (2011) 627-631. [23] H.J.Snaith, C.Ducati, Nano Lett.10 (2010) 1259-1265. [24] J.Gajendran, V.Rajendran, Adv.Nat.Sci.:Nanosci.Nanotechnology 2 (2011) 015001015004. Figure captions Fig. 1. XRD pattern of the SnS2 nanoparticles via heating the mixture of SnCl4.5H2O and thiourea at different temperature. Fig. 2. FT-IR spectrum of SnS2 nanoparticles synthesised at 300°C. Fig. 3. Raman spectrum of SnS2 nanoparticles synthesised at 300°C.

Fig. 4. (a) SEM image (b) EDS spectrum (c) low and (d) high magnification FESEM image of SnS2 nanoparticles synthesised at 300°C. Fig. 5. UV-Vis absorption spectrum of SnS2 nanoparticles synthesised at different reaction temperature. Fig. 6. The plot of (αhυ)2 versus hυ. Fig. 7. PL spectrum of SnS2 nanoparticles synthesised at 300°C. Fig. 8. Photocatalytic degradation of RR 120 over SnS2 under UV irradiation.

Table captions Table 1 Structural parameters of the products at different reaction temperature. Table 2 Elemental composition of the products at different reaction temperature.

Fig. 1.

Fig. 2.

Fig. 3.

Fig. 4.

Fig. 5.

Fig. 6.

Fig. 7.

Dark 1h

2h 3h

Fig. 8.

Table 1 Temp (°C)

2θ (degree)

Lattice parameters a

c

c/a ratio

Crystallite size (nm)

Micro strain ε (x10-3)

Dislocation density δ (1018)

150

32.30

2.1484

5.4448

2.5343

1.64

21.1

0.3718

200

32.20

2.3056

5.4825

2.3779

1.76

19.6

0.3228

250

32.55

2.8820

5.6375

1.9561

2.20

15.6

0.2066

300

32.50

3.6155

5.8806

1.6264

2.76

12.5

0.1312

350

32.07

8.6338

8.3878

0.9715

6.59

5.26

0.1386

Table 2

Temp (°C)

Atomic percentage of the elements (%) Sn

S

150

42.23

57.77

200

44.30

55.70

250

43.78

56.22

300

37.71

62.29

350

36.68

63.32

Graphical abstract

Highlights

• It is intended to prepare nanometer sized SnS2 particles by solid state synthesis at low temperature. • The functional groups and the chemical bonding of Sn-S are confirmed by FT-IR and FTRaman. • Tuning of optical band gap by controlling the size of SnS2 shows its suitability for solar cells and opto-electronic devices.

• The obtained band gap value (3.41eV) is expected to facilitate electron injection from photo-excited dye molecules in dye sensitized solar cells.

Solid state synthesis and spectral investigations of nanostructure SnS2.

Nanometer sized SnS2 particles were synthesised by solid state reaction between tin chloride and thiourea in air at 150-350°C. The structural, morphol...
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