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Synthesis of tetrapod like PbS microcrystals by hydrothermal route and its optical characterization
Journal of Alloys and Compounds 481 (2009) 806–810
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Synthesis of tetrapod like PbS microcrystals by hydrothermal route and its optical characterization S. Jana a , S. Goswami b , S. Nandy a , K.K. Chattopadhyay a,b,∗ a b
Thin Films and Nanoscience Laboratory, Department of Physics, Jadavpur University, Kolkata 700032, India Centre for Nanoscience and Technology, Jadavpur University, Kolkata 700032, India
a r t i c l e
i n f o
Article history: Received 11 December 2008 Received in revised form 18 March 2009 Accepted 20 March 2009 Available online 31 March 2009 Keywords: Lead sulfide Hydrothermal Crystal morphology Optical properties
a b s t r a c t Microcrystalline PbS tetrapod like structure has been successfully prepared by a hydrothermal route using Pb(CH3 COO)2 ·3H2 O and thiourea as reagents and acrylamide (AA) as a surfactant. X-ray diffraction (XRD) studies confirmed the highly crystalline cubic PbS phase formation. Scanning electron microscopic study revealed the tetrapod like structures with length 1.5 m and breadth 400–500 nm. Detail morphology and structure have been investigated by transmission electron microscopic studies also. Selected area electron diffraction and HRTEM lattice image confirmed the single crystallinity and preferential growth direction of the as-obtained samples. UV–vis spectrophotometric study showed the direct band gap of the PbS tetrapod ∼4.9 eV. The photoluminescence measurements showed that the emission peaks were located at 440 nm for all the excitation wavelength of 275–325 nm and are explained due to radiative recombination from the surface states. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Many fundamental properties and applications of crystals depend strongly on their shapes, sizes and as well as on dimensionalities. Because of the correlation between the shapes and properties, a lot of effort has been made in recent years to prepare various inorganic, organic or inorganic–organic composite materials with novel morphologies and to study their properties. So far, a variety of one dimensional nanostructures with different morphologies, such as nanowires, nanorods, nanocables, nanobelts, nanotubes etc. have been prepared due to their unique electronic, electrical and optical properties and as well as for important application in mesoscopic physics and fabrication of nanoscale devices [1]. Various nanodevices including nanologic circuits [2], nanolasers [3], nanosensors and nanothermometers [4] etc. have been assembled using one dimensional nanoscale materials. Quantum wires of semiconductors [5] and metallic alloys [6] have been found to exhibit interesting magnetic and electrical properties. For example, ZnO and GaN nanowires have been used to construct blue and ultraviolet emitters and lasers [7–9]. Therefore, the controlled growth of semiconductor nanostructures has received considerable attention from the scientific communities.
∗ Corresponding author at: Thin Films and Nanoscience Laboratory, Department of Physics, Jadavpur University, Kolkata 700032, India. Tel.: +91 9433389445; fax: +91 33 2414 6007. E-mail address: kalyan [email protected] (K.K. Chattopadhyay). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.03.110
Well defined structures of CdS such as nanorods [10], urchin like nanoflowers, branched nanowires [11] etc. have been prepared by facile solvothermal approach. ZnS nanobelt production via thermal evaporation method without using any catalyst was reported by Zhu et al. [12]. Qian’s group has developed a mild solvothermal route for the synthesis of belt like SnS2 crystals [13]. A relatively less studied but equally interesting material is PbS, an attractive – semiconductor with relatively narrow band gap (0.41 eV at 300 K) [14] and the larger Bohr radius of 18 nm [15]. It has wide applications in many fields such as in Pb2+ ion selective sensors [16], IR detectors [17], display devices [18], and solar cells [19]. Moreover, as a consequence of its carrier confinement an exceptional third order nonlinear optical properties of PbS nanoparticles has been found, which may be useful in optical devices such as optical switch [20]. The size and shape dependent optical properties of nanocrystalline semiconductors can be utilized in optoelectronic nanodevices and for bioengineering applications. Several methods have been used for the preparation of PbS nanocrystals with various morphologies. For example PbS nanowire and nanorods dispersed in mesoporous silica [21] and polymer films [22] have been prepared by template and surfactants assisted methods. PbS nanowire arrays and network have been prepared by a chemical vapour deposition method [23]. Xiu et al. [24] reported synthesis of PbS nanocrystals with rod-like structures by a sonochemical technique. PbS nanoparticles with different shapes such as of triangular pyramid, square plate, cube, cuboid and rod have been produced at the air/water interface via reaction between Pb++ and H2 S gas under thin films of PVK [25]. Furthermore, Liang et al. [26] used surfactant-
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assisted hydrothermal method to produce large scale uniform lead sulfide nanorods with high yield. Recently ear-like, hexapod and dendritic PbS nanostructures have also been prepared using cyclic microwave assisted process [27]. There are not many reports on the tetrapod like PbS structures and detail studies on its optical properties. In the present study, we report fabrication of PbS crystals with a special morphology (tetrapod) using Pb(CH3 COO)2 ·3H2 O, thiourea and acrylamide (AA) as precursors by a simple hydrothermal method. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and selected area electron diffraction (SAED) are used for the morphological and structural characterization of the obtained products. X-ray diffraction (XRD) pattern and HRTEM images revealed the preferential growth of the asprepared PbS tetrapods along [1 1 0] direction which is different from the preferential [1 0 0] growth directions of star-shaped PbS single crystal reported by Ji et al. [28]. Optical properties of this kind of interesting PbS morphology have been less studied. Here we also present the detail study of UV–vis and PL spectra of PbS tetrapod. 2. Experimental procedures 2.1. Synthesis of PbS tetrapod crystals The synthesis of PbS microstructures was carried out as follows: firstly 0.0201 mol (1.43 gm) of solid acrylamide was dissolved into 40 ml de-ionized water. Then 0.00079 mol (0.543 gm) lead acetate Pb(CH3 COO)2 ·3H2 O and 0.00285 mol (0.217 gm) thiourea CS(NH2 )2 were dissolved into 40 ml and 20 ml de-ionized water separately. Thiourea solution was added with lead acetate sol drop wise under constant stirring condition at room temperature followed by acrylamide. Then the solution was transferred to a Teflon lined stainless steel autoclave and 80% of the total volume (120 ml) was filled. Then the sealed autoclave was heated to 140 ◦ C and maintained at this temperature for 14 h in an air oven. After the autoclave was cooled down to room temperature naturally, the products were separated by centrifugation, then washed with water and absolute alcohol several times and finally dried in air at 50 ◦ C for 3 h. For understanding the effect of surfactant, another experiment was performed using only lead acetate and thiourea without using any acrylamide keeping all other conditions the same. 2.2. Characterizations X-ray diffraction pattern of the as prepared products were recorded with an x-ray diffractometer (Bruker D8 Advance) in the 2 range 20–60◦ using Cu K␣ radiation of wavelength = 1.54 Å, operated at 40 kV and 40 mA. The morphology of the products was examined by a JEOL-JSM-6360 scanning electron microscope. The composition of PbS samples was analyzed by an energy dispersive analysis of X-rays (EDX) system equipped with the SEM. The detail morphological and structural features were investigated using HRTEM images with a JEOL-JEM-2100 transmission electron microscope operated at an accelerating voltage of 200 kV. Transmittance measurement was carried out with a UV–vis spectrophotometer (Shimadzu UV-3101PC) with the samples dispersed in alcohol at room temperature. Photoluminescence (PL) spectra were taken with a fluorimeter (Shimadzu FL 4500) at room temperature.
3. Results and discussion 3.1. XRD and EDX study The proper phase formation of the synthesized product was investigated by X-ray diffraction studies. Fig. 1 shows the XRD patterns of the as prepared PbS samples. The patterns show various diffraction peaks at 2 values 26.02◦ , 30.09◦ , 43.07◦ , 50.98◦ and 53.36◦ . All these diffraction peaks can be assigned to the reflections from (1 1 1), (2 0 0), (2 2 0), (4 0 0) and (2 2 2) planes of a face-centre-cubic rocksalt structured PbS with a lattice constant a = 0.594 nm (JCPDS card File No. 78-1901). The appeared peaks were very intense and sharp, which implies that the samples were well crystallized. No characteristic diffraction peaks of any other impurities can be detected. The elemental composition of the crystals was obtained using energy dispersive X-ray (EDX) spectrum as shown in Fig. 2. The strong peaks of Pb and S are clearly present in the spectrums and no other peaks from impurities were detected, confirming the high purity of the synthesized PbS samples. Moreover, according to quantitative analysis of EDX, the molar ratio of
Fig. 1. X-ray diffraction (XRD) pattern of PbS tetrapod like structure prepared by hydrothermal route.
Pb to S was found 1:1.09, which is almost consistent with the stoichiometric PbS within experimental error. 3.2. Morphology and structure analysis The detail morphology and structure of the as-prepared product have been investigated by SEM and TEM images. Typical SEM images of the as prepared sample are shown in Fig. 3a and b indicating tetrapod like morphology. The SEM image at relatively higher magnification (Fig. 3b) shows clearly the terapod morphology. The diameters of single pods/branch are about 400–500 nm and their lengths are upto 1.5 m. From the SEM images it can be seen that the separation angle between the adjacent pods is 90◦ and the diameter of each pods are nearly identical. For better understanding the role of surfactant (acrylamide) in the growth of this morphology the previous experiment was performed without using AA and keeping all other conditions unaltered. As a result, a mixed structure of PbS containing both tetrapod and rod-like structure was observed as shown in SEM images of Fig. 3c. TEM image of a single tetrapod is shown in Fig. 4a which confirms the morphology obtained from the SEM images. The corresponding SAED pattern (inset of Fig. 4a) of the individual tetrapod displays symmetric spotty pattern. The diffraction spots can be indexed as (2 0 0) and (2 2 0) planes of fcc rocksalt structured PbS and symmetric dotted pattern confirms their single crystalline nature. The detail crystal structure characterization of these tetrapods was carried out using HRTEM. Fig. 4b shows the HRTEM image at the junction of two adjacent arms of a single tetra-
Fig. 2. EDX spectrum of individual PbS tetrapod.
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Fig. 4. (a) TEM image of a single tetrapod like PbS structure and the corresponding SAED pattern (inset) and (b) HRTEM image of adjacent arms of a single PbS tetrapod (inset: magnified lattice image).
Fig. 3. SEM images of PbS samples prepared by hydrothermal method with acrylamide as a surfactant: (a) lower magnification; (b) higher magnification and (c) without any surfactant.
pod, clearly revealing the good crystalline and lattice fringes. The image (inset of Fig. 4b) clearly shows the regular fringes with a lattice spacing of about 2.09 A´˚ which corresponds to (2 2 0) planes of fcc phase of PbS and indicating that the crystal growth of the arm is preferentially in the [1 1 0] direction. HRTEM image taken from the ´˚ All these other arm also showed the fringe spacing of about 2.09 A. results suggest that the individual PbS tetrapods are single crystals and have the identical crystal orientation in both arms.
The XRD pattern (Fig. 1) suggested preferential orientation along (2 2 0). The result is in accordance with the result obtained from the HRTEM image. The faster growth rate on the {1 1 0} faces compared to other faces drives the formation of tetrapod like PbS structure. The process of crystal growth may be divided into two stages: an initial nucleation stage and a subsequent crystal growth stage. Initially PbS nucleation formation is kinetically controlled. As the solubility product of PbS in the above synthetic solution at 140 ◦ C is small, the supersaturation condition has reached and a large scale of uniform small cubic shaped PbS was formed which served as seeds. The PbS nanocubes are thermodynamically unstable as they have a considerably increased surface energy due to their total extended surface. When the synthesis duration was ∼10 h, the cubes were prolonged to form tetrapods by oriented attachment [29] to decrease the surface energy. The formation of tetrapod like architecture in the present synthesis method follows the growth mechanism as suggested by Zhang et al. [30] in their dendritic PbS structure. Surfactant acrylamide also plays a crucial role in the present anisotropic crystal growth process. From the SEM image
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Fig. 5. Transmittance vs. wavelength plot of crystalline PbS nanostructure sample.
Fig. 6. Plot to determine the direct band gap of PbS nanostructure sample.
(Fig. 3c) it is observed that a mixture of tetrapod and rod like structure were obtained from a non-AA assisted hydrothermal process. AA may act as a size tunable reagent for the formation of uniform PbS tetrapods. The new born nuclei cannot quickly aggregate due to absorption of AA which further preferentially grow along a certain direction. The source of sulfur is also one of the influencing factors. Here thiourea was used as a sulfur source which slowly releases S− ions. This also controls the nucleation and growth process. The exact mechanism for the formation of tetrapod like structure is still under investigation.
PbS also helps in observing size effect in its larger nanocrystals.
3.3. Optical transmittance spectra UV–vis–NIR spectroscopy is a useful technique to monitor the optical properties of the quantum sized particles. Fig. 5 shows the UV–vis transmittance spectra of the as prepared PbS tetrapod sample. Absorption coefficient (˛) for the as prepared PbS products has been calculated using the experimentally observed transmittance of the PbS products. The optical band gap energy of the products can be determined from the following equation. ˛h = K(h − Eg )
n
(1)
where h is the incident photon energy, K is a constant and Eg is the gap energy between the conduction and valance band of the nanoparticles. The (˛h)1/n versus h plot for n = 1/2, shown in Fig. 6, indicates the presence of direct band gap in the as prepared product. The optical band gap energy can be determined by extrapolating the curve to the energy axis for zero absorption coefficients. The optical band gap energy of PbS tetrapod samples has been estimated to be 4.9 eV which shows a large increment compared with that of bulk PbS (0.41 eV) [14,31,32]. Although the sizes of tetrapod PbS crystals are considerably larger than the exciton Bohr radius (18 nm) of PbS nanocrystals, a large blue shift is observed in the band gap energy of the as-prepared PbS nanostructure compared with the bulk PbS. Such increment of band gap energy possibly occurs due to small effective mass of carriers in PbS. Cao et al. [33] also observed a large blue shift of the absorption edge in the UV region (5.04–4.57 eV) for their synthesized PbS nanocubes. The quantum confinement may occur near the tip edge of the PbS tetrapods, which are smaller or comparable to PbS Bohr radius. The band gap energy can be increased from the inner thicker part to the tip edge of the crystal due to the position-dependent quantum-size effect also [34]. The large value of Bohr radius of
3.4. Photoluminescence study Photoluminescence emission spectra of the as-prepared PbS products are shown in Fig. 7. The sample kept at room temperature was excited with light of wavelength 275 nm, 300 nm and 325 nm using a xenon lamp. The energy of excitation used here is less than the band gap energy of the PbS tetrapod sample as determined from the optical transmittance spectra. So the emission corresponding to fundamental recombination is not expected to originate whereas emissions from defect related trap states, which generally appear at energies less than the band gap energy is expected to appear. All the PL spectra showed only one emission peak centered at 440 nm which is almost independent of the excitation wavelengths used. This result indicates that the PL emission comes from the PbS tetrapod sample and not from any other impurities. The PL peak around 440 nm wavelength may be ascribed due to the transition of electrons from the conduction band edge to holes trapped at surface states located
Fig. 7. PL spectra of tetrapod like PbS nanostructure.
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within the band gap energy as previously reported by Cao et al. [33]. 4. Conclusions In summary, we have synthesized single crystalline PbS tetrapod like structure by a conventional hydrothermal process using easily available reactants. XRD and HRTEM studies confirmed highly crystalline phase formation and a preferential growth along [1 1 0] direction which is different from the preferential [1 0 0] growth directions of other star-shaped PbS structure. The UV–vis transmittance spectra of the tetrapod like PbS crystals showed a large shift in the band gap energy compared to bulk PbS crystals (0.41 eV). The product also showed a photoluminescence peak at 440 nm due to radiative recombination from the surface states. These PbS crystals may find potential applications in fundamental studies of nanostructure as well as for the fabrication of semiconductor devices based on these structures. Acknowledgements The authors wish to thank the University Grants Commission (UGC), the Government of India and the Department of Science & Technology (DST), the Government of India, for financial support. References [1] X. Duan, Y. Huang, Y. Cui, J. Wang, C.M. Lieber, Nature 409 (2001) 66. [2] A. Bachtold, P. Hadley, T. Nakanishi, C. Dekker, Science 294 (2001) 1317. [3] M.H. Huang, S. Mao, H. Feick, H.Q. Yan, Y.Y. Wu, H. Kind, F. Weher, R. Russo, P.D. Yang, Science 292 (2001) 1897. [4] Y.Q. Gao, Y. Bando, Nature 415 (2002) 599.
[5] S.T. Lee, Y.F. Zhang, N. Wang, Y.H. Tang, L. Bello, C.S. Lee, Y.W. Chung, J. Mater. Res. 14 (1999) 4503. [6] H.J. Blythe, M.V. Fedosynk, O.I. Kasyutich, W. Schwarzacher, J. Magn. Mater. 208 (2000) 251. [7] Y.C. Kong, D.P. Yu, B. Zhang, W. Fang, S.Q. Feng, Appl. Phys. Lett. 78 (2001) 407. [8] Y. Zhang, R.E. Russo, S.S. Mao, Appl. Phys. Lett. 87 (2005) 043106. [9] G.S. Cheng, L.D. Zhang, Y. Zhu, G.T. Fei, L. Li, C.M. Mo, Y.Q. Mao, Appl. Phys. Lett. 75 (1999) 2455. [10] H. Wang, P. Fang, Z. Chen, S. Wang, J. Alloys Compd. 461 (2008) 418. [11] W.T. Yao, S.H. Yu, S.J. Liu, J.P. Chen, X.M. Liu, F.Q. Li, Phys. Chem. B 110 (2006) 11704. [12] Y.C. Zhu, Y. Bando, D.F. Xue, Appl. Phys. Lett. 82 (2003) 1769. [13] C.H. An, K.B. Tang, G.Z. Shen, C.R. Wang, Q. Yang, B. Hai, Y.T. Qian, J. Cryst. Growth 244 (2002) 333. [14] D.E. Aspnes, M. Cardona, Phys. Rev. 173 (1968) 714. [15] J.L. Machol, F.W. Wise, R.C. Patel, D.B. Tanner, Phys. Rev. B 48 (1993) 2819. [16] H. Hirata, K. Higoshiyama, Bull. Chem. Soc. Jpn. 44 (1971) 2420. [17] E. Pentia, L. Pintilie, I. Matei, T. Boltia, E. Ozbay, J. Optoelectron. Adv. Mater. 3 (2001) 525. [18] P. Yang, C.F. Song, M.K. Lu, X. Yin, G.J. Zhou, D. Xu, D.R. Yuan, Chem. Phys. Lett. 345 (2001) 429. [19] S. Gunes, K.P. Fritz, H. Neugebauer, N.S. Sariciftci, S. Kumar, G.D. Scholes, Sol Energy Mater. Sol. Cells 91 (2007) 420. [20] A. Martucci, J. Fick, J. Schell, G. Battaglin, M. Guglielmi, J. Appl. Phys. 86 (1999) 79. [21] F. Gao, Q. Lu, X. Liu, Y. Yan, D. Zhao, Nano Lett. 1 (2001) 743. [22] S. Wang, S. Yang, Langmuir 16 (2000) 389. [23] M. Afzaal, P. O’Brien, J. Mater. Chem. 16 (2006) 1113. [24] Z. Xiu, S. Liu, J. Yu, F. Xu, W. Yu, G. Feng, J. Alloys Compd. 457 (2008) L9. [25] C.W. Wang, H.G. Liu, X.T. Bai, Q. Xue, X. Chen, Y.I. Lee, J. Hao, J. Jiang, Cryst. Growth Des. 8 (2008) 2660. [26] D. Liang, S. Tang, J. Liu, X. Lv, L. Kang, Mater. Lett. 62 (2008) 2426. [27] A. Phuruangrat, T. Thongtem, S. Thongtem, Mater. Lett. 63 (2009) 667. [28] Y. Ji, X. Ma, H. Zhang, J. Xu, D. Yang, J. Phys: Cond., Mater 15 (2003) 7611. [29] A.P. Alivisatos, Science 289 (2000) 736. [30] W. Zhang, Q. Yang, L. Xu, W. Yu, Y. Qian, Mater. Lett. 59 (2005) 3383. [31] K.K. Nanda, F.E. Kruis, H. Fissan, M. Acet, J. Appl. Phys. 91 (2002) 2315. [32] M.F. Acosta, M.S. Lerma, H.A. Chavez, F.F.C. Barraza, R.R. Bon, Solid State Commun. 128 (2003) 407. [33] H. Cao, G. Wang, S. Zhang, X. Zhang, Nanotechnology 17 (2006) 3280. [34] W.S. Chae, H.W. Shin, E.S. Lee, E.J. Shin, J.S. Jung, Y.R. Kim, J. Phys. Chem. B 109 (2005) 6204.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Synthesis of tetrapod like PbS microcrystals by hydrothermal route and its optical characterization S. Jana a , S. Goswami b , S. Nandy a , K.K. Chattopadhyay a,b,∗ a b
Thin Films and Nanoscience Laboratory, Department of Physics, Jadavpur University, Kolkata 700032, India Centre for Nanoscience and Technology, Jadavpur University, Kolkata 700032, India
a r t i c l e
i n f o
Article history: Received 11 December 2008 Received in revised form 18 March 2009 Accepted 20 March 2009 Available online 31 March 2009 Keywords: Lead sulfide Hydrothermal Crystal morphology Optical properties
a b s t r a c t Microcrystalline PbS tetrapod like structure has been successfully prepared by a hydrothermal route using Pb(CH3 COO)2 ·3H2 O and thiourea as reagents and acrylamide (AA) as a surfactant. X-ray diffraction (XRD) studies confirmed the highly crystalline cubic PbS phase formation. Scanning electron microscopic study revealed the tetrapod like structures with length 1.5 m and breadth 400–500 nm. Detail morphology and structure have been investigated by transmission electron microscopic studies also. Selected area electron diffraction and HRTEM lattice image confirmed the single crystallinity and preferential growth direction of the as-obtained samples. UV–vis spectrophotometric study showed the direct band gap of the PbS tetrapod ∼4.9 eV. The photoluminescence measurements showed that the emission peaks were located at 440 nm for all the excitation wavelength of 275–325 nm and are explained due to radiative recombination from the surface states. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Many fundamental properties and applications of crystals depend strongly on their shapes, sizes and as well as on dimensionalities. Because of the correlation between the shapes and properties, a lot of effort has been made in recent years to prepare various inorganic, organic or inorganic–organic composite materials with novel morphologies and to study their properties. So far, a variety of one dimensional nanostructures with different morphologies, such as nanowires, nanorods, nanocables, nanobelts, nanotubes etc. have been prepared due to their unique electronic, electrical and optical properties and as well as for important application in mesoscopic physics and fabrication of nanoscale devices [1]. Various nanodevices including nanologic circuits [2], nanolasers [3], nanosensors and nanothermometers [4] etc. have been assembled using one dimensional nanoscale materials. Quantum wires of semiconductors [5] and metallic alloys [6] have been found to exhibit interesting magnetic and electrical properties. For example, ZnO and GaN nanowires have been used to construct blue and ultraviolet emitters and lasers [7–9]. Therefore, the controlled growth of semiconductor nanostructures has received considerable attention from the scientific communities.
∗ Corresponding author at: Thin Films and Nanoscience Laboratory, Department of Physics, Jadavpur University, Kolkata 700032, India. Tel.: +91 9433389445; fax: +91 33 2414 6007. E-mail address: kalyan [email protected] (K.K. Chattopadhyay). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.03.110
Well defined structures of CdS such as nanorods [10], urchin like nanoflowers, branched nanowires [11] etc. have been prepared by facile solvothermal approach. ZnS nanobelt production via thermal evaporation method without using any catalyst was reported by Zhu et al. [12]. Qian’s group has developed a mild solvothermal route for the synthesis of belt like SnS2 crystals [13]. A relatively less studied but equally interesting material is PbS, an attractive – semiconductor with relatively narrow band gap (0.41 eV at 300 K) [14] and the larger Bohr radius of 18 nm [15]. It has wide applications in many fields such as in Pb2+ ion selective sensors [16], IR detectors [17], display devices [18], and solar cells [19]. Moreover, as a consequence of its carrier confinement an exceptional third order nonlinear optical properties of PbS nanoparticles has been found, which may be useful in optical devices such as optical switch [20]. The size and shape dependent optical properties of nanocrystalline semiconductors can be utilized in optoelectronic nanodevices and for bioengineering applications. Several methods have been used for the preparation of PbS nanocrystals with various morphologies. For example PbS nanowire and nanorods dispersed in mesoporous silica [21] and polymer films [22] have been prepared by template and surfactants assisted methods. PbS nanowire arrays and network have been prepared by a chemical vapour deposition method [23]. Xiu et al. [24] reported synthesis of PbS nanocrystals with rod-like structures by a sonochemical technique. PbS nanoparticles with different shapes such as of triangular pyramid, square plate, cube, cuboid and rod have been produced at the air/water interface via reaction between Pb++ and H2 S gas under thin films of PVK [25]. Furthermore, Liang et al. [26] used surfactant-
S. Jana et al. / Journal of Alloys and Compounds 481 (2009) 806–810
807
assisted hydrothermal method to produce large scale uniform lead sulfide nanorods with high yield. Recently ear-like, hexapod and dendritic PbS nanostructures have also been prepared using cyclic microwave assisted process [27]. There are not many reports on the tetrapod like PbS structures and detail studies on its optical properties. In the present study, we report fabrication of PbS crystals with a special morphology (tetrapod) using Pb(CH3 COO)2 ·3H2 O, thiourea and acrylamide (AA) as precursors by a simple hydrothermal method. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and selected area electron diffraction (SAED) are used for the morphological and structural characterization of the obtained products. X-ray diffraction (XRD) pattern and HRTEM images revealed the preferential growth of the asprepared PbS tetrapods along [1 1 0] direction which is different from the preferential [1 0 0] growth directions of star-shaped PbS single crystal reported by Ji et al. [28]. Optical properties of this kind of interesting PbS morphology have been less studied. Here we also present the detail study of UV–vis and PL spectra of PbS tetrapod. 2. Experimental procedures 2.1. Synthesis of PbS tetrapod crystals The synthesis of PbS microstructures was carried out as follows: firstly 0.0201 mol (1.43 gm) of solid acrylamide was dissolved into 40 ml de-ionized water. Then 0.00079 mol (0.543 gm) lead acetate Pb(CH3 COO)2 ·3H2 O and 0.00285 mol (0.217 gm) thiourea CS(NH2 )2 were dissolved into 40 ml and 20 ml de-ionized water separately. Thiourea solution was added with lead acetate sol drop wise under constant stirring condition at room temperature followed by acrylamide. Then the solution was transferred to a Teflon lined stainless steel autoclave and 80% of the total volume (120 ml) was filled. Then the sealed autoclave was heated to 140 ◦ C and maintained at this temperature for 14 h in an air oven. After the autoclave was cooled down to room temperature naturally, the products were separated by centrifugation, then washed with water and absolute alcohol several times and finally dried in air at 50 ◦ C for 3 h. For understanding the effect of surfactant, another experiment was performed using only lead acetate and thiourea without using any acrylamide keeping all other conditions the same. 2.2. Characterizations X-ray diffraction pattern of the as prepared products were recorded with an x-ray diffractometer (Bruker D8 Advance) in the 2 range 20–60◦ using Cu K␣ radiation of wavelength = 1.54 Å, operated at 40 kV and 40 mA. The morphology of the products was examined by a JEOL-JSM-6360 scanning electron microscope. The composition of PbS samples was analyzed by an energy dispersive analysis of X-rays (EDX) system equipped with the SEM. The detail morphological and structural features were investigated using HRTEM images with a JEOL-JEM-2100 transmission electron microscope operated at an accelerating voltage of 200 kV. Transmittance measurement was carried out with a UV–vis spectrophotometer (Shimadzu UV-3101PC) with the samples dispersed in alcohol at room temperature. Photoluminescence (PL) spectra were taken with a fluorimeter (Shimadzu FL 4500) at room temperature.
3. Results and discussion 3.1. XRD and EDX study The proper phase formation of the synthesized product was investigated by X-ray diffraction studies. Fig. 1 shows the XRD patterns of the as prepared PbS samples. The patterns show various diffraction peaks at 2 values 26.02◦ , 30.09◦ , 43.07◦ , 50.98◦ and 53.36◦ . All these diffraction peaks can be assigned to the reflections from (1 1 1), (2 0 0), (2 2 0), (4 0 0) and (2 2 2) planes of a face-centre-cubic rocksalt structured PbS with a lattice constant a = 0.594 nm (JCPDS card File No. 78-1901). The appeared peaks were very intense and sharp, which implies that the samples were well crystallized. No characteristic diffraction peaks of any other impurities can be detected. The elemental composition of the crystals was obtained using energy dispersive X-ray (EDX) spectrum as shown in Fig. 2. The strong peaks of Pb and S are clearly present in the spectrums and no other peaks from impurities were detected, confirming the high purity of the synthesized PbS samples. Moreover, according to quantitative analysis of EDX, the molar ratio of
Fig. 1. X-ray diffraction (XRD) pattern of PbS tetrapod like structure prepared by hydrothermal route.
Pb to S was found 1:1.09, which is almost consistent with the stoichiometric PbS within experimental error. 3.2. Morphology and structure analysis The detail morphology and structure of the as-prepared product have been investigated by SEM and TEM images. Typical SEM images of the as prepared sample are shown in Fig. 3a and b indicating tetrapod like morphology. The SEM image at relatively higher magnification (Fig. 3b) shows clearly the terapod morphology. The diameters of single pods/branch are about 400–500 nm and their lengths are upto 1.5 m. From the SEM images it can be seen that the separation angle between the adjacent pods is 90◦ and the diameter of each pods are nearly identical. For better understanding the role of surfactant (acrylamide) in the growth of this morphology the previous experiment was performed without using AA and keeping all other conditions unaltered. As a result, a mixed structure of PbS containing both tetrapod and rod-like structure was observed as shown in SEM images of Fig. 3c. TEM image of a single tetrapod is shown in Fig. 4a which confirms the morphology obtained from the SEM images. The corresponding SAED pattern (inset of Fig. 4a) of the individual tetrapod displays symmetric spotty pattern. The diffraction spots can be indexed as (2 0 0) and (2 2 0) planes of fcc rocksalt structured PbS and symmetric dotted pattern confirms their single crystalline nature. The detail crystal structure characterization of these tetrapods was carried out using HRTEM. Fig. 4b shows the HRTEM image at the junction of two adjacent arms of a single tetra-
Fig. 2. EDX spectrum of individual PbS tetrapod.
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Fig. 4. (a) TEM image of a single tetrapod like PbS structure and the corresponding SAED pattern (inset) and (b) HRTEM image of adjacent arms of a single PbS tetrapod (inset: magnified lattice image).
Fig. 3. SEM images of PbS samples prepared by hydrothermal method with acrylamide as a surfactant: (a) lower magnification; (b) higher magnification and (c) without any surfactant.
pod, clearly revealing the good crystalline and lattice fringes. The image (inset of Fig. 4b) clearly shows the regular fringes with a lattice spacing of about 2.09 A´˚ which corresponds to (2 2 0) planes of fcc phase of PbS and indicating that the crystal growth of the arm is preferentially in the [1 1 0] direction. HRTEM image taken from the ´˚ All these other arm also showed the fringe spacing of about 2.09 A. results suggest that the individual PbS tetrapods are single crystals and have the identical crystal orientation in both arms.
The XRD pattern (Fig. 1) suggested preferential orientation along (2 2 0). The result is in accordance with the result obtained from the HRTEM image. The faster growth rate on the {1 1 0} faces compared to other faces drives the formation of tetrapod like PbS structure. The process of crystal growth may be divided into two stages: an initial nucleation stage and a subsequent crystal growth stage. Initially PbS nucleation formation is kinetically controlled. As the solubility product of PbS in the above synthetic solution at 140 ◦ C is small, the supersaturation condition has reached and a large scale of uniform small cubic shaped PbS was formed which served as seeds. The PbS nanocubes are thermodynamically unstable as they have a considerably increased surface energy due to their total extended surface. When the synthesis duration was ∼10 h, the cubes were prolonged to form tetrapods by oriented attachment [29] to decrease the surface energy. The formation of tetrapod like architecture in the present synthesis method follows the growth mechanism as suggested by Zhang et al. [30] in their dendritic PbS structure. Surfactant acrylamide also plays a crucial role in the present anisotropic crystal growth process. From the SEM image
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Fig. 5. Transmittance vs. wavelength plot of crystalline PbS nanostructure sample.
Fig. 6. Plot to determine the direct band gap of PbS nanostructure sample.
(Fig. 3c) it is observed that a mixture of tetrapod and rod like structure were obtained from a non-AA assisted hydrothermal process. AA may act as a size tunable reagent for the formation of uniform PbS tetrapods. The new born nuclei cannot quickly aggregate due to absorption of AA which further preferentially grow along a certain direction. The source of sulfur is also one of the influencing factors. Here thiourea was used as a sulfur source which slowly releases S− ions. This also controls the nucleation and growth process. The exact mechanism for the formation of tetrapod like structure is still under investigation.
PbS also helps in observing size effect in its larger nanocrystals.
3.3. Optical transmittance spectra UV–vis–NIR spectroscopy is a useful technique to monitor the optical properties of the quantum sized particles. Fig. 5 shows the UV–vis transmittance spectra of the as prepared PbS tetrapod sample. Absorption coefficient (˛) for the as prepared PbS products has been calculated using the experimentally observed transmittance of the PbS products. The optical band gap energy of the products can be determined from the following equation. ˛h = K(h − Eg )
n
(1)
where h is the incident photon energy, K is a constant and Eg is the gap energy between the conduction and valance band of the nanoparticles. The (˛h)1/n versus h plot for n = 1/2, shown in Fig. 6, indicates the presence of direct band gap in the as prepared product. The optical band gap energy can be determined by extrapolating the curve to the energy axis for zero absorption coefficients. The optical band gap energy of PbS tetrapod samples has been estimated to be 4.9 eV which shows a large increment compared with that of bulk PbS (0.41 eV) [14,31,32]. Although the sizes of tetrapod PbS crystals are considerably larger than the exciton Bohr radius (18 nm) of PbS nanocrystals, a large blue shift is observed in the band gap energy of the as-prepared PbS nanostructure compared with the bulk PbS. Such increment of band gap energy possibly occurs due to small effective mass of carriers in PbS. Cao et al. [33] also observed a large blue shift of the absorption edge in the UV region (5.04–4.57 eV) for their synthesized PbS nanocubes. The quantum confinement may occur near the tip edge of the PbS tetrapods, which are smaller or comparable to PbS Bohr radius. The band gap energy can be increased from the inner thicker part to the tip edge of the crystal due to the position-dependent quantum-size effect also [34]. The large value of Bohr radius of
3.4. Photoluminescence study Photoluminescence emission spectra of the as-prepared PbS products are shown in Fig. 7. The sample kept at room temperature was excited with light of wavelength 275 nm, 300 nm and 325 nm using a xenon lamp. The energy of excitation used here is less than the band gap energy of the PbS tetrapod sample as determined from the optical transmittance spectra. So the emission corresponding to fundamental recombination is not expected to originate whereas emissions from defect related trap states, which generally appear at energies less than the band gap energy is expected to appear. All the PL spectra showed only one emission peak centered at 440 nm which is almost independent of the excitation wavelengths used. This result indicates that the PL emission comes from the PbS tetrapod sample and not from any other impurities. The PL peak around 440 nm wavelength may be ascribed due to the transition of electrons from the conduction band edge to holes trapped at surface states located
Fig. 7. PL spectra of tetrapod like PbS nanostructure.
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within the band gap energy as previously reported by Cao et al. [33]. 4. Conclusions In summary, we have synthesized single crystalline PbS tetrapod like structure by a conventional hydrothermal process using easily available reactants. XRD and HRTEM studies confirmed highly crystalline phase formation and a preferential growth along [1 1 0] direction which is different from the preferential [1 0 0] growth directions of other star-shaped PbS structure. The UV–vis transmittance spectra of the tetrapod like PbS crystals showed a large shift in the band gap energy compared to bulk PbS crystals (0.41 eV). The product also showed a photoluminescence peak at 440 nm due to radiative recombination from the surface states. These PbS crystals may find potential applications in fundamental studies of nanostructure as well as for the fabrication of semiconductor devices based on these structures. Acknowledgements The authors wish to thank the University Grants Commission (UGC), the Government of India and the Department of Science & Technology (DST), the Government of India, for financial support. References [1] X. Duan, Y. Huang, Y. Cui, J. Wang, C.M. Lieber, Nature 409 (2001) 66. [2] A. Bachtold, P. Hadley, T. Nakanishi, C. Dekker, Science 294 (2001) 1317. [3] M.H. Huang, S. Mao, H. Feick, H.Q. Yan, Y.Y. Wu, H. Kind, F. Weher, R. Russo, P.D. Yang, Science 292 (2001) 1897. [4] Y.Q. Gao, Y. Bando, Nature 415 (2002) 599.
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