Cent. Eur. J. Chem. • 7(2) • 2009 • 215–221 DOI: 10.2478/s11532-009-0005-3
Central European Journal of Chemistry
PbS nanostructures synthesized via surfactant assisted mechanochemical route SSC-2008
Peter Baláž , Parviz Pourghahramani , Erika Dutková , Martin Fabián , Jaroslav Kováč3, Alexander Šatka3 1*
2
1
1
Institute of Geotechnics, Slovak Academy of Sciences, 043 53 Košice, Slovakia
1
Mining Engineering Department, Sahand University of Technology, P.O.Box 51 335/1996 Tabriz, Iran
2
Department of Microelectronics, Slovak University of Technology and International Laser Centre, 812 19 Bratislava, Slovakia
3
Received 02 July 2008; Accepted 21 December 2008
Abstract: P bS nanocrystals using surfactant assisted mechanochemical route has been successfully prepared. The methods of XRD, SEM, surface area and particle size measurements were used for nanocrystals characterization. The XRD patterns confirmed the presence of galena PbS (JCPDS 5-592) whatever treatment conditions were applied. The strong observable peaks indicate the highly crystalline nature in formation of PbS nanostructures where preferential crystal growth in the (200) direction after chelating agent (EDTANa2•2H2O) addition has been observed. The mean volume weighted crystallite size 4.9 nm and 35 nm has been calculated from XRD data using Williamson-Hall method for PbS synthesized without and/or with chelating agent, respectively corresponding with surface weighted crystallites sizes of 2.9 and 18.8 nm. The sample prepared without surfactant yields the smaller crystallites and the higher microstrain compared with surfactant assisted synthesis. The obtained results illustrate a possibility to manipulate crystal morphology by combining effect of milling and surfactant application. Keywords: Lead sulphide • Mechanochemistry • Nanostructure • Surfactant © Versita Warsaw and Springer-Verlag Berlin Heidelberg.
1. Introduction Semiconductor nanocrystals, also called quantum dots, appear to be interesting objects for studying basically novel properties of matter, generally described as ”size quantization effects”. In principle, the electronic and optical properties of semiconductor nanocrystals are tunable by varying their shape and size, so it is one of the desired goals in materials science to realize precise control of the morphology of semiconductor materials [1,2]. The semiconductors with a narrow band gap have the possibility to be engineered from the point of view of their band gap modification. Lead sulphide, PbS, as
a representant of IV-VI semiconductors has a narrow band gap of 0.41 eV and an exciton Bohr radius of 18 nm at room temperature. PbS has extraordinary optical properties and is used in many fields such as solar absorbers, IR detectors, Pb2+ ion-selective sensors and optical switches. Many synthesis methods have been reported for the preparation of PbS nanoparticles in various morphology and under different synthesis conditions [3-11]. There are several approaches to synthesize nanosized materials starting from vapor, liquid and/ or solid state. The approach starting from solid state applies high-energy milling and is governed by mechanochemical methodology [12-14]. In this case,
* E-mail:
[email protected] 215
PbS nanostructures synthesized via surfactant assisted mechanochemical route
the solid state reactions occur at the interfaces of the nanometer size grains that are continuously regenerated during milling. As a consequence, reactions that would normally require high temperatures to occur, due to separation of the reacting phases by the product phases, can occur under mild conditions. Ambient temperatures and atmospheric pressure are usually satisfactory for performing mechanochemical reactions with a total conversion. It is the aim of this paper to synthesize nanocrystalline lead sulphide PbS from organometallic and inorganic reaction precursors. The novelty of the reaction is that addition of chelating agent EDTANa2•2H2O in the solid form directly into milling process greatly modifies physico-chemical properties of the produced PbS nanocrystals.
2. Experimental Procedures PbS nanocrystals were synthesized from lead acetate (CH3COO)2Pb•3H2O and sodium sulphide Na2S•9H2O supplied by ITES (Vranov, Slovakia). Ethylenediaminetetraacetatic disodium salt-2-hydrate (EDTANa2•2H2O) supplied by Fluka-Riedel-de Haën (Hannover, Germany) has been applied as the chelating agent (CA). Mechanochemical solid state synthesis of nanocrystalline lead sulphide, PbS, was performed in a laboratory planetary mill Pulverisette 6 (Fritsch, Germany). The following milling conditions were used: loading of the mill with 50 balls from tungsten carbide of 10 mm diameter; rotation speed of the planet carrier 500 rpm; protecting atmosphere in the mill argon. Milling strategy is illustrated by Table 1. The XRD measurements were performed by employing the X-ray diffractometer Siemens, D5000 bearing Bragg-Brentano geometrical configuration. The CuKα radiation (λ = 1.5406 Å) was used. The counting time was 5 second per step and the step size was 0.3 degree. The PROFILE software supplied by
Bruker/Socabin was used in the pattern decomposition (profile fitting) procedures and in the extraction of parameters. The standard LaB6 proposed by NIST was applied for removing instrumental broadening of the XRD patterns. To extract the microstructure characteristics, two conventional methods, Williamson-Hall and Warren-Averbach, were used. The profile fitting procedure and principles of the methods were discussed in detail in previous works [15,16]. The dispersion and the particle size analysis of samples were measured on NANOPHOX particle sizer (Sympatec, Germany) using the photon cross correlation spectroscopy method. The equipment works with the build-up He-Ne laser diode with the maximum output 10 mW and the wavelength of radiation λ = 0.6328 μm. The samples were taken directly from the mill and after sedimentation the images of fine particles present in the liquid state were taken. The measured results have been processed using Windox 5 software. The synthesized samples were also analyzed using FE-SEM LEO 1550 scanning microscope. The samples were left uncovered from any conductive material in order to keep their original properties. The specific surface area was determined by the low temperature nitrogen adsorption method in a Gemini 2360 sorption apparatus (Micromeritics, USA).
3. Results and discussion 3.1 Changes in particle size
The dispersion and the particle size of PbS nanoparticles formed via mechanochemical synthesis were characterized by photon cross correlation spectroscopy, as shown in Figs. 1, 2. The plots indicate well isolated nanoparticles without aggregates. In the case of chelating agent addition, the size of the particles is uniform with average hydrodynamic diameter d = 211 nm (Fig. 2). Three individual peaks are visible for the PbS nanoparticles without addition of chelating agent (Fig. 1). The first one, which belongs
Table 1. Milling strategy Milling system (milling time in parenthesis)
Milling stage I
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Without surfactant (CH3COO)2Pb•3H2O + Na2S•9H2O (10 min)
I
-
II
-
III
-
With surfactant (CH3COO)2Pb•3H2O (5 min) Product from stage I + EDTANa2•2H2O (2 min) Product from stage II + Na2S•9H2O (5 min)
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Figure 1.
Particle size distribution of mechanochemically synthesized PbS, no CA.
Figure 2. Particle size distribution of mechanochemically synthesized PbS with addition of
CA into milling.
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PbS nanostructures synthesized via surfactant assisted mechanochemical route
to the average hydrodynamic diameter of particles d = 67 nm, presents only 16% of total composition of synthesized powder. Almost 61% of mechanochemically prepared PbS powder have average hydrodynamic diameter up to 3200 nm. These particles are not visible in the distribution diagram in the case of the chelating agent addition probably due to the surface protection of forming these particles. The difference may be mainly due to the effect of EDTANa2•2H2O addition as well as high viscosity of the measured solution. The average hydrodynamic diameter for synthesized nanoparticles in the case of the chelating agent addition was shifted from 211 nm to 263 nm. It is due to the strong adsorption of the chelating agent on the surface of PbS nanoparticles. Its addition into the milling process can stabilize and make the nanopowder particles uniform.
3.2 Changes in morphology of particles
FE-SEM micrographs of mechanochemically synthesized PbS nanoparticles are given in Fig. 3. There is a strong tendency for cluster formation by milling without addition of the chelating agent
(Fig. 3A, 3B). The smaller particles can be seen in this case in contrast with the milling in presence of EDTANa2•2H2O (compare Fig. 3A with 3C). This observation is supported by measuring the particle surface area: the values 31.3 and 13.8 m2 g-1 were obtained for PbS without and/or with the chelating agent, respectively. Milling with the chelating agent show good morphology of the synthesized grains. However, there is a tendency to form plate-like particles instead of expected cube-like ones (Fig. 3D).
3.3 Bulk changes
The XRD pattern of the synthesized PbS (without chelating agent) is shown in Fig. 4. All the diffraction peaks could be indexed to cubic phase with the cell constant a = 0.5920 nm, which was very close to the value of JCPDS card No. 5-592 which is cubic PbS. No other impurities could be detected in the XRD-pattern. The diffraction peaks show that PbS is well crystallized. When the milling is performed in the presence of chelating agent, the qualitatively same pattern is shown in Fig. 5. However, by thorough peak shape inspection,
Figure 3. FE-SEM of mechanochemically synthesized PbS: A, B - no CA, C, D - with addition of CA into milling. 218
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Figure 4. XRD pattern of the mechanochemically synthesized PbS,
Figure 5.
Figure
Figure 7.
no CA.
6.
Williamson-Hall plots for the mechanochemically synthesized PbS, no CA. Dashed line refers to direction (200) and solid line to all intensive reflections.
the differences can be observed. The most important feature is that the sample milled without chelating agent shows the minimum reflection intensity and maximum line broadening (Fig. 4) compared with the sample milled with chelating agent (Fig. 5). This phenomenon suggests more structural refinement and smaller crystallites. Moreover, the differences in (200)/(111) intensities ratio (r) can be observed by these two samples. While this value is 1.01 for PbS characterized by Fig. 4, the value 1.29 has been calculated for PbS synthesized in presence of the chelating agent. It follows that the crystal growth is preferential in the (200) direction when using chelating agent in mechanochemical synthesis. A similar tendency has been observed by Wang et al. [17]. However, they prepared PbS nanowires of ~ 7 nm diameter with lengths of several hundreds nanometers. In their case, the different reaction conditions were used: lead acetate and thiourea were applied as reaction precursors, phenyl ether as solvent and oleic acid as surfactant. The reaction proceeded at 140°C for 30 minutes under N2 atmosphere without intervention of milling.
XRD pattern of the mechanochemically synthesized PbS with addition of CA into milling.
Williamson-Hall plots for the mechanochemically synthesized PbS with addition of CA into milling. Dashed line refers to direction (200) and solid line to all intensive reflections.
To ascertain the nature of structural imperfections in synthesized samples the Williamson-Hall plots were applied in Figs. 6 and 7. From the Williamson-Hall plots, lines for all intensive reflections and separately for (200) direction have nonzero and different slopes and intercepts. This suggests that the strain and size contributions exist simultaneously in the mechanochemically synthesized PbS. The scatter of the β*2f values indicates that the crystallite shape differs from a spherical one. Besides, the (200) and (400) reflections show smaller broadening compared with other reflections. There is also a systematic deviation between the line connecting the two orders of (200) and (400) reflections and the line connecting all intensive reflections, the line relating to the (200) direction lie below it. This may be understood by considering the anisotropy in the elastic properties of the single-crystal galena, indicating that galena crystal is ”hard” in (200) crystallographic direction. For evaluation, the strain and crystallite size of the samples were estimated from the slope and intercept of plots [15]. The related results are summarized in
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PbS nanostructures synthesized via surfactant assisted mechanochemical route
Table 2. It can be observed that the sample synthesized without EDTANa2•2H2O yields the smallest crystallites and slightly higher microstrain compared with the other samples. The largest crystallites were calculated for the sample treated with the chelating agent. In addition, the values of microstructure characteristics for direction (200) have a slight difference with the average values. Table 2.
The mean volume weighted crystallite size, DV and maximum strain, e for the mechanochemically synthesis samples obtained using Williamson-Hall plots
Application of chelating agent
(200) direction DV(nm)
e×10-3
No
4.9
Yes
35.0
Other directions DV(nm)
e×10-3
4.2
4.8
7.0
2.2
28.2
3.2
The Warren-Averbach analysis based on Fourier analysis provides detail information regarding to crystallite size, lattice strain and their distributions. The results for the mechanochemically synthesized samples using the Warren-Averbach approach are given in Table 3. The surface weighted crystallite size and the root mean square strain at L = 10 nm (1/2) are given. The results depict the nature of progressive evolution of the microstructure of the mechanochemically synthesized samples. The results of Warren-Averbach method are in line with the results of Williamson-Hall method [18]. Once more, the Warren-Averbach method confirms that the mechanochemically synthesized PbS (without chelating agent) contains the smallest crystallites and slightly higher values of microstrain in comparison with the second sample. This sample which is PbS prepared in presence of EDTANa2•2H2O have the larger crystallites. The role of chelating agent in growing the crystallites during mechanochemical treatment should be noted. Table 3. The mean surface weighted crystallite size, DS and root mean square strain, 1/2.10-3 for the mechanochemically synthesized PbS obtained using Warren-Averbach method
Application of chelating agent
(200) direction
Other directions DS (nm)
1/2×10-3
1.79
3.3
2.98
1.49
13.9
1.31
DS (nm)
1/2×10-3
No
2.9
Yes
18.8
3.4. Mechanism of the PbS formation
The nanocrystalline PbS formation can be described by a three-stage mechanism. In all three stages the reactants have been applied into milling in a solid state. In the first stage, the crystalline water from the lead acetate is loosened. The new bonds are created and
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the water-free acetate has the greater predisposition for further reactivity. It has been observed in literature that the crystalline water can influence reactivity of the coordination sites for attack, thus reducing the activation energy and increasing the reaction rates [19,20]. In our case, these consequences are strengthened by the applied effect of mechanical activation as can be supported by strain data in Tables 1 and 2. In the second stage, the lead acetate forms complex with EDTANa2•2H2O. This compound acts as the chelating agent as well as surfactant in this case. The effect of the crystalline water loosening is again observed in the third stage. However, NaSS•9H2O has no complex ability [21] and therefore reacts immediately with PbEDTA. In addition, the steric effect of EDTA compound cannot be supressed in the reaction space of the mill pot. There is a difference in intensity of (200) and (111) planes for mechano-chemically synthesized PbS with or without EDTA agent, see Figs. 4, 5. It has been found that the preferential adsorption on different crystal faces directs the growth of nanoparticles into various shapes thus controlling the growth rates along different crystal axes. Based on XRD (Figs. 4, 5) and FE-SEM (Fig. 3) data the anisotropic growth character of nanocrystalline PbS can be confirmed. EDTA and Pb2+ ions can form a very stable complex [22]. This steric stabilization has negative effect on the total specific area formation as observed in our experiments: the values 13.8 and 31.3 m2 g-1 have been obtained for milling with and without EDTANa2•2H2O, respectively. Furthermore, the “arrested” growth of PbS nanoparticles has been observed with a tendency to form belt-like instead of cube-like nanostructures. Generally, these structures can be further developed into more pronounced 1D – structures. It follows that there is a possibility to manipulate crystal morphology by smart combination of milling and in-situ surfactant application.
4. Conclusions The presented paper describes surface, morphology and structural properties of PbS nanocrystals successfully synthesized using surfactant assisted mechanochemical route. The novelty of this synthesis was that addition of chelating agent EDTANa2•2H2O directly into milling process greatly modified physicochemical properties of the prepared PbS nanocrystals. The average hydrodynamic diameter for PbS nanocrystals mechanochemically synthesized using
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chelating agent was shifted from lower to higher values. It is due to the strong adsorption of the chelating agent on the PbS surfaces. The PbS particles prepared with addition surfactant are larger and they have tendency to form plate-like particles. The cubic PbS (galena) is the only product of the mechanochemical synthesis. XRD pattern reveals the highly crystalline nature of PbS nanostructures. PbS synthesized with surfactant yields the larger crystallites and the smaller microstrain compared without surfactant. The role of chelating agent in growing the crystallites during mechanochemical treatment was proven. The three-stage mechanism of the nanocrystalline PbS formation was described. It follows that there is a possibility to manipulate crystal morphology by combining effect of milling and surfactant application.
Acknowledgements The support through the Slovak Research and Developing Agency (projects APVV-0347-06, VVCE-0049-07), the Slovak Grant Agency (projects VEGA-2/0035/08, 1/689/09) and Center of Excellence of Slovak Academy of Sciences (NANOSMART) is gratefully acknowledged.
References [1] P.D. Yang, C.M. Lieber, Science 273, 1836 (1996) [2] Y. Ni, H. Liu, F. Wang, Y. Liang, J. Hong, X. Ma, Z. Xu, Cryst. Res. Technol. 39, 200 (2004) [3] R. Houbertz, W. Krauss, R. Birringer, U. Hartmann, Nanostruct. Mater. 9, 339 (1997) [4] D. Chen, K. Tang, G. Shen, J. Sheng, Z. Fang, X. Liu, H. Zheng, Y. Qian, Mater. Chem. Phys. 82, 206 (2003) [5] J. Wang, X. Chen, Z. Wang, W. Yu, Y. Qian, Mater. Chem. Phys. 88, 217 (2004) [6] U.K. Gantam, R. Seshadri, Mater. Res. Bull. 39, 669 (2004) [7] Q. Liu, Y. Ni, G. Yin, J. Hong, Z. Xu, Mater. Chem. Phys. 89, 379 (2005) [8] R.R. Hawalder, K.G. Kanade, K.R. Patil, S.D. Sathaye, U.P. Mulik, D.P. Amalnerkar, Mater. Chem. Phys. 91, 447 (2005) [9] S.F. Wang, F. Gu, M.K. Lü, G.J. Zhou, A.Y. Zhang, J. Cryst. Growth 289, 621 (2006) [10] C. Li, G. Shi, H. Xu, S. Guang, R. Yin, Y. Song, Mater. Lett. 61, 1809 (2007) [11] Z. Xiu, S. Liu, J. Yu, F. Xu, W. Yu, G. Feng, J. Alloy. Compd. 457, L9 (2008) [12] P. Baláž, E. Boldižárová, E. Godočíková, J. Briančin, Mater. Lett. 57, 1585 (2003)
[13] E. Dutková, P. Baláž, P. Pourghahramani, A.V. Nguyen, V. Šepelák, A. Feldhoff, J. Kováč, A. Šatka, Solid State Ionics 179, 1242 (2008) [14] P. Baláž, Mechanochemistry in Nanoscience and Minerals Engineering (Springer Berlin, Heidelberg, 2008) [15] P. Pourghahramani, E. Forssberg, Int. J. Miner. Process. 79, 120 (2006) [16] G.K. Williamson, W.H. Hall, Acta Metall. Mater. 1, 22 (1953) [17] Y. Wang, B. Zhao, F. Zhang, W. Mao, G. Qian, X. Fan, Mater. Lett. 61, 3733 (2007) [18] P. Pourghahramani, E. Forssberg, Int. J. Miner. Process. 79, 106 (2006) [19] X.R. Ye, D.Z. Jia, J.Q. Yu, X.Q. Xin, Z.L. Xue, Adv. Mater. 11, 941 (1999) [20] X. Wang, Ch. Xu, Z. Zhang, Mater. Lett. 60, 345 (2006) [21] D. Liang, S. Tang, J. Liu, J. Liu, X. Lu, L. Kang, Mater. Lett. 62, 2426 (2008) [22] S.M. Zhou, X.H. Zhang, X.M. Meng, X. Fan, S.T. Lee, S.K. Wu, J. Solid State Chem. 178, 399 (2005)
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