Structural characterization of mechanochemically synthesized PbS nanocrystals

A. Zorkovská, P. Baláž, M. Baláž

Institute of Geotechnics, Slovak Academy of Sciences, Watsonova 45, Košice, Slovakia

zorkovska@saske.sk

Keywords: mechanosynthesis, X-ray diffraction, nanocrystals, semiconductors

Introduction

Lead sulfide PbS is a IV-VI semiconductor with a narrow band gap (0.41 eV), which can be tuned to the visible range by nanostructure formation. Nanocrystalline PbS exhibits unique optical, electronic, and photovoltaic properties [1-3] desirable for potential applications in electroluminescent devices, such as light-emitting diodes (LED).

In this study we report the direct solid state mechanosynthesis of nanocrystalline PbS from lead acetate ((CH₃COO)₂Pb.3H₂O) and two different sulfur containing precursors, and we compare the structural properties of the products. In the first process lead acetate with L-cystine (C₆H₁₂N₂O₄S₂) amino acid has been co-milled, in the second process eggshell membrane (ESM) was used as sulfur source. L-cystine has a characteristic disulfide linkage (-S-S-) and it occurs in many natural proteins, also in the constituents of the ESM.

Experimental

The mechanochemical synthesis of PbS nanocrystals was performed in a planetary mill Pulverisette 6 (Fritsch, Germany). The following milling conditions were used - loading of the mill: 50 balls of 10 mm diameter; ball charge in the mill: 360 g; material of milling chamber and balls: tungsten carbide; rotation speed of the planet carrier: 500 rpm; argon atmosphere as protective medium in the mill; laboratory temperature; milling time t_M 15-240 minutes. In the first process, the lead acetate to L-cystine ratio was 2:1, because of the presence of two sulfur atoms in the structure of L-cystine and only one lead atom in the structure of lead acetate (stoichiometric ratio). In the second process 15 g of ESM and 2.39 g of lead acetate were co-milled. The amounts were calculated according to the assumption, that the ESM contains 4% of cystine and that two atoms of Pb are needed for the interaction with one molecule of cystine. The products were washed with 0.003M HCl and dried.

X-ray diffraction measurements were carried out using a D8 Advanced diffractometer (Bruker, Germany) equipped with a θ/ θ goniometer, CuKα radiation (40 kV, 40 mA), secondary graphite monochromator and scintillation detector. For the data treatment and analysis the commercial Bruker processing tools have been used. Concretely, for the phase identification the Diffrac plus Eva and for the Rietveld analysis and microstructure characterization the Diffrac plus Topas software have been utilized. The crystalline size was estimated by „double-Voight“ method, using the integral breadth, since this characteristic is the most comparable to values observed by TEM.

The electron microscope study was performed on Field Emission Scanning Electron Microscope – FE SEM (JEOL JSM-7600F and Hitachi S-4800) at accelerating voltage 5kV. Transmission Electron Microscope (TEM) images were recorded by JEOL JEM 2000FX microscope at 160 kV.

Results

The progress of the synthesis in case of both mechanochemical routes is demonstrated by XRD patterns in Figure 1. From Figure 1a (left) it can be seen, that PbS (face-centered cubic structure, space group Fm-3m) was obtained after 60 min of milling of lead acetate and L-cystine, however, the reaction was not completed and the precursors are also present in the product. After 105 min of milling the patterns do not change considerably, the mechanochemical conversion reaches equilibrium at around 80 % of PbS obtained. The diffraction peaks show that PbS is well-crystallized, nevertheless, L-cystine is still present, despite the acidic post-treatment of the product. This fact suggests that L-cystine is somehow bound to the PbS particles.

Figure 1. XRD patterns (left) and microphotographs (right) documenting the mechanosynthesis of PbS from lead acetate and either L-cystine (a) or eggshell membrane (b) precursors. The microphotographs corresponding to the selected stages of milling are tagged by arrows. The topmost picture is a TEM image, below the corresponding SEM pictures are shown.

The dependence of crystallite size, D, and of the refined lattice parameter, a, on milling time is summarized in Table 1. After t_M = 105 min the size of the PbS nanocrystals gets stabilized at around 20 nm. From the corresponding TEM picture (Figure 1, right top) as well as from the SEM image below, it can be seen, that the sample consists of aggregates of rather uniform, well-faceted cubic nanocrystals.

In the second process the PbS formation on the of considerable amorphous organic background becomes visible after 120 min of milling, as it is documented by XRD patterns in Figure 1 b (left). The morphology of the product is noticeably different from the one obtained in the previous process. From the fine fibrous structure with the diameter of fibers around 1-2 μm new particles with sharp and angular edges develop, with relatively wide size distribution, but on rather large, micrometer scale (from 5-150 μm). Nevertheless, the crystallite size, i.e. the structurally coherent size is even finer than in the first process. The estimated average crystallite size after milling for 180 min is approximately 9 nm [4].

Table 1. Microstructural characteristics of the mechanosynthesized PbS. The values are obtained from Rietveld analysis of XRD data

	L-cystine						ESM
Milling time (min)	60	105	120	150	180	240	180
Lattice parameter a (nm)	0.5938	0.5939	0.5938	0.5939	0.5939	0.5940	0.595
Crystallite size D (nm)	25	18.5	20	21	19	19	9

Summary

Nanocrystalline semiconductor PbS has been mechanosynthesized from organic precursors using a simple, solvent-free technique. Lead acetate was co-milled with either (i) L-cystine or (ii) eggshell membrane in a planetary mill. The nanocrystals are formed in both cases on the background of organic residuum (serving as sulfur source), which may play the role of potential surface-functionalizing agent in future studies. The morphology of particles prepared by the two different routes is different: well-shaped, rather uniform particles with crystalline size around 20 nm can be obtained when using L-cystine, particles with sharp and angular edges on micrometer scale with crystalline size of about 9 nm are formed in the synthesis with the eggshell membrane.

Acknowledgements.

The SEM and TEM microphotographs have been kindly provided by M. J. Sayagués, M. Čaplovičová and Ľ. Čaplovič.

References

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