ISSN 00201685, Inorganic Materials, 2011, Vol. 47, No. 8, pp. 837–843. © Pleiades Publishing, Ltd., 2011. Original Russian Text © S.I. Sadovnikov, N.S. Kozhevnikova, A.A. Rempel, 2011, published in Neorganicheskie Materialy, 2011, Vol. 47, No. 8, pp. 929–935.
Stability and Recrystallization of PbS Nanoparticles S. I. Sadovnikov, N. S. Kozhevnikova, and A. A. Rempel Institute of Solid State Chemistry, Ural Branch, Russian Academy of Sciences, Pervomaiskaya ul. 91, Yekaterinburg, 620990 Russia email:
[email protected] Received September 20, 2010
Abstract—The recrystallization and thermal stability of nanocrystalline lead sulfide have been studied by Xray diffraction and scanning electron microscopy. PbS nanoparticles ranging in size from 10 to 20 nm were prepared by chemical precipitation from aqueous solutions. To assess the thermal stability of the size of PbS nanoparticles, the nanocrystalline powders were annealed in air or under dynamic vacuum (10–3 Pa) at a tem perature varied from 433 to 930 K in 50K steps. Annealing at temperatures of up to 700 K increases the par ticle size only slightly but relieves the lattice strain, suggesting that the nanocrystalline state of lead sulfide is thermally stable in this temperature range. The temperature range 700–900 K, where the particle size increases by a factor of 5–10, corresponds to the secondary recrystallization of nanocrystalline PbS. The tem perature 700 K is half the melting temperature of macrocrystalline PbS, indicating that PbS nanoparticles have higher thermal stability in comparison with other nanomaterials. DOI: 10.1134/S0020168511080176
INTRODUCTION Coarsegrained lead sulfide, PbS, is a narrowgap semiconductor with a band gap of about 0.4 eV [1, 2] and a cubic (B1) structure [3]. PbS particles less than 100 nm in size [4, 5] differ in electronic properties from bulk PbS [6], which has led to increased interest in nanoparticulate lead sulfide, because PbS nano films and nanopowders can be used to extend the spec tral range of infrared detectors, as well as in nearIR lasers and solar cells [7–9]. It is worth pointing out that the most widespread devices that take advantage of macrocrystalline lead sulfide are fire sensor systems, flame sensors, and heat source detection systems [10, 11], so the thermal stability of the nanocrystalline state determines to a significant degree the feasibility of using nanoparticulate PbS in such devices. The ability to extend the application field of nanocrystalline PbS depends crucially on knowledge of the temperature range where PbS particles do not grow, that is, where no transition from a nanocrystalline to a macrocrystal line state occurs. Such data are not available in the lit erature. This led us to study the thermal stability of the nanocrystalline state of lead sulfide and PbS recrystal lization in vacuum and air and to determine the recrystallization temperature of PbS nanoparticles. EXPERIMENTAL To produce nanocrystalline lead sulfide powders with a particle size of 10–20 nm, the synthesis reaction should be instantaneous, preventing growth of nuclei [8]. For this reason, nanopowders were synthesized by
reacting lead acetate, Pb(OAc)2, with sodium sulfide, Na2S. The initial reactant concentrations were in the stoichiometric ratio and were varied in the range [Pb(АсO)2] = [Na2S] = 0.005–0.25 M. The synthe sized nanoparticulate PbS was washed with a solution of a complexing agent by decantation, filtered off, and again washed with distilled water. The complexing agent used was Trilon B (0.005 M solution). To assess the thermal stability and crystallization temperature of nanocrystalline lead sulfide, powders with a particle size in the range 10–20 nm were annealed in air or under dynamic vacuum (10–3 Pa) at a temperature varied from 423 to 930 K in 50K steps. The air anneals were performed in a muffle furnace. Before each anneal, the muffle furnace was heated to a predetermined temperature, and then a PbS nanop owder sample (3 mg) enclosed in a quartz crucible was placed in the furnace. For the vacuum anneals, we used a system that incorporated an Ilmvac Type 203099 roughing pump, TV 70D turbomolecular pump, R12A 1000 vacuum gage, and MTP2M tunnel furnace. The temperature was monitored with a Pt/Pt–Rh thermocouple, and thermopower was mea sured using a V721A generalpurpose voltmeter. A weighed amount of PbS nanopowder in a quartz cru cible was placed in a quartz ampule with one end sealed. The quartz ampule was connected to a vacuum system and pumped down to 10–3 Pa. Next, the ampule was introduced into the furnace, preheated to a predetermined temperature. At each temperature, the nanopowders were annealed for 1 h. After each anneal, the powders were characterized by Xray dif fraction (XRD).
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Table 1. Qualitative and quantitative phase analysis data for PbS nanopowders vacuumannealed at different temperatures Annealing temperature, K 433 529 613
705
810
930
Weight percent in the nanopowder with the average particle size below Phase PbS PbSO3 PbS PbSO3 PbS PbO ⋅ PbSO4 4PbO ⋅ PbSO4 PbS PbO ⋅ PbSO4 4PbO ⋅ PbSO4 PbS PbO ⋅ PbSO4 PbSO4 PbS Pb
12 ± 5 nm
15 ± 3 nm
18 ± 3 nm
84 16 93 7 76 11 13 70 16 14 ×
97 3 99
