| Due to their size and shape-dependent physical and chemical properties, nanostructured materials with different shapes and morphologies have exhibited promising application in many fields such as optics, biology, magnetics, energy conversion, etc. Therefore, controlled synthesis of nanocrystals is essential to fabricate nanodevices. Lead telluride is a very important narrow bandgap semiconductor showing great promise in the fields of thermoelectric (TE) and Infrared (IR) photoelectric devices. Both theoretical calculations and experimentals prove that nanostructured PbTe material can significantly improve the thermoelectric and optoelectrical properties.In this work, different PbTe nanostructures, including nanocubes, nanorods, nanosheets, nanoboxes and hierarchical superstructures, have be synthesized via a low-temperature wet chemical method and an alkaline hydrothermal method. We investigated controllable synthesis of PbTe nanostructures by adjusting some factors, such as reaction time and temperature, concentration of NaOH, surfactants, reductants, etc. The formation mechanisms of different PbTe nanocrystals were proposed based on time dependent experiments. Some innovative results of our present work are listed as fellows:(1) 1D PbTe polycrystalline nanorods with rough surfaces have been successfully synthesized via a low-temperature wet chemical route, using Pb(CH3COO)2·3H2O and Na2TeO3 as the precursors and NaBH4 as the reductant. Microstructural analyses show that these nanorods range from 50 to 200 nm in diameter with lengths up to 1 mm. The component nanoparticles have particle sizes of approximately 30-50 nm, comparable with the excitonic Bohr radius of PbTe (46 nm). Such rough polycrystalline nanorods may show enhanced TE performance.Different from the oriented attachment mechanism suggested for single-crystalline PbTe nanorods, PbTe polycrystalline nanorods was designed by using tellurium nanorods as templates: PbTe nucleates on Te nanorod templates and grows up by consuming Te nanorods. At last rough PbTe polycrystalline nanorods with rough surfaces were formed.(2) Hierarchical PbTe structures, such as hopper cubic, flowerlike, and dendritic structures, have been successfully synthesized by a surfactant-free hydrothermal method. Pb(NO3)2 and Na2TeO3 were used as the precursors and NaBH4 was used as the reductant. These symmetrical structures are single crystals. The band gap value of hierarchical PbTe structures was calculated to be about 0.29 eV from the FTIR absorption spectrum.The formation of PbTe hierarchical structures should be determined by the nucleation and the subsequent growth stage, which are associated with the energy of the exposed facets (kinetics) and the diffusion effect, respectively. Firstly, as an face-centered cubic(f.c.c) crystal, PbTe grow into a cubic structure with {100} facets exposed. When the growth of PbTe cubes proceeds, the process will be controlled by the Berg effect. The concentration of Pb and Te ions is higher at the edges and corners, which is favorable for the formation of the hopper structure. NaOH plays a crucial role in the formation process. With increasing reaction time, the growth of PbTe crystals are determined by diffusion effect, thus dendritic structures are formed.(3) PbTe nanosheets, nanoparticles have been synthesized via an alkaline hydrothermal method. Pb(NO3)2 and Na2TeO3 were used as the precursors, PVP was used as the surfactant and NaBH4 was used as the reductant. These nanosheets are of 20-80 nm in thickness and 0.2-5μm in-plane sizes.Due to the f.c.c. structure, PbTe will naturally grow into nanocubes. So far there is no report on PbTe nanosheets. It is found that PVP, NaOH and N2H4·H2O play important roles in the growth of PbTe nanosheets. A "soft template" may be formed due to the effect of hydrogen bonds between -NH2 in Hydrzine and OH-. Then PbTe nucleates among this 2D network array. Meanwhile, PVP acts as a capping agent during the process, which also leading to the oriented arrangement of PbTe nuclei. |
