Synthesis and structure of undoped and indium-doped thermoelectric lead telluride nanoparticles
© Kadel et al.; licensee Springer. 2014
Received: 20 February 2014
Accepted: 27 April 2014
Published: 8 May 2014
Undoped and indium (In)-doped lead telluride (PbTe) nanostructures were synthesized via solvothermal/hydrothermal route. The crystalline structure of the as-prepared undoped and In-doped PbTe samples was examined by X-ray diffraction (XRD) which indicated the formation of face-centered single-phase cubic crystal. A first principle calculation on indium doping shows that the indium atoms are more likely to replace lead (Pb) rather than to take the interstitial sites. Laser-induced breakdown spectroscopy (LIBS) analysis confirms that indium is incorporated into the PbTe matrix of the indium-doped PbTe samples. The effects of surfactant and synthesis temperature on the structure and morphology of the undoped PbTe were also investigated; it was found that PbTe nanostructures synthesized with the addition of surfactants exhibited uniform shapes and their size increased with the synthesis temperature.
Though solid-state thermoelectric (TE) materials are considered as potential candidates for their application in power generating and refrigerating devices, the low efficiency of the TE materials limits their practical application. Nanostructured materials are drawing more attention due to their potential applications in thermoelectrics with high efficiency. Theoretical predictions and experimental results indicate that low-dimensional TE materials can exhibit high thermoelectric efficiency[3–5]. The efficiency of TE materials can be defined by dimensionless thermoelectric figure of merit (ZT), ZT = (S 2 σ/κ)T, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the absolute temperature at which the figure of merit is measured. The quantity S2σ is most commonly referred as power factor. Increase in power factor and decrease in thermal conductivity are required to enhance the ZT value. Nanostructures can induce the reduction of thermal conductivity due to the enhanced phonon scattering by the interface or the boundary and the increment in power factor via quantum confinement of electrons. According to Slack, semiconductors having narrow band gap and high mobility carriers are best suited for thermoelectric materials. Lead telluride (PbTe) is a narrow band gap semiconducting material and has great applications in thermoelectric devices, IR photoelectrics, and IR laser devices. PbTe is considered as one of the best thermoelectric materials which can be efficiently employed as a power generator in the medium and high temperature range (450 to 800 K). It is shown theoretically and experimentally that the TE property of PbTe can be improved by doping it with some donor or acceptor atoms. Recently, there has been renewed research interest in PbTe after Heremans et al. reported the enhancement of the Seebeck coefficient of PbTe through the distortion of electronic density of states by doping it with thallium. The electric property of PbTe can vary significantly when it is doped with group IIIA elements, such as In and Ga, which generate a deep lying impurity level in IV-VI compounds. A previous work by Dashevsky et al. reported a higher ZT value of about 0.92 at 700 K for a functionally graded indium-doped single crystal of PbTe.
PbTe nanostructures have been synthesized using various techniques. Beyer et al. reported an enhanced thermoelectric efficiency of molecular beam epitaxially (MBE) grown superlattices based on PbTe. Palchik et al. synthesized PbTe from solutions under microwave radiations. Earlier works also reported the synthesis of 3-D structures of PbTe such as dendrite-like structures via electrochemical deposition and sponge-like structures from sonochemistry. Among the various synthesis techniques employed for the formation of PbTe nanostructures, the solvothermal/hydrothermal process has attracted much interest due to the advantage of high yield, low synthesis temperature, high purity, and high crystallinity. Zhu et al. reported the synthesis of PbTe powders using alkaline reducing solvothermal route and the synthesis of PbTe three-dimensional hierarchical superstructures via an alkaline hydrothermal method. The solvothermal/hydrothermal technique produces various PbTe nanostructures such as nanotubes[18, 19], nanospheres, and nanoboxes. In this work, we report the synthesis of undoped and In-doped PbTe nanostructures using the solvothermal and hydrothermal routes in alkaline solution medium with or without a surfactant at different temperatures and reaction time durations. We have explored the synthesis of the undoped and In-doped PbTe nanostructures using a water/glycerol mixture as a solvent, which, to the best of our knowledge, has not been previously reported. The morphology and crystal structure of the as-synthesized undoped and In-doped PbTe nanostructures have been discussed in detail. Laser-induced breakdown spectroscopy (LIBS) analyses were conducted to investigate the indium incorporation into the PbTe matrix. A pseudo-potential first principle calculation was conducted to study the mechanism of indium doping into the PbTe matrix. In-doped PbTe is expected to enhance the thermoelectric property due to the increase in Seebeck coefficient through the distortion electron density of states near the Fermi level.
Analytically pure lead nitrate (PbNO3), indium chloride (InCl3), and tellurium (Te) powder were used as precursor materials for the synthesis of PbTe and In-doped PbTe. These materials were put in the Teflon liner in the appropriate molar ratios according to the formula In x Pb1-xTe, where x = 0, 0.005, 0.01, 0.015, and 0.02. Then, 6.25 mmol of sodium hydroxide (NaOH) as a pH controlling agent, 2.6 mmol of sodium borohydrate (NaBH4) as a reducing agent, and 1 mmol of ethylenediaminetetraacetic acid (EDTA) as a shape-directing additive were added. Water was used as a solvent in the hydrothermal process; either ethanol or a mixture of glycerol and water in 1:3 volume ratio was used as solvent for the solvothermal route. Later, the Teflon liner was filled up to 80% of its total volume with the solvent and was placed in an ultrasonicator for 30 min to obtain a uniform reaction mixture. After sonication, the Teflon liner was placed in an autoclave and sealed tightly. Then, the autoclave was heated in the furnace at 140°C and 200°C for 24 h. After synthesis, the autoclave was allowed to cool down to room temperature naturally. A black precipitate was collected, and then vacuum filtered, rinsed with ethanol and distilled water several times repeatedly, and dried at 120°C in vacuum for 4 h. The above synthesis process was repeated with the addition of 1 mmol each of cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and Triton X-100 as cationic, anionic, and non-ionic surfactants/capping agents, respectively, at 140°C for 24 h in water/glycerol solution (3:1 volume ratio). The PbTe nanostructures synthesized without surfactants at 140°C and 200°C for 24 h in ethanol are termed as PbTe-1 and PbTe-3 and in the water/glycerol mixture are named as PbTe-2 and PbTe-4, respectively. In x Pb1-xTe (x = 0.005, 0.01, 0.015, and 0.02) synthesized at 140°C for 24 h in water/glycerol solution are named as In005PbTe, In01PbTe, In015PbTe, and In02PbTe, respectively.
X-ray diffraction (XRD) measurements were carried out using a Siemens D5000 diffractometer equipped with a Cu anode operated at 40 kV and 40 mA (Siemens AG, Karlsruhe, Germany). The XRD patterns were collected with a step size of 0.01° and a scan rate of 1 step per second. Surface morphology analysis was performed by a field emission scanning electron microscope (SEM, JEOL JSM-6330 F, 15 kV; JEOL Ltd., Tokyo, Japan). Transmission electron microscopy (TEM), selected-area electron diffraction (SAED) patterns, and energy dispersive X-ray spectroscopy (EDS) spectrum were obtained from a FEI Tecnai F30 apparatus (FEI Co., Hillsboro, OR, USA) operated at an accelerating voltage of 300 kV with a point-to-point resolution of 2 Å. LIBS analyses were conducted on a RT100HP system (Applied Spectra, Fremont, CA, USA), equipped with a 1,064-nm ns-Nd:YAG laser. The detector has a CCD linear array (Avantes, Broomfield, CO, USA) with possible gate delay adjustment from 50 ns to 1 ms with 25-ns step resolution and a fixed integration time of 1.1 ms. Data interpretation and data analysis were conducted with TruLIBS™ emission database and Aurora data analysis software (Axiom 2.1, Applied Spectra, CA, USA).
A first principle calculation was conducted to investigate the indium doping into the PbTe matrix. We first calculated the lattice constant of PbTe in its NaCl structure. Then, we constructed a simple cubic (SC) 2 × 2 × 2 supercell with 32 PbTe units and used the same lattice constant for further calculation of substitution energy and interstitial insertion energy.
Results and discussion
Lattice constants of undoped and In-doped PbTe samples
Lattice constant, Å
6.423 ± 0.017
6.452 ± 0.019
6.437 ± 0.014
6.418 ± 0.013
6.441 ± 0.015
To further investigate the doping mechanism, we studied the favorability of indium atom to substitute Pb by conducting the pseudo-potential first principle calculations using a single cubic 2 × 2 × 2 supercell with 32 units of PbTe. We first started with 64-atom Pb32Te32 cell to calculate the lattice constant of PbTe crystal. The calculated value of the lattice constant is found to be 6.33 Å which is in close agreement with the reported value for cubic PbTe, 6.454 Å (JCPDS: 78-1905). This is followed by calculation of the formation energy for substitution with one indium in the 2 × 2 × 2 supercell (1.5 at% of In) which is slightly higher in indium level compared to our highest doped experimental sample In0.02Pb0.98Te (1.0 at%). The formation energy of the substitution is defined as Esub = E(Pb32Te32) + E(In) - E(InPb31Te32) - E(Pb). The calculated value of the formation energy of the substitution is 3.21 eV which is larger than the calculated cohesive energy of indium crystal (Ein), 2.52 eV. Since Esub > Ein, we can conclude that indium is highly favorable to substitute Pb into the PbTe for 1.5 at% doping level. This conclusion is consistent with the result we got from the XRD analysis of our In-doped PbTe samples. No indium phase is detected by XRD in our sample. We further calculated the formation energy of substitution for InPb15Te16 (3.12 at% of In) and InPb7Te8 (6.24 at% of In) in order to investigate the solubility of the indium into PbTe. It is found that formation energy for substitutions reduced to -0.6 and -1.17 eV, respectively, for 3.12 and 6.24 at% of indium doping. The reduced value of substitution energy indicates that substitution of Pb with indium becomes less favorable with the increased In doping concentration. The very large negative substitution energy, -1.17 eV for 6.24 at% of In doping, suggests that it is almost impossible for In to substitute Pb at such high doping level. This corresponds well with the solubility limit of In in PbTe. We have also tested In doping into interstitial sites of the PbTe lattice. At the most likely (0.25, 0.25, 0.25) interstitial site, the insertion energy comes to be 0.068 eV. From these energy calculations, as well as from our X-ray measurement, we can conclude that In doping, at our level of 1.5 at%, allows substitution on the Pb site.
Our conclusion is consistent with a previous first principle calculation of aluminum (Al) doping on PbSe, which also concluded that Al atoms prefer to replace Pb rather than to take interstitial sites. The reported band structure and density of states (DOS) calculation showed that upon low-level doping of Al, the enhanced density of states of PbSe near the Fermi energy is responsible for enhanced carrier density, which leads to higher conductivity. Since In doping to our PbTe sample allows substitution on the Pb site, we expect a similar effect on electronic properties of our PbTe samples upon doping.
Undoped and In-doped PbTe nanoparticles were synthesized via the solvothermal and hydrothermal routes with or without surfactant at different preparation conditions. It is found that the solvent plays a very important role in the size and shape of the PbTe and In-doped PbTe nanoparticles. A water/glycerol mixture used as solvent yields nanoparticles with relatively uniform shapes and narrow size distribution, while water used as the solvent will result in nanoparticles with irregular shapes and wide range size distribution. Absence of any impurity phase of indium in the XRD pattern indicated that indium was likely doped into the lattice sites of Pb in PbTe. The presence of multiple indium lines in the LIBS emission spectra for indium-doped PbTe samples, In01PbTe and In02PbTe, confirms the incorporation of indium into the PbTe matrix. The theoretical calculation also indicates that indium is likely to replace lead during the doping process for the smaller concentration of indium (<3 at%) which complements the results obtained from LIBS and XRD analyses. The In-doped and undoped PbTe nanostructures are intended to be utilized in future thermoelectric applications. In-doped PbTe is expected to exhibit enhanced thermoelectric property due to improved electronic properties upon indium doping.
This work is supported by the Florida International University under the Bridge Grant AWD000000001773 and the American Chemical Society Petroleum Research Foundation under grant 51766-ND10. This work was performed, in part, at the Center for Integrated Nanotechnologies at Sandia National Laboratories under the user proposals U2009B032 and C2011A1022.
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