Edited by: Elizabeth J. Podlaha, Clarkson University, United States
Reviewed by: Avinash Raj Kola, Applied Materials, United States; Edward Gillan, University of Iowa, United States; Despina Davis, Raytheon, United States
This article was submitted to Electrochemistry, a section of the journal Frontiers in Chemistry
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Lead telluride (PbTe) nanofibers were fabricated by galvanic displacement of electrospun cobalt nanofibers where their composition and morphology were altered by adjusting the electrolyte composition and diameter of sacrificial cobalt nanofibers. By employing Co instead of Ni as the sacrificial material, residue-free PbTe nanofibers were synthesized. The Pb content of the PbTe nanofibers was slightly affected by the Pb2+ concentration in the electrolyte, while the average outer diameter increased with Pb2+ concentration. The surface morphology of PbTe nanofibers was strongly dependent on the diameter of sacrificial nanofibers where it altered from smooth to rough surface as the Pb2+ concentration increased. Some of thermoelectric properties [i.e., thermopower (S) and electrical conductivity(σ)] were systematically measured as a function of temperature. Energy barrier height (Eb) was found to be one of the key factors affecting the thermoelectric properties–that is, higher energy barrier heights increased the Seebeck coefficient, but lowered the electrical conductivity.
The restriction of non-renewable resources along with the threat of environmental and ecological degradation is a key driver for improving energy generation and efficiency. Various renewable energy technologies including solar cells (Oregan and Gratzel,
Lead telluride (PbTe) is a V-VI semiconductor with a narrow band-gap energy of 0.31 eV at room temperature with a rock-salt crystal structure. By adjusting the composition, PbTe can be either an n- or p-type semiconductor. For example, Te-rich PbTe results in p-type semiconductor whereas Pb-rich PbTe results in n-type semiconductor (Dughaish,
Various methods including chemical deposition (Lokhande,
Electrospinning is a technique that can produce ultra-long nanofibers by continuously stretching and whipping viscoelastic jets in a high electric field. Various nanofibers of polymer (Xiao et al.,
In this paper, hollow PbTe nanofibers with controlled dimension and morphology were synthesized for the first time. Electrospinning was exploited to fabricate sacrificial cobalt nanofibers. Various dimensions and morphologies of the PbTe hollow nanofibers were synthesized by tuning the electrolyte concentrations in the galvanic displacement reactions. Additionally, thermoelectric properties were characterized and correlated to their materials properties.
The procedure of electrospinning of cobalt nanofibers is based on previous reported nickel nanofibers (Park et al.,
The galvanic displacement of cobalt to PbTe nanofiber mat was conducted at room temperature for 30 min by dipping a freestanding Co nanofiber mat of the desired amount into 10 ml solution. The solution consisted of X mM lead nitrate (Pb(NO3)2, Fisher Chemical), 0.1 mM tellurium oxide (TeO2, 99+%, Acros Organic) and 0.1 M nitric acid (HNO3, Certified ACS Plus, Fisher Chemical) The pH of solution was controlled to be 2 by adding nitric acid (HNO3, Certified ACS Plus, Fisher Chemical). The effects of Pb+2 concentration on the morphology and dimension of the nanofiber were conducted by altering the Pb2+concentrations from 10 to 100 mM. After galvanic displacement, the mats were rinsed with de-ionized water five times follow by air dried.
Various solution properties including electrical conductivity, solution viscosity, and surface tension were measured with Accumet AB-200 benchtop electrical conductivity meter, Brookfield DV-I Prime viscometer, and Interfacial tensiometer (CSC-Denouy 70545), respectively.
Transmission electron microscopy (TEM), selected area electron diffraction (SAED), field emission-scanning electron microscopy (FE-SEM, FEG-Philips XL30), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD, D8 Advance Diffractometer, Bruker) were used to characterize morphologies, compositions, crystal structures and crystallinity of the nanofiber mats.
Single fiber-based devices were fabricated by a standard photolithography process with a gap size of 3 μm between two Au pads. Nanofiber mats based devices were formed by sputtering Pt to form electrodes using shadow mask technique with the fixed electrode gap distance of 1 mm.
Temperature-dependent electrical properties including current-voltage (I-V) and field-effect transient (FET) measurements were characterized based on the single fiber-based devices at a temperature ranging from 293 to 353 K, while thermopower (S) was measured based on mats by a home-built instrument. The temperature range was varied from 300 to 360 K.
Table
Solution properties of electrospinning precursors.
Solution 1 (Resulted in smaller Co nanofiber) | 82.5 | 1.0 | 37 |
Solution 2 (Resulted in larger Co nanofiber) | 130.3 | 1.1 | 37 |
SEM images of electrospun PVP/acetic acid/Co acetate nanofibers with average diameters of
Co was chosen as the sacrificial material for galvanic displacement reaction for the synthesis of PbTe due to its ability to provide an appropriate electrochemical driving force (difference in materials' redox potential) for the metal chalcogenide displacement. Since the redox potential of Co2+/Co pair (-0.28 V vs. SHE) is more cathodic than that of the Pb2+/Pb (−0.13 V vs. SHE) and
Control over the dimension and morphology of the PbTe nanofibers was achieved by varying the diameter of Co nanofibers (
Figure
SEM images of synthesized PbTe hollow nanofibers using 124-nm (top row,
The synthesized PbTe nanofiber mats were then sonicated and dispersed in IPA to obtain single nanofiber-based suspension solutions. These solutions were drop-casted on Si/SiO2 chips for the EDS characterization. The composition of over six individual fibers were measured and averaged for each condition. The composition of PbTe as a function of Pb2+ concentration (Table
Effect of [Pb2+] on the Pb content in the PbTe nanofibers using two different Co nanofibers (i.e., 52 nm and 124 nm).
10 | 42 ± 4.1 | 42 ± 6.3 |
50 | 42 ± 5.9 | 38 ± 6.2 |
100 | 37 ± 6.8 | 44 ± 3.3 |
A quantitative assessment of the effect of the Pb2+ concentrations on the average outer diameter of the PbxTey nanofibers is shown in Figure
Effect of [Pb2+] on the outer diameter of PbTe nanofibers. The outer diameters of Co nanofibers are shown in Figure at [Pb2+] = 0. Yellow squares and blue circles indicate PbTe nanofibers from Co nanofibers with the average diameter of 52 and 124 nm, respectively.
Figure
XRD pattern of
High-resolution transmission electron microscopy (HR-TEM) with EDS and SAED were utilized to characterize the morphology, composition, and crystal structure of nanofibers (Figure
Temperature-dependent I-V characterizations were carried out based on single nanofibers where the temperature was varied from 295 to 360 K (Figure
Temperature-dependent
Figure
Temperature-dependent
Temperature-dependent Seebeck coefficient of PbxTey nanofiber mats with various x are shown in Figure
In our experiments, higher Pb content (i.e., Pb43Te57 and Pb42Te58) smaller nanofibers show the thermopower increased with temperature (Figure
Figure
Figure
PbTe nanofiber mats were fabricating using electrospun cobalt nanofibers as the sacrificial materials. Control over the dimension and morphology of the nanofibers were achieved by applying sacrificial material with various diameters and tuning the concentration of Pb2+ in the electrolytes during galvanic displacement reaction. Hollow PbTe nanofibers were synthesized in all the conditions. The fibers with larger outer diameter were obtained from thicker Co nanofibers. For the larger PbTe nanofibers, hollow and smooth surfaces were achieved using electrolytes containing low concentrations of Pb2+, whereas rough surfaces were observed from using concentrated electrolytes. The formation of rough surface in the latter case may be due to the faster reaction rate. On the other hand, for the smaller PbTe nanofibers, no clear differences in morphology were observed with various [Pb2+]. The smaller PbTe nanofibers were rougher than the larger PbTe nanofibers, which might be due to the rougher surface of small Co nanofibers. The Pb2+ concentration had negligible effects on the fibers' composition in the studied range. No residue of Co was observed in the fibers after the galvanic displacement reactions, which indicated a complete reaction. The outer diameter of PbTe nanofibers increased with the Pb2+ concentration. XRD analysis showed that all synthesized PbTe samples were polycrystalline in nature.
The temperature-dependent I-V characterization was conducted based on single PbTe nanofibers. The electrical conductivity decreased as the Pb content in the nanofibers decreased. It could be suggested that the excess Te created barriers in the nanofibers, increasing the barrier height while decreasing the electrical conductivity.
MZ, JK, and MN conducted the experiments. SK conducted TEM analysis. S-DP, YC, JL, and NM provided funding and inputs to the manuscript.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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