Thermoelectric Properties of Ultralong Silver ... - ACS Publications

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Chemistry of Materials

Thermoelectric Properties of Ultra-long Silver Telluride Hollow Nanofibers Miluo Zhang1, Hosik Park1, Jiwon Kim1, Hyounmyung Park1, Tingjun Wu1, Seil Kim2, Su-Dong Park3, Yongho Choa2, and Nosang V. Myung1,* 1

Department of Chemical and Environmental Engineering and Winston Chung Global Energy Center, University of California-Riverside, Riverside, California 92521. 2 Department of Fusion Chemical Engineering, Hanyang University, Ansan 426-791, Korea. 3 Advanced Materials and Application Research Division, Korea Electrotechnology Research Institute, Changwon 642-120, Korea. ABSTRACT: Ultra-long AgxTey nanofibers were synthesized for the first time by galvanically displacing electrospun Ni nanofibers. Control over the nanofiber morphology, composition and crystal structure was obtained by tuning the Ag+ concentrations in the electrolytes. While Te-rich branched p-type AgxTey nanofibers were synthesized at low Ag+ concentrations, Ag-rich nodular AgxTey nanofibers were obtained at high Ag+ concentrations. The Te-rich nanofibers consist of coexisting Te and Ag7Te4 phases, and the Ag-rich fibers consist of coexisting Ag and Ag2Te phases. The energy barrier height at the phase interface is found to be a key factor affecting the thermoelectric power factor of the fibers. A high barrier height increases the Seebeck coefficient, S, but reduces the electrical conductivity, σ, due to the energy filter effect. The Ag7Te4/Te system was not competitive with Ag2Te/Ag system due to its high barrier height where the increase in S could not overcome the severely diminished electrical conductivity. The highest power factor was found in the Ag-rich nanofibers with an energy barrier height of 0.054 eV.

INTRODUCTION The rising cost of compliance to laws and regulations (from the Clean Air Act to Geologic Sequestration) for consuming non-renewable energy resources is the key driver to improve the efficiency of environmentally-friendly and renewable energy generation.1 Thermoelectric materials offer simple, silent and reliable solid-state energy conversion devices due to their unique ability to directly convert heat into electricity and viceversa without moving parts or bulk fluids. The efficiency of a thermoelectric device can be determined by the thermoelectric figure-of-merit (ZT), given by = /. Here S, σ, κ, T, and S2σ are the Seebeck coefficient, electrical conductivity, thermal conductivity, temperature, and power factor, respectively. Nonetheless, the interrelationships of these key parameters in a bulk material tend to offset one another making it difficult to improve ZT.2 Recent research in low-dimensional, especially one-dimensional (1-D) thermoelectric nanostructures, has invigorated the field by identifying quantum confinement, the energy filtering effect, and stronger phonon scattering effects to enhance S2σ and reduce κ.3-5 Quantum confinement shifts the Fermi level away from the conduction band, creating a greater energy difference between them thereby improving the power factor.6 Energy filtering enhances the average carrier energy by filtering out low energy carriers at grain boundaries or interfaces, thus increasing the Seebeck coefficient and optimizing the power factor.7 Stronger phonon scattering at the grain boundaries and interfaces is anticipated in 1-D nanostructures due to their larger surface-to-volume ratio, which can significantly decrease the thermal conductivity. Among 1-D thermoelectric materials, nanotubes benefit from their unique wall thickness, which provides an extra de-

gree of freedom for tuning and optimizing thermoelectric properties.8 Nevertheless, owing to the challenges in synthesis processes as well as device fabrication, only limited research has been performed on the thermoelectric properties of nanotubes. Silver telluride has three solid phases including Ag2Te, Ag1.9Te and Ag7Te4. Among them, Ag2Te and Ag7Te4 are thermally stable at room temperature.9 Ag2Te is a narrow-band gap semiconductor (i.e. ~ 0.2 eV10) that has drawn considerable attention due to its wide applications in electronic, optical, magnetic and thermoelectric devices.11-13 Especially for the thermoelectric application, the material possesses many unique advantages13 such as: (1) low thermal conductivity (close to the amorphous limit value) due to the vibration of the Te2- sublattices and the collision between randomly distributed Ag+ ion and Te2- sublattices;10, 14 (2) high electron mobility due to its low effective mass thereby high electrical conductivity;10 (3) tunable carrier concentration by doping; (4) a reversible crystal phase transition from the low-temperature monoclinic structure (β-Ag2Te) to the high-temperature fcc structure (αAg2Te) at about 403 K to 423 K.15 The β-Ag2Te is a narrow band gap semiconductor with high electron mobility and low lattice thermal conductivity, whereas the α-Ag2Te is a superionic conductor because Ag+ cations can easily move through the tellurium anion sublattices.16 Since the semiconductorsuperionic conductor phase transition has been reported to be a new way to alter the electric transport property across the phase transition, the crystal phase transition in Ag2Te may be an effective way to optimize its power factor.17 Ag7Te4 can be found in nature as mineral stuetzite and has been described variously as Ag3Te2, Ag12Te7, Ag5Te3, and Ag1.64Te.9 Howev-

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er, only limited work about Ag7Te4 and its electrical and thermoelectric properties has been reported. No electrical or thermoelectric properties of single crystal Ag2Te have been reported so far because the material experiences a crystal phase transition at a relatively low temperature, which introduces difficulties in the growth of single crystals.18 Almost all measurements are based on polycrystalline bulk samples, and the highest ZT reported is 0.6 at 576 K.19 Even through Ag2Te has a lower thermal conductivity than most state-of-the art thermoelectric materials, such as Bi2Te3 and PbTe, its relatively low band gap energy (~ 0.2 eV) gives a low Seebeck coefficient which prevents it from achieving high ZT.10 Therefore, several attempts have been made to enlarge its band gap through alloying with other foreign materials, such as PbTe10. However, few works have been done on the low-dimensional especially 1-D Ag2Te nanostructures13, 20, where quantum confinement, energy filtering and stronger phonon scattering may be triggered to enhance the overall thermoelectric performance. Several methods have been reported to synthesize 1-D AgxTey nanostructures. For example, Ag2Te nanofibers and nanorods were synthesized by a topochemical reaction using Te as self-sacrificing templates15, 21; Ag2Te nanotubes were prepared by the hydrothermal process with a rolling-up mechanism22; Ag2Te nanowires were synthesized by the compositehydroxide-mediated method23 and solvothermal co-reduction method24. In addition, heterojunction double dumb-bell Ag2Te-Te-Ag2Te nanowires were synthesized by a solution phase growth process with a post heat treatment25. Compared to Ag2Te, fabrication of 1-D Ag7Te4 nanostructures has been less reported and mainly though sonochemical synthesis26 and template-directed electrodeposition27. Electrospinning is based on the uniaxial stretching of a viscoelastic jet derived from a polymer solution or a melt by electrical force.28 Ultra-long polymer29, metal30, and metal oxide31 nanofibers with controllable diameters, compositions and structures can be fabricated by this technique in a simple and versatile way. Although various nanofibers have been synthesized using electrospinning, limited work has been reported on the synthesis of chalcogenide nanofibers due to the incompatibility of the precursor solutions. To address this issue, galvanic displacement reaction was applied as a post-electrospinning step to convert the electrospun materials to the desired hollow chalcogenide nanofibers. This method has been successfully exploited to synthesize Te32, BixTey33 and PbxSeyNiz34 hollow nanofibers in our group. In this work, the synthesis of AgxTey hollow nanofibers was first demonstrated by combining electrospinning with galvanic displacement reaction. Ni nanofibers with controlled dimension and morphology were synthesized by electrospinning to provide a large quantity of sacrificial materials for the subsequent galvanic displacement reaction for AgxTey. Composition, morphology and crystal structure of the AgxTey nanofibers were controlled simply by tailoring the Ag+ concentration in the displacement reaction. The temperature dependent electrical conductivity, Seebeck coefficient, and power factor of these nanofiber mats were characterized and correlated. EXPERIMENTAL SECTION Electrospinning of Ni nanofibers. The electrospinning procedure has been reported previously.34 An aqueous solution

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containing 2 g of nickel acetate (C4H6NiO4. 4H2O, 98%, Sigma-Aldrich) and 3 g of nanopure water was added into a polymer solution consisting of 10 wt. % of PVP (Polyvinylpyrrolidone, MW= 360,000 g/mol) in 9 g of anhydrous ethanol (Fisher Scientific, PA). The solution was stirred at 300 RPM for 1 h at 60 oC for a complete dissolution. The obtained clear solution was fed though a metallic needle with an inner diameter of 0.25 mm by a peristaltic pump at a constant feed-rate of 0.5 ml/h. Ten-kV was applied to the system by a high voltage power supply connected to the needle tip. The nanofibers were electrospun to a 3 cm by 3 cm Si/SiO2 chip attached to a grounded collector 10 cm away from the needle tip for 15 min. The collected nanofiber mats were first aged at 60 oC overnight followed by calcination at 500 oC in air at 3 oC/h for 3 hours to form NiO nanofibers. These oxides were then reduced at 400 oC in 5% H2/N2 at 5 oC/h for 3 hours to obtain Ni nanofibers. Finally, the metal nanofiber mats were cut into a rectangular shape with 1 cm in length and 0.2 cm in width for the consequent galvanic displacement reaction. GDR of AgxTey nanofiber mats. The galvanic displacement reaction of AgxTey was carried out by dipping a sacrificial Ni nanofiber mat into the electrolyte containing tellurium oxide (TeO2, 99+%, Acros Organic), silver nitrate (AgNO3, Certified ACS Plus, Fisher Chemical), and nitric acid (HNO3, Certified ACS Plus, Fisher Chemical) at room temperature for 1h. A 1 cm by 1 cm glass slide was used as a substrate and placed underneath the nanofiber mat in the solution. The effect of electrolyte concentration change on the mass transfer profile of the ions in the reaction was minimized by controlling the amount of sacrificial materials to be 10 % of the necessary quantity for a complete consumption of chalcogen ions present in the electrolytes. The effects of electrolyte concentrations on the morphology, composition, dimension, and crystal structure of the hollow AgxTey nanofiber mats were investigated by varying the concentrations of Ag+ from 5 µM to 10 mM. After the reaction, the nanofiber mats were washed by nanopure water three times and air dried. Material characterization. Morphology, composition, crystal structure and crystallinity of the obtained nanofibers was confirmed using emission-scanning electron microscopy (SEM, FEG-Philips XL30) with energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD, D8 Advance Diffractometer, Bruker) and transmission electron microscopy (TEM, JEOL JEM-2100F). The Ni content residue was also analyzed by inductively coupled plasma mass spectrometry (ICP-MS). Electrical and thermoelectric property characterizations. Electrical contacts were obtained by sputtering four Pt electrodes with both the gap width and the electrode size of 1 mm onto the nanofiber mats using a homemade shadow mask. Electrical conductivity and Seebeck coefficients were measured by a homemade system at a temperature range from 300 K to 340 K. This system was calibrated by a standard single crystal Bi2Te3 bulk material.

RESULTS AND DISCUSSION Synthesis and material characterization of AgxTey hollow nanofibers. Ni was chosen as the sacrificial material to form AgxTey nanofibers due to the more negative cathodic


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standard reduction potential of Ni2+/Ni pair (-0.257 V vs. standard hydrogen electrode, SHE) than that of the Ag+/Ag (0.799 V vs. SHE), and HTeO2+/Te (0.551 V vs. SHE) pairs. The electrospun Ni nanofibers are shown in Figure S.1c, with an average diameters of 152 nm. Mixing of the Ni nanofibers and the acidic electrolyte that contains Ag+ and HTeO2+ led to the oxidation of Ni to Ni2+ (Equation 1) as well as the deposition of either Te (Equation 2) or Ag (Equation 3) or both. Then the produced Te and Ag atoms react with each other to form Ag7Te4 (Equation 4) or Ag2Te (Equation 5), depending on their atomic percentage.27 Negative Gibbs free energy in both equations indicates that these reactions are spontaneous.9 The overall reactions for the deposition of AgxTey are shown in Equation 6 and 7. The deposition mechanism of AgxTey may be different from that of BixTey or PbxSey.33, 34 It is believed that the deposition of BixTey or PbxSey begins with the deposition of Te followed by an underpotential deposition (UPD) of Bi or Pb to form BixTey or PbxSey. However, due to the high redox potential of Ag, deposition of AgxTey is a codeposition process of Ag and Te. 2 →

E0=-0.257 V vs. SHE (1)

4 3 → 2 E0 = 0.551 V vs. SHE (2) →

E0 = 0.779 V vs. SHE (3)

7 4 → !

∆Gf0=-114.0 kJ/mol

(4)

2 →

∆Gf0= - 45.4 kJ/mol

(5)

14 8 24 23 → 2 ! 16 23

(6)

2 3 3 → 2 3

(7)

Control over the morphologies (Figure 1) and compositions (Figure 2) of the AgxTey nanofibers was achieved by varying the concentration of Ag+ in the electrolytes from 5 µM to 10 mM. Branched, continuous nanofibers were obtained at a low Ag+ concentration (i.e. 0.01 mM, Figure 1a). These nanofibers are expected to have a high Te content since their morphology is similar to that of pure Te nanofibers reported previously.32 The growth of the branched structures may be attributed to the limited mass transfer of HTeO2+ ions, leading to the preferential deposition in the (0 0 1) direction.32 Nodular nanofibers (Figure 1b, c) were observed with higher Ag+ concentrations of 0.1 and 0.2 mM, while dendritic pine leaf-like structures (Figure 1d) were obtained at the highest concentrations of Ag+ (i.e. 1 and 10 mM).

Figure 1. SEM images (inserted low magnification pictures) of synthesized (a-c) AgxTey hollow nanofibers and (d) dendritic structures by using Ni as a sacrificial material. The electrolytes contain a fixed concentration of 1 mM HTeO2+ and 1 M HNO3 with various concentration of Ag+ of (a) 0.01 mM, (b) 0.1 mM, (c) 0.2 mM and (d) 1 mM. The reactions were conducted at room temperature for 1 h.

The composition change as a function of the Ag+ concentration is shown in Figure 2. The Ag content increased from 5 to 100 at.% when the concentration of Ag+ increased from 0.005 to 1 mM. The Ag content saturated at 100 at. % with a further increase of Ag+ concentration to 10 mM. Near-stoichiometric nanofibers were obtained with 0.1 mM Ag+. Significant competition between the Ag/Ag+ and Te/HTeO2+ deposition since both the deposition consumes the same sacrificial materials. Increasing the concentration of Ag+ increased the Ag deposition which diminish the Te deposition since Ag/Ag+ (Eo = 0.779 V vs SHE) is more noble than Te/HTeO2+ (Eo = 0.551 V vs SHE). At low concentrations of Ag+ (i.e., 0.005 to 0.1 mM), the GDR seems to be dominated by the deposition of Te because the nanofibers were Te-rich.” “These nanofibers are expected to have a high Te content since their morphology is similar to that of pure Te nanofibers reported previously.32 The growth of the branched structures may be attributed to the limited mass transfer of HTeO2+ ions, leading to the preferential deposition in the (0 0 1) direction.32” When the concentration of Ag+ is between 0.1 and 0.2 mM, the nanofiber has a composition similar to Ag2Te where compounds (i.e., Ag7Te4 and Ag2Te) formation is preferred over elemental constituents (i.e., Ag and Te) which may resulted in nodular growth. When the concentration of Ag+ was further increased to 1 and 10 mM, no Te deposition may occur since only pure Ag dendrites were obtained. The formation of Ag dendrites consisting of planar, highly symmetrical branches and leaves by GDR have also reported by several groups.35, 36” The nickel residue after GDR was determined by inductively coupled plasma mass spectrometry (ICP-MS). The residue nickel content ranges from 0.2 to 1.8 wt. % independent of electrolyte composition which is significantly lower to GDR PbSe nanofibers34. The standard redox potential of Ag+/Ag pair (0.799 V vs. SHE) is more positive than HTeO2+/Te pair (0.551 V vs. SHE). This means that the deposition of Ag is thermodynamically preferred than that of Te. This theoretical predication is confirmed by the deposition of only pure Ag but no Te in the



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electrolyte that contains the same concentration of Ag+ and HTeO2+ of 1 mM. The deposition of Ag+ is preferred than other metallic ions, such as Bi3+ and Pb2+, since no overpotential deposition (OPD) of metal was observed either in the deposition of BixTey or PbxSey.32, 33 In addition, the higher deposition content of Ag (100 at.%) than that of Bi (21 at.%) from the same metal ion concentration also indicates the preference of Ag deposition over Bi.33

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dominant phase co-existing with Ag2Te. Only Ag peaks were obtained in the Ag nanostructures (Figure 3e).

Figure 2. Effect of [Ag+] on the Ag content in the AgxTey nanofibers.

Figure 3. XRD patterns of synthesized (a) Ag13Te87, (b) Ag38Te62, (c) Ag64Te36, (d)Ag73Te27 hollow nanofibers and (e) Ag nanostructures. Te peaks are indicated in red open circle referred to JCPD card #36-1452, Ag7Te4 peaks are labeled in blue circle referred to JCPD card #18-1187, Ag2Te peaks are marked in green triangle referred to JCPD card #00-034-0142, Ag peaks are labeled in orange rectangular referred to JCPD card #01-0040783.

XRD characterizations of AgxTey nanofiber mats with various Ag content are shown in Figure 3. Ag7Te4 and Te peaks can be identified from both Ag13Te87 (Figure 3a) and Ag38Te62 (Figure 3b) nanofibers. Since a stronger peak intensity was observed at 29.6o in Ag38Te62, a higher Ag7Te4 or Ag content is expected in Ag38Te62 than that in Ag13Te87, which is consistent with the EDS analysis. No Ag2Te or Ag peaks were observed in these Te-rich AgxTey samples. Therefore, it can be predicted that the Ag13Te87 nanofibers may actually contain 3 molar % Ag7Te4 and 97 molar % Te, and the Ag38Te62 sample may be the mixture of 12 molar % of Ag7Te4 and 88 molar % Te. These Ag7Te4 structures may present themselves as nanoinclusion in the Te matric. All the peaks of the near stoichimetric Ag64Te36 nanofibers (Figure 3c) belong to Ag2Te. No preferred orientation was detected and the sample is polycrystalline in nature. According to the phase diagram, Ag7Te4 is a stable phase in a Te-rich Ag-Te system at room temperature. Phase segregation to Ag7Te4 and Te should be observed at any Ag content lower than 66.7 at.%. However, only Ag2Te phase (with no Ag7Te4) was detected in our nanofibers even with 64 at. % Ag. This may be due to the imprecise analysis of the Ag content by EDS characterization, since over 2 at.% error can be created especially on a porous and uniform sample. No XRD peaks arising from Te could be observed in the sample even though it is slightly rich in Te. This result may be additional evidence showing the imprecise composition analysis by EDS, or it may be attributed to the small content of Te compared to the high signal-to-noise level resulting from the low intensity of XRD signal. When the Ag content further increased to 73 at.%, Ag element peaks were detected in addition to Ag2Te, indicating that the sample contains two mixed phases of 59 molar % of Ag2Te and 41 molar % of Ag. The significant molar percentage of Ag in Ag73Te27 may no longer act as nanoinclusions in the Ag2Te matrix but act as another

More detailed material properties were characterized by transmission electron microscopy (TEM). Figure 4 shows the TEM analysis of a nanofiber synthesized by the electrolyte containing 0.1 mM Ag2+ and 1 mM HTeO2+ in 1 M HNO3. The as-formed nanofiber (Figure 4a) shows a nodular surface and hollow structure. High resolution TEM (HR-TEM) images with fast Fourier transform (FFT)-converted selected area electron diffraction (SAED) patterns (Figure 4b) indicated the distances between lattice planes are 0.317, 0.245, and 0.203 nm, which confirmed the formation of monoclinic Ag2Te with crystal orientation of (1 2 1), (2 0 2), and (-4 0 2) direction. The line-scan EDS (Figure 4c) further confirms the nanofiber’s hollow structure by showing a greater Ag and Te intensity at the edges of the nanofiber. The EDS spectrum (Figure 4d) identified the main elemental components as Ag and Te. The composition of the nanofiber was found to be Ag64Te36, which is consistent with the EDS analysis in the SEM characterization. The nanofibers’ dimensions including outer diameter, inner diameter and wall-thickness were measured based on brightfield TEM images and shown in Figure 4. Examples of the bright-field TEM images of Ag38Te62, Ag63Te37, and Ag72Te28 nanofibers are shown in Figure S.2. Dimension of Te (Ag% = 0) nanofibers were adapted from reference32 and were also included in the figure. The outer diameter, inner diameter and wall-thickness of the nanofibers increased with the Ag content (Figure 7a). The outer diameter (blue triangle) increased by 91 % from 137 ± 8 nm to 262 ± 23 nm and the inner diameter (black rectangular) increased by 40 % from 94 ±12 nm to 132 ± 20 nm. It is known that the average outer diameter of Ni nanofibers is 152 nm. However, the inner diameters of resulting Te and AgxTey nanofibers are even smaller than the Ni sacrificial material, which may be attributing to the continuous shrinking of the template nanofibers during GDR. The smaller


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expansion in the fibers’ inner diameter than the outer diameter results in an increase in the wall thickness. As shown in Figure 5a, the wall thickness increased from 22 ± 4 nm to 67 ± 12 nm.

Figure 4. (a) TEM (inserted low magnification pictures), (b) HRTEM image (Inserted FTT image) (c) Line-scan EDS and (d) EDS analysis of Ag64Te36 hollow nanofibers. The electrolyte consisted of 0.1 mM Ag+, 1 mM HTeO2+, and 1 M HNO3 at room temperature.

The formation of Te and Ag phase in Ag7Te4 and Ag2Te systems are also confirmed by the HRTEM analysis (Figure S.3 and S.4) As shown in Figure S.3, the Te-rich AgxTey (i.e., Ag38Te62) nanofiber contains three phases including Ag2Te, Ag7Te4, and elemental Te phases which is consisted with XRD analysis (Figure 3). This observation aligned with literature works where Te segregated from silver telluride phases. The Ag-rich AgxTey (i.e., Ag73Te27) nanofibers show the formation of elemental Ag phase (Figure S.4) in addition to Ag7Te4 and Ag2Te which is also consistence with the XRD analysis (Figure 3). The observation of elemental Ag or Te phases as a function of the chemical composition of AgxTey represents the formation of interfaces among Ag/Te phase and Ag2Te and Ag7Te4 phases. This detail crystallographic analysis provides the extra evidence to support the energy barrier height dependent electrical conductivity behavior of the nanofibers.

Figure 5. (a) Effect of Ag content on wall-thickness (red circle), inner diameter (black rectangular), and outer diameter (blue trian-

gle) of AgxTey nanofibers. (b) Effect of Ag content on volume expansion (VAgxTey/VNi) of AgxTey hollow nanofibers. The red lines are calculated values. The electrolytes contain a fixed concentration of 1 mM HTeO2+ and 1 M HNO3 with various concentration of Ag+ of 0, 0.05 mM, 0.1 mM, and 0.2 mM. Data of Te Nanofibers with 0% Ag is referred to Park’s work.

Both the experimental and theoretical volume ratio of AgxTey to Ni were plotted against the Ag content (Figure 5b). The experimental data are calculated from the dimensions of these four samples shown in Figure 5a, and the theoretical volume ratio was calculated based on an assumption of 100 % GDR efficiency. Since the deposition of Ag-rich and Te-rich AgxTey requires different amounts of electrons thereby consuming different amounts of Ni, two calculated lines are shown in the figure. They are separated at the point of stoichiometric Ag content. The volume ratio increase with the Ag content, which may be due to the heavier molecular weight of silver chalcogenide than that of tellurium element. In addition, all the experimental data are smaller than the calculated data, especially for the nanofibers with a low Ag content (i.e. 0% and 38%). A similar phenomenon was observed in the deposition of PbxSeyNiz nanofibers, which can be attributed to the lower GDR efficiency in a real GDR system due to the hydrogen gas evolution reaction (HER). When the Ag content increased, the difference between the experimental and theoretical data became smaller, which may indicate a higher reaction efficiency in the Ag deposition than that of Te. Overall, the results of Figure 1 through 5 show that the morphology, composition and crystal structure of AgxTey could be well controlled by tuning the concentration of Ag+ in the electrolytes. Electrical and thermoelectric properties of AgxTey nanofiber mats. Temperature-dependent I-V characterizations of various AgxTey nanofibers were conducted in a temperature range of 300 K to 340 K. Figure 6a shows typical I-V curves of a Ag64Te36 nanofiber mat. Linear I-V curves are observed in the temperature range indicating an ohmic contact between mat and electrodes. The temperature-dependent electrical conductivity (σ) of the same sample was calculated and shown in Figure 6b, with the plot of nature log of σT0.5 verse of 1/kT inserted. The nanofiber mat shows a typical semiconductor behavior since the electrical conductivity increased linearly when the temperature increased. If we assume that the electrical transport properties of these nanofiber mats are governed by carrier trapping at the grain boundary or the interfaces between the two materials, the temperature dependent electrical conductivity should follow Equation (8)3, 37: = % &

'

(∗ *+,

'⁄

exp 1

23

+,

4

(8)

where σ is electrical conductivity, L is grain size, q is element charge, p is average carrier concentration, m* is effective mass, k is Boltzmann’s constant, T is absolute temperature and Eb is barrier height at the grain boundary or the interfaces. Therefore, the plot of ln(σT0.5) vs. 1/kT should be a straight line with – Eb as its slope3. By fitting the inserted plot in Figure 6b, a barrier height of 240 meV was yielded. The barrier height of different samples were calculated and plotted against the Ag content (Figure 6c). The Te-rich or near-stoichiometric AgxTey nanofibers have a higher barrier height than the Ag-rich sample. Ag38Te62 has the highest barri-



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er height of 0.3 eV and Ag72Te28 has the lowest barrier height of 0.054 eV. In a bi-phase system, the barrier height can by affect by several parameters including the electron affinities, working functions, as well as the volume ratio, size, and shape of the two mixed phases.38 Higher Eb in Ag38Te62 than Ag13Te87 may be attributed to the larger amount of Ag7Te4 inclusion in the Ag38Te62 sample. However, a higher Eb was also found in Ag5Te95 than Ag13Te87, even though the former has a less amount of Ag7Te4 inclusion. This may be attributed to the higher interface potential at the junctions of branches and chunks in Ag13Te87 nanofibers (Figure 1a). Ag-rich Ag72Te28 nanofibers have the lowest barrier height. This may be attributed to the disordered structure of the Ag atoms in the lattice of Ag2Te due to the high mobility of Ag atoms. However, owing to the limited study on the electron affinity and working function of Ag2Te and Ag7Te4, the theoretical band bending and interfacial potential in Ag7Te4-Te and Ag2Te-Ag systems are not clear.

modulation in the energy barrier height will cause an exponential change in materials’ conductance. In the case of AgxTey nanofiber, the electrical conductivity increased by more than 3 orders with a decrease of energy barrier height by 6 folds. The Seebeck coefficients of the nanofiber mats with various compositions were measured in a homemade system in vacuum in a temperature range of 300 K to 340 K. A temperature difference of -1 ~ 6 K was maintained across the length of the samples and the as-generated open circuit voltage was recorded. The linear relationship between the voltage and the applied temperature gradient was observed over the entire temperature range, resulting in the Seebeck coefficient by fitting the slope. Figure 7a shows ∆V vs. ∆T characterization of Ag64Te36 nanofibers. Positive slopes indicate positive Seebeck coefficients and the p-type nature of this sample. This is consistent with the p-type nature of Te-rich Ag2Te. The temperaturedependent Seebeck coefficients of Ag64Te36 nanofibers are showed in Figure 7b. The Seebeck coefficient gradually increased in the temperature range of 300 K to 320 K, followed by a sharp decrease when the temperature exceeded 320 K. The Seebeck coefficient in two-type carrier transport, where both electrons and holes contribute, given by39 S=

Figure 6. Temperature dependent (a) I-V characterization, (b) electrical conductivity of Ag64Te36 nanofiber mat (Ln(σT0.5) as a function of 1/kT is inserted). (c) Energy barrier height Eb as a function of Ag content. Electrical conductivity of AgxTey nanofiber mats as functions of (d) Ag content and (e) energy barrier height Eb.

The electrical conductivity as a function of Ag content is shown in Figure 6d. Generally, the Te-rich (i.e. Ag5Te95, Ag13Te87, Ag38Te62) and the near stoichiometric (i.e. Ag64Te36) AgxTey nanofibers have lower electrical conductivity compared to that of Ag-rich sample (i.e. Ag72Te28). The highest electrical conductivity was found in Ag72Te28 and the lowest one was found in Ag38Te62. This trend is opposite to that in Figure 6c, which indicates the inverse relationship between electrical conductivity and energy barrier height (Figure 6d). Dominated by a thermionic emission transport mechanism, electrical conductivity is exponentially proportional to barrier height at a fixed temperature (Equation 8). Thus, a small

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67 87 69 89 87 89

(9)

Where S is Seebeck coefficient, Se and Sh are the Seebeck coefficient contributed by electrons and holes, respectively, and σe and σh are electrical conductivity contributed by electrons and holes, respectively. The partial Seebeck coefficients of electrons and holes have opposite sign, therefore the magnitude of total Seebeck coefficient becomes smaller than the absolute number of those partial values. The Seebeck coefficient increases with temperature because Fermi level moves away from the edge of the band, giving a larger difference between the average energy and the the Fermi level.39 However, when the temperaure is over a certain value, a rapid decrease in the Seebeck coefficient can be observed due to the higher mobility of electrons than holes. This will increase the absolute value of the : : term in Equation 9, thereby decrease the total Seebeck coefficient. The onset temperature for Ag64Te36 nanofibers seems to be 320 K, above which the Seebeck coefficient drops thereafter. The Seebeck coefficients of AgxTey nanofiber mats with various Ag contents were measured and the number at 33 oC was plotted against the Ag content (Figure 9c). Nanofibers with a low Ag content (< 67 at.%) were p-type while the nanofibers with a high Ag content (> 67 at.%) were n-type. The ptype characteristic may be attributed to the p-type nature of both Te and Ag7Te4 phases as well as Te-rich Ag2Te, and the n-type characteristic may be attributed to the n-type nature of Ag-rich Ag2Te. The absolute number of the Seebeck coefficient decreased with increasing Ag content, with the highest value of 589 µV/K from the Ag5Te95 sample. This is the first time reporting the Seebeck coefficient of Ag7Te4 systems, showing absolute values higher than that of the Ag2Te systems (-75.6 µV/K).


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Figure 7. Temperature dependent (a) ∆V-∆T characterization, (b) Seebeck coefficient of Ag64Te36 nanofiber mat. (c) Seebeck coefficient of AgxTey as functions of (c) Ag content and (d) energy barrier height Eb.

The scattering of carriers due to energy band bending at a grain boundary or at an interface between two materials strongly depends on the carriers’ energy level. Charge carriers with energies greater than the barrier height (eV) can transport through the material while the ones with lower energy can’t. The barrier energy thus acts as a filter that filters out the low energy carriers-thus increasing the average carrier energy. Since the Seebeck coefficient is directly proportional to the difference in the average energy of the charge carriers and the Fermi level, increasing the average charge energy directly increases the Seebeck coefficient. This phenomenon is known as the energy filtering effect, which has been proved to be one of the key strategies to improve materials’ thermoelectric properties. The absolute value of the Seebeck coefficient as a function of energy barrier height is shown in Figure 9d. Higher Seebeck coefficients were obtained from samples displaying the higher barrier heights, which is consistent with the theoretical prediction.38

Figure 8. Power factor of AgxTey nanofiber mats as functions of (a) Ag content and (b) energy barrier height Eb.

The power factor of AgxTey nanofiber mats as functions of Ag content is shown in Figure 8a. The power factor stays below 1 µW/K2m when the Ag content is less than 67 at. %. A dramatic increase in the power factor was observed in Ag72Te28, showing a value about 30 times higher than that of the near-stoichiometric nanofibers. This may be attributed to the significantly higher electrical conductivity of Ag72Te28 nanofibers than the other fibers (by 2-3 orders), which can

complement its relatively low Seebeck coefficient. Figure 8b shows that the power factor decreased with the barrier height. The highest power factor of 3.94 µW/K2m was obtained with the lowest barrier height of 0.054 eV. With a proper barrier height, the power factor can be optimized by the energy filtering effect because the Seebeck coefficient can be greatly improved without significantly suppressing the electrical conductivity. Several research studies based on inorganic and organic thermoelectric material have suggested that the optimized barrier height ranges from 0.05 eV to 0.1 eV.9, 40, 41 Our lowest barrier height lays in this window and gives the highest power factor. These results also suggest that the Ag7Te4/Te system may not be competitive with the Ag2Te/Ag system at roomtemperature due to the large energy barrier height and the low power factor. The power factor of this work is much lower than other reported 1-D Ag2Te nanostructures.3, 5, 13 This may be due to first the under estimation of the fiber mats’ electrical conductivity. Since these nanofibers are randomly orientated, the fiber mats possess a large porosity and significant uniformity. However, the calculation of their electrical conductivity was based on the assumption of a dense and homogeneous film, which brings substantial error to the calculation. It has been experimentally proven that the electrical conductivity of a single PbTe nanofiber can be 2-3 orders higher than that of the nanofiber mat, mainly due to an unsecured electrical contact between the fibers and a significant contact resistance between the fibers and electrodes. (unpubilished work) The low power factor in this work can also be attributed to the much lower packing density of our nanofiber sample than other hot pressed nanostructured pellets.3, 5, 13 In our samples, charge can mainly transport through 1-D channels along the nanofibers, which will significantly reduce the effective conducting channels thereby the electrical conductivity and power factor.42 CONCLUSIONS Ultra-long AgxTey nanofibers were synthesized by combining electrospinning and the galvanic displacement reaction. Control over the morphology, composition and crystal structure of the nanofibers was achieved by adjusting the Ag+ concentration in the electrolytes. The Ag content of the nanofibers increased with Ag+ concentration in the electrolytes, with Terich branched AgxTey nanofibers synthesized at low Ag+ concentrations and Ag-rich nodular AgxTey nanofibers obtained at high Ag+ concentrations. Clear phase separation was observed within the samples. The Te-rich AgxTey samples consist of Ag7Te4 and Te phases, while the Ag-rich fibers are a mixture phase of Ag2Te and Ag. Ag dendritics were observed at the highest Ag+ concentration. Temperature dependent electrical conductivity and Seebeck coefficients of various AgxTey nanofibers were characterized. Nanofibers with a low Ag content (i.e. < 66.7 %) possessed ptype transport properties, while the ones with a high Ag content (i.e. > 66.7 %) expressed n-type semiconductor behaviors. The p-type characteristics can be attributed to the p-type nature of both Ag7Te4 and Te, while the n-type nature of Ag2Te may contribute to the n-type transport behavior of the composite fibers. The Ag7Te4/Te system may not be competitive with Ag2Te/Ag system at room-temperature due to its lower power factor. Energy barrier height was calculated and shown to have a significant effect on both the materials’ electrical conductivity and Seebeck coefficient: higher barrier height will lead to a



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lower electrical conductivity and a higher Seebeck coefficient. The highest power factor was found in the nanofibers with an energy barrier height of 0.054 eV.

ASSOCIATED CONTENT Supporting Information: SEM images of electrospun PVP/Ni acetate, NiO and Ni nanofibers, TEM and HRTEM images, SAED of AgxTey nanofibers. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Nosang V. Myung. Electronic mail: [email protected], phone: 1-951-827-7710, Department of Chemical and Environmental Engineering at University of California-Riverside, Bourns Hall B353, 900 University Avenue, Riverside, CA 92521

ACKNOWLEDGMENT This work was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Trade, Industry & Energy, Republic of Korea (#10050890) and Semiconductor Research Corporation (SRC).

ABBREVIATIONS GDR, galvanic displacement reaction.

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