ARTICLE pubs.acs.org/JPCC
Tellurization: An Alternative Strategy to Construct Thermoelectric Bi2Te3 Films Zhengliang Sun,†,‡ Shengcong Liufu,† Xihong Chen,*,† and Lidong Chen† †
CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Science, 1295 Dingxi Road, Shanghai 200050, P.R. China ‡ Graduate School of the Chinese Academy of Science, 19 Yuquan Road, Beijing 100049, P.R. China
bS Supporting Information ABSTRACT: An alternative tellurization strategy has been introduced into the construction of thermoelectric Bi2Te3 films by a facile chemical vapor transport process, in which presynthesized bismuth films made by a solution route were used as the platform. The synthesized Bi2Te3 films are highly crystallized with (00l) orientation, which is attributed to embedded (00l) oriented nanoplates. On the basis of the experiment phenomena, a possible mechanism was proposed for the nucleation and growth of bismuth telluride. The thickness of the embedded nanoplates can be easily adjusted by changing the reaction temperature in the chemical vapor transport process. N-type conduction behavior as well as the adjustable transport properties for the Bi2Te3 films were discussed in detail. The optimized power factor reaches 20.4 μW cm1 K1 at room temperature, which is 1 order higher than the value of the solution-based films, and even comparable to the value of the vacuum-based Bi2Te3 films.
1. INTRODUCTION Novel film growth processes have been of considerable interest in the area of thermoelectrics for both the chemists and materialists.13 According to theoretical prediction, the conversion efficiency can be greatly enhanced through introducing the “nano” technology into thermoelectric (TE) materials. A depression of thermal conductivity (k) due to grain boundary scattering and/or an increase of Seebeck coefficient (S) caused by quantum effects without deteriorating the electrical conductivity (σ) in nanosized and/or nanocomposites will contribute to the enhancement of the thermoelectric figure of merit, ZT = S2σT/k (T, absolute temperature).4,5 Theoretical investigation has suggested that low-dimensional materials, such as Bi2Te3 quantum well may exhibit ZT values much larger than 1.0 as a consequence of the modulated transport of the carrier and phonon by the above courses.68 These predictions have stimulated the development of the numerous methods to prepare Bi2Te3 films over the past decades. According to previous work, the transport properties of the Bi2Te3 films depend greatly on their crystallinity and microstructure. Good transport properties often go with delicate processes with some special conditions in previous reports.1,2 Special equipment and/or special starting materials based techniques, such as cosputtering,9 coevaporation,10,11 and MOCVD,12 provide considerable control on the material composition and phase profiles, which endow good transport properties. Recently, a new method was developed for the synthesis of Bi2Te3 films by drop-casting the presynthesized Bi2Te3 nanostructures, including nanoparticles,13 nanorods,14 and nanoplates.15 The merit of delicate control on the shape and the size r 2011 American Chemical Society
of nanostructures, as well as no limit for the substrate materials, are often enshrouded by poor electrical conductivity of the prepared Bi2Te3 films. Besides the above methods, electrodeposition seems to supply an excellent platform to construct Bi2Te3 films,16 but unfortunately, the introduction of indispensable metal substrate makes the measurement of the TE parameters more complicated. Thus, the development of a novel process for desirable materials without any complicated manipulation or high-cost procedure is still a challenge. Herein, we introduce an alternative strategy, “tellurization”, to grow nanostructured Bi2Te3 films by combining the solution route and the easily operated chemical vapor transport (CVT). As indicated in Scheme 1, first, Bi films were prepared on a silicon substrate by a facile redox solution route, in which bismuth nitrate, EDTA, and ascorbic acid were used as bismuth precursor, coordinative reagent, and reductive reagent, respectively, and then, the Bi films were transferred into a horizontal tube furnace and used as platforms for the preparation of Bi2Te3 films via tellurization in the CVT process. The introduction of Bi films simplifies the subsequent vapor process and cuts the cost significantly. As for the CVT process, typically, a single tellurium stream is demanded instead of the conventional double streams (bismuth and tellurium), and the duration in the CVT process is only about 30 min. High crystallization, obtained at relatively high temperature (285350 °C), oriented growth due to the Received: April 11, 2011 Revised: June 10, 2011 Published: July 14, 2011 16167
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Scheme 1. Sketch Map of the Reaction Process: First, Deposition of Bismuth Films by a Facile Redox Solution Route; Second, Transformation of Bismuth Films into Nanostructured Bi2Te3 Films by a Chemical Vapor Deposition Process (Scale Bar: 100 nm)
Figure 1. XRD patterns of as-prepared Bi (a) and Bi2Te3 (b) films, indicating the pure phase of both films. The relative strong (00l) peaks in the Bi2Te3 profile indicate the (00l) dominant orientation for the Bi2Te3 films.
intrinsic crystal structure of bismuth telluride during the CVT process will endow good transport properties for the present Bi2Te3 films. The nucleation and growth process as well as the transport properties were discussed in detail.
2. EXPERIMENTAL SECTION Synthesis of Bi Films. A typical precursor solution (50 mL) was composed of 0.2 mmol of Bi(NO3)3, 0.5 mmol of ethylenediaminetetraacetic acid (EDTA), and 0.5 mmol of ascorbic acid (AA). The pH value of the solution was adjusted to about 12 by the addition of a few drops of diluted ammonia. All of the reagents were of analytical grade and used without further purification. The Si substrates, after being oxidized by piranha solution, were immersed in the middle of the solution to favor the nucleation and growth of Bi films. The solution was kept covered with polyethylene film to prevent the evaporation of water and put in an aqueous bath with the temperature maintained at 70 °C for 12 h. At the end of the experiment, the films were removed from the bath and rinsed carefully with deionized water, and then dried naturally. Synthesis of Bi2Te3 Films. The above Bi films were transferred into a horizontal tube furnace and placed in the position about 10 cm downstream from the center, where the tellurium powder was placed (tellurium: 99.999%, Alfa Aesar) to construct the Bi2Te3 films. Then, the furnace was heated up to ∼400 °C at a rate of 10 °C/min with a continuous flow of 95% Ar/5% H2 (cleanout and carrier gas). The tellurium vapor was formed at 400 °C and transported to the Bi films by the carrier gas with a flow rate of 40 sccm, where the corresponding temperature is 350 °C, and reacted with bismuth to produce the nanostructured Bi2Te3 films. Before starting the CVT process, the quartz tube was evacuated to about 104 bar and then flushed with the carrier gas three times to prevent possible oxidation of the films, and during the CVT process, the pressure in the tube was kept at ∼104 bar by continuous pumping. Film Characterization. The crystalline structures of the films were analyzed by an X-ray diffractometer (XRD; Rigaku RINT 2000) operating with Cu KR radiation at 40 kV/100 mA. The morphology of the samples was analyzed by scanning electron
Figure 2. SEM images of the Bi films (a, the inset is the corresponding cross-sectional profile) and EDX spectrum of a Bi nanoparticle (b, the insets are the TEM image and SAED pattern of a Bi nanoparticle).
microscopy (JSM-6390). The synthesized film was scraped from the Si substrate and dispersed onto a carbon coated Cu grid, and then placed into a JEOL200CX instrument for phase and composition analysis (including TEM, HRTEM, SAED, and EDX). The dark electrical resistivity and the carrier concentration of films were measured by a four-point probe method in van der Pauw configuration with an Accent HL5500 Hall System. Silver paste was applied to provide ohmic contact with the Bi2Te3 films. For the Seebeck coefficient measurement, two PtPt/Rh thermocouples were attached to both ends of the thin films, and temperature gradients of 35 K were generated by a film heater at one end. The Seebeck coefficient was then obtained from the slope of thermoelectromotive force to the temperature difference.
3. RESULTS AND DISCUSSION As a typical XRD result indicated in Figure 1a, the films obtained by the solution route displayed a pure rhombehedral phase of bismuth with lattice constants of a = b = 4.535 Å, c = 11.834 Å, which are consistent with the literature values (a = b = 4.547 Å, c = 11.862 Å, JCPDS 44-1246). The Bi films (with a thickness of ∼2 μm, inset in Figure 2a) are constructed by islandlike structures with nanoparticle constituents about of 50100 nm (Figure 2a). The EDX spectrum (Figure 2b) confirms the composition of pure element bismuth, and the SAED pattern indicates the single crystal of the nanoparticle. Island-like structures as well as the high activity of the nanoscaled particles make the Bi films excellent platforms for the synthesis of Bi2Te3 films. In the present work, a low pressure CVT process (∼104 bar) 16168
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Figure 3. SEM image of the Bi2Te3 films obtained at 350 °C (a) and TEM images of the Bi2Te3 nanoplate (b, the inset is the corresponding SAED pattern).
Figure 4. Surface and cross-sectional SEM images obtained at 350 °C (sample S1, a, d), 320 °C (sample S2, b, e), 285 °C (sample S3, e, f). The thickness of the embedded nanoplates is ∼80 nm for S1, ∼150 nm for S2, and ∼220 nm for S3.
was chosen to realize “tellurization” for the above Bi films, in which the chemical process at the surface of the wafers can be controlled by the reaction rate rather than the mass transfer process.17 The rate controlled CVT process will facilitate the heterogeneous nucleation and growth of the bismuth telluride. After the “tellurization” process, the phase of the films has been transformed into hexagonal with the Bi2Te3 phase (Figure 1b) having lattice constants of a = b = 4.385 Å, c = 30.445 Å, which are consistent with the literature values (a = b = 4.385 Å, c = 30.483 Å, JCPDS 15-0863). The dominant 006 and 0015 diffraction peaks suggest that the Bi2Te3 films have a preferred orientation along the (00l) direction. The SEM results (Figures 3a and 4a,d) suggest that the as-prepared Bi2Te3 films (obtained at 350 °C, with a thickness of ∼0.9 μm) are made up of platelet-like crystals 300500 nm in length and ∼80 nm in thickness. The SAED pattern suggests the single crystal characteristic of the nanoplate with (00l) orientation (Figure 3b), which is responsible for the (00l) orientation of the films. The formation of the (00l) orientations of nanoplates might be attributed to the intrinsic anisotropic structure of hexagonal Bi2Te3. The Bi2Te3 crystal is composed of repeated sets along the c-axis, and each set consists of five layers of atoms (Te1BiTe2BiTe1). The bonding within the Te1BiTe2BiTe1 layer is considered to be covalent, while the bonding between the sets is the van der Waals force. Due to this anisotropic bonding environment, the rate of
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crystal growth along the (00l) plane should be much greater than that in the perpendicular direction. The intrinsic crystal properties dominate the shape and orientation of Bi2Te3. Furthermore, the (00l) orientation was also acknowledged to facilitate the transport of the carriers (for the present films, electrons are the dominating carriers, determined by the negative Seebeck coefficient).18,19 As for the nucleation and growth of bismuth telluride, the position of bismuth films, namely, the temperature of wafers, played a vital role in the tellurization process. If the bismuth films were placed about 18 cm downstream from the center, where the corresponding temperature was ∼250 °C, only mixed products of Bi, Bi2Te3, and Te (Figure S1, Supporting Information) were obtained even by prolonging the reaction duration (∼5 h). On the other hand, when the bismuth films were placed about 10, 14, and 16 cm downstream from the center, where the corresponding temperatures were about 350 (sample S1), 320 (sample S2, with a thickness of 1.2 μm from Figure 4e), and 285 °C (sample S3, with a thickness of 1.2 μm from Figure 4f), films with a pure hexagonal Bi2Te3 phase could be obtained. Differently, the thickness of the nanoplates embedded in the films decreased along with the increase of the reaction temperature (Figure 4). The pure Bi2Te3 phase was only obtained above the melting point of bismuth (∼271 °C), suggesting the determinative role of the formation of the melting bismuth. On the basis of these experiment phenomena, a possible nucleation and growth mechanism was proposed. First, Te vapor was formed at the center of the tube and transported to the surface of the Bi films. Second, Te vapor was adsorbed on melting Bi islands and then BiTe eutectic alloys were formed, during which the transformation of the Bi crystal structure happened due to the diffusion between Bi and Te; this can explain the morphology change between Bi and Bi2Te3 films (Figures 2 and 3). The last stage is the nucleation of Bi2Te3 and the formation of Bi2Te3 plates according to the lowest-energy principle. At higher reaction temperature (350 °C), the vapor pressure of tellurium is higher, which suggests a larger amount of Te vapor is absorbed into the Bi liquid islands, which results in a larger amount of nuclei. Higher temperature results in a more uniform and more compacted microstructure20 (Figure 4). As a result, the thickness of nanoplates formed at higher temperature is thinner than the thickness of nanoplates formed at lower temperature. Thermoelectric properties of the above Bi2Te3 films are also characterized to evaluate the merit of the present “tellurization” process. As indicated in Table 1, the electrical resistivity of the Bi2Te3 films is in the range 0.001950.00261 Ω cm, which is much lower than those TE films obtained by a wet chemical process.1316 The much lower resistivity is mainly a benefit from the clean surface without capping of insulating ligands. The S value is negative, indicating n-type conduction, which is expected for Bi2Te3 films.21,22 It should be pointed out that the value and the sign (p or n character) of the Seebeck coefficient for bulk Bi2Te3 have been reported to vary depending on the crystal nonstoichiometry (Bi or Te excess).2325 However, when deposited in the form of thin film, the reported Bi2Te3 is always to be n-type, which is also confirmed in our results. As the recent result indicated in ref 25, n-type conduction behavior is dominative with both Bi and Te excess (3845%Bi). Here, we propose a plausible mechanism for the formation of n-type conduction behavior in the film form of bismuth telluride. It is suggested that two main structure defects (vacancy, antisite defects) are responsible for the dominant conduction behavior. 16169
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Table 1. Experiment Parameters and the Electric Transport Properties of the Bi2Te3 Films at Room Temperature
a
sample
Tsub (°C)
Bi/Tea (atomic)
n (1019 cm3)
μ (104 m2/(V s))
S (μV/K)
F (Ω cm)
PF (μW/(K2 cm))
S1
350
40.5/59.5
2.1
135
212
0.0022
20.4
S2
320
40.6/59.4
2.1
114
221
0.00261
18.7
S3
285
42.0/58.0
3.7
87
174
0.00195
15.5
EPMA results.
Figure 5. Temperature dependence of the electrical resistivity (b), Seebeck coefficient (0), and power factor (2) for sample S1 synthesized by the tellurization process.
Previous reports suggested that the formation of antisite defects dominated over the formation vacancies caused by the energy difference between the above mechanisms.26 With the decrease of the size, the formation energy of the vacancy will decrease caused by the increase of the surface to volume, especially in the nanoscaled structures.27 As in the form of thin films, the formation of vacancies will become prevalent, and the vacancy interacts with antisites to produce n-type conduction behavior:28,29 2V Bi 000 þ 3V Te •• þ BiTe 0 ¼ V Bi 000 þ BiBi x þ 4V Te •• þ 6e0
Comparing the electric properties of sample S1 with those of sample S2, an anomalous conduction behavior can be observed: larger embedded crystallite size results in larger electrical resistivity. This phenomenon might be related to the reaction temperature, as higher reaction temperature can enhance the contact between the particles, which will increase the carrier mobility.20 As a result, the higher mobility for sample S1 (135 104 m2/(V s)) contributes to the lower electrical resistivity. As for sample S3, because of the low amount of Te and low diffusion rate of bismuth and tellurium, the sample obtained at 285 °C contains a large amount of Te vacancies (Table 1). As a result, sample S3 displayed lower electrical resistivity and lower absolute value of Seebeck coefficient compared with samples S1 and S3 because of the higher carrier concentration (3.7 1019 cm3) for sample S3. At room temperature, the power factor reaches 20.4 μW cm1 K1 for sample S1. This value is among the best results obtained by either the solution-based or vacuumed-based techniques (Table S1, Supporting Information), indicating that the present facile and low-cost “tellurization” process is a promising route to construct the Bi2Te3 films. Furthermore, temperature dependent
transport properties for sample S1 are also illustrated in Figure 5. A linear increase of F with temperature is observed in Figure 5, suggesting the metallic conduction behavior of the present Bi2Te3 films. The absolute value of Seebeck coefficient initially increases and then decreases after reaching the maximum value of 277 μV K1 at 400 K. The shift of maximum S from room temperature for the bulk Bi2Te3 to 400 K might be related to the quantum confinement effect existing in the film that increases the difference between the Fermi level and the average mobile carrier energy.30 The calculated PF value displays similar temperature dependence to the Seebeck coefficient and reaches the maximum value of 31.3 μW cm1 K1 at 400 K. Furthermore, across a wide temperature range, the PF is above 20 μW cm1 K1 (an average PF of 24.7 μW cm1 K1 in the temperature range of 300500 K). This delightful result makes the present Bi2Te3 films much more practical in the microheating and cooling areas, as the TE materials are required to have a wide service temperature range.
4. CONCLUSION In summary, an alternative strategy of “tellurization” was established for the synthesis of thermoelectric Bi2Te3 films. The tellurization process was accomplished by a chemical vapor transport technique on presynthesized Bi films. The Bi2Te3 films were made up of plate-like crystals with (00l) dominant orientation. The temperature of the substrate plays a vital role in the nucleation and growth of the Bi2Te3 films, and the detailed process and corresponding transport properties were discussed. Furthermore, the optimized power factor of 20.4 μW cm1 K1 at room temperature and an average power factor of 24.7 μW cm1 K1 (300500 K) make the present tellurization process attractive in microheating and cooling applications. In 16170
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’ ASSOCIATED CONTENT
bS
Supporting Information. Figure showing the XRD pattern of the films obtained at 250 °C and table showing the room temperature thermoelectric properties of Bi2Te3 films prepared by other methods. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: +862152412520. Fax: +862152413122. E-mail: xhchen@ mail.sic.ac.cn.
’ ACKNOWLEDGMENT Financial support from the Science and Technology Commission of Shanghai Municipality (08DZ2210900), Program of Shanghai Subject Chief Scientist (No. 09XD1404400), and Shanghai Rising-Star Program (No. 10QA1407700) is gratefully acknowledged.
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