Cellulose Fiber-Based Hierarchical Porous Bismuth Telluride for High

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Cellulose Fiber-based Hierarchical Porous Bismuth Telluride for High-Performance Flexible and Tailorable Thermoelectrics Qun Jin, Wenbo Shi, Yang Zhao, Jixiang Qiao, Jianhang Qiu, Chao Sun, Kaiping Tai, Hao Lei, and Xin Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16356 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 20, 2017

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ACS Applied Materials & Interfaces

Cellulose

Fiber-based

Bismuth

Telluride

Hierarchical

for

Porous

High-Performance

Flexible and Tailorable Thermoelectrics Qun Jina,b, Wenbo Shic,d, Yang Zhaoa,c, Jixiang Qiaoa,c, Jianhang Qiua, Chao Sund, Hao Leid,*, Kaiping Taia,*, and Xin Jianga,e,* a

Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese

Academy of Sciences, Shenyang 110016, China. b

University of Chinese Academy of Sciences, Shenyang 110016, China.

c

School of Materials Science and Engineering, University of Science and Technology of

China, Shenyang 110016, China. d

Surface Engineering of Materials Division, Institute of Metal Research, Chinese Academy

of Sciences, Shenyang 110016, China. e

Institute of Materials Engineering, University of Siegen, Paul-Bonatz-Str. 9-11, Siegen

57076, Germany. KEYWORDS Flexible and tailorable thermoelectrics, bismuth telluride, cellulose fiber, high-performance, hierarchical porous structures

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ABSTRACT

Porous modification is a general approach to endowing the rigid inorganic thermoelectric (TE) materials with considerable flexibility, however, by which the TE performances are severely sacrificed. Thus, there remains ongoing struggle against the trade-off between TE properties and flexibility. Herein, we develop a novel strategy to combine Bi2Te3 thick film with ubiquitous cellulose fibers (CFs) via unbalanced magnetron sputtering technique. Owing to the nano-micro hierarchical porous structures and the excellent resistance to crack propagation of the Bi2Te3/CF architectures, the obtained sample with nominal Bi2Te3 deposition thickness of tens of micrometers exhibit excellent mechanically reliable flexibility, of which the bending deformation radius could be as small as a few millimeters. Furthermore, the Bi2Te3/CF with rational internal resistance, tailorable shapes and dimensions are successfully fabricated for practical use in TE devices. Enhanced Seebeck coefficients are observed in the Bi2Te3/CF as compared to the dense Bi2Te3 films and the lattice thermal conductivity is remarkably reduced due to the strong phonon scattering effect. As a result, the TE figure of merit, ZT, is achieved as high as ~0.38 at 473 K, which competes the best flexible TEs and can be further improved by optimizing the carrier concentrations. We believe this developed technique not only opens up a new window to engineer flexible TE materials for practical applications, but also promotes the robust development of the fields, such as paper-based flexible electronics and thin-film electronics.


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1. Introduction The proliferation of the demand on developing new strategy to provide power sources stimulates world-wide interests in novel renewable energy technology.1 Thermoelectrics (TEs) enable direct conversion of waste heat into electricity without maintenance, which have captured rapid growing attention for sustainable energy harvesting technology. Recently, milliwatt-level electrical energy was successfully extracted by TE conversion from a small temperature gradient between human body and surrounding environment, revealing their great potential for powering portable electronics.2-3 To fully utilize the heat sources, such as human body and curved pipes with complex geometries, the TE materials are necessary to be constructed with high flexibility, so that they can closely contact the body surface to avoid heat lost.2 The widely used inorganic TE materials, such as Bi2Te3,4 PbTe,5 SnSe6 etc., have already exhibited the best TE performances under a wide range of working temperatures. However, their flexibilities are still seriously restrained by the intrinsic bonding features which determine their native rigidity and brittleness. Organic TE materials with the unique advantages of flexibility, light weight and easy processing, are developed dramatically during the past few years.7 Nevertheless, their practical use remains largely limited by the deficiencies, such as relative low efficiency, high contact resistance with metal electrode, difficulty for n-type doing, poor air/thermal stability, narrow range of operating temperatures, and corrosion issues.7-9 Therefore, fabricating flexible TEs with high performance remains a great challenge. Searching effective solutions to improve the flexibility of inorganic TE materials have been attracting great efforts from various interdisciplinary fields.2 One approach is to fabricate porous inorganic materials on flexible organic substrates by solution-process and drop-cast methods. The porous structures can accommodate the deformation, thus offering important



contributions to the flexibility.10 For example, Sun et al. fabricated flexible paper-based nanocomposite with room temperature (RT) ZT value of ~7×10-3;11 Dun et al. prepared flexible self-assembled Te nanorods on polyvinylidene fluoride substrate showing power factor (PF) of 45.8 µW/m·K2 at RT;12 Lu et al. synthesized thin-film TE device of Bi2Te3based nanoparticles on polyimide with maximum PF of ~180 µW/m·K2 at 523 K;13 We et al. reported screen-printed n-type Bi2Te3 and p-type Sb2Te3 powders with filling of poly (3,4ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT:PSS) and obtained ZT value of ~0.2 at RT.14 Moreover, it has also been found that the nano-micro porous structures can effectively scatter the phonons and reduce the thermal conductivity,15-17 and could give rise to enhanced Seebeck coefficients.18-19 Nevertheless, their electrical conductivities are severely deteriorated as compared to the bulk counterparts, which could be caused by contaminations, poor interconnection and serious carrier scattering, etc.2, 11, 13 Alternatively, porous inorganic TE materials can be fabricated via using flexible porous template as substrate and physical vapor deposition (PVD) technique which is typically employed for high-quality TE film growth. For instance, Kato et al. fabricated porous Bi2Te3-based thin film on polymer substrate via arc plasma gun method.20 Of note is that the film should be thick enough to lower the internal resistance for any practical use. However, the arrayed nanopores synthesized by etching or nanoimprinting methods will be buried during the thick film deposition process. Once the pores are filled by TE films, the flexibility is seriously degraded and the phonon-pore scattering effect is inhibited. Therefore, flexible substrates with randomly oriented/distributed and nano-micro hierarchical pores are highly preferred for fabricating thick TE film with duplicated porous structure to significantly reduce thermal conductivity,16, 21-22 as shown in Figure 1. In addition, the substrate must resist destruction caused by the high temperature and/or plasma bombardment in the film deposition process. In this vein, it is very important to develop an appropriate PVD method to synthesize high4 ACS Paragon Plus Environment

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quality TE films on porous substrate. Herein, we introduce unbalanced magnetron sputtering (UBMS) technique, which have demonstrated high deposition rate and good adhesion to the substrates21 for TE materials in our earlier work,22 to fabricate flexible and tailorable TEs by depositing Bi2Te3 thick film on the CFs-paper as hierarchical porous template. Benefited from the nano-micro porous structures and the excellent resistance to crack propagation of the Bi2Te3/CF architectures, the prepared Bi2Te3 films exhibit nominal deposition thickness of tens of micrometers and remarkable flexibility. The bending deformation radius could be as small as ~4 mm. Meanwhile, the thermal conductivities are significantly reduced owing to the strong phonon scattering effect, giving rise to high PFs of ~250 to 375 µW/m·K2 from RT to 473 K, which could be further improved by optimizing the carrier concentrations. These PFs are comparable to the most elaborated n-type organic intercalated TiS2 single crystal,8 and higher than other reported values.2 As a result, the TE figure of merit, ZT, of as high as ~0.38 at 473 K is obtained and competitive to the best n-type flexible TEs values.23 Moreover, the synthesized Bi2Te3/CF can be facilely tailored into any configuration by a laser micro-cutting machine to fit the contact area to the heat source with any shapes and dimensions. We thus construct both n- and p-type TE legs on the double sides of CFs-paper sheet as prototype of flexible TE generator (TEG) which can efficiently recover the waste heat in our daily life. This work strongly demonstrates that our developed technique has great potential for fabricating high performance flexible TE devices for thermal energy harvesting from heat sources with arbitary geometries. 2. Experimental Section 2.1. Fabrication of thermoelectric materials and devices A home-built unbalanced direct current (DC) magnetron sputtering system was employed in this work, which had been described in our previous study.22 The CFs-paper used in this work



is commercial CFs printer paper which is abundant and cheap. The CFs-paper has good temperature resistance up to ~573K (in Figure S1a), which is composed of randomly oriented CFs with diameters from hundred nanometers to dozens of microns. Both n- and ptype Bi2Te3 alloys were deposited on the CFs-paper as substrate at 473 K. For comparison of TE performances and flexibility, samples deposited on the SiO2/Si and polyimide substrates under the same experimental conditions (deposition power, duration, temperature, etc.) were also prepared. K-type thermocouple located on the substrate surface was used to monitor the heating temperatures and the fluctuation was ≤ ± 2 K. During the growth process, the operating Ar pressure was maintained at ~0.5 Pa. Commercial 4 inches Bi2Te3 target (99.99%) and Te target (99.99%) were used for co-sputtering to tune the doping carrier concentrations for n-type TE film deposition. (Bi, Sb)2Te3 target (99.99%) was used for ptype film deposition in fabrication of TE device. The distance between target and substrate was ~100 mm. The sputtering power was applied at ~40-80 W, and therefore, a high deposition rate of ≥ 5 µm/h was achieved in this work. A calibration for sputtering power dependent deposition rate is provided in Figure S1b. Negative bias voltages were maintained at the substrate to increase the energy of the incident ions, which can benefit the film coating adhesion and penetration depth into the porous CFs-paper substrate.21 A home-built laser micro-cutting system (in Figure S2) with beam resolution better than ~2 µm was employed to tailor the TEG with precisely controlled shapes and dimensions. 2.2. Characterization The microstructures of the samples were systematically investigated by X-ray diffraction (XRD, D8 Discover, Bruker), three-dimensional X-ray micro/nano-tomography (3D-XRT, Xradia 800 Ultra, Zeiss) and field emission scanning electron microscopy (FESEM, Supra 55, Zeiss). The 3D-XRT technique was used to reveal the depth profile of the Bi2Te3/CF sample microstructures for more details. The real deposition thickness of the Bi2Te3 on the


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porous CFs-paper was determined by measuring the film thickness on flat SiO2/Si substrate under the same deposition experimental conditions from the SEM cross-section image. The actual deposition depth was the penetration depth of the depositing Bi2Te3 film into the porous CFs-paper, which was the mixture layer thickness of the Bi2Te3 and CFs-paper and measured from the nano 3D-XRT depth profile. Energy-disperse spectrum (EDS) was employed to ascertain the compositions of the films and check their uniformity along depth with a relative measuring error less than 3%. The acceleration voltage of 20 kV was used in the EDS analysis. X-ray photoelectron spectroscopy (XPS, ESCALAB 250, Thermo) was utilized to analyze the bonding energy of the deposited films. NETZSCH SBA-458 system was used to measure the in-plane electrical conductivity (σ) and Seebeck coefficient (S) simultaneously with Ar (99.999%) gas protection from RT to 473 K. The measurement error for electrical conductivity and Seebeck coefficient was less than 3% and 5%, respectively. Thermal conductivity ( κ ) was calculated through κ = ρ ⋅ D⋅Cp , where ρ, D and C p are the density, thermal diffusivity and specific heat capacity, respectively. D was measured by a laser flash method using NETZSCH LFA-467 with multilayer sample holder. Differential scanning calorimetry (DSC, NETZSCH STA 449 F3) was used to determine C p . The sample density was measured by weighing method. The uncertainty for the measurements involved in thermal conductivity is around 15%. The combined uncertainty for the ZT values calculation is approximately ~25%. The Hall coefficient was measured by the HMS-3000 Hall system using the Van der Pauw method. The magnitude of the magnetic field is 0.558 T. The carrier concentrations n at RT were determined on the assumption Hall coefficient RH equals to 1 / ne and the Hall mobility µH was calculated based on µH=σ/ne. The flexible bending tests were conducted on a home-made experimental setup as shown in Figure S3. The samples were attached to the surface of glass tubes with different radii. The relative



resistance changes of the samples with various bending radii were used to evaluate the bending flexibility. 3. Results and Discussion 3.1. Microstructural Characteristics The XRD patterns of the samples and the sputtering target are provided in Figure S4. For comparison, we also deposited Bi2Te3 films on SiO2/Si (Bi2Te3/SiO2/Si) and polyimide (Bi2Te3/polyimide) substrates using the same preparing parameters. All of the patterns display characteristic diffraction peaks of the rhombohedral Bi2Te3 phase (JCPDS #15-0863). The Bi2Te3/SiO2/Si and Bi2Te3/polyimide samples are highly out-of-plane (015)-textured, which are generally obtained for the Bi2Te3 thin films prepared by PVD method.24-25 For the Bi2Te3/CF samples, the XRD patterns show that the texture strength is much weaker than those in the other two samples (in SI Note 1). The full widths at half maximum (FWHM) of the (015) diffraction peaks for all samples are only ~0.2-0.3 degree, indicating good crystallinity of the deposited films owing to the unique properties of our UBMS technique.22 The chemical compositions of all samples are determined by EDS to be ~Bi40Te60 and the composition uniformity across the thickness is confirmed by the cross-section sample. Figure 2 and S5 depict the top-view and cross-section SEM images of the deposited samples which have real deposition thickness of ~10-20 µm without micro-crack and/or peeling off from the substrates (The real deposition thickness is determined by measuring the thickness of samples deposited on the SiO2/Si substrate under the same experimental conditions). It can be seen that dense Bi2Te3 films of typical columnar structures with average grain size of ~800 nm and smooth surface morphology are synthesized on the SiO2/Si substrate (in Figure S5). The average grain size for the Bi2Te3/polyimide is ~500 nm and there is no perceivable porous structure in the films. It is evident that both the grain sizes and film density are much better than those fabricated by other PVD techniques at similar deposition temperatures. This could


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be ascribed to the enhanced atomic diffusion effects during the UBMS process.26 The synthesized Bi2Te3/CF samples exhibit porous network architectures, as shown in Figure 2bc. High magnification SEM analysis demonstrates that the surfaces of the CFs are covered by the Bi2Te3 films with average grain size of ~100 nm, which is much smaller than those of the films deposited on the SiO2/Si and polyimide substrates. It is noticed that the Bi2Te3/CF sample contains high density of nano- to micrometer sized pores with irregularly shaped, randomly distributed and orientated properties (in Figure 2b-e). Further back scattered electron (BSE) images (in Figure 2f) reveals that the fibers underneath the surface are coated by the Bi2Te3 films with porous structure as well, suggesting that such technique is favorable to fabricating thick film with high electrical conductance for practical use in TE devices. Such network architectures and hierarchical porosities are also expected to play important roles in the bending flexibility9, 28 and reduction of thermal conductivity due to the phonon scattering at the Bi2Te3/CF interfaces and pore boundaries.15 Figure S6 gives the morphologies of the surfaces of Bi2Te3/CF and Bi2Te3/polyimide samples tailored by the paper scissors across the thickness. One can clearly see that around the cut edge, the Bi2Te3/polyimide is split severely into small pieces with obvious crack propagation due to the intrinsic brittleness of Bi2Te3. In contrast, the cut edge of the Bi2Te3/CF sample with only a few of tiny flaws indicates the vital contributions of the unique microstructures to the excellent resistance to crack propagation.27-29 There are three primary mechanisms dominating the tailorability of the TE samples. Firstly, the CFs-paper consist of high density of hierarchical pores, from nano- to micrometer, giving rise to a very large specific surface area of 1.72 m2/g for film deposition (in Figure S7). Furthermore, the pores are irregularly shaped, randomly distributed and oriented, which is considered helpful to persist porous structure during thick film deposition. As a result, the thickness of the deposited Bi2Te3 film coated on each CF is much less than that on the polyimide substrate. Therefore, the induced



strain in the Bi2Te3 film coated on each fiber is small, because the bending strains scale linearly with the film thickness. Secondly, the porous structures could accommodate the bending deformation, thus improving the energy dissipation/absorption capacity of the inherently brittle materials, as compared to the solid state.10 Moreover, the high porosity could weaken the stress-intensity factors of the crack tip and prevent the crack propagation.27 Thirdly, the Bi2Te3/CF network is an excellent crack arrestor, offering high damage tolerance.28 It can be seen from Figure 2, a large number of CFs orientated randomly and tangled with each other in the sample, which can effectively prevent the catastrophic crack propagation that occurred in the Bi2Te3 films.28 Consequently, no destructive fraction in the Bi2Te3/CF sample can be observed even under severe cutting deformation (in Figure S6). To give an further insight into the three-dimensional structures, 3D-XRT was introduced to reveal the depth profile of the Bi2Te3/CF sample.30 Figure S8 shows the 3D-XRT images of the Bi2Te3 films deposited on the double sides of the CFs-paper with high (~80 W) and low (~40 W) deposition powers, respectively. Interestingly, the penetration depth of the depositing Bi2Te3 film into the hierarchical porous CFs-paper directly depends on the deposition power which is related with the energy of the incident ions/atoms during deposition.21 The maximum depth of incident ions/atoms with high deposition power is ~50 µm. The depth profile of the Bi2Te3 deposition has two peaks of distribution intensity, i.e. ~10 µm or ~45 µm away from the paper surface (in Figure S8b), owing to the distribution of the CFs as well as the deposition self-shadowing effects. The penetration depth profile of incident ions/atoms with low deposition power is only ~10 µm, which is derived from the Bi2Te3 films coated on the CFs near the paper surface, where the self-shadowing effects become dominant. Thus, other than the solution-processes,11, 31 the results indicate that the cross-plane distribution of the Bi2Te3 films deposited into the porous CFs-paper can be controlled through finely tuning the deposition power. So, it is really advantageous to avoid


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blocking the pores during thick film deposition and the hierarchical porous structure can survive, as shown in Figure 2. 3.2. Thermoelectric Properties 2

The TE performance of materials is typically defined by the figure of merit, ZT=α σT/κ , where α is the Seebeck coefficient, σ and κ are electrical conductivity and thermal conductivity, respectively, and T is the average operational temperature. The temperature dependent TE properties from RT to 473 K are shown in Figure 3. It should be pointed out that the CF is electrically insulating and has no effect on the carrier transport properties. The nominal thicknesses of the Bi2Te3/CF (the thickness of Bi2Te3/CF mixture layer, i.e. the Bi2Te3 penetration depth, as shown in Figure S9) are used to measure the in-plane electrical conductivities and carrier concentrations. The thickness values are determined from the 3DXRT depth profile. It can be seen that the σ values of the deposited samples exhibit typical metallic-like transport properties. The conductivities decrease with the rising temperatures because of the well-known scattering effects.32 Their in-plane Seebeck coefficients are negative, suggesting the n-type electron carrier transport mechanism in the deposited films due to the Te defects.24 The values of |α| exhibit typical behaviors of heavily-doped degenerate semiconductors and increase with the temperature rise. The carrier concentrations for the Bi2Te3/SiO2/Si, Bi2Te3/polyimide and Bi2Te3/CF are ~5.4×1020 cm-3, ~8.2×1020 cm-3 and ~3.9×1020 cm-3, and their carrier mobilities are ~20 cm2/Vs, ~17 cm2/Vs and ~10 cm2/Vs, respectively. It is obvious that the carrier mobility of the Bi2Te3/CF sample is about ~50% lower than the values of the dense Bi2Te3 films. Such decrease is primarily derived from the more serious carrier scattering effects caused by the smaller grain size, the Bi2Te3/CF interfaces and the nano-micro porous structures, as shown in Figure 2. The grain boundary, pore boundary and interface strongly scatter carrier transport and reduce carrier mobility, which can be qualitatively explained by the Matthiessen’s rule.32 Such scattering 11 ACS Paragon Plus Environment


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effects are enhanced with increasing density of these defects, i.e. smaller grain size and larger porosity. One can see that the Bi2Te3/CF demonstrates highest values of |α| in all prepared samples at the measured temperatures. For the degenerate semiconductors under assumption of a simple parabolic electronic band structure, the relationship between the Seebeck coefficient and the carrier concentration could be expressed by the Mott formula: 2

α=

8π 2 k B2 m ∗T π 3 ⋅ ( ) ⋅ (1 + r ) 3n 3eh 2

(1)

where n is the carrier concentration; m* is the carrier effective mass, depending on the specific band structures; r is the scattering parameter which determines the energy dependence of the carrier relaxation time and mobility. From the |α| and n, the calculated values of m* for the Bi2Te3/SiO2/Si and Bi2Te3/polyimide samples are really comparable, revealing that the kinds of deposition substrates have no influences on the Bi2Te3 band structure variations. Because the CF is insulator material that gives no contribution to the α values, the real carrier concentration of the Bi2Te3 films in theBi2Te3/CF sample, which is ~7.9×1020 cm-3 determined from the real thickness, should be used to evaluate the α values dependence on the n. It is noticeable that, the Seebeck coefficients of the Bi2Te3/CF are almost ~60% higher than the values of the Bi2Te3/polyimide sample, although the carrier concentrations of the Bi2Te3/polyimide and Bi2Te3/CF are very close. This enhanced Seebeck coefficients could be related to the interfacial barrier scattering effect,33 which might be induced by the thin layer of native oxidation of Bi2Te3 nanograins near the film surface.34-36 Figure 4a-d shows the XPS results on the Bi2Te3/SiO2/Si and Bi2Te3/CF samples, where the surface oxidations of Te and Bi atoms without other impurities are detected. It can be seen that the peak area ratios of the oxidation layer (Te-O and Bi-Te-O) to the Bi2Te3 in the Bi2Te3/CF sample are 1.33 and 1.07. These values are higher than those (0.88 and 0.79) of the Bi2Te3/SiO2/Si samples, indicating more serious oxidation in the Bi2Te3/CF. The quantity of the oxidation layer in the Bi2Te3/CF could be seriously underestimated because of the surface 12 ACS Paragon Plus Environment

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roughness effect on the XPS results, although the Bi2Te3/CF sample has much larger specific surface area than those of the dense Bi2Te3 films on the SiO2/Si and polyimide substrates.37 The thin oxidation layers give rise to the misalignment of the band structures, thus essentially scattering the charge carriers when they transport across the interfaces of the Bi2Te3 films coated on neighboring CFs.38 Such effect could be enhanced with increasing amount of oxidation layer. This enhancement in Seebeck coefficients is similar to the grain-boundary potential barrier scattering effect in the polycrystalline TE materials, where the grain boundary thickness is ~0.5 nm.33 The schematic of carrier energy scattering effect at the interface is illustrated in Figure 4e-f. Because of the formation of crystal defects at relative low deposition temperatures, the carrier concentrations of our deposited samples are almost one magnitude higher than the optimized values reported in the literatures.4 Such high carrier concentrations restrain the Seebeck coefficients in low range as compared with the state-ofthe-art of their bulk counterparts. Through tuning deposition power of the co-sputtering Bi2Te3 and Te targets during deposition, the carrier concentrations of the Bi2Te3 films can be finely controlled. With carrier concentration of ~1.5×1020 cm-3, the sample average value of |α| is increased up to ~130 µV/K, which could be further optimized in future work (in Figure 3b). In contrast to the other three samples, this Seebeck coefficient presents no significant change with the increased temperature, mainly due to the bipolar effect.23, 39 The maximum value of |α| is generally limited by the onset of bipolar conduction, which involves thermal excitation of minority carriers across the band gap. The Seebeck coefficiants from majority and minority carriers have opposite signs, making the total Seebeck coefficient cancel each other.39 This bipolar effect can be suppressed by a higher majority carrier concentration, as shown in Figure 3b. As a result, the power factors of the Bi2Te3/CF are as high as ~250 to 375 µW/m·K2 from RT to 473 K (in Figure S10), exhibiting remarkable performance as ntype flexible TE nanocomposite.2 13 ACS Paragon Plus Environment


The laser-flash technique is employed to determine the in-plane thermal conductivity of the Bi2Te3/CF sample. Note that the incident laser beam is configured to be parallel with the inplane direction of ~one hundred of such samples stacking up along the thickness direction, as shown in Figure S9 and SI Note2. The thermal conductivities of the blank CFs-paper are comparable with the values of the Bi2Te3/CF whole sample. The mixture law (in SI Note2 equation S5) is adopted to evaluate the thermal conductivity of the Bi2Te3/CF mixture layers (in Figure S11 and Note 2). This equation was employed for bulk nanocomposite and could not underrate their values.40 The thermal conductivities of the Bi2Te3/CF are notably lower than the values of the bulk Bi2Te3, which could be partially derived from the contribution of the CF.23 To investigate the fundamental mechanism for the reduction in thermal conductivity, the lattice ( κl ) and electron ( κe ) thermal conductivities are calculated based on the Wiedemann-Franz relationship (in Figure 3c). One can see that the reduction in thermal conductivity is derived from the lattice thermal conductivity that approaches to the theoretical minimum lattice thermal conductivity of 0.14-0.28 W/m·K predicted for Bi2Te341-42 and even smaller than the thermal conductivity of the CFs-paper ( κ of CF has no contribution from

κe , as shown in Figure S11). In recent studies, the contributions to the lattice thermal conductivity of the phonons with different mean free paths (MFPs) have been calculated for the dense Bi2Te3.43 It is found that around ~90% of the total κ l are contributed by the phonons with MFPs shorter than ~20 nm characteristic dimension size. Therefore, nanostrucutres with approximately equal to or less than this characteristic size can intensively decrease the lattice thermal conductivity, which can be attributed to the phonon scattering by a combination effect of point defects, dislocations, stacking faults, and grain boundaries as well.32 As compared to the dense Bi2Te3, the hierarchical nano-micro pores in Bi2Te3/CF sample that are believed to effectively suppress both the short- and long-wavelength phonon transports.15, 17 The pores with irregular shapes, random distributions and orientations will 14 ACS Paragon Plus Environment

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diffusively scatter phonons in different directions, resulting in a combination effect of ballistic-diffusive phonon scattering which may account for the significant decrease in the lattice thermal conductivity.44 The CF is naturally insulator material with low thermal conductivity, which can prevent the short circuits of heat and current. The thermal conductivity of the substrate could play a significant role to determine the energy conversion efficiency, especially for the thin film TE device with in-plane configuration.45 The suspending cold-side in the ambient air is typical and ideal to maximize the temperature differences across the device, instead of tightly attaching the whole device on the heat source surface, by which the TE efficiency could be greatly reduced by the thermal energy loss though the substrate.46 Thus, the ZT values of ~0.23 to 0.38 from 330 K to 473 K (in Figure 3d) are obtained from the measured power factors and thermal conductivities, which are comparable with the best values of previous studies, such as ~0.3 for the n-type flexible hybrid TiS2/hexylamine superlattices8,

47

and

~0.42 for the p-type dimethylsulphoxide-mixed PEDOT:PSS.48 3.3. Measurements of Bending Flexibility To evaluate the bending flexibility of the Bi2Te3/CF and the Bi2Te3/polyimide, the resistance changes with respect to the bending deformation radius were measured by a home-made apparatus (in Figure S3). The real deposition thickness on one side of CFs-paper is ~13 µm and the corresponding nominal thickness is ~30 µm determined by the 3D-XRT depth profile. Figure 3e plots the results of the resistance tests on one-side and double-side deposited samples as function of various bending radii. One can see that the Bi2Te3/CF exhibits much improved bending flexibility, as compared to the Bi2Te3/polyimide (in Figure 3f) sample with similar real deposition thickness. For the Bi2Te3/CF sample with one-side deposition, the resistance decreases ~50% with the concave-bending radius up to ~4 mm. This reduction could derive from the compression of the porous structures under bending deformation,



leading to more contact areas and facilitating the carrier transport between the Bi2Te3/CF. The resistances are maximally increased by ~two-fold under the convex-bending deformation with ~4 mm bending radius. Over 100 bending cycles at concave-bending radius of ~10 mm, the increase of flat state resistance after cyclic bending test is ~6%, as compared to the original flat state (in Figure S12). On the contrary, the resistances of the Bi2Te3/polyimide samples are seriously deteriorated under the same bending test. For instance, with convexbending radius of ~6 mm (in Figure 3f), the Bi2Te3/polyimide is in tensile stress/strain state and cracks into small pieces. The bending resistances are increased by a factor of ~7. For the samples with double-side deposition, the resistances of Bi2Te3/CF are also reduced by ~20% with decreasing bending radius from ~10 to ~4 mm. To highlight the notable bending flexibility, a piece of Bi2Te3/CF sample with double-side deposition was tailored with lateral dimensions of ~15×20 mm and tested vividly, as shown in Figure 5. The initial resistance of the sample strip in original flat state is as small as ~ 9.3 Ω. When the bending radius is ~5 mm, the resistance is remarkably dropped to ~5.2 Ω and quite stable. Such excellent bending flexibility also gives prospect to continuously manufacture such flexible TE sample by the roll-to-roll technique for scale-up production. To evaluate the TE performance of our sample under bending state, the Seebeck coefficients were also measured with concave-bending radii (in Figure S13), which are close to the results of the original flat state. Thus, the bending state ZT value could be estimated, if the lattice thermal conductivity is presumed not sensitive to bending state. 3.4. Thermoelectric Devices for Thermal Energy Harvesting The Bi2Te3/CF sample for potential application as flexible devices for electric power generation is also evaluated in this work. The Bi2Te3/CF samples can be simply cut by scissors into any shapes and dimensions (in Figure 5). To demonstrate the ability of harvesting thermal energy from human body, one end of the strip was clamped by alligator


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clip at RT and the other end was hold by fingers. The temperature difference between the two ends was thus maintained at ~ 15 K. The corresponding open voltage is as high as ~2 mV (in Figure 5e) measured by the voltmeter, indicating the Seebeck coefficient is ~133 µV/K that is compatible well with the TE properties measurement (in Figure 2b). When the load resistance matches the TE sample resistance, the maximum output electrical power (Pmax) can be approximately estimated by the expression:23, 49 ܲ௠௔௫ =

ሺఈ·௱்ሻమ

(2)

ସோ

Where α is the Seebeck coefficient, ∆T is the temperature gradient, R is the TE sample resistance. In this work, α·∆T ≈ 2 mV and Rmin ≈ 5 Ω (bending state, in Figure 5), and thus, the small pieces of n-type TE samples could deliver a Pmax of ~0.2 µW to a matched external load, which is really competitive to the values of the flexible TE materials synthesized by other techniques.2 To highlight the versatility of this technology in efficiently utilizing heat source, such as, the typical waste heat of light bulbs in our daily life, a flexible radial-pattern TE device with 12 pairs of n- and p-type (TE performances are shown in Figure S14) legs in series was precisely tailored from a piece of double-side deposited sample by the laser micro-cutting system without any cracked edges (in Figure 6 and Figure S15). To maximize the temperature differences across the device, the hot-side of the TE device was stuck on the surface of hemispherical light-bulb shield using the thermal paste and the cold-side was suspended in the ambient air. With the temperature gradient of ~50 K, the output voltage reached ~0.144 V and quite stable without any decline during the test for several hours. The time dependence of temperature distributions of the TE legs on the light-bulb on and off states is shown in Figure 6f. Utilizing the air convection as heat sink, the temperature gradient can be quickly established in a few minutes after the light-bulb turn-on and the distributions settle down to a steady-state along the hot-side to the cold-side, which can 17 ACS Paragon Plus Environment


enable a high efficiency of heat energy recovery. The purpose of fabricating n-/p-type legs on each side of CFs-paper respectively is to demonstrate the capability of high integration to fully use the surface of heat source; besides, the configuration of n-/p-type legs on both sides could decrease the bending state resistance (in Figure 3). These results strongly demonstrate the versatile capabilities of this technique to fabricate flexible and tailorable TE devices with high performances. 4. Conclusions In summary, we develop a novel approach to fabricate flexible and tailorable Bi2Te3/CF TE material using UBMS method. Distinct enhancement of Seebeck coefficients is observed in the Bi2Te3/CF, which can be attributed to the carrier scattering effect caused by the Bi2Te3 surface oxidation layer. The thermal conductivity of the Bi2Te3/CF mixture layer is extremely lower than those of the bulk Bi2Te3 counterparts owing to strong phonon scattering effect, giving rise to a high ZT value of ~0.38 at 473 K. The Bi2Te3/CF sample including tensmicrometer thickness of Bi2Te3 exhibits excellent flexibility, due to the unique architectures. Through employing a home-built laser micro-cutting system, we successfully fabricate a flexible TE generator based on the developed Bi2Te3/CF, which can harvest thermal energy from heat source with arbitrary geometries. Our approach is applicable to manufacture both n- and p-type high-quality flexible TE materials. We are convinced this developed technique not only sheds a light on the thermoelectrics community, but also promotes the robust development of the fields, such as paper-based flexible electronics and thin-film electronics.


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Figure 1. Schematic illustration of flexible TE materials with hierarchical porous structures.



a

200 µm

d

1 µm

b

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c

200 µm

40 µm

e

f

10 0 nm

4 µm

Figure 2. Cross-section (a) and top-view (b-e) SEM images of the Bi2Te3/CF sample. Cross-section BSE images of the Bi2Te3/CF sample (f).


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c

24

12 8

κML

e

Bi2Te3/SiO2/Si

Bi2Te3/CF

Bi2Te3/polyimide

Optimized Bi2Te3/CF

0.3 0.2

κl κe

1.0

0.1

0.0

0

330 360 390 420 450 480 T (K)

300 330 360 390 420 450 480 T (K)

d

Rcv Rd

-70

0.2

-130 -140

0.1 300 330 360 390 420 450 480 T (K)

R / R0

ZT

-60

6

7

8

9

8 6

0.3

5

Radius (mm)

0.4

-50

Rav

4

f

0.5

-30 -40

-1

1.5

0.5

4

b

2.5 2.0

0.4

16

κ (W m-1K-1)

4

-1

σ (×10 S m )

20

0.5

R / R0

a

α (µV K )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Rcv

4 Rd

2 0 330 360 390 420 450 480 T (K)

Rav

6

7

8

9

Radius (mm)

Figure 3. Temperature dependence of (a) electrical conductivity and (b) Seebeck coefficient of the Bi2Te3/SiO2/Si sample, Bi2Te3/polyimide sample, Bi2Te3/CF sample and Bi2Te3/CF sample with optimized carrier concentration. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Temperature dependence of the thermal conductivity (κML), lattice thermal conductivity (κl) of the Bi2Te3/CF mixture layer (c), κ l = κ ML − κ e , κ e = L ⋅ σ ⋅ T , L is the Lorenz number and L=1.75 was used in the calculations.50 The figure-of-merit ZT of the Bi2Te3/CF sample (d). The relative resistance change as a function of bending radius for the Bi2Te3/CF and Bi2Te3/polyimide samples are shown in (e) and (f), respectively. R0 is the resistance of the original flat state before bending, Rcv is the value under convex-bending, Rca is the value under concave-bending, Rd is the value of the sample with double-side deposition.



Intensity (a.u.)

a

Te-O

e

Bi2Te3

3d5/2

b

Te-O

Bi2Te3

f 590

585

580

575

570

565

Binding Energy (eV)

Bi-Te-O

c

OH HO

Bi2Te3

Bi-Te-O

Bi 2 Te 3

O

OH

Bi2Te3

Bi-Te-O

VB 165

162

159

156

n

Eb Ef

CB

168

O O

OH

4f7/2

d

OH

O

Cellulose Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Bi 2Te 3

Te-O

e-

Interfacial Energy Filter

153

Binding Energy (eV)

Figure 4. XPS analysis results of the Bi2Te3/CF (a), (c) and Bi2Te3/SiO2/Si (b), (d). (a), (b) show the peaks of Te 3d and the oxide state. (c), (d) shows the peaks of Bi 4f and the oxide state. Schematic of the carrier transport in the Bi2Te3/CF sample (e) and the energy barrier scattering effect at the Bi2Te3/CF interfaces due to the existence of thin oxide layers (f).


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a

b

c

d

R0

R1

R2

R3

e

f

g

h

V

R6

R5

R4

Figure 5. Vivid demonstration of the Bi2Te3/CF sample bending performances (a-d) and (f-h). The output voltage by harvesting thermal energy from human body (e).



a

d

b

f

e

c

Figure 6. Flexible thermoelectric device with radial-pattern tailored by laser micro-cutting system (a-b). Thermoelectric generator measurements (c-e), where the flexible thermoelectric device was tightly stuck on the hemispherical light-bulb shield surface as the heat source using thermal paste. The temperature gradient is ~50 K and the output voltage is ~0.144 V. The time dependence of temperature distributions of the TE legs on the heat source on and off states (f).


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Supporting Information The Supporting Information is available free of charge. Additional experimental results, the calculation of orientation factors and thermal conductivities (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] AUTHOR INFORMATION Author Contributions All authors have given approval to the final version of the manuscript. Funding Sources National Natural Science Foundation of China (Grant No. 51402310, 51571193), Hundred Talents Program of Chinese Academy of Sciences and Innovation Foundation of Institute of Metal Research. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Financial supports from the National Natural Science Foundation of China (Grant No. 51402310, 51571193), Hundred Talents Program of Chinese Academy of Sciences and Innovation Foundation of Institute of Metal Research are gratefully acknowledged. The



experiments were carried out in part in the Institute of Metal Research, Chinese Academy of Sciences. Supporting Information is available online.


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ABSTRACT GRAPHIC Flexible TEG

TEs Deposition e

-

N P on 0

Laser Tailoring

∆ U（ mV）

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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off 10

20

30

Time（ min）


40

120 80 40

50

Cellulose Fiber-Based Hierarchical Porous Bismuth Telluride for High

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