Tunable Synthesis and Thermoelectric Property of Bi2S3 Nanowires

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Tunable Synthesis and Thermoelectric Property of Bi2S3 Nanowires Qi Yang, Chenguo Hu,* Shuxia Wang, Yi Xi, and Kaiyou Zhang Department of Applied Physics, Chongqing University, Chongqing 400044, PR China ABSTRACT: Bismuth sulfide (Bi2S3) is a specific thermoelectric material that thermoelectric properties are anisotropic. One dimensional Bi 2 S 3 structure would improve the conductivity and Seebeck coefficient because of the anisotropic electron transmission and phonon scattering. The Bi2S3 nanostructures with different morphologies are prepared by a modified composite molten salt method. The Bi2S3 nanowires with lengths up to 20 μm and high crystallization are obtained. The thermoelectric properties of the synthesized samples with different morphologies are comparatively investigated. We find that the power factor of the film made from the Bi2S3 nanowires is much larger than that of the film made by the Bi2S3 nanosheets or nanowires mixed with sheets due to its lower resistivity and larger Seebeck coefficient. The low resistivity of the Bi2S3 nanowires film is a result of the high carrier concentration and high carrier mobility due to the high orientation degree and better crystallization. The Bi2S3 nanowires orientated along the film plane gives fast electron transmission along the a−c or b−c planes (electron crystal), and efficient phonon scattering between the cleaved a−c planes or b−c planes and between grains of the nanowires (phonon glass). The introduction of many interfaces from smaller size of grains, which scatter phonons more effectively than electrons, or serve to filter out the low-energy electrons at the interfacial energy barriers, allows the enhancement of Seebeck coefficient.

1. INTRODUCTION As the consumption of traditional energy increased exponentially in the near decades because of rapid development of human society, more and more researchers have come to pay close attention to new energy sources. It is an effective way to utilize waste heat in daily life to produce electricity through thermoelectric (TE) devices that can directly achieve the conversion between heat and electricity. However, to find and prepare suitable thermoelectric materials is a challenging work.1,2 The efficiency of TE devices is determined by the dimensionless figure of merit (ZT), which is defined as ZT= (S2/ρκ)T, where S, ρ, κ, and T denote Seebeck coefficient, electrical resistivity, thermal conductivity, and absolute temperature, respectively.3 The compound A2B3 (where A = Bi, Sb, Pb, and B = S, Se, Te) is considered to be the most promising for TE applications. Bi−Te-based and Pb−Te-based compounds show best TE properties at room and moderate temperatures.4,5 Telluride-based materials usually exhibit good TE properties and hold dominant market shares in TE materials, but tellurium is rare and toxic, so the development of alternative materials is necessary. Bismuth sulfide (Bi2S3) is an important semiconductor with the direct gap band energy 1.3 eV and has potential application in thermoelectric field.6−8 Unfortunately, TE development of Bi2S3 is hindered from its high electrical resistivity. Many efforts have been paid on the modification of the structure Bi2S3 to improve its TE properties. It is quite encouraging that the electrical resistivity of Bi2S3 can be reduced by 2−3 orders of magnitude through introducing sulfur vacancies in the lattice.9 On the other hand, thermoelectric properties of Bi2S3 crystals are anisotropic. Bi2S3 © 2013 American Chemical Society

nanostructure would improve the conductivity and Seebeck coefficient because of the anisotropic to electron transmission and phonon scattering. Theoretical and experimental explorations demonstrated that significant improvement in TE efficiency could be achieved in nanostructured systems owing to both a high electronic density of states near the Fermi level and an increased phonon scattering.10−14 Especially, onedimensional (1D) structure of Bi2S3 crystals can efficiently transport electrical carriers along c-axis and are suitable for building films with crystal orientation. The preparation of Bi2S3 nanorods, nanowires, and nanotubes has been reported by means of solvothermal,15−17 hydrothermal,18−20 sonochemical21 methods, solventless thermolysis,22 colloidal chemistry,23 vapor deposition,24 and microwave irradiation25 routes. We have ever synthesized the Bi2S3 nanowires for the first time by the composite-saltmediated-method8 which is quite different from the methods above. However, the obtained nanowires were short. In this article, we modified the composite-salt-mediated-method with adding a small amount of water to achieve long and uniform nanowires with high crystallization. On the other hand, although TE properties were studied based on bismuth sulfide polycrystals,26 or sulfide composite material,27,28 it is surprised that few research has been reported on the TE properties of Bi2S3 nanowires. Therefore, thermoelectric power factors of the films made by the Bi2S3 nanowires, nanosheets, and nanowires Received: August 4, 2012 Revised: February 24, 2013 Published: February 28, 2013 5515

dx.doi.org/10.1021/jp307742s | J. Phys. Chem. C 2013, 117, 5515−5520

The Journal of Physical Chemistry C

Article

cold ends of the film were measured by the thermocouples contacted on the surfaces. The carrier concentration and mobility of the samples were measured at room temperature using a Hall Effect Measurement System (HMS-3000).

mixed with sheets were compared. The Seebeck coefficient, carrier concentration and mobility of electrons were discussed.

2. EXPERIMENTS All the analytically pure chemical reagents, including bismuth nitrate (Bi(NO3)3·5H2O), potassium nitrate (KNO3), and lithium nitrate (LiNO3), were purchased from Chongqing Chemical Company; sodium sulfide (Na 2 S·9H2 O) was purchased from Aladdin, and they were used as received without further purification. The conditions for the synthesis of Bi2S3 nanostructures via the M-CMS method are listed in Table 1. In a typical synthesis

3. RESULTS AND DISCUSSION To obtain uniform and long Bi2S3 nanowires, different conditions were tried. Figure 1 shows XRD patterns of the samples synthesized in the conditions listed in Table 1.

Table 1. Experimental Conditions for the Preparation of Bi2S3 Nanowires sample

temp (°C)

time (h)

water (mL)

morphology

P1 P2 P3 P4 P5 P6 P7 P8 P9 P10

200 200 200 200 140 160 180 200 200 200

8 24 48 72 48 48 48 48 48 48

2.5 2.5 2.5 2.5 2.5 2.5 2.5 0 1 5

NWs and sheets NWs and sheets NWs NWs irregular NPs nanosheets NWs nanosheets NWs and sheets NWs

phase Bi2S3 Bi2S3 Bi2S3 Bi2S3 Bi2S3 Bi2S3 Bi2S3 Bi2S3 Bi2S3 Bi2S3

and BiS and BiS

and and and and

BiS BiS BiS BiS

Figure 1. XRD patterns of part of the samples.

From the patterns, we can see that all the peaks can be identified as the orthorhombic phase of Bi2S3 (JCPDS 170320). The diffraction peaks of P3, P7, and P10 are stronger and sharper than that of P1, P5, and P8, suggesting the better crystallization in a condition of higher temperature, longer synthesis time and with 2.5−5 mL water. However, the XRD patterns of P1, P5, P7 and P8 show a little impurity phase (BiS) as labeled in triangles. The pure orthorhombic phase of Bi2S3 can be synthesized under the condition (1−5 mL water, ≥200 °C, ≥48h) as listed in Table 1. The SEM images in Figure 2 show the morphology of the Bi2S3 samples. Figure 2a−c show the Bi2S3 samples (P1, P2, P3) synthesized at 200 °C for different time (8, 24, 48 h) with 2.5 mL water, from which we can see that the P1 and P2 consist of nanowires mixed with sheets (Figure 2a and b), while P3 is of well dispersed nanowires (Figure 2c), indicating well crystalline nanowires can be obtained as the reaction time is prolonged. Figure 2d−f show the samples (P5, P6, and P7) synthesized for 48 h with 2.5 mL water at different temperatures (140 °C, 160 °C, 180 °C), which reveal that the nanowires can be only obtained at temperature higher than 180 °C. Figure 2g-i show the samples (P8, P9, and P10) synthesized at 200 °C for 48 h with different amount of water (0, 1, and 5 mL), from which we can see that the P8 consists of nanosheets (Figure 2g), and P9 consists of nanowires mixed with sheets (Figure 2h), while P10 is of well dispersed nanowires (Figure 2i), indicating that a suitable amount of deionized water in the melts benefits the growth of highly crystallized and long Bi2S3 nanowires. In conclusion, the better crystallized nanowires with length more than 20 μm could be obtained at temperature of 200 °C for 48−72 h with 2.5−5 mL deionized water (Table 1), which agrees with the results of XRD. As the samples are direct synthesized under the ionic state, we propose the following three relevant chemical reactions for the growth process of the Bi2S3 samples

of the Bi2S3 nanocrystals, 9 g of mixed nitrate (LiNO3/KNO3 = 1:2) was put in a Teflon vessel, and 0.1 mmol Bi (NO3)3·5H2O and 4 mmol Na2S·9H2O were added into the mixed nitrates. Although the melting point is 337 °C for KNO3 and 255 °C for LiNO3, the eutectic point is only 125 °C at mole ratio of 58:42. Then 0−5 mL deionized water was added into the Teflon vessel. The vessel was sealed and put in a furnace preheated to 140−200 °C. After reacting for 8−72 h, the vessel was taken out and let cool down to room temperature naturally. The black product was separated by centrifugation, washed several times with deionized water and absolute alcohol, and dried at 60 °C for 3 h. The products were characterized by X-ray diffraction measurement (XRD-6000, Shimadzu) with the use of Cu Kα radiation (λ = 1.5418 Å) at a 2°/min scanning speed in the 2θ range from 10° to 70°, a field emission scanning electron microscopy (FESEM, Nova 400 Nano SEM), a transmission electron microscopy (TEM, TECNAI20, Philips). An UV−visNIR Spectrophotometer (U-4100, Hitachi) was used to measure the reflection spectrum of the samples. Energy dispersive X-ray fluoresence (EDX) spectrometer was used to detect the atomic ratio of the samples from individual crystals in TEM chamber and the average atomic ratio was calculated by several detection results. To measure the Seebeck coefficient and electrical conductivity of the products, a film made from the samples was carried out as previous report method,14 at the pressure of 25 MPa. The dimension of the film is 28 mm in length, 7 mm in width, and 0.1 mm in thickness. The measurement of electrical properties was performed by a computer-controlled multifunctional measuring system (Keithley 2400 source meter). The thermoelectromotive force (thermo emf) was measured by changing the temperature at the hot end, while keeping the cold end at room temperature. The temperatures of the hot and 5516

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Figure 2. SEM images of Bi2S3 samples synthesized at different time (a = P1, b = P2, c = P3), different temperature (d = P5, e = P6, f = P7) and different deionized water (g = P8, h = P9, i = P10).

Bi(NO3)3 · 5H 2O → Bi 3 + + 3NO3− + 5H 2O

(1)

Na 2S ·9H 2O → 2Na + + S2 − + 9H 2O

(2)

2Bi 3 + + 3S2 − → Bi 2S3

(3)

At higher temperature, the ions get enough energy to react, and a small amount of water in the reaction would influence viscosity in the melts,29 which could contribute to the transfer of the reaction ions, and with a suitable transfer speed uniform Bi2S3 nanowires could be obtained after a long growth period. SEM images of the broken shallow surfaces of the thin films made from P1, P3, and P8 are given in Figure 3. From Figure 3a−c, we can see that the films are pressed tightly, and the morphology of the three samples can be observed clearly. In Figure 3b, it is clear that the nanowire structure with a highaspect-ratio is maintained in the film after pressing. The nanowires orientated along the film plane may be attributed to that the nanowires are fairly uniformly distributed on the substrate surface with their lateral orientations random by dispersing a droplet of the suspended nanowire solution onto the substrate during the film preparation before pressing. The grains of film made from P1 and P8 are much larger than that of film made from P3. Some micrometer scale crystals are formed in the film made from nanosheets (P8) after pressing. The insets are the corresponding EDX of the three samples which are taken from individual crystals in TEM chamber. The EDX of the three samples display the elements of Bi and S (C and Cu signals are from TEM grids and carbon film). The Bi/S atomic ratio is about 2:2.75 (P1), 2:2.95 (P3), and 2:2.84 (P8),

Figure 3. SEM images of the fractured surfaces of the films of P1 (a), P3 (b), P8 (c), and EDX (insert a, b, c) of the sample P1, P3, P8, and HRTEM (d and insert) images of sample P3.

suggesting a better stoichiometry of P3. The sulfur-deficient Bi2S3 in P1 and P8 may attribute to the impurity phase of BiS as detected by XRD. From selected area electric diffraction and 5517

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interplanar spacing in HRTEM image, we can see the nanowire grows along [001] direction as shown in Figure 3d and insets. The UV−visible diffuse reflectance spectrum and Kubelka− Munk function of the P1, P3, and P8 are shown in Figure 4; the

Figure 5. Electrical resistivity (a) and Seebeck coefficients (b) of the Bi2S3 nanowire films made from sample P1, P3, and P8.

obviously anisotropic electrical transport properties. It is wellknown that the chemical bonds in Bi2S3 crystals are very anisotropic. Covalent bonds are directed along the c-axis, while along the a and b directions, bonding is made by weak ionic and van der Waals forces.30 Therefore, bismuth sulfide is featured with a layered structure, and the crystal is easily cleaved parallel to the longitudinal direction. The interplane parallel to the a−c or b−c plane provides a good path to electron transport. Cantarero et al.31 have proved that the carrier mobility in the c-axis direction is higher than that in the a-axis direction because of the electron effective-mass anisotropy. Therefore, the P3 film displays a low resistivity due to that the nanowire lateral on the P3 film plane parallel to the a−c or b−c plane.32 However, as the film of P1 and P8 presents a low orientation degree, the resistivity is larger than that of the P3 film. The Seebeck voltage measured between two Cu electrodes at the ends of the film varies linearly with the temperature difference as are shown in Figure 5b. Seebeck coefficient can be obtained from the slope of the curve. The Seebeck coefficient of P3, P8, and P1 film is about −1.05, −0.64, and −0.51 mV/K, respectively, exhibiting that the Seebeck coefficient of the uniform nanowire film is much larger than that of the nanosheets and the mixed structure film. It is well-known that the electrical resistivity is determined by carrier concentration and mobility as described by the relationship ρ = 1/neμ, where ρ, n, and μ are the electrical resistivity, carrier concentration, and the carrier mobility, respectively. The electrical resistivity, carrier concentration and the carrier mobility measured from P1, P3, and P8 films are listed in Table

Figure 4. Reflection spectrum and corresponding Kubelka−Munk function of the sample P1 (a), P3 (b), and P8 (c).

energy gap can be estimated by extrapolating the linear part of Kubelka−Munk function. Obviously, the band gap is about 1.25 eV for P1 (a), 1.1 eV for P3 (b), and 1.21 eV for P8 (c) evaluated from the Kubelka−Munk function. To explore the thermoelectric property of the Bi2 S 3 nanowires and the advantages of 1D structure, the electrical resistivity ρ and Seebeck coefficient S of the film electrode made from the Bi2S3 nanowires (P3) and compared its properties with that of the films made from the nanosheets (P8) and the mixed structures (P1) are measured. The linear current−voltage (I−V) curves in Figure 5a are measured at the both ends of the films. Each film has 28 mm ×7 mm ×0.2 mm for its length × width × thick in appearance, and 5.26 ± 0.15 g/cm3, 5.40 ± 0.10 g/cm3, 5.12 ± 0.20 g/cm3 for the density of the P3, P8 and P1 film, respectively. The film resistivity of P3 (ρ=1.76 Ω·cm) is much smaller than those of P1 (ρ = 28.02 Ω·cm) and P8 (ρ = 6.93 Ω·cm) as shown in Figure 5a. As the nanowire structures are maintained in the P3 film after pressing, the high orientation degree contributes to 5518

dx.doi.org/10.1021/jp307742s | J. Phys. Chem. C 2013, 117, 5515−5520

The Journal of Physical Chemistry C

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temperature of 200 °C for 48−72 h with 2.5−5 mL deionized water added. The direct band gap of Bi2S3 nanowires is about 1.1 eV. From the thermoelectric measurements, the power factor S2/ρ of the film made by the Bi2S3 nanowires is 63.01 μW/mK2, which is much larger than that of the film made by the Bi2S3 nanosheets (5.91 μW/mK2) or nanowire mixed with nansheets (0.93 μW/mK2) due to its lower resistivity and larger Seebeck coefficient. The low resistivity of the Bi2S3 nanowire film is a result of the high carrier concentration and high carrier mobility due to the high orientation degree and better crystallization. The Bi2S3 nanowires orientated along the film plane gives fast electron transmission along the a−c or b−c planes (electron crystal), and efficient phonon scattering between the cleaved a−c planes or b−c planes and between grains of the nanowires (phonon glass). The introduction of many interfaces from smaller size of grains, which scatter phonons more effectively than electrons, or serve to filter out the low-energy electrons at the interfacial energy barriers, allows the enhancement of Seebeck coefficient.

2, from which we can see that P3 film possesses the largest carrier concentration and mobility, which agrees with the Table 2. Power Factor (S2/ρ), Carrier Concentration (n), and Mobility (μ) for the Bismuth Sulfide Samples sample no.

S2/ρ (μW/mK2)

n (1016/cm3)

μ (cm2/(V s))

P1 P3 P8

0.93 63.01 5.91

0.87 5.53 3.84

55.6 210.3 108.7

lowest resistivity measured in Figure 5a. Among the three samples, the band gap of P3 is the smallest (Figure 4), which would lead the higher carrier concentration because a small band gap makes it easier for electrons to be irritated to the conduction band. Therefore, the lowest resistivity of the P3 film is a result of the high carrier concentration and high carrier mobility because of the higher orientation degree and better crystallization. The power factor of the film made by the Bi2S3 nanowires is much larger than that of the film made by the Bi2S3 nanosheets or nanowires mixed with sheets because of its lower resistivity and higher Seebeck coefficient, demonstrating that Bi2S3 with one-dimensional structure can greatly improve its thermoelectric properties (Table 2). Usually Seebeck coefficient is proportional to electric resistivity. However, the Seebeck coefficient of the P3 film is the highest, while its resistivity is the lowest in the P1, P3, and P8 films. To understand this abnormal phenomenon, we take careful check of composition of the P1, P3, and P8 samples. EDX detections reveal the sulfur deficit in the P1 and P8 samples, which are further proved by XRD in existence of BiS impurity phase. The BiS impurity phase in Bi2S3 could result in abnormal phenomenon that the lower Seebeck coefficient is obtained with higher electric resistivity in comparison with the pure Bi2S3 phase from room temperature to 100 °C as reported previously.5 The more BiS impurity phase is in the Bi2S3 sample the lower Seebeck coefficient is obtained. The BiS impurity phase is formed under the condition of low temperature (