Supporting Information for
Supporting Information for n-type Nanostructured Thermoelectric Materials Prepared from Chemically Synthesized Ultrathin Bi2Te3 Nanoplates Jae Sung Son,†, ‡ Moon Kee Choi,† Mi-Kyung Han,§ Kunsu Park,† Jae-Yeol Kim,∥ Seong Joon Lim,⊥ Myunghwan Oh,† Young Kuk,⊥ Chan Park,∥ Sung-Jin Kim,§ and Taeghwan Hyeon*,†
Experimental Section Materials. Bismuth neodecanoate (technical grade), tellurium (99.8%), tri-noctylphosphine (TOP, 90%), 1-dodecanethiol (DDT, ≥ 98%), oleic acid (90%), and ammonia solution (7N in methanol) were purchased from Aldrich Chemical Co. Oleylamine (80~90%) was purchased from Acros Organics. All experimental procedures were conducted under an argon atmosphere using standard Schlenk techniques. Synthesis of Bi2Te3 Nanoplates. First, 5 ml of oleylamine was degassed at 120 oC for 2 h in a vacuum to remove water and oxygen, and then cooled to room temperature. Then, 0.3 mmol of bismuth neodecanoate was added into the oleylamine solution at room temperature. The mixture solution was then heated to 70 oC with vigorous stirring, followed by the addition of 0.072 ml of 1-dodecanethiol (DDT), and then maintained at this temperature for 5 min. Upon the addition of DDT, the initially colorless solution turned yellow, indicating the formation of a bismuth dodecanethiolate complex.S1 Tri-noctylphosphine-tellurium (TOP-Te) was used as a tellurium source, which was prepared by the addition of 0.45 mmol of tellurium powder into 1 ml of tri-n-octylphosphine and subsequent vigorous stirring at room temperature. The TOP-Te solution was injected S1
into the bismuth dodecanethiolate complex solution at 70 oC, and the resulting solution was further aged for 1 h at 70 oC. The Bi2Te3 nanoplates were precipitated by the addition of acetone. The powdery form of Bi2Te3 nanoplates was obtained by centrifugation and the powder was washed several times with ethanol. When the synthesis was performed using 15 times more quantities of reagents (4.5 mmol of bismuth neodecanoate), as much as 1.12 g of Bi2Te3 nanoplates was obtained in a single batch (Figure S4). Removal of Surfactant Coating on Bi2Te3 Nanoplates. The organic surfactant coating on the surface of the nanoplates was removed by the modified version of the method reported by the Weller group.10e First, 15 ml of oleic acid was degassed at 120 oC for 2 h in a vacuum to remove water and oxygen, and cooled to room temperature. Next, the powder of Bi2Te3 nanoplates was added to degassed oleic acid. The mixture was then heated to 60 oC and maintained for more than 12 h at this temperature with vigorous stirring. The mixture was precipitated by the addition of hexane and the powdery form was obtained by the subsequent centrifugation of the precipitate, followed by washing three times with hexane. This powder was added to a solution composed of 15 ml of 7N ammonia in a methanol solution, and then, the resulting mixture was stirred for more than 12 h at room temperature. The precipitate was obtained by centrifugation and subsequent removal of the supernatant. This precipitate was washed once with 7.5 ml of 7N ammonia in a methanol solution, followed by washing several times with methanol. The resulting precipitate was dried for more than 12 h in a vacuum. Typically, ~1.0 g of the surfactant-removed Bi2Te3 nanoplates in a powder form was obtained after the entire procedure (Figure S5). Spark Plasma Sintering of Surfactant-removed Bi2Te3 Nanoplates. To prepare Bi2Te3 nanostructured bulk materials, the surfactant-removed Bi2Te3 nanoplate powder was sintered by SPS. Typically, ~1.0 g of the powder was loaded into a graphite die having an inner diameter of 10 mm in a glove box. They were sintered into pellets using SPS with a pressure of 30 MPa under 660 torr of N2. The sintering temperature was controlled from room temperature to 325 oC at a heating rate of 100 oC/min, and the holding time was 5 min at the sintering temperature. After sintering, disk-shaped pellets with a diameter of 10 mm and thickness of 1~2 mm were obtained (Figure S6).
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Characterization of Materials. The Bi2Te3 nanoplates were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron diffraction (ED), X-ray diffraction (XRD), atomic microscopy spectroscopy (AFM) and elemental analysis. The TEM images and ED patterns were collected on a JEOL JEM-2010 electron microscope. The SEM images were collected on a JEOL JSM-6701F scanning electron microscope. The powder XRD patterns were obtained with a Rigaku D/Max-3C diffractometer equipped with a rotating anode and a Cu Kα radiation source (λ= 0.15418 nm). The AFM images and the height profile were obtained with home-made AFM operated in non-contact mode. Elemental analysis was performed by using a CHNS analyzer (CHNS-932, LECO Corp). Characterization of Thermoelectric Properties. To measure the electrical properties, the pellet was ground into a bar having a length of ~10 mm and width of ~4 mm. The electrical conductivity of the sample was measured by a four-point dc-current switching technique, and the Seebeck coefficient was measured by a static dc method based on the slope of the voltage versus the temperature-difference curves using commercial equipment (ZEM-3, Ulvac-Riko) under a low-pressure helium atmosphere. The carrier concentration was measured by a Hall measurement system (BIO-PAD, HL5500PC) at room temperature in the air. The thermal conductivity was calculated using thermal diffusivity, specific heat capacity, and density. The thermal diffusivity was measured by a laser flash analysis (LFA 457, Netzsch) on the disk-shaped pellets. The specific heat capacity was measured using a differential scanning calorimeter (DSC 204 F1, Netzsch). The density was calculated by measuring volume and mass.
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Figure S1. AFM image and height profile of a single Bi2Te3 nanoplate.
Figure S2. TEM images of Bi2Te3 nanostructures synthesized (a) in the reaction condition of dodecanethiol/bismuth neodecanoate ratio of 3 and by using (b) octadecene and (c) oleic acid as a solvent.
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Figure S3. The room-temperature electrical conductivities of the Bi2Te3 nanostructured bulk samples sintered at various temperatures.
Figure S4. (a) A photograph showing 1.12 g and (b) TEM image of Bi2Te3 nanoplates synthesized in large scale. S5
Figure S5. A photograph showing 1.06 g of the surfactant-removed Bi2Te3 nanoplates.
Figure S6. A photograph showing Bi2Te3 nanostructured bulk pellet prepared by SPS of the surfactant-removed Bi2Te3 nanoplates at 300 oC.
References S1. Romann, T.; Grozovski, V.; Lust, E. Electrochem. Comm. 2007, 9, 2507.
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Experimental Section Materials. Bismuth neodecanoate (technical grade), tellurium (99.8%), tri-noctylphosphine (TOP, 90%), 1-dodecanethiol (DDT, ≥ 98%), oleic acid (90%), and ammonia solution (7N in methanol) were purchased from Aldrich Chemical Co. Oleylamine (80~90%) was purchased from Acros Organics. All experimental procedures were conducted under an argon atmosphere using standard Schlenk techniques. Synthesis of Bi2Te3 Nanoplates. First, 5 ml of oleylamine was degassed at 120 oC for 2 h in a vacuum to remove water and oxygen, and then cooled to room temperature. Then, 0.3 mmol of bismuth neodecanoate was added into the oleylamine solution at room temperature. The mixture solution was then heated to 70 oC with vigorous stirring, followed by the addition of 0.072 ml of 1-dodecanethiol (DDT), and then maintained at this temperature for 5 min. Upon the addition of DDT, the initially colorless solution turned yellow, indicating the formation of a bismuth dodecanethiolate complex.S1 Tri-noctylphosphine-tellurium (TOP-Te) was used as a tellurium source, which was prepared by the addition of 0.45 mmol of tellurium powder into 1 ml of tri-n-octylphosphine and subsequent vigorous stirring at room temperature. The TOP-Te solution was injected S1
into the bismuth dodecanethiolate complex solution at 70 oC, and the resulting solution was further aged for 1 h at 70 oC. The Bi2Te3 nanoplates were precipitated by the addition of acetone. The powdery form of Bi2Te3 nanoplates was obtained by centrifugation and the powder was washed several times with ethanol. When the synthesis was performed using 15 times more quantities of reagents (4.5 mmol of bismuth neodecanoate), as much as 1.12 g of Bi2Te3 nanoplates was obtained in a single batch (Figure S4). Removal of Surfactant Coating on Bi2Te3 Nanoplates. The organic surfactant coating on the surface of the nanoplates was removed by the modified version of the method reported by the Weller group.10e First, 15 ml of oleic acid was degassed at 120 oC for 2 h in a vacuum to remove water and oxygen, and cooled to room temperature. Next, the powder of Bi2Te3 nanoplates was added to degassed oleic acid. The mixture was then heated to 60 oC and maintained for more than 12 h at this temperature with vigorous stirring. The mixture was precipitated by the addition of hexane and the powdery form was obtained by the subsequent centrifugation of the precipitate, followed by washing three times with hexane. This powder was added to a solution composed of 15 ml of 7N ammonia in a methanol solution, and then, the resulting mixture was stirred for more than 12 h at room temperature. The precipitate was obtained by centrifugation and subsequent removal of the supernatant. This precipitate was washed once with 7.5 ml of 7N ammonia in a methanol solution, followed by washing several times with methanol. The resulting precipitate was dried for more than 12 h in a vacuum. Typically, ~1.0 g of the surfactant-removed Bi2Te3 nanoplates in a powder form was obtained after the entire procedure (Figure S5). Spark Plasma Sintering of Surfactant-removed Bi2Te3 Nanoplates. To prepare Bi2Te3 nanostructured bulk materials, the surfactant-removed Bi2Te3 nanoplate powder was sintered by SPS. Typically, ~1.0 g of the powder was loaded into a graphite die having an inner diameter of 10 mm in a glove box. They were sintered into pellets using SPS with a pressure of 30 MPa under 660 torr of N2. The sintering temperature was controlled from room temperature to 325 oC at a heating rate of 100 oC/min, and the holding time was 5 min at the sintering temperature. After sintering, disk-shaped pellets with a diameter of 10 mm and thickness of 1~2 mm were obtained (Figure S6).
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Characterization of Materials. The Bi2Te3 nanoplates were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron diffraction (ED), X-ray diffraction (XRD), atomic microscopy spectroscopy (AFM) and elemental analysis. The TEM images and ED patterns were collected on a JEOL JEM-2010 electron microscope. The SEM images were collected on a JEOL JSM-6701F scanning electron microscope. The powder XRD patterns were obtained with a Rigaku D/Max-3C diffractometer equipped with a rotating anode and a Cu Kα radiation source (λ= 0.15418 nm). The AFM images and the height profile were obtained with home-made AFM operated in non-contact mode. Elemental analysis was performed by using a CHNS analyzer (CHNS-932, LECO Corp). Characterization of Thermoelectric Properties. To measure the electrical properties, the pellet was ground into a bar having a length of ~10 mm and width of ~4 mm. The electrical conductivity of the sample was measured by a four-point dc-current switching technique, and the Seebeck coefficient was measured by a static dc method based on the slope of the voltage versus the temperature-difference curves using commercial equipment (ZEM-3, Ulvac-Riko) under a low-pressure helium atmosphere. The carrier concentration was measured by a Hall measurement system (BIO-PAD, HL5500PC) at room temperature in the air. The thermal conductivity was calculated using thermal diffusivity, specific heat capacity, and density. The thermal diffusivity was measured by a laser flash analysis (LFA 457, Netzsch) on the disk-shaped pellets. The specific heat capacity was measured using a differential scanning calorimeter (DSC 204 F1, Netzsch). The density was calculated by measuring volume and mass.
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Figure S1. AFM image and height profile of a single Bi2Te3 nanoplate.
Figure S2. TEM images of Bi2Te3 nanostructures synthesized (a) in the reaction condition of dodecanethiol/bismuth neodecanoate ratio of 3 and by using (b) octadecene and (c) oleic acid as a solvent.
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Figure S3. The room-temperature electrical conductivities of the Bi2Te3 nanostructured bulk samples sintered at various temperatures.
Figure S4. (a) A photograph showing 1.12 g and (b) TEM image of Bi2Te3 nanoplates synthesized in large scale. S5
Figure S5. A photograph showing 1.06 g of the surfactant-removed Bi2Te3 nanoplates.
Figure S6. A photograph showing Bi2Te3 nanostructured bulk pellet prepared by SPS of the surfactant-removed Bi2Te3 nanoplates at 300 oC.
References S1. Romann, T.; Grozovski, V.; Lust, E. Electrochem. Comm. 2007, 9, 2507.
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