Large-scale synthesis and characterization of the size-dependent thermoelectric properties of uniformly sized bismuth nanocrystals.

Highly efficient thermoelectric materials have attracted tremendous attention because of various technological applications such as power generation from waste heat and environmentally friendly refrigeration. The efficiency of thermoelectric materials is generally evaluated in terms of thermoelectric figure of merit ZT= (sS/k)T, where s is the electrical conductivity, S is the Seebeck coefficient, k is the thermal conductivity, and T is the absolute temperature. Recently, various nanostructured thermoelectric materials have been reported to exhibit high ZT values. This increase in thermoelectric efficiency was attributed to the decrease of thermal conductivity caused by the increased interfaces to scatter phonons or the enhancement of power factor (sS) by quantum confinement effects. However, most of the highZT nanostructured materials were prepared by costly and complicated processes, making it very difficult to inexpensively synthesize a large quantity of nanostructured materials. More recently, several kinds of nanostructured bulk materials with high ZT values were fabricated in large quantity by a ball-milling process and subsequent hot-press process. Recently, colloidal chemical methods have been used to synthesize large quantities of uniform-sized nanocrystals. These chemical methods can synthesize uniform-sized nanocrystals in a size-controlled manner, allowing the characterization of size-dependent properties, which is very difficult to perform using top-down physical methods, such as the ballmilling process. Over the past few decades, intensive research has attempted to characterize the electrical properties of bulk bismuth (Bi), because it is semimetallic with a small band overlap and has high carrier mobility and extremely small carrier effective mass. Furthermore, thermoelectric properties of Bi nanocrystals were intensively studied, because theoretical calculations predicted that Bi nanocrystals can exhibit a ZT value as high as 10 at 77 K. Moreover, Bi costs around one tenth of the price of bismuth telluride, which is one of the most popular thermoelectric materials. However, a high ZT value has not yet been realized experimentally for Bi nanostructured materials. Although several chemical syntheses of Bi nanocrystals have been reported, the thermoelectric properties of spherical Bi nanocrystals have rarely been studied. Herein, we report a simple and largescale synthetic method to produce uniform-sized Bi nanocrystals with controlled sizes and characterized their sizedependent thermoelectric properties. The size-dependant electrical and thermal properties were clearly demonstrated using uniform Bi nanocrystals with controlled particle sizes. Interestingly, the ratio of electrical to thermal conductivity increased with decreasing particle size, which leads to the enhancement of the ZT values. Bi nanocrystals were synthesized by reducing bismuth dodecanethiolate, which was generated by the reaction of dodecanethiol and bismuth neodecanoate in octadecene. Bismuth thiolate was so reactive that Bi nanocrystals could be readily produced by injecting the mild reducing agent tri-noctylphosphine (TOP) into bismuth dodecanethiolate solution at a temperature as low as 80 8C. The sizes of Bi nanocrystals could be easily tuned by varying the aging temperature and time. Transmission electron microscopy (TEM) images (Figure 1a–f) show uniform-sized Bi nanocrystals with sizes ranging from 6 to 27 nm. All of the nanocrystals exhibited narrow size distribution with standard deviation of less than 10%. The electron diffraction patterns (Figure 1a–f, insets) revealed the highly crystalline nature of the Bi nanocrystals. The X-ray diffraction (XRD) patterns (Figure 1g) showed that all of the nanocrystals had a rhombohedral Bi structure (JCPDS 85-1331), while the peaks became broader as the size decreases. To demonstrate large-scale production, we performed the reaction with 20 mmol of bismuth precursor and obtained as much as [*] J. S. Son, K. Park, Prof. T. Hyeon National Creative Research Initiative Center for Oxide Nanocrystalline Materials World Class University (WCU) Program of Chemical Convergence for Energy & Environment (C2E2) School of Chemical and Biological Engineering Seoul National University, Seoul 151-744 (Korea) Fax: (+ 82)2-886-8457 E-mail: thyeon@snu.ac.kr