| Thermoelectric materials have attracted tremendous attention and research interest for their ability to convert energies between heat and electricity directly, without needing any moving parts, the working medium, or any poisonous and harmful substances directly emissions, in the background that environment pollution and energy shortage being the two major challenges in this rapid developing society. Among all the thermoelectric materials, the alloys of V-VI binary compounds, such as Bi2Te3 and Bi2Se3, has a similar lattice structure of rhombohedral layered structure. These materials of a binary alloy can be mixed by arbitrary proportion to form a continuous solid solution alloy, and can be reduced the lattice thermal conductivity and achieved enhanced thermoelectric properties by fine adjusting its carrier concentration. Therefore, Bi2Se3 is one of the most promising thermoelectric materials and is suitable for work near the room temperature range from 300 K to 500 K. It is an effective way to enhance the thermoelectric performance of materials by lowering thermal conductivity or enhancing the Seebeck coefficient.The rhombohedral structure with slabs of five shifted Se and Bi atomic layers stacked along the c-axis, and the adjacent Se(2)-Se(2) atomic layers are bound by van der Waals faces. This is similar to the layer-structured TiS2 where the van der Waals gap allows for intercalation by a wide range of both organic and inorganic materials and leads to the reduction of the thermal conductivity and an improvement of the thermoelectric performance. Hence, the introduction (doping) of guest atoms into the van der Waals gap of Bi2Se3 appears as promising way to enhance the thermoelectric performance.However, it is impossible that the phonon mean free path be less than the interatomic distance even lowering thermal conductivity by all kinds of ways. Therefore, there is a limitation to enhance the thermoelectric performance of materials by reducing the lattice thermal conductivity when the lattice thermal conductivity is small enough. In addition, it will inevitably lead to the decrease of mobility that the introduction of scattering center or scattering interface into the conventional semiconductor materials either by doping or nanostructuring. Therefore, to increase the conductivity of materials is full of challenge. Beside the properties above, Bi2Se3 is proved to be one of the three dimensional topological insulators recently, which has a robust topological protected surface state even up to room temperature. This new quantum state of matter is characterized by gapped insulating bulk states and gapless conducting surface states that are not affected by non-magnetic detects. Studies have shown that three dimensional topological insulators have been theoretically proposed as good thermoelectric materials. The results showed that by utilizing the unique properties of TIs, i.e., topologically protected conducting-surface states, the thermoelectric efficiency of TI materials can be improved markedly. Among the several kind of three dimensional topological insulators materials, Bi2Se3 has received much attention recently due to its several point of advantages as following:first and foremost, topological insulator Bi2Se3 single crystal is much easier to be synthesized by precise control the consistent elements Bi and Se in stoichiometric ratios; besides, The strong three dimensional topological insulators Bi2Se3 was found to have a single Dirac cone with a Dirac point at the surface Brillouin zone center, which is similar to that of the ideal topological insulators; last but not least, Bi2Se3 has a relatively large band-gap that Eg=0.3 eV, which is equivalent to 3600 K, this is much higher than the energy scale of room temperature (about 0.026 eV). This means that Bi2Se3 is much suitable for utilization near or above room temperatures than the other three dimensional topological insulators. Here we employed the approach of decreasing the grain size of bulk Bi2Se3 to nanometer scale to increase the surface to bulk ratio, whose effect is threefold:(1) to increase mobility (or electrical conductivity) due to an increased contribution of conducting surfaces; (2) to suppress lattice thermal conductivity through enhanced phonon scattering of the numerous interfaces or the grain boundaries; (3) to increase thermopower through energy filtering effect due to carrier scattering at the interface potentials.In this work, we firstly prepared bulk samples of CuxBi2Se3 with different Cu contents (x=0,0.005,0.010,0.015,0.020) and studied their thermoelectric properties in the temperature range from 300 K to 590 K. Our results show that the thermal conductivity of the moderately doped CuxBi2Se3 samples (x=0.010,0.015) is remarkably reduced; at the same time, the electrical resistivity is sizably decreased. Consequently, these doped compounds exhibit excellent thermoelectric performance. The largest ZT of about 0.54 at 590 K is obtained for Cu0.01Bi2Se3.After that, the nanostructured Bi2Se3 samples with different average grain size were also fabricated by melting, milling and hot pressing, and the thermoelectric properties has been studied, the results show that a high value of ZT=～0.6 is obtained for the Bi2Se3 samples at T>450 K due to increased contribution of conduction surfaces upon decreasing grain dimensions. As grain size decreases from microns to ～80nm in thickness, the electron mobility μ increases steeply from 12-15cm2V-1s-1 to ～600cm2V-1s-1,owing to the contribution of increased topologically-protected conducting surfaces. Simultaneously, its lattice thermal conductivity is lowered by ～30-50% due to enhanced phonon scattering from the increased grain boundaries. As a result, thermoelectric figure of merit, ZT, of all the fine-grained samples is improved. Specifically, a maximum value of ZT=-0.63 is achieved for Bi2Se3 at T=～570 K. |
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