LETTER pubs.acs.org/NanoLett
Al-Doped Zinc Oxide Nanocomposites with Enhanced Thermoelectric Properties Priyanka Jood,†,|| Rutvik J. Mehta,†,§ Yanliang Zhang,‡ Germanas Peleckis,|| Xiaolin Wang,|| Richard W. Siegel,†,§ Theo Borca-Tasciuc,‡,§ Shi Xue Dou,|| and Ganpati Ramanath*,†,§ †
Materials Science and Engineering Department, ‡Department of Mechanical, Aerospace and Nuclear Engineering, and Rensselaer Nanotechnology Center, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, New York 12180, United States Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2519, Australia
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bS Supporting Information ABSTRACT: ZnO is a promising high figure-of-merit (ZT) thermoelectric material for power harvesting from heat due to its high melting point, high electrical conductivity σ, and Seebeck coefficient α, but its practical use is limited by a high lattice thermal conductivity kL. Here, we report Al-containing ZnO nanocomposites with up to a factor of 20 lower kL than non-nanostructured ZnO, while retaining bulklike α and σ. We show that enhanced phonon scattering promoted by Al-induced grain refinement and ZnAl2O4 nanoprecipitates presages ultralow k ∼ 2 Wm 1 K1 at 1000 K. The high α∼ 300 μV K1 and high σ ∼ 1104 Ω1 m1 result from an offsetting of the nanostructuring-induced mobility decrease by high, and nondegenerate, carrier concentrations obtained via excitation from shallow Al donor states. The resultant ZT ∼ 0.44 at 1000 K is 50% higher than that for the best non-nanostructured counterpart material at the same temperature and holds promise for engineering advanced oxide-based high-ZT thermoelectrics for applications. KEYWORDS: Zinc oxide, nanostructured thermoelectrics, aluminum doping, microwave synthesis, heat-harvesting, high figure of merit
lectrical power generation from waste heat,1 for example, in cars, aircrafts, power plants, and thermophotovoltaic cells, requires high stability thermoelectric materials with high figures of merit ZT at temperatures above 600 K, where ZT = α2σT/k, α is the Seebeck coefficient, σ the electrical conductivity, k the thermal conductivity, and T the absolute temperature. Nontoxic and low-cost oxides such as ZnO are promising for such applications because their excellent charge carrier transport properties2,3 are tunable via doping.4,5 However, decreasing k is a major challenge because of the high lattice thermal conductivity kL of these materials due to the noncomplex Wurtzite structure. For example, Al-doped non-nanostructured ZnO is one of the best n-type thermoelectric oxides5 with ZT ∼ 0.17 at 1000 K (reaching a peak 0.3 at 1273 K), but k1273K = 5 Wm1 K1 is factorially higher than k for chalcogenide alloys at their operational temperatures.1 Nanostructuring6 has been shown to be more effective than incorporating dopants and/or alloying elements7 or nanopores8 to decrease kL. Since the electronic component of the thermal conductivity ke is 10- to 100-fold lower than kL in ZnO,9 kL diminution directly translates to a lower k, which promotes ZT enhancement. Here, we report the first-time creation of bulk n-type ZnObased nanocomposites through the assembly of Al-doped ZnO nanocrystals synthesized by a rapid scalable approach (see Figure 1). The bulk nanocomposite pellets obtained by cold-pressing and sintering the nanocrystals consist of ZnO nanograins and ZnAl2O4
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nanoprecipitates and exhibit more than 20-fold lower k300K to values as low as ∼1.5 Wm1 K1. This presages ultralow thermal conductivities of k1000K∼ 1 to 2 Wm1 K1 at 1000 K, while retaining high electrical conductivities comparable to non-nanostructured ZnO and large Seebeck coefficients as high as α300K ∼ 300 μV K1, consistent with theoretical predictions10 for nanocomposites. As a consequence, one can attain ZT ∼ 0.44, which is 50% higher than the best reported value for a non-nanostructured counterpart, thereby paving the way for further ZT enhancements through a combination of doping and nanostructuring. Multigram quantities of Al-doped ZnO nanocrystals were synthesized by a rapid (>2 g/min) and scalable microwaveactivated thermal decomposition of zinc and aluminum acetates in pentanediol with oleylamine as a surfactant (scheme in Figure 1). While the oleylamine/acetate ratio and temperature are key factors in conventional ZnO synthesis via aminolytic reaction,11 microwave dose also allows control over nanocrystal shape and size.12 Microwave exposures of e3 min, corresponding to microwave dose of ∼16 kJ/g, result in ZnO nanocrystals with average sizes of davg e 25 nm with a ∼50% yield (Figure 2a). Increasing the microwave dose to ∼4860 kJ/g increases the yield to ∼6575% and davg to ∼35 nm accompanied by up to Received: July 17, 2011 Revised: September 4, 2011 Published: September 12, 2011 4337
dx.doi.org/10.1021/nl202439h | Nano Lett. 2011, 11, 4337–4342
Nano Letters
LETTER
Figure 1. Schematic illustration of the fabrication of high ZT Alcontaining ZnO nanocomposites by the cold-pressing and sintering of Al-doped ZnO nanocrystals synthesized by microwave-stimulated solvothermal synthesis. This approach results in 20-fold lower thermal conductivity than non-nanostructured Al-ZnO composites due to enhanced phonon scattering at nanograin boundaries and ZnAl2O4 nanoprecipitates, and the retention of bulklike power-factors due to Al doping.
Figure 3. (a) Nanocrystal dimensions normal to the (1010) and (0002) planes as a function of Al doping. Insets show a schematic sketch of the ZnO unit cell illustrating the relative orthogonal orientation of the two crystallographic planes and the nanocrystal aspect ratio calculated from their relative peak widths as a function of Al-doping. (b) Optical absorption spectra from as-synthesized Al-doped ZnO nanocrystal solutions showing the shift in absorption band edge with Al-doping. (c) Plots of the bandgap change (red circles, left axis) and the quantum confinement contribution ΔEQC (blue squares, right axis), and BursteinMoss contribution ΔEBM (green triangles, right axis) to it, as a function of Al-doping.
Figure 2. SEM micrographs from ZnO nanocrystals synthesized with microwave doses of (a) 16 and (b) 48 kJ/g. Insets show the nanocrystal size distributions. (c) Bright-field TEM micrograph showing faceted ZnO nanocrystals doped with 0.25 atom % Al. (d) A representative electron diffraction pattern showing Bragg rings consistent with the wurtzite structure.
3-fold aspect ratio increase (Figure 2b). A possible mechanism for anisotropic growth at higher microwave dose could be extended interactions between pentanediol13 and positive charges14 on the (0001) facets of ZnO. Transmission electron microscopy (TEM) and selected-area electron-diffraction from the as-synthesized ZnO nanostructures (see Figure 2c,d) indicate that each nanoparticle is a single-crystal
with the wurtzite structure, as confirmed by X-ray diffraction (XRD) from the nanocrystal powders. The increase in the (0002) peak width with Al doping (Supporting Information Figure S1) corresponds to about a factor of 2 decrease in grain size from 30 to 15 nm as Al doping is increased from 0.5 to 2 atom % (see Figure 3a). In contrast, the (1010) peak width decreases upon Al doping, but is insensitive to the extent of Al doping. These results indicate that Al doping decreases nanocrystal size and promotes shape anisotropy (Figure 3a inset), likely via Alinduced variations in oxygen stoichiometry on the (0001) facets.15 Optical absorption spectra (see Figure 3b) from colloidal ZnO nanocrystal solutions reveal a monotonic bandgap increase due to Al doping, as reported for non-nanostructured ZnO16 (see Figure 3c). The bandgap increase is attributable to quantum confinement-induced bandgap increase16,17 and BursteinMoss effects. The quantum confinement effect ΔEQC increases to 0.21 eV upon 2 atom % Al doping. The BursteinMoss contribution is significant at low doping levels, that is, 0.35 < ∼ΔEBM < ∼0.45 eV for e0.5 atom % Al doping, but diminishes to ∼0.04 eV with increasing doping. The absence of red shifts, characteristic of a semiconductor-to-metal Mott transition18 in degenerately doped ZnO, implies subdegenerate doping in the Al-doped nanocrystals. While high carrier concentrations favor high σ, the subdegenerate carrier concentrations are conducive for high α, as described below. 4338
dx.doi.org/10.1021/nl202439h |Nano Lett. 2011, 11, 4337–4342
Nano Letters Bulk nanostructured pellets with 90 ( 3% theoretical density were fabricated by cold-pressing 0.32 g of the ZnO nanocrystals, followed by sintering in air at 950 °C (see Figure 1) to preserve O/Zn stoichiometry. X-ray spectroscopy diffractograms from the pellets indicate the retention of the wurtzite structured ZnO, but reveal a secondary ZnAl2O4 phase for >2 atom % Al doping (see Figure 4a). Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) measurements show ZnAl2O4 precipitates for all Al-doping levels investigated, indicating that the precipitate fraction for
