Thermoelectric properties of Al-doped ZnO ...

Vol. 37, No. 9

Journal of Semiconductors

September 2016

Thermoelectric properties of Al-doped ZnO: experiment and simulation S. Jantrasee1; Ž , P. Moontragoon2 , and S. Pinitsoontorn2 1 Materials

Science and Nanotechnology Program, Faculty of Science, Khon Kaen University, Khon Kaen, Thailand of Physics, Faculty of Science, Khon Kaen University, Khon Kaen, Thailand

2 Department

Abstract: Advancement in doping other elements, such as Ce, Dy, Ni, Sb, In and Ga in ZnOŒ1 , have stimulated great interest for high-temperature thermoelectric application. In this work, the effects of Al-doping in a ZnO system on the electronic structure and thermoelectric properties are presented, by experiment and calculation. Nanosized powders of Zn1 x Alx O (x D 0; 0:01; 0:02; 0:03 and 0.06) were synthesized by hydrothermal method. From XRD results, all samples contain ZnO as the main phase and ZnAl2 O4 (spinel phase) peaks were visible when Al additive concentrations were just 6 at%. The shape of the samples changed and the particle size decreased with increasing Al concentration. Seebeck coefficients, on the other hand, did not vary significantly. They were negative and the absolute values increased with temperature. However, the electrical resistivity decreased significantly for higher Al content. The electronic structure calculations were carried out using the open-source software package ABINITŒ2 , which is based on DFT. The energy band gap, density of states of Al-doped ZnO were investigated using PAW pseudopotential method within the LDA C U . The calculated density of states was then used in combination with the Boltzmann transport equationŒ3 to calculate the thermoelectric parameters of Al-doped ZnO. The electronic band structures showed that the position of the Fermi level of the doped sample was shifted upwards in comparison to the undoped one. After doping Al in ZnO, the energy band gap was decreased, Seebeck coefficient and electrical conductivity were increased. Finally, the calculated results were compared with the experimental results. The good agreement of thermoelectric properties between the calculation and the experimental results were obtained. Key words: ZnO; doping; thermoelectric property DOI: 10.1088/1674-4926/37/9/092002 PACS: 77.55.hf

1. Introduction As the world’s demand for energy consumption increases, energy conservation is central to prolong our finite resources. The energy challenge has become one of the most essential problems to our world nowadays. Waste heat from automobiles, factories, and other similar sources totals to about 70% of the total primary energy produced, but is very difficult to reclaim. Thermoelectric materials and devices offer the only approach to recycling this waste heat. Also, thermoelectric technology is a friendly solution to the environment, and it operates without mechanical moving parts of the use of CFC (chlorofluorocarbon) gas. In addition, thermoelectric materials can be incorporated as part of the solid-state devices that convert solar energy into electricity. Despite their low efficiency (7%–8%) of current materials and devices, the development of high-performance thermoelectric materials becomes an important research area that receives great attention. Thermoelectric power generation had been identified as a major conservation technique by increasing energy conversion efficiency. In a conventional car, 80% of the fuel’s energy will be lost as heat and thermoelectricity hold the key to convert this large amount of waste heat back into useful electricity. Power generation by Seebeck effect was proposed in 1940 and was under intensive research until the 1960s. Practical applications were not established except in aerospace. Recently,

EEACC: 2520E

as the world’s oil prices soar and there are more concerns about energy resources and the environment, there has been much attention and funding given to clean energy globallyŒ4 . Thermoelectric materials are more efficient at the higher temperatures due to higher Carnot efficiency. Although inter-metallic compounds are mainly used as thermoelectric material, they are not stable at high temperatures in air. Therefore, an oxide semiconductor can be expected to be a good candidate of thermoelectric material with high performance. Ce, DyŒ5 NiŒ6 , SbŒ7 , In and GaŒ8 in ZnO are of great interest for high temperature thermoelectric application with highest power factor of 1.5, 4.5, 6.0, 3.9, 5.5 and 6.2  10 4 W/(m
K2 /, respectively. Recently, aluminum-doped zinc oxide has been identified to exhibit good thermoelectric properties and has received considerable attention thereafter. These oxide-based thermoelectric materials have many advantages compared to the inter-metallic alloys, such as nontoxicity, thermal stability, oxidation resistance, etc. Among the best n-type thermoelectric oxides, Aldoped ZnO has shown the most promising potential because of its high stability and distinguished thermopower. The potential of ZnO as the thermoelectric material was recognized in 1996 by Ohtaki et al.Œ9 and the current highest power factor reading of Al-doped ZnO was 16  10 4 W/(m
K2 /Œ10 from the same research group. ZnO is a semi-conductive oxide but when Zn was substituted by Al, the electrical conductivity increased. However above 2 mol%,

† Corresponding author. Email: [email protected] Received 4 March 2016, revised manuscript received 11 May 2016

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a ZnAl2 O4 spinel phase was detected and this decreased the electrical conductivity. Researchers are currently trying different element co-doping and varying compositions to optimize the thermoelectric properties. It was a fast-paced experimentalbased research. In this work, the effects of Al doping on the electronic structure and thermoelectric properties of the ZnO system were investigated using the local density approximation with the Hubbard model (LDA C U ) to calculate the energy band structure. The Boltzmann transport equation within the constant scattering time approximation was then used to calculate the thermoelectric parameters of Al-doped ZnO. The calculated results were compared with the thermoelectric properties of Aldoped ZnO which was synthesized by hydrothermal method. Figure 1. XRD patterns of ZnO and various Al-doped ZnO.

2. Experiment 2.1. Sample preparation First, we mixed aqueous solutions of zinc and aluminum acetates in de-ionized water solution in the ratio of Alx Zn1 x (C2 O4 /
2H2 O (x D 0, 0.01, 0.02, 0.03 and 0.06). Second, the solution was filled with 1 mol NaOH solution to precipitate in the autoclave. Third, the precursor was synthesized by hydrothermal method. After that, the precipitates were washed with copious quantities of de-ionized water. Then the precipitates at each Al concentration were dried at 80 o C for 6 h. Finally, the Alx Zn1 x O materials were kept in humidity controlled cabinet at room temperature before characterizations. 2.2. Characterization All samples were analyzed using an X-ray diffractometer (Phillips, PW3020) with Cu anode (K˛/ at 40 kV, 200 mA, and step scan of 0.01ı . Before thermoelectrical characterization, all samples were cut using a precision diamond cutter from the conversion-pressed pellets as rectangular bars of 10  2:5 2 mm3 and polished with SiC emery paper of grits 180, 600 and 1000. All faces were polished to ensure a high reproducibility in measurements. Morphology and particles’ size of samples were characterized by Scanning Electron Microscopy (LEO, SEM 1450VP). Seebeck coefficient and electrical resistivity were measured by static DC method (ULVAC, ZEM-3) under He atmosphere from room temperature (RT) up to 400 ıC and temperature gradients of 20, 30, and 40 ıC. All measurements were repeated twice and in both cases, similar values of Seebeck coefficient and electrical resistivity were found, suggesting that at least after short intervals at 400 ıC, the samples are chemically and thermally stable. The thermal conductivity was determined from the thermal diffusivity obtained by the laser flash method (ULVAC, TC-7000) and the specific heat capacity calculated by the Dulong-Petit relation.

3. Computation method At ambient temperatures and pressures, the stable phase of ZnO is a hexagonal wurtzite structure, which belongs to the P63 mc space group, each primitive cell contains two O atoms and two Zn atoms, each Zn atom is located in the center of the tetrahedron made up of four O atoms, i.e. Zn–O4 tetrahedron, and the arrangement of the O atom is similar to that of

ZnŒ11 . The lattice constants are as follows: a D b D 3:250 Å, c D 5:212 Å and ˛ D ˇ D 90ı , D 120ı of wurtzite structure are used as initial values of the original unit cell. In this work, all the calculations were carried out using the opensource software package ABINIT codeŒ2 , which is based on density functional theory (DFT), using the local density approximation (LDA) as the exchange-correlation function plus U (LDACU / with the U D 10 eV on-site Coulomb interaction energy. A ZnO structure was simulated by using 32-atom 2  2  2 supercell. An Al-doped ZnO was constructed by substituting one of the Zn atoms in the supercell with an Al atom. In order to simulate the ordered Zn1 x Alx O alloys with wurtzite structure, we employed 32-atom supercells, which corresponds to a supercell that is twice the size of a primitive wurtzite unit cell in base plane direction. The ions were described using a self-consistent PAW pseudopotential scheme, in which the orbitals of Al(3s2 3p1 /, Zn(3d10 4s2 /, and O(2s2 2p4 / were treated as valence electrons. To ensure the convergence of our calculation, we carefully tested the total energy depending on the cutoff energy and k-point sampling. The final set of cutoff energy is chosen to be 15.0 Hatree (408 eV) so that the wave functions are expanded in a planewave basis set up, self-consistent convergence precision is 5  10 6 eV for each atom. All thermoelectric properties reported here were calculated with BoltzTraP, a code developed by Madsen and SinghŒ3 . Some of the calculation of specific parameters used by BoltzTraP were seen in a previous workŒ12 .

4. Results and discussion X-ray diffraction analysis for these samples showed that most of the observed XRD peaks are assigned to those of pure ZnO phase (JCPDF #75-0576) (Figure 1). All samples contain ZnO as the main phase and ZnAl2 O4 (spinel phase, JCPDF # 05-0669) as a minority secondary phase. The success of Aldoped ZnO synthesized by hydrothermal method overcame the limit of Al doping 3 at% with no second phase. However, when Al additive concentrations were just 6 at% (x D 0.06), then the ZnAl2 O4 diffraction peaks were visible, which is in good agreement with the first result reported by BransonŒ13 . The detailed mechanisms of the ZnAl2 O4 reaction were shown and pointed out that the formation is a result of Zn ions diffusing in Al2 O3 . The particle size and lattice parameter of samples are

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Table 1. Variation of the particle size, lattice parameter and c/a lattice ratio shown as a function of Al additive concentration. Lattice parameter (Å) Materials Particle size (nm) c=a ratio a c ZnO 105.72 ˙ 9.2 3.249 5.212 1.604 1% Al 75.92 ˙ 8.1 3.250 5.211 1.603 2% Al 66.12 ˙ 8.8 3.251 5.210 1.603 3% Al 57.88 ˙ 8.5 3.251 5.207 1.602 6% Al 56.61 ˙ 9.3 3.252 5.208 1.601

Figure 2. The particle size and c=a ratio of ZnO and various Al-doped ZnO.

showed in Table 1. The particle size of samples decrease with increasing of Al. The beneficial effect of aluminum has been attributed to aluminum decreasing the c/a ratio of the crystal. In general, lattice parameter a tends to increase while c tends to decrease with respect to ZnO standard sample with amount of Al doping. Figure 2 shows the total distortion of ZnO lattice, estimated from the c/a lattice ratio, as a function of Al additive concentration. In spite of the scattering in a and c values, a clear correlation between c/a ratio and doping concentration is observed. In hexagonal structures, c/a lattice ratio is a well-known index for characterizing doping effects because c/a ratio refers to the most energetically stable configuration for a given hexagonal structure even under different loading conditions. Shirouzu et al.Œ14 studied the distribution and solubility limit of Al in sintered ZnO ceramics (3 at%), and although their samples were prepared by a colloid method and at a given additive concentration (1 at%), their lattice parameters follow a similar c/a tendency with those observed in this study: 1.601 and 1.604 for doped and undoped samples, respectively. A correlation between c/a lattice ratio and the maximum power factor for this kind of ceramic was also found. Finally, the results suggest that in an ion-doped ZnO system, a high compression of c/a ratio due to heavy doping could be a key to improve the power factor. Some results from the micro structure analysis by SEM are presented in Figures 3(a)–3(e). Scanning electron microscope analysis for the powders synthesized under hydrothermal conditions show the influence of the aluminum dopant content on the nanopowders morphology and distribution. SEM images are collected at high magnifications to investigate the morphology and size of particles. The magnification of this image is 500

and 3 000 times and reference bar of 1 m, the average particle sizes are around 100–200 nm. With increasing aluminum content the morphology changes from hexagonal prism shape to spherical shape with a homogeneous distribution of the particles, agreement with Zhang and MuŒ15 . ZnO is a polar crystal, O2 is in hexagonal closest packing, and each Zn2C lies within a tetrahedral group of four oxygen ions. Zn and O atoms are arranged alternatively along the c-axis and a polar top face (0001) consisting of tetrahedral zinc having a terminal OH ligand. The optical properties of the doped ZnO were measured and compared with undoped ZnO to investigate the effect of doping on the band gap. The UV-VIS absorption spectra of the doped ZnO powders in Figure 4 shows similar characteristic with the typical pure ZnO phase. The absorption edge of the Al-doped samples ( D 380 nm) is also close to that of the undoped sample with the absorption edge at 376 nmŒ16 . The Tauc method was implemented for calculating the optical band gap of the samples using the EquationŒ13 : .˛h/n D A.h

Eg /;

(1)

where ˛, h, A and Eg are absorption coefficient, photon energy, relation constant and optical band gap, respectively. For semiconductors with the direct band gap, n D 2, and for the indirect band gap, n D 0:5. In this study, the optical band gap is calculated with the direct band gapŒ17 . Plotting .˛h/2 versus photon energy .h/ and extrapolation of the linear part to the photon energy axis results in the band gap value. The optical band gap of ZnO is 3.15 eV, while Al-doped ZnO band gaps decrease from 3.07, 3.02, 3.00 and 2.95 eV when Al doping increases, respectively. The redshift of the band gap with incorporating Al into ZnO was observed. The decrease in the energy gap appears to originate from the active transitions involving 3d levels in Al3C ions and strong sp-d exchange interaction between the travelling “sp” carriers (band electrons) and the localized “d” electrons of the dopantŒ14 . Besides, the narrowing of the band gap can improve electrical conductivity. 4.1. Thermoelectric properties The Seebeck coefficient (S/ and electrical conductivity (/ (the conversion of the electrical resistivity (// of Al-doped ZnO were measured by ZEM-3 technique. Figure 5 shows the temperature and Al-content dependence of the Seebeck coefficient for Zn1 x Alx O ceramics prepared by the hydrothermal method. They have distribution of Seebeck coefficient when the Al-doping increased from 1% to 6% by mole. The sign of Seebeck coefficient of all samples is negative (n-type), the absolute value of the Seebeck coefficient increases with increasing x up to x D 0:03 and maximum value is 597 V/K at 100 ıC. While x D 0:06, it is almost stable around 365 to

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Figure 3. SEM images of ZnO, 1%, 2%, 3% and 6% Al-doped ZnO at magnitude of 500  and 3,000  , respectively.

Figure 4. (Color online) The optical band gap of ZnO and Al-doped ZnO.

Figure 5. The Seebeck coefficient of ZnO and various Al-doped ZnO.

390 V/K and minimum from others. The experimental results are in agreement with those reported by Tsubota et al.Œ9 , whose samples were prepared by the conventional solid-state

Figure 6. The electrical conductivity (/ of ZnO and Al-doped ZnO.

reaction method. The temperature dependence of the electrical conductivity (/ of ZnO and Al-doped ZnO samples show in Figure 6. All the investigated samples showed a semiconducting behavior over the whole measured temperature range, i.e. the electrical conductivity increases with increasing temperature. The maximum of electrical conductivity is 75.48  1 m 1 at 400 ıC (6% Al), while  of ZnO is 16.01  1 m 1 at the same temperature. The power factor of the samples are calculated by PF D S 2 , and showed in Figure 7. The maximum power factor (PF) is 16.310 4 W/(m
K2 / at 400 ıC (3% Al) and this value is higher than pure ZnO by about fourfold. When compared with Al-doped ZnO thin films (4.8710 4 W/(m
K2 //Œ18 , the thermopower of bulk nanomaterials is measured 12.010 4 W/(m
K2 / and higher from the same concentration of aluminum doping. The thermal conductivity was lowest from 3% Al doping at 400 ıC and equal to 1.59 W/(m
K), while  values of 2% Al doping showed 2.15 W/(m
K) lower than the results from

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Figure 7. The power factor of ZnO and various Al-doped ZnO.

Figure 9. The dimensionless figure of merit ZT of ZnO and Al-doped ZnO.

Figure 8. The thermal conductivity of pure ZnO and Al-doped ZnO.

Ohtaki et al. (13 W/(m
K) at 400 ıC)Œ10 . The thermal conductivity considerably decreases with Al contents, which may arise from the random disorder phonon scattering induced by the Al deficient sites, see Figure 8. The thermal conductivity  is dominated by the lattice thermal conductivity l because of the relatively low electrical conductivity of the materials. This result indicates a possibility to further improve the thermoelectric performance of Al-doped ZnO ceramics by enhancing the Al-doping in ZnO through processing modification. Nevertheless, the total thermal conductivity of Al 6 at% in ZnO increasing from 3% doping might cause more free electrons and induce the electronic thermal conductivity rather than lattice thermal conductivity. Owing to the low thermal conductivity, high absolute Seebeck coefficient and high electrical conductivity of the Al-doped ZnO compound, the dimensionless fig2 ure of merit ZT (ZT D S T , T is absolute temperature) of Al-doped ZnO is much higher than that of pure ZnO over the whole temperature range (as shown in Figure 9). The obtained maximum ZT for Zn0:97 Al0:03 O is 0.28 at 400 ıC, in spite of its low thermal conductivity. To compare with other research, the highest ZT of Zn0:98 Al0:02 O at 400 ıC is 0.21, while the ZT values at the same temperature are 0.12Œ9 , 0.08Œ10 , 0.015Œ19 . It should be noted that the hydrothermal method can report for effective reduction of thermal conductivity by overcoming a large disadvantage due to the markedly high thermal conductivity with a predominant contribution from phonons.

Figure 10. (Color online) Total density of states (DOS) of ZnO and Al-doped ZnO.

4.2. Calculation The electronic structure of a semiconductor can be strongly disturbed in the presence of impuritiesŒ20 . Figure 10 gives the density of states (DOS) of Al-doped ZnO system. For pure ZnO, it is evident that the conduction band minimum (CBM) and the valence band maximum (VBM) are in the same kpoint (G/, i.e., ZnO is a typical direct band gap semiconductorŒ7 . According to the density of states (DOS), shown in Figure 10, calculated by LDA C U , it indicates that pure ZnO has energy band gap about 1.75 eV, which is in good agreement with the other calculationŒ21 . However, the energy band gap is still under-estimated when compared with the experimental value (a direct wide band gap of 3.15 eV). Meanwhile,

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Figure 11. Partial density of states of Al-doped ZnO (Zn0:94 Al0:06 O): (a) Zn, (b) O, and (c) Al.

all of the energy bands move toward the low energy region. This is the result from the interaction between Al3p and Zn4s orbitals, which combined with CBM. Take the position of the peak of O2p as reference, 6 at% Al doping makes the energy band move 5.01 eV toward the low energy region. The energy band gap of calculation Al-doped ZnO is 1.41 eV lower than 2.95 eV from experiment. To illustrate why the band structures change in going to Al doping, the partial density of states (PDOS) are investigated. As shown in Figure 11, the valence band can be divided into two regions, the lower valence band between –7.5 and –6.5 eV and the upper valence band between 6:5 and 5:2 eV. Moreover, the lower valence band contributed by Zn3d states and the higher valence band is mainly from O2p states. The domi-

Figure 12. Thermoelectric properties of Al-doped ZnO: (a) Seebeck coefficient, (b) electrical conductivity, and (c) electric thermal conductivity.

nant contributors to the conduction band are the Zn4s and Zn4p states, mixed with a few O2s and O2p states. Following the replacement of a Zn atom with an Al atom, the electrons occupying the lowest level of the conduction band cause the Fermi level to move upward into the conduction band, which produces typical n-type metallic characteristics. This section presents the results of the thermoelectric calculations in comparison with experimental studies (as shown in Figure 12). The Seebeck coefficient, electrical conductivity and electric thermal conductivity of Al-doped ZnO from the BoltzTraP simulations were compared with values for 6% Al-doped ZnO samples from the nanomaterial synthesized by hydrothermal method. The Seebeck coefficient comparison is

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shown in Figure 12(a); it can be seen that the sign of Seebeck coefficient from the calculation and experiment is negative and the magnitude trend is to stability with increasing temperature. The magnitudes from calculation are significantly lower, this may be characteristic of the oversimplified model used to estimate the transport properties. The temperature dependence of the electrical conductivity compared with the experiment is shown in Figure 12(b). The BoltzTraP calculation of Al doping yields a relatively constant value of  of the order of 50  1 m 1 from 50 to 400 ıC, in contrast to the experiment result where  increases over this temperature range. Also, the magnitude of the electrical conductivity from BoltzTraP was found to be similar to the experimental data. The temperature dependence of thermal conductivity, clearly verifies that the  can be decreased with nanocrystallization. The total thermal conductivity (tot / can be written as a sum of the electronic (e / and lattice thermal conductivity (l /, which is given by the classical kinetic theoryŒ22 : l D 1=3Cv lvs , where Cv is the specific heat at constant volume, l is mean free phonon path, and vs is the average velocity sound. The e is directly proportional to the electrical conductivity through the Wiedermann-Franz relationŒ23 , e D LT , where L is the Lorenz number,  from experiment data. The electric thermal conductivity plot is shown in Figure 12(c), both e from BoltzTraP and experiment increases with increasing temperature. Owing to poor electrical transport properties, the shown total thermal conductivity of the nanostructured material is very close to the lattice contribution. As the temperature increases, the e raised due to the  increased.

5. Conclusion In summary, the enhanced thermoelectric properties of Zn1 x Alx O (x D 0.0, 0.01, 0.02, 0.03 and 0.06) were investigated from the nanoparticles of the wurtzite structure synthesized by hydrothermal method of zinc and aluminum salts. Al doping decreases nanocrystal size. Bulk pellets, obtained by cold-pressing and sintering the nanocrystals, consist of ZnO particles with no ZnAl2 O4 phase that lead to thermal conductivity to ultralow values as low as 1.59 W/(m
K) at 400 ıC, which is considered very low reported for oxides. The Seebeck coefficient and the electrical conductivity increase significantly with temperature resulting in large bulk-like power factors above 400 ıC. The resultant ZT 0.28 of 3% Al-doped ZnO expected at 400 ıC is two orders of magnitude higher than pure ZnO at the same temperature. In this work, ZnO and Aldoped ZnO are prepared a path for obtaining high ZT for lowcost practical waste heat harvesting. We used DFT techniques to calculate the electronic structure of ZnO and Al-doped ZnO. The results showed that the conduction band near the Fermi energy was a combination of hybridized Zn sp-states and Al sstates. The calculated energy band structure was used in combination with the Boltzmann transport equation to calculate the thermoelectric parameters of ZnO and Al-doped ZnO. The calculated thermoelectric properties were compared with experimental results, showing some agreement. For the Al-doped ZnO system, the Seebeck coefficient was shown to be negative and the magnitude trend is to stability with increasing temperature. The electrical conductivity and electronic thermal conductivity followed the trend of the experimental results.

Acknowledgments The authors are grateful for the financial support from the National Research University Project of Thailand, Office of the Higher Education Commission, through the Advanced Functional Materials Cluster of Khon Kaen University and the Nanotechnology Center (NANOTEC), NSTDA, Ministry of Science and Technology, Thailand, through its program of Center of Excellence Network.

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