Thermoelectric Properties of Al-Doped Mesoporous ZnO Thin Films

Hindawi Publishing Corporation Journal of Nanomaterials Volume 2013, Article ID 131537, 6 pages http://dx.doi.org/10.1155/2013/131537 Research Article Thermoelectric Properties of Al-Doped Mesoporous ZnO Thin Films Min-Hee Hong,1 Chang-Sun Park,1 Won-Seon Seo,2 Young Soo Lim,2 Jung-Kun Lee,3 and Hyung-Ho Park1 1 Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Republic of Korea Korea Institute of Ceramic Engineering and Technology, Seoul 153-801, Republic of Korea 3 Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA 15261, USA 2 Correspondence should be addressed to Hyung-Ho Park; Received 7 June 2013; Accepted 25 August 2013 Academic Editor: Chan Park Copyright © 2013 Min-Hee Hong et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Al-doped mesoporous ZnO thin films were synthesized by a sol-gel process and an evaporation-induced self-assembly process. In this work, the effects of Al doping concentration on the electrical conductivity and characterization of mesoporous ZnO thin films were investigated. By changing the Al doping concentration, ZnO grain growth is inhibited, and the mesoporous structure of ZnO is maintained during a relatively high temperature annealing process. The porosity of Al-doped mesoporous ZnO thin films increased slightly with increasing Al doping concentration. Finally, as electrical conductivity was increased as electrons were freed and pore structure was maintained by inhibiting grain growth, the thermoelectric property was enhanced with increasing Al concentration. 1. Introduction Development of alternative energy is very important because fossil fuels are being rapidly depleted. Thermoelectricity is a very important green energy conversion technique, as waste heat could be used. When materials have a temperature gradient, electric current is generated in the materials. To maintain a high thermoelectric property, factors of Seebeck coefficient, electrical conductivity, and thermal conductivity need to be controlled individually. However, among thermoelectric factors, electrical conductivity and thermal conductivity have a proportional relationship. To overcome this problem, studies on structure change have been performed, including studies on nanostructures, nanowires, and superlattices [1]. Of the structures studied, mesoporous structures could be adapted to thermoelectrics because mesoporous structures have low thermal conductivity. Mesoporous materials have pores that range between 2 and 50 nm [2]. Because of their pore structure, mesoporous structures have properties such as a high specific surface area, low dielectric constant, and low thermal conductivity. In the evaporation-induced selfassembly (EISA) process, a micelle structure is formed and an ordered pore structure is synthesized after the annealing process. When the surfactant concentration exceeds the critical micelle concentration, the EISA process progresses. Good thermoelectric materials have high Seebeck coefficient and electrical conductivity and low thermal conductivity. The order of the pore structure is very important to controlling the electrical conductivity and the thermal conductivity individually because the inelastic mean free path of electrons and phonons is different. This difference is generally referred to as the phonon-glass electron crystal (PGEC) effect. Because the inelastic mean free path of an electron is longer than that of a phonon, phonons are scattered effectively in ordered mesoporous structures [3, 4]. In order to maximize the PGEC effect, a doping source with an inhibited grain growth property is used because its ordered pore structure is less prone to collapse. Zinc oxide (ZnO) is an important n-type wide band gap semiconductor material used in various applications such as photovoltaic devices [5], gas sensors [6], and solar cells [7]. By using physical vapor deposition, chemical vapor deposition, and the sol-gel process [8–10], ZnO thin films can be easily synthesized. In this work, the sol-gel process was Intensity (a.u.) ∗ 112 ∗ 103 102 4 at.% 110 Journal of Nanomaterials 100 002 101 2 2 at.% 1 at.% 0 at.% 20 30 40 50 2𝜃 (deg) 60 70 Figure 1: Wide-angle XRD patterns of Al-doped mesoporous ZnO thin films with various Al doping concentrations (∗ represents a typical diffraction peak position of ZnAl2 O4 at high angle 2 theta region). selected to adapt the EISA process. ZnO could be doped with various group III materials such as In, Ga, and Al. Among the many doping materials, Al-doped ZnO thin films are most important for this application. Because Al-doped ZnO thin films have high thermal stability and are inexpensive, they have been studied as possible next-generation materials to replace indium tin oxides [11]. In this work, Al was doped to increase its thermoelectric properties. When Al was doped in ZnO, the electrical conductivity was increased [12]. Moreover, the grain growth of ZnO was effectively inhibited [13]. In Al doping, Al ions are substituted at Zn ion sites. In this process, the hexagonal wurtzite structure is distorted because of the difference in the radius of Al3+ (0.054 nm) and Zn2+ (0.074 nm). Finally, the crystalline structure deteriorates because when stress forms in thin films, the grain growth of the hexagonal wurtzite structure is inhibited. In a mesoporous structure, high temperature stability is essential to maintaining an ordered pore structure. An Al-doped mesoporous ZnO thin film could have good thermoelectric properties because it has a high electrical conductivity and high temperature stability due to inhibited grain growth. In this paper, the effects of Al concentration on the pore arrangement in mesoporous ZnO thin films and on the crystallization, pore structure, porosity, and thermoelectric property of the film were analyzed. 2. Experimental Procedure Al-doped mesoporous ZnO thin films were prepared on SiO2 /Si substrates by sol-gel and spin coating processes. Zinc acetate dihydrate [Zn(CH3 COO)2 ⋅H2 O], n-propanol, Brij-76 (C58 H118 O21 , Aldrich, MW 711), aluminum nitrate nonahydrate [Al(NO3 )⋅9H2 O], and monoethanolamine (MEA) were used as Zn precursor, solvent, surfactant, dopant source, and complex agent, respectively. The molar ratios of MEA/Zn precursor and surfactant/Zn precursor were fixed at 1 and 0.05 in this work. The molar ratio of zinc acetate dihydrate : Brij76 : MEA : n-propanol was 1 : 0.05 : 1 : 34.5. The atomic ratio of Al precursor and Zn precursor was changed from 0 at.% to 4 at.%. The precursor solutions were spin coated onto SiO2 /Si substrate. Solvent was evaporated during the spin coating process. Then, the EISA process was started in the thin films and a micelle structure was synthesized. Asprepared thin (...truncated)