Electrodeposition of ZnO from Acetate Bath for Thermoelectric Devices

Abstract

Among various fields where ZnO is applied, thermoelectric application has received considerable attraction because metal oxide materials have high thermal stability. Up to date, various methods to prepare ZnO have been studied. Among them, electrochemical process has several advantages such as low reaction temperature and process simplicity. Moreover, micro-device fabrication is applicable by using electrodeposition due to the selectivity of deposition. In numbers of reports, nitrate-based electrolytes were used and have been extensively investigated [1]. However, some drawbacks have still remained that ZnO films from nitrate solution have rough surface mainly due to gas evolution derived from nitrate ion, such as nitrogen. For the approach to solve this, the application of acetate bath was proposed. The merit of using acetate solution is the higher pH value in the bath than nitrate to prevent generation of hydrogen. Acetate is also advantageous for no gas evolution derived from nitrate. Therefore it is suggested the quality of ZnO films is improved by the acetate bath. We have attempted to obtain smooth ZnO films using acetate bath, however the bath was not stable and few papers have been published for the acetate bath. Moreover, for the application to the thermoelectric devices, the characterization of electrodeposited ZnO has not been performed yet. In this study, we demonstrated electrodeposition of ZnO films from not only nitrate but acetate bath and investigated the effect on morphology and deposition behavior by difference of Zn precursors. We further show the thermoelectric characterization of obtained ZnO by fabricating the thermoelectric device. Electrodeposition was conducted in a three-electrode system with a 700-μm-thick sheet of glass covered with 100 nm Au/10 nm Cr as working electrode, a Pt plate as counter electrode and Ag/AgCl as reference electrode, respectively. In the case of nitrate bath, the electrolytes were the aqueous solution containing 0.05 M Zn(NO3)2･6H2O and the applied potential was modified from -0.8 to -1.4 V. The bath temperature was controlled at 65, 75 and 85 oC. The deposition duration was 60 min. On the other hand, the acetate solution is containing 0.05 M Zn(OCOCH3)2･2H2O, 0.1 M CH3COONa and 0.1 M CH3COOH. The potential was applied from -0.8 to -1.3 V and the bath temperature was fixed at 65, 75 and 85 oC. The deposition was performed for 30 min. Finally using the solution mixed the Zn(NO3)2･6H2O and CH3COONa, the ZnO films were electrodeposited into patterns with the diameter of 200 μm. The morphology and crystallinity of the sample were characterized using FESEM, Raman spectrometer and XRD. Thermoelectric properties were measured by generating a thermal gradient using focused light from a halogen lamp as a heat source. First the electrodeposition process was carried out from the nitrate bath and the ZnO films were obtained. However, by using nitrate bath, hydrogen or other gases were generated resulting in the films with a lot of pits and low adhesion. In order to solve this problem, the acetate bath was applied. Although the electrodeposition was carried out from the acetate solution changing the applied potential or bath temperature, introducing the oxygen gas or additives, annealing the deposited films, Zn tended to deposit instead of ZnO under all conditions. The fact that ZnO was obtained using only nitrate was caused by the shift of the reduction potential of NO3 - ion towards less negative values in the presence of Zn2+. Therefore, the bath mixing both nitrate and acetate ion was used. As a result, Raman spectra showed the films electrodeposited from mixing bath was consisted of ZnO. The ZnO electrodeposited from the bath adding 0.05 M Zn(NO3)2･6H2O and 0.01 M CH3COONa has smooth surface and the adhesion was improved. Then using the same mixing bath, the ZnO films were electrodeposited at -0.9 V and measured the thermoelectric property, resulting that the film showed Seebeck effect as n-type semiconductor. Finally, the ZnO was electrodeposited into the patterns using the above mixing bath, and it was found that ZnO with smooth morphology was electrodeposited into patterns. These results indicated that ZnO electrodeposited from the mixing bath using Zn(NO3)2･6H2O and CH3COONa has smooth morphology and high adhesion, and ZnO was promising for thermoelectric material due to its thermoelectric property. [1] M. Skompska, K. Zarebska, Electrochimica Acta, Vol. 127, pp. 467-488, 2014