Polarization effect of external electric field on Raman activity and gas sensing of nano zinc oxide

Abstract

Control and administration of various dangerous gases existing in the environment is very important both for safety in the workplace and for quality of daily life, such as acetone and ethanol, etc. Zinc oxide, a well-known n-type semiconductor with a direct wide band-gap of 3.37 eV, is a very promising gas sensing material. However, zinc oxide's limited selectivity, relatively long response/recovery time, high-power consumption, and lack of long-term stability have restricted its applications in high-standard gas detection. Therefore, increasing gas sensing selectivity is a crucial issue for ZnO application in the gas sensing field. So far, many researches have reported and discussed the effects of morphologies, structures, doping of gas sensing materials, on its sensing performance. In this work, we intend to investigate and theoretically analyze how the polarization of the external electric field affects gas sensing performance and selectivity. Zinc oxide nanoparticles, as a testing gas sensing material, are synthesized by simple precipitation method. Then they are pressed into a disc and polarized under an external electric field with different electric field intensities at different temperatures. The structure and Raman activity for each of the unpolarized ZnO and the polarized ZnO are characterized using X-ray diffraction and Raman spectrometry, respectively. The gas sensing performances of unpolarized and polarized ZnO based sensors to ethanol and acetone are carefully examined using a chemical gas sensing system. The mechanism of external electric field polarization effect on gas sensitivity is discussed. The results reveal that there exists a threshold value for each of voltage and temperature for ZnO polarization under an external electric field. When the voltage and temperature are over 9375 V·cm-1 and 150℃, respectively, the leakage of electricity in ZnO disk happens and the polarization effect gradually disappears. Within the above voltage and temperature limits, Raman peak intensity of the polarized ZnO at 437 cm-1 obviously decreases after external electric field polarization. The response of the polarized ZnO sensor to acetone increases with external electronic field and polarization temperature increasing, while the response to ethanol decreases, which indicates that external electric field polarization can effectively adjust the gas sensing selectivity of nano zinc oxide. Raman analysis indirectly shows that the enhanced gas sensing selectivity of ZnO by the polarization effect of the external electric field is due to oxygen vacancy and zinc vacancy directionally moving under the action of an external electric field. Thus it can be seen that the polarization of the external electric field acting on gas sensing material is a promising effective method to improve gas sensing selectivity.