Size effect on oxygen vacancy formation and gaseous adsorption in ZnO nanocrystallites for gas sensors: a first principl
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Size effect on oxygen vacancy formation and gaseous adsorption in ZnO nanocrystallites for gas sensors: a first principle calculation study Jianqiao Liu1 · Fengjiao Gao1 · Liting Wu1 · Haipeng Zhang1 · Wusong Hong1 · Guohua Jin1 · Zhaoxia Zhai1 · Ce Fu1 Received: 20 February 2020 / Accepted: 16 May 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract The first principle calculation based on density function theory was used to investigate the crystallite size effect of ZnO semiconductor, the size of which was controlled from 0.325 to 1.625 nm. The formation energy of oxygen vacancy increased from 3.855 to 6.256 eV, showing incremental difficulties in the formation of defects. The densities of states of ZnO super cells with complete and defective (110) surface were calculated, concluding positive dependences of carrier mobility and conductivity on crystallite size. The oxygen species of O − and O2− were considered as adsorbates on ZnO super cells with defective (110) surface. In case of crystallite size below 1.3 nm, the adsorption energy of O− increased with crystallite size, while the one of O2− kept constant, inferring that the oxygen adsorption was dominated by O−. A competitive adsorption between both species was found when crystallite size was over 1.3 nm. The adsorption of O− was inhibited by the incremental adsorption energy of O2−, which was the dominated species on ZnO crystallite surface. Keywords Zinc oxide · First principle calculation · Crystallite size effect · Vacancy formation energy · Density of states · Adsorption energy
1 Introduction Gas sensors have important applications in a variety of fields [1–3]. As one of the earliest and most widely used gas-sensing materials, zinc oxide (ZnO) is a semiconductor with band gap of 3.44 eV as well as exciton binding energy of 60 meV, which provide good characteristics in the development of ZnO gas sensors [4]. It has a simple structure of hexagonal wurtzite with good stability at natural temperature and pressure. It is highly sensitive to a variety of gases at the optimized operating temperature. Many types of ZnO gas sensors have been fabricated and some of them are put into practical use [5, 6]. ZnO gas sensors hold the advantages of high response, small volume and easy integration with
* Jianqiao Liu [email protected] * Ce Fu [email protected] 1
College of Information Science and Technology, Dalian Maritime University, Linghai Road 1, Ganjingzi District, Dalian 116026, Liaoning, People’s Republic of China
microelectronics technology [7, 8]. Recent development of gas sensors focused on incorporation of foreign elements [8–10], design of nanostructures [11] and control of surface vacancies [12, 13]. In the ZnO semiconductor, there are interstitial Zn and oxygen vacancies, both of which are fundamental to ZnO grains. However, in a gas-sensitive ZnO grain, the oxygen defects are more active to determine the performances of gas sensors [14, 15]. It is known that oxygen plays an important role in ZnO gas sensors. The behav
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