High Sensitivity Humidity Sensors Based on Zn_1−xSn_xO Nanostructures and Plausible Sensing Mechanism

Abstract

Humidity sensors have become a widespread and important integrated device for monitoring the environmental humidity in people's daily life to improve human comfort. They have already widely used for online real-time detection of automobile industry, medical field, construction, meteorology, and food processing. [1] Therefore, it is necessary to develop various humidity sensors with simple manufacturing, high sensitivity, and high-cost performance. Through domestic and overseas studies, it was found that metal oxides such as ZnO, SnO 2 , TiO 2 , In 2 O 3 , and WO 3 can meet the requirements of producing high-quality humidity sensors due to their small nanostructure size and large specific surface area. [2][3][4][5][6] Among the materials mentioned above, ZnO is a promising sensing material due to its wide energy bandgap (3.37 eV at room temperature) of n-type semiconductors, excellent chemical/physical stability, and unique electrical and optical properties. [7] Related studies have shown that 1D ZnO nanostructures have excellent sensing properties, such as high sensitivity, fast response/recovery, and have been used in humidity detection fields. [8][9][10][11][12][13][14] In order to meet the actual demand of production and life, the sensing characteristics of ZnO sensors need to be further improved. The chemical sensing ability of metal oxide materials can be enhanced by increasing the specific surface area and improving the surface state. Doping can effectively adjust the material characteristics such as specific surface area, surface state, and lattice structure strain, and it is considered as a valid method to modulate and enhance the performance of nanostructure humidity sensors. [15,16] Some groups have obtained remarkable sensing performance promotion through N, Co, and In doping in ZnO, respectively, as well as our previous works on the Ni-and K-doped ZnO sensors. [17][18][19][20][21] Sn (Sn 4þ , 0.69 Å) has similar ionic radii with Zn (Zn 2þ , 0.74 Å). Hence, the Zn 2þ can be easily substituted by Sn 4þ during the growth of materials. [22] With the doping process, the lattice structure of ZnO will be adjusted and more defects (e.g., oxygen vacancies) will be introduced, and the doped ions and defects can be directly or indirectly involved in the humidity-sensitive reaction, which enhances the humidity-sensitive properties.Among the methods for preparing materials, nonvacuum techniques or solution-based techniques such as hydrothermal, sol-gel, and spray pyrolysis have attracted a lot of attention from the scientific community due to their inexpensive nature. [23][24][25] In the above techniques, hydrothermal is the best choice for the preparation of nanowires because of its cost-effective and easy synthesis. [26] Precise positioning of nanostructures between electrodes is also necessary in the construction of sensors. Dielectrophoresis (DEP) has been widely used in many fields of research since its invention, such as biological cell separation, inorganic particle separation, material detection, manipulat...