Study of Highly Sensitive Formaldehyde Sensors Based on ZnO/CuO Heterostructure via the Sol-Gel Method.

Yan Chen,Jing Liu,Hongyan Zhang

doi:10.3390/s21144685

Abstract

Formaldehyde (HCHO) gas sensors with high performance based on the ZnO/CuO heterostructure (ZC) were designed, and the sensing mechanism was explored. FTIR results show that more OH− and N–H groups appeared on the surface of ZC with an increase in Cu content. XPS results show that ZC has more free oxygen radicals (O*) on its surface compared with ZnO, which will react with more absorbed HCHO molecules to form CO2, H2O and, electrons, accelerating the oxidation-reduction reaction to enhance the sensitivity of the ZC sensor. Furthermore, electrons move from ZnO to CuO in the ZC heterostructure due to the higher Fermi level of ZnO, and holes move from CuO to ZnO until the Fermi level reaches an equilibrium, which means the ZC heterostructure facilitates more free electrons existing on the surface of ZC. Sensing tests show that ZC has a low detection limit (0.079 ppm), a fast response/recovery time (1.78/2.90 s), and excellent selectivity and sensitivity for HCHO detection at room temperature. In addition, ambient humidity has little effect on the ZC gas sensor. All results indicate that the performance of the ZnO sensor for HCHO detection can be improved effectively by ZC heterojunction.

Highlights

ZnO is an n-type metal oxide semiconductor material of the II-VI groups with high exciton binding energy (60 meV) and a wide direct band gap (3.4 eV), which has attracted a lot of attention in the field of gas sensors due to its rich morphology and excellent chemical and thermal stability [1,2,3,4,5]
HCHO gas sensors with high performance based on ZnO/CuO heterostructure (ZC) were designed, and the sensing mechanism was investigated
The prepared ZnO/CuO powder and deionized water (DI) were mixed into paste

Summary

Introduction

ZnO is an n-type metal oxide semiconductor material of the II-VI groups with high exciton binding energy (60 meV) and a wide direct band gap (3.4 eV), which has attracted a lot of attention in the field of gas sensors due to its rich morphology and excellent chemical and thermal stability [1,2,3,4,5]. The performance of ZnO gas sensors can be improved by doping [3,8], hybridization [9,10], surface modification [11,12], and heterostructure [13,14,15,16]. It has been reported that ZnO/CuO heterostructure (ZC) sensors can improve the sensing performance and result in the formation of a p-n heterojunction [17,18]. Some literature reported that the presence of functional groups (carboxyl, hydroxyl, amino, carbonyl groups, etc.) on the material’s surface can improve the performance of gas sensors. HCHO gas sensors with high performance based on ZnO/CuO heterostructure (ZC) were designed, and the sensing mechanism was investigated. The molar ratios of Cu2+ to Zn2+ in ZnO, ZC1, ZC2 and ZC3 are 0 mol%, 10 mol%, 30 mol% and 50 mol%, respectively

Gas Sensing Device Preparation and Measurement

Findings

Sensing Mechanism