An Enhanced Gas Sensor Based on Zig-Zag TiO₂ Nanorods (1D)–BiVO₄ Nanosheet (2D) Heterojunction

The detection of low-concentration gases and odors in fields such as healthcare, mobility, and indoor environment control is attracting tremendous attention. Among various gases, oxidizing gas can cause damage to the human respiratory tract even quite low concentration. At present, oxidizing gas can be tested by gas sensors of all types, including semiconductor metal oxide (SMO), electrochemical, organic compound, optical and carbon-based gas sensor. Within them, SMO gas sensor have been widely concerned due to their advantages, such as simple device structure and manufacturing process, easy integration and suitable for online measurement. Various nanostructures have been investigated for oxidizing gas sensing performance, primarily in single oxides, such ZnO, SnO, Fe₂O₃, TiO₂ and BiVO₄ combining with different modification skills including constructing of complex multi-dimensional structures, loading of noble metal catalysts and doping of MOSs, etc. However, there are still great challenges for constructing high-performance oxidizing gas sensors. Representatively, poor active surface area of nanostructure and large energy band gap of SMO resulting low detection limit.<br/>Recently, many researchers have attempted to overcome these issues. There are some strategies such as increasing active sites for adsorption of target gas and design of novel nanostructure via formation of heterojunction. Zigzag structure is one of the most effective structures that can expand the surface to volume ratio compared to other one-dimensional (1D) nanostructure like nanowires and nanorods. In addition, n-n heterojunction structure leads to energy band bending and high potential barrier at material interface, which improving resistance changes.<br/>Herein, we demonstrated a gas sensor enhanced by fabricating heterojunction between 1D TiO₂ zigzag nanorod and two-dimensional (2D) BiVO₄ nanosheet on Si wafer. The 1D TiO₂ zigzag nanorods were realized on 2D BiVO₄ nanosheet by glancing angle deposition (GLAD) method using e-beam evaporation. Compared to the single TiO₂ zigzag nanorods, the TiO₂-BiVO₄ n-n heterostructures can induce a large quantifiable variation in resistance when the oxidizing gas adsorbed at the surface even at room temperature. This improvement is induced by increased depletion layer width at the surface and energy band bending between TiO₂ and BiVO₄ interface. Additionally, we performed the analysis about sensing properties including stability and response time at the room temperature.<br/>We believe that our study will be a cornerstone for the application of functional nanoscale heterostructures based on SMO to gas sensor.