SnO₂ has been extensively used in the detection of various gases. As a gas sensing material, SnO₂ has excellent physical-chemical properties, high reliability, and short adsorption-desorption time. The application of the traditional SnO₂ gas sensor is limited due to its higher work-temperature, low gas response, and poor selectivity. Nanomaterials can significantly impact gas-sensitive properties due to the quantum size, surface, and small size effects of nanomaterials. By applying nanotechnology to the preparation of SnO₂, the SnO₂ nanomaterial-based sensors could show better performance, which greatly expands the application of SnO₂ gas sensors. In this review, the preparation method of the SnO₂ nanostructure, the types of gas detected, and the improvements of SnO₂ gas-sensing performances via elemental modification are introduced as well as the future development of SnO₂ is discussed.

Hazardous gases have been strongly associated with being a detriment to human life within the environment. The development of a reliable gas sensor with high response and selectivity is of great significance for detecting different hazardous gases. TiO₂ nanomaterials are promising candidates with great potential and excellent performance in gas sensor applications, such as hydrogen, acetone, ammonia, and ethanol detection. This review begins with a detailed discussion of the different dimensional morphologies of TiO₂, which affect the gas sensing performance of TiO₂ sensors. The diverse morphologies of TiO₂ can easily be tuned by regulating the manufacturing conditions. Meanwhile, they exhibit unique characteristics for detecting gases, including large specific surface area, superior electron transport rates, extraordinary permeability, and active reaction sites, which offer new opportunities to improve the gas sensing properties. In addition, a variety of efforts have been made to functional TiO₂ nanomaterials to further enhance sensing properties, including TiO₂-based composites and light-assisted gas sensors. The enhanced gas sensing mechanisms of multi-component composite nanomaterials based on TiO₂ include loaded noble metals, doped elements, constructed heterojunctions, and compounded with other functional materials. Finally, several studies have been summarized to demonstrate the comparative sensing properties of TiO₂-based gas sensors.