Zhijun Liu, Jianfeng Jia, Guozheng Zhao
Formaldehyde (HCHO), a toxic volatile organic compound (VOC) widely used in indoor construction and furnishing materials, poses significant health risks even at trace concentrations. The development of highly sensitive, selective, and low-cost detection techniques for formaldehyde has become an urgent priority. Among various sensing platforms, indium oxide (In 2 O 3 ), an n-type semiconductor with a wide bandgap and abundant oxygen vacancies, has emerged as a promising material for gas sensor applications due to its high chemical stability, surface reactivity, and electronic tunability. This review systematically summarizes recent advances in In 2 O 3 -based gas sensors for formaldehyde detection, with a particular focus on the impact of material dimensionality (0D, 1D, 2D, and 3D) and surface modification strategies—including doping elements, noble metal loading, oxide hybridization, constructing heterojunctions, and light activation. The mechanisms underlying each enhancement approach are discussed in detail, including modulation of oxygen vacancies, bandgap engineering, heterojunction formation, and charge transfer dynamics. Emerging photoactivation strategies that enable room-temperature sensing are also highlighted for their potential in developing low-power, highly responsive gas sensors. Finally, key performance metrics of representative materials are compared and analyzed, and future directions are proposed to guide the rational design of next-generation In 2 O 3 -based formaldehyde sensors.