From the Surface Reaction Control to Gas-Diffusion Control: The Synthesis of Hierarchical Porous SnO 2 Microspheres and Their Gas-Sensing Mechanism

A series of hierarchical porous SnO 2 microspheres (SnO 2 -Ms) with same sizes of nanoparticles were fabricated through increasing the reaction time of the one-step hydrothermal method. Especially, these SnO 2 -Ms also have the different specific surface areas and pores sizes. When they are applied in sintering type thick film gas sensors, through comparing the gas-sensing property of the as-prepared SnO 2 -Ms, it can clearly demonstrate that the surface chemical reaction (SCR) control of the sensing properties of sensors is gradually replaced by gas diffusion control with the increasing operation temperature ( T o ). For the first time, this dual control is discovered through contrast experiments. According to the testing results, the sensing mechanism of sensors can be explained by many factors, such as the reaction rate constant of the SCR, the Knudsen diffusion coefficient of the target gas, the T o, the specific surface area, the pore size, and the change of the H 2 O, etc. A pore canal model and a hollow sphere model are introduced, which can effectively explain the sensing mechanism of gas sensors. This discovery can make up for the inadequacy of the surface-control and the diffusion-control theory, and expound their interrelationship. This discovery also provides a novel strategy for studying the sensing mechanism of sensors, which is expected to open up exciting opportunities for improving the sensing properties of the gas-sensing materials and studying some gas–solid catalytic phenomena.