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The lower gas sensitivity, humidity dependence of the gas sensing properties, and long recovery times of room-temperature gas sensors severely limit their applications. Herein, to address these issues, a series of 3D inverse opal (IO) InO-ZnO heterogeneous composite microspheres (HCMs) are fabricated by ultrasonic spray pyrolysis (USP) employing self-assembled sulfonated polystyrene (S-PS) spheres as a sacrificial template. The 3D IO InO-ZnO HCMs possess highly ordered 3D inverse opal structures and bimodal (meso-scale and macro-scale) pores, which can provide large accessible surface areas and rapid mass transfer, resulting in enhanced gas sensing characteristics. Furthermore, the 3D IO architecture and n-n heterojunctions can extend the photoabsorption range to the visible light area, effectively prolonging the lifetimes of photo-generated charge carriers, and can increase separation of visible light-generated charges. As a result, the as-prepared 3D IO InO-ZnO HCMs deliver excellent NO sensing performance under visible light irradiation at room temperature, such as high sensitivity (R/R = 54.3 to 5 ppm NO), low detection limit (250 ppb), fast recovery time (188 s), excellent selectivity and humidity independence. These enhanced photo-electronic gas sensing properties are attributed to the combination of highly ordered 3D IO microspheres and InO-ZnO heterogeneous composites.

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Understanding the effect of substitutional doping on gas-sensing performances is essential for designing high-activity sensing nanomaterials. Herein, formaldehyde sensors based on gallium-doped In 2 O 3 inverse opal (IO-(Ga x In 1−x ) 2 O 3 ) microspheres were purposefully prepared by a simple ultrasonic spray pyrolysis method combined with selfassembled sulfonated polystyrene sphere templates. The well-aligned inverse opal structure, with three different-sized pores, plays the dual role of accelerating the diffusion of gas molecules and providing more active sites. The Ga substitutional doping can alter the electronic energy level structure of (Ga x In 1−x ) 2 O 3 , leading to the elevation of the Fermi level and the modulation of the band gap close to a suitable value (3.90 eV), hence, effectively optimizing the oxidative catalytic activity for preferential CH 2 O oxidation and increasing the amount of adsorbed oxygen. More importantly, the gas selectivity could be controlled by varying the energy level of adsorbed oxygen. Accordingly, the IO-(Ga 0.2 In 0.8 ) 2 O 3 microsphere sensor showed a high response toward formaldehyde with fast response and recovery speeds, and ultralow detection limit (50 ppb). Our findings finally offer implications for designing Fermi leveltailorable semiconductor nanomaterials for the control of selectivity and monitoring indoor air pollutants.