Co3O4 hollow spheres prepared by ultrasonic spray pyrolysis were converted into Co3O4-SnO2 core-shell hollow spheres by galvanic replacement with subsequent calcination at 450 °C for 2 h for gas sensor applications. Gas selectivity of the obtained spheres can be controlled by varying the amount of SnO2 shells (14.6, 24.3, and 43.3 at. %) and sensor temperatures. Co3O4 sensors possess an ability to selectively detect ethanol at 275 °C. When the amount of SnO2 shells was increased to 14.6 and 24.3 at. %, highly selective detection of xylene and methylbenzenes (xylene + toluene) was achieved at 275 and 300 °C, respectively. Good selectivity of Co3O4 hollow spheres to ethanol can be explained by a catalytic activity of Co3O4; whereas high selectivity of Co3O4-SnO2 core-shell hollow spheres to methylbenzenes is attributed to a synergistic effect of catalytic SnO2 and Co3O4 and promotion of gas sensing reactions by a pore-size control of microreactors.

Pure and 3–12 at. % Pr-doped In2O3 macroporous spheres were fabricated by ultrasonic spray pyrolysis and their acetone-sensing characteristics under dry and humid conditions were investigated to design humidity-independent gas sensors. The 12 at. % Pr-doped In2O3 sensor exhibited approximately the same acetone responses and sensor resistances at 450 °C regardless of the humidity variation, whereas the pure In2O3 exhibited significant deterioration in gas-sensing characteristics upon the change in the atmosphere, from dry to humid (relative humidity: 80%). Moreover, the 12 at. % Pr-doped In2O3 sensor exhibited a high response to acetone with negligible cross responses to interfering gases (NH3, CO, benzene, toluene, NO2, and H2) under the highly humid atmosphere. The mechanism for the humidity-immune gas-sensing characteristics was investigated by X-ray photoelectron and diffuse reflectance infrared Fourier transform spectroscopies together with the phenomenological gas-sensing results and discussed in relation with Pr3+/Pr4+ redox pairs, regenerative oxygen adsorption, and scavenging of hydroxyl groups.

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Solar fuel production, water splitting, and CO2 reduction by sunlight‐assisted catalytic reactions, are attractive and environmentally sustainable approaches used to generate energy. Since many different parameters, including energy band structures, electronic conductivity, surface area, porosity, catalytic activity, and stability of photocatalytic materials, determine the photocatalytic reaction, a single photocatalytic material is often insufficient to fulfill all the requirements. Hybridization to complement the limitations of two or more component materials can provide a viable solution. Particularly, hybridization with metal organic frameworks (MOFs), a new class of materials with excellent controllability of topology, surface area, porosity, morphology, band structure, electrical conductivity, and composition, enables the on‐demand design of a myriad of high‐performance photocatalysts. Moreover, hybrids formed by MOF‐derived materials inherit the distinctive merits from the MOF and offer further diversification for hybrid photocatalysts. Here, the rational design of MOF‐based hybrid photocatalysts for solar fuel production is discussed. The synthetic strategies of diverse MOF‐based hybrids, the key physicochemical parameters of hybrids to determine photocatalytic and photoelectrochemical reactions, and the mechanisms underlying the synergistic enhancement of solar fuel production are reviewed. Moreover, remaining challenges and future perspectives are addressed.

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