In this paper, a hierarchical structure of graphite carbon nitride (g-C₃N₄) modified ZnO (g-C₃N₄/ZnO) was synthesized using a simple precipitation-calcination method. Through this method, g-C₃N₄ nanosheets with a controlled content were successfully decorated on the petals of flower-like ZnO. Various techniques were used to confirm the successful formation of the g-C₃N₄/ZnO hierarchical structure. The methane (CH₄) sensing properties of g-C₃N₄/ZnO sensor were investigated. The result exhibited that after decorating ZnO with g-C₃N₄, the CH₄ sensing performances of the fabricated sensor were remarkably improved. At the optimum operating temperature of 320 °C, the response of the sensor fabricated with CNZ-3 (the sample with an optimum content of g-C₃N₄) towards 1000 ppm CH₄ was as high as 11.9 (R_a/R_g), which was about 2.2 times higher than that of the pure ZnO sensor (5.3). In addition, the CNZ-3 sensor also exhibited a fast response/recovery speed (15/28 s) and outstanding long-term stability. The enhancing CH₄ sensing mechanism may be contributed to enlarged surface area, pore structure, and g-C₃N₄-ZnO n-n junction.

UV light-assisted gas sensors based on metal oxide semiconductor (MOS) have attracted much attention in detecting flammable and explosive gases at room temperature. In this paper, graphite-based carbon nitride (g-C₃N₄) nanosheets-decorated ZnO porous hollow microspheres (PHMSs) with the size about 3~5 μm in diameter were successfully synthesized by annealing the solvothermally-synthesized Zn₅(CO₃)₂(OH)₆ PHMSs together with g-C₃N₄. The synthesized samples were characterized by XRD, SEM, TEM, FT-IR and XPS. The results indicated that the prepared g-C₃N₄/ZnO PHMSs were constructed by numerous loosely stacked ZnO nanoparticles of 20~30 nm in diameter. Gas sensing tests indicated that under UV light (365~385 nm) illumination, the sensors fabricated with g-C₃N₄/ZnO HPMSs showed an enhanced response and faster response speed than the pure ZnO counterpart at room temperature. In addition, the g-C₃N₄/ZnO sensor also exhibited good repeatability and long-term stability for CH₄ detection.

A highly sensitive and selective detection of volatile organic compounds (VOCs) by using gas sensors based on metal oxide semiconductor (MOS) has attracted increasing interest, but still remains a challenge in gas sensitivity and selectivity. In order to improve the sensitivity and selectivity of In₂O₃ to triethylamine (TEA), herein, a silver (Ag)-modification strategy is proposed. Ag nanoparticles with a size around 25–30 nm were modified on pre-synthesized In₂O₃ PNSs via a simple room-temperature chemical reduction method by using NaBH₄ as a reductant. The results of gas sensing tests indicate that after functionalization with Ag, the gas sensing performance of In₂O₃ PNSs for VOCs, especially for TEA, was remarkably improved. At a lower optimal working temperature (OWT) of 300 °C (bare In₂O₃ sensor: 320 °C), the best Ag/In₂O₃-2 sensor (Ag/In₂O₃ PNSs with an optimized Ag content of 2.90 wt%) shows a sensitivity of 116.86/ppm to 1–50 ppm TEA, about 170 times higher than that of bare In₂O₃ sensor (0.69/ppm). Significantly, the Ag/In₂O₃-2 sensor can provide a response (R_a/R_g) as high as 5697 to 50 ppm TEA, which is superior to most previous TEA sensors. Besides lower OWT and higher sensitivity, the Ag/In₂O₃-2 sensor also shows a remarkably improved selectivity to TEA, whose selectivity coefficient (S_TEA/S_ethanol) is as high as 5.30, about 3.3 times higher than that of bare In₂O₃ (1.59). The sensitization mechanism of Ag on In₂O₃ is discussed in detail.

A nest-like ZnO morphology was successfully prepared by a reliable solvothermal route. The obtained nest-like ZnO nanostructure was about 200 nm in size and densely assembled with numerous nanoparticles (NPs) of about 8.6 nm in size. Through a simple impregnation method, different amounts of Au NPs were evenly decorated on the as-prepared ZnO nanostructure. It was found that after introduction of Au, the size of primary ZnO NPs in the nest-like structure increased from 8.6 nm to 43.2 nm, however, the sensor based on Au/ZnO composites still revealed strikingly enhanced TEA sensing properties at its optimum working temperature of 320 oC. The gas response value of the NZA-2 (molar ratio between Au and ZnO is 2:100) sensor towards 200 ppm TEA is as high as 625 (Ra/Rg), which is about 6 folds that of the pure ZnO sensor (97). More importantly, the NZA-2 sensor also showed good selectively, low detection limit (1ppm), fast response/recovery speed (200 ppm TEA, 4s/26s), and good repeatability and long-term stability. These sensing properties made the Au-decorated ZnO nest-like nanostructure a promising candidate for fabricating TEA sensor. The increased gas sensitivity of Au/ZnO was illustrated by the good catalytic properties of Au NPs.