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.

Recently, semiconducting metal oxide (SMO) gas sensors have attracted the attention of researchers for high conductivity, labile features by environment, low cost, easy preparation, etc. However, traditional SMOs have some defects such as higher operating temperature and lower response value, which greatly limit their application in the field of gas sensor. In this work, the carbon nitride decorated ball-flower like Co₃O₄ composite was successfully synthesized via a facile hydrothermal method, the composition and morphology of the as-synthesized samples were studied by the techniques of X-ray powder diffraction (XRD), Field-emission scanning electron microscopy (FESEM), Transmission electron microscopy (TEM), Fourier transform infrared spectrometer (FT-IR) and N₂-sorption. As a consequence, the pure Co₃O₄ and the carbon nitride decorated Co₃O₄ both possess ball-flower like structure, and the as-synthesized carbon nitride decorated Co₃O₄ composite exhibits significant sensing properties to ethanol which is 1.6 times higher than that of pure Co₃O₄, furthermore, the composite possesses high selectivity and stability towards ethanol detection.

The SnO₂/g-C₃N₄ composites were synthesized via a facile calcination method by using SnCl₄·5H₂O and urea as the precursor. The structure and morphology of the as-synthesized composites were characterized by the techniques of X-ray diffraction (XRD), the field-emission scanning electron microscopy and transmission electron microscopy (FESEM and TEM), energy dispersive spectrometry (EDS), thermal gravity and differential thermal analysis (TG-DTA), and N₂-sorption. The analysis results indicated that the as-synthesized samples possess the two dimensional structure. Additionally, the SnO₂ nanoparticles were highly dispersed on the surface of the g-C₃N₄ nanosheets. The gas-sensing performance of the as-synthesized composites for different gases was tested. Moreover, the composite with 7 wt % g-C₃N₄ content (SnO₂/g-C₃N₄-7) exhibits an admirable gas-sensing property to ethanol, which possesses a higher response and better selectivity than that of the pure SnO2-based sensor. The high surface area of the SnO2/g-C3N4 composite and the good electronic characteristics of the two dimensional graphitic carbon nitride are in favor of the elevated gas-sensing property.

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.

Flower-like SnO₂/g-C₃N₄ nanocomposites were synthesized via a facile hydrothermal method by using SnCl₄·5H₂O and urea as the precursor. The structure and morphology of the as-synthesized samples were characterized by using the X-ray powder diffraction (XRD), electron microscopy (FESEM and TEM), and Fourier transform infrared spectrometer (FT-IR) techniques. SnO₂ displays the unique 3D flower-like microstructure assembled with many uniform nanorods with the lengths and diameters of about 400–600 nm and 50–100 nm, respectively. For the SnO₂/g-C₃N₄ composites, SnO₂ flower-like nanorods were coupled by a lamellar structure 2D g-C₃N₄. Gas sensing performance test results indicated that the response of the sensor based on 7 wt. % 2D g-C₃N₄-decorated SnO₂ composite to 500 ppm ethanol vapor was 150 at 340 °C, which was 3.5 times higher than that of the pure flower-like SnO₂ nanorods-based sensor. The gas sensing mechanism of the g-C₃N₄nanosheets-decorated SnO₂ flower-like nanorods was discussed in relation to the heterojunction structure between g-C₃N₄ and SnO₂.

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.