Graphene-based gas/vapor sensors have attracted much attention in recent years due to their variety of structures, unique sensing performances, room-temperature working conditions, and tremendous application prospects, etc. Herein, we summarize recent advantages in graphene preparation, sensor construction, and sensing properties of various graphene-based gas/vapor sensors, such as NH3, NO2, H2, CO, SO2, H2S, as well as vapor of volatile organic compounds. The detection mechanisms pertaining to various gases are also discussed. In conclusion part, some existing problems which may hinder the sensor applications are presented. Several possible methods to solve these problems are proposed, for example, conceived solutions, hybrid nanostructures, multiple sensor arrays, and new recognition algorithm.

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Cuprous oxide (Cu2O) is an attractive photocatalyst because of its visible-light-driven photocatalytic behavior, abundance, low toxicity, and environmental compatibility. However, its short electron diffusion length and low hole mobility result in low photocatalytic efficiency, which hinders its wider applications. Herein, we report an in situ method to introduce nitrogen-doped carbon dots (N-CDs) into Cu2O frameworks. It is interestingly found that the introduction of N-CDs drives the morphology of N-CDs/Cu2O to evolve from rough cube to sphere, and the most encouraging result is that all of the obtained N-CDs/Cu2O composites exhibit better photocatalytic activities than pure Cu2O cubes. The optimal N-CDs/Cu2O photocatalyst is synthesized with 10 mL of N-CDs solution, which shows the best degradation ability (100%, 70 min), far superior to pure Cu2O cubes (∼5%, 70 min) and P25 (∼10%, 70 min). Beside the photodegradation of methyl orange, N-CDs/Cu2O(10) composites also exhibit excellent photocatalytic activities in the photodegradation of methyl blue and rhodamine B. It is demonstrated that the excellent photocatalytic performance of N-CDs/Cu2O composites can be attributed to the highly roughened structure and the suppression of electron-hole recombination as a result of the introduction of N-CDs. These findings demonstrate that the conjugation of CDs is a promising method to improve the photocatalytic activities for traditional semiconductors.

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High-performance gas sensors based on metal oxides operated at room temperature are of great interest due to their energy saving and cost effective characteristics. How to improve the sensitivity of metal oxide gas sensors and enable their room-temperature operation are challenging for their realistic applications. In this work, we have designed and fabricated Al-doped NiO nanosheets for greatly enhanced NO detection at room temperature. Different amounts of Al were doped into two-dimensional (2D) NiO nanosheets via a fast and facile microwave assisted solvent-thermal technique. Sensing tests of the as-fabricated devices indicated that Al doping could significantly affect the gas-sensing properties of the NiO nanosheets due to increased oxygen vacancies as well as the formation of Lewis acid and base sites. When 12 at% of Al was added to the raw materials, the response value of the device to 10 ppm NO was enhanced more than 35 times compared with those of pure NiO nanosheets. In addition, when the amount of Al reached 20 at%, it took only 200 s for the gas sensor to achieve full recovery, which was a breakthrough for room temperature gas sensors based on metal oxides. Above all, the excellent performances of the as-fabricated devices make Al-doped NiO nanosheets a potential candidate for NO sensing applications. This design strategy can also give guidance for designing high-performance gas sensors based on other similar 2D sensing materials.

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