Detection of hazardous volatile organic compounds (VOCs) by metal oxide nanostructures-based gas sensors: A review

Semiconductor polymeric graphitic carbon nitride (g-C3N4) photocatalysts have attracted dramatically growing attention in the field of the visible-light-induced hydrogen evolution reaction (HER) because of their facile synthesis, easy functionalization, attractive electronic band structure, high physicochemical stability and photocatalytic activity. This review article presents a panorama of the latest advancements in the rational design and development of g-C3N4 and g-C3N4-based composite photocatalysts for HER application. Concretely, the review starts with the development history, synthetic strategy, electronic structure and physicochemical characteristics of g-C3N4 materials, followed by the rational design and engineering of various nanostructured g-C3N4 (e.g. thinner, highly crystalline, doped, and porous g-C3N4) photocatalysts for HER application. Then a series of highly efficient g-C3N4 (e.g., metal/g-C3N4, semiconductor/g-C3N4, metal organic framework/g-C3N4, carbon/g-C3N4, conducting polymer/g-C3N4, sensitizer/g-C3N4) composite photocatalysts are exemplified. Lastly, this review provides a comprehensive summary and outlook on the major challenges, opportunities, and inspiring perspectives for future research in this hot area on the basis of pioneering works. It is believed that the emerging g-C3N4-based photocatalysts will act as the “holy grail” for highly efficient photocatalytic HER under visible-light irradiation.

Semiconductor polymeric graphitic carbon nitride photocatalysts: the “holy grail” for the photocatalytic hydrogen evolution reaction under visible light

Xin Zhou,†,‡ Songyi Lee,† Zhaochao Xu,* and Juyoung Yoon*,† †Department of Chemistry and Nanoscience, Ewha Womans University, Seoul 120-750, Republic of Korea ‡Research Center for Chemical Biology, Department of Chemistry, Yanbian University, Yanjii 133002, People’s Republic of China Key Laboratory of Separation Science for Analytical Chemistry, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Shahekou, Dalian, Liaoning, People’s Republic of China

Recent Progress on the Development of Chemosensors for Gases.

Quasi-one dimensional metal oxide semiconductors: Preparation, characterization and application as chemical sensors

Two-dimensional (2D) nanostructures are highly attractive for fabricating nanodevices due to their high surface-to-volume ratio and good compatibility with device design. In recent years 2D nanostructures of various materials including metal oxides, graphene, metal dichalcogenides, phosphorene, BN and MXenes, have demonstrated significant potential for gas sensors. This review aims to provide the most recent advancements in utilization of various 2D nanomaterials for gas sensing. The common methods for the preparation of 2D nanostructures are briefly summarized first. The focus is then placed on the sensing performances provided by devices integrating 2D nanostructures. Strategies for optimizing the sensing features are also discussed. By combining both the experimental results and the theoretical studies available, structure-properties correlations are discussed. The conclusion gives some perspectives on the open challenges and future prospects for engineering advanced 2D nanostructures for high-performance gas sensors devices.

Two-Dimensional Nanostructured Materials for Gas Sensing

Aligned zinc oxide nanorods were synthesized directly via a two-step solution approach on an Al2O3 tube, and were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The zinc oxide nanorods prepared were uniform with diameters of 10–30 nm and lengths about 1.4 μm. The response Sr (= Ra/Rg) of the aligned zinc oxide nanorod sensor reached 18.29 and 10.41 to 100 ppm ethanol and hydrogen, respectively, which was a two-fold increase compared with that reported in literature, demonstrating the potential for developing stable and sensitive gas sensors.

Nanopillar ZnO gas sensor for hydrogen and ethanol

Graphitic carbon nitride (g-C3N4) behaving as a layered feature with graphite was indexed as a high-content nitrogen-doping carbon material, attracting increasing attention for application in energy storage devices. However, poor conductivity and resulting serious irreversible capacity loss were pronounced for g-C3N4 material due to its high nitrogen content. In this work, magnesiothermic denitriding technology is demonstrated to reduce the nitrogen content of g-C3N4 (especially graphitic nitrogen) for enhanced lithium storage properties as lithium ion battery anodes. The obtained nitrogen-deficient g-C3N4 (ND-g-C3N4) exhibits a thinner and more porous structure composed of an abundance of relatively low nitrogen doping wrinkled graphene nanosheets. A highly reversible lithium storage capacity of 2753 mAh/g was obtained after the 300th cycle with an enhanced cycling stability and rate capability. The presented nitrogen-deficient g-C3N4 with outstanding electrochemical performances may unambiguously promot...

Nitrogen-Deficient Graphitic Carbon Nitride with Enhanced Performance for Lithium Ion Battery Anodes

Novel g-C3N4/CoO nanocomposite application for photocatalytic H2 evolution were designed and fabricated for the first time in this work. The structure and morphology of g-C3N4/CoO were investigated by a wide range of characterization methods. The obtained g-C3N4/CoO composites exhibited more-efficient utilization of solar energy than pure g-C3N4 did, resulting in higher photocatalytic activity for H2 evolution. The optimum photoactivity in H2 evolution under visible-light irradiation for g-C3N4/CoO composites with a CoO mass content of 0.5 wt % (651.3 μmol h–1 g–1) was up to 3 times as high as that of pure g-C3N4 (220.16 μmol h–1 g–1). The remarkably increased photocatalytic performance of g-C3N4/CoO composites was mainly attributed to the synergistic effect of the junction or interface formed between g-C3N4 and CoO.

Novel g-C3N4/CoO Nanocomposites with Significantly Enhanced Visible-Light Photocatalytic Activity for H2 Evolution.

Zinc oxide (ZnO) nanosheets were directly synthesized using a facile precipitation method at room temperature without any template, surfactant or organic solvent. X-ray diffraction (XRD) confirms that the ZnO nanosheets belong to hexagonal wurtzite structure. Scanning electron microscope (SEM) and transmission electron microscope (TEM) reveals the morphology and structure of the ZnO nanosheets, showing the predominantly exposed non-polar {100} planes and an average thickness of about 20 nm. In order to regulate and control the intrinsic surface defect contents, improving the thermal stability, the samples were calcined at different temperatures (200 °C, 400 °C and 600 °C respectively), showing that the sheet-like structures can be maintained below 400 °C. Photoluminescence (PL) analysis shows that abundant intrinsic surface defects exist on the ZnO crystal surfaces. Gas sensors based on ZnO nanosheets calcinated at 200 °C exhibits high response, fast response-recovery and good selectivity to 5–1000 ppm acetone vapor at 300 °C. The response value to acetone vapor is correlated with the surface defect contents, namely, the more defects, the higher sensor response. Thus, it is considered that the improved acetone sensing property, especially enhanced response value, is mainly originated from the increased intrinsic defect content on the surface of ZnO nanosheets. Developed precipitation method is facile for synthesis of ZnO nanosheets, which demonstrate an effective strategy for surface defect engineering to improve the metal oxide semiconductor gas sensing performance.

Acetone sensing of ZnO nanosheets synthesized using room-temperature precipitation

A facile approach for synthesis of flower-like p-CuO/n-ZnO heterojunction nanorods was reported. The CuO/ZnO nanorods were prepared by co-precipitation of CuO nanoparticles on the hydrothermally grown ZnO nanorods. The obtained samples were characterized by X-ray diffraction and transmission electron microscopy, which confirms that the heterogeneous nanostructure of the CuO/ZnO nanorods was highly crystalline. The ethanol gas-sensing properties of CuO/ZnO nanorods were evaluated with different ethanol vapor concentrations at the working temperature of 300 °C. The response of 0.25:1 CuO/ZnO nanorod sensor to 100 ppm ethanol was 98.8, which is 2.5 times that of ZnO only sample, with a response and recovery time of 7 s and 9 s, respectively. Good selectivity and long-term stability can also be achieved and the response of low concentration as 1 ppm ethanol can reach the value of 9.68 using the flower-like p-CuO/n-ZnO heterojunction nanorods as sensing material. The enhanced ethanol response is mainly attributed to a wider depletion layer on the CuO/ZnO surface resulted from the formation of p–n heterojunctions between p-CuO nanoparticles and n-type ZnO nanorods.

Li-Jian Bie

Papers

Nanopillar ZnO gas sensor for hydrogen and ethanol

Nitrogen-Deficient Graphitic Carbon Nitride with Enhanced Performance for Lithium Ion Battery Anodes

Novel g-C3N4/CoO Nanocomposites with Significantly Enhanced Visible-Light Photocatalytic Activity for H2 Evolution.

Acetone sensing of ZnO nanosheets synthesized using room-temperature precipitation

Enhanced ethanol gas-sensing properties of flower-like p-CuO/n-ZnO heterojunction nanorods