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Air pollution is becoming an increasingly important global issue. Toxic gases such as ammonia, nitrogen dioxide, and volatile organic compounds (VOCs) like phenol are very common air pollutants. To date, various sensing methods have been proposed to detect these toxic gases. Researchers are trying their best to build sensors with the lowest detection limit, the highest sensitivity, and the best selectivity. As a 2D material, graphene is very sensitive to many gases and so can be used for gas sensors. Recent studies have shown that graphene with a 3D structure can increase the gas sensitivity of the sensors. The limit of detection (LOD) of the sensors can be upgraded from ppm level to several ppb level. In this review, the recent progress of the gas sensors based on 3D graphene frameworks in the detection of harmful gases is summarized and discussed.

such as graphene-based materials, boron nitride, and transition metal dichalcogenides, which have been predicted to show piezoelectric properties by density functional theory analysis. [3] Aerogels are porous materials with low density, high porosity, and high specific area. [4] Graphene aerogels (GAs), also known as 3D graphene, are macroscopic architectures assembled from graphene nanosheets. [5] GAs have high surface utilization and are suitable for surface functionalization. [6] The preparation methods of GAs are relatively simple, including self-assembly, [7] sacrificial template, [8] and 3D printing methods. [9] GAs have been widely investigated for the photothermal conversion, [10] electromagnetic shielding, [11] gas sensing, [12] surface catalysis, [13] and wearable devices. [14] GAs are assembled from the graphene sheets via van de Waals forces during the synthesis processes, where the physical connection between the graphene sheets provides macroscopic electrical conductivity. [15] In general, deformation occurs when an external pressure is applied to the GAs, which increases the area of contact between the graphene sheets, and increases the electrical conductivity. [16] It is this mechanism that makes GAs be suitable for piezoresistive sensors. The high sensitivity, wide linear range, good stability, low detection limit, and short response time are all critical features required for a piezoresistive sensor. [17] Although GAs meet most of these requirements due to their 3D interconnected architectures, the disordered microstructure of most isotropic GAs leads to a poor stability and a limited linearity, which severely limits their practical applications. [18] Innovative attempts to control the architectures with the introduction of orientation, reinforcing, wrinkling, and hierarchical pores can help to overcome these limitations. GAs with ordered microstructures can be prepared via air foaming, [19] 3D printing, and freeze-drying. [20] In the case of freeze-drying, the ice crystals act as a solid template, and the graphene sheets can aggregate along the ice crystals. [21] Parallel stacked lamellar architectures with a periodic arrangement could be produced via freeze-drying. [22] The unique architecture could result in Graphene-based aerogels (GAs) have been extensively studied for pressure sensing applications due to their high compressibility and conductivity. But the usually-adopted high-temperature treatment generally damages the flexibility and stability. It is still challenging to prepare piezoresistive sensors with a wide linear range, and reliable cyclic performance. Herein, we demonstrate an excellent wide-range piezoresistive sensor based on an aerogel of methylcellulose reinforced reduced graphene oxide (MC/GA). A steaming reduction at 120 °C is proposed to properly control the intermolecular forces and the chemical bonding between MC and GA. The finite element analysis indicates that wrinkled lamellae with hierarchical pores are beneficial to minimize the stress concentrations. As a result ...

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and low-cost method, surface wrinkling of thin films on soft substrates has been developed to generate various functional micro/nanostructures (e.g., wrinkles, creases, crumples, and folds) on planar and curved substrates. [13][14][15][16] These wrinkling structures with tunable surface-wetting, electronical, and mechanical properties bring great potentials for emerging applications including flexible electronics, [17][18][19] reversible patterning, [20][21][22] smart wetting surfaces, [23][24][25] and actuators. [26,27] Compared with traditional microfabrication methods, surface wrinkling shows advantages (e.g., template-free, simple, and low-cost) in the fabrication of microarchitecture arrays on curved substrates. [15] Many efforts have been put into the controlled wrinkling of metal coatings, polymer films, and oxide layers on curved surfaces through thermal shrinkage, [28,29] negative pressure, [30,31] and swelling/ deswelling. [32,33] Due to fascinated thermal, electrical, and optical properties, strain engineering of 2D materials has become a hot research topic. [34,35] Graphene oxide (GO), because of excellent flexibility and rich oxygen-containing groups, is suitable to form conformal coatings on various curved substrates. [36,37] Diverse functional wrinkled structures have been achieved via the wrinkling of GO films on spherical and cylindrical substrates, such as highly-convoluted patterns for deforming actuators, [27] hierarchical ridges for strain sensors, [38,39] and multigenerational crumples for anticounterfeiting patterns. [40] Recently, inspired by the hierarchical papillae of the rose petal, we obtained superhydrophobic papillae arrays by the wrinkling of nonuniform GO films coated on spherical latex substrates. [8] Although it provides a feasible method for fabricating hierarchical architecture arrays on curved substrates, there are still some issues: i) Only millimeter-scale architectures (>0.6 mm) could be obtained because of the restriction of the coating technique, which limits their applications in largearea micro/nanofabrication of architecture arrays on curved substrates. ii) The previous work mainly focuses on the surfacewetting properties of the papillae whereas mechanical stability of the wrinkled papillae arrays has not yet been demonstrated. iii) Most importantly, although the wrinkling of single-layer stiff or soft films on curved substrates has been investigated, [15,27,31] the folding of stiff/soft bilayers with large modulus contrast Although controlled wrinkling is demonstrated to be a powerful tool for micro/nanofabrication, large-area fabrication of microarchitecture arrays on curved substrates by surface wrinkling still remains challenging. Inspired by the cortical folding, a facile method for transforming graphene oxide (GO) patterns into multiscale microarchitecture arrays on curved substrates is developed. Mass production of hierarchical GO papillae arrays can be realized by homogeneous compression of patterned GO/rubber bilayers. The reduced GO (rGO) papillae arrays sho...