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Two-dimensional (2D) ultrathin and mesoporous Co3O4 nanomeshes modified by ZnO nanocrystals (ZnO/Co3O4) was achieved via in-situ growth of ZIF-8 polyhedral particles on ultrathin ZIF-67 nanosheets, which was synthesized by a salt-template strategy followed by calcination. The ZnO/Co3O4-2 (the molar ratio of Zn/Co is 2) ultrathin nanomeshes heterostructure based sensor exhibits a much higher response (Ra/Rg) of 3.2–5 ppm triethylamine with a shorter response/recovery time of 30/55 s at a low operating temperature of 100 °C compared with pristine Co3O4 and ZnO. The enhanced gas sensing performances of ZnO/Co3O4-2 are attributed to its ultrathin mesoporous nanostructure, the presence of rich oxygen vacancy defects and unique heterojunctions at the ZnO/Co3O4 interface. This research not only provides a solid and universal design strategy for the synthesis of 2D ultrathin nanomeshes but also offers valuable guidance to optimize their gas sensing properties by heterostructure design.

Biomass derived nitrogen-doped porous carbon (NPC) has received considerable interest in various fields, resulting from that biomass is abundant, renewable, fast generation and low cost. However, few studies were focused on the gas sensing properties of NPC. In this work, by simple calcination and hydrothermal method, chrysanthemum tea derived NPC is originally introduced on ZIF-8 derived ZnO polyhedrons to form ZnO/NPC heterostructure as a novel acetone sensor. Remarkably, the response of ZnO/NPC towards 100 ppm acetone (25.47) is 8 times more than that of ZnO (3.33) and 14 times higher than that of NPC (1.85). DFT calculations results demonstrate that the adsorption energy between acetone molecules and ZnO/NPC heterostructure is higher than the adsorption energy between acetone molecules and pure NPC (or pure ZnO). These experimental and theoretical results can be adopted for the fundamental investigation of other gas sensing materials based on biomass derived NPC.

One-dimensional cobalt oxide (Co3O4) exhibits great potential in gas sensing, but the application is severely hindered because of the agglomeration or close packing during the normal synthesis process. Herein, 3D radial Co3O4 assembled with nanorods is fabricated by employing cobalt-based layered hydroxide metal salt (Co-LHMS) as a self-template. Meanwhile, Co3O4 with flake floral and dumbbell radial structure obtained by tuning the hydrothermal reaction time of precursors is used as a control group for gas sensing tests. The sensitivity of 3D radial Co3O4 to sub-ppm acetone is 4 times higher than other structures and the detection limit is 100 ppb at a relatively low working temperature of 200 ℃, which is excellent among intrinsic acetone sensors based on pure Co3O4. The satisfactory gas sensing performance can be ascribed to the large specific surface, numerous transmission pathways, and low grain boundary resistance of the material. Furthermore, this work provides an idea for the further improvement of one-dimensional gas sensors.

The demand of acetone gas sensors with great response value and fast response/recovery time to low concentration acetone is increasing in practical diagnosis for diabetes. In this work, metal-organic frameworks (MOFs) derived flower-like ZnO/Co3O4 heterojunctions based sensor is prepared by a simple solution mixing method at room temperature. The sensor shows high acetone response value, short response/recovery time, excellent selectivity and good stability. For example, the response value of ZnO/Co3O4 heterojunctions based sensor is up to 120 % towards 500 ppb acetone at 300 ℃, which is about 30-fold enhancement than these of intrinsic ZnO and Co3O4. Moreover, its minimal detection limitation is 100 ppb acetone and it shows ultra-fast response/recovery time for ppb-level acetone (22 s/27 s@500 ppb). The excellent acetone sensing properties of flower-like ZnO/Co3O4 heterojunctions based sensor is attributed to the sharp resistance variation of heterojunction interface between ZnO and Co3O4 and the ultra-fast gas transport channel provided by ZnO nanosheets. This finding provides important insights and useful guidance for the detection of ppb-level acetone.

It is challenging for current metal oxides based acetone sensors to realize excellent ppb-level acetone sensing properties at low working temperature. Herein, the hollow ZnO/MoS2 nanosheets (HZnO/MoS2) core/shell heterogeneous structures are originally produced. Fast gas transport channels and n–p heterojunctions are produced by the decoration of MoS2 nanosheets on HZnO surface, resulting in high acetone response and fast response/recovery speed. Ultraviolet (UV) light is introduced to further improve acetone response and drastically reduce working temperature. The light diffraction and reflection caused by the decoration of hierarchical MoS2 nanosheets could significantly enhance light harvesting. Hence, the excellent acetone sensing properties are obtained by the synergistic effect of UV light and HZnO/MoS2 core/shell heterogeneous structures. Exactly, at 100 °C, the HZnO/MoS2 possesses a stable response (1.52) to 100 ppb acetone with UV irradiation while it exhibits no response to 100 ppb acetone without UV. Even at room temperature (30 ℃), UV activated HZnO/MoS2 still exhibits a stable response (∼1.33) and fast recovery/recovery time (19 s/97 s) to 50 ppm acetone. Furthermore, the DFT calculations are performed to demonstrate the underlying acetone sensing mechanism. The results demonstrate that UV activated HZnO/MoS2 heterostructure could achieve trace-acetone detection at low temperature.

It is imperative to explore an accurate ppb-level acetone sensor for noninvasive detection of diabetes. In this work, two dimensional p-type MoS2 nanosheets are introduced on the surface of p-type ZnO derived from metal-organic frameworks (MOFs) to produce ZnO@MoS2 core/shell heterojunctions as a novel acetone sensor, showing a great enhancement of acetone response, about two orders of magnitude than that of pure ZnO derived from MOFs. For example, the ZnO@MoS2 exhibits about 80 times enhancement in response to 100 ppb acetone than that of pure ZnO. More importantly, this ZnO@MoS2 heterojunctions sensor exhibits an ultra-fast response/recovery to ultra-low concentration acetone (60 s/40 s @ 5 ppb), which is the best acetone sensing performance for the metal oxide-based materials reported to date. Moreover, the acetone sensing performances are negligibly affected by humidity and other gas, which is suitable for exhaled acetone detection. Finally, it is elucidated that the sharp increase of negative heterojunction interface resistance, ultra-fast gas diffusion rates in MoS2 nanosheets and strong interaction energy are key factors for the excellent acetone sensing properties of ZnO@MoS2. This work opens up a novel and efficient way for ultra-low concentration gas detection.

For the first time, Co-MOF-74 derived Co3O4/graphene heterojunction nanoscrolls (Co3O4/GNS) structure is synthesized by a simple one-step hydrothermal strategy and subsequent calcination process. Theoretically, the growth mechanism of Co3O4/GNS is revealed by molecular dynamic simulation, demonstrating that a high loading of Co-MOF-74 nanoparticles is needed for effective rolling of GO. The sensor based on Co3O4/GNS exhibits high response of 58.1 (34-fold enhancement compared with that of Co3O4), short response/recovery time of 12 s/66 s (157 s/200 s for Co3O4) toward 1 ppm acetone at 190 °C and superior selectivity, which are much better than those of most reported acetone sensors. Moreover, the Co3O4/GNS can even detect as low as 50 ppb acetone with a notable response (1.24). The appreciable response together with rapid response/recovery of Co3O4/GNS is attributed to the large heterojunction interface between Co3O4 and graphene nanoscroll and high-speed gas transfer pathways provided by nanoscroll. This unique structure design is believed to offer guidance for generalizable synthesis of metal oxide/GNS that can be effectively applied in the fields of ppb-level gas detection.