This review organizes and introduces several common gas sensing mechanisms of metal oxide semiconductors in detail and classifies them into two categories. The scope and relationship of these mechanisms are clarified.

We report synthesis of hierarchical nanostructures of SnO₂ nanosphere functionalized TiO₂ nanobelts as a novel sensing material by a simple hydrothermal technique. A systematic comparison study reveals an enhanced gas sensing performance for the sensor made of SnO₂ and TiO₂ toward volatile organic compounds of several species over that of the commonly applied undecorated TiO₂ nanobelts. The improved gas sensing properties are attributed to the pronounced electron transfer between the hierarchical nanostructures and the absorbed oxygen species as well as to the heterojunctions of the SnO₂ nanospheres to the TiO₂ nanobelts which provide additional reaction rooms. The results represent an advance of hierarchical nanostructures in further enhancing the functionality of gas sensors, and this facile method could be applicable to many sensing materials.

Since MXene (a two-dimensional material) was discovered in 2011, it has been favored in all aspects due to its rich surface functional groups, large specific surface area, high conductivity, large porosity, rich organic bonds, and high hydrophilicity. In this paper, the preparation of MXene is introduced first. HF etching was the first etching method for MXene; however, HF is corrosive, resulting in the development of the in situ HF method (fluoride + HCl). Due to the harmful effects of fluorine terminal on the performance of MXene, a fluorine-free preparation method was developed. The increase in interlayer spacing brought about by adding an intercalator can affect MXene’s performance. The usual preparation methods render MXene inevitably agglomerate and the resulting yields are insufficient. Many new preparation methods were researched in order to solve the problems of agglomeration and yield. Secondly, the application of MXene-based materials in gas sensors was discussed. MXene is often regarded as a flexible gas sensor, and the detection of ppb-level acetone at room temperature was observed for the first time. After the formation of composite materials, the increasing interlayer spacing and the specific surface area increased the number of active sites of gas adsorption and the gas sensitivity performance improved. Moreover, this paper discusses the gas-sensing mechanism of MXene. The gas-sensing mechanism of metallic MXene is affected by the expansion of the lamellae and will be doped with H₂O and oxygen during the etching process in order to become a p-type semiconductor. A p-n heterojunction and a Schottky barrier forms due to combinations with other semiconductors; thus, the gas sensitivities of composite materials are regulated and controlled by them. Although there are only several reports on the application of MXene materials to gas sensors, MXene and its composite materials are expected to become materials that can effectively detect gases at room temperature, especially for the detection of NH₃ and VOC gas. Finally, the challenges and opportunities of MXene as a gas sensor are discussed.

MIL-53(Co–Fe)/NF behaves as a durable catalytic electrode for water oxidation, offering 100 mA cm ⁻² at an overpotential of only 262 mV.

Aqueous sodium‐ion battery of low cost, inherent safety, and environmental benignity holds substantial promise for new‐generation energy storage applications. However, the narrow potential window of water and the enlarged ionic radius because of hydration restrict the selection of electrode materials used in the aqueous electrolyte. Here, inspired by the efficient redox reaction of biomolecules during cellular energy metabolism, a proof of concept is proposed that the redox‐active biomolecule alizarin can act as a novel electrode material for the aqueous sodium‐ion battery. It is demonstrated that the specific capacity of the self‐assembled alizarin nanowires can reach as high as 233.1 mA h g⁻¹, surpassing the majority of anodes ever utilized in the aqueous sodium‐ion batteries. Paired with biocompatible and biodegradable polypyrrole, this full battery system shows excellent sodium storage ability and flexibility, indicating its potential applications in wearable electronics and biointegrated devices. It is also shown that the electrochemical properties of electrodes can be tailored by manipulating naturally occurring 9,10‐anthroquinones with various substituent groups, which broadens application prospect of biomolecules in aqueous sodium‐ion batteries.