Hierarchical three-dimensional (3D) molybdenum disulfide (MoS2) nanospheres with diameter of about 1 μm have been successfully synthesized via a facile hydrothermal process by controlling the concentration of sulfur and molybdenum sources. The results show that the MoS2 nanospheres are self-assembled by numerous nanosheets, and the concentration of reactants plays a key role in the formation of the hierarchical MoS2 nanospheres. Specifically, the thickness of the nanosheets decreases gradually with the increasing concentration of reactants within certain limits. In addition, the possible mechanism of MoS2 nanospheres formation is proposed through analyzing the experimental results. Firstly, a fast nucleation of amorphous MoS2 particles occurs in the hydrothermal process by the oxidation–reduction reaction. Subsequently, the MoS2 nanoparticles start to spontaneously aggregate into MoS2 nanosheets according to their crystal growth habit. In the case of the appropriate concentration of reactants, the nanosheets are self-oriented and assembled by degrees to form the nanospheres driven by reducing surface energy. Finally, well-defined MoS2 nanospheres are formed through the Ostwald ripening process. The research will open a window to prepare novel hierarchical 3D MoS2 assembled by low dimensional nanoscale building blocks and the analysis of mechanism can also be used to discuss other transition-metal sulfides.
Paper is known to have good flexible and hydrophilic characteristics, making it a potential candidate material to fabricate flexible humidity sensor. The key technology for fabricating paper-based (PB) humidity sensor is to prepare electrodes on the surface of paper. However, the preparation of electrodes on the flexible paper usually involves complex processes and expensive raw materials. In this work, inspired by the traditional writing art with carbon ink, a facile PB humidity sensor is fabricated by drawing the daily available carbon ink on paper as the electrodes. The results show that the PB humidity sensor exhibits an excellent humidity sensing response of more than 1000 as well good linearity (R2 = 0.9981) within the humidity range from 18.7% to 91.5% relative humidity (RH). Owing to the good flexibility and fast response rate (~7.5 nA/s, current change rate of the humidity sensor from18.7% to 91.5% RH), endowing the PB humidity sensor has multifunctional applications for breath rate, baby diaper wetting, and vertical space humidity distribution monitoring. Moreover, the PB humidity sensor can be directly disposed by a simple and cost-saving combustion method. This work provides a helpful guidance for the preparation of flexible, low-cost, eco-friendly and multifunctional humidity sensor, and expands the application of daily available carbon ink in the field of electrodes.
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