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Defect engineering has received extensive attention as an effective method to tune the gas sensing properties of semiconductor materials. Here, defective WO 3 (D-WO 3 ) nanosheets were obtained by a simple hydrogenation process with a detection limit as low as 5 ppb for dimethyl trisulfide (DMTS) and a response of 2.3 times that of the initial WO 3 nanosheets to 100 ppb DMTS. Importantly, X-ray photoelectron spectroscopy and Raman spectroscopy confirmed the partial loss of oxygen atoms in D-WO 3 nanosheets, and density functional theory calculations found that the W sites near the oxygen defect showed higher adsorption energy for DMTS and transferred more electrons during the gas interaction, indicating that the active W site caused by oxygen atom loss can effectively enhance the reactivity of twodimensional WO 3 nanosheets. Different from the traditional oxygen defect model, this work reveals the positive effect of active metal sites on gas sensing for the first time, which is expected to provide an effective reference for the sensing application of defect engineering in metal oxides.

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Two-dimensional (2D) transition metal oxides (TMOs) have attracted much attention for various applications, owing to the abundance of active sites, rapid ion transmission speed, and short carrier migration distance. However, the current preparation strategies are usually limited to producing intrinsically layered compounds or sacrificing template. Here, we report a universal strategy for preparing ultrathin porous 2D TMO nanosheets by chemical topological transformation of the corresponding transition metal selenides. We observed that the as-prepared 2D TMO nanosheets not only perfectly inherit the transition metal selenides' 2D topological structure, but also possess numerous pore structures formed as a way to release the structural stress associated with lattice growth. As a proof of concept, ultrathin porous WO 3 , MoO 3 , and Co 3 O 4 nanosheets were successfully prepared based on the in-situ oxidation of the corresponding ultrathin WSe 2 , MoSe 2 , and Co 0.85 Se, respectively. The outstanding sensing properties and photodetector performance displayed by the as-prepared porous 2D WO 3 nanosheets further indicate the promising prospects of topology transformation for the preparation of porous 2D TMO nanosheets.

The formation path of hollow complex nanocages prepared via Na2WO4·2H2O etching, using Prussian blue as a template, is tracked, which confirms the existence of a central cross structure inside the etching products.

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