It is well known that the metal atoms of metal oxide semiconductor (MOS) exhibit significant activity in gas sensing. However, limited by the shielding effect of the outer oxygen atom layer, layered MoO 3 is often difficult to show ideal gas adsorption activity. Hence, the MoO 3 microporous nanoflowers (MPNFs) assembled by porous two-dimensional nanosheets were successfully synthesized and exhibited excellent gas sensing performance to H 2 S, and the response was 7.2 times higher than that of simple MoO 3 nanosheets. The abundant pores of MoO 3 MPNFs were due to the influence of the crystal cell shrinkage effect on the atomic arrangement, while the significantly enhanced gas sensing performance was attributed to the positive effect of the microporous structure on gas diffusion and the exposed edge Mo atoms. This was confirmed by DFT calculation results that, compared to the Mo atoms on the surface of MoO 3 nanosheets, the Mo atoms around the pores were exposed because they broke through the shielding effect of the oxygen atom layer and exhibited higher adsorption activity for H 2 S and O 2 molecules. Therefore, this work can shed a light on the design of high-performance gas sensors based on metal oxides .
Limited by the dense arrangement of oxygen atoms in the outer layer, the active Mo sites of MoO 3 ultrathin nanoflowers are shielded by oxygen, making it difficult for efficient gas adsorption. Here, oxygen atoms on the surface of MoO 3 nanoflowers (MoO 3 NFs) were knocked out based on a simple hydrogenation strategy to obtain MoO 3 defective nanoflowers (MoO 3 DNFs) with abundant molecular traps and exposed Mo sites. Notably, the response of MoO 3 DNFs-10 to 2 ppm H 2 S is 3 times that of MoO 3 NFs. Oxygen atom knockout was verified based on Laser Raman Spectroscopy, Photoluminescence (PL), UV–Visible Spectroscopy (UV–Vis), and X-ray photoelectron energy Spectroscopy (XPS), while calculations based on density functional theory (DFT) were used to reveal the sensing enhancement of molecular traps and exposed Mo sites. On the one hand, molecular traps can effectively trap H 2 S molecules to facilitate further adsorption. On the other hand, exposed Mo sites have higher charge activity and exhibit higher adsorption capacity for H 2 S gas. This work breaks through the limitations of the traditional oxygen vacancy model and innovatively discusses the synergistic effect of molecular traps and exposed Mo sites on enhanced sensing, which can provide a reference for the gas sensing by metal oxide semiconductors.
The inert gas adsorption of outer oxygen atoms is becoming an important factor limiting the gas sensing activity of two-dimensional MoO 3 nanoflowers, which makes it necessary to modify the surface activity of MoO 3 nanoflowers. For a long time, the unique activity of Co atoms has attracted much attention in gas sensing. In this work, Co metal sites were introduced into MoO 3 defective nanoflowers (MoO 3 DNFs) by doping, which further improved the H 2 S gas sensing performance of MoO 3 DNFs. Based on H 2 S gas sensing experiment, the relationship between Co doping amount and gas sensing performance was constructed, and the optimal Co doping ratio was obtained. Finally, the adsorption reaction process of H 2 S gas molecules on MoO 3 -Co DNFs was simulated by DFT calculation, and the activation effect of Co metal sites on outer oxygen atoms was revealed.
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