| Online monitoring and analyzing of sulfur hexafluoride(SF6) decomposed components is one of the most promising and effective methods to investigate incipient faults in SF6 insulated equipment. Gas sensing technology is the core content in SF6 insulated equipment online insulation monitoring. It has an important theoretical and practical significance in online detection, fault diagnosis and evaluation to make sustained research on gas sensing properties of sensing materials and their sensing mechanism, which can further improve the accuracy and sustainability of online monitoring technology. Because high working temperature, weak response, poor selectivity and stability were popular baffle in sensing application, graphene, a new sensitive material was conducted in SF6 decomposed components detection. Meanwhile, the corresponding first-principles study on immanent sensing mechanism was followed to provide further theoretical guidance and technical support for high performance sensor devices development.Supported by the National Natural Science Foundation of China, this paper aimed to explore the sensing performances of hydrogen sulfide(H2S), sulfur dioxide(SO2), thionyl fluoride(SOF2) and sulfuryl fluoride(SO2F2), which were four characteristic products of SF6 decomposed components, on pristine and Au-doped graphene sensors at room temperature. Sensitivity response, repeatability, stability and selectivity properties were dominated in the experimental discussion part. Besides, Au-doped graphene model and the corresponding gas adsorption models were built based on density functional theory(DFT) and first-principles approach for the purpose of simulating the doping property and the gas adsorption properties from the atomic and electronic level. Finally, the immanent sensing mechanism was deeply discussed based on the macro experimental sensing properties combined with micro simulating calculations. The main innovative achievements are as follows:① Pristine graphene and graphene films doped with Au nanoparticles were synthesized via chemical reduction method. Their corresponding gas sensors were both fabricated by the traditional drop coating method and then used as an adsorbent for the detection of H2 S, SO2, SOF2 and SO2F2 at room temperature. Results demonstrated that the sensitivity of SO2, SOF2 and SO2F2 are insignificant on pristine graphene, whereas the pristine graphene yields good response sensitivity to H2 S. Therefore, pristine graphene is considered a promising adsorbent for H2 S selective detection. Compared with the performance on pristine graphene films, Au-doped graphene emerges significant responses to H2 S, SOF2 and SO2F2 but weak interaction to SO2, with the sequence of SO2F2>H2S>SOF2>SO2. Among them, only H2 S shows the opposite response with its resistance increase, while SO2, SOF2 and SO2F2 decreases the resistance of Au-doped graphene.② Pristine and Au-doped graphene models were built by Materials Studio software. Similarly, H2 S, SO2, SOF2 and SO2F2 adsorption models were built separately on pristine and Au-doped graphene surface. DFT calculation results demonstrated that gases on pristine graphene are all physisorption effect with no charge transfer phenomenon and van der Waals dominates among the interaction process. Au-doped graphene exhibits relatively strong chemisorption effect to H2 S, SOF2 and SO2F2 with obvious charge transfer, indicating the old bond breaking or the new bond formation. Interaction strength order is SO2F2>H2S>SOF2. As to SO2, its poor interaction is the intermediary between physisorption and chemisorption.③ In order to interpret the adsorption processes between Au-doped graphene and gas molecules, this paper discussed the charge transfer mechanism on the adsorption surface for further investigation. Our preliminary judgment is that the key parameter that determines the direction of charge transfer is the energy difference △ between work function of Au-doped graphene(Au-GF) and HOMO/LUMO molecular orbits. The micro process of charge transfer can be summarized as: as for n-type adsorption system, Au-doped graphene acts as electronic acceptor with its positively charged donor formation energy state(E+) larger than Au-GF, then its Fermi level up-shited; as for p-type adsorption system, Au-doped graphene acts as electronic donor with its negatively charged acceptor formation energy state(E-) smller than Au-GF, then its Fermi level down-shifted.④ For the purpose of providing guidance for selective detection based on mechanism theory, this paper combined the experimental sensing properties and the DFT calculation results to investigate the correlation between n/p-type adsorption effects and the change trend of surface resistance. The correlation can be concluded as: the p-type adsorption gases(SO2, SOF2 and SO2F2) lead to the resistance decrease on adsorbent while the n-type adsorption gas(H2S) results in the resistance increase.⑤ According to our DFT calculation results, the physisorption effect can explain the experimental sensing performances of SO2, SOF2 and SO2F2 on pristine gaphene. Meanwhile, the relatively strong chemisorption effect of H2 S, SOF2 and SO2F2 on Au-doped graphene confirms their macroscopic response properties. Therefore, our first-principles study basis on DFT is a feasible and correct approach to investigate the experimental sensing property of graphene-based sensors. |
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