Substitutional Doping of MoS₂ for Superior Gas-Sensing Applications: A Proof of Concept

Two-dimensional layered materials (like MoS₂ and WS₂) those are being used as sensing layers in chemoresistive gas sensors suffer from poor sensitivity and selectivity. Mere surface functionalization (decorating of material surface) with metal nanoparticles (NPs) might not improve the sensor performance significantly. In this respect, doping of the layered material can play a significant role. Here, we report a simple yet effective substitutional doping technique to dope MoS₂ with noble metals. Through various material characterization techniques like X-ray diffraction, scanning tunneling spectroscopy images, and selected area electron diffraction pattern, we were able to put forward the difference between surface decoration and substitutional doping by Au at S-vacancy sites of MoS₂. Lattice strain was found to exist in the Au-doped MoS₂ samples, while being absent in the Au NP-decorated samples. Surface chemistry studies performed using X-ray photoelectron spectroscopy showed a shift of Mo 3d peaks to lower binding energies, thus realizing p-type doping due to Au. The blue shift of the peaks as observed in Raman spectroscopy further confirmed the p-type doping. We found that gold-doped MoS₂ was more sensitive and selective toward ammonia (with a response of 150% for 500 ppm of ammonia at 90 °C) as compared to gold NP-decorated MoS₂. The advantages of substitutional doping and the gas-sensing mechanism were also explained by the density functional theory study. From the first principles study, it was found that the adsorption of Au atoms on the S-vacancy site of a monolayer of the MoS₂ sheet was thermodynamically favorable with the adsorption energy of 2.39 eV. We also successfully doped MoS₂ with Pt using the same technique. It was found that Pt-doped MoS₂ gives huge response toward humidity (60,000% at 80% relative humidity). Thus, various noble metal doping of MoS₂ selectively improved the sensing response toward specific analytes. From this work, we believe that this method could also be useful to dope other layered nanomaterials to design gas sensors with improved selectivity.