Paper 2D+AS+MI+NS-TuM11
Surface Reactivity of MoS₂ by ambient pressure X-ray Photoelectron Spectroscopy

Tuesday, October 22, 2019, 11:20 am, Room A216

Molybdenum disulfide (MoS₂) has potential applications as a low-cost catalyst for the hydrogen evolution reaction (HER). Defect sites in MoS₂ have been demonstrated to have high catalytic activities, where edge sites and sulfur vacancies are the major active sites for HER. Intentionally inducing defects offers a simple way to enhance the reactivity of MoS₂ and other 2D materials. In this study, we have characterized the surface reactivity and the catalytic activities of bulk MoS₂ samples using ambient pressure X-ray photoelectron spectroscopy (APXPS). The pristine surface was exposed to 1 mbar of H₂O vapor for temperatures ranging from 300 to 573 K. APXPS Mo 3d, S 2p, and O 1s core levels do not show any significant changes under these reaction conditions due to the inert nature of the MoS₂ surface. To activate the MoS₂ basal plane to improve surface reactivity, we have formed well-controlled densities of defects using Ar⁺ sputtering. The defective surfaces were exposed to 1 mbar of H₂O vapor for temperatures ranging from 300 to 600 K. Changes in the APXPS Mo 3d, S 2p, and O 1s core levels indicate that the surface is much more reactive to H₂O, with the formation of Mo-O bonds. These results are consistent with the reduction in the H₂O gas phase which was measured by operando mass spectrometry. We have found that the reactivity strongly depends on the temperature and the size and density of defects. Following this first report of APXPS on MoS₂ acquired at more realistic pressure, we will also report the HER activity and X-ray absorption spectroscopy (XAS) on the pristine surface and compare it with defective surfaces. Our findings demonstrate that the reactivity and the catalytic activity of MoS₂ are significantly improved through the formation of defects.