Search

Oxygen reduction reaction (ORR) is a key microscopic process in many electrochemical applications of materials, where the requirements of their ORR performances may vary strikingly, for example, during the uses of MoS 2 as an electrocatalyst and anticorrosion/lubricating coating in aqueous/humid environments, ORR should be activated and inhibited, respectively. To reveal a complete ORR profile of MoS 2 , using first-principles calculations, we examine the stabilities of various possible zero-dimensional point defects on the surface and one-dimensional edge defects and comprehensively explore the ORR activities on pristine MoS 2 surface and those defects in acid/alkaline solutions. It is found that the ORRs on the pristine surface and surfaces with point defects always require large overpotentials (>1.0 V), indicating a defect-immune resistance of the planar MoS 2 surface against the ORR. However, the ORR overpotentials on edge defects can reach as low as 0.66 V, corresponding to a relatively high activity close to that of the prototypical catalyst Pt (overpotential ∼0.45 V). Such contrasting ORR behaviors of point and edge defects are also understood in depth by analyzing the underlying thermodynamic and electronic-structure mechanisms. This work not only quantitatively explains the performances of MoS 2 in both galvanic corrosion and electrochemical catalysis but also provides a useful structure-ORR map that can facilitate adapting the realistic MoS 2 to versatile electrochemical applications.

Two-dimensional (2D) semiconductors SnP 3 are predicted, from first-principles calculations, to host moderate band gaps (0.72 eV for monolayer and 1.07 eV for bilayer), ultrahigh carrier mobility (∼10 4 cm 2 V -1 s -1 for bilayer), strong absorption coefficients (∼10 5 cm -1 ) and good stability. Moreover, the band gap can be modulated from an indirect character into a direct one via strain engineering. For experimental accessibility, the calculated exfoliation energies of monolayer and bilayer SnP 3 are smaller than those of the common arsenic-type honeycomb structures GeP 3 and InP 3 . More importantly, a semiconductor-to-metal transition is discovered with the layer number N  >  2. We demonstrate, in remarkable contrast to the previous understandings, that such phase transition is largely driven by the correlation between lone-pair electrons of interlayer Sn and P atoms. This mechanism is universal for analogues phase transitions in arsenic-type honeycomb structures (GeP 3 , InP 3 and SnP 3 ).