Nitrogen-doping induced oxygen divacancies in freestanding molybdenum trioxide single-layers boosting electrocatalytic hydrogen evolution

High electrical conductivity and large amount of active sites are the two prerequisites for an efficient electrocatalyst. Doping engineering is widely utilized to tailor the electrical property, while investigation on the dopant-induced modification of active sites remains elusive. To address this issue, we construct an ideal model of atomically-thin layers and hence deliberately create element doping on their surface. As an example, freestanding N-doped MoO3 single-layers are first synthesized by virtue of a lamellar intermediate. Positron annihilation spectrometry discloses that N-doping leads to increased (V0V0 center dot center dot)-V-center dot divacancies relative to intact MoO3 single-layers, while the similar electrochemical active area implies that the increased (V0V0 center dot center dot)-V-center dot divacancies accounts for the former's 6 times higher H-2 evolution activity. In addition, density-functional calculations revealed that the presence of (V0V0 center dot)-V-center dot center dot divacancies results in increased states density near the valence band edge, which favors their improved electronic conductivity, verified by electrochemical impedance spectroscopy. This work demonstrates that N-doping confined in MoO3 atomic-layers increase the concentration of (V0V0 center dot center dot)-V-center dot center dot divacancies, which are first verified to be highly active sites for H-2 evolution. Thus, doping engineering in atomic-layers opens new opportunities for achieving efficient catalytic performances through tailoring the catalytically active sites. (C) 2016 Elsevier Ltd. All rights reserved.