On the other hand, intrinsic p-type defects, especially vacancies, are inevitably introduced during the synthesis and processing of BP. [15][16][17] Recently, a number of theoretical investigations employing either first-principles ground-state DFT calculations or tight-binding propagation method reported the effects of vacancies on the local geometric and electronic structures in phosphorene. [18][19][20][21][22][23][24] For the p-type characteristics of phosphorene, either DFT or hybrid functional calculations show a spin-polarized state at about 0.10-0.22 eV above the valence band maximum (VBM). [21,23] The most important concern is how these intrinsic vacancies affect the degradation process. A recent study found that vacancy-containing phosphorene shows a stronger oxygen affinity than the perfect phosphorene lattice site, and the energy barrier for oxygen molecule from physisorption to chemisorption surrounding a vacancy on phosphorene can be significantly reduced by the presence of single vacancies (SV). [22] These results suggest that oxygen chemisorbed on defected phosphorene may lead to the formation of vacancy-oxygen complexes. However, the structural properties of defect-complexes in phosphorene are still far from being completed. In particular, vacancy-oxygen complexes in phosphorene should have profound influences on their electronic properties and device characteristics.In this work, we focus on the effects of O defects on the structural and electronic properties of pristine and vacancy-containing phosphorene. We have carried out ground-state DFT calculations to determine the structural properties of defective phosphorene. To evaluate how the electronic structures of phosphorene may be affected by different defect environments, such as oxygen species, vacancies, and vacancy-oxygen complexes, we have performed ab initio GW calculations to determine the quasiparticle band structures. The GW (Green's function G times the screened coulomb interaction W) approximation derived by Hedin [25] has become the state-of-the-art approach to study the quasiparticle electronic properties. Results and DiscussionThe oxidative degradation of BP in ambient conditions involves the chemical reaction with O 2 molecules. To understand how the oxidation of phosphorene deteriorate its performance, we first need to verify whether O impurity in pristine phosphorene affects its electronic structures. Although previous DFT or hybrid functionals reported O defects in pristine phosphorene, Degradation of phosphorene due to oxidation in air is extremely harmful to its performance in electronics and optoelectronics. Origin of this phenomenon is still debated and remains elusive. Through the state-of-the-art firstprinciples theoretical approach, GW method, it is believed that it is actually the vacancy-oxygen complex, formed by a single vacancy and an oxygen impurity nearby, that could lead to deep defect states in the bandgap and therefore deteriorate the performance of phosphorene, while isolated oxygen defects are electronically ...

The electrocatalytic nitrogen reduction reaction (NRR) is currently constrained by sluggish reaction kinetics and poor selectivity because of the difficulties in activating inert N≡N triple bonds and the existence of competing hydrogen evolution reaction (HER). Therefore, electrocatalysts with high activity, selectivity, and stability are highly desired. Herein, by means of first-principles calculations, we investigated the electrocatalytic NRR performance of a series of transition metal atoms (e.g., 3d, 4d, and 5d) embedded in defective hexagonal MBene nanosheets [ h- Zr(Hf) 2 B 2 O 2 ] and identified that h- Zr(Hf) 2 B 2 O 2 could be an excellent platform for electrocatalytic NRR. On the basis of our proposed screening criteria, 16 candidates are efficiently selected out from 50 systems, among which, Zr 2 B 2 O 2 -Cr stands out with high selectivity to NRR against HER and the ultralow limiting potential (−0.10 V). The value is much lower than that of the well-established stepped Ru(0001) surface (−0.43 V). The origin of the high activity toward NRR is attributed to the synergistic effect of the single atom (SA) and the M atoms in the substrate. More impressively, a composition descriptor is further proposed on the basis of the inherent characteristics of the catalysts [number of valence electrons of SA and electronegativity of the SA and Zr(Hf) atoms], which helps to better predict the catalytic performance. Our work not only contributes to the development of highly efficient NRR electrocatalysts but also extend the application of h -MBenes in electrocatalysis.

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The major challenge of black phosphorus (BP) is its fast-oxidative degradation in air. Organic covalent chemical modification of BP flakes could suppress the chemical degradation. Here we focus on the effects of covalent chemical modification on the electronic structures of single layer BP. We employed the state-of-the-art first principles GW method, which is based on the many body green functions, to study the covalent modification of single layer BP by forming P-C bonds. Our results show that the functional group chemisorbed at perfect lattice site results in deep in-gap state, which may deteriorate its performance, while the P-C bonds forming at single vacancies are electrically inactive. In addition, our results show that functional groups at low concentration would be preferably chemisorbed at single vacancies. At high concentration, functional groups would lead to the breaking of P-P bonds, and hence, oxygen impurity will easily insert in the interstitial position. Finally, we propose that functional groups at low concentration would protect BP from deterioration, and at high concentrations, functional groups would accelerate the deterioration of BP performance.

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