Non-precious metal activated MoSi2N4 monolayers for high-performance OER and ORR electrocatalysts: A first-principles study

We have systematically investigated a family of newly proposed two-dimensional MA2N4 materials (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W; A = Si, Ge) using first-principles calculation. We categorize the potential of these materials into three different applications based on accurate simulation of band gap (using Hybrid HSE06 functional) and the associated descriptors. Three candidate materials (MoGe2N4, HfSi2N4, and NbSi2N4) turn out to be extremely promising for three different applications. MoGe2N4 and HfSi2N4 monolayers show strong optical absorption in the visible range, including high transition probability from the valence to conduction band. The GW+BSE calculations confirm a strong excitonic effect in both the systems. With a band gap of 1.42 eV, multilayer MoGe2N4 shows reasonably large simulated efficiency (∼15.40%) and hence can be explored for possible photovoltaic applications. High optical absorption, suitable band gap/edge positions, and the CO2 activation make HfSi2N4 monolayer a promising candidate for photocatalytic CO2 reduction. NbSi2N4, on the other hand, belongs to a new class of spintronic material called a bipolar magnetic semiconductor, recommended for spin-transport-based applications.

Novel Two-Dimensional MA2N4 Materials for Photovoltaic and Spintronic Applications.

Motivated by the recent synthesis of MA2Z4 family materials, we perform a first-principles investigation on the structural stability and electronic properties of VN2X2Y2 nanosheets (X = B-Ga, Y = O-Te), which are the isostructural and isoelectronic analogues of the VSi2N4 system. We find that among the 16 possible III–VI combinations, there are four XY compositions, i.e., XY = BO, BS, AlO, and GaO, that can form stable VN2X2Y2 nanosheets with robust dynamic and thermal stabilities. Intrinsic ferromagnetism appears in these VN2X2Y2 nanosheets, among which the VN2B2S2 and VN2Ga2O2 ones possess an easy out-of-plane magnetization. Thus, unlike the VSi2N4 system, the VN2B2S2 and VN2Ga2O2 nanosheets exhibit spontaneous valley polarization with a sizeable polarization value in the bottom conduction band. Through the strain modulation, the valley polarization can be switched from the bottom conduction band to the top valence band, which is closely related to the strain-induced band inversion. Due to the existence of valley polarization, the band inversion occurs at different strains for the two valleys. Therefore, in the critical strain region, an intriguing quantum anomalous Hall state is present in the strained VN2X2Y2 nanosheets, which is characterized by a nonzero Chern number of C = 1. Such nontrivial topology is further confirmed by a quantized Hall conductance and a single gapless edge state in the bulk gap. Our study demonstrates as the cousin of MA2Z4 materials, the III–VI VN2X2Y2 nanosheets possess robust structural stability, peculiar electronic and topological properties, which have potential applications in electronics, spintronics, and valleytronics.

Switchable valley polarization and quantum anomalous Hall state in the VN2X2Y2 nanosheets (X = group-III and Y = group-VI elements)

Fixation of dinitrogen into ammonia is an essential biological process for the evolution of life, and at the same time ammonia is an essential component for many industrial processes, including fertilizers, plastics, and so on. NH3 is also known as the best alternative to H2 in fuel cells. However, due to chemical inertness, direct conversion of N2 into NH3 is not possible; a suitable catalyst is required. The century-old Haber–Bosch process, utilizing an Fe-based catalyst, is extremely energy-intensive and eco-unfriendly due to the consumption of fossil fuels. In recent years, huge development has taken place in the design and applications of suitable electro- and photocatalysts for artificial N2 fixation. First-principles calculations have been considered as a powerful avenue for theoretical screening of promising catalysts through rational design and analysis of the plausible mechanisms or pathways of reactions. The present review focuses on recent theoretical developments of various unsupported nanoclusters and supported single-atom and cluster catalysts for application in electro- and photochemical N2 reduction reactions. The support substrates include oxides, carbides, nitrides of metal-based 2D materials, porous carbonaceous materials, and metal-free 2D materials containing main-group elements like B, C, N, and P. Although some reviews have already been made, focusing on the research related to either electro- or photocatalytic N2 fixation on different catalysts, here we give a comprehensive account of both electrochemical and photochemical nitrogen reduction reaction (NRR) activities and selectivities of various supported single-atom and polyatomic cluster catalysts. Additionally, a comparative assessment is made on the basis of free energy of adsorption, most favorable pathway, potential limiting step, and corresponding limiting potential of the NRR. 

Recent Progress in Computational Design of Single-Atom/Cluster Catalysts for Electrochemical and Solar-Driven N2 Fixation

Monolayer MoSi2N4 (MoSiN) was successfully synthesized last year [Hong et al., Science369, 670 (2020)]. The MoSiN monolayer exhibited semiconducting characteristics and exceptional ambient stability, calling for more studies of its properties. Here, we conduct first-principle calculations to examine the structural, magnetic, and electronic properties of substitutional doping of MoSiN monolayers with transition metals (TM) at the Mo site (TM-MoSiN). We find that the Sc-, Y-, Ti-, and Zr-MoSiN are metallic systems, while Mn-, Tc-, and Ru-MoSiN are n-type conducting. The Fe-MoSiN is a dilute magnetic semiconductor, and the Ni-MoSiN is a metal (or half-metal). The inclusion of spin-orbit coupling turns them into a half-metal and a semimetal, respectively. We also find that the work function of TM-MoSiN and the bond lengths between the TM and neighbor atoms increase as the atomic radius and electronegativity of the TM atom increase, respectively. The Fe-, Co-, and Ni-MoSiN may be used in spintronic devices, while Mn-, Rh- and Pd-MoSiN could be utilized for spin filter applications. 

Shuaishuai Gao

Papers

Enhancing the hydrogen evolution reaction by non-precious transition metal (Non-metal) atom doping in defective MoSi2N4 monolayer

TM3 (TM = V, Fe, Mo, W) single-cluster catalyst confined on porous BN for electrocatalytic nitrogen reduction