Zihe Zhang

Ti2CO2 MXene: a highly active and selective photocatalyst for CO2 reduction

Photocatalytic reduction of carbon dioxide (CO2) into hydrocarbons could promote the CO2 utilization and retard the greenhouse effect, which has gained much attention. Due to high surface–bulk ratio, two-dimensional materials can be promising candidates for catalysis. In this study, by means of first-principles computations, we have investigated the reduction of CO2 at the oxygen vacancy on MXene monolayers. Among Ti2CO2, V2CO2 and Ti3C2O2, Ti2CO2 has exhibited the best catalytic performance for the reduction of CO2. The reaction pathway CO2 → HCOO → HCOOH was found to be favorable with an energy barrier of 0.53 eV. The energy barriers of the reaction pathways for other single-carbon organic products were much higher, indicating high selectivity for HCOOH. Moreover, we have proposed that CO and H2 can introduce sufficient oxygen vacancies on O-terminated MXene. This study provides new insights into the design of catalysts for the reduction of CO2 and further widens the applications of MXene-based materials.

Tetragonal-structured anisotropic 2D metal nitride monolayers and their halides with versatile promises in energy storage and conversion

Here we propose a series of novel two-dimensional tetragonal-structured metal nitride (t-MN, M = Ti, Zr or Hf) monolayers mainly bonded with strong hybridization of N-p and M-d orbitals. These monolayers exhibit metallic properties which make them promising candidates as electrode materials for Li-ion batteries. The computations disclosed that t-TiN has a high theoretical Li-storage capacity of 432 mA h g−1 and a low Li ion diffusion barrier of 0.25 eV. After halogenation, the t-MNX (X = Cl, Br and I) monolayers transform to semiconductors with high carrier mobility and direct band gaps ranging from 0.41 to 3.26 eV. Moreover, the 2D nature and strong light absorption endow them with wide applications to photocatalysis. The appropriate band edge position indicates that t-ZrNX and t-HfNX can be applied as 2D photohydrolytic catalysts. Finally, a top-down method followed by high-temperature treatment is proposed to prepare t-MN monolayers.

An effective method to screen sodium-based layered materials for sodium ion batteries

Due to the high cost and insufficient resource of lithium, sodium-ion batteries are widely investigated for large-scale applications. Typically, insertion-type materials possess better cyclic stability than alloy-type and conversion-type ones. Therefore, in this work, we proposed a facile and effective method to screen sodium-based layered materials based on Materials Project database as potential candidate insertion-type materials for sodium ion batteries. The obtained Na-based layered materials contains 38 kinds of space group, which reveals that the credibility of our screening approach would not be affected by the space group. Then, some important indexes of the representative materials, including the average voltage, volume change and sodium ion mobility, were further studied by means of density functional theory computations. Some materials with extremely low volume changes and Na diffusion barriers are promising candidates for sodium ion batteries. We believe that our classification algorithm could also be used to search for other alkali and multivalent ion-based layered materials, to accelerate the development of battery materials.Sodium-ion batteries: searching for layered electrode materialsSodium-ion batteries are one step closer to market thanks to a computer program that screens for promising electrode materials. A team led by Zen Zhou at Nankai University in China developed a rapid, ‘high-throughput’ computational approach to search the vast online ‘Materials Project’ database for layered sodium-based materials. Once identified, the authors assessed the candidate materials’ potential for use in sodium-ion batteries by calculating their energy storage properties, such as volume change and sodium ion mobility, finding that a number of them-including Na(CuO)2, NaTiF4, Na2Zr(CuS2)2, Na3Co2SbO6, and Na2Cu(CO3)2–are potentially suitable for use as ‘insertion-type’ cathode materials. The authors hope their method could also be used to search for other types of layered compound for electrode materials, and accelerate the development of next-generation batteries.

Transition metal anchored C2N monolayers as efficient bifunctional electrocatalysts for hydrogen and oxygen evolution reactions

Developing highly active non-noble catalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is essential for overall water splitting. In this work, by means of first-principles computations, we screened a series of transition metal atom anchored C2N monolayers (TMx@C2N, TM = Ti, Mn, Fe, Co, Ni, Cu, Mo, Ru, Rh, Pd, Ag, Ir, Pt, or Au) as electrocatalysts for both HER and OER. Almost all TMx@C2N composites show metallic properties, indicating outstanding charge transfer for efficient electrochemical procedures. Ti1@C2N, Mn1@C2N, Co2@C2N, Ni2@C2N, Cu2@C2N, Mo1@C2N, Ru2@C2N and Ir1@C2N exhibit high catalytic activity toward the HER. Among them, Ti1@C2N would be the best HER catalyst since both N and Ti atoms are active sites. Unfortunately, Ti1@C2N exhibits no OER activity. Instead, only Mn1@C2N could perform as a bifunctional electrocatalyst with N and Mn atoms as active sites for the HER and OER, respectively. This work would open a new door for the development of non-noble metal bifunctional electrocatalysts for overall water splitting and also shed light on C2N-supported nanomaterials as advanced catalysts.

First-principles computational studies on layered Na2Mn3O7 as a high-rate cathode material for sodium ion batteries

By first-principles computations, we propose Na2Mn3O7 as a high-rate cathode material for sodium ion batteries. Na2Mn3O7 has a voltage window of 3.6–3.1 V and a theoretical reversible capacity of 124 mA h g−1. Na2Mn3O7 is a semiconductor and turns metallic after Na extraction; moreover, the calculated Na migration barrier in Na2Mn3O7 is 0.18 eV, ensuring ideal conductivity and rate capability.

Double-atom catalysts: Transition metal dimer anchored C2N monolayers as N2 fixation electrocatalysts

Nitrogen fixation is one of the most essential processes in chemistry. Developing more effective nitrogen fixation systems to catalyze the reaction under mild conditions is one of the most attractive but long-standing challenges. In this work, by means of first-principles computations, we systematically investigated the catalytic performance of transition metal atom-anchored C2N monolayer electrocatalysts (TMx@C2N, x = 1 or 2; TM = Ti, Mn, Fe, Co, Cu, Mo, Ru, Rh, Pd, Ag, Ir, Pt, or Au) for N2 fixation. The natively N edged uniform holes could fix the TM atoms firmly in TM–Nx configurations, which is beneficial for the N2 fixation. Among them, Mo2@C2N exhibits the best catalytic performance for the reduction of N2 to NH3 with a maximum free energy change of 0.41 eV and an energy barrier of 0.51 eV, indicating that Mo2@C2N is a promising catalyst with high catalytic activity for the reduction of N2 to NH3. We believe that this work could provide a new idea for the design of N2 fixation catalysts and shed light on C2N monolayers as excellent substrates for catalysis.

Computational screening and first-principles investigations of NASICON-type LixM2(PO4)3 as solid electrolytes for Li batteries

Li-containing NASICONs (Na super ionic conductors) usually have high ionic conductivity and can serve as solid electrolytes for Li batteries. In this work, we screened the Materials Project database and found seven kinds of LixM2(PO4)3 (M = Bi, Ge, In, Mo, Sb, Sc, and Zr) which could be applied as solid electrolytes for Li batteries. The most thermodynamically stable phases of each series of LixM2(PO4)3 were investigated through density functional theory computations. The results indicate that all these materials are wide-band-gap semiconductors with the band gap ranging from 3.55 to 4.88 eV. Meanwhile, the width of their electrochemical window is not directly related to the band gap; instead, it is mainly determined by the chemical potential of Li ions in these materials. Ab initio molecular dynamics simulations indicate that NASICONs with more Li ions or monoclinic symmetry possess higher Li ion conductivity at room temperature, among which Li3Bi2(PO4)3 has the highest conductivity of ∼10−3 S cm−1. Comprehensively considering the stability, the width of the electrochemical window, the band gap and the ionic conductivity, we propose that Li3Sc2(PO4)3 could be a very promising solid electrolyte for Li batteries.

Rational design of C2N-based type-II heterojunctions for overall photocatalytic water splitting

Photocatalytic water splitting is a promising method for the production of clean energy and searching for efficient photocatalysts has received extensive attention. Fabricating type-II heterojunctions is an effective approach to improve the photocatalytic efficiency. Based on the band edge positions and lattice parameters, we found that several kinds of monochalcogenide monolayers can be used to fabricate type-II heterojunctions with C2N monolayers. C2N/GaTe and C2N/InTe van der Waals (vdW) heterojunctions were investigated as potential photocatalysts for water splitting by means of first-principles computations. Both are type-II heterojunctions, and could promote the efficient spatial separation of electron–hole pairs. Their band edges straddle water redox potentials, satisfying the requirements for photocatalytic water splitting. Besides, the high carrier mobility of C2N/GaTe and C2N/InTe heterojunctions implies that the transfer of carriers to reactive sites is easy, and the recombination probability of photo-generated carriers is reduced. The Gibbs free energy calculations indicate that C2N/GaTe and C2N/InTe heterojunctions, especially C2N/InTe, exhibit high catalytic performance towards hydrogen and oxygen evolution reactions. Particularly, C2N/InTe exhibits a direct band gap with strong absorption in both visible and near ultraviolet regions, indicating that it is a very promising candidate for photocatalytic water splitting. This work would provide a new idea for the development of type-II heterojunctions for photocatalytic water splitting.