Linggang Zhu

MXene: a promising photocatalyst for water splitting

Identifying suitable photocatalysts for photocatalytic water splitting to produce hydrogen fuel via sunlight is an arduous task by the traditional trial-and-error method. Thanks to the progress of density functional theory, one can nowadays accelerate the process of finding candidate photocatalysts. In this work, by ab initio calculations, we investigated 48 two-dimensional (2D) transition metal carbides also referred to as MXenes to understand their photocatalytic properties. Our results highlight 2D Zr2CO2 and Hf2CO2 as the candidate single photocatalysts for possible high efficiency photocatalytic water splitting. A significant property of 2D Zr2CO2 and Hf2CO2 is that they exhibit unexpectedly high and directionally anisotropic carrier mobility, which may effectively facilitate the migration and separation of photogenerated electron–hole pairs. Meanwhile, these two MXenes also exhibit very good optical absorption performance in the wavelength ranging approximately from 300 to 500 nm. The stability of 2D Zr2CO2 and Hf2CO2 in liquid water is expected to be good based on ab initio molecular dynamics simulations. Finally, the adsorption and decomposition of water molecules on the 2D Zr2CO2 surface and the subsequent formation process of hydrogen were studied, which contributes to the unravelling of the micro-mechanism of photocatalytic hydrogen production on MXenes. Our findings will open a new way to facilitate the discovery and application of MXenes for photocatalytic water splitting.

Microscopic origin of MXenes derived from layered MAX phases

Two-dimensional transition metal carbides/nitrides Mn+1Xns labeled as MXenes derived from layered transition metal carbides/nitrides referred to as MAX phases attract increasing interest due to their promising applications as Li-ion battery anodes, hybrid electro-chemical capacitors and electronic devices. To predict the possibility of forming various MXenes, it is necessary to have a full understanding of the chemical bonding and mechanical properties of MAX phases. In this work, we investigated the chemical bonding changes of MAX phases in response to tensile and shear stresses by ab initio calculations using M2AlC (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W) as examples. Our results show that the M2C layer is likely to separate from the Al layer during the tensile deformation, where the failure of M2AlC is characterized by an abrupt stretch of the M–Al bonds. While under shear deformation, the M2C and Al layers slip significantly relative to each other on the (0001) basal planes. It is found that the ideal strengths of M2AlC are determined by the weak coupling of the M2C and Al layers, closely related to the valence-electron concentration. Our results unravel the possibility as well as the microscopic mechanism of the fabrication of MXenes through mechanical exfoliation from MAX phases.

2D Intrinsic Ferromagnets from van der Waals Antiferromagnets

Intrinsically ferromagnetic 2D semiconductors are essential and highly sought for nanoscale spintronics, but they can only be obtained from ferromagnetic bulk crystals, while the possibility to create 2D intrinsic ferromagnets from bulk antiferromagnets remains unknown. Herein on the basis of ab initio calculations, we demonstrate this feasibility with the discovery of intrinsic ferromagnetism in an emerging class of single-layer 2D semiconductors CrOX (CrOCl and CrOBr monolayers), which show robust ferromagnetic ordering, large spin polarization, and high Curie temperature. These 2D crystals promise great dynamical and thermal stabilities as well as easy experimental fabrication from their bulk antiferromagnets. The Curie temperature of 2D CrOCl is 160 K, which exceeds the record (155 K) of the most-studied dilute magnetic GaMnAs materials, and could be further enhanced by appropriate strains. Our study offers an alternative promising way to create 2D intrinsic ferromagnets from their antiferromagnetic bulk counterparts and also renders 2D CrOX monolayers great platform for future spintronics.

Design principles of tuning oxygen vacancy diffusion in SrZrO3 for resistance random access memory

Resistance random access memory (RRAM) is known to be a promising candidate for next generation non-volatile memory devices, in which the diffusion of oxygen vacancies plays a key role in resistance switching. Based on first principles calculations and transition state theory, using SrZrO3 (SZO) as an example, we found that the diffusion energy of an oxygen vacancy strongly depends on its charge states and V2+O contributes mostly to the resistance switching due to its lowest activation energy. To adjust the performance of SZO RRAM, the effects of dopants (Y, V, Nb and Ta) were revealed according to their modifications on the diffusion of V2+O. We found that doping of Y or V has the most significant effect on the performance of RRAM devices. Furthermore, for dopants with various numbers of valence electrons and atomic radius, general design principles were proposed based on their different effects on the RRAM characteristics. Our results will guide the experimentations and pave a new way for the optimization of RRAM devices.

Realization of a reversible switching in TaO2 polymorphs via Peierls distortion for resistance random access memory

Transition-metal-oxide based resistance random access memory (RRAM) is a promising candidate for next-generation universal non-volatile memories. Searching and designing appropriate materials used in the memories becomes an urgent task. Here, a structure with the TaO2 formula was predicted using evolutionary algorithms in combination with first-principles calculations. This triclinic structure (T-TaO2) is both energetically and dynamically more favorable than the commonly believed rutile structure (R-TaO2). The metal-insulator transition (MIT) between metallic R-TaO2 and T-TaO2 (band gap: 1.0 eV) is via a Peierls distortion, which makes TaO2 a potential candidate for RRAM. The energy barrier for the reversible phase transition is 0.19 eV/atom and 0.23 eV/atom, respectively, suggesting low power consumption for the resistance switch. The present findings about the MIT as the resistance-switch mechanism in Ta-O system will stimulate experimental work to fabricate tantalum oxides based RRAM.

Origin of the abnormal diffusion of transition metal atoms in rutile

Diffusion of dopant in rutile is the fundamental process that determines the performance of many devices in which rutile is used. The diffusion behavior is known to be highly sample-dependent, but the reasons for this are less well understood. Here, rutile is studied by using first-principles calculations, in order to unravel the microscopic origins of the diverse diffusion behaviors for different doping elements. Anomalous diffusion behavior in the open channel along the [001] direction is found: larger atoms including Sc and Zr have lower energy barrier for diffusion via interstitial mechanism, apparently contradicting their known slow diffusion rate. To resolve this, we present an alternate model for the overall diffusion rate of the large-size dopants in rutile, showing that parallel to the [001] channel, it is limited by the formation of the interstitial states, whereas in the direction perpendicular to [001], it proceeds via a kick-out mechanism. By contrast, Co and Ni prefer to stay in the interstitial site of rutile, and have conventional diffusion with a very small migration barrier in the [001] channel. This leads to highly anisotropic and fast diffusion. The diffusion mechanisms found in the present study can explain the diffusion data measured by experiments.

Metastable Stacking-Polymorphism in Ge2Sb2Te5

Metastable rocksalt structured Ge2Sb2Te5 is the most widely used phase-change material for data storage, yet the atomic arrangements of which are still under debate. In this work, we have proposed metastable stacking-polymorphism in cubic Ge2Sb2Te5 based on first-principles calculations. Our results show that cubic Ge2Sb2Te5 is actually polymorphic, varying from randomly distributed vacancies to highly ordered vacancy layers; consequently, the electrical property varies between metallic and semiconducting. These different atomic stackings of cubic Ge2Sb2Te5 can be obtained at different experimental synthetic conditions. The concept of stacking-polymorphic Ge2Sb2Te5 provides important fundamentals for metastable Ge2Sb2Te5 and is useful for tuning the performance of the phase-change materials.

Reversible formation-dissociation of polaron in rutile driven by electric field

ABSTRACT Density functional theory with Hubbard U correction is employed to study the polaron in rutile under external electric fields. It is found that electric-field-induced displacements of the ions enhance the sampling of the polaron configurations, which is helpful to find the most stable structure among local minima. More importantly, we demonstrate that with increasing electric field, the polaron may vanish, i.e. the excess electron can dissociate from the bounded Ti3+, and this process is reversible with the opposite electric field. The present findings shed light on more complex mechanisms for the polaron-related conductivity in oxides and corresponding devices. Impact statement The first ab initio demonstration of the polaron dynamics under electric fields provides a complete picture of the polaron in rutile, deepening the understanding of the conductivity in the oxides. GRAPHICAL ABSTRACT

Photoinduced reversible lattice expansion in W-doped TiO2 through the change of its electronic structure

External stimulations of applied force or voltage have been reported to induce crystal lattice dimension changes with the order of 0.1% or above by imposing external mechanical or electric forces on atoms forming the lattice for various types of materials, including oxides, metals, polymers, and carbon nanostructures. As far as we know, however, no report is available for similar level changes in oxides from their internal electronic structure changes induced by photoirradiation. We show that reversible lattice expansion comparable to those by applied force or voltage can be induced by UV-irradiation on an oxide of W-doped TiO2 nanotubes through the reversible changes of its internal electronic structure by the accumulation and release of photogenerated electrons in W-dopants when UV-illumination is on and off. This photoirradiation-induced reversible lattice expansion and subsequent optical, electric, and magnetic property changes may also be present in other material systems by proper material design if...