Xiangying Meng

Enhanced photoelectrochemical activity for Cu and Ti doped hematite: The first principles calculations

To improve photoelectrochemical (PEC) activity of hematite, the modification of energy band by doping 3d transition metal ions Cu and Ti into α-Fe2O3 were studied via the first-principles calculations with density function theory (DFT)+U method. The results show that the band gap of hematite is ∼2.1u2002eV and n-type dopant Ti improves the electric conductivity, confirmed by recent experiments. The p-type dopant Cu enhances the utilization ratio of solar energy, shifts both valance, and conduction band edges to a higher energy level, satisfying hydrogen production in the visible light driven PEC water splitting without voltage bias.

Chemical synthesis of faceted α-Fe2O3 single-crystalline nanoparticles and their photocatalytic activity

Faceted hematite nanocrystals have been synthesized via a hydrothermal route and their different morphologies can be tuned by appropriate stabilizer molecules. Detailed observation by high-resolution transmission electron microscopy and atomic force microscopy has revealed many terraces, steps, and kinks on the faceted surface of hematite nanoparticles, and thus, one growth mechanism of the terrace-step-kink model has been suggested to play a major role in determining the equilibrium morphology, together with effect of surface chemistry via the interaction between outer surfaces of iron and oxygen ions and functional groups. The photocatalytic activities were evaluated by decomposing rhodamine B dye. It has been shown that polyhedron hematite particles enclosed by high-index surface planes exhibited higher photoactivity. Density functional theory calculations revealed that the higher photoactivity originates from the more flat band edge in directions normal to the surface planes.

Dependence on the structure and surface polarity of ZnS photocatalytic activities of water splitting: first-principles calculations

It has been reported that phase structure and surface polarity largely affect the photocatalytic efficiency of semiconductor nanostructures. To understand the chemical activity of ZnS at the electronic level, we investigate electron structures and carrier transportation ability for bulk intrinsic zinc blende (ZB) and wurtzite (WZ) ZnS, as well as the reaction pathway of hydrogen generation from water splitting on Zn- and S-terminated polar surfaces. The electron structure calculations prove that the WZ phase possesses a higher reducing ability than the ZB phase. The conductivity of the bulk ZB phase surpasses that of the WZ phase at or above room temperature. As the temperature increases, the asymptotic conductivity ratio of WZ/ZB is close to the Golden Ratio, 0.62. Reaction kinetics studies indicate that Zn-terminated polar surfaces are more chemically active than S-terminated polar surfaces in the reaction of hydrogen generation from water splitting. The calculation results suggest that the first H splitting from water on Zn-terminated polar surfaces can occur with ground state electronic structures, while photo-assistance is necessary for the first H splitting on the S-terminated surfaces. Electronic triplet states calculations further show that Zn-terminated surfaces are more photosensitive than S-terminated surfaces.

4d transition-metal doped hematite for enhancing photoelectrochemical activity: theoretical prediction and experimental confirmation

To explore the photoelectrochemical efficiency of hematite as a photoanode, we comprehensively investigate the electronic structures of hematite doped with 4d transition-metal X (X = Y, Zr, Mo, Tc, Rh, and Ru) based on the density-functional theory (DFT). The results indicate that the bandgap of hematite can be reduced by doping with the transition metal atoms, which leads to the enhanced absorption coefficient of long-wavelength photons in the visible light region. In addition, the carrier concentration can be improved by Zr, Mo, Tc, and Ru dopants. More interesting, the incorporation of Ru can also modify the conduction band edge and hence reduce the effective electron mass, leading to better electron mobility. Subsequent experiments confirm that the photoelectrochemical (PEC) activity of Ru doped hematite film is significantly improved. For example, the highest photocurrent density value of 9 at.% Ru doped hematite is 4.7 times that of the undoped material at E = 1.23 V. Based on both calculations and experiments, the enhanced PEC activities of Ru doped hematite are derived from the improved electrical conductivity and increased visible light absorption coefficient.

Composition-dependent structural and magnetic properties of Ni–Mn–Ga alloys studied by ab initio calculations

We have revealed the influence of composition doping (Ni2+xMn1−xGa, Ni2+xMnGa1−x, and Ni2Mn1+xGa1−x) on lattice constants and atomic magnetic moments of austenite, 7xa0M and NM martensite, by ab initio calculations. It is demonstrated that Ni-doping decreases the volume, whereas Mn-doping increases it. The total magnetic moment of the three series of alloys is mainly dominated by their Mn content with little phase-state dependence. The perturbation of the magnetic moments by atom substitution is mainly dominated by the Mn environment. This study is expected to provide information on composition-related structure and magnetic properties of Ni–Mn–Ga alloys that could not be obtained by experiments.

Novel stable hard transparent conductors in TiO2-TiC system: Design materials from scratch

Two new ternary compounds in the TiO2-TiC system, Ti5C2O6 and Ti3C2O2, are reported for the first time based on ab initio evolutionary algorithm. Ti5C2O6 has a tube-structure in which sp1 hybridized carbon chains run through the lattice along the b-axis; while in the Ti3C2O2 lattice, double TiO6 polyhedral are separated by the non-coplanar sp2 hybridized hexagon graphite layers along the c-axis, forming a sandwich-like structure. At ambient conditions, the two compounds are found to be mechanically and dynamically stable and intrinsic transparent conductors with high hardness (about twice harder than the conventional transparent conducting oxides). These mechanical, electronic, and optical properties make Ti5C2O6 and Ti3C2O2 ternary compounds be promising robust, hard, transparent, and conductive materials.

Stability of NNO and NPO Nanotube Crystals

We combine the USPEX evolution searching method with density functional theory using dispersion corrections (DFT-ulg) to predict the crystal structure of the NNO extended solid at high pressures (from 100 to 500 GPa). We find that the NNO nanotube (with diameter ≈ 2.5 Å) is the most stable form above 180 GPa. We report here the stability, electronic properties, and mechanical properties of this novel nanotube and show that it is stable above 20 GPa. To find a similar structure that might be stable at ambient conditions, we considered the NPO tube and show that it is stable at zero pressure. The NPO phase leads to an insulator to metal transition at 25 GPa, where the PP van der Waals distance approaches the covalent bond distance. The energy content of this NPO nanotube crystal is 10.6 kJ/g, which is 152% higher than that of TNT and 86% higher than that of the HMX energetic material. This is the first example of a structural energetic material, which could have important applications in igniters, incendiaries, screening smoke ammunition, and similar devices. This process illustrates how materials discovery in extreme conditions can be used to discover and stabilize novel structures.

First principles calculation on thermal stability of metastable precipitates in Mg-Gd binary alloys

Abstract First principles thermodynamic models based on the cluster expansion formalism, lattice dynamic calculations and quantum mechanical total energy calculations are employed to compute the thermal stability of metastable hardening precipitations in hcp structure α-Mg–Gd binary alloys. It shows that vibrational entropy reverses the energetic preference and plays a critical role in hardening precipitation at different aging temperatures in Mg–Gd binary alloys. In addition to energetics, the analysis of bonding charge density reveals that the metastable β′ phase is responsible for the high strength during subsequent isothermal treatment. Our results are found to be in good agreement with experimental measurements and helpful in clarifying the metastable precipitation sequence in Mg–Gd binary alloys.

Band engineering of multicomponent semiconductors: a general theoretical model on the anion group

Development of energy conversion semiconductor materials has attracted increasing interest over the past three decades, but most successful semiconductors are unary or binary, rather than multicomponent semiconductors (MCSCs). There is a several orders of magnitude wider variety of MCSCs than unary and binary semiconductors, but very few electronic energy theories have been able to deal with more than two composition variables so far, and thus desired MCSCs are hard to predict. In this work, we propose a universal anion group model based on the analysis of electronic structures in an ABO3 perovskite prototype. Under a first order approximation, that is, the ‘A’ cation and the (BO6) anion group have very little hybridization, we find that the band gap of the ABO3 semiconductor is mainly determined by the (BO6) anion group and is very similar to that of binary compounds consisting of the same anion group constituents, while the band edges can be adjusted by the ‘A’-site cation. When more intense hybridizations exist, the predicted results can be amended by considering the higher order approximation. Using this model, the band gaps and edges of quaternary AgxNa(1−x)NbO3 perovskites and ZnxMg(1−x)Fe2O4 spinels have been predicted and are consistent with reported experiments and first principles calculations, further confirming the validity of the proposed model. Therefore, an anion group model on MCSCs can not only promote the probability of success in band engineering, but can also pave the way for speeding up the design of novel and desired MCSCs using known binary semiconductors for use in the field of energy conversion materials as photocatalysts, light-emitting materials, complementary light-absorption materials for solar cells, etc.

Electronic and optical properties of MoS2/α-Fe2O3(0001) heterostructures: a first-principles investigation

Experimentally, MoS2/α-Fe2O3 composites have exhibited excellent photocatalytic activity in photoelectrochemical tests. However, the micromechanism of their improved photocatalytic efficiency is not clear. In this research, we use MoS2/α-Fe2O3(0001) heterostructures as a case to study the effects of interfacial atomic structure, electronic structure, and optical properties on the enhanced photocatalytic performance. Our results show that the MoS2/Fe–Fe–O3–R heterostructure is the most promising one in improving the photocatalytic performance. First, there is a strong coupling between the MoS2 film and Fe–Fe–O3–R surface, which could provide the channel for photo-generated carrier transfer. Moreover, the increase of carrier concentration in the MoS2/Fe–Fe–O3–R system can be predicted according to the emergence of occupied electronic states below the conduction band edge (CBE) and the enhancement of the Fermi level. Additionally, the MoS2/Fe–Fe–O3–R heterostructure can exhibit better optical absorption in the visible light range. In summary, these electronic features may shed light on the experimental phenomenon, and be helpful in further improving the photocatalytic performance of MoS2/α-Fe2O3 composites.