Xiaodong Zhang

C/B codoping effect on band gap narrowing and optical performance of TiO2 photocatalyst: a spin-polarized DFT study

The electronic and optical properties of several possible C/B-codoped models of anatase and rutile TiO2 have been investigated systematically using spin-polarized density functional theory calculations. Our calculated results show that the synergistic effect of C/B codoping can induce some hybridized energy states appearing in the forbidden gap and the band gap has a narrowing in anatase and rutile TiO2, which can lead to a decrease of the photon excitation energy and an obvious red-shift of the optical absorption edge. These results lead to an excellent photocatalytic activity in C/B-codoped TiO2. Moreover, with the increase of C and B impurities’ concentration in anatase and rutile TiO2, we find that the intensity of impurity states has a strengthening in the band gap, the position of impurity states changes, and the visible-light absorption performance improves gradually.

Enhanced optical absorption and photocatalytic activity of anatase TiO2 through (Si,Ni) codoping

The electronic and optical properties of (Si,Ni)-codoped anatase TiO2 are investigated using the density functional theory. The calculated results indicate that the synergistic effects of (Si,Ni) codoping can effectively extend the optical absorption edge, which can lead to higher visible-light photocatalytic activity than pure anatase TiO2. To verify the reliability of our calculated results, nanocrystalline (Si,Ni)-codoped TiO2 is synthesized by a sol-gel-solvothermal method, and experimental results also show that the (Si,Ni)-codoped sample exhibits better absorption performance and higher photocatalytic activities than pure TiO2.

Elastic and vibrational properties of monoclinic HfO2 from first-principles study

The elastic and vibrational properties of crystalline monoclinic HfO2 have been investigated using density functional perturbation theory. Using the Voigt and Reuss theory, we estimate the bulk, shear and Youngs modulus for polycrystalline HfO2, which agree very well with the available experimental and theoretical data. Additionally, we present a systematic analysis of the elastic properties of HfO2 polymorphs and find the trends in the elastic parameters for the HfO2 structures are consistent with those for the ZrO2 structures. The choice of exchange-correlation functional has an important effect on the results of elastic and vibrational properties. The utilization of Hartwigzen?Goedecker?Hutter type functional is a great improvement on calculation of the zone-centre phonon frequencies, and shows the root-mean-square absolute deviation of 7?cm?1 with experiments. A rigorous assignment of all the Raman modes is achieved by combining symmetry analysis with the first-principles calculations, which helps us to identify the main peak and some other features of Raman spectra. Furthermore, the Raman spectrum of HfO2 powder has been simulated for the first time, providing a theoretical benchmark for the interpretation of the unresolved problems in experimental studies.

The electronic and optical properties of Eu/Si-codoped anatase TiO2 photocatalyst

The electronic and optical properties of Eu/Si-codoped anatase TiO2 are investigated using the density functional theory. The calculated results show that the synergistic effects of Eu/Si codoping can effectively extend the optical absorption edge, which can lead to higher visible-light photocatalytic activities than pure anatase TiO2. To verify the reliability of our calculated results, nanocrystalline Eu/Si-codoped TiO2 is prepared by a sol-gel-solvothermal method, and the experimental results also indicate that the codoping sample exhibits better absorption performance and higher photocatalytic activities than pure TiO2.

Elastic properties of NaXH4 (X = B, Al)

Elastic properties of NaXH(4) (X = B, Al) have been studied by first-principles calculations using a projected augmented plane-wave approach. The calculated elastic constants compare favorably with experimental values. Our calculations show that the theoretically calculated elastic constants and bulk moduli have small values compared with those of typical metals and intermetallic compounds, which indicates that NaXH(4) (X = B, Al) are highly compressible. Comparison of bulk moduli B of different complex hydrides shows a correlation between B and the decomposition temperatures. Also, we calculated the elastic anisotropies and the Debye temperatures from the elastic constants.

Theoretical study of the NO, NO2, CO, SO2, and NH3 adsorptions on multi-diameter single-wall MoS2 nanotube

On the basis of first-principles calculations, the ability of a single-wall MoS2 nanotube to detect NO, NO2, CO, SO2, and NH3 gas molecules is studied. The most stable adsorption configurations, adsorption energies, and charge transfers are calculated. Among these gas molecules, the NO molecule has the biggest interaction with the MoS2 nanotube. The adsorption energy of NO on the nanotube is 129.3 meV, which is almost double compared to that for the monolayer (74.4 meV). The charge density difference calculation shows that all the molecules on the MoS2 nanotube act as electron acceptors except NH3. The charge transfer between NO and the nanotube is still one order of magnitude higher than that for the monolayer. Compared with the results for these molecules adsorbed on the MoS2 monolayer, the nanotube is more sensitive, especially for the NO molecule. Moreover, a comparative study of MoS2 nanotubes with different diameters (curvatures) indicates that the NO adsorption capability of the outer surface decreases on the increasing radius. It is predicted that the MoS2 nanotube with a smaller diameter should increase the stability and sensitivity of MoS2-based field-effect-transistor (FET) sensors.

Effects of physiological integration on photosynthetic efficiency of Trifolium repens in response to heterogeneous UV-B radiation

Several studies have found the photosynthetic integration in clonal plants to response to resource heterogeneity, while little is known how it responses to heterogeneity of UV-B radiation. In this study, the effects of heterogeneous UV-B radiation (280–315 nm) on gas exchange and chlorophyll fluorescence of a clonal plant Trifolium repens were evaluated. Pairs of connected and severed ramets of the stoloniferous herb T. repens were grown under the homogeneity (both of ramets received only natural background radiation, ca. 0.6 kJ m−2 d−1) and heterogeneity of UV-B radiation (one of the ramet received only natural background radiation and the other was exposed to supplemental UV-B radiation, 2.54 kJ m−2 d−1) for seven days. Stomatal conductance (gs), intercellular CO2 concentration (Ci) and transpiration rate (E) showed no significant differences in connected and severed ramets under homogenous and heterogeneous UV-B radiation, however, net photosynthetic rate (PN) and maximum photosynthetic rate (Pmax) of ramets suffered from supplemental increased UV-B radiation and that of its connected sister ramet decreased significantly. Moreover, additive UV-B radiation resulted in a notable decrease of the minimal fluorescence of dark-adapted state (Fo), the electron transport rate (ETR) and photochemical quenching coefficient (qP) and an increase of nonphotochemical quenching (NPQ) under supplemental UV-B radiation, while physiological connection reverse the results. In all, UV-B stressed ramets could benefit from unstressed ramets by physiological integration in photosynthetic efficiency, and clonal plants are able to optimize the efficiency to maintain their presence in less favourable sites.

Structural, elastic, electronic, and thermodynamic properties of MgAgSb investigated by density functional theory

The structural, elastic, electronic, and thermodynamic properties of thermoelectric material MgAgSb in γ,β,α phases are studied with first-principles calculations based on density functional theory. The optimized lattice constants accord well with the experimental data. According to the calculated total energy of the three phases, the phase transition order is determined from α to γ phase with cooling, which is in agreement with the experimental result. The physical properties such as elastic constants, bulk modulus, shear modulus, Youngs modulus, Poissons ratio, and anisotropy factor are also discussed and analyzed, which indicates that the three structures are mechanically stable and each has a ductile feature. The Debye temperature is deduced from the elastic properties. The total density of states (TDOS) and partial density of states (PDOS) of the three phases are investigated. The TDOS results show that the γ phase is most stable with a pseudogap near the Fermi level, and the PDOS analysis indicates that the conduction band of the three phases is composed mostly of Mg-3s, Ag-4d, and Sb-5p. In addition, the changes of the free energy, entropy, specific heat, thermal expansion of γ-MgAgSb with temperature are obtained successfully. The obtained results above are important parameters for further experimental and theoretical tuning of doped MgAgSb as a thermoelectric material at high temperature.

From orientation disordered to ordered--an ab initio simulation on ammonia borane phase transition within van der Waals corrections.

In this work, we report a detailed theoretical investigation of the phase transition of ammonia borane (NH3BH3; AB), from a tetragonal I4mm ( C4v9 ) phase with disordered orientation of hydrogen to an orthorhombic phase with Pmn21 ( C2v7 ) symmetry, as a function of temperature based on Density Functional Theory calculations with semiempirical dispersion potential correction. We define a series of substructures with the NH3BH3 moiety always in C3v symmetry and the partially occupied high temperature state can be described as a continuous transformation between these substructures. To understand the role of the van der Waals corrections to the physical properties, we use the empirical Grimmes dispersion potential correction (PBE‐D2). Both Perdew–Burke–Emzerhof (PBE) and PBE‐D2 functional yield almost the same energy sequence along the transition path. However, PBE‐D2 functional shows obvious advantage in describing the lattice parameters of AB. The rigid rotor harmonic oscillator approximation is used to compute the free energy and the entropies contribution along the transition pathway. With knowledge of free energy surfaces along rotations of the uf8ff[NH3] and uf8ff[BH3] groups, complete transformation paths are mapped out. The phase transition is found to follow the sequence of partially occupied tetragonal system (I4mm) of a mixture of states with monoclinic (Cc), (CM) and orthorhombic (Pmn21) symmetries to fully occupied quasitetragonal system (the intermediate phase, Pmn21) to fully occupied orthorhombic system (Pmn21).

Elastic constants of NaBH4 and LiBH4: Instability of β-LiBH4

The structural properties and mechanical stabilities of α-NaBH4 and β-LiBH4 have been investigated using first-principles calculations. Elastic constants of α-NaBH4 and β-LiBH4 are calculated from the second derivative of total energy as a function of strain. The calculated structural parameters and elastic constants of α-NaBH4 are in good agreement with the available experimental data. Our calculations show that β-LiBH4 with P63mc space group is unstable under the distortions (0, 0, 0, δ, δ, 0) and (0, 0, 0, 0, 0, δ) and these distortions lead to a Martensitic transformation into Cc and Cmc21 structures, respectively. Thermodynamically, β-LiBH4 with P63mc space group is more likely changed into a Cmc21 structure, which is consistent with experimental results.