Wen Lai Huang

DFT Calculations on the Electronic Structures of BiOX (X = F, Cl, Br, I) Photocatalysts With and Without Semicore Bi 5d States

The electronic structures of BiOX (X = F, Cl, Br, I) photocatalysts have been calculated with and without Bi 5d states using the experimental lattice parameters, via the plane‐wave pseudopotential method based on density functional theory (DFT). BiOF exhibits a direct band gap of 3.22 or 3.12 eV corresponding to the adoption of Bi 5d states or not. The indirect band gaps of BiOCl, BiOBr, and BiOI are 2.80, 2.36, and 1.75 eV, respectively, if calculated with Bi 5d states, whereas the absence of Bi 5d states reduces them to 2.59, 2.13, and 1.53 eV successively. The calculated gap characteristics and the falling trend of gap width with the increasing X atomic number agree with the experimental results, despite the common DFT underestimation of gap values. The shapes of valence‐band tops and conduction‐band bottoms are almost independent of the involvement of Bi 5d states. The indirect characteristic becomes more remarkable, and the conduction‐band bottom flattens in the sequence of BiOCl, BiOBr, and BiOI. Both O 2p and X np (n = 2, 3, 4, and 5 for X = F, Cl, Br, and I, respectively) states dominate the valence bands, whereas Bi 6p states contribute the most to the conduction bands. With the growing X atomic number, the localized X np states shift closer toward the valence‐band tops, and the valence and conduction bandwidths evolve in opposite trends. Atomic and bond populations have also been explored to elucidate the atomic interactions, along with the spatial distribution of orbital density.

Electronic structures and optical properties of BiOX (X = F, Cl, Br, I) via DFT calculations.

Based on the density functional theory (DFT), the lattice constants and atomic positions of BiOX (X = F, Cl, Br, I) species have been optimized, and the electronic and optical properties of the relaxed species have been calculated, with Bi 5d states considered or not. Relaxation generally results in the shrinkage in a and the expansion of c. Relaxed BiOCl, BiOBr, and BiOI present indirect band gaps, whereas BiOF exhibits a direct or somewhat indirect band‐gap feature corresponding to the relaxation and calculation with the Bi 5d states or not. The bottom of the conduction band is quite flat for relaxed BiOI, and apparently flat in BiOBr, and shows observable flatness in BiOCl as well when considering the Bi 5d states. The top of the valence band is rather even as well for some species. The obtained maximum gaps for relaxed BiOF, BiOCl, BiOBr, and BiOI are 3.34, 2.92, 2.65, and 1.75 eV, respectively. The density peak of X np states in the valence band shifts toward the valence band maximum with the increasing X atomic number. The bandwidths, atomic charges, bond orders, and orbital density have also been investigated along with some optical properties.

Petascale molecular dynamics simulation of crystalline silicon on Tianhe-1A

An efficient and highly scalable bond-order potential code has been developed for the molecular dynamics simulation of bulk silicon, reaching 1.87 Pflops (floating point operations per second) in single precision on 7168 graphic processing units (GPUs) of the Tianhe-1A system. Furthermore, by coupling GPUs and central processing units, we also simulated surface reconstruction of crystalline silicon at the sub-millimeter scale with more than 110 billion atoms, reaching 1.17 Pflops in single precision plus 92.1 Tflops in double precision on the entire Tianhe-1A system. Such simulations can provide unprecedented insight into a variety of microscopic behaviors or structures, such as doping, defects, grain boundaries, and surface reactions.

Efficient GPU-accelerated molecular dynamics simulation of solid covalent crystals

Different from previous molecular dynamics (MD) simulation with pair potentials and many-body potentials, an efficient and highly scalable algorithm for GPU-accelerated MD simulation of solid covalent crystals is described in detail in this paper using sophisticated many-body potentials, such as Tersoff potentials for silicon crystals. The algorithm has effectively taken advantage of the reordering and sorting of atoms and the hierarchical memory of a GPU. The final results indicate that, about 30.5% of the peak performance of a single GPU can be achieved with a speedup of about 650 over a contemporary CPU core, and more than 15 million atoms can be processed by a single GPU with a speed of around 2 ns/day. Furthermore, the proposed algorithm is scalable and transferable, which can be applied to other many-body interactions and related large-scale parallel computation

Evolution of pore and surface characteristics of silica xerogels during calcining

This paper has investigated the effect of calcining temperature (600-1000degreesC) on the mesopore, structure and surface fractal dimension of silica xerogels derived from two-step acid-base-catalyzed TEOS (tetraethyl orthosilicate) systems. Two base contents were adopted in the base step. FE-SEM (field emission gun-scanning electron microscopy) observation and XRD (X-ray diffraction) analysis indicate that gel coarsening, agglomeration and sintering occurred successively with increasing temperature, and those prepared with higher base content exhibit easier agglomeration, lower sintering temperature and weaker crystallizability. Percent mesoporosity and surface fractal dimensions evaluation via image analysis methods shows that the mesoporosity of those prepared with lower base content increases below 700degreesC and decline at higher temperatures, while the surface fractal dimension appears to decrease above 800degreesC

Effect of HCl in a two-step sol–gel process using TEOS

Abstract This paper introduces and investigates a new two-step method by further addition of acid in the basic step. By this method high Rw (molar ratio of water/TEOS) systems (Rw reaches 250) can be gelled without volume loss. Below the coagulation point, a larger amount of HCl addition in the basic step (the system remains basic) and higher ultimate base concentration lead to faster gelation, which might be attributed to the change of electrolyte concentration, ion strength and pH value of the system, while a larger amount of water causes longer gelation times due to dilution effects.

Calcining silica gels at different drying stages

Silica gels derived from an acid-catalyzed tetraethyl orthosilicate (TEOS) system were calcined after different duration of ambient drying, and their mesopore structure and surface fractal dimensions are investigated. The largest mesopore porosity is found in the sample with long period of ambient drying, while the sample calcined without drying exhibits the lowest surface fractal dimensions.

Influence of ultrasonic vibration on the mesoporosity and surface fractal dimensions of resultant silica xerogels

Abstract The present work has investigated the influence of ultrasonic vibration during sol preparation on the mesoporosity and surface fractal dimensions of resultant silica xerogels, in comparison with magnetic stirrings. The xerogels are derived from hydrochloric acid-catalyzed TEOS (tetraethyl orthosilicate) systems (acetone as solvent). The sols were agitated under ultrasonic vibration (of high or low intensity) or by magnetic stirring in sealed containers for 30 minutes, and then gelled in the air with solvent evaporation. The resultant wet gels were dried in ambient conditions after 11 days of aging or not, and became xerogels. Image analyses based on the FE-SEM (field emission gun-scanning electron microscopy) images of these xerogels showed that high intensity of ultrasonic vibration caused a distinct mesoporosity decrease of the resultant xerogels, and that despite the different ultrasonic intensities, samples prepared under ultrasonic vibrations showed similar surface fractal dimensions which were different from those of classic samples. The influence of aging is also presented.

Influence of calcining temperature on the mesopore structures and surface fractal dimensions of MgO-Al2O3-SiO2 xerogels

This investigation deals with the effect of calcining temperature on the mesopore structures and surface fractal dimensions of MgO-Al2O3-SiO2 xerogels prepared via a two-step acid-base-catalyzed sol-gel process with/without alcohol washing before drying. The mesoporosity and surface fractal dimensions are evaluated by image analysis methods, based on FE-SEM (field emission gun-scanning electron microscopy) images. The results show that samples with alcohol washing before drying exhibit finer textures and lower sintering temperature. The mesoporosity increase before 700 degreesC might be attributed to the microporosity reduction during large extent of further condensation (detectable weight loss), and continuous further condensation and sintering resulted in the monotonic decrease of mesoporosity above that temperature. Smoothing and sintering effects also contributed to the general decreasing trend of surface fractal dimensions with increasing temperatures. Complication of surface textures caused by heterogeneous distributions of pores resulted in the abnormal high surface fractal dimensions

Structures and Energetics of SrFeO2.875 Calculated within the GGA + U Framework.

The energetics and electronic properties of SrFeO2.875 have been systematically calculated with the fully relaxed atomic positions at both GGA and GGA + U levels, and different spin-polarized configurations have been considered. Many atoms besides the nearest neighbors of the oxygen vacancy have been found to be influenced by the vacancy in terms of positions and electronic structures. The obtained magnetic moments suggest the high-spin character of the Fe 3d electrons, in combination with the larger exchange splitting compared with the crystal field splitting. The local states at the Fermi level are found to be situated within the π* band in the nonmagnetic case and the σ* bands in the ferromagnetic and antiferromagnetic cases. The energy values concerning the oxygen-vacancy formation from SrFeO3 to SrFeO2.875 have been deduced with the correction of the O2 overbinding error and the consideration of the oxygen partial pressure and the temperature.