Jincheng Du

Preparation, Microstructure and Photocatalytic Activity of the Porous TiO2 Anatase Coating by Sol-Gel Processing

In this study the porous TiO2 anatase coatings are prepared from alkoxide solutions containing polyethylene glycol (PEG) by a dip-coating technique. The effects of PEG addition to the precursor solution on the photocatalytic activity and microstructure of the resultant coatings are studied. The larger amount and the larger molecular weight of PEG, the larger size and more pores produced in the resultant coatings on the decomposition of PEG during heat-treatment. The adsorbed hydroxyl content of such porous coatings is found to increase due to the larger size and more pores in the coatings. However, the transmittance of the coatings decreases due to the scattering by the larger size and more pores. Photocatalytic degradation experiments show that organophosphorous insecticide, dimethyl-2,2-dichlorovinyl phosphate (DDVP), was efficiently degraded in the presence of the porous TiO2 coatings by exposing the DDVP solution to sunlight. Photocatalytic degradation rate was related to the adsorbed hydroxyl content, transmittance and morphology of the resultant coatings.

Three-dimensional structure determination from a single view

The ability to determine the structure of matter in three dimensions has profoundly advanced our understanding of nature. Traditionally, the most widely used schemes for three-dimensional (3D) structure determination of an object are implemented by acquiring multiple measurements over various sample orientations, as in the case of crystallography and tomography, or by scanning a series of thin sections through the sample, as in confocal microscopy. Here we present a 3D imaging modality, termed ankylography (derived from the Greek words ankylos meaning ‘curved’ and graphein meaning ‘writing’), which under certain circumstances enables complete 3D structure determination from a single exposure using a monochromatic incident beam. We demonstrate that when the diffraction pattern of a finite object is sampled at a sufficiently fine scale on the Ewald sphere, the 3D structure of the object is in principle determined by the 2D spherical pattern. We confirm the theoretical analysis by performing 3D numerical reconstructions of a sodium silicate glass structure at 2 Å resolution, and a single poliovirus at 2–3 nm resolution, from 2D spherical diffraction patterns alone. Using diffraction data from a soft X-ray laser, we also provide a preliminary demonstration that ankylography is experimentally feasible by obtaining a 3D image of a test object from a single 2D diffraction pattern. With further development, this approach of obtaining complete 3D structure information from a single view could find broad applications in the physical and life sciences.

Molecular dynamics simulations of soda–lime–silicate glasses

Molecular dynamics simulations of a series of sodium calcium silicates are reported. The results are discussed in terms of the local co-ordination of the sodium and calcium cations, and how the replacement of sodium with calcium changes the medium range order.

Alkali ion migration mechanisms in silicate glasses probed by molecular dynamics simulations

The application of the molecular dynamics computer simulation technique to the problem of elucidating alkali ion migration mechanisms in alkali silicate glasses is reviewed. Some new results are presented that help to clarify the processes and their timing. In particular, it is shown that alkali ions jump into empty sites; that is, the mechanisms owe more in character to their crystalline vacancy counterpart rather than their interstitial cousins.

Workfunction tuning of zinc oxide films by argon sputtering and oxygen plasma: an experimental and computational study

Zinc oxide (ZnO) films were grown by radio frequency magnetron sputter deposition and the changes to its surface composition and workfunction induced by argon sputter cleaning and oxygen plasma treatments were characterized using x-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and density functional theory modelling. Compared with a workfunction of 3.74?eV for the as-deposited ZnO films, a workfunction of 3.95?eV was obtained after Ar sputter cleaning and 4.21?eV after exposure to oxygen plasma. The data indicate that oxygen plasma treatment leads to a more negative ZnO surface. The dipole induced by this charge redistribution reinforces the original surface dipole, which results in an increase in the surface dipole moment and an increase in workfunction. The reverse is true for hydrocarbon contamination of ZnO surfaces. Excellent qualitative agreement between the experimental results and computational modelling was obtained. The results suggest that specific surface functionalization may be a viable method of controlling the workfunction of ZnO for use as the transparent conducting oxide in optoelectronic applications such as solar cells and organic light-emitting diodes.

Sodium ion migration mechanisms in silicate glasses probed by molecular dynamics simulations

Abstract Molecular dynamics simulations have been carried out on a silica glass containing 1 mol% soda. The Na distribution is not the same as that found in more highly concentrated sodium containing glasses. Long-range diffusive motion appears to be correlated with the absence of a neighboring non-bridging oxygen. Those Na ions remaining localized often show backward–forward hops that are not readily seen in glasses with larger sodium content.

87Sr solid-state NMR as a structurally sensitive tool for the investigation of materials: antiosteoporotic pharmaceuticals and bioactive glasses.

Strontium is an element of fundamental importance in biomedical science. Indeed, it has been demonstrated that Sr(2+) ions can promote bone growth and inhibit bone resorption. Thus, the oral administration of Sr-containing medications has been used clinically to prevent osteoporosis, and Sr-containing biomaterials have been developed for implant and tissue engineering applications. The bioavailability of strontium metal cations in the body and their kinetics of release from materials will depend on their local environment. It is thus crucial to be able to characterize, in detail, strontium environments in disordered phases such as bioactive glasses, to understand their structure and rationalize their properties. In this paper, we demonstrate that (87)Sr NMR spectroscopy can serve as a valuable tool of investigation. First, the implementation of high-sensitivity (87)Sr solid-state NMR experiments is presented using (87)Sr-labeled strontium malonate (with DFS (double field sweep), QCPMG (quadrupolar Carr-Purcell-Meiboom-Gill), and WURST (wideband, uniform rate, and smooth truncation) excitation). Then, it is shown that GIPAW DFT (gauge including projector augmented wave density functional theory) calculations can accurately compute (87)Sr NMR parameters. Last and most importantly, (87)Sr NMR is used for the study of a (Ca,Sr)-silicate bioactive glass of limited Sr content (only ~9 wt %). The spectrum is interpreted using structural models of the glass, which are generated through molecular dynamics (MD) simulations and relaxed by DFT, before performing GIPAW calculations of (87)Sr NMR parameters. Finally, changes in the (87)Sr NMR spectrum after immersion of the glass in simulated body fluid (SBF) are reported and discussed.

Structure and properties of sodium aluminosilicate glasses from molecular dynamics simulations.

Addition of alumina to sodium silicate glasses considerably improves the mechanical properties and chemical durability and changes other properties such as ionic conductivity and melt viscosity. As a result, aluminosilicate glasses find wide industrial and technological applications including the recent Corning(®) Gorilla(®) Glass. In this paper, the structures of sodium aluminosilicate glasses with a wide range of Al∕Na ratios (from 1.5 to 0.6) have been studied using classical molecular dynamics simulations in a system containing around 3000 atoms, with the aim to understand the structural role of aluminum as a function of chemical composition in these glasses. The short- and medium-range structures such as aluminum coordination, bond angle distribution around cations, Q(n) distribution (n bridging oxygen per network forming tetrahedron), and ring size distribution have been systematically studied. In addition, the mechanical properties including bulk, shear, and Youngs moduli have been calculated and compared with experimental data. It is found that aluminum ions are mainly four-fold coordinated in peralkaline compositions (Al∕Na < 1) and form an integral part of the rigid silicon-oxygen glass network. In peraluminous compositions (Al∕Na > 1), small amounts of five-fold coordinated aluminum ions are present while the concentration of six-fold coordinated aluminum is negligible. Oxygen triclusters are also found to be present in peraluminous compositions, and their concentration increases with increasing Al∕Na ratio. The calculated bulk, shear, and Youngs moduli were found to increase with increasing Al∕Na ratio, in good agreement with experimental data.

Fundamental mechanisms of oxygen plasma-induced damage of ultralow-k organosilicate materials: The role of thermal P3 atomic oxygen

Fourier transform infrared (FTIR) spectroscopy, x-ray photoelectron spectroscopy (XPS), and ab initio density functional theory-based molecular dynamics simulations demonstrate fundamental mechanisms for CH3 abstraction from organosilicate films by thermal O(P3). Ex situ FTIR analysis demonstrates that film exposure to thermal O(P3) yields chemical changes similar to O2 plasma exposure. In situ XPS indicates that exposure to thermal O(P3) yields O/OH incorporation in the organosilicate film concurrent with carbon loss from the surface region. These results are consistent with simulations indicating specific low kinetic barrier (<0.1 eV) reactions resulting in concurrent Si–C bond scission and Si–O bond formation.

Structure, dynamics, and electronic properties of lithium disilicate melt and glass

Ab initio molecular dynamics simulations within the framework of density functional theory have been performed to study the structural, dynamic, and electronic properties of lithium disilicate melt and the glass derived from quenching the melt. It is found that lithium ions have a much higher diffusion coefficient and show different diffusion mechanisms than the network forming silicon and oxygen ions in the melt. The simulated lithium disilicate glass structure has 100% four coordinated silicon, close to theoretical nonbridging oxygen to bridging oxygen ratio (2:3), and Q(n) distributions of 20.8%, 58.4%, and 20.8% for n=2,3,4, respectively. In the melt there are considerable amounts (10%-15%) of silicon coordination defects; however, the average silicon coordination number remains about 4, similar to that in the glass. The lithium ion coordination number increases from 3.7 in the glass to 4.4 in the melt mainly due to the increase of bridging oxygen in the first coordination shell. The bond length and bond angle distributions, vibrational density of states, and static structure factors of the simulated glass were determined where the latter was found to be in good agreement with experimental measurement. Atomic charges were obtained based on Bader and Hirshfeld population analyses [Atoms in Molecule: A Quantum Theory (Oxford University Press, Oxford, 1990); Theor. Chim. Acta 44, 129 (1977)]. The average Bader charges found in lithium disilicate glass were -1.729, 3.419, and 0.915 for oxygen, silicon, and lithium, respectively. The corresponding Hirshfeld charges were -0.307, 0.550, and 0.229. The electronic densities of states of the melt and glass were calculated and compared with those of crystalline lithium disilicate.