Development of Stable Boron Nitride Nanotube and Hexagonal Boron Nitride Dispersions for Electrophoretic Deposition.

Boron nitride nanotubes (BNNTs) represent a relatively new class of materials that provides alternative electrical and thermal properties to the carbon analogue. The high chemical and thermal stability and large band gap combined with high electrical resistance make BNNTs desirable in several thin-film applications. In this study, stable BNNT and hexagonal boron nitride (hBN) particle dispersions have been developed using environmentally friendly advanced oxidation processing (AOP) that can be further modified for electrophoretic deposition (EPD) to produce thin films. The characterization of the dispersions has revealed how the hydroxyl radicals produced in AOP react with BNNT/hBN and contaminant boron nanoparticles (BNPs). While the radicals remove the carbon contaminant present on BNNT/hBN and increase dispersion stability, they also oxidize the BNPs and the boron oxide produced, which, conversely, reduces the dispersion stability. The use of high- or low-powered ultrasonication in combination with the AOP affects the rate of the competing reactions, with low-powered sonication and AOP providing the best combination for producing stable dispersions with high concentrations. BNNT/hBN dispersions were functionalized with polyethyleneimine to facilitate EPD, where films of several micrometer thickness were readily deposited onto stainless steel and glass-fiber fabrics. BNNT/hBN films produced on glass fabrics by EPD exhibited a consistent through-thickness macroporosity that was facilitated by platelet and nanotube stacking. The film macroporosity present on the coated fabrics was suitable for use as separator layers in supercapacitors and provided improved device robustness with a minimal impact on electrochemical performance.

Theoretical insight on the origin of anelasticity in zinc oxide nanowires.

Anelasticity of nanowires has recently attracted attention as an interesting property for high efficiency mechanical damping materials. While the mechanism of anelasticity has so far been analysed using continuum mechanical models based on defect diffusion, the mechanisms behind anelasticity have not yet been determined on an atomic level. Such information is needed in order to be able to design and synthesise new nanomaterials having desired mechanical properties. Here we determine the potential mechanism of anelasticity in narrow zinc oxide nanowires by analyzing the bond stretching and compression within the nanowire structure based on density functional theory. Our approach shows that different local minimum structures are created when different strain patterns are applied which give rise to the anelastic behavior. These findings can be applied for the prediction of potential anelasticity of other nanowire materials.

Adsorption of toxic gases on silicene/Ag(111).

Silicene is a two-dimensional nanomaterial, composed of Si atoms arranged into a buckled honeycomb network. It has become of great interest in recent years due to its remarkable properties such as its natural compatibility with current silicon-based technology. Due to its extreme thinness on the nanoscale, and large lateral dimensions, it has potential applications in gas sensing, gas storage and components in modern electronic devices. In this work, density functional theory calculations and ab initio molecular dynamics simulations are used to examine the reaction of SO2, NO2 and H2S on the Si/Ag(111) surface. It was shown that each gas will adsorb on the surface in different orientations and adsorption sites. SO2 and NO2 were found to chemisorb on the surface, whereas H2S was found to physisorb. SO2 and H2S adsorb associatively, whereas NO2 readily dissociates, producing adsorbed oxygen, and gaseous NO. At elevated temperatures, the SO2 and NO2 remain strongly bound to the surface, resulting in poisoning of the silicene, while H2S readily desorbs. Ab initio molecular dynamics also show that NO2 will selectively bind before SO2 when both gases are present in the same environment. This work shows that Si/Ag(111) may provide useful properties for gas sensing and storage applications.

Tuning the work function of the silicene/4 × 4 Ag(111) surface.

Silicene, the silicon analog of graphene, is an atomically thin two-dimensional material with promising applications in gas sensing, storage and as components in modern electronic devices. Silicene epitaxially grown on the Ag(111) surface can expand the utility of the silver surface by enabling the tuning of its work function through the functionalisation of silicene. Here we examine the electronic and structural properties and the thermodynamic stability of functionalised silicene/4 × 4 Ag(111) using density functional theory calculations coupled with ab initio molecular dynamics (AIMD) simulations. We focus on 11 functional groups, namely phenyl, methyl, hydroxyl, cyano, methoxyl, amino and ethylmethylamine, in addition to 4 halogen atoms. These functional groups are shown to endow the Si/Ag(111) surface with a large variation in the work function. Our AIMD simulations confirm the thermodynamic stability of these 11 functionalised structures. This work shows the possibility of tuning the electronic structure of silicene by functionalisation, which could then be utilized in polymer solar cells and nanoelectronic circuit components.

How ionic species structure influences phase structure and transitions from protic ionic liquids to liquid crystals to crystals.

The phase behaviour of n-alkylammonium (C6 to C16) nitrates and formates has been characterised using synchrotron small angle and wide angle X-ray scattering (SAXS/WAXS), differential scanning calorimetry (DSC), cross polarised optical microscopy (CPOM) and Fourier transform infrared spectroscopy (FTIR). The protic salts may exist as crystalline, liquid crystalline or ionic liquid materials depending on the alkyl chain length and temperature. n-Alkylammonium nitrates with n ≥ 6 form thermotropic liquid crystalline (LC) lamellar phases, whereas n ≥ 8 was required for the formate series to form this LC phase. The protic ionic liquid phase showed an intermediate length scale nanostructure resulting from the segregation of the polar and nonpolar components of the ionic liquid. This segregation was enhanced for longer n-alkyl chains, with a corresponding increase in the correlation length scale. The crystalline and liquid crystalline phases were both lamellar. Phase transition temperatures, lamellar d-spacings, and liquid correlation lengths for the n-alkylammonium nitrates and formates were compared with those for n-alkylammonium chlorides and n-alkylamines. Plateau regions in the liquid crystalline to liquid phase transition temperatures as a function of n for the n-alkylammonium nitrates and formates are consistent with hydrogen-bonding and cation-anion interactions between the ionic species dominating alkyl chain-chain van der Waals interactions, with the exception of the mid chained hexyl- and heptylammonium formates. The d-spacings of the lamellar phases for both the n-alkylammonium nitrates and formates were consistent with an increase in chain-chain layer interdigitation within the bilayer-based lamellae with increasing alkyl chain length, and they were comparable to the n-alkylammonium chlorides.