Current Nanoscience - Volume 11, Issue 3, 2015

Sparks are frequently observed during dielectric breakdown on valve metal oxide films, and several papers investigated their correlation with the oxide crystallization. In this work, the effect of high temperature of a spark was studied using finite element method. An important result is that the oxide area heated above the oxide fusion temperature is more than 50 times larger than the diameter of the channel generated during the breakdown event, meaning that just a few numbers of sparks could lead to a large area of oxide crystallization. Besides, the heated area is proportional to the spark temperature and to the oxide thickness. Finally, using a factorial design, a cross effect was also detected between the channel temperature and the barrier oxide thickness.

Titanium dioxide (TiO2) is one of the most important materials to be employed as a photocatalyst for environmental protection and other applications. Owing to its certain important combination of properties, TiO2 in the form of thin film has been found to be more attractive for a great variety of applications including photocatalytic degradation of organic pollutants in water as well as in air, dye sensitized solar cells (DSSCs), anti-fogging, superhydrophilic, photochromic, and optical applications. Although, a great number of techniques have been employed so far to fabricate TiO2 thin films, the cost of these thin films has not only been found to be dictated by the sophistication of the preparation method involved but also by the quick recombination of photo-generated electron/hole pairs, backward reactions involved, and the poor response of TiO2 to the visible light. Even though, metal loading, metal ion doping, anion doping, dye sensitization, composite semiconducting phenomenon, metal ion-implantation, addition of sacrificial reagents and carbonate salts to the reaction mixtures, etc., have been employed to improve the efficiency of the photocatalytic applications of TiO2 thin films, a clear-cut relationship between the properties of TiO2 thin films and their performance in a given application is yet to be established. In this article, some of the latest developments accomplished in the fabrication techniques, in the characterization and in the understandings of property and performance relationship of TiO2 thin film have been presented and discussed while citing certain of important references.

A review of the electrowetting EW phenomenon and its applications is provided. The state of the art includes already implemented technologies like EW displays or variable focus lenses but also laboratory level researches in the lab-on-chip and fluid manipulation field. In the second part of the work some preliminary experimental results are provided for the electrowetting on fluoropolymer coated anodized aluminum, as example of EWOD electrowetting on dielectrics. Anodization was performed in weakly acidic or neutral 2% wt. boric acid solution in water with an applied voltage range between 10 and 40 V, then coating the surface with AF 1600. Experimental electrowetting data fit with the Young-Lippmann relation, confirming aluminum as a promising material for electrowetting applications.

Physicochemical characterization of annealed TiO2 nanotubes (TNTs) was conducted by using photocurrent spectroscopy and differential capacitance techniques. It has been shown that the geometry and architecture of nanotubes determine how photogenerated electrons and holes are separated and transferred. Photocurrent generation in TNTs is a consequence of two phenomena; drifting of holes into the electrolyte and diffusion of electrons toward the substrate. These two processes have been shown to be independent of the anodic polarization. The capacitance of TiO2 nanotubes is also affected by their geometry. In anodic potentials, with respect to the flat band potential of the underlying barrier layer, the capacitance is mainly controlled by the barrier layer because nanotubes are almost inactive. In the cathodic potential region, electrons injected from the substrate into the conduction band of TiO2 induce nanotubes to behave more like porous metallic electrodes. As a consequence, the electrochemical double layer along TNTs large surface area causes high values of capacitance.

All valve metals are potential candidates for anodic oxidation, an electrochemical technique that can produce the growth of nanometric oxide films at the metal surface, exhibiting fascinating colors due to their interference with light. Yet, comprehensive works comparing their oxidation kinetics and the characteristics and functionalities of oxides produced are hardly found, in spite of the technological importance of such information. In fact, these films lie in the field of optical coatings, whose engineering opens applications in technological areas such as communication, sensing, imaging, and display. Moreover, the possibility to modify their electronic structure allows to produce dielectrics, as well as ionic conductors or semiconductors, or memristors. The availability of a wide range of properties can increase the variety of functions that can be covered by such films, which requires not only different oxide thicknesses, but also different compositions, and therefore know-how on the widest set of valve metal oxides possible. This review article will present the main characteristics of anodic oxide films formed on the most common valve metals – i.e., titanium, aluminum, niobium, tantalum and zirconium, with reference to the relationship between the properties achieved and the potential applications. Additionally, a comparison will be presented among oxides produced on different metals in same anodizing conditions to discuss the effects of anodic oxidation parameters on the resulting oxide main features.

Anodizing of aluminum generates a nanoporous alumina layer comprising cylindrical nanopores extending essentially perpendicular to the substrate. Visual observation of such nanoporous layers shows a colored specular reflectance consistent with reflectance spectrum measurements. Such structural colors raise interest in particular for producing non-fading colors. However the optical behavior is complex and requires specific characterization setups and modelling approaches. A morphological and optical characterization of nanoporous anodic alumina has been performed and enabled to draw the chromatic paths. Moreover, the optical behavior has been compared to numerical simulations of optical properties using the Fourier Modal Method, thus demonstrating the impact of short-range order and long-range disorder on the formation of structural colors in these partially ordered nanostructures.

The changes of morphology, crystal structure and fluoride concentration of TiO2 nanotubes were investigated systematically as a function of heat-treatment temperature and time using SEM, XRD and EDS. The TiO2 nanotubes were grown electrochemically on the Ti surface by anodizing in ethylene glycol containing 0.1 M NH4F + 2% H2O. Decomposition of the TiO2 nanotubes was found to begin first inside of the nanotubes into fine particles with sizes of 50 ~ 150 nm and then it occurred in the surface region. The particles were randomly distributed on the surface region while they were aligned into straight lines within the interior region of the oxide after heat-treatment for 3 h at 450 ~ 700 °C. The TiO2 nanotubes were transformed from amorphous into anatase crystal without destruction of the nanotublar structure after 10 h at 250 °C in air and anatase crystallization was also observed to occur with the formation of fine particles after 1 h at 450 °C. The fluorine concentration in TiO2 nanotubes was found to decrease linearly with log t, which was attributed to the removal of fluorine from the nanotubes by diffusion process during the heat-treatment in air.

We discuss the electrochemical formation of metal oxide films as candidate materials for the primary dielectric layer in electrowetting-on-dielectric (EWOD) systems. The ability to produce significant contact angle reduction at low voltage requires thin (<100nm), high dielectric strength films that also exhibit a high breakdown field; these requirements are met in a wide variety of metal oxides. A detailed understanding of materials as well as electronic and ionic transport in dielectrics are essential for the development and performance enhancement of these devices; we describe the process of oxides in film formation, the peculiar defect structure, electrical transport processes, as well as degradation and breakdown. Additionally, examples of low-voltage EWOD systems based on oxide/hydrophobic bilayers are discussed along with current efforts to improve performance of the metal oxide layers.

ZnO/montmorillonite (ZnO/MMT) nanocomposite was prepared using MMT as a support, zinc chloride as ZnO synthesis precursor and cetyltrimethylammonium bromide as a surfactant. The prepared ZnO/MMT nanocomposite was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscope, Fourier transform infrared spectroscopy and N2 adsorption/desorption analysis. Results indicated the appropriate immobilization of the ZnO nanoparticles with 20-40 nm width on the surface of the MMT. The effect of ZnO immobilization on its adsorption and photocatalytic activity was evaluated by the removal of basic yellow 28 (BY28) in aqueous solution under UV light irradiation. The ZnO/MMT nanocomposite was more effective in adsorption, and photocatalytic degradation processes than pure ZnO. The performance of used light source for photocatalytic degradation of BY28 was found to be in the order of UV-C (wavenumber region of 200-280 nm) > UV-B (wavenumber region of 280-315 nm) > UV-A (wavenumber region of 315-400 nm). An artificial neural network (ANN) model was developed to predict the photocatalytic degradation process under UV-C radiation. The ANN model with reasonable predictive performance (R² = 0.999) indicated that the influence of initial concentration of BY28 on decolorization process (49.66%) was higher than that of nanocomposite dosage (36.44%) and UV radiation time (13.09%). Results of reusability tests indicated that the ZnO/MMT was stable and appropriate for long time application.

ZnO nanoparticles have been synthesized for utilization as UV filter in sunscreens. Optical characteristics of the synthesized ZnO nanoparticles have been investigated. Two methods were used for stabilizing of nanoparticles to prevent coagulation and stabilization of disperse systems: deposition of inert polymer (PVA) during synthesis and surface modification by 3-aminopropyltriethoxysilane (APTES). Presences of PVA layer of 2nm and APTES have modified the optical properties. It was shown that both UV-A and UV-B can be completely blocked by modified ZnO nanoparticles. Surface modification by APTES leads to aggregative stability of the sol dispersion in solution. Level of hemolysis and cytotoxicity activities of all samples was also studied, and the influence of surface modification has been analyzed in details. Addition of PVA (more than 3 wt%) leads to a significant increase in cytotoxicity. Surface modification of nanoparticles by APTES also leads to an increase in level of hemolysis and cytotoxicity, however this level is quite low. Notably we have achieved the stabilized and non-harmful ZnO nanoparticles by not only depositing the PVA but by modifying the PVA capped nanoparticles with APTES. Stabilized ZnO nanoparticles act as UV filters and were transparent in the visible region of the spectrum, which makes the present material promising for use as UV filters in sunscreens, while the effect on cells in case of penetrating the layer of epidermis may be minimal.

In this study, a novel ZnS/ZnWO4 nanocomposite photocatalyst was synthesized through a facile hydrothermal synthesis method. The as-prepared samples were characterized by energy dispersive X-ray analysis (EDS), X-ray powder diffraction (XRD), transmission electron microscopy (TEM), UV-vis diffuse reflectance spectroscopy (UV-vis DRS) and so on. The ZnS/ZnWO4 nanocomposite shows an enhanced photocatalytic activity for the degradation of rhodamine B (RhB) under simulated solar light. The results show that ZnS/ZnWO4 nanocomposite photocatalysts have better photocatalytic activity than the single ZnWO4 or ZnS. In the case of ZnS compounding, ZnS/ZnWO4 photocatalyst led to the lower recombination ratios of photo-generated electron hole pairs and then higher photocatalytic performance. In addition, it has the potential to become a useful technology for environmental cleanup without secondary pollution.

Glycyrrhizic acid (GA) is a triterpenoid saponin glycoside from licorice roots and rhizomes, having a variety of pharmacological activities, but its solubility in water is poor. In this study, we got GA nanoparticle suspensions about 180 nm by SAS (supercritical antisolvent) process. Comparative anti HSV-1 (herpes simplex virus type 1) studies were undertaken using nano-GA (glycyrrhizic acid nanoparticles) with its ammoniated salt--- MAG (glycyrrhizic acid ammonium salt). The maximum noncytotoxic concentration of both nano-GA and MAG were 187.5 μM for BHK cells. MTT assay showed GA nanoparticle suspensions exhibited better anti-HSV activities compared to MAG, especially during replication period. Morphology of HSV-1 observed by TEM (transmission electron microscopy) significantly changed with nano-GA treatment. TEM images showed that the envelope of HSV-1 was damaged and shedded.

This manuscript studies the effect of defects like large vacancies which can be categorised as pinholes, on the vibration characteristics of double walled carbon nanotubes (DWCNT). Simulations are performed on chiral, zigzag and armchair nanotubes with cantilever and bridged conditions using molecular structure mechanics approach. Pinholes are the larger vacancies (through which a C60 molecule can pass) consisting of a no. of missing atoms, which are formed during the manufacturing process. Researchers have reported that variation in the current and voltage along with a proper dose of electrons leads to development and migration of vacancies which further increases in size. In this manuscript, the authors have categorized these larger vacancies into two types i.e. 6 missing atoms and 24 missing atoms on the outer wall of DWCNT. Moreover, it has been reported by many researchers that the location of defect affects the vibrational characteristics of nanotubes. Hence considering the vibrational aspect, the effect of number of such defects and their location along the length of nanotube has been studied. The simulation results indicate that the resonant frequency of defective DWCNT is reducing with the increase in chiral angle. Further, it is also observed that the fundamental frequency reduces with the increase in the number of pinhole defects in DWCNT and maximum frequency reduction takes place when the defect is located nearer to the fixed end of DWCNT.

The study is aimed at development of nanocrystals of Nelfinavir Mesylate, an anti-HIV drug to overcome the drawback associated with drug such as poor solubility and oral bioavailability. Nanocrystals were attempted by combination technique and ultrasonication method using polyvinyl alcohol (PVA) and poloxamer 407 as stabilizers at various concentrations. The solid-state characteristics of optimized nanocrystals were studied by XRD, FTIR, DSC and SEM analysis. The release behavior of the drug was studied by in vitro dissolution. The in vivo pharmacokinetics was assessed in Wister Albino rats by administering nanocrystals orally. Nelfinavir nanocrystals were obtained with the narrow size distribution and mean particle size was ranged from 216 to 360 nm for the nanocrystals obtained from combination technique. At 0.5% w/w of PVA, particle size was 236 ± 19.23 nm and zeta potential was 18.34 ± 2.0 mV. Freeze dried Poloxamer 407 and PVA nanocrystals showed very good solubility than the freeze dried powder with cryoprotectant and pure drug. However, nanocrystals with PVA (NC PVA) showed high dissolution velocity as compared to nanocrystals with Poloxamer 407 and were supported by high saturation solubility of NC PVA. SEM and other solid-state studies indicated the crystalline nature of developed nanocrystals. Compared to pure drug, NC PVA formulation had decreased Tmax and increased Cmax and AUC0-24. Thus pharmacokinetic study revealed significant increase in oral absorption of the drug with nanocrystals which could be attributed to the increase in dissolution velocity of the Nelfinavir in nanocrystal form.

It is evident that surface effects dealing with decreasing characteristic size play an extremely important role in the mechanical behaviors of nanoscale structures, from the one-dimensional nanowire to complex micro/nano device. In the present study, the impact of surface effects on the internal forces of a nanobeam is evaluated in terms of the action of the residual surface stress and surface elasticity. Through deriving the expressions of the shear force and bending moment analytically, we depicted the updated internal force diagrams accordingly. Unlike the classical ones in mechanics of materials, the nanobeams take on a strong size-dependent feature. These analyses hold great potential for modulating the mechanical performance of micro/nano materials, and the obtained results provide new insight into the education innovation on mechanics of materials.

This communication deals with the convective flow of third grade nanofluid when the flow is induced by the stretching of a sheet in linear manner. Brownian motion and thermophoresis effects are considered in the transport phenomenon to describe the nanofluidics. Heat generation and absorption effects are also analyzed under the action of magnetic field. Similarity transforms are invoked for the conversion of system of partial differential equations into nonlinear coupled ordinary differential equations. Analytic solutions for the momentum, energy and mass fraction fields are computed and examined for the different physical parameters including the third grade parameter, heat source/sink parameter, nanofluidics parameters, Biot numbers etc. Numerical results for different sundry parameters are computed to present the different rheological effects. Comparison with an already published data is made in the limiting sense which shows a nice agreement between the two solutions.