Enhanced efficiency of tri-layered dye solar cells with hydrothermally synthesized titania nanotubes as light scattering outer layer

In the present study dye sensitized solar cells (DSSCs) have been fabricated with a tri-layer photo anode consisting of hydrothermally prepared titania nano tubes (TNT) having a diameter of 9-10 nm and length of several micrometers as outer layer, P25 TiO2 powder as transparent light absorbing middle layer and a compact TiO2 inner layer to improve the adhesion of different layers on a transparent conducting oxide coated substrate. In comparison to cells fabricated using TNTs or P25 alone, the tri-layer DSSCs exhibit an enhanced efficiency of 7.15% with a current density of 17.12 mA cm(-2) under AM 1.5 illumination. The enhancement is attributed to the light scattering generated by TNTs aggregates, reduction in electron transport resistance at the TiO2/dye/electrolyte interface and an improvement in electron life-time. (c) 2012 Elsevier B.V. All rights reserved.

On the Presumed Kinetic Consequences of Pre-equilibrium. Implications for the Michaelis-Menten Equation

The overall rate equation for a reaction sequence consisting of a pre-equilibrium and rate-determining steps should not be derived on the basis of the concentration of the intermediate product (X). This is apparently indicated by transition state theory (as the path followed to reach the highest energy transition state is irrelevant), but also proved by a straight-forward mathematical approach. The thesis is further supported by the equations of concurrent reactions as applied to the partitioning of X between the two competing routes (reversal of the pre-equilibrium and formation of product). The rate equation may only be derived rigorously on the basis of the law of mass action. It is proposed that the reactants acquire the overall activation energy prior to the pre-equilibrium, thus forming X in a high-energy state en route to the rate-determining transition state. (It is argued that conventional energy profile diagrams are misleading and need to be reinterpreted.) Also, these arguments invalidate the Michaelis-Menten equation of enzyme kinetics, and necessitate a fundamental revision of our present understanding of enzyme catalysis. (The observed ``saturation kinetics'' possibly arises from weak binding of a second molecule of substrate at the active site; analogous conclusions apply to reactions at surfaces).

Relaxation and jump dynamics of water at the mica interface

The orientational relaxation dynamics of water confined between mica surfaces is investigated using molecular dynamics simulations. The study illustrates the wide heterogeneity that exists in the dynamics of water adjacent to a strongly hydrophilic surface such as mica. Analysis of the survival probabilities in different layers is carried out by normalizing the corresponding relaxation times with bulk water layers of similar thickness. A 10-fold increase in the survival times is observed for water directly in contact with the mica surface and a non-monotonic variation in the survival times is observed moving away from the mica surface to the bulk-like interior. The orientational relaxation time is highest for water in the contact layer, decreasing monotonically away from the surface. In all cases the ratio of the relaxation times of the 1st and 2nd rank Legendre polynomials of the HH bond vector is found to lie between 1.5 and 1.9 indicating that the reorientational relaxation in the different water layers is governed by jump dynamics. The orientational dynamics of water in the contact layer is particularly novel and is found to undergo distinct two-dimensional hydrogen bond jump reorientational dynamics with an average waiting time of 4.97 ps. The waiting time distribution is found to possess a long tail extending beyond 15 ps. Unlike previously observed jump dynamics in bulk water and other surfaces, jump events in the mica contact layer occur between hydrogen bonds formed by the water molecule and acceptor oxygens on the mica surface. Despite slowing down of the water orientational relaxation near the surface, life-times of water in the hydration shell of the K ion are comparable to that observed in bulk salt solutions. (C) 2012 American Institute of Physics. http://dx.doi.org/10.1063/1.4717710]

Gel Sculpture: Moldable, Load-Bearing and Self-Healing Non-Polymeric Supramolecular Gel Derived from a Simple Organic Salt

An easy access to a library of simple organic salts derived from tert-butoxycarbonyl (Boc)-protected L-amino acids and two secondary amines (dicyclohexyl- and dibenzyl amine) are synthesized following a supramolecular synthon rationale to generate a new series of low molecular weight gelators (LMWGs). Out of the 12 salts that we prepared, the nitrobenzene gel of dicyclohexylammonium Boc-glycinate (GLY.1) displayed remarkable load-bearing, moldable and self-healing properties. These remarkable properties displayed by GLY.1 and the inability to display such properties by its dibenzylammonium counterpart (GLY.2) were explained using microscopic and rheological data. Single crystal structures of eight salts displayed the presence of a 1D hydrogen-bonded network (HBN) that is believed to be important in gelation. Powder X-ray diffraction in combination with the single crystal X-ray structure of GLY.1 clearly established the presence of a 1D hydrogen-bonded network in the xerogel of the nitrobenzene gel of GLY.1. The fact that such remarkable properties arising from an easily accessible (salt formation) small molecule are due to supramolecular (non-covalent) interactions is quite intriguing and such easily synthesizable materials may be useful in stress-bearing and other applications.

Influence of Bottom Bubbling Rate on Formation of Metal Emulsion in Al-Cu Alloy and Molten Salt System

In steel refining process, an increase of interfacial area between the metal and slag through the metal droplets emulsified into the slag, so-called ``metal emulsion'', is one prevailing view for improving the reaction rate. The formation of metal emulsion was experimentally evaluated using Al-Cu alloy as metal phase and chloride salt as slag phase under the bottom bubbling condition. Samples were collected from the center of the salt phase in the container. Large number of metal droplets were separated from the salt by dissolving it into water. The number, surface area, and weight of the droplets increased with the gas flow rate and have local maximum values. The formation and sedimentation rates of metal droplets were estimated using a mathematical model. The formation rate increased with the gas flow rate and has a local maximum value as a function of gas flow rate, while the sedimentation rate is independent of the gas flow rate under the bottom bubbling condition. Three types of formation mode of metal emulsion, which occurred by the rupture of metal film around the bubble, were observed using high speed camera. During the process, an elongated column covered with metal film was observed with the increasing gas flow rate. This elongated column sometimes reached to the top surface of the salt phase. In this case, it is considered that fine droplets were not formed and in consequence, the weight of metal emulsion decreased at higher gas flow rate.

Study of n-ZnO/p-Si (100) thin film heterojunctions by pulsed laser deposition without buffer layer

ZnO/Si heterojunctions were fabricated by growing ZnO thin films on p-type Si (100) substrate by pulsed laser deposition without buffer layers. The crystallinity of the heterojunction was analyzed by high resolution X-ray diffraction and atomic force microscopy. The optical quality of the film was analyzed by room temperature (RT) photoluminescence measurements. The high intense band to band emission confirmed the high quality of the ZnO thin films on Si. The electrical properties of the junction were studied by temperature dependent current-voltage measurements and RT capacitance-voltage (C-V) analysis. The charge carrier concentration and the barrier height (BH) were calculated, to be 5.6x10(19) cm(-3) and 0.6 eV respectively from the C-V plot. The BH and ideality factor, calculated using the thermionic emission (TE) model, were found to be highly temperature dependent. We observed a much lower value in Richardson constant, 5.19x10(-7)A/cm(2) K-2 than the theoretical value (32 A/cm(2) K-2) for ZnO. This analysis revealed the existence of a Gaussian distribution (GD) with a standard deviation of sigma(2)=0.035 V. By implementing the GD to the TE, the values of BH and Richardson constant were obtained as 1.3 eV and 39.97 A/cm(2) K-2 respectively from the modified Richardson plot. The obtained Richardson constant value is close to the theoretical value for n-ZnO. These high quality heterojunctions can be used for solar cell applications. (C) 2012 Elsevier B.V. All rights reserved.

Modeling of Ascending/Descending Velocity of Metal Droplet Emulsified in Pb-Salt System

Metal-slag emulsion is an important process to enhance the reaction rate between the two phases; thus, it improves the heat and mass transfer of the process significantly. Various experimental studies have been carried out, and some system specific relations have been proposed by various investigators. A unified, theoretical study is lacking to model this complex phenomenon. Therefore, two simple models based on fundamental laws for metal droplet velocity (both ascending and descending) and bubble velocity, as well as its position at any instant of time, have been proposed. Analytical solutions have been obtained for the developed equations. Analytical solutions have been verified for the droplet velocity, traveling time, and size distribution in slag phase by performing high-temperature experiments in a Pb-salt system and comparing the obtained data with theory. The proposed model has also been verified with published experimental data for various liquid systems with a wide range of physical properties. A good agreement has been found between the analytical solution and the experimental and published data in all cases.

Self-Assembly and Catalytic Activity of Metal Nanoparticles Immobilized in Polymer Membrane Prepared via Layer-by-Layer Approach

Densely packed nanoparticles distributed in a stable and robust thin film is a highly preferred system for utilizing the various applications of nanoparticles. Here, we report covalent bond mediated layer-by-layer (LbL) self-assembled thin films of nanoparticles embedded in polymer membrane. Polymer with complementary functional group is utilized for fabrication of thin film via covalent bonding. UV-visible spectroscopy, atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to monitor the growth of LbL thin film. Subsequently, the composite thin film is used for catalysis of an organic electron transfer reaction of p-nitrophenol to p-aminophenol by sodium borohydride. The catalytic activity of these composite films is assayed multiple times, proving its applicability as a catalyst. The kinetic data obtained by monitoring reduction of p-nitrophenol suggest that the reaction rates are directly related to the sizes of the nanoparticle and porosity of the membrane.

Ionic conductivity of bis(2-cyanoethyl) ether-lithium salt and poly(propylether imine)-lithium salt liquid electrolytes

We report here a multiple-nitrile based lithium-salt liquid electrolyte. The ionic conductivity of poly (propyl ether imine) (abbreviated as PETIM) lithium salt dendrimer liquid electrolyte was observed to be a function of dendrimer generation number, n=0 (monomer)-3. While the highest room temperature ionic conductivity value (similar to 10(-1) Sm-1) was recorded for the bis-2cyanoethyl ether monomer (i.e. zeroth generation; G(0)-CN), conductivity decreased progressively to lower values (similar to 10(-3) Sm-1) with increase in generation number (G(1)-CN -> G(3)-CN). The G(0)-CN and higher dendrimer generations showed high thermal stability (approximate to 150 to 200 degrees C), low moisture sensitivity and tunable viscosity (similar to 10(-2) (G(0)-CN) to 3 (G(3)-CN) Pa s). The linker ether group was found to be crucial for ion transport and also eliminated a large number of detrimental features, chiefly moisture sensitivity, chemical instability associated typically with prevalent molecular liquid solvents. Based on the combination of several beneficial physicochemical properties, we presently envisage that the PETIM dendrimers especially the G(0)-CN electrolytes hold promise as electrolytes in electrochemical devices such as lithium-ion batteries.

Designing carboxymethyl cellulose based layer-by-layer capsules as a carrier for protein delivery

Stable hollow microcapsules composed of sodium carboxymethyl cellulose (CMC) and poly (allylamine hydrochloride) (PAH) were produced by layer-by-layer adsorption of polyelectrolytes onto CaCO 3 microparticles. Subsequently the core was removed by addition of chelating agents for calcium ions. Zeta potential studies showed charge reversal with deposition of successive polyelectrolyte layers, indicating that the alternate electrostatic adsorption of polyelectrolytes of opposite charge was successfully achieved. The size and surface morphology of the capsules was characterized by various microscopy techniques. The pH responsive loading behavior was elucidated by confocal laser scanning microscopy (CLSM) studies using fluorescence labeled dextran (FITC-dextran) and labeled BSA (FITC-BSA). CLSM images confirmed the open (pH ≤ 6) and closed state (pH ≥ 7) of the capsules. A model drug bovine serum albumin (BSA) was spontaneously loaded below its isoelectric point into hollow microcapsules, where BSA is positively charged. The loading of the BSA into the microcapsules was found to be dependent on the feeding concentration and pH of the medium. 65 of the loaded BSA was released over 7h of which about 34 was released in the first hour. These findings demonstrate that (CMC/PAH) 2 hollow capsules can be further exploited as a potential drug delivery system.

Vaporization and collision modeling of liquid fuel sprays in a co-axial fuel and air pre-mixer

Droplet collision occurs frequently in regions where the droplet number density is high. Even for Lean Premixed and Pre-vaporized (LPP) liquid sprays, the collision effects can be very high on the droplet size distributions, which will in turn affect the droplet vaporization process. Hence, in conjunction with vaporization modeling, collision modeling for such spray systems is also essential. The standard O'Rourke's collision model, usually implemented in CFD codes, tends to generate unphysical numerical artifact when simulations are performed on Cartesian grid and the results are not grid independent. Thus, a new collision modeling approach based on no-time-counter method (NTC) proposed by Schmidt and Rutland is implemented to replace O'Rourke's collision algorithm to solve a spray injection problem in a cylindrical coflow premixer. The so called ``four-leaf clover'' numerical artifacts are eliminated by the new collision algorithm and results from a diesel spray show very good grid independence. Next, the dispersion and vaporization processes for liquid fuel sprays are simulated in a coflow premixer. Two liquid fuels under investigation are jet-A and Rapeseed Methyl Esters (RME). Results show very good grid independence in terms of SMD distribution, droplet number distribution and fuel vapor mass flow rate. A baseline test is first established with a spray cone angle of 90 degrees and injection velocity of 3 m/s and jet-A achieves much better vaporization performance than RME due to its higher vapor pressure. To improve the vaporization performance for both fuels, a series of simulations have been done at several different combinations of spray cone angle and injection velocity. At relatively low spray cone angle and injection velocity, the collision effect on the average droplet size and the vaporization performance are very high due to relatively high coalescence rate induced by droplet collisions. Thus, at higher spray cone angle and injection velocity, the results expectedly show improvement in fuel vaporization performance since smaller droplet has a higher vaporization rate. The vaporization performance and the level of homogeneity of fuel-air mixture can be significantly improved when the dispersion level is high, which can be achieved by increasing the spray cone angle and injection velocity. (C) 2012 Elsevier Ltd. All rights reserved.

Role of Specific Cations and Water Entropy on the Stability of Branched DNA Motif Structures

DNA three-way junctions (TWJs) are important intermediates in various cellular processes and are the simplest of a family of branched nucleic acids being considered as scaffolds for biomolecular nanotechnology. Branched nucleic acids are stabilized by divalent cations such as Mg2+, presumably due to condensation and neutralization of the negatively charged DNA backbone. However, electrostatic screening effects point to more complex solvation dynamics and a large role of interfacial waters in thermodynamic stability. Here, we report extensive computer simulations in explicit water and salt on a model TWJ and use free energy calculations to quantify the role of ionic character and strength on stability. We find that enthalpic stabilization of the first and second hydration shells by Mg2+ accounts for 1/3 and all of the free energy gain in 50% and pure MgCl2 solutions, respectively. The more distorted DNA molecule is actually destabilized in pure MgCl2 compared to pure NaCl. Notably, the first shell, interfacial waters have very low translational and rotational entropy (i.e., mobility) compared to the bulk, an entropic loss that is overcompensated by increased enthalpy from additional electrostatic interactions with Mg2+. In contrast, the second hydration shell has anomalously high entropy as it is trapped between an immobile and bulklike layer. The nonmonotonic entropic signature and long-range perturbations of the hydration shells to Mg2+ may have implications in the molecular recognition of these motifs. For example, we find that low salt stabilizes the parallel configuration of the three-way junction, whereas at normal salt we find antiparallel configurations deduced from the NMR. We use the 2PT analysis to follow the thermodynamics of this transition and find that the free energy barrier is dominated by entropic effects that result from the decreased surface area of the antiparallel form which has a smaller number of low entropy waters in the first monolayer.

Effect of high gold salt concentrations on the size and polydispersity of gold nanoparticles prepared by an extended Turkevich-Frens method

The Turkevich-Frens synthesis starting conditions are expanded, ranging the gold salt concentrations up to 2 mM and citrate/gold(III) molar ratios up to 18:1. For each concentration of the initial gold salt solution, the citrate/gold(III) molar ratios are systematically varied from 2:1 to 18:1 and both the size and size distribution of the resulting gold nanoparticles are compared. This study reveals a different nanoparticle size evolution for gold salt solutions ranging below 0.8 mM compared to the case of gold salt solutions above 0.8 mM. In the case of Au3+]<0.8 mM, both the size and size distribution vary substantially with the citrate/gold(III) ratio, both displaying plateaux that evolve inversely to Au3+] at larger ratios. Conversely, for Au3+]>= 0.8 mM, the size and size distribution of the synthesized gold nanoparticles continuously rise as the citrate/gold(III) ratio is increased. A starting gold salt concentration of 0.6 mM leads to the formation of the most monodisperse gold nanoparticles (polydispersity index<0.1) for a wide range of citrate/gold(III) molar ratios (from 4:1 to 18:1). Via a model for the formation of gold nanoparticles by the citrate method, the experimental trends in size could be qualitatively predicted:the simulations showed that the destabilizing effect of increased electrolyte concentration at high initial Au3+] is compensated by a slight increase in zeta potential of gold nanoparticles to produce concentrated dispersion of gold nanoparticles of small sizes.

Unusual Salt-Induced Color Modulation through Aggregation-Induced Emission Switching of a Bis-cationic Phenylenedivinylene-Based pi Hydrogelator

The synthesis, hydrogelation, and aggregation-induced emission switching of the phenylenedivinylene bis-N-octyl pyridinium salt is described. Hydrogelation occurs as a consequence of pi-stacking, van der Waals, and electrostatic interactions that lead to a high gel melting temperature and significant mechanical properties at a very low weight percentage of the gelator. A morphology transition from fiber-to-coil-to-tube was observed depending on the concentration of the gelator. Variation in the added salt type, salt concentrations, or temperature profoundly influenced the order of aggregation of the gelator molecules in aqueous solution. Formation of a novel chromophore assembly in this way leads to an aggregation-induced switch of the emission colors. The emission color switches from sky blue to white to orange depending upon the extent of aggregation through mere addition of external inorganic salts. Remarkably, the salt effect on the assembly of such cationic phenylenedivinylenes in water follow the behavior predicted from the well-known Hofmeister effects. Mechanistic insights for these aggregation processes were obtained through the counterion exchange studies. The aggregation-induced emission switching that leads to a room-temperature white-light emission from a single chromophore in a single solvent (water) is highly promising for optoelectronic applications.