The aim of this study was to investigate the nucleation, growth, and surface deposition of poly(2,2,2-trifluoroethyl methacrylate) [poly(TFEMA)] from the one-phase, cloud point, and two-phase regions of a supercritical CO2–toluene solvent. A ternary mixture of 20 wt% toluene + 79 wt% scCO2 + 1 wt% poly(TFEMA) at 40.0 °C was exposed to a fluorine-doped tin oxide (FTO) surface for 30 min at pressures placing the solution in (i) a one-phase region (15.86 MPa), (ii) the cloud point (12.37 MPa), and (iii) a two-phase region (8.96 MPa). Using the Altunin–Gadetskii–Haar–Gallagher–Kell (AG–HGK) equation of state (EOS), the corresponding CO2 densities are 793.9, 729.2, and 477.8 kg m−3. Scanning electron microscopy (SEM) and particle-size analysis (sample sizes N = 852–1177) show particle-size distributions (PSDs) that are well described by the following lognormal form: the mean diameter increases monotonically with a decrease in pressure (1.767 μm → 2.605 μm → 2.863 μm), while dispersion tightens slightly near the cloud point (coefficient of variation, CV: ≈0.47 → 0.44) and then broadens strongly in the two-phase region (CV ≈ 1.02). Morphologies transition from sparse, compact islands (one-phase) to agglomerated, necked spheres (cloud point) and finally hierarchical populations containing hollow/pitted large particles (two-phase). These outcomes are consistent with a phase-state-controlled shift in nucleation pathways, as follows: from heterogeneous surface nucleation in the one-phase regime to homogeneous nucleation with agglomeration at the cloud point, and to homogeneous nucleation with coalescence and solvent capture in the two-phase regime. The results provide a mechanistic basis and practical design rules for pressure-programmable control of fluoropolymer coatings prepared from scCO2/aromatic-cosolvent systems.

A new process is reported for the incorporation of a fluoropolymer into a solid perovskite film. Poly(trifluoroethyl methacrylate) [CH2C(CH3)(CO2CH2CF3)]n was delivered to methylammonium lead iodide (CH3NH3PbI3) perovskite films by crystallizing the film in supercritical carbon dioxide/ethanol containing the dissolved fluoropolymer. The surface was characterized before and after fluoropolymer exposure using scanning electron microscopy, Raman spectroscopy, and contact angle measurements. The results indicate that the fluoropolymer was incorporated into the perovskite film during the supercritical fluid crystallization process. The incorporation of a hydrophobic fluoropolymer into perovskite has the potential to improve resistance to environmental degradation.

Perovskite-based solar cells with high power conversion efficiency were produced by annealing the perovskite layer at low temperature in the presence of supercritical carbon dioxide (scCO(2)). Photovoltaic devices were produced with perovskite layers annealed at either 50 degrees C or 100 degrees C, with and without the assistance of scCO(2). For devices annealed at 50 degrees C, the use of scCO(2) resulted in an 88% increase in the power conversion efficiency (from 9.17% to 17.22%) in comparison to devices annealed without scCO(2). For devices annealed at 100 degrees C the use of scCO(2) resulted in a smaller (3%) improvement in power conversion efficiency (from 16.94% to 17.52%). The quality of the perovskite layer at each condition was analyzed using x-ray diffraction, scanning electron microscopy and optical absorption. It was determined that the improvement in the observed device power conversion efficiency corresponds to an improvement in the quality of the photoactive perovskite layer.

Performance degradation under environmental conditions currently limits the practical utility of perovskite-based solar cells. The moisture stability of CH3NH3PbI3 perovskite films and solar cells was measured during exposure to three different levels of relative humidity. The films were crystallized at two different temperatures with and without simultaneous exposure to supercritical carbon dioxide. The film crystallinity, optical absorption, and device photoconversion efficiency was measured over time for three relative humidity levels and both crystallization methods. It was determined that film crystallization in supercritical CO2 resulted in significant improvement in moisture stability for films processed at 50 °C, but negligible improvement in stability for films processed at 100 °C.

Crystallization of CH3NH3PbI3 perovskite films was performed in supercritical carbon dioxide with and without organic cosolvents. Post deposition crystallization of the films was performed in a binary, single phase supercritical fluid at constant conditions (45 degrees, 1200 psi) but with varying cosolvent volume fractions up to 2 %. Organic cosolvents with varying polarity, propensity for hydrogen bonding and strength of solvation were used and the resulting perovskite film morphology, crystal structure and optical absorption spectra were measured. It was determined that the cosolvents can provide selective interactions with one or both of the perovskite precursor compounds resulting in different film morphologies ranging from uniform films containing large grains to films exhibiting large cubic or hexagonal crystals or preferential crystallographic orientations. The use of supercritical fluids to enhance or tune crystallization in solid-state thin films could have broad applications toward the realization of high efficiency photovoltaic devices. (C) 2019 Elsevier B.V. All rights reserved.

A new method for the production of high-quality CH3NH3PbI3 perovskite films at low temperature is reported. It is shown that CO2 above the critical point (31 degrees, 7.38 MPa (1071 psi)) acts as an anti-solvent to perovskite precursors and significantly enhances the solid-state film crystallization rate at low temperatures and also assists in the removal of residual solvent from within the film. Post deposition treatment in supercritical carbon dioxide (scCO(2)) resulted in complete conversion of perovskite compared to incomplete conversion at the same temperatures using thermal annealing alone. Complete crystallization and an average crystal grain size of 408 nm at 45 degrees was observed using scCO(2) treatment compared to incomplete crystal formation with average grain size of 175 nm at 45 degrees with thermal annealing alone. (C) 2019 Elsevier B.V. All rights reserved.

Gamma spectroscopy was used to quantify the accumulation of the uranyl ion (UO22+) into mesoporous silica gel in an aqueous solution under static and pressure-driven flow conditions. The amount of uranyl accumulated into silica gel under static conditions does not trend with the surface area reported by the manufacturer, but itis controlled by the silica gel permeability. Under flow conditions, the amount of uranyl deposited within mesoporous silica gel increases with pore size and ion removal efficiencies ranging from 1.8 to 7.0% were observed. Uranium transport and accumulation within mesoporous silicates is important in environmental monitoring, waste management and remediation.

The fluorescence lifetime of uranyl adsorbed within nanoporous silica gel was measured as a function of pore size at two different pH values and both wet and dry. it was determined that for uranyl adsorbed within pores larger than 4 nm, the lifetime is relatively independent of pore size, whereas below 4 nm, the lifetime increases with decreasing pore size. A blue shift in the emission spectra was observed at the smallest pore size (2.2 nm) and is believed to be caused by quantum confinement. The lifetime was found to be longer at a neutral pH than in an acidic pH, and this is caused by the formation of a uranyl hydroxyl complex at higher pH values. The presence of water within the pores is found to increase the fluorescence lifetime at all pore sizes and pH values studied in this paper; this is caused by the formation of a uranyl silicate bond in the absence of water. An understanding of the parameters that influence the fluorescence lifetime of uranyl within silica gel is important for the development of more sensitive detection methods. (C) 2018 Elsevier Ltd. All rights reserved.

The fluorescence intensity and transport kinetics of uranyl into mesoporous silica gel was measured in the presence of six naturally occurring cations. It was shown that the presence of the cations can reduce the fluorescence intensity of the uranyl through collision quenching and through competition for the silica gel surface sites. Stern-Volmer quenching coefficients were obtained by measuring the uranyl fluorescence as a function of cation concentration. The cations compete with uranyl to occupy silica gel surface sites and cause a decrease in uranyl fluorescence intensity and a reduction in the uranyl saturation time constant. Energy-dispersive x-ray spectroscopy (EDS) was used to measure the weight percentage of uranium and the cations in the silica gel samples and these results correlated well with the results of the saturation time constant measurements. The results of this study show that, at high concentrations, the presence of cations in water can influence the fluorescence intensity and transport kinetics of uranyl into mesoporous silica gel.

Trace quantities of a uranyl dissolved in water were measured using a simple optical method. A dilute solution of uranium nitrate dissolved in water was forced through nanoporous silica gel at fixed and controlled water flow rates. The uranyl ions deposited and accumulated within the silica gel and the uranyl fluorescence within the silica gel was monitored as a function of time using a light emitting diode as the excitation source and a photomultiplier tube detector. It was shown that the response time of the fluorescence output signal at a particular volumetric flow rate or average liquid velocity through the silica gel can be used to quantify the concentration of uranium in water. The response time as a function of concentration decreased with increasing flow velocity.

Uranyl transport into nanoporous silica gel is limited in a static aqueous solution by slow natural diffusion to the open bonding sites. In order to make this process faster, the diffusion dependence was eliminated using pressure driven fluid flow. Uranyl transport and adsorption within nanoporous silica gel was measured using time-dependent fluorescent measurements in an aqueous solution. The transport kinetics was measured under two different conditions: static solution in a standard cuvette and flowing solution through the silica gel. It was determined that the kinetics of uranyl uptake within nanoporous silica gel depends strongly on the liquid flow velocity. Above a certain velocity, the adsorption kinetics increased by at least two orders of magnitude (from about 40min to 2s) in comparison to a static solution. In a static solution, the kinetics depends on the porosity of the silica gel, but this dependence was not observed when the liquid flow velocity exceeded a certain value. Flow-enhanced adsorption kinetics has potential applications for fast detection of trace levels of uranyl in water.

The fluorescence of organic fluorophore molecules is enhanced when they are placed in contact with certain metals (Al, Ag, Cu, Au, etc.) whose surface plasmon waves couple into the radiative modes of the molecules and increase the radiative efficiency. Here, we report a hitherto unknown size dependence of this metal-enhanced fluorescence (MEF) effect in the nanoscale. When the molecules are deposited in nanoporous anodic alumina films with exposed aluminum at the bottom of the pores, they form organic nanowires standing on aluminum nanoparticles whose plasmon waves have much larger amplitudes. This increases the MEF strongly, resulting in several orders of magnitude increase in the fluorescence intensity of the organic fluorophores. The increase in intensity shows an inverse superlinear dependence on nanowire diameter because the nanowires also act as plasmonic "waveguides" that concentrate the plasmons and increase the coupling of the plasmons with the radiative modes of the molecules. Furthermore, if the nanoporous template housing the nanowires has built-in electric fields due to space charges, a strong molecule-specific red- or blue-shift is induced in the fluorescence peak owing to a renormalization of the dipole moment of the molecule. This can be exploited to detect minute amounts of target molecules in a mixture using their optical signature (fluorescence) despite the presence of confounding background signals. It can result in a unique new technology for biosensing and chemical sensing.

Hybrid solar cells (HSCs) with water soluble polythiophene sodium poly[2-(3-thienyl)-ethyloxy-4-butylsulfonate] (PTEBS) thin films produced using electrospray deposition (ESD) were fabricated, tested, and modeled and compared to devices produced using conventional spin coating. A single device structure of FTO/TiO 2 /PTEBS/Au was used to study the effects of ESD of the PTEBS layer on device performance. ESD was found to increase the short circuit current density (Jsc) by a factor of 2 while decreasing the open circuit voltage (Voc) by half compared to spin coated PTEBS films. Comparable efficiencies of 0.009% were achieved from both device construction types. Current-voltage curves were modeled using the characteristic solar cell equation and showed a similar increase in generated photocurrent with an increase by two orders of magnitude in the saturation current in devices from ESD films. Increases in Jsc are attributed to an increase in the interfacial contact area between the TiO 2 and PTEBS layers, while decreases in Voc are attributed to incomplete film formation from ESD.

ABSTRACTHydrophobic materials with tunable wettability were developed by electrospinning aligned polystyrene (PS) fibers onto the surface of a unimorph composite piezoelectric substrate. An electric field was used to modify the curvature of the substrate resulting in a corresponding change in the morphology of the electrospun coating. Contact angle measurements were performed on droplets deposited onto the surface before and after application of the electric field. The water droplet contact angle was observed to change in response to the applied voltage. Contact angle measurements were performed as a function of surface fiber density and suggest that the change in contact angle is caused by a transition from Wenzel to Cassie–Baxter wetting. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41592.

A novel electrospun polymer-fiber solar cell was synthesized by electrospinning a 1 : 2.5 weight% ratio mixture of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) resulting in bulk heterojunctions. Electrospinning is introduced as a technique that may increase polymer solar cell efficiency, and a list of advantages of the technique applied to solar cells is discussed. The device achieved a power conversion efficiency of %. The absorption and photoluminescence of MEH-PPV nanofibers are compared to thin films of the same material. Electrospun nanofibers are discussed as a favorable structure for application in polymer solar cells.

ABSTRACTPolystyrene (PS) fiber mats incorporating iron (Fe) particles were fabricated by electrospinning and the hydrophobicity of the resulting magnetic fabrics was investigated with and without an applied magnetic field. The results show that the hydrophobicity increases in the presence of a magnetic field and the hysteresis in the advancing/receding contact angle decreases in the presence of a magnetic field. It is also shown that the contact angle and hysteresis increase with decreasing fiber diameter. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 2352–2358, 2013

Advances in nanofiber fabrication techniques (e.g., electrospinning) have come to allow control over fiber distribution and orientation such that an ordered coating with fibers arranged in layers orthogonal to one another can potentially be produced. Such coatings can serve as a nano-sieve that can be designed and placed on the downstream side of a conventional nonwoven fibrous filter to enhance its performance (collection efficiency for a given pressure drop). This paper presents a thorough analysis of the performance characteristics of these thin coatings to guide the fabrication process in terms of the coatings' microstructural properties. In particular, we have found a correlation such that, for coatings composed of a given fiber size, there exists a corresponding particle size for which a coating's performance becomes independent of variations in fiber-to-fiber spacing (i.e., coating's non-homogeneity). We have also found that a coating's performance improves when its mass is distributed across more than one pair of orthogonal layers.

We report the fabrication and characterization of thin films from the water soluble polymer sodium poly[2-(3-thieny1)-ethyloxy-4-butylsulfonate] (PTEBS) by electrospray deposition (ESD). Contiguous thin films were created by adjusting the parameters of the electrospray apparatus and solution properties to maintain a steady Taylor cone for uniform nanoparticle aerosolization and controlling the particle water content to enable coalescence with previously deposited particles. The majority of deposited particles had diameters less than 52 nm. A thin film of 64.7 nm with a root mean square surface roughness of 20.2 nm was achieved after 40 min of ESD. Published by Elsevier B.V.

Mesh-like fiber mats of polystyrene (PS) were deposited using DC-biased AC-electrospinning. Superhydrophobic surfaces with water contact angles greater than 150° and gas fraction values of up to 97% were obtained. Rheological study was conducted on these fiber surfaces and showed a decrease in shear stress when compared with a noncoated surface (no slip), making them excellent candidates for applications requiring the reduction of skin-friction drag in submerged surfaces. We have also shown that addition of a second, low-surface energy polymer to a solution of PS can be used to control the fiber internal porosity depending on the concentration of the second polymer. Contact-angle measurements on mats consisting of porous and nonporous fibers have been used to evaluate the role of the larger spaces between the fibers and the pores on individual fibers on superhydrophobicity. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

In this paper, we present a numerical study devised to investigate the influence of microstructural parameters on the performance of fibrous superhydrophobic coatings manufactured via dc and ac electrospinning. In particular, our study is focused on predicting the resistance of such coatings against elevated hydrostatic pressures, which is of crucial importance for submersible applications. In our study, we generate 3D virtual geometries composed of randomly or orthogonally oriented horizontal fibers with bimodal diameter distributions resembling the microstructure of our electrospun coatings. These virtual geometries are then used as the computational domain for performing full morphology numerical simulations to establish a relationship between the coatings’ critical pressure (pressure beyond which the surface may depart from the Cassie state) and their microstructures. For coatings with ordered microstructures, we have also derived analytical expressions for the critical pressure based on the balance of forces acting on the water–air interface. Predictions of our force balance analysis are compared with those of our FM simulations as well as the equations proposed by Tuteja et al. [Proc. Natl. Acad. Sci. U.S.A. 105, 18200 (2008)]10.1073/pnas.0804872105, and discussed in detail. Our numerical simulations are aimed at providing useful information with regards to the tolerance of fibrous superhydrophobic coatings against elevated pressures, and helping with the design and optimization of the coatings’ microstructures. Our results show considerably higher pressure tolerance for the case of coatings with orthogonally oriented fibers as compared to those with randomly laid fibers when other microstructural parameters are held constant. Moreover, it is demonstrated that thickness of the coating has less influence on performance in the case of orthogonal microstructures. Coatings’ responses to other variations favor those that yield smaller-sized inter-fiber spaces. Studies are also performed investigating the effect of subtle permutations in the layer configurations of our ac-electrospun coatings, as well as the use of a hybrid coating that utilizes advantages from both dc and ac electrospinning.