Electrochemical oxidation of pharmaceuticals in synthetic fresh human urine: Using selective radical quenchers to reveal the dominant degradation pathways and the scavenging effects of individual urine constituents

Electrochemical oxidation of fresh human urine is a promising method to prevent pharmaceuticals from being discharged into the environment. Here, we evaluate the importance of electro-generated oxidants and direct anodic oxidation for degradation of four pharmaceutical (cyclophosphamide (CP), carbamazepine (CBZ), sulfamethoxazole (SMX) and ibuprofen (IBP)) accounting for the scavenging effect of urine constituents using boron-doped diamond (BDD) and IrO₂ electrodes. Allyl alcohol and tert-butanol were used as selective quenchers for adsorbed and dissolved radicals, respectively. In electrolyte containing only chloride and pharmaceuticals, we found that CBZ and SMX are primarily oxidized by electro-generated Cl₂ in the fluid boundary layer , and CP and IBP are primarily oxidized by physisorbed ^•OH or chemisorbed chlorine (IrO₃-Cl). Regarding the effects of other fresh urine constituents, urea, creatinine, and uric acid quench the dissolved reactive chlorine species (Cl^•/Cl₂^•‒, HOCl, Cl₂, etc.). However, SO₄^2‒ shows no effect on pharmaceutical degradation while H₂PO₄^‒ and citrate ions quench IrO₃-Cl resulting in a mixed kinetic and mass-transfer limiting oxidation of pharmaceuticals on IrO₂. Citrate ions only quench the dissolved oxidants (surface adsorbed radicals are the dominant oxidants) leading to the pharmaceutical degradation limited by the mass transfer of pharmaceutical to BDD surface. This work provides an understanding of the significance of various pathways for pharmaceutical degradation, scavenging effect of urine constituents, and strategies for rapid pharmaceutical degradation in human urine.

Water-Accelerated Photooxidation of CH3NH3PbI3 Perovskite

Halide perovskites have the potential to disrupt the photovoltaics market based on their high performance and low cost. However, the decomposition of perovskites under moisture, oxygen, and light raises concerns about service lifetime, especially because degradation mechanisms and the corresponding rate laws that fit the observed data have thus far eluded researchers. Here, we report a water-accelerated photooxidation mechanism dominating the degradation kinetics of archetypal perovskite CH₃NH₃PbI₃ in air under >1% relative humidity at 25 °C. From this mechanism, we develop a kinetic model that quantitatively predicts the degradation rate as a function of temperature, ambient O₂ and H₂O levels, and illumination. Because water is a possible product of dry photooxidation, these results highlight the need for encapsulation schemes that rigorously block oxygen ingress, as product water may accumulate beneath the encapsulant and initiate the more rapid water-accelerated photooxidative decomposition.

Dilution effect for highly efficient multiple-component organic solar cells

Although the multiple-component (MC) blend strategy has been frequently used as a very effective way to improve the performance of organic solar cells (OSCs), there is a strong need to understand the fundamental working mechanism and material selection rule for achieving optimal MC-OSCs. Here we present the 'dilution effect' as the mechanism for MC-OSCs, where two highly miscible components are molecularly intermixed. Contrary to the aggregation-induced non-radiative decay, the dilution effect enables higher luminescence quantum efficiencies and open-circuit voltages (V_OC) in MC-OSCs via suppressed electron-vibration coupling. The continuously broadened bandgap together with reduced electron-vibration coupling also explains the composition-dependent V_OC in ternary blends well. Moreover, we show that electrons can transfer between different acceptors, depending on the energy offset between them, which contributes to the largely unperturbed charge transport and high fill factors in MC-OSCs. The discovery of the dilution effect enables the demonstration of a high power conversion efficiency of 18.31% in an MC-OSC.

Selective oxidation of pharmaceuticals and suppression of perchlorate formation during electrolysis of fresh human urine

Urine comprises only a small (~1%) volumetric fraction of municipal wastewater, but represents a dominant source of pharmaceuticals, many of which may pass through conventional wastewater treatment and pose risks to aquatic ecosystems. Point-source treatment of source-separated urine presents a unique opportunity to degrade pharmaceuticals before dilution with wastewater, and electrochemical advanced oxidation processes are one increasingly investigated option. However, they often lead to the formation of oxidation byproducts including chlorate, perchlorate at very high concentrations. Here, we show that the high urea content of fresh human urine suppresses the formation of oxychlorides by inhibiting formation of HOCl/OCl^‒ during electrolysis, while still enabling pharmaceutical degradation due to the slow rate of urea oxidation by ^•OH. This results in improved performance compared to equivalent treatment of hydrolyzed aged urine. This electrochemical oxidation scheme is shown to degrade the model contaminants cyclophosphamide and sulfamethoxazole with surface-area-to-volume-normalized pseudo-first-order rate constants greater than 0.08 cm/min in authentic fresh human urine. It results in ~100 × decrease in pharmaceutical concentrations in 2 h while generating ~1000 × lower oxychloride byproduct concentrations in synthetic fresh urine than synthetic hydrolyzed aged urine matrixes. Importantly, this proof-of-principle shows that simple and safe electrochemical methods can be used for point-source-remediation of pharmaceuticals in fresh human urine (before storage and hydrolysis), without formation of significant oxychloride byproducts.

Solution-Processed BiI3 Films with 1.1 eV Quasi-Fermi Level Splitting: The Role of Water, Temperature, and Solvent during Processing

We present a mechanistic explanation of the BiI3 film formation process and an analysis of the critical factors in preparing high-quality solution-processed BiI3 films. We find that complexation with Lewis bases, relative humidity, and temperature are important factors during solvent vapor annealing (SVA) of films. During SVA, water vapor and higher temperatures limit the formation of the BiI3-dimethylformamide coordination complex. SVA with an optimized water content and temperature produces films with 300-500 nm grains. Films that formed solvent coordination compounds at lower temperatures showed preferential crystal orientation after solvent removal, and we elucidate its implications for carrier transport. Addition of dimethyl sulfoxide to highly concentrated tetrahydrofuran-BiI3 inks prevents film cracking after spin-coating. We have measured a quasi-Fermi level splitting of 1.1 eV and a diffusion length of 70 nm from films processed with optimal temperature and humidity. The best device produced by optimized SVA has a power conversion efficiency of 0.5%, I sc of ∼4 mA/cm2, and V OC of ∼400 mV. The low photocurrent and voltage we attribute to the low diffusion length and the unfavorable band alignment between the absorber and the adjacent transport layers. The deep understanding of the relationship between morphology/crystal structure and optoelectronic properties gained from this work paves the way for future optimization of BiI3-based solar cells.

Photoluminescence and Photoconductivity to Assess Maximum Open-Circuit Voltage and Carrier Transport in Hybrid Perovskites and Other Photovoltaic Materials

Photovoltaic (PV) device development is much more expensive and time-consuming than the development of the absorber layer alone. This Perspective focuses on two methods that can be used to rapidly assess and develop PV absorber materials independent of device development. The absorber material properties of quasi-Fermi level splitting and carrier diffusion length under steady effective 1 Sun illumination are indicators of a material's ability to achieve high V_OC and J_SC. These two material properties can be rapidly and simultaneously assessed with steady-state absolute intensity photoluminescence and photoconductivity measurements. As a result, these methods are extremely useful for predicting the quality and stability of PV materials prior to PV device development. Here, we summarize the methods, discuss their strengths and weaknesses, and compare photoluminescence and photoconductivity results with device performance for four hybrid perovskite compositions of various bandgaps (1.35-1.82 eV), CISe, CIGSe, and CZTSe.

Overcoming the Photovoltage Plateau in Large Bandgap Perovskite Photovoltaics

Development of large bandgap (1.80-1.85 eV E_g) perovskite is crucial for perovskite-perovskite tandem solar cells. However, the performance of 1.80-1.85 eV E_g perovskite solar cells (PVKSCs) are significantly lagging their counterparts in the 1.60-1.75 eV E_g range. This is because the photovoltage ( V_oc) does not proportionally increase with E_g due to lower optoelectronic quality of conventional (MA,FA,Cs)Pb(I,Br)₃ and results in a photovoltage plateau ( V_oc limited to 80% of the theoretical limit for ∼1.8 eV E_g). Here, we incorporate phenylethylammonium (PEA) in a mixed-halide perovskite composition to solve the inherent material-level challenges in 1.80-1.85 eV E_g perovskites. The amount of PEA incorporation governs the topography and optoelectronic properties of resultant films. Detailed structural and spectroscopic characterization reveal the characteristic trends in crystalline size, orientation, and charge carrier recombination dynamics and rationalize the origin of improved material quality with higher luminescence. With careful interface optimization, the improved material characteristics were translated to devices and V_oc values of 1.30-1.35 V were achieved, which correspond to 85-87% of the theoretical limit. Using an optimal amount of PEA incorporation to balance the increase in V_oc and the decrease in charge collection, a highest power conversion efficiency of 12.2% was realized. Our results clearly overcome the photovoltage plateau in the 1.80-1.85 eV E_g range and represent the highest V_oc achieved for mixed-halide PVKSCs. This study provides widely translatable insights, an important breakthrough, and a promising platform for next-generation perovskite tandems.

Highly Efficient Perovskite-Perovskite Tandem Solar Cells Reaching 80% of the Theoretical Limit in Photovoltage

Organic-inorganic hybrid perovskite multijunction solar cells have immense potential to realize power conversion efficiencies (PCEs) beyond the Shockley-Queisser limit of single-junction solar cells; however, they are limited by large nonideal photovoltage loss (V _oc,loss ) in small- and large-bandgap subcells. Here, an integrated approach is utilized to improve the V _oc of subcells with optimized bandgaps and fabricate perovskite-perovskite tandem solar cells with small V _oc,loss . A fullerene variant, Indene-C₆₀ bis-adduct, is used to achieve optimized interfacial contact in a small-bandgap (≈1.2 eV) subcell, which facilitates higher quasi-Fermi level splitting, reduces nonradiative recombination, alleviates hysteresis instabilities, and improves V _oc to 0.84 V. Compositional engineering of large-bandgap (≈1.8 eV) perovskite is employed to realize a subcell with a transparent top electrode and photostabilized V _oc of 1.22 V. The resultant monolithic perovskite-perovskite tandem solar cell shows a high V _oc of 1.98 V (approaching 80% of the theoretical limit) and a stabilized PCE of 18.5%. The significantly minimized nonideal V _oc,loss is better than state-of-the-art silicon-perovskite tandem solar cells, which highlights the prospects of using perovskite-perovskite tandems for solar-energy generation. It also unlocks opportunities for solar water splitting using hybrid perovskites with solar-to-hydrogen efficiencies beyond 15%.

Stabilized Wide Bandgap Perovskite Solar Cells by Tin Substitution

Wide bandgap MAPb(I_1-yBr_y)₃ perovskites show promising potential for application in tandem solar cells. However, unstable photovoltaic performance caused by phase segregation has been observed under illumination when y is above 0.2. Herein, we successfully demonstrate stabilization of the I/Br phase by partially replacing Pb²⁺ with Sn²⁺ and verify this stabilization with X-ray diffractometry and transient absorption spectroscopy. The resulting MAPb_0.75Sn_0.25(I_1-yBr_y)₃ perovskite solar cells show stable photovoltaic performance under continuous illumination. Among these cells, the one based on MAPb_0.75Sn_0.25(I_0.4Br_0.6)₃ perovskite shows the highest efficiency of 12.59% with a bandgap of 1.73 eV, which make it a promising wide bandgap candidate for application in tandem solar cells. The engineering of internal bonding environment by partial Sn substitution is believed to be the main reason for making MAPb_0.75Sn_0.25(I_1-yBr_y)₃ perovskite less vulnerable to phase segregation during the photostriction under illumination. Therefore, this study establishes composition engineering of the metal site as a promising strategy to impart phase stability in hybrid perovskites under illumination.

Synthesis of Ligand-free CdS Nanoparticles within a Sulfur Copolymer Matrix

Aliphatic ligands are typically used during the synthesis of nanoparticles to help mediate their growth in addition to operating as high-temperature solvents. These coordinating ligands help solubilize and stabilize the nanoparticles while in solution, and can influence the resulting size and reactivity of the nanoparticles during their formation. Despite the ubiquity of using ligands during synthesis, the presence of aliphatic ligands on the nanoparticle surface can result in a number of problems during the end use of the nanoparticles, necessitating further ligand stripping or ligand exchange procedures. We have developed a way to synthesize cadmium sulfide (CdS) nanoparticles using a unique sulfur copolymer. This sulfur copolymer is primarily composed of elemental sulfur, which is a cheap and abundant material. The sulfur copolymer has the advantages of operating both as a high temperature solvent and as a sulfur source, which can react with a cadmium precursor during nanoparticle synthesis, resulting in the generation of ligand free CdS. During the reaction, only some of the copolymer is consumed to produce CdS, while the rest remains in the polymeric state, thereby producing a nanocomposite material. Once the reaction is finished, the copolymer stabilizes the nanoparticles within a solid polymeric matrix. The copolymer can then be removed before the nanoparticles are used, which produces nanoparticles that do not have organic coordinating ligands. This nascent synthesis technique presents a method to produce metal-sulfide nanoparticles for a wide variety of applications where the presence of organic ligands is not desired.

Enhanced Carrier Lifetimes of Pure Iodide Hybrid Perovskite via Vapor-Equilibrated Re-Growth (VERG)

Solution deposition of planar films of the hybrid perovskite (HP) methylammonium (MA) lead iodochloride (MAPbI3-xClx) often results in very low surface coverage, small grain size, and high density of defects, particularly for the pure iodide HP. These decrease the optoelectronic quality of MAPbI3 (minority lifetimes all less than 10 ns) and creates pinholes that may result in shunt pathways that severely degrade the efficiency of photovoltaic devices. The poor morphology is usually attributed to the formation of large disconnected grains of PbI2 that nucleate first and set the morphology of the final HP layer. As a result, many use PbCl2 as a lead source. The PbCl2 is less soluble, forms smaller grains, and promotes more continuous HP films. Here, we show a highly reproducible deposition method for pure iodide MAPbI3 that yields continuous films with large grain sizes and minority carrier lifetimes greater than 200 ns. The method consists of thermal evaporation of PbI2 and a post-deposition Vapor-Equilibrated Re-Growth (VERG) step at 110 °C in a closed vessel.

Sulfur copolymer for the direct synthesis of ligand-free CdS nanoparticles

Aliphatic ligand-free CdS nanoparticles were synthesized using a sulfur copolymer that operates as both a high-temperature solvent and sulfur source concurrently.

Nanoscale surface potential variation correlates with local S/Se ratio in solution-processed CZTSSe solar cells

Thin film solar cells made from Cu, Zn, Sn, and S/Se can be processed from solution to yield high-performing kesterite (CZTS or CZTSSe) photovoltaics. We present a microstructural study of solution-deposited CZTSSe films prepared by nanocrystal-based ink approaches using scanning probe microscopy (SPM) and scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS). We correlate scanning Kelvin probe microscopy (SKPM) maps of local surface potential with SEM/EDS images of the exact same regions of the film, allowing us to relate observed variations in surface potential to local variations in stoichiometry. Specifically, we find a correlation between surface potential and the S/(S + Se) composition ratio. In particular, we find that regions with high S/(S + Se) ratios are often associated with regions of more negative surface potential and thus higher work function. The change in work function is larger than the expected change in the valence band position with these small changes in sulfur, and thus the data suggest an increase in acceptor-like defects with increasing sulfur. These findings provide new experimental insight into the microscopic relationships between composition, structure, and electronic properties in these promising photovoltaic materials.

Tricontinuous Cubic Nanostructure and Pore Size Patterning in Mesostructured Silica Films Templated with Glycerol Monooleate

The fabrication of nanostructured films possessing tricontinuous minimal surface mesophases with well-defined framework and pore connectivity remains a difficult task. As a new route to these structures, we introduce glycerol monooleate (GMO) as a template for evaporation-induced self-assembly. As deposited, a nanostructured double gyroid phase is formed, as indicated by analysis of grazing-incidence small-angle x-ray scattering data. Removal of GMO by UV/O(3) treatment or acid extraction induces a phase change to a nanoporous body-centered structure which we tentatively identify as based on the IW-P surface. To improve film quality, we add a co-surfactant to the GMO in a mass ratio of 1:10; when this co-surfactant is cetyltrimethylammonium bromide, we find an unusually large pore size (8-12 nm) in acid extracted films, while UV/O(3) treated films yield pores of only ca. 4 nm. Using this pore size dependence on film processing procedure, we create a simple method for patterning pore size in nanoporous films, demonstrating spatially-defined size-selective molecular adsorption.

DNA translocation through an array of kinked nanopores

Synthetic solid-state nanopores are being intensively investigated as single-molecule sensors for detection and characterization of DNA, RNA and proteins. This field has been inspired by the exquisite selectivity and flux demonstrated by natural biological channels and the dream of emulating these behaviours in more robust synthetic materials that are more readily integrated into practical devices. So far, the guided etching of polymer films, focused ion-beam sculpting, and electron-beam lithography and tuning of silicon nitride membranes have emerged as three promising approaches to define synthetic solid-state pores with sub-nanometre resolution. These procedures have in common the formation of nominally cylindrical or conical pores aligned normal to the membrane surface. Here we report the formation of 'kinked' silica nanopores, using evaporation-induced self-assembly, and their further tuning and chemical derivatization using atomic-layer deposition. Compared with 'straight through' proteinaceous nanopores of comparable dimensions, kinked nanopores exhibit up to fivefold reduction in translocation velocity, which has been identified as one of the critical issues in DNA sequencing. Additionally, we demonstrate an efficient two-step approach to create a nanopore array exhibiting nearly perfect selectivity for ssDNA over dsDNA. We show that a coarse-grained drift-diffusion theory with a sawtooth-like potential can reasonably describe the velocity and translocation time of DNA through the pore. By control of pore size, length and shape, we capture the main functional behaviours of protein pores in our solid-state nanopore system.

Dependence of carrier mobility on nanocrystal size and ligand length in PbSe nanocrystal solids

We measure the room-temperature electron and hole field-effect mobilities (micro(FE)) of a series of alkanedithiol-treated PbSe nanocrystal (NC) films as a function of NC size and the length of the alkane chain. We find that carrier mobilities decrease exponentially with increasing ligand length according to the scaling parameter beta = 1.08-1.10 A(-1), as expected for hopping transport in granular conductors with alkane tunnel barriers. An electronic coupling energy as large as 8 meV is calculated from the mobility data. Mobilities increase by 1-2 orders of magnitude with increasing NC diameter (up to 0.07 and 0.03 cm(2) V(-1) s(-1) for electrons and holes, respectively); the electron mobility peaks at a NC size of approximately 6 nm and then decreases for larger NCs, whereas the hole mobility shows a monotonic increase. The size-mobility trends seem to be driven primarily by the smaller number of hops required for transport through arrays of larger NCs but may also reflect a systematic decrease in the depth of trap states with decreasing NC band gap. We find that carrier mobility is independent of the polydispersity of the NC samples, which can be understood if percolation networks of the larger-diameter, smaller-band-gap NCs carry most of the current in these NC solids. Our results establish a baseline for mobility trends in PbSe NC solids, with implications for fabricating high-mobility NC-based optoelectronic devices.

How to dip-coat and spin-coat nanoporous double-gyroid silica films with EO19-PO43-EO19 surfactant (Pluronic P84) and know it using a powder X-ray diffractometer

Previously, the synthesis of highly oriented pure double-gyroid nanoporous silica films has been demonstrated using evaporation-induced self-assembly (EISA) and dip-coating with a specialty triblock surfactant (PEO-PPO-alkyl) as the template. For these films, grazing-incidence small-angle X-ray scattering (GISAXS) was used to determine orientation and structure. However, GISAXS is not widely available, and we have observed significant batch-to-batch variability in the PEO-PPO-alkyl surfactants used. Here, we show for the first time: (1) synthesis of highly oriented pure double-gyroid nanoporous silica films using freely available EO(19)-PO(43)-EO(19) surfactant (Pluronic-P84) as the nanostructure-directing agent, (2) the use of spin-coating and dip-coating EISA to fabricate the double-gyroid films, and (3) the use of theta-theta X-ray diffractometers (commonly available and typically used for powder X-ray diffraction, PXRD) to identify the double-gyroid phase. Processing diagrams for P84 using dip-coating and spin-coating are shown in order to map the dependency of the nanostructure on solution composition, relative humidity, and solution aging time. In addition, an effect of the rate of evaporation during EISA is observed via dependence on the angular velocity in spin-coating. Also, through quantitative comparison of the GISAXS patterns with corresponding PXRD patterns, previously unexplained diffraction peaks in the PXRD patterns are shown to result from diffraction from crystallographic planes that are not parallel to the substrate (typically not observed in PXRD) due to the small angles involved and the nonzero acceptance angle of the PXRD Soller slits. These peaks provide a means to distinctly identify the double-gyroid phase using PXRD. The same trends relating aging-time-before-coating to the phase that forms via EISA are observed with EO(19)-PO(43)-EO(19) as was the case in previous studies using EO(17)-PO(14)-C(12). This shows the generality of use of aging time to synthesize nanoporous silica films with nonionic surfactants. Finally, a list of "tips and tricks" is provided to facilitate easy reproducible synthesis of double-gyroid nanoporous silica thin films in other laboratories.

Synthesis of Cu2ZnSnS4 nanocrystal ink and its use for solar cells

Cu(2)ZnSnS(4) (CZTS) and Cu(2)ZnSnSe(4) (CZTSe) based solar cells are promising candidates for low cost solar cells due to the natural abundance and low toxicity of the constituent elements. Here, we present the first reported synthesis of colloidal CZTS nanocrystals using a simple solution-phase method. Solar cells fabricated using selenized CZTS nanocrystal inks had a power conversion efficiency of 0.74% under AM1.5G illumination.

Characterization of lipid-templated silica and hybrid thin film mesophases by grazing incidence small-angle X-ray scattering

The nanostructure of silica and hybrid thin film mesophases templated by phospholipids via an evaporation-induced self-assembly (EISA) process was investigated by grazing-incidence small-angle X-ray scattering (GISAXS). Diacyl phosphatidylcholines with two tails of 6 or 8 carbons were found to template 2D hexagonal mesophases, with the removal of lipid from these lipid/silica films by thermal or UV/O3 processing resulting in a complete collapse of the pore volume. Monoacyl phosphatidylcholines with single tails of 10-14 carbons formed 3D micellular mesophases; the lipid was found to be extractable from these 3D materials, yielding a porous material. In contrast to pure lipid/silica thin film mesophases, films formed from the hybrid bridged silsesquioxane precursor bis(triethoxysilyl)ethane exhibited greater stability toward (both diacyl and monoacyl) lipid removal. Ellipsometric, FTIR, and NMR studies show that the presence of phospholipid suppresses siloxane network formation, while actually promoting condensation reactions in the hybrid material. 1D X-ray scattering and FTIR data were found to be consistent with strong interactions between lipid headgroups and the silica framework.

Sulfide nanocrystal inks for dense Cu(In1-xGa(x))(S1-ySe(y))2 absorber films and their photovoltaic performance

Recent developments in the colloidal synthesis of high quality nanocrystals have opened up new routes for the fabrication of low-cost efficient photovoltaic devices. Previously, we demonstrated the utility of CuInSe(2) nanocrystals in the fabrication of CuInSe(2) thin film solar cells. In those devices, sintering the nanocrystal film yields a relatively dense CuInSe(2) film with some void space inclusions. Here, we present a general approach toward eliminating void space in sintered nanocrystal films by utilizing reactions that yield a controlled volume expansion of the film. This is demonstrated by first synthesizing a nanocrystal ink composed of Cu(In(1-x)Ga(x))S(2) (CIGS). After nanocrystal film formation, the nanocrystals are exposed to selenium vapor during which most of the sulfur is replaced by selenium. Full replacement produces a approximately 14.6% volume expansion and reproducibly leads to good dense device-quality CIGSSe absorber films with reduced inclusion of void space. Solar cells made using the CIGSSe absorber films fabricated by this method showed a power conversion efficiency of 4.76% (5.55% based on the active nonshadowed area) under standard AM1.5 illumination.

Development of CuInSe2 nanocrystal and nanoring inks for low-cost solar cells

The creation of a suitable inorganic colloidal nanocrystal ink for use in a scalable coating process is a key step in the development of low-cost solar cells. Here, we present a facile solution synthesis of chalcopyrite CuInSe 2 nanocrystals and demonstrate that inks based on these nanocrystals can be used to create simple solar cells, with our first cells exhibiting an efficiency of 3.2% under AM1.5 illumination. We also report the first solution synthesis of uniform hexagonal shaped single crystals CuInSe 2 nanorings by altering the synthesis parameter.

Mass transport and electrode accessibility through periodic self-assembled nanoporous silica thin films

Ordered nanoporous silica films have attracted great interest for their potential use to template nanowires for photovoltaics and thermoelectrics. However, it is crucial to develop films such that an electrode under the nanoporous film is accessible to solution species via facile mass transport through well-defined pores. Here, we quantitatively measure the electrode accessibility and the effective species diffusivity for nearly all the known nanoporous silica film structures formed by evaporation-induced self-assembly upon dip-coating or spin-coating. Grazing-angle of incidence small-angle X-ray scattering was used to verify the nanoscale structure of the films and to ensure that all films were highly ordered and oriented. Electrochemical impedance spectroscopy (EIS) was then used to assess the transport properties. A model has been developed that separates the electrode/film kinetics and the film transport properties from the film/solution interface and bulk solution effects. Accounting for this, the accessible area of the nanoporous film coated FTO electrode (1-theta) is obtained from the high-frequency data, while the effective diffusivity of the ferrocene dimethanol (D(FDM)) redox couple is obtained from intermediate frequencies. It was found that the degree of order and orientation in the film, in addition to the symmetry/topology, is a dominant factor that determines these two key parameters. The EIS data show that the (211) oriented double gyroid, (110) oriented distorted body center cubic, and (211) distorted primitive cubic silica films have significant accessibility (larger than 26% of geometric area). However, the double-gyroid films showed the highest diffusivity by over an order of magnitude. Both the (10) oriented 2D hexagonal and (111) oriented rhombohedral films were found to be highly blocking with only small accessibility due to microporosity. The impedance data were also collected to study the stability of the nanoporous silica films in aqueous solutions as a function of pH. The distorted primitive silica film showed much faster degradation in pH 7 solution when compared to a blocking film such as the 2D hexagonal. However, silica films maintained their structure at pH 2 for at least 12 h.

Controlling interfacial curvature in nanoporous silica films formed by evaporation-induced self-assembly from nonionic surfactants. II. Effect of processing parameters on film structure

The double-gyroid phase of nanoporous silica films has been shown to possess facile mass-transport properties and may be used as a mold to fabricate a variety of highly ordered inverse double-gyroid metal and semiconductor films. This phase exists only over a very small region of the binary phase diagram for most surfactants, and it has been very difficult to synthesize metal oxide films with this structure by evaporation-induced self-assembly (EISA). Here, we show the interplay of the key parameters needed to synthesize these structures reproducibly and show that the interfacial curvature may be systematically controlled. Grazing angle of incidence small-angle X-ray scattering (GISAXS) is used to determine the structure and orientation of nanostructured silica films formed by EISA from dilute silica/(poly(ethylene oxide)-b-poly(propylene oxide)-b-alkyl) surfactant solutions. Four different highly ordered film structures are observed by changing only the concentration of the surfactant, the relative humidity during dip-coating, and the aging time of the solution prior to coating. The highly ordered films progress from rhombohedral (Rm) to 2D rectangular (c2m) to double-gyroid (distorted Iad) to lamellar systematically as interfacial curvature decreases. Under all experimental conditions investigated, increasing the aging time of the coating solution was found to decrease the interfacial curvature. In particular, this parameter was critical to being able to synthesize highly ordered, pure-phase double-gyroid films. The key role of the aging time is shown via processing diagrams that map out the interplay between the aging time, composition, and relative humidity. 29Si nuclear magnetic resonance (NMR) spectroscopy and solution-phase small-angle X-ray scattering (SAXS) of the aged coating solutions presented in part I of this series are then used to interpret the effects of aging prior to dip-coating. Specifically, it was found that a predictive model based on volume fractions and the silica cluster stoichiometry obtained from 29Si NMR qualitatively explains the trends observed with composition and aging. However, apart from the effects of relative humidity, a quantitative comparison of the predicted phase with the experimental processing diagram suggests that less-condensed silica clusters are more effective at swelling the EO blocks at early aging times. This enhanced swelling decreases with aging time and results in lower-curvature nanostructures such as the double-gyroid. The decrease in swelling is attributed to the decreased thermodynamic driving force for the more-condensed silica clusters to mix with the EO block of the surfactant.

Controlling interfacial curvature in nanoporous silica films formed by evaporation-induced self-assembly from nonionic surfactants. I. Evolution of nanoscale structures in coating solutions

The double-gyroid phase of nanoporous silica films formed by evaporation-induced self-assembly (EISA) has been shown to possess facile mass-transport properties and may be used as a robust template for the nanofabrication of metal and semiconductor nanostructures. Recently, we developed a new synthesis of double-gyroid nanoporous silica films where the aging time of the coating solution prior to EISA was the key parameter required to control the interfacial curvature that results upon self-assembly of the film. Here, we use 29Si nuclear magnetic resonance (NMR) and small-angle X-ray scattering (SAXS) to investigate the nanoscale structure of the coating solutions used to obtain double-gyroid nanoporous silica films. NMR and SAXS were carried out on the water, ethanol, silica, and poly(ethylene oxide)-b-poly(propylene oxide)-b-alkyl (EO17-PO12-C14) surfactant coating solutions as well as similar solutions that excluded either the silica or the surfactant. NMR data reveal that the silica monomers in the coating solution condense very rapidly to form rings and connected ring species. After 1 day of aging, all monomers and dimers have disappeared, and the distribution is dominated by Q2 and Q3 species, where the superscript in Qn describes the number of silicon atoms in the second coordination shell of the central silicon. Over the course of the next 9 days, the Q3 population slowly rises at the expense of the Q2 and Q3t populations. Absolute intensity SAXS measurements reveal that the size of the silica clusters increases steadily during this aging period, reaching an average radius of gyration of 9.0 A after 9 days of aging. Longer aging results in the continued growth of clusters with a mass fractal dimension of 1.8. Absolute intensity SAXS data also reveals that micelles are not present in the coating solution. At 9% volume fraction of surfactant, the coating solution is far above the aqueous critical micellar concentration. However, even a small amount of ethanol inhibits micellization. SAXS data also shows that when surfactant is present the radius of gyration is larger but increases more slowly. This indicates that there are weak associative interactions between the silica clusters and surfactant in solution that reduce the cluster-cluster growth rate. In part II of this work, we use the results discovered here to interpret the effects of aging on interfacial curvature in the nanostructured films that self-assemble from these solutions.

Synthesis of ordered mesoporous zirconium phosphate films by spin coating and vapor treatments

Ordered mesoporous zirconium phosphate films were prepared on a silicon substrate by spin coating using a mixture of zirconium isopropoxide, triethyl phosphate, Pluronic P123 triblock copolymer, nitric acid, ethanol, and water. The spin-on film was consecutively treated with vapors of phosphoric acid and ammonia. The post-vapor treatments effectively enhanced the thermal stability of an ordered mesostructure when heated to 500 degrees C. XRD and TEM analyses show that the calcined zirconium phosphate film has a hexagonal structure with straight channels parallel to the film surface. The zirconium phosphate film exhibited high proton conductivity of 0.02 S/cm parallel to the film surface at 80% RH and 25 degrees C.

Direct Probing of Sorbent−Solvent Interactions for Amylose Tris(3,5-dimethylphenylcarbamate) Using Infrared Spectroscopy, X-ray Diffraction, Solid-state NMR, and DFT Modeling

The sorbent-solvent interactions for amylose tris(3, 5-dimethylphenylcarbamate) (ADMPC) with five commonly used solvents, hexane, methanol, ethanol, 2-propanol (IPA), and acetonitrile (ACN), are studied using attenuated total reflection infrared spectroscopy (ATR-IR) of thin sorbent films, X-ray diffraction (XRD) of thin films, (13)C cross polarization/magic angle spinning (CP/MAS) and MAS solid state NMR of polymer-coated silica beads (commercially termed "Chiralpak AD"), and DFT modeling. The ADMPC-polymer-coated silica beads are used commercially for analytical and preparative scale separations of chiral enantiomers. The polymer forms helical rods with intra- and inter-rod hydrogen bonds (H-bonds). There are various nm-sized cavities formed between the polymer side-chains and rods. The changes in the H-bonding states of the C=O and NH groups of the polymer upon absorption of each of the five solvents at 25 degrees C are determined with ATR-IR. The IR wavenumbers, the H-bonding interaction energies, and the H-bonding distances of the polymer side-chains with each of the solvent molecules are predicted using the DFT/B3LYP/6-311+g(d,p) level of theory. The changes in the polymer crystallinity upon absorption of each solvent are characterized with XRD. The changes in the polymer crystallinity and the H-bonding states of C=O groups are also probed with (13)C CP/MAS solid-state NMR. The changes in the polymer side-chain mobility are detected using (13)C MAS solid-state NMR. The H-bonding states of the polymer change upon absorption of each polar solvent and usually result in an increase in the polymer crystallinity and the side-chain mobility. The polymer rods are reorganized upon solvent absorption, and the distance between the rods increases with the increase in the solvent molecular size. These results have implications for understanding the role of the solvent in modifying the structure and behavior of the polymer sorbents.

Ion Transport in the Microporous Titanosilicate ETS-10

Impedance spectroscopy was used to investigate ion transport in the microporous crystalline framework titanosilicate ETS-10 in the frequency range from 1 Hz to 10 MHz. These data were compared to measured data from the microporous aluminosilicate zeolite X. Na-ETS-10 was found to have a lower activation energy for ion conduction than that of NaX, 58.5 kJ/mol compared to 66.8 kJ/mol. However, the dc conductivity and ion hopping rate for Na-ETS-10 were also lower than NaX. This was found to be due to the smaller entropy contribution in Na-ETS-10 because of its high cation site occupancy. This was verified by ion exchanging Na(+) with Cu(2+) in both microporous frameworks. This exchange decreases the cation site occupancy and reduces correlation effects. The exchanged Cu-ETS-10 was found to have both lower activation energy and higher ionic conductivity than CuX. Zeolite X has the highest ion conductivity among the zeolites, and thus the data shown here indicate that ETS-10 has more facile transport of higher valence cations which may be important for ion-exchange, environmental remediation of radionucleotides, and nanofabrication.

Versatile Fabrication of Distorted Cubic Mesoporous Silica Film Using CTAB Together with a Hydrophobic Organic Additive

We have succeeded in the fabrication of distorted cubic R-3m mesoporous silica film with 3D open mesostructure providing high pore accessibility from the surface-air interface of the film. The film fabrication involves a unique approach of adding 1,3,5- triisopropylbenzene (TIPB) to the silica/cetyltrimethylammoniumbromide (CTAB) coating sol during its preparation. The addition of TIPB induces the formation of spheroid micelles, promoting R-3m mesostructure in the film. This phase does not exist in the corresponding binary phase diagram of CTAB surfactant.

Simulation and Interpretation of 2D Diffraction Patterns from Self-Assembled Nanostructured Films at Arbitrary Angles of Incidence: From Grazing Incidence (Above the Critical Angle) to Transmission Perpendicular to the Substrate

A method to calculate the location of all Bragg diffraction peaks from nanostructured thin films for arbitrary angles of incidence from just above the critical angle to transmission perpendicular to the film is reported. At grazing angles, the positions are calculated using the distorted wave Born approximation (DWBA), whereas for larger angles where the diffracted beams are transmitted though the substrate, the Born approximation (BA) is used. This method has been incorporated into simulation code (called NANOCELL) and may be used to overlay simulated spot patterns directly onto two-dimensional (2D) grazing angle of incidence small-angle X-ray scattering (GISAXS) patterns and 2D SAXS patterns. The GISAXS simulations are limited to the case where the angle of incidence is greater than the critical angle (alpha(i) > alpha(c)) and the diffraction occurs above the critical angle (alpha(f) > alpha(c)). For cases of surfactant self-assembled films, the limitations are not restrictive because, typically, the critical angle is around 0.2 degrees but the largest d spacings occur around 0.8 degrees 2theta. For these materials, one finds that the DWBA predicts that the spot positions from the transmitted main beam deviate only slightly from the BA and only for diffraction peaks close the critical angle. Additional diffraction peaks from the reflected main beam are observed in GISAXS geometry but are much less intense. Using these simulations, 2D spot patterns may be used to identify space group, identify the orientation, and quantitatively fit the lattice constants for SAXS data from any angle of incidence. Characteristic patterns for 2D GISAXS and 2D low-angle transmission SAXS patterns are generated for the most common thin film structures, and as a result, GISAXS and SAXS patterns that were previously difficult to interpret are now relatively straightforward. The simulation code (NANOCELL) is written in Mathematica and is available from the author upon request.

Order and orientation control of mesoporous silica films on conducting gold substrates formed by dip-coating and self-assembly: a grazing angle of incidence small-angle X-ray scattering and field emission scanning electron microscopy study

Grazing-angle of incidence small-angle X-ray scattering (GISAXS) and high-resolution field emission scanning electron microscopy have been used to characterize the mesophase symmetry, orientation, and long-range order in PEO20-PPO70-PEO20 (Pluronic P123) templated mesoporous silica thin films on conducting gold substrates as a function of silica-to-ethylene oxide (Si/EO) block ratio and relative humidity (RH). The films are formed by dip-coating followed by evaporation-induced self-assembly under tightly controlled RH. The general evolution of the mesophase follows the trends that are expected based on shape factors due to swelling of the PEO block. However, changes in orientation of the nanostructure relative to the substrate and the degree of long-range order are found to depend on Si/EO ratio. These effects are likely due to the dynamics of evaporation and self-assembly. Generally, at Si/EO ratios lower than 3.29, the films contained regions where the nanostructure was not oriented relative to the plane of the substrate. However, for Si/EO ratios greater than 3.62, conditions were found where the nanostructure of the film was highly oriented relative to the plane of the substrate. This is true over the range of RH studied, independent of the nanostructure symmetry. For low Si/EO ratios at the highest RH levels, the films were composed of a mixture of spherical and cylindrical pores. At high Si/EO ratios and high RH levels, the films had a highly oriented R-3m nanostructure but displayed streaking perpendicular to the substrate in the Bragg spots on GISAXS patterns. This streaking is interpreted as faulting along planes parallel to the substrate.

Synthesis of Thermally Stable Highly Ordered Nanoporous Tin Oxide Thin Films with a 3D Face-Centered Orthorhombic Nanostructure

Thin films of nanoporous tin oxide with a 3D face-centered orthorhombic nanostructure have been synthesized by self-assembly that is controlled by post-coating thermal treatment under controlled humidity. In contrast to the conventional evaporation-induced self-assembly (EISA), the films here have no ordered nanostructure after dip-coating. However, the initial coatings are formed under conditions that inhibit significant hydrolysis and condensation for extended periods. This allows the use of postsynthesis thermal vapor treatments to completely control the formation of the nanostructure. With EO106-PO70-EO106 (Pluronic F127) triblock copolymer as the template, highly ordered nanostructures were generated by exposing the disordered films to a stream of water vapor at elevated temperature, which rehydrates the films and allows the formation of the thermodynamically favored phase. Further exposure to water vapor drives the condensation reaction through the elimination of HCl. The X-ray diffraction pattern from the nanostructure was indexed in the space group Fmmm as determined by analysis of 2D small-angle X-ray scattering patterns at various angles of incidence. The nanostructure is then stabilized and made nanoporous by extended controlled thermal treatments. After self-assembly and template removal, the films are thermally stable up to 600 degrees C and retain an ordered, face-centered orthorhombic nanostructure.

A titanosilicate molecular sieve with adjustable pores for size-selective adsorption of molecules

Zeolites and related crystalline microporous oxides-tetrahedrally coordinated atoms covalently linked into a porous framework-are of interest for applications ranging from catalysis to adsorption and ion-exchange. In some of these materials (such as zeolite rho) adsorbates, ion-exchange, and dehydration and cation relocation can induce strong framework deformations. Similar framework flexibility has to date not been seen in mixed octahedral/tetrahedral microporous framework materials, a newer and rapidly expanding class of molecular sieves. Here we show that the framework of the titanium silicate ETS-4, the first member of this class of materials, can be systematically contracted through dehydration at elevated temperatures to 'tune' the effective size of the pores giving access to the interior of the crystal. We show that this so-called 'molecular gate' effect can be used to tailor the adsorption properties of the materials to give size-selective adsorbents suitable for commercially important separations of gas mixtures of molecules with similar size in the 4.0 to 3.0 A range, such as that of N2/CH4, Ar/O2 and N2/O2.