Air-solution Interface Research Articles

Preparation of efficient and stable FA0.75MA0.25SnI3 perovskite solar cells using passivation materials with multiple F hydrophobic group

Although Sn-based perovskite solar cells (PSC) have impressive conversion efficiency, their stability needs to be improved. Herein, 3,4,5-trifluorophenol (C6H3F3O) containing –F and –OH groups was introduced to improve the film stability and quality of FA0.75MA0.25SnI3. –F groups with low surface energy cause C6H3F3O to spontaneously migrate to the air-solution interface and attract the SnI64− octahedron to the solution-air surface through hydrogen bond interactions between –OH and I. This leads to preferential nucleation of dense, high-quality perovskite films at the solution-air surface and orderly growth from top to bottom. Additionally, C6H3F3O is bonded to the surface of the film, and the outward-facing hydrophobic –F groups effectively shield FA0.75MA0.25SnI3 from water infiltration. The C6H3F3O-doped PSC exhibited a champion efficiency of 10.47 % and long-term stability of over 1000 h, retaining 80 % of its initial efficiency (in N2).

Trapping light, revealing properties: Laser trapping as a powerful tool for photoluminescence spectroscopy

Laser trapping, a non-contact technique for manipulating microscopic objects, has gained prominence in scientific research. When coupled with fluorescence spectroscopy, it offers a powerful tool for exploring material properties. This study demonstrates the application of laser trapping in the crystallization of MAPbBr3 perovskites and its simultaneous use for photoluminescence imaging. MAPbBr3 perovskites are a class of materials with exceptional optical properties, making them attractive for various optoelectronic applications. However, conventional excitation methods using UV light can lead to phase segregation and mechanical distortion of these materials. Two-photon excitation, on the other hand, offers advantages such as deeper penetration and reduced scattering interference. In this research, we utilize a 1064 nm continuous wave laser for both trapping and excitation purposes. The MAPbBr3 perovskite, with its absorption band ranging from 400 to 540 nm, exhibits two-photon excitation at 532 nm. By focusing the laser beam at the air-solution interface, we successfully crystallize MAPbBr3 perovskites from an unsaturated precursor solution. Simultaneously, the same laser source is used for photoluminescence imaging, allowing for real-time analysis of the crystal's emission properties. This approach eliminates the need for additional excitation sources and simplifies the experimental setup. The combination of laser trapping and two-photon excitation opens up new possibilities for studying perovskite materials. It provides a gentle and non-invasive method for manipulating and characterizing hybrid perovskites materials, paving the way for advancements in various fields such as optoelectronics and energy harvesting.

Optical trapping-induced crystallization promoted by gold and silicon nanoparticles.

This study investigates the promotion of sodium chlorate (NaClO3) crystallization through optical trapping, enhanced by the addition of gold nanoparticles (AuNPs) and silicon nanoparticles (SiNPs). Using a focused laser beam at the air-solution interface of a saturated NaClO3 solution with AuNPs or SiNPs, the aggregates of these particles were formed at the laser focus, the nucleation and growth of metastable NaClO3 (m-NaClO3) crystals were induced. Continued laser irradiation caused these m-NaClO3 crystals to undergo repeated cycles of growth and dissolution, eventually transitioning to a stable crystal form. Our comparative analysis showed that AuNPs, due to their significant heating due to higher photon absorption efficiency, caused more pronounced size fluctuations in m-NaClO3 crystals compared to the stable behavior observed with SiNPs. Interestingly, the maximum diameter of the m-NaClO3 crystals that appeared during the size fluctuation step was consistent, regardless of nanoparticle type, concentration, or size. The crystallization process was also promoted by using polystyrene nanoparticles, which have minimal heating and electric field enhancement, suggesting that the reduction in activation energy for nucleation at the particle surface is a key factor. These findings provide critical insights into the mechanisms of laser-induced crystallization, emphasizing the roles of plasmonic heating, particle surfaces, and optical forces.

Gold Nanoparticles at a Liquid Interface: Towards a Soft Nonlinear Metasurface

Optical second-harmonic generation (SHG) is achieved using adsorbed gold nanoparticles (AuNPs) with an average diameter of 16 nm at the aqueous solution–air interface in reflection. A detailed analysis of the depth profile of the SHG intensity detected shows that two contributions appear in the overall signal, one arising from the aqueous solution–air interface that is sensitive to the AuNP surface excess and one arising from the bulk aqueous phase. The latter is an incoherent signal also known as hyper-Rayleigh scattering (HRS). The results agree with those of an analysis involving Gaussian beam propagation optics and a Langmuir-like isotherm. Discrepancies are revealed for the largest AuNP concentrations used and indicate a new route for the design of soft metasurfaces.

Aromatic salt induced modulations in micellization behavior of amide functionalized surface-active ionic liquids: The role of cationic head group

The present study highlights the effect of aromatic salt i.e. sodium benzoate (NaBz) at fix concentration (1 mM), on the micellization behavior of four amide functionalized surface-active ionic liquids (SAILs) having different cationic head groups in aqueous medium. The adsorption behavior of the SAILs at the air-solution interface has been investigated by tensiometry, where improved surface activity was observed in all the investigated SAILs in the presence of added hydrotrope. The conductivity, steady-state fluorescence, dynamic light scattering (DLS), and zeta-potential measurements have been performed to probe the micellization of the SAILs in bulk solution. The position and orientation of aromatic anion, Bz-, at the micelle-water interface surface has been investigated by 1H NMR spectroscopy. The thermodynamic aspects of micellization have been examined by Isothermal titration calorimetry (ITC). It has been observed that the nature, geometry, and steric factor of the cationic head group of SAILs govern their interactions with Bz- and this influences the characteristic properties of micellization.

Pressure Changes Across a Membrane Formed by Coacervation of Oppositely Charged Polymer-Surfactant Systems.

We investigate the mass transfer and membrane growth processes during capsule formation by the interaction of the biopolymer xanthan gum with CnTAB surfactants. When a drop of xanthan gum polymer solution is added to the surfactant solution, a membrane is formed by coacervation. It encapsulates the polymer drop in the surfactant solution. The underlying mechanisms and dynamic processes during capsule formation are not yet understood in detail. Therefore, we characterized the polymer-surfactant complex formation during coacervation by measuring the surface tension and surface elasticity at the solution-air interface for different surfactant chain lengths and concentrations. The adsorption behavior of the mixed polymer-surfactant system at the solution-air interface supports the understanding of observed trends during the capsule formation. We further measured the change in capsule pressure over time and simultaneously imaged the membrane growth via confocal microscopy. The cross-linking and shrinkage during the membrane formation by coacervation leads to an increasing tensile stress in the elastic membrane, resulting in a rapid pressure rise. Afterward, the pressure gradually decreases and the capsule shrinks as water diffuses out. This is not only due to the initial capsule overpressure but also due to osmosis caused by the higher ionic strength of the surfactant solution outside the capsule compared to the polymer solution inside the capsule. The influence of polymer concentration and surfactant type and concentration on the pressure changes and the membrane structure are studied in this work, providing detailed insights into the dynamic membrane formation process by coacervation. This knowledge can be used to produce capsules with tailored membrane properties and to develop a suitable encapsulation protocol in technological applications. The obtained insights into the mass transfer of water across the capsule membrane are important for future usage in separation techniques and the food industry and allow us to better predict the capsule time stability.

Coiling of Secondary Tubes Formed from the Colloidal Exhaust of Primary Chemical Gardens.

Chemical gardens are self-organized precipitate structures such as thin-walled tubes and membrane-bound cells reminiscent of biological shapes. These usually inorganic precipitates compartmentalize the reaction system and allow the study of materials synthesis in very steep concentration gradients. We create such tubes by steadily injecting a mixture of MnCl2 and CuSO4 solutions into a large reservoir of sodium silicate solution. The growing tube is open at its tip and ejects a stream of colloidal particles that aggregate to form a secondary tube above the original one. This secondary tube can coil into a tightly wound nest-like structure, freely suspended underneath the solution-air interface. Using three-dimensional image reconstruction, we analyze the onset of coiling and show that the structure is helical with a helix radius that increases in the vertical direction. The height at which the coiling begins is lowered with each successive repeat of the growth experiment, suggesting that coiling is induced by small variations in the density of the silicate solution.

Enantioselective control in chiral crystallization of ethylenediamine sulfate using optical trapping with circularly polarized laser beams.

In this study, we conducted successful experiments on ethylenediamine sulfate (EDS), an organic compound, to investigate its enantioselectivity in chiral crystallization. We employed optical trapping with circularly polarized laser beams, using a continuous wave laser at 1064nm. By focusing the laser at the air-solution interface of a heavy water-saturated EDS solution, the formation of sub-micrometer-sized chiral EDS crystals was verified. Two generated enantiomorphs (d-crystal and l-crystal) were identified by the rotating analyzer method. The enantioselectivity in the chiral crystallization of EDS was assessed through 30 to 60 times experiments conducted under various conditions of laser powers and polarization modes, utilizing the count of generated crystals for each enantiomorph in the evaluation. Circularly polarized lasers at a specific power created an imbalance in the generation probability of the enantiomorphs, resulting in crystal enantiomeric excess values of 23% and -30%. The enantioselectivity mechanism was explored from two perspectives: refractive index differences of two enantiomorphs and 3D helical optical forces. Study of the thermodynamic mechanism was insufficient to explain the outcomes. Conversely, the 3D helical optical force mechanism revealed that the forces acting on EDS clusters in solution induced helical fluid motion, driving EDS nucleation, with the helicity of fluid motion determining the crystal's chirality. This approach will present new insights into chirality in industrial and research fields, with potential applications in regard to improving optical resolution and addressing the origin of homochirality.

Levitating Droplet Electroanalysis.

Chemical reactions that occur in droplets proceed much differently compared to bulk phases. For instance, many groups have studied droplets during levitation by mass spectrometry and fluorescence to gain more detailed mechanistic insight. Such droplets maximize the probability of solution species interacting with the solution-air interface, an interface that is inherently difficult to probe electrochemically. In this Technical Note, we overcome this limitation by developing a laser-pulled dual-barrel electrode. Having two microwires sealed within the same glass capillary allows one to make two-electrode measurements. We show that the electrode can be positioned within a levitating water droplet and that the voltammetry of a redox indicator (hexacyanoferrate (II/III)) can be observed in real-time. Such foundational measurement tools are important to probe a variety of chemical reactions at complex interfaces.

Microrheology of gemini surfactants at interfaces and in solutions in the dilute and semidilute regimes.

Gemini surfactants are ideal systems to study a wide range of rheological behaviours in soft matter, showing fascinating analogies with living polymers and polyelectrolytes. By only changing the concentration, the shear viscosity can vary by 7 orders of magnitude in the bulk when transitioning through the semidilute regime. In order to elucidate on the intrinsic shear viscosity profile at the interface in soft matter systems manifesting various concentration regimes and morphological transitions, we performed microrheology and adsorption experiments under a wide range of experimental conditions. The surface shear viscosity has been characterized by passive microrheology, tracking Brownian particles trapped at the air-solution interface, under particle wetting conditions precisely characterized by interferometry. We observe that a steep increase in bulk shear viscosity as a function of the concentration does not translate at the interface, which may show a negative surface shear viscosity. By comparing macrorheology and microrheology, we measure significant differences both at the interface and in the bulk in the semidilute regime, where wormlike micelles start to entangle. The disparity in rheological measurements can be attributed to notable depletion effects near both the air-solution and particle-solution interfaces.

The power of microdroplet photochemistry.

This study presents compelling evidence demonstrating that irradiation of the air-solution interface, whether achieved through the spraying of microdroplets into the air or by bubbling air through a solution, significantly accelerates the rate of photochemical reactions by orders of magnitude compared to identical reaction conditions in bulk solutions. We propose this approach as a novel and versatile method for harnessing solar energy in chemical transformations.

Threshold bandgap of lead halide hybrid perovskites for making stable single crystal by NIR laser trapping

The remarkable optoelectronic properties of hybrid halide perovskites position them as pivotal materials for the next generation of electronic devices, LEDs, lasers, and solar cells, potentially revolutionizing these technological domains. In this study, we harness the capabilities of contemporary laser trapping technology—a non-contact method for manipulating particles—to elucidate the conditions necessary for stable perovskite crystal formation. Using a 1064 nm near-infrared (NIR) continuous-wave (CW) laser, commonly selected for trapping experiments, we establish the threshold bandgap essential for the stable crystallization of MAPbX3 perovskite crystals. Our research reveals that MAPbI3, with a bandgap of 1.59 eV, undergoes rapid explosive crystallization when subjected to the 1064 nm laser, making it unsuitable for stable crystal growth. In contrast, MAPbBr3 with a bandgap of 2.26 eV forms stable single crystals within 6 min at the air-solution interface, when exposed to the same 1064 nm NIR laser at 1.0 W output power. Notably, a minor reduction in bandgap to 2.23 eV in the mixed halide MAPbBr2.75I0.25 results in an abrupt transition to explosive crystallization. These findings introduce a novel insight into the bandgap dependency for the crystallization process, delineating a threshold bandgap of 2.26 eV for MAPbBr3, below which the crystal growth becomes unstable under a 1064 nm laser. The study provides a significant advancement in the field by defining the precise bandgap necessary for the formation of robust, stable perovskite crystals using laser trapping techniques. This threshold bandgap knowledge is crucial for developing controlled synthesis protocols for perovskites, optimizing their optoelectronic properties for application in modern technologies. Moreover, the investigation suggests that to achieve stable crystallization of perovskites with a bandgap below 2.26 eV, laser trapping with wavelengths longer than 1064 nm may be required, opening new avenues for material processing and device fabrication.

Evaporation of Concentrated Polymer Solutions Is Insensitive to Relative Humidity.

A recent theory suggests that the evaporation kinetics of macromolecular solutions is insensitive to the ambient relative humidity (RH) due to the formation of a "polarization layer" of solutes at the air-solution interface. We confirm this insensitivity up to RH≈80% in the evaporation of polyvinyl alcohol solutions from open-ended capillaries. To explain the observed drop in evaporation rate at higher RH, we need to invoke compressive stresses due to interfacial polymer gelation. Moreover, RH-insensitive evaporation sets in earlier than theory predicts, suggesting a further role for a gelled "skin." We discuss the relevance of these observations for respiratory virus transmission via aerosols.

Mechanochemical Synthesis of Ethoxyaminohumic Acids and Surface-Active Properties of Their Solutions at Solution–Air Interface

Mechanochemical Synthesis of Ethoxyaminohumic Acids and Surface-Active Properties of Their Solutions at Solution–Air Interface

Fabrication of Oriented MOF-Based Mixed Matrix Membrane via Ion-Induced Synchronous Synthesis.

Developing a facile strategy for constructing oriented mixed matrix membranes (MMMs) with uniformly dispersed and high-loading metal-organic frameworks (MOFs) is a crucial scientific challenge in probing the enhanced capability and potential applications of MOF-polymer MMMs. Herein, a novel synchronous synthetic method for constructing oriented CuBDC/poly(m-phenylenediamine) (CuBDC/PmPD) MMM with uniform MOF dispersion at high loading at the air-solution interface via the dual function of metal ions is reported. The resulting MMM exhibits excellent separation performance in ion sieving and seawater desalination due to the structural integrity of the proposed membrane and the highly interconnected channels created through the oriented distribution of MOF in a polymer matrix. Such a cutting-edge approach may provide promising insights into the development of advanced MMMs with optimized structure and superior performances.

Mechanochemical Synthesis of Ethoxyaminohumic Acids and Surface-Active Properties of Their Solutions at Solution–Air Interface

Ethoxyamine derivatives of humic acids have been obtained by mechanochemical synthesis via the simultaneous interaction of humic acids with poly(ethylene glycol) (PEG-6000 and PEG-1500) and an aminating reagent (urea, hydroperitum, or cyanoguanidine) in a vibrating apparatus. Reaction products have been characterized by IR spectroscopy, acid–base potentiometric titration, and viscometry. Tensiometric and rheological characteristics of the surface layers of solutions of salts of the synthesized derivatives of humic acids have been studied by the pendant drop and oscillating pendant drop methods. The solutions of the salts of ethoxyaminohumic acids have been found to exhibit a pronounced surface activity at the air–water interface. The experimental dependences of the viscoelastic modulus on the surface pressure and the concentration of the solutions of ethoxyaminohumic acid salts are in satisfactory agreement with the functions calculated in terms of the theoretical model of bimolecular adsorption. The presence of amino groups in the structure of ethoxyaminohumic acids predetermines their high solubility in the acidic pH region. The simultaneous incorporation of ethoxy and amino groups into humic acid macromolecules makes it possible to obtain a novel type of surfactants, which combine three functions, i.e., the functions of anionic, cationic, and nonionic surfactants.

Aqueous Radical Initiated Oxidation of an Organic Monolayer at the Air-Water Interface as a Proxy for Thin Films on Atmospheric Aerosol Studied with Neutron Reflectometry.

Neutron reflectometry has been used to study the radical initiated oxidation of a monolayer of the lipid 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) at the air-solution interface by aqueous-phase hydroxyl, sulfate, and nitrate radicals. The oxidation of organic films at the surface of atmospheric aqueous aerosols can influence the optical properties of the aerosol and consequently can impact Earth's radiative balance and contribute to modern climate change. The amount of material at the air-solution interface was found to decrease on exposure to aqueous-phase radicals which was consistent with a multistep degradation mechanism, i.e., the products of reaction of the DSPC film with aqueous radicals were also surface active. The multistep degradation mechanism suggests that lipid molecules in the thin film degrade to form progressively shorter chain surface active products and several reactive steps are required to remove the film from the air-solution interface. Bimolecular rate constants for oxidation via the aqueous phase OH radical cluster around 1010 dm3 mol-1 s-1. Calculations to determine the film lifetime indicate that it will take ∼4-5 days for the film to degrade to 50% of its initial amount in the atmosphere, and therefore attack by aqueous radicals on organic films could be atmospherically important relative to typical atmospheric aerosol lifetimes.

Dimethylpiperazine on Air-Water Interface: Vibrational Sum frequency Generation study of a cyclic tertiary amine

Dimethyl piperazine (DMP) is a weakly basic, cyclic, tertiary diamine. Being a saturated 6-membered ring, it may attain a number of energetically close conformational orientations in the ground state. In a particular environment, its conformational distribution depends on the stabilising factors present in the surroundings. Though soluble in water, it is expected to have substantial surface propensity due to presence of multiple –CH2 and –CH3 bonds. Detection of Bohlmann bands in the C–H stretch region of the Vibrational Sum Frequency Generation (VSFG) spectrum revealed that comparatively higher energy chair conformer with one axial and one equatorial –CH3 bond is present on the pure liquid DMP-air interface. At the air-aqueous solution interface, those bands were found to disappear as the lone electron pair on N-atom is engaged in hydrogen bonding with water molecule. Due to this hydrogen bonding, the peak of “dangling free OH” bonds disappeared from VSFG spectrum of water molecules at air–water interface. Alongwith this hydrogen bonding, the hydrophobic nature of the –CH3 bonds of DMP molecules cause disorganisation of native hydrogen bonded network of water molecules in the air–water interface. Structure and orientation of DMP is expected to be pH-responsive since the chemical conditions of the surrounding medium directly affect the extent of protonation of the amine group. Upon addition of HCl to the bulk, protonated DMP ions, too, could be detected on the air-solution interface by their VSFG spectrum which were considerably different from unprotonated DMP. As the bulk pH of solution changed towards more acidic, spectral features of VSFG spectra of DMP on the surface showed significant change, implying variation in the surface pH, as structure and orientation changes at different degrees of protonation. Since the VSFG spectral features of DMP was found to be pH-responsive, it can be utilised to ascertain the pH of any surface that has DMP absorbed on it. pH-sensitivity of the DMP molecules on air–water interface also suggest that orientation of headgroups will have some effect on the surface orientation of cationic surfactants with amine based headgroups.

Surface enrichment of ionomer in fuel cell catalyst layer prepared using slot-die coating method

The surface enrichment process of the ionomer during the drying of catalyst ink film produced by the slot-die coating method and its effect on fuel cell performance were investigated, wherein the solvent composition of the ink was considered. The slot-die-coated catalyst layer (CL) surface was observed using helium ion microscopy, conductive atomic force microscopy, and X-ray photoelectron spectroscopy, and ionomer enrichment was observed on the slot-die-coated CL surface. Furthermore, spontaneous adsorption of the ionomer at the air–solution interface was observed in the water-rich dispersion by measuring the surface tension of water–alcohol mixed solvents with and without an ionomer. Therefore, two ionomer enrichment mechanisms were proposed based on the alcohol content of the ink: an evaporation-dominated mechanism for alcohol-rich ink and an adsorption-dominated mechanism for water-rich ink. The effect of surface ionomer enrichment on proton conduction depends on the alcohol content of the catalyst ink. In addition, ionomer enrichment is conducive to the reduction of the oxygen transport resistance because of the reduction in the ionomer distributed inside the CL. These findings provide novel insights into the structural evolution and engineering of the CL.

Interfacial Effects during Phase Change in Multiple Levitated Tetrahydrofuran Hydrate Droplets

Recent strategies developed to examine the nucleation of crystal structures like tetrahydrofuran (THF) hydrates without the effects of a solid interface have included acoustic levitation, where only a liquid-gas interface initially exists. However, the ability now exists to levitate and freeze multiple droplets simultaneously, which could reveal interdroplet effects and provide further insight into interfacial nucleation phenomena. In this study, using direct digital and infrared imaging techniques, the freezing of up to three simultaneous THF hydrate droplets was investigated for the first time. Nucleation was initiated at the aqueous solution-air interface. Two pseudo-heterogeneous mechanisms created additional nucleation interfaces: one from cavitation effects entraining microbubbles and another from subvisible ice particles, also called hydrate-nucleating particles (HNPs), impacting the droplet surface. For systems containing droplets in both the second and third positions, nucleation was statistically simultaneous between all droplets. This effect may have been caused by the high liquid-solid interfacial pressures that developed at nucleation, causing some cracking in the initial hydrate shell around the droplet and releasing additional HNPs (now of hydrate) into the air. During crystallization, the THF hydrate droplets developed a completely white opacity, termed optical clarity loss (OCL). It was suggested that high hydrate growth rates within the droplet resulted in the capture of tiny air bubbles within the solid phase. In turn, light refraction through many smaller bubbles resulted in the OCL. These bubbles created structural inhomogeneities, which may explain how the volumetric expansion of the droplets upon complete solidification was 23.6% compared with 7.4% in pure, stationary THF hydrate systems. Finally, the thermal gradient that developed between the top and bottom of the droplet during melting resulted in a surface tension gradient along the air-liquid interface. In turn, convective cells developed within the droplet, causing it to spin rapidly about the horizontal axis.