Quantitative in situ observation of evaporation-induced micellization in EISA mesoporous oxide films: sub-optical template sizes and realistic deposition regimes

Evaporation-induced self-assembly (EISA) is the standard route to ordered mesoporous oxide films, yet its earliest step — evaporation-triggered formation of micelles in the liquid precursor — was directly imaged in real space only once, by Kimura, Shintate and Miyamoto (Chem. Commun. 2015), using label-free resonant-scanner confocal laser scanning microscopy (CLSM) on a very high-molecular-weight PS-b-PEO/titania sol–gel droplet. The demonstration works only for apparent in-liquid sizes of 150–200 nm: lower-molecular-weight block copolymers and common surfactant templates fall below the ~200–250 nm diffraction-limited resolution, apparent sizes are biased by unquantified artifacts (finite optical slice thickness, Brownian-motion blurring, focal-depth dependence), and the observation was made on a static evaporating droplet rather than under rapid spin-coated film formation. The surviving open problem is to make the liquid-phase micellization step of EISA directly observable and quantitatively measurable for the sub-optical template size range actually used in mesoporous film fabrication — with non-perturbing contrast strategies, artifact correction recovering true in-liquid micelle size distributions, and explicit attention to the gap between slow-droplet and rapid-deposition processing regimes. No later work is known to discharge this objective, so the core remains open.

Background

Ordered mesoporous oxide films (e.g., titania) are typically made by sol–gel chemistry combined with supramolecular templating: a metal alkoxide precursor is hydrolyzed in an acidic solution containing an amphiphilic template — a diblock copolymer such as poly(styrene)-block-poly(ethylene oxide) (PS-b-PEO) or a commercial surfactant — and the solution is deposited on a substrate. As solvent evaporates, the dissolved amphiphiles are concentrated past their effective critical micelle concentration, form micelles that incorporate inorganic species into 'composite micelles', pack into ordered arrays, and the inorganic network condenses around them; calcination removes the template and leaves an ordered pore network. This scenario is called evaporation-induced self-assembly (EISA) and underpins films used in photocatalysis, sensing, photovoltaics, and separation. The most industrially used templates are low-molecular-weight surfactants: alkyltrimethylammonium halides ($C_nTMA$), alkyl-ethylene-oxide nonionics ($C_nEO_m$), and Pluronic-type triblock copolymers ($EO_nPO_mEO_n$), which template pores of roughly 2–10 nm; high-molecular-weight PS-b-PEO can template tens-of-nanometers pores.

Until 2015 the EISA mechanism rested on indirect evidence: ex situ electron microscopy of dried or calcined films, dye-probe polarization, and in situ scattering. The reference point for the latter is the study of Grosso et al. (Chem. Mater. 14, 931, 2002), who followed cetyltrimethylammonium bromide (CTAB)–silica film formation during dip coating by time-resolved small-angle X-ray scattering (SAXS — scattering at small angles from nanometer-scale electron-density periodicities) combined with interferometry; such synchrotron methods are ensemble-averaged, so transient, localized events like the birth of individual micelles are not resolved in real space. Yang et al. (Langmuir 27, 1700, 2010) observed EISA in drying droplets under controlled pressure and temperature, again without single-micelle real-space resolution.

Kimura, Shintate and Miyamoto (Chem. Commun. 51, 1230, 2015) filled this gap partially with confocal laser scanning microscopy (CLSM — laser-scanning optical microscopy in which a pinhole rejects out-of-focus light, enabling optical sectioning of the liquid). Using a resonant (rotating-mirror) scanner achieving 30 frames per second, 405 nm excitation, and label-free detection of scattered light, they imaged a 2 µL static droplet of a titania sol–gel precursor containing $PS_{100000}$-b-$PEO_{150000}$ with pre-hydrolyzed titanium isopropoxide. Spherical scattering objects appeared 35–45 s into evaporation, while dynamic light scattering (DLS) and synchrotron SAXS confirmed the initial solution was free of aggregates — direct real-space evidence that micellization is induced by evaporation. The objects showed apparent diameters of 150–200 nm, versus ~100 nm pores in the calcined film.

The demonstration is bounded by optical physics and processing realism. First, the diffraction-limited lateral resolution at 405 nm is ~200–250 nm, so lower-molecular-weight copolymers ($PS_{58600}$-b-$PEO_{71000}$ and $PS_{40000}$-b-$PEO_{53000}$, which template ~60 nm and ~40 nm pores) and all common surfactant templates could not be resolved at all; whether and how they micellize in the liquid precursor therefore remains unobserved. Second, the apparent 150–200 nm sizes are known to be inflated by unquantified artifacts — the finite confocal optical slice thickness (~500 nm for the pinhole setting), blurring from Brownian motion of the micelles during the fast raster scan, and dependence on the focal ($Z$) position — and no deconvolution or calibration was performed, so the true in-liquid size distribution is unknown. Third, the suggested remedies (higher-numerical-aperture objectives — the dimensionless measure of light-gathering angle that sets resolution —, shorter excitation wavelengths, and fluorescent labels confined to micelle cores) each risk perturbing the chemistry they are meant to observe, and none has been validated. Fourth, the imaging was done on a quiescent droplet evaporating over tens of seconds, whereas actual films are made by spin coating, where a thin layer dries in seconds under strong shear and convection; whether the droplet-observed pathway operates during spin coating is unestablished. Real-space imaging of polymer micelles has since advanced in other contexts — liquid-cell transmission electron microscopy (LC-TEM — TEM through a thin liquid cell, enabling nanometer-scale imaging of solvated objects) has shown block-copolymer micelle fusion, growth, and fragmentation in quiescent solutions (Parent et al., JACS 2017; Early, Yager and Lodge, ACS Macro Lett. 2020) — but not in evaporating sol–gel film precursors, where beam damage and confinement geometry are unresolved obstacles. In situ spectroscopic ellipsometry (Bilchak et al., ACS Macro Lett. 2020) now tracks block-copolymer self-assembly kinetics, but only as an ensemble-averaged optical observable. Deep-learning analysis of noisy CLSM images has been demonstrated for detecting colloidal nanosheets (Fujioka et al., Key Eng. Mater. 804, 11, 2019) but not for sub-resolution micelle sizing in EISA. The combination of sub-optical resolution, quantitative size recovery, non-perturbation, and processing-regime transfer for the EISA micellization step therefore remains an open, coherent objective.

Problem Statement

Make the early-stage, liquid-phase micellization step of evaporation-induced self-assembly (EISA) directly observable and quantitatively measurable for the template systems and deposition regimes actually used in mesoporous oxide film fabrication, as left open by Kimura, Shintate and Miyamoto (2015). As framed by the limitations of that demonstration, the goal has three linked core requirements. (1) Sub-optical observation: extend direct, time-resolved, real-space observation of evaporation-induced micellization in sol-gel film precursors below the ~200-250 nm diffraction-limited resolution of the demonstrated label-free 405 nm resonant-scanner CLSM, reaching the tens-of-nanometers assemblies of lower-molecular-weight diblock copolymers (e.g., $PS_{58600}$-b-$PEO_{71000}$ and $PS_{40000}$-b-$PEO_{53000}$, which template ~60 nm and ~40 nm pores) and common surfactant templates ($C_nTMA$, $C_nEO_m$, and Pluronic-type $EO_nPO_mEO_n$), provided that the contrast strategy used does not alter the intrinsic evaporation-driven self-assembly and inorganic condensation it is meant to observe (e.g., unchanged micellization onset and unchanged final film structure relative to unlabeled references). (2) Quantitative size recovery: establish how finite optical slice thickness (~500 nm), Brownian-motion blurring during fast scanning, and focal ($Z$) position dependence bias apparent in-liquid sizes, and provide a correction scheme that recovers true in-liquid micelle size distributions, so that quantitative connections between in-liquid micelle sizes, including framework-shrinkage effects, and the pore sizes of the final calcined films become possible. (3) Processing-regime transfer: in situ observation has so far been demonstrated only under slow, static-droplet evaporation; establish whether the observed in-liquid assembly pathway also governs film formation under rapid, shear-dominated spin-coated deposition, and how assembly pathways link to final film microstructure across deposition conditions. All three requirements are mandatory and jointly required rather than alternative options: the corrected size distribution of (2) presupposes the independent real-space observation of (1) and does not by itself discharge it, (2) is to be applied to the same template systems for which (1) was achieved, and (3) presupposes both under the deposition regime it addresses, so the task is resolved only when all three are met. The template families named in (1) delimit the scope of the sub-optical class to be reached; a demonstration in at least one representative of the lower-molecular-weight diblock copolymers and at least one representative of the common surfactant templates is required, whereas a single-family demonstration does not discharge (1).

References

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