Ordered Mesoporous to Macroporous Oxides with Tunable Isomorphic Architectures: Solution Criteria for Persistent Micelle Templates

Abstract

Porous and nanoscale architectures of inorganic materials have become crucial for a range of energy and catalysis applications, where the ability to control the morphology largely determines the transport characteristics and device performance. Despite the availability of a range of block copolymer self-assembly methods, the conditions for tuning the key architectural features such as the inorganic wall-thickness have remained elusive. Towards this end we have developed solution processing guidelines that enable isomorphic nanostructures with tunable wall-thickness. A new poly(ethylene oxide-b-hexyl acrylate) (PEOb-PHA) structure directing agent (SDA) was used to demonstrate the key solution design criteria. Specifically, the use of a polymer with a high Flory-Huggins effective interaction parameter, χ, and appropriate solution conditions leads to the kinetic entrapment of persistent micelle templates (PMT) for tunable isomorphic architectures. Solubility parameters are used to predict conditions for maintaining persistent micelle sizes despite changing equilibrium conditions. Here the use of different inorganic loadings controls the inorganic wall-thickness with constant pore size. This versatile method enabled a record 55 nm oxide wall-thickness from micelle coassembly as well as the seamless transition from mesoporous materials to macroporous materials by varying the polymer molar mass and solution conditions. The processing guidelines are generalizable and were elaborated with three inorganic systems, including Nb 2 O 5 , WO 3 , and SiO 2 that were thermally stable to 600 o C for access to crystalline materials. access to extensive pore size regimes that seamlessly span from mesopores to macropores. This concept is demonstrated with a new PEO-b-PHA SDA. The use of a polymer with sufficiently high Flory-Huggins interaction parameter is needed to inhibit micelle re-equilibration that would otherwise change the final pore size with different inorganic loadings. Both micelle fusion-fission and unimer expulsion-insertion reactions may be slowed with appropriate solution conditions that inhibit micelle changes. 68-72 The demonstrated broad range of tunable pore sizes fills the gap typically found between block copolymer approaches and colloidal approaches. The resulting materials were stable to high temperatures and enabled the formation of multiple crystalline oxide frameworks. Experimental Methods Reagents: Anhydrous, inhibitor free THF (>99.9%, Aldrich), Niobium (V) ethoxide (99.9%, Fisher) and Tungsten (VI) chloride (99.9%, Acros) were stored inside a glove box and used as received. Concentrated hydrochloric acid (37 wt% ACS grade, VWR) and Tetraethoxysilane (98%, Alfa Aesar) were used as received. Poly(ethylene glycol) methyl ether (M n 20, 000 g mol-1 , Aldrich) was dried by azeotropic distillation with toluene before use.

Cavitation-enabled rapid and tunable evolution of high-χN micelles as templates for ordered mesoporous oxides

“…Micelles of diverse dimension are needed for different applications. 16 The use of kinetic entrapment to prevent micelles from responding to changing solutions conditions is crucial to decouple the resulting pore size from an adjustable wall thickness. [13][14][15] For micelle templated bioelectrodes, the pore size must balance the need for surface area enhancement against the need for large enough pores to accommodate biological photoactive species.…”

“…For drug delivery, the micelles size determines both the loading capacity 11,12 as well as the capability to cross cell membranes. 16 Block copolymer micelles generally evolve through singlechain exchange, micelle fusion/fission or combination of these processes. 4 In this later example, high molar mass block copolymer micelles generated ∼80 nm macropores in antimony-doped tin oxide electrodes.…”

“…[17][18][19][20] The rate of exchange varies strongly with the thermodynamic barrier for rearrangement. 16 The application of ultrasonic waves to solutions induces continuous cavitation events to form <200 μm diameter bubbles (varies with sonic power and solution) that collapse on the microsecond timescale and can produce ephemeral conditions with pressures of several hundred atmospheres and temperatures up to 5500°C. [21][22][23][24][25] For example, a study by Bates et al found that a high-χN system of poly (ethylene oxide-b-butadiene) in water exhibited negligible chain exchange after 8 days.…”

“…25 A recent study by Epps et al with the same high-χN system showed that stirring over many days activated a novel exchange process that resulted in a bimodal distribution of micelles. 16 The polymer (22.4 mg) was dissolved in 2 mL of anhydrous THF (>99.9%, Aldrich) and 100 μL of 37 wt% HCl(aq.) 27 Here we note that chain exchange is a much faster processes than micelle size equilibration.…”

“…20,28 A key finding in this study was that AICE was surface-limited and scaled with the rate of turn-over of the solution-air interface. 16 The films were calcined at 600°C and the resulting porous niobia films were observed by scanning electron microscopy (SEM). 16 The application of ultrasonic waves to solutions induces continuous cavitation events to form <200 μm diameter bubbles (varies with sonic power and solution) that collapse on the microsecond timescale and can produce ephemeral conditions with pressures of several hundred atmospheres and temperatures up to 5500°C.…”