Hard templating of symmetric and asymmetric carbon thin films with three-dimensionally ordered mesoporosity

Sacrificial colloidal crystal templating of porous carbon films of tunable thickness is demonstrated using a facile thin-film assembly and hard-template-based nanoreplication process. Convectively assembled, colloidal crystal films composed of size-tunable silica nanoparticles (ca. 10-50 nm) serve as scalable sacrificial scaffolds for the formation of thickness-tunable, structurally robust, and flexible porous carbon films. Both precursor vapor infiltration (PVI) and precursor immersion/spin-off (PIS) techniques, suitable for replication by various carbon sources (e.g., furfural/oxalic acid, phenol-formaldehyde, resorcinol-formaldehyde, sucrose), result in continuous, crack-free porous replica films. Systematic PVI-based underfilling of the template film or PIS-based complete spin-off of excess carbon replica precursor results in porous carbon films endowed with a symmetric three-dimensionally ordered mesopore (3DOm) topology uniformly distributed across the film thickness. Alternatively, by tuning the nanoparticle crystal film thickness and the degree of overfilling (PVI) or rate of spin-off of the carbon replica precursor (PIS), films bearing an asymmetric structure composed of 3DOm-supported ultrathin carbon layers can be realized. The stability of the silica templates under polymerization and carbonization conditions helps bolster mesopore robustness within the replica films, eliminating uniaxial pore shrinkage upon template sacrifice. The decoupling of the template assembly and its replication enables film formation from a wide range of carbon sources and possibly a further expanded materials palette. Realization of porous carbon films on various substrates without degradation of the mesostructure is enabled by robustness of the coating/replication process to characteristic surface roughness at scales several-fold larger than the template particle size as well as to polymer-mediated film transfer. Among various possible applications, we demonstrate how properties of the symmetric 3DOm films in particular (e.g., high surface area, large pore volume) enable their exploitation as potential low-cost alternatives to costly Pt-based electrodes for dye-sensitized solar cell (DSSC) technologies.