Enhanced Photoelectrochemical Energy Conversion in Ultrathin Film Photoanodes with Hierarchically Tailorable Mesoscale Structure

Highly porous electrode designs are often employed for photoelectrochemical energy conversion applications. “Inverse opal” structures generate high surface area electrodes to enhance light absorption in semiconductors with short carrier collection lengths, effectively increasing the optical depth of ultrathin film photoelectrodes. Here, the fabrication of hierarchically structured, “host–guest” photoelectrodes based on selective atomic layer deposition of ZnO in composite polystyrene–SiO2 nanosphere films is described. Nanostructured scaffolds for ultrathin film photoanodes are prepared with a facile, continuously tunable solution‐phase synthesis. The characteristic length scales for absorption, carrier collection, and mass transport can be independently engineered into the electrode by choosing appropriate colloidal components for the composite scaffold. 20 nm ZnO photoanode layers based on the “host–guest” architecture exhibit roughly 500 times the photocurrent generated on an equivalent planar electrode and a 430% increase over a photoanode structured by a scaffold comprised of a close‐packed assembly of identical SiO2 nanospheres. This results from an improved balance of reactant mass transport and the locus of light absorption throughout the electrode. This approach offers a facile route for preparing strategically nanostructured photoelectrodes based on strategies developed from more complex fabrication techniques. A simple, hierarchical nanostructure based on self‐assembled colloidal composites can generate large improvements for photoelectrochemical energy conversion for ultrathin film photoanodes.