Chemical Epitaxy of Iridium Oxide on Tin Oxide Enhances Stability of Supported OER Catalyst.

Significantly reducing the iridium content in oxygen evolution reaction (OER) catalysts while maintaining high electrocatalytic activity and stability is a key priority in the development of large-scale proton exchange membrane (PEM) electrolyzers. In practical catalysts, this is usually achieved by depositing thin layers of iridium oxide on a dimensionally stable metal oxide support material that reduces the volumetric packing density of iridium in the electrode assembly. By comparing two support materials with different structure types, it is shown that the chemical nature of the metal oxide support can have a strong influence on the crystallization of the iridium oxide phase and the direction of crystal growth. Epitaxial growth of crystalline IrO2 is achieved on the isostructural support material SnO2, both of which have a rutile structure with very similar lattice constants. Crystallization of amorphous IrOx on an SnO2 substrate results in interconnected, ultrasmall IrO2 crystallites that grow along the surface and are firmly anchored to the substrate. Thereby, the IrO2 phase enables excellent conductivity and remarkable stability of the catalyst at higher overpotentials and current densities at a very low Ir content of only 14 at%. The chemical epitaxy described here opens new horizons for the optimization of conductivity, activity and stability of electrocatalysts and the development of other epitaxial materials systems.

Trap-Free Heterostructure of PbS Nanoplatelets on InP(001) by Chemical Epitaxy.

Semiconductor nanocrystalline heterostructures can be produced by the immersion of semiconductor substrates into an aqueous precursor solution, but this approach usually leads to a high density of interfacial traps. In this work, we study the effect of a chemical passivation of the substrate prior to the nanocrystalline growth. PbS nanoplatelets grown on sulfur-treated InP (001) surfaces at temperatures as low as 95 °C exhibit abrupt crystalline interfaces that allow a direct and reproducible electron transfer to the InP substrate through the nanometer-thick nanoplatelets with scanning tunnelling spectroscopy. It is in sharp contrast with the less defined interface and the hysteresis of the current-voltage characteristics found without the passivation step. Based on a tunnelling effect occurring at energies below the bandgap of PbS, we show the formation of a type II, trap-free, epitaxial heterointerface, with a quality comparable to that grown on a nonreactive InP (110) substrate by molecular beam epitaxy. Our scheme offers an attractive alternative to the fabrication of semiconductor heterostructures in the gas phase.

Role of Penetrability into a Brush-Coated Surface in Directed Self-Assembly of Block Copolymers.

High-density and high-resolution line and space patterns on surfaces are obtained by directed self-assembly of lamella-forming block copolymers (BCPs) using wide-stripe chemical guiding patterns. When the width of the chemical pattern is larger than the half-pitch of the BCP, the interaction energy between each BCP domain and the surface is crucial to obtain the desired segregated film morphology. We investigate how the intermixing between BCPs and polymer brush molecules on the surface influences the optimal surface and interface free energies to obtain a proper BCP alignment. We have found that computational models successfully predict the experimentally obtained guided patterns if the penetrability of the brush layer is taken into account instead of a hard, impenetrable surface. Experiments on directed self-assembly of lamella-forming poly(styrene- block-methyl methacrylate) using chemical guiding patterns corroborate the models used in the simulations, where the values of the surface free energy between the BCP and the guiding and background stripes are accurately determined using an experimental method based on the characterization of contact angles in droplets formed after dewetting of homopolymer blends.