Thin Films of Thermally Stable Ordered Mesoporous Rh₂O₃(I) for Visible-Light Photocatalysis and Humidity Sensing

The preparation of highly ordered mesoporous thin films of rhodium sesquioxide (Rh2O3) on silicon and quartz glass substrates by dip-coating via solution-based block copolymer self-assembly is reported. Hydrated rhodium chloride is shown to be an effective precursor for sol–gel processing of Rh2O3. Heating in air to 600 °C crystallizes single-phase Rh2O3(I) while fully preserving the cubic symmetry 3D pore-solid architecture, whereas at 750 °C only the in-plane pore order is retained. Interestingly, there is no crystalline–crystalline transformation upon heating to 1000 °C, despite distinct microstructure coarsening. The thermal stability of the sol–gel-derived material in terms of mesostructure retention can be significantly enhanced by atomic layer deposition surface treatment, making the block copolymer-templated thin films interesting for demanding applications. Furthermore, the results of preliminary humidity sensing and photochemical activity measurements indicate that the nanocrystalline Rh2O3(I) shows promising properties as a sensor material and a visible-light photocatalyst.