Photocatalysis using semiconductor materials like titania (TiO2) is a key method for environmental purification or solar fuel generation. Nanostructures that maximize incident light absorption are highly desired to enhance depollution rate or solar‐to‐fuel conversion efficiency in limited volumes of catalysts. Here, we report on structural and optical properties of metasurfaces based on a 20 nm thick anatase layer conformally deposited onto a wavelength‐scale two‐dimensional periodic photonic lattice. We investigate the NO degradation using such metasurfaces, and evaluate the impact of the patterning on photocatalytic activities between 340 and 400 nm. In the 380–385 nm range, the mean photochemical efficiency is increased by a factor up to 5.7 compared to flat references, with an overall three‐fold enhancement within the whole spectral range of interest. This approach can be applied to numerous types of systems by varying active materials, leading to substantial improvements in air/water depollution, water splitting or artificial photosynthesis processes.

MagnØli phasesT i n O 2nÀ1 (3 < n 10) are mixed Ti 4 + /Ti 3 + oxides with high electrical conductivity.W hen used for water remediation or electrochemical energys toragea nd conversion, they are nanostructured and exposed to various environments. Therefore, understandingt heir surfacer eactivity is of prime importance.S uch studies have been hindered by carbon contamination from syntheses. Herein, this synthetic and characterization challengei sa ddressed throughan ew approacht o5 0nmc arbon-free Ti 4 O 7 and Ti 6 O 11 nanoparticles. It takes advantage of the differentr eac-tivitieso fr utile and anatase TiO 2 nanoparticles towards H 2 , to use the former as precursor of Ti n O 2nÀ1 and the latter as a diluting agent. This approachi sc ombined with silica templatingt or estrain particle growth. The surface reactivity of the MagnØli nanoparticles under different atmospheresw as then evaluated quantitatively by synchrotron-radiationbased X-ray photoelectron spectroscopy,w hichr evealed oxidized surfaces with lower conductivity than the core. This findings heds an ew light on the charget ransfer occurring in these materials.Supporting information and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.