Novel silicon oxide microtube microstructures for photocatalytic reactions: application to hydrogen photoproduction

• Microstructure of free-standing SiO 2 microtubes anchored to a silicon substrate. • 15 · 10 6 microtubes of ∼ 3 μm in diameter, arranged in a square form. • Exceptionally high specific contact area of 7 · 1 0 5 m ch 2 · m − 3 • Microstructure coated with TiO2 via ALD and decorated with Au nanoparticles. • Outstanding H 2 photoproduction of 3.7 · 10 2 mols H 2 · h −1 · cm −3 . Current challenges related to environmental preservation and energy requirements have pushed the research field of solar fuels, where solar light is used to carry out chemical transformations using photocatalysts. Achieving optimal reaction yields requires uniform light distribution throughout the active photocatalyst volume, as well as effective interaction of the reactants within that volume. Consequently, the design of the support structure is as crucial as the choice of photocatalyst. In this work, we present a novel structure of micromonoliths consisting of a high-density array of free-standing SiO 2 microtubes, obtained via controlled oxidation of macroporous silicon. This structure provides a fabulous surface-to-volume ratio of ∼ 7 × 1 0 5 m ch 2 m − 3 and is transparent to light, enabling efficient support of the photocatalysts while allowing the flow of reactants and products across. The structure is functionalized by coating the walls of the microtubes with a suitable photocatalyst, which in this work consist of Au nanoparticles anchored on TiO 2 . Owing to the optical transparency of SiO 2 , incident light penetrates the structure and is effectively absorbed by the photocatalyst, thereby enhancing photoreaction efficiency. In addition, the structure exhibits excellent thermal, mechanical and chemical stability. As a proof of concept, we report results of the photoproduction of hydrogen by flowing gaseous reactants through the microtubes under UV irradiation. The system exhibited outstanding hydrogen photoproduction rates, reaching up to 3.7 × 10 2 molH 2 h −1 cm −3 , with no observable damage or blockage after extended operation, and showing only a slight initial deactivation rate of 1.1 % h -1 . These values represent an improvement of over six orders of magnitude in hydrogen production per unit reactor volume compared to other structures reported in the literature under identical experimental conditions. This constitutes a significant advancement in the engineering of microstructured reactors designed to enhance photocatalytic efficiency.