Design of an alkaline pyridyl acceptor-based calix[4]arene dye and synthesis of stable calixarene–TiO2 porous hybrid materials for efficient photocatalysis

Abstract

A porous organic–inorganic hybrid material consisting of calix[4]arene dye C4BTP with pyridine as an acceptor and titanium dioxide, denoted as C4BTP–TiO₂, has been prepared by a facile sol–gel method. The C4BTP–TiO₂ materials show expandable photocatalytic activities for H₂ evolution from water and CO₂ reduction to CO when they are loaded with Pt nanoparticles or rhenium complex ReCl(bpy) (CO)₃ (ReP) as co-catalysts, respectively. The optimized Pt/C4BTP–TiO₂ (5.0 wt%) catalyst exhibits a high H₂ production activity (11.99 mmol g⁻¹ h⁻¹ based on catalyst mass) and excellent recyclable durability (a turnover number of 12 981 within 50 h after 10 rounds) under visible-light (λ > 420 nm), presenting one of the best performances among all the heterogeneous photocatalytic systems based on metal-free sensitizers. As a contrast, the surface sensitized catalyst Pt/gel-TiO₂/C4BTP only shows far lower activities than hybrid Pt/C4BTP–TiO₂ under the same conditions. In addition, the optimized ReP/C4BTP–TiO₂ (5.0 wt%) catalyst obtained by loading ReP exhibits a selective photoactivity for CO₂ reduction to CO, giving a turnover number of 112 after 25 h of visible-light irradiation. The good photoactivity and durability of the as-prepared hybrid materials can be attributed to the firm Ti–N linkage and efficient electron transfer between metal-free dye C4BTP and the substrate TiO₂, while the alkaline-stable Ti–N bond improves the stability of the photocatalysts under basic aqueous conditions. This study offers a good example for the design and preparation of stable organic/inorganic hybrid materials applied in solar energy-solar fuel conversion.