Synthesis of Co₃O₄-modified sulfur-doped polyimide for photocatalytic reduction of 4-nitrophenol via ion-exchange technique

Photocatalytic reduction of 4-nitrophenol to 4-aminophenol has a broad development prospect and potential economic value. In this study, we prepared for the first time Co₃O₄-modified sulfur-doped polyimide (Co₃O₄/SPI) catalyst using a facile ion-exchange technique. The chemical structure, morphology and photoelectronic performance of the catalysts were analyzed by different characteristics. The results showed that Co₃O₄ displayed high dispersibility; increasing the specific surface area of SPI provided more reactive sites. Meanwhile, the Co₃O₄ modification successfully constructed a Co₃O₄/SPI type-II heterostructure with a suitable band gap, which significantly improved the absorption of visible light and the photogenerated electron–hole pair separation efficiency of SPI. These factors caused Co₃O₄/SPI to exhibit higher photocatalytic activity than pure SPI, which facilitated the photocatalytic reduction of 4-nitrophenol to 4-aminophenol by Co₃O₄/SPI. At 5 min of visible light irradiation, the conversion of 4-nitrophenol was 100%, while that of SPI was 23%. Moreover, the photocatalyst showed good stability after five cycles. Finally, the mechanism of photocatalytic reduction of 4-nitrophenol by Co₃O₄/SPI was also discussed. The Co₃O₄/SPI provides a new approach for the cost-effective reduction of 4-nitrophenol and obtains high-value-added 4-aminophenol.