Graphene–semiconductor photocatalysts are of utmost interest for achieving visible light activity but there is ambiguity about the role played by graphene in these photocatalysts. Such photocatalysts exhibit both adsorptive and photocatalytic activities towards pollutant degradation. Herein, we report nitrogen doped CaTiO₃ (NCT) perovskite coupled with reduced graphene oxide (RGO), RGO–NCT, composites for the first time and utilize them for environmental remediation as an efficient adsorptive photocatalyst. The adsorption and photocatalytic activity were evaluated by investigating the adsorption/degradation of a model dye, methylene blue (MB). In comparison with bare CT and NCT, the RGO–NCT composites exhibit enhanced photocatalytic activity, which could be attributed to the synergistic effect of the adsorption of dye molecules on the RGO surface followed by their degradation under visible light irradiation. In the degradation mechanism, we propose that the degradation of MB is not only due to the dye photosensitization process but also involves the true photocatalytic action of the RGO–NCT composites. The clear role of graphene as an electron acceptor is demonstrated by photoluminescence spectroscopic study. Also, the degradation of a colorless pollutant thiabendazole (TBZ) under visible light irradiation supports our hypothesis. In addition, the commendable stability and recyclability of RGO–NCT photocatalysts demonstrates that these materials can be used as potential, viable and stable photocatalysts for environmental remediation. In principle, we anticipate that our work paves the way for tailoring the photocatalytic activity of perovskite-type semiconductor materials by coupling with graphene for the design of recyclable bifunctional photocatalysts.

A Cd(II)-based metal–organic framework (MOF) {[Cd(PA)(4,4′-bpy)₂](H₂O)}_n (where, PA = pamoic acid, and bpy = bipyridine) has been demonstrated to have trifunctional properties, namely as (i) an efficient and selective adsorbent for dyes, (ii) a visible-light-active photocatalyst for the degradation of dyes and (iii) a photocatalyst for Cr(VI) reduction. Hence, this material has a very good potential for application in environmental remediation. In particular, the MOF exhibits excellent selectivity toward cationic methylene blue (MB) dye adsorption and its separation over neutral (neutral red) and anionic (methyl orange) dyes. In addition, the catalyst has also been explored for the photocatalytic degradation of less adsorbed dyes, such as methyl orange (MO) and rhodamine B (RhB). Subsequently, the potential of the presented MOF for the photocatalytic reduction of Cr(VI) under visible light irradiation has also been investigated and the catalyst was found to reduce 95% of Cr(VI) within 3 h. Moreover, simultaneous one-pot photocatalytic reduction of Cr(VI) and dye degradation has been demonstrated with high efficiency using the same catalyst. The mechanism of adsorption could be correlated to the presence of peripheral hydroxyl groups in the MOF and the photocatalytic activity observed under visible light could be attributed to the optical properties of Cd-MOF. In principle, this work could broaden the utilization of single metal organic platform for multifunctional applications toward environmental remediation.

A highly efficient, non-toxic, metal-free, low-cost, acid–base bifunctional sulfonated graphitic carbon nitride (S-g-C ₃ N ₄ ) nanosheets for Knoevenagel condensation and multicomponent tandem reactions under environmentally benign conditions.