A detailed transformation process of cation exchange (CE) chemistry from hexagonal phase CdS to CdS-Cu7S4 core-shell heterostructures nanorods and eventually to monoclinic phase Cu7S4 nanorods is reported. Applications of the nanorods in environmental pollution removal, photothermal conversion and light-induced water evaporation were investigated. The influence of the reaction time, morphology, and crystal phase on the photocatalysis, photothermal conversion and light-induced water evaporation was discussed. The results showed maximum removal value of rhodamine B by 97.8% at 1 h timepoint. Aqueous dispersion of the core-shell CdS-Cu7S4 heterostructures nanorods prepared with 2 h (1.0 g/L) exhibited a good photothermal conversion performance as it saw increase in sample temperature by 37.8 C-circle in 300 s under the irradiation of 980 nm laser. The exposure of as-obtained sample film to simulated sunlight irradiation at 2 h (8 mg) increased the weight loss of pure water by 0.62 g in 30 min with the water evaporation efficiency up to 48.4%. We are reporting an excellent light-induced water evaporation performance potential of the nanorods discussed.