Cationic defect-engineered CuMn2O4 photothermal membranes to leverage interfacial solar steam generation

Abstract

Herein, we report efficient and cost-effective utilization of CuMn₂O₄ (CMO) nanostructures to generate drinkable pure water from saline and wastewater via interfacial solar steam generation (ISSG). Defect-tunable cubic crystalline CMO nanoparticles (CMO_NP) and nanoflakes (CMO_NF) were synthesized using the co-precipitation method by varying the Mn-concentration (CuMn_xO₄, where x = 1.5 and 2). These CMO nanostructures comprising mixed oxidation states of Cu⁺/Cu²⁺ and Mn³⁺/Mn⁴⁺ and O²⁻ exhibited distinct morphologies and optical band offsets. The CMO_NF obtained with reduced Mn content showed a high surface area (43.5 m² g⁻¹), lower bandgap (∼0.9 eV), and excellent hydrophilicity compared to CMO_NP, enabling rapid and effective spectral absorbance. The CMO_NF based photothermal membrane generated an interfacial temperature of ∼37.5 °C under 1 Sun illumination, leading to a steam generation rate of 1.61 kg m⁻² h⁻¹. Under direct sunlight, a rate of 1.21 kg m⁻² h⁻¹ was recorded with stable performance maintained up to 40 consecutive cycles. The CMO_NF membrane delivered excellent purification performance for 3.5 wt% saline water and 100 ppm RhB and MB dyes, with evaporation rates of ∼1.41, 1.37, and 1.24 kg m⁻² h⁻¹, respectively. The remarkable NIR/IR absorption activity of CMO_NF resulted in a maximum surface temperature of ∼49.7 °C under IR illumination, exhibiting an evaporation rate of 3.64 kg m⁻² h⁻¹, which demonstrates its strong potential for ISSG. In addition, the CMO_NF membrane maintained its structural integrity and chemical composition even after multiple ISSG cycles, highlighting its durability and suitability as a cost-effective and efficient material for continuous solar-driven steam generation.