Enhancing Solar Steam Generation Using Oxidation of Carbonaceous Materials: Expanded Graphite and Aerogels

The growing global demand for sustainable clean water solutions has intensified research into solar-driven interfacial evaporation (SDIE) systems. This work investigates the photothermal performance of expanded graphite (EG)-based materials modified via hydrogen peroxide (H₂O₂) oxidation and noncovalent functionalization with 1-pyrenebutyric acid (PBA) for enhanced solar steam generation. A double-layer architecture, comprising a photothermal top layer (expanded graphite or aerogel based) and a thermally insulating carbon foam substrate, was employed to promote heat localization and efficient water delivery. Surface treatments were designed to improve water affinity while preserving the structural integrity of the graphitic framework. H₂O₂ treatment introduced oxygen-containing groups, while PBA functionalization leveraged π–π stacking to achieve surface carboxylation. Material characterization using SEM, XPS, and XRD confirmed successful surface modification without disrupting the lattice structure. Photothermal testing under 1 sun illumination revealed that untreated EG achieved an evaporation rate of 0.97 kg/m²·h. In comparison, H₂O₂-treated EG reached 1.26 kg/m²·h (78.72% efficiency), while PBA-EG achieved 1.3 kg/m²·h (81.61% efficiency), demonstrating the role of tailored surface chemistry in enhancing water uptake and thermal confinement. Additionally, GO-aerogel and GnP/GO aerogel-based configuration were explored for their lightweight and porous structure, though performance was moderate which was limited by their structural stability and interfacial coupling. The GO-aerogel showed an evaporation rate of 1.12 kg/m²·h, while the GO-GnP aerogel showed an evaporation rate of 1.22 kg/m².h, respectively. Furthermore, X-ray photoelectron spectroscopy and Raman analyses showed that the PBA treated EG has a high C/O ratio of 9.48 and low Raman ID/IG ratio of 0.037 so it has very low structural defects. The study also explored the thickness effect on the evaporation rate and the overall efficiency, and we found that certain optimum thickness of solar absorber is necessary to have a better performance. These findings highlight the potential of low-cost, surface engineered EG as a scalable photothermal material, and provide insight into design strategies for solar driven interfacial systems.