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In this work, photothermal materials are integrated with a temperature-sensitive hydrogel and structural color for visually detecting solar intensity. Inspired by the functional performance of beetles, the photothermal layer is constructed by depositing candle soot on a film of Cu nanoparticles, while the temperature-sensitive colored hydrogel is fabricated by self-assembling colloidal photonic crystals on poly­(N-isopropylacrylamide) (PNiPAM). The deposition of candle soot not only improves the photothermal performance but also leads to a superhydrophobic surface with a self-cleaning function. The photothermal layer absorbs sunlight and converts it into heat, which is then transferred to the hydrogel. The structural color of the hydrogel changes due to the heat-induced volume shrinkage. As the solar intensity increases from 0.62 to 1.27 kW/m2, the structural color conspicuously changes from red to orange, yellow, green, cyan, and blue, with reflection peaks shifting from 640 to 460 nm accordingly. The color change is highly apparent, which can be easily observed by the naked eye, suggesting that the solar intensity can be easily detected by reading out the structural color. This power-free and self-cleaning solar sensor can work for a long period without maintenance, which is suitable for a wide application prospect, such as smart home and agriculture.

To develop solar steam generation (SGG) for water treatment, light absorbers have been widely synthesized by carbonization of plants at high temperature. However, the thermal treatment has high energy‐consumption and is environmentally unfriendly. Thus, it is urgent to explore new green approaches to convert biomasses to carbon materials for SSG. Herein, a concentrated acid‐induced dehydration method is developed to convert fallen leaves to carbon powders, fallen‐leaf photothermal film prepared by dehydration (FLPD). The resultant FLPD exhibits a strong absorption covering a wide spectrum. It also contains sufficient oxygen‐containing groups on the surface, leading to low water evaporation enthalpy. To meet the demands of practical applications, the FLPD membrane is modified with polyvinyl alcohol (PVA) to improve the mechanical stabilities and the surface wettability is further enhanced by an oxygen plasma treatment. An SSG device based on the modified membrane attains a competitive evaporation rate of 1.355 kg m−2 h−1 under 1 sun irradiation. The evaporation rate remains approximately constant when evaporating the seawater. Moreover, the salt deposited on the surface could be self‐cleaned due to the high wettability. Therefore, herein, it is demonstrated that concentrated acid‐induced dehydration is an effective and green approach to convert cheap biomasses to carbon materials for SSG applications.