The ultrahighly nanoporous aerogel is recognized as a state of matter rather than as a functional material, because of its qualitative differences in bulk properties, transitional density and enthalpy between liquid and gas, and diverse chemical compositions. In this review, the characteristics, classification, history and preparation of the aerogel were introduced. More attention was paid to the sol-gel method for preparing different kinds of aerogels, given its important role on bridging the synthetic parameters with the properties. At last, preparation of a novel single-component aerogel, design of a composite aerogel and industrial application of the aerogel were regarded as the research tendency of the aerogel state in the near future.

Many scientists have devoted themselves to the study of the interaction between subwavelength structures and electromagnetic waves. These structures are commonly composed of regular arrays of subwavelength protuberances, which can be artificially designed. However, extending from 2D periodic patterns to 3D disordered subwavelength structures has not been studied yet. In this study, we studied the total diffuse reflectivity of carbon aerogels with various 3D networks of randomly oriented particle-like nanostructures by using normally incident visible light (430-675 nm). We observed that the different 3D network nanostructures of carbon aerogels, especially for the structures with the minimum size, reduced the reflectivity effectively. It was found that the key mechanism for the subwavelength-structure-induced ultralow reflectivity property is due to the decrease of the amplitude of electron vibration forced by the electromagnetic wave, which provides a simple method for designing perfect black materials.

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Solar steam generation has been extensively studied for its potential application in power generation and water treatment. Although some efficient evaporators have been developed, the challenge of the abrupt drop in the evaporator performance under outdoor environments remains to be overcome. The heteroblasty of Monstera and other climbers allows them to grow rapidly under the extreme shade of a tropical rainforest, inspiring the design of a high-efficiency evaporator that can function even in weak light environments. Herein, artificial trees that imitate the leaf fenestration of Monstera combined with the Chinese paper cutting technique exhibit the highest evaporation rate of 2.30 kg m −2 h −1. Moreover, under oblique incidence (from 0° to 75°) and dappled sunlight, the evaporation rates of artificial trees with leaf fenestration are 1.08-1.26 and 1.34-2.78 times those of artificial trees without leaf fenestration and a 2D evaporator, respectively. The excellent performance is attributed to high-efficiency light absorption, photothermal conversion, high evaporation area, and excellent light and thermal management abilities, which are achieved through leaf fenestration and efficient thermal recovery through multiple reflections of light and thermal radiation between the leaves. The design of the 3D hierarchical structure and leaf fenestration are also applicable for various light absorbents.

Ecofriendly, highly effective, and low-cost solar steam generation has great potential in the applications of power generation, seawater desalination, and industry wastewater treatment. Solar steam generation requires an evaporator that has strong light absorption over a wide-frequency band (200–3000 nm), high photothermal conversion efficiency, and good thermal insulation to avoid excessive heat loss. Herein, foam-strengthened ultrablack carbon aerogels (CAs) with micropores, mesopores, and macropores were prepared using freeze drying. The small-size conductor effect in ultrablack CAs could increase the surface electron concentration, leading to the increase of light absorption and ultimately enhancing photothermal conversion efficiency. Consequently, under 1 sun illumination, dried CA-5 exhibited a fast temperature rise rate and the highest thermal equilibrium temperature of 87.6 °C among CAs, the equilibrium temperature is 20.8 °C higher than that of the foam. Besides, CA-5 exhibited a high water evaporation rate of 1.29 kg m–2 h–1 under only 1 sun illumination. To further increase the hot-electron effect, CO2 activation was implemented. The highest water evaporation rate among activated CAs (ACAs) was 1.37 kg m–2 h–1, which is about 2.85 times higher than that of pure water. The highest water evaporation efficiency was 87.51%, which is better than most of the previously reported evaporation efficiencies. Besides the hot-electron enhancement effect, ACAs with thermal insulation, mechanical strength, and thermal stability show great potential for producing fresh water from seawater, industrial wastewater, and even concentrated acidic or alkaline solutions.