Effective and efficient utilization of solar energy has drawn dramatically increased attention due to the urgent request from energy crisis and sustainable development. Photothermal conversion of solar energy is one of the most promising approaches by which solar energy is firstly converted into heat and then heat energy is utilized for different applications, such as water purification, desalination, electric power generation, catalysis conversion, bacterial killing, and actuators.[1] One of the most important challenges for the photothermal conversion is to develop efficient broadband light absorber.
Here we have developed two excellent broadband light absorbers based on hierarchical plasmonic nanostructures. One is Ag/SiO2 hybrid nanostructure (hierarchical SiO2-scaffold loaded with many Ag nanoparticles) which has a very high absorption above 99 % from 220 nm to 2500 nm.[2] The Ag/SiO2 hybrid nanostructure is fabricated by using a novel atomic layer deposition (ALD) technology, namely metastable atomic layer deposition, in which 3D hierarchical SiO2 nanostructure is evolved in a self-assembly way assisted by cyclic formation and decomposition of the metastable silver oxides.[3, 4]
Another one is noble-metal-free Al/AlN plasmonic nanostructure fabricated by a limited reactive magnetron sputtering at the elevated temperature of 200 °C, [5] showing a highly efficient (96.8%) and broadband (full solar spectrum) absorption. Different from conventional reactive sputtering of AlN thin films using a large amount of N2 in the mixture of N2 and Ar gases for a complete reaction, here the N2 flow rate in the mixture is strictly controlled so that the co-sputtering of both Al and AlN occurs with the single Al target. In addition, 3D structure will be evolved at an elevated temperature (such as 200 °C) due to the different diffusion kinetics between Al and AlN.
The results from finite-difference time-domain (FDTD) simulation reveal that forwards scattering and multiple scatting are very favored in the 3D hierarchical structures, and there is a strong collective effects of absorption from many plasmonic nanoparticles, leading to an excellent broadband absorption.[2, 6] Due to the excellent broadband absorption, both types of absorbers demonstrate great potentials for the applications of photothermal conversion, such as solar water evaporation, photothermal imaging and photo-thermoelectric generation.[6]

References
[1] P. Cheng; D. Wang; P. Schaaf Advanced Sustainable Systems, 2022, 6, 2200115.
[2] M. Ziegler et al. ACS nano, 2020, 11, 15023.
[3] M. Ziegler et al. Adv. Mater. Technol., 2017, 2, 1700015.
[4] M. Ziegler et al. Materials Today Chemistry 2018, 10, 112-119.
[5] Q. Yuan et al. ACS Appl. Nano Mater., 2018, 1, 1124-1130.
[6] P. Cheng et al. Applied Materials Today 2021, 25, 101238.


Funding by DFG within SPP “Tailored Disorder” (DFG grant Scha 632/24) is gratefully acknowledged.