Structured graphene metamaterial selective absorbers for high efficiency and omnidirectional solar thermal energy conversion

An ideal solar-thermal absorber requires efficient selective absorption with a tunable bandwidth, excellent thermal conductivity and stability, and a simple structure for effective solar thermal energy conversion. Despite various solar absorbers having been demonstrated, these conditions are challenging to achieve simultaneously using conventional materials and structures. Here, we propose and demonstrate three-dimensional structured graphene metamaterial (SGM) that takes advantages of wavelength selectivity from metallic trench-like structures and broadband dispersionless nature and excellent thermal conductivity from the ultrathin graphene metamaterial film. The SGM absorbers exhibit superior solar selective and omnidirectional absorption, flexible tunability of wavelength selective absorption, excellent photothermal performance, and high thermal stability. Impressive solar-to-thermal conversion efficiency of 90.1% and solar-to-vapor efficiency of 96.2% have been achieved. These superior properties of the SGM absorber suggest it has a great potential for practical applications of solar thermal energy harvesting and manipulation.

The development of plasmonic structures provides a promising approach for photothermal modulation technology [5][6][7][8][9][16][17][18][19][20] . Many metamaterial-based structures are being extensively investigated and have demonstrated tailorable high absorbance over broad bandwidth 16,17,19 . In general, metallic metamaterial-based resonators are threelayered structures that create a resonant cavity in the structure. The three-layered structures comprise sophisticated isolated metal nanostructures on a dielectric layer/ flat metal film stack for fine-tuning of the electromagnetic (EM) properties of such structures. These isolated metal structures result in undesired high absorption of the photo-generated thermal energy trapped locally by the discontinuous metallic structures, which could not be transported to other sites for effective conversion or dissipation. In addition, metamaterials operation mechanism requires the dimensions of the metallic structures to be much smaller than the EM absorbance wavelengths in the visible to near-infrared (NIR) region, which limits the feature sizes to be ~ 10 to 100 nm 16,17,20 , producing stringent requirements for fabrication process based on focused ion beam (FIB) or electron beam lithography (EBL) 17-20 -both of which are not scalable and of high cost. Therefore, the metallic metamaterial approach is less practical for real-life thermal management.
Recently, low-cost and scalable EM absorbers based on chemically synthesized adsorbing metallic/dielectric nano-particles (NPs), such as gold (Au) and aluminum (Al), onto a porous membrane or polymer matrix 21,22 have been demonstrated. These NPs-stacked absorbers exhibited high and broadband absorbance from visible to NIR regime (wavelength from 0.4 to 2.5 µm). Nevertheless, the tunability and selectivity of the absorption bands of these structures are limited by localized surface plasmon resonance (LSPR) and the porous supporting membrane. Furthermore, the thermal transport inside the volumetric stacks is not effective for heat conversion due to the in accessible surface areas. Therefore, the performance of such absorbers is significantly limited.
These graphene-based absorbers featuring high absorption over broad wavelength regime means they do not have spectral selective mechanism and face the high thermal emission loss. Furthermore, the large thickness prevents the effective transport of the photothermal energy generated on the surface. Accordingly, the overall photo-generated heat could not be efficiently used and stored. Besides, the optical behaviors of the thick carbon-based absorbers would hardly be manipulated by the plasmonic structures due to the effective zone of near-field phenomena of metallic nanostructures, which only limits to several tens to hundreds nanometres.

Supplementary Note 3: Thermal conductivities of various metals and graphene
How to efficiently exploit the photogenerated heat is also one of the important factors in the design of solar-thermal absorber. In general, the graphene-based materials exhibit much better thermal conductivity than other metals, including Au, silver (Ag), Al, copper (Cu), and nickel (Ni). As displayed in Table S1, the thermal conductivities of suspended graphene and graphene on substrate are approximately 9.3 to 48.3 times and 1.4 to 7.2 times, respectively, larger than those of other metals [23][24][25] . In addition, we based on the densities and specific heats of metals and graphene-based materials to calculate the temperature increments in the metal films with respect to that in the graphene-based materials under consideration of the same volume and at the same thermal energy.
Compared with the other metals, the graphene provides the highest temperature increments, which are 1.5 to 2.5 times higher than those of metals. Therefore, the graphene-based materials feature not only high temperature increments under illumination but also active function to efficiently transport the photo-generated heat due to the excellent thermal conductivity.    Figure 12. Calculated solar-to-thermal efficiencies at working situations of TA=373 K, and C=1 and 10 of fabricated SGM absorber before, after 12 hours, and after 24 hours of heating at 100 °C in air. Table 4. Total solar absorbances (Eα), thermal radiation loss (ER), and solar-to-thermal conversion efficiencies (ƞsolar-thermal) of ideal H2 SGM absorber with 30-nm GM layer, which are calculated based on simulated absorbance spectrum (Fig.   1c), at working situations of TA=373 K, and C=1 and 10.