Hierarchical WO3- x Ultrabroadband Absorbers and Photothermal Converters Grown from Femtosecond Laser-Induced Periodic Surface Structures

Abstract

Oxygen-vacancy-rich WO_3-x absorbers are gaining increasing attention because of their extensive absorbance-based applications in near-infrared shielding, photocatalysis, sterilization, interfacial evaporator and electrochromic, photochromic, and photothermal fields. Thermal treatment in an oxygen-deficient atmosphere enables us to prepare WO_3-x but lacks the capacity for finely manipulating the grown structures. In this work, we present that laser-induced periodic surface structure (LIPSS) obtained by femtosecond laser ablation is a good template to grow various hierarchical WO_3-x ultrabroadband absorbers and photothermal converters by thermal oxidation annealing in air. Increasing annealing temperature from 600 to 1000 °C allows the manipulation of WO_3-x crystal sizes from ∼70 nm to ∼4 μm, accompanied by a color transition from brown to dark blue and finally to yellow. Benefiting from annealing-induced surface cracks and phase transition into WO_3-x (containing both WO₃ and W₁₈O₄₉) at 600 °C, excellent UV-vis-NIR-MIR ultrabroadband absorbers were produced: >90% UV-NIR absorbance (0.3-2.5 μm) and 50-90% MIR absorbance (2.5-16 μm), much better than most W-based metamaterial absorbers. The higher the annealing temperature (1000 > 800 > 600 °C), the better the photothermal performances (sample temperature as the indicator) of annealed interfaces due to the increased oxidation rates and resultant thicker oxide layers (6, 150, and 507 μm), a trend which is more apparent upon the irradiation of high-density (3160 mW/cm²) and ultrabroadband (200-2500 nm) light but much less apparent for shorter-band (200-800, 420-800, 800-2500 nm, etc.) and less-intensity (1694, 1540, 1460 mW/cm², etc.) light irradiation. This phenomenon indicates that (1) higher-performance ultrabroadband absorbers possess a higher photothermal conversion capacity; (2) thicker-WO_3-x oxide layer converters are more effective in preserving photothermal heat; and (3) both the W-LIPSS and metal tungsten substrate can quickly dissipate the photothermal heat to inhibit heat accumulation in the oxide photothermal converters. It is also proved that ablation-induced high-pressure shockwaves can produce deformation layers in the subsurfaces to release annealing-induced stresses, beneficial for the formation of less-cracked non-stoichiometric WO_3-x interfaces upon annealing. High-pressure shockwaves are also capable of inducing grain refinement of LIPSS, which facilitates a homogeneous growth of small non-stoichiometric metal-oxide crystals upon annealing. Our results indicate that femtosecond laser ablation is a convenient upstream template-fabrication technique compatible with the thermal oxidation annealing method to develop advanced functional oxygen-vacancy metal-oxide interfaces.

Keywords: LIPSS; femtosecond laser ablation; oxygen-vacancy tungsten oxide; photothermal; thermal annealing; ultrabroadband absorber.