Strong Resonant Light Absorption Band on Sub-Oxidized Metal Titanium Nanostructures
Authors: Cheng Chaoqun , Wang Kaiying 
Published: July 2022

Nanopatterned or roughed metallic structures have received considerable attention due to the effective light absorption capability via optical effects of light confinement or photonic excitation of surface plasmon polaritons.1, 2, 3, 4, 5 However, individual metallic structures have limited light capture. To augment the solar light utilization, coupling of different absorption modes was developed within three-dimensional architectures.6, 7 Among these, the absorbers using dielectric-nanostructures interactions showed effective light harvesting, which exhibited enhanced photocatalysis and water splitting activities. Here, we present a solid visible light absorber made of proper sub-oxidation of metal Ti through a facial approach.
First, surface nanostructures were prepared using a chemical etching process on cleaned metal Ti foils. Then, a thermal sub-oxidation was employed to diffuse oxygen into the sub-surface region of the metal Ti, forming titanium-oxides nanocomposites. The titanium-oxides are composed of titanium suboxides on the sub-surface of metal Ti nanostructures. The surface sharpness of nanostructures was checked by atomic force microscopy (AFM) and scanning electron microscopy (SEM) (Figure 1), showing favorable surface nanostructures. The nanostructures of quasi-prisms with sharp nano-curvatures allow light concentration in a more vigorous intensity. The affluent sharp edges of nanostructures could behave as “hot spots”. In Figure 2, the UV-vis absorption spectra of the as-prepared specimen disclose a strong resonant light absorption in the visible region with an almost four times enhancement. The resonant absorption band position and intensity are adjustable by controlling thermal diffusion parameters and surface structures.
Our observation is quite distinctive from the absorption spectra of single phases, anatase or rutile phase TiO2, and the cases of mixed-phase TiO2-x nanoparticles. To our best knowledge, this is the first demonstration of uniquely strong resonant visible absorption band by sub-oxidized metal Ti nanostructures. The absorber is of particular interest in a wide range of extensive photo-responsive applications, such as optoelectronic devices, photovoltaics, and photocatalysis. The approach represents a new pathway to enable photonic absorption within nanostructures, not only for metal Ti but also for other related metallic structures.

Acknowledgments: The authors acknowledge the research grants from EEA (European Economic Area)-Norway-Romania project# GRAFTID, RO-NO-2019-0616, EEA-Poland-NOR/POLNORCCS/PhotoRed/0007/2019-00, and the Research Council of Norway projects #314012 and #245963/F50.
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Figure 1