Photocatalytic water splitting to produce molecular hydrogens and oxygen in artificial photosynthetic system has been practiced in laboratories for decades. So far, many materials have been suggested until now, however, titanium dioxide (TiO₂) is still one of attractive photoelectrodes due to its strong optical absorption, high chemical stability, environmental benignity, and low cost. There have been many reports which show the photoconversion efficiencies to enable the development and commercialization of TiO₂-related materials. However, the TiO₂-based photoelectrodes still suffer from many drawbacks that limit the successful commercialization such as the predominant absorption in the UV region due to the wide bandgap (~3.4eV) and the slow water oxidation kinetics due to the valence band far away (~1.17eV) below the water oxidation level. These resulted in a dramatic decrease in the energy conversion efficiency.

Among the challenges of overcoming the issues is the development of photoelectrodes with various nanostructures such as 0-D nanodots, 1-D nanowires, 2-D ultrathin films and 3-D nanostructures. … Here, we demonstrate a simple approach for maximizing the light absorption in the entire UV–visible region, in which the photoelectrode consists of three layers of Au film, TiO₂ film, and Au nanoparticles (NPs). The layers are carefully produced using thin-film deposition technique, followed by the post annealing process. Although the electrode does not have a large surface-area-to-volume ratio, compared with previous nanostructures, we will show that a significant increase of the photoelectrochemical water oxidation rate can be achieved by optimal design of the three-layered photoelectrodes for broadband enhancement over the UV and visible part of solar spectrum. This ascribes to the increase of the light absorption in UV region due to the light reflected at TiO₂/Au thin film and in visible region due to the electric field enhancement at the TiO₂ film resulting from LSPR of Au NPs.

—Park et al.

The Au and TiO2 films are produced by e-beam evaporation and atomic layer deposition, respectively, in a carefully controlled way to minimize surface roughness.

The Au film increases the light absorption in the UV region with TiO₂ acting as an impedance-matching layer, while the Au NPs increase the light absorption in the visible region due to the plasmonic resonance effects, increasing the photocurrent under visible light.

3D numerical simulation results suggest that the Au film also plays an important role in enhancing the electrical field intensity at the TiO₂ film in contact with Au NPs, by efficient excitation of localized surface plasmon resonances, thereby, contributing to the enhanced photoactivity of the film in the visible range.

Prof. Jeong Min Baik of UNIST (School of Materials Science and Engineering) said that while several attempts have been made to use UV-based photoelectrodes for hydrogen production, the new photoelectrode represents the first use of the metal-dielectric hybrid-structured film with TiO₂ for oxygen production.

The metal-dielectric hybrid-structured film can further reduce the overall cost of producing hydrogen, as it doesn’t require complex operation processes, said Prof. Heon Lee (Korean University).

This work was supported by the Pioneer Center Program through the National Research Foundation of Korea (NRF) grant, funded by the Korean government (MSIP). It has been also equivalently funded by the 2014 Research Fund of UNIST (Ulsan National Institute of Science and Technology), as well as by the KIST-UNIST partnership program.

Resources

• Joonmo Park, Hee jun Kim, SangHyeon Nam, Hyowook Kim, Hak-Jong Choi, Youn Jeong Jang, Jae Sung Lee, Jonghwa Shin, Heon Lee, and Jeong Min Baik (2016) “Two-dimensional metal-dielectric hybrid-structured film with titanium oxide for enhanced visible light absorption and photo-catalytic application” Nano Energy doi: 10.1016/j.nanoen.2016.01.004