Photoelectrochemical water splitting using sunlight is expected to be used as a technology to produce clean energy in the form of hydrogen. As key materials for this technology, photoelectrodes must have low overpotential against water splitting reactions, in addition to high solar absorption and charge transfer efficiencies. For practical applicability, this technology cannot use rare metals as primary materials, and the fabrication process must be industrialized; however, materials that satisfy these requirements have not yet been developed.

Accordingly, the research team solely focused on the zinc oxide nanopagoda array, as such arrays are inexpensive to produce, feature high electron conductivity, and are not vulnerable to raw material depletion. Initially, zinc oxide nanopagoda arrays were considered difficult to fabricate with good reproducibility. Led by Marwa Abouelela – a third-year doctoral student who is also the lead author of this paper - the team first optimized the synthesis process to ensure high reproducibility. When the photoelectrochemical properties of the obtained photoelectrode were evaluated, a relatively large photocurrent was observed to emerge under pseudo-sunlight irradiation. In addition to the high charge transfer efficiency associated with low defect density and high surface chemical reaction activity due to its many steps, an electromagnetic field analysis has revealed that the nanopagoda's unique nanostructure can efficiently capture ultraviolet rays contained in the incident light.

To ensure the effective utilization of visible light, which accounts for 55% of sunlight, the research team further improved the photoelectrochemical properties by decorating the zinc oxide nanopagoda surface with silver nanoparticles that exhibit localized surface plasmon resonance, increasing the photocurrent by approximately 1.5-fold. The action spectrum of the photocurrent value indicates that this improvement is primarily attributed to the hot electron transfer caused by visible light absorption by the localized surface plasmon resonance of silver nanoparticles. By optimizing the application of silver nanoparticles, it became possible to improve the photoelectrochemical properties without producing adverse effects on the properties of the nanopagoda itself.

Associate Professor Go Kawamura, one of the corresponding authors, stated the following: “Zinc oxide nanopagodas were previously only considered suitable for use in electron gun emitters, which made the most of their high charge transfer efficiency. However, because the structure has many steps, our initial idea was that it suitable for photoelectrochemical reactions due to its high activity for surface chemical reactions. After successfully fabricating the nanopagodas, we modified them with silver nanoparticles that exhibit surface plasmon resonance in order to improve the efficiency of sunlight utilization, and evaluated the effect of the modification by electromagnetic field analysis..What we found however, was that the zinc oxide nanopagoda has the property of absorbing incident light, especially ultraviolet rays, into the zinc oxide. This was a completely unexpected, but fortuitous discovery, as this property contributes to the improved photoelectrochemical properties.”

Currently, Marwa and students of the same laboratory are investigating the effects of precise structural control of zinc oxide nanopagodas, as well as surface decoration with other materials, on their photoelectrochemical properties. Since zinc oxide is prone to photocorrosion due to its inability to withstand long-term sunlight irradiation, improving its durability via surface decoration is a major current theme. Upon achieving both high photoelectrochemical properties and durability, we plan to carry out water splitting hydrogen production in a real environment (decomposition of river water or seawater by sunlight) to extract real issues.

This research was supported by JSPS Grants-in-Aid for Scientific Research 21K18823, 21K18824, and 22K04737, the Casio Science Promotion Foundation, the ENEOS TonenGeneral Research/Development Encouragement & Scholarship Foundation, and the Ministry of Higher Education of the Arab Republic of Egypt PD-071.