Since scientists first discovered the adverse effects of carbon-based fuels, there have been immense efforts to decarbonize our energy systems. Such a bold step has had its own set of challenges but recent inputs have shown positive results. Amongst numerous technologies and techniques that have been put forth, the use of photo-electrochemical processes to split water into hydrogen and oxygen gasses has been seen as the holy grail of the scientific and engineering community. It has earned such a prestigious title credit to the fact that the technique enables the capture and storage of solar energy by means of hydrogen gas energy carriers. Nonetheless, a fundamental challenge has plagued this approach, particularly regarding the identification and development of photo-anode semiconductors with both the appropriate band gap so as to straddle the thermodynamic voltage for water splitting reaction and the most efficient crystal structure that facilitates the fast charge transfer, thus minimizing the recombination losses.
Consequently, efforts have been directed towards the development of one-dimensional (1-D) nanostructures – such as nanorods/nanowires, to replace nanoparticle-based films mainly made using metal oxide semiconductors such as those of TiO2, ZnO, WO3 and Fe2O3. ZnO has attracted much attention, however, there still lacks in-depth understanding regarding the exact mechanism and electrokinetics of its photoelectrochemical water splitting properties and its stability in alkaline electrolytes at pH<13.
In this view, researchers at the Foundation for Research and Technology Hellas namely Dr. Katerina Govatsi, Dr. Andreas Seferlis and Research Directors Spyros Yannopoulos and Stylianos Neophytides investigated in depth the photoelectrochemical properties of Zinc oxide nanorod arrays. Their goal was to shed light on the stability of ZnO nanorods arrays and photo-electrokinetics of oxygen evolution reaction on the Zinc oxide surface interfaced with 0.1 M aqueous sodium hydroxide solution. Their work is currently published in the research journals, Electrochimica Acta and International journal of Hydrogen Energy.
They started by growing Zinc oxide (ZnO) nanorod arrays, a step that involved the application of a hydrothermal method. During this process, Fluorine-doped tin oxide and Indium-doped tin oxide conductive glasses were used as substrates. The prepared Zinc oxide samples had their morphology characterized using field-emission scanning electron microscopy, X-ray diffraction, RAMAN and photoluminescence spectroscopy – among other intricate characterization techniques. Finally, photoelectrochemical characterization was undertaken coupled with the detection of oxygen and hydrogen peroxide by use of permanganate analytical method.
The authors observed for the first time that the ZnO surface is highly selective to the oxygen evolution reaction that took place on the side-wall (110) surface of the nanorods and its high reactivity and selectivity could be attributed to the photo-induced oxygen vacancies, which were seen to possess the ability to be oxidized and photo-reduced with relatively high reaction rates. Further, the researchers reported that the electrocatalytically active sites were the zinc surface atoms.
In summary, the Greek study was based on the potentiodynamic and potentiostatic measurements as well on specific surface area analysis data for Zinc oxide nanorods array. The team reported a rather efficient electrocatalytic surface (Zinc oxide prepared by chemical bath deposition) as compared to other photo-excited semiconductors e.g. TiO2. Overall, the transient kinetic analysis of the potentiodynamic measurements resulted in the determination of the electrokinetic parameters, symmetry factors and exchange current densities of the hydroxide- based species, thus providing significant insight regarding their binding energies and their reactivity on the Zinc oxide surface.
This work opens up new perspectives for the use of ZnO 1-D nanostructures for the development of novel nanocomposite more efficient photo-anodes for water splitting and oxygen evolution.
Katerina Govatsi, Andreas Seferlis, Spyros N. Yannopoulos, Stylianos G. Neophytides. The photo-electrokinetics of the O2 evolution reaction on ZnO nanorods. Electrochimica Acta, volume 298 (2019) page 587-598.
Katerina Govatsi, Andreas Seferlis, Stylianos G. Neophytides, Spyros N. Yannopoulos. Influence of the morphology of ZnO nanowires on the photoelectrochemical water splitting efficiency. International journal of Hydrogen Energy, volume 43, (2018) page 4866-4879.