New perspectives of ZnO nanorods as efficient water splitting and O2 evolution photo-electrocatalyst

Significance 

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.

New perspectives of ZnO nanorods as efficient water splitting and O2 evolution photo-electrocatalyst - Advances in Engineering

About the author

Katerina Govatsi received her B.S. degree in Physics from University of Patras in 2011. She was awarded her M.S. degree in solid-state Physics at the same department in 2013 and completed her doctoral studies at the Institute of Chemical Engineering Sciences (FORTH/ ICE-HT) and the Department of Chemistry (University of Patras) in 2019 under the supervision of Dr. S. N. Yannopoulos and Dr. S. G. Neophytides. Her doctoral research focuses on ZnO semiconductor photo-electrochemistry for water splitting, synthesis and characterization of ZnO nanowires and nanoparticles and their heterostructures using earth-abundant transition metals or materials with narrow band gap. A part of her PhD studies had been funded by Stavros Niarchos Foundation. She has published 8 papers in peer reviewed journals and 1 chapter in book.

About the author

Andreas K. Seferlis obtained his Diploma in Chemical Engineering in 2003 from University of Patras. After his military service he continued his studies with Dr. S. G. Neophytides at the Institute of Chemical Engineering Sciences (FORTH/ ICE-HT) and obtained his MSc and PhD in 2010 from University of Patras. He continues his research as post-doc in FORTH/ ICE-HT. Since 2015 he also works as S.E.M. operator in the University of Patras.

His research interests are focused on semiconductor photo-electrochemistry, nanomaterial synthesis, characterization, optimization and imaging. He is particularly interested of nanostructure composites of Titanium Dioxide, Zinc oxide and their electrochemical promotion through reductive doping for photo-electrochemical water splitting.

About the author

Dr. Spyros N. Yannopoulos is Research Director at FORTH/ ICE-HT. He graduated from the Physics Department and received his PhD in Chemical Physics from the Department of Chemical Engineering (University of Patras, 1996). His main research activities are focused on two directions, the synthesis and applications of novel functional nanostructures and exploration of advanced amorphous materials. Efforts in his research group are directed towards understanding the fundamental issues of nanostructured assemblies and atomic arrangement in disordered solids, which will enable the rational control of materials’ properties and functionalities. A primary goal is the advancement of scalable synthesis routes beyond the laboratory scale, with particular emphasis on the development of simple, inexpensive synthetic approaches, e.g. laser-assisted fabrication of graphene and graphene-based structures, large-scale CVD grown few-layer transition metal dichalcogenides.

Current interests also include low dimensional nanostructures, 1-D and 2-D crystals, with potential applications in energy conversion/storage/harvesting, (photo)-catalysis, gas sensing, nanotribology, etc., as well as in amorphous semiconductors with potential applications in photonics and optoelectronics. He has published more than 135 refereed papers (ISI), 12 invited chapters in books and more than 50 articles in international proceedings.

About the author

Dr. Stylianos Neophytides obtained his Diploma in Chemical Engineer in 1985 from University of Patras and completed his doctoral studies in the Chemical Engineering Department of University of Patras in 1990 on the catalysis and electrocatalysis field. He continues his research as post-doc at the University of Ghent, Belgium. Since 2005 he is Research Director at FORTH/ ICE-HT. His research interest are focused in the field of heterogeneous catalysis, electrochemistry, fuel cells and electrolysers and photoelectrochemical water splitting, as well as the study of surfaces and adsorbed reaction species using spectroscopy (XPS, FTIR). He has coordinated more than 15 international and national research projects and participated in total 25 projects. He is the co-founder of ADVENT TECHNOLOGIES, which was established in 2005 to commercialize new fuel cell materials that had been invented by his group at FORTH/ICE-HT. He has published 194 papers in international refereed journals, including books and chapters, which received more than 4400 citations (h index: 38).

References

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.

Go To Electrochimica Acta

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.

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