Anions and cations distribution in M5+/N3- co-alloyed TiO2 nanotubular structures for photo-electrochemical water splitting

Significance Statement

Developing new materials for solar energy conversion has been the aim of most researchers. Solar energy can be considered a decentralized and an inexhaustible natural resource. Solar energy reaching the earth’s surface is thousands times more than the energy actually consumed in our societies. Apart from the photovoltaic technologies that convert solar energy into electricity, it has been found that it is necessary to come up with devices that use solar energy to initiate a chemical reaction that generates energetic molecules that can be stored and used on demand as a fuel.

Hydrogen is a storable chemical fuel that can be implemented in various processes including electricity production in fuel cells or even chemical preparation. Hydrogen comes with specific energy density that is three times more than that of methane while its combustion product is water only. Therefore, coming up with an eco-friendly procedure to generate hydrogen is a big challenge. Water splitting is one of the solar to hydrogen energy conversion process that depends on the use of a semi-conductor as a photo electrode to convert photons into electron-hole pairs that are in charge of oxidation and reduction of water into oxygen and hydrogen.

Water splitting demands a particular semiconducting material having particular attributes in a bid to realize effective solar to chemical energy conversion. The semiconductor conduction band energy should be above the hydrogen evolution reaction energy while its valence band energy should be below the oxygen evolution energy to thermodynamically realize the two reactions.

Titanium oxide is a material that has been investigated most for electrochemical water splitting. It possesses appropriate energy band positions along with superior features such as stability against photo-corrosion, non-toxicity, and low cost manufacturing. In a bid to improve its charge transport attributes and reactivity, it is necessary to control the preparation of titanium oxide at the nano-metric scale and control its electronic properties.

Thomas Favet, Dris Ihiawakrim, Valérie Keller, and Thomas Cottineau at the University of Strasbourg in France, synthesized (Nb, N and Ta, N) co-alloyed aligned titanium oxide nanotubes. This co-alloying approach aims to introduce large quantities of cationic (Nb5+ or Ta5+) and anionic (ex: N3-) hetero atoms species in a stoechiometric ratio in order to significantly improve the light absorption without creating unwanted defects in the structure. Cation insertion is achieved through an elementary anodization process. This was then followed by a thermal treatment in ammonia in order to introduce nitrogen. Their work is published in journal, Materials Science in Semiconductor Processing.

The authors tested various samples in a photo-electrochemical water splitting experiment. Cyclic voltammetry exhibited varying behavior dictated by the thermal treatment applied. At a preliminary annealing step at 450 °C under nitrogen led to black titanium oxide nanotubes that exhibited capacitive behavior as well as a low photo-activity. When the first heating step was achieved under airflow, the samples indicated a faradic characteristic with a distinct photocurrent at the positive potentials.

The photoelectrochemical response of the samples indicated that the co-alloyed samples presented an activity in the visible improved more than four times as opposed to N-doped titanium oxide nanotubes. The authors realized that this activity was maximal with an optimized thermal treatment encompassing the first step at 450 °C under airflow, which was followed by a 12 h thermal treatment at 500 °C under ammonia.

These conditions, the incorporation of cations and anions in the alloyed samples tends towards a perfect balance of the charge between N3- and M5+ anions. The co-alloying with high concentration allowed for an effective band gap narrowing followed by a charge compensation process. These two modifications of the features of titanium oxide enhanced the photo-electrochemical activity of the materials.

Anions and cations distribution in M5+N3- co-alloyed TiO2 nanotubular structures for photo-electrochemical water splitting. Advances in Engineering

About the author

Thomas Favet is a PhD student involved in a joint project between ICPEES (CNRS, Strasbourg, FR) and INRS-EMT (Montréal, CA). His PhD research aims at developing titanium dioxide based nano-material for photo-electrochemical production of hydrogen via the water splitting process. Prior to this, he graduated a master degree in Material Sciences at Paul Sabatier University (Toulouse, FR) and worked one year at ICPEES as an research engineer, in the “Photocatalysis and Photoconversion” team. He had the opportunity to present his first promising results in a peer review journal and in two international conferences.

About the author

Dris Ihiawakrim works as a research engineer at the Institute de Physique et Chimie des Materiaux de Strasbourg since 2008. Currently he is in charge of several projects in different areas related with electron microscopy for the characterization of nanostructured organic, inorganic and biological materials. He has lead the implementation and development of cryo and in situ electron microscopy techniques in his research group.

His expertise includes the preparation and characterization of thin-layered and biological samples, nanoparticles, mesoporous and graphene-like materials, zeolites, and other nanometric systems. He has more than 30 publications high impact journals. He collaborates in various international projects with Morocco, Germany, Canada and Brazil. His results were presented in several international conferences and he has been invited to present his work in foreign universities. Additionally, he has teaching experience in Physics for high-school and master students.

About the author

Valérie Keller is a senior scientist at ICPEES in Strasbourg. She received her Ph.D. degree in Chemistry and Catalysis from the University Louis Pasteur of Strasbourg in 1993. In 1996 she returned to Strasbourg and was appointed as researcher in CNRS, where she is now responsible of the Team “Photocatalysis and Photoconversion”.

Her main research activities concern photocatalysis for environmental, energy and health applications, and the synthesis and characterization of nanomaterials for photoconversion purposes. She is the author of over 100 publications in peer-reviewed journals and more than 90 oral communications in international conferences and symposium. She is also the author of 15 patents. In 2013 she was awarded the 1st Price of the Strategic Reflection (awarded by the French prime Minister).

About the author

Thomas Cottineau is a researcher at the IPCEES CNRS laboratory in Strasbourg since 2014. He received a MSc degree in material science and engineering at the Polytechnique school of the University of Nantes (2004) and obtain his PhD in material chemistry from the University of Nantes (2007). Following postdoctoral work with Pr Daniel Bélanger at the University of Québec in Montréal, he worked as a postdoctoral researcher at the LMSPC (CNRS/University of Strasbourg).

His research interests concerns the design and synthesis of nanostructured materials and surfaces for energy conversion, chemical detection and biomedical applications. He is the author of over 20 publications in peer-reviewed journals, 2 patents and more than 15 oral communications in international conferences and symposium

Reference

Thomas Favet, Dris Ihiawakrim, Valérie Keller, Thomas Cottineau. Anions and cations distribution in M5+/N3- co-alloyed TiO2 nanotubular structures for photo-electrochemical water splitting. Materials Science in Semiconductor Processing, volume 73 (2018), pages 22–29.

 

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