Laser-Treated Transition Metal Oxides for Water Splitting

The civilizational development of the past century has enabled steady consumption of fossil fuels to meet humanity's energy requirements. However, their depleting reserves along with climate change urge us to develop cleaner, more efficient fuels of the future. The most prominent fuel so far is hydrogen due to potentially unlimited availability, energy density, and – as it burns to water – sustainability. Unfortunately, nowadays hydrogen production is rarely achieved through eco-friendly means due to the availability of cheaper, albeit dirtier techniques.<br/>The high-surface titania nanotubes (TNTs), known for their corrosion resistance, are often synthesized via a scalable electrochemical anodization process. Through the adjustments in the process parameters, such as electrolyte composition, temperature, and potential applied, their geometry can be tailored, changing their properties depending on the specific needs. Although they exhibit photoactivity, their 3.2 eV wide-bandgap limits their efficiency under visible light. Nevertheless, the TNTs can be regarded as an excellent material for further modifications.<br/>The transition metal oxides, as a part of the non-noble family, are becoming increasingly popular due to vast availability and, thanks to the tailored synthesis pathways, are often almost as feasible as more expensive alternatives. The transition metals from the 4^th period especially, are one of the most abundant elements on Earth, and although iron, cobalt, nickel, and copper have all been used as catalysts for water splitting, the obtained results vary significantly depending on the synthesis pathways. It is known, however, that the activity of both titania and the transition metal oxides benefit from the presence of defects, such as oxygen vacancies, therefore, to achieve a simple and efficient synthesis pathway, methods promoting structural disordering should be considered.<br/>We, therefore, propose a titania-based electrode modified with 4^th group transition metals (Fe, Co, Ni, Cu) and treated with pulsed laser radiation (Nd:YAG, 355 nm) in a vacuum. The proposed approach eliminates the usage of metal liquid precursors while employing scalable and well-controlled fabrication steps. Moreover, the performance of the electrodes towards water splitting in alkaline media is investigated, and the reaction overpotentials are calculated for the best-performing samples. Furthermore, the mechanism responsible for the enhancement is proposed based on a semiconductor charge-transfer theory.<br/>This work was supported by the Polish National Science Centre. Grant Number: 2017/26/E/ST5/00416.