Abstract
Sub‐5 nm cobalt oxide nanoparticles are produced in a flowing water system by pulsed laser fragmentation in liquid (PLFL). Particle fragmentation from 8 nm to 4 nm occurs and is attributed to the oxidation process in water where oxidative species are present and the local temperature is rapidly elevated under laser irradiation. Significantly higher surface area, crystal phase transformation, and formation of structural defects (Co2+ defects and oxygen vacancies) through the PLFL process are evidenced by detailed structural characterizations by nitrogen physisorption, electron microscopy, synchrotron X‐ray diffraction, and X‐ray photoelectron spectroscopy. When employed as electrocatalysts for the oxygen evolution reaction under alkaline conditions, the fragmented cobalt oxides exhibit superior catalytic activity over pristine and nanocast cobalt oxides, delivering a current density of 10 mA cm−2 at 369 mV and a Tafel slope of 46 mV dec−1, which is attributed to a larger exposed active surface area, the formation of defects, and an increased charge transfer rate. The study provides an effective approach to engineering cobalt oxide nanostructures in a flowing water system, which shows great potential for sustainable production of active cobalt catalysts.
Highlights
As the energy crisis and environmental pollution increase, public interest and research topics have increasingly focused on developing a sustainable energy conversion/storage system with high efficiency and economical scalability.[1]
Water splitting provides a promising path for converting renewable energy into hydrogen, a clean fuel that can be consumed for various energy demands.[2]
Accepted manuscript online: November 22, 2019 Version of record online: December 30, 2019
Summary
As the energy crisis and environmental pollution increase, public interest and research topics have increasingly focused on developing a sustainable energy conversion/storage system with high efficiency and economical scalability.[1]. Laser-induced engineering of materials has been adopted to effectively reduce particle sizes with a narrow size distribution.[18] As no surfactants are required during the process, with pure water employed as the solvent, the products from laser synthesis expose a clean surface without the surface-blocking effect of molecular ligands or residues of chemical precursors.[18] In addition, high temperature and pressure can be generated locally upon laser irradiation, followed by a rapid cooling down of the products Such a process normally induces formation of defects and structural disorder in the obtained product, resulting in reduced particle size and modulated electronic states.[18,19] A large range of starting materials could be subjected to laser irradiation for efficient structural engineering, including metal colloids, ionic crystal powders, and semiconductors.[18] Remarkably, this technology is appealing for material synthesis owing to its simple preparation process compared with chemical synthesis.[18,20] laser irradiation is a desirable technology to prepare cobalt oxide based electrocatalysts for OER. The fragmented and oxidized cobalt oxide shows a much higher OER activity compared with those of the initial oxides and the ordered mesoporous Co3O4 that were nanocast from ordered mesoporous silica
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