Ceramic that changes from white to blue while absorbing and releasing oxygen: Discovery by Kanagawa University

A research group at Kanagawa University and other institutions has discovered a ceramic material that can absorb and release oxygen simply through temperature changes, turning from white to deep blue depending on the amount of oxygen it has taken in. While conventional materials often require special gases to release oxygen, this new ceramic can perform the oxygen exchange under mild conditions, such as in normal air or an oxygen-filled environment. Potential applications include oxygen storage material, oxygen sensors, and color-changing pigments.

Professor Teruki Motohashi, who focusses on material chemistry at the Faculty of Chemistry and Biochemistry, Kanagawa University, has spent 20 years with the aim of applying ceramics that efficiently absorb oxygen as oxygen storage materials. Under his supervision, Researcher Kosaku Ohishi (also in material chemistry) systematically investigated the properties of materials with a "melilite-type structure." Named after the natural mineral melilite, this crystal structure is characterized by three elements (cations), including metals, each of which is associated with an oxygen ion in a layered arrangement.

To find an optimum combination of elements for oxygen uptake, the team tested various ceramics based on manganese, which is a known element for oxygen storage. They prototyped several ceramics with a melilite-type structure by swapping and recombining strontium and barium, as well as silicon and germanium. Their oxygen uptake properties were examined and it was found that ceramics composed of barium, manganese, and germanium (BMG), baked at 1,100 degrees Celsius, were the best, storing up to 1.2% of their weight in oxygen.

While conventional manganese-based oxygen storage materials have the property of constantly absorbing oxygen whenever it is present in the surroundings, they require highly reducing gases (gases with the power to reverse the oxidized state) such as hydrogen, ammonia, or carbon monoxide to release that oxygen. In contrast, BMG was found to take in oxygen at around 200 degrees Celsius when heated in an oxygen-filled environment, and at 400-500 degrees Celsius, it releases oxygen rapidly without the presence of a strong reducing gas. It has a property of turning white when it absorbs the least amount of oxygen and becoming darker blue as the amount of oxygen increases.

The crystal structure of BMG was investigated by X-ray absorption spectroscopy, synchrotron in situ X-ray diffraction, single crystal X-ray diffraction, and powder neutron diffraction. The results revealed that while the structure is a tetrahedron (triangular pyramid) with four oxygen ions surrounding each manganese ion when oxygen is released, it transforms into a trigonal bipyramid (a shape formed by two triangular pyramids joined at the base) with five oxygen ions surrounding each manganese ion upon oxygen absorption. The resulting melilite-type structure of the crystals was different from the basic form.

Motohashi said, "The reversible oxygen absorption and release properties under mild conditions (such as in air or filled with oxygen) could be used in the future to separate oxygen from air. It could also be used as a catalyst-like material that exhales oxygen to purify carbon monoxide and other substances contained in automobile exhaust gas."

This research was conducted in collaboration with Tohoku University, Kyoto Institute of Technology, and Kindai University, and was published in the American Chemical Society's journal, Chemistry of Materials, on January 24.