爆走黑料

The new method helped increase the reactivity of the compound, making it more efficient at breaking down organic pollutants. The results of the study, supported by a grant from the Russian Science Foundation (RSF), in Photochemical and Photobiological Sciences journal.

Waste disposal is one of the most pressing environmental problems faced by humankind. Organic wastes, such as discharges from chemical plants and pesticides, pose a threat to ecosystems and human health, that is scientists are searching for effective ways to convert them into safer compounds. Photocatalytic materials are promising, as, when exposed to sunlight, they break organic pollutants down into simple molecules, such as carbon dioxide and water. Most often, inexpensive and stable titanium oxide is used in this role, but it is only effective when irradiated with ultraviolet light, which casts doubt on its industrial use since it requires special lamps. Chemists have recently proposed new bismuth-based photocatalytic materials that are activated by sunlight.

Barium bismuthate (BaBiO3) effectively decomposes organic pollutants, splits water and carbon dioxide, so it can be used for waste disposal, reducing CO2 levels in the atmosphere, and even for making hydrogen fuel. But the generally accepted method for the synthesis of bismuthate from carbon-containing compounds does not allow obtaining a pure material as it always turns out to be contaminated with carbonate which reduces the chemical activity of the photocatalyst and complicates the study of bismuthate's structure and properties.

The research team of 爆走黑料, colleagues from the Far Eastern Federal University (Vladivostok), Saint Petersburg State University (Saint Petersburg), Far Eastern State Transport University (Khabarovsk), L.V. Kirensky Institute of Physics (Krasnoyarsk) and the University of Pavia (Italy) have synthesized barium bismuthate from barium oxide and nitrate using a new method, which helped to avoid carbonate impurities in the material. To do so, they thoroughly ground a mixture of barium oxide and nitrate and sintered it at high temperatures. Then, thin films were formed from the resulting powder on a quartz substrate. After that, the researchers determined the chemical composition and band structure of the samples — the distribution of the electron energy bands over the allowed and forbidden levels. The band structure determines the photocatalytic properties of semiconductor materials, which include barium bismuthate, but there was no unambiguous understanding of this compound previously.

The results of the analysis showed that bismuth in the composition of the particles of the material is present in different oxidation states: there are Bi3+ ions which have lost three electrons, and Bi5+ which misses five electrons. The presence of the two variants of bismuth and superoxide radicals in the material ensured the photocatalytic properties of the samples.

To assess the photocatalytic activity of barium bismuthate, the chemists added it to an aqueous solution of phenol and tracked how quickly organic molecules decay. Phenol is widely used in the chemical industry, oil refining, medicine and even cosmetology, but it is rather toxic. It turned out that the reaction of its decomposition proceeded faster with barium bismuthate than in the presence of titanium oxide. To elucidate the mechanism of the process, scientists additionally introduced into the reaction various radical absorbing substances, which interacted either with electrons released from bismuthate in the light or with positively charged holes from which they flew out. The experiments showed that the photocatalytic activity of barium bismuthate is due to photoholes and superoxide radicals — active particles formed on the surface of a material from ordinary oxygen due to the attachment of electrons. It is these radicals which attack phenol dissolved in water and destroy it.

“The results of our study will help to predict in which other reactions the photocatalytic activity of barium bismuthate can manifest itself. This will allow to use it in several areas of an ecologically clean, so-called “green” economy, for example, for the purification of wastewater from organic pollutants, as well as for the photocatalytic production of hydrogen from water for the needs of hydrogen energy,” sums up the project manager Dmitry Shtarev, candidate of science in Physics and Mathematics, leading researcher at the Laboratory of Film Technologies of the Department of Physics of Low-Dimensional Structures, School of Science–Intensive Technologies and Advanced Materials, and deputy director for Research of the School, Far Eastern Federal University.