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Influence of the Size of Coal Ash FAU Zeolites Used as Dopants on the Sensing Properties of Nb2O5 Thin Films
* 1 , 2 , 3 , 4 , 3
1  Institute of Optical Materials and Technologies “Acad. J. Malinowski”, Bulgarian Academy of Sciences, Akad. G. Bonchev str, bl. 109, 1113 Sofia, Bulgaria
2  Technical University of Sofia, Department of Thermal and Nuclear Power Engineering, Sofia, Bulgaria
3  Institute of Optical Materials and Technologies “Acad. J. Malinowski”, Bulgarian Academy of Sciences
4  Technical University of Sofia, College of Energy and Electronics, Sofia, Bulgaria.


FAU (faujasite) is a rare natural zeolite which has its synthetic counterpart zeolite X. The sodium form of the synthetic zeolite X - Na-X, is widely used because of its structural supercage with large pore size and high specific surface area. It usually finds application for gas adsorption, separation, ion-exchange, etc.

In this study solid waste from coal combustion in thermal power plants (TPP) was used for synthesis of zeolite Na-X samples. They were prepared by long-term alkaline atmospheric conversion of coal ash collected from the electrostatic precipitators in TPP “AES Galabovo” supplied by lignite coal from the “Maritza East” basin. When used in form of thin films/layers, optical detection of VOCs (Volatile organic compounds) is possible due to change of color of the sample. In order to improve the sensing properties of synthesized zeolites, they were wet-milled for 60 seconds and both, milled and not milled, were used as a dopant for the niobium oxide matrix in form of thin film deposited by the spin-coating method on silicon substrate.

The surface morphology and structure of both zeolites powders were studied by scanning electron microscopy, while their size is determined from DLS (Dynamic Light Scattering) spectra. Optical constants (refractive index, n and extinction coefficient, k) and thickness of the films were calculated. The change in the reflection coefficient ∆R of the films was determined from measured reflectance spectra prior to and after exposure to probe acetone molecules. An increase in the reaction of the films with milled zeolites to acetone compared to the samples with not milled zeolites is demonstrated.


The financial support of Bulgarian National Science Fund (BNSF) under the project DN 17/18 (12.12.2017) is highly appreciated. Research equipment of distributed research infrastructure INFRAMAT (part of Bulgarian National roadmap for research infrastructures) supported by Bulgarian Ministry of Education and Science under contract D01-284/17.12.2019 was used in this investigation.

Keywords: thin films; Nb2O5; FAU; zeolites; coal ash; optical sensing; acetone.