Influence of the oxygen concentration on the formation of crystalline phases of TiO2 during the low-pressure arc-discharge plasma synthesis

Abstract

Abstract. The synthesis of titanium dioxide (TiO 2 ) nanoparticles with different percentage of anatase and rutile phases is investigated. The synthesis is performed by controlling the oxygen percentage in the gas mixture in the plasmachemical evaporation-condensation process employing a low-pressure arc discharge. In all our experiments, the pressure in the plasmachemical reactor and the average size of particles remain constant and are 60 Pa and 6 nm, respectively. The crystal structure of synthesized TiO 2 is studied using X-ray diffraction; the morphology of the particles is analyzed employing transmission electron microscopy. Using Xray phase analysis, it is established that the concentration of the TiO 2 anatase phase decreases upon a decrease in the oxygen concentration in the gas mixture. It is shown that the TiO 2 anatase phase is more efficient for photocatalytic decomposition of methylene blue than the rutile phase. IntroductionTitanium dioxide (TiO 2 ) is one of the most studied materials; it has many applications in industry and research, especially in photocatalysis [1,2]. A comprehensive review of the surface properties of TiO 2 , in which the interrelation between the structure and properties is emphasized, is given in [3]. The photoactivity of TiO 2 is pointed out in [4] as one of the most interesting and attractive properties. Experimental and theoretical investigations of the structural and electronic properties of TiO2 are described in [5]. TiO 2 can exist in a few stable crystalline forms the most widespread of which are anatase, rutile, and brookite [6]. The energy properties of TiO 2 polymorphs (relative to the rutile phase) was studied in [7], where it was found that they are lower by 0.71 kJ/mol for the brookite and are higher by 2.61 kJ/mol for the anatase. The brookite phase is stable only at very low temperatures and, therefore, is almost of no practical importance. The rutile and anatase phases usually coexist as mixed phases; that is why some efforts are necessary for obtaining the required pure phase. In recent years, the anatase phase of TiO 2 has aroused great interest owing to the technological applications in paints, pigments, and photocatalysis. It was reported that anatase has a higher photocatalytic activity than rutile [8]. Thus, the synthesis of TiO 2 with a preset crystalline phase on a large scale is of great practical importance. Although the chemical methods make it possible to obtain TiO 2 in large amounts [9], their structure, however, is mainly amorphous. It is shown in [10] that the presence of amorphous structure in TiO 2 reduces its photacatalytic activity and, consequently, the synthesis of nanocrystalline TiO 2 is an urgent problem. The results of chemical synthesis show that the time and the temperature of calcination greatly affect the TiO 2 nanopowder [11]. Although the phase transformations of TiO 2 during annealing of chemically synthesized TiO 2 have been studied by many researchers [12], the data on the temperatures in the reactions are controversial. Th...

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