Photoluminescence and Raman spectroscopy studies of low-temperature γ-Al2O3 phases synthesized from different precursors
Graphical abstract
Raman and photoluminescence spectra of γ-Al2O3 samples synthesized from crystalline boehmite and nanodispersed pseudoboehmite.
Introduction
Alumina is widely applied in various fields of science, engineering and industry [1], [2]. For example, alumina is used in optics and optoelectronics as optical windows, coatings and active elements for solid-state lasers. It is also used for production thermoluminescent detectors, abrasives and a wide set of catalysts for different chemical processes [2]. In all cases, during synthesis of alumina the starting compounds, theirs phase and impurity compositions are of great importance. For such practical applications low-temperature (γ-, η-, χ-) and high-temperature (θ-, δ-, κ-, α-) phases of alumina are used. It is known that γ-Al2O3 can be used to produce θ-, δ- and α-Al2O3 [3], [4]. Investigation of physicochemical properties of γ-Al2O3 as a promising nanostructured material remains quite topical. It is known, particularly from XRD data, that depending on the initial compound (crystalline boehmite or pseudoboehmite), γ-Al2O3 is represented by one of two spinel-like structures with different unit cell parameters: γ- and γ∗-Al2O3 [ICSD Collection Code 66559; ICSD Collection Code 953020]. It has been shown previously that the properties of such γ-Al2O3 phases strongly differ in some structural and morphological characteristics [5]. This result was obtained by various physicochemical methods and, particularly, by photoluminescence (PL) spectroscopy using Cr3+ and Fe3+ ions as the luminescent probes in their natural impurity concentration. However, for verification of the result obtained with these nanostructured materials, it was necessary to acquire additionally the Raman spectroscopy data and highlight the differences in the properties of these phases using other fluorescent probes.
In this work, we aimed to perform a Raman spectroscopy study for revealing differences in the local structure of γ- and γ∗-Al2O3 phases synthesized from crystalline boehmite and pseudoboehmite, respectively, and search of characteristic differences in the luminescence of each phase.
Section snippets
Experimental
The study was performed with nanosized powders of γ-Al2O3 phases synthesized from crystalline boehmite and pseudoboehmite (without the 020 reflection) and α-Al2O3 phases obtained by thermal treatment of γ-Al2O3 at 1250 °C for 4 h. Methods and conditions of the synthesis were described in detail in Ref. [5]. The γ-Al2O3 samples are denoted as γ (from crystalline boehmite) and γ∗ (from pseudoboehmite), while the α-Al2O3 samples as α and α∗, respectively.
X-ray diffraction analysis (XRD) of the
Physicochemical study
According to XRD data, the tested samples are the well-crystallized γ- and α-phases of alumina, respectively. The γ- and γ∗-Al2O3 phases are the spinel-like species that differ from each other, particularly, in the ratios of octahedral and tetrahedral positions and the Me–O distances. In distinction to γ∗-Al2O3 sample, γ-Al2O3 has a defect spinel structure and is described by crystallochemical formula Al8[□2.67Al13.33]O32. It contains Al3+ ions in tetrahedral (Al3+Td) and octahedral (Al3+Oh)
Discussion
As shown in [5], the γ- and γ∗-Al2O3 samples strongly differ from each other. According to the works [7], [8], [9], [10], ‘γ-phase of Al2O3’ does not reveal any Raman features in the frequency region 200–1200 cm−1. Despite this fact, the obtained Raman spectra of γ- and γ∗-Al2O3 not only demonstrate a number of peculiarities in this frequency region but also reveal some differences between these samples. As seen from Fig. 1, the differences are most pronounced in the frequency regions of
Conclusion
The nanosized spinel-like structures γ- and γ∗-Al2O3 differing in the unit cell parameters were studied. Samples of γ- and γ∗-Al2O3 were synthesized from crystalline boehmite and pseudoboehmite, respectively. According to X-ray fluorescence analysis, the content of impurity 3d elements in each of the synthesized γ- and γ∗-Al2O3 phases does not exceed 10−3 wt.%. The luminescence studies showed that all the tested samples possess photoluminescence. Depending on the excitation condition, PL
Acknowledgements
The study was supported by the Russian Foundation for Basic Research (Projects No. 14-03-31704 young_a and 16-38-00353 young_a) and Basic budgetary financing. The authors are grateful to A.P. Yelisseyev for his support of the work.
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