Luminescence of photoactivated pristine and Cr-doped MgAl2O4 spinel
Introduction
Photoactivation of metal oxides leads to various relaxation processes on the surface and in the bulk of the solids. Such relaxation processes may result in the emission of photons, (i.e. luminescence), which is an example of the physical pathway of system relaxation. Such physical relaxation can be either complete resulting in restoration of the initial ground state, or else incomplete yielding a metastable excited state of the solids due to formation of electronic states within the energy band-gap in the metal oxides. The latter can manifest itself as formation of photoinduced color centers or photo-coloration [1], [2]. Concomitantly, a chemical relaxation pathway can also be realized in surface chemical processes occurring at the interface of the heterogeneous system [3]. Complete relaxation through the chemical pathway corresponds to the photocatalytic process, while incomplete relaxation in the heterogeneous system results in such surface chemical modification as photostimulated adsorption. Both chemical and physical relaxation pathways are closely inter-connected to each other.
The Photo-Induced Chemisorption Luminescence (PhICL [4], [5], [6], [7], [8], [9]) is a remarkable and attractive phenomenon to observe in such interconnection between chemical and physical relaxation pathways in a single process when chemical relaxation through interaction of electron-donor molecules (H2, CH4, H2O, NH3) with photoactivated surface active sites triggers the physical relaxation process of luminescence observed as a flash of light (the PhICL phenomenon). This phenomenon was originally observed by Andreev and Kotel’nikov on the photoactivated surfaces of Al2O3, BeO and MgO [4], [5] as a result of dissociative adsorption of either water or dihydrogen. From the very first observation of PhICL phenomenon, which has attracted considerable attention, a question arose as to the source of the luminescence flash. Andreev and Kotel’nikov initially proposed that this luminescence was caused by relaxation of the electronically excited surface-adsorbed species such as the OH groups (Eq. (1)).Os− + RH → (OHs−)* + R → OHs− + hνwhere Os−is the photoinduced surface-active center of dissociative adsorption of hydrogen-containing electron-donor molecules RH, and the (OHs−)* and OHs− are the surface hydroxyl groups formed as a result of dissociative adsorption in their electronically excited and ground states, respectively; R denotes the radical species as products of dissociative adsorption, and hν is the luminescence emission. However, in our study of the PhICL phenomenon occurring on the photoactivated surfaces of ZrO2 and γ-Al2O3 we observed that the PhICL spectra were very similar to the emission spectra of the photo- and/or thermo-luminescence of the metal oxides [7], [8]. This inferred that the physical pathway of relaxation occurred in solids, and that the primary chemical process of dissociative adsorption of H-bearing molecules led to the additional excitation of the solids. To further confirm this hypothesis, the present study examines the pristine and the Cr-doped MgAl2O4 spinel to explore this PhICL phenomenon for two principal reasons: (1) the pristine MgAl2O4 spinel is a well-known photoactive material displaying its photochemical activity in various surface chemical processes, including dissociative adsorption of hydrogen-containing electron-donor molecules (H2, CH4, NH3), that are then followed by the PhICL phenomenon [9]; and (2) various types of luminescence of Cr-doped MgAl2O4 spinel have been studied extensively and luminescence characteristics of Cr states in spinel are well established both theoretically and experimentally [10], [11], [12], [13], [14], [15]. Accordingly, if the PhICL spectra included spectral features related to Cr-states it would demonstrate that dissociative adsorption results in the excitation of the metal oxide.
Section snippets
Experimental
Powdered pristine spinel was obtained from S.I. Vavilov State Optical Institute (Russia). It was doped using a thermochemical method by heating the mixture of spinel and chromium(III) chloride in spinel at 1100 °C for 10 h to obtain 1 at.% of Cr3+-states. It was characterized by X-ray diffraction which showed the peculiar spinel crystallographic phase; X-ray photoelectron spectroscopy (XPS) provided an estimation of the Cr3+ concentration (0.8 at.%). The BET specific surface areas of the samples
Photoluminescence (PhL)
Spectra of photoluminescence (PhL) of pristine and Cr-doped spinels are displayed in Figure 1, which shows that Cr-doping the spinel results in almost complete quenching of the luminescence band with maximum at 420 nm typical of pristine spinel (curve 1), and the appearance of a strong luminescence band with maximum at 680 nm associated with Cr3+ states in the spinel (curve 2).
Such observations infer that luminescence spectrum of pristine spinel is associated with intrinsic lattice defects. The
Concluding remarks
This short Letter has shown that chemical relaxation processes occurring through dissociative adsorption of molecular hydrogen on a photoactivated surface can lead to secondary electronic excitation of the metal oxide, particularly to the formation of excited states of the intrinsic defects and Cr3+-states, which decay through luminescence relaxation pathways, i.e. by the PhICL phenomenon. Observation of R-line emission associated with Cr3+-states shows unambiguously the excitation of the
Acknowledgments
The present study was performed within the Project “Establishment of the Laboratory of Photoactive Nanocomposite Materials” supported by a Grant (No. 14.Z50.31.0016) from the Government of the Russian Federation. This work was also supported by Saint-Petersburg State University within the Fundamental Project Support program (Grant No. 11.38.207.2014) and the University Postdoctoral program (Grant No. 11.50.1595.2013). F.M.A and A.V.E. are also grateful to the Russian Foundation for Basic
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