.I. Chem. SOC., Faraday Trans. I, 1985,81, 2913-2919 On the Nature of the Luminescence Centres on MgO Surface BY VLADISLAV A. SHVETS,* ALEKSEY V. KUZNETSOV, VIKTOR A. FENIN AND VLADIMIR B. KAZANSKY N. D. Zelinsky Institute of Organic Chemistry of Academy of Sciences of the U.S.S.R., Leninsky pr. 47, 117913 Moscow, U.S.S.R. Received 9th July, 1984 The effect of the conditions of the oxygen pretreatment and the adsorption of various molecules on the characteristics of the photoluminescence of thermoevacuated powder MgO have been investigated. The data obtained are shown to be in better agreement with the attribution of the luminescence at 415 nm to surface F l centres and that at 530 nm to surface F: centres, rather than to the coordinatively unsaturated surface 02- anions. Surface defects play an important role as active centres in adsorption and catalysis on magnesium oxide.According to ref. (1)-(5), heating large-surface-area MgO in vacuo at 600-900 "C results in photoluminescence (LM) from the low-coordinated lattice oxygen 02- on the surface. A similar conclusion was also drawn in the case of LM of highly dispersed CaO, SrO and Ba0.3 Investigation of small-surface-area bulk alkaline-earth-metal oxides, including MgO, e.g. single crystals, shows that their LM is caused by defects in the crystalline lattice, namely by F+ and F" centres, i.e. the oxygen vacancies that have captured one or two electrons, respectively.sy The LM of these centres in the bulk of these oxides is due to the photoexcitation and emission of light by these electrons, which have wavefunctions resembling those of the s orbitals of hydrogen but with an expanded Bohr r a d i ~ s .~ Theoretical calculations*, show that the excitation and emission energies of the LM of the surface F,+ centres must be close to the corresponding energies of the bulk F+ centres. In agreement with theory, the excitation energy of the F i centre on the surface of KCl was found to be very close (0.04 eV) to the corresponding bulk va1ue.l" The characteristics of the LM of the bulk F+ centres of the alkaline-earth-metal oxides coincide with those of the surface LM described in ref. (1)-(5). For instance, the LM of the bulk F+ centres in MgO has emission and excitation maxima at 400 and 250 nm, respectively, and the surface LM of MgO, according to ref.(1)-(5), at 400-410 and 250-270 nm. This paper reports a reinvestigation of the LM properties of powdered MgO in order to explore the alternative explanation of the nature of the centres responsible for its appearance. The possibility of the attribution of the LM appearing after thermovacuum treatment of oxides, which do not contain transition- metal ions, to anion-vacancy-type surface defects with captured electrons has been discussed previously for CaOll. l2 and Th02.13 29132914 LUMINESCENCE CENTRES ON MgO SURFACES EXPERIMENTAL The magnesium oxide was prepared by the decomposition of basic carbonate of the 'especially spectrally pure' grade in vacuo at 600 "C for 3 h. For convenience the powder of the starting material was pressed, ground and a fraction of size 0.5-2 mm was selected.Before the experiments the MgO was placed in quartz ampoules, heated in 0, (5-10 kPa, 500 "C, 20-30 min), and thermoevacuated (10-3-10-4 Pa) at a selected temperature in the range 300-1000" for 1-3 h. y-Irradiation of the samples was carried out in quartz ampoules with 6oCo at room temperature. Treatment with CO or 0, to determine the degree of a reduction of the samples was carried out in a circulating system equipped with a liquid-nitrogen trap for freezing of any CO, formed. The amount of CO, was determined by a volumetric method after evacuation of CO or 0, from the system and subsequent warming of the trap up to room temperature . Hydrogen was purified by diffusion through the walls of a hot palladium thimble. Oxygen was liquefied at - 196 "C, one-third of it was then pumped off and the middle fraction was collected.LM spectra were recorded, mainly at room temperature, using MPF-4 Hitachi spectro- fluorimeter. The diffuse reflectance i.r. spectra were recorded at room temperature using a ' Perkin-Elmer 580B ' spectrophotometer provided with a special accessory. RESULTS AND DISCUSSION The LM spectrum of the samples thermoevacuated in the range 300-1000 "C is similar to that observed earlier in ref. (1)-(4). It consists of a wide line with the maximum at 415 nm and a shoulder at 480 nm [fig. 1 (a)] at both 20 and - 196 "C (at the latter temperature the LM is ca. 2.5 times more intense). The excitation spectrum has the maximum at 275 nm [fig. 1 (c)]. We did not observe an increase of the intensity in the excitation spectrum at short wavelengths, this being ascribed in ref.(3) and (5) to the existence of the second excitation maximum at < 230 nm. In our opinion the reason for this spectroscopic difference is poor reproducibility of the shape of the excitation spectrum at the very short wavelengths, as pointed out in ref. (2)-(4). Note that according to ref. (1) the excitation spectrum of MgO does not have any bands at ;1 < 230 nm. With increasing activation temperature up to 1000 "C the intensity of the LM grows, as is shown in fig. 2(a). The form of the spectrum remains unchanged. After heat treatment in CO for the same time as in the case of thermoevacuation and subsequent outgassing for 10 min, MgO samples show LM spectra identical to those shown in fig.1 (a). However, the minimum temperature of treatment required for its appearance is ca. 100 "C lower, and the intensity at each temperature of pretreatment is higher than that after thermoevacuation [fig. 2 (b)]. Moistening the samples before their thermoevacuation also results in higher intensity LM. In the course of the CO treatment the formation of CO, is observed. For example, at a pressure 27 kPa and temperatures of 500 and 700 "C 1.3 x 1019 and 1.9 x 1019 molecule g-l, respectively, is formed. After subsequent treatment at these temperatures more CO, is formed but in smaller quantities (0.3 x 1019 and 0.4 x 1019 molecule g-l at 500 and 700 "C, respectively). These data show that in the CO atmosphere two processes take place: reduction of MgO and disproportion of CO according to the Boudouard reaction into C and CO,.Increasing the temperature of the MgO vacuum treatment up to 1000 "C results in the appearance of new LM with the emission maximum at 530 nm and the excitation maximum at 310 nm (fig. 3). With increasing thermoevacuation time at 1000 "C the intensity of this new LM grows [fig. 2(c)], but the LM at 41 5-480 nm remains almost unchanged [fig. 2 (a)]. The LM in the range 415-480 nm is quenched at 20 "C by 0, (ca. 0.1 kPa). ThisV. A. SHVETS, A. V. KUZNETSOV, V. A. FENIN AND V. B. KAZANSKY 2915 1 LOO 5 00 300 X/nm 200 Fig. 1. LM spectra (Aex = 275 nm) of MgO: (a) thermoevacuated at 800 "C for 1 h, (b) after exposure of (a) to an H,+O, mixture at room temperature and (c) the excitation spectrum of (a) at Aern = 415 nm.1 . o / n e v) +A .r( d m' 0.5 W - 0 . 5 - s '2, 200 LOO 600 800 1000 1000 (4 h) TI°C Fig. 2. Variation of integral intensity of the LM spectra of MgO with temperature of: (a) thermoevacuation (Am = 415 nm, AeX = 275 nm), (b) reduction by CO (Aern = 415 nm, Aex = 275 nm) and (c) thermoevacuation (Aern = 530 nm, Aex = 310 nm). effect is fully reversible, since outgassing of the sample at room temperature restores its initial intensity. The LM at 530 nm is also reversibly quenched by 0, at 20 "C, but it is less sensitive to the oxygen pressure. For example, at 30 kPa its intensity decreases by only a factor of two. Thus the LM at 530nm appearing after the higher-temperature treatment is associated, along with that in the 415-480 nm range, with centres on the surface of MgO since these interact with gaseous oxygen.As already mentioned above, the LM in the range 415-480 nm was previously ascribed to the low-coordinated 0,- ani~ns.l-~ However, the following data allow us2916 LUMINESCENCE CENTRES ON MgO SURFACES to revise this attribution and to ascribe it to surface oxygen vacancies that have captured one electron (FL centres). Moreover, according to these data, LM at 530 nrn should be ascribed to surface anion vacancies with two captured electrons (F," centres). As has been shown above, the reduction of MgO in CO increases the LM intensity at 415-480 nm and shifts the threshold of appearance of the LM toward lower temperatures (fig. 2). These results can be explained by the following scheme of reduction resulting in a formation of oxygen vacancies, 0, which capture electrons: (1) The amount of CO, which is formed at temperatures of 600-800 "C in the course of the reduction shows that the concentration of F,+ centres can reach the value of (1-3) x 1019 g-l when the electrons are not captured by impurities.A similar mechanism is valid for a formation of F,+ centres in the course of the thermoevacuation of MgO: 1 I MgO+xCO -+ MgO,-.,+.xCO,+.uC]+2rre 0 +e -+ F,+. (2) The amount of F,+ centres formed with thermoevacuation is certainly lower than with reduction in CO. Therefore the LM intensity of thermoevacuated samples is lower than that of the reduced ones and appears at higher temperatures. In contrast, reduction by CO should decrease the intensity of LM if it is connected with low-coordinated 0,- anions, because these anions are more reactive than the regular surface 0,-.Heating samples with LM in the range 41 5-480 nm at 550 "C in oxygen (30-40 kPa, 40 min) results in its complete disappearance (the LM spectra were measured after outgassing of 0, at room temperature). At the same time heating of MgO in 0, at higher temperatures (700-800 "C) only decreases the LM intensity without its complete disappearance. Such behaviour should be also explained by the formation of oxygen vacancies according to reaction (2), which is reversible in an oxygen atmosphere. At 500 "C and 30-40 kPa of 0, reaction (2) if likely to be shifted to the left, and at higher temperatures to the right, even at 30-40 kPa of 0,.Thus the disappearance of LM after calcination in oxygen is in agreement with our suggestion about the nature of the emission centres. Moreover, it is not clear why the coordinatively unsaturated 02- ions localized at corners and edges of microcrystals of MgO should disappear after treatment in oxygen. Therefore this phenomenon contradicts the attribution of the LM to low-coordinated 0,- ions. The appearance of the LM at 530 nm after heating MgO in vacuo at 900-1000 "C also substantiates the attribution of both types of LM (at 530 and 415-480 nm) to surface oxygen vacancies that have captured electrons. Indeed, the appearance of this LM is most likely connected with two processes. First, at high temperatures the amount of electrons trapped at anion vacancies increases in accordance with reaction (2).Secondly, simultaneous sintering of the surface takes place leading to a decrease in the concentration of surface oxygen vacancies. Both processes must, apparently, cause an increase in the concentration on the anion vacancies that have captured two electrons, i.e. FZ centres. Consequently, the LM data for MgO heated in vacuo at 900-1000 "C allow the high-temperature LM at 530 nm to be assigned to surface FZ centres and the low-temperature LM in the range 41 5-480 nm to be assigned to surface MgO+Mg0,-z+~0,+xO+2xe n+e+F,+. 2v. A. SHVETS, A . v. KUZNETSOV, v. A. FENIN AND v. B. KAZANSKY 2917 t Xlnm Fig. 3. LM spectra (Aex = 310 nm) of MgO: (a) thermoevacuated at 1000 "C for 1 h, (b) after adsorption on (a) of BC1, at room temperature and (c) the excitation spectrum of (a) at Aem = 530 nm.F: centres. This attribution also agrees with the literature data,14 which show that emission of the bulk F" centres takes place at longer wavelength than that of the bulk F+ centres (530 and 400 nm, respectively). The surface F: centres have strong basic properties, since their EM is not quenched by adsorption of H,O and NH,. At the same time it is fully quenched at room temperature by the adsorption of gaseous Lewis acids such as BCl, [fig. 3(c)]. The LM of F: centres is more easily annealed in 0, than that of F,+ centres. For example, at an 0, pressure. of 10 kPa it begins to decrease at 200 "C and disappears fully following treatment at 400 "C for 10 min. It is well known that F; centres on MgO surface are formed in the course of irradiati~ltl.l~-~~ In this connection we also irradiated the oxidized MgO samples with y-rays at room temperature in ziacuo with a dose of ca.30 Mrad and investigated their photoluminescence. After the irradiation the LM in the range 41 5-480 nm appeared with intensity corresponding to that for the samples thennoevacuated at 500-550 "C. Adsorption of 0, leads to its almost complete quenching. The LM reappears after outgassing of 0, at room temperature with an intensity of CQ. 90% of the initial intensity. Thus the irradiation showed that surface F; centres as well as bulk F;' centres can be luminescent. Moreover, most of them are stable in oxygen at room temperature, although some irreversibly donate electrons to adsorbed oxygen molecules.This leads to the formation of 0; radicals detected by e.p.r.I*, *O We have also observed by e.p.r. the formation of 0; radicals after adsorption of (I2 on the thermoevacuated luminescencing samples of MgO. Their concentration is ca. 10ls g-l, in agreement with ref. (21) and (22). This i s 2-3 orders of magnitude 'less than the concentration of F,t centres determined from the amount of CO,. Thus only a. small part of the thermally induced F$ centres can form 0; radicals. In thermoevacuated samples there is only a weak e.pr. signal assigned to F: This shows that the majority of thermally induced F,' centres are not observed by e.p.r. at 20 and - 196 "C. There are several possible reasons for this, for example F$ centres can be located close to each other and their e.p.r. signal will be broadened by dipole-dipole interaction.However, this problem needs further detailed i 11 vest i g a t i on .2918 3600 LUMINESCENCE CENTRES ON MgO SURFACES 5 % II 3700 fi/cm-' 3800 Fig. 4. 1.r. spectra of MgO: (a) thermoevacuated at 800 "C for 1 h and (b) after exposure of (a) to the H, + 0, mixture at room temperature for 10 min. Note that irradiation-induced bulk and surface F+ centres are annealed at high temperatures: bulk F+ centres disappear near 500 "C in neutron- or electron-irradiated Mg024 and surface F+ centres at 200-300 "C in y-irradiated samples,l5? l6 but in additively coloured crystals annealing takes place at 900-1000 0C.24 The annealing is a result of recombination of electrons of F+ centres with hole centres.In this connection the thermally induced surface F i centres are stable at high temperatures because of the absence of hole centres. The adsorption of H, or 0, at room temperature and their subsequent outgassing for 15-20 min Pa) do not change the LM spectrum of the surface F,S. centres. This is in disagreement with the observation of the influence of hydrogen on the emission spectrum of Mg0.4 In our opinion the difference in the result could be caused by impurities in H, interacting with F,+ centres, for example CO, the adsorption of which strongly affects the LM spectrum of Mg0.25 At the same time exposure of the samples at this temperature to an H, + 0, mixture (6 kPa H,, 1.3 kPa 0,) for 10 min and subsequent evacuation causes the disappearance of the LM band at 4 15 nm, but does not change the shoulder at 480 nm, which is then seen as a clear band [fig.1 (b)]. Simultaneously there is an increase in the intensity of the absorption band at 3626 cm-l in the i.r. spectrum (fig. 4) due to the surface hydrogen-bonded hydroxyl groups.26 The intensity of the band at 3730 cm-1 attributed to the isolated OH groups26 remains unchanged. Note that the interaction of H, + 0, mixture with non-luminescent samples, for example with those oxidized at 550 "C, does not lead to an increase in the bands of both types of hydroxyl groups. The appearance of surface OH groups on MgO after interaction with the H, + 0, mixture and the simultaneous disappearance of the LM at 41 5 nm are also consistent with our suggestion that it is connected with the surface F,S.centres and not with the low-coordinated anions 0,-, because the formation of the surface OH groups takes place only in the presence of oxygen. Apparently, the surface F,+ centres responsible for the LM band at 415 nm react with the H,+O, mixture according to the scheme: 20+2e+0,+H2 -+ 20H;u,,,,,. (3) Consideration of the H,+02 mixture adsorption data shows also that the LM in the range 415-480 nrn comes from the two types of centres with different chemical properties. The LM band at 480 nm may be assigned to F: centres whose structure differs from that of the centres showing luminescence at 410 nm. For example they may be formed on different crystallographic planes. The LM spectra observed in this work allow us to compare the energy parametersv .A. SHVETS, A. v . KUZNETSOV, v. A. FENIN AND v. B. KAZANSKY 2919 of the F," and FZ centres with those characteristic of the corresponding bulk centres. The positions of the maximum in the emission spectra of the bulk and surface F" centres coincide and lie at 530 nm. However, the maximum of the LM excitation of the bulk F" centres lies at 250 nm,14 but that from the corresponding surface defects at 310 nm [fig. 3(b)]. At the same time the maximum of the LM excitation of the F,f centres is observed at 275 nm [fig. 1 (c)] and that of the bulk centres at 250 nm.s, Note that previously27 the FZ centres of MgO were associated with an absorption band at 540 nm, but recently2s it was shown that this band belongs to surface V; centres. CONCLUSIONS The experimental data on the LM of thermoevacuated samples of MgO are in better agreement with the attribution of the LM at 415 nm to the surface FZ centres and that at 530 nm to the Fi centres than to coordinatively unsaturated surface oxygen anions.Of course, it is not necessary to consider these data as a negation of the existence of such anions on the surface of MgO and other oxides. These anions do not luminesce at the excitation with light, but they have absorption bands in the ultraviolet region of the spectrum and have been widely studied by a reflectance spe~troscopy.~~-~~ A. J. Tench and G. T. Pott, Chem. Phys. Lett., 1974,26, 590. S. Coluccia, A. M. 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