J. CHEM. SOC. FARADAY TRANS., 1994, 90(14), 2107-2111 Photoluminescence Spectra Resulting from Hydroxy Groups on Magnesium Oxide supported on Silica Hisao Yoshida, Tsunehiro Tanaka, Takuzo Funabiki and Satohiro Yoshida" Department of Molecular Engineering, Kyoto University, Kyoto 606-01,Japan Photoluminescent excitation and emission spectra resulting from hydroxy groups on magnesium oxide have been investigated using highly dispersed magnesium oxide species supported on silica. Since this sample has almost only one kind of Mg-0 species, reaction of the Mg-0 species with H,O produces hydroxy groups uniformly coordinated to Mg ions. Therefore, clear photoluminescent excitation spectra were obtained. Coordi- nation states of the hydroxy groups were elucidated from excitation spectra.The hydroxy group attached to the surface Mg ion is excited by 255 nm light. The other hydroxy groups which are coordinated to Mg ions within the silica matrix are excited by 265 nm light. Hydroxy groups on the sample exhibit almost the same broad emission spectra centred at 440 nm regardless of their coordination. The excited triplet states of photoactive sites on solid cata- lysts often play an important role in photocatalysis, and phosphorescence emitted from triplet states allows the photoactive sites to be studied. Consequently, photo-l~minescence'-~ and the photocatalytic activity6 of MgO have been studied extensively. We cannot disregard hydroxy groups on MgO when we examine the photoluminescence of MgO.These hydroxy groups are directly related to photoluminescent emission, and interfere with or mask the emission from Mg-0 ion pairs on the surface.lV2 Investigations into the properties of MgO bulk have been of particular interest'-7 and have revealed that hydroxy groups were removed by evacuation at high tem- peratures ; however, the assignment of luminescence by hydroxy groups remained unclear. Several coordination states of surface ions exist in the MgO bulk. The type of coordination affects the surface properties, for example, the basicity' or surface band-ga~.~ The properties of hydroxy groups on the surface may also be influenced by the coordination state. There have been some studies on the photoluminescence of dehydrated magnesium hydroxide7 and also of the hydrated surface of MgO.'V2 However, in these studies, coordination of the hydroxy groups, especially the surface ones, was not well defined, with the exception of the intrinsic hydroxy groups in Mg(OH), because of the variety of states involved.' In order to clarify the coordination, it is necessary to prepare a sample which contains uniform luminescent sites. Recently, we have found that highly dispersed magnesium oxide on silica exhibits a new type of photol~minescence~ and concluded that new Mg-0 bonds are the emission sites.The fine structure on the photoluminescence emission spec- trum reveals that the emission site, an Mg-0 bond, is dis-tributed uniformly. When hydroxy groups coordinated to these Mg ions are produced, or the 0 ions in Mg-0 bonds convert to hydroxy groups, the hydroxy group is expected to be highly uniform.In this paper, we describe the change in photoluminescence caused by adding water to this sample, and discuss the coordination states of the hydroxy groups produced. Experimental In this study, we used MgO supported on silica of 1 wt.% loading, MgO/SiO, (MS), because it gives a clear photolu- minescence spectrum due to the Mg-0 bond.g Silica (568 m2g-'), used as a support material, was prepared by the sol-gel method as described elsewhere." The sample was prepared as described in ref. 9. Prior to measurements, the sample was treated with 50 Torr 0, for 1 h at 1073 K, fol-lowed by 1 h evacuation at 1073 K. Deionized H,O was distilled, and purified by several freeze-pumpthaw cycles in a vacuum line before adsorption experiments.Photoluminescence spectra were recorded at 77 K with a Hitachi 850 fluorescence spectrometer using a UV filter (permitted wavelength >300 nm) to remove scattered light from the UV source. An in situ sample cell made of quartz (0.5 mm x 10 mm x 44 mm) was used. The amount of the sample in the cell was 200 mg, therefore the total amount of magnesium ions contained in the MS sample was 50 pmol. H,O was introduced to the sample cell at room temperature. Results The photoluminescent emission spectra of the sample vary with the wavelength of the excitation light. The characteristic spectra of the MS sample can be classified into two sets: A, emission spectra produced by excitation of the sample with light of 240 nm and excitation spectra monitored at 520 nm emission; B, emission spectra of the sample excited by 265 nm light and excitation spectra monitored at 430 nm emis- sion. Photoluminescence of SiO, Photoluminescence spectra of the silica support were re-corded as a blank test.The pretreatment was the same as that for the MS sample. Fig. 1 shows the photoluminescent emission spectra of SiO, excited by light at 240 nm (Fig. 1A) and 265 nm (Fig. 1B). In Fig. lA, a broad band centred at 480 nm was observed after evacuation at 1073 K. Upon adsorption of H,O, a component at ca. 440 nm was detected. Its intensity increased with further addition of H,O.(Note that the scales are different.) The emission at 440 nm was quenched by contact with 100 Torr He, indicating that the luminescent sites are on the silica surface. Therefore, it should be assigned to hydroxy groups produced on the silica surface. In Fig. lB, a band centred at 440 nm was already observed after evacuation at 1073 K. The peak position suggests that the band results from residual hydroxy groups on the silica. Fig. 2 shows photoluminescent excitation spectra moni- tored at 520 nm (Fig. 2A) and 430 nm (Fig. 2B) emission. After evacuation at 1073 K, a peak was observed at 250 nm. After adsorption of H,O, a peak at 240 nm appeared instead. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 A 6 "4,4Y I I I 1 I I I 1 0 400 500 600 700 0 400 500 600 700 wavelengt h/nm wavel en g t h/n m Photoluminescent emission spectra of SO, excited by A, 240 nm and B, 265 nm light; (a)after evacuation at 1073 K, (b)in the presence of 20 pmol H,O, (c) in the presence of 50 pmol H20, (d)exposed to excess H20, (e)followed by evacuation at room temperature.Intensities of recorded spectra are: A, (a) x 1.0, (b) x 1.0, (c) x 0.47, (d)x 0.40, (e) x 0.40;B, (a)1.4, (b)2.7, (c) 1,2, (d)1.2 and (e) 0.78. These results show that the luminescence, which was observed after the addition of H,O, is clearly attributed to the hydroxy groups on the SiO, surface. Therefore, we con- clude that the hydroxy groups on silica are excited by 240 nm light and that luminescence is emitted at 440 nm.A B nv) c.-C 4 v .-).c v) a3 c .-I I I L 0 300 400 200 300 400 wavelengt h/nm Fig. 2 Photoluminescent excitation spectra of SO, recorded by monitoring the emission at A, 520 nm and B, 430 nm; (a)-(e) see caption to Fig. 1. Intensities of recorded spectra are: A, (a) x 1.0, (b) x 1.3, (c) x0.80, (d)x 1.2, (e) x 1.1; B, (a) x 1.4, (b) x 1.0, (c) x0.60, (d)x 0.58 and (e) x 0.48. The luminescent site exhibiting a component of the emis- sion band at around 500 nm after evacuation at 1073 K, and the site exhibiting the excitation peak at 250 nm, are identi- cal. The luminescence was not quenched by the presence of 100 Torr He, indicating that the luminescent site exists in the silica matrix and not on the surface.This site presumably results from internal residual hydroxy groups or a radical species such as an oxygen dangling bond. Photoluminescence of tbe MS Sample: Set A In this section, the emission spectra of the MS sample excited by 240 nm light (Fig. 3) and the excitation spectra monitored at 520 nm emission (Fig. 4) are discussed. Fig. 3(a) shows the emission spectrum recorded after evacuation of the sample at 1073 K. It exhibits a band centred at 520 nm with fine structure due to the vibrational levels of the Mg-0 bond.g The excitation spectrum in Fig. 4(a)reveals a shoulder peak at 240 nm, suggesting that this photoactive Mg-0 band was excited by 240 nm light and emitted lights at 520 nm. Immediately after 20.3 pmol H,O was added at room tem- perature, the emission spectrum of the MS sample excited by 240 nm light changed; a broad band was observed with a maximum at 440 nm [Fig.3(b)] and its intensity was reduced to one third. Furthermore, the fine structure disappeared. These results suggest that H,O molecules interact with the Mg-0 bonds and new photoluminescent sites are produced giving rise to a broad peak. Subsequently, the sample was annealed at room temperature and left for 30 min. The emis- sion spectrum was recorded again at 77 K [Fig. 3(c)], and exhibits a broad band at 440 nm whose tail extends to the original band at ca. 520 nm. When a further 29.7 pmol H,O were added [Fig. 3(d)], the 440 nm band was not quenched but grew larger, whereas the intensity of the band at CQ.520 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 I I I I 300 400 500 660 700 wavelengthlnm Fig. 3 Photoluminescent emission spectra of the MS sample excited by 240 nm light; (a)after evacuation at 1073 K, (b) immediately after addition of 20 pmol H20, (c)30 min later, (d) in the presence of 50 pmol H20, (e) exposed to excess H20 vapour followed by evacuation at room temperature. Intensities of recorded spectra are: (a) x 1.0,(b) x 2.9, (c) x 0.86, (d) x 1.3 and (e) x ca. 0.4. nm decreased. The sample was exposed to saturated H20 vapour at room temperature for 10 min and then evacuated. The spectrum in Fig. 3(e) changed to that comprising a single broad band centred at 440 nm, and the original band at 520 nm completely disappeared.These results indicate that the photoluminescent peak at 440 nm relates to hydroxy groups. The hydroxy groups would exist not only on the surface of magnesium oxide, but also on the surface of the silica support, and both would exhibit such photoluminescent emission spectra. It is quite probable that the Mg-0 species, which exhibits the emis- sion spectrum centred at 520 nm, reacts to give hydroxy groups. Fig. 4 shows the photoexcitation spectra. The spectrum after the pretreatment [Fig. 4(a)]exhibits a shoulder peak at 240 nm. On addition of a small amount of H,O (20 or 50 pmol: less than or comparable to the amount of Mg ions), the spectrum did not change as shown in Fig. 4(b) and (c). After an excess of H,O was added, followed by evacuation at room temperature, the shoulder shifted to 260 nm, as shown in Fig.4(d). Since silica does not exhibit such a peak at 260 nm, this excitation peak is assignable to hydroxy groups coordinated to Mg ions. On addition of a small amount of H20, the excitation spectra did not change although the emission spectra changed as mentioned above. On the other hand, when excess H20 was added, a new excitation peak appeared. From these results, we suppose that at least two kinds of OH groups are formed; one is formed initially by the adsorption of water molecules comparable to the number of Mg ions, and the other is subsequently formed by adsorption of an excess of water molecules. The spectral change clearly shows that they are distinguishable.Since magnesium ions in this sample exist as isolated monoatomic or raft-like crystallite species interacting at the 1 I 300 I 400 wavelengthlnm Fig. 4 Photoluminescent excitation spectra of the MS sample recorded by monitoring the emission at 520 nm: (a)after evacuation at 1073 K, (b) in the presence of 20 pmol H20, (c)in the presence of 50 pmol H,O, (d) exposed to excess H20 followed by evacuation at room temperature. Intensities of recorded spectra are (a) x 1.0, (b) x 1.5, (c) x 2.3 and (d) x ca. 0.4. silica surface," it is likely that at least two types of hydroxy groups coordinated to Mg ions are present on the surface; one is coordinated to only one magnesium ion and the other is bridged to Mg and Si.In this sample, it is probable that isolated Mg-0 bonds exist and that their oxygen is more reactive than the oxygen coordinated to both Mg and Si, so we suggest that the H,O molecule first reacts with this iso- lated Mg-0 bond to produce a new Mg-(OH) species. The result that the luminescence of the hydroxy groups replaces the luminescence of Mg-0 supports this assumption. It is likely that excess H20 molecules react with the oxygen bridging Mg and Si, or with MgO microcrystallites. Thus, we speculate that hydroxy groups produced by the addition of an excess of H,O are coordinated to Mg and Si, or that Mg(OH), microcrystallites are formed. The excitation wavelength of these hydroxy groups is 260 nm.Photoluminescence of the MS Sample: Set B In this section, the emission spectra of the MS sample excited by 265 nm light (Fig. 5) and the excitation spectra monitored at 430 nm emission (Fig. 6) are discussed. After pretreatment of the MS sample at 1073 K [Fig. 5(a)], the emission peak at 440 nm and the excitation peak at 265 nm [Fig. qa)] were observed. Such a peak in the excitation spectra is not reported for evacuated magnesium oxide at a 21 10 1 360 460 500 660 760 wavelength/nm Fig. 5 Photoluminescent emission spectra of the MS sample excited by 265 nm light: (a) after evacuation at 1073 K, (b) exposed to 50 pmol H,O vapour followed by evacuation at room temperature, (c) exposed to excess H,O followed by evacuation at room temperature.Intensities of recorded spectra are (a) x 1.0, (b) xO.85, and (c) x ca. 0.1 I I 3 300 400 wavelength/nm Fig. 6 Photoluminescent excitation spectra of the MS sample recorded by monitoring the emission at 430 nm: (a)after evacuation at 1073 K, (b)in the presence of 20 pmol H,O, (c)in the presence of 50 pmol H,O, (d)exposed to excess H,O followed by evacuation at room temperature. Intensities of recorded spectra are (a) x 1.0, (b) x 0.68, (c) x 1.0,and (6)x ca. 0.2. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 high temperature, and is not observed for silica (Fig. 2). The emission peak at 440 nm is assigned to hydroxy groups as described in a previous section in the case of silica. Therefore, this luminescence which is produced by excitation at 265 nm, and which emits at 440 nm is presumably due to hydroxy groups coordinated to Mg ions remaining even after evacu- ation at 1073 K for 1 h.Duley' reported that the excitations of the OH-in Mg(OH), are promoted by 267-276 nm light, and the emis- sion peaks are seen in the range 435-443 nm, and that OH- ions remain when Mg(OH), was evacuated for 1 h at lo00 K. These values are in good agreement with our observations. We found that luminescence was not quenched by contact with 100 Torr He, indicating that the luminescent species are not present on the surface. Therefore, the luminescence observed in the present work could be due either to hydroxy groups in the magnesium hydroxide crystallite which has been encapsulated in the SiO, matrix, or to hydroxy groups located on the inner interface of magnesium oxide species and silica.Hence, both hydroxide species attached to an Mg ion and the new Mg-0 species coexist in the MS sample evacuated for 1 h at 1073 K. Addition of a small amount of H20, less than, or compar- able to the Mg content, caused the excitation peak to shift to 255 nm as shown in Fig. 6(b) and (c').However, addition of an excess of H20 caused the peak to shift to 260 nm [Fig. 6(6)]. As described above, the Mg-0 species, which exhibit the fine structure in the photoluminescence excited at 240 nm, reacted with H20 molecules to produce a new Mg-(OH) species. Hence, we conclude that this new Mg-(OH) species is specifically excited by 255 nm light.On addition of an excess of H20 molecules, the peak in the excitation spectrum was replaced by a small broad maximum at 260 nm. It is likely that the excess of H,O molecules inter- feres with the localization of photoexcitation of the new Mg-(OH) species, and hence, the peak at 255 nm in the exci- tation spectrum vanished. This suggests that all Mg species may react with excess H20 molecules to form Mg(OH),-like species. Although the peak in the excitation spectrum changed, the emission spectra in Fig. 5 did not change on addition of H20. As the concentration of Mg ions in the MS sample is only 1 wt.%, one may conjecture that the emission bands centred at 440 nm are mainly due to the hydroxy groups coordinated to the silica surface.However, the presence of such obvious peaks in the excitation spectra indicates a certain amount of emission from hydroxy groups coordinated to Mg ions. Therefore, regardless of whether hydroxy groups are coordinated to Mg ions or Si ions, we can conclude that all hydroxy groups show the same photoemission spectra at 440 nm. Discussion After evacuation at 1073 K, the component of the emission band centred at 440 nm, excited by 240 nm light, resulting from hydroxy groups on the silica support was observed in a blank test, although it was scarcely observed on the MS sample, as shown in Fig. 3(a).The loading of MgO in the MS sample was 1 wt.% which corresponds to 0.25 mmol g- '. The population of residual hydroxy groups on silica calcined at 773 K was evaluated at 0.211 mmol g-'.? The photolumin- ~ t The population of surface hydroxy groups was estimated as follows.Silica was soaked in hexane. Hexane solution of n-butyl lithium was introduced into the mixture under an N, atmosphere. Evolved butane was estimated volumetrically. Si-OH + C,H,Li -,Si-OLi + C,HIof. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 escence results in this work suggest that luminescence-active silanols are not present on the surface of the MS sample. These silanols have presumably been lost by the reaction of OH with Mg(OCH,), in methanol during MS sample prep- aration. Si-OH + Mg(OCH,), --* Si-0-MgOCH, + CH,OH The photoluminescent emission spectra of other MS samples evacuated at 773 and 1073 K, which contain 20 and 1 wt.% of MgO, respectively, were reported in ref.9. The spectra of the sample evacuated at 773 K and excited by 240 nm light are the same as those of the hydroxy groups re- ported in the present paper. We mentioned in the previous paperg that the spectra result from three-coordinated Mg-0 ion pairs at the corners in MgO crystallites. Therefore, we concluded that crystallites of MgO are present on the silica surface in the sample evacuated at 773 K. Since the spectrum of the sample evacuated at 1073 K was assigned to the new Mg-0 bond, we reported that the MgO crystallites react with silica under evacuation at 1073 K, to result in the forma- tion of new Mg-0 bonds which interact with the silica.From the result described in the preceding sections, hydroxy groups excited by 240 nm light emit a broad band centred at 440 nm. Taking this into account, there are two possible considerations for the change of the photolumin- escent site due to evacuation at 1073 K. One is that the sample, after evacuation at 773 K, contained crystallites of MgO with surface hydroxy groups, and both dehydration and solid-solid reactions occurred during the evacuation at 1073 K to produce the new Mg-0 species. The other is that Mg ions were already dispersed on the silica surface and the new Mg-0 bonds, or the precursor, were formed after the evacuation at 773 K. In the latter case, the strong emission from the hydroxy groups on silica masked the emission from the Mg-0 bonds, and the dehydration at 1073 K promoted the removal of hydroxy groups to result in the spectrum exhibiting fine structure. Duley7 reported that hydroxide ions in a specific low- coordination site on Mg(OH), evacuated for 1 h at 1200 K exhibited 472 nm (2.63 eV) emission when it was excited by 270 nm light, and that this emission can be excited by the trapping of excitons created by absorption at a variety of 0:; and OH,, sites.The basis of his assignment was the change of the spectrum by subsequent evacuation of the sample for 6 h at 1200 K. The appearing emission excited by the 268 nm (4.63 eV) light with the emission spectrum centred at 386 nm (3.21 eV). He suggested that the excitation peak could be attributed to absorption by three-coordinated O2-ions3v4 on the MgO surface, and the emission band was assigned to the characteristic blue-violet emission of MgO.Almost all excitation spectra in his paper show a single peak at ca. 270 nm, regardless of the evacuation temperature. Duley argued that the emission site was excited by the trap- ping of excitons created at different absorption sites. However, the explanation is not clear because of discrep- ancies between the emission sites with the excitation sites. In the preceding sections, we have clearly assigned the peak at 255 nm in the excitation spectra and the band at 440 nm in the emission spectra to the surface hydroxy groups attached to an Mg ion. In our sample, hydroxy groups func- tion as both excitation sites and emission sites.Ordinarily, MgO crystallite has several kinds of hydroxy groups on the surface, and therefore the photoluminescence of hydroxy groups had not previously been assigned in detail because of the complexity and low resolution of photolumin-escent spectroscopy. We should make it clear that in the present paper, the sample MgO/SiO, has essentially only one kind of Mg-0 species. Conclusion Hydroxy groups coordinated to Mg or Si in MgO/SiO, are distinguishable from their pho toluminescen t excitation spectra. We conclude that for the new Mg-OH species, the hydroxy group attached to the surface Mg ion is excited by 255 nm light, and that hydroxy groups coordinated to Mg ions which are within the silica matrix are excited by 265 nm light.Surface hydroxy groups coordinated to Mg and Si, or that of Mg(OH), microcrystallites, produced by addition of an excess of H20 are excited by 260 nm light. 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