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Nov 16, 2008 - Guylène Costentin: Phone: +33 1 44 27 60 05. ... This paper deals with the assignment of photoluminescence excitation spectra of hydro...
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J. Phys. Chem. C 2008, 112, 19710–19717

Assignment of Photoluminescence Spectra of MgO Powders: TD-DFT Cluster Calculations Combined to Experiments. Part II. Hydroxylation Effects Ce´line Chizallet,*,†,‡ Guyle`ne Costentin,*,†,‡ He´le`ne Lauron-Pernot,†,‡ Jean-Marc Krafft,†,‡ Michel Che,†,‡,§ Franc¸oise Delbecq,| and Philippe Sautet| UPMC UniV Paris06, UMR 7609, Laboratoire Re´actiVite´ de Surface, F-75005 Paris, France, CNRS, UMR 7609, Laboratoire Re´actiVite´ de Surface, F-75005 Paris, France, Institut UniVersitaire de France, and UniVersite´ de Lyon, Institut de Chimie de Lyon, Laboratoire de Chimie, Ecole Normale Supe´rieure de Lyon and CNRS, 46 alle´e d’Italie, 69364 Lyon Cedex 07, France ReceiVed: July 30, 2008; ReVised Manuscript ReceiVed: October 7, 2008

This paper deals with the assignment of photoluminescence excitation spectra of hydroxylated MgO powders thanks to a combined theoretical and experimental approach. Experimentally, despite the numerous types of hydroxyl groups formed upon hydroxylation of bare MgO surfaces, only three types of luminescent species are observed. TD-DFT excitation energy calculations are performed on hydroxylated MgO clusters modeling planes, edges, corners, kinks, and divacancies. The results indicate that the photoluminescence excitation properties do not depend primarily on the coordination of oxygen of OLCH groups (LC) low coordination), which can be satisfactorily explained by theoretical calculations. The hydroxylation state of the surface can thus be more accurately followed by the thermal evolution of the bands characteristic of bare surface irregularities. In addition, OLCH groups are themselves directly implied in the excitation process only if they are located on convex areas of the surface and if L e 3, L being defined as the number of magnesium neighbors. Otherwise, the observed excitation modes of OH groups rather correspond to perturbations of excitation modes of the related dehydroxylated surface. The emission process is followed experimentally by lifetime measurements, which enable one to visualize energy transfer between surface species. 1. Introduction The study of MgO as a model of basic oxides enables us to define the influence of structural parameters, like coordination numbers of the surface ions Mg2+LC and O2-L′C (L and L′ ) 3: corners and kinks, 4: edges of steps, 5: terraces), on the basicity of an inorganic surface.1 Photoluminescence spectroscopy which associates an emitting species to an excitation and emission wavelengths couple {λexc; λem} is considered as one of the few methods suited to characterize the coordination of surface ions, particularly oxide ions.2-12 In a previous paper (part I),13 we focused on the study of dehydroxylated surface irregularities of MgO. A set of five types of species (called A, B, B′, C, and D) was identified and assigned thanks to TD-DFT excitation energy calculations performed on various clusters modeling planes, edges, monatomic steps, corners, kinks, and divacancies. This work underlined the complexity of excitation spectra because a single defect can lead to several bands, and a single band can contain contributions of several defects. It also showed that the coordination number is not the only parameter governing photoluminescence properties, insofar as local topology also plays a major role. It was possible to assign species A {λexc ) 240 nm; λem ) 380 nm} to O2-4C ions of edges (independently of the step height * Corresponding authors. Ce´line Chizallet. Current address: IFP-Lyon, Direction Catalyse et Se´paration, BP3, 69360 Solaize, France. Phone: +33 4 78 02 55 42. Fax: +33 4 78 02 20 66. E-mail: [email protected]. Guyle`ne Costentin: Phone: +33 1 44 27 60 05. Fax: +33 1 44 27 60 33. E-mail: [email protected]. † UPMC Univ Paris06, UMR 7609, Laboratoire Re´activite´ de Surface. ‡ UMR 7609, Laboratoire Re´activite´ de Surface. § Institut Universitaire de France. | Institut de Chimie de Lyon.

and of the location near a corner). Species B and B′ that exhibit the same excitation and emission energies {λexc ) 280 nm; λem ) 470 nm} were differentiated from lifetime measurements together with the calculated localization of the electronic state at the origin of the excitation: O3C and Mg3C corners were associated to species B since they can be more efficiently excited by energy transfer from edges than O3C and Mg3C kinks associated to species B′. Species C {λexc ) 320 nm; λem ) 530 nm} was assigned to Mg3C-terminated kinks and O3C-terminated corners and species D {λexc ) 350 nm; λem ) 605 nm} specifically to O3C-terminated kinks. Under realistic conditions (treatment at moderate temperature), the surface is able to dissociate water to form hydroxyl groups (eq 1), of which interest in catalysis was evidenced.1

Two main categories of hydroxyls are expected: (i) the OL′CH groups generated by protonation of oxide anions O2-L′C and (ii) OH groups coming from the hydroxylation of surface magnesium cations Mg2+LC. It was shown that the local topology of the surface is the first parameter governing the strength of the interaction between the hydroxyl groups and the surface:14 due to the bridging ability of OH in concave areas of the surface, the related O2,3CH groups are thermally more stable than the O1CH formed on convex areas (Figure 1). Recently, we also showed that the IR and 1H NMR spectroscopic fingerprints of the OH groups of MgO were primarily driven by their

10.1021/jp8067602 CCC: $40.75  2008 American Chemical Society Published on Web 11/16/2008

Photoluminescence Spectra of MgO Powders

Figure 1. Schematized representation of local topology: on convex areas, OH groups formed by hydroxylation of surface Mg2+ are O1CH, whereas they are able to bridge between several Mg2+ on concave areas, thus giving rise to O2C-H and O3C-H.

involvement in a hydrogen bond and, if they are not isolated, by their hydrogen-bond acceptor or donor character.15,16 Compared to bare surfaces, hydroxylated MgO powders17 evidenced three additional luminescent species characterized by {λexc ) 250 nm; λem ) 410 nm}, {λexc ) 350 nm; λem ) 470 nm}, and {λexc ) 370 nm; λem ) 470 nm}. From their relative thermal stabilities, the former one was assigned to isolated OH groups and the two latter to OH in hydrogen bonding with other hydroxyl or nondissociated water, respectively. Theoretical calculations consistent with experiments showed however that very few isolated OH groups could exist.14 The predominant role played by local topology on the excitation energies was evidenced on dehydroxylated surfaces.13 Its influence as well as that of the coordination of the oxide ion of the hydroxyl group is questionable on hydroxylated surfaces and should be discussed as done earlier for the IR and 1H NMR features.15,16 Molecular modeling, used with success so far for these spectroscopic techniques, appears as an accurate tool to reach this goal. We recently used embedded clusters to model surface defects of MgO,15,16 hydroxylated in a realistic way, by adsorption of a whole water molecule described by concomitant adsorption of H+ and OH- ions. The validity of those clusters was checked by comparison of geometric and spectroscopic data with their periodic analogs and with experimental data. The same approach was taken to study their electronic properties. To the best of our knowledge, there is no report in the literature on the calculation of electronic excitation energies for hydrated MgO surfaces. Data on calculated optical properties of MgO in interaction with H2 exist,18-20 which can be compared with experiments performed on MgO systems modified by pre- or postirradiation by UV light,18,19 but they are limited to only one type of defects, protonated corners,20 without considering the impact of the hydride moiety. The present work opens interesting perspectives to the assignment of experimental spectra by applying this method to hydrated MgO clusters, considering the impact of the two OH groups formed on irregularities known to dissociate water. It aims at proposing a structural interpretation of the complexity of photoluminescence excitation and emission spectra of hydrated MgO surfaces on the basis of TD-DFT excitation energy calculations, together with the experimental modifications of excitation spectra observed upon hydroxylation. The excitation and emission processes are finally discussed on the basis of the localization of the initial state of the main calculated transitions and of experimental luminescence decays.

J. Phys. Chem. C, Vol. 112, No. 49, 2008 19711 2. Experimental Section 2.1. Samples Preparation. MgO-sol-gel and MgOprecipitation samples were prepared by thermal decomposition of Mg(OH)2 precursors, prepared by sol-gel route and by precipitation from Mg(NO3)2 solution, respectively. In both cases, the precursor samples were treated in vacuum (10-6 Torr) (1 Torr ) 133.32 Pa) up to 1273 K (ramp 1 K · min-1) and kept at this temperature for 2 h. For sol-gel preparation, a subsequent treatment of the sample under 100 Torr of oxygen at 673 K for 30 min was applied to eliminate remaining methoxy groups. 2.2. In Situ Photoluminescence Experiments. The photoluminescence cell21 was connected to a vacuum system, allowing in situ thermal pretreatments and adsorption-desorption experiments. Samples were outgassed (ramp 1 K · min-1) up to 1273 K and kept at this temperature for 2 h to reach a pressure of 10-6 Torr to remove carbonates and hydroxyl groups. Hydroxylated surfaces were obtained by subsequent adsorption of distilled water (previously purified by the freezepump-thaw technique) at 373 K at an equilibrium pressure of 1 Torr. After heating the samples at 673 K under static vacuum, desorption of hydroxyl groups is followed by progressive outgassing up to 1273 K. Photoluminescence spectra were registered after outgassing (10-6 Torr) under dynamic vacuum, using a spectrofluorophotometer Spex Fluorolog II from Jobin-Yvon (450W Xe lamp, band-passes: 1.1 nm). To avoid difficulties relative to irreproducible positioning of the cell in the quartz dewar filled with liquid nitrogen that could influence the intensities, all the spectra presented here were recorded at room temperature. Note that it was checked that the shape of the spectra was not influenced by the lowering of the recording temperature to 77 K for hydroxylated surfaces. Lifetime decays of the emitting species were also followed at room temperature (pulsed Xe lamp, frequency 20 Hz, flash FWMH 3 ms, band passes: 4.5 nm), following the procedure described earlier.22 The relative amount of different luminescent species was determined using a procedure described earlier17 based on the extraction from deconvoluted excitation spectrum of the component relative to the contribution of a given luminescent species. To optimize the optical yield for each species, its excitation spectrum has to be recorded at the emission energy. Because of the exploitation of excitation spectra that are collected at fixed emission energy, species emitting via an energy transfer along the surface can not be taken into account in this procedure. 3. Computational Methods 3.1. Systems and Methods. Embedded cluster geometry optimizations were performed at the DFT level, within the B3LYP23,24 hybrid exchange correlation functional (which provides better results than GGA, see ref 13), using the Gaussian03 code.25 Hydroxylated clusters (one water molecule per cluster, hereafter referred to as -1w in the cluster nomenclature) were previously used to obtain their vibrational15 and nuclear properties.16 The 6-311+G** basis set was chosen for geometry optimization, except for the third-order neighbors of hydroxyl groups (magnesium atoms at the border of the clusters noted Mg*), for which Mg LANL2 effective core potential was used to avoid unphysical polarization with embedding species, which consist in an array of point charges (from 913 to 2100, see Table 2 of ref 15). These clusters exactly correspond to the dehydrated ones studied in part I.13 4C ions are described at the edge of monatomic or higher steps, whereas 3C ions are depicted by corners, step divacancies,

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Chizallet et al. TABLE 1: Experimental Fingerprints, Effective Lifetime τ at 300 K, and Assignment of Luminescent Species Observed on Hydroxylated MgO-sol-gel species λexc (nm) λem (nm) E

250

410

F G

350 370

410 470

a

τ (10-6s)

assignment17

nonexponential OH and neighboring decay O2-/Mg2+ 12a OH at high coverage 13a OH in interaction with H2O

Values measured for outgassing performed both at 300 and 673

K. Figure 2. Evolution of excitation and emission spectra of MgOprecipitation recorded at λem) 380 nm and λexc ) 240 nm (species A), respectively (- initial treatment at 1273 K, -0- after hydration and further heating at 673 K, -4- and further outgassing at 773 K).

and kinks. Monatomic steps are modeled by the S1-1w cluster and edges and valleys by the S2-ON-1w and S2-IN-1w systems, respectively. Corners (exhibiting 3C ions) are described by the C-1w-O3C (O2-3C-terminated corners) and the C-1w-Mg3C (Mg2+3Cterminated corners) clusters, and 3C ions in concave environments by a divacancy performed in the edge of the monatomic step, called D-1w-Mg3C-O3C, and by kinks: K-1w-O3C (O2-3Cterminated kinks) and K-1w-Mg3C (Mg2+3C-terminated kinks). The related hydroxylated clusters are depicted in Supporting Information S1. The hydrated (100) plane was not studied because it was shown that no water is adsorbed on the terraces in the pressure and temperature range under consideration.14 The size of the cluster chosen was shown to be sufficient for a converged description of vibrational,15 nuclear,16 and electronic13 properties of the surface sites. 3.2. Calculation of Electronic Properties. TD-DFT was then used to calculate the energies associated to electronic transitions, with the 6-31G** basis sets (see ref 13). The energies and oscillator strengths of the 15 singlet f singlet transitions of lower energy were calculated, except for D-1w-Mg3C-O3C and K-1w-O3C where 30 and 25 transitions were calculated, respectively, to account for the experimental energy range. The calculated absorption spectra are given by the position of the electronic transitions weighted by the oscillator strengths. They are smeared out by Gaussian functions (sigma of 0.2 eV). The localization (atoms of the clusters with the highest coefficient of the orbitals) of the initial and final states of the transitions was determined by a population analysis to visualize the atoms at the origin of a given transition. 4. Results 4.1. Experimental Excitation and Emission of Hydroxylated MgO Powders. The modification of photoluminescence excitation and emission spectra related to species A upon hydration of MgO-precipitation is presented in Figure 2. As reported earlier,17 hydration followed by evacuation at 300 K does not modify the spectra (spectrum not shown). Further heating in vacuo above 673 K changes the intensity and shape of the spectra. Recording the spectra at 77 K does not significantly change the position of the excitation and emission bands. While five types of luminescent species were identified on dehydroxylated surfaces13 (referred to as species A, B, B′, C and D, see the Introduction), only three types related to hydroxylation of the surface were observed with λexc) 250, 350, and 370 emitting at λem) 410, 410, and 470 nm,respectively, as reported earlier.17 They are labeled species E, F, and G, respectively (Table 1). Note that in Figure 2 the recording

conditions of the spectra were kept fixed for all the series presented (λem ) 380 nm and λexc ) 240 nm), corresponding specifically to the optimized visualization of A species. This is why species E appears at 255 nm instead of 250 nm. The intensity increases with the subsequent outgassing temperature (spectra shown for outgassing temperature at 773 K in Figure 2), in relation with the decrease of the quenching role played by water molecules and/or OH due to competitive nonradiative processes. The relative amount of the different types of luminescent species (A, B, B′, E, F, and G) present on hydroxylated MgO precipitation outgassed at increasing temperature has been determined following the procedure given in the Experimental section, and the change in intensity in the range of 673-1273 K is reported in Figure 3. Because of their too low intensity, species C and D have not been considered. Species B and B′, related to corners and kinks, respectively, have been grouped because of their unique fingerprint (they can only be discriminated by lifetime measurements).22 Globally, the contributions of species E, F, and G formed upon hydroxylation decrease for increasing outgassing temperature, at the benefit of those observed on dehydroxylated surfaces (A,B-B′). Among the species formed upon hydroxylation, F and G are the less stable, disappearing around 773 and 873 K, respectively. Species E which exhibits the most intense signal is the most stable. Species A and B related to bare oxide ions are only detected from 873 K. Moreover, species B (and B′) are fully recovered at lower temperature than that of species A, which can be linked to the lower thermal stability of OH formed on a corner (species B) than on monatomic step S1 (species A).14 The decrease of signal intensity from 1173 K for A and from 1073 K for B-B′ is probably related to a sintering effect.9,10 Contrary to species F and G whose lifetimes are reported in Table 1 for MgO-sol-gel, the intensity of the band of species E does not follow an exponential luminescent decay, as evidenced in Figure 4. By analogy with earlier work,22 it can be inferred that the related emission at 410 nm could originate both from species directly excited at 250 nm, consistently with the initial decrease of intensity, and from species emitting via energy transfer. 4.2. Calculated Excitation Energies. The calculated absorption profiles, resulting from the convoluted oscillator strengths of the 15 transitions (or more) calculated for each hydroxylated/ dehydroxylated cluster, are reported in Figure 5. The main components, oscillator strength and localization of the transitions, are given in Table 2. It appears that hydroxylation greatly modifies the excitation energies. In several cases, S2-ON-1w, C-1w-O3C, D-1w-Mg3C-O3C, K-1w-Mg4C-O3C, and K-1w-Mg3CO4C hydroxylation results in the disappearance of the highest wavelength part of the spectra. This feature is consistent with XPS and MIES results, concluding to a shift toward higher energy levels upon hydroxylation of MgO.26-29 In a few cases,

Photoluminescence Spectra of MgO Powders

J. Phys. Chem. C, Vol. 112, No. 49, 2008 19713

Figure 3. Areas of the excitation band relative to species A, B-B′, E, F, and G as a function of the outgassing temperature for MgO precipitation hydrated at 300 K and further heated in static conditions at 673 K.

Figure 4. Decay of the intensity of the signal of species E {λexc ) 250 nm, λem ) 410 nm} for the MgO-sol-gel sample ougassed at 300 K. The decay observed for species A{λexc ) 240 nm, λem ) 380 nm} is presented for comparison (outgassing at 1273 K).

less energetic levels difficult to be observed were also invoked,28,29 possibly corresponding to S1-1w, S2-IN-1w, and C-1w-Mg3C configurations. The result for hydroxylated corners has to be compared to protonated O2-3C-terminated corners resulting from dissociative adsorption of H2.20 In the present work, calculated optical transitions indicate a shift from 261 to 247 nm of the main excitation maximum upon hydroxylation of the C-O3C system by a water molecule, whereas they were found to shift from 258 nm (4.8 eV) to 275 nm (4.5 eV) upon simple protonation of O2-3C corners.20 These opposite shifts of calculated excitation energies may be due to the fact that the formation of the accompanying hydride entity has not been considered by Mu¨ller et al.9 underlining the crucial importance played by the other moiety of the dissociated molecule. 5. Discussion There have been few reports on the role of OH groups on the photoluminescence properties of MgO.17 In the present work, in situ photoluminescence allows us to discriminate the luminescent signals present on dehydroxylated MgO from those (E, F, G) appearing upon hydroxylation of the MgO surface (section 4.1). In view of the great number of different types of hydroxyl groups expected upon water dissociation on MgO,14 these experimental bands probably gather several types of hydroxyls. The subject of this work (part II) is to determine which structural

parameters govern luminescence properties of hydroxylated MgO surfaces. Together with lifetime measurements and TDDFT calculations, these results enable us (i) to describe the influence of hydration on excitation spectra, (ii) to propose an assignment for these bands, (iii) and to describe the mechanism of the excitation and emission processes. 5.1. Influence of Hydration on Excitation Spectra. The first aim of our study is to check whether experimental excitation spectra are consistent with features obtained by calculations. Figures 6a and b allow us to evidence the influence of hydroxylation on the calculated positions and intensities of the bands on a real system that simultaneously involves various types of irregularities (here considered in equal proportions). As stated earlier,13 the convolution of calculated absorption profiles cannot be directly compared to experimental spectra due to the fact that the distribution of the various irregularities in a real sample is morphology dependent, which should affect the relative weight of the various contributions. Focusing on the energy windows restricted to what can be experimentally accessible (λexc > 220 nm), it appears that the relative contribution weight of the various defects in experimental spectra is expected to be modified upon hydroxylation. Indeed, the contribution the S2-ON system is shifted outside of our experimental window and is thus no longer observed. Moreover, the intensities of absorption profiles largely depend on the nature of the considered sites. As a result, for a given distribution of irregularities, the contribution of S2-ON greatly decreases, whereas that of C-O3C increases upon hydroxylation. Note that, experimentally, hydroxylation is not expected to significantly modify the morphology of samples prepared by humid routes,17 but if any, it would rather also increase the proportion of defects such as corners.5 The relative contribution of clusters representative of 3C corner ions (C-O3C and C-Mg3C) are increased and globally shifted from 265 to 250 nm (or lower) upon hydroxylation. The resulting convoluted absorption profile appears to be bimodal with a strong contribution at ∼ 250 nm and a weaker one at 290 nm, whereas it was rather trimodal for dehydroxylated clusters at 240, 265, and 290 nm. Taking into account the fact that the lower energy contributions of these two absorption profiles are related to kinks defects that are expected to be in low amounts, this description is consistent with the experimental

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Figure 5. Calculated absorption profiles for hydroxylated (-9-) and dehydroxylated clusters (-) S1, S2-ON, S2-IN, C-O3C, C-Mg3C, D-Mg3C-O3C, K-Mg4C-O3C, and K-Mg3C-O4C. Intensities have been weighted with the corresponding oscillator strengths.

excitation spectra (Figures 6a and 6b). Indeed, the experimental excitation spectrum that results in two bands at around 240 and 270 nm for dehydroxylated MgO (Figure 6a) leads to a unique broadband at around 255 nm after hydroxylation (Figure 6 b). The agreement between the experimental spectra and the calculated features occurring upon hydroxylation is thus satisfactory. 5.2. Assignment of Excitation Bands for Hydroxylated Samples. 5.2.1. First group: Excitation Energy >4.3 eV (λexc < 290 nm). We discard hydrated edges, the excitation energy of which is not experimentally accessible with our set up, and hydrated O3C-terminated kinks that give a contribution at a low energy range ( Excitation Energy > 3.75 eV (290 nm < λexc < 330 nm). Monohydrated O3C-terminated kinks lead to calculated maxima at 290 and 330 nm and to a weak band at 385 nm (Table 2). Taking into account the low associated oscillator strength and the relatively low abundance of such defects on real systems, as well as the high thermal stability expected for the related OH groups,14 it can be inferred that these calculated contributions cannot be responsible for the experimental bands observed at 350 and 370 nm, which correspond to less stable OH groups than those formed on kinks. Thus, no transition calculated for monohydrated systems fits the experimental bands at 350 and 370 nm (species F and G). The similar lifetime values and thermal stabilities of species F and G suggest that their nature also is similar. Moreover, their lifetime values are the lowest of all those measured, indicating that a nonradiative deexcitation process is promoted for the related structures. It could thus be inferred that species F and G imply structures with high water coverage that could not be taken into account in our molecular modeling. Since there is no relationship between the coordination of O2-LC of OLCH groups and the luminescent signature of the latter, the interconversion protonation process O2-LC / OLC H cannot be simply monitored following the selective transformation of the related photoluminescence signals. By photoluminescence, such protonation process is better de-

Photoluminescence Spectra of MgO Powders

J. Phys. Chem. C, Vol. 112, No. 49, 2008 19715

TABLE 2: Calculated Excitation Energy Maxima, Main Components, Oscillator Strength, and Localization of the States Corresponding to the Main Transitions Implied Related to Hydroxylated Clustersa

system S1-1w

main localization of the initial electronic state

maxima (nm)

main components (nm)

oscillator strength

236

233 237 245 257

0.0492 0.0258 0.0182 0.0123

O2-4C, O2-4C, O2-4C, O2-4C,

255

O2-5C, O2-6C (near O2C-H and O4C-H) O2-5C, O2-6C (near O2C-H and O4C-H) O2-5C (near O2C-H) O2-5C (near O2C-H)

main localization of the final electronic state Mg2+5C, Mg2+5C, Mg2+5C, Mg2+5C,

Mg2+6C Mg2+6C Mg2+6C Mg2+6C

S2-ON-1w

223

220 223 226

0.0962 0.0505 0.1294

O1C-H O1C-H O1C-H

Mg2+4C, Mg2+6C Mg2+4C, Mg2+6C Mg2+4C, Mg2+6C

S2-IN-1w

237

236 247

0.0567 0.0699

O2-4C, O2-5C, O2-6C (near O2C-H and O5C-H) O2-4C, O2-5C, O2-6C (near O2C-H and O5C-H)

Mg2+5C, Mg2+6C Mg2+5C, Mg2+6C

C-1w-O3C

247

240 248 252

0.0535 0.0895 0.0763

O3C-H O1C-H O1C-H

Mg2+6C Mg2+6C Mg2+6C

C-1w-Mg3C

214 231

212 229 230 232

0.0615 0.0726 0.0074 0.0835

O2-4C, O2-5C (near O1C-H and O4C-H) O1C-H O1C-H O1C-H

Mg2+4C Mg2+4C Mg2+4C Mg2+4C

D-1w-Mg3C-O3C

253 273

250 275

0.014 0.0253

O2-4C, O2-5C inside D (near both O3C-H) O2-4C, O2-5C inside D (near both O3C-H)

Mg2+3C, Mg2+4C, Mg2+5C Mg2+3C, Mg2+4C, Mg2+5C

K-1w-Mg4C-O3C

290

282 288 315 385

0.0215 0.0089 0.0057 0.0005

O2-4C (near O2C-H) O2-4C (near O2C-H) O2-4C, O2-5C (near O2C-H) O2-4C (near O2C-H)

Mg2+5C, Mg2+5C, Mg2+5C, Mg2+5C,

249 253 256 275

0.0118 0.0123 0.009 0.0176

O2-4C, O2-5C, O2-6C (near O3C-H and O4C-H) O2-5C (near O3C-H) O2-4C, O2-5C, O2-6C (near O3C-H and O4C-H) O2-5C (near O3C-H)

Mg2+6C Mg2+5C, Mg2+6C Mg2+5C, Mg2+6C Mg2+5C, Mg2+6C

330 (shoulder) 385 (weak) K-1w-Mg3C-O4C

253 273

a

Mg2+6C Mg2+6C Mg2+6C Mg2+6C

When second-order neighbours O2- are involved, the OH concerned is mentioned in brackets.

Figure 6. Experimental excitation spectra (corresponding to MgO precipitation) and calculated absorption profiles between 220 and 340 nm by TD-DFT. Intensities have been weighted with the corresponding oscillator strengths: (a) dehydroxylated systems (adapted from Figure 6 in part I13) and (b) hydroxylated systems. Open circle S1, solid triangle pointing up S2-ON, open triangle pointing down S2-IN, solid diamond C-O3C, open triangle pointing left C-Mg3C, solid triangle pointing right D, solid hexagon K-Mg4C-O3C, solid star K-Mg3C-O4C, s experimental spectrum.

scribed by considering the evolution of signals characteristic of dehydroxylated surfaces (species A-D) (Figure 3). 5.3. Mechanism of Excitation and Emission Processes. The similarity of emission spectra of our hydroxylated MgO samples (Table 1) but also of several hydroxylated oxides30-33

with a band at λem ) 410 nm leads to the following questions: (i) Are OH groups excited directly or via energy transfer from other surface excitons? (ii) Are OH groups themselves emitting species, with λem poorly depending on the nature of the ionic lattice?

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5.3.1. Excitation Process. The examination of the localization of the initial state, reported in Table 2, leads one to distinguish two series of hydroxylated systems. In the first one, the OH groups are directly participating to the initial state of the transition leading to the most intense transitions. Their excitation mode is thus related to hydroxyl groups themselves. They correspond to O1CH or O3CH belonging to convex areas (edges, O3C, and Mg3C-terminated corners). In the second, i.e., all other OH groups, those of concave areas and O4CH of convex areas, the observation of the excitation of the hydroxylated surface results from an indirect process. Indeed, the initial state is localized mainly on oxygen ions O2- of the second-order neighborhood. The corresponding excitation modes observed are thus perturbations (of the intensity and/or energy) of excitation modes of parent dehydroxylated surfaces. It can thus be concluded that coordination together with local topology play a key role in the ability of the OH group to be directly excited. Indeed, these convex areas (corners typically) correspond to a weaker Madelung field on the OH group and hence to a poorly stabilized occupied state on the oxygen. These situations correspond to the cases where the highest-energy occupied states are localized on the OH group, explaining their strong participation in the transition. In the concave areas (or in the case of high coordination OH), the OH related energy levels are stabilized by the Madelung field and hence do not contribute to the transition. 5.3.2. Emission Process. The luminescence decay relative to species E {250 nm; 410 nm} after treatment at 300 K of MgO-sol-gel, which is clearly nonexponential (Figure 4), is very similar to that obtained for species excited at 240 nm and emitting at 475 nm that was shown to be associated both to energy transfer and to direct emission of species B:11 the emission at 410 nm could also be partly related to energy transfer resulting from the excitation of the surface MgO pair modified by hydroxyl groups that further emit via hydroxyl groups. The hydroxyls directly emitting at 410 nm without any energy transfer could be located in convex areas, which can be excited directly according to TD-DFT calculations. Both emission modes, intrinsic excitation, and emission of OH groups themselves and emission by OH after energy transfer could thus explain why most hydroxylated oxides exhibit a single emission at 410 nm.30-33

view, on the high-energy range. Thus, a calculated band centered at ∼250 nm results from the convolution of the contributions of monohydrated irregularities. This is consistent with the band at 250 nm experimentally observed that is formed upon hydroxylation at the expense of those at 240 and 270 nm observed on dehydroxylated surfaces, which were shown to originate from defects involving 4C ions of edges and 3C ions of corners or kinks, respectively.13 Thus, while the coordination number of oxide ions is only one among the parameters that influence the excitation energy of dehydroxylated surfaces, on hydroxylated surfaces, it has no more influence on the excitation energy of the related sites. By contrast, the excitation energy calculated for the considered clusters is lower than that of the two bands observed for species F and G. From their relative low thermal stability and lifetime values, it is concluded that these bands are related to structures with a water coverage too high to be handled with our molecular modeling. As shown earlier for the IR stretching frequency of OH groups,15 the local topology of MgO appears as an important parameter which determines the spectroscopic properties of hydroxylated surfaces. Indeed, from the localization of the electronic state at the origin of excitation, the OLCH groups are directly implied in the excitation process only if they are localized on convex areas of the surface and if L e 3. Otherwise, the observed excitation modes of OH rather correspond to perturbations of excitation modes of the parent dehydroxylated surface. From lifetime measurements, it is also concluded that the emission at 410 nm results from direct emission from OH groups of convex areas but also from energy transfer, by excitation of surface Mg2+ and O2- ions modified by hydroxyl groups that further emit via those hydroxyl groups. The implication of OH groups themselves in the emission process may explain why several hydroxylated oxides exhibit emission at 410 nm. As a result, it appears that monitoring the O2-LC / OLCH- protonation by photoluminescence spectroscopy is more accurately achieved by following the evolution of the excitation bands characteristic of bare surface irregularities. Photoluminescence is thus a tool very complementary to IR or 1H NMR because it allows us to identify the nature of oxide ions O2-LC acting as Brønsted basic sites.

6. Conclusion

Acknowledgment. The authors thank the ANR Project: BASICAT, ANR-05-JCJC-0256-01, for its financial support of the experimental work.

Hydroxylated MgO surfaces have been investigated by photoluminescence, with a combined experimental and theoretical approach. Only three types of luminescent species (labeled E, F, and G) are observed experimentally on hydroxylated surfaces and characterized by {λexc ) 250 nm; λem ) 410 nm}, {λexc ) 350 nm; λem ) 410 nm}, and {λexc ) 370 nm; λem ) 470 nm}, respectively. The results show that the assignment of these species cannot simply be rationalized on the basis of the coordination of oxygen ions. This is confirmed by following the disappearance/appearance of luminescent species upon in situ hydroxylation of MgO powders and studying the relative thermal stabilities of the related species. Excitation energies were calculated by TD-DFT using clusters modeling monohydrated surface irregularities, on which dissociative adsorption of water takes place.14 The influence of hydroxylation by a single water molecule on absorption profiles is calculated, and the results are consistent with the experimental features from a qualitative point of

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