Assignment of Photoluminescence Spectra of MgO Powders: TD-DFT

Sep 27, 2008 - Ecole Normale Supérieure de Lyon and CNRS, 46 allée d'Italie, 69364 Lyon Cedex 07, France,. ReceiVed: May 21, 2008; ReVised Manuscript ...
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J. Phys. Chem. C 2008, 112, 16629–16637

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Assignment of Photoluminescence Spectra of MgO Powders: TD-DFT Cluster Calculations Combined to Experiments. Part I: Structure Effects on Dehydroxylated Surfaces Ce´line Chizallet,*,†,‡ Guyle`ne Costentin,*,†,‡ He´le`ne Lauron-Pernot,†,‡ Jean-Marc Krafft,†,‡ Michel Che,†,‡,§ Franc¸oise Delbecq,| and Philippe Sautet| UPMC UniVersite´ Paris 06 and CNRS, UMR 7609, Laboratoire Re´actiVite´ de Surface, F-75005 Paris France, Institut UniVersitaire de France, and UniVersite´ de Lyon, Laboratoire de Chimie, Institut de Chimie de Lyon, Ecole Normale Supe´rieure de Lyon and CNRS, 46 alle´e d’Italie, 69364 Lyon Cedex 07, France, ReceiVed: May 21, 2008; ReVised Manuscript ReceiVed: July 30, 2008

Photoluminescence excitation spectra of dehydroxylated MgO powders are assigned from a combined theoretical and experimental approach. Experimentally, a set of five species is identified, among which are two new species, by recording the spectra with enhanced resolution and at 77 K so as to minimize energy transfer along the surface. Performed on various clusters modeling terraces, edges, monatomic steps, corners, kinks, and divacancies, TD-DFT calculations show that the excitation energy depends not only on the coordination of surface ions, but also on the local topology (kinks versus corners for example). This, together with the fact that a single defect can be excited at several wavelengths, explains the complexity of the experimental spectra and the difficulty encountered so far for their assignment. Several groups of calculated excitation energies can be identified which enable to rationalize the assignment of the experimental spectra. 1. Introduction Monitoring the surface state of solids with basic properties is of prevailing importance in the fields of surface science, corrosion, and heterogeneous catalysis. Catalysts exhibiting basic sites are promising for fine chemistry and biomass conversion.1,2 Because of its simple rock salt structure, MgO appears as a model basic oxide suited for the fundamental study of structural parameters governing the basicity of oxide ions. On a highly divided powder, the form of solids generally used in catalysis, the (100) faces expose many irregularities such as edges of steps, corners, kinks, or divacancies, which are represented in Figure 1. They involve surface in low coordination Mg2+LC and O2-L′C ions (L and L′ being the coordination number). Photoluminescence is one of the few techniques enabling to characterize oxide ions as a function of their coordination.3,4 With this technique, luminescent species are characterized by a couple of excitation (λexc) and emission (λem) wavelengths, corresponding to intensity maxima on emission spectra (emission wavelength recorded at fixed excitation wavelength λexc) and excitation spectra (excitation wavelength recorded at fixed emission wavelength λem), respectively. In the case of alkaline earth oxide powders, photoluminescence bands are broader and at lower energy than those observed for the single crystals,5,6 and assigned to surface sites. However, because of energy transfer processes,7,8 the assignment of the excitation and emission features to surface defects appears to be a complex task, because of the composite nature of the broad bands observed. * Corresponding authors. Ce´line Chizallet: current address is 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, email: [email protected]. Guyle`ne Costentin: phone +33 1 44 27 60 05, fax +33 1 44 27 60 33, email: [email protected]. † UPMC Universite ´ Paris 06. ‡ CNRS. § Institut Universitaire de France. | Ecole Normale Supe ´ rieure de Lyon and CNRS.

Figure 1. Schematic representation of irregularities on the MgO surface, adapted from Figure 11 of ref 3. The terminologies used for the cluster systems are given in brackets; see section 3.1.

The assignments proposed so far for spectra of the bare MgO surface fall into two main groups. In relation to the first group, some authors invoke the {λexc ) 240 nm; λem ) 380 nm} and {λexc ) 280 nm; λem ) 470 nm} couples, which they assign to O2-4C and O2-3C ions, respectively, on the basis of comparisons of samples of various morphologies,6,9-14 the most recent works corresponding to spectra recorded in optimized dynamic vacuum conditions.15,16 Lifetimes of photoluminescent emitting species were recently measured and a kinetic model was obtained.8 The related deactivation mechanism confirmed the former assignments and enabled one to distinguish between O2-3C on corners (involved in energy transfer from O2-4C) and on kinks (not involved in energy transfer from O2-4C). In the following, these three species assigned to O2-4C ({λexc ) 240 nm; λem ) 380

10.1021/jp8045017 CCC: $40.75  2008 American Chemical Society Published on Web 09/27/2008

16630 J. Phys. Chem. C, Vol. 112, No. 42, 2008 nm}), O2-3C terminated corner, and O2-3C terminated kink ({λexc ) 280 nm; λem ) 470 nm} for both) will be labeled A, B, and B′, respectively in a first time, as their assignment has to be confirmed. In relation to the second group, other authors evidence the {λexc ) 280 nm; λem ) 380 nm} couple which they assign to corner O2-3C ions17-19 on the basis of spectra recorded in static vacuum on cubic samples of various particles sizes. The assignment of photoluminescence spectra of bare MgO surfaces appears thus to be rather controversial. Molecular modeling appears to be an efficient tool to rationalize the assignment. The calculation of electronic transitions has already been performed in the case of cluster models of the MgO surface, in most cases at the Hartree-Fock or semiempirical level, by configuration interaction (CIS, CISD).20,21 TD-DFT,22-25 known to provide better agreement with experiments,26-28 has only been scarcely used in the case of MgO. Satisfactory results were obtained for MgO single crystal.29 Avdeev and Zhidomirov30 used very small nonembedded clusters and did not manage to accurately reproduce experimental spectra. On the contrary, the work of Sushko et al.31 reports on TD-DFT excitation energies of various irregularities modeled with a high level of embedding. They, however, report only the lowest-energy excitation transition, even if its probability is low. More recently, the same group32 investigated a wide variety of electron traps at the MgO bare surface by the same techniques and reported the strongest excitation peaks calculated among the 10 lowest singlet-singlet electronic transitions. We recently used embedded clusters to model surface defects of hydroxylated MgO (Figure 1) and their spectroscopic properties, in good agreement with experiments.33,34 These clusters can be further used in their dehydroxylated form with the aim at studying their electronic properties. A dependence of calculated excitation energies upon the size of the clusters was, however, demonstrated.20,21 DFT with GGA exchange correlation functionals is also known for underestimating the band gap of oxides35 like MgO, which is partially improved by the use of hybrid functionals,36 so that trends between various types of defects will be considered, rather than absolute excitation energy values. The present study thus aims at proposing a structural assignment of photoluminescence excitation and emission spectra of dehydroxylated MgO surfaces. First, a more accurate experimental identification of excitation and emission wavelengths is proposed from spectra recorded at 77 K allowing the identification of new luminescent species. The assignment is proposed on the basis of TD-DFT excitation energies calculations performed on embedded clusters modeling various irregularities of the MgO surface. The heaviest calculation of emission energies requires a set of additional assumptions21 and is beyond the scope of the present work. The emission processes are investigated by recording experimental photoluminescence decays. Finally, an identification of associated excitation and emission wavelengths and a model for their assignment is given, on the basis of this combined experimental and theoretical approach. The present article will be followed by a complementary description of hydroxylation effect (part II), also using the same approach. 2. Experimental Section 2.1. Sample Preparation. As reported elsewhere,16 MgO sol-gel and MgO precipitation samples were prepared by thermal decomposition of Mg(OH)2 precursors. The Mg(OH)2 precursor of MgO sol-gel was prepared by a sol-gel route,

Chizallet et al. i.e., by hydrolysis of magnesium methanolate Mg(OCH3)2 (Mg: Aldrich, 99.5%; methanol: Acros Organics, H2O < 0.005%). The Mg(OH)2 precursor for MgO precipitation was obtained by reacting Mg(NO3)2 (Aldrich, 99.99%) solution with ammonium hydroxide (Aldrich, 8 M). In both cases, the precursor samples were treated in vacuum (10-6 Torr) up to 1273 K (ramp 1 K.min-1) and maintained at this temperature for 2 h. For sol-gel preparation, the sample was subsequently treated under 100 Torr of oxygen at 673 K for 30 min to eliminate remaining methoxy groups. 2.2. In Situ Photoluminescence Experiments. Procedure. The photoluminescence cell, described earlier,15 was connected to a vacuum system, for thermal pretreatments to be performed in situ. Samples (about 30 mg) were outgassed (ramp 1K.min-1) up to 1273 K and kept at this temperature for 2 h under dynamic vacuum to a final pressure of 10-6 Torr to remove carbonates and hydroxyl groups. Spectra Recording. All the photoluminescence spectra were registered after evacuation (10-6 Torr) under dynamic vacuum, using a spectrofluorophotometer Spex Fluorolog II from JobinYvon (equipped with 450 W Xe lamp as excitation source and color filters to eliminate scattered light). The excitation and emission band passes were both set to 1.1 nm, to enable a gain in resolution compared to previous experiments where band passes were set to 5 and 1.9 nm for excitation and emission, respectively.15,16 The spectra were recorded at 77 K, using a quartz Dewar filled with liquid nitrogen. Luminescence Decay Measurements. Lifetime measurements of the emitting species were determined at room temperature (for quantitative purposes, the setup used at 77 K is not accurate enough) using a pulsed Xe lamp (frequency 20 Hz, flash fwhm 3 ms) adapted on the spectrofluorometer, following the procedure already described8 and increasing the excitation and emission band passes to 4.5 nm in order to collect enough signal to observe significant intensity changes. Intrinsic and effective lifetimes were deduced with the help of the methodology reported earlier.8 3. Computational Methods 3.1. Systems and Methods. Embedded cluster geometry optimizations were performed at the DFT level, within the B3LYP hybrid exchange correlation functional,37,38 using the Gaussian03 code.39 As mentioned in the introduction, part II is devoted to hydroxylated systems. To allow rigorous comparison of TD-DFT results between hydroxylated and dehydroxylated clusters, the simulated systems differ only by the presence/ absence of the adsorbed water molecule (eventually dissociated). Genuine clusters were constructed in their hydrated form (single water molecule), with the rules given earlier.33,34 The clusters are embedded in an array of point charges (from 913 to 2100; see Table 1) distributed in a parallelepiped including from 4 to 5 layers in all directions starting from the border of the quantum mechanically (QM) described cluster, and excluding the region of this QM cluster. In most cases, atoms of the water molecule, atoms on which water is adsorbed (including those on which bridging of the HO- ion is formed), and their nearest neighbors are relaxed (6-311+G** basis set). The next-nearest neighbors are described by the same basis set, and were kept fixed during geometry optimization. When next-nearest (second-order) neighbors are oxygen atoms, their third-order neighbors (magnesium atoms) are described by a LANL2 effective core potential, with no additional basis set, so as to prevent their artificial polarization by the embedding species.40-44 The dehydrated clusters were deduced from removal of the water molecule and further geometry optimization.

Assignment of Photoluminescence Spectra of MgO Powders

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TABLE 1: Label, Formula (Mg* Represents Mg Atoms Described by Effective Core Potentials during Geometry Optimization) and Embedding (Number of Point Charges) of Model Clusters Describing the Surface Sites

nature (100) plane

label

T T′ monatomic step S1 diatomic step: edge S2-ON S2-ON′ diatomic step: valley S2-IN corner: O2-3C-terminated C-O3C corner: Mg2+3C-terminated C-Mg3C divacancy D-Mg3C-O3C kink: O2-3C-terminated K-O3C kink: Mg2+3C-terminated K-Mg3C

formula

number of embedding point charges

Mg13O14Mg*17 OMg*5 Mg20O14Mg*10 Mg10O10Mg*10 Mg10O16Mg*22 Mg22O9 Mg13O13Mg*12 Mg7O9Mg*10 Mg9O10Mg*13 Mg19O13Mg*9 Mg21O14Mg*10

3156 3194 2100 1470 1452 913 962 974 2096 2098 2091

4C ions are located at the edge of monatomic or higher steps, whereas 3C ions are found at corners, step divacancies, and kinks. Monatomic steps were modeled by the S1 cluster (Mg20O14Mg*10), and edges and valleys by the S2-ON (Mg10O10Mg*10) and S2-IN (Mg22O9) systems, respectively. For this latter cluster, we were interested in probing the electronic properties of the 6C and 5C ions in the vicinity of the valley. Corners (exhibiting 3C ions) are described by the C-O3C (O2-3C-terminated corners, Mg13O13Mg*12) and the C-Mg3C (Mg2+3C-terminated corners, Mg7O9Mg*10) clusters, and 3C ions in concave environments45 by a divacancy performed in the edge of the monatomic step, called D-Mg3C-O3C (Mg9O10Mg*13), and by kinks: K-O3C (O2-3C-terminated kinks, Mg19O13Mg*9) and K-Mg3C (Mg2+3C-terminated kinks, Mg21O14Mg*10). Fundamentally, kinks and corners differ in the fact that corners are the crossing points between three extended edges, whereas in the case of the kink, one at least of the three edges is “short” (one or two Mg-O units at most). The kink clusters exhibit two short edges (contrary to the clusters of McKenna at al.32 where kinks are the crossing points of two extended edges and a single short one). Clusters modeling the (100) terraces were added to provide a more detailed description of optical properties of the surface and described by the T system (Mg13O14Mg*17). Table 1 reports the label, formula, and characteristics of all clusters considered in the present study. Figure 2 shows two representative clusters, whereas the structure of all other dehydroxylated ones is depicted in Supporting Information S1. In the case of S2-ON and T, the impact of the size of the cluster on excitation energies is studied by means of the S2-ON′ (Mg10O16Mg*22) and T′ (OMg*5) clusters, respectively. 3.2. Calculation of Electronic Properties. TD-DFT was used to calculate the energy of electronic transitions. The energies and oscillator strengths of the 15 transitions of lower energy were calculated, except for D-Mg3C-O3C, K-O3C, and K-Mg3C, where 30, 40, and 20 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 curves (0.2 eV, chosen on the basis of comparison with experiments; see Supporting Information S2). Because nonzero oscillator strengths are obtained in the case of singlet f singlet transitions only, no forbidden singlet f triplet transitions have been considered. The localization (atoms of the clusters on which the coefficient of the orbitals are the highest) of the initial and final states of the singlet f

Figure 2. Outermost surface layer of clusters depicting (a) S1 (monatomic step), (b) K-O3C (O2-3C-terminated kinks). Green spheres, Mg; blue spheres, Mg* (described by LANL2 core potential during geometry optimization); red spheres, O. Embedding charges are omitted for the sake of clarity.

singlet transitions was determined by a population analysis, to visualize the atoms at the origin of a given transition. From preliminary tests (see Supporting Information S3), the PW91 functional was shown to underestimate the energy band gap between occupied and unoccupied states, in line with literature data.46 In agreement with earlier results,25,27,28,47 the hybrid B3LYP functional provides better agreement with experiments and was thus preferred for excitation energy calculation. Moreover, the basis set has to be lightened in comparison to geometry optimizations, since calculations of excited states are much more CPU intensive. Results obtained with the 6-31G* basis20,21,31,32,48,49 were compared to 6-31G, largely used in literature for excitation energy calculation.20,21,31,32,48,49 The former, which is more accurate, induces changes in oscillator strengths for the highest energy transitions calculated (due to additional d polarization functions; see Supporting Information S4). More complete basis sets did not lead to significant evolutions or led to excessive calculation time required for convergence. The 6-31G* was thus used for the calculations. It should be emphasized that the curve resulting from the summation of all the considered transitions cannot formally be directly compared to experimental excitation spectrum, since no hypothesis on the resulting emission is made and because some of these excitation components may not lead to radiative emissions. It would thus rather correspond to absorption spectrum recording conditions. 4. Results 4.1. Experimental Excitation and Emission Features of MgO Powders. Spectra. In line with spectra recorded at room temperature,8,16 typical photoluminescence spectra at 77 K (Figure 3a) of MgO sample outgassed at 1273 K evidence species which emit at 380 and 470 nm, when excited at 240 and 280 nm, respectively. Kinetic modeling of energy transfer has shown that they correspond to three contributions related to species A and B/B′ (see Introduction).8 Species excited at higher energy cannot be observed with the available UV source. In addition to these luminescent species (excitons), two other species corresponding to less intense bands are observed, but

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Figure 4. Emitted light intensity at 300 K at 380 nm (0) and 475 nm (2) upon 240 nm excitation, and corresponding numerical fit (lines, according to the model described in ref 8) for MgO precipitation.

Figure 3. MgO sol-gel photoluminescence spectra recorded at 77K (a) emission spectra excited at 240 and 280 nm and corresponding excitation spectra (emission at 380 and 470 nm, respectively); (b) emission spectra excited at 320 and 350 nm and corresponding excitation spectra (emission at 530 and 605 nm, respectively).

they appear much more clearly in the case of the more defective MgO sol-gel sample, as reported in Figure 3b. Excitations at 320 and 350 nm, corresponding to emissions at 530 and 605 nm, respectively, are evidenced and will be hereafter referred to as species C and D. Their observation was made possible due to adequate recording conditions (low temperature, low residual pressure, and thin slits). The high sensitivity of the spectra to O2 atmosphere (intensity of the bands reduced up to 70% under 1 mbar of O2; spectra not shown) indicates that they are related to surface species. Luminescence Decay Measurements. Measurements of photoluminescence lifetimes τ can be a complementary tool to assign photoluminescence spectra,8 and it was recently used to study a highly regular (cubic-shaped) MgO sample prepared by CVD.17,50 A kinetic model8 was applied to luminescence decays observed for species A, B, and B′. The two latter species

are distinguished by their ability to be excited by energy transfer: B is able to be excited after 240 nm excitation in contrast to B′. This leads to nonexponential decay for A, and for B under 240 nm excitation. Intrinsic lifetimes for each species as well as the energy transfer kinetic constant were deduced8 from the following: • simultaneously fitting the A and B decay emission curves obtained under 240 nm excitation, following the numerical resolution of differential equations governing the emission of A, the energy transfer from A to B, and the subsequent emission of B. Intrinsic lifetime of A and B can be deduced (assuming no energy transfer). The effective lifetime of A (obtained by assuming monoexponential decay) is lowered compared to the intrinsic one, due to its partial deexcitation by energy transfers through B. • then, fitting by biexponential decay the 470 nm emission curve upon 280 nm excitation, enclosing intrinsic emissions of B and B′. The lifetime of B being known from the previous fit, that of B′ could be deduced. This method was applied here to MgO precipitation, and the numerical fits for A and B decay under 240 nm excitation are reported in Figure 4. The fact that the fit reported in Figure 4 for MgO precipitation is not as accurate as for CVD-MgO8 demonstrates the more complex nature of the MgO precipitation surface. The results obtained for MgO sol-gel did not prove satisfactory, probably because of the great variety of defects involved in a narrow excitation range. Consequently, only the global decay for B/B′ (assuming monoexponential decay) is reported for MgO sol-gel. For species C and D, luminescence follows an exponential decay, so that lifetimes can be directly deduced. The intensity is quite low (especially on MgO precipitation where no lifetime could be measured for D), so that the poor sensitivity could hide more complex profiles. All recorded data are summarized in Table 2. 4.2. Calculated Excitation Energies. Construction of absorption spectra is exemplified in the case of the edge S2-ON on Figure 5a. Despite its composite character, the shape of the calculated spectrum appears quite simple due to the overlap of different Gaussians curves in relation with appropriate value chosen for fwhm parameters (see Supporting Information S2). The Gaussians curves are calculated as a function of energy, so that they appear asymmetrically shaped as a function of wavelength.

Assignment of Photoluminescence Spectra of MgO Powders

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TABLE 2: Experimental Signature, Effective Lifetimes at 300 K, and Assignment of Luminescent Species Observed on Dehydroxylated MgO Sol-Gel and MgO Precipitation Samples τ (10-6 s) species A B B′ C D

λexc (nm) 240 280 280 320 350

λem (nm) 380 470 470 530 605

ref 8 and this work 8 and this work 8 and this work this work this work

MgO precipitation 370 71 9

global decay: 17 39 not detectable

Influence of Cluster Size. Due to the delocalization of excitons on extended defects (edges, terraces),31 the size of the embedded cluster might influence the calculated excitation energies. As our family of clusters is built from the same

Figure 5. Absorption profile evaluated by TD-DFT: (a) Construction of the absorption curve (dashed line) for S2-ON, from summation of all Gaussian curves corresponding to calculated contributions (straight lines, intensities have been weighted with the corresponding oscillator strength). (b) Cluster size effect on the absorption profile of the edge. (c) Cluster size effect on the excitation profile of the terrace. (T ) terrace, S2-ON ) edge).

MgO sol-gel 562 global decay: 17 70 85

proposed assignment 2-

O 4C of edges and close to Mg3C-corners O2-3C of corners O2-3C of kinks and O2-4C close to corners and kinks O2-3C of corners and O2-4C close to Mg2+3C-kinks O2-3C of kinks

strategy, it leads to consistent results which can be compared. The size effect on the excitation profile has been studied in the case of terrace and edge (Figure 5b,c) and will thus be extrapolated to other clusters. In the case of the terrace, 15 excited states were calculated for clusters T and T′. Figure 5c shows that the calculated spectra are incompatible to each other. The smaller cluster T′ does not enable the description of the lowest energy transitions evidenced by the larger cluster T. Although close to that reported earlier,31,32 the calculated value for the T cluster (Eexc ) 5.64 eV, λexc ) 220 nm) remains very different from EELS experimental data, Eexc ∼ 6.5 eV (λexc ∼ 190 nm).51-55 This discrepancy may be related to the fact that not enough surface is exposed in the clusters (0.8 nm2 for atoms of cluster T described by the 6-31G** basis set), compared to the real system, such as cubic particles of CVD sample, with edges of ∼5 nm.17 Thus, a much bigger cluster would be required to obtain a converged absorption profile. However, the related excitation wavelength cannot be reached experimentally with the UV source available; this limitation will not impact the spectra assignments. For the profiles reported for S2-ON and S2-ON′ edges (Figure 5b), 15 and 20 excited states were calculated, respectively. As a matter of fact, the number of excited states increases with the number of atoms for increasing cluster size. More than five additional states would have been required for S2-ON′: its calculated spectrum does not cover the wavelength range 240-225 nm where a maximum appears in the case of S2-ON. It can thus be concluded that extending the cluster prevented the observation of the lowest wavelengths absorptions. According to Susko et al.,31 excited states on corners and kinks are even more localized than on edges, so that the latter will not need a larger size than for S2-ON. As a conclusion, the size of our set of clusters corresponds to the best possible compromise. Smaller clusters (such as T′) do not correctly describe the extension of the states (mainly the final one). Larger clusters (such as S2-ON′) would in principle be better but would require inclusion of a large number of excited states, which is not feasible within reasonable CPU time. With a limited number of excited states, these bigger clusters in fact yield an inferior description than the initial size. Calculated Absorption Profiles of Surface Irregularities. Calculated absorption curves are reported in Figure 6 for each dehydroxylated cluster (except S2-ON′ and T′); the relative intensity only takes into account the various oscillator strengths. Gaussian band position corresponding to maxima or shoulders, oscillator strength, and localization of the electrons involved in the corresponding transitions are reported in Table 3. Note that, on real samples, if excitation spectrum of a powdered MgO sample effectively results from the convolution of the contributions of the various defects modeled in this work, their relative weight is also particle morphology dependent, in relation with preparation procedures,16 which govern in turn the relative amounts of irregularities.

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Figure 6. Absorption profiles calculated between 180 and 400 nm by TD-DFT. Inset: extended view of the 300-400 nm range. Intensities have been weighted with the corresponding oscillator strengths.

TABLE 3: Calculated Excitation Energy Maxima and Localization of the States Corresponding to the Main Transitions Implieda maximum (nm)

main components of the band (nm)

S2-IN

209 220

0.10 0.11 0.16 0.43

(O2-5C) f (Mg2+4C, Mg2+5C, Mg2+6C) (O2-4C, O2-5C, O2-6C) f (Mg2+4C, Mg2+5C, Mg2+6C) (O2-6C) f (Mg2+6C) (O2-5C) f (Mg2+6C)

out of range

T

208 217 212 221

C-Mg3C

234

231 227

0.03 0.03

(O2-4C) f (Mg2+3C, closer Mg2+5C) (O2-4C) f (Mg2+3C, closer Mg2+5C)

λexc ∼ 240 nm

S2-IN

234

234

0.09

(O2-4C) f (Mg2+5C, Mg2+6C)

S2-ON

235

229 234 248

0.07 0.19 0.13

(O2-4C, O2-5C) f (Mg2+4C, Mg2+5C, Mg2+6C) (O2-4C, O2-5C) f (Mg2+4C, Mg2+5C, Mg2+6C) (O2-4C) f (Mg2+4C, Mg2+5C, Mg2+6C)

system

shoulder 247

oscillator strength (a.u.)

localization of the transition

corresponding experimental range

S1

244

228 237 245 253

0.04 0.12 0.13 0.04

(O2-4C, O2-5C, O2-6C) f (Mg2+6C) (O2-4C) f (Mg2+6C) (O2-4C) f (Mg2+6C) (O2-4C) f (Mg2+6C)

C-Mg3C

259

252 256 261

0.04 0.05 0.09

(O2-4C) f (Mg2+3C, Mg2+5C) (O2-4C) f (Mg2+3C, Mg2+5C) (O2-4C) f (Mg2+3C, Mg2+5C)

C-O3C

261

255 267

0.10 0.10

(O2-3C, O2-4C) f (Mg2+4C, Mg2+5C, Mg2+6C) (O2-3C, O2-4C) f (Mg2+4C, Mg2+5C, Mg2+6C)

K-O3C

287

282 288 308

0.01 0.03 6 × 10-3

(O2-3C) f (Mg2+6C) (O2-4C outside of the kink) f (Mg2+6C) (O2-3C) f (Mg2+6C)

K-Mg3C

289

283

0.08

(O2-4C in the valley of the kink) f (Mg2+6C)

K-Mg3C C-O3C

327 328

327 327

0.01 0.03

(O2-4C in the valley of the kink) f (Mg2+3C) (O2-3C) f (Mg2+4C, Mg2+5C, Mg2+6C)

λexc ∼ 320 nm

K-O3C

354

348

4 × 10-3

(O2-3C) f (Mg2+6C)

λexc ∼ 350 nm

1355 786 430 449

3× 7 × 10-4 4 × 10-3 3 × 10-3

(O2-

1355 795 D-Mg3C-O3C 430

10-4

2+

λexc ∼ 280 nm

2+

3C and neighboring Mg ) f (Mg 5C in valley of D) (O2-3C) f (Mg2+5C in valley of D) (O2-6C in valley of D) f (Mg2+5C in valley of D) (O2-6C in valley of D) f (Mg2+5C in valley of D)

out of range

T ) terrace, S2-ON ) edge, S1 ) monatomic step, S2-IN ) valley, C-Mg3C and C-O3C ) Mg2+3C and O2-3C corner, respectively, K-Mg3C and K-O3C ) Mg2+3C and O2-3C kink, respectively, D-Mg3C-O3C ) monatomic step divacancy. a

It can be seen that (i) most systems exhibit several maxima, indicating that a given species can be characterized by several bands, and (ii) due to overlapping contributions, the same experimental feature may imply various systems. Note that S2IN also exhibits O2-4C ions in order to lighten the modeling of the valley (see structure in Supporting Information S1), which by nature should only exhibit 5C and 6C ions. Thus, those ions do not have to be taken into consideration to discuss the intrinsic electronic properties of valleys.

5. Discussion In the present work, improving spectra resolution and lowering acquisition temperature down to 77 K enabled observation of luminescent species (denoted A, B, B′, C, and D; see Table 2), among which are two new species (C and D).16 Together with lifetime measurements and TD-DFT calculations, these results enable us to propose an overall picture of excitons on dehydroxylated MgO powders.

Assignment of Photoluminescence Spectra of MgO Powders 5.1. Surface Irregularities versus Color Centers. Surface structural irregularities are at the origin of species A, B, and B′.5,12 Their relative amount was indeed shown to depend on the morphology of the sample.16 In particular, it was observed that the more defective the sample (as deduced from TEM pictures), the more intense the relative contribution of the 280 nm excitation band. Two additional species C and D are evidenced, more clearly on the most defective sample. While species D has never been reported, species C was already mentioned5,56 and assigned to F centers.56 However, it has been theoretically shown that vacancies, such as those involved in F centers, easily react with water molecules,57 with an important energy of -3.18 eV (-307 kJ.mol-1), suggesting that such a reaction will occur at very low water pressures. If present at any stage of the sample synthesis, F centers, for which a detailed assignment of luminescent properties has been theoretically proposed,58 must thus have reacted before photoluminescence analysis in our experimental setup. Moreover, the quenching of species C and D by oxygen (section 4.1) demonstrates that they are located on the MgO surface. To analyze the calculated excitation energies in relation with experimental features, the maxima calculated by convolution of the different components relative to the transitions of a given system, weighted with the corresponding oscillator strength, were ordered following increasing excitation wavelength and reported in Table 3. If the initial state usually implies O2-LC oxide ions, the final state mainly concerns magnesium cations and becomes much more delocalized. To enable comparison with experimental data, four groups of decreasing energy observed experimentally are considered in sections 5.2-5.5. The final assignments of the luminescent species are reported in Table 2. 5.2. Species A. The calculated excitation energies between 5 and 5.4 eV, e.g., 230 nm < λexc < 250 nm, are likely to correspond to the experimental band around 240 nm (5.2-5.4 eV), associated with species A. This band is widely reported and assigned to O2-4C excitation.6,9,10,16-18,59,60 The present results, in line with earlier data,21,32 confirm this assignment: O2-4C belonging to edges of monatomic (S1) and higher steps (S2-ON) and close to Mg3C corners (but with lower oscillator strength) are implied in the initial state of all related transitions. However, participation of ions of higher coordination is also implied in a few transitions, showing that the assignment of the bands depend more on the type of defect than on the coordination of ions. 5.3. Species B and B′. Another group of bands corresponds to calculated excitation energies between 4.3 and 5 eV, e.g., 250 nm < λexc < 290 nm. It has to be related to the experimental band ∼280 nm, e.g., to species B and B′. If, as expected from experimental literature data,6,9,10,16-19,59,60 O2-3C ions of O3C corners and of O3C kinks are involved in this excitation band, calculations show that O2-4C of Mg2+3C containing systems (Mg3C-terminated corners and kinks) are also implied. If the results reported by McKenna et al.32 are quite consistent in the case of O3C corner, lower energies are reported for the Mg3C corner and for kinks (referred to as step corners). The differences in the clusters chosen can explain this slight discrepancy. Note that Trevisanutto et al.48 have calculated emission energy for O3C-terminated corners at 480 nm, from a triplet state: this value is consistent with the experimental emission at 470 nm, and with the order of magnitude of lifetime reported in Table 2 for the species B/B′, typical of phosphorescence. This confirms the existence of the {280 nm; 470 nm} couple and the contribution of O3C corners in those species.

J. Phys. Chem. C, Vol. 112, No. 42, 2008 16635

Figure 7. Scheme of plausible energy transfer along edges leading to excitation of corners and kinks.

Figure 8. Assignment of experimental excitation bands on the basis of TD-DFT calculations. The experimental spectra correspond to excitation spectra of MgO sol-gel with λem fixed at 380, 470, 530, and 605 nm (relative intensities have been changed for the sake of clarity).

In previous work, we distinguished B and B′ by the following property: B is efficiently excited by energy transfer from edges (species A), whereas B′ is not.8 On the basis of EPR results reported by Diwald and al.,61 we proposed that B could be identified with O3C corner and B′ to O3C kinks. The present work shows that species B and B′ have to be assigned to O3C and Mg3C-terminated corners and kinks: Mg3C containing species have to be introduced in the assignment. Figure 7, deduced from the localization of the electronic state at the origin of the transitions (Table 3), illustrates that corners only can be excited by simultaneous energy transfer along three edges converging to the same corner. Kinks can be involved in such an energy transfer with only one edge, e.g., with a much lower probability.

16636 J. Phys. Chem. C, Vol. 112, No. 42, 2008 B could thus be assigned to all corners, and B′ to all kinks. The lowest lifetime reported in Table 2 for species B′ than B is in line with this assignment, insofar as kinks might be more quickly nonradiatively de-excited due to the highest coordination number of their second neighbors compared to corners, leading to more efficient coupling with surface phonons. Note that oscillator strengths are much lower for kinks than for corners (Table 3), which means that the effect of the morphology of the sample on the relative intensities of the 380 and 470 nm emission bands is not proportional to the kinks/ corners ratio. 5.4. Species C. We now can consider calculated excitation energies between 3.8 and 4.1 eV, e.g., 300 nm < λexc < 330 nm. This third group of bands may correspond to the experimental band described as species C observed at 320 nm. It appears to be associated with irregularities involving 3C ions: either directly as O2-3C of corners, or via O2-4C of Mg3Cterminated kinks. Note that, generally, calculation results concerning O3C and Mg3C-terminated corners for 240, 280, and 320 nm excitations are in good agreement with those reported by Trevisanutto et al.48 5.5. Species D. Finally, calculated excitation energies lower than 3.5 eV, e.g., 350 nm < λexc, indicate that the experimental band observed around 350 nm and assigned to species D could be related to O3C-terminated kinks. The assignment of species C and D to corners and kinks is consistent with their higher concentration for MgO sol-gel, which is the more defective sample. It is also consistent with the lifetime values (Table 2) which are quite close to those determined for species B and B′. The low oscillator strengths calculated for species C and D explains the difficulty to detect them, in spite of the irregular morphology of the MgO sol-gel sample. Note that excitation energies of transitions involving a divacancy, which in fact represents the limit case of a kink from a geometrical point of view, appear at wavelengths higher than 400 nm and correspond to very low oscillator strength: they can therefore not be observed using a conventional UV source. 5.6. Synopsis of Assignments. Sections 5.2-5.5 evidence that bands of excitation spectra can thus not simply be assigned considering only the coordination number of oxide ions. For a unique coordination number, O2-4C, for example, different transitions may be obtained depending on their location, resulting in the expected contribution at 230 but also at 280 and 320 nm. In the same way, O2-3C are involved in three experimental excitation bands (280, 320, and 350 nm). It can thus be concluded that several bands can be associated to a given type of oxide ion, which is not usually considered in literature. Moreover, due to convolution effects, the same band may also be related to several species. Despite the composite character of the experimental bands, they gather defects of the same type (the band at 240 nm is characteristic of extended defects (O2-4C belonging to edges of step or close to corner C-Mg3C)), whereas the other bands are characteristic of more localized defects. The assignment should thus be made on the basis of the local configuration rather than simply on coordination number, as summarized in Figure 8. Generally, oscillator strengths are much more important for defects implying high delocalization, like edges and planes. Among more localized excitons (in practice, those involving 3C ions), convex areas (corners) exhibit higher oscillator strengths than concave areas (kinks; see ref 45 for the definition of concave and convex areas). Consequently, the latter species would then be quite difficult to observe by photoluminescence under conventional conditions.

Chizallet et al. 6. Conclusion Dehydroxylated MgO powder surfaces have been investigated by photoluminescence spectroscopy, with a combined experimental and theoretical approach. Experimentally, spectra have been acquired under dynamic vacuum conditions minimizing the quenching by gaseous atmosphere, at 77 K in order to reduce energy transfer, and with high resolution. Five species have been characterized, each by an excitation/emission wavelengths couple. The existence of the {λexc ) 240 nm; λem ) 380 nm} (species A) and {λexc ) 280 nm; λem ) 470 nm} (species B and B′) couples, already observed in previous studies, has been theoretically and experimentally confirmed. Two new species are identified: {λexc ) 320 nm; λem ) 530 nm} (species C) and {λexc ) 350 nm; λem ) 605 nm} (species D), only on the more defective sample. TD-DFT has been used to assign excitation spectra, by modeling surface irregularities by a set of embedded clusters depicting terraces, edges of monatomic and higher steps, corners (O3C and Mg3C-terminated), kinks (O3C and Mg3C-terminated), and monatomic step divacancies. These calculations evidence that a single defect can lead to several bands, and a single band can contain contributions of several defects. It is however possible to assign species A to O2-4C ions of edges (independently of the height of the step, and of the location near a corner). Species B and B′ are assigned to corners and kinks, respectively. The experimental study of their emission properties (lifetime measurements and subsequent modeling of the luminescence decays) together with the calculated localization of the electronic state at the origin of the excitation enable us to distinguish corners (species B, efficiently excited by energy transfer from edges) from kinks (species B′, much less efficiently excited by such an energy transfer). Species C is assigned to kinks and species D specifically to O3C-terminated kinks. This assignment opens up new perspectives in the use of photoluminescence for monitoring surface irregularities. In a following paper, we will report results involving water adsorption: as photoluminescence features are significantly affected by hydroxylation, the same combined experimental and theoretical approach will help to rationalize the experimental spectra and the observed phenomena. Acknowledgment. The authors thank the ANR BASICAT (project: ANR-05-JCJC-0256-01) for its financial support. Supporting Information Available: Structures of dehydroxylated clusters (S1), Gaussian smearing of absorption profiles (S2), influence of the exchange-correlation functional on calculated electronic excitations (S3), influence of the basis set on calculated electronic excitations (S4) and complete reference 39 (S5). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Barrault, J.; Pouilloux, Y.; Clacens, J. M.; Vanhove, C.; Bancquart, S. Catal. Today 2002, 75, 177. (2) Huber, G. W.; Iborra, S.; Corma, A. Chem. ReV. 2006, 106, 4044. (3) Che, M.; Tench, A. J. AdV. Catal. 1982, 31, 77. (4) Anpo, M.; Che, M. AdV. Catal. 2000, 44, 119. (5) Tench, A. J.; Pott, G. T. Chem. Phys. Lett. 1974, 26, 590. (6) Coluccia, S.; Deane, A. M.; Tench, A. J. J. Chem. Soc., Faraday Trans. 1 1978, 74, 2913. (7) Coluccia, S. Stud. Surf. Sci. Catal. 1985, 21, 59. (8) Chizallet, C.; Costentin, G.; Krafft, J. M.; Lauron-Pernot, H.; Che, M. ChemPhysChem 2006, 7, 904. (9) Coluccia, S.; Tench, A. J.; Segall, R. L. J. Chem. Soc., Faraday Trans. 1 1979, 75, 1769.

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