Surface Features of P-Doped Silica Explored with CD3CN Adsorption

The presence of hydroxyl groups and Lewis acid centers was tested with ... These results were compared with ν(CN) and ν(OH) harmonic frequency shift...
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Chem. Mater. 2005, 17, 1416-1423

Surface Features of P-Doped Silica Explored with CD3CN Adsorption: Can Si Atoms Act as Lewis Centers? Marta Cerruti, Claudio Morterra,* and Piero Ugliengo Department of Chemistry I.F.M. and Center of Excellence NIS, UniVersity of Turin, Consortium INSTM, Research Unit of Turin UniVersity, Via P. Giuria 7, 10125 Torino, Italy ReceiVed NoVember 8, 2004. ReVised Manuscript ReceiVed January 17, 2005

Surface features of a P-doped nonporous pyrogenic silica (A200P) were analyzed by means of in-situ FTIR spectroscopy. P-doped silica finds application in different fields and can be considered a reference system for silica-based P-containing bioactive glasses. The presence of hydroxyl groups and Lewis acid centers was tested with perdeuterated acetonitrile (CD3CN), a medium strength Lewis base often used as probe molecule in acid/base IR analyses. Experimental results were compared with computational ones, obtained with ab initio calculations on small Si/P-containing clusters. An IR band at ∼2290 cm-1 was observed upon CD3CN adsorption on A200P outgassed at increasing temperatures up to ∼500 °C. On A200P outgassed at T g 500 °C, another band, located at ∼2340 cm-1, was also observed. These results were compared with ν(CN) and ν(OH) harmonic frequency shifts computed on molecular clusters. The comparison suggested that the occurrence of the band located at ∼2290 cm-1 is due to the formation of both linear and “tilted” H-bonds between POH and acetonitrile molecules. The assignment of the intriguing spectral ν(CN) feature found at ∼2340 cm-1 has been attributed to CD3CN interacting with Si atoms directly bound to at least three PO4 groups. The strong nucleophilic character of the O atoms of the PO4 groups allows for the expansion of the Si atom coordination upon CD3CN adsorption (from fourfold to sixfold), which increases its CN frequency by a sizable amount. At temperatures of sample treatment higher than 700 °C, a fraction of Si atoms already bound to PO4 groups may become sixfold coordinated by extra bonds to the available surface PO4 groups, inhibiting further interaction with CD3CN. It is hypothesized that a similar phenomenon may occur at the surface of silica-based bioactive glasses, when a layer of hydroxycarbonated apatite precipitates on their surface.

Introduction P-doped silica, silicophosphates, and P2O5-SiO2 glasses find application in many different fields: fiber optic technology,1 microelectronics,2 nuclear waste retaining,3 and fast proton conductors,4 and also because P-doping can increase the strength of interaction between silica and polar adsorbates.5 Some compositions of SiO2-P2O5 glasses can also be used as bioactive materials6 and, in addition to CaO and Na2O, are the basis for most bioactive glasses (e.g., see refs 7-9). Bioactive glasses are used clinically as bone regenerative materials in several dental and orthopedic applications:7 when immersed in physiological fluids, they partially dissolve, and a layer rich of silica is formed on their surface. * All correspondence should be sent to this author. E-mail: claudio.morterra@ unito.it.

(1) Lu, P.; Bao, X.; Kulkarni, N.; Brown, K. Radiat. Meas. 1999, 30, 725. (2) Pacchioni, G.; Erbetta, D.; Ricci, D.; Fanciulli, M. J. Phys. Chem. B 2001, 105, 6097. (3) Massiot, P.; Centino, M. A.; Gouriou, M.; Dominguez, M. I.; Odriozola, J. A. J. Mater. Chem. 2003, 13, 67. (4) Matsuda, A.; Kanzaki, T.; Tadanaga, K.; Tatsumisago, M.; Minami, T. Solid State Ionics 2002, 154-155, 687. (5) Turov, V. V.; Gun’ko, V. M.; Zarko, V. I.; Bogatyr’ov, V. M.; Dudnik, V. V.; Chuiko, A. A. Langmuir 1996, 12, 3503. (6) Hench, L. L. J. Am. Ceram. Soc. 1991, 74, 1487. (7) Hench, L. L. Biomaterials 1998, 19, 1419. (8) Vallet-Regi, M.; Romero, A. M.; Ragel, C. V.; LeGros, R. Z. J. Biomed. Mater. Res. 1999, 44, 416. (9) Vallet-Regi, M.; Izquierdo-Barba, I.; Salinas, A. J. J. Biomed. Mater. Res. 1999, 46, 560.

After a short time, a Ca/P-rich layer precipitates on the silicarich layer and further crystallizes as hydroxycarbonate apatite (HCA). Collagen molecules are then incorporated in the HCA, osteoblasts arrive at the interface, and a bond between the material and the living tissue is formed. This process has been at first explained by Clark and Hench,10 and several researchers confirmed some of these steps (e.g., see refs 1113). Still, the structure of specific sites for HCA deposition at the surface of bioactive glasses is unknown, so molecular modeling can be used as a tool to characterize the nature and properties of these sites. In this work, ab initio calculations of some P-containing SiO2 clusters have been carried out as a first step toward the modeling of more complex bioactive glass systems. The cluster models have been designed following the IR spectroscopic results obtained on a sample of P-doped silica (A200P), a material already used by us as a reference system for the study of surface properties of bioactive glasses.14,15 Similar P-containing clusters were presented in a previous paper,14 which addressed the effects (10) Clark, A. E.; Hench, L. L. J. Biomed. Mater. Res. 1976, 10, 161. (11) Pereira, M. M.; Clark, A. E.; Hench, L. L. J. Biomed. Mater. Res. 1994, 28, 693. (12) Nakamura, T.; Yamamuro, T.; Higashi, S.; Kokubo, T.; Ito, S. J. Biomed. Mater. Res. 1985, 19, 685. (13) Cerruti, M.; Greenspan, D.; Powers, K. Biomaterials 2005, 26, 1665. (14) Cerruti, M.; Morterra, C.; Ugliengo, P. J. Mater. Chem. 2004, 14, 3364. (15) Cerruti, M.; Bianchi, C.; Bonino, F.; Damin, A.; Perardi, A.; Morterra, C. J. Phys. Chem. B, submitted for publication.

10.1021/cm0480420 CCC: $30.25 © 2005 American Chemical Society Published on Web 02/24/2005

Surface Features of P-Doped Silica

of thermal treatments and water adsorption on the A200P system. In the present study, the adsorption of acetonitrile on the same sample and its in-situ FTIR spectroscopic features have been compared with vibrational frequency shifts and with binding energies computed on the molecular clusters. Acetonitrile has been often used as a specific probe molecule in in-situ FTIR spectroscopic studies, because of its medium-strength Lewis base character, which allows interaction with both hydroxyl groups and Lewis acid surface sites. The changes of the CN stretching vibration frequency is a function of the type of interaction.16 A reference work relative to acetonitrile adsorption on both A200P and on some bioactive glasses has been recently published.17 For IR spectroscopic measurements, the perdeuterated form of acetonitrile (CD3CN) was used to avoid the wellknown splitting of the CN stretching mode by Fermi resonance (see ref 18 and references therein) that renders the spectra of adsorbed species difficult to read, especially when more than one adspecies is formed. For theoretical calculations, the CH3CN molecule was used, since in this case no such effect is present in the fully harmonic calculation of the vibrational frequencies. On the basis of both spectroscopic and modeling results, a thorough discussion of the different H-bond types formed between CD3CN and surface hydroxyl groups, as well as the role played by the coordination of Si atoms to the PO4 groups, will be addressed in the following. Materials and Methods The P-doped silica sample termed A200P was obtained by impregnation of A200 (pyrogenic silica, provided by Degussa, Frankfurt A. M., Germany) with the method of incipient wetness (i.e., using an amount of solution just sufficient to wet all the powder). A titrated dilute H3PO4 aqueous solution was used, and after impregnation the sample was dried at ∼100 °C for 30 min. The final overall composition of A200P was 97% SiO2 and 3% P2O5 (mol %), but all the P introduced was at the surface as a consequence of the impregnation method. For IR measurements, the powders were compressed in the form of self-supporting pellets of ∼10 mg cm-2. All spectra were obtained with an FTIR spectrometer (Bruker IFS 113v, equipped with a MCT criodetector). The homemade quartz infrared cell, equipped with KBr windows, was connected to a conventional vacuum line (residual pressure ≈10-5 Torr) and was allowed to perform in strictly in-situ conditions both thermal treatments on the sample pellets and probe molecules adsorption/desorption cycles on the activated samples. All IR spectra were recorded at beam temperature (BT), that is, the temperature reached by (white) sample pellets in the IR beam. BT is estimated to be some 20-30 °C higher than the actual room temperature (RT). (16) (a) Plemenschikov, A. G.; Morosi, G.; Gamba, A.; Coluccia, S.; Martra, G.; Paukshtis, E. A. J. Phys. Chem. 1996, 100, 5011. (b) Janchen, J.; Peeters, M. P. J.; van Wolput, J. H. M. C.; Wolthuizen, J. P.; van Hooff, J. H. C. J. Chem. Soc., Faraday Trans. 1994, 90, 1033. (c) Chen, J.; Thomas, J. M.; Sankar, G. J. Chem. Soc., Faraday Trans. 1994, 90, 3455. (17) Cerruti, M.; Bolis, V.; Magnacca, G.; Morterra, C. Phys. Chem. Chem. Phys. 2004, 6, 2468. (18) Kno¨zinger, H.; Krietenbrink, H.; Mu¨ller, H. D.; Schultz, W. Proceedings of the International Congress on Catalysis, 6th, Imperial College, London, July 12-16, 1976; Royal Society of Chemistry: 1977; paper A.10.

Chem. Mater., Vol. 17, No. 6, 2005 1417

Figure 1. Background IR spectra of A200P outgassed for ∼1 h at (a) room temperature, (b) 200 °C, (c) 500 °C, and (d) 700 °C. Inset: IR spectra of A200 outgassed for 1 h at (a) room temperature and (b) 700 °C.

Calculations on model clusters were carried out using GAUSSIAN-9819 software package, using B3LYP functional with 6-31+G(d,p) basis set. This level of theory has been proved to properly describe the geometrical and vibrational features of various molecules.20 When computing binding energies, no correction for the basis set superposition error has been taken into account. This is justified because in the present paper no comparison with experimental microcalorimetric data is carried out, and the computed binding energies are used only for internal comparisons. After geometry optimization, harmonic vibrational frequencies were calculated at the same level of theory, using the standard Wilson method as encoded in GAUSSIAN-98. All considered structures have been confirmed to be minima by checking that no imaginary frequencies were present.

Results and Discussion IR spectra of A200P outgassed at increasing temperatures are shown in Figure 1. The most relevant features are summarized in Table 1. For more detailed explanation and discussion, see Cerruti et al.14 (19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision A.1; Gaussian, Inc.: Pittsburgh, PA, 2003. (20) Koch, W.; Holthausen, M. C. A Chemist’s guide to Density Functional Theory; Wiley-VCH: Weinheim, 2000.

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Figure 2. IR spectra of CD3CN adsorption/evacuation on A200 (Section A) and A200P (Section B). All absorbance spectra are presented as differentials, i.e., normalized against the background spectrum of the corresponding bare samples of A200 (Section A) or A200P (Section B). In both sections, solid-line spectra refer to 1 Torr CD3CN adsorption, dashed/dotted-line spectra to 30 Torr CD3CN adsorption, dotted spectra to CD3CN evacuation at BT for 1 min, and dashed spectra to CD3CN evacuation at BT for 30 min. Section A: CD3CN adsorption/evacuation on A200 outgassed for 1 h at (a) room temperature and (b) 700 °C. Section B: CD3CN adsorption/evacuation on A200P outgassed for 1 h at (a) room temperature, (b) 200 °C, (c) 500 °C, and (d) 700 °C. Table 1. Peak Assignment of Background Spectra of A200P and A200 (Figure 1) peak position (cm-1) and aspect

assignment

3747, sharp

free SiOH groups

3674/3660, sharp

free POH groups

3700-2200, broad band

H-bond interacting hydroxyl groups

2000, 1870, 1640

overtone and combination modes of bulk Si-O stretching vibrations

spectra where the peak is visible

comments14

A200 at RT and 700 °C (inset); A200P at RT and higher temperatures, decreasing in intensity A200P at RT and higher temperatures, up to 700 °C A200P at RT and 200 °C; far less broad (only 3700-2900 cm-1), on A200 at RT.

the decrease observed in A200P is due to the condensation with POH groups the red shift and the decrease in intensity is due to pyrophosphate formation on A200P, the maximum centered at low frequency indicates a strong H-bonding system, involving PdO groups.The decreasing intensity of the band with increasing temperature is due to hydroxyl condensation. see Legrand, ref 34

A200 and A200P at all temperatures

Figure 2 reports IR spectra relative to the admission of CD3CN on A200 and A200P and subsequent evacuation. When 1 Torr CD3CN is admitted on A200 treated at either RT or 700 °C (see Figure 2A), a peak centered at ∼2275 cm-1 appears. This is relative to the ν(CN) mode of acetonitrile H-bonded to SiOH groups.17 When a high pressure of CD3CN is allowed (∼30 Torr), a shoulder appears at ∼2267 cm-1, indicating the formation of a liquidlike (i.e., physisorbed) layer of acetonitrile on the surface of the sample. When acetonitrile is allowed onto A200 treated at 700 °C (Figure 2A-b), the overall intensity of the spectrum of adsorbed acetonitrile increases remarkably. This intensity change must be considered a genuine one, because of the strictly in-situ procedure adopted. Indeed, the high-temperature treatment transforms long chains of interacting silanols (not available for H-bonding with acetonitrile) into isolated ones (see spectrum b in the inset of Figure 1). These can interact with CD3CN, thus causing an intensification of the 2275 cm-1 band. Acetonitrile H-bonded at the surface of

plain A200 silica activated either at RT or 700 °C does not resist outgassing at BT, as no peak remains visible at ∼2275 cm-1 after a short evacuation (∼1 min). When CD3CN is admitted on A200P treated at RT (Figure 2B-a), a broad peak centered at ∼2290 cm-1 is observed. This ν(CN) frequency is quite high and is usually found on bioactive glasses when acetonitrile is coordinated to mediumstrength Lewis acid centers, such as Ca2+.17 In this case, however, no such cations are present at the surface of the siliceous material, and the band has to be related to CD3CN H-bonded to POH groups. The large ∆ν(CN) shift indicates a strong H-bonding interaction. A small fraction of CD3CN adsorbed on A200P treated at RT resists 30 min outgassing: the peak at ∼2290 cm-1 nearly disappears, and the hydroxyl zone remains barely modified with respect to the sample before acetonitrile contact. The same peak at ∼2290 cm-1 is observed for CD3CN adsorbed on A200P treated at 200 °C (Figure 2B-b), 500 °C (Figure 2B-c), and 700 °C (Figure 2B-d). The overall intensity of the H-bonded CD3CN band

Surface Features of P-Doped Silica

declines rapidly with activation temperature, in line with the similar decline of all free OH species at the surface of P-doped silica shown in Figure 1. In these cases, however, a fraction of H-bonded nitrile more resistant to evacuation remains adsorbed, as shown by the peak at ∼2290 cm-1 still clearly visible in Figure 2B-b and 2B-c after 30 min outgassing. The increase in the strength of H-bonding interaction of CD3CN on A200P treated at higher temperatures is supposed to be due to the formation of pyrophospate species, which carry POH groups of increased acidity.14 When A200P is outgassed at increasingly high temperatures (see Figure 2B), the intensity of the shoulder/peak at ∼2267 cm-1 (relative to the liquid like physisorbed acetonitrile) increases faster in comparison with the main peak at ∼2290 cm-1. This is due to the rapid decrease of surface sites suitable for specific interactions with acetonitrile, brought about by the thermal condensation of both SiOH and POH groups to give SiOP and POP groups.14 Unlike that observed with the pure silica system (Figure 2A), not only the H-bonded hydroxyl groups, but also the isolated ones, are eliminated by thermal treatment (e.g., see the spectrum of A200P treated at 700 °C, Figure 1-d). This implies that, at such a high T, virtually all SiOH and POH groups disappear from the A200P surface. For this reason, on A200P treated at 700 °C, the band at ∼2267 cm-1 overwhelms the tiny, barely visible peak at ∼2290 cm-1 (Figure 2B-d). Remarkably, on the sample treated at 500 °C (Figure 2Bc), a broad band centered at ∼2340 cm-1 appears and intensifies with increasing CD3CN pressure (30 Torr). The nature of this band is quite puzzling. In the literature, ν(CN) bands at such a high frequency have been exclusively reported upon nitrile adsorption on strongly acidic samples, such as Al-exchanged zeolites or transition aluminas. In those cases, the high-frequency band has been assigned to CD3CN interacting either with Brønsted sites confined in side pockets21 or to the formation of a protonated form of acetonitrile, stabilized by pore walls;22 a further possibility is the interaction of CD3CN with very strong Lewis sites such as coordinatively unsaturated tetrahedral Al3+ ions.23 A200P sample is a nearly nonporous material (only macropores of ∼400 Å diameter are present)24 and, being free from cations, does not possess surface Lewis acid sites that could justify the 2340 cm-1 feature. Despite the large ∆ν(CN) shift, which usually correlates with a large interaction energy, the present band easily disappears, after just 1 min evacuation. The ∼2340 cm-1 band is also visible on A200P treated at 700 °C (Figure 2B-d), though with a definitely lower intensity. CD3CN interactions with hydroxyl groups are clearly visible in Figure 3, which shows the high-frequency region of the differential spectra relative to CD3CN admission on (21) Marie, O.; Thibault-Starzyk, F.; Lavalley, J.-C. Phys. Chem. Chem. Phys. 2000, 2, 5341. (22) Thibault-Starzyk, F.; Travert, A.; Saussey, J.; Lavalley, J.-C. Top. Catal. 1998, 6, 111. (23) Escalona-Platero, E.; Penarroya Mentruit, M.; Morterra, C. Langmuir 1999, 15, 5079. (24) Cerruti, M.; Magnacca, G.; Bolis, V.; Morterra, C. J. Mater. Chem. 2003, 13, 1279.

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Figure 3. Differential IR spectra of CD3CN adsorption/evacuation on A200P outgassed for 1 h at (a) room temperature and (b) 500 °C and (c) on A200 outgassed for 1 h at 700 °C. Inset: blown-up segments in the 1500-1300 cm-1 spectral region of CD3CN adsorption/evacuation on A200P outgassed for 1 h at 500 °C. Dashed-line spectra refer to 30 Torr CD3CN adsorption, and solid-line spectra refer to CD3CN evacuation at BT for 30 min.

A200P treated at RT (spectrum a) and 500 °C (spectrum b) and on A200 treated at 700 °C (spectrum c). As for pure silica, the interactions with silanol groups are shown by the decrease of the free SiOH peak and by the simultaneous appearance at ∼3420 cm-1 of a broad band due to H-bonded OH species (Figure 3c). The relevance of the interaction with POH groups when CD3CN is admitted on A200P is shown by the decrease in intensity of the free POH peak at ∼3670 cm-1 (Figure 3a). The higher strength of this interaction compared with the interaction with silanol groups is confirmed by the fact that the maximum of the band due to interacting hydroxyl groups is shifted to much lower frequencies (∆ν(OH) ≈ -500 cm-1, Figure 3a) than in the CD3CN contact with A200 (∆ν(OH) ≈ -300 cm-1, Figure 3c). The increase in the strength of interaction of CD3CN on A200P treated at increasing temperatures can be noted by the fact that the maximum of the band of interacting hydroxyl groups has a broad tail that extends to frequencies definitely lower on A200P treated at 500 °C with respect to those observed after CD3CN adsorption on A200P treated at RT (compare spectra a and b in Figure 3). Furthermore, the inset in Figure 3 shows that, on A200P treated at 500 °C, some complex bands centered at ∼1394 cm-1 decrease in intensity upon nitrile uptake. These bands have been assigned to the vibrations of PdO groups belonging to surface pyrophosphate species formed during the vacuum thermal treatment,14 and their decrease in intensity indicates that adsorbed acetonitrile can interact also with surface Pd O groups, as previously hypothesized. To understand which surface features of A200P can give rise to the labile high-frequency band, and to better analyze

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Figure 4. Models of CH3CN H-bonded to Si/P clusters. Distances reported on the figures are expressed in Å.

the H-bonding system formed between acetonitrile and POH groups, a series of P-containing SiO2 molecular models have been designed, in line with those reported in a previous work.14 A linear H-bond between CH3CN and POH groups is predicted with both LIN1 and LIN2 models (see Figure 4A). The computed binding energy (BE) and the ∆ν(CN) shift, calculated with respect to the ν(CN) mode of the free CH3CN molecule, are shown in Table 2. The ∆ν(CN) shift values calculated for these models exceed 20 cm-1. From the experiments (band at ∼2290 cm-1), a ∆ν(CN) shift of +11 cm-1 resulted, with reference to the ν(CN) vibration of the gas-phase CD3CN (ν(CN) ) 2279 cm-1 for the gas-phase CD3CN, according to NIST database). The calculated ∆ν(CN) values for models LIN1 and LIN2 are almost twice as large as the experimental one. A possible reason for such a discrepancy may be tracked down to the oversimplified linear configuration of CH3CN on both LIN1 and LIN2 clusters, in which CH3CN exclusively interacts with the OH group via its N atom. Indeed, it is well established25 that CH bonds become slightly acidic when near electronwithdrawing groups (such as CN in CH3CN), so that they can give weak CH‚‚‚O intermolecular interactions. The involved oxygen atoms may be part of the siloxane framework, even if this is very unlikely as a consequence of the very weak basicity of the oxygen atom in Si-O-Si bonds. More probably, oxygen atoms belonging to either SiOH or POH groups or even to the PdO groups show enough basic (25) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford University Press: New York, 1997.

Cerruti et al.

character to be relevant sites for the CH‚‚‚O interaction. The latter kind of interaction (expected to be the strongest) is modeled in Figure 4B, in which tilted H-bonds resulted from the optimization. This type of weak H-bonding interaction is very likely to occur on systems such as A200P, where all the P has been loaded on the silica surface and PdO surface density is relatively high. The CH‚‚‚OdP interaction is in agreement with the observed decrease of the 1394 cm-1 PdO band once CD3CN is adsorbed on A200P treated at 500 °C (see inset in Figure 3). The ∆ν(CN) values calculated for models TILT1 and TILT2 are much smaller than those calculated for models LIN1 and LIN2 (see Table 2) and are in better agreement with the experimental values. The reason for the drastic reduction of the ∆ν(CN) is rather involved. The bathochromic shift is due to a combination of electrostatic and charge-transfer effects. When the OH‚‚‚Nt C bond is assisted by the extra and weak CH‚‚‚OH bond, a complex charge transfer occurs between the NtC bond (from the molecule towards the surface) and the CH one (from the surface towards the molecule). The fluxes are opposite, and that from the molecule toward the surface is the largest one. Then, the final value of ∆ν(CN) is a compromise between the two, and it is always smaller for the TILT structures when compared with the LIN ones. Model TILT3 (Figure 4B) shows the OH‚‚‚NtCCH2H‚‚ ‚OdP interaction between CH3CN and POH/PdO groups both belonging to the same phosphate moiety. In this case, the electronic effects are so relevant that the calculated ∆ν(CN) becomes negative. Such a situation is unlikely to exist on the surface of the real material, where many other PdO groups belonging to different phosphate moieties are close enough to easily interact with acetonitrile. The formation of a tilted H-bond between CH3CN and SiOH groups has been also modeled (Figure 4E, model TILT_SI). The possible existence of a similar situation, where acetonitrile engages CH‚‚‚O interactions with the oxygen of SiOH groups, is demonstrated in Figure 3 by the simultaneous disappearance of both Si-OH and P-OH free hydroxyls. Calculated ∆ν(OH) for SiOH group interacting with CH3CN in model TILT_SI is similar to that observed on pure A200 (Table 2; Figure 3c). BE for this model is much lower than that calculated for models TILT1 and TILT2 (see Table 2), thus explaining the lability of CD3CN adsorption observed on pure silica systems such as A200. Calculated ∆ν(CN) in model TILT_SI is analogous to the values obtained for models TILT1 and TILT2. On the surface of A200P samples outgassed at temperatures at which dehydroxylation has not yet occurred completely (i.e., T < 700 °C), a variety of situations similar to those of models LIN1, LIN2, TILT1,TILT2, and TILT_SI are likely to be present. All these configurations are likely to occur simultaneously on the surface of the real material as shown by the relatively large bandwidth of the band centered at ∼2290 cm-1; indeed, the experimental ∆ν(CN) value (reported as ∼+11 cm-1) may as well result as an average of the ∆ν(CN) values of surface configurations presenting both linear and tilted H-bonds. That this may be the case is shown by a model in which two CH3CN molecules interact with two POH groups, giving rise to a

Surface Features of P-Doped Silica

Chem. Mater., Vol. 17, No. 6, 2005 1421

Table 2. Calculated Binding Energies, ∆ν(CN), and ∆ν(OH) of Models Presented in Figures 4 and 5, Compared with Experimental ∆ν(CN) and ∆ν(OH) Observed on the Reference Samples calculated binding energy (kJ/mol)

model

calculated shifts (cm-1) ∆ν(CN) ∆ν((OH)

LIN1

42

+26

-468

LIN2 TILT1 TILT2 TILT_SI BOTH

46 50 57 40 50

PYRO TILT3 LEW1_ AC

63 40 44

+23 +7 +9 +8 +23 (CN1), +6 (CN2) +16 -6 +75

-506 -465 -592 -230 -508 (POH1), -464 (POH2) -655 -367

13 (without deformation +69 energy: 104) LEW2_ 2AC 24 (without deformation +68 and +76 energy: 158)

experimental shifts that the model could refer to (cm-1) ∆ν(CN) ∆ν((OH) +11 (broad band ∼-600 for POH groups (maximum of the broad band at ∼2290 cm-1) at ∼3700-2200 cm-1, Figure 3a and 3b); ∼-300 for SiOH groups (maximum of the band at ∼3700-3000 cm-1, Figure 3c)

+60 (broad band at ∼2340 cm-1)

LEW2_ AC

linear and a tilted H-bond structure (see Figure 4C). The calculated ∆ν(CN) values are +23 cm-1 and +6 cm-1, for the linear and tilted configurations, respectively. Because the binding energies are moderately larger for the TILT complexes than for the LIN ones, it is expected that a larger population of the former will be present at the surface, which fully justifies the experimental +11 cm-1 figure. TILT3 has a significant lower BE, which allows to exclude this configuration from the set of the most probable at the A200P surface. Both BE and ∆ν(CN) are sensitive to the pyrophosphate moiety: model PYRO (see Figure 4D) envisaging a fourmembered pyrophosphate chain shows the highest BE with a relatively high ∆ν(CN) shift. The relationship between pyrophosphate chain length and the CH3CN BE is also shown by the data computed for TILT and LIN complexes in which BE(TILT2) > BE(TILT1) and BE(LIN2) > BE(LIN1). This is in agreement with the experimental observation that, on A200P samples treated at temperatures higher than RT (favoring pyrophosphate structures formation14), the ν(CN) band at 2290 cm-1 is more resistant to outgassing. The ∆ν(POH) shifts calculated for all models shown in Figure 4A-4D are very large, spanning the range ∼-370600 cm-1. Such a strong red shift for the POH stretching vibration is in agreement with the experimental corresponding OH band seen in the spectra of A200P (Figure 3a and 3b). The smaller ∆ν(SiOH) shift calculated for model TILT_SI (∼-230 cm-1) is in fair agreement with the experimental shift observed for the OH band on A200 (∼300 cm-1, see Figure 3c). The calculated ∆ν(POH) for model PYRO (Figure 4D) is higher than that calculated for all other models. This agrees with the observation that, when A200P is treated at higher temperatures, that is, when pyrophosphates are abundantly present on the surface,14 the maximum of the band relative to interacting hydroxyl groups presents a broad tail that extends down to fairly low frequencies (Figure 3b). The agreement reached so far between experimental and computational data is quite satisfactory, but none of the calculated models involving H-bonds between CH3CN and the A200P system presents a ∆ν(CN) shift high enough to

explain the experimental band observed at ∼2340 cm-1. As previously mentioned, such a high frequency band is usually due to the interaction of CD3CN with very strong Lewis acid sites. The case in which Si may act as a Lewis center has been explored using models shown in Figure 5. At first, the very rigid and symmetric LEW1 model, shown in Figure 5A, has been designed in such a way that the Si atom is linked to three PO4 groups. This model has C3V symmetry.

Figure 5. Section A-E: models of CH3CN adsorption on Si acting as Lewis acid center. Section F: reproduction of part of Si5(PO4)6O crystal cell.28 Distances reported on the figures are expressed in Å.

1422 Chem. Mater., Vol. 17, No. 6, 2005

Once CH3CN molecule interacts with LEW1 model (see LEW1_AC in Figure 5B), the central Si atom bonds CH3CN, expanding its coordination from 4 to 5, and behaving as a Lewis acid center. The calculated BE is large (44 kJ/ mol), and the corresponding ∆ν(CN) of +75 cm-1 compares well with the experimental value of +60 cm-1 (experimental reference is again ν(CN) mode of CD3CN gas). The expansion of Si valence shell only occurs when Si atom is linked to at least three PO4 groups. Indeed, CH3CN does not bind to models topologically equivalent to LEW1 containing Si atoms only (data not reported for brevity). Clearly, the nucleophilic character of the oxygen atoms belonging to PO4 groups is much higher than that of the corresponding atoms of SiO4. This accounts for the change in coordination of Si atom, which can easily reach hexacoordination in the presence of strong bases. It is likely that Si atoms surrounded by at least three P atoms indeed exist on the surface of A200P treated at 500 °C, because of the condensation of POH and SiOH groups brought about by the high temperature. BE calculated for model LEW1_AC is high enough (44 kJ/mol; see Table 2) to be in contrast with the labile nature of the experimental band at 2340 cm-1, which easily disappeared after 1 min of outgassing. The reason for this inconsistency is linked to the exceedingly large rigidity of the proposed LEW1 model, which is already in a “prepared” configuration to accept the CH3CN molecule without the need for large structural changes. That this is the case has been proved by designing a nonsymmetric LEW2 model (Figure 5C). Upon CH3CN interaction, the extension of the coordination of Si atom is computed to still occur (Figure 5D, model LEW2_AC), and the calculated ∆ν(CN) is +69 cm-1, in fair agreement with the experiments. However, BE results are much smaller than that of model LEW1_AC (i.e., only 13 kJ/mol), because the system undergoes a large structural reorganization upon CH3CN coordination (compare models LEW2 and LEW2_AC in Figure 5D). The cost of deformation energy was almost negligible for model LEW1_AC because of the rigidity of LEW1 model. The energy cost of the structural deformation can be estimated by single-point energy calculation using as a reference point the LEW2 in the geometry of the LEW2_AC complex. A value of 104 kJ/mol resulted for BE in this case, which is an indication of a strong Lewis acid/base interaction. Still, ∼90 kJ/mol out of it are spent as deformation energy, and the overall strength of the interaction is low. This explains the easy vacuum reversibility of the experimental ν(CN) band at ∼2340 cm-1, if the same mechanism is supposed to occur also at A200P surface. This last point seems to be conceivable, considering that the amorphous nature of A200P is unlikely to favor the formation of highly symmetric and strained structures such as model LEW1. Further expansion of Si coordination shell (from 5 to 6) is still possible: model LEW2 can accept an extra CH3CN molecule, as shown in Figure 5E (model LEW2_2AC). Calculated frequencies are in good agreement with those observed before on models LEW1_AC and LEW2_AC and with the experimental ∆ν(CN). BE is again relatively low (24 kJ/mol; see Table 2), because of the higher deformation

Cerruti et al.

energy required. As observed for model LEW2_AC, BE calculated without considering the deformation energy is very high (158 kJ/mol; see Table 2). The ability of Si to coordinate two CH3CN molecules may help explaining the observed increase in intensity of the 2340 cm-1 band as the pressure of CD3CN increased. The definitely lower intensity observed for the band at ∼2340 cm-1 on A200P treated at 700 °C may be due to surface reconstruction. This phenomenon is very likely to occur at such a high temperature and can bring some Si atoms to be fully surrounded by six PO4 groups. Should this happen, those Si atoms would be no longer available as Lewis centers, since the maximum extension of the coordination sphere has already occurred. Indeed, it is known that all silicon pyrophosphate polymorphs can be considered as ionic salts of formula Si4+ P2O7,4- where P atoms are fourfold coordinated and Si atoms are sixfold coordinated.26,27 This has been attributed to the higher strength of the P-O bond with respect to the Si-O bond, so that, in silicophosphates, Si is forced to get a sixfold coordination for the stereochemical requirements of P to be met.28 An example of a possible pyrophosphate structure is shown in Figure 5E, where a part of the crystal cell of pentasilicon hexakis(phosphate) oxide (Si5(PO4)6O, ICSD structure n.7299929) is reproduced. The possible existence of sixfold coordinated Si atoms is known also in metallo-organic Si structures (see, for instance, ref 30 and references therein) and in inorganic complexes in which Si is surrounded by strongly electronegative elements, as in SiF62-, trans-SiF4py2, cis-SiF4bipy, and SiCl4L2 (L ) py, PMe3).31 The existence of sixfold coordinated Si has been shown with NMR spectroscopic analysis also in SiO2-P2O5 sol-gel glasses, with high P loading and treated at high temperatures.32,33 Results obtained in the present paper on A200P system show that the extension of Si coordination from four to six is possible also in small Si/P clusters (representing specific surface sites of the material), when Si is surrounded by PO4 groups. Such a phenomenon may happen also on the surface of a bioactive glass reacted in body fluids: after a short period, the silicarich layer first formed is covered by an apatite layer, so that surface Si atoms are surrounded by a large amount of PO4 groups. Conclusions Surface features of P-doped silica (A200P) have been studied by comparing experimental results of CD3CN (26) Corbridge, D. E. C. Phosphorus 2000: chemistry, biochemistry & technology; Elsevier: New York, 2000; p 205. (27) Poojary, D. M.; Borade, R. B.; Campbell, F. L.; Clearfield, A. J. Solid State Chem. 1994, 112, 106. (28) Fleet, M. E.; Muthupari, S.; Kasrai, M.; Prabakar, S. J. Non-Cryst. Solids 1997, 220, 85. (29) Poojary, D. M.; Borade, R. B.; Clearfield, A. Inorg. Chim. Acta 1993, 208, 23. (30) Seiler, O.; Penka, M.; Tacke, R. Inorg. Chim. Acta 2004, 357, 1955. (31) Cotton, F. A.; Wilkinson, G. AdVanced inorganic chemistry; J. Wiley & Sons: New York, 1980; pp 393-394. (32) Bernestein, T.; Fink, P.; Mastikhin, V. M.; Shubin, A. A. J. Chem. Soc., Faraday Trans. 1 1986, 82, 1879. (33) Clayden, N. J.; Esposito, S.; Pernice, P.; Aronne, A. J. Mater. Chem. 2001, 11, 936. (34) Legrand, A. P. The surface properties of silicas; J. Wiley & Sons: New York, 1999.

Surface Features of P-Doped Silica

adsorption, monitored with in-situ FTIR spectroscopy, with theoretical results obtained by modeling CH3CN interactions on specifically designed P-containing SiO2 clusters. A band at ∼2290 cm-1, corresponding to a ∆ν(CN) of ∼+11 cm-1, was observed when CD3CN was allowed on A200P and was attributed to CD3CN H-bonded to POH groups. Calculated models showed that there are many possible configurations for this H-bond: (i) if the POH group is not close to PdO groups, a linear -POH‚‚‚NCCH3 bond is formed, bringing to a high ∆ν(CN) (∼23 cm-1) and to low binding energies (∼40-45 kJ/mol); (ii) if PdO groups are close enough to the POH group, CH3CN can form extra CH‚‚‚OdP weak interactions. In this case, a smaller ∆ν(CN) is observed, but the additional H-bond also brings about higher binding energies. It has been thus hypothesized that the experimental ν(CN) band observed at ∼2290 cm-1 corresponds to an average of all these possible configurations weighed by their relative Boltzman factor. Calculated BEs increased on models showing complex, long pyrophosphate chains. These chains are likely to be formed at high T, thus explaining the increased resistance to outgassing exhibited by the band at ∼2290 cm-1 on A200P treated at T > 500 °C. The unexpected ν(CN) band measured at ∼2340 cm-1, corresponding to a ∆ν(CN) of ∼+60 cm-1, has been observed when CD3CN was allowed on A200P treated at 500 °C and, to a lower extent, on A200P treated at higher temperatures. This band has been interpreted as the interaction of CD3CN with Lewis acid centers, that is, Si atoms

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surrounded by PO4 groups that can cause an expansion of the Si coordination shell. It has been shown that, when Si is surrounded by at least three PO4 groups, it can expand its coordination from four- to five- or sixfold, upon interaction with one or two CH3CN molecules. Moreover, it has been shown that a Si atom can extend and fully saturate its coordination sphere by means of PO4 groups only, preventing any further interaction with CH3CN. It was suggested that similar processes may happen at the surface of A200P treated at temperatures higher than 500 °C, as this would explain the observed decrease in intensity of the band at ∼2340 cm-1 when CD3CN was allowed on A200P treated at 700 °C. The lability of the band at ∼2340 cm-1 was due to the large energetic cost brought about by the structural rearrangement of local geometry of surface Si atoms (after valence shell expansion from four up to sixfold coordination). The formation of sixfold Si atoms may also be relevant in some of the steps occurring at the surface of bioactive glasses, when a layer of apatite precipitates on the silicarich surface layer. An analysis with NMR of both A200P and bioactive glasses will be the object of a forthcoming paper to confirm the possible existence of such species. Acknowledgment. This research was partly financed with funds of the Italian Ministry MIUR (Project COFIN2003, Prot. 2003032158), and of the Center of Excellence for Nanostructured Interfaces and Surfaces (University of Turin). CM0480420