Reaction Mechanisms in Irradiated, Precipitated, and Mesoporous Silica

Article pubs.acs.org/JPCA

Reaction Mechanisms in Irradiated, Precipitated, and Mesoporous Silica D. Dondi,*,† A. Buttafava,*,† A. Zeffiro,† S. Bracco,‡ P. Sozzani,‡ and A. Faucitano*,† †

Department of Chemistry, University of Pavia, V.le Taramelli 12, 27100 Pavia, Italy Department of Materials Science, University of Milano Bicocca, Via R. Cozzi 53, 20125 Milano, Italy

‡

S Supporting Information *

ABSTRACT: A matrix EPR spectroscopy study of the low temperature γ radiolysis of precipitated (Zeosil) and mesoporous high surface silica has afforded evidence of the formation of trapped H-atoms, H-atom centers, siloxy radicals Si−O•, anomalous silyl peroxy radicals Si-OO• with reduced g tensor anisotropy, siloxy radical-cations (Si−O− Si)+•, E′ centers, and two species from Ge impurity. Coordination of peroxyl radicals with diamagnetic Si+ centers is proposed and tested by DFT computations in order to justify the observed g tensor. Coordination of H-atoms to Si+ centers is also proposed for the structure of the H-atom centers as an alternative model not requiring the intervention of Ge, Sn, or CO impurities. The DFT method has been employed to assess the electronic structure of siloxy radical-cations and its similarity with that of the carbon radical-cation analogues; the results have prompted a revision of the structures proposed in the literature for ST1 and ST2 centers. The comparison between the two types of silica has afforded evidence of different radiolysis mechanisms leading to a greater yield of trapped H-atoms and H-atom centers in zeosil silica, which is reckoned with the 4-fold greater concentration of silanol groups. Parallel radiolysis experiments carried out by using both types of silica with polybutadiene oligomers as adsorbate have afforded evidence of free valence and energy migration phenomena leading to irreversible linking of polybutadiene chains onto silica. Reaction mechanisms are proposed based on the detection of SiO2-bonded free radicals whose structure has been defined by EPR.

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INTRODUCTION The effects of ionizing radiations and thermal and mechanical treatments on crystalline quartz and glasses have been extensively investigated in the past decades, the major aims being the identification of defects and the elucidation of their role in affecting the performance of microelectronics devices (MOSFET) and optical fibers.1−16 As a consequence of such practical objectives, the research has been mainly focused on the most stable species, like the E′ type centers, capable of surviving above room temperature. Much less attention has been devoted to low temperature studies suited for the characterization of the labile intermediates. Reactive intermediates like free radicals, ions, and excited species are bound to play a key role in radiation damage mechanisms and as initiators of radiation induced surface reactions with gases, vinyl monomers, specific organic functional groups, oligomers, and high molecular weight polymers.17−28 Beside the theoretical interest related to solid state and interface chemistry, the reactions with adsorbed substrates can be the source of modified silica with altered surface energy and reactivity suited for use as a catalyst, stationary phase in HPLC chromatography and as filler with enhanced compatibility in silica rubber blends. Pursuing a research line initiated in our laboratories,29−33 this work is aimed at contributing to the elucidation of the low © 2013 American Chemical Society

temperature radiolysis mechanism of pristine silica and silica containing unsaturated oligomers as adsorbate. Two types of silica, which were prepared by precipitation from a silicate solution (Zeosil 1165) and by a template synthesis (mesoporous silica), have been investigated on a comparative basis in order to obtain information about the role played by the surface area and structure in the radiolysis mechanism.

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EXPERIMENTAL AND THEORETICAL METHODS Precipitated Silica Zeosil 1165. Precipitated silica Zeosil 1165 was purchased from Rhodia and used as received. The total surface area, determined by BET (N2), is 160−165 m2/g.34 The water content, determined by TGA from the weight loss in the 100−130 °C range, is 3−4%. The dispersive and specific components of the surface energy, measured in previous work by inverse gas chromatography (IGC) are γsd = 96.0 mJ/m2 and Isp = 142.6 mJ/m2, respectively.32 Information concerning Zeosil 1165 morphology are available from the recent paper by Schaefer et al.35 By TEM, light scattering, and ultrasmall-angle X-ray scattering (USAXS), a demonstration is given of a four Received: October 29, 2012 Revised: February 27, 2013 Published: March 22, 2013 3304

dx.doi.org/10.1021/jp310683e | J. Phys. Chem. A 2013, 117, 3304−3318

The Journal of Physical Chemistry A

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(Sigma Aldrich, Mn = 5000, 80% 1−4 cis, 20% vinyl double bonds) as adsorbate were prepared by evaporation (rotavapor) of silica dispersions in CHCl3 solutions of the oligomer followed by 24 h high vacuum drying at 10−5 Torr. The irradiations were performed under vacuum (10−5 Torr) at the liquid nitrogen temperature in a 60Co cell with total doses of 7 kGy at a dose rate of 0.25 kGy/h. The radiation dose was determined by Fricke dosimetry41 by applying a correction factor of 0.90 for the greater electron density of the Fricke solution with respect to pristine silica (0.534 and 0.500 mols of electrons/g, respectively). The prerequisite for such a correction is the dominance of the Compton effect for 60Co γ rays of 1.17 and 1.33 Mev, which implies a linear dependence of the radiation interaction probability with the electron density of the material. The change in electron density of silica, caused by the presence of 15−20% of adsorbed polybutadiene, was neglected. The EPR measurements were carried out on a Bruker EMX/ 12 spectrometer equipped with data acquisition and temperature control systems. The spectra simulations were made using the Hamiltonian including nuclear and electron Zeeman terms with isotropic and anisotropic components of the g and hf tensors. Diffusion reflectance FTIR spectra were recorded with a Perkin-Elmer 1600 spectrometer equipped with a PE L127 5000 device. Thermogravimetric analysis (TGA) was performed on a Mettler TGA/SDTA 851 instrument at a heating rate of 10 °C/min in air in the temperature range 25−800 °C. It is known from literature that, in this temperature range, the combined effects of the temperature and oxygen lead to the quantitative degradation/elimination of the organic material adsorbed onto silica.42 The TGA curve shows a first weight decrease in the range 100−130 °C, which is reckoned to the loss of adsorbed water (3−4%); a second, more important weight loss is observed between 400 and 700 °C, which is diagnostic of the pyrolytic oxidative degradation of grafted polybutadiene. DFT Computations. DFT calculations43 concerning the species SiO•, SiOO•, and (SiOSi)•+ in the silica matrix were made under the cluster model approach, which has been successfully applied for investigating a series of point defects in silica.44,45 The molecular models (CH3O)3SiO•, (CH3O)3SiOO•, and (CH3O)3SiOSi(OCH3)3]•+ were adopted with UB3LYP double and triple split unrestricted basis set with diffuse and polarization functions on both H and heavy atoms. The choice of the molecular models is justified by the fact that the paramagnetic species investigated have essentially localized unpaired electron orbitals with most of the spin density at the radical center; as a consequence, little effects are expected from the expansion of the model structure by insertion of additional (−O)3Si units as well as from the type of capping (with H-atoms or CH3 groups) of the end oxygen atoms. The strategy adopted for the models choice has been preliminarily tested by DFT calculations and an example concerning the g tensor of siloxy radicals computed by using the different formulas (CH3O)3Si−O•, (HO)3Si−O•, and [(HO)3SiO]3Si−O• is available in the Supporting Information. The calculations pertaining the H-atom center and the anomalous silyl peroxy radical have been made by using the molecular models (CH3O)3Si+···H and (CH3O)3SiOO•···+Si(OCH3)3 and by imposing variable Si+···H and O•···Si+ interatomic distances in the optimized fragments.

level fractal hierarchical structure consisting of primary particles of ca. 10 nm forming particle aggregates of ca. 300 nm giving in turn hard agglomerates; the latter structure ultimately forms soft agglomerates of 100−200 μm average size. Soft aggregates are disrupted by stirring in solvent dispersions. No evidence of a mesoporous structure is given. Mesoporous Silica. Mesoporous silica was prepared according to a previously described procedure36−38 by a template synthesis on micelles with elimination of the surfactant by thermal treatment at 600 °C (details of the procedure and the qualitative scheme of the template synthesis are given in Supporting Information). The BET (N2) absorption isotherm at 77 K belongs to type IV class being thus consistent with a mesoporous material (Supporting Information). The surface area, as measured with the BET method, is 798 m2/g, while the pore volume, as determined by the BJH method, is 0.71 cm3/g; the pores diameter measured by BET is 43 Å. The water content, as measured from the adsorption isotherms at room temperature and 0.45 p/p0, is about 6%. Solid-State NMR of Silica and Determination of Silanols Content. The solid-state 29Si NMR spectra were run at 59.6 MHz on a Bruker Avance 300 instrument operating at a static field of 7.04 T equipped with 4 mm double resonance MAS probe. The samples were spun at the magic angle at a spinning speed of 15 kHz, and a ramped-amplitude crosspolarization (RAMP-CP) transfer of magnetization was applied. The 90° pulse for proton was 2.9 μs. Quantitative 29Si singlepulse excitation (SPE) experiments were run using a recycle delay of 150 s and cross-polarization (CP) MAS experiments were performed using a recycle delay of 10 s and contact times of 8 ms. The 29Si MAS NMR spectra of precipitated silica Zeosil 1165 and mesoporous silica are given in the Supporting Information and in previous papers.36−38 The silicon chemical shifts are sensitive to the distinct condensed silica species, namely, the signals at about −110, −100, and −90 ppm are assigned to silicon atoms without hydroxyl groups Si(Si−O)4 (Q4), to silicon atoms bearing one hydroxyl group (Si−O)3Si−OH (Q3), and to geminal silanols (Si−O)2Si-(OH)2 (Q2), respectively. The concentration of the different silanol groups was obtained by the combined analysis of spectra collected with quantitative 29Si SPE-MAS experiments and with the 29Si CP-MAS technique.36−39 A concentration of 3.5 OH/nm2 was obtained for mesoporous silica at the value of 0.43 p/p0. A much greater concentration of 17.1 OH/nm2 was obtained with the same method for zeosil 1165 silica. The latter value is close to the upper limit in the range 7.7−16.9 OH/nm 2 determined by Leonardelli et al.39 in the NMR study of a variety of amorphous silica samples. The silanols content, as determined by 29Si NMR, is usually found greater than the one determined with other techniques. According to Zhuravlev,40 the surface density of OH groups in amorphous silica, determined by deuterium exchange and mass spectrometric analysis, is a physicochemical constant for the fully hydroxylated surface, and its average value is 4.9 OH/nm2. According to Leonardelli et al., an explanation for this discrepancy is that both the surface and internal OH groups are determined by NMR; furthermore, an underestimation of the silica surface with the BET method when using N2 must be taken into account. In the present work, only the relative abundance of silanol groups in the two types of silica is needed as a key for a qualitative explanation of the different EPR signal intensities observed for trapped H-atoms and the H-atom centers. Silica with Polybutadiene Oligomer As Adsorbate. Silica samples containing 15−20% of polybutadiene PB-5000

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RESULTS AND DISCUSSION Irradiation of Pristine Zeosil 1165 Silica. Figures 1 and 2 show the experimental and computed EPR spectra obtained

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main radiolytic path leading to H-atoms is the dissociation of excited silanol according to the reaction47,48 3

excitons + ≡SiOH → ≡SiO• + H

A minor contribution is also expected from the dissociative electron capture of silanols49 according to the reaction ≡SiOH + e− → ≡SiO− + H

The major mechanism of decay of H-atoms is proposed to be based on the reactions with silica radiolytic defects. H-Atom Centers. A second signal detected in irradiated silica consists of an asymmetric doublet with an average splitting of 11.7 mT (Figure 1d,e) and the hf and g tensors given in Figure 2d. Doublets of approximately 7.4, 11.9, and 15.0 mT, referred to in the literature as H(I), H(II), and H(III) centers, respectively, have been detected in various types of irradiated fused silica, but their structure is still a matter of debate.14,44,50−56 Also, a doublet of 13.3 mT was observed and assigned to formyl radicals HC•O arising from the reaction of trapped CO with H-atoms.5 According to its characteristics, the doublet of 11.7 mT detected in this work can be reckoned with an H(II) center. The 7.4 mT doublet was proposed to be generated by (−O)2Si•-H radicals, presumably arising from the reaction of H-atoms with precursor silylene centers (−O)2Si:; this assignment is further supported by the 29-Si coupling of 23.8 mT detected by Tsai and Griscom.51 The analogue species (−O)2Ge•−H and (−O)2Sn•−H, arising from Ge and Sn substitutional impurities, respectively, were considered as possible sources of H(II) and H(III) centers.44,50−53 Ge impurities are indeed known to be present in silica, quartz, and glasses,2,15,57−59 and as it will be discussed in a latter paragraph, two EPR signals in the irradiated zeosil silica spectrum are attributed to Ge species. The assignment of H(I) and H(II) centers to silyl and germyl radicals, as well as the possibility of alternative interpretations not requiring the intervention of impurities, is discussed in this

Figure 1. EPR spectra of pristine zeosil 1165 silica after γ irradiation at 77 K (irradiation dose, 7 kGy). The spectra were recorded at 77 K after annealing at higher temperature.

from zeosil 1165 silica after gamma irradiation at 77 K. The major species, which have been identified and characterized, are described below. Trapped H-Atoms. Trapped H-atoms are detected from their characteristic 50.55 mT doublet splitting, which is diagnostic of 100% spin density in the 1s orbital (Figure 1a,b). This signal is invariably observed in 77 K irradiated glasses, silica, and quartz,46 and the mechanism of formation and decay of H-atoms has been the object of several studies, which suggest that the

Figure 2. Computer simulation of the silica zeosil 1165 EPR spectrum recorded at 77 K after γ irradiation at the same temperature (irradiation dose 7 kGy). The species identified and the EPR parameters of the corresponding EPR signals are given aside. 3306



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Table 1. DFT 6.311G++(2d,p) Computation of Hα Couplings in the Model Radicals (CH3O)2Si•−H And (CH3O)2Ge•−H; the Capability of the DFT Method to Correctly Predict EPR Parameters in the Class of Radicals under Study Is Assessed by Comparison with Experimental Dataa Aiso 29Si (mT)

gav

AisoHα(mT)

radical

computed

exptl

computed

exptl

computed

H3Si• (refs 61, 62) F3Si• (refs 61, 63) (CH3O)3Si• (refs 61, 64) (CH3O)2Si•−H model for H(I) center (refs 46, 53) (CH3O)2Ge•−H H(II) center

2.0041 2.0006 2.0016 2.0020 2.0014

2.0032 2.0003 2.0012 n.d. n.d.

17.1 50.0 33.3 23.9 10.4 (Aiso Ge)

18.9 49.8 33.9 23.8 (ref 51) n.d.

1.08 11.9 (F)

0.79 13.6 (F)

7.0 9.7

7.4 11.7 −11.9 (this work and refs 50−56)

a

exptl

n.d. = not available in the literature.

work on the light of the current knowledge about the EPR properties of low molecular weight Si and Ge radical analogues and the results of DFT computation. Silyl radicals of structure R2Si•H (R = H or alkyl group) have isotropic Hα couplings in the range from 0.796 mT (H3Si•) to 1.699 mT (Me2Si•H).60,61 Such couplings are too small for being compatible with the proposed H(I) center structure; however, larger couplings cannot a priori be excluded because of the substituent’s electronegativity effect. The 29Si hf constant is a suitable probe for this effect since it increases from 6.4 to 49.8 on changing from methyl to F substituents in the species Me2Si•H53 and F3Si•60,62 and with alkoxyls substituents having an intermediate electronegativity, it takes the value of ca. 33.0 mT.60,63 The growth of the 29Si coupling is diagnostic of an increase of piramidality of the radical center, which is reckoned with the destabilization effect caused by the overlap between the unpaired electron orbital and the lone pairs of substituent atoms.64 This change of the radical center geometry is also expected to be accompanied by an increase of the Hα coupling contants.60,61 As no literature information are available concerning species of formula (−O)2Si•−H and (−O)2Ge•−H representing the H(I) and H(II) structures, the advanced DFT UB3LYP 6.311G++(2d,p) method, coupled with the lower rank UB3LYP 6.31G(d) for molecular geometry optimization, was used. The method was first tested with respect to the reference radicals H3Si•, (CH3O)3Si•, and F3Si•60−63 and subsequently used to predict Hα couplings in the model species (HO)2Si•−H and (HO)2Ge•−H. The results in Table 1 confirm the efficacy of the DFT method for the class of radicals under study and, in agreement with previous theoretical work,53 confirm that the species of formulas (−O)2Si•−H and (−O)2Ge•−H can be reasonable candidates for the structure of H(I) and H(II) centers. However, while the identification of the H(I) center can be considered reasonably assessed, the interpretation of the H(II) and H(III) centers in terms of the analogue species from Ge and Sn impurities is still uncertain because of the lack of direct experimental information. For this reason, it was thought justified to exploit the possibility of alternative models not based on the intervention of impurities. Satisfactory results have been obtained by applying the DFT theory to models consisting of trapped H-atoms interacting with diamagnetic Si+ centers. In such models, the decrease of the H-atom coupling below the maximum value of 50.5 mT is caused by spin transfer to the diamagnetic center modulated by the Si+···H interatomic distance

Table 2. DFT UB3LYP Computations on the H-Atom Center Model (CH3O)3Si+···H; the Isotropic H Couplings Are Computed As a Function of the Si···H Interatomic Distance Si−H distance (nm)

calcd isotropic H coupling (mT) DFT UB3LYP 6.311G++(2d,p)

exptl isotropic H-atom coupling (mT)

0.145a 0.210 0.215 0.218

0.125 7.7 10.2 11.7

n.db 7.4 (refs 50−56)

0.220 0.221 0.225

12.8 13.3 15.6

11.7 (this work) 11.9 (refs 50−56) 13.3 (ref 5)

a Corresponding to the Si−H bond length in the silane radical-cation (CH3O)3Si−H)+•. bn.d. = not available in the literature. In ref 66, the H coupling of 1.8 mT is reported for alkyl silane radical-cations

distances in the (CH3O)3Si+···H model. The results (Table 2) show that a matching with the experimental couplings of 7.4, 11.7, 13.3, and 15.6 mT is obtained at the Si···H distances of 0.210, 0.218, 0.221, and 0.225 nm, respectively. The minimum energy is obtained instead at the Si···H distance of 0.145 nm, corresponding to the formation of a siloxane radical-cation [(−O)3Si−H]•+. For the latter species, in agreement with literature reports,65 a proton hf coupling too small for being assigned to H-atom centers is predicted by calculations (Table 2). The lack of detection of siloxane radical-cations [(−O)3Si−H]•+ is reasonably justified by considering their expected tendency to decay by proton transfer to neighbor oxygen donors thus generating Si• radical centers: [(−O)3 Si−H]•+ + (≡Si)2 O → (−O)3 Si• + [( ≡ Si)2 O−H]+

If this reaction is fast with respect to the lifetime of the H-atom centers, only the latter will be observable by EPR. We are thus led to the conclusion that H-atoms trapped near diamagnetic Si+ defects can be proposed as reasonable alternative models, not requiring the intervention of Ge, Sn, and CO impurities, for explaining the origin of the variety of H doublets detected in irradiated silica and glasses. Eγ′ Centers. The spectra of Figure 1d,e show the presence of a narrow singlet centered at gav = 2.0007, due to E′γ centers, which is better visible at higher temperature. At low microwave power and modulation amplitude, this signal shows an approximately axially symmetric g tensor with g∥ > g⊥. The E′γ species have been widely investigated, and several hypothesis concerning their structure have been formulated, the most widely accepted one being that of an oxygen vacancy with an sp3 Si radical center associated with a diamagnetic Si cation Si•···+Si.3,7,9,11,13−16,66

( −O)3 Si+ ··· H

DFT UB3LYP 3.11G+(2d,p) calculations were thus performed by imposing a stepwise change of the Si···H interatomic 3307



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Table 3. Experimental and DFT UB3LYP 6-311G++(2d,p) Computed g Tensors for Silyl Peroxy Radicals SiOO•, Siloxy Radical SiO•, and Their Carbon Analogues radical SiO• (CH3O)3SiO• (model) RR′R″C−O• CH3O• (model)

DFT UB3LYP 6-311++G(2d,p) computed g tensora,b

exptl g tensor g1 = 2.0782; g2 = 20095; g3 = 1.999; giso = 2.0289 (T = 300 K) (refs 6,11) g1= 2.0602; g2 = 2.0108; g3 = 1.9999; giso = 2.0236 (T = 77 K) (this work, Figure 2b)

g1 = 2.0688 (2.0438); g2 = 2.0115 (2.0116); g3 = 2.0025 (2.0032); giso = 2.0276 (2.0195) g1 = 2.054−2.093; g2 = 2.005−2.009; g3 = 1.995−2.000 (T = 77 K) (refs 73,74) g1 = 2.0578; g2 = 2.0088; g3 = 2.0021; giso = 2.0229 g1 = 2.0520; g2 = 2.0086; g3 = 2.0019; giso = 2.0208

(CH3)3SiOO•

g1 = 2.0587; g2 = 2.0079; g3 = 2.0022; giso = 2.0229 (T = 77 K) (refs 81,82)

SiOO•

g1 = 2.0670; g2 = 2.0078; g3 = 2.0018; giso= 2.0257 (T = 300 K; refs 6,11) g1 = 2.0680; g2 = 2.0074; g3= 2.0014; giso = 2.0256 (T = 110 K) (ref 6)

(CH3O)3SiOO• (model) R3COO• (polyethylene peroxy radical) CH3OO• (model)

g1 = 2.0513; g1 = 2.0083; g1 = 2.0021; giso = 2.0205 g1 = 2.0366; g2 = 2.0092; g3 = 2.0024; giso = 2.016 (T = 77 K) (ref 78) g1 = 2.0322; g2 = 2.0080; g3 = 2.0022; giso = 2.0141

a The g tensors measured at different temperatures are comparable since the temperature induced motional modulation of g tensor anisotropy is negligible in rigid quartz/glass matrixes (see silyl peroxy radical as an example). bResults from the fully optimized molecular geometry between brackets.

radicals Si−O• on the basis of the comparison with their carbon alkoxy radicals analogues and the results of UB3LYP DFT computations (Table 3). Such species have similarity with nonbonded oxygen hole centers or oxygen associated hole centers (NBOHC or OHC) detected in irradiated glasses and quartz.6,9,11,12,72 In oxygen centered radicals, line broadening by g tensor anisotropy is so large to prevent their EPR detection except in the solid state under the influence of asymmetric H bond structures, which remove orbital degeneracy. This is the situation first encountered with alkoxy radicals (R)3C−O• and thiyl radicals R−S•, which are the carbon and sulfur analogues of siloxy radicals. Alkoxy radicals were first detected in methanol and carbohydrate matrixes showing a gmax principal value in the range 2.054−2.093.73,74 An even larger g tensor anisotropy with gmax in the range 2.49−2.01675−77 was found for thyil radicals due to the greater sulfur spin−orbit coupling constant. Judging from the experimental and computed g tensors shown in Table 3, a similar electronic structure can be envisaged for siloxy radicals where the oxygen orbital degeneracy might be removed by hydrogen bonding with neighbor silanol groups within the silica matrix. Judging from the similarity of the electronic structures, gmax in siloxy radicals should be oriented along the Si−O bond, gmin along the symmetry axis of the unpaired electron orbital, and gint perpendicular to the gmin/gmax plane. In such a structure, the major spin−orbit coupling effect is expected to be on gmax, which is thus strongly deviated toward the low field (gmax ≫ 2.0023), and it becomes quite sensitive to the H-bond structure; the principal values gint and gmin instead are relatively unaffected (Table 3). The UB3LYP DFT method is found to be adequate for correctly predicting the anomalous increase of gmax, which is of key importance for the radical identification, but it fails to reproduce the minor spin−orbit effects causing the gmin value to be slightly shifted toward high field with respect to 2.0023 (Table 3). Test computations (results shown in Supporting Information) point to this specific failure not arising from the choice of the basis set or the molecular geometry parameters but rather from an intrinsic limitation of the DFT method.

This hypothesis is supported by the 29Si hf coupling of ca. 40 mT.7 Two type of Eγ′ centers have been identified showing different thermal stabilities.15 As it will be shown later, Eγ′ centers surviving the warming at room temperature do not react with adsorbed polybutadiene. This lack of reactivity is at variance with the chemistry of Si radicals, which are known to be even more reactive toward oxygen and double bond addition than their C analogues.61,67,68 A possible explanation is that the stable fraction of E′γ centers is buried out of the reach of reactants within the bulk of the silica structure. Eδ′ Centers. In addition to E′γ centers, other E′ type defects in irradiated quartz and amorphous silica are described in the literature, all consisting of sp3 Si centered radicals in different structural environments and showing the common feature of a giso value slightly smaller than 2.0023.11,12,66,69 This latter feature is diagnostic of the presence of oxygen substituents at the Si center.60 A signal used as a component (1.7%) in the computer simulation of the low temperature spectrum (Figure 2e) is assigned to Eδ′ centers on the base of the close similarity of the g tensors (this work, g1 = 2.0016, g2 = 2.0019, g3 = 2.0019; gav = 2.0018; from refs 69 and 70, g1 = 2.0018, g2 = 2.0021, g3 = 2.0021; gav = 2.0020). The E′δ centers were first detected by Griscom and Friebele69 in 100 keV X-ray irradiated bulk silica containing chlorine impurities and later in γ and X irradiated high purity silica glasses.70,71 The structure of such species is still a matter of discussion; delocalization of the spin over a number of Si atoms is a prerequisite in order to explain the 29Si hf coupling of 10.0 mT, which is much smaller than that expected for a 100% sp3 hybridization. In the model proposed by Griscom and Friebele,69 the unpaired spin is delocalized over 4 Si atoms, while in other models, spin delocalization is envisaged to involve a number of Si atoms ranging from 2 to 5. According to recent Q.M. calculations by Karma et al., the Eδ′ center consists of a trapped hole shared between two Si atoms adjacent to an oxide vacancy.66 Siloxy Radicals Si−O•. The species with a positive gmax shift of 0.0579 with respect to the free electron value and giso = 2.0236 (Figure 2b and Table 3) have been identified as siloxy 3308



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Silyl Peroxy Radical SiO-O•. The EPR signal in Figure 2c, which is simulated with the g tensor g1 = 2.0286, g2 = 2.0076; g3 = 2.0036 (giso = 2.0133), can be considered, at first glance, diagnostic of siloxy radicals SiOO• due to the similarity with their carbon, sulfur, and nitrogen peroxy radicals analogues. In fact, peroxy radicals of general formula X−OO• are expected to have quite similar electronic structures as a consequence of the dominant role played by the peroxidic group, which attracts more than 90% of the overall spin density.78−80 However, the few experimental data available from literature strongly suggest that silyl peroxy radicals are peculiar being characterized by a more pronounced g tensor anisotropy. According to Bennett and Howard,81 trimethyl silyl peroxy radical (CH3)3Si−OO• have gmax = 2.0587 and giso = 2.0229, and the other congruent giso values of 2.0294, 2.025, 2.0277, and 2.026 were detected by Howard for the tri-t-butylsilylperoxyl, tri-n-butylsilyl peroxyl, triphenysilylperoxyl, and diphenylmethyl peroxy radicals, respectively.82 The results of DFT UB3LYP calculations shown in Table 3 confirm the peculiarity of silyl peroxyradicals predicting for the model species (CH3O)3Si−O−O• gmax = 2.051 and giso = 2.020. We are then led to the conclusion that the signal detected in this work (Figure 2c), despite its similarity with C, S, and N peroxy radicals analogues, cannot be assigned to normal silyl peroxy radicals since the differences gmax − gmin and giso − 2.0023 are significantly smaller than expected. In order to explain this difference, coordination of the silyl peroxy radical with an adjacent diamagnetic Si+ cation is assumed with a loss of spin density justifying the attenuation of g tensor anisotropy. As it is shown in Figure 3, this anomalous silyl peroxy radical may be

envisaged to result from the reaction of trapped molecular oxygen (present as impurity and/or produced in the radiolytic degradation of silica) with accessible E′γ centers. This hypothesis is validated by the results of UB3LYP DFT computations shown in Table 4, which also predict an interatomic distance of 1.88 nm, close to the energy minimum, for matching the experimentally observed gmax and giso values. Two EPR signals with g tensors 2.067, 2.0078, 2.0018 (signal I, giso = 2.025) and 2.027, 2.0085, 2.0028 (signal II, giso = 2.0125), both assigned to silyl peroxy radicals SiOO•, are reported in the literature.6,11 According to the discussion above, the species I has the characteristics expected for a normal peroxy radical, while the species II can be reckoned with the proposed variant structure shown in Figure 3. Radical-Cation (Si−O−Si)•+. The radical-cation ( Si−O−Si)•+ is identified on the base of the satisfactory matching between experimental and computed g tensors and the comparison with the properties of the carbon radical-cations analogues (Figures 2a and Table 5). This species is seen to be thermally unstable since its doublet signal, visible in the center of the spectrum, disappears rapidly on warming above 140 K (Figure 1a−d). As compared to siloxy radicals SiO•, the radical-cation is characterized by a smaller deviation of giso toward the low field and a smaller gmax − gmin difference. This difference of electronic structure is confirmed by the results of DFT computations (Tables 3 and 5) and it is congruent with that experimentally assessed for the carbon analogue species. In fact, ether radical-cations (C−O− C)•+ are characterized by a partial delocalization of charge and spin density, which causes an attenuation of the g tensor anisotropy, and it gives rise to relatively large β-proton hf couplings.73,74,84 This is a major difference with respect to alkoxy radicals C−O• where the spin density is concentrated on the oxygen atom. Thus, for instance, the g tensor for the dimethyl ether radical-cation (CH3−O-CH3) •+ is gmax = 2.0138, gint = 2.0072, and gmin = 2.0045 (giso = 2.0085),84 while, for alkoxy radicals, the g principal values are found in the range

Figure 3. Molecular model for the anomalous peroxy radical where the decrease of g tensor anisotropy is explained by partial spin transfer to a neighbor diamagnetic Si+ center.

Table 4. DFT UB3LYP 6-311++G(2d,p) Calculations on the Molecular Model Representing the Anomalous Silyl Peroxy Radical Shown in Figure 3; the Model Is Aimed at Accounting for the Loss of g Tensor Anisotropy with Respect to the Normal Species (Table 3); Calculations Are Performed As a Function of the O•···+Si Interatomic Distance molecular model (CH3O)3Si−OO•···+Si(OCH3)3 O•···Si+ interatomic distance (nm)

computed g tensor

exptl g tensor

0.183 (optimized geometry) 0.188

g1 = 2.0195; g2 = 2.0077; g3 = 2.0073; giso = 2.0115 g1 = 2.0249; g2 = 2.0082; g3 = 2.0028; giso = 2.0120

0.190 0.200 distance > 4 nm (negligible interaction)

g1 = 2.0250; g2 = 2.0082; g3 = 2.0027; giso = 2.0119 g1 = 2.0260; g2 = 2.0083; g3 = 2.0025; giso = 2.0122 g1 = 2.0513; g2 = 2.0083; g3 = 2.0020; giso = 2.0020

g1 = 2.0286; g2 = 2.0076; g3 = 2.0013; giso = 2.0133 (this work); g1 = 2.027; g2 = 2.0085; g3 = 2.0020; giso = 2.0125 (refs 6,11)

g1 = 2.0670; g2 = 2.0078; g3 = 2.0018; giso = 2.025 (ref 6, 11)

Table 5. Experimental and 6.311G++(2d,p) DFT UB3LYP Computed g Tensors for the Siloxy Radical-Cation (≡Si−O−Si≡)•+; Computations Are Carried out on the Molecular Model [(CH3O)3Si−O−Si-(OCH3)3]•+ species

computed g tensor

(≡Si−O−Si≡)•+

[(CH3O)3Si−O−Si(OCH3)3]•+ (model)

exptl g tensor

computed spin density distribution (%)

g1 = 2.0085 g2 = 2.0032 g3 = 2.0014 giso = 2.0044 g1 = 2.0137 g2 = 2.0086 g3 = 2.0037 giso = 2.0086

(CH3) 27.5 (Si) 28.4 (O) 6.4 O(CH3) 37.7 3309



Article

Figure 4. EPR spectra of mesoporous silica recorded as a function of the temperature after γ irradiation at 77 K with a dose of 7 kGy. The spectrum f shows the computer simulation of the hf component assigned to the radical-cation (Si−O−Si)•+.

gmax = 2.054−2.093, gint = 2.005−2.009, and gmin = 1.995− 2.000.73,74 The results of UB3LYP DFT computations confirm that a similar difference of electronic structure changes is to be expected between siloxy radical-cations (Si−O−Si)•+ and siloxy radicals Si−O•, the former being characterized by a reduced g tensor anisotropy (Tables 3 and 5). Unstable oxygen species detected in irradiated glasses and quartz are described in the literature as self-trapped holes ST1 and ST2 having the g tensors 2.043, 2.0082, 2.0027 (gav =2.0179) and 2.013, 2.0078, 2.0054 (gav = 2.0087), respectively;11,12,83 the ST1 center is defined as a hole trapped on a normal bridging oxygen within the SiO4 network, and its electronic structure is assumed to be very similar to that of a siloxy radical;84 the ST2 center is defined as a trapped hole delocalized over two bridging oxygen. The comparison with the carbon radical-cation analogues and the results of DFT UB3LYP computations rise doubts about such proposed structures; the properties of the ST2 centers should indeed be considered diagnostic of normal (Si−O− Si)•+ radical-cations, while a different hypothesis should be envisaged for the structure of the ST1 centers. Ge Impurity Centers. The simulation of the high field part of the experimental spectrum near g = 1.9930 in Figures 1 and 2 has been greatly improved by adding a linear combination of two signals with g tensors g1 = 2.0006, g2 = 1 9931, g3 = 1.9931 (0.6%) and g1 = 2.0007, g2 = 1 9994, g3 = 1.9930 (0.3%), respectively. Judging from the similarity with the g tensors reported in the literature, these signals are considered diagnostic of species arising from Ge(1) and Ge(2) impurities. As previously stated, Ge is a common isovalent substitutional impurity in quartz, and the formation of Ge paramagnetic species is well assessed in irradiated quartz and glasses.2,15,57−59 The yield of such species and their contribution to the overall EPR spectrum is often enhanced beyond the expected direct radiolysis contribution by the charge scavenging properties of Ge.44,85 Friebele, Griscom, and Sigel85 have observed four different Ge centers in irradiated Ge doped glasses, namel, y Ge(0), Ge(1), Ge(2), and Ge(3), which were proposed to

differ only in the number of Ge atoms in the second coordination sphere. The Ge(3) center, which is the most stable one since it is detected also in not-irradiated samples, was associated to the tetrahedral species Ge• similar to the E′ centers in quartz (g1 = 2.0011, g2 = 1.9945 and g3 = 1.9945). A similar structure was assigned to the Ge(0) center with an axially symmetric g tensor (g1 = 2.0009, g2 = 1.9943 and g3 = 1.9943). The Ge(1) center was identified as an anionic species [(−O)4Ge]•− arising from the electron capture by a tetrahedral GeO4 site (g1 = 2.0007, g2 = 1.9994 and g3 = 1.9930). The structure of the Ge(2) center, with g1 = 2.0010, g2 = 1.9978 and g3= 1.9868, is still a matter of discussion; the hypothesis of a formyl radical type structure −O− Ge•O has been forwarded, but recently, its assignment to the bivalent cationic species Ge•+ has been proposed.44,57 As stated above, judging from the similarity of the reported g tensors and their thermal instability, the species Ge(1) and Ge(2) are considered the most likely candidates for the assignment of the EPR components detected in this work (Figure 2f). Irradiation of Pristine Mesoporous Silica. The EPR spectra from mesosporous silica recorded at low temperature after irradiation at 77 K (Figure 4a,b) are very similar to those obtained from pristine zeosil silica (Figure 1a) exception being made for a lower yield of trapped H-atoms and H-atom centers. Allowance being made for minor effects on the EPR parameters caused by different matrix environments, the computer simulations in Figure 5 do confirm the presence of the same species observed in zeosil silica. It is therefore inferred that the main features of the radiolysis mechanism in the two type of silica are similar. The observed lower yield of H-atoms and H-atom centers in mesoporous silica is explained with the experimentally assessed lower content of precursor silanol groups. Decay Behavior of the Silica Species. Most of the radiolytic species produced by low temperature γ irradiation in zeosil silica are very stable at 77 K but decay rapidly on warming above 120 K, the exception being made for the stable fraction of E′γ centers, which persists at room temperature. 3310



Article

Figure 5. Computer simulation of the EPR spectrum of irradiated mesoporous silica recorded after annealing at 137 K. The hf components belonging to silyl peroxy radical, siloxy radicals, and the H-atom center are shown together with their pertinent EPR parameters.

of the presence of ionic intermediates in the radiolysis of silica has been obtained by scavenging experiments with organic adsorbates like N,N-dimethyl aniline, pyrene, alkyl halides, galvinoxyls, naphthalene, tetracyano ethylene, and aryl amines.10,18,23 Furthermore, the participation of ionic processes in irradiated controlled pore glasses has been proven by NMR spectroscopy.49 In agreement with such information, both the ionic and excitonic pathways are considered in the scheme of Figure 6 as possible concurrent sources for radiolytic intermediates. The Si−O and O−H bonds rupture in excited silanols is expected to be a prominent source of H-atoms, silyl, and siloxy radicals. In the ionic pathway, holes and electrons escaping geminate recombination can be envisaged to contribute H-atoms, silyl, and siloxy radicals by dissociative electron capture of SiO−H and Si−O bonds and through the reactions of precursor radical-cations. The simplest hypothesis for the formation of silyl peroxy radicals is the addition of trapped molecular O2 to Si radicals and E′γ centers yielding normal and anomalous peroxy radicals, respectively. In agreement with literature reports, molecular oxygen needed for such reaction can be provided by the radiolytic decomposition of silica itself91−93 as well as be present as trapped impurity. The reaction scheme in Figure 6 also includes trapping of H-atoms near diamagnetic Si+ centers as a hypothesis for the structure of the H-center, which is alternative to that based on Ge impurities. The scheme in Figure 6 does not show reactions concerning the radiolysis of adsorbed H2O, which is suggested in the literature to contribute OH radicals (not detected) and H-atoms via a prominent excitonic mechanism:90

Table 6. Decay Behavior of the Radiolytic Species Produced by γ Irradiation at 77 K of Silica Zeosil 1165 and Mesoporous Silica residual radicals concentration (%) annealing temp (K)

zeosil 1165

mesoporous silica

77 117 137 159 177

100 43 24 18 6

100 35 26 12

The data in Table 6 show that only ca. 6% of the initial radical population is left after warming for a few minutes at 177 K, while, for the mobile H-atoms, a complete decay is already observed slightly above 120 K. Since mass translational diffusion is hindered at low temperature in a rigid solid matrix, spin migration phenomena characterized by low or negligible activation energy must be assumed in order to account for bimolecular encounters needed for the decay of paramagnetic centers. Electron and hole transfer and H-atom hopping between neighbor silanols can be envisaged as possible mechanisms. Charge migration in irradiated silica is theoretically assessed1 and experimentally supported by scavenging experiments using high electron affinity and low ionization potential organic adsorbates.10,18,23 Further evidence of the existence of free valence migration mechanisms in irradiated silica will be later shown in connection with the radiolysis of silica with adsorbed polybutadiene oligomer. According to the results in Table 5, the radical decay kinetics in irradiated zeosil and mesoporous silica are quite similar, thus supporting the conclusion that mechanisms of free valence migration are not significantly influenced by silica surface area and morphology. Radiolysis Mechanism of Pristine Silica. Although the exciton chemistry is reported to be of major importance for the generation of H-atoms and molecular H2 in irradiated dried silica and silica containing adsorbed water,18,48,49,86−90 evidence

3

excitons + H 2O → OH + H

Irradiation of Silica with Adsorbed Polybutadiene Oligomers. EPR Spectra. The EPR spectra obtained from irradiated zeosil impregnated with polybutadiene oligomer PB-5000 (15−20% by weight) are shown in Figure 7. 3311



Article

Figure 6. Proposed partial reaction scheme for the low temperature radiolysis of pristine silica.

Figure 7. EPR spectra of zeosil 1165 silica impregnated with polybutadiene oligomer PB-5000 recorded as a function of the temperature after γ irradiation at 77 K (radiation dose, 7 kGy) The computer simulation of the room temperature spectrum c was obtained by a linear combination of the signals belonging to the A1 radical and the E′γ center.

of species chemically anchored to the silica surface. In fact, the linkage to silica, while preventing radical decay by inhibiting their translational diffusion, does not hinder fast intramolecular rotational motions, which lead to the averaging of hf and g tensor anisotropies.19,20,22,24,26−28,94 In agreement with this conclusion, no stable radicals are obtained from neat polybutadiene after irradiation near room temperature. For the assignment of the spectra in Figure 7, the three species A1, A2, and A3, all bearing the same number and type of interacting protons and arising from the reaction of silica species with polybutadiene double bonds, can be considered (Figure 8). As the magnetic equivalence of the 3 protons holds up to a resolution of 0.02−003 mT (tested by computer simulation), the attention is driven toward a radical with a freely rotating

Starting from the lowest temperature, the dominant spectral component is a partially resolved binomial quintet with average peak to peak separation of ca. 2.1 mT. All the species from irradiated pristine silica are absent, the exception being made for a minor amount of trapped H-atoms, H-atoms centers (low field feature at g = 1.9636), and E′γ centers; the latter species are better identified in the room temperature spectrum from the narrow singlet at g = 2.0007 (Figure 7c). The quintet pattern is thermally stable, and when recorded near room temperature, it changes reversibly into a well resolved doublet of binomial quartets, which is interpreted in terms of 3a(H) = 2.27 mT and 1a(H) = 1.53 (Figure 7e,f). The almost solution like resolution of the EPR spectra near room temperature coupled with negligible radical decay are characteristics 3312



Article

methyl group. However, the splitting of 1.53 mT is too small for being assigned to an α type proton in silica-bonded species; in fact, the Hα couplings for radicals of type Si−CH•(R),  Si−O−CH•(R), and Si-CH(R)CH•(R) are reported to be in the range 2.02−2.280,67,95 2.0,19,26 and 1.94−2.24 mT, respectively.67 Therefore, the smaller coupling is to be assigned to a β type proton. It is inferred that the radical structure A1 bearing three equivalent methyl protons of 2.27 mT and one β-proton of 1.53 mT is the best suited to justify the observed EPR spectrum. The alternative structures A2 and/or A3 are disregarded because the additional, less likely, hypothesis is required that one Hα and two Hβ protons become magnetically equivalent by motional averaging of g and hf tensor anisotropy. The coupling constant of Hβ protons in alkyl radicals ranges from near zero to ca. 5.0 mT depending on the orientation of the C−H bond with respect to the unpaired electron orbital. This angular dependence is embodied in the McConnell

Figure 8. Chemical structures for the SiO2-bonded radicals. The structure A1 with the conformation shown aside is the best suited to account for the experimental EPR spectrum.

Figure 9. EPR spectra from irradiated mesoporous silica impregnated with the polybutadiene oligomer PB-5000: identification of the SiO2-bonded radical A1 and its corresponding peroxy radical formed by reaction with trapped O2.

Figure 10. FTIR-diffuse reflectance spectra demonstrating the irreversible radiation induced attachment (grafting/cross-linking) of polybutadiene onto zeosil silica. 3313



Article

equation a(H)β = A + B cos2 θ, where A and B are constants and θ is the dihedral angle between the C−Hβ bond and the symmetry axis of the unpaired electron orbital.96 From the average methyl proton coupling of 2.27 mT, ⟨cos2 θ⟩ = 1/2 is obtained for a freely rotating methyl group; by neglecting the A constant (normally

Reaction Mechanisms in Irradiated, Precipitated, and Mesoporous Silica

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