Journal of Non-Crystalline Solids 362 (2013) 152–155

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Journal of Non-Crystalline Solids

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Investigation on the generation process of HO2• radicals by γ-ray irradiation inO2-loaded fumed silica

G. Buscarino a,⁎, S. Agnello a, A. Parlato b, F.M. Gelardi a

a Department of Physics, University of Palermo, Via Archirafi 36, 90123, Palermo, Italyb Dipartimento dell' Energia, Universita' di Palermo, Viale delle Scienze, Edificio 6, 90128 Palermo, Italy

⁎ Corresponding author. Tel.: +39 091 238 91725; faE-mail address: gianpiero.buscarino@unipa.it (G. Bu

0022-3093/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.jnoncrysol.2012.11.040

a b s t r a c t

a r t i c l e i n f o

Article history:Received 12 September 2012Received in revised form 24 October 2012Available online 23 December 2012

Keywords:HO2•;O2;Fumed silica;Electron paramagnetic resonancespectroscopy;Raman spectroscopy

We report an experimental investigation on the effects of γ-ray irradiation in three types of fumed silica pre-viously loaded with O2 molecules. Our data indicate that the main effect of irradiation in these systems is togenerate a very large concentration of HO2• interstitial radicals (about 1018 molecules/cm3). Furthermore,the number of generated HO2• was found to be larger in the samples with higher O2 contents before irradi-ation. This correlation suggests that HO2• radicals are induced by reaction of interstitial O2 molecules with ra-diolytic H atoms, as previously suggested for O2-loaded bulk amorphous silicon dioxide (a-SiO2 or silica)samples. However, at variance with respect to bulk materials, in fumed silica the radiolytic H does notarise from SiOH or SiOOH groups, as no EPR signal due to non-bridging oxygen hole centers (NBOHC) or toperoxy radicals (POR) is detected in the spectra of irradiated samples. As a reasonable alternative we proposethat radiolytic hydrogen atoms could arise from a radiation induced breaking of interstitial H2O molecules,indicating that fumed silica in its pristine form could possess a very large concentration of interstitialwater molecules.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Hydroperoxy radical (HO2•) is one of the most important interme-diates in reactions in acidic solutions and its formation is usually con-trolled by various catalysts [1]. Among them, a-SiO2 plays a key role,thanks to its interstitial voids which are able to encapsulate small dia-magnetic and paramagnetic molecules [2–4]. In particular, HO2• radi-cal has been studied in O2-loaded bulk a-SiO2 irradiated by F2-laser atT=77 K [5,6]. In those studies it has been shown that irradiation in-duces radiolytic hydrogen by breaking O\H bonds in SiOH and SiOOHgroups. Once formed, these H atoms promptly react with interstitialO2 molecules generating the HO2• radicals. It is worthy to note thatin this scheme, together with the hydroperoxy radicals, a comparableamount of NBOHC plus POR is also induced, as experimentally ob-served by Kajihara et al. [5,6].

Here we report a study on the HO2• radical induced by γ-ray irra-diation in O2-loaded a-SiO2 nanoparticles, with particular attentionon its spectroscopic properties and on its possible generation mecha-nism. Furthermore, the analogies and the differences with respect tothe case of bulk silica, considered in a previous experimental investi-gation [5,6], are examined. This comparison could help to clarify thephysical origin of the peculiar properties which pertain to a-SiO2

when it is spatially confined to a nanometric scale. These peculiaritiesare very interesting for both fundamental physics, in relation to the

x: +39 091 6162461.scarino).

rights reserved.

properties to the amorphous solid systems, and the numerous applica-tions which could potentially involve nanostructured silica. This ap-proach has been already successfully applied in previous experimentalinvestigations focused on different paramagnetic centers, such as E′centers, oxygen hole centers, methyl radicals, induced by differenttypes of irradiation in nanostructured a-SiO2 [7–10].

2. Experimental

Thematerials considered here are three different types of fumed silicaproduced by Evonik [11]. Their commercial names are Aerosil300®(nickname AE300), Aerosil150® (AE150) and AerosilOX50® (AEOX50),and differ in the average diameters of the constituting nanoparticles,which are 7 nm, 14 nm and 40 nm, respectively. All the pristine mate-rials were preventively loaded with O2 molecules by treating them atT~473 K in oxygen atmosphere (P=50 bar) [12]. Subsequently thematerials were subjected to γ-ray irradiation at the dose of 130 kGy ina 60Co source (dose rate of 0.5 kGy/h).

In order to passivate NBOHC and POR in the irradiated samples,they were also exposed to H2 atmosphere (P=140 bar) for 13 h atT=300 K in a PARR reactor.

FT-Raman measurements were carried out by a Bruker RAMIIspectrometer equipped with a Nd:YAG laser source at 1064 nm(9398 cm−1) that enabled detection of both the intrinsic vibrationalRaman modes of a-SiO2 and the photoluminescence emission fromO2 molecules [12]. This latter optical activity was used to estimatethe O2 content of loaded samples, as described in detail elsewhere

Table 1Concentration of O2 and HO2• estimateda in the materials considered.

Material O2 HO2•

AEOX50 1.2×1019 3×1018

AE150 3.6×1018 1.3×1018

AE300 2.0×1018 4×1017

a The concentration of O2 molecule was estimated by photoluminescence emissiondetected in Raman spectra [8], whereas that of HO2• radical by double integration ofits EPR line.

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[12–14]. The experimental uncertainty associated to the estimation ofO2 concentration is about ±10%.

EPR measurements were carried out at T=300 K and at T=77 Kwith a Bruker EMX-micro-Bay spectrometer working at a frequencyof about 9.8 GHz (X-band) and with a magnetic-field modulationfrequency of 100 kHz. Low temperature spectra were obtained byputting the sample into a Dewar flask filled with liquid nitrogen.Concentrations of defects were determined, with a relative accuracyof ±10%, by comparing the double numerical integral of the EPRspectra with that of the E′ centers in a reference sample.

3. Results

In the O2-loaded samples before γ-ray irradiation no EPR signalwas detected. At variance, in the Raman/PL spectra the typical bandthat peaked at 1538 cm−1 related to the photoluminescence of in-terstitial O2 and excited by the 1064 nm laser line is easily detected,as shown in Fig. 1, together with the bands associated with the vibra-tional modes of silica (with a Raman shift lower than about1200 cm−1). In line with previous experimental works, by comparingthe amplitude of the 1538 cm−1 band with that of the intrinsicRaman bands of silica, a quantitative estimation of O2 molecules canbe obtained [12–14]. As reported in Table 1, with this method an O2

content increasing from 2.0×1018 up to 1.2×1019 molecules/cm3

on increasing the nanoparticles diameter from 7 nm to 40 nm, isfound.

After irradiation a strong EPR signal is observed in all the investigat-ed samples, whose spectroscopic features are compatible with thoseattributed to the HO2• radicals [5,6,15]. The EPR spectra obtained at

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Fig. 1. Raman/PL spectra of the O2-loaded materials (a) AE300, (b) AE150, and(c) AEOX50. The spectra are scaled to have equal amplitude of the silica Raman bandat about 440 cm−1.

T=300 K and T=77 K for the O2-loaded and irradiated AEOX50mate-rial are compared in Fig. 2. As shown, at T=300 K the EPR spectrum isbroad and structureless, whereas at T=77 K it reveals a structure andexhibits the typical spectral features of the HO2• radicals [5,6,15]. Thenarrow small line indicated by the arrow in Fig. 2 is attributable to theE′γ centers [16] and is discussed later. This strong effect of the temper-ature of the sample on the EPR line shape of the paramagnetic center isdue to the fact that, since the radical is in the form of an interstitial mol-ecule, it suffers a relevant tumblingmotion, which affects the linewidthand is intrinsically dependent on the temperature. This property is alsoin agreement with previous observations on bulk a-SiO2 [6]. Similar re-sults were also obtained for AE150 and AE300 materials.

To further characterize the HO2• paramagnetic center observed innanosized a-SiO2 (fumed silica) we have also investigated the depen-dence of its EPR signal on the microwave power (Pin) at T=77 K. Theresults of these studies are reported in Fig. 3 and show that the EPRsignal of HO2• radicals saturate for values of the microwave powerabove ~5 mW for all the materials considered, i.e., independently onthe nanoparticle size. This value of power is relatively high, indicatingthat at T=77 K the molecule possesses efficient mechanism of ener-gy relaxation.

By double integration of the EPR signal we have estimated theHO2• radical concentration in the various materials considered. Asreported in Table 1, this concentration was found to change from4×1017 spins/cm3 to 3×1018 spins/cm3 going from the materialwith the smallest nanoparticles to that with the largest ones. Thesedifferences approximately reflect the O2 content of the differentnanoparticles measured before irradiation (Table 1). An interestingpoint of our experimental investigation is that we have observedthat, in spite of these differences among the various materials inves-tigated, the line shape of the EPR resonance is found to be virtually in-distinguishable. This result is shown in Fig. 4, where the normalizedEPR spectra obtained for AEOX50 and for AE300 are compared.These results and the strict correspondence between the spectra ob-served in the present work and those reported for HO2• in D2O [15],suggest that the signal reported in Fig. 4 essentially represents the

Fig. 2. Comparison between the normalized EPR spectra obtained at T=300 K and T=77 K for O2-loaded and irradiated AEOX50 material.

Fig. 3. Dependence of the EPR signal (normalized to the spin number) on the micro-wave power incident in cavity (Pin) obtained at T=77 K for O2-loaded and irradiatedAEOX50, AE150 and AE300 materials.

Fig. 5. Comparison between the normalized EPR spectra obtained for O2-loaded andirradiated AEOX50 before and after H2 loading.

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“isolated” HO2• EPR line shape. This point is relevant because, at var-iance to previous experimental investigations focused on bulk a-SiO2,in the observed spectra no signals ascribable to NBOHC and PORradicals are present, so suggesting that these latter defects are notinduced in fumed silica upon irradiation.

To further investigate this point the irradiated samples weresubjected to H2 loading. The results of these experiments are reportedin Figs. 5 and 6, where the normalized EPR spectra obtained forAEOX50 before and after H2 loading are compared for T=77 K andT=300 K, respectively.

4. Discussion

NBOHC radicals are known to be very reactive with H2 [17–19].Consequently, the treatment of H2 loading is expected to cancel outthe contribution of these centers to the overall EPR spectrum, if any.As it is evident in Figs. 5 and 6, the treatment does not affect the res-onance line shape at all, strongly supporting the conclusion that noNBOHC (and similarly no POR) are induced in O2-loaded fumed silicaupon γ-ray irradiation. Furthermore, as it is evident in Fig. 5, by com-paring the EPR spectra obtained before and after hydrogen treatment,a relevant reduction of the amplitude of the E′γ centers is observed(inset), whereas the main contribution to the EPR signal, due toHO2• radicals, is essentially unchanged (main figure). It is worthy tonote that the disappearance of E′γ centers shown in Fig. 6 indicatesthat the exposure to hydrogen atmosphere worked properly andthat the H2 molecules have actually diffused into the core of thea-SiO2 nanoparticles. Similar results were also obtained for AE150and AE300 materials.

Fig. 4. Comparison between the normalized EPR spectra obtained for O2-loaded andirradiated AEOX50 and AE300 materials.

The absence of NBOHC and POR radicals in the EPR spectra of theirradiated materials indicates that radiolytic H atoms, which interactwith O2 molecules to give HO2• radicals, do not arise from SiOH orSiOOH groups, as in the case of bulk a-SiO2. Furthermore, it is also un-reasonable that radiolytic hydrogen could arise from H2 moleculesdissolved into the as-grown material, as they are expected to escapeefficiently from the material by rapid diffusion at room temperature.

It is worthy to note that, in principle, irradiation could induceradiolithic hydrogen by breaking O\H bonds in facing Si\OH defectspairs, such as those discussed in Ref. [24]. In this case, in fact, due toDipolar interaction, the resulting center is expected to have an EPRline so broad to become experimentally undetectable. However, al-though this process is possible in principle, we believe it is quite im-probable, and will not be further considered here.

At variance, a reasonable alternative we propose here is that the ra-diolytic hydrogen atoms could arise from the radiation induced break-ing of the interstitial H2O molecules. In principle, water moleculesshould also diffuse rapidly in a-SiO2 at room temperature [19]. This con-clusion suggests that a relevant concentration of H2O molecules couldreside into the fumed silica network in interstitial positions. The originof this large interstitial water content is presumably related to the factthat a-SiO2 nanoparticles (fumed silica) are completely covered by awater film, due to air humidity [20]. This conclusion points out thatthe different generation mechanisms of HO2• centers by irradiation ob-served in the fumed silicawith respect to the bulk silica is essentially re-lated to the very high specific surface of the former with respect to thelatter material. This property, in fact, gives to the former system a veryhighwatermolecule availabilitywhich strongly influences the chemicalreactions taking place upon the irradiation in the material. The specialrole played by the high specific surface of the material has been also

Fig. 6. Comparison between the ESR spectra obtained at T=300 K for O2-loaded andirradiated AEOX50 material before and after H2 loading. Inset: zoom on the region ofthe spectra showing the resonance due to E′γ centers.

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pointed out in previous experimental investigations which focusedon the structural and optical properties of fumed and porous a-SiO2ma-terials [21–29]. In theseworks, in fact, it has been shown that the obser-vation of specific photoluminescence (PL) activities is strictly connectedto the high specific surface of the system, and that many properties ofthese PL bands are directly related with the species adsorbed on its sur-face [21–29].

5. Conclusion

We have reported an experimental investigation focused on theHO2• radicals induced by room temperature γ-ray irradiation inthree types of O2-loaded a-SiO2 nanoparticles, with diameters of7 nm, 14 nm and 40 nm. Our results have permitted to obtain a char-acterization of the HO2• EPR spectrum and to explore a generationmechanism of this interstitial molecule. In particular, concerningthis latter point, we have found reasonable evidences suggestingthat HO2• radicals are generated under room temperature γ-ray irra-diation by the reaction of interstitial O2 molecules with H atoms, aris-ing from the radiation induced breaking of interstitial H2O molecules.These results strongly suggest that the core of a-SiO2 nanoparticleshas a very large water content.

This interesting conclusion could help in future works the under-standing to a very high level the physical and chemical properties ofthe fumed silica and of other similar nanodimensional high-surfacematerials.

Acknowledgments

The authors thank the people of the LAMP group (http://www.fisica.unipa.it//amorphous) at the Physics Department of the Universityof Palermo for useful discussions and technical assistance by G. Napoliand G. Tricomi.

The authors would like also to acknowledge K. Kajihara, A. Alessiand G. Iovino for useful comments and suggestions.

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