ARTICLE pubs.acs.org/JPCC
Structure of Amorphous SiO2 Nanoparticles Probed through the E0γ Centers G. Vaccaro,*,† G. Buscarino,† S. Agnello,† A. Sporea,‡ C. Oproiu,‡ D. G. Sporea,‡ and F. M. Gelardi† † ‡
Dipartimento di Scienze Fisiche ed Astronomiche, Universita di Palermo, Via Archirafi 36, I-90123 Palermo, Italy National Institute for Lasers, Plasma and Radiation Physics, 409 Atomistilor St., RO-077125 Magurele, Romania ABSTRACT: We report an experimental investigation by electron paramagnetic resonance (EPR) spectroscopy on the properties of the E0 γ centers induced by β-ray irradiation in nanoparticles of amorphous SiO2 (fumed silica) with mean diameters from 7 up to 40 nm. We found that the E0 γ centers are induced in all the fumed silica types in the dose range 4400 kGy. They are characterized by an EPR line shape similar to that observed in common bulk silica materials independently on the particle diameter. Moreover, the E 0 γ center concentration decreases on decreasing of the particle size for each given dose. Our findings are interpreted in terms of a shell-like model of nanoparticles in which it is assumed that stable E0 γ centers can be mainly induced in the inner part of the particles, whereas those induced in the surface shell are supposed to be essentially unstable and rapidly disappear after irradiation. Furthermore, we have found that the same shell-like model naturally explains the actual radiation resistance observed for nanoparticles which has been previously tentatively attributed to the high O2 content of the materials.
’ INTRODUCTION Nanomaterials usually exhibit properties that can significantly differ from those observed in larger scale materials (bulk materials). Furthermore, at variance to the latter materials, many physical properties of the former ones usually exhibit a strong size dependence.13 The nanomaterials have also recently attracted attention for many potential applications in different scientific areas such as optics, mechanics, electrical devices, reactivity, and biomedicine.19 Besides, nanomaterials play an important role to deepen the knowledge of the solid state matter properties because they represent a bridge between extended and atomic scale systems. For the above reasons, nanotechnology research has been growing rapidly worldwide during the past decade, with an increasing number of nanotechnology products becoming commercially available. In this context, amorphous SiO2 (or silica) plays a fundamental role among amorphous nanomaterials. Indeed, the investigation of this archetype amorphous structure may help to understand and deepen how the macroscopic properties of a system are modified when it is confined into nanometric scale and, at the same time, can help to exploit the potentialities of nanometric silica systems.4,5,8,10,11 In the last years, numerous simulative and experimental investigations have shown that silica nanoparticles are characterized by uncommon properties as compared to bulk silica materials. One of the major factors that gives the nanoparticles of amorphous SiO2, the so-called fumed silica, these properties is their very large surface area to mass ratio (specific surface area or S).13 Indeed, for these systems S can assume values up to ∼400 m2/g, whereas for bulk silica materials of typical size 5 5 1 mm3, S ∼ 103 m2/g. As a consequence, the number of atoms nearby to the particles surface becomes r 2011 American Chemical Society
dominant on decreasing the nanometric size. For example, a spherical particle of 30 nm diameter has 5% of its atoms on the surface, whereas a particle of 3 nm size has 50% of its atoms on the surface. By contrast, for bulk materials, the amount of atoms at the surface is negligible compared to the number of atoms inside the material. The predominance of surface effects can drastically change the structural properties of the particle and its interactions with the surroundings, opening many interesting questions. For example, a question of concern is the influence of the nanometric nature of fumed silica on the creation efficiency of points defects by irradiation and on their structural features. Among the point defects of bulk silica, the so-called E0 γ center is one of the most known and investigated.1213 It is a paramagnetic point defect, which can be detected by the electron paramagnetic resonance (EPR) technique after opportune irradiation of the sample by X-ray, γ-ray, β-ray, laser, etc. The most accepted microscopic model for this defect assumes that it is a positively charged oxygen vacancy with an atomic structure characterized by an unpaired electron highly localized in a sp3 hybrid orbital of one of the silicon atoms constituting the vacancy: OtSi•+SitO (where t represents the bonds with the three distinct oxygen atoms and • is the unpaired electron).12,1417 It is well-known that the irradiation dose from which the E0 γ centers begin to be detectable by EPR measurements strongly depends on the type of bulk silica material considered.18 Besides, it has been recently shown that the EPR line shape of the E0 γ center in bulk silica materials can change during irradiation.19,20 Received: August 2, 2011 Revised: November 15, 2011 Published: November 30, 2011 144
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Table 1. List of the Employed Materialsa
Table 2. List of Doses and Dose Rates Employed for the β-ray Irradiation
commercial name
specific surface
particle diameter
(nickname)
[m2/g]
[nm]
dose [kGy]
Aerosil 380 (AE380)
380
7
1 4 10
dose rate [kGy/min] 1 1.6 2.5
Aerosil 300 (AE300)
300
7
Aerosil 200 (AE200)
200
12
Aerosil 150 (AE150)
150
14
40
3.3
20
400
3.3
Aerosil 90 (AE90)
90
a
Commercial name, nickname, specific surface, and primary particle mean diameter are reported.
In particular, two different line shapes can be detected, denominated L1 and L2 by the authors. The former, observed at low irradiation doses, is characterized by an almost axial symmetry g tensor, whereas the latter, observed at higher irradiation doses, tends toward a more orthorhombic g. A gradual change from L2 toward L1 was also evidenced after opportune thermal treatments. This line shape change was attributed to a structural variation of the defect.19,20 Only few works have been carried out on the EPR properties of nanometer-sized silica materials, such as the fumed silica, a powder of silica nanoparticles with diameter from a few up to tens of nanometers. For example, it has been recently shown that fumed silica, with particles of 7 nm mean diameter, photon irradiated at ∼10 eV (VUV irradiation), is characterized by an EPR spectrum in which the E0 γ centers line is recognizable.2124 In particular, the spectrum acquired at 4.2 K, immediately after VUV irradiation, showed a line shape resembling that of the E0 γ centers of bulk silica, but it was significantly broadened.24 However, the signal properties were found to be significantly different when the spectrum of the same sample was acquired several days later. In this latter case, in fact, no unusual broadening was present, and the spectrum showed a line shape virtually indistinguishable with respect to that of the common E0 γ center. This line shape modification is accompanied by the decrease of the signal intensity by a factor ∼2. These findings were attributed to the presence of E0 γ centers with two distinct structures in fumed silica: one localized in the inner part of nanoparticles and the other on their surface. The point defects of the interior of the nanoparticles exhibit a “bulk” like structure, resulting in an E0 γ center line shape very similar to the common E0 γ centers of bulk silica (bulk E0 γ centers). On the other hand, the E0 γ centers localized on the surface of the particles (surface E0 γ centers) are characterized by a broader line shape as compared with that of the core. Clemer et al. suggested that the E0 γ centers are induced in the whole silica nanoparticle by VUV irradiation, and the EPR spectrum acquired immediately after the irradiation is characterized by the superposition of signals arising from bulk and surface E0 γ centers.24 On the other hand, on increasing the time after the irradiation, the surface E0 γ centers, experiencing physical and chemical interactions with the environment, are inactivated (passivated) leaving only the bulk E0 γ centers. Besides, by the observation that the E0 γ concentration induced in fumed silica, after VUV irradiation prolonged for daysweek, was about 100 times smaller than that observed in typical bulk silica materials, the authors suggested that fumed silica is more radiation resistant. This property was supposed to represent an indirect evidence of the oxygen richness of the materials. As a consequence, fumed silica materials are assumed to possess a very low density of oxygen vacancies, which are commonly considered as E0 γ center precursors.2527
To shed new light on the actual origin of the silica nanoparticles radiation resistance and on a possible particle size dependence of EPR properties of the E0 γ, in the present work we report a detailed study by EPR measurements of the creation efficiency of the E0 γ center at different irradiation doses in five fumed silica types, differing for the particle mean diameter (from 7 to 40 nm) and for S (from 380 to 50 m2/g). Raman photoluminescence spectra, able to give evidence of the oxygen richness of the materials, were also acquired to estimate the content of O2 molecules in the various fumed silica types,28 to clarify the actual role played by oxygen in the irradiation resistance of such materials.
’ EXPERIMENTAL SECTION Fumed silica samples of commercial origin were used (Aerosil by Evonik-Industries).29,30 They were synthesized by SiCl4 oxidation in O2/H2 flame at 11001400 °C. As-received fumed silica powders were pressed in a uniaxial press at ∼0.3 GPa, forming a cylindric self-supporting powder tablet from which rectangular shape samples with size 5 5 2 mm3 were obtained and used in all the following reported measurements. In Table 1, the commercial name, nickname, specific surface, and primary particle mean diameter are specified for each of the fumed silica materials considered. The samples were β-ray irradiated in the 4400 kGy dose range. The irradiations were carried out with mean electron energy 6 MeV; mean beam current 2 μA; pulse frequency 100 Hz; pulse duration 3 μs; and (5% spot uniformity over a 12 mm diameter area; and dose rates for each accumulated dose are detailed in Table 2. The sample temperature during all the irradiations increases only by 23 °C above room temperature. EPR measurements were carried out at room temperature at frequency ∼9.8 GHz and 100 kHz magnetic field modulation frequency with a Bruker EMX-micro-Bay spectrometer working in standard first-harmonic mode (FH-EPR) and in high-power second-harmonic mode (SH-EPR).31 The FH-EPR measurements were carried out employing low microwave power and low magnetic field modulation amplitude to avoid line shape distortion and to allow accurate defect concentration estimations. Usually, the concentration of the E0 γ centers is estimated by the comparison of the double integral of the main resonance line acquired in FH-EPR with that of a reference sample with known E0 γ center concentration. In the present work, a γ-ray irradiated bulk silica sample was used as a reference for the line shape and for the concentration of the E0 γ centers, and the latter was determined by the spin echo technique.32 When the concentration of the E0 γ centers was too low to be detectable by FH-EPR measurements, we estimated it by SH-EPR signal, which gives higher sensitivity than the FH-EPR. These estimations were obtained by multiplying the integral of the SH-EPR spectrum by an empirical factor deduced by the ratio between the concentration of the E0 γ centers, estimated from FH-EPR measurements, 145
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Figure 2. Amplitude normalized SH-EPR spectra of the E0 γ center main resonance line acquired for different fumed silica types and for a bulk silica sample. All the fumed silica samples were irradiated at the same dose of 400 kGy.
Figure 1. FH-EPR spectra of the E0 γ center main resonance line acquired for the AE90 and bulk silica samples. The spectra are normalized to the peak-to-peak signal amplitude and are horizontally shifted to make the first maximum coincide.
and the integral of the SH-EPR spectrum, determined in a sample where both the FH- and SH-EPR signals were detectable. Since irradiations were done in Romania, EPR measurements were acquired only starting from two weeks since irradiation. Raman measurements were performed at room temperature by a Bruker RAMII Fourier Transform Raman spectrometer, employing a 500 mW Nd:YAG laser source at 1064 nm. The spectral resolution was fixed at 5 cm1. This instrument enabled us also to detect the photoluminescence emission from the molecular oxygen singlet state and to obtain a quantitative estimation of its content, as explained previously.28 Indeed, it is known that O2 molecules in bulk and the fumed silica network are characterized by a weak near-infrared (NIR) phosphorescence emission at about 1272 nm, excited around 1069 nm.28,3335 Since our Raman spectrometer is equipped with a laser source at 1064 nm, the O2 luminescence activity can be induced and measured. These latter measurements will be named Raman/ PL hereafter.
Figure 3. Growth curves of the E0 γ center concentration estimated for AE90 (filled circles) and AE200 (filled triangles) fumed silica types two months after the irradiation. The data for the AE90 sample (open circles) acquired two weeks after the irradiation are also reported. The experimental error bars are comparable with the symbol size. The dashed lines passing through the data points indicate the square root law.
’ RESULTS In all of the fumed silica samples considered, no EPR signal was detected before the irradiation. At variance, in the irradiated samples a weak signal ascribed to the main resonance line of the E0 γ center was detected around g = 2. In Figure 1, the FH-EPR spectrum acquired two months after irradiation at 400 kGy of the AE90 fumed silica is shown. A typical spectrum of the E0 γ center with a L2 line shape acquired in the bulk silica sample is also included in the same figure, for comparison.19,20 The fumed silica sample spectrum is characterized by a very low signal-to-noise ratio, as compared with that of the bulk sample, but shows a line shape very similar to the L2 line shape. These findings are qualitatively in agreement with the results of the previous works discussed in the Introduction section, in which the VUV irradiation of the AE380 fumed silica was able to induce only a very low concentration of the E0 γ centers.24,21 Besides, the E0 γ center line shape of the fumed silica spectrum in Figure 1 is very similar to that reported in these previous works, in which the bulk-like E0 γ line shape was only observed in the spectrum of the fumed silica sample measured several days after the irradiation. From this point of view, it is important to underline that all of the EPR measurements reported in the present work were acquired starting from two weeks since irradiation.
To enhance the signal-to-noise ratio in the fumed silica sample spectra, we acquired the main resonance line of the E0 γ center by the SH-EPR method.31 The normalized spectra for the fumed silica considered here and for the reference bulk silica sample (the same showed in Figure 1) are compared in Figure 2. No evident changes are observed in the line shape among the fumed silica types, whereas just a minor difference around 346.1 mT can be recognized between fumed silica and bulk silica spectra. The growth curves of the E0 γ center concentration as a function of the dose for AE90 and AE200 samples, acquired two months after irradiation, are shown in Figure 3 (filled symbols). A monotonic concentration increase is found in both samples. Besides, we found that the signal amplitude is not constant in time after irradiation. Indeed, it is observed that the signal recorded two weeks after the irradiation is larger by a factor 23 than the signal recorded two months after the irradiation, for all of the doses considered and for all of the fumed silica samples. This feature is evidenced by the data reported for the sample AE90 in Figure 3 (compare the filled and open symbols). On the contrary, we verified that the concentration of defects remains stable for times longer than two months since the 146
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Figure 4. Concentration of the E0 γ centers, estimated in each fumed silica sample two weeks after the irradiation (filled circles) and after two months (open circles), as a function of the specific surface. All the samples were irradiated at the same dose of 400 kGy. The dashed lines are the best fit curve discussed in the text.
irradiation. In particular, we found that the concentration of defects, independent of fumed silica type and irradiation dose, recorded two and nine months after the irradiation are the same within the experimental error. For simplicity, the data acquired nine months after irradiation are not shown because they are superimposed to those acquired two months after irradiation. Furthermore, the line shape does not change in time, suggesting a simple defect removal process, which happens only during the first two months since the irradiation. For these reasons we will distinguish henceforth the data acquired during the period (within two months since the irradiation) in which the E0 γ center concentration tends to decrease from the data acquired during the period (two months after the irradiation) in which the concentration of defects remains stable. It is important to note that all of the data reported in Figure 3 are characterized by a sublinear growth law, with a square root dependence on dose. We verified that the same growth law is valid independently of the fumed silica type and the time since irradiation. This result suggests that the growth of the E0 γ centers is related to the creation of point defects from the intrinsic SiOSi network, as suggested in some previous works on bulk silica materials.36,37 The data reported in Figure 3 evidence that the concentration of the E0 γ centers is higher in AE90 samples as compared with AE200, suggesting that the more the specific surface is large (or in other words the more the silica particles are small) the smaller is the concentration of the E0 γ centers induced in the material. To elucidate this property, the concentration of the E0 γ centers as a function of the specific surface acquired after two weeks and after two months since the irradiation is shown in Figure 4 for each fumed silica type irradiated at 400 kGy. The data show that the E0 γ center concentration linearly decreases on increasing the specific surface, suggesting a direct connection between the E0 γ center generation efficiency and the nanometric nature of fumed silica. Since it was tentatively suggested that the radiation resistance of fumed silica originates from oxygen excess in the SiO2 nanoparticles, we estimated by Raman/PL measurements the content of interstitial O2 molecules in all of the fumed silica types considered. In the inset of Figure 5 the Raman/PL spectrum acquired for the AE90 sample is reported. The bands observed for the Raman shift below 1200 cm1 are the typical Raman signals attributed to vibrational modes of the silica matrix,14,3840
Figure 5. Area of the luminescence band of the O2 molecules, estimated by the normalized Raman spectra of fumed silica samples, as a function of the specific surface. The experimental error bars are comparable with the symbol size. In the inset, the Raman/PL spectrum of the AE90 sample is shown. The arrow indicates the luminescence band of the interstitial O2 molecules.
whereas the band peaked at about 1538 cm1 is associated to the luminescence band of O2 molecules.28,3335 Indeed, its position corresponds to the absolute wavelength ∼1272 nm (7862 cm1). It is important to note that this signal is originated by the O2 molecules introduced in the silica nanoparticles during their preparation process,35,28 whereas the same signal is typically observed in bulk silica materials only after opportune treatments in the O2 atmosphere.33,34 By the Raman/PL spectra normalized to the intrinsic vibrational modes of silica, as discussed in detail in our previous work,40 we calculated the area under the luminescence band peaked at about 1538 cm1 from which a comparison of the content of O2 molecules in the fumed silica samples is possible. The data obtained with this method are reported in Figure 5 as a function of the specific surface. As it is evident, the amplitude of the luminescence band decreases on increasing of the specific surface, suggesting that the bigger the particles the more the O2 molecules are introduced during the preparation process. The data reported in Figure 5 were estimated by the asgrown fumed silica samples. However, we verified that the content of O2 is not modified in the irradiation dose range here considered.
’ DISCUSSION The spectra reported in Figure 1 and Figure 2 show that the main resonance line of the E0 γ center is independent of the particles size of each fumed silica type. Indeed the line shape is the same for all the fumed silica samples, and it is characterized by an orthorhombic g tensor typically observed in the bulk silica materials irradiated at high doses (L2 line shape).19,20 On the other hand, the data shown in Figure 4 suggest that the creation efficiency of the E0 γ centers strongly depends on the considered fumed silica type, and a direct correlation between the E0 γ center concentration and the specific surface exists. In this context, if one supposes that the E0 γ centers were induced by irradiation in the whole volume of each fumed silica particle, then it is natural to expect that the concentration of the E0 γ centers should not depend on the particle size, and then it should be the same in all the fumed silica types. However, the data reported in Figure 4 clearly disagree with this expectation. To interpret our findings, we assume, in agreement with the previous EPR investigation, that most stable E0 γ centers, characterized by a bulk-like central 147
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line shape, can be induced only in a part of the particle volume. In particular, we considered a shell-like model of the fumed silica particles, discussed in detail in our previous work40 and in some simulative works,41,42 in which each particle is supposed to consist of a surface shell, characterized by a network more strained as compared with that of the bulk silica network, and a core region, with a network more similar to that of bulk materials. On the basis of this model, we suggest that the signal shown in Figure 1 and Figure 2 is originated from the E0 γ centers induced mainly in the core region. Indeed, if they were localized in the surface shell, their concentrations should be expected to increase with the specific surface. To verify this supposition, we calculated the expected concentration of E0 γ centers in each fumed silica sample as CE0γ ¼ k
VCS mp
specific surface. As a consequence, if one assumes that the presence of O2 molecules prevents the generation of the E0 γ centers, then the concentration of these point defects should be lower in the fumed silica samples with higher O2 content. On the contrary, as discussed above, we found that the concentration of the E0 γ centers is higher in the samples characterized by a smaller specific surface, that is, with higher O2 content (compare Figure 4 and Figure 5). Another information that we extracted from the fit is the thickness of the surface shell: δ ∼ 1 nm. (This value was obtained putting F ∼ 2.2 g/cm3 as reported by the producers.29,30) The reasonableness of this value, considering that the fumed silica particles have a mean diameter from 7 to 40 nm, further corroborates our interpretation. It is worth noting that our work reports the first experimental estimation of 00 δ00 , whose value was predicted to be about 0.20.3 nm by simulations.41,42 To conclude, we also applied the above-discussed model to the E0 γ center concentration estimated two weeks after irradiation (unfilled symbols of Figure 4), that is, during the “transient” period of time in which the concentration of point defects tends to decrease. We fitted the experimental data reported in Figure 4 with eq 2, obtaining again a good agreement with the best fit curve (the gray dashed line). We estimated in this case that CE0 γ(0) = k/F = (2.79 ( 0.15) 1016 spins/cm3, whereas the obtained thickness of the surface shell is again ∼1 nm, further supporting our approach. In other words, the two data sets shown in Figure 4 differ only for a multiplicative factor (∼2.2), that is, a different k value, referring to eq 2. These findings suggest that our model can be also applied to the “transient” period. The transient effect, that deserves further investigation, is not associated to the instability of the shell network, but it could be attributed to fading of irradiation induced defects, a phenomenon already observed in bulk silica and caused by diffusion-limited reactions of the E0 γ centers with interstitial species released during irradiation.44,45
ð1Þ
where k is a constant; VCS is the volume of the core region; and mp is the particle’s mass. It is possible to substitute VCS = Vparticle VSS, Vparticle and VSS being the volume of the whole particle and the surface-shell volume, respectively. Since, as suggested by simulative works,41,42 the thickness of the surface shell (δ) is small as compared with the dimension of the particles (and it is independent of the particles size), the term VSS/mp can be approximated to δ S, where S is the specific surface. As a consequence, eq 1 can be rewritten as follows 1 ð2Þ CE0γ ðSÞ≈k δ S F where F is the mean density of the particle. The value of F can be considered constant as compared with the variation of CE0 γ and S that change about 1 order of magnitude (see Figure 4). At variance F, as reported in our previous work,40 changes at most of 1020% occur in all the fumed silica types. We initially focused our attention on the data acquired after two months since the irradiation because, as discussed in the previous section, after this period the E0 γ concentration becomes stable in time. We fitted the experimental data reported in Figure 4 (filled symbols) with eq 2, where k/F and k δ were considered as fit parameters. The very good agreement between the best fit curve (the gray dashed line) and the experimental data confirms the initial supposition: the fumed silica particles can be described in terms of the shell-like model, where the E0 γ centers are induced mainly in the core. From the intercept value CE0 γ(0) = k/F = (1.25 ( 0.03) 1016 spins/cm3, important information can be obtained. Indeed, this value represents the expected concentration of defects induced in the core of the particles in the limit of zero specific surface, which means in the limit of very large (bulk) particles. Interestingly, the value we obtained for this quantity is of the same order of magnitude with respect to that known for bulk materials.43 This result strongly suggests that the radiation resistance of the nanoparticle core is comparable with that of ordinary bulk materials. Consequently, the higher radiation resistance observed for fumed silica materials is solely attributable to the high specific surface of the system, which implies that a portion of the material is involved in the surface shell, and consequently it is not able to accommodate stable E0 γ defects. Previous works, focused on the AE380 material, attributed the irradiation resistance of fumed silica to the oxygen excess of the particles.2124 However, as shown in Figure 5, the O2 content decreases on increasing the
’ CONCLUSIONS The EPR investigation presented in this work shows that the E0 γ center concentration induced by irradiation in fumed silica exhibits a strong dependence on the nanometric nature of this material, linearly decreasing with the specific surface. Notwithstanding, we showed that the E0 γ center line shape is the same in all the fumed silica types considered and is characterized by an orthorhombic symmetry of g tensor, analogous to that typically observed in bulk silica materials. Besides, we verified by Raman/ PL measurements that the higher content of O2 molecules in fumed silica as compared with bulk silica materials is not responsible for the irradiation resistance of the fumed silica network. The reported findings are interpreted by a shell-like model of the silica nanoparticles, in which the E0 γ centers are mainly induced in the core region of the particles. It clarifies that the silica nanoparticle radiation sensitivity scales with the particle diameter due to the fact that not the whole volume of the particles is able to generate stable E0 γ centers by irradiation. Finally, by our experiments we were able to estimate a thickness of 1 nm for the surface shell of the nanoparticles, independently of their mean diameter. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. 148
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’ ACKNOWLEDGMENT The authors thank the people of the LAMP group (http:// www.fisica.unipa.it//amorphous) at the Physics Department of the University of Palermo for useful discussions and technical assistance by G. Napoli and G. Tricomi. Partial financial support by POR Sicilia 2000/2006 Misura 3.15-Sottoazione C and by the Romanian Ministry for Education, Research, Sport and Youth in the frame of the research grant 12084/2008 is acknowledged. The work was done in the frame of the bilateral collaboration existing between University of Palermo and National Institute for Laser, Plasma and Radiation Physics in Bucharest.
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dx.doi.org/10.1021/jp2073842 |J. Phys. Chem. C 2012, 116, 144–149
