Entrapping of O2 Molecules in Nanostructured Silica Probed by

Article pubs.acs.org/JPCC

Entrapping of O2 Molecules in Nanostructured Silica Probed by Photoluminescence A. Alessi,* G. Iovino, G. Buscarino, S. Agnello, and F. M. Gelardi Dipartimento di Fisica e Chimica, Università degli Studi di Palermo, Via Archirafi 36, I-90123 Palermo, Italy ABSTRACT: We studied the emission of the O2 molecules embedded in fumed silica (amorphous silicon dioxide) nanoparticles differing for diameters and specific surface. By using a 1064 nm laser as a source we recorded both the O2 emission and the Raman signal of silica. Our experimental data show that the O2 emission/Raman signal (at 800 cm−1) ratio decreases with increasing the specific surface for both the asreceived and the loaded samples. By performing a thermal treatment (600 °C for 2 h) we modified the structure and the water content of the smallest nanoparticles without observing any significant change in the O2 emission/Raman signal ratio. Our data are explained by a shell model showing that the O2 emission is essentially due to the molecules entrapped in the core of the nanoparticles, whereas the contribution due to the surface shell, having a thickness of about 1 nm, is negligible because of its high content of Si−OH groups that introduce nonradiative relaxation channels or because of the very low content of molecules trapped in this thin region.

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INTRODUCTION The interest for nanomaterials with emission properties is continuously growing due to the great number of applicative opportunities that they provide.1,2 Among these materials a relevant role is covered by SiO2-based nanostructures and nanoparticles due to their chemical and physical properties,2,3 the versatility and availability of the root material, and simplicity of preparation.2 A possible pathway to obtain luminescent SiO2-based materials is constituted by the entrapping of luminescence molecules as the O2. On these bases, recently a significant number of investigations were focused on the characterization of the O2 molecules in bulk and nanosilica (amorphous silicon dioxide) materials.4−11 In general, wide interest was devoted to the O2 molecule electronic excited states, optical properties, and interaction with neighboring molecules since they have a fundamental role in physical, chemical, and biological processes.3,12,13 It is well-known that the decay from the excited singlet state (1Δg) toward the triplet ground state of O2, 3Σg (0), originates a near-infrared (NIR) phosphorescence14 and that a quenching of emission from interacting molecules can take place.4,12,15 The lifetime of this emission is about one hour for the unperturbed O2,14 whereas it diminishes in embedded O2 depending on its surrounding environment.15−18 In bulk silica such molecules can be interstitial in the network as a consequence of the material production technique,8 as an irradiation product,9 or as a consequence of thermal treatments performed in a controlled atmosphere (loading process).10,11 These studies showed that in bulk silica the O2 emission is peaked at about 1272 nm and that its lifetime is about 0.84 s,4 whereas it decreases in the silica nanoparticles to about 0.46 s © 2013 American Chemical Society

(in the biggest nanoparticles) and 0.3 s (smallest nanparticles), maintaining its spectral position.7 For the data reported in the following, it is important to summarize the effects of the Si−OH groups on the emission of the O2 molecules embedded in silica materials. We remind that it was reported that the lifetime of the O2 in bulk samples with Si−OH contents below 1017 cm−3 is ∼0.84 s and that it monotonically decreases as the Si−OH concentration increases.4 In the same study the authors suggested that the Si−OH groups whose distances from O2 are less than 1 nm accelerate the nonradiative decay from the excited O2 molecules.4 In the present investigation we study the influence of the nanoparticle structure and of the water content on the O2 emission measured before and after the loading process. Furthermore, we investigate the emission of O2 as a function of the particle diameter, and we propose an explanation for the different O2 emission/silica Raman signal ratio already observed.7

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EXPERIMENTAL SECTION Commercial aerosil fumed silica samples produced by Evonik Industries were used. They were synthesized by SiCl4 oxidation in an O2/H2 flame at 1100−1400 °C. The starting powders of nanoparticles were pressed in a uniaxial mechanical press at about 0.3 GPa to obtain tablets. In Table 1 we report the Received: October 18, 2012 Revised: January 21, 2013 Published: January 22, 2013 2616

dx.doi.org/10.1021/jp310314t | J. Phys. Chem. C 2013, 117, 2616−2622

The Journal of Physical Chemistry C

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Table 1. Nickname (Name), Specific Surface, and Particle Diameter nickname (name) AEOX50 (AerosilOX50) AE90 (Aerosil90) AE150 (Aerosil150) AE200 (Aerosil200) AE300 (Aerosil300) AE380 (Aerosil380)

specific surface

particle average diameter

m2/g

nm

± ± ± ± ± ±

40 20 14 12 7 7

50 90 150 200 300 380

15 15 15 25 30 30

nickname, name of the samples, their specific surface (S), and the primary particle mean diameter (the distributions of the particle diameters can be found in an industry report19). The specific surface and the particle diameter were estimated by the physical absorption of N2 (BET method) and transmission electron microscopy measurements, respectively.19 For some aspects of the present work, it is worth noting that the nanoparticles were prepared about four years ago and that they were kept in ambient atmosphere. Raman spectra were recorded at room temperature by using a Bruker RAMII Fourier transform Raman spectrometer supplied with a 500 mW Nd:YAG laser at 1064 nm. The applied experimental conditions lead to a spectral resolution of 5 or 15 cm−1 (in this case the acquisition time of the spectrum is reduced allowing a more precise investigation of the O2 content as a function of the time). IR spectra were obtained at room temperature with a nitrogen-purged Bruker spectrophotometer (model VERTEX70) with a spectral resolution of 1 cm−1. To eliminate the effect of residual water in air, the absorption spectrum of the empty beamline was subtracted from the spectrum of each sample, after suitable normalization. To detect eventual morphological modifications of the nanoparticles,20 we acquired tapping mode amplitude modulation AFM measurements by using a Multimode V (Veeco Metrology) scanning probe microscope. It was equipped with a piezoscanner (maximum xy range ∼14 μm and maximum z range ∼3.6 μm) and a four-segment photodetector for cantilever deflection monitoring. PointProbePlus Silicon-SPMprobes were employed with Al backside reflex coating, resonance frequency ∼300 kHz, and tip apical diameter ∼10 nm. All the scans were executed at room temperature in N2 atmosphere (flux ∼2 L/min). The inert gas ambient was required to decrease the probability of adhesion of the nanoparticles to the tip. The oxygen molecules were inserted in the nanoparticles using thermal treatments at fixed temperatures and pressures of a pure O2 atmosphere with maximum time of 5 h. This procedure is called loading in the following. Before the O2 loading the as-received samples were put in a furnace at the temperature of 300 °C for 5 min to unload the O2 present in the as-received nanoparticles.21 Another set of samples was thermally treated in air for 2 h at the temperature of 600 °C before O2 loading to induce structural modifications and water drying of the nanoparticles.22

Figure 1. Raman spectra recorded in the AEOX50 (black line), AE150 (dark gray line), and AE300 (light gray line), before any treatment (panel a) and after O2 loading at 130 °C, 75 bar (panel b). The inset of panel a shows a zoom of the spectral range 300−700 cm−1.

stretching.23,24 We note that the shape of this band is equal for each investigated sample. This normalization provides the possibility to compare the different spectra since it was shown that for fixed scattering volume25,26 the 800 cm−1 band amplitude is independent from the sample after the division of the spectra for the densities of various samples. This indicates that the 800 cm−1 band allows us to normalize for the masses of the samples, which determine also the Raman amplitude of the other band and of the O2 emission since they are inside the nanoparticles. Furthermore, such normalization allows overcoming the lack of control on the scattering volume. In all the spectra we note the presence of the silica related bands.23−27 The inset of Figure 1a shows the zoom of the spectral range 300−700 cm−1. These data highlight the differences in the Raman spectra of the different silica nanoparticles. More in detail, with increasing the specific surface we note a shift of the R band (at ∼440 cm−1, Si−O−Si vibration23) toward higher energies and an increase of the relative amplitudes of the D1 (at ∼495 cm−1, four-membered ring23) and D2 (at ∼605 cm−1, three-membered ring23) bands. Furthermore, at ∼980 cm−1 we observe the presence of the Raman band that was related to the Si−OH group in fiber and bulk silica materials.28,29 It is important to note that the amplitude of the Si−OH related band depends on the specific surface. Such dependence will be more deeply examined in the following of this section.

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RESULTS In Figure 1a we report the Raman spectra of the AEOX50, AE150, and AE300 nanoparticles before any treatment. All the spectra were normalized to the amplitude of the Raman band peaked at 800 cm−1, attributed to the Si−O−Si symmetric 2617



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In addition, at about 1540 cm−1 we observe the presence of the O2 luminescence activity as clearly visible in Figure 1a.5−11 The relative amplitude of this band decreases with increasing the specific surface. It is important to note that the amplitude of the O2 emission measured in the spectra recorded for the AEOX50 and AE380 is equal, within the experimental error, to the one measured in the spectra previously reported7 and recorded about 2 years ago. This suggests that the O2 emission of the samples kept in ambient atmosphere reached an equilibrium value. The as-received samples were then O2 loaded at different temperatures and pressures for different times. Fixing the temperature and the pressure for each sample the O2 emission reaches a maximum value as a function of the time. Moreover, in all the samples, by changing the pressure the O2 emission increases linearly increasing the pressure, and then it tends to a limit value that is almost independent from the loading temperature in the range 100−130 °C. Further and more detailed studies will be performed to characterize the dependence on the loading conditions and to more deeply understand the kinetic features, whereas in the present investigation we focus on the maximum values of such kinetics and on the equilibrium values observed in the asreceived samples kept in ambient atmosphere for long times (about 4 years). Figure 1b illustrates the Raman spectra recorded when the O2 emission tends to be independent in each sample from the loading time and pressure. We note that the ratio between the amplitude of the O2 emission and the 800 cm−1 depends on the nanoparticle type. More in detail, similarly to the unloaded samples, it decreases, increasing the specific surface. In Figure 2 we report the area of the Si−OH group Raman activity as a function of the specific surface and a linear fit with

reported, this treatment induces structural changes of the nanoparticles.22 The effect on the AE300 is illustrated by Figure 3. After the thermal treatment, the R band peak shifts to lower

Figure 3. Raman spectra of the as-received AE300 (black line) and AEOX50 (dark gray line) samples and the AE300 (light gray line) sample thermally treated for 2 h at 600 °C in ambient atmosphere.

energy values, becoming similar to the one observed in the AEOX50 nanoparticles, and we also observe a significant increase of the D2 band. Because of its partial overlap with the R band, it is more complicated to clearly understand if the D1 amplitude changes. Together with these variations, we note that the Raman activity of the Si−OH groups decreases as a consequence of the thermal treatment. The thermal treatment induces also a strong decrease of the H2O molecule content, as illustrated by the decrease of the IR absorption activity in the range 5000−5500 cm−1 (see insets of Figure 4) where the absorption bands related to combinations of the fundamental vibrational modes of the water molecules are located.30 The water absorption increases, keeping the sample in ambient atmosphere after 600 °C treatment; in fact, after 12 min the absorption activity at 5250 cm−1 is about twice the one observed after 2 min, and after 1 week it goes back to half the value measured before any thermal treatment. We remark that, in agreement with the Raman measurements, the IR spectrum (Figure 4) indicates a decrease of the Si−OH groups’ content, which is, however, lower than the one of the water molecules. Clear examples of this peculiarity are the behavior of the free Si−OH groups responsible for the narrow IR absorption band at about 3740 cm−131 and the comparison of the IR absorption activities in the range 4200− 4800 cm−1 where the combinations of the vibrational modes of the Si−OH are located.30 The same oxygen loading conditions were chosen to compare the O2 emission in as-received and pretreated (dried and structural modified) samples, when O2 emission amplitude reaches about its maximum in both types of samples. For this reason we used a temperature of 100 °C, a pressure of 75 bar, and different times, from 1 to 5 h, to widen the tests. Figure 5 clearly evidences that, comparing the data of the asreceived and pretreated samples, there are not significant differences in the O2 loaded molecules. In the insets of the

Figure 2. Area of the 980 cm−1 Raman band as a function of the specific surface of the nanoparticles (●). A linear fit is superimposed to the data.

intercept of 10 ± 3 and a slope of (0.31 ± 0.04) g/m2. From this data one can infer that the great part of the Si−OH group is located in the nanoparticle surface. To more deeply understand the differences in the O2 band amplitude we performed thermal treatment of the AE300 nanoparticles at 600 °C for two hours, and then we performed the O2 loading of the thermally treated sample. As already 2618



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Figure 4. IR spectra of the as-received AE300 (black line) and of the AE300 sample thermally treated for 2 h at 600 °C after 2 min (light gray line) and after 12 min (dark gray line) in ambient atmosphere. Inset (a): zoom of the spectral range 4000−5500 cm−1. Inset (b): water absorption bands after the subtraction of a background in the two thermally treated samples.

Figure 6. Top: AFM image recorded for the AE300 nanoparticles before any treatments. Bottom: AFM image recorded on the AE300 nanoparticles after a thermal treatment of 14 h at 600 °C.

14 h at 600 °C) do not induce significant sintering processes. Such a conclusion, expected on the basis of previous studies,20 is also supported by the lack of significant variations in the size distribution obtained from the quantitative analysis of the AFM images recorded for the two samples.

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DISCUSSION The data of Figures 3 and 5 indicate that strong modifications in the SiO2 associated bands of Raman spectra of the nanoparticles do not imply a great difference in the value of O2 molecules that can be incorporated into the nanoparticle network. As a consequence, the differences in the O2 emissions reported in Figure 1 can not be justified on the basis of the structural differences evidenced by the Raman spectra of the various nanoparticles. As previously reported,30 we observed a relevant content of physisorbed water. The adsorption of the water on the silica surface was previously investigated, and it was proposed that it starts near surface Si−OH groups and proceeds, forming clusters of water molecules around these sites.30 For the present investigation, it appears important to understand if the water molecules inside the nanoparticles can occupy a significant number of network interstices, subtracting them to the O2 molecules. From this point of view we note two previous results regarding the water diffusion in silica materials: (i) coefficient of diffusion is much larger at 600 °C than at room temperature32,33 and (ii) the equilibrium content of water molecules dissolved in the silica network is larger at room temperature than at 600 °C.32,33 So the thermal treatment is expected to be able to significantly decrease not only the water content outside (layers of water on the surface) but also the one trapped in different parts inside the nanoparticles. We noted that the few minutes (∼2) necessary to start the O2 loading, in which the sample is kept in ambient atmosphere, are not sufficient to recover the H2O released by the thermal treatment. Despite this fact we did not observe variation of the O2 loaded value. On the basis of these considerations, it can be

Figure 5. Raman spectra of the AE300 samples after O2 loading at 100 °C of (black line) as-received and (light gray) pretreated at 600 °C for 2 h in ambient atmosphere samples. Inset (a): ratio between the O2 emission amplitude and the Raman signal amplitude at 800 cm−1 as a function of the loading time. Inset (b): ratio between the O2 emission amplitude and the Raman signal amplitude of the R band as a function of the loading time.

same figure we report the ratios between the O2 emission amplitude and the 800 cm−1 (inset a) and the R band (inset b) amplitudes, respectively, as we observed a variation of the ratio between the two silica related bands. To be sure that the thermal treatment does not induce sintering processes on the AE300 nanoparticles, 20 we performed a longer thermal treatment (14 h at 600 °C) and compared the AFM images recorded for this sample with the one recorded for the untreated sample. These data, reported in Figure 6, suggest that the employed thermal treatments (2 or 2619



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To explain our data we consider a shell model, which divides the particles in the core and in the surface zones. In this framework the recorded Raman spectra are the results of the sum of the Raman signal of the core and of the surface so one can write the following equation m m IRP = C IRC + S IRS mP mP (1)

supposed that for nanoparticles kept in air and within the tested loading conditions the H2O content, inside different parts of the particles, does not affect at all the maximum value of the O2 emission. The large variation of the water-related IR absorption suggests that the great part of the water molecules is adsorbed on the exterior part of the particle surface.30 At variance, for the Si−OH groups (especially the free ones), the difference before and after the thermal treatment appears inferior, and it requires further considerations. Anyway, the increase of the D2 band and the decrease of the Si−OH groups (principally located on the surface) can suggest that in the surface an increase of the three-membered ring concentration takes place.22 Our experimental data show that the O2 emission amplitude before and after loading decreases with increasing the specific surface and that the relative amplitudes (O2 emission/800 cm−1) depend on the specific surfaces in almost the same way in as-received and loaded specimens. An example of this similarity is reported in Figure 7a, which illustrates the data

where IPR and mP are the Raman spectrum of the nanoparticle and its mass, whereas ICR , mC, ISR, and mS are those of the core and of the surface shell, respectively. If, as in our case, the laser source and detection system permit us to measure in the same spectrum the O2 emission and the Raman signal of SiO2, one can write the following equation m C m S P Oemission = C Oemission + S Oemission mP mP (2) where OPemission, OCemission, and OSemission are the (O2 PL)/800 cm−1 ratios of the particles, of the core, and of the surface shell, respectively, so they stand for the PL for silica mass unit. Before continuing in the analysis of our data, it is important to recall the main results obtained by the simulation.34 In that investigation the authors found that the particles can be divided in three zones named core, transition, and surface shell. Furthermore, the structures of these zones and the thicknesses of the transition (0.3 nm) and surface (0.5 nm) shells are independent from the particle sizes. In comparison with the present analysis the difference stands in the number of zones used to describe the nanoparticle, but it does not represent a great problem if our surface shell is considered as the sum of the surface and the transition zones of the simulative investigation.34 Now if we consider that decreasing the nanoparticles sizes the relative mass (mS/mP) of the surface shell increases because its thickness is constant,34 our data indicate that for the O2 emission the surface shell contribution is much lower than the one of the core and can be neglected, so eq 2 can be rewritten as

Figure 7. (a) O2 emission amplitudes recorded in the various samples before (●) and after loading (○) normalized to the value measured in the AEOX50. (b) O2 emission amplitude as function of the specific surface (●) and fit law A(1 − B × S) (gray line).

⎛ m ⎞ P C Oemission ≈ Oemission ⎜1 − S ⎟ mP ⎠ ⎝

(3)

P C Oemission ≈ Oemission (1 − ρS × δ × S)

(4)

where the term VSS/mP (VSS stands for surface shell volume) was approximated to δ × S, with S, δ, and ρS being the specific surface, the thickness, and the density (both independent from the nanoparticle sizes34) of the surface shell. This approximation is valid for a small value of δ which appears to be applicable to our data, since the data of Figure 7 can be described by a linear law, although such an approximation can induce an error that increases decreasing the nanoparticle radius, and that is not considered in the present investigation. For these reasons we fitted the experimental data using the law (gray curve of Figure 7a): A(1 − B × S). More in detail, by using this law we obtained A ∼ 7 and B ∼ 2.46 × 10−7 g/cm2. Now since the surface shell density should be higher (at maximum ∼20%34) than the core one, using a density of about 2.43 g/cm3 (10% larger than normal silica) we can obtain a reasonable value of the thickness of the surface shell, which is ∼1 nm. It appears important to note that this estimation is in good agreement with that previously reported35 using electron paramagnetic resonance data and with the value of 0.8 nm

recorded in both cases as a function of the specific surface. The data are normalized to the signal of the AEOX50 to plot and compare the ratios between the different nanoparticles before and after loading. To obtain information on the O2 contents from the emissions observed in the different nanoparticles, it is necessary to consider the quantum yield Φ = kr × τ where kr and τ are the radiative decay rate and the emission lifetime, respectively.6 In this context it is important to note that for the emission of the interstitial O2 in silica kr can be considered as a constant.5,6 Furthermore, the AEOX50 and the AE380 nanoparticles were O2 loaded up to O2 emission/800 cm−1 values comparable with the ones here reported, and the lifetimes were measured.7 It was found that in the AEOX50 the lifetime is ∼0.46 s, whereas in the AE380 it is ∼0.32 s. By taking this into account for the other nanoparticle types, we used an average lifetime value of ∼0.39 s. The data of the as-received samples corrected for the lifetime are reported in Figure 7b. 2620



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even more interesting and valid in the application fields, it is necessary to find other parameters that are more effective in inducing changes of the intensity of the luminescence. By using a shell model we can explain our data with the following scheme: the emission signal recorded is attributed essentially to the O2 molecules inside the core of the nanoparticles, whereas the contribution from the surface shell, having a thickness of about 1 nm, is about 1 order of magnitude lower because of the high Si−OH group content of the surface shell or because a very low content of O2 molecules can be trapped in the surface shell.

obtained by the sum of thicknesses of the transition and surface zone reported in the simulative investigation.35 We note that for all the nanoparticles, but for the AE380, by using the geometrical estimation of the surface volume in spherical approximation the expressions (VSS/VP) = (r3 − (r − δ)3)/r3 and ρP = (VC/VP)ρC + (VSS/VP)ρS should be inserted in eq 3 to obtain eq 5, where r is the average nanoparticles radius obtained from Table 1. P Oemission

≈

⎛ ⎜

C ⎜1 Oemission

⎜⎜ ⎝

r 3 − (r − δ)3 r3

− 1+

r 3 − (r − δ)3 r3

×

(

ρS ρC

⎞ ⎟ ⎟ − 1 ⎟⎟ ⎠

ρS

ρC

)

■

(5)

AUTHOR INFORMATION

Corresponding Author

By fitting (see Figure 8) the data with the obtained new equation and considering ρS/ρC =1.1, one obtains a surface

*E-mail: [email protected]. Phone: 003909123891761. Fax: 00390916162461. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the people of the LAMP group (http://www.fisica.unipa.it/amorphous/) for useful discussions. Technical assistance by G. Napoli and G. Tricomi is acknowledged. The authors acknowledge the financial support provided by Assessorato Regionale BB.CC.AA.

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REFERENCES

(1) Feldmann, C. Nanoscale 2011, 3, 1947−1948. (2) Schärtl, W. Nanoscale 2010, 2, 829−843. (3) Couleaud, P.; Morosini, V.; Frochot, C.; Richeter, S.; Raehm, L.; Durand, J.-O. Nanoscale 2010, 2, 1083−1095. (4) Kajihara, K.; Kamioka, H.; Hirano, M.; Miura, T.; Skuja, L.; Hosono, H. J. Appl. Phys. 2005, 98, 013528. (5) Kajihara, K.; Hirano, M.; Uramoto, M.; Morimoto, Y.; Skuja, L.; Hosono, H. J. Appl. Phys. 2005, 98, 013527. (6) Kajihara, K.; Kamioka, H.; Hirano, M.; Miura, T.; Skuja, L.; Hosono, H. J. Appl. Phys. 2005, 98, 013529. (7) Agnello, S.; Cannas, M.; Vaccaro, L.; Vaccaro, G.; Gelardi, F. M.; Leone, M.; Militello, V.; Boscaino, R. J. Phys. Chem. C 2011, 115, 12831−12835. (8) Skuja, L.; Guttler, B. Phys. Rev. Lett. 1996, 77, 2093−2096. (9) Skuja, L.; Guttler, B.; Schiel, D.; Silin, A. R. Phys. Rev. B 1998, 58, 14296−14304. (10) Nuccio, L.; Agnello, S.; Boscaino, R. Appl. Phys. Lett. 2008, 93, 151906. (11) Kajihara, K.; Miura, T.; Kamioka, H.; Hirano, M.; Skuja, L.; Hosono, H. Appl. Phys. Express 2009, 2, 056502. (12) Kearns, D. R. Chem. Rev. 1971, 71, 395−427. (13) Krasnovsky, A. A. J. Membr. Cell. Biol. 1998, 12, 665−690. (14) Schweitzer, C.; Schmidt, R. Chem. Rev. 2003, 103, 1685−1758. (15) Carraway, E. R.; Demas, J. N.; DeGra, B. A. Langmuir 1991, 7, 2991−2998. (16) Cantau, C.; Pigot, T.; Manoj, N.; Oliveros, E.; Lacombe, S. Chem. Phys. Chem. 2007, 8, 2344−2353. (17) Schmidt, R. Chem. Phys. 2004, 304, 315−316. (18) Wessels, J. M.; Rodgers, M. A. J. J. Phys. Chem. 1995, 99, 17586−17592. (19) Basic Characteristics of Aerosil, 4th ed.; Degussa: Frankfurt, 2001. (20) Buscarino, G.; Ardizzone, V.; Vaccaro, G.; Agnello, S.; Gelardi, F. M. J. Appl. Phys. 2010, 108, 074314. (21) Iovino, G.; Agnello, S.; Gelardi, F. M.; Boscaino, R. J. Phys. Chem. C 2012, 116, 11351−11356. (22) Vaccaro, G.; Agnello, S.; Buscarino, G.; Gelardi, F. M. J. Phys. Chem. C 2010, 114, 13991−13997. (23) Galeener, F. L.; Geissberger, A. E. Phys. Rev. B 1983, 27, 6199− 6204.

Figure 8. O2 emission amplitudes recorded in the various samples before loading (●) as a function of the specific surface and fit (gray line) with eq 5; see the text.

shell thickness of 1.3 nm and an OCemission of about 8, which are in agreement with the values obtained using eq 4. We also note that the use of the average nanoparticles radius does not consider the size distribution, whereas the specific surface obtained by the BET measurements reflects the entire distribution of the nanoparticle size for each typology. At this point one could explain the lack of the contribution of emission from the surface shell oxygen using the high content of Si−OH groups ∼2.5 for nm2 in the surface,19 which, as above-reported, introduces a nonradiative channel if the Si− OH is at a distance of about 1 nm.4 Otherwise, one could tentatively suggest that there is not a significant content of molecular oxygen in this part of the nanoparticles. To be confirmed, such a hypothesis needs further investigations: the study of irradiation products of the O2, other emissionindependent measurements of the O2 content, and the extension to nanoparticle types differing for production technique.

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CONCLUSIONS We studied the emission of O2 molecules trapped in silica nanoparticles with different diameters (from 7 to 40 nm) and specific surface. Our data suggest that in both the as-received and the loaded samples the O2 emission signal decreases by increasing the specific surface. We changed the structure and the H2O content of one of the small nanoparticles without observing an increase of the O2 emission. Consequently, to increase the O2 emission amplitude and make these systems 2621



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(24) Liang, Y.; Miranda, C. R.; Scandolo, S. J. Chem. Phys. 2006, 125, 194524. (25) Burgin, J.; Guillon, C.; Langot, P.; Vallée, F.; Hehlen, B.; Foret, M. Phys. Rev. B 2008, 78, 184203. (26) Hehlen, B. J. Phys.: Condens. Matter 2010, 22, 025401. (27) Galeener, F. L.; Lucovsky, G. Phys. Rev. Lett. 1976, 37, 1474− 1478. (28) Stolen, R. H.; Walrafen, G. E. J. Chem. Phys. 1976, 64, 2623− 2631. (29) Hartwing, C. M.; Rahn, L. A. J. Chem. Phys. 1977, 67, 4260− 4261. (30) Burneau, A.; Barres, O.; Gallas, J. P.; Lavalley, J. C. Langmuir 1990, 6, 1364−1372. (31) Humbert, B.; Burneau, A.; Gallas, J. P.; Lavalley, J. C. J. NonCryst. Solid 1992, 143, 75−83. (32) Diffusion of reactive molecules in solids and melts; Doremus, R. H., Ed.; John Wiley and Sons, Inc.: New York, 2002. (33) Zouine, A.; Dersch, O.; Walter, G.; Rauch, F. Phys. Chem. Glasses: Eur. J. Glass Sci. Technol., Part B 2007, 48, 85−91. (34) Roder, A.; Kob, W.; Binder, K. J. Chem. Phys. 2001, 114, 7602− 7614. (35) Vaccaro, G.; Buscarino, G.; Agnello, S.; Sporea, A.; Oproiu, C.; Sporea, D. G.; Gelardi, F. M. J. Phys. Chem. C 2012, 116, 144−149.

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Entrapping of O2 Molecules in Nanostructured Silica Probed by

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