Effects of Pressure, Thermal Treatment, and O2 Loading in MCM41

J. Phys. Chem. C , 2015, 119 (49), pp 27434–27441 ... Publication Date (Web): November 23, 2015. Copyright © 2015 American ... Phone: +390912389170...
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Effects of Pressure, Thermal Treatment, and O2 Loading in MCM41, MSU-H, and MSU‑F Mesoporous Silica Systems Probed by Raman Spectroscopy A. Alessi,* G. Buscarino, S. Agnello, F. Messina, L. Sciortino, M. Cannas, and F. M. Gelardi Department of Physics and Chemistry, University of Palermo, Via Archirafi 36, I-90123 Palermo, Italy ABSTRACT: We present a Raman study of the effects induced by pressure, thermal treatments, and O2 loading in MCM41, MSU-H, and MSU-F representative mesoporous silica. We compared the starting powders with the mechanically pressed tablets produced applying pressures of ∼0.2 and ∼0.45 GPa. The spectra of the three untreated tablets evidence that the main value of the Si−O−Si angle decreases and that in the MCM41 and the MSU-H Si−O−Si hydrolysis occurs, whereas such a process is absent or much less efficient in the MSU-F. Despite their different networks, the three powders tend to crystallize in cristobalite when treatments are at 1000 °C. The MCM41 and MSU-H tablets exhibit behavior similar to their starting powders, whereas the MSU-F tablets tend to form the tridymite crystalline phase. Such a finding could be related by the differences in the Si−O−Si hydrolysis, occurring during the tablets production. Finally, we inserted O2 molecules within the interstices of the SiO2 wall only in MSU-F. By monitoring the O2 content as a function of the delay time from the loading end, we provide evidence that the O2 remains in the interstices for a sufficiently large time to modify the material properties by inserting small molecules and not only through the surface functionalization.

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INTRODUCTION In the last years there was a great interest in the synthesis of mesoporous silica systems.1−7 These materials can be functionalized and employed in different technology fields: they can be used for catalysts,8,9 as adsorbents,5 for fixing bioactive molecules such as enzymes and drugs10,11 or other organic molecules useful for uranium extraction,12 and as hard templates or fillers.10 For all these materials mechanical and/or thermal stability is relevant for applications.13,14 These two physical properties, for MCM-41, have been previously studied.13,14 Additional investigation regarding the thermal, hydrothermal, and mechanical stabilities was reported15 also for MCM-48, HMS, FSM-16, KIT-1, PCH, and SBA-15. Such investigations evidenced that these materials collapse at the pressure of about 0.40 GPa. Furthermore, in these systems different thermal stabilities were highlighted since such a property is affected by the wall thickness as well as by the silica type employed as a precursor during synthesis.15 It is also worth noting that for MCM41, MCM48, and FSM-16 some previous investigations16−18 based on IR (infrared adsorption), XRD (Xray diffraction), BET analysis, and NMR (nuclear magnetic resonance) suggested that under pressure in air the collapse of such ordered structure, with thin walls, takes place as a consequence of the Si−O−Si hydrolysis (Si−O−Si + H2O → 2Si−OH).17,18 Nevertheless, the modifications undergone by the walls seem not to be deeply understood yet. In addition, it appears interesting to study the effects of successive thermal treatments to highlight differences among the as-received © 2015 American Chemical Society

powders and the mechanically produced tablets and to determine possible phase changes of SiO2. In fact, some previous investigations on tablets of silica nanoparticles19 or on MCM41 systems14 evidenced the possibility to obtain crystallization in phases as cristobalite and tridymite. This kind of topic not only can be interesting for basic research but also can be notable for some applications.20 In addition, considering the small thickness of such walls one could study the entrapping of small molecules inside the wall. This type of investigation can be useful for basic research since the mesoporous systems are ideal to study the properties of thin layers of amorphous matter. On the other hand, such studies can be useful also for applicative reasons since they could reveal further possibilities to tune the features of the final material adapting not only the surface but also the interior part of the walls. An interesting case for this type of investigation is constituted by the study of the entrapping process of the O2. Such a molecule, in fact, is fundamental in many physical, chemical, and biological processes. 21,22 Furthermore, O2 molecules exhibit a near-infrared emission, related to the transition from the first excited singlet state to the ground state,23 which could be of interest for probing applications, for example, to monitor the mesoporous material motion inside the biological tissues in drug deliverers. This emission was Received: August 10, 2015 Revised: November 23, 2015 Published: November 23, 2015 27434

DOI: 10.1021/acs.jpcc.5b10206 J. Phys. Chem. C 2015, 119, 27434−27441

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Figure 1. Raman spectra recorded for MCM41 (panel a), for MSU-H (panel b), and for MSU-F (panel c) samples; in all panels from top to bottom, powder and tablet pressed at 0.2 and at 0.45 GPa are separately shown, whereas the black line indicates the peak position of the R band in the asreceived powders. All the spectra are vertically shifted for viewing purposes. Comparison of the Raman spectra recorded in the starting powders and in the tablets obtained with a pressure of 0.45 GPa are reported in (d) MCM41 samples, zoom of the range 250−650 cm−1 (black line) powder and tablet (gray line); (e) MCM41 samples, zoom of the range 850−1350 cm−1 (black line) powder and tablet (gray line); (f) MSU-H samples, zoom of the range 250−650 cm−1 (black line) powder and tablet (gray line); (g) MSU-H samples, zoom of the range 850−1350 cm−1 (black line) powder and tablet (gray line); (h) MSU-F samples, zoom of the range 250−650 cm−1 (black line) powder and tablet (gray line); (i) MSU-H samples, zoom of the range 850−1350 cm−1 (black line) powder and tablet (gray line).

studied in bulk silica materials23−25 and in nanoparticles26−28 but not in mesoporous silica. For the data reported in the following it is important to underline that these previous studies evidenced that (i) the emission can be excited by using a 1064 nm laser source and (ii) it is possible to detect the O2 emission at about 1272 nm with a Raman spectrometer, which reveals it as a line at ∼1540 cm−1. Beyond the archetypal mesoporous silica MCM41, having a hexagonal structure,1 in recent times, large pore hexagonal and cellular foam-like mesoporous materials named MSU-H and MSU-F, respectively, were produced under neutral pH conditions.29 These materials appear to be attractive for commercial production of mesostructured silicas29,30 since for their synthesis low-cost sodium silicates are employed as a source of silica29 and since they do not need any postsynthesis treatments, in contrast to other cases.30 In addition, the three materials present some similarities or differences in the structure and in the synthesis, which can be used to evidence the occurrence or the absence of relation between these features and the thermal and pressure effects. In the present study we report an experimental investigation focused on the effects of pressure and temperature on MCM41, MSU-H, and MSU-F materials, highlighting the pressureinduced processes and the phase changes induced at 1000 °C in both as-received and pressed materials. We further evidence that it is possible to insert O2 molecules on the MSU-F pore walls31 and that a relevant amount of the O2 content measured after the insertion is still present after 1 week. This result

suggests the possibility to use this route as a further method to manipulate the physical-chemical features of this material. Finally, we evidenced various other peculiar features of the MSU-F materials as compared to the other mesoporous systems. We also note that the present investigation is based on Raman scattering measurements which, differing from those recorded with the above cited experimental techniques, are poorly available in previous studies, are not invasive, and enable a fast study of structural aspects of the materials.

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EXPERIMENTAL SECTION

For the present study we used three different commercial mesoporous materials (MCM41, MSU-H, and MSU-F) acquired from Sigma-Aldrich (Italy). The MCM41, hexagonal type, has a unit cell of 4.5−4.8 nm, pore size of 2.1−2.7 nm, and specific surface of about 1000 m2/g. The MSU-H, hexagonal type, has a unit cell of ∼11.6 nm, pore size of ∼7.1 nm, and specific surface ∼750 m2/g. The MSU-F, cellular foam type, has a unit cell of ∼22 nm, cell window of ∼15 nm, and specific surface ∼562 m2/g.32 For each of these materials we investigate both the asreceived powders and tablets produced by using an axial press. For each material, two tablets were obtained by using ∼0.2 and ∼0.45 GPa, respectively. We acquired Raman spectra at room temperature using a Bruker RAMII Fourier transform Raman spectrometer pumped with a 500 mW Nd:YAG laser at 1064 nm. The applied experimental conditions lead to a spectral resolution of 5 cm−1. 27435

DOI: 10.1021/acs.jpcc.5b10206 J. Phys. Chem. C 2015, 119, 27434−27441

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The Journal of Physical Chemistry C We carried out thermal treatments at 1000 °C in air for 5 and 272 h. For the shorter treatment, the samples were inserted in the oven at the temperature of 200 °C, and the temperature increase was set to reach 1000 °C in 200 min. After 5 h at 1000 °C the samples were kept inside the oven during its natural cooling time down to room temperature, which takes place in about 9 h. For the longer thermal treatment (272 h) the samples were inserted in the oven at room temperature; the temperature of 1000 °C was reached in about 180 min; and the samples were kept in the oven for the required treatment time and successively during its natural cooling at room temperature for a time of about 9 h. Finally, we exposed some samples to O2 pure atmosphere (maximum pressure 70 bar) for about 11 h at the temperature of about 100 °C. This latter procedure is indicated as loading in the following.

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Figure 2. Raman spectra recorded after thermal treatments in air at 1000 °C for 5 h: (a) MCM41 0.2 GPa (dark gray line), 0.45 GPa (light gray line), and powders (black line); (b) MSU-H 0.2 GPa (dark gray line), 0.45 GPa (light gray line), and powders (black line), (c) MSU-F 0.2 GPa (dark gray line), 0.45 GPa (light gray line), and powders (black line); (d) Raman spectra, in the range 300−700 cm−1, of the starting MCM41 powder (dark gray line) and of the one treated at 1000 °C for 5 h (black line), and (e) Raman spectra in the range 900− 1300 cm−1 of the same samples.

RESULTS Pressure Effects. In Figure 1 we report the Raman spectra of the untreated powders of MCM41 (panel a), of MSU-H (panel b), and of MSU-F (panel c). All the spectra have been normalized to the amplitude detected at 800 cm−1 since this band features the smaller variations and since this procedure is coherent with previous investigations.26 The comparisons between the Raman spectra recorded for the three starting powders and the three tablets produced applying a pressure of 0.45 GPa are reported in the panels from d to i. In particular, panels d, f, and h illustrate the zoom of the spectral region 250−650 cm−1, whereas the panels e, g, and i illustrate the zoom of the spectral range 850−1350 cm−1. In the panels d and e we reported the spectra of the MCM41, in the panels f and g the ones acquired for the MSU-H, and in the panels h and i those recorded for the MSU-F samples. The red lines in panels a and c are the tangents to the Raman signal at about 570 and 650 cm−1 subtracted from the spectra to evaluate the relative (with respect to the one measured at 800 cm−1) amplitudes of the D2 band measured in the spectra of the different samples. In the spectra recorded for MCM41 we detected all the Raman bands associated with silica,33,34 although the fine spectral features differ from the ones observed in the bulk materials. In the MCM41 the R band, predominantly originated by the oxygen vibration along the Si−O−Si angle bisecting direction,33,34 is peaked at ∼420 cm−1 (see also Figure 1d and Figure 2). In this spectral range we also observe the presence of the D1 band and of the D2 band related to the breathing vibration of the four- and three-membered rings, respectively.35 At higher wavenumbers we detected two other bands at ∼800 and ∼980 cm−1. The former is a silica network vibration33−35 involving the motion of oxygen, defined symmetric stretching (SS)33 (also indicated as bending36,37), and a significant amount of motion of the Si,33,34 whereas the latter has been related to the OH vibration in the SiOH groups.38−41 Finally, in the spectral range 1060−1200 cm−1 where the silica Raman bands due to asymmetric stretching are found,33 we observe a single band peaked at ∼1100 cm−1 (see also Figure 1e). Notably, the spectra recorded for the tablets produced with ∼0.2 and ∼0.45 GPa strongly differ with respect to the one recorded for the powders. In particular, we note a blue shift of the R band and a decrease of the amplitude of the D2 that is more clearly evidenced from the zoom of the spectral range 250−650 cm−1, which is reported in Figure 1e. To quantify the

decrease of the D2 band we subtracted a tangent from the Raman signal at 570 and 650 cm−1. By performing this analysis we evaluated that the D2 relative amplitude is ∼1.3, ∼0.9, and ∼0.6 in the powder, in the 0.2 GPa, and in the 0.45 GPa tablets, respectively. As regards to the D1 band, the overlap with the R band, which shifts toward higher wavenumbers, prevents a quantitative analysis, but we guess that its amplitude is unchanged or slightly increased. Furthermore, we note a significant variation of the Raman signal in the range 900−1000 cm−1; in fact, near the 980 cm−1 band, whose amplitude is larger or unchanged, we clearly note an additional band at ∼920 cm−1, especially after 0.45 GPa pressure, which has been attributed to SiOH groups39−41 (the zoom of this spectral range is reported in Figure 1e). Finally, we note that on pressing the powder the 800 cm−1 band gets slightly narrower and that in the range of the silica asymmetric stretching we observe two bands at ∼1060 and ∼1170 cm−1 as highlighted by the zoom of Figure 1e. In Figure 1b the spectra acquired for the powders and the tables of the MSU-H are reported. In the spectrum of the powders the R band is peaked at ∼460 cm−1 (see also Figure 1f). Similarly to the MCM41, we note an intense Raman band at 980 cm−1 and a broad feature at about 1100 cm−1 (see also Figure 1g). The spectra recorded for the tablets evidence the blue shift of the R band and a decrease of the amplitude of D2, which is more clearly evidenced in Figure 1f. To quantify the decrease of the D2 band we used the above-reported procedure. In this case we evaluated that the D2 relative amplitude is ∼0.8, ∼0.6, and ∼0.4 in the powder, in the 0.2 GPa, and in the 0.45 GPa tablets, respectively. Similarly to the MCM41, the Raman signal, in the tablets of the MSU-H, in the range 900−1000 cm−1 (zoom reported in Figure 1g) increases, and two Raman bands are measured in the range of the asymmetric stretching. Finally, in Figure 1c we report the spectra recorded for the MSU-F samples. In the one acquired for the powders we detected the peak of the R band at ∼420 cm−1, and in this 27436

DOI: 10.1021/acs.jpcc.5b10206 J. Phys. Chem. C 2015, 119, 27434−27441

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The Journal of Physical Chemistry C material this band appears broader than in the spectra recorded for MCM41 and MSU-H. In addition, the D1 band, located at ∼490 cm−1 (see Figure 1h), has a very low relative amplitude, whereas the D2 band is absent and the band at 980 cm−1, if present, very small. As for the MCM41 and the MSU-H a broad band is detected in the spectral range of the asymmetric stretching vibration bands. After tablet production of MSU-F the R band shifts toward higher wavenumber (see also Figure 1h); the D2 band is still absent or has an amplitude near the detection limit; and we do not observe an efficient, in comparison with those previously observed, increase of the Raman signal in the range 900−1000 cm−1. Finally, in the range of the asymmetric stretching vibration bands we find two bands at about 1030 and 1170 cm−1 (the zoom of this spectral range is reported in Figure 1i). Thermal Treatment Effects. In Figure 2a we report the spectra recorded for the MCM41 powder and the tablets pressed at ∼0.2 and ∼0.45 GPa, as measured after a thermal treatment of 5 h at 1000 °C in air. The three spectra are very similar, and they are constituted by an R band peaked at about 410 cm−1, by a D1 and D2 band (Figure 2d illustrates the Raman spectra recorded in the range 300−700 cm−1 for the starting MCM41 powder and for the powder treated for 5 h at 1000 °C), by the 800 cm−1 band and the 1060 and 1170 cm−1 bands. In the three spectra the SiOH Raman band at 980 cm−1 is absent. A further comparison between the spectra of the starting and treated powders is reported in the panel e (zoom of the spectral range 900−1300 cm−1) of Figure 2. It is worth noting that these spectra resemble those of bulk silica. Figure 2b illustrates the spectra recorded for the MSU-H powder and the tablets obtained with a pressure of ∼0.2 and ∼0.45 GPa after a thermal treatment of 5 h at 1000 °C in air. Differing from the data of the MCM41 materials, we do not observe the Raman signal of silica but the one associated with the cristobalite crystalline form,42,43 even if the three spectra present some spectroscopic differences. The reasons for such differences, which are out of the aim of the present investigation, could be better clarified by isothermal and isochronal thermal treatments. The MSU-F powder and its tablets pressed at ∼0.2 and ∼0.45 GPa were thermally treated in the same way, and the data acquired for such samples are reported in Figure 2c. We note that the spectra of the tablets are comparable among them but different with respect to the one of the powder. In fact, this latter has spectral features similar to that of the MSU-H, whereas the spectra of the tablets contain additional bands. Applying a much longer thermal treatment to the powders and to the tablets produced with the pressure of ∼0.45 GPa we observed that the spectra, reported in Figure 3 a and b, of the MCM41 and the MSU-H feature the Raman bands associated with the cristobalite phase of SiO2. The spectrum of the similarly treated powders of MSU-F (black line in Figure 3c) features similar bands, whereas the spectrum recorded for the tablet (gray line in Figure 3c) is characterized by very different bands, which are comparable to the ones of the tridymite phase of the SiO2.42,44 Finally, by comparing the spectra recorded after the short and the long treatments we can suggest that the above-reported additional bands detected after the short treatment of MSU-F (Figure 2c) can be attributed to the signal of tridymite on the basis of the spectral range where they are detected. O2 Loading Effects. We performed O2 loading of the powders of the nontreated investigated materials. With this

Figure 3. Raman spectra recorded after thermal treatments at 1000 °C for 272 h: (a) MCM41 tablet (gray line) and powders (black line), (b) MSU-H tablet (gray line) and powders (black line), (c) MSU-F tablet (gray line) and powders (black line). All the tablets were obtained by applying a pressure of 0.45 GPa.

experiment we aimed to test the possibility of trapping the O2 NIR emitting molecules in the silica matrix of the walls of the mesoporous systems. As we reported in a previous study,31 O2 loading was successfully achieved only in the MSU-F powders where, as illustrated in the inset of Figure 4, after the loading

Figure 4. Ratio between the intensity I(t) of the O2 emission at a given time after loading and the starting intensity I(0). All the emission amplitudes are evaluated after the normalization for the silica Raman signal. Inset: O2 emission recorded after O2 loading of the MSU-F powder.

procedure we detected the O2 emission activity. We remark that previous studies evidenced the possibility to detect contemporarily the O2 PL emission and the silica Raman signals by using a 1064 nm laser excitation in a Raman spectrometer.23,26,27 Such a finding allows a direct comparison between the O2 emission and the Raman signal. In this way it is 27437

DOI: 10.1021/acs.jpcc.5b10206 J. Phys. Chem. C 2015, 119, 27434−27441

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O−Si linkages that become stressed as a consequence of pressure. Because of this process, the pressure does not only imply a reorganization of the networking through the variation of the distribution of the Si−O−Si angle and the ring statistics. In the present scheme, in fact, also the process Si−O−Si + H2O → 2Si−OH is active and overcomes the formation of the threemembered rings reported for the densified bulk materials.45 Furthermore, in the mesoporous materials because of their low wall thickness not only the surface of the wall is affected but also the wall structure itself could be broken. In this context, we mention that based on XRD and BET experiments it was suggested that a part of the pores can be destroyed,18 and the idea that some of the pores could be blocked by particles of the same materials has been proposed,15 implying the breakage of some of the pore wall parts. An exhaustive study as a function of the mechanical pressure is desirable to conclusively clarify in detail the interplay between the processes induced by the pressure, and the study of the pressed samples with HRTEM (high-resolution transmission electron microscopy) is also desirable to determine the breakage of the wall. We also remark that the reported considerations agree with the previous suggestion that the pressure induces the Si−O−Si hydrolysis together with the collapse of the mesoporous ordered structure and the reduction of the specific surface.13,15−18 We consider that the above dynamics is not in contrast with the specific surface decrease estimated through N2 adsorption.50 In this respect we remind that different studies describing N2 adsorption on the silica surface consider the surface oxygen atoms as the principal absorbents.51,52 Furthermore, it was evidenced that the surface roughness plays a relevant role,51 whereas the surface curvature effect is small for pore diameters larger than 2 nm.52 Taking into account these results we can suggest that if the breakage of the wall takes place, as it seems, and even if it implies a geometrical increase of the surface the number of surface oxygen atoms that contribute to N2 adsorption is decreased by the generation of pairs of SiOH groups, which have been considered the origin of the Raman signal at about 920 cm−1, interacting through O···H bridges.39 The N2 adsorption can be further decreased by the fact that the breakage of the Si−O−Si and the SiOH formation should be greater where the surface roughness is larger, implying more stressed bonds. Such considerations imply that BET experiments should be interpreted taking into account the specific process of N2 absorption since a geometrical surface in which this adsorption process is modified can differently contribute to the specific surface estimation. On the basis of the Raman data it appears that the SiOH mechanical formation is less efficient in the MSU-F material, suggesting a different efficiency of the two proposed processes. In this case, further differences of the effects induced by the pressure are highlighted by the spectra recorded after the thermal treatments of the powders and the tablets, and they will be commented on in the following. The data of Figures 2 and 3 indicate that it is possible to obtain different structures of silicon dioxide, including crystalline phases, by thermally treating the mesoporous materials. More in detail, as illustrated by the spectra of Figure 3, we note that the powders of all the materials became cristobalite after a sufficiently long thermal treatment regardless of the mesostructural order and the differences in the features of the silica walls evidenced by the Raman spectra. In particular, since MSU-F is not characterized by mesostructural order, the

possible to monitor the ratio between the O2 emission and the silica Raman signal to evaluate the O2 outgassing. The amplitude of the PL emission is 1.7 times higher than the amplitude of the silica Raman R band after the end of the loading procedure, and as previously reported, the O2 loading process does not induce variation of the Raman signal related to the silica.31 To consider the O2 loading an effective route for applicative aims, the O2 should stay in silica at least for some days. For this reason we acquired measurements as a function of the delay time from the end of the loading to determine the time scale of the outgassing process. In Figure 4, we report the ratio between the amplitude I(t) and the starting amplitude I(0) of the O2 emission. Each value of I(t) is estimated after the normalization for the Raman signal of the R band of silica. These data clearly evidence that after 1 week the amplitude of the O2 emission is still about one-half the initial one.

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DISCUSSION The reported results have shown clear spectral differences in the Raman bands of the as-received investigated mesoporous materials. We noted dissimilarity in the R band, which should reflect changes in the distribution of the Si−O−Si angle, dissimilarities in the ratios between the amplitudes of the D1 and D2 and the other silica network bands, indicating changes in the ring statistics, and modification in the line shape of the 800 cm−1 band. All these differences should highlight relevant differences in the Si−O−Si network; in addition, the difference in the relative intensity of the 980 cm−1 band suggests the variability of SiOH group content. All these features denote strong differences between the amorphous matrixes of the mesoporous systems considered. By applying a mechanical pressure to the powders the R band of the three materials shifts toward higher wavenumbers (see Figure 1), while the 800 cm−1 band remains unchanged or is poorly affected (MCM41). We clearly measured the decrease of the D2 band (see Figure 1 panels d and f), where present, and an increase of the SiOH associated band in MCM41 and MSUH, and this latter process appears absent or much less efficient in MSU-F (see Figure 1 and the zooms reported in panels e and g). In fact, the Raman signal in the range 900−1000 cm−1 remains quite low in the spectra of the latter material tablets. Using the relation between the R band and the Si−O−Si angle, the blue shift of the R band, which has been observed in densified bulk materials,45−48 can be interpreted as a decrease of the peak value of the Si−O−Si angle distribution. This effect has already been observed in silica densified materials but without the concurrent decrease of the D2 band and the formation of the SiOH groups, so we can suppose that in the investigated samples the pressure effects are the nontrivial result of at least two processes occurring simultaneously. One of these processes should be comparable to the one taking place in the bulk during densification, in which we note that the 800 cm−1 band is also almost unaffected.48 This process consists of the filling of the free voids of the network by the puckering of the rings,45 which implies a reduction of the Si− O−Si angle, which causes in turn the blue shift of the R band. In this process it has been shown that the tetrahedrons are almost unchanged,45,49 and it has been suggested that a slight expansion of the Si−O linkages does not induce significant deformations of the tetrahedron.47 The other process is specific to the mesororous silica system and consists of the mechanically induced hydrolysis of the Si− 27438

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experimental data prove the contrary. Such a result could be related to the microporosity of the pore walls reported for the MSU-H material5 or to differences in their synthesis affecting the chemical composition of the final materials. Similarly to the MSU-H, in the MSU-F powders we observed only the formation of the cristobalite. However, the spectra recorded for MSU-F tablets indicate that the tridymite phase is preceded in this case by the formation of the cristobalite. In fact, the two phases are present after 5 h at 1000 °C, whereas after ∼270 h we detected only the Raman peak of the tridymite, these findings indicating that the complete transition of all the material requires more than 5 h. Among the peculiar properties of the MSU-F material we observe that by keeping the powders of the three materials at 100 °C for 11 h in O2 atmospheres (50−70 bar) we found the emission related to the O2 molecules trapped in silica interstices23 only in the MSU-F sample.31 This finding is in agreement with previous results,26,31 which evidenced that the content per mass unit of O2 molecules trapped within silica systems decreases with increasing the specific surface, since a near surface layer of silica of about 1 nm has a low capability to trap O2 within its interstices, and the specific surface of MSU-F is the smallest of the investigated mesoporous materials. This result highlights that in the MSU-F regions far from the pore surface, enabling us to trap O2, are present, and this aspect could play a relevant role in determining the pressure effects and as a consequence in the evolution during thermal treatments of the tablets. By contrast, the MCM41 does not entrap O2, due to the low thickness of the pores walls, whereas in the MSU-H, the wall microporosity reduces the presence of regions far enough from surface to trap O2. In this context, we noted that the amplitude of the O2 emission decreases by about a factor 2 after 1 week from the end of the treatments. Such a finding indicates that the interstices of the silica walls of the MSU-F materials can be used to trap small molecules, which modify the material properties. As final remarks we would like to summarize some of the peculiar and relevant properties of the MSU-F material. On the basis of our Raman data we clearly evidenced very low content or the absence of the three-membered rings. Differing from the other materials the SiOH groups are present in a very low content or are absent, and we do not have clear evidence of mechanically induced Si−O−Si hydrolysis. Considering the data reported by Zhuravlev,58 which indicate an almost constant value of SiOH groups per nm2, the reasons for the very low content of SiOH need to be more deeply understood. Furthermore, by applying mechanical pressure and a successive thermal treatment at 1000 °C it is possible to induce the formation of the tridymite crystalline phase. Finally, differing from the MCM41 and from the MSU-H it is possible to trap O2 molecules in the interstices of the silica wall of the MSU-F for a considerable time.

present data indicate that an ordered spatial disposition is not required to obtain a crystalline phase. This finding is in agreement with previous studies performed on tablets of silica nanoparticles.19 Anyway, the phase change depends on many parameters, so that in other cases reported in the literature53 the thermal treatment of tablets of silica nanoparticles did not induce the formation of crystalline phases. A similar variability has been observed also in the case of the thermal treatments of silica gels.54−56 Further details on the thermally induced processes that generate the detected modifications on the investigated materials will be obtained by isochronal and isothermal treatments performed on samples having specific and controlled physical-chemical features. In this context, anyway, the differences in the SiOH content of the three nontreated materials suggest that these groups do not play a relevant role in determining the final phase of the starting material, even if, where present, the formation of a continuous network of large extension comes through their elimination, as indicated by the removal of their Raman bands after the thermal treatments. We remind that SiOH elimination, through the reaction 2SiOH → Si−O−Si + H2O, was reported in the standard silica synthesis from porous sol−gel materials.54 In the MSU-F material we note a different behavior between tablets and powders (see Figure 3c) after prolonged thermal treatment. In fact, the former tend to form tridymite, whereas the latter form cristobalite. This could suggest an effect of the pressure in determining the thermal response of the mesoporous materials. Anyway, in the MCM41 and the MSU-H samples, where the pressure induces also the Si−O− Si hydrolysis, no differences are found by comparing the tablets and the starting powders. These findings indicate that the pressure does not have a unique role. It can be supposed that the mechanically induced hydrolysis could also contribute to the thermal response, affecting the neighborhood around the broken Si−O−Si linkages. Further studies, for example in controlled atmospheres avoiding the presence of H2O molecules to avoid the hydrolysis, should be performed to deepen this aspect. Our data clearly evidence that in MCM41, independently on the powder or tablet form, the formation of the cristobalite by thermal treatment is preceded by a phase in which the material features a Raman spectrum with a silica-like line shape and with the R band peaked at 410 cm−1. This latter finding should indicate a low density of the material, if we assume that the relation between the R band peak position and the density is equal to the one observed for the bulk.48 This phase has not been evidenced in the other two materials, but it could appear perhaps for shorter treatment times. The data reported on MCM41 seem in agreement with previous studies. In fact, as previously reported14 the hexagonally ordered MCM41 became cristobalite after an annealing at 1000 °C for 4 h followed by a further treatment at 700 °C for 5 h, whereas the authors of the previous investigation57 did not report the formation of a crystalline phase after a treatment of 1050 °C for 4 h for the hexagonal SBA-15 mesoporous material even if the porosity was removed by a geometrical contraction of the cylindrical pores. In the MSU-H, which has a hexagonal order, as the MCM41, we only observed cristobalite. We note that the thickness of the walls of the here investigated MSU-H is larger than the one of the MCM41, so that, even if one could expect that the thermally induced process occurs earlier in the MCM41, the

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CONCLUSIONS We have studied the pressure and thermal effects on the MCM41, MSU-H, and MSU-F mesoporous silica systems by Raman spectroscopy. Our data indicate that the walls of the asreceived powders are constituted by very different silica networks. By applying a pressure of about 0.2 and 0.45 GPa we measured a blue shift of the R band and a decrease of the D2 band, where present (MCM41 and MSU-H). Furthermore, in the MSU-H and MCM41 materials we detected an increase 27439

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The Journal of Physical Chemistry C

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of the SiOH groups that can be attributed to the Si−O−Si mechanically induced hydrolysis. Such a process seems to be much less efficient or absent in the MSU-F material where the SiOH starting content is very low. Despite their differences in the Raman spectra the three powders tend to form the cristobalite crystalline phase after thermal treatment at 1000 °C. After similar treatments, the mechanically pressed tablets of the MCM41 and of the MSU-H undergo phase changes that are comparable to the ones observed in their respective powders. By contrast, the MSU-F tablets tend to form the tridymite crystalline phase. The dissimilar behavior of the MSU-F tablets could be associated with the lack of an efficient formation of SiOH groups during the production of the tablets, which differentiates the pressure effects on the MSU-F material with respect to the MSU-H and the MCM41 ones. Finally, among the various peculiarities of the investigated MSU-F powders we demonstrate that only in this sample is it possible to trap O2 molecules in the interstices of the pore walls, suggesting that this material features regions far enough from the pore surfaces.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +3909123891703. Fax: 00390916162461. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the people of the LAMP group (http:// www.fisica.unipa.it/amorphous/) for useful discussions and technical assistance by G. Napoli and G. Tricomi. Partial financial support by the FAE-PO FESR SICILIA 2007/2013 4.1.1.1 and by the FFR 2012/2013 project of the University of Palermo is acknowledged.

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DOI: 10.1021/acs.jpcc.5b10206 J. Phys. Chem. C 2015, 119, 27434−27441