Analysis of the flake sample

The small thickness of the flake sample allowed its analysis in a single operation (from the first gel layer to the pristine glass core to the second gel layer, Fig. 8). In the following section, the flake sample profiles are presented in E direction (from the first gel layer to the pristine glass core) and L direction (from the pristine glass core to the second gel layer). Please refer to the methods section for more information.

Elements from the glass

The profile obtained for Zr+ during analysis 1 is not perfectly flat (Fig. 1a), which is a result of the matrix effect. The other profiles displayed in Figure. 1 are devoid of these matrix effects due to Zr+ normalization.52 Al+ profile displays a very similar shape as that of Zr+ (Supplementary Material Fig. S1a), and as a result Al+ normalized profile is perfectly flat (Fig. 1c)28.Si+ profile appears to vary lightly in shape compared to that of Zr+ and Al+ (Supplementary Material Fig. S1a), and its normalized profile does not appear to be perfectly flat (Supplementary Material Fig. S1c). At first impression, one could be tempted to say that the gel layer is faintly depleted in Si due to acid leaching conditions undergone by the flake sample. However, when comparing 28Si+ profile with that of 29Si+ and 30Si+ (Supplementary Material Fig. S2a) it appears that 29Si+ and 30Si profiles are similar to that of Zr+. Moreover, the 29Si+/28Si+ and 30Si+/28Si+ ratios in the both gel layers are slightly above that of natural abundance found in the pristine glass, while the 30Si+/29Si+ one stays constant (Supplementary Material Fig. S2b). Considering that no enrichment of 29Si nor 30Si was brought by the solution, it is thus possible to affirm that the 28Si+ profile shape is abnormal due to saturation effect of the detector. Therefore, the Si profiles presented here (Fig. 1b) is that of 29Si+. Like Al+29,Si+ normalized profile is flat, which is consistent with previous studies.27,40,53

Fig. 1 Analysis 1 performed on the flake sample. Profiles of the glass cations. All profiles (except the Zr+ one) are normalized to that of Zr+, and the resulting data are normalized to the mean value found in the pristine glass. The position of the pristine glass/gel interface, determined from B+ profiles, is represented by a black dotted line in each graph. a Zr+ profile, b Si+/Zr+, c Al+/Zr+, d B+/Zr+, e Na 2 +/Zr+, and f Ca+/Zr+ normalized profiles Full size image

Normalized B+, Na 2 +, and Ca+ concentrations drop from a value of 1 in pristine glass to a value below 0.15 in gel layers, and the sharp drop in the concentration of those three elements occurs at the same position, meaning that these elements were congruently dissolved under the studied conditions (Fig. 1d–f). All these results are also coherent with previous studies.27,40,53

The E and L profiles for normalized 29Si+ and Al+ are identical, meaning that these profiles, often displayed and discussed in the literature, were not affected by ion beam sputtering artifacts (Fig. 1b, c).27,28,30,42 This is coherent with their role of glass former making covalent bonds with O atoms. B, while being a network former, is also known to be preferentially leached out even in Si-saturated conditions.27,53 As for Na+ and Ca+, they are known to act mostly as charge compensators for [BO 3 ]−, [AlO 4 ]−, and [ZrO 6 ]2− units in ISG glass, and, to a lesser extent, as glass modifiers.54 As these cations are less strongly bonded to the structure than the glass formers, their distributions within the gel could be impacted by the O or Cs beam. B+ and Ca+ profiles appear to be fairly symmetrical (Fig. 1d, f), but the same plots in logarithm scale (Supplementary Material Fig. S3a,b) reveal that both elements are slightly pushed forward, as a slight tail can be observed in the L direction. Na2+ profile is not symmetrical at all and displays differences in both shape and width at the pristine glass/gel interface (Fig. 1e). When analyzed in the E direction, the Na 2 + interface is very sharp, similar to a Heaviside step function. On the contrary, when analyzed in the L direction, the Na profile is larger with a decrease in the Na 2 + concentration in the pristine glass and a tail in the gel layer (Fig. 1e). Alkali profiles can be distorted when analyzing insulating materials due to the formation of an electric field.1,55 Here, a low-energy electron flux was used to neutralized the surface of the sample in order to avoid such artifact. Notwithstanding, the shape of Na 2 + L profile is not surprising. The difficulty to observe the diffusion of an element from a layer 1 to a layer 2 when the element is present at high concentration in layer 1 and as a trace in layer 2 is the reason backside analyses were developed in the first place.56,57 However, it shows the tendency of some elements to be pushed forward by the ion beam in normal analysis condition. Na in particular is the most impacted, which is consistent with its ionization yield that is higher than that of B and Ca.

Exogenous species

All the exogenous species studied here (i.e., K18,O, and H) are assumed to be mostly mobile inside the gel structure. K is known to exchange with Ca as a charge compensator for [AlO 4 ]− and [ZrO 6 ]2− units.53 Concerning 18O and H, some uncertainties currently remain as to whether water molecules diffuse as such, or dissociate into H and O that then diffuse separately.58 The H profile tends to be shifted compared to the 18O profile, which could be an indication that water molecules dissociate and then react with the silicate network. It is reported that H profile also tends to be shifted compared to other species profiles (e.g., B, Na), which may indicate that hydration precedes ion exchange.42

Here41,K+ seems to have been pushed forward by the sputtering beam; its profile in the E direction is very sharp and anti-correlated with that of B+, while it is shifted by ~70 nm in the L direction (Fig. 2b). Moreover, the shape of the 41K+ profile differs depending on the sputtering direction; the enrichment in the interfacial area in E layer corresponds to a depletion in the same area of L layer (in blue).

Fig. 2 Analysis 1, 2, and 3 performed on the flake sample. Profiles of exogenous elements and 18O41.K+ profile is normalized to that of Zr+. H−, and 18O− profiles are normalized to that of their respective Si− profiles. The position of the pristine glass/gel interface, determined from B+ (Fig. 1d) and BO2- profiles (Supplementary Material Fig. S4a, b), is represented by a black dotted line in each graph. a Zr+ profile from analysis 1, b 41K+/Zr+ profile (analysis 1), c Si− profile from analysis 2, d 18O−/Si− profile (analysis 2), e Si− profile from analysis 3, and f H−/Si− profile (analysis 3) Full size image

Regarding 18O and H, our results confirm that these elements were not pushed forward by the sputtering beam despite being fairly mobile elements59; their profiles widths at the interface are similar regardless of the sputtering direction (Fig. 2d, f). This validates the determination of the diffusion coefficient from the width of the H interface.42 Moreover, H− profiles are perfectly correlated with the 18O− profile and anti-correlated with the BO2− profiles (Supplementary Material Fig. S4c, d), regardless of the sputtering direction, proving that, in the studied conditions, H does not diffuse inside the pristine glass as a separate proton species.

Still, the E layer seems to be slightly more depleted in both 18O and H. Depletion in 18O can be attributed to exchange with the moisture of the atmosphere during the sample preparation and handling, as the surface of the E layer remained in contact with air for 3 min before being introduced into the vacuum chamber.59 However, as H was similarly impacted as 18O, this could be the result of the vacuum needed for ToF-SIMS profiling (1.3 × 10−8 mbar) that leads to some confined water in the sample evaporation. The depletion in E layer is on a similar order of magnitude within experimental error for both H and 18O (Table 1). H− and 18O− profiles display similar shapes near the pristine glass/gel interface. However, H− is slightly more depleted than 18O−, especially near the external part of the gel (see the red dashed line in Fig. 3). This suggests that H could have diffused toward the carbon tape as a proton, as proved by the enrichment of H when reaching the flake/carbon tape interface (Supplementary Material Fig. S5).

Table 1 Comparison between data obtained in E and L layer Full size table

Fig. 3 Analysis 2 and 3 performed on the flake. H− and 18O− profiles normalized to that of their respective Si− profiles. Those data are similar to that presented in Fig. 2b, c, but are plotted as a function of total depth of the sample. The position of the pristine glass/gel interfaces, determined from BO2− profiles (Supplementary Material Fig. S4c, d), is represented by black dotted lines. The red dashed line indicates the depth at which the profiles shapes differ Full size image

Analysis of the monolith samples

The previous observations suggested that the sputtering beam does not impact network formers such as Si and Al, but could push forward some mobile species such as B, K, Na, and Ca. This could reduce the interfacial profile width usually presented in the literature (profiles obtained in the E direction) and, in turn, lead to some uncertainties regarding the profile shapes at the pristine glass/gel interfaces. To further investigate this idea, the choice was made to analyze two similar samples with and without cryo-treatment, prior to the ToF-SIMS analysis. Cryo-treatment has previously been used to study the migration of alkalis in thin SiO 2 layers,60 showing drastic differences between alkalis profiles at ambient temperature and low temperatures (down to −120 °C).

Despite being altered in the same vessel, the monoliths used for cryo- and non-cryo-analysis do not display the same alteration depth (~545 nm and ~860 nm, respectively). However, it has been previously demonstrated above and in other studies that the layer formed in Si-saturated conditions are homogeneous.27,40,53 It means that while they have different alteration depths, both monoliths should display similar profiles for each elements. Indeed, all network former profiles, including that of B, are similar (Supplementary Material Fig. S6a to d) and can be perfectly superimposed when aligning the pristine glass/gel interfaces (Supplementary Material Fig. S6e to g). However, the cryo-treatment is expected to reduce the mobility of the cation species. Here, the differences observed between the cryo- and non-cryo-profiles suggest that some cations were impacted (Fig. 4), although the resulting effect differs depending on their origin (from the pristine glass or from the solution).

Fig. 4 Profiles obtained on the monolith samples. The positions of the pristine glass/gel interfaces, determined from the B+ profile (Supplementary Material Fig. S6c), are represented by vertical dotted lines. The profiles displayed are those of a Zr+, b Ca+/Zr+, c Na 2 +/Zr+, d Li+/Zr+, e 41K+/Zr+, and f Cs 2 +/Zr+. For Ca and Na, the data resulting from Zr+ normalization are normalized to the mean value found in the pristine glass Full size image

Na and Ca are two elements that come from the pristine glass and are depleted inside the gel layer. Na 2 + profiles are fairly similar in shape, even if the cryo profile is slightly above its non-cryo counterpart (Fig. 4a). This element does not appear to be much pushed forward in this experiment. Ca+ profiles, on the contrary, differ depending on the thermal treatment of the sample (Fig. 4b). Ca amount rises linearly in the cryo profile (from 0.20 to 0.45 after the initial bump) until it reaches a fairly narrow pristine glass/gel interface. For the non-cryo experiment, Ca+ amount also rises linearly, although with a greater slope (from 0.00 to 0.70), and the interface is less pronounced. This suggest that the sputtering beam pushes Ca atoms forward, a result that is more visible than in the previous experiment on the flake sample because of Ca higher mean concentration inside the gel layer.

Fig. 5 Profiles obtained on the monolith samples. The positions of the pristine glass/gel interfaces, determined from the BO2− profile (Supplementary Material Fig. S4e), are represented by vertical dotted lines. The profile displayed are those of a Si−, b H−/Si−, and c 18 O− Full size image

Profiles obtained for exogenous species such as Li, K, and Cs also differ depending on the sample treatment. The non-cryo-profiles of these species display an artificial defect with a decreasing slope and a shape that is less flat than its cryo- counterpart. This artifact is more pronounced for light elements such as Li and K, and nearly nonexistent for Cs, proving that heavy elements were less displaced by the ion beam due to reduced mobility in this type of material. These results are consistent with that obtained by Krivec, et al.60 in thin SiO 2 layer. In addition, one might note that Li cryo profile is still less flat than that of other alkalis, suggesting that this light element might still be pushed forward by the beam despite the cryo-treatment, as previously observed by Krivec, et al.60. All Li+ and 41K+ profiles show a dramatic enrichment at the pristine glass/gel interface, which suggest that this is an actual feature of this type of gel layer. It can be explained by the fact that this area was still very reactive, with negatively charged boron units still requiring charge compensation. The rest of the gel layer, which has had some time to reorganize, was completely boron-depleted and globally homogeneous in composition, explaining why the cation profiles are mostly flat.59 However, the absence of this enrichment in the cryo experiment for Cs 2 +—the less impacted alkali—profile raises uncertainties on this affirmation. The enrichment observed for Li+ and 41K+ could still be an artifact of the ion beam even during cryo experiments, as observed by Krivec, et al.60 This needs to be further studied.

In this type of material (nanoporous gel layer formed in Si-saturated conditions), previous cryo experiments suggested that no water evaporation was caused by the ToF-SIMS vacuum.59 The results obtained here do not allow us to draw any further conclusions regarding H 2 O evaporation caused by the ToF-SIMS vacuum (1.3 × 10−8 mbar). While the H/Si value is higher in the cryo experiment, the 18O/Si value is lower (Fig. 5, Table 2). To address this apparent contradiction, further investigation with model porous materials is needed. Nevertheless, the value found for each elements are close enough that it is possible to rule out any significant water evaporation due to the vacuum.

Table 2 Comparison between data obtained on monolith samples with different heat treatment Full size table

18O/16O profiles

During this study, especially for the flake sample where a low 18O− signal was expected, our analytical conditions were adjusted to optimize the sensitivity of 18O measurement. However, this was done to the detriment of 16O measurement, resulting in a saturated non-linear response of the detector. This makes the determination of the isotopic ratio impossible for our samples: values that differ from the natural abundance for the 18O/16O ratio are found inside the pristine glass (up to 3.7-fold higher, Fig. 6). ToF-SIMS lower sensitivity compared to that of D-SIMS thus constrain the domain of isotopic enrichment that can be use if quantities data are required: a too low enrichment for the minor isotope is not favorable, as the adjustment of the detector often lead to saturation effect for the major isotope.

Fig. 6 18O−/16O− profiles. Results obtained on the flake sample (Analysis 2). The experimental value obtained in the pristine glass is higher than that expected. The position of the pristine glass/gel interfaces, determined from the BO2− profiles (Supplementary Material Fig. S4c), is represented by black dotted lines. The red dashed line represents the natural abundance Full size image

In summary, our study demonstrates that, despite careful sample preparation and analysis, ToF-SIMS data could be incorrectly interpreted due to several analytical artifacts. This should be considered in future studies on glass corrosion, as well as any other studies considering material corrosion and/or tracing experiments. Some caveats from this study are: