Goethite Mineral Dissolution to Probe the Chemistry of Radiolytic Water in Liquid‐Phase Transmission Electron Microscopy

Abstract Liquid‐Phase Transmission Electron Microscopy (LP‐TEM) enables in situ observations of the dynamic behavior of materials in liquids at high spatial and temporal resolution. During LP‐TEM, incident electrons decompose water molecules into highly reactive species. Consequently, the chemistry of the irradiated aqueous solution is strongly altered, impacting the reactions to be observed. However, the short lifetime of these reactive species prevent their direct study. Here, the morphological changes of goethite during its dissolution are used as a marker system to evaluate the influence of radiation on the changes in solution chemistry. At low electron flux density, the morphological changes are equivalent to those observed under bulk acidic conditions, but the rate of dissolution is higher. On the contrary, at higher electron fluxes, the morphological evolution does not correspond to a unique acidic dissolution process. Combined with kinetic simulations of the steady state concentrations of generated reactive species in the aqueous medium, the results provide a unique insight into the redox and acidity interplay during radiation induced chemical changes in LP‐TEM. The results not only reveal beam‐induced radiation chemistry via a nanoparticle indicator, but also open up new perspectives in the study of the dissolution process in industrial or natural settings.


Figure S2 Pourbaix diagram
Eh/pH Pourbaix diagram was calculated using the software core Geochemical workbench [11] .The plots in Figure S2 reveal the thermodynamically stable phases at 25 °C, under atmospheric conditions and in the presence of 4.8 mM of dissolved Fe 3+ .The data shows that hematite (Figure S2a) should be the stable phase under our standard conditions.However, in our study we focused on goethite and at our specific LP-TEM settings (imaging parameters, electron flux, liquid cell thickness) we only observed the dissolution of goethite particle, and did not expect any transformation of the goethite to the hematite.
To show this we excluded hematite as a stable phase from our Pourbaix diagram shows (Figure S2b) that indeed goethite is the predominant phase over a large range of conditions and only at pH lower than 7, the predominant species can be aqueous: either Fe 2+ or Fe 3+ depending on solution potential.However, it must be emphasized that one must be careful in transferring and applying such pH-dependent thermodynamic representations to conditions present in LP-TEM.

Figure S3 Relative change in mineral width and length in situ
The dimensions were measured on 26 width and length from 4 different particles in 2 distinct independent experiments.Red and black marks correspond to length and width dimensions, respectively.According to the volume loss during electron beam induced dissolution at high electron fluxes, it is possible to estimate an amount of iron released in the surrounding medium.To do so, geometric approximations were made from the 2D projection images and FIB-SEM transversal section.The first one concerns the layout of the particle settling on the substrate surface.As seen in Figure S6a, the isolated measured particles were settled on their longest crystalline face on the substrate.Then, TEM only provides the 2D projection of the object of interest on a projection screen or on the CCD detector.Therefore, it only gives access to projected length and width but not the equivalent of height necessary to calculate a volume of particles.To overcome this, TEM analysis of a FIB cross section of the particles aimed at accessing that missing dimension.Figure S6a present the representative perpendicular cut of goethite particle.The transversal section of the particles is a parallelogram.Therefore, the volume of each particle can be calculated by multiplying the area of the transversal section to the length of the particles.

Figure S4 and S5 Length and width histograms of bulk experiments in acidic, basic and reductive conditions
The length is directly estimated by the 2D projection of the particle length on the images, however the calculation of the transversal section is more complex.As detailed in Figure S6b, the area of a parallelogram is calculated by the edge times the height of the parallelogram.In our case, we can only access the 2D projection of particle width w.However, from this measure it is possible to obtain an approximated area of the transversal section.First the 2D projection of width can be related to the actual longer diagonal of the section though Pythagorean theorem, then, the height h in the goethite present particle can be approximated to half of the longer diagonal of the section.Using these parameters, the section area is estimated as A = w•w/4.The estimation of area through this calculation was confirmed by the error estimation of the actual area measures on the particle section.The calculation error when measuring area though the approximation was between 3-12% when compared to the theory of a perfect parallelogram area.
Through this method, we estimated the amount of iron released in the liquid cell image Figure 5b after 100 seconds of irradiation to 2.8 10 -18 mol of iron released to the surrounding medium.

Figure S1 :
Figure S1: Temporal evolution of the species concentrations at the electron flux densities used during LP-TEM in this study.

Figure S2 :
Figure S2: Pourbaix Diagram of iron at room temperature using Geochemical workbench including (a), or excluding (b) hematite.Eh refers to the oxidation-reduction potential.

Figure S3 :
Figure S3: relative change in length and width dimension of particles observed in situ with an electron flux density of (a) 21 e -Å -2 s -1 and (b) 167 e -Å -2 s -1 .

Figure S4 :
Figure S4: Frequency distribution of length and width dimensions of particles exposed to solutions of ultrapure water at pH 2, 5, 8, and 11 for 24 h

Figure S6 :
Figure S6: FIB-SEM transection of goethite nanoparticles observed in (a) STEM and (b) TEM and (c) scheme of geometric approximations of the section surface estimation

Figure S7 &
Figure S7 & S8 Analysis of π*and Kw* and simulated steady state concentrations

Figure S8 :
Figure S8: Simulated steady state concentrations of radical, ionic and molecular species for aqueous solutions containing 4.8 mM Fe 3+ .Dotted and dashed lines represent respectively an electron flux density of 21 e -Å -2 s -1 and 167 e -Å -2 s -1 .