Evaluation of correlated studies using liquid cell and cryo-transmission electron microscopy: Hydration of calcium sulphate and the phase transformation pathways of bassanite to gypsum

Abstract

Insight into the nucleation, growth and phase transformations of calcium sulphate could improve the performance of construction materials, reduce scaling in industrial processes and aid understanding of its formation in the natural environment. Recent studies have suggested that the calcium sulphate pseudo polymorph, gypsum (CaSO₄ ·2H₂ O) can form in aqueous solution via a bassanite (CaSO₄ ·0.5H₂ O) intermediate. Some in situ experimental work has also suggested that the transformation of bassanite to gypsum can occur through an oriented assembly mechanism. In this work, we have exploited liquid cell transmission electron microscopy (LCTEM) to study the transformation of bassanite to gypsum in an undersaturated aqueous solution of calcium sulphate. This was benchmarked against cryogenic TEM (cryo-TEM) studies to validate internally the data obtained from the two microscopy techniques. When coupled with Raman spectroscopy, the real-time data generated by LCTEM, and structural data obtained from cryo-TEM show that bassanite can transform to gypsum via more than one pathway, the predominant one being dissolution/reprecipitation. Comparisons between LCTEM and cryo-TEM also show that the transformation is slower within the confined region of the liquid cell as compared to a bulk solution. This work highlights the important role of a correlated microscopy approach for the study of dynamic processes such as crystallisation from solution if we are to extract true mechanistic understanding.

Keywords: calcium sulphate; cryogenic TEM; crystallisation; liquid phase TEM.

FIGURE 1

Characterisation of synthesised bassanite nanorods. (A) TEM image showing rods of 200–400 nm in length and SAED of the particle indicated by the arrow was indexed to bassanite. (B) XRD analysis shows characteristic peaks at d ₁₁₀ 6.00 Å, d ₃₁₀ 3.47 Å, d ₄₀₀ 3.00 Å, d _–114 2.80 Å, d _–514 1.85 Å. (C) Raman spectroscopy indicates strong peaks at 1015 cm⁻¹ associated with the υ ₁ SO₄ band along with peaks at 427, 489, 628, 668 and 1128 cm⁻¹ associated with the υ ₂/ υ ₃/ υ ₄ (SO₄) vibrational modes (see Table 1)

FIGURE 2

(A) Image series taken from Video S1 at different time points showing the transformation of bassanite to gypsum through hydration using a 9:1 [12 mM CaSO₄(aq)]:[ethanol] solution. Dissolution of bassanite occurred followed by gypsum nucleation and continued dissolution and reprecipitation of bassanite on growing gypsum crystals. All bassanite nanorods transformed after ∼200 s. (B) Post‐mortem SAED analysis carried out after pushing air through the LC confirmed gypsum crystals had been formed. The circled region indicates the region from which the SAED pattern was obtained and the scale bar is 500 nm

FIGURE 3

Verification that the transformation of bassanite to gypsum was not solely beam induced. (A) Initial HAADF STEM image of bassanite nanorod precursor (5.8 e ^–/Ų) alongside EDX spectra showing large C Kα peak indicative of the nanoparticles being dispersed in ethanol. (B) HAADF STEM image taken after 5 min of flowing a 9:1 [12 mM CaSO₄(aq)]:[ethanol] solution though the LC chip and an additional 2 min with beam blanked, alongside EDX spectra clearly showing a large O Kα peak and reduced C Kα peak indicative of the cell being filled with an aqueous solution

FIGURE 4

Images taken from Video S3 where ethanol was preflowed through the LC to ‘loosen’ the bassanite nanorods prior to the reaction taking place. Some movement of the nanorod precursor was observed within the circled regions, but the overall mechanisms of transformation was via dissolution and reprecipitation

FIGURE 5

in situ Raman spectral series of the transformation of bassanite nanorods exposed to an undersaturated CaSO_4(aq) solution inducing hydration and a transformation to gypsum. The bottom spectrum is the area highlighted in the dashed box in the upper spectrum. The coloured lines denote the varying time points where, black refers to time = 0 s; red, time = 90 s; blue, time = 180 s; green, time = 270 s; purple, time = 360 s; gold, time = 450 s and cyan, time = 540 s

FIGURE 6

(A) Time resolved cryo‐EM images taken at 1, 5, 10 and 20 s after spraying aqueous 9:1 [12 mM CaSO₄(aq)]:[ethanol] on a TEM grid loaded with predried bassanite nanorod precursor (on‐grid mixing) (scale bar 1 μm). Transformation to gypsum appears to occur between 5 and 10 s. Gypsum crystals were confirmed by SAED showing distinctive rings at 3 Å (solid line) and 4.2 Å (dashed line) d‐spacings attributed to the (041) and (021) lattice planes respectively (B). EDX confirms Ca (red) and S (green) rich crystals (C). Spherical particles observed in S/TEM images are frost contamination. (D) Time resolved cryo‐EM images taken at 2.7, 5, 15 and 20 s after mixing aqueous 9:1 [12 mM CaSO₄(aq)]:[ethanol] and bassanite nanorods in ethanol and spraying onto a plasma cleaned TEM grid (in‐flow mixing) (scale bar 1 μm). Gypsum begins to form after 15 s. Gypsum crystals were confirmed by SAED showing distinctive 4.7 Å d‐spacing (‐111) (E). EDX confirmed Ca (red) and S (green) rich crystals, oxygen (blue) carbon (yellow) (F)