Figure 4

Transmission electron microscopy (TEM), selected area electron diffraction (SAED), and fast Fourier transform (FFT) analyses of amorphous calcium carbonate precipitated at high pH. (a) Bright-field TEM overview image of the ACC structure dispersed on a Lacey carbon support grid (scale bar: 200 nm). The inset shows a SAED ring pattern indicating arbitrarily oriented crystallites (scale bar: 5 nm⁻¹), with diffuse peaks at 3.549 nm⁻¹ and 5.315 nm⁻¹ corresponding to real space distances of about 0.28 nm and 0.18 nm respectively. (b) High-resolution (HR) TEM micrographs of an edge region of the ACC nanoparticles (top scale bar: 5 nm, bottom scale bar: 2 nm), which had been irradiated with as little electron dose as possible, showing crystalline regions with a few nm in size with a lattice spacing of 0.24 nm, as indicated with white lines. (c,d) HR-TEM images of ACC after further electron beam irradiation; larger crystalline regions of 10–30 nm in size form (top scale bar: 20 nm); the bottom micrographs are enlarged views of the marked regions (bottom scale bar: 2 nm). The upper insets represent FFTs of the upper micrographs (scale bar: 2 nm⁻¹). In the lower and enlarged micrographs, the banded areas are FFT-filtered areas using the FFT signals accentuated by yellow rings. The observed lattice spacings in subfigures (c,d) can be assigned to calcite (d₁₀₄ = 0.30235 nm) and vaterite (d₋₃₁₂ = 0.32983 nm), respectively.