Orientation and Mg-incorporation of calcite grown on functionalized self-assembled monolayers: A synchrotron X-ray study

Calcite crystals were nucleated from MgCl2/CaCl2 solutions onto functionalized self-assembled monolayers adsorbed onto E-beam evaporated Au films. Synchrotron x-ray scattering studies of the crystals reveal new information about preferred orientation and Mg incorporation. The Au [111] axis is distributed within 2.6° of the film surface normal, but the oriented crystals may be tilted up to 6° away from this axis. For low Mg2+ content, SO3- functionalized films nucleated primarily near the (106) calcite face, odd-chain length carboxylic acid terminated alkanethiols films nucleated near the (012) face, and even-chain length carboxylic acid terminated alkanethiol films nucleated near the (113) face. [Mg2+]/[Ca2+] concentration ratios of 2 and greater defeated this preferred orientation and created a powder texture. Diffraction patterns within the layer plane from the coarse calcite powders indicated a shift to higher two-theta accompanied by peak broadening with increasing [Mg2+]/[Ca2+] ratio. For 0.5 [Mg2+]/[Ca2+] 3.5, a double set of calcite peaks is observed, showing that two distinct Mg-calcite phases form: one of comparatively lower Mg-content, derived from the templated crystals, and a Mg-rich phase derived from amorphous precursor droplets. According to the refinement of lattice parameters, Mg incorporation of up to 18 mol % occurs for [Mg2+]/[Ca2+] = 4, independent of film functionality. We discuss the differences between the differently functionalized monolayers, and also introduce the hypothesis that two separate routes to Mg-calcite formation occur in this system.