Calcite Microneedle Arrays Produced by Inorganic Ion‐Assisted Anisotropic Dissolution of Bulk Calcite Crystal

Besides studies on the mineralization process, research on the demineralization of minerals provides another way to understand the crystallization mechanism of biominerals and fabricate crystals with complicated morphologies. The formation of ordered arrays of c‐axis‐oriented calcite microneedles with a tri‐symmetric structure and lengths of more than 20 μm was realized on a large scale for the first time through anisotropic dissolution of calcite substrates in undersaturated aqueous solution in the presence of ammonium salts. The lengths and the aspect ratios of the calcite microneedles can be tuned by simply changing the concentrations of the ammonium salts and the dissolution time. The shape of the transverse cross sections of the calcite microneedles obtained in the presence of NH₄Cl and NH₄Ac is almost regularly triangular. The tri‐symmetric transverse cross‐section geometry of the calcite microneedles could be attributed to the tri‐symmetric feature of rhombohedral calcite atomic structures, the synergetic interactions between electrostatic interaction of ammonium ions and dangling surface carbonate groups, and the ion incorporation of halide ions.