Physico-chemical Characterization

The powder X-ray diffraction (XRD) profiles of the coral before and after hydrothermal conversion were measured by powder XRD analysis (RINT- Ultima-III, Rigaku Co., Japan; CuKα radiation, 40 kV, 40 mA). The step scanning was performed with an integration time of 1 min at intervals of 2° (2θ) and matched with JCPDS database. The chemical composition of the crushed sample powder was investigated by fourier transform infrared spectroscopy (FTIR). Samples were ground with 1% KBr in an agate mortar, and analyzed under nitrogen atmosphere from 2000 to 400 cm⁻¹ using a Nicolet IR 760. Inductively coupled plasma-mass spectroscopy (ICP-MS) was used to measure the chemical composition of the samples by using approximately 0.3 g of sample which was digested with 0.25 mL of HNO₃ and 0.25 mL of H₂O₂. Once the digestion was completed the sample volume was made up to 5 mL with H₂O. The samples underwent a further 1100 dilution before ICP-MS analysis. Samples were diluted further as needed. The surface morphology was characterized by scanning electron microscopy (JEOL JSM-7600F, Field Emission SEM, 10 KV). The internal architectural structure was characterized by a micro-CT scanner (InspeXio; Shimadzu Science East Corporation, Tokyo, Japan) with a voxel size of 70 mm/pixel as a non-destructive method. Tri/3D-Bon software (RATOC System Engineering Co. Ltd, Tokyo, Japan) was used to generate a complete 3D reconstruction of the sample. The surface area was measured by using a Quantachrome Monosorb™ B.E.T. surface area analyzer and the pore size distribution profile was measured by nitrogen volumetric adsorption measurements (Quantachrome Autosorb pore size analyser).