Effects of the properties of Mg(OH)2 precursor on the hydrothermal synthesis of magnesium oxysulfate whiskers

Magnesium oxysulfate (MOS) whiskers are commonly synthesized through a dissolution-precipitation mechanism, wherein Mg(OH)2 dissolves to form Mg(OH)64-, followed by reactions with SO42- and H2O to yield MOS whiskers. However, the impact of Mg(OH)2 precursor properties on the formation process of MOS whiskers has been insufficiently explored, and sector-like and needle-like morphologies were both observed in previous studies. In this study, we systematically investigated how the properties of Mg(OH)2 precursors influenced the crystalline structure and morphology of MOS whiskers. Under various experimental conditions, MgSO4·5Mg(OH)2·2H2O (152MOS) whiskers were consistently obtained, regardless of Mg(OH)2 morphology and size. The size of Mg(OH)2 emerged as a critical factor in shaping the morphology of 152MOS whiskers. Smaller-sized Mg(OH)2 (≤106±37 nm) favored the formation of sector-like whiskers, while larger-sized Mg(OH)2 (≥206±98 nm) encouraged the development of needle-like whiskers. This distinction was attributed to the slower dissolution rate exhibited by larger-sized Mg(OH)2, maintaining a smaller number of 152MOS nuclei and promoting the growth of needle-like whiskers instead of the aggregation of 152MOS nuclei into sector-like structures. In addition, the effects of the molar ratio of Mg2+ to SO42- and hydrothermal temperature were also studied. The average length and diameter of the needle-like whiskers prepared with Mg(OH)2 of 331±145 nm under optimized conditions were 77±32 and 0.39±0.10 μm, respectively. This study presented an effective strategy for controlling the morphology of 152MOS whiskers.