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Background  For decades, copper sulphide has been renowned as the superior optical and semiconductor materials. Its potential applications can be ranged from solar cells, lithium-ion batteries, sensors, and catalyst systems. The synthesis methodologies of copper sulphide with different controlled morphology have been widely explored in the literature. Nevertheless, the understanding on the formation chemistry of CuS is still limited. The ultimate approach undertaking in this article is to investigate the formation of CuS hexagonal plates via the optimization of reaction parameters in hydrothermal reaction between copper (II) nitrate and sodium thiosulphate without appending any assistant agent.    Results  Covellite (CuS) hexagonal plates were formed at copper ion: thiosulphate ion (Cu   2  +    :    S   2    O  32  −) mole ratio of 1:2 under hydrothermal treatment of 155°C for 12 hours. For synthesis conducted at reaction temperature lower than 155°C, copper sulphate (CuSO 4 ), krohnite (NaCu 2 (SO 4 )(H 2 O) 2 ] and cyclooctasulphur (S 8 ) were present as main impurities with covellite (CuS). WhenCu   2  +    :    S   2    O  32  −mole ratio was varied to 1: 1 and 1: 1.5, phase pure plate-like natrochalcite [NaCu 2 (SO 4 )(H 2 O)] and digenite (Cu 9 S 5 ) were produced respectively. Meanwhile, mixed phases of covellite (CuS) and cyclooctasulphur (S 8 ) were both identified whenCu   2  +    :    S   2    O  32  −mole ratio was varied to 1: 2.5, 1: 3 and 1: 5 as well as when reaction time was shortened to 1 hour.    Conclusions  CuS hexagonal plates with a mean edge length of 1 μm, thickness of 100 nm and average crystallite size of approximately (45 ± 2) nm (Scherrer estimation) were successfully synthesized via assisting agent- free hydrothermal method. Under a suitableCu   2  +    :    S   2    O  32  −mole ratio, we evidenced that the formation of covellite (CuS) is feasible regardless of the reaction temperature applied. However, a series of impurities were attested with CuS if reaction temperature was not elevated high enough for the additional crystallite phase decomposition. It was also identified thatCu   2  +    :    S   2    O  32  −mole ratio plays a vital role in controlling the amount of cyclooctasulphur (S 8 ) in the final powder obtained. Finally, reaction time was recognized as an important parameter in impurity decomposition as well as increasing the crystallite size and crystallinity of the CuS hexagonal plates formed.

Optimization of reaction parameters in hydrothermal synthesis: a strategy towards the formation of CuS hexagonal plates