title: Optimization of reaction parameters in hydrothermal synthesis: a strategy towards the formation of CuS hexagonal plates.
abstract: 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 ([Formula: see text]) 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 (CuSO4), krohnite (NaCu2(SO4)(H2O)2] and cyclooctasulphur (S8) were present as main impurities with covellite (CuS). When [Formula: see text] mole ratio was varied to 1: 1 and 1: 1.5, phase pure plate-like natrochalcite [NaCu2(SO4)(H2O)] and digenite (Cu9S5) were produced respectively. Meanwhile, mixed phases of covellite (CuS) and cyclooctasulphur (S8) were both identified when [Formula: see text] 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 suitable [Formula: see text] 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 that [Formula: see text] mole ratio plays a vital role in controlling the amount of cyclooctasulphur (S8) 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.
authors.affiliation: COMBICAT Laboratory, Nanotechnology & Catalysis Research Centre (NANOCAT), University of Malaya, Kuala Lumpur 50603, Malaysia ; Chemistry Department, Faculty of Science, University of Malaya, Kuala Lumpur 50603, Malaysia.
authors.forename: Yow Loo
authors.initials: YL
authors.lastname: Auyoong
authors.affiliation: COMBICAT Laboratory, Nanotechnology & Catalysis Research Centre (NANOCAT), University of Malaya, Kuala Lumpur 50603, Malaysia ; Chemistry Department, Faculty of Science, University of Malaya, Kuala Lumpur 50603, Malaysia.
authors.forename: Pei Lay
authors.initials: PL
authors.lastname: Yap
authors.affiliation: Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
authors.forename: Xing
authors.initials: X
authors.lastname: Huang
authors.affiliation: COMBICAT Laboratory, Nanotechnology & Catalysis Research Centre (NANOCAT), University of Malaya, Kuala Lumpur 50603, Malaysia.
authors.forename: Sharifah Bee
authors.initials: SB
authors.lastname: Abd Hamid