Dispersion stability analysis of copper sulphide nanoparticles in different medium

Nanoparticles (NPs) of covellite CuS have been synthesised using a modified polyol method at 160ºC - 165ºC without adding any extra surfactant. These NPs were charecterized by X-ray diffraction (XRD), Raman spectroscopy, Field Emission Scanning Electron Microscope (FESEM), Energy Dispersive Analysis of X-ray (EDAX), Zeta potential and Dynamic Light Scattering (DLS) techniques. XRD data and three Raman modes confirm P63/mmc space group and hexagonal crystal structure of CuS NPs. EDAX data of Cu:S was slight differed from nominal 1:1 ratio due to extra S taken in synthesis. FESEM images show sheet-like NPs. The zeta potential (ξ) data analysis shows that the stability of the NPs is dependent on the dispersion medium, with high stability observed above pH 8.93 in an acidic medium. At pH 10.95, a highest ξ value of -37.64 mV is recorded in de-ionized water (DIW), while the nanoparticles are found to be unstable in ethanol and propanol as dispersion mediums. According to the hydrodynamic diameter (HD) data, the particles tend to agglomerate more quickly in ethanol and propanol compared to DIW. These findings are expected to be useful in various applications such as pigments in paints, batteries, pharmaceuticals, ceramics, and waste water treatment.


Introduction
Chalcogenides are substances that are bound through covalent bonds; contain one or more chalcogen elements, which may include sulfur (S), selenium (Se), or tellurium (Te), as a significant component.These materials are primarily semiconductors, with a band gap of 1 to 3 electronvolts (eV) [1].Their outstanding characteristics and diverse applications in electronic, electrical, magnetic, optical, and optoelectronic devices have garnered significant interest.Copper sulphides, are a type of metal chalcogenides that are chemically stable under ambient conditions, such as at room temperature.Examples of copper sulfides include CuS 2 (villamaninite), Cu 9 S 8 (yarrowite), Cu 1.8 S (digenite), Cu 1.75 S (anilite), Cu 1.96 S (djurleite), CuS (covellite), and Cu 2 S (chalcocite) etc [2].CuS is of particular interest among these sulfides due to its low toxicity, affordability, natural abundance, and metallic properties down to ~5 K.With an energy band gap ranging from 1.55 to 2.15 eV, it functions as an indirect ptype semiconductor [2].
The zeta potential (ξ -potential) refers to the electrical potential difference that exists at the interface of liquids and solids.This measurement is used to determine the surface charge in a liquid medium of suspended NPs.Greater the repulsion between the particles, higher the surface charge, which in turn indicates the particle stability within the suspension medium and provides information about NPs' surface state.Here, we have reported the synthesis of CuS NPs using polyol method at 160ºC -165ºC.The next step is to investigate the stability of these particles, using the DLS technique, at different pH values, in various dispersing media.This method involves exposing the sample to a monochromatic light wave and detecting the signal using an avalanche photodiode.

Sample synthesis
A standard procedure for synthesizing CuS NPs using the modified polyol method involves combining 2 mmol of Cu(NO 3 ) 2 .3H 2 O (99% , MERCK) and 5 mmol of CH 4 N 2 S (99%, MERCK) in a roundbottom flask with three necks with 40 ml of ethylene glycol (EG) (99%, MERCK).An excess amount of CH 4 N 2 S was used in the synthesis due to the volatile nature of sulfur [2].The mixture was heated to a temperature of 160-165ºC for a period of 2 hours, during which continuous argon gases flow of was maintained.The resulting reaction produced a precipitate (ppt) of black colour.The ppt was left to cool down to room temperature, after completion of the reaction.The product was subjected to centrifugation for 10 minutes at 12,000 rpm.After centrifugation, the resulting supernatant was thrown away, and the black ppt was washed using ethanol along with 4 minutes of probe ultrasonication.The above steps were repeated one more time to ensure the complete removal of EG.After the NPs had fully settled, the ethanol was poured off after one hour.The resulting ppt was then vacuum-dried at a temperature of 60ºC for 2 hours, thereby obtaining a powder sample of CuS NPs.

Characterization techniques
The synthesized CuS sample was subjected to Powder XRD analysis with Cu K α radiation (1.54 Å) in Bruker D8 Advance X-ray diffractometer in the angle range from 20° to 70°.Additionally, room temperature Raman spectroscopy measurements were performed at with the Jobin Yvon Horiba LABRAM-HR.The particle size distribution of the synthesized CuS sample was determined using FESEM in secondary emissions mode with Carl Zeiss AURIGA FIBSEM.For performing EDAX measurements, a JEOL JSM 5600 SEM equipped with EDAX was used.The DLS-based Zeta/nanoparticle analyser (NanoPlus-3) was used for particle size and zeta potential measurements at room temperature, with a detection angle of 15º.Before the measurements, nanoparticles were dispersed separately in DIW, ethanol, and propanol and subjected to probe sonication for 8 minutes before analysis.

Results and discussion
The XRD pattern showed that the synthesized covellite CuS NPs with a hexagonal crystal structure, having lattice parameters of a = 3.79 Å, b = 3.79 Å and c = 16.41Å and belong to the P6 3 /mmc space group, which is consistent with the, JCPDS-79-2321, standard hexagonal CuS phase of (figure 1a).The diffraction peaks observed in XRD analysis for the CuS nanoparticles are broad and centered  216), ( 102), ( 103), (006), ( 105), ( 110), ( 108) and (203) crystal planes of hexagonal CuS phase, respectively.The absence of additional peaks suggests that the sample is singlephase in nature.The average crystalline size of the nanoparticles was determined to be approximately 22 ± 0.5 nm by using the Debye-Scherrer formula [1,3].
Three peaks in the micro Raman spectrum further confirmed the hexagonal covellite phase of NPs.The strong peak detected at 474.6 cm -1 in figure 1b is due to S-S stretching vibrational mode of A 1g symmetry for S atoms at 4e sites, which indicates that 2/3 of sulfur atoms in the covellite structure of CuS form covalent S−S bonds.In addition, two weaker peaks are identified at 139 cm -1 and 268 cm -1 (associated with the Cu-S vibrational mode), which is in agreement with previous studies [2].
FESEM images of the CuS NPs are shown in Figure 2(a, b), which exhibit a sheet-like shape with varying breadth sizes with an average of 30 nm.In figure 2c, the EDAX data of the NPs indicates an atomic percentage ratio of 47.77 for Cu and 52.23 for S, revealing a slight deviation from the expected 1:1 ratio, likely because of the additional sulfur utilized during synthesis of the NPs.The EDAX spectrum showed no presence of any other elements.To determine the stable dispersion medium, particle size and zeta potential analyses were conducted with DIW, ethanol, and propanol as the chosen dispersion media.The intensity  In figure 3(b, c), a noticeable peak shift in the second run towards larger HDs can be observed, indicating a rapid agglomeration of smaller particles [4].This trend was observed for both ethanol and propanol as dispersion media, in comparison to DIW The ξ data obtained at pH 7 corroborate these findings and verify the greater stability, when DIW is used as the dispersion medium, with a highest ξ = -19.09mV (figure 3d).Conversely, when ethanol and propanol were used as the dispersion media, the solution exhibited high instability, with maximum ξ of -18.85 mV and -6.47 mV, respectively (figure 3(e, f)).Based on these results, it can be inferred when particles become more aggregated, their mobility within the dispersion medium decreases, resulting in lower ξ values and less stable colloidal dispersion.As a result, to further investigate the impact of varying pH values (from 3 to 11) on ξ, we have opted to use DIW as the dispersion medium.Figure 4 illustrates the influence of changing pH values on zeta potential for CuS NPs dispersed in DIW.The absence of an isoelectric point indicates a highly stable nature of the system, with a minimal agglomeration of nanoparticles.To avoid agglomeration, a colloidal system needs to possess a high ξ exceeding ±30 mV [5].The results indicate that the NPs exhibit high stability when the pH is above 8.93, with a ξ value of -30.08 mV.As the pH value increases, the ξ value also increases and reaches a maximum value of -37.64 mV at pH 10.95.In contrast, the NPs exhibit low stability at pH 3.09 with a lowest ξ = -9.13mV.The ξ value of a particle in a particular medium represents the overall charge acquired by the particle; hence the negative ξ values observed in this study suggest that the NPs' surface in DIW creates a negative double layer.The findings from this study are expected to be valuable in various applications such as pigments in paints, batteries, pharmaceuticals, waste water treatment and ceramics.

Conclusion
Nanoparticles (NPs) of covellite CuS have been successfully synthesised via a modified polyol method at ~160 ºC-165 ºC.Absence of any impurity peaks and formation of the hexagonal crystal structure with P6 3 /mmc space group has been confirmed by XRD data.Additionally, their stoichiometric formation has been further validated by the presence of three Raman modes.The EDAX spectrum has confirmed that the as-synthesized NPs contain only Cu and S, with no traces of other elements present.FESEM shows the sheet-shaped NPs have an average thickness of 30 nm.According to the HD distribution curve, the agglomeration of particles occur more rapidly in ethanol and propanol, which are used as dispersion mediums, when compared to DIW.The analysis of the zeta potential data indicates that at pH above 8.93 these NPs are highly stable, with the highest ξ zeta value at pH 10.95 (-37.64 mV) is observed in DIW.However, they are found to be unstable in ethanol and propanol.This information is anticipated to have significant value for numerous industrial applications.

Figure 1 .
Figure 1.(a) Powder XRD pattern and (b) Raman spectrum of CuS NPs

Figure 3 .
Figure 3. Intensity distribution of (a-c) HDs and (d-f) ξ-potential data of CuS NPs at pH 7 in DIW, ethanol and propanol for first and second run.
diameter (HDs) and zeta potential at pH 7 for the first run and second run of the NPs is presented in figure 3.

Figure 4 .
Figure 4. Effect of pH on ξ potential of CuS NPs dispersed in DIW.