The Effect of Washing on the Structural, Morphological, and Compositional Properties of CuS Nanopowder Synthesis via Chemical Bath Technique

1 Introduction

The importance of CuS nanopowder in the last years come through the using in the wide applications for different technological fields such as: light emitting [1], supercapacitor [2], treatment and purification of water and air [3,4], diverse biological uses and treatment [5,6,7], pharmaceutical and industrial [8], photocatalytic [9], photoelectrodes material in photovoltaic [10], solar cells [11], antibacterial device [12,13], optoelectronic [14], ammonia gas sensor [15], photoelectrochemical water splitting [16]. There are many methods to prepare CuS powder such as: wet chemical route [17,18], solvothermal method [19,20,21], hydrothermal method [22], magnetic grinding method [23], solid state grinding method [24], industrial milling [25], facile one pot microwave assisted [26], facile chemical conversion, double crucible sulfurisation [27], precipitation reaction [28], and precipitated from chemical bath deposition method [29,30,31,32]. It is known that the CBD technique was used to prepare thin films on the substrates. Regardless of whether the films are deposited on the substrates, the remaining solution after the experiment is discarded, resulting in a significant loss of the prepared solution in each experiment. In this study, the entire solution was utilised to produce pure nanopowder with minimal loss. In this study, the washing process was one of the purification methods used to remove unwanted impurities and surface contamination from the synthesised nanopowder [33]. The effect of washing on the structural, morphological, and compositional properties of the prepared CuS nanopowder was studied.

2 Experimental Details

The procedure of all prepared solutions of nanopowder such as solutions, molar concentrations, sources of copper sulphate, sodium thiosulphate, EDTA complex agent, and characterisation techniques which were used to test the structural, morphological, and concentration ratios of precipitated CuS nanopowder were mentioned in previous our paper in details [34]. The best synthesised parameters have been applied in this study (deposition time 45 min, water bath temperature 50°C, and pH 2). All details of the experimental procedure are illustrated in the following flowchart Scheme 1.

Scheme 1: The flowchart represents the experimental procedure for synthesising the CuS nanopowder in this work.

The prepared nanopowder samples are symbolised as “w-CuS and n-CuS”, indicating the washed and unwashed samples, respectively.

The prepared solution was slowly filtered through filter paper to obtain nanopowder. For the first sample, n-CuS, the nanopowder was lifted without washing; for the second sample, w-CuS, it was washed.

The washing method for precipitated nanopowder included six total times: two with absolute chilled distilled water, two with a mixed solution of 50% ethanol and 50% distilled water, and two with absolute ethanol.

A wash bottle was used to carefully spray the washing solution onto the precipitate nanopowder to prevent nanoparticles loss.

After each washing process, the precipitate was allowed to settle by gravity for 10 to 20 min, and the supernatant was decanted. The nanopowders from both samples were annealed at 150°C for 1 h in air. The percentage value of the yield was 98% and 91.59% for n-CuS and w-CuS, respectively.

In this study, the effects of washing and nonwashing on the structural, morphological, and compositional properties of precipitated CuS nanopowder were investigated.

3 Results and Discussion

3.1 X-Ray Diffraction Analysis

XRD patterns of precipitated CuS nanopowder without washing (n-CuS) and with washing (w-CuS) are shown in Fig. 1. Both without washing and with washing of CuS nanopowder revealed a hexagonal covellite nanostructure. The number of peaks of XRD patterns, and crystallographic planes for without washing n-CuS nanopowder indexed along (101), (102), (006), (103), (105), (110), (108), (116), and for washing w-CuS nanopowder indexed along (101), (102), (006), (110), (116) which belong to the hexagonal crystal structure of CuS according to JCPDS card No. 06-0464. Peaks at 2θ = 13.92, 18.48, 22.45 and 34.8° which corresponding Miller indices (002), (111), (112) and (220) in the XRD pattern for n-CuS indicate the presence of secondary phase Na2EDTA-Cu complex impurity according to the ICDD card No. 00-050-2051 and CuSO4·5H2O with 2θ = 17.19, 24 and 26° which corresponding Miller indices (111), (020) and (120) according to the JCPDS 01-077-1900 which may be unreacted, and this occurs on the surface of the nanopowder before washing and the intensity reduce after washing. Also, the (311) and (222) peak at 2θ = 73.8 and 77.5° in both n-CuS and w-CuS indicates a secondary phase Cu2O according to the JCPDS card No. 05-0667, due to partial oxidation or initial precursor ratios.

There are two main diffraction preferred peaks with high intensity corresponding to the Miller indices (103) and (110). From Fig. 1, the high intensity corresponding to (103) in sample n-CuS, which changes to (110) in sample w-CuS, means changing the preferred orientation plane because the precipitated nanopowder was affected by the washing, where any residual salts, ionic impurities, unreacted precursors, and contamination were removed, and the resultant nanopowder has been purified.

Also, the number of low intensity peaks in the sample w-CuS after washing is reduced, due to the enhanced in crystallinity of the prepared nanopowder.

Crystallite size ( C . S ) for both samples was calculated by using the Debye-Scherrer formula [35] C.S=0.9λβcosθ(1) where λ = 0.154 nm is the wavelength of the CuKα X-ray, θ is the Bragg angle (half of the diffraction angle 2 θ ) at which the peak is located, β is the full width at half maximum ( FWHM ) of the diffraction peak.

Lattice constants a = b , and c for both samples, are calculated by the following equation [36] 1d2=43h2+hk+k2a2+l2c2(2) where d is the interplanar spacing lattice distance ( Å ), ( hkl ) are Miller indices, and (a and c) are lattice constants in ( Å ).

2 θ for standard and experimental values, FWHM , d -spacing, Relative intensity %, Miller indices ( hkl ), and crystallite size ( C . S ) are tabulated in Table 1 and Table 2 for without washing n-CuS, and with washing w-CuS, respectively.

Lattice constants were calculated as (a = 3773, c = 16.398) Å, and (a = 3.792, c = 16.534) Å for sample n-CuS and w-CuS, respectively. The lattice constants are in very good agreement with previous studies [37,38]. The lattice constants for the sample w-CuS with washing are greater than those of the sample n-CuS without washing, due to residual strain, defects or compositional variations rather than increased crystallinity of the prepared nanopowder. The average crystallite sizes were found as to be 11.4 nm , and 4.5 nm for n-CuS and w-CuS, respectively. These values of C . S are lower than those reported [39,40,41], and agree with those reported [42,43] in previous studies. The crystallite size decreases due to the washing effect which removes or reduces agglomeration, impurities, capping agents, and surface contaminants on the prepared nanopowder [44].

Figure 1: XRD patterns for CuS nanopowder without washing (n-CuS) and with washing (w-CuS).

3.2 Surface Morphological Analysis

3.2.1 FESEM Characterization

Morphological properties of precipitated CuS nanopowder were characterised by field emission scanning electron microscopy (FESEM). Images of precipitated CuS nanopowder (without washing n-CuS and with washing w-CuS) are shown in Fig. 2 and Fig. 3, respectively. it is clear the nanoparticles in both samples n-CuS and w-CuS have been aggregated with hollow microspheres like structures which agreement with [45], and the aggregation of nanoparticles like as a Bucky spheres with thin shell, these microspheres were bonded between them such as hexagonal structure, and in the sample w-CuS the formed nanoparticles with the hexagonal structure have been observed with more purity in microstructure compare with that in the sample n-CuS, and this was observed in diffraction patterns in XRD analysis. In fact, the nanopowder consists of many nanoparticles, and each nanoparticle consists of many tiny primary nanoparticles. FESEM images, the nanoparticles in both n-CuS and w-CuS nanopowder are composed of many grains or primary nanoparticles that agglomerate to form nanoparticles or polycrystalline spheres. Fig. 4 shows the nanoparticle size distribution, indicating that w-CuS has a higher proportion of smaller nanoparticles compared to n-CuS. The average nanoparticles sizes are 62 nm for n-CuS and 58 nm for w-CuS. Consequently, the average nanoparticle exceeds the crystallite size determined by XRD analysis. Additionally, nanoparticle agglomeration is more pronounced in w-CuS than in n-CuS nanopowder. All nanoparticles sizes were analysed and determined by using the free ImageJ program.

Figure 2: FESEM images with scale (2 μm, 1 μm, and 200 nm) for CuS nanopowder without washing n-CuS.

Figure 3: FESEM images with scale (2 μm, 1 μm, and 200 nm) for CuS nanopowder with washing w-CuS.

Figure 4: Distribution of nanoparticle size for CuS nanopowder without washing (n-CuS) and with washing (w-CuS).

3.2.2 Compositional Analysis

The elemental compositions, the colour map of element distribution, and the SEM images obtained by EDS analysis are shown in Fig. 5. The percentage ratio (%) of the prepared nanopowder was taken only for elements that form the CuS nanopowder. The other elements O, C and Cl were found in both samples of prepared nanopowder due to the precipitated nanopowder technique in air. The percentage ratio (%) of elements Cu and S only was calculated through (Atomic At%) in Table 3. The precipitated nanopowders n-CuS (without washing) and w-CuS (with washing), showed real-phase values of 15%, which agree with the XRD pattern analysis. The best precipitated nanopowder was obtained after washing, indicating greater crystallisation of the formed nanopowder.

Figure 5: EDS plot included compositions of elements, a map for distribution of formed nanopowder, and an SEM image with scale (10 μm) of elements for CuS nanopowder with washing and without washing.

4 Conclusions

In this work, CuS nanopowder was precipitated using the CBD technique. The effects of washing and nonwashing on the structural, morphological, and compositional properties of precipitated CuS nanopowder were studied. The conclusion of the results is based on the following:

Both washed and unwashed CuS nanopowders revealed a hexagonal covellite nanostructure. Lattice constants were found as (a = 3773, c = 16.398) Å, and (a = 3.792, c = 16.534) Å, and the average crystallite size was found as 11.4 nm, and 4.5 nm for CuS nanopowder without washing and with washing, respectively.

Secondary phases common in the CBD technique come from varied stoichiometry of copper or sulphur, and oxygen in air can lead to the formation of Cu2S, CuSO4·5H2O and Na2EDTA-Cu complex, and the intensity of these secondary phases reduce after the washing process.

Precipitated nanopowder has been aggregated into hollow microsphere-like structures, termed Bucky spheres, with a thin shell, without washing and with washing, respectively, using CuS nanopowder. The average nanoparticles sizes are 62 nm for n-CuS and 58 nm for w-CuS.

The precipitated CuS nanopowder, both without and with washing, showed real and secondary phase values of 15% and 85%, respectively.

The two processes without and with washing are of high purity, but the process with washing was more pure and had more advantages in different applications.

In this study, the entire solution was used to produce pure nanopowder with minimal loss; therefore, the resulting CuS nanopowder with high purity can be utilised in various photovoltaic applications.