1 Introduction

Dithiolates are well-studied ligands in coordination chemistry due to their remarkable applications in various fields and capable of forming very stable complexes with transition metals [1, 2]. Particularly, transition-metal complexes involving dithiocarbamate ligands have received much attention in contemporary research due to their excellent applications in organic synthesis, medicine, analytical chemistry, biology and material science [3,4,5,6,7]. The study of coordination chemistry of copper complexes has been impacted by the roles played by copper in the ligands used in the Wilsons disease therapy due to their affinity for copper [8]. Modifications in the N-bound organic moiety of the dithiocarbamate ligand can lead to changes in structure and properties of complexes [9, 10]. Three different structural types have been reported for copper(II) dithiocarbamate complexes. Most of them form monomers and contain a square planar copper(II) centre [9, 11, 12]. While few complexes adopt centrosymmetric dimers with one terminal and one bridging dithiocarbamate ligand per copper centre [13, 14]. Bis(N,N-dimethyldithiocarbamato-S,S’)copper(II) is different from other in that the structure is polymeric chain [15]. The copper centre adopts a distorted octahedral coordination environment.

In addition to these structural variations and several applications of copper(II) dithiocarbamates, they were used to prepare various phase, size, shape and composition of copper sulfide nanoparticles [16,17,18,19]. Copper sulfide has wide range of applications in various fields, including thermoelectric, photocatalysis, photovoltaic, lithium ion batteries, hydrogen generation and chemical sensing applications [20,21,22,23]. Various dyes are widely used in different industries due to the low cost of their synthesis and their special properties. Congo red is one of the dyes that is commonly used in textile, paper, chemicals and cosmetics industry [24, 25]. Because of its intensive use, a considerable amount of congo red dye is found in industrial effluents. Congo red is resistant to degradation caused by natural processes. Congo red is slightly toxic and carcinogenic anionic dye [26]. Hence, the presence of congo red industrial waste water causes negative effects on all organisms in the ecosystem [27]. Therefore, it is necessary to design efficient methods to remove congo red dye from industrial effluents.

Photocatalysis is one of the most promising techniques among the oxidation techniques for the treatment of wastewater from dye industries [28]. Furthermore, various photocatalytic materials have been investigated for waste water treatment under UV and visible light irradiation [29]. Among various photocatalysts, copper sulfide is considered one of the promising materials because it is cheap and non-toxic p-type semiconductor [30, 31]. Todate, some important developments have been made in the field of CuS serving as photocatalyst. Recently, several authors fabricated copper sulfide nanomaterials, which have promising application in industrial waste water purification [32, 33]. In this paper, we report on the synthesis, spectral, structural and DFT studies on Bis(N-hexyl-N-(4-methylbenzyl)dithiocarbamato-S,S’)copper(II) (1) and bis(N-dodecyl-N-(4-methylbenzyl)dithiocarbamato-S,S’)copper(II) (2). Preparation of copper sulfides from 1 and 2 and their characterization are also presented. Photocatalytic activity of copper sulfides is also evaluated.

2 Experimental

2.1 Materials

Dodecylamine (98%, SRL chemicals), Hexylamine (99%, sigma aldrich chemicals), 4-methylbenzaldehyde (98.0%, Himedia chemicals), Sodium borohydride (98%, Avra chemicals) and copper sulphate pentahydrate (CuSO4.5H2O, 98.0%, EMPLURA) were used as received without further purification.

2.2 Instrumentation

Complexes were characterized by the physicochemical methods involving carbon, hydrogen, nitrogen elemental analysis, IR and UV–Visible spectroscopic techniques. Perkin Elmer 2400 seies(II) elemental analyzer was used for C, H and N analysis. Electronic spectra were recorded in the wavelength range of 200–800 nm using SHIMADZU UV-1650 PC spectrophotometer. Agilent-Cary 650 IR spectrophotometer was used to capture the infrared spectra from 4000 cm−1–600 cm−1. FESEM (Gemini zeiss Sigma 300, Germany) provided morphological details and the results of energy dispersive X-ray spectroscopy (EDS) were employed to detect the constituents of the samples. The powder X-ray diffraction measurements were conducted using Cu Kα (λ = 1.5418 Å) X-ray diffractometer (BRUKER).

2.3 X-ray crystallography

The Bruker D8 Quest diffractometer was used for the determination of cell parameters and intensity data collection at 298 K using MoKα radiation (λ=0.71073 Å). The APEX-III software is used for the collection of diffraction data. The structures were solved (SHELXL-2018) and refined with full matrix least square methods based on F2 (SHELXL-2018) [34, 35] All the atoms heavier than the hydrogen atoms are refined using anisotropic displacement parameters, while the hydrogen atoms are assigned by relative isotropic parameters. ORTEP-3 [36] and Mercury software [37] are employed to express the findings of diffraction results.

2.4 Photocatalytic activity experiment

Photocatalytic reactions were carriedout in Heber multilamp photoreactor with Sankyo Denki (Japan) 8 W mercury UV lamp. Photocatalytic activity of copper sulfide was assessed using congo red. Typically 50 mg of copper sulfide was added in 40 mL solution of congo red (1 × 10–4 M) in a reaction vessel. The mixture was stirred in the dark for 30 min to achieve adsorption–desorption equilibrium before by taking the sample solution at every 10 min. The concentration of dye was estimated by recording the UV–Visible spectra of the samples at a maximum absorption wavelength (496 nm).

2.5 Theoretical studies

Based on the literature [38, 39], the Density Functional Theory (DFT) was used at the B3LYP/ LanL2DZ level performed by Gaussian 09 code set for computational calculations [40]. SOMO, LUMO and molecular electrostatic potential (MEP) were created using Gaussview 5.0 program [41]. Hirshfeld surface and 2D fingerprint plot were determined using the crystal explorer 21.0 software [42, 43]. CIF files were used as input files to do the calculations.

2.6 Synthesis of Bis(N-hexyl-N-(4-methylbenzyl)dithiocarbamato-S,S’)copper(II) (1) and bis(N-dodecyl-N-(4-methylbenzyl)dithiocarbamato-S,S’)copper(II) (2)

4-methylbenzaldehyde (5.3 mmol, 0.5 mL) was dissolved into 20 mL of methanol in a 100 mL beaker. Another solution of hexylamine or dodecylamine (4.6 mmol) in 20 mL methanol was prepared and both were mixed. The reaction mixture was stirred for a period of 4 h at room temperature. After that solution was allowed to evaporate. Oily product was obtained. This was dissolved in methanol and sodium borohydride (13.18 mmol, 0.5 g) was added under stirring. The solvent was evaporated. The resulting viscous liquid was washed with water and dichloromethane was added to extract the product. This product was dissolved in 20 ml of ethanol. To this solution, sodium hydroxide (4 mmol, 0.2 g) was added and mixture was stirred for 2 h in ice cold condition (5 ºC). Yellow colour solution was obtained. To this reaction mixture, 20 mL aqueous solution of copper sulphate pentahydrate (CuSO4.5H2O) (2 mmol, 0.49 g) was added with continuous stirring for 30 min. The dark brown precipitate was collected. The precipitate was washed with ethanol and then dried (scheme 1).

Scheme 1
scheme 1

Preparation of complexes 1 and 2

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3 Complex 1

Yield 73%, mp 98-100ºC. IR (KBr, cm−1): 1494ν(C–N); 963ν(C–S). UV–Vis (CHCl3, nm): λ = 612, 434, 274, and 232. Anal. Calcd. For Chemical Formula: C30H44N2CuS4 (%): C, 57.70; H, 7.10; N, 4.49; Found:C, 57.48; H, 7.12; N, 4.42.

4 Complex 2

Yield 78%, mp 94-96ºC. IR (KBr, cm−1): 1497ν(C–N); 967ν(C–S). UV–Vis (CHCl3, nm): λ = 606, 436, 276 and 232.); Anal. Calcd. For Chemical Formula: C42H68N2CuS4 (%): C, 63.63; H, 8.65; N, 3.53; Found: C, 63.70; H, 8.60; N, 3.51.

4.1 Preparation of copper sulfide (Cu7S4-1) from 1

0.5 g of Bis(N-hexyl-N-(4-methylbenzyl)dithiocarbamato-S,S’)copper(II) (1) was dissolved in 15 mL of diethylenetriamine in a round bottom flask and then heated to 240 ºC for 10 min. The reaction was carried out under open air. The solid separated was filtered off and washed with methanol. Final mass: 0.0381 g, Yield: 59%.

4.2 Preparation of copper sulfide (Cu7S4-2) from 2

A method similar to that described for the preparation of Cu7S4-1 was adopted to prepare Cu7S4-2. 0.5 g of 2 was refluxed with triethylenetriamine (270 ºC) for 15 min. The reaction was carried out under open air. The black solid was filtered, washed with methanol and dried. Final mass: 0.0321, Yield: 63%.

5 Results and discussion

5.1 Spectral studies

FTIR and electronic spectra of 1 and 2 are displayed in Figs. S1–S4. In IR spectra of complexes 1 and 2, the appearance of νC–N and νC–S bands indicate the formation of dithiocarbamate complex. The strong νC–N band was observed at 1494 and 1497 cm−1 for 1 and 2, respectively. This stretching vibration reveals the carbon–nitrogen partial double bond character due to the delocalization of electrons in NCS2M moiety [44]. A single band found at 963 and 967 cm−1 for 1 and 2, respectively, has been attributed to the stretching vibration of νC–S suggested a bidentate coordination of dithiocarbamate ligands [44].

The absorptions < 300 nm in the electronic spectra of 1 and 2 are assigned to intraligand π-π* transition within the NCS2 group [45, 46]. A band observed at 434 and 436 nm for 1 and 2, respectively, is propably due to ligand to metal charge transfer (LMCT) transitions [45]. A less intense absorption band, responsible for the brown colour and due to d-d transitions is present at 612 and 606 nm, suggesting square planar coordination geometry around copper(II) [13]

5.2 Structural analysis of 1 and 2

A perspective view of 1 and 2 are shown in Fig. 1 together with the atom numbering scheme. Data collection and refinement parameters are given in Table 1. Table 2 reports the selected bond lengths and angles. Both copper complexes 1 and 2 are monoclinic belonging to the P21/n and C2/c space groups, respectively. Complexes are centrosymmetric with half of the molecule in the asymmetric unit. Figure 1 exhibits that both complexes are mononuclear neutral species with a central copper(II) ion coordinated to four sulfur atoms from two dithiocarbamate ligands. Four coordinated copper(II) ions adopt a distorted square planar geometry (sp2d hybrid state of copper(II)). The tight S–Cu–S chelating angles 77.56(3)º and 77.58(2)º contribute to the distortion of the square plane. Dithiocarbamate ligands in bis(dithiocarbamato-S,S’)copper(II) complexes are considered as stereo chemically rigid and sterically non-demanding because the planarity of the C2NCS2Cu determines the structures since all the seven atoms lie in a plane in both complexes. The Cu–S bond lengths are almost equal (2.2846(8) Å to 2.3144(9) Å) in both complexes, which indicates that the dithiocarbamate ligands are symmetrically coordinated to copper(II) ion. Similar bond lengths were reported in copper(II) dithiocarbamate complexes [9, 11, 12]. The observed C–S (1.715(2) Å to 1.726(3) Å) and C–N (1.319(3) Å) bond distances are longer than double bonds and shorter than single bond distances. The observed intermediate value indicates partial double bond character of C–S and C–N bonds and the delocalization of π-electrons in the backbone of the dithiocarbamate ligand (NCS2) [9, 10].

Fig. 1
figure 1

ORTEP diagram of a 1 and b 2

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Table 1 Crystal data, data collection and refinement parameters for complexes 1 and 2
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Table 2 Selected bond distances (Å) and bond angles (º) by X-ray diffraction and theoretical (DFT) calculations for complexes 1 and 2
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The packing of 1 is consolidated by intermolecular C–H∙∙∙π and C–H∙∙∙S interactions. The C–H∙∙∙π interaction has C–H∙∙∙centroid distances of 3.054 Å with C–H∙∙∙centroid angle of 126.6º (Fig. S5, Table S1). The intermolecular C–H∙∙∙π interactions link the molecule into chains. Sulfur atom S2 hosts an intermolecular hydrogen bond with H4 (Fig. 2a). Molecules of 2 are also held together in the crystal by intermolecular C–H∙∙∙S interactions. On each molecule, six hydrogen atoms of dodecyl group are engaged intermolecular C–H∙∙∙S hydrogen bonds with all sulfur atoms of dithiocarbamate moiety (Fig. 2b, Table S2).

Fig. 2
figure 2

Intermolecular C–H∙∙∙S interactions in a 1 and b 2

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5.3 Hirshfeld surface analysis

Hirshfeld surface analysis is an effective method to quantify the nature of the intermolecular interactions. The colour coding is used to visualize the interactions. Red and blue regions indicate the strong and weak hydrogen bond interactions. White regions represent the contacts larger than van der Waals radii [42, 43]. The 3D dnorm surfaces enable the identification of previously characterized contacts using X-ray crystallography. The deep red spots in 1 and blue regions in 2 exhibit the S∙∙∙H interactions observed in crystal structure (Fig. 3). Furthermore, 2D fingerprint plots are used to quantify the intermolecular contacts experienced by the molecules in the crystal packing. Figure 3 shows the 2D fingerprint plot 1 and 2. The H∙∙∙H, S∙∙∙H and C∙∙∙H are the important interatomic contacts that have stronger contribution to crystal packing. The H∙∙∙H interaction contributes 66.4 and 75.9% to the overall crystal packing of 1 and 2, respectively. The contribution of H…H interactions to the total Hirshfeld surfaces in 2 is greater than those of 1 due to the presence of more hydrogen atom hydrocarbon chain (dodecyl) in 2. Further, S… H/H…S (18.18 and 13.6% for 1 and 2, respectively) and C…H/H…C (11.6 and 8.1% for 1 and 2, respectively) fingerprint plots also reveal the information regarding the intermolecular hydrogen bonds with contribution towards crystal packing. The other interatomic contacts that have less contribution to crystal packing are such as Cu…H/H…Cu, C…C and C…N/N…C with percentage contributions in the range 2.5% to 0.1%.

Fig. 3
figure 3

The two-dimensional fingerprint plots for 1 (a) and 2 (b) showing all interactions, and delineated into H…H, S…H and C…H interactions

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5.4 DFT Studies

The optimized molecular structures of 1 and 2 are displayed in Fig. S6. The obtained geometrical parameters with experimental data are presented in Table 2. The calculated Cu–S bond lengths are longer (0.11–0.15 Å) than those of experimental results. The C–S and C–N bond lengths are also slightly longer (0.03–0.07 Å) than the experimental values. These differences could be attributed to the fact that theoretical calculation focuses on an isolated molecule, neglecting the intermolecular forces existing in crystals which influence the overall configuration [47]. Correlation graphs were drawn between the experimental and calculated bond parameters (Fig. S7). R2 values found for 1 and 2 are 0.999 reflects the good correlation between theoretical and experimental bond parameters.

5.5 SOMO-LUMO analysis

SOMO and LUMO energy levels and SOMO-LUMO energy gap are important parameters to understand the chemical stability and electronic properties of the compounds. Since Cu(II) has an odd number of electrons (d9 configuration) in both complexes, the SOMOs (singly occupied molecular orbitals) and LUMOs (lowest unoccupied molecular orbitals) were obtained for both α and β cases. α and β basically denote spin-up (+ 1/2 for electrons) and spin-down (− 1/2 for electrons), respectively [48, 49]. Frontier orbitals of complexes 1 and 2 are shown in Fig. 4. The molecular orbital energies of the α/β-SOMO and α/β-LUMO for 1 -5.3449/-5.9558 eV and − 1.0919/− 3.2036 eV, respectively, and for 2, − 5.3380/− 5.9492 eV and − 1.0868/− 3.1973 eV, respectively. Figure 4 exhibited that SOMO and LUMO of α in both complexes are mainly concentrated on S4Cu and (NCS2)Cu, respectively, whereas SOMO and LUMO contours of β in both complexes can be seen over S4 and S4Cu atoms. In both the complexes, the SOMO and LUMO energies of β-spin are lower than those of α-spin. The energy gap between SOMO and LUMO of α-spin (4.2529 and 4.2512 eV for 1 and 2, respectively) is greater than that of β-spin (2.7523 and 2.7519 eV for 1 and 2, respectively). Hence the global reactivity parameters for both complexes were discussed on the basis of only beta ESOMO—ELUMO energy gap. The energy gap of 1 is slightly greater than those of 2 which indicates complex 1 is less reactive than 2. Chemical reactivity indices like absolute electronegativity (χ = I + A/2), absolute softness (σ = 1/η), chemical hardness (η = I–A/2), chemical potential (μ =–χ), electrophilicity (ω = μ2/2η), ionization potential (IP = -EHOMO) and electron affinity (EA = – ELUMO) are calculated using DFT. The calculated global reactivity descriptors of both complexes are given in Table 3. The softness and hardness of both complexes are almost same which indicates that the substitution of hexyl in dithiocarbamate ligand by dodecyl group does not affect the hardness and softness of the complexes. However, the electrophilic character of 1 as indicated from the electrophilicity index (ω) is slightly higher than 2.

Fig. 4
figure 4

SOMO-LUMO plots of a 1 and b 2

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Table 3 Global chemical reactivity parameters for complexes 1 and 2
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5.6 Molecular electrostatic potential (MEP) map

MEP has been used primarily for predicting sites and reactivity towards electrophilic and nucleophlic attack. The MEP surface of red, blue, yellow and green colour region on molecules exhibits the electron rich, electron deficient, slightly electron deficient and neutral region, respectively. MEP map is shown in Fig. 5. The red region is localized on S atoms in both complexes. This supports the existence of intermolecular S∙∙∙H interactions. The S∙∙∙H interactions are observed in single crystal X-ray diffraction analysis and Hirshfeld surface analysis of both complexes. However, the positive region (blue) is localized on most of the hydrogen atoms.

Fig. 5
figure 5

The electrostatic potential surface of a 1 and b 2.

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5.7 Characterization of copper sulfide

XRD analysis was used to analyze the crystal phase of the sample. Powder X-ray diffraction pattern of 1 and 2 are displayed in Fig. 6. Several well-defined characteristic peaks at 2θ values 22.52º, 23.94º, 27.79º, 29.13º, 32.16º, 35.05º, 36.93º, 39.64º, 42.14º, 46.37º, 54.65º and 66.57º were obtained corresponding to (2 0 0), (2 0 1), (2 0 2), (1 1 3), (2 2 0), (3 0 1), (3 1 1), (3 1 2), (3 2 1), (2 2 4) and (2 3 6) planes. The XRD profile of the as-prepared copper sulfide nanoparticles were indexed to orthorhombic phase referenced to the standard Cu7S4 phase (JCPDS card No: 72–0617). In Cu7S4, Cu2+ is bonded to four S2− atoms to form tetrahedra that shares corners with two equivalent Cu7S4 tetrahedra and an edge with Cu7S4 tetrahedra [50].The average grain size of the material was estimated using the Debye-Schrrer’s formula (D = 0.9λ/ βcosθ) [51]. Where D is the crystallite size (nm), β is fullwidth at half maximum and θ is the angle of diffraction. The grain sizes of Cu7S4-1 and Cu7S4-2 were found to be 30 and 24 nm, respectively.

Fig. 6
figure 6

Powder X-ray diffraction patterns of a Cu7S4-1 and b Cu7S4-2

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Energy dispersive X-ray spectroscopy was used to analyze the chemical composition of as-prepared copper sulfide nanoparticles and the obtained spectra of both samples are shown in Fig. 7. Both the spectra exhibit relatively strong peaks for the elements of copper and sulfur from the emission of energies confirming the formation of copper sulfides. The other peaks in the spectra are due to the presence of capping agent (diethylenetriamine and triethylenetetramine in Cu7S4-1 and Cu7S4-2, respectively).

Fig. 7
figure 7

EDS spectra of a Cu7S4-1 and b Cu7S4-2

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The morphology of as-prepared copper sulfide nanoparticles were investigated using field emission scanning electron microscopy (FESEM). Figure 8 displays the FESEM micrographs of Cu7S4-1 and Cu7S4-2. FESEM pictures of both nanoparticles exhibited spherical nanoparticles with an average size of ̴̴ 10 nm. Due to the low size of the particles, the particles agglomerated to give sharp edged rods and irregular shape sheets with micrometer size in Cu7S4-1 and Cu7S4-2, respectively. PXRD patterns of both copper sulfides indicate the formation of the same phase in Cu7S4-1 and Cu7S4-2. However, FESEM images indicate the shape and size of copper sulfide particles are different. This reveals that the shape and size of particles are affected by the precursor and capping agent.

Fig. 8
figure 8

FESEM images of a and b Cu7S4-1 and c and d Cu7S4-2

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Metal sulfide nanoparticles were prepared using metal dithiocarbamate as single source precursors [52]. Different copper(II)dithiocarbamate complexes are also used to prepare copper sulfides. Solvent, composition, morphology and phase of copper sulfide obtained from solvothermal decomposition of some bis(dithiocarbamato)copper(II) complexes are displayed in Table 4. Table 4 shows that various composition, morphology and phase copper sulfide nanoparticles can be prepared using different bis(dithiocarbamato)copper(II) complexes and different solvents [18, 19, 46, 53, 54].

Table 4 Comparison of the dithiocarbamate precursor of as-prepared copper sulfide nanoparticles with the formerly reported data for the composition, phase and morphology
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5.8 UV-DRS analysis

UV-DRS spectra were recorded for both copper sulfides to determine the optical band gap energy. We can estimate the optical band gap energy (Eg) of copper sulfides using Kubelka–Munk function F(R) [55]. According to the Tauc model:

$$F(R)hv)^{1/2} = A\,(hv - Eg),$$

where h, ν, Eg and A are Plank’s constant, light frequency, band gap energy and constant, respectively [56]. The plots of F(R)hν)2 versus hν shown in Fig. 9 provide the optical band gaps of both copper sulfides. By extrapolating the smooth linear part of the curves toward the x-axis, we can determine the band gap energies. The band gap energy of Cu7S4-1 and Cu7S4-2 is found to be 5.04 eV and 4.96 eV, respectively.

Fig. 9
figure 9

UV-DRS analysis of a Cu7S4-1 and b Cu7S4-2

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Based on the calculated band gap energies from UV-DRS, the values of valence band (VB) and conduction band (CB) potentials were calculated using the following equations [57,58,59]:

$$E_{CB} = \chi - E_{e} - 0.5\,E_{g}$$
$$E_{VB} = E_{CB} + E_{g},$$

where χ is the Mulliken electronegativity of Cu7S4 (5.06 eV) [60], Ee is the free electrons energy of normal hydrogen electrode scale (ENHE) of the order of 4.5 eV [61]. Eg is the obtained Kubelka–Munk band gap energy (5.04 and 4.96 eV for Cu7S4-1 and Cu7S4-2, respectively).

Table 5 displays the band gap, VB and CB potentials. Semiconductors with more negative CB and more positive VB levels have wide band gaps which will enhance the effective utilization of UV light. As CB values become more negative, there is typically an increase in the generation of reductive photoexcited electrons and a reduction in electron hole recombination due to the enhanced potential for charge transport. Furthermore, the enhanced photo oxidation efficiency and swift transfer of generated holes can also be attributed to the positive VB values. CB and VB band positions of Cu7S4-1 and Cu7S4-2 indicate that these are good photocatalysts for the degradation of organic pollutants [62, 63].

Table 5 Band gap, VB and CB potentials of the as-prepared Photocatalyst
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5.9 Photocatalytic activity

Photocatalytic activity of as-prepared copper sulfide nanoparticles was evaluated via the photodegradation of congo red under UV irradiation. Figure 10 shows the UV–Visible absorption spectra of congo red absorbance with respect to time for Cu7S4-1 and Cu7S4-2. It can be seen that the intensity of the absorption maxima at 496 nm decreases with increases in the irradiation time. This result exhibits that the photodegradation of congo red increases with increasing irradiation time. The degradation efficiency was calculated using the following equation

$${\text{\% }}\,{\text{of}}\,{\text{degradation}}\,{\text{efficiency}}\, = \,({\text{A}}_{{0}} - {\text{A}}_{{\text{t}}} {\text{/A}}_{{0}} ) \times 100$$
Fig. 10
figure 10

Time-dependent UV–Vis absorption spectra for photodegradation of congo red in the presence of a Cu7S4-1 and b Cu7S4-2

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The degradation efficiency of Cu7S4-1 and Cu7S4-2 was found to be 91 and 71%, respectively, after 70 min irradiation time under UV light irradiation. A relative change in congo red concentration (C/C0) as a function of time in the presence of photocatalysts is shown in Fig. 11. In order to further understand the photocatalytic reaction, kinetic studies were carried out [64]. The relationship between lnCt/C0 and UV light irradiation time t presented in Fig. 12 illustrates linear relationship. Hence, the pseudo first order kinetics model is used to explain the photodegradation of congo red using as-prepared catalysts. The slope of the plots (Fig. 12) was used to calculate the rate constant. The rate constant was found to be 0.0871 min−1 and 0.0657 min−1 for Cu7S4-1 and Cu7S4-2, respectively. When UV light is passed, copper sulfides absorb photons, resulting in the formation of electron–hole. The electrons transferred to VB absorb oxygen to give superoxide radical anions (O2–⋅) while the holes in the VB reacted with water to yield hydroxyl (OH⋅). These highly oxidizing species attack the dye and produce H2O and CO2 to complete the degradation process [65].

Fig. 11
figure 11

Photodegradation (C/C0 vs Time) of congo red in the presence of a Cu7S4-1 and b Cu7S4-2

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Fig. 12
figure 12

Kinetics analyses of degradation of congored using a Cu7S4-1 and b Cu7S4-2

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The photodegradation of different dyes such as methylene blue, rhodamine B, eosin yellow, methyl orange, Malachite green and congored using copper sulfide nanomaterials is reported by several authors [32, 66]. The degradation efficiency of photocatalyst depends on various factors such as band gap energy, size and shape of the nanomaterials, mass of the catalyst, properties of dye, concentration of dye, reaction time and light intensity. The photocatalytic efficiency of copper sulfide prepared in the present study was compared with those reported in the literature [30, 31, 67, 68] for the degradation of congo red (Table 6). The degradation efficiency of as-prepared copper sulfides is comparable. The degradation efficiency of Cu7S4-1 is superior to those of Cu7S4-2 and some of previously examined copper sulfide catalysts listed in Table 6.

Table 6 Comparison of the photocatalytic activity of as-prepared copper sulfide nanoparticles with the formerly reported data for the photodegradation of congo red
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6 Conclusion

Two copper(II) complexes 1 and 2 based on long chain dithiocarbamate ligands were synthesized and characterized using elemental analysis, spectroscopy (FTIR and UV–Visible) and single crystal X-ray diffraction. The central copper atoms in 1and 2 are four coordinated, forming distorted square planar geometry. Hirshfeld surface analysis shows that the contribution of H…H interactions to the crystal packing increases with increasing aliphatic hydrocarbon chain length of the dithiocarbamate ligands. DFT calculations revealed that complex 1 is more electrophilic than 2. The PXRD, FESEM and EDS studies showed that copper sulfides were effectively synthesized from 1 and 2. The FESEM analysis revealed that spherical nanoparticles agglomerated to give sharp edged rod and sheet shaped copper sulfides prepared from 1 and 2, respectively. Based on the dye degradation study, the effectiveness of the photocatalysts (copper sulfides) was confirmed. Copper sulfide prepared from complex 1 showed the higher photocatalytic activity than the other prepared from 2.