*Review* **CuS-Based Nanostructures as Catalysts for Organic Pollutants Photodegradation**

**Luminita Isac 1,2 , Cristina Cazan 1,2 , Luminita Andronic <sup>1</sup> and Alexandru Enesca 1,\***


**Abstract:** The direct or indirect discharge of toxic and non-biodegradable organic pollutants into water represents a huge threat that affects human health and the environment. Therefore, the treatment of wastewater, using sustainable technologies, is absolutely necessary for reusability. Photocatalysis is considered one of the most innovative advanced techniques used for pollutant removal from wastewater, due to its high efficiency, ease of process, low-cost, and the environmentally friendly secondary compounds that occur. The key of photocatalysis technology is the careful selection of catalysts, usually semiconductor materials with high absorption capacity for solar light, and conductivity for photogenerated charge carriers. Among copper sulfides, CuS (covellite), a semiconductor with different morphologies and bandgap values, is recognized as an important photocatalyst used for the removal of organic pollutants (dyes, pesticides, pharmaceutics etc.) from wastewater. This review deals with recent developments in organic pollutant photodegradation, using as catalysts various CuS nanostructures, consisting of CuS NPs, CuS QDs, and heterojunctions (CuS/ carbon-based materials, CuS/organic semiconductor, CuS/metal oxide). The effects of different synthesis parameters (Cu:S molar ratios, surfactant concentration etc.) and properties (particle size, morphology, bandgap energy, and surface properties) on the photocatalytic performance of CuS-based catalysts for the degradation of various organic pollutants are extensively discussed.

**Keywords:** CuS nanostructures; heterojunctions; photocatalysis; organic pollutants; wastewater treatment

## **1. Introduction**

The global population growth has resulted in constantly increasing environmental pollution, with serious consequences for human health. Harmful impacts on humans, animals, and the environment are due to the water resource contamination with inorganic, i.e., heavy metals [1–3], and organic, pollutants. Toxic, non-biodegradable, and recalcitrant organic pollutants, such as dyes [4–7], pesticides [8], pharmaceutical active compounds (PhACs) [9,10], and other organic pollutants [11,12], are discharged into water from various industries: textile, leather, food, pharmaceutical, cosmetic, agriculture, plastic, etc. Consequently, the removal of these pollutants from wastewater, via appropriate technology, still remains a great challenge for worldwide researchers.

In this context, many technologies, such as sedimentation, reverse osmosis (RO), coagulation, flotation, solvent extraction, electrolysis, biodegradation, ozonation, sonolysis, sonophotocatalysis, gammaeradiolysis, photo-Fenton, photo-, electro-, and photoelectrocatalysis and chemical catalysis, and combined anaerobic photocatalysis with membrane techniques, have been developed [10,13–15]. Among these technologies, photocatalysis, an advanced oxidation process (AOP), has attracted considerable attention as a green and sustainable technology with promising prospects in global environmental issues remediation [16–18]. Although the photocatalytic degradation of organic pollutants has been

**Citation:** Isac, L.; Cazan, C.; Andronic, L.; Enesca, A. CuS-Based Nanostructures as Catalysts for Organic Pollutants Photodegradation. *Catalysts* **2022**, *12*, 1135. https:// doi.org/10.3390/catal12101135

Academic Editor: John Vakros

Received: 1 September 2022 Accepted: 25 September 2022 Published: 28 September 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

proven to have significant results, its application at the industrial level faces certain deficiencies related to the efficient use of solar energy, mainly due to the high photo-generated carriers recombination rate [19,20]. Thus, the design and development of low-cost and highly efficient catalysts is the main topic in photocatalysis research. Semiconductor materials, with wide solar selective spectral response, high activity, and increased chemical and physical stability, are the most frequently used photocatalysts. As wastewater treatment technology, semiconductor-based photocatalysis has numerous advantages, such as simplicity, ease of handling, good reproducibility, high efficiency, and low costs. Moreover, it is an ecological, non-toxic and energy-free technology [5].

An important class of semiconductor photocatalysts is that of metal sulfides. For these materials, the band gap energy can be easily tuned by simply controlling the particle size without changing the chemical composition of the metal sulfide.

Copper sulfides (CuxS, x = 0.5–2), one of the most significant metal chalcogenide representatives, have been intensively studied in recent decades, due to their particular properties (optical and electrical), that result from their various chemical compositions and morphologies. Considering the chemical composition (x variation), at least eight crystalline phases of CuxS have been reported to date, ranging from the "copper low" sulfide CuS<sup>2</sup> (copper disulfide, x = 0.5) to the copper-rich phase Cu2S (chalcocite, x = 2).

Adjusting the chemical composition (x value), optoelectronic properties are modified, affecting the photocatalytic performance of CuxS catalyst. Generally, the increase of x causes a decrease in the CuxS photocatalytic activity. Copper sulfides with x = 1.8–2 act as more efficient materials for solar cells and optoelectronic devices than as photocatalysts [21–23]. Accordingly, it was reported [24] that Cu2S NPs (0.08 g/L pollutant solution), obtained by a template free polyol reaction, degraded only 51% of dye ABRX-3B (100 mg/L) under Vis light (300W Xe lamp), after 100 min. More recently, the photocatalytic activity of Cu2S-metal oxide(s) heterostructures, prepared by a simple two-step sol–gel procedure, was studied under UV and UV–Vis irradiation scenarios using herbicide S-MCh (30 mg/L) as the reference pollutant [25]. The results showed that the three-component heterostructure of Cu2S/TiO2/WO<sup>3</sup> had higher photocatalytic efficiency (61%) compared with two-component heterostructures, 30% for Cu2S/TiO2, and 28% for Cu2S/WO3, respectively, after 8 h in UV-Vis light irradiation. A lower photocatalytic efficiency (~10%) was reported for Cu9S<sup>5</sup> (Cu1.8S) MCs in the degradation of MB solution (5.8 mg/L), under Vis light irradiation, after 175 min [26].

Another potential non-stoichiometric copper sulfide catalyst is Cu7S<sup>4</sup> (Cu1.75S). However, its photocatalytic application is reduced, due to both the high recombination rate of the photogenerated electron/holes, and also to the powder recycling issues. Thus, as an efficient alternative to powders, flexible Cu mash/Cu7S<sup>4</sup> films were developed via a facile in situ anodization technique [16]. Using Cu/Cu7S<sup>4</sup> films of 4 cm<sup>2</sup> area as catalysts, the highest efficiency in the photocatalytic Fenton-like dye MB (10 mg/L) degradation was 98.4%, under simulated sunlight, after 140 min.

Over recent years, the CuS semiconductor has been shown to be a promising candidate for visible light photocatalysis, due to its narrow band gap and good optical absorption properties in the Vis-NIR region. Based on the increased photocatalytic performances of CuS, compared with CuxS ( x = 1.8–2) and Cu1.75S (more efficient and more environmentally friendly as films), therefore, a large amount of research was conducted for this review, which is focused on the use of CuS nanostructured powders as photocatalysts for organic pollutant degradation in wastewater.

Due to its special properties, e.g., increased conductivity at high temperature, superconductivity at 1.6 K, (electro)chemical-sensing capabilities, low-toxicity, nonlinear optical and ideal solar energy absorption capacity, CuS shows a versatile range of applications, including solar thermal collectors [27], artificial photosynthesis [28], gas sensors [29–32], biosensors [29,33,34], photodynamic therapy of cancer [29,32,34], lithium-ion batteries [29,35], supercapacitors [29,36], photoconductors [37], water disinfection [38–41], hydrogen production via water splitting [37,42], thermoelectric materials [29], and photocatalysis [4,37]. hydrogen production via water splitting [37,42], thermoelectric materials [29], and photo‐ catalysis [4,37].

optical and ideal solar energy absorption capacity, CuS shows a versatile range of appli‐ cations, including solar thermal collectors [27], artificial photosynthesis [28], gas sensors [29–32], biosensors [29,33,34], photodynamic therapy of cancer [29,32,34], lithium‐ion bat‐ teries [29,35], supercapacitors [29,36], photoconductors [37], water disinfection [38,39–41],

*Catalysts* **2022**, *12*, x FOR PEER REVIEW 3 of 19

According to literature [10,14,43,44], the lack of chemical stability, fast recombination of photogenerated charge carriers, and particle aggregation in solution are still significant drawbacks of a single semiconductor photocatalyst, such as CuS. An effective solution to these impediments is heterojunction construction with additional advantages, such as improving solar energy absorption and carriers charge transfer rate. The photocatalytic performance of CuS nanostructures (NSs) and CuS-based hetero-nanostructures depends on particles size, surface area, morphology, and the interface properties, that can be tailored by tuning the synthesis methods and/or conditions. According to literature [10,14,43,44], the lack of chemical stability, fast recombination of photogenerated charge carriers, and particle aggregation in solution are still significant drawbacks of a single semiconductor photocatalyst, such as CuS. An effective solution to these impediments is heterojunction construction with additional advantages, such as im‐ proving solar energy absorption and carriers charge transfer rate. The photocatalytic per‐ formance of CuS nanostructures (NSs) and CuS‐based hetero‐nanostructures depends on particles size, surface area, morphology, and the interface properties, that can be tailored by tuning the synthesis methods and/or conditions.

In recent years, many studies have been developed on CuS NSs with applications in environmental issues, especially in wastewater treatment. In this review, the photocatalytic activity of various CuS-based nanostructures for degradation of different organic pollutants, such as organic dyes, pesticides, phenol and phenolic compounds, pharmaceutical active compounds, etc., under simulated (UV, Vis) and sunlight irradiation, is extensively discussed. This study aims to identify the photocatalytic performances of recently reported CuS-based catalysts, for possible future improvements so as to ensure it is as efficient as possible for the degradation of industrial organic pollutants in wastewater effluents. In Figure 1 is illustrated the schematic representation of this review regarding CuS-based nanostructures photocatalysts obtaining, their specific properties (morphology, band gap energy, specific surface area), and application in organic pollutants degradation from wastewater. In recent years, many studies have been developed on CuS NSs with applications in environmental issues, especially in wastewater treatment. In this review, the photocata‐ lytic activity of various CuS‐based nanostructures for degradation of different organic pollutants, such as organic dyes, pesticides, phenol and phenolic compounds, pharma‐ ceutical active compounds, etc., under simulated (UV, Vis) and sunlight irradiation, is ex‐ tensively discussed. This study aims to identify the photocatalytic performances of re‐ cently reported CuS‐based catalysts, for possible future improvements so as to ensure it is as efficient as possible for the degradation of industrial organic pollutants in wastewater effluents. In Figure 1 is illustrated the schematic representation of this review regarding CuS‐based nanostructures photocatalysts obtaining, their specific properties (morphol‐ ogy, band gap energy, specific surface area), and application in organic pollutants degra‐dation from wastewater.

**Figure 1.** Synthesis methods and specific properties of CuS‐based nanostructures photocatalysts used in organic pollutants degradation from wastewater. **Figure 1.** Synthesis methods and specific properties of CuS-based nanostructures photocatalysts used in organic pollutants degradation from wastewater.

#### **2. CuS Nanostructures with Different Morphologies as Catalysts for Organic Pollu‐ tant Photodegradation 2. CuS Nanostructures with Different Morphologies as Catalysts for Organic Pollutant Photodegradation**

At room temperature, CuS (covellite) has a hexagonal crystalline structure (space group P63/mmc, a = 3.8020 Å, c = 16.430 Å) consisting of unit cells, in which layers of planar CuS3 (triangles) and CuS4 (tetrahedrons) with S−S bonds alternate. At 55 K, the CuS hexagonal structure changes to orthorhombic, due to second‐order phase transition re‐ sulting in orthorhombic distortion of the Cu‐S and S‐S bond lengths [45,46]. At room temperature, CuS (covellite) has a hexagonal crystalline structure (space group P63/mmc, a = 3.8020 Å, c = 16.430 Å) consisting of unit cells, in which layers of planar CuS<sup>3</sup> (triangles) and CuS<sup>4</sup> (tetrahedrons) with S−S bonds alternate. At 55 K, the CuS hexagonal structure changes to orthorhombic, due to second-order phase transition resulting in orthorhombic distortion of the Cu-S and S-S bond lengths [45,46].

The energy band alignments and total–partial density of states, calculated with the generalized gradient approximation (GGA) of the density functional theory, showed that CuS has significant metallic behavior due to p(S)−d(Cu) orbital interactions up to Fermi The energy band alignments and total–partial density of states, calculated with the generalized gradient approximation (GGA) of the density functional theory, showed that CuS has significant metallic behavior due to p(S)−d(Cu) orbital interactions up to Fermi level [45]. The Fermi level, an empty energy band induced by both copper vacancy and sulfur vacancy on the top of VB, is an important factor in the evaluation of CuS photoelectronic properties, mainly related to the potential local surface plasmon resonance (LSPR) [47]. Due to its deficiency in Cu atoms (comparing with Cu2S), CuS exhibits the highest concentration of free carriers (holes) in the VB, resulting in LSPR bands in the NIR region, hence having extended light absorption [48].

As a copper sulfides class representative, CuS is a p-type semiconductor with a bandgap energy ranging from 1.7 eV to 3.46 eV [33,49] but, for most CuS, the Eg values are in the range of 1, 7–2.2 eV. Controlling the CuS semiconductor morphology (shape and particle size), the band gap can be tailored without changing the chemical composition of the material [50]. For example, the band gap energy for CuS nanoparticles (NPs) can vary from 1.7 to 2.14 eV, depending on morphology: 1.7 eV for hollow spheres [51], 1.87 eV for nanoflowers [52], 1.97 eV for microspheres [53], 2.1 eV for nanoplates [54] and 2.12 eV for flake-like nanostructures [55].

Thus, many works have reported the preparation of CuS with different morphologies, from quantum dots (QDs) to 3D hierarchical CuS architectures consisting of 1D nanotubes, using methods such as hydrothermal [50,56,57], solvothermal [58–63], coprecipitation [64,65], photochemical precipitation [66], mechanochemical [67,68], thermolysis [49], chemical reduction [69], microwave assisted growth [70] and solid-state reaction routes [71]. However, the preparation of uniform CuS nanostructures, through simple, fast, ecological and low-cost technologies, still remains a significant challenge for all researchers in the field.

Covellite nanoparticles (CuS NPs) with remarkable chemical, structural and surface properties, significantly different from those of bulk, are considered promising photocatalytic materials [55]. The photocatalytic properties of CuS catalyst can be tailored by changing preparation method parameters. In addition, the photocatalyst dosage, the pollutant and its concentration, the type and intensity of the irradiation source are other important factors to be considered in the photodegradation process. The photocatalytic activity of CuS-based nanostructures for the degradation of different organic pollutants is selectively presented in Table 1.


**Table 1.** Representative studies on organic pollutants photodegradation using nanostructured CuSbased catalysts.


#### **Table 1.** *Cont.*

η\* is the efficiency degradation of pollutant after t min of irradiation.

The Vis and natural light-driven photocatalytic degradation of cationic azo dye methylene blue (MB) using CuS nanoparticles (NPs) with different morphologies, has been demonstrated to be an attractive topic for many research. Flower-like [52], hollow microspheres [53], and flake- and spherical-like CuS NPs [55], prepared by simple aqueous solution route, facile PVP assisted solvothermal process and biological sulfate reduction, were investigated as photocatalysts in the degradation of MB, in the absence/presence of H2O2.

The CuS nanostructured flowers, with diameters of about 800–1200 nm, Eg = 1.87 eV and BET surface areas of 61.55 m2/g, obtained by using a simple aqueous solution route without any surfactant addition, removed only 39% MB after 90 min, under Vis light irradiation [52]. The addition of H2O<sup>2</sup> in the photocatalytic system significantly increased the degradation efficiency to 92%, after the same irradiation time. This increase was due to the presence of H2O<sup>2</sup> molecules, which enhanced the dye degradation by accelerating the formation of hydroxyl radicals (·OH), the active species which promote the oxidation of MB into smaller, non-toxic molecules [52,81].

An enhanced photocatalytic efficiency (94%) in the degradation of MB under natural light (sunlight), in the presence of H2O2, was reported for CuS hollow microspheres

mesoporous structures with a bandgap of ~1.97 eV and surface area of 36 m2/g [53]. The CuS hollow microspheres nanostructures were obtained by the solvothermal process, varying the amounts of PVP surfactant (0–2 g), the copper/sulfur precursors, and solvents, while the precursor ratios were maintained at constant during the experiments. The average diameter of microspheres in the nanosheets-based hierarchical structure was about 2.3 µm. The studied photocatalysts showed excellent stability and recyclability, with 96.5% of the dye removed after 6th cycle. The photocatalytic performance of CuS catalyst was attributed to its hollow microsphere morphology, which favors the absorption of more MB molecules, promotes the light generated charge carriers transfer to the reactive surface and allows rapid diffusion of the reactants and products during the oxidation/reduction reaction [53].

CuS NPs with two different morphologies were synthesized via the sulfate reducing bacteria (RBS) method by changing the copper precursor concentration: lower concentrations favored the obtaining of flake-like nanoparticles (Eg = 2.12 eV) with an average width of 25 nm and average length of 130 nm, while spherical-like CuS NPs (Eg = 2.14 eV), with average diameter in the range 30–50 nm, were synthesized at higher concentrations [55].

Both the flake-like and spherical-like CuS NPS showed high photodegradation efficiency for the MB + H2O<sup>2</sup> system, respectively 94% after 5.5 min and 93% after 1 min illumination with a halogen lamp (600 W). Although these photocatalysts have the advantage of being used in applications that require short irradiation time or short catalyst–pollutant contact time, their reusability is still an open-issue for further studies.

The studies mentioned above confirm that the photocatalytic activity of CuS NPs with hollow microspheres or spherical-like morphologies, with consistent shape and size of spherical particles, was higher than that of CuS NPs with non-spherical structures.

The MB dye photodegradation mechanism, through oxidation/reduction reactions, using CuS NPs as the photocatalyst, in the absence/presence of H2O<sup>2</sup> molecules, is illustrated in Figure 2. *Catalysts* **2022**, *12*, x FOR PEER REVIEW 7 of 19

**Figure 2.** The photocatalytic degradation mechanism of MB dye using CuS NPs photocatalyst under sunlight irradiation. **Figure 2.** The photocatalytic degradation mechanism of MB dye using CuS NPs photocatalyst under sunlight irradiation.

The adsorption and photocatalysis performance of CuS nanostructures with different morphologies of particles, prepared by a simple co‐precipitation method using Cu(NO3)2 and thioacetamide as precursors, for degradation of the dye Rhodamine B (RhB) aqueous solutions (20 mg/L) under Vis light irradiation (150 W Xe lamp equipped with a glass filter) were evaluated by Li and Wang [51]. It was reported that CuS NPs with different morphologies (flakes, rods and hollow spheres), and Eg = 1.7 eV had good photodegrada‐ The adsorption and photocatalysis performance of CuS nanostructures with different morphologies of particles, prepared by a simple co-precipitation method using Cu(NO3)<sup>2</sup> and thioacetamide as precursors, for degradation of the dye Rhodamine B (RhB) aqueous solutions (20 mg/L) under Vis light irradiation (150 W Xe lamp equipped with a glass filter) were evaluated by Li and Wang [51]. It was reported that CuS NPs with different morphologies (flakes, rods and hollow spheres), and Eg = 1.7 eV had good photodegradation

tion efficiency for RhB, with removal rates higher than 85% within 120 min. As was ex‐ pected, CuS hollow spheres, with 400–3500 nm outer diameter and 10–400 nm thickness,

mL dye solution) had the result of halving the time necessary for photodegradation of 99% RhB with high concentration (50 mg/L) in wastewater. The increased photocatalytic activity of the CuS NPs, attributed to the combination of the adsorption and photocataly‐ sis processes, demonstrates the potential application of CuS NPs photocatalyst in high dye

The photocatalytic activity and stability of CuS QDs in degradation of RhB solutions under visible light illumination (150 W Xe lamp) were studied by Li et al. [67]. Using a low‐cost and simple mechanochemical ball milling method, CuS powders containing ul‐ trafine crystals with uniform size (1–5 nm) and an average diameter of 2.9 nm, were pre‐ pared. The calculated BET surface area of about 90.0 m2/g, which was significantly higher than that of CuS NPs (30.6 m2/g, [51]), was expected to improve the photocatalytic activity of the CuS QDs. Higher specific surface area, with more reactive sites and shorter migra‐ tion distance, reduced the recombination rate of photogenerated electron/hole pairs. Ac‐ cordingly, the H2O2 addition increased the degradation efficiency of RhB from about 60% to 95%, after 30 min exposure in Vis light, due to the quantum effect of CuS QDs which

Based on previous studies, both CuS NPs and CuS QDs are excellent photocatalysts for RhB degradation, depending on the dye concentration in wastewater: CuS QDs is more efficient in wastewater with a low content of dye, while CuS NPs shows good efficiency

favors ultra‐fast transfer of charge carriers from CuS QDs to RhB dye.

concentration wastewater treatment.

in dye‐concentrated wastewater as well.

efficiency for RhB, with removal rates higher than 85% within 120 min. As was expected, CuS hollow spheres, with 400–3500 nm outer diameter and 10–400 nm thickness, formed by self-assembly of nanoparticles with sizes in the range 1–2 µm, showed a RhB photodegradation of 99% in 120 min. The addition of a small amount of H2O<sup>2</sup> (1mL/100 mL dye solution) had the result of halving the time necessary for photodegradation of 99% RhB with high concentration (50 mg/L) in wastewater. The increased photocatalytic activity of the CuS NPs, attributed to the combination of the adsorption and photocatalysis processes, demonstrates the potential application of CuS NPs photocatalyst in high dye concentration wastewater treatment.

The photocatalytic activity and stability of CuS QDs in degradation of RhB solutions under visible light illumination (150 W Xe lamp) were studied by Li et al. [67]. Using a low-cost and simple mechanochemical ball milling method, CuS powders containing ultrafine crystals with uniform size (1–5 nm) and an average diameter of 2.9 nm, were prepared. The calculated BET surface area of about 90.0 m2/g, which was significantly higher than that of CuS NPs (30.6 m2/g, [51]), was expected to improve the photocatalytic activity of the CuS QDs. Higher specific surface area, with more reactive sites and shorter migration distance, reduced the recombination rate of photogenerated electron/hole pairs. Accordingly, the H2O<sup>2</sup> addition increased the degradation efficiency of RhB from about 60% to 95%, after 30 min exposure in Vis light, due to the quantum effect of CuS QDs which favors ultra-fast transfer of charge carriers from CuS QDs to RhB dye.

Based on previous studies, both CuS NPs and CuS QDs are excellent photocatalysts for RhB degradation, depending on the dye concentration in wastewater: CuS QDs is more efficient in wastewater with a low content of dye, while CuS NPs shows good efficiency in dye-concentrated wastewater as well.

3D nanostructured CuS have been remarked on as materials with high solar catalytic efficiency due to their large surface areas with sufficiently active sites, which improve CuS surface-reactants contact [62].

The hierarchical 3D CuS nanostructures, prepared by low-temperature solvothermal grow of 1D CuS nanotubes on a self-assembled 3D Cu(MAA)<sup>2</sup> precursor, not only provided the advantage of a large number of active sites on the surfaces, but also the advantages of improved molecular/ionic transport (through the 1D nanotubes), and of good mechanical stability (due to the 3D structure) [74]. The as-prepared 3D hierarchical CuS architectures showed a degradation of almost 99% of RhB within 45 min, in the presence of visible light and oxidizing agent (H2O2). The stability and reusability of the photocatalysts proved to be quite good, the removal rate of RhB reaching 70% after 5 cycles.

A simple and one-step in situ heating sulfuration procedure was used to prepare the CuS photocatalyst with a 3D hierarchical nanostructure with CuS nanoplates formed on the copper foam precursor structure in [75]. According to this study's results, the excellent photocatalytic performance of 3D nanostructured CuS catalyst resulting in 99% degradation of RhB + H2O<sup>2</sup> solution when exposed in simulated solar light for 25 min, was attributed to the synergistic effects of high optical absorption (Eg = 1.58 eV), and large specific surface area (12.06 m2/g), resulting in sufficient reaction active sites. Moreover, the photocatalytic activity and stability of the studied catalysts did not alter after 4 photocatalytic cycles, which made them ideal recyclable catalysts.

CuxS nanoparticles with different compositions (x = 1, 2) and morphologies were synthesized via simple and environmentally friendly methods, e.g., one-step hydrothermal and thermal chemical reduction of S precursor with/without surfactant, in [56,69].

By tuning the molar ratios Cu:S (1:0.1–1:3) of copper acetate and sublimed sulfur precursors in polyethylene (PEG-400) surfactant, CuS, Cu2S, and CuS–Cu2S NPs with various morphologies and particle sizes, and, therefore, different band gap energies, were obtained: Cu2S spherical and irregular nanoflakes (Eg = 3.5 eV) for Cu:S = 1:0.25 Cu2S-CuS irregular flakes with 30 nm thickness (Eg = 2.72 eV) for Cu:S = 1:0.75 CuS irregular nanoflakes with particle sizes between 200 and 300 nm and thickness less

than 30 nm (Eg = 2.01 eV) for Cu:S = 1:1.

tained:

[56].

The photocatalytic experiments showed that photocatalyst efficiency in RhB degradation under Vis light (250W cold xenon lamp with cut-off wavelength of 420 nm), in the presence of H2O2, increases with band gap energy decrease, thus CuS NPs degraded 96% RhB in 5 min, while Cu2S and Cu2S-CuS NPs degraded 87% and 92% RhB respectively [56]. dation under Vis light (250W cold xenon lamp with cut‐off wavelength of 420 nm), in the presence of H2O2, increases with band gap energy decrease, thus CuS NPs degraded 96% RhB in 5 min, while Cu2S and Cu2S‐CuS NPs degraded 87% and 92% RhB respectively

The RhB photodegradation reaction mechanism in presence of CuS NPs under visible light irradiation is schematically presented in Figure 3. The RhB photodegradation reaction mechanism in presence of CuS NPs under visible light irradiation is schematically presented in Figure 3.

$$\begin{array}{rcl} \mathbf{2CuS} + \mathbf{h}\mathbf{v} & \rightarrow & \mathbf{CuS} \text{ (e}\cdot\text{)} & + & \mathbf{CuS} \text{ (h}\cdot\text{)}\\ & & \downarrow + \mathsf{H}\_{2}\mathsf{O}\_{2} & \downarrow + \mathsf{H}\_{2}\mathsf{O}\\ & \mathsf{CuS} + \mathsf{HO}\cdot + \mathsf{HO}^{\cdot} & \mathsf{CuS} + \mathsf{HO}\cdot + \mathsf{H}^{+}\\ \mathsf{RhB} + \mathsf{h}\mathbf{v} & \rightarrow & \mathsf{RhB}^{\cdot +} \xrightarrow{\mathsf{H}\times\mathsf{S}} \mathsf{CuS} \text{ (e}\cdot\text{)} + \mathsf{RhB}^{+}\\ \end{array}$$

$$\begin{array}{rcl} \mathbf{RhB}^{+}\cdot + \mathsf{HO}^{\cdot}\cdot \rightarrow \mathsf{CO}\_{2} + \mathsf{H}\_{2}\mathsf{O} + \mathsf{NO}\_{3}^{-} + \mathsf{Cl}^{-} \end{array}$$

*Catalysts* **2022**, *12*, x FOR PEER REVIEW 8 of 19

lytic cycles, which made them ideal recyclable catalysts.

less than 30 nm (Eg = 2.01 eV) for Cu:S = 1:1.

surface‐reactants contact [62].

3D nanostructured CuS have been remarked on as materials with high solar catalytic efficiency due to theirlarge surface areas with sufficiently active sites, which improve CuS

The hierarchical 3D CuS nanostructures, prepared by low‐temperature solvothermal grow of 1D CuS nanotubes on a self‐assembled 3D Cu(MAA)2 precursor, not only pro‐ vided the advantage of a large number of active sites on the surfaces, but also the ad‐ vantages of improved molecular/ionic transport (through the 1D nanotubes), and of good mechanical stability (due to the 3D structure) [74]. The as‐prepared 3D hierarchical CuS architectures showed a degradation of almost 99% of RhB within 45 min, in the presence of visible light and oxidizing agent (H2O2). The stability and reusability of the photocata‐

A simple and one‐step in situ heating sulfuration procedure was used to prepare the CuS photocatalyst with a 3D hierarchical nanostructure with CuS nanoplates formed on the copper foam precursor structure in [75]. According to this study's results, the excellent photocatalytic performance of 3D nanostructured CuS catalyst resulting in 99% degrada‐ tion of RhB + H2O2 solution when exposed in simulated solar light for 25 min, was at‐ tributed to the synergistic effects of high optical absorption (Eg = 1.58 eV), and large spe‐ cific surface area (12.06 m2/g), resulting in sufficient reaction active sites. Moreover, the photocatalytic activity and stability of the studied catalysts did not alter after 4 photocata‐

CuxS nanoparticles with different compositions (x = 1, 2) and morphologies were syn‐ thesized via simple and environmentally friendly methods, e.g., one‐step hydrothermal

By tuning the molar ratios Cu:S (1:0.1–1:3) of copper acetate and sublimed sulfur pre‐ cursors in polyethylene (PEG‐400) surfactant, CuS, Cu2S, and CuS–Cu2S NPs with various morphologies and particle sizes, and, therefore, different band gap energies, were ob‐

lysts proved to be quite good, the removal rate of RhB reaching 70% after 5 cycles.

and thermal chemical reduction of S precursor with/without surfactant, in [56,69].

Cu2S spherical and irregular nanoflakes (Eg = 3.5 eV) for Cu:S = 1:0.25

Cu2S‐CuS irregular flakes with 30 nm thickness (Eg = 2.72 eV) for Cu:S = 1:0.75 CuS irregular nanoflakes with particle sizes between 200 and 300 nm and thickness

The photocatalytic experiments showed that photocatalyst efficiency in RhB degra‐

**Figure 3.** The reaction mechanism proposed for RhB photodegradation by CuS NPs catalyst. **Figure 3.** The reaction mechanism proposed for RhB photodegradation by CuS NPs catalyst.

When CuS photocatalyst absorbs photons from solar light or an irradiation source, with energy equal or higher than its band‐gap, the electrons from CB are transferred to When CuS photocatalyst absorbs photons from solar light or an irradiation source, with energy equal or higher than its band-gap, the electrons from CB are transferred to VB, resulting in photogenerated electrons CuS (e−) and holes CuS (h<sup>+</sup> ). The H2O<sup>2</sup> molecules capture these photogenerated electrons and rapidly generate hydroxyl radicals (HO·) and hydroxide ions (OH−), while the photogenerated holes react with H2O forming HO·. In the meantime, RhB dye absorbs Vis light and undergoes a transition to its excited state (RhB\*), which transfers electrons to CuS, resulting in CuS(e−) and RhB<sup>+</sup> . Then, the highly reactive radical HO· oxidizes and decomposes RhB<sup>+</sup> to CO2, H2O and other salt ions [4,56].

Shamraiz et al. synthesized CuS–Cu2S NPs, with an average size less than 30 nm, by chemical reduction of copper thiourea complex, without any surfactant, at moderate temperature [69]. The CuS–Cu2S NPs were tested as photocatalysts in the degradation of different dyes under direct sunlight (outdoor lightening), without H2O<sup>2</sup> addition. The results showed that the photodegradation process was faster for cationic dyes, such as MV (85.03% in 60 min), MG (90,25% in 40 min) and RhB (70.16% in 80 min), but almost negligible for the anionic dye MO (9.4% in 3 h). This behavior of the CuS–Cu2S catalyst in cationic dye photodegradation was attributed to the presence of active negative charges (OH- ) on the catalyst surface, which were electrostatically attracted by cationic dye molecules, thus facilitating the electron transfer under direct sunlight irradiation [69].

The photocatalytic performance of CuS nanoparticles, with size < 20 nm and specific surface area of 34.37 m2/g, in the degradation of organic dye pollutants, MB, RhB, EY and CR, under various light (UV, Vis and solar) irradiations, was evaluated by Ayodhya et al. [61]. For CuS NPs synthesis, a simple, relatively fast and green (using xanthan gum as a capping agent) solvothermal method was proposed. The photodegradation of the MB, RhB, EY and CR dyes in the absence and presence of CuS NPs were studied under similar experimental conditions, using UV, Vis and solar light sources. The photocatalytic performances of CuS NPs are shown in Figure 4.

mances of CuS NPs are shown in Figure 4.

presence of CuS catalyst.

**Figure 4.** The photocatalytic activity of CuS NPs in dyes degradation under different light irradia‐ tions, for 4 h. **Figure 4.** The photocatalytic activity of CuS NPs in dyes degradation under different light irradiations, for 4 h.

VB, resulting in photogenerated electrons CuS (e−) and holes CuS (h+). The H2O2 molecules capture these photogenerated electrons and rapidly generate hydroxyl radicals (HO∙) and hydroxide ions (OH−), while the photogenerated holes react with H2O forming HO∙. In the meantime, RhB dye absorbs Vis light and undergoes a transition to its excited state (RhB\*), which transfers electrons to CuS, resulting in CuS(e−) and RhB+. Then, the highly reactive

Shamraiz et al. synthesized CuS–Cu2S NPs, with an average size less than 30 nm, by chemical reduction of copper thiourea complex, without any surfactant, at moderate tem‐ perature [69]. The CuS–Cu2S NPs were tested as photocatalysts in the degradation of dif‐ ferent dyes under direct sunlight (outdoor lightening), without H2O2 addition. The results showed that the photodegradation process was faster for cationic dyes, such as MV (85.03% in 60 min), MG (90,25% in 40 min) and RhB (70.16% in 80 min), but almost negli‐ gible for the anionic dye MO (9.4% in 3 h). This behavior of the CuS–Cu2S catalyst in cati‐ onic dye photodegradation was attributed to the presence of active negative charges (OH‐ ) on the catalyst surface, which were electrostatically attracted by cationic dye molecules,

The photocatalytic performance of CuS nanoparticles, with size < 20 nm and specific surface area of 34.37 m2/g, in the degradation of organic dye pollutants, MB, RhB, EY and CR, under various light (UV, Vis and solar) irradiations, was evaluated by Ayodhya et al. [61]. For CuS NPs synthesis, a simple, relatively fast and green (using xanthan gum as a capping agent) solvothermal method was proposed. The photodegradation of the MB, RhB, EY and CR dyes in the absence and presence of CuS NPs were studied under similar experimental conditions, using UV, Vis and solar light sources. The photocatalytic perfor‐

It can be observed that CuS NPs showed good photocatalytic activity for dye degra‐ dation in sunlight irradiation, ranging from 69.2% for RhB to 92% for EY. The exception was CR which degraded better in UV light (75.5% photodegradation efficiency) in the

radical HO∙ oxidizes and decomposes RhB+ to CO2, H2O and other salt ions [4,56].

thus facilitating the electron transfer under direct sunlight irradiation [69].

**3. CuS‐Based Heterostructures as Catalysts for Organic Pollutant Photodegradation** Although previous studies confirmed the successful applicability of CuS NP photo‐ catalysts in the degradation of organic contaminants (especially organic dyes), the photo‐ catalytic efficiency of single CuS is quite low for the complete degradation of persistent, It can be observed that CuS NPs showed good photocatalytic activity for dye degradation in sunlight irradiation, ranging from 69.2% for RhB to 92% for EY. The exception was CR which degraded better in UV light (75.5% photodegradation efficiency) in the presence of CuS catalyst.

#### much more toxic, organic compounds (dyes, pharmaceuticals, pesticides) from wastewater. **3. CuS-Based Heterostructures as Catalysts for Organic Pollutant Photodegradation**

Although previous studies confirmed the successful applicability of CuS NP photocatalysts in the degradation of organic contaminants (especially organic dyes), the photocatalytic efficiency of single CuS is quite low for the complete degradation of persistent, much more toxic, organic compounds (dyes, pharmaceuticals, pesticides) from wastewater.

## *3.1. CuS/Carbon-Based Materials Heterostructures as Catalysts for Organic Pollutant Photodegradation*

Carbon quantum dots (CQDs), with sizes under 10 nm and fluorescent properties, have large surface areas, high porosity, excellent electrical conductivity, relatively low cost and toxicity, and high aqueous stability. Due to these properties, CQDs can act as absorbents for water impurities, but also as supporting catalyst (co-catalyst) for the main photocatalyst (CuS) [50,78].

Another promising supporting catalyst for CuS is reduced graphene oxide (rGO) with high surface area, efficient electron transfer, and superior conductivity [39,52].

Therefore, the CuS/CQD and CuS/rGO composites could be attractive solutions for photocatalytic degradation of persistent organic contaminants under Vis light irradiation.

Recent investigations on the degradation of brilliant green (BG) highly toxic dye, usually used in textiles and paper industries but also in poultry feed to avoid fungi and parasite contagion, and panadol (PAN), one of the top three most commonly prescribed drugs in the world, were done using CuS/CQD photocatalysts [50,73]. In both studies, CQDs were obtained by carbonization of vegetative wastes (water hyacinth weed, peanut shells), while CuS NPs were prepared via sol–gel [78], respectively, hydrothermal [50] methods. The structural, optical, surface and photocatalytic properties of CuS/CQD composites and CuS NPs, together with organic pollutant details, are given in Table 2.


CuS/rGO composites and CuS NPs are presented in Table 2.

*Catalysts* **2022**, *12*, x FOR PEER REVIEW 10 of 19

*Photodegradation*

5.

ment (i.e., wastewater).

photocatalyst (CuS) [50,78].

*3.1. CuS/Carbon‐Based Materials Heterostructures as Catalysts for Organic Pollutant*

Carbon quantum dots (CQDs), with sizes under 10 nm and fluorescent properties, have large surface areas, high porosity, excellent electrical conductivity, relatively low cost and toxicity, and high aqueous stability. Due to these properties, CQDs can act as absorbents for water impurities, but also as supporting catalyst (co‐catalyst) for the main

Another promising supporting catalyst for CuS is reduced graphene oxide (rGO)

Therefore, the CuS/CQD and CuS/rGO composites could be attractive solutions for photocatalytic degradation of persistent organic contaminants under Vis light irradiation. Recent investigations on the degradation of brilliant green (BG) highly toxic dye, usu‐ ally used in textiles and paper industries but also in poultry feed to avoid fungi and par‐ asite contagion, and panadol (PAN), one of the top three most commonly prescribed drugs in the world, were done using CuS/CQD photocatalysts [50,73]. In both studies, CQDs were obtained by carbonization of vegetative wastes (water hyacinth weed, peanut shells), while CuS NPs were prepared via sol–gel [78], respectively, hydrothermal [50] methods. The structural, optical, surface and photocatalytic properties of CuS/CQD com‐

Due to improved morphologies, large surface areas and reduced band gap energies, and therefore, superior capacity for light absorption, these composites were shown to be the most effective catalysts for dyes and active pharmaceutical photodegradation. The CuS/CQDs catalyst act as a scavenger during the photocatalytic process,reducing the elec‐ tron‐hole recombination rate, and the possible redox reactions are summarized in Figure

Both CuS/CQDsq photocatalysts were prepared via green techniques, representing an easy and low‐cost way to remove organic pollutants and plant waste from the environ‐

The photocatalytic performance of CuS/rGO nanocomposites was evaluated for the degradation of toxic persistent organic pollutants, malachite green (MG) and atrazine (ATZ), under direct/simulated sunlight radiance [14,43]. Malachite green is a carcinogenic and non‐biodegradable dye used in numerous industrial applications. Atrazine, a com‐ monly used herbicide in the agriculture and food industries, is one of the most stable toxic contaminants in polluted water. The molecular formulae and structures of the organic pollutants, together with the structural, optical, surface and photocatalytic properties of

with high surface area, efficient electron transfer, and superior conductivity [39,52].

posites and CuS NPs, together with organic pollutant details, are given in Table 2.

**Table 2.** The structural, optical, surface and photocatalytic properties of CuS NPs and CuS/carbonbased materials composites. **Table 2.** The structural, optical, surface and photocatalytic properties of CuS NPs and CuS/carbon‐ based materials composites.

η\* is the efficiency degradation of pollutant after t min of irradiation. η\* is the efficiency degradation of pollutant after t min of irradiation. η is the degradation pollutant after t η\* is the efficiency degradation of pollutant after t min of irradiation.

The experimental results revealed that the addition of various wt% of rGO in CuS had significant contributions to persistent organic pollutant photodegradation with CuS/rGO composite, such as the following: (a) enhanced light absorption and decreased bandgap energy values from 2.08 eV to 1.76−1.9 eV; (b) ensured relatively large specific surface area which provided more active reaction sites exhibiting strong photo‐absorption of pollutant molecules under solar/Vis light irradiations; (c) allowed strong adsorption of The experimental results revealed that the addition of various wt% of rGO in CuS had significant contributions to persistent organic pollutant photodegradation with CuS/rGO composite, such as the following: (a) enhanced light absorption and decreased bandgap energy values from 2.08 eV to 1.76−1.9 eV; (b) ensured relatively large specific surface area which provided more active reaction sites exhibiting strong photo‐absorption of pollutant molecules under solar/Vis light irradiations; (c) allowed strong adsorption of The results that various rGO CuS hadsignificantcontributionsto persistentorganicpollutantphotodegradation withCuS/rGO composite,such asthefollowing: (a)enhancedlightabsorptionanddecreasedbandgap energy values from 2.08 eV to 1.76−1.9 eV; (b) ensured relatively large specific surface which provided more active reaction sites exhibiting strong photo‐absorption ofpollutant underlightirradiations;(c) strong of Due to improved morphologies, large surface areas and reduced band gap energies, and therefore, superior capacity for light absorption, these composites were shown to be the most effective catalysts for dyes and active pharmaceutical photodegradation. The CuS/CQDs catalyst act as a scavenger during the photocatalytic process, reducing the electron-hole recombination rate, and the possible redox reactions are summarized in Figure 5.

pollutant molecules due to the oxygen‐containing functional groups on the surface of rGO. The mechanism of ATZ degradation by CuS/rGO photocatalyst is illustrated in Fig‐ ure 6. The absorption of light photons with energy (hʋ) higher than the band gap energy pollutant molecules due to the oxygen‐containing functional groups on the surface of rGO. The mechanism of ATZ degradation by CuS/rGO photocatalyst is illustrated in Fig‐ ure 6. The absorption of light photons with energy (hʋ) higher than the band gap energy pollutantmoleculesdueto oxygen‐containing functionalgroups of rGO.The mechanismofATZdegradation byCuS/rGO photocatalystis illustratedin Fig‐ ure 6. The absorption of light photons with energy (hʋ) higher than the band gap energy Both CuS/CQDsq photocatalysts were prepared via green techniques, representing an easy and low-cost way to remove organic pollutants and plant waste from the environment (i.e., wastewater).

of CuS (Eg = 2.07 eV) generated photo‐excited electrons (e−) on CB, and holes (h+) in the VB. The rGO nanosheets, which were anchored to the surface of the assembled CuS NPs, could receive the photo‐excited e−, favoring the separation of photogenerated charge car‐ riers and oxidized species (HO∙, ∙O2−) formation. These active radicals reacted with the ATZ molecules adsorbed on the photocatalyst active sites, resulting in CO2, H2O and other non‐harmful ions [43]. of CuS (Eg = 2.07 eV) generated photo‐excited electrons (e−) on CB, and holes (h+) in the VB. The rGO nanosheets, which were anchored to the surface of the assembled CuS NPs, could receive the photo‐excited e−, favoring the separation of photogenerated charge car‐ riers and oxidized species (HO∙, ∙O2−) formation. These active radicals reacted with the ATZ molecules adsorbed on the photocatalyst active sites, resulting in CO2, H2O and other non‐harmful ions [43]. of CuS 2.07 eV) excited electrons (e−) on CB, in the VB.The rGO which were to surfaceof assembled CuSNPs,couldreceive photo‐ <sup>e</sup>−,favoring separation of charge‐ riers and oxidized species(HO∙, ∙O2)formation.Theseactiveradicals reacted withtheATZmolecules adsorbedon the photocatalyst activesites,resultingin CO2,H2Oand othernon‐harmful ions [43]. The photocatalytic performance of CuS/rGO nanocomposites was evaluated for the degradation of toxic persistent organic pollutants, malachite green (MG) and atrazine (ATZ), under direct/simulated sunlight radiance [14,43]. Malachite green is a carcinogenic and non-biodegradable dye used in numerous industrial applications. Atrazine, a commonly used herbicide in the agriculture and food industries, is one of the most stable toxic contaminants in polluted water. The molecular formulae and structures of the organic pollutants, together with the structural, optical, surface and photocatalytic properties of CuS/rGO composites and CuS NPs are presented in Table 2.

**Figure 5.** The photocatalytic degradation mechanism of PAN drug using CuS/CQDs photocatalyst

**Figure 5.** The photocatalytic degradation mechanism of PAN drug using CuS/CQDs photocatalyst

**Figure5.**Thephotocatalyticdegradationmechanismof PANdrugusing CuS/CQDsphotocatalystunder Vis light irradiation (400 mercury lamp).

under Vis light irradiation (400 W mercury lamp).

under Vis light irradiation (400 W mercury lamp).

CuS/CQDs small nano‐flowers with

CuS/rGO uniform CuS NPs distrib‐

urchin‐like structure and some irregular hexagonal NPs

CuS NPs

CuS NPs

CuS/CQDs small nano‐flowers with

CuS/rGO uniform CuS NPs distrib‐

urchin‐like structure and some irregular hexagonal NPs

nano‐petals ‐ 96.5

uted on rGO nanosheets 1.9 34.4 99.2

[43] CuS/rGO separated hexagons of CuS assembled on rGO 1.76 <sup>155</sup> <sup>100</sup> <sup>20</sup>

η\* is the efficiency degradation of pollutant after t min of irradiation.

MG, C23H25ClN2

The experimental results revealed that the addition of various wt% of rGO in CuS had significant contributions to persistent organic pollutant photodegradation with CuS/rGO composite, such as the following: (a) enhanced light absorption and decreased bandgap energy values from 2.08 eV to 1.76−1.9 eV; (b) ensured relatively large specific surface area which provided more active reaction sites exhibiting strong photo‐absorption of pollutant molecules under solar/Vis light irradiations; (c) allowed strong adsorption of pollutant molecules due to the oxygen‐containing functional groups on the surface of rGO. The mechanism of ATZ degradation by CuS/rGO photocatalyst is illustrated in Fig‐ ure 6. The absorption of light photons with energy (hʋ) higher than the band gap energy of CuS (Eg = 2.07 eV) generated photo‐excited electrons (e−) on CB, and holes (h+) in the VB. The rGO nanosheets, which were anchored to the surface of the assembled CuS NPs, could receive the photo‐excited e−, favoring the separation of photogenerated charge car‐ riers and oxidized species (HO∙, ∙O2−) formation. These active radicals reacted with the ATZ molecules adsorbed on the photocatalyst active sites, resulting in CO2, H2O and other

621

222.5

92

60 50

90 [14]

90 [14]

2.08 20.25

CuS NPs hexagonal 2.07 130 ATZ, C8H14ClN5

non‐harmful ions [43].

*Catalysts* **2022**, *12*, x FOR PEER REVIEW 11 of 19

**Figure 5.** The photocatalytic degradation mechanism of PAN drug using CuS/CQDs photocatalyst under Vis light irradiation (400 W mercury lamp). **Figure 5.** The photocatalytic degradation mechanism of PAN drug using CuS/CQDs photocatalyst under Vis light irradiation (400 W mercury lamp). η\* is the efficiency degradation of pollutant after t min of irradiation.

The experimental results revealed that the addition of various wt% of rGO in CuS had significant contributions to persistent organic pollutant photodegradation with CuS/rGO composite, such as the following: (a) enhanced light absorption and decreased bandgap energy values from 2.08 eV to 1.76−1.9 eV; (b) ensured relatively large specific surface area which provided more active reaction sites exhibiting strong photo-absorption of pollutant molecules under solar/Vis light irradiations; (c) allowed strong adsorption of pollutant molecules due to the oxygen-containing functional groups on the surface of rGO. The mechanism of ATZ degradation by CuS/rGO photocatalyst is illustrated in Figure 6. The absorption of light photons with energy (h The experimental results revealed that the addition of various wt% of rGO in CuS had significant contributions to persistent organic pollutant photodegradation with CuS/rGO composite, such as the following: (a) enhanced light absorption and decreased bandgap energy values from 2.08 eV to 1.76−1.9 eV; (b) ensured relatively large specific surface area which provided more active reaction sites exhibiting strong photo‐absorption of pollutant molecules under solar/Vis light irradiations; (c) allowed strong adsorption of pollutant molecules due to the oxygen‐containing functional groups on the surface of rGO. The mechanism of ATZ degradation by CuS/rGO photocatalyst is illustrated in Fig‐ ure 6. The absorption of light photons with energy (hʋ) higher than the band gap energy of CuS (Eg = 2.07 eV) generated photo‐excited electrons (e−) on CB, and holes (h+) in the VB. The rGO nanosheets, which were anchored to the surface of the assembled CuS NPs, could receive the photo‐excited e−, favoring the separation of photogenerated charge car‐ riers and oxidized species (HO∙, ∙O2−) formation. These active radicals reacted with the ATZ molecules adsorbed on the photocatalyst active sites, resulting in CO2, H2O and other non‐harmful ions [43]. ) higher than the band gap energy of CuS (Eg = 2.07 eV) generated photo-excited electrons (e−) on CB, and holes (h<sup>+</sup> ) in the VB. The rGO nanosheets, which were anchored to the surface of the assembled CuS NPs, could receive the photo-excited e−, favoring the separation of photogenerated charge carriers and oxidized species (HO·, ·O<sup>2</sup> −) formation. These active radicals reacted with the ATZ molecules adsorbed on the photocatalyst active sites, resulting in CO2, H2O and other non-harmful ions [43]. *Catalysts* **2022**, *12*, x FOR PEER REVIEW 12 of 19

**Figure 6.** The photocatalytic degradation mechanism of ATZ herbicide using CuS/rGO photocata‐ lyst under Vis light irradiation (300 W Xe lamp with λ < 410 nm cut‐off sieve). actants in further decomposition of TC on the catalyst surface (Figure 7). **Figure 6.** The photocatalytic degradation mechanism of ATZ herbicide using CuS/rGO photocatalyst under Vis light irradiation (300 W Xe lamp with λ < 410 nm cut-off sieve).

The reusability and photo‐stability studies showed that CuS/rGO heterojunction nanostructure had the capacity to regenerate several times (e.g., 5 times), without a sig‐ nificant loss in its photodegradation efficiency, therefore exhibiting a high stability under

PDI, one of the most studied organic fluorescent dyes, has excellent optoelectronic properties, specific to an n‐type semiconductor, and, therefore, is used in solar cells and sensor applications. Supramolecular architectures (NS 1D) formed by self‐assembled PDI are of particular interest in photocatalytic materials development due to their high photo‐

A recent study [79] reported the synthesis of a novel CuS/PDI p‐n heterojunction photocatalyst for removal of tetracycline (TC) from wastewater. The CuS/PDI composites were prepared using a two‐step self‐assembly procedure, varying the CuS concentration (5%, 10%, 15%) in self‐assembled PDI. By comparison with pure self‐assembled PDI and CuS nanosheets, higher efficiency in photodegradation of TC, under simulated visible light, after 2 h, was obtained for the CuS10%/PDI catalyst. Thus, before coupling, both pure CuS (Eg = 2.01 eV, SBET = 1.87 m2/g) and PDI (Eg = 1.67 eV, SBET = 0.76 m2/g) cata‐ lysts showed poor photocatalytic performance, meaning about 40% and 60%,respectively, within 120 min. After coupling, and p‐n heterojunction formation, the CuS/PDI composite (Eg = 1.71 eV, SBET = 39.43 m2/g) exhibited higher efficiency (about 90%) for TC degrada‐ tion. The enhancement of CuS/PDI photocatalytic activity was mainly attributed to the highly ordered H‐type p‐p stacking in the PDI structure and the p‐n junction, which al‐ lowed the fast separation of the photo‐generated charge carrier pairs. After the formation of a p‐n junction, the photo‐electrons were transferred from CuS CB to self‐assembled PDI CB, under the action of the internal electric field, while photo‐holes migrated in the oppo‐ site direction, from the self‐assembled PDI VB to that of CuS. The transferred electrons further reacted with O2, resulting in peroxide ion radicals ∙O2‐ and holes, which produced active radicals ∙OH after the reaction with H2O. Both active oxidized radicals acted as re‐

thermal stability, charge mobility, and electron affinity [79,82].

under Vis light irradiation (400 W mercury lamp).

light irradiation [14,43].

*Photodegradation*

**Figure 5.** The photocatalytic degradation mechanism of PAN drug using CuS/CQDs photocatalyst

The reusability and photo-stability studies showed that CuS/rGO heterojunction nanostructure had the capacity to regenerate several times (e.g., 5 times), without a significant loss in its photodegradation efficiency, therefore exhibiting a high stability under light irradiation [14,43].

## *3.2. CuS/Organic Semiconductor Heterostructures as Catalysts for Organic Pollutant Photodegradation*

PDI, one of the most studied organic fluorescent dyes, has excellent optoelectronic properties, specific to an n-type semiconductor, and, therefore, is used in solar cells and sensor applications. Supramolecular architectures (NS 1D) formed by self-assembled PDI are of particular interest in photocatalytic materials development due to their high photo-thermal stability, charge mobility, and electron affinity [79,82].

A recent study [79] reported the synthesis of a novel CuS/PDI p-n heterojunction photocatalyst for removal of tetracycline (TC) from wastewater. The CuS/PDI composites were prepared using a two-step self-assembly procedure, varying the CuS concentration (5%, 10%, 15%) in self-assembled PDI. By comparison with pure self-assembled PDI and CuS nanosheets, higher efficiency in photodegradation of TC, under simulated visible light, after 2 h, was obtained for the CuS10%/PDI catalyst. Thus, before coupling, both pure CuS (Eg = 2.01 eV, SBET = 1.87 m2/g) and PDI (Eg = 1.67 eV, SBET = 0.76 m2/g) catalysts showed poor photocatalytic performance, meaning about 40% and 60%, respectively, within 120 min. After coupling, and p-n heterojunction formation, the CuS/PDI composite (Eg = 1.71 eV, SBET = 39.43 m2/g) exhibited higher efficiency (about 90%) for TC degradation. The enhancement of CuS/PDI photocatalytic activity was mainly attributed to the highly ordered H-type p-p stacking in the PDI structure and the p-n junction, which allowed the fast separation of the photo-generated charge carrier pairs. After the formation of a p-n junction, the photo-electrons were transferred from CuS CB to self-assembled PDI CB, under the action of the internal electric field, while photo-holes migrated in the opposite direction, from the self-assembled PDI VB to that of CuS. The transferred electrons further reacted with O2, resulting in peroxide ion radicals ·O<sup>2</sup> - and holes, which produced active radicals ·OH after the reaction with H2O. Both active oxidized radicals acted as reactants in further decomposition of TC on the catalyst surface (Figure 7). *Catalysts* **2022**, *12*, x FOR PEER REVIEW 13 of 19

**Figure 7.** The photocatalytic degradation mechanism of TC antibiotic using CuS/PDI catalyst under Vis light irradiation (300 W Xe lamp with a cut filter with λ > 420 nm). **Figure 7.** The photocatalytic degradation mechanism of TC antibiotic using CuS/PDI catalyst under Vis light irradiation (300 W Xe lamp with a cut filter with λ > 420 nm).

The stability and reusability experiments encountered some problems with the pow‐ der catalyst, therefore PDI/CuS composite was coupled with the modified cotton fibers through electrostatic adsorption. The newly‐designed photocatalytic fabric was tested un‐ The stability and reusability experiments encountered some problems with the powder catalyst, therefore PDI/CuS composite was coupled with the modified cotton fibers

der simulated actual water quality conditions and, after 5 cycles, the degradation rate was

Wide band gap semiconductors, such as TiO2 (Eg = 3.25 eV [83]), SnO2 (Eg = 3.4 eV [84]) and ZnO (Eg = 3.2 eV [85]), perform better as photocatalysts in the UV region, which causes their limited use in industrial applications, and, thus, a significant increase in pro‐ cess costs. Another factor that limits their photocatalytic efficiency is the rapid recombi‐ nation of charge carriers. To extend the photocatalytic response in the Vis spectral region, and to reduce the recombination processes, many heterojunctions have been developed in recent years by coupling a wide band gap semiconductor with a suitable narrow band gap semiconductor, such as CuS. Most of the previously published review articles on CuS nanostructured materials have focused on synthesis methods, special properties, and pro‐ spective applications [28,29,36]. More recently, in a mini‐review, we published the devel‐ opments on dye photodegradation using various copper sulfide‐based heterojunctions (copper sulfide/metal oxide, copper sulfide/metal sulfide, copper sulfide/graphene, cop‐

Recently, Sudhaik et al. [46] published an extensive literature study (review) on CuS and different CuS‐based heterostructured materials as photocatalysts for wastewater treatment. Thus, in this part of the article, approaches related to recently developed CuS‐ based heterojunctions as catalysts for organic pollutant photodegradation are highlighted,

The enhanced visible light photocatalytic efficiency of type II heterojunction CuS/ZnO in MB and toluidine blue (TB) degradation was reported by Khausik B et al. [80]. The CuS/ZnO photocatalyst was obtained by assembling p‐type CuS NPs on n‐type ZnO heterostructures, using hydrothermal method. For the photocatalysis experiments, 35 mg of CuS/ZnO catalyst and 50 mL aqueous solution of MB (3 × 10−<sup>5</sup> M), respectively TB (6 × 10−<sup>5</sup> M), were used. The photocatalytic experiments showed that the tandem struc‐ ture of CuS/ZnO had excellent photocatalytic efficiency for MB and TB, with 93% and 87.5% dyes degradation within 16 respectively 18 min, under visible light irradiation. The

issues that were not considered in the previously mentioned publications.

*3.3. CuS/Metal Oxide Heterostructures as Catalysts for Organic Pollutant Photodegradation*

per sulfide/organic semiconductors) as catalysts [4].

through electrostatic adsorption. The newly-designed photocatalytic fabric was tested under simulated actual water quality conditions and, after 5 cycles, the degradation rate was maintained at about 80%, which indicated good reusability [79].

#### *3.3. CuS/Metal Oxide Heterostructures as Catalysts for Organic Pollutant Photodegradation*

Wide band gap semiconductors, such as TiO<sup>2</sup> (Eg = 3.25 eV [83]), SnO<sup>2</sup> (Eg = 3.4 eV [84]) and ZnO (Eg = 3.2 eV [85]), perform better as photocatalysts in the UV region, which causes their limited use in industrial applications, and, thus, a significant increase in process costs. Another factor that limits their photocatalytic efficiency is the rapid recombination of charge carriers. To extend the photocatalytic response in the Vis spectral region, and to reduce the recombination processes, many heterojunctions have been developed in recent years by coupling a wide band gap semiconductor with a suitable narrow band gap semiconductor, such as CuS. Most of the previously published review articles on CuS nanostructured materials have focused on synthesis methods, special properties, and prospective applications [28,29,36]. More recently, in a mini-review, we published the developments on dye photodegradation using various copper sulfide-based heterojunctions (copper sulfide/metal oxide, copper sulfide/metal sulfide, copper sulfide/graphene, copper sulfide/organic semiconductors) as catalysts [4].

Recently, Sudhaik et al. [46] published an extensive literature study (review) on CuS and different CuS-based heterostructured materials as photocatalysts for wastewater treatment. Thus, in this part of the article, approaches related to recently developed CuS-based heterojunctions as catalysts for organic pollutant photodegradation are highlighted, issues that were not considered in the previously mentioned publications.

The enhanced visible light photocatalytic efficiency of type II heterojunction CuS/ZnO in MB and toluidine blue (TB) degradation was reported by Khausik B et al. [80]. The CuS/ZnO photocatalyst was obtained by assembling p-type CuS NPs on n-type ZnO heterostructures, using hydrothermal method. For the photocatalysis experiments, 35 mg of CuS/ZnO catalyst and 50 mL aqueous solution of MB (3 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M), respectively TB (6 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M), were used. The photocatalytic experiments showed that the tandem structure of CuS/ZnO had excellent photocatalytic efficiency for MB and TB, with 93% and 87.5% dyes degradation within 16 respectively 18 min, under visible light irradiation. The remarkable photocatalytic activity was attributed to the CuS/ZnO p-n heterojunction formation, which favored the efficient separation of photoinduced charge carriers.

Based on scavenging experiments using different trappers, the mechanism proposed for the degradation of MB and TB under Vis light corresponds to type II heterojunction CuSbased photocatalyst (Figure 8), allowing the transfer of electrons and holes to the other semiconductor from heterojunction (ZnO), when spatial charge carriers separation occurred.

To summarize, in the photocatalytic degradation of dyes by CuS/ZnO catalyst, the photo-excited electrons and holes generate highly reactive hydroxyl radicals (HO·) which decompose pollutants into CO2, H2O and other environmentally friendly compounds that do not require any other chemical or physical treatment.

Recently, WO<sup>3</sup> with activated carbon (AC, 1% and 2%) and co-doped with CuS (10%, 15%) composites, WO3-AC/CuS, were prepared via a facile hydrothermal method [80].

The hydrothermal method was used because it allows the control of the composite's structural, morphological and optical properties, in order to increase photocatalytic activity, by varying the synthesis parameters, such as temperature and time. The nano-rod-like structure (500 nm with size of nano-rods of 80–89 nm) of WO<sup>3</sup> became sharper and clear after doping with AC and was covered with small nanoflowers of hexagonal CuS in WO3-AC/CuS composites. This morphology demonstrated itself to be suitable for the degradation of organic pollutants, in this case aspirin. In addition, the UV–Vis analysis results showed that increasing the AC and CuS amounts in WO<sup>3</sup> caused the bandgap energy to decrease from 2.49 eV (WO3) to 2.3 eV (WO3-2% AC), and to 1.92 eV for WO3-2% AC/15% CuS. This reduction of band-gap energy confirmed that the addition of CuS enhanced the optical properties, and, therefore, the photocatalytic performance of WO3-Ac material.

The photocatalytic experiments were focused on the comparative degradation of aspirin (10 mg/L), as an active pharmaceutical contaminant model, under Vis light (homemade photocatalytic reactor with a 400 W metal halide lamp) in the presence of catalysts WO3, WO3-AC and WO3-AC/CuS. mation, which favored the efficient separation of photoinduced charge carriers. Based on scavenging experiments using different trappers, the mechanism proposed for the degradation of MB and TB under Vis light corresponds to type II heterojunction CuS‐based photocatalyst (Figure 8), allowing the transfer of electrons and holes to the

remarkable photocatalytic activity was attributed to the CuS/ZnO p‐n heterojunction for‐

*Catalysts* **2022**, *12*, x FOR PEER REVIEW 14 of 19

The results showed that ASP photodegradation increased from 40% (WO3) to 97.6% when WO3-AC/CuS heterostructure was used as catalyst, after 150 min of Vis light illumination. This high photocatalytic activity was due to the increase of the photo-generated charges recombination ratio and the decrease of the photon depth penetration. The proposed mechanism for the degradation of ASP by WO3-AC/CuS photocatalyst is schematically presented in Figure 9. other semiconductor from heterojunction (ZnO), when spatial charge carriers separation occurred To summarize, in the photocatalytic degradation of dyes by CuS/ZnO catalyst, the photo‐excited electrons and holes generate highly reactive hydroxyl radicals (HO∙) which decompose pollutants into CO2, H2O and other environmentally friendly compounds that do not require any other chemical or physical treatment.

**Figure 8.** The photocatalytic degradation mechanism of toluidine blue (TB) dye using CuS/ZnO cat‐ alyst under Vis light irradiation. **Figure 8.** The photocatalytic degradation mechanism of toluidine blue (TB) dye using CuS/ZnO catalyst under Vis light irradiation. posed mechanism for the degradation of ASP by WO3‐AC/CuSphotocatalyst is schemat‐ ically presented in Figure 9.

showed that increasing the AC and CuS amounts in WO3 caused the bandgap energy to decrease from 2.49 eV (WO3) to 2.3 eV (WO3‐2% AC), and to 1.92 eV for WO3‐2% AC/15% **Figure 9.** The reaction mechanism for ASP photodegradation by WO3‐AC/CuS catalyst. **Figure 9.** The reaction mechanism for ASP photodegradation by WO<sup>3</sup> -AC/CuS catalyst.

#### CuS. This reduction of band‐gap energy confirmed that the addition of CuS enhanced the **4. Conclusions 4. Conclusions**

optical properties, and, therefore, the photocatalytic performance of WO3‐Ac material. The photocatalytic experiments were focused on the comparative degradation of as‐ pirin (10 mg/L), as an active pharmaceutical contaminant model, under Vis light (home‐ made photocatalytic reactor with a 400 W metal halide lamp) in the presence of catalysts WO3, WO3‐AC and WO3‐AC/CuS. The results showed that ASP photodegradation increased from 40% (WO3) to 97.6% when WO3‐AC/CuS heterostructure was used as catalyst, after 150 min of Vis light illumi‐ nation. This high photocatalytic activity was due to the increase of the photo‐generated Covellite (CuS), a p‐type semiconductor, is a promising Vis light‐responsive photo‐ catalyst due to its narrow bandgap (Eg = 1.7–2.2 eV) and good optical absorption proper‐ ties in the Vis to NIR region. In this review, the influence of different CuS properties (mor‐ phology, particle size, bandgap energy, and surface properties) and synthesis parameters (Cu:S molar ratios, precursor concentration, surfactant amount in precursors solutions, etc.) on the photocatalytic activity of CuS‐based catalysts (CuS nanostructures and CuS‐ based heterostructures) for various toxic, non‐biodegradable, and recalcitrant organic pol‐ lutants (dyes, herbicides, active pharmaceuticals compounds) degradation was discussed. Covellite (CuS), a p-type semiconductor, is a promising Vis light-responsive photocatalyst due to its narrow bandgap (Eg = 1.7–2.2 eV) and good optical absorption properties in the Vis to NIR region. In this review, the influence of different CuS properties (morphology, particle size, bandgap energy, and surface properties) and synthesis parameters (Cu:S molar ratios, precursor concentration, surfactant amount in precursors solutions, etc.) on the photocatalytic activity of CuS-based catalysts (CuS nanostructures and CuS-based heterostructures) for various toxic, non-biodegradable, and recalcitrant organic pollutants (dyes, herbicides, active pharmaceuticals compounds) degradation was discussed.

Photocatalytic performances of CuS nanostructures and CuS‐based heterostructures in organic pollutants removal from wastewater contribute to the further development of various CuS‐based photocatalysts with enhanced solar energy conversion efficiency for

**Author Contributions:** Conceptualization, L.I; methodology, L.I.; literature investigation, L.I., C.C., L.A. and A.E.; writing—original draft preparation, L.I.; writing—review and editing, L.I. and A.E.; visualization, L.I., C.C., L.A. and A.E.; supervision, A.E. All authors have read and agreed to the

**Funding:** This work was supported by a grant of the Ministry of Research, Innovation, and Digiti‐ zation, CNCS–UEFISCDI, project number PN‐III‐P4‐PCE‐2021‐1020 (PCE87), within PNCDI III.

**Conflicts of Interest:** The authors declare no conflict of interest.

published version of the manuscript.

**Abbreviations**

EY eosin Y CR congo red MoO mordant orange SO safranine orange AO acridine orange

MCs microcrystals HT hydrothermal ST solvothermal SG sol–gel

CoPp Co‐precipitation MV methyl violet MG malachite green MO methyl orange

Photocatalytic performances of CuS nanostructures and CuS-based heterostructures in organic pollutants removal from wastewater contribute to the further development of various CuS-based photocatalysts with enhanced solar energy conversion efficiency for environmental remediation and green energy production.

**Author Contributions:** Conceptualization, L.I; methodology, L.I.; literature investigation, L.I., C.C., L.A. and A.E.; writing—original draft preparation, L.I.; writing—review and editing, L.I. and A.E.; visualization, L.I., C.C., L.A. and A.E.; supervision, A.E. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by a grant of the Ministry of Research, Innovation, and Digitization, CNCS–UEFISCDI, project number PN-III-P4-PCE-2021-1020 (PCE87), within PNCDI III.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **Abbreviations**

