Eco-Friendly Gelatin–Cerium–Copper Sulphide Nanoparticles for Enhanced Sunlight Photocatalytic Activity

Abstract

Using a semiconductor catalyst with sunlight can make the photodegradation of pollutants an economically viable process since solar energy is an abundant natural energy source. Solar photocatalysis can provide clean and green eco-friendly technology for the analysis of industrial effluents. Photocatalytic deterioration of the aqueous solution of malachite green oxalate dye (MGO dye) was studied using gelatin–cerium–copper sulphide (Ge-Ce-CuS) nanoparticles under the sunlight source. The nanoparticles were synthesised by a hydrothermal process. The structural properties of the nanoparticles have been characterised by XRD, SEM, EDS, HR-TEM, and XPS. The effects of the initial concentration of dye, dosage of photocatalyst, reaction time, and pH on dye removal efficiency were studied. The mineralisation of MGO dye has been confirmed by chemical oxygen demand (COD) measurements. The reusability of the catalyst was proved. The antibacterial activity has been studied for the synthesised nanoparticles. The higher photocatalytic degradation efficiency of Ge-Ce-CuS is explained by its reduced electron-hole recombination and sunlight activity.

1. Introduction

The environmental society has been impacted by modern industrial advances. Copious industries, such as the textile industry, utilise dyes to colour their goods, which results in wastewater that contains organics with bright colours. Due to poor levels of dye-fibre fixation, 50 percent of the dye used in dyeing processes ends up in wastewater [1]. People who utilise these effluents for daily activities including drinking, bathing, and washing are impacted by the discharge of these dyes [2]. As a result, it is crucial to check the water quality, especially since drinking water with a dye concentration of just 1.0 mg/L can colour the water significantly and render it unsafe for human and animal consumption [3]. Aquatic plants can also be impacted by dyes since they decrease sunlight transmission through water. Additionally, dyes may be harmful to aquatic life, mutagenic, carcinogenic, and dangerous to human health, including by causing renal, reproductive, liver, brain, and central nervous system malfunction [4,5,6]. Due to the harmful and highly apparent effects of even small amounts of dye in water, the removal of colour from waste effluents becomes crucial for the environment [7]. There is a continuing need for an efficient method that can remove these dyes since doing so is thought to be an environmental problem and because government legislation mandates that textile wastewater be cleaned [8]. As a result, the recent utilisation of adsorption by by-products of nanocomposites as an economical and feasible technique for the removal of various pollutants has been shown to be effective at doing so for a variety of pollutants, including heavy metals [9,10] and dyes [11,12,13]. Malachite green oxalate irritates the eyes, lungs, and digestive system. It is extremely difficult to remove malachite green oxalate dye from the aquatic environment. Toxic dyes like malachite green oxalate can be removed from the environment through the process of photodegradation. In this work, we have reported the photodegradation of the aqueous solution-based dye, malachite green oxalate, under direct sunlight [14]. Numerous photocatalytic materials, including metal oxides, metal sulphides, metal phosphides, and metal-organic frameworks (MOFs), have been researched for the photodegradation of pollutant molecules in water purification systems [15,16,17,18]. The electrochemical behaviour of metal oxide in an aqueous lithium hydroxide electrolyte is reported [19]. Gelatine is a biopolymeric substance that acts as an adsorbent for dye removal in the current work. Gelatin is a molecularly heavy polypeptide. It is created by carefully hydrolysing collagen. Gelatin’s ability to be used to create biocompatible materials is its most significant quality [20]. Due to its propensity to gel, gelatin is a very alluring raw material for making hydrogels. Additionally, it is simple to cross-link because of its abundance of functional groups [21]. In the medical industries, gelatin hydrogels are employed as biodegradable materials [22,23,24,25]. These materials have a wide range of uses in numerous industries, including solar cells, catalysis, electroluminescence devices, optical filters, and more [26,27,28,29,30]. We selected cerium as the metal dopant because of its superior Ce³⁺/Ce⁴⁺ redox combination and substantial electron trapping efficiency, which helps to separate the electrons and holes [31]. Minakshi et al. reported the effect of CeO₂ additions on an aqueous rechargeable lithium battery [32]. Li et al. reported that cerium element doping can diminish the band gap of TiO₂, which results in enhanced photocatalytic activity [33]. Zhang and Liu suggested that cerium doping may well prohibit the recombination of the photo-generated electron-hole pairs [34]. Copper (II) sulphide (CuS) is one of the most promising semiconductor materials with excellent optical, electronic, chemical, and thermal properties [35,36,37,38,39,40,41,42]. Many other processes have been used to create CuS, including solvo/hydrothermal, chemical bath deposition (CBD), wet deposition, microwave, sonochemical, and others [43,44,45,46,47,48]. CuS has been synthesised using several synthesis techniques, resulting in morphologies such as sheet, flower, rod, and hollow spheres [48,49,50]. Metal-doped CuS nanostructures have an extensive application area in modern optoelectronic devices [51,52,53,54,55,56]. A hydrothermal technique was used to create the Co-doped CuS nanocrystals for solar photocatalytic activity [57]. Because of its unique optical and electrical properties, great chemical stability, affordability, ease of regeneration, biocompatibility, and environmental friendliness, copper sulphide is being investigated as a photocatalyst [58,59,60]. This increases its efficiency in the removal of dyes from wastewater. For more environmentally friendly applications, semiconductor photocatalysis is a key method [61,62]. In this study, the hydrothermal technique was employed to synthesise gelatine-assisted cerium co-doped CuS nanoparticles. When compared to pure and mono-doped CuS, the resultant gelatine-assisted cerium co-doped CuS exhibits improved photocatalytic activity. The prepared catalyst was characterised by XRD, SEM, EDS, HR-TEM, and XPS. The activity was tested by photodegradation of malachite green oxalate (MGO) in an aqueous solution under sunlight irradiation. Based on the results of various effects of catalyst, a catalytic mechanism was also discussed.

2. Experimental

2.1. Materials

The commercial azo dye, malachite green oxalate (MGO) dye, was obtained from Oxford Lab Fine Chemicals manufacturer, in Maharashtra. Gelatine was purchased from (SDFCL) Pondicherry and was used as such. Cerium ammonium nitrate (99%), sodium sulphide (99%), and copper (II) acetate monohydrate (98%) were obtained from Himedia Chemicals, and were used exactly as they were received. ZnS, CuO from Aldrich, ZnO from HiMedia, and TiO₂ anatase from Merck were employed. For the COD analysis, the following substances were utilised as received: ferrous ammonium sulphate (Qualigens 98.5%, Mumbai, India), potassium dichromate (SD fine 99.5%, Mumbai, India), silver sulphate (AR-HiMedia, Mumbai, India), and mercury sulphate (Merck, Bengaluru, India). To make the experimental solutions, double-distilled water was employed. Using H₂SO₄ (or) NaOH, the pH of the solution was adjusted before irradiation.

2.2. Synthesis of Gelatin-Assisted Cerium-Loaded CuS by Hydrothermal Method

A Gelatin assisted cerium loaded CuS (Ge-Ce-CuS) was synthesized using a hydrothermal method (Scheme 1). Cerium ammonium nitrate (0.05 M) was dissolved in 100 mL of distilled water for this operation. Gelatin (0.3 g) was added to hot water to prepare gelatin gels. The two solutions were mixed together, then underwent stirring for 10 min and allowed to settle. Gelatin-cerium pale yellow precipitate was formed. Afterwards, 100 mL of distilled water was used to individually to dissolve 0.2 M copper acetate and 0.2 M sodium sulphide after that mixing them. Then stirred for a further 2 h. The formed copper sulphide precipitate was mixed with the gelatin-cerium precipitate. pH is adjusted to 10 with the help of NaOH, then stirred for 6 h at 350 rpm. This was subjected to a sonication process for 2 h. This solution was treated for 6 h hydrothermally in a stainless steel autoclave lined with Teflon. The precipitate (Ge-Ce-CuS) collected was filtered, well washed with distilled water, dried in an oven at 100 °C, and then calcined at 450 °C for 6 h. The dried Ge-Cerium-CuS catalyst was cooled then used. The bare CuS was prepared using the same procedure without the addition of gelatin and cerium.

2.3. Analytical Method

The crystalline phase of the gelatin–cerium–CuS and copper sulphide nanoparticles were characterised by X-ray powder diffraction (XRD) patterns on a Bruker USA D8 Advance (Bruker, Germany), Davinci X-ray diffractometer equipped with a Cu tube for CuK radiation (wavelength 1.5406) at kV, 25 mA. Peak positions, as well as standard files, were compared to identify the crystalline phases. Using the Debye–Scherrer equation (D = Kλ/β cos θ), the size of the Ge-Ce-CuS crystals was determined. SEM-EDS was taken with gold-coated samples using a Jeol apparatus model JSM-IT200 equipped with a Bruker EDS with an LN2 probe for energy-dispersive spectra. The crystalline phase of the gelatin–cerium–CuS and copper sulphide nanoparticles were examined using a high-resolution transmission electron microscope (HR-TEM) (the grids were dried under natural conditions and examined using an HR-TEM model: FEI-TECNAI G²-20 twin-operating voltages of 200 kV). X-ray photoelectron spectra of the catalysts were recorded in a ULVAC-PH1, INC: Model: PH15000 version probe III (Hagisono, Japan); the spectra were referenced to the binding energy of C1s (285 eV).

2.4. Irradiation Experiment

For the photolysis experiment, the dye solution (MGO dye) of the desired molar ratio was freshly prepared from the stock solution of the dye. MGO dye stock solution was used as synthetic wastewater. All photocatalytic tests were conducted on sunny days between the hours of 11 am and 2 pm, under analogous settings. An open borosilicate glass tube of 50 mL capacity, 40 cm height, and 20 mm diameter served as the vessel. To achieve adsorption–desorption equilibrium between dye and Ge-Ce-CuS, the suspension was magnetically swirled in the dark for 30 min. The radiation process was done outside. By using a pump, 50 mL of dye solution containing Ge-Ce-CuS was constantly aerated to provide oxygen and ensure that the reaction solution was thoroughly mixed. The first sample was obtained after dark adsorption; 2–3 mL of the sample was taken at predetermined intervals and centrifuged to separate the catalyst; 1 mL of the centrifugate was appropriately measured to check the dye level. Figure 1 depicts the chemical composition and absorption spectra.

2.5. Sunlight Strength Measurements

Every 30 min, the intensity of the sunlight was monitored and the average light intensity for the length of each experiment was calculated. The sensor was always placed in the most aggressive condition. The strength of sunlight was measured using an LT Lutron Lx-10/A Digital Lux Meter and the strength was 1250 × 100 ± 100 lx. During the experiments, the level of sunlight strength was almost consistent.

2.6. Chemical Oxygen Demand (COD) Measurements

By applying the following process, the COD was assessed. The dye sample was refluxed for two hours with HgSO₄, a known amount of standard K₂Cr₂O₇, AgSO₄, and H₂SO₄, and then titrated with standard ferrous ammonium sulphate (FAS), using ferroin as the indicator. Instead of a dye sample, distilled water was used for a blank titration. To calculate the COD, the following equation was used:

$$\mathrm{COD}=\frac{ \left(\mathrm{Blank} \mathrm{titre} \mathrm{value}-\mathrm{dye} \mathrm{sample} \mathrm{titre} \mathrm{value}\right) \times \mathrm{normality} \mathrm{of} \mathrm{FAS} \times 8 \times 1000 }{\mathrm{Volume} \mathrm{of} \mathrm{the} \mathrm{sample}}$$

(1)

3. Results and Discussion

3.1. XRD Analysis

The XRD pattern of bare CuS and Ge-Ce-CuS nanocomposites are displayed in Figure 2a,b. From the bare CuS sample (Figure 2a), the XRD peaks observed 2θ at, 33.2°, 35.6°, 39.1°, 48.9°, 53.7°, and 59.0° correspond to (006), (103), (105), (110), (108), and (116) crystal planes of CuS [63], identifying the hexagonal structure [64,65] (card No. 01-078-0876). In the XRD spectrum of Ge-Ce-CuS (Figure 2b), additionally, a new minor peak with a 28.6 value appeared, analogous to the CeO₂ (111) plane, [66] (card No. 898436). This proves the cerium loading. The addition of gelatin (template) does not change the phases of CuS in Ge-Ce-CuS, and also, there are no other peaks observed except the cerium and CuS peaks, confirming the purity of Ge-Ce-CuS [67].

3.2. SEM and EDS

SEM is used to examine the morphologies of the prepared compounds. The surface morphologies of bare CuS and the gelatin-cerium co-doped (Ge-Ce-CuS) nanocomposite are observed on SEM and their images are given in Figure 3a–f. From the figures (Figure 3a–c), it is noticed that the bare CuS surface has an agglomerated structure. However, the co-doped sample (Figure 3d–f) is homogeneous, quasi-spherical in shape, and has minor agglomerates with small particles that are subjected to the surface. The structure and morphology of the photocatalyst have the most significance in controlling catalytic activity [68]. EDS analysis (Figure 4a,b) is performed for CuS and Ge-Ce-CuS nanocomposites, which indicated the presence of cerium, copper, and sulphur in the nanocomposites.

3.3. HR-TEM Analysis

HR-TEM analysis was employed to measure the shape and size of the nanoparticles. HR-TEM images of the as-prepared pure CuS nanoparticles are presented (Figure 5a–c). HR-TEM images of Ge-Ce-CuS nanoparticles are given in Figure 5e–g. From the figure, it is noticed that the nanoparticles were randomly distributed with the size of different nm with hexagonal shapes. The selected area electron diffraction (SAED) of the as-prepared CuS (Figure 5d) and Ge-Ce-CuS nanoparticles (Figure 5h) showed a brightly dispersed dotted pattern, which is attributed to the highly crystalline nature of the nanoparticles [69]. However, the scattered spots indicate that the nanoparticles are not monodispersed [70]. The results are in good agreement with XRD analysis.

3.4. X PS

Bonding states and surface chemical composition of Ge-Ce-CuS nanoparticles were measured by the XPS instrument. Figure 6a shows the survey spectrum of Ge-Ce-CuS nanoparticles that exhibit peaks like Cu2p, S2p, and Ce3d. The binding energy peaks of copper at 934.71 and 954.72 eV (Figure 6c) correspond to Cu 2p_3/2 and Cu 2p½, respectively, which indicates the presence of Cu²⁺ ions. In addition, the two corresponding “Shake up” satellites were noticed at 942.6 and 963.1 [71,72,73]. Figure 6d shows the presence of an S2p peak at 169.06 eV, which indicates the presence of metal sulphides [74]. Ce 3d appeared at 885.9 (Ce3d_5/2) and 904.8 (Ce3d_3/2) (Figure 6b), which indicates the presence of Ce³⁺ and Ce⁴⁺ ions [75,76,77,78].

3.5. Photodegradability of MGO Dye Using Sunlight

The photo degradability of MGO dye using various photocatalysts under sunlight irradiation is described in Figure 7. A 4.1% decrease in dye concentration occurred because of adsorption by Ge-Ce-CuS in the absence of sunlight (curve-c). The dye is resistant to self-photolysis (3.1%) (curve-a). Simultaneous irradiation and aeration in the presence of Ge-Ce-CuS catalyst caused a 90.7% deterioration (curve-b) in 120 min. Based on these findings, we can say that sunlight and a catalyst are required for the successful deterioration of MGO dye. When different photocatalysts such as CuS, ZnO, TiO₂ (anatase), CuO, and ZnS are therefore utilised under the same circumstances, deterioration occurred in percentages of 60.4% (curve-g), 53.6 (curve-h), 78.2 (curve-d), 64.1 (curve-f), and 72.9 (curve-e), respectively. This displays that the sunlight/Ge-Ce-CuS process is more efficient than other methods for degrading MGO dye. Figure 8 shows the UV-visible spectra of MGO dye (3 × 10⁻⁴ M) solution at various irradiation times. The intensity at 315 and 617 nm gradually diminishes during the deterioration, although the UV-visible spectra are not significantly altered by the radiation. Its decrease shows that the dye has degraded.

3.6. Effect of Catalyst Loading

One of the key criteria for the studies on deterioration is the amount of catalyst. It is important to investigate the ideal loading for effective pollution removal to prevent the use of extra catalysts. In the photocatalytic degradation process, several authors have investigated the reaction rate as a function of catalyst loading [79,80,81]. From using 0.5 to 2.5 g L⁻¹ of the catalyst, the effect of catalyst weight (Ge-Ce-CuS nanoparticles) on the percentage removal of MGO dye was investigated. The results are given in Figure 9. The outcomes demonstrate that the dye degradation employing Ge-Ce-CuS under solar light increases from 54.9% to 88.6% when catalyst weight is increased from 0.5 to 1.5 g L⁻¹ in 90 min. This is brought on by a rise in the number of catalyst particles, which boosts photon absorption and pollutant (dye) molecule adsorption. Ge-Ce-CuS loading that is increased further (above 1.5 g L⁻¹) lowers the elimination rate. Beyond 1.5 g L⁻¹, the catalyst loading may rise and result in a screening effect. These consequences lower the catalyst’s particular activity. Particle aggregation may also diminish the catalytic activity when the catalyst is loaded heavily. The optimum amount of catalyst loading is found to be 1.5 g L⁻¹ of the deterioration of MGO dye.

3.7. Effect of Solution pH

A fundamental factor of the process of degradation has been the initial pH of the solution. For the MGO dye deterioration by the Ge-Ce-CuS nanocomposite, the pH effects have been studied in the pH 3–11 range and the corresponding values are 43.0, 66.8, 71.7, 88.6, and 60.1%, as exhibited in Figure 10. The effective deterioration is performed at pH 9; at the same time the pH is higher (or) lower than 9, the degradation decreases. Generally, the dye degradation in alkaline pH is greater than in acidic pH [82]. At low pH, Ge-Ce-CuS nanocomposite dissolution reduces the MGO dye adsorption and photoabsorption. So at an acidic pH, the efficiency is minimum. The anionic dye adsorption at above pH 9 decreases because the Ge-Ce-CuS nanocomposite surface is negatively charged by the adsorption of OH⁻ ions. So at high pH (>pH 9), the MGO dye degradation decreases. (Deterioration efficiency of a catalyst depends upon the adsorption of dye molecules). To discover the dark MGO dye adsorption under different pHs, the degradation experiment was conducted. The MGO dye adsorption percentages at pH 3, 5, 7, 9, and 11 were established at 18.3, 24.6, 35.8, 46.4, and 41.6, respectively, after the attainment of adsorption equilibrium. Hence pH 9 is observed to be an effective pH for this deterioration process.

3.8. Effect of Initial Dye Concentration

Investigations have been done into the impact of varied initial dye concentrations on the breakdown of MGO dye on the Ge-Ce-CuS catalyst surface. The rise of pollutant (dye) consolidation from 1 to 5 × 10⁻⁴ M reduces the degradation from 94.8% to 69.7% in 90 min of sunlight exposure (Figure 11). This behaviour could be explained by the reason that at high starting dye concentrations, the path length of a photon entering the solution similarly shortens. Thus, the photocatalytic degradation capability decreases, while at lower concentrations, the opposite effect is seen, increasing the number of photons absorbed by the catalyst [83]. A large amount of adsorbed dye also has a competing effect on the adsorption of O₂ and OH⁻ onto the surface of the catalyst.

3.9. Stability of the Catalyst

MGO dye deterioration in sunlight has been used to test for the stability of catalyst Ge-Ce-CuS (Figure 12). This figure displays the deterioration of MGO dye using the same Ge-Ce-CuS nanocatalyst for four consecutive runs. In the first run, approximately 90.7% of the dye was removed at 120 min. The dye deterioration percentages in the second and third runs are 83.7% and 74.4%, respectively. The catalyst displays a 65.9% degradation capability even in the fourth run. This demonstrates that the catalyst is more stable and reusable when exposed to sunlight. The efficiency of Ge-Ce-CuS is compared with other catalysts (

Table 1. A comparison of Ge-Ce-CuS efficiency with other reported modified CuS-based photocatalysts.

S.No	Catalysts	Pollutants/Light Used	Concentration	Catalyst Amount	% of Degradation/Time (Min)	References
1	rGo-ZnS/CuS	Methyl orange/Visible light (500 W xenon arc lamp)	20 mg L−1	20 mg	81.2/150 min	[36]
2	Ni/CuS	Rhodamine B/Sun light, (150 W xe lamp)	10 mg L−1	100 mg	98.4/60 min	[54]
3	CO/CuS	Rhodamine B/Solar irradiation (150 W Xe lamp)	10 mg L−1	100 mg	74/60 min	[56]
4	CuS-ZrO2	Tetracycline/UV light (14 W UV lamp)	100 mg/L	50 mg	96.5/60 min	[60]
5	Cr(VI) CuS	Adipic acid/Visible light (0.41 W Visible fiber lamp)	0.5 mM	3 mg	80/240 min	[64]
6	Ge-Ce-CuS	Malachite green oxalate/Direct sunlight	3 × 10−4	1.5 g L−1	90.7/120 min	[Present work]

).

4. Chemical Oxygen Demand (COD) Analysis

Mineralisation (deterioration) of MGO dye is confirmed by using COD values. After 120 min of radiation, there was a 71.4% decrease in COD, indicating dye mineralisation had taken place (

Table 2. Chemical oxygen demand measurements.

Time (min)	COD Values (mg/L)	COD Reduction (%)
0	1225.3	0
30	787.7	35.7 (%)
120	350.0	71.4 (%)

[MGO] = 3 × 10⁻⁴ M, catalyst suspended = 1.5 g L⁻¹, pH = 9, airflow rate = 8.1 mL s⁻¹, I_solar = 1250 × 100 ± 100.

). The generation of CO₂ during photodegradation is also displayed by the mineralisation of MGO dye. By injecting the gas formed during photodegradation into lime water, carbon dioxide production was examined.

4.1. Antibacterial Activity

The Baur–Kirby disc diffusion method was used to test the bactericidal activity of the produced CuS and Ge-Ce-CuS catalysts [84]. In this experiment, three gram-positive and three gram-negative microbes, including Proteus mirabilis, Staphylococcus aureus, Bacillus subtitles, Pseudomonas sp., Salmonella typhi-A, and Escherichia coli, were used for evaluation of antibacterial activities for 30 and 50 g of catalyst, and Amoxicillin was used as a standard. Test pathogens were propagating on Muller–Hinton agar (MHA) plates during the antibacterial measurement. All of the wells were made using a sterile cork borer, loaded with the necessary amount of nanocatalysts (CuS and Ge-Ce-CuS), and then put over the agar. The test plates were incubated for 24 h at 37 degrees Celsius. The activity against the test pathogen was determined by reading the zone of inhibition (mm in diameter). In mm, the measured zone of inhibition is specified. In

Table 3. Antibacterial zone of growth inhibition capacities owing to the prepared bare CuS and Ge-Ce-CuS nanocomposite with the different bacterial strain.

Microorganisms (Bacteria)	Zone of Growth Inhibition
	Amoxicillin (p)	30 µg/mL		50 µg/mL
		Bare CuS (A)	Ge-Ce-CuS Nanocomposite (C)	Bare CuS (B)	Ge-Ce-CuS Nanocomposite (D)
Gram-positive
Bacillus subtilis	31	9	14	12	18
Proteus mirabilis	25	7	11	10	15
Staphylococcus aureus	30	10	15	12	19
Gram-negative
Salmonella paratyphi A	35	7	11	9	14
Escherichia coli	30	8	12	10	15
Pseudomonas sp.	30	10	14	12	17

you can find the values for the zone of inhibition. Table 3 shows that Ge-Ce-CuS has higher antibacterial activity than bare CuS, and Staphylococcus aureus and Pseudomonas sp. have better activity than other microorganisms.

4.2. Mechanism

The photocatalytic process is started by lighting up sunlight on a semiconductor catalyst (Ge-Ce-CuS). In the conduction band (CB) and valence band (VB), as indicated in Scheme 2, this irradiation produces electrons (e⁻) and holes (h⁺), respectively. Typically, these electron holes are combined again to decrease the semiconductor’s catalytic activity. However, cerium (Ce) suppresses the electron from CB of Ge-CuS from recombining with the hole by trapping it. The electron transfer to O₂ may be the rate-determining step in semiconductor [85] photocatalysis if the oxygen adsorbed on the photocatalyst’s surface can trap the photogenerated electrons [86]. However, Ce⁴⁺ easily traps the photoexcited electron in the system of Ge-Ce-loaded CuS catalyst. A Ce⁴⁺ ion, such as in Lewis acid, has more ability in trapping the electron than an oxygen molecule (O₂) [87]. The trapped electrons are transferred to the adsorbed O₂ synergistically to form superoxide radicals so that the recombination between electrons and holes is diminished [88].

Ce⁴⁺ + e⁻ → Ce³⁺

(2)

Ce³⁺ + O₂ → O₂^•− + Ce⁴⁺

(3)

Therefore, it is clear that the Ce 4f level in Ge-Ce-CuS is crucial for interfacial charge transfer and the prevention of electron-hole recombination, which enables the Ge-Ce-CuS photocatalyst to increase its photocatalytic activity.

Since Ce⁴⁺ traps the electron easily, it acts as the scavenger of the electron. Hence, Ce³⁺ and Ce⁴⁺ co-existing in Ge-Ce-CuS change the electron-hole pair recombination, which influenced the photoreactivity. Furthermore, the Ce^4+/Ce³⁺ site’s trapping capabilities are subsequently transferred to the nearby adsorbed O₂ to produce more superoxide radical anion O₂^•−, while at the same time, the VB holes of Ge-CuS react with water to produce a highly reactive hydroxyl (^•OH) radical. The highly reactive superoxide radical anion and hydroxyl radical are used for the degradation of pollutants.

4.3. Conclusions

We have prepared the Ge-Ce-CuS catalyst by a hydrothermal method. The presence of cerium is evidenced by XRD. Under optimal reaction conditions, the Ge-Ce-CuS nanocatalyst has a high solar photocatalytic activity for the degradation of MGO dye. A Ce⁴⁺ ion, as Lewis acid, traps the electrons and transfers them to the adsorbed O₂ synergistically to form superoxide radicals, so that the recombination between electrons and holes is reduced. Recycling experiments clearly show the stability of Ge-Ce-CuS. The prepared nanocomposite (Ge-Ce-CuS) showed better antibacterial activity. This analysis applies to water treatment and the outcomes can produce an important modification in industrial effluent treatment. The outcomes can be used for the designing of more efficient treatment plants in the dyeing industry.