Abstract
Environmental remediation of toxic organic pollutants on catalytic degradation has gained much attention. Organic dyes and fossil fuels as pollutants are the two major problems nowadays. The efficient and targeted eradication of organic dye from water systems is a critical global concern for the treatment of both drinking water and wastewater. In this study, ZnO–ZnS–CdO–CdS quaternary core–shell nanocomposites (NCs) were synthesized using Ricinus communis as a stabilizing agent and hydrazine hydrate as a reducing agent. UV-visible spectroscopy and photoluminescence confirmed the formation of NCs. Fourier transform infrared spectroscopy confirmed the presence of functional groups, while scanning electron microscopy analysis revealed that the morphology of nanomaterials was spherical and poly distributed. X-ray powder diffraction confirmed the crystalline nature of prepared samples. The prepared nanocatalysts were used in the production of hydrogen gas from green sources of the Brassica campestris leaf extract and the degradation of Congo red and methyl red dyes. Overall, the photocatalytic performance of NCs and their design was successful. The prepared catalysts were not only active in the degradation of a single substrate but also in the degradation of a mixture of dyes.
1 Introduction
Environmental pollution, particularly water pollution, is increasing with each passing day as a result of massive industrial growth. Textile dyes, pharmaceutical metabolites, and discrete organic solvents make up the majority of industrial waste [1]. Large numbers of industrial effluents are poisonous, non-biodegradable, and harmful to living things. The accumulation of these contaminants in wastewater, particularly organic dyes, can cause both acute and chronic health problems [2,3].
Organic dyes are commonly used in the textile, printing, cosmetic, painting, baking, and plastic industries, as well as in acid–base titration and biological research as indicators and tracers [4]. During the coloring process, the textile and paint industries discharge a large number of organic dyes into wastewater [5]. Among the different dyes used in various industries, azo dyes are regarded to be the largest group of synthetic organic dyes. Because of the azo groups, azo dyes are exceptionally stable [6,7]. More than 2,000 different azo dyes in various natures and forms are in use, with about 1 million tons of planned global azo dye production per year [8,9]. During the painting and synthesizing process, 10–15% of dyes were nearly released into the wastewater. This wastewater containing colors raises concerns about public health and also disrupts aquatic life by lowering gas solubility and light penetration, thereby impacting phytoplankton photosynthetic activities [8,9]. At present, dyes are produced and used widely in many different industries, including printing, garments, paper and pulp, etc. However, large amounts of dyestuffs are discharged into aqueous habitats, endangering both human health and aquatic life [10]. As a result, wastewater containing industrial dyes must be cleaned using modern procedures before being discharged into the aquatic environment. Metallic nanoparticles (NPs) and nanocomposites (NCs) have been used as excellent photo-catalysts for the removal of organic dyes due to their distinctive localized surface plasmon resonance, increased surface area, and OH− generation. As a result, it is necessary to eliminate or convert these toxic contaminants into non-poisonous forms [11]. Adsorption, coagulation–flocculation, reverse osmosis, and oxidation are common wastewater treatment methods used, which are less effective for this purpose. Such processes turn dissolved pollutants into solid trash, which can then be released into the environment in one or the other form [12,13]. In this case, the photo-catalysis procedure has been shown to be quite effective in removing such dyes. The presence of a free electron–hole pair in photo-catalysis can generate free radical (OH˙) species with advanced oxidation properties, which can lead to the complete breakdown of organic pollutants into mineral acids, CO2, and H2O [14].
In recent years, in order to achieve long-term industrial benefits, there is a demand for a tremendous increase in clean and green energy production. An environmentally friendly, renewable, and green energy source can help not only to reduce environmental issues but also to alleviate the global energy crisis that is on the horizon. Recently, hydrogen production has received a lot of attention. However, during industrial hydrogen production, a lot of fossil fuels are wasted, resulting in large amounts of NO x and CO2 emissions, which are the main causes of the greenhouse effect and industrial smog. The use of photo-catalysts for hydrogen fuel production from green sources can solve this problem because it is a renewable, abundant, clean, and green sources of electricity [15].
The development of suitable photo-catalysts is a key research topic to attain higher photo-catalytic activity. Various types of photo-catalysts have been documented. Metal sulfides and semiconductor NPs, particularly those in groups II–VI, are noteworthy in this regard due to their outstanding chemical and physical features [16]. Generally, researchers are concerned not only with the nanocrystals, structural features, such as crystallography, composition, morphology, and mean size, but also with their surfaces, because nanostructures’ surfaces contain a wide range of atoms, the electronic states formed on their surfaces may be critical, influencing the physical properties of the nanostructures. Coating nano-crystals with a shell of an unusual species is one of the strategies for influencing their surface features [17]. Zinc oxide (ZnO), zinc sulfide (ZnS), cadmium oxide (CdO), and cadmium sulfide (CdS) NPs are essential semiconductors in this regard. At room temperature, ZnO has a band gap of 3.7 eV. Solar cells, light-emitting diodes (LED), gas sensors, and field emission are only a few of the technological applications [18]. Similarly, the band gap of ZnS is 3.3 eV, which is sufficient. ZnS has unique properties, such as resistance to high electric fields, large band-gap strength, and photo- and electroluminescence residents. UV LED, infrared purple windows, solar cells, sensors, bio-catalysis, and imaging are only a few of the applications of ZnS [19]. CdO has a band gap of ∼2.7 eV, making it a good candidate for photo-catalytic research when combined with other NPs from this group [20]. The band gap of CdS is 2.42 eV and due to its unique properties, CdS NPs have a wide range of applications, including solar cells, LED, photo-catalysis, and optical instruments [21].
Because of excellent optical characteristics, ZnO and ZnS nanomaterials have been extensively employed as photo-catalysts [22]. They are, however, only active in UV light. As a result, attempts have been made to make them active in the visible zone in order to make the best use of solar energy [23]. On the other hand, CdS is an excellent visible light photo-catalyst but it has poor quantum performance due to low stability charge recombination in solution. As a result, significant attempts have been made to increase the photo-catalytic stability of CdS, despite its relatively high photoactivity [24]. Controlling morphology, modifying the structural surface of CdS NPs, doping transition metal ions with CdS [25], applying CdS to graphene sheets or carbon nanotubes to obtain homogeneously allotted CdS QDs [26], and combining CdS with other semiconductors [27] are some of the methods used. In the foregoing attempts, a combination of various band gap semiconductors generating a continuous chain of electron–hole pair is accomplished. So, due to their optoelectronic and structural properties, ZnO, ZnS, and CdO NPs are good semiconductor materials for CdS, forming a series of chains of electron–hole pairs, where their efficiency is increasing [24]. The various methods used for the synthesis include precipitation, solvothermal, hydrothermal, co-precipitation, sol–gel, microwave-aided, and sono-chemical techniques. Over the other approaches, precipitation methods have a distinct advantage since it is easy, inexpensive, and more energetic [28,29]. Here, the successful fabrication of ZnO–ZnS–CdO–CdS core–shell quaternary NCs was demonstrated by using a novel method of chemical precipitation at 80°C.
Despite its low energy density and significant moisture content, biomass offers a potential sustainable fuel source for the production of H2. However, a number of processes, including gasification, pyrolysis, and super-/subcritical hydrothermal treatment, have been developed to efficiently convert biomass to hydrogen [30,31]. In this study, the prepared nanocatalysts were used in the production of hydrogen gas from green sources of the Brassica campestris leaf extract and the degradation of Congo red (CR) and methyl red (MR) dyes.
2 Experimental
2.1 Materials
The chemicals, zinc sulfate heptahydrate (ZnSO4·7H2O), cadmium sulfate octahydrate (CdSO4·8H2O), sodium sulfide (Na2S), and potassium hydroxide (KOH), were purchased from BDH Chemicals, England. Castor (Ricinus communis) oil was purchased from a local Kohat market and used as a capping agent. Hydrazine monohydrate was purchased from Daejung, Korea. Azo dyes, CR, MR, and methyl orange, were from Merck.
2.2 Synthesis of quaternary ZnO–ZnS–CdO–CdS NCs
First, 50 mL of 0.05 M aqueous ZnSO4·7H2O was taken in a round bottom flask and stirred with a magnetic stirrer. The capping agent solution was prepared by dissolving 1 mL of castor oil in 10 mL of absolute ethanol. Then, the castor oil solution was added dropwise in a round bottom flask containing zinc sulfate solution and mixed. For the synthesis of ZnO, 1 mL of hydrazine monohydrate was added dropwise to the zinc sulfate solution and mixed continuously with a magnetic stirrer until white precipitates were obtained. The formation of white precipitate confirmed the synthesis of ZnO NPs. After washing three times with distilled water, precipitates were suspended in 50 mL of distilled water.
In the second step, 50 mL of 0.05 M aqueous ZnSO4·7H2O was added to the ZnO suspension. Then, 50 mL of 0.05 M sodium sulfide pentahydrate and 5 mL of ethanolic castor oil were added dropwise. After that, 1 mL of hydrazine monohydrate was added dropwise and stirred until turbid white precipitates were obtained, which confirmed the formation of ZnO–ZnS NCs. The product was washed with distilled water, filtered, and suspended in 50 mL of distilled water.
The same experimental procedures were followed for the synthesis of ternary ZnO–ZnS–CdO and quaternary ZnO–ZnS–CdO–CdS NCs using CdSO4·8H2O as a precursor of cadmium. The NCs were washed with distilled water, followed by washing with ethanol, then dried, and calcined in a furnace at 300°C for 2 h, and stored for future use.
2.3 Photo-catalytic degradation of dyes
Organic dyes CR and MR degradation were carried out using synthesized NCs. First, 50 mL of 0.04 mM dye solution was taken in a round bottom flask, and its spectra were recorded using a UV-visible (UV-Vis) spectrophotometer in a quartz cuvette. Then, 50 mg of NCs was added and kept in the dark for 5 min to allow the system to reach equilibrium. Then, the reaction mixture in the flask was placed under the solar light and stirred. The spectra of the same solution were then recorded using a UV-Vis spectrophotometer at different time intervals. During the test, the dye concentrations were kept constant at 0.04 mM. The maximum drop in λ max was recorded at 500 nm for CR and 490 nm for MR. The following equation was used to compute the % degradation of the degraded dye:
where the starting absorbance is denoted by A 0 and the absorbance at different time intervals is represented by A t .
2.4 Gas generation from the biomass
The biomass of Brassica campestris leaves was collected and dried at room temperature. The leaves were then pulverized into powder. Following that, 6 g of the leaf powder was dissolved in 0.05 M aqueous KOH solution. The extract was filtered through filter paper after 1 h. The production of gas (H2) from the biomass was carried out in a round bottom flask as a reaction vessel. Reactions were carried out both with and without a catalyst. In the presence of the catalyst, a specific amount of biomass was placed in a round-bottom flask and stirred using a magnetic stirrer. The flask was connected to a 500 mL graduated cylinder through a pipe. The entire reaction was carried out in the light (100 W tungsten lamp). After a few minutes, the generated gas started to cause a drop in the water level in the graduated cylinder. The amount of the alkaline biomass solution used in the experiment ranged from 10 to 50 mL, while the amount of the catalyst used ranged from 10 to 50 mg.
2.5 Characterization
A UV-Vis spectrophotometer (Shimadzu, UV-1800), with a wavelength range of 200–700 nm was used for optical absorption spectrum monitoring of ZnO–ZnS–CdO–CdS NCs. The photo-catalytic degradation analysis of the organic dyes was also performed using the same apparatus. A NICOLET iS50 Fourier transform infrared (FTIR) spectrophotometer was used to determine the different functional groups that act as stabilizers on the surface of NCs. Average crystalline structures of NPs were analyzed using powder X‐ray diffraction (XRD) with a PAN (JDX‐3532 JEOL, Japan) analytical diffractometer. An energy dispersive X-ray spectrometer (EDX) 6490 (LA) linked to a MIRA3-TESCAN scanning electron microscope (SEM) was used to examine the elemental composition of the prepared NCs. The DV 420_OE model was used to study the photoluminescence (PL).
3 Results and discussion
3.1 Synthesis of quaternary core–shell NCs
The synthesis of NPs and ZnO–ZnS–CdO–CdS NCs is presented in Figure 1. The formation of white precipitates (Figure A1 in Appendix) after mixing zinc sulfate with hydrazine hydrate confirmed the synthesis of ZnO NPs. Similarly, the formation of turbid white precipitates confirmed the formation of ZnO–ZnS NCs (Figure A1). The product was washed with distilled water, filtered, and suspended in 50 mL of distilled water. The chemical procedure for the configuration of ZnS and CdS NPs is as follows:
Schematic representation of the NC synthesis.
In the first phase, sodium sulfide was converted to hydrogen sulfide, which then interacted with Cd2+ and Zn2+ to form CdS and ZnS, respectively. A pictorial representation is shown in Figure A1.
3.2 Characterization
3.2.1 UV-Vis analysis
The UV-Vis spectra of the prepared NPs and NC samples were recorded using a UV-Vis spectrophotometer. The UV-Vis spectra of the prepared samples are presented in Figure 2a.
(a) UV-Vis and (b) PL of the synthesized NPs and NCs.
The ZnO NPs showed a broad absorption band at around 280 nm. CdO NPs showed an absorption band at 285 nm and a small peak around 305–315 nm [32]. A small peak was observed in ZnS NPs near 280 nm. CdS NPs displayed an absorption band at 440 nm. The oil NCs displayed an absorption band at 420 nm, which indicated a blue shift in comparison to pure CdS NPs due to quantum confinement. The above absorption bands confirm the configuration of ZnO, ZnS, CdO, CdS NPs, and ZnO–ZnS–CdO–CdS NCs [27,28,33].
3.2.2 PL study
PL spectroscopy is an influential tool to calculate approximately the recombination of photo-induced charge carriers (e− and h+). PL not only explains the dependence of photo-catalytic activity but also on the transfer rate and separation efficiency of the photo-generated charges as well as improved surface chemistry characteristics of the photo-catalyst [34]. The PL spectra of the prepared NCs were recorded at room temperature in order to examine the optical properties. The electronic structure of the semiconductor nanomaterials may be very clearly seen in the PL spectra [35]. Thus, PL spectra are also very helpful in demonstrating the effective growth of ZnO, and also later the ZnS, CdO, and CdS shells on ZnO NPs to synthesize ZnO–ZnS–CdO–CdS NCs. Figure 2b displays the visible room-temperature PL spectra of ZnO–ZnS–CdO–CdS NCs [36,37].
3.2.3 FT-IR analysis of nanomaterials
Figure 3 shows the FT-IR spectra of the synthesized NPs and NCs, which demonstrate distinct peaks of the functional groups included in the stabilizer in the region 500–3,500 cm−1. The O–H vibration of stretching is visible in the sharp band between 3,200 and 3,600 cm−1 [38,39]. The C–H group stretching vibration has a peak at 2,900–3,100 cm−1. The R–N═C═S bonding is visible on the peaks from 2,086 to 2,093 cm−1 [40]. The peaks ranging from 1,009 to 1,150 cm−1 indicate stretching vibrations of C–O and C–N, whereas peaks at 1,220 and 1,140 cm−1 exhibited bending vibrations of aliphatic amines, respectively. The peaks between 600 and 800 cm−1 are attributed to the formation of ZnS and CdS. Furthermore, zinc and CdO vibrations were found to have high peaks at 600 and 500 cm−1, respectively [41,42,43]. Thus, from the above discussion, it can be concluded that biomolecules of castor oil (organic acids) serve as a stabilizing agent for the prepared nanomaterials.
FTIR spectra of the synthesized NPs and NCs.
3.2.4 XRD patterns of nanomaterials
Figure 3 shows the XRD patterns of prepared ZnO, ZnS, CdO, CdS NPs, and ZnO–ZnS–CdO–CdS NCs. It showed the main characteristic peaks with indices of 31.28 (101), 33.88 (002), 48.18 (102), and 63.88 (103), indicating that ZnO NPs had a hexagonal structure [44]. Similarly, major peaks with indices of 28.31 (111), 47.69 (220), and 56.69 (311) were identified, confirming the cubic zinc-blended structure of ZnS NPs [45,46]. Similarly, key feature peaks with indices of 33.37 (111), 38.8 (200), and 55.88 (220) are shown, indicating the cubic structure of CdO [47]. The [111], [220], and [311] diffraction peaks, indicative of the CdS cubic system [48], are also validated in Figure 4.
XRD pattern of the synthesized NPs and NCs.
A mixed structure of ZnO–ZnS–CdO–CdS composites was synthesized, as shown in Figure 4. This validated the hexagonal structure of ZnO and the cubic structures of ZnS, CdO, and CdS, which were unaffected by the coating methods [49]. The Debye–Scherer formula, L = kλ/β ½cos θ, was used to calculate the size of nanocrystallites [50], where θ is the Bragg angle, and λ =1.5418 Å is the X-ray wavelength (Cu source utilized), β = 1/2 is the full width at half-maximum (FWHM) on a 2θ scale, and k is constant nearly equal to unity. According to Scherer’s equation, the average crystallite sizes of ZnO, ZnS, CdO, CdS NPs, and ZnO–ZnS–CdO–CdS NCs were 3, 3.87, 2.96, 3, and 4.72 nm, respectively.
3.2.5 EDX and SEM analyses of nanomaterials
Through energy dispersive X-ray analysis elemental compositions of ZnO NPs, ZnS NPs, CdO NPs, CdS NPs, and ZnO–ZnS–CdO–CdS NPs using castor oil as a catalyst were determined and are shown in Figure 5. The spectra revealed fine signals for Cd and O in CdO, Cd and S in CdS, Zn and O in ZnO, Zn and S in ZnS, and Zn, CdO, and S in ZnO–ZnS–CdO–CdS NPs, demonstrating the reduction of Zn2+ into ZnO and ZnS nanomaterials and Cd2+ into CdO and CdS nanomaterials. EDX examination revealed the presence of Zn, Cd, O, and S in the formed NPs and NCs [51].
EDX investigation of (a) NCs, (b) ZnS NPs, (c) ZnO NPs, (d) CdO NPs, and (e) CdS NPs.
SEM was used to examine the morphology and agglomerated sizes of ZnO, ZnS, CdO, CdS NPs, and ZnO–ZnS–CdO–CdS NCs produced with castor oil, as shown in Figure 6.
SEM analysis of (a) NCs, (b) ZnS NPs, (c) ZnO NPs, (d) CdO NPs, and (e) CdS NPs.
The electrical and optical characteristics of NPs and NCs are dependent on their forms [52]. SEM has been utilized by various researchers to determine the size and morphology, as well as to characterize NPs [53]. The synthesized ZnO, ZnS, CdO, CdS NPs, and ZnO–ZnS–CdO–CdS NCs were polydistributed and spherical in shape, according to SEM analysis, and the sizes of NPs and NCs ranged from 1–10 µm up to 100 nm.
3.3 Photo-catalytic degradation of dyes
In recent years, commercial dyes like CR and MR have been successfully degraded using nanomaterials [54]. Two different azo dye solutions (CR and MR) were utilized to investigate the photo-catalytic performance of ZnO, CdO, CdS, and ZnO–ZnS–CdO–CdS NCs.
First, control studies for the two different dyes were carried out without the use of a catalyst, using only 3 mL of each dye in a cuvette. It was exposed to sunlight for varied periods of time and its spectrum was recorded using a UV-Vis spectrophotometer. Figure 4 illustrates control studies that reveal a small fraction of dye degradation, a process known as photolysis response. Figures 5–8 show a decrease in absorption intensities with time exposure, indicating photo-catalytic degradation of CR and MR dyes [55]. With increasing irradiation time, the absorbance maximum (495 nm for CR and 488 nm for MR) gradually dropped. The degradation percentage and ln C t /C o of all the dyes in the presence of the produced NPs and NCs are also shown in Figures 7–10. The degradation percentage of the dyes was calculated by using the following equation:
where A 0 represents the initial absorbance of the dye solution and A t represents the absorbance at different intervals [56].
UV-Vis spectra of the CR dye degradation in the presence of (a) ZnO, (b) CdO, (c) CdS, and (d) NCs.
Degradation percentage of the CR dye: (a) ZnO, (b) CdO, (c) CdS, and (d) NCs.
UV-Vis spectra of the MR dye degradation in the presence of (a) ZnO, (b) CdO, (c) CdS, and (d) NCs.
Degradation percentage of the MR dye: (a) ZnO, (b) CdO, (c) CdS, and (d) NCs.
As evident from Figure 7, the degradation percentage of the CR dye was 83% by ZnO NPs, 71% by CdO NPs, 68% by CdS NPs, and 86% by the prepared NCs.
In the case of MR degradation, the degradation percentage was 82% by ZnO NPs, 97% by CdO NPs, 77% by CdS NPs, and 93% by the prepared NCs (Figure 9).
It is noteworthy that the prepared NCs had the maximum efficiency in the case of CR and noticeable efficiency in the case of MR.
3.3.1 Kinetics of photodegradation
We can understand dye degradation behaviors using the Langmuir–Hinshelwood (L–H) model. Therefore, the pseudo‐first‐order equation can be expressed as follows:
Since the reaction is colored, the rate of reaction can be determined and expressed in adsorption spectra at time t:
where C o is the initial concentration and C t is the final concentration.
The half-life (t ½) is calculated from Eq. 5 as
where C 0 is the initial concentration and C t is the concentration at time (t) of the dye.
Figures A2 and A3 show a linear relationship in plots of ln (C t /C 0) versus time of degradation. So, the degradation of dyes followed the first-order kinetics. From Eq. 5, the slopes of ln(C t /C 0) vs time and half-life (t 1/2) of the degraded dye samples were calculated (in min) in the presence of ZnO, CdO, CdS NPs, and ZnO–ZnS–CdO–CdS NCs for CR and MR dyes, respectively [57,58].
The slope and half lifetime for CR dyes were 0.1112 and 6.2 in the presence of ZnO, 1.11 and 0.62 for CdO, 0.1335 and 5.16 for CdS, and 0.2425 and 2.85 for oil NCs, respectively. Similarly, for MR dyes, the slope and half lifetime were 0.4129 and 1.67 for ZnO, 0.9021 and 0.76 for CdO, 0.2231 and 3.10 for CdS, and 0.2231 and 3.10 for oil NCs, respectively.
Thus, the above kinetic study exposed that ZnO–ZnS–CdO–CdS NPs and NCs are efficient photo-catalysts for the removal of the given dyes. The synthesized NCs showed the highest efficiency for the degradation CR dyes and a well-defined result for MR. CR was chosen for further investigation because it had the highest efficiency when compared to MR.
3.3.2 Degradation of a mixture of MR and CR
In order to check the catalytic degradation activity of the prepared NCs, the catalyst was tested in this work against a solution of a mixture of dyes. For the degradation of the mixed solution of MR and CR, 25 mL of each 0.04 mM dye was mixed in a reaction beaker. Then, 50 mg of NCs was added into the reaction vessel, and its spectra were recorded using a UV-Vis spectrophotometer. The reaction mixture was placed under a magnetic stirrer in solar light. The spectra of the solution mixture were then recorded using a UV-Vis spectrophotometer. A strong absorption peak is observed at 491 nm for the aqueous MR and CR mixture, as shown in Figure A4.
It is well known that an absorption peak is observed at 495 nm for CR, while that of MR is observed at 488 nm. Thus, the appearance of the peak at 491 nm was due to the chemical interaction of the two dye solutions. The peak intensity at 491 nm decreases after the addition of 10 mg of the catalyst in sunlight. UV-Vis spectra were continuously recorded at a time interval of 15 min, as shown in Figure A4. With time, the intensity of the peak decreases at 491 nm. The decrease in the peak intensity at 491 nm after the addition of the catalyst was due to the initiation of dye degradation [59,60]. The prepared NCs show excellent catalytic degradation activity, and the 74% degradation of the mixture of CR and MR takes 105 min. Thus, the prepared NC catalyst has the best catalytic degradation activity not only for a single substrate but also for a mixture of dyes.
3.3.3 Reusability of NCs
The reusability of effective catalysts and their uses for the next cycle is a major concern in catalysis. The majority of catalysts are deactivated after just one or two cycles of usage. In addition to catalytic efficiency, other important factors to take into account when assessing catalysts’ performance include photocorrosion, versatility, compostability, and stability since they may significantly lower the overall expenses of the process [61]. Heterogeneous NP-based catalysts have been effectively documented in the literature due to their photocatalytic activity; however, the problem is that they must recover from the reaction media in order to be reused. Thus, stable and fully separable catalysts are essential to the processes of recycling and reuse.
The recyclability of catalysts was investigated using CR in this study. The degradation of CR was evaluated for 105 min for consecutive three cycles. The recyclability was tested after washing the catalyst three times in double distilled water when it was used for degradation and utilized for the next reaction at the same time.
As shown in Figure A5, ZnO degrades the CR dye by 83%, 75%, and 71% for the first, second, and third cycles, respectively. Similarly, CdO degrades the CR dye by 71%, 60%, and 65% for the first, second, and third cycles, respectively. Degradation percentage of CdS were 68%, 64%, and 59% for the first, second, and third cycles, respectively. However, degradation efficiencies of NCs were 86%, 80%, and 76% for the first, second, and third cycles, respectively.
3.3.4 Mechanism of degradation
A schematic representation of organic dye photocatalysis by the prepared NCs is illustrated in Scheme 1. It provides a plausible mechanism for the degradation reaction. The proposed degradation mechanism of the dye reaction is as follows. Photoelectrons are elevated from the valence band (VB) of the NC catalyst to the conduction band (CB), which often initiates photocatalytic activities. The energy of the absorbed light has now reached or exceeded the photocatalyst band gap energy. Photoelectron excitation causes a hole to form in the VB. As a result, a pair of electron and hole (e−/h+) is produced, as shown in the following equation:
Possible mechanism of photodegradation of the dye for ZnO/ZnS/CdO/CdS NCs.
The generation of hydroxyl radicals (OH˙) occurs when water ionizes and combines with the newly generated hole in the VB to produce a hydroxyl radical. This process is implicated in the destruction of the dye molecule:
This OH˙ radical on the surface of NCs is a very potent oxidizing agent, and dye molecules that are close to the catalyst surface mineralize molecules to varying degrees depending on their structure and stability. These hydroxyl radicals can destroy any existing bacteria in the media in addition to attacking organic contaminants [62]. Similarly, the excited electrons in the conducting band now interact with the oxygen in the area to generate the anionic superoxide radical (
Superoxide radicals contribute to the oxidation process while also preventing electron and hole recombination, preserving electron neutrality inside the catalyst.
Superoxide is protonated in this step, which results in the production of H2O2. This further splits apart and produces a hydroxyl radical, which is very reactive. The oxidation and reduction processes occur at the same time on the photo-excited catalyst surface in photo-catalysis. Scheme 1 shows the procedure and plausible mechanism of dye degradation.
3.4 Hydrogen gas generation from the biomass
The prepared nanocatalysts were used in the production of hydrogen gas from green sources of the Brassica campestris leaf extract. Hydrogen produced through gasification exhibits an economic efficiency compared to other renewable energy sources.
3.4.1 Gas generation from the biomass using ZnO NPs
The experiment for gas generation from the biomass was carried out in a round bottom flask used as a reaction container as presented in Figure A5, where in each case 100 W tungsten lamp is used as a source of radiation. In the absence of the prepared catalyst, the flask was filled with only 10 mL of 0.05 M KOH solution. Through rubber tubing, the flask was connected to a 500 mL graduating cylinder. The flask was placed on the magnetic stirrer to be stirred. The entire assembly was placed in a tungsten lamp, and the gas production was measured. After 1 h, 91 µmol·g−1·min−1 gas was generated. In another trial, 10 mL of the biomass extract alone was added, and 122 µmol·g−1·min−1 gas was generated in 1 h.
Similarly, in the presence of a catalyst, the round bottom flask was filled with 20 mL of the biomass extract and then 20 mg of ZnO NPs was added. The process was initiated and gas production started, and produced 550 µmol·g−1·min−1. The amount of the biomass extract was constant at 20 mL, while the catalyst amount increased from 20 to 50 mg. With an increase in the amount of catalyst, the gas production increased from 550 to 672 µmol·g−1·min−1 in 1 h (Figure 11a and b).
(a) H2 generation in the presence of ZnO NPs. (b) The rate of H2 evolution with various amounts of ZnO NPs. (c) H2 generation in the presence of CdS NPs. (d) The rate of H2 evolution with various amounts of CdS NPs. (From 10 mL 0.05 M KOH, 10 mL biomass extract, 20 mL biomass + 10 mg NPs, 20 mL biomass + 20 mg NPs, 30 mg NPs, and 20 mL biomass + 50 mg NPs, 60 min at 100 W tungsten lamp, respectively).
In order to increase H2 production, Reddy et al. have shown that sodium hydroxide can be utilized as an effective additive in glucose gasification and the supercritical water reaction of cellulose [63]. At a comparatively low temperature of 573 K, Ishida et al. showed that the addition of sodium hydroxide could greatly increase the H2 output from cellulose [64]. They proposed the following alkaline thermal treatment reaction stoichiometry:
Sodium hydroxide's effectiveness in generating H2 is due to its capacity to promote the dissolving of cellulose [65] and the disintegration of cellulose [66]. Additionally, the solid carbonate formed during this reaction traps the carbon dioxide formed during it, producing H2 with little to no carbon dioxide. The hydroxyl group may enable H2 synthesis from cellulose at lower pressures and temperatures according to a comparison of the literature regarding the supercritical water reaction with sodium hydroxide and alkaline thermal treatment reaction [67].
The mechanisms of photo-electrochemical hydrogen production from the biomass (presented as C x H y O z ) and water over prepared NCs are shown in Eqs. 10–13. Under irradiation with sunlight (photon energy), the electrons (e ̄) from the VB will be excited and transferred to the CB, and then finally react with a proton (H+) to produce hydrogen gas. Photo-excited h+ generated in the VB of the NCs will oxidize the C x H y O z molecules to carbon dioxide and water. In actuality, the process by which C x H y O z degrades into CO2 and water is incredibly complicated, as explored in more detail below. The following equations succinctly describe the chemical processes [68]:
3.4.2 Gas generation from the biomass by using CdS NPs
About 20 mL of the biomass extract prepared in 0.05 M KOH solution and 10 mg of CdS NPs were taken in a round bottom flask. The reaction was started; 216 µmol·g−1·min−1 H2 gas was collected in the graduated cylinder. Then, the amount of the biomass extract was kept constant at 20 mL, while the amount of the photo-catalyst was changed to 20, 30, 40, to 50 mg. The H2 gas concentrations collected were 274, 580, 641, and 733 µmol·g−1·min−1, respectively, in 60 min, while the rate of H2 evolution is shown in Figure 11c and d.
3.4.3 Hydrogen gas generation from the biomass by using oil NCs
About 10 mL of the biomass extract and 10 mg of oil NCs were taken in a round bottom flask. The process was initiated, and the graduated cylinder was filled with 214 µmol·g−1·min−1 H2. The catalyst concentration was kept constant at 10 mg, and the biomass extract concentration was varied from 20 to 50 mL, resulting in H2 gas concentrations of 305, 366, 550, and 580 µmol·g−1·min−1, respectively, in 60 min, as shown in Figure 12. In another experiment, the biomass value was held constant at 20 mL, while the oil NC values were adjusted to 10, 20, 30, 40, and 50 mg in that order. As shown in Figure 12, the H2 generated was 305 µmol·g−1·min−1 for 10 mg NCs, 397 µmol·g−1·min−1 for 20 mg, 458 µmol·g−1·min−1 for 30 mg, 611 µmol·g−1·min−1 for 40 mg, and 763 µmol·g−1·min−1 for 50 mg NCs.
(a) H2 generation from the biomass in the presence of oil NCs and NPs. (b) The rate of H2 evolution with various amounts of oil NCs.
4 Conclusions
In this work, quaternary NCs were successfully developed through an in situ precipitation method. Here, castor oil was used as a capping agent in deionized water. The synthesized pure NPs and NCs were characterized by UV-Vis, PL, FTIR, XRD, EDX, and SEM analyses to study their optical, structural, and morphological data. UV-Vis spectroscopy showed the redshift in the NCs as compared to CdS NPs. This indicates the presence of more CdS NPs in NCs and hence exhibits better visible light photo-catalytic activity. PL spectroscopy reveals evidence for interaction between ZnO, ZnS, CdO, and CdS. The photo-catalytic properties of NCs and hydrogen gas generation are encouraging as compared to pure ZnO, ZnS, CdO, and CdS NPs due to efficient charge separation in ZnO–ZnS–CdO–CdS NCs.
Acknowledgements
This study is based on the work conducted by Mr. Yousaf Khan as part of his PhD thesis under the supervision of Prof. Dr. M. I. Khan and Prof. Dr. Murad Ali Khan.
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Author contributions: Yousaf Khan, Uzma Sharafat, Rafia Younus, Saima Gul: conceptualization, investigation, formal analysis, methodology, visualization, and validation; Murad Ali Khan, Muhammad Ismail: formal analysis, investigation, writing – original draft, methodology, validation, writing – review and editing; Sher Bahadar Khan, M. I. Khan: project administration, visualization, supervision, resources, data curation, writing – review and editing.
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Conflict of interest: The authors state that there are no conflict of interest.
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Data availability statement: The authors confirm that the data supporting the findings of this study are available within the article.
Appendix
Synthesis of (a) ZnO NPs, (b) ZnO/ZnS NCs, (c) ZnO/ZnS/CdO NCs, and (d) ZnO/ZnS/CdO/CdS NCs.
Plot of ln(C t /C o) versus time for the CR dye for (a) ZnO, (b) CdO (c), CdS, and (d) NCs.
Plot of ln(C t /C o) versus time for the MR dye for (a) ZnO, (b) CdO, (c) CdS, and (d) NCs.
Photocatalytic degradation of a mixture of MR and CR dyes (25 mL of each 0.04 mM dye + 50 mg of NCs).
Reusability of the CR dye for ZnO, CdO, CdS, and NCs.
Set up for hydrogen generation from the biomass and photo-catalyst reaction.
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