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Article

Facile Fabrication of Pd-Doped CuO-ZnO Composites for Simultaneous Photodegradation of Anionic and Neutral Dyes

by
Sumalatha Bonthula
1,
Muna Farah Ibrahim
2,
Aisha Omar Al-Jaber
3,
Al-Dana Faisal Al-Siddiqi
3,
Ramyakrishna Pothu
4,*,
Tauqeer Chowdhury
5,
Yusuf Siddiqui
6,
Rajender Boddula
1,*,
Ahmed Bahgat Radwan
1 and
Noora Al-Qahtani
1,7,*
1
Center for Advanced Materials (CAM), Qatar University, Doha 2713, Qatar
2
Department of Chemistry and Earth Science, College of Arts and Science, Qatar University, Doha 2713, Qatar
3
Al Arqam Academy, Doha 23148, Qatar
4
School of Physics and Electronics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China
5
Department of Chemical Engineering, Qatar University, Doha 2713, Qatar
6
Department of Computer Engineering, Qatar University, Doha 2713, Qatar
7
Central Laboratories Unit (CLU), Qatar University, Doha 2713, Qatar
*
Authors to whom correspondence should be addressed.
Physchem 2024, 4(3), 181-196; https://doi.org/10.3390/physchem4030014
Submission received: 1 May 2024 / Revised: 14 June 2024 / Accepted: 20 June 2024 / Published: 27 June 2024
(This article belongs to the Section Catalysis)
Browse Figures
Versions Notes

Abstract

:
This study explores the synthesis and application of Pd-doped CuO-ZnO composites for the simultaneous photodegradation of anionic and neutral dyes. The nanocomposite was synthesized using a hydrothermal technique and characterized using XRD, FTIR, and UV-Vis absorption spectra. Photocatalytic degradation experiments were conducted with varying catalyst loadings, revealing optimal conditions for enhanced degradation performance. The nanocomposite exhibited a synergistic effect on the degradation of the dye mixture, following pseudo-first-order kinetics with significant efficiency under sunlight exposure. Moreover, the study evaluated the influence of pH on the degradation process, showing improved efficiency in neutral and basic conditions. Overall, the findings highlight the efficacy of the Pd-doped CuO-ZnO catalyst in degrading complex dye mixtures, offering potential applications for wastewater treatment in various industrial settings.

Graphical Abstract

1. Introduction

Dyes are micropollutants that are discharged into water from various industries like textiles, leather, paint, food, and pharmaceuticals, and they pose a significant threat to aquatic life and the environment. Of particular concern is the presence of dye molecules in water channels, which is increasingly alarming to environmental researchers [1,2]. Most artificial food colorings use azo dyes, organic dyes that contain aromatic rings in their molecular structure. Due to the toxicity of these compounds, degradation or metabolism of these compounds can result in a variety of harmful compounds such as aromatic amines, benzoquinones, benzidines, and benzene sulfonic acids. These carcinogens, mutagens, and DNA-damaging agents can be especially dangerous in extreme cases, leading to serious health problems or, in extreme cases, death [1,2].
The micropollutants generated from textile dyeing processes contain dyestuff (comprising about 8–20% of the total pollution load) along with numerous auxiliary chemicals, resulting in substantial water contamination [3]. With the widespread use of different dyes for various purposes, excess unused dyes are discharged as industrial effluents, contributing to water pollution. These synthetic dye effluents, which are often aromatic, xenobiotic, and carcinogenic, are resistant to sunlight and impair the water’s reoxygenation capacity [4]. So, removing the dyes from the water bodies is essential to minimize the negative impacts on living organisms and the environment. Traditional methods of wastewater treatment often fall short of addressing the challenges posed by complex dye mixtures. The dye removal methods that are adopted are classified into two types: non-destructive methods, such as coagulation, flocculation, membrane separation, and adsorption, and destructive methods, such as biodegradation, photocatalysis, electrochemical oxidation methods, etc., that are used commonly for effective removal [5]. The eco-friendly method for removing these organic pollutants from wastewater is photocatalysis, which is a very effective process of degrading pollutants in waste effluents [6]. Hussin et al. reviewed the photocatalytic degradation of dispersed azo dyes in textile wastewater using green zinc oxide (ZnO) nanoparticles synthesized in plant extract. It was found that the photocatalysis using ZnO/TiO2 as a photocatalyst recorded 70 to 80% of the dyes [7]. The presence of azo dyes in water through the utilization of nanotechnology and photocatalytic approaches has been explored through polyaniline (PANI), which also provided a promising potential solution to treat azo dyes. The fabrication of PANI composites has been examined, and their capacity to effectively treat water for the removal of toxic or persistent dyes has been evaluated [8].
Recent studies have explored various methods for catalytic degradation of azo dyes in aqueous media. For instance, Singla et al. synthesized bimetallic (Ag/Ni) nanoparticles loaded in smart polymer microgels, demonstrating their effectiveness in degrading azo dyes under different conditions [9]. Similarly, Manish et al. found that Zn-doped TiO2 nanoparticles were found to exhibit enhanced degradation of azo dyes under UV-Visible light, while Mn-doped ZnO nanoparticles showed significant degradation of commercial dyes [10]. Mn-doped ZnO nanoparticles capped with polyvinylpyrrolidone (PVP) were synthesized via co-precipitation and showed a reduced band gap, enhanced photocatalytic activity, and increased reusability via degradation of crystal violet dye. PL and UV-Visible spectroscopy studies were conducted to analyze the effect of UV light on the nanoparticles (NPs) [11]. Another study investigated the photocatalytic degradation of a commercial azo reactive dye in a simple design reusable miniaturized reactor with interchangeable TiO2 nanofilm. The study found that about 98% degradation of the commercial dye was achieved after 100 min in a stopped-flow system and 15% in a continuous-flow system [12]. Copper nanomaterials have been used for the photocatalytic degradation of azo dyes. For example, a study found that cupric oxide nanoparticles (CuO NPs) were investigated as efficient photocatalysts for degrading azo dyes. The energy band gap of CuO NPs was found to be about 1.76 eV. Azo dyes like methylene blue, acid yellow 23, and reactive black 5 obey pseudo-first-order kinetics [13]. BiVO4 and CuO-BiVO4 photocatalysts have been synthesized and studied for their physical properties, band gap energy, and optimal conditions for photocatalytic activity. Results show that 1wt% CuO-BiVO4 achieved complete degradation of methylene blue dye under visible-light irradiation [14]. Another study found that bio-engineered copper nanoparticles synthesized from Ficus carica extract showed favorable degradation efficiency of alizarin yellow R dye under solar irradiation, and the maximum degradation performance of alizarin yellow R was 89.71% [15].
Zinc oxide nanoparticles can be used for the photocatalytic degradation of azo dyes. This research study explored the effects of an agitation rate of 1900 rpm on the molecular size and morphology of ZnO Nano-swirlings (ZNsw). The characterization results showed that the ZNsw formed a large cluster of folded long threads. The study tested the photocatalytic and antifungal properties of the ZNsw. Their findings revealed that the ZNsw achieved 79% azo dye AR183 dye discoloration after an 80 min UV light irradiation [16]. A study synthesized graphene/nitrogen-doped ZnO (G/N-doped ZnO) nanocomposites using the combustion method. Characterization of the nanocomposites showed the presence of hexagonal wurtzite structures and increased surface area. The photocatalytic activity of the nanocomposites was evaluated for the degradation of carmine dye, achieving a maximum of 66.76% degradation over 185 min of UV light irradiation. It was concluded that photoelectrochemical carmine dye degradation is more effective than photocatalytic carmine dye degradation [17].
Another study demonstrated the biosynthesis of sodium alginate (SA)-mediated Pd nanoparticles for the catalytic degradation of azo dyes. SA serves as a reducing and stabilizing agent without using any toxic chemicals. The Pd nanoparticles were found to accelerate the degradation rate of mono-azo and di-azo dyes by more than 80 and 10 times, respectively [18]. Another study showed that PdNPs synthesized with carboxymethyl cellulose showed excellent catalytic activity in reducing the degradation of azo dyes such as p-Amino azobenzene, acid red 66, acid orange 7, scarlet 3G, and reactive yellow 179 [19].
The recent surge in the use of azo dyes in various industrial processes (such as dyeing and printing of textiles) has caused great concern regarding the environmental impact of their disposal. In order to address this issue, many researchers have focused on the development of efficient catalysts for the degradation of azo dyes. Many promising catalysts (Pd, Ti, Ag, Ni, Zn, Cu, Fe) catalysts have proven to be effective in degrading azo dyes and have also been found to be capable of achieving enhanced degradation through a synergistic effect between different catalysts [20,21,22]. The degradation of azo dyes requires more energy and similarly degradation of their mixture needs far more energy.
In this study, two dyes, namely, azocarmine, which is an anionic dye, and neutral red, which is a neutral dye, were chosen, and degradation studies were performed for their mixture. The degradation of complex dye mixtures in wastewater is a significant challenge in environmental remediation. While several studies have explored the use of photocatalysts for dye degradation, the simultaneous degradation of anionic and neutral dyes remains less studied. Moreover, the influence of pH on the degradation process and the optimization of catalyst loadings are often overlooked. Consequently, this study endeavors to fill this research lacuna by probing the synthesis and application of Pd-doped CuO-ZnO composites for the simultaneous photodegradation of anionic and neutral dyes. The palladium/zinc oxide-copper oxide combination has good activity toward these molecules due to its strong Lewis acidity. The zinc oxide also functions as a basic support, contributing to the hydrogen evolution reaction, whereas the copper oxide assists with the oxidation of the dye molecules. These properties make the combination especially suitable for the degradation of dye mixture. Hence, palladium-doped copper oxide-zinc oxide is chosen as the efficient solution for the degradation studies because it has the advantages of high efficiency, selectivity, low cost, and being environmentally benign. In this study, we focus on the synthesis and characterization of a palladium-doped CuO-ZnO bimetallic catalyst and investigate its potential for the degradation of both anionic and neutral dye mixtures.
The potential features such as the introduction of unique composite material with enhanced photocatalytic performance with simultaneous degradation of anionic and neutral dyes. Many studies focus on the degradation of one type of dye at a time, and facile fabrication offers a simple method of fabrication, which is reproducible.

2. Experimental

2.1. Materials and Methods

Palladium nitrate (Pd(NO3)2·2H2O), copper nitrate (Cu(NO3)2), zinc nitrate hexahydrate (Zn(NO3)2·6H2O), sodium hydroxide (NaOH), azocarmine G (C28H18N3NaO6S2), and neutral red (C15H17ClN4) were required for this research study, and they were purchased from the Sigma Aldrich (St. Louis, MO, USA), which is 99% pure. These were of high-purity, analytical-grade compounds, so no further purification was performed. All the experiments were carried out using distilled water.
To examine the synthesized palladium nanomaterial’s composition, structure, and morphology, UV-Visible spectroscopy, Fourier transform infrared spectroscopy (FT-IR), and X-ray diffraction (XRD) were performed. The Fourier transform infrared spectrum was obtained from a PerkinElmer Frontier spectrometer with an attenuated total reflectance (ATR) sampling methodology. The FTIR spectrum is measured at a range of 400–4000 cm−1. UV-Vis analysis for the samples is performed on a Biochrom libra S50 series spectrophotometer to measure the UV-Visible absorption spectra ranging from 250 to 800 nm. XRD analysis of the synthesized nano powder was performed with (PANalytical, EMPYREAN) generator settings of 45 kV/40 mA—X-ray with anode material as Cu at 25 °C. Data were taken for the 2θ range of 10 to 99.99 degrees with a step of 0.0130 degrees (K-Alpha1 [Å] 1.54060; K-Alpha2 [Å] 1.54443, K-Beta [Å] 1.39225).

2.2. Synthesis of CuO-ZnO Nanomaterials

A total of 0.6 g of copper nitrate and 0.28 g of zinc nitrate hexahydrate are taken and dissolved in deionized water to form a transparent precursor solution. A total of 2.5 M sodium hydroxide solution is prepared and added dropwise to the above precursor at 80 °C, and the pH is maintained at 9.0 for one hour. A blackish-grey precipitate is obtained. This precipitate further continued to synthesize the desired palladium-doped nanomaterial via hydrothermal synthesis. However, to prepare the CuO-ZnO nanomaterial, the precipitate is transferred into the autoclave, and hydrothermal treatment is performed at 140 °C for 16 h, which is further centrifuged and dried for calcination, and procedure is given below.

2.3. Synthesis of Pd-Doped CuO-ZnO Nanomaterial

A total loading of 2wt%, 6wt%, and 10wt% of Pd-based catalyst Pd/CuO-ZnO was synthesized via a one-step hydrothermal technique. A total of 2wt%, 6wt%, and 10wt% of palladium nitrate solution (Pd(NO3)2·2H2O) was dissolved in deionized water to form a solution that was slowly added to the above precipitate (Section 2.2) and stirred constantly for another half an hour at 60 °C [23]. Now the above solution is transferred into an autoclave, and hydrothermal treatment is performed at 140 °C for 16 h. Thus, the obtained precipitate, after hydrothermal synthesis, is subjected to centrifugation thrice while washing with deionized water, and the precipitate dried at 100 °C for 2 h and finally calcined at 400 °C for 4 h.

2.4. Catalytic Degradation of Dye Mixture

The performance of the Pd-doped CuO-ZnO nanocomposite in degrading a mixture of azocarmine and neutral red dyes was carefully monitored. In this reaction, a mixture of two dyes with 50 mL of 50 ppm solution of each dye solution was taken and mixed with 3 mg of catalyst. The reduction in dyes with time was carried out during day sunlight by taking down the absorbance of the dye mixture solution at 433 nm by using UV-Visible spectroscopy. Under similar conditions, the concentration effect was monitored by varying the catalyst loading, which is examined by changing the nanocomposite amount from 0.025 g/L to 0.25 g/L for the degradation of the dye mixture using UV-Visible spectroscopy. Keeping the similar experimental conditions, the effect of the pH on the photocatalytic degradation of the dye mixture was carefully studied.

3. Results and Discussion

3.1. Surface Morphology and Elemental Analysis

Scanning electron microscopy (SEM) is an imaging technique used to find the growth and morphological properties of the nanoparticles, which scans the surface of a sample, providing images of microscopic features [24]. It is an effective technique for examining the morphology and microstructure of a Pd/CuO-ZnO composite. Particle size and shape distributions, surface area, porosity, and other parameters can be determined from the obtained SEM images of different magnifications. In Figure 1, the Pd/CuO-ZnO nanocomposite micrographs that were acquired are shown; agglomeration is the key contributor for very large numbers of densely packed, irregular amorphous particles. In Figure 1a,c, the SEM micrograph shows that they are mostly composed of rod-like and disc-like structures, and the particle size was in the range of 150 to 200 nm. In Figure 1b, the SEM micrograph shows a small disc-like pattern with uneven sizes, from 50 nm to 150 nm.
For further understanding the composition of these materials, EDX analysis for catalyst is carried out. EDX can measure elemental compositions and properties at the nanoscale level, providing more detailed information on the physical and chemical characteristics of the composite. Figure 1d–f shows the spectra for the EDX analysis of 2%, 6%, and 10% Pd/CuO-ZnO. The EDX spectra showed the respective elemental constituents of palladium, copper, zinc, and oxygen. It has been shown by the presence of oxygen that copper and zinc are in their oxidized state. Additionally, the purity of the synthesized nanomaterial is confirmed with no impure peaks in the spectra. The weight percentage for the elemental constituents of the nanocomposite is summarized in Table 1.

3.2. FT-IR Analysis

Figure 2 displays the synthesized nanomaterial that has been scanned over 4000 to 400 cm−1 for studying the chemical and structural nature of nanoparticles for the 2wt%, 6wt%, and 10wt% palladium-doped CuO-ZnO. The peak at 1782 cm−1 is attributed to the vibrational frequency of N=O stretching and peaks at 1149, 1362, and 1425 cm−1 and shows NO3 asymmetric and symmetric stretching, respectively [25]. The peak in the region of ~667 cm−1 and 683 cm−1 may be due to the presence of Pd nanoparticles [26]. FTIR analysis of CuO nanoparticles reveals distinct peaks at various levels, corresponding to different functional groups and stretches within the material. Examination of the FTIR spectrum indicates that bands appearing at 606 and 627 cm−1 were attributed to Cu–O stretching vibrations [26,27] and a strong stretching vibration at 596 cm−1, which confirms the existence of CuO [27]. A small shift in the bands at 495, 524, and 863 cm−1 is an indication of Zn-O stretching vibration in the nanocomposite. From the FTIR study, it is confirmed that palladium and copper ions are substituted into the ZnO lattice synthesized by hydrothermal process [27,28,29].

3.3. UV-Visible Analysis

In order to evaluate the effect of doping on optical properties, the UV-Vis absorption spectra have been observed, and the band gap of the samples calculated. The absorption spectrum of the 2wt%, 6wt%, and 10wt% palladium-doped CuO-ZnO is shown in Figure 3. The nanostructure’s absorption properties for the band gap were recorded in the 250–800 nm range. In a study, the absorption peaks for palladium NPs are reported at 280 nm [30]. In another study for the CuO NPs, the absorption peak was observed at 390 nm, which moves toward the visible region with zinc doping, and the optical band gap is measured at 1.82 eV [31]. The absorption in Figure 3a–c is enhanced and broadened due to the presence of CuO-ZnO and measured at 377 nm for 2wt% and measured at 340 nm for 6wt% and 10wt%, respectively. As shown in Figure 3d–f, by plotting (αhν)2 versus the energy of incident photons, the optical band gap was measured (i.e., E = hν). From the tauc plot, the band gap for 2wt%, 6wt%, and 10% Pd-doped CuO-ZnO is measured at 1.78 eV, 2.66 eV, and 2.89 eV, respectively. The values show a gradual increase in the bandgap with an increase in the concentration of Pd in CuO-ZnO samples.

3.4. XRD Analysis

The crystallinity of the obtained Pd-doped CuO-ZnO nanomaterial was confirmed by using XRD and the diffractograms are shown in Figure 4. The peaks identified in the Pd-doped CuO-ZnO nanocomposite’s diffraction pattern at 2θ = 35.48, 35.57, 36.46, 38.74, 48.78, 61.57, 68.1, 73.54 are attributed to the (0 0 2), (1 1 −1), (1 1 1), (1 1 0), (2 0 −2), (1 1 −3), (2 2 0), (1 1 3) lattice indices, respectively. It is well matched with the standard diffraction peaks for CuO (Tenorite) nanoparticles (JCPDS: card no. 48-1548) [32]. The obtained diffraction peaks at 2θ = 40.36, 46.95, and 87.20 are ascribed to the indices (1 1 1), (0 0 2), and (2 2 2), respectively (JCPDS card no. 05-0681) [33]. The peaks identified at 2θ = 31.80, 34.45, 36.28, 47.57, and 89.73 are attributed to (0 1 0), (0 0 2), (0 1 1), (0 1 2), (0 2 3) lattice indices, respectively. These diffraction patterns are well matched with the standard peaks of JCPDS no. 00-036-1451 for ZnO (Zincite) nanoparticles [34,35]. The slight variation in the peaks is observed due to the presence of other nanoparticles in the nanocomposite. The intensities of the peaks associated with the CuO in the nanocomposite diffraction pattern seemed higher than those of ZnO and PdNPs, respectively. The (1 1 1), (0 0 2) planes have a much higher intensity than the other peaks, suggesting that the nanocrystals are preferentially aligned along this direction.
The crystallite size can be calculated from the full width at half maximum (FWHM) of the diffraction peaks. The diffraction pattern of X-rays is a function of the crystallite size D, which can be calculated from Debye Scherrer’s Equation (1) [36].
D = K λ β · C o s θ
The calculated particle size for ZnO, CuO-ZnO, 2% Pd/CuO-ZnO, 6% Pd/CuO-ZnO, and 10% Pd/CuO-ZnO were found to be 45, 34, 28, 20, and 24 nm, respectively. The smaller crystalline size of 20 nm is observed in the 6%Pd/CuO-ZnO composites and is attributed to the quantum confinement effect [37].

3.5. Photodegradation of Two Dye Mixture by Pd/CuO-ZnO Catalyst

The complex combination of anionic and neutral dyes is challenging to break down because anionic compounds are positively charged and tend to retain their charge even in solutions, making them difficult to remove when mixed with a neutral compound. Additionally, most degrading processes and the design of the catalyst are specific and effective for one type of compound but not for the other. This is because different compounds require different reagents/conditions for successful degradation. Here are some of the catalysts that performed best for the degradation of neutral red and azocarmine dyes.
As shown in Table 2, in most of the cases, the dye concentration is typically around 1 ppm to 20 ppm concentration. So, here the hybrid and the experimental conditions are designed such that they will degrade the dyes with concentrations greater than 50 ppm. For the photocatalytic experiment, 2wt%, 6wt%, and 10wt% of Pd-based catalyst Pd/CuO-ZnO are synthesized and investigated on the anionic and neutral dye mixture. Further examination revealed that the 6% palladium-doped CuO-ZnO catalyst performed the highest azo dye degradation efficiency, making it a better choice than its contemporary 2% and 10% palladium catalysts.
The photocatalytic activities of the prepared hybrid nanocomposites were evaluated by the degradation of a dye mixture of azocarmine and neutral red (NR) in an aqueous solution under sunlight. In the given photocatalytic degradation experiment, the photocatalyst (3 mg) was added to an azocarmine and neutral red dye mixture solution (50 mL, 50 ppm each), which was magnetically stirred for 15 min, sonicated to leave the photocatalyst-solution system, and placed in the dark for one hour. Under sunlight irradiation, the photocatalytic activity was evaluated along with the adsorption-analytical equilibrium. All the experiments were carried out during daylight after 9 a.m. in the month of May at 37 °C temperature, and the degradation efficiency of the dyes was measured by using UV-Vis spectrophotometer. The maximum absorbance observed was 540 nm for azocarmine and predominantly 512 nm for neutral red, respectively, for the experimental conditions with the nanocomposite. The absorption maximum observed for the azocarmine and neutral red dye mixture is 433 nm, and there was 80.61% degradation efficiency for the dye mixture with 6% palladium-doped CuO-ZnO as a nano catalyst. The significant increase in the photodegradation rate of anionic and neutral dye mixture is observed due to the presence of more wt% of palladium that enhances the absorption of visible light, while CuO and ZnO facilitate efficient charge separation and transfer.

3.5.1. Effect of Catalyst Loading

The effects of catalyst loading on photocatalytic degradation of dye mixture were studied by using the following amounts of catalyst: 0.025 g/L, 0.05 g/L, 0.1 g/L, 0.15 g/L, 0.2 g/L, and 0.25 g/L, respectively. The photocatalytic degradation of the dye mixture with respect to the different catalyst loading is shown in Figure 5a,b. Figure 5a,b also shows the concentrations of dye mixtures with respect to the amount of catalyst, which declined gradually. The complete degradation of the dye mixture is shown by 0.15 g/L, 0.20 g/L, and 0.25 g/L, respectively, taking two hours to complete, and it had the steepest slope. On the other hand, there is less prominent degradation observed with other catalyst loading within the given irradiation time. Figure 5c represents the percentage of azocarmine, neutral red, and their mixture removal with respect to different catalyst loading values of 0.05 g/L, 0.1 g/L, 0.15 g/L, 0.20 g/L, and 0.25 g/L, respectively. In Figure 5c, there is a remarkable increase in the percentage removal of azocarmine and neutral red dyes. However, in the case of the dye mixture, there is an increase until 0.15 g/L, and then it remains the same for other catalyst loading values of 0.2 g/L and 0.25 g/L, respectively. The results reveal a proportional increase in the degradation performance with higher catalyst loading and remain constant for higher catalyst loadings, and hence, the optimum loading value for dye mixtures of 0.15 g/L photocatalyst is considered with solar irradiation as the light source, and further studies were continued.

3.5.2. Effect of pH of the Solution

The pH value is vital in photocatalytic studies, as it affects the reactions taking place during the degradation of dyes and organic compounds. The generation of hydroxyl and other radicals depends on the pH of the solution [47]. The effect of pH on the photocatalytic degradation of the dye mixture is tested for pH 3, 7, and 10, respectively, in the presence of a synthesized photocatalyst, which was studied under conditions (concentration of each dye in the dye mixture = 50 ppm, contact time = 120 min, and catalyst dose = 0.15 g/L) without and with sunlight, as shown in Figure 6.
It is clearly evident that the photocatalytic degradation took place in neutral and basic mediums at pH 7.0 and pH 9.0 rather than in an acidic medium at pH 3.0 (Figure 6). Of the two dyes, one was anionic and the other neutral, so the dye mixture was predominantly anionic and did not undergo any kind of degradation in sunlight in the absence of the catalyst, and there was an excess of H+ ions at pH 3.0, respectively. Even in the presence of the catalyst, the degradation is not very noticeable at pH 3.0, which is not a favorable condition for degradation. In a neutral medium, the anionic nature decreased, which made it a favorable condition for degradation, and in alkaline conditions, OH radicals will be more and can initiate the photocatalytic degradation process.

3.5.3. Time Degradation

The photocatalytic reaction of the dye mixture under sunlight irradiation is examined after the absorption in the dark for 30 min incubation with the 6% palladium-doped copper zinc photocatalyst, which is depicted in Figure 7, indicating that the dye mixture concentration is significantly decreased as the reaction progresses. A substantial decrease in the absorption spectra of azocarmine and neutral red dye mixture is observed under exposure to sunlight irradiation for time intervals of 15 min for two hours, respectively. The obtained data have an agreement with the calculated photodegradation efficiency depicted in Figure 7.
The decolorization and photocatalytic degradation efficiency have been calculated using Equation (2) as follows:
Efficiency   ( % ) = C 0 − C e C 0 × 100
The rate constant is measured by using the pseudo-first-order kinetic Equation (3) shown below.
l n C 0 C = K 1 × t
where C0 and Ce correspond to the initial and degraded concentration of dye mixtures before and after solar irradiation, respectively. K1 (s−1) is the rate constant of the first order [48].

3.6. Possible Microscopic Mechanism of the ZnO/CuO-Pd Nanocomposites

Figure 8 illustrates the photodegradation mechanism of the azocarmine and neutral dye mixture using Pd/CuO-ZnO nanocomposites. It is well known that pristine ZnO and CuO cannot be excited by visible light irradiation; thus, the photodegradation of organic dyes can mainly be attributed to photo-generated hole oxidation and photo-reduction processes. During the photodegradation process, visible light excitation causes ZnO and CuO to generate electrons and holes at the conduction band (CB) and the valence band (VB), respectively. In the Pd-ZnO/CuO nanocomposites, upon exposure to sunlight, the photogenerated electrons (e−) in the CB of CuO migrate to the CB of ZnO, while the holes (h+) in the VB of ZnO transfer to the VB of CuO. This process results in numerous photoexcited electron–hole pairs in the CuO/ZnO heterostructures, effectively separating the photogenerated carriers through the p-n heterostructures and extending the light response range of ZnO from the UV region to the visible region. Additionally, Pd nanoparticles (PdNPs) broaden the light utilization range of CuO/ZnO, exciting CuO/ZnO to form more photogenerated electron–hole pairs. Moreover, PdNPs not only enhance electron (e−) transfer to the surface of CuO/ZnO heterostructures but also transfer excess electrons from the CuO/ZnO heterostructures to PdNPs, thus improving the separation rate of electron–hole pairs. Subsequently, through redox reactions, oxygen (O2) adsorbed on the catalyst surface captures electrons to produce superoxide radicals (·O2−), and holes combine with water (H2O) to generate hydroxyl radicals (·OH), which oxidize organic pollutants into carbon dioxide (CO2) and water (H2O), resulting in higher photocatalytic activity.

4. Conclusions

In conclusion, the 6wt% palladium-doped CuO-ZnO bimetallic catalyst demonstrated a synergistic effect in degrading neutral red and azocarmine dye mixtures. Synthesis of 2wt%, 6wt%, and 10wt% palladium-doped copper oxide-zinc oxide nanocomposites were investigated, confirming the presence of palladium, Cu-O, and Zn-O stretching vibrations. Increased palladium doping resulted in a slight increase in crystallite size. The nanocomposite showed significant photocatalytic activity under sunlight exposure, achieving 80.61% degradation efficiency for the dye mixture. Moreover, the catalyst exhibited enhanced performance in neutral and basic pH conditions. These findings suggest the potential of the Pd-doped CuO-ZnO catalyst as a cost-effective and eco-friendly solution for degrading complex dye mixtures, thereby reducing waste disposal and treatment costs.

Author Contributions

Conceptualization, R.B.; Data curation, R.P. and A.B.R.; Formal analysis, M.F.I., A.O.A.-J. and A.-D.F.A.-S.; Investigation, S.B.; Software, T.C. and Y.S.; Supervision, N.A.-Q.; Validation, R.P.; Writing—original draft, S.B. and R.B.; Writing—review and editing, R.P. and R.B.; project administration, N.A.-Q.; funding acquisition, N.A.-Q. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by Qatar University through a National Capacity Building Program grant (NCBP) [QUCP-CAM-22/24-463]. Statements made herein are solely the responsibility of the authors.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Physchem 04 00014 g001
Figure 1. SEM images (upper) and EDS spectra (down) of (a,d) 2%Pd/CuO-ZnO, (b,e) 6%Pd/CuO-ZnO, and (c,f) 10%Pd/CuO-ZnO samples.
Figure 1. SEM images (upper) and EDS spectra (down) of (a,d) 2%Pd/CuO-ZnO, (b,e) 6%Pd/CuO-ZnO, and (c,f) 10%Pd/CuO-ZnO samples.
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Figure 2. FTIR spectrum for (a) ZnO, CuO-ZnO, 2wt%, 6wt%, and 10wt% of Pd/CuO-ZnO nanocomposite and (b) 6wt% of Pd/CuO-ZnO samples.
Figure 2. FTIR spectrum for (a) ZnO, CuO-ZnO, 2wt%, 6wt%, and 10wt% of Pd/CuO-ZnO nanocomposite and (b) 6wt% of Pd/CuO-ZnO samples.
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Figure 3. (a–c) UV-Visible absorption spectrum and (d–f) tauc plot (inset) for 2wt%, 6wt%, and 10wt% Pd-doped CuO-ZnO nanomaterial.
Figure 3. (a–c) UV-Visible absorption spectrum and (d–f) tauc plot (inset) for 2wt%, 6wt%, and 10wt% Pd-doped CuO-ZnO nanomaterial.
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Figure 4. XRD pattern for ZnO, CuO-ZnO, 2wt%, 6wt%, and 10wt% Pd/CuO-ZnO samples.
Figure 4. XRD pattern for ZnO, CuO-ZnO, 2wt%, 6wt%, and 10wt% Pd/CuO-ZnO samples.
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Figure 5. (a) Degradation efficiency rate and (b) different catalyst loading of 6% Pd-doped CuO-ZnO for dye mixture under solar irradiation and (c) a bar plot of the percentage of dye removal of two dyes and its mixture at different loadings of the nanocomposite.
Figure 5. (a) Degradation efficiency rate and (b) different catalyst loading of 6% Pd-doped CuO-ZnO for dye mixture under solar irradiation and (c) a bar plot of the percentage of dye removal of two dyes and its mixture at different loadings of the nanocomposite.
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Figure 6. The effect of pH on the degradation of azocarmine and neutral red dye mixture in (a) acidic pH 3.0, (b) neutral pH 7.0, and (c) alkaline pH 9.0 mediums.
Figure 6. The effect of pH on the degradation of azocarmine and neutral red dye mixture in (a) acidic pH 3.0, (b) neutral pH 7.0, and (c) alkaline pH 9.0 mediums.
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Figure 7. Photocatalytic degradation of azocarmine and neutral dye mixture under solar irradiation in the presence of 6% palladium-doped copper zinc nanocomposite at different time intervals. The UV-Vis spectra for decolorization of the azo dye’s mixture under experimental conditions: (azocarmine concentration = 50 ppm, neutral red = 50 ppm, Pd/CuO-ZnO dose = 0.15 g/L, time = 2 h, and pH = 7). (a) Study of dye degradation of dye mixture with respect to time. (b) The gradual decolorization of azo dye mixture with 6% Pd/CuO-ZnO catalyst. (c) Normalized absorbance of dye mixture in sunlight in the presence of catalyst. (d) Kinetic study of dye mixture degradation. (e) Dye mixture removal efficiency with reaction time.
Figure 7. Photocatalytic degradation of azocarmine and neutral dye mixture under solar irradiation in the presence of 6% palladium-doped copper zinc nanocomposite at different time intervals. The UV-Vis spectra for decolorization of the azo dye’s mixture under experimental conditions: (azocarmine concentration = 50 ppm, neutral red = 50 ppm, Pd/CuO-ZnO dose = 0.15 g/L, time = 2 h, and pH = 7). (a) Study of dye degradation of dye mixture with respect to time. (b) The gradual decolorization of azo dye mixture with 6% Pd/CuO-ZnO catalyst. (c) Normalized absorbance of dye mixture in sunlight in the presence of catalyst. (d) Kinetic study of dye mixture degradation. (e) Dye mixture removal efficiency with reaction time.
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Figure 8. Possible microscopic mechanism of the ZnO/CuO-Pd nanocomposites for degradation of organic pollutants.
Figure 8. Possible microscopic mechanism of the ZnO/CuO-Pd nanocomposites for degradation of organic pollutants.
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Table 1. The percentage weight content of the different elemental compositions of (a) 2%Pd/CuO-ZnO, (b) 6%Pd/CuO-ZnO, and (c) 10%Pd/CuO-ZnO.
Table 1. The percentage weight content of the different elemental compositions of (a) 2%Pd/CuO-ZnO, (b) 6%Pd/CuO-ZnO, and (c) 10%Pd/CuO-ZnO.
SamplesElemental Constituent of the Sample in Weight %
PdCuZnO
2%Pd/CuO-ZnO0.6555.1221.1523.09
6%Pd/CuO-ZnO2.5850.9716.7429.71
10%Pd/CuO-ZnO5.6158.4422.2613.69
Table 2. The favorable conditions for photocatalytic degradation of neutral red and azocarmine dyes with palladium, copper, and zinc nanomaterials.
Table 2. The favorable conditions for photocatalytic degradation of neutral red and azocarmine dyes with palladium, copper, and zinc nanomaterials.
S.NoNanomaterial UsedLight SourceSynthesis MethodPollutantCatalyst AmountDye ConcentrationDegradation TimeDegradation Efficiency Ref.
1Pd/W18O49 nanowiresSunlightHydro
thermal		Methylene Blue10 mg15 mg/L40 min98.4%1 February 2021 [38]
Neutral red10 mg10 mg/L40 min96.1%
2Sulfur Cu-TiO2UV lightSol-gelNeutral red0.75 g/L20 mg/L120 min98.4%1 July 2014[39]
3PS/ZnOSunlightElectrospunAzocarmine G10 mg10 mg/L6 h95%15 June 2022[40]
4PVDF/ZnOSunlightElectro spinningAzocarmine5 mg10 mg/L120 min85%11 April 2022[41]
Malachite green5 mg10 mg/L240 min90%
5 KA@CP-SSunlightHydrothermalNeutral red0.025 mmol10 mg/L150 min72%31 October [42]
6Al/ZnOSunlightAuto combustionAzocarmine0.1 g/L0.15 g/L140 min93%20 March 2023[43]
7ZnOSunlight Azocarmine0.1 g/L0.15 g/L140 min68.13%20 March 2023[43]
8Cu,N-CDs/Ag3PO4Visible lightThermolysisNeutral red50 mg12 mg/L60 min95.5%3 July 2019[44]
9CuO HydrothermalNeutral red0.4 g/L 120 min96%28 March
2022		[45]
10CuO-ZnO tetrapodal hybridSolar irradiationHydrothermal synthesisBV-31 g/L40 mg/L100 min86%23 July
2020		[46]
RY-1451 g/L40 mg/L80 min80%
PS/ZnO—Porous nanocomposite of polystyrene; KA@CP-S: Mixed-Ligand H-bonded Cu coordination polymer {[Cu2(Or)2(Bimb)3]·4H2O}n; Cu, N-CDs/Ag3PO4: Cu, N co-doped carbon dots/Ag3PO4 nanocomposite; BV-3—Basic violet-3; RY—Reactive yellow.
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Bonthula, S.; Ibrahim, M.F.; Al-Jaber, A.O.; Al-Siddiqi, A.-D.F.; Pothu, R.; Chowdhury, T.; Siddiqui, Y.; Boddula, R.; Radwan, A.B.; Al-Qahtani, N. Facile Fabrication of Pd-Doped CuO-ZnO Composites for Simultaneous Photodegradation of Anionic and Neutral Dyes. Physchem 2024, 4, 181-196. https://doi.org/10.3390/physchem4030014

AMA Style

Bonthula S, Ibrahim MF, Al-Jaber AO, Al-Siddiqi A-DF, Pothu R, Chowdhury T, Siddiqui Y, Boddula R, Radwan AB, Al-Qahtani N. Facile Fabrication of Pd-Doped CuO-ZnO Composites for Simultaneous Photodegradation of Anionic and Neutral Dyes. Physchem. 2024; 4(3):181-196. https://doi.org/10.3390/physchem4030014

Chicago/Turabian Style

Bonthula, Sumalatha, Muna Farah Ibrahim, Aisha Omar Al-Jaber, Al-Dana Faisal Al-Siddiqi, Ramyakrishna Pothu, Tauqeer Chowdhury, Yusuf Siddiqui, Rajender Boddula, Ahmed Bahgat Radwan, and Noora Al-Qahtani. 2024. "Facile Fabrication of Pd-Doped CuO-ZnO Composites for Simultaneous Photodegradation of Anionic and Neutral Dyes" Physchem 4, no. 3: 181-196. https://doi.org/10.3390/physchem4030014

APA Style

Bonthula, S., Ibrahim, M. F., Al-Jaber, A. O., Al-Siddiqi, A. -D. F., Pothu, R., Chowdhury, T., Siddiqui, Y., Boddula, R., Radwan, A. B., & Al-Qahtani, N. (2024). Facile Fabrication of Pd-Doped CuO-ZnO Composites for Simultaneous Photodegradation of Anionic and Neutral Dyes. Physchem, 4(3), 181-196. https://doi.org/10.3390/physchem4030014

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