**Structured Semiconductors in Photocatalysis**

Editors

**Jorge Bedia Carolina Belver**

MDPI • Basel • Beijing • Wuhan • Barcelona • Belgrade • Manchester • Tokyo • Cluj • Tianjin

*Editors* Jorge Bedia Universidad Autonoma de Madrid Madrid, Spain

Carolina Belver Universidad Autonoma de ´ Madrid Madrid, Spain

*Editorial Office* MDPI St. Alban-Anlage 66 4052 Basel, Switzerland

This is a reprint of articles from the Special Issue published online in the open access journal *Catalysts* (ISSN 2073-4344) (available at: https://www.mdpi.com/journal/catalysts/special issues/ Photocatalysis Semiconductors).

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## **Contents**

Reprinted from: *Catalysts* **2022**, *12*, 1099, doi:10.3390/catal12101099 ................. **143**

## **About the Editors**

#### **Jorge Bedia**

Dr. Jorge Bedia is an Industrial Engineer and obtained his PhD in Chemical Engineering at the University of Malaga (Spain). He has been an Assistant Professor at the Chemical Engineering Department of the Autonomous University of Madrid (Spain) since 2013. His research interests include (i) the synthesis and characterization of carbon-based materials for adsorption and catalysis; (ii) synthesis and applications of MOFs; (iii) water purification by advanced oxidation processes, especially photocatalysis; (iv) gas-phase hydrodechlorination of chlorinated volatile organic compounds; and (v) separation and catalytic processes with ionic liquids and supported ionic liquids. Dr. Bedia is the co-author of around 110 journal publications (more than 4800 citations, H factor of 42), 3 book chapters, and 2 Spanish patents. He has presented over 150 works to national and international conferences with over 20 oral presentations. He has been involved in several research contracts with private companies and more than 20 research projects with different entities: European, national, integrated or cooperative with other countries (Russia, Mexico, Germany, Peru, USA). He is an Editor of the *Separation and Purification Technology* and *Catalysis Communications* journals. He is also a member of the Editorial Board of *Chemical Engineering* and Associate Editor of *Separation and Purification Technology*.

#### **Carolina Belver**

Dr. Carolina Belver is Associate Professor at the Department of Chemical Engineering at the Universidad Autonoma de Madrid (Spain). She received a B.Sc. in Chemistry from ´ the Universidad Autonoma de Madrid in 1997 and a Ph.D. in 2004 from the University of ´ Salamanca (Spain). She enjoyed competitive national postdoctoral contracts at the Catalysis and Petrochemistry Institute (Spain) (2005-2007) and the Material Science Institute of Madrid (2008-2011). Dr. Belver's specialization deals with the design, processing, and evaluation of novel heterostructures for applications in environmental remediation, mainly focused on heterogeneous catalysis, photocatalysis, and adsorption. She is the co-author of more than 100 referred publications and 13 book chapters (more than 5000 citations, H factor of 40), and in 2013 she received a Fulbright Fellowship recognizing her accomplishments as a researcher. Dr. Belver is the Editor-in-Chief of *Applied Catalysis B*, the Executive Editor of *Chemical Engineering*, and a member of the Editorial Advisory Boards of Materials Science for the journals *Energy* and *Catalysts*.

## *Editorial* **Structured Semiconductors in Photocatalysis**

**Carolina Belver \* and Jorge Bedia \***

Departamento de Ingeniería Química, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain **\*** Correspondence<EMAIL_ADDRESS>(C.B<EMAIL_ADDRESS>(J.B.)

#### **1. Introduction**

Photocatalysis appears as an interesting approach for different applications, with the possibility of using sunlight as a sustainable and renewable source of energy. This technology is based on the use of a semiconductor that can be excited by light with an energy higher than its band gap inducing the formation of energy-rich electron-hole pairs, which can be involved in redox reactions. Recent progress explored the chemical nature of structured semiconductors with the object to improve their electronic and optical properties, enhancing their photoresponse under different conditions. This Special Issue collects original research papers, reviews, and commentaries focused on the challenges for the design of structured semiconductors with photocatalytic applications, thus including synthesis, characterization of new photocatalysts, studies of activity and stability, and the mechanisms of photocatalytic reactions.

#### **2. Structured Semiconductors in Photocatalysis**

This Special Issue includes outstanding studies focused on structured semiconductors in photocatalysis for different applications. In this sense, Bobape et al. [1] synthesized CuO-TiO2 nanocomposites using *C. Benghalensis* plant extracts. They analyzed the effect of the CuO to TiO2 ratio on the morphological, optical, electrochemical, and photodegradation efficiency. The XRD data confirmed the tenorite structure of the CuO and the anatase phase of the TiO2. The voltammogram of the CuO-TiO2 30/70 electrode showed the highest response current density, suggesting a higher specific capacitance in this structured semiconductor. This sample also showed the highest photocatalytic degradation efficiencies against methylene blue, ciprofloxacin, and sulfisoxazole, with hydroxyl radicals being the primary species responsible for the photodegradation. In another study, Matussin et al. [2] synthesized CeO2 and palladium–doped CeO2 photocatalysts via the microwave-assisted synthesis method. The authors observed mixed phases of CeO2/Ce2O3, although the Ce2O3 phase gradually disappeared upon doping with a higher percentage of Pd. The presence of Pd resulted in a decrease in the band gap energies, and the photoluminescence intensities were also quenched with Pd doping. Pd–CeO2 NPs showed enhanced activities under visible light irradiation in the photodegradation of methylene blue and photoantibacterial activities. Chue et al. [3] prepared a hexagonal wurtzite ZnO photocatalyst via precipitation method. CuS nanoparticles and PbS quantum dots were loaded onto ZnO via a hydrothermal method to obtain a CuS/PbS/ZnO heterojunction photocatalyst. The CuS/PbS/ZnO photocatalyst showed significant absorption capabilities in the ultraviolet to near-infrared spectral regions, and effectively reduced the recombination of electron–hole pairs during a photocatalytic reaction. This catalyst demonstrated the best water splitting effect. Furthermore, after adding a 0.25 M mixed solution of Na2S and Na2SO3 as the sacrificial reagent, the hydrogen production efficiency from water splitting reached 6654 μmol g−1 h−1 after 5 h. Tien et al. [4] synthesized a new binary MoS2/Co3O4 nanohybrids. The heterojunction presented an S-scheme structure that acted as electron traps and promoted light absorption capacity for the degradation of methyl orange under visible light. The photocatalyst also showed excellent stability and recyclability over five

**Citation:** Belver, C.; Bedia, J. Structured Semiconductors in Photocatalysis. *Catalysts* **2023**, *13*, 1111. https://doi.org/10.3390/ catal13071111

Received: 8 July 2023 Accepted: 12 July 2023 Published: 17 July 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

consecutive cycles, without noticeable changes in the nanocomposite structure. The boosted photocatalytic degradation and redox activities of MoS2/Co3O4 can be attributed to the created S-scheme heterostructure to facilitate the separation of and delay recombination of photoinduced charge carriers. In another study, Rezgui et al. [5] studied a heterogeneous photo-electro-Fenton process, which is an attractive technology for the removal of recalcitrant pollutants. To better exploit the presence of an irradiation source, a bifunctional catalyst with TiO2 nanoparticles embedded into an iron–chitosan matrix was developed. The catalytic activity of the catalyst was improved by the optimization of the loaded TiO2 content. The prepared composite catalysts based on TiO2, Fe3O4, and chitosan were called TiO2/Fe3O4-CS beads. The best catalyst with an optimal ratio TiO2/Fe = 2 exhibited a high efficiency in the degradation and mineralization of chlordimeform insecticide. Under the optimum conditions, a real effluent loaded with 30 mg L−1 of the insecticide was efficiently treated, leading to 80.8 ± 1.9% TOC reduction after 6 h of treatment, with total removal of the pollutant after only 1 h. Hudandini et al. [6] loaded Ag onto ZnO by an ultrasonic spray pyrolysis system at different Ag contents (1, 5, and 10 wt%). An increase in the ZnO-Ag activity compared with pristine ZnO was observed at a carrier gas ratio of 0:1 with reaction rate constants of 0.0059 and 0.0025 min−1, respectively, in the degradation of textile wastewater under UV light irradiation. Babyszko et al. [7] analyzed the modification of titanium dioxide with fumed silica. The SiO2/TiO2 photocatalysts were obtained by the sol-gel method and were then calcined under an argon atmosphere. Various SiO2 weights (2–17.2 wt.%) were prepared. The modification of titanium dioxide with SiO2 inhibited the increase in crystallite size of anatase and brookite during calcination and the decrease in specific surface area values. The photocatalytic activity was determined based on the decomposition of methylene blue under UV irradiation. All the obtained SiO2/TiO2 photocatalysts showed higher activity compared to the starting TiO2. Dies et al. [8] prepared fluoride-doped TiO2 (F-TiO2) photocatalysts by an efficient and simple one-step synthesis and tested them in the UV-photo-degradation of methylene blue and bisphenol A. F-TiO2 defeated commercial TiO2, and almost complete pollutant removal was achieved within 30 min. The energy consumption was reduced as a result of the suitable reactor set-up, which reduced light scattering, and by the application of a long-pulse radiation procedure, where the lamp was switched off during periods of continued degradation. This enhanced the overall photocatalytic performance. Under these conditions, 80% of dye removal was attained within 15 min of radiation with an energy consumption of only 0.070 Wh min−1, demonstrating a much better efficiency when compared to previously reported data. The catalyst was reusable, and its performance can be improved by the addition of H2O2. The results were validated by BPA degradation and the treatment of real wastewater with both pollutants. Harris et al. [9] investigated ZnO powders prepared by alkali precipitation using different [Zn(acetate)2(amine)x] compounds to alter the particle size and aspect ratio. Slow precipitations from 95 ◦C solutions produced micron-scale particles with morphologies of hexagonal plates, rods, and needles, depending on the precursor used. Powders prepared at 65 ◦C with rapid precipitation yielded particles with minimal morphology differences, but the particle size was dependent on the precursor used. The smallest particles were produced using precursors that yielded crystals with low aspect ratios during high-temperature synthesis. Particles produced during rapid synthesis had sizes ranging from 21 to 45 nm. The materials prepared using precursors with less volatile amines were found to retain more organic material than ZnO produced using precursors with more volatile amines. The amount of organic material associated with the nanoparticles influenced the photocatalytic activity of the ZnO, with powders containing less organic material producing faster rate constants for the decolorizing of malachite green solutions under ultraviolet illumination, independent of particle size. [Zn(acetate)2(hydrazine)2] produced ZnO with the fastest rate constant and was recycled five times for dye degradation studies that revealed minimal to no reduction in catalytic efficiency. Akitunde et al. [10] studied disinfection and photocatalytic degradation of organic contaminants using visible light-activated GCN/Ag2CrO4 nanocomposites. The organic pollutants studied were 2,4dichlorophenoxyacetic acid and methyl chlorophenoxy propionic acid present in Killex®, a commercially available herbicide, bovine serum albumin (BSA) protein, and SARS-CoV-2 spike protein. The disinfection experiments were conducted on wastewater secondary effluent. Overall, the results indicate that GCN/Ag2CrO4 nanocomposite is a promising photocatalyst in degrading organic pollutants and disinfecting microorganisms under visible light irradiation within a reasonable time. Lang et al. [11] evaluated a TiO2–CNT–Ag ternary composite film synthesized using the plasma-enhanced chemical vapor deposition method by simultaneously feeding a carbon nanotube (CNT)/Ag suspension and titanium tetraisopropoxide gas. The performance of the TiO2-composite film for the degradation of rhodamine 6G under simulated solar light irradiation was evaluated. The rate constant of the prepared TiO2–CNT–Ag for rhodamine 6G degradation was approximately 1.8 times greater than that of the prepared TiO2. This result indicates that the addition of CNT and Ag significantly improved the photocatalytic activity of the prepared films. This same research group prepared TiO2–Ag nanoparticle composite films by plasma-enhanced chemical vapor deposition (PECVD) using a mixture of aerosolized AgNO3, which was used as an Ag nanoparticle precursor, and titanium tetraisopropoxide, which acted as the TiO2 precursor [12]. Notably, the use of PECVD enabled a low process temperature and eliminated the need for pre-preparing the Ag nanoparticles, thereby increasing the process efficiency. The photocatalytic activity of the deposited films was determined by assessing the degradation of methylene blue under UV light irradiation. The Ag ions were successfully reduced to metallic nanoparticles and were embedded in the TiO2 film. The best photocatalytic activity was achieved for a 1 wt% Ag-loaded TiO2 composite film, which was 1.75 times that of pristine TiO2. Finally, Sanni et al. [13] studied the synthesis approach (thermal polyol and deposition–precipitation) regarding the dispersion of Ag/AgBr nanoparticles dispersed on activated carbon prepared from chemically impregnated pinecone to increase their photocatalytic efficiency on the degradation of tetracycline.

This Special Issue also includes two reviews. The first one is a work by Venezia et al. [14] that described the state of the art of the current design strategies and emerging trends to hierarchical catalysts. Nature mimicking implies the design of nanostructured materials, which can be assembled into a hierarchical structure, thus outperforming the features of the neat components because of their multiple length scale organization. This approach can be effectively exploited for the design of advanced photocatalysts with superior catalytic activity for energy and environment applications with considerable development in the recent six years. The review presented different synthesis strategies, including template-free structuring, and organic, inorganic, and hybrid templating. Furthermore, emerging approaches based on hybrid and bio-waste templating were also highlighted. Finally, a critical comparison among available methods was carried out based on the envisaged application. The second review by Sreedhar and Noh [15] described the advancements in solar desalination of seawater by various Ti3C2 MXene-based morphologies for freshwater generation. The study explained key features such as light absorption, reflection, multiple internal reflection, hydrophilicity, lower thermal conduction, light-to-heat generation, and salt rejection for achieving efficient desalination output throughout the visible and broadband region. Specifically, it explored the self-floating and salt rejection nature of various state-of-the-art 2D Ti3C2 MXene structures. Among the different morphologies, Ti3C2 MXene in the form of a membrane is believed to be a promising morphology that effectively desalinates seawater into freshwater.

It can be concluded that structured semiconductors have key characteristics for many kinds of applications and purposes [16]. Probably, due to their high tunability and versability, we will assist in the future with a fast development of these amazing materials that can be fundamental for many applications.

**Author Contributions:** C.B. and J.B. have equally contributed to the writing and revision of this editorial. All authors have read and agreed to the published version of the manuscript.

**Acknowledgments:** We would like to thank the authors, reviewers, and editorial team of *Catalysts* for the work devoted to this Special Issue.

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

#### **References**

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## *Article* **Green Synthesis of CuO-TiO2 Nanoparticles for the Degradation of Organic Pollutants: Physical, Optical and Electrochemical Properties**

**Dineo A. Bopape 1, Sarah Mathobela 1, Nolubabalo Matinise 2, David E. Motaung 3 and Nomso C. Hintsho-Mbita 1,\***

**\*** Correspondence<EMAIL_ADDRESS>
**Abstract:** CuO-TiO2 nanocomposites were successfully synthesized using the *C. benghalensis* plant extracts. The effect of the composition of CuO to TiO2 on the morphological, optical, electrochemical, and photodegradation efficiency in the composites was studied. SEM, XRD, UV-vis, FTIR, TGA, BET, and CV were used to characterize these materials. The XRD data reported the tenorite structure of the CuO and the anatase phase of the TiO2. SEM showed the spherical morphologies for all the CuO-TiO2 NPs, and these were also mesoporous in nature, as depicted by BET. The voltammogram of the CuO-TiO2 30/70 electrode showed a higher response current density compared to the other two samples, suggesting a higher specific capacitance. Upon testing the photocatalytic efficiencies of the CuO-TiO2 nanocomposites against methylene blue (MB), ciprofloxacin (CIP), and sulfisoxazole (SSX), the highest degradation of 94% was recorded for SSX using the CuO-TiO2 30/70 nanocomposites. Hydroxyl radicals were the primary species responsible for the photodegradation of SSX, and the material could be reused once. The most active species in the photodegradation of SSX has been identified as OH•. From this study, it can be noted that the CuO-TiO2 nanocomposites were more selective toward the degradation of antibiotics (sulfisoxazole and ciproflaxin) as compared to dyes (methylene blue).

**Keywords:** CuO-TiO2 nanocomposites; green synthesis; photocatalysts; sulfisoxazole

#### **1. Introduction**

Water pollution is one of South Africa's most serious environmental problems, adding to the water scarcity the country is facing [1–3]. Despite this, contaminants such as dyes and antibiotics are being detected in natural water sources, and the number of contaminants detected has increased in recent years as new emerging pollutants are reported each year [4,5]. The textile sector contributes significantly to water pollution in the environment by dumping dye-bearing waste effluents [6–9]. Even at low concentrations, the presence of these dyes (which can be anionic, cationic, or non-ionic) in water is highly apparent and undesired [10]. Humans can be exposed to these toxic dyes through direct or indirect consumption of polluted water via the food chain [11]. The after effect has been reported to be mutagenic to human health and damaging to the aquatic ecology [12].

Methylene blue (MB) is a cationic dye that has been classified as a toxic colorant [11]. This pollutant is non-biodegradable due to its complex aromatic composition, and it damages septic tanks during wastewater treatment [13].

Sulfisoxazole (SSX) is an endothelin receptor antagonist that protects retinal neurons against ischemia/reperfusion and lipopolysaccharide damage [14]. On the other hand,

**Citation:** Bopape, D.A.; Mathobela, S.; Matinise, N.; Motaung, D.E.; Hintsho-Mbita, N.C. Green Synthesis of CuO-TiO2 Nanoparticles for the Degradation of Organic Pollutants: Physical, Optical and Electrochemical Properties. *Catalysts* **2023**, *13*, 163. https://doi.org/10.3390/catal 13010163

Academic Editors: Jorge Bedia and Carolina Belver

Received: 3 November 2022 Revised: 12 December 2022 Accepted: 17 December 2022 Published: 10 January 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

excessive usage of SSX may leave traces of the medicine in animal products [15]. However, wastewater treatment plants that treat pharmaceutical wastes transfer the SSX into freshwater environments [14]. SSX residues in the aquatic environment may cause allergic responses, drug resistance, and endocrine problems [16].

Ciprofloxacin (CIP) is the most commonly prescribed fluoroquinolone antibiotic [17,18]. CIP is effective against a wide range of Gram-negative and Gram-positive bacteria and is one of the emerging pharmaceutical contaminants found in water [17–19]. It is often found in the environment and has been demonstrated to be genotoxic and it is a primary metabolite of enrofloxacin, a popular fluoroquinolone used in veterinary medicine [18]. CIP is also difficult to completely remove using conventional treatment methods [18,19]. Thus, some of the CIP has remained in the water following treatment in concentrations measured in parts per million (g/mL) or even parts per billion (ng/mL), its unpredictable toxicological effects have sparked growing concern [20]. As a result, there is a strong need to develop materials that can remove various pollutants such as MB, SSX, and CIP from effluents before they are discharged into freshwater systems [13–19].

Several methods such as adsorption, electrocoagulation, and membrane filtration have been used for the elimination of these pollutants in wastewater streams [11,14,18]. Most of these approaches have drawbacks such as excessive sludge generation, formation of by-products, high operational costs, and poor selectivity, thus researchers are moving toward Advanced Oxidation Processes in particular the photocatalysis process as its been shown to be an efficient method for the degradation of multiple pollutants. However, in the treatment of organic MB, SSX, and CIP, these approaches fall short of the dye wastewater discharge criteria. Given the current environmental issues, finding a costeffective and environmentally friendly technique to breaking down organic contaminants is critical [21–23].

Metal oxide semiconductor catalysts have received much press in recent years because of their capacity to convert solar energy and clean up pollution [24]. The process of photocatalytic degradation using metal oxides is frequently utilized in water treatment due to its efficacy in removing pollutants and has no secondary pollution [24,25]. Researchers are currently focusing on metal oxide nanostructured composites due to their unique features, such as a high surface to volume ratio and quantum confinement effects [22]. Various metal oxides such as ZnO, CuO, NiO, TiO2, etc., are highly explored but nanosized transition metal oxides, such as copper oxide (CuO) and titanium oxide (TiO2), have attracted scientific interest due to their potential applications in medicine, sensing, catalysis, etc. [22,24,26]. CuO has gained much attention among transition metal oxides because of its intriguing photochemical and photomagnetic capabilities [26]. It is a photoconductive p-type semiconductor with a narrow bandgap (1.2 eV) and has been used in a variety of applications in gas sensing, catalysis, super capacitors, and field emission, to name a few [24,27]. CuO has a monoclinic structure and is employed in a variety of device fabrication units depending on its properties [22]. It is one of the most important catalysts used to remove industrial effluents from the environment [21].

On the other hand, TiO2 is an n-type semiconductor with a large band gap that ranges from 3.2 to 3.6 eV and it is widely used in cosmetics, coatings for paper and medical devices, as well as gas sensors [24]. Although TiO2 is a promising material for photocatalytic applications, its wide bandgap energy of 3.2 eV limits its absorption to UV light at ʢmax = 365–387 nm [23,25,28]. It was recorded that TiO2 can only utilize 5–8% of the UV light and for the vis light it was recorded to be relatively low [6,25,29]. One significant benefit is that when TiO2 reacts with another metal oxide, it forms heterojunctions [22,24,29]. However, metal doping of TiO2 has a low thermal stability and is prone to photo corrosion [25]. CuO has been frequently employed to enhance the photocatalytic properties of TiO2 [25–27]. This is because it can reduce the photogenerated electrons and holes from recombining while simultaneously lowering the energy of the band gap [24–28]. The disadvantage with the synthesis of such CuO-TiO2 nanocomposite, is the use of harmful chemicals. Thus, it is critical to synthesize CuO-TiO2 nanocomposites using a simple, eco-friendly, and

cost-effective method that does not require the use of hazardous reagents. In recent years, the synthesis of CuO-TiO2 using biological methods such as plant extracts has sparked considerable interest in the field of nanotechnology [30]. Some of the required properties in these materials have been reported to have been added from bimetal oxides synthesized from plant extracts, microorganisms, and enzymes. Plant-mediated nanocomposites for example, can be used to degrade a wider range of pollutants, such as pharmaceutical and dyes strains, due to the phytochemicals they contain.

Various studies have been conducted on the photodegradation of organic pollutants using the green-derived CuO/TiO2 nanocomposites. Lu et al. [29] prepared the CuO/TiO2 nanocomposite. In their study, UV light was irradiated and 100% of phenol was degraded after 80 min. Approximately, 98 and 88% of MB and MO, respectively, was degraded by CuO/TiO2 nanocomposite after 150 min irradiation by Khodadadi et al. [30]. Based on the previous studies discussed, the photocatalytic efficiency of TiO2 were improved by coupling the material with CuO. Hence there is a need to prepare the CuO-TiO2 nanocomposites as photocatalyst semiconductors.

In this study various ratios of CuO-TiO2 nanocomposites were synthesized using the *Commelina benghalensis* (*C. benghalensis*) plant extracts for the first time. The plant is rich in phytochemicals that can act as trapping and capping agents during bimetallic synthesis [25]. The physical, optical, surface, and electrochemical properties were investigated. The effect of the different compositions of the photocatalysts were also explored for the degradation of various pollutants, methylene blue, ciprofloxacin, and sulfisoxazole.

#### **2. Results and Discussions**

#### *2.1. FTIR Characterization*

Figure 1a,b shows the IR spectrum of the *C. benghalensis* plant extract, CuO, TiO2- NPs, and CuO-TiO2 nanocomposites. An intense band at 1641 cm−1 for *C. benghalensis* extracts was noted. This corresponds to the amide band, which is caused by a carbonyl stretch and N-H deformation vibrations in the amide linkage of *C. benghalensis* proteins [31]. These could be from alkaloids and tannins, which were identified from this plant, both of which are known to be stabilizing and reducing agents [32,33]. For the CuO-NPs, a Cu-O bond vibration at around 516, 832, and 1052 cm−1, which shows the formation of the CuO metal oxide, was noted [29,34]. The hydroxyl groups (–OH) from the *C. benghalensis* plant extract were responsible for the absorption bands at 1663 cm−1 (bending mode) and 3388 cm−1 (stretching mode), whereas the Cu-O stretching vibration mode was responsible for the formation of the absorption bands at 510, 832, and 1052 cm−1 [35,36]. Peaks associated with the bending and stretching vibrations of hydroxyl groups (OH) were seen in CuO-TiO2 samples at ~1651 and 3356 cm−1, respectively, and the Ti-O and Ti-O-Ti bands were also detected vibrating at 400–900 cm−1 [32]. In addition, when identifying the stretches of the composites (Figure 1b), a peak at 1376 cm−1 representing the stretching vibrations of Cu-O-Ti groups confirmed the presence of CuO structures on the surface of TiO2 nanoparticles [37,38]. This verifies the development of a chemical link at the CuO-TiO2 interface. The CuO-TiO2 nanocomposite at the 50/50 composition had more intense peaks, which corresponded to the *C. benghalensis*, CuO, and TiO2 peaks. The intensity of the peaks increased in the order CuO-TiO2 30–70 < CuO-TiO2 70–30 < CuO-TiO2 50–50. Table 1 shows the detailed peak formations of the compounds.

**Table 1.** The IR peaks depicted *for C. benghalensis* plant extract, *C. benghalensis*-mediated CuO, and TiO2 nanoparticles.

**Figure 1.** (**a**) FTIR diagram for the *C. benghalensis* extract, TiO2, and CuO and the (**b**) CuO-TiO2 nanocomposites with different compositions.

#### *2.2. Optical Properties of CuO-TiO2 Nanocomposites*

The UV-vis absorption spectra of CuO-TiO2 30/70, CuO-TiO2 50/50, and CuO-TiO2 70/30 were explored in Figure 2a to determine the properties of the material in the 200–800 nm spectrum. For all the CuO-TiO2 nanocomposites, the absorption band was found to be 301 nm. The intensity of the absorption peak increased with the composition of CuO on the TiO2 material. The adsorption edge of all the prepared samples was the same, indicating that CuO was deposited on the surface of TiO2 rather than doped into the TiO2 crystalline [39,40]. This absorbance band makes the CuO-TiO2 materials better photocatalysts in the visible light region. Zhang et al. [39] obtained an absorption band at 380 nm, whereas Shi et al. [40] obtained it at 385 nm, and Yu et al. [41] at 388 nm. When compared to the literature, it shows that the absorbance peaks of all green-synthesized CuO-TiO2 in this study were redshifted. This could be due to the difference in the synthetic process of the CuO-TiO2 nanocomposites. In this study, a plant extract of *C. benghalensis* was utilized as a reducing agent, as opposed to the NaOH, NaBH4, and C2H6O2 that was used by Zhang et al. [39], Shi et al. [40], and Yu et al. [41], respectively. The band gap energies (Eg) of materials were determined by extrapolating and intersecting the linear portion of absorbance to the energy axis [28]. The Eg values for the CuO-TiO2 30/70, CuO-TiO2 50/50, and CuO-TiO2 70–30 were found to be 3.76, 3.72, and 3.79 eV, respectively. The optical band gap energy (Eg) for all the CuO-TiO2 nanocomposites was calculated based on the absorption spectrum of the samples according to the equation of Eg = 1240/wavelength [23]. From the literature, Gnanasekaran et al. [35] obtained a band gap of 2.98 eV, Muzakki et al. [36] recorded 3.01 eV, and Chowdhury et al. [27] obtained 3.17 eV. These studies used NaOH and urea as reducing agents. Therefore, in this study, maybe due to the phytochemicals present in the *C. benghalensis* plant extract, which was used as a reducing and capping agent, may have led to the higher band gap values of the CuO-TiO2 nanocomposites.

**Figure 2.** (**a**) UV-vis curves for CuO-TiO2 (30/70, 50/50, and 70/30) and the corresponding (**b**) Tauc's plot.

#### *2.3. Structural and Phase Composition Analysis*

To obtain the structural properties, crystallite size, and phase composition of the nanocomposites, XRD analysis (Figure 3) was conducted. The XRD data of pure CuO were assigned with 2θ diffraction peaks at 35.6◦ (002), 38.9◦ (200), 48.65◦ (−202), 58.23◦ (202), 61.53◦ (−113), and 66.2◦ (−311) agreed to the JCPDS card no. 48-1548, which identified the tenorite structure of CuO [42–44]. The TiO2 was identified by the characteristic diffraction peaks at 25.3◦ (101), 36.9◦ (103), 37.8◦ (004), 48.05◦ (200), 53.9◦ (105), 62.12◦ (213), 68.7◦ (116), 70.311◦ (220), 75.03◦ (215), and 76.02◦ (301) corresponding to the anatase phase of TiO2 (according to the JCPD 21-1272) [44]. The relative broad peaks at 35.3◦, 38.9◦, and 48.37◦ were indexed to the diffraction of the TiO2 planes (103), (004), and (200), and CuO planes (−103), (200) and (−202) demonstrating that TiO2 and CuO coexist in the CuO-TiO2 heterojunction [45]. This is reasonable given that the anatase TiO2 lattice constants are identical to those of CuO. The intensity of the peaks of CuO and TiO2 decreased in the CuO-TiO2 bimetallic. It is interesting to note that the visibility of CuO peaks (plane −111 and 200) was more visible in the CuO-TiO2 (70/30) than the (30/70) and the TiO2 peaks

(plane 101 and 200) were more intense in the CuO-TiO2 (30/70), vice versa. This showed the difference in the composition of the two metal oxides. Then on the TiO2-CuO (50/50), the peaks from both CuO and TiO2 were more intense and visible as compared to those of CuO-TiO2 (70/30) and CuO-TiO2 (30/70). These data corroborate with the FTIR data because the presence of Ti-O, Cu-O, and C-H. N-H peaks were also intense on the FTIR data for the 50/50 composition. The Scherrer Equation (2) was used to calculate the crystalline sizes of materials, and the sizes were found to be 17.03, 17.13, 39.72, 47.2, and 39.14 nm for the TiO2, CuO, and CuO-TiO2 30/70, 50/50, and 70/30, respectively. This demonstrates that the formation of the heterojunctions led to an increased crystallite sizes [41].

$$\mathbf{D} = \mathbf{K}\lambda / \beta \cos \theta \tag{1}$$

where: K = Scherrer constant (i.e., 0.94),

λ = X-ray wavelength (0.154060 nm)

β = Full width at half-maximum of the (101) XRD peak

θ = The Bragg diffraction angle (degree).

**Figure 3.** TiO2, CuO, and CuO-TiO2 (30/70, 50/50 and 70/30) XRD patterns.

#### *2.4. Morphological Analysis of the Various Materials*

Figure 4a,b shows the SEM images of the biosynthesized CuO nanoparticles. The materials are shown to be thick tubes in shape, as shown in the literature previously, with some aggregation, and have a particle size distribution of 50–250 nm, with dominant particle sizes being between 100 and 200 nm (Figure 4k) [24]. For the TiO2, the SEM images in Figure 4c,d showed spherically shaped nanoparticles with a particle size distribution of (Figure 4l) 50–400 nm with dominating particles ranging in size from 100 to 300 nm. The lesser agglomeration that was observed on the TiO2 may be due to the optimum concentration of the plant extract that was utilized [37]. Upon analyzing the three bimetallic materials, the more we increased the composition of CuO on TiO2, the less spherical the composites became, which is evidence that Cu, Ti, and O species are strongly demonstrated to be homogeneously distributed throughout the entire selected area, and the formation

of the p-n heterojunction between CuO and TiO2 is revealed [40]. The particle sizes of the spherical nanocomposites increased as we increased the composition of CuO. The particle size distribution of CuO-TiO2 30–70, 50–50, and 70–30 was 100–200 nm, 125–250, and 105–250, respectively. The larger particle sizes may be due to the agglomeration that is observed from CuO-TiO2 30–70 to 70–30. To further confirm the formation of these materials, EDS analysis was conducted as reported in Figure S1a–e.

**Figure 4.** (**a**–**j**) SEM images for TiO2, CuO, and CuO-TiO2 (30/70, 50/50, and 70/30) and their (**k**–**o**) particle size distributions.

#### *2.5. TGA and DTA Analysis*

For thermal analysis studies on the CuO-TiO2 nanocomposites (Figure 5a), two mass losses were observed arising from the elimination of water. All mass losses were linked to the endothermal transformation, according to the DTA curves (Figure 5b). At the molar ratios CuO-TiO2 = 30:70, 50:50, and 70:30, total mass losses of the nanocomposites were 6.5%, 1.8%, and 8.3%, respectively. When compared to the TiO2 reference sample, a 50% increase in copper (II) oxide content enhanced the thermal stability in the temperature range of 20–1000 ◦C. All of the materials studied had suitable thermal stability up to 1000 ◦C, which is in line with previous studies by Ashok et al. [42] where weight loss percentages were 9.0, 4.5, 3.0, and 2.5 for CT-5, CT-6, and CT-7, respectively. The 50/50 CuO-TiO2 nanocomposite was the most thermally stable composite of all the materials

**Figure 5.** (**a**) TGA diagrams and (**b**) the derivatives diagrams of TiO2, CuO, and CuO-TiO2 (30/70, 50/50, and 70/30).

#### *2.6. N2 Adsorption-Desorption Studies, BJH Pore Volume, and BET Surface Area*

The surface area, pore diameter, and pore volume analysis through N2 adsorption/desorption isotherms for all the CuO-TiO2 nanocomposites are demonstrated in Figure 6a–e. All the samples exhibited a type III hysteresis loop except for the TiO2 material, which had an H1 hysteresis loop [43]. The BET surface areas of the photocatalysts generally decreased as the CuO loading increased. The BET surface area of the materials formed at the molar ratios CuO-TiO2 30/70, 50/50, and 70/30 was 5.58, 5.74, and 3.22 m2/g, respectively. On the other hand, the surface area of TiO2 and CuO was 13.18 m2/g and 1.613 m2/g, respectively. All the materials, excluding TiO2, were mesoporous in nature [44]. The mesoporous nature of these materials could have been influenced by the manner of the pore diameter distribution of copper (II) oxide [45].

**Figure 6.** The adsorption-desorption isotherms (**a**–**e**) with the pore volume distribution curve (inset) and (**f**) the surface area diagrams for TiO2, CuO, and CuO-TiO2 (30/70, 50/50, and 70/30).

#### *2.7. Electrochemical Characterization of the Composites*

Cyclic and linear swept voltammetry were used to test the electrochemical properties to quickly determine information about the thermodynamics of redox processes and the electron transfer between an electrolyte and a modified working electrode. CV and linear curves of bare (unmodified) glassy carbon electrodes modified with different samples (S1–S3) at the scanning rate of 2 mV/s are displayed in Figure 7A,B. According to the graph observed, there was no electrochemical performance revealed by the bare GCE electrode. An electrochemical reaction was revealed after the modification of GCE with the three different samples noted due to the peak potential and high current density. All the CV curves for the modified electrode displayed a similar shape, a redox peak, but the different positions of the redox peaks were dependent on the type of sample. A couple of redox peaks and an increase in the current density can be seen clearly in the voltammogram of each sample. Voltammogram curves generally reveal redox peaks and high current density, indicating a pseudo-capacitance behavior [11,12]. An anodic peak appeared to the positive sweep around 0 to 0.2 V due to the oxidation of CuO, and cathodic peaks appeared to the negative ongoing from −0.2 to −0.3 due to the reduction of Cu2+/Cu1+. Noticeably, the voltammogram of the CuO-TiO2 30/70 (S1) electrode showed a higher response current density compared to the other two samples, suggesting a higher specific capacitance, and it could be due to more titanium dioxide, as there is a synergistic effect that increases the electrochemical properties of copper oxide. In addition, this may be attributed to the high conductivity of the electrode due to the p-n junction between CuO and TiO2 nanoparticles, which enhances the electrochemical activities. This implies that more TiO2 nanoparticles added to the CuO, which is an n-type semiconductor, play an important role in increasing the electrochemical activity of the electrode to show the best

response in terms of voltammetry. The electrochemical results can make the electrode to be the best candidate for electrochemical applications due to their best voltammetric response and possible photocatalytic applications. The specific capacitance was calculated from the area under the curve of cyclic voltammetry using Equation (3) and was found to be 164, 134, and 113 for samples 1, 2, and 3, respectively.

$$\mathcal{C}\_{\mathbb{P}} = \frac{\mathcal{A}}{2 \text{km} \Delta \mathcal{V}} \tag{2}$$

A—area under I-V curve, k—scan rate, m—active mass, ΔV—potential window.

**Figure 7.** Cyclic (**A**) and linear (**B**) voltammetry curves of bare GCE and modified A: GCE/ CuO-TiO2 30/70 (S1) B: GCE/ CuO-TiO2 50/50 (S2): GCE/ CuO-TiO2 70/30 (S3) in 0.5 M H2SO4 as an electrolyte.

Figure 8 reveals CV at different scan rates of 5–50 mV/s for three different samples. The cyclic voltammogram curves showed that the current density increased with increasing the scan rates; thus, the voltammetric current is directly proportional to the scan rate and gives an indication of the excellent rate capability of the modified electrode. There was a slight shift of redox peaks (oxidation and reduction) toward a more positive and negative potential, illustrating that the electrochemical kinetics are controlled by the diffusion process and displaying a rapid charge transfer [38].

The EIS was also used to study the electrochemical impedance properties of the samples (S1–S3). It is a prevailing tool to investigate the capacitive behavior of the samples. It is used to study the charge transfer resistance and diffusion process from the solution to the modified electrode surface and to the resistance of the electrolyte [14,45–47]. In Figure 9, the Nyquist plots of CuO-TiO2 30/70 (S1), CuO-TiO2 50/50 (S2), and CuO-TiO2 70/30 (S3) that were modified in the glassy carbon electrode are shown. As seen from the graph, the results show that all samples revealed a semi-circle in the high-frequency range, which is related to the charge transfer resistance caused by faradic reactions. The intersection of the curve signifies solution resistance, which is an indication of the resistance of the electrolyte and modified electrode [48,49]. A straight line in the low-frequency resistance, which corresponds to Warburg impedance, indicates an ion diffusion process on the modified electrode and electrolyte [15,16]. The EIS data suggest that the CuO-TiO2 electrode material showed faster charge transport ability, leading to suitable electrochemical properties. Therefore, the electrochemical results showed that the modified CuO-TiO2 electrode (30:70) is an outstanding electrode material for electrochemical application due to its great voltammetric response, suitable electrochemical properties, and best charge transfer resistance.

**Figure 8.** Cyclic voltammetric at different scan rates (2–50 mV/s). (**A**): GCE/ CuO-TiO2 30/70 (S1) (**B**): GCE/ CuO-TiO2 50/50 (S2) and (**C**) GCE/ CuO-TiO2 70/30 (S3) in 0.5 M H2SO4 as an electrolyte.

**Figure 9.** Nyquist plot of GCE/ CuO-TiO2 30/70 (S1) GCE/ CuO-TiO2 50/50 (S2): GCE/ CuO-TiO2 70/30 (S3) in 0.5 M H2SO4 as an electrolyte.

#### *2.8. Photodegradation of MB Dye Using the CuO-TiO2 Nanocomposite*

#### 2.8.1. Photodegradation of MB UV Light

The green-synthesized CuO, TiO2, CuO-TiO2 30/70, 50/50, and 70/30 photocatalysts were used for the degradation of MB dye (Figure 10A). From the analysis, 18%, 65%, 17%, 33%, and 14% of the MB pollutant was degraded. It can be noted that there was a drastic drop in the degradation efficiencies of these materials, in particular with the 30/70 CuO-TiO2 and the 70:30 CuO-TiO2, which recorded 17% and 14% degradation. Sankar et al. [46], using the CuO photocatalyst, were able to degrade 50% of Coomassie brilliant blue R-250 after 240 min exposure to sunlight. Lingaraju et al. [50] utilized a green-synthesized CuO photocatalyst to degrade ~100% of trypan blue dye after 140 min irradiation under UV light. The green-synthesized CuO/TiO2 prepared by Lu et al. [29] degraded 100% of phenol after 160 min exposure to UV light. Khodadadi et al. [30] synthesized CuO/TiO2 nanocomposite, which was able to degrade 100% of MB and methyl orange (MO) after 10 min irradiation with UV light. It can be noted that for the green-derived CuO and TiO2 NPs, for their improved degradation to occur, extended time periods (360 min) were noted even though in one study, the degradation was still only 50% using CuO. In addition, the choice of the pollutant could play another major role. Using CuO/TiO2, for all the pollutants there was a complete degradation for all the mentioned pollutants, and thus it is the complete opposite of what has been obtained in our study, with a maximum degradation of 33% for MB, considering that optimum conditions from other similar studies were used. Thus, it is suspected that the phytochemicals from the various plant extracts used for the synthesis of these materials also play a major in their optical properties. In addition, their activity could be pollutant dependent; thus, it was important that we also investigate other pollutants, such as sulfisoxazole and ciprofloxacin, which are pharmaceutical pollutants. From the kinetics study (Figure 10B), it was noted that the CuO-TiO2 30/70 nanocomposite had a better fitting than the first-order kinetics as the other two materials did not fit completely. The rates of reactions were 0.014, 0.0029, and 0.001 min−1 for the CuO-TiO2 30/70, 50/50, and 70/30, respectively.

**Figure 10.** (**A**) Photodegradation of MB by CuO, TiO2, and various ratios of CuO-TiO2 and (**B**) kinetics study of the CuO-TiO2.

#### 2.8.2. Photodegradation of CIP and SSX Using UV light

For the degradation of antibiotics (CIP and SSX), similar conditions as with MB, but the concentration of SSX and CIP was 10 mg/L (Figures S2 and S3), and from Figure 11A, all the CuO-TiO2 nanocomposite showed a high degradation efficiency against SSX than for CIP. The CuO-TiO2 photocatalyst degraded 51% (30/70), 55% (50/50), and 67% (70/30) of CIP, while CuO and TiO2 only degraded 58.2 and 62.7%, respectively. From the kinetics diagram (Figure 11B), the rates of reactions were 0.044, 0.0069, and 0.009 min−1 for the CuO-TiO2 30/70, 50/50, and 70/30, respectively, meaning the rate of the reaction for CuO-TiO2 was faster compared to the other two materials. On the other hand, the percentage degradation of SSX by CuO-TiO2 30/70, 50/50, and 70/30 was recorded to be 93.6%, 93.2%, and 91.2%, which is the highest for these composites compared to MB and CIP, respectively. While the degradation of CuO and TiO2 was 44.8% and 50.2%, respectively. Comparing the results with previous studies, Mofokeng et al. [48] prepared a CuO/TiO2@GCN, which was able to degrade 94% of 10 mg/mL ketoprofen (KP) after 30 min irradiation of visible light. Using UV light, Castaneda et al. [49] degraded 90% of caffeine within 3 h using 100 mg of CuO-TiO2-F photocatalyst. Hajipour et al. [50] utilized 5 mg of TiO2/CuO photocatalyst to degrade 80% of amoxicillin. The CuO-TiO2-GO prepared by Cosma et al. [51] degraded 95% and 98% of amoxicillin and ciprofloxacin, respectively. The photodegradation process was conducted over 120 min. Li et al. [52] used green-synthesized TP-TiO2 to degrade 90% of CIP in 60 min under the UV-vis light source. These nanocomposites are suitable photocatalysts for the degradation of pharmaceutical drugs.

**Figure 11.** (**A**) Photocatalytic degradation of various materials against SSX and CIP, (**B**) kinetics of CIP, and (**C**) kinetics of SSX.

2.8.3. Effect of Trapping and Mechanism of Degradation

In an effort to test the cost-effectiveness of a material, reusability studies were conducted using the 30/70 CuO-TiO2 NPs for the degradation of SSX. It is important for synthesized materials to not only have a high degradation efficiency but to ensure their material are cost-effective should they be upscaled. In this study, the 30/70 CuO-TiO2

material was tested for its efficiency at least four times. As it can be noted (Figure 12B), the material was not reusable as the efficiency significantly dropped. Upon testing the species responsible for the degradation, it can be noted that the •OH species were the main active species for the degradation of SSX as the degradation dropped to 59% as these species were trapped during the photodegradation process. Other studies have also shown the •OH radicals to be the species responsible for their degradation.

**Figure 12.** (**A**,**B**) Reusability studies, (**C**,**D**) effect of reactive.

On the basis of the observed photocatalytic activities and the characterizations shown above, a probable mechanism for the photooxidation of SSX is illustrated in Scheme 1. TiO2 is a wide band gap semiconductor; thus, due to its limited access to the wider spectrum, low bandgap materials such as CuO are coupled with it to reduce the recombination rate and fast generation of radicals. Upon irradiating enough light on the photocatalysts, the electrons were excited and moved up to the conduction band of TiO2. These photoexcited electrons reacted with the O2 species that were adsorbed on the TiO2 surface, thus forming the O2 radical anion and the OH radical through the protonation process [43]. Therefore, the CuO, which is a low bandgap material (1.2 eV), assisted in the reduction of the holes and electron recombining. Furthermore, environmentally friendly materials are potentially formed.

**Scheme 1.** For the mechanism of degradation.

#### **3. Experimental Procedure**

#### *3.1. Chemicals and Materials*

Analytical grade chemicals such as Cu(NO3)2·3H2O, propan-2-ol (C3H8O), ethylenediaminetetraacetic acid (EDTA), p-benzoquinone (C6H4O2) and ciprofloxacin (CIP), methylene blue (MB), Titanium Fluoride (TiF4) were purchased from Sigma Aldrich, Germany. The pharmaceutical antibiotic sulfisoxazole (SSX) was obtained from the organic group at the University of Limpopo's Department of Chemistry. Methylene blue dye was purchased from Merck chemicals, Germany. The *C. benghalensis* plant was harvested, dried, and ground at the University of Limpopo. No further purifications were performed in all the chemicals used.

#### *3.2. Preparation of the Plant Extracts*

The *C. benghalensis plants* were collected in the fields of University of Limpopo between September–November (yearly basis). The plant was washed with tap water, then the leaves and flowers were removed in order to work with only the roots and stem. The plant was dried (indoors) for 6–8 weeks. The dry plant was then crushed (using a blender) and stored in an airtight container. The plant extracts of *C. benghalensis* (See Scheme 2) were prepared in a 500 mL flask containing 250 mL of hot water. A total of 10 g *C. benghalensis* powder was added and further heated at 80 ◦C for 15 min. The resulting green-brownish extract was cooled (at room temperature) and then collected by vacuum filtration. The extracts were reserved in a refrigerator for further use.

**Scheme 2.** *C. benghalensis* plant extract preparation.

#### *3.3. Preparation of C. benghalensis-Mediated TiO2 Nanoparticle Synthesis*

Green-medisted TiO2-NPs were prepared by mixing 50 mL of 0.5 M TiF4 with an aliquot of 50 mL of 10 g *C. benghalensis* extract, respectively. The mixture was then heated for two hours at 90 ◦C until it turned brownish-yellowish color. The mixture was allowed to cool at room temperature before being centrifuged for 30 min at 4000 rpm. It was then washed and centrifuged in distilled water for another 15 min at 4000 rpm. After that, the solid precipitate was transferred to the crucible and calcined for 2 h in a 400 ◦C furnace before cooling. The product was gathered, crushed, and stored for later.

#### *3.4. Preparation of C. benghalensis-Mediated CuO-NPs*

An aliquot of 50 mL of *C. benghalensis* extract was added to 5 g of cupric nitrate (Cu(NO3)2·3H2O) and then boiled at 80 ◦C for 1 h until a brown-colored paste was observed. The paste was collected in a ceramic crucible and heated in an air furnace at 400 ◦C for 2 h. A black powder was obtained, and the product was carefully collected and packed for characterization.

#### *3.5. Synthesis of CuO-TiO2 Nanocomposite*

The CuO-TiO2 nanocomposites were synthesized (Scheme 3) by adding an aliquot of 50 mL of the *C. benghalensis* to the Cu(NO3)23H2O and TiF4 at ratios of 70:30, 50:50, and 30:70, separately. The mixture was heated at 80 ◦C for an hour to produce a paste. The paste was then transferred into a crucible and calcined in a furnace for 2 h. The products were collected and crushed for further characterization and applications.

#### *3.6. Characterization of CuO-TiO2 Materials*

FTIR, over a range of 4000–500 cm−1, was used to study the stretching bond frequencies of the synthesized CuO-TiO2 nanocomposites, phytochemical identification, and nanoparticle synthesis. The optical parameters of the generated materials, as well as the concentration of the dye and pharmaceutical component, were determined using a PerkinElmer Spectrum SP-UV 500VDB spectrophotometer. The bandgap energy (Eg) of CuO-TiO2 nanocomposites was calculated using Tauc's plot. The crystallinity and phase composition of the materials were determined by scanning for 15 min in the range of 20–90 2θ degrees on a Bruker AXS D8 Advance X-ray Diffractometer (XRD) (Germany). Scanning electron microscopy (SEM), coupled with Image J, was used to determine the particle size distribution and morphology of the samples. The electron-dispersive X-ray (EDX) analysis was used for elemental identification and chemical composition of the nanocomposites. Pore sizes, specific surface area, and pore volume were measured at 196 ◦C using the Bruner–Emmet–Teller (BET) surface analyzer, with which the sample was heated to 150 ◦C for 2 h using N2 gas flow. A Perkin Elmer Pyris Thermogravimetric Analyzer (TGA) was

used to test the thermal stability, with a thermal analyzer, SDT Q600, heating 10 mg of the sample to 900 ◦C at a rate of 10 ◦C/min.

#### *3.7. Electrochemical Experiment*

Cyclic voltammetry (CV), linear swept voltammetry (LSV), and electrochemical impedance spectroscopy (EIS) techniques were evaluated to characterize the electrochemical properties of the samples, using an Autolab electrochemical workstation with three electrodes electrochemical cell, namely the modified glassy carbon electrode as a working electrode, Ag/AgCl as a reference and platinum wire as a counter electrode in 0.5 M H2SO4 as an electrolyte. Glassy carbon was modified with 3 different samples, namely CuO-TiO2 (30/70), CuO-TiO2 (50/50) CuO-TiO2 (70/30). The samples were renamed as CuO-TiO2 (30/70): sample 1 (S1), CuO-TiO2 (50/50) sample 2, and (S2) CuO-TiO2 (70/30): sample 3 (S3). The voltametric tests were recorded in the potential range of −0.6 V to 0.6 V at a different scan rate from 2 to 50 mV/s. Before the modification of the glassy carbon electrode, the electrode was polished with 1, 0.5, and 0.03 μm polishing powder and then rinsed with ethanol and distilled water after each polish.

#### *3.8. Preparation of the Modified Electrode*

An amount of 3 mg of different powders (S1–S3) was dissolved in 1000 μL of ethanol, and 3 μL of 5% Nafion solution was added; thereafter, the mixtures were ultra-sonicated to make a slurry. Then 3 μL of the solution was drop-coated on the surface area of the glassy carbon electrode and allowed to dry using an oven at 35 ◦C for 15 min. The electrochemical impedance spectroscopy was recorded over the frequency range of 0.1–100 kHz. Before each electrochemical measurement, the solution was purged using Ar gas for deoxygenation.

#### *3.9. Photocatalytic Degradation of Dyes and Antibiotics*

The 30 mg of CuO-TiO2 powder was added in 300 mL of 20 ppm methylene blue (MB), then stirred in a photochemical reactor in the absence of light for 30 min to reach the adsorption-desorption equilibrium. Then, the mixture was exposed to UV light (450 W) for 2 h. Upon exposure to the light, at 30 min intervals, 3 mL of the samples were collected for analysis using the UV-vis spectrophotometer. The ʢmax of 668 nm for MB was used to monitor the degradation over time. The efficiency of the degradation was calculated by the following equation:

$$\% \text{ Depradation } = \left(\frac{\text{Ao} - \text{Af}}{\text{Ao}}\right) \times 100\% \tag{3}$$

where:

Ao = is the initial absorbance at 0 min (adsorption-desorption).

Af = is the final absorbance of 30, 60, 90, and 120 min of photodegradation

Similar steps were followed for the degradation of the antibiotic CIP and SSX, except upon analysis, ʢmax values of 275 and 290 were used, and the concentration was 10 ppm.

#### *3.10. Recyclability Studies of SSX*

To determine both the stability and recyclability of the CuO-TiO2 nanocomposites, then 4 cycle experiments were conducted following similar conditions for normal photocatalytic degradation experiments. After every cycle, the photocatalyst was thoroughly washed with distilled water and recovered by membrane filtration. Then, it was dried at 60 ◦C to obtain a powder-like material for the next cycle.

#### *3.11. Effect of Scavengers on the Degradation of SSX*

To determine the effect of scavengers on the degradation of the SSX, holes (h+), hydroxyl radicals (OH·), and superoxide (O2 −) were investigated during the photodegradation process. A measure of 2 mL (5 mmol) of EDTA, isopropyl alcohol, and p-benzoquinone

was added, respectively, for each individual experiment. The photodegradation of similar conditions for normal photocatalytic degradation experiments was followed.

#### **4. Conclusions**

CuO-TiO2 p-n heterostructured photocatalysts were prepared in this study and tested for the degradation of MB, CIP, and SSX pollutants under UV light irradiations. This research was motivated by the need for highly efficient photocatalysts for the degradation of pollutants in wastewater. The materials were found to be spherical in shape with varied particle sizes, thermally stable, and electrochemically active, which could potentially assist in the photodegradation of the organic pollutants. These materials also displayed high degradation potential against antibiotics CIP and SSX when exposed to UV light. The degradation efficiency of CIP and SSX were recorded to be 67.3% and 93.64%, respectively. Furthermore, it was shown that the heterostructured photocatalysts could not be reused. Additionally, the species that is most active in the photodegradation of SSX was identified as OH•. These heterojunctions were shown to be selective toward antibiotic degradation as compared to dyes.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/catal13010163/s1, Figure S1. (a) EDS of *C. Benhghanlensis* TiO2; (b) EDS of *C. Benhghanlensis* CuO NPs; (c) EDS of *C. Benhghanlensis* CuOTiO2(30/70); (d) EDS of *C. Benhghanlensis* CuOTiO2(50/50); € EDS of *C. Benhghanlensis* CuOTiO2(70/30); Figure S2: SSX photodegradation curves by (a) 30/70 CuO-TiO2 (b) 50/50 CuO-TiO2 (c) 70/30 CuO-TiO2 (d) TiO2 and (e) CuO nanomaterials; Figure S3: CIP photodegradation curves by (a) 30/70 CuO-TiO2 (b) 50/50 CuO-TiO2 (c) 70/30 CuO-TiO2 (d) TiO2 and (e) CuO nanomaterials.

**Author Contributions:** Conceptualization, N.H-M. and D.E.M.; methodology, D.A.B., S.M., N.M.; formal analysis, D.A.B., S.M., N.M.; investigation, D.A.B., N.M.; resources, N.C.H.-M. and D.E.M.; writing—original draft preparation, D.A.B., N.M.; writing—review and editing N.C.H.-M.; supervision, N.C.H.-M. and D.E.M.; funding acquisition, N.C.H.-M. and D.E.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** The National Research Foundation and the DSI NRF Centre of Excellence in Strong Materials (grant no. R808-CoE Mbita) provided funding for this work and are greatly acknowledged.

**Data Availability Statement:** The data can be made available upon request from the authors.

**Conflicts of Interest:** The authors affirm that they have no known financial or interpersonal conflicts that would have appeared to have an impact on the research presented in this study.

#### **References**

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## *Article* **Effect of Pd-Doping Concentrations on the Photocatalytic, Photoelectrochemical, and Photoantibacterial Properties of CeO2**

**Shaidatul Najihah Matussin 1, Fazlurrahman Khan 2,3, Mohammad Hilni Harunsani 1, Young-Mog Kim 2,3,4 and Mohammad Mansoob Khan 1,\***

**Abstract:** Cerium oxide (CeO2) can exhibit good photocatalytic and photoantibacterial activities. However, its light-harvesting property is rather limited due to its large band gap. In order to boost these properties, doping with metal ions can improve light absorption and charge mobility. In this report, CeO2 and palladium−doped CeO2 (Pd−CeO2) NPs were synthesized via the microwaveassisted synthesis method. The structural, optical, and morphological studies of CeO2 and Pd−CeO2 NPs were carried out using various techniques. Mixed phases of CeO2/Ce2O3 were observed in pure CeO2 (S−CeO2) and Pd−CeO2. However, the Ce2O3 phase gradually disappeared upon doping with a higher percentage of Pd. Almost spherical particles were observed with average sizes between 6 and 13 nm. It was found that the incorporation of Pd reduced the particle size. Moreover, band gap energies of S−CeO2 and Pd−CeO2 NPs were reduced from 2.56 to 2.27 eV, and the PL intensities were also quenched with more Pd doping. The shifts in the conduction band and valence band were found to cause the reduction in the band gap energies of S−CeO2 and Pd−CeO2 NPs. In the case of photocatalytic degradation of methylene blue, photoelectrochemical, and photoantibacterial activities, Pd−CeO2 NPs showed enhanced activities under visible light irradiation. Therefore, Pd−CeO2 NPs have been shown to be a visible-light active material.

**Keywords:** cerium oxide; CeO2; Pd−doped CeO2; noble metal; oxygen vacancies; MB degradation; antibacterial studies

#### **1. Introduction**

Antibiotics have been used to treat bacterial infections that are major causes of chronic infections and mortality. However, studies found that the widespread use of antibiotics has led to the emergence of multidrug-resistant bacterial strains [1]. In general, the major groups of antibiotics that are currently in use have three bacterial targets, namely: cell wall synthesis, translational machinery, and DNA replication machinery [2]. However, bacterial resistance can develop at each mode of action. Nanoparticles (NPs) have demonstrated broad-spectrum antibacterial properties against Gram-positive and Gram-negative bacteria. Studies have revealed that NPs do not show antibiotic resistance mechanisms because NPs contact directly with the bacterial cell wall, which would be less prone to promoting resistance in bacteria. Therefore, NPs can be potentially used as an alternative antibiotic. According to research, the major processes underlying the antibacterial effects of NPs are disruption of the bacterial cell membrane, generation of ROS, penetration of bacterial cell

**Citation:** Matussin, S.N.; Khan, F.; Harunsani, M.H.; Kim, Y.-M.; Khan, M.M. Effect of Pd-Doping Concentrations on the Photocatalytic, Photoelectrochemical, and Photoantibacterial Properties of CeO2. *Catalysts* **2023**, *13*, 96. https://doi.org/10.3390/ catal13010096

Academic Editors: Jorge Bedia and Carolina Belver

Received: 1 December 2022 Revised: 22 December 2022 Accepted: 25 December 2022 Published: 3 January 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

membrane, and induction of intracellular antibacterial effects, including interactions with DNA and proteins [2,3].

Organic dyes, which generally consist of non-biodegradable, highly poisonous, and colored pigments, have been reported to be present and widely spread in industrial wastewater originating from the paper, textile, and apparel industries [4,5]. Even at low concentrations, dyes can be clearly seen and pollute various aquatic environments [6]. These dye-polluted effluents are harmful to living organisms [7]. Dyes such as methylene blue (MB) can also cause serious threats to humans, for example, abdominal disorders, respiratory problems, and digestive and mental disorders [8,9]. Hence, the removal of dyes from wastewater is important. Apart from that, redox reactions have shown promising water treatment activity as the reaction products can be conveniently separated and removed [10]. Owing to this, researchers have applied electrochemical systems to diverse functional applications, including photovoltaics [11], fuel cells [12], supercapacitors [13], as well as sensors [14]. It is reported that heterostructure exhibits interesting electrochemical properties owing to the possibility of a dual charge storage contribution from both materials [15]. Nevertheless, materials with high porosity and large surface areas are highly suitable for electrochemical applications [16].

Metal oxides such as TiO2 [17], ZnO [18], Fe2O3 [19], CeO2 [20], etc., have shown remarkable activities in biological and photocatalytic activities. Among these, CeO2 has shown extensive industrial applications in medicine, catalysis, solid oxide fuel cells, luminescence, optical, and sensor technologies [21–25]. Despite being a highly abundant material, its redox ability to change between Ce3+ - Ce4+ has also attracted significant attention. CeO2 has been synthesized using hydrothermal, sol-gel, precipitation, microwave, and green synthesis methods where multiple morphologies such as cube, rod, spherical, and flower-like particles have been produced [26–30]. In catalytic applications, the activity of CeO2 primarily arises from the reduction of Ce4+ to Ce3+ and the formation of oxygen vacancies [31,32]. The modification in shape and size results in the formation of surface defects such as Vo, which endows it with the ability to exist in either the Ce3+ or Ce4+ state on the surface of the particle [33,34].

In addition, dopants with different ionic states incorporated into CeO2 can generate more structural defects to gain charge neutrality, enhancing physical properties and biocompatibility. Many studies have emphasized a strong synergy between CeO2 and noble metals, which can significantly enhance catalytic activities [35]. Among the noble metals, palladium is increasingly used in electrical equipment, dental materials, implanted medical devices and automobiles as catalysts [36]. The incorporation of Pd2+ into the CeO2 crystal lattice has shown better catalytic activity than the palladium-supported CeO2 catalyst [37]. It is believed that the unique properties of Pd-doped CeO2 depend on the active components of palladium and the interaction between palladium and CeO2 [38].

Therefore, in this study, CeO2 (S−CeO2) and Pd−doped CeO2 (Pd−CeO2) NPs were synthesized using a microwave-assisted synthesis method. To the best of the authors' knowledge, a study on Pd−CeO2 with mixed phases of CeO2 and Ce2O3 has not yet been explored. Moreover, studies on the photocatalytic degradation of methylene blue (MB) were conducted using synthesized-CeO2 (S−CeO2) and (0.5, 1, 3 and 5%) Pd−CeO2 under visible light irradiation for 5 h. The photoelectrochemical studies, namely, linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS), were performed under dark and visible light conditions using NaCl as the electrolyte. Moreover, the photoantibacterial/antibacterial properties of S−CeO2 and (0.5, 1, 3 and 5%) Pd−CeO2 NPs were also carried out for 24 h using bacterial strain *Staphylococcus aureus* (*S. aureus*) KCTC 1916 (Gram-positive) and *Pseudomonas aeruginosa* (*P. aeruginosa*) KCTC 1637 (Gram-negative).

#### **2. Results and Discussion**

#### *2.1. Structural Analysis of CeO2 and Pd*−*CeO2 NPs*

XRD provides a broad range of information related to the crystallographic nature and chemical structure of materials. The XRD patterns of the S−CeO2 and Pd−CeO2 NPs are shown in Figure 1a, with diffraction peaks in the range of 20◦ to 80◦. The peak positions at about 2θ = 28.56◦, 33.27◦, 47.36◦, 56.40◦, and 59.10◦ of each sample correspond to the (111), (200), (220), (311), and (222) planes of CeO2 cubic fluorite phase (JCPDS no. 00-004-0593) (Figure S1). In S−CeO2, the diffraction peak of Ce2O3 was observed, suggesting that mixedphase CeO2/Ce2O3 (JCPDS no. 00-023-1048) was present. It was found that gradually, the peak corresponding to Ce2O3 decreased with higher Pd doping. Hence, the peak intensity at 28.56◦ was seen to increase. This suggests that Pd doping inhibited the formation of Ce2O3 and suggested that the incorporation of Pd2+ lowered the appearance of the Ce2O3 phase. The ionic radii of Pd2+ and Ce4+ are 86 and 101 pm, respectively. No other peaks were observed that contribute to the presence of PdO. Therefore, it can be concluded that Pd2+ was successfully incorporated into the CeO2 lattice.

**Figure 1.** (**a**) XRD patterns and (**b**) FTIR-spectra of S−CeO2 and (0.5, 1, 3, and 5%) Pd−CeO2 NPs.

Furthermore, the average crystallite sizes were estimated using Debye-Scherrer's Formula (1):

$$D = \frac{0.9\lambda}{\beta \cos \theta} \tag{1}$$

where *β* is the full width at half maximum (FWHM), in radians, of the peak for a given (*hkl*) value, *λ* = 1.5406 Å for the CuKα radiation used, and *θ* is the diffracting angle. The average crystallite sizes were found to be 34.67, 37.25, 16.25, 23.03, and 19.17 nm for S−CeO2, 0.5%, 1%, 3%, and 5% Pd−CeO2 NPs, respectively (Table 1).

**Table 1.** The average crystallite size (D), lattice parameter, cell volume, and average lattice strain of S−CeO2 and Pd−CeO2 NPs.

The average lattice strain was also calculated using Equation (2):

$$
\varepsilon = \frac{\beta\_{hkl}}{4tan\theta} \tag{2}
$$

The lattice parameters of S−CeO2 and Pd−CeO2 were observed to be comparable with the bulk CeO2 (5.411 Å). The average lattice strain was decreased significantly when 0.5% and 1% Pd were incorporated. However, as more Pd2+ were incorporated into the lattice, it would cause higher lattice strain. This can also be seen that as more Pd was doped, the cell volume increased slightly.

#### *2.2. Fourier Transform-Infrared Spectroscopy of CeO2 and Pd*−*CeO2 NPs*

FT-IR spectroscopic studies of S−CeO2 and Pd−CeO2 NPs were carried out within the range of 450–4000 cm−1 at room temperature (Figure 1b). The broad band at approximately 3400 cm−1 for all samples is attributed to the stretching mode of absorbed O-H in the samples. A broad, intense peak at 450 cm−1 corresponds to the O-Ce-O stretching mode. At approximately 900 cm−1, a broad peak was observed in S−CeO2 and Pd−CeO2 NPs, which corresponds to the bending of the intercalated Ce-O [39]. Moreover, a peak at about 1730 cm−1 was also observed in all samples, and it could be assigned to the H-O-H bending mode.

#### *2.3. X-ray Photoelectron Spectroscopy*

XPS was performed at room temperature to investigate the chemical state and the electronic structure of the elements in S−CeO2 and 0.5 and 5% Pd−CeO2 NPs (Figure 2). Figure 2a shows the complete survey scan spectra of the samples, which confirmed the presence of Pd 3*d*,O1*s*, and Ce 3*d*. Figure 2b shows the six typical peaks for Ce 3*d*. The peaks positioned at 883.43, 886.80, 896.26, 898.89, 905.59, and 914.75 eV is characteristic of Ce4+ [40]. Peaks at 898.89, 905.59, and 914.75 eV correspond to 3*d*3/2, whereas peaks at 883.43, 886.80, and 896.26 eV correspond to 3*d*5/2 [41]. No obvious shift in the binding energy of Ce 3*d* spectra was observed.

Figure 2c shows the Pd 3*d* spectra of 0.5% Pd−CeO2 and 5% Pd−CeO2 NPs. As a result of the spin-orbit coupling, the Pd 3*d* spectra are split into two peaks of 3*d*5/2 and 3*d*3/2 at 335.03:335.03 eV and 340.40:340.31 eV, respectively. The XPS spectrum of O 1*s* can be seen in Figure 2d, in which all peaks exhibit two asymmetrical peaks, indicating the presence of O2−, OH−, and O− at the surface of the nanostructures. The peak at higher binding energy, i.e., 531–532 eV, is attributed to O2− vacancies and adsorbed -OH or H2O, while the peak at lower binding energy, 529 eV, is attributed to the metal-oxygen binding [42]. S−CeO2 showed a dominant peak at 531–532 eV, suggesting that O2− vacancies or adsorbed -OH were mainly found in the sample. On the other hand, 0.5 and 5% Pd−CeO2 showed a dominant peak at 529 eV [42]. The difference might be due to the formation of the Ce2O3 phase in S−CeO2 and the gradual decrease of the Ce2O3 phase in both 0.5 and 5% Pd−CeO2, as shown in XRD. The typical C 1*s* were observed in the spectra (Figure 2e), which were derived from the carbon coating used in the analysis. The atomic concentration of C 1*s*,O1*s*, Ce 3*d*, and Pd 2*p* can be found in Table 2. The estimated Pd content in both 0.5% and 5% Pd−CeO2 is lower than the expected value.

**Figure 2.** XPS spectra of S−CeO2, 0.5% Pd−CeO2, and 5% Pd−CeO2: (**a**) Survey scan, (**b**) Ce 3*d*, (**c**) Pd 3*d*, (**d**)O1*s*, and (**e**)C1*s*.

**Table 2.** Atomic concentration of C 1*s*,O1*s*, Ce 3*d*, and Pd 3*d* of S−CeO2, 0.5% Pd−CeO2, and 5% Pd−CeO2.

The band gap reduction of S−CeO2 and Pd−CeO2 NPs might be due to the development of mid-gap states either above the valence band (VB) or below the conduction band (CB). Therefore, to study the cause of the reduction of band gap energy, VB-XPS was carried out for S−CeO2 and Pd−CeO2 NPs (Figure 3a).

**Figure 3.** (**a**) Valence band XPS spectra of S−CeO2, 0.5% Pd−CeO2, and 5% Pd−CeO2 NP; (**b**) zoomed valence band spectra of (**i**) S−CeO2, (**ii**) 0.5% Pd−CeO2, and (**iii**) 5% Pd−CeO2 to estimate the band gap; and (**c**) proposed density of electronic states (DOS) for (**i**) S−CeO2, (**ii**) 0.5% Pd−CeO2, and (**iii**) 5% Pd−CeO2 NPs.

The zoomed area of VB-XPS is shown in Figure 3b. The VB maximum of S−CeO2, 0.5% Pd−CeO2, and 5% Pd−CeO2 NPs were found to be at 0.39, 0.16, and −1.63 eV, respectively (Figure 3b(i–iii)). The optical band gap energy obtained from the Tauc plot was 2.56, 2.50, and 2.27 eV for S−CeO2, 0.5% Pd−CeO2, and 5% Pd−CeO2 NPs, respectively. Consequently, based on the materials' optical band gap and VB maxima, the CB minima would follow at −2.17, −2.34, and −3.90 eV for S−CeO2, 0.5% Pd−CeO2, and 5% Pd−CeO2 NPs, respectively. The shifts of VB and CB result in the band gap energy reduction, as proposed in Figure 3c.

*2.4. Optical Studies of CeO2 and Pd*−*CeO2 Using UV-vis-DRS and Photoluminescence Spectroscopy*

UV-Vis DRS analysis is used to estimate the band gap energies of S−CeO2 and Pd−CeO2 NPs. The Tauc plot derived from the Kubelka–Munk function is shown in Figure 4a. The band gap energies of all samples were estimated using the Kubelka–Munk Equation (3):

$$\mathcal{F}(R) = \left(\frac{(1-R)^2}{2R} \times h\nu\right)^{\frac{1}{2}}\tag{3}$$

where *R* is the measured absolute reflectance of the samples. The band gap was obtained from the plots of [F(*R*)*hv*] 1/2 versus *hv*, as the intercept of the extrapolated linear part of the plot at [F(*R*)*hv*] 1/2 = 0, assuming that the absorption coefficient (α) is proportional to the Kubelka-Munk function F(*R*).

**Figure 4.** (**a**) Tauc plot derived from Kubelka-Munk function for band gap energy estimation, and (**b**) photoluminescence spectra of C−CeO2, S−CeO2, and Pd−CeO2 NPs.

The band gap energy of C−CeO2, S−CeO2, 0.5% Pd−CeO2, 1% Pd−CeO2, 3% Pd−CeO2, and 5% Pd−CeO2 NPs were found to be decreasing (3.10 to 2.27 eV) as shown in Table 3. This shows that Pd doping is proven to decrease the band gap energy of the CeO2. Pd doping is reported to help in reducing the energy difference between a CB and VB of CeO2, which enhances the electronic conductivity of CeO2. Moreover, oxygen vacancies (Vo) might be produced at the interface between the grains of Pd−CeO2 NPs. Furthermore, the absorption edge of C−CeO2, S−CeO2, 0.5% Pd−CeO2, 1% Pd−CeO2, 3% Pd−CeO2, and 5% Pd−CeO2 NPs were greatly shifted into the visible region with the Pd doping as shown in Figure 4a.

**Table 3.** Band gap energy of C−CeO2, S−CeO2, and Pd−CeO2 NPs.

PL is used to evaluate the optical study of the crystals, defects on the surface and excitation fine structure of the semi-conducting materials. PL intensity describes the lifetime of electron relaxation from the VB to the CB, as well as the separation efficiency of photogenerated e−/h+ [43]. From Figure 4b, the PL intensity was decreased with more Pd doping concentration. This indicates that Pd doping can efficiently inhibit the e−/h+ recombination process due to the electron or hole trap levels (in this case, Pd2+/Pd3+ or Pd+/Pd2+) in the band structure of CeO2. Moreover, the reduction in peak intensity might be due to the formation of more oxygen defects, increased surface defects, as well as maximum separation of charge carriers [44].

#### *2.5. Transmission Electron Microscopy Analysis of CeO2 and Pd*−*CeO2 NPs*

Figure 5 exhibits the TEM, HR-TEM, and SAED images of S−CeO2, 0.5%, and 5% Pd−CeO2 NPs. Both S−CeO2 and Pd−CeO2 (Figure 5a,d and g) have almost spherical morphology. At the same time, a rod-like structure was observed for S−CeO2 NPs only. The average particle sizes of S−CeO2, 0.5% Pd−CeO2, and 5% Pd−CeO2 are about 13, 9, and 6 nm, respectively. This shows that the incorporation of Pd into the lattice of CeO2 decreased the particle size. The *d*-spacing value of the lattice planes was also determined from the HR-TEM images (Figure 5b,e,h). The *d*-spacing values for S−CeO2, 0.5% Pd−CeO2, and 5% Pd−CeO2 NPs are estimated to be around 0.3, 0.2 and 0.1 nm, which corresponds to the (111), (200), and (220) planes of a fluorite-structure of cubic CeO2 [45]. This is in accordance with the XRD analysis (Section 2.1).

Moreover, the nano-crystallinity of S−CeO2 and Pd−CeO2 NPs were examined by selected area electron diffraction (SAED) analysis, as shown in Figure 5c,f and i. It was observed that S−CeO2, 0.5%, and 5% Pd−CeO2 exhibit four prominent broad rings, which can be attributed to the (111), (200), (220), and (311) reflections of the fluorite cubic CeO2 structure. This observation is supported by the XRD patterns reported earlier (Section 2.1).

The EDX mapping images shown in Figure S2a–c confirmed the presence of Ce, O, and Pd in S−CeO2, 0.5%, and 5% Pd−CeO2 NPs, respectively. The percentage of Ce was seen to decrease with more Pd content. The percentage mass of Ce, O, and Pd can be found in Table S1. This shows that Pd has been successfully incorporated into the CeO2 lattice.

**Figure 5.** TEM, HR-TEM, and SAED patterns of (**a**–**c**) S−CeO2, (**d**–**f**) 0.5% Pd−CeO2, and (**g**–**i**) 5% Pd−CeO2 NPs.

#### **3. Applications**

#### *3.1. Photocatalytic Degradation of MB*

The photocatalytic degradation of MB using S−CeO2 and Pd−CeO2 NPs was carried out under visible light irradiation in a total experiment duration of 5 h. The activity of C−CeO2 against MB dye under visible light was also investigated to compare the photocatalytic efficiency with the synthesized materials. Figure 6a shows the average C/C0 of the photocatalytic degradation of MB using C−CeO2, S−CeO2, and Pd−CeO2 NPs, while the average percentage of the photocatalytic degradation of MB and the absorption spectra of MB within 5 h can be seen in Figures S3 and S4, respectively. Based on the study, adsorption-desorption equilibrium was reached in 30 min, in which C−CeO2 showed a slight adsorption capacity. About 39–61% adsorption was reached for the synthesized materials suggesting that the addition of Pd effectively increases the affinity of MB molecules towards photocatalysts [46]. The enhanced adsorption is also driven by the increase in the surface area contributed by significantly reduced particle size [46].

**Figure 6.** (**a**) The average C/C0 of photocatalytic degradation of MB using C−CeO2, S−CeO2, and Pd−CeO2 NPs under visible light irradiation, (**b**) LSV, and (**c**) EIS Nyquist plots of S−CeO2, 0.5%, 1%, 3%, and 5% Pd−CeO2 photoelectrode in the dark and under visible light irradiation.

The effectiveness of the photocatalysts in the photocatalytic degradation of MB was seen as follows: 1% Pd−CeO2 > 5% Pd−CeO2 > S−CeO2 > 0.5% Pd−CeO2 > 3% Pd−CeO2 > C−CeO2. A small response from C−CeO2 might be due to the larger band gap energy, which was found to be 3.10 eV. Moreover, 3% Pd−CeO2 and 0.5% Pd−CeO2 also showed lower responses despite their lower band gap energies. This might be due to variations in particle size, which resulted in lower responses in the photocatalytic activity. In addition to that, a smaller surface area might also lead to less active sites present on the surface of the photocatalysts [47]. S−CeO2 and 5% Pd−CeO2 showed almost similar activities. The activity of S−CeO2 was higher than some of the materials, which might be due to the mixed CeO2/Ce2O3 phases, as shown in Figure 1a. The presence of both phases can ease the redox reaction between Ce3+ and Ce4+, which causes the photocatalytic activities to increase [48–50]. Choudhury et al. also stated that an electron from Ce3+ can be transferred to adsorbed oxygen to form reactive oxygen species (ROS), i.e., superoxide radicals (O2 •−), whereas holes interact with water to form hydroxyl radicals (•OH) [51]. In addition, the e− in the CB can be trapped by oxygen vacancies and then react with O2 to form O2 •−. These radicals are involved in the degradation of MB into harmless products [52]. Recombination of photogenerated e− and h+ was restrained through the Ce3+/Ce4+ redox and oxygen vacancies which finally improved the photocatalytic performance of the Pd−CeO2 materials [53]. Moreover, smaller particle size as well as low band gap energy also effectively increased the photocatalytic activity [54]. Nevertheless, 5% Pd−CeO2 showed a slightly better performance than S−CeO2 due to its smaller particle size and band gap energy. Accordingly, 1% Pd−CeO2 showed the highest response, which might be due to the optimum Pd loading. This shows that a further increase in doping does not enhance the photocatalytic degradation of MB [46]. Table 4 summarizes the average percentage of photocatalytic degradation of MB activities using S−CeO2 and Pd−CeO2 NPs under visible light irradiation.

**Table 4.** The average percentage MB dye degradation using C−CeO2, S−CeO2, and Pd−CeO2 under visible light irradiation.

### *3.2. Photoelectrochemical Studies of Pd*−*CeO2 NPs*

The electrochemical and photoelectrochemical studies of S−CeO2 and Pd−CeO2 NPs were analyzed using LSV and EIS. Figure 6b shows the LSV analysis carried out in the dark and under visible light at 100 mV/s in the range of −0.7–1.5 V. S−CeO2 showed the lowest photocurrent, and 5% Pd−CeO2 showed the highest photocurrent. As the doping percentage increases (Figure 6b), the photocurrent response also increases. Each material, respectively, showed a higher response under visible light irradiation as compared to their response in the dark, suggesting that the materials were light-responsive. The improvement in the photocurrent response in Pd−CeO2 was attributed to the light absorption ability due to the creation of a mid-gap state which lowered the band gap energy [55]. The band gap energy of Pd−CeO2 was narrowed with higher Pd doping, in which the valence electrons can be excited to the conduction band state by absorbing light. It is stated that the e−/h+ pairs will recombine unless they are separated quickly [56]. In general, a high photocurrent suggests that the material has a strong ability for the generation and transfer of the photoexcited charge carrier under light irradiation [57]. Therefore, in this case, 5% Pd−CeO2 exhibits better electron-hole separation, and it might be excited easily by visible light.

EIS was performed in the dark and under visible light irradiation at 0.0 V with a frequency ranging from 1–106 Hz (Figure 6c). The EIS Nyquist plots show the charge transfer resistance and separation efficiency between the photogenerated electrons and holes [58]. In general, a small arc radius and low resistance indicate higher charge transfer efficiency [55]. S−CeO2 showed the highest resistance, followed by 0.5% and 1% Pd−CeO2 indicating the slow interfacial charge-transfer process. This response might be due to their high band gap energies. In each case, there was no significant decrease in the resistance under visible light, which suggests that the transfer efficiency of the photogenerated electrons and holes was not accelerated under light irradiation. On the other hand, 3% Pd−CeO2 and 5% Pd−CeO2 showed significant responses, especially under visible light irradiation. This suggests that lower resistances derived from both 3% and 5% Pd−CeO2 show a faster interfacial charge-transfer process. Other factors can also facilitate charge separation and transfer efficiency, such as the presence of Ce3+ and oxygen vacancies [55].

### *3.3. Antibacterial Activities of Pd*−*CeO2 NPs*

The effect of bactericidal activities of S−CeO2 NPs and Pd−CeO2 NPs was examined against one Gram-positive bacteria (i.e., *S. aureus*) and another against Gram−negative pathogenic bacteria (i.e., *P. aeruginosa*). Since the synthesized NPs are light sensitive, the antibacterial activities were performed in the absence and presence of light. The

antibacterial effect of Pd−CeO2 and S−CeO2 NPs under light and dark conditions is shown in Figure 7. The results showed that both S−CeO2 NPs and Pd−CeO2NPs exhibit concentration-dependent inhibitions of *S. aureus* growth under light or dark conditions (Figure 7a,b). At a high concentration of 2048 μg/mL in the dark, the growth inhibition of *S. aureus* cells was found to be higher (reduction of 1.3 log CFU) in the presence of 5% Pd−CeO2 NPs as compared to the CeO2 NPs (reduction of 1.2 log CFU) (Figure 7a). Similarly, in the presence of light at 2048 μg/mL, the antibacterial effect of 5% Pd−CeO2 NPs was also found to be higher (reduction of 1.6 log CFU) as compared to the S−CeO2 NPs (reductions 1.0 log CFU) (Figure 7b). The bactericidal effect of CeO2 NPs was found to be higher in the presence of light as compared to dark conditions. This finding suggests that light enhances the bactericidal effect of CeO2 NPs towards Gram-positive bacteria *S. aureus*.

**Figure 7.** Log reduction of the *S. aureus* treated with different concentrations of S−CeO2 NPs and Pd−CeO2 NPs incubated under (**a**) dark and (**b**) light conditions. Log reduction of the *P. aeruginosa* treated with different concentrations of S−CeO2 NPs and Pd−CeO2 NPs incubated under (**c**) dark conditions and (**d**) light conditions, where \*\*\*, \*\*, and \* imply significance at *p* < 0.0001, *p* < 0.01, and *p* < 0.05, respectively, while ns means non-significant.

The antibacterial effect of S−CeO2 NPs and Pd−CeO2 NPs on *P. aeruginosa* was found to be different as compared to *S. aureus* (Figure 7c,d). There was no concentration-dependent inhibition of *P. aeruginosa* cell growth in the presence of light and dark conditions. The growth inhibition at the higher concentration (2048 μg/mL) and dark conditions was found to be non-significant in the presence of 5% Pd−CeO2 NPs as compared to the S−CeO2 NPs (Figure 7c). The results in the presence of light were found to be significantly effective at all tested concentrations of S−CeO2 NPs and Pd−CeO2 NPs (Figure 7d).

In the presence of light and high concentration (2048 μg/mL), the 5% Pd−CeO2 NPs showed a significant reduction of *P. aeruginosa* cells (1.0 log CFU) compared to the S−CeO2 NPs (0.6 log CFU). Based on the findings of the present study, it is clear that light acts as a sensitizing agent, which results in the enhanced antibacterial activity of CeO2 NPs as compared to dark conditions. Previous research has shown that UV light irradiation causes the formation of reactive oxygen species (ROS) such as •OH, O2 •−, and singlet oxygen (O−) from metal-NPs [59–62]. This evidence can be explained in the current study by the possibility of ROS generation during the photocatalytic degradation of the MB experiment (Section 3.1). In addition, Pd-doping will also result in the enhanced antibacterial effect of CeO2 NPs, and the effect was increased with the increasing concentration of Pd. Overall, these effects were found to be more effective towards Gram-positive bacteria *S. aureus* than the Gram-negative pathogen *P. aeruginosa*.

In general, the antibacterial activity of metal oxides is associated with the release of metal ions from metal oxides. It is reported that metal ions are involved in the destruction of the bacterial cell and membrane [63]. This could lead to the possibility of metal oxides penetrating the cell. In the case of CeO2, CeO2 dissociates into Ce4+ ions (most of the time under irradiation of visible light) and interacts with the bacterial cell and penetrates the cell [64]. This might lead to changes in cell integrity, lactate dehydrogenase leakage and cell death. Moreover, the generation of ROS is also responsible for antibacterial activity [65]. When CeO2 is irradiated with light, e− are promoted to CB and reacted with molecular oxygen to form O2 •−. The photogenerated h+ reacts with H2O molecules on the surface of CeO2 to form •OH radicals. These two radicals are believed to be contributing significantly to the destruction of the bacterial cell membrane [60].

#### **4. Experimental**

#### *4.1. Chemicals, Bacterial Strain, Culture Media, and Growth Conditions*

Cerium(III) nitrate hexahydrate 98% (Ce(NO3)3·6H2O) and commercial CeO2 (C−CeO2) were purchased from Sigma-Aldrich whereas, palladium(II) chloride anhydrous (PdCl2) was obtained from Fluka. Water was purified using Aquatron (England) prior to use. For photocatalytic degradation of MB and photoelectrochemical studies, MB and NaCl were obtained from Merck. For electrode preparation, ethanol (95%), ethyl cellulose 48–49.5% (*w*/*w*) ethoxy basis, and alpha-terpineol were obtained from Daejung Chemicals and Metals Co., Ltd., Sigma-Aldrich and Merck, respectively. The bacterial strain *S. aureus* KCTC 1916 (Gram-positive) and *P. aeruginosa* KCTC 1637 (Gram-negative) were purchased from the Korean Collection for Type Cultures (KCTC, Daejeon, Korea). The bacteria were cultivated using Tryptic soy broth (TSB) and a TSB agar plate (Difco Laboratory Inc., Detroit, MI, USA). The growth temperature of the bacterial strain was 37 ◦C under aerobic conditions.

#### *4.2. Instrumentations*

CeO2 and (0.5, 1, 3, and 5%) Pd−doped CeO2 NPs were synthesized using a microwaveassisted method (Anton Paar Monowave 400, Graz, Austria). Fourier Transform-Infrared Spectroscopy (FT-IR) was used to identify the possible functional groups present in the synthesized CeO2 and Pd−CeO2 NPs using Shimadzu IRPrestige-21 FT-IR Spectrophotometer. The stretching frequencies obtained were plotted as %transmittance mode on the y-axis and wavenumber (cm−1) on the x-axis from 450 to 4000 cm−1. The morphology, structure, and elemental mapping were analyzed using field emission transmission electron microscopy (FE-TEM) and selected area electron diffraction (SAED) using JEM-F200 (JEOL Ltd., Tokyo, Japan). The determination of band gap energy of S−CeO2 and Pd−CeO2 NPs was investigated using UV-Vis diffuse reflectance spectroscopy (DRS) (Shimadzu, UV-2600, Tokyo, Japan). A photoluminescence (PL) study was carried out using F-7000 Fluorescence spectroscopy (Hitachi High Tech, Tokyo, Japan) with an excitation wavelength of 370 nm. X-ray photoelectron spectroscopy (XPS) and valence band (VB)-XPS were

performed (Kratos Analytical, AXIS Nova, Manchester, UK). Photocatalytic activities of MB dye degradation were carried out using a photochemical reactor Toption (TOPT-V) having a 300 W Xenon lamp, and the photocatalytic degradation of MB was monitored using UV–visible spectrophotometer (Shimadzu UV-1900, Japan). Photoelectrochemical studies were performed using Autolab (MetroHm, Herisau, Switzerland) under dark and visible light irradiation (Simon FL30 LED Floodlight, 100 W, Jiangsu, China). The measurements were carried out and taken from NOVA software. Photoantibacterial studies of S−CeO2 and Pd−CeO2 NPs were investigated using a 96-well microplate in the presence of the LED light (One & One Plus OP-0303 Nape Slim LED Stand).

#### *4.3. Microwave-Assisted Synthesis of Cerium Oxide Nanoparticles*

CeO2 NPs were synthesized using the microwave-assisted method. Briefly, 0.05 M of Ce(NO3)3·6H2O solution (15 mL) was prepared in a microwave vessel. Exactly 2.4 mL of 1 M NaOH was added dropwise into the solution. Subsequently, the microwave reaction was carried out at 180 ◦C for 15 min at 850 W microwave power. The precipitate formed was centrifuged and washed three times with water before it was dried at 80 ◦C. The product was stored and labeled as S−CeO2.

### *4.4. Microwave-Assisted Synthesis of Pd*−*CeO2 Nanoparticles*

Pd−CeO2 NPs were synthesized using the same method as mentioned above. A 15 mL of 0.05 M Ce(NO3)3 solution was prepared. A specific amount of PdCl2 was then added to prepare 0.5, 1, 3, and 5% Pd−CeO2. Subsequently, 1 M NaOH was added dropwise into the solution. The microwave reaction was carried out at 180 ◦C for 15 min at 850 W microwave power. The precipitate was formed, centrifuged, and washed three times with water before it was dried at 80 ◦C. The products were stored and labeled as 0.5% Pd−CeO2, 1% Pd−CeO2, 3% Pd−CeO2, and 5% Pd−CeO2 for further use.

#### *4.5. Electrode Preparation*

S−CeO2, 0.5%, 1%, 3%, and 5% Pd−CeO2 electrodes were prepared using the doctor blade method. In brief, 25 mg of the respective sample was mixed with 0.5 mL ethanol and 0.5 mL α-terpineol. The mixture was sonicated for 10 min. Then, 25 mg of ethyl cellulose was added to the mixture and stirred at 80 ◦C for ~2 h to produce a thick paste. The paste was then spread on fluorine-doped tin oxide (FTO) glass electrode by 2 cm × 1 cm using a doctor blade. The glass electrode was dried at 80 ◦C for 24 h prior to use.

#### *4.6. Photocatalytic Degradation of MB Dye*

The photocatalytic degradation of MB dye using S−CeO2 and Pd−CeO2 NPs under visible light irradiation was monitored using UV-Vis spectroscopy. Exactly 10 mg of S−CeO2 and 0.5, 1, 3, and 5% Pd−CeO2 NPs were dispersed in 50 mL of 10 ppm MB dye solution. The sample mixture was stirred in the dark for 30 min to reach adsorptiondesorption equilibrium. Then, the reaction tubes were irradiated with visible light (300 W) for 5 h in which the absorbance of the solution was measured every 1 h. The percentage of photocatalytic dye degradation of MB was obtained using Equation (4):

$$\% \text{ photocatalitytic MB} \text{ dye degradation} = \frac{\left(\text{A}\_{\text{blank}} - \text{A}\_{\text{sample}}\right)}{\text{A}\_{\text{blank}}} \times 100\tag{4}$$

where Ablank is the absorbance of MB only and Asample is the absorbance of MB dye after photocatalytic degradation by the photocatalyst.

#### *4.7. Photoelectrochemical Studies*

The photoelectrochemical response of S−CeO2 and Pd−CeO2 NPs was examined through linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) experiments. The experiments were carried out under ambient conditions in the dark and under visible light irradiation (LED, 100 W) in 100 mL of a 1 M NaCl aqueous electrolyte solution. The distance between the light and reactor vial was 22 cm. The prepared glass electrode, Ag/AgCl electrode, and Pt electrode were used as the working electrode, reference electrode, and counter electrode, respectively. For each electrode, LSV and EIS were performed in the dark and later under visible light irradiation at 100 mV/s in the potential range of −0.7–1.5 V and at 0.0 V with a frequency ranging from 1–106 Hz, respectively.

#### *4.8. Assays for Antibacterial Activities of Pd*−*CeO2 Nanoparticles*

A single colony from the agar plate of each bacterium was inoculated in the TSB and incubated for 12 h at 37 ◦C under shaking conditions (250 rpm). The seed culture was diluted into fresh TSB in order to achieve the final OD600 ~0.05. The diluted cell culture (300 μL) was placed in a 96-well microtiter plate and treated with different concentrations of S−CeO2 and 0.5, 1, 3, and 5% Pd−CeO2 NPs. The concentrations of these NPs ranged from 265 to 2048 μg/mL. Two sets of microtiter plates were prepared to contain different concentrations of NPs. One set of microplates was incubated in the dark for 24 h at 37 ◦C. The second set of the microplates was incubated in the presence of LED light that was set at the height of 27 cm for 24 h at 37 ◦C. After incubation, the cell culture (100 μL) was transferred into another 96-well microplate containing 200 μL sterile TSB. A two-fold serial dilution (up to 10−8 dilutions) of the cell culture was carried out. The serially diluted cell culture (100 μL) was spread-plated on the TSA plates. The TSA plates were incubated at 37 ◦C for 24 h, followed by the counting of the total colonies. The log colony forming unit (CFU) values of treated and untreated cells were calculated. All the experiments were carried out in triplicate.

#### **5. Conclusions**

S−CeO2 and 0.5, 1, 3, and 5% Pd−CeO2 NPs were successfully synthesized using the microwave-assisted method. Mixed phases of CeO2/Ce2O3 were obtained in S−CeO2 and 0.5, 1, and 3% Pd−CeO2. However, there were no mixed phases observed in 5% Pd−CeO2 NPs. Average crystallite sizes were found to be between 16.25–37.25 nm. Based on the TEM images, irregularly spherical shaped S−CeO2 and Pd−CeO2 NPs were observed with an average particle size between 6 and 13 nm. The band gap energy of S−CeO2 decreased with the addition of higher Pd content, in which the band gap narrowing phenomena was illustrated through the DOS scheme. One percent of Pd−CeO2 NPs showed enhanced responses under visible light irradiation in photocatalytic degradation of MB. Meanwhile, 5% Pd−CeO2 showed enhanced responses in photoelectrochemical and photoantibacterial activities. Therefore, Pd−CeO2 has shown visible light active properties and can be potentially used in photocatalysis and photoantibacterial applications.

**Supplementary Materials:** The following supporting information can be downloaded at: https://www. mdpi.com/article/10.3390/catal13010096/s1. Figure S1. Standards XRD patterns of CeO2, Ce2O3, and PdO; Figure S2. EDX-mapping of Ce, O, and Pd elements in (a) S−CeO2, (b) 0.5% Pd−CeO2, and (c) 5% Pd−CeO2 NPs; Figure S3. The average percentage of photocatalytic degradation of MB dye using C−CeO2, S−CeO2, and Pd−CeO2 NPs under visible light irradiation; Figure S4. Absorbance spectra of photocatalytic degradation of MB using (a) C−CeO2, (b) S−CeO2 (c) 0.5%, (d) 1%, (e) 3%, and (f) 5% Pd−CeO2 NPs; Table S1. Percentage mass of Ce, O, and Pd elements in S−CeO2, 0.5% Pd−CeO2, and 5% Pd−CeO2 NPs using EDX-Mapping.

**Author Contributions:** S.N.M.: Methodology, Investigation, Data curation, and Writing—original draft. F.K.: Methodology, Investigation, Data curation, and Writing. M.H.H.: Supervision, Writing—review and editing. Y.-M.K.: Resources, Formal analysis. M.M.K.: Supervision, Conceptualization, Funding acquisition, Writing—review and editing. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors would like to acknowledge the FRC grant (UBD/RSCH/1.4/FICBF(b)/2022/046) received from Universiti Brunei Darussalam, Brunei Darussalam. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded by the Ministry of Education (2021R1A6A1A03039211 and 2022R1A2B5B01001998).

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

#### **References**

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**Ming-Huan Chiu 1, Cheng-Ching Kuo 1, Chao-Wei Huang 2 and Wein-Duo Yang 1,\***

**Abstract:** A hexagonal wurtzite ZnO photocatalyst was prepared via a precipitation method. CuS nanoparticles (NPs) and PbS quantum dots (QDs) were loaded onto ZnO via a hydrothermal method to obtain a CuS/PbS/ZnO heterojunction photocatalyst. The CuS/PbS/ZnO photocatalyst obtained via the abovementioned method has significant absorption capabilities in the ultraviolet to nearinfrared spectral regions, and effectively reduced the recombination of electron–hole pairs during a photocatalytic reaction. Electron microscope images showed that in the CuS/PbS/ZnO photocatalyst prepared at 130 ◦C, the particle size of the PbS QDs was approximately 5.5–5.7 nm, and the bandgap determined from the Tauc plot was 0.84 eV; this catalyst demonstrated the best water splitting effect. Furthermore, after adding a 0.25 M mixed solution of Na2S and Na2SO3 as the sacrificial reagent in photocatalysis for 5 h, the hydrogen production efficiency from water splitting reached 6654 μmol g−1 h−1.

**Keywords:** heterojunction; CuS/PbS/ZnO; photocatalyst; hydrogen production; sacrificial reagent

#### **1. Introduction**

With industrial development and rapid population growth, humanity has accelerated the consumption of the Earth's resources, especially fossil fuels and forest resources. Some scholars have pointed out that, among the energy sources on Earth, petroleum can be used for about 40 years, natural gas for 60 years, and coal for 200 years. The concentration of carbon dioxide in the atmosphere due to the heavy usage of fossil fuels has reached a record high, exacerbating the greenhouse effect and causing rapid melting of the Arctic ice cap and global climate change. These phenomena are extremely detrimental to the sustainable development of human civilization. Therefore, all countries worldwide are actively undertaking research and development to secure new energy resources, with the aim of discovering an energy system to replace petroleum.

The calorific value of hydrogen is very high. Hydrogen can generate 142 million joules of heat energy per kilogram, which is three times that of gasoline and three and a half times that of natural gas. Hydrogen combustion can generate higher energy density, along with water, and will not produce greenhouse gases such as carbon dioxide, rendering it a form of clean energy. If hydrogen can be produced sustainably, combined with safe and efficient storage, transportation, and utilization, a platform for the sustainable use of hydrogen energy can be established to transform the current "petroleum economy" society into a clean and zero-carbon "hydrogen economy" society.

Solar energy can be regarded as an inexhaustible resource. The average light energy reaching the Earth is approximately 1.7 × 1017 W, and 1% of this energy is equivalent to providing the world's 7.4 billion people with 200 days of energy consumption. If this solar energy can be effectively harnessed, the efficiency of energy usage can be improved, and huge progress can be made towards realizing the vision of establishing low-carbon nations and sustainable development. In recent years, the utilization of semiconductor

**Citation:** Chiu, M.-H.; Kuo, C.-C.; Huang, C.-W.; Yang, W.-D. Preparation of CuS/PbS/ZnO Heterojunction Photocatalyst for Application in Hydrogen Production. *Catalysts* **2022**, *12*, 1677. https:// doi.org/10.3390/catal12121677

Academic Editors: Jorge Bedia and Carolina Belver

Received: 29 October 2022 Accepted: 12 December 2022 Published: 19 December 2022

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

photocatalysts for converting solar energy has become an important research direction. Semiconductor photocatalysts are used for water splitting to produce hydrogen [1,2], as well as in photocatalysis [3], the degradation of organic pollutants [4], and biomass energy conversion [5], all of which can be used to find reliable and stable solutions for environmental issues.

The current shortage of traditional energy requires the emergence of alternative energy sources. Among them, hydrogen energy is one of the high-profile energy sources, and there are more and more reports on the production of H2. The advantage of this work is that the materials used are not expensive, which reduces the cost of synthesis. The photocatalyst synthesis method is a hydrothermal method, and the process is simple.

Photocatalytic water splitting using solar energy to produce hydrogen was first proposed by Japanese scholars Fujishima and Honda in 1972. The titanium dioxide electrode of the anode absorbs light, and its valence band electrons excite the conduction band. Electrons come to the platinum electrode along the circuit to generate hydrogen. The holes on the titanium dioxide of the anode react with water to generate oxygen. Concurrently, hydrogen gas is generated on the platinum cathode; this phenomenon is the well-known Honda–Fujishima effect [6].

The method of electrochemically producing H2 from H2O is to put the cathode, anode and membrane in water and apply voltage to anode to produce H+, which then reacts with the electrons on the cathode to produce H2. Moreover, the strategy of photochemical production of H2 is to place a material that can absorb light energy in water, and through the irradiation of sunlight, the electrons of the material jump from the valence band to the conduction band, generating electron–hole pairs, and H+ reacts to produce H2. These two methods of producing H2 are the methods that are currently being discussed and researched, and it is expected that they can replace the traditional methods of producing H2 in the future. These two methods can be linked together. In fact, photocatalyst degradation, photocatalyst hydrogen production, solar cells, etc., are all combined and used together [7].

In photocatalytic systems, the main function of the photocatalyst is to absorb the solar energy of different wavelengths to conduct reactions. Photocatalytic materials, such as TiO2 [8], ZnO [9], CdS [10], and BiVO4 [11], have been developed for hydrogen production via water splitting. As ultraviolet radiation only accounts for 4% of sunlight, the wavelength of visible light accounts for a much larger percentage. In order to further extend the absorption range of the photocatalyst to the visible region of sunlight and adjust the redox potential to increase the efficacy of hydrogen production, different metal ions such as Pt, Au, Cu, and other transition metals can be added to [12–14] or combined with other semiconductor molecular structures to form heterojunction composite photocatalytic compounds [15–17], such as CdS@ZnO, CuS/ZnO, ZnO/ZnS, etc., which have heterojunction photocatalytic potential. In this manner, the original bandgap of the semiconductor can be altered to achieve effective electron–hole separation and enhance the efficiency of hydrogen production. For the CuS material, the reduction potential is −0.48 eV and the oxidation potential is +1.18 eV, while the reduction potential of water is 0 eV and the oxidation potential is +1.23 eV. It is revealed that the reduction potential of CuS is lower than that of water. When CuS receives energy, it is suitable for the reduction reaction of water to generate hydrogen; the energy gap of CuS is 1.66 eV, which belongs to the visible light absorption range. However, due to the poor optical stability of semiconductors with narrow bandgaps, the high rate of electron–hole recombination in the photocatalytic hydrogen production process leads to poor hydrogen production efficiency. In order to improve the hydrogen production efficiency, in their research, Wang et al. added Na2S and Na2SO3 as sacrificial reagents to effectively reduce electron–hole recombination [18]. In 1996, Alivisatos reported on the use of quantum dots (QDs) to control and adjust the size of semiconductor materials [19] in order to improve the solar absorption and bonding problems encountered with narrow bandgap semiconductors, and increase the photocatalytic hydrogen production efficiency.

The PbS QDs and CuS loaded onto ZnO in this study can absorb light in the nearinfrared range during the photocatalytic reaction, and thus the photocatalyst is not limited

by the range of UV and visible wavelength. The size of the PbS QDs can also be adjusted by changing the light wave absorption position of the photocatalyst.

When the conduction band potential of the photocatalyst is lower than the reduction potential of water (0 V vs. NHE, pH = 0), and the valence potential is higher than the oxidation potential of water (1.23 V vs. NHE, pH = 0), the photocatalyst absorbs light energy greater than the bandgap value, resulting in electron–hole pair separation. The electron transitions to the conduction band to reduce the hydrogen ions in the water to hydrogen, and the holes left in the valence band oxidize the oxygen ions in the water to generate oxygen; this process is termed photolysis by the semiconductor photocatalyst. Due to its bandgap of 3.2 eV, TiO2 can only absorb in the UV region of the solar spectrum: its photoelectric conversion efficiency is limited, and it is subject to electron–hole recombination, which affects the overall photocatalytic performance.

Nandi et al. loaded ZnO NPs and CdS NPs onto a CuS photocatalyst and adjusted the energy band relationship of the photocatalyst through the heterostructure, which effectively improved the efficiency of photocatalytic degradation [20]. Chang et al. prepared dendritic PbS@CuS photocatalysts by a hydrothermal method and used NaCl, Na2S, and Na2SO3 as sacrificial reagents for photocatalytic hydrogen production, achieving a hydrogen production efficiency of 1736 μmol h−1 g−1 [3]. Shi et al. first synthesized PbS with QDs and then prepared a PbS@ZnO/graphene oxide photocatalyst. Through the multiple exciton generation of PbS QDs, a good cooperative effect with graphene oxide was produced, which effectively facilitated electron generation and electron–hole separation and improved the efficiency of photocatalytic hydrogen production [21]. Liu et al. prepared a (ZnS-PbS)/Au/TiO2 photocatalyst via a hydrothermal method, where the loaded PbS QDs enhanced the absorption range of the photocatalyst from the near-infrared zone to the visible light region. Moreover, 20 wt% methanol was used as a sacrificial reagent for hydrogen production, which afforded an efficiency of 5011 μmol h−1 g−1 [22]. Wang et al. electrochemically modified a ZnO rod/reduced graphene (rGO)/CdS composite with CuS at room temperature to obtain a good heterostructure material. These studies show that CuS nanoparticles play a key role in enhancing the visible light response, and exhibit excellent catalytic performance. A visible-light photocatalytic H2 production rate of 1073 μmol h−1 g−1 was obtained with a CuS-ZnO/rGO/CdS heterostructure containing 0.23% CuS and 1.62% CdS [23].

Thus far, many ZnO-based substrates have been prepared by doping with transition metals or rare earth metal nanoparticles to reduce the electron–hole recombination rate and extend the light response into the visible region [24,25]. The generation of composites with other semiconductors has also been used to form heterostructures that combine bandgaps and different properties to enhance photocatalytic activity. However, if energy can be absorbed in the near-infrared region, the efficiency of photocatalytic hydrogen production could be further improved [26].

The aforementioned research results demonstrate that the photocatalytic efficiency of photocatalysts can be improved by loading a narrow bandgap semiconductor on the photocatalyst substrate to form a heterojunction. Therefore, in this study, CuS NPs and PbS QDs were loaded onto the surface of a ZnO photocatalyst to improve the bandgap relationship of the photocatalyst by utilizing sulfide with a narrow bandgap and increasing the absorption range of the photocatalyst in the visible light region and the near-infrared region, so as to improve the photocatalytic hydrogen production efficiency.

#### **2. Results and Discussion**

#### *2.1. Analysis of Material Properties*

The prepared heterojunction photocatalyst was analyzed using transmission electron microscopy (TEM), as is shown in Figure 1. Figure 1a shows that ZnO prepared via the precipitation method has a rectangular shape and a particle size of approximately 323.8 nm. The sulfide CuS NPs prepared via the hydrothermal method at 130 ◦C had a size of approximately 35.7 nm, and presented a sheet-like morphology similar to the shape reported by Saranya et al. [27]. The particle size of the PbS QDs was approximately 5.5–5.7 nm, and the particle shape was approximately spherical, which is similar to that of the PbS QDs prepared by Liu and colleagues [22].

**Figure 1.** TEM analysis of CuS/PbS/ZnO heterojunction photocatalyst. (**a**) Analytical images of CuS/PbS/ZnO photocatalyst, (**b**) image analysis of the lattice spacing for CuS NPs loaded onto ZnO, (**c**) image analysis of PbS QDs and ZnO lattice spacing, and (**d**) EDX analysis of CuS/PbS/ZnO photocatalyst.

The high magnification TEM image of the photocatalyst (Figure 1b) shows a lattice spacing of 0.247 nm for ZnO, corresponding to the (101) crystal face of hexagonal wurtzite; the lattice spacing of the CuS (101) crystal plane was 0.326 nm. From Figure 1c, the lattice spacing of the (200) plane of PbS QDs was 0.296 nm, which is consistent with the results of Sun and Chang et al. [3,7] and is consistent with the XRD results presented hereinafter. The TEM image of the prepared CuS/PbS/ZnO photocatalyst was analyzed by energydispersive X-ray spectroscopy (EDX) (as shown in Figure 1d), whereby Cu, Pb, S, Zn, and O were clearly observed, in addition to C originating from the carbon-coated copper mesh itself. Thus, it was confirmed that the CuS NPs and PbS QDs were successfully loaded onto the ZnO photocatalyst.

The XRD data for the prepared CuS/PbS/ZnO photocatalyst are shown in Figure 2. The characteristic peaks of hexagonal wurtzite ZnO were observed at 2 *θ* values of 31.8◦, 34.5◦, 36.3◦, 47.6◦, 56.6◦, 62.9◦, 66.4◦, 68.0◦, and 69.1◦, corresponding to the (100), (002), (101), (102), and (110) crystal faces; the data are consistent with JCPDS card no. 36-1451 [28]. The characteristic peaks of the loaded CuS NPs were observed at 27.3◦, 27.8◦, 29.4◦, 31.9◦, 48.1◦, 52.7◦, and 59.1◦, corresponding to the (100), (102), (103), (110), (108), and (116) crystal faces, consistent with JCPDS card no. 06-0464 [29]. The characteristic peaks of PbS were apparent at 26.0◦, 30.1◦, 43.1◦, 51.0◦, 53.5◦, 62.6◦, 68.9◦, 70.9◦, and 78.9◦, corresponding to the (111), (200), (220), (311), (222), (400), (331), (420), and (422) crystal faces, consistent with

JCPDS card no. 05-0592 [3]. Thus, CuS NPs and PbS QDs were successfully loaded onto the ZnO photocatalysts at 100 ◦C, 130 ◦C, 150 ◦C, and 170 ◦C by the hydrothermal method.

**Figure 2.** XRD analysis of CuS/PbS/ZnO heterojunction photocatalysts prepared under different hydrothermal temperatures.

As is shown in Figure 2, the characteristic peaks of the loaded CuS at 31.9◦ and 48.0◦ overlapped with the characteristic peaks of ZnO at 31.8◦ and 47.5◦, which intensified the peaks. The characteristic peaks of the loaded PbS at 62.6◦and 68.9◦ overlapped with those of ZnO at 62.9◦ and 69.1◦, leading to higher intensities. This indicates the presence of CuS and PbS in the photocatalyst. Moreover, these peaks were more intense for the CuS/PbS/ZnO photocatalyst prepared at higher hydrothermal temperature because the formation of sulfides was more complete under the hydrothermal environment [30].

#### *2.2. Spectral Properties of Prepared Heterostructure Photocatalysts*

Table 1 shows ZnO, PbS/ZnO, CuS/ZnO, and CuS/PbS/ZnO photocatalysts incident at a single wavelength light to detect the absorbance of the photocatalysts. It can be found that single composition of ZnO photocatalyst absorbs UV (300–350 nm), but ZnO does not absorb from VIS. to NIR (400–1800 nm). Comparing PbS/ZnO and CuS/ZnO, it can be seen that ZnO-loaded PbS QDs can enhance the absorption of 400–1800 nm; while the ZnO-loaded CuS can enhance the absorption of 350–1800 nm, the above two carriers can improve the absorption of ZnO. Further comparison can be found that for the wavelength of 350–600 nm, CuS is more improved; as for PbS QDs, it is more improved at 800–1800 nm.

The UV-VIS-NIR spectra of the CuS/PbS/ZnO photocatalysts prepared at various hydrothermal temperatures are shown in Figure 3, and the dotted line in Figure 3 represents the position of the maximum absorption wavelength of sulfide quantum dots synthesized by different hydrothermal temperatures, which can confirm the quantum confinement effect of quantum dots. When the hydrothermal temperature is lower, the smaller size of synthesized quantum dots is produced and spectrum present the blue-shift of the absorption wavelength is occurred. The CuS/PbS/ZnO photocatalysts prepared at all the hydrothermal temperatures had significant absorption bands from the near-infrared to the visible light region. The photocatalysts prepared at 100 ◦C and 130 ◦C absorbed more strongly at wavelengths of 380–760 nm. In comparison, the photocatalysts prepared at 150 ◦C and 170 ◦C had weaker absorption in the visible spectral region. The maximum absorption wavelength was shifted from 1435 nm for the sample prepared at 170 ◦C to 817 nm for the congener prepared at 100 ◦C, which suggests that the absorption of the photocatalyst prepared using the lower-temperature hydrothermal conditions shifted towards the shorter wavelength. Hence, it was concluded that the sulfide QDs prepared at lower temperatures had a smaller particle size, and the bandgap was widened due to the quantum confinement effect, which resulted in a blue-shift of the absorption [31].

**Figure 3.** UV-VIS-NIR spectra of CuS/PbS/ZnO photocatalysts prepared at various hydrothermal temperatures. Dotted lines indicate the maximum absorption wavelength of photocatalysts.

The bandgaps of the sulfide QDs and ZnO photocatalysts were obtained via an analysis of the Tauc plots. Figure 4 shows the Tauc plot for the CuS/PbS/ZnO photocatalysts prepared at different hydrothermal temperatures. The CuS/PbS/ZnO photocatalyst had two bandgaps: a lower bandgap of PbS, and a higher bandgap of ZnO. As can be deduced from Figure 4, the bandgap of ZnO increased from 2.95 eV to 3.10 eV and the bandgap of PbS QDs decreased from 0.71 eV to 0.55 eV for the CuS/PbS/ZnO photocatalysts prepared in the hydrothermal temperature range of 100–170 ◦C. The bandgap of the PbS QDs prepared at a hydrothermal temperature of 130 ◦C was approximately 0.84 eV, which corresponds to PbS QDs of approximately 5.5 nm in the TEM image in Figure 1a. The relationship between the particle size and bandgap of PbS QDs is consistent with the study of Liang and co-workers [32]. Guchhait et al. also indicated that smaller PbS QDs particles should have a larger bandgap, which could shift the absorption from the near-infrared region to the visible light region, resulting in a blue-shift phenomenon [33].

**Figure 4.** Tauc plot of CuS/PbS/ZnO photocatalysts prepared at various hydrothermal temperatures.

Electron–hole pair recombination in the photocatalyst was investigated by PL spectral analysis. When the photocatalyst absorbs light energy, the electrons will transition from the ground state to the excited state, and the electrons will fall back to the ground state and generate fluorescence. This process is termed electron–hole pair recombination. The degree of electron–hole recombination in the photocatalyst determines the fluorescence intensity [29].

Figure 5 shows the PL spectrum of the prepared photocatalyst. Figure 5a shows the PL spectrum of the ZnO photocatalysts loaded with different sulfides. Fluorescence was generated in the wavelength range of approximately 400 nm to 800 nm, and the strongest fluorescence intensity was at a wavelength of 600 nm. Non-loaded ZnO produced the strongest fluorescence intensity, at a wavelength of 600 nm, whereas the fluorescence intensity decreased significantly for the ZnO photocatalyst loaded with the CuS NPs and PbS QDs, indicating that the loaded sulfide QDs effectively reduced the electron–hole recombination rate of the photocatalyst [34]. In comparison with the PbS/ZnO and CuS/ZnO photocatalysts, the fluorescence intensity generated by the CuS/PbS/ZnO photocatalyst was the lowest, which indicates that when CuS NPs and PbS QDs were simultaneously loaded onto the ZnO photocatalyst, the sulfide QDs and ZnO photocatalysts approached each other more closely, which effectively reduced the recombination of electron–hole pairs [35]. Yendrapati et al. [29] pointed out that the higher the peak value of the PL, the stronger the fluorescence intensity generated by the photocatalyst, corresponding to a faster electron–hole recombination rate. In contrast, the lower the peak value of the PL, the weaker the fluorescence intensity generated by the photocatalyst, indicating a slower electron–hole recombination rate.

Figure 5b shows the PL spectrum of the CuS/PbS/ZnO photocatalysts prepared at different hydrothermal temperatures. The CuS/PbS/ZnO photocatalysts prepared at different hydrothermal temperatures also emitted the maximum fluorescence intensity at 600 nm. The fluorescence intensity of the photocatalysts prepared at temperatures from 170 to 130 ◦C decreased gradually, where the photocatalyst prepared at 130 ◦C had the lowest fluorescence intensity. This is because the particle size of the sulfide QDs prepared at a lower hydrothermal temperature was smaller, and the bandgap of the sulfide QDs increased due to the quantum confinement effect, which effectively facilitated the separation of the electron–hole pairs and reduced the incidence of recombination. Notably, the highest PL peak intensity was observed for the photocatalyst prepared at 100 ◦C. This might be due to the hydrothermal temperature being too low, the crystallinity of the sulfide QDs being low, or crystal defects, which may have led to a high electron–hole pair recombination rate [36].

**Figure 5.** PL spectra of the prepared photocatalysts at 579 nm as the excitation wavelength (**a**) Various sulfides supported on ZnO photocatalyst, and (**b**) CuS/PbS/ZnO photocatalysts prepared at different hydrothermal temperatures.

#### *2.3. Hydrogen Production by Water Splitting with Photocatalysts*

Figure 6 shows the hydrogen production efficiency of the respective photocatalysts in a 0.25 M aqueous mixed solution of Na2S and Na2SO3 as sacrificial reagents. The photocatalytic efficiency of the photocatalyst was studied by irradiating the aqueous solution containing the photocatalyst and sacrificial reagents with light generated by a solar simulator for 5 h. Figure 6a shows that the hydrogen production of the ZnO photocatalyst loaded with sulfide QDs was better than that of the non-loaded ZnO photocatalyst, and the hydrogen production achieved with the respective photocatalysts increased with time. CuS and PbS of the CuS/ZnO and PbS/ZnO photocatalysts facilitated energy absorption from the near-infrared to visible light region. PbS and CuS are prone to photocorrosion; thus, the hydrogen production rates of the CuS/ZnO and PbS/ZnO photocatalysts were lower. As is shown in Figure 6b, the hydrogen production achieved with the ZnO photocatalyst without any loading was 203 μmol g−1 h−1, whereas the hydrogen production of the CuS/PbS/ZnO photocatalyst was 6654 μmol g−1 h−1, which is 32 times that of the non-loaded ZnO photocatalyst. The CuS/PbS/ZnO photocatalyst had a higher hydrogen production rate, because the simultaneous loading of PbS QDs and CuS NPs not only facilitated absorption in the near-infrared and visible light regions, but the prepared ternary CuS/PbS/ZnO heterojunction photocatalyst was also more efficient. The effective electron–hole separation reduced the incidence of recombination [35].

**Figure 6.** Hydrogen production efficiency of respective photocatalysts in aqueous solution containing 0.25 M Na2S and Na2SO3 as sacrificial reagents. (**a**) Relationship between hydrogen production and time. (**b**) Hydrogen production rate in μmol g−1 h−1.

Wang et al. reported that CuS/ZnS nanomaterials exhibit high visible light-induced H2 generation activity. The H2 generation rate increases with increasing Cu2+ ions. However, as with other cocatalysts, when the maximum amount of Cu2+ is reached (above 7 mol %), the hydrogen evolution rate decreases significantly. This is due to light shielding by excess CuS, which reduces the number of active sites on the surface [37].