Fabrication of ZnCo2O4-Zn(OH)2 Microspheres on Carbon Cloth for Photocatalytic Decomposition of Tetracycline

Zinc cobalt oxide-zinc hydroxide (ZnCo2O4-Zn(OH)2) microspheres were successfully fabricated on carbon cloth via a sample hydrothermal method. The surface morphology of these microspheres and their efficacy in degrading methyl violet were further modulated by varying the thermal annealing temperatures. Adjusting the thermal annealing temperatures was crucial for controlling the porosity of the ZnCo₂O₄-Zn(OH)₂ microspheres, enhancing their photocatalytic performance. Various analytical techniques were utilized to evaluate the physical and chemical properties of the ZnCo2O4-Zn(OH)2 microspheres, including field-emission scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction, field-emission transmission electron microscopy, X-ray photoelectron spectroscopy, and UV-vis spectroscopy. Compared to untreated ZnCo2O4-Zn(OH)2 microspheres, those subjected to thermal annealing exhibited increased specific surface area and light absorption capacity, rendering them highly effective photocatalysts under UVC light exposure. Subsequent studies have confirmed the superior performance of ZnCo2O4-Zn(OH)2 microspheres as a reusable photocatalyst for degrading methyl violet and tetracycline. Furthermore, trapping experiments during the photodegradation process using ZnCo₂O₄-Zn(OH)₂ microspheres identified hydroxyl radicals (·OH) and superoxide radicals (·O₂⁻) as the primary reactive species.


Introduction
Spinel-structured photocatalysts are gaining attention for their cost-effectiveness, durability, and strong photoelectrochemical response, making them prime for enhancing solar energy capture [1][2][3].Various AB 2 O 4 spinels, such as ZnFe 2 O 4 and ZnCo 2 O 4 , have been explored for applications ranging from gas sensing to energy storage and degradation of pollutants under visible light [4][5][6].ZnCo 2 O 4 is classified as a p-type semiconductor due to its spinel crystal structure and is notable for its versatility, including roles in Li-ion batteries, catalysis, and supercapacitors [7,8].ZnCo 2 O 4 -based sensors have shown exceptional sensitivity to various gases, likely due to their high surface area [9].The morphology of the nanoparticles significantly impacts gas sensing performance, highlighting the importance of optimizing ZnCo 2 O 4 sensors for enhanced sensitivity and lower operating temperatures [10,11].Diverse synthesis methods like hydrothermal and microwave-assisted heating techniques have been developed, offering efficiency and cost-effectiveness for oxide material structures [12][13][14][15].ZnCo 2 O 4 ′ s effective use in breaking down organic pollutants showcases its potential for environmental cleanup [16][17][18].The evolution from bulk to porous structures, offering more active sites and better light absorption, marks a significant advancement [19].The annealing-based self-sacrificial templating emerges as a cost-effective, high-performance method for fabricating these porous photocatalysts, emphasizing the importance of precursor selection in achieving superior product quality [16,20,21].Previous studies have demonstrated the successful preparation of ZnCo 2 O 4 nanostructures on carbon cloth for electrodes for supercapacitors and lithium-ion batteries [22][23][24].However, their applications on carbon cloth for photocatalytic degradation remain infrequent.
Carbon cloth, a carbon filament textile, offers high conductivity, mechanical strength, and flexibility, making it ideal for flexible energy storage systems [25][26][27].Despite a low surface area and few electroactive sites, carbon cloth is a flexible substrate for electrode materials [28][29][30].Notably, Wang et al.'s oxidative method with hydrogen peroxide and sulfuric acid introduces oxygen-containing groups onto carbon cloth, enhancing its function [31].Kordek et al., Liu et al., and Zhao et al. employed various complex activation methods to enhance the oxygen electrocatalytic activities of carbon cloth, each achieving improved performance through surface modification techniques, such as etching, calcining, plasma treatment, and doping with heteroatoms [32,33].Therefore, surface modification technology can further improve carbon cloth's specific surface area and active sites, improving its application in various fields.Past research has shown that combining carbon cloth and ZnCo 2 O 4 nanostructures creates a high-capacity, flexible anode with excellent cycle stability and rate performance, thus forming a highly flexible lithium-ion battery with excellent electrochemical properties [23,34].In this study, combining carbon cloth and ZnCo 2 O 4 -Zn(OH) 2 microspheres is expected to be further used in the photocatalytic degradation of methyl violet and tetracycline, simplifying the subsequent recycling process.
ZnCo 2 O 4 -Zn(OH) 2 microspheres fabricated on carbon cloth through a simplified hydrothermal process demonstrated enhanced photocatalytic degradation of pollutants such as methyl violet, attributed to optimized porosity achieved by varying thermal annealing temperatures.Comprehensive characterization showed that annealed microspheres possess superior surface area, enhanced light absorption, and improved charge carrier separation, making them highly effective under UVC light.These microspheres demonstrated efficiency as reusable photocatalysts, with superoxide and hydroxyl radicals identified as the primary reactive species in pollutant degradation.

Results and Discussion
Figure 1 is a detailed schematic diagram illustrating the growing ZnCo 2 O 4 -Zn(OH) 2 microspheres on a carbon cloth substrate.Initially, carbon cloth undergoes a meticulous etching process using a mixture of sulfuric acid and hydrogen peroxide.This critical step creates a series of micropores on the surface, enhancing its texture and providing anchoring points that facilitate the subsequent nucleation and growth of ZnCo 2 O 4 -Zn(OH) 2 microspheres.Following surface treatment, the carbon cloth underwent hydrothermal processing, forming and adhesion of ZnCo 2 O 4 -Zn(OH) 2 microspheres on the etched surface.The reaction process was performed at a controlled temperature of 120 • C and maintained for 2 h, ensuring uniform growth of microspheres.This meticulously designed process successfully integrated ZnCo 2 O 4 -Zn(OH) 2 microspheres onto carbon cloth, enhancing surface properties and leveraging the synergistic effects between the carbon cloth and the microspheres, thus laying the foundation for potential applications in photocatalytic degradation.Figure 2a,b shows FESEM images of carbon cloth before and after soaking in a solution containing hydrogen peroxide and sulfuric acid.Before shaking, the carbon cloth surface is quite smooth.However, after soaking, the surface of the carbon cloth displays holes of various sizes.Subsequently, the two types of carbon cloth are placed in a ZnCo2O4-Zn(OH)2 reaction precursor at a reaction temperature of 120 °C for 2 h by a facile hydrothermal process, as shown in Figure 2c,d.These images reveal that after soaking in the sulfuric acid and hydrogen peroxide mixture, the ZnCo2O4-Zn(OH)2 microspheres on the surface of the carbon cloth exhibit a higher density.This result proves that soaking the carbon cloth in a solution containing hydrogen peroxide and sulfuric acid creates more pores on its surface and aids in the subsequent growth of ZnCo2O4-Zn(OH)2 microspheres.Additionally, high-resolution FESEM images (as seen in Figure 3a) depict the presence of ZnCo2O4-Zn(OH)2 microspheres.The distribution of specific elements within these  Figure 2a,b shows FESEM images of carbon cloth before and after soaking in a solution containing hydrogen peroxide and sulfuric acid.Before shaking, the carbon cloth surface is quite smooth.However, after soaking, the surface of the carbon cloth displays holes of various sizes.Subsequently, the two types of carbon cloth are placed in a ZnCo2O4-Zn(OH)2 reaction precursor at a reaction temperature of 120 °C for 2 h by a facile hydrothermal process, as shown in Figure 2c,d.These images reveal that after soaking in the sulfuric acid and hydrogen peroxide mixture, the ZnCo2O4-Zn(OH)2 microspheres on the surface of the carbon cloth exhibit a higher density.This result proves that soaking the carbon cloth in a solution containing hydrogen peroxide and sulfuric acid creates more pores on its surface and aids in the subsequent growth of ZnCo2O4-Zn(OH)2 microspheres.Additionally, high-resolution FESEM images (as seen in Figure 3a) depict the presence of ZnCo2O4-Zn(OH)2 microspheres.The distribution of specific elements within these microspheres is further elucidated via FESEM-EDS elemental mapping, as shown in Additionally, high-resolution FESEM images (as seen in Figure 3a) depict the presence of ZnCo 2 O 4 -Zn(OH) 2 microspheres.The distribution of specific elements within these microspheres is further elucidated via FESEM-EDS elemental mapping, as shown in Figure 3b-d.Examination of these images reveals that the microspheres are composed of zinc (Zn), cobalt (Co), and oxygen (O), with these components being uniformly distributed throughout.Consequently, this highlights the specific elemental composition of the ZnCo 2 O 4 -Zn(OH) 2 microspheres.Figure 4 shows FESEM images of ZnCo2O4-Zn(OH)2 microspheres subjected to different thermal annealing temperatures for 2 h.The thermal annealing temperatures are (a) without, (b) 450 °C, (c) 550 °C, and (d) 650 °C, respectively.As the annealing temperature rises, there is a progressive emergence of porosity on the surface.The surface morphology of the ZnCo2O4-Zn(OH)2 microspheres remains unchanged compared to the unannealed samples, with the only noticeable difference being the appearance of porosity at an annealing temperature of 550 °C for 2 h.When the annealing temperature rises to 650 °C, it becomes apparent that some microspheres have collapsed.This occurrence drastically decreases the reactive surface area, negatively impacting the efficiency of subsequent photocatalytic reactions.The structural collapse observed at higher temperatures underscores the material's thermal instability, highlighting the necessity for meticulous optimization of the thermal annealing process to preserve the desired functional properties of the microspheres.Hence, the BET analyzer can assess the specific surface area of ZnCo2O4-Zn(OH)2 microspheres before and after thermal annealing at 550 °C for 2 h.The surface area of the microspheres was measured at 20.78 m 2 g −1 before thermal annealing, and following the annealing process, this value rose to 31.29 m 2 g −1 .This result indicates that thermal annealing effectively enhances the specific surface area of the microspheres.As the annealing temperature rises, there is a progressive emergence of porosity on the surface.The surface morphology of the ZnCo 2 O 4 -Zn(OH) 2 microspheres remains unchanged compared to the unannealed samples, with the only noticeable difference being the appearance of porosity at an annealing temperature of 550 • C for 2 h.When the annealing temperature rises to 650 • C, it becomes apparent that some microspheres have collapsed.This occurrence drastically decreases the reactive surface area, negatively impacting the efficiency of subsequent photocatalytic reactions.The structural collapse observed at higher temperatures underscores the material's thermal instability, highlighting the necessity for meticulous optimization of the thermal annealing process to preserve the desired functional properties of the microspheres.Hence, the BET analyzer can assess the specific surface area of ZnCo 2 O 4 -Zn(OH) 2 microspheres before and after thermal annealing at 550 • C for 2 h.The surface area of the microspheres was measured at 20.78 m 2 g −1 before thermal annealing, and following the annealing process, this value rose to 31.29 m 2 g −1 .This result indicates that thermal annealing effectively enhances the specific surface area of the microspheres.The FETEM image in Figure 6a reveals a microspherical structure of ZnCo2O4-Zn(OH)2, which aligns with the SEM results.This configuration is characterized by the arrangement of numerous sheets stacked on each other.The typical SAED pattern (Figure 6b) further confirms the polycrystalline nature of the ZnCo2O4-Zn(OH)2 microsphere.The major diffraction ring closely matches the orthorhombic Zn(OH)2 (PDF No. 00-020-1437) and cubic ZnCo2O4 (PDF No. 00-023-1390) crystal structures.The HRTEM image of the ZnCo2O4-Zn(OH)2 microsphere (Figure 6c) displays crystal lattice fringes characterized by two discernible interplanar spacings: 0.302 nm and 0.244 nm.These can be attributed to the (031) crystallographic plane of the orthorhombic phase of Zn(OH)2 and the (311) crystallographic plane of the cubic phase of ZnCo2O4. Figure 6d    To analyze the elemental composition and valence state distribution on the surface of ZnCo2O4-Zn(OH)2 microspheres, X-ray photoelectron spectroscopy (XPS) was employed.Figure 7a displays the full range XPS spectrum of ZnCo2O4-Zn(OH)2 microspheres annealed at 550 °C, exhibiting distinct peaks corresponding to C, Zn, Co, and O.These peaks are consistent with the TEM-EDS observations, further confirming the presence of these elements.The carbon element is believed to have its source in the pump oil present in the vacuum system of the XPS equipment, carbon cloth, or an organic layer that has been applied to the surface of the sample.The high-resolution XPS spectrum (Figure 7b)   To comprehend the correlation between the photocatalytic efficiency of ZnCo2O4-Zn(OH)2 microspheres under different annealing temperatures, the photocatalytic activity in degrading methyl violet (MV), an organic pollutant commonly found in the textile industry was evaluated [39].ZnCo2O4-Zn(OH)2 microspheres were grown on a 2.5 cm × 1.5 cm carbon cloth substrate as photocatalytic samples.The photocatalytic efficiency of ZnCo2O4-Zn(OH)2 microspheres at different annealing temperatures was evaluated by the degradation of MV by UVC light (253 nm, 10 W), as shown in Figure 8a.The time variation of MV concentration was monitored by examining the change in maximum absorbance at 587 nm in UV-vis spectroscopy.The photodegradation percentages of MV were 58.0%(without annealing), 89.8% (450 °C), 91.7% (550 °C), and 78.6% (650 °C).When the annealing temperature was below 550 °C, a decrease in maximum absorbance was observed with increasing irradiation time and annealing temperature.ZnCo2O4-Zn(OH)2 microspheres (550 °C) exhibited the highest photocatalytic activity in MV decomposition.The photo- To comprehend the correlation between the photocatalytic efficiency of ZnCo 2 O 4 -Zn(OH) 2 microspheres under different annealing temperatures, the photocatalytic activity in degrading methyl violet (MV), an organic pollutant commonly found in the textile industry was evaluated [39].ZnCo 2 O 4 -Zn(OH) 2 microspheres were grown on a 2.5 cm × 1.5 cm carbon cloth substrate as photocatalytic samples.The photocatalytic efficiency of ZnCo 2 O 4 -Zn(OH) 2 microspheres at different annealing temperatures was evaluated by the degradation of MV by UVC light (253 nm, 10 W), as shown in Figure 8a.The time variation of MV concentration was monitored by examining the change in maximum absorbance at 587 nm in UV-vis spectroscopy.The photodegradation percentages of MV were 58.0%(without annealing), 89.8% (450 • C), 91.7% (550 • C), and 78.6% (650 • C).When the annealing temperature was below 550 • C, a decrease in maximum absorbance was observed with increasing irradiation time and annealing temperature.ZnCo 2 O 4 -Zn(OH) 2 microspheres (550 • C) exhibited the highest photocatalytic activity in MV decomposition.The photocatalytic degradation process conformed to pseudo-first-order kinetics, and the plot of −ln(C/C 0 ) versus irradiation time (t) showed a pseudo-first-order linear relationship (Figure 8b), where C 0 is the initial concentration of MV and C is the actual concentration of MV at time t.The slope of the pseudo-first-order linear line is the apparent rate constant (k, min -1 ) of the photocatalytic reaction.The rate constants of ZnCo 2 O 4 -Zn(OH) 2 microspheres at different annealing temperatures were calculated to be 0.03298 (without annealing), 0.05002 (450 • C), 0.08519 (550 • C), and 0.07241 min -1 (650 • C), respectively.ZnCo 2 O 4 -Zn(OH) 2 microspheres (550 • C) displayed the highest photocatalytic efficiency in MV photodegradation under UVC light irradiation.The rate constant (k) of ZnCo 2 O 4 -Zn(OH) 2 microspheres (550 • C) was about 2.58 times higher than that of the non-annealed ones.This phenomenon is attributed to the formation of porosity on the surface of ZnCo 2 O 4 -Zn(OH) 2 microspheres after annealing, which increases the active sites.However, when the annealing temperature is too high, it can cause the structure to collapse, leading to a decrease in active sites, which is consistent with the observations from SEM results.We chose tetracycline (TC) as an antibiotic to illustrate that ZnCo2O4-Zn(OH)2 microspheres can also be used for photocatalytic antibiotic degradation.Tetracycline (TC), a widely used antibiotic effective against various infections, is prevalent in water bodies due to its use as a growth promoter in aquaculture and insufficient removal by traditional wastewater treatments [40,41].As demonstrated in Figure 9a, we observe the degradation rates of ZnCo2O4-Zn(OH)2 microspheres before and after annealing when subjected to UVC light.The findings indicated that the microspheres that did not undergo the annealing process had a degradation rate of 75.6%.In contrast, those annealed at a temperature of 550 °C exhibited a rate of 83.3%. Figure 9b represents the pseudo-first-order linear relationship of the ZnCo2O4-Zn(OH)2 microspheres in the non-and annealed process.The reaction constants, corresponding to TC degradation over the non-annealed microspheres and those annealed at 550 °C, were determined to be 0.00794 and 0.00986 min −1 , respectively.Notably, the microspheres that underwent annealing at 550 °C showed superior photocatalytic activity, with their reaction constant being 1.24 times greater than their non-annealed counterparts when exposed to UVC light.This suggests an enhancement in photocatalytic efficiency due to the annealing process.We chose tetracycline (TC) as an antibiotic to illustrate that ZnCo 2 O 4 -Zn(OH) 2 microspheres can also be used for photocatalytic antibiotic degradation.Tetracycline (TC), a widely used antibiotic effective against various infections, is prevalent in water bodies due to its use as a growth promoter in aquaculture and insufficient removal by traditional wastewater treatments [40,41].As demonstrated in Figure 9a, we observe the degradation rates of ZnCo 2 O 4 -Zn(OH) 2 microspheres before and after annealing when subjected to UVC light.The findings indicated that the microspheres that did not undergo the annealing process had a degradation rate of 75.6%.In contrast, those annealed at a temperature of 550 • C exhibited a rate of 83.3%. Figure 9b represents the pseudo-first-order linear relationship of the ZnCo 2 O 4 -Zn(OH) 2 microspheres in the non-and annealed process.The reaction constants, corresponding to TC degradation over the non-annealed microspheres and those annealed at 550 • C, were determined to be 0.00794 and 0.00986 min −1 , respectively.Notably, the microspheres that underwent annealing at 550 • C showed superior photocatalytic activity, with their reaction constant being 1.24 times greater than their non-annealed counterparts when exposed to UVC light.This suggests an enhancement in photocatalytic efficiency due to the annealing process.
reaction constants, corresponding to TC degradation over the non-annealed microspheres and those annealed at 550 °C, were determined to be 0.00794 and 0.00986 min −1 , respectively.Notably, the microspheres that underwent annealing at 550 °C showed superior photocatalytic activity, with their reaction constant being 1.24 times greater than their non-annealed counterparts when exposed to UVC light.This suggests an enhancement in photocatalytic efficiency due to the annealing process.The recyclability of ZnCo2O4-Zn(OH)2 microspheres (550 °C) was examined through repeated experiments involving the degradation of MV and TC solutions under UVC light irradiation, as shown in Figure 10.In the case of the MV solution (Figure 10a), the The recyclability of ZnCo 2 O 4 -Zn(OH) 2 microspheres (550 • C) was examined through repeated experiments involving the degradation of MV and TC solutions under UVC light irradiation, as shown in Figure 10.In the case of the MV solution (Figure 10a), the photocatalytic efficiency remained consistently high across four cycles, with efficiencies of 91.7%, 90.5%, 89.9%, and 87.4%, respectively.Similarly, the photocatalytic efficiency maintained a steady rate for the TC solution (Figure 10b), with 82.8%, 82.4%, 81.7%, and 80.9% across the four cycles.Even after four cycles of use, the decline in the photocatalytic efficiency of the ZnCo 2 O 4 -Zn(OH) 2 microspheres was insignificant, demonstrating their durability and consistent performance.This result suggests that the ZnCo 2 O 4 -Zn(OH) 2 microspheres, heated at 550 • C, possess a long lifespan as photocatalysts, maintaining high activity and reusability.The ZnCo 2 O 4 -Zn(OH) 2 microspheres were also directly cultivated on a carbon cloth.In the meantime, the tested ZnCo 2 O 4 -Zn(OH) 2 microspheres (Figure 11) displayed remarkable consistency in the XRD patterns before and after degradation tests, validating their high resistance to photo-corrosion and stability.This result suggests that ZnCo 2 O 4 -Zn(OH) 2 microspheres have the promising potential for repeated use in practical applications, a highly desirable characteristic for sustainable and efficient photocatalysts.This resilience ensures their longevity and enhances their cost-effectiveness, making them a compelling choice for environmental applications.This unique growth method simplifies recycling and provides a stable and economical photocatalyst platform.photocatalytic efficiency remained consistently high across four cycles, with efficiencies of 91.7%, 90.5%, 89.9%, and 87.4%, respectively.Similarly, the photocatalytic efficiency maintained a steady rate for the TC solution (Figure 10b), with 82.8%, 82.4%, 81.7%, and 80.9% across the four cycles.Even after four cycles of use, the decline in the photocatalytic efficiency of the ZnCo2O4-Zn(OH)2microspheres was insignificant, demonstrating their durability and consistent performance.This result suggests that the ZnCo2O4-Zn(OH)2 microspheres, heated at 550 °C, possess a long lifespan as photocatalysts, maintaining high activity and reusability.The ZnCo2O4-Zn(OH)2 microspheres were also directly cultivated on a carbon cloth.In the meantime, the tested ZnCo2O4-Zn(OH)2 microspheres (Figure 11) displayed remarkable consistency in the XRD patterns before and after degradation tests, validating their high resistance to photo-corrosion and stability.This result suggests that ZnCo2O4-Zn(OH)2 microspheres have the promising potential for repeated use in practical applications, a highly desirable characteristic for sustainable and efficient photocatalysts.This resilience ensures their longevity and enhances their cost-effectiveness, making them a compelling choice for environmental applications.This unique growth method simplifies recycling and provides a stable and economical photocatalyst platform.The optical properties of the ZnCo2O4-Zn(OH)2 microspheres, both with and without thermal annealing, were analyzed using UV-visible spectroscopy.As shown in Figure 12a, the ZnCo2O4-Zn(OH)2 microspheres that underwent thermal annealing at 550 °C demonstrated a significantly superior light absorption capacity within the spectral range spanning from 250 to 800 nm compared to those that did not undergo thermal annealing.The improved light absorption spectrum observed in the annealed microspheres offers advantages for maximizing solar energy utilization, thereby enhancing the photocatalytic degradation process.The energy band gaps (Eg) were established utilizing the Tauc relationship, represented by the equation: αhν = A (Eg − hν) 1/n [42,43].In the provided equation, A, α, ν, Eg, and h represent constants: the constant, the absorption coefficient, the frequency of light, the band gap energy, and Planck's constant, respectively.The variable "n" denotes a property of the semiconductor material, taking a value of 2 for indirect bandgap semiconductors and 1/2 for direct bandgap semiconductors, as illustrated in Figure 12b.The energy band gap value of the ZnCo2O4-Zn(OH)2 microspheres, both with and without annealing, was computed to be approximately 2.42 eV.These data validate that the energy gap remains relatively constant throughout the thermal annealing process.Moreover, a straightforward thermal annealing technique can offer a high specific surface area and a more extensive optical absorption spectrum at a suitable temperature.This method significantly enhances the photocatalytic degradation of MV or TC solutions, improving their overall performance and effectiveness.The optical properties of the ZnCo 2 O 4 -Zn(OH) 2 microspheres, both with and without thermal annealing, were analyzed using UV-visible spectroscopy.As shown in Figure 12a, the ZnCo 2 O 4 -Zn(OH) 2 microspheres that underwent thermal annealing at 550 • C demonstrated a significantly superior light absorption capacity within the spectral range spanning from 250 to 800 nm compared to those that did not undergo thermal annealing.The improved light absorption spectrum observed in the annealed microspheres offers advantages for maximizing solar energy utilization, thereby enhancing the photocatalytic degradation process.The energy band gaps (E g ) were established utilizing the Tauc relationship, represented by the equation: αhν = A (E g − hν) 1/n [42,43].In the provided equation, A, α, ν, E g , and h represent constants: the constant, the absorption coefficient, the frequency of light, the band gap energy, and Planck's constant, respectively.The variable "n" denotes a property of the semiconductor material, taking a value of 2 for indirect bandgap semiconductors and 1/2 for direct bandgap semiconductors, as illustrated in Figure 12b.The energy band gap value of the ZnCo 2 O 4 -Zn(OH) 2 microspheres, both with and without annealing, was computed to be approximately 2.42 eV.These data validate that the energy gap remains relatively constant throughout the thermal annealing process.Moreover, a straightforward thermal annealing technique can offer a high specific surface area and a more extensive optical absorption spectrum at a suitable temperature.This method significantly enhances the photocatalytic degradation of MV or TC solutions, improving their overall performance and effectiveness.Four radical scavengers were introduced into the photocatalytic reaction to investigate the underlying mechanism of the photocatalysts of ZnCo2O4-Zn(OH)2 microspheres during the photodegradation of MV or TC solution, as shown in Figure 13a,b.Isopropyl alcohol (IPA), L-ascorbic acid (AA), triethanolamine (TEOA), and silver nitrate (AgNO3) were utilized as scavengers to impede hydroxyl radicals (•OH), superoxide radical anions (•O 2− ), holes (h + ), and electrons (e -), respectively [44][45][46][47].Adding IPA and AA scavengers to the photocatalytic reaction leads to a notable reduction in the photocatalytic efficiency.This outcome provides evidence that hydroxyl radicals (•OH) and superoxide radicals (•O 2− ) are the primary active species involved in the photodegradation of TC.Potential reactions that may occur during the photocatalytic degradation of MV or TC solutions over ZnCo2O4-Zn(OH)2 microspheres can be outlined as a schematic diagram, as shown in Figure 13c.When the ZnCo2O4-Zn(OH)2 microspheres are exposed to UVC light with photon energy (hv) exceeding their band gap, an electron (e -) in the valence band (VB) can be promoted to the conduction band (CB), creating a hole in the VB and generating electron-hole pairs.Photogenerated electrons can interact with oxygen molecules on the surface, forming superoxide radical anions (•O2 -).These can then react with water molecules absorbed on the surface, producing hydroxyl radicals (•OH).Moreover, the photogenerated holes may combine with H2O molecules, causing their dissociation into••OH radicals.These superoxide radical anions and hydroxyl radicals are recognized as potent oxidants responsible for the decomposition of MV or TC molecules.

Fabrication of ZnCo 2 O 4 -Zn(OH) 2 Microspheres
The carbon cloth was cut into (2.5 cm × 1.5 cm) sizes, soaked in a volume ratio of hydrogen peroxide and sulfuric acid (3:7) for 15 min, then thoroughly rinsed with DI water, and dried in an oven at 75 • C for 5 h.ZnCo 2 O 4 -Zn(OH) 2 microspheres were prepared using a facile hydrothermal approach on carbon cloth.A solution consisting of 0.75 mmol of Zn(NO 3 ) 2 •6H 2 O, 1.5 mmol of Co(NO 3 ) 2 •6H 2 O, 1.5 mM of NH 4 F, and 3.75 mM of urea was prepared in 30 mL of DI water and stirred magnetically for 20 min.Subsequently, the carbon cloth, having undergone prior preparation, was immersed in this homogenous mixture and maintained at 120 • C for 5 h.Once the temperature returned to ambient, the ZnCo 2 O 4 -Zn(OH) 2 microspheres were washed with DI water and ethanol and desiccated at 75 • C for the entire night.The specimen was calcinated at varying temperatures for 2 h under atmospheric pressure.

Characterization
Various analytical techniques thoroughly examined the microstructures and elemental composition of the as-synthesized ZnCo 2 O 4 -Zn(OH) 2 microspheres.Field-emission scanning electron microscopy (FESEM) was performed utilizing a Hitachi S-4800 instrument from Japan.Field emission transmission electron microscopy (FETEM) utilized a JEOL-2100F instrument manufactured in Japan, outfitted with energy-dispersive X-ray spectroscopy (EDS) to analyze the primary components.X-ray diffraction (XRD) analysis was conducted utilizing a Bruker D2 instrument in the United States to examine the crystal structures of the fabricated substrates.The elemental chemical compositions of the ZnCo 2 O 4 -Zn(OH) 2 microspheres were analyzed through X-ray photoelectron spectroscopy (XPS) utilizing a ULVAC-PHI PHI 5000 VersaProbe instrument manufactured in Japan.

Photocatalytic Measurement
The efficiency of photocatalytic materials was assessed through the disintegration of a methyl violet solution (0.01 mM) and tetracycline (0.1 mM) without altering the pH levels.During the standard photocatalytic activity, a UVC lamp emitting 253.7 nm with a power of 10 W from Philips in Amsterdam served as the illumination source.Changes in the distinctive absorption bands of the methyl violet and tetracycline solutions were monitored via a UV-vis spectrophotometer (Hitachi U-2900, Tokyo, Japan).When exposed to UVC lamp illumination, these photocatalysts' effectiveness was quantified by the ratio C/C 0 , where C 0 represents the solutions' initial concentrations, and C denotes their concentrations at specific instances.

Conclusions
ZnCo 2 O 4 -Zn(OH) 2 microspheres were effectively created on a carbon cloth using a facile hydrothermal method.The surface features of these microspheres and their ability to break down methyl violet were enhanced by adjusting the thermal annealing temperatures.

Figure 1 .
Figure 1.Describe the reaction mechanism and how to prepare ZnCo2O4-Zn(OH)2 microspheres with the ratio of Zn/Co 1:2 on the carbon cloth.

Figure 1 .
Figure 1.Describe the reaction mechanism and how to prepare ZnCo 2 O 4 -Zn(OH) 2 microspheres with the ratio of Zn/Co 1:2 on the carbon cloth.

Figure 17 Figure 1 .
Figure2a,b shows FESEM images of carbon cloth before and after soaking in a solution containing hydrogen peroxide and sulfuric acid.Before shaking, the carbon cloth surface is quite smooth.However, after soaking, the surface of the carbon cloth displays holes of various sizes.Subsequently, the two types of carbon cloth are placed in a ZnCo 2 O 4 -Zn(OH) 2 reaction precursor at a reaction temperature of 120 • C for 2 h by a facile hydrothermal process, as shown in Figure2c,d.These images reveal that after soaking in the sulfuric acid and hydrogen peroxide mixture, the ZnCo 2 O 4 -Zn(OH) 2 microspheres on the surface of the carbon cloth exhibit a higher density.This result proves that soaking the carbon cloth in a solution containing hydrogen peroxide and sulfuric acid creates more pores on its surface and aids in the subsequent growth of ZnCo 2 O 4 -Zn(OH) 2 microspheres.
Figure 3b-d.Examination of these images reveals that the microspheres are composed of zinc (Zn), cobalt (Co), and oxygen (O), with these components being uniformly distributed throughout.Consequently, this highlights the specific elemental composition of the ZnCo2O4-Zn(OH)2 microspheres.

Figure 4
Figure 4 shows FESEM images of ZnCo 2 O 4 -Zn(OH) 2 microspheres subjected to different thermal annealing temperatures for 2 h.The thermal annealing temperatures are (a) without, (b) 450 • C, (c) 550 • C, and (d) 650• C, respectively.As the annealing temperature rises, there is a progressive emergence of porosity on the surface.The surface morphology of the ZnCo 2 O 4 -Zn(OH) 2 microspheres remains unchanged compared to the unannealed samples, with the only noticeable difference being the appearance of porosity at an annealing temperature of 550 • C for 2 h.When the annealing temperature rises to 650 • C, it becomes apparent that some microspheres have collapsed.This occurrence drastically decreases the reactive surface area, negatively impacting the efficiency of subsequent photocatalytic reactions.The structural collapse observed at higher temperatures underscores the material's thermal instability, highlighting the necessity for meticulous optimization of the thermal annealing process to preserve the desired functional properties of the microspheres.Hence, the BET analyzer can assess the specific surface area of ZnCo 2 O 4 -Zn(OH) 2 microspheres before and after thermal annealing at 550 • C for 2 h.The surface area of the microspheres was measured at 20.78 m 2 g −1 before thermal annealing, and following the annealing process, this value rose to 31.29 m 2 g −1 .This result indicates that thermal annealing effectively enhances the specific surface area of the microspheres.

Figure 5 .
Figure 5.The XRD patterns of ZnCo2O4-Zn(OH)2 microspheres grown on the carbon cloth (a) without thermal annealing and (b) thermal annealing at 550 °C for 2 h, respectively.
reveals the corresponding elemental mapping images of the ZnCo2O4-Zn(OH)2 microsphere, showing the distribution of Zn, Co, and O elements.This result indicates that Zn, Co, and O define the ZnCo2O4-Zn(OH)2 microsphere composition.

Figure 5 .
Figure 5.The XRD patterns of ZnCo 2 O 4 -Zn(OH) 2 microspheres grown on the carbon cloth (a) without thermal annealing and (b) thermal annealing at 550 • C for 2 h, respectively.Molecules 2024, 29, x FOR PEER REVIEW 7 of 17

Figure 6 .
Figure 6.The (a) FETEM image, (b) SAED pattern, (c) HRTEM image, (d) EDS-mapping images of ZnCo 2 O 4 -Zn(OH) 2 microspheres grown on the carbon cloth under the annealing temperature of 550 • C.To analyze the elemental composition and valence state distribution on the surface of ZnCo 2 O 4 -Zn(OH) 2 microspheres, X-ray photoelectron spectroscopy (XPS) was employed.

Figure
Figure 7a displays the full range XPS spectrum of ZnCo 2 O 4 -Zn(OH) 2 microspheres annealed at 550 • C, exhibiting distinct peaks corresponding to C, Zn, Co, and O.These peaks are consistent with the TEM-EDS observations, further confirming the presence of these elements.The carbon element is believed to have its source in the pump oil present in the vacuum system of the XPS equipment, carbon cloth, or an organic layer that has been applied to the surface of the sample.The high-resolution XPS spectrum (Figure 7b) of Zn 2p, showing peaks at 1020.9 eV and 1044.1 eV for Zn 2p 3/2 and Zn 2p 1/2, respectively, confirms the presence of Zn 2+ in the ZnCo 2 O 4 -Zn(OH) 2 structure [35,36].Figure 7c reveals the high-resolution XPS spectrum of Co 2p of ZnCo 2 O 4 , which can be deconvolved into four different states: Co 3+ at 779.5 eV (2p 3/2 ) and 794.5 eV (2p 1/2 ), and Co 2+ at 780.6 eV (2p 3/2 ) and 795.7 eV (2p 1/2 ).Additionally, two vibrational satellite peaks for Co 2+ are located at 789.6 eV near the Co 2p 3/2 band and 804.8 eV near the Co 2p 1/2 band, consistent with previous literature [37,38].The O 1 s spectrum of the synthesized ZnCo 2 O 4 -Zn(OH) 2 microspheres (Figure 7d) reveals a primary peak at 529.5 eV corresponding to lattice oxygen (O L ), along with shoulder peaks at 530.8 eV and 532.1 eV attributed to surface hydroxyl groups (O OH ) and chemisorbed oxygen (O C ) [37,38].Molecules 2024, 29, x FOR PEER REVIEW 8 of 17
29, x FOR PEER REVIEW 9 of 17 leading to a decrease in active sites, which is consistent with the observations from SEM results.

Figure 8 .
Figure 8.(a) Photocatalytic efficiency and (b) kinetic plot of as-prepared photocatalysts for MV solution under the UVC light irradiation.

Figure 8 .
Figure 8.(a) Photocatalytic efficiency and (b) kinetic plot of as-prepared photocatalysts for MV solution under the UVC light irradiation.

Figure 9 .
Figure 9. (a) Photocatalytic efficiency and (b) kinetic plot of the photocatalysts in treating TC solution under UVC light irradiation.

Figure 9 .
Figure 9. (a) Photocatalytic efficiency and (b) kinetic plot of the photocatalysts in treating TC solution under UVC light irradiation.

Figure 12 .Figure 13 .
Figure 12.(a) UV-vis spectra and (b) Tauc plot of ZnCo 2 O 4 -Zn(OH) 2 microspheres and ZnCo 2 O 4 -Zn(OH) 2 microspheres (550 • C).Four radical scavengers were introduced into the photocatalytic reaction to investigate the underlying mechanism of the photocatalysts of ZnCo 2 O 4 -Zn(OH) 2 microspheres during the photodegradation of MV or TC solution, as shown in Figure 13a,b.Isopropyl alcohol (IPA), L-ascorbic acid (AA), triethanolamine (TEOA), and silver nitrate (AgNO 3 ) were utilized as scavengers to impede hydroxyl radicals (•OH), superoxide radical anions (•O 2− ), holes (h + ), and electrons (e -), respectively [44-47].Adding IPA and AA scavengers to the photocatalytic reaction leads to a notable reduction in the photocatalytic efficiency.This outcome provides evidence that hydroxyl radicals (•OH) and superoxide radicals (•O 2− ) are the primary active species involved in the photodegradation of TC.Potential reactions that may occur during the photocatalytic degradation of MV or TC solutions over ZnCo 2 O 4 -Zn(OH) 2 microspheres can be outlined as a schematic diagram, as shown in Figure 13c.When the ZnCo 2 O 4 -Zn(OH) 2 microspheres are exposed to UVC light with photon energy (hv) exceeding their band gap, an electron (e -) in the valence band (VB) can be promoted to the conduction band (CB), creating a hole in the VB and generating electronhole pairs.Photogenerated electrons can interact with oxygen molecules on the surface, forming superoxide radical anions (•O 2 -).These can then react with water molecules absorbed on the surface, producing hydroxyl radicals (•OH).Moreover, the photogenerated holes may combine with H 2 O molecules, causing their dissociation into••OH radicals.These superoxide radical anions and hydroxyl radicals are recognized as potent oxidants responsible for the decomposition of MV or TC molecules.Molecules 2024, 29, x FOR PEER REVIEW 13 of 17