The Synthesis of carbon dots//zincoxide (CDs/ZnO-H400) by using hydrothermal methods for degradation of ofloxacin antibiotics and reactive red azo dye (RR141)

The development of photocatalytic powders to remove contaminants from air solutions is an important field of research in the field of environmental conservation. CD/ZnO-H400, a heterogeneous photocatalytic production, is utilized to degrade the reactive red dye and the antibiotic ofloxacin found in wastewater. This study explains the synthesis of carbon dots (CDs) derived from coconut air and zinc oxide (ZnO) using a hydrothermal method at a temperature of 180 °C with a duration of 4 h and subsequently calcinated at a 400 °C temperature for 4 h. This shows a significant improvement in photocatalytic performance due to improved delivery efficiency at the interface. The cost-efficient use of solar energy allows the comprehensive elimination of harmful pollutants through detoxification. The removal of the contaminant takes place through the first-order reaction, with RR141 showing the highest constant rate at 0.03 min−1, while ofloxacin has a constant speed at 0.01 min−1. The photocatalytic stability is measured after five cycles. The study also tested the impact of sunlight on degradation, showing a degrading rate of 98% for RR141 and 96% for ofloxacin. This study displays a new catalyst powder synthesized from carbon dots derived from the air, coconut and ZnO, showing remarkable photoactivity to completely remove harmful dyes and antibiotics from the surrounding environment.


Methods
Synthesis the carbon dots (CDs) By following Nugroho et al. 14 in the previous literature, carbon dots were synthesized by using 20 mL of coconut water combined with 20 mL of ethyl alcohol, and the hydrothermal synthesis process was carried out in an oven at 180 °C for 4 h.After synthesis, the product was centrifuged at 10,000×g rpm for 10 min.After the process, it was filtered through a 0.22 µm nylon filter membrane and dried in an oven for 24 h.Synthesis CDs/ZnO by hydrothermal methods (CDs/ZnO-H) 10 mL of CDs and mixed with 0.1-g ZnO.The synthesis process was performed by a hydrothermal method, by heating in an oven with temperatures 180 °C and times 4 h After synthesis, dry in the oven for 24 h to become powder.
Synthesis CDs/ZnO by precipitation methods (CDs/ZnO-P) 10 mL of CDs and mixed with 0.1-g ZnO.The synthesis process was performed by stirrer methods in the Room temperature for 24 h, after mixing the CDs/ZnO dry in the oven for 24 h to become powder.
Synthesis CDs/ZnO-H400 and CDs/ZnO-P400 After synthesized the CDs/ZnO-P and CDs/ZnO-H, 1 g of each powder has been prepared and heated in the 400 °C for 4 h under calcination methods by using close system.

Photodegradation of pollutants RR141 azo dye and ofloxacin antibiotics
The photocatalytic activity under UV light (a mercury lamp, 125W), The catalyst's photoactivity was assessed through the degradation of the RR141 azo dye and Ofloxacin antibiotic (OFL Antibiotics).50 mg of the photocatalyst that had been produced was combined with a pollutant solution that had been prepared with a final volume (200 mL and a concentration of 10 mg L −1 ).The suspension was agitated in a dark room environment for a duration of 60 min to establish an equilibrium state of adsorption-desorption for the photocatalyst.Following a 60 min period under dark conditions for the photo irradiation process, a 5 mL sample was collected and put through centrifugation at 5000 RPM for 10 min.This centrifugation step was performed to obtain the supernatant, which was subsequently analyzed for concentration using the UV-vis spectrophotometric method.The concentration was determined by measuring the absorbance at wavelengths of 544 nm (RR141) and 293 nm (OFL Antibiotics) 1, 26,27 , respectively.The photodegradation efficiency was assessed using the equation for photodegradation efficiency, photoactivity (%) = ((1 − C/C 0 ) × 100%), which is calculated as the percentage of degradation based on the difference between the initial concentration (C 0 ) and the concentration of the pollutant aqueous solution (C) after a given period of irradiation.The degrading response of the pollutant through photocatalysis follows a kinetic model that may be described by the first-order reaction rate equation, ln (C 0 /C) = k1t, where k1 represents the rate constant associated with the first-order reaction.The investigation focused on the impact of several experimental parameters, such as the initial solution pH, photocatalyst loading, and initial pollutant content, on the photoactivity during the examination of dye and antibiotic degradation 1,5,6,9 .In order to investigate the key species responsible for the elimination of the pollutant, individual scavengers (at a concentration of 5 mM) including isopropyl alcohol (IPA), EDTA-2Na, and K 2 Cr 2 O 7 were introduced in this study.These scavengers were employed to specifically quench hydroxyl radicals, superoxide anion radicals, holes, and electrons, respectively.Furthermore, KI was also employed as a scavenger of surface hydroxyl radicals and holes.The individual scavengers were integrated individually in the presence of the prepared photocatalyst 1,9,68 .

Photodegradation of RR141 and ofloxacin antibiotic under sunlight
This study aimed to analyze the photodegradation of RR141 and OFL antibiotics in sunlight.In March 2023, the RR141 (22 March, 11.00 a.m.) and OFL Antibiotics (26 March, 11.00 a.m.) solution conducted sunlight exposure on the rooftop of Science Building Number 04, located inside the Faculty of Science at Khon Kaen University, Thailand.The geographical coordinates of the exposure site were recorded as latitude 16°28′ 33.7″N and longitude 102°49′ 26.2″E (Fig. 1).

Characterization of CDs and various conditions CDs/ZnO
The XRD Data of CDs/ZnO in the various conditions of synthesis is shows that CDs/ZnO-H, CDs/ZnO-P, CDs/ ZnO-H400, and CDs/ZnO-P400 (Fig. 2A) show have a strong crystal peak and shows diffraction peaks of the ZnO at 2θ = 31.7°,34.6°, 36.2°,47.7°, 56.6°, 62.7°, and 67.8°, due to (100), (002), (101), (102), (110), (103), and (112), reflection planes, peaks of CDs = 20.8(002) the crystallites (grains) size from XRD data has been analyst by using formula D = (kλ/β cos θ) and the average of crystallites of CDs, CDs/ZnO-H, CDs/ZnO-P, CDs/ZnO-H400, and CDs/ZnO-P400 is 0.2 nm, 42.3 nm, 37.1 nm, 41.69 nm and 38.9 nm (Table 2).The UV-vis diffuse reflectance spectra of the samples are displayed in Fig. 2B.The Kubelka-Munk formula was applied to figure out the band gap energy of each sample 5 .Figure 2C illustrates this process.Consequently, the absorption edge values of 387 nm were observed in the case of ZnO.The band gap values of bare CDs/ZnO-H, CDs/ZnO-P, CDs/ZnO-H400, and CDs/ZnO-P400 were determined to be 3.16, 3.17, 3.15, and 3.07 eV, respectively.The data indicates that CDs/ZnO synthesized using various methods exhibit a heterojunction structure, as evidenced by the presence of only one energy bandgap that have been show in the result, Based on the photoluminescence (PL) spectra illustrated in Fig. 2D, it can be observed that the CDs/ZnO-H400 photocatalyst exhibited the least  intensity in the PL spectra when compared to the other experimental conditions.This observation shows that CDs/ZnO-H400 photocatalyst had the lowest rate of charge carrier recombination.The morphological structures of CDs, CDs/ZnO-H400 has been analyst by using transmission electron microscopic (TEM) and field emission scanning electron microscopic (FE-SEM) technique.The FE-SEM of CDs, CDs/ZnO-H400 shown in Fig. 3, CDs from coconut water has the characterization and it's show that the carbon dots has the characterize is melting in the room temperature (Fig. 3C), but after mixed with the ZnO by using hydrothermal methods, and calcination in the temperature 400 °C and 4 h it's had unstructured spherical morphology and CDs has been coating onto the ZnO structured that can be seen in Fig. 3D. the particle size of CDs shown in Fig. 3A with the average particles sized of ± 2 nm. Figure 3B illustrates the deposition of ZnO following treatment with CDs, resulting in significantly higher particle size compared to the crystallite size determined through XRD analysis.This increase in size can be attributed to the aggregation of several crystallites.in Fig. 3.In addition, the composition of the CDs/ZnO-H400 and CDs was also analyst for elemental structured by used EDX.The spectrum of the CDs (Fig. 3E) and CDs/ZnO-H400 (Fig. 3F), Fig. 3E indicated the CDs has element existence of Carbon (C), oxygen (O), and copper (Cu) elements with the weight% of C, O and Cu are 72.8%,25.0% and 2.3%, respectively.The element of CDs/ZnO-H400 shown on Fig. 3F is Zinc (Zn), Carbon (C), and Oxygen (O), with atomic% are 59.8%, 29.6% and 10.6%, respectively.The stoichiometry CDs/ZnO-H400 result of element confirmed that the 2:1 between Zn and C and 5.5:1 atomic ratio of Zn with O. X-ray photoelectron spectroscopy (XPS) was employed to ascertain the elemental composition and chemical oxidation state of CDs/ ZnO-H400 at the surface.The spectra that illustrated in Fig. 4. The survey scan spectra depicted in Fig. 4A demonstrate the presence of Zn, O, and C elements inside the CDs/ZnO-H400.The spectra depicted in Fig. 4B performed deconvolution, resulting in the identification of two distinct peaks originating from the pristine ZnO material.These peaks were found to be positioned at 1021.1 eV and 1044.2 eV, corresponding to the Zn 2p3/2 and Zn 2p1/2 states, respectively 65 .From these XPS results we can find out that CDS/ZnO-H400 has a heterostructure elemental composition.The observed difference in binding energies, around 23 eV, suggests the presence of Zn in the ZnO sample in the form of Zn 2+ .The O 1s spectrum displayed in Fig. 4C exhibited a notable level of resolution.Through deconvolution analysis, it was determined that this spectrum consisted of two distinct peaks located at 530.4 eV and 531.6 eV.These peaks were attributed to the presence of adsorbed oxygen originating from the surface hydroxyls 28 .Figure 4D illustrates the resolution of the C 1s spectrum, wherein the data was subjected to deconvolution resulting in the identification of two different peaks at 285.4 eV and 288.9 eV.These peaks can be attributed to the presence of C-C and O-C=O bonds, respectively 51 .

Photoactivity
The evaluation of the photocatalytic effectiveness of each photocatalyst was performed by observing the photodegradation of the RR141 azo dye and the ofloxacin antibiotic.The concentrations RR141 and OFL antibiotic had measured by utilizing their respective λmax values of 544 nm and 293 nm, respectively 6 .Additionally, an investigation was conducted on the photodegradation process of OFL Antibiotics and RR141 in the presence of sunlight.

Photodegradation of RR141 and OFL antibiotics under Uv-light
The analysis includes the examination of the photodegradation of RR141 dye, specifically focusing on the decrease in dye concentration (C/C 0 ) over time (t) following exposure to UV light as shown in Fig. 5A, the adsorption The concentration of OFL antibiotics (C/C 0 ) after UV light irradiation was examined in order to determine the photodegradation of the antibiotic OFL Antibiotics, the removal of the dye OFL antibiotic via adsorption process has been studied.The integration of CDs/ZnO-H, CDs/ZnO-P, CDs/ZnO-H400, and CDs/ZnO-P400 resulted in a decrease in the C/C 0 ratio with increasing UV irradiation time (Fig. 5A).Specifically, the application of CDs/ZnO-H400 corresponded to a decrease in the C/C0 ratio from 1 to 0.004 after 240 min.the degradation efficiency of OFL antibiotic by using CDs/ZnO-H400 shows 99%.The ofloxacin was photodegraded 99% in the presence of the nanocomposite.The photodegradation rate constant (k) of CDs/ZnO-H400 shows in the Fig. 5D, photocatalyst provided the UV-driven photodegradation rate constant (k) of 0.01 with R 2 0.99.

Photodegradation of RR141 and OFL antibiotics under sunlight
The concentration (C/C 0 ) of RR141 by uv irradiation under sunlight has been examined in order to determine the photodegradation of the RR141, as shown in Fig. 6, the removal of the dye RR141 is relatively fast.The incorporation of CDs/ZnO-H, CDs/ZnO-P, CDs/ZnO-H400, and CDs/ZnO-P400 resulted in a decrease in the C/C 0 ratio with increasing UV irradiation time (Fig. 6A), reaching values of 1 and 0.04 after 240 min, respectively.The degradation efficiency of RR141 using CDs/ZnO-H, CDs/ZnO-P, CDs/ZnO-H400, and CDs/ZnO-P400 was found to be 92%, 94%, 98%, and 97%, respectively.The photoactivity of the catalysts was assessed by measuring the rate of the photodegradation reaction.In Fig. 6B, the UV-driven photodegradation rate constants (k) of CDs/ZnO-H, CDs/ZnO-P, CDs/ZnO-H400, and CDs/ZnO-P400 photocatalysts were found to be 0.01, 0.02, 0.01, and 0.01, respectively.The effect of Uv irradiation under sunlight was examined in order to determine the photodegradation of the OFL Antibiotics, as shown in Fig. 6C.In addition, removal of the OFL Antibiotics is moderate, after incorporation of CDs/ZnO-H, CDs/ZnO-P, CDs/ZnO-H400, and CDs/ZnO-P400, lowering of C/C 0 with UV irradiation time (Fig. 6C) from 1 to 0.07, 0.09, 0.03 and 0.06 respectively, was detected after 240 min.the degradation efficiency of antibiotic OFL Antibiotics by using CDs/ZnO-H, CDs/ZnO-P, CDs/ZnO-H400, and CDs/ZnO-P400 shows 92%, 90%, 96% and 93% it's mean the optimum condition is by using CDs/ZnO-H400.The determination of the catalysts' photoactivity was also conducted based on the rate of the photodegradation reaction.The photodegradation rate constant (k) of CDs/ZnO-H, CDs/ZnO-P, CDs/ZnO-H400, and CDs/ZnO-P400, in Fig. 5D shown According to the theoretical framework, the valence band (VB) and conduction band (CB) potentials of ZnO were determined to be 2.81 and − 0.23 electron volts (eV), respectively (Fig. 7).In this study, it was observed that the formation of a heterojunction resulted in a measured conduction band potential of ZnO at − 0.22 eV.The band gap energy is measured to be 3.04 electron volts (eV) 1,5,6,28 .

The impact of experimental parameters on photoactivity.
The various conditions to carry out the test at the optimum condition were carried out under various conditions, including the effect of pH, the effect of catalyst concentration, and the effect of concentration of RR141 (Fig. 8). Figure 8A shows that the condition of adding pH7 buffer has optimum conditions compared to the others, with an efficiency of 97.4%.Meanwhile, the optimum condition of various catalyst concentrations is 50 mg (Fig. 8B), and the optimum condition of concentration RR141 is 5 ppm (Fig. 8C).Studying the impact of the experimental factors on OFL antibiotic degradation is shown in Fig. 8D.The pH natural of the antibiotic OFL solution is around 7. Contrarily, in the fundamental pH range between 9 and 11, the anionic OFL antibiotic and the presence of a negative charge on the surface of the photocatalyst cause them to repel one another.a decrease in the extent of adsorption of OFL antibiotics onto the photocatalyst's surface could potentially lead to a decline in the catalytic activity of the photocatalyst.The enhanced adsorption of anionic OFL on the photocatalyst's positively charged surface was ascribed to the heightened photoactivity seen at a pH 5 5,6 .It should be noted that under extremely acidic conditions (pH of roughly 3), the photocatalyst can dissolve.It was also investigated how the photocatalyst content affected the photodegradation of OFL (Fig. 8E).Due to an increase in the number of OFL molecules adsorbed on the photocatalyst's surface and an increase in photocatalyst particle density per unit area  of photo irradiation, photoactivity increases as photocatalyst content increases 7 .However, the increase in solution turbidity, which in turn causes a decrease in photo penetration, is what causes the photoactivity to decline at a photocatalyst level of 75 mg.The result will be a decrease in photoactivity.Additionally, it was established how the concentration of OFL affected the photoactivity (Fig. 8F).The performance of the photocatalytic system is decreased by the addition of OFL concentration.The amount of light absorbed by the OFL molecules increases with the concentration of OFL.This could result in a reduction in the amount of light that reaches the surface of the photocatalyst, which would diminish its effectiveness 8 .With 5 ppm of OFL.The produced nanocomposite photocatalyst's photoactivities towards the breakdown of the antibiotic OFL under sunlight were also has been studied 7 .

The effect of trapping agent and reusability
The trapping experiment by using various trapping agents has been studied for RR141 and OFL Antibiotics.In this study investigated the impact of certain reactive species on the breakdown process of the OFL antibiotic.Specifically, the effectiveness of different scavengers, including EDTA, IPA, K 2 Cr 2 O 7 and KI were used as a quencher of ⋅OH, h + , and e -, respectively.Figure 9 illustrates a significant decline in photoactivity subsequent to the photo chat reaction with KI.The efficiency of the reaction was observed to be 14%, accompanied by a notably low rate constant when compared to the absence of a scavenger mechanism.In general, the photogenerated hole is a crucial factor in the process of photodegradation of OFL Antibiotics.Similar to an OFL antibiotic, an OFL Antibiotics antibiotic exhibits comparable characteristics and properties.The efficacy of different scavengers was evaluated in the RR141 experiment.It was observed that the photoactivity significantly decreased following the photo chat interaction with EDTA.The effectiveness of the scavenger process was determined to be 21%, with a notably low corresponding rate constant compared to the absence of any scavenger process..This investigation also represents a study of the powder's catalytic cycle capabilities.Antibiotic elimination of OFL and RR14 dye was tested in five consecutive trials.After the initial round of photocatalytic investigation, the catalyst underwent the separation process using the whatman 1 paper filter, was cleaned with DI water, and dried in a 60 °C oven for 12 h before being used in the next round until the fifth round.The fifth run of the photocatalyst CDs/ZnO-H400, as illustrated in Fig. 10, demonstrates its remarkable cycling ability.The degradation performance of RR141 by the photocatalyst shows a minimal decrease from 96% (4th run) to 98% (1st run).Similarly, the degradation performance of OFL Antibiotics by the photocatalyst exhibits a minimal decrease from 96% (5th run) to 99% (1st run).The nanocomposite photocatalyst exhibited a high level of cycling performance, thereby confirming its good ability in cycling.Furthermore, Fig. 11 presents evidence of the structural stability of the catalyst that was synthesized.The FE-SEM analysis, as shown in Fig. 11A-C, reveals that the SEM images obtained before and after the photodegradation investigation show a similarity result.The X-ray diffraction (XRD) spectra (Fig. 11D) indicates that the sample morphology of both the fresh and used CDs/ZnO-H400 is comparable, suggesting their stability.This observation is further supported by the similarity in the XRD spectra before and after the photocatalytic process, as well as the FTIR spectra (Fig. 11E).

Conclusion
Carbon dots from coconut water have been synthesized using hydrothermal methods, and mixed with Zinc Oxide (ZnO) using the same method, after being powdered, the powder is put into the calcination process at a temperature of 400 °C and takes 4 h to become the final powder.In this experiment, CD/ZnO-H400 showed a promising catalyst to reduce the RR141 and OFL antibiotics.The study also investigated the photocatalytic performance under sunlight and revealed a maximum degradation efficiency of 98% for RR 141 and 96% for the OFL Antibiotic.The decay of pollutants through photolysis follows the first order reaction.The optimum conditions

Figure 7 .
Figure 7. Mechanism scheme photocatalytic degradation of organic pollutants (RR141 and Ofloxacin antibiotics) under solar light irradiation.

Figure 8 .
Figure 8.The effect of various conditions of pH (A), catalyst concentration (B) and (C) concentration of RR141, various conditions of pH (D), catalyst concentration (E) and (F) concentration of ofloxacin.

Figure 9 .
Figure 9.The Concentration (C0/C) by photodegradation on ofloxacin (A) the constant rate obtained from degradation of ofloxacin after adding various scavenger (B) degradation efficiency on ofloxacin (C) The Concentration (C0/C) by photodegradation on RR141 (D) the constant rate obtained from degradation of RR141 after adding various scavenger (E) degradation efficiency on RR141 (F) (SD, 3 replication).

Figure 10 .
Figure 10.Repetition of photocatalyst toward degradation of RR141 dye and OFL antibiotic by using CDs/ ZnO-H400 (a) Uv light (sun light).

Table 2 .
Crystallite size of catalyst powder.